Transactions of the Geological Society of South Africa, 77 (1), 1-15
An intrusion of gabbroic rocks with a longer axis of 13,5 km and the form of a cauldron subsidence occurs in the North-eastern Cape. Emplacement, accompanied by uplift of Stormberg Group sediments, was followed by collapse and fragmentation of the roof. Huge xenoliths now occur as much as 700 metres above their normal level in undesturbed areas. Metasomatism and metamorphism culminated in extensive vitrification in places. Fusion of Cave Sandstone produced sanidine-bearing granophyres. Intersertal tholeiites chemically akin to the iron-enriched dolerites of New Amalfi or melanogranophyres of the Skaergaard intrusion were emplaced in the earliest stages. Fractionation of later, olivine-normative, Kokstad-type magma led to pegmatitic derivatives. The area was a centre of late-Karoo volcanic activity in which volcanic breccias and minor basaltic flows accumulated above the Cave Sandstone prior to the effusion of Stormberg basalts. Emplacement of the complex, at a high level, appears to have pre-dated the main phase of flood basalt effusion.
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Transactions of the Geological Society of South Africa, 77 (1), 17-29
The geology of an area in the southwestern Bushveld Complex between Rustenburg and Pretoria is described. The mapping was carried out by the multiple exposure method. Further evidence is presented of the transformation of sedimentary rocks of the Transvaal System into "igneous" rocks of the Bushveld Complex. It is postulated that quartzite is transformed into norite whereas shale and limestone pass into pyroxenite and serpentinite. Magnetite bands in the upper part of the succession are found on the same horizons as outcrops of limestone north-east of Pretoria. Variations in thicknesses are used in support of the transformation theory. Chemical analyses of sedimentary, intermediate and basic rocks are presented and also indicate that transformation has taken place. It is suggested that silica, alumina and alkalies were transferred to higher levels in a three- dimensional continuum while new oxides such as magnesia, lime and iron were introduced from below. It is further suggested that the transformation of the sediments disturbed the stability in the infra- and super-structures and caused sagging, slumping, fracturing or folding of the rocks under the influence of gravity.
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Transactions of the Geological Society of South Africa, 77 (1), 31-36
The mineralogy, the petrology, and the petrochemistry of ten kimberlites from Pofadder in Namaqualand are described. It is shown that these pipes are kimberlites similar to those from the Kimberley area. The Pofadder kimberlites can be classified as basaltic kimberlites after the nomenclature of Wagner (1914), and as autolite kimberlite breccias after the terminology of Rabhkin and others (1962). The compositions of the garnets from the Pofadder kimberlites are slightly more Fe-rich and Mg-poor than the average of the Kimberley kimberlites. The ilmenites are consistently more Fe3+ and Mg-rich than the ilmenites from kimberlites as was determined by Frick (1973). The presence of kimberlites as far north as Pofadder in Namaqualand adds the distinct possibility that not only were the diamonds along the west coast of South Africa derived from kimberlites in this area (De Villiers and Söhnge, 1959; Keyser, 1972), but that the diamonds in the Orange River at Sendelingsdrift and Kheis, and also at Alexanderbaai (Keyser, 1972), could have been derived from kimberlites in Namaqualand.
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Transactions of the Geological Society of South Africa, 77 (1), 37-51
The significance of tridymite in geobarometric and geothermometric studies is investigated. It is concluded that in lavas, or where its deposition is associated with a varpour phase, the presence of tridymite is not meaningful. In the sanidinite facies quartz inverts readily to tridymite, which may also occur in the associated intrusives; evidence points to its being a reliable indicator of temperatures above 870°C and low pressures in this environment. Electron microprobe analyses establish that Na, K and Al are taken into the lattice on conversion of quartz to tridymite in contact-metamorphised rocks. On cooling, tridymite inverts to quartz, and this may leave two genetically different varieties of quartz with different levels of impurities in the same host. The melting behaviour of two sediments and a partially vitrified dolerite clast in metamorphosed volcanic breccia is traced by analyses of whole-rock samples and their glass fractions. P-T conditions are deduced for the contact-metamorphosed rocks of the Birds River Complex and it is concluded that tridymite formed here in its field of stability. Field work indicates that the base of the Stormberg Group defined the approximate position of the limiting isobar for the formation of the formation of tridymite in the Upper Karoo sequence of the North-eastern Cape.
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Transactions of the Geological Society of South Africa, 77 (1), 53-57
The Cape fold-belt represents an orogeny of Alpine style with vergence to the north; the folding is of similar type and is associated with pervasive shear movement along a regional plane of transport, which has produced a continuously mappable axial-plane cleavage or foliation. During the orogeny the pre-Cape rocks, including both metasediments and granites, also responded by shear along the regional plane of the transport and now show folding and cleavage or foliation parallel to an of the same age as those in the Cape cover-rocks. Whereas the latter were folded on nearly horizontal axes during the orogeny, the plunges of folds in the older sediments range from horizontal to nearly vertical; the axes of these folds reflect the intersection of the Cape plane of transport with pre-Cape bedding whose attitude ranged from horizontal to vertical depending on its position on earlier broad folds with steep limbs and a west-north-westerly trend, which register a pre-Cape period of deformation.
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Transactions of the Geological Society of South Africa, 77 (1), 59-64
The shape of pebbles deposited in beach or fluviatile environments is considered to be diagnostic. Pebbles from beach deposits tend to be statistically flatter and less spherical than those from river deposits. To overcome the use of arbitrary measurements the flatness and the sphericity of the pebbles were determined from measurements of the length of the long, intermediate, and short axes of pebbles.
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Transactions of the Geological Society of South Africa, 77 (1), 65-67
Certain aspects of the geochemistry of the platinum-group elements are briefly discussed. It is shown that discrete platinum-group minerals occur as disseminations in ultramafic rocks which are the source of some placer concentrations of platinum-group minerals. The uncertainty of quantitatively assessing which are the principal platinum-group minerals in the Sudbury, Noril'sk, and Monchegorsk deposits is discussed. Present published data suggest that sperrylite, michenerite, and possibly palladian melonite, are the principal platinum-group-bearing minerals in the Sudbury deposits. Palladium predominates over platinum at the Taimyr Mine, Noril'sk, where the principal palladium minerals are "solid solutions of palladium with platinum, lead, and tin" as well as "intermetallic combinations of palladium with lead and tin", and the principal platinum minerals are iron-bearing platinum, native platinum, and sperrylite (Genkin, 1968). The disseminated and breccia-ores underlying the Noril'sk 1 intrusive are, on the other hand, richer in platinum and palladium sulphides. The Cu-Ni ores of Monchegorsk are different from both of these as they contain mainly tellurides of platinum and palladium (Genkin, 1968). It is concluded, from data presently available, that there is a low probability that significant quantities of platinum-group elements occur in solid solution in the sulphides of the Cu-Ni deposits of Sudbury and Noril'sk. The lower tenor of palladium is placer deposits, compared to that in Cu-Ni sulphide deposits, may be due, in part, to its greater solubility and subsequent dispersal and, in part, due to its scarcity in the ultramafic source rocks.
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Transactions of the Geological Society of South Africa, 77 (1), 69-70
Pinnacle form subsurface solution phenomena, similar to those of the North-west Cape Province, are reported from Lyttelton and Irene, south of Pretoria. The exposure of stratified Ecca Grits at 6 m depth is considered to indicate sinkhole collapse contemporaneous with pinnacle formation by active karst solution. The implication for the dating of the period of karst solution and the beveling of the present surface, as post-Karoo, are indicated.
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Transactions of the Geological Society of South Africa, 77 (1), 71-72
Rb-Sr age measurements have been made on six samples of basic lavas from the Hartley Hill Stage of the Matsap Formation of the Northern Cape. Five of these define a linear isochron yielding an age of 2 070 plus-minus 90 million years (λ=1,39 x 10-11 / years).
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Transactions of the Geological Society of South Africa, 77 (2), 105-108
A "diorite porphyrite" dyke for which a Damaran age had been assumed is described. According to its petrology it should be called a dacite porphyry. Geological and petrological evidence suggest a Karoo age for this dyke.
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Transactions of the Geological Society of South Africa, 77 (2), 109-111
Petrographic examination of some Ecca and Beaufort beds has shown that most of the sandstones contain subordinate amounts of volcanic detritus and zeolite. One mudstone from the Ecca includes replaced glass shards; this is the first report of contemporaneous volcanic activity during Ecca time. Evidence of contemporaneous igneous activity in the Gondwanide Orogen from South America to Antarctic is reviewed briefly.
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Transactions of the Geological Society of South Africa, 77 (2), 113-116
Graywackes rich in volcanic fragments and tuff layers are described in the Uppermost part of the Dwyka Series, the Ecca Series and the Lower part of the Beaufort Series of the southern part of the Karoo basin. They indicate Permian volcanic activity which was situated to the south of the African Craton. An origin in Permian Patagonia is postulated, when this part of South America was close to Southern Africa before continental drift. An hypothesis concerning the relationship of this volcanism with the Cape Orogeny is proposed.
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Transactions of the Geological Society of South Africa, 77 (2), 117-129
Recent mapping of the lower part of the Drakensberg Subgroup in the District of Barkly East indicates that normal stratigraphic techniques can be applied to the study of the volcanic rocks. A number of lithostratigraphic units, some formal and some informal, are recognised and described. The stratigraphic studies are preliminary to the elucidation of the regional structure and of the complex volcanic history of the area.
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Transactions of the Geological Society of South Africa, 77 (2), 131-140
Middle Ecca sandstones - mainly subarkoses - from boreholes in the northern part of the northern Karoo basin often have good porosity and permeability in contrast with those from farther to the south. A petrographic study of these rocks has shown that the effective porosity and related permeability are of secondary origin and have been produced by leaching of carbonate cement. This took place after the destruction by cementation, compaction and other diagenetic changes of almost all intergranular porosity. The occurrence of secondary porosity is associated with kaolinisation of feldspars and micas in the sandstones, and the presence of kaolinite in associated shales. Its geographic distribution coincides roughly with that of the Middle Ecca coal deposits. It is suggested that carbonates were dissolved and kaolinite formed by the action of solutions produced during the diagenetic transformation of montmorillonite to illite, and which contained CO2 derived from the diagenesis of coal and coaly organic matter.
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Transactions of the Geological Society of South Africa, 77 (2), 141-149
The Pleistocene Port Durnford Formation, which is intermittently exposed in seacliffs along the southern Zululand coast, consists of a Lower Argillaceous Member up to 10 m thick, containing mammalian remains, marine fossils, crustacean burrows and wood debris, separated by a persistent lignite bed averaging 1,3 m thick from an upper cross-bedded Arenaceous Member up to 15 m thick. This vertical succession, which has its base below lowest spring tide, is interpreted as representing the intermittent landward accretion of a barrier complex (cross-bedded sands) over back-barrier swamps (lignite) and lagoonal- shallow marine facies (fossiliferous clays with subordinate sandstones). Deposition was associated with a marine transgression which culminated in a sea-level approximately 8 m above the present, an event most probably related to the Sangoan (Eem) Interglacial Stage. This barrier-lagoon complex provided a framework around which later Pleistocene and Holocene deposition took place. It is mantled by aeolian coversands, presumably deposited during the Wisconsin (Weichsel) Marine regression. The Port Durnford Formation affords important stratotypes for transgressive shore-zone deposition of Pleistocene age in south- east Africa, and a valuable field laboratory for studying the sedimentation patterns associated with barrier-lagoon complexes, probably comparable with similar Holocene features at present forming across Natal estuaries.
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Transactions of the Geological Society of South Africa, 77 (2), 151-158
Die dreineringsdigthede en stroomfrekwensies in die twee dreineerstelsels word bespreek met betrekking tot die litologie, gronde en gebiedsgradiënte. Hieruit blyk dit dat daar wel 'n verband bestaan tussen al drie die veranderlikes en die betrokke eienskappe van die dreinering. Die gevolktrekking word egter gemaak dat die gebiedsgradiënte waarskynlik die belangrikste enkele faktor is in die bepaling van die dreineringsdigthede en die stroomfrakwensie in die navorsingsgebied.
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Transactions of the Geological Society of South Africa, 77 (2), 159-168
Sedimentary ores are those formed primarily by gravitative settling of mineral particles at or near the surface of the earth, followed by diagenetic cementation within the parent formation. Modern classifications of mineral deposits are based on the combined parameters of environment and process within the framework of the geosynclinal cycle. In this context, research has branched out into diverse interconnected fields. Tectonic Setting: with respect to craton and orogen or specific parts of a developing geosyncline, the localization of stratiform ore-types is much more varied than portrayed in individual models. Paleophysiography: The lower slopes of former islands and seamounts are favoured sites for stratabound base-metal deposits. Intraformational karst development provides openings in which diagenetic ore may be trapped much later. Sedimentary Petrology: The mineral make-up, texture and primary structures of the associated sediments are more basic to understanding the environment and the concentration process than the ore itself. Submarine Volcanism: Many layered base-metal deposits are directly or indirectly related to specific volcanic episodes. Petrographic evidence is easily overlooked or may be found hundreds of kilometres away from the ore bed. Biogeochemistry and Paleoecology: Some of the largest Precambrian iron, manganese and copper deposits have been generated with the help of metabolic processes of micro-organisms on land and in the oceans. Stratiform limestone-lead-zinc ores of Phanerozoic age are widely associated with algal reefs of epicontinental seas. Diagenesis: Migrating pore-waters may collect and concentrate metals dispersed through the strata, or alter the original sediment to valuable mineral matter. Some water-saturated beds have been mobilised to intrusive slurry associated with sulphide deposition. Diverse sedimentary ore-types of the Damara geosyncline illustrate some of the principles set out in the text.
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Transactions of the Geological Society of South Africa, 77 (2), 169-183
The geology, the mineralogy, the petrology and the petrochemistry of the sillimanite occurrences west of Pofadder are described, and fifty-three new chemical analyses presented. Two types of sillimanite mineralization can be distinguished, namely, a corundum-sillimanite type, which is confined to a pelitic succession, and a folded sillimanite type which occurs as lens-shaped bodies in a psammitic succession. The data indicate that the former group represent well preserved fossil bauxites which were metamorphosed in the almandine amphibolite facies. The rocks were transformed into corundum-pyrophyllite-bearing schists in the greenschist facies of metamorphism, and were not deformed during tectonic activity owing to the rigidity of corundum, resulting in lens-shaped bodies of corundum-sillimanite in a micaceous schist. The folded sillimanite deposits, on the other hand, represent contaminated bauxite deposits which were metamorphosed to muscovite pyrophyllite schists under greenschist conditions. Subsequent tectonic activity caused the micas to be concentrated in the crests of anticlines and the troughs of synclines as a result of flow-folding of competent mica schist in the psammatic succession. Progressive metamorphism transformed the pyrophyllite and the muscovite into sillimanite and water, in which the alkalies may have been dissolved. The latter solution either metasomatized the country-rock or accummulated as a mica pegmatoid at the contacts of the sillimanite body with the country-rocks. Guide-lines for future exploration for concealed sillimanite deposits of both types are mentioned.
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Transactions of the Geological Society of South Africa, 77 (2), 177-129
Recent mapping of the lower part of the Drakensberg Subgroup in the District of Barkly East indicates that normal stratigraphic techniques can be applied to the study of volcanic rocks. A number of lithostratigraphic units, some formal and some informal, are recognised and described. The stratigraphic studies are preliminary to the elucidation of the regional structure and of the complex volcanic history of the area.
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Transactions of the Geological Society of South Africa, 77 (2), 185-190
Chemical, X-ray diffraction, and infra-red absorption data are given for pelletal phosphorite, rock phosphorite and phosphatised bone from late Tertiary sediments of the south-western Cape Province. The apatite, the major component of phosphorite, has in all cases a high carbonate content. The fluorine content for all samples except the Hoedjiespunt apatite is also high and defines the mineral as francolite. The a-cell dimension for the francolite ranges from 9,34 to 9,36Å. At Hoedjiespunt the low fluorine content, less than 1 per cent, defines the apatite as dahllite, a carbonate-hydroxyapatite. The a-cell dimension for the dahllite is 9,42Å. There appears to be a contraction of the a-cell dimension in the francolite with increasing carbonate substitution for phosphate.
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Transactions of the Geological Society of South Africa, 77 (2), 191-199
Coarse (~1,0mm) grains of quartz sand of Kalahari type from widespread localities in central and southern Africa were examined under the scanning electron microscope, and their surface textures recorded. In spite of extensive surface reworking by wind and water, the earliest texture seen on all grains is upturned plating of aeolian origin. This persists in re-entrants as a palimpsest texture even when overprinted by later events. It is concluded that this evidence supports the view that the original distribution process was aeolian. Quaternary redistributions of Kalahari sand around the Victoria Falls have also been examined. Again the original aeolian texture is present, but largely obliterated by mechanical polishing in a subaqueous environment, confirming the optical results of Bond (1955). Sharp triangular etch-pits are developed on all subaqueous samples from this area in spite of the high parity and neutral pH of Zambezi water. A few samples from outside the main area of Kalahari sand are included for comparison.
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Transactions of the Geological Society of South Africa, 77 (2), 201-206
Selection of suitable projection planes and recognition of trend lines in the CMAS tetrahedron and its subprojections is facilitated by stereoscopic viewing of pairs of suitably orientated tetrahedra. Construction of such tetrahedra is readily accomplished by computer methods.
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Transactions of the Geological Society of South Africa, 77 (2), 207-209
Newton (1973) has presented a much-needed synthesis of the tectonics of the Cape fold-belt and has raised some interesting questions concerning the genesis of the mountain range. His analysis stresses the absence of synorogenic magmatism and metamorphism and demonstrates the décollement nature of the folding. He ascribes the folding to gravitational sliding from the uplifted southern margin of the Cape-Karroo sedimentary basin and he questions the relevancy of modern plate tectonic theory in the structural interpretation of the Cape fold-belt. Instead he favours a regime of essentially vertical tectonics to the south of the fold-belt with related downslope gravity-folding. My comments are concerned mainly with the broader implications of Mr. Newton's model. Two major aspects of the problem are considered in the following discussion: (1) Was the Cape-Karroo sedimentary basin intracratonic or was it located on a developing continental margin? (2) Did the southern land mass evolve during late Paleozoic-early Mesozoic time as a youthful island-arc type of orogenic welt? In view of the available evidence, I suggest that the Cape fold-belt can be readily interpreted in terms of lateral plate motions as predicted by the new global tectonics.
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Transactions of the Geological Society of South Africa, 77 (2), 73-77
A detailed gravity survey over an outcrop of greenschists of the Nyanzian System in northern Tanzania is described. The outcrop measures nearly 100 by 60km, and is found to be associated with a positive Bouguer gravity anomaly of 330 g.u. Interpretation of the anomaly suggests that the greenschists have the shape of shallow basin, with a maximum depth of nearly 6km. The contacts between the greenschists and the surrounding granites are most likely intrusive in character.
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Transactions of the Geological Society of South Africa, 77 (2), 79-84
Gravity surveys in northern Tanzania and southern Kenya have revealed a circular positive Bouguer anomaly near Mara, northern Tanzania (1,6° S, 34°E). The anomaly has an amplitude of +380 g.u. (10 g.u. = 1 mgal.), a diameter of about 44 km, and interpretation shows that the causative body is probably a gabbroic laccolith similar to the Trompsburg Igneous Complex in South Africa. The Mara intrusion can be shown to be about 2 000 m.y. old, and lies on the projection of the "Bushveld line of intrusives" described by Cousins (1959). The significance of this is discussed.
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Transactions of the Geological Society of South Africa, 77 (2), 85-91
Current classifications of kimberlites are briefly reviewed, and the conclusion is reached that neither chemical nor petrographical classifications appear to be of economical value. An investigation of kimberlitic heavy mineral concentrates indicates that economically interesting kimberlites contain less than 12 percent of chrome diopside and less than 3 percent of zircon when the heavy mineral analyses are expressed in terms of garnet, opaque oxide minerals, chrome diopside, and zircon only. The ratio of garnet to opaque oxide minerals is not a suitable parameter of classification. Representative samples of garnet from payable kimberlites have a lower modal refractive index than those from provisionally payable and unpayable kimberlites. Ilmenite shows a similar relationship as far as the reflectivity of the ordinary ray is concerned, but the variation in properties of chrome diopside appears to be too small to serve as a basis of classification. It is concluded that only magnesium-rich garnet and magnesium-rich ilmenite crystallized under conditions approaching those of diamond nucleation. These diamond nuclei were probably enlarged by secondary grain growth by the reduction of thermal decomposition of carbon-rich gases, but this process was not necessarily accompanied by physical and chemical changes in the indicator minerals so that a quantitative relationship between the properties of these minerals and the diamond content of the parent kimberlite cannot be expected.
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Transactions of the Geological Society of South Africa, 77 (2), 93-98
Statistical comparison of mean compositions of Bushveld granite and Rooiberg felsite shows that they form two distinct geochemical populations that are probably genetically unrelated. Bushveld granite is homogeneous in composition and conforms closely with the thermal valley in the quartz-orthoclase-albite system. Rooiberg felsite displays complex petrochemical trends not readily explainable by normal processes of crystallization-fractionation. Petrochemical evidence of compositional heterogeneity and magmatic superheating are consistent with an origin of the felsite by meteorite-impact melting although direct evidence of shock-metamorphism has not yet been reported.
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Transactions of the Geological Society of South Africa, 77 (2), 99-104
A basaltic extrusive formation, previously mapped as a sill intrusive into the Pretoria Group, is described from the Transvaal Supergroup, 2 200 million years old, in the eastern Transvaal. The volcanic nature of the formation has been indicated by the pillow-structures found in the basalts in some key-exposures. Three chemical analyses of the pillow-lava are presented. The lavas are basaltic and, chemically, bear a very strong resemblance to the family of oceanic tholeiites which have been dredged off the present-day ocean floor. The presence of volcanics with this chemistry is puzzling, since they are found in a sedimentary pile deposited in a basin which rests on an older sialic crust. The lavas must have been injected through the crust with little or no contamination from this source.
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Transactions of the Geological Society of South Africa, 77 (3), 211-222
Eight formations are defined in the former Transvaal Dolomite from an area north-west of Johannesburg. With the underlying Black Reef Formation these form the Malmani Subgroup as defined here. Simple properties such as colour, and the presence or absence of chert were used as the primary basis for the mapping of these units. In more detail this mapping also involves the distinction of stromatolitic assemblages and other primary structures. Much fine detail within the formations has lateral persistence and numerous marker-horizons, often dominated by differing large domical stromatolitic structures, can be traced over distances of at least 40km. Stratotypes are established for the formations. The Subgroup is cut across by the Fountains formation, a chert breccia of basin-wide distribution in relation to the former Transvaal Dolomite. Uplift and subaerial exposure resulted in the formation of this unit, and there is evidence of greater localised uplift in the area studied. At least five other significant chert breccias and related rocks can be traced through the area, and there is a third group of chert breccias of more local significance.
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Transactions of the Geological Society of South Africa, 77 (3), 223-227
Sandstones of the Cape Supergroup in Natal and the Transkei were probably deposited in an elongate trough trending approximately parallel to the present-day coastline. Detrital influx was mainly from the north, with fluvial conglomerates and arkoses in Zululand and Natal grading southward into more mature sandstones. In the Transkei, where the Cape Supergroup is over 1 000m thick, the compositional and textural characteristics, the patterns of arrangement of current-produced sedimentary structures and the nature of certain trace-fossil occurrences all point to a shallow marine origin. Three different varieties of lithogenetic units are recognised in the succession. Each is characterised by a particular internal and external geometry and is attributed to a specific set of sedimentary processes. Sheet-sandstone units, consisting of horizontal, cross-bedded and cross-laminated sandstones, are remarkably persistent laterally and in places have oxidised and bioturbated upper surfaces. They are interpreted as having accumulated under sheltered back-bar conditions. The second variety, referred to as lenticular sandstone units, is composed of cross-bedded cosets, some of which display evidence of possible tidal influence. These units may have originated as offshore sand bars. Finally, channel-sandstones occur in three subtypes, all of which have a preferred north-east to south-west orientation, and are thought to represent the filling of tidal, rip or estuarine scours.
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Transactions of the Geological Society of South Africa, 77 (3), 229-237
The Table Mountain Group, which is Ordovician/Silurian in age, can on lithology and areal distribution be subdivided into a Cape, a Pondoland and a Natal Facies. The two facies first-mentioned consist of about 90 percent of supermature quartz arenites while the Natal Facies is represented by immature quartzofeldspathic and argillaceous sediments. The sequence with a maximum thickness of about 3 500 m and a known areal extent of about 1500 km by 350 km is probably one of the largest quartz accumulations in the world. The sediments were deposited on a broad stable shelf following the present coastline roughly, with the trough axis probably lying far to the south. Transgression of the area probably started in the south-west, but its northern limit is unknown except in Natal where marine incursions reached up to the Tugela Fault. The source-areas of the sediments were located to the north of the basin and consisted mainly of gneissose rocks with subordinate sedimentary and volcanic rocks. The material was transported southwards by fast-flowing rivers and was mainly deposited in the littoral environment. Extensive reworking of the sediment by the waves and the wind resulted in the elimination of the feldspar and the clay fraction, whilst the bulk of the supermature residue was laid down during stormy periods, and to a lesser extent by grain flow and migrating sand bars on the beaches. The Table Mountain sandstone largely represents first cycle sediments and its supermature nature can be ascribed to the fresh material supplied to the basin, the extensive reworking on a very stable shelf and the very low rate of sedimentation. It is concluded that the factors necessary for the deposition of thick extensive deposits of quartz arenite are a quartzose source, reworking and sorting of the material, and large-scale stability in the depositional area. It can be proved that these factors occurred simultaneously in the past resulting in periodic thick quartz accumulations. This took place during stable periods following on "granite-forming episodes" in the crust. Uplift and erosion of the granitic terrains yielded quartz sands deposited in nearby stable shelf areas during transgressive phases.
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Transactions of the Geological Society of South Africa, 77 (3), 239-245
Cape and Karoo sediments from the Cape fold-belt and its northern fringes have been subjected to advanced diagenesis or incipient metamorphism as a result of deep burial. Porphyroblastic micaceous minerals which grew on bedding have been abraded, disrupted and concorted by movement along a cleavage associated with the Cape folding. The folding clearly post-dates the mineralogical changes associated with diagenesis and metamorphism.
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Transactions of the Geological Society of South Africa, 77 (3), 247-252
The J(c)-1 bore-hole was drilled on the continental shelf 24km east of Stanger on the Natal coast. It intersected 4 560 feet of marine Tertiary sediments (Pliocene-Paleocene) and 2 405 feet of marine Upper Cretaceous sediments. The bore-hole was abandoned in quartzitic sandstones of the Cape Supergroup after penetrating 160 feet of Dwyka Tillite. Five informal lithostratigraphic units are recognised in the Cretaceous-Tertiary sequence which appears undeformed, exhibits a mild seaward dip, and has an over-all rate of sedimentation of about 100 feet per million years. The sedimentation rate during the lower Paleocene and Oligocene is relatively slow by comparison with the Eocene. The Upper Cretaceous sediments are interpreted as an onlapping marine transgressive succession and the Paleocene and Eocene sediments as an upward coarsening prograding subaqueous deltaic sequence. A marked unconformity is present at the base of the transgressive marine Oligocene unit which includes the Pecten shelly limestone. This unconformity is correlated with the Oligocene hiatus in Joides DSDP bore-holes and with the unconformity below the Pecten bed at Uloa.
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Transactions of the Geological Society of South Africa, 77 (3), 253-260
The effect of basement structure and tectonics on the pattern of sedimentation over the western continental margin of Southern Africa since the time of continental brack-up are discussed. For the purposes of this paper the margin is divided into three bathymetric provinces: north of 28°S, 28°S to 32,5°S and south of 32,5°S. The southern province has experienced restricted sediment accumulation for which the Cape Granites-Agulhas Arch high may be partly responsible. Elsewhere, the post-rifting sediment wedge is well developed and uninterrupted by large sediment dams of basement-rock lying parallel to the margin; maximum sediment accumulation has occurred in the 28°S to 32,5°S province. It is suggested that the absence of post-rifting marine sedimentary basins in the coastal regions north of the Cape Granites is related to a steep seaward gradient in basement maintained by repeated uplift of the Great Escarpment "hinge line". The boundaries between the three bathymetric provinces may coincide with basement features crossing the margin, which developed during the rifting of South Africa from Africa and have since acted as axes of differential subsidence. Similar features may also be present at 21°S and 31,5°S.
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Transactions of the Geological Society of South Africa, 77 (3), 261-264
The first lithofied marine phosphorites recovered from modern seas were dredged by HMS Challenger from the Agulhas Bank, South Africa in 1873. Early work on the rocks was mainly descriptive, though several proposals for lithogenesis were considered. Recent work on ths slag and cobble size phosphorite nodules and pelletal material has produced modifications of the earlier hypotheses. These entail the partial replacement of lime mud (i.e. micrite) by phosphatic solutions producing francolite, a carbonate fluorapatite. The source of these solutions is considered to be zoo- and phytoplankton protoplasm, the production of which is concentrated under zones of intensive upwelling of cold, nutrient-rich oceanic water. Two main phases of Tertiary phosphoritic formation are postulated, both associated with regressive sea level movements (late Eocene and late Miocene/early Pliocene or late Pliocene). Although minor amounts of contemporary authigenic apatite may be forming on the western Agulhas Bank, present-day hydrologic and sedimentologic conditions are not considered analogous to those responsible for the Tertiary deposits. This conclusion was reached early in the history of investigations. Possible mechanisms for the establishment of the necessary environments of deposition are considered.
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Transactions of the Geological Society of South Africa, 77 (3), 265-283
The Varswater Formation is essentially a basin deposit, best developed on the farm Langeberg near Langebaanweg, and on the farms Witteklip and Sandheuwel north-east of Saldanha. It lies unconformably upon a Miocene phosphate rock which has been tilted to the south-west. Deposition of the Varswater Formation was initiated by a marine transgression in the Pliocene. A temporary stillstand of the sea resulted in accumulation of freshwater estuarine sands behind a barrier. These deposits are now situated at 30 m above sea level. The estuarine sediments are rich in mammal remains. Final transgression carried the sea inland, the beach zone now being situated at 50-55m above sea level. At the base of the Varswater Formation is a black kaolinitic clay. This is followed by the Beach Gravel Member consisting of beach gravel, waterworn cobbles and silty sand with molluse shell casts. Overlying these sediments in the Fluvial Sand Member, in which an estuarine facies and a fluvial facies are recognized. The final transgression resulted in accumulation of the Pelletal Phosphorite Member of quartzose sand and pelletal phosphorite. It is shown that the Miocene and Pliocene deposits reflect considerably warmer conditions at that time. The pelletal phosphorite is a detrital component, being derived from the miocene basal bed where it occurs as the matrix material. Textural analyses, predominantly of the Pelletal Phosphorite Member, show that at Langeberg deposition has occurred mainly in a sheltered environment behind a beach bar which is situated on the outer edge of a terrace. At this outer edge more turbulent conditions and consequent winnowing resulted in better sorted sediments. Textural analyses were also performed on the pelletal phosphorite fraction. Tests for hydraulic equivalence show that the pelletal phosphorite approaches hydraulic equivalence with the saltation population of the quartz fraction and that departures are readily explained by location of the sample within the sequence. The evidence for two transgressions, Miocene and Pliocene, is in good accord with other evidence of sea level movement from the south and east coasts and the continental shelf. It would appear that the west coast has been epeirogenically unstable in the late Cainozoic.
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Transactions of the Geological Society of South Africa, 77 (3), 285-289
An extensive examination of quartz grain surface textures by scanning electron microscopy has been made of sediments from the Pliocene Varswater Formation and Late Pleistocene cave sediments from Die Kelders, Cape Province, South Africa. Post-depositional diagenesis in a highly active chemical environment has all but obliterated the mechanical textures of the grain surfaces and has replaced them by chemical textures, viz. solution features controlled by internal symmetry, solution features independent of internal symmetry and reprecipitation features. Diagenesis appears to depend more upon physicochemical conditions than upon time.
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Transactions of the Geological Society of South Africa, 77 (3), 291-294
Microprobe analyses of uranium and thorium in uraninite grains from the Witwatersrand, South Africa, and Blind River, Ontario, reveal that, although individual grains are fairly homogeneous, the assemblage of grains is quite heterogeneous. This heterogeneity appears to favour genetic concepts advocating a detrital, placer origin for the uraninite.
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Transactions of the Geological Society of South Africa, 77 (3), 295-300
The Doringberg fault is a major tectonic feature of the northern Cape, and can be traced for a distance of some 200-250 km from De Aar in the south-east to the Matsap synclinorium of the Eselberge in the north-west. It separates the greater part of the nonmetamorphosed Transvaal sequence in the north-east from the older pre-Ventersdorp granitoids and Kheis metamorphites in the south-west. It is an oblique-slip fault, the origin of which is related to the second post-Matsap folding (the Namaqua tectogenesis, which culminated some 1,0 Ga B.P.) The movement along the fault is consistently dextral. The part of the fault striking north-westerly is a first-order shear; in the north-west it passes into a second-order splay-fault striking north-north-westerly and finally fades out in the vicinity of the major refolds in the Matsap synclinorium. Although some other lower-order fractures and associated fold structures are present in the zone of the Doringberg fault, they represent only a small part of all the wrench-fault structural elements theoretically defined by Moody and Hill (1956). The Doringberg fault, together with several other major fractures of the same magnitude and nature, is situated in a major zone of crustal weakness, for which the name Doringberg lineament is proposed. Along this lineament, still seismically active, the two segments of the Namaqua-Natal Mobile Belt are dextrally coupled.
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Transactions of the Geological Society of South Africa, 77 (3), 301-308
The Lebombo volcanic belt (south-eastern Africa), which runs almost straight for 700km in a north-south direction, is associated with a pronounced positive isostatic gravity anomaly. Earlier geophysical studies have attributed the whole of this anomaly (about + 100mgal) to the relatively high-density basalts present. Reinterpretation of the anomaly, using new data, shows that only a part of the observed anomaly is produced by the basalt, and that the remainder is a "regional" or "crustal" effect. Models are presented for the structure of the Lebombo volcanic belt which show that the most important consequences of the reduced gravity anomaly are much reduced estimates for both the thickness of basalts and the depression of the sub-Karoo basement, east of the Lebombo Range.
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Transactions of the Geological Society of South Africa, 77 (3), 309-322
The structural and physiographic development of Natal is difficult to decipher, as is demonstrated by the many opinions on the subject that have been advanced in the past. Significant changes over the years in the views of some workers, particularly Prof. L.C. King, are briefly reviewed. This paper represents an attempt to reconcile earlier views with the results of regional photogeomorphological interpretation and offshore drilling. The framework for landscape development in southern Africa since the late Jurassic was imposed during the rift-faulting which marked the commencement of the dismemberment of Gondwanaland. As in modern rift-valleys, the shoulders of the Jurassic graben were raised above the general level of the land, and this caused the formation of extensive inland basins on the African protocontinent. A short active drainage was initiated on the graben side of the shoulders, and a much longer drainage, with more gentle gradients, developed towards the interior basins. Deposition of sediments commenced almost simultaneously in the marginal graben and the interior basins, as is shown by their comparable ages (late Jurassic). The ancestral fault- or warp-line scarps of the graben were gradually driven back by erosion and are now represented by the extremely irregular Great Escarpment of southern Africa. Concomitant offshore deposition led to continued but spasmodic seaward tilting and the development of a coastal arch from which a very great thickness of crust was removed. The formation of major river systems, such as the Zambezi and the Orange, may be attributed to late capture of the inland drainage. Africa always stood relatively high, and as a result of the duality of the drainage over a very long period it is difficult if not impossible to correlate inland and coastal erosion surfaces. Erosion surfaces in Natal can best be described in relation to four physiographic features - the Drakensberg escarpment, the Pietermaritzburg step, the Natal hingeline and the straight coastline. Two polycyclic erosion surfaces can be recognised. The youngest is the product of very intense, deep erosion, which extends on average about 80 kilometres inland along the main streams but seldom beyond the Pietermaritzburg step. The older surface is polycyclic east of the step, but to the west only one erosion surface, currently active, can be recognised; above it mesas capped by resitant rock-types rise to varying heights. The existence of remnants of two Tertiary erosion surfaces in a constant relationship over the whole of Natal, recorded in the literature, could not be verified. On top of the high Drakensberg a surface with wide valleys is being incised by two very active erosion cycles; there is no evidence for correlating it with any coastal surfaces or sediments, and the sharp interfluves on its cannot be construed as remnants of an ancient, smooth erosion surface. The main post-mid-Jurassic structural features of Natal are the Lebombo monocline, the Empangeni fault system, the Vryheid and Natal arches, the arcuate coastal fault system, the Natal hingeline, and the straight continental shelf break. The Lebombo monocline represents the edge of a
late Jurassic graben or basin in which the contemporaneous surface of Gondwanaland was downwarped and in which some of the earliest coastal Mesozoic sediments were laid down. It ends at the Empangeni fault system, south of which no Upper Jurassic or Lower Cretaceous sediments have yet been recognised in Natal, but where the Upper Cretaceous rests on Karoo, Cape and probably basement-rocks up to at least 24 kilometres from the present coast. In this area the marginal monoclinal flexure (or fault) of Africa must be situated far to the east of the coast, possibly even beyond the shelf break; it apparently developed in different positions on the opposite sides of the Empangeni fault zone. The dominant structure in the coastal hinterland of central and southern Natal is therefore an arch and not a monocline. The comparable Vryheid structure, and perhaps the Natal arch as well, seem to have developed at a distance of about 150 kilometres from the ancestal African coast. Accelerated uplift along the Natal arch in the Paleocene appears to have led to the development of a scarp or "treppe" on the older polycyclic erosion surface, which is now represented by the Pietermarizburg step. Further uplift, associated with the Natal hingeline, was responsible for the carving of the lower phase of the older surface, and renewed movement, possibly assisted by climatic change and eustatic change in sea level, initiated the intense erosion associated with the younger polycyclic surface. The drainage of Natal shows marked structural control around the Natal arch. Dating of erosion surfaces in Natal with reference to interior surfaces and coastal sedimentary successions rests on very insecure grounds. Drilling results off Stanger suggest that the movement that produced the Pietermaritzburg step was followed by accelerated deposition from the mid-Paleocene to the Mid-Eocene, and finally by regression in the late Eocene. A transgression commenced in the Oligocene and reached the present coast about the early Miocene. The upper phase of the earliest polycyclic surface - corresponding to the African surface of much of the recent literature - probably reached its greatest "perfection" at the time of maximum transgression, and its comparative age may therefore be appreciable younger than early Miocene. The results of the present study indicate that the currently accepted geomorphological framework for southern Africa require extensive revision.
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Transactions of the Geological Society of South Africa, 77 (3), 323-328
The granitic phase of the Bushveld Complex in the area north of Rustenburg and Brits contains variable amounts of granophyre. Linear zones of abundant granophyre occur (a) along the contact between the granite and the Bushveld mafic sequence and (b) midway between this contact and the Crocodile River Fragment. It is concluded that the second mode of occurrence of granophyre is identical to the first and that its exposure is the result of folding of the Bushveld Complex along north-west trending axes. This conclusion is based on: (1) the distribution of the granophyre within the Bushveld granite and (2) the interpretation of aeromagnetic and gravity observations.
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Transactions of the Geological Society of South Africa, 77 (3), 329-338
The area mapped consists of approximately 1 250 sq. km covering parts of the Main and Upper Zones and the epicrustal phase of the Bushveld Complex in the Eastern Transvaal. The oldest rocks are quartzite, feldspathic quartzite, hornfels and dolomite of the Pretoria Series. These rocks are generally sandwiched between the Layered Mafic Sequence and the epicrustal and/or granitic rocks of the roof of the Complex. Apart from the sediments, the rocks in the roof are granophyre, leptite and granite. Coarse-grained granite is the dominant acid rock in the central and northern parts of the area, where the underlying Layered Sequence is thickest; with lateral thinning southwards of this sequence there is a corresponding transition in the roof from granite, through granophyre at Paardekop, to Rooiberg felsite at Bothasberg which is south of the area. The present work has yielded further evidence in support of Willemse's placing (1964, p. 97) of the epicrustal rocks as older than the Layered Sequence which in turn predates at least some of the Bushveld Granite. There is field evidence of granitisation of sediments and it is likely that some of the granophyre was derived by that process. However, most of the granophyre (and leptite) was probably originally Rooiberg felsite which was altered to granophyre by heat and volatiles emanating from the crystallizing Layered Sequence. Finally, at least some of the granite is likely to have been produced through anatexis and palingenesis of granophyre. The Main Zone of the Layered Sequence is approximately 2 860 m and the Upper Zone some 1 790 m thick. In these zones layering is remarkably persistent and uniform. The base of the Main Zone is at the footwall of the Merensky Reef and the division between the Main and Upper Zones is taken as the base of the mottled magnetite anorthosite 30 m below the lowest magnetite seam. Differentiation is most pronounced in the Upper Zone and least developed in the middle of the Main Zone. Spotted anorthosite is restricted to the Main Zone but the mottled variety occurs throughout the sequence. Iron-rich olivine and widespread cumulus titanomagnetite are very largely confined to the Upper Zone. Cumulus titanomagnetite coexists with olivine in the compositional range Fo60-0, orthopyroxene En63-29 and plagioclase, An60-43. Plagioclase varies in composition from An70 at the Merensky Reef to An 43 in the quartz diorite at the top of the Layered Sequence. In this interval orthopyroxene changes in composition from En78 to En29 and cumulus olivine, which occurs almost exclusively in the Upper Zone, from Fo60 to Fo0. Biotite is prominent throughout the Upper Zone whereas apatite is restricted to the diorite constituting the top of the Layered Sequence. Hornblende, which occurs only near the roof, may be a primary mineral in the mafic rocks, but quartz in bulk and orthoclase appear to have been formed only as a result of hybridisation between the roof-rocks and magma of the Layered Sequence. A surge of parent-magma was probably introduced into the chamber prior to the formation of the Merensky Reef and the magma then embarked on a trend of differentiation which continued with little
interruption for about 2 200 m up to the footwall of the Pyroxenite Marker. Reversals in the compositions of orthopyroxene and plagioclase indicate that there was a major influx of magma before the formation of the Pyroxenite Marker. Differentiation then probably proceeded with little further interruption, the end product being the diorite constituting the top of the Upper Zone. Above the Pyroxenite Marker some 750 m of layered rocks were deposited before the magma returned to its earlier state and this interval may thus indicate the vertical dimension of magma introduced. After the introduction of this probable final surge of magma the internal depth of the chamber is likely to have been of the order of 2 400 m which is the present vertical distance between the Pyroxenite Marker and the roof.
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Transactions of the Geological Society of South Africa, 77 (3), 339-345
This paper reviews the geology of Namaqualand and Bushmanland, which consists mainly of gneissic rocks broadly similar to those of gneissic terrains in other parts of the world. A general geological succession consisting of porphyroblastic granite gneiss and pink gneiss at the base, biotite and sillimanite schist, metaquartzite and magnetic quartzite, quartz-muscovite schist and gneiss and grey gneiss at the top is established for the area. The rocks are exposed in a broad belt which trends north-west to south-east but towards the south of the area the strike changes towards the east. Regarding the origin of some of the gneisses, attention is focused particularly on the following: (a) The basal gneisses, formerly regarded as representing the platform on which the sequence of paragneisses and metavolcanic rocks had been deposited, should also be looked upon as representing remobilised basement or a granite of batholithic proportions. (b) The pink gneisses, clearly intrusive into the older rocks in many places, are part of the volcanic-sedimentary sequence and have apparently, been derived, by metamorphic processes, from eruptive rocks which originally formed the roof of the batholith. (c) The aluminous schists and the sillimanite-bearing rocks, formerly regarded as metamorphosed bauxites, might also have been derived from leached eruptive rocks rather than from weathered crusts. (d) The mafic intrusives, mainly confined to the zone of grey gneiss, resemble the "globuliths" present in the gneiss complex of South-eastern Norway, but in the present area they have apparently been intruded prior to the emplacement of the granitic gneiss. The Namaqua deformational event consists of four, possibly five, episodes of which the second which gave rise to recumbent isoclinal folds, was the most important. It was accompanied or followed by the highest degree of metamorphism attained in the area and is characterized by the ubiquitous development of sillimanite. The succeeding episodes of deformation were less intense and culminated in shear deformation. Pegmatites are most frequently associated with the fourth episode. The age of the deformation is uncertain but from the information available it would appear that this event was terminated 1000-900 million years ago, after which deformation by shearing followed.
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Transactions of the Geological Society of South Africa, 77 (3), 347-351
Of the four isotopes of sulphur, S32 constitutes about 95,1 percent of the total whereas S34 accounts for about 4,2 percent. With S32/S34 _ 22,22 in troilite from the Canyon Diablo meteorite taken as standard, per mil differences from the standard ratio are expressed in terms of SS34. Samples of sedimentary sulphides are commonly enriched in S32 and have negative SS34 values, whilst sedimentary sulphates are enriched in S34 and have positive SS34 per mil values. Sulphides associated with intrusions tend to have a narrow range (q 5 per mil) of deviation of SS34 from the meteoritic standard. Samples of sulphide minerals obtained from twelve deposits in central South-West Africa were analysed for their sulphur-isotope content. SS34 values range from -33,6 at Klein Aub Mine to +10,18 at the Hope and Gorob Prospects. High negative SS34 values are believed to reflect the original syngenetic sulphur-isotopic ratios in Proterozoic stratabound sulphide deposits that have been only slightly metamorphosed. Nearly zero per mil SS34 values in pegmatites may result either from a magmatic source or from partial to complete homogenisation of sulphur by anatexis during the Damara Orogeny. The 8,1 per mil range of SS34 values at Onganja Mine suggests that the deposits are secretion pegmatites and that the copper and sulphur contained therein have a sedimentary provenance. High positive SS34 values of sulphides in metasedimentary rocks closely associated with the Matchless amphibolite probably result from intensive thermal isochemical metamorphism of cupriferous Proterozoic volcanic rocks.
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Transactions of the Geological Society of South Africa, 77 (3), 353-361
Recent studies concerned with the post-Karoo kimberlites of South Africa and Lesotho are summarized and reviewed. The majority of mantle-derived xenoliths in kimberlite are considered to be accidental inclusions derived from relatively restricted mantle zones and were incorporated in the kimberlite fluid during its emplacement. The mantle sampled is of variable mineralogical and chemical composition and includes peridotite, garnet peridotite, garnet pyroxenite, eclogite and ilmenite-bearing rocks. The evidence presented indicates that peridotites (dunites, harzburgites, lherzolites, garnet harzburgites, garnet lherzolites) are the most common rocks in the mantle sampled by the South African kimberlites. Three important groupings have been noted: (1) The commonest peridotite xenoliths are granular rocks which are thought to be derived from depths of approximately 140-160km beneath the earth's surface, where they have equilibrated at temperatures of approximately 950-1 050°C. (2) Many of the mantle-derived xenoliths show evidence of a complex metamorphic history involving varying degrees of recrystallization and in some cases metasomatic alteration. Certain sheared rocks appear to be derived from depths of 160-220 km at temperatures of 1 050-1 400°C. It has been suggested that the shearing was produced in these rocks because they were at the base of a moving lithospheric plate during a period of active continental drift. Some ilmenite-bearing rocks are placed at depths similar to the sheared rocks (160-190 km). (3) In other mantle xenoliths the stress-induced recrystallization features are not noticeably associated with greater depths of origin or high temperature equilibration and these stress features have been interpreted as due to mantle creep. It has been suggested that an increase in the temperature of the mantle at the base of a continental plate (lithosphere) was produced by friction during lateral movement of the plate relative to the underlying mantle (asthenosphere). The low-velocity zone of the mantle contains an interstitial fluid in which dissolved water, CO2, other volatiles and incompatible elements are important constituents. The presence of this fluid and the rise in temperature accompanying plate movement facilitate partial melting of the mantle and kimberlitic fluids are formed. After a complex emplacement history these fluids breach the earth's surface, utilizing zones of deep tensional stress.
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Transactions of the Geological Society of South Africa, 77 (3), 363-367
The Naauwpoort Formation, located near the northern margin of the Damara eugeosynclinal trough, has been subdivided into upper and lower units. The lower unit consists of more than 6 600 m of welded pyroclastic flows of potash-rich alkali rhyolite. It underlies rocks of the Damara Group conformably or with a slight disconformity only. Rhyolite and bostonite extrusions of the upper unit follow the rocks of the lower unit conformably and interfinger with the carbonate-rocks of the Damara Group that underlie the glaciogenic Chuos Formation. The Lower Naauwpoort Formation bears a stratigraphic relationship to the Damara Group that is very similar to the relationship between the Nosib and Damara Groups elsewhere in the Damara geosyncline. The whole formation forms a local volcanic sequence within the Nosib Group and is late Nosib to early Damara in age.
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Transactions of the Geological Society of South Africa, 77 (3), 369-370
Many biogenic structures (trace-fossils) occur in a band of sandstone 4 m thick in the Nardouw Formation (middle Silurian?) of the Table Mountain Group at Milner Peak, Cape Province. Tracks and "tube-form" casts are common. The tracks are similar to the so-called "scribbling grazing traces" described from certain Palaeozoic and older sediments and may be due to brittle stars (Ophiuroida or Auluroida). The "tube-forms", some of which show bilateral symmetry, are probably due to polychaete worms, although they may be coprolites. The resemblance between the "tube-forms" and Phycodespedum Seilacher may be important, since Phycodes has never been noted above the lower Ordovician. Palaeobathymetric indications are uncertain, but a shallow marine environment of deposition is favoured.
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Transactions of the Geological Society of South Africa, 77 (3), 371-372
The contact between the Ecca and Beaufort Series is transitional from pre- dominantly argillaceous to an alternation of sandstone, siltstone and mudstone. This indicates a change in depositional environment from deep marine through deltaic to fluvial and when this criterion is used in conjunction with other features the contact can be more accurately defined during mapping.
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Transactions of the Geological Society of South Africa, 77 (3), 373-374
A bed of crystal tuff, half a metre thick, has been traced for approximately one hundred metres on the farm Geelhoutboom, fifteen kilometres east of Lake Mentz, in the eastern Cape Province. The tuff is composed of plagioclase (now pure albite) and minor quartz, and is one of the few examples of associated volcanicity within the Cape Fold belt.
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Transactions of the Geological Society of South Africa, 77 (3), 377-380
I should like to congratulate Dr. Van Eeden on a very interesting paper, on a matter with which I have been concerned for some time. There can be little doubt that many of the lithostratigraphic units of the Karroo Group are markedly diachronous, and Dr. Van Eeden's timely paper can only serve to direct further attention to this perplexing problem. However, without a known isochronous horizon the only convincing method of proving diachronism is by a study of the fossil content of the various lithologic units. It is in this respect that some of Dr. Van Eeden's correlations fail.
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Transactions of the Geological Society of South Africa, 77 (3), 381-382
Dr. Ferguson has given a valuable contribution on the Pilanesberg Alkaline province, in particular on its petrology. However, I only wish to comment on his structural interpretations because several members of the Geological Survey of South Africa, including myself, have been engaged on research in the north-western Transvaal mainly on the Waterberg basins and their structural and volcanological features. Our interpretations differ considerably from those of Dr. Ferguson, illustrated in Figs. 1 and 2 and they will be presented in a paper on Precambrian basins and their structural and sedimentological features. My main objections are against the north-west-striking anticlinal and synclinal structures in Figs. 1 and 2, the existence of which, in my opinion, is very doubtful. Before embarking on details I venture to make a general statement, namely that the linking of certain structural features is a rather subjective procedure, which applies to all investigators. The importance of each structure must be carefully evaluated in magnitude, shape, origin and age before major structural trends are drawn. To illustrate this, a major anticlinal structure, the "Johannesburg anticline", links the Johannesburg and Makoppa domes. Reasons will be given in the sequel why the latter is not a dome and of an origin different from that of the Johannesburg dome.
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Transactions of the Geological Society of South Africa, 77 (3), 383-384
Any attempt to treat statistically the observed differences between pebbles from fluviatile and littoral environments is to be applauded, especially where the results indicate a "magic line" that differentiates between the two. There are, however, some points that need to be raised. 1. Coefficient of flatness. 2. Sampling. 3. Statistical treatment.
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Transactions of the Geological Society of South Africa, 77 (3), 385-388
The paper by De Swardt et al. (1974) adds some significant data to our knowledge of the Cape fold-belt, but many more such studies are needed before we arrive at a full understanding of its history and mechanism. My recent suggestion (Newton, 1973) of a gravity mechanism for the fold-belt was obviously not available to the authors at the time they were preparing their paper, and since their model is somewhat different from mine, particularly as regards the degree of basement involvement, there are a number of points on which I would value their comments. Origin and significance of cleavage. General style of the Cape orogeny.
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Transactions of the Geological Society of South Africa, 77 (3), p375
Bonaccordite was found in the mineral assemblage trevorite-liebenbergite-nephouite-nimite-bunsenite-gaspeite-violarite-millerite. Trevorite and liebenbergite were described in earlier papers (De Waal, 1972, and De Waal and Calk, 1973, respectively), and this note gives the mineralogical data on the bonaccordite, which is the nickel analogue of ludwigite. The nickel-rich mineral assemblage occurs in the Bon Accord area in the Barberton Mountain Land. The name bonaccordite, which is for the locality, has been accepted by the Commission on New Minerals and Mineral Names of the International Mineralogical Association.
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Transactions of the Geological Society of South Africa, 78 (1), 1-10
Rocks correlated with the Waterberg System of the Republic of South Africa are relatively well exposed in three main sectors in eastern Botswana. They are also known to underlie extensive tracts of southern Botswana but here are mainly obscured by the cover of Kalahari Beds. Throughout Botswana the rocks of the Waterberg System consist predominantly of sedimentary rocks of "continental" aspect. Arenaceous rocks are the commonest type but rudaceous and argillaceous rocks are also abundant. The rocks are characteristically red or purple in colour but other colour varieties exist. A brief review of the lithostratigraphic succession in each of the main areas in which the Waterberg System outcrops in the eastern part of the country is given and some observations are made concerning the interrelations between occurrences of the formation elsewhere in Botswana and in South Africa. The sedimentary rocks of the Waterberg System within eastern Botswana are considered to have been laid down under lacustrine or very shallow marine conditions. No evidence has been found of deposition under subaerial conditions although in some areas newly deposited sediments may have been exposed to the atmosphere temporarily. Consideration is given to the question of the tectogenetic environment in which the Waterberg System was laid down. The hypothesis is presented that sedimentation took place mainly in a series of graben-like rectilinear basins whose margins were defined by a number of sets of persistent major faults. The corresponding uplifted areas adjacent to these basins show only minor and thinly developed examples of Waterberg sedimentation.
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Transactions of the Geological Society of South Africa, 78 (1), 101-109
Geotechnical investigations in the Richards Bay area have furnished an opportunity for obtaining information and material relating to the Post-Karoo sediments present in this area. From the micropalaeontological study of borehole core material it has been possible to establish more closely the relationship between the Cretaceous and Palaeocene marine sediments in the area and to extend the area of known occurrence of the Palaeocene sediments. Boreholes in the area have also disclosed the presence of thin Late Tertiary Miocene sediments overlying the Cretaceous and the Palaeocene sediments. From micropalaeontological study, the lower coquina and upper calcarenite members of the Miocene Formation have both been found to be of later Miocene age. The Pleistocene Port Durnford Beds and a Pleistocene calcarenite are also found in the Richards Bay area, and borings have also disclosed the presence of an alluvium-infilled, deeply incised former valley system of the Mhlatuze River beneath its present estuarine flood plain and the Richards Bay lagoon.
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Transactions of the Geological Society of South Africa, 78 (1), 11-23
The geology, the mineralogy and the petrology of the Kaffirskraal Igneous Complex south-west of Heidelberg (Transvaal) are described, and 36 new mineral and rock analyses are presented. The rocks from the Kaffirskraal Complex display variations in the compositions of the minerals within their solid solution series, isomodal layering (Jackson 1967), planar lamination (Jackson 1967) and cumulate textures (Wager and others 1960; Jackson 1961), indicating that the Kaffirskraal complex can be considered as a layered intrusion. However, it also exhibits features which differ significantly from those usually found in layered complexes, such as an inverted succession of rock-types, an ultramafic marginal phase, a disproportionately large metamorphic aureole, and specific rock-types with diagnostic mineral and chemical compositions similar to the rock-types from the zoned ultramafic complexes (Taylor and Noble 1969). It has therefore been classified as a zoned ultramafic complex, but in contrast with this type of complex, no dunite or peridotite have been encountered at the Kaffirskraal Complex. This may be due to insufficient outcrops. It is concluded that the Kaffirskraal complex formed as a consequence of the multiple intrusion of magmas derived from a parent-magma by crystallization-differentiation in depth. Each of these magmas crystallized after emplacement by the processes of crystallization- differentiation and crystal settling to give rise to the layered succession of rock-types encountered. The first intrusion at the Kaffirskraal Complex was apparently by a differentiated magma which gave rise to the gabbro and norite. The second magma appears to have been more mafic, and resulted in the formation of at least the magnetite clinopyroxenite clinopyroxenite and magnetitite. The geological evidence suggests that the marginal phase, which can be classified as hornblende peridotitic according to the chemical classification after Buff, (1959, p. 95), may represent the second intrusive magma, but the mineralogical the second intrusive magma, but the mineralogical and geochemical data are not in accordance with this interpretation. The data do, however, indicate that the second intrusion was characterized by a high PH2O which was responsible for the absence of plagioclase and the presence of magnetite.
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Transactions of the Geological Society of South Africa, 78 (1), 111-121
The Epembe alkaline carbonatite complex intrudes the Basement-complex which contains both felsic and mafic units; both units have been fenitised for distances up to 1 km from the contacts with the carbonatite and the nepheline syenite masses. Shattered and jointed rocks adjacent to the intrusive bodies show the highest grades of fenitisation but screens of massive unmetasomatised rocks are present within the main fenite aureole. An essentially similar degree and style of fenitisation is associated with both the carbonatite and nepheline syenite intrusives. In the case of the felsic rocks, new minerals are readily nucleated during the fenitisation process, whereas in the mafic fenites their formation appears remarkably inhibited. Petrographic evidence indicates a series of desilication reactions in the felsic fenites, whereas the mafic fenites show silication reactions. The first mineralogical changes involved in the felsic fenites are partial replacement of microcline, plagioclase and quartz by a white mica. Albitisation of plagioclase and new generation of albite and biotite are present along fractures. In the highest grade both the microcline and plagioclase are albitised and can crinitised; albite and white mica occupy the areas vacated by quartz. Biotite, Napyriboles, hematite and calcite can be found in vein fillings. In the mafic rocks biotite is replaced by chlorite, epidote and zoisite in the lowest grades of fenitisation; hematite is introduced along fractures. In the highest grades chlorite gives way to biotite, sericite disappears and the plagioclases are further albitised and in rare cases show minor replacement by cancrinite. In the felsic fenites Si decreases and so does K, to a very minor degree; at the same time the following elements are introduced: Al, Fe2+, Fe3+, following elements are introduced: Al, Fe2+, Fe3+, Mg, Ca, Na and C. In contrast, the mafic fenites show an increase in Si, Al and C. In contrast, the mafic fenites show an increase in Si, Al and Na and a depletion in Ti, Fe3+, Ca and C. The mafic and felsic fenites display a convergent chemistry for most elements, with the focus coinciding approximately with the composition of the nepheline syenites from Epembe. A feature of the felsic rocks in the Qz-Ne-Ks system is that the fenitised equivalents breach the Ab-Or thermal barrier of crystal-melt equilibria and enter the undersaturated field; this behaviour supports the concept of solid-solution equilibria as accounting for the fenitisation process. The fenitising solution was essentially an alkali brine in the proportions Na:K:H of about 60:40:1 and a molar concentration in excess of 40. It is likely that fenitisation took place at a P < 4 kb and at a temperature of 450° - 680°C. It is thought that the carbonatite and nepheline syenite magmas are an immiscible pair of liquids, in equilibrium, and thus producing similar styles of fenitisation.
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Transactions of the Geological Society of South Africa, 78 (1), 123-128
A Bouguer gravity-anomaly map of the Barberton Land has been compiled, using data from the Republic of South Africa and Swaziland, which show clearly that the mafic and ultramafic rocks comprising the Onverwacht Group of the Swaziland Sequence are associated with a positive gravity anomaly of 20 to 30 mgal. Interpretation of this anomaly indicates that the Barberton greenstone belt has a probable depth extent of 3 to 4 km, with the possibility of depths reaching 6 km beneath the deeply infolded sediments of the Fig Tree and Moodies Groups, near Barberton.
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Transactions of the Geological Society of South Africa, 78 (1), 129-136
The Soutpansberg trough, though intracratonic, differs in its characteristics from intracratonic basins on the Transvaal craton. The latter developed in depressions or undulations in the cratonic crust, whereas the Soutpansberg trough developed by rifting and downwarping, spreading from east to west on the recently stabilized Limpopo mobile belt, contemporaneous faulting and post- Soutpansberg but pre-Karroo faulting and tilting are largely obscured by post- Karroo faulting. An attempt is made to reconstruct the faulting and the downwarping during the formation of the trough and the original boundaries of the latter. The trough is compared with aulacogens, large graben-like structures spreading from the edge of cratons into the interior. These structures are commonly associated with large geosynclines along the boundaries of cratons and a large hidden Precambrian geosyncline is postulated under the Mozambique coastal plains. Regarding the Waterberg-Soutpansberg-Umkondo correlation, it is concluded that these successions are partly contemporaneous, but were formed under different conditions, the Waterberg succession in intracratonic basins, the Soutpansberg one in an aulacogen and the Umkondo in a miogeosyncline, both the aulacogen and miogeosyncline being associated with a pericratonic geosyncline.
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Transactions of the Geological Society of South Africa, 78 (1), 137-151
The country-rocks of the fluorite deposits comprise felsite, leptite and Bushveld Granite. Towards the west and the south-east these rocks are covered by sandstone and conglomerate of the Karoo System. Quartz veins of post-Bushveld and of post-Karoo age are also present, as well as diabase of post-Waterberg age. Based on variations in texture and/or mineralogical composition four varieties of leptite and five varieties of granite were distinguished. However, no intrusive relationships could be ascertained for the granite varieties, and they are probably comagmatic. The leptite is considered to represent metamorphosed felsite. Mineable fluorite is confined to veins in the leptite which occurs as roof-pendants in coarse-grained granite. These veins generally have a fairly constant orientation, except in the so-called East Workings, where they resemble a stockwork. This consistency in strike and dip of the veins is ascribed to original bedding in the leptite which represents relict flow-banding inherited from the felsite. Apart from the common rock-forming constituents the principal minerals in the veins are fluorite, monazite, allanite, apatite, and bastnaesite. These are described in detail and a complete chemical analysis of the monazite is given and discussed. The monazite is partly metamict and rather rich in calcium, but does not resemble cheralite. Partial analyses indicate that phosphorus, and probably some other elements, have been leached preferentially from metamict monazite which may explain the high calcium content of the monazite. Bastnaesite is formed as an alternation product of metamict monazite. The fluorite varies in colour from almost white to dark purple. The purple variety has a larger unit cell, a higher magnetic susceptibility, a higher specific gravity and a higher yttrium content than the light-coloured variety. Textural evidence suggests that apatite, monazite and allanite were amongst the first minerals to crystallize whereas fluorite was amongst the last. Mineralisation took place predominantly by emplacement in open fissures, although evidence of replacement also exists. Decrepitation tests on the fluorite suggest a crystallization temperature of at least 390±50°C, so that the deposits are probably of pneumatolytic origin. As the granite has an intrusive contact with the fluorite veins, it seems as if the fluorite crystallized from a gas phase which separated early from the granite magma, invaded the overlying roof-rocks and was sealed off by the consolidation of the granite.
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Transactions of the Geological Society of South Africa, 78 (1), 153-159
A small, oval-shaped igneous complex was discovered in the Messina area during recent mapping. This complex consists of an olivine basalt plug, which has been brecciated and veined by carbonate. Chemical and petrological examination of the basalt indicates a similarity with the basalts of the Nuanetsi Igneous Province. The carbonate was observed to occur as dolomitic and calcitic vein filling, and as a flow-banded dolomitic and sideritic variety containing orthoclase and quartz. Extensive sodium metasomatism was observed, radiating away from this igneous centre. Metasomatic alteration of the olivine basalt is also well developed. The chemical composition of the banded carbonate-rock as well as its geological setting suggest that it is a carbonatite.
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Transactions of the Geological Society of South Africa, 78 (1), 161-165
Recent artificial exposures in Grahamstown reveal details of Upper Dwyka rocks which are already largely obliterated by infilling. The structure is broadly synclinal and includes two phosphate beds, one francolite rock and the other a crandallite rock. There are also two horizons of chert. Leaching has affected these rocks to a depth of 40 m below overlying silcrete.
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Transactions of the Geological Society of South Africa, 78 (1), 167-169
Paolovite (Pd2Sn) has been identified in a concentrate from the Atok Mine in the Merensky Reef. The mineral is very similar in its optical properties and chemical composition to a mineral from the Oktyabr Deposit, U.S.S.R. It has a creamy colour when observed with a dry objective, and is pinkish-brown under oil immersion - similar to niccolite. It is fairly anisotropic, ranging from greyish-blue to dark-pink. The chemical composition is Pd 59,9 mass percent, Pt 5,03 mass percent, and Sn 34,4 mass percent, which has the recalculated formula (Pd1,941 Pt0,090)2,031Sn.
The reflectance varies from between 40,59 and 42,43 percent at 436 nm to between 57,43 and 58,28 percent at 656 nm, and the Vickers microhardness is 410,99 kg/mm² when a 5 g load is used. The mineral is orthorhombic with a=8,157, b=5,653, and c=4,313 Å.
Paolovite occurs in the Atok Mine associated with pentlandite and other platinum minerals - mainly braggite, cooperite, laurite - and minor amounts of Pt-Fe alloys, serryfite, moncheite, atokite, As-Sn-Pd and Sb-As-Pd minerals.
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Transactions of the Geological Society of South Africa, 78 (1), 171
Statements such as "The Cape Fold-belt shows ... a complete lack of igneous and metamorphic phenomena" were used recently as a partial justification for rejecting a plate tectonic model for this orogenic belt (Newton, 1973). This view can no longer be supported, as Elliot and Watts (1974), Martini (1974) and Lock and Johnson (in press) have all demonstrated the presence of volcanic materials in lower Karroo strata in the southern Cape. It is our purpose, in this communication, to report some of our own observations on the rocks described by Elliot and Watts, and to show that evidence for volcanism within the Cape fold-belt is found on a regional scale. This is particularly true of the stratigraphic vicinity of the Dwyka/Ecca contact. Elliot and Watts (page 110) remark that the most interesting sample they studied was from one of the green mudstone beds at Zwartskraal, north of Oudtshoorn. It should be noted that although the top of the Matjiesfontein Chert is conventionally accepted as the top of the Dwyka (and therefore the sample mentioned was from the Dwyka, rather than the Ecca, as stated by the authors), the strata between the top of the White Band and the base of the Ecca greywackes have a consistent character. The Matjiesfontein Chert lies near the middle of this interval in the southern and western Cape. A large majority of the mudstones in the topmost Dwyka/lowermost Ecca interval contains some proportion of volcanic debris, and for convenience of reference we will here use the informal term "volcanic interval" for this stratigraphic unit, which usually has an average thickness of about 50 m. We found that the mudstone type of greatest significance was yellow, rather than green.
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Transactions of the Geological Society of South Africa, 78 (1), 25-33
New data on basins or troughs of Waterberg age are presented. The Waterberg basins are compared with the Soutpansberg, Transvaal and Witwatersrand basins, all of which were formed on the Precambrian platform in Southern Africa. Sedimentological and structural features of the basins are not always similar to those of basins which came into existence in late-Precambrian to more recent times. It is demonstrated that conventional terms such as geosyncline, miogeosyncline, flysch, molasse, orogenic belt, etc., are not always applicable to early and middle Precambrian conditions, which are considered to represent a transitional phase between the Archaean and late-Precambrian to more recent times. In the structural control of the basins by the underlying crust, the east-north-east Murchison direction and the north-north-east-trending abyssal fracture zone in the Transvaal and Rhodesian cratons are considered to have played an important role. The Murchison direction is displayed by belts of Swaziland rocks. On the abyssal fracture zone, which was active for a long period, a chain of layered intrusions, alkaline intrusions and extrusions and centres of subsidence were formed. Linear elements in Waterberg tectonism were partly inherited from the Murchison direction and non-linear elements from dome-like or ovoid Archaean structures, which were reactivated in early Precambrian times. The shifting centres of subsidence of the basins and the structural patterns, in particular of Waterberg age, are attributed to subcrustal flow of magma under a relatively unstable craton. The basins were formed in depressions of large crustal undulations, which slowly spread to the north and the west, centrifugally from the cluster of domes in the Southern Transvaal.
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Transactions of the Geological Society of South Africa, 78 (1), 35-41
The mineral composition of eight large lithium-beryllium pegmatites in Rhodesia has been estimated from measurements on bulk samples and in situ. Samples of one to two tons representative of single sections through the bodies were classified mineralogically and the composition of cleaned rock surfaces was estimated visually using a wire graticule. There is close agreement between the results given by each method after estimating the composition of fine-grained mineral assemblages microscopically. Albite is the dominant constituent of the pegmatites, followed by quartz and variable amounts of potassium feldspar, lithium micas, muscovite and spodumene. The average grain size of the lithium-beryllium pegmatites is typically a few centimetres, mainly because of the very fine-grained character of much of the albite and lithium micas. Beryl, the chief beryllium mineral, may be concentrated in zones, but in general the internal structure of the pegmatites is not well defined and beryl is widely dispersed. Studies facilitated by the use of portable beryllium detectors show that although much of the beryl is very fine grained it is not homogeneously distributed through fine-grained pegmatite. The alkali content and refractive indices of beryl from lithium pegmatites in Rhodesia are higher, and the beryllium content lower, than those of beryl from non-lithium pegmatites.
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Transactions of the Geological Society of South Africa, 78 (1), 43-55
The Table Mountain Sandstone in southern Natal consists of two distinctly different lithologic associations, called the Hibberdene facies in the north and the Margate facies in the south. The former consists of four newly named formations and one informally named unit, namely: The Hibberdene Sandstone Formation (at the top), The micaceous sandstone zone The Wood Grange Arkose Formation The Mkunya Orthosandstone Formation The Mtwalume Arkose Formation (at the base). The Margate facies consists of four newly named formations, namely: The Shelly Beach Arkosic Sandstone Formation (at the top) The Mhlangeni Grit Formation The Uvongo Sandstone Formation The Manaba Conglomerate Formation (at the base). An abrupt facies change takes place between the red bed to arkose association (Hibberdene facies) and a quartzose association (Margate facies). The greatest change in lithology takes place between Hibberdene and Umhlangakulu River mouth and it is postulated that a beach zone was situated there during the sedimentation of the Table Mountain Group. North of this beach zone, sediments were deposited in a fluvial environment while south of Hibberdene a shallow high-energy marine environment on a stable platform prevailed.
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Transactions of the Geological Society of South Africa, 78 (1), 57-65
Palaeomagnetic measurements are reported from twenty-eight drilled sites in the Umvimeela Dyke, East Dyke, Sebanga Poort Dyke and the Southern Satellite Dykes of the Great Dyke. No significant differences in the direction of magnetization were observed over the length of the Umvimeela Dyke and its mean direction (D = 220,6°, I = 162,7° with k = 128,6) is very close to that of the Great Dyke and 220,4°, I = -58,3°) (2 532±89 m.y.). The angular dispersion of directions of the Umvimeela Dyke (7,1°) is small compared with the secular variation which suggests that the whole of the dyke was emplaced over a very short period of time. Similar results were obtained for the East Dyke with a mean direction of D = 204,6°, I = -56,2° with k = 101,2. The northernmost member of the Main Swarm, the Msiningira Dyke, shows a direction of magnetization (D = 209,8°, I = -61,1° with k = 25,0) close to that of the Great Dyke, supporting the hypothesis that these dykes are correlatives. The sites in the southern Parts of the Main Swarm, the Umvimeela Dyke Extension and East Dyke Extension give virtual geomagnetic poles which are strung out between the Bushveld gabbro pole and the Mashonaland dolerite pole. It is suggested that these parts of the Main swarm and the Extensions were emplaced in Great Dyke times but completely remagnetized during a widespread thermal event in the Limpopo Belt which ended approximately 1 900 m.y. ago. The directions of magnetization of the Bubi Swarm (D = 151,2°, I = -43,3°) and the Crystal Springs Swarm (D = 152,8°, I = -40,4°) are very similar and it is suggested these swarms were emplaced subsequent to the Mashonaland dolerites. The direction of magnetization of the Sebanga Poort Dyke (D = 146,5°, I = -58,8°) suggests a time of emplacement close to that of the Mashonaland dolerites.
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Transactions of the Geological Society of South Africa, 78 (1), 67-70
Peculiar structures present in the upper part of the Tsumeb Subgroup of the Otavi Group near Tsumeb, South-West Africa, previously described as stromatolites are now shown to be silicification structures. The occurrence of chert in this group may elucidate the puzzling origin of pebbles of red jasper, and black oolitic and stromatolitic chert of the conglomerates of the upper part of the Nama Group. At certain times a dry climate could have existed in the provenance areas which supplied these pebbles.
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Transactions of the Geological Society of South Africa, 78 (1), 71-74
Granophyre developed along the contact beween Bushveld granite and Transvaal quartzite in the Stavoren area is demonstrably of magmatic origin and formed as the roof facies of the Bushveld granite batholith. The granophyre sheet shows intrusive contacts with the overlying quartzites and grades downward into normal Bushveld granite. The presence of phenocrysts and interstitial patches of microgranite in the granophyre reflects stages in the progressive crystallization of the magma. Recognition of a magmatic origin of this granophyre places constraints on the use of the term "Stavoren granophyre" as previously defined by other workers.
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Transactions of the Geological Society of South Africa, 78 (1), 75-82
Ecca strata exposed to the north-east of Vryheid were subdivided into six recurring facies, each of which is readily identified in the field on the basis of composition, texture and sedimentary structures. Statistical analysis of facies arrangement in eight measured sections reveals the presence of Markovian tendencies which are expressed in vertical sequences of two distinct types. These tend to occur below and above the lowermost coal bed respectively. The lower statistically-derived sequence partially confirms a qualitative model of upward-coarsening deltaic deposits. Cross-laminated sandstone is overlain by coarser-grained cross-bedded sandstone, together representing distributary mouth-bar accumulation at the top of a cycle. This is followed by alternating siltstones and sandstones of the basal prodelta portion of the overlying cycle. The anticipated upward transition from these alternating beds into cross- laminated sandstone is not confirmed statistically, possibly because of interference from a number of smaller bay-fill and crevasse sequences, some of which are apparently non-cyclic. Markovian tendencies are far stronger in the rocks above the lowermost coal. Here erosively-based massive or conglomeratic sandstone is overlain successively by cross-bedded sandstone, cross-laminated sandstone and alternating beds of siltstone and sandstone. These are interpreted as fluvial channel deposits. An additional independent transition of siltstone or mudstone into coal probably represents interfluvial accumulation. The majority of the coals appear to have had an autochthonous origin.
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Transactions of the Geological Society of South Africa, 78 (1), 83-88
The Mogalakwena Formation of the Precambrian Waterberg Supergroup (Northern Transvaal) consists predominantly of sandstone but also of coarse and widely distributed conglomerates. The sandstone shows trough type cross-bedding throughout and lacks other types of sedimentary structures. By comparison with both modern and ancient fluviatile sediments it is concluded that the Mogalakwena Formation was deposited by a braided river system which drained a youthful mountain range in the area that is now known as the Limpopo Belt.
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Transactions of the Geological Society of South Africa, 78 (1), 89-95
Two bodies of biotite-rich granite, here named the Makhutso Granite, occur north-west and south-west of Dennilton, Transvaal. The southern body displays a fine-grained marginal phase, suggesting that the Makhutso Granite plutons were intruded after solidification of the Main Granite batholith. The Makhutso Granite cannot be distinguished with confidence from other Bushveld granites on the basis of textural analysis, nor does the major-element geochemistry offer a unique criterion for recognition of the granite. Its minor element geochemistry (Zn, Zr, Ba, Rb, and Sr), however, is significantly different from the Main and Bobbejaankop Granites. The Makhutso Granite probably intruded as a discrete magma in late-Bushveld time.
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Transactions of the Geological Society of South Africa, 78 (1), 97-100
In this report the field relations and sampling in the area concerned are described. The analytical methods and results are figured, tabled and discussed. It is concluded that the Hoedjiespunt granite has a zircon age of approximately 600 My while the Vredenburg granite dates in the vicinity of 530 My. Preliminary field results indicate that a type of granite designated as HPG4 might represent an intrusive phase slightly younger than the main Hoedjiespunt granite. The postulated extension of the Colenso fault from the Darling area into the Saldanha area is corroborated by the available geochronological data. Investigations are continuing.
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Transactions of the Geological Society of South Africa, 78 (2), 173-183
Measurement of 833 cross-bed foresets in the Dwaal Heuvel Formation, Transvaal Supergroup (2 224±21 million years old) at 41 localities along the eastern and north-eastern outcrop rim of the Transvaal Basin indicate transport of sediment towards the south and south-south-west. The formation becomes thinner and finer- grained in the down-current direction, and eventually feathers out into an assemblage of shale with oolitic ironstone lenses. The latter are considered to represent a more distal facies of the Dwaal Heuvel arenites. Some of the clastic material which makes up the Dwaal Heuvel was probably derived by the deroofing of the once deeply buried metamorphics of the Limpopo Mobile Belt. This detritus was carried to the northern limit of the basin by rivers, there to be distributed laterally by marine currents moving consistently to the south and south-west.
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Transactions of the Geological Society of South Africa, 78 (2), 185-190
Granophyre occurs as a sheet between Rooiberg felsite and Bushveld granite in the Moloto area. Statistical analyses of phenocrysts in the felsite and the granophyre suggest a new technique for determining the origin of the granophyre. Similarity in size and frequency of phenocrysts in the felsite and the granophyre indicate that granophyre phenocrysts are inherited from the felsite. Recrystallization of felsite to produce granophyre, with a slight difference in the parameters of the phenocrysts, as a low grade metamorphic and probably a partially metasomatic product, was probably caused by the intrusion of the Bushveld granite magma.
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Transactions of the Geological Society of South Africa, 78 (2), 191-199
The Precambrian Makgabeng Formation of the Waterberg Supergroup (northern Transvaal) exhibits large bedding and pronounced lamination. These, together with additional features described in the text, lead to the conclusion that the Makgabeng Formation is of aeolian origin and shows many of the characteristics of transverse dunes.
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Transactions of the Geological Society of South Africa, 78 (2), 201-213
The geology, mineralogy and petrology of the syenites of the Phalaborwa Complex are described, and 61 new chemical analyses are presented. The syenites occur as small plug-shaped bodies, and can be classified into four textural groups which include a hypidiomorphic-, a granular-, a gneissic- and a orphyritic-type of syenite. These four groups of syenites are interpreted as the result of the timing of emplacement with respect to crystallization of the original magma. Around the western periphery of the pyroxenite at Phalaborwa, fenites were encountered which appear to have formed as a consequence of the introduction of Al, K, Ca, Mg and Fe, and the removal of Si and Na from the Archaean granite. The chemical data show that the syenites form a succession of differentiated rocks, which appear to have crystallized from a fractionating trachytic magma. The chemical data also suggest that the parent magma from which the complex formed may have been shonkinitic. It is concluded that the carbonatite magma formed as an immiscible carbonate magma from the shonkinitic magma at the time that the pyroxenite crystallized. The remaining magma then fractionated towards a series of differentiated trachytic magmas from which the syenites crystallized. The association of syenites with carbonatite at Phalaborwa is in marked contrast with the normal association of ijolites with carbonatite. This contrast is explained by the fact that the magmatic lineage at Phalaborwa became K-enriched and not enriched in Na, and thus fractionated towards a trachytic end point rather than a phenolitic or nephelinitic end point. It is concluded that the former trend results from the dissociation of the diopside molecules in the relatively unfractionated parent magma, whereas the latter trend results from the dissociation of the anorthite molecules, in a more fractionated magma.
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Transactions of the Geological Society of South Africa, 78 (2), 215-224
The Granitberg foyaite complex is one of three known alkaline igneous complexes, which, together with a dyke swarm, constitute the Luderitz Alkaline Province of Lower Cretaceous age. The near-circular complex is emplaced into dolomites and sandstones of the Bogenfels Formation. A large fragment of these sedimentary rocks, originally forming the roof of the intrusion, is preserved in the centre of the complex. Three major intrusive phases can be recognized. The first phase resulted in the magma chilling at the roof to form porphyritic nepheline syenites. The second phase crystallized as the inner Foyaite - a cylindrical zoned plug capped by layered, laminated, and xenolith-rich foyaites. The third phase was the emplacement of the Outer Foyaite magma into which the Roof Zone - Inner Foyaite unit foundered. The Outer Foyaite is zoned with a miaskitic core and agpaitic outer zone. A dyke swarm is developed in the vicinity of the complex and the dominant dyke rocks are tinguaites and quartz-bostonites.
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Transactions of the Geological Society of South Africa, 78 (2), 225-233
Reactions between the Outer Foyaite magma of the Granitberg complex and the sedimentary country rocks are striking. At sandstone contacts the magma has assimilated quartz-rich sedimentary material and a suite of pulaskites, peralkaline nordmarkites, and peralkaline granites has been generated by a complex process involving both assimilation and crystal fractionation. The sandstones have suffered Ca-Mg metaso transformed into rocks containing perthite, diopside and some times quartz. At dolomite contacts, assimilation has resulted in the formation of a widely developed shonkinite with minor amounts of leucocratic, pegmatitic nepheline syenite and a suite of ijolite-melteigite rocks. Assimilation reactions have also generated in fenitizing fluid phase which is responsible for the observed fenitization in some of the contact rocks. Metamorphic mineral assemblages in the calc-silicate rocks indicate that temperatures of 500°C and greater prevailed in the contact zone, and that the fluid phase accompanying metamorphism had a low CO2 content.
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Transactions of the Geological Society of South Africa, 78 (2), 235-249
Metasedimentary sequences in the Tantalite Valley area of the Namaqualand Gneiss Complex have experienced a complex metamorphic and tectonic history. Rocks to the north of a major tectonic lineament (the Tantalite Valley Lineament) have been affected by upper-amphibolite facies metamorphism, while those to the south show mineral associations consistent with lower grades. There is evidence that right-lateral-strike-slip movement has taken place along the Tantalite Valley Lineament which, in this area, is expressed as a wide zone (2-8 km) of highly deformed rocks containing mylonites and faults characterized by brittle fracture (i.e. "shear zone"). The Tantalite Valley Complex is a poorly mineralised (Cu and Ni sulphides) body of approximately concentric peridotite-gabbroid intrusions which has been partly affected by the metamorphism and deformation within the Tantalite "shear zone". It has some similarities with the zoned, or concentric, complexes found in orogenic belts elsewhere in the world. The distribution of economic, mineralised pegmatites in the area is related to the presence of the Tantalite Valley Complex during deformation in the "shear zone".
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Transactions of the Geological Society of South Africa, 78 (2), 251-256
A plot of the (FeO + MgO)/(CaO + MnO) ratio versus the cell-edge for nine garnets from a variety of granulite facies rocks in the Limpopo Folded Belt outlines a well defined hyperbolic curve. Although the garnets are from a number of different host-rocks and show a wide range of composition, they reflect the same general P/T conditions of metamorphism; the use of Nandi's correlation curves (Nandi, 1967, p. 89-93) for cell-edge and composition to determine metamorphic grade is therefore suspect.
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Transactions of the Geological Society of South Africa, 78 (2), 257-264
The middle Miocene Saldanha Formation is encountered at isolated localities onshore between Cape Town and Port Nolloth. Offshore the equivalent deposits are more widespread. The formation is characterized by shallow marine phosphatic sandstone and a marine microsphorite. At Ysterplaat the Saldanha Formation bears casts of intertidal and infratidal mollusc shells. Deposition is thought to have taken place in warmer, transgressive Neogene seas.
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Transactions of the Geological Society of South Africa, 78 (2), 265-273
Arthropod trackways have previously been described from glacial varves on the eastern side of South Africa. On the western side the glacial deposits in which the trackways are preserved show sole structure evidence of subaqueous mass- transport. They are tentatively regarded as small-scale turbidity current deposits. Umfolozia sinuosa Savage 1971 and Maculicbna varia gen. et sp. nov. are the most common trackways in the Dwyka. However, the new ichnogenus is scarcely represented in the east. It is also much less conspicuous than Umfolozia in the overlying non-glacial Ecca deposits. As the glacial deposits in the east are younger than those in the west, it is probable that the creatures responsible for the Maculicbna varia trackways were already on the decline by the time the glacial sediments in the east were accumulating. As such, the association between M. varia and the "turbidites" in the west must be co- incidental.
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Transactions of the Geological Society of South Africa, 78 (2), 275-292
The geological considerations in this address concern the following: 1. Exploration for uranium in quartz-pebble conglomerates of the Dominium Reef Group and the Witwatersrand Supergroup, in the Karoo Supergroup and in the western part of the Damara Orogenic Belt; 2. Characteristics and criteria affecting uranium mineralization in (a) the Witwatersrand Supergroup (b) the Ventersdorp Supergroup and the Ventersdorp Contact Reef, (c) the Transvaal Supergroup, particularly the Black Reef Group in various parts of its occurrence; and (d) the intrusive pipes at Palabora, Tweerivier (Brits), Glenover (Thabazimbi), Pilanesberg and Ondurakoruma in South-West Africa.
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Transactions of the Geological Society of South Africa, 78 (2), 293-311
The changing gold price, the variation in world gold production, the relationship of the gold price to the price of other commodities as well as to the South African balance of trade are reviewed. The conclusion is reached that the price will continue to rise and that the discovery of new goldfields is essential to South Africa's continued prosperity. The concept of a fluvial origin for the Witwatersrand auriferous conglomerates is supported; a comparison is made with sediments deposited by a present-day river on the coastal plain of the eastern United States. The conclusion is reached that new Witwatersrand type goldfields still await discovery in or around the Witwatersrand sedimentary basin and that exploration for them should be vigorous.
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Transactions of the Geological Society of South Africa, 78 (3), 313-322
Black Rock is situated on the northern Zululand coastline, 20 km south of the Mozambique Border. It consists of a 20 m high headland, virtually surrounded by intertidal platforms. The intertidal platforms are typically developed with raised rims, shallow tidal pools and concave sea-level notches profiled into the headland. An ill-defined emergent platform exists at 4 m above the present mean sea-level. Densely spaced, perfectly cylindrical solution pipes originated from a now completely eroded emergent platform penetrate the headland downwards to sea-level. The main rock-type constituting the headland and intertidal platforms is strongly cross-bedded calcareous dune-rock (aeolianite). It consists of well- rounded quartz grains and shell fragments cemented together in a finely crystallized calcium carbonate matrix. The overall carbonate content is of the order of 50 percent. Sedimentological and geomorphological similarities exist between the aeolianite of Black Rock, the Bluff Beds of Durban and other occurrences along coastal Natal and Zululand. It is suggested that the aeolianite formed during a marine regression in the past. The dunes then became cemented by downward percolation of ground-water, containing dissolved calcium carbonate.
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Transactions of the Geological Society of South Africa, 78 (3), 323-333
The occurrence of chylindrical pipes in coastal aeolianite at Black Rock and Pomene, situated respectively along the Northern Zululand and Moçambique coastlines, is described in detail. The diameters of these pipes vary between 300 mm and 400 mm and they extend vertically downward to present mean sea-level; their maximum depth is approximately 20 m. The genesis of the pipes is discussed considering various possible hypothese: a pothole hypothesis, a hypothesis relating to the decay of buried tree trunks and lastly a solution hypothesis. Of these the last hypothesis is preferred and a genetic model is presented. The observations made at Black Rock and Pomene conform to this mode of origin.
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Transactions of the Geological Society of South Africa, 78 (3), 335-348
Seven boreholes drilled through the cover of Karroo rocks in the Bethal district were studied; six of the boreholes intersected a northward-dipping extension of the Bushveld Igneous Complex. Except for 170 m of the Layered sequence, which was not intersected, the complete section of about 1 900 m could be constructed. The Layered Sequence was divided into five zones: these consisted of a Basal Zone of peridotite and pyroxenite which rests on a diabase of the early sill phase of the Bushveld Complex. Associated with the diabase are thermally metamorphosed sediments of the Pretoria Series. The Basal Zone passes upwards into Zone I which contains abundant anorthosite and mafic pegmatoid, both of which occasionally contain significant amounts of pyrrhotite and pyrite. Minor amounts of chalcopyrite and pentlandite are usually associated with the sulphides.
A possible fresh injection of magma may have been responsible for a break in the mineralogical trend in the plagioclase of Zone I, from An45 to An56 over only 35 m, which has given rise to a very persistent mineralogical marker horizon. Zone II consists mainly of norite and gabbro and forms a transition between Zone I, which is poor in magnetite, and Zone III which is characterized by magnetite-rich gabbro, a prominent anorthosite layer and six magnetite layers. Zone IV is predominantly dioritic in composition; two anorthosite layers and eleven magnetite seams were intersected in this zone. Trends established from modal analyses and determinations of the composition of the plagioclase, orthopyroxene and olivine show that cyclical crystallization had occurred. There is, however, an overall trend from An55 and Fa32 in the middle of Zone I to An38 and Fs62 at the top of Zone IV. The olivine and orthopyroxene of the Basal Zone are poor in iron, with values ranging from Fa6 and Fs17 to Fa15 and Fs22 respectively. Apart from its occurrence in the Basal Zone, olivine reappears only at the top of Zone IV, where it is greatly enriched in iron at Fa75. Fluorapatite enters as a prominent cumulus phase near the top of Zone IV. Zones I to IV of the Bethal area can be correlated with the Upper Zone of the Bushveld Complex, and the Basal Zone with the equivalent horizon in other parts of the Bushveld Complex. The absence of both the Main and Critical Zones in the Bethal area may be due to the migration of fractionated magma from the main magma chamber. It is considered more probable, however, that the Bushveld Complex rocks in the Bethal area represent a separate injection centre. Crystallization occurred under conditions of faster cooling and higher oxygen fugacity in the case of rocks of the Upper Zone in the eastern Bushveld Complex.
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Transactions of the Geological Society of South Africa, 78 (3), 349-357
The geology of part of the south-eastern margin of the Damara geosyncline east of Windhoek is described. In the south-east a fault-bounded wedge of strata has, on the basis of similarities in lithology as well as the presence of copper mineralisation, been equated with the Tsumis Group. To the south-east these sediments abut against the Buschmannsklippe Formation, while in the north-west they have been overridden by rocks of the Damara Supergroup. This consists of the essentially phyllitic Nosib Group, overlain paraconformably by the Swakop Group, comprising schists, quartzites and carbonate rocks, with subordinate amphibolite intercalations. A pebble conglomerate within the sequence has been tentatively equated with the Chuos Tillite. Intrusive rocks comprise the Dachsberg granodiorite and various dolerites, now extensively metamorphosed. The Damara Supergroup has undergone at least three periods of deformation, and is of low metamorphic grade. Economic interest is centred on numerous small occurrences of copper mineralisation.
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Transactions of the Geological Society of South Africa, 78 (3), 359-360
Mafic dykes of diabasic and doleritic composition are described from the Messina District, Transvaal and their bearing on the age of copper mineralization discussed. Since the diabase dykes can be demonstrated to precede copper mineralization and some dolerite dykes post-date copper mineralization three dykes were selected for K-Ar age determination in order to set limits on the age of copper mineralization. It was not possible to date the metallization with accuracy. The copper mineralization is definitely older than 185 My and younger than 1 800 My and it is also probably younger than 700 My.
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Transactions of the Geological Society of South Africa, 78 (3), 361-365
Cenozoic dolostones are rare in South Africa. This note records a Pleistocene dolostone outcrop located in the intertidal zone at Saldanha Bay. The rock is mostly composed of subhedral dolomite crystals, with lesser amounts of quartz, feldspar and glauconite. The dolomite is stoichiometric and well ordered, suggesting formation under hypersaline conditions. This dolostone is interpreted as having formed during a desiccation phase in the history of Saldanha Bay. Onset of the Wurm II glaciation caused the sea to withdraw from its high stand about 35 000 YBP. As the sea receded, it left behind an isolated body of saline water. Evaporation eventually concentrated the brine in this saline lake to a level which allowed precipitation of dolomite. This happened about 20 300-630 + 600 YBP. The subsequent post-glacial transgression has removed salts precipitated after and seaward of the dolostone.
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Transactions of the Geological Society of South Africa, 78 (3), 367
Grey, thickly-bedded sandstone of the Table Mountain Group is the oldest rock-type in the area. Cross-beds indicate a major source area to the north-north-west. Following intra-Karroo folding of this sandstone the Cretaceous conglomerate of the Enon Formation was deposited. The clasts in this unit show a better degree of sorting and smaller average size towards the south where several layers of sand and clay-sized material are intercalated. The Brenton beds, of Upper Jurassic age (Dingle and Klinger, 1972), consist of fossiliferous silty clay with minor sand and conlomerate. The Enon and Brenton beds occur mainly in fault troughs, the bounding faults of which show late to post-Cretaceous movement. Subsequent marine encroachment cut a bevel which now reaches back to the Outeniqua Mountains. A very thin post-Enon cobble conglomerate exposed on the west bank of the Goukamma River, rests directly on this bevel and probably represents reworked Enon conlgomerate. Re-emergence was followed by deposition of the dominantly aeolian Knysna beds. A 1,0 m thick intercalation of lignite occurs 4,3 m east of Knysna on the southern side of the road to Port Elizabeth. During the Pleistocene local beach deposits formed and dunes more than 60 m in height accumulated in the western part of the area. Subsequent cementation by calcite transformed much of this material into sandy limestone. Recent dune sand now forms a thin discontinuous cover to the limestone nearer the coast.
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Transactions of the Geological Society of South Africa, 78 (3), 369-370
A simple set of curves is presented which is designed to aid the interpreter in the choice between a "2D" or "3D" approach to gravity interpretation. Use of the curves as an initial test shows that many apparent "3D" anomalies could be successfully treated in a "2D" manner if certain conditions are met.
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Transactions of the Geological Society of South Africa, 78 (3), 371-372
In a recent paper (Darracott, 1974), I described the discovery of a basic igneous intrusion at Mara, northern Tanzania, and showed how the intrusion lies on the northward extension of the "Bushveld line of intrusions" described originally by Cousins (1959). At the same time I pointed out that, although this was quite an extrapolation of Cousins' hypothesis (Fig. 1), it may be more than just fortuitous, but that in any case this could not be firmly tested until igneous intrusions of comparable age and type are discovered along the line. Mr. J.K. Whittingham, of the Geological Survey's Cape Town office, has recently informed me (pers. comm., February, 1975) that such intrusions do in fact exist. In two works, of which I was unaware at the time (Whittingham, 1963 and 1965), he describes Precambrian, post-granite dykes of altered gabbro in the upper Nzombe Valley, north of Chunya, southern Tanzania, which lie on the projected extension of the "Bushveld line", approximately midway between the Great Dyke and the Mara intrusion (Fig. 1). These north-east to north-north-east trending dykes are remarkably similar petrographically to the Mara intrusions, and to some of the gabbros of the Great Dyke and, more particularly, its associated dykes as described by Worst (1960). The similarity to the Rhodesian dykes led Whittingham (1965) to suggest that they were part of the same dyke system; in his personal communication he notes that, as far as he is aware, no other comparable dykes or intrusions have been located in southern Tanzania or any part of Zambia. The existence of the Nzombe dykes adds far greater credance to the suggestion that the "Bushveld line" may extend as far north as Lake Victoria, and possibly beyond.
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Transactions of the Geological Society of South Africa, 78 (3), 373-375
Research on the phosphate-rich sediments of the South-Western Cape Coast has hitherto largely been concentrated on the off-shore deposits of the Agulhas Bank. Mr. Tankard's paper on the on-shore, phosphate-rich sediments therefore fills a notable gap in the published literature. My work on these on-shore, phosphate-rich sediments (Smith, 1971) was obviously not available to the author when he was preparing his paper. Mr. Tankard's views on the genesis of the "Varswater Formation" parallel mine. There are however a number of points on which I would appreciate his comments.
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Transactions of the Geological Society of South Africa, 79 (1), 13-21
Recent studies of the acid rocks of the western Bushveld Complex have led to the conclusion that there are here varieties of granophyric rocks that include both magmatic and metamorphic granophyres. Metamorphic granophyres are considered to have been formed by metasomatic and anatectic processes acting on the roof rocks of the layered mafic sequence. Magmatic granophyre intruded subsequently and precedes the intrusion of the granite to which it is not genetically related. Within this intrusive framework it is possible to obtain an estimate of the date of formation of the Brits graben (about 1 940 M.a.), which is believed to have formed after the intrusion of the magmatic granophyre and very shortly before, or contemporaneous with, the intrusion of the granite.
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Transactions of the Geological Society of South Africa, 79 (1), 22-26
Quantitative interpretation of the gravity data of the western Bushveld Complex has been carried out. The results tend to support an earlier, qualitative interpretation consisting of a system of anticlines and synclines having north- west trending fold axes. They also indicate that the folding may extend farther to the south-west than previously suspected and between Rustenburg and the acid phase - mafic sequence contact of the Bushveld Complex, the layering of the Complex may be horizontal for some distance.
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Transactions of the Geological Society of South Africa, 79 (1), 27-30
Rhyolite vitric and lithic tuffs, accretionary lapilli-tuff, volcaniclastic sandstone and tuffaceous siltstone and shale attain a thickness of about 40 m on Hereford 202 in the Herbert District. The sequence for which the informal name "Hereford formation" is proposed rests directly on the Basement Complex and is unconformably overlain by the basal sedimentary unit of the Ventersdorp Group. Although these beds can be correlated with the Zoetlief Formation for stratigraphic reasons, evidence from adjacent acid volcanic sequences indicates that the tuffs and sediments belong to the Ventersdorp Group. Ash-flow giving rise to ignimbrites built the largest part of the succession, while ash-falls and reworked volcanic material are subordinate. The volcanism was of an explosive nature with the centre of eruption located nearby to the south-west. The acid volcanism was followed by a lengthy period of erosion before the outflow of the andesitic lava of the Ventersdorp Group.
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Transactions of the Geological Society of South Africa, 79 (1), 3-12
In the eastern Transvaal, the mafic phase of the Bushveld Complex rests on eight different formations, six of them situated stratigraphically above the Magaliesberg Quartzite. The latter formations, which are usually largely obliterated by the Bushveld layered sequence around the rest of the Transvaal Basin, are exceptionally well preserved in the study area. Their stratigraphy, depositional environments, and stratigraphic relations are outlined. Most of the formations were deposited in marginal-marine situations, including the offshore, nearshore-subtidal, beach, and intertidal mud flat environments. The stratigraphic data collected was used to compile a panel-diagram on which two field-established isotherms were plotted. The quartzite recrystallization line and the hornfels line are located, respectively, some 500-1 500 metres and 3 500 metres below the complex. Both isotherms diverge from the basal contact of the Layered Sequence when traced to the north. The deeper penetration of metamorphic effects in this direction is related to a thicker pile of mafic igneous rocks and a greater proportion of ultramafics in that direction. From south to north across the area, the basal contact of the Layered Sequence truncates up to 5 000 metres of the floor stratigraphy. The truncated pile is inferred to have been jacked up on top of the mafic phase, there to be metamorphosed by the subsequent intrusion of the granitic phase of the Bushveld. The highly altered array of sediments that have been mapped along the contact of the Bushveld mafic rocks and the overlying acid phase are thus thought to represent the strata "missing" from the Bushveld floor.
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Transactions of the Geological Society of South Africa, 79 (1), 31-48
The geological setting of the Ventersdorp succession and the age of this early Proterozoic sequence on the South African shield are discussed, and a comprehensive review of the literature on stratigraphic aspects is presented. Bothaville is selected as the type area, being the only region where the complete succession is present, and where the stratigraphy can be unravelled from cores of numerous boreholes drilled in the search for new gold mines between the Klerksdorp and Orange Free State goldfields. A formal lithostratigraphic classification, in compliance with the South African Code of Stratigraphic Terminology and Nomenclature, is set up. The Ventersdorp succession is subdivided as follows: the andesitic Klipiviersberg Group at the base, subdivided into the Westonaria, Alberton Porphyry, Orkney, Jeanette Agglomerate, Loraine and Edenville Formations; the Platberg Group comprising the Kameeldoorns Formation, consisting wholly of immature sediments; the Makwassie Quartz Porphyry Formation with distinctive quartz porphyries as well as green- grey dense and porphyritic lavas and intercalated beds of immature sediments; and the Rietgat Formation with alternating dense and porphyritic quartz-free lavas and sediments. A cycle of clastic sub-greywacke deposition the Bothaville Formation, and finally the Allanridge Andesite Formation, are correlated with the former Pniel Series of A.L. du Toit.
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Transactions of the Geological Society of South Africa, 79 (1), 49-52
A few small, isolated lenticular deposits of limestone (stromatolitic in places and with varying proportions of arkosic clastic particles) appear in a highly variable succession of intercalated arkose lithofacies and volcanic rocks. The best exposure of the local limestone basins contains several beds of domal stromatolites and one of columnar stromatolite at the base. Domal stromatolites occur in all the limestone outcrops in the T'Kuip Hills area and vicinity. This points to ideal conditions for growth of algae and to the primary nature of the limestone. A new isotopic age determination of 2 310 M.a. for an acid lava near the base of the sedimentary-volcanic succession, previously correlated with the Soetlief Formation, now confirms the Ventersdorp age of the succession, which is correlated with the middle division of the Ventersdorp Supergroup in the Bothaville area.
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Transactions of the Geological Society of South Africa, 79 (1), 53-57
In the lower part of the Ongeluk Andesite Formation a road-cutting west of Griquatown exposed a sequence of lava with very distinctive textures. At the bottom lies a massive lava of andesitic or basaltic composition. This is overlain conformably by a unit with large oval fragments of massive lava surrounded by smaller, dark-grey, angular glassy fragments in a fine-grained matrix of angular palagonitised glass shards. The large fragments of massive lava are interpreted as the remains of pillows, the outer portions of which were shattered by quenching in water and which formed the smaller angular fragments surrounding the large ones. The matrix probably represents aquagene tuff, and the unit as a whole is therefore a hyaloclastite. Non-vesicular pillow lavas, devoid of concentric zoning, abound in the top unit. Their dimensions vary from 1 m to 14 m horizontally and less than 1 m to 3 m vertically. Red jasper fills the tricuspate interpillow spaces and is probably a major source of the larger fragments of red jasper rubble on surface.
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Transactions of the Geological Society of South Africa, 79 (1), 58-60
In the area to the north-west of Upington (Area 2820B) a close relation exists between volcanic rocks of the Koras Formation and the Blauwbosch Granite. A remarkable resemblance in mineralogical and chemical composition is demonstrated. The Blauwbosch Granite, which was hitherto mapped as Granite Gneiss (Ag 7) of the Namaqualand Belt of metamorphism and granitization, can therefore be classified as the granite phase of the Koras Formation.
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Transactions of the Geological Society of South Africa, 79 (1), 61-75
The area is underlain by arenaceous and argillaceous sediments, limestone and andesite of the Pretoria Group. Thermal metamorphism is responsible for their alteration to hornfels, slate, quartzite, marble, and amphibolite and they are composed of minerals including andalusite, cordierite, biotite, garnet, amphibole, diopside, calcite, feldspar, and a few more. All mineral assemblages plot on ACF and AKF diagrams typical of the hornblende-hornfels facies. Mesonorms and Niggli values were calculated for eight chemically analysed metamorphic rocks. Several variation diagrams are drawn so as to compare them mutually and with existing data. An estimate of physical conditions prevailing during metamorphism is made.
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Transactions of the Geological Society of South Africa, 79 (1), 76-80
The capacity of a mafic magma to dissolve sulphur has been shown by Haughton and others to decrease in melts with low ferrous iron and fS2 and a high fO2. In order to establish the chemical environment that encouraged both sulphide immiscibility and the precipitation of the sulphide phase in the diorites of the Upper Zone of the Bushveld Complex in the Bethal area, the aFeO, fS2, and fO2 would have to be determined.
Although there is insufficient thermodynamic data to calculate aFeO in Bushveld rocks, temperature, fO2 and fS2 can be determined from magnetite-ilmenite and pyrite-pyrrhotite pairs. Minimum temperature of crystallization calculated from coexisting silicate phases, however, ranged from 859°C to 1 020°C, compared with a range of 585°C to 685°C established for the oxides and 440°C to 680°C for the sulphides. It is evident, therefore, that the oxide and sulphide phases have undergone extensive subsolidus re-equilibration.
During fractional crystallization the FeO in a Bushveld magma would combine with SiO2 to form pyroxene and olivine. Increases in fO2 would, therefore, be reflected in an increase in aSiO2 and in the amount of magnetite precipitated, as can be shown in the following reactions:
FeSiO3 + 1/6O2 = 1/3FeO.Fe2O3 + SiO2 (liquid)
in pyroxene magnetite
Fe2SiO4 + 1/3O2 = 2/3FeO.Fe2O3 + SiO2 (liquid)
in olivine magnetite
Calculations show that increases in aSiO2 in rocks above the sulphide-enriched zones are associated with sulphur undersaturation.
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Transactions of the Geological Society of South Africa, 79 (1), 81-145
Geochemical analysis, porosity and permiability determinations and petrographic studies were carried out on borehole and on some surface material from various parts of the Cape-Karroo basin. The results for different parameters show trends that are consistent with a general decrease in diagenesis from south to north over the basin as a whole. In the southern fold belt and for some distance to the north Cape and Karroo argillites which could have acted as source rocks for hydrocarbons are in a state of incipient or very early metamorphism, which makes them prospective, at best, for dry gas only. The associated sandstones, which could have formed reservoirs, have almost no porosity and permeability and likewise show indications of incipient metamorphism. This is attributed mainly to deep burial and associated high temperatures prior to the Cape folding. On available evidence shales in the central part of the Karroo basin, roughly between latitudes 31°30' and 29°S, have very low organic contents, which make them unattractive as source rocks for oil; they are also in a state of very strong diagenesis bordering on early metamorphism, due partly to the effects of dolerite intrusions and partly to relatively deep burial. Sandstones are virtually absent below the base of the Beaufort sequence; those of the Beaufort which have been analysed or studied petrographically show poor reservoir characteristics. Good results were, however, recorded for one shallow borehole drilled in the Cave Sandstone near Barkly East. In the northern part of the Karroo basin, roughly to the north of latitude 29°S, the Middle Ecca sandstones and associated organic-rich shales constitute fair to adequate reservoir and source rocks, respectively. Their state of diagenesis were not influenced by dolerite intrusions is consistent with the preservation of wet gas in the south and south-east, and oil in the north and north-west. In the southern part of the basin the regional incipient or low-grade metamorphism has obliterated almost all local variations in clay mineralogy, porosity and source- rock potential which may once have existed in the Karroo sediments. In the northern part, on the other hand, there is much greater variability, and relationships are therefore much more complex. This paper includes only a generalised summary of results for the northern area; more detailed reports on some aspects of our studies may be prepared at a later stage.
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Transactions of the Geological Society of South Africa, 79 (1), 81-145
Dr Jansen's paper on the Soutpansberg must be regarded with caution for the following reasons: 1. A proposal of a basin model should be based on a considerable amount of sedimentological and stratigraphical data, e. g. Button [1973]. The field work should be related to detailed mapping of the basin and its surrounding areas. As admitted by Dr Jansen (page 136) this work has not been completed. 2. By definition, an aulacogen is developed at a continental margin and represents a failed geosyncline, [Hoffman, 1970]. No evidence is available to support this. In fact it is probable that the Limpopo Mobile Belt was located well within Gondwanaland and that the Soutpansberg lavas and sediments were developed in an intracratonic, rift-like basin long before continental break-up occurred. An age determination, quoted by Dr Jansen, [Burger and Coertze, 1973], indicates the lavas at the base of the succession to be about 2025 m.y. old. 3. The absence of any chemical and marine clastic sediments indicates that the dominant environment was non-marine during the deposition of most of the rocks exposed at present. Thus there is no evidence to indicate that the Soutpansberg rocks were deposited in a marine environment on a continental margin (and thus possibly related to a geosyncline). 4. Without detailed stratigraphic sections to support his contention, Dr Jansen would appear to be speculating as to the eastward thickening of the basin. Current work [Barker, in prep., 1971 and 1970] has indicated that the lavas thin outwards from the centre of the basin, west of Sibasa, to the east and west. 5. A statement regarding transportation directions not based on large numbers of palaeocurrent measurements, must be regarded with some reservation. One of the definitive aspects of an aulacogen is the dominance of transportation of sediments along its axis [Hoffman, 1970]. Evidence accumulated so far in the Soutpansberg [De Villiers, 1967 and Barker, in prep.] indicates a predominantly north-west to north-east provenance for the Soutpansberg sediments. This agrees well with a rifted basin environment. 6. Not one of Dr Jansen's 10 points in support of the aulacogen model would contradict a rift or yoked basin. The observed clastic, non-marine sediments and interbedded basaltic lavas [Barker, in prep.], indicate an essentially intercratonic, clastic wedge environment, related to rifting parallel with the axis of the mobile belt. Should not Dr Jansen have waited until full information was available and the mapping of the Soutpansberg completed before he undertook to propose the model that he has? Finally, a discussion at this stage regarding the relation of the Umkondo to the Soutpansberg Group must be regarded as pure speculation. Until a detailed analysis of the evolution of the Limpopo and Mocambique belts and their related rocks is completed, no serious scientific debate is really possible.
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Transactions of the Geological Society of South Africa, 79 (1), 149-151
Comment upon two papers concerning the geomorphology of Natal. (a) Structure and physiographic development of Natal since the late Jurassic by A.M.J. de Swardt and G. Bennet, (b) The J(c)-1 Borehole on the continental shelf near Stanger, Natal by S.R. du Toit and M.J. Leith, both of which appear in the Trans. geol. Soc. S. Afr. vol 77 part 3. Detailed study has shown that the several planations of Natal, though agreeing with the Dixeyan system in style and age, were not a set of erosional steps upon a uniformly uplifted continent, but that the terrain had been intermittently tilted seaward about a hingeline situated near the present coast. Whereas western Natal had been uplifted by thousands of metres, the coastline had moved up (or down) by only small measures. These cumulative changes were clearly explained in (a) "A Reappraisal of the Natal Monocline" King & King [1959], (b) the 1963 edition of South African Scenery, and (c) The Natal Monocline published in 1972. The contoured map obtained from relicts of the early Cenozoic surface remains the basic document on the geomorphology of Natal. Perhaps because of the limitations of vertical air photo interpretation, Dr de Swardt and Miss Bennet did not identify any of the relict geomorphic features of middle and western Natal which are clearly displayed in profile - with accordance of summit levels. The Gondwana and post-Gondwana landsurfaces of high Lesotho, for instance, are strikingly evident on horizontal photographs obtained by flying at 3 600-4 250 m in front of the Drakensberg. Photographs from two such flights were specially taken for me by the Land Survey Department of the University of Natal, from whom the relevant photographs are obtainable. After noting the presence along the coast of both the African and post-African landsurfaces, the former very smooth and separated by pronounced scarps from the younger, rolling surface, Dr de Swardt and Miss Bennet opine: "However in our view these do not continue to rise at a constant rate westwards so that they go over the top of the Pietermaritzburg step". As the accompanying section (Figure 1) shows, however, they do continue, firstly in the Natal Midlands and secondly at the foot of the Drakensberg escarpment. In the Natal Midlands relicts of the smooth African planation, standing above wider areas of the rolling post-African landscape, may be seen - 1. Around Eastwolds on the long spur between Ixopo and Donnybrook; 2. On the Mooi River Heights (Currys Post, Nottingham Road, Griffins Hill); 3. Qudeni; and 4. Louwsberg Plateau south of the Pongola River. The examples all stand somewhat below 1 600 m in a zone extending the full length of the Province. All the examples quoted stand on or near main roads and can be readily inspected. The Drakensberg examples are known collectively as the Little Berg. They form long, even-crested spurs bevelled at about 2 000 m, which extend from the base of the Drakensberg escarpment across the lower basaltic lavas. Thus the African planation is a truly erosional planation, bevelling in succession across the full width of Natal all formations from the Table Mountain Series (including the quartzites) to
the Stormberg lavas. The easiest example to reach by car in the Drakensberg is Highmoor, below Giant's Castle. It should be visited by all who are interested in Natal geomorphology.
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Transactions of the Geological Society of South Africa, 79 (1), p. 152
After some four years' usage by the South African Committee for Stratigraphy and its nine Working Groups it is perhaps now a convenient time to list some errors and comment briefly on some other aspects of the Code. Most of the printing errors, spelling mistakes and others are reasonably obvious but a few perhaps more acute matters need to be pointed out. In the introductory paragraph to the Code reference should have been made to "Subcommission" and not "Subcommittee". In section 4.9 the examples of formal biostratigraphic names should have been printed correctly as follows: ".... Lystrosaurus Range-zone, Homalonotus herscheli Acmezone but Brachiopod Assemblage-zone ...". In the Code the terms isochronous and synchronous are used in the same sense as used by the ISSC (1970, 1971, 1974). This usage implies that isochronous refers to a plane representing everywhere the same instant in time (sections 3.12 and 5.1 of the South African Code), while the term synchronous refers to a sedimentary body which everywhere represents a certain fixed efflux of geological time (section 3.7 of the Code). However, this is directly opposite to the meanings accorded to these terms by Krumbein and Sloss [1963]. Mann [1970] has drawn attention to the fact that this confusion is wide-spread and pointed out that "The differences between the two adjectives frequently are ignored, unrecognized, and misunderstood by many geologists" [ibid., p. 750]. Having analysed the meaning of the Greek prefixes iso and syn Mann concluded that the correct usage is that of Krumbein and Sloss, namely that an isochronous unit is bounded by two synchronous surfaces. SACS has decided to accept Mann's interpretation of the meaning of these terms. Accordingly the two terms isochronous and synchronous should be interchanged where they occur in the Code. This has already been done in the Afrikaans version of the Code. A synchronous surface is a plane which everywhere represents the same instant in time. An isochronous rock body is a rock unit which everywhere represents the same simultaneous efflux of geological time; it must, therefore, be bounded by a lower and upper synchronous boundary.
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Transactions of the Geological Society of South Africa, 79 (2), 155-167
The importance of mineral deposits to the economy and future of South Africa is stressed. Since mineral deposits are wasting assets, new deposits must be discovered at an increased rate. During exploration it is often a problem of where to look for new deposits. Gold is where you find it - so it is said. However, the author believes that when the origins of deposits are well understood, we should know where to look. Today, the theory of Plate Tectonics since the Mesozoic is generally accepted and as many ore deposits are related to plate boundaries, it is of extreme importance to determine whether plate movements also took place during the early history of the earth. With regard to spreading zones the direction and rate of movement can be deduced from the magnetic striping of the oceanic crust. However, when plates move toward one another oceanic crust is digested and the only evidence available to determine whether plate boundaries existed in the past lies in the comparison of the sedimentary, igneous and metamorphic rocks as well as the structure at plate boundaries in action at present. For this reason the structure and rocks of the Transvaal are discussed and it is suggested that the Swaziland Supergroup in the Barberton Mountain Land, the Murchison Range and the Sutherland Range represent ancient subduction zones. So also the Witwatersrand Basin was caused by irregularities in plate boundaries. The rocks as well as the ore deposits of the Transvaal Supergroup, although originally still influenced by subduction zones, reflect later east-west tensional strain conditions. This tensional strain also led to the intrusion of the Great Dyke in Rhodesia and after that to the intrusion of the Bushveld Complex along NW and NE faults. The original east-west subduction zones then acted as weak zones along which transform faulting took place. The tensional strain also controlled the intrusion of the Bushveld granites and associated ore deposits as well as the formation of the Rooiberg felsites and the sediments of the Waterberg Group. Even the later deposition of the Karoo sediments, as illustrated by the distribution of coal fields, was also influenced by this faulting. It is realised that the suggestions presented in this paper are speculative and perhaps an oversimplification but it is trusted that they will stimulate thought and will lead to a better understanding of the geological history and the origin of ore deposition in the country. Only then will gold no longer be where we find it but where we look for it.
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Transactions of the Geological Society of South Africa, 79 (2), 168-176
The Pomona ring complex consists of several intrusive masses of syenitic magma that define a differentiation trend towards silica oversaturation. Volume relations, sequence of intrusion, and broad chemical characteristics are consistent with the syenites being derived by feldspar fractionation from an immediate syenitic parent. A plug of nepheline syenite and tinguaite dykes indicate the availability of silica-undersaturated magma generally in the Luderitz Province. Quartz-feldspar porphyry dykes near the contact probably developed by partial melting of the gneissic basement at the contact or by gross contamination of the syenite magma as their chemistry is not consistent with them being on the main differentiation trend of the syenites. The Drachenberg complex is very poorly exposed and relations between the various syenite types are obscure. The syenites display a crude annular arrangement and also define a trend towards silica-oversaturation. The correlation between degree of differentiation, as indicated by chemical characteristics, and volume relations and sequence of intrusion is not as well defined as at Pomona. Undersaturated rocks appear to be absent from the Drachenberg intrusion. The Luderitz Province is smaller than the related Angola and Damaraland Provinces and is also devoid of basic and carbonatitic rocks, which characterize these other two Provinces. At least three distinct batches of syenitic magma were available for differentiation and tapping during the evolution of the province. The ultimate source of these batches is believed to be alkaline basaltic or basanitic magma generated in the mantle.
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Transactions of the Geological Society of South Africa, 79 (2), 177-178
Halite casts have been discovered in the Upper Argillaceous Series of the Umkondo System (ca 1 100 m.y.) in southeastern Rhodesia. The casts were encountered in red, mid-cracked siltstones and shales, interpreted as overbank (and possibly interdistributary) sediments in a meandering stream assemblage. A low-lying, arid-climate, marginal-marine depositional environment is envisaged.
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Transactions of the Geological Society of South Africa, 79 (2), 179-182
The circumstantial evidence of overall composition of the preserved sequence, tholeiitic trend, composition of cumulus olivine and othopyroxene in lowermost rocks, tenor of chromium, and compositions of contemporaneous satellite dykes strongly indicates that the parental magma of the Great Dyke was akin to high magnesium basalt.
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Transactions of the Geological Society of South Africa, 79 (2), 183-185
A new stratigraphic subdivision of the Rooiberg felsite into two formations is proposed. The lower formation consists of massive, homogeneous felsite and varies in thickness from 0 to more than 1 000 m owing to transgression of the intrusive members of the Bushveld Complex. The upper formation exceeds 1 000 m and is characterized by fluidal textures in lavas and by minor intercalated Pyroclastic and sedimentary beds. A zone of large quartzite xenoliths occurs near the top of the lower formation and is overlain by a persistent volcanic breccia marking the first eruption of the upper formation. This stratigraphic subdivision appears to be recognizable over most of the Bushveld Complex.
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Transactions of the Geological Society of South Africa, 79 (2), 186-190
Dykes and sills of Karoo dolerite in north-western Damaraland represent a younger period of intrusion than those in South Africa. The emplacement of the dykes was controlled by the breaking-up of Gondwanaland, whereas the sills were injected at shallow depth along the contact between the Damara and Karoo Supergroups, transgressing upwards through the Karoo sediments. The dolerites are mineralogically similar to those in South Africa, except of the absence of Pigeonite and the occurrence of quartz dolerites south-west of Brandberg.
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Transactions of the Geological Society of South Africa, 79 (2), 191-196
A whole-rock geochemistry and electron microprobe investigation of some of the exotic xenoliths (augitite, carbonatite, syenite, biotite-aegirine porphyry) occurring within the Pretoria Saltpan has been carried out. The highly alkalic nature of the clinopyroxenes and feldspars, the alkalic nature of the rocks and the presence of carbonatite indicate that the xenoliths are derived from a complex alkaline-carbonatite magma. Analyses of the phases in a gabbro of Bushveld Igneous Complex type from the Saltpan indicates that this sample was derived from a depth of 7-9,5 km. This depth may represent a minimum depth from which high velocity eruption occurred.
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Transactions of the Geological Society of South Africa, 79 (2), 197-212
Late Mesozoic sediments in the southern Cape Province, South Africa, are virtually confined to a series of small fault-controlled basins extending for 700 km along the Cape fold belt. Only the Algoa basin and the small outcrops near Knysna and Plettenberg Bay contain fossiliferous marine sediments, but elsewhere plant leaves, wood, fresh-water crustaceans, ?insects, ?fish scales, and reptile bones occur in non-marine facies. The literature has been reviewed with the object of bringing together all pertinent palaeontological information and also to assess its stratigraphic significance. In the Algoa basin the Sundays River Formation is conservatively considered to range in age from Upper Valanginian to Hauterivian; this assessment is based mainly on information from ammonites. The Kirkwood and Enon Conglomerate Formations are thought to fall within the lower Valanginian to Berriasian. The presence of sediments of Jurassic age has not yet been demonstrated. The review provides an alternative to the facies model followed by most authors. It is suggested that rather than interfingering with the Kirkwood Formation, the Sundays River Formation overlies it unconformably and by progressive truncation comes close to resting on marine beds of the Colchester member in the outcrops of the Sundays River valley near Dunbrodie. The Robberg Formation is seen as a distal facies of the Enon Conglomerate; it was deposited partly under marginal marine conditions and was probably associated with the same early marine transgression that resulted in the deposition of the Colchester member. Microfauna and palynological biostratigraphic correlation demonstrates an equivalence of the Brenton Formation at Knysna with the Sundays River Formation in the adjacent PB-A/1 well and also in the Algoa basin and an Upper Valanginian age is indicated.
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Transactions of the Geological Society of South Africa, 79 (2), 213-216
While the majority of 15 new K/Ar ages of biotite from the Damara-Orogen uniformly lie around 485 M.a., those in the westernmost part (Khan area) are 40-50 M.a. lower. The distribution pattern of the ages suggests that the uplift of the southern part of the orogen (Khomas highland), relative to the central zone, occurred prior to 485 M.a., at a time when the region had not yet cooled below or 300-350°C.
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Transactions of the Geological Society of South Africa, 79 (2), 217-224
The compositions of ten Ca-rich and eight Ca-Poor pyroxenes from the western Bushveld Complex were determined by x-ray fluorescence spectrometry. The Ca-rich pyroxene trend is from Wo42,0:En49,4:Fs8,6 to Wo38,2:En25,0:Fs36,8 whereas the coexisting Ca-poor pyroxenes trend from bronzite (Wo3,1:En78,9:Fs18,0) to inverted pigeonite and ferropigeonite with a composition of Wo8,6:En49,7:Fs41,7. On the basis of the compositional trend found and the general petrography, of the succession studied, two successive phases of emplacement are distinguished. These phases, which are represented by the Norite-Gabbro Unit and the Upper Gabbro Unit, correspond roughly to the Main and Upper Zones of the eastern Bushveld Complex. Each of the two phases have differentiated in situ under approximately the same pressure and temperature conditions. Slight differences between the trends of the Mg-rich clinopyroxenes from the eastern and the western Bushveld Complex and the Skaergaard intrusion were observed. This is seen as the result of a displacement of the liquidus and solidus surfaces relative to the solvus surfaces in the pyroxene system in one intrusion as compared with the other. There is also a significant difference between the ferroaugite trends of the western and the eastern Bushveld Complex which is far from clear. The explanation of this important difference is complicated by the fact that the composition of one ferroaugite from the eastern Bushveld lies on the ferroaugite trend of the western Bushveld.
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Transactions of the Geological Society of South Africa, 79 (2), 225-231
Decisions concerning the exploitation of a coal deposit should not be taken before sufficient geological information on which these decisions are based, is available. For this reason it is important that exploration results are not represented merely as a collection of observations in reports, but that the data are interpreted systematically and rationally and explained in accordance with an acceptable geological theory. Furthermore, the interpretations should continually be tested and re-adjusted in the light of the latest exploration data. In this context, coal exploration is seen as a dynamic process that will continue even during exploitation of a coal deposit, a dictum that, in fact, is true also for mineral deposits in general.
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Transactions of the Geological Society of South Africa, 79 (2), 232-238
This paper attempts to define some tectonic and igneous provinces for which new geochronological data are reported elsewhere in this volume. The Proterozoic Richtersveld, Namaqua and Gariep tectonic provinces are characterized by their contrasting evolution and by their specific style of deformation reflecting different crustal levels. Their approximate geographic extent is delineated. Various plutonic complexes of Late Precambrian to Early Paleozoic age are grouped into the Richtersveld and Kuboos - Bremen igneous provinces respectively and their origin is discussed.
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Transactions of the Geological Society of South Africa, 79 (2), 239-241
The Saldanha quartz porphyry occurs as a pluton to the west and south-west of the town of Saldanha, and is intrusive into the coarsely porphyritic Hoedjiespunt granite. The quartz porphyry consists of phenocrysts of microcline microperthite and plagioclase in a crytocrystalline matrix. Analytical data for lead and uranium are reported for five zircon samples from different localities. A. isochron yields a linear array, the slope of which indicates an age of 522±12 M.a. It is concluded that the Saldanha quartz porphyry has the same age as the porphyritic Vredenburg granite within the experimental error of 2%.
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Transactions of the Geological Society of South Africa, 79 (2), 242-252
After an account of the Stopes-Heerlen nomenclature for coal petrography the microscopical properties and the mode of occurrence of the macerals and associated minerals in South African coals are described. An indication is also given of the variations in the petrographical composition. In the light of the petrographical data conclusions are drawn regarding the physical-chemical conditions during peatification. The relationship between the spore associations and the petrographical composition indicates that the South African coals are of autochthonous origin. A review is also given of the factors which cause carbonification (geochemical coalification). In conclusion the role played by quality, type and rank in determining the properties of coal is mentioned.
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Transactions of the Geological Society of South Africa, 79 (2), 253-280
In 1885 Edwin Bray found gold in the Sheba Hills, a rugged tract of country to the north-east of the town of Barberton. His discovery eventually led him to the famous Golden Quarry orebody and sparked off a resurgent gold rush that culminated in the establishment of Eureka City (now a ghost town). Ever since the discovery of gold in the region, mining operations have continued unabated, and today most of the small workings have been amalgamated to form either the Sheba or Fairview gold mines. The area, with its colourful historical and mining background is equally fascinating geologically and has been subjected to the scrutiny of a number of investigators in the past. This paper attempts to synthesize the regional geology of the Sheba Hills, special attention being devoted to the stratigraphy, structure, and gold mineralization of the area as a whole, and of the Eureka Syncline in particular. The Moodies stratigraphy in the Eureka Syncline is subdivided into three formations, the latter overlain by a unit afforded member status. Sedimentological criteria suggest that the Moodies stratigraphy was deposited mainly under shallow water, fluvio-deltaic conditions, in sharp contrast to the deeper water, turbidite, depositional style of the older Fig Tree sediments. Structural investigations confirmed the presence of at least four phases of deformation in the Sheba Hills. Minor structures and pebble deformation data suggest that granite diapirism was largely responsible for much of the tectonic activity in the region, particularly the arcuation of the Eureka and Ulundi synclines. Finally, the relation of gold mineralization to the structural history of the region is discussed, and gold production data for mines in the area are tabulated.
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Transactions of the Geological Society of South Africa, 79 (2), 281-291
The Blouberg area is the only area where the isolated late-Waterberg basin and the Soutpansberg trough are developed in close proximity; in other words, where a rift zone and an aulacogen extend into the domain of normal intracratonic basins. The stratigraphic outline of the area is partly obscured by intensive block-faulting. The pre-Soutpansberg sedimentary and volcanic rocks in the block-fault zone are grouped into the Blouberg Formation, which does not represent a lithologic unit nor a continuous succession in contrast with the Waterberg and Soutpansberg sequences. Correlations between members of the Blouberg Formation and formations of the Waterberg and Soutpansberg Groups and age relations are based on palaeogeographical data, mode of deposition of the formations, and radiometric datings on the basement (Limpopo belt). It is concluded that the Blouberg Formation is partly of middle and partly of late- Waterberg age and the overlying middle portion of the Soutpansberg Group (Wyllies Poort Formation) of very late- to post-Waterberg age whereas the lower portion (Sibasa Formation) in the Soutpansberg may be of early- to late- Waterberg age. The complex structural pattern of the Blouberg area originated during different periods. Intensive block-faulting in middle Waterberg times along a strongly uplifted rim of the late-Waterberg basin gave rise to the formation of partly isolated yoked basins or troughs. Block-faulting continued in the late-Waterberg times and the block-fault zone widened to the north. The dominant structural feature of this period is the lava-filled Lebu trough, which is also a prototrough of the Soutpansberg trough. Subsequently, the Soutpansberg trough developed from east to west into the area. Faulting, partly by reactivation of older faults, was contemporaneous with deposition of the Soutpansberg succession and also post-Soutpansberg and post-Karoo in age. The distribution and thickness of lava flows in the Soutpansberg and Lebu troughs indicate.
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Transactions of the Geological Society of South Africa, 79 (3), 293-300
The area presently under study comprises a shallow sheltered lagoon which is drained by tidal channels, and a deep (> 10 m) bay more exposed to oceanic conditions. Slightly organic (< 1,0% C) fine terrigenous sediment mantles the deeper water (< 40 m) the mouth area, whereas fine quartzose sands cover the floor of the Bay. The sediment becomes more calcareous towards the centre of the Bay and minor amounts of coarse quartz occur in the northern and southern extremities of the region. The marshland areas lying above the mean high water line in Langebaan Lagoon are composed of non-calcareous highly organic (> 10% Corg) muddy sediment. These regions are fringed by intertidal sandbanks comprising well-sorted medium quartzitic sands, which, in turn, are dissected by deep (> 3 m) tidal channels containing coarse sand-size quartz. The sediment contains increasing amounts of coarse quartz towards the western shore of the Lagoon. Sediment throughout the study area is phosphate-poor (< 0,8%, P2O5). The Saldanha Bay-Langebaan Lagoon region was eroded to a maximum depth of 70 m during a low stand of the sea, possibly in Tertiary times. A combination of a powerful northerly littoral drift and a strong prevailing south-easterly wind has provided the Bay and Lagoon with plentiful supplies of sediment at least throughout the Quaternary. Extensive (7 to 10 m) marine terracing during the last interglacial (Eem) was almost certainly followed by further infilling during the warm glaciation by the unvegetated and easily transportable sands that were left behind by the retreating sea. The Lagoon developed behind a northerly prograding spit during the subsequent recovery of the sea and eventually a tombolo was formed between Ysterfontein and the Konstabelskop granite outlier. Wave-cut nicks in limestone and raised boulder beaches are possibly evidence of a recent (4 000-6 000 yrs B.P.) high stand of the sea.
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Transactions of the Geological Society of South Africa, 79 (3), 301-303
In 1966 a storm exposed new outcrops at the mouth of the Umbogintwini River, Natal, for a few weeks. These included a thin sandstone bed that contained abundant fragmental bones and ruminant teeth. Nearby outcrops, which are exposed at normal times, contain fossil crab burrows and occasional fossil crabs resembling Ocypode ceratophthalmus, which is found on the present beach. Aeolian ripple, marks are also preserved. The evidence implies a Quaternary age and a depositional environment similar to that prevailing on the present beach between the lagoon and the sea.
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Transactions of the Geological Society of South Africa, 79 (3), 304-311
Some 14 000 km in the vicinity of Kenhardt, Marydale, Groblershoop, and Upington was mapped by staff and students of the University of the Orange Free State. A number of new interpretations regarding the stratigraphic sequence and associated phenomena were reached. The Kheis sequence rests with a sedimentary contact on the Draghoender granite which therefore acted as basement for the Kheis. The Kheis and Matsap sequences are conformable and the Kheis sequence is shown to be the equivalent of the Upper Matsap. The Waterval Member of Vajner's (1874) Seekoebaard Formation is now correlated with the Hartley Andesite Formation of the Middle Matsap sequence which probably represents the lowest stratigraphic horizon in the Matsap to the north of the basement high formed by the Draghoender granite. The conformable stratigraphic sequence in the vicinity of Boegoeberg is therefore: Hartley Andesite Formation (bottom) followed by Volop Quartzite Formation followed by Dabep Formation followed by Kaaien Quartzite Formation followed by Groblershoop Formation (top). The Marydale sequence to the immediate north of the town of Marydale is correlated with the Hartley Andesite Formation; the basal amphibolites of the Marydale may, however, represent highly altered and foliated Ventersdorp volcanics. The Top Dog Member of the Volop Quartzite Formation (Matsap) in part is the equivalent of the Kaaien quartzite. The schistose Dabep Formation (a new proposed lithostratigraphic name) is in part the equivalent of Vajner's (1974) Groblershoop Formation. A study of cross-bedding proves that the Dabep Formation lies conformably on the Volop Quartzite Formation, and in turn is conformably overlain by the Kaaien Quartzite Formation. Therefore it follows that the Groblershoop Formation overlies the Kaaien Quartzite Formation. Rogers and Du Toit's (1908) "Wilgenhout Drift Beds" now form the main part of the schistose Wilgenhout Drift Formation, which is considered a facies of the Groblershoop Formatio.
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Transactions of the Geological Society of South Africa, 79 (3), 312-320
Pegmatoid nodules of pyroxenitic and noritic composition were studied in the Bafokeng Leasehold area, Western Transvaal. The mineralogy, chemistry, and mode of formation are considered. Nine chemical analyses are presented. The nodules have similar mineralogy and chemistry as the overlying Merensky Reef and the underlying pegmatoid pyroxenite, which is found associated with the UG2 chromitite layer. The nodules would explain the absence of a pseudoreef at the Bafokeng Mine. Owing to this absence, their mineralogy, and their shape the nodules are considered to have formed during the break-up of a pyroxenite layer produced by movements within the magma chamber. The presence of depressions in layers that occur above and below the nodules are taken as further evidence of magma movements. Several possibilities, including current action, earthquakes, fracturing of the roof, and a new addition of magma are suggested to explain movements in the magma.
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Transactions of the Geological Society of South Africa, 79 (3), 321-340
Mbotyi and Mngazana lie 40 km apart on the Transkei coast near Port St Johns. The Cretaceous deposits consist of poorly bedded conglomerates with minor amounts of lignitic sandstone and mudstone, and represent the earliest sediments within small half-grabens. The Mbotyi and Mngazana formations appear to be genetically equivalent units, with the latter occupying a relatively more distal position. Apart from wood, no other fossils have yet been found at Mbotyi, but the sediments at Mngazana yield plants, a diverse macrofauna (including ammonites) and a microfauna. Fifty species of foraminifera and fourteen species of ostracods are described and illustrated. They allow a biostratigraphic correlation with the lower part of the Sundays River Formation in both the Algoa basin and the PB-A/1 well on the Agulhas bank. An Upper Valanginian age is indicated for the Mngazana formation and a similar age is inferred for the Mbotyi deposits. This information provides a dating for the early stages of basin formation.
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Transactions of the Geological Society of South Africa, 79 (3), 341-370
The outcrops of marine beds at Knysna are described and the Brenton Formation is proposed as a formal lithostratigraphic unit. The adjacent PB-A/1 borehole on the continental shelf intersected a lithological sequence very similar to that of the Algoa basin and provides a reference section against which the Brenton Formation can be compared. A total of 44 species (1 new) of foraminifera and 33 species (1 new) of ostracods from the two sites are described and illustrated. They indicate a biostratigraphic correlation of the Brenton Formation with the lower part of the Sundays River Formation in PB-A/1 and also in the Algoa basin. The Brenton microfauna suggests an Upper Valanginian age. The Colchester member in PB-A/1 represents an initial marine transgression of early Cretaceous age associated with an episode of faulting and basin formation some time prior to continental separation. Following on the regressive terrestrial and marginal marine sediments of the Kirkwood Formation, the Sundays River Formation (Upper Valanginian to Hauterivian) records a widespread transgression. The dramatic increase in microfaunal diversity in the shaly upper part of the formation is tentatively thought to represent a major transgression associated with the separation of the Falkland plateau from the African continent during the Hauterivian or perhaps slightly later.
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Transactions of the Geological Society of South Africa, 80 (1), 1-8
The glacial beds of the Makganyene Formation which forms part of the Middle Precambrian Griqualand West Supergroup were examined in detail in cores from four boreholes drilled along a north-south line. Petrographic investigation shows that the sedimentary succession consists of a rhythmic alternation of arenaceous and argillaceous diamictite, conglomerate, clayey sandstone, subgraywacke, cherty iron carbonate, dolomitic limestone, banded jaspilite and a cherty rock. The clastic rocks consist predominantly of grains of quartz and chert with minor amounts of feldspar, carbonate and quartzite. Calcite/dolomite, siderite, chamosite, stilnomenlane and chlorite occur disseminated in the groundmass of the clastic rocks. The clasts in the diamictite consist largely of chert and quartzite. The diamictite was probably deposited from floating ice, while the arenaceous beds resulted from subaqueous gravity mass flows. The non- clastic beds were deposited during interglacial periods. The underlying Gamagara Formation and dolomite of the Campbell Group acted as sources for the sediments. The Ongeluk volcanism commenced before sedimentation had ceased and volcanic material was incorporated in the upper sedimentary beds.
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Transactions of the Geological Society of South Africa, 80 (1), 9-16
The stratigraphic sequence of the late Precambrian Sinclair Group occurring in southern South-West Africa is defined. The succession consists essentially of five distinct major volcanic, volcanoclastic and/or clastic units invaded extensively at intervals by masses of high level granite, basic to intermediate bodies, a large syenite intrusion and dense swarms of felsic and basic dykes. The volcanics are of both felsic and basic to intermediate types and many of the latter are characteristically rich in potassium. The Sinclair Group constitutes part of a prominent late Precambrian but pre-Damara, pre-Nama, belt of volcanic, clastic and plutonic units in southern and central South-West Africa and Botswana - named the Rehoboth Magmatic Arc. The age of the arc falls within the approximate age range of 1 400 - 950 Ma and it is regarded as a feature developed during immediately pre-Damara times along the margin of the stable Kalahari Plate as a result of subduction processes. The correlation of the various units comprising the Sinclair Group with other units of the Rehoboth Magmatic Arc is proposed.
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Transactions of the Geological Society of South Africa, 80 (1), 17-28
The geology, mineralogy and the geochemistry of the Roodekrans Greenstone Complex, situated 10 km north-east of Krugersdorp, are described, and 11 new rock analyses are presented. The successions in the area can be divided into two principal components - rocks belonging to a layered ultramafic sequence (the Roodekrans Ultramafic Complex) and rocks forming part of a succession of Mg-rich pillow basalts and subordinate peridotitic interlayers. The rocks of the layered ultramafic sequence have been serpentinized and deformed whereas the metabasalts show signs of contact metamorphism resulting from the intrusion of a porphyritic granodioritic phase of the Johannesburg granite dome. Consideration is given to the nature of the layered ultramafic sequence and chemical evidence is presented, suggesting that the rocks do not constitute part of a differentiated complex but that they may rather represent an eruptive succession of ultramafic flow units like those described by Pyke et al. from Munro Township in the Abitibi greenstone belt of north-eastern Ontario or those described from the Komati Formation in the Barberton area by Viljoen and Viljoen. The chemical data relating to the ultramafic and mafic rocks in the Roodekrans area is discussed with the aid of ternary and binary variation diagrams. The high-Mg basalts are shown to constitute a distinctive class of Archaean volcanic rocks transitional between basaltic komatiites and oceanic tholeiites. Supplementary geochemical data from the Barberton Mountain Land is combined with that of the Roodekrans area and is used to demonstrate an almost complete evolutionary trend of magma development involving olivine, orthopyroxene, and clinopyroxene fractionation. The fractionation of these minerals is considered responsible for the generation of a wide range of cumulus-enriched liquids and melt composites ranging from dunites, orthopyroxenites, websterites, and gabbroic igneous rocks to peridotitic and basaltic komatiites, high-Mg basalts and oceanic tholeiites. Finally it is suggested that the generation of the ultramafic-mafic assembleges, like those in the basal units of the southern African greenstone belts, occurred as a result of unique P-T conditions in the Archaean. These distinctive geotectonic conditions have only rarely been simulated in post-Archaean times.
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Transactions of the Geological Society of South Africa, 80 (1), 29-30
The Waterberg thrust fault in South-West Africa is discussed from the point of view of its palaeontological age and possible mode of initiation. One explanation provides for it to have been active in Early Middle Triassic times as a normal fault associated with an ancient line of crustal weakness recently described by Fuller or as part of a triple junction: other explanations involve crustal warping associated with alkaline volcanic centres of an age later than the Upper Triassic.
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Transactions of the Geological Society of South Africa, 80 (1), 31-42
Published and unpublished radiometric age determinations available up to the end of 1976 for material from Botswana have been compiled, tabulated and recalculated, where necessary, to conform to the same decay constants. A total of four C14, 25 K-Ar, two U-Pb, one Pb-Pb, 25 single Rb-Sr whole rock and 12 Rb- Sr isochron/errorchron analyses are quoted. Most of the data pertain to south- east Botswana. The Gaborone Granite-felsite complex may represent two events at about 2 750 Ma and around 2 380 Ma; the former involves extrusive and intrusive felsic igneous activity and the latter (Ventersdorp) volcanism accompanied by widespread thermal activity. However, the age data are ambiguous and other authors (e.g. Crampton, 1972) have interpreted the felsic complex as a single c. 2 300 Ma event. Major granitic emplacement ceased in the Rhodesian Craton within Botswana at around 2 540 Ma. This is the age obtained from a nine point isochron of the post-tectonic (G5) Timbale Granite. A ten point isochron from a typical layered gneiss complex at Pikwe in the Limpopo Mobile Belt gives an age of 2 640 Ma which may be related to the generation of the gneissose fabric. A 2 000 Ma mineral age is widespread in eastern Botswana and is not confined to the Limpopo Mobile Belt as previously thought. The limited data available indicate that calcrete development in eastern Botswana took place continuously between 10 000 and 34 000 years before present. There is no direct evidence for the ages of the extensive non-metamorphic sedimentary units of Botswana.
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Transactions of the Geological Society of South Africa, 80 (1), 43-45, 1 fig.
A gravity survey of the Shawa Carbonatite Complex restrains the thickness of the dunite core to between 500 and 800 metres. This result supports a petrochemical model whereby dunite formed at the bottom of a deep magma chamber and was subsequently uplifted to its present level.
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Transactions of the Geological Society of South Africa, 80 (1), 47-48
Contents: 1. Laminar viscous flowage in ignimbrites. 2. Graded bedding in ignimbrites. 3. Physical environment. 4. Concluding remarks.
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Transactions of the Geological Society of South Africa, 80 (2), 101-109
The Matsap sequence, which is the equivalent of the Kheis and Namaqualand stratigraphic sequences, rests unconformably on the Griqualand West sequence. The latter underwent very mild, open folding prior to deposition of the Matsap. In contrast, the Matsap and its equivalents were subjected to three distinct sets of deformation as deduced from a study of the major structural features. The regional distribution of rock-types, some geomorphological features and the structural style can be explained by superposed folding. A major role was played by the rigid basement high in the south-eastern part of the area. During F,, the first macrofolding event, isoclinal folds overturned to the east were developed probably parallel to the Langeberg Range. The effect of F, on the Transvaal and older sequences is conspicuous. A pronounced foliation and greenschist to amphibolite facies of metamorphism developed during F, which is tentatively dated at 1 200 Ma based on a single U-Pb isotope age determination on the syntectonic Kalkwerf granite. F2 folding produced steeply dipping axial surfaces and northward-plunging b-lineations. In some domains, e.g. west of the Kaaien Hills, F, and F2 appear to be coaxial and cannot be distinguished from each other. In other domains, the directions of F, and F2 differ and the effect of F2 on F, is responsible for the development of some of the major structural features, notably the Orange River syncline and other superposed fold patterns between Groblershoop and Sultanaoord. The F2 folding dies out east of the Kaaien Hills and its influence on pre-Matsap rocks is negligible. F2 was probably post- metamorphic as no foliation parallel to F2 axial traces are present. F, axial planes strike north-east and are vertical. The oblique orientation relative to F1 and F2 produced a sigmoidal fold pattern west of Kenhardt, but its effect dies out rapidly eastward. F, is also post-metamorphic and probably contemporaneous with late-tectonic granite. A tentative age of 900 Ma is proposed for this episode. Fractures of post-F, and post-F2 age are particularly abundant in the Kheis hills. They are mainly normal faults with small displacements, but appear to be slightly deformed by F in the Kenhardt area. A younger megashear (left-lateral wrench fault? runs from a point north-west of Upington and traverses the central part of the area and appears to be the continuation of the Doornberg fault to the south. Faulting produced graben-like structures into which the Koras Group was deposited. Some movement along the faults persisted to post-Koras times. The Matsap domain appears to represent the shelf to represent the shelf facies of the basin, the Kheis, the hinge zone between shelf and trough, while the Namaqua sequence is representative of a yoked type of basin. Deposition took place in a cratonic basin, possibly on a thin portion of continental crust where sedimentation was subjected to epeirogenic movements. This would imply the presence of a sialic crust - some of the highly foliated, porphyritic granites near Kenhardt could represent such an original basement.
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Transactions of the Geological Society of South Africa, 80 (2), 111-115
Markovian tendencies are detected in the analysis of cores from the Middle Ecca and Lower Beaufort subgroups in the north-eastern part of the Karroo Basin. The two preferred vertical arrangements of lithofacies are basically upward-coarsening and upward-fining. These are interpreted as deltaic and fluviatile deposits respectively. The lowest part of the Middle Ecca penetrated by the cores is predominantly deltaic. Above this is a fluviatile interval which contains most of the economically important coal seams. The overlying delta-dominated succession is interrupted by two transgressive shoreline deposits. Advanced textural and mineralogical maturity characterize these sediments of "marine" affinity. A change in Palaeoslope from southward to westward occurred between these two transgressive events. Rapid regional subsidence and transgression was followed by accumulation of the Upper Ecca mudstones. Renewed delta progradation deposited the basal portions of the Lower Beaufort. This was succeeded by the establishment of fluviatile conditions, which prevailed through the remainder of the section investigated.
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Transactions of the Geological Society of South Africa, 80 (2), 117-123
En the Sekhukhune Plateau in the Malope area basic rocks of the layered sequence of the Bushveld Complex rest on an updomed floor of hornfels and quartzite of the Transvaal Supergroup. The basic rocks are transgressively overlain by the Nebo granite which in places rests directly on the floor rocks. The basic rocks are typical of the Critical, Main and Upper Zones of the Complex. The rocks of the Upper Zone seem to have a transgressive relationship to the rocks of the lower zones. Chemical analyses as well as Pt + Pd and Ni values indicate that the chromitite developed here might be correlated with the UG2 layer in other parts of the Complex. The magnetite layers have compositions typical of those in Subzones D of the Upper Zone. Leached, limonitic rocks are developed at various places throughout the area.
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Transactions of the Geological Society of South Africa, 80 (2), 125-130
A furrowed tillite, near the base of the Dwyka succession in the Nqutu District of northern Natal, is interpreted as representing a subglacial lodgement deposit, sculptured by an overriding ice sheet. The grooves are believed to have been temporarily exposed in the proglacial zone during the oscillatory retreat of a south-easterly flowing glacial mass.
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Transactions of the Geological Society of South Africa, 80 (2), 49-52
Fractionation trends established for Bushveld Complex rocks of the Bethal area indicate that normal gravity differentiation has been modified by cyclical crystallisation and multiple injection. This has given rise to chemical breaks and reversals in the cryptic layering. There is, however, an overall trend from Fs14 to Fs62 in the Ca-poor pyroxene and En50 Fs7 Wo43 to En 29 Fs34 Wo37 in the Ca-rich pyroxene. Microprobe analyses of plagioclase gave a range of values from An79 to An41. Apart from its occurrence in the Basal Zone with values ranging from Fa17 to Fa21, olivine only occurs at the top of the sequence, greatly enriched in iron at Fa76. Ilmenite, magnetite and pyrrhotite have undergone substantial sub-solidus re-equilibration and apatite is present as fluorapatite.
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Transactions of the Geological Society of South Africa, 80 (2), 53-65
Preliminary sedimentological studies on the stratigraphic succession in the Rooiberg fragment indicate a grouping of the strata into two formations based on the presence of two major sedimentary sequences. The lower Leeuwpoort Formation, a perfect fining-upward succession, is thought to be one large fluvially dominated sedimentary cycle. A thick accumulation of immature braided stream deposits at the base of the cycle grades vertically through a series of cyclothems into mudflat shales. Inconclusive evidence for the paleoenvironment of the sediments comprising the cyclothems is lacking. Sedimentary structures indicative of both fluvial and estuarine deposits could imply a marginal marine environment. The cyclic pattern of the transition from wedge arkoses to shales in the Leeuwpoort Formation is interpreted as a product of fluctuating rates of subsidence in a yoked basin and a steadily decreasing supply of clastics from the north-east. The overlying Smelterskop Formation, consisting primarily of fluvial deposits interbedded with andesitic lava flows, is envisaged as a separate, incomplete cycle of sedimentation. An ill-defined cyclic pattern interrupted by periodic outbursts of volcanic activity in the Smelterskop Formation is thought to be indicative of increasingly unstable sedimentary conditions caused by tectonism that heralded the initial episodes of the Bushveld Complex. It is suggested that the formation of a clastic wedge at Rooiberg might have been a response to block-faulting along an almost east-west linear zone of crustal weakness in the northern margin of the Transvaal Supergroup basin.
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Transactions of the Geological Society of South Africa, 80 (2), 67-78
Cassiterite mineralization, confined to definite stratabound planes in the Rooiberg Tin-field, is directly related to intrusion of the Bushveld acidic phase in and around the Rooiberg fragment. Compression during multilateral igneous emplacement gave rise to horizontally disposed forces and the manifestation of superimposed folding and thrusting within the fragmented roof-rocks. The structural deformation and associated tin mineralization in the Rooiberg fragment were controlled primarily by northward-directed compressional forces in Bushveld times. Various phases of mineralization were introduced during deformation, resulting in open space-filling bedded lodes associated with incipient thrusting on one flank of a regional arch structure, and metasomatic replacement deposits in a cross-trending syncline located along the hinge-zone of the antiform. Relationships between the different compressional episodes and the type of tin-deposit formed in response to specific structural modifications are discussed. Confinement of stanniferous ore-deposits to well-fissured sites on a transitional stratigraphic contact zone between cross-bedded arkosites and overlying shales is attributed to differences in the degree and nature of fracturing above and below the contact. Certain primary sedimentary features provided planes of fracture and impervious barriers for channelling hydrothermal fluids and deposition of cassiterite in bedded fissures and stratabound tin-zones. After reviewing relevant structural relationships in the Bushveld, a discussion on the structural formation of various Bushveld tin-deposits is offered. It is proposed that northward compression and local thrusting were major factors in determining the distribution of most Bushveld tin-deposits in a broad, ill-defined, east-west belt where it is intersected by a north-north-west aligned abyssal fracture zone.
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Transactions of the Geological Society of South Africa, 80 (2), 78-86
Additional data concerning the direction of late Palaeozoic (Dwyka) ice flows in the western part of the country are presented and known information is reviewed. In southern South-West Africa it is confirmed that deposits from the southerly-trending Namaland Ice Sheet are overlain by deposits from the westwardly flowing Transvaal Ice Sheet. In the area south of Loeriesfontein it has been established that there was an easterly ice flow direction produced by the Atlantic Ice Sheet and also a westerly flow of the Transvaal Ice Sheet. Evidence is led to demonstrate that deposition from the Atlantic Ice Sheet preceded that from the Transvaal Ice Sheet. From this it is postulated that the eastwardly flowing Atlantic Ice Sheet first covered most of the area south of Loeriesfontein. Subsequently the westwardly flowing Transvaal Ice Sheet overrode the area, in parts covering the deposits from the Atlantic Ice Sheet and in other parts obliterating evidence of its existence.
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Transactions of the Geological Society of South Africa, 80 (2), 87-92
An alignment of igneous centres along the trace of a great circle passing through the Bushveld Igneous Complex and Great Dyke was recognised by C.A. Cousins in 1959. Subsequently additional complexes were added and the line extended, so that twenty-five such bodies are now known over a distance of 3 800 km between the Orange Free State and Ethiopia from 30°S to 6°N latitude. The location, age and geological setting of these complexes is reviewed briefly and it is noted that they occur in a variety of stable cratons, mobile orogenic belts and rift valleys of differing period and trend. Despite apparent different ages of emplacement commencing about 2 600 Ma ago, the alignment of centres has persisted, thus reflecting a remarkably permanent and fundamental phenomenon, unaffected by subsequent tectonic events.
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Transactions of the Geological Society of South Africa, 80 (2), 93-95
The structural sequence in the Namaqualand Mobile Belt to the west of Upington in the Gordonia district consists of quartz-muscovite schist at the base, overlain by quartzo-felspathic gneiss (pink gneiss), calc-silicate rocks, garnet-sillimanite-cordierite-biotite gneiss, and biotite-gneiss at the top. The gneissic sequence is intruded by syn-and late-tectonic granites. The gneisses are the product of high grade regional metamorphism under low pressure-high temperature conditions. The stable mineral assemblages indicate that temperatures between 650°C and 780°C at a relatively low pressure of the order of 3,5 to 4,5 kbar existed during the main metamorphic event. The regional structure of the area is characterised by north-west-trending, overfolded, isoclinal folds giving rise to a very prominent north-west-orientated grain in the structural pattern. The area can be divided into two structural domains, which are separated by a prominent north-west-trending shear zone, named the Cnydas Shear Zone. The north-eastern domain is characterised by large, isoclinal folds with rounded hinge zones. Only one period of deformation could be established in this area. The late-tectonic Colston granite and the granite of the Cnydas Complex, are intrusive into the metasediments of this domain. The south-western domain is marked by a highly complicated fold pattern, which is the result of the interaction of three main periods of deformation of which the first period seems to have been the dominant event. A change in the direction of the dip of the foliation across a north-west-trending line is an interesting structural feature of this domain. The south-western domain is marked by the intrusion of the syntectonic Bokvasmaak and Kourop River granites.
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Transactions of the Geological Society of South Africa, 80 (2), 97-99
The Tsaun Formation, a supposed pre-Damara basement unit in the Damara belt south of the Brandberg, comprises a metamorphosed sequence of banded gneisses, schists, calc-silicate rocks, amphibolites, marbles and metavolcanic rocks and is conformably overlain by marbles and schists of the Damaran Swakop Group. A re-evaluation of its lithology and stratigraphic position strongly indicates a correlation of this formation with the upper Nosib Group and not with the pre-Damara basement as previously suggested.
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Transactions of the Geological Society of South Africa, 80 (3), 139-144
Certain mineralogical features suggest that the crystallisation of the eastern Bushveld Complex occurred at several kilometres depth. These are: (1) The composition of the calcic pyroxene becomes much less calcic than in all other tholeiitic intrusions before the calcium-poor pyroxene disappears. (2) Cumulus olivine and orthopyroxene occur in this sequence unlike other layered complexes. (3) The silica content of the liquid at which point olivine ceases to crystallise, and pyroxenes and plagioclase precipitate, was probably lower than in other intrusive bodies. Consequently, silica was not enriched in residual liquids of the Bushveld Complex by pyroxene-plagioclase fractionation to the same extent as in other complexes. (4) Some metamorphic rocks in the floor of the Bushveld Complex indicate very high pressures were generated during emplacement of the magma. All these differences between the eastern Bushveld Complex and other tholeiitic bodies are readily interpreted if higher pressure crystallisation (approximately 4 kb) of the Bushveld Complex was maintained. Phase diagrams in the system forsterite-diopside-SiO2 under 4 kb pressure are presented to substantiate these observations.
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Transactions of the Geological Society of South Africa, 80 (3), 145-156
The evolution of basins in Transvaal and Waterberg times follows the same pattern, namely from protobasins to large depositories and finally to shrinking basins. Contrary to previous interpretations the two major sedimentary cycles are not entirely separated in time by the emplacement of the Bushveld Complex. During the latter event basin formation and sedimentation continued, though on a minor and very local scale. These processes were continuous in Botswana (Ootse basin) but discontinuous in the Transvaal with time intervals of less than 180 Ma (central Transvaal) and exceeding 180 Ma (south-eastern Transvaal). The final stage of the Transvaal cycle is represented by the Loskop and Glentig Formations which were laid down in remnants of the shrinking Transvaal basin. This took place subsequent to the extrusion of the Rooiberg lavas and prior to intrusion of the Bushveld Complex. The Rooiberg group represents the last volcanic phase in the evolution of the Transvaal basin and the Loskop Formation the last sedimentary phase. The initial stage of the Waterberg cycle in the central Transvaal is represented by the Nylstroom protobasin in which the lower portion of the Swaershoek Formation was laid down, the lowermost beds contemporaneously with the intrusion of the acid Bushveld magma or rest magmas. The Nylstroom protobasin subsequently evolved into the Alma trough, which is occupied by the upper portion of the Swaershoek Formation and the Alma Formation. Stratigraphic implications are the introduction of the Loskop Formation as a stratigraphic unit and the following correlations: the Glentig Formation with the Upper Loskop Formation and the Upper Swaershoek Formation (Nylstroom protobasin and Alma trough) with the Wilgerivier Formation (Cullinan-Middelburg basin). No attempt is made at this stage to correlate the late-Transvaal and early-Waterberg formations with the succession in the distant Ootse basin in which Transvaal-Waterberg sedimentation was continuous.
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Transactions of the Geological Society of South Africa, 80 (3), 157-165
Granite samples have been recovered offshore of the coastal granite exposures at Groot Haelkraal between Danger Point and Quoin Point. They include coarse biotite granites from the east of the outcrop and micro-adamellites from the west. It is tentatively concluded that the eastern side of the batholith is more deeply denuded than the west. The coastal exposures contain large tourmaline-rich clots whose major element geochemistry has been determined. This shows that the clots are depleted in K2O and Na2O whilst the immediately surrounding granite is considerably enriched in K2O.
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Transactions of the Geological Society of South Africa, 80 (3), 167-175
The Koras Group is composed of six alternating sedimentary and volcanic formations with the upper three unconformably overlying the lower ones. The lower two sedimentary formations are characterized by immature sediments, probably deposited as alluvial-fan deposits while the upper one is a more mature red-bed type. The volcanics are composed of two formations of quartz-porphyry and one of basalt to K-rich basaltic andesite with some pyroclastic facies, all forming part of a calc-alkaline association. The Koras Group unconformably overlies the intensely folded Kheis Group composed of quartzites and schists which occupy the hinge zone adjacent to the Namaqualand Mobile Belt. The largest distribution of Koras rocks was deposited in graben-like repositories on a floor which through interference folding was deeply depressed, a situation which might have had genetic significance for the outpouring of the Koras lavas and the accumulation of completely undeformed sediments. The Koras volcanics were emplaced in a newly stabilised orogenic region probably shortly after the cessation of the last period of folding, so that the age of the Koras will also date this period of deformation. The Koras Group resembles the Sinclair Group of southern South-West Africa lithologically and geochemically.
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Transactions of the Geological Society of South Africa, 80 (3), 177-181
Pink and augen gneisses, which form the base of the stratigraphy at Aggeneys (and throughout much of the Namaqualand granite-gneiss complex), exhibit petrological and geochemical characteristics very similar to those of the granulites (leptites) and augen-gneisses of the Nababeep District. A correlation is proposed between these respective rock-types. Results from a literature search have indicated that this correlation might be extended further afield in the complex.
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Transactions of the Geological Society of South Africa, 80 (3), 183-191
Inhaca Island comprises a calcareous sandstone base overlain by N-S trending dune ridges of semi-lithified and unconsolidated sand. The calcarenites resemble those of the Natal-Zululand coast, and are thought to represent aeolian deposition during the penultimate glaciation, and beach and estuarine sedimentation corresponding with the high sea-level of the last (Eem) interglacial. Terraces at 1-2,5 m and 5-6 m are also believed to be Eemian. Old dune cordons display advanced diagenesis, and possibly accumulated during an early phase of the Wurm glaciation. The high coastal dunes are more youthful and may be related to the upper Wurm glacial maximum 18 000 years ago. Post glacial transgression of the incised shelf flooded Delagoa Bay and isolated Inhaca Island. As the sea approached its present level about 4000 years ago modern patterns of sedimentation became established. Today there is a unique assemblage of diverse environments including coral reefs, tidal sand bars, tidal flats, high energy beaches and spits, all in close proximity.
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Transactions of the Geological Society of South Africa, 80 (3), 193-196
The roles of carbon dioxide and water in the genesis of sulphide and spinel concentrations in mafic rocks are examined with special reference to the Bushveld Complex. A new hypothesis - that of a carbon dioxide water fluid fountain - is proposed to explain many of the features of the Vlakfontein nickel pipes. The localization of these fluid fountains was controlled by active fracture zones in the floor of the Complex. It is suggested that pin-pointing of areas where flushing by carbon dioxide and water probably took place could be of assistance in the search for base metal sulphide deposits in all mafic rocks.
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Transactions of the Geological Society of South Africa, 80 (3), 197-209
In the Cape Province of the Republic of South Africa occur approximately 25 high level granitic plutons, intrusive into late Precambrian metamorphites. The major element geochemical features of the granites are investigated with the aid of two graphical representations of De la Roche (1978), for which purpose 160 whole rock chemical analyses from 18 plutons are employed. After grouping the plutons according to their geochronological relationships each group exhibits dominant trends which differ from those of other groups. The older granite magmas appear to have been subjected, prior to consolidation, to the influence of two major petrogenetic processes: magmatic differentiation and contamination by interaction with xenoliths of metasedimentary wall rocks. The younger granites, having compositions in the leucogranite range, display no significant petrochemical trends. It is suggested that differences in the depth of magma genesis could have given rise to these two different types of granite plutons.
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Transactions of the Geological Society of South Africa, 80 (3), 211-234
A re-interpretation of the available palaeopole data for the Carboniferous to Jurassic of Gondwanaland and North America is presented. Particular emphasis is placed on refining the interregional correlations of the host strata and in distinguishing between intervals of movement and relative stability of the participant landmasses. In order that these irregularities in plate movement should not be smoothed over in sequential reconstructions the concepts palaeopole clan, clan mean and clan mean path are introduced and applied. Palaeopole paths are plotted for the eight different crustal units comprising Gondwanaland and North America. Although these paths are subject to much improvement with incoming palaeopole data and improved stratigraphic correlations they appear sufficiently distinct to suggest marked relative lateral movement within Gondwanaland. Two sequences of 12 reconstructions are presented - one for Gondwanaland and one showing the West Gondwana/North American contact. The Gondwana reconstructions are briefly assessed with respect to their elucidating the tectonic history of southern Africa and the spread and retreat of the Lower Permian glaciation.
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Transactions of the Geological Society of South Africa, 80 (3), 235-239
The chromitite of the Marico occurrence forms seams that dip inward shallowly in a basin-shaped cumulate assemblage of pyroxene and olivine representing the Basal Zone of the Bushveld Igneous Complex. As many as five seams have been recorded from this area, and the present study centres around the second and third seams from the base of the succession. The gangue phase in the chromitite is well dispersed and clearly of secondary nature, consisting of montmorillonite and opal, with minor quartz, illite, and carbonates. The crystals of chromium spinel are intensely brecciated and strongly oxidized, especially along grain boundaries. Unaltered kernels are invariably present in the core areas of the larger grains. Chemically, the original chromium spinels in the relevant seams are not much different from the LG2 and LG3 seams in the main body of the complex, and they evidently do not have an excessively high content of Al2O3. Where the chromium spinel is in contact with alnoitic rocks of the Goudini volcano, it shows a loss of ferric iron and chromium, minor increases in ferrous iron, magnesium, and aluminium and major increases in silicon, titanium, vanadium, and nickel. Possible reasons for the refractory character of the ore can be seen in the distribution of the gangue phase, and the oxidized state of the constituent chromium spinel.
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Transactions of the Geological Society of South Africa, 80 (3), 241-252
Cross-bedding patterns in the fluviatile Upper Triassic Molteno Formation of the Karoo (Gondwana) Supergroup in South Africa and Lesotho are consistent with deposition by braided low sinuosity streams that locally may have been meandering in character. Statistical analysis of the vector mean azimuths and their partitioning into regional components reveals the presence of two major flow systems directed towards the north-west and north-north-east. Overlapping and interaction between two flow systems is held responsible for the local complexity of cross-stratal azimuths along the Orange River area, although the overriding direction of transport is towards the north-west concomitant with basinal trend. Since the aggregate data for the entire basin provides a unimodal distribution, the presence of these two flow components may go undetected without adequate statistical appraisal. This might explain why previous studies in the southern part of the basin have stressed the north-easterly component of flow which is not representative of the basin as a whole. Statistical analysis of the variance of cross-stratification azimuths at three different sampling levels shows a pronounced decrease in variability with increase in hierarchical order. This ordering of variance components is attributed to the relative contributions of each of a series of hierarchically arranged environmental parameters present in alluvial rivers. The exceptionally small overall basinal variance suggests that the rivers were not only of low sinuosity but formed part of an unusually strong and essentially unidirectional flow system, under the control of a tectonically active source area.
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Transactions of the Geological Society of South Africa, 80 (3), 253-265
Assessment of style and intensity of deformation are the main objectives of this investigation in an effort to find evidence for the cause of the post-Cape tectogenesis and the amount of crustal shortening. The upper part of the quartzitic Table Mountain Group slides over the lower along the southern backlimbs of mega-anticlines to be thrown into stacks of lower order folds on the overturned north-limb. This latter effect is probably aided locally by gravity forces. Two prominent parallel arrays of intensely buckled mega-anticlines are separated by a wide synclinorium of less buckled but more internally deformed rocks. Isopach hingelines in the most competent units controlled the sites of mega-anticlines and their comparatively high rate of deformation. Horizontal shortening by flexural-slip for, competent units in anticlines and by homogeneous bi-axial strain for incompetent strata in the synclinorium both amount to about 25 percent of the original length in a north-south direction. The homogeneity of structure and strain intensity in the Cape Fold Belt, and a probable participation of the basement in the post-Cape tectogenesis, are strong arguments in favour of a tangential crustal shortening model as against gravity sliding along an unconformity décollement.
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Transactions of the Geological Society of South Africa, 80 (3), 267-277
In this paper the locality and two new species of Zosterophyllum are described from a new Devonian phytostratigraphical zone in the Bokkeveld Group of the Warmwaterberg area of the Little Karoo. The zone was located by Dr J.N. Theron of the Geological Survey in the course of a lithostratigraphical survey of the area in 1973. Since then the farmer A.H. de Vries has enlarged the collection of fossil plants considerably and in one area of fossil fish from the same zone. The plants were submitted to the author for description. Only the Zosterophyllum is included at this stage because it has not been recorded previously from Africa and is important both from an evolutionary point of view and in geochronology since it has a limited time range. The zone should contribute to the long-felt need for biostratigrapical evidence in the Cape Supergroup.
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Transactions of the Geological Society of South Africa, 80 (3), 279-281
Obituary. Biographical memoir.
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Transactions of the Geological Society of South Africa, 80 (3), 283-285
A new occurrence of lacustrine stromatolites is described from Urwi Pan, Botswana. The spheroidal morphology of the stromatolites indicates that they were formed in permanent to seasonal wave-agitated waters. The existence of such water bodies is regarded as providing further evidence for a major late Pleistocene humid phase in the Kalahari.
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Transactions of the Geological Society of South Africa, 80 (3), 287-288
The paper on tectonic and igneous provinces by Kröner and Blignault merits applause for its aim and comment on its content. There is probably no other school where the knowledge of tectonic cycles of a region is as concentrated as that of PRU in the North-west Cape and southern South West Africa. EGRU has concentrated on a single cycle at Barberton. Leeds, with all of Africa as its bailiwick, may have been in business long enough to synthesize the scattered data (but it is the PRU authors who made an attempt). All three schools are leaders in our understanding of sedimentation, vulcanism, plutonism, deformation, metamorphism, and metallogenesis as components of the tectonic cycle. The authors have the best of credentials. None of the compilers of tectonic maps of large countries or continents had available to them data on all the component parameters of the tectonic cycle, though Scheibner, for one, may now have such data available. So the PRU workers are not only the logical authors of a paper on the concept of tectonic cycles applied to the Northern Cape, but also on the development of the concept itself. Unfortunately, their technology has developed so much since Schatzki and Bogdanov (1957), King (1969), Stockwell (1969) and Scheidner (1972) that an exposition in the terms of those authors has become inadequate; incorporation of the new technology is mandatory. But this they failed to do. Let me say at once, that I could not do the job myself; any knowledge that I have is of the similarities and variables in geological sequences. I hope eventually to progress to the dispositional characteristics of similar sequences, at which time tectonic provinces and their boundaries will become my bailiwick. I find I like the proposed definition of a tectonic province as "a geographic region that is characterized by a combination of such parameters as lithology, structure, metamorphism" but would drop "and predominant radiometric age differing significantly from those of adjacent areas." There is a growing awareness that not only K-Ar dates are unreliable evidence of emplacement, but that, as the authors acknowledge: "U-Pb and whole rock Rb-Sr ages ... may represent the onset of closed system behaviour associated with late tectonic uplift", for example. More elsewhere. I also find I like the amplifications of the definition of a tectonic province in the Introduction. And yet I had expected more. What are the natures of the boundaries of sedimentation, vulcanism, plutonism, metamorphism, deformation, and metallogeny for each of the tectonic provinces, for both emplacement and overprinting? It is here that the authors must surely have data to offer. The descriptions of the Tectonic Provinces are largely unstructured and therefore incomplete. For example, the Richtersveld Tectonic Province "lithologically, tectonically, and geochronologically, differs from the adjacent provinces. It is characterized by a discontinuously developed foliation of cataclastic nature and low to medium grade of metamorphism". That is all; other tectonism described here relates to younger Proterozoic phenomena. For at least ten years the PRU school has routinely mapped
deformational and metamorphic histories. Is the discontinuously developed foliation of cataclastic nature D1, D2 or D3? What happened during the other deformational episodes? To what episode does the low to medium grade of metamorphism relate? How do the episodes as a whole compare and contrast with those of other tectonic provinces? A table of geological events for each area, comprising sedimentation, vulcanism, plutonism, metamorphism, deformation, and metallogenesis, would aid discussion. A change in style evident at the start of the discussion on Namaqua Province would also have been found to be unnecessary. If we are considering the Alpine tectonic period of the last 200 m.y., any and all older provinces are infrastructural. The authors appear to have reverted to the old idea that high-grade metamorphics are inherently old. But the rocks of the Richtersveld Tectonic Province, though low grade, are older (= infrastructural) than those of the Namaqua Province. And the "supracrustal Kheis and Kaapvaal Provinces" are certainly older than (= infrastructural to) them all. Should we not acknowledge that, as the authors describe them, each of the tectonic provinces includes supracrustals and intrusives? In any mobile belt I know of we have pre-tectonic supracrustals, and a variety of intrusives and metamorphism accompanied deformation. These are the components of mobile belts, which commonly comprise one or more sequences recognizable as tectonic cycles. The rocks of Namaqua Province appear to form one or more tectonic cycles - a matter I leave to PRU - and in describing them must be considered as a structure, though in describing the Gariep, the Namaqua rocks are infrastructural. I think the problem arises in part because the word province has been applied to both the extent or rocks of one mobile belt (though arguably according to the authors) and to specific parts, e.g., igneous suites, of mobile belts. Which left the Kaapvaal supracrustals high and dry unless themselves considered as a province. Reply to Dr. Wilfred Walker. The so-called deficiencies of our paper (Kröner and Blignault, 1976) as pointed out by Dr. Walker are the lack of a detailed exposition of stratigraphic, structural and metamorphic correlation between the proposed tectonic domains (provinces). We consider such an exposition beyond the scope and intent of our paper which was meant as a geological background within a crustal framework for geochronological data reported elsewhere (Allsopp et al.; Welke et al., in press). The definition of tectonic provinces, at any rate, should be independent of tectogenetic considerations and should be based on geological data unburdened by tenuous interpretation. As such, the tectonic classification becomes a powerful analytical tool. Subsequently, correlation and interpretation of parameters such as stratigraphy, structure, metamorphism and geochronology may lead to the formulation of tectogenetic models. At that stage it can be decided whether e.g. the Richtersveld Province is supracrustal or superstructural with respect to an infrastructural Namaqua Province (see Blignault, 1977, for details). In this respect we were in error in using
such terms without adequate motivation and in incorporating tectogenetic terminology in the definition of the provinces. According to our usage though, the term infrastructural (sensu stricto Gary et al., 1972) does not carry the connotation "older" as is apparent from Dr. Walker's discussion, and would clarify some of his objections. We share Dr. Walker's need to have a full synthesis incorporating the new technologies, but fear he sought too much from a mere classification of tectonic domains.
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Transactions of the Geological Society of South Africa, 80 (3), 289-298
Dr Jansen is to be congratulated on his contribution on the Blouberg area, in which he tried to unravel a complicated chapter of the geological history of the northern Transvaal. That this history was one of tectonic and stratigraphic chaos, did not in the least deter him to try and come up with a synthesis However, my own conclusions, based on my data collected while mapping the area, are not entirely in agreement with some views expressed by Dr Jansen and it is for this reason that I dare to tread in Dr Jansen's footsteps. Contents: Structural geology, stratigraphy, correlations, conclusions.
Author's reply to discussion: Reply to B. Meinster. Mr. Meinster no doubt raises important issues and adduces much field evidence to stave his arguments. His objections to my interpretations have been previously discussed, already during the period of mapping. They have been duly considered by myself and other members of the Geological Survey. His proposals regarding a different stratigraphic terminology were also considered, but not accepted by SACS. Mr. Meinster's conclusions are radically different from mine. They are summarized in his "logical succession in Blouberg" and by his statement that the Soutpansberg Group is much older than the Waterberg Group, hinting even at a Ventersdorp age. This entirely contradicts global evidence on the lower age limits of red beds (Cloud, 1976, pp. 16, 18). Portions of the Soutpansberg sequence display red beds characteristics, in particular the argillaceous beds, which are in all subdivisions partly red, brown, or purple in colour, even those interbedded with lavas in the lower portion of the sequence (Sibasa Formation). Morever, some strata of the underlying Blouberg Formation ("Lebu and Blouberg Complexes") also display red beds characteristics, for instance those which are lithologically similar to Waterberg beds. In South Africa the first red beds appear on a small scale in the uppermost Transvaal sequence (upper Rooiberg Group, Loskop Formation) approximately 2100 m.y. ago, which accordingly represents the lowermost age limit of the Soutpansberg and Blouberg successions. In my opinion all Mr. Meinster's arguments are already invalidated by the red beds in these successions. However, I take the opportunity to discuss some of his arguments in support of his proposed stratigraphy. Mr. Meinster's age relationships also clash with age determinations on the Limpopo belt. His statement that recrystallization in the belt could have occurred close underneath a thick pile of Soutpansberg beds is to me entirely unacceptable. As Dr. Burger pointed out, the zircons on which the datings were carried out, crystallized from a melt, thus from a magma or remobilized rocks. The mobilized portions would have caused locally intensive metamorphism of the overlying Soutpansberg beds, which, in fact, were only subjected to a regional, very low-grade burial metamorphism. Moreover, none of the mobilized portions have intruded into the Soutpansberg and Blouberg successions. These successions were of course laid down on an erosional surface already occupied by mobilized and non-mobilized portions of the belt.
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Transactions of the Geological Society of South Africa, 81 (1), 1-11
Recent mapping of the acid rocks of the Bushveld Complex and older formations has revealed a number of mappable granite and granophyre bodies. Uranium-lead zircon determinations provide five different ages for these rocks: granophyre, granophyric gneiss and acid lava of the Dennilton dome - 2 460±120 m.y. (older than Bushveld Complex); the Rashoop Granophyre Suite - 2 050±30 m.y.; the Nebo Granite - 1 920±40 m.y.; the Makhutso Granite - 1 670±70 m.y. and the Klipkloof and Klipvoor (Bobbejaankop-type) granites - 1 400±190 m.y. Good correspondence exists between the results of the age determinations and the field evidence of the granophyre and the Nebo and Makhutso Granites. In the case of the Klipkloof and Klipvoor granites contradictory evidence is found and the actual age obtained must be regarded with caution although a younger age for these granites seems evident.
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Transactions of the Geological Society of South Africa, 81 (1), 101-104
The upper part of the Bokkeveld and lower part of the Witteberg Groups have been studied in detail in the Montagu-Touws River area and that part of the succession which straddles the boundary, between the two groups has been divided into three new formations. As a result of this study it has been possible to draw a consistent and easily recognisable dividing line between the groups throughout the area involved. It is acknowledged, however, that lateral variations of the formations will necessitate similar detailed studies in the regions to the south and east of the area before this dividing line can be continued with confidence.
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Transactions of the Geological Society of South Africa, 81 (1), 105-107
The Groot Haelkraal granite crops out along the coast between Danger Point and Quoin Point, South Africa, and occupies an area of 120 sq. km offshore. Cape Peninsula granites have been dated at around 550 Ma and the original formation of the Groot Haelkraal granite is assumed to be of a similar age. However, age spectrum analysis of biotite from a dredged sample from station 1824 (34°48 S, 19°29'E) has shown that the granite was involved in a tectonic event around 248±2 Ma, in Upper Permian times. This event is considered to have been of major importance and to represent Cape Fold Belt overprinting of the basement.
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Transactions of the Geological Society of South Africa, 81 (1), 109-114
The Witwatersrand Supergroup shows a number of coherent trends when traced eastwards from the Central Rand. The gross thickness decreases markedly. The unconformity separating the upper from the lower division truncates successively lower units towards the east; projections suggest the Upper Witwatersrand will come to rest on the Archaean basement east of the Evander Goldfield. The size of the pebbles, and the proportion of coarse-grained material in the Upper Witwatersrand, decreases. The thickness of the Bird lavas increases appreciably. An Upper Witwatersrand equivalent east of the Evander basin is likely to rest directly on the Archaean basement, to be relatively thin and fine-grained, and to comprise a greater proportion of volcanic material than is usual for that unit. In the Kaapsehoop area of the Great Escarpment, a succession of arenaceous and volcanic rocks rests unconformably on the Archaean basement, and is unconformably covered by the Black Reef Quartzite. This succession, known as the Godwan Formation, has suffered pre-Black Reef folding, faulting and sill intrusion. Its basal conglomerate is locally auriferous, while the overlying Black Reef Quartzite contains pyrite, gold and uranium mineralization in conglomerates reworked out of an older formation. The evidence assembled suggests that the Godwan Formation is the easternmost outcrop of the Upper Witwatersrand basin. The covered region between Kaapsehoop and Evander is thus potentially underlain by structural basins of auriferous-uraniferous upper Witwatersrand material. This region is largely inaccessible to drill-hole exploration, except in a triangular region between Kaapsehoop, Machadodorp and Moedigsiding, where the cover formations will range in thickness from 0 to about 2 500 metres.
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Transactions of the Geological Society of South Africa, 81 (1), 115-121
The Pretoria Group of the Transvaal Supergroup best outlines the gently dipping domes, basins, anticlines and synclines of the rim synclinorium of the Vredefort Dome. These structures are separated from the core of the dome by a collar of steeply dipping and overturned sedimentary and volcanic rocks of the Witwatersrand and Ventersdorp grous. Penetrative cleavage is absent from the rim synclinorium and the deformation is most intense in the north-eastern area adjacent to the collar rocks. All bedding planes are warped and many fold hinge zones are broken and/or faulted and brecciated. All the major fold hinges and axial surfaces are concentric about the collar and about a near-vertical axis. There is no evidence for more than one phase of deformation and most of the folding and overturning was due to a single event which occurred simultaneously with the updoming of the Vredefort structure. The rocks were deformed in the brittle state and were folded by a flexural slip mechanism. Most of the strike-slip and normal faults which are concentric about the collar were synchronous with the folding. However, radiating normal faults which downthrow towards the collar are post-folding and post-overturning in age. Shatter cones occur on fault breccia associated with the downfaulting of already overturned rocks and are therefore post-overturning in age, which is incompatible with an origin for the Vredefort Dome by meteorite impact cratering.
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Transactions of the Geological Society of South Africa, 81 (1), 123-126
Data for arsenic in southern African argillaceous rocks are lacking. The element can be readily determined by atomic absorption spectrophotometry using the NaBH4-hydride method (detection limit 50 ng/g). The mean arsenic content of 91 southern African fine-grained sedimentary rocks is 10 ug (world average shale -13) and the range is typically limited (1-29 ug/g). The Cango formation (7 ug), the Dwyka series (4 ug/g), the Western Ecca series (6 ug/g) and the Beaufort Series (8 ug/g) seem to be particularly depleted. The Fig Tree Series (17 ug/g) may be relatively enriched in arsenic. In the southern African samples arsenic is not correlated with iron, organic matter or pyrite. Arsenic-rich provinces may be indicated by the concentration of the element in shales, and it is notable that the Fig Tree rocks, which are associated with gold deposits, may be enriched in arsenic.
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Transactions of the Geological Society of South Africa, 81 (1), 127
This note mentions the catalytic effect of ferrous-ferric redox reactions on manganese precipitation as previously, described by J.D. Hem. The chemical reactions are given and the conditions that favour MnO2 precipitation are briefly described.
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Transactions of the Geological Society of South Africa, 81 (1), 13-22
A Middle Ecca sedimentary sequence some 320 m thick is described from the Mateku River in Zululand. The succession falls into three main parts. In the lower one, a prograding delta front is succeeded by minor coarsening-upward cycles which are transitional into fluvial associations with thin coal seams. The second phase comprises a repetition of the first, though the basal, prograding delta front is more subdued, and coal seams (still thin) are more strongly developed. The uppermost division consists of two upward-coarsening deltaic cycles, the top one being intruded by a thick (90 m) dolerite sill. Sedimentary structures show that the palaeodrainage was, in the initial stages, almost entirely derived from the east. This is in accord with existing accounts of Dwyka glacial trends in the region (Stratten, 1968). Comparison with Middle Ecca successions at Tugela Ferry and in northern Natal, where sedimentary facies are essentially similar, shows that the Mateku succession possesses intermediate characteristics with respect to the other localities. Repetitive prograding delta fronts are not as evident as at Tugela Ferry while fluvial cycles and associated coal seams are not as strongly developed as in northern Natal.
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Transactions of the Geological Society of South Africa, 81 (1), 23-34
The clay mineral assemblage on the continental margin off the west coast of South Africa is characterised by abundant illite (50-90 percent), subordinate smectite (10-30 percent) and kaolinite (5-20 percent), and minor chlorite (<5 percent). Although more marked regional variations occur in onland sediments, approximately the same proportion of clay minerals is found in the adjacent shales and in the sediments of the rivers draining the hinterland. A well-developed seaward distribution of clay minerals is observed. Illite is richest close to the coast, followed by kaolinite which is most abundant on the outer shelf, whereas the highest concentration of smectite occurs in the slope sediments. This distribution is in keeping with the known settling and flocculation characteristics of these minerals.
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Transactions of the Geological Society of South Africa, 81 (1), 35-40
The Beaufort Group in north-western Natal can be subdivided into three formations and are informally, designated the Estcourt Formation (bottom), the Belmont Formation (middle) and the Otterburn Formation (top). All three are typical fluviatile deposits. The Estcourt Formation is built of carbonaceous shale and subordinate sandstone which overlie the shaly mudrocks of the Volksrust Shale Formation of the Ecca Group. The formation has no direct correlate in other parts of southern Africa, but shows some resemblance to the Balfour Formation of the Eastern Cape Province. The Belmont Formation consists of sandstone with subordinate mudstone and is probably the stratigraphic equivalent of the Katberg Sandstone Formation of the Eastern Cape. The Otterburn Formation has the same lithological characteristics as the Burgersdorp Formation.
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Transactions of the Geological Society of South Africa, 81 (1), 41-46
The Ritchie Quartz Porphyry Formation (new lithostratigraphic unit, first named in this paper) consists of acid volcanics, including ignimbrites, rheoignimbrites and rhyolite lavas, which outcrop 40 km south of Kimberley. The geology, structure and age of the Formation are discussed. The correlation of the rocks in question has previously been uncertain. In 1906, Du Toit mapped these acid volcanics as equivalents of the Zoetlief Series, while the overlying andesitic volcanics were identified as an extension of the Pniel Series; an unconformable contact was assumed. Later workers, however, believed that the Zoetlief Series rocks were far older than those of the Ventersdorp System with which they had previously been included, and reassigned them to the Dominion Reef System (aged about 2 800 Ma). This correlation is incorporated in the current geological map of South Africa. The present investigation has produced evidence which justify a reappraisal of this view: (i) there is a well-exposed concordant contact between the Ritchie Formation and the overlying andesites of the local Ventersdorp System; and (ii) comparing the recent work done by Winter (1976), a possible correlation of the Ritchie Quartz Porphyry Formation with the Makwassie Quartz Porphyry Formation is suggested and the former thus related to the Platberg Group. Thus it is concluded that the Ritchie Formation represents the initial phase of Ventersdorp volcanicity in the area, and is considered an integral component of the Ventersdorp sequence, an argument consistent in stratigraphic concept to regard the Ritchie Formation as a possible lithostratigraphic equivalent of the Makwassie Quartz Porphyry Formation.
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Transactions of the Geological Society of South Africa, 81 (1), 47-54
Thirty-five mineral species forming in the cave environment have been found in the Transvaal. Genetically they may be classified in four groups: 1. Ca and Mg carbonates, plus smectite, resulting from the action of seepage water (8 species); 2. resistate minerals (3 species); 3. phosphates, nitrates, chlorides, sulphates and Mn oxides, resulting directly or indirectly from the action of bat guano (25 species); and 4. brushite forming on bones. The conditions of formation are discussed.
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Transactions of the Geological Society of South Africa, 81 (1), 55-59
The Jennerville Chaotic Pyroclastic Member of the Moshesh's Ford Formation, in the area around Barkly East, comprises poorly graded, unstratified sedimentation units of which less than 20 percent by volume consist of essential fragments. The author's earlier suggestion that these rocks are products of volcanic mudflow is supported by the new investigation. It is found that fractures of the type which characterise many of the included blocks can be reproduced in the laboratory only by heating similar rocks to temperatures in the region of 800°C, followed by quenching in cold water. The experiments indicate that eruption of hot pyroclastic material through crater lakes may have resulted in the formation of lahars. The radial fracture of clasts in certain mudflows has not previously been described and its significance appreciated. Palaeomagnetic studies show that included blocks of dolerite tend to have a consistent orientation. This implies rapid deposition before the blocks cooled throughout, so that at least the central parts of the larger blocks came to rest while still at temperatures above the Curie point. It is concluded that palaeomagnetic fabrics alone are not adequate to distinguish hot volcanic mudflow deposits from those of, for example, nuées ardentes.
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Transactions of the Geological Society of South Africa, 81 (1), 61-73
The Palapye Group comprises five formations which from base to top are as follows: Selika, Moeng, Tswapong, Lotsane and Shoshong. The Lotsane and Shoshong formations are chronostratigraphically equivalent and locally there is a gradational contact between the Tswapong and Shoshong formations. The succession from the Selika to the Lotsane Formation comprises a two-fold sequence of coarse (conglomeratic) to fine (argillaceous) grained sediments. Near the top of the Selika Formation are several manganese-horizons while near the base of the Tswaong Formation several pisolitic ironstone horizons are developed. The Shoshong Formation comprises a basal and discontinuously developed greywacke, followed by a more extensively distributed two-fold succession of quartzite and shale in turn overlain by limestones. Although the age and correlation of the Shoshong Formation has long been problematical, it is suggested that the formation is an upward continuation of the Palapye Group. The group lies unconformably on rocks of the tectono-metamorphic complex of the southern part of the Limpopo Belt. The top of the group is an erosion surface or is unconformably overlain by Karoo strata. The Selika, Moeng, Tswapong and Lotsane formations lie adjacent to and north of the post-tectonic Mahalapye plutonic block from which much of the detritus of the Palapye Group is thought to have been derived. The Shoshong Formation, however, partly surrounds the massive Mahalapye dome which is one of the products of reactivation along the southern edge of the Limpopo Belt at around 2 200 Ma ago. The development of the sedimentary depository is related to isostatic adjustment of the ancient crust along pre-existing zones of weakness reactivated peripherally to the post-tectonic Mahalapye Granite. In the east where the Selika Formation contains lava beds, the Group is thought to occupy a graben which gradually dies out westwards. During the final stages of deposition of the Group the Lotsane and Shoshong formations were laid down in non-graben type depositories. The strata progressively overstep lower Group rocks on to granitic basement westward. As such the Palapye Group is probably a localised stratigraphic succession and cannot yet be correlated on a detailed scale with any other sequence in southern Africa. However, generally, the Palapye Group is part of the pattern of granite/sedimentary basin pairs developed on the Kaapvaal Craton and is probably chronostratigraphically correlatable to the Soutpansberg Group of the northern Transvaal.
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Transactions of the Geological Society of South Africa, 81 (1), 75-86
A set of ten en echelon felsite sills is exposed in the East Rand Proprietary Mines, Boksburg. These sills are part of a larger occurrence of felsite sills which occur in the Witwatersrand, and they may be genetically related to some similar sills in the north-western Transvaal and Botswana. The average thickness of the various sills ranges between 1,5 and 4 m, with an aerial extent that ranges from 125 to 18 x 10(6) m}. These sills contain unusual non-laminar structures and nodules of unknown origin. Rb-Sr isotopic analysis of the sills shows that the magma was emplaced - 2 100 Ma ago, after the Ongeluk lavas of the Transvaal System and before the solidification of the basic phase of the Bushveld Igneous Complex. The depth of emplacement is deduced as - 9 km, and the magma is thought to have been derived by melting of ancient crustal rocks. The magma produced was alkali granitic in composition, based on the normative mineralogy. Structural features such as extension joints, quartz veins and possible Luders' bands indicate that a near horizontal, north-south orientated maximum principal stress controlled the orientation of the sills during their emplacement.
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Transactions of the Geological Society of South Africa, 81 (1), 87-93
A number of felsite sills, occurring in the sediments of the Witwatersrand Group, have been dated at 2 150±26 (1σ) m.y. with an initial 87Sr/86Sr ratio of 0,7 26 0±0,001 4 (I/0). The plotted positions of the normative quartz, albite and orthoclase compositions of samples from the central zones of these sills closely approximate the quartz-albite cotectic line and the minimum melting point of Luth and others (1964) under 500 MPa pressure and water saturated conditions. This, together with the isotopic data, is interpreted as resulting from a partial melting of sodium-and silica-rich crustal material, the age of which is deduced to be 3 000-3 300 Ma old. The postulated anatexis probably took place at temperatures in excess of 660°C and at depths in excess of 18 km.
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Transactions of the Geological Society of South Africa, 81 (1), 95-99
A detailed mapping programme of Permo-Triassic rocks in the eastern Cape Province revealed dolerite intrusions with annular outcrop patterns.These intrusions have shallow (2 degrees-35 degrees) centroclinal dips and a drilling programme has shown that the dolerites are continuous beneath the central area of horizontal sediments that is enclosed by the annular outcrops. Mapping has shown that these basins are connected to dome structures and that they form parts of more extensive undulating dolerite sheets. Several theories that may explain similar types of intrusions are discussed and it is shown that the compensation surface theory (after Bradley, 1965) is applicable to the present case. According to this thesis magma pressure and alacotopography at the time of intrusion have an important influence on the shape of intrusion in a closed system. Applying this theory it is possible to explain features such as the regional and stratigraphic distribution of the rings as well as their size, shape, orientation and variations in dip.
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Transactions of the Geological Society of South Africa, 81 (2), 129-142
Results of deep and ultradeep Schlumberger soundings as well as magnetometer array studies show that the resistivity structure of southern Africa correlates very well with the tectonic structure. The Schlumberger sounding results have identified two kinds of terrain depending on the amount of fracturing present. Massive Terrains invariably, indicate the presence of vast amounts of massive, strongly, consolidated, unfractured granitoid material in the form of granitic intrusions, gneisses and migmatites while Fractured Terrains are associated with intensely deformed, fractured metamorphic rocks. The Archaean cratons are Massive Terrains and are characterized by a highly, resistive zone 1 which has an average resistivity of about 100 000 m. and extends downwards from the surface to a maximum depth of ten km. The Limpopo and Damara Mobile Belts are Fractured Terrains as manifested by the presence of a moderately resistive zone 2 with an average resistivity of 5 000 m. which extends from the surface to a depth of 25 to 30 km. An exception is the Namaqua-Natal Mobile Belt which shows the characteristics of a Fractured Terrain only in its central core. An important feature, deduced from the sounding results, is that the moderately resistive zone 2 which at the surface is associated with Fractured Terrains, continues laterally into the Masssive Terrains where it underlies the highly, resistive zone 1. This would imply that the lower crust is continuous and is fractured and metamorphic in character. The moderate resistivity of zone 2 suggests that free water is present. Near the base of the crust, in the depth range 25 to 40 km, a conductive zone 3 with a variable conductance has been detected under both Massive and Fractured Terrains with the exception of the cental core of the Namaqua-Natal Mobile Belt. It is contended that water played an important role in its formation.
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Transactions of the Geological Society of South Africa, 81 (2), 143-154
Two arrays of magnetometers were operated in southern Africa in 1971 and 1972 to supplement deep Schlumberger resistivity studies. A magnetometer array that operated in central southern Africa revealed an extensive conductive structure that underlies the deep Karoo basin and the Cape Fold Belt. Although the lateral extent of the conductor still has to be determined, it could be related to the Malmesbury Geosynclinal Belt. A second magnetometer array revealed a lithospheric conductor that crosses South-West Africa between latitudes 20°and 21°S and curves north-east in the vicinity of the Okavango Delta in Botswana to join the Luangwa-Middle Zambezi valley. This conductive zone is related to an old lithospheric fracture system along which present-day incipient rifting occurs.
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Transactions of the Geological Society of South Africa, 81 (2), 155-163
The age of the Ventersdorp volcanic rocks is of importance in establishing a terminal age for the Witwatersrand sedimentary cycle. The Ventersdorp Supergroup is currently subdivided into three main groups (Winter, 1976). The middle Platberg Group contains prominent acidic volcanic rocks suitable for U-Pb dating. Initial U-Pb dating of quartz porphyry from this group yielded an age of 2 300±100 Ma (Van Niekerk and Burger, 1964). Acid volcanic rock correlated with the Ventersdorp in Botswana yielded a Rb-Sr age of 2 083±210 Ma (Harding et al. 1974). Samples of acidic volcanic rocks from three major locations - the farthest over 300 km apart - yielded the current U-Pb measurements. In relation to concordia the analytical values follow a trend of natural continuous diffusion lead loss from an initial crystallization 2 643±80 Ma ago. The age now obtained for the Platberg acid porphyries supports Winter's (1976) correlation of the Makwassie Quartz Porphyry Formation with the acid lavas of the Soetlief succession.
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Transactions of the Geological Society of South Africa, 81 (2), 165-171
The problem of developing significant "tectonic overpressures" to account for certain metamorphic phenomena is examined with the aid of a simple rheological model. The model used reveals that "tectonic over-pressure" cannot be of any real significance. A mechanism which may, be more significant in producing pressure gradients at the same level in the earth's crust is a viscosity gradient which can be coupled with a heat gradient. At deep levels pressure gradients of the order 2 kbar and more could be developed by these circumstances.
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Transactions of the Geological Society of South Africa, 81 (2), 173-178
Many organic-rich sedimentary materials have high concentrations of certain trace elements and this is true for the Permian Ecca shales of South Africa. The insoluble organic fractions of 33 Ecca samples were isolated by flotation in a benzene/water/ammonia mixture during ultrasonic bombardment. Analysis of the organic separates showed that the concentrations of Rb, Cs, Ca, Co, Ni, Cu, Zn and Pb are strongly influenced by the organic matter, whereas Li, Na, K, Mg, Sr, Cr, Mn, Fe and Al are associated mainly with the inorganic minerals. A plot of the organophilic tendency of any element Y - (10 x organic IYD/(whole rock [Yb against the logarithm of the oceanic residence time of Y, for 13 elements, yields a good straight line. Metals which are readily hydrolysed (Al, Mn, Fe, Cr) deviate uniformly from the line. It has been suggested that the trace element content of the organic fraction of shales might be used as a paleosalinity indicator. However, this method is apparently unreliable, since the criteria established elsewhere for distinguishing between salt and fresh water environments of deposition give false results when applied to the lacustrine Ecca shales.
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Transactions of the Geological Society of South Africa, 81 (2), 179-184
Geological evidence suggests that the manganese ores that occur within the Table Mountain Group, at Houtbaai and elsewhere, are a result of the deposition in shallow fissures of manganese compounds leached from the surrounding arenites by percolating groundwaters ("lateral secretion mechanism"). There has been no previous geochemical study of the origin of these ores, or of the Table Mountain Group in general, and several samples of Table Mountain Group arenites from the Cape Peninsula were therefore analysed. The compositions of the Graafwater and Peninsula Formations of the Table Mountain Group are in many respects quite different. It is possible to compute an estimated mean composition of the arenites of the Houtbaai area and mass balance calculations based on these data indicate that the quantities of manganese and other elements present in the Houtbaai lodge could easily have been derived from the surrounding sedimentary rocks of the Group. The major components of the arenites and of the ores are the same, and the ore does not contain known concentrations of any elements which are rare in the country rocks. Three important components of the ore - iron, manganese and phosphorus - are much more abundant in the Table Mountain Group arenites of the central Cape Peninsula than in the "world average sandstone". The analysis of soils from around the Houtbaai mine shows that this deposit exhibits no marked manganese pedogeochemical dispersion pattern although it occurs on a steep slope. This suggests that the exposed ore is not the root of an eroded orebody but is a feature controlled by the present topography. The geochemical data collected are therefore entirely consistent with the geological model for the origin of these ores.
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Transactions of the Geological Society of South Africa, 81 (2), 185-191
Lithological and palaeontological evidence as well as the delta facies model are used to establish the water depth in the partly enclosed "sea" during Ecca sedimentation in the Karoo basin. Although the presence of turbidite deposits is inconclusive as regards water depth, conditions were favourable for the existence of a stratified water body during lengthy periods, and the black shales could have been deposited under shallow conditions. This conclusion is substantiated by the association of carbonate lenses, phosphorite lenses and nodules and ripple-marked siltstones which indicate a very restricted water depth. Trace fossils include the entire range from Nereites, through Zoophycos and Cruziana, to the Skolithos ichnofacies. While the last named indicates very shallow water, the first facies is today associated with extreme depths, but in the Ecca both Nereites and Cruziana occur in the same lithological unit which makes depths of thousands of metres highly unlikely. Remnants of bone fragments, Mesosaurus, leaves and fossil tree trunks in the beds indicate deposition in shallow water and proximity to land. A delta model in which the combined thickness of the delta front and prodelta deposits is approximately equal to the water depth during deposition, was applied at 11 localities in the basin and the results indicate depths of less than 70 m during the greater part of Ecca sedimentation. Water depth in the Ecca basin probably reached a maximum of about 500 m during deposition of the turbidite sequence in the early stages of sedimentation, but afterwards became progressively shallower and never exceeded about 100 m. Open-water sedimentation ended when deltas encroached from the south-west, south-east and north-east and the Beaufort beds were subsequently deposited.
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Transactions of the Geological Society of South Africa, 81 (2), 193-203
The Muldersdrif Ultramafic Complex is located in an Archaean greenstone remnant situated on the western edge of the Johannesburg granite dome. The layered igneous body, which consists of cyclically repetitive units of serpentinized dunites, harzburgites and pyroxenites, together with metagabbroic rocks, is similar to many of the apparently unique layered ultramafic complexes found in the Lower Ultramafic Unit in the Barberton Mountain Land. The Muldersdrif Ultramafic Complex has been intensely deformed by several stages of cross-folding and is made up of an involved series of interfering synclines and anticlines, the fold axes of which plunge at various angles either to the east or to the west. The interfering folds produce a complex outcrop pattern and the deformation was mainly responsible for the development of chrysotile asbestos mineralization in the serpentinized dunites found throughout the region. The paper provides an account of the geology of the Muldersdrif Ultramafic Complex as well as some of the surrounding areas and includes petrological and geochemical descriptions of some of the principal rock types encountered in the region.
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Transactions of the Geological Society of South Africa, 81 (2), 205-209
Four phases of deformation are recognized in the southern Damara Orogenic Belt near Omitara, SWA/Namibia. The east-west-trending F2 folds are restricted to a broad belt trending more or less north-east-south-west and it is suggested that these folds resulted from a ductile shear zone of left-lateral displacement. Detailed investigation of a major structure has indicated that the F2 folds are similar folds and that the regional tectonic transon was east-west in a horizontal plane. This direction of tectonic transport coincides with the expected stretching fabrics in a north-east-south-west-trending sinistral shear zone. Unfortunately it is very difficult, if not impossible, to determine the relative horizontal displacement across this shear zone, but several kilometres are probably required to account for the tight F2 folds.
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Transactions of the Geological Society of South Africa, 81 (2), 211-218
A marundite occurrence, situated in an Archaean greenstone xenolith in the trondhjemitic gneisses south of the Barberton greenstone belt is described. The corundum-bearing unit, which is about 3 metres wide, outcrops poorly over a strike length of approximately 500 metres, and is associated with metamorphosed mafic and ultramafic rocks (amphibolites, talc schists, serpentinites). Migmatites occur in the areas flanking the xenolith and are considered to have developed as a result of the interaction of the trondhjemitic gneisses and the greenstones. The marundites occur as heavy, massive, granulite textured, blue-grey rocks and consist mainly of corundum, margarite, biotite and sericite. Accessory components include apatite, tourmaline, garnet, epidote, rutile, magnetite, muscovite and hydromuscovite. Three new chemical analyses of marundites are presented and compared with similar rocks found in the north-eastern Transvaal. The marundites are characterized by high percentages of alumina, and relatively low amounts of silica. Margarite, the calcium-rich 'brittle mica', is largely, responsible for the relatively, high CaO values reflected in the analyses. Marundite genesis is briefly reviewed and, according to Hall (1922), these rocks are intimately linked with plumasites (corundum-feldspar pegmatites), but represent the pneumatolytically altered equivalents - the feldspar having been converted to margarite. Although this origin for the Barberton occurrence can not be discounted, it is suggested, in view of the absence of signs of pegmatites in the area, that the marundites described in this paper might represent a metamorphosed or metasomatized alumina-rich stratigraphic interlayer, similar to those reported in the Theespruit Formation of the Onverwacht Group.
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Transactions of the Geological Society of South Africa, 81 (2), 219-221
Erratic boulders of Cape Granite having a mass of up to 700 kg occur in a Table Mountain Group sandstone boulder-rampart at Smitswinkelbaai. The erratics must have been transported by ocean waves. The source area is situated in 20 m of water and about one km seawards, if measured along the curved path of the approaching wave front. However, calculation indicates that the movement of such large boulders by ocean waves at a depth of 20 m is not feasible under prevailing hydrodynamic conditions. It is concluded that the boulders must have been carried along in the breaker zone during a marine transgression.
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Transactions of the Geological Society of South Africa, 81 (2), 223-224
Biographical memoir.
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Transactions of the Geological Society of South Africa, 81 (2), 225-226
Contents: The Rift System of Eastern Africa. The deep lineament. The Great Dyke-Bushveld line in Southern Africa. The age of the rift lineament of Eastern Africa. The Mara and Seronera ultramafic bodies. Conclusions: The Basement structures of the northern sector of the Eastern Rift System in Kenya and Ethiopia are largely buried beneath the Cainozoic volcanic cover associated with the rift faulting, and so do not afford evidence of possible protorift dislocations. There are signs that some rift faulting may continue in a northerly direction from Lake Turkana (Rudolf), but the importance of the great Ethiopian Rift Valley dislocation, and the fault pattern east of Lake Turkana relaying it to the Gregory rifting, indicate clearly that the main axis of the Eastern Rift System continues on the north-north-east strike to the Afar depression, as has been generally accepted (e.g. Baker and others, 1972). Although the volcanic cover prevents the observation of possible taphrogenic structures cutting the Basement trends, such as those observed further south, the continuity of the overall north-north-east south-south-west strike between the Gregory and Ethiopian rifts suggests a similar age pattern, and hence the probability that protorift dislocations in the northern sector would also date back to the Late Archaean. In this case the great dislocation system of Eastern Africa would extend on a constant north-north-east south-south-west great circle strike for at least 5 500 km from Trompsburg in the south to the Afar depression, and constitute one of the greatest of the early dislocation systems to affect the earth's crust.
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Transactions of the Geological Society of South Africa, 81 (2), 227-228
In reading Mr. Vermaak's critique I came to the conclusion that our main point of difference is with regard to the age of the faulting, which, by all appearances, controls the pipe mineralization. My interpretation of this issue is that two major zones of floor instability existed during the time of emplacement of the pipes and that these zones are outlined by the present-day distribution of the pipes. Reference to Figs. 3 and 4, and especially Fig. 6 of Vermaak (1976) shows the one linear zone to extend from Vlakfontein 207JP in the south to the boundary of Turflaagte 163JP and Vlakfontein 164JP in the north. This zone of pipes is parallel to, and roughly coincides with, the Rustenburg fault which at present appears to be a very old fracture zone that originated prior to the intrusion of the Bushveld Complex but remained active over a very long span of geological time. The second zone of pipes is found on the farms Bierkraal 134JP and Groenfontein 13SJP, again straddling two major faults parallel to and probably related in age to the Rustenburg fault. These two zones mark the general directions of floor instability but the fracturing in the cumulate pile may still have been conjugate in nature. These fracture zones then connected the carbonate-rich aureole with the magma chamber and thus acted as conduits for the H2O-CO2 fluid. At this point I may also explain that I expect partial melting along these conduits to have taken place at levels normally deeper than the actual level of pipe formation This melting would happen in partly consolidated cumulates so that by all appearances the phenomenon could be mistaken to be "post Bushveld" as Mr Vermaak put it. This phenomenon may also explain the depth expression of the magnetic linear zones. To round off the picture, the Pilanesberg intruded and in addition to possible thermal effects, also caused physical rejuvenation of existing faults and weak zones (basified zones) and even new faulting. So, in fact, I advocate the idea that the Pilanesberg contributed to, but is not solely responsible for, the structural damage we see today; rather what we see is the effect of weak zones in the earth's crust that remained active from pre-Bushveld to post-Karoo times. I do hope that this explanation clarifies my views on the fault-control mechanism and I will now proceed to the phenomenon of enrichment of iron in olivine and chromite as one proceeds from the edge towards the centre of the pipes. I think the solution lies in one or all of three possibilities. First, the sulphide is tapped from a more differentiated magma (higher level) and may finally become trapped in cumulates that precipitated from a less differentiated magma (lower level). Reaction between the sulphide and the surrounding cumulate material is therefore expected. Second, one should take into account that cooling of the protopipe (molten sulphide body in partly consolidated cumulate) took place very slowly and that during this process further reactions could be expected. In the Fe-S-O system (Naldrett, 1969) it was shown that on cooling excess oxygen dissolved in sulphide melts, finally
extracting some iron to form magnetite (or wüstite). I suggest that this iron oxide, which expectedly was more abundantly released in the more massive parts of the sulphide pipes, partitioned into the olivine and chromite to cause the iron enrichment Third, according to the experimental work of Holloway and Burnham (1972) the CO2-H2O fluid will tend to extract an iron-rich partial melt from the mafic rock Therefore, all three factors strongly indicate possible enrichment of iron in the oxide and silicate material towards the centres of the pipes.
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Transactions of the Geological Society of South Africa, 81 (2), 229-231
The gravity concentrates prepared at Witwatersrand gold and uranium mines contain both alloys of the platinum group elements (e.g. iridosmine and osmiridium), and mineral compounds such as arsenides, sulpharsenides, sulphides, tellurides, antimonides and bismuthides of the platinum group elements. In particular sperrylite (PtAS2) accounts for a substantial proportion of the total platinum content of the concentrates. It is common practice on the mines to acid treat the concentrates in order to remove impurities such as common sulphides, tramp metal and uraninite. Laboratory investigations prove that, in addition, this practice causes significant dissolution of the precious metals, and it is therefore recommended that steps be taken to recover the precious metals from the solutions. Examination of leach solutions and residues from one mine has substantiated this prediction.
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Transactions of the Geological Society of South Africa, 81 (2), 233-240
The Bushveld and Kaffirskraal ores consist of multiphase titaniferous magnetite grains containing crystallographically oriented ilmenite, ulvospinel and pleonaste microintergrowths. Minor coarser-grained ilmenite is also present. The Usushwana ores are texturally similar, but contain abundant lamellar ilmenite in place of the ulvospinel. The ores from these three complexes cannot be beneficiated by conventional ore-dressing techniques and require direct metallurgical treatment for the recovery of Fe, Ti and V. The metamorphosed Mambula and Rooiwater ores have been recrystallised to varying degrees and consist of multiphase titaniferous magnetite grains containing modified ilmenite and pleonaste micro-intergrowths. Variable amounts of coarse granular ilmenite are also present and their development is related to the metamorphic grade and degree of recrystallisation. These ores can be partially beneficiated to yield ilmenite- and lower-TiO2 magnetite concentrates in which the V2O5 contents of the magnetic fractions are higher than those of the original ores. The main titaniferous magnetite layer of the Bushveld Complex and the lower titaniferous magnetite layer of the Rooiwater intrusion contain sufficient V2O5 to warrant recovery while the upperrmost magnetite layers in both these intrusions might be suitable for high-TiO2 slag production. The Mambula ores are amenable to beneficiation, but the Usushwana, Kaffirskraal and Trompsburg ores cannot be utilised at present.
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Transactions of the Geological Society of South Africa, 81 (2), 241-248
A review is given of the chemical composition of coal ash with respect to both major and minor elements. From this it is concluded that of the major elements only alumina is present in sufficient concentration to warrant its eventual extraction. Of the minor elements mainly germanium and uranium seem to be of any potential value at this stage. A review is also given of the possible uses of fly ash in the building industry, as a possible source of alumina and as a soil conditione. The occurrence of the more important minerals in coal is described briefly, and special attention is given to pyrite (and marcasite) and uranium-bearing minerals and mineraloids. The possibility of recovering purite from coal waste for the production of sulphuric acid and of uranium from coal is discussed. No data is presently available on the uranium content of South African coal or coal ash, and a plea is made for more research on the minerals and minor elements in South African coals, lest possible resources of metals should be squandered by using the wrong fly ash for construction purposes.
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Transactions of the Geological Society of South Africa, 81 (2), 249-253
Apatite is recovered by a flotation process at the Dorowa Mine in Rhodesia. A small fraction of the apatite does not respond to this treatment and is consequently lost in the tailings. The loss is due to the non-reactive surface of the apatite and does not appear to be due to liberation or to the different processes which are used in the beneficiation of the ore. Surface texture analysis using a scanning electron microscope shows that the apatite grains in the concentrate are angular and have smooth surfaces with sharp edges. These features are characteristic of freshly fractured material. By contrast apatite grains in the tailings are weathered and have pitted, uneven textures. Improved recoveries are achieved by grinding the weathered apatite to produce fresh surfaces. This is confirmed by laboratory and plant scale tests.
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Transactions of the Geological Society of South Africa, 81 (2), 255-260
The presence of significant amounts of brannerite in Witwatersrand mine products affects the extraction of uranium in that brannerite is not only more refractory than uraninite, but frequently occurs intimately associated with silicates or leucoxene and only occasionally as discrete grains. Examination of leach residues almost invariably shows uraninite in carbon (kerogen), and brannerite to be mainly responsible for the residual uranium values. The mineralogical associations of brannerite are illustrated and clearly show that these associations inhibit recovery of the uranium in brannerite and thus are of economic significance.
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Transactions of the Geological Society of South Africa, 81 (2), p227
I must congratulate Dr. de Waal on a very lucid and well-conceived paper and certainly one which gives food for thought. I am especially enamoured by the attractive alternative which his model of the Vlakfontein nickel pipes offers in contrast to my own. Perhaps he will allow me to offer some facts which may not exactly fit his theory to perfection, in order that he may ponder and possibly utilize these to make his theory even more acceptable. Dr. de Waal envisages the faulting of the area to be the pathway or conduit for his mineralizing fluids. I agree that there can be no doubt regarding the control of faulting or conjugate dilatent fracturing for the nickel mineralization. On the other hand, it should be pointed out that the bulk of the faulting in the area is related to the emplacement of the Pilanesberg Alkaline Complex, from which the faults radiate as a result. It follows that if the nickel mineralization is controlled by faulting which is ca. 500 m.y. post-Bushveld, then some agent other than the Bushveld must be chosen to have produced the metamorphism to have distilled water and CO2 from the Transvaal sediments. Naturally the Pilanesberg intrusion could have provided the necessary heat (and possibly even the necessary volatiles) for this process but then one would have expected the incidence of the nickel pipes to be greatest close to the Pilanesberg, with a lesser apportionment away from it, but there is no evidence for such a distribution. Naturally basification (i.e. silica removal from orthopyroxenites to produce olivine-rich rocks) by hot circulating waters is no new concept and was naturally considered, but I was also struck by the lack of a source of heat to produce this mechanism which would also explain the spatial distribution of the nickel pipes in the Bushveld Complex. As a result I chose the mechanism of overpressure along the faults which obviously were a major control to the distribution of the nickel mineralization, suggesting that the overpressure was caused by the "room problem" related to the intrusion of the Pilanesberg Complex. This is in keeping with the work of Avias and Sorensen (References, see Vermaak, 1976. Econ. Geol. V.71, pp. 284 286) who suggest that overpressure along faults can produce temperatures far in excess of normal metamorphism. It was also considered that pressure would enhance the removal of silica from orthopyroxene to produce olivine, therefore accommodating the pressure by producing a smaller-volumed mineral and rock in accordance with the Le Chateliers' principle. It would also favour the removal of large atoms such as K and the combination with the silica would produce the abundant graphic granite veins in the area although the amount of potash and phosphorus in those rocks is problematical unless this was contributed by the water as Dr. de Waal suggests. After logging the many shafts, pits and boreholes during the mapping of the area west of the Pilanesberg, the impression gained was that there were two seemingly unrelated events, namely an earlier "basification" or harzhurgite formation along the faults and the later mineralizing event to produce the nickel
pipes. In the final analysis, however, these temporally separated events are intimately linked in the overall mineralization process since (a) nickel, copper, chrome, platinoids, etc. were concentrated by the basification process along faults and (b) the fault channels were utilized by fluids causing sulphurization of the preconcentrated metals. This does not appear to conflict with the postulates of Dr. de Waal except that he would have many of the reactions occur while the Bushveld was still fluid although the evidence suggests that the faulting and mineralization were post-Bushveld. In the light of his postulates and those of McLean (1969). Haughton and others (1974) and McLean and others (1976). I wonder if Dr. de Waal would care to speculate on the cause of the iron-enrichment of olivine and chromite as one proceeds from the edge to the centre of a nickel pipe?
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Transactions of the Geological Society of South Africa, 81 (3), 261-270
A petrological study of volcanic rocks from the Ventersdorp sequence at T'Kuip in the Prieska district shows that, although their igneous textures are preserved, they have greenschist metamorphic facies mineral parageneses. They are also found to be chemically altered, with compositions that suggest exchange of material with the surrounding sediments. A 1 920±100 m.y. Rb-Sr isochron defined by four of the samples reflects the age of metamorphism and metasomatism of these rocks which were probably extruded at about 2 600 m.y. Other work, on Ventersdorp and older formations from Lobatse, Botswana and from the Transvaal and Orange Free State goldfields is reviewed, and it is shown that these formations also were affected by metamorphism and in some cases by metasomatism about 2 000 m.y. ago. Little penetrative deformation accompanied this event. The widespread metamorphism and metasomatism thus documented is considered to be due to burial of these formations under several kilometres of Transvaal sequence rocks prior to 2 000 m.y. Increased mantle activity at that time, which led to the intrusion of the Bushveld Complex and other tectonic events, may have resulted in elevation of the geothermal gradient over a wide area. The hypothesis is made that a widespread metamorphic and metasomatic event affected pre-Transvaal (and to a lesser extent, Transvaal) formations over the vast area covered by the Transvaal volcano-sedimentary pile about 2 000 m.y. ago.
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Transactions of the Geological Society of South Africa, 81 (3), 271-276
A gravity survey on the farm Volspruit, south of Potgietersrus, has confirmed a far more complicated structure to the Bushveld Complex and floor rocks than previously envisaged. Major horst and graben tectonics which run approximately parallel to the adjacent floor contact with the complex appear to have developed during crystallization of the magma and presumably reflect readjustment of the floor rocks in response to sudden overloading by a massive magma body. The structure is further complicated by the existence of transgressive relationships within the layered sequence and between it and the floor metasediments. The chromitite layers probably have a great lateral extent, but a combination of faulting and transgressions terminate the seams before they intersect the present erosional level, except for a small section where they are mined on the farms Grasvally and Zoetveld.
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Transactions of the Geological Society of South Africa, 81 (3), 277-317
Geobotanical, biogeochemical and geochemical investigations carried out on the western and northern fringes of the Kalahari Basin in South-West Africa and Botswana, as part of a multi-disciplinary mineral exploration programme, were aimed at delineating a hitherto unknown but inferred mineralized province in Proterozoic sediments and associated extrusive rocks. The investigations were based on a geological appreciation of the possible presence of Proterozoic sediments of comparable age and type to those of the Katanga System of Zambia that carries important stratiform copper deposits. Because the widespread cover of Tertiary and Quaternary, Kalahari sediments and the scarcity of bedrock outcrops limited geological mapping, emphasis was placed on photo-geological interpretation, geobotany, biogeochemistry, geochemistry, and also geophysics in a research based exploration programme. Initial air and ground reconnaissance revealed that distinctive vegetation associations distinguished areas of near surface bedrock from those with thick cover of sand and calcrete, and that their lineations reflected the geological structure and stratigraphy. The recognition of anomalous plant communities of Helic brysum leptolepis DC during ground reconnaissance near Witvlei led to the discovery, beneath cover of sand and calcrete, of copper mineralization in argillite bedrock that was subsequently exposed by trenching. This early success, coupled with equally promising geochemical results, prompted exploration over some 115 000 square kilometres of country. The detailed investigations focused on four main areas, namely Witvlei and Dordabis (Gamma) in South-West Africa and Ghanzi and Ngwaku in Botswana, each of which is characterized by specific environmental features posing particular exploration problems that required the development and application of appropriate techniques. The investigations which included basic geomorphological and soil studies necessary for the interpretation of geobotanical, biogeochemical and geochemical data led to the location of copper deposits in all four areas. In the Witvlei area where there are zones of near-surface subcropping steeply dipping strata planed by erosion and veneered by shallow sand, as well as areas of thick sand and calcrete cover, the investigations established relationships between the distribution of plant communities and bedrock geology, and determined the distributional aureole of plant species over and adjacent to mineralized bedrock. The strike continuities of mineralized zones could thus be traced, and the most suitable plant species be isolated for biogeochemical application. Furthermore, investigations of particle size distribution, and analyses of different mesh size fractions of surface and profile sod samples, revealed that copper mineralization in bedrock beneath sand cover could preferentially be detected by analyses of the -270 mesh fraction of surface soil samples as opposed to coarser fractions. In the Gamma grant area, which is characterized by a succession of parallel sand dunes and intervening swales aligned at right angles to the inferred geological strike, by colluvial and alluvial cover and by
abandoned water courses, the investigations concentrated on the interpretation of geobotanical, biogeochemical and geochemical data in the context of the geomorphology, on tracing the origin of transported geochemical anomalies and on assessing the role of biogeochemistry in detecting copper mineralization in bedrock beneath thick overburden. The investigations led to the location of copper mineralization in bedrock far from the sites of the transported geochemical anomalies. In the Ghanzi area the investigations concentrated on establishing the relationships between vegetation associations and concealed bedrock geology, and in extrapolating the strike continuities of curiferous strata. Spectacular fold patterns visible on aerial photographs faithfully reflect bedrock structures and lithology, that in turn control the distribution and nature of plant species. In the Ngwaku area, an appreciation of the direct relationship between lithology and overlying vegetational types enabled the strike trends and structural complications of strata to be determined with ease. Furthermore, the recognition within the prevailing low savanna woodland of an anomalous shrub layer dominated by Ecbolium lugardae N.E. Brown enabled reliable predictions being made of the presence of cupriferous argillite and limestone bedrock beneath as much as 30 m of calcrete. The investigations succeeded in locating sub-outcropping copper mineralization in seven different localities, and demonstrated the value of geobotany and biogeochemistry in exploration provided the relationships of vegetation distribution to environmental factors are understood.
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Transactions of the Geological Society of South Africa, 81 (3), 319-326
The Kirkwood and Sundays River formations were deposited in the Algoa basin during what is considered to have been a single major episode. Facies profiles constructed from measured stratigraphic sections indicate lateral gradation between the two formations. Bed thickness distributions for the Kirkwood and Sundays River formations are log normal and coincident, which suggests a complete record of deposition without major erosional breaks for the two units. The sandstone petrography and palaeocurrent trends of the Kirkwood and Sundays River formations are markedly similar. There is no evidence of an unconformity between the Kirkwood and Sundays River formations. The depositional environment for the Kirkwood Formation ranged from high energy fluvial to estuarine. The lithology, sedimentary structures and palaeocurrent trends of the Sundays River Formation are consistent with deposition in a tidal environment.
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Transactions of the Geological Society of South Africa, 81 (3), 327-329
Giant Cruziana have been found in the Beaufort Group at Burgersdorp. The tracks suggest that huge trilobite-like arthropods inhabit the sediments which are believed to be fluvial in origin.
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Transactions of the Geological Society of South Africa, 81 (3), 331-338
The Archaean granitic terrane between Nelspruit and Bushbuckridge in the eastern Transvaal has recently been the subject of study under the auspices of the South African Geodynamics Project. Six distinct granitic phases, some of them hitherto unrecognized, are found to underlie the area. These are described as 1. The Tonalite Gneisses and Migmatites; 2. The Nelspruit Migmatite and Gneiss Terrane; 3. The Nelspruit Porphyritic Granite; 4. The Hebron Granodiorite; 5. The Cunning Moor Tonalite; and 6. The Mpageni Granite Pluton. Each granitic type is briefly described and a model is presented which attempts to synthesize the current views on the evolution of the granitic crust in the region.
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Transactions of the Geological Society of South Africa, 81 (3), 339-352
Geological and geophysical investigations are being conducted on Swartvlei in the Wilderness Lakes region of South Africa and in the adjacent nearshore zone offshore. Swartvlei is mantled by two distinct types of sediment. Virtually pure quartzose sand comprises the lake margin, whereas highly organic (up to 17 percent organic matter) muds cover the floor of the vlei. Two well-defined calcified aeolinite ridges on land and two additional severely eroded remnants off the coast form a system of concentric lime-cemented dune ridges in the Wilderness Embayment. These dunal structures mark pauses (or minor transgressive pulses) in the general retreat of the sea from 120 000 (Eem) to 20 000 (Wurm) years B.P. Interdunal depressions were broadened and deepened by rivers that were deflected into the intervening valleys as the sea-level dropped during this period. The low-lying regions were then flooded during the following Flandrian transgression. Since -4 000 years B.P. the sea has slowly withdrawn to its present-day level. The data acquired during the present study have been used to construct a sedimentation model for Swartvlei. Coarse lag material accumulated on the floors of interdunal valleys during the Wurm regression. Initially, the majority of sediment entering the flooded valleys after the Flandrian transgression would have been blown in from the surrounding poorly-vegetated dunes, but pelagic sedimentation (organic and terrigenous fines) probably became increasingly, important, especially in the central lake region, as the dunes stabilised. Most of the aeolian material would be deposited close to the lake edge and outward progradation of these sands has resulted in the development of a shallow littoral margin around the shores of Swartvlei. An increasing bromine content with depth in two cores taken in the bottom sediments of Swartvlei suggests a diminishing marine influence in recent times.
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Transactions of the Geological Society of South Africa, 81 (3), 353-357
Very little is known about the geochemistry of the Table Mountain Group. Twenty-one samples from the Graafwater Formation (Chapman's Peak and Graafwater Village) have been analysed for major and 18 trace elements. The arenaceous samples are generally depleted in trace elements (chemically mature) and this shows that it will be difficult to apply geochemical provenance techniques to the bulk of the rocks of the Table Mountain Group. The Chapman's Peak shales are quite strongly enriched in typical "granitic" elements and belong to the U-Th-Zr-Pb geochemical province of southern Africa. It therefore appears that these shales were derived entirely from source rocks similar in composition to granites. However, it is difficult to decide which formation was the parent because several of the older Formations in the western and north-western Cape are very similar to one another in composition. The western Table Mountain Group appears to be unusually enriched in phosphorus.
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Transactions of the Geological Society of South Africa, 81 (3), 359-368
The purpose of the investigation was utilising available geological and geophysical data from the eastern mafic lobe of the Bushveld Complex to attempt to interpret its form at depth, to locate its possible feeders and to define the buried western margin, and also to determine the relationship of the Lebowa Granite Suite to the rest of the Complex. With these objectives in view, four sections were modelled from geological, gravity and aeromagnetic information and on these models the likely form of the Complex is indicated. The principal fault systems in the area consist of the Steelpoort, Sekhukhune, Wonderkop and Laersdrif fault zones. The locations of these faults almost certainly influenced, and were influenced by the intrusion of both the mafic and acid components of the Bushveld Complex. There are also important folds of which the more important are the Olifantspoort Antiform, the Malope, Marble Hall and Dennilton Domes. The profiles derived from the magnetic and gravity data generally indicate that the Layered Suite is sill-like with maximum thickness of about 3 km for the combined Lower, Critical and Main Zones and some 2 km for the Upper Zone. Rocks of the Upper Zone seem to be confined to the central part of the area as aeromagnetic data indicate a westerly termination of Upper Zone rocks some 20 km west of the Phokwane Plateau. A large gravity anomaly over the Marble Hall Dome indicates the presence of a thick sequence of heavy, probably mafic rocks beneath the sediments of the Transvaal sequence. Gravity modelling shows that the buried portion of the Layered Suite becomes gradually thinner westwards from the location at outcrop of the Main Zone and that no feeders of basic rocks need be invoked to explain the observed anomalies. Profiles across the Phokwane Plateau indicate a thickness of Bushveld granite of about 2,5 km overlying the magnetite gabbro of the Upper Zone. There is no convincing geophysical evidence of the presence of a granitic feeder, though one might exist beneath an area of low gravity values east of the Marble Hall and Malope Domes.
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Transactions of the Geological Society of South Africa, 81 (3), 369-371
The predominantly arenaceous basal beds of the Griqualand West sequence range in thickness from about 10 m in the south to 25 m in the north and unconformably overlie Basement granite-gneiss and lavas of the Ventersdorp Supergroup. The beds on the contact vary from arkosic to immature argillaceous sandstone, but upwards grade into orthoquartzite which is in places coarse-grained to conglomeratic. Shale clasts and planar cross-bedding are developed in the orthoquartzite. Virtually no transition to the Ghaap Plateau Dolomite Formation is present in the south, but in the north the transition beds which consist of calcarenite, quartzite and siltstone are from 5 to 10 m thick. These basal beds thin rapidly to the south and west and pinch out later on.
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Transactions of the Geological Society of South Africa, 81 (3), 373-378
Sills of rare diabase containing "cone-structures" occur within the Transvaal sequence, immediately above the Magaliesberg Quartzite in the Lydenburg district, eastern Transvaal. They comprise an interlocking system of radial and pyramidal clusters of striae, up to 0,4 m in diameter; this structure being formed by sheaves of fine-grained plagioclase, pyroxene, magnetite and uralite, containing scattered, elongate, euhedral orthopyroxene crystals. Four samples of this "cone-type" diabase, taken along a strike length of 45 km, cluster closely and are broadly comparable geochemically to a group of eight samples of "reference" diabase taken from elsewhere in the Lydenburg area. Statistically, the "cone-type" diabases are high in SiO2 and the "reference" diabases are high in TiO2, Al2O3, CaO and H2O, most other components being within the range of one standard deviation of the reference set. It is assumed that these relative enrichments were acquired by differential assimilation by the original diabase magma when traversing either quartzite or shale. The high SiO2 content of the cone-diabase magma is that property which allowed this liquid to freeze more rapidly than magma which traversed shales, giving rise to rapid nucleation of plagioclase and pyroxene, subsequent flash crystallisation and the production of the quench textures observed.
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Transactions of the Geological Society of South Africa, 81 (3), 379-391
Calcretes are mobile, heterogeneous mixtures of several ages of host and authigenic minerals. Fundamental aspects of calcrete formation such as sequence of development, fabric and composition must therefore be considered if meaningful dates and correlations are to be obtained. Fossils, artefacts and geomorphology have been the most useful methods so far, but are usually only capable of fixing maximum ages. Reasonable 14C and Th/U dates have been published, while, of a number of other possibly suitable methods, the Pa/U, U/H, K/Ar, thermoluminescene, and electron spin resonance techniques appear to be especially worthy of trial. Several techniques exist whereby relative ages and rough estimates of age can be assigned. Dating of certain other pedocretes also appears possible.
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Transactions of the Geological Society of South Africa, 81 (3), 393-399
Lenticular and persistent sandy interbeds are common throughout the argillaceous sequences that dominate cyclothems of the Lower Beaufort Group (Adelaide Subgroup). One of the thin, persistent sandstones near Beaufort West contains a set of sedimentary structures which resemble some of those currently forming on modern tidal flats and other low-energy shorelines. Diagnostic environmental criteria imply that sedimentation of the sandstone unit took place in a fluvial environment during ephemeral flooding along the lower reaches of an arid flat.
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Transactions of the Geological Society of South Africa, 81 (3), 401-402
This small unvegetated island, 200 x 100 m, consists of two humps of granite, connected by a neck of large angular blocks. There are two small hollows filled with vein quartz and Table Mountain Sandstone pebbles and the island is covered by a thin layer of guano. Beneath these the rock proves to be a homogeneous, non-porphyritic granite, showing no signs of shearing or contamination. Only 2 thin quartz veins are present. The fresh granite shows it to be a pale grey, medium-grained, markedly leucocratic rock with dark quartz grains clearly visible against the grey mass of feldspar. Muscovite and fluorite are present. It is therefore markedly different from the coarsely porphyritic biotite granite of the Cape Peninsula Pluton but resembles quite closely the micro granite marginal phase of the latter, although a chemical analysis shows it to be more siliceous. A thin section description and a chemical analysis are provided. From the data a possible contamination by the guano is inferred. In his conclusion the author considers the apparent closeness to or even the circumnavigation of Seal Island by Malmesbury metasediments and the possibility that the island could represent the tip of an isolated cupola of microgranite approximately 4 km east of the main Cape Peninsula Pluton.
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Transactions of the Geological Society of South Africa, 82 (1), 1-5
The sedimentation of the Matsap and, by inference, that of the Kheis an Namaqua sequences commenced around 2 100 m.y. ago. These sequences were deformed and metamorphosed during the Namaqua tec togenesis, which is currently dated around 1 000 m.y. The Koras Group rests with a sharp, angular unconformity on Kheis and Nama qua rocks, but previous geochronological work indicated an age of 1 085 m.y. for the former. The Leeuwdraai Formation of the Koras Group was, therefore, resampled in three different localities and six new U-Pb isotopic ages are presented. The new ages cluster around 1 200 m.y. and confirm the earlier word. This entails a re-interpretation of previous geochronological data in Eastern Namaqualand and the conclusion is reached that some of the Namaqualand granites in fact represent the intrusive equiva lents of the Koras lava.
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Transactions of the Geological Society of South Africa, 82 (1), 109-131
Precambrian igneous rocks underlie the mountainous tract that straddles the lower Orange River near Vioolsdrif. The oldest rocks are lavas and related fragmental rock types of the Haib Volcanic Subgroup. Total rock isotopic ages reported elsewhere indicate that the volcanics were erupted about 2 000 m.y. ago. The earliest volcanics were predominantly andesitic-rhyolitic, being made up of two distinct components: (1) non-porphyritic rhyolites (?ignimbrites) and related pumice sheets and bedded tuffs, and (2) a differentiated suite of porphyritic lavas ranging in composition from andesite to rhyolite, with andesite dominant. Later extrusive activity was characterised by an increased proportion of basaltic-andesitic and andesitic material, in the form of porphyritic lavas, pyroclastic beds and volcanogenic sediments. The differentiated suite of porphyritic lavas follows a high-K calc-alkaline trend, ranging in composition from basaltic andesite, through andesite and dacite, to rhyolite. An attempt has been made to explain the chemical variation in terms of a stepwise crystal fractionation model. Results for both major and trace elements are encouraging and indicate that the more mafic lavas of the Haib Volcanic subgroup (basaltic andesite to dacite) could have been produced by progressive removal of ortho- and clinopyroxene (augite), plagioclase and titanomagnetite. The porphyritic rhyolites could have been produced by removal of hornblende, biotite, plagioclase and titanomagnetite from a parental dacite. The most basic lava (basaltic andesite) is probably not a primary magma and a more basic precursor is preferred. The Haib basaltic andesites could have been derived from this primary basaltic precursor by five to ten per cent olivine fractionation. The early non-porphyritic rhyolites do not appear to be related to the porphyritic lava suite through fractional crystallisation and may represent a separate acid magma. The nature and source of this magma is not well-defined at present. If the crystal fractionation model for the origin of the porphyritic lava suite is accepted, then at least 60 per cent of the Haib Volcanic Subgroup constitutes juvenile addition from the upper mantle. A major mid-Proterozoic crust-producing event in the Lower Orange River region is therefore indicated.
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Transactions of the Geological Society of South Africa, 82 (1), 133-147
Exposed to the east and west of the Marico River in the vicinity of Derdepoort is a sequence of sediments, basalts and acid lavas correlated with the Ventersdorp Supergroup. The 70 km long succession is divided into 3 belts, the Tshwene-Tshwene, Seokangwana and Derdepoort belts, all of which are unconformably overlain by the Black Reef Quartzite. The Derdepoort belt is preserved in a post-Black Reef graben. The writer considers that almost the full Ventersdorp succession is preserved in the area discussed, represented by a basal basaltic zone over 2 000 metres thick (correlated with the Klipriviersberg Group), a 1 000 metre thick chaotic alluvial-fan deposit (correlated with the Kameeldoorns Formation), a zone of acid volcanics (correlated with the Makwassie Quartz Porphyry Formation), and an upper meandering-stream, flood-plain sedimentary zone (correlated with the Bothaville Formation). The basal volcanics of the Derdepoort and Tshwene-Tshwene belts have a major element geochemistry that compares fairly well with continental tholeiitic basalt, except that the Ventersdorp basic lavas are relatively enriched in K2O but deficient in Al2O3. The acid lavas of the Derdepoort and Seokangwana belts correspond, in a general way, to the average composition of rhyolite and to the composition of Lebombo rhyolite, although differences in the major-element chemistry of the various felsites are present. A trend towards extreme alumina oversaturation is present in the acid lavas of the Derdepoort and Seokangwana belts, which are also enriched in K2O but deficient in Na2O and CaO. It is concluded that the felsites have undergone alteration, possibly during lower greenschist metamorphism, in which the loss of Na and Ca from the system accounts for the Al-oversaturation. The name Swartruggens Trough is proposed for the south-east downwarp in which the rocks of the Ventersdorp were deposited. Because the distribution of the Kanye Volcanic Group of the Republic of Botswana is apparently restricted to the limits of the Swartruggens Trough, it is tentatively suggested that the Kanye volcanics may constitute the proto-basinal phase of the Ventersdorp Supergroup.
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Transactions of the Geological Society of South Africa, 82 (1), 149-154
One incomplete and two complete fluviatile megacycles (160+ metres in thickness) were recognised in the upper part of the Lower Beaufort (Balfour Formation) and the lower part of the Middle Beaufort (Katberg Formation) sequence. Each cycle consists predominantly of medium-grained sandstone with very subordinate mudstone at the base, grading upwards into mudstone with minor sandstone and siltstone lenses. The basal arenaceous units contain sedimentary structures indicative of braided-stream deposits while the upper part of the cycle represents sedimentation in a low kinetic energy environment. The finer-grained facies is interpreted as point bar deposits with associated overbank and flood-basin sediments of a meandering river. The cause of the cyclicity is interpreted as tectonic. It is postulated that sporadic uplift in the source area led to an increase in stream gradient and sediment input which resulted in largely braided-stream deposits. Denudation of the source caused the sourceward migration of facies and the return to a meandering-river system depositing predominantly argillaceous material in large flood basins. Recognition of successive tectonic events in the Beaufort beds could be an aid in solving correlation problems.
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Transactions of the Geological Society of South Africa, 82 (1), 155-160
A report is presented of a red sandstone that underlies much of the Namib Desert west of the escarpment. It extends from the Kuiseb River in the north to Luderitz and in places outcrops along the coast. Dissection has resulted in characteristic tabular landforms along the eastern margin. Elsewhere the sandstone is overlain by blown sand and alluvial deposits. The massive, well-jointed strata are both horizontal and cross-bedded. Calcium carbonate inclusions are frequent. Unsorted quartz grains, sub-angular or rounded, pitted or clear, are the predominant constituent. Although the matrix is rarely solely calcium carbonate, the calcium content of all samples is high. Deposition under seasonally arid conditions is postulated. The Namib succession consists of sandstone resting on older basement rocks, overlain by a limestone conglomerate and then by dune sands derived by weathering from the sandstone. This succession has affinities with the Kalahari Formation of eastern Namibia, Botswana, Angola and the Congo but no firm analogy can as yet be drawn.
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Transactions of the Geological Society of South Africa, 82 (1), 161-167
The > 2 700 m.y. old Singelele granitoid gneiss which outcrops at several localities in the Limpopo mobile belt has been mapped in detail in the "type area". Previous workers have regarded this gneiss to be of magmatic or metasomatic origin. It is more heterogeneous than previously described and contains deformed amphibolite dykes. The gneiss has undergone four main tectonic events, the most important of which generated an intense quartz-leaf fabric during F2. In addition, two ages of deformed felsic veins and dykes, regarded as local derivatives of the gneiss, are present, one of which is older than the amphibolite dykes, the other youn ger. During F4 the gneiss was refolded into a major synformal structure rather than antiformal as previously described, and it has a similar tectonic history to the supracrustal gneisses in the area. The lack of intrusive relationships and its heterogeneous nature suggest that the Singelele gneiss may be a sedimentary or volcano-sedimentary unit within the gneissic supracrustal sequence which is comprised largely of quartzite, amphibolite, pelitic gneiss and various leucocratic quartzo-feldspathic gneisses.
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Transactions of the Geological Society of South Africa, 82 (1), 169-178
Recent geochemical studies in the Archaean granitic terrane between Nelspruit and Bushbuckridge have provided the controls nec essary for establishing the relationships that exist between granite composition and topography in the area. The rate of weathering of granites is shown to be partially dependent on their composition in that granisic rocks of high K2O/Na2O ratios are generally more resistant than those with lower K2O/Na2O ratios. Porosity is also shown to be an important feature in the weatherability of granitic rocks. The distribution of granitophile elements in the Nelspruit Porphyriric Granite is also illustrated in trend surface maps, which are thought to reflect the geological processes related to the mode of formation of this granite. The concentric or semi-concentric distribution of certain granitophile elements provides semi-quantitative evidence for the progressive fractional crystallization of mineral phases from the margins of the body, inwards.
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Transactions of the Geological Society of South Africa, 82 (1), 179-181
This paper criticises some of the conclusions reached by the authors of a map of the Malmesbury and Ceres areas published by the Geological Survey of South Africa in 1975, especially in so far as the rocks of the Koringberg-Hermon area are concerned, and presents a simplified geological map of this area. After detailing a number of criticisms of some of the conclusions reached by the Geo logical Survey authors, the writer states "these authors have drawn an oversimplified picture of the geology of the area. The scope and power of the sources of deformation, metamorphism and of surface alteration have not been adequately taken into account and as a result they repeatedly interpreted the effects of such alteration as primary features of the rock".
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Transactions of the Geological Society of South Africa, 82 (1), 183-184
Any contribution to the petrography and geochemistry of Ventersdorp rocks - a subject which has long been in need of a systematic modern petrologic study - is welcome. It is thus with reluctance that one has to point out that Dr Cornell's study, based on only seven new samples, of which five were taken from a sedimentary unit correlated with the largely sedimentary Kameeldoorns Formation of the type area, can hardly be a substantial basis for a new and sweeping hypothesis. I refer to his hypothesis that the process of metasomatism was important in the alteration of the rocks of the Kaapvaal Craton toward the end of the Transvaal depositional epoch. The alteration of Ventersdorp lavas and tuffs can be attributed to a number of factors: (a) chemical weathering of these terrestrial lava flows, ash flows, tuffs and agglomerates after their emplacement and before burial, including such processes as have been described as deuteric; (b) deep weathering accompanying and following upon erosion during the hiatus represented by the Pniel unconformity; (c) a "burial" metamorphism reaching the greenschist facies; and (d) a period of diastrophism that occurred towards the end of the Transvaal, and perhaps relates to the Bushveld igneous events. The variable effects of areally differing degrees of weathering and diagenesis, followed by a thermal episode, is expected to produce a complex pattern of retrogressive and progressive metamorphic imprints. The task of the geologist to recognise the original lithology in many of the altered rocks of this volcanosedimentary pile is no mean one. Many secondary products of weathering and erosion of volcanic rocks have been confused with their primary sources.
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Transactions of the Geological Society of South Africa, 82 (1), 23-36, 2 folders
This paper comprises an investigation into the structural evolution of one of the "fragments" of sedimentary rocks within the Bushveld Complex - the Moos River Fragment - with special reference to the origin of the adjacent Dennilton Dome and other related structures. Intersection of two saddle anticlines in the Blood River Valley between Tauteshoogte and Bothasberg gave rise to a synclinal structure which may be considered as a continuation of a major syncline, concentrically located around the eastern part of the Dennilton "uplift". The evaluation of the style and orientation of the original stress-fields that gave rise to these local structural phenomena has included a study of the major structural lineaments such as faults, fractures and dykes and, in addition, a detailed study of the regional joint patterns. Statistical methods were applied to the joint measurements in order to evaluate differences between the predominant joint trends. Eight distinct joint trends were distinguished, and were subdivided into master and minor joint systems, each system comprising four sets of joints. The orientation of the stress ellipsoids derived from the joint measurements indicates that they were produced by two near-horizontal stresses intersecting at 62°. A reasonable agreement was attained between the directions of tectonic forces deduced from the pattern of jointing and those obtained by analysis of the other structural trends in the area. The analysis of the regional joint pattern and structural lineaments indicates that the Dennilton Dome was produced by interference between two fold phases. The principal compressive stress direction deduced from the master joint system is 104°, which corresponds approximately to the Vryburg arch trend and to the Laersdrif fault direction. The trend of the Minor joint system is 042°, which approximates to that of the Great Dyke and Steelpoort fault trends. In this study it has been deduced that structural irregularities in the depositional basin of the Transvaal Supergroup were reactivated by the intrusion of the Bushveld Complex and gave rise to megascopic domes and basins, specifically the Dennilton Dome. This conclusion precludes the Dennilton and also, by inference, the Elandslaagte and Malope Domes, having been formed by magmatic updoming.
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Transactions of the Geological Society of South Africa, 82 (1), 37-41
The Tsaun Formation of the Nosib Group exposed along the lower reaches of the Omaruru River is composed of quartz-feldspar gneiss, garnet-biotite and garnet-biotite-cordierite schist and amphibole gneiss, with subordinate amphibolite, calc-silicate rock and marble. The sequence can be divided into a lower psammitic Tsaur Member and an upper, more pelitic and calcareous, Autseib Member. The sequence is conformably overlain by schist and marble of the Swakop Group. Both sequences were deformed and metamorphosed simultaneously.
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Transactions of the Geological Society of South Africa, 82 (1), 43-53
A detailed petrographic and electron microprobe investigation has revealed an intimate association of glauconite and apatite minerals in phosphatic rocks mantling the continental margin of South Africa. High biological productivity associated with the zone of divergent upwelling over the outer shelf during the Pliocene provided a plentiful supply of calcareous foraminifera to the bottom sediment. The quantity of phosphorus supplied to the sea-floor by this biogenous process was sufficient to cause alteration of the calcium carbonate mud (micrite) mantling the bottom to carbonate apatite (francolite). A concomitant enrichment of iron and potassium in the sediment resulted in the alteration of minor quantities of finely divided illitic clay to interstitial glauconite. The phosphatic rocks therefore comprise a heterogeneous mixture of allochthonous constituents, e.g. quartz, calcareous debris, residual glauconite pellets and a crytocrystalline cement of apatite and glauconite. Alternating layers of glauconite and apatite were precipitated on to the walls of fractures and in voids from solutions permeating through the sediment after partial lithification of the upper layers (~ 0,5 m) of the sea-floor had taken place. The fine repetitive layering resulted from slight, but repeated, shifts in the physico-chemical conditions in the microenvironment. These data support a theory of contemporaneous glauconite and apatite formation by both alteration/replacement and precipitation mechanisms.
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Transactions of the Geological Society of South Africa, 82 (1), 55-66
The metamorphic mineral parageneses in the Matsap and Grobershoop sequence of the Kheis domain and in the Namaqua sequence of the Namaqua domain in the area between Groblershoop and Kenhardt were investigated. As a general rule, greenschist facies rocks are present in the Kheis domain and along the eastern edge of the Namaqua domain and amphibolite facies in the greater part of the Namaqua domain. The metamorphic mineral assemblages present in mafic, calcareous and pelitic and semi-pelitic rocks of the Matsap, Groblershoop, Wil genhoutdrif, Hartebeespan, N'Rougas, Koker Berg and Kenhardt sequences show that the area can be divided into four successive metamorphic zones. The contact between low-grade and medium-grade metamorphism lies well within the Namaqua domain. The low-grade area was subdivided into three sub-zones on the basis of the "hornblende-in", the "hornblende An17-in" and the "kyanite-in" isograds. The change from low-grade to medium-grade metamorphism is defined by the "diopside-in" (or "blue-green hornblende-out") isograd and is located north-west of Putsonderwater. The greater part of the Namaqua domain is characterized by medium-grade metamorphism, with high-grade metamorphism confined to the central part. The boundary between medium-grade and high-grade metamorphism is defined by the "mus covite-out, isograd. The central high-grade area was subdivided into three zones on the basis of the "brown hornblende-in" and the "hypersthene-in" isograds. The conclusion is reached that the area had acted as a single unit during metamorphism and that there was no separation of micro-continental blocks. The metamorphism was thus the result of intracrustal, vertical movements induced by mantle upwelling and convected heat along hot rise-ridge systems.
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Transactions of the Geological Society of South Africa, 82 (1), 67-80
The Tantalite Valley Complex, which now consists largely of metagabbro, intruded medium-grade metamorphic rocks of the Namaqualand Metamorphic Complex during the later part of a cycle of regional metamorphism. This intrusion produced a narrow aureole which includes orthopyroxene plus cordierite hornfels, and garnet plus cordierite pelitic hornfels containing staurolite. These assemblages have allowed estimates to be made for the conditions prevailing at the peak of metamorphism and these range from approximately 800°C near the contact with the metagabbro to about 650°C at about 5 kbar pressure, with XH2O << 1, for a pelitic hornfels near the outer edge of the aureole. Such conditions are supported by the presence of a partial melt texture in psammitic hornfels and by the development of cordierite + anthophyllite rocks, considered to represent residue following partial melting. Later deformation and metamorphism, which may have been of regional extent or localised along the Tantalite Valley shear zone, resulted in retrograde effects in the aureole, including the breakdown of staurolite and the formation of andalusite. The hydration of the gabbroic body to metagabbro and the intrusion of mineralised pegmatites was probably associated with this metamorphic cycle. Conditions in the aureole at this stage are reflected in compositional changes in garnet rims which indicate conditions at about 525°C and 3,5 kbar. At some later stage low-grade metamorphism led to the development of prehnite, chlorite, saussurite, epidote and other alteration products of minerals within the aureole and the metagabbro.
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Transactions of the Geological Society of South Africa, 82 (1), 7-22
Granitic rocks of the composite Chinamora Batholith fail naturally into three groups (based on field criteria), the Old Gneisses, Gneissic Granites and Late Granites. The oldest rocks, the Old Gneisses, consist mainly of diffusely banded migmatites. In contrast the Gneissic Granites include several younger intrusions, each texturally homogeneous and distinct from each other. They are all either tonalites or granodiorites and exhibit relatively high mafic mineral contents (5 per cent to 40 per cent) and obvious tectonite fabrics. The Late Granites, comprising the youngest intrusions, mainly granites with some granodiorites, have low mafic mineral contents (< 5 per cent). The Gneissic Granites, some of which are only a few km in outcrop extent, were emplaced over a protracted period of time. Simple field inspection as well as a study of the variations in the strain of inclusions shows that there was a deformation event (F1) after or during the emplacement of the earliest Gneissic Granites. The largest of the Late Granites, the sheet-like Chinamora Porphyritic Granite, was intruded during the next deformational event (F2). Remnants of greenstone-belt type rocks are found in most granites and gneisses of the batholith. The Old Gneisses contain remnants of higher metamorphic grade (granulite in at least one area) than that of inclusions in the Gneissic and Late Granites. Inclusions in these granites can be matched with specific lithologies in the greenstone belts (dated at 2 730±16 m.y.) surrounding the batholith. The Old Gneisses are probably older than the greenstone belts. The Late Granites are cut by fractures, some of which are filled with quartz gabbro related to the Great Dyke magmatism (2 514±16 m.y.). Thus granite emplacement had ceased by this time. The F1 deformation imposed a flattening fabric on the greenstone-belt rocks, the Old Gneiss and some of the Gneissic Granites. The events, F2 and F3, were folding episodes of regional extent and caused the development of large fold interference structures in the batholith. Extensive fracturing developed during the F3 folding and is considered to be related to the Great Dyke fracture system which extends across the Rhodesian craton. The present outcrop distribution of the various rock types in the Chinamora Batholith is strongly influenced by the disposition of major antiforms and synforms. In gross structure the batholith is a competent updomed body which has evolved as a result of the interference of F2 and F3 folding. The granite-greenstone belt configuration reflects the effect of the horizontal tectonics and not a diapiric origin for the batholith.
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Transactions of the Geological Society of South Africa, 82 (1), 81-108
The problem of the classification of the Triassic Dicynodontia, a notoriously difficult group of fossil Theraspida, is reviewed in depth and Cox's classification is revised accordingly. From this a suite of characters emerges which serves to define the unified character of the larger Triassic genera, the Kannemeyeridae Heune, typified by Kannemeyeria Seeley. These characters are: large size, with skulls normally exceeding 25 cm in overall length and often being more than 50 cm; possession of a long snout and pala-tal region, with concomitant shortening of the temporal openings; short fenestra medio-palatinalis, which migrates out with the lim its of the choanal depression in some late forms, and may or may not have raised flap-like margins; dorsally deepened inter temporal bar; fused anterior elements in the brain-case; a reflected lamina of the angular which is close to or contacts the lateral con dyle of the articular; (associated with the latter case, the qua drate foramen is seen to disappear); the forward growth of a ptery goid process of the quadrate; the epipterygoid with a dorsal process on the anterior footplate; an intertemporal region always narrower than the interorbital; highly reduced postfrontals; absence or extreme reduction of the ectopterygoid; a short "x"- or "v"-shaped interclavicle; a separately ossified olecranon ulnae. Using these characters, Triassic dicynodonts can be seen to be derived from a form similar to Rhachiocephalus dubius Boonstra, which is shown to be close to the genera Daptocephalus and Dinano modon of Daptocephalus zone age (uppermost Permian). Because of this relationship, a new genus is created for Rhachiocephalus dubius, viz Odoncocyclops, which is of Cistecephalus zone age (Upper Permian). Daptocephalus is believed to be ancestral to the newly defined, broad-snouted, subfamily Dinodontosaurinae and Dinanomodon is believed to be ancestral to the narrow-snouted, subfamily Kanne meyerinae Lehman emend. The very large Upper Triassic genera are classified in the subfamily, Stahleckerinae non sensu Lehman. The very late Triassic subfamily, Jachelerinae Keyser, is retained. Lystrosaurus, Myosaurus and Kombiusia are included in the final revised classification of Triassic dicy- nodonts which is seen to be made up of 26 genera and 47 species.
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Transactions of the Geological Society of South Africa, 82 (2), 185-204
Late Precambrian to Early Palaeozoic rocks, exposed in the north-western Cape Province (South Africa) and southern South-West Africa, comprise the mainly sedimentary Nama and Gariep groups, the Kuboos-Bremen line of intrusives and the Richtersveld and Bremen Igneous Complexes. Known stratigraphic relationships between these rock-units are enumerated, and the stratigraphic history of the area is described briefly. In this paper we present new Rb-Sr (mainly whole- rock) and U-Pb zircon age-determinations, involving a total of 145 samples on these rocks. A complementary paper is concerned with the Middle-Proterozoic rocks from the same area (Welke et al., 1979). Three bodies were studied from the Kuboos-Bremen line of plutons. Ages of ~500 m.y. (possibly a minimum age), ~520 m.y. and ~550 m.y. were obtained, and multiphase intrusion is indicated. Data for the post-Nama younger Bremen intrusives establish a minimum age of 518±15 m.y. (and probably 553±13 m.y.) for the Nama Group. Felsic dykes that post-date the Richtersveld Igneous Complex underlie the Nama Group, but reliable data on these dykes was not obtained either by extensive Rb-Sr work or by a single U-Pb zircon determination. No correlation was found between Rb-Sr data points from widely separated dyke samples; data for groups of samples collected across the width of individual dykes each defined a separate line on an isochron plot. Resetting of the Rb-Sr systems on a localised scale ~480 m.y. ago is suggested. The best estimate that can be made for the maximum age of the Nama Group is 667±212 m.y. U-Pb zircon data establish ages of 920±10 m.y. for the Richtersveld Complex and for the older Bremen intrusives and 780±10 m.y. for the Lekkersing granite. The latter result is a minimum age for the Stinkfontein Formation, Gariep Group, but it is suggested that the Stinkfontein Formation may be older than the Richtersveld Complex. A whole-rock Rb-Sr age of 911±39 m.y. for the older Bremen syenites is in good agreement with the U-Pb age. However, Rb-Sr data on the Richtersveld Complex and on the Lekkersing granite did not provide reliable ages; resetting of the Rb-Sr systems is indicated by the high apparent initial Sr ratios obtained. Rb-Sr measurements on the Gariep Group rocks (Stinkfontein lavas, Hilda and Kapok felsites and Numees shales) all yielded results that are interpreted as resetting ages. Coupled with the Rb-Sr evidence from other rocks in the area these data provide evidence for two widespread resetting events at ~700 m.y. and ~500 m.y. ago. Both events seem confined to a belt extending down the west coast, and it is suggested that the ~700 m.y. event is related to Gariepian (i.e. Pan-African) orogenic deformation and that the ~500 m.y. event is related to subsequent thermal activity.
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Transactions of the Geological Society of South Africa, 82 (2), 205-214
Using the U-Pb and Rb-Sr methods, age measurements were made on 29 whole rock and 24 mineral samples from the Middle Protero zoic rocks of the Orange River Group and the granitoids of the Vioolsdrif igneous suite in the north-western Cape Province and southern South West Africa. The samples are briefly described; a short account is given of the geological setting and the stra tigraphic relationship in this geologically complex area, characterized by multiphase deformation and metamorphism. The age patterns are also complex and show the effects of the ~1 100 m.y. Namaqua and ~500 m.y. Pan-African metamorphic events. Metalavas of the lower De Hoop Subgroup in the Orange River Group yield a Rb-Sr whole rock isochron age of 1 000±30 m.y. The high initial 87Sr/86Sr ratio of 0,722±0,002 indicates resetting at that time. Analyses of Vioolsdrif granitoid zircons from six localities give two U-Pb ages of 1 830±30 m.y. and 1 730±20 m.y. The latter age was obtained for only one sample (a composite zircon and three size fractions). This sample comes from close to the boundary between the Richtersveld Province and the Namaqua Province and U-Pb data for shene and apatite give an ~1 100 m.y. age. Reservations are expressed about the validity of the younger 1 730±20 m.y. zircon age, although it agrees closely with the 1 740±40 m.y. whole rock Rb-Sr age for granitoid samples from the Klein Helskloof area. The somewhat elevated 87Sr/86Sr initial ratio 0,708±0,001 for the Klein Helskloof isochron is sugges tive of resetting, perhaps associated with anatexis and the pre ferred interpretation is that the age of the entire Vioolsdrif suite is 1 885±20 m.y. U-Pb data for zircons from the Obib leucogranite and the Gumchavib foliated granodiorite yield an age of 1 870±20 m.y. This age suggests that these granitic inliers within the Gariep belt were emplaced as part of the Vioolsdrif igneous suite. The Rb-Sr whole rock data for the Obib leucogranite give a resetting age of 950±30 m.y. and a 87Sr/86Sr initial ratio of 0,713±0,002. The least altered of these samples, however, falls very much above the isochron; if an initial Sr ratio of 0,705 is assumed, the indicated age of this sample is ~1 600 m.y. A 1 070±300 m.y. errorchron, with a high initial Sr ratio of 0,720±0,008, is obtained for the Gumchavib gneiss Rb-Sr whole rock data. Minerals separated from this gneiss, and whole rock samples of the inter-banded leucogranite indicate a resetting age of ~500 m.y. attributed to the Cariepian, i.e. Pan-African, metamorphism. The companion paper (Allsopp et al., this vol.) deals with isotopic evidence that the Pan-African event has affected the lower Orange River area on a regional scale. It is noted that the age pattern for the Vioolsdrif suite closely resembles that of the Fransfontein granitic rocks in northern South West Africa. This may indicate that these two regions belong to the same crustal entity and therefore point to the possible existence of an ~ 1 900 m.y. old basement underlying extensive areas of south-western Africa.
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Transactions of the Geological Society of South Africa, 82 (2), 215-226
Outcropping below the Black Reef Quartzite to the west of Thabazimbi, between Sentrum and Ganskuil in the west-central Transvaal, is a succession of early-Proterozoic sediments and volcanics which unconformably rest upon the Archaean granites and greenstone rem nants of the Makopa Dome. This early-Proterozoic volcanic-sedimentary assemblage for which the name Buffalo Springs Group is proposed, commences with the Hampton Formation, a sedimentary unit with a maximum thickness of over 1 200 metres, in which braided-stream, meandering-stream, and near-shore sedimen tation have been recognized. Overlying the Hampton Formation is the 200 metres thick Waterval Formation, dominantly composed ot basaltic lavas. Conformably succeeding the Waterval Formation is a 850 metres thick hete-rogeneous pile of altered acid volcanics, pyroclastics, sediments and basic lavas that constitute the Witfonteinrant Formation. Arenites and wackes of the 40 metres thick Kransberg Quartzite cap the Buffalo Springs Group succession and grade upwards into the base of the Black Reef Quartzite. In view of the gradational contact between the Kransberg Quartzite and the base of the Black Reef Quartzite, the Buffalo Springs Group is assigned to the base of the Transvaal Supergroup and is corre lated with the Wolkberg Group that outcrops along the Drakensberg Escarpment in the north-eastern Transvaal.
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Transactions of the Geological Society of South Africa, 82 (2), 227-230
New volcano-stratigraphic nomenclature is proposed for the Jurassic volcanics of the Lebombo Monocline. Nomenclature is based on field criteria since all major rock types are characterised by distinctive lithologies and can be mapped as individual units. "Lebombo Group" is redefined to include all the basic and acid volcanics of the monocline. Brief descriptions of the main extrusive and intrusive rock types are presented.
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Transactions of the Geological Society of South Africa, 82 (2), 231-241
Three bodies of calc-silicate rock which occur on approximately the same stratigraphic horizon, i.e. below the Beynestpoort quartzite of the Rayton Formation of the Transvaal Supergroup, were investigated. Body A is surrounded by magnetite-gabbro of the Rustenburg Layered Suite whereas body B is surrounded by diabase. Hybrid rocks crop out around body C. The calc-silicate body A is zoned and consists of marmorite, wollastonite-garnetfels, garnetfels, diopside-garnetfels and plagioclase- diopsidefels. Body B consists only of marble with an associated fluorite-bearing rock and body C consists of marmorite and a prehnite-hydrogrossularite rock. Metamorphism took place in two stages, the first stage being isochemical metamorphism and the second metasomatism which caused the formation of sphene, andradite and grossularite as a result of the addition of titanium, iron and aluminium. The mineral assemblages of body A belong to the pyroxene-hornfels facies. A temperature of approximately 780°C and a pressure of approximately 500 bar were probably reached during metamorphism. Decrepitation tests on calcite from body B indicate that a temperature of 550°C and a minimum pressure of 500 bar prevailed at body B. The mineral assemblages of body C belong to the hornblende-hornfels facies and apparently temperatures and pressures during metamorphism were the same as at body B.
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Transactions of the Geological Society of South Africa, 82 (2), 243-249
Geochemical analysis of soil samples taken from the area underlain by the Regenstein alkaline diatreme indicated potential areas for Pb-Zn-Ag mineralization, and these were subsequently proved by drilling. The pipelike body, emplaced into quartzites of the Damara Supergroup, consists of lithic and volcanic breccias. The breccias have been intruded, first by phonolite dykes, and then by numerous bodies of alkaline mafic and ultramafic rocks. The final phases of volcanic activity are represented by hydrothermal alteration of the pipe-filling and by Pb-Zn-Ag mineralization. The mineralization occurs in the form of fine disseminations and irregular replacements and as thin veinlets of open-space filling type along fracture zones in the volcanic breccia. The nature of the mineralization indicates that it was emplaced under epithermal-mesothermat conditions and the deposits exhibit considerable simitarities to those of Cripple Creek, Colorado.
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Transactions of the Geological Society of South Africa, 82 (2), 251-256
A study of the outliers of Dwyka Tillite south of latitude 33°15'S has revealed the presence of a disconformity at the base of the tillite. Seven detailed sections of the Witteberg Group, from Pieter Meintjies in the north-east to Genadendal in the south-west, are presented to show that the hiatus represented by this disconformity increases in magnitude towards the south until, at Genadendal, most of the Witteberg Group is absent. Modifications to the generally accepted depositional model of tee Dwyka Tillite (Du Toit, 1921) are discussed as the relevance of the disconformity in the light of palaeontological dating. It is shown that a considerable gap exists in the sequence at the base of the Dwyka Tillite even where this unit appears to be conformable to the underlying rocks.
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Transactions of the Geological Society of South Africa, 82 (2), 257-258
Two facies are defined in the Clarens Formation at Kamberg. Facies 1 consists of medium-coarse-grained sandstone characterized by planar cross-bedding and channel systems. At the base of facies 2 is a lens of fine-grained sandstones and siltstones exhibiting convolute bedding, convolute laminations, fine laminations, small sets of cross-bedding, compaction structures, scour-and-fill structures. This is contained within silty fine-grained sandstone showing thin bedding, fine and wavy laminations, ripples, compaction structures, runzel marks, tool marks, mud cracks and trace fossils. Facies 1 is interpreted as having been deposited by braided wadi channels and lateral sheetflow on an alluvial plain. Facies 2 was probably laid down by low-energy pulses of sheetwash, much of it into ephemeral playa lakes. These processes took place as the result of infrequent rainfall in a desert.
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Transactions of the Geological Society of South Africa, 82 (2), 259-269
The results of Rb-Sr and Pb-Pb whole-rock and U-Pb zircon isotopic age studies are presented of the Singelele and Bulali Gneisses of the Central Zone of the Limpopo Mobile Belt. The whole-rock results from the Singelele Gneiss yield ages that are younger than the emplacement age of that unit while the whole-rock results from the Bulai Gneiss yield ages that are consistent with the emplacement age of that unit. The zircons from both units are widely discordant and of little use as age indicators. Under conditions of metamorphism such as have affected the Singelele Gneiss, individual isotopic systems behave independently of one another. Geochronological studies in polymetamorphic terranes require an integrated approach involving age studies by multiple techniques on numerous rock units, linked with detailed structural and stratigraphic studies. Age determinations by single methods in isolation can be very misleading in deciphering the history of such a terrane.
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Transactions of the Geological Society of South Africa, 82 (2), 271-273
Two U-Pb zircon ages from granitic rocks near Prieskapoort are discussed. Evidenceis presented suggesting the occurrence of two different granitic rock types, hitherto regarded as being of the same age. A minimum age of 2 600 m.y. for the "Steenkoppie" granite was obtained. Evidence, in the form of restites of Marydale Group rocks, confirms the intrusive character of the second granitic rock type, herein referred to as "migmatite". This "migmatite" yielded an age of 1 170 m.y.It is suggested that the "migmatite" represents the initial phase of the Namaqualand deformation period.
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Transactions of the Geological Society of South Africa, 82 (2), p275
It was with some interest that I read Mr Marchant's paper on the metal content of the Ecca shales. As regards the palaeosalinity and depositional environment of the Ecca Group it seems, however, that he would have done better to trust in his own results and the palaeosalinity criteria established by Degens et al (1957) rather than the work of Danchin (1970). It would seem, that both the palaeontological data and Mr Marchant's studies on the metal contents of the Ecca shales provide strong evidence to suggest that Ecca sedimentation occurred in an atypical, brackish "marine" environment similar to the present-day Caspian Sea. Thus, contrary to Mr Marchant, the application of trace element studies of organic shales may well be a reliable palaeosalinity indicator; it is frequently the human interpretations which are not.
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Transactions of the Geological Society of South Africa, 82 (3), 277-285
The previously unmapped quarter degree sheet 1830C was reconnaissance mapped for the purpose of a wider regional study. The oldest known rocks here are the tonalitic gneisses and their Sebakwian Group schist inclusions. The gneisses are intruded by irregular shaped plutons of fine-grained Rhodesdale Tonalite (G1). The Milota Porphyritic Adamellite (G2) and associated rocks were intruded into this tonalite complex as a large batholith that stretches north-east for 40 km across the area. This was, itself, subsequently intruded by the Wanganui Porphyritic Adamellite (G3), which was in turn followed by the Ngezi Granite Structures in the gneiss exposed mainly along the Umniati River, show early north-trending isoclinal folds (F1), overprinted by a more open north-west-plunging set (F2). Intrusion of the batholith (G2) was associated with F3 warps and doming. The existence of an intrusive batholith at the domed centre of the Rhodesdale gneissic terrane suggests a mantled gneiss dome structure. Large shear zones with lateral displacements of several kilometres pre-date the Great Dyke. Various ages of dolerite dykes and large, saucer-shaped Mashonaland Dolerite sills post-date the Great Dyke. Outliers of Karoo and Kalahari sediments occur in the east.
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Transactions of the Geological Society of South Africa, 82 (3), 287-303
Amphibolites occur as large bodies and lenses within the gneisses of the Namaqua Metamorphic Belt. The amphibolites are divided into an Eastern and Central Domain for practical reasons. Lithological descriptions of the various amphibolite bodies are submitted and their relationships to the surrounding gneisses illustrated on detailed maps of the individual sequences. Structural and textural features which may be of interest regarding the origin of the amphibolites, are described in the text. Petrochemical analyses of 50 selected samples reveal a meta-igneous parentage for most of the amphibolites; however, some may have sedimentary precursors. A Comparative study indicates that the amphibolites of the Eastern Domain resemble basalt and andesite of the calc-alkaline series, while the amphibolites of the Central Domain resemble tholeiite and tholeiitic andesite. The parental magma of the ampibolites of the Eastern Domain form part of a calc-alkaline suite possibly associated with an active continental margin (of the Andes-type) along the eastern margin of the Namaqua Mobile Belt, while the amphibolites of the Central Domain resemble tholeiitic rocks intruded into a continental environment which might have served as a foreland to the subduction zone developed along the eastern margin of the metamorphic belt.
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Transactions of the Geological Society of South Africa, 82 (3), 305-311
Plutonic rocks of the Vioolsdrif composite batholith form a significant component of the Precambrian basement that underlies the lower Orange River region in southern Africa. Field relationships between the rocks of the batholith suggest that the order of intrusion was basic to acid. Whole rock Rb-Sr and Pb/Pb isotopic data presented indicate the following ages of emplacement: Intermediate rock types (diorite, tonalite, granodiorite) - 1 900±30 m.y. ago Acid rock types (adamellite, leucogranite) - 1 731±20 m.y. ago These ages are consistent with the deduced intrusive relations but finer resolution of the intrusion chronology is precluded by the analytical uncertainty (40-60 m.y.). The Vioolsdrif batholith, together with its envelope of volcanic rocks, constitute the oldest known rocks in the lower Orange River region.
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Transactions of the Geological Society of South Africa, 82 (3), 313-327
The Ghaap-Group of the Transvaal Supergroup consists from the base upwards of the Schmidtsdrif Subgroup (a mixed siticiclastic-carbonate unit), the Cambellrand Subgroup (a carbonate unit), the Asbesheuwels Subgroup (an iron-formation unit), and the Koegas Subgroup (iron-formation and siliciclastic rocks). In turn the Schmidtsdrif Subgroup is subdivided into the Vryburg Formation (feldspathic arenites, quartz arenites, quartz wackes, argillites, carbonate rocks and lavas), the Boomplaas Formation (carbonate rocks and argillites), and the Lokammona Formation (argillites with subordinate amounts of carbonate rocks and chert). The Vryburg Formation rests unconformably on the Ventersdorp Supergroup whilst the Lokammona Formation is followed conformably by the Cambellrand Subgroup. The Schmidtsdrif Subgroup thins towards the north over the Ganyesa dome whilst facies and thickness changes reach a maximum along the Griekwastad fault zone (a synsedimentary growth fault).
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Transactions of the Geological Society of South Africa, 82 (3), 329-333
The Khomas Subgroup along the lower reaches of the Omaruru River overlies the Nosib Group conformably, indicating non-deposition of the Ugab Subgroup. The Khomas sequence can be subdivided into five formations, i.e. a basal Geluk formation consisting of white marble, overlain conformably by the Oberwasser and Hoopverloor formations consisting of immature clastics and impure calcareous rocks, followed conformably by white marble of the Jun-Mon Formation, which in turn is conformably overlain by a monotonous succession of schists of the Rietkuil Formation. A definite change in sedimentary facies, represented by immature clastics and minor calcareous rocks in the west and impure marbles farther eastwards, is present in the lower units of the Khomas Subgroup. The sequence as a whole was subjected to a single, prolonged metamorphism during which the PF conditions reached a minimum of 600° at 3,5-5 kb.
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Transactions of the Geological Society of South Africa, 82 (3), 335-342
Equilibrium thermodynamic calculations indicate that the Bon Accord nickel deposit was in situ during the intrusion of the Nelspruit Granite some 3,2 giga-years ago. Thermal effects of this granite caused the formation of a liebenbergite-bunsenite (XFe = 0,065) - trevorite assemblage at temperature T approximating 1 000 K, pressure P(total) less than 2 x 10(8) Pa, and fO2 approximately equal to 10(-14). In the waning stages of this metamorphic event, retrograde hydration of the liebenbergite to nepouite took place at very low fH2O (less than 10(-10)) and a temperature estimated to have been ca. 700 K. This estimate is based on the sliding equilibrium between trevorite and bunsenite. Apparently, at an even later stage, at temperatures of 500 to 600 K, increased CO2 activity in the fluid led to the inversion of nepouite to willemseite. These calculated physical conditions are in good agreement with the geology of the country rocks as described by earlier workers. Furthermore, the added constraint on the origin of the nickel seems to support the meteorite hypothesis propounded by the author in earlier papers. However, mantle derivation cannot be excluded.
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Transactions of the Geological Society of South Africa, 82 (3), 343-348
The Lebombo monocline is a large flexure of late-Karoo age (late-Jurassic) extending along the eastern South African border from Rhodesia to northern Natal. It consists mainly of a thick sequence of Karoo basalts and rhyolites which overlie Archaean rocks and dip eastwards beneath the Cretaceous cover at angles of up to 50°. The lowest volcanic unit exposed in Swaziland is the Sabie River Basalt Formation. It consists of a thick (c. 5 km) sequence of basalts and includes two groups of interbedded rhyolites: the Mkutshane Beds and the Twin Ridge Beds. A 5 km thickness of rhyolites overlies the Sabie River Basalt, the contact marked by a steep escarpment. The succession has been divided into the Jozini and Mbuluzi Rhyolite formations. The rhyolites are thought to be pyroclastic or ignimbritic in origin. Detailed mapping of individual flows has enabled the stratigraphy of the rhyolite sequence to be determined. Structurally the area is simple with broad warps superimposed on the monoclinal flexure. A number of granopyre plutons intrude the basalts close to the base of the rhyolites. A large dolerite dyke-swarm intrudes the basalts; it is associated with a zone of strike-faulting and represents the locus of tension developed along or parallel to the monoclinal axis.
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Transactions of the Geological Society of South Africa, 82 (3), 349-361
The Okahandja Lineament forms the northern border of the 25-40 km wide Okahandja Lineament Zone in the north-east-trending arm of the Damara Orogen. The lineament has been compared to the median tectonic line separating paired metamorphic belts (Sawyer, 1978) which are here represented by the high temperature/medium pressure Central Zone north of the lineament and the low temperature/high pressure Southern Zone, the two being separated by the Okahandja Lineament Zone. Central and Southern Zones each contain their own distinctive granites and only the uppermost unit of the stratigraphic succession, the schistose Kuiseb Formation, is common to both zones. Southern and Okahandja Lineament Zones, though lithologically and aeromagnetically indistinguishable, are structurally distinct. The Okahandja Lineament is a huge monocline-like downfold of the whole Damara succession that exceeds 530 km in length. The lineament is clearly defined by remote sensing techniques but is difficult to pinpoint in the field for few of the rocks types in it show evidence of shearing. The Okahandja Lineament is believed to have been a fundamental line of weakness in the crust throughout the history of the Damara Orogen which
(i) was the locus of late Nosib block faulting and acid volcanism,
(ii) coincided with the margin of a deep early Khomas depository,
(iii) separated regions in which the intensity and orientation of folds that formed during each of the three main phases of deformation differ
(iv) formed the margin of the Central Zone during uplift relative to the Southern Zone of possibly 24 km,
(v) largely delineates the northern limit of the Donkerhuk Granite, and
(vi) was the locus of strike-parallel post-Karoo faulting.
The last Damaran movements on the lineament ceased prior to intrusion of the Donkerhuk Granite 523±8 m.y. (Ma) ago. The Purros Lineament in north-western South West Africa Namibia is similar in several respects to the Okahandja Lineament.
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Transactions of the Geological Society of South Africa, 82 (3), 363-370
Present day landscapes in Zimbabwe Rhodesia are the resultant of uplift and erosion since Palaeozoic time. Vestiges of the Pre-Karoo landscape are particularly numerous across the resistant banded iron-stones and jaspilites of the ancient Sebakwian and Bulawayan groups, and a major watershed of that age can be traced parallel to, but about 100 km south of the present Zambezi-Limpopo divide Post-Karoo time is recorded geomorphologically by the progression of the six major erosion cycles that coincide with those seen in the adjacent countries Fragments of the oldest surfaces are best seen along the border between Zimbabwe Rhodesia and Mocambique, which forms the Eastern Highlands and which has been the site of repeated uplift. Measurements taken across the African (early Tertiary) landsurface show a tilt due west which markedly steepens in the proximity of the Eastern Highlands As a result of this tilting and encroaching of younger erosion cycles along the Zambezi and Limpopo rivers and their tributaries, certain changes of drainage have occurred during Cainozoic time. In particular, the Zambezi has lengthened to its present course by capturing an earlier and now defunct river which drained southwards from western Zambia and south-eastern Angola, through eastern Botswana possibly to join with the proto-Orange River. Several tributaries of this river drained westwards from Zimbabwe Rhodesia and have since been diverted to their present-day positions and directions of flow. Changes during the Quaternary era in Zimbabwe Rhodesia include renewed erosion and spasmodic and patchy deposition of river alluvium The filling of Lake Kariba has caused not only shoreline features to develop, but also the lake floor to be slightly depressed in movements which caused earthquakes in 1963.
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Transactions of the Geological Society of South Africa, 82 (3), 371-375
We read with considerable interest R. W. Shone's paper on giant Cruziana from the Beaufort (Burgersdorp Formation of the Beaufort Group). However, we should like to discuss some of his conclusions and the premises on which they are based.
1. It is inferred in the paper that Cruziana represents the "burrowing" activities of trilobites. This is a misconception, since trilobites are generally considered to have "furrowed" when producing Cruziana. Examples of Cruziana burrows have been recorded (Lessertisseur, 1955; illustrated in Seilacher, 1964; Crimes, 1970; Baldwin, 1977, Plate 2e) but these appear to be the exception rather than the rule (Crimes, 1975). The trilobites that produced Cruziana were usually animals that were at all times exposed to the water body.
2. Shone invokes Seilacher's (1970) model to explain the preservation of the giant Cruziana on the sole of a sandstone bed. This model has been questioned by Crimes (1975) who favours the mode of preservation proposed by Radwanski and Roniewicz (1963, 1972) in which the excavation of the Cruziana trackway preceded the deposition of the sand layer that preserved it. Thus the producer organism furrowed in the mud which acted as a mould to the later casting effect of sand deposition, both mediums being ideal for preservation of the traces. This view was supported by Crimes (1975) on the basis of several criteria, including sediment stability which he discusses in some detail. Since then additional evidence has been found to substantiate this model (Crimes et al., 1977; Gutschick and Rodriguez, 1977) which is now generally accepted by most workers. The difference between the two models becomes crucial when considering the environment in which the animal that produced the Burgersdorp Cruziana lived.
If Seilacher's model is correct then the producer organism was living on a crevasse-splay. If the second model is correct then the animal inhabited part of the flood-plain which was later covered by crevasse-splay sands.
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Transactions of the Geological Society of South Africa, 82 (3), 377-378
Dr Cornell is to be congratulated upon his interesting study of the petrology of the Ventersdorp rocks of the T'Kuip area. His paper serves to emphasise our urgent need for more study and, eventually, a regional synthesis of the nature of the volcanogenic environment of the Ventersdorp Supergroup. Dr Cornell's hypothesis of a pervasive episode of burial metamorphism and metasomatism that affected Ventersdorp rocks in those areas subsequently covered by volcanosedimentary piles of Transvaal Supergroup age is worthy of close attention particularly as he extends his hypothesis to rocks in Botswana that may be of pre-Ventersdorp age. In south-eastern Botswana the earlier view of the Geological Survey in that country (see, for example, Crockett, 1971) was that the pre-Transvaal rocks of the area Gaborone-Kanye-Lobatse were divisible into sediments and volcanics of Ventersdorp age and a much older complex association of the Kanye Volcanic Group felsites and the Gaborone Granite. This view was supported by the somewhat limited number of geochronological data available at that time. This interpretation had to be re-examined in the light of new geochronological data published by Harding et al. (1974). This study broadly confirmed an age in the range 2 000-2 250 m.y. for rocks correlated with the Ventersdorp Supergroup and therefore not incompatible with the generally accepted limits for the succeeding Transvaal Supergroup. A real problem, however, was created with the new determinations for the Gaborone Granite. Earlier whole rock Rb/Sr dates had pointed to an age perhaps as old as 2 800 m.y. but eight new determinations on the so-called Inner Marginal Assemblage of the Gaborone Granite, which had not previously been sampled, appeared to be clearly related to an isochron giving an age of 2 328±44 m.y. These are noted in Table II of Dr Cornell's paper. The question arises as to whether this is a true age for the Inner Marginal Assemblage (now renamed Kgale Formation by the Geological Survey of Botswana - Dr R. M. Key, personal communication, 1979) or whether, as Dr Cornell suggests, such an age is a function of an episode of partial chemical and isotopic homogenisation that occurred as late as Transvaal time.
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Transactions of the Geological Society of South Africa, 83 (1), 1-4
Groundwater is a valuable resource in Swaziland and occurs in both crystalline Precambrian rocks and the Karoo Supergroup. Groundwater abstraction is limited to areas in which secondary permeability has developed or where basins of decomposition have formed on the crystalline rocks. Average borehole yield is 0,9 l/s and only 19 per cent of all the water boreholes yield more than 1,5 l/s. Chemically the groundwater is fresh to moderately mineralised; it is dominated by the bicarbonate ion.
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Transactions of the Geological Society of South Africa, 83 (1), 107-111
Having looked forward to the appearance of this paper for some time, it was with much interest that I read the above contribution and Drs Keyser and Cruickshank are to be congratulated on their review. One of the more disturbing features which I have encountered in the recent change from invertebrate to vertebrate palaeontology is the tendency of taxonomists in this latter field to use the generic rank for the species category. This has the effect of creating numerous monogeneric taxa which tend to obscure both biological and stratigraphical relationships, as well as introducing a host of superfluous generic names. I can thus only whole-heartedly agree with Keyser and Cruickshank that Tetragonias, Vinceria, Dolichuranus, Rhopalorhinus and probably Rhinodicynodon should be included in the synonymy of Shansiodon, the differences being those between species. Similarly, Proplacerias, Uralokannemeyeria, Rechnisaurus and Rabidosaurus should be considered junior subjective synonyms of Kannemeyeria, Chanaria a synonym of Dinodontosaurus and Parakannemeyeria a synonym of Sinokannemeyeria. Moreover, as noted by Keyser and Cruickshank (1979, p. 96), the differences between Ischigualastia and the sympatric Stahleckeria "are hardly more than those used by Cox (1965) and Cruickshank (1967) to demonstrate possible sexual dimorphism in Dinodontosaurus and Tetragonias respectively", whilst Barysoma is very close to Ischigualastia. In view of this, for the purpose of the following discussion, Stahleckeria, Ischigualastia and Barysoma are referred to as the "Stahleckeria plexus". As regards the taxonomic and phylogenetic interpretations of Keyser and Cruickshank, I should like to make the following observations: (a) The family Shansiodontidae was validly introduced by Cox (1965) but, because Keyser and Cruickshank considered that "so little is known about the detailed morphology of Shansiodon .. we do not propose to base any higher taxon on this unit", they introduced the conceptually virtually identical new subfamily Dinodontosaurinae. This, however, is contrary to Article 23 of the International Code of Zoological Nomenclature which states that "the valid name for a taxon is the oldest avail- able name applied to it, provided that the name is not in- validated by any provision of the Code or has not been suppressed by the Commission". Thus, the subfamily Dinodontosaurinae Keyser and Cruickshank must become a junior subjective synonym of the subfamily Shansiodontinae (nom. transl. herein ex family Shansiodontidae Cox, 1965). (b) According to Keyser and Cruickshank (1976, p. 96), "it is now clear that Dolichuranus and Rhopalorhinus represent the extremes of a morphological series and that the two genera are synonymous". Since these two genera occur side-by-side in the Omingonde Mudstone, surely this morphological cline should also imply that but a single species is present, and that Dolichuranus etionensis (Keyser) is a synonym of D. primaevus Keyser? (c) I somehow feel that Keyser and Cruickshank (1979) have unnecessarily complicated the suprageneric classification of their Kannemeyeriidae, since the outstanding feature of this group is the fundamental
division into tusked and tuskless forms. Using this criterion and with the synonymy suggested in the introduction to this discussion, the kannemeyeriids may be listed as follows: Tusked: Kannemeyeria, Shansiodon, Sinokannemeyeria, Dinodontosaurus, Rhinocerocephalus, Rhadiodromus, Jacheleria. Tuskless: Wadiasaurus, Sangusaurus, Zambiasaurus, Stahleckeria plexus, Placerias.
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Transactions of the Geological Society of South Africa, 83 (1), 113-114
I am at present carrying out a petrographic study in the Kielder area some 40 km south-south-west (lat. 29°50', long. 22°10') of the area discussed by Botha et al. (1979). The samples used have been taken from over 7 000 metres of diamond-drill core drilled during 1977-1979 in the course of a base metal exploration programme. This enables not only the study of fresh rock below the weathering depth of 60-70 metres (sulphides are oxidized down to a depth of 100 metres) but also observation of the change in mineral assemblages on mesoscopic and macroscopic scales (the area in question stretches some 6 km along strike). The major mineral assemblages which I have encountered to date are as follows: 1. quartz-biotite-plagioclase (An35)-almandine (biotite-garnet gneiss), 2. diopside-plagioclase (An35)-hornblende (diopside granulite), 3. plagioclase (An60)-hornblende (green brown)-hypersthene-diopside-biotite (brown)-magnetite (hypersthene-diopside granulite), 4. quartz-biotite (green-brown)-hypersthene-plagioclase (biotite gneiss), 5. hypersthene-plagioclase-biotite-almandine-quartz (hypersthene garnet gneiss), 6. quartz-perthite-plagioclase (An35)-sillimanite (sillimanite gneiss), 7. quartz-cordierite-hypersthene-biotite (brown)-plagioclase (hypersthene-cordierite granulite), 8. hornblende (green)-plagioclase (An60)-calcite (amphibolite). These assemblages put the area unequivocally in the Regional Hypersthene Zone (or Granolite High Grade) as defined by Winkler (1976). Botha et al. (1979) state that: "different plagioclase compositions lie side by side in the same general area and the plagioclase composition is a very unreliable indicator of metamorphic grade". Although the phrase "same general area" is rather vague I have found this statement applies to my area as well. In fact, it applies over a distance of millimetres over a contact between diopside granulite (plagioclase An32) and granodioritic gneiss (An60), and over a distance of metres between diopside granulite (An50) and hypersthene-diopside granulite (An40-50). It would appear, therefore, that metasomatism, at least with respect to sodium and calcium, is very limited indeed notwithstanding the temperatures and pressures reached at this grade. However, not only is the plagioclase composition an unreliable marker but so also is the colour of the hornblende and biotite. Again over fewer than 10 metres green-brown biotite can become brown biotite and dark green hornblende change to green-brown hornblende. No brown hornblende was observed in any of the assemblages. These colour variations are present within rocks with the same mineral assemblages. In fact, in one case both green and brown biotite were observed in the same thin section. Another important factor is that retrograde alteration has occurred. The uralitization of pyroxenes to green hornblende, although not extensive, has taken place preferentially along grain edges and cleavage cracks of diopside (and hypersthene), resulting, in places, in complete replacement of the pyroxene. There is also ample evidence of biotite being replaced by chlorite and magnetite. I therefore sound a warning on drawing up isograd maps in this
area without taking cognisance of the above phenomena. Particularly in view of the poor outcrop and the biased surface sampling one is forced to do if no drill core is available. In fact, unless extremely detailed (plane table) mapping was done in the Kielder Area a petrographic study (of surface samples) would probably not reveal the true nature of the metamorphic processes.
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Transactions of the Geological Society of South Africa, 83 (1), 17-21
Conspicuous heavy mineral layers up to 5 cm thick, and laterally continuous for up to 100 metres, occur frequently throughout the vertical profile of the fluviatile, Lower Triassic, Katberg Sandstone. These layers are upper flow regime features and were created by large traction currents m shallow, low sinuosity streams at shooting flow stage. Polished specimens were prepared and examined initially using conventional ore microscopy. The opaque minerals in order of abundance were titanomagnetite, the polymorphs geothite/lepidocrocite, maghemite, hematite and ilmenite. Almandine was the most common non-opaque mineral. Selected phases were investigated with the electron microprobe and analysed for iron and titamium. The titanium contents of the magnetites indicate that they originated from low-temperature granitic rocks where temperatures of formation were between 560 and 650°C.
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Transactions of the Geological Society of South Africa, 83 (1), 23-38
The geology of a part of the Kaaimans Group and some of the granites in the George area is described. The Kaaimans Group is a metamorphosed sedimentary sequence, which comprises primarily fine-grained pelitic to psammitic rocks. Four major periods of deformation have been recognized. Metamorphism associated with F1 reached temperatures of approximately 600°C at 2 to 3,5 kb. The F2 metamorphism reached upper low-grade conditions. Syntectonic granite intrusions during F1 and probably F2 contributed considerably to the heat flow, with the result that metamorphic grades are higher than those of any of the other late Precambrian sequences in the Cape Province. The third period of deformation, F3, has been recognized in both Cape and pre-Cape rocks. Retrogressive metamorphism during F3 down-graded the pre-Cape rocks to lower low-grade. The first three periods of deformation are characterized by north-south compression. The fourth period of deformation, F4, is defined by tensional features with no evidence of associated thermal metamorphism. The model proposed by Urien et al. (1976) relating to the break-up of Gondwanaland fits the deformation of both Cape and pre-Cape sequences. No evidence for the presence of palaeo-subduction zones has been found.
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Transactions of the Geological Society of South Africa, 83 (1), 39-45
Petrography, radioactivity distribution and fluid inclusions have been studied in three samples of the Rössing alaskites from the S.H. anomaly outside of the Rössing deposit. After the crystallization of the alaskitic magma which involved oligoclase, biotite, orthoclase and quartz, a deuteric alteration produced a quartz, microcline, albite, muscovite, calcite ± chlorite mineral association similar to those observed in several French uraniferous granites. Uranium redistribution occurred during this alteration. From fluid inclusions data, the pressure at the time of the intrusion is estimated to have been at least 6 kbar for a temperature of 625°C. Reaction of the magma with the marbles of the Rössing formation led to the boiling of the magma by an increase in the CO2 partial pressure as well as to its crystallization. Immiscibility between a dense saline (>30 per cent NaCl) and a CO2-rich fluid is proposed to have occurred simultaneously or after the fluid oversaturation of the magma. Part of uraninite is formed at the magmatic stage, another part crystallized from the magmatic fluids in the biotite-rich selvages of the alaskites or in the uraninite-fluorite veins. Uraninite crystallization appears to be mainly controlled by the oxygen fugacity prevailing in the magma and in the surrounding rocks.
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Transactions of the Geological Society of South Africa, 83 (1), 47-53
Recent field work and the availability of good maps and air photos have led to a comprehensive picture of the Grahamstown Peneplain. The original surface was boat-shaped in plan, 60 Km long and 8 Km wide as outlined in the diagram and cuts across a synclinal sequence of sediments ranging in age from Witteberg Quartzite to Upper Dwyka. It is almost entirely surrounded by the resistant quartzite which would have tended to produce a region of internal drainage. The peneplain surface is one of erosion without river deposits but is capped largely by metasomatic silcrete up to 8 metres thick and ferricrete. Where it cuts across argillaceous formations the latter are leached to a white clay which may exceed 30 metres in thickness especially on the Upper Dwyka. The various remnants of the peneplain surface isolated by erosion sometimes carry later terraces on the slopes below. Precise dating of peneplanation is impossible. In places it predates the inner portion of the Coastal Peneplain and could be Early Tertiary. The article is intended to serve as a guide to the area but not a substitute for the original literature.
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Transactions of the Geological Society of South Africa, 83 (1), 5-16
The Pan African Damara orogenic belt continues from north-western Namibia across the Kunene River into south-western Angola. Its northern margin is characterized by miogeosynclinal rock assemblages of the Damara Supergroup which were deposited in late Precambrian times along the margin of the Congo Craton and which rest unconformably on ca. 1 060 Ma old or older basement. The Nosib Group constitutes the basal sequence of the Damara Supergroup in northern Namibia and its lithology, stratigraphy and depositional environment are described from five selected areas surrounding the Etosha basin. These rocks as well as the overlying strata of the Otavi and Mulden groups can be followed from the Baines Mountains of the Kaokoveld into the Tchamalindi Mountains of southern Angola. In view of the excellent lithostratigraphic correlations we propose to include the Tchamalindi sequence into the Damara Supergroup and to adopt the nomenclature introduced in Namibia for the equivalent strata, at least at subgroup and group level. Rocks of the Chela Group and overlying Leba Formation constitute mixed sedimentary-volcaniclastic platform deposits which are exposed m four areas of south-western Angola. The lithology, stratigraphy and age of these strata are reviewed on the basis of detailed work in the Humpata Plateau near Sa da Bandeira and we propose a correlation with the Nosib and Otavi groups of northern Namibia. Our proposed correlations imply that the Damara belt extends well into south-western Angola, perhaps as far as Porto Alexandre, and that the Chela Group was deposited in a stable cratonic environment east of the area affected by Pan African tectono-thermal activity.
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Transactions of the Geological Society of South Africa, 83 (1), 55-62
Four sandstone models based on texture, sedimentary structures, geometry, contact relationships and depositional environment were recognised in the Elliot Formation. The Type A sandstone represents a crevasse channel deposit occurring predominantly near the base of the sequence. The Type B sandstone, which usually overlies the Type A sandstone, represents a typical point-bar deposit. The Type C sandstone, of which three subtypes were recognised, represents variations of crevasse splay deposits. The Type D sandstone, which occurs at the top of the sequence, represents a sheet sand deposit of a flood fan with the upper part reworked by wind. The vertical distribution of the sandstone types led to the recognition of a meandering channel-flood-plain facies, a flood-basin facies and a flood fan-dune facies. The close relationship between Types A, B and C sandstones in the meandering channel-flood-plain facies is explained by migration of a high-sinuosity meandering stream, while the deposition of the Type D sandstone is related to a change in climatic conditions. The combined sequence of the Molteno, Elliot and Clarens formations represents a north-south, upward-fining clastic wedge with time-lines cutting across the formational boundaries. This is the result of rapid basin subsidence and an active tectonic source in the south.
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Transactions of the Geological Society of South Africa, 83 (1), 63-67
The Malmesbury Group in the area mapped, comprises a thick succession of sediments, mostly greywacke and shale, which has been intensely folded and deformed. The strike is fairly uniform (approximately N30°W) and no evidence was found that the rocks were affected by more than one orogenic cycle. It is concluded that the Franschhoek beds are part of the Malmesbury Sequence and that the stratiform quartz porphyry bodies present in these beds are an integral part of the succession, antedating the Cape granite. Large plutons of this granite outcrop in the south-western Cape but they were, for the most part, not included in the area surveyed. Intense faulting and brecciation movements occurred early in the Palaeozoic Era, in a zone extending from Klipheuwel to Simonsberg, and they resulted in the formation of large bodies of mylonite and breccia. The Klipheuwel beds were deposited in the valley of the main fault. Cape and Karoo sedimentation took place later in the Palaeozoic and Mesozoic eras. After the South American and African continents had drifted apart during the Triassic Period the area again rose and was subjected to long-continued erosion.
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Transactions of the Geological Society of South Africa, 83 (1), 69-86
The Kuruman and Griquatown iron-formations are described in detail and a new lithostratigraphic nomenclature is suggested for them. The Kuruman Iron- formation (135-750 metres thick) consists essentially of alternating stilpnomelane lutite-siderite-microbanded chert-siderite or siderite-silicate bandrhythmite - siderite-magnetite or siderite-silicate-magnetite bandrythmite - magnetite-hematite or magnetite-siderite ribbonrhythmite macrocycles. The basal part of the sequence consists of carbonaceous claystones alternating with ankerite-banded chert. It represents a transition zone from the underlying Cambellrand carbonate sequence. The top of the formation is formed by the Ouplaas member which consists of orthochemical (siderite lutite) and allochemical (grainlutites, disclutites, grainstones) iron-formation. Riebeckite and crocidolite are preferentially concentrated next to stilpnomelane lutite mesobands and macrobands and thus adopt specific positions in stilpnomelane lutite-magnetite rhythmite macrocycles with crocidolite only developed in magnetite-bearing rhythmite. Macrocycles are correlatable over tens of kilometres and in this way potential crocidolite-bearing zones can be accurately defined in the Kuruman iron-formation. The Griquatown Iron-formation (250 metres thick) follows conformably on the Kuruman Iron-formation. It consists essentially of endoclastic iron-formation which is arranged in a number of upward-coarsening orthochemical - allochemical iron-formation cycles. The orthochemical units include siderite lutites and greenalite lutites whereas the allochemical units consist of siderite grainlutites, siderite-greenalite grainlutites, disclutites and grainstones. Peloids and intraclasts are the most common grain types.
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Transactions of the Geological Society of South Africa, 83 (1), 87-91
A 1,9 metre thick sandstone within thick grey siltstones in the lower part of the Ecca Group shows a tripartite subdivision. The base of the sandstone is mildly erosive. The tops of the three component beds are well-defined and each shows bioturbation. The uppermost bed is replete with the trace fossil Skolithos. Fossil wood and plant fragments are also present. The combined evidence suggests a shallow water, near coastal origin and it is concluded that the three beds represent an offshore sand tongue derived by storm wave action from littoral deposits. Modern analogues have been documented from the North Sea. Intercalation of the sand tongue in grey siltstone typical of the lower Ecca Group in Natal shows that the latter (at least in this locality) was also of shallow water origin.
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Transactions of the Geological Society of South Africa, 83 (1), 93-106
Numerous greenstone enclaves or xenoliths occur in the trondhjemitic/tonalitic gneiss terrane south of the Barberton greenstone belt. One of these xenoliths outcrops prominently south of the Boesmanskop syenite pluton and has been intruded by two discrete diapiric gneiss plutons. These diapiric plutons have caused extensive deformation and metamorphism in the main xenolithic body (the Weergevonden greenstone remnant) and have, in addition, been responsible for the extensive development of migmatites and gneisses in areas flanking the supracrustal sequences as well as in the vicinity of the numerous small greenstone rafts found scattered throughout the granitic terrane. Also intruded into the region are bodies of syenite or quartz syenite approximately 2 850 m.y. old which were emplaced into a north-west-south-east-striking zone of crustal weakness along which subsequent mafic dyke intrusions took place during early- to middle-Proterozoic times. Aspect of the geology of the greenstones as well as the granitic rocks are described with the aid of major and trace element geochemistry. Numerous photographs illustrate the complexities encountered in the migmatites exposed in the area. The conclusion is drawn that the greenstones represent scattered remnants of the lowermost formations of the Onverwacht succession (Sandspruit and Theespruit formations of the Tjakastad Subgroup) of the Barberton greenstone belt. The migmatites are interpreted as having been formed as a result of granite-greenstone interaction, the tonalitic/trondhjemitic magma being responsible for the rafting off of greenstone xenoliths and their subsequent modification by processes involving granitization, metasomatism, resorption (assimilation) and anatectic melting.
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Transactions of the Geological Society of South Africa, 83 (2), 115-124
A 15 km coastal section to the south of Port Elizabeth provides a detailed cross-section through the rocks of the mid-Palaeozoic Table Mountain Group. These sedimentary rocks underwent regional metamorphism and were subjected to polyphase deformation during the Gondwanide orogeny. The rocks represent part of the steeply-dipping northern limb of a major asymmetrical anticline. The Table Mountain Group unconformably overlies the late-Precambrian Malmesbury Group. It comprises the Sardinia Bay Formation, a 950 metres thick sequence of sedimentary rocks deposited in a tidal environment, and the overlying Cape Recife Formation, an 8 500 metres thick sandstone succession deposited in a shallow marine environment. Interfering minor deformation structures in the Sardinia Bay Formation provide evidence that the Table Mountain Group was subjected to polyphase deformation. The first phase was accompanied by low-grade to very-low- grade regional metamorphism. It produced structures indicative of movement of a thrust sheet in a north-north-easterly direction. The second phase produced large and small-scale north-west-south-east-trending folds. These folds are predominantly overturned towards the north-east and are accompanied by a well- developed south-west-dipping axial-planar cleavage. These structures probably resulted from a north-east directed compressive stress accompanied by a shearing component produced either by overthrusting or by gravity acting on steeply- inclined strata. The third phase of deformation is characterised by upright minor folds trending north-west-south-east and a near-vertical cleavage indicative of a horizontally directed compressive stress.
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Transactions of the Geological Society of South Africa, 83 (2), 125-133
The Bouguer anomaly pattern of the area comprises a number of gravity highs and lows. The gravity highs are caused by mafic rocks of the Bushveld Complex, while the gravity lows over the western part of the area are interpreted as being caused by a body of Bushveld granite with a vertical thickness of 6 km. Two- dimensional interpretations of the gravity data were carried out along three selected profiles. From the gravity models of the profiles it is evident that the body of mafic rocks of the Bushveld Complex has a synformal structure. The largest portion of these mafic rocks is overlain by Bushveld granite in the north and by Rooiberg Felsite and sediments of the Waterberg Group in the south. According to interpreted gravity values on the flank of the Dennilton dome, rocks of the Transvaal Supergroup which outcrop on the south-eastern flank of the dome, have been intruded by mafic rocks of the Bushveld Complex. Further to the south-east of the Dennilton dome rocks of the Transvaal Supergroup outcrop in an area where they are practically surrounded by mafic rocks. These rocks have been interpreted from the gravity data to be a protuberance of the floor of the Complex and not an isolated xenolith.
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Transactions of the Geological Society of South Africa, 83 (2), 135-139
In the Vryheid-Piet Retief area in the south-eastern Transvaal the Mozaan Group is the upper, predominantly sedimentary, unit of the 3 000 Ma old Pongola Supergroup. In this the type area, the Mozaan Group comprises a sequence of arenites and argillites with minor volcanics and has a gradational contact with the underlying Nsuze Group. The Mozaan Group is exposed in two open structural basins produced by interference between north-westerly and easterly fold trends. The succession locally displays evidence flow-grade regional metamorphism and an axial cleavage is developed in areas of intense deformation. Contact aureoles are associated with mafic intrusives. A reviewed stratigraphic column reveals that the Mozaan Group is approximately 3 000 metres thick as compared with a previous estimate of 5 000 m. Regional stratigraphic and textural trends imply a northern provenance for the Vryheid-Piet Retief area which accords with palaeocurrent data.
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Transactions of the Geological Society of South Africa, 83 (2), 141-170
The Cambellrand Subgroup consists essentially of dolomite and limestone. It follows conformably on the Schmidtsdrif Subgroup (siliciclastic and carbonate rocks) and is overlain by the Asbesheuwels Subgroup (iron-formation) of the Ghaap Group. A stromatolitic carbonate facies (the Ghaap Plateau facies), situated on the north-eastern side of the Griekwastad fault zone (a synsedimentary growth fault), and a clastic-algal laminated, ferruginous carbonate facies with carbonaceous shale, banded ferruginous chert and basic tuffs (the Prieska facies) are present. The Ghaap Plateau facies consists, from the base upwards, of the Reivilo Formation (manganiferous dolomite containing columnar stromatolites and giant domes, and capped by a banded ferruginous chert); the Fairfield Formation (dolomite with columns and giant domes, fenestrated dolomite and light grey coarse-grained dolomite); the Klipfonteinheuwel Formation (light grey, coarse-grained, cherty dolomite); the Papkuil Formation (dark grey, algal laminated dolomite); the Klippan Formation (light grey, coarse-grained, cherty dolomite) and the Kogelbeen Formation (algal laminated dolomite capped by Lime Acres limestone). The Prieska facies consists of the Nauaga Formation (clastic-algal laminated carbonaceous dolomite and limestone with shale, banded ferruginous chert and basic tuffs) and the Naute shale (carbonaceous shale and chert). The Monteville and Gamohaan formations represent two tongues of the Prieska facies which occur respectively at the base and top of the Ghaap Plateau facies on the north-eastern side of the Griekwastad fault zone. Facies and thickness variations reach a maximum intensity along this fault zone.
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Transactions of the Geological Society of South Africa, 83 (2), 171-177
The Plat Sjambok anorthosite suite, which is intrusive into high-grade metamorphic gneisses along the eastern margin of the Namaqua metamorphic belt, comprises very coarse-grained anorthosite, leucogabbro, gabbro and jotunite. A comparison with existing, well examined, anorthosite complexes of the world reveals a remarkable resemblance to andesine-type massif anorthosite.
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Transactions of the Geological Society of South Africa, 83 (2), 179-192
When viewed from the polar positions of either aragneisses or orthogneisses, field relationships displayed by the granitic rocks surrounding the Aggeneysberge, are equivocal. Examination of the gneiss geochemistry reveals that the small range in SiO2 content (70,87-77,67 percent) is inconsistent with an arkose parentage. Binary diagrams of Mg vs Si and K2O vs Na2O indicate an igneous rather than sedimentary origin, as do the discriminant function (DF) and correlation signs of available element pairs. The augen gneiss and pink (granoblastic) gneiss, which constitute the bulk of the local rock types, have abuttine linear major and trace inter-element trends, implying a common, igneous genetic link. The pervasive D2 imprint on the pink (granoblastic) gneiss indicates generation of this rock type prior to the only metamorphic episode approaching anatectic P-T conditions, i.e. M2. An in situ partial melt model for the pink (granoblastic) gneiss is thus unlikely. Rather, the poorly developed post - D2 Aggeneys granite has the composition of a minimum melt derived from an augen gneiss parent which was subjected to M2 metamorphic conditions. The gneisses at Aggeneys display an absolute geochemistry strikingly similar to the high potassium rapakivi granites of Fennoscandia. This is borne out by a ratio of K2O/Na2O > 2, as well as in plots of K2O vs SiO2, Al2O3 vs An/An + Ab, A-F-M and Ab-An-Or, where the Aggeneys gneisses lie within or very close to the rapakivi fields. Attention is drawn to rapakivi granite as a member of the "anorthosite suite". The presence of the remaining members of the suite in the Namaqualand Metamorphic Complex, viz. charnockite and anorthosite, suggests a gross analogy with the anorthosite suite of Fennoscandia.
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Transactions of the Geological Society of South Africa, 83 (2), 193-205
Ductile zones shear and fault zones with lateral displacements of many kilometres are an important feature of the Rhodesian craton and were investigated by mapping, photogeology, ERTS imagery and a literature search. Eight regional systems (A-H), were distinguished on the basis of their field relationships and orientation analysis. Estimates of the orientation of the maximum compressive stress in the horizontal plane were made for each system. In the field a realistic distinction between the early systems of shear zones and fault zones is not always possible. The Coulomb criterion was found to be a useful approximation for both in terms of their angular relationships to the maximum compressive stress directions ((sigma)1). Systems A and B developed during the last phase of late granite intrusion (2 625±25 Ma) and reflect a N68E line of (sigma)1, (system A) that changed to N15E (system B). Systems C and D are associated with regional development of the gold-bearing quartz veins and indicate a N20E line of (sigma)1 (system C) that changed to a simple dextral shear (system D) acting parallel to a N65E line. System E is associated with emplacement of the Great Dyke (2 514±16 Ma) and systems F and G followed this event during the mid-Precambrian. Post-Karoo age rift faulting occurred on system H fractures which might have developed initially in a wrench system in the Late-Precambrian basement.
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Transactions of the Geological Society of South Africa, 83 (2), 207-212
The Kuboos-Brennen line is regarded as a post-Gariepian tensional feature, and has low-grade hypogene mineralisation and alteration associated with some of the alkaline intrusions. The Marinkas Kwela alteration body. associated with the Tatasberg Complex, is characterised by phyllic alteration and anomalous Cu, Mo, Pb and Zn geochemistry. The occurrence is classed as a porphyry system of the alkaline suite, but has affinities with the plutonic rather than the volcanic subclass.
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Transactions of the Geological Society of South Africa, 83 (2), 213-219
In mid-eastern Botswana, Karoo strata forming easterly extensions of the main body of Karoo rocks underlying the Kalahari region. were laid down in graben-like fault-controlled troughs. The location and north-west-south-east or east-north-east-west-south-west alignment of these troughs were in turn controlled by very old resurgent trends. Pre-Karoo, intra-Karoo and post-Karoo movements along these fractures can be inferred. An example of the reversal of the direction of fault movement in post-Karoo times is provided. The Karoo-filled troughs themselves tend to be resurgent expressions of older troughs in which the thicker developments of the strata of the Waterberg Supergroup were laid down. The general tectono-sedimentary pattern appears to have had its inception at least as long ago as Ventersdorp Supergroup times. It may even in part reflect such fundamental features as the "sutures" between the Rhodesian and the Kaapvaal cratons and the intervening Limpopo Mobile Belt. Folding, in both post-Waterberg and post-Karoo times, can be inferred from the trend of outcrops within the areas of the troughs: recognition of a class of "subsequently-arched basins" is suggested accordingly. The geographical localization of foci of continued net subsidence within an "anorogenic province" may bear on the problem of nuclear waste disposal.
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Transactions of the Geological Society of South Africa, 83 (2), 221-230
The vanadium-bearing titaniferous magnetite layer of the Kaffirskraal Intrusion consists essentially of closely packed and polygonised multiphase titaniferous magnetite crystals. Between 2 and 6 percent granular ilmenite is present and not more than 15 percent intercumulus augite is irregularly distributed throughout the ore. A wide variety of Ti-and Al-rich phases is present within the magnetite and their development is explained in terms of the available data on phase relationships within this mineralogical system. The constituent titaniferous magnetite crystals are dominated by the characteristic cloth-like texture formed by the exsolution of ulvospinel. This texture is indicative of relatively low oxygen fugacities during the later stages of subsolidus cooling. The local development of fine lamellar ilmenite and modified oxidised ulve-spinel intergrowths in the vicinity of grain boundaries, fractures and other permeable features indicate the presence of somewhat higher oxygen fugacities in these areas. These higher oxygen fugacities are sharply localised and are believed to be due to hydrogen loss from entrapped late-stage aqueous fluids. The Kaffirskraal ores are both chemically and texturally similar to the titaniferous iron ores of subzone C of the Bushveld complex. The coexisting granular and intergrown ilmenite types are characteristically Mg-rich and the reported values are amongst the highest obtained for ilmenite of igneous origin from non- kimberlitic sources. The high Mg contents of this ilmenite reflects the preferential partitioning of this element into the rhombohedral phase during re-equilibration under slow subsolidus cooling conditions. The ore-rich layer formed as a typical cumulus rock during fractional crystallisation of the Kaffirskraal Intrusion and its textural relationships can be interpreted in terms of adcumulus growth and annealing processes while the resulting microstructural development was dependent on factors such as oxygen fugacity, mineralogical composition and cooling rate.
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Transactions of the Geological Society of South Africa, 83 (2), 231-238
Arenaceous sediments, with intercalated lenses of volcaniclastites and lavas comprise an 800 metres thick sequence at the base of the circa 3 000 Ma Nsuze Group. The volcanics range in composition from basaltic andesite to rhyolite and the sandstones are characterized by trough and planar cross-stratification displaying a unimodal palaeocurrent distribution. The sandstones are predominantly quartz wackes which contain lenticular matrix-supported conglomerates. Deposition is thought to have taken place in a distal braided stream environment which derived sediment from a granitic source terrane located towards the west. Rapid lateral migration of the facies tract suppressed the development of vertical accretionary deposits. Away from the major channels sheet flood sequences accumulated in response to high precipitation. The presence of volcanogenic sediments and volcanic bombs, lapilli, and chert stringers, in the sedimentary rocks, provides evidence of coeval sedimentation and extrusion.
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Transactions of the Geological Society of South Africa, 83 (2), 239-241
A new mineral, sveite, occurs as white crusts and efflorescences on the walls of Autana Cave, Venezuela, excavated in the Roraima Group quartzite. It has been deposited from seepage water. Under the microscope it consists of aggregates of contorted flakes. Its formula is KAl7(NO3)4Cl2(OH)16.8H2O. The X.R.D. pattern can be indexed with the monoclinic cell a = 10,89; b = 13,04, c = 30,71 Å; β= 92.10°; Z = 6, based on calculated density 2,1
observed 2,0). The extreme refractive indices are 1,503 and 1,535, the extinction is parallel to the perfect cleavage {001} and 2V is positive, small.
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Transactions of the Geological Society of South Africa, 83 (2), 243-253
Data based on 246 microprobe analyses of spinels in dolerites, gabbros, picrites and silicic differentiates demonstrate an unbroken range of compositions between chromite and titanomagnetite. Four stages of spinel crystallization are recognized. The early stage preceded nucleation of plagioclase feldspar and was one of weak Al enrichment. The succeeding middle stage shows strong depletion of Al in spinel, concomitant with feldspar crystallization, while the late stage followed nucleation of pyroxene, when Cr was scavenged from the melt. Crystallization of Cr-spinels was completed after approximately one third of the magma had crystallized. Quantitative use of whole-rock V, Ti and Zr data, and the application of a Rayleigh fractionation model, show that the magnetite stage of crystallization was deferred until two thirds of the magma had solidified. The existence of a "spinel gap" is consistent with a shift in the spinel liquidus as Cr is removed from the liquid. Ti contents of spinels increase as Al decreases, and crystallization at high temperatures and low fO2 yielded, at Birds River, titanomagnetites with 75 mole percent ulospinel, close to the theoretical maximum under geologically realistic conditions. Spinels show lower Mg/(Mg+Fe2+) ratios in chilled margins of intrusions than within their interiors, a feature attributed to crystallization at lower temperatures. Some analogies are drawn with spinel crystallization in the Bushveld and Skaergaard complexes.
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Transactions of the Geological Society of South Africa, 83 (2), 255-283
Calcretes and other authigenic carbonate accumulations in the regolith in southern Africa are described in relation to their occurrence in the profile and their sequence of development. On the basis of secondary (chemical) structure, such materials can be classified in the field into calcareous soils, calcified soils, powder calcretes, glaebular calcretes, cutans, pedotubules, honeycomb calcretes, hardpan calcretes, calcrete boulders and calcrete cobbles. Each of the major varieties possesses a significantly different range of geotechnical properties as well as representing a particular stage of calcrete development. Several variants of some of these varieties are also recognised on the basis of morphology, mineralogy, origin, and other properties. With simplification and addition of standard descriptors the classification is also suitable for geotechnical and other uses.
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Transactions of the Geological Society of South Africa, 83 (2), 285-290
Iron-formation consists of allochems (ooids, peloids, etc.), massive femicrite (felutite) mesobands, microbanded femicrite (ferhythmite) mesobands and chert mesobands. The latter are defined as ribbons, waves and pods when thinner than 10 cm and bands, billows and pillows when thicker. These components are used to define iron-formation lithotypes. Lutite mesobands which contain allochems are disclutites or grainlutites and allochemical chert mesobands are called discstones, grainstones, etc. Macrobands of iron-formation, consisting of rhythmically alternating femicrite mesobands and chert mesobands, are defined by a combination of chert and femicrite mesoband names such as in ribbon- rhythmite, ooidbandlutite, etc. An adjective is used to describe the mineralogical composition of the femicrite. Metamorphic iron-formation is defined and rock types the roof are named through a system similar to the one applied to relatively unmetamorphosed iron-formation.
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Transactions of the Geological Society of South Africa, 83 (2), 291-296
Investigation of the clay mineralogy of the Dwyka tillite of southern Africa using X-ray diffraction and electron microscope techniques has shown that chlorite and not, as was previously thought, kaolinite, is the predominant clay mineral. Dissolution in heated hydrochloric acid is required to differentiate these two minerals by X-ray diffraction techniques. In addition to chlorite, illite/muscovite is present in all samples, with traces of smectite and kaolinite present in most samples. Examination of the clay matrix with an electron microscope indicates the absence of a cementing agent, and induration is apparently due to bonds between the clays formed under compactive pressures during lithification. The presence of the clay minerals has an important effect on the engineering properties of the material. As chlorite is the predominant clay mineral, smectites are possible products of chemical weathering. The fine- grained nature of the clays and the lack of cementing agent affect the effective and absolute porosity respectively, and result in an extremely low permeability (4 x 10(-12) mm/s), rendering the rock material resistant to rapid chemical weathering. Once weathering is initiated, expansive clays (smectites) form, which may be eventually leached to form non-expansive clays (kaolinites).
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Transactions of the Geological Society of South Africa, 83 (2), 297-299
The marginal faults and monoclines of the Malawi Rift are known to be seismically active; but all the earthquakes beneath Lake Malawi may not be of tectonic origin. High heat flow values in the central parts of Lake Malawi may indicate active magmatism at depth. It is of interest that a sediment covered fault scarp in the south-west arm of the lake discovered during an echosounding profile in 1972, was the epicentre of the 1st June 1978 earthquake felt by Monkey Bay residents. The seismic wave arrivals of this event at local seismograph network stations are consistent with a normal fault down-throwing to the west. This earthquake at least was of tectonic as opposed to magmatic origin.
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Transactions of the Geological Society of South Africa, 83 (3), 301-311
The erection of a Logical-letter Coding System, or LLC, for facies nomenclature facilitates a rapid but standardised description and interpretation of sedimentary deposits and their internal structures within the Witbank Coalfield. This scheme is applicable to a broad spectrum of sedimentary deposits and provides a foundation for the construction of genetic stratigraphy. The LLC System entails the grouping of letters (initial) that are representative of a sedimentary rock's physical attributes. A designated lithofacies code comprises a cluster of letters that respectively constitute: the sediment composition of grain size (upper-case lettering), internal sedimentary structure and optionally subordinate qualifying criteria, such as distinctive mineralogical components and the nature of the basal contact (all lower-case lettering). Subtle variations to a facies code are appended as a script after the code. The principal lithofacies comprise: conglomeratic facies, which are subdivided into clast- and matrix-supported forms respectively constituting gravel (G) and diamictite (D); sandstone facies (s); fine-grained (argillaceous) facies (F) and organochemical facies which includes coal (C), and carbonate facies (limestone) (L). Sedimentary structure codes comprise a lower-case initial or logical letter for the structural term. The facies basal contact codes are erosive (e), sharp (s) and graded (g). The LLC System provides a simple but comprehensive foundation for detailed subsurface and surface mapping studies, with particular application to the exploitation of coal and other sedimentary ore deposits, and should largely alleviate problems currently experienced from descriptive inconsistencies in field mapping and core logging.
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Transactions of the Geological Society of South Africa, 83 (3), 313-326
Subsurface data from over 1 200 boreholes in the Witbank Basin Coalfield provided information for determining the coalfield stratigraphy and the palaeoenvironmental controls on coal distribution. The inadequacies of existing coalfield lithostratigraphy are largely obviated by the erection of a genetic stratigraphy. A total of ten areally extensive defined marker surfaces resolve the sedimentary succession into ten defined fundamental genetically related stratal increments, each termed a Genetic Increment of Strata (GIS); these are grouped into four defined Genetic Sequences of Strata (GSS), termed from the base upwards, the Witbank, Coalville, Middelburg and Van Dyksdrift GSS which are respectively grouped to comprise a single genetic "Generation" of strata, termed the Ogies Generation. Coalfield genetic stratigraphy is summarised in graphic mode in a composite stratigraphic column. It is anticipated that the basic genetic stratigraphy resolved for the Witbank Coalfield should prove broadly applicable within the Highveld and Eastern Transvaal coalfields, and possibly even into the Northern Natal Coalfield. Genetic stratigraphy provides an excellent mechanism for detailed computer based subsurface mapping of coalfield strata, and should prove a useful aid in the exploration and exploitation of coal, particularly in sedimentologically complex areas.
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Transactions of the Geological Society of South Africa, 83 (3), 327-332
Borehole data from an area close to the northern margin of the Karoo Basin reveal a 110 m thick coal-bearing succession of the Vryheid Formation overlying Dwyka tillite. The lowermost sediments reflect processes of deglaciation with a complex array of glaciolacustrine, glaciofluvial and alluvial-outwash fan deposits. Above this paraglacial milieu, tundra-type peat bogs developed in inactive areas and account for the two thick basal coal seams (No. 1 and 2 seams). During accumulation of peat which was later to form the extensive No. 2 seam, active clastic sedimentation was confined to laterally restricted river channels which incised into the underlying peat. Lateral migration was inhibited by vegetation stabilized river banks and channel deposits are characterized by vertically accreted upward-fining cycles. Channel fill consists of coarse- grained bedload sediment deposited in anastomosing streams. Flood episodes are marked by widespread, but thin (< 1 m), shale zones that intercalate with the channel sandstones. Both sandstone and shale units are completely enveloped by No. 2 coal seam. This clastic parting influences No. 2 coal seam distribution and ash content.
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Transactions of the Geological Society of South Africa, 83 (3), 333-344
Trace fossils are described from the Lower Permian Vryheid Formation (middle Ecca) in the Transvaal. They are identified from surface outcrop in the Witbank Coalfield and next to the Smithfield Ridge, a pre-Karoo granitic ridge that periodically acted as an island archipelago. In this area, the trace fossils are found between the 2 and 4 coal seams. They are Diplocraterion parallelum, Skolithos, Monocraterion, Scalarituba, and a new species of Rhizocorallium. Trace fossils associated progradational phase of delta out-building can be distinguished from those associated with an abandonment phase, whereby the delta subsided due to compaction and was probably transgressed by marine waters. In the case of the latter, the position of a transgressive sand barrier can be recognised, which protected swamp areas that are now preserved as a coal seam of enhanced thickness. A beach environment can be recognised as part of this barrier system, which fronted one of the islands of the Smithfield Ridge. Wave and probable tidal processes operated on this beach. In the Eastern Transvaal Coalfield from a borehole core comprising most of the Vryheid Formation (Middle Ecca), several trace fossils are identified. These include Skolithos, Planolites and a new trace fossil named here Siphonichnus eccaensis ichnogen and ichnos. nov. The latter is a vertical tube burrow, with backfill laminae which indicated fairly rapid burrowing in response to erosion. A central tube which pierces these laminae probably accommodated a siphon for breathing and feeding from suspension. Similar traces from the Recent are ascribed to the bivalve Mya arenaria, and if S. eccaensis similarly represents the burrow of a bivalve, this bears the important implication that diagenetic removal of shell material was an important process in the Ecca Group. Various bioturbated layers can be recognised in the borehole core, the majority of which occur just above a coal seam. These are interpreted preted as representing abandonment phases in agreement with recent work on ancient delta complexes. S. eccaensis is associated with two of these abandonment phases and this would agree with the persistent erosion implied by the structure of this trace fossil. Glauconite pellets are associated with one of the bioturbated layers, interpreted as glauconitised faecal pellets. Again, the marine nature of the abandonment phase is indicated.
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Transactions of the Geological Society of South Africa, 83 (3), 345-351
A regional structural and sedimentological study of the Ecca Group was undertaken in northern Zululand where at least 1 029 m of Ecca Group sediments were deposited within the newly identified Nongoma graben. Within the graben, vertical displacement contemporaneous with sedimentation resulted in fluvio- deltaic patterns of cyclic sedimentation. The sediments consist of lower regressive deltaic sequences which are succeeded by a fluvial depositional phase characterized by fining-upward sequences which show a gross fining-upward trend. Early fluvial deposition is characterized by low sinuosity channels with relatively thin floodplain deposits. During later fluvial deposition, after initial low sinuosity channel development in each sequence, stabilization of source and basin and the resulting low gradient, caused high sinuosity channel development and the deposition of thick, fine-grained floodplain sediments, which include coal. Fluvial deposition was terminated by a transgressive deltaic deposition phase.
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Transactions of the Geological Society of South Africa, 83 (3), 353-360
Measured sections through middle Beaufort sandstones exposed in a shallow syncline to the south-west of East London and again to the north-east of the city, have revealed upper flat-bed lamination with associated primary current lineation to be the dominant sedimentary structure. This is mostly associated with trough cross-bedded units but planar cross-beds are also locally abundant. These and the generally coarse-grained nature of the sandstone, with scattered pebbles, are taken to represent deposition from swift-flowing, shallow, ephemeral braided streams on the distal reaches of an alluvial fan. This notion is supported by the scar city of fine siltstone and mudstone which would have been the result of vertical accretion during slack water stages. Typical sequences and facies models based on Markov analysis are presented for the sections. Correlation of these Middle Beaufort sandstones with the Katberg Sandstone of the Winterberg area to the west is tested and found to be valid, with the coastal sandstones being interpreted as more proximal equivalents of the Katberg sediments.
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Transactions of the Geological Society of South Africa, 83 (3), 361-374
In Permo-Triassic times the provenance area of the main Karoo basin underwent rapid uplift. This is shown by the presence of a prominent clastic wedge (the Katberg sandstone), located in the upper third of the Beaufort Group. Stratigraphic relationships indicate that the uppermost beds of the Balfour Formation and lower part of the Burgersdorp Formation are correlatable with, and represent distal facies of the Katberg Sandstone. Progradation of arenaceous sediment, deposited mainly as channel fill upper flat beds, was effected by swiftly flowing shallow braided streams. Sedimentation was flashy and took place almost exclusively under an arid climate. Provenance rocks comprised a range of igneous and metamorphic granitic assemblages as well as quartz-mica schists and sedimentary rocks. The presence of silicified wood as pebbles implies that some of the source area sediments were Devonian or younger in age. Mutually supporting evidence suggests that the Falkland Plateau's crystalline basement, and the Cape Fold Belt, shed sediment into the basin via a large fan-channel complex with its apex roughly 100 km off the present south-east coast, along the boundary between east and west Gondwanaland.
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Transactions of the Geological Society of South Africa, 83 (3), 375-390
The sedimentology of some uranium-bearing sandstones from the Beaufort Group in the Beaufort West area was studied by use of some 116 vertical profiles measured across and adjacent to 14 mineralized deposits. The vertical profiles consist of 91 field sections and 25 borehole logs. The sandstones are usually multistorey and alternate with a mudstone and/or siltstone succession. The depositional environment is considered to be fluvial meandering with the sandstones representing channel deposits and the mudstones and/or siltstones representing floodplain deposits. The vertical profiles basically consist of a succession of facies. Some 19 facies were recognised within the sandstones on the basis of texture and sedimentary structure. The facies transitions within the sandstone sequence were subjected to a one-step Markov chain analysis. The results of this analysis indicate that the massive facies are genetically related to the trough cross-bedded facies. A palaeoenvironmental interpretation of the markov- dependent facies transitions and associations shows that upward-fining point bar and/or channel bar deposits are predominant. Natural levee, channel-fill, and swale-fill deposits can also be recognised. The coarser-grained facies (grain sizes in excess of very fine) comprise 82 percent of the total facies occurrences and there is a lack of continuity, i.e. Markov-dependent transitions, between these facies and the finer-grained facies. This is apparently a result of reworking of the finer-grained facies by each succeeding storey in the sandstone sequence. The multistorey sandstone sequence is probably a result of deposition by successive laterally migrating channels under conditions of slow subsidence. There is evidence that the sinuosity of these channels is both low and high. The low sinuosity channels tend to be more frequent. A vector mean azimuth of 047 was derived from a palaeocurrent analysis using some 843 readings. This indicates a provenance to the south-west. The cumulative thickness of uranium mineralization of each sedimentary facies within the sandstone sequence was measured. Some 99 percent of the total cumulative thickness occurs within the coarser-grained facies (grain sizes in excess of very fine), which suggests that permeability was an important control on the mineralization. The coarser-grained facies, which mostly represent lower point bar or channel bar deposits near the base of each storey, probably acted as suitable aquifers for the transport of uraniferous solutions. Irregularities in the base of each storey may have interrupted the flow of these solutions and allowed sufficient time for precipitation of the uranium. Carbonaceous debris is frequently associated with the mineralized deposits and most likely acted as an indirect reductant for this precipitation. Mineralization decreases upwards in the sandstone sequence and some 40 percent of the total cumulative thickness is restricted to the initial storey. This may be related to an abundance of carbonaceous debris derived from plants, which colonized the floodplain. These plants would have been eroded and subsequently deposited in the initial storey, following major
avulsion of the stream system into the floodplain. The high proportion of mineralization in the initial storey may also be a result of more frequent irregularities in the base, caused by differential compaction of the entire sandstone sequence with respect to the underlying mudstone. The horizontally bedded facies contain a high proportion of the total cumulative thickness of mineralization (45 percent) and this again may be related to a more abundant content of carbonaceous debris. These facies were most likely deposited under upper flow regime conditions and the inferred higher stream power and rate of erosion would have resulted in an increase in the accumulation of carbonaceous material that was derived from plants colonizing the natural levee and floodplain areas.
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Transactions of the Geological Society of South Africa, 83 (3), 391-398
Sheet-like and lenticular sandstone bodies in the Lower Beaufort Group (Adelaide Subgroup) uranium district occur in megacyclic repetition as superimposed systems of ephemeral fluvial channels that display characteristics of complex lateral and vertical accretion. Channel sandstone bodies are defined on morphological grounds into two types. Sheet sandstone bodies are the commonest type and comprise the bulk of sandstone packages in arenaceous zones of megacycles. Composite sandstone sheets result from multi-lateral coalescence of individual sandstone bodies. Isolated lenticular sandstone units in argillaceous zones of megacycles comprise sheet and ribbon sandstone types. Multi-storeying is a prominent feature of most channel sandstone bodies and often results in local sandstone thickening. Bedforms relate to the formation of compound bars and record periods of dynamic accretion and erosion. Rarely preserved palaeosurfaces vividly illustrate the fluctuating hydrodynamic conditions that typified ephemeral fluvial sedimentation in a semi-arid environment during Lower Beaufort times.
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Transactions of the Geological Society of South Africa, 83 (3), 399-413
Flood-plain deposits are defined as the mainly argillaceous sediments accumulated by vertical accretion on the low-lying areas flanking major channels. In the Lower Beaufort (Adelaide Subgroup), such deposits comprise some 80 percent of the total stratigraphic succession. A detailed study of the lithology, sedimentology and taphonomy of these rocks was made on the farm Bran dewyns Gat 214, near Beaufort West, Cape Province. Siltstone and mudstone with minor sandstone beds are the major rock types. Included in the argillaceous rocks are: calcareous, haematitic and siliceous nodules; calcareous concretionary layers; "chert" bands and gypsum rosettes; along with reptile, amphibian and fish fossils. The calcareous nodules and concretionary layers are interpreted to be of pedogenic origin. Four distinct sedimentary facies distinguished within the flood-plain deposits are linked with geomorphological features of modern alluvial plains and named accordingly: levee deposits, crevasse splay deposits, proximal flood-basin deposits, and distal flood-basin deposits. Fundamental to the facies distinction is the decreasing periodicity and competence of flow over the flood plain with increasing distance from the main channel. Taphonomic consideration of fossils collected from each facies is used to help reconstruct the palaeohydraulics of sedimentation.
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Transactions of the Geological Society of South Africa, 83 (3), 415-424
Nine lithofacies are described in the western part of the Permian Ecca Group (thickness 300-2 000 m). A black shale, an alternating shale-siltstone and a greywacke-shale facies constitute the shelf association including distal and proximal turbidites. A rhythmically banded silty shale and a mudstone-siltstone facies from the prodelta association. The lower delta plan association consists of a combination of fine-grained sandstone, siltstone and mudstone lithofacies. A mudstone-sandstone and a large-scale trough cross-bedded sandstone facies form the upper delta plain association. These facies are arranged in upward- coarsening and upward-fining sequences depending on the depositional environment. Three stages of coarser-grained sediment input which resulted in upward-coarsening megacycles are recognised. The lowermost event is represented by a high-slope turbidite fan complex indicative of delta progradation south of the present basin margin. This is followed upwards by a destructive delta phase having its maximum development in the south-west and which is finally overlain by a low-slope high-constructive delta phase, prograding largely from the west and which completed the filling of the Ecca basin.
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Transactions of the Geological Society of South Africa, 83 (3), 425-431
The upper Triassic (Carnian) Molteno Formation forms a northward thinning intracratonic clastic wedge comprising a series of large-scale fining-upward sequences deposited by braided streams. The streams drained an alluvial plain which was probably built on to the distal slopes of a glacial outwash fan, initiated by uplift and faulting in the south and south-east. The hydrology of the molteno streams has been reconstructed based on the empirical relationships of modern rivers defined by Schumm (1968a, b, 1969, 1972). Although these provide only reasonable approximations because of the many assumptions that have to be made in the analysis, they allow for greater refinement in palaeogeographic interpretation. The results of the analysis show that during deposition of the middle of the formation (Indwe Sandstone Member) when the Molteno basin attained it maximum extent the rivers had a discharge of about 774 m³/s, a drainage area of some 39 400 km² and estimated stream lengths for major drainage channels of up to 800 km. This information has important palaeogeographic implications. Provenance location and estimates of drainage area provide a means of determining the extent of the watershed for the Molteno drainage system. This suggests the existence of a long narrow linear ridge-like feature rather than an extensive highland provenance, with the possibility of subsidence and deposition to the south-east. The length of major drainage channels implies that the drainage basin probably extended some 200 km farther north than it does at present, and that it was not continuous with the smaller Karoo drainage basins in the northern Transvaal where lithological correlatives of the Molteno Formation occur.
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Transactions of the Geological Society of South Africa, 84 (1), 1-6
The Klinghardt Mountains in South West Africa (Namibia) were the site of an important volcanic outburst late in the Eocene or early in the Oligocene. Over a hundred endogenous domes and several lava flows of the couleé type were emplaced: all analysed samples (more than 60) comprise varieties of phonolite. A sequence of ash-flow tuffs of limited areal extent is present on Glasrücken. With the exception of Höchster and Feldspatberg, each volcano was probably active only for a short period before becoming extinct. Höchster and Feldspatberg have a more complex structure, with a greater proportion of pyroclastic rocks but are still relatively small volcanoes. The Klinghardt Mountains provide a textbook example of an areal volcanic field.
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Transactions of the Geological Society of South Africa, 84 (1), 19-25
A sedimentological study of the Upper Ecca and Lower Beaufort in the western Karoo Basin indicates that a gradual transition in the environment of deposition from deltaic to fluviatile took place during deposition of these sediments. Four associations of sediments reflecting deposition in the prodelta, delta front, delta plain and flood plain environments were identified. Four options for a stratigraphic Ecca-Beaufort contact emerged and these include: the base of the Waterford Formation, the base of the delta plain deposits, the top of the delta plain deposits, and the first prominent, alluvial channel sandstone. The top of the delta plain deposits represents a distinctive break in lithology and sedimentary environments in the western Karoo Basin and is suggested as the Ecca-Beaufort contact in those parts of the Karoo Basin which have delta plain deposits present.
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Transactions of the Geological Society of South Africa, 84 (1), 27-40
A basin analysis of the Ecca and lower Beaufort groups near Grahamstown in the Eastern Cape Province has revealed a sequence of basin plain, turbidity, deltaic and fluvial deposits. Facies analyses and new palaeocurrent data indicate that coalescing deltas prograded from the south and south-east over sediments deposited from suspension and gravity flows on a deep basin floor. Turbidites generated by the slumping and sliding of sediment down delta fronts, which occurred in water depths shallower than previously considered, are overlain by deltaic sediments. Palaeocurrent data from fluvial meander-belt sediments at the top of the sequence studied substantiate a southern source area which, from petrological examination, was a low-grade metamorphic terrain and which may have been part of the southern extension of the Cape Supergroup. These facts do not support Ryan's hypothesis of a south-eastern opening of the Karoo Basin to the sea but rather that the basin was an inland sea or lake which occupied a eugeosyncline that was fed with sediments from the landmass situated to the south and south-east of the present subcontinent. The interpretation of volcaniclastic sediments in the sequence lend weight to this type of tectonic setting. The basin model devised here for the sedimentation of the Ecca and Beaufort groups associates pelagic deposition, turbidity currents which flow down delta fronts, deltaic deposition and fluvial sedimentation together in a single sedimentary model.
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Transactions of the Geological Society of South Africa, 84 (1), 41-50
The very large partition coeefficient for Cr between magnetite and liquid makes Cr a very sensitive indicator of magmatic fractionation. We have analysed a large number of pure magnetite mineral separates from four vertical sections through the Main magnetite layer in the Bushveld Complex to test various hypotheses concerning the formation of layered complexes. Several patterns of Cr distribution emerge from plots of the variation in cr content as a functional of height in the layers: 1 - massive depletion of Cr may occur over a short, vertical interval, especially near the base of the layer; 2 - often this flattens out to give a constant Cr content over considerable vertical intervals; 3 - rapid reversals in Cr content occur; 4 - such reversals and changes in mineralogy in the comulate pile are ot interdependent. These data are ot reconcilable in terms of crystall ization from a single, large homogeneous magam chamber, not by mul tiple magma addition. Bottom crystallization and convective ovearturn in a large magma could produce the compositional profiles observed. Diffusion controls the magma composition at the crystallization interface and the magma becomes inhomogeneous. The concentration gra dients are not steep enough to have resulted from pure ionic diffu sion. Ra[id minor eddies associatd with the masjor convective over turns masy be important in stirring the magma close to the base, and increasing the effctive rates of diffusion. A mathematical treatment is presented which expresses these processes and is quantitavely con sistent with the observed data. the average Cr content of the Main magnetite layer varies laterally according to the number of convec tive cycles operating in any specific area. By analogy, this process may produce the lateral variation in V2O5 content previously reported from magnetite layers. Our data on magnetite samples from the Potgietersrus limb seem chaotic and not reconcilable with the other sections. The reason for this is unknown.
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Transactions of the Geological Society of South Africa, 84 (1), 51-66
New isotopic data are presented from three areas of Precambrian outcrop through the "Kalahari Beds" in west and north-west Botswana. The existence of a continuous c. 2 000 km long north-east- south-west-trending Irumide Belt extending across southern Africa from South West Africa to Zambia is confirmed. In Botswana volcanic activity at c. 980 Ma took place in this belt and followed intracratonic rifting which occurred between about 1 200 and 1 000 Ma. The south-eastern limit of this Irumide Belt in Botswana is defined by the Makgadikgadi Line - a near-vertical fault downthrowing to the north-west. The same feature defines the north-west boundary of the Rhodesian Craton. The north-west limit of the Irumide Belt is not well-defined but extends beyond the Xangwa Valley where Rb-Sr ages of around 890 Ma have been identified. The Damaran orogen was confined to north-west Botswana to the area west of the Goha Hills and north of the Okwa Valley. Within this area tectonothermal activity occurred between 700 and 650 Ma. The Okwa valley area contains dioritic rocks in excess of 2 000 Ma old and gneiss formation took place at c. 1 810 Ma. This, together with aeromagnetic data, suggests that this area forms a northerly extension of the Kheis Belt of South Africa.
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Transactions of the Geological Society of South Africa, 84 (1), 67-73
The Dominion Group is developed in the south-western Transvaal where it rests unconformably on a granite-greenstone basement. The sequence comprises a basal clastic unit overlain by a considerable thickness of volcanic rocks. The sediments, which are up to 120 m thick, are mainly arkosic arenites with numerous interbedded conglomerates. The majority of these conglomerates are laterally impersistent, although two of them have attracted attention due to the presence of placer mineralization. Limited palaeocurrent data display a unimodal distribution, suggesting that deposition was controlled by a braided fluvial environment, which derived sediment from an eastern highland. A gradual upward increase in the proportion of volcanic material indicates a period of coeval sedimentation and volcanicity. The upper, predominantly volcanic succession attains a maximum thickness of 2 600 m. It may be subdivided into a lower formation dominated by basic to intermediate lavas and pyroclastics, overlain by an upper sequence of felsic lavas with interbedded tuffs. In the Ottosdal area, a number of prophyllite-rich horizons are developed in the upper stratigraphy. These are interpreted as beds of volcanic ash, which were subjected to secondary removal of silica by percolating ground water.
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Transactions of the Geological Society of South Africa, 84 (1), 7-17
The Jurassic Clarens Formation was studied in part of the Natal Drakensberg. The methods of investigation included the measuring and analysis of sedimentary sections, and the petrographic (including staining and point-counting) and scanning electron microscopic analysis of the sediments. Petrological analysis of the rock samples revealed these sediments to be fine-grained greyw ackes with poor sorting, subangular grain shapes and a matrix made up mainly of chloritic mud, which infrequently contains considerable quantities of calcite cement. Statistical evaluation of sand grain surface textures observed under the scanning electron microscope revealed that three important factors contribute to the formation of these sediments: diagenesis, high-energy chemical activity, and sediment source material. Aeolian and glacial textures are also present on sand grains. From the scanning electron microscopy it was concluded that these sediments were deposited in an environment dominated by chemical and diagenetic processes. A wet desert margin would give the necessary combination of high temperatures and the presence of water. Distal glacial source material would have been responsible for much of the sediment formed, and this was to some extent reworked by desert margin and dry aeolian conditions in the depositionary basin. Four sedimentary facies were defined on the basis of lithology, sedimentary structures and fossils. The vertical and lateral distribution of these facies points to an environmental change from south to north along a direction consistent with the major palaeocurrent trend for the postulated wet desert and fluvial facies. This change is from a wetter palaeoenvironment dominated by fluvial and wet desert processes (in the south) to a more arid one characterized by mass flow and aeolian processes (in the north). The climate was probably semi-arid with a low rainfall which precipitated as short, very intense storms, which provided the water necessary for desert margin.
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Transactions of the Geological Society of South Africa, 84 (1), 75-83
The sills which intrude the Transvaal sequence in the Lydenburg District may be divided into three groups. A noritic to pyroxenitic group is equated with the Bushveld Complex, but the majority comprise metabasites of widely-varying compositions. Doleritic sills are also present. Within the Waterberg Group the most abundant sill lithology is dolerite. Two sets of samples were taken from the eastern Transvaal: a group of 32 metabasites, classified as pre-Bushveld or "diabase" sills which intrude the Transvaal strata, and 32 sills which were emplaced into the Waterberg Group in the Cullinan-Middelburg trough. The petrography and geochemistry of both groups are described and comparisons drawn between the two sills of the pre-Bushveld suite comprise mostly amphibole and saussuritised plagioclase and have a wider compositional range than the relatively fresh Waterberg sills; both possess tholeiitic affinities. The two samples groups may be separated on the basis of age, petrography and bulk chemistry, aided by statistical methods such as discriminant analysis.
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Transactions of the Geological Society of South Africa, 84 (1), 87-89
Two trilobites (superfamily dalmanitacea; family, genus et species indet.) have been recovered from near the base of the Soom member of the Cedarberg Formation of the Table Mountain Group. The fossils were found at the Buffels Dome, a rugged and isolated peak in the Hex River Mountains near Worcester in the Cape Province. Only one other trilobite is known from the Table Mountain Group and this occurs higher up in the Cedarberg Formation, in the Disa Member. The Disa member was hitherto the only known source of body fossils in the Table Mountain Group and the new trilobites are thus the oldest known body fossils in South Africa.
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Transactions of the Geological Society of South Africa, 84 (1), p85, 1 map
A regional gravity survey covering an area of approximately 70 000 km² in the North-western Cape. Province and the central Karoo was recently finished. The data supplement those obtained from previous surveys based on a rather sparse network of gravity stations in this area. The results are presented in the form of a Bouguer anomaly map with a contour interval of 50 g.u.
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Transactions of the Geological Society of South Africa, 84 (2), 107-114
A geophysical study, forming part of a geohydrological investigation, was undertaken in the Cape Flats to determine the thickness of the porous overburden and the nature of the underlying bedrock. The seismic refraction and electrical resistivity techniques were employed during the geophysical survey. The seismic refraction survey was undertaken first and produced travel-time curves of good quality which were assumed to represent a three-layer model. The second layer of this model was considered to comprise water-bearing unconsolidated sand with a relative restricted range of velocities (between 1550 and 1800 m/s) throughout the study area. A number of boreholes were sunk on the basis of the seismic results and in most cases the borehole findings confirmed the interpreted overburden thickness. However, in some areas in situ weathered bedrock was present which has a seismic velocity that differs so little from that of the water-bearing sand as to produce no expression of its presence on a travel-time curve. This problem which resulted in erroneous estimates of the amount of groundwater in storage prompted the use of the resistivity sounding technique to detect those areas where the water-bearing sand is underlain by weathered bedrock. The weathered bedrock is a conductive unit which makes it recognizable on a sounding curve. The electrical method could differentiate between the water-saturated sand and the clay, giving a more conservative but also a more realistic estimate of the available groundwater in storage. This study is a good example of how two geophysical techniques can complement each other to yield more accurate results.
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Transactions of the Geological Society of South Africa, 84 (2), 115-122
An alluvium-filled delta around the mouth of the Omaruru River was surveyed by means of the Schlumberger sounding technique. This geoelectrical survey method proved to be well-suited for studying the extent and thickness of the extensive fresh-water aquifer that occurs in the alluvium. The study shows conclusively that there is only one main channel containing good quality groundwater. This channel runs along the southern half of the old delta. The thickness of the fresh-water aquifer in this channel in some places exceeds 60 m. In the northern part there is a second deep channel which contains saline material.
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Transactions of the Geological Society of South Africa, 84 (2), 123-133
This paper describes the results obtained from 955 Schlumberger electrical soundings which were carried out over an area of roughly 350 km² of the Breë River Valley between the Brandvleidam in the south and Witbrug in the north. The resistivity results complemented by a limited amount of borehole information, pumping tests, conductivity measurements of groundwater samples, seismic refraction measurements and geophysical borehole logging made it possible to obtain valuable information on the distribution and geohydrological nature of the alluvial deposits and the underlying bedrock types. Five aquifers where the alluvium attains thicknesses of between 20 m and 40 m were identified. The aquifers occur in the upper reaches of valleys leading into the Breë River Valley and consist mainly of sand. In two aquifers a well-developed boulder layer overlies the sand layer. The aquifers and their associated valleys are areas of recharge as shown by water quality distribution. The upper reaches of the aquifers are also the most permeable containing very little or no argillaceous material Lower down in the valleys where more argillaceous material is present there is a correlation between the resistivity of the water-saturated alluvium (corrected for water quality) and permeability on the one hand and between porosity and permeability on the other. The quality of the water in the alluvium is good but in areas underlain by Cretaceous bedrock the groundwater is brackish. The geology of the bedrock which consists of rocks of the Malmesbury, Cape, Karoo and Cretaceous systems is complicated. According to the geophysical data the trace of the Worcester fault lies well to the south of its inferred position deduced from mapping.
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Transactions of the Geological Society of South Africa, 84 (2), 135-144
Mathematical models are now widely used in the design of well fields for the regulation of river flows by the intermittent abstraction of groundwater. It is particularly important in such models to correctly represent the distribution in time of baseflow discharge to the river system, and where the water table is deep it will be necessary to model the delay properties of the unsaturated zone. For a Chalk aquifer in eastern England the delay properties were represented by a linear response function and coupled to an exponential recession of water levels to provide an autoregressive, moving average model of water transfer through the unsaturated zone. The model was fitted to observed water levels and calculated surface infiltrations to provide estimates of infiltration to the water table and aquifer specific yield. The mean moisture pulse velocity through the unsaturated zone was calculated to be about 1 m/d. Aquifer transmissivities were calculated by flow net analysis. Spring discharge from the Chalk. and induced leakage from the rivers into the Chalk were modelled differently, to take account of the presence of low permeability deposits in the river bed. Using the calculated water table infiltrations the model was able to successfully reproduce the observed groundwater levels and river flows.
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Transactions of the Geological Society of South Africa, 84 (2), 145-151
A portion of a coastal aquifer was considered as a source of municipal water. It was planned to prevent brine from entering the well-field by inserting a barrier of fresh water, derived from purified sewage, on its seaward side. Sections of the boundary were porous, and because it was not known previously whether the water would be mined, fixed external potentials could not be specified. In order, therefore, to predict its long-term response to the planned abstraction, a mathematical model was built of the actions of the boundaries as well as those of the internal aerated and saturated zones. Measured rainfall figures acted as inputs, and initial estimates of transmissivity, vertical delay, soilwater saturation and agricultural pumpage were automatically and separately refined so as to minimize differences between individual aspects of the measured and simulated water-tables. In order to reduce ambiguity in parameter estimates, the quadrilateral computation elements were aligned to the groundwater contours and streamlines. The accuracy of the processing was checked by referring the model output back to measurements at borehole locations. When drawdowns due to postulated municipal abstraction and recharge were integrated with the simulated water-table over a long period, it was deduced that there was a danger of salt intruding from the sea into the well-field. Assessments were also made of the effects of the abstraction on an agricultural area, a recreational facility and a residential neighbourhood. It was confirmed that the safe abstraction rate depended critically on the rate of recharge of the fresh-water barrier, as well as on predictions of rainfall and agricultural pumpage.
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Transactions of the Geological Society of South Africa, 84 (2), 153-160
The basic premise of the models is that groundwater recharge could be estimated by means of a simple relationship, involving the average precipitation, actual annual precipitation and climatic indices that accentuate the annual variability of the water balance. Three deterministic groundwater recharge models are discussed. Annual values representing the equivalent rise in water level due to recharge, were used in calibrating the models. In order to convert the recharge values obtained from the water balance (models) to an equivalent rise in water level, the porosity of the aquifer had to be incorporated. The average recharge from the model divided by the average value of the reference recharge water levels, was taken as the best value for the porosity of the aquifer. In the first two models, the recharge was calculated as the difference between precipitation and evaporative losses - the latter consisting of interception and evapotranspiration as separate components. In the third model the evaporative losses were combined as a single loss factor. Model 3 produced the best simulation of annual variability in recharge, but cannot determine the annual recharge values quantitatively, unless an average value of recharge is available. By extrapolating from an area for which recharge is known, it is illustrated how model 3 could be used in estimating the potential groundwater recharge for a similar area experiencing different precipitation.
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Transactions of the Geological Society of South Africa, 84 (2), 161-167
Groundwater with its influence on stress in soil and rock leads to the failure of many cut slopes. In local design practices, groundwater is seldom studied in any detail and its control is thus usually first considered after failure or signs of failure of a cut slope. This paper summarizes preliminary recognition of groundwater problems and the influence of groundwater on cut slopes and discusses the range of remedial measures used to control excessive pressures or flows of groundwater.
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Transactions of the Geological Society of South Africa, 84 (2), 169-175
Intensive geohydrological investigations were carried out to establish the water supply potential of the Stampriet Artesian Basin for rural purposes. Hydrogeochemistry was used as a research tool to unravel the water quality problems posed by extreme variations in the chemical composition of the groundwater. It is demonstrated that the groundwater in the Auob aquifer undergoes a chemical evolution indicative of a dynamic basin environment, ion exchange being the most important modifying phenomenon. In this confined aquifer the water is also subjected to naturally occurring bacterial denitrification. The hydrochemical development pattern corroborates the existence of two recharge areas for the Auob aquifer. It was also established that defective borehole casings could lead to serious impairment of the water quality in certain areas. It is concluded that a thorough knowledge of the hydrogeochemistry in all the superimposed aquifers is imperative for solution of geohydrological problems in the Stampriet Artesian Basin.
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Transactions of the Geological Society of South Africa, 84 (2), 177-190
A geohydrological study of the sand-covered area between the Kuruman and Orange rivers has resulted in a better understanding of the general geology, hydrology and hydrochemistry of the area. The central feature of the basement geology is a prominent granite-gneiss ridge which has a north-south strike. To the east occur steep-dipping quartzitic rock of the Wilgenhoutdrift Group and the Groblershoop Formation and in the west the Koras and Nama groups are overlain by the Dwyka Formation. Tertiary and Quaternary sediments of the Kalahari Group blanket large portions of the area. Water-level contour maps were compiled from which the regional groundwater flow pattern could be deduced. Several subsidiary groundwater basins were delineated. It was concluded that the Kuruman River on the northern perimeter of the area constitutes the main recharge area, with minor contributions from the southern and eastern rims. The salt pan zone in the central-west is an area of groundwater discharge. Hydrochemical studies substantiated the inferred flow pattern of the groundwater. A geochemical interpretation of the water quality data confirmed that the groundwater follows a geochemical cycle, whereby bicarbonate/carbonate- and calcium/magnesium-rich water from the recharge areas changes to chloride/sulphate- and sodium-rich groundwater towards the discharge area.
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Transactions of the Geological Society of South Africa, 84 (2), 191-192, 1 map
Since 1976 the Geological Survey Department of Botswana has been working on a Hydrogeological Reconnaissance Map series at a scale of 1: 500 000. The maps are being produced to illustrate ground- water occurrence and groundwater quality within Botswana for planning purposes. The country is covered by 11 map sheets; 4 are already printed and 4 are awaiting final drawing. The remainder should be finalized by the year 1983 (Map sheet No. 3 "Ghanzi" in rear pocket).
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Transactions of the Geological Society of South Africa, 84 (2), 91-97
The concentrations of gaseous nitrogen, argon, oxygen and helium dissolved in groundwater are often different from their concentrations in rain and surface waters. These differences reflect changes in the gas content occurring after rain or surface water, having infiltrated into the ground, become isolated from equilibrium contact with the atmosphere. A study of these changes can give insight into the origin and subsequent subsurface history of groundwater. Nitrogen and argon concentrations for many groundwaters in southern Africa indicate that "excess air" is added to water during infiltration. The amount of excess air is believed to reflect the physical structure of the unsaturated zone and the climate of the recharge area. Since nitrogen and argon are essentially conservative in many aquifer environments in South Africa, their concentrations can be used in distinguishing groundwaters of different recharge origins. In some areas the high helium content of the groundwater suggests that much of the helium is derived through migration from a source outside (e.g. below) the aquifer itself. Radiogenic helium concentrations nevertheless show, in two artesian aquifers, a close linear relationship to the radiocarbon age of the groundwater. This indicates a uniformity in the factors responsible for the accumulation of helium, and suggests that in these circumstances helium data can be used to give information on the age of very old groundwater. In some groundwaters dissolved oxygen concentrations are found to decrease with increasing groundwater age. Whilst the rate of decrease may be very different for different aquifers, the field measurement of oxygen may be useful in preliminary surveys directed toward the location of recharge areas.
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Transactions of the Geological Society of South Africa, 84 (2), 99-105
The evolution of the major ion concentrations of groundwater (Na, K, Ca. Mg, HCO3, SO4, Cl and NO3) can be described as the consequence of a number of competing chemical reactions. With the aid of the naturally occurring radioactive and stable isotopes some of these reactions can be separated, identified and followed in space and time. In some field studies, especially of artesian water, the rates of reactions can be estimated. A number of processes observed in South African sandstone aquifers are discussed and the variable reaction rates demonstrated. Reactions that can be identified include carbonate solution, chemical weathering, salt leaching, cation exchange and redox processes.
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Transactions of the Geological Society of South Africa, 84 (3), 193-197
The formation of low angle, laterally persistent cross-strata and horizontal lamination in sandstones of the high sinuosity channel facies association of the Beaufort Group in the southern Karoo Basin is primarily a function of grain size and the suspension load. Because fine-grained sands are indicative of a much higher suspension load than coarser sands the viscosity and density of the water increases and the effective particle fall velocity decreases. Under these conditions dune length increases and the angle of the upstream and downstream faces decreases, giving rise to laterally persistent low angle sets of cross-strata. The origin of the horizontal lamination is more difficult to assess because of its variable mode of formation. However, the available evidence suggests that in the Beaufort the horizontally laminated sandstones were probably deposited as upper flow regime plane beds during decelerating flows. Boundaries between successive flows are marked by internal erosion surfaces or by the laminations themselves, thereby overcoming the depth-velocity problem associated with thick (> 1 m) apparently unbroken sequences of horizontally laminated sands of upper flow regime origin. Other factors which may have influenced the development of horizontal laminations are temperature, storm intensity and sediment concentration, shallow depths, the unsteady nature of the flow, and aridity of the environment.
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Transactions of the Geological Society of South Africa, 84 (3), 199-203
The Karbonat-Bombe is a small (15,5 x 0,6 cm), light-weight (0,52 kg) and robust instrument for determining CaCO3 in sediments and rocks. Analyses can be carried out on site, in the field, or aboard ship. The device is cheap (~R150) and running costs are negligible (~R0,10/sample). Precision is 2 per cent for CaCO3 values > 5 per cent and < 4 per cent for smaller concentrations, whereas absolute accuracy is 2 per cent for > 10 per cent CaCO3. Because only one reading per sample is required and no expertise or sample preparation is needed, at least ten crushed samples can be routinely processed for CaCO3 per hour. These attributes make the Karbonat-Bombe suitable for small institutions handling large numbers of samples for CaCO3 analysis.
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Transactions of the Geological Society of South Africa, 84 (3), 205-206
Three whole rock Rb/Sr analyses of Kanye Volcanic Group rocks plot nearly colinearly on a B.P.I. plot. It is suggested that the age of these rocks is greater than 2,4 x 10(9) yr.
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Transactions of the Geological Society of South Africa, 84 (3), 207-216
The test area of some 20 km by 15 km was chosen as representing typical highveld conditions and containing a good variety of rock types and structures. Image types obtained were (1) panchromatic photographs, (2) black-and-white infrared photographs, (3) colour photographs, and (4) false colour infrared photographs. Comparison of the photo types showed that, for the particular rock types present and under the particular seasonal conditions (mid-winter), the following conclusions could be drawn: 1. Black-and-white infrared is inferior to panchromatic film when vegetation is largely inactive. 2. Colour film showed no more geological detail than panchromatic photography in highveld winter conditions. 3. False colour film showed maximum clarity and good colour range, but in winter conditions its particular properties are not best exploited. 4. Most major stratigraphic units could be recognised on all 61m types, although lithologies could not often be identified from photos alone. 5. All film types contained a large amount of structural data which was not recorded on the reference map. 6. It is suggested that the colour and false colour/IR film types would show up to much greater advantage if the photography was obtained under early summer conditions.
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Transactions of the Geological Society of South Africa, 84 (3), 217-225
A comparison was made of different types of LANDSAT imagery for geological interpretation of a test site of known geology. Available images consisted of summer and winter prints from NASA negatives and from computer-compatible tapes using computer processing by principal component analysis and rationing. As the test area is small the images were enlarged to about 1:90 000 and at this scale details are poor and inherent image limitations and defects are magnified. The prints from negatives which are 5th or 6th generation products contain 6th-line banding, scratches and dust spots. Images produced from computer-compatible tapes and preprocessed by de-striping before printing are of better quality and geologically more useful. Seasonal effects are very marked. Very little geological information is present in winter images, although some topography is emphasised by a low sun elevation. Summer images show a greater colour range and are preferable for interpretation. The combination of processes used in producing the digitally enhanced image did not succeed in improving lithological discrimination. The main use of LANDSAT imagery remains in its display of the regional geological framework.
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Transactions of the Geological Society of South Africa, 84 (3), 227-232
It may be necessary to compare the relative efficacy of several types of imagery. With standard air-photo prints, comparisons should always be made stereoscopically, since the spatial properties of the image may be more important than the spectral ones. The use of "mixed-pair" stereoscopy is advocated, and a disadvantage of multispectral additive projection is that this method is not possible. Comparisons of air photos with linescan and pulse systems such as thermal infrared and radar should be done with particular awareness of the fundamental differences in the data sensed, method of recording and image geometry. With digitally recorded systems such as LANDSAT, there is the additional possibility of automating the comparative process.
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Transactions of the Geological Society of South Africa, 84 (3), 233-237
The south coast of South Africa between Cape Seal and Cape Recife is characterized by a rugged, cliffed coastline, broken by two half heart-shaped bays (spiral beaches). The bays occupy east-west structurally-controlled valleys which were eroded from folded, softer Cretaceous rocks. A narrow (3-14 km) coastal rise (~ 1°) offshore is associated with the occurrence of basement rocks (Table Mountain Group), whereas a smooth, flat sea bed in the nearshore is underlain by relatively steeply dipping (2-3°) Cretaceous strata. Ridge and vale topography to seaward is related to gently dipping (< 1°) Tertiary sediments. Two high (relief ~ 50 m) ridges close to the Cretaceous/Tertiary contact off the Tzitzikamma River are probably basement outliers at the edge of the deep Lower Cretaceous Pletmos Basin.
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Transactions of the Geological Society of South Africa, 84 (3), 239-244
The layered mafic component of the Bushveld Complex crops out in four discrete compartments. Each compartment possesses at least one supposed feeder, as determined by gravity data. There are seven feeders, which, together with the main Bushveld satellitic intrusions, fall on two concentric elliptical traces, each of which possibly originated by large-scale conical fracture of the crust. These ellipses are, in turn, broadly concentric to the supposed margins of the Transvaal basin, and their locus falls on the "Bushveld-Great Dyke lineament". It is proposed that the location of the feeders along these ellipses was controlled by the regional grain of the basement. The size and location of individual right inverted conical magma chambers sited above these feeders were major influences on the shape of the compartments. Possible sites for exploration activity are suggested at the intersection between elliptical traces and certain regional lineament trends.
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Transactions of the Geological Society of South Africa, 84 (3), 245-250
Discontinuous lenses and wisps of garnetite occur in the Houtenbek Formation in the eastern Transvaal, close to the contact of the main zone of the Bushveld Complex. The lenses consist of 96 percent grandite (Ad69Gr28) containing traces of hydrogrossular, Ti-garnet and spessartine components, with four percent clinopyroxene and traces of magnetite. Narrow veins of azurite and malachite ramify through the lenses. Pressure and temperature conditions for the skarn-producing reactions are calculated from known P-T fixes elsewhere in the eastern Transvaal. Bracketed calculations indicate P-T conditions of ~3,5 kb and ~560°C during thermal metamorphism of impure stromatolitic limestones to garnetite. An estimate of XCO2=0,15 is consistent with these data. The Cu mineralization appears to originate from the oxidation of primary copper sulphides. The sulphides were formed by reduction of primary copper carbonates that were trapped by algal mats during stromatolite formation and later reduced to sulphides.
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Transactions of the Geological Society of South Africa, 84 (3), 251-259
Airborne particles derived from mineralized rocks and from residual soils are collected on 0,1 mm diameter nylon threads of 300 m length, wound on 33 cm² aluminium frames, towed by aircraft at 100 m above the ground. The collected particles are analysed for metallic elements by emission spectrometry. The mathematical theory of the collection efficiency of thin threads is presented and the limitations and possibilities of the technique are discussed. A case history is given from a survey which covered 17 000 km² of the Limpopo Mobile Belt in South Africa. Airborne copper anomalies were confirmed by geochemical soil samples and analyses and geological mapping. Copper anomalies ranged from 15 to 500 ppm in the air, against regional background of 3 ppm. The low background was due to rain. Soil samples gave Cu concentrations ranging from 100 to 1 100 ppm. Rock chips gave 0,1 per cent Cu. The geological survey discovered old borehole sites. Subsequent information from these disclosed intersections of copper mineralization of the order of 1,4 per cent Cu over 3 m. The mineralization host is a shear zone in the Bulai Granite, an intrusive of the Archaean Complex. The particle cloud covered 62 km², the length of the mineralized shear zone was 3 km.
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Transactions of the Geological Society of South Africa, 84 (3), 261-269
Several layers and numerous discontinuous lenses of titaniferous magnetite ore are exposed locally within ferrogabbros of the Piet Retief Layered Suite. These ores consist essentially of closely packed, polygonal titaniferous magnetite crystals that are commonly separated from each other by a narrow selvage of chlorite. The titaniferous magnetite is characterized by the development of abundant lamellar ilmenite that is indicative of subsolidus cooling under relatively high oxygen fugacities. The microstructural features of the ores are described and interpreted in terms of the available data on the FeO-Fe2O3-TiO2 system. The ores have been subjected to greenschist-facies metamorphism which resulted in the widespread development of chlorite at the expense of the original silicates. This episode also resulted in the extensive fracturing of the ores and the development of small amounts of sphene at the expense of the titaniferous magnetite. Minor modifications to the titaniferous magnetite microstructures also occurred. Chemically, the ores are characterized by low TiO2 (12-14 per cent) and very low V2O5 (0,25 per cent) contents and cannot be regarded as a potential source for either of these commodities.
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Transactions of the Geological Society of South Africa, 84 (3), 271-279
A Fortran IV computer program for rapid graphic display of sedimentary log (or measured section) data on a line printer is described and presented. The display consists of a column, coded according to lithology and scaled according to measured stratigraphic thicknesses. Sedimentary structures and grain sizes are indicated by a coded offset field to the right of the column. A comment field is provided to the right of this information. To the left of the column, stratigraphic thicknesses and depths. elevations and the nature of contacts is printed. Dolerites (intrusives) may be excluded from the graphic log, with depths and elevations appropriately adjusted, thus simplifying stratigraphic correlation.
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Transactions of the Geological Society of South Africa, 84 (3), 281-291
In classical debates about the origin of granitic batholiths, granites were derived by (1) differentiation of parent basaltic magmas, (2) partial fusion of the crust, or (3) metasomatic transformation of the crust by aqueous emanations or solid state diffusion. The source materials were mantle peridotite for basalt, or the base of metamorphosed piles of sediments and lavas. With formulation of the theory of plate tectonics, the eruption of andesites and the intrusion of calc-alkaline batholiths was clearly associated with a subducted oceanic lithosphere plate. This introduced a third potential source of silicic magmas: the subducted oceanic crust (hydrated basalt-amphibolite-quartz eclogite-serpentinite-pelagic sediment). The results of experimental petrology provide a framework for interpretation of the relationships between the erupted or intruded magmas and the possible source rocks. There are now sufficient experimental data to provide constraints for testing petrological and geophysical models for the origin of andesites and batholiths. Our knowledge of the geophysics and thermal structure in subduction zones, which is critical for petrogenetic interpretations, remains uncertain. The constraints of experimental petrology and geochemistry appear to be consistent with processes and products as follows. The basalts and andesites are not primary magmas from mantle or subducted oceanic crust. Dehydration of subducted oceanic crust provides aqueous fluids for metasomatism or partial melting of overlying mantle. These fluids may also initiate partial melting of oceanic crust yielding hydrous siliceous melts that leak into the overlying mantle. It is unlikely that melts from subducted oceanic crust yield andesites by fractionation, except by melting of amphibolite in especially warm subducted crust. Partial melting of mantle peridotite modified by fluids or magmas yields H2O-undersaturated basic melts that may differentiate. Andesitic melts can be generated from mantle peridotite only under restricted oceanic conditions not attainable beneath continental crust. The normal product of partial melting of the continental crust is H2O-undersaturated rhyolite melt, or syenite in the deepest levels. Andesite melts could be produced from continental crust only by extreme heating. Underplating of the crust by basic magmas gives the prospect of such heating, and also for the mixing of basalt and rhyolite magmas. Physical aspects, such as the behaviour of partially molten rocks, are intimately related to the compositions of magmas varying from source to emplacement or eruption site. Batholiths include contributions of material and heat from less siliceous magmas generated in subducted oceanic crust and mantle peridotite, and granitic components from crustal anatexis. The calc-alkaline batholiths and eruptive rocks are products of complex, multi-stage processes.
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Transactions of the Geological Society of South Africa, 84 (3), 293-312, 2 figs
Although uranium is ubiquitous, it needs a certain geological setting in order to accumulate and it has been demonstrated that the migration and concentration of uranium depends primarily on its oxidation state and on the presence of certain elements which constitute the earth's crust. The uranium provinces of the globe are distinctly time-bound and occur in a series of five clearly defined mega-rhythms ranging from the early Proterozoic to the Recent. A different type, or a combination of different types, of mineralization is found to be characteristic of each epoch, and study of these variations has in the recent past led to a better understanding of the behaviour of uranium under wide-ranging conditions. This paper reviews the time-bound characteristics of the uranium provinces of southern Africa in the context of their global setting and their relationship to other uranium provinces. The nature and origin of the hiatus between each of the major pulses of uranium mineralization are also reflected on.
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Transactions of the Geological Society of South Africa, 85 (1), 1-12
Petrographic descriptions and mineral analyses are presented for a vertical banded kimberlite dyke associated with the main kimberlite pipe in the De Beers Mine, Kimberley, South Africa. The emplacement history of the dyke is interpreted in terms of multiple intrusion of at least three pulses of kimberlitic magma that are shallow differentiates from a common parent. A characteristic feature of the dyke is the presence of zones of abundant elongate amygdales, filled by carbonate and crytocrystalline serpentine and oriented horizontally, normal to the dyke walls. The textural features of the dyke are distinct from those of kimberlite pipes and document the existence of kimberlite magma, present as a mixture of crystals plus carbonatitic liquid, at shallow levels in the crust.
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Transactions of the Geological Society of South Africa, 85 (1), 13-28
In undisturbed natural terrain the vegetation, soils, superficial and bedrock geology each produce individual spectral responses which together give rise to distinctive density levels, tones and colours on imagery acquired by remote sensors. Where bedrock is relatively near surface and particularly in semi-arid areas the distributions of individual vegetation associations and plant communities are closely related to geology. Depending on the terrain and on the type and scale of the imagery, changes in the spectral reflectances produced by the vegetation may discriminate structural features, distinguish lithological/stratigraphical sequences favourable for mineralization and even in rare cases identify the presence of mineral deposits. The phased use of satellite and air survey imagery, geobotanical, geochemical and geophysical techniques in exploration for stratiform copper, lead-zinc and phosphate deposits, for kimberlite pipes and for carbonatites is illustrated from case studies in southern Africa. northern Australia and Scandinavia.
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Transactions of the Geological Society of South Africa, 85 (1), 29-41
Stratigraphic and geometrical evidence shows that structures in quartzites and hornfelses of the Transvaal Sequence within and adjacent to the eastern compartment of the Bushveld Complex may be divided into three groups. First, there are folds and fragments which are attached to the floor of the Complex and restricted to the lower part of the Complex: their deformation ended prior to the crystallization of the upper zone and Nebo Granite. Secondly, there are large xenolithic inclusions forming gently dipping sheets or disoriented blocks which appear to be restricted to the upper zone and roof regions. The third group includes demal structures within the Nebo Granite. The floor-attached structures are interpreted as the result of regional compression and high grade metamorphism related to subsidence of the floor and intrusion of the Bushveld Complex. The large xenolithic structures are interpreted as flotation phenomena which formed during the early evolution of the Bushveld intrusion and remained at the top of the chamber when denser magmas intruded beneath them during progressive filling of the Complex. The third group of structures may be related to either the floor-attached structures of the xenoliths.
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Transactions of the Geological Society of South Africa, 85 (1), 43-57
At least two periods of medium- to high-grade metamorphism are recognisable in the Vredefort Dome. The oldest period in which the amphibolite and granulite facies are attained, is displayed by the rocks of the Basement Complex. The younger thermal event is related in time to the Bushveld Complex and took place at the closing stages of dome formation. Rocks of the Witwatersrand Supergroup were metamorphosed in the hornblende hornfels facies and some of the Basement granulites in the central part of the dome were retrograded to the pyroxene hornfels facies. Apart from this increase in metamorphic grade towards the central part of the dome, there is also an increase in grade towards the alkali granite plutons which are intrusive into the sedimentary collar. Several metamorphic zones, of which the staurolite zone is the most prominent, are related to these plutons. The metamorphosed basic igneous rocks (epidiorites) also show an increase in grade towards the central part of the dome. The actual heat source for the metamorphism is not obvious, but it is possible that concealed intrusions related to the dioritic and granitic plutons in the collar of the dome were responsible for the contact metamorphism.
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Transactions of the Geological Society of South Africa, 85 (1), 59-60
Palaeocurrent and maximum-pebble-size analyses of the Beaufort Group indicate that substantial topographic relief is present in the floor of the Karoo Basin in its northern present-day extensions. Domes protruded above the surface of accumulation at the onset of Beaufort sedimentation and these provided sediments to the basin. In general these sediments are recognized by their coarse and arkosic nature in the proximity of these localized provenance areas. These surveys indicate the presence of a dome or domes to the south-east of Kroonstad.
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Transactions of the Geological Society of South Africa, 85 (2), 105-110
Further isotope and radiometric age data are reported for plutonic rocks of the Proterozoic Vioolsdrif batholith. A two stage emplacement history for the batholith, whereby an early phase of basic to intermediate plutonism was followed by a later phase of felsic plutonims has been confirmed by consistently younger radiometric ages obtained for the latter units. Rb-Sr, Pb-Pb and Th-Pb whole-rock isochron ages for the Vioolsdrif felsic rocks point to intrusion 1 730±20 Ma ago, approximately 170 Ma after the basic to intermediate rocks. Previous uncertainty over the initial 87Sr/86Sr ratio for the basic rocks in the batholith has been reduced by analysing recently discovered fresh material. The initial 87Sr/86Sr ratio of 0,70313±13 is indistinguishable from that previously determined for the intermediate plutonic rocks (0,7031±3). A Rb-Sr mineral study on fresh granitoids from within the undeformed core of the batholith along the Orange River reveals the effects of resetting -900-1 000 Ma ago. Kibaran metamorphic resetting of the Rb-Sr system on a mineral scale was therefore very extensive and not only confined to the margins of the Vioolsdrif batholith. It follows that the so-called Richtersveld Province, which was originally considered to be free of Kibaran metamorphic effects, should rather be regarded as a very low-grade domain within the much larger Namaqua Province.
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Transactions of the Geological Society of South Africa, 85 (2), 111-116
Tonalitic gneiss samples collected from various localities in the southern portion of the Johannesburg-Pretoria granite dome were subjected to age dating of the constituent zircons by the U-Pb isotopic method. A variety of zircon concentrates yielded a discordant trend with a concordia intercept age of 3 170 1 34 Ma (10). The least discordant isotopic data points also closely, conform to a 3 200 Ma Wasserburg-type diffusion curve indicating that the upper intercept with concordia is not much dependent upon either the "episodic" or "continuous diffusion" lead loss models. Field relationships led Anhaeusser (1973) to conclude that tile tonalitic gneisses immediately north of the Lower Witwatersrand successions in the Central Rand are among the oldest granitic components on the dome. A previously published Rb-Sr isochron age of the granodiorites developed north of the tonalite gneisses and the U-Pb zircon age reported here are, however, virtually indistinguishable suggesting some degree of cogenetic behaviour for the granitic phases. This interpretation is evaluated and it is concluded that additional isotopic studies are needed to clarify evolutionary events on the Johannesburg-Pretoria granite dome.
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Transactions of the Geological Society of South Africa, 85 (2), 61-69
Recently, discovered base metal (Zn>Cu>Pb) massive sulphide deposits in the Northern Cape District of Prieska occur at Kielder 12 km north-west of a similar Zn-Cu deposit, the Copperton mine. Three massive sulphide bodies on Kielder, known as K3, K1 and K6, occur as stratabound massive sulphide lenses within granulite grade quartz-feldspar gneisses, basic granulites and amphibolites. Extensive exploratory core-drilling provided specimens of the massive sulphides, their disseminated pyritic haloes and the enclosing wall rocks in the area of poor outcrop with extensive calcrete and sand cover and, in places, in situ Karoo Supergroup Dwyka formation varved shales. Geothermometry and geobarometry using garnet-biotite, garnet-cordierite, garnet-hypersthene, and the FeS content of sphalerites showed a gradual metamorphic gradient from east to west, with the K3 area suffering P-T conditions of 695°C and 6,0 kb; the K1 area 686°C and 5,8 kb and K6 area 590°C and 5,6 kb. Metamorphic mineral assemblages correspond to the Regional hypersthene or granulite high-grade zone. The presence of amphibolites and lower-grade metamorphic mineral assemblages indicate retrograde metamorphism which, from geothermometre, or cordierite grain edges, occurred at a temperature of 530°C. The precursor lithologies of the Kielder wall rocks indicated by the metamorphic mineral assemblages are pelites, pelitic greywacke, tholeiitic basalt and magnesium enriched pelite.
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Transactions of the Geological Society of South Africa, 85 (2), 71-79, 7 figs
Drilling operations revealed a deep sediment-filled glacial valley running down the south-facing escarpment of the Transvaal Highlands. Seven lithofacies are recognised in the glacial sequence. The diamictite facies is subdivided into a conglomeratic, a massive, a banded and a bedded heterogeneous diamictite subfacies, which respectively represent reworked basal till, massive basal till, basal till showing flow banding, and flow till. The conglomerate and sandstone facies were formed by glacial outwash and minor deltaic sedimentation. Non-glycogenic gravity flow deposits constitute the mudstone-sandstone and part of the pebbly mudstone facies. The micaceous mud shale and shale facies originated by suspension setting of mud and silt. Ice-rafted debris is associated with the varved shale subfacies and the pebbly mudstone facies. The glacial history of the area comprises a pre-Depositional Stage, an early Valley Glaciation Stage and a late Valley Glaciation Stage. During the valley glaciation stages two sequences of basal till with associated glacial outwash, flow tills and varved clays, separated by argillaceous beds containing ice-rafted debris and distal turbidites were deposited. A post-glacial marine transgression inundated the entire area whereafter dark-coloured mud, clay, silt and sand were deposited.
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Transactions of the Geological Society of South Africa, 85 (2), 81-86
Sr isotope analyses of 14 whole rock samples of shoshonitic lavas and intrusives of the Barby Formation, a member of the Sinclair Group in Namibia, indicate that the age of emplacement of these units is 1 392±33 Ma with an initial (87)Sr/(86)Sr ratio of 0.7023±0,0002. Resetting of the Rb-Sr isotopic systems appears to have resulted in some of the samples defining a second isochron giving an age of 1 151±32 Ma. This re-equilibration may be related to the final stages of metamorphism and deformation associated with the formation of the Namaqua Mobile Belt lying to the south of the shoshonite occurrences. The low initial ratio is suggestive of a mantle origin for the K-rich magmas and insignificant contamination by crustal material indicates that the highly potassic character of these rocks is a primary magmatic feature.
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Transactions of the Geological Society of South Africa, 85 (2), 87-90
Clasts of gneiss, vein quartz, granite and limestone contain pyrite grains and stringers at seven localities along the southern margin of the Karoo Basin. At an eighth locality, Elandsvlei, along the western margin of the basin brochantite was found in a quartzite clast. The clasts were mostly from coarse tillite close to the basal contact of the Dwyka Formation. Glacial erosion of rocks of the Namaqualand Metamorphic Complex, the Natal Metamorphic Province and probably a tectonic land-mass to the south and south-east of the present continental margin supplied the mineralized debris which was transported and deposited by ice sheets in the southern part of the basin.
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Transactions of the Geological Society of South Africa, 85 (2), 91-103
Sedimentological investigations and continuous seismic reflection profiling have been undertaken on the continental margin and upper slope between the Msikaba River and Rame Head (vicinity of Port St Johns) off the east coast of southern Africa. A thick, extensive deposit of mainly terrigenous sediment (2 930 x 10/6 m³) has been able to accumulate between the Mbotyi and St Johns canyons because the Agulhas Current bypasses the shelf south of the structural offset at Waterfall Bluff. Two sources of sediment are the Mzimvubu River, which supplies 60 percent of the bedload material to the area (0,21 x 106 m³ annually), and the Agulhas Current which entrains bedload sediment (0,16 x 10/6 m³ annually) disgorged by rivers between Illovo and the Msikaba River to the north. The source of the suspended material mantling the shelf is probably the Mzimvubu River which discharges 4 x 10/6 m³ of fine detritus annually. If modern sedimentation resumed on this shelf 10 000 years B.P., then a considerable quantity of sediment has been lost from the region. Sediment is stripped from the foot of the sand sheet by the Agulhas Current south of the Mngazi River and unconsolidated material is observed spilling over the shelf break into the Mzimvubu Canyon and probably, moves down the St Johns Canyon as well. Sediment, probably mainly derived from the north, has prograded from Waterfall Bluff southwards to produce the Mfihlelo Spit. Irregularities in the coastline and in the bathymetry in this region are anomalous for the eastern margin. Rift-related structural offsets at Waterfall Bluff and at the Mngazi River have produced large dislocations of the coastline, and less pronounced, subparallel faulting has resulted in a "step-like" coastal profile. The location of river mouths and submarine canyons is possibly related, directly or indirectly, to faulting. Seismically, two bedrock units are discernible; an older, more stratified unit and a younger, less structured sequence. Erosion of the latter bedrock type has given rise to broad outer shelf terraces and shelf breadth is probably, also largely, determined by the thickness of this sequence.
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Transactions of the Geological Society of South Africa, 85 (3), 117-126
Mapping of the Ventersdorp Supergroup in the Northern Cape and Western Transvaal has revealed problem areas where Rietgat and Allanridge lavas are juxtaposed and need to be distinguished. An examination of their petrographic and field characteristics has revealed that a combination of such criteria as degree of alteration, presence of dark green spots (glomeroporphyritic textures formed by mafic mineral aggregates) exclusively, in Allanridge lavas thickness of lava flows, bleaching and oxidation of flow tops, vesicularity, distribution of amygdales in flows and the colour and nature of weathered outcrops could be used to distinguish the Rietgat from Allanridge lavas in exposures of relatively complete successions. Correlation by major element composition is a useful tool and the only one possible in the case of incomplete successions and small isolated outcrops. Both P2O5 and TiO2 content of the Rietgat lavas is significantly higher than the Allanridge, while a Jensen plot reveals two fields which overlap only slightly. The Allanridge Andesite Formation is composed of basaltic to andesitic lava types and it is therefore proposed that the name be changed to simply Allanridge Formation.
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Transactions of the Geological Society of South Africa, 85 (3), 135-139
A new Rb-Sr whole rock isochron age of 1 454±59 (20) Ma has been determined for previously, undated gabbroic rocks which crop out in the western portion of the Kruger National Park in the eastern Transvaal, South Africa. These rocks form prominent koppies and ridges which are in part sheet-like and in part dyke- like and have been designated as the Tsange Gabbros. They are strongly tholeiitic and are depleted in incompatible element abundances relative to the younger Lebombo basalts which crop out to the east.
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Transactions of the Geological Society of South Africa, 85 (3), 141-153
Both foliated and unfoliated and unfoliated granite are present in the eastern marginal zone of the Namaqualand Metamorphic Complex. The regional field, structural and chemical aspects of these granites are discussed with their mode of emplacement and possible geochronology. Field petrographic and chemical data indicate that these granites are derived from crustal sources; that the foliated type is syntectonic to F1 deformation while the unfoliated type is post-tectonic and probably associated with Koras volcanism.
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Transactions of the Geological Society of South Africa, 85 (3), 167-178
The northern and central Lebombo consists of a major succession (7,5 km) of Karoo sedimentary and volcanic rocks. The rocks overlie granitoid rocks of the Kaapvaal craton and Limpopo Mobile Belt and a close relationship exists between Precambrian basement structure and the geological configuration of the northern and central Lebombo. The volcanic succession consists of a diverse assemblage of nephelinites, picrite basalts, basalts and rhyolites and associated intrusive rocks. Picrite basalts and basalts are the dominant rock types, attaining thicknesses in different areas, of 4 and 4,5 km respectively. The diversity of extrusive and intrusive rock types indicates a complex history, of volcanism and tectonism ranging in age from middle Jurassic (-195 Ma) to Cretaceous (-137 Ma). Present-day outcrops of Karoo sediments and volcanics mark the down-warped and faulted western margin of a rift system which developed in response to fragmentation and rifting of eastern Gondwanaland. For the most part volcanism preceded rifting and it is suggested that processes which initiated Lebombo volcanism may have also been the cause of rifting.
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Transactions of the Geological Society of South Africa, 85 (3), 179-187
The lowermost recognizable unit (basal tillite) of the Dwyka For mation, which has a maximum thickness of about 100 m is developed along an east-west zone in the southern Karoo and consists of predominantly fine massive tillite was subordinate shale and diamictic beads. Both extrabasinal and intrabasinal clasts up to 2.5 m in diameter are present in the tillite. In the west the tillite rests on striated and soft-sediment pavements, while southwards and east wards locally deformed argillaceous beds of the Witteberg Group form the bedrock. A hiatus separates the Cape and Karoo sequences and during advance of the ice minor modification of this erosion surface occurred. At the onset of the glaciation a lowland area bounded in the north by the Cargonian Highlands, in the south by an Alpine-type highland and in the east by a large water body forming an embayment into the depository, existed over this part of Gond wana. Ice flowed overland into the basin from both the north and south, but changed direction down the slight palaeoslope towards the embayment where bedded tills were deposited by, basal melt-out. In the west lodgement tills were laid down by predominantly, warm-based ice. As climatic cooling proceeded, build up of the ice mass in the basin caused a reversal of ice flow and subsequent outlet of the ice to the West where glacial erosion of the Cape Supergroup then took place. Extensive till sheets were laid down by lodgement processes over the entire area during this stage.
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Transactions of the Geological Society of South Africa, 85 (3), 189-201
The geology of Langerman's Kop, situated in Kensington, Johannesburg, has had a profound influence on concepts of Witwatersrand sedimentation since E.T. Mellor reported the presence of an unconformable relation between the Mondeor Formation and the Government Subgroup at this locality. Remapping of the area reveals that contacts between the Government Subgroup and the Mondeor Formation are faulted with several ages of faults being developed. Folding of Mondeor Formation strata has also occurred. The chronological tectonic history involves a period of normal faulting, in early Ventersdorp times, which juxtaposed Mondeor Formation and Government Subgroup strata, followed by low angle faulting. This tensional period was succeeded by north-south compression which caused folding of the Mondeor Formation and thrusting of the Government Subgroup over the Mondeor Formation. A tectonic melange developed in the vicinity of the thrust plane during movement on the thrust fault, and dips of strata along the contact underwent steepening. The melange was subsequently deformed during continued compression and strong cleavage and shearing developed in the melange and surrounding rocks. The cleavage-forming event was outlasted by low-grade regional metamorphism. North-striking faults occurred after this compressive event and were probably responsible for the breccias which were regarded by Mellor as part of the unconformity.
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Transactions of the Geological Society of South Africa, 85 (3), 203-210
Two types of laumontite (CaAl2Si4O12.4H2O) occurrences are present in the Clarans and Drakensberg formations. The first (Type 1) shows cross-cutting relations, while the second (Type 2) is concordant with respect to the bedding. Both are interpreted as having originated due to geothermal activity, before and during the outpouring of the Drakensberg basalts. The presence of laumontite at this stratigraphic level in the Karob Sequence is used to infer the original thickness of the Drakensberg Formation in the type area and it also serves as indirect evidence that the upper acid lavas, as developed in the Lembombo Formation, did not erupt in the main Karoo Basin. In view of the growing industrial demand for natural zeolites and the anticipated search for these minerals in South Africa, the types of laumontite occurrences described in this paper may be useful as models for further and future exploration for these commodities in the upper parts of the Karoo Sequence.
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Transactions of the Geological Society of South Africa, 85 (3), 211-214
Small subidioblastic porphyroblasts of kyanite have been identified in a horizon of ferruginous metapelite within the Orange Grove Quartzite at Northcliff, Johannesburg. The remaining minerals are quartz, muscovite, hematite, accessory pyrophyllite, and a large proportion of secondary kaolinite. Coarse blades of kyanite also occur, together with secondary pyrophyllite, in quartz veins dissecting quartzites of the Central Rand Group near Krugersdorp, that is stratigraphically some 5 000 m above the Orange Grove Quartzite. Whereas the Northcliff metapelite has just attained conditions of the reaction pyrophyllite - kyanite + quartz + H2O, the temperatures in the Krugersdorp 17 veins were higher than those defined by the univariant curve of this reaction, that is above at least 390°C. Minimum total pressures were close to 3 kb. The kyanite- producing event may be related to some sort of modified burial metamorphism in connection with tectonic dislocations. The age of this event could be post- Transvaal, like the formation of the Johannesburg Dome, but also pre-Transvaal, or even pre-Ventersdorp. Abnormally high geothermal gradients for a cratonic area are required if kyanite formation occurred at pressures not much above 3 kb.
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Transactions of the Geological Society of South Africa, 85 (3), 215-219
Observations made on a point bar on the Great Fish River of the Eastern Cape Province, revealed a layer of ripple cross-lamination in which foreset dip directions indicate currents of opposing direction. Several possible explanations are discussed but the most likely is that during bankfull discharge large-scale flow separation occurs and ripple cross-lamination with upstream dipping foresets is deposized in the zone of separation.
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Transactions of the Geological Society of South Africa, 86 (1), 1-10
Field investigations in the vicinity of the Msauli Mine have shown that a number of porphyry stocks occurring in this area are intrusive into the pre-folded metavolcanic formations of the upper Onverwacht Group. The age of one of these porphyry stocks has been determined by means of Rb-Sr whole rock isotopic analyses and yielded an age of 2 766 Ma with an initial 87Sr/86Sr ratio of 0,705 9. Geochemically, the Msauli porphyries are classified as granitic to granodioritic in composition and thus differ from many other porphyries in the Barberton greenstone belt. Trace element modelling, using K, Rb, Sr and Ba, was undertaken in an attempt to distinguish between two potential source rocks, namely, tonalite/trondhjemite or amphibolite (tholeiite basalt). None of the calculations, however, resulted in a model magma similar to the porphyry rock. Instead indications are that mechanical mixing of the two potential source rocks resulted in a magma whose composition approaches that of the porphyries. The latter process, therefore, is favoured by the authors for the petrogenesis of the Msauli porphyries. The trace element modelling further suggests that the porphyries were formed by between 0 and 30 per cent fractional crystallization. This process is reflected in the existence of isolated phenocrysts within a fine-grained matrix.
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Transactions of the Geological Society of South Africa, 86 (1), 11-18
Reconnaissance mapping of an 8 000 m-thick arenaceous sequence, presently correlated with the Kamtsas Formation (Damara Sequence), has led to an informal three-fold subdivision of the succession. The lower unit can be further subdivided into a tidally reworked zone, containing trace fossils, probably Planolites and Treptichnus, and an upper zone of rocks deposited mostly under fluvial conditions. The middle unit is less mature than either the underlying or overlying sediments and may represent an alluvial fan. The poorly-exposed upper zone may have formed under similar conditions to those prevailing during the deposition of the lower unit. The lower part of the sequence, which correlates with the upper part of the Ghanzi Group in Botswana, may belong to the Duruchaus or Doornpoort formations. The sequence was probably deposited in grabens related to continental rifting of the pre-Damara basement sometime during the period 900-760 Ma. The sequence has been deformed into open flexural slip folds with an associated rough cleavage, which postdate the deposition of the Kuibis Subgroup.
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Transactions of the Geological Society of South Africa, 86 (1), 19-35
Near Johnston's Leap in the Sterkspruit Valley, Barkly East, an arcuate southward-dipping intrusive sheet of Karoo dolerite has fused the overlying Cave sandstone at some 40 places along its upper contact which has been traced over a distance of about 17 Km. The buchites are found mainly where the sheet attains its highest elevations on either side of the valley and are confined to within 2 m from the intrusive contact. The vitrification seems to be mainly localized in roof pendants and xenolithic slabs or lenses of sandstone in the dolerite. The changes caused by this pyrometamorphism and the subsequent alteration of the buchites are described with the aid of petrographic data derived from a study of about 100 samples taken from various outcrops along the upper contact of the dolerite sheet in addition to 19 whole-rock chemical analyses and one of glass separated from one of these samples. The sandstones are very fine-grained feldspathic arkosic wackes consisting of quartz and feldspar in a microcrystalline matrix. Chemical analyses show that they contain roughly equal amounts of Na2O and K2O. The first melt to form in these sandstones develops in the microcrystalline matrix and most specifically along the boundaries between quartz and feldspar grains. The composition of this first-formed melt is not known, but it most probably lies on the cotectic phase boundary between Q and ab-an and close to the temperature minimum for the appropriate H2O pressure. The buchites that have resulted from this vitrification contain varying amounts of recognizable glass up to about 98 per cent by volume. On the average the buchites and especially those with more than 80 per cent of glass, contain more Na2O, less K2O, and considerably more H2O than the sandstones. The specific cause of this apparent enrichment in Na2O in the most highly vitrified rocks cannot be ascertained without further detailed chemical data on the composition of the glass in the buchites, but may perhaps be ascribed.
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Transactions of the Geological Society of South Africa, 86 (1), 37-49
Banded iron-formation clasts appear in quantity in the alluvial grav els on high level Orange River terraces some 30 km downstream from the Vaal River confluence at a point a considerable distance upstream from where this rock type becomes a significant constitu ent of the catchment bedrock. The point of first appearance of banded iron-formation is associated with an impressive valley on the right bank of the Omnge River, extending northwards towards Griekwastad and beyond, and contaning vast gravel deposits in which banded iron-formation is the dominant clast type. Comparison of the clast assemblages of these gravels with those along the Omnge River both upstream and downstream of the right bank valley indicates that the change in Omnge River gravel composition was caused by entry into the Omnge of a major river. Over the time period recorded in the high level terraces this river was at least equal to, or may have contained as much as four times the volume of water of the Orange. The river appears to have ceased flowing very suddenly, its demise apparently related to crustal tectonism along the Transvaal- Griqualand Axis. Cessation of discharge from this river may have coincided with the commencement of the infilling of the Kalahari Basin,.
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Transactions of the Geological Society of South Africa, 86 (1), 51-54
Cruziana acacensis, an index-trace fossil from Silurian strata in North Africa, occurs in the middle of the Peninsula Formation (Cape Supergroup). On the strength of this, and in the absence of body fossils in this formation, a Silurian age is suggested for the Peninsula, as well as for the overlying Pakhuis and Cedarberg formations.
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Transactions of the Geological Society of South Africa, 86 (1), 55-61
Additions and revisions are made to the volcano-stratigraphic nomenclature of the Lebombo monocline, including the Bumbeni Complex. This nomenclature is extended to correlative Karoo lavas found to the north of the Lebombo in south-east Zimbabwe. Brief descriptions of the more important intrusive rocks are found in the Lebombo and adjoining areas and are presented and possible relationships between intrusive and extrusive rock types noted.
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Transactions of the Geological Society of South Africa, 86 (1), 63-64, 1 fig
The bochiantid genus Umgazaniceras is recorded from the Sundays River Formation for the first time. This record strengthens the palae ontological correlation of the Mngazana and Sundays River Formatins, as well as emphasizing the importance of the terminal Valanginian eustatic transgression in south-east Africa.
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Transactions of the Geological Society of South Africa, 86 (1), 65-70
Dr Sidney Henry Haughton was born in Bethnal Green, London, on 7 May 1888 and died in Johannesburg on 24 May 1982, just 17 days after his 94th birthday. So passed one of South Africa's most able, productive and distinguished earth scientists. All his school career took place in London and he matriculated from the Essex County Technical Institute, Walthamstow, at the age of 14 years and six months. He later sat various examinations of the City and Guilds Institute of London where he obtained the intermediate B.Sc. of London before being awarded the science scholarship at Trinity Hall, Cambridge where he read the Natural Sciences Tripos, specializing in geology under Woods and Harker.
He graduated B.A. in 1909 and, after teaching for two years at Clayesmore School in Berkshire, he was recommended by the Cambridge University Appointment's Board for the vacant post of geologist-palaeontologist at the South African Museum in Cape Town. He accepted the post and arrived in South Africa on 15 September 1911. During his early years in the museum the young Haughton received constant guidance, friendship and assistance from A.W. Rogers and A.L. du Toit who were members of the Cape Geological Commission (later incorporated into the Geological Survey of the Union of South Africa).
He remained at the museum for nine years, becoming Assistant Director in 1915. During this period he was awarded the Murchison Medal by the Geological Society of London. His love of music and the possession of a fine base singing voice, led him to meet Edith, a pianist of great sensitivity and the daughter of W.T. Hoal, the first Postmaster-General of the Union of South Africa, and they were married in 1914. In 1920 he resigned from the South African Museum and joined the Geological Survey in Cape Town serving under the directorship of one of his earlier mentors, Dr A.W. Rogers.
The Cape Town offices of the Geological Survey were located in the grounds of the South African Museum and he continued to be honorary keeper of geology and palaeontology, adding continuously to the substantial fossil collections which he continued to help build up. His research work on fossil fish, amphibians, reptiles and mammals from all over Africa - South Africa, Central Africa, and Malagasy - is amply reflected in the Museum's annals. Influenced by Robert Broom, another of the giants of early South African science, Haughton initially focused his attention on the fossils of the Karoo sequence. He concentrated on what were then known simply as Tapinocephalus and Cistecephalus zones of the Beaufort Group, and on the "Stormberg Series", eventually submitting a thesis on the fauna and stratigraphy of the "Stormberg Series" to the University of Cape Town.
He was awarded the D.Sc. in 1921 by the University's Chancellor, General J.C. Smuts, who was also then Prime Minister of the Union of South Africa. In 1929 the XV International Geological Congress was held in South Africa. This highly successful geological gathering had important consequences for South African geology, as it resulted in the establishment of the Sub-Commission on the Gondwana System, whose successor body, under the IUGS, has been responsible for the series of outstanding international "Gondwana Symposia" held since the mid-sixties. Haughton was elected first Secretary of the original sub-commission, and later served also as its President. Later still he served as chairman of the organizing committee of the memorable Second Gondwana Symposium, held in South Africa in 1970, and he was Editor of the voluminous proceedings of that meeting. While in Cape Town he took an active interest in the Mountain Club of South Africa and was President of the club from 1926-1928 and Honorary Editor of the Annual from 1915-1930. He was presented with the Gold Badge of the club in 1932 and was elected an Honorary Member in 1964. In January 1934 Haughton was appointed Director of the Geological Survey and moved to Pretoria. Here he remained in this capacity until his official retirement, at the age of 60, in 1948.
During his term of office the complement of trained geologists on the staff increased from ten to about 70, and several major innovations were introduced in the work of the Survey. During the war years of 1939-45 the Geological Survey and its director were involved in a number of activities of strategic importance. Haughton was appointed by Smuts to a body known as the Industrial and Agricultural Commission, and he was appointed Controller of Non-Ferrous Minerals to regulate these resources under war conditions. In 1943 he was appointed head of the South African Scientific Mission which was attached to the South African Embassy in Washington. It was a parallel of similar scientific missions from other allied countries established in the US capital during the war. This appointment lasted until the end of the war, and it, too, later had a number of important consequences. One of them was the instigation of a renewed and intensified search for uranium ores in South Africa, a task
assigned to a specially formed unit of the Geological Survey. Haughton was so personally involved in this assignment that on his retirement from the directorship of the Survey in 1948, he was immediately re-employed as officer in charge of the uranium sub-unit. Later, a Uranium Research Committee was formed, to be replaced ultimately by the Atomic Energy Board (now the Nuclear Development Corporation of South Africa - NUCOR). The Fuel Research Institute and the CSIR were two other major statutory bodies with which he had close ties, as a member of their boards of control. Indeed, the very establishment of the CSIR was at least partly attributable to Haughton, following discussions which he and Sir Basil Schonland had on the matter with General Smuts, then Prime Minister.
In 1954 Dr Haughton played an important part as Geological Correspondent of the Commission for Technical Co-operation in Africa South of the Sahara, co-ordinating scientific research throughout this vast area from his Pretoria office. His duties for the eight years he held this post enabled him to travel very widely in Africa and, as a direct result of his experiences he was able to compile his book The Stratigraphic History of Africa South of the Sahara, which was published in 1963. Other books with which he was directly associated included his 1954 edition of Du Toit's Geology of South Africa and the Geological History of South Africa published in 1969. In the most complete list of publications available he has seven books, 218 substantive papers and 321 abstracts to his credit, as well as 13 maps. Some of the most significant works are listed in the Bibliography which accompanies this obituary. Dr Haughton's role in the Geological Society of South Africa is also noteworthy. He contributed many papers to the Transactions of the Society, served for many years on Council and was twice elected President of the Society in 1925 and 1967 a distinction shared only by R.B. Young who held this office in 1910 and 1926 and A.L. du Toit who was President in 1917 and 1927. He was elected an Honorary Member of the Society and in 1953 was chosen as the third Alex. L. du Toit Memorial Lecturer.
He also received the Draper Memorial Medal from the Society in 1941. In the sixties Dr Haughton was President of the South African Geographical Society and was appointed Honorary Fellow of the Royal Society of London in 1960. He was also elected an Honorary Fellow of the Geological Society of America, the Geological Society of Belgium and the Royal Society of South Africa (elected in 1918). In 1951-52 he was President of the Associated and Technical Societies of South Africa. In 1965 he was attached to the CSIR as Geological Adviser and as organizer of the Second Gondwana Symposium and Editor of the proceedings of that Symposium. He later acted as Chairman of the CSIR-sponsored Upper Mantle Project and edited the symposium volume which was published in 1969.
Dr Haughton had a long and close association with the University of the Witwatersrand. He was appointed Honorary Director of the Bernard Price Institute for Palaeontological Research at the University in 1951, in which post he served until
1973, and he remained intimately associated with the BPI as Honorary Editor of the Institute's journal - Palaeontologia Africana - and consultant to the director until his death. He received honorary degrees from three South African universities; the University of Cape Town, Hon. LL.D. (1947); the University of the Witwatersrand, Hon. D.Sc. (1964); and the University of Natal, Hon. D.Sc. (1967). His life was one of unstinting service to his profession, country, and community.
His application of acquired knowledge clearly demonstrated his profound wisdom. This, combined with his great personal integrity, earned him the deep respect and admiration of all who knew him and worked with him. Those who had the honour to be associated with him, apart from his contributions to palaeontology and geology, will remember Dr Haughton as a fine public speaker, a man of great dignity laced with humour and gifted with the piercing ability to separate the wheat from the chaff. South African science has lost one of the last of its pioneers, and one of the greatest.
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Transactions of the Geological Society of South Africa, 86 (2), 105-116
Two heavy mineral-bearing sandstone beds occur in the upper part of the Weltevrede Formation of the Witteberg Group, 35 Km east-north-east of Willowmore, Cape Province. The upper or main bed has a mean thickness of 2.3 m and a lateral extent of 1.1 Km, whereas the lower bed has a mean thickness of 1.8 m and a lateral extent of 0.3 Km. The beds are separated by lenticular-bedded mudstone or siltstone and flaser-bedded sandstone which are also present throughout the upper part of the Weltevrede Formation and represent subtidal deposits. They generally form the lower portion of upward-coarsening sequences, the upper portion of which consists of fine-grained, quartz arenite. These sequences have a maximum thickness of 30 m and probably represent prograding subtidal sand bars. The heavy mineral-bearing beds are composite, consisting of up to five separate units. The base of each successive unit rests unconformably on the underlying one with the result that only the uppermost unit is fully preserved and displays an upward-fining sequence. Each unit represents deposition from a storm- surge ebb current and the upward-fining sequence suggests that waning- energy conditions were active. The lithofacies sequence within each unit is laterally consistent. Planar cross-bedded and parallel bedded, very fine- to fine-grained sandstones are predominant in the lower units and in the lower part of the uppermost unit. These lithofacies represent deposition from traction currents. Parallel bedded, very fine-grained sandstone and siltstone, which contain pinch-and-swell structures and gentle undulations in places, are ubiquitous in the upper part of the uppermost unit of each bed. These lithofacies represent deposition from suspension under the influence of oscillatory wave motion. Symmetrical ripple marks on the top surface of the upper bed provide further evidence of wave activity. The heavy minerals are, in order of decreasing abundance, rutile, zircon ilmenite, magnetite, monazite, staurolite, and sphene and the relative proportion of these minerals does not vary significantly between samples. The radiometric response of the samples, determined with a portable gamma-ray scintillometer, is directly related to the heavy-mineral content. The heavy minerals are concentrated in the upper 20 cm of each bed and their grain sizes are smaller than those of the light minerals, of which quartz is predominant. Computation of the settling velocities of each mineral, from grain-size data, indicates that hydraulic equilibrium was achieved during sedimentation. Both a lithofacies analysis of the upper 20 cm of the beds and a grain-size distribution analysis of two samples, show that the bulk of the heavy minerals were deposited from suspension during waning storm-wave surge. The heavy minerals were most likely entrained by storm-surge ebb currents from a beach and/or shallow marine deposit situated approximately 100 Km north of the study area. Water depth during deposition of the heavy mineral sands was between 50 m and 100 m.
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Transactions of the Geological Society of South Africa, 86 (2), 117-125
A tunnel driven through the Berea Ridge in Durban provided fresh exposures of the Berea Formation which were studied to erect a sedimentary model for the deposition of the Berea Formation. The study showed that there are three recognizable sand facies in the Formation; facies A is typical of the surface outcrops of the Berea Formation and is a clay-rich, structureless, dark red or reddish-brown sand with heavy minerals randomly distributed throughout. Facies B is a coarser-grained, yellowish-red, partially cemented sand which contains prominent, discontinuous, heavy mineral laminae, bioturbated layers, water escape structures and a ferricrete horizon. Facies C is similar to facies B, but is less well cemented and exhibits fewer sedimentary structures, though bioturbation and discontinuous heavy mineral laminae are present. Facies C sand contains a number of large, carbonate-cemented, lenticular, fossiliferous sand bodies. The fossils are marine. Numerous, smaller, irregularly branching carbonate-cemented concretions are also found in this sand and contain a similar assemblage of marine fossils. It is thought that facies B and C sands were deposited in a beach environment during a regression of the Pleistocene sea. The carbonate-cemented bodies represent fossil beach-rock which was exposed to subaerial erosion and later covered by aeolian sand to form the present-day Berea Ridge.
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Transactions of the Geological Society of South Africa, 86 (2), 127-135
In glacial sequences subaqueous debris flow deposits consist of bedded diamicite, small-pebble conglomerate and coarse-grained sand stone intercalated with tillite, laminated diamicite, fine- grained sandstone, siltstone, mudstone and shale. The debris flow deposits have a matrix- to clast-supported texwture and are characterized by the presence of abundant mudstone clasts, normal and inverse grading, a crude fabric, a layering, soft-sediment deformation and commonly displaced fossil species. Commonly the distinction between debris flow diamictite, laminated diamictite formed by debris rain, and tillite causes problems where outcrops are poor. The small pebble conglomerate and coarse-grained sandstone can be distinguished from subaqueous glacial outwash by the absence of tractional structures and the presence of abundant mudstone clasts.
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Transactions of the Geological Society of South Africa, 86 (2), 137-141
Trace element data from the Losberg Complex, Transvaal, South Africa are reassessed. A sudden increase in incompatible trace element content and a decrease in certain ratios, such as K/Rb, Ba/Rb, Ba/Zr and Zr/Cu at the base of the gabbro zone are not consistent with fractional crystallization from a single, homogeneous magma. Changes in initial strontium isotope ratio at the base of the gabbro zone argue against unmixing of a magma controlling trace element data. An injection of new magma of different composition from the first magma causes these chemical changes and the appearance of plagioclase as a cumulus mineral. A further injection of magma is postulated close to the top of the intrusion, based on reversals of such parameters as Ti, Ni and Cr contents and Mg/Fe ratio. The trace element geochemistry of the first magma to the Losberg Complex and to the Rustenburg Layered Suite of the Bushveld Complex are similar, supporting their genetic relationship.
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Transactions of the Geological Society of South Africa, 86 (2), 143-153
The Lake Mentz and Kommadagga subgroups are characterized by a heterogeneous sequence of sediments, ranging in grain size from clays to coarse sands, which were deposited in a complex of nearshore and shallow marine environments. The sandstones in the sequence range in composition from greywackes to quartz arenites and were derived from a mixed metamorphic-igneous-sedimentary source terrane. A decrease in pore space, mineral overgrowths, formation of silica and calcite cements and development of authigenic minerals such as opal, albite, stilpnomelane, analcite, prehnite and various clay minerals are the characteristic diagenetic features of the sediments. The mineralogical evidence suggests the maximum temperature and depth of burial were 150 C and 5 to 6 Km respectively.
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Transactions of the Geological Society of South Africa, 86 (2), 155-159
Previous studies of class shapes from the Dwyka tillite nave been highly qualitative. Despite the imprecision and subjectivity of visual estimation, a number of size-shape relationships have been postulated which are now becoming entrenched in the literature via repetition. This pilot study was undertaken to attempt to quantify class shape in terms of roundness, flatness, sphericity, elongation and oblate-prolate index. Additional observations on faceting and striations were also obtained. Clasts measured were within the size range 0,015 m to 0,387 m. The percentage of striated clasts from the 11 sample points varied between 14,29 and 29,55 whilst the mean number of facets were within the range 1,52 to 3,07. Mean shape indices were relatively uniform but standard deviations often varied substantially between sites. The results show that the hitherto qualitative size-shape relationships suggested for the whole of the Dwyka could not be substantiated. Those relationships which were found to exist are difficult to explain by an approach of this type and are, in themselves, worthy of detailed study.
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Transactions of the Geological Society of South Africa, 86 (2), 161-163
Professor Geoffrey Bond, Fellow of the Zimbabwe Scientific Association and former Professor of Geology and Vice-Principal of the University of Zimbabwe, died suddenly at home in Harare on 19 June 1983 aged 71 years. Born in Brighton, England on 27 May 1912 Geoffrey Bond spent much of his childhood and youth in Yorkshire. At the age of 18 he left school in Cumberland to face the frustrating task of job hunting in the depression years of the thirties. He eventually found work as a schoolteacher in London. His formative years among the largely limestone country of the Yorkshire dales had instilled in him, from an early age, a love of geology and of the wide open spaces. As a schoolteacher he further pursued his hobby of geology in evening classes at the Chelsea Polytechnic. Friends eventually persuaded him to write the examinations for a Royal Scholarship in which, much to his astonishment, he found himself successful. Geology had suddenly become something more than a mere hobby and, thus financed, he embarked in 1937 on a 4-year B.Sc. course at the Imperial College of Science and Technology in London. World War II intervened and from September 1940 to December 1945 he served with the Royal Air Force. He returned to Imperial College in January 1946 and, after a supreme two-term effort, graduated in July of that year with First Class Honours. Most of his wartime service was spent in what was then Southern Rhodesia. Geoffrey Bond always maintained that his wartime posting to Africa, and that of the several hundred others who accompanied him, was the result of a clerical error where "400 airscrews" became "400 aircrew". This posting changed the whole course of his subsequent life; it instilled in him a love of the bush and a determination to return to this country after the war. So, in September 1946, he arrived in Bulawayo to take up the post of Keeper of Geology at the National Museum. Though palaeontology was still his prime interest Geoffrey Bond, with the late H.H. Read.
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Transactions of the Geological Society of South Africa, 86 (2), 165-166
Alfred Frost, doyen of South African mining geology, passed away on 26 July 1983 at the age of 90 years. He was born in Burnley, Lancashire, England, on 2 October 1892. During the First World War he served in the Royal Navy as a radio operator on trawlers converted to minesweepers. He spent the period 1920 to 1924 at the Imperial College, Royal College of Science, London, where he graduated as an Associate of the Royal College of Science (A.R.C.Sc.) and B.Sc. (Hons.), London University, in geology. He started his geological career in Devonshire where he introduced "Applied Geology" in the ball clay industry in the Bovey Tracey Basin. This work made him decide to make applied geology his career. In 1925 he accepted a post in Southern Rhodesia as curator of the Bulawayo Museum from where he supplied geological information and advice to the mining community of Matabeleland. In 1927 he accepted an invitation to join Union Corporation Limited in Johannesburg as their Consulting Geologist. He created the Mines Geological Department at Union Corporation and, together with Eric Marland, pioneered mining geology on the Witwatersrand. They started the Geology Department of Union Corporation and many of the methods and techniques introduced by them still form the basis on which this department, now part of the Gencor Exploration and Geology Department, is operating.
During the mid 1930s to 1940s he played a major part in the exploration for and eventual discovery of gold-bearing strata in the Orange Free State. Subsequently he also played an important role in the discovery and evaluation of the Evander Gold Field. He retired as Consulting Geologist in 1957 but was retained as a Consultant to Union Corporation until 1972 when he finally retired at the age of 80 years. Alfred Frost was very methodical and tenacious, he had a totally practical approach to all problems and had the ability to reduce the most complicated problem to its bare essentials. He always reverted to first principles, an approach which must have contributed greatly to his very considerable successes. Alfred Frost was a member of the Council of the Geological Society of South Africa from 1936 to 1965 and served as President of the Society in 1947. In 1957 he was awarded the Draper Memorial Medal of the Geological Society, the highest award bestowed by the Geological Society. He also served on the Controlling Executive of the Associated Scientific and Technical Societies. He was a Fellow of the Institution of Mining and Metallurgy (London). Alfred Frost served on the Board of Control of the Bernard Price Institute of Geophysical Research at the University of the Witwatersrand from 1948 to his death in 1983. He read extremely widely and as a result was invited by the Johannesburg City Council to serve on the Johannesburg Library Consultative Committee. He served on this committee as well as on the Consultative Committee for the Johannesburg Geological Museum up to his death in 1983. He seldom missed any meetings of these bodies where his views were greatly respected. In 1982 he was nominated by the Geological Society to become the first earth scientist to be accepted as a Registered
Scientist in terms of the Natural Scientists' Act of 1982. This honour was personally bestowed upon him by the South African Minister of Constitutional Development Mr Chris Heunis.
He was widely respected by all who came in contact with him. His friendly nature made him a very popular figure wherever he went. Probably one of his outstanding characteristics was his tremendous ability to stimulate discussion and motivate people, something that can be testified to by the many geologists who were fortunate enough to come in close contact with him. He married Florence Margaret Eriksen, a very talented concert pianist, in 1928. She died in 1975. It is to be regretted that this highly talented couple was not blessed with children.
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Transactions of the Geological Society of South Africa, 86 (2), 167-168
Eric Frank Marland was born on 16 April 1910 and grew up on the Witwatersrand where his parents were employed at the Ferreira Deep Mine. He qualified at the University of the Witwatersrand with a B.Sc. degree in chemistry and geology but after experiencing laboratory work during vacations he decided that he preferred an open air life in geology and went on to obtain an Honours and Masters Degree in geology at the same university. Emerging from university during the depression years it was difficult to obtain work so that his first employment was with Gold Fields of South Africa as a topographic surveyor, where he was part of a team carrying out surveys of the Far West Rand in preparation for magnetometer surveys. This pioneer magnetic survey eventually led to the discovery of the Far West Rand goldfields. In 1933 he approached Mr Alfred Frost for employment as a geologist but because such a position was not available he accepted an offer of employment as an Assistant Sampler on East Geduld Mines Limited. The association with Alfred Frost grew into a very close friendship which lasted throughout their lives. Shortly after starting in the sampling department a magnetometer survey was required on Van Dyk Consolidated Mines Limited and Eric was put on the project where he worked both as a surveyor and magnetometer observer. During this time he was able to give geological advice at both Van Dyk and Modder Deep and the realization arose that trained geologists could make a contribution to solving problems in the mines. Accordingly, in 1936 he was appointed on Van Dyk Mine as a geologist with the brief to assist with any geological problems encountered in any of the Union Corporation controlled mines on the East Rand. As the number of problems grew, and the demand for a larger geological input in solving them became apparent, the geological offices were moved to East Geduld Mines Limited and additional staff were employed. In this way the Mines Geological Department was formed.
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Transactions of the Geological Society of South Africa, 86 (2), 71-80
Rb-Sr whole rock and mineral isotopic analyses of granitoid units in Swaziland have yielded the following ages:
A. Granodiorite Suite ......... 3 350±57 Ma.
B. Mponono Intrusive Suite ......... approximately 2 100 Ma.
C. Sphene-bearing Orthogneiss .......... 2 734±266 Ma.
D. Ngwempisi Granite .......... 2 802±56 Ma.
E. Sinceni Granite ......... approximately 2 800 Ma.
F. Kwetta Granite .......... 2 520±422 Ma.
G. Mhlosheni Granite .......... approximately 2 290 Ma.
H. Hlatikulu Granite .......... approximately 2 420 Ma.
I. Nhlangano Gneiss ......... approximately 2 240 Ma.
Of these ages, only those from the Granodiorite Suite, sphere-bearing orthogneiss and Kwetta Granite are defined by whole rock isochrons. The age for the Ngwempisi Granite is a biotite-whole rock age. The other ages are defined by whole rock errorchrons and are believed to reflect times of imperfect Sr isotopic homogenization in magmas or during metamorphism. The ages of the Granodiorite Suite, sphere-bearing orthogneiss and Ngwempisi, Sinceni, Kwetta, Mhlosheni and Hlatikulu granites are believed to reflect times of emplacement. The ages of the Mponono Intrusive Suite and Nhlangano Gneiss are believed to reflect metamorphism. As such, these age data suggest that a major, previously unrecognized, tectonic event (or events) affected the Precambrian rocks in southern Swaziland between about 2 500 Ma and about 2 100 Ma ago. The full significance of this event has yet to be evaluated.
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Transactions of the Geological Society of South Africa, 86 (2), 81-85
The geology, mineralogy and geochemistry of a number of impure black quartzite layers in the pink gneiss of the Kokerberg Formation are described. All the evidence indicates that these black quartzite layers represent a fossil black sand deposit similar to those often observed on recent beaches. Since the black quartzite layers constitute an integral part of the pink gneiss succession it would appear that this particular pink gneiss is a paragneiss. The provenance for the pink gneiss was mainly granitic with almost no volcanic contribution. As this particular pink gneiss appears to represent the stratigraphically lowest pink gneiss yet recognized it is concluded that the arkosic sandstone which eventually gave rise to the pink gneiss may have been deposited on a pre-existing granitic crust.
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Transactions of the Geological Society of South Africa, 86 (2), 87-91, 2 figs
Environmentally and recreationally undesirable sediment accumulation is taking place in the floodtide dominated Keurbooms Estuary, located in the southern Cape Province. The sedimesnt consists mainly of marine calcareous sand and enters the estuary by barrier over wash and through the tidal inlet. This sediment accumulation could be countered by stabilizing the overwash channels and by fixing the position of the migratory inlet in a position where the longshore current sediment supply to the inlet is reduced. Undesirable side-effects could effectively be countered.
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Transactions of the Geological Society of South Africa, 86 (2), 93-98
Uranium mineralization was found in the Pristerognathus-Diictodon Assemblage Zone of the Teekloof Formation, Beaufort Group, west of Beaufort West, Cape Province, South Africa. All the anomalies can be related to a single mineralization model. Mineralization is found at the termination of a silt parting between two coalescing sandstones and lies in the lower sandstone as an inclined zone dipping down flow from the termination of the silt parting. The existence of a primary Eh-pH gradient is indicated by a uranium-molybdenum zonation, the molybdenum lying above the uranium mineralization. The upper sandstone was an oxidizing fluvial channel in an arid environment through which uranyl carbonate was being transported in solution. Carbonaceous material undergoing anaerobic bacterial breakdown generated a weakly reducing fluid int he lower sandstone. Carbonaceous material at the REDOX front developed between the two mixing fluids at the point of sandstone coalescence reduced uranyl carbonates in solution. Once reduced the uranium minerals remained stable because the conditions in the REDOX front were only very weakly oxidizing. As floodplain aggradation continued, the upper sandstone was buried and the entire sandstone couplet became reducing, permanently stabilizing the uranium mineralization.
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Transactions of the Geological Society of South Africa, 86 (2), 99-104, 5 figs
A systematic survey of bauxite-bearing areas in Natal was initiated by the Geological Survey during August 1978 and this paper contains some of the results. Promising areas were delineated according to three parameters which control the lateritization process, viz, rainfall, geomorphology and geology. The distribution of bauxite on any target was found to be controlled by a sub-surface bedrock topography which is the result of chemical weathering and leaching along joints in the doleritic parent rock. Leaching on a microscopic scale, however, is manifested by a reaction zone between the dolerite and the bauxitic weathering product. The efficiency with which cations are being leached int he reaction zone is dependent on the ability of the joint systems to provide adequate drainage for the cation-enriched solvent. Cyclic variations int he climatological history of the region are proposed to illustrate the relationship between the rates of chemical weathering and erosion which resulted in the observed variation in soil-profile thicknesses between neighbouring targets and on a regional scale.
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Transactions of the Geological Society of South Africa, 86 (3), 169-187
The Insizwa Complex, Transkei, consists of four intrusions showing different levels of erosion. The Tabankulu intrusion consists of a sequence of mafic and ultramafic rocks which occupy an asymmetric basinal depression close to the contact of the Jurassic Ecca and Beaufort group sediments. Emerging from the south-western margin, a dyke 200 m wide penetrates the Ecca Group sediments with a dip of 80° for a distance of 10 Km along strike. The sequence of rock types is divided, from bottom to top, into: (1) the basal olivine gabbro (10 m of cumulus olivine in a groundmass of olivine, plagioclase, and bronzite), (2) the picrite (0 to 300 m of cumulus olivine and spinel and intercumulus plagioclase, bronzite, and augite), (3) the troctolite unit (250 m thick, with nets of cumulus olivine enclosing clusters of cumulus plagioclase crystals), and (4) the upper (Central Zone) gabbros (150 m thick, with varying modal proportions of cumulus and intercumulus olivine, plagioclase, bronzite, and augite). The dyke consists of a marginal olivine gabbro and a core of picrite similar to that of the main intrusion. Mineral compositional data illustrate: (1) an increase over a distance of 10-25 m from the base of the Mg/(Fe + Mg) ratios of the olivine and pyroxene, Ni content of the olivine, and the anorthite content of the plagioclase, (2) a progressive decrease in the Mg/(Fe + Mg) ratio of the olivine and pyroxenes, Ni content of the olivine, and anorthite content of the plagioclase through the troctolite and Central Zone gabbros, and (3) a uniformly high forsterite and relatively low nickel content of olivine within the picrite unit, combined with high Mg/(Mg + Fe) ratios of the intercumulus pyroxenes and low anorthite content of the intercumulus plagioclase. The mineral compositional data are best explained by a combination of: (1) supercooling, gravitational settling, and resorption of olivine in the basal gabbro to account for the reversed fractionation trend at the base, (2) continuous in situ fractional crystallization to account for variation in the troctolite and Central Zone gabbros, (3) equilibration of the magma parental to the basal olivine gabbro and picrite with varying proportions of sulphide to account for the depletion of the olivine in nickel, and (4) multiple intrusion of magmas to permit variable degrees of saturation in sulphur within the basal olivine gabbro, picrite, troctolite. and Central Zone gabbros. Emplacement of the basal olivine gabbro and picrite appears to have been effected up the dyke, which is believed to run the length of the intrusion as a keel. The troctolite and Central Zone parent magmas may have been emplaced laterally, or up an unexposed portion of the feeder. The degree of the depletion of the olivine in nickel has important implications for the exploration for magmatic nickel sulphide mineralization. It is suggested that a large tonnage (140 million tonnes) of sulphide of low grade (0,7 % Ni by weight and undetermined Cu and platinum group elements) may have formed from the magma parental to the picrite. Saturation is not believed to have occurred in situ, but this does not preclude the possibility that the
picrite magma carried some of the sulphide to the surface, and deposited it in depressions along the base of the sheet.
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Transactions of the Geological Society of South Africa, 86 (3), 189-197
The Proterozoic "Composite Reef" on the Randfontein Estates Gold Mine, is a proximal Witwatersrand braided-stream placer, in which pyrite, chromite, zircon, uraninite. and gold are the more common detrital minerals. They range in concentration from a few ppm to over 3 per cent. Optimum concentration of these minerals occurs on scour- and pebble-armoured surfaces, in conglomerate gravel bars, and in trough cross-bedded quartz-arenites. The distribution of gold is, however, complex and the relative proportions of the detrital minerals chance from one depositional situation to another. The abundance of detrital and other related minerals was monitored geochemically and quantitatively indicates the prevalence of optimal placer concentration situations in preserved depositional subenvironments of the "Composite Reef". The relationships between 20 elements were determined by using an R-mode factor-analysis of the geochemical data. The elements load on to chalcophile, detrital oxide, hydrothermal and clay factors, suggesting the consanguinity of four subsets of elements. A multiple linear regression of gold against the other elements provides the framework for an improved prediction of gold where only very small or single samples are available. The method uses many elements in a single sample to achieve statistical reliability, as opposed to the geostatistical method where many samples of a single element are analysed. The regression equation demonstrates the geochemical validity of the geological-response model for optimum gold mineralization. The potassium concentration reflects sericite, which is the main clay mineral, and is antipathetic with gold. Chromium and zirconium, representing detrital chromite and zircon, are also antipathetic with gold. Iron and sulphur, which mainly represent detrital concentrations of colloform pyrite, and uranium and radiogenic lead, representing detrital uraninite, are sympathetic with gold.
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Transactions of the Geological Society of South Africa, 86 (3), 199-209
The upper part of the Dwyka Formation in the eastern Karoo Basin, south of the Tugela River, is represented by a stratified sequence comprising sandy diamictite, siltstone and shale which is comformably followed by carbonaceous shale of the Ecca Group. The sequence is underlain by a homogeneous and massive diamictite which constituents the bulk of the Dwyka succession in the region. Whereas this massive diamictite is interpreted as a subglacial depo sit the stratified rocks am considered to be the products of subaqueous gravity flows, suspension settling and deposition of ice- rafted debris along the terminus of a marine ice sheet. A widespread occurrence of the stratified sequence is attributed to a withdrawal of the ice sheet towards the close of the Late Palaeozoic glaciation. This recession was accelerated by a eustatic rise in sea level which coincided with climatic amelioration and melting of ice. Decouping and uplift of grounded ice by buoyant forces promoted iceberg calving and release of basal debris near the retreating grounding line. Basinward flow of ice and transport of glacial detritus was sustained during the frontal retreat causing the released sediments to blanket subglacial debris which was being exposed in front of the receding ice and which is now represented by the massive diamictite overlain by the stratified sequence. The upward facies change from this sequence to the Ecca shale marks an incursion of wanner water, possibly related to post-glacial isostatic readjustments.
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Transactions of the Geological Society of South Africa, 86 (3), 211-220
Karoo dolerites contain between 0,5 and 4,0 volume per cent of ilmenite and titaniferous magnetite, both of which are relatively late-crystallizing phases. Ilmenite forms discrete euhedral or subhedral crystals as well as more complex subgraphic or vermiform intergrowths with biotite, chlorite or amphibole. The ilmenite is very near to the theoretical FeTi03, end-member composition and the measured unit cell dimensions (hexagonal axes) range between the following: a = 5,0869-5,0892±0,0005 A and c = 14,066-14,077±0,001 A. The reflectivity range in air at a wavelength of 546 mn is R0 = 19,4-20,0 per cent and R1 = 16,4-17,2 per cent. Microindentation hardness is slightly anisotropic and lies within the range VHN = 538-640. None of these parameters are sensitive enough to reflect the small compositional variation shown by the ilmenite. The Ti-magnetite forms anhedral aggregates and commonly displays extensive subsolidus exsolution of ulvospinel and contemporaneous oxidation-exsolution of ilmenite in the form of external granules and lamellae that are oriented parallel to (111). The Ti-magnetite crystals are intermediate members of the magnetite-ulvospinel solid solution series and show variable TiO2-contents (6,0-25,0 %) that reflect the degree to which exsolution has taken place. Measured cell dimensions of the Ti-magnetite range between 8,3950 and 8,3998±0,0005 Å due to variations in the amounts of Fe2TiO4 remaining in solid solution and the variable degree of maghemitization. The effects of subsolidus oxidation and near-surface weathering preclude the use of unit cell dimensions or certain d-spacings as a method for determining Ti-magnetite compositions in Karoo dole rites. The Ti-magnetite displays a wide variety of lamellar ilmenite intergrowths, the development of which is related to variations in fO2 during subsolidus cooling. The sequential development of them contemporaneous oxidation-exsolution bodies in Ti-magnetite provides petrogenetically important information on the cooling histories of their host rocks. The Ti-magnetite also shows varying degrees of late-stage deuteric reaction and alteration. Ti-magnetite is commonly rimmed by biotite which has formed by maction with late-stage fluids in areas shere it is in contact with late-stage residua. Vermiform and subgraphic intergrowths of ilmenite with chlorite and other hydrous silicates represent the decomposition products of pre-existing Ti-magnetite crystalls that contained lamellar ilmenite intergrowths. The Ti-magnetite-bearing component of the intergrowths is replaced by chlorite during late-stage alter ation and the released Ti is precipitated as ilmenite overgrowths on the ilmenite lamellae thus giving rise to the vermiform morphologies.
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Transactions of the Geological Society of South Africa, 86 (3), 221-236
Twelve kimberlite pipes and more than 50 dykes in East Griqualand and neighbouring portions of Transkei and Lesotho are intruded through the Namaqua-Natal Mobile Belt (~1,3 Ga), south-east of the well-known, diamond-bearing kimberlites of the Kaapvaal Craton (>2,5 Ga). The craton boundary is covered by Karoo rocks, but granulite xenoliths are equated with the Namaqua-Natal Mobile Belt. These occur not only in East Griqualand kimberlites but also in diamondiferous kimberlites further north, and it is conjectured that the mobile belt has been overthrust on to the craton. Zircon ages for the Ramatseliso pipe (150 Ma) and the Clarkton pipe (194 Ma) suggest that the eruption of East Griqualand kimberlites spanned the interval of outpouring of the Stormberg basalts (155-187 Ma, Fitch and Miller, 1970). A single carbonate tuff cone (63,4 Ma), associated with incipient rifting, is markedly younger than the kimberlites. The kimberlites are mainly minor intrusions and are variable petrographically and chemically. They are enriched in CaCO3 and Ti but are poorer in silica than the intracratonic kimberlites further north. They are also low in K, Ba, LREE and La/Yb. Minerals that are believed to have crystallized from the kimberlite magma include olivine, mica, ilmenite, spinel group phases, rutile, perovskite, sphere, calcite, apatite and zircon. The spinels (ulvöspinel-spinel-magnetite solid solutions) and perovskites (0,64-0,78 wt % FeO; 0,19-0,75 wt % Nb2O5; 0,66 1,18 wt % Ce2O3) are compositionally closer to equivalent phases in some alnöites than to those in other kimberlites. The olivine (Fe85-92), ilmenites (up to Gk73.6) and spinels are zoned or have overgrowths, and demonstrate crystallization trends that probably resulted from mixing of precursor magmas (protokimberlites). Undisputed diamonds have not been found in any of the kimberlites. A search for low-Ca, pyrope-knorringite (diamond-indicator garnet) in mineral concentrates from Clarkton, Ramatseliso and Zeekoegat has also yielded negative results. The scarcity or absence of these minerals reflects a different tectono-thermal environment of the kimberlite source region from that which existed on the craton.
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Transactions of the Geological Society of South Africa, 86 (3), 238-262
Knowledge of the distribution and timing of continental ice sheets upon Gondwanan continents has expanded since the time of Alex. L. du Toit. In going back in geologic time from the present, the Late Cenozoic glaciation is now flourishing in Antarctica, and was well started by Oligocene times. The Late Palaeozoic ice age ended in mid-Permian times about 260 Ma ago, and commenced at least within the early part of the Carboniferous Period (about 350 Ma ago) and may have begun in the Late Devonian (about 370 Ma ago). This major glaciation therefore lasted for at least 90 and perhaps 110 Ma. Late Ordovician and Early Silurian ice caps thrived between 410 and 440 Ma. Late Proterozoic glaciations are documented upon all of the continents later assembled into Gondwana, but their dating and extent are unsatisfactory. Previous to about a thousand million years ago for another thousand million years, no continental glaciation has yet been documented. Before this long non-glacial interval, and reaching back into latest Archaean time, additional glacial episodes have left a piecemeal record. Palaeozoic ice sheets on Gondwana waxed when the united supercontinent drifted into south polar regions in late Ordovician time, a factor in bringing on the Ordovician-Silurian ice age. Gondwana glided across the pole so that by mid-Silurian time it had moved away from the pole, which then lay beneath palaeo-Pacific waters. By Late Devonian time, the supercontinent had again entered polar regions. In mid-Permian times, it moved away again and rotated and the ending of glaciation is probably related in part to these movements. These interpretations are supported by published Palaeozoic palaeomagnetic data as well as by the glacial record assuming that glaciation occurs in circumpolar regions. With the break-up of Gondwana during the Mesozoic, however, such a simple relationship between polar position and glaciation is no longer discernible, nor can it be documented for Late Proterozoic and Archaean-Early Proterozoic ice ages. An eclectic hypothesis to explain major ice ages on earth seems most reasonable at present, at least for the last thousand million years. Most important are terrestrial influences rooted in the tectonic mobility of the crust, such as the gliding about of continental blocks, as their positionings affect the circulation of ocean and air currents. Complex feedback systems involving albedo changes, raising and lowering of sea level both tectonically and glacially induced, cold and saline bottom-water changes, the opening and closing of gateways between continents and the raising and lowering of basin sills to guide ocean currents, and changes in volcanic and plate-tectonic regimes are among the terrestrial parameters involved. In addition, occasional bolide impacts and perhaps other extra-terrestrial influences may have had their climatic effect. Similar complex interactions are preferred to explain glaciations during the Late Proterozoic. In addition, as we reach for understanding into remote times, changes in atmospheric composition and different mixes of parameters, both terrestrial and extra-terrestrial, will require evaluation.
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Transactions of the Geological Society of South Africa, 86 (3), 263-271
The role played by gravity in the deformation of the Cape Fold Belt of South Africa has been the subject of much discussion, with several workers suggesting that at least some of the deformation was gravity induced. These suggestions stem from the existence of zones of tight, angular northward-verging second-order folds progressively stepped down to the north in the Table Mountain Group of the Cape Supergroup. An investigation of one such zone of folding in the east-central part of the fold belt reveals a picture of second-order folds, with wave lengths of about 10 to 30 m, on the steep northern limb of a first-order antiform. Second-order antiforms are upward facing and second-order enveloping surfaces dip north at about 50 degrees. During the same episode of deformation that led to this folding the Table Mountain Group was thrust northwards about 4 to 5 Km on south-dipping thrust planes to overlie, in places, the younger Bokkeveld Group. The sequence of events in the area of study appears to have been: formation of small angular second-order folds verging north followed by the development of first-order mega-antiforms and synforms; thrusting occurred during the formation of the first-order folds. All deformation was caused by compression directed from the south and gravity played no part in the folding although it may have played some part in the thrusting.
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Transactions of the Geological Society of South Africa, 86 (3), 273-279
Petrographic and whole rock analytical data suggest that obsidian flows (now represented by perlitic pitchstone), rhyolitic breccias, air-fall tuffs and flow-banded rhyolites which crop out in the vicinity of the Nxwala Perlite Mine were derived from a single felsic magma body. Reconstruction of the geological history of these rocks indicates that the Nxwala pyroclastics and lavas represent products of a plinian eruption associated with a period of rhyolitic dome building and dome collapse.
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Transactions of the Geological Society of South Africa, 86 (3), 281-291
In 1978, a small collection of vertebrate fossils was made in the Late Jurassic to Early Cretaceous Kirkwood Foffnation of the Algoa Basin, Cape Province. The specimens included sauropod dinosaurs similar to the North American and European genera Astrodon and Pleurocoelus, an iguanodontid dinosaur similar to the European Mochlocon, and the first African records for the Sphenodontia in the Late Jurassic or Early Cretaceous. In addition, this collection confirmed the presence of camarasairod sauropods in Africa. Other faunal elements previously unreported from the Kirkwood Fo MI ation include Lepisosteidae (garfish), Testudines (turtles), Crocodilia and Theropoda (carnivorous dinosaurs). To the degree now known, the fauna of the Kirkwood Formation has many similarities to Late Cretaceous faunas of the Hell Creek and Lance Creek formations of the northern United States. This probably reflects both the relative ease of terrestrial dispenal during the Mesozoic and physical similarities between the three formations. All three formations appear to be predominantly fluviatile deposits laid down on flood plains near sea level. All three collection areas were roughly the same distance from the palaeoequator. The collections from these two North American faunas are three to four magnitudes larger, and this may explain in part why several groups present in them are missing from the Kirkwood sample. It is, therefore, reasonable to expect that with further collecting, many of these missing groups will be found.
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Transactions of the Geological Society of South Africa, 86 (3), 293-299
The discovery of an angular unconformity in the Sout River area, south-west of Bitterfontein, between the steep-dipping Namaqualand basement and more gently-dipping cover rocks, previously recorded as all belonging to a pre-Nama post-gneiss formation, necessitated re-interpretation of the geology in that area. Two post-gneiss regional metamorphic imprints can be recognized; one pre-Nama of kyanite grade and one post-Nama which gave rise to mineral assemblages consistent with temperatures of 400 to 500°C at a pressure of at least 3 kb. The earlier event was associated with large-scale sinistral rotation of basement structure and increases in degree of metamorphism towards the west. The two events are thought to correlate with the Pan-African isotopic resetting episodes dated at 700 and 500 Ma, respectively.
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Transactions of the Geological Society of South Africa, 86 (3), 301-306
Meta-igneous and metasedimentary rocks, cropping out in a belt about 22 Km long and 5,5 Km wide, are complexly folded as a consequence of three deformational events. Mineral assemblages indicate that the first two deformations occurred under increasing temperature conditions that reached a maximum of the order of 550-600°C, subsequent to D2. The presence of andalusite sets a maximum pressure of about 4 kb for this phase of metamorphism. The supracrustal rocks were intruded by a sheet of leucotonalite prior to D2, shown by the fact that the leucotonalite, the Assegaai supracrustals and their mutual contact share the same pronounced planar foliation. The third deformation was of a less ductile nature and was accompanied by retrogression under lower greenschist facies conditions. Stocks of isotropic, pegmatitic granite intrude the supracrustal sequence and are probably related to complex pegmatites carrying minerals such as beryl, spodumene and monazite. The latter mineral has been dated at 2 960 Ma, providing a minimum age for the supracrustal rocks and the leucotonalite.
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Transactions of the Geological Society of South Africa, 86 (3), 307-309
Professor Eric Stanley Wynne Simpson died suddenly on 28 June 1983 and, with his passing, South Africa lost one of its most distinguished ambassadors in international science - a man well known and highly respected for his leadership, enthusiasm and dedication to the fostering of science and international co-operation. Eric Simpson, son of H.S.K. Simpson, a land surveyor of Dundee, Natal, was born on 23 January 1924. He received his early education at Cordwalles Preparatory School in Pietermaritzburg and between 1937-1941 he was a pupil at Michaelhouse, Balgowan. His love for the sea was doubtlessly reinforced during World War II when he saw active service between 1942-1946 as a naval officer in the South Atlantic, Indian and Pacific oceans. Following the cessation of hostilities he enrolled at the University of Cape Town where it was recognized from the start that he was an outstanding student. He graduated with a B.Sc. degree in 1948 and in 1949 was awarded an M.Sc. degree for his thesis entitled The geology of Mauritius. He then studied at the University of Cambridge from 1950-1952 and obtained a Ph.D. (Cantab.) degree for his thesis entitled The Okonjeje Igneous Complex, South West Africa. During these early years he was awarded several scholarships testifying to his outstanding abilities as a research student. These included the H.B. Webb Scholarship in 1941 and the Elsie Ballot and Union Scholarships in 1950. In addition he was Exhibitioner of Trinity College, Cambridge. Following his studies in England Eric Simpson returned to Cape Town where he took up the post of Lecturer in the Department of Geology, a post he held from 1952 to 1956 under Professor F. Walker who he succeeded as Philipson-Stow Professor and Head of the Department of Mineralogy and Geology in 1957. This post he held until 1974 during which time he established a Sub-Department of Geochemistry which later became an autonomous department in 1961. In 1963 he was instrumental in establishing the Chamber of Mines Precambrian Research Unit and from 1964-1965 was Acting Head of the Department of Oceanography at the University of Cape Town. From 1965-1974 he established and directed the Marine Geophysical Unit (with the Geological Survey) and the Marine Geology Research Group (with the National Committee for Oceanographic Research). His connections with oceanographic research continued to strengthen and from 1974-1976 he held the post of Director, National Research Institute for Oceanology of the CSIR, Stellenbosch. Professor Simpson returned to the University of Cape Town in 1977 and held the post of Professor of Oceanography. During his long and productive career Eric Simpson engaged himself in a wide variety of activities. He held honorary memberships of numerous editorial boards.
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Transactions of the Geological Society of South Africa, 86 (3), 311-312
Henry Clement Melvill Whiteside died quietly and peacefully in the evening of 24 September 1983 in the presence of his immediate family at the age of 72. He had been frail and a semi-invalid for several years before his death but throughout retained his sharp mental faculties and his unfailing courtesy and gentleness. Larry, as he was universally known, was born on 24 January 1911 at Batu Gajah, Malaya where his parents were stationed on a rubber plantation. Batu Gajah is 150 Km north-west of Kuala Lampur. He was an only child and most of his school days were spent in England far away from home where he was sent, as was the custom in those days of expatriates of the British Empire, to board at the preparatory school, Cheam School, from 1920-1925 and to Bradfield College from 1925-1929. His holidays were spent with relatives because the journey each way took several weeks so he rarely went back to Malaya. His academic record was good and he was a more than average sportsman, particularly in swimming and rugby. Whilst at prep school he learnt the art of carpentry and his home in South Africa was adorned by a number of well-made pieces of quality furniture such as a dining room table and a display cabinet. From 1929 to 1934 he attended the Royal School of Mines in London where he obtained a second-class Honours B.Sc. in Mining Geology and won the Boswell Prize for Geological Mapping. He started his career by joining Bancroft's Circus in Northern Rhodesia in 1934 where he worked as a field geologist in the mapping and testing of the Copper Belt. During this time he learnt the basics of bush lore and became a good shot with a wide selection of game in his bag because of the need to shoot for the pot for himself and his field party. In consequence of this he became conservation conscious and gave up game shooting altogether. He worked in both the Rhodesia Minerals Concession Area and in the Luangwa Concession Area. In 1938 he was transferred to Springs where he was to set up the geological laboratory at Springs Mines and to initiate the provision of direct full-time geological input to the operations of the Anglo American Corporation gold mines on the East Rand. It was here that he met Kathlyn Christine Cranny, better known as Paddy, whom he married in 1939. The establishment of the laboratory was quite an innovation in the gold mining industry as, up until that time, geological input on the gold mines had been very much on an ad hoc basis. Larry worked on the East Rand from 1938-1954 and a number of Witwatersrand geologists were inducted into the complexities and difficulties of Witwatersrand geology by him. The laboratory also provided services to the other activities of the Anglo American Corporation and Larry was involved in a lot of outside work. During this time he furthered his studies by submitting an M.Sc. thesis to the University of the Witwatersrand in 1943 entitled Heavy Mineral Analysis of Two Reefs from the Far East Rand, and in 1950 he submitted a Ph.D. thesis to London University entitled The Geology of a Part of the Far East Rand with Special Reference to the Economic Horizons of the Kimberley-Elsburg Series and the Footwall Beds.
His Ph.D. thesis was one of the first to come to grips with the geology of specific mines and areas of the Witwatersrand. About the time the atomic bombs were dropped on Hiroshima and Nagasaki, Larry got involved with uranium and atomic energy. His work on the reefs of the East Rand made him aware of the presence of uranium and its possible importance so it was no surprise to him when early in 1945, Dr George Bain from the United States and Dr C.F. Davidson from the United Kingdom arrived, in great secrecy, to carry out a survey of the uranium potential of the Witwatersrand. It was, however, a surprise to the two visitors that he should have deduced so quickly the objective of their visit. After visits to many mines, an initial request was made by these two for 200 lb. of selected ore from each of four mines, Blyvooruitzicht, Vogelstruisbult, Western Reefs and East Daggafontein. This initial request was followed shortly thereafter by a request for five tons from each of the four mines; the sample had to be of a much higher uranium grade than the run of mine ore. He and Dr Norman Schindler therefore collected five tons from East Daggafontein by visually selecting and checking by geiger counter each and every piece that went into making up the five tons, as did other geologists for the other mines. In 1954, on the same lines, Larry mapped and tested the monazite deposit at Vanrhynsdorp. In 1954 he was appointed a Technical Assistant in Head Office and in 1957 an Assistant Consultant. During the eight years 1957 to 1965 he was involved with AAC's expanding exploration and mining efforts throughout Africa which involved a lot of travelling in diverse geological terranes. Special assignments included trips to Brazil, Australia, Spain and Portugal as well as an investigation of the Phalaborwa copper deposit which was subsequently taken up by Rio Tinto. Gradually his work focused more and more on the gold mines in the Republic so that, on his appointment as Consulting Geologist in 1966, he became responsible for the geological input to the AAC mines on the East Rand, the West Wits line, Klerksdorp and the OFS. Larry retired at the end of 1972 but thereafter for some years he practised as an independent consultant which involved him in quite a diverse panorama of assignments. For example he prepared the updates for quite a number of sections of the 5th Edition of the Mineral Resources of the Republic of South Africa as well as being appointed a Professor in Mineral Exploration at the Universidade de Brazilia from 1976-1978. He was elected to Council of The Geological Society of South Africa in 1957, and appointed Junior Vice-President in 1963. He was President during the 1966/1967 year and he remained on as a Member of Council until 1970. His inaugural Presidential Address was a review of the history of uranium, its discovery in South Africa, its occurrence in the Witwatersrand rocks and its exploitation therein. He also served for many years on the Advisory Committee to the Geological Museum of the Johannesburg Municipal Library. In his private life, Larry was admired and respected for his Christian outlook, for his friendliness and compatibility with
all and sundry. He was adored by his two daughters and his beloved wife and had a most happy relationship with them. His home was a home-from-home for many young geologists who had been recruited overseas by Anglo American Corporation and who were establishing themselves in South Africa. Contacts and friendships made in this manner lasted throughout his life. His friends included people from all walks of life and he was able to maintain contact with them even though they lived in other countries. His passing in the fullness of time was a sadness to all who knew him and we remember him with affection.
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Transactions of the Geological Society of South Africa, 87 (1), 1-10
A sedimentary sequence more than 250 m thick is preserved in the 3 000-Ma Mozaan Group of the White Mfolozi inlier. These relatively undeformed rocks have tectonic dips of 5 to 15 degrees, and rest unconformably on volcanics and subordinate sedimentary rocks of the Nsuze Group. This investigation was initiated because of known gold mineralisation in conglomerates at the base of the Mozaan Group. Data were collected from borehole cores, mine adits, and surface exposures. The sequence is divided into six laterally extensive units. The lower four have a combined thickness of 40 to 60 m, and comprise conglomerates and coarse- to very coarse-grained quartz-arenites. Typically, the conglomerate clasts are well rounded and sorted, and poorly stratified or horizontally bedded with minor graded bedding. The arenites are very mature and are characterised by planar and trough cross-bedding. Palaeocurrent measurements indicate flow to have been predominantly to the north-east. A braided-stream environment is envisaged for this sequence, possibly in the form of a braid-plain or a humid-type alluvial fan. An overall upward-fining trend implies basin transgression during accumulation of these deposits. They are succeeded by a thin mudrock-ironstone unit that is persistent over a distance of more than 15 km, Deposition by suspension setting and chemical precipitation in a shallow shelf setting is postulated. Thin, extensive, matrix-supported conglomerates which overlie this unit are attributed to storm events. These are in turn overlain by a thick fining-upward sequence of sandstones with subordinate mudrock, alternating sand stone and mudrock, and finally mudrock. Sedimentary structures include herringbone cross-bedding and sand-filled desiccation cracks. A regressive sequence from a subtidal environment, through mid-tidal and high-tidal is envisaged.
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Transactions of the Geological Society of South Africa, 87 (1), 11-18
A suite of nine marine benches in the Alexandria karst area of the Eastern Cape, was found to extend from Kei Mouth to Cape St Fran cis. The evidence was then related to that from carlier studies. The conclusion reached is that the suite of benches is widespread and of more than local significance, that the benches are of undoubted marine origin and that they span the entire Cenozoic period -that at 240 M being the oldest as it supports Palaeogene sediments. The three highest benches arc all Tertiarv in age and were created by three transgressive-regressive phases responsible for the deposition of the coastal limestone, Archaeological evidence, with supporting C14 dates, indicates that the 8- to 5-m bench was cut between 25 000 and 30 000 years B.P. Absence of differential movement along the coast was not established.
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Transactions of the Geological Society of South Africa, 87 (1), 19-33
Sixty-three samples of drift pumice, collected on the coasts of South Africa, East Africa, Madagascar, Mauritius, the Cocos Islands, Australia, Indonesia, Brazil, Marion Island and Bouvet Island, were investigated Petrographically and geochemically with a view to establishing the possible source areas. Geochemically five distinct groups could be distinguished and some could be linked to specific eruptions in the Indian, Atlantic and Pacific Oceans. Group-A pumice originated from a submarine eruption off Zavodovski Island in the South Sandwich Island Group in 1962. Pumice from this eruption was distributed to Marion, Bouvet and Kerguelen Islands, and then to Tasmania and New Zealand. The pumice of Group B occurs mainly, on beaches bordering the Atlantic Ocean, and was found on the west coast of South Africa, on the sea-floor south-west of South Africa, and in Brazil, Two specimens drifted up the coast as far as East London on the south coast of South Africa. The source of this group is unknown, but all the evidence indi-cates that it must have been from the Mid- Atlantic Ridge in the South Atlantic Ocean. The Group-C pumice was found only on the west coast of Australia. The source is in the southern Indian Ocean, probably from the Mid-Indian Ridge. The fourth group originated from a submarine eruption along the Tonga Trench in the Pacific Ocean, and this was distributed to the east coast of Aus tralia; one sample crossed the Indian Ocean and was found on a South African beach. Group E, which is by far the most homogeneous, includes samples from Australia, Indonesia, the Indian Ocean islands, East Africa and South Africa. It also also includes three samples of undisputed Krakatoan origin. No doubt exists that this group origi nated from the 1883 eruption of Krakatoa. After one century, speci mens from this eruption are still found on the beaches of the coun tries bordering the Indian Ocean, and it is still the most abundant type of drift pumice that can be found.
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Transactions of the Geological Society of South Africa, 87 (1), 35-39, 4 figs
Geikia elginensis, a highly specialised edentulous dicynodont from the Scottish Upper Permian (Tatarian = Dicynodon lacerticeps Assem blage Zone), is reconsidered and shown not to be a member of the family Lystrosauridae, as was considered to be the case in a recent publication. Geikia may be related to Dicynodon locustices Huene from Tanzania and, therefore, may be a member of the family Aulacephalodontidae, and closely related to Pelanomodon. The masticatory apparatus of Geikia is very different from that of Lys trosaurus and reinforces the belief that the two genera are not at all closely related.
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Transactions of the Geological Society of South Africa, 87 (1), 41-44
An unusual primary sedimentary structure, believed to have been produced by a small mudflow, was found in the Gross-Aub Formation, Fish River Subgroup, South West Africa/Namibia. This subgroup spans the Cambrian-Precambrian boundary. The structures appear in swarms and resemble flattened "mudstone-fingers" some 20 cm long, 5 to 10 cm wide and 2 to 4 cm thick. They frequently have ridges on their upper surface and "snout". The mudstone-fingers are enveloped in a silty mudstone package believed to be the parent material for the individual "fingers".
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Transactions of the Geological Society of South Africa, 87 (1), 45-51
Lake Malawi is the southernmost major water-body in the Cenozoic rift valley system of Africa. Despite its potential value, particularly as an analogue of ancient sedimentary troughs, little is known about sedimentary processes within the lake. Echosounding in the vicinity of the North Rukuru River defines a narrow littoral shelf with water depths of less than 10 m, an escarpment with slopes of up to 20° descending to depths of 100 m, and a deep lake floor continuing to depths of at least 200 m on gradients of less than 1°. The escarpment and deep lake floor directly off the river mouth show marked irregularities with amplitudes of up to 12 m and jagged profiles. This rough bottom is considered to be due to gravitational movement of unstable sediment deposited directly from the river - an interpretation supported by terrestrial plant debris, including a large tree trunk, trawled from a depth of 100 m on the irregular lake floor. Consideration of factors affecting sedimentation in the large tropical freshwater rift lakes indicates that these systems are particularly prone to mass-gravity transport processes.
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Transactions of the Geological Society of South Africa, 87 (1), 53-55
Potgieter and Oelofsen (1983) illustrate spectacular examples of the trace fossil Cruziana acacensis from some 400 m below the top of the Peninsula Formation, thereby suggesting a Silurian age for much of the latter formation. Their conclusions, however, are questionable on a number of grounds. Originally, the marine faunule from the Cedarberg Formation was dated as late Ashgillian on the basis of the Brachiopoda (Cocks et al., 1970) and the Chitinozoa (Cramer et al., 1974), but is now generally accepted as early Llandovery in age (Berry and Boucot, 1973a, b; McKerrow, 1979). However, because Potgieter and Oelofsen (1983) choose to regard Cruziana acacensis as a Silurian index, these determinations are thrown out of the window. Trace fossils are notoriously facies controlled, nor can it be said with conviction that the stratigraphic range of a single ichnospecies is known to perfection. Thus, any local zonation based upon trace fossils is largely a reflection of depositional environments in the area concerned. To extend such local biostratigraphy to a global scale is unwise and likely to prove erroneous. Moreover, it is most improbable that a trace fossil of unknown trilobite origin is likely to have greater biostratigraphic (i.e. chronostratigraphic) significance than a rapidly evolving group such as the Brachiopoda. On these grounds alone the Silurian age inferred for the Peninsula Formation examples of Cruziana acacensis must be rejected. Potgieter and Oelofsen (1983) note that Crimes (1975) regards C. acacensis as diagnostic of the late Silurian, but they merely refer to it as a Silurian index, presumably because they are not prepared to accept a late Silurian age for the middle of the Peninsula Formation. Unfortunately, they do not say as much. Thus, although Crimes (1975) regarded C. acacensis as a late Silurian index, Potgieter and Oelofsen (1983) have arbitrarily extended its range downwards so as to include the early Silurian as well.
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Transactions of the Geological Society of South Africa, 87 (1), 59-67
Although South Africa is committed to a system of free enterprise, Government plays an important role in the mining industry because it not only performs many regulatory functions, but also supplies some vital ingredients necessary to stimulate and encourage investment for the optimum utilization of its large mineral wealth. By following its enlightened policy, South Africa has succeeded in mobilizing its vast and varied mineral resources into one of the most dynamic and successful mineral industries in the world and has also earned the reputation of being a reliable and consistent supplier of minerals. This paper describes the philosophy of the South African Government on developing its mineral resources and the policy it pursues to encourage development. It also describes the mining laws, tax structure and the role the Government plays in providing infrastructure, sophisticated research and technical services in the fields of geology, metallurgy, marketing, health, and safety.
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Transactions of the Geological Society of South Africa, 87 (1), p57
Biographical memoir and bibliography.
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Transactions of the Geological Society of South Africa, 87 (2), 101-109
Sediments deposited by a high-constructive lobate delta are described from the Lower Permian Vryheid Formation in the Transvaal. These deposits are well exposed at Rietspruit coal mine in the southern Witbank Coalfield north of the Smithfield Ridge. Upward-coarsening cycles typify the interval between the No. 4 & 5 coal seams. Basal carbonaceous siltstone grades upward into interlaminated siltstone & sandstone, medium- to coarse-grained sandstone and, finally, coal which caps the sequence. A lower upward-coarsening cycle is overlain by two clastic wedges composed of asymptotically based, low-angled, giant foresets. The lower giant foresets erosively overlie strata below, while upper foresets are not erosively based but are tangentially interbedded with siltstone below. Coarse-grained to gravelly trough cross-bedded & flat-bedded sandstone terminates the sequence with basal scour surfaces eroding the underlying sandstone wedges. Facies assemblages, sedimentary structures, and geometry are analogous with lobate, high-constructive, shallow water delta systems. Prodelta siltstones grade vertically into distal bar deposits of interlaminated siltstone & sandstone which display steep depositional slopes. Sand dominated distributary mouth bar sediments overlie these distal bar deposits, and erosively based, coarse-grained, distributary channel sandstone and gravel end the sequence.
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Transactions of the Geological Society of South Africa, 87 (2), 111-124
Stilpnomelane, ferri-annite and riebeckite are important alkali-bearing silicates from which physico-chemical conditions of riebeckitisation in very low-grade metamorphosed iron-formations can be deduced. The chemistry, mineral assemblages and occurrences of these alkali-bearing silicates are described from the Kuruman Iron Formation and compared with those of the Brockman Iron Formation in Western Australia. The phase relations of the alkali-bearing silicates are constructed in the system Na-K-Fe-Mg-Si-O-H on the basis of available and predicted thermodynamic data and the mineralogical-petrological data from this study, ideal chemical formulae used in the construction of the phase diagrams. K-rich stilpnomelane is probably stable at the boundary region between stilpnomelane and ferri-annite. Ferri-annite and/or K-rich stilpnomelane is, therefore, a good indicator of riebeckitisation in iron formations. At fO2 as low as the magnetite-pyrite-pyrrhotite buffer, riebeckite and ferri-annite are seldom found; instead Fe-rich ferrostilpnomelane is dominant. This phenomenon is well explained by the phase relations of this study.
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Transactions of the Geological Society of South Africa, 87 (2), 125-140
Material in the author's collection is used for presenting the biostratigraphy and palaeogeographic distribution of the faunas of the Bokkeveld Group in tabular form. No significant temporal changes in the fauna can be detected, but differences appear to exist between the assemblages of the eastern & western parts of the sedimentary basin. The biostratigraphic distribution of trilobites is controlled by lithology.
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Transactions of the Geological Society of South Africa, 87 (2), 141-159
The Archaean granitic basement of the Barberton region, Eastern Transvaal, represents an ideal locality for an integrated study of the relationships between geology & geomorphology. The degree of erosion and the nature of landforms in tis granitic terrane are clearly shown to be related to a number of parameters including granite type (i.e. compositon & mineralogy), texture, grain size, and porosity. Three distinct erosion surface (termed the Highveld, Intermediate and Lowveld surfaces) are shown to transect the crystalline basement in this region. Significant differences in the depths of weathering mantles occur between the various surfaces, this being a function of the age of the erosion surface and the climatic conditions prevailing during the formation of the regolith. The evolution of the planation surfaces is related to sporadic episodes of epeirogenic uplift with respect to the continental margin; an amended chronology for the formation of the surfaces is suggested in the light of recent sedimentological information obtained from off-shore drilling programmes. A compete understanding of the geomorphological evolution of the Eastern Transvaal basement can only be obtained if consideration is given to the diversity of geological variables that characterize what has up until fairly recently been regarded as a relatively homogeneous granitic terrane.
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Transactions of the Geological Society of South Africa, 87 (2), 161-168
The Kruitfontein boulder pavement, with its veneer of grooved tillite, overlies a massive lodgement tillite. The pavement was formed by selective lodgement processes during palaeo-ice flow from the east. The grooved tillite was also deposited subglacially and the water-saturated sediment, largely derived from abrasion of the boulder bed, deformed plastically during ice movement. The laminated shaly diamictite, which conformably covers the grooved tillite, was deposited in either a subglacial or proglacial lake by suspension settling of clays, debris rain of coarse particles, and minor debris flows during stagnation or a temporary recession of the ice front. When the ice re-advanced, arenaceous tillite was deposited unconformably on top of the lake sediments.
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Transactions of the Geological Society of South Africa, 87 (2), 169-179
The case for recognition of tectonostratigraphy as an independent scheme of stratigraphic subdivision is presented. These unconformity-bounded units of genetically related depositional facies are shown to be the ideal stratigraphic scheme for basin analysis and for synthesis of the geological history of depositional basins because tectonic control and depositional response can be related. Models of tectonostratigraphic units are analysed, elucidating the principle that every depositional discontinuity contains an isochronous element within its hiatus equal to the least break in time between contiguous units. This principle enables the analyst to establish numerous relative time-surfaces through any sedimentary basin without resorting to fossils or other methods of age-dating, thus creating the essential time datums without which the plaeogeology cannot be accurately reconstructed. Practical basin analysis is done from a combination of structural and chronostratigraphic sections across the basin. A tectonostratigraphic unit is an unconformity-bound chronostratigraphic unit and is basin-restricted unless the bounding unconformities are controlled by eustatic sea-level fluctuations. It follows from the known hierarchy in dimensions of cyclic deposition and from chronostratigraphic experience that a ranking of tectonostratigraphic units exist. Rank terms are recommended for formal use of this stratigraphic scheme by SACS and that body will have to decide on the nature of the specific names of units. The Phanerozoic geological history is then reviewed employing tectonostratigraphical analytical techniques on a regional scale to demonstrate the power of this analytical tool.
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Transactions of the Geological Society of South Africa, 87 (2), 181-198
The ancient mineral explorers, although having similar objectives, could not have been committed to the stringent modern requirement for an acceptable financial return on exploration expenditure. National minerals policies are set variously to secure metal supplies, to encourage local production and to generate foreign exchange. These policies depend largely on the countries' commitment to free enterprise, socialism or a development away from a subsistence economy. Discovery statistics for Canada, Australia & the USA are available from government agencies and have been summarized by various authors. Discovery success has been computed from these costs in terms of the ratio of exploration expenditure to the number of economic mineral discoveries and the expected value of, and the rate of return on, this investment. It is demonstrated that the multiplier effect of the exploration dollar is such that mineral exploration is successful in specific regions, but that on average the return on all exploration expenditure in a region, by virtue of discovery value, is not adequate when considering the risks involved. Thoroughbred mining companies, however, show an exceptionally good rate of return on this investment and a relatively low overall expenditure per discovery, easily offsetting the risk aspect endemic in mineral exploration. Statistics quoted show this risk to be considerably less than generally perceived if the individual mining company beats the average success ratio by applying a totally professional approach to the discovery procedure. South African discovery statistics have been assembled largely from a survey of company annual reports. The ratio of discovery success, or the average cost per discovery, is demonstrated as being considerably better for South Africa than the other countries studied. Although Canada has achieved considerably more mineral discoveries at lower cost than Australia, the eventual value of Australian discoveries is on average higher. South Africa's remarkable success ratio must, to a certain measure, be due to a mineral-rich geological environment where exposure is relatively good and where the regular stratigraphy of the Witwatersrand and Karoo Sequences, and the Bushveld Complex, has led to numerous major discoveries. Exploration expenditure expressed as a percentage of pre-tax profits, of selected mining groups, indicate that the conglomerate South African mining companies spend, on average, approximately 5 to 6% of their pre-tax profits on mineral exploration. This percentage is obviously diluted by non-mining revenue, whereas exploration expenditure of the true mining companies averages approximately 20 to 25 % of profits. The latter, relatively higher expenditure, is an indication of investor interest in achieving higher returns for larger risk, with an eye on the windfall discovery. Successful mineral discovery is only achieved when undertaken on a continuing basis and by the correct management of a selected and highly motivated professional exploration team. Corporate philosophy, objective policy, and strategy should be clearly determined. The criteria recognized for successful exploration are, generally:
money, experience, perseverance, technical excellence, imagination, ideas, flexibility, time, confidence, and security. The company that dissipates a good discovery orientated geological team, or cuts back radically on exploration expenditure, is taking the decision to get out of the mineral business. The corporate risk of not taking the exploration is taking the decision to get out of the mineral business. The corporate risk of not taking the exploration risk is high. The understanding of geological concepts of ore deposition is now fundamental to the discovery process. Companies by non- professional organizations is fraught with danger. Mineral discovery will, however, continue and there is enough discovery potential on land to make ocean mining inappropriate for some time to come, & space mining is only for dreamers.
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Transactions of the Geological Society of South Africa, 87 (2), 73-78
U-Pb isotopic data indicate that the sporadic high concentrations of uranium (up to approximately 250 ppm), found in pegmatites and granitoids in the Precambrian rocks which comprise part of the Natal basement, were introduced during very extensive metamorphic and igneous activity which occurred in southern Natal about 1 050 Ma ago. The uraniferous pegmatites and granites probably represent highly differentiated products that were initially derived from a source region characterized by a bulk-earth U/Pb ratio. Uranium lost recently from the pegmatites and granites has been reconcentrated along veins and in weathered basement rocks which are characteristically biotite-rich or carbonaceous. There is no indication of the presence of more extensive zones of U enrichment beneath the present erosional surface. Apart from the recent secondary redistribution, there is no evidence to suggest that the uranium in the pegmatites and granites has been mobilized since 1 050 Ma ago.
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Transactions of the Geological Society of South Africa, 87 (2), 79-85
A comparison of the true bearings derived from sun compass readings & magnetic bearings recorded in the vicinity of a banded iron-formation at Gamsberg, North-western Cape is made as a case study on the reliability of the magnetic compass for recording bearings in the vicinity of a magnetic body. The difference between the true & magnetic bearing is an estimate of the local magnetic declination for that locality & values of local magnetic declination thus derived are divided into seven domains based on distance from the iron-formation exposure. The distributions for these seven domains are analysed for kurtosis & skewness using "g" statistics and for dispersion using the Mood test. The iron-formation itself shows negative kurtosis (flat-topped) while those of all other domains show positive kurtosis (sharp peaked), except that furthest from the iron-formation which shows a normal distribution. The results show that the Gams Iron Formation has a significant effect on the accuracy of the magnetic compass up to 800 m from its exposure. Comparison of true bearings derived from sun compass readings and magnetic bearings can be used to determine the distance from magnetic lithologies at which the magnetic compass can be reliably used to record bearings.
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Transactions of the Geological Society of South Africa, 87 (2), 87-99
The structural geology of the West Rand Group at Swartkops, situated 10 km to the north of Krugersdorp, has been re-investigated. Palinspastic reconstructions of the geology reveal that stratigraphic thicknesses of various formations & members are less than the values obtained from the Central & West Rand. These differences can be accounted for, in part, by tectonic thinning associated with nappe- and thrust-formation. The structure of the are can best be accounted for by means of a system of thrust faults, as was suggested by Hendriks (1961). The general vergence of the system is towards the north. A uniformly oriented fabric is associated with this deformation and is a result of a simple shear strain. The fabric has an east-west orientation ad dips towards the south at 30°, indicating that the overlying layers, or thrust slices, have moved in a northerly direction. A fold axis & lineations are orientated in a north-south direction and lie in the fabric plane, indicating that both pre- and syn-deformational linear structures tend to orient themselves int he direction of the tectonic transport. Tensile phenomena, revealed by boundins, foliation boudins, and quartz veins, are orientated in a dominantly east-west direction. The combined data are suggestive of a tectonic movement which was directed from the south towards the north. observations in the rocks surrounding Swartkops indicate that this deformational period also involved successions which are currently correlated with the Ventersdorp Supergroup, but did not affect the Transvaal Supergroup. Estimates of strain suggest shortening of, at least, 6 to 7 Km. If the same degree of shortening were applied to the entire area from Swartkops, through the Witwatersrand outcrops at Honingklip, to the West Rand Syncline, a contraction of 40 Km would be involved. These estimates of strain are much greater than have been considered previously applicable to Witwatersrand rocks.
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Transactions of the Geological Society of South Africa, 87 (3), 199-210
Diamictite between Pringle Bay and the coast east of Cape Hangklip has previously been correlated with the Pakhuis Formation on lithological grounds. Structural and stratigraphic evidence indicates that the diamictite is a separate unit of restricted extent within the Peninsula Formation. The assemblage differs from the Pakhuis in respect of deformed sandstone megaclasts, tongues of pebbly sandstone especially towards the base, and absence of a "fold zone" immediately below it. The poorly exposed overlying quartz wacke and siltstone are lithologically unlike shales of the Cedarberg Formation. Clastic dykes in the upper part of the diamictite relate to fissuring while pore water was being expelled from the semi-compacted mass. The formation probably represents an accumulation of subaqueous debris flows originating under a floating ice shelf. Scanty fossil evidence combined with roughly calculated sedimentation rates suggests that the Hangklip diamictite is 8 to 10 Ma older than the pakhuis Formation. A glacial eposide in the mid-Caradocian Epoch of the Ordovician Period is implied.
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Transactions of the Geological Society of South Africa, 87 (3), 211-223
The Timbavati Gabbro is a group of shallowly dipping sills of ultrabasic to basic rocks intruded along conical fractures in the Swazian granite-gneiss basement rocks of the eastern Transvaal Lowveld, South Africa. A study has been made of the geochemistry of the sills and the data obtained leave little doubt that the Timbavati Gabbro was emplaced as a number of separate intrusive pulses involving at least three different rock types. These were derived by periodically tapping magma from a deep-water chamber in which A was crystallising fractionally. The compositional variations of the magma in the chamber are reflected by the average compositions of the individual intrusions which also show evidence of mixing with pre-existing cumulates. Superimposed on these variations are the post-emplacement fractionation trends of each of the intrusions, which generally parallel the overall trend, but show divergences in certain cases.
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Transactions of the Geological Society of South Africa, 87 (3), 225-232
Clasts from platform and valley-facies provinces of tillite in Southern Africa were considered with respect to their size-shape-lithology relationships. Generalised shape values, with respect to lithology, indicated a difference in shape, particularly roundness, between the two facies. A lithologic control of clast size was noted and size-shape relationships were seen to vary greatly with lithology. A variety of glacial transport paths and the added effects of reworking further complicate analysis, particularly in the platform facies where the final deposit may be the product of several glacial stages plus interglacial subaerial processes. At one site a knowledge of the sediment genesis was needed to help interpret the shape indices. Detailed analysis of clast size and shape with respect to lithology, although time consuming, is considered a useful tool in the investigation of tillites and far more work is needed on the glaciogenic sediments of Southern Africa. The results show that, with respect to previous studies on Dwyka sediments, the suggested qualitative size-shape relationships are not valid and that much more rigorous approaches are needed in future.
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Transactions of the Geological Society of South Africa, 87 (3), 233-236
The mean bulk density of 40 sediment samples from the shallow-marine environment around Southern Africa was found to be 1,55 g/cm³, with a mean porosity of 40,39%. Bulk densities decreasing mean diameters of the sediments and also decrease with increasing CaCO3 contents. No significant correlation was found between bulk density and sediment sorting in this particular data set. On the basis of these results it is recommended that conversions from sediment volumes (e.g. m³) to sediment masses (e.g. metric tons), or vice versa, should make use of a mean bulk density of 1,5 g/m³. Thus, 1 m³ of unconsolidated marine sand would have a mass of 1,5 metric tons, or 1 metric ton would have an equivalent volume of 0,667 m³. Strictly, these values apply only to quartz-carbonate mixtures commonly found around the South African coastline. If bulk densities for specific particle sizes are required, then the regression equation y = 1,77-0,1 x should be used, shere y is the bulk density and x is the mean particle size.
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Transactions of the Geological Society of South Africa, 87 (3), 237-244
The Upper Triassic Molteno Formation of the Natal Drakensberg forms part of the Upper Karoo Sequence. It rests with an erosional base on the Beaufort Group and has a gradational upper contact with the Elliot Formation. Four lithofacies are defined in the study area. Basal coarse-grained, trough-crossbedded sandstones, interpreted as in-channel dune and bedload deposits, are commonly overlain by fine- to medium-grained, planar-cross-stratified sandstones. The latter probably reflect the migration of bar bedforms within braided fluvial systems. These sediments normally pass up into fine- to medium-grained sandstones with trough and planar cross-beds, laid down by sand-flat processes which reworked emergent bars. The succeeding argillites of the lithofacies are postulated to be overbank or inter-channel floodplain sediments. The sequence is normally terminated by a second development of the bar facies. The vertical lithofacies arrangement probably represents deposition in low-sinuosity channel systems, which derived detritus from east-north-east and south-east-south source areas. Although this palaeoenvironment is very similar to the depositional models of I.C. Rust (1962) and Turner (1970 to 1983), bar migration and sand-flat processes appear to have been more important in the north-east of the basin.
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Transactions of the Geological Society of South Africa, 87 (3), 245-256
The Witwatersrand basin was filled with sediments during the Early Proterozoic when vegetation was absent and the atmosphere was probably deficient in oxygen. During the erosion-sedimentation cycle, and after burial, the sediments were subjected to a number of consecutive mechanical and geochemical processes each changing or even erasing the records of the preceding one. Although the information provided by the current mineral assemblages indicate that most of the mineralogical transformations required reducing conditions, some of them could also have occurred in a weakly oxidising environment. Notwithstanding the difference in terrestrial, atmospheric, and hydrospheric conditions during the Early Proterozoic, it is possible from the mineralogical records to establish the processes and chemical mechanisms that operated before and after the Witwatersrand sediments were deposited.
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Transactions of the Geological Society of South Africa, 87 (3), 257-272
The Hotazel Formation of the Proterozoic Griqualand West Sequence is a relatively undisturbed unit of mixed volcanogenic-chemical sediments, containing the world's largest land-band repository of manganese. Fifteen drill cores, sampled lithologically at intervals of approximately 1 m through the total sedimentary sequence, were studied by microscope, X-ray diffraction, and electron-microprobe methods. The mineralogy was studied on a regional scale, with the emphasis on the manganese minerals. A brief lithological description of the manganese ore and the relate rocks is given. Mineralogically the Kalahari basin can be subdivided into two specific types of areas where there are differences in mineral distribution and ore composition, but no obvious sedimentological differences throughout the entire rock sequence. Two-thirds of the basin comprises diagenetic to very low-grade metamorphic sediments, the so- called Mamatwan-type environment, in which the minerals show virtually no alteration other than diagenetic influences. The diagenetic minerals encountered are kutnahorite, bementite, braunite, hematite, rhodochrosite, jacobiste, and hausmannite. The remaining one-third, the north-western portion of the basin, has partially been altered by hydrothermal solutions. Evidence for the hydrothermal alteration is found in the presence of the minerals acmite, andradite, baryte, bixbyite, and the braunite group of minerals, while kutnahorite is completely absent in this environment. This area has been called the Wessels-type environment. Mineral assemblages commonly found in the Hotazel Formation during this investigation are given in the appendix.
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Transactions of the Geological Society of South Africa, 87 (3), 273-279
The supergene transformation reaction: pyrrhotite → marcasite + Fe(2+) + 2e was simulated in the laboratory using a modified soxhlet apparatus. In geological terms the reaction is very fast and the first signs of the transformation is visible within 14 days. The reaction is apparently coupled with an electron-consuming reaction in which pyrrhotite is directly altered to goethite. The latter reaction only occurs under highly oxygenated conditions, whereas the marcasite formation requires very low oxygen partial pressures. Thermodynamic calculations as well as observations in nature support the experimental results. The deferration* of sphalerite could not be detected in the same series of experiments, which may indicate low rates of diffusion, or alternatively, the presence of other chemical or electrochemical barriers.
* Used in this paper to indicate the loss of iron.
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Transactions of the Geological Society of South Africa, 87 (3), 281-286
The future exploitation of low-grade nickel resources in the Republic of South Africa may depend on the availability of low-cost beneficiation technology such as acid-bacterial leaching. However, the application of such a process is largely dependent upon the mineralogical suitability of the ore. Important factors include the mode of occurrence of nickel (and other economic metals), permeability of the ore, ore mineralogy, ore textures, and gangue mineralogy. The type and degree of hypogene and supergene alteration experienced by magmatic nickel-sulphide deposits is the principal controlling influence in the development of these parameters.
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Transactions of the Geological Society of South Africa, 87 (3), 287-295
This paper describes the role of the mineralogist in the development of a copper-mining venture, and demonstrates how geological and metallurgical problems can be solved with the aid of mineralogical techniques. It is shown that, at each stage of development, namely exploration, laboratory and pilot plant testwork and, finally, full-scale plant operation, determination of mineralogical parameters is important. The mineralogical aspects of exploration, mining, milling, smelting and refining, are emphasized. Although the examples given apply specifically to the copper industry the techniques and methods described are applicable to most mining ventures.
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Transactions of the Geological Society of South Africa, 87 (3), 297-302
X-ray diffraction continues to be a useful technique for the qualitative determination of crystalline phases in both industrial and geological materials. Despite dramatic improvements in instrumentation, X-ray diffraction cannot yet be regarded as a truly quantitative technique, since accuracy of results is strongly dependent on crystallite perfection and size. The authors examine a few of the factors influencing analytical accuracy, and present experimental results to emphasize the importance of controlling particle size and particle orientation when preparing samples for X-ray diffraction analysis. Used in conjunction with other analytical techniques, X-ray diffraction nevertheless remains a very powerful supplementary analytical tool.
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Transactions of the Geological Society of South Africa, 87 (3), 303-314
Chromite-bearing and related peridotitic rocks are present in the Southern Marginal zone of the Limpopo Metamorphic Complex. The regional, field, structural and chemical aspects of these rocks are discussed in relation to their mode of emplacement. Field, petrographic, and chemical data indicate that them peridotites, which have strong alpine characteristics, may represent relics of a pre-existing greenstone belt that has survived a granulite-grade tectonometamorphic imprint.
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Transactions of the Geological Society of South Africa, 87 (3), 315-325
The Murchison greenstone belt is characterized by heterogeneous deformation and a dominantly steep, east-north-east-trending schistosity. Upright, tight to isoclinal folding (F1) about steep east-north-east-plunging axes forms part of the earliest recognizable deformation event. Subsequent deformation episodes are of more local significance ranging from a (S2) crenulation cleavage related to F2 "s" asymmetric folding, to late kink band formation (S(3) cleavage) related to box folding. The final deformation comprises a series of brittle fractures. Two areas studied in detail, the Gravelotte Shaft Quarry and the Free State Ore Body, part of the Antimony Line at Monarch Mine, demonstrate the strain heterogeneity of D(1). The quarry occurs within the competent metasediments of the "Antimony Bar" where sedimentary structures are still recognizable. In contrast the Antimony Line is an upright shear zone where D(1) structures overprint S(0) bedding fabrics. Stress-related solution and subsequent precipitation of quartz, carbonate, and sulphide minerals in tensional fractures in competent lithological horizons along the Antimony Line played a part in ore body formation.
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Transactions of the Geological Society of South Africa, 87 (3), 327-333
The antimony-gold deposits of the Murchison Antimony Line show several different styles of mineralization. Minor mineralization occurs as disseminated stibnite grains within massive quartz-carbonate rock, and may represent a pre-metamorphic concentration. The major style of mineralization is as quartz-carbonate-stibnite veins occupying brittle fractures in massive quartz-carbonate rock; in the Monarch Mine, most structures are of D(1) age, whereas in the Alpha Mine, D2 age structures appear to dominate. Most of these veins show features indicative of crystallization from hydrothermal fluids, with limited superimposed deformation and annealing recrystallization features. Carbonate-bearing schist adjacent to massive quartz-carbonate rock also contains stibnite-bearing veins. Some of these were emplaced during D(1), but others are transgressive to D(1) age structures and represent secondary hydrothermal remobilization of early mineralization during later deformation events. Local bodies of massive sulphide ore show extensive development of deformation features (foliation, tectonic brecciation, mylonitization), but these cannot be related to specific deformation events. The study has not been able to resolve the question of whether the major mineralizing event was pretectonic or syntectonic.
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Transactions of the Geological Society of South Africa, 87 (3), 335-345
Maranda J is one of several Archaean volcanogenic copper-zinc deposits along the "Copper-Zinc Line" of the Murchison greenstone belt. The deposit consists of a stratiform lens of massive sulphide rock stratigraphically above "stringer" mineralization in a chloritic alteration zone. The deposit has been deformed and metamorphosed at middle greenschist facies. Wallrock structures indicate two main phases of deformation: D(1) deformation is characterized by isoclinal folds, a strongly developed spaced cleavage, flattened pyroclastic fragments, and syntectonic quartz veins; D(2) deformation resulted in more open folding and development of a spaced cleavage. Two main varieties of massive sulphide rock can be recognized. Banded massive sulphide rock, rich in pyrite, is a relatively competent rock; although grain shapes are a result of metamorphic crystallization, the rock retains an original bedding. Where the rock composition is dominated by ductile minerals, especially by chalcopyrite and pyrrhotite, plastic flow has destroyed original structures and produced a tectonic breccia consisting of fragments of brittle rock in a matrix of fine recrystallized sulphides which possess a secondary foliation. While the structures in this foliated massive sulphide rock cannot, with confidence, be correlated with wall rock structures, the brecciation and foliation probably developed during the D(1) deformation event. Competency contrast between massive sulphide rock and silicate wall rock has resulted in narrow zones of shearing along their interfaces. Some sulphide rock has been injected plastically into the wall rocks to produce dyke-like and irregular apophyses. The generation of a fluid phase during metamorphism has resulted in partial solution of mineral constituents and the development of secondary hydrothermal (lateral secretion) sulphide-bearing quartz veins.
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Transactions of the Geological Society of South Africa, 87 (3), 347-359
The structure of the Murchison greestone belt has been investigated using deep geoelectrical soundings and gravity data. Twenty-one Schlumberger soundings with maximum current electrode spacings of 4 to 30 Km were carried out. The Gravelotte Group is found to have an electrical resistivity between 1 000 and 3000 ohm metres whereas values for the Rooiwater Complex and the granite-gneiss complex exceed 10 000 ohm metres. Although the limited width of the belt complicates the interpretation of the resistivity data, the results indicate that the maximum depth extent for the Gravelotte Group is between 8.8 and 12.3 Km, and that most of the belt has a thickness of less than 4.5 Km, depth extent and geophysically determined thickness being coincident. The gravity data combine a regional survey (about 700 stations) by the Council for Scientific and Industrial Research with a detailed investigation (about 2 000 stations) of the Geological Survey. The Bouguer anomaly map shows that positive gravity anomalies are associated with the Murchison Sequence, the maximum residual being over the Novengilla Gabbro Suite of the Rooiwater Complex. Interpretation of the bouguer gravity data along profiles across the schist belt shows that the greenstone assemblage reaches its greatest thickness a few kilometres north-east of Gravelotte. The absolute maximum was found to be 12 Km which is in agreement with the geoelectrical interpretation. It is also clear that the thickest part of the Rooiwater Complex is offset in a left-lateral sense with respect to the thickest part of the Gravelotte Group and that the entire Murchison Sequence seems to be affected by left-lateral shearing. The Free State Diorite and the late-stage Maranda and Baderoukwe granite plutons seem to be intruded into shear zones, the one associated with the Maranda Granite showing a close correlation with the "Antimony Line". It may be significant that the producing antimony mines occur where the schist belt extends deepest.
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Transactions of the Geological Society of South Africa, 88 (1), 1-9
Modern columnar-layered and tabular "crinkled" stromatolites, which are presently being formed, have been found in the Wondergat sinkhole (25 km east of Mafikeng) in the Malmani Subgroup of the Transvaal Sequence. In this paper the two types of stromatolites are compared and their environment of development is examined in detail.
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Transactions of the Geological Society of South Africa, 88 (1), 109-133
The genesis of the most important types of Archaean gold deposits in Zimbabwe - namely goldbearing iron-formations, auriferous volcaniclastic sediments, and vein and shear-zone deposits - can be interpreted within the framework of the evolving Rhodesian craton for which previous workers have identified three periods of greenstone-belt development - the 3,5 Ga Sebakwian Group, the 2,9 Ga Lower Greenstones (Bulawayan Group), and the 2,7 Ga Upper Greenstones (Bulawayan Group) and Shamvian Group. Syngenetic and epigenetic gold mineralization occurs in the banded iron-formations. In general, the level of gold production from these chemical sediments, quantified in terms of kg Au/km BIF, bears no relationship to the type of intercalated volcanic and clastic rocks. However, a time-disparity is evident with a much greater gold production coming from iron-formations within ultramafic (dominantly komatiitic) and tholeiitic sequences of late Archaean (ca 2,7 Ga) age compared to chemical sediments in similar volcanic settings in the early and mid-Archaean (3,5-2,9 Ga). Epigenetic gold deposits in ultramafic rocks are relatively rare, reflecting in part the high ductility of such lithologies, and also their low potential as source-rocks due to removal of a significant proportion of the gold by magmatic sulphide-segregation and/or sea-floor alteration processes prior to metamorphic or magmatic hydrothermal leaching. A close association between gold mineralization and intrusion of a late-Archaean trondhjemite-tonalite-granodiorite suite (the Sesombi Suite) is explained by a number of complex and inter-related variables including the source(s) of the granitoids, the diapiric nature and synkinematic timing of emplacement, and the generation of high f(O2), high SO2/H2S fluids capable of efficient gold transport. The importance of gold mineralization in Archaean terranes can be explained by the favourable combination of many of the above factors, particularly high mantle heat flow and related fumarolic activity throughout much of the Archaean, followed by widespread low- to medium-grade metamorphism and synkinematic intrusion of granitoids in the late-Archaean, and the optimum interaction of these processes in rift zones which developed in response to Archaean extensional tectonics.
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Transactions of the Geological Society of South Africa, 88 (1), 11-17
Lenticular bodies of sugilite skarn are described from the Wessels Mine. The skarn forms a zoned sequence and the respective zones are characterized by the presence of sugilite, andradite, wollastonite, pectolite, vesuvianite, quartz, and glaucochroite. Locally layers of portlandite are also developed. The zoned bodies are classified as infiltration metasomatic skarns, formed by the action of highly alkaline hydrothermal fluids on the lower manganese ore body and siliceous banded iron-formation at Wessels Mine. A temperature of between 400 and 600°C and a pressure of less than 1 kb are postulated for their formation.
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Transactions of the Geological Society of South Africa, 88 (1), 135-148
The majority of Archaean gold production from Western Australian greenstone belts (ca. 2100 t Au) has come from mineralization in shear zones, quartz veins or stockworks in volcanic or intrusive host rocks, particularly tholeiitic basalts or dolerites, in sub-amphibolite facies metamorphic domains. Significant production has also come from deposits hosted by banded iron-formation (BIF), and from volcanic/intrusive hosts in amphibolite facies domains; the latter includes the Big Bell deposit. Most deposits have a strong structural control, and an important feature is the great depth extension (> 1,5 km) of lodes even where strike length is relatively short. A common characteristic of most deposits is extensive wall rock alteration involving carbonation, K-metasomatism, and pyritization with contemporaneous gold deposition. There is commonly sub-horizontal, but rarely vertical, zonation of this alteration. Available constraints on gold mineralization in both volcanic/intrusive and BIF hosts (e.g. (1) timing of mineralization, (2) nature of fluid, (3) physicochemical conditions of metal transport and deposition, and (4) nature of fluid channelways), are compatible with a metamorphic-replacement model for gold mineralization. In this model, the ore fluid and ore components were derived by devolatilization, in the absence of significant melting, of dominantly volcanic sequences during high-grade metamorphism at the base of the greenstone pile. Gold was transported as a reduced sulphur complex (e.g. HAu(HS)2) in reduced, neutral to slightly alkaline, H2O-CO2 fluids of low salinity and low density. Fluid access to depositional sites was via faults and shear zones. Deposition occurred at minimum P-T conditions of 1 to 2 kb and 300 to 400°C, largely in response to fluid-wall rock interaction which resulted in sulphidation of Fe-rich host rocks, with synchronous gold and Fe-sulphide deposition; other reactions that control pH and fO2 may be important and explain the enormous range in characteristics of some large and most smaller deposits. The origin of metamorphosed gold deposits in amphibolite facies domains is more equivocal, and genetic models are strongly debated. On the regional scale, the Norseman-Wiluna belt, interpreted to be a young (ca. 2,8 Ga) rift-phase greenstone belt, hosts most of the larger volcanic/intrusive-hosted gold deposits. Younger (ca. 3 Ga) platform-phase greenstone belts (Murchison and Southern Cross Provinces) and older (ca. 3,SGa) platform-phase greenstone belts (east Pilbara) are progressively less well mineralized. Most of the BIF- hosted gold deposits occur in the younger platform-phase belt. The great intensity of gold mineralization in rift-phase greenstones appears to be related to the greater extension and shorter interrelated volcanic/tectonic history of the belts. Important aspects appear to be: (1) the greater eruption of komatiites (potentially gold-enriched source rocks), (2) generation of thick, rapidly buried and relatively unaltered, volcanic sequences, (3) high thermal gradients during regional metamorphism, and (4) the presence of
major reactivated syn-volcanic faults that focused fluid flow during the gold mineralization event. There is a marked concentration of large gold deposits along the axis of the interpreted rift.
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Transactions of the Geological Society of South Africa, 88 (1), 149-158
A detailed study of fluid inclusions from vein quartz associated with Archaean gold deposits in Western Australia indicates that gold deposition was from low salinity (< 2 wt % NaCl equiv.), H2O-CO2-rich (ca. 20-30 mol % CO2), alkaline to near-neutral fluids with densities of 0,7-0,8 g/cm³. Data from Red Hill suggest that Na/K ratios were ca. 9 and that H2S and N2 were present in the ore fluid. Vein quartz was deposited over a temperature range of 250 to 400°C and at pressures between 1 and 2 kb; the main gold mineralization event could have been at slightly higher temperatures and pressures. Early, minor phase separation occurred due to fluctuating P-T conditions, and later phase separation took place in limited, late-stage vugs at pressures as low as ca. 500 bars. Fluid/wall rock interaction is indicated by the inferred presence of CH4 only in those inclusions from veins adjacent or in proximity to carbonaceous host rocks. Importantly, there are no detectable vertical temperature gradients over at least 200 m of exposed single lode systems, and no evidence from fluid inclusion studies of other physical controls on gold deposition; fluid/wall rock interaction is the most likely major mechanism. The fluid inclusions, in isolation, cannot define unequivocal genetic models for gold mineralization. However, when combined with other constraints, they define a metamorphic-replacement model for gold genesis in which metamorphic fluids derived by devolatilization of lowermost greenstones under conditions of high-grade metamorphism were channelled upwards via faults and shear zones to the depositional sites; gold and associated sulphides were deposited by fluid/wall rock interaction at elevated P and T. The fluid inclusions in vein quartz of Archaean gold deposits may provide an exploration tool; data from barren veins are generally absent.
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Transactions of the Geological Society of South Africa, 88 (1), 159-173
Gold deposits are relatively uncommon in higher metamorphic grade parts of Archaean greenstone belts, but this setting does contain some major deposits. Referred to as "Big Bell/Hemlo-type", these deposits have common characteristics that may include: occurrence in high strain domains near granitoids and greenstone belt margins, minor to negligible carbonate alteration haloes, lack of original textures, and a lack of thick quartz veins. Metamorphic and geochemical studies are of particular importance in deciphering Big Bell/Hemlo deposits; and retrogression, partial melting, and decarbonation should all be considered as possible processes in their evolution. These processes may be rather difficult to recognize. There is considerable difficulty deciphering original rock types around the Big Bell deposit, and there are strong reasons to believe many rocks were altered before peak metamorphism. In particular, the main host rocks to gold mineralization appear to be altered mafic rocks, based on their immobile element ratios. Metamorphism was accompanied by dehydration, desulphidation, and probably decarbonation. Minor partial melting of selected parts of the sequence is suggested by the inferred P-T regime for the area, low Na2O, high and erratic K/Rb ratios and quartz-feldspar veinlets. The waning stages of metamorphism involved introduction of fluids and retrogression of the prograde assemblages, especially of the more permeable micaceous units. A full chronology is unavailable due to the lack of critical timing evidence. Caution is needed in making genetic interpretations where precursors and timing are not fully resolved. A metamorphic replacement model is favoured for Big Bell on the basis of supportive geochemical evidence, and the significant problems with alternative models. It appears likely that many features of the deposit are the result of metamorphic and deformational processes, subsequent to mineralization. The Hemlo deposit of Canada has many similarities.
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Transactions of the Geological Society of South Africa, 88 (1), 175-177
The Helderberg, located near Stellenbosch, Cape Province, is a near-linear topographical feature which is notable mainly because it lies cross wise to the grain of the basement. It is an erosion relic of sandstone of the Table Mountain Group lying unconformably on a basement of Malmesburgy Group rocks and granite. The sandstone was originaly continuous with the sandstone of the Cape Peninsula. Recently the ridge and slopes of the Helderberg were surveyed by the author in an attempt to establish the reason for its apparently anomalous position and orientation. Two possible reasons for the preservation of the Helderberg outlier have been considered. Firstly, the Table Mountain sandstone building the upper part of the mountain may have been rendered resistant to weathering due to physical features such as folds, faults, joints, or metamorphic effects. Secondly, the nature of the basement may have determined the attitude of the mountain.
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Transactions of the Geological Society of South Africa, 88 (1), 179-182
Nodules consisting of a mixture of siderite and calcite from a banded iron-formation lithofacies and chamosite nodules from an ironstone lithofacies have, in common, non-gradational boundaries, barrel and drop-like shapes, associated granules (of similar composition to the nodule), and soft-sediment deformation of the host-sediment. The soft-sediment deformation which includes bending, shearing, displacing, and folding of laminae can be attributed to nodule growth in uncompacted sediment. This implies that the nodules are of very early diagenetic age.
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Transactions of the Geological Society of South Africa, 88 (1), 183-185
Discussion by J.N.J. Visser. The application of tectonostratigraphy to South African geology, especially in large basins where stratigraphic units are diachronous, is well illustrated in this paper by Winter (1984). However, it is in his analysis of the Cape and Karoo basins where other interpretations can also be made with the available field data. Author's reply to discussion: The points raised by Professor Visser are welcomed in that certain aspects of the paper can do with clarification or re-emphasis. In his concluding remarks, Visser repeats my statements at the end of the chapter dealing with relationship to units of other classifications, namely the integration with other geological disciplines. Interpretation of any basin analysis remains subjective, and is coloured by the data available to the analyst, as well as loss of data caused by basin destructive agencies, hence the quotation presented under the title of the paper. We can only approach the goal of the ultimate answer, as Visser puts it very aptly, but in attempting to attain that objective, we need to understand and use available analytical tools logically and correctly. In my paper, the tectonostratigraphy scalpel was modelled and honed to fit essential requirements as one of the analytical tools necessary for any type or size of stratified depositional body.
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Transactions of the Geological Society of South Africa, 88 (1), 187-188
Discussion by M.J. Jordaan. The paper by Winter (1984) is a welcome contribution and an attempt to clarify and promote a concept which has been neglected in the past, but which is now being applied successfully in certain fields of South African stratigraphy. Inconsistencies in the classification, correlation, and ranking of stratigraphic units can be resolved by observing their genetic relationships. The Karoo Sequence allows much room for discussion in this regard and, unfortunately, in this section of his paper Winter raises a number of debatable points which require further discussion. Author's reply to discussion. I appreciate the encouraging remarks about the tectonostratigraphic concept. Let us leave the details of nomenclature to be resolved by competent workers such as yourself and discuss the principles involved, whereby decisions are to be taken as to where isotime surfaces occur on which the series of palaeogeographic maps upon which geological history is to be based, must be drawn. In my paper, it is concluded that key beds and unconformities provide such levels. The latter also provides information about the tectonics involved in the development of a depositional basin margin. Every stratigrapher agrees that regional stratigraphic analysis mut be based on time-stratigraphy (chronostratigraphy).
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Transactions of the Geological Society of South Africa, 88 (1), 189-194
The South African rock record contains a rich heritage of pre-Devonian formations, some of which may require reinterpretation with regard to environment of deposition. Modern analogues of fluvial systems operating in a vegetation-free environment on a scale to be expected in pre-Devonian times do not exist. Palimpsest geomorphological aspects of modern systems further reduce their usefulness. Pre-Devonian conditions would have favoured developmeth of braided, bedload-dominated, fluvial systems. Theoretical analysis of form suggests that channels would have been wide and shallow. Imperfect candidate modern analogues, Platte River, Icelandic Sandurs, and Yellow River, support theoretical considerations. Coastal aggrading terminal areas of large pre-Devonian bedload-dominated rivers may have no modern counterparts, but features of the Indo-Gangetic Plain may be relevant. Du Toit's views on the origin of the Table Mountain Group are re-examined in the light of the above considerations and a fluvial origin is supported for most of the succession.
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Transactions of the Geological Society of South Africa, 88 (1), 19-26
The Upper Triassic Elliot Formation of the Natal Drakensberg and north-eastern Orange Free State forms part of the Upper Karoo Sequence. It has gradational contacts with both the underlying Molteno Formation and the overlying Clarens Formation. Three lithofacies are distinguished within the study area. Subordinate cross-bedded sandstone lenses of Facies 2 and sheet-like Facies 3 sandstones are randomly arranged within the predominant structures argillites of Facies 1. The Facies 2 sandstones are thought to represent channel deposits and the argillites interchannel sediments, within an arid area anastomosing fluvial palaeoenvironment. Facies 3 lenses may be sheet-flood deposits, laid down under periodically more arid conditions. Petrographic examination of Facies 1 samples suggest that aeolian processes also played a role within the proposed palaeoenvironment. Deflation of sedimentary material in and around the Elliot basin possibly added loessic dust to the fluvial systems; alternatively, fine interchannel sediments may have been subjected to aeolian reworking. Fluvial source regions lay to the south-south-east and appear to have been characterised by sedimentary material, possibly belonging to pre-existing Karoo units. The palaeoenvironment suggested for the present study area is in general agreement with the fluvial and aeolian depositional processes proposed for the Elliot Formation by previous workers who have postulated that aeolian deposition only occurred in the Upper Elliot sediments. In contrast, this study indicates that fluvial and aeolian processes interacted throughout deposition of the formation at least in the Natal Drakensberg.
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Transactions of the Geological Society of South Africa, 88 (1), 27-32
Heavy mineral distribution patterns in the sandy soils of the area drained by the upper reaches of the Harts River constitute a fabric which points to a multifold provenance for these soils. A partly saprolitic, partly aeolian, and partly palaeofluvial origin is suggested. From the latter, the possible existence of a Tertiary watershed, north-east of the andalusite-bearing hornfelses of the Western Bushveld Complex, is deduced for the study area.
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Transactions of the Geological Society of South Africa, 88 (1), 33-36
3 figs, 2 tables. Although large-dimension tufa deposits am widely distributed through out the drier karst areas of Southern Africa, current active tufa precipitation is rare. This report records a site of limited, but rapid, tufa deposition at Gorrop on the Ghaap Plateau escarpment. The relationships between slope gradient and water chemistry were moni tored in January 1984. The factors enabling the maintenance of wfa activity at this semi-arid site are considered.
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Transactions of the Geological Society of South Africa, 88 (1), 37-48
17 figs. The poorly exposed Dwyka Formation in Bushmanland and Namaqualand consists of massive, clast-rich, arenaceous and clast-poor, aargillaceous diamictite, bedded diamictite, massive carbonate-rich diamictite, basement-derived breccias, dropstone argillite, and fine- to coarse-grained sandstone. These lithofacies represent former lodgement and subglacial melt-out tills, dropstone diamicton deposited by basal melting of a floating ice shelf, debris-rain deposits from icebergs, debris-flow diamicton, subglacial outwash beds, and turbidite sand deposits. The waterlain diamictons accumulated exclusively in the wide Sout River and Krom valleys which were at least 300 m deep, and the Namaqua basin. An older ice-flow pattern, when glaciers flowed southwards, is apparent from the erosional features in the area. Deposition, related to a younger ice-flow pattern, was primarily by an ice sheet with major lobes in the north and east. A large interlobe area extended from east of Kenhardt to Loeriesfontein. The northern source supplied largely basement rock debris, whereas the ice flowing from the east transported mostly sedimentary rock detritus from the Ghaap Plateau and surroundings. The complex lithofacies relationships can thus be attributed to differences in ice-flow direction, transportation paths, depositional processes and basement topography, and post-depositional alteration.
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Transactions of the Geological Society of South Africa, 88 (1), 49-59
Mafic gneisses ("amphibolites") form a minor, but distinctive, part of the amphibolite-granulite facies terrane of the Broken Hill Block, Australia. Along with inferred felsic metavolcanics, they are restricted to parts of the Willyama Supergroup stratigraphically equivalent to and below the position of the Broken Hill massive sulphide deposit. Clastic metasediments (mainly K-rich) comprise the bulk of the sequence. The mafic gneisses dominantly consist of deformed and metamorphosed flows and/or sills in which magmatic features have been mostly destroyed. Hornblende-plagioclase amphibolites form a mineralogically and chemically uniform subgroup of regional extent, and are compositionally equivalent to metamorphosed Fe-rich tholeiites (up to 15 wt % FeO). Around the Champion Mine, major element compositions vary systematically and suggest a tholeiitic differentiation trend. Similar Fe-rich tholeiites are found in continental rift environments-the setting favoured by the authors for Broken Hill. Exceptionally Fe-rich mafic gneisses (15-25 % FeO) form a distinctive subgroup of mafic gneisses that is spatially related to areas of sulphide mineralization, and has no modern analogue among volcanic rocks. As many of these rocks have prograde metamorphic assemblages, they are inferred to have undergone pre-metamorphic alteration. Their unusual composition cannot be attributed to partial melting. The exceptionally Fe-rich mafic gneisses are footwall to the major Broken Hill Pb-Zn-Ag ore body and extend for several kilometres along strike. These rocks have higher Na2O contents (1,5-2,7 %) than other mafic gneisses of the Willyama Supergroup, and lower MgO and CaO contents. It is suggested that these rocks formed synchronously with mineralization, by alteration that probably involved seawater. Similar mafic gneisses are known from only one other locality in the Broken Hill Block (Silver King), 40 km north-north-west of Broken Hill. Another type of exceptionally Fe-rich mafic gneiss is common for tens of metres around many sub-economic occurrences of base metal sulphides. These rocks have high Fe, Mn, K, Ti, and P, and low Na (less than 1% Na20), Mg, and Ca contents, compared to other mafic gneisses of the Willyama Supergroup. Their origin is attributed to early metamorphic alteration by fluids that reflect the overall chemical nature of the hosting sequence. It is not possible to determine the alteration assemblages that led to the exceptionally Fe-rich compositions, because of probable devolatilization during regional metamorphism. Chlorite, carbonates, and muscovite are likely, with additional albite or paragonite involved in the alteration around the Broken Hill deposit. Neither the Kuroko nor Canadian Archaean massive sulphide models provide ideal lower metamorphic grade analogues for the Broken Hill deposit.
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Transactions of the Geological Society of South Africa, 88 (1), 61-64
The object and assumptions of geostatistics are discussed, in particular assumptions about the mean and variance of a regionalized random variable. This leads to a discussion of the variogram and its properties, and the relation between geostatistics and classical statistics. The experimental variogram is introduced and, finally, kriging is discussed as a method to obtain unbiased estimates and variances for those estimates. This estimation technique utilizes the dependence between observations, whereas dependence between observations is regarded as a "nuisance" in classical statistics.
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Transactions of the Geological Society of South Africa, 88 (1), 65-67
The use of geostatistical methods in the estimation of ore reserves for feasibility studies and later for selective mining and grade control are well known. Applications of geostatistics in mineral exploration such as the design of sampling campaigns (i.e. sample size, spacing, ect), the design of sample preparation methods, and the selection of analytical techniques, are often neglected. This paper highlights the importance of the use of geostatistical concepts in the various exploration stages of a mining venture through the analysis of two case studies.
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Transactions of the Geological Society of South Africa, 88 (1), 69-72
The underlying statistical concepts in defining confidence limits for grade estimates are explained briefly for two situations. The first situation is where the data used is confined to a statistical sample with the population, i.e. the ore body concerned, and the second where additional data, not drawn from the ore body itself, but correlated with it, is introduced. The latter case corresponds to the use of the geostatistical technique of kriging; it also provides a probability distribution for the unknown mean grade of the ore body which can be used in a risk analysis and feasibility study for the exploitation of the ore body. The principals are demonstrated for a case study of the hige-grade section of the Free State Geduld Mine which was originally valued on five borehole results, including the famous Geduld No. 1 borehole. It is shown that, compared with the results of subsequent underground development in this section, valuations based on the five boreholes only gave poor results. A considerably improved estimate would, however, have been obtained had the additional data from the adjacent Western Holdings Mine, and from the "super" population forming the block of six mines from Free State Geduld southwards, been introduced via a geostatistical kriging procedure.
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Transactions of the Geological Society of South Africa, 88 (1), 73-76
The present investigation is mostly concerned with the contribution that the method of kriging can make towards the interpolation of water levels and reducing the number of observed gravity measurements necessary for a reliable estimate of the volume of weathered dolomite in the dolomitic areas of South Africa. Firstly, a number of universal kriging procedures such as: (a) universal kriging with polynomial and trignometric base functions; (b) Fletcher's penalty function applied to an intrinsic function of order 1; and (c) the trend assumed to be known a priori, are applied to the Toppenish Creek water levels. It turns out that the use of a pre-determined polynomial trend yields the best results. Secondly, the kriging method is applied to the Bouguer anomalies obtained from a study on a part of the Zuurbekom compartment. A reduction of 90 % of the number of measurements gave rise to only a 1,1 % loss in accurancy of the calculated volume of weathered dolomite.
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Transactions of the Geological Society of South Africa, 88 (1), 77-79
It is shown, using a simulated deposit, that a generalized indicator kriging approach renders an estimated distribution that improves remarkably upon the raw histogram in an exploration situation with sparse data.
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Transactions of the Geological Society of South Africa, 88 (1), 81-82
Geostatistics has been applied to measure regionalization of both physical and quality variables important in mining coal. That is, variograms have been used to measure spatial correlation of coal in all directions. This has been successfully implemented: 1. For evaluation of sampling programme effectiveness; and 2. For estimating tonnage and quality of mining panels. There are a large number of variables associated with coal mining, and each has an individual regionalization requiring independent treatment. Suggestions are put forward to control this situation.
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Transactions of the Geological Society of South Africa, 88 (1), 83-97
This paper deals primarily with the methods used by the Atomic Energy Corporation of South Africa, Limited (formerly NUCOR) in arriving at the assessment of the South African uranium resources. The Resource Evaluation Group of NUCOR's Geology Department is responsible for this task, which is carried out on a continuous basis. A major reassessment exercise is undertaken every second year to coincide with the compilation of South Africa's contribution to the joint Nuclear Energy Agency (NEA)/International Atomic Energy Agency (IAEA) publication "Uranium Resources, Production and Demand". The latest South African assessment was completed during 1983 and consisted of the evaluation of 44 mines and more than 200 deposits and occurrences. The evaluation is done on a property-by-property basis and relies upon data submitted to NUCOR by the various companies involved in uranium mining and prospecting in South Africa. Resources are classified into Reasonably Assured (RAR), Estimated Additional (EAR) and Speculative (SR) categories as defined by the NEA/IAEA Steering Group on Uranium Resources. Each category is divided into three cost categories, viz. resources exploitable at less than $80/kg uranium, at $80-130/kg uranium and at $130-260/kg uranium. Resources are reported in quantities of uranium metal tat could be recovered after mining and metallurgical losses have been taken into consideration. Resources in the RAR and EAR categories exploitable at costs of less than $130/kg uranium are now estimated at 460 000 t uranium. The evaluation of a uranium venture is carried out in various steps, of which the most important, in order of implementation, are: geological interpretation, assessment of in situ resources using techniques varying from manual contouring of values, geostatistics, feasibility studies and estimation of recoverable resources. Because the choice of an evaluation method is, to some extent, dictated by statistical considerations, frequency distribution curves of the uranium grade variable are illustrated and discussed for characteristic deposits.
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Transactions of the Geological Society of South Africa, 88 (1), 99-107
A pilot gold ore reserve system was initialy developed to calculate ore reserves using geostatistical techniques. It soon became apparent that the real power of the system lay in having all the available data captured in a convenient form in a centralized data-base from which relevant information could easily be extracted. Uses of the pilot system, to date, include ore reserve calculations, evaluation of different geostatistical techniques, contour maps as an aid to geological interpretation, histograms and statistical information, identification of off-reef mining, and control of external waste.
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Transactions of the Geological Society of South Africa, 88 (2), 195-206
The Central Zone of the Limpopo Belt was intruded at ca. 2 650 Ma by a suite of mantle-derived dykes with basanite to phonolite compositions. Subsequently, these dykes were deformed and metamorphosed during a regional retrogressive amphibolite event. This suite may be indicative of a period of extensional tectonics that followed a rapid uplift of the region which had been caused by rebound of a subducted slab. The dykes possibly represent manifestations of a late Archaean metasomatic event in the upper mantle beneath southern Africa.
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Transactions of the Geological Society of South Africa, 88 (2), 207-214
Clinopyroxenites, which comprise over two-thirds of the Phalaborwa Complex are made up of clinopyroxene, phlogopite, apatite and microcline. These four minerals vary in proportion forming a gradational series of rock types from glimmerite (100% phlogopite) to apatite-rich and apatite-poor massive pyroxenite (100% clinopyroxene). Flow textures, inch-scale layering, and microscopic textures suggest that magmatic processes dominated in the formation of these rocks. Phlogopite primarily crystallized from a melt and did not form from metasomatism of clinopyroxene or olivine. Apatite cocrystallized with clinopyroxene, phlogopite and microcline and, therefore, was not introduced later hydrothermally. A history of multiple intrusion of magma with a high proportion of crystals is indicated by the numerous inclusions and cross-cutting relationships.
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Transactions of the Geological Society of South Africa, 88 (2), 215-223
The Pienaars River Alkaline Complex (PRAC) comprises a group of volcanic and subvolcanic centres which crop out to the north and north-east of Pretoria, South Africa. A variety of rock compositions, from peralkaline foyaites to oversaturated quartz syenites, are represented. Sixty new whole rock Rb-Sr isotopic analyses are presented for eight of the complexes. The results indicate that the PRAC was emplaced over a protracted time period beginning with the development of the Leeuwfontein volcanic-subvolcanic complex at 1430 Ma, and closing with a significant igneous event at 1330 to 1300 Ma when the compositionally diverse centres at Leeuwkraal, Franspoort, Buffelsdrift, Klipdrift and Haakdoornfontein formed. The intrusion of the Premier group of kimberlites probably represents the closing phases of this igneous event. There is no apparent relationship between age of emplacement and geographic location or chemical composition within the PRAC units. Initial Sr ratios for these alkaline units are low and it is argued that the involvement of continental crust in their genesis is unlikely.
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Transactions of the Geological Society of South Africa, 88 (2), 225-243
Circular magnetic anomalies are present between Pretoria and Warmbad below a cover of Karoo sediments and volcanics. Drilling on one of these anomalies on the Farm Elandskraal 71JR in the Warmbad district has revealed that at least one of these anomalies is due to a pre-Karoo alkaline volcano, similar to the post- Waterberg Roodeplaat caldera north-east of Pretoria. A geological map of the Elandskraal volcano has been compiled based on magnetic data together with the available borehole information. Data show that the Elandskraal volcano is a typical shield volcano formed by multiphase eruptions which were extruded on a floor of Bushveld granite or rhyolite of the Rooiberg Group. A comparison between the Elandskraal volcano and the Roodeplaat caldera shows that the former also belongs to the Pienaars River Complex of alkaline intrusions. It lies on the northern extension of the "Franspoort line" and its emplacement was controlled by the same structural feature that facilitated the emplacement of other members of the Pienaars River Complex. Petrographically the rock types at the Elandskraal volcano can be divided into alkaline basalts, trachytes and pyroclastic breccias. The latter are interbedded with the lava flows. The trachytes can be further subdivided into quartz-normative variolitic trachyte and nepheline-normative prophyritic, and glassy trachytes. The alkaline basalts can be divided into microcrystalline and glassy basalts. Major and trace element data show that all the lavas are distinctly alkaline in composition. These data demonstrate the existence of two different magmatic lineages similar to those at the Roodeplaat volcano. The one, a basalt-trachyte lineage, shows a systematic chemical variation in both major and trace element compositions, which is consistent with accepted models for fractional crystallization, and which can be explained by the early fractionation of clinopyroxene, magnetite, ilmenite, apatite and possibly orthoclase or leucite solid solution. The accumulation of this mineral assemblage in a subvolcanic magma chamber could give rise to the formation of a pluton of alkaline pyroxenite. The second magmatic lineage is represented by the variolitic trachyte, which shows very little evidence of fractional crystallization. The data show that a relationship by fractional crystallization between the variolitic trachyte series and the basalt-trachyte series is extremely unlikely, and that while the latter series can be explained by fractional crystallization in the mantle the variolitic trachyte must have formed by some other process. The formation of immiscible liquids and crustal anatexis of previously fenitized zones surrounding earlier magma chambers are two mechanisms that have been considered to account for the variolitic trachyte lineage. Both mechanisms can account for the observed data but the latter is referred as it is also able to account for the north-south chemical zonation of the intrusives of the Pienaars River Complex.
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Transactions of the Geological Society of South Africa, 88 (2), 245-248
The Colossus kimberlite is one of several such occurrences situated in central Zimbabwe. A relationship with alluvial diamond deposits in the same area has been suggested, and on this basis a pre-Permian age has been inferred. The general geology and petrography of the Colossus kimberlite pipe are described. Rb-Sr age measurements on somewhat weathered kimberlite micas are reported, and the best estimate for the age of the kimberlite is reported as 502±47 Ma. A model Rb-Sr age of 2630 Ma for the Formona granite, which forms the country rock to the kimberlite, is also reported.
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Transactions of the Geological Society of South Africa, 88 (2), 249-266
Rb-Sr pholopite age determinations, interpreted as emplacement ages, are reported for 15 southern African kimberlites. Jagersfontein and Rietfontein (85 en 95 Ma) have ages typical of the majority of well-known Cretaceous kimberlites, whereas somewhat older ages of about 118 to 125 Ma have been obtained for localities in the Postmasburg, Barkly West and Boshoff districts (Finsch, Bellsbank, Poortjie and the New Elands-Blaaubosch-Roberts Victor Group). Previous zircon ages of 90 Ma for Finsch and Roberts Victor are believed to be incorrect. Two other localities in the Barkly West area, Frank Smith and Newlands, have significantly younger emplacement ages of about 114 Ma relative to most Barkly West occurrences. Two off-craton kimberlites, Uintjiesberg and Mzongwana, are 100 and 150 Ma in age respectively. Swartruggens (western Transvaal) and Elandskloof (eastern Transvaal) have ages of 150-160 and 165 Ma, respectively. A Barkly West occurrence, Klipfontein, also has an apparent age of 160 Ma, but this result cannot be considered reliable. The emplacement ages and initial 87Sr/86Sr ratios of southern African Jurassic and Cretaceous kimberlites that have been dated define two distinctive populations in accord with previous whole-rock isotopic studies. With the exception of the East Griqualand occurrences (150 and 190 Ma), Group I kimberlites are generally less than about 114 Ma in age with 87Sr/86Sr ratios of about 0,704 whereas Group II kimberlites are generally older than about 114 Ma with initial 87Sr/86Sr ratios of about 0,708. The two varieties of kimberlite must be derived from distinct types of sources, possibly by distinct processes. It is shown that old crustal biotite grains present as contaminants in the New Elands kimberlite are only partially reset during emplacement, resulting in spurious ages if not detected. Since very fine-grained crustal biotite is not isotopically re-equilibrated with kimberlite, it is premature to conclude that all phlogopite xenocrysts.
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Transactions of the Geological Society of South Africa, 88 (2), 267-280
Major and trace element compositions of southern African kimberlite samples previously analysed for isotopic compositions confirm that isotopically defined Group I (basaltic) and Group II (micaceous) variants have distinctive geochemical signatures. These signatures are generally consistent with geochemical variation in petrographically defined types (e.g. Dawson, 1967). Stepwise discriminant function analysis is used to define the most important geochemical distinctions at the group level and to derive a procedure which successfully classifies a large number of unknowns based on chemical composition only. In comparison to Group I, Group II kimberlites have consistently higher abundances of SiO2, K2O, Pb, Rb, Ba and LREE, and lower abundances of TiO2 and Nb. In conjunction with isotopic results, the distinctions in incompatible element contents in particular are believed to reflect broad differences in source rock character. Results are consistent with derivation of Group I kimberlites from asthenospheric-like sources similar to those from which oceanic island basalts are produced. In contrast, Group II kimberlites are inferred to originate from sources within ancient stabilized subcontinental lithosphere characterized by time-averaged incompatible element enrichment. Group I kimberlites can be further subdivided into two isotopically similar types to some degree correlative with tectonic environment. Compared to subgroup IA (on-cration), IB kimberlites have lower SiO2 and higher CaO, FeO + Fe2O3 and volatile contents in addition to somewhat greater TiO2, P2O5, Nb, Zr and Y abundances, and tend to occur outside the inferred boundaries of the Kaapvaal Craton though exceptions are present and new unpublished data suggest that this group may be relatively common on the craton.
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Transactions of the Geological Society of South Africa, 88 (2), 281-294
An association of early Tertiary sediment and breccia-filled diatremes, olivine melilitite plugs, kimberlite pipes of doubtful status and a glimmerite-carbonatite complex is related to a craton-margin tectonic setting. Four different populations of olivine, distinguished on textural and geochemical grounds point towards a complex crystallization history for the olivine melilitites. Spherical inclusions in olivine phenocrysts are interpreted as evidence of multiphase liquid immiscibility: apart from silicate magma at least three other liquids (sulphide, carbonate and hydrous fluids) appear to have existed at an early stage of crystallization. The differences between olivine melilitites, "kimberlites", glimmerites and carbonatites are tentatively ascribed to differing proportions of volatile components retained or exsolved from the magma at shallow levels.
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Transactions of the Geological Society of South Africa, 88 (2), 295-299
New Rb-Sr age measurements are reported for a number of intrusives from Angola. Data for the Njoio and Tchivira nepheline syenite bodies yield mineral isochrons indicating ages of 104,3±0,8 Ma and 130,8±1,4 Ma respectively. Palaeomagnetic studies on the same occurrences gave marginal and scattered results respectively. Micas from the Camafuca crater-facies kimberlite yielded an apparent age of 1 822±151 Ma, a result that is far in excess of the Tertiary (or younger) age inferred for this pipe. Similarly conflicting data were obtained for the Nova Lisboa kimberlite. It is likely that older crustal micas incorporated in the kimberlite breccias are responsible for the anomalous ages reported on the kimberlites. Satisfactory palaeomagnetic data are reported for the Zenza and Bailundu occurrences, not dated by the Rb-Sr method. A convenient K-Ar age of 80±0,8 Ma was obtainable for Zenza.
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Transactions of the Geological Society of South Africa, 88 (2), 301-310
The mineral inclusions in Premier Mine diamonds, fall predominantly into the peridotitic and eclogitic inclusion suites reported from diamond world-wide. Sulphides are the commonest inclusions overall and occur with other minerals from both suites. The eclogitic suite is more abundant by approximately 3:2 and apart from sulphides, consists of garnet, clinopyroxene and very rare kyanite and coesite. The garnets have a wide range in compositions which are mainly due to substitution of iron and calcium for magnesium. They contain sosium up to 0,39 weight percent Na2O. The clinoproxenes also vary in composition in a systematic manner in which tie-lines between coexisting garnet-clinopyroxene do not cross. The largest variations in chemistry result from substitution of Na-Al for Ca-Mg in the pyroxenes as the Ca-Fe increases in the coexisting garnets. Potassium is present in trace amounts up to 0,33 weight percent K2O. Calculated equilibration temperatures for coexisting mineral pairs are in the range 1199-1363°C at 50 kb. The mineral compositions of the majority of a small suite of eclogite xenoliths are not the same as those of the inclusions in the diamonds. The inclusions and the diamonds are nevertheless considered to have crystallized together in eclogite which precipitated from a mantle derived magma on cooling. Sampling and disaggregation of the eclogite host rock by the kimberlite is inferred to have postdated diamond formation. Sub-solidus re-equilibration of the eclogite is inferred to account for the differences in composition of the minerals in the rocks and the diamonds. The alternative possibility that the rocks and the inclusions are derived from two distinct environments is considered less likely. Olivine followed by orthopyroxene are the most abundant peridotitic minerals apart from sulphides. Both silicates show a bimodal distribution which is interpreted as evidence for two distinct harzburgitic parageneses for diamonds. Garnet, chromite and particularly clinopyroxene are rare. The presence of the latter suggests a very minor garnet lherzolite field. Limited geothermometry indicates equilibration temperatures less than 1200°C. Some of the peridotitic diamond inclusions show similarities to two categories of harzburgites present as xenoliths in the kimberlite but not to the high temperature recrystallized harzburgite and lherzolite xenoliths also found at Premier Mine.
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Transactions of the Geological Society of South Africa, 88 (2), 311-334
Alkaline rocks are known from all major tectonic provinces in Australia except for the Precambrian Pilbara Block. Most occurrences are intrusions of limited extent and, with the exception of the Cainozoic alkali basalts of the Tasman Fold Belt of Eastern Australia, are volumetrically insignificant compared with the volume of subalkaline rocks observed in each province. The alkaline rocks range in age from late Archaean to Holocene. Where reliable isotopic dates are available the intrusive ages mostly appear to be broadly synchronous with, or closely postdate ages of, cratonization for each of the provinces, a noteable exception being the Tertiary lamproites of the West Kimberley region. The emplacement of many of the pre-Cainozoic rocks, particularly the basic-ultrabasic suites, appears to be influenced by, and in some cases clearly controlled by, initiation or reactivation of deep crustal fractures. The distribution of many of the Cainozoic, and more recently some of the Mesozoic, igneous rocks of eastern Australia has been ascribed to northward movement of the Australian continent over hot-spots, and is associated with regional uplift. The alkaline rocks of Australia include carbonatites and a spectrum of silicate rocks belonging to one of three series: an ultra-potassic (K2O/Na2O>3) series of dominantly ultrabasic to basic rocks which include kimberlite, lamprophyre and lamproite (some of which are diamondiferous); a series of potassic (K2O/Na2O = 1-3), dominantly basic to intermediate rocks (shoshonite association), including monzonites, latites and lamprophyres; a sodic series (Na2O > K2O) of dominantly alkali basalt composition but ranging from mafic, e.g. olivine nephelinites, to felsic compositions, e.g. syenites, phonolites, trachytes. The dramatic increase in the volume of alkaline compared to subalkaline rocks in the Cainozoic may reflect decreasing degrees of partial melting of the modern mantle and lower crust with time, perhaps as a response to falling.
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Transactions of the Geological Society of South Africa, 88 (2), 335-340
The Argyle pipe occurring in the East Kimberley Province of Western Australia is a unique, highly-diamondiferous lamproite. Although it resembles other lamproites located in the West Kimberley Province with respect to its setting, structure, petrography and geochemistry, it is probably Proterozoic in age and hence substantially older than Tertiary occurrences of the West Kimberley Province. Rb-Sr measurements on whole rock and phlogopite samples from magmatic olivine-phlogopite lamproite, reveals a two point model age of 1 126±9 Ma for the Argyle pipe. This age is consistent with ages of other, similar volcanic igneous rocks occurring in several localities worldwide. The widespread occurrence of Proterozoic kimberlites and lamproites suggests that this was an important period of worldwide alkalic intrusive activity.
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Transactions of the Geological Society of South Africa, 88 (2), 341-345
Rb-Sr ages are presented for four lamproite intrusions (Mount North, Old Leopold Hill, Mount Rose and Seltrust Pipe 2) located in the west Kimberley region of West Australia. The data are in agreement with the early Miocene ages previously obtained for the lamproites of this area. The lamproites are characterized by high initial-Sr ratios, indicative of derivation from an enriched source. Localized and regional mantle heterogeneity is indicated by new and existing data.
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Transactions of the Geological Society of South Africa, 88 (2), 347-354
Xenolithic alnoïtes (ca. 34 Ma) intrude an unusually thickened portion of the Pacific Plate, known as the Ontong Java Plateau. Lherzolite xenoliths from the Kwaikwai intrusion on Malaita Island contain spinels with garnet rims; both primary and secondary clinopyroxenes and amphiboles are present. On the evidence of mineral composition and textures, several events are postulated to have affected the mantle below Malaita. Cooling (during separation and formation of the thickened plate) resulted in spinel reacting with clinopyroxene and amphibole to form garnet. During reaction Cr increases (along with Fe) in the spinel, thus enlarging its stability field to greater pressures co-existing with garnet at 959 to 1039°C and 89 to 95 km. A late reheating mantle event possibly associated with alnoïte eruption, produced a limited reversal of the reaction to form slivers of secondary clinopyroxene and amphibole, and aluminium and magnesium-rich rims in the spinels.
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Transactions of the Geological Society of South Africa, 88 (2), 355-377
Alkaline rocks, ranging in age from Precambrian to Cenozoic, are widespread in the Cordillera. They may be divided into kimberlite, mafic ultrapotassic syenite-pyroxenite-ijolite-carbonatite, alkali basalt, felsic syenite, and peralkaline granite-syenite-gabbro assemblages. Many of these alkaline rocks were emplaced in extensional tectonic environments, including rifts and back-arc basins. Important precious metal deposits associated with alkaline rocks include: (1) epithermal gold deposits; (2) porphyry copper-precious metal deposits; and (3) submarine exhalative deposits. Epithermal gold deposits are related to felsic (generally silica-undersaturated) syenites, trachytes, and phonolites forming epizonal intrusions and vent complexes. They are characterized by K-feldspar-quartz- carbonate-fluorite-telluride veins, relatively low S abundance, and Au > Ag. Porphyry copper- precious metal deposits are related to alkali gabbro and syenite (both silica-saturated and -undersaturated) plutons. They are characterized by Cu-sulphides in disseminations, pegmatites, veins, and local immiscible blebs; relatively high S abundance; and Cu > Ag > Pt metals or Au. Both types of deposits exhibit pervasive carbonatic, K-metasomatic, redox, and phyllic alteration. Epithermal gold deposits may grade downward into porphyry copper-precious metal deposits. The difference in gold concentration between the two types may reflect changes in the dominance of thiosulphide-Au and telluro-Au complexes as transporting agents. To date submarine exhalative deposits have not been important producers of precious metals in the Cordillera, except where the exhalite metals have been mobilized and concentrated by plutons. Epithermal silver and silver-base metal deposits are associated with calc-alkaline rocks. In cases where the calc-alkaline rocks are coeval with alkaline rocks, the calc-alkaline rocks probably represent partial crustal melting by alkaline magmas. Using an electronic computer and a data-base of over 4 200 analysed alkaline and related rocks from the Cordillera, the authors developed a series of chemical fingerprints to recognize areas with a high exploration potential for discovery of alkaline rock-related precious metal deposits. These criteria define potential source-host rocks as: (1) alkaline (wt % Na2O + K2O > 0,3718 (wt % SiO2)-14,5); (2) members of the alkali basalt (basaltic rocks with normative olivine and nepheline (or leucite)), or the felsic syenite (Na2O + K2O > 10 wt % and MgO < 2 wt %) assemblages; (3) showing one or more of the following alteration assemblages-K-metasomatism (K2O > Na2O), redox (Fe2O3 > 1,5 FeO), or carbonatic (CO2 > 0,5 wt %), (4) showing local anomalies in some of the following - Au (> 10 ppb), Ag, As, Bi, Ce, Cu, F, Hg, La, Mo, Nb, Pb, S, Sb, Te, T1 (low in porphyry systems), U, and V; and (5) showing high Ba and Sr in Ba : Sr : Rb ratios. Such fingerprints may be effective in other areas and may be
particularly useful in prospecting for invisible epithermal and submarine exhalative gold deposits.
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Transactions of the Geological Society of South Africa, 88 (2), 379-394
The Tyrrhenian border of the Italian peninsula is the site of recent to active potassic alkaline volcanism which makes up the well-known Roman Comagmatic Province. The available petrological and geochemical data indicate the presence of four groups of potassic rocks represented by: (a) trachybasalts to trachytes, near-saturated in silica with K2O/Na2O around unity, relatively less enriched in potassium and incompatible elements, making up the so-called Potassium Series; (b) leucite tephrites to leucite phonolites, strongly silica undersaturated, with higher K2O, K2O/Na2O ratios and incompatible element contents, making up the so-called High-Potassium Series; (c) ultrabasic leucite and kalsilite bearing melilitites, more strongly undersaturated in silica, with very high K2O/Na2O ratio, representing hyperpotassic magmas with a kamufugitic affinity; (d) minettes with intermediate silica contents, slightly oversaturated in silica, representing hyperpotassic magmas with an orenditic affinity. Orenditic and kamafugitic rocks are volumetrically largely subordinate with respect to the potassium and high- potassium series. Based on the results of experimental petrology, it is suggested that the parental magmas of each one of these groups were generated at different pressure in a potassium-rich, phlogopite-bearing peridotitic mantle in conditions of CO2 saturation with the presence of small amounts of water. The primary mantle melts have undergone a complex evolutionary history involving simultaneous crystal fractionation, mixing among different batches of magmas, interaction with crustal material and wall rock aqueous fluids. These processes were more effective in volcanic centres characterized by well-developed edifices with summit calderas. The mafic potassic rocks from Italy display important chemical affinities with rocks from volcanic arcs and have patterns of incompatible elements as well as ratios of some discriminant elements similar to those of leucite-bearing rocks from the Aeolian arc. This is considered as strong evidence for the hypothesis that the composition of the mantle source where the potassic magmas were generated, was modified by addition of material, possibly sediments, carried down by subduction processes which were active during Tertiary under the Apennine chain.
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Transactions of the Geological Society of South Africa, 88 (2), 395-401
Kimberlites of the north-eastern Siberian platform show a certain regularity in space and time. Many of the kimberlites are diamondiferous. Alkalic rocks, aside from kimberlite, are also found on the Siberian platform. Emplacement of all these rocks is related to reactivation of various deep-seated faults and fractures. Many of these fault systems were established during formation of the buried aulocogens of the Upper Proterozoic (Riphean) period. A model relating alkalic magmatism to deep-seated faults and fractures is presented on the basis of studies involving geophysics, plutonism and metallogeny of the Siberian platform, particularly of the region adjacent to the Anabar shield.
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Transactions of the Geological Society of South Africa, 88 (2), 403-409
A new textural-genetic classification based primarily on macroscopic features of kimberlites is proposed and discussed.
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Transactions of the Geological Society of South Africa, 88 (2), 411-437
Lamproites are recognized as a distinct petrological clan of rocks possessing mineralogical characteristics that set them apart from other leucite-bearing potassic rocks. Lamproite mineralogy clearly reflects the peralkalinity (sodium and aluminum deficiency) and high titanium content of the parent magmas. As a group they are characterized by the presence of one or more of the following; titanian potassian richterite, titanian-Al-poor phlogopite, titanian tetraferriphlogopite, priderite, wadeite, Na and Al-deficient leucite and/or sanidine and Al-poor diopside. Minerals characteristic of the Roman province or West African Rift potassic rocks, e.g. nepheline, melilite, kalsilite, alkali feldspar, plagioclase and Al-rich augites, are absent. A new definition of lamproites and a revised nomenclature are presented. The new terminology, based upon a modal-textural scheme, recognizes two broad groups of lamproites; (1) phlogopite-sanidine-leucite-diopside lamproites (wyomingites, orendites, fitzroyites, cedricites, verites, etc. ) characterized by the occurrence of resorbed phenocrystal phlogopite; (2) madupitic lamproites (madupite, jumillite, wolgidite, etc.) characterized by the presence of poikilitic phlogopites. The paragenesis and composition of titanian phlogopite, titanian tetraferriphlogopite, titanian potassian richterite, titanian potassian arfvedsonite, diopside, enstatite, forsteritic olivine, leucite, analcite, sanidine, spinels, priderite, jeppeite, armalcolite, ilmenite, anatase, wadeite, schcherbakovite, perovskite and apatite are discussed. It is concluded that different lamproite provinces appear to be characterized by phlogopites, amphiboles, sanidines, leucites and priderites of distinctly different compositions. Phlogopites, spinels and amphiboles exhibit sufficient compositional variation that they can be used to assess the relative degree of evolution of lamproitic rocks within and between provinces. Thus on a mineralogical basis madupitic lamproites (madupite, jumillite) are more evolved than phenocrystal phlogopite and leucite lamproites (orendite, verite).
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Transactions of the Geological Society of South Africa, 88 (2), 439-447
Lamproites and kimberlites exhibit a wide range in Nd, Pb and Sr-isotope ratios which is at present much greater than that reported from mantle xenoliths and continental basalts. They contain at least a contribution from old (1-2,5 Ga) upper mantle source regions which had low Sm/Nd, but variable Rb/Sr and U/Pb ratios. Contrasting Pb-Pb and Sm-Nd whole rock ages for lamproites suggest that, on average, Sm/Nd was reduced by approximately 45 per cent during melting and fractionation, which in the simplest model implies very small (<1%) degrees of melting. High Rb/Sr ratios, and hence with time, high (epsilon)Sr values can occur in rocks with both high and low Rb/Ba. The majority tend to have Rb/Ba = 0,1-0,03, and the inferred enrichment of trace elements in their source regions is consistent with the introduction of small volume partial melts. The comparatively rare high Rb/Ba rocks, in contrast, may reflect mantle metasomatism in the presence of H2O-rich fluids. A discussion of the fractionation of trace elements that might result from mantle metasomatism further suggests that it has been responsible for the development of distinctive trace element features in relatively few mantle-derived rocks.
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Transactions of the Geological Society of South Africa, 88 (2), 449-457
The grazing incidence relation of shield geotherm and mantle solidus indicates that kimberlite is the limiting case of cratonic magmatism. Other alkaline ultramafic magmas form by volatile fluxing along steeper geotherms which permit melt migration and accumulation. The nature of the erupted magma is determined by earlier source mantle, the stability of phlogopite, the composition of incoming fluids, the rate and period of influx, the pre-existing geotherm, and the extent and manner of fluid and melt movement. Assembly of the available evidence shows that in addition to the obvious alkalis, H, C and S, the magmatism requires provision of significant P, Ti and Fe, and especially Ca. These constituents would not be present in the required amounts or ratios in lithosphere that had experienced previous temperatures significantly higher than the vapour-present peridotite solidus, and must therefore have been added by a new influx, prior to melting. Along geotherms close to shield, the influx gives rise to kimberlites (Groups I and II), lamproites and melilitites; on steeper geotherms, nephelinites and basanites. Percolation of flux-induced melts along the existing geotherm leads to intensive lithophile enrichment near the solidus, and metasomatism in the sub-solidus. The end products of metasomatism, melt enrichment and melting converge on alkali clinopyroxenite composition. Near-solidus melts can penetrate to the surface initially only by flash-over into a high-speed fluidized eruption, giving rise to typical ultramafic breccia diatremes. At the other extreme the magma generating system also must be capable of erupting liquid to the surface, sometimes at high velocities; this requires melt accumulation at temperatures well above solidus. Rising melts are impelled into high speed eruption when they pass through a liquid structure transition during their passage towards the surface. Basanitic melts are accelerated by CO2 exsolution on passing through the carbonate stability boundary, around 80 km. From this point on they acquire their characteristic spinel lherzolite nodule suite. Melts richer in carbonate, accumulating on shallower geotherms, achieve "lift-off" at greater depths when the melt rises through a transition zone related to the carbonate out boundary. Alkaline ultramafic melts achieve their distinctive eruption chemistry by interaction with enriched mantle below the point of lift-off; and pick up their nodule suite from this level and above.
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Transactions of the Geological Society of South Africa, 88 (2), 459-470
Maars are small explosive volcanoes and diatremes are in many places their volcanic subsurface structure. Their volcanological characteristics, ejecta beds, the rocks filling the diatremes, and their mode of formation are discussed as well as their relationship with the hydrogeology of the respective areas. Maars and diatremes are associated with any type of magma involved in volcanic activity, i.e. acid to ultrabasic, non-alkalic and alkalic magmas. Most maars and diatremes are not the result of explosive exsolution of large amounts of juvenile volatile phases from magmas rising to the earth's surface. In contrast they are the result of phreatomagmatic activity, i.e. a complex magma/external water (mostly groundwater) interaction, giving rise to powerful water vapour explosions. These explosions are responsible for hydraulic fragmentation not only of the magma, but also of large amounts of country rocks which form the wall rocks of the diatremes. The wall rock fragments characteristically make up a very high proportion of the produced pyroclastic rocks. The hydrostatic pressure of the water column above the explosion site and the availability or influx capacity of groundwater is assumed to control propagation or prevention of downward penetration of diatremes. Explosive eruptions in shallow marine and in diverse continental environments are discussed in relation to a model of hydrostatically controlled phreatomagmatism.
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Transactions of the Geological Society of South Africa, 88 (2), 471-472
Field studies of carbonatite occurrences (e.g. Tuttle and Gittins, 1966; Le Bas, 1977) leave little doubt that carbonatites can be magmatic in origin. Numerous experimental investigations over the past two decades have shown that carbonatitic magmas may be derived directly or indirectly (e.g. via liquid immiscibility) from liquids generated by melting of carbonated peridotitic mantle materials at depths in the order of 70-90 km (Eggler , 1976; summary in Wyllie, 1980). Sr isotope compositions were determined for 22 carbonatite samples from six of the younger intrusions representing an east-west traverse of over 400 km across the central Kaapvaal Craton from the Ystervarkkop (Goudini) Complex north of Zeerust in the west to the Spitskop Complex north-east of Groblersdal in the east. All the samples have high Sr contents. The Spitskop samples represent a differentiation sequence ranging in Sr concentration from 600-2 000 ppm whereas the carbonatite samples from the other centres all have > 1 000 ppm Sr. The present data indicate the existence of a homogeneous mantle reservoir of primitive Rb-Sr composition below the central Kaapvaal Craton during the mid-Proterozoic. As carbonatites appear to be derived from peridotitic starting materials at high pressures, this reservoir probably underlies the (heterogeneous) lithospheric mantle sampled to produce the tholeiitic magmas of the Karoo volcanic cycle (Hawkesworth et al., 1983).
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Transactions of the Geological Society of South Africa, 88 (2), 472-473
It is proposed that alkali rocks be classified on the basis of a threefold division according to the relative proportions of silicon, aluminium and alkalis as follows: 1. Ekeritic-silicon adequate or excessive; aluminium deficient. 2. Miaskitic-silicon deficient; aluminium adequate or excessive. 3. Agpaitic - silicon deficient; aluminium deficient.
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Transactions of the Geological Society of South Africa, 88 (2), 473-474
Two unusual silica-poor (< 36 wt %), alumina-rich (> 21 wt %) specimens (35A and 35B) occur some 5,5 km south of the farmhouse Rietfontein in Namaqualand (Zelt, 1979). The specimens lie within Proterozoic granulite facies terrain which has approached equilibrium conditions at temperatures and pressures of about 830° and 7 kb (Zelt, 1980). The unique chemistry and mineralogy (Table I) of these two independent specimens dictates a singular parent, a phenomenon which may be explained by tectonic dismemberment (Jackson and Zelt, 1984).
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Transactions of the Geological Society of South Africa, 88 (2), 474-475
Ilmenite grains containing more than three percent MgO are typically associated with kimberlite magmas, either as xenocrysts or complexly intergrown microphenocrysts. Such mineral compositions have also been reported from alkali basaltic magmas. In magmas of tholeiitic affinity ilmenite is usually a late- stage mineral in the crystallization sequence, appearing as an interstitial phase in dolerites or at the end-stages of differentiation in layered complexes. Such minerals contain less than 2 percent MgO. Most of the intrusive and extrusive rocks of tholeiitic affinity in the Karoo Sequence of southern Africa conform with this generalization. However, two exceptions, one extrusive and one intrusive, have been recognized. The Lebombo monocline is a major flexure of lower Cretaceous age extending from Empangeni in Natal to the Limpopo River. Within the volcanic stratigraphy recognized in the monocline is the Letaba Basalt Formation (Bristow and Saggerson, 1983) which consists of picrite lavas with up to ˜24% MgO. These lavas typically contain olivine and ilmenite grains in a glassy matrix. The olivine crystals may be subrounded or develop varying hopper textures due to in situ growth. Ilmenite crystals are prismatic to elongated to skeletal and may display a parallel growth pattern. These observations suggest a crystallization sequence of olivine followed by ilmenite, which is very unusual for a tholeiitic magma. The second occurrence is in the Insizwa Complex in the Transkei, where a very thick intrusive body has differentiated and in which Ni-Cu sulphide mineralization has been found (Tischler et al., 1981). The olivine-rich rocks at the base of the body contain euhedral crystals of ilmenite enclosed in plagioclase and orthopyroxene as well as sub-rounded olivine grains. The textural relations again suggest that ilmenite crystallized before plagioclase and pyroxene. Disseminated blebs of sulphide are also found in the lowest portion of the sheet and as well as containing pyrrhotite, pentlandite and chalcopyrite, ilmenite grains are also found in the ore.
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Transactions of the Geological Society of South Africa, 88 (2), 475-476
Phonolites of Tertiary age occur as eroded tholoids, lava flows, ignimbrites, and coulees in the Klinghardt Mountains of southern Namibia. Sixty samples have been analysed for major and trace elements and 14 of these for 87Sr/86Sr. The phonolites lie close to the low pressure cotectics in Q-Ne-Ks, in keeping with their petrography which indicates that most samples have phenocrysts of both nepheline and sanidine. Sodium has been variably lost from the rocks during crystallization and devitrification/alteration of hypocrystalline specimens. The concentrations of other elements except perhaps LREE remain unaffected. The compositional variation within the phonolites is consistent with fractional crystallization of nepheline, sanidine and minor clinopyroxene to produce a number of near-parallel lineages from similar, but not identical, parent compositions. Fractional crystallization results in increasing SiO2, Na2O, Zr, Nb, Ce, Rb, Th, Y, Zn, Pb and REE, but decreasing K2O, TiO2, CaO, MgO, P2O5, Ba and Sr with differentiation. Enrichment factors for Zr and Nb indicate that the most evolved phonolites represent < 20 per cent residual liquids, a result which is broadly consistent with major element modelling. In the evolution of the most differentiated phonolites, fractional crystallization was accompanied by an egress of a vapour phase causing depletion of Th, Y, REE, Zn, Pb but not Zr and Nb. Initial 87Sr/86Sr for most of the phonolites lie in the range 0,7050-0,7057, rising to 0,7080 in the low-Sr samples. This is suggestive of some interaction with crustal rocks, but this interaction has no detectable influence on the fractionation controlled compositional evolution of the suite. The phonolites are associated on a regional scale with nephelinite and melilitite rocks but there is no favourable evidence for an evolutionary link between these and phonolite. Alternatively, the primitive phonolites may have originated through anatexis of metasomatized, nepheline-normative crust.
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Transactions of the Geological Society of South Africa, 88 (2), 476-477
The islands of Tristan da Cunha and Gough are located in the South Atlantic some 500 km to the east of the Mid-Atlantic Ridge, with Gough Island being 400 km to the south-west of Tristan da Cunha. Gough Island has a maximum elevation of ~ 1000 m, is elongate in shape and is characterized by being heavily eroded, particularly on the eastern side of the island. In contrast Tristan is circular in form and rises to a well defined volcanic peak ~3000m above sea level in the centre of the island. Lavas on Gough Island range from picrite basalt through alkali olivine basalts and trachyandesite to trachyte. The latter can be subdivided into trachyte per se and more evolved aegirine-augite trachyte (Le Maitre, 1962). Trachyte is extremely abundant on Gough and covers over 50 per cent of the surface, while the aegirine-augite trachytes occur as volumetrically minor intrusive plugs. Tristan lavas range from ankaramite through alkali olivine basalt and trachyandesite to trachyte (Baker et al., 1964). Trachyte is extremely rare on Tristan and occurs only as intrusive plugs. Despite the similarities in lava types found on these two islands, some significant differences occur in the petrography and mineral chemistry of the lavas. Olivine and clinopyroxene are the earliest crystallizing phases in the primitive Gough Island basalts and are followed by plagioclase and then ilmenite and finally titanomagnetite. Apatite, alkali feldspar and, in some samples, pseudobrookite occur as accessory phases in the most evolved basaltic lavas and trachyandesites. In contrast, olivine, clinopyroxene, titanomagnetite and kaersutitic amphibole are all early crystallizing phases in the primitive Tristan da Cunha lavas and are followed by plagioclase. Accessory apatite, biotite and, in some samples, leucite occur in the most evolved basalts and trachyandesites. The less evolved trachytes from both islands comprise alkali feldspar, clinopyroxene, olivine, biotite, apatite, titanomagnetite and sodic plagioclase and, in the case of Tristan, also amphibole. In addition to the above, the more evolved trachytes from Tristan can contain zircon, sphene and nepheline as accessory phases while the most evolved lavas from Gough contain sodalite as an accessory phase.
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Transactions of the Geological Society of South Africa, 88 (2), 479-482
Biographical memoir and bibliography.
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Transactions of the Geological Society of South Africa, 88 (2), p477
Monastery Mine is situated close to the inferred eastern margin of the Kaapvaal Craton and the general geology has been described by Whitelock (1973). The mineral inclusions in diamonds from the Monastry Mine have been found to be almost entirely of the eclogitic paragenesis and this abstract is concerned only with the garnets from this suite. Fifty-six eclogitic garnets have been analysed for their major element compositions by electron microprobe. Two distinct populations of garnets are present in the diamonds. One group of ten (termed Group A) have compositions similar to those described from diamonds worldwide (e.g. Harris and Gurney, 1979). The second group of 46 (termed Group B) form a related suite of compositions that appear to reflect the effect of pyroxene solid solution in the garnets. Particularly, extreme concentrations of SiO2 and Al2O3 range to as high as 47,43 wt% and as low as 11,29 wt% respectively. The low Al2O3 values are not supported by high Cr2O3 concentrations as is typical for peridotitic garnets; the maximum Cr2O3 concentrations in these garnets being 0,37 wt%. Sodium can be present in extremely high concentrations, ranging up to 1,03 wt% Na2O. These are the highest sodium concentrations ever recorded in mantle-derived garnets. Samples spanning the compositional range of the garnets have been analysed by X-ray diffraction using a 57,4 mm Gandolfi camera. The results unequivocally confirm a cubic garnet structure for both groups of garnets.
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Transactions of the Geological Society of South Africa, 88 (3), 483-493
The only permanent water resource in Hereroland comprises ground water stored within the Kalahari and underlying formations. The ground water conditions in Hereroland have been investigated by means of Schlumberger electrical soundings, as well as hydro chemical, hydrological, and petrophysical studies, i.e. investigation of drilling (core) samples. The resistivity survey comprised some 900 Schlumberger electrical soundings. Where possible, soundings were carried out at boreholes for calibration purposes. A comparison of electrical sounding curves with applicable borehole records showed that the Kalahari Group can be subdivided into three zones, viz. the Upper Kalahari, the Middle Kalahari and the Lower Kalahari. The geoelectrical data, interpreted in conjunction with the available borehole data and the measured distribution of piezometric levels, enabled the identification of two distinct aquiferous units forming part of the Kalahari Group within the western part of Hereroland. Without a regionally well distributed set of aquifer parameter values, a full assessment of the amount of water stored in the Middle Kalahari aquifer and of the recharge to the region is not possible.
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Transactions of the Geological Society of South Africa, 88 (3), 495-499
Borehole stimulation and rehabilitation techniques are commonly used overseas to increase the yields from boreholes. Trials were done to stimulate two boreholes chemically. The yield from one of the holes was improved by more than 50 per cent. The technique appears to be cost-efficient and directions for the application are given.
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Transactions of the Geological Society of South Africa, 88 (3), 501-515
A geophysical and hydrogeological survey was made of a strip of country along the lower Kuruman River in the Gordonia and Kuruman districts to locate areas with thick Kalahari deposits occurring below the water table and to evaluate the ground-water potential of such regions. The geophysical survey comprised 707 Schlumberger electrical soundings and, in addition, as many boreholes were logged geophysically. Pumping tests of varying duration were conducted to determine the geohydrological parameters of the aquifer and estimates for the storage coefficient and transmissivity were obtained. The geoelectrical survey proved to be very successful in delineating the shape of the Kalahari Basin, in determining the thickness of the Kalahari Group sediments, and in detecting and outlining the brackish to salt water occurrences in the Kalahari strata. The combined drilling and geophysical results show that the pre-Kalahari topography is gentle and is associated with basins containing up to 133 m of sedimentary strata and that broad valleys lead into these basins. These results were integrated with the ground-water head levels to delineate the areas where water- saturated Kalahari strata occur. Combining this information with the data on ground-water resistivity and chemistry, volumes of Kalahari strata saturated with fresh water amounting to 10,1 x 10(9) m³ in the area east of the Tellerie Pan Road and 19,3 x 10(9) m³ to the west of the road were calculated. Although the eastern aquifer contains no brackish water, it is thin and spread out over a large area, making ground-water abstraction on a large scale impractical. The fresh-water aquifer in the western sector has a much smaller area, but has almost double the volume and is therefore more suitable for large-scale ground- water exploitation. It was calculated that this aquifer can yield 91 x 10(6) m³ of fresh water. The most suitable abstraction area is the approximately 25 km- long zone along the Kuruman River situated on the farms Erin, Askham, Cromdale, Murray, Colinton, Eerste Rust, and Mara where the fresh-water aquifer is thicker than 30 m. The study further showed that floods in the Kuruman River represent the major fresh-water recharge mechanism.
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Transactions of the Geological Society of South Africa, 88 (3), 517-522
Environmental isotope observations were conducted on ground-waters from approximately 50 boreholes covering a substantial part of Gordonia. The quality of these waters ranges from fresh to saline. The observed isotope ratios cover a wide range of values, indicating varied hydrological conditions. The most important conclusions arrived at by this study are: 1. no important regional movement of ground-water occurs at present; 2. there is widespread evidence of diffuse rainfall recharge; and 3. an important part of ground-water salinity is derived from the unsaturated zone, during such recharge.
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Transactions of the Geological Society of South Africa, 88 (3), 523-528
The dewatering of Sishen Mine in the northern Cape Province supplies good quality water for the mine and surrounding areas. Using various approaches, attempts are made to quantify the remaining storage of ground-water. Geohydrological observations provide an estimate based on extrapolating the thickness of dewatered rock. Environmental isotope observations on various borehole outputs show contrasts between different ground-water bodies and their mixtures and allows for some extrapolations of observed trends. Indications are that previous estimates of storage, based on ground-water level changes, are conservative.
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Transactions of the Geological Society of South Africa, 88 (3), 529-540
An airborne radiometric anomaly near the Kuruman River in the Vanzylsrus area of the Northern Cape was recorded and ground follow-up investigations established that the anomaly was caused by low-grade uranium accumulations in organic-rich diatomaceous earth deposits. Following this discovery a ground-water sampling programme was initiated and a significant uranium anomaly was found situated within a palaeodrainage channel. Together, the radiometric and ground-water uranium anomalies constituted a likely target area for the location of a hidden uranium deposit. Geohydrology, ground-water chemistry, isotopic data and radiometric borehole logging were used to evaluate its potential. It was concluded that low-grade uranium deposits formed in pans and vleis during the young stage of the drainage. Subsequently, these deposits were covered by Kalahari sands. Isotopic evidence indicates local infiltration and uranium enrichment of the ground-water with the highest concentration in the surficial material and the ground-water at the lower end of the drainage.
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Transactions of the Geological Society of South Africa, 88 (3), 541-544
Underground extraction of coal is taking place on an increasing scale around the world. This is partly the result of a realization that the coal reserves are limited, but also because of better mining techniques. As a result of increased extraction, the strata overlying the coal collapse and ground-water contained within these rocks is released into the mines. This creates a two-fold problem in that significantly large areas may become depleted in ground-water resources; also this water may be contaminated within the mines. This paper deals with the present status of increased underground extraction of coal in South Africa and its relationship to ground-water disturbances. The degree of this disturbance is discussed and it is pointed out that careful planning will be necessary to ensure that this problem is dealt with efficiently.
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Transactions of the Geological Society of South Africa, 88 (3), 545-548
Conventional deep-well dewatering systems are not feasible on strip coal mines in South Africa due to the low permeability of the Middle Ecca coal formation. Alternative methods of dewatering were sought by modelling the complex multi- aquifer system using permeability values derived from packer testing. Blasting a pre-split line approximately 60 m back from and parallel to the crest of the highwall is suggested as a practical method of dewatering. The whole pre-split block some 300 m long is moved laterally towards the open pit. A crack is thus opened up along the line of the pre-split which prevents further ground-water recharge to the pre-split block. Existing tight joints may also be opened up by the pre-split blast, thus accelerating the process of gravity drainage. When the main blast holes are drilled some three weeks after the pre-split, these holes are commonly found to be dry and may be charged with less costly non-waterproof explosives. Other benefits of pre-splitting include a cleaner, straighter highwall, increased coal extraction, and improved mine operating efficiency.
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Transactions of the Geological Society of South Africa, 88 (3), 549-552
The efficiency of modelling a ground-water system depends mainly on the physical data available and the mathematical method used to solve the governing equations. The effect that any deficiencies in the physical data has on the solution is discussed and appropriate action recommended. A new method which is very efficient on computer memory space and time for solving the governing equations is also discussed.
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Transactions of the Geological Society of South Africa, 88 (3), 553-559
Parameter identification is always a problem in modelling ground-water systems. The present investigation is concerned with what useful contribution the method of kriging can make towards solving this problem. After a brief introduction to the method and a particular implementation of it, based on least square spline approximation of the semi-variogram, the method is applied to a study of the transmissivities and storage coefficients of the Cape Flats aquifer. It is shown that kriging not only yields the best estimates of these parameters required in modelling a system, but that the associated error map can also be used to advantage in pointing out deficiencies in the set of data used. Moreover, since the error map depends only on the position of sample points and the error variance of the regionalized variable, the error map can also be quite helpful in reducing drilling costs when starting with a new aquifer.
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Transactions of the Geological Society of South Africa, 88 (3), 561-580
Most continents coalesced during the last part of the Palaeozoic Era, to form the supercontinent of Pangaea. The Mesozoic Era saw the break-up and dispersal of the supercontinent. This great drama of alternating coalescence and dispersal of continents defines an extended Wilson cycle. This paper is concerned with the physical basis for the extended Wilson cycle. Because the dispersal, affecting most of the continents, and the movements of plate boundaries exhibit strong regularities when they are viewed in the hotspot reference frame, the hotspot family is reviewed. Relationships between hotspot concentrations, the geoid, deep mantle velocity structure, and the movements of continents, are reviewed. The degree of dispersal of the continents within the extended Wilson cycle correlates with four geophysical parameters displaying cyclical variation: the mean velocity of the continents in the hotspot reference frame, sea level, 87Sr/86Sr ratio in seawater and the cumulative geomagnetic polarity record. The interrelationships between sea level, cumulative geomagnetic polarity, and degree of dispersal in the extended Wilson cycle indicate that a new core-mantle coupling scheme is required. Dynamo models based on bistable solidification of the core lie at the heart of the problem. In the Schloessin-Jacobs model, each polarity of the geomagnetic field corresponds to crystallization of the liquid core at one of its two spherical boundaries. To relate this model to the extended Wilson cycle, the contrast in the "face of the earth" during the Permian (Reversed) and the Late Cretaceous (Normal) magnetically quiet epochs is examined. Use is made of a coupled mechanical system, in which the core liquid solidifies either on its outer spherical boundary or on its inner spherical boundary, depending on the positive or negative sign of secular change in external pressure applied to the core. Cyclical behaviour of a poroelastic carapace within the mantle provides a plausible source for such pressure changes. When key features of the physics are examined side by side with the geological record, the full ensemble of data indicates that Normal polarity intervals are intervals when metal solidifies on the inner core boundary, and Reversed polarity intervals are intervals when slag (probably FeO) solidifies on the mantle-core boundary. Normal polarity intervals are the consequence of a net fluid release from the poroelastic carapace and resulting increase of pressure on the core, while reversed polarity intervals are the result of a net inflation of the poroelastic carapace and decrease of pressure on the core. The fluids released from subducting slabs (as they move into the asthenosphere and mesosphere) are likely to make up the major fraction of the fluids (vapour plus magma) which inflate the carapace. The concluding section of this paper deals with the dynamics of continent-bearing plates. Coalescence of such plates and inflation of the carapace reinforce each other, as do dispersal of continent-bearing plates and outward expulsion of fluids from the carapace. This mutual reinforcement is the key to coalescence and dispersal of the continents. During the later
Palaeozoic, mutual reinforcement led to steady diminution in the amount of heat and fluid reaching the base of the lithosphere. During the late Mesozoic, mutual reinforcement led to a steady increase in the expulsion of heat and fluid from the poroelastic carapace. Inflation and deflation of a poroelastic carapace adds a useful tool to theoretical geophysics: it modulates the throughput of the heat which originates in the lower mantle and core. Consequently, it modulates the amount of heat and fluid reaching the base of the plates-parameters which govern the summed length of plate boundary and annual turnover of plate surface.
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Transactions of the Geological Society of South Africa, 89 (1), 1-8
Palaeomagnetic data were acquired from eight sampling sites situated 3 to 10 m below the Merensky Reef at widely spaced localities in the Bushveld Complex, Transvaal, South Africa. The specimens were subjected to alternating field and thermal demagnetization. The mean magnetization directions of six sites are well grouped, have mixed polarity, and yield a virtual geomagnetic pole at 39,5°S/133°W. The position of this pole on the apparent polar wander path (APW) for Africa indicates that the mafic portion of the Bushveld Complex cooled from the floor upwards. The cooling of the Merensky footwall rocks occurred before, or contemporaneously with the formation of the Phalaborwa Complex and the onset of deposition of Waterberg Group sediments.
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Transactions of the Geological Society of South Africa, 89 (1), 17-28
Samples representing the major lithological units between Copperton and Prieskapoort in the Cape Province have been analysed for Sm and Nd concentrations and Nd isotopic compositions. The ~3 000 Ma Marydale Group is shown to extend without major chronostratigraphic breaks from slightly metamorphosed basalt and arkose west of the Doornberg Fault at Prieskapoort to highly metamorphosed gneisses east of the Brakbosch Fault at Copperton. Evidence is found of plutonic alkaline igneous activity, and possibly volcanism, which occurred during the ~1 900 Ma Kheis tectogenesis. Early Proterozoic material apparently dominates the lower crustal region of the eastern Namaqua Province sampled by kimberlites. Sm-Nd for the Copperton Formation of the Namaqua Province place an upper limit of -1 514 Ma and lower limit of -1 350 Ma on its origin. Omission of basaltic samples from the isochron regression yields the younger age, which is more easily reconciled with Pb isotope data. Volcanism is envisaged from a mantle source with a history of light rare-earth depletion relative to the chondritic earth model. By analogy with modern tectonic environments, the Copperton Formation probably formed in an active continental margin or island arc environment. Pretorius's (1974) recognition of the Brakbosch Fault zone as a fundamental crustal break between crustal provinces is confirmed. However, the age relationships revealed by this work contradict his model of the boundary in which progressively older, deeper levels of the crust are exposed from east to west. The Brakbosch Fault represents either an unconformity between Kheis-Kaapvaal basement and Copperton cover rocks, or alternatively, the thrust plane on which Namaqua crust was tectonically superposed on the Kheis-Kaapvaal craton. Late-tectonic right-lateral movements reactivated the suture plane.
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Transactions of the Geological Society of South Africa, 89 (1), 29-34
Microprobe analyses of coexisting magnetite-ilmenite grains from the predominantly dioritic Koperberg Suite, Namaqualand, indicate oxide compositions near to the end members. The magnetite contains between 0.1 and 5,0 mole percent ulvospinel, while the haematite content of ilmenite varies from 1,2 to 5.8 mole percent. Application of the classical iron-titanium oxide geothermometer/oxygen barometer originally calibrated by Buddington and Lindsley (1964), yields temperatures and oxygen fugacities just beyond the lower experimental limits. Temperatures cluster in the region between 450 and 5000C, while oxygen fugacities range from 10\22 to 10\29 atm. These results represent distinctive low-grade conditions when compared with data reported in the relevant literature. The fO2-conditions, determined for some ore-bearing members of the Koperberg Suite, are slightly lower than the values for barren rock types. However, the present data set is inadequate to provide meaningful discriminants. The unusually low temperatures are interpreted as representing equilibration conditions resulting from extremely slow cooling.
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Transactions of the Geological Society of South Africa, 89 (1), 35-55
The rich Golden Mile deposits at Kalgoorlie occur at the top of an Archaean mafic volcanic pile, overlain by the products of felsic volcanism and sedimentation. The principal host rocks are carbonated greenstones, the lithologies of which have been the subject of various interpretations. Though all appear to be concordant, some may be intrusive; others obviously are extrusive. In effect, a series of repetitive dominantly mafic volcanic cycles is represented, each commencing with a basal ultramafic and becoming generally less mafic. Each cycle concludes with an episode of gold deposition, apparently coeval with deposition of mostly cherty and/or carbonaceous sediments and minor felsics. The Golden Mile deposits appear to represent the culmination of these auriferous episodes. Underground mapping suggests that the bedded bonanza ores (so-called Green Leader) of the Golden Mile are folded volcanogenic or volcanically derived deposits that are associated with, but are separate from, pyrite-telluride shoots of high gold grade occurring on several series of steep tabular lode structures; the structures fault each other and are in turn all faulted by later flat reverse faults. These mineralized (internal) shears and faults are distinct from barren (external) semi-regional faults of moderately steep dip, one of which has had a pronounced influence on interpretations of the stratigraphic succession. Specifically, although brought almost into juxtaposition by this fault, a North End Series is now indicated at a lower stratigraphic level than the Golden Mile series. A prominent host member of the former-the North End Dolerite - lacks fragmentals, tuffites and enclosed carbonaceous sediments characteristic of its Golden Mile counterpart and is devoid of any rich ores. Although the Golden Mile mineralization is consistent with the model of Anhaeusser (1976) for Archaean metallogeny, a precise volcanic centre as a source for mineralization at Kalgoorlie cannot be defined.
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Transactions of the Geological Society of South Africa, 89 (1), 57-73
Broken Hill and Mt Isa-type deposits are regarded as two contrasting fundamental types of Proterozoic submarine exhalative Pb-Zn deposits. Examples of Broken Hilltype (BHD) are Broken Hill and Pegmont (Australia) and examples of Mt Isatype (MID) are Mt Isa, Lady Loretta, Dugald River, and McArthur River (Australia). A number of deposits are regarded as intermediate between the two end members, examples being Gamsberg and Aggeneys (South Africa), Franklin and Sterling (USA), deposits of the Bergslagen area (Sweden) and the Ramura-Agucka deposit (India). BHD characteristically occur in high metamorphic grade, Early- Middle Proterozoic, deep-water, distal, turbidite-metavolcanic sequences. MID occur in Middle Proterozoic, low metamorphic grade, shallow-water, hypersaline, emergent shale-carbonate sequences and Palaeozoic, low metamorphic grade, shallow- to deep-water, shale-carbonate sequences. Both deposit types form immediately after sudden deepening of an ensialic rift. The most common associated igneous rocks are tholeiitic to subalkaline basalts and, with BHD, silicic volcanism is not uncommon. A great diversity of unusual rock types with no sedimentary or igneous precursors, which derive from water-rock interaction or competing chemical-clastic sedimentation, are associated with BHD. Exhalite facies for both types are zoned from proximal siliceous and manganiferous facies to distal calcareous and boron-rich facies with no regular distribution for ferruginous, zincian and barian facies. Exhalative Cu-Zn, Cu-Zn, W and Sn deposits are associated with BHD, whereas Mississippi Valley Pb-Zn and brecciated sediment Cu deposits are associated with MID. Zonation from Cu-Zn-Pb- Mn for BHD is in contrast to Mn-Pb±Zn-Cu-Pb-Zn zonation for MID. The non- sulphide fraction of the FeS2-(FeS)-ZnS-PbS MID is identical to the enclosing sediments, whereas the non-sulphide fraction of BHD is enriched in Mn, Ca, P, Fe, F, and CO2. Primordial S, Sr, and Pb isotopic compositions S-poor phases, and base metal-bearing non-sulphides from BHD, contrast with crustal/biogenic compositions and S-rich phases for MID. It is suggested that MID form as a result of aborted rifting of thick piles of sediments where convective seawater geothermal systems are established, or hypersaline formation waters are exhaled. In contrast, BHD form as a result of successful rifting of thin crust, intrusion/extrusion of mafic volcanics, consequent elevation of geothermal gradient, and ascent of CO2-, F- and P-rich mantle fluids. These fluids induce lower crustal sediment melting and resultant silicic volcanism. Mixing of seawater with these mantle fluids occurs at or near the sea floor.
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Transactions of the Geological Society of South Africa, 89 (1), 9-16
The data set of satellite total field (F) magnetic measurements obtained during the Magsat mission, is reduced to a long wavelength magnetic anomaly map for Southern Africa, covering the region 15°S to 38°S and 8°E to 36°E. A detailed investigation of the latitudinal variation due to external influences (e.g. ring current, ionospheric currents, and magnetic secular variation) on the satellite measurements shows that accepted standard correction procedures can be applied effectively to these measurements over the Southern African continent. However, the standard procedures, which include the potential function correction for ring-current effects and the "track-to-track" correction for ionospheric currents and secular variation, need further investigation before application to satellite measurements south of the continent. Standard procedures, such as the equivalent source technique, were not used to correct the F measurements for altitude variations of the satellite. Instead, the data were divided into data cells centred at 1 degree latitude and longitude intersections and a multiple linear regression analysis was performed on the data in each cell in terms of altitude, latitude, and longitude. The resulting vertical and horizontal gradients were used to reduce individual measurements to a common altitude of 350 km and to the centre of the data cell. The total field anomaly map derived from the corrected satellite data shows a large positive anomaly south of the continent and coincident with the Agulhas Plateau and the Mozambique Ridge and also with that part of the Transkei Basin lying between the two features. Another positive anomaly off the West Coast coincides with the Walvis Ridge. On the continent a band of negative anomalies runs from the South-Western Cape to the Transvaal. Another negative anomaly occurs over northern South-West Africa and a positive anomaly over southern Botswana.
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Transactions of the Geological Society of South Africa, 89 (2), 103-116
An enigmatic topic in the Namaqualand Metamorphic Complex of southern Africa is the elusive "floor" on to which the mid-Proterozoic Aggeneys Subgroup was deposited. In Bushmanland, grey gneisses were considered suitable candidates for basement around the Namiesberg and have been termed the Achab Gneiss. Further work has revealed this lithology to be a suite of quartzo-feldspathic gneisses which is more widespread than previously suspected and, in part, may be correlated with the augen gneiss around the Aggeneysberge. Geochronological data demonstrate that the Achab Gneiss is older than the Aggeneys Subgroup, but structural evidence indicates that a thrust zone separates these two units. Such a horizontal tectonic regime has been recognized as responsible for juxtaposing rocks of different ages elsewhere in Namaqualand. The referred interpretation in this region is that the Achab Gneiss intruded a "pre-Bushmanland" sequence and represents a "floor" to the Aggeneys Subgroup. Subsequently, an early complex fold-and-thrust event resulted in the development of imbricate structures by utilizing contacts between units of contrasting viscosity as zones of movement.
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Transactions of the Geological Society of South Africa, 89 (2), 117-142
This study presents the results of a detailed palaeomagnetic sampling of the Haib metavolcanics and the adjacent Vioolsdrif Suite combined with intensive laboratory demagnetization including thermal demagnetization, petrographic examination to identify the carrier and gene sis of the remanence, and a more accurate estimate of the age of magnetization using the 41Ar/39Ar dating method.
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Transactions of the Geological Society of South Africa, 89 (2), 143-170
Palaeomagnetic samples were collected from sites along a 500 km traverse from the high-grade metamorphic rocks of the central area of the Bushmanland Subprovince near Okiep, to amphibolite facies rocks of the eastern area of the belt, along the western border of the Kaapvaal Craton. This traverse crossed several major shear zones, including the Brakbosch Fault, west of the western limit of the Kaapvaal Craton. The Namaqua zone pole (Long. = 328°E, Lat. = 8°N, N = 6, K = 20, α95 = 15) is consistent with previously published palaeomagnetic poles for the Okiep Norite and the Noncai (Konkoonsie) Gabbro, although opposite in polarity to the Okiep remanence. Also reported is a new palaeomagnetic site pole from the Port Edward Charnockite (Long. = 149°E, Lat. = 5° N, N = 6, K = 57, α95 = 9), which is also consistent with the Okiep results. Rocks from four sites distributed over 200 km along the eastern margin of the Gordonia Subprovince yield a completely different palaeomagnetic pole (Long. = 203°E, Lat. 41°S, N = 4, K = 14, α95 = 26). The drastic change in the direction of remanent magnetization seems to occur between Kakamas and Upington. A 1,2 Ga 40Ar/39Ar plateau date on cummingtonite from the Jannelsepan amphibolite, suggests that the metamorphic rocks of the eastern area may have cooled through their magnetic blocking temperatures at a significantly earlier time than the granulites of the central area. Additional 40Ar/39Ar analyses are in progress to delineate more accurately the cooling history of these two subprovinces of the Namaqua Province. The abrupt shift in the palaeomagnetic vector either corresponds to a very rapid migration of the assembled Gondwanaland cratons towards the pole, or it reflects ~3 000 km left-lateral displacement of the Kaapaal Craton with respect to the Namaqua Province during the late stages of the Namaqua Orogeny at approximately ~1 050 Ma. A more extensive palaeomagnetic sampling programme will be required to ascertain which of these.
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Transactions of the Geological Society of South Africa, 89 (2), 171-183
Palaeomagnetic samples were collected from two dykes belonging to the Gannakouriep dyke swarm, one located in the centre of the Richtersveld Subprovince (8215), the other further south in the Steinkopf Domain (8212), where it has been clearly overprinted by a later metamorphic episode. Detailed thermal and A.F. demagnetization indicate that the primary remanence has been extensively over printed in both dykes. The palaeomagnetic pole corresponding to the overprint directions are 80°N, 209°E (N = 6, K = 23, α95 = 14) for 8215 and 37°N 353°E (N = 6, K = 72, α95 = 8) for 8212. Analyses of the thermal demagnetization data by using convergent remagnetization circles suggest a possible primary component with a corresponding palaeomagnetic pole 22°S, 67°E. 40Ar/39Ar step-heating analyses of a whole 8212 yield a plateau spectrum with an integrated age of 542±4 Ma; whereas the 41Ar/39Ar spectrum for a whole-rock sample of 8215 yields a saddle-shaped spectrum with the minimum at ~400 Ma. The palaeo-magnetic pole and the 41Ar/39Ar result for 8212 are both consistent with a metamorphic overprinting during the Pan-African Orogeny. The secondary remanence for 8215 appears to be a present- day field direction. The 41Ar/39Ar spectrum for 8215 may reflect partial 41Ar loss during Karoo times, as suggested by 41Ar/39Ar results from the Vioolsdrif diorite. The palaeomagnetic pole for the suggested primary component does not coincide with previously published versions of the 500 to 900 Ma portion of the African apparent polar wander path, but does lie near the 700 Ma portions of proposed Gondwanaland APW paths.
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Transactions of the Geological Society of South Africa, 89 (2), 185-198
Recent structural mapping along the Orange River between Prieska and Upington has shown that the Kheis Subprovince folds (KF1-3) were overprinted by the main (NF2) Namaqua folds. The 1 800 Ma-old Kheis Subprovince diverges northwards away from the 1 300 to 1 000 Ma-old Namaqua Province and extends for at least 350 km beneath the dune-covered southern Kalahari. Low metamorphic grade quartzites, phyllites, and chloritic amphibolites of the Olifantshoek Sequence were formerly deposited in fluvial to shallow marine settings parallel to the Kaapvaal Craton margin. The sequence was folded and thrust eastwards as a thin-skinned thrust belt and is subdivided into six tectonostratigraphic domains, separated by probable thrust zones with mylonites in some places. Large and small-scale early KF1 recumbent folds affected both the Matsap and the overlying Groblershoop formations though the former was deformed mainly by bedding-parallel thrusts. In the west the KF2 folds are also east-vergent and associated with a second set of thrusts. These structures are correlated with the early NF1 folds and closure of the Wilgenhoutsdrif volcanic basin in the Namaqua Province. Large, open, north- plunging KF3 folds are confined to discrete zones mainly along the east side of the Skeurberge. Transition to the Namaqua Province occurs across a 20 to 50 km- wide zone, designated the Upington Terrane, and consisting of mainly Kaaien Group quartzites, overprinted by the north-west-trending Namaqua Province NF2 folds and metamorphic zonation. Early thrust domains of the Groblershoop Formation within the Kheis Subprovince, extend westwards into the Upington Terrane, now folded around the NF2 folds, turned up on edge and disrupted along the main Brakbos-Dagbreek-Straussburg shear zone that forms the south-west boundary with the adjacent Kakamas Terrane. The main feature of the Kakamas Terrane is the central axis of high-grade gneiss, granulite, charnockite and late-tectonic granite across which the steeply-dipping, divergent foliation fan was arched (ND4) in a transcurrent shear regime. The north-east boundary with the Upington Terrane is marked by calc-alkaline amphibolites of the Jannelsepan Formation. In the south-west of the Kakamas Terrane the lithologically and structurally different Korannaland Sequence may be part of an accreted microcontinent behind the Jannelsepan "arc". Thrust mechanisms, possible accretion, and obduction of the early precursor Wilgenhoutsdrif volcanic basin, are discussed.
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Transactions of the Geological Society of South Africa, 89 (2), 199-213
The Port Shepstone-Port Edward area of Natal contains extensive bodies of granitoid intrusive into older, high-grade, supracrustal gneisses. This paper describes the petrographic, Rb-Sr isotopic and geochemical characteristics of four of these granitoids, namely the Glenmore granite, Nicholson's Point granite, Port Edward enderbite and Bomela charnockite. The Glenmore, Port Edward, and Bomela granitoids contain abundant feldspar megacrysts whilst the Nicholson's Point granite is coarse grained. Orthopyroxene is common in the enderbite and charnockite and is also developed in certain exposures of the Nicholson's Point granite. Rare fayalite has also been reported from the Bomela charnockite. Chemical variation in these units can be ascribed mainly to changing proportions of the feldspars. Rb-Sr isotope data indicate that these granitoids were formed in the period 1 011 to 859 Ma. The youngest age, dating the crystallization of the Bomela charnockite, indicates that this portion of the Namaqua-Natal Mobile Belt was active for longer than previously thought. At least two of the four intrusives (Port Edward and Bomela) have chemical characteristics (e.g. high HFS element concentrations and FeO*/(FeO* + MgO) ratios) which are typical of A-type granitoids. Their mineralogy, chemistry, and Proterozoic age provide convincing evidence that they are members of the charnockite-rapakivi granite association. Furthermore, field relationships and the A-type character of these two units constrain the age of D2 tectonism in southern Natal to between 987 and 859 Ma. Comparison of the data presented in this paper with that from mineralogically and chemically suitable source rocks on the Kaapvaal Craton and the older supracrustal gneiss succession in southern Natal indicates that it is very unlikely that these four granitoids were derived by reworking of Archaean crustal material or of metasediments like those presently exposed in the region. It is, however, possible that the granitoids were derived from -1 400 Ma mafic to intermediate material like the granulite xenoliths caught up in kimberlites in Lesotho.
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Transactions of the Geological Society of South Africa, 89 (2), 215-222
A model of lithosphere delamination and suture progradation is proposed to explain the complex structural and metamorphic history of the southern Damara Orogen. After continental collision of the Congo and Kalahari cratons, the gravitational forces of the formerly subducted oceanic lithosphere and the subcontinental lithosphere initiated delamination at the crust-mantle boundary below the Khomas Trough. Asthenospheric mantle material intruded the delaminated region resulting in uplift of the buoyant continental crust. The absence of syntectonic granites and high-grade metamorphic rocks in the Khomas Trough implies cold-mode delamination whereby the mantle intrusion cooled and inverted to garnet lherzolite. This material, of excessively high density above the descending lithospheric slab, resulted in an increase of the slab-pull force. Another consequence of continental collision between the Congo and Kalahari cratons is that the ridge-push force increased due to elevated heat flow below the spreading centres elsewhere. Thus, both driving forces increased after continental collision which resulted in suture progradation involving subduction of continental crust along a new major thrust in the southern Damara Orogen. This caused a decrease in both temperature and pressure after the first metamorphic event. Suture progradation was eventually terminated by the buoyancy of the excessively thickened continental crust of the southern Damara Orogen and isobaric heating led to the post-tectonic M2 metamorphic event. Finally, complete detachment of the previously delaminated subcrustal lithosphere concluded orogenesis and initiated rapid uplift and erosion.
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Transactions of the Geological Society of South Africa, 89 (2), 223-232
Recent studies of young rift systems have contributed much to our knowledge of continental rifting and its complexities. Furthermore, palaeorifting environments are being increasingly documented in Proterozoic terranes. Despite uncertainty regarding the relative importance of Wilson cycle tectonics versus ensialic orogeny in the formation of most Proterozoic orogenic belts, careful geologic work in many such belts, notably the Damarides, suggests that initially rifting environments were dominant. Continental rifting tends to produce distinctive lithologic assemblages, petrochemical signatures, and basement-cover rock relationships, and also tends to spawn a distinctive spectrum of metal deposits. However, the intense deformation and high-grade metamorphism that characterize the interior portions of Proterozoic orogens can grossly obscure these early geologic and metallogenic relationships. However, insights gained from careful study of less complex belts, and even certain Palaeozoic terranes can be utilized in exploration for metals in complex Proterozoic terranes. Two ore types, sediment-hosted massive sulphide deposits and stratiform copper deposits, that are generated in continental rift settings, are of particular importance in terms of mid- to late-Proterozoic metallogeny. Examples of such deposits are known in both little metamorphosed and highly metamorphosed Proterozoic terranes and represent a major fraction of the base metal resources of the southern hemisphere.
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Transactions of the Geological Society of South Africa, 89 (2), 233-241
Pyritic base metal sulphide and lode gold occurrences within the Virginia Piedmont of the Appalachian orogen are hosted by the Chopawamsic Formation. This late Proterozoic to early Palaeozoic succession of thin-banded mafic and felsic metavolcanic rocks, interlayered with epiclastic plus chemical metasedimentary rocks, formed in a volcanically active rift setting. An evolutionary model is proposed in which laterally extensive fissure volcanism during latest Proterozoic through Cambrian produced a western volcanic suite dominated by felsic pyroclastic rocks, a central suite of interlayered epiclastic sedimentary and bimodal volcanic rocks, and an eastern mafic volcanic suite. This initial stage culminated in high level intrusions, felsic pyroclastic volcanism, and regional deposition of ironstone. The rift was a non-volcanic shale basin during most of the Ordovician. After fore-shortening during late Palaeozoic metamorphism, it remained stable until the Mesozoic when widespread intrusion of diabase dykes and the formation of deep molassic graben accompanied the opening of the Atlantic. The base metal occurrences are spatially associated with ironstones rich in alumino-silicate porphyroblasts interlayered with epiclastic metasediment and bimodal metavolcanics of the central suite. The lode gold occurrences are spatially associated with a lithologically diverse garnet-grunerite iron-formation, cummingtonite-tschermakitic hornblende ironstone, aluminous quartzite, and ultramafic metavolcanic assemblages occurring along the base of the Ordovician metashale. Heat flow within the rift was greatest during the period of high level intrusion and pyroclastic volcanism. Hydrothermal activity at this time was responsible for alteration of the volcanic rocks, deposition of the diverse ironstone-aluminous quartzite association, and concentration of base metals and gold. The metallogeny of the Chopawamsic belt suggests an initial high water/rock system in which massive base metal sulphides were deposited near discharge sites. Subsequent to compaction and dewatering a low water/rock system concentrated gold in mineralogically complex ferro-aluminous beds.
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Transactions of the Geological Society of South Africa, 89 (2), 243-251
The recently investigated 10 km-thick Hatches Creek Group is exposed over 15 000 km² in the southern part of the Proterozoic Tennant Creek Inlier, central Australia. It unconformably overlies the 1870 Ma old Warramunga Group to the north, and was deformed and later intruded by post-tectonic granite plutons, one of which has been dated at about 1 640 Ma. Rock types include quartzose, lithic, and feldspathic sandstones, basaltic and dacitic to rhyolitic lavas and pyroclastics, and subordinate pebbly sandstone, conglomerate, siltstone, shale, mudstone, and carbonates which in places are stromatolitic. Sedimentary structures indicate fluvial to shallow marine deposition, with rapid sedimentation keeping pace with subsidence. The volcanics are largely subaerial. Granophyre and dolerite sills, probably comagmatic with the volcanics, intrude the lower part of the sequence. The felsic volcanics and associated intrusives (63-77 % SiO2) are potassic. The basalt lavas and dolerite sills (44-57% SiO2) have continental tholeiite affinities. The Hatches Creek Group has been compressed into major upright anticlines and synclines, mostly concentric in style, and regionally metamorphosed to mainly greenschist facies. It is cut by gold-bearing quartz veins, which are possibly syntectonic, and by tungsten-bearing quartz veins containing minor copper, bismuth, and molybdenum, probably related to post-tectonic granites. The prevalence of shallow-water sediments and subaerial volcanics and the bimodal composition of the volcanics indicate crustal downwarping and thinning during a tensional regime. Underlying sialic rocks indicate an intracratonic setting. Sedimentation and volcanism do not appear to be associated with rift structures, and the middle and upper parts of the group were probably laid down in an extensive epicontinental sea.
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Transactions of the Geological Society of South Africa, 89 (2), 253-262
Within the Mount Isa Inlier, crystalline basement rocks, older than about 1840 Ma, are overlain unconformably by Middle Proterozoic cover sequences dated at 1810 to 1670 Ma or possibly 1600 Ma. These cover rocks are many kilometres thick and represent predominantly shallow-water sediments and largely subaerial bimodal volcanics. They were probably laid down in an intracratonic setting, rather than at an active continental margin, during an essentially tensional tectonic regime, when subsidence more or less kept pace with deposition. Complex folding, faulting, and thrusting, and regional metamorphism to greenschist and amphibolite facies took place between about 1620 and 1550 Ma. Pre-tectonic and post- tectonic granites and several generations of mafic dykes intrude the cover sequences. The felsic igneous rocks include both I and A-types, and the mafic igneous rocks are mainly of continental tholeiite type.
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Transactions of the Geological Society of South Africa, 89 (2), 263-283
This paper discusses the occurrence of an early Proterozoic deep crustal basic granulite-anorthosite suite and its anatectic derivates in central Australia.
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Transactions of the Geological Society of South Africa, 89 (2), 285-304
This paper reviews apparent similarities in the Pb-isotope patterns of galenas from South Africa and Western Australia. As prelude, the discussion is preceded by a brief summary of the geological information contained within these uranium- free leads, and of the all-important question of "pseudoisochrons": how do we decide when an apparent lineation on the standard isotopic variation diagrams has no geochemical meaning, but is merely an expression of analytical error? Discussion proceeds by means of examples culled from the literature, augmented for Western Australia by a summary table of some new precise data. A growing body of successful correlations between presumed "Pb-source rock" and displacement from the "ore-lead growth curve" point to a possibly major difference between the neighbouring Murchison and Barberton regions in South Africa. It is concluded that the similarities between the post-Archaean of Western Australia and the Aggeneys-Gamsberg locality in the North-West Cape Province is likely to be a reflection of no more than a similarity in past geological processes. Modelling of such a process seems best left until more detailed locality studies can justify selection of the geologically relevant data.
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Transactions of the Geological Society of South Africa, 89 (2), 79-96
In the absence of proof provided by the fossil record and inadequate palaeomagnetic data, verification that the Namaqualand Metamorphic Complex was accreted to the Kaapvaal Craton is sought in a study of the distribution of various lithologies and lithological assemblages as well as in the tectonic relationships. It has been demonstrated that the rocks of the Richtersveld Subprovince are, on the whole, older than, and completely different from, those of the Bushmanland Subprovince. These two areas are separated by a strongly tectonized zone and, in part, by a chain of mafic/ultramafic bodies. It is thought that these two subprovinces were accreted to the African continent as a unit approximately 1 400 to 1 200 Ma ago. The collision zone is marked by a wide belt of many mafic/ultramafic bodies and dyke swarms as well as by structures usually found associated with sutures of Cenozoic age.
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Transactions of the Geological Society of South Africa, 89 (2), 97-102
Assemblages in pelitic gneisses reveal a symmetrical pattern of east-west-trending zones in the western Namaqualand Metamorphic Complex. Within the granulite facies region a central zone of highest grade contained hercynite-rich spinel coexisting with quartz at the metamorphic peak over an area of about 10 000 km², suggesting T>=800°C at P <=5 kb.
The metasedimentary rocks experienced constant or increasing pressure during both the prograde and retrograde high T history. Uplift, therefore, did not play a part in achieving the very high thermal gradients of the metamorphic peak. Low variance mineral assemblages and evidence for local gradients in aH2O indicate that moving fluids were not responsible for the heat flux. The P-T evolution can be explained by the addition of voluminous acid magmas at or above the present erosion level. The metamorphic history rules out any tectonic model for the Namaqualand Metamorphic Complex which would be grossly out of isostatic equilibrium during the metamorphic climax.
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Transactions of the Geological Society of South Africa, 89 (3), 305-310
Soft-sediment deformation structures of an unusually large size have been recorded in the Carbon Leader, a Witwatersrand braided-stream placer, on East Driefontein gold mine. Two types of structure were recorded: dish structures and diapirs. The dish structures underlie the diapirs which comprise metre-high intrusions of quartz arenite which have invaded an overlying conglomerate. The deformation structures are thought to have formed during water-escape as a result of differential loading by the gravel bed on a water-saturated, underlying, planar cross-bedded sand bed.
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Transactions of the Geological Society of South Africa, 89 (3), 311-323
A study has been undertaken of the deformation of the Black Reef Formation in the vicinity of the northern portion of the Witwatersrand Basin to establish the nature of Post-Transvaal Sequence structural features which may have affected the Witwatersrand Basin. Deformation of the Black Reef Formation has produced major and minor open to overturned folds and penetrative cleavage, all of which are heterogeneously developed through simple-shear processes. Strikes of cleavages and fold trends are parallel and they are tangential to the Vredefort structure. Cleavage dips towards this structure and at a shallower angle than that of the axial planes of folds. The authors conclude that these features may be genetically related to the development of the Vredefort structure. Cleavage in the Transvaal Sequence is regionally associated with the presence of underlying Witwatersrand Supergroup rocks (which are also cleaved), but not Klipriviersberg volcanics. This suggests that shear was transmitted predominantly through Witwatersrand Supergroup strata, giving rise to cleavage and folding. This shearing may locally have led to the development of bedding sub-parallel faults. Movement of these faults may have involved an initial outward movement followed by later retraction.
The cleavage deforms a sill associated with the Bushveld Complex and an associated thrust has been dated as post-Bushveld and pre-Pilanesberg in age. Emplacement of the Johannesburg Dome appears to have dispersed the cleavage and reactivated older structures and, therefore, may postdate the Vredefort event. The cleavage is a useful time datum in relative dating of the structural events in the vicinity of the Witwatersrand Basin. In addition to the above, the authors have also recognized a set of easterly striking folds and associated cleavage which are developed in the north-western portion of the study area, and a set of east-west-striking right-lateral faults. The regional significance of these latter events is not known.
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Transactions of the Geological Society of South Africa, 89 (3), 325-334
A correlation is suggested between aeolian sand belonging to the Kalahari Formation and sandy deposits of Cenozoic age along the south-eastern banks of the Vaal and Orange River, between Orkney and Prieska. The pronounced gap between these deposits is explained in terms of post-Tertiary erosion. The preservation of sandy deposits to the south-east of the Vaal and Orange Rivers and the removal of equivalent deposits to the north-west of these rivers are ascribed to differential erosion in response to recent crustal warping along the Griqualand-Transvaal Axis. This axis of diastrophism lies within the gap area. In the Lichtenburg and Delareyville districts a small-scale model is afforded in which pattern of grain size and sorting in the soil can be interpreted in terms of differential erosion.
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Transactions of the Geological Society of South Africa, 89 (3), 335-345
The Lease Granite, a fine-grained member of the Lebowa Granite Suite, is well exposed at the Zaaiplaats Tin Mine. It is intimately associated with the Bobbejaankop Granite and is restricted to roof zones of Bobbejaankop Granite stocks. Field investigations indicate that the Lease Granite is intrusive into granophyric granite of the Rashoop Granophyre Suite. Petrographically and geochemically the Lease and Bobbejaankop Granites are almost identical, however, they contain more fluorite, sulphides, and sericite than the granophyric granite, but less biotite, apatite, and sphene. The Lease and Bobbejaankop Granites are enriched in Ti, Fe, Mn, and P relative to the granophyric granite, but depleted in Ba, Zr, and Zn. The Lease and Bobbejaankop Granites are also highly metasomatized and more differentiated than the granophyric granite. Field relations as well as geochemical and petrographical evidence indicate that the Bobbejaankop and Lease Granites probably crystallized from the same magma and that the latter possibly represents a quenched volatile enriched portion of the Bobbejaankop Granite.
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Transactions of the Geological Society of South Africa, 89 (3), 353-360
Five unconformity-bounded sequences of Middle Proterozoic red beds, all previously thought to be portions of the Waterberg Group, occur in north-western Transvaal. The oldest sequence is equivalent to the Loskop Formation; the upper four are parts of the Waterberg Group. Because each of the sequences within the Waterberg rests not only on one or more older sequences, but also on crystalline basement rocks, each is bounded by major unconformities. The five sequences overlap northward, so that four basal unconformities exist in the Waterberg. Consequently, the actual thickness of the Waterberg Group in most places is less than the composite stratigraphic thickness: the succession is usually less than 3 km thick, and probably nowhere as thick as 5 km. The upper two sequences of the Waterberg, at least, are younger than 1 770±60 Ma. The distributional patterns of the five sequences are probably preservational, not depositional. Therefore, the original thickness, depositional centre, and areal extent of each sequence cannot be determined. Because such comparisons cannot be made between sequences, protobasins cannot be defined, and the migration of depositional axes with time cannot be determined. The so-called Nylstroom protobasin consist of two unconformity-bounded sequences preserved in a syncline. On a larger scale, the four Waterberg sequences are preserved in a synformal downwarp.
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Transactions of the Geological Society of South Africa, 89 (3), 361-365
The Ngoye Granite-Gneiss Formation is located in the Natal sector of the Namaqua-Natal Mobile Belt. Eight different granite types have been recognized within the formation. These range from per aluminous to peralkaline in composition. The peralkaline granites underlie about 10 percent of the formation and are essentially restricted to its southern contact with surrounding amphibolitic country rocks. Peraluminous granites are typically muscovite- and garnet-bearing whereas metaluminous varieties contain pale-green biotite and hornblende. Riebeckite and aegerine, with zircon and fluorite as common accessory minerals, are characteristic of the peralkaline granites. Geochemically, the peralkaline samples analysed display a range in agpaitic index (A.I. = mole Na2O + K2O/Al2O3) from 1,02 to 1,08. They are generally depleted in Al, Ca, and Mg relative to average granites, whereas Na and K are relatively high. Trace element analyses show enrichment of Nb, Zr, and Zn and depletion of Ba and Sr in the peralkaline granites. Stream sediment samples have delineated several mineralization targets around the margin of the massif. Those along the southern contact are associated with magnetite-quartz-rich rocks, which are highly enriched in Nb, Zr, Y, Zn, U, Th and to a lesser extent Sn and W. One target on the north-eastern margin of the formation has enhanced Sn contents. The Ngoye Granite-Gneiss Formation is similar in size, morphology and composition to the younger anorogenic granite complexes of Nigeria, Sudan, and Saudi Arabia. It may be classified as an "A" type or "within-plate" granite complex.
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Transactions of the Geological Society of South Africa, 89 (3), 367-372
Close examination of recently identified heavy mineral layers in the quartzites of the Bushmanland Group near Aggeneys suggests a clastic sedimentary origin for these rocks. The application of stereoscopic X-ray radiography could lead to establishing facing directions in these highly metamorphosed and deformed rocks. This would greatly benefit structural-stratigraphic mapping in the area.
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Transactions of the Geological Society of South Africa, 89 (3), 373-383
A study of 43 stratigraphic field and boreholes sections of the glaciogene Dwyka Formation in the western and central parts of the Karoo Basin shows that a lithofacies approach with lithofacies codes similar to that in use for fluvial sediments, can be successfully applied to illustrate facies changes. Analysis of local and regional facies variations, based on the presence of marker beds, indicates that inter-unit correlation, even over short distances (< 1 km), is poor as a result of the depositional processes involved, climatic and sea-level changes, and the plaeotopography. Lateral facies variations can be sufficiently quantified by means of triangular plots of mutually related lithofacies and transition count matrices followed by the reconstruction of facies relationship sequences, to define a northern and a southern facies region in the basin. In the northern region the random facies arrangement is attributed to deposition largely in valleys and inlets on a dissected highland, whereas the southern region is represented by a platform facies association with a higher order of facies arrangement. The break between the two domains coincides with an east- west palaeoescarpment. The lateral facies variation also emphasizes shortcomings in the present stratigraphic nomenclature of the glaciogene deposits in the western and central parts of the Karoo Basin and it is suggested that the Dwyka Group should consist of a Mbizane Formation in the north and a Rhemhoogte Formation in the south.
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Transactions of the Geological Society of South Africa, 89 (3), 385-388
Along the Ghaap Escarpment near Boetsap a Permo-Carboniferous glacial pavement, partly overlain by fluvioglacial Dwyka deposits, occurs on carbonate rocks of the Griqualand West Sequence. The pavement exhibits large grooves and poorly preserved striations and crescentic fractures, suggesting ice flow to the south-south-west. The elevation of the pavement and the presence of dark-grey Karoo shales in minor valleys along the flank of the large Permo-Carboniferous Kaap Valley imply that the Ghaap escarpment was a preglacial topographic feature and that palaeoice flow was entirely controlled by the topographic grain of the region.
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Transactions of the Geological Society of South Africa, 89 (3), 389-393
Investigations of the Upper Triassic Clarens Formation in the Natal Drakensberg have led to relatively minor changes in sedimentary facies descriptions, confirmed the palaeocurrent trends, and promoted a better understanding of facies relationships previously suggested by Eriksson (1981). Wadi-channel, sheetflood, playa-lake, and debris-flow deposits proposed in the earlier study are assigned to an overall distal alluvial-fan depositional environment. The postulated dominance of aeolian sediments to the north of the initial field area is confirmed. Widespread aeolian deposition apparently succeeded alluvial-fan sedimentation in the south and east of the Natal Drakensberg study region.
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Transactions of the Geological Society of South Africa, 89 (3), 395-400
Dispersed agglomerations and tubular root fillings of syn- to epigenetic vivianite occur in an olive-dark-grey silt layer forming part of a 20 000 years Late Pleistocene sequence north of Clarens. In fresh exposures the unoxidized vivianite is white, but the mineral oxidizes within hours to various shades of blue. The dark-coloured, sandy, silty, and clayey host sediments with subordinate gravel lenses were interpreted as meander channel and poorly drained swamp deposits formed in a relatively cool, wet climate. Biologically concentrated phosphate was released to the depositional environment and enabled the lithofacies-related formation of vivianite which was controlled by a very low sedimentation rate, reducing environment, Fe" and phosphate-enriched interstitial waters, a pH of approximately 7,4 and cavities in the sediment.
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Transactions of the Geological Society of South Africa, 89 (3), 409-414
A Fortran IV computer program which displays detailed coal-seam data is described. The graphic log consists of a lithological column displaying codes for coal and rock descriptions, with corresponding thickness, elevation and depth values and associated contact relationships, a structure/qualifier column appears to the right of the lithology column wherein predetermined grain-size offsets for coal and non-coal units are displayed. Coal sample numbers and widths, together with certain quality parameters, are listed adjacent to the stratigraphic and structure-qualifier columns. A common field is provided to the right of this information.
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Transactions of the Geological Society of South Africa, 89 (3), 415-418
Coarse volcaniclastic deposits, comprising poorly sorted, subrounded to subangular volcanic-rock clasts set in a fine matrix, outcrop in the Hekpoort Formation west of Pretoria. The coarse deposits form a lens, set within a larger body of finer volcaniclastic rocks. A subhorizontal planar fabric in the coarse volcaniclastic deposits appears to have formed largely by compaction. Petrographic examination of these rocks reveals extensive alteration with poorly preserved remnants of primary plagioclase and clinopyroxene. The coarse volcaniclastic rocks are thought to have been emplaced as a volcanic debris flow, or lahar.
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