1 (1), 1-10
Measurements of the stress orientation are directly relevant to a tectonic analysis and can be related to both visible structures and to the kinematic pattern. The most widely used method for obtaining stress orientation data is by analysing earthquake focal mechanisms using P-wave first motion analysis. Other tectoniques include the study of shear wave polarization, direct measurement by overcoring, or else indirectly by measuring borehole breakout or from a study of fault slip data. The origins of tectonic stresses are discussed and a composite stress map for Southern and East Africa is presented. Once the orientation of the neotectonic (present day) stress field has been determined it is possible to predict which pre-existing structures are favourably orientated for re-activation by strike-slip or normal motion. It is postulated that these structures would be the most favourable aquifers. A case study of the application of this technique in the Swartwater area is presented.
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1 (1), 11-20
As part of the National Water Master Plan study for Botswana the country's ground water resources were assessed. Borehole data were used and processed to determine aquifer representative values for borehole yield, success rate, aquifer thickness, ground water level and transmissivity. Aquifer basins were delineated from geological and hydrogeological maps. Extractable ground water resources were determined on a regional scale assuming a wellfield of 3 x 3 wells spaced 500 m in a regular and. Over a pumping period of 30 years a certain restricted drawdown was allowed. Recharge was calculated using different methods as a soil water balance modelling, chloride mass balance and ground water modelling. The main aquifer formations of Botswana were described and the available resources were calculated. Ground water quality data were processed to determine principal hydrochemical characteristics and to assess potability. There is sufficient ground water of good quality to meet the predicted demand to the year 2020. However, locally, overdevelopment of the ground water will be unavoidable if Botswana's economic development is to be sustained at the current pace.
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1 (1), 21-26
Rehabilitation is the process whereby a borehole is restored to its optimum production-level/condition or better. This can be achieved by either physical/mechanical, chemical or other means. However, the rehabilitation of a borehole, admits to the fact that basic house-keeping procedures have been neglected or ignored and/or that especially the subsurface component of a ground water supply system are poorly understood. The basic house-keeping procedures are appropriate and proper monitoring and maintenance. It is information from these that will indicate which method/s will produce the desired result of restoring/improving production. This paper looks at the three basic aspects for the proper management of ground water resources, viz. monitoring, maintenance and rehabilitation, as well as the hydrofracture stimulation technique and the Blended Chemical Heat Treatment (BCHT) rehabilitation technique, and highlights their value to the ground water scientist and the end user.
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1 (1), 27-35
The search for ground water in the Rural Areas and adjoining farms in the North West Cape, at the time of writing under the jurisdiction of the House of Representatives, had prior to 1989 been undertaken purely on an ad hoc basis. No integrated database existed and plans showing borehole localities were incomplete and out of date. To rectify the situation, a hydro-census was initiated in 1990, which entailed the location and sampling of all water points, dry and abandoned boreholes, using a Magellan Global Positioning System. This is currently being followed by detailed geological mapping and geophysical investigations involving resistivity and magnetic profiling, before carrying out drilling operations. Because of demographic considerations, it is not always possible to drill at the optimum site. Nevertheless a success rate of 80% has been attained since the inception of the programme in 1990 which has involved the drilling of some 80 boreholes totalling 8 408 m. A management programme, which involves the monitoring of the water supply to 26 villages on a regular basis and the supervision of six desalination stations, has been initiated. A public awareness programme which entails regular meetings with the communities and illustrated lectures to schools, involving various competitions and the distribution of a specially designed pamphlet, is under way.
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1 (1), 37-43
The philosophy of presenting South Africa's ground water situation on a national map is discussed. Borehole prospects and depth of ground water level below the surface based on a statistical analysis of some 70 000 boreholes, are to be shown on a scale of 1:2 million against a background of simplified lithostratigraphy. The map will be complemented by two smaller maps: a ground water availability and a quality map. Recharge in relative terms is to be shown against four main (saturated) hydrogeologic types: fractured rock; fractured and decomposed/disintegrated rock; karst; and strata with primary porosity. Quality will be depicted by electrical conductivity. Areas where nitrate and fluoride reach critical values are to be outlined.
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1 (1), 45-61
As part of a hydrogeological project in Botswana, BRGM has carried out a multimethod geophysical survey of a large sedimentary basin (50 x 30 Km) in the Kalahari desert. The aim of the survey was first to map the basement topography, affected by horst and graben structures, and then to accurately site the reconnaissance boreholes needed for evaluating the water resources of the basin. The regional mapping of the basement was performed using audio-magnetotellurics (AMT), gravimetry and magnetometry. AMT proved by far the most effective method (much more suitable than magnetometry, more cost-effective and quantitative than gravimetry) as a result of certain favourable conditions (good resistivity contrast between the basement and the cover rocks, regional uniformity of the sedimentary structure, and absence of disturbing "static" effects). The high productivity obtained with AMT was due mainly to the discovery of a fast quantitative interpretation procedure based on reading a single frequency (10 Hz). This procedure used an experimental correlation, established from 22 boreholes, between the depth to basement and the apparent resistivity at 10 Hz. Satisfactory theoretical justification for this correlation was obtained through 1D modeling with models that reflect the specific sedimentary organization of the basin. This correlation is thus mainly a consequence of the regional sedimentary history. 2D modeling did not provide a better explanation of the phenomenon and, moreover, it is more difficult to handle because the results depend on each particular geometry. At the end of the regional survey, 20 favourable areas covering the junctions between horsts and grabens were selected for detailed HLEM and DC profiling, the purpose of which was to precisely locate the fractured zones at the edges of deep grabens, which are ideal targets for hydrogeological exploration drilling. This approach was successful since six out of the 20 boreholes gave an air-lift yield greater than 30 m³/h.
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1 (1), 63-70
It is commonly accepted that under natural conditions and without abstraction, there exists a dynamic equilibrium between recharge and drainage in the aquifers of eastern Botswana. For the Kalahari Basin, the question as to whether there is such a dynamic equilibrium has not yet been satisfactorily resolved. Progress has been made recently with the development of lumped-parameter recharge simulation models which make explicit use of the dynamic equilibrium between recharge and drainage, using a simple linear reservoir to simulate aquifer drainage characteristics. Precipitation and evapotranspiration measurements are used as input to these models which may be calibrated with observed soil moisture and water level values. Statistical analysis of the available data, coupled with the results of such a recharge simulation model, indicates that the distribution of recharge events can be modelled by a compound Poisson process with a simple exponential event distribution. The first and second moment of natural groundwater level fluctuations can then be calculated through the theory of random differential equations, yielding expressions relating water level changes to specific yield, drainage resistance, frequency of recharge events and long-term average recharge values.
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1 (1), 71-79
Ground water for supplying the demand of Otjiwarongo town is abstracted from a marble aquifer. Expansions to the existing abstraction facilities were preceded by geohydrological investigations, facilitating characterisation and conceptualisation of this heterogeneous and fissured water table aquifer. Application of a water balance model, based on the saturated volume fluctuation (SVF) method, indicated a ground water recharge potential of three percent of average annual rainfall and an average storativity value of 2.7/10³, assuming no inflow to and outflow from the aquifer compartment. Despite aquifer heterogeneity, a satisfactory simulation of ground water levels in the oldest production area, the so-called Omatjenne Compartment, was achieved by an in tandem utilisation of the inverse model AQUA-INV and the 2-D finite element model AQUAMOD. Calibration and verification of the model did, however, indicate certain limitations to the present model's prediction capabilities, especially concerning transient recharge anomalies and prolonged recharge events associated with the dissipation of a recharge mound resulting from dam-bed infiltration from the Omatjenne Dam.
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1 (1), 81-88
The Mueda plateau in northern Mozambique is a Cretaceous sandstone erosional feature rising 200-300 m above the surrounding countryside. Its plane summit is densely wooded and is largely devoid of surface drainage. The recent sharp increase in population of the plateau has put pressure on water supplies. A pumped reticulation system deriving its water from some of the many springs which emerge at the foot of the plateau has repeatedly failed due to the erratic fuel supply. The alternative of a borehole water system is being investigated. Environmental isotope data from several springs show that the rain recharge over the plateau is very uniform, and that ground water residence times are of the order of a century. Calculated nett rain recharge to the deep ground water mound is much larger than the total flow of known spring occurrences, implying major losses due to diffuse ground water discharge. The alternative of a borehole-based water supply on the plateau itself may therefore be considered as: (1) yields will be more reliable than springflow, (2) water can be supplied where required and (3) various power sources could be employed.
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1 (1), 89-94
Ground water is a major source for the water supply in Botswana and elsewhere in Africa. The major part of all villages and all mining centres of Botswana depend on wellfields and their future relies on the availability of reliable water. Therefore good monitoring schedules are required as warning systems for the water supply and the economic development of Botswana. There are currently 27 major wellfields in Botswana. The geological set-up for these wellfields varies from place to place, but the major aquifers are weathered or fractured Basement Complex, Transvaal Dolomites, Fractured Waterberg, Karoo Sandstones and Arkoses, Kalahari Beds and Sand river beds. All wellfields are monitored by the Department of Water Affairs (DWA) and Department of Geological Surveys (DGS). Ground water level, abstraction rates and ground water quality are monitored on a regular basis. Each wellfield is equipped with a number of automatic water level recorders with charts being changed every 32 days. It is a convenient system considering the wide areal distribution of the wellfields. All recorders are installed in observation boreholes to monitor the general ground water level fluctuations in aquifers. In addition manual water levels are taken in production boreholes using standard electrical dippers. To ensure that water levels do not drop to pump intake levels without the knowledge of the operator, two measurements are taken in the production borehole each day of operation; before production starts and before it is switched off. These water levels are taken together with readings of the amount of ground water abstracted. The data collected during the monitoring of the wellfields are stored in a database developed by DWA in co-operation with DGS. Data in the database can be easily retrieved, displayed and used to evaluate the ground water situation. Periods of drought have had an effect on water level fluctuations in the wellfields with high abstraction. Basically the ground water abstraction in these wellfields exceeds the natural replenishment and the wellfields are operating under mining conditions. The most affected areas are Maun to the northwest of Botswana, Moshupa within the Gaborone area and Orapa in the central part of the country. DWA regards the water resources monitoring as an issue of extreme importance and the monitoring system is continuously upgraded including additional water level recorders, improved operational schemes and personnel training.
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1 (1), 95-99
In special conditions of arid, semi-arid and wet and dry regions, ground water is one of the most important African natural resources. In semi-humid parts of the African tropics ground water also plays an important role since many surface resources are intermittent or polluted and infected and thus local supply needs, cattle watering, irrigation and even industry depend on ground water sources. An effective ground water management becomes one of the prerequisite conditions of the sustainable development in Africa. In addition, a proper ground water management is an efficient tool in the restoration and conservation of the African environment. Effective management and development of ground water resources and restoration of the damages done to the ground water resources and aquifers elsewhere in Africa are important and challenging tasks for modern African research and technology.
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1 (1), 101-106
The population of Maputo and its suburbs mainly depend on shallow ground water for its water supply. Deep aquifers are not suitable for exploitation due to their high salinity. A sedimentary sequence hosts a shallow aquifer beneath the city; several hundreds of water wells, both for private and public use, are present. The shallow aquifer is highly heterogeneous, with transmissivities ranging from 4 m²/d to 1200 m²/d. A transmissivity map is presented and an analysis of the correlation of transmissivity (T) and specific capacity (Qs) is attempted. Ground water quality is affected both by salinity levels and nitrate concentrations. Salinity is related to past marine ingressions and to present day salt water intrusion. Nitrates appear to be related to high population density and to current sanitation practices; the NO3 map shows good correspondence of high nitrate areas with the longest and most densely inhabited districts of the city. Recharge to the shallow aquifer is predominantly local, and recharge areas generally coincide with high NO3, densely populated areas. An analysis of frequency distribution of NO3 concentrations indicates that 27.7% to 48.5% of ground water samples are above the 50 mg/l W.H.O. limit for potable water. Both local infant-feeding practices and existing literature suggest that the W.H.O. limit may be an unnecessarily strict goal in specific situations. Though no data are currently available on industrial/chemical ground water contamination, this should be considered as a potential high risk factor for both ground water quality and human health.
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1 (1), 107-111
The majority of water resources in Southern Africa are principally surface water and have resulted in larger scale development. However, water supplies of up to 5 million cubic metres per year can be adequately supplied by means of ground water. Distinction is made between primary and secondary aquifers. Analysis is made in terms of capital and operating costs of surface and ground water schemes. This paper assesses merits and relative costs of ground and surface water as a resource in southern Africa.
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1 (1), 113-118
The importance of ground water as a supply source for rural communities in South Africa, and the need for the management of the resource to ensure its sustainability in terms of both quantity and quality have been emphasized by recent droughts. Records of failed supply systems due to the malfunctioning of equipment and/or over-utilisation is considered to be attributable to a large degree to poor management. A prerequisite for the management of any system is an understanding of that system. Basement aquifers, which constitute the main aquifer in large parts of South Africa, are particularly difficult to define because of their great variability in terms of storage capacity and transmissivity. They can, however, be characterised broadly on the basis of geological environment, and recharge. Input data for the ongoing management of a rural ground water supply system must be easily obtainable at a local level and must also not require expensive or sophisticated equipment. Basic data requirements include rainfall figures, usage volumes, water level fluctuations, water quality and equipment status. This information must be fed into a central database for the district or region and must be evaluated, interpreted and acted upon by qualified people at regular intervals. The maintenance of the quality of the resource must be a responsibility at local level, particularly with regard to the uncontrolled disposal of domestic and other waste. This paper proposes a basic approach for the development of a ground water management system for rural communities. Experience gained in the rural areas of the Northern Transvaal and the Transkei are used to illustrate the feasibility of the approach.
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1 (1), 119-123
In many regions of Africa, consumers are dependent on ground water or surface-water supplemented by ground water. An important part of water supply management in such regions is assessing the reliability of the ground water resource being exploited. This paper proposes a Monte Carlo simulation method assessing the reliability given the uncertainty of the physical characteristics of an aquifer and the unpredictability of rainfall as the source of recharge.
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1 (1), 125-130
About 62 percent of the Mozambican territory is underlain by the Precambrian crystalline basement, frequently covered by a weathered overburden. Ground water forms the principal water source for the population in these areas. The national programme for rural water supply (PRONAR) has started since 1984 to execute boreholes, equipped with hand pumps, to improve the actual coverage of about 25%. Since these boreholes have relatively modest yields (Q < 1.5 m³/h), they can be supported by small local systems in the weathered overburden, provided the saturated weathered thickness is sufficient. Geoelectrical surveys (Schlumberger configuration) have proven to be a rapid and efficient tool to site boreholes. No conclusive relations between geoelectrical parameters (specific resistivity and depths) and the hydraulic potential (specific capacity and yield) were derived. Combining of the geoelectrical methods with other methods (especially geomorphological surveys through aerial photographs and field observations) contribute importantly to the effectiveness of the method. Examples in the Manica provinces show an increase of success rate from 60% to over through siting.
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1 (2/3), III-IV
The West African sub-Saharan Iullemmeden basin, covers most of the western half of the Niger Republic extending into Mali, Benin and North-west Nigeria. The relatively stable intracratonic setting of the basin is ideal for the study of eustatic changes. The near-horizontal topography of the flanks of the basin amplifies the lateral displacement of shallow marine invasion along the shore-line (Rat et al., 1991). Four new major observations are presented: (1) During the Upper Cretaceous, the Tethys already extended as far south as eastern Niger with the deposition of alternating sedimentary sequences consisting of (i) marine deposits (mainly limestones and sandy shales with Mesogean fauna) confirming warm epicontinental marine conditions, and (ii) sub-continental deposits made up of shales and sandstones, rich in fossil wood and vertebrate fossils (Dikouma et al., 1993). The following trangressions occurred: (a) The Upper Cenomanian - Lower Turonian transgression typified by Neolobites and Nigericeras (Mathey et al., 1991; Courville et al., 1991; Alzouma, 1992; Pascal et al., 1993). The theory proposed by Greigert (1966), of a regression ending up with the replacement of the marine environment by a highly salinated transitional environment with shales and primary gypsum, should be abandoned. Isotopic analysis confirm that the gypsum is secondary and resulted from the re-oxidation of sulphur originating most probably from sulphato-bacterial reduction processes within a confined environment (Mathey et al., 1991). (b) The Upper Turonian to Coniacian transgression which is still not adequately studied but is typified by the presence of the ammonite Coilopeceras inflatum of Upper Turonian and probably Coniacian age (Meister et al., 1992). (c) The Upper Maastrichtian transgression with two fluctuations typified by the presence of Libycoceras and Laffiteina. The first Maastrichtian fluctuation in southern Niger corresponds with the Farin-Doutchi Formation also known as "Mososaurus Shales" or Dukumaje Formation in NW Nigeria. The second fluctuation is related to the deposition of a lateral equivalent in NW Nigeria known as the Wurno Formation (Dikouma et al., 1993). The marine sedimentation at the beginning of the Tertiary was biochemical in character as suggested by the dominance of limestones, phosphates and palygorskites. The transgression was typified by the presence of the foraminifera Ranikothalia and Lockhartia. The deposits include the Garadoua Formation and the lateral equivalent in N-W Nigeria (Gamba Formation, Kalambaina Formation and Dange Formation). They were considered by previous authors as the first phase of the (Ct1) sedimentary cycle dated late Cenozoic. They are now considered in this volume by Alzouma and Dikouma as the end of a sedimentary cycle during the Eocene (Fig. 2). The Continental Terminal sensu stricto (Ct2 and Ct3) consists of a ferrugineous facies overlying the first level of oolites discordantly. It represents a different sedimentary cycle (Dubois and Lang, 1981; Lang et al., 1986,1990). (2) The Lower Paleocene is absent in the Iullemmeden basin as new palynologic and micropaleontologic data confirm the Upper Maastrichtian age for the Farin-Doutchi and In-Wagar Formations. A large part of the Garadoua Formation has been attributed to the Upper Paleocene by Carbonnel et al. (1990), Dikouma (1990), Dikouma et al. (1993). These confirm the existence of a break in sedimentation during the Lower Paleocene. The Lower Paleocene unconformity has also been recognised in the coastal basins of Benin-Togo (Bio Lokoto, 1991) and Côte D'Ivoire (Goua Tokpa, 1993a and b, Goua Tokpa et al., 1994). (3) The Iullemmeden basin, located within the heart of the African continent, was linked up by marine transgressions with the Tethys during Upper Cretaceous and Lower Tertiary periods. The Mesogean fauna originating from the Cenomanian-Turonian transgression include the nautilus Angulithes lapparenti and ammonites Neolobites vibrayeanus, Exogyra olisiponensis, Mecaster pseudofourneli and Heterodiadema Libycum (Neraudeau, 1991; Mathey et al., 1994; Neraudeau et al., 1994). The basin was also occasionally linked up with the South Atlantic during two well-dated periods (Greigert, 1966; Reyment, 1971, 1980; Petters, 1977; Kogbe, 1981; Dufaure et al., 1984; Bellion, 1989; Dikouma, 1990; Alzouma, 1992). (a) Paleontologic evidence based on the presence of ammonites from the Iullemmeden basin, Nigerian coastal basins and the Benue Trough (Courville et al., 1991; Courville, 1993; Meister et al.), confirm the linkage by a "seaway" during the end of the Cenomanian. The transgression attained a maximum development by the Cenomanian-Turonian passage corresponding with Horizon IV (Gadeni) and probably Horizon Va (Cauvini) of the global scale of transgressions. The Cenomanian-Turonian boundary corresponds to a sequence boundary (Pascal et al., 1993). Horizon III ("dalle à Exogyres") confirm the linkage of both seas with the higher calcareous content resulting from an increase in the volume of sea water. The higher content of Sr and Ni in Horizons III and IV suggests the opening-up of the sea-way (Pascal et al., 1991). (b) Similarity between the Iullemmeden basin, the North African basins (Mali, "Détroit Soudanais", Algerie, Tademait and Tinrhert) and the Nigerian sedimentary basins (Sokoto, Benue, Niger Delta) is confirmed by the same species of macrofossils found in all the basins. These include echinoderms (Plesiolampas, Linthia sudanensis, etc.); bivalves (Pseudomiltha chavani, Ostrea aviola, Ostrea multicostata, Pycnodonta senegalensis) and gastropods (Gisortia brevis, Heligmotenia molli, Chenopus etc...) - Dikouma, 1992. These similarities further confirm a linkage of the Tethys with the South Atlantic during the Upper Paleocene. The discovery of the Paleocene nautilus Deltoidonautilus sudanensis, Deltoidonautilus chudeaui in the basin provide additional favourable evidence (Dikouma et al., 1987). The ostracod fauna tend to be a useful evidence of a mixed fauna from both the Algerian coastal basin and the north African (Lybian) and middle-eastern basins (Carbonnel et al., 1990). (4) Cenomanian-Turonian marine fluctuations coincide with global eustatic movements which are closely related to regional climatic changes brought about by cyclic sea level changes (Mathey et al., 1991). The abundance of palygorskite during the Upper Paleocene suggests a much drier climate than during the Upper Cretaceous as the equator was slightly displaced southwards.
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1 (2/3), 131-281
The Iullemmeden basin is an intracratonic basin covering an immense area extending from the Republic of Mali in the west to the western borders of the Chad Republic (1000 x 800 Km² approximately). It is, however, relatively shallow with an Upper Cretaceous marine sequence not exceeding a hundred meters in thickness. The continental surfaces over which the transgressions occurred were relatively flat. The marine transgressions were therefore rapid as evidenced by the Cenomanian-Turonian transgression overlying the "Continental Intercalaire" (Rat, Lang, Alzouma, Dikouma et al., 1991). The facies succession confirmed a rapid progression of the sea over an alluvial plain. The slightest fluctuations of the sea level produced noticeable geographic repercussions involving important displacement of the coast-line. They also influenced the migration and re-organisation of the littoral biosedimentary systems resulting in lateral facies changes.
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1 (2/3), 283-424
The Campano-Maastrichtian transgressions arrived earlier along the coastal basins of West Africa during the Upper Campanian or Lower Maastrichtian. They were only recorded in the Iullemmeden basin during the Upper Maastrichtian. Similarly, the Paleocene transgression occurred in the coastal basins during the Danian but was recorded in the Iullemmeden basin only in the Thanetian. The Eocene was regressive in the Iullemmeden basin whereas in the coastal basin the marine transgression attained its maximum development during the same period. It is important to note that the slightest changes in sea-level resulted in very important paleogeographic modifications within the Iullemmeden basin as a result of important displacements of the shoreline and the sedimentary systems.
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1 (4), 425-447
East African Karoo rifts underwent four evolutionary stages. Each rift cycle produced characteristic sediments, i.e. postglacial, lacustrine shales, coals and associated fluvio-lacustrine sediments and synrift sequences. Rifting in intracratonic settings started either during the Permian and continued until Middle to uppermost Triassic. Prolific coal fields are located in the SW-Tanzanian Ruhuhu rift where peat swamps developed during early rift stages. During the main rift stage, deep water anoxic and turbiditic sequences prevailed, the post-rift thermal subsidence stage is characterized by several kilometre-thick fluviatile successions, correlated on a regional scale as a quasi synchronous event. A similar development is recorded from rifts outside of Tanzania (Zimbabwe, Mozambique and Malawi). Pericratonic rifts developed in coastal areas, starting after the termination of the Karoo cycle in the Lower to Middle Jurassic leading from an initial rift to a drift stage. Coals represent the major energy resources of the Karoo rifts. Organic geochemistry, coal petrography and mineralogy indicate fair source rocks for the postglacial Asselian sequence, the Lower Permian coal-bearing and the Upper Permian lacustrine sequences. Similar observations are made in time-equivalent horizons from neighbouring countries, i.e. Zimbabwe, Zambia, Malawi and Mozambique. Coals are gas prone vitrinite-inertinite-rich, high ash varieties, but occasionally, liptinites indicate a marginal potential for the generation of liquid hydrocarbons.
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1 (4), 449-453
I hereby propose an explanation of the discordance observed in the magnitudes determined in Lwiro (Eastern Zaïre) as pointed out by Ambraseys (1972) and Bäth (1975). I also propose a correction for magnitudes determined at Lwiro in order to constitute an earthquake catalogue for the Western Branch of the African Rift Valley. Moreover, an evaluation of the anelastic attenuation of Lg waves is done. A set of 117 regional earthquakes with magnitudes determined both at the Lwiro (Zaïre) and the Bulawayo (Zimbabwe) seismographic station constitutes the basic data to this paper.
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1 (4), 455-467
The Miocene to Subrecent volcanic rocks of Jordan are part of the widespread Cenozoic volcanic province within the Arabian Plate. The magmas have changed in composition with time from older hawaiites to younger and more voluminous alkali-olivine basalts and basanites, with the later showing a primitive character and containing a wide variety of crustal and mantle xenoliths. The alkali-olivine basalt was derived from a peridotitic source by a higher degree of partial melting (~12%) than basanite (~5%). The alkali olivine basalt and hawaiite have also been modified by limited open system fractionation of olivine and clinopyroxene. The volcanic activity occurred in phases corresponding in time to two stages of rifting and later sinistral displacement along the Jordan Rift. The tectonism relates to the opening of the Red Sea in the south, and the subduction of the Arabian plate to the north.
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1 (4), 469-474
A 250 Km Bouguer gravity anomaly profile (southwestern tip at 26.5°N, 55.7°E, NE tip at 27.2°N, 56.6°E) was made to evaluate the thickness and configuration of the sedimentary section (to the pre-Permian basement) at the southernmost part of the Arabian Gulf and the Strait of Hormuz. A relatively steep gravity gradient decreasing towards the Makran (Lran) at -0.2 mGal/Km may be related to the increasing thickness of the post-collisional Arabian plate sediments near the edge of the Arabian and central Iran Plates. The residual gravity anomalies pattern indicates several positive and negative anomalies. Calculations show that some of these are related to thickness variation of the saliferous section of the Mid. Upper Eocene, others related to anomalous thickening at the edge of an interpreted high angle thrust fault. Broader anomalies are found caused by lithology variation (from dolomite to limestone) of the Upper Jurassic to Lower Triassic Permian Khuff and by thickness variation of the salt members of the Infracambrian Hormuz Series. Larak and Hormuz Islands are shown occupied by piercement type salt domes arising from the Hormuz Series. The calculated gravity pattern shows shortening of the section which is in agreement with geologic theorems advocating accretionary sedimentary columns which constitute the continental margin of the Makran. Although other subsurface models are plausible (for the given gravity distribution), it is felt that the presented one is the most feasible geologically.
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1 (4), 475-494
New chemical analyses data are presented for basic lavas covering the plateaus on either sides of the Ethiopian rift. On the Southeastern plateau, the Bale basalts are 2-30 Ma old and consist of alkalic lavas that are intercalated with tholeiites in the lower part. Coeval basalts extending from Muger to the rift margin on the Western plateau are uniformly transitional in composition. Major elements melting systematics indicate that the silica-undersaturation of the alkalic magmas relative to the transitional ones is only due to their higher depth of segregation than the latter, which is estimated to be (greater than or equal to) 50 Km. Both types of basalts bear clear evidence for a decrease in the degree of partial melting with time. This was accompanied by changes in the source regions of the alkalic basalts from an early one with recycled oceanic crust and mantle-wedge type geochemical signatures and located within the lithospheric mantle, to a later deep-seated convecting mantle similar to that of the Tristan da Cunha ocean island basalts (OIBs). The transitional and tholeiitic basalts were exclusively derived from the lithospheric and Tristan OIB - type sources, respectively. These observations suggest that Tertiary magmatism in Ethiopia was triggered by the rise of a hot blob of material lacking an underlying plume and that alkalic and tholeiitic magmas were generated by melting of adiabatically upwelling mantle diapirs in areas of normal crustal thickness. It appears that transitional basalts were formed at the expense of alkalic magmas where mantle upwelling coincided with crustal extension.
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1 (4), 495-502
A detailed study of structural history of the metamorphic rocks of Haramsam area has shown that they have been subjected to folding of five generations and shearing of various types. Folds of first generation (F1) are isoclinal with well developed axial planar foliation (S1). The second generation folds range from close to isoclinal, resulting in folding of the axial planes of F1 and S1 foliation. Folds of the third generation (F3) are represented by synforms and antiforms with NS to NE striking axial planes. The fourth generation folds (F4) are open to close generally with N-S axial planes. These have been overprinted by gentle to open folds (F5) which resulted in a swing in the strike of the foliations and axial planes of the earlier folds. Ductile and brittle-ductile shearing events synchronous with F1, F2 and F4 folding, and brittle shears related with the folds of fifth generation have affected the rocks. The F1-2 folds and associated structures are interpreted to be related to accretion-subduction processes. The F3 and F4 folds are the results of E-W oriented compression and shearing respectively; and the F5 fold and associated brittle structures are formed during N-S oriented compression accompanied by uplift.
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1 (4), 503-514
The Nubian Shield is built up of metavolcanics, metasediments and plutonic rocks of oceanic lithosphere, accretionary prisms, subduction zones and evolved island arcs of Neo-Proterozoic, Pan-African age. Shear-zones are a characteristic structural feature and are developed both within and between the two major structural tiers of gneissic (tier 1) and low grade (tier 2) sequences. In places, they form linked faults and antiformal stacks, in the core of which the gneissic rocks are exposed. The largest of these antiformal structures in Egypt is the Wadi Hafafit Culmination (WHC), which is investigated here with emphasis on shear-zones and mineral deposits. After a brief general characterization of shear-zones the paper deals with the Gabal Um Siwirab Cu-Zn sulphide occurrence which is related to the shear-zone at the WHC's southern margin. Microscopic studies revealed that the primary magmatic fabric with fine-grained ore minerals is overprinted by shearing and, in turn, overprinted by coarse-grained recrystallization. Accordingly, shearing released the economic elements from the primary fabric and allowed concentration and recrystallization. A review of published data and our observations show a majority of the Pan-African mineral deposits in the WHC and environs to be either genetically related to shearing or to have acquired the present texture, economic mineral concentration and geometry during deformation. In contrast to earlier views, the low angle nature of the shear-zones at a regional scale favours the interpretation that recrystallization and remobilization occurred at relatively high crustal levels and potentially present fluids cannot be directly related to high-angle faults tapping deeper crustal or mantle sources.
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1 (4), 515-521
Many placers containing potentially useful minerals occur along the Somali coast of the Gulf of Aden, especially at the estuaries of watercourses draining areas where the crystalline basement outcrops nearby. The most abundant heavy minerals are Ti-magnetite and ilmenite, but Th-monazite (sometimes exceeding 2%), zircon and rutile are also locally plentiful. The beach sands of Batalaleh contain about 38% of heavy minerals. Ti-magnetite and ilmenite are very abundant in them and their fabrics suggest that they are mainly derived from gabbroid rocks and metabasites; zircon and rutile are moderately widespread and show more varied lithological origin. Monazite contains up to 61% of REE2O3 and up to 2.9% of Y2O3 Ce2O3 accounts for about 29%; La2O3 ranges between 14% and 19% and ThO2 between 3% and 12%. The lower La and Th contents correspond to the most significant enrichments in Nd, Sm, Gd and Y. Monazite is essentially derived from the small outcrops of eroding granitoids and pegmatites in the basin feeding the Batalaleh beach. The abundant monazite reported in the coastal sands slightly west of Berbera is due to the widespread occurrence in the adjacent western basin of both masses Pan-African granitoids and pegmatitic swarms with Nb, Ta, Be and REE minerals.
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1 (4), 523-532
Two types of phosphatic sand crusts were found in the shallow, southern part of Lake Malawi. The laminated sand crust is brown with patches of purple and green coloration, medium to coarse grained, with subrounded to angular quartz and feldspars as the major minerals while the micronodular crusts are also brown with patches of about a millimetre in diameter, blue sphaeroids covering over 70% of its upper surface. XRD results for the sand crusts show that the dominant detrital mineral is quartz, which constitutes about 48% of the crust. Feldspars are second in abundance at about 24% while the main authigenic minerals are carbonate hydroxyapatite [Ca10(PO4)3(CO3)3(OH)2] (francolite) at 12%, carbonate fluorapatite [Ca10(PO4)5CO3F.5(OH)], vivianite (Fe3(PO4)2.8H2O) and anapaite [Ca2Fe(PO4)2.4H2O]. The phosphate in the sand crusts is from fish bones. Anoxic conditions introduced by cutting off of oxygen supply to the sediments due to the "blanketing" of the sands by finer sediments and/or cementing together of the sand particles by iron oxides and clays have led to the formation of vivianite and anapaite. The crusts were formed by desiccation in a period of a lower lake level.
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1 (4), 533-545
In Northeast Tunisia, two contemporaneous types of deformation are demonstrated. The first is deformation combining strike-slip and reverse faulting in the northwestern part of the study area, the second is a combination of strike-slip and normal faulting in the southeastern part of Northern Tunisa. The orientation of principal stresses σ1 > σ2 > σ3 and the ratio R = σ2-σ3/σ1-σ3 which characterises the shape of the stress ellipsoid is calculated using microtectonic data. From the Oligocene to Messinian rocks, contemporaneous types of stress tensor can be determined, one is associated with a horizontal maximum compressive stress σ1 striking N14°E in the north, the other one is related to an extensive stress regime in the southern part Kechabta and surrounding areas: (sub-vertical σ1 and horizontal N130-140° striking σ2. The interchange between σ1 and σ2 resulted from a decrease in the value of the maximum horizontal stress (σh max) from Northwest to Southeast. Field investigations show that the compressive deformation propagated in space and time from northwest to southeast from the Late Oligocene to Villafranchan. The maximum horizontal stress has a constant NW-SE direction.
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1 (4), 547-556
Chemical, X-ray fluorescence and X-ray diffraction analyses of sixteen samples from the upper part of Eocene Gercus Formation (Zawita, Iraq) indicate that these sediments are poor in the major oxides (SiO2O2, FeO and K2 relative to average shales. This is a consequence of the fact that the rocks are carbonates, rich in clay minerals and consequently poor in quartz and clay minerals. Normative mineralogy indicates that the Gercus sedimentary rocks consist of clay minerals (45.73%), carbonates (41%), quartz (10.27%), haematite (2.5%) and heavy minerals (0.5%). The clay and clay-like minerals, in their order of abundance are palygorskite, chlorite, monmorillonite and antigorite. The Gercus sedimentary rocks are depleted in the trace elements sulphur and strontium, whereas they are enriched in chromium, nickel and cobalt. Sulphur is partitioned between a sulphide and a sulphate phase. Strontium is mostly found within carbonate and sulphate phases replacing calcium. The elements Cr, Co and Ni are almost exclusively contained within the clay minerals. In addition, a correlation has been found between the intensity of the red colour and the Fe3/FeO ratio of these sediments.
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1 (4), 557-566
A total of 1488 events reported in the Dead Sea region by the Jordan Seismological Network are used to study the earthquake recurrence of the area. The events cover the period from September 1983 to September 1986 and lie in a magnitude range between 1.4 and 5.1. The frequency-magnitude relation for the considered period has been derived in the form, log N = a - bML, where N is the cumulative number of events per year for events with magnitude (greater than or equal to) ML, A = 5.271 and b = 1.025. The b-value is considered to be representative of the region since in the process of its evaluation it was found that incorporating events of the high magnitude end of the range utilized tends to produce a more stable and constant b factor. The relation seems to fit well with the pattern of the historical earthquakes in the area. It also characterizes the region as a highly active one for the continental rift and platform-block seismotectonic zone. The geographic distribution of both historical and instrumental events show, in many cases, a relation to geologically known surface faults. Earthquakes follow NW-SE faults in the southern parts, while in the central and northern parts they are associated with the main Dead Sea rift direction and the E-W and NE-SW fault systems.
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1 (4), 567-578
The Dead Sea rift forms the NW tectonic boundary of the Arabian plate, from the northern Red Sea to SE Anatolia. The rift was considered a prime example of a transform fault, due to its association with geological indications for 105 Km of sinistral displacement. Recent investigations cast doubt on the magnitude of that lateral offset, and bring forth new evidence for extension across the rift. Noteworthy are the arcuate salt diapirs in the northern Red Sea, that show the swing of the tectonic axis from the Red Sea into the Gulf of Elat. Also important is the age determinations that date the initiation of the Red Sea and the Suez Rift in the Miocene, and the Dead Sea rift in the Plio-Pleistocene. Consequently, it is suggested that the Dead Sea rift is an excellent example of continental breakup, that seems to grow into an oblique oceanic spreading center.
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1 (4), 579-592
The Current Bibliography of Middle East Geology contains recently published material on the geology of the region (including Egypt). The bibliography lists books, articles, abstracts, maps, Ph.D. and M.Sc. theses, reports, etc. Special emphasis is given to local material which does not reach international bibliographies or databases. Most of the material in the bibliography is available at the library of the Geological Survey of Israel, which is accessible to the scientific community. The bibliography will, henceforth, appear in each issue of the Africa Geoscience Review.
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2 (1), 1-8
Audiomagnetotelluric (AMT) measurements in the frequency range 8-5000 Hz were carried out on profiles across two small anorthosite-bearing Abontorok and Tagueï ring complexes in central Aïr, in order to determine the electrical character of the rocks and the possible subsurface shapes of the intrusions. The AMT responses from sites ranged from isotropic to strongly anisotropic, sometimes with parallel behaviour between the measured apparent resistivity curves. Static shift effects due to local near-surface inhomogeneities are present at some of the sites. An interpretation of the data considered to be free of static distortion was obtained with one-dimensional models consisting of three or four layers. Modeling indicates a thin, moderately conductive zone at a depth of 1.3 Km under Abontorok, except near its centre where the rocks are characterized by high resistivities of the order of 20,000 Ohm.m, possibly related to a feeder pipe. No clear electrical signature was associated with the Tagueï complex, the absence of contrast with the country rocks being related to the occurrence of monzo-anorthosite and the suggested presence of granite below it. The large anisotropy observed at some sites at Abontorok may be explained by the two-dimensionality of the structure.
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2 (1), 9-23
Structural control of the dry valley systems within the Kalahari of Botswana and eastern Namibia has previously been noted in the literature but its importance has never been demonstrated. The extent of such structural control is mapped using a specially adapted technique of network orientation analysis which identifies valleys showing close alignment with geological lineaments. In many cases there appears to be structural control of valley location even in areas where bedrock is buried by thicknesses of Kalahari Group sediments. A link between valley development and geological structures is proposed for many valleys, invoking the process of deep-weathering as an erosive mechanism by the movement of groundwater along preferential subsurface flow paths. Deep weathering is proposed as an important factor in the early stages of Kalahari valley development, controlling the location of many valley systems, but only appears to operate where the sedimentary cover is less than 30 m thick.
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2 (1), 25-35
New geochemical data (major elements) on birimian volcanic rocks from Toumodi area, confirm the existence of a bimodal volcanism, without intermediate terms, which may be approaching to modern island arc. The correlations with the bordering and same age (approximately equal to 2.1 Ga) volcano-sedimentary complexes enable to propose a geodynamic evolution model for all the country.
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2 (1), 37-60
On the basis of a new geological map, the magmatic evolution of a sector of the Main Ethiopian Rift, South of Addis Abeba, has been studied. In the area, between the two villages of Dera and Nazret the volcanic rocks have been subdivided into 8 Units. From the oldest unit they are: Eastern Margin and Nazret (basaltic flows and ignimbrites associated to rhyolitic lava flows and domes), Keleta and Boku-Tede (ignimbrites and pumice falls associated with a few acid lava domes), Bofa (basaltic flows), Dera-Sodore and Gedemsa (ignimbrites, pumice falls and acid domes) and Wonji (lava flows, cinder and spatter cones). In this area, flows and tuff of basaltic and rhyolitic composition have been erupted closely spaced in time. Intermediate products are practically absent. The basaltic units are mainly transitional, whereas the acid rocks show a predominantly peralkaline character. The major and trace element variation diagrams show chemical variations within the basic and acid members that could be related to fractional crystallization processes. The REE patterns show an enrichment in LREE with a negative Eu anomaly for the acid members. These patterns seem to confirm an important role of the fractional crystallization process. However, the 87Sr/86Sr ratios (0.704-0.710) and some incompatible elements ratios (Ba/Rb and Rb/Sr) suggest that this process cannot act alone. The geochemical diversity of the basaltic and acid products can be interpreted as a result of assimilation-fractional crystallization (AFC) processes. The diversity of basalts seems to derive from a assimilation of lower crustal rocks during fractionation. The petrogenesis of acid rocks is more questionable. Rhyolites can be derived from a basaltic liquid by a moderate upper crustal assimilation rate (r = 0.1), whereas the peralkaline rocks require a very low assimilation rate (r = 0.04) and a more significant fractionation rate (75%< F< 85%).
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2 (1), 61-76
The evolution of the calco-carbonic system in the groundwaters in Côte d'Ivoire reveals that the different families of waters identified in the ISC-ISD diagram according to their relative age, are located on sensibly parallel lines. In water samples belonging to the different families, values of Ca/Mg and Ca+Mg are inversely proportional to water age: thus Ca/Mg decreases as Ca+Mg increases. Consequently, these two parameters may be used as cationic indices for age determination in groundwaters. However, only Ca+Mg values present a constant evolution in all groundwaters collected from various regions of Côte d'Ivoire. In general, variations of this parameter follow an arithmetical series particularity characteristic of groundwaters from Dabou area. On the contrary, since Ca/Mg values depend on the heterogeneity of enclosing rocks, they are more of an index of the age of waters within homogeneous rocks or within those rocks with identical weathering behaviour.
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2 (1), 77-89
The Late Oligocene(?) - Middle Miocene rift sediments in Wadi Wizr-Wadi Assal area south of Quseir could be subdivided into four lithofacies: Facies A: Dominantly alluvial, coarse clastic, fans (lower part of Ranga Formation). Facies B: Fluviatile coarse and fine siliciclastics, with marine evaporites near the top (upper part of Ranga Formation). Facies C: Open marine, mixed coralgal carbonate-siliciclastic sediments (lower part of Um Mahara Formation). Facies D: Restricted marine, algal laminated carbonate and siliciclastic sediments (upper part of Um Mahara Formation). Synsedimentary tectonics have played a great role in the distribution of these facies in the studied area. Thus by Late Oligocene (?) time, the intersection of NW and SE faults created two structural depressions trending NW and N separated by a NW trending high. Facies "A" was deposited on the lows. No sedimentation took place on the high. By Early Miocene time, Facies "B" was deposited on both the high and lows but with thicker and coarser siliciclastics in the lows. By middle Miocene time rejuvenation along NW trending faults resulting in the formation of NW fault blocks, thus giving rise to new highs and lows with NW orientation influencing the deposition of Facies "C" and "D".
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2 (1), 91-99
Sediments of the Dubaydib Formation and Tubeiliyat Member (Mudawwara Formation) of Early Ordovician age (previously known as the Sabellarifex and Conularia sandstones) in the southern desert of Jordan, comprise at least 330 m of siliciclastic sediments. These sediments were divided into three major lithofacies; hummocky cross-stratified sandstone, channelled sandstone and silty shale. The most striking sedimentary features in these sediments are the hummocky cross-stratification and the subaqueous channels, which are considered to be diagnostic of inner shelf storm deposits, and have been attributed to the action of strong storm-wave surges. The interaction of intermittent storm-wave events and fair weather conditions, associated with eustatic sea-level changes and minor transgressive-regressive phases, were responsible for the accumulation of several coarsening-upwards sequences, in an inner to mid shelf setting.
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2 (1), 101-105
The Kabba-Okene granodiorite gneiss, a widely occurring orthogneiss unit in the basement complex of Nigeria, has yielded an U-Pb zircon age of 2103±8 Ma. The age supports the claim of an Eburnean (early Proterozoic) thermotectonic event in Nigeria. More importantly also, because the gneiss contains xenolithic inclusions of the associated low grade metasedimentary schists (younger metasediments) hitherto believed to be late Proterozoic supracrustals, the age information constrains the minimum age of the metasediments to early Proterozoic. One obvious implication of this is the urgent need for a review of the existing chronology of the basement complex of Nigeria.
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2 (1), 107-119
During the late Cenozoic (Miocene, Pliocene and Quaternary) Israel was structurally located between two subsiding basins in which sediments accumulated at a considerable rate, the Jordan - Dead Sea Rift to the east and the Mediterranean to the west. Both basins had been drilled in search of natural gas or oil. Many of the penetrated sections have been subject to pollen analysis in order to obtain palynostratigraphic scheme of the region. The Quaternary sequence was subdivided by Horowitz and Horowitz (1985) into ten palynozones: QI-QII of preglacial Pleistocene age, QIII through QIX of Glacial Pleistocene age and QX representing the Holocene. In this study sedimentological methods were employed to investigate the processes of deposition that prevailed in the preglacial Pleistocene QI palynozone of boreholes Amiaz-1 (1100-1600 m) and Melekh-Sdom 1 (2410-3280 m) at the southern part of the Dead Sea. The QI palynozone envisaged in the Amiaz-1 borehole comprises varying proportions of very coarse to fine clastic beds alternating with clays, marls and subordinate amounts of salt and anhydrite. The studied interval of Melekh-Sdom 1 borehole consists alternating layers of sandstone and limey mudstone. Textural parameters derived from the sand fraction of this palynozone in the Amiaz-1 borehole imply, to some extent, conditions characterizing braided stream depositional environments but they are most similar to sands deposited by aeolian processes. The mudstone layers of this interval probably precipitated in partly confined aquatic basins under arid climate. In the Melekh-Sdom 1 borehole the lithological and textural characteristics of the investigated rock sequence imply depositional processes that commonly prevail in lake environments. The sand layers have settled from distal parts of currents originated from river mouth during relatively humid periods. The mudstone layers reflect more arid phases when ionic concentrations in the lake rose. The Dead Sea Rift at the preglacial QI palynozone time span showed only slight subsidence which made some of its parts intermediate lakes on the way of rivers from Transjordan to the Mediterranean during a rather wet, temperate climate with a somewhat drier period in its middle part.
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2 (1), 121-139
Cotonou town is located alongside the Atlantic Ocean on a margin-coastal complex composed of Quaternary formations revealing a sand gravel-stone levels and clay levels cycle in continuous layers or lens-shaped. Their morphological evolution is indicated by the presence of a table land to the north and littoral plain to the south. The hydrogeological work deals with the coastal zone and the south border of the table land. Besides the superficial aquifer, three other main aquifer horizons have been identified about 150 to 200 metres deep. Their hydrodynamic parameters are exposed and are characterized by some considerable spatial variations showing the heterogenous nature of reservoirs. Water has a favourable chemical quality for human consumption and for current agricultural and industrial uses, except those with high levels of nitrate. Nevertheless their bacteriological quality requires disinfection before consumption. Having a clear-cut idea about the geometry, the functioning and the hydrochemical quality of worked aquifers for water supply in Cotonou town, some relevant suggestions for the exploitation of existing workable resources, important though vulnerable, have been mentioned.
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2 (1), 141-149
A seismotectonic map is drawn up by using recent seismological, geological and neotectonic data. The comparison of the seismological with the neotectonic data shows a certain relationship between the seismic activity and the active faults. The focal mechanism of some earthquakes in the Rift indicates a correlation between the sub-meridian direction of the pressure axis and the main Plio-Quaternary tectonic stress owing to the Africa-Europe plates convergence.
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2 (1), 151-162
Lake Malawi occupies the southernmost part of the East African Rift system. Ferromanganese nodules and ooids occur at water depths of 80 to 160 m at the facies boundary between sands and muds. In most cases, the nodules have been found at the water/sediment interface. The nodules/micronodules range in colour from dark brown to rusty brown or yellowish. Some tend to be of metallic lustre on the outside. The size of the nodules range from about a millimetre to about a centimetre in diameter, but on average most are about 3 mm. The shapes of the nodules may be spherical, mamillated, potato-like and pea-like. Internally, the nodules may be banded therefore have a laminated cortex or homogeneous. The nodule core may be a central cellular portion with a distinct or unclear central nucleus and tends to have voids of variable sizes and shapes. The physical characteristics of the nodules suggest both stable and unstable environments of formation influenced by the nature of bottom water currents. The organic matter content and trace element composition of the nodules indicate the possible influence of bacteria in their formation.
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2 (1), 163-178
Seismic reflection and well velocity data have been used to study the structure of the FF-GG area in the central part of the Gulf of Suez. Five seismic reflectors have been identified and correlated with the prominent reflectors in the Gulf area and aged. The youngest one of them is correlated with the top Zeit Formation, while the oldest is correlated with the pre-Miocene unconformity. Five seismic units between these reflectors have been described seismically. Four structure contour maps have been produced (in T. W T.) for top Zeit, top south Gharib, top Belayim and top Kareem Formations. The study reveals that the pre-Miocene high block have been eroded forming the pre-Miocene unconformity. This unconformity is well represented to the east of the study area and stopped at the west forming an edge or escarpment. The pre-Miocene unconformity has been followed by intensive normal faulting. These faults have the Gulf of Suez trend and affected most of the Miocene. The faulting system have been crossed by a small shear component probably at the late Miocene time (or younger) mostly with a N20°E trend (Gulf of Aqaba trend). The study indicates that the faulting system of the Suez Gulf and the Red Sea rift activated during the Late Oligocene-Early Miocene was rejuvenated during the rest of the Miocene. The main structural features in the investigated area comprise (from west to east); a half graben, a relatively high or uplifted block and monocline or eastward tilting block.
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2 (1), 179-188
The Current Bibliography of Middle East Geology contains recently published material on the geology of the region (including Egypt). The bibliography lists books, articles, abstracts, maps, Ph.D. and M.Sc. theses, reports, etc. Special emphasis is given to local material which does not reach international bibliographies or databases. Most of the material in the bibliography is available at the library of the Geological Survey of Israel, which is accessible to the scientific community.
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2 (1), 189-203
PANGIS network, supported by UNESCO, ACCT and bilateral French and Belgium Cooperation since 1989, is a pool of African and European documentation centers working in the field of African Geology, all using the same tools for micro-computerized information management and connected together for data supply, information exchange, training support and publishing. PANGIS is also a micro-computerized bibliographic database co-compiled by the 20 members (10 more members are expected for 1995) of the network gathering about 30,000 references (both published and unpublished documents) with analytical descriptions of documents. The material in this bibliography on African Sedimentary Basins is available in the libraries mentioned at the end of each reference or at the library of CIFEG (see address above); CIFEG being the coordinating center of PANGIS.
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2 (2), 205-213
Seismicity due to the compressional motion (0.9 cm/yr.) in North Africa has an important effect on the instability of the Pliocene-Quaternary sediments on the Algerian continental margin. The 1954 and 1980 Ech-Cheliff earthquakes induced turbidity currents that cut several submarine telephone cables at the foot of the continental slope. The bathymetry and the seismic reflection data reveals the existence of a cone-shaped sedimentary accumulation (2600 m depth). This sediment body is interpreted as formed by the high earthquake activity of the Ech-Cheliff area.
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2 (2), 215-217
In the District of Kogo in Equatorial Guinea (N1°11', E10°1'), a morphological semi-circular feature corresponding to an astrobleme can be recognized both on radar and thematic mapper imagery. It is located on the Congo Craton (Central Africa) with a diameter of 4,670 m. Since Neocomian faulting cross-cuts the circular feature, the impact of the astrobleme must be older than 145 Ma.
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2 (2), 219-225
Sm-Nd and Rb-Sr whole-rock data on gneisses from SW Nigeria are in agreement with the Late Archaean-Early Proterozoic crustal growth, with known geology and existing U-Pb and Rb-Sr ages. In contrast, similar gneisses from northern Nigeria yield consistent Archaean (TNd, TSr) model ages of 3.5 Gal, which are in agreement with U-Pb and Rb-Sr crystallisation ages from that region. The lack of Eburnean ages in northern Nigeria by the same radiochronologic methods calls into question the spatial significance of the Eburnean event and its tectonothermal importance vis-a-vis the widespread Late Proterozoic Pan-African orogeny. The U-Pb zircon ages of 2.5-2.4 Ga on grey gneisses from northern and SW Nigeria are in agreement with Sm-Nd and Rb-Sr data presented herein and suggest a distinct crustal initiation event with significant crustal growth at the Archaean-Proterozoic boundary. This event is correlated with the development of the Birrimian system on the West African craton.
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2 (2), 227-245
In the Uwaynat region, there are hundreds of volcanic plugs, dykes and lava flows, ranging in composition from carbonatites to olivine mela-nephelinites, alkali-basalts and minor phonolites and trachytes. Olivine mela-nephelinites, basanites and alkali basalts are the most abundant rock types. These rocks are from ring complexes and mafic plugs located between the Uwaynat Inlier and the Al Kufrah basin, in the Libyan desert. These volcanic rocks range from 28.2 to 26.7 Ma in age. Olivine mela-nephelinites and basanites are characterised by extremely high titania (up to 6 wt.%) and incompatible trace elements contents. Minor ferro-carbonatites are also present and they are also characterised by high content of titania and related high field strength elements and higher content in incompatible trace and rare earth elements than olivine mela-nephelinites and basanites. No large variations in the initial Sr and Nd isotope compositions are observed. This may suggest none or very little contribution of crustal rocks-magma interaction. A similar isotopic signature for the source of olivine mela-nephelinites and ferro-carbonatites is inferred. Olivine mela-nephelinites and basanites are not genetically related to each other by fractionation but they represent primitive magmas generated by different degrees of partial melting of a metasomatised upper mantle source. Incompatible trace elements in olivine mela-nephelinites may be modelled with small degrees of partial melting of such a source. Close relationships are also observed with the slightly older volcanic rocks from Gilf Gebir (SW Egypt), suggesting that Uwaynat magmatism is possibly related to a northeastward migration of the African plate above an intracontinental hot spot.
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2 (2), 247-265
During and after the very wet 1991/1992 winter experienced in Jordan, significant slope failures occurred along several sections of the new Irbid-Amman Highway in Jordan. The topography over most of the route is hilly, and significant amounts of cut and fill were used to construct the highway. Many of the failures appeared to display classic, rotational slip geometries which are typical of failures in soil and fill profiles. One such slope instability is an embankment foundation failure that lies at km 40+700 on the highway south of Jerash City. This paper deals with the overall stability of the slope at km 40+700. It presents the geological and geotechnical studies carried out at this site, identifies the causes and mechanism of instability, and presents appropriate remedial measures. Also, slope stability back analysis was carried out for the slope to assess the conditions at time of failure, and estimate most representative shear strength parameters of foundation materials. Slope stability analysis was also carried out for proposed remedies, using the computer programme STAB5M (Siegel, 1975). As Jordan is an earthquake prone area, probabilistic seismic hazard analysis was carried out for the landslide site using the computer programme FRISK (McGuire, 1978). A Peak Ground Acceleration (PGA) value of 0.2 g was estimated for design. This corresponds to a 90% probability of not occurring in a 50-year-design life of the highway. Dynamic slope stability analysis was also carried out. The study concluded that the landslide movement occurred within the foundation colluvium material. It resulted primarily from the excessive load of the embankment and excess piezometric pressures generated within the slope. Most representative shear strength parameters of colluvium materials at time of failure were found to be residual values: cohesion c' = 0 and angle of friction φ' = 20.0. The most feasible remedial measure to stabilize the area was partial removal of existing embankment, and reconstruction using imported rockfill of a split level embankment. Also, a surface and subsurface drainage system was recommended at the site.
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2 (2), 267-278
The geological surroundings of the horst of N'diass during the Campano-Maastrichtian period was a detrital platform oscillating between the upper limit of the littoral one to the down limit of the middle one, under subtropical climate. It was locally affected diverging subtidal chenals. The detrital material come from an uniform sedimentary stock which has suffered several phases of abrasion. The campano-maastrichtian sea level variation upon which depend the conditions of transportation and sedimentation determines the textural and mineralogical characteristics of sediments and their space and temporal distribution inside the platform.
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2 (2), 279-310
The micromammal fauna from the Neogene continental deposits of Aït Kandoula basin have been studied. 23 species distributed among 10 genera of rodents are found in seven fossiliferous localities. The localities extend from Middle Miocene to Late Pliocene. The Western basin has yielded a new fauna of middle Astaracian age, situated below breccia, which allows us to identify and precisely date one of the Atlas orogenic phases. The basin is subdivided into two subbasins, Oued Tabia to the West and the other to the East named "new village". The gradation occurred from W to E, the mean sedimentation rate was estimated to correspond to about 130 m/Ma. The sedimentation series in the basin is syntectonic. The periods corresponding to a lacustrine or marshy sedimentation could have corresponded to relative periods of low tectonic activity. They contrast with periods of high detritic sedimentation, which attest to an orogenic pulse. Palaeomagnetic analysis permit to give to the Oued Tabia locality an age of 10.9 Ma. The Argon/Argon dating of a volcanic ash layer from the eastern basin indicates an age of 5.9±0.5 Ma. The two western European species of micromammals discovered beneath that ash level indicate that transmediterranean terrestrial faunal exchanges occurred before the Messinian crisis and are therefore not directly related to that event.
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2 (2), 311-321
The Current Bibliography of Middle East Geology contains recently published material on the geology of the region (including Egypt). The bibliography lists books, articles, abstracts, maps, Ph.D. and M.Sc. theses, reports,, etc. Special emphasis is given to local material which does not reach international bibliographies or databases. Most of the material in the bibliography is available at the library of the Geological Survey of Israel, which is accessible to the scientific community.
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2 (3/4), VII-VIII
This volume on climatic changes and sedimentary records during the Quaternary in the central part of the Sahel (Niger and Chad Republics) is the product of two complementary methods of investigation. One approach is to start with the past and move towards the present as was exemplified by the IGCP-IUGS project 210 on the continental sediments in Africa (C. A. Kogbe, J. Lang, E. Klitzsch, 1983-1989) applying the principles of stratigraphy, sedimentology and geochemistry to investigate the "Continental Terminal" in Africa. - The second approach is to move from the present to the past as exemplified by the PALHYDAF - INSU project (J.Ch. Fontes and F. Gasse) which involved the study of recent geodynamics. Analysis of climatic evolution with time is based on phenomena resulting from breaks in the equilibrium of a system dominated by energy exchanges. These breaks produce effects (particularly with regards to temperature and humidity) the intensity and duration of which are important with regards to the capacity for record preservation. The analysis is made from geosystems that are best adapted to sedimentary records that reveal climatic changes once their functions are understood (research of CNRS Unite 157, J. Chaline and B. David). The approach is centered around three important questions: - which climatic phenomena were precisely recorded and what is the role of the sedimentary environment in these records? - what degree of confidence and precision should be attributed to a poorly preserved original record? - which stratigraphic events can be reconstituted from sedimentary records and subsequently, - what are the controlling roles of local and global factors on the recorded events? For recent periods, it is possible to recognise slight climatic anomalies of no stratigraphic importance; important climatic crises of global importance, and a chronologic reference (eolian sand dynamics; variation in the lake levels; palaeolandscapes etc). The classical question of autochtony and allochtony in analysis of different sedimentary facies (bauxites, carbonates, coal etc) should also be relevant to eolian sands. In order to evaluate the real importance of eolian sands at different periods of the late Quaternary with regards to the culmulative character of wind actions, it is important and relevant to initially define the inherited component from preceding times. The Kanemian (20,000-12,000 years 14C BP) will no longer appear to be a long period of aridity of more than 10,000 years but will be recognised as a succession of several periods of which some are even more humid than present times. Another important question relates to the differentiation of lacustrine and palustrine facies. The palaeolake Megachad of 6000 years ago characterising a very humid period could very well have been the juxtaposition of many palustrine and lacustrine entities. This will tend to contest the existence of large deltas and a vaste "peri-lacustrine ridge" with an altitude of 325 m. The "peri-lacustrine ridge" would have a structural, fluviatile or eolian origin and did not result from major fluctuations in the level of the lake. In other words, emphasis should be on the predominance of eolian and alluvial conditions within an unstable structural setting rather than the lacustrine deposits representing marker horizons within a stable structural setting.
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2 (3/4), 323-614
The study areas lie in the heartland of Africa, in the Republics of Niger and Chad which range approximately from 0° - 24° East and 8° - 23° North. These areas are part of two major hydrological, geomorphological and geological systems: the Chad Basin in the East (by reference to Lake Chad) and the Iullemmeden Basin in the West (by reference to the Touareg community in its centre). Instead of this latter relatively recent name (Racier, 1959), we shall refer to the "Eastern Basin of Middle Niger" (Chudeau, 1910). The Middle-Niger stretches from the outlet of the Inland Niger Delta in Mali to northern Nigeria (ORSTOM, 1970). The eastern part of this basin includes all those areas potentially drained by the Niger's left-bank tributaries in Niger. It also includes the River Sokoto of which half the catchment area is in Nigeria. At the time I first undertook this study, earlier works had reached three main conclusions: - The surface area of Lake Chad, recently some 24 000 Km² for an average depth of 4 m, had attained more than 330 000 Km² during the Holocene with depths in excess of 160 m locally: Lake Megachad. - The period from approximately 20 000 to 12 000 years 14C BP in the Sahel had been hyperarid: the Kanemian. The Sahara had spread significantly during this period and aeolian sands had been deposited as South away as the present day Sudanian zone. - Winter rains, now exceptional in the Sahel, and directly related to the advection of polar air masses, were allegedly major features at several times during the Late Quaternary. Now my early sedimentological field work in the Chari "deltas" revealed neither traces of an "inland sea" nor any record of "hyperarid periods", but fluctuations apparently comparable with those observed since the turn of the century. Nor was there any indication of advections of polar air masses. What could be more unsettling for a fledgling researcher than to quickly reach conclusions radically opposed to those of his seniors, or to have to admit his failure to understand. The organisation of this work retraces the course followed in responding to this challenge and in reviewing these issues from zero, this time with the necessary tools. An understanding of the present-day model is basic to this process. Therefore, after defining the geographical and geological framework of the study, we shall briefly introduce the concepts and facts concerning the climate and the present-day environment. The following chapters reflect this determination to take a fresh look at two major questions. Hyperaridity, deduced primarily from the occurrence of identical facies (azoic sands) and the extent of gaps in the sedimentary records, seemed to be essentially a problem of methodology and stratigraphy, viz. how can aridity be quantified and dated, how can aridity and aeolian phenomena be related? In fact, in order to judge the true significance of aeolian sands at various times in the late Quaternary, given the cumulative character of aeolian processes, we first had to specify to what extent they were the legacy of earlier periods. The question of what criteria could be used for recognising aridity was all the more prominent as in recent decades situations heretofore unobserved have occurred, ascribed to hyperaridity in keeping with the criteria used in earlier works: drying up of Lake Chad, almost complete interruption of flow of the Chari and the Niger, formation of a new erg (sand sea) near Timbuktu, etc. By contrast, the problem of the Holocene Megachad characterising a very humid period proved to be of a completely different order. Evidence of Megachad deposits and their age was not convincing. The hypothesis of neotectonic phenomena, however, seemed an interesting lead. The neotectonics hypothesis was not new in Chad, but the slow growth of the Megachad hypothesis over 150 years had gradually led to formal opposition to any suggestion of recent movement. Beyond the geological facts, the historical and conceptual aspects were surely important points to be verified. These different topics will therefore be covered on the basis of those geosystems beatable to provide sedimentary records that can account for the climatic dynamics once we have understood how they work. The presentation of analytical questions is kept to the strict minimum required for supporting the interpretations proposed. A description of the 100 sections made and the 1600 samples that, inter alia, were systematically subjected to grain size analysis would have burdened the work without adding anything. Moreover, upon analysis many sections taken from wells turned out to be composed entirely of uniform sands and so of no interest here. On the other hand, most of the 800 mineral and chemical analyses (heavy minerals, carbonate contents, X ray diffractometry, organic matter, radiocarbon, k13C and k18O) and most of the 200 palaeobiological analyses (pollen, diatoms and molluscs) are set out in the report where necessary. Some findings have been presented more exhaustively either in earlier publications (e.g. Durand, 1978 or Durand et al., 1984) or as part of a different area of research by the co-worker in charge of the analyses (Lesieur, 1982; Icole et al., 1983; Gasse, 1987b). A few dozen analyses only have not been presented. As, for instance, the pollen analyses by E. Schulz and C. Caratini, in view of the lack of significant results because of the extreme paucity of the samples. This is also the case of quartz surface textures analysed in conjunction with I. Zimmerlin and P. Hommeril, but here because the results were not relevant to the objective pursued. These sands have been subjected to alternating arid and humid conditions for sometimes more than one million years. Accordingly they are far too similar and cannot readily be interpreted in terms of palaeo-environmental and stratigraphic markers.
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2 (3/4), 615-626
The Current Bibliography of Middle East Geology contains recently published material on the geology of the region (including Egypt). The bibliography lists books, articles, abstracts, maps, Ph.D. and M.Sc. theses, reports, etc. Special emphasis is given to local material which does not reach international bibliographies or databases. Most of the material in the bibliography is available at the library of the Geological Survey of Israel, which is accessible to the scientific community.
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2 (3/4), 627-639
The Current Bibliography of Middle East Geology contains recently published material on the geology of the region (including Egypt). The bibliography lists books, articles, abstracts, maps, Ph.D. and M.Sc. theses, reports, etc. Special emphasis is given to local material which does not reach international bibliographies or databases. Most of the material in the bibliography is available at the library of the Geological Survey of Israel, which is accessible to the scientific community.
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3 (1), 1-16
Site-specific techniques of seismic hazard assessment suitable for large engineering projects, such as radioactive and toxic waste disposal sites, require a detailed knowledge of the geological structures that are active over the region under scrutiny. Neotectonic faults and other seismogenic structures were identified across South Africa by means of several techniques including seismic databases, Landsat, SEASAT, and GEOSAT satellite imagery, aerial photography and detailed field mapping. On-shore neotectonic activity is currently taking place in the southwestern Cape, in Namaqualand, and over a broad area extending from the Free State to the Northern Transvaal and to Natal. Neotectonic activity in southern Africa is analyzed in terms of known stress fields. NW-SE trending maximum horizontal compression directions (SHmax) developed from southern Angola to the offshore Transkei basin are defined as the Wegener stress anomaly (WSA). The interaction between the WSA and the other stress fields acting on the African plate (linked to ridge-push, and to the southern propagation of the African Rift) causes neotectonic faults and intraplate seismicity, possibly including some of the largest events experienced in the South African mines. Neotectonic activity could also constrain the long-term performance of waste disposal facilities.
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3 (1), 17-30
Following intensive regional and site investigations, the National Radioactive Waste Disposal Facility for intermediate- to low-level radioactive waste was established at Vaalputs in Bushmanland. The investigations were applied to study the rocks as a natural analogue to waste disposal. The more important radionuclides present in the waste are 60Co, 90Sr, 137Cs and 134Cs, while some 238U may also be present. The geological investigations led to the identification of an arenaceous and argillaceous surficial succession overlying a crystalline basement, comprising mainly granitic gneiss. Clay minerals derived from the hydrolysis of basement rocks constitute the major components of the surficial rocks, and are important in the concentration of certain elements through ion-exchange reactions. It is concluded that eight major processes have been operating h the geological environment at Vaalputs to give rise to the present distribution of elements. It is shown that mechanisms exist to significantly retard the mobility of elements, including the radionuclides present in the radio-active waste. The extent to which these elements are adsorbed and retarded are controlled by the physico-chemical conditions prevailing h the sedimentary environment. The approach adopted in this study may be applied to the selection of sites for other waste types. The geological integrity of the site may adequately compensate for the absence of other selection criteria.
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3 (1), 31-38
The general objective of a safety assessment is to estimate the impact of radioactive waste disposal on human and the surrounding environment The heterogeneity in the geosphere and biosphere, as well as the time scale of interest in safety assessments, introduce a lot of uncertainties into the process. One can distinguish two approaches to address uncertainties in predictive simulation: (1) deterministic simulation using sensitivity analysis, and (2) probabilistic simulation. In this paper a safety assessment methodology is proposed using probabilistic simulation. It is based on a revised decision-making model proposed by Freeze et al. (1990), taking into consideration geological, geohydrological, hydrological and repository parameters, in a Bayesian statistical framework. The application of the decision-making model in safety assessment can be described as a sequence of decisions between alternative safety assessment scenarios under conditions of uncertainty. It takes the cost and benefits of each alternative into account but give also weight to the associated risk, which are influenced by the uncertainty. Three models can be identified in the methodology: a decision model based on a risk-cost-benefit objective function, a predictive simulation model, and an uncertainty model. The iterative nature of decision analysis makes it particularly attractive for implementation into a safety assessment methodology. The proposed methodology can be divided into two stages. The first being the site characterization and problem formulation, and the second the implementation of the decision model.
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3 (1), 39-46
As specified in almost all permit conditions, some form of groundwater monitoring is required at waste disposal sites. Minimum requirements have been developed for this type of monitoring in order to ensure a co-ordinated and meaningful monitoring programme. These requirements are based on the BATNEEC principle. However, one of the real dangers of setting generalised monitoring requirements is that a "data rich - information poor" syndrome will result. Effective monitoring requires a systems approach which entails far more than merely sinking a borehole and grabbing a sample. A proper understanding of the prevailing site-specific geohydrological regime is required before valid monitoring results can be obtained. Two case studies are presented to illustrate the dangers of not adopting a systems approach and the need for a proper understanding of prevailing groundwater conditions. Both waste sites are located on primary aquifers, the dynamics of which are usually considered fairly easy to understand. In the first case, initial monitoring results showed that a garden and building rubble site had seemingly caused marked groundwater contamination to occur. More detailed investigation later showed that the contamination was in fact caused by other sources. In the second study, conflicting groundwater chemistry data was obtained from four boreholes. Full consideration of borehole position, geohydrological regime, borehole construction and sampling protocol was required to identify that a valuable aquifer, which had been developed for water-supply, was in fact in the process of being destroyed.
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3 (1), 47-52
The preservation of South Africa's groundwater resources is of critical importance if the future water demand of this country is to be satisfied. The impact that waste disposal sites have on these resources is an area of serious concern. A recently completed project, funded by the Water Research Commission, developed a systematic, physically based method to evaluate the suitability of existing or proposed sites as waste disposal facilities. The evaluation only considers geohydrological criteria Following the examination of over 29 methods used elsewhere in the world, a conceptual methodology was developed. Waste site permit application reports as well as field investigations performed at 10 waste sites throughout South Africa were used to quantify the method and ensure that it is valid, appropriate and accurate under South African conditions. The method comprises of three distinct but interrelated factors, namely the Threat Factor, the Barrier Factor and the Resource Factor. An individual score is assigned to each Factor and, by means of a nomogram, the Factor scores combined to obtain an index. The obtained index relates to the suitability of the site, based on geohydrological considerations only, for waste disposal activities.
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3 (1), 53-74
The mining sector plays a very important role in the sustenance and development of the economy of many African states. Foreign exchange earnings from export of mineral resources, increased government revenue from taxes, employment opportunities for nationals and provision of necessary infrastructures, are some of the major benefits that the mining industry has generated in the continent. Despite these benefits, serious environmental damages resulting from mining activities have been overlooked. The environmental assessment system during the mining project cycle consisting of identification, preparation, appraisal, implementation and post-evaluation phases is described in this paper in order to promote an effective environmental impact monitoring and management. Guidelines on environmental assessment during the pre-mining, operational and rehabilitation phases are outlined for small-, medium- and large-scale mining activities. The importance of adequate legislative measures to guarantee adherence to the environmental mining guidelines is emphasised and an environmental impact check-list and sensitivity index score sheet to facilitate impact assessment by supervising officials is provided. The paper concludes with annexes including internationally approved values for air and water pollutions as well as noise and vibrations recommended limits.
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3 (1), 75-82
Mining activities in Africa are closely associated with environmental degradation which should be avoided as far as possible. This objective can be attained if prior to implementation of the mining project, careful planning design and management guidelines are established. African countries must maintain a balance between the economic and environmental impacts of mining projects. Possible adverse effects of mining should be forecasted and mitigation measures established. Compensation for natural resources that will be affected by the project and rehabilitation of degraded land should be envisaged. Adequate funding should be set aside for post-mining re-habilitation process. Priority environmental issues to be considered should include, water and air pollution control, toxic and hazardous waste control, soil disposal control, tailing disposal control, deforestation, erosion, sediment control, visual impact and visible health hazards. Environmental monitoring should be facilitated by the establishment of a training programme for staff who can monitor and enforce standards and regulations effectively.
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3 (1), 83-96
The Current Bibliography of Middle East Geology contains recently published material on the geology of the region (including Egypt). The bibliography lists books, articles, abstracts, maps, Ph.D. and M.Sc. theses, reports, etc. Special emphasis is given to local material which does not reach international bibliographies or databases. Most of the material in the bibliography is available at the library of the Geological Survey of Israel, which is accessible to the scientific community.
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3 (1), 97-156
PANGIS network, supported by UNESCO, ACCT and bilateral French and Belgium Cooperation is a pool of African and European documentation centers working in the field of African Geology, all using the same tool for micro-computerized information management and connected together for data supply, information exchange, training support and publishing. PANGIS is also a micro-computerized bibliographic database co-compiled by the 30 members of the network gathering about 35.000 references (both published and unpublished documents) with analytical descriptions of documents. The references listed below have been extracted from a printed bibliography published by CIFEG in 1994 called "Africa and Environment: A selective bibliography, 1990-1993" sponsored by the French Ministry of Foreign Affairs. The material in this bibliography is available in the libraries mentioned at the end of each reference or at the library of CIFEG (see address above, CIFEG being the coordinating center of PANGIS). Global changes: climatic and human influences. African regions. African countries. Water Management: Quantity, quality. Natural hazards. Surface and underground planning.
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3 (2), 157-174
Diamonds were discovered in the aeolian system of the Namib Desert on the west coast of southern Africa in 1908. The economic significance of aeolian placers was later surpassed by the discovery of onshore marine diamond deposits from 1925 onwards. The onshore diamond deposits extending both onshore and offshore over some 600 Km of coastline represent the world's greatest secondary diamond deposit. The geological history of the marine deposits on the continental shelf off southwestern Africa is complex, involving the interaction of fluvial, marine and aeolian systems. The rock record of events is incomplete due to the extensive erosion of sequences by the marine and aeolian systems. Diamonds were introduced to the continental shelf via several river systems draining the interior of southern Africa. The offshore deposits contained in composite marine lag gravels are the product of repeated reworking of material on the continental shelf during numerous regressions and transgressions which are overprinted by events during the last glacial maximum. The formation of the offshore deposits has largely been controlled by marine coastal and nearshore processes, but in Namibia aeolian processes reworked palaeoshorelines whilst they were subaerially exposed during regression(s). This enabled the high-energy, unidirectional aeolian system operating within the narrow, coastally located deflation basin to transport diamonds up the continental shelf, resulting in the formation of aeolian diamond placers. Offshore shallow-water marine mining commenced in the early 1960s. Deep-water exploration to the edge of the continental shelf {200 m below sea-level (mbsl.)} was initiated in 1971. De Beers Marine (Pty) Ltd. is currently the world's deepest-water mining operation, and produced some 400 000 carats during 1994. This is more than double the entire shallow-water production. The success of the deep-water mining operation has led to widespread deep-water exploration activity by numerous companies.
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3 (2), 175-189
The diamond production figures and geological framework of the main alluvial diamond deposits in South Africa occurring above the escarpment, are reviewed. The oldest post-Gondwana deposits are Late Cretaceous in age and have been subjected to deep chemical weathering processes. Younger deposits are of Miocene, Plio-Pleistocene and Late Pleistocene/Holocene age and can all be mineralised to some degree. The older terraces, particularly along the Vaal and Orange Rivers which are characterised by terrace-stepped development, have historically been the more economic deposits. Concentration of diamonds has occurred close to, but off, the horizontally-bedded sediments of the Karoo Supergroup and, with the exception of three isolated small deposits, most alluvial diamond deposits are found along the north-western rim of the Karoo basin. This can be explained by preferred trapsites as a result of bedrock competence and coarse matrix, the latter derived from weathering of the basal Karoo tillites and exfoliated, Ventersdorp lava bedrock. However, despite the presence of these trapsites, less than 1% of the diamonds eroded from known kimberlites during Late Mesozoic and Tertiary degradation have been captured within these alluvial systems.
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3 (2), 191-201
Group II kimberlite dyke complexes have proved to be viable diamond deposits in certain cases in South Africa. They persist to depths greater than current mining levels (-600 metres) showing no evidence of grade depletion. Typically not all the dykes in an individual swarm will be viable to mine and some may even be completely barren of any mantle minerals. Closely related dykes and different parts of the same dyke can contain widely differing samples of mantle minerals as well as different diamond grades. In the case of Helam Mine, grade variations are routinely monitored by reference to these variations. A barren dyke at Helam is completely devoid of high pressure (upper mantle) phases.
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3 (2), 203-216
Comparison of chemical compositions of chromite macrocrysts (390 grains) and chromites in mantle-derived xenoliths and xenocryst assemblages (78 samples) from diamond-bearing kimberlites in southern African kimberlites reveals that virtually all chromite macrocrysts are xenocrysts derived by disaggregation of mantle peridotites. Ti-rich chromites (up to 4.0 wt% TiO2), previously interpreted as early formed phenocrysts in Group II kimberlite magmas, are abundant as macrocrysts at Kamfersdam and Bultfontein Group I kimberlites in Kimberley, South Africa, where essentially complete compositional overlap exists between macrocrysts and chromites in garnet-bearing and garnet-free mantle assemblages. Garnet-free peridotites may be the dominant source of low-Al (< 6 wt% Al2O3) and high Al2O3 (> 15 wt% Al2O3) chromite macrocrysts. Comparison of chromite xenocryst populations with chromites included in diamonds from Finsch, Bultfontein and Roberts Victor (South Africa) and Orapa (Botswana) reveals little to no compositional overlap between included chromites and xenocrysts. The lower Cr2O3 and MgO of the xenocryst populations indicates substantial post-diamond formation chemical reequilibration in the open system of peridotite outside of the diamond armoring. Similar core to rim decreases in MgO and Cr2O3 in chromites intergrown with garnet xenocrysts is suggestive of generally operating processes producing low Cr/Mg chromites, likely cooling and/or uplift. The high Cr-Mg "diamond inclusion chromite field" may not be a realistic target in diamond exploration programs due to post-diamond reequilibration of unarmored chromites in peridotites.
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3 (2), 217-230
Located in two main areas, the alluvial diamond deposits in the Central Africa Republic (C.A.R.) are in spatial relationship with two detrital Mesozoic formations, the Carnot Formation in the west and the Mouka-Ouadda Formation in the east. These two fluvial formations act as the diamond host rocks. The origin and history of the diamonds are discussed on the basis of sedimentological data and knowledge about diamond geology in Central Africa. The primary diamond sources probably date from the Kibarian age. Located in the northern Congo and Zaire, they belong to the diamond fields of the Congo and Gabon which extend towards the San Francisco craton in Brazil.
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3 (2), 231-246
The Kéniéba diamond province in Mali was discovered only a few decades ago, although the region has been known to contain gold deposits exploited by small-scale miners since centuries. The geology consists of rocks of Palaeoproterozoic age overlain in the east by sub-horizontal sandstones of Upper Proterozoic age. The presence of diamonds in Late Quaternary palaeoplacers was revealed by local small-scale miners. A study of associated minerals and airborne magnetometric investigations, backed-up by field investigations, confirmed the presence of about 20 kimberlite bodies arranged in a step-like form with occasional scattered relic craters within the Birrimian or Upper Proterozoic. The distribution of the kimberlite bodies seem to be regionally controlled by SSW-NNE lineaments and locally controlled by a system of N105°-115°E fractures. Ilmenite and chromite are the most frequent minerals; garnet is rare, but diamond is present. This kimberlitic magmatism could also be of probable Lower Cretaceous age but the diamond characteristics suggest the contributory role of a Proterozoic kimberlitic magmatism towards the mineralisation of detritic Birrimian facies. Geomorphologic studies of the Quaternary reveal a probable extension of the palaeoplacers.
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3 (2), 247-260
This brief review of the occurrences of diamonds in the republic of Zaïre, provides basic information on the successive historical milestones marking the geological research and exploration that constitute the basis of the discovery of the major diamond fields in Zaïre. The origin of most of the deposits is associated with kimberlite satellites situated further south in the republic of Angola. The direction of transport of the depositional current is from south to north as evidenced by the decrease in size and density of the diamond grains in alluvial beds and terraces.
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3 (2), 261-272
Ghanaian diamond deposits originate from two major diamond fields: the Bonsa and the Birim fields. The Bonsa diamonds occur in Early Proterozoic Tarkwaian fluvial sediments known as the Kawere and Banket conglomerates. The Birim field is underlain by Birimian rocks which consist of nearly vertical grey, blue and black phyllites with interbedded tuffaceous greywackes and schists. There are occurrences of rocks of "ultramafic" composition (Junner et al., 1943). Superficial deposits are gravels, sands, clays, laterite and soils. The diamonds in the Bonsa have better grades (3 ct/m³, 15 stones/carat) but production in the Birim field exceeds that of the Bonsa field. The Birim field has produced over 50 million carats of diamonds from 1923 to the present. Sizes of diamonds vary from 4 mm to 0.5 mm and the average grade of the gravel ranges from 1.2 to 2.33 ct/m³. Field and analytical work has shown that what Junner et al., 1943 termed as ultramafic rocks in the Birim area are actually intensively hydrothermally altered and regionally metamorphosed kimberlitic rocks that have now become actinolite schists. The major and minor element chemistry fit well with kimberlite that is characterized by high concentrations of REE enriched in the light REE. The rock appears to be a composite of kimberlite and country rock as do other kimberlite intrusions. The name metakimberlite is therefore an appropriate term for the rock.
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3 (2), 273-293
A review of deposits in Tanzania is presented. The deposits like other similar deposits worldwide are characterised by their size, clarity, colour and shape. Tanzanian diamonds are of exceptionally high quality as exemplified by the world famous "Williamson pink" diamond. Statistics on size distribution from the Mwadui diamond mine over several years constitute the basis for predicting prices and experiments confirm that it is more satisfactory to use a single class boundary of -14 diamond screen mesh than to use the median value ranging between -17 + 3. Valuation of diamonds from five different locations are presented and it is demonstrated that diamond categories and value vary from one location to the other and from type to type.
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3 (2), 295-316
Graphite aggregates occur as octahedra and other forms of cubic symmetry within garnet pyroxenites in the Beni Bousera orogenic peridotite massif, N. Morocco. These aggregates have been interpreted as graphitised diamonds and their occurrence, together with a similar one at Ronda, S. Spain, is evidence that diamonds can form in the mantle away from cratons, possibly by recycling of crustal carbon via subduction zones. The tectonic setting of the Beni Bousera massif is clearly different to African diamond occurrences with obvious kimberlite parentage. This difference is also evident in the chemistry and age of the associated peridotites. The graphitic garnet pyroxenites at Beni Bousera bear some resemblance to eclogite xenoliths found in kimberlite pipes that commonly host diamonds. Subtle mineral chemical differences exist between these two rock types that may ultimately reflect fundamental differences in petrogenesis. However, the crustal isotopic signatures observed in some Beni Bousera pyroxenites and eclogite xenoliths in kimberlites imply that members of both suites may ultimately originate from subducted oceanic crustal protoliths plus sediment-derived material. Consideration of the physics of diamond graphitisation, the apparent absence of any remnant, ungraphitised diamonds, together with the thermal history of the Beni Bousera and other orogenic peridotite massifs suggests that diamond survival during emplacement is unlikely in similar massifs. If, as seems likely, many so-called "anomalous" diamond occurrences in continental collision zones originate from tectonically emplaced mantle fragments, their pressure-temperature-time trajectories must have been very different from that experienced by the Beni Bousera massif. However, the occurrence of graphitised diamonds within the pyroxenites at Beni Bousera provides new constrains on regional tectonic models and provides an important insight into diamond genesis in a non-cratonic setting.
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3 (2), 317-329
The River Sarabaya, a headwater tributary of the Niger drainage basin in SE Guinea, is diamondiferous throughout its length. Diamond is preferentially concentrated in fluviatile gravels found on the northern flanks of the valley as well as underlying low terraces and gravels associated with flood plains in the valley bottom. The valley has evolved under a long-term tectonic movement such that it has migrated progressively southwards. There has been a relatively recent increase in valley incision, possibly due to Quaternary climatic change, and the valley-bottom gravels contain higher concentrations of diamonds than those along the northern valley slopes. The Sarabaya has a unique population of diamonds which are thought to have been derived from local kimberlite dykes within its drainage basin.
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3 (2), 331-342
PANGIS network, supported by UNESCO, ACCT and bilateral French and Belgium cooperation since 1989, is a pool of African and European documentation centers working in the field of African Geology, all using the same tools for micro-computerized information management and connected together for data supply, information exchange, training support and publishing. PANGIS is also a micro-computerized bibliographic database co-compiled by the 31 members (11 new members in 1995) of the network gathering about 30.000 references (both published and unpublished documents) with analytical descriptions of documents. The material in this bibliography on Diamonds is available in the libraries mentioned at the end of each reference or at the library of CIFEG (see address above); CIFEG being the coordinating center of PANGIS.
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3 (2), 343-354
The Current Bibliography of Middle East Geology contains recently published material on the geology of the region (including Egypt). The bibliography lists books, articles, abstracts, maps, Ph.D. and M.Sc. theses, reports, etc. Special emphasis is given to local material which does not reach international bibliographies or databases. Most of the material in the bibliography is available at the library of the Geological Survey of Israel, which is accessible to the scientific community.
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3 (3/4), 355-366
The Pan-African terrain of Eritrea has been known for its gold potential since at least the 17th century AD. However, over thirty years of warfare precluded the systematic geological investigations that relate mineralization with either tectonics or rock types so as to guide exploration programs. In an attempt to contribute to the solution of this problem, the available geological data have been assessed and integrated with new field observations. The Precambrian rocks of Eritrea consist of two major terrains. These are the Gneiss Terrain and the Metavolcano-Sedimentary-Ophiolite Assemblage intruded by calc-alkaline syn-, to post-tectonic granitoids. The nature and orientation of mesoscale gold-bearing quartz veins relates to regional scale N-S/NNE-SSW and NW-SE trending ductile to brittle-ductile Pan-African strike-slip shear zones. Two mineralized zones, the Ugaro and the Asmara zones are broadly located, respectively, within the releasing bends or transtensional sites in the major shear zones. This indicates that the vein-type hydrothermal gold and associated mineralization of Eritrea is largely tectonically controlled. Fluids accompanying late Pan-African and/or Tertiary granitization might have also played a role in concentrating and redistributing earlier mineralization.
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3 (3/4), 367-384
Based on extensive field and literature surveys together with radiometric dating evidence, the coastal geology of Kenya consists of the Karoo (Upper Carboniferous-Lower Jurassic) to Recent sedimentary sequences with minor local intrusions of igneous rocks of post-Jurassic age. The oldest sedimentary rocks found in coastal Kenya belong to the Karoo sequence and are exposed in the Mombasa coastal basin. In East Africa, the Karoo grabens were initiated by passive extensional regimes with normal-fault displacements being the extensional mechanism. In coastal Kenya, the Karoo graben was initiated in the Early Permian with the possible existence of pre-rift tillitic Dwyka equivalent sediments of Late Carboniferous sediments. The extensional regime of coastal Karoo basin terminated in the Jurassic when the basin was tectonically sealed off by the Late Jurassic faults which heralded the drifting away of Madagascar from the African continent. The Karoo basin is elongated in a broadly north-south direction and has a basin-fill of approximately 6,500 - 7,000 m comprising alluvial, fluvial and lacustrine sequences. The Karoo sediments in coastal Kenya constitute the Duruma Group which is equivalent to the Karoo Supergroup seen elsewhere in East Africa and they crop out onshore west of Mombasa town parallel to the coastline. The western boundary of the coastal Karoo basin is relatively more complex than the eastern margin, with both faulted and unconformable contact with the basement, which is of the Precambrian Mozambique Belt. The eastern margin of the basin is broadly defined by the post-Karoo Middle Jurassic marine limestones of the Kambe Formation. The limestone has both faulted and unconformable relationships with the underlying Duruma Group. The rest of the post-Karoo sediments are mainly prograding marginal to deep marine and shoreline sequences deposited in cycles separated by well-defined tectonic phases. The uplift of the eastern shoulders of the Tertiary East African Rift during the Pliocene led to downwarping of the Kenya coastal region resulting in the deposition of fluviatile sediments of the Marafa Formation. The coastal area emerged later in the Late Pliocene to Early Pleistocene, at which time dune sands of the Magarini Formation and other lagoonal sands were deposited. In the Quaternary period, coastal Kenya was the deposition of dune and lagoonal sands, and the growth of coral limestone, intercalated with continentally derived fluvial channel sands.
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3 (3/4), 385-393
A pedostratigraphic succession, consisting of a modem surface soil and two buried paleosols, was analyzed to determine if D/L (Dextrorotatory/Levorotatory) ratios of amino acids are useful for relative age determinations. The site, located in the Hagenia woodland of the Virunga Mountains at approximately 3000 m a.s.l., is considered representative of transported and residual regolith weathered during the late Pleistocene and Holocene. D/L ratios of aspartic acid and alanine from paleosols in this profile indicate relatively slow racemization rates that provide useful relative age controls. The variations in ratios for valine, leucine and glutamic acid were less but still useful in relative age determination. Valine and glutamic acid, which are known to behave erratically in tropical forest soils on Mt. Kenya, were not detected in the youngest soil, and show relatively low concentrations in the lower and older profiles. Phenylalanine was not detected in the Virunga soil and paleosols, and its absence is taken to indicate minimal aeolian contamination. When combined with other relative age indicators, such as stratigraphic position, strength of development and percent clay, amino acid D/L ratios for aspartic acid assist in refining age controls on buried paleosols. This is particularly true with older paleosols for which 14C ages are not available. Based on the data for aspartic acid, there is a considerable difference in age between the lower and upper buried paleosols, which are considered to separate LGM (Last Glacial Maximum) and Kalambo Interstade sediments from pre-LGM sediments.
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3 (3/4), 395-405
Three Mesozoic to Recent phases of distensive stress are observed in the Archaean Congo Craton of Continental Equatorial Guinea: a Neocomian phase with stress oriented N035E strike developed the Eastern Gabon Basin; an Albian phase oriented N120E developed the Douala Basin; and a Miocene to Recent phase oriented N-S developed the Uoro Rift Basin by oblique extension, and associated half-graben basins system formed along E-W faults dipping south. This third phase created the Uoro Rift; the associated half-graben basins are described for the first time in this paper. The Archaean foliation of the basal cratonic complex presents three prevailing strikes: N125E, N030E and N050E which coincide respectively with the strikes of the Eastern Gabon Basin, Douala Basin and Uoro Rift. With the exception of the half-graben system of the third phase, all the structures generated in the three Mesozoic-to-Recent extensional phases were controlled by the Archaean foliation.
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3 (3/4), 407-411
An area with several ductile faults of various importance has been recognized in the western part of Côte d'Ivoire. These accidents affect both Archaean rocks and formations of the transitional zone between Palaeoproterozoic and Archaean. They form a major South-North 2.1 Ga transpression structure.
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3 (3/4), 413-427
The Takoradi Shale Formation of the Sekondian Group (Late Ordovician to Early Cretaceous age) outcrops mainly along the coastal area of the Sekondi-Takoradi city in the Western Region of Ghana with an estimated thickness of 200 m. The formation has been dated as Devonian or Early Carboniferous age by various workers. The present study records a well preserved plant microfossil assemblage from the formation outcropping at Essipon beach. The miospore assemblages are assigned to three biozones namely Retispora lepidophya - Indotriradires explanatus (LE), Retispora lepidophyta - Verrucosusporites nitidus (LN) and Vallatisporites verrucosus - Retusotriletes incohatus (VI) biozones of the miospore zonation scheme established for the Tournaisian rocks of southern Ireland, dated as Late Devonian (Strunian) to Early Carboniferous (Lower Tournasian) age. The Devonian/Carboniferous boundary has been located within this section of the formation. From a phytogeographic point of view, some species from the formation imply some floral links with Europe, North America, South America, North Africa and Australia in their Late Devonian-Early Carboniferous palynoflora.
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3 (3/4), 429-437
Geochemistry of fluoride in Lake Natron is studied to elucidate processes controlling fluoride concentration and removal during brine evaporation. The range of fluoride concentrations (0.0004-2.75 g/l) in brines from Lake Natron is the greatest found anywhere. So fluoride ion increases by evaporation until saturation with respect to kogarkoïte is attained. Kogarkoïte Na3SO4F occurs in relatively considerable amounts about 4.5% in salt crust of Lake Natron. It has been identified by X-ray diffraction and scanning electron microscopy and it is the first occurrence recognized in the recent evaporitic deposits. The presence of the kogarkoïte salt can explain the behaviour of fluoride element in alkaline brines by evaporation.
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3 (3/4), 439-449
Results from a core sample (435) and grab samples 717 and 722 collected from the southern end of Lake Malawi indicate that rephosphatisation of bones from fish is taking place both at the sediment/water interface and below this interface. X-ray diffraction (XRD) and scanning electron microscope and electron microprobe (SEM/microprobe) results show that carbonate fluorapatite {Ca10(PO4)5(CO3)F.5(OH)} and carbonate hydroxyapatite Ca10(PO4)5(CO3)3(OH)2 at about 5% and 10% respectively are the dominant authigenic minerals at the sediment/water interface. These phosphatic minerals occur either in massive and microcrystalline forms coating clays or as endocasts of fish bone. Vivianite {Fe3 (PO4).8H2O), which occurs in both fibrous and rosette forms, is the dominant phosphate mineral at 7 cm beneath the sediment/water interface in core 435. Results from the two grab samples indicate a richness in biogenic material including fish bone, faecal pellets, molluscan shells, plant debris, fragmentary diatom frustules and arthropod exoskeletons while minerals include quartz, feldspars and clays. Several controls appear to be responsible for the formation of the phosphate minerals encountered in the present study which include: the presence of fish bone concentrates; prevalence of oxic or anoxic sedimentary environment; availability of crucial ions such as iron, bicarbonate and fluoride; and persistent mild alkaline water solution.
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3 (3/4), 451-459
In general the geologic formations of the crystalline basement of northern Côte d'Ivoire are unproductive because they have little or no primary porosity. They are useful aquifers only when a sufficient depth of weathered bedrock exists, or in some cases, in areas with intense open fracturing. A structural and geomorphological approach based on photo-interpretation is used to obtain a detailed description of the drainage networks and to observe fracture and lineament directions. The results show that fractures trending NE, NW and NS related to the Precambrian orogenesis of West Africa, are open fractures. Results also indicate that well productivity is only poorly related to geomorphological characteristics. The crossing of fractures can be used to suggest zones likely to provide high groundwater yields.
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3 (3/4), 461-470
The Ebrie lagoon system, a privileged place for human settlement (implantation) is a most extensive brackish environment in West Africa. It is composed of a central basin of 1 to 7 kilometres wide, orientated E-W and numerous arms or bay orientated N-S covering hundreds of square kilometres. This lagoon system which is the pride of the population of Abidjan is becoming asphyxiated by pollutants of diverse origins because of lack of adequate environmental protection policy. All urban effluents are thrown out into the lagoon without prior treatment causing severe consequences to both the ecosystem and the p