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Wed, Jul 23, 2014

Sinkholes and Subsidences


Dolomite and Limestone occurrences in South Africa
Rsadol.jpg

The main sedimentary carbonate units, as indicated in the Home Building Manual Part 1 & 2, on page 21, comprise:

  1. Malmani Subgroup, Chuniespoort Group
  2. Campbell Rand Subgroup, Ghaap Group
  3. Mzimkulu Group
  4. Gariep Supergroup
  5. Gamtoos Group
  6. Cango Group
  7. Malmesbury Group
  8. Nama Group
  9. Vanrhynsdorp Group
  10. Mzamba Formation
  11. Algoa Group
  12. Bredasdorp Group
  13. Maputaland Group
  14. Sandveld Group

In Gauteng Province, the carbonate formations comprise the Malmani Subgroup  of the Chuniespoort Group (Transvaal Supergroup), which is ~ 2600–2400 Ma. The Subgroup is subdivided into various formations of which some are chert-poor and some are chert-rich.  The dolomitic formations are, in places, overlain by younger rocks, of the Pretoria Group (2350–2100 Ma), Transvaal Supergroup, and/or the Karoo Supergroup (300–200 Ma), and/or mantled by unconsolidated material of Cenozoic age (≤ 65 Ma). 

In Kwazulu-Natal Province, the carbonate rock is hosted in the high-grade metamorphic rocks of the Marble Delta Formation of the Mzimkulu Group, which is ~ 1300–1250 Ma, which occurs as an oval-shaped outcrop, 10 km north-west of Port Shepstone.  Carbonate rock is also found in the Tugela Group, which is 1300 Ma, located near Kranskop and east of Greytown.

In Mpumalanga Province, the carbonate formations comprise the Malmani Subgroup (Chuniespoort Group, Transvaal Supergroup).  Alteration of dolomite to limestone (de-dolomitization) has occurred in many places due to the intrusion of the Bushveld Complex.

In the North West and Limpopo Provinces, the carbonate formations comprise the Malmani Subgroup of the Chuniespoort Group of the Transvaal Supergroup.  Alteration of dolomite to limestone due to the intrusion of the Bushveld Complex is particularly evident north-east of Mokopane (formerly known as Potgietersrus).

In the Northern Cape Province, carbonate rocks mainly comprise the Campbell Rand Subgroup (Ghaap Group, Transvaal Supergroup), which is ~ 2600–2400 Ma.  Carbonates can also be found in various Groups of the Gariep Supergroup, which is ~ 700 Ma, near Eksteenfontein, as well as in the Schwarzrand Formation of the Nama Group, which is ~ 550 Ma, near Vioolsdrif.

In the Western Cape Province, listed according to location, the carbonate rocks form part of the following:

Vanrhynsdorp-Bitterfontein area – Knersvlakte and Kwanous Formations of the Vanrhynsdorp Group, which is ~ 550 Ma,
Langebaan area - Langebaan Formation of the Sandveld Group, which is of Quaternary age (≤ 2 Ma),
Piketberg area – various formations of the Malmesbury Group, which is ~ 700 Ma,
Hermanus to Mossel Bay area - Bredasdorp Group, which is of Cenozoic age (≤ 65 Ma),
Area north of Oudtshoorn - Cango Caves Group, which is ~ 700 Ma. 

In the Eastern Cape Province, the carbonate rocks form part of:

Gamtoos Group, which is ~ 700 Ma, located near Patensie
Cenozoic Algoa Group, which is ≤ 65 Ma, in the Port Elizabeth-East London area
Mzamba Formation, which is 100 Ma, situated south of Port Edward

Although dolomite (carbonate) related instability may take place in any karstic terrain, most instability features have been noted in the Malmani Subgroup and the Campbell Rand Subgroup.  However, the risk of sinkhole and doline formation in the other groups cannot be excluded.

Areas on Dolomite
Is your town on dolomite?

Approximately 25% of Gauteng province, as well as parts of Mpumalanga, Limpopo and Northern Province are also underlain by dolomite which poses a potential risk to the safety of many people and the structures in which they work and live.

Distribution of dolomite in South Africa Distribution of dolomite in South Africa

See the list below:

COMMON NAME:
NEW NAME:
BARBERTON
UMJINDI
BARKLEY WEST
THUSANANG MUNICIPALITY
BELFAST
HIGHLANDS
BENEDE
BENEDE
BO KAROO
BO KAROO
BRITS
NW372 LOCAL COUNCIL
BRONKHORSTSPRUIT
KUNGWINI LOCAL MUNICIPALITY
BRUERSFORT/ OHRIGSTAD/EASTERN TUBATSE
GREATER TUBATSE MUNICIPALITY
BUSHBUCKRIDGE
BUSHBUCKRIDGE MUNICIPALITY
CARLTONVILLE
MERAFONG CITY LOCAL MUNICIPALITY
DANIELSKUIL
DAN-LIME MUNICIPALITY
DELMAS
DELMAS
DIAMONDFIELDS
DIAMONDFIELDS
DMA LOWVELD
DMA LOWVELD
EAST RAND
GREATER EAST RAND METRO
ELLISRAS
LEPHALALE
ELUKWATINI/CAROLINA
ALBERT LUTHULI
GANYESA
KAGISANO LOCAL MUNICIPALITY
GRIEKWASTAD
SIYANCUMA MUNICIPALITY
GROBLERSDAL
GREATER GROBLERSDAL MUNICIPALITY
HEIDELBERG
LESEDI LOCAL MUNICIPALITY
HOEDSPRUIT
DRANKENSBERG MUNICIPALITY
HOPETOWN
ORANJE-KAROO MUNICIPALITY
JOHANNESBURG
CITY OF JOHANNESBURG
KALAHARI CBDC
KALAHARI CBDC
KATHU
GAMMAGARA MUNICIPALITY
KGALAGADI
SEGONYANA MUNICIPALITY
KLERKSDORP
KLERKSDORP LOCAL MUNICIPALITY
KOSTER
NW374 LOCAL MUNICIPALITY
KROONSTAD
MOOHAKA LOCAL MUNICIPALITY
KRUGERSDORP
MOGALE CITY LOCAL MUNICIPALITY
KURUMAN
KURAMAN-MOTHIBISTAD MUNICIPALITY
LEBOWAKGOMO
LEPELLE-NKUMPI
LICHTENBRUG
LICHTENBURG LOCAL MUNICIPALITY
MAFIKENG
MAFIKENG LOCAL MUNICIPALITY
MARBLE HALL
GREATER MARBLE HALL MUNICIPALITY
MEYERTON
MIDVAAL LOCAL MUNICIPALITY
MOGWASE
MANKWE-MADIKWE LOCAL MUNICIPALITY
NELSPRUIT
MBOMBELA
NYLSTROOM
MODIMOLLE
PARYS
NGWATHE LOCAL MUNICIPALITY
PIETERSBURG
POLOKWANE
POMFRET
MOLOPO LOCAL MUNICIPALITY
POSTMASBURG
RE A IPELA MUNICIPALITY
POTCHEFSTROOM
POTCHEFSTROOM LOCAL MUNICIPALITY
POTGIETERSRUS
MOGALAKWENA
PRETORIA
TSHWANE METROPLITAN MUNICIPALITY
PRIESKA
PRIEMANDAY MUNICIPALITY
RANDFONTEIN
RANDFONTEIN LOCAL MUNICIPALITY
REIVILO
GREATER TAUNG LOCAL MUNICIPALITY
RUSTENBURG
RUSTENBURG LOCAL MUNICIPALITY
SABIE
THABA CHWEU
SASOLBURG
METSIMAHOLO LOCAL MUNICIPALITY
SCHUINSDRAAI NATURE RESERVE
SCHUINSDRAAI NATURE RESERVE
STERKFONTEIN
STERKFONTEIN
THABAZIMBI
THABAZIMBI
TZANEEN
GREATER TZANEEN MUNICIPALITY
VENTERSDORP
VENTERSDORP LOCAL MUNICIPALITY
VEREENIGING
EMFULENI LOCAL MUNICIPALITY
VRYBURG
NALEDI LOCAL MUNICIPALITY
WARMBATH
BELA BELA
WESTONARIA
WESTONARIA LOCAL MUNICIPALITY
ZEERUST
ZEERUST LOCAL MUNICIPALITY

DOLOMITIC GROUND?

Quite frequently devClassification of Dolomitic and Non-dolomitic Landelopments take place on the edge of the dolomite ground, which raises the question as to whether a dolomite stability investigation is required. This obviously affects the quotation process resulting in jobs being won or lost. It also results in unnecessary costs to the client or delays, if deep drilling is later required.

Dolomite stability investigations are, by definition, required where dolomite bedrock is either <60mbs (non dewatering scenario) and <100mbs (dewatering scenario). In addressing this problem the Council for Geoscience (CGS) has produced a ‘dolomitic’ map which includes the marginal areas around the dolomite lands. It utilizes the 100m depth to dolomite bedrock definition and was produced by utilizing the typical dips of the younger layers. The buffer area thus varies in width depending on dip. It is hoped to improve the accuracy of this map by regularly feeding in borehole data that the CGS receives. Detail clips from this map of particular sites can be obtained from Lindy Heath ().