Overberg Geoscientists Group
310 Mya
Karoo Sedimentation
Location and Stratigraphy. The Karoo Supergroup ranges in age from Late Carboniferous to Early Jurassic and is made-up of the main five groups with local sub-groups and formations. It occurs in the large, complex Main Karoo sedimentary basin in South Africa, extending across nearly two-thirds of the country.
Basin Evolution. Following the deposition of the Cape Supergroup sediments 290 million years ago the Falklands plateau started to compress the Agulhas Sea that continued until the basin was closed. The Karoo Basin evolved into an intra-continental foreland basin concurrent with the development of the Cape Fold Belt. Sediments eroding from these mountains were deposited in the Karoo Basin filled with sedimentary strata over 10 km thick. These were deposited in glacial, deep marine (including turbidite), shallow marine, deltaic, meandering and braided fluvial, lacustrine and aeolian environments. Fossil-bearing sediments with plants, invertebrates, fishes, mammal-like reptiles and reptiles such as the dinosaurs occur throughout. The deposition of the Karoo sediments ended some 180 Myr ago with the outpouring of the Drakensberg flood basalts.
Geology. During the global Upper Carboniferous Ice Period, around 290 mya vast, up to 4 km thick, ice sheets melted to leave behind a thick layer of glacial sediments. These being the earliest Karoo sediments of the Dwyka Group glaciogenic diamictite deposits. Striated pavements caused by the action of boulders frozen into the base of the glaciers can be seen in basement outcrops along most of the margin of the Main Karoo Basin.
As the Dwyka ice-sheets melted during the Early Permian 270 mya, there was a change to shallow water deltas and wetlands along the northern shoreline of the Karoo Sea where organic-rich mudstones, siltstones and sandstones of the Ecca Group were deposited under warmer conditions. Tree-like plants colonised the deltas where the genus Glossopteris formed massive swamps responsible for producing South Africa’s vast coal deposits.
In contrast the southwestern portion of Karoo Sea was very deep, with steep slopes leading up to the shoreline. Underwater sediment avalanches known as turbidites, carried coarse and fine-grained material out into the distant, deep water forming large fan-shaped accumulations on the sea floor in the Laingsburg and Tanqua Sub-basins. Further proof of the deep-water setting comes from the aquatic reptile Mesosaurus and fossil fish found in the thin White Hill Formation. This organic-rich black shale present throughout the southwestern part of the Karoo Basin may be the best prospect for South African shale gas.
The transition into terrestrial environments marks the boundary between the Ecca and the Beaufort Group. Inboard of the rising Cape Fold Belt, the upward fining continental succession of the Beaufort sediments was deposited as several fluvial systems. The rocks are mostly mudstone dominated up until the sandstones of the lower Tarkastad Subgroup in the north. Concurrent volcanic activity that took place with the foreland tectonics provided the thin volcanic ash beds known as tuffs. By about 250 million years the Karoo Sea became largely silted up. Vertebrate and plant fossils are common with the former used to subdivide the Beaufort Group into eight assemblage zones.
The Late Triassic-Early Jurassic (220 to 200 mya) fluvial, lacustrine and aeolian Stormberg Group deposits reflect a gradual change to arid conditions because of a global warm period. Carnivorous and herbivorous dinosaurs became well established with the first true mammals appearing in the Early Jurassic Elliot Formation. By the time of the deposition of the Clarens Formation true desert conditions prevailed with the development of an extensive sand sea much like the Namib Desert.
The Drakensberg Group of a thickness of more than 1,000 m of Jurassic lavas form the top of the Karoo Sequence that marks the end of Karoo sedimentation and the beginning of the break-up of Gondwana. The 183 ± 2Ma continental flood basalts of intrusive and extrusive rocks occur over a very extensive area of southern Africa. The associated Karoo Dolerites developed as an interconnected network of dykes, sills and sheets that represent the shallow feeder system to the flood basalt eruptions of the Drakensberg Group.
Further reading
​
Catuneanu, O. Wopfer, H., Eriksson, P.G., Cairncross, B., Rubidge, B.S., Smith, R.M.H., and Hancox. P.J. 2005. The Karoo basins of south-central Africa. Journal of African Earth Science. 43.
​
Hancox, P.J. and Götz, A.E. 2014. South Africa’s coalfields – a 2014 perspective. International Journal of Coal Geology. 132, 170-254.
Johnson, M.R., van Vuuren, C.J., Hegenberger, W.F., Key, R., and Show, U. 1996. Stratigraphy of the Karoo Supergroup in southern Africa: an overview. Journal of African Earth Sciences Volume 23 (1).
​
Johnson, M.R., van Vuuren, C.J., Visser J.N.J., Cole D.J., Wickens H. de V., Christie, A.D.M., Roberts D.L. and Brandl, G. 2002. Sedimentary Rocks of the Karoo Supergroup. Geology of South Africa. Council Geoscience, Pretoria, 461-481.
McCarthy, T. and Rubidge, B. 2005. The story of earth & life. A southern Africa perspective on a 4.6 billion year journey. Struik Publishers.
​
Rubidge, B. and Hancox, J. 2002. The Karoo Supergroup. In Rocks and Minerals, 77 (1).
J Malan Sept 2023