The Karoo Supergroup is the most widespread stratigraphic unit in Africa south of the Kalahari Desert. The supergroup consists of a sequence of units, mostly of nonmarine origin, deposited between the Late Carboniferous and Early Jurassic, a period of about 120 million years.
In southern Africa, rocks of the Karoo Supergroup cover almost two thirds of the present land surface, making part of the 75% of sediments or sedimentary rocks covering the earth including all of Lesotho, almost the whole of Free State, and large parts of the Eastern Cape, Northern Cape, Mpumalanga and KwaZulu-Natal Provinces of South Africa. Karoo supergroup outcrops are also found in Namibia, Eswatini, Zambia, Zimbabwe and Malawi, as well as on other continents that were part of Gondwana. The basins in which it was deposited formed during the formation and breakup of Pangea.McCarthy, T., Rubridge, B. (2005). The Story of Earth and Life. pp. 161, 187–241. Struik Publishers, Cape Town The type area of the Karoo Supergroup is the Great Karoo in South Africa, where the most extensive outcrops of the sequence are exposed.Geological map of South Africa, Lesotho and Swaziland (1970). Council for Geoscience, Geological Survey of South Africa. Its strata, which consist mostly of shales and sandstones,Hamilton, G.N.G. and Finlay, J.G. (1928). Outline of Geology for South African Students, Central News Agency Ltd., Johannesburg. record an almost continuous sequence of marine glacial to terrestrial deposition from the Late Carboniferous to the Early Jurassic. These accumulated in a retroarc foreland basin called the "main Karoo" Basin. This basin was formed by the subduction and orogenesis along the southern border of what eventually became Southern Africa, in southern Gondwana. Its sediments attain a maximum cumulative thickness of 12 km, with the overlying basaltic lavas (the Drakensberg Group) at least 1.4 km thick.Adelmann, D. and Kerstin Fiedler, (1996). "Sedimentary development of the Upper Ecca and Lower Beaufort Groups (Karoo Supergroup) in the Laingsburg subbasin (SW Karoo Basin, Cape Province/South Africa)" , Schriftenreihe der Deutschen Geologischen Gesellschaft, 1: 88–89, Bonn.
Fossils include plants (both macro-fossils and pollen), rare insects and fish, common and diverse tetrapods (mostly therapsids, temnospondyl amphibians, and in the upper strata dinosaurs), and ichnofossils. Their biostratigraphy has been used as the international standard for global correlation of Permian to Jurassic nonmarine strata.Hancox, P. J. and Bruce S. Rubidge (1997). The role of fossils in interpreting the development of the Karoo basin, Palaeontologica Africana, 33: 41–54.
Geological origin
Origins of sediment deposition
About 510 million years ago a rift valley developed across Southern Gondwana just south of Southern Africa, but extending westward into South America, and eastward into Eastern Antarctica and possibly even into Australia. An 8 km thick layer of sediment, known as the Cape Supergroup, accumulated on the floor of this rift valley. Closure of the rift valley, starting 330 million years ago, resulted from the development of a subduction zone along the southern margin of Gondwana, and the consequent drift of the Falkland Plateau back towards Africa, during the Carboniferous and Early Permian periods. After closure of the rift valley, and compression of the Cape Supergroup into a series of parallel folds, running mainly east–west, the continued subduction of the paleo-Pacific Plate beneath the Falkland Plateau and the resulting collision of the latter with Southern Africa, raised a mountain range of immense proportions to the south of the former rift valley. The folded Cape Supergroup formed the northern foothills of this mountain range.
The weight of the Falkland-Cape Supergroup mountains caused the continental crust of Southern Africa to sag, forming a retroarc foreland system, which became flooded to form the Karoo Sea. Sedimentation, beginning with glacial deposits from the north, but later from the Falkland Mountains to the south, into this depression formed the Karoo Supergroup.
The Dwyka group
About 330 million years ago Gondwana had drifted over the South Pole,Norman, N., Whitfield, G. (2006). Geological Journeys. pp. 28, 122. Struik Publishers, Cape Town. with the result that an ice sheet several kilometers thick covered much of Africa, and other parts of Gondwana. The glacial deposits from this ice sheet were the first of the sediments to be deposited to the north of the Cape Fold Mountains (and partially over these incipient mountains). The basin into which these sediments settled was deepest immediately north of the Cape Fold Mountain ranges. The ice sheet therefore floated on an inland lake, termed the Karoo inland sea, into which icebergs which had calved off the glaciers and ice sheet to the north deposited vast quantities of mud and rocks of various sizes and origins. Such deposits are known as tillite. Further north, the ice sheet was grounded also leaving diamictite deposits whenever it partially melted, but, in addition, it scoured the bedrock, leaving behind striations (scratch marks) which can be seen near Barkly West in the Northern Cape, and in the grounds of the University of KwaZulu-Natal. This layer of tillite, traces of which can be found over a wide area of Southern Africa, India, and South America provided crucial early evidence in support of the Theory of Continental Drift. In South Africa the layer is known as the Dwyka Group. It is the earliest and lowermost of the Karoo Supergroup of sedimentary deposits.
The Ecca group
As Gondwana drifted away from the South Pole, the glaciers melted, leaving a vast inland sea, extending across South Africa, and neighboring regions of Gondwana. It might have had an opening to the ocean (similar to the Black Sea) but tidal effects were small. Rivers draining mountains to the north of the Karoo Sea formed large swampy deltas in which plants belonging to the Glossopteris flora flourished. This dense vegetation accumulated as peat, which eventually turned into coal. The coal deposits are confined to the northern shores of the early Permian Karoo Sea, and is mined today in the Highveld and KwaZulu-Natal.
These sedimentary deposits are termed the Ecca Group of the Karoo Supergroup. They consist largely of shales and sandstones, and extend over the entire former Karoo Sea, but the southerly deposits do not contain coal, even though rivers from the Cape Fold Mountains formed small deltas. Although the vegetation in the south was not as dense as on the northern shores of the inland sea, several early reptiles such as Mesosaurus are found in these Ecca deposits. This is a fossil reptile found only in Southern Africa and Brazil providing important paleontological evidence of the existence of the Gondwana supercontinent.
The northern shores contain mainly fossil plants, pollens and spores. Fossils of a cephalopod and some echinoids are also found in the north.
During the Ecca period the Falklands Plateau collided and then fused with Southern Africa, forming a vast range of mountains to the south of the Cape Fold Belt. This new mountain range was comparable in size to the Himalayas. The northern slopes of these mountains generally dipped steeply into the Karoo Sea which was at its deepest at this point. The earthquakes that accompanied the formation of the Cape Mountains therefore initiated frequent underwater mud- and rock-slides, forming fan-shaped accumulations of turbidites, which can be seen in the south west corner of the Karoo today (see photograph lower down on the right) the Ecca group also has its members that are named based on the lithologies of the locality they are in . Turbidites have for some time been recognized as petroleum producing rocks, because the underwater avalanches that cause these deposits often carry organic matter from close to the coastline, especially near river estuaries and deltas, into the anoxic depths of adjoining troughs. Here it is buried in the turbidite and turns into hydrocarbons, particularly petroleum and gas. The turbidites in the Ecca formation of the Tanqua and Laingsburg Karoo regions have thus, recently, come under scrutiny by the petroleum industry and geologists, who have found them to have rich and readily accessible deposits of oil and gas. Thus the north-eastern Ecca basin is rich in coal, while its south-western corner is becoming renowned for its oils reserves.
Beaufort Group
With the formation of the Falkland Plateau and Cape Fold Mountain ranges, rivers from the south began to dominate the sedimentation in the Karoo Sea, which began to silt up. (The highlands to the north of the Karoo Sea had, by this time, been leveled by erosion and begun to be buried under newer sediments.) Several Mississippi-like rivers flowed over the silted up Karoo Basin from the south, creating rich new habitats for a variety of flora and fauna. The terrestrial (as opposed to lacunar or marine) deposits created by these rivers gave rise to the Beaufort Group. It is composed of a monotonous sequence of shales and mudstones, with some interbedded lenticular sandstones. The Beaufort Group is rich in reptilian, and to a lesser extent, amphibian remains. There is a plethora of both herbivorous and carnivorous reptile fossils. The Beaufort rocks are internationally famous for its rich record of therapsid synapsids (mammal-like reptiles), which mark an intermediary stage in the evolution of the mammals from reptiles. The most abundant herbivores were the anomodonts, whose most primitive forms are also known from the Beaufort rocks. The dinocephalians (terrible head) are so named because of their extraordinary thick boned skulls, which were probably used for head butting during territorial fights. With their 3-meter body length, they were the first large animals to live on land.
During the course of the laying down of the 6 km thick Beaufort deposits, the massive end-Permian mass extinction, 251 million years ago, extinguished about 96% of all species alive at that time. The global event can clearly be seen in the Beaufort rocks. A few members of the genus Lystrosaurus were the only mammal-like reptiles that survived this event. The Beaufort sediments that were laid down after this event tend to be coarser than the ones that preceded them, probably because of a massive die-off of vegetation, which had protected the surface against erosion. These early Triassic sandstone-dominated strata are known as the Katberg formation (within the Beaufort Group), which accumulated to a thickness of 1 km. With time the Beaufort deposits became more fine-grained once again, probably indicating a recovery of the vegetation in the Karoo, and with it the appearance of a wide range of new species, including the dinosaurs, and true mammals during the late Triassic – early Jurassic.
Stormberg group
As Gondwana drifted north, conditions in the portion that was to become Southern Africa became increasingly hot and arid. Sandstones were the predominant rocks that formed. However, in some areas there was sufficient water to form swamps with consequent coal formation, but the quality is poor. The landscape probably resembled the Kalahari desert of today, with rivers like the present-day Orange River or the Nile running through it, sustaining localized areas of Triassic flora and fauna. The Stormberg Group contains South Africa's earliest dinosaur fossils. It also contains the fossil remains of the shrew-sized Megazostrodon, the oldest mammal in Africa. A remarkable array of insect and plant fossils are found in some of the strata.