Karoo Supergroup

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. The type area of the Karoo Supergroup is the Great Karoo in South Africa, where the most extensive outcrops of the sequence are exposed. Its strata, which consist mostly of shales and sandstones, 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.

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.

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.

About 330 million years ago Gondwana had drifted over the South Pole, 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.

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.