The Great Karoo

A schematic geological map of the outcrops (surface exposures) of the Karoo Supergroup rocks in Southern Africa. The location and approximate structure of the Cape Fold Mountains are also diagrammatically indicated for reference purposes

Ingeological terms, the Karoo Supergroup refers to an extensive and geologically recent (180–310 million year old) sequence of sedimentary and igneous rocks, which is flanked to the south by the Cape Fold Mountains, and to the north by the more ancient Ventersdorp Lavas, the Transvaal Supergroup and Waterberg Supergroup. It covers two-thirds of South Africa and extends in places to 8000m below the land surface, constituting an immense volume of rocks which was formed, geologically speaking, in a short period of time. Although almost the whole of the Great Karoo is situated on Karoo Supergroup rocks, the geological Karoo rocks extend over a very much larger area, both within South Africa and Lesotho, but also beyond its borders and onto other continents that formed part of Gondwana.

Geological history of the Karoo Supergroup

Extinct Karoo vertebrates

Bradysaurus

The Karoo Supergroup was formed in vast inland basin starting 320 million years ago, at a time when the part of Gondwana which would eventually become Africa lay over the South Pole. Icebergs that had carved off the glaciers and ice sheets to the north deposited a kilometre thick layer of mud containing dropstones of varying origins and sizes into this basin. This became the Dwyka Group consisting primarily of tillite, the lowermost layer of the Karoo Supergroup. As Gondwana drifted northwards the basin turned into an inland sea with extensive swampy deltas along its northern shores. The peat in these swamps eventually turned into large deposits of coal which are mined in KwaZulu-Natal and on the Highveld. This 3 km thick layer is known as the Ecca Group, which is overlain by the 5.6 km thick Beaufort Group, laid down on a vast plain with Mississippi-like rivers depositing mud from an immense range of mountains to the South. Ancient reptiles and amphibians prospered in the wet forests, and their remains have made the Karoo famous amongst palaeontologists. The first of these Karoo fossils was discovered in 1838 by Scots-born Andrew Geddes Bain at a road cutting near Fort Beaufort. He sent his specimens to the British Museum, where fellow Scotsman Robert Broom recognised the Karoo fossils' mammal-like characteristics in 1897.

After the Beaufort period, Southern Africa (still part of Gondwana) became an arid sand desert with only ephemeral rivers and pans. These sands consolidated to form the Stormberg Group, the remnants of which are found only in the immediate vicinity of Lesotho. Several dinosaur nests, containing eggs, some with dinosaur fetal skeletons in them, have been found in these rocks, near what had once been a swampy pan.

Finally about 180 million years ago, volcanic activity took place on a titanic scale, which brought an end to a flourishing reptile evolution. These genera represent some of the extinct, mainly pre-dinosaur, animals of the Karoo:

Flat topped hills (called Karoo Koppies) are highly characteristic of the southern and south western Karoo landscape. These hills are capped by hard, erosion resistant doleritesills. This is solidified lava that was forced under high pressure between the horizontal strata of the sedimentary rocks that make up most of the Karoo’s geology. This occurred about 180 million years ago, when huge volumes of lava were extruded over most of Southern Africa and adjoining regions of Gondwana, both on the surface and deep below the surface between the sedimentary strata. Since this massive extrusion of lava, Southern Africa has undergone a prolonged period of erosion exposing the older softer rocks, except where they were protected by a cap of dolerite.

•           Mesosaurus, aquatic Dwyka carnivore

•           Bradysaurus, Beaufort Group herbivore

•           Diictodon, Permian mammal-like reptile

•           Rubidgea, Permian predator

•           Lystrosaurus, Triassic mammal-like herbivore

•           Thrinaxodon, Triassic mammal-like carnivore

•           Euparkeria, early dinosaur

•           Massospondylus, late Triassic to early Jurassic herbivorous, bipedal dinosaur

•           Megazostrodon, early mammal

A diagrammatic 400 km north-south cross-section through the southern portion of the country at approximately 21° 30’ E (i.e. near Calitzdorp in the Little Karoo), showing the relationship between theCape Fold Mountains (and their geological structure) and the geology of the Little and Great Karoo, as well as the position of the Great Escarpment. The colour code for the Karoo rocks is the same as those used in the above diagram. The heavy black line flanked by opposing arrows is the fault that runs for nearly 300 km along the southern edge of the Swartberg Mountains. The Swartberg range owes some of its great height to upliftment along this fault line. The subsurface structures are not to scale.

The lava outpourings that ended the Karoo deposition of rocks, not only covered the African surface, and other parts of Gondwana with a 1.6 km thick layer basaltic lava,[13] but it also forced its way, under high pressure, between the horizontal layers of sedimentary rocks belonging to the Ecca and Beaufort groups, to solidify into dolerite sills. The long vertical fissures through which the lava welled up solidified into dikes which resemble the Great Wall of China from the air.[4] From about 150 million years ago the South African surface has been subjected to an almost uninterrupted period of erosion, particularly during the past 20 million years,[13] shaving off many kilometers of sediments. This exposed the dolerite sills, which were more resistant to erosion than the Karoo sediments, forming one of the most characteristic features of the Karoo landscape, namely the flat topped hills, called "Karoo Koppies".