Karoo Supergroup Sedimentation

The Karoo is a large, complex sedimentary basin in South Africa, extending across nearly two-thirds of the country. The basin is filled with Carboniferous to Early Jurassic sedimentary strata over 5 km thick, known as the Karoo Supergroup. A component of the Karoo Supergroup is the Early Permian-age Ecca Group, which consists primarily of organic-rich mudstones, siltstones, sandstones, and minor conglomerates deposited in shallow water deltas and wetlands on the northern shoreline of the Karoo Sea under warm climate conditions (Catuneanu et al., 2005). This marshy or “paludal” depositional environment produced abundant coals within the Ecca Group. These coal deposits are confined to the northern part of the basin and do not occur to the south, but the Ecca Group contains almost all of South Africa's coal resources.

The southwestern portion of Karoo Sea was very deep, with steep slopes leading up to the shoreline. This environment was favorable for underwater sediment avalanches known as turbidites to carry coarse and fine-grained material out into distant, deep water. Each turbidite layer consists of a fining-upward sequence grading from coarse sandstone at the base to siltstone and shale at the top. Organic material carried along with the turbidite remained in suspension well into the deepest part of the basin, where it settled out under anoxic bottom conditions, before being buried and eventually converted to oil and gas (Raseroka and McLachlan, 2009). The Ecca Group is quite thick in the southwestern part of the Karoo basin, approaching 1300 m (4300 ft) in some locations.
Tectonic and climatic shifts from the southern to the northern margins of the basin during the deposition of the Karoo Supergroup altered the lithologic character of the rocks in both space and time. Tectonic activity in the Cape Fold Belt at the southern boundary of the Karoo basin produced a series of igneous intrusions into the sedimentary sequence known as dolerite sills (Coetzee and Kisters, 2016). The most prominent and thickest sills are concentrated within the upper Ecca Group rocks, and have resulted in off-gassing, compartmentalization, and mine stability problems in the Ecca coalfields.

Reference: Leslie Robert Brown, Hugo Bezuidenhout, in Encyclopedia of the World's Biomes, 2020