Dwyka Group
| Stratigraphy of the Karoo Supergroup in the Karoo Basin | ||||
|---|---|---|---|---|
| Period | Group | Formation west of 24°E | Formation east of 24°E | Assemblage Zone |
| Jurassic | Drakensberg | Hiatus | Drakensberg | |
| Stormberg | Clarens | |||
| Triassic | Elliot | |||
| Molteno | ||||
| Beaufort | ||||
| Burgersdorp | Cynognathus | |||
| Katberg | Lystrosaurus | |||
| Balfour | ||||
| Permian | Dicynodon | |||
| Teekloof | ||||
| Cistecephalus | ||||
| Middleton | ||||
| Tropidostoma | ||||
| Pristerognathus | ||||
| Abrahams-Kraal | Abrahams-Kraal | |||
| Tapinocephalus | ||||
| Eodicynodon | ||||
| Ecca | Waterford | Waterford | ||
| Tierberg / Fort Brown | Fort Brown | |||
| Laingsburg / Ripon | Ripon | |||
| Collingham | Collingham | |||
| Whitehill | Whitehill | |||
| Prince Albert | Prince Albert | |||
| Carboniferous | Dwyka | Elandsvlei | Elandsvlei | |
| References: Rubidge (2005),[1] Selden and Nudds (2011).[2] | ||||
The Dwyka Group is the group of sedimentary geological formations laid down in the Karoo Basin of southern Africa in the Late Carboniferous and possibly extending into the Asselian of the early Permian. It consists mainly of tillites, laid down along the sandy shorelines of swamplands. The Dwyka is the oldest and lowermost unit of the Karoo Supergroup that is recognized throughout sub-Saharan Africa.
Formation
In the Carboniferous, southern Africa was part of Gondwana. During the Late Carboniferous the lithosphere underlying what is now the Karoo Basin migrated over the South Polar Region. This resulted in southern Gondwana being covered by a major ice sheet. As the ice sheet and subsequent glaciers melted, the sediments of the Dwyka Group were deposited in the newly formed basin. These glacial deposits include diamictite, varved shale and mudstone with dropstones, fluvioglacial gravel and conglomerates. The total thickness of the group ranges from 600 to 750 metres (1,970 to 2,460 ft).[3]
The Dwyka Group is considered to be Permo-Carboniferous in age, but due to ambiguities in the fossil record, more precise dating is not available. Maximum age inferred from fossils found in underlying strata is Late Devonian or Early Carboniferous, and minimum age inferred from fossils in the upper glacial deposits is Early Permian.[4]
Stratigraphic position
In the Eastern Cape Province, the Karoo Basin fill commenced with the deposition of the Dwyka Group, followed by the Ecca Group, the Beaufort Group, the Molteno, Elliot, and Clarens Formations and the igneous Drakensberg Group. The basin followed the typical evolution of foreland basins, with the Ecca Group representing the ‘flysch’ component and the Beaufort Group, the overlying Molteno and Elliot Formations representing the ‘molasse’-fluvial type sediments.[5][6]
In the Nama or Kalahari Basin of southeastern Namibia, the Dwyka Group contains the fossiliferous Ganigobis Formation.
See also
- Geology of Namibia
- Geology of South Africa
- List of fossiliferous stratigraphic units in Namibia
- List of fossiliferous stratigraphic units in South Africa
References
- ^ Rubidge, B.S. (2005). "Re-uniting lost continents – Fossil reptiles from the ancient Karoo and their wanderlust". South African Journal of Geology. 108 (1): 135–172. doi:10.2113/108.1.135.
- ^ Selden, P.; and Nudds, J. (2011). "Karoo". Evolution of Fossil Ecosystems (2 ed.). Manson Publishing. pp. 104–122. ISBN 9781840761603.
- ^ SACS (South African Committee for Stratigraphy) (1980) Stratigraphy of South Africa. Part 1 (Comp. L.E. Kent). Lithostratigraphy of the Republic of South Africa, South West Africa/Namibia, and the Republics of Bophuthatswana, Transkei and Venda: Handbook Geological Survey of South Africa, 8, 690p.
- ^ M. J. Hambrey, W. B. Harland, Earth's pre-Pleistocene glacial record, International Geological Correlation Programme. Project 38: Pre-Pleistocene Tillites, p76
- ^ Johnson, M. R. (1991). Sandstone petrography, provenance and plate tectonic setting in Gondwana context of the south-eastern Cape Karoo basin, South African Journal of Geology, 94, 137-154.
- ^ Catuneanu, O. (2004) Retroarc foreland systems – evolution through time, Journal of African Earth Sciences, 38, 225-242.