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| Dwyka Formation | |
|---|---|
| Name | Dwyka Formation |
| Type | Geological formation |
| Region | Karoo Basin |
| Country | South Africa |
Dwyka Formation The Dwyka Formation is a Late Carboniferous to Early Permian geological succession within the Karoo Supergroup of South Africa and parts of Namibia and Antarctica. It records glacial, glaciomarine, and postglacial deposits associated with the Gondwanan glaciation and underlies the Ecca Group and overlies the Table Mountain Group in parts of the Cape Fold Belt. The formation is central to interpretations of Permo-Carboniferous paleoclimate, paleogeography, and the tectono-sedimentary evolution of southern Gondwana.
The Dwyka Formation crops out across the Karoo Basin, Ceres, Prince Albert (Karoo), Beaufort West, Sutherland, Northern Cape, and parts of Namaqualand and Namibia. Its recognition influenced early mapping by the Geological Survey of South Africa and later synthesis by workers at the Council for Geoscience and universities such as the University of Cape Town, University of the Witwatersrand, and University of Pretoria. The unit is correlated with glacial deposits in Gondwana including successions in Brazil, Argentina, Australia, India, and Antarctica.
Lithologies include diamictite, tillite, rhythmites, dropstones, mudstone, siltstone, and sandstone. Diamictites and massive tillites occur near Prince Albert (Karoo) and Beaufort West localities, whereas laminated rhythmites and mudstones dominate marginal basins near Sutherland, Northern Cape and the Namaqua-Natal Mobile Belt. Dropstone-bearing laminated mudstones are noted at exposures studied by teams from University of Cape Town and Universidade de São Paulo. Glacial pavements and striated clasts have been described in sections mapped by the British Geological Survey-linked researchers and the Council for Geoscience.
The Dwyka Formation is the basal unit of the Karoo Supergroup in many parts of the basin and is succeeded by Ecca Group shales and sandstones and, regionally, by Beaufort Group deposits. Stratigraphic subdivision into members—such as the Elandsvlei, Vryheid, and Suid Bokkeveld equivalents in different basins—has been proposed by researchers at the Council for Geoscience and the South African Museum. Correlations have been drawn with the Ponta Grossa Formation of Brazil, the Gondwana sequences of India (for example, the Krol Belt correlative successions), the Permian glacial deposits of Antarctica such as the Sirius Group, and Australian glacial units including the Hallett Cove Formation and Beacon Supergroup equivalents.
Interpretations emphasize glaciomarine, proglacial, and subglacial depositional settings linked to the Late Paleozoic Ice Age. Evidence for ice-sheet advances and retreats is supported by diamictites, striated clasts, and glacial rhythmites documented in field studies by teams from University of Cape Town, Universidade de São Paulo, Australian National University, and Smithsonian Institution collaborators. Paleoclimatic significance ties the unit to the assembly of Gondwana during the Variscan Orogeny and global carbon-cycle perturbations recorded in contemporaneous successions in Europe, North America, and South America. Glacial-interglacial cycles inferred from rhythmites have been compared with cyclothems in the Carboniferous of England and the Midcontinent Rift records of United States researchers.
Macro- and microfossils are rare but include dropstone-transported wood fragments, pollen and spore assemblages, and microbial mats. Palynological studies from samples analyzed at the Council for Geoscience, University of the Witwatersrand, and University of Pretoria yielded spores correlated with Permian assemblages known from Argentina and India. Microbial textures and biofilms analogous to those described from the Falkland Islands and East Antarctica glacial successions are present. Vertebrate remains are generally absent in most sections, whereas trace fossils and soft-sediment deformation structures have been documented in detailed facies studies by teams at the South African Museum and international collaborators from University of Texas and University of Melbourne.
Although the Dwyka Formation itself is not a major hydrocarbon reservoir, it forms an impermeable basal seal for overlying Ecca Group shale-gas and coal-bearing intervals and influences groundwater flow exploited by municipalities including Beaufort West and Sutherland, Northern Cape. Diamictites and tills have been used locally as roadstone and construction aggregate by regional authorities and by contractors associated with the National Roads Agency South Africa. The formation’s glacial pavements and spectacular exposures also support geotourism in areas promoted by the South African National Parks and local tourism bodies in the Karoo.
Early recognition came from 19th-century geologists mapping the Cape Fold Belt and later formal description by workers affiliated with the Geological Survey of South Africa. Key modern syntheses and stratigraphic revisions were published by researchers at the Council for Geoscience, University of Cape Town, University of Pretoria, and international teams from Universidade de São Paulo, Australian National University, Smithsonian Institution, British Geological Survey, and University of Texas. Notable studies include palynological analyses linking the unit to Permian successions in Argentina and India, sedimentological work documenting glacigenic facies transitions, and glaciomarine models advanced in collaborative papers presented at meetings of the International Geological Congress and the South African Geological Society.
Category:Geologic formations of South Africa Category:Karoo Supergroup