A simplified geological map of the outcrops of Karoo Supergroup rocks in Southern Africa. The Dwyka Group deposits are represented by the red key on the map
At the commencement of the deposition of the Dwyka Group, it is thought that the development of the Karoo supergroup
foreland system had begun approximately 30 million years prior. This
foreland system was caused by
crustal uplift that had previously begun to take course due to the
subduction of the Palaeo-pacific
plate beneath the Gondwanan
plate. This resulted in the rise of the Gondwanide mountain range in what is known as the
Gondwanide orogeny. The continuation of the
orogenic pulses from the growing Gondwanides mountain chain and associated
subduction created accommodation space for sedimentation in the
Karoo Basin which ran along an east to west trending foreland trough. The formation of the Karoo Basin resulted in the preservation of the Dwyka Group rocks and all succeeding rocks that make up the greater
Karoo Supergroup.
Southern Africa at this time was part of the
supercontinent,
Gondwana, and was positioned over the
antarctic circle at this time. The result was the development of the
Permo-Carboniferous glacial environment where massive
ice sheets entombed the early Karoo Basin in the surrounding highlands and permanent, floating
glaciers in the lowlands.[5][6][7][8][9][10]
Dwyka Group deposits are also found outside of and north of the
Karoo Basin. These deposits found north of the Karoo Basin are found as the lowermost
geological formation of the Springbok Flats, Tshipise, northern Lebombo, Tuli, and
Ellisras (
Lephalale) Basins of north-northeastern
South Africa.
In its southern, western, and eastern deposits, the Dwyka Group conformably overlies rocks of the
Cape Supergroup, which includes the
Cape Fold Belt, and the Natal Supergroup. It also unconformably overlies the Namaqua-Natal Metamorphic Province in some localities in the west-northwest of
South Africa. Its north and northeastern
Karoo Basin deposits and all deposits found north of the Karoo Basin
unconformably overlie the
Transvaal Supergroup, Ventersdorp Group, or
Archean and
Proterozoic basement rocks. In all South African localities, the Dwyka Group underlies rocks of the
Ecca Group.
The geographical range of the Dwyka Group is large with its deposits also being found in other localities in
southern Africa. Dwyka-aged deposits that are considered to correlate in age to those found in
South Africa have been located in the southern Karasburg and
Kalahari Basins of southern
Namibia – in and around the
Fish River Canyon – in the Huab Basin of northern-western
Namibia, the Waterberg and
Owambo Basins of northern Namibia, the Dukwi Formation of the
Kalahari Basin of
Botswana, and the Save Basin of southeastern
Zimbabwe.[11]
Stratigraphic units
The Dwyka Group deposits have been categorized by those found as part of the
Karoo Basin and smaller formations found in different basins north of the Karoo Basin. In the
Karoo Basin, the Dwyka Group is known by two distinctive
lithologicalfacies. These two
facies are represented in its northern and southern deposits respectively and are recognized as the two
geological formations below:
Elandsvlei Formation is the southern
facies and it is considered to represent the earliest
foredeep deposits of the Karoo foreland system. This portion is restricted to the southern areas of South Africa and is characterized by uniform, high, massive, and
clast-poor
diamictites and rarer
mudrock deposits.
The
diamictites are highly compacted and overlie stratified diamictites and mudrocks. The southern facies is interpreted as suspension or melt-out deposits that were deposited in low energy
subglacial or subaqueous rain-out from either semi-grounded or floating
ice sheets. All sedimentation occurred below the water surface in a deep
marine environment. More evidence to support this is that
turbidites are often found in Elandsvlei Formation deposits.[12][13]
North of the
Karoo Basin outcrops of another
geological formation that correlates in age to the main Karoo Basin's Dwyka-aged deposits. This formation is the lowermost unit of the Springbok Flats, Tshipise, northern Lebombo, Tuli, and
Ellisras (
Lephalale) Basins. This geological formation is recognized and differentiated below due to its unique
lithologicalfacies from its main
Karoo Basin counterparts:[20][21][22]
Tshidzi Formation : Deposits of this formation represent
backbulge basin deposits of the Karoo
foreland system. In the Tshipise and Tuli Basins, this formation is known as the Madzaringwe Formation. This formation consists of
pebblymudstones that are
interbedded with mainly coarse-grained
diamictites. These deposits are considered to be
lacustrinefacies, which show that the rock sediments were deposited in
glacial or
periglacial lakes. The northernmost Dwyka Group deposits are considered to have been most proximal to the shoreline of the shallow interior sea that was present at this time in southern Africa.[23]
^Visser, J.N.J. (1986). "Lateral lithofacies relationships in the glacigene Dwyka Formation in the western and central parts of the Karoo Basin". Transactions of the Geological Society of South Africa. 89: 373–383.
^Smith, R.M.H., Eriksson, P.G., Botha, W.J. (1993-01-01). "A review of the stratigraphy and sedimentary environments of the Karoo-aged basins of Southern Africa". Journal of African Earth Sciences (and the Middle East). 16 (1–2): 143–169.
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1993JAfES..16..143S.
doi:
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^Bangert, Berthold; Stollhofen, Harald; Lorenz, Volker; Armstrong, Richard (1999-07-01). "The geochronology and significance of ash-fall tuffs in the glaciogenic Carboniferous-Permian Dwyka Group of Namibia and South Africa". Journal of African Earth Sciences. 29 (1): 33–49.
Bibcode:
1999JAfES..29...33B.
doi:
10.1016/S0899-5362(99)00078-0.
ISSN1464-343X.
^Theron, J.N. and Blignault, H.J., 1975. A model for the sedimentation of the Dwyka glacials in the southwestern Cape. Gondwana Geology, pp.347–356.
^Visser, Johan N.J.; Young, Grant M. (1990-12-01). "Major element geochemistry and paleoclimatology of the Permo-Carboniferous glacigene Dwyka Formation and postglacial mudrocks in southern Africa". Palaeogeography, Palaeoclimatology, Palaeoecology. 81 (1–2): 49–57.
Bibcode:
1990PPP....81...49V.
doi:
10.1016/0031-0182(90)90039-A.
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^Von Brunn, V. and Visser, J.N.J., 1999. Lithostratigraphy of the Mbizane Formation (Dwyka group). South African Committee for Stratigraphy. Lithostratigraphic Series, (32), p.10.
^Huber, Heinz; Koeberl, Christian; McDonald, Iain; Reimold, Wolf Uwe (2001-06-15). "Geochemistry and petrology of Witwatersrand and Dwyka diamictites from South Africa: search for an extraterrestrial component". Geochimica et Cosmochimica Acta. 65 (12): 2007–2016.
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2001GeCoA..65.2007H.
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^Gravenor, C.P.; von Brunn, V.; Dreimanis, A. (1984-03-01). "Nature and classification of waterlain glaciogenic sediments, exemplified by Pleistocene, Late Paleozoic and Late Precambrian deposits". Earth-Science Reviews. 20 (2): 105–166.
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1984ESRv...20..105G.
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^von Brunn, V. (1996-10-01). "The Dwyka Group in the northern part of Kwazulu/Natal, South Africa: sedimentation during late Palaeozoic deglaciation". Palaeogeography, Palaeoclimatology, Palaeoecology. 125 (1–4): 141–163.
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^Smith, R.M.H., Eriksson, P.G., Botha, W.J. (1993-01-01). "A review of the stratigraphy and sedimentary environments of the Karoo-aged basins of Southern Africa". Journal of African Earth Sciences (and the Middle East). 16 (1–2): 143–169.
Bibcode:
1993JAfES..16..143S.
doi:
10.1016/0899-5362(93)90164-L.
ISSN0899-5362.{{
cite journal}}: CS1 maint: multiple names: authors list (
link)