The Bokkeveld Group is the second of the three main subdivisions of the Cape Supergroup in South Africa. It overlies the Table Mountain Group and underlies the Witteberg Group. The Bokkeveld Group rocks are considered to range between Lower Devonian (Lochkovian) to Middle Devonian (Givetian) in age.
Background
The Cape Supergroup rocks were deposited in a purely Ocean setting, within a wide passive margin basin known as the Cape Basin. The rocks were deposited over a 170-million-year period ranging from approximately 485 Ma (Tremadocian) to the Carboniferous (about 330 Ma; late Mississippian). Up to of Stratum were preserved throughout. The Cape Supergroup rocks later underwent deformation during the Cape orogeny, in which the rocks were folded and thrust upwards. The Cape orogeny formed the Cape Fold Belt and the mountains that range along the Cape and the southern parts of South Africa.[Truswell, J.F., 1977. The geological evolution of South Africa. Purnell.]
Geographic extent
Bokkeveld Group and exposures range from the Breede River Valley in the west to Port Alfred near Grahamstown in the east. The group displays lateral continuity throughout the length of the Cape Fold Belt. The Msikaba Formation rocks appear north-northeast of Port St. Johns in the Eastern Cape.
Stratigraphic units
The Bokkeveld Group is subdivided into three subgroups: the Ceres Subgroup and Bidouw Subgroup that are found West of 24ºE, and the Traka Subgroup found East of 24ºE.
Ceres Subgroup:
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Gydo Formation: Composed mainly of mudrock and siltstone that contain minor sandstone layers. The rock sediments of the Gydo were deposited in low-energy, offshore-prodeltaic environments.
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Gamka Formation: Sandstone-rich in contrast to the Gydo, composed of fine to medium-grained Feldspar wacke, and some subordinate mudstone. The are arranged in coarsening upward cycles - a greater geological feature seen in the Bokkeveld Group as a whole. Hummocky cross-bedding structures are frequently observed in this formation. The exhibit coarsening-upward cycles and hummocky cross-bedding. These features indicate that the depositional environment was a high-energy storm and wave reworked River delta front or delta plain environment. The formation is abundant in invertebrate , especially .
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Voorstehoek Formation: Mudstone and siltstone. Same depositional environment as the lower Gydo Formation.
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Hex River Formation: arranged in coarsening-upward sequences much like the Gamka Formation. of Planolites, Skolithos, and Arenicolites are found.
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Tra-Tra Formation: Mudstone and siltstone-rich.
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Boplaas Formation: Fine to medium-grained Feldspar and with minor and siltstone.
Bidouw Subgroup (West of 24ºE):
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Waboomberg Formation: Mudstone and siltstone interbedded with fine-grained .
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Wuppertal Formation: Fine to medium-grained and minor .
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Klipbokkop Formation: and with subordinate fine-grained sandstone.
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Osberg Formation: Westernmost of this formation are dominated by sandstone.
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Karoopoort Formation: Shale and siltstone with some minor sandstone layers.
Traka Subgroup (East of 24ºE):
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Karies Formation: Dark-coloured shale and are dominant. Deep marine (pelagic zone) depositional environment.
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Adolphspoort Formation: Dark, wavy-bedded siltstone and micaceous . Eastern correlate of the Osberg Formation.
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Sandpoort Formation: Reddish, lenticular bedded with minor . Plant fossils are common.
Paleontology
The bulk of the found in the Cape Supergroup are eroded fragments of Benthic zone invertebrate Malvinokaffric fauna, particularly that of various such as Australocoelia,[Boucot, A.J. and Gill, E.D., 1956. Australocoelia, a new Lower Devonian brachiopod from South Africa, South America, and Australia. Journal of Paleontology, pp.1173-1178.] Australospirifer, and Chonetidina. are also found, although their dis-articulated ossicles are more common, as are such as worm burrows and feeding trails left by other invertebrates. Rarer are fossils of , Bivalvia, , Gastropoda, Brittle star, Hyolitha, echinoids, , Conulariida, cricoconarids, and .[Almond, J.E., 2005. PALAEONTOLOGICAL IMPACT ASSESSMENT: Exceptional fossil starfish bed, Prince Albert District, Western Cape. John E. Almond (Natura Viva cc, Cape Town) and Derek Ohland (Iziko Museums, Cape Town). January 2005.][Almond, J.E., 2013. PALAEONTOLOGICAL SPECIALIST STUDY: FIELD ASSESSMENT. Expansion of an existing Borrow Pit in the Prince Albert townlands, Prince Albert District, Western Cape. John E. Almond (Natura Viva cc, Cape Town). March 2013.][Anderson, M.E., Long, J.A., Evans, F.J., Almond, J.E., Theron, J.N. and Bender, P.A., 1999. Biogeographic affinities of Middle and Late Devonian fishes of South Africa. Records of the Western Australian Museum, Supplement, 57, pp.157-168
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/ref>[Becker, G., Bless, M. and Theron, J., 1994. Malvinokaffric ostracods from South Africa (Southern Cape; Bokkeveld Group, Devonian). Courier Forschunginstitut Senckenberg, 169, pp.239-259.]
In the upper Bidouw and Traka Subgroups, plant and trace fossils are more common than invertebrate fossils. Lycophyte and of Spirophyton have been recovered. Rare Osteichthyes have also been found, mainly of Placodermi fishes, although placoderm fish are mainly known from rocks of the overlying Witteberg Group.
