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Devonian sedimentary rocks in South Africa From Wikipedia, the free encyclopedia
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.[1][2]
| Bokkeveld Group | |
|---|---|
| Stratigraphic range: Early-Middle Devonian ~ | |
| Type | Geological group |
| Sub-units | Gydo, Gamka, Voorstehoek, Hex River, Tra-Tra, Boplaas, Waboomberg, Wuppertal, Klipbokkop, Osberg & Karoopoort Formations |
| Underlies | Witteberg Group |
| Overlies | Table Mountain Group |
| Lithology | |
| Primary | Sandstone, mudstone, siltstone, shale, and conglomerates |
| Other | Calcite |
| Location | |
| Region | Western & Eastern Cape |
| Country |  South Africa |
| Type section | |
| Named for | Bokkeveld mountains |
Schematic diagram of a west-east (left - right) geological cross section through the Cedarberg portion of the Cape Fold Belt (South Africa). The rock layers represent the three main subdivisions of the Cape Supergroup. The Bokkeveld Group rocks are represented by the pale purple layer. | |
The Cape Supergroup rocks were deposited in a purely marine setting, within a 1,300 kilometres (810 mi) 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 Early Carboniferous (about 330 Ma; late Mississippian). Up to 10 kilometres (33,000 ft) of strata 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.[3] An additional geological formation, the Msikaba Formation, found north of Port St. Johns in the Eastern Cape is considered to correlate with the Witteberg Group of the Cape Supergroup.[4]
Bokkeveld Group outcrops 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.
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.[5] The Ceres Subgroup is found throughout the extent of the lower Bokkeveld Group exposures. The Bokkeveld Group contains five complete coarsening-upward cycles and is arranged into three distinctive facies arrangements represented by the subgroups. The geological formations are also distinguished by their sedimentology of alternating mudstone/siltstone and sandstones.[6][7][8][9][1] The Bokkeveld Group subgroups and their respective geological formations are listed below (from oldest to youngest):
Ceres Subgroup:
Bidouw Subgroup (West of 24ºE):
Traka Subgroup (East of 24ºE):
The bulk of the fossils found in the Cape Supergroup are eroded fragments of benthic invertebrate Malvinokaffric fauna, particularly that of various brachiopods such as Australocoelia,[11] Australospirifer, and chonetids. Crinoids are also found, although their dis-articulated ossicles are more common, as are trace fossils such as worm burrows and feeding trails left by other invertebrates. Rarer are fossils of trilobites, bivalves, cephalopods, gastropods, ophiuroids, hyoliths, echinoids, echinoderms, conulariids, cricoconarids, and corals.[12][13][14][15][16][17]
In the upper Bidouw and Traka Subgroups, plant and trace fossils are more common than invertebrate fossils. Lycopods and trace fossils of Spirophyton have been recovered. Rare bony fish fossils have also been found, mainly of placoderm fishes, although placoderm fish are mainly known from rocks of the overlying Witteberg Group.
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