Overberg Geoscientists Group
400 Mya
Formation of Bokkeveld Sediments
Introduction and location. The around 400 million year ago Devonian age Bokkeveld Group represents a series of fine-gained (muddy) formations alternating with coarser (sandy) layers. It makes up the 1,5 to 4 km thick middle section of the Cape Supergroup. The Bokkeveld is defined by its relatively subdued weathering in comparison to the over- and underlying resistant sandstone units of the Table Mountain Sandstones and the Witteberg Group. Bokkeveld outcrops form the valleys (i.e., the Hex River Valley) and lows (such as the rolling topography of the Rûens) and follow the bordering high mountain ranges. These can be seen from Clanwilliam (north of Cape Town), Ceres, Villiersdorp, past Oudtshoorn, Willowmore to Gqeberha in the east.
Geology. The Bokkeveld Group is formed by the three subgroups, the Ceres, Bidouw and Traka. The five more resistant and coarser formations (Q₁ to Q₅) give rise to hogsback topography. Good exposures of the six intervening softer, muddy formations are rare. However, fossils are preserved relatively well in these Bokkeveld units. The bulk of the well-known invertebrate fossils (animals without a backbone) of the Cape Supergroup are found within these fine-grained rocks.
The cyclic nature of the Bokkeveld basin and its source is reflected in the alternation of the arenitic (sandy) and politic (muddy) units. The five coarsening-upward cycles, feather out southwards into a relatively uniform mudstone-siltstone sequence. They represent successive southward advances of deltas along a mixed tidal-and wave-dominated coastline, under the influence of repetitive rise (transgression) and fall (regression) cycles in sea-level.
Fossils. The Bokkeveld sediments were deposited in the Agulhas Sea during the Devonian period, implying that you should find fossils of marine animals from that time in these rocks. The most abundant fossils and therefore the ones you’re most likely to find include Trilobites, Brachiopods, Criniodea as well as some fossil fish. The Devonian is well known as a 60-million-year period marred by extinction events on a global scale linked to changes in climate causing shifts in sea-level.
Colours and importance. The Bokkeveld shales show a range of colours from red, orange, green, yellow, purple through to grey and black. Examples of which can be seen in road cuttings along the N2 and side roads such as the R43 to Hermanus and the R316 between Caledon and Bredasdorp. The red shales are iron-rich, though only by but a few percent. The green to purple colours is a function of the amount of iron and its state of oxidation. The dark grey to black colours are due to the presence of organic material. Of importance are the fine clay minerals in soils derived from the Bokkeveld shales essential for the retention of moisture and plant nutrients. Thus making these soils prime areas of farming and planting of vineyards, wheat, canola and deciduous fruit.
References
Penn-Clark, C. 2020. A modern interpretation of the Bokkeveld geology. Overberg Geoscientists Group talk. 2 July 2020. You-tube presentation. Geological Society of South Africa.
Penn-Clarke, C. 2021. Towards a complete picture of the Devonian world: the Bokkeveld Group of South Africa. U3A Overberg. You-Tube presentation.
Gresse, P.G. and Theron, J.N., 1992. The geology of the Worcester area. Explanation of Sheet 3319. Scale 1:250 000. Council for Geoscience.
MacRae, C. 1999. Life etched in stone. Fossils of South Africa. Geological Society of South Africa.
McCarthy, T. & Rubidge, B. 2005. The story of earth and life: 1–333. Cape Town: Struik.
Norman, N. and Whitfield, G. 2006. Geological journeys. A traveller’s guide to South Africa’s rocks and landforms. Struik Publishers.