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The Witpoort Formation
The Famennian (~372- 358 Ma) aged Witpoort Formation, forms the middle unit of the Witteberg Group and conformably overlies the Wletervrede Formation. In general, the Witpoort Formation comprises a stack of about 350 metres of clean quartzites, with a number of thin bands of mudstones/shales. The quartzites represent predominantly fluvial deposits, with lagoonal to estuarine deposits represented by the thin interbedded bands. West of 25° E the Formation is subdivided into 3 members; namely the Rooirand, Perdepoort, and Skitterykloof members, with the Rooirand forming the bulk of the succession (~ 250 to 300 m thick). The Witpoort acts as a stratigraphic bridge between shallow marine sediments of the underlying Weltevrede Subgroup, and lacustrine deposits of the overlying Lake Mentz Subgroup. The Witpoort Formation, deposited during a global sea-level low stand, represents an ancient fluvial (channel and sheet-braided) deposit, influenced by sea-level fluctuations. At times these resulted in short lived flooding of the continental margin and development of extensive estuarine and coastal lagoon deposits. Body fossils from the latter, including those of lingulid brachiopods (low in the sequence), and presumed marine fish and 'seaweeds' (higher in the sequence), evidence connections with the sea. No open marine deposits are, however, recognised (1). Dating of this formation is based on correlation with global changes in sea level, supported by plant and vertebrate diversity. The top of the Witpoort Formation correlates with the end of the Devonian Period. This is, for example, supported by absence of placoderms higher in the Witteberg Group, reflecting their extinction during the End Devonian Mass Extinction Event (3).
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Lithostrat coloumn for the Witteberg Group with the Witpoort Fm highlighted. ( WF- Waterloo Farm, RR- Rabbit Ridge, HP- Howison's Poort)
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