Abstract

Devonian rocks form part of the clastic Cape Supergroup which borders the western and southern coast of South Africa. A threefold subdivision can be distinguished, with the predominantly arenitic Table Mountain Group at the base, overlain successively by the more argillaceous Bokkeveld and Witteberg Groups.

Fossil evidence indicates an Emsian age for the uppermost formation of the Table Mountain Group and a Givetian-Frasnian age-range is recognized for brachiopods from the lower part of the Witteberg Group.

Devonian sedimentation concurred with a change in erstwhile stable shelf conditions. The pulsatory nature and tectonic unrest of the Cape basin during Bokkeveld sedimentation are well documented in alternating upward coarsening clastic wedges, which interpose southwards into a homogeneous mudstone-sub-greywacke sequence. The rate of subsidence towards the east was about double that in the western part of the basin and resulted in accumulation of about 4000 metres of sediment in the east. Initially the subsidence and sedimentary cycles of the western and eastern portions of the basin were reasonably in phase and the various formations (Ceres Subgroup) extended throughout the basin. However, this did not apply during deposition of the upper part of the Bokkeveld Group or the lower Witteberg sequence. Gradual return to greater stability of the shelf occurred during sedimentation of the Witteberg Group, especially towards the west.

Successive progradation of major lobate deltas onto the shallow shelf was affected by marine transgressions and regressions. The upward coarsening cycles represent shelf-prodelta, distributary mouth bar, barrier and/or beach, tidal and/or interdistributary bay deposits. Variable interplay of tidal-and wave-reworked delta-front sands usually culminate the cycles.

Extensive bioturbation occured and the typical Malvinokaffric Realm fossils are represented.