Abstract

The Lower Devonian clastic Bokkeveld Group of South Africa likely resulted from the vertical stacking of a number of wave dominated deltaic complexes. Concentrating on the lowermost of these deltaic cycles, comprising the Gydo and Gamka formations, it is possible to recognize a number of benthic invertebrate fossil communities. These can be correlated with the various depositional sub-environments within a delta complex.

The deepest water environment was the prodelta-shelf area in which shale was deposited. This area was home to the most diverse fossil assemblage, dominated by thin-shelled, free lying brachiopods but which also contained infaunal bivalves, gastropods, trilobites, crinoids and hyoliths. Higher in the sequence the sediment becomes coarser grained and sandstone becomes interbedded with siltstone. In this delta slope environment thicker shelled brachiopods predominate and trilobites become progressively scarce. At the top of the deltaic pile shallow water depositional environments were developed, including distributary mouth bars and tidal flats. In such areas low diversity communities existed. Distributary mouth bars were dominated by large thick-shelled brachiopods fixed to the substrate by functional pedicles; tidal flats were dominated by infaunal bivalves and inarticulate brachiopods.

Similar benthic assemblages to those detailed for the lowest deltaic cycle can be found in some of the overlying cycles but the topmost part of the Bokkeveld Group is apparently devoid of invertebrate remains. However, in the basal part of the overlying Witteberg Group a few representatives of the shallow water communities have been found.