Geochemistry and palaeogeography of upper Ordovician glaciogenic sedimentary rocks in the Table Mountain Group, South Africa

Evidence for a late Ordovician glaciation is best known from northern Africa and locally in South Africa, and western South America. The South African glacial deposits (Pakhuis Formation of the Table Mountain Group) are present in the western part of the Cape Fold Belt. Two thin diamictites are succeeded by a coarsening upward shale-siltstone unit known as the Cedarberg Formation. These glaciogenic rocks are associated with one of the worldʹs greatest accumulations of mature sandstones. The diamictites are under- and overlain by shales and sandstones that have undergone extensive alteration, presumably as a result of chemical weathering. Some diamictite samples have low values for a Chemical Index of Alteration (CIA), in keeping with their inferred glacial origin. Immediately above the glaciogenic diamictites, black shales, comprising the basal Soom Member of the Cedarberg Formation preserve an unusual soft bodied fauna and are enriched in Mo, U and other trace elements. The unusual chemical composition of these shales is attributed to starved basin conditions and a reducing environment during a rapid sea level rise that accompanied demise of the late Ordovician glaciers. The late Ordovician glacial deposits of western South Africa are widely regarded as the distal deposits of a large continental ice sheet centred on North Africa but they may have been partly derived from piedmont glaciers that descended from highlands formed as a result of late Ordovician orogeny.