PROVENANCE OF MUDSTONES IN THE KAROO SUPERGROUP OF THE ELLISRAS BASIN, SOUTH-AFRICA - GEOCHEMICAL EVIDENCE

The Karoo Supergroup in the Ellisras basin consists of sediments that were deposited predominantly during the Permian Period. Mudstone sampl es of the Swartrant and Grootegeluk Formations of the Ecca Group and t he succeeding Eendragtpan Formation of the Beaufort Group were collect ed from borehole core. Based on the concentration and ratios of relati vely immobile elements that have been shown to be directly related to their source rocks, the Grootegeluk Formation and Eendragtpan Formatio n mudstones have provenances which are approximately granodioritic in composition and have similar Late Proterozoic Sm-Nd model t(nd-DM) age s (similar to 1.2 Ga). The provenance of the Swartrant Formation was r elatively more mafic and has Early Proterozoic Sm-Nd model t(Nd-DM) ag es (similar to 2.1 Ga). The model t(Nd-DM) ages for all the sediments are too low to support a cratonic origin. It is not possible to identi fy a unique source for the Swartrant sediments from only the Sm-Nd iso topic data, but a source from approximately the northwest of the coalf ield in the Damaran mobile belt or from Antarctica to the east, in its Late Proterozoic to Late Palaeozoic palaeo-position, cannot be discou nted. Palaeo-current data for the Swartrant Formation indicates a sour ce from the east. This is consistent with palaeo-drainage patterns fro m Antarctica and therefore a fluvio-glacial source from Antarctica, in its Late Paleozoic geographic position, is favoured. The Grootegeluk and Eendragtpan mudstone Formations are most compatible with a source from retreating continental ice-sheets to the north of the Ellisras ba sin at the time of Early Permian deglaciation. Although the Swartrant and Grootegeluk Formations have been assigned to the same stratigraphi c group (Ecca), the Grootegeluk Formation is geochemically distinct fr om the Swartrant Formation and indistinguishable from the overlying Ee ndragtpan Formation of the Beaufort Group. Copyright (C) 1996 Elsevier Science Ltd.