SEDIMENT TRANSPORT OF THE LATE PALEOZOIC GLACIAL DWYKA GROUP IN THE SOUTHWESTERN KAROO BASIN

Sediment, which was fed by ice streams into the southwestern part of t he Karoo Basin, was redistributed by sediment gravity flows, turbidity currents, bottom currents and sediment-laden meltwater plumes, and ra in-out from icebergs. Together these deposits formed large subaqueous fans and sediment aprons which were controlled by the ice sheet dynami cs, sediment flux, substrate relief, and geotechnical properties of th e debris. Glacial valleys, striated and eroded bedrock surfaces, soft- sediment pavements, ice-thrust features, cross-bedding, flow folds, cl ast fabrics and composition, and facies changes were used as direction al indicators in reconstructing the sediment transport system. The str atigraphic framework of the Dwyka Group is comprised of an imbricate s tacking of four deglaciation sequences (DS), each representing a time slice of the Permo-Carboniferous glacial history. The sediments of the Late Carboniferous DS 1, which were deposited in a small incipient ex tensional basin, were mainly derived from the northeast (Northern Cape source) with minor inputs from the north and south. All ice streams w ere deflected westwards as a result of the prominent regional palaeosl ope. During deposition of DS 2, when the basin expanded laterally, pal aeoslopes were largely inclined towards the southwest where the bathym etric axis was-located, as well as to the west-northwest. The major Se diment input sources were in the north, northeast, and east, whereas t he southern source progressively declined. Interference of sediment tr ansport systems occurred along the northern margin of the study area w here the different ice streams merged. During deposition of DS 3, pala eoslopes in the basin remained largely unchanged. The Namaland source became irrelevant in the study area and sediment was fed into the basi n primarily by ice flowing from the northeast and east. Where the ice streams merged in the basin, sedimentation rates increased, resulting in a northeastwards shift in the basin depo-axis. An uneven basin floo r caused interfingering of gravity flow fans. Ice streams from the sou theast and south were the sediment sources during the Late Sakmarian-A rtinskian DS 4 event, and glacial debris and post-glacial muds were di stributed approximately parallel to the basin depo-axis. During the +/ - 40 Ma glaciation period there was thus a clockwise shift in the grow th of ice-spreading centres within the Gondwana Ice Sheet.