Sediment transport of the late Palaeozoic glacial Dwyka Group in the southwestern Karoo Basin

University of the Orange Free State, Department of Geology, Bloemfontein, South Africa

Sediment, which was fed by ice streams into the southwestern part of the Karoo Basin, was redistributed by sediment gravity flows, turbidity currents, bottom currents and sediment-laden meltwater plumes, and rain-out from icebergs. Together these deposits formed large subaqueous fans and sediment aprons which were controlled by the ice sheet dynamics, sediment flux, substrate relief, and geotechnical properties of the debris. Glacial valleys, striated and eroded bedrock surfaces, soft-sediment pavements, ice-thrust features, cross-bedding, flow folds, clast fabrics and composition, and facies changes were used as directional indicators in reconstructing the sediment transport system. The stratigraphic framework of the Dwyka Group is comprised of an imbricate stacking 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 extensional basin, were mainly derived from the northeast (Northern Cape source) with minor inputs from the north and south. All ice streams were deflected westwards as a result of the prominent regional palaeoslope. During deposition of DS 2, when the basin expanded laterally, palaeoslopes were largely inclined towards the southwest where the bathymetric axis was located, as well as to the west-northwest. The major sediment input sources were in the north, northeast, and east, whereas the southern source progressively declined. Interference of sediment transport systems occurred along the northern margin of the study area where the different ice streams merged. During deposition of DS 3, palaeoslopes in the basin remained largely unchanged. The Namaland source became irrelevant in the study area and sediment was fed into the basin 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 floor caused interfingering of gravity flow fans. Ice streams from the southeast and south were the sediment sources during the Late Sakmarian-Artinskian DS 4 event, and glacial debris and post-glacial muds were distributed approximately parallel to the basin depo-axis. During the + or -40 Ma glaciation period there was thus a clockwise shift in the growth of ice-spreading centres within the Gondwana Ice Sheet.

This record provided courtesy of AGI/GeoRef.

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