THE DWYKA GROUP IN THE NORTHERN PART OF KWAZULU NATAL, SOUTH-AFRICA -SEDIMENTATION DURING LATE PALEOZOIC DEGLACIATION/

Multiple sedimentary sequences, up to and over 300 m thick, and compri sing the Late Palaeozoic Dwyka Group in the southeastern part of the A frican subcontinent, were deposited during deglaciation of the Gondwan a ice sheet. Attention is focused on the northern part of KwaZulu/Nata l in South Africa where the Dwyka Group, represented by glaciogenic de posits, rests on an uneven surface of Archaean basement rocks. These d eposits occur on the northwestern flank of an elongate pre-Dwyka crust al downwarp which had a significant influence on the mode of sedimenta tion as it progressively subsided. Stratigraphic sections from selecte d localities in northern KwaZulu/Natal display the thickness variation s, and marked vertical and lateral lithofacies changes that characteri se the Dwyka Group of this region. Diamictite consistently occurs at t he base of the Dwyka sequences but is also interstratified with mainly arenaceous and conglomeratic lithotypes which predominate in the uppe r 60-160 m. These heterolithic sequences reflect sedimentation from a retreating marine ice sheet during a rise in relative sea-level which commenced towards the close of the Carboniferous Period in southwester n Gondwana. As the margin of the ice sheet receded towards an Archaean upland region in the northwest, this ice mass was locally grounded an d stabilised on bathymetric highs of the irregular basement topography . Release of glacial debris produced diachronous sedimentary sequences as the ice sheet was temporarily pinned on successive basement highs during its episodic retreat. Matrix- and clast-supported conglomerates and associated sandstones testify to a plexus of sediment gravity flo w and subaqueous outwash processes that occurred near the grounding li ne. Thick, homogeneous, blanket-like diamictite facies, in the southea stern and southern part of the region, suggest an unimpeded retreat of the sediment-laden marine sheet in the deeper parts of the basin. Rap id dissipation of the ice in the closing stages of deglaciation was fo llowed by accumulation of postglacial muds which mantled the glaciogen ic sediments. In the northernmost part of the study area proximal prod ucts of deglaciation, represented by conglomeratic deposits and associ ated sandstones, are preserved in deep partly exhumed glaciated valley s and depressions along the southern periphery of the Archaean upland. The distinctive character of the Dwyka Group in northern KwaZulu/Nata l is partly attributable to the complex tectonic setting in which it o ccurs and reflects an interplay of various factors that controlled the styles of sedimentation during deglaciation.