The Dwyka Group in the northern part of Kwazulu/Natal, South Africa: sedimentation during late Palaeozoic deglaciation

Abstract

Abstract Multiple sedimentary sequences, up to and over 300 m thick, and comprising the Late Palaeozoic Dwyka Group in the southeastern part of the African subcontinent, were deposited during deglaciation of the Gondwana ice sheet. Attention is focused on the northern part of KwaZulu/Natal in South Africa where the Dwyka Group, represented by glaciogenic deposits, rests on an uneven surface of Archaean basement rocks. These deposits occur on the northwestern flank of an elongate pre-Dwyka crustal downwarp which had a significant influence on the mode of sedimentation as it progressively subsided. Stratigraphic sections from selected localities in northern KwaZulu/Natal display the thickness variations, and marked vertical and lateral lithofacies changes that characterise the Dwyka Group of this region. Diamictite consistently occurs at the base of the Dwyka sequences but is also interstratified with mainly arenaceous and conglomeratic lithotypes which predominate in the upper 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 southwestern Gondwana. As the margin of the ice sheet receded towards an Archaean upland region in the northwest, this ice mass was locally grounded and 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 flow and subaqueous outwash processes that occurred near the grounding line. Thick, homogeneous, blanket-like diamictite facies, in the southeastern and southern part of the region, suggest an unimpeded retreat of the sediment-laden marine sheet in the deeper parts of the basin. Rapid dissipation of the ice in the closing stages of deglaciation was followed by accumulation of postglacial muds which mantled the glaciogenic sediments. In the northernmost part of the study area proximal products of deglaciation, represented by conglomeratic deposits and associated sandstones, are preserved in deep partly exhumed glaciated valleys and depressions along the southern periphery of the Archaean upland. The distinctive character of the Dwyka Group in northern KwaZulu/Natal is partly attributable to the complex tectonic setting in which it occurs and reflects an interplay of various factors that controlled the styles of sedimentation during deglaciation.