Abstract

Depositional styles of the Late Palaeozoic Dwyka Group in the eastern part of South Africa were governed by the nature of the palaeotopography which, in turn, was largely controlled by the tectonic setting of the region. Much of the Dwyka succession is preserved in an Early Palaeozoic linear downwarp that trended subparallel to the present-day coastline and which originally developed as an incipient crustal rupture preceding the break-up of Gondwana. During the Dwyka glaciation ice flowing south-southwest down the axial gradient of the trough-shaped depression, which continued to subside in Dwyka time, was augmented by glaciers entering the depression from its elevated flanks. Following the Mesozoic fragmentation of Gondwana, only the northwestern flank of the trough remained on the African subcontinent. The sloping region, situated on the southeastern part of the stable Kaapvaal craton, is characterized by an uneven pre-Dwyka surface related to complexly deformed Archaean basement rocks, and a northwest–southeast structural lineament. Limited tectonic activity prevailed during the Dwyka sedimentation. Ice-flow to the southeast was controlled by the palaeoslope while the irregular topography profoundly influenced the nature of sedimentation. Debrisfilled glacially incised valleys, excavated along zones of crustal weakness, extended northwards into the periphery of a bordering highland region down which ice flowed and which was later subjected to limited post-glacial isostatic rebound.

The predominant lithotype of the Dwyka Group is a bluish-black diamictite which originated in a marine setting primarily by icerafting and settling of suspended fines, on the floor and flanks of the subsiding regional trough.