Controls on Early Permian shelf deglaciation in the Karoo Basin of South Africa
Abstract The Late Carboniferous to Early Permian glacigene Dwyka Group has at its top three types of transitions from diamictite to postglacial mudrock in the southern Karoo. The type 1 transition sequence consists of clast-poor carbonate-rich diamictite having a sharp contact with the overlying mudrocks. This diamicton accumulated slowly by rain-out and sediment gravity flow processes. During sedimentation the polar ice front was stabilised by a possible ice shelf. Collapse of the ice shelf caused rapid ice margin retreat. The type 2 transition sequence consists of clast-poor diamictite with sandstone bodies overlain by mudrock with ice-rafted debris or thin interbedded debris-flow diamictites. Deposition of the diamictite facies occurred during oscillations of a subpolar ice front with short stable periods of surface melting of the ice resulting in the formation of small subaqueous outwash fans. The type 3 transition sequence consists of bouldery diamictite interbedded with massive or sandstone-bearing diamictite overlain by mudrock with ice-rafted debris or thin interbedded debris-flow diamictites. The sequence represents highly unstable ice front conditions and fluctuating sea-levels with periods of rapid deposition probably by collapsing ice streams. Climatic conditions may have alternated between polar and subpolar. The type 1 transition sequence is predominant in the western part and the type 3 sequence in the eastern part of the Karoo Basin. Long-term controls on deglaciation was climatic warming as Gondwana drifted over the pole as well as changes in land-sea configuration. Superimposed on these were local controls such as relative sea-level changes, presence of deformable substrate and meltwater production.