CONTROLS ON EARLY PERMIAN SHELF DEGLACIATION IN THE KAROO BASIN OF SOUTH-AFRICA

The Late Carboniferous to Early Permian glacigene Dwyka Group has at i ts top three types of transitions from diamictite to postglacial mudro ck in the southern Karoo. The type I transition sequence consists of c last-poor carbonate-rich diamictite having a sharp contact with the ov erlying mudrocks. This diamicton accumulated slowly by rain-out and se diment gravity flow processes. During sedimentation the polar ice fron t was stabilised by a possible ice shelf. Collapse of the ice shelf ca used rapid ice margin retreat. The type 2 transition sequence consists of clast-poor diamictite with sandstone bodies overlain by mudrock wi th ice-rafted debris or thin interbedded debris-flow diamictites. Depo sition of the diamictite facies occurred during oscillations of a subp olar 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 wit h massive or sandstone-bearing diamictite overlain by mudrock with ice -rafted debris or thin interbedded debris-flow diamictites. The sequen ce represents highly unstable ice front conditions and fluctuating sea -levels with periods of rapid deposition probably by collapsing ice st reams. Climatic conditions may have alternated between polar and subpo lar. 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. Superimpo sed on these were local controls such as relative sea-level changes, p resence of deformable substrate and meltwater production.