Carboniferous-Permian glaciation in the main Karoo Basin, South Africa; stratigraphy, depositional controls, and glacial dynamics

Pennsylvanian and Permian glacigenic deposits of the Dwyka Group occur within Karoo basins throughout southern Africa. The largest, the main Karoo Basin, evolved into a foreland basin during Dwyka accumulation. Tectonism along the convergent margin of Gondwana resulted in the formation of a foreland basin bounded by southern (Cape fold belt) and northern (Cargonian Highlands) uplands. Glaciers carved deep paleovalleys into the northern highlands that were later filled by glacigenic and postglacial strata. Within this basin, a platform facies association composed of four deglaciation sequences occurs. These sequences, which are hundreds of meters thick, consist of thick, massive, basal diamictite lithofacies that grade upward into stratified lithofacies (stratified diamictites, dropstone-bearing mudrocks, and rhythmites). Interpretations depict grounded ice advancing into the basin followed by gradual retreat of the ice front resulting in ice-proximal followed by ice-distal glaciomarine sedimentation. Sensitive high-resolution ion microprobe (SHRIMP) dates of juvenile zircons obtained from tuff beds indicate that the deglaciation cycles were 3.6-8.2 m.y. in duration. Such cycles were likely the result of tectonic development of the foreland basin. Paleocurrent and provenance studies indicate that Dwyka glaciation asynchronously emanated from multiple glacial centers in upland areas, and in Antarctica. Therefore, southern Africa was not covered by a single ice sheet, but instead, smaller ice sheets, ice caps, and alpine glaciers waxed and waned along basin margins during the late Paleozoic. Despite a long history of study, many questions concerning Dwyka glaciation remain.