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The sedimentary fill of the structural Karoo Basin consists of a valley facies association, deposition of which was controlled by a highland topography, and a shelf facies association. The latter, locally overlying glaciotectonized bedrock, consists of four diamictite and two mudrock facies. Their distribution is attributed to glacial sediment input from the eastern highlands, from the south along the basin axis, and from a tectonic arc in the west, and changes in climatic setting.

The predepositional phase in the glacial history of the basin comprises expansion of the ice sheet onto the continental shelves. No sedimentary deposits are preserved in the Karoo Basin, but a polar setting is visualized. During the grounded ice sheet phase, predominantly basal lodgement and melt-out diamictons were deposited in a probable polar setting. The largest volume of glacial sediment in the basin accumulated during the succeeding ice shelf phase. Deposition was primarily by rain-out of basal ice debris, subordinate fall-out from subaqueous meltwater streams, suspension settling, and resedimentation. Rising sea levels, regional decoupling of the marine ice sheet from its substrate, and rapid disintegration of the ice front controlled glacial sedimentation. Highly fluctuating climatic settings from polar (stable ice shelves and very little sedimentation) to brief subpolar intervals (maximum sediment input) prevailed in the region. The final mountain ice cap phase was characterized by the accumulation of marine muds on the shelf and formation of glacial valley fills and subaqueous outwash fans in front of tidewater glaciers along the highlands. The climatic setting was temperate.

The climatic setting for the ice sheet in the Karoo Basin was complex. The region experienced polar conditions from the mid-Carboniferous to the Carboniferous-Permian boundary, but the change to a temperate setting during the early Permian was characterized by extremely rapid fluctuations from polar ice shelves to disintegrating subpolar ice fronts. Deposition of many ancient massive diamictites probably occurred during such unstable ice sheet conditions.

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