SEA-LEVEL CHANGES IN A BACK-ARC FORELAND TRANSITION - THE LATE CARBONIFEROUS-PERMIAN KAROO BASIN OF SOUTH-AFRICA

The late Carboniferous to early Permian succession in the Karoo Basin consists in ascending stratigraphic order of diamictites, mudrocks, greywackes and immature sandstones. The diamictites are interpreted as glacigene deposits laid down by a marine ice sheet on a shelf. Post-glacial suspension-settling of predominantly mud occurred on the basin floor during open marine and later brackish conditions. Basin floor fan deposits accumulated in the foredeep, whereas deltaic mud, silt and sand finally infilled the basin. Glacigene sedimentation occurred in a back-arc basin during the late Carboniferous and early Permian. The post-glacial succession accumulated in a foreland basin in which the rising orogenic belt was the predominant sediment source. Relative sea-levels, following the major Namurian regression, apparently remained low during the extensive Gondwana glaciation. Marine ice sheets are highly sensitive to sea-level fluctuations; and minor sea-level rises, probably climatically controlled, were recorded as interglacials when parts of the marine ice sheet collapsed. Final collapse of the marine ice sheet was associated with a major transgression. Post-glacial mudrock deposition was characterised by relatively high sea-levels, except for the period of basin floor fan deposition when there was a sea-level lowstand. Final infilling of the basin by prograding deltas was accompanied by a major regression. The late Carboniferous to early Permian sea-level fluctuations in the Karoo Basin show correspondence with global eustatic cycles reflecting low tectonic interference with sea-level patterns. However, late Permian sea-levels were predominantly tectonically controlled as a result of foreland basin evolution. Possible global eustatic events complemented apparent sea-level changes within the basin during this period.