Lateral Variability of Basin Margin Clinothems from the Karoo Basin, South Africa
The lateral variability of two basin-margin scale clinothems from the Permian, lower Waterford Formation has been studied in detail across a 1000 km2 area. Exposure of these two clinothems occurs along the limbs of a series of east-west trending, post-depositional synclines and anticlines that trend parallel to depositional dip. Correlating logged sections by walking out parasequence-scale flooding surfaces has established 2D shelf to slope profiles for two successive clinothems along the northern limb of the Baviaans syncline. Cycle C is a fluvial dominated mouth bar parasequence in its most proximal exposures and exhibits a large scale zone (5 km wide) of extensional deformation at the clinoform rollover with limited delivery of sediment beyond the shelf-edge. In contrast, the overlying Cycle D comprises wave/storm dominated shoreface deposits in its most proximal exposures. This system was able to prograde beyond the clinoform rollover onto the upper slope and bypass sediment farther into the basin via closely spaced gullies at the shelf-edge and a large upper slope channel.
Extensive regional correlations to the north and south of the Bavianns panel reveal that lateral exposures of Cycle C and D comprise wave/storm dominated shoreface deposits that lack any compelling evidence for progradation or bypass of sediment beyond the shelf-edge break. Evidence for a single sediment input point and the presence of extensive shoreface deposits is consistent with the interpretation that this is a wave and storm-dominated fluvial-influenced system.
Distinguishing wave/storm influenced shoreface deposits from the location of sediment input points along ancient shelf margins is critical in understanding the partitioning of sediment between the shelf, slope and basin floor. Localities subject to strong fluvial input are considered more likely to supply sediment to the shelf and basin floor, whereas wave/storm influenced shorefaces have limited potential for the delivery of sediment beyond the shelf-edge. In this case study, however, the fluvial dominated Cycle C did not supply sediment beyond the shelf-edge, whereas the wave/storm dominated Cycle D shows significant evidence for sediment bypass beyond the shelf-edge.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California