Contrast in the process response of stacked clinothems to the shelf-slope rollover

Abstract

Two stacked parasequences exposed continuously along a 35-km dip section in the Permian lower Waterford Formation, Karoo Basin, South Africa, form basin margin–scale clinothems, and their internal facies distributions have been mapped out from shelf to upper submarine-slope settings. Sedimentary facies changes have been determined by walking out key surfaces between measured sections. The two parasequences (Waterford clinothems WfC 3 and WfC 4) share progradational profiles, but WfC 3 is characterized by a strong fluvial influence, whereas overlying WfC 4 is a wave-storm–dominated delta front system. When correlated basinward, the two clinothems exhibit stratigraphic thickening as well as differing process responses to the increased gradient at the shelf-edge rollover. WfC 3 exhibits synsedimentary, wedge-shaped rotational growth faults. These growth faults trapped sand at the shelf-edge rollover, so minimal sand was delivered to the upper slope; therefore, the clinothem downlaps into the slope mudstones within 7 km of the shelf-edge rollover. In contrast, the top of WfC 4 is marked by closely spaced gullies cut into deformed delta front deposits. The delta front deposits pass into sand-prone slope turbidites 3 km downdip. Locally, these turbidites are truncated by a 60-m-thick turbidite-sandstone–filled slope channel fill. In this case, most of the slope delivery is associated with a wave-dominated process regime. It is important to consider the sequence stratigraphic setting of clinothems in such analyses; WfC 4 represents a minimum accommodation point in a depositional sequence and is overlain by a Type 2 sequence boundary. Despite wave and storm dominance, the low accommodation and high sediment supply at that time is interpreted to have driven sand beyond the gullied shelf-edge rollover. Therefore, the delivery of sediment to deep-water settings is governed by parameters other than the presence and proximity of a fluvial point source, which is heavily advocated in current models for shelf construction.