Sedimentary processes and stratigraphic record of the shelf to slope transition : an example from the Karoo Basin, South Africa

• Main Author: Jones, George
• Other Authors: Marshall, Jim
• Published: University of Liverpool 2013
• Subjects: 550; GC Oceanography
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.592836

Exposures of the lower Waterford Formation, Karoo Basin, South Africa provide rare three-dimensional control to an exhumed low-gradient basin margin, which permits increased understanding of sub-seismic scale process-regime and lateral variability. The study section overlies a 500-m-thick channelized submarine slope succession and is subdivided into eight mappable parasequences clinothems. The central 40 km 2D reference profile parallel to depositional dip (i.e. west-to-east) enables characterization of the shelf-to-slope transition for two successive clinothems and the establishment of robust criteria for identifying the shelf edge at outcrop. The two clinothems exhibit differing process responses to the gradient increase at the shelf edge rollover. The fluvial-dominated, mouth-bar clinothem of WfC 3 exhibits a 5 km dip-parallel zone of extensional growth faulting at the shelf edge with limited delivery of sediment beyond the shelf edge rollover. In contrast, WfC 4 is a wave and storm dominated shoreface-clinothem that supplied a thick upper slope turbidite succession via closely spaced gullies at the shelf edge and a large upper slope channel. This suggests that 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. It would be common practice to attribute variability between successive clinothems on a single 2D dip profile to lateral variability across the basin margin. However, two additional, sub-parallel dip profiles across depositional strike to the north and south of the reference profile indicate significant 3D variability in the nature of the clinothems. A greater supply of sediment to the upper slope in the north of the region, in the absence of a clear fluvial driver, suggests that the deltaic/shoreface system was able to transit quickly to the shelf edge and establish itself in that position for an extended period. Increasing parasequence thickness toward the north indicates greater accommodation in this area throughout the lower Waterford succession. It is therefore likely that differential subsidence across the margin controlled the position of the shelf edge and maintained a narrower shelf in the north, which would have had significant influence on sediment routing paths, diverting them to the north. Abundant shelf-confined soft sediment deformation in successive clinothems along all three dip margin profiles is attributed to instability on the frontal slopes of inner shelf deltas, due to high rates of sediment supply. The basinward splitting and abrupt thinning of deformed packages beyond the inferred shelf edge rollover shows that the presence of soft-sediment deformation alone is not an adequate criterion with which to define the shelf edge rollover. The significant thickness of shelf deposits without subaerial exposure indicates that the margin was subject to a relatively high subsidence rate but that the rate of sediment supply was sufficient to drive the system to the shelf edge and deliver sand to the slope without the development of incised valleys. Therefore, the lower Waterford Formation is considered a high accommodation / high supply system. This study provides a high-resolution outcrop-based dataset with three-dimensional constraints. It enables greater understanding of the controls on basin margin construction, and the sub-seismic scale processes that control the spatial and temporal variability of sediment delivery to the slope and basin floor.