Lateral variability in clinoform trajectory, process regime, and sediment dispersal patterns beyond the shelf-edge rollover in exhumed basin margin-scale clinothems

Abstract

Sediment supply rate and accommodation regime represent primary controls on the depositional architecture of basin margin successions, but their interaction is commonly inferred from 2D dip profiles and/or with limited constraints on sedimentary facies. In this study, three parallel (>40 km long) 2D depositional oblique-dip profiles from outcrops of the lower Waterford Formation (Karoo Basin, South Africa) have been correlated. This data set provides a rare opportunity to assess the lateral variability in the sedimentary process record of the shelf-to-slope transition for eight successive clinothems over a 900 km2 area. The three profiles show similar shelf-edge rollover trajectories, but this belies significant along-margin variability in sedimentary processes and down-dip sediment supply. The depositional architecture of three successive clinothems (WfC 3, 4 and 5) also show along-shelf physiographic differences. The reconstructed shelf-edge rollover position is not straight, and a westward curve to the north coincides with an area of greater sand supply to the slope beyond a shelf dominated by wave and storm processes. All the clinothems thicken northwards, indicating an along-margin long-term increase in accommodation that was maintained through multiple shoreline transits across the shelf. The origin of the differential subsidence cannot be discriminated confidently between tectonic or compaction processes. The interplay of basin margin physiography, differential subsidence rate and process regime resulted in significant across-strike variability in the style and timing of sediment dispersal patterns beyond the shelf-edge rollover. This study highlights the challenge for accurate prediction of the sediment partitioning across the shelf-edge rollover in subsurface studies.