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Contrasting Processes During Fine-Grained Progradation Along a Regional Shelf Margin, Karoo Basin, South Africa


Shelf accommodation has been emphasized as a major control on progradation of shelf margin systems and delivery of sand to deep water. However, a complicated interplay of factors including rate of sediment supply, shelf physiography, and lateral variability in dominant process regime challenge simple predictive models for slope sediment bypass and the development of basin-floor fans. Three-dimensional datasets can help to deconvolve these factors, but need to be regional in scale to capture basin margin architecture, with detailed facies observations to capture process regimes and the distribution of depositional environments. Here, the evolution of a shelf margin in the Permian Karoo basin, South Africa is investigated. More than 1000 regionally distributed sedimentary logs along a >150 km strike section, from shelf to basin-floor in both the Tanqua and Laingsburg depocentres provide data for stratigraphic panels that show the position of the clinoform rollover and the base-of slope through time. This helps to constrain water depth estimates, and to capture changes in slope length and gradient. This dataset has also been integrated with absolute age control and isopach mapping. A differential subsidence across the basin led to a thinner and progradational ramp type setting in the Tanqua depocentre, whereas a thicker slope succession in the Laingsburg depocentre features an aggradation followed by progadational stacking pattern. Late stage shelf margin progradation in the Laingsburg area operated by the accretion of silt through multiple parasequence cycles without the sandy part of each clinothem reaching the clinoform rollover. The downdip correlation of the Laingsburg deltas show that their associated slope deposits are generally less sand-rich than the underlying basin-floor stratigraphy of the Fort Brown Formation, which points to either physiographic change in the basin margin architecture and/or process regime through time. Detailed field observations have provided criteria to define a muddy shelf to slope transition. These findings provide a facies basis for equivalent seismic expressions in cases of margin progradation via fine grained material. These investigations have demonstrated that basin margin progradation can occur without development of basin-floor fans, and address the processes responsible for transporting fine grained material across the shelf and beyond the shelf edge rollover during relative sea-level highstands.