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Stratigraphic Record of a Sediment Bypass Zone on a Stepped Slope Profile, Karoo Basin, South Africa


Submarine sediment bypass zones have been interpreted on many ramps of stepped slope profiles from reflection seismic datasets, but are seldom identified in exhumed ancient deep systems. This means that the sedimentological characteristics and sub-seismic stratigraphic architecture of these zones and the nature of their down-dip transition into sand-rich deposits is poorly constrained. Regional mapping of Unit E, Fort Brown Formation, permits the physical correlation of a sandstone-detached system from updip feeder deposits through perched lobes and bypass-dominated channels to lobe complexes on the basin-floor, consistent with a stepped-slope profile. This has led to the identification of a kms scale tract on the lower submarine slope with limited sand deposition recorded updip of sandstone-detached lobe complexes. Here, the depositional architecture of the transition from bypass-dominated channels to their genetically related sand-rich lobe complexes is investigated along three sub-parallel dip-oriented sections that provide 3D constraint. Indicators of significant sediment bypass include composite erosion surfaces, thin bedded mudstone clast conglomerates, and megaflutes, with paleocurrent indicators following regional downslope trends. In all sections the basinward thickening of sand-rich lobes is abrupt with rates of 7cm/m and 3cm/m in parallel sections 4km apart. Combined with an increase in the proportion of sand, and the presence of stoss-side preserved climbing ripple lamination, these data point to an abrupt change in flow behavior from bypassing to depositing of sand. This change in all three sections is recorded over relatively short distances (<1km) suggesting an abrupt change in flow confinement and/or break-in-slope. This study has allowed greater vertical resolution and analysis of bed scale processes for the first time in deep-water bypass zones, which can be applied to subsurface data sets to provide better constraints to geologic models. Identification and characterisation of the sub-seismic elements in a seismic scale setting is crucial in understanding stratigraphic trap potential of these settings, and improving the prediction of down-dip sands.