Abstract

The Laingsburg depocenter of the SW Karoo Basin, South Africa, includes a series of sandstone-dominated deepwater clastic deposits (units A and B of the Laingsburg Formation and Unit C to Unit G of the overlying Fort Brown Formation) separated vertically by regional mudstones and records basin-floor to upper-slope deposition during the Permian icehouse climate. Unit C provides nearly continuous exposures over tens of kilometers, and the presence of regionally persistent internal mudstone markers (lower and upper C mudstones) allows the distribution of sedimentary facies and architectural elements of a slope-turbidite system to be documented for more than 30 km downslope and 20 km across slope. The spatial and temporal distribution of architectural elements and interpreted depositional environments (external levees, channel belts confined by a combination of basal erosion and overbank aggradation along the margins, and distributive frontal splays) reveals distinct changes in the sedimentation pattern and stratal architecture of the turbidite system through time. Unit C evolved in a stepwise manner from a weakly incised, levee-confined channel belt and its downdip distributive frontal splays (C1), through a more entrenched and sinuous channel–levee complex set that fed submarine fans farther into the basin (C2), to a regionally backstepping package of thin-bedded deposits of a distal distributive system (C3). Unit C is interpreted as a lowstand sequence set, composed of three depositional sequences, each of which includes a sandstone-dominated lowstand systems tract (C1, C2, and C3) and a mudstone-prone interval that is taken to represent the transgressive and highstand systems tracts. The overlying combined transgressive and highstand sequence set is marked by the 30-m-thick C–D mudstone interposed between Unit C and Unit D. Unit C and the C–D mudstone together form the Unit C composite sequence. At the scale of the Unit C lowstand sequence set, the evolutionary trend from C1 to C3 reflects an overall basinward then landward stepping of the system across the depositional profile, which is interpreted as the product of an overall waxing then waning of flow energy (volume and efficiency).

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