Shelf-margin clinothem progradation, degradation and readjustment: Tanqua Depocentre, Karoo Basin (South Africa)

Degradation of basin-margin clinothems around the shelf-edge rollover zone may lead to the generation of conduits through which gravity flows transport sediment downslope. Many studies from seismic-reflection datasets show these features, but they lack small-scale (centimetre to metre) sedimentary and stratigraphic observations on process interactions. Exhumed basin-margin clinothems in the Tanqua depocentre (Karoo Basin) provide seismic-reflection-scale geometries and internal details of architecture with depositional dip and strike control. At the Geelhoek locality, clinothem parasequences comprise siltstone-rich offshore deposits overlain by heterolithic prodelta facies and sandstone-dominated deformed mouth bars. Three of these parasequences are truncated by a steep (6 to 22°), 100 m deep and 1.5 km wide asymmetrical composite erosion surface that delineates a shelf-edge canyon. The fill, from base to top comprises: (i) thick-bedded sandstone with intrabasinal clasts and multiple erosion surfaces; (ii) scour-based interbedded sandstone and siltstone with tractional structures; and (iii) inverse-graded to normal-graded siltstone beds. An overlying 55 m thick coarsening-upward parasequence fills the upper section of the canyon and extends across its interfluves. Younger parasequences display progressively shallower gradients during progradation and healing of the local accommodation. The incision surface resulted from initial oversteepening and high sediment supply triggering deformation and collapse at the shelf edge, enhanced by a relative sea-level fall that did not result in subaerial exposure of the shelf edge. Previous work identified an underlying highly incised, sandstone-rich shelf-edge rollover zone across-margin strike, suggesting that there was migration in the zone of shelf-edge to upper slope incision over time. This study provides an unusual example of clinothem degradation and re-adjustment with three-dimensional control in an exhumed basin margin succession. The work demonstrates that large-scale erosion surfaces can develop and migrate due to a combination of factors at the shelf-edge rollover zone, and proposes additional criteria to predict clinothem incision and differential sediment bypass in consistently progradational systems.