River-dominated, shelf-edge deltas: delivery of sand across the shelf break in the absence of slope incision



Shelf-edge deltas record the potential magnitude of sediment delivery from shallow water shelf into deep water slope and basin floor and, if un-incised, represent the main increment of shelf-margin growth into the basin, for that period. The three-dimensional complexity of shelf-edge delta systems and along-strike variability at the shelf edge in particular, remains understudied. The Permian–Triassic Kookfontein Formation of the Tanqua Karoo Basin, South Africa, offers extensive three-dimensional exposure (>100 km2) and therefore a unique opportunity to evaluate shelf-edge strata from an outcrop perspective. Analysis of stratal geometry and facies distribution from 52 measured and correlated stratigraphic sections show the following: (i) In outer-shelf areas, parasequences are characterized by undeformed, river-dominated, storm-wave influenced delta mouth-bar sandstones interbedded with packages showing evidence of syn-depositional deformation. The amount and intensity of soft-sediment deformation increases significantly towards the shelf edge where slump units and debris flows sourced from collapsed mouth-bar packages transport material down slope. (ii) On the upper slope, mouth-bar and delta-front sandstones pinch out within 2 km of the shelf break and most slump and debris flow units pinch out within 4 km of the shelf break. (iii) Further down the slope, parasequences consist of finer-grained turbidites, characterized by interbedded, thin tabular siltstones and sandstones. The results highlight that river-dominated, shelf-edge deltas transport large volumes of sand to the upper slope, even when major shelf-edge incisions are absent. In this case, transport to the upper slope through slumping, debris flows and un-channellized low density turbidites is distributed evenly along strike.