Abstract

The Holocene shelf sediment wedge has been elucidated by high resolution seismic reflection profiling augmented by side-scan sonar imagery, box cores and grab samples. The quasi-continuous sediment wedge extends over 1500 km from the narrow east coast shelf (12 km) to the broad south coast shelf (100 km). The wedge varies in width from 4 km up to 35 km, while thicknesses can be up to 40 m. The wedge lies on a gently-dipping erosion surface which truncates folded Tertiary and Cretaceous strata and is frequently incised by channels. The inner margin may be banked against steep coastal cliffs, merge with shoreface sands or be flanked locally by nearshore rocky outcrops. The water depth of the outer margin can be up to 100 m and may be draped in mud, dammed by Pleistocene aeolianite dune ridges, or flanked by geostrophic current-generated bed forms on the mid-shelf of the east coast. These large bed forms are sometimes superimposed on the wedge, and in other cases, starved bed forms lie seaward of the main sediment wedge. Whereas a single complex dune cordon occurs on the east coast, multiple aeolianite ridges interfere with the sediment prism on the south coast. In both areas relict sediments lie seaward of Holocene sediments, except where the wedge locally reaches the shelf edge near Port St. Johns. Locally near headlands, embayments or coastal offsets, where bed load sediments converge in sediment traps, submerged spit bars form at water depths of 40 to 90 m within the wedge. Sediments with these features are partly derived from longshore drift and are texturally and compositionally similar to adjacent beach sediments. Large progradational cross-beds seen in seismic profiles and small cross-beds seen in box cores characterize these features. Mud-draped sand bodies, such as the Robberg submerged spit bar that exhibits acoustic anomalies suggestive of gas accumulation, may provide potential hydrocarbon reservoirs. Narrow linear sand bodies, such as those that extend along the South Africa shelf and include submerged spit bars, may be mistaken in the fossil record for shoreface sands including beaches, sub-aerial spits and shallow bars. We suggest that a drowned sand body in the Norwegian Sea, which has been interpreted as a beach, may be a submerged fossil spit bar.