Cenozoic Regional Erosion of the Abyssal Sea Floor Off South Africa
The Cape, Agulhas, and Mozambique basins off South Africa have a well-defined and mostly continuous erosional zone along their perimeters between 4 and 5 km (2.5 and 3.1 mi) water depth. This zone lies beneath a deep boundary current of Antarctic Bottom Water. However, current speeds are generally less than 15 to 20 cm/sec (5.9 to 7.9 in/sec) and part of the erosional zone is armored by authigenic manganese deposits, so that only limited erosion is presently occurring. Erosion and corrosion of sea-floor sediments by abyssal currents probably are in dynamic equilibrium with sediment supply. The present erosional zone is largely a relict feature inherited from late Miocene time when strongly increased glaciation of West Antarctica produced large volumes of bottom water that scoured the sea floor. A deeper unconformity dating to the early Oligocene marks the onset of significant abyssal circulation in the basins, and current-controlled deposition of sediments is well defined above this unconformity. In contrast, an underlying basal Eocene unconformity shows no marked effects of control by abyssal circulation, although erosion/corrosion by weak, deep currents could have occurred. The unconformity formed primarily because of reduced sediment supply, caused by elevated sea level and probably low productivity in surface waters.