FY 1984

Cenozoic regional erosion of the abyssal sea floor off South Africa

Tucholke, B.E., and R.W. Embley

In Interregional Unconformities and Hydrocarbon Accumulation, Am. Assoc. Petr. Geol. Mem., 36, 145–164 (1984)

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.