Post-Eocene seismic stratigraphy of the deep ocean basin adjacent to the southeast African continental margin: a record of geostrophic bottom current systems

Abstract

A high-resolution seismic-reflection survey of the Transkei Basin and Natal Valley permits the first recognition of three major reflectors that mark basin-wide unconformities across the continental rise and deep abyssal plain off the southeast African continental margin. Reflector O marks a change in acoustic reflectivity, coincident with a change in sedimentary bedforms from generally parallel bedding below to large-scale lenticular and clinoform shapes above. Reflector O probably marks the onset of cold, abyssal current circulation around the Eocene–Oligocene boundary. The overlying O sequence records deposition of a contourite drift (Oribi Drift) by northeast flowing abyssal currents at ∼4000 m water depths along the continental rise of the northeastern Agulhas Fracture Zone. This water depth is shallower than present-day Antarctic Bottom Water (AABW). The M reflector unconformity (possibly lower Middle Miocene) marks seafloor erosion in 4500 m water depth in the Transkei Basin and the cessation of drift construction along the continental rise. Above reflector M in the abyssal plain, a contourite drift (M-Drift) records deposition from an east-flowing bottom current in a location similar to, but slightly shallower than present-day AABW. The stagnation of bottom current activity in the northern Natal Valley and/or a rapid influx of sediment accumulation is marked by M sequence turbidite sediments (the Mzimkulu apron) deposited against and burying the Oribi Drift on the continental rise. Reworking of M sequence sediment along the continental rise to form low mounds (M 4) and sediment waves in the northern Natal Valley indicate that a shallow, bottom current flowed at depths of 3800 to 3600 m. The coeval current-molding of the slope and abyssal plain indicates a two-layered structure of the bottom water may have commenced in the Miocene. Reflector P is the most pronounced unconformity in the deep abyssal plain, where it truncates M Sequence reflectors, and marks the base the Agulhas Drift which stands approximately 200 m above the surrounding seafloor. The P Sequence sedimentation is estimated to have begun in the Pliocene prior to or concurrent with an expansion of Southern and Northern polar ice-caps. Major slumping of the continental slope in the Natal Valley also began at this time, probably triggered by a combination of onland neotectonic activity and erosion of the base of the slope by vigorous bottom currents (possibly North Atlantic Deep Water, NADW).