Large scale excavation of outer shelf sediments by bottom currents during the Late Miocene in the SE Atlantic

Abstract

An unusual erosional surface has been identified on seismic data from the outer shelf on the northern Namibian margin. The undulating surface defines a depression extending for 1300 km2 from which fine-grained Miocene sediments have been excavated to a depth of up to 160 m. The depression has a kidney-shaped planform, with three deep pits marking its basinward extent. It is an entirely closed feature, with no evident channels feeding in or leading out. Subsequent sedimentation has completely filled in the depression with the earliest infilling units exhibiting unusual depositional geometries. Well correlation yields a date of 9.9 ± 1.0 Ma for the erosional surface. No analogous features are known, but of all the possible causal mechanisms, it is considered most likely that the erosional depression was created by the extraordinary action of eddying bottom currents. These were probably associated with the well-documented Late Miocene onset of intensification of the Benguela Upwelling System, and were possibly directed by a small local variation in sea bed relief.