Pleistocene Seascape Evolution Above a “Simple” Stepped Slope—Western Niger Delta

Abstract

The morphology of a 1250 km2 portion of the middle slope off the western Niger Delta shows that gradients on the Pleistocene slope vary both spatially and at different stratigraphic levels. In the deeper section, three lower-gradient steps are connected by three higher-gradient ramps, generating a stepped-slope morphology. Through time, preferential accumulation of slope aprons, composed of mass-transport deposits, compensationally stacked lobes, and overbank deposits (wedge-shaped outer levees), helped fill slope accommodation, smoothing over the gradient change across ramps and steps, and vice versa. Consequently at the local scale, the stepped slope evolved into a smoother slope that is nearly graded at the modern seafloor. As in other studies, preferential accumulation of sediment on the slope is believed to reflect in part the deceleration of sediment gravity flows (both turbidity currents and debris flows) as they encountered lower-gradient steps. Down-slope changes in slope morphology also caused variations in the amount, and presumably rate, of erosion along the axes of canyons in the study area—with increased incision depth where knickpoints cut through positive-relief bathymetric structures in an attempt to establish a graded profile. Along the Benin-major Canyon there is an inverse linear relationship between the thickness of deposits that accumulate on the slope adjacent to the canyon and the amount of vertical erosion along its axis. The thickest outer levee deposits coincide with canyon segments that have the shallowest incision, in turn corresponding to slope segments showing a sharp decrease in pre-incision gradient. This implies that the increase in sediment flux to outer levees on some parts of the stepped slope results from a combination of increased overspill from flows passing through shallower canyon reaches, and increased sedimentation caused as mud-dominated flows decelerated on lower-gradient slope segments immediately adjacent to the canyon. Thus there appears to be an intimate relationship between slope morphology, canyon incision depth, and the thickness of overbank deposits adjacent to canyons.