Large-Scale Sediment Waves (SW) on the Continental Slope of West Africa (Cenozoic): Their Link with Mass-Transport Deposits (MTDs)

We focus on the Cenozoic strata offshore West Africa, where large-scale SW developed along continental slope, reaching wavelengths of the order of 2000 m and height of around 40 m. Their crests are parallel to the bathymetric contours, suggesting that their most likely mechanism of formation would be downslope flowing currents. The SW fields are laterally separated by wide (4–12 km wide), relief (49–64 m deep) and mostly rectilinear channel features.

Here we use very high-resolution 3D seismic analysis to perform a geomorphological analysis of the SW fields, to understand their genetic relationship with the adjoining channels, to discuss their likely mechanisms of formation and to assess their lithological composition. Our results show that: 1) The presence or absence of SW seems to be related to the morphology of the margin which controls the hydrodynamic conditions. SW are only observed when the channels become less erosive and shallower, resulting in smaller levees promoting turbidity current overflow.2) The wide, straight deep-sea channels adjoining SW fields are associated with chaotic seismic geomorphologies suggesting that they are carved and filled by debris flows. They can be viewed as straight, “early-stage” or “immature” channelized systems carved by MTDs. At places the mass transport deposits contained within those channels overflows and accumulates in the troughs of the SW, resulting in “fishbone” amplitude pattern that could be mistaken for sand lenses on lower-resolution data.3) An estimation of SW lithology can be made since they are largely dominated by polygonal faulting, indicating a clay-rich lithofacies.