Mid- to late Quaternary environments and stratigraphy of the southern Sierra Leone shelf, West Africa

Abstract

The middle to upper Quaternary succession of the southern Sierra Leone shelf appears to have been deposited in response to three glacial sea-level cycles, the last of which has left the most complete sedimentary record. The unconformable base of the succession is deeply incised off larger rivers inshore but has little relief toward the shelf-break. This difference results from the steep gradient of the inner shelf and the gentle gradient of the valley's long profile. Incision was caused by high fluvial discharge during sea level fall. Decline of discharge and sediment supply during and after glacial maximum resulted in cessation of incision and in the formation of cemented beach ridges at the shelf-break. The lowstand wedge of the last glacial cycle is well developed beyond the shelf-break and where the pre-existing shelf was deep. Where sampled it is deposited below wave-base. Its preservation is apparently the result of protection from wave and shelf-current action during regression. The subsequent transgressive wedge entirely dominates shelf deposition, infilling incised valleys, and contains vertically stacked fluvial, estuarine, littoral and sub-littoral elements. It is thin or absent beyond the shelf as a result of active oceanic currents hindering sedimentation. Active oceanic currents also result in only weak development of the highstand wedge. Energy regime, as represented by moderate shelf currents and variable fluvial discharge, has been the fundamental control on sediment architecture. Sediment supply has been of only secondary importance.