Sedimentation in a fluvially infilling, barrier-bound estuary on a wave-dominated, microtidal coast: the Ouémé River estuary, Benin, west Africa

The Oueme River estuary is located on the seasonally humid tropical coast of Benin, west Africa. A striking feature of this microtidal estuary is the presence of a large sand barrier bounding a 120 km2 circular central basin, Lake Nokoue, that is being infilled by heterogeneous fluvial deposits supplied by a relatively large catchment (50 000 km2). Borehole cores from the lower estuary show basal Pleistocene lowstand alluvial sediments overlain by Holocene transgressive–highstand lagoonal mud and by transgressive to probably early highstand tidal inlet and flood-tidal delta sand deposited in association with non-preserved transgressive sand barriers. The change in estuary-mouth sedimentation from a transgressive barrier-inlet system to a regressive highstand barrier reflects regional modifications in marine sand supply and in the cross-barrier tidal flux associated with barrier-inlet systems. As barrier formation west of the Oueme River led to an increasingly rectilinear shoreline, the longshore drift cell matured, ensuring voluminous eastward transport of sand from the Volta Delta in Ghana, the major purveyor of sand, to the Oueme embayment, 200 km east. Concomitantly, the number of tidal inlets, and the tidal flux associated with a hitherto interlinked lagoonal system on this coast, diminished. Complete sealing of Lake Nokoue has produced a large, permanently closed estuary, where tidal intrusion is assured through the interconnected coastal lagoon via an inlet located 60 km east. Since 1885, tides have entered the estuary directly through an artificial outlet cut across the sand barrier. Although precluding the seaward loss of fluvial sediments, permanent estuary-mouth closure has especially deprived the highstand estuary of marine sand, a potentially important component in estuarine infill on wave-dominated coasts. In spite of a significant fluvial sediment supply, estuarine infill has been moderate, because of the size of the central basin. Estuarine closure has resulted in two co-existing highstand sediment suites, with limited admixture, the marine-derived, estuary-mouth barrier and upland-derived back-barrier sediments. This situation differs from that of mature barrier estuaries characterized by active fluvial-marine sediment mixing and facies interfingering.