Morphosedimentary evolution of a delta-sourced, drift-aligned sand barrier–lagoon complex, western Bight of Benin

Abstract

The coast of eastern Ghana and Togo is bounded by a major Holocene sand barrier complex hinged on the Volta Delta to the west and subject to high rates of longshore sand drift (1–1.5×10 6 m 3/yr). In spite of its proximity to this major deltaic sediment source, this barrier system shows marked longshore variations in progradation following the middle Holocene eustatic stillstand. It is a `hybrid' system in terms of internal facies composition and plan-view morphology, in that it has evolved from an essentially regressive to a stationary (synonymous with cessation of progradation) system. Both transgressive and barrier-inlet facies have also developed through time and space. These temporal and spatial changes in patterns of development reflect variations in longshore sand transport and deposition over the last 5000 years. Such variations have been controlled by sediment supply from the Volta, by antecedent topography, by lagoonal and river drainage, by sediment cell development and maturation, and in the very recent past, by anthropogenic perturbation. Barrier progradation in the western Bight of Benin diminished considerably or ceased probably as early as 3400 yr B.P. as a single major equilibrium `drift aligned' bight cell matured, permitting transport of Volta sand toward the rest of the eastern bight in Benin and Nigeria. The eastward longshore supply from the Volta has involved both direct inputs of upland-derived sand by the Volta river channel and reworking of nearshore deltaic deposits. A recent reduction in direct fluvial supply, exacerbated by the Akosombo Dam, completed in 1961, has induced a wave of erosion of the downdrift delta-mouth barrier and shoreface deposits to fulfil the strong drift requirements. In Togo, construction of a deepwater port in 1967 has also affected the barrier system. Both of these changes have had considerable impacts on longshore sediment dynamics. The most significant impact has been segmentation of the former unique sediment cell characterising the 400-km-long Bight of Benin coast, resulting in differential longshore erosion, progradation or equilibrium. These changes have also induced new patterns of barrier behaviour that include transgressive and regressive dynamics at various locations alongshore.