BEACH-RIDGE DEVELOPMENT AND SEDIMENT SUPPLY - EXAMPLES FROM WEST-AFRICA

Beach ridges form the dominant mode of Holocene coastal progradation i n West Africa. They vary from minor accumulations to massively prograd ed plains over 100 km long. Variations in the patterns and morphologic al expression of these sandy deposits record spatial and temporal chan ges in various environmental parameters. Among these, sediment supply stands out. The importance of this factor is illustrated by the morpho logical patterns and the current sediment dynamics of minor and major beach-ridge plains in this region. The simplest beach ridges are relat ed to just one or two sources of sand supply from a nearby source. In these examples, sand has been supplied by the nearshore zone or by ero ding cliffs. In many areas, beach-ridge formation ceased as the sedime nt supply dwindled, because of cliff stabilization or establishment of an equilibrium nearshore profile. Major beach-ridge plains in souther n Sierra Leone and in the Eight of Benin exhibit complex patterns of d evelopment related to a bigger sediment supply from nearshore sources and rivers. In southern Sierra Leone, the beach-ridge plains have been constructed essentially from sands derived from the nearshore zone. L ocally, as in the proximal part of Sherbro Island, shallow nearshore s hoals encroached upon by the beach-ridge front have led to more massiv e progradation while engendering complex patterns of beach-ridge devel opment. The sediment supply has been locally supplemented by sands dri fting alongshore from the Sewa and Moa rivers. The sediment supply inv olved in beach-ridge development in the Eight of Benin highlights the role of longshore drift of sands from the Volta Delta. Early stages of progradation were characterized by the development of segmented inner beach-ridge barriers separated by still infilling coastal re-entrants . Nearshore sources were particularly important in these early stages of beach-ridge progradation. As coastal re-entrants and their estuarie s became infilled following the middle Holocene eustatic highstand, th e longshore drift system evolved from a segmented to a unicellular one . The resulting conditions of through-drift east of the Volta Delta al lowed for longshore transport of sands from this river for the constru ction of outer beach-ridge barriers over a distance of over 250 km. Wh ile rivers still actively supply sand to the coast, current patterns o f beach-ridge development depend essentially on the coastal sand trans port system which determines either the confinement of incoming sand t o the vicinity of the very source area itself, its regular delivery to the beach-ridge front or its through-drift to the local downdrift ter minus.