COASTAL EVOLUTION IN SENEGAL AND MAURITANIA AT 10(3)-YEAR, 10(2)-YEARAND 10(1)-YEAR SCALES - NATURAL AND HUMAN RECORDS

Previous works have shown that the range of marine level eustatic chan ges on the coast of the Senegalo-Mauritanian basin after 5500 BP must have been relatively small. On the basis of regional geological mappin g this position is confirmed in two littoral regions in Mauritania (Ba le de Saint Jean, Cape Timiris) and in Senegal (Saloum Delta). The evo lution of the shoreline is linked mainly to the balance between deposi tion and reworking by the sea. In Mauritania, the reworking capacity i s high before 4000 BP, under a more humid climate than the present one . It becomes insufficient around 3000 BP, as areas of aeolo-marine pro gradation develop in the sebkhas at the inner part and the shorelines of the Bale de Saint Jean. The deposits are subjected to a constant se a level and are therefore formed by the rhythm of recurrent aridity cr ises on the millenary or the centenary scale. In the Saloum delta, the evolution of the shoreline results from several processes involving m ore varied timescales. An essential factor is the positioning of litto ral sand barriers, shaped like long sandy spits which divert the flow of the river towards the South and are arranged according to a basal s urface also adjusted to a constant sea level. The development of sand barriers, such as the present Sangomar spit and also the Langue de Bar barie, occurs at a centennial rhythm. Higher probability events (at th e decade scale) are likely to perturb this development by inducing gap s in the sand barrier and, as the river goes beyond the sand barrier, the latter becomes incorporated in the deltaic plain, forcing the litt oral drift to build a new sand barrier further forward. These events a re linked to a conjunction of factors involving different timescales: aridity crises at the 10(3) or 10(2)-year scales, sedimentation waves at the 10-year scale, seasonal exceptional swells, river floods, varia tions of the tidal range determined by modifications in the estuarian morphology.