LATE QUATERNARY SEDIMENTS IN INCISED COASTAL VALLEYS IN BENIN - A PRELIMINARY SEQUENCE-STRATIGRAPHIC INTERPRETATION

The incised coastal valley fill of the Mono and Oueme Rivers in Benin comprises several Late Pleistocene and Holocene facies that have been interpreted in sequence-stratigraphic terms. The study shows the exist ence of lowstand, transgressive and highstand systems tracts related t o the last 120,000 year eustatic cycle and the subsequent still-stand since around 5500 BP. The lowstand systems tract consists of Late Plei stocene alluvial sediments overlying an earlier sequence in the Oueme valley. This tract has not been identified in the Mono Valley mouth wh ere the lower sequence boundary directly separates the transgressive s ystems tract from the underlying substratum. The transgressive systems tract is represented by lagoonal mud and bay-mouth shoal/flood-tidal delta sands. This tract may also subsist as a thin shoreface lag or ov erwash sandy gravel deposit. The lagoonal muds are separated in places from the overlying estuary-mouth sands by tidal or wave ravinement su rfaces. The highstand systems tract comprises the following: prograded sand barrier beach/shoreface sands, a non-prograding barrier associat ed with a migrating inlet, lagoonal muds and bay-head delta sands and muds. This tract has been partly reworked by tidally-influenced migrat ion of the Mono channel and includes highstand fluvial-tidal, tidal an d wave ravinement surfaces. Fine-scale differentiation of early highst and and late transgressive systems tracts and the maximum flooding sur face, especially in the Oueme Valley, is rendered difficult by the ess entially similar nature of their facies. This is notably the case of m uddy backbarrier lagoonal sediments. It is hoped that future work will enable a finer resolution of these sequence tracts and stratigraphic surfaces.