A PETROLOGICAL AND GEOCHEMICAL STUDY OF THE CALCAREOUS SANDSTONE OF WEST-AFRICAN MARINE PLEISTOCENE (CAP DES BICHES, SENEGAL)

In Senegal, the marine Pleistocene is represented by several outcrops of calcareous sandstone slabs which underly the recent sandy shoreline south of Dakar. These outcrops show dissolution and recrystallization phenomena increasingly evident as one moves south. They disappear at Toubab Dialao, i.e, approximately 40 km from Dakar. The outcrop at Cap des Biches has been studied mainly because of the exceptional preserv ation of some Ostrea calcitic tests which allowed isotopic U/Th measur ements. This outcrop of metric thickness was subjected to petrographic and mineralogical examination and to several geochemical analyses (tr ace elements, stable isotopes, amino acids) which lead to the conclusi on that the base of the deposit has been preserved. Two U/Th apparent ages obtained (178 ka and > 240 ka) are older than the only age of the isotopic stage 5c obtained in the area. A post-depositional mobility of U cannot be rejected, and ages may not be reliable. However, a mini mal age, considerably older than finite C-14 ages previously published , may be proposed (Eemian Interglacial, i.e. isotopic stage 5). In the se environments, geochemical analyses show high Pleistocene sea levels of either warmer and/or more diluted waters than the present levels, i.e. a probable Northern extension of Guinean waters.