Dating of West African planation surfaces thanks to paleomagnetic approaches

International audience; Lateritic planation surfaces cover a predominant part of the West African landscape. The geomorphological framework and its evolution since Late Jurassic has been described as a succession of 3 bauxitic surfaces followed by an alumino-ferruginous surface, followed itself by 3 ferruginous duricrustsurfaces attributed to Tertiary age. There is little time constraint as conventional geological or geochemical methods cannot be applied to lateritic weathering profiles. This paper presents the first ever relative paleomagnetic dating results obtained from a weathering profile in Africa. The studied profile was sampled at the Léro gold mine in the North of Guinea. A full 100 m thick weathering profile has been sampled from the weathering front up to the surface. The saprolite in Léro recorded good quality paleosignals carried mostly by hematite. Our results confirm the vertical model oflateritic weathering profile development, with the oldest levels in the upper part of the profile and the most recent ones at the bottom. Surprisingly, the bottom part of the profile close to the weathering front, did not recorded any recent signal, suggesting that the deepening of the weathering front has greatly slowed down or was stopped for a long period. The bottom part that represents a layer of about 50 m thick shows likely a Miocene age with a mean pole falling between the 10 to 20 Ma poles of the African APWP. The upper part of the saprolite shows a Late Cretaceous age of 70 Ma. Finally, the top part of the saprolite, close to the ferruginous duricrust, indicates a recent magnetization phase that could be related to an ongoing duricrust forming process. Based on the landsurface correlation, the predominant alumino-ferruginous duricrust surface in the region may probably be attributed to a Late Cretaceous paleosurface rather than Oligocene as supposed from the literature. This leads to reconsider the general geomorphological framework of the West African region.