Geological significance of two smectite-rich beds from lower Cenomanian sediments, northern Aquitaine Basin, France

Two clay-rich beds (AHY and RPC) are interstratified in the Lower Cenomanian sediments of the northern domain of the Aquitaine basin (France). The paleobiological associations indicate a change in the sedimentation environments: a lagoon for the oldest one (AHY) and a sea internal platform for the most recent one (RPC). Although the mineralogical composition of both is dominated by smectite-rich random I/S mixed-layers, chemical and mineralogical differences were observed. The AHY clay material is richer in Zr, Th and REE. These chemical components indicate the presence of zircon and monazite or allanite. As no free crystals were observed, it is probable that these mineral species are disseminated as solid inclusions in the clays. The RPC sediment contains detrital (feldspars, micas, pyrophyllite) and post-sedimentary minerals (gypsum and pyrite). The AHY and RPC clay-rich beds are respectively a bentonite derived from a volcanic ash deposit and a sediment formed by the re-working of this bentonite during sea-invasion. The volcanic source is probably related to the opening of the Gascogne gulf and the large Cenomanian trachytic series of northern Spain (Bilbao area).