Abstract
About 70 rock samples have been collected in theBarremian-Aptian of the «Vocontian trough and in several sites of the surrounding Urgonian platform. Geochemical analyses (mineralogy, major and trace elements) have been performed on these materials, in oder to verify and specify the hypothesis previously proposed on the factors controlling the sedimentation, especially in the platform-basin transitional zone. These results confirm the relevance of the sedimentologicalmodel established for the Lower Cretaceous, and they afford the following additional data. 1. The variations of the boron concentration of theclay fraction is in keeping with the paleoecological and sedimentological characteristics, confirming the reliability of this element as paleosalinity indicator. The other elements usually sensitive to paleosalinity (e.g. fluorine and strontium) are more dependent on the nature of their mineralogical bearers, or on diagenetic processes. 2. The elements directly linked to the terrigenous supply can be split in two main groups: -an association involving Li, Al, Ti, which is related to alteration minerals, notably kaolinite; -another association appears to be, partly atleast, representative of inherited aluminous smectites (Ca, Mg, Ti, Al, V, Zn, loss on ignition). 3. Another lithophile association, including K,Ba, B, obviously presents a statistical correlation with illitic minerals; it reflects a certain aggradation of these clay minerals during their transportation to the basin. 4. A siderophile factor (Fe, Ga, Ni, V, Cr) canbe observed in the hydroxide-rich horizons (e.g. hard-grounds) and in the glauconitic beds. 5. Lastly, an assemblage of elements (Cu, Mo,Zn, Ni, V, Cr) is definitely linked to sulphides and organic matter: it particularly exists in inner platform-type environments and in the distal part of bioclastic accumulations. All these data enable us to emphasize the opposition between a tremendous expansion of life on the Urgonian platform, and the adverse restricted and hypersaline environment in the next «Vocontian trough.
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