Depositional dynamic of glaucony-rich deposits in the Lower Cretaceous of the Nice arc (Southeast France).

A sedimentological and stratigraphical analysis has been applied to the Early Cretaceous glaucony-rich deposits of the southeastern margin of the Vocontian Basin. Thin sections, X-ray diffraction on random powders analyses, and data from geochemical analyses, performed on pure disaggregated glauconite grains by wavelength dispersion spectroscopy (WDS), allow the distinction of two populations of highly evolved glauconite grains. The first population is interpreted to be autochthonous (i.e., grains that have not experienced any transport from their place of origin). The second is interpreted to be parautochthonous (i.e., grains that have been removed from their place of origin and concentrated landward and seaward within nearly coeval deposits). Palaeoenvironmental information has been deduced mainly from (1) the characteristics of glauconitic grains, (2) meso- and microscopic analyses performed on the named lithozones, and (3) their lateral changes on a kilometre scale. Basin palaeogeography implies a southern area belonging to an outer platform and a northern zone with the characteristics of a distal ramp (Hauterivian). This depositional setting changed during the Barremian–Aptian owing to tectonics; fault systems led to a drowning of the western segment, but the previous environmental pattern presented again during Albian–Early Cenomanian times. Four different depositional events have been distinguished in the Early Cretaceous, and the relative sea-level changes have been reconstructed. We have traced relative sea-level fall during the Late Valanginian, the Early Barremian and the Albian (the second and third of these corresponding to the tectonic uplift outlined by Wilpshaar et al. (Cretac. Res. 18 (1997) 457)), while the general subsidence is confirmed by the transgression that led to the formation of glauconitic minerals. During the Lower Cretaceous, two second-order cycles have been recognized; although of the second order, the first sequence (Late Valanginian–Barremian) is made up like a third-order depositional sequence where different system-tracts are enhanced. The compared relative sea-level change corresponds to the curve of the second-order cycles of Haq et al. (1988, SEPM Spec. Publ. 42, 71–108).