Geochemical evolution and paleoenvironments of the eocene continental deposits in the Paris Basin

Abstract

The continental and lacustrine deposits of Eocene age in the Paris Basin form a geochemical sequence extending from clastic sediments characterized by aluminous clay minerals to chemical sediments with dolomite, gypsum and magnesian clay minerals. During the Early Eocene, flood plains and lakes with large deltas extended across the southern margin of the Paris Basin. The clays were derived from the weathered Mesozoic rocks. Calcretes and smectitic vertisols developed within the flood plain deposits, indicating that the climate was warm with an alternation of wet and dry seasons. During the Middle Eocene, lacustrine limestones and palygorskite clays developed in small lakes subject to periodic desiccation and pedogenesis. Landwards, pedogenetic silcretes developed on the early Eocenc pediments. Palygorskite is indicative of environments with high concentrations of silica and magnesium, and the presence of opal in silcretes confirms a high silica concentration in the soils solutions. The paleoclimate was warm with distinct dry seasons or periods. During the Late Eocene, gypsum was deposited in the basin center, surrounded by a marly deposit in which dolomite and sepiolite are the characteristic minerals. Lacustrine limestone with alumino-magnesian smectite and palygorskite formed on the margin of the basin. The development of the evaporitic sequence was favored by the flatness of the palaeolandscape and its silcrete capping which restricted erosion and detrital input to the basin. The climate was warm with a tendency to aridity. Thus the continental deposits of Eocene age in the Paris Basin reflect a progressively more aric climate. Moreover, each depositional, pedogenetic and encrusting phase modified the palaeolandscape and thus influenced the next sedimentary episode.