DIVERSITY OF SMECTITE ORIGINS IN LATE CRETACEOUS SEDIMENTS - EXAMPLE OF CHALKS FROM NORTHERN FRANCE

According to XRD, DTA, geochemistry and electron microscopy data of a few selected samples, three categories of smectitic clay minerals occu r in the Cenomanian and Turonian chalks sampled in several sections of northern France. The first one corresponds to detrital illite-smectit e mixed-layers reworked from pedogenic blankets. The second category, characterized by crystalline overgrowths around detrital particles, is especially well-developed in slowly deposited sediments. The third ca tegory, located in thin marry horizons, corresponds to Cheto-montmoril lonite derived from the submarine alteration of volcanic glass. Smecti tic minerals having distinct origins may therefore occur at different levels of a given formation. From these examples, the smectite-rich cl ay sedimentation of the Late Cretaceous is tentatively explained by th e combination of favourable factors induced mainly by highstand of the sea-level. The climatic conditions, the reduction of continental area s, the development of carbonate platforms and the decrease in sediment ation rates constitute the most relevant of these factors.