Diversity of smectite origins in Late Cretaceous sediments: example of chalks from northern france

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Abstract
According to XRD, DTA, geochemistry and electron microscopy data of a few selected samples, three categories of smectitic clay minerals occur in the Cenomanian and Turonian chalks sampled in several sections of northern France. The first one corresponds to detrital illitesmectite 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 category, located in thin marly horizons, corresponds to Cheto-montmorillonite derived from the submarine alteration of volcanic glass. Smectitic minerals having distinct origins may therefore occur at different levels of a given formation. From these examples, the smectite-rich clay sedimentation of the Late Cretaceous is tentatively explained by the combination of favourable factors induced mainly by highstand of the sea-level. The climatic conditions, the reduction of continental areas, the development of carbonate platforms and the decrease in sedimentation rates constitute the most relevant of these factors.