Caractérisation et extension de niveaux repères de bentonites dans le Turonien supérieur du Bassin de Paris (Boulonnais, Aube)

Petrographic, mineralogical and geochemical analyses have been performed on thirteen samples coming from marly layers intercalated in upper Turonian chalks from the Boulonnais and Aube (Paris Basin, France). Two samples characterized by abundant illite and kaolinite correspond to terrigenous horizons, while the other samples, characterized by the occurrence of volcanic glass shards, zircon and apatite, are of volcanogenic origin. The clay fraction is dominantly composed of smectite formed by submarine weathering of volcanic glass; therefore, the marly layers correspond to bentonites. The initial mineralogical and geochemical composition of the marly layers is disturbed by common burrowing. Consequently, it is unfortunately impossible to characterize and distinguish one bentonite layer from another using mineralogical and geochemical data only. However, correlations based on lithological and paleontological data are proposed between the sections exposed in the Boulonnais and Aube. The bentonites allow to refine these correlations and show that the upper Turonian in the Aube region is about three times thicker than in the Boulonnais which was probably less subsident. The bentonites layers also occur in England and Germany. They reflect instantaneous events and are therefore very useful for long distance correlations and much more precise than palaeontological zones.