Characterization and correlation of Upper Jurassic (Oxfordian) bentonite deposits in the Paris Basin and the Subalpine Basin, France

• Published in: Sedimentology
• Main Authors: Pellenard, Pierre, Deconinck, Jean‐Francois, Huff, Warren D., Thierry, Jacques, Marchand, Didier, Fortwengler, Dominique, Trouiller, Alain
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2003
• Subjects: North Atlantic
• Online Access: http://dx.doi.org/10.1046/j.1365-3091.2003.00592.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1046%2Fj.1365-3091.2003.00592.x; https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-3091.2003.00592.x

Abstract Explosive volcanic activity is recorded in the Upper Jurassic of the Paris Basin and the Subalpine Basin of France by the identification of five bentonite horizons. These layers occur in Lower Oxfordian ( cordatum ammonite zone) to Middle Oxfordian ( plicatilis zone) clays and silty clays deposited in outer platform environments. In the Paris Basin, a thick bentonite (10–15 cm), identified in boreholes and in outcrop, is dominated by dioctahedral smectite (95%) with trace amounts of kaolinite, illite and chlorite. In contrast, five bentonites identified in the Subalpine Basin, where burial diagenesis and fluid circulation were more important, are composed of a mixture of kaolinite and regular or random illite/smectite mixed‐layer clays in variable proportions, indicating a K‐bentonite. In the Subalpine Basin, a 2–15 cm thick bentonite underlain by a layer affected by sulphate–carbonate mineralization can be correlated over 2000 km 2 . Euhedral zircon, apatite and biotite crystals have been identified in all the bentonites. The geochemical composition of the bentonites in both basins is characterized by high concentrations of Hf, Nb, Pb, Ta, Th, Ti, U, Y, Zr and low concentrations of Cr, Cs and Rb. Biostratigraphical and geochemical data suggest that the thick bentonite in the Paris Basin correlates with the thickest bentonite in the Subalpine Basin, located 400 km to the south. These horizons indicate that significant explosive volcanic events occurred during the Middle Oxfordian and provide potential long‐distance isochronous marker beds. Immobile element discrimination diagrams and rare‐earth element characteristics indicate that the original ash compositions of the thickest bentonites correspond to a trachyandesitic source from a within‐plate alkaline series that was probably related to North Atlantic rifting.