Characterization and correlation of Upper Jurassic (Oxfordian) bentonite deposits in the Paris Basin and the Subalpine Basin, France |
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| Authors: | Pierre Pellenard Jean-Francois Deconinck Warren D Huff† Jacques Thierry‡ Didier Marchand‡ Dominique Fortwengler§ Alain Trouiller¶ |
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| Institution: | Morphodynamique continentale et côtière, UMR 6143 CNRS, University of Rouen, Department of Earth Sciences, 76821 Mont St Aignan Cedex, France (E-mail: );University of Cincinnati, Department of Geology, PO Box 210013, Cincinnati, OH 45221-00, USA;Biogéosciences-Dijon, UMR 5561CNRS, Department of Earth Sciences, University of Burgundy, 6 bd Gabriel, 21000 Dijon, France;Le Clos des Vignes, Quartier Perry, 26160 La Bégude de Mazenc, France;ANDRA, Parc de la Croix Blanche, 1–7 rue Jean Monnet, 92298 Châtenay-Malabry, France |
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| Abstract: | 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 km2. 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. |
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| Keywords: | Bentonites clay minerals correlations Oxfordian palaeovolcanism Paris Basin Subalpine Basin of France |
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