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

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.

Clay Mineralogy of Coal-Hosted Nb-Zr-REE-Ga Mineralized Beds from Late Permian Strata, Eastern Yunnan, SW China: Implications for Paleotemperature and Origin of the Micro-Quartz

“…In fact, the clay fractions in this study, especially kaolinite and I/S have been described within volcanic-ash originated depositions of coal-bearing strata [30]. I/S has been reported in most Mesozoic bentonites [30], the lower Cretaceous bentonites, British Columbia [43][44][45], Oxfordian bentonites from the Subalpine Basin, Turonian bentonites of France [46], and Ordovician Kinnekulle K-bentonites, France [47]. Furthermore, I/S-enriched Late Permian coals were reported in the Changxing Mine, eastern Yunnan, southwest China (closely located to the drill holes present in this study) [42].…”

The origin and timing of multiphase cementation in carbonates: Impact of regional scale geodynamic events on the Middle Jurassic Limestones diagenesis (Paris Basin, France)

“…On the borders of the Paris Basin, Toarcian to Kimmeridgian sections do not exhibit a progressive decrease in smectite and kaolinite with depth (Uriarte, 1997;Pellenard et al, 1999;Pellenard and Deconinck, 2006;Dera et al, 2009), which suggests only slight burial-related modification. The smectite-rich composition of a bentonite layer, detected in the Callovian-Oxfordian clays 100 m above the top of the Middle Jurassic neretic carbonates in the HTM102 borehole, also confirms the negligible influence of burial diagenesis (Pellenard et al, 1999;Pellenard et al, 2003;Pellenard and Deconinck, 2006).…”

Milankovitch and sub‐Milankovitch forcing of the Oxfordian (Late Jurassic) Terres Noires Formation (SE France) and global implications

“…1). They are present in continuously exposed intervals with a well‐constrained ammonite biostratigraphic framework, encompassing the Early–Middle Oxfordian (Fortwengler & Marchand, 1994; Gaillard et al , 1996, 2004; Pellenard et al , 2003). The lithologies are mainly composed of grey marls with a slight enrichment in carbonate up to the Middle Oxfordian.…”