Geochemical signature of paleofluids in microstructures from Main Fault in the Opalinus Clay of the Mont Terri rock laboratory, Switzerland

The present study reports on elemental and Sr isotopic analyses of calcite and associated celestite infillings of various microtectonic features collected mostly in the Main Fault of the Opalinus Clay from Mont Terri rock laboratory. Based on a detailed microstructural description of veins, slickensides, scaly clay aggregates and gouges, the geochemical signatures of the infillings were compared to those of the leachates from undeformed Opalinus Clay, and to the calcite from veins crosscutting Hauptrogenstein, Passwang and Staffelegg Formations above and below the Opalinus Clay. Vein calcite and celestite from Main Fault yield identical 87Sr/86Sr ratios that are also close to those recorded in the Opalinus Clay matrix inside the Main Fault, but different from those of the diffuse Opalinus Clay calcite outside the fault. These varied 87Sr/86Sr ratios of the diffuse calcite evidence a lack of interaction among the associated connate waters and the flowing fluids characterized by a homogeneous Sr signature. The 87Sr/86Sr homogeneity at 0.70774 ± 0.00001 (2σ) for the infillings of most microstructures in the Main Fault, as well as of veins from nearby limestone layer and sediments around the Opalinus Clay, claims for an “infinite” homogeneous marine supply, whereas the gouge infillings apparently interacted with a fluid chemically more complex. According to the known regional paleogeographic evolution, two seawater supplies were inferred and documented in the Delémont Basin: either during the Priabonian (38–34 Ma ago) from western Bresse graben, and/or during the Rupelian (34–28 Ma ago) from northern Rhine Graben. The Rupelian seawater that yields a mean 87Sr/86Sr signature significantly higher than those of the microstructural infillings seems not to be the appropriate source. Alternatively, Priabonian seawater yields a mean 87Sr/86Sr ratio precisely matching that of the leachates from diffuse calcite of the Opalinus Clay inside the Main Fault, as well as that of its microstructures and the same features of the sediments above and below. To envision a Priabonian seawater supply, there is a need for its storage without a significant evolution in its Sr isotopic composition until the final deformation of the area. The paleo-hydrogeological context calls for a possible infiltration of the seawater into a limestone karst located above the Opalinus Clay that could have acted as the storage reservoir. The karstic nature of this reservoir also explains why the 87Sr/86Sr of the fluids was not modified significantly until expulsion. An alternative storage could have been provided by the regional faulting system that developed during the contemporary regional rifting of the Rhine Graben. The fluid expulsion started along these extensional faults during the further Upper Eocene–Lower Oligocene rifting phase. Later, the thin-skinned deformation of the Jura Belt affected the Mont Terri region in the form of the Main Fault, probably between approximately 9 and 4 Ma on the basis of preliminary K–Ar ages of nanometer-sized authigenic illite crystals recovered from gouge samples.