Isotope investigations combined with U-Pb geochronology show that, during the Eocene – Oligocene, downward migration of meteoric surface waters affected the Malm Group – Dogger Group above Opalinus Clay interval. At the clay pit of Frick, similar negative δ¹⁸O values are observed for calcite-forming fluids in the Lias Group below the Opalinus Clay. Neither the origin nor the process(es) by which these signals were transported in the vertical dimension are fully understood. In any case, evidence from U-Pb dating, the general geodynamic evolution and structural mapping suggest that the fluid evolution at Frick is not necessarily representative for the siting regions further east.

Downward migration of Burdigalian seawater during deposition of the Upper Marine Molasse Group is evidenced for the top 100 – 200 m of the Malm section at the Oftringen (OFT) and NL boreholes. In contrast, at JO, mineral-forming fluids from similar depth and age do not show any seawater signals. Most likely, such regional differences reflect the location of the palaeo-shoreline during OMM deposition. Müller et al. (2002) show that the transgression of the Burdigalian sea covered both NL and ZNO, whereas JO was largely unaffected (Section 3.4.4). Moreover, during late Burdigalian and Langhian times an erosive phase affected the JO area, eroding parts of the older Molasse deposits and in places cutting channels into Mesozoic units (Diebold et al. 2006). It is thus likely that possible marine porewaters would have been rapidly displaced by meteoric waters.

For one of the youngest Pleistocene structures in the Malm Group, a hydraulic connection with the groundwater in the Malm aquifer (i.e. as sampled at present) is evidenced. Minimum transport distances estimated from the isotope profiles are in the range of 80 – 100 m. Advection was the likely transport mechanism, however, transient fluid fluxes must have been short-lived, affecting only the porewater in close proximity to the water-conducting structures, otherwise anomalies in the porewater isotope profiles would be expected, which is not the case.

For the time period pre-dating the evolution times of the porewater tracer profiles (Section 4.6), the hydrogeological history of the Opalinus Clay remains poorly constrained, because veins are infrequent, thin and U-Pb geochronology was unsuccessful. In general, a similar evolution as for the confining units is likely, involving modified seawater-type mineral-forming fluids prior to the Eocene, followed by a change towards meteoric waters in the Cenozoic. However, the underlying transport process(es) of this change are not exactly known. In general, δ¹³C and ⁸⁷Sr/⁸⁶Sr values of vein calcite are consistent with an internal evolution of the mineral-forming fluids, however, the isotope signals could also result from an effective rock buffering by which external signals were possibly obliterated. Thus, thin veins in the Opalinus Clay can be explained by external or by internal fluid sources. The underlying mechanisms for the latter include local pressure drops in response to slip induced by tectonic deformation (Diamond 1998) or mixing of chemically different waters (Sanz et al. 2011).

Based on the observed regional cooling at around 5 Ma (Section 4.3.5), clumped isotope temperatures of vein calcite in the Opalinus Clay (Akker et al. 2023) suggest a minimum Pliocene age for these veins, thus documenting for at least this time the efficient barrier properties of the Opalinus Clay. Further constraints on the timing of vein formation in the Opalinus Clay derived from rock-mechanical considerations are presented in Section 5.7.6.