The study is based on core samples from the recently drilled, 719 m deep borehole at Oftringen (near Olten), located in the northwestern Molasse basin, 1.5 km from the frontal thrust of the Folded Jura (Waber, 2008). Veins of calcite (±celestite, pyrite) occur in the whole Malm sequence (up to 8 veins/m), including the more clay-rich Effingen Member (marl and argillaceous limestone). Such an intensity of veining in the Effingen Member has never been found to date in other deep boreholes located in the Molasse basin. Most of the veins are related to tectonic activity, but clay-filled karst structures are recognized in the overlying Geissberg Member limestone, and a few structures probably related to diagenetic processes are documented in the Effingen Member. Fluid inclusions show average salinities between 3.3 and 4.4 wt% eq. NaCl in vein celestite and 2.7 wt% eq. NaCl in vein calcite. Average homogenization temperatures in calcite fluctuate between 56 and 68 °C, with a broad increase with depth and no correlation with salinity. Malm whole-rock carbonates have δ18O values fluctuating within a narrow range, probably determined by equilibrium with seawater (Fig. 1A). Their 87Sr/86Sr ratios follow a well-defined depth profile with minimum values in the middle part of the Effingen Member, fitting with Oxfordian seawater (McArthur et al., 2001; Fig. 1B). No correlation is observed between 87Sr/86Sr and clay content, and values higher than contemporary seawater might be related to the incorporation of radiogenic detrital carbonate. The δ18O values of vein calcite are systematically lower than the corresponding whole rock carbonate (Fig. 1A), consistent with precipitation from seawater at 50 - 70 °C (homogenization temperatures of fluid inclusions). The δ34S and δ18O values of vein celestite follow a bacterial reduction trend pointing to Miocene seawater sulfate. Two vein pyrites gave negative δ34S values consistent with bacterial sulfate reduction. Calcite and celestite of diagenetic origin have 87Sr/86Sr ratios that are indistinguishable from the corresponding whole rock carbonate fraction. In contrast, the 87Sr/86Sr ratios of epigenetic vein calcite and celestite show a systematic enrichment in radiogenic Sr compared to the corresponding whole-rock carbonate and require an external Sr source. Only Burdigalian seawater, at the time of Upper Marine Molasse (OMM) deposition, had an 87Sr/86Sr ratio high enough to explain the highest value obtained (Fig. 1B). The rocks of the Malm-Dogger sequence were not pervasively affected by fluids post-dating burial diagenesis, and the influence of such fluids was restricted to open structures. The Molasse basin subsided in the Burdigalian (Kuhlemann and Kempf, 2002; Mazurek et al., 2006), and the veins might record tectonic activity related to this process. Calcite and celestite precipitated from descending seawater due to heating to 50-70°C, while precipitation of pyrite resulted from bacterial reduction of part of the seawater sulfate. This study shows that, in a specific geotectonic situation, a limestone-marl formation like the Effingen Member can behave transiently as an open system for external fluids (through fractures). At Oftringen, the precipitation of vein minerals led to the sealing of the permeable structures through which the paleo-fluids circulated, and the current hydraulic conductivity below the Geissberg Member is 1E-11 m/s or less (Fisch et al. 2008). References: Fisch, H. R., Rösli, U., Reinhardt, S., Yeatman, B., Senger, R. and Dale, T. 2008: Oftringen borehole - Hydraulic packer testing. Nagra Arbeitsbericht NAB 08-15, Nagra, Wettingen, Switzerland. Kuhlemann J., & Kempf, O. 2002: Post-Eocene evolution of the North Alpine Foreland Basin and its response to Alpine tectonics. Sed. Geol. 152, 45-78. McArthur, J.M., Howarth, R.J., & Bailey, T.R. 2001: Strontium isotope stratigraphy: LOWESS Version 3: Best fit to the marine Sr-isotope curve 0-509 Ma and accompanying look-up table for deriving numerical age. J. Geol. 109, 155-170. Mazurek, M., Hurford, A. J. & Leu, W. 2006: Unravelling the multi-stage burial history of the Swiss Molasse Basin: Integration of apatite fission track, vitrinite reflectance and biomarker isomerisation analysis. Basin Res. 18, 27-50. Waber, N., ed. 2008: Borehole Oftringen: Mineralogy, Porosimetry, Geochemistry, Pore Water Chemistry. NAGRA Arbeitsbericht NAB-08-18, Nagra, Wettingen, Switzerland.