Geochemical constrains on dolomitization pathways of the Upper Jurassic carbonate rocks in the Geneva Basin (Switzerland and France)

Abstract

The Kimmeridgian-Tithonian carbonate rocks of the Geneva Basin represent potential reservoirs for geothermal energy exploitation. Based on petrographic data, a previous study (Makhloufi et al. 2018) reported three different stages of dolomitization affecting these carbonate rocks, followed by dedolomitization. The present study focuses on the geochemical characterization of these stages based on O, C and Sr isotopes of dolomites and dedolomites. The oxygen isotopic values of early dolomite are depleted compared to the values of Late Jurassic marine cements. This is interpreted to reflect dolomite precipitation from an oxygen-enriched fluid, likely evaporitic. Carbon isotopic values are close to the composition of well-preserved Late Jurassic cements suggestive of abiotic precipitation. These findings are consistent with a scenario of reflux type dolomitization induced by high-frequency sea-level changes producing pulses of dolomitizing brines. Late dolomitization represents an advanced level of replacement. Isotopic data exhibits depleted oxygen composition pointing towards burial diagenesis and is interpreted as the results of shallow burial over-dolomitization. Ages provided by radiogenic strontium isotopes data are consistent with an early first stage of dolomitization followed by late burial dolomite. Dedolomitization is observed at different orders of magnitude and might results from the interaction with meteoric water initiating the dissolution of both early and late dolomites. This dedolomitization would have taken place during long-term emersion events or after the exhumation. The results presented in this work provide further understanding of the processes involved in dolomitization under the influence of high-frequency sea-level fluctuations and the evolution of dolomitic fabrics during burial.