Hypersaline – Meteoric Waters; Elucidating the Complex Dolomitization History of the Upper Muschelkalk, Switzerland

Dolomitized zones of the middle-Triassic Upper Muschelkalk of Switzerland are of interest for their aquifer properties in the context of fresh water, geothermal energy and CO2-sequestration resources. Our studies of drill core... [ view full abstract ]

Dolomitized zones of the middle-Triassic Upper Muschelkalk of Switzerland are of interest for their aquifer properties in the context of fresh water, geothermal energy and CO2-sequestration resources. Our studies of drill core from 10 regionally distributed wells in N. Switzerland reveal an extensive matrix dolomitization trend, from complete dolomitization near Triassic paleo-shorelines to less dolomitization towards the centre of the paleo-seaway. The same trend is recognizable in literature data throughout ~100 000 km2 of the German Basin, the setting of a large epeiric sea during the Anisian–Ladinian.
The strontium isotopes of the dolomites range from 0.70844 to 0.7117, significantly higher than contemporaneous seawater 87Sr/86Sr (0.70775), with low Sr concentrations from 30 to 80 µg/g and Fe concentrations of ~400 µg/g Fe. Values of δ18OPDB span from -6.56 to -1.05‰, δ13CPDB exhibits a narrow range of -0.04 to 1.21‰ and Mg/Ca ratios are mostly stoichiometric. These geochemical signals display consistent trends within each well: 87Sr/86Sr ratios, δ13C and dolomite disorder increase with depth, whereas Sr concentrations and δ18O decrease with depth. Furthermore, average δ18O increases with increasing distance from the paleo-shoreline, which correlates with a decrease in the average 87Sr/86Sr ratio.
These values and correlations with proximity to the paleo-shoreline suggest that the matrix dolomite precipitated from an influx of meteoric or groundwaters from the Vindelician High in the East, mixing with hypersaline refluxing brines from the overlying Gispkeuper.
Following recrystallization of dolomite I, extensive thin layers of porous sucrosic dolomite formed both within the matrix dolomite and the underlying Hauptmuschelkalk limestone. These later dolomites have higher Fe concentrations (x = 1200 µg/g), a narrow range of δ18OPDB (-7.85 to -5.93‰), and slightly elevated δ13CPDB values (0.99 to 2.03‰) compared to matrix dolomites. Strontium isotope analysis is underway and will shed light on the source of these dolomitizing fluids.
Whereas previous petrographic and C–O stable isotope studies have attributed dolomitization in the Upper Muschelkalk to penecontemporaneous (Anisian–Ladinian) processes, our combined petrographic, geochemical and isotopic analyses of numerous boreholes show that dolomitization occurred via infiltration of hypersaline brines from the overlying Keuper evaporites mixing with near-shore brines with a significant meteoric component. Keuper paleogeography therefore exerted a strong control on today's aquifer properties of the Upper Muschelkalk.