Bulk rock geochemical and mineralogical characterisation of clay-rich sediments from drill core analysis of the Staffelegg Formation

The long-term integrity of argillaceous formations as host rocks for deep geological disposal of nuclear waste depends significantly on hydrogeological factors. A 58 m long borehole was drilled at the Mont Terri Rock Laboratory (Switzerland) from the Opalinus Clay through the entire underlying Staffelegg Formation, which includes two water-bearing sections. Groundwater was found locally in the Beggingen and Rietheim Members, otherwise known as Gryphaea Limestone and Posidonia Shale, respectively. We want to identify indicators to determine the impact of groundwater flow in clay- and carbonate-rich sediments by characterising mineralogical and geochemical alteration. The groundwater influence is restricted to the water-bearing zones and can be identified by fractures and calcite veins. Additional indicators like mineral assemblage, rock geochemistry and the combination of different redox proxies, show constrained groundwater influence in the clay-rich areas. To investigate in particular the water-bearing horizons on a smaller scale (mm- μ m) would help to detect further alteration and mobilisation features. • Fracturing and calcite veins show groundwater flow in Rietheim and Beggingen Members. • Chemical buffer capacity and sealing prevent alteration in adjacent clay-rich members. • Redox proxies to assess disturbances of depositional and diagenetic fingerprints.