Key points:

• The Opalinus Clay host rock shows thicknesses of about 100 to 120 m in the three siting regions. Lithological characteristics are similar due to a common, relatively homogeneous marine depositional environment characterised by clay-mineral-rich input and high sedimentation rates.
• The units directly below and above the Opalinus Clay are, in general, also characterised by high clay-mineral contents, but they show more variable thickness and facies. This reflects locally more varying depositional environments.
• While the marine Staffelegg Formation directly below the host rock is similar between the siting regions, some prominent differences occur in the underlying partly shallow marine to mostly terrestrial Klettgau Formation.
• The marine deposits of the Dogger Group above Opalinus Clay are characterised by lateral thickness and facies changes. The deposits include decametre-thick clay-mineral-rich units as well as intercalated sandy-calcareous beds, condensed horizons and carbonate build-ups reflecting changes in sediment input, water depth and ocean currents.