The Opalinus Clay exhibits similar characteristics in all the siting regions, with high clay-mineral content and relatively small vertical and lateral variability. This can be attributed to deposition in an epicontinental marine basin characterised by clay-mineral-rich input, generally high sedimentation rates and a relatively homogeneous environment. The thickness of the Opalinus Clay is similar in all three siting regions (104 – 122 m). In NL, the Opalinus Clay is slightly thinner compared to JO and ZNO, but the layers directly above the Opalinus Clay have a higher clay-mineral content compared to the other siting regions (> 35% over large sections). In the eastern part of NL, the clay-mineral-rich zone including Opalinus Clay and these overlying rocks reaches thicknesses > 130 m.
In all the siting regions, the lower and upper confining units of the Opalinus Clay have an overall high clay-mineral content but are more variable (e.g. intercalated sandy, calcareous, or dolomitic interlayers).
Underlying the Opalinus Clay, the Staffelegg Formation is dominated by strata rich in clay minerals, including more calcareous intercalations. These units reflect marine conditions with changing depositional environments. The lateral extent of the individual units is large, therefore the differences between the siting regions and the variability within the siting regions are small in this formation.
The stratigraphic succession further below (Klettgau Formation) is more variable and preserves intercalated terrestrial and marine sedimentary rocks. The stratigraphically uppermost, potentially water-conductive unit is the Seebi Member, which is thickest and most porous in ZNO (sandstone and dolomitic breccia), thinner and less porous in NL (mainly dolostone) and absent in JO. The next prominent unit with low clay-mineral content is the Gansingen Member. It is thickest, dolomite-rich and most porous in JO and increasingly thinner and more anhydritic towards the east. Sandy intercalations lower in the succession (Ergolz Member) were deposited in fluvial channels in a floodplain environment. These facies are local, difficult to predict, and can be present in all the siting regions.
Further down-section, above the regional Muschelkalk aquifer (Schinznach Formation), the Bänkerjoch Formation is represented by an evaporitic to clay-mineral-rich succession, mainly deposited in a sabkha environment.
The successions above the Opalinus Clay differ between the three siting regions. In JO and ZNO, the generally clay-mineral-rich units following the Opalinus Clay contain numerous calcareous and sandy beds. In NL, the decametre-thick interval above the Opalinus Clay exhibits a higher clay-mineral content compared to JO and ZNO. In the eastern part of NL, this clay-mineral-rich succession is interpreted to result from a depositional positive relief formed by marine bottom currents during the Bajocian. This topographically high area favoured the growth of an isolated carbonate platform several square kilometres in size («Herrenwis Unit»). Up-section, clay-mineral-rich deposits dominate in NL and ZNO with thickness variations reflecting the filling of the basin around the carbonate platform. As a result of this depositional succession, the «Herrenwis Unit» represents an isolated body embedded in clay-rich rocks.
In contrast, in JO carbonate deposition is represented by strata in the upper part of the Dogger Group (Hauptrogenstein). The easternmost part of the carbonate platform and its margin are recognised in the JO siting region (platform in the west, basin towards the east).
The uppermost unit (Wildegg Formation) below the regional Malm aquifer is represented by successions of limestone and calcareous marl with a pronounced reduction in thickness from west to east in the study area.
