The Bänkerjoch Formation («Gipskeuper») is a thick aquitard of regional extent (Fig. 4‑10) between the Keuper aquifer and the regional Muschelkalk aquifer (Fig. 4‑84). It consists mainly of anhydrite (which at shallow levels may be hydrated to gypsum), clay minerals and dolomite (Section 4.2.3). Where the Keuper aquifer is not active, it is part of the low-permeability confining units below the Opalinus Clay.
At the relevant depths, the hydraulic conductivities are very low: tests on drill cores showed values of < 10-11 m/s and a packer test in the BEN borehole indicated 1.1 × 10-13 m/s (Klemenz et al. 2000). The anhydrite-rich sections have good self-sealing properties because of the volume expansion caused by reaction with groundwater resulting in the hydration to gypsum (T < ca. 42 °C). The profiles of the natural tracers indicate diffusion as the dominant transport mechanism between the Keuper and the Muschelkalk aquifer (Section 4.6). Particularly steep concentration gradients are mostly observed in the lowermost Bänkerjoch Formation related to the occurrence of massive anhydrite units which are an efficient transport barrier. In this subunit, porosity trends to zero if the anhydrite content exceeds ca. 60 to 80 wt.-% (Fig. 5‑8; Mazurek et al. 2023a).
The interpretation of 3D seismic data provides indications for dissolution processes in the Bänkerjoch and / or the Schinznach Formation probably related to subaerial exposure (Nagra 2024a, 2024b, 2024c). Under current conditions, the development of gypsum karst is restricted to the outcrop zone (Plum et al. 2008, Ufrecht 2017).