In the Central Alps, emplacement of the Penninic nappes occurred in the Early Eocene, whereas the Helvetic domain was affected by N- to NNE-directed thrusting from the Late Eocene onwards (Pfiffner 2009). In the Late Eocene to Early Oligocene, the opening of the Upper Rhine Graben, part of the European Cenozoic Rift System (ECRIS), was accompanied by subsidence and associated with uplift of the graben shoulders (Rotstein & Schaming 2011). This extensional system extended from the Bohemian Massif to the Mediterranean Sea (Bourgeois et al. 2007). The development of the Upper Rhine Graben can be subdivided into an early extensional phase which was followed by a transtensional phase. The transtensional phase is characterised by a NW‑SE-oriented maximum horizontal stress axis and is thus comparable to the present day. The closure of the Piemont – Liguria and Valais Oceans that initiated in the Late Cretaceous was completed in the Eocene, marking the onset of continental collision (Handy et al. 2010). Lithospheric folding resulting from the continental collision is proposed to result in surface uplift of the North Alpine Foreland (Fig. 3‑4; Dèzes et al. 2004, Bourgeois et al. 2007).

During the Eocene (Fig. 3‑5e), a humid and warm (sub-) tropical climate formed an extensive karst landscape on the exposed Mesozoic bedrock. The residual material of the strong chemical weathering, consisting of mostly clay minerals, quartz sand, and ironhydroxide pisoids, was accumulated in palaeokarst depressions and infills (Siderolithic Group). The surface topography was probably hills and valleys partly covered by tropical forest with a species-rich fauna including primates and crocodiles (Rosselet 1991).

The change to erosive conditions between the Late Cretaceous and Early Eocene preluded a ca. 50-Myr-long period of recharge of meteoric waters in Northern Switzerland. This caused karstification extending several decametres into the Malm Group (filled palaeokarst). Over the long timescale, the freshwater conditions in the Malm aquifer caused an overprinting of the initial marine porewaters in the Opalinus Clay and the confining units (Section 4.6.5).