The Oligocene represents the time of major deformation in the Helvetic domain of the Central Alps with the emplacement of the Helvetic nappes (Pfiffner 2009). The development of the Alpine Foreland basin can be explained by flexural bending of an elastic lithospheric plate as a response to crustal loading resulting from internal build-up of the Alps (e.g. Willett & Schlunegger 2010) or slab roll-back and associated buoyancy-driven uplift (Schlunegger & Kissling 2015).

During the Early Oligocene, Northern Switzerland was still characterised by a karst landscape. Locally, nodular and crustal limestone formed under arid to semiarid conditions («Ältere Krustenkalke»; Hofmann et al. 2000). At this time, the emerging Alps led to an increased sedi­ment supply and deposition in the northern Alpine foredeep, firstly as deep marine North-Helvetic Flysch Group and then as shallow marine to littoral sediments of the Lower Marine Molasse Group (UMM; e.g. Kuhlemann & Kempf 2002).

During the Late Oligocene (Fig. 3‑5f), the increasing crustal loading or slab dynamics resulted in foredeep migration and migration of Molasse deposition from southeast to northwest (Allen et al. 1991, Sinclair & Allen 1992, Kuhlemann & Kempf 2002, Ford et al. 2006, Schlunegger & Kissling 2015). The adjacent forebulge also migrated northwards and was associated with erosion and normal faulting (e.g. Laubscher 1985, Jordan et al. 2015).

The former marine Alpine Foreland basin was progressively infilled, passing into terrestrial fluvial systems. The Lower Freshwater Molasse Group (USM) was deposited on large debris fan systems, which drained towards northeast to east. Deposition from Alpine River systems migrated over time towards the northwest. In a NNW-SSE cross-section, a wedge-shaped sedimentary body developed with coarse clastic debris fan series with conglomerates («Nagelfluh») in the near-Alpine, proximal area and silty-sandy-marly sediments of rivers and floodplains in the distal area (Schlunegger et al. 1997). No deposits of USM occur in the JO siting region (eroded or not deposited), except for the southeastern-most part near Villnachern. USM is preserved in distal facies in the NL and ZNO siting regions.