Continued uplift of the Upper Rhine Graben rift shoulders (i.e. Black Forest and Vosges Massifs) in the Miocene caused local extensional reactivation of inherited structures. Furthermore, it resulted in tilting of the pre-Mesozoic basement towards the south, which was further enhanced by the flexural lithospheric bending and folding (e.g. Laubscher 1992, Dèzes et al. 2004, Berger et al. 2005). As a consequence, the Mesozoic sedimentary cover of the Black Forest area was emergent and eroding. The eroded material was deposited in conglomeratic units containing Mesozoic detritus in parts of Northern Switzerland and adjacent Germany (Jura Nagelfluh). As a result of the uplift and erosion in the Black Forest area, meteoric waters could directly infiltrate into increasingly deep aquifers.
At the beginning of the Miocene, deposition of USM continued with interruptions. In Northern Switzerland, USM, dominated by Alpine material, is found up to a line from Brugg via Schaffhausen into the Hegau (Fig. 3‑2), where it partly interfingers with conglomerates of the Malm Group limestone from the Black Forest area («Ältere Juranagelfluh»).
In the Early Miocene (since ca. 21 Ma), subsidence led to transgression of the sea from the Rhone Valley. Eventually, it formed the narrow, around 100-km-wide Burdigalian seaway connecting the Mediterranean in the southwest (Lyon) with the Paratethys in the northeast (Vienna). The Upper Marine Molasse Group (OMM) was deposited in a shallow marine coastal environment. It comprises glauconitic sandstone and marl with scattered units of fossiliferous conglomerate («Quarzit Nagelfluh», «Austernnagelfluh»), some containing pebbles of Jurassic origin («Mittlere Juranagelfluh»; Jost et al. 2016). The OMM is characterised by facies changes over small distances and several erosional unconformities. In the latest Early Miocene (after ca. 17 Ma), the sea had largely retreated, and a river system formed running SW across the Hegau and Schaffhausen (Fig. 3‑2). This locally incised up to 100 m deep into the older Molasse deposits. At this time, extensive quartz-rich sandstone was deposited in the so-called "Graupensandrinne" in a brackish depositional environment (Bolliger 1999). Overall, the OMM is commonly missing in the JO siting region, presumably because this area was located outside or near the northern coastline of OMM and affected by later erosion. Relicts of OMM can also be found in the eastern part of JO (Diebold et al. 2006).
The marine to brackish conditions during the Upper Marine Molasse Group (OMM) deposition were the latest marine phase in Northern Switzerland. Remnants of these marine waters can be sampled in deep boreholes from the Molasse (OMM, USM) and from the Malm aquifer (Section 4.5.5.2). Additionally, calcite veins in the Upper Jurassic Effingen Member in the Oftringen (OFT) borehole are related to the infiltration of OMM groundwater (de Haller et al. 2011, Mazurek et al. 2018).
From the beginning of the Middle Miocene, continental conditions prevailed again throughout the Alpine Foreland in Northern Switzerland and the terrestrial deposits of the Upper Freshwater Molasse Group (OSM) were deposited. Large debris fans built up at the Alpine front, flowing N and NW into a broad southwest-flowing stream system, the "Glimmersandrinne" (Zaugg & Vogel 2014). The NL and ZNO siting regions are situated in a distal setting of these debris fans ("Hörnli-Schuttfächer"), which are dominated by marl, silt- and sandstone. In JO, fluvial conglomeratic layers («Jüngere Juranagelfluh») with pebbles of Malm and Dogger Groups from the southern Black Forest were also deposited. These are intercalated with marly, silty and sandy sedimentary rocks (Diebold et al. 2006).
Activity of the Hegau – Bodensee Graben located northeast of the study area is dated to Early to Middle Miocene based on correlation of OMM sedimentary rocks across the Randen Fault (Schreiner 1992, Hofmann et al. 2000, Egli et al. 2017, Ibele 2015). The main tectonic activity in the Hegau – Bodensee Graben occurred between the Late Miocene and the Pliocene in a transtensional manner (e.g. Ibele 2015, Egli et al. 2017). Volcanic activity dated to 19 to 6.9 Ma accompanied the early extension-dominated phase of the Hegau – Bodensee Graben (Ibele 2015 and references therein).
Early to Middle Miocene (20 to 13 Ma) thrusting of the External Crystalline Massifs over the sedimentary cover resulted in their exhumation (Herwegh et al. 2017, 2020). It is a likely cause for the NW propagation of the Alpine thrust front along the Triassic evaporites, which are generally considered as décollement horizons (Fig. 3‑6d; Marro et al. 2023, Burkhard 1990, Sommaruga et al. 2017, Jordan & Nüesch 1989). The front of the detached Mesozoic strata strikes north of the JO and NL siting regions and runs through the southern part of ZNO. Thermochronological data from the Schafisheim (SHA) borehole located south of the Jura Main Thrust show that tectonic activity along the basal décollement is constrained to have started during the Middle Miocene (Fig. 3‑6d; 14.3 Ma; Looser et al. 2021). Similar age ranges were documented for the western Jura (Smeraglia et al. 2021). No evidence for shortening related to the Alps in the distal foreland is known east of Zürich; the shortening was most probably accommodated within the Subalpine Molasse (Burkhard 1990, von Hagke et al. 2012, Ortner et al. 2024).
The combination of age dating and 2D geometric balancing of selected cross-sections indicated maximum shortening rates during the main thin-skinned Jura Fold-and-Thrust Belt formation of around 0.5 mm/yr (for the time period 14.3 to 4.5 Ma, i.e. linked to the décollement activity in the SHA borehole) for the western part of JO (Jordan et al. 2015, Madritsch et al. 2024).
The youngest preserved Molasse sedimentary rocks in Northern Switzerland are approximately 11.5 Ma old (Rahn & Selbekk 2007). However, sedimentation continued within the Swiss Molasse Basin that evolved from a flexural foreland basin to a wedge-top basin as a result of the formation of the Jura Fold-and-Thrust Belt (e.g. Cederbom et al. 2004, Schlunegger & Mosar 2011, Schlunegger & Kissling 2015, Willett & Schlunegger 2010). Deposition of the Cenozoic sedimentary rocks resulted in a second burial of the Mesozoic strata (Mazurek et al. 2006), whereby maximum temperatures decreased with increasing distance from the Alpine front, reflecting the wedge geometry of the Molasse deposits. For the distal margin of the Molasse, thermal history modelling based on vitrinite reflectance and thermochronological data suggests that, before the onset of Miocene erosion, the Molasse thickness in the Weiach – Benken area was approximately 1 km greater than today (e.g. Mazurek et al. 2006).