The geological framework of Northern Switzerland is largely controlled by the Pre-Mesozoic basement and the Variscan orogeny. The Pre-Mesozoic basement is composed of crystalline basement and overlying Permo-Carboniferous sedimentary troughs (Fig. 3‑3).
The crystalline basement is composed of polymetamorphosed, pre-Variscan sedimentary rocks and intrusive granites preserved as orthogneisses (Thury et al. 1994). Variscan metamorphism was accompanied by anatexis and intrusion of granite bodies and numerous dykes. Crystalline rocks from the Black Forest outcrops as well as from boreholes in Northern Switzerland show evidence for substantial post-metamorphic deformation and polycyclic hydrothermal alteration (Mazurek & Peters 1992).
The Variscan orogeny overprints the crystalline basement and was the result of the Late Paleozoic collision of Gondwana in the south and Laurussia in the north (Lorenz & Nicholls 1984, Stampfli & Borel 2002, Raumer et al. 2009). The main shortening phase of the Variscan orogeny occurred during the Carboniferous by strike-slip-dominated deformation, which produced NNE-SSW-trending faults (Rhenish orientation; Thury et al. 1994, Schori 2021). In addition, NW-SE-trending dextral shear zones formed (Hercynian orientation; e.g. Schori 2021 and references therein). Inherited NW-SE-trending faults also acted as zones of weakness during the later (i.e. Middle Miocene) subsidence of the Hegau – Bodensee Graben (e.g. Egli et al. 2017).
During into two phases. An initial strike-slip-dominated phase occurred after the Late Carboniferous and Early Permian, post-Variscan orogenic collapse gave rise to wrench tectonics and the formation of the Permo-Carboniferous Konstanz – Frick Trough in Northern Switzerland (Fig. 3‑4, Fig. 3‑6a; Diebold et al. 1991, Marchant et al. 2005, Naef & Madritsch 2014, Schori 2021). The kinematic evolution of the trough can be subdivided 334 Ma, recorded for instance by the offset of the Albtal granite (Madritsch et al. 2018). This was followed by a main subsidence phase with normal faulting localised along inherited structures (e.g. Vorwald Fault). With the formation of ENE-WSW-trending faults (i.e. Erzgebirgian trend) related to the graben formation, three fault orientations occur within the Paleozoic basement of Northern Switzerland and acted as zones of weakness localising future deformation (NW-SE-trending Hercynian, ENE-WSW-trending Erzgebirgian, NNW-SSE-trending Rhenish).
The central Konstanz – Frick Trough preserves a sedimentary succession reaching up to several km in thickness (Fig. 3‑2; Naef & Madritsch 2014, Madritsch et al. 2018), but significantly thinner on its shoulders. Based on boreholes drilled between the Upper Rhine Graben and the Bodensee area, a multiphase depositional history can be outlined. However, the detailed sedimentary succession remains poorly defined and is laterally variable. Volcanic activity also occurred during Permo-Carboniferous times, resulting in deposition of tuffites (Matter et al. 1988). Based mainly on the findings in the Weiach (WEI) borehole, three depositional phases were differentiated in the sedimentary succession (Matter et al. 1987, 1988, Diebold 1988, Madritsch et al. 2013):
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Late Carboniferous times saw varying humid tropical conditions and deposition from anastomosing fluvial systems on externally drained fan complexes in the Konstanz – Frick Trough («Pre-Weitenau Formation deposits»). Fine- to coarse-grained sediments were deposited in fluvial channels, overbanks and floodplains. Organic-rich sediments were deposited in mires or shallow bog lakes on floodplains (later transformed into a few coal beds).
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Late Carboniferous to Early Permian deposition («Pre-Weitenau Formation deposits») was characterised by anastomosing fluvial and lacustrine systems in a varying humid climate. Towards the end of this period, the relief surrounding the Konstanz – Frick Trough was generally flattened, leading to reduced sediment input.
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During the Early to Middle Permian (Weitenau Formation; Fig. 3‑5a), the relief and erosion at the margin of the Konstanz – Frick Trough increased (Fig. 3‑6a) and vast alluvial plains developed with the deposition of locally reddish siltstone, fine- to coarse-grained sandstone and fanglomeratic breccias in an arid climate. Towards the end of the Permian, the landscape was generally flattened, forming a peneplain.