The Quaternary landscape evolution is driven mostly by tectonics, climate, and river drainage reorganisations in Northern Switzerland (Nagra 2024k). The tectonic setting is determined by ongoing Alpine orogeny, transtensional deformation within the Upper Rhine Graben, tectonic activity of the Jura Fold-and-Thrust Belt and the Hegau – Bodensee Graben. The dominant Alpine-related uplift produced a general uplift gradient oriented perpendicularly to the Alpine front with rates that decline towards the NNW (Nagra 2024l).
The Quaternary climate is globally characterised by a series of glacial and interglacial periods. These glacial/interglacial cycles are mainly driven by oscillations in the Earth’s orbit and axial tilt, which affect the distribution of solar radiation. Cooling and warming periods have been divided into marine isotope stages (MIS) based on isotope data derived from deep-sea core samples (Lisiecki & Raymo 2005). Alpine glaciers advanced and retreated dynamically in concert with the global climate cycles and largely depended on local Alpine temperature and humidity. During pronounced global glacial periods, the Alpine glaciers reached the Alpine Foreland (Fig. 3‑5g, Fig. 3‑11; Nagra 2024j). During the so-called Mid-Pleistocene Transition (ca. 1.25 – 0.7 Ma; Clark et al. 2006), the principal glacial/interglacial cycle changed from a dominant 41‑kyr to a 100-kyr period (Fig. 3‑11). The cause for this transition remains debated, potential candidates include ice-dynamic, oceanic and atmospheric feedbacks (Berends et al. 2021). Large glaciations have reached the foreland since the Early Pleistocene (Schlüchter et al. 2021) but probably had their largest erosion potential only within the last one million years, when cyclicity changed to a high-amplitude 100-kyr glacial/interglacial cycle (e.g. Preusser et al. 2011). At that time, glaciers eroded into the bedrock below the fluvial baselevel, creating major overdeepened valleys in the Swiss Plateau Molasse.
During the Quaternary, the Rhine River system evolved in several phases of drainage reorganisation that resulted in significant drops in the local baselevel (Fig. 3‑11). The drainage divide between the Rhine and the Danube system continued to shift eastwards because of the headward erosion of the Rhine River and its precursors. This shift was probably facilitated by glacial erosion and deposition. Since late Early Pleistocene to Middle Pleistocene, the Alpenrhein River no longer drains northwards into the Danube River, but towards the Hochrhein River (Ziegler & Fraefel 2009, Heuberger et al. 2014).
In the Early Pleistocene (2.6 – 0.77 Ma), the Alpine glaciers advanced into the Alpine Foreland for the first time. Rare findings of ice-contact sediments indicate that some Early Pleistocene glaciers advanced to similar extents to that reached, for instance, during the Last Glacial Maximum (LGM; Fig. 3‑5g). Downstream of these glaciers, extensive plains with fluvioglacial gravels were aggraded in wide channel systems (Graf 1993). Fluvial incision resulted in the development of gravel terraces, leaving behind flat-topped hills in the landscape, the so called Höhere and Tiefere Deckenschotter ("Higher and Lower Cover Gravels"; Fig. 3‑11). In this progressively incising fluvial system, older gravel terraces are generally preserved at higher elevation, while younger terraces are at lower elevation (Burbank & Anderson 2012). Many of the Deckenschotter terrace bodies are further characterised by a composite internal structure (including cut-and-fill sequences) interpreted as climatically controlled depositional–erosional phases (Graf 1993). Continued fluvial baselevel lowering during the Early Pleistocene into the Middle Pleistocene caused incision into accumulated gravels and, more importantly, into bedrock (Kuhlemann et al. 2013, Dieleman et al. 2022a, Thew et al. 2024).
During the Middle Pleistocene (0.77 Ma – 130 ka), global glacial/interglacial cycles (now dominated by the 100-kyr periodicity) continued to cause expansion and retreat of Alpine glaciers including the most extensive glaciation (Fig. 3‑11). During this period, large volumes of fluvioglacial gravel were deposited in the forefield of the glaciers and the downstream valleys, together forming the Hochterrasse ("High Terrace"). Interglacial sediments are rarely preserved.
The most extensive glaciation of the Swiss Alpine Foreland is represented by the Möhlin Glacial (Graf 2009b; Fig. 3‑11), which is tentatively assigned to MIS 16 to MIS 12 (676 – 424 ka; Preusser et al. 2021, Dieleman et al. 2022b). It left behind extended overdeepened troughs in the Alpine Foreland (Buechi et al. 2024). The subsequent glaciations during the Habsburg Glacial (ca. MIS 10, 374 – 337 ka) had a smaller extent, but also eroded overdeepened troughs. Fewer constraints exist for the glaciations of the Hagenholz Glacial (ca. MIS 8, 300 – 243 ka), that had an even smaller extent and left comparatively minor traces in the landscape (Graf 2009b). The main glaciations of the following Beringen Glacial (MIS 6, 191 – 130 ka; Lowick et al. 2015) were again extensive and eroded new and reactivated existing overdeepened troughs.
During the Birrfeld Glacial (corresponding to the Late Pleistocene after the Eemian, 115 – 12 ka), several periods of glacier advance and retreat are well documented in Northern Switzerland, forming the Niederterrasse ("Low Terrace"). An early and limited glacier advance into the foreland occurred during MIS 4 (71 – 57 ka). The Rhine glacier reached its maximum extent (LGM) between ca. 26 and 22 ka ago (Gaar et al. 2019, Kamleitner et al. 2023; Fig. 3‑11). The LGM ice extent is reduced compared to the maximum extents of the Beringen Glacial. It did not reach JO, only reached into the southeastern part of NL but covered ZNO completely.
Probably as a result of gravel accumulation in front of the small Black Forest ice cap, the Wutach River formerly draining to the Danube was diverted after ca. 18 ka towards the Rhine system, triggering a rapid incision of "up to 150 m" within only 6 kyr and producing a deep gorge (Einsele & Ricken 1995, Hebestreit 1995). This incision has potentially impacted the recharge into the Muschelkalk aquifer (Section 4.5.5.5).
With the disappearance of the glaciers in the foreland around 16 – 17 ka ago, fluvial, lacustrine and hillslope processes dominated on a pre-sculptured, locally glacially overdeepened landscape (12 ka – recent). The Rhine catchment is still growing at the expense of the Danube River (Winterberg & Willett 2019).
Total bedrock incision since deposition of the Deckenschotter reaches ~ 180 m (since HDS deposition) and ~ 100 m (since TDS deposition) at the location of the Aare confluence (Fig. 3‑11). Incision values in the realm of the siting regions generally decrease from east to west and from south to north (Fig. 6‑6c and d) with values generally decreasing from east to west and from north to south (Nagra 2024k) with regionally averaged incision between ~ 200 – 300 m since HDS and ~ 130 – 200m since TDS deposition, respectively. The incision probably occurred stepwise, with the highest values achieved as a response to the Mid-Pleistocene Transition (ca. 1.25 – 0.7 Ma). The reason for this incision is probably a combination of (1) a colder climate with frequent extensive glaciations reaching the Alpine Foreland, (2) drainage reorganisations and (3) rock uplift.
Quaternary incision and the related eastward enlargement of the Rhine catchment with the new drainage of the Alpenrhein from the Bodensee area towards the west (Hochrhein) probably shifted the discharge areas for the Malm aquifer north of the ZNO and NL siting regions in an east- to southeastward direction. This may have influenced the groundwater circulation to the north of the siting regions and potentially also in the northwestern part of ZNO.
The glaciations of the Pleistocene led to locally increased subglacial recharge; for example, numerous groundwater samples with cold climate signatures (Section 4.5.5.2) from the Bodensee area have been preserved in the Malm aquifer and in the Molasse (Fig. 3‑3). Here, the ice cover caused a temporary reversal of the hydraulic gradient, whereby water flowed across the USM into the Malm aquifer (see Section 4.5.5.2 or Waber & Traber 2022). Many of the deep Molasse groundwater samples represent a mixture between a Pleistocene cold-climate water and an OMM marine component.