Glaciofluvial sequences recording the Birrfeld Glaciation (MIS 5d–2) in the Bern area, Swiss Plateau
Swiss Journal of Geosciences volume 115, Article number: 12 (2022)
In this paper, we document that glaciofluvial gravel sequences and glacial till deposits that are exposed in the Müntschemier and Finsterhennen gravel pits (Swiss Plateau west of Bern) record three glacial advances during the Birrfeld Glaciation, which corresponds to the last glacial cycle. Sedimentological logging shows that both gravel pits expose deposits of glaciofluvial braided river systems. These sediments are overlain by a till that was deposited during the Last Glacial Maximum (LGM). The results of the provenance analysis imply that the sediments were mainly supplied by the Valais Glacier, which originated in the Central Alps. A minor contribution of the material was supplied by the Saane Glacier with sources in the northern parts of the Alps. In addition, the morphometric analysis particularly of quartzite clasts in the till deposits indicate that while some clasts (the angular ones) were eroded and transported by the Valais Glacier from the Central Alps to the depositional site, the majority of the quartzite constituents (the rounded ones) were most likely reworked from the Molasse bedrock or older gravels. This implies that a large fraction of the sediments in the Müntschemier and Finsterhennen gravel pits could represent recycled material from older fluvial gravels and conglomerates that were then reworked by the glaciers as they advanced to the foreland. Based on the sedimentological data and considering published and new optically stimulated luminescence (OSL) chronological data, we propose a landscape evolution scenario where the first glacial advance occurred during Marine Isotope Stage (MIS) 5d. The second glacier advance followed during MIS 4, while the last one during the Last Glacial Maximum (LGM), which corresponds to the MIS 2. The MIS 5d advance is recorded by the lowest unit of the Müntschemier gravel pit and consists of a fining upward sequence made up of an alternation of gravel and sand beds. The MIS 4 advance is recorded by the unit beneath the LGM till at Müntschemier and by the lowermost layer in the Finsterhennen gravel pit. It comprises an alternation of gravel and sand beds, which coarsens and thickens upwards. The LGM advance, finally, resulted in the deposition of amalgamated gravel beds at Finsterhennen, which ended with the construction of a till that is encountered on the top of both gravel pits. Sediments related to the interstadial conditions between MIS 5a and MIS 5b and MIS 3 were not encountered, which suggests that the warmer periods were characterised by non-deposition and/or erosion, which possibly resulted in the observed sedimentary hiatus. Although the chronological results are still preliminary, the available information allows us to suggest that during the Birrfeld Glaciation, the Valais lobe advanced several times to the Swiss Plateau. In addition, the facies associations imply that the eastward expansion of the Valais lobe during the MIS 5d and MIS 4 were most likely shorter than during the LGM.
During the Quaternary period, the Swiss Plateau and its adjacent mountainous regions experienced at least 15 advances of Alpine glaciers (e.g., Monegato & Ravazzi, 2018; Muttoni et al., 2003; Preusser et al., 2011; Schlüchter, 1986, 2004). The history of Quaternary glacial advances into the Swiss Plateau is built on a combination of geomorphological observations, detailed logging of numerous sedimentological sections, and chronologies established by various dating techniques including 14C, surface exposure dating with terrestrial cosmogenic nuclides, and optically stimulated luminescence (OSL) (e.g., Ivy-Ochs et al., 2008; Preusser et al., 2011; Schlüchter, 2004). Among the numerous foreland glaciations, the last glacial cycle, which is locally referred to the Birrfeld Glaciation (Graf & Müller, 1999; Graf, 2009a, b), is most probably the best investigated one. During this glaciation, which spans approximately the last 120 ka, at least three glacial advances occurred in the Swiss Plateau (Ivy-Ochs et al., 2008; Preusser et al., 2007, 2011). The third and most recent one was dated to the time interval between 30 and 17.5 ka including the period between 24 and 21.5 ka when the Alpine glaciers with sources in the central part of this orogen reached their maximum extents (Ivy-Ochs et al., 2008; Preusser, 2004; Schlüchter et al., 2021). This latter glacial advance occurred during the global Last Glacial Maximum (LGM) (Shakun & Carlson, 2010), which has been correlated with the maximum in global ice volume during Marine Isotope Stage (MIS) 2 (Ivy-Ochs et al., 2008; Schlüchter, 2004). The LGM has been extensively investigated in the Alps (e.g., Bini et al., 2009; Graf et al., 2015), thanks to the clear and well preserved glacial landforms and deposits, such as moraines, erratic boulders and glaciofluvial gravels that could be related to the LGM. In contrast, except for a few chronological studies (e.g., Gaar et al., 2019; Preusser, 1999), the history of the previous Alpine glacial advances during the Birrfeld Glaciation has been less explored, mainly because of the poor preservation of the related deposits and the difficulty to access the corresponding stratigraphic archives. Nevertheless, based on lithostratigraphic evidence and the results of luminescence dating, Ivy-Ochs et al. (2008) suggested that two glacial advances occurred during MIS 5d and 4, both of which preceded the LGM.
The gravel pits at Finsterhennen and Müntschemier, which are situated in the Swiss Plateau to the northwest of Bern (FIH and MÜN on Fig. 1), offer potential sites where sedimentary archives preceding the LGM are exposed. In particular, the results of previous chronological investigations suggest that the deposition of the sediments at Finsterhennen most likely occurred in response to two glacial advances during MIS 4 and MIS 2 times (Preusser et al., 2007; Schlüchter, 2004). Therefore, it is possible that further investigations of the Finsterhennen (FIH) and Müntschemier (MÜN) gravel pits will allow us to reconstruct the early advances of the Alpine glaciers onto the Swiss Plateau during the Birrfeld Glaciation, and to compare them with the extent of the LGM glaciation (Fig. 1a). Such information has been missing so far (Preusser et al., (2011), see their Fig. 19), and we expect that the MÜN and FIH sites offer ideal archives to fill this knowledge gap. Accordingly, we logged in detail the stratigraphic sections in both gravel pits. We determined the petrographic and morphometric properties of the clasts and the paleo-flow directions at these sites in an effort to gain information on the provenance of the sediments, the dominant transport mechanisms and the depositional environment. Finally, we determined the depositional ages of a sand layer in the MÜN gravel pit through the application of the infrared stimulated luminescence (IRSL) dating technique on two feldspar samples. These ages complete the chronological record of the Birrfeld Glaciation for the MÜN and FIH ensemble, which provides geologic evidence for a further archive where material of the last glacial period is potentially preserved.
2 Study site
The gravel pits at Müntschemier (MÜN) and Finsterhennen (FIH) are situated in the Swiss Plateau ca. 20 km west of Bern and ca. 15 km southeast of the Jura Mountains (Fig. 1). The region surrounding the pits is characterised by three lakes (Biel, Neuchâtel and Murten) and a flat alluvial plane made up of peat, which extends over several kilometres (Fig. 1b). This area was affected by environmental engineering practices in the framework of two so-called “Juragewässerkorrektionen” (1868–1891 and 1962–1973), with the result that ca. 400 km2 of peat was transformed to agricultural land (Eugster and Schneider, 2006). Seismic surveys and multiple drillings have shown that this region is underlain by several hundreds of metres-deep overdeepenings that were carved into the Molasse bedrock (Lower Freshwater Molasse and Upper Marine Molasse) by subglacial processes throughout repeated Quaternary glacial advances (Dehnert et al., 2012; Dürst Stucki & Schlunegger, 2013; Magrani et al., 2020; Reber & Schlunegger, 2016). These troughs were then filled by glacio-lacustrine sediments or host modern lakes (Dehnert et al., 2012; Buechi et al., 2017; Gegg et al., 2021; Schwenk et al., 2022). The lakes and alluvial planes are bordered by drumlins and roches moutonées that are covered by a till probably of a LGM age, and that have a SW–NE extension, which parallels the flow direction of the Valais Glacier during the LGM (Dürst Stucki & Schlunegger, 2013). This glacier was sourced in the central Alps and reached its maximum position ca. 45 km farther NE of the study area, where terminal moraines mark the position of the former ice margin (Ivy-Ochs et al., 2008). South of the study area and also during LGM times, two additional glaciers (Saane and Aare, Fig. 1a) with sources in the northern part of the Alps advanced to the Swiss Plateau and converged with the Valais Glacier near Fribourg (Saane Glacier) and Bern (Aare Glacier) (Preusser et al., 2011). The bedrock underneath the Quaternary sediments consists of Early Miocene sandstones and mudstones of the Lower Freshwater Molasse (USM) and Upper Marine Molasse (OMM).
The MÜN and FIH pits are located on one of these drumlins (Fig. 1b). They comprise a till cover that was most likely deposited during the LGM (Preusser et al., 2007). The diamicts at these sites are underlain by a sequence of sand and gravel layers that have not been dated yet (Müntschemier), and by a suite of gravels at Finsterhennen, which were already dated. In particular, a mammoth tusk, which was found embedded in pro-glacial meltwater deposits below the uppermost till layer at Finsterhennen, has an originally reported age of 25.4 ± 0.2 14C ka BP (Schlüchter, 2004), which was then corrected to an age of 29.5 ± 0.6 ka (cal.) BP using a more recent 14C calibration (Graf et al., 2015). This age assignment was then complemented by two OSL ages of 28.5 ± 2.3 ka and 28.9 ± 2.5 ka within the same glaciofluvial unit (Preusser et al., 2007). Consequently, the pro-glacial meltwater deposits and the overlying till at this site have been attributed to the LGM glaciation (Preusser et al., 2007). In addition, OSL dating of glaciofluvial sediments below a till in the lower part of the same gravel pit yielded an age of 76.0 ± 6.0 ka (Preusser et al., 2007), which suggests that these sediments were possibly associated with an earlier glacial advance into the lowlands during MIS 4 (Ivy-Ochs et al., 2008; Preusser et al., 2007, 2011). Because of ongoing gravel mining, the sites for which these ages were determined in the Finsterhennen gravel pit do no longer exist, and precise information about where the chronological samples were taken is not available, unfortunately.
We used several methods including techniques in sedimentology and geochronology to reconstruct the depositional processes, the sedimentary environments, and the evolution of the landscape in this part of the Swiss Plateau.
3.1 Remote sensing
High-resolution digital elevation models of the MÜN (Fig. 2a) and FIH (Fig. 2b) gravel pits were generated to gain an overview of the investigation area, to allocate the sections for the subsequent logging, and to precisely determine the location where the sedimentological logs were taken. Such an allocation is required because the sections will disappear as gravel mining proceeds. The aerial photographs were taken with the unmanned air vehicle (or drone) DJI Mavic 2 Pro®. These images then served the basis to generate, for each gravel pit, a high-resolution orthomosaic (2.4 cm/pix and 2.07 cm/pix, for MÜN and FIH respectively). This was accomplished through combining a total of 253 and 716 images that were taken at MÜN and FIH, respectively with the software Agisoft Metashape Professional®.
3.2 Sedimentological analysis
3.2.1 Facies description and logging
Sedimentological logging of the sediments at Müntschemier and Finsterhennen (sites indicated with crosses on Fig. 2) was accomplished following the scheme proposed by Evans and Benn (2014) and Buechi et al. (2017). The larger gravel pit (FIH) is subdivided into a northern (FIH-N, 19.5 m thick) and a southern section (FIH-S, 18.0 m thick), while in the smaller gravel pit (MÜN) only one section was logged (20 m thick). For each of these three sections, in-situ observations and digital images were used to collect data on grain size, to identify the sedimentary structures and to measure the thicknesses of individual units, as well as to determine paleo-flow directions (see below). The results of sedimentological analyses are used to identify 11 different lithofacies elements (or lithofacies, LF; Table 1), which are grouped into 5 lithofacies associations (LFA; Table 2) in an effort to reconstruct the depositional paleo-environments following Rust (1978), Miall (1985), Buechi et al. (2017) and Schwenk et al. (2022). The occurrence of these lithofacies associations (LFA) was then used as a basis to develop a scenario of how the landscape of the study area evolved through time.
3.2.2 Paleo-flow direction analysis
The orientation of elongated and flat pebbles in alluvial sediments can be used to infer the paleo-flow direction at the time of deposition (Krumbein, 1939; Rust, 1972). This can be accomplished by measuring either the azimuth of the longest a-axis of an elongated clast, or the azimuth of the a-b-plane of disc-shaped or flat pebbles (Krumbein, 1939; Millane et al., 2006; Rust, 1972) where the b-axis is the clast’s intermediate axis. For elongated clasts, it is important to note that the orientation of the a-axis depends on the transport mechanism. In particular, if such pebbles bounce upon transport, they tend to be deposited with their a-axis parallel to the flow (e.g. Schlee, 1957), while rolling on the bed surface results in an orientation where the a-axis is aligned perpendicular to the flow (e.g. Doeglas, 1962; Kelling & Williams, 1967), which is commonly the case in a fluvial setting (Johansson, 1963; Millane et al., 2006). In a till, however, the long (a-) axis of elongated clasts is oriented parallel to the ice flow (Evans & Benn, 2014; Nichols, 2009).
For disc-shaped or flat pebbles, the a-b-plane tends to be orientated dipping upstream, resulting in imbrication (Doeglas, 1962; Johansson, 1963; Kauffman & Ritter, 1981; Millane et al., 2006; Nichols, 2009; Rust, 1972; Schlunegger & Garefalakis, 2018). Because orientations of a-b-planes of flat pebbles yield generally a more consistent picture on the paleo-flow direction than measurements of the a-axes of elongated pebbles (Millane et al., 2006), we preferentially conducted our measurements on flat pebbles particularly for fluvial gravels. Furthermore, because the orientation of smaller clasts largely depends on the transport and deposition of the larger clasts (Potter & Pettijohn, 1977), only the larger clasts (i.e. > mean grain size) are considered for our measurements. Please note that we avoided measuring the orientation of those clasts that occur in cross-bedded layers because pebbles are lying flat on the bedform surface in such deposits, which could add a bias. The paleo-flow dataset was then complemented by information provided by orientations of cross-beds, which also allow to determine the directions of the paleo-flow at the time of sediment accumulation (Nichols, 2009). All measurements were conducted with a compass in the field.
3.2.3 Pebble petrography
Investigations on the pebble petrography are an important part of the provenance analysis of sediments as the resulting information can be used to determine the source area of the sediments (e.g., Claude et al., 2017a; Graf & Müller, 1999; Graf et al., 2015; Meichtry, 2016). In addition, such data bear information on the transport, erosion, weathering, sorting and recycling mechanisms prior to deposition (Schlüchter, 1989; Weltje & von Eynatten, 2004). In this context, special attention is focussed on identifying the key lithologies because they indicate a spatially constrained source area (e.g., Claude et al., 2017b, 2019). We followed the analysis protocol provided by Claude et al. (2017a) and randomly collected 250 pebbles within 4 m2-large areas along the sections at MÜN and FIH using a bucket. The pebbles were then grouped into the following lithological classes using a hammer, HCL (3.2%), and macroscopic criteria: Vein quartz, quartzite, granite and metamorphic rock (granite, gneiss and crystalline schist, and amphibolite), serpentinite and green crystalline, sandstone, dark-coloured limestone (dark-coloured limestone and siliceous limestone), light-coloured limestone (light- and flesh-coloured limestone and oosparite), Niesen- and Gurnigel-Flysch, and others (volcanic rock, conglomerate, claystone, and unknown) (Table 3 and Fig. 3). Please note that there are more advanced ways of determining the clast composition of gravels (Schiemenz, 1960). We justify the use of our simple approach since it is sufficiently well suited to identify first order patterns and changes of the petrographic composition of coarse-grained deposits, which is the scope of our paper (e.g., Schlunegger et al., 1993).
3.2.4 Pebble morphometry
The shape of pebbles provides information about the transport mechanisms (e.g., fluvial versus glacial), the depositional environment, and particularly about the origin of the clasts (e.g. Claude et al., 2017a; Lukas et al., 2013; Schlüchter, 1989). In the context of the material’s provenance, it has been reported that rounded quartzite pebbles represent products of reworked fluvial sediments such as the Miocene Molasse conglomerates or older unconsolidated gravels, whereas sub-angular pebbles are interpreted to indicate rather fresh erosional products from the Alps (e.g. Claude et al., 2017a; Du Pasquier, 1891). In our case, sub-angular quartzite pebbles are likely to represent clasts that were eroded from the St. Bernard nappe in the Central Alps (Spicher, 1972) and transported by the Valais Glacier to the study area. We thus selected quartzite clasts for the morphometric analysis because of their abundance in the gravel pits and because research on the relationships between their morphometry and transport mechanisms has already been conducted (see above). We collected 100 quartzite pebbles from multiple 4 m2-large outcrops and determined the pebble morphometry by calculating the roundness (Zi) (1) and flattening (Ai) index (2):
where L is the length, w the width, E the thickness of the pebble and 2r the smallest curvature radius in the L/w plane, respectively (Cailleux, 1947). Here, low Zi values indicate angular pebbles, whereas high values are characteristic for rounded pebbles. Likewise, low Ai values point to spheroidal pebbles, and high values represent elongated and flat pebbles (Schlüchter, 1989).
The resulting Zi and Ai values were grouped into bins with an interval of 50 following Schlüchter (1989). These large groups also consider the uncertainties, which are usually associated upon estimating the required radii of the pebbles from the measured variables. From this dataset, we then created an Ai–Zi plot following Schlüchter (1976) to infer a possible glacial or fluvial imprint on the shape of the clasts and thus the origin of the material (Fig. 9).
3.3 Luminescence dating
Because the deposits at MÜN have not been dated yet, we determined a depositional age using optically stimulated luminescence (OSL) dating technique (Aitken, 1998). Two samples referred to as GUG20-01 and GUG20-02 were collected in black plastic tubes from the top and the bottom of a ca. 5.5 m-thick unit of amalgamated sand beds (LFA 2; Fig. 4a). Samples were prepared under red light laboratory conditions at the Institute of Geological Science, University of Bern. The 150–200 μm-size fraction, which was then used for luminescence measurements, was extracted by wet sieving. The samples were treated with 10% HCl and 30% H2O2 to remove carbonates and organic components, respectively. After chemical treatment, density separation was performed with heavy liquid LST (sodium heteropolytungstate solution) to isolate quartz and feldspar minerals (density of 2.70 and 2.58 g/cm3 for quartz and feldspar, respectively). Quartz separates were etched with HF (40% for 1 hour) but were not used for further analyses due to the dim luminescence signal and the high feldspar contamination that was revealed by preliminary quartz OSL measurements. Feldspar grains were fixed on 10 mm diameter stainless steel discs using a silicon spray and a 1-mm aliquot mask.
Luminescence measurements were conducted with a TL/OSL DA-20 Risø reader equipped with a calibrated 90Sr/90Y beta source at the Institute of Geology, University of Bern. Feldspar infrared (IR) stimulation of 1-mm aliquots was performed with IR LEDs emitting at 875 nm. The generated luminescence signal (called infrared stimulated luminescence, IRSL) was detected in the blue wavelength by the photomultiplier tube EMI 9235QA with a Schott BG-39 and L.O.T.-Oriel D410/30 nm filter combination.
Equivalent doses (De) were measured for each 1 mm aliquot (n = 24 per sample) using a post-IR IRSL 150 °C protocol similar to the one of Reimann and Tsukamoto (2012). In this pIRIR150 protocol, a preheat at 180 °C is followed by a first IRSL stimulation at 50 °C (IR50) and a second post-IRSL stimulation at 150 °C (pIRIR150). The suitability of the pIRIR150 protocol was assessed with dose-recovery and residual tests (Wintle & Murray, 2006), carried out on samples that were previously bleached under sunlight. After removing the residual signals, recovery ratios were within 10% of unity for all the samples. For data analysis, IRSL signals were integrated over the first 1.2–10 s minus the last 90–99 s, and dose–response curves were constructed using exponential fitting. Acceptance criteria for each individual aliquot measurement data include a recycling ratio within 15% of unity and a recuperation within 10% of the natural dose.
Final De per sample used for the age calculation was determined using the Central Age Model (CAM; Galbraith & Roberts, 2012) or the Finite Mixture Model (FMM; Galbraith & Green, 1990). We selected the FMM model for the case where the overdispersion (OD) of the measured 1 mm-aliquot De distribution was significantly larger than the reported OD values for well bleached (i.e. fully reset luminescence signal prior to deposition) glacio-fluvial deposits in the Alps with similar measurements (OD ~ 19%, Gaar et al., 2014). The FMM was run using multiple components (testing 2 to 4 components) with sigma-b values (σb, which corresponds to expected OD for well-bleached sample) of 0.2 (Gaar et al., 2019).
Finally, the De values that resulted from the CAM or FMM models were then corrected for fading (i.e. spontaneous loss of feldspar luminescence signal with time; Wintle, 1973) using fading rates measured on the same aliquots as those that were used for De measurements. This was accomplished following Auclair et al. (2003) and by applying the fading correction procedure of Huntley and Lamothe (2001), which is implemented in the Luminescence R package developed by Kreutzer et al. (2012).
U, Th, and K concentrations, which were used for the determination of the environmental dose rate, were measured on the bulk sediment collected around the target samples. The measurements were conducted using high-resolution gamma spectrometry (Department of Chemistry and Biochemistry, University of Bern, Switzerland; Preusser et al., 2001). The resulting radionuclide concentrations, an assumed water content of 10 ± 5%, an internal K-content of 12.5 ± 0.5% (Huntley & Baril, 1997), and an alpha efficiency value of 0.15 ± 0.05 (Balescu & Lamothe, 1994) were then considered upon employing the Dose Rate and Age Calculator (DRAC) by Durcan et al. (2015). The ages were obtained by dividing the fading corrected final CAM/FMM De by the calculated dose rate.
4.1 Lithofacies and facies associations
The MÜN and FIH gravel pits comprise well-sorted gravel and sand beds (Fig. 4). They are overlain by ca. 2–5 m thick diamicts that have a poor sorting. The contact with the underlying Molasse bedrock is not observed in both gravel pits. The gravel and sand beds are dominated by horizontal- and cross-beddings as well as by massive structures. In general, the sediments at MÜN (Fig. 5) are mainly characterised by the occurrence of sand beds, whereas the sediments at FIH predominantly comprise medium- and coarse-grained gravel beds (Figs. 6, 7). The 11 lithofacies (LF) elements, which were identified in these sections (Table 1), were then grouped into 5 lithofacies associations (LFA, numbered from bottom to top), which are summarised in Table 2 and described with more details regarding their vertical stacking in the following chapters.
4.1.1 LFA 1—Alternation of gravel and sand beds, fining upward
Lithofacies association 1 (LFA 1) consists of an alternation of massive and cross-bedded gravel layers (Gm and Gc; Table 2) and massive- and horizontal- as well as cross-bedded sand units (Sm, Sh, and Sc; Table 2). It is only found in the MÜN gravel pit (Fig. 5). In the lower part, the gravel beds are coarse-grained and have a thickness of approximately 1.5 m. Towards the top of the LFA 1 succession, the grain size and the thickness of individual beds decrease to fine-grained gravel beds that are few decimetres thick. The fabric is mostly clast-supported. The matrix consists of grey-coloured medium- to coarse-grained sand, and only a few crushed pebbles are observed (Fig. 4b). Furthermore, some metres-wide and several decimetres-thick lenses consisting mostly of medium- to coarse-grained sand are interbedded within the gravels. While these sand beds forming horizontal layers are rare in the lower part of LFA 1, their thickness and frequency increase towards the upper part of LFA 1. This trend is also associated with a general fining-upward trend (Fig. 5). Measurements of the orientation of clasts and cross-beds (a total of 70) indicate a transport direction towards the NE to S albeit with a large spread.
The results of the petrological investigations reveal that the clast suite of LFA 1 is dominated by (i) light-coloured (32%) and dark-coloured (15%) limestone clasts, (ii) quartzite constituents (19%), and (iii) granite and metamorphic rocks (16%). The roundness index values (Zi) of quartzite pebbles range between 50 and 750. They have a median value of ca. 340. Lower values (angular shape) are rare in comparison to the higher values (rounded shape). The flattening index values (Ai) of the same pebbles have a median of 157 (Fig. 5).
4.1.2 LFA 2—Amalgamation of sand beds
Lithofacies association 2 (LFA 2) consists of an amalgamation of massive- and horizontal- as well as cross-bedded sand layers (Sm, Sh, and Sc; Table 2) (Fig. 4c). This LFA is encountered in the middle of the MÜN section (Fig. 5) and at the base of the FIH gravel pit in its northern part (Fig. 6). At both sites, the bed thickness varies between the centimetre- to the metre-scale. The beds mainly consist of fine- to medium-grained sand with a low amount of silt. In general, they are brownish-beige-coloured. Few gravel beds are embedded preferentially in Sc at the bottom and the top of LFA 2. The contacts to the underlying LFA 1 and the overlying LFA 3 are gradual. No paleo-flow and morphometric data was collected for this LFA.
4.1.3 LFA 3—Alternation of gravel and sand beds, coarsening upward
Lithofacies association 3 (LFA 3), which occurs in both gravel pits (Figs. 5, 6, 7), consists of an alternation of massive and cross-bedded gravel beds that have a clast- and matrix-supported fabric (Gm and Gc; Table 2). At both sites, decimetre-thick horizontal- and cross-bedded sand interbeds (Sh and Sc; Table 2) and a few centimetres-thick laminated clay layers also occur (Fl; Table 2). These fine-grained sediments are mainly interbedded within the sand beds. In the lower part of LFA 3, gravel beds are rare, thin (approximately 20 cm), and dominated by fine-grained clasts. This lower part is thus characterised by the occurrence of fine- and medium-grained, cross-bedded sand layers that are > 1 m thick. Towards the upper part of LFA 3, the thickness of the gravel beds increases to 0.5–1.5 m, whereas the sand layers thin to between 0.5 and 1 m. LFA 3 can thus be characterised by a coarsening upward trend particularly at FIH-S. The beds of LFA 3 are often affected by deformation features such as dykes consisting of clay fillings, clay pebbles, and faults. Figure 4d shows the occurrence of a thrust fault at the centimetre-scale at MÜN, which indicates an E-directed paleo-shear movement. Measurements of the orientation of clasts and cross-beds indicate a large spread in flow directions that are mainly oriented towards the NE.
The results of the petrological investigations reveal that whereas the clast types in LFA 3 are the same at the three sites (MÜN, FIH-N and FIH-S) and similar to LFA 1, their relative abundance is quite different, particularly if the LFA 3 clast composition at the three sites is compared with each other. For instance, quartzite clasts contribute to > 25% to the clast suite at MÜN and 18% at FIH-N (Figs. 5, 6), but they are less frequent at FIH-S (9%; Fig. 7). In the same sense, whereas dark-coloured limestone clasts contribute to 40% at FIH-S (Fig. 7), their abundance is ca. 20% at FIH-N and MÜN (Figs. 5, 6) and thus ca. 50% less than in FIH-S. In general, the clast composition of the LFA 3 unit is remarkably different between both FIH sections (Figs. 6, 7), but similar between FIH-N and MÜN (Figs. 5, 6). In the same sense, the distribution of clast types is similar between LFA 3 and LFA 4 at FIH-S (Fig. 7), but different between LFA 3 and LFA 4 at FIH-N (Fig. 6). As will be pointed out in the discussion, this pattern complicates a stratigraphic correlation between the sections.
Within LFA 3, the median of the roundness index values (Zi) measured for the quartzite clasts ranges between 250 (FIH-S) and ca. 315 (MÜN, FIH-N), which is thus slightly lower than in LFA 1. The median of the flattening index Ai values is ca. 155 for all sites, which is the same as in LFA 1.
4.1.4 LFA 4—Amalgamation of gravel beds, coarsening upward
Lithofacies association 4 (LFA 4), which is only encountered at FIH (Figs. 6, 7), consists of an amalgamation of massive and cross-bedded gravel layers that coarsen upward (Gm and Gc; Table 2). The sedimentary structures of the gravel beds vary with the orientation of the outcrops and the location of observations. In general, the lower part is dominated by medium-sized gravel beds, while towards the upper part, the dominant grain size changes to coarse-grained gravels. Occasionally boulders with diameters up to 30 cm occur within this unit. The fabric is mostly clast-supported, and some pebbles are crushed. The matrix consists of medium- to coarse-grained sand. Rarely the matrix is completely washed out. A few metres-wide and few decimetre-thick sand lenses occur in the lower and middle part of LFA 4. The contact to the underlying LFA 3 is erosional, and the depths of the erosional scours vary laterally at the decimetre-scale. The contact to the overlying LFA 5 is sharp and characterised by a horizontal plane. Measurements of the orientation of clasts point to a paleo-flow direction that was generally oriented towards the N with a large spread between W and E. A small population of paleo-flow data even points towards the SW.
The results of the petrological investigations reveal that the clast types in LFA 4 are identical at both FIH sections. Limestone clasts clearly dominate the petrographic composition (ca. 50%), followed by ca. 20% of sandstone constituents (including Flysch clasts). The other clast types include quartzite, igneous and metamorphic lithologies. Interestingly, whereas the clast suite of LFA 4 is similar to that in LFA 3 at FIH-S (Fig. 7), it is quite different to the LFA 3 unit in FIH-N (Fig. 6; see also section above).
Similar to LFA 3, the distribution of the roundness index (Zi) values of the quartzite clasts reveals a large spread between 50 and 600. The mean Zi ranges between ca. 250 (FIH-S) and ca. 290 (FIH-N), which is nearly the same as in LFA 3 at FIH-S (250), but smaller than in LFA 3 at FIH-N (ca. 315) and at MÜN (310), and also smaller than in LFA 1 (ca. 340). The flattening index values (Ai) of the same pebbles reveal a median value Mdn (Ai) of 159 and 166, which is comparable to LFAs 1 and 3.
4.1.5 LFA 5—Matrix-supported diamict
Lithofacies association 5 (LFA 5) represents the uppermost layer in all sections and consists of an unsorted and matrix-supported diamict (Dmm; Table 2), which is partially underlain by a sheared sand and clay bed (Sfl(s); Table 2). The thickness of LFA 5 varies spatially between 2 and 5 m. The Dmm consists of pebbles, cobbles, and up to 40 cm large boulders, which are embedded in a brown-beige-coloured silty and sandy matrix (Fig. 4e). The colour of this matrix thus contrasts to the grey colour of the underlying LFA 3 and LFA 4 deposits at MÜN and FIH, respectively. The contact of LFA 5 to the underlying units is sharp. In some places, these sediments show faults at the mm-scale, which indicate a paleo-shear movement. Polished, striated, and elongated clasts can be found in the scree deposits in front of the outcrop (Fig. 4e). Such pebbles were most probably transported by a glacier (Boulton, 1978). Finally, the orientation of the clasts suggests that the paleo-transport occurred towards the NNW (MÜN) and the NE (FIH-N and FIH-S).
The results of the clast counts reveal that the LFA 5 petrographic composition at MÜN is dominated by granite and metamorphic clasts (30%), followed by quartzite components (24%) and limestone lithologies (20%). This composition is quite different to the LFA 5 at FIH, where the clast suite is dominated by limestone constituents at both sites (ca. 50–60%), followed by sandstone and flysch clasts (10–20%) and quartzite components (between 10 and 15%). The other clast types include igneous and metamorphic lithologies including serpentinite clasts.
The distribution of the roundness index values (Zi) of the quartzite clasts reveals a large spread that ranges between 50 and nearly 700. The median of the Zi values clusters between ca. 200 and 240 at the three sites, which is the lowest value that was determined for the sediments at these sites. Similar to the other LFAs, the flattening index values (Ai) of the same pebbles have a median Mdn (Ai) of around 160.
4.2 Luminescence dating
The results of feldspar IR50 and pIRIR150 measurements (1-mm aliquots) of samples GUG20-01 and GUG20-02 that were collected from unit LFA 2 at MÜN are summarised in Table 4 and Fig. 8. They show that for sample GUG20-01, 1 mm- aliquot De distributions for both signals are multimodal and significantly dispersed (OD values of 27.8% for IR50 and 30.7% for pIRIR150) compared to similar well-bleached glaciofluvial deposits investigated in the Alps (OD ~ 19%, Gaar et al., 2014). For both signals, the calculated De using the FMM model results in a moderate decrease (< 15%) of the final De that is obtained through applying the CAM model. Final ages calculated from FMM De and corrected for fading are 65.7 ± 9.1 (IR50) and 112.3 ± 9.5 ka (pIRIR150). Distributions of 1 mm- aliquot De for sample GUG20-02 are moderately dispersed (OD around 20% for both IR50 and pIRIR150 measurements). Fading-corrected ages from calculated CAM De are 93.4 ± 8.2 ka (IR50) and 144 ± 11.0 ka (pIRIR150).
The large difference between final fading corrected IR50 and pIRIR150 ages for both samples (ratio IR50/pIRIR150 ages of ~ 0.59 for GUG20-01, and ~ 0.65 for GUG20-02) suggest that the pIRIR150 signal, at least, is significantly affected by partial bleaching (i.e. limited exposure to light prior to deposition), due to its lower resetting properties compared to the IR50 signal (Kars et al., 2014). We therefore focus for the rest of the discussion only on the FMM (for GUG20-01) and CAM (for GUG20-02) fading corrected IR50 depositional ages (i.e., 65.7 ± 9.1 ka and 93.4 ± 8.2 ka, respectively).
5 Interpretation of sedimentary environments
5.1 Lithofacies association LFA 1: braided river system with a trend from a proximal towards a more distal setting
LFA 1 is encountered at MÜN only, where the related sediments are interpreted to have been deposited by a braided river system within a glaciofluvial environment. In such a setting, a large proportion of the gravelly bedload is transported by rolling and saltation along the channel bed, which results in the formation of longitudinal and transverse gravel bars with massive to cross-bedded structures (Miall, 1978; Nichols, 2009). In general, the gravel deposits are products of high-water stages, during which high-energy conditions prevail, whereas the sandy components are deposited during the waning stages of a flood. The structures of the sand beds display evidence for planar bed flow, scour fills, longitudinal bars, transverse bars, sand waves and dunes (Table 1; Miall, 1978). The fining upward trend of LFA 1 is interpreted to record a transition from large runoff with high-energy conditions to a lower waterflow with less transport energies. The crushed pebbles within the grain supported sections most likely resulted from high stress conditions at the grain to grain contact during the subglacial deformation at any time after deposition (van der Meer, 1993). The large spread of the paleo-flow directions is typical for a braided river system (Nichols, 2009). The median of the roundness index (Mdn (Zi) = 342) established for the quartzite clasts suggests that this facies association contains the roundest pebbles that were encountered in the investigated sediments and, therefore, reflects the largest fluvial signal in comparison to the other gravel containing LFAs (Fig. 9).
5.2 Lithofacies association LFA 2: braided river system with a distal sediment source
LFA 2 is encountered both at MÜN and in the northern segment at FIH (i.e. FIH-N). This unit exposes sedimentary features that are diagnostic for a low energy braided river system, which was possibly sourced at some distance from the MÜN and FIH-N sites. We base this interpretation on the distinct assemblage of lithofacies elements and their sedimentary properties (Tables 1, 2). In particular, the structures encountered in LFA 2, which are characterised by the occurrence of planar beds, point to the deposition in the upper flow regime. Additionally, features that are indicative for scour fills, longitudinal bars, transverse bars, dunes, and sand waves are also found (Miall, 1978). Because LFA 2 mainly comprises coarse- and medium-grained sand, the sediment source (most likely the front of a glacier, see below) was situated farther away from the study area compared to the other LFAs where the material comprises much coarser material.
5.3 Lithofacies association LFA 3: braided river system with a trend from a distal towards a more proximal sediment source
Similar to the lowermost LFA 1, LFA 3 is interpreted to have been deposited by a braided river, possibly in a glaciofluvial environment. The gravel beds are considered as products of longitudinal bars, gravel bars, and channel fills (Miall, 1978; Nichols, 2009), and the cross-bedded sand layers probably represent scour fills and transverse bars. In addition, upper flow regime conditions might have contributed to the formation of planar beds (Miall, 1978). The coarsening upward trend encountered in this LFA is interpreted as evidence towards a decreasing distance from the material source. This is thus the major difference to LFA 1, which shows a fining- and thinning-upward trend, suggesting an increasing distance to the material source. The centimetre-thick laminated clay layer in the lower part of LFA 3 at MÜN could be interpreted as overbank deposits (Miall, 1978). Also at MÜN, the centimetre-scale thrust fault is interpreted as a result of glaciotectonic deformation, induced by the pressure and shear stress of an advancing glacier (Hart & Boulton, 1991).
5.4 Lithofacies association LFA 4: braided river system close to a glacier’s front
LFA 4 is interpreted as a deposit of a braided river close to a glacier’s front. This interpretation is based on the predominantly massive structure and clast-supported fabric of the coarse-grained material that additionally coarsens upward, thus pointing towards a decreasing distance to the sediment source (Miall, 1977, 1985). This suggests a short transport distance and a possible material supply from a glacier (Etienne et al., 2003). In addition, the large spread of the paleo-flow directions is characteristic for a braided river system (Nichols, 2009). Crushed pebbles within the grain supported fabric indicate high stress conditions during subglacial deformation (van der Meer, 1993), hence, the glaciofluvial sediments were overridden by the Valais Glacier after deposition (Preusser et al., 2007).
5.5 Lithofacies association LFA 5: till
The diamict layers encountered in LFA 5 are interpreted as glacial till. This interpretation is corroborated by the occurrence of a matrix-supported fabric and by the roundness data, which indicates that most pebbles have an angular and sub-angular shape, thus pointing to a transport predominantly by a glacier. In addition, the lower part of the LFA 5 at FIH-S bears evidence for glaciotectonic deformation. Because such deformation is not encountered at the other sites, the occurrence of such a feature might vary spatially. The transport direction and thus the ice flow was oriented towards the NE and thus parallel to the long axis of the roche moutonnées (Dürst Stucki & Schlunegger, 2013) and the orientation of the Swiss Plateau.
6.1 IRSL ages
The IRSL ages established for LFA 2 at MÜN have to be considered with caution. Indeed, the significant difference (~ 30–40%) between the IR50 and pIRIR150 ages for both samples after fading correction suggests a moderate time of exposure to light before the sediments were deposited, which is typically the case in a glaciofluvial environment (Fuchs & Owen, 2008). Furthermore, although we used the signal with the best bleaching properties (IR50) for the final age calculation and despite the fact that we employed the FMM model to isolate the best bleached sub-population of the De measurements, we cannot exclude the persistence of an inherited signal in our measurements. If such as signal would occur, it cannot be isolated at the measurement scale (i.e. 1 mm aliquot) that is applied in this work. Considering these aspects, the IRSL ages that we obtained for the sand layers at MÜN might possibly represent an overestimate and would require further confirmation using alternative approaches (e.g., measurements at the single grain scale; e.g. Gribenski et al., 2021) or independent dating techniques (e.g. radiocarbon) if possible. Nevertheless, because the IRSL ages established for the LFA 2 sand layer at MÜN are consistent with the lithostratigraphic framework (see section 6.2) and the results of previous chronologies established for the gravely sequences at FIH, we tentatively use them for our interpretation.
6.2 Correlation of sections
The correlation between the FIH und MÜN sections is accomplished based on lithostratigraphic and chronological evidence. From a lithostratigraphic perspective, we correlate the suite with sand layers at the base of the northern FIH gravel pit, which is assigned to LFA 2 in this work, with the sandy unit in the middle of the MÜN section, which corresponds to the identical facies and thus to the same LFA (Fig. 10a). Such a lithostratigraphic correlation is justified by the position of this LFA in both gravel pits and based on the chronological constraints. First, at FIH, the top of LFA 2 is encountered at an elevation of ca. 450 m a.s.l.. At MÜN, the top of the same LFA occurs at an elevation that is ca. 10 m higher. If we consider a general sediment transport direction towards the NE, as revealed by the paleo-flow data of the under- and overlying units (Figs. 5, 6, 7), then an elevation difference of ca. 10 m over a downstream length of ca. 3.5 km yields a gradient (ca. 0.2°). Second, our IRSL data imply an age of 93.4 ± 8.2 ka and 65.7 ± 9.1 ka for the base and the top of LFA 2 at MÜN, respectively, which is consistent (within errors) with the OSL age of 76.0 ± 6.0 ka (Preusser et al., 2007) established for the base of the FIH gravels.
The interpretation of the litho- and chronostratigraphic position of LFA 3, however, is less straightforward if we consider the petrographic and morphometric data. In particular, the LFA 3 of FIH-N reveals a clast suite that is different to the LFA 3 unit in the southern part of the FIH gravel pit, but quite similar to the LFA 3 at MÜN. In contrast, at FIH-S the LFA 3 reveals a petrographic composition that is comparable to the overlying gravels that we assigned to LFA 4. Based on these observations, we tentatively group the LFA 3 unit at FIH-S with the overlying LFA 4 gravels (Fig. 10a). In contrast, the LFA 3 at FIH-N could be an equivalent to the identical unit at MÜN. These lithostratigraphic correlations have further implications for the assignment of the OSL (Preusser et al., 2007) and 14C ages (Schlüchter, 2004) to one of these units, mainly because we cannot precisely locate the sample sites of these authors based on published information. Nevertheless, based on the position of these sites relative to the glacial till, and because the LFA 3 gravels of FIH-N are more similar to the LFA 3 at MÜN than at FIH-S, we tentatively consider (i) the OSL age of 76.0 ± 6.0 ka by Preusser et al. (2007) to chronicle the accumulation of the LFA 3 at FIH-N, and (ii) the ca. 30 ka-old 14C and OSL ages by Preusser et al. (2007) and Schlüchter (2004) to calibrate the deposition of the LFA 4 (Fig. 10a).
Finally, the chronological constraints allow us to assign the till LFA 5 at FIH to the LGM. As a consequence, the till at MÜN was most likely also formed during the LGM because it underlies a drumlin that has a same shape and occurs at the same elevation as the drumlin at FIH (Fig. 1b). We acknowledge that the clast composition of LFA 5 at MÜN is different to the petrographic composition of the FIH tills particularly if we consider the relative abundance of quartzites and granites/metamorphic clasts (54% at MÜN and ca. 21–22% only at FIH). In fact, because the clast compositions of the till deposits at both sites are quite similar to those of the underlying gravel units (LFA 4 at FIH-S and FIH-N, and LFA 3 at MÜN), it is possible that the petrographic composition of the LGM tills was largely achieved through erosion of the underlying gravel units with some admixture of fresh material from the Central Alps.
6.3 Origin of the material
The allocation of the origin of the clasts in the gravel pits is not straightforward because the material could have been derived from the Alps and the Jura Mountains through erosion by e.g., a glacier. Alternatively, and possibly more likely, a very large fraction of the clastic material particularly in LFA 1, 3, and 4 could have been derived either through erosion of Miocene Molasse conglomerates, or through recycling of older Quaternary gravels, all of which share similar petrographic compositions (e.g., Matter, 1964; Schlüchter, 1976). Yet, the occurrence of angular quartzite and serpentinite clasts in the till deposits (LFA 5) that overlay the gravels at both pits does point towards the Valais Glacier as an important sediment source. In this context, we can use the morphometric properties such as the roundness values of quartzite clasts as a proxy for estimating either a relative distance to the glacier’s front or a relative importance of recycling of older gravelly units on the construction of the LFAs. This is the case because a high Zi-value, which indicates a high roundness and thus a high textural maturity of the clasts, implies a relatively long fluvial transport time and/or a low admixture of glaciogenic material, whereas a low Zi-value results from a clast population where the fluvial transport time is short and/or admixture of glaciogenic material is significant, thereby resulting in a low textural maturity. Accordingly, the highest mean Zi-value that we determined for the oldest LFA 1 unit suggests a high textural maturity where a large portion of the clasts was probably recycled from older gravels, or where the distance to the glacier’s front was the largest of all gravelly units encountered in this study. In either case, the high textural maturity of the LFA 1 clasts points towards the lowest glacial signal in comparison to the overlying gravelly units. However, because the LFA 2 mainly consists of sand, the glacial signal recorded by these deposits is even less.
The nearly identical mean Zi-values that we determined for the quartzite clasts for the LFA 3 gravels at FIH-N and MÜN confirms the lithostratigraphic correlation where both units were possibly deposited at the same time at ca. 65–75 ka. At both sites, the LFA 3 unit chronicles a situation where the distance to the glacier’s front was probably significant, yet shorter than during deposition of the LFA 1, or where a very large portion of the material was recycled from older gravel deposits. The situation then changed during deposition of the pre-LGM gravels of LFA 3 and 4 at FIH-S and LFA 4 at FIH-N where the relative admixture of angular quartzite clasts increased, finally resulting in a composition of clasts where the largest portion was supplied by a glacier as recorded by the low mean Zi-values of the youngest LFAs.
In summary, based on the morphometric properties of the clasts (Fig. 9) and the correlation of the sections (Fig. 10a), we assign the lowest glacial signal to the oldest gravel unit (LFA 1) that has an age older than ca. 95 ka. The subsequent package of gravel layers, which was deposited at ca. 65–75 ka and which includes the LFA 3 at FIH-N and MÜN, records evidence for a glacial signal more pronounced than at ca. 95 ka but significantly lower than during pre-LGM and LGM times between ca. 30 and 20 ka. This time span is recorded by the LFA 3 and LFA 4 units at FIH-S and by the LFA 4 gravels at FIH-N, and finally by the till deposits overlying both sites (Fig. 10a). This interpretation is consistent with the conclusion by Preusser et al. (2007) who considered the material of these LFAs to have been mainly supplied by the Valais Glacier. Furthermore, the occurence of Niesen- and Gurnigel-Flysch point to a minor sediment supply by the Saane Glacier.
6.4 Evolution of sedimentary environment
The fining upward trend of the lowermost unit LFA 1 (MÜN) is interpreted to reflect a retreat of the sediment and water source, which was possibly a glacier. We infer such a source because a large sediment flux and a large water runoff was required to transport and accumulate coarse-grained material in such a braided fluvial environment (Miall, 1985; Rust, 1978). In addition, since the water and sediment transport occurred towards the NE, it is very likely that the sourcing ice was the Valais Glacier, which advanced to the Swiss Plateau most likely during MIS 5d (Fig. 10b) as the IRSL age of 93.4 ± 8.2 ka in the overlying sandy unit (LFA 2) suggests. Because the transition to the overlying LFA 2 unit is characterised by a continuous fining-upward trend, we infer that the LFA 1 to LFA 2 succession records the retreating stage of this glacier and the transition to a warmer period. Accordingly, we consider LFA 1 to record the first glacial advance of the Birrfeld Glaciation (Fig. 10b).
The age interval between the base (93.4 ± 8.2 ka) and the top of the LFA 2 (65.7 ± 9.1 ka) spans approximately 30 ka and is thus quite long. This time span also corresponds to the time interval between ca. MIS 5c and MIS 5a, during which, except for MIS 5b, warmer conditions prevailed (Helmens, 2014; Wohlfarth, 2013). These age constraints thus suggest that LFA 2 chronicles a period with low sedimentation and/or erosion. This interpretation is consistent with the inferences by Dehnert et al. (2012), who dated lake sediments and peat material in the Wehntal overdeepening (north of Zürich, Switzerland) with luminescence dating and palynology. Based on the resulting age pattern, the aforementioned authors considered the occurrence of a sedimentary hiatus during MIS 5a in their sedimentary succession. The large amount of sand within the LFA 2 unit, which was likely to have been deposited mainly during interstadial conditions between MIS 5c and MIS 5a, could partly be explained by re-deposition of reworked Molasse material. This is similar to the observations by Preusser and Schlüchter (2004), who suggest that the sandy material in the western part of the Swiss Plateau, which was deposited during the last interglacial period (Eemian, MIS 5e), could be explained by weathering and erosion of Molasse bedrock.
The coarsening upward sequence of the overlying and, therefore, younger LFA 3 at MÜN and FIH-N is interpreted as resulting from a larger sediment and water supply. Although no evidence for a direct ice contact was encountered within this unit, we infer that this unit recorded the second advance of the Valais Glacier on the Swiss plateau during the Birrfeld Glaciation. We sustain this interpretation by (i) the coarse-grained nature of the LFA 3 units at MÜN and FIH-N, (ii) the NE-directed sediment transport, and the (iii) the OSL age of 76.0 ± 6.0 ka (Preusser et al., 2007) that suggests that this unit was most likely deposited during the relatively cold period MIS 4 (Fig. 10b). In support of this interpretation, Preusser et al. (2007) described the occurrence of a residual till in the FIH pit close to the site where the OSL age was established. These authors considered this till to have been condensed to a boulder lag and to record a glacial advance into the lowlands during MIS 4. The presence of a large erratic boulder of Allalin gabbro in the till, as reported by Preusser et al. (2007) apparently confirms the interpretation of an ice contact and the origin of the sediment from the Valais during these times (Preusser et al., 2007). However, we have not found a sedimentary layer within LFA 3, which we could interpret as a glacial till. Accordingly, in case that such a sedimentary layer was indeed deposited, then it most likely had a low preservation potential and was eroded during the subsequent advance of the LGM Valais Glacier. In either case, the occurrence of gravel deposits that were deposited by a braided stream most likely during MIS 4 suggests that this archive chronicles a second possible advance of the Valais Glacier towards the foreland.
The third and possibly most extensive advance of the Alpine glaciers occurred during the LGM. In this context, we consider that: (i) the LFAs 3, 4 and 5 at FIH-S, (ii) the LFAs 4 and 5 at FIH-N, and (i) the till at MÜN (also LFA 5) were deposited during the advance (LFAs 3 and 4) and final extension (LFA 5) particularly of the Valais Glacier. Such a scenario is consistent with the chronological data established for LFA 4 in the northern part of the FIH pit, and it also corresponds to the interpretation of the Finsterhennen archives by Preusser et al. (2007). Furthermore, we interpret that the extent of the Valais Glacier was greater during the LGM than during MIS 4 and MIS 5d. We particularly base this interpretation on the large contribution of dark-coloured limestone clasts in the sediments recording the LGM advance. These clasts could have been derived from the northern part of the Alps where such a lithology is largely exposed in the Penninic and Helvetic sedimentary nappes (Spicher, 1972). This suggests that glaciers with a small catchment such as the Saane Glacier could also have contributed material to the depositional site. The growth and advance of such glaciers with a relatively small basin indeed require a period of a major glaciation, which was the case during the LGM (Bini et al., 2009).
Note that we correlated the LFA 3 unit in the northern part of FIH with MIS 4 based on an OSL age of 76.0 ± 6.0 ka (Preusser et al., 2007) and because this unit shares many similarities with the LFA 3 unit at MÜN, but not with the same LFA that is exposed in the southern part of FIH. We consider this heterochronicity in the facies relationships as a document for the high sedimentary and erosional dynamics that are usually encountered in a glacial setting, and it emphasizes the complications that are associated when analysing and correlating glaciofluvial sediments. In summary, the sedimentological and chronological data yield a highly dynamic picture of glacial advances and retreats during the past 100 ka on the Swiss Plateau, which is consistent with the results of numerical models (Seguinot et al., 2018).
7 Summary and conclusions
Considering the IRSL ages at Müntschemier and the pre-existing radiocarbon and OSL ages at Finsterhennen, the succession at the former gravel pit records significantly older deposits, even though the gravel pits are located only ca. 3.5 km away from each other. Nevertheless, both pits record a scenario where the Valais Glacier approached the research site from the SW and sourced the braided rivers with a discharge direction towards the NE. As mentioned in the introduction and suggested by several authors before (Ivy-Ochs et al., 2008; Preusser et al., 2007, 2011), three independent glacial advances probably reached the Alpine Foreland during the last (Birrfeld) glacial cycle, which are likely to be recorded in our study sites (Fig. 10b): These are (1) the first advance likely during MIS 5d (LFA 1 at MÜN), (2) a second advance during MIS 4 (LFA 3 at MÜN and FIH-N, and (3) a third one during MIS 2, corresponding to the LGM (LFA 3, LFA 4, and LFA 5 in FIH-S, LFA 4 and LFA 5 in FIH-N, and LFA 5 at MÜN; Fig. 10b). The missing ice contact, related to the first and second glacial advances, the lower thicknesses of the gravel beds in comparison to the LGM gravels and particularly the absence of material from smaller glaciers imply that these former glaciations were probably less pronounced in this area than the LGM glaciation (Fig. 10b). According to the petrographic investigations, the first and second glacial advances were probably associated with a larger sediment supply from the southern Valaisan Alps, whereas the LGM advance was characterised by a substantial material supply from both the Valais and the catchment of the Saane Glacier. This might confirm the interpretation that the LGM advance was more extensive and therefore also recorded a petrologic signal of lower-elevated catchments. Finally, this study documents that correlations of gravel layers are complicated by the high dynamics of erosional and sedimentary processes that are usually associated with the construction (and erosion) of Quaternary sediments in Alpine forelands.
Availability of data and materials
All material that has been used in this work has been presented in the figures and tables.
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Special thanks go to Peter Thomet for guiding through the study area, and Christian Schlüchter for providing previous research material. Many thanks to the Gugger Kies und Immobilien AG and Vigier Beton Nordwest for giving access the gravel pits at any time.
This research has been funded through the University of Bern.
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Pfander, J., Schlunegger, F., Serra, E. et al. Glaciofluvial sequences recording the Birrfeld Glaciation (MIS 5d–2) in the Bern area, Swiss Plateau. Swiss J Geosci 115, 12 (2022). https://doi.org/10.1186/s00015-022-00414-z