SSP3.8

Overdeepened valleys in the Alps allow to probe the glacial sedimentation record, which in turn can illuminate the climatic history. In particular, seismic reflections can be used to extend punctual borehole data (for instance a number of boreholes are to be drilled into Alpine glacial overdeepened valleys as part of the DOVE ICDP project) in the second dimension or even survey a region before drilling begins. Thus, we use detailed, 2-D seismic P-wave profiles to reveal the shape and infill of an overdeepened Rhine glacier valley in the area of Basadingen, near to the German/Swiss border. We acquired two profiles nearly perpendicular to the valley strike, approximately 500 m apart. The first profile was 1246 m long, and consisted of a single spread of 624 geophones. The second profile was 1120 m long and was acquired using 200 3-component geophones using a roll-along method. For both profiles we used a vibro-source with a 12 s linear sweep of 20-240 Hz at every second geophone (two metre spacing), which produced a high fold.

Both seismic images reveal that the overdeepened basin at this location is asymmetrical and circa 260 m deep, although the deepest part (220 m wide) covers only a small portion of the broader main valley. The infill is characterised by at least three unconformities and distinct onlap and erosive boundaries between the sedimentary units. We interpret the infill to represent a highly dynamic sedimentary system. The lower part, within the deepest part of the basin is filled with chaotic sediments and slumping. Above a major unconformity, the upper part contains strongly-dipping reflectors that probably represent a prograding point-bar in a glacio-fluviatile environment that migrated toward the north-east. Beneath the deepest part of the basin we see evidence for faults in the Tertiary Molasse basement, which correlate with known faults at the surface. The faults most likely caused the valley to be sited at this location and they were probably also the cause of the ‘valley in valley’ shape.

A new DOVE research borehole will be drilled in the centre of the valley in 2021. This will bring more light on the sedimentary history and OSL-dating of the material will bracket the timing of the infill.

How to cite: Brandt, A.-C., Tanner, D. C., Buness, H., Burschil, T., and Gabriel, G.: The shape and infill of the Basadingen overdeepened glacial valley from P-wave seismic reflections, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5996, https://doi.org/10.5194/egusphere-egu21-5996, 2021.

Subglacial overdeepenings are a common element of once glaciated mountain forelands and have considerable implications for society, e.g. in construction projects, water production and radioactive waste disposal. Yet the processes of overdeepening erosion, especially the influence of bedrock lithology and structure, are poorly understood. We present a case study of the Gebenstorf-Stilli Trough in northern Switzerland, a unique overdeepening with complex underlying geology: In contrast to the Molasse-hosted majority of the Alpine foreland overdeepenings, it is to a large part incised into Upper Jurassic limestones and marls. In order to constrain its morphology in 3D, it was targeted with scientific boreholes as well as a seismic campaign based on analysis of surface waves. The results reveal unexpected trough morphology with two nested sub-basins that appear to be closely related to the bedrock geology. We suggest that this morphology is a product of low erosional efficiency in Jurassic limestones in comparison with both underlying marls and overlying Molasse deposits as well as secondary paleoglaciological effects. We further infer that the glacier’s basal drainage system was the main driver of subglacial erosion of the Gebenstorf-Stilli Trough.

How to cite: Gegg, L., Buechi, M. W., Deplazes, G., Madritsch, H., Keller, L., Spillmann, T., and Anselmetti, F. S.: 3D overdeepened trough morphology revealed as a product of bedrock geology (northern Switzerland), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5899, https://doi.org/10.5194/egusphere-egu21-5899, 2021.

Glaciotectonic structures commonly include thrusting and folding, often as multiphase deformation. Here we present the results of a small-scale 3-D P-wave seismic reflection survey of glacial sediments within an overdeepened glacial valley in which we recognise unusual folding structures in front of push-moraine. The study area is in the Tannwald Basin, in southern Germany, about 50 km north of Lake Constance, where the basin is part of the glacial overdeepened Rhine Valley. The basin was excavated out of Tertiary Molasse sediments during the Hosskirchian stage, and infilled by 200 m of Hosskirchian and Rissian glacioclastics (Dietmanns Fm.). After an unconformity in the Rissian, a ca. 7 m-thick till (matrix-supported diamicton) was deposited, followed by up to 30 m of Rissian/Würmian coarse gravels and minor diamictons (Illmensee Fm.). The terminal moraine of the last Würmian glaciation overlies these deposits to the SW, not 200 m away.

We conducted a 3-D, 120 x 120 m², P-wave seismic reflection survey around a prospective borehole site in the study area. Source/receiver points and lines were spaced at 3 m and 9 m, respectively. A 10 s sweep of 20-200 Hz was excited by a small electrodynamic, wheelbarrow-borne vibrator twice at every of the 1004 realized shot positions. We recognised that the top layer of coarse gravel above the till is folded, but not in the conventional buckling sense, rather as cuspate-lobate folding. The fold axes are parallel to the terminal moraine front. The wavelength of the folding varies between 40 and 80 m, and the thickness of the folded layer is on average about 20 m. Cuspate-lobate folding is typical for deformation of layers of differing mechanical competence (after Ramsay and Huber 1987; µ₁/µ₂ less than 10), so this tell us something about the relative competence (or stiffness) of the till layer compared to the coarse clastics above. We also detected small thrust faults that are also parallel to the push-moraine, but these have very little offset and most of the deformation was achieved by folding.

Ramsay, J.G. and Huber, M. I. (1987): The techniques of modern structural geology, vol. 2: Folds and fractures: Academic Press, London, 700 pp.

How to cite: Tanner, D., Buness, H., and Burschil, T.: Near-surface glaciotectonic structures in the sediments of an overdeepened glacial valley revealed by a shallow 3D seismic survey, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8195, https://doi.org/10.5194/egusphere-egu21-8195, 2021.

The modern basin of trinational Lake Constance, between Switzerland, Germany, and Austria, represents the underfilled northern part of a glacially overdeepened trough. It is over 400 m deep and reaches well into the Alps at its southern end. The overdeepening was formed by the numerous glacial advance-retreat cycles of the Rhine Glacier throughout the Middle to Late Quaternary. A seismic survey of Lake Constance revealed a Quaternary sediment fill of over 150 m thickness under the modern lake floor in a maximal water depth of >250 m. This sedimentary sequence represents at least the last glacial cycle with ice-contact deposits at the base on top of the Molasse bedrock overlain by glaciolacustrine to lacustrine sediments. During the successful field test of a newly developed mid-size coring system ("HIPERCORIG"), the longest core ever taken in Lake Constance was recovered with an overall length of 24 m. The drill core, taken in a water depth of 200 m, consists of a nearly continuous succession of lacustrine sediments including over 12 m of pre-Holocene sediment at the base. The entire core was petrophysically and geochemically analyzed, sedimentologically described, and 14 lithotypes were identified. In combination with a ¹⁴C- and OSL-based age-depth model, the core was divided into three main chronostratigraphic units. The basal age of ~13.7 ka BP places the base of the section back into the Bølling-Allerød interstadial whereas the overlying strata represent a complete Younger-Dryas and Holocene section.

The sediments offer a high-resolution insight into the evolution of Paleolake Constance from a cold postglacial to a more productive warm Holocene lake. The Late Glacial sections are dominated by massive, m-thick sand beds reflecting episodic sedimentation pulses. They are most likely linked with a subaquatic channel system that is still apparent in today's lake bathymetry despite the Holocene drape. This channel system was fed from a Late Glacial river from the north; provenance analysis of the initially unexpected sands together with hydrologic considerations will document whether this inflowing high-discharge river represented a local catchment (i.e. northern lake shore) or an Alpine signal (i.e. from the south) provided by the Rhine glacier. Tentative pore water hydrogeochemical and isotope analyses indicate a still active flow system at depth. The overlying Holocene section reveals a prominent, several cm-thick double-turbiditic event layer representing the most distal impact of the "Flimser Bergsturz", the largest known rock slide of the Alps that occurred over 100 km upstream the Rhine River at ~9.5 ka BP. Furthermore, lithologic variations in the Holocene section document the varying sediment load of the Rhine and of the endogenic production representing a multitude of environmental changes.

How to cite: Schaller, S., Boettcher, M. E., Buechi, M. W., Epp, L. S., Fabbri, S. C., Gribenski, N., Harms, U., Krastel, S., Liebezeit, A., Lindhorst, K., Raschke, U., Schleheck, D., Schmiedinger, I., Schwalb, A., Vogel, H., Wessels, M., and Anselmetti, F. S.: Retreat of the Rhine Glacier from Lake Constance: Sedimentological and geochemical evidences from a deep lake-basin drillhole, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7098, https://doi.org/10.5194/egusphere-egu21-7098, 2021.

A scientific drilling was conducted into a bedrock trough in Bern-Bümpliz, a branch of the Aare Valley overdeepening (Switzerland). It is the first scientific drilling in the Bern area that reached the bedrock. We analyzed the 208.5 m-thick succession of Quaternary sediments recovered in this scientific drilling and present the sedimentological results of the campaign. In the retrieved sediments 12 different lithofacies were identified, which were grouped into 5 facies assemblages, and 2 major sedimentary sequences (A = lower, B = upper), which transition into a minor sequence C. Generally, the sedimentary successions of sequences A and B are similar. The lowermost facies assemblage of each sequence consists of a till that was deposited during a period of ice cover. However, the two tills differ from each other. In particular, while the till at the base of sequence A is dominated by large clasts derived from the underlying Molasse bedrock, the till at the base of sequence B has no such Molasse components. Furthermore, the till in sequence A bears evidence for glaciotectonic deformations. Both tills are overlain by thick facies assemblages of subaqueous, most likely glaciolacustrine and lacustrine sediments. Sequence A is characterized by cross-bedded and steeply inclined sand, gravel and diamictic beds which we interpret as deposits of density currents in a subaqueous ice-contact fan system in a proglacial lake. In contrast, the lacustrine sediments in sequence B are considered to record a less energetic environment where the material was most likely deposited in a prodelta setting that gradually developed into a delta plain. Towards the top, sequence B evolves into the fluvial system of sequence C, where large sediment fluxes of a possibly advancing glacier resulted in a widespread cover of the region by a thick gravel unit. Additionally, feldspar luminescence dating was performed on two samples from a sand layer at the top of sequence B. The dating in combination with lithostratigraphic correlations with the sequences encountered in the neighboring scientific drillings to the north (Meikirch) and south of Bern (Thalgut) suggests that sequence B was deposited during Marine Isotope Stage 8 (MIS 8; 300–243 ka).

How to cite: Schwenk, M., Schläfli, P., Bandou, D., Gribenski, N., Douillet, G. A., and Schlunegger, F.: Glacial erosion and subsequent shallowing-up sequence, evidence for two glacial advances into the overdeepened Aare Valley, Switzerland, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14551, https://doi.org/10.5194/egusphere-egu21-14551, 2021.

The evolution of the Alpine mountain belt during the Quaternary is strongly controlled by periodic glaciations and deglaciations. The significant erosion during these glacial/interglacial cycles has left very few sedimentary archives to study the surface dynamics within the mountain belt over the last hundreds of thousands of years. Valleys within the periglacial zone are the best candidates to target long-term geological archives in the Alps because they potentially preserve ancient fluvial deposits that have been preserved from glacial abrasion. The Drac River in the French western Alps preserves the alluvial fills of three generations of paleo-valleys, which were filled in response to glacial damming of the river and subsequently re-incised during glacial retreat. Detailed 3D mapping of the paleo-valleys was carried out to constrain their geometry and reconstruct the evolution of the Drac fluvial profile over time. The age of the fills of the three paleo-valleys was constrained by measuring the luminescence signal of feldspars, targeting sandy intervals within the coarse fluvial deposits. Dating these fills allows to quantitatively constrain the alluviation and incision dynamics of the Drac paleo-valleys. The onset of alluviation of the most recent paleo-valley occurred before the Last Glacial Maximum, between 40 ka and 90 ka BP (MIS 3 -5). The fill of the intermediate paleo-valley is dated to the previous cold period at 134±20 ka BP (MIS 6). Finally, the oldest paleo-valley was filled more than 200 ka ago. The filling periods correspond to the global climatic cooling stages and are much longer than the incision phases, which took place during global warm intervals. The pattern of sedimentary filling implies it is controlled by an increase in sediment flux in the context of glacial advance, while the incision phases are due to rapid base-level lowering linked to the retreat of glaciers damming the Drac basin. Complementary luminescence dating is currently carried out on the terraces, at the tops of the fillings, and will lead to a better understanding of the control of glaciations on the dynamics of alluvial deposits in the periglacial zone.

How to cite: Mai Yung Sen, V., Valla, P., and van der Beek, P.: The Drac paleo-valleys: a long-term archive of fluvial dynamics in the periglacial zone of the western French Alps, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12416, https://doi.org/10.5194/egusphere-egu21-12416, 2021.

Glacial erosion processes shape the Earth’s surface. Nevertheless, the processes that drive glacial erosion and the subsequent export of sediments are poorly understood and quantified. These processes include ice sliding, which controls erosion by abrasion and quarrying, and meltwater availability, which is essential to flush out sediment stocks that form a protective layer of sediments impeding bedrock erosion. Mapping glacial erosion rates can help understand the role of these different processes through the spatial relationships between the subprocesses and erosion rates. Here we report timeseries of glacial erosion rate maps inferred from the inversion of suspended sediment loads and their provenance. Geographically, we focus on the Gornergletscher complex (VS, Switzerland) where we collected data for the summer of 2017. The erosion rate timeseries are then compared to records of temperature, precipitation and estimates of discharge and turbidity of the meltwater river. Erosional activity seems to increase with rising temperatures and meltwater discharge, leading to an increased proportion of suspended sediments coming from the north-eastern (and occasionally western) side of the glacier. Interestingly, the peak in sediments from the north-eastern side is always preceded by a peak in sediments from the western side of the glacier. Sediments of these two zones are predominant in the suspended load signal when the maximal temperature at the Equilibrium Line Altitude (ELA) is above 10°C and on the rising limb of the hydrograph. Furthermore, the obtained erosion rate maps suggest that sliding velocities are not the only explanatory factor of the erosion rate patterns. We therefore postulate from these preliminary results that the present-day sediment output of the Gornergletscher complex is largely influenced by short term variations in temperature and meltwater availability.

How to cite: De Doncker, F., Herman, F., Prasicek, G., Adatte, T., Mettra, F., and Belotti, B.: Present-day sediment dynamics and provenance of the Gornergletscher, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6207, https://doi.org/10.5194/egusphere-egu21-6207, 2021.