Gravel sequences at Seeland, west of Bern: Record of glacier advances during the Birrfeld glaciation Jonathan Pfander, Elena Serra, Natacha Gribenski, Philippos Garefalakis, Naki Akçar, Fritz Schlunegger University of Bern, Institute of Geolocigal Sciences, Balzerstrasse 1+3, 3012 Bern,

Introduction Methods During the Quaternary, glaciers reached the Swiss foreland at least 15 times (Schlüchter, 2004). This is also the case for the Bernese Seeland, which was formed and affected by these glacial advances and which hosts thick glacial and fluvioglacial deposits. The gravel pits in Müntschemier and provide outcrops of gravel and sand sequences (Fig. 1), recording glacier advances during the Birrfeld glaciation (105–25 ka). Schlüchter (2004) and Preusser et al. (2007) dated glaciofluvial deposits in the middle and lower part of the gravel pit MÜN FIH Finsterhennen to the MIS 2 and MIS 4 (Fig. 4). In this study, the gravel pits in Müntschemier and Finsterhennen serve as basis to determine the provenance of the pebbles, the transport mechanism and the deposition environment. Furthermore, luminescence dating was applied on two samples from Müntschemier to compare the two gravel pits in terms of the depositional age. a b Study site Fig. 3. Orthomosaic was generated in Agisoft Metashape Professional with images taken by the drone DJI Mavic Pro in (a) Müntschemier The gravel pits in Müntschemier and Finsterhennen are situated on hills covered by till (Fig. 1) and (b) Finsterhennen. and clearly within the extent of the Last Glacial Maximum (LGM) glaciation (Fig. 2). Remote sensing was performed using a drone with an effort to gain an overview. (Fig. 3). Sedimentological logging in MÜN and FIN build the basis to reconstruct the sedimentary

Finsterhennen environment (Fig. 4). Paleocurrent direction analysis includes investigations of pebble fabrics Müntschemier and crossbeds (Fig. 4). Therefore, dip directions of a-axis (elongated pebbles), a-b-planes (flat pebbles) and crossbeds were measured. The pebble petrography contains information about FIH Legend the source area. Accordingly, 250 pebbles were collected and grouped into lithological classes MÜN USM OMM (Fig. 4). Information on the pebble morphometry was gathered through measuring the length Gravel Till Valais Glacier Glacier Peat (L), the width (l), the thickness (E) and the smallest radius of curvature (2r) of 100 quartzite Water Paleoriver pebbles. The calculated roundness (Zi=2r/L*1000) and flattening index (Ai=(L+l)/2E*100) lead to Gravel pits Investigation area

10 km Saane Glacier an interpretation of the material transport mechanism and the depositional environment (Fig. 5). Infrared stimulated luminescence (IRSL) dating at 50 °C was applied on two samples in Fig. 1. Geological setting of the gravel pits in Müntschemier (MÜN) Fig. 2. Estimated ice extent during the LGM (24 ka). and Finsterhennen (FIH). (https://map.geo.admin.ch, modified) Müntschmier (GUG20-01 and GUG20-02) to obtain the sediment burial age.

Results

MÜN FIH

Paleocurrent Paleocurrent Sand Gravel Sand Gravel Clay Silt Diamict Clay Silt Diamict Depth (m) Depth (m) Lithology Facies assemblage Facies assemblage Lithology m a.s.l m a.s.l direction Pebble petrography direction Pebble petrography Pebble morphometry Others 2 % 468 0 N and gurnigel flysch 6 % Vein quartz 6 % 468 0 Others 1 % N Vein quartz 4 % Light limestone 7 % Niesen and gurnigel flysch 7 % Quartzite 24 % Quartzite 12 % Light limestone 15 % I Dark limestone 13 % Granite and gneiss 9% 250

25 flat Sandstone 6 % Serpentinite and green rocks 3 % predominantly predominantly 25 Sandstone 3 % glacial fluvial Serpentinite and green rocks 6 % I Granite and gneiss 30 % Others 6 % N Dark limestone 45 % Niesen and gurnigel flysch 1 % Vein quartz 7 % 200 Light limestone 18 % III Quartzite 28 % 5 5 N Others 0 % Vein quartz 2 % Niesen and gurnigel flysch 10 % Mdn (Ai) 50 Dark limestone 20 % Quartzite 11 % Granite and gneiss 9 % Light limestone 5 % 150 Granite and gneiss 4 % Sandstone 8 % Serpentinite and green rocks 3 % Serpentinite and green rocks 6 % GUG20-01 compact IRSL: 65.7 ± 9.1 ka 73 Sandstone 12 % angular round Dark limestone 46 % 100 II 150 200 250 300 350 400 IV OSL: 28.5 ± 2.3 ka Mdn (Zi) OSL: 28.9 ± 2.5 ka 10 10 C14: 30.2 ± 0.3 ka (cal.) BP (Preusser et al., 2007 and Schlüchter, 2004) GUG20-02 Sections IRSL: 93.4 ± 8.2 ka N Others 1 % MÜN Diamict FIH Diamict ? Niesen and gurnigel flysch 9 % Vein quartz 7 % Quartzite 9 % Light limestone 6 % MÜN Upper gravel FIH Upper gravel Granite and gneiss 9 % MÜN Lower gravel FIH Middle gravel Serpentinite and green rocks 3 % FIH Lower gravel Sandstone 12 % 55 Dark limestone 44 % N Others 3 % Vein quartz 7 % N 15 15 III Others 1 % Vein quartz 5 % Niesen and gurnigel flysch 12 % Light limestone 32 % Quartzite 19 % Quartzite 12 % III 70 Light limestone 16 % Granite and gneiss 11 % Fig. 5. Pebble morphometry in MÜN and FIH. 25 Serpentinite and green rocks 0 % Granite and gneiss 16 % The sections Diamict, Upper gravel, Middle Sandstone 8 % gravel and Lower gravel refer to the same Dark limestone 15 % Serpentinite and green rocks 2 % Sandstone 6 % Dark limestone 35 % locations as the paleocurrent direction and pebble petrography investigations in figure 4. IV Each section includes measurements of OSL: 76 ± 6.0 ka 20 (Preusser et al., 2007) 100 quartzite pebbles. Modified after Schlüchter (1976).

Mdn (Zi) = Median of the roundness index Facies assemblage Lithology Sedimentary structures Mdn (Ai) = Median of the flattening index Alternation of gravel and sandbeds, Unsorted boulders Crossbedding Massive bedding I Diamict III deposited by a braided stream Diamict Sand in matrix Amalgamation of gravel beds, Amalgamation of sandbeds, Horizontal Trough Clay/ silt Lenses II deposited by a braided stream IV deposited by a braided stream Gravel bedding crossbedding

Fig. 4. Sedimentological logs show the sedimentary record in Müntschemier (MÜN) and Finsterhennen (FIH) and the locations of the IRSL, paleocurrent direction analysis and pebble petrography investigations. IRSL: Samples GUG20-01 and GUG20-02. Paleoflow direction: Rose diagrams with paleoflow directions and amount of measurements. Pebble petroghraphy: Pie charts show the distribution of 250 pebbles, grouped into lithological classes.

Conclusion Smaller grain sizes and sedimentary structures of the Müntschemier section indicate a distal braided river system which was possibly sourced by a glacier. In contrast, larger grains, sedimentary structures and the morphometry in the younger gravel layers in Finsterhennen point to a braided river system close to a glacier's snout. The occurence of serpentinite pebbles within the gravel and a dominant paleoflow towards northeast imply glacial advances of the Valais glacier during the Birrfeld glaciation, as testified by the IRSL ages. The correlation of the IRSL dates in Müntschemier with the OSL date from the lower part of the Finsterhennen section suggest a paleoslope of approx. 0.2° between Müntschemier and Finsterhennen. Further, the absence of the facies assemblage II in Müntschemier could be the consequence of erosion by the LGM glaciers. The different petrological composition of the diamict in Müntschemier and Finsterhennen shows either a local petrolocigal difference of the LGM-till (MIS 2) or the diamict in Müntschemier is interpreted as a till of an earlier MIS 4 glacier advance.

References Preusser, F., Blei, A., Graf, H., Schlüchter, C., 2007. Luminescene dating of Würmain (Weichselian) proglacial sediments from Switzerland: Methodological aspects and stratigraphical conclusions. Boreas 36, 130–142. Schlüchter, C., 1976. Geologische Untersuchungen im Quartär des Aaretals südlich von Bern. Beiträge zur Geol. Karte der Schweiz NF148. Schlüchter, C., 2004. The Swiss glacial record - A schematic summary. Dev. Quat. Sci. 2, 413–418. https://doi.org/10.1016/S1571-0866(04)80092-7.

Institut für Geologie, Baltzerstrasse 1+3, CH-3012 Bern, [email protected] Februar 2021