The Mid-Pleistocene landscape history of the Lower Aare Valley with emphasis on subglacial overdeepening

Throughout the most recent part of geological history, glaciers repeatedly built up in the Alps and advanced into the mountain foreland, episodically covering the majority of Switzerland and neighbouring regions with ice. This had severe geomorphic impacts that include the subglacial erosion of overdeepenings. These closed basins are not only a potential source of geohazards, but also contain valuable resources and archives of past environments, and therefore deserve our closer attention. However, our knowledge of i) the erosional mechanisms and the subglacial conditions that lead to the formation of overdeepenings, and ii) of the number, the timing, and the extent of Alpine glaciations, is still very limited. The present thesis is centred around four scientific boreholes in the Lower Aare Valley in northern Switzerland and addresses the above-mentioned uncertainties. The study area hosts the overdeepened Gebenstorf-Stilli Trough, whose subsurface morphology is the first major focus. It is constrained by borehole and geophysical data that together reveal a complex trough shape controlled by the particular local bedrock architecture comprising rocks of varying subglacial erodibility. The results further highlight the important role of basal water in overdeepening erosion, which is further corroborated in a second case study. There, surficial brecciation of the walls of a paleokarst network in limestone underlying the overdeepening is presented. Following detailed macro- and microscopic analysis, it is interpreted as the result of subglacial hydrofracturing, and thus illustrates the extreme water pressures below the glacier ice. Finally, the focus is shifted towards the sedimentary archives of the Gebenstorf-Stilli Trough and its surrounding. Based on a multi-method sedimentological approach, the diverse Quaternary deposits are characterised and their depositional history is reconstructed. Several phases of glacial and glaciofluvial reactivation of the major drainage pathways as well as contributions from confluent glaciers are identified. An integration of luminescence data indicates that overdeepening erosion in the Lower Aare Valley dates back to MIS 10 or earlier, and that the local sedimentary record spans large parts of the Middle and Late Pleistocene

Rinikerfeld Palaeolake (Northern Switzerland) – a sedimentary archive of landscape and climate change during the penultimate glacial cycle

While timing and ice extent of the last glacial maximum are generally well known, the courses of earlier glaciations have remained poorly constrained, with one of the main reasons being the scarcity of sedimentary archives. This study introduces a new palaeolake record from a Mid‐Pleistocene glaciofluvial channel system in the Lower Aare Valley (Northern Switzerland). The record of Rinikerfeld comprises a >40 m long succession of Quaternary deposits that are targeted by multi‐method sedimentological analysis. Sedimentary facies together with geochemical and geotechnical parameters, pollen content, as well as luminescence ages allow the reconstruction of the establishment, evolution and infilling of the early Marine Isotope Stage 6‐aged Rinikerfeld Palaeolake. A drastic change in lake sediment composition and structure indicates cessation of the initial glacially derived input, which is explained by landscape modification and drainage rerouting during the Penultimate (Beringen) Glaciation. Geochemical and palynological data further reveal cold, initially periglacial but slightly ameliorating, climate conditions, while the lake was progressively filled up by local runoff, before being buried by periglacial colluvial diamicts, and potentially overridden by ice. It is therefore concluded that the onset of the Beringen Glaciation was an environmentally as well as geomorphically dynamic time period in the Northern Alpine Foreland

The Mid-Pleistocene landscape history of the Lower Aare Valley with emphasis on subglacial overdeepening

Throughout the most recent part of geological history, glaciers repeatedly built up in the Alps and advanced into the mountain foreland, episodically covering the majority of Switzerland and neighbouring regions with ice. This had severe geomorphic impacts that include the subglacial erosion of overdeepenings. These closed basins are not only a potential source of geohazards, but also contain valuable resources and archives of past environments, and therefore deserve our closer attention. However, our knowledge of i) the erosional mechanisms and the subglacial conditions that lead to the formation of overdeepenings, and ii) of the number, the timing, and the extent of Alpine glaciations, is still very limited. The present thesis is centred around four scientific boreholes in the Lower Aare Valley in northern Switzerland and addresses the above-mentioned uncertainties. The study area hosts the overdeepened Gebenstorf-Stilli Trough, whose subsurface morphology is the first major focus. It is constrained by borehole and geophysical data that together reveal a complex trough shape controlled by the particular local bedrock architecture comprising rocks of varying subglacial erodibility. The results further highlight the important role of basal water in overdeepening erosion, which is further corroborated in a second case study. There, surficial brecciation of the walls of a paleokarst network in limestone underlying the overdeepening is presented. Following detailed macro- and microscopic analysis, it is interpreted as the result of subglacial hydrofracturing, and thus illustrates the extreme water pressures below the glacier ice. Finally, the focus is shifted towards the sedimentary archives of the Gebenstorf-Stilli Trough and its surrounding. Based on a multi-method sedimentological approach, the diverse Quaternary deposits are characterised and their depositional history is reconstructed. Several phases of glacial and glaciofluvial reactivation of the major drainage pathways as well as contributions from confluent glaciers are identified. An integration of luminescence data indicates that overdeepening erosion in the Lower Aare Valley dates back to MIS 10 or earlier, and that the local sedimentary record spans large parts of the Middle and Late Pleistocene

Luminescence properties and dating of glacial to periglacial sediments from northern Switzerland

Abstract. Luminescence dating has become a pillar of the understanding of Pleistocene glacial advances in the northern foreland of the Swiss Alps. However, both quartz and feldspar from the region are equally challenging as dosimeters with anomalous fading and partial bleaching being some of the obstacles to overcome for the establishment of decisive chronologies. In this study, luminescence properties of coarse- and fine-grained quartz, feldspar, and polymineral fractions of eight samples from a palaeovalley, Rinikerfeld in northern Switzerland, are systematically assessed. Standard performance tests are conducted on all four fractions. Deconvolution of luminescence signals of the quartz fractions is implemented and shows the dominance of stable fast components. Reader-specific low preheat temperatures are investigated on the infrared stimulated luminescence (IRSL) signal of feldspar. Thermal stability of this signal is found for low preheats, and thermal quenching could be excluded for higher preheats. However, anomalous fading is observed in the feldspar and polymineral IRSL signals and two correction approaches are applied. For one approach, fading corrected coarse-grained feldspar ages are consistent with those derived from quartz. In general, coarse-grained quartz and feldspar, as well as the fine-grained polymineral fraction of one sample, are in chrono-stratigraphic agreement and present negligible evidence for partial bleaching. However, ages derived from fine-grained quartz are found to underestimate those of the coarse-grained quartz fractions. Hence, the impact of alpha efficiency and water content on the dose rate and thus the ages are assessed. A finite explanation for the observed discrepancies remains lacking, but this systematic investigation of different luminescence signals allows for the establishment of a chronology for the palaeovalley fill dating back to at least Marine Isotope Stage 6 (MIS 6). </jats:p

Brecciation of glacially overridden palaeokarst (Lower Aare Valley, northern Switzerland): result of subglacial water‐pressure peaks?

Water pressures at the base of active glacial overdeepenings are known to fluctuate strongly on various time scales. Rapid peaks in basal water pressure can lead to fracturing of the glacier bed, a process that has been described at numerous sites around the world, mostly based on large hydrofracture systems. This article presents drill-cores from the base of a >100-m-deep glacial overdeepening in the Lower Aare Valley in northern Switzerland that were investigated with high-resolution imaging (including X-ray computed tomography) as well as compositional and microstructural analyses. The drill-cores recovered Jurassic limestones hosting palaeokarst voids infilled with blue clay. We identify this clay, based on its kaolinitic composition, as siderolithic Bolus Clay but in a rather atypical variety formed under reducing conditions. The surfaces of the palaeokarst walls show smoothly undulating as well as brecciated sections with form-fit interlocking clasts, which are the result of an in situ brecciation process. We argue that these particular fractures are not related to (glacio-)tectonics or frost action. Instead, we suggest an explanation by involving water-pressure peaks that were transmitted to the void walls by the clayey karst filling, resulting in hydrofracturing. In addition to pervasive karstification and tectonic overprinting, this water pressure-driven fracturing may have enhanced the deep incision of the overdeepening into the rheologically competent bedrock

3D morphology of a glacially overdeepened trough controlled by underlying bedrock geology

Subglacial overdeepenings are common elements of mountain forelands and have considerable implications for human infrastructure. Yet, the processes of overdeepening by subglacial erosion and especially the role of bedrock geology are poorly understood. We present a case study of the Gebenstorf-Stilli Trough in northern Switzerland, a foreland overdeepening with a regionally unique, complex underlying bedrock geology: in contrast to other Swiss foreland overdeepenings, it is incised not only into Cenozoic Molasse deposits, but also into the underlying Mesozoic bedrock. In order to constrain the trough morphology in 3D, it was targeted with scientific boreholes as well as with seismic measurements acquired through analysis of surface waves. Our results reveal an unexpectedly complex trough morphology that appears to be closely related to the bedrock geology. Two sub-basins are incised into calcareous marls and Molasse deposits, and are separated by a distinct ridge of Jurassic limestones, indicating strong lithological control on erosional efficiency. We infer generally relatively low glacial erosion efficiency sensu stricto (i.e. quarrying and abrasion) and suggest that the glacier’s basal drainage system may have been the main driver of subglacial erosion of the Gebenstorf-Stilli Trough

Luminescence dating of glacially sourced deposits from northern Switzerland: Comparing multigrain aliquots and single grains of quartz and feldspar

A complex picture of the Pleistocene glaciation history of northern Switzerland has been identified over the last three decades. To gain further insights into the long-term landscape evolution, numerical dating is required. In the absence of alternative dating techniques, luminescence dating is the key method for establishing chronological constraints of past glaciations. However, this is presented with complex challenges, especially in regard to the resetting of the luminescence signal prior to deposition, the components contributing to the signal as well as the signal intensity and stability. In this study, the luminescence properties of glacially sourced deposits from northern Switzerland are assessed using single aliquot (SA) and single grain (SG) measurements of feldspar (F) and quartz (Q). While no obvious connection between bleaching and distal or proximal deposition in relation to the proposed ice margin is observed, most samples seem to reveal a partially bleached signature in F SG De measurements. This appears to be masked in the respective F SA measurements even though only few grains emit luminescence signals. In addition, comparisons between fading corrected infrared stimulated luminescence (IR) and post-infrared infrared stimulated luminescence (pIR) De values appear to be unreliable indicators of bleaching, even though these signals bleach at different rates. Hence, it is recommended to conduct both IR and pIR investigations in combination with Q measurements on a SG level. The dating potential of the investigated deposits remains rather limited, yet, in the sedimentologic context the presented results reveal that several glacial advances occurred prior to the Last Interglacial in the study area

Pyroclastic dune bedforms: macroscale structures and lateral variations. Examples from the 2006 pyroclastic currents at Tungurahua (Ecuador).

Pyroclastic currents are catastrophic flows of gas and particles triggered by explosive volcanic eruptions. For much of their dynamics, they behave as particulate density currents and share similarities with turbidity currents. They occasionally deposit dune bedforms with peculiar lamination patterns, from what is thought to represent the dilute, low concentration, and fluid-turbulence supported end member of the pyroclastic currents. Here, we present a high resolution dataset of sediment plates (lacquer peels) with several closely spaced lateral profiles representing sections through single pyroclastic bedforms from the August 2006 eruption of Tungurahua (Ecuador). Most of the sedimentary features contain backset bedding and preferential stoss-face deposition. From the ripple scale (few cm) to the largest dune bedform scale (several m length), similar patterns of erosive-based backset beds are evidenced. Recurrent trains of sub-vertical truncations on the stoss side of structures reshape and steepen the bedforms. In contrast, sporadic coarse-grained lenses and lensoidal layers flatten bedforms by filling troughs. The coarsest (clasts up to 10 cm), least sorted and massive structures still exhibit lineation patterns that follow the general backset bedding trend. The stratal architecture exhibits strong lateral variations within tens of centimeters, with very local truncations both in flow-perpendicular and flow-parallel direction. We infer that the bedforms' sedimentary patterns result from four formation mechanisms: "differential draping", "slope-influenced saltation", "truncative bursts", and "granular-based events". Whereas most of the literature makes a straightforward link between backset bedding and Froude-supercritical flows, we reconsider this interpretation. Indeed, features that would be diagnostic of subcritical dunes, antidunes, and "chute and pools" can be found on the same horizon and in a single bedform, only laterally separated by short distances (10s of cm). Our data stress the influence of the pulsating and highly turbulent nature of the currents and the possible role of coherent flow structures such as Görtler vortices. Backset bedding is interpreted here as a consequence of a very high sedimentation environment of weak and waning currents that interact with the pre-existing morphology. Quantification of near-bed flow velocities are made via comparison with wind tunnel experiments. We estimate that shear velocities of ca. 0.30 m.s-1 (equivalent to pure wind velocity of 6 to 8 m.s-1 at 10 cm above the bed) could emplace the constructive bedsets, whereas the truncative phases would result from bursts with impacting wind velocities of at least 30-40 m.s-1

Pyroclastic dune bedforms: macroscale structures and lateral variations. Examples from the 2006 pyroclastic currents at Tungurahua (Ecuador).

Pyroclastic currents are catastrophic flows of gas and particles triggered by explosive volcanic eruptions. For much of their dynamics, they behave as particulate density currents and share similarities with turbidity currents. They occasionally deposit dune bedforms with peculiar lamination patterns, from what is thought to represent the dilute, low concentration, and fluid-turbulence supported end member of the pyroclastic currents. Here, we present a high resolution dataset of sediment plates (lacquer peels) with several closely spaced lateral profiles representing sections through single pyroclastic bedforms from the August 2006 eruption of Tungurahua (Ecuador). Most of the sedimentary features contain backset bedding and preferential stoss-face deposition. From the ripple scale (few cm) to the largest dune bedform scale (several m length), similar patterns of erosive-based backset beds are evidenced. Recurrent trains of sub-vertical truncations on the stoss side of structures reshape and steepen the bedforms. In contrast, sporadic coarse-grained lenses and lensoidal layers flatten bedforms by filling troughs. The coarsest (clasts up to 10 cm), least sorted and massive structures still exhibit lineation patterns that follow the general backset bedding trend. The stratal architecture exhibits strong lateral variations within tens of centimeters, with very local truncations both in flow-perpendicular and flow-parallel direction. We infer that the bedforms' sedimentary patterns result from four formation mechanisms: "differential draping", "slope-influenced saltation", "truncative bursts", and "granular-based events". Whereas most of the literature makes a straightforward link between backset bedding and Froude-supercritical flows, we reconsider this interpretation. Indeed, features that would be diagnostic of subcritical dunes, antidunes, and "chute and pools" can be found on the same horizon and in a single bedform, only laterally separated by short distances (10s of cm). Our data stress the influence of the pulsating and highly turbulent nature of the currents and the possible role of coherent flow structures such as Görtler vortices. Backset bedding is interpreted here as a consequence of a very high sedimentation environment of weak and waning currents that interact with the pre-existing morphology. Quantification of near-bed flow velocities are made via comparison with wind tunnel experiments. We estimate that shear velocities of ca. 0.30 m.s-1 (equivalent to pure wind velocity of 6 to 8 m.s-1 at 10 cm above the bed) could emplace the constructive bedsets, whereas the truncative phases would result from bursts with impacting wind velocities of at least 30-40 m.s-1

Rinikerfeld Palaeolake (Northern Switzerland) – a sedimentary archive of landscape and climate change during the penultimate glacial cycle

While timing and ice extent of the last glacial maximum are generally well known, the courses of earlier glaciations have remained poorly constrained, with one of the main reasons being the scarcity of sedimentary archives. This study introduces a new palaeolake record from a Mid‐Pleistocene glaciofluvial channel system in the Lower Aare Valley (Northern Switzerland). The record of Rinikerfeld comprises a >40 m long succession of Quaternary deposits that are targeted by multi‐method sedimentological analysis. Sedimentary facies together with geochemical and geotechnical parameters, pollen content, as well as luminescence ages allow the reconstruction of the establishment, evolution and infilling of the early Marine Isotope Stage 6‐aged Rinikerfeld Palaeolake. A drastic change in lake sediment composition and structure indicates cessation of the initial glacially derived input, which is explained by landscape modification and drainage rerouting during the Penultimate (Beringen) Glaciation. Geochemical and palynological data further reveal cold, initially periglacial but slightly ameliorating, climate conditions, while the lake was progressively filled up by local runoff, before being buried by periglacial colluvial diamicts, and potentially overridden by ice. It is therefore concluded that the onset of the Beringen Glaciation was an environmentally as well as geomorphically dynamic time period in the Northern Alpine Foreland.NATIONALE GENOSSENSCHAFT FÜR DIE LAGERUNG RADIOAKTIVER ABFÄLL