Subaquatic moraine amphitheatre in Lake Thun

The combination of a recently acquired high-resolution multibeam bathymetric dataset with 2D multichannel reflection seismic data from perialpine Lake Thun reveals new insights into the evolution of the lake basin upon deglaciation and a so far unknown subaquatic moraine. These new data improve our socomprehension of the landforms associated with the ice-contact zone, the facies architecture of the sub- to proglacial units, the related depositional processes, and thus the retreat mechanisms of the Aare Glacier. The overdeepened basin of Lake Thun was formed by a combination of tectonically predefined weak zones and glacial erosion during the last glaciation periods. Seismic stratigraphic analysis of the new data indicates that below the outermost edge of a morphologically distinct platform in the southeastern part of the lake basin (‘Bödeli’), a complex ridge structure marked by strong reflection amplitudes occurs. This structure is interpreted as a stack of several subaquatic terminal moraine crests, most likely created by a slightly advancing or stagnant and grounded Aare Glacier during its overall retreat phase. Packages of overridden moraine crests are distuinguishable, which smoothly transform downstream into prograding clinoforms with foresets with internally recognisable layering. They dip steeply towards the deepest part of the basin, eventually transforming into bottomsets. This stacked succession of subaquatic glacial sequences is overlain by lacustrine deposits formed by Late-Glacial and Holocene laminated muds comprising intercalated turbidites (Wirth et al. 2011). Little is known about the exact timing and behaviour of the retreating Aare Glacier between its recessional phase from the Alpine foreland to the deglaciation of the inner-Alpine ice cap, mostly due to the lack of well-developed moraines that indicate glacier stabilization or slight readvance. Radiocarbon-dated calcareous clay gyttja of Late-Glacial Lake Amsoldingen, located adjacent to the water outlet of Lake Thun, shows a ~16.3 ka BP age (Lotter, 1985), providing a minimum age for the formation of the postglacial small lake. Higher up in the catchment, the oldest 10Be exposure ages from the Grimsel area, the accumulation area of the Aare Glacier, indicate ice-free conditions around 14-11.3 ka BP (Kelly et al., 2006; Wirsig et al., 2016). The emplacement of the subaquatic moraine complex of the Aare Glacier must have occurred between these age constraints, implying high sedimentation rates in the lake basin

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

A subaquatic moraine complex in overdeepened Lake Thun (Switzerland) unravelling the deglaciation history of the Aare Glacier

To investigate the history of the Aare Glacier and its overdeepened valley, a high-resolution multibeam bathymetric dataset and a 2D multi-channel reflection seismic dataset were acquired on perialpine Lake Thun (Switzerland). The overdeepened basin was formed by a combination of tectonically predefined weak zones and glacial erosion during several glacial cycles. In the deepest region of the basin, top of bedrock lies at ~200m below sea level, implying more than 750m of overdeepening with respect to the current fluvial base level (i.e. lake level). Seismic stratigraphic analysis reveals the evolution of the basin and indicates a subaquatic moraine complex marked by high-amplitude reflections below the outermost edge of a morphologically distinct platform in the southeastern part of the lake. This stack of seven subaquatic terminal moraine crests was created by a fluctuating, “quasi-stagnant” grounded Aare Glacier during its overall recessional phase. Single packages of overridden moraine crests are seismically distuinguishable, which show a transition downstream into prograding clinoforms with foresets at the icedistal slope. The succession of subaquatic glacial sequences (foresets and adjacent bottomsets) represent one fifth of the entire sedimentary thickness. Exact time constraints concerning the deglacial history of the Aare Glacier are very sparse. However, existing 10Be exposure ages from the accumulation area of the Aare Glacier and radiocarbon ages from a Late-Glacial lake close to the outlet of Lake Thun indicate that the formation of the subaquatic moraine complex and the associated sedimentary infill must have occurred in less than 1000 years, implying high sedimentation rates and rapid disintegration of the glacier. These new data improve our comprehension of the landforms associated with the ice-contact zone in water, the facies architecture of the sub- to proglacial units, the related depositional processes, and thus the retreat mechanisms of the Aare Glacier

Multiple Quaternary erosion and infill cycles in overdeepened basins of the northern Alpine foreland

The cumulative effect of repeated extensive glaciations represents a poorly constrained component in the understanding of landscape evolution in mid-latitude mountain ranges such as the Alps. Timing, extent, and paleo-climatic conditions of these glaciations are generally poorly understood due to the often-fragmentary character of terrestrial Quaternary records. In this context, the sedimentary infills of subglacial basins may serve as important archives to complement the Quaternary stratigraphy over several glacial--interglacial cycles. In this study, sedimentary facies, valley-fill architecture, and luminescence dating are used to describe nine erosional and depositional cycles (Formations A--I) in the Lower Glatt valley, northern Switzerland. These cycles can be related to the `Birrfeld' Glaciation (~ MIS2), the `Beringen' Glaciation (~ MIS6), and up to three earlier Middle Pleistocene glaciations that can be tentatively correlated to the regional glaciation history. Evidence suggests that deep bedrock trough incision and/or partial re-excavation last occurred mainly during the `Beringen' and `Habsburg' Glaciations. Second-order, 'inlaid' glacial basins document separate glacier re-advances during the Beringen Glaciation. The arrangement of subglacial basins in the Glatt valley with different sub-parallel or bifurcating bedrock troughs, re-excavated segments, and inlaid basins document changes in the magnitude and the spatial focus of subglacial erosion over time. The Glatt valley may thus serve as a key example for the glacial landscape evolution in many other repeatedly glaciated forelands

Deglaciation history of the Aare Valley (Switzerland) revealed by seismic stratigraphy of subaquatic moraine complex

ISSN:1029-7006ISSN:1607-796

Subaquatic moraine amphitheatre in Lake Thun

The combination of a recently acquired high-resolution multibeam bathymetric dataset with 2D multichannel reflection seismic data from perialpine Lake Thun reveals new insights into the evolution of the lake basin upon deglaciation and a so far unknown subaquatic moraine. These new data improve our socomprehension of the landforms associated with the ice-contact zone, the facies architecture of the sub- to proglacial units, the related depositional processes, and thus the retreat mechanisms of the Aare Glacier. The overdeepened basin of Lake Thun was formed by a combination of tectonically predefined weak zones and glacial erosion during the last glaciation periods. Seismic stratigraphic analysis of the new data indicates that below the outermost edge of a morphologically distinct platform in the southeastern part of the lake basin (‘Bödeli’), a complex ridge structure marked by strong reflection amplitudes occurs. This structure is interpreted as a stack of several subaquatic terminal moraine crests, most likely created by a slightly advancing or stagnant and grounded Aare Glacier during its overall retreat phase. Packages of overridden moraine crests are distuinguishable, which smoothly transform downstream into prograding clinoforms with foresets with internally recognisable layering. They dip steeply towards the deepest part of the basin, eventually transforming into bottomsets. This stacked succession of subaquatic glacial sequences is overlain by lacustrine deposits formed by Late-Glacial and Holocene laminated muds comprising intercalated turbidites (Wirth et al. 2011). Little is known about the exact timing and behaviour of the retreating Aare Glacier between its recessional phase from the Alpine foreland to the deglaciation of the inner-Alpine ice cap, mostly due to the lack of well-developed moraines that indicate glacier stabilization or slight readvance. Radiocarbon-dated calcareous clay gyttja of Late-Glacial Lake Amsoldingen, located adjacent to the water outlet of Lake Thun, shows a ~16.3 ka BP age (Lotter, 1985), providing a minimum age for the formation of the postglacial small lake. Higher up in the catchment, the oldest 10Be exposure ages from the Grimsel area, the accumulation area of the Aare Glacier, indicate ice-free conditions around 14-11.3 ka BP (Kelly et al., 2006; Wirsig et al., 2016). The emplacement of the subaquatic moraine complex of the Aare Glacier must have occurred between these age constraints, implying high sedimentation rates in the lake basin