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

Abstract

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 ~200 m below sea level, implying more than 750 m 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 ice-distal 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 10 Be 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 1,000 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.

Combined On-Fault and Off-Fault Paleoseismic Evidence in the Postglacial Infill of the Inner-Alpine Lake Achensee (Austria, Eastern Alps)

“…The Pertisau basin seismic-stratigraphic unit succession closely compares to the type stratigraphy of glacigenic lake sequences as established for other alpine and peri-alpine lakes (Van Rensbergen et al, 1998;Ndiaye et al, 2014;Daxer et al, 2018;Fabbri et al, 2018). Accordingly, we interpret the basal unconformity as a glacially scoured valley into a sedimentary succession (U0) deposited before the Last Glacial Maximum.…”

Depositional setting of the southern arms of Lago Argentino (southern Patagonia)

“…The overall complex morphological and acoustic characteristics of the seismic sequence bounded by U2, i.e. the Glacial Unit, suggest that it represents glacial deposits, with the mound-shaped reliefs being moraines, similar to how crested morphologies identified in the Lago Fagnano has been interpreted (Waldmann et al, 2010), as well as in alpine lakes (Fabbri et al, 2018) and other perialpine lakes (Dowdeswell et al, 2016;Hilbe et al, 2016). However, we cannot differentiate seismically whether they represent frontal or recessional moraines.…”

Late-glacial fluctuations of two southern Patagonia outlet glaciers revealed by high-resolution seismic surveys

“…This would indicate that they originated from subglacial meltwater at the grounded glacial front and were produced in large amounts during deglaciation, as is also hypothesized for perialpine lake moraines (e.g., Hilbe et al, 2016). Studies carried out in alpine lakes (Fabbri et al, 2018) and in lakes of the British Islands (Pinson et al, 2012) using similar geophysical methods as those used in this work have proposed a diversification between the various glacial deposits based on specific seismic facies and architecture and analogies with available ground outcrops. However, it is often difficult to find correspondences between very different depositional and glacial settings, which prevents a general classification that applies to all environments.…”