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Titel |
Overdeepened glacigenic landforms in Lake Thun (Switzerland) revealed by a multichannel reflection seismic survey |
VerfasserIn |
Stefano Fabbri, Marco Herwegh, Fritz Schlunegger, Christian Hübscher, Benedikt J. Weiss, Cedric Schmelzbach, Heinrich Horstmeyer, Marius W. Buechi, Flavio S. Anselmetti |
Konferenz |
EGU General Assembly 2016
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Medientyp |
Artikel
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Sprache |
en
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 18 (2016) |
Datensatznummer |
250132272
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Publikation (Nr.) |
EGU/EGU2016-12763.pdf |
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Zusammenfassung |
Recently acquired high-resolution multibeam bathymetry, in combination with a 2D
multichannel reflection seismic campaign on perialpine Lake Thun (Switzerland) reveals new
insights into the diverse geometry of the lake basin and a so far unknown subaquatic moraine
crest with unprecedented clarity. These new data will improve our comprehension concerning
the retreat phases of the Aare glacier, the morphology of its proximal deposits and the facies
architecture of the subglacial units.
The overdeepened basin of Lake Thun was formed by a combination of tectonically
predefined weak zones and glacial erosion during the last glacial periods. The new data
indicate that below the outermost edge of a morphologically distinct platform in the south
eastern part of the lake basin, a ridge structure marked by strong reflection amplitudes occurs.
This structure is interpreted as a subaquatic terminal moraine crest, most likely created by a
slightly advancing or stagnant grounded Aare glacier during its major retreating phase. The
terminal moraine smoothly transforms downstream into well distinguishable foresets with
internally recognisable layering, which dip steeply towards the deepest part of the basin,
eventually transforming into bottomsets. This depositional sequence formed by the fore- and
bottomsets represents ∼50% of the overall sediment volume that fills the basin and was
deposited while the glacier was stagnant, interpreted to represent a rather short period of time
of a few hundreds of years. This sequence 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 retreating glaciers between their
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 glacial stabilization or slight
readvance. Findings from pollen analyses by Ammann (1994) hint at a completely ice-free
Northern Alpine foreland during the Oldest Dryas. 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) while 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). The deposition of the subaquatic moraine of the Aare glacier
hence has to fit temporally between these age constraints, implying rather high
sedimentation rates, which will be integrated in an appropriate sedimentological concept
quantifying subaquatic moraine formation in a recessional overdeepened setting. |
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