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CGC-11 Lacustrine records as archives of climate change
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A sedimentological view on the complex glacial and Holocene history of Lago Villarrica
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Katrien Heirman, Ghent University (Belgium)
Sofie Lahousse, Ghent University (Belgium)
Jasper Moernaut, Ghent University (Belgium)
Maarten Van Daele, Ghent University (Belgium)
Stephen Roberts, British Antarctic Survey (United Kingdom)
Marc De Batist, Ghent University (Belgium)
Mario Pino, Universidad Austral de Chile (Chile)
Robert Brümmer, Universidad Austral de Chile (Chile)
Roberto Urrutia, Universidad de Concepción (Chile)
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Lago Villarrica (39°S) is one of the lakes that were created by glaciers with a piedmont lobe in the Región de los Ríos in South Central Chile. The locally poorly developed plain of the central valley and the proximity of several active volcanoes prevented the glacier lobe from free expansion and constrained it to previously eroded glacial and fluvial valleys.
The Villarrica glacier was located at the northernmost edge of the former Patagonian Ice Sheet making it very sensitive to all types of ice sheet fluctuations. The sedimentary infills of the different glacial lakes in the Región de los Ríos and los Lagos show strong resemblances. These similarities were revealed thanks to the acquisition of dense grids of high-resolution reflection seismic profiles that provide a complete view of the sedimentary infill of the lake basins. As in all of the lakes in this region, the acoustic penetration in the eastern part of Lago Villarrica is limited due to gas blanking. Fortunately, the western part of the lake has a very good penetration.
A striking characteristic of the lake infill is the existence of a sequence of erosional unconformities, which all occur in the area where the glacier most likely lost its restrictions enforced by the bedrock geology of the area. These unconformities separate units that are acoustically sub-horizontally stratified, sometimes with an acoustically chaotic unit in between them. These lower stratified units are virtually undisturbed by faulting or folding, except for some fluid-escape structures. Such extensive, high-relief erosional surfaces in this context are most likely created by glacial erosion. Their presence in the seismic stratigraphy therefore suggests that the lake-basin infill has recorded multiple periods of glacier advance and retreat. We postulate that these stratified facies probably represent deposits from a previous interstadial period, since no interglacial deposits have been observed near the lakes. A volcanic origin of these sediments can nonetheless not totally be excluded considering the geodynamic setting of the region.
In January 2008, a 9-m-long core was taken on a ca. 80-m-deep plateau in the western part of Lago Villarrica. Analyses and dating of this core are underway, but we expect the core to cover the entire Holocene history of the lake (based on core studies from similar environments in similar lakes in the region). The preliminary sedimentological results of this core are here presented.
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