The Late Quaternary sedimentary infill of Lake Annecy (northwestern Alps):an overview from two seismic-reflection surveys

The sedimentary fill of Lake Annecy (northwestern Alps) - related to the la st glacial/post-glacial episode - was investigated through high resolution (sparker) and very high resolution (2.5 kHz) seismic-reflection surveys. A seismostratigraphic approach led to subdivision of a 150 m-thick pile (maxi mum thickness in axial part) into five units. Basal units (1 and 2) represe nt an imbrication of subglacial and glacio-lacustrine deposits, close to th e grounding line of the glaciers' fronts (respectively at the northern and southern terminations of the lake). The first acoustically well-stratified unit (3) developed during a fast retreat of the glaciers fronts far from th e lake basin, and a progradational alluvial regime, with abundant underflow s, in a lake larger than the present one. Unit 4 represents the progressive decrease of this elastic input mixed with the progressive development of i n situ bio-induced production. As in many other alpine lakes, a topmost uni t (5), relatively thin (about 8-10 m) and with a conspicuous drape configur ation, is the signature of the Holocene interglacial climatic conditions wi th a sedimentation rate of about 1 mm/yr. On the lacustrine basin slopes, s lumps and debris flow occurred mainly within Unit 3; they may be due to, ei ther climate-induced high rate terrigenous sedimentation, or/and to a perio d of increased seismo-tectonic activity.