High-resolution seismic stratigraphy of late quaternary fill of Lake Annecy (northwestern Alps): evolution from glacial to interglacial sedimentary processes

Lake Annecy is a peri-alpine lake of glacial origin, remnant of the last glaciation. The lake basin accumulated over 150 m of sediment since the Last Glacial Maximum. A dense grid of high-resolution seismic data, acquired during a 1993 sparker survey, revealed the glacial and post-glacial sedimentary infill, the depth and morphology of the Wurm erosion surface and penetrated the Riss-Würm interglacial deposits as deep as the underlying Riss erosion surface. This paper focuses on the interpretation of the high-resolution seismic facies of the infilling sediments of the lake basin in correlation with the results of the Annecy core. The quality and density of the seismic data allow, in this case, detailed analysis of the seismic facies in their three-dimensional organisation and permit reliable interpretation in terms of depositional process and palaeo-environmental setting. The study reveals that the geometry and seismic facies of the infilling deposits are controlled essentially by the basin morphology and the characteristics of the sediment supply. The sediment supply to the lake changes throughout the deglaciation from subglacial drainage, to proglacial meltwater streams, to alluvial streams and finally to the present-day interglacial condition where authigenic sedimentation prevails. The depositional patterns change accordingly from chaotic outwash deposits and glacio-lacustrine muds at the base, over axially aggrading lacustrine fans, prograding alluvial fan deltas at the lake margins to, finally, a drape of lacustrine chalk and marl. The interpreted sedimentary environment and deduced character of the sediment source are used to reconstruct the changing palaeo-environment in the catchment area.