3D MAPPING OF GLACIAL DEPOSITS IN WESTERN LAKE GENEVA USING HIGH-RESOLUTION WATER-BORNE SEISMIC REFLECTION

During the last glaciation maximum (~25 to 18 ky BP), the Geneva area was covered by more than 1000 m of ice belonging to the Alpine ice cap. In the Western Lake Geneva area (Switzerland and France), the geometry of glacial sedimentary bodies and their mode of deposition were poorly understood. Lithological correlations between the existing numerous boreholes is difficult because of the rapid lateral variations and the similarity of deposits resulting from various glacial processes leading to a complex geometry of porous sediments deposited in sub- and pro-glacial environments. To increase our understanding of the depositional processes and the geometry of the sediments associated with the last glaciations, we have acquired a dense grid of lacustrine high-resolution seismic data to create a 3D image of the sedimentary fill.

The Molasse bedrock is deeply incised by a network of subglacial tunnel-valleys, filled with gravel and diamictons of glacial origin. Dome-shaped elongated gravel bodies, initially interpreted as eskers (Moscariello et al., 1998) are recognized in the lower unit. Based on geometrical arguments, we demonstrate that: 1) Many internal reflectors within these sedimentary gravel bodies are truncated laterally, which points out to the presence of a major erosion surface. 2) The 3D mapping of this erosion surface is corresponding to the top of the gravel deposits and displays more like a network of streamlined hills than a meandering pattern of eskers.

Consequently, we reinterpret these elongated gravel bodies as mega-flutes or elongated drumlins, although their transverse cross-sections mimics the shape of eskers. Laterally, they pinch out between the till units. Longitudinal cross-sections show an alternation of depositional sequences and erosion surfaces. These mega-flutes are probably the result of powerful subglacial currents concentrated in channels. The description of the 3D geometry of these deposits, as well as the evidences for important subglacial erosion surfaces, will help to interpret the complex sedimentary record of the densely populated Geneva area.

This research is supported by the Swiss National Science Foundation (grant number 20-68091.02). Interpretation was performed with the Kingdom Suite software from SMT.