Déglaciation Würmienne dans des conditions lacustres à la terminaison occidentale du bassin lémanique ( Suisse occidentale et France )

At the end of the last glacial cycle, one single moraine ridge has been reported by literature in the alpine Rhone system to indicate a short stop (or a stage) between the Last Glacial Maximum (LGM) near Lyon and the Alps , over a distance of 200 km. The glacier margin deposits of this so-called Laconnex- stage in the Geneva basin have been investigated and are compared here with the sediments in the western part of Lake Geneva, known from geotechnical drilling and high resolution reflection seismic records. The main results of this research are the following: 1. The Laconnex stage and a further stage up valley in the town of Gene- va indicate deglaciation in lacustrine environments, in a large lake covering a much wider area at a higher level than the actual Lake Geneva. Late Würmian lake levels linked ta the Rhone glacier have been identified at altitudes of 470 m, close to 430 m and 405 m. Little is known about the position and nature of the lake dams. 2. The subbottom of Lake Geneva shows particular sedimentary geome- try of the deglaciation. Over sub-glacial tunnel-channels, a sequence of glacial diamicts has been deposited below an ice tangue of a thickness close to the floating equilibrium. Esker deposits are interstratified in the diamicts. Pro- glacial turbidites with dropped boulders and till layers indicate two short glacial stops and advances at about 12 and 18 km up Geneva, near Coppet and the town of Nyon. 3. The large volume of glacial sediments accumulated in the central part of the basin suggests that the Rhone glacier is affected by a rapid melting under relatively warm and wet climatic conditions, as indicated by palynologi- cal evidence and high lake levels. Also , the fading glacier tangue was actively fed with rock debris and water from precipitation and melting in the high areas of the drainage basin. A new 14Cage (18,900 yr. B.P.) indicates early melting of the Rhone glacier system