Interprétation de la déglaciation rhodanienne au Würm, des moraines « internes » à la cuvette lémanique

Abstract

Between the "inner moraines" and the upperside of the Leman Lake, we observe the following elements : — two generations of "inner moraines" of as yet undetermined age, the oldest being over 241 10 ±900 B.P.; — a series of over-deepened basins now either wholly filled up with lacustrine deposits (Malville, Le Bouchage) or not (Le Bourget, Annecy, Léman), with thicknesses of several hundred meters; — a series of paraglacial or glacio-lacustrine supra-morainic deposits directly related to the déglaciation, which are disposed "en echelon" from the downstream side to the upstream side; — on single till cover to the downstream side of the Geneva basin, directly superposed to the substratum or to a wurmian ante-morainic complex; — a thick complex of superposed morainic, lacustrine and marshy deposits in which several interstadials were dated from 34550 to 23500 B.P. in the Leman basin only (Chablais, Vaud). Present chrono-stratigraphic conclusions, notably concerning the age of the maximum glacial advance, differ between the frontal area (early Wùrm) and the upstream side of the Leman basin (late Wùrm). Now, we note that : — undoubted, dated interstadial deposits only occur in the Leman basin, far upstream from the "inner moraines"; — between the "inner moraines" and the Leman basin, there is only one single till cover as well as déglaciation deposits, without any evidence of interstadial formations or of true glacial stops. Therefore, in all likelihood, the morainic deposit of the Plateau Savoyard represents one single glacial phase, predating the oldest Leman interstadials (34600 ± 2700 B. P.). One indétermination still subsists, regarding its age, in relation with both the sedimentary gaps in the glacial-interstadial series and the duration of the sedimentation process of the déglaciation and of later lacustrine deposits.