The Würmian glaciation in the Geneva basin ; reconstitution of the vegetation dynamics in a context of successive ice retreats
by
Frédéric Guiter
a IMEP, UMR 6116 CNRS, case 451, Faculté Saint Jérôme,F-13397 Marseille cedex 20
Coauthors: Valérie ANDRIEU-PONEL (IMEP, UMR 6116 CNRS, case 451, Faculté Saint Jérôme,F-13397 Marseille cedex 20), Jacques-Louis de BEAULIEU (IMEP), Boris VANNIERE (Lab. de Chrono-écologie, UMR 6565, 16 route de Gray ,F-25030 Besançon cedex), Gilles BOSSUET (Lab. de Chrono-écologie), Anne TRIGANON (Lab. d'Hydrogéologie, Faculté des Sciences, 33 rue Louis Pasteur, F-84000 Avignon), Bernard BLAVOUX (Lab. d'Hydrogéologie), Gérard NICOUD (dLab. Géologie Structurale et Appliquée, Université de Savoie, BP 1104,F-73011 Chambéry cedex)

The glacial Geneva basin has been investigated since the outset of the glacial theory and was one of the first alpine regions where the plurality of the Würmian alpine glaciations has been evidenced [1] [2]. Recent multi-proxy studies [3] [4] [5] have been carried out on interstadial and glacial sediments covering the last 80, 000 years. They indicate that the maximum extent of the Rhodanian glacier [6] is prior to the Last Glacial Maximum (LGM) as evidenced formerly in the Pyrénées [7] [8], the Vosges [9] and recently in the Cantabriques Mountains [10]. The aim of our new investigations is 1) to validate the hypothesis of an ancient Würmian maximum advance thanks to pollen, paleomagnetic and isotopic (delta¹⁸O) data ; 2) to reconstruct the evolution of both vegetation and climate during the Würmian deglaciation. Our work is subdivided into two parts : first, the study of interstadial deposits interlayered in morainic formations. Pollen assemblages of organic facies of the SP3 morainic profile dominated by Pinus and Poaceae are the testimony of cold and dry vegetation in the surroundings of the Geneva basin. This sequence is dated > 54, 000 years BP. Study of further organic sequences interlayered in Würmian morainic deposits is still in progress. Second, the study of four paleolacustrine sites located on the Plateau Gavot, north of Evian between 580 m and 970 m altitude. We intent to reconstruct vegetation and climate changes that prevailed at the end of the ultimate deglaciation. The pollen record of La Beunaz peaty sequence (971 m altitude) begins during the middle Holocene, suggesting that the development of the peat-bog is not contemporaneous with the Rhodanian in situ glacier melting. Moreover, pollen and paleomagnetic data provided by Les Blaves and Roseire profiles, show that two paleolakes were present respectively at 580 m and 890 m altitude since the beginning of the Lateglacial Interstadial. The palaeoenvironmental record of the last 15, 000 years is as well evidenced in other profiles. Their analysis is still in progress.

References

[1] Morlot A., 1858 : Sur le terrain quartaire du bassin du Léman -Bull. Soc. Vaudoise Sci. Nat., 6, 101-108. [2] Gagnebin E., 1937 : Les invasions glaciaires dans le bassin du Léman -

Bull. Labo. Géol., univ. Lausanne 58, 82 . [3] Montjuvent G. Nicoud G., 1988 : Interprétation de la déglaciation rhodanienne au Würm, des " moraines internes " à la cuvette lémanique - Bull. AFEQ, Paris 2-3, 129-140. [4] Nicoud G., Coddet E., Blavoux B. Dray M., 1993 : Les complexes détritiques de marge glaciaire active dans le Bas Chablais (bassin lémanique, France). Implications hydrogéologiques - Quaternaire, 2-3, 69-76. [5] Schoenheich P., 1993 : Correlation of the alpine LGM and deglaciation with the Greenland isotopic record - Bull. AFEQ, vol. 5, n. 3, 203-215. [6] Mandier P., 1984 : Le relief de la moyenne vallée du Rhône au Tertiaire et au Quaternaire : essai de synthèse paléogéographique - Thèse doct., Lyon 654 + 217 p. [7] Andrieu V., Hubschman J., Jalut G. Hérail G., 1988 : Chronologie de la déglaciation des Pyrénées françaises. Dynamique de sédimentation et contenu pollinique des paléolacs : application à l'interprétation du retrait glaciaire - Bull. AFEQ, Paris 213, 55-67. [8]Andrieu-Ponel V., Beaulieu J.L., Ponel P. Guiter F., 2000 : Environnement glaciaire et végétations dans les Pyrénées françaises entre 25 ka cal. BP et 15 ka cal. BP - Rapport du Programme National d'Etude de la Dynamique du Climat (PNEDC). [9] Seret G., Dricot E. Wansard G., 1990 : Evidence for an early glacial maximum in the French Vosges during the last glacial cycle - Nature, 346, 453-456. [10] Jimenez-Sanchez and P. Farias Aquer, in progress: Late Quaternary Glacier Evolution in Tedes Natural Park, Cantabrian Mountains, NW Spain -Accepted in Geodinamica Acta .

Date received: May 22, 2001

Copyright © 2001 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # cahr-12.