A radiocarbon dating of the Moras drilling (30 800 cal BP) is located at the bottom of lacustrine deposits on the Crémieu plateau. A former ice-dammed lake is attributed to the ponding of meltwaters in the Moras River valley by a diffluence of the Rhône glacier It allows to assert that the Rhône glacier did invade the piedmont of the Alps dunng the Late Würm and that this recurrence can be correlated with the so-called stade D (Mandier, 1988a).
This stratigraphy is confirmed elsewhere in the fluvio-glacial alluvium of the Rhône valley m the Péage-de-Roussillon area (24 000 cal BP and 22 250 cal BP) and m the alluvium of the Isère valley at Saint-Hilaire-du-Rosier (32 500 cal BP and 25 500 cal BP). The last Würm terraces above the Holocene levels are positioned at two thirds of the amount of total incision following stade D. All of these terraces are comprised in between 17 000 cal BP and 18 250 cal BP in the Rhône and in the Isère valley. They are correlated with stade E (Morestel stade). A tentative correlation with Greenland GRIP drilling is proposed : the Rhône LGM of stade A > 45 000 cal BP (considering the presence of Mousterian industry on top of a terrace) is better correlated with isotopic stade 4 (Heinrich 6 event). For stades B and C, two solutions are proposed. The first one links these stades to the double cold event occurring between GRIP interstadial 4 and 5. The second one, correlates stades B and C with Heinrich 4 and Heinrich 5 events. Stade D can be clearly attributed to the severe cold event, which occurred between GRIP interstadial 3 and 2 (Heinrich 2 event). At last, stade E (Morestel) must be attnbuted to the cold event which follows interstadial 2 and which ends at 1 8 250 cal BP.
Depuis longtemps, les phases de récession würmiennes du Bas-Dauphiné et de la Région Lyonnaise ont été étudiées en détail par les auteurs, mais sans aucun calage en chronologie absolue. En 1984, nous avons proposé d'attribuer le maximum d'extension glaciaire au Würm ancien (stade A de Béligneux > 35 000 B.P.) et les autres phases (stades B, C, D, E) à un Würm récent calé seulement par des corrélations avec les terrasses du Péage-de-Roussillon (fig. 1) datées de 19 000-20 000 à 14 000-15 000 B.P.
Plus récemment des sondages effectués dans le bassin de Malville et une datation absolue obtenue dans l'un d'eux constituaient des faits nouveaux susceptibles d'être contradictoires avec les résultats stratigraphiques rappelés ci-dessus. D'où la présentation de deux hypothèses de travail :
— la première consiste à admettre une déglaciation complète du piedmont avant le Pleni-Würm supérieur, mais cette solution, outre les problèmes de dynamique qu'elle pose quant aux profils en long des terrasses, est en contradiction avec le dispositif sédimentaire du bassin de Malville;
— la seconde, celle de culots de glace persistant dans les grands ombilics, notamment celui du Bouchage, permet de lever la contradiction et de rectifier partiellement notre ancienne façon de voir.
Two cores were drilled in the river Drômes's vast and thick (10-12 m) holocène alluvial cone, where it flows into the river Rhône. The presence of alterning layers of terrigenous silty-clayey sediments and organic peaty loam made it possible - thanks to sedimentology and radiocarbon dating - to identify the different evolving phases of the river. The period of post-maxi wurmian river-bed formation ends before 11 850 B.P. It is followed until 7000 B.P. by a period of low accumulation (average rate: 10 centimeters per 1000 years) and then between 7000 B.P. and 3000 B.P., by a stage of accelerated accumulation (2 meters per 1000 years). Subsequently and up to the present the average rate drops markedly (50 centimeters per 1000 years), although this doesn't necessarily indicate a decrease in erosion phenomena nor a slowdown in downstream accumulation : increase in the cone's surface area and retention of recent upstream accumulation.
We would suggest an attempted correlation between on the one hand the detrital accumulation crisis from 7000 B.P., and on the other, the early and large anthropisation of the mountain basin during the Middle and perhaps even Early Neolithic periods equally demonstrated by pollen analysis. Therefore could be explained the importance of both upstream erosion phenomena and downstream accumulation.
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