Chronostratigraphie des accumulations würmiennes dans la moyenne vallée du Rhône. L'apport des dates radiocarbone

Abstract

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.