Palaeogeographical relationships between Alpine and Jura glaciers during the two last Pleistocene glaciations

Two main moraine complexes are usually defined in the northwestern Alpine piedmont: the External Moraine Complex (EMC) dated to the penultimate glaciation (“Riss” sensu lato) and the Internal Moraine Complex (IMC) dated to the last glaciation (“Wurm”). Until recently the prevalent concept was that these two complexes had been built by glaciers originating in the central Alpine zone. This paper presents a new palaeogeographical diagram, based on mapping and petrographical studies of these two moraine complexes, and especially on the relationships between Alpine and Jura tills. We reach the following conclusions. 1. (1) The Jura glaciers were independent of the Alpine glaciers during the two last Pleistocene glaciations. 2. (2) The Jura Mountains formed a topographical barrier blocking and modifying the Alpine glacier advance towards the northwest, with the result that ◦ -during the Wurmian glaciation, the maximum elevation (1200 m a.s.l.) of the Alpine glaciers on the eastern margin of the Jura was too low to allow Alpine ice to enter the Jura; and ◦ -during the “Rissian” glaciation, the greater thickness of the Alpine glaciers resulted in their penetration only into the lower marginal zone of the Jura. 3. (3) Thus, in the studied area, the central Alps were not the only centre of ice dispersal during the two last Pleistocene glaciations: on the northwestern Jura slopes, the corresponding moraine complexes were mainly built by glaciers emanating from the central Jura.