Cave sediments in the Western Bauges karst: A record of Middle and Upper Pleistocene glacial advances in the French Alps

Previous studies point to a discrepancy in the timing of the maximum extent of the Alpine glaciation during the last glacial cycle (Würmian) in the Western Alps. Work carried out on the Lyon palaeoglacier lobe in France suggests that the glacial maximum occurred during Marine Isotope Stage (MIS) 4, while more recent studies on the Rhône palaeoglacier lobe in Switzerland provide evidence of a maximum during MIS 2. Here, we present new chronological data from the karst region of the Western Bauges (French Alps). Between 500 m and 1200 m a.s.l. several karst systems were in lateral position to the Isère palaeoglacier. This palaeoglacier invaded the Bauges limestone massif and the Chéran valley by a transfluence controlled by the Frêne pass at 980 m a.s.l. Allochthonous glacier-derived gravel and sand interbedded with speleothems are present in caves, providing terminus ante quem and post quem age constraints on the timing of several glacial advances. Other speleothems were submerged by rising water levels in the karst due to damming by the glacier. Finally, some speleothems show shatter marks caused by the impact of pebbles transported by highly turbulent meltwaters. 25 U/Th dates of speleothems cover MIS 9 to 1. For MIS 4 to 2, they allow, indirectly, to constrain the different glacial advances into the Bauges limestone massif and to re-evaluate the timing of the maximum Alpine glaciation during the Würmian in this part of the Western Alps. Although the new data and observations do not exclude the possibility of a maximum glacier advance during MIS 2, they argue for a maximum advance during MIS 4 and a local, smaller one during MIS 2.