Tectonics and fluviodynamics at the Saone-Rhone confluence from the Wurm to the Holocene (France).

Tectonics and fluviodynamics at the Saone-Rhone confluence from the Wurm to the Holocene (France). Classical studies dealing with the downstream course of the Saone River have enlighted the fact that the altitude of the gravelly Wormian alluvial level is lower than the modern river bed. Moreover, this level stands lower than the Wormian fluvio-glacial terrace of the Rhone at Lyon. The common opinion is that tectonics is responsible for the subsidence of the alluvial plain of the Saone and for the surrection of the Lyon area. New field evidence and the use of neglected ancient observations allow to propose a new set of interpretations about postglacial evolution. The low position of Wurmian alluvium in the region of Villefranche-sur-Saone is explained by fluvial incision during the Lateglacial period, the lowered Rhone River being the base level of the Saone. The early Holocene accumulation in the Rhone valley induced a regressive accumulation in the lower Saone valley. Moreover, the Azergues River, a tributary of the Saone flowing from the Massif Central and delivering a high bedload, may have plaid a role in the control of the long profile of the lower Saone. It may have dammed the Saone upstream its confluence and increased the deposition of fine material by floods in the alluvial valley of the Saone. The probable important contribution of fluvial processes to the history of sedimentation does not preclude the role of tectonics, the efficiency of which having been proved during the Quaternary era. Then this contribution combines fluvial processes and tectonics and reappraises the weight of fluvial processes in the geomorphology of the lower Saf,ne River since the Upper Wurmian maximum.