The Pliocene Succession of Lyon Metropolis (SE France): an Overfill of a Messinian Incised-Valley

The Pliocene ria, a narrow seaway running up the Rhône Valley, has been mapped for a while by field geologists. Only much later, after the DSDP Leg 13 in 1970, a consensus was reached that this unique geological feature of the Rhône Valley was created by the major Mediterranean sea-level drop associated with the Messinian Salinity Crisis, followed by a sudden sea-level rise caused by the breach of the Strait of Gibraltar and the invasion of the Mediterranean Basin by the Atlantic waters. At the regional level of the Lyon Metropolis in the upper Rhône Valley, main issues were however remaining about the course and depth of the Messinian valley, and about the valley fill, namely where and how do the Pliocene marine strata of the Rhône Valley pass to the continental deposits of the Bresse Basin to the north? These are key-questions in that the Plio-Pleistocene makes up a large fraction of a basement that holds up a large city, not free from potential geological hazards and subject to problems of groundwater management, high-cost tunneling projects, etc. Our work is based on a review of the historical researches - descriptions of the outcrops and fossil assemblages, followed by an unprecedented analysis and physical correlation of a thousand boreholes drilled over the last decades. Sections across the Messinian valley reveal a proper canyon morphology for the segments that cut the crystalline basement. The magnitude of the incision has been calculated as 335 m to a minimum, which yields a local erosion rate of about 2,400 m.Myr-1. Three major depositional systems are distinguished for the Pliocene - Lower Pleistocene succession. The valley that initially ran much farther north of Lyon was occupied in the Zanclean by a series of pounded lakes, dammed by transverse local alluvial fans, filled by minor Gilbert-type deltas, and repeatedly flooded by marine ingressions. The valley was then overfilled during the Piacenzian by a major transverse fluvial system that onlapped the valley wings. At the level of Lyon, the Alps-sourced, fluvial deposits were deflected to the north (Sables de Trévoux) and to the south (Alluvions jaunes inférieures et supérieures) as a dichotomy. The Pliocene was then capped by an Early Pleistocene, gravel-dominated alluvial fan (Alluvions jaunes sommitales) that spread out from the north-east to the south, and intersected the previous overfill due to the sudden shift to the north, i.e., South Jura, of the feeder stream. The depositional and current elevations of the marine-influenced episodes, i.e. marker bands that punctuate the regional Neogene succession, are used to bring out alternating uplift and subsidence phases we tentatively relate to the unload - reload of the Mediterranean Basin by the marine waters. Finally, we interpret the major shift in the depositional patterns of the Pliocene - Pleistocene transition in the Lyon area as reflecting a major change from an inactive thrust belt to vertical uplift in the Western Alps.