Reappraisal of the Messinian canyons morphology of the Rhône and Ardèche (Rhône valley): new insights from seismic profiles

Abstract

12 available two-way time high-resolution seismic reflexion profiles located on the central part of the middle Rhône valley have been interpreted. In addition, one of the profiles was reprocessed to determine the P-wave velocities of the main geological units and to convert this line into a depth cross-section. On all these profiles, the Lower and Upper Cretaceous units are clearly identifiable, as is the Messinian Erosion Surface (MES) carved out during the Messinian Salinity Crisis (MSC) by the paleo Rhône and its tributaries, the Ardèche and Cèze paleo-canyons. The Plio-Quaternary fill of these paleo-canyons shows at least 4 main units showing an overall transgression. The combination of geological data from geological maps, geological field surveys and borehole data has made it possible to model the MES in 3D at the scale of the region, and in depth/altitude. This surface is compared to the velocity contrast surface that could be derived from the H/V method, highlighting some differences in interpretation between the two geophysical methods. From a geological point of view, the interpretation of the seismic lines led to propose a reconstruction of the stages in the sub-aquatic filling of the Messinian-Pliocene aggradation of the paleo-river. Several occurrences of Mass Transport Deposits (MTDs) were identified during the fall of the Mediterranean sea level as well as during the rise of sea level. From a geomorphological point of view, this study provides new information on the route and longitudinal profile of paleo-rivers and, in particular, it deepens the profile of the Paleo-Rhône at the latitude of the Tricastin region (up to 700m bsl) and significantly modifies the course of the Ardèche as proposed in previous studies. The Paleo-Ardèche river, known to develop a karstic system during the MSC, is connected to a deep canyon, most likely through a karstic pocket valley as suggested by the very steep longitudinal profile of the MES. Finally, from a structural point of view, interpretation of the seismic lines shows a broad ENE-trending anticlinal structure associated with a normal fault apparently not affecting the Mio-Pliocene fill. In the southern part of the area, near the Uchaux anticline, the imaged structures suggest the presence of a recent (syn to post-Pliocene) fold propagation fault. In addition to all the new information provided by this study on the geology, morphology and methods of excavation and filling of the Messinian paleocanyon, the proposal of a topographic model of the paleo-canyon is crucial for modelling seismic movement in the context of a basin with a complex geometry and, in particular, for the numerical assessment of site effects in a context of low seismicity.