Sediment routing and palaeogeographic evolution of the Western Alpine Foreland Basin during the early collisional stage

In the Western Alpine Foreland Basin (WAFB), Late Eocene and Miocene periods were characterized by longitudinal sediment routing systems: The first one was situated within the turbidite basin during the underfilled phase and exhibited a northward orientation toward the Swiss Basin, whereas the second was located in the Rhône Valley during the overfilled phase and was directed southward toward the Mediterranean Sea. The transition between these two periods occurred during the Oligocene, which corresponds to both the underfilled/overfilled transition and the early overfilled period. In this study, we provide new fieldwork observations, seismic and well data interpretations, biostratigraphic analyses and a literature synthesis to reconstruct the palaeogeographic and source-to-sink evolution of the WAFB from Priabonian to Aquitanian. The aim is to discuss this reorganisation of sediment routing in relation to the evolution of the Alpine orogenic wedge, as well as the structural inheritance and the suite of geodynamic events that affected southeastern France during the mid-Cenozoic. We divided the WAFB sedimentary formations into four depositional sequences (S1 to S4). During the deposition of the first two sequences (Priabonian to early late Rupelian; ~37.4–28.8 Ma), the WAFB routing system was influenced by the end of the Pyrenean-Provençal orogeny, the European Cenozoic Rifting System (controlling the Rhône Valley s.l.) and the Alpine orogenic wedge (controlling the Alpine foredeep). The very first connection between the Alpine domain and the Rhône Valley is established at ~30 Ma, during the late Rupelian (S2 highstand), controlled by E -W inherited Pyrenean-Provençal structures implying a ‘broken foreland’. In the meanwhile, from the Dévoluy Basin and northward, the orogenic wedge controlled a classical, although thin, foreland basin characterized by a northward sediment routing connected to the Northern Alpine Foreland Basin. Most of the S3 sequence (Latest Rupelian to middle Chattian; ~28.8–23.25 Ma) corresponds to a decrease of clastic Alpine inputs throughout SE France caused by a reorganisation of the drainage network related with the exhumation of the southern External Crystalline Massifs. S3 highstand and S4 sequence (late Chattian to Aquitanian; from ~23.25 Ma) correspond to the establishment of a longitudinal sediment routing system in the Rhône Valley, with material flowing southwards toward the Gulf of Lion, and supplied by the Palaeo-Isère to the north and potentially by the Palaeo-Durance to the south. This final stage in the reorganisation of the drainage network is clearly associated with the post-rift phase of the Gulf of Lion, which facilitated the opening of a new sink and the ultimate southward migration of the sedimentary area.