Palaeoenvironmental and drainage network reconstitution of the Oligocene Western Alpine Foreland Basin

The Western Alpine Foreland Basin ("French Molasse Basin") is located along the western Alps and is composed of Oligo-Miocene formations resulting from the erosion of the alpine range. Although the Miocene molasse basin have been widely described since the last decades, Oligocene basins lack documentation in terms of palaeoenvironmental evolution and source to sink approach. Most of these basins formed under both the Alpine influence and the European Cenozoic Rift System influence and developed in lacustrine environment with local sedimentation next to active normal faults. Several fluvial formations with exotic materials have been briefly described and could correspond to a transport from the internal parts of the Alps, where collision started at the Eocene/Oligocene boundary. Here, we propose a new tectono-sedimentary study of these fluvial deposits based on extensive fieldwork (facies analysis, sequence stratigraphy, palynological analysis) and reinterpretation of available subsurface data (seismic profile, well data). The goal is to provide a new palaeoenvironmental reconstitution of the Oligocene molasse basin(s) with regional correlations and to determine the evolution of early alpine drainage network. We focus on the entire Western Alpine Foreland Basin from the Rhone Valley (Bas-Dauphiné, Valréas, Mormoiron) to the Digne Thrust where Oligocene molasse is called « Red Molasse » (Dévoluy, Faucon-du-Caire, St-Geniez, Esclangon, Barrême). First results show that Red Molasse is composed of massive meandering deposits, which evolve to braided river and alluvial fan in a regressive continental sequence following the flysch formation. Transition from marine distal turbidites is often missing except in Dévoluy syncline where tidal and shoreface deposits precede fluvial molasse. Exotic material from the internal alps is very common and indicates high landforms nearby. In the Rhone Valley, a massive fluvial system has been identified on seismic and well log data in the Bas-Dauphiné and we documented a 900 m field section with two meandering formations with exotic minerals in the Mormoiron basin. Paleocurrents and channels direction indicate a major divide located east of Diois-Baronnies range with Dévoluy fluvial systems flowing to the north and other Red Molasse sites located south of the divide converging to St-Geniez system. On a regional scale, it may be possible that early salt tectonic which has been widely described caused this particular drainage network. South of the divide, converging fluvial formations may have flowed in an Est-West valley between Diois-Baronnies range and Ventoux-Lure Montain where tectonic and Eocene landforms link to the Pyreneo-Provençal orogen have been documented. These deposits where probably connected with Mormoiron and Bas-Dauphiné fluvial formations and formed a major drainage system located in the Rhone Valley.