Palaeoenvironmental and drainage network evolution 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, at least to some extent, from the erosion of the alpine range. The distribution of sedimentation area, drainage network and sedimentary sources have strongly varied during its development. Late Eocene and Miocene marine formations are well-constrained as longitudinal basins with some transverse sedimentary transfer: the Eocene turbiditic basin was fed from the South, whereas the Miocene molasse basin was flowing southward. The Oligocene time period corresponds to the beginning of continental collision and to the exhumation of internal crystalline massifs. The erosion of first Alpine landforms causes the transport of sedimentary materiel in the basin with the transition from flysch (underfilled) to molasse (overfilled) deposits. The paleoenvironment is mainly continental and sediments are preserved in both internal and external position, which attests of a complex drainage network. Oligocene is therefore an important period of reorganisation in the foreland basin but has been poorly studied at the scale of the whole Western Alps and remains under-documented, mainly because of scarce outcrops probably due to early deformation in the basin. Here, we provide a new tectono-sedimentary study of these deposits based on new field work, seismic and well data interpretations, palynological analyses and bibliographic synthesis. This work led us to propose an exhaustive synthesis of the Oligocene foreland basin (or sub-basins) with synthetic logs and detailed palaeoenvironmental maps. Our results show that the Oligocene Western Alpine Foreland Basin can be divided in two main sedimentation areas: (1) an internal area which is mainly influenced by the alpine range evolution and (2) the Rhône Valley which has been structured by both the European Rift and the Pyrenean orogeny ("Pyrenean-Provence phase") and receives autochthonous sediments but also erosional products from the Massif Central, the Pyrenean Chain and the Alps. Palaeoenvironments and nature of sedimentation have strongly changed during the entire Oligocene. The internal basins (i.e., in the footwall of the Penninic Frontal Thrust) are connected with the South Rhône Valley since the early Rupelian thanks to E-W transverses valleys possibly inherited from the Pyrenean orogeny. Sedimentary supply remains mixt (Massif Central/Alps) until the end of Oligocene. A final longitudinal system set up at the beginning of the Aquitanian in which all the Alpine material was flowing to the south and the Mediterranean Sea. Two episodes of marine incursion have been identified (Early Rupelian and Early Chattian) thanks to biostratigraphy in the Rhone Valley which was probably already connected to the Mediteranean Sea before the Miocene. To sum up, the West Alpine Foreland Basin experienced during Oligocene (and Early Miocene) times transient basin dynamics with sub-basins controlled by westward propagation of the wedge front due to frontal accretion, a complex transverse routing system along with global flow inversion from north to south.