Huge Foreland Basin Incised-Valleys Systems (IVSs) as Witnesses of Compressional Stages: Case Study of the Western Alps Foreland Basin, Southeast France

David Besson¹, Olivier Parize¹, Robert W Dalrymple², Noel James², Jean­Yves Reynaud ³, Jean-Loup Rubino ⁴ (1) Ecole Nationale Superieure des Mines de Paris, Fontainebleau Cedex, France (2) Queen’s University, Kingston (3) Museum National d’Histoire Naturelle, Paris, France (4) Total, Pau Cedex, France

Ten depositional sequences can be recognized in the Miocene South East Alps foreland basin. They belong to the Neogene cycle, which evolved from peri-reefal environments to sili­ciclastic coastal and fluvial systems. Geometrical relations of these sequences allow us to identify three huge IVSs, respectively dated Lower, Middle and Upper Miocene. These huge IVSs (up to 100 km long and up to 300 m deep) are related to geodynamic foreland basin evolution and constitute major potential stratigraphics traps. The Lower Miocene IVS cuts most of the East-West Middle Eocene (Provençal) folds and are generally localized along Oligocene extensional structures. This first IVS is linked to the basal foreland unconformity. It presents a compound cool-water carbonates Incised-Valleys Fill (IVFs)with deep erosional sequence boundaries underlined by diagenetic features. Lower Miocene deposits were also inverted and the associated compound IVS was re-incised between Burdigalian and Langhian. This younger incision stage occurs perpendicularly to the first and is linked to a sea level drop, strongly enhanced by regional uplift. This foreland basin uplift is explained by shorten­ing rate increase, shown by fold growing, coeval with the end of the Gulf of Lion opening. Despite these geodynamic links, the timing of the incisions seems to fit with eustatic sequence boundaries. Similarly, Upper Miocene deposits were shifted and nested within pre­vious deposits. This study shows that in overfilled foreland basins, uplifts related to major thrust activation stages, in addition with eustatic variations, could generate wide networks of compound IVSs which have a high potential for stratigraphic trap formation.