Post-orogenic sedimentary evolution of the retro-foreland basin of Aquitaine (Priabonien to today)

Abstract : The Mesozoic Aquitaine Basin (alternations through time of rifts and intracratonic to passive margin basins) is inverted during Cenozoic times to become the retro-foreland of the Pyrenees Mountain Belt. A lot of studies were carried out on the paroxystic phase of the retro-foreland activity, few paid attention on the latest-post orogenic-stage, the so-called "Molassic" periods. This can be explained by the monotonous facies distribution along poor outcrops with few dating available. For a better understanding of the last stage evolution of both the Pyrenees and Massif central relief-and their consequences on the sediment routing system-we performed a stratigraphic study of the "Molassic" deposits and their marine equivalent, from the upstream part (Lannemezan area) to the distal part (Landais Plateau and Biscaye Bay abyssal plain). This study was mainly based on subsurface data, seismic lines and wells (industrial and BSS) provided by the BRGM and TOTAL. Wells correlation was based on the principles of stacking pattern calibrated in age and facies on cuttings and clabs. The seismic lines were interpreted using the principles of the shoreline trajectory and calibrated in age and facies on wells. This study is part of frame of the s2s project funded by TOTAL and the BRGM. The main results are as follows. (1) A sharp transition between continental and marine environments characterized by mixed carbonate-siliciclastic (clays and silts) platforms. The Bartonian to Priabonian shelves are reefal platforms with a major downward shift at the Bartonian-Priabonian boundary. The exact location of the Eocene-Oligocene boundary is unknown (ongoing biostratigraphic studies) but the time interval between the base Priabonian and the Rupelian eustatic maximum flooding is characterised by a nice lowstand (LNR) wedge followed by an aggrading highstand (HNR) wedge. The Late Rupelian is progradational agradational and the Rupelian-Chattian boundary is a major unconformity. (2) Continental environments (Priabonian to Chattian) are made up of palustrine to carbonate lacustrine sediments passing to mixed to suspended-load rivers, local base level of conglomeratic alluvial fans located in the extreme inner part of the basin at the feet of the Pyrenees. (3) A major erosion occurred in the Chattian (ongoing biostratigraphic dating for a more precise age) with present-day preservation of the Chattian in the inner part of the basin along the Pyrénées (500 to 1500 m-thick wedge). Chatiian sediments were partly removed by erosion in the western part of the Lannemezan Plateau area and are mostly missing along the Aquitaine Basin. (4) Lower to Middle Miocene sediments (with the same facies than above) are quite thin (50 to 200 m-low preservation). They record a major fall in the subsidence pattern. (5) A two phases major uplift of the Lannemezan and Ger Plateaus occurred during Serravallian and base Tortonian times sealed by weatherings and fluvio-lacustrine sediments (Orignac lignites) (6) From Late Miocene to today, the Aquitaine Basin is a sediment transit zone with deposition on the Landais Plateau as thick siliciclastic prograding wedges.