Closing and continentalization of the South Pyrenean foreland basin (NE Spain): magnetochronological constraints

This paper presents new magnetostratigraphic results from a 1100‐m‐thick composite section across the marine to continental sediments of the central part of the SE margin of the Ebro basin (NE Spain). Integration with existing marine and continental biochronological data allows a robust correlation with the geomagnetic polarity time scale. The resulting absolute chronology ranges from 36.3 to 31.1 Ma (Priabonian to Rupelian), and yields an interpolated age of ∼36.0 Ma (within chron C16n.2n) for the youngest marine sediments of the eastern Ebro basin. This age is in concordance with a reinterpretation of earlier magnetostratigraphic data from the western South Pyrenean foreland basin, and indicates that continentalization of the basin occurred as a rapid and isochronous event. The basin continentalization, determined by the seaway closure that resulted from the uplift of the western Pyrenees, was probably coincident with a mid‐amplitude eustatic sea level low with a maximum at 36.2 Ma. The base level drop that followed the basin closure and desiccation does not appear associated to a significant sedimentary hiatus along the margins, suggesting a late Eocene shallow marine basin that rapidly refilled and raised its base level after the seaway closing. Rapid basin filling following continentalization predates the phase of rapid exhumation of the Central Pyrenean Axial Zone from 35.0 to 32.0 Ma, determined from the thermochronology data. It is possible then that sediment aggradation at the front of the fold‐and‐thrust belt could have contributed to a decrease in the taper angle, triggering growth of the inner orogenic wedge through break‐back thrusting and underplating. Contrasting sedimentation trends between the western and eastern sectors of the South Pyrenean foreland indicate that basin closing preferentially affected those areas subjected to sediment bypass towards the ocean domain. As a result, sediment ponding after basin closure is responsible for a two‐fold increase of sedimentation rates in the western sector, while changes of sedimentation rates are undetected in the more restricted scenario of the eastern Ebro basin.