Lithostratigraphic and biostratigraphic revision of the Oligocene of Western Aquitaine (France)

The Oligocene stratigraphy of western Aquitaine is reviewed through detailed literature screening and re-examination of 93 boreholes, 60 of which have been dated on the basis of foraminifera or calcareous nannofossils. The revision of these boreholes allowed reconstructing the sedimentary evolution of Western Aquitaine in relation to its tectonic history. The small benthic foraminifera enabled estimating the variations in water depth along the sections, ranging from epibathyal to brackish water settings. Around 60% of the Priabonian foraminifera went extinct during this stage and at the Eocene/Oligocene boundary. During the Oligocene, species appeared and disappeared progressively in the Aquitaine Basin, allowing some of them to be used as stratigraphic markers. The foraminifera of the Aquitaine Basin show close affinities with those of the Central Paratethys, indicating that during this period both regions were interconnected through the Strait of Gibraltar and the Betic zone. Seven formations have been retained in the marine Oligocene of the Adour Basin, of which one is newly introduced (the Capcosle Formation of RupelianAquitanian age) and two are redefined (the Biarritz Formation of early Rupelian age and the Escornebeou Formation of late Chattian age). Three are distinguished in the continental domain (the Jurancon and Campagne Formations and finally the Agenais Formation). The Oligocene of the North Aquitanian shelf includes two marine (the Bel-Air Formation and the Calcaire a Asteries Formation with the Crassostrea longirostris Member at the base) and three continental formations (in ascending order the Fronsadais, the Castillon and the Agenais Formations). Three major sedimentary areas are differentiated in the Aquitaine region during the Oligocene. The first area, between Labenne and Arcachon, is characterized by thick (up to 1700 m) bathyal clayey deposits. The second area, forming a circular arc around the first, represents the shelf zone with a much larger variety in sedimentation, including bioclastic limestone, clay and shelly sand, reaching up to 400 to 500 m in thickness. The third area includes the continental sediments from the east and south of the basin. The Pyrenean tectonic events influenced the sedimentation, as shown, firstly, by the middle Rupelian transgression, which was vaster in the north than in the south, and by the reverse phenomenon during the late Rupelian, and, secondly, by the early and late Chattian transgressions, which were conditioned by local subsidence and reactivation of ancient structures.