A not so tranquil basin: recording of the west-European geodynamics amidst marine incursions and retreats in the Paris Basin.

Around the Late Priabonian-Early Rupelian, the Paris Basin is characterized by an incomplete succession of sediments deposited at the marine-continent interface. In the overall marine record, this interval is marked by the Eocene-Oligocene Transition (EOT), characterized by a climate deterioration and a significant sea level drop, associated with the permanent establishment of the Antarctic ice cap. Nevertheless, the EOT is poorly documented and understood in terrestrial areas.

Located between the active tectonic regions of the Pyrenean and Alpine orogens and the West-European Cenozoic Rift Systems, the lagoon to lacustrine deposits of the Paris Basin therefore enable to acutely record both global and local processes (glacio-eustasy, climate, tectonic). A detailed stratigraphic framework is consequently necessary to estimate the contribution of each of these controls. This study is based on: 1) a large-scale correlation of boreholes in order to study the 3D organization of deposits and their lateral and vertical variations, and 2) an elementary and isotopic geochemical, mineralogical, and paleontological study to clarify the depositional environments and the causes of the observed variations (sea level, tectonic and hydrological changes). The analyzed sites are located around tectonic structures (the Bray, Beynes-Meudon, and Remarde anticlines and the Saint-Denis syncline) and in various areas, ranging from the edges to the center of the Paris Basin.

We established a correlation between lagoon-marine deposits of the center of the basin and lacustrine deposits of its southern and eastern edges. Detailed sedimentological studies of the sites reveal a two-steps evolution. The first step is marked by marls deposited during the latest Priabonian. Their mineralogical and chemical composition indicates a deposition evolving from a clastic to a chemical-dominated system in a wetter to drier climate. The second step, during Early Rupelian times, shows the return to detrital deposition in a wetter climate. More specifically, the sections show a mineralogical, chemical and environmental separations. The Priabonian cycle is influenced by sea level variations (marine incursion, then confinement of the basin) and a climate changing from wetter to drier. The Rupelian cycle shows a global transgression in a wetter climate, briefly interrupted by a confinement of the basin, but above all the reactivation of tectonic structures linked to the Pyrenean compression, which caused palustrine deposits on the anticlines and marine deposits in the synclines.

The Paris Basin shows to a lesser extent the same record of the EOT as several marine sites. The major regression is only illustrated by the confinement and partial emersion of the basin in the latest Priabonian; the cooling seems to be recorded by the progressive increase in oxygen isotope values, and the aridification by mineralogical proxies and the known floral evolution. The basin also reflects the west-European regional geodynamics with the recurrence of tectonic structures in the Early Rupelian associated with the African-Eurasian convergence, illustrated for instance as well by the inversion of the Cotentin and Hampshire basins, further north of the Paris Basin.