Astrochronology of the Valanginian Stage from reference sections (Vocontian Basin, France) and palaeoenvironmental implications for the Weissert Event.
Résumé
High-resolution gamma-ray measurements performed on five biostratigraphically well-dated reference sections from the Vocontian Basin (south-eastern France) are used to develop a new astrochronology of the Valanginian Stage and its subdivisions (i.e. ammonite and calcareous nannofossil zones and subzones). Spectral analyses show a pervasive dominance of 405-kyr eccentricity cycles with the expression of 100-kyr eccentricity, obliquity and precession. Previous rough estimates of Valanginian Stage duration ranged from 3.9 to 6.5 myr but were generally based on less reliable or indirect methods. This study provides a precise duration of 5.08 myr, tuning the series to the 405-kyr eccentricity cycle, thus further enhancing the accuracy of the Geologic Time Scale. New durations of magnetochrons are proposed based on intercalibration between ammonite and calcareous nannofossil biozones, cyclostratigraphy and carbon-isotope stratigraphy. After integrating orbitally calibrated durations and available radio-isotopic ages for the Berriasian-Hauterivian interval, a link between theWeissert Event and the emplacement of the Paraná-Etendeka Traps now appears unlikely. Strontium, carbon and oxygen isotope stratigraphy, sedimentological and palaeontological features, clay mineralogy, and the orbital record during the Upper Valanginian all suggest that a transient cool, arid climate occurred after the carbon isotope event, possibly associated with limited polar ice.