Astronomical calibration of the Valanginian “Weissert” episode: The Orpierre marl–limestone succession (Vocontian Basin, southeastern France)

A high-resolution, biostratigraphic (calcareous nannofossils, calpionellids), chemostratigraphic (C-isotope) and cyclostratigraphic (magnetic susceptibility) study was performed on the marl–limestone alternations of the Upper Berriasian–Valanginian Orpierre section, deposited in the hemipelagic setting of the Vocontian Basin (SE France). The main aims of this study were to detect orbital forcing, to estimate the duration of the Valanginian “Weissert” episode, and to discuss the palaeoenvironmental implications. Detailed calcareous nannofossil biochronology allowed the recognition of Upper Berriasian–Lower Hauterivian biohorizons. The general trends of the δ13C curve and the major positive C-isotope excursion (amplitude of 1.8‰) recorded at the Orpierre section are very similar to those found in other sections worldwide. Spectral analysis applied on high-resolution magnetic susceptibility (MS) variations coupled with frequency ratio method reveals a strong cyclic pattern related to the Earth's orbital parameters (precession, obliquity and eccentricity). The prominent 405 kyr eccentricity cycle in the MS signal and has been used as a geochronometer to time calibrate the section. The duration of the Weissert episode was estimated as 2.08 Myr. This duration is coherent with those obtained from the Umbria Marche Basin (∼2.3 Myr) and from the Angles section (∼2.14 Myr). Durations of more than 2 Myr suggest that a long perturbation in the dynamic of the global carbon cycle that is not compatible with the rapid and intense volcanic activity. Our cyclostratigraphic study indicates a minimal duration of 4.29 Myr for the Valanginian stage. The comparison between the MS signal at Orpierre and the spectral gamma-ray signal recently obtained in two composite sections of the Vocontian Basin allows to propose a revised duration for the Valanginian stage of 4.695 Myr instead of 5.08 Myr.