Astronomical Calibration of the Valanginian “Weissert” Episode: The Orpierre Marl–Limestone Succession (Vocontian Basin, Southeastern France)

Abstract

A high-resolution biostratigraphic (calcareous nannofossils and calpionellids), chemostratigraphic (C and O isotopes), 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 (southeastern France). The main aims of this study were to demonstrate that orbital forcing was the primary driver of the formation of the marl–limestone alternations and to estimate the duration of the Valanginian “Weissert” episode. The detailed calcareous nannofossil biochronology allowed upper Berriasian–lower Hauterivian Tethyan standard biozones and subbiozones to be recognised. The general trends of the δ13C curve and the major positive C isotope excursion (amplitude of 1.8 ‰) recorded at the Orpierre section are similar to those for other sections worldwide. Spectral analysis applied to high-resolution magnetic susceptibility (MS) variations, coupled with the frequency ratio method, reveals a strong cyclic pattern related to Earth’s orbital parameters (precession, obliquity, and eccentricity). The 405-kyr eccentricity cycle is prominent in the MS signal and has been used as a geochronometer to temporally calibrate the section. Such orbital calibration provided a minimum duration of 4.4 Myr for the Valanginian Stage. The duration of the Weissert episode, as defined by the δ13C curve, is estimated at 2.08 Myr. This duration is consistent with that obtained from the composite section in the Umbria Marche Basin (~2.3 Myr), but contrasts with the estimate (1.5 Myr) proposed from the composite section of the Vocontian Basin.