Astronomical calibration of the Early Oxfordian (Vocontian and Paris basins, France): Consequences of revising the Late Jurassic time scale.
Résumé
Magnetic susceptibility (MS) variations record Earth's orbital parameters in the clays and marls of the Early Oxfordian in the Paris and Vocontian basins of France. Climatically driven weathering of surrounding emergent areas and the resulting detrital input to the basins is a significant source of the MS signal. MS proves to be an effective tool for deciphering the orbital forcing signal in these sediments, and for assessing the chronology of these formations. The 405-kyr orbital eccentricity cycle is clearly visible in the MS signal and, consequently, is a valuable geochronometer for this portion of the Jurassic time scale. Astronomical calibration of the Mariae ammonite zone (basal Oxfordian stage) indicates a duration of ~2.2 myrs, whereas current time scales assign only 0.6 myrs to this biozone. However, the Late Jurassic time scale has large uncertainties in stage boundary ages (±4 myrs), hence of interval durations. These results could lead to significant revisions in the M-sequence magnetic anomaly block model, and greatly improve the resolution of the Jurassic time scale.
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