International Geologiical Congress - Oslo 2008


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HPS-05 Recent developments in the Geologic Timescale


Astronomical calibration of the Oxfordian time scale (Terres Noires Formation, Vocontian Basin, France)


Slah Boulila, Université Pierre et Marie Curie (France)
Bruno Galbrun, Université Pierre et Marie Curie (France)
Linda A. Hinnov, Johns Hopkins University (United States)
Pierre-Yves Collin, Université Pierre et Marie Curie (France)
Emilia Huret, Université Pierre et Marie Curie (France)
Dominique Fortwengler, Le Clos des Vignes (France)
Didier Marchand, Université de Bourgogne (France)


Magnetic susceptibility (MS) variations record Earth's orbital parameters in the marls of the Lower and Middle Oxfordian Terres Noires Formation from three sections that crops out at Aspres-sur-Buëch, Oze, and Trescléoux (Vocontian Basin, southeastern France). Climatically driven weathering of surrounding emergent areas and detrital input to the basin are the principal 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 formation's chronology.
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 Lower and Middle Oxfordian indicates a duration of 4.1 myrs which is in agreement with the current time scale GTS2004, which assigns a duration of 3.8 ±0.4 myrs. However, estimated ammonite zone durations within the substages are potentially different to those in GTS2004. In particular, Mariae ammonite zone indicates a duration of ∼2.2 myrs, whereas GTS2004 assigns only 0.6 myrs to this biozone. However, the Late Jurassic geologic time scale has large uncertainties in stage boundary ages (± 4 myr), hence of interval durations. These results urgently appeal to a revision of the M-sequence model through a combination of astronomical calibration and radiometric dating of bentonites recently discovered in the Terres Noires Formation. These innovations will improve the resolution of the Jurassic time scale and the M-sequence block model by an order of magnitude.


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