Lower Cretaceous stage durations combining radiometric data and orbital chronology: Towards a more stable relative time scale?

We propose an alternative calibration of Lower Cretaceous stage durations constrained by direct absolute dating of each stage combined with orbital chronology. Ten glauconitic horizons sampled in the Vocontian basin (SE, France) from the base of the Lower Hauterivian to Upper Albian, yielded K–Ar ages from 123.3 ± 1.7 Ma to 96.9 ± 1.4 Ma, respectively. The relative duration of each stage has been derived by cyclostratigraphy previously obtained in the south-east France and central and south Italy basins. Using the GL-O standard from the Albian–Cenomanian boundary at 95.3 Ma as the anchor point, a cyclostratigraphic age for each stage boundaries has been extrapolated and thus compared with the K–Ar ages. This shows a very well-defined linear correlation which demonstrates the robustness of the proposed durations of the Lower Cretaceous stages. The estimated durations are 5.3 ± 0.4 my, 5.1 ± 0.3 my, 6.8 ± 0.4 my and 11.6 ± 0.2 my for the Hauterivian, Barremian, Aptian and Albian stages, respectively. It also shows that glauconite minerals are powerful radiochronometric tools, when precisely stratigraphically defined and carefully selected. Moreover, the large discrepancy of the estimated Aptian duration of more than 6 my between the most recent published time scale and this study highlights the problem of the radiometric calibration of the M-0 magnetic chron. Finally, the stage durations and boundary ages proposed here bring strong constraints towards the calibration of the Lower Cretaceous time scale. Such accurate temporal calibration is required before any relationship between major biological crises and magmatic emplacement, for instance, could be further investigated.