Emilia Huret, Johns Hopkins University (United States)
Linda Hinnov, Johns Hopkins University (United States)
Bruno Galbrun, Université Pierre et Marie Curie (France)
Pierre-Yves Collin, Université Pierre et Marie Curie (France)
Slah Boulila, Université Pierre et Marie Curie (France)
The Late Jurassic Time Scale has large uncertainties in stage boundary ages (+/- 4 myr), hence in interval durations. To improve interval duration estimates, a cyclostratigraphic study based on high resolution magnetic susceptibility (MS) data was conducted on the Callovian-Oxfordian homogeneous marly formations situated in the eastern Paris Basin (Bure, Haute-Marne) where an underground research laboratory (ANDRA) was created to investigate the feasibility of a deep geological radioactive waste repository. Four boreholes oriented on a 40 km SW-NE transect with precise biostratigraphic data were analyzed. MS measurements were made directly on the cores with a Bartington Instruments MS2E1 sensor every 4 centimeters. The boreholes cover a 100 m interval from the middle Callovian to lower Oxfordian (Mariae ammonite zone). Sedimentological data suggest that variations in clay content influence the evolution of MS, and are correlated to sea level changes.
High frequency variations of MS were subjected to spectral analysis using quantitative signal processing tools. Cycles with preferred thicknesses of 0.5 m, 1 m, 2.5 m and 10 m were resolved, which when reported by the method of frequency ratios indicate the presence of precession, obliquity and eccentricity. Time-frequency analysis shows a persistent evolution of closely spaced frequency components in the 1/2.5 m band indicative of forcing by short eccentricity. Minimal tuning of these components to the short (100-kyr) eccentricity frequency focuses significant power into the other Milankovitch frequencies; a secondary tuning of the 41-kyr band restores the dual-frequency behavior of the short eccentricity, while simultaneously concentrating significant power into the obliquity and precession bands. These tuning experiments demonstrate that during the Jurassic the orbital eccentricity and obliquity variations had similar time-frequency behavior to the high-precision La2004 model predicted for the past 60 million years. These tuning results indicate a duration of 2.4 myr for the Mariae zone, whereas the Geologic Time Scale 2004 assigns only 0.6 myr.
These MS records also hold great promise as a high-resolution tool for regional stratigraphic correlation. We demonstrate that the eccentricity cycles recorded in these cores can be correlated among all 4 boreholes in the Paris Basin, as well as to the Aspres sur Buëch section in the Southeastern Basin, approximately 500 km away. These correlations point to differences in sedimentation rate and stratigraphic completeness among these depositional records.