Abstract
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.
Highlights
The Geologic Time Scale 2004 (GTS2004, Gradstein et al, 2004) reveals significant and numerous uncertainties, which impact the interpretation of major geologic events: velocities of plate motions, sediment accumulation rates, paleoclimatic change, biological evolution, and geochemical cycling
A poorly constrained Phanerozoic geological interval is the Late Jurassic, which has the largest uncertainties in the Phanerozoic time scale, with stage boundary ages estimated with 2σ uncertainties on the order of ±4 myrs (Hinnov and Ogg, 2007)
The Mariae Zone is only 25-m thick and was recently biostratigraphically defined to the horizon scale, with both lower and upper boundaries precisely recognized at the cm scale (Thierry et al, 2006)
Summary
The Geologic Time Scale 2004 (GTS2004, Gradstein et al, 2004) reveals significant and numerous uncertainties, which impact the interpretation of major geologic events: velocities of plate motions, sediment accumulation rates, paleoclimatic change, biological evolution, and geochemical cycling. We tackle the problem of the virtually unconstrained time scale of the Mariae ammonite zone that defines the lower boundary of the Oxfordian stage. We compare the Aspressur-Buëch section with its equivalent in the EST342 ANDRA core from the eastern Paris Basin, ~400 km to the northwest (Fig. 1). In this core, the Mariae Zone is only 25-m thick and was recently biostratigraphically defined to the horizon scale (the thinnest stratigraphic unit characterized by a distinctive ammonite association within which no finer subdivision is possible), with both lower and upper boundaries precisely recognized at the cm scale (Thierry et al, 2006)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
