A MILANKOVITCH TIMESCALE FOR THE CRETACEOUS (Invited Presentation)

Recognition of the influence of Earth’s orbital parameters on climate and sedimentation underlies much of the Geologic Time Scale for the Cenozoic, and the astronomically tuned time scale now extends into Mesozoic. Recent advances in radioisotopic dating, intercalibration with cyclostratigraphy, and the availability of newly drilled deep-sea sedimentary sequences have allowed reproducible dating of the PETM and the K/Pg boundary, and the potential closing of the Eocene tuning gap.

However, constructing an astronomically tuned time scale for the Cretaceous (66–145 Ma) is still impeded by the limitations of the astronomical target curve and the nature of the rock record. The latest astronomical solution is reliable until 50–55 Ma and, beyond that, only the 405-kyr periodicity of eccentricity can be considered stable. Identifying each 405-kyr cycle downward in the Cretaceous is challenging, starting with the Campanian (83.6–72.1 Ma), for which few continuous rhythmic sequences are identified, and correlation to radioisotopically dated levels is hampered by provincialism of key biota and relative climatic quiescence.

To move forward, or in this case backward, we propose that cyclostratigraphic studies of the Mesozoic should focus on testing for the presence of multi-million year periodicities of eccentricity. The presence of these long-period cycles has been demonstrated for large parts of the Cretaceous. Minima in the 2.4-Myr and 1.2-Myr cycle, preferably in sections with independent radioisotopic age control, can provide suitable anchors for correlation.

The onset of Oceanic Anoxic Event 2 at ~94 Ma has been shown to coincide with a 2.4-Myr minimum in eccentricity. Extending the 2.4-Myr tuned time scale into the Early Cretaceous–Late Jurassic would greatly improve age control as few radiometric dates exist for this interval. Long, continuous rhythmic sections of Valanginian, Hauterivian and Barremian age in southern France show great potential for this purpose. Complications may arise from apparent chaotic transitions between the 2.4-Myr and 1.2-Myr pacing of eccentricity that need carefully evaluation.

We propose that correlation to the rock record rather than tuning to the astronomical solution may provide the best way to extend the orbitally tuned time scale further back in time.