Abstract:
A global composite of depositional orbital rhythms interpreted from land-based marine records, and several ODP cores provides an Astronomical Time Scale (ATS) for the Early Cretaceous. Paleolatitude, paleobathymetry, proximity to strandlines, sea level change, and Oceanic Anoxic Events affect the recorded orbital parameters differently. High-resolution cyclostratigraphic proxies for the stratigraphy of some of the lower stages were constructed from lithologic or facies coding, or by greyscale scans of section photographs. Geochemical series (carbonate, TOC, and other elemental measurements, e.g., Al, Si, etc.) were used to characterize sedimentary cyclicity in many of the other records; and high-resolution downhole logs proved to be extremely useful for the ODP sites. Each measured stratigraphic series was analyzed by spectral analysis for significant stratigraphic frequencies; and spectrograms, filters and quadrature techniques were used to evaluate the time-frequency behavior of detected signals. The application of multiple analytical techniques was crucial not only for interpretation of depositional frequencies, but also for identification of diagnostic modulations and effects from variable sedimentation rate. Preliminary assignment of depositional cyclicity to periodicities of the orbital parameters allowed application of fine-scale tuning for temporal correction of frequencies due to variable sedimentation rate. The 405-kyr long-eccentricity modulation of precession is presently believed to be the most stable orbital period in deep time. When detected, this orbital component was tracked and used as the basis for determination of stage durations and inclusive time intervals. The final succession of tuned series was spliced at stage boundaries to create a composite Cretaceous ATS.