The Cretaceous Astronomical Time Scale

Many offshore marine sequences of the Cretaceous Period are characterized by decimeter-scale bedding cycles, which typically are expressed by alternations of relatively carbonate-rich (chalk, marl) and carbonate-poor beds (marl, shale, chert). A number of these Cretaceous successions have now been analyzed with spectral techniques to test for hypothesized orbital forcing of these depositional rhythms. The results have facilitated the development of an Astronomical Time Scale (ATS) through extended intervals of the Cretaceous marine record. The high resolution of this ATS provides a means to address rates of change for key Earth processes (seafloor spreading, biotic turnover, etc.) that have been otherwise difficult to evaluate based solely on available radiometric constraints. The ATS also enables an independent assessment of radiometric techniques and can be used for global correlations.

Compilation of a continuous record of Cretaceous orbital rhythms was accomplished by time series analysis of rhythmically bedded marine strata from localities across the globe, and stitched together by global chronostratigraphic datums. Multiple high-resolution geochemical and petrophysical time series were constructed from outcrops and cored sections, and from DSDP/ODP cores and boreholes. The most useful measurements were CaCO3, TOC, and well log data (FMS/FMI, gamma ray, magnetic susceptibility, photolog scans). Time series from each locality were analyzed at the stage level, and evaluated for stratigraphic completeness, biostratigraphic and magnetostratigraphic constraints.

Some of the challenges encountered in the study included the presence of major unconformities (e.g., Campanian and upper Turonian), condensed sections resulting in stratigraphic under-sampling for key orbital parameters (precession), poor core recovery, thus, poor biostratigraphic constraints, poorly resolved records, and lack of multiple robust, time-equivalent series for parallel investigation. Nevertheless, suitable time series were identified for every Cretaceous stage with the exception of the Campanian. Case studies for the Maastrichtian and Albian Stages will be highlighted with a discussion of applications.