Development of from Astronomical Time Scale for the Cretaceous Period
The recent development of the cyclostratigraphic methods and theoretical astronomical solutions has permitted unparalleled resolution of the Cenozoic time scale. It is crucial now to pursue the investigation as far as possible down the Cretaceous and through the Mesozoic Era. The main objective is to create an Astronomical Time Scale (ATS) for the Cretaceous, as a global composite of depositional orbital rhythms recorded in outcrops and boreholes from land-based and DSDP/ODP sites. A large number of Cretaceous sedimentary formations have now been analyzed for orbital forcing. Challenges have included the presence of major unconformities (e.g., Campanian and upper Turonian), condensed sections resulting in stratigraphic under-sampling for key orbital parameters (esp. precession), poor core recovery, and poor biostratigraphic and magnetostratigraphic constraints. The feasibility of testing for orbital forcing at a given locality is also linked to available cyclostratigraphic proxies (FMS/FMI, gamma ray, magnetic susceptibility, etc.) and sampling resolution, and availability of time-equivalent series for parallel investigation. The application of multiple analytical techniques is crucial for interpretation of depositional frequencies, and identification of diagnostic modulations and effects from variable sedimentation rate. The 405 kyr eccentricity variation is presently believed to be the most stable orbital period in deep time. Thus, when possible, tuning of the data to the theoretical variations of this parameter has been performed. The tuned series are further analyzed by the same methods as for the untuned series to test the age model, and compared to astronomical solutions defined for the given time interval. The Cretaceous ATS is linked to absolute time by compromise to the inter-calibrated, astronomically tuned Cretaceous-Paleogene boundary from above (i.e., the Paleogene) and tuned cyclic data from below the boundary. At present, the Cretaceous stages have astronomically tuned estimated durations as follows: 5.27 Myr (Maastrichtian), 14.2 Myr (Campanian), 2.4 Myr (Santonian), 3.4 Myr (Coniacian), Turonian (5.3 Myr), Cenomanian (5.1 Myr), 11.8 Myr (Albian), 2.6 Myr (Aptian), 5.06 Myr (Barremian), 5.6 Myr (Hauterivian), 2.3 Myr (Valanginian). The Berriasian is only partially resolved at this time. It should be emphasized that these estimates will likely change (and improve) with the future discovery and analysis of duplicate stratigraphies.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009