International Geologiical Congress - Oslo 2008

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HPS-05 Recent developments in the Geologic Timescale

 

Construction of the Cretaceous Astronomical Time Scale. Part 2: Late Cretaceous

 

Robert Locklair, Johns Hopkins University (United States)
Linda Hinnov, Johns Hopkins University (United States)
James Ogg, Purdue University (United States)
 

 

An Astronomical Time Scale (ATS) for the Late Cretaceous was generated as a global composite of depositional orbital rhythms recorded in outcrops and boreholes from land-based and DSDP/ODP sites. As with the Lower Cretaceous sections, paleolatitude, paleobathymetry, proximity to strandlines, sea level change and Oceanic Anoxic Events influenced depositional rhythmicity. The feasibility of testing for orbital forcing in any locality is also linked to available cyclostratigraphic proxies (e.g., bulk % carbonate, TOC, greyscale scan, gamma log, magnetic susceptibility, etc.) and sampling resolution. Global correlation opportunities were greatly expanded from those of the Lower Cretaceous, owing in large part to the aggressive campaigns of ODP. The recovered data series were analyzed by spectral analysis for significant stratigraphic frequencies; filters and spectrograms were applied to evaluate time-frequency behavior of detected signals. These techniques aided in the interpretation of depositional frequencies, their modulations, and effects from changes in 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 tuned series were further analyzed by the same methods for the untuned series in order to test the age model and were compared to results of orbital solutions for the given time interval. If present, the 405-kyr (long eccentricity) modulation of precession was tracked and used as the basis for determination of stage durations and inclusive time intervals. The placement of stage boundaries onto the final succession of tuned series completes the composite Cretaceous ATS.

 

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