INTEGRATED CYCLOSTRATIGRAPHIC AND CHEMOSTRATIGRAPHIC ANALYSIS OF CENOMANIAN-TURONIAN DEPOSITS OF THE WESTERN INTERIOR BASIN

Integration of astrochronologic and chemostratigraphic data sets with new radioisotopic age data has significantly improved the resolution of geologic time scales. Augmenting recent reductions in uncertainties of radioisotopic age estimates and better constraints on sources of uncertainty in time scale development, chemostratigraphic data have contributed to the identification, and in some cases quantification, of hiatuses within stratigraphic successions. One such interval is the Cenomanian-Turonian GSSP in south-central Colorado. Although the site was chosen based on excellent lithostratigraphy, biostratigraphy, chemostratigraphy, and radioisotopic age control, the presence of hiatuses in the record has long been suspected. In this study, a combination of carbon (δ¹³C) and osmium (Os_i) isotope data from several Western Interior sections (USGS #1 Portland, Aristocrat Angus, and Smoky Hollow #1 cores) was analyzed within an astrochronologic framework. A major shift in Os_i to highly unradiogenic values, which is expressed virtually globally at marine sites, has been interpreted to indicate the onset of massive submarine volcanism, presumably the trigger of Ocean Anoxic Event 2 (OAE2, ~94 Ma). This shift provides a particularly effective datum within the section. Combined with d¹³C data, it helps to confirm the presence of a hiatus in the GSSP at the onset of OAE2 and constrains its duration to about 50 ka. Additional geochemical data reveal the presence of similar paraconformities near the onset of OAE2 elsewhere in the Western Interior, and it is hypothesized that these are distal expressions of sea level lowstands. In the proximal part of the basin (Smoky Hollow #1 locality, S. Utah: SH#1), where significantly higher sedimentation rates and foreland subsidence resulted in an expanded and conformable succession spanning OAE2, we have developed a high resolution d¹³C profile. Analysis of this record using advanced spectral techniques shows strong evidence of orbital frequencies in δ¹³C_carb oscillations. A floating astrochronologic time scale for SH#1 is compared to a similar record from the GSSP, and provides an even more refined basis for chronostratigraphic analysis of OAE2.