GLOBAL APPLICATION OF REVISED LATE CRETACEOUS TIME SCALE USING INTEGRATED CHEMOSTRATIGRAPHY, BIOSTRATIGRAPHY AND GEOCHRONOLOGY

This study builds on the seminal work of Obradovich (1993), whose Ar-Ar dating of volcanic ash beds (bentonites) from the Western Interior U.S. provided the definitive radioisotopic time scale for the Late Cretaceous. We have determined 34 new ⁴⁰Ar/³⁹Ar ages from sanidines separated from Western Interior bentonites dated by Obradovich, including re-analysis of his legacy samples as well as analysis of fresh material. These age determinations span Albian through Maastrichtian time. From eight of the dated bentonites we also determined the ²³⁸U-²⁰⁶Pb ages of zircons using the CA-IDTIMS method and EARTHTIME tracer solution. Intercalibration of the two radioisotope systems supports use of the 28.201 Ma age for the FCs standard and the decay constants proposed by Kuiper et al. (2008), as adopted in GTS2012. The new radioisotopic data significantly reduce uncertainties on individual age determinations and differ from recent published ages by an average of ~0.36 Ma. In order to improve age interpolation of significant boundaries and events, the new radioisotopic data has been integrated with floating astronomical times scales (ATS) developed from the mid-Cenomanian through Early Campanian (with a gap in the mid- to Late Turonian). Because they are based on evolutive assessments of orbital signals preserved in the rhythmically bedded strata of the Western Interior, the ATS provide more accurate interpolation of datums between dated horizons. Finally, detailed analysis of d¹³C_org values from the same cores used to develop the ATS has provided a means to compare the new Late Cretaceous time scale from the Western Interior of North America to other Late Cretaceous sections where similar chemostratigraphic frameworks have been developed. This is a critical step toward building a more consistent global Late Cretaceous chronostratigraphy with lower uncertainty, and it will allow important advances in the interpretation of global carbon isotope data and Late Cretaceous carbon cycling.