THE CRETACEOUS TIME SCALE: PROGRESS AND PROSPECTS FROM RADIOISOTOPIC DATING AND ASTROCHRONOLOGY

With a primary focus on the Western Interior Basin of the USA, we have determined new 40Ar/39Ar ages from sanidine in 25 of the 32 bentonites that formed the basis of John Obradovich’s classic time scale of 1993. During this effort, we have refined the way our mass spectrometer data are analyzed. The result is that the 95% confidence level uncertainties due to analysis are typically in the 1-2 permil range for individual bentonite beds, about an order of magnitude better than was possible at the time Obradovich (1993) was published. Taking full advantage of this improved age resolution requires that we understand not only the ammonite biozone within which any given bentonite occurs, as did Obradovich (1993), but also that we stratigraphically constrain where within each biozone the ash beds were deposited. This is because we have correlated the ash horizons to other sections in the basin where we have developed astrochronologic frameworks that allow the age of stage boundaries, biozone datums and formation tops to be more accurately interpolated. The correlation introduces a new source of uncertainty when linking radioisotope ages to the astrochronlogy, and we propose a method to address this uncertainty. From 8 of the dated bentonites we have also determined the 238U-206Pb ages of zircons using the CA-ID-TIMS method and EARTHTIME tracer solution. The pairs of 40Ar/39Ar and 238U/206Pb ages span from 94.4 to 66.0 Ma, i.e., upper Turonian to lowermost Paleogene, and are brought into agreement with one another using the age of the FCs standard of 28.201 Ma and the 40K decay constants proposed by Kuiper et al. (2008) and adopted in GTS2012. Using these radioisotopic data, we have anchored 3 astrochronologic age models derived from study of drilled cores that now span most of the interval between the middle Cenomanian and the lower Campanian. Radioisotopic ages remain sparse from: the middle and upper Albian, the Campanian from about 81.8 to 76.1 Ma, and the Maastrichtian from about 72.4 Ma to the K-Pg boundary. Current focus is on filling these gaps, anchoring new astrochronologic age models under development, and defining several stage boundaries with an accuracy and precision of 2-3 permil. We dedicate this work to the memory of our colleague John Obradovich whose pioneering studies made this work possible and have lit the way forward.