GEOCHRONOLOGY OF THE MAASTRICHTIAN-PALEOCENE ROCKS OF THE DENVER BASIN: AN OPPORTUNITY FOR HIGH-PRECISION CALIBRATION OF CLIMATE CHANGE AND EXTINCTION CA. 69-64 MA

Late Cretaceous through Paleocene rocks of the Rocky Mountain region contain the best and most sampled terrestrial record of this time interval due to a rich sequence that includes evidence of climatic fluctuations, biotic diversity, marine to terrestrial transition, and the K-T extinction and subsequent recovery. In the Denver Basin, the combination of fossiliferous K-T boundary sections, an established magnetostratigraphy, and numerous volcanic ash beds (>40 horizons in the first 2 My of the Paleocene) provides an unparalleled opportunity for high-precision sequencing and calibration of this interval of time. The current time-scale for this interval relies on 40Ar/39Ar calibration of the Geomagnetic Polarity Time Scale (GPTS) with interpolation using cyclostratigraphy and other curve-fitting techniques. The West Bijou K-T boundary section in the Denver Basin contains an Ir anomaly closely bounded by ash beds, which if precisely dated, will provide an absolute constraint on a globally recognizable isochronous boundary. High-precision geochronology will be a major step forward in understanding the rate of global climate change leading up to the Cretaceous-Paleocene boundary and through the post-extinction recovery. It will also provide an independent test for cyclostratigraphy by internally dating the magnetic polarity subchrons adjacent to the K-T boundary.

State-of-the-art 40Ar/39Ar sanidine and U-Pb zircon geochronological techniques can be used to date ca. 70 Ma ash beds with an uncertainty of <100 kyrs and in some cases less than 40 kyrs. This allows an unprecedented opportunity to compare U-Pb and Ar-Ar geochronology of identical ash beds and to evaluate potential systematic differences as well as issues of diagenetic alteration (40Ar/39Ar) and magmatic residence time (U-Pb). We expect the Denver Basin to become a test-bed for high-precision geochronology applied to the stratigraphic record with the aim of gaining a better understanding of environmental and biological change during late Cretaceous to Paleocene time. In addition, the public visibility and accessibility of Denver Basin research offers an excellent opportunity to teach the general public about geochronology, paleontology, and understanding climate change in deep time, which in turn will raise consciousness about human impact on the present day Denver Basin.