40AR/39AR CHRONOSTRATIGRAPHY OF CRETACEOUS AND PALEOCENE STRATA IN THE SAN JUAN BASIN, NEW MEXICO: ACCURACY LIMITATION OF HIGH PRECISION MEASUREMENTS

Via the EARTHTIME initiative great advances have been made to accurately calibrate the geological time scale. During the past decade the U-Pb and Ar/Ar geochronology communities have moved from ca. ±0.5% accuracy and precision issues to new levels of offsets at the ±0.1% level. Despite this excellent progress, especially the Ar/Ar community, continue to be unable to fully implement high precision measurements towards solving problems that require comparisons of absolute time. Also, Ar/Ar age calibrations to the astrochronological (ACTS) and geomagnetic polarity (GPTS) timescales pose additional limitations in constraining geological and evolutionary processes on regional and global scales.

A rich mammal, dinosaur and plant fossil record is contained within the Cretaceous and Paleocene rocks of the San Juan Basin (SJB) in NM. Recent geochronology, paleontology and magnetostratigraphy in the SJB highlight both successes and limitations for basin-wide correlations of strata, as well as linkages among regional fossil records. Ultra high precision Ar/Ar geochronology from ash and detrital sanidine obtained on the new ARGUS VI multi-collector mass spectrometer provides unprecedented results that indicate: 1) first occurrence of the Puercan 2 and 3 faunas occurred within the first ~500 ka of the Paleocene; 2) the dinosaur bearing Naashoibito Member rocks are complex with multiple disconformities that provide no unequivocal evidence for Paleocene dinosaurs; 3) the Ojo Alamo Sandstone may represent a time transgressive depositional sequence; and 4) precise dates on detrital sanidine yield meaningful estimates of rock depositional age. Despite these useful conclusions, accuracy limitations arise from comparison of our data to the existing K-Pg geochronology. These limitations stem from multiple sources including interlab and intermethod bias, unknown uncertainties of the GPTS and ACTS timescales and now recognized complexity of sanidine argon systematics. Work is underway to combine U-Pb zircon and Ar/Ar sanidine data sets within the context of SJB geology. Preliminary data indicate that the best agreement between NM Tech Ar/Ar dates and other age estimates are obtained utilizing the Kuiper et al. (2008) calibration of the Fish Canyon Tuff sanidine fluence monitor.