TESTING DIFFERENT CALIBRATIONS OF THE EOCENE GEOMAGNETIC POLARITY TIME SCALE USING NEW PALEOMAGNETIC RESULTS FROM GREEN RIVER BASIN TUFFS

New paleomagnetic results from thirteen radiometrically dated tuff layers from the Green River and Bridger Formations in the southern Green River Basin of Wyoming provide independent temporal constraints that can be used to test different calibrations of the Eocene part of the Geomagnetic Polarity Time Scale (GPTS). Of the twelve reliable paleomagnetic sites, ten sites show normal polarity whereas two stratigraphically adjacent sites are characterized by reverse polarity, indicating at least one polarity reversal event. These paleomagnetic data are combined with published ⁴⁰Ar/³⁹Ar ages from the same beds and interpreted in the context of known biostratigraphy to test five different published calibrations of the Eocene GPTS. A calibration model that shifts Chrons C22 to C24n. 1n ~ 2 m.y. younger compared to the most recent GPTS is tentatively favored as it shows the best fit to the currently available geochronological information from the basin, and it can largely eliminate an existing discrepancy between magnetostratigraphy from the basin margin and ⁴⁰Ar/³⁹Ar ages from the basin center. The Eocene part of the current GPTS is calibrated based on five tie points that are widely spaced, leaving the interpolated segments poorly constrained. The new results from the Green River Basin provide potential additional control points at a higher resolution than is currently available. This new correlation suggests a longer duration for the early Eocene, the Wasatchian NALMA (North American Land Mammal Age), and the early Eocene climatic optimum, all of which could have significant implications for understanding the timing, rate and pattern of mammalian evolution, lake evolution and climate fluctuations during this interval. This study also demonstrates the usefulness of ash-fall deposits as reliable paleomagnetic recorders that can be directly combined with ⁴⁰Ar/³⁹Ar ages to calibrate the GPTS at a higher resolution than has previously been achieved.