TESTING DIFFERENT CALIBRATIONS OF THE EARLY TO MIDDLE EOCENE GEOMAGNETIC POLARITY TIMESCALE: NEW PALEOMAGNETIC RESULTS FROM TUFF DEPOSITS IN THE GREEN RIVER BASIN

New paleomagnetic results from eleven radiometrically dated ash-fall tuffs from the Green River and Bridger Formations in the greater Green River Basin of Wyoming provide independent constraints to test different age calibrations of sea-floor magnetic anomalies from Chron 24 through Chron 20. These paleomagnetic results are derived from stratigraphically well defined volcanic tuffs. Of the eleven tuffs, eight tuffs showed reliable paleomagnetic results; six tuffs exhibited normal polarity whereas two stratigraphically adjacent tuffs showed reverse polarity, indicating at least one polarity reversal. When combined with published ⁴⁰Ar/³⁹Ar age ranges, these results support an age calibration that makes Chron 22 younger by ~ 1 to 2 m.y. and increases the relative durations of the early Eocene, the Wasatchian NALMA (North American Land Mammal Age) and the Early Eocene Climatic Optimum compared to the most recent Geomagnetic Polarity Timescale (GPTS). This calibration has important implications for understanding the timing, rate and pattern of mammalian evolution, lake evolution and climate fluctuations during this unusually warm interval.

The Eocene part of the current GPTS is constructed based on a spline fit to five radiometric tie points. As a result, the majority of the Eocene Epoch is calibrated via interpolation assuming smoothly varying rates of sea-floor spreading. Although this assumption is probably valid over long time scales, this simplification may lead to inaccurate calibration for intervals between tie points due to erratic relative plate motions. This study demonstrates that such irregular changes in the sea-floor spreading can be identified with the simultaneous use of paleomagnetic and isotopic dating of closely-spaced tuffs from terrestrial sections.