Calibration of the Latest Eocene-Oligocene Geomagnetic Polarity Record Using Paleomagnetism and 40Ar/39Ar Dating ofIgnimbrite Sequences in New Mexico, Colorado, and Texas

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Published radiometric calibrations of the mid-Tertiary magnetic polarity time scale (MPTS) vary in age by as much as 20%. PI's will improve this calibration by determining a well- dated magnetic polarity record from volcanic sequences in New Mexico, Colorado, and Texas, using a combination of paleomagnetic analyses and high-precision 40 Ar/39 Ar dating of sanidine. The combination of precisely dated polarity records from these volcanic fields, together with similar data from the San Juan volcanic field, will allow development of a single integrated polarity record spanning much of latest Eocene and Oligocene time. Although discontinuous, this integrated record will offer very precise constraints on the age and duration of polarity intervals from several segments of the mid-Tertiary MPTS. Accurate calibration of the MPTS will tightly constrain mid- Tertiary sea floor spreading rates and improve correlations between marine and terrestrial biostratigraphic records.