MAGNETOSTRATIGRAPHY ACROSS THE WASATCHIAN BRIDGERIAN NALMA BOUNDARY (EARLY TO MIDDLE EOCENE) IN THE WESTERN GREEN RIVER BASIN, WYOMING/

New paleomagnetic and biostratigraphic data from the western Green Riv er Basin of Wyoming provide a magnetobiostratigraphic framework to cor relate the late Wasatchian through early Bridgerian North American Lan d Mammal Ages (NALMA) to the Geomagnetic Polarity Time Scale (GPTS). T hree-hundred and twelve paleomagnetic samples were collected from 73 s ites within a 350-m continuously exposed flat-lying section. The secti on is characterized by alternating packages of well-developed paleosol mudstone horizons (Wasatch Formation) and lacustrine shale/limestone horizons (Green River Formation). Fossil mammal assemblages are known from various levels within the section and constrain the location of t he Wasatchian/Bridgerian NALMA boundary to lie between 60 and 120 m. P aleomagnetic results of paleosol horizons are generally characterized by two components, a present-day field component with low unblocking t emperatures up to 400 degrees C, and a characteristic component with u nblocking temperatures up to 680 degrees C. Characteristic magnetizati ons have either N-NW and steeply down directions or antipodal S-SE and up directions. The characteristic component is interpreted as primary based on antipodal polarity directions and congruence with the expect ed Eocene direction for western Wyoming. Results indicate that the fir st 30 m of section are characterized by normal polarity, the next 230 m by dominantly reversed polarity, followed by a 70 m interval of alte rnating polarity. Given radiometric ages for the late Wasatchian (earl y Lostcabinian ''subage'') and middle Bridgerian, as well as previous Wasatchian and Bridgerian magnetostratigraphic results, two potential correlations to the GPTS are possible. Correlation 1 places the Wasatc hian/Bridgerian NALMA boundary within Chron C23r at about 52 Ma. Corre lation 2 places the boundary within Chron C22r at about 50.5 Ma. Corre lation 2 is tentatively preferred as it agrees most closely with previ ous estimates for the age of the boundary and assumes more uniform sed iment accumulation rates.