MAGNETOSTRATIGRAPHY OF THE WASATCHIAN/BRIDGERIAN NALMA BOUNDARY IN NORTHEASTERN WIND RIVER BASIN, WYOMING

The Wasatchian/Bridgerian (Wa/Br) North American Land Mammal Age (NALMA) boundary represents a significant early Eocene faunal turnover that is likely the result of global climatic forcing during the Early Eocene Climatic Optimum (EECO). This understudied, roughly 2 Myr peak (~52-50 Ma) of the greatest warming trend in the Cenozoic, was a time in which average North American continental interior mean annual temperature (MAT) and ocean surface temperature (OST) rose to ~23 °C (8°C greater than the 2012 global average). Increased MAT and mean annual precipitation (MAP) of 150 cm/y resulted in escalated floral diversity and habitat complexity. Paratropical conditions made the expansion into high latitudes by tropical foraminifera, broad-leaved evergreens, crocodiles and arboreal mammals possible. Mammalian diversification intensified in the Artiodactyla, Perissodactyla, Rodentia, and Primates, while members of the previously successful order, the Pantodonta, suddenly became extinct.

Whether the Wasatchian/Bridgerian boundary directly coincided with the EECO is still uncertain. Until the exact timing of this terrestrial boundary is known, precise correlation with marine climate proxy records and understanding the role of climate change in mammalian evolution leading up to it is not possible. In order to better constrain the age of the Wa/Br boundary, we have developed a magnetostratigraphic framework within the Wind River Fm. of northeastern Wind River Basin (WRB), WY. The Wind River Fm., comprised of terrestrial mudstone deposits and interbedded sandstones, contains the Wa/Br boundary stratotype section. Previous work in the adjacent Green River Basin places the boundary at either Chron 22r or 23r, a temporal range in uncertainty of nearly 2 Myr. No similar work is published for the WRB. This magnetostratigraphic framework will provide further age constraint on the boundary, allowing selection of either Chron 22r or 23r, and eliminate up to 2 Myr of error in its age.