U/PB DETRITAL ZIRCON GEOCHRONOLOGY OF THE WILLWOOD FORMATION IN THE BIGHORN BASIN, WY: IMPLICATIONS FOR EARLY EOCENE PALEOGEOGRAPHY

As much as 1500 m of Willwood Formation (Eocene) mudstone, sandstone, and conglomerate was deposited in the Bignorn Basin during the waning stages of the Laramide orogeny. The surrounding Beartooth, Bighorn, and Owl Creek basement cored uplifts have long been viewed as the likely source areas for Willwood sediment. As part of this research, we used single crystal U/Pb detrital zircon geochronology (#645) of seven Willwood outcrops along the western margin of the Bighorn Basin (Clarks Fork Canyon, McCullough Peaks, Gooseberry Creek, and Meeteetse areas).The samples collected from the base of the section (Clarks Fork Canyon and Gooseberry Creek) have significant Proterozoic age zircon populations whereas the samples taken from the McCullough Peaks and Meteetsee regions have significant Cretaceous age zircon populations. All data sets also have only minor Archean zircon populations. The Proterozoic and Cretaceous populations are interpreted to be derived from westerly sources. The Cretaceous zircons are primary zircons derived from the newly uplifted Idaho Batholith. The Proterozoic zircons, which are largely Yavapai-Mazatzal in age, are interpreted to be recycled from Neoproterozoic quartzite units of the Targhee uplift. These data indicate that the basin was open to the west during the early stages of Willwood deposition, and that the Beartooth uplift was not a principal sediment source area. Samples collected from higher stratigraphic positions have significant zircon populations derived from westerly sources, but have a lower proportion of primary Archean zircons than the samples collected from the base of the section, indicating that the proximal Laramide Beartooth uplift was was not a significant source area during the latter stages of Willwood deposition.