PROVENANCE ANALYSIS OF THE LOWER EOCENE WILLWOOD FORMATION IN THE ABSAROKA BASIN USING U/PB DETRITAL ZIRCON GEOCHRONOLOGY

The Willwood Formation is an Eocene synorogenic succession of mudstone, sandstone and conglomerate that was deposited in the Bighorn and Absaroka basins during the waning stages of the Laramide Orogeny. The Willwood Formation crops out beneath volcanic rocks of the Absaroka Volcanic Supergroup throughout the Absaroka Basin. The purpose of this research is to determine the provenance of the Willwood Formation and reconstruct the paleogeography of the region at the onset of volcanism and Heart Mountain faulting. In this study, five samples from the South Fork (n=401) and two samples from the North Fork (n=169) of the Shoshone River Valley in the Absaroka Basin were collected for U/Pb geochronology on detrital zircons. Six of the samples were taken from sandstones near the top of the section; one was sampled near the base. The detrital zircon patterns for each sample were remarkably similar; 74% of the South Fork and 64% of the North Fork samples were dominated by Archean zircon, with other frequency peaks relating to Mesozoic, Grenville, Yavapai-Mazatzal, and Trans-Hudson orogens. The principal source area for the Willwood Formation here was likely the Beartooth uplift to the north, but some sediment was also contributed from the Idaho Batholith and and recycled from the ancestral Targhee uplift to the west. All but one of the samples included Eocene zircon, which indicates that the transition from early Eocene syntectonic sedimentation to middle Eocene volcanic sedimentation was more gradual than evident in the broader physical characteristics of the units.