DETRITAL ZIRCON U/PB PROVENANCE OF THE LOWER CRETACEOUS CLOVERLY FORMATION, BIG HORN AND POWDER RIVER BASINS, WYOMING

The Cloverly Formation of Wyoming represents the first synorogenic strata shed from the Sevier highlands of ID and MT. Detrital zircon U-Pb geochronology was used to determine the provenance of these rocks to better understand the early stages of unroofing of the Sevier highlands. Lowermost Cloverly Formation Sandstones were sampled at three localities in the Big Horn Basin (#283 zircon total) and two localities in the western Powder River Basin (#185 zircon total). The Big Horn Basin detrital zircon spectra are dominated by Proterozoic ages (78-83%), which are mainly Grenville age zircons. Paleozoic zircons represent 10-15% of the spectra of each sample. Lesser amounts of Mesozoic (1-3%) and Archean (2-6%) also were present. The Powder River zircon spectra also were dominated by Proterozoic zircons, but the proportion was smaller (55-69%) and the age distribution was much more diverse. Mesozoic zircons were more prominent (12-23%), Paleozoic (11-13%), and Archean (9-10%) were more abundant as well. The Proterozoic and Archean zircons were likely recycled from cratonic Mesozoic and Paleozoic strata that was eroded as the Highlands were unroofed. The Mesozoic zircons were derived from the underlying Idaho Batholith and related plutonic rocks in the region. It is curious that the more distal Powder River Basin Cloverly localities are more enriched in younger zircons. This may indicate that distal Cloverly sandstones are comprised of sediment derived from more deeply eroded areas of the Sevier Highlands.