COMPARISON OF U-PB DETRITAL ZIRCON PROVENANCE BETWEEN UPPER JURASSIC MORRISON FORMATION AND LOWER CRETACEOUS CLOVERLY FORMATION (BIGHORN BASIN, NORTHWEST WYOMING, USA)

The initiation of the Sevier Orogeny in the Western Interior of North America and associated continental-scale tectonic processes are poorly understood in comparison to later stages of the orogeny. This is due to weaker age control, diverse lithofacies patterns, and the uncertain stratigraphic position of unconformities among Late Jurassic and Early Cretaceous strata. Constraining sediment routing patterns and provenance within the incipient foreland basin is a clear step towards addressing these issues. We evaluate changes in provenance between the Morrison Formation and the Pryor Conglomerate of the Cloverly Formation within northwest Wyoming. This transition is key in assessing the early uplift history of the Sevier Orogeny and addressing contrasting models of foreland basin development. Our dataset includes 1000 new U-Pb detrital zircon age analyses derived from fluvial sandstones in each formation as well as a compilation of records spanning the length of the Cordillera. Within northwest Wyoming the age spectra are broadly similar between formations and contain distinct peaks at 160-168, 227-258, ~415, 584-610, ~1040, ~1154, ~1440, 1641-1875, and 2700 Ma. These peaks are consistent with recycled zircons from Mesozoic and Paleozoic strata in the hinterland. The major difference between formations is a substantial increase in the proportion of Grenville-aged zircons (1040 and 1154 Ma peaks) in the Cloverly Formation at the expense of younger Appalachian-aged zircons (~530-610 Ma). This shift, combined with chert-rich gravel deposition, is consistent with greater exposure and erosion of Upper Paleozoic strata and potentially exposure of Neoproterozoic strata in the hinterland. Potentially this is related to movement on the Paris-Willard thrust system ca. 140 Ma. The increase in Grenville-aged zircons appears to be largely restricted to Wyoming, as equivalent strata in the American southwest show no such change between the Late Jurassic and Early Cretaceous. Similarly, strata to the north in Montana and Canada document a consistently strong contribution from the Trans-Hudson Orogeny grains (1800-2000 Ma).