COMPREHENSIVE PROVENANCE ANALYSIS OF THE EARLY PALEOGENE STRATA IN THE BIGHORN BASIN (NORTHWEST WYOMING, U.S.A.)

In the Western Interior of North America, sedimentary basins preserve a record of Late Cretaceous through latest Eocene sedimentation derived from Laramide uplifts surrounding intramontane basins. The Bighorn Basin (Wyoming, U.S.A.) studied here contains some of the most extensively studied nonmarine early Paleogene strata in the world. Over a century of research has produced a highly resolved record of early Paleogene terrestrial climatic and biotic change as well as extensive documentation of spatiotemporal variability in the depositional environments represented in the stratigraphy. However, a comprehensive provenance analysis of the Fort Union and Willwood formations is missing. It is unclear to what extent basin-scale stratigraphy and depositional environments are controlled by provenance, tectonism, and climate. This study uses paleocurrent analysis (n=553 measurements), U-Pb detrital zircon geochronology(n= 2,258 ages), and sandstone composition (n=75 thin sections) of fluvial sandbodies from sites across the basin to constrain spatiotemporal changes in provenance and paleodrainage. We link these data with independent constraints on paleoclimatic change and tectonism. Petrographic data of the Fort Union and Willwood formations show that Fort Union samples tend to be slightly more enriched in lithic grains compared to the Willwood samples which are more enriched in Quartz and feldspar. The detrital zircon spectra for the Fort Union and the Willwood formations show diagnostic zircon peaks at ~98 Ma, ~1760 Ma and, 1800-2700 Ma with only noticeable minor changes to the spectra. Willwood data shows more minor age peaks between 200-800 Ma compared to the Fort Union spectra.