PROVENANCE ANALYSIS OF THE PALEOGENE STRATA AT PUMPKIN BUTTES, POWDER RIVER BASIN, WYOMING

The Powder River basin is among several yoked inter-montane basins that occur within the Laramide foreland. More than 2000 m of Paleogene synorogenic strata fill the basin, which consists of the Fort Union and Wasatch Formations. Pumpkin Buttes are in the southwestern area of the Powder River basin; ten to twenty meters of conglomeratic sandstone and mudstone of the post-Laramide Oligocene White River Formation caps the buttes. Our broad goal is to characterize the provenance of White River Group strata of the Rocky Mountains and Great Plains and the late Paleogene burial of this region. Here we present detrital zircon data (LA-ICPMS at the Arizona Laserchron Center) for the matrix of the basal conglomeratic sandstone at Pumpkin Buttes (z=104). The youngest zircon fraction is middle Eocene in age, indicating a maximum age of deposition of ~46 Ma. These zircons were likely sourced by the Absaroka volcanic field 250 km to the west. The principal age peak is ~1460 Ma, which was likely sourced from basement-cored Laramide uplifts in central Colorado more than 500 km to the southwest. The age spectrum also includes smaller Yavapai (geon 17) and Archean (geon 27 and >geon 30) age peaks. The Yavapai zircons were likely derived from central Colorado, whereas the Archean zircons were likely sourced from the Beartooth and Tobacco Root Mountains of southwest Montana more than 350 km to the northwest. Thus, the zircon age spectrum reveals a variety of distal sediment source areas, mainly from the tops prominent mountain ranges. Our data set supports the hypothesis that the Laramide ranges in this area were buried by the early Oligocene, with regional high topography supplying the bulk of the sediment to the Powder River basin at this time.