FAUNAL AND ENVIRONMENTAL CHANGE ACROSS THE PALEOCENE-EOCENE BOUNDARY IN THE NORTH AMERICAN CONTINENTAL INTERIOR: BIGHORN BASIN, WYOMING

The Bighorn Basin of northwestern Wyoming is a large NW to SE-trending Laramide intermontane basin, overthrust from the west, with a thick and virtually continuous succession of richly fossiliferous upper Paleocene through lower Eocene continental strata. Two formations are represented: Fort Union and Willwood. The first is lignite-bearing and predominantly Paleocene. The second is red-banded and predominantly Eocene. Each is on the order of 1000 m thick. Paleosols are present through both formations but these are more oxidized and red in the Willwood Fm. Faunas from three land-mammal ages, Tiffanian late Paleocene, Clarkforkian latest Paleocene, and Wasatchian early Eocene, have been the subject of a succession of Ph.D. dissertations. Mammalian intercontinental dispersal events mark the boundaries of these, and coordinated first appearances enable subages and biochrons to be recognized within each land-mammal age. The best studied part of the Bighorn Basin stratigraphic section is the intensely-colored interval spanning the Paleocene-Eocene boundary. The Paleocene-Eocene carbon isotope excursion (CIE) marking the Paleocene-Eocene thermal maximum (PETM) was first found on land at Polecat Bench, and it is now known at four additional sites around the Bighorn Basin. At Polecat Bench, the CIE is about 30 m thick, with a latest Clarkforkian Cf-3 mammalian fauna occupying the lowest 2 m at the base and an earliest Wasatchian Meniscotherium or Wa-M fauna present for the next 3 m. Together these span the onset of the CIE. An early Wasatchian Wa-0 fauna fills the next 25 m of section, during the body and initial recovery phases of the CIE. The later recovery phase of the CIE coincides with the appearance of a more normal Wa-1 Wasatchian mammalian fauna. This faunal succession is found in all parts of the basin, although it is expressed a little differently and represented by different thicknesses of strata in different places. Faunal responses to PETM environmental warming across the Paleocene-Eocene boundary include immigration of new taxa, and body size dwarfing of existing taxa. There is some evidence that paleosol development and carbon isotope ratios are modulated by orbital precession, offering the potential for precise age correlation and calibration.