Stratigraphy and mammalian paleontology of the McCullough Peaks, northern Bighorn Basin, Wyoming: Implications for biochronology, basin development, and community reorganization across the Paleocene-Eocene boundary.

The McCullough Peaks area of the northern Bighorn Basin, Wyoming, preserves a thick ($\sim$2700 m) sequence of highly fossiliferous upper Paleocene (Fort Union Formation) to lower Eocene (Willwood Formation) fluvial strata. Four hundred and seven fossil localities are known from this area, 255 of which have been correlated to measured stratigraphic sections. These fossil localities provide a biostratigraphic framework for McCullough Peaks sections and indicate that mammalian faunal zones Ti-3 (Tiffanian land mammal age) to Wa-7 (Wasatchian land mammal age) are preserved in superpositional relationship in this area. Stable isotope analysis of paleosol carbonates corroborates the existence of a carbon isotope excursion coincident with highly mature paleosols at the base of the Willwood Formation and a Clarkforkian-Wasatchian transient mammalian assemblage (zone Wa-0). This excursion provides an important point for regional and global correlation, and represents further evidence for a severe atmospheric perturbation at the end of the Paleocene. Magnetostratigraphic analysis indicates that the Wasatchian land mammal age spans approximately 5 my of geologic time ($\sim$55.5 to 50.5 Ma), and that zone Wa-7 represents about one-half of that period. Sediment accumulation rates calculated from various sections throughout the basin show periods of high sediment accumulation during the late Clarkforkian and middle Wasatchian. Sediment accumulation rates tapered off at the beginning of zone Wa-7, probably signifying the end of local Laramide tectonism. Faunal analysis of mammalian assemblages from the northern Bighorn Basin shows that significant changes in diversity, body size structure, and guild structure at the species and individual levels of organization coincide with the latest Paleocene isotope excursion and coeval earliest Wasatchian immigration event. A short-term decrease in average individual body-size may be a direct effect of the latest Paleocene thermal maximum (LPTM). Increases in mammalian diversity and average species size, as well as significant reorganization of trophic structure all seem to be direct effects of the Wasatchian immigration event, which was likely triggered by the LPTM. A scenario is developed that ultimately links sedimentological and biotic changes across the Paleocene-Eocene boundary in the Bighorn Basin to coincident changes in the deep ocean.PhDEarth SciencesGeologyPaleoecologyPaleontologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/130450/2/9732059.pd