AN EARLIEST EOCENE CONGLOMERATE IN THE BIGHORN BASIN, WYOMING, AND POSSIBLE RELATIONSHIP TO THE PALEOCENE-EOCENE THERMAL MAXIMUM

Willwood Fm. strata in the Bighorn Basin representing the Paleocene-Eocene Thermal Maximum (PETM) have been the subject of intense paleontological, paleopedological and stratigraphic study to characterize the accompanying carbon isotope excursion (CIE) and shifts in climate, depositional systems and biota. The vast majority of this work has been performed along an ~160-km-long transect from the southeast to nearly the northwest limit of Willwood outcrop. The Paleocene-Eocene transition in the south-central part of the basin is much less studied.

Recent field work near Winchester, Wyoming has recovered fossils of the earliest Eocene equid Sifrhippus (characteristic of biozone Wa0, associated with the PETM) in the lowest variegated paleosol horizons of the Willwood Fm. Plant macrofossils and pollen from mud lenses within a fluvial channel sandstone between the paleosols also are typical of the body of the CIE. The PETM flora and fauna occur in the basal 20 m of the Willwood Fm., and are overlain by variegated paleosols containing mammals of the succeeding Wa1 biozone. The conformably underlying Fort Union Fm. contains diagnostic late Paleocene pollen.

The sequence of paleosols can be traced 10 km to the southernmost exposures of the Willwood Fm. Here ~15m of Willwood Fm. containing two variegated red-purple paleosols also rests conformably on the Fort Union Fm., which contains late Paleocene pollen. The two Willwood paleosols are overlain by up to 20 m of quartzite cobble conglomerates and coarse sandstone. The lower contact of this unit is sharp but not deeply eroded into the paleosols. Quartzite clasts range from pebbles to cobbles up to 20 cm long and the conglomerate is largely matrix supported by coarse sand. The conglomerate beds thin rapidly basinward, and are not seen north of the valley of Cottonwood Crk., about 5 km away. Though previously described, the probable earliest Eocene age of the conglomerates was not known. We suggest that the progradation of these conglomerate fans into the Bighorn Basin may have been caused by enhanced erosion and high depositional energy associated with vegetation change and extreme rainfall events during the PETM. These deposits may be a North American analog to the better-known Claret Conglomerate of northern Spain.