BIOSTRATIGRAPHICALLY SIGNIFICANT PALYNOFLORAS FROM THE PALEOCENE–EOCENE BOUNDARY OF THE USA

Pollen and spores were recovered from the Paleocene Fort Union Formation and Paleocene–Eocene Willwood Formation of the Bighorn Basin (BHB), northwestern Wyoming, USA. In many local stratigraphic sections in the BHB, the base of the Eocene has been identified by the characteristic negative carbon isotope excursion (CIE) that is the best global marker for the base of the Eocene and beginning of the Paleocene–Eocene Thermal Maximum (PETM). The palynotaxa from outcrop samples were examined using light microscopy (LM) and scanning electron microscopy (SEM). Seven new species are formally described (Tricolpites vegrandis, Rousea spatium, Striatricolporites astutus, Striatopollis calidarius, Friedrichipollis geminus, Retistephanocolporites modicrassus and Retistephanocolporites pergrandis). The temporal and geographic distributions of many of these palynotaxa suggest that hotter and more seasonally dry climates associated with the PETM facilitated their northward range shifts from the tropics or subtropics of the USA. For the temperate palynotaxa, the hotter and seasonally dry conditions resulted in local extirpation. A re-evaluation of the palynostratigraphic schemes established for the Paleocene–Eocene boundary of the USA confirms that the first appearance of Platycarya platycaryoides denotes the Paleocene–Eocene boundary in the Rocky Mountain region. A new Striatopollis calidarius Biozone, associated with early Wasatchian (Wa) Wa-0 faunas, is recognized for PETM localities in the BHB that contain palynofloral assemblages that cannot be placed in the existing palynostratigraphic schemes.