Paleocene Palynostratigraphy in the Hanna Basin: Friend or Foe?

The Hanna Basin, located in south-central Wyoming is a Laramide basin that contains strata deposited during the Late Cretaceous through the early Eocene. The Ferris and superposed Hanna Formation contain a diversity of fossilized plants and animals. Because no volcanic ashes from the basin have been recognized or radiometrically dated, ages for the rock units have been determined through biostratigraphy. Vertebrate-bearing rock units deposited during the Paleocene contain faunas assignable to the Puercan, latest Torrejonian and early Tiffanian North American Land Mammal Ages (NALMAs). Palynological samples collected from mammal sites provide correlation points between NALMAs and the well-established Paleocene palynostratigraphic scheme where pollen zones P1-P6 occur in ascending order. All of the six Paleocene pollen zones occur in the Hanna Basin. Pollen Zone P2 occurs along with Puercan (Pu2 and Pu3) mammal assemblages and Zone P3 occurs in latest Torrejonian (To3) through middle Tiffanian-aged (Ti3) strata.

While previous palynological work in the Ferris and Hanna coalfields reported that all six of the Paleocene pollen zones occur in superposition, more recent samples from an exposure known as The Breaks have yielded Zone P5 palynomorphs stratigraphically below Zone P2. This finding has drawn criticism about the validity of the pollen zonation. However, several lines of evidence suggest that the "out-of-order" pollen sequence is confined to The Breaks and is the result of post-depositional processes. Such evidence comes from on-going efforts to refine Paleocene time scales on a regional level as well as comparisons of local stratigraphy and micro- and megafloral localities in The Breaks. Taken together, these lines of evidence strongly suggest that the palynostratigraphic zonation scheme is sound, but that the stratigraphic section in The Breaks is in need of revision.