PALYNOLOGICAL RECONSTRUCTIONS OF EARLY EOCENE FLORA OF THE WIND RIVER BASIN, WYOMING

From the end of the Cretaceous through the early Eocene, the Laramide Orogeny gave rise to a series of well-established small basins spanning western North America. Studies of the late Paleocene through early Eocene climate also record warming to a sustained Cenozoic maximum called the Early Eocene Climatic Optimum (EECO). This resulted in increased species diversity and habitat complexity. Each basin has a distinct history, environment and taxa, which can be contrasted to explore responses to these changes. This study focuses on palynological occurrences in the Wind River Basin, Wyoming, during early Eocene, specifically the North American Land Mammal Zones Wasatchian 6 to Bridgerian 1a (Wa6-Br1a, 53 Ma- 51 Ma). Pollen is very responsive to environmental changes; therefore it can be hypothesized that pollen diversity and complexity should increase with warming until the end of the EECO as similar patterns are seen in the adjacent basins. The EECO is the warmest period of the last 65 million years and thus can aid in the understanding of current climate issues as well as how floras respond to environmental variations.

Samples were collected from the pollen-bearing lithologies of the Wind River Formation, which is composed of three members: the Lysite, the Lost Cabin and the Red Creek Members. These facies are dominated by lacustrine and overbank deposits, mudstones with channel sands and grey mudrocks, respectively. Stratigraphic sections were measured to assess depositional environments associated with recovered pollen. Nine samples were analyzed from the Wa6, eight from the Wa7 and four from the Br1. Pollen counts were conducted for all 21 samples, and quantitative analyses were used to compare diversity, evenness, abundance distribution and composition among palynofloras.

Results show fluctuations in plant composition and species’ abundances in line with the onset of the EECO. This flora includes long-ranging types (Pinaceae), Eocene markers (Lygodium, Platycarya), and warm climate markers (Bombax, Anacolosa). Modern examples of this flora correlate with types seen in temperate and tropical environments. Areas for future studies include comparison of these data to those of the macroflora as well as the investigation of two new species only previously recorded from the PETM in the Bighorn Basin.