PALEOBOTANICAL ASSESSMENT OF THE HANNA BASIN, SOUTH-CENTRAL WYOMING: CORRELATING PALEOCENE LEAF MEGAFOSSILS AND PALYNOLOGICAL DATA WITH NORTH AMERICAN LAND MAMMAL AGES

The Hanna Basin, in south-central Wyoming is a structurally complex, Laramide basin that contains over 11 km of fossiliferous, nonmarine strata from Late Cretaceous through early Eocene time. Vertebrate paleontologists have established an extensive mammalian biostratigraphy for the Hanna Basin, utilizing North American Land Mammal Ages (NALMAs); hence Lancian through Wasatchian time is well constrained in the superimposed Ferris (Cretaceous–Paleocene) and Hanna (Paleocene-early Eocene) formations. Leaf-bearing strata in close association with fossil mammal localities are common throughout the section. This unusual condition offers the opportunity to stratigraphically correlate leaf megafossils and fossil pollen with NALMAs.

Fossil leaf specimens and palynological samples from over 30 fossil leaf localities have been collected throughout the section. Changes in floral diversity and composition are evident from Puercan through Tiffanian time. They indicate changing climatic conditions, inferred from mean annual temperature and mean annual precipitation estimates. A stratigraphic range chart constructed using first and last appearance datums for leaf morphotypes indicate that fossil plant associations may be linked in a biostratigraphically useful fashion similar to the existing vertebrate-based zonation. Samples taken for palynological analysis from Paleocene aged localities, were assigned a P-zonation based upon Nichols and Ott (1978). These data are important for constraining the timing and duration of P-zones in respect to NALMAs, and may be applicable across other Tertiary basins in the Rocky Mountain region that lack mammal zonation.