Integrating sedimentological information collected in northwestern Wyoming during the summer field season of 1999 with an existing database of terrestrial vertebrate taxonomic abundance information provides insight into the paleoecology of the Wasatchian-3 Land Mammal Age Zone of the early Eocene. The Willwood Formation is exposed as badlands topography in the Bighorn Basin of northwest Wyoming and preserves abundant terrestrial fauna in paleosol deposits and other fluvial sediments that formed on the floodplains of an ancient river system (below). A previous study of the Willwood Formation in the south central region of the Bighorn Basin showed a significant relationship between mammalian faunal abundance and paleosol maturity but little subsequent research has explored this hypothesis. Maturity of a paleosol is thought to be indicative of it's lateral position on the ancient floodplain. Mature paleosols are thought to indicate distal floodplain environments and immature paleosols are thought to indicate areas adjacent to the ancient riverbed.

Topographic exposure of the Willwood Formation in the McCullough Peaks. Arrow points to 1.5 meter Jacob Staff (blue) on left, for scale.

Results of Pearson correlation coefficient analysis and principal components analysis on taxonomic abundance information indicate certain groupings of taxa are preserved in the Willwood Formation sediments. However, further statistical analysis indicates only a weak relationship exists between sedimentological information and taxonomic abundances.

Sedimentological information was also used to examine fluvial depositional patterns during the approximately 350-kyr time interval of Wasatchian-3. Results of Markov chain analysis indicate lithologies are dependent on the preceding lithology within these ancient fluvial deposits.According to the analysis, channels of the early Eocene avulsed to mature areas of the floodplain and once they avulsed, remained in that area for an extended period of time.

Schematic representation of the results of the Markov chain analysis

The observed lithological sequence follows an expected avulsion pattern where the channel shifts to distal, topographically low, areas of the floodplain.