PREDICTING FOSSIL PRESERVATION USING RATES OF ROCK ACCUMULATION: A CASE STUDY IN THE EOCENE WASHAKIE FORMATION, WY

Fossil preservation is thought to be strongly controlled by facies type, and thus up-section variation in facies can create artefactual shifts in species diversity. The frequency of facies in nonmarine systems can be largely controlled by rates of rock accumulation. Here, I propose using rates of rock accumulation (a function of subsidence and sediment supply) to predict the preservation of vertebrate skeletal material. I test this approach in the intermontane Washakie Basin in Wyoming using Eocene mammalian fossil material collected during recent fieldwork.

Preservation and abundance of mammalian fossils should vary with respect to rates of supply of siliciclastic sediment into a basin and the basin's ability to store that sediment (accommodation). Intervals with high rates of both accommodation and sediment supply would promote rapid aggradation and burial of both small and large-bodied animals with a high likelihood of preserving articulated material. Fossils in these records should be sparsely distributed due to dilution with sediment, with a low degree of time averaging per assemblage. Where accommodation is high, but sediment supply is low, preservation would favor smaller elements in patchy, concentrated localities, with greater time averaging per assemblage. Low rates of accommodation should reduce preservation potential regardless of sediment supply, given greater reworking of previously deposited sediments. The only taxonomically identifiable material in these records should be small, durable material.

These predictions are explored using new fossil collections and measured sections of the middle Eocene Washakie Formation in Wyoming (~1000 m thick, ~49-46 Ma). Relative changes in rates of accommodation (A) and sediment supply (S) were inferred from lithologic evidence such as the frequency of lacustrine facies, multi-story sandstones, floodplain fines, and mature paleosols. Observations of associated vertebrate material reveal strong variation in preservation with this three-phase history of rock accumulation consistent with the above predictions. The rate of rock accumulation, which controls the frequency of facies, rather than facies alone, plays a strong role in determining the composition and nature of mammal assemblages in this inland basin, providing insight into the occurrence and preservation of fossils in fluvial records.