THE BALANCE OF CLIMATE VS. GEOMORPHOLOGICAL DRIVERS INFLUENCING PATTERNS OF FAUNAL AND TAPHONOMIC CHANGE IN THE WHITE RIVER SEQUENCE

The fauna of the White River Sequence (WRS) of the Great Plains of North America has long been used as a proxy to study the influence of the Eocene-Oligocene Climate Transition away from low-latitude marine settings. A key problem with this approach (which applies to most existing terrestrial taphonomic and paleoecological studies) is the lack of a suitable facies model within which to understand the background geomorphological/sedimentological change occurring during the deposition of the WRS. The Distributive Fluvial System (DFS) model provides such a framework.

DFS (encompassing alluvial fans, fluvial fans and megafans) appear to comprise the bulk (>80%) of modern terrestrial deposits that will be preserved for geologically long periods of time, and hence should dominate the terrestrial sedimentary record. One key prediction of the DFS framework is that DFS sequences should show drying upwards trends as more proximal, coarser (hence well drained) facies prograde over more distal, finer (hence poorly drained) facies. These paleoenvironmental changes will influence the structure and preservational style of the WRS fauna, independent of any climatic drivers.

A comparison of the taphonomic patterns and the faunal structure of the WRS assemblage along a proximal-distal transect through the putative WRS DFS to new quantitative models built within a DFS framework shows good congruence. Much of the faunal variation that has previously been ascribed to climate change can be attributed to DFS-growth related geomorphological change. Similarly, much of the thus far unexplained variation in taphonomic pattern across the WRS corresponds to that expected under a DFS framework. This supports the importance of the DFS model and suggests that a wider reinterpretation of other terrestrial assemblages under the DFS framework is necessary.