GEOLOGIC CONTEXT IN TAPHONOMIC STUDIES: AN UNDER-APPRECIATED APPROACH TO TAPHONOMY AND GEOLOGY

Summaries of vertebrate taphonomy correctly list geology as one in a group of taphonomic factors that needs to be addressed to fully appreciate the taphonomic history of a given site or assemblage. In practice, "geology" often refers to a site's stratigraphic position, host facies sedimentology (e.g., grain size analysis, facies analysis of host matrix, orientation analysis), as well as some geochemical aspects and diagenesis of the matrix and its fossils. In contrast, the overall geologic context of a site in relation to the host formation or region -- here regarded as distinct from the concepts of taphonomic mode or taphofacies -- is often overlooked, sometimes due to limitations in resources, exposure, or both. This is unfortunate because an assessment of geologic context provides the opportunity to judge how typical or atypical a site's geology is on the scale of the host formation or region. A study of the Dalton Wells dinosaur bonebeds exemplifies how a thorough analyses of geologic context can (1) greatly improve our taphonomic understanding of a site and, (2) elevate the importance of the site in the context of the geologic history of the host formation or succession.

The Lower Cretaceous Dalton Wells bonebeds, Eastern Utah, occur at the base of the Cedar Mountain Formation, and lie unconformably on the Upper Jurassic Morrison Formation. The bonebeds are exceptional examples of stacked subaerial debris flow deposits that most likely accumulated on the eastward-dipping slopes of a backbulge in a newly-forming foreland tectonic setting. The depositional slope required to generate flows may have been established by bulge crustal flexure, but could also have been maintained or amplified by the high rates of local subsidence. The consistent expression and location of marginal- and fully-lacustrine facies in both the upper Morrison and lower Cedar Mountain formations may explain why the area attracted vertebrates. The stable expression of a lacustrine margin in this area further indicates long-term expression of high rates of subsidence before and after the J-K transition, and suggests that there was long-term to episodic and localized influence on subsidence -- possibly salt-diapir-related faulting.