OXYGEN-CONTROLLED BIOFACIES OF THE MIDDLE CAMBRIAN WHEELER SHALE AND EQUIVALENTS, EASTERN BASIN AND RANGE

The Wheeler Shale contains a diverse Middle Cambrian biota including many soft-bodied members of the Burgess Shale fauna, and the well-known trilobite, Elrathia kingi, in addition to an abundance of other skeletonized taxa. Despite this relatively well-documented richness, very little is known about the paleoecology of this unit. This study combines two approaches in order to understand the faunas of the Wheeler Shale in their proper ecological context: millimeter-scale laboratory analysis of continuously collected 1-5 meter sections of mudstone, and outcrop-scale study to identify recurrent fossil communities, with the goal of providing an environmental context. Wheeler Shale localities with the highest taxonomic diversity occur in the central House Range. These exposures represent deposition in an oxygen-deficient fault bounded trough, which was geographically localized. Exposures of the Wheeler Shale in the northern House Range and adjacent ranges reveal profound lateral facies changes providing an opportunity to examine in-situ paleoecology across an environmental transect from anoxic basin to shallower ramp environments. Results indicate that bottom water oxygenation was the primary control on the distribution of faunas within the Wheeler Shale. Four biofacies are present: Soft-bodied biofacies (SBF), Elrathia biofacies (EBF), Asaphiscus biofacies (ABF), and Bolaspidella biofacies (BBF). SBF strata represent exceptional preservation of allochthonous fossils in an anoxic basin. Within SBF strata, the fossil content is regulated by the proximity of habitable environments (i.e., source of fossils): distal SBF beds are the most basinal of the study, and contain a pelagic fauna of algae and agnostids. Proximal SBF beds contain more common metazoans. EBF occurs at the transition from laminated (i.i.1) to bioturbated (i.i.2) strata and contains dense, monospecific accumulations of Elrathia, which were apparently adapted to life in low oxygen settings. ABF occurs in uniformly weakly bioturbated (i.i.2-dysoxic) strata and contains a diverse trilobite community dominated by Asaphiscus. BBF beds are uniformly well-bioturbated (i.i.3-4), and contain a depauperate fauna dominated by the trilobite Bolaspidella.