Trace fossils and fluvial-lacustrine ichnofacies of the Eocene Uinta and Duchesne River Formations, northern Uinta Basin, Utah

Abstract

Trace fossil assemblages in a fluvial-lacustrine sequence stratigraphic context hold significant poten-tial for expanding our understanding of environmental controls and continental basin-fill history. The succession of the Eocene Uinta Formation and four members of the Duchesne River Formation is ex¬tremely well-exposed in the Uinta Basin of northeastern Utah, revealing a robust stratigraphic framework to document broad-scale fluvial-lacustrine facies architectures and associated trace fossil assemblages. Greenish- and gray-colored mudstone beds with interbedded tabular sandstone representing lacustrine environments contain the trace fossils Arenicolites and Gordia (= Haplotichnus). In contrast, red mudstone beds with interbedded channelized sandstone representing upstream fluvial and alluvial environments contain a variety of insect trace fossils, including Scoyenia, Ancorichnus, and nest structures. Transitional, interfingering lithologies of wetland or shallow, short-lived lacustrine environments on the alluvial plain contain the trace fossil Steinichnus. Although there are many small-scale (bed-scale) physical sedimen¬tary structures and trace fossils from continental subenvironments, this study focuses on the large-scale (member-scale) change in trace fossil assemblages, with results indicating that the ichnofacies corroborate continental sequence stratigraphic interpretations in a fluvial-lacustrine setting.