FLUVIAL-LACUSTRINE ICHNOFACIES OF THE EOCENE UINTA AND DUCHESNE RIVER FORMATIONS, NORTHERN UINTA BASIN, UTAH

Sequence stratigraphic concepts and ichnologic studies have been well researched and documented in marine settings. In contrast, sequence stratigraphy in continental environments and the corresponding continental ichnology have been studied less thoroughly, and together they hold significant potential for expanding our understanding of environmental controls and basinal history. The succession of the Eocene Uinta Formation and four members of the Duchesne River Formation is extremely well-exposed in the Uinta Basin of northeastern Utah, revealing a robust sequence stratigraphic framework where the detailed basin-scale, fluvial-lacustrine facies architecture and the associated trace fossil assemblages record the late-stage fill of Eocene Lake Uinta. Specifically, the uppermost Uinta Formation and third member of the overlying Duchesne River Formation (Dl) indicate lacustrine-dominant sedimentary facies and trace fossil suites (e.g., Arenicolites, Gordia/Haplotichnus). In contrast, the basal member of the Duchesne River Formation (Db) exhibits a fluvial-dominant facies and a variety of insect trace fossils (e.g., Scoyenia, Ancorichnus, unidentified nest structures). The second member of the Duchesne River Formation (Dd) contains transitional facies between the basal and third members with frequent interventions of wetland or shallow, short-lived lacustrine environments on the alluvial plain, where intermediate trace fossil assemblages (e.g., Steinichnus) are present. Although this study cannot offer either rigorous bed-by-bed analyses or interpretations on specific continental environments, such as channel, bar and levee, the large-scale (i.e., member scale) change in trace fossil assemblages shows how the ichnofacies can corroborate continental sequence stratigraphic interpretations and can serve as a valuable indicator of paleoenvironmental change in a fluvial-lacustrine setting.