LITHOSTRATIGRAPHIC CORRELATIONS AND VERTEBRATE PALEONTOLOGY OF THE LATE TRIASSIC CHINLE FORMATION IN SOUTHWESTERN UTAH

The Chinle Formation is a Late Triassic, fluvial unit composed of complexly interfingering members and diverse lithologies, including channel fill, floodplain, lacustrine, and dunal facies. It is widely distributed, and its geographic range spans the Four Corners region of the southwestern United States and extends as far west as southeastern Nevada. The formation has been the subject of a considerable amount of research and has been particularly well-studied in northwestern New Mexico and at Petrified Forest National Park, Arizona. However, comparatively little literature exists concerning the Chinle Formation in southwestern Utah, and the stratigraphic assignments made within the existing literature are inconsistent between authors. We present three stratigraphic sections of the Chinle Formation in southwestern Utah, propose tentative correlations between them, and provide preliminary descriptions of vertebrate fossil material from these sections. Two of the sections were measured on Hurricane Mesa west of Zion National Park, in Washington County, and the third was measured just east of Cedar City, in Iron County. The Shinarump Conglomerate, Cameron Member, and “purple pedogenic beds”, a locally occurring paleosol unit bearing distinctive carbonate nodules, of Martz et al. (2015) are present at each of the sections. Additionally, the “lower sandstones” of Gregory (1950), the “upper gray beds” and the “friable sandstone” of Martz et al. (2015) are present on Hurricane Mesa but absent in Cedar City. Vertebrate fossil material is present in the Chinle Formation at Cedar City and Hurricane Mesa, but has been better sampled and appears to be more abundant on Hurricane Mesa. The material collected in both areas is fragmentary and includes metoposaur cranial and clavicular elements and phytosaur teeth found in float. Additionally, we collected and screen-washed approximately five gallons of matrix on Hurricane Mesa from which we have identified numerous scales and teeth likely from semionotiforms and palaeonisciforms, as well as a single tooth bearing potential venom grooves. Coprolites are also abundant at both localities. The majority of the collected coprolites are too fragmentary to be assigned to ichnotypes, however several may be assigned to Heteropolacopros texaniensis and Alococopros triassicus.