A Regionally Consistent Ichnofacies Associated with Marginal and Offshore Marine Rocks of the Early Triassic Moenkopi Formation, Utah

Regional ichnological and sedimentological analysis of the Torrey and Moody Canyon Members of the Early Triassic Moenkopi Formation in central and northern Utah indicate that these rocks contain a diverse set of trace fossils that together constitute an ichnofacies. Vertebrate, invertebrate, and plant ichnites all coexist in marginal marine (principally tide-dominated channel systems) and offshore marine settings. The ichnites are absent from continental strata of the Black Dragon Member of the Moenkopi Formation in the same region.

We have identified the types of depositional environments represented by the rocks at various locations as well as the specific ichnites present. Because the depositional environment of the rocks can be determined, it is possible to use information about the presence or absence of different types of trace fossils in different lithofacies to identify the correlation between specific trace fossils and different depositional environments. The vertebrate ichnites Chirotherium, Rhynchosauroides, Rotodactylus, and Undichna,, along with the invertebrate ichnites Kouphichnium, Rotamedusa, Palaeophycus, Fuersichnus, and Arenicolites, and rare Equisetum(?) plant traces, and body traces of horseshoe crabs, shrimp-like crustaceans, and fish are consistently associated with rocks deposited in marginal marine (primarily tidal) and offshore marine environments. The assemblage is absent from continental rocks, and it appears to be a good indicator of shoreline to offshore depositional settings. Previously, some of the vertebrate ichnites in this assemblage were thought to occur in continental rocks, but the results of this study suggest that the vertebrate traces may in fact be limited to marginal marine and offshore environments. The presence of these ichnites together constitutes and ichnofacies that can be used as an indicator of shoreline and offshore settings in Early Triassic strata.