Ichnopedologic Assemblages in the Upper Triassic Chinle Formation, Stevens Canyon, Southeastern Utah: Implications for Paleohydrology and Paleoclimate During Chinle Deposition

Abstract

Regional climate models for the southwest United States interpret aridification during deposition of the Upper Triassic Chinle Formation (CF). Few studies, though, have dealt with interpreting local climate variations during Chinle deposition and fewer have looked at ichnopedologic associations. This study uses ichnofossils, paleosols, and sedimentary features to interpret environmental, paleohydrologic, and climatic conditions of Chinle deposits in Stevens Canyon, southeast Utah. Here, the CF is a complex fluvial-lacustrine-deltaic-floodplain depositional system composed of the Monitor Butte, Moss Back, Petrified Forest, Owl Rock, and Church Rock members. Although the CF displays large lateral discontinuity and variability (e.g., transition from lake shore to subaerial paleosol in the course of a dozen meters in the Petrified Forest member.), ichnopedologic associations can be traced and correlated across SC. Ichnofossils found at SC include Naktodemasis, Camborygma, Planolites, Steinichnus, stromatolites, insect cocoons (wasp or beetle), large therapsid burrows, reptile tracks, rhizoliths, rhizohaloes, and rhizocretions, and dominant paleosols are calcisols, argillisols, and inceptisols. Combining trace fossils with pedofacies, we erected ichnopedologic associations that encompass the variability present in these complex continental successions and establish better controls for continental facies models, environmental interpretations, sedimentation rate, and determining the length of environmental stability and subaerial exposure. For example, Naktodemasis, Camborygma, and therapsid burrows were often associated with calcareous angular blocky siltstone paleosols and are interpreted as floodplain deposits. Preliminary results of this study match the overall aridification trend shown in previous research, but the incorporation of ichnopedologic associations allows for higher resolution environmental interpretations. The utility of ichnopedologic associations in continental deposits is significant as the CF is a depositional analog to fluvial/alluvial reservoirs which are becoming increasingly important (e.g., Permian Basin). Our ichnopedologic associations are a valuable tool for evaluating lateral and vertical fluid conductivity, subsurface facies architecture, and reservoir characterization in other continental settings which contain hydrocarbons, such as the Bakken Formation and Paradox Basin.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015