Stratigraphic Changes in Ichnopedofacies of the Upper Triassic Chinle Formation, Northeast Chinle Basin, Southeastern Utah: Implications for Depositional Controls and Paleoclimate

Abstract

The Upper Triassic Chinle Formation in the Stevens Canyon area, southeastern Utah, represents fluvial, palustrine, and lacustrine strata deposited in a continental back-arc basin on the western edge of Pangea. Previous investigations interpreted a megamonsoonal climate with increasing aridity for the Colorado Plateau towards the end of the Late Triassic. In this study, we systematically integrate ichnologic and pedologic features of the Chinle Formation into ichnopedofacies to interpret paleoenvironmental and paleoclimatic variations in the northeastern part of the Chinle Basin. Seventeen ichnofossil morphotypes and six paleosol orders were combined to form 12 ichnopedofacies. Ichnopedofacies development was controlled by autocyclic and allocyclic processes, and hydrology. In the northeast Chinle Basin, annual precipitation was ~1100–1300 mm in the Petrified Forest Member. Precipitation levels were 1300 mm/yr at the base of the lower Owl Rock Member, decreased to ~700–1100 mm/yr, and then to ~400–700 mm/yr. Two drying upward cycles from ~1100 mm/yr to ~700 mm/yr are observed in the middle and upper Owl Rock members. In the Church Rock Member, precipitation decreased from ~400 mm/yr at the base of the unit to ~25–325 mm/yr at the end of Chinle Formation deposition. Ichnopedofacies indicate monsoonal conditions persisted until the end of the Triassic Period with decreasing precipitation the result of the northward migration of Pangea. Ichnopedofacies in the northeast Chinle Basin indicate both long-term drying of climate and short-term, wet-dry fluctuations. Wet-dry cycles occur at other locations in the Chinle Basin, but variation exists in interpreted precipitation levels across the Chinle Basin due to the use of different climate indices. Overall trends of decreasing paleoprecipitation between the northeast and southeast Chinle Basin during Petrified Forest Member deposition, and between the northeast and central part of the Chinle Basin basin during Owl Rock Member deposition are recognized. Climate was not consistent across the Chinle Basin during the Late Triassic; there were complex variations in precipitation.