Ichnofossil assemblages and palaeosols of the Upper Triassic Chinle Formation, south-eastern Utah (USA): Implications for depositional controls and palaeoclimate
The Upper Triassic Chinle Formation in the Stevens Canyon area in south-eastern 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 Triassic. In this study, we systematically integrate ichnological and pedological features of the Chinle Formation into ichnopedofacies to interpret palaeoenvironmental and palaeoclimatic variations in the north-eastern part of the Chinle Basin. Seventeen ichnofossil morphotypes and six palaeosol orders are combined into twelve ichnopedofacies, whose development was controlled by autocyclic and allocyclic processes and hydrology. Ichnopedofacies are used to estimate palaeoprecipitation in conjunction with appropriate modern analogue latitudinal and geographic settings. In the north-east 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 occurred in the middle and upper part of the Owl Rock Member. In the overlying 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 with decreasing precipitation that resulted from the northward migration of Pangea. Ichnopedofacies in the northeast Chinle Basin indicate both long-term drying of climate and short-term, wet-dry fluctuations.
