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

Abstract

In this study, we systematically integrate ichnologic and pedologic features of the Upper Triassic Chinle Formation into ichnopedofacies (IPF) to interpret stratigraphic variations in paleoenvironments, physiochemical conditions, and paleoclimate in the NE Chinle Basin. The Chinle Formation in the Stevens Canyon area, SE Utah, represents fluvial, palustrine, and lacustrine strata deposited in a continental back-arc basin on the western edge of Pangea under a megamonsoonal climate with increasing aridity towards the end of the Late Triassic. Seventeen ichnofossil morphotypes and six paleosol orders were combined to form 12 IPF, whose development was controlled by autocyclic and allocyclic processes, and hydrology. IPF in the NE Chinle Basin indicate both long-term drying of climate and short-term, wet-dry fluctuations. Lateral distribution of IPF indicate depositional energy, sedimentation rate, and height of the water table decrease with increasing distance from fluvial systems, and indicate the presence of incised paleovalleys. Landscape degradation, basin subsidence, and salt tectonics influenced paleovalley formation. Valley incision was additionally controlled by the shifting from wetter to drier climates, while valley fill likely occurred during shifts back to wetter intervals. Sedimentation varied during Chinle Formation deposition: 1) high, nonsteady in the Moss Back Member; 2) low, nonsteady in the Petrified Forest and lower Owl Rock members; 3) high, nonsteady in the middle Owl Rock Member; 4) high, steady in the upper Owl Rock Member; 5) high, nonsteady in the Church Rock Mbr. The water table was controlled by precipitation levels and increases in depth up section. High water tables in the upper Chinle Formation are present in proximal fluvial deposits from losing rivers. Lacustrine deposits were uplifted by salt tectonism and reworked by fluvial systems. The only evidence of lake systems in Stevens Canyon area are oncoid clasts in laterally accreted conglomerate beds. IPF indicate megamonsoonal circulation continued until the end of the Triassic Period. IPF enable higher resolution reconstructions of paleoenvironments and physiochemical conditions, and can aid identification of sand-in-sand and silt-in-sand paleovalleys. IPF are valuable tools for evaluating lateral and vertical fluid conductivity, subsurface facies architecture, and reservoir characterization in continental settings which contain hydrocarbons.