Abstract The Upper Triassic Chinle Formation of the Colorado Plateau in the western United States was deposited as a continental fluvial-floodplain-lacustrine-eolian depositional system near the west coast of Pangea just north of the paleoequator. The deposystem evolved in response to regional tectonics and long-term climate change. Clastic and volcanic sediment was supplied by uplifted highlands and arc magmatism on the margins of the basin. Climate changed from a Pangean megamonsoon regime in the early Late Triassic to an arid climate setting at the close of the Triassic. Chinle fluvial discharge and sediment load varied through time in response to climate, producing degradational and aggradational cycles. Degradation eroded paleovalleys, and aggradation filled them with both meandering and braided fluvial systems, as well as varying amounts of floodplain mudstones. Paleosols developed on paleovalley fills, interfluves, and floodplain siltstones and mudstones, all of which combine to define several scales of degradational and aggradational cycles. An upward stratigraphic trend in Chinle paleosols from Oxisols and Gleysols, to Alfisols (Argillisols) and Vertisols, culminating in Inceptisols (Prototsols) and Aridisols (Calcisols), supports previous interpretations that climate evolved from monsoonal or humid conditions at the beginning of Chinle deposition to progressively more arid climate conditions at the end of deposition.

Abstract This five-day field trip examines Pennsylvanian to Jurassic strata in the Paradox Basin on the Colorado Plateau in southeastern Utah. The trip will emphasize four major themes: 1) Permian-Triassic stratigraphy, 2) characteristics of non-marine depositional systems, 3) paleogeography, and 4) paleoclimate. The trip follows depositional facies in the Permian and Triassic section from proximal continental settings near the Ancestral Rocky Mountains in the Uncompahgre Highlands to distal marine settings within the Paradox Basin. Evolution of these depositional systems along the west coast of Pangea from the late Paleozoic to the early Mesozoic is a key indicator of the paleogeographic and paleoclimatic history of the region. Late Paleozoic red beds encountered during the trip provide additional, high-resolution paleoclimate information. The trip will highlight unconformities, and their origin as a result of sea-level change, regional tectonics, or salt diapirism. The Permian-Triassic unconformity, which is expressed worldwide, will be examined during the trip. In southeastern Utah, the Permian-Triassic unconformity influenced the diagenetic history of the White Rim Sandstone, resulting in formation of the Tar Sand Triangle—the largest tar sand deposit in the U.S. Rocks examined in detail during the trip include the Permian Cutler Group (informal lower Cutler beds, Halgaito Formation, Cedar Mesa Sandstone, Organ Rock Formation, White Rim Sandstone, and De Chelly Sandstone), the overlying Lower Triassic Moenkopi Formation, and locally the Upper Triassic Chinle Formation. The trip also passes through the underlying Pennsylvanian section and the overlying Jurassic section, including eolianites exposed in and around Canyonlands National Park. This field guide includes several sections. The introduction and following text sections present short discussions of the major rock units and topics to be presented on the field trip. The field trip section of the field guide begins with a brief list of significant geologic features encountered during the drive across the Rocky Mountains from Denver to Grand Junction. We will not stop at any of the sites in the list. The remainder of the field trip section provides brief descriptions of the individual field stops that will be visited.

More than one-third of the United States is estimated to have high geologic radon potential. A high radon potential area is defined as an area in which the average indoor radon screening measurement is expected to be 4 pCi/L or greater. Geologic terrains of the United States with high radon potential include: 1. Uraniferous metamorphosed sediments, volcanics, and granite intrusives that are highly deformed and often sheared. Shear zones in these rocks cause the highest indoor radon problems in the United States. 2. Glacial deposits derived from uranium-bearing rocks and sediments and glacial lake deposits. Clay-rich tills and lake clays have high radon emanation because of high specific surface area and high permeability due to desiccation cracking when dry. 3. Marine black shales. The majority of black shales are moderately uraniferous and have high emanation coefficients and high fracture permeability. 4. Soils derived from carbonate, especially in karstic terrain. Although most carbonates are low in uranium, the soils derived from them are very high in uranium and radium. 5. Uraniferous fluvial, deltaic, marine, and lacustrine deposits. Much of the nation’s reserve uranium ores are contained within these sedimentary deposits, which dominate the stratigraphy of the western U.S.

Abstract The Upper Triassic Chinle Formation in southeastern Utah is a sequence of continental strata deposited in a back-arc cratonic basin. Archi tectural-faries analysis of exposures in sub-parallel canyons and in cliffs surrounding structural uplifts reveals an intricately interbedded fluvial-deltaic-lacustrine system characterized by: 1. mobile fluvial-channel belts consisting of stacked, high- and low-sinuosity channel complexes; 2. overbank deposits, including levees, paleosols, marshes, and small floodplain lakes interiingered with crevasse splays and lacustrine deltas; and 3. extensive lacustrine-basin, lacustrine-mudflat, and eolian sandsheet strata deposited throughout the entire study area. Stratigraphic panels of closely-spaced measured sections oriented both perpendicular and parallel to depositional dip depict the facies architecture both within the extra-channel strata and the relations of the extra-channel deposits to adjacent channel complexes. In the Shinarump and Monitor Butte Members in the lower part of the Chinle Formation, fluvial-channel complexes interfinger with extra-channel facies. These extra-channel facies include levees, crevasse splays, and wetland complexes consisting of lacustrine deltas, lakes, and marshes. In the Moss Back and Petrified Forest Members in the middle part of the Chinle, fluvial-channel complexes interfinger with floodplain strata characterized by paleosols and crevasse splays. Locally, paleosol horizons and lake and marsh deposits serve as marker beds in three dimensions. The Owl Rock and Church Rock Members in the upper part of the Chinle are dominated by laterally extensive lacustrine-basin, lacustrine-mudflat, and minor eolian sandsheet strata that extended throughout the study area.