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  • Thumbnail for The Geochronology and Stratigraphy of Comanche National Grasslands, CO
    The Geochronology and Stratigraphy of Comanche National Grasslands, CO by Hager, Alexander Olin

    Little was previously known for certain about the age and depositional history of the strata exposed in Picket Wire Canyon in Comanche National Grasslands, CO, as prior work in developing a comprehensive geochronology and stratigraphy of the area has been minimal. Previous age constraints for the strata have been largely speculative, as the canyon contains the only outcrop of Paleozoic¬–Mesozoic strata within 100 km, and correlating local strata to regional formations is troublesome. Prior chronostratigraphic constraints were based entirely on fossils in Late Triassic and younger strata, but there have not previously been age constraints for strata older than the Late Triassic Chinle Formation. Radiometric U/Pb dating of detrital and volcanic zircon grains, along with carbon and oxygen stable isotope geochemistry, were employed in this study to constrain ages of the strata in Picket Wire Canyon, as well as determine depositional conditions. A 77 m thick eolianite unit disconformably underlies the Chinle Formation and was of unknown age; however, detrital zircons extracted from the base of the unit yielded a youngest grain of 245.5 ± 5.9 Ma, indicating the eolianite is correlative to the Triassic Red Draw Member of Jelm Formation. Additionally, this data suggests the oldest strata in the canyon, those stratigraphically below the eolianite, correlate to the Permian¬–Triassic Lykins Formation. U/Pb dating of volcanic ash in the Ralston Creek Member of the Morrison Formation yields an age of 152.987 ± 0.063 Ma, which is the most precise age for the Morrison Formation known to date. δ13C and δ18O values of Morrison carbonate beds indicate deposition in an arid, hydraulically open lacustrine setting, and are consistent with previous results from other Morrison localities. The Lower Morrison Formation at this locality contains one of the world’s largest continuously mapped dinosaur trackways. These footprints are atypical in that they are preserved in oolite, which due to its granular texture should not theoretically preserve footprints. Thin section analysis of the oolite depicts microbial films both within and surrounding the ooids, along with cementation structures indicative of meteoric vadose diagensis. It is hypothesized that the presence of microbial films and diagenetic cements allowed for the preservation of the footprints by increasing cohesion between grains, thus permitting the imprints to maintain their mold indefinitely.