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Water samples from the Colorado River and its tributaries provide a present-day 87Sr/86Sr profile of the Colorado River system from Moab, Utah, down to the Gulf of California. 87Sr/86Sr values around 0.7097 are prevalent above the Grand Canyon, and values of 0.7103 or higher are observed below the Grand Canyon. 87Sr/86Sr values close to the seawater ratio of 0.70917 are seen near the Colorado River delta. All tributaries except the Green River contribute Sr that is lower in 87Sr/86Sr than the main Colorado. The cause of the increase in 87Sr/86Sr through the Grand Canyon and Lake Mead area has not been found. Samples of carbonate and gypsum from the lower lacustrine member of the mid-Miocene to Pliocene Bidahochi...
In 1965 a debris flow from Warm Springs Draw, Colorado, deposited approximately 15,000 metric tons of sediment in the channel of the Yampa River, constricting the river against the opposite cliff and forming Warm Springs Rapid. Radiocarbon ages associated with several older debris-flow deposits bisected by the 1965 flow indicate a time interval of approximately 1000 years between large, channel-constricting debris flows from Warm Springs Draw. The older debris-flow deposits probably constricted the Yampa River in a manner analogous to that of the 1965 flow. The configuration of these tributary debris-fan rapids, which occur along canyon rivers throughout the western U.S., is a function of the sediment characteristics...
U/Pb zircon/titanite geochronology, in situ monazite geochronology, and 40Ar/39Ar thermochronology provide an unusually complete data set for reconstructing the tectonic history of Proterozoic rocks exposed in the Black Canyon, Gunnison, Colorado. These new geochronologic data record three protracted orogenic episodes and an exhumation event between orogenic pulses: (1) Yavapai orogeny (1741–1689 Ma), (2) exhumation marked by an angular unconformity beneath post�Yavapai, pre�Mazatzal quartzites, (3) Mazatzal orogeny (postquartzite deposition), and (4) 1434–1403 Ma intracratonic tectonism. Supracrustal rocks of the Black Canyon succession were deposited or crystallized at or prior to Ma and were intruded...
New paleomagnetic and biostratigraphic data from the western Green River Basin of Wyoming provide a magnetobio-stratigraphic framework to correlate the late Wasatchian through early Bridgerian North American Land Mammal Ages (NALMA) to the Geomagnetic Polarity Time Scale (GPTS). Three-hundred and twelve paleomagnetic samples were collected from 73 sites within a 350-m continuously exposed flat-lying section. The section is characterized by alternating packages of well-developed paleosol mudstone horizons (Wasatch Formation) and lacustrine shale/limestone horizons (Green River Formation). Fossil mammal assemblages are known from various levels within the section and constrain the location of the Wasatchian/Bridgerian...