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Joint 70th Rocky Mountain Annual Section / 114th Cordilleran Annual Section Meeting - 2018

Paper No. 29-4
Presentation Time: 9:25 AM

CONTROLS ON PATTERNS OF UPPER COLORADO RIVER BEDROCK INCISION


ASLAN, Andres, Department of Physical and Environmental Sciences, Colorado Mesa University, 1100 North Avenue, Grand Junction, CO 81501, KARLSTROM, Karl E., Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, HEIZLER, Matthew, New Mexico Geochronology Research Laboratory, New Mexico Bureau of Mines & Mineral Resources, 801 Leroy Place, New Mexico Tech, Socorro, NM 87801, KIRBY, Eric, College of Earth, Ocean and Atmospheric Sciences, Oregon State University, Wilkinson 202D, Corvallis, OR 97331, GRANGER, Darryl E., Earth Atmospheric and Planetary Sciences, Purdue University, 550 Stadium Mall Dr., West Lafayette, IN 47907, HANSON, Paul R., Conservation and Survey Division, School of Natural Resources, University of Nebraska, 612 Hardin Hall, Lincoln, NE 68583, FEATHERS, James K., Anthropology, University of Washington, Raitt Hall Room 206, Seattle, WA 98117 and MAHAN, Shannon A., U.S. Geological Survey, Denver Federal Center, Box 25046, MS 974, Denver, CO 80225

New age constraints (40Ar/39Ar, burial cosmogenic, U-series, and luminescence dates) on basalt flows and down-stepping flights of fluvial terraces consisting of 5 to 10 individual terrace units provide insights on controls on Colorado River incision in western Colorado measured over timeframes of 104-107 years. The oldest Colorado River deposits are ca. 11 Ma and are present ~1500 m above the modern river. These sediments, along with late Miocene basalt flows at Grand Mesa (near Grand Junction), Battlement Mesa (upstream near Rifle), and Little Grand Mesa (further upstream near Glenwood Canyon), constrain long-term (~107 yrs) bedrock incision rates. Long-term incision rates are fastest (~160 m/Ma) downstream (Grand Junction, Rifle), and are slowest (~100 m/Ma) upstream (Glenwood Canyon, a bedrock knickzone). Similarly, over shorter timescales (~105-106 yrs) rates are fastest (~150 m/Ma) downstream (Grand Junction), and slower (<100 m/Ma) upstream (Rifle and Glenwood Canyon). Exceptions to this pattern include anomalously rapid incision rates (~1200 m/Ma) calculated near Rifle based on ca. 2.1-1.7 Ma cosmogenic burial ages, and 218-277 m/Ma in Glenwood Canyon based on published speleothem ages (Polyak et al., 2013). Over the shortest timescale (~104 to <105 yrs), bedrock incision rates calculated using luminescence ages are 2-4 times (290-640 m/Ma) long-term rates (~160 m/Ma), and the rates increase upstream.

Our present interpretation is that semi-steady, long-term bedrock incision in the upper Colorado Basin largely reflects regional tectonic uplift of the Rockies relative to the Colorado Plateau. Climatically controlled changes in sediment and water discharge, probably related to ca. 100 ky eccentricity cycles, dominate short-term (104 to <105 yrs) rates and patterns of river incision, and are superimposed on regional uplift. Moreover, the observed patterns of bedrock incision support the idea that waves of incision have propagated upstream in response to base-level fall. Whether or not base-level fall and the observed patterns of erosion in the upper Colorado Basin are connected in any way to Grand Canyon integration remains debatable.

Session No. 29

T6. Colorado Plateau Landscape Evolution -- Grand Canyon- and Upper Basin-Focused Colorado River Evolution I
Wednesday, 16 May 2018: 8:20 AM-10:05 AM
Coconino Room (DoubleTree by Hilton)
Geological Society of America Abstracts with Programs. Vol. 50, No. 5
doi: 10.1130/abs/2018RM-313816
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