Geological Society of America

29 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

CONTROLS ON PATTERNS OF UPPER COLORADO RIVER BEDROCK INCISION

Paper number: 29-5
Program: Theme Sessions
Time: 9:25 AM – 9:45 AM

Authors

Andres Aslan Presenting Author
Colorado Mesa University, Grand Junction, CO
Karl E. Karlstrom
University of New Mexico, Albuquerque, NM
Matthew Heizler
New Mexico Bureau of Mines & Mineral Resources, Socorro, NM
Eric Kirby
Oregon State University, Corvallis, OR
Darryl E. Granger
Purdue University, West Lafayette, IN
Paul R. Hanson
University of Nebraska, Lincoln, NE
James K. Feathers
University of Washington, Seattle, WA
Shannon A. Mahan
Geosciences and Environmental Change Science Center, Denver, CO

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 50, No. 5

doi: 10.1130/abs/2018RM-313816

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