Resolving Time-Space Histories of Late Cenozoic Bedrock Incision along the Upper Colorado River, USA

Authors

Andres Aslan, Colorado Mesa UniversityFollow
Karl E. Karlstrom, University of New MexicoFollow
Eric Kirby, Oregon State UniversityFollow
Matthew T. Heizler, New Mexico TechFollow
Darryl E. Granger, Purdue UniversityFollow
James K. Feathers, University of WashingtonFollow
Paul R. Hanson, University of Nebraska-LincolnFollow
Shannon A. Mahan, US Geological SurveyFollow

Document Type

Article

Date of this Version

12-15-2019

Citation

Geomorphology 347 (2019), 106855.

doi: 10.1016/j.geomorph.2019.106855

Comments

This article is a US government work and is not subject to copyright in the United States.

Abstract

The upper Colorado River basin drains the western slope of the Rocky Mountains province of North America and hosts a rich record of fluvial terraces and volcanic deposits that provide an archive of bedrock river incision since the Late Miocene. Here we present new geochronology from fluvial deposits and associated volcanic units using multiple methods (luminescence, cosmogenic-burial, 40Ar/39Ar basalt and detrital sanidine, and U/Pb detrital zircon) to reconstruct a detailed record of bedrock incision in the upper Colorado River over timescales ranging from Miocene to Late Pleistocene. Late Miocene (ca. 11–8 Ma) basalt flows perched 800–1700 m above the Colorado River, and the oldest known ancestral Colorado River deposits preserved beneath basalt flows on Grand Mesa, provide the basis for average, long-term (∼107 yr) incision rates. Rates range from ∼110–160 m/Ma between Grand Junction (downstream) and Glenwood Canyon (upstream), with rates decreasing upvalley. Intermediate-scale (∼105–106 yr) incision rates are ∼200–320 m/m.y. over the past 1–3 Ma, and these rates are faster than the average over the past 8–11 Ma. Short-term (ka) bedrock incision rates are highly variable, and range from ∼250–725 m/m.y. The large variation in rates is interpreted to reflect the inherently unsteady nature of climate-driven incision when measured over time intervals (103–104 yr) that are shorter than the duration of ∼100 ky climate cycles. Finally, episodes of anomalously rapid (∼2000–3700 m/m.y.) short-term bedrock incision appear to be the consequence of stream piracy and drainage reorganization and are confined to specific reaches of the system. Collectively, these data provide a substantially refined picture of the history of incision through space and time that is consistent with headward erosion of the Colorado River into the uplifting flanks of the Rocky Mountains since at least the Late Miocene. We argue that the long-term pace of incision is not completely accounted for by isostatic rebound in response to exhumation, and that ongoing differential uplift driven by mantle buoyancy sets the background rates of fluvial incision (100–150 m/m.y.). Pulses of more rapid incision are a consequence of non-steady incision related to climatically-driven changes in erosion efficiency and/or drainage reorganization.

Highlights:

—The ancestral upper Colorado River existed by ca. 11 Ma.

—Basalt flows support ∼800–1700 m of Neogene fluvial bedrock incision.

—Incision was paced by mantle-driven uplift (∼0.5–1.0 km).

—The Colorado River eroded headward into the uplifting Rockies.

—Climatic episodes and stream-piracy events are superimposed on mantle-driven uplift.