CRevolution 2: Origin and Evolution of the Colorado River System II themed issue New incision rates along the Colorado River system based on cosmogenic burial dating of terraces: Implications for regional controls on Quaternary incision Andrew L. Darling1, Karl E. Karlstrom2, Darryl E. Granger3, Andres Aslan4, Eric Kirby5, William B. Ouimet6, Gregory D. Lazear7, David D. Coblentz8, and Rex D. Cole4 1Arizona State University, School of Earth and Space Exploration, Interdisciplinary Science and Technology Building 4, Room 795, Tempe, Arizona 85287-1404, USA 2University of New Mexico, Earth and Planetary Sciences, Northrop Hall 141, MSC 032040, Albuquerque, New Mexico 87131, USA 3Purdue University, Earth and Atmospheric Sciences, 550 Stadium Mall Drive, West Lafayette, Indiana 47907, USA 4Colorado Mesa University, Department of Physical and Environmental Sciences, 1100 North Avenue, Grand Junction, Colorado 81501, USA 5The Pennsylvania State University, Department of Geosciences, 336 Deike Building, University Park, Pennsylvania 16802, USA 6Department of Geography, 215 Glenbrook Road, U-4148, University of Connecticut, Storrs, Connecticut 06269-4148, USA 720508 Brimstone Road, Cedaredge, Colorado 81413, USA 8Geodynamics Group, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA ABSTRACT the upper Colorado River are 150 m/Ma ences on the river over this time period may over 0.64 and 10 Ma time frames. Higher include regional epeirogeny (Karlstrom et al., New cosmogenic burial and published incision rates, gradient, and discharge along 2008), tectonic offset on faults (Pederson et al., dates of Colorado and Green river terraces the upper Colorado River relative to the 2002a; Karlstrom et al., 2007), salt tectonics are used to infer variable incision rates Green River are consistent with differential (Huntoon, 1988; Kirkham et al., 2002), and per- along the rivers in the past 10 Ma. A knick- rock uplift of the Colorado Rockies relative haps mantle-driven uplift via long-wavelength, point at Lees Ferry separates the lower and to the Colorado Plateau. whole-mantle fl ow (with similarities to the Ara- upper Colorado River basins. We obtained bian case, Daradich et al., 2003; Moucha et al., an isochron cosmogenic burial date of INTRODUCTION 2008; Liu and Gurnis, 2010), or upper mantle 1.5 ± 0.13 Ma on a 190-m-high strath ter- convection (Schmandt and Humphreys, 2010; race near Bullfrog Basin, Utah (upstream The Colorado River system is established van Wijk et al., 2010; Karlstrom et al., 2012; cf. of Lees Ferry). This age yields an average across complex lithology, climate, and uplift King and Ritsema, 2000). incision rate of 126 +12/–10 m/Ma above gradients. What processes have been the most The modern longitudinal profi le of the Colo- the knickpoint and is three times older than signifi cant in forming features such as Grand rado River is shown in Figure 2. Along this pro- a cosmogenic surface age on the same ter- Canyon and the relief of the western Colorado fi le, knickpoints, i.e., convexities in the profi le, race, suggesting that surface dates inferred Rockies (Fig. 1)? Focusing on the main features have several hypothesized origins. In regions by exposure dating may be minimum ages. of this river system in its longitudinal profi le of nonuniform rock type, erosion-resistant Incision rates below Lees Ferry are faster, (Fig. 2), we study the primary geomorphic and substrates may affect long-profi le develop- ~170 m/Ma–230 m/Ma, suggesting upstream tectonic processes that have acted on the river ment; studies show that channel narrowing and knickpoint migration over the past several system over the past fi ve to six million years, increased gradient correlate with harder rocks million years. A terrace at Hite (above which is the likely time for integration of Colo- in the river substrate (Moglen and Bras, 1995; Lees Ferry) yields an isochron burial age rado Plateau drainages through Grand Can- Grams and Schmidt, 1999; Stock and Mont- of 0.29 ± 0.17 Ma, and a rate of ~300–900 yon to the Gulf of California (e.g., Karlstrom gomery, 1999; Duvall et al., 2004; Turowski m/Ma, corroborating incision acceleration et al., 2008; Dorsey, 2010). The upper Colorado et al., 2008). At short timescales, signifi cant in Glen Canyon. Within the upper basin, River, on the other hand, has a history reaching sediment input from debris fl ows in ephem- isochron cosmogenic burial dates of 1.48 ± back to ca. 11 Ma (Larson et al., 1975; Aslan eral tributaries is observed throughout the arid 0.12 Ma on a 60 m terrace near the Green et al., 2008, 2010). Evolution of the river sys- Colorado Plateau, and these also can create River in Desolation Canyon, Utah, and tem since 5–6 Ma has likely involved climati- convex reaches through bed armoring and chan- 1.2 ± 0.3 Ma on a 120 m terrace upstream cally infl uenced variations of discharge and nel fi lling (Schmidt and Rubin, 1995; Grams of Flaming Gorge, Wyoming, give incision sediment fl ux that are often presumed to drive and Schmidt, 1999; Hanks and Webb, 2006). rates of 41± 3 m/Ma and 100 +33/–20 m/Ma, episodic periods of downcutting and aggrada- Regionally, recent debate has focused on the respectively. In contrast, incision rates along tion in the river (Bull, 1991). Tectonic infl u- extent to which steep reaches and the Lees Ferry Geosphere; October 2012; v. 8; no. 5; p. 1020–1041; doi:10.1130/GES00724.1; 14 fi gures; 2 tables. Received 1 April 2011 ♦ Revision received 18 May 2012 ♦ Accepted 22 May 2012 ♦ Published online 18 September 2012 1020 For permission to copy, contact [email protected] © 2012 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/8/5/1020/3341664/1020.pdf by guest on 29 September 2021 Burial dating Colorado River terraces knickpoint refl ect bedrock competence (Mack- magnitudes of Laramide versus mid-Tertiary River Profi les ley and Pederson, 2004; c.f. the Desolation/ and Neogene epeirogenic uplift of the Rockies Gray canyons case, Roberson and Pederson, and Colorado Plateau continue to be debated. At The longitudinal profi les of the Colorado and 2001) and/or transient incision (c.f. Kirby et al., one end member, the modern high-relief land- Green rivers are shown in Figure 2. The predomi- 2007; Karlstrom et al., 2008; Cook et al., 2009; scape developed from a Laramide plateau via nant feature of the longitudinal profi le of the Pelletier, 2010). Discussions of hypotheses later erosional processes (Gregory and Chase, Colorado River is a knickpoint near Lees Ferry regarding knickpoint formation must take into 1994; McQuarrie and Chase, 2000; Huntington that separates a high gradient reach through account unique features of each reach studied to et al., 2010). An alternative uplift model hypoth- Grand Canyon from lower gradient reaches in discern the big-picture importance of the knick- esizes Tertiary epeirogeny that may have coin- Glen Canyon and above (Fig. 2). The Lees Ferry point to the broader river system. cided with the Tertiary ignimbrite “fl are-up” knickpoint divides the upper Colorado River This paper explores the long-term inci- due to magmatism (Roy et al., 2004; Lipman, hydrologic basin from the lower basin and is sion history of the Colorado River system in 2007) and mantle-driven thermal topography the boundary between two distinct portions of order to help evaluate the fi rst-order controls (Eaton, 2008; Roy et al., 2009). At the other the profi le. Additional minor knickzones and on river evolution. This work is part of the end member, evidence for post–10 Ma tilting of convexities exist within Grand Canyon (Hanks Colorado Rockies Experiment and Seismic sediments draped along the Rocky Mountains and Webb, 2006), but these are minor perturba- Transect (CREST) collaborative effort and (Leonard, 2002; McMillan et al., 2002) suggests tions at the regional scale and long time frames is summarized in Karlstrom et al. (2012), in a young component of rock uplift. Probably of interest here. There are also several other which interdisciplinary research efforts com- more realistic models involve several episodes prominent knickpoints in the upper basin. There bine to increase understanding of the Colorado of uplift (e.g., Karlstrom et al., 2012; Liu and is a distinct knickzone through Cataract Canyon, Rockies and the Colorado Plateau. For this Gurnis, 2010). a short distance downstream from the confl u- paper, we fi rst present new estimates of long- ence of the Green and Colorado rivers. Farther term Quaternary incision rates at six key locali- Regional River Systems upstream, the Green River has two large knick- ties along the upper Colorado River and its trib- zones, one in Desolation Canyon and the other utaries. We utilize a relatively new approach to The Colorado River below Lees Ferry (the where the Green River crosses the Uinta Moun- dating fl uvial deposits by cosmogenic burial lower basin) and through Grand Canyon began tains. Upstream of the Green-Colorado confl u- dating isochron analysis (Balco and Rovey, to carry Rocky Mountain water and detritus to ence, the Colorado River has smaller knickpoints 2008). This method, although costly, over- the Gulf of California after 6 Ma (House et al., located in Glenwood Canyon, Gore Canyon, and comes some of the limitations of traditional 2008; Howard and Bohannon, 2000; Karlstrom Black Canyon (Gunnison River), all shown as cosmogenic burial dating (e.g., Granger and et al., 2008; Dorsey, 2010). At this time, a paleo– stars in Figure 1. The profi le depicts a river that Muzikar, 2001) such that it may be applied to Colorado River already existed in the Colorado is not uniformly graded. This is either a result deposits that experienced signifi cant postburial Rockies as shown by ca.