CRevolution 2: Origin and Evolution of the River System II themed issue

New incision rates along the 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, 85287-1404, USA 2University of , 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 separates the lower and to the . 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, (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- 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 (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 . 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 , 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, , 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

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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 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- 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 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 , 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 after 6 Ma (House et al., located in , , and comes some of the limitations of traditional 2008; Howard and Bohannon, 2000; Karlstrom Black Canyon (), 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. 11 Ma river gravels of resistant rock locally steepening slope, or production during either shallow burial and/or near Grand Mesa, Glenwood Canyon, and Gore another perturbation or perturbations that the later exhumation. Our new dates and incision Canyon (Fig. 1; Larson et al., 1975; Kunk et al., river is still adjusting to. Due to the expected rates are then presented in the context of a 2002; Czapla and Aslan, 2009; Aslan et al., high rate of transient knickpoint retreat (Whipple regional synthesis of incision rates through- 2010; Cole, 2011). Little physical evidence for and Tucker, 1999; Berlin and Anderson, 2007), it out the Colorado River system. Comparison of where the Colorado River system fl owed has is likely that any transient features are relatively incision rates with the shape of the longitudinal been documented from the time period of ca. young. Thus, many potential causes of the knick- profi le reveals information about the convolved 11 Ma to ca. 6 Ma. However, erosion since ca. points of the Colorado River are recent pertur- effects of regional uplift, climate change, and 10 Ma has been dramatic (>1.5 km in places) bations (105–106 years). We attempt to test the drainage reorganization that, if resolved, can as the Colorado River and its tributaries began youth of these perturbations by discussing pat- help elucidate the still-controversial uplift and to carve deep canyons (e.g., Aslan et al., 2010). terns of incision rates revealed by a compilation denudation history of the western U.S. (e.g., In contrast, the Green River is a younger of regional data, supplemented by new estimates Pederson et al., 2002b; McMillan et al., 2006; system. Basin filling continued within por- that exploit cosmogenic burial dating. Moucha et al., 2008; Huntington et al., 2010; tions of the upper Green River watershed until Liu and Gurnis, 2010). at least 8 Ma as shown by Miocene deposits of METHODS the Browns Park Formation in Browns Park, GEOLOGIC BACKGROUND Colorado. These deposits are mostly older than Cosmogenic Burial Dating 8.25 Ma (Luft, 1985) and provide a maximum Tectonic Setting limit on the age of the Green River between The objective of this project was to identify Flaming Gorge and the Gates of Lodore. Neo- old, high terraces with thick gravel deposits suit- The modern landscape of the Colorado Pla- gene subsidence and graben collapse played a able for cosmogenic burial dating. Such sites are teau and Rocky Mountains is the result of ero- key role in the early development of the Green scarce in the erosional landscape of the region; sion and fl uvial incision acting on a region with River (Izett, 1975; Hansen, 1986). Sometime however, we report six new terrace dates using a protracted uplift history. Deformation during after ~8 million years ago, the Green River cosmogenic nuclide burial ages. Five of these Laramide time (70–45 Ma) resulted in local began eroding the low-relief region north of the ages from fi ve different locations are deter- uplifts of Precambrian basement juxtaposed Uinta Mountains as a result of drainage integra- mined using the relatively new method of iso- with deep basins with structural relief greater tion events that diverted surface waters south chron burial dating (Balco and Rovey, 2008), than 10 km (MacLachlan et al., 1972; Dickinson toward and eventually across the Uinta Moun- which requires only a few meters of burial. The et al., 1988). Paleoelevations at the end of the tains, beyond which they join the Colorado River sixth date uses simple burial dating of amalgam- Laramide are not well known, and the relative system (Hansen 1986; Munroe et al., 2005). ated clasts, which requires a much deeper burial

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Green River Basin UT Rock Springs Airport

108° W WY N Creston Junction Peru Bench

Flaming Gorge Brown’s Park Canyon of Lodore Uinta Mountains CO

Gore Canyon Tabyago Canyon Morrisania Figure 1. Map of rivers and Mesa Glenwood Canyon locations throughout the Colo- Westwater rado Plateau, including new Desolation Canyon Rifle sample locations, marked with Canyon Battlement Mesa black squares. Stars are knick- 39° N 39° N points in the longitudinal pro- fi le. Inset shows context of the Gunnison River

Colorado River major Colorado River tributar- River Green Bostwick Park Uncompahgre ies. Three-second digital eleva- River tion model generated by Chalk Butte, Inc., 1995. Black Canyon Trachyte Cataract San Juan Mountains Creek Canyon Hite Bullfrog N

Bluff Glen Canyon San Juan River

AZ Map area Grand Lees Ferry Canyon

250 km 108° W

(>10 m). Burial dates are calculated from the (6.28 half-lives for 26Al), corroborated by dated erator mass spectrometry (AMS) at the time of differential decay of cosmogenic 26Al and 10Be overlying basalt, have been reported on the measurement; (2) rapid, deep (~5–10 m) sample in quartz (e.g., Granger, 2006). Cosmogenic Colorado River System (Matmon et al., 2011). burial for adequate shielding from postdeposi- nuclides (such as 10Be and 26Al) are produced Many other sites have been successfully burial tion nuclide production; (3) a sample within when secondary cosmic ray particles interact dated in this range (e.g., Stock et al., 2004), and the age range that provides measurable quanti- with target nuclei in minerals. Secondary cos- ages from 0.5 to 3 Ma are routinely reported ties of 26Al and 10Be (i.e., maximum ca. 5 Ma); mic ray neutrons penetrate only a few meters (e.g., Granger et al., 2001; Haeuselmann et al., (4) samples that were not previously buried beneath the ground surface, while less inter- 2007; Craddock et al., 2010, 2011). Thus, burial within the past 10 million years or so, and (5) a active muons continue to be important at depths dating can provide age control in a time frame stable environment to ensure continued shield- to tens of meters. from 1 to 5 Ma and in deposits otherwise devoid ing until excavation. Preferred sample sites Cosmogenic burial dating (Granger and of datable material such as volcanic ash. include gravel deposited in caves (Anthony and Muzikar, 2001) relies on the different decay Dating deposition of river gravel by cosmo- Granger, 2007; Granger et al., 2001), quarries 26 rates of Al (t1/2 = 0.717 Ma; Granger, 2006) genic burial dating requires: (1) sufficient in alluvium (Wolkowinsky and Granger, 2004), 10 and Be (t1/2 = 1.387 Ma) (Chmeleff et al., 2010; nuclide production before burial to ensure con- and landslide and/or fl uvially eroded scarps of Korschinek et al., 2010). Dates as old as 4.5 Ma centrations are above the detection limit of accel- very recent exposure (this study) where depth

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3500

3000 San Juan River Green River Green Little Colorado Gore Canyon Gore Gunnison River 2500 Canyon Glenwood Virgin

2000 Green River

1500 Uinta Elevation (m) Mountains International Bounday Bill Williams River Williams Bill

1000 Ferry Lees Glen Canyon

500 White River Price River Price Grand Canyon Colorado River Green River, UT River, Green Canyon Cataract Desolation Canyon Flaming Gorge Flaming 0 of Lodore Canyon 0 500 1000 1500 2000 2500 3000

Distance upstream from Gulf of California (km)

Figure 2. Longitudinal profi le of the Colorado and Green rivers as determined from elevation data from U.S. Geological Survey 1:24,000 topographic maps, with distances measured along main channel.

of shielding exceeds ~4 m for isochrons (see the intercept of the line. The fact that postburial important to realize that the York (1966) method below) and 10 m for simple burial dates. Field production can be accounted for in the isochron underestimates uncertainty, if the deviation of parameters relevant to the cosmogenic shielding technique allows for a critical advantage in many data about the regression is small. We calculate for our samples are outlined in Table 1. geologic settings. The dating of samples with as uncertainty according to measurement data, if Two analytical techniques for determining little as 3–4 m (Table 1) of vertical shielding is the mean square of weighted deviates (MSWD) burial ages via cosmogenic nuclide concentra- now possible, although very recent exposure is about the line is less than one. tions were implemented in this study. First, still required. However, in some ways isochron Burial dating can be contrasted to cosmo- simple burial ages of deeply buried samples burial dating is not as simple as isochron dating genic surface exposure dating, which measures were analyzed via AMS as an amalgamation of in other radiometric dating methods. The initial the buildup of cosmogenic nuclides in rocks that several clasts crushed and processed together as 26Al/10Be ratio at the time of burial is dependent are exposed at the surface. In geologically active described by Granger and Muzikar (2001). How- on average erosion rates in the drainage basin. landscapes such as the western U.S., cosmo- ever, many of the deposits in this study consist of Thus, a graph of 26Al versus 10Be is not perfectly genic surface exposure dating tends to yield thin (<10 m) fl uvial sand and gravel atop bedrock linear, and the data must be linearized prior to minimum exposure ages. While exposure dating straths, and may have been subject to signifi cant regression of the slope. This is done using an can be used to date river terraces, the method postdepositional production of cosmogenic 26Al iterative process. First, any postburial production assumes that there has been zero erosion, and and 10Be that complicates simple burial dating. is estimated from the intercept of the regression, that the terrace has never been covered by addi- Thus, we also used the isochron technique that and this value is subtracted from measured 10Be tional sediment, such as eolian sands. Surface involves AMS analyses of several clasts individu- concentrations. Then, an initial age estimate from erosion and burial both effectively reset expo- ally and sampled from a single depth. In the ideal the regression is used to decay-correct 10Be con- sure ages. Thus, exposure ages provide a mini- case, decay after burial from a range of initial centrations to their pre-burial values. The initial mum age for the terrace, and a maximum river cosmogenic nuclide concen trations leads to sys- 10Be values are then used to estimate the 26Al/10Be incision rate, especially for terraces older than tematic changes in concentra tion, such that one ratios at the time of burial, and the 10Be values ca. 100 ka (Wolkowinsky and Granger, 2004). can evaluate the age of burial from an isochron are adjusted to account for lowered 26Al/10Be plot (Balco and Rovey, 2008), similar to those ratios that occur with slow erosion rates. The age Incision Rate Calculation and Compilation used in traditional geochronology (cf. Dickin, is recalculated, and the process repeated until 2004). In isochron burial dating, 26Al is plotted convergence. For details, see Balco and Rovey The rate of erosion of the bedrock channel is against 10Be. The burial age can be calculated (2008). We use the method of York (1966) to cal- the net result of a river cyclically aggrading and from the slope of a line regressed through the data culate the regression of the isochrons and deter- then incising through its alluvial cover, conse- and postburial production can be estimated from mine the uncertainty in the burial age, but it is quently eroding the bed and aggrading again.

Geosphere, October 2012 1023

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/8/5/1020/3341664/1020.pdf by guest on 29 September 2021 Darling et al. ) continued ( ows and ows Field notes river gravel: clastsriver gravel: ows, but no gravel Central Rockies. down by landsliding. Rate nonslumped is for higher, gravel, but evaporite collapsed derived from exposed basement rocks from the near the bottom of basalt, rather than the top as reported in the abstract. No river gravel has been Lava Creek B ash; one appears to have dropped block with ash. reported here. Valley evaporite collapseValley region. Elevations reported here are Site has two locations of This sample is within Eagle notes rock collected fromrock Geochronology complex. reworked fall deposits data. reworked fall deposits presumed to closely postdate fall. eroding surface. Height reported is probably the tread of the alluvial fan reworked fall deposits presumed to closely postdate fall. original result from one sample was reported in Aslan et al., 2007, GSA abstract. presumed to closely presumed to closely reworked fall deposits postdate fall. postdate fall. reworked fall deposits presumed to closely postdate fall. Not sure of source of Ar/ArNot sure of source 5 1 2 . 8 0 1 – 5 4 0 . 9 38.683 –106.897 Basalt on on 38.683 –106.897 Basalt 39.509 –107.786 Single 38.858 –108.480 dates, burial two of Mean 3 t l a s a B Sample cobbles material Latitude Longitude cuttings 5 3 9 2 4 5 5 (m) Elevation 1 Strath height 884 2652 Basalt 39.575 –107.154 fl Basalt 1640 3200 Basalt 36.386 –107.868 730 2500 Basalt 39.542 –107.261 fl basalt Intact 90 65 Ash 457 40.066 1884 –108.014 Ash age given; deposits are Ash Drill 40.538 –107.413 Ash age given; deposits are 85 1950 Ash 39.646 –107.058 are deposits given; age Ash 55 Ash 39.395 –107.236 are deposits given; age Ash 64 2100 Ash 39.654 –106.791 are deposits given; age Ash (m) depth Burial Error (2 sigma) (2 sigma) (2 sigma) (2 sigma) (2 sigma) (2 sigma) (1 sigma) (2 sigma) (2 sigma) (2 sigma) (std. dev., (std. dev., if reported) age (Ma) Measured rate Min. rate Max. 64 64 64 10 Not reported 640 3078 Basalt 75 75 75 9.72 0.05 86 86 86 0.639 .002 rate 114 115 113 7.74 0.06 179 180 178 9.16 0.06 133 133 133 0.639 .002 128 214 100 1.77 0.71/0.51 1 227 1860 Quartz-rich 144 148 141 10.76 0.24 102 102 101564 0.639 802 435 .002 0.81 0.24 141 141 140 0.639 .002 Incision location Canyon, Colorado Colorado Colorado Colorado Colorado Colorado Colorado Colorado Mountain, Mountain, Geographic Carbondale, Mesa, Colorado Mesa, Colorado Mesa, Colorado TABLE 1. INCISION RATES COMPILED THROUGHOUT THE COLORADO RIVER SYSTEM ON THE COLORADO PLATEAU AND COLORADO ROCKIES COMPILED THROUGHOUT THE COLORADO RIVER SYSTEM ON PLATEAU 1. INCISION RATES TABLE Fork river Nearest 2 Colorado Mesa, Grass 1 Colorado Unaweep Ar 2 Colorado Battlement Ar 3 Colorado Lookout Ar 2 Colorado Little Grand Ar 1 Colorado Grand Ar 2 Gunnison Flat Top 39 39 39 39 39 K/ K/ K/ Ar/ Ar/ burial Burial Dating 40 40 40 method Rating 40 40 cosmogenic Cosmogenic Tephrochronology 1 Colorado Dotsero, Tephrochronology 1 Yampa Creek, Elk Tephrochronology 1 Eagle Eagle, Colorado 100 100 100 0.639 .002 Tephrochronology 1 White Meeker, Tephrochronology 3 Roaring 2007; Kunk et al 2002 2008 2007; Kunk et al., 2002 2008 2007; Kunk et al., 2002 2008 al., 2010 abstract Kirkham, 2007 2007 2008 2008 2008 Brown et al., Berlin et al., Data source Berlin et al., Aslan et al., Aslan et al., Brown et al., Aslan et al., Aslan et Brown et al., Aslan and Aslan et al., Aslan et al.,

1024 Geosphere, October 2012

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 ) ow basalt ow continued ( Field notes g. location is location g. ne-grained sediment ne-grained below locally derived gravel. This package is in fi on top of Uncompahgre gravel close to River-like the modern Gunnison River. cutbank in an ephemeral approximate. gravel mainly volcaniclastics from West quartzite, granite, and schist clasts likely from Central Rockies. place with channels cut in it by Kirkham et al., 2002 approximate. approximate. Sample location is a channel. approximate. Elk Mountains. Sparse boulders; reported as in approximate. approximate. Weakly preservedWeakly ashfall Lat./long. location is Lat./long. location is Abandoned meander. Basalt on river gravel: river on Basalt Lat./long. location is Probably debris-fl Lat./long. location is Lat./long. location is /s water level. /s water level. ) 3 3 notes Geochronology /s water level. 3 report those closest to all clasts share a similar reworked fall deposits presumed to closely postdate fall. report those closest to the reported; we chose to the strath in terrace and report the rate to 380 reported; we chose to strath and report the rate to 380 m report those closest to the to 380 m burial history. m reported; we chose to strath and report the rate to Hite (Darling, this volume) exaggerates incision rate uncertainty. continued Similar in height and age and height in Similar 29 –107.658 37.960 –110.59237.960 Lat./lon –110.592 37.960 –110.592 terrace low Very gravel gravel gravel Sample material Latitude Longitude (m) Elevation Strath height 96 1571 Ash 38.724 –108.178 are deposits given; age Ash 62 110Quartz-rich Quartz-rich 837 2605 Basalt 39.604 –107.109 7Quartz-rich 4 60 1475 Cobbles 39.768 –109.905 isochron; datapoint Four (m) depth Burial ER SYSTEM ON THE COLORADO PLATEAU AND COLORADO ROCKIES ( ER SYSTEM ON THE COLORADO PLATEAU Error (1 sigma) (2 sigma) (1 sigma) (1 sigma) (2 sigma) (1 sigma) (std. dev., (std. dev., if reported) age (Ma) Measured rate Min. rate Max. 41 44 38 1.48 0.12 rate 150 151 150 0.639 .002 221 221 221 0.077 ? 17 985 Sand 36.853 –111.610 points chronology Multiple 107 108 107 7.8 0.04 150 150 150 0.12 ? 18 986 Sand 36.853 –111.610 points chronology Multiple 289 289 289 0.142 ? 41 1009 Sand 36.853 –111.610 points chronology Multiple 348 414412 301 448 0.178 381 0.267 .0283 .0214 538 1522 327 0.013 0.0084 171 171 171 10.381775Basalt39.0 Incision Utah Utah Utah Utah Ferry Ferry Ferry, Ferry, Arizona location Canyon, Colorado Colorado Colorado Geographic Mount Darline, river Nearest of Gunnison TABLE 1. INCISION RATES COMPILED THROUGHOUT THE COLORADO RIV 1. INCISION RATES TABLE 1 Green Tabyago 1 Colorado Lees 1 Colorado Lees 2 Trachyte2 Creek, Trachyte Trachyte Creek, Trachyte 2 Trachyte Creek, Trachyte 1 Colorado Lee’s le le le Ar 1 Fork N. Ar 3 Colorado Spruce Ridge, 39 39 K/ Ar/ burial Dating 40 method Rating 40 Isochron Be cosmogenic Be cosmogenic Be cosmogenic cosmogenic 10 10 10 luminescence luminescence exposure profi exposure profi exposure profi Al/ Al/ Al/ Tephrochronology 1 Gunnison Sawmill Mesa, luminescence (OSL) 26 26 26 2009 this volume 2009 2007; Kunk et al., 2002 USGS Open-File Report 2009 2009 Cook et al., Cragun, 2007 Optically stimulated Darling et al., Cragun, 2007 Optically stimulated Darling et al., Data source Cole, 2011 Cragun, 2007 Optically stimulated Brown et al., Cook et al., Cook et al.,

Geosphere, October 2012 1025

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/8/5/1020/3341664/1020.pdf by guest on 29 September 2021 Darling et al. ) continued ( Field notes uence of Dirty samples are from the from the northerly terraces. Cosmogenic bottom of a gravel pit. estimate as strath appears to have relief, higher elevation. Elevation of river bottom taken from 1921 survey. confl Devil and Colorado River. Sample taken one of two road cuts. reaching ~10–15 m was not reported. deposited under local gravel that includes Lava Creek B ash. exact sample location One terrace in a suite of Strath elevation a minimum Terrace near the Terrace 10-m-deep river gravel Location approximate; nd this ash. ) notes Geochronology poor precision. Combining this with other bore-hole samples would be very helpful. clasts creating a slope near production rate ratio. Indicates little deposition. date, reasonable linearity. yields fresh exposure for sampling. Isochron shows good linearity with six clasts. Region potentially local streams reworking Canaan Peak gravels as well. decay has occurred since contains relatively vast deposits of river gravel, Cosmo date is younger than Mesa Falls by at least 200 ka at 1 sigma. Dickenson to include by Aslan ca. 2007 Search failed to fi on the same terrace as date is 1/3 as old. Tread reported as 204 m above river. reported; oldest reported here. Minimum age. reported; oldest reported reported; oldest reported here. Minimum age. Mesa Falls ash (1.2 Ma). our Bullfrog date; surface here. Minimum age. continued 37.520 –110.700 scarp wasted Mass 39.467 –107.989 Nicely shielded; however 37.917 –110.398 Isochron date with several 41.587 –109.580 isochron Four-datapoint 38.494 –107.729 1960s by Reported 37.542 –110.712 surface Cosmogenic date surface Sample boulder cobbles cobbles cobbles cobbles material Latitude Longitude cuttings (m) Elevation Strath height 120 1865 Quartz-rich 189from Quartz 170ages 1160surface Boulders 37.120of –110.960 Array 60 260ages 1251 surface Boulders Boulders 37.120of 37.070 –110.960 –110.940 Array Array of surface ages 7 189 1207 Quartz-rich 5 107 1167 Quartz-rich 10+ 335 2073 Quartz-rich 110 94 1655 Drill (m) depth Burial SYSTEM ON THE COLORADO PLATEAU AND COLORADO ROCKIES ( SYSTEM ON THE COLORADO PLATEAU Error (1 sigma) (1 sigma) (1 sigma) (1 sigma) (1 sigma) (1 sigma) (1 sigma) (1 sigma) (1 sigma) (std. dev., (std. dev., if reported) age (Ma) Measured rate Min. rate Max. rate 369 892 233 0.29 0.17 100 133 80 1.20 0.3 126 138 116 1.50 0.13 214 671 127 0.44 0.3 639 825 521 0.266 0.06 492 632459 403 591 0.122 375 0.027 0.567 0.127 395 405 385 0.479 0.012 385 515 307 0.87 0.22 Incision Hite, Utah Utah Utah Utah Utah Utah Island, Bench, location surface, Colorado Wyoming Geographic Mesa, Colorado river Devil Nearest TABLE 1. INCISION RATES COMPILED THROUGHOUT THE COLORADO RIVER 1. INCISION RATES TABLE 1 Dirty 1 Green Peru 1 Colorado Bullfrog, 1 Gunnison Park, Bostwick burial burial burial burial Dating method Rating Isochron Isochron Isochron Isochron Burial date 3 Colorado Morrisania Be exposure 2Be exposure Colorado Bridge Canyon, 2Be exposure Colorado 2 “4103” Colorado Oak cosmogenic cosmogenic cosmogenic cosmogenic 10 10 10 Cosmogenic surface 3 Colorado Bullfrog, this volume this volume this volume this volume this volume 2005 2001 2005 2005 Darling et al., Darling et al., Darling et al., Darling et al., Data source Davis et al., Garvin et al., Darling et al., Garvin et al., Garvin et al.,

1026 Geosphere, October 2012

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 ; ) a e s t r l e A a o s d s f s t o e a n h e t continued r d ( e i o s d l t n i a u Field notes c o W l o y s l v p b o d a l T b e t i eld check. Rate is 67 m/ o Utah State thesis. 10 km downstream of . Details in Counts, 2005, no river gravel. Slow rate is a function of the ~6–10 Ma. from publication, changed by Aslan during fi Ma in Izett and Wilcox, 1982. old age, prior to incision collapse; no gravel not have any known relationship to river gravel. a r h l P F S le. Deposit is ) notes Geochronology are reworked fall deposits presumed to closely postdate fall. material in sediment, toward the river, the top toward the river, of the canyon walls at is heavily 4100 ft. Terrace eroded. deposits presumed to closely postdate fall. only allows the assumption of an age bracket. techniques on a shallow depth profi bracketed by 98 ka and 77 ka OSL dates (see Cragun, 2007). are reworked fall Formation continued 9 4 7 2 9 6 6 9 3 0 4 9 . . . . 7 7 7 6 0 0 0 0 1 1 1 1 – – – – 3 3 5 9 7 2 1 0 4 8 4 4 . . . . 9 9 9 9 37.123 –110.876 The Cha surface projected 3 3 38.548 –110.130 Magnetically reversed 36.853 –111.610 Dated using Monte Carlo 3 3 t t t t l l l l a a a a s s s s a a a a B B B B surface Sample cobbles material Latitude Longitude carbonate 3 0 6 0 8 0 2 7 2 9 9 8 3 2 2 2 9 2 8 2 6 9 5 0 3 1 1 1 (m) Elevation 1 1 1 1 Strath height 61 2354deposits given; Ash 38.448age –107.300 Ash 250 1400 Boulder 34 2234 Ash 42.896 1.000 #2 Elevation of strath different 79 2103 Ash 39.928 –106.715 Ash age given; deposits (m) depth Burial ER SYSTEM ON THE COLORADO PLATEAU AND COLORADO ROCKIES ( ER SYSTEM ON THE COLORADO PLATEAU Error (2 sigma) (1 sigma) (2 sigma) (2 sigma) (2 sigma) (2 sigma) (2 sigma) (2 sigma) (std. dev., (std. dev., if reported) age (Ma) Measured rate Min. rate Max. 6 7 6 8.25 (?) 0.7 52 1683 Ash 40.827 –108.906 Ash in Browns Park 88 88 87 15.6 0.09 95 96 95 0.639 0.002 14 15 14 9.9 0.4 (?) 140 1768 Ash 40.727 –108.761 Ash in Browns Park Little Hole Day Use Area 53 53 53 0.639 0.002 rate 500 625 417 0.5 0.1 115 116 114 10.38 0.12 105 106 104 10.49 0.07 114 117 111 10.14 0.26 250 571 160 1.6 0.9 400 1620 Pedogenic 124 124 123 0.639 0.002 203 262 173 0.083917985 Quartz-rich Incision Utah Utah Utah Baldy Ferry, Ferry, Knoll, Gulch, Navajo Arizona Arizona location Colorado Colorado Colorado Colorado Colorado Wyoming Mountain, Mountain, Mountain, Geographic Fork, Colorado Loaf Mountain, river Juan Nearest TABLE 1. INCISION RATES COMPILED THROUGHOUT THE COLORADO RIV 1. INCISION RATES TABLE 2 San 1 Colorado Lees 3 Green Park, Brown’s 3 Green Goodman Ar 2Ar Colorado Basalt 2Ar Colorado 1 Little Ar Colorado Sugar 1 Colorado Sunlight Peak, le 39 39 39 39 Ar/ Ar/ Ar/ Ar/ Zircon Zircon profi Dating method Rating Surface 40 40 40 40 exposure Be cosmogenic ssion track ssion track fi fi 10 Paleomagnetism 3 Green Keg Al/ Tephrochronology 1 Colorado McCoy, Tephrochronology 1 Gunnison Lake Tephrochronology 1 Green of Pinedale, W. 26 1985 2002 2008 Naeser et al., 1980 2010 Wilcox, 1982 2002 1975; Aslan et al., 2007 2002 2002 Harden et al., Kunk et al., Kunk et al., Data source Aslan et al., Izett, 1975; Hidy et al., Izett and Luft, 1985 Zircon Kunk et al., Kunk et al., Larson et al.,

Geosphere, October 2012 1027

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Therefore, the rates inferred from dates of

s s bedrock strath formation proxies yield an aver- i i n n

o o age rate of change of the bedrock, which fi l- i i t t a a

c c ters short-term climate oscillations and can be Field o o notes l l . .

g g associated with rock uplift in steady-state ero- n n o o l l / /

. . sion conditions (Whipple and Tucker, 1999). t t in Westwater Creek, in Westwater deposit. Erosional on a hill in pediment surface is extrapolated toward the river. approximate. approximate. gravel deposit. a a Ash deposit is upstream L L Preserved in large outwash Bedrock incision rates can be calculated from a single dated strath, if depth to modern bedrock can be estimated (e.g., Burbank et al., 1996; Pederson et al., 2002a; Karlstrom et al., 2007). le.

) If multiple datable strath terraces are present, notes a preferred method is to calculate variation Geochronology in incision rates through time using strath-to- closely postdate fall. closely postdate fall. are reworked fall deposits presumed to Not a true incision rate although shows that landscape has not denuded much. are reworked fall deposits presumed to closely postdate fall. et al., 2005. in Marchetti burial profi in Marchetti et al., 2005. in Marchetti continued strath comparisons (Pederson et al., 2006; Karl- 5 5 7 8 8 8 strom et al., 2007). Because of the diffi culty in 9 9 7 . . . 0 0 7 obtaining age control on preserved deposits 1 1 0 1 1 1 – – – that doesn’t violate any critical assumptions, 8 8 4 most published incision rates rely on a single 9 4 4 3 3 5 . . . 8 8 8 dated deposit. In addition, because average 3 3 38.555 –111.591 Detailed chronology notes 3 37.293 –109.545 Gravel quarry dated with a river depth and (especially) depth to bedrock

h are rarely known, strath heights are commonly s A

gravel gravel reported relative to the modern river (usually surface Sample cobbles material Latitude Longitude the water level shown on U.S. Geological Sur- 0

0 vey [USGS] maps). The Bureau of Reclama- 0 2 tion has used mid-channel drilling to assess dam-site feasibility. The depth to bedrock pro- 5 2 3

(m) Elevation vided by these data is valuable for incision rate Strath height calculations where available; however, drill- 105 1426 Ash 39.226 –109.184 Ash age given; deposits 20Quartz-rich 130 Quartz-rich 150 1450 Quartz-rich 155 2565 Boulder 65 1685 Ash 40.913 –109.305 Ash age given; deposits 190 Ashnotes 37.217chronology –107.847 Detailed ed 1 through 3, being most reliable based on analytical uncertainty and in geologic context. ing data are very limited on the Colorado and (m) depth Burial tributaries, precluding reach-to-reach compari- sons of bedrock depth. Drilling data combined

Error with sonar sounding studies (which measures (2 sigma) (2 sigma) (2 sigma) (1 sigma) (1 sigma) (1 sigma) (1 sigma) (2 sigma) (std. dev., (std. dev., if reported) only water depth) suggest that bedrock is com- monly on the order of ~10 m below the river

age surface, but can be 30 m or more (e.g., Miser, (Ma)

Measured 1924; Woolley, 1930; Hanks and Webb, 2006; Karlstrom et al., 2007). For the purposes of rate Min. this paper, however, and comparison with pub- lished incision rate data, Table 1 reports terrace rate Max. height as calculated from the difference in ele- vation between a strath and the water surface of 4 4 4 11.8 0.4 49 1708ParkBrowns Ash 40.912in –109.148 Ash 27 30 25 8.6 0.8 (?) 232 1982 Ash 40.641 –108.537 Ash in Browns Park. rate 164 165 164 0.639 0.002 509 510 507 0.639 0.002 333 392861 290 1024 0.06 743 0.151 0.009 0.024 258 310 221 0.6 0.1 297 298 296 0.639 0.002 102 102 101 0.639 0.002 110 124 96 1.36the 0.15/0.2 nearest river. Hence, reported incision rates Incision are underestimates of bedrock incision rates, but at the long timescales of most of our ages en geographic location. The qualitative assessment of reliability rank Utah Utah Utah Utah Utah Utah Utah (1–3 Ma), this is only a small (~10%) under- Park, Bluff, Draw, Draw, Creek, Boone location Plateau, Plateau, Canyon, Canyon, Colorado Colorado Wyoming

Geographic estimate. Ewing Canyon, The compilation in Table 1 reports incision rates as a range based on the maximum and river Juan

Nearest minimum date reported for each date where

TABLE 1. INCISION RATES COMPILED THROUGHOUT THE COLORADO RIVER SYSTEM ON THE COLORADO PLATEAU AND COLORADO ROCKIES ( COMPILED THROUGHOUT THE COLORADO RIVER SYSTEM ON PLATEAU 1. INCISION RATES TABLE available. Geologic uncertainty such as the 1San 1San 2 Fremont2 Caineville Fremont Caineville 3 Little Snake East 2 Fremont Ivy amount of time between strath development and deposition of overlying datable sediment is

le le often unknown. To address this source of error,

Ar 3 Green Jesse we apply a relative quality rating (“1”–“3”) for 39 K/ burial

Dating incision rate data, where “1” is most reliable. 40 method Rating Be cosmogenic Be cosmogenic ssion track fi Cosmogenic 10 10

He cosmogenic The quality rating is based on the following cri- exposure profi exposure profi 3 surface exposure Al/ Al/ Tephrochronology 1 Gunnison Redrock Tephrochronology 1 Animas Durango, Tephrochronology 1 Colorado Westwater Tephrochronology 2 Green Browns

26 26 teria: methods that date material directly asso- ciated with the fl uvial system (e.g., analytically

We sort the data by author and then nearest river th We precise burial dates, interfi ngered lava fl ows,

Note: and fl uvial gravel) are reported as reliable (“1”). Damon, 1970 al., 2005 Biek, 2001; Aslan et al., 1997 2007 2005 and Granger, 2004 1991 1997 2008 Winkler, 1970; Winkler, Marchetti et Marchetti Willis and Data source Wolkowinsky Wolkowinsky Repka et al., Sandoval, Luft, 1985 Zircon Munroe et al., Patton et al., Repka et al.,

1028 Geosphere, October 2012

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Certain rates are analytically or geologically each terrace. Additional samples would likely Paleocene Canaan Peak Formation (T.C. Hanks, uncertain and rated “2” or “3” depending on improve the dating, and can be done in future 2011, personal commun.) to the north and degree of perceived or quantifi ed uncertainty. work. At present, we interpret the results as the brought down along the paleo–Bullfrog Creek. Chronology points from the literature (such as best available ages on the terraces and attempt The river and streams in this area shared a basalt dates) that do not have direct fi eld rela- to place the new ages in the context of incision common base level, meaning that if the main tionships to river gravel are less reliable and are rates obtained by other methods. stem changed incision rate, the tributaries not necessarily reported in this compilation. We As an empirical test of this method, one of would respond directly. Therefore, data from report some rates as “apparent incision rates” our samples was taken from Bostwick Park, the clasts yield information on Colorado River in central Colorado because of numerous loca- Colorado, where fl uvial gravels contain a incision regardless of clast source area. The tions where basalt fl ows and gravel deposits are deposit of Lava Creek B ash (Figs. 2 and 5). nuclide inventories in the gravel are dependent offset by normal faults activated by salt dissolu- More detailed discussion of the geology at on the duration of burial of the clasts and the tion and/or deformation (Kirkham et al., 2002). Bostwick Park is in Sandoval (2007) and San- paleoerosion rate, both of which can be solved Locally faulted incision rates are considered doval et al. (2011) with an overview in Aslan for using the 26Al/10Be nuclide pairs, if the low (“3”) quality rates for the purpose of under- et al. (2008). At this site, ~10 m of channel nuclides are detectable. Thus, dating the deposit standing regional bedrock incision patterns (the gravel was deposited by a paleotributary to the at Bullfrog provides a constraint on the history focus of this study); however, the dated deposits Gunnison River within a confi ned valley. The and timing of incision along the main-stem can yield fault-slip rates calculated from appar- paleotributary was captured and the channel Colorado River. ent incision rates (Pederson et al., 2002a; Karl- abandoned such that the river gravel is over- The Bullfrog terrace has a strath ~190 m strom et al., 2007). lain by tens of meters of locally derived gravel above the pre– river eleva- and sand that, near their base, contain layers of tion (Birdseye et al., 1922) and a tread ~204 m RESULTS reworked Lava Creek B ash (639 ± 2 ka; Lan- above the river. Gravel exposed at the base of phere et al., 2002). Approximately 10 m strati- one landslide scarp (suggesting very recent graphically below Lava Creek B ash (exposed exposure) was sampled (depth of ~7 m; Fig. 7) Incision rate estimates presented here are in a gravel pit, Fig. 5), several quartzite clasts for burial dating and analyzed using the iso- organized within regional context. Incision data were collected and analyzed using the cosmo- chron technique. Six cobbles of quartzite were reported and compiled in this paper are plot- genic isochron method for burial dating. The collected, and each cobble was analyzed sepa- ted for both short-term (<1 Ma) and long-term isochron estimated age for deposition of the rately. The sampling locality was estimated to (>1 Ma) time frames (Figs. 3 and 4, respec- gravel is 870 ± 220 ka. The slope of the line for be within a few meters (<3 m) of the bedrock tively). Rates determined from dates that are this isochron is controlled by the 26Al/10Be con- strath, which was not exposed. Five points less than ca. 200 ka may reveal complex pat- centrations from one clast, while the other data yielded 26Al/10Be ratios with errors less than terns due to glacial oscillations that alter inci- are clustered (Fig. 6). Geologic relationships 10% and produced an isochron cosmogenic sion rate (Hancock and Anderson, 2002; Pan suggest the basal gravels must predate 0.64 Ma burial date of 1.5 ± 0.13 Ma (Fig. 6). The sixth et al., 2003); hence, we concentrate on longer- by an unknown duration such that the 870 ± sample did not yield 26Al data and was therefore term bedrock incision. 220 ka burial age is a reasonable, albeit impre- not included in the analysis. All fi ve samples lie cise, estimate for the basal Bostwick gravel and within error of the regressed line and therefore Uncertainties and Empirical Evaluation of hence a positive empirical test for the isochron have a shared burial history. There is a small the Isochron Method technique. but statistically signifi cant amount of postburial production, indicated by the intercept. The One of the disadvantages of cosmogenic Glen Canyon Burial Dates resulting incision rate is 126 +12/–10 m/Ma burial dates is their large analytical uncertain- (Table 1). The terrace tread (204 m above the ties. In addition, there are also uncertainties Two samples were taken from upstream of river) was previously dated with a cosmogenic about the geologic history of the gravels. Many Lees Ferry at Bullfrog Bay and Hite Cross- surface date of 479 ± 12 ka (Davis et al., 2001; of the isochrons presented below are strongly ing, ~50 km apart, in Glen Canyon (Fig. 1). Table 1), yielding apparent incision rates higher leveraged by a single point that happened to These sites were analyzed with the isochron by a factor of three. The surface data may differ have relatively high 10Be and 26Al concentra- technique due to relatively shallow burial (7 m because of two possibilities. Either the deposit tions. There is nothing inherently suspect about and 5 m, respectively; Table 1). At Bullfrog, represents ~1 Ma of stability or aggradation on these points, but for all of the clasts, there is a we sampled a large gravel deposit ~4 km north the Colorado and its graded tributaries, or the small but signifi cant possibility that they have of the modern river with a somewhat complex surface date is biased by erosion and resetting been reworked from a paleoterrace and hence and debated geologic history. As visible in of the exposure age. have compound histories involving multiple Figure 7A, the deposit is interbedded gravel The Hite Crossing terrace exposure is a phases of production and burial (e.g., Hu et al., and fi ne-grained sediment. Much of this fi ne roadcut along Highway 95 (Fig. 1). The cur- 2011). If this is the case, then one would expect material is probably from local streams (e.g., rent exposure is vertically shielded ~5 m (Fig. that the reworked clast would lie below the iso- paleo–Bullfrog Creek). We interpret the exten- 7B). The sample at Hite consists of seven ana- chron defi ned by the other samples. It is thus sive gravel deposit to indicate a temporarily lyzed clasts. Six clasts lie on a clearly defi ned helpful to have as many single clasts analyzed aggrading condition in the Colorado River that isochron, with the seventh well below the line as possible. On the other hand, cosmogenic caused the river and tributaries to backfi ll a and inferred to be reworked (Fig. 6). The iso- nuclide analyses are expensive and time con- few tens of meters at most. There are two pos- chron yields a very young burial age of 0.29 ± suming, placing a practical limit on the number sible sources for far-traveled quartzite (much 0.17 Ma. In this case, we limit the isochron to of samples in any given isochron. We chose to of the coarse fraction): the Colorado River have a positive intercept, and there is no evi- analyze four to seven individual clasts from and/or clasts eroding out of the Cretaceous– dence for signifi cant postburial production.

Geosphere, October 2012 1029

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Darling et al.

Elevation in m in Elevation Divide

0 Continental 500 4000 3500 3000 2500 2000 1500 1000 Wind Wind Rivers

3000

67/ 0.64 67/

s; een River een

to rate Gr Tg Tw posit (i.e.,

Green River Basin Green Flaming Gorge Flaming

102/ 0.64 102/ Canyon of Lodore of Canyon Uinta Uplift Uinta Mountains Tg

Tw

Distance from Gulf of California (km) Gulf of California from Distance De solat ion Canyon ion

Tw Uinta Basin Bookcliffs 2000 2500 Tavaputs Plateau 0

500

4000 3500 3000 2500 2000 1500 1000 Elevation in m in Elevation

V.E. ~500x V.E.

Divide 0 Continental 500

4000 3500 3000 2500 2000 1500 1000

iver

Grand Lake Grand 2500 en R en

Gore Canyon Gore River Green

Gre 124/0.64

Rocky Mountains 132/0.64 Glenwood Canyon Glenwood

Rifle

214/0.44

Grand Junction Grand

ver

Westwater Canyon Westwater 150/0.64 Ri

ado ado

lor

Co iver

2000

Juan R Juan

n

Sa Confluence with Green with Confluence

Cataract Canyon Cataract

369/0.29(Hite)

412/0.267 (Trachyte Ck) (Trachyte 412/0.267 (right panel) rivers with schematic bedrock les of the Colorado (left panel) and Green

395/0.48 (Bullfrog) 395/0.48

500/0.5 (Navajo Mtn.) (Navajo 500/0.5

lorado lorado

Lees Ferry Lees

289/0.14 1500

iver

R

Little Co Little 94/0.39

123/0.49

27/0.49 54/0.57

Grand Canyon Canyonlands Grand Wash Cliffs Wash Grand 1000

Distance from Gulf of California (km) Gulf of California from Distance Needles

500 Intern. Boundary Intern. Figure 3. Short-term rates on longitudinal profi Figure Compiled short-term (less than 1 Ma) incision igneous and metamorphic rocks. pink—Precambrian blue—Paleozoic sedimentary rocks; proportional are Arrows publication. publication; yellow arrows—this rates (including new data points); orange arrows—previous strath elevation of dated terraces. Numbers indicate rate and date the de represent magnitude; horizontal bars within arrows geology and canyon names. Vertical exaggeration (V.E.) 500×. Yellow—Tertiary sedimentary rocks; green—Mesozoic sedimentary rock green—Mesozoic sedimentary rocks; Yellow—Tertiary 500×. exaggeration (V.E.) Vertical geology and canyon names. rate/date = 369/0.29 for the new Hite date). Vector length: 1 km; elevation change = 100 m/Ma. Vector the new Hite date). rate/date = 369/0.29 for Gulf of California River Colorado Lower 0 0

500

4000 3500 3000 2500 2000 1500 1000 Elevation in m in Elevation

1030 Geosphere, October 2012

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Burial dating Colorado River terraces

Elevation in m in Elevation Divide

0 Continental 500 4000 3500 3000 2500 2000 1500 1000 Wind Wind Rivers

3000 er

on

iv

R

rate Green

Tg Tw 100/1.2 he deposit

Green River Basin Green Flaming Gorge Flaming

6/8.25 Canyon of Lodore of Canyon Uinta Uplift Uinta Mountains

Tg 41/1.5

Tw

Distance from Gulf of California (km) Gulf of California from Distance Desolation Canyon Desolation

Tw Uinta Basin Bookcliffs 2000 2500 Tavaputs Plateau 0

500

4000 3500 3000 2500 2000 1500 1000 Elevation in m in Elevation

V.E. ~500x V.E.

Divide 0 Continental 500

4000 3500 3000 2500 2000 1500 1000

er

iv

Grand Lake Grand 2500

Gore Canyon Gore River Green Green R Green

Rocky Mountains

Glenwood Canyon Glenwood 114/10

Rifle 170/9.1

Grand Junction Grand

er 150/10

Westwater Canyon Westwater

rado Riv rado

lo

o er

C Riv

2000

San Juan Juan San Confluence with Green with Confluence

110/1.36 Cataract Canyon Cataract

126/1.5

les of the Colorado (left panel) and Green (right panel) rivers with schematic bedrock geol- (right panel) rivers with schematic bedrock les of the Colorado (left panel) and Green

Lees Ferry Lees

le Colorado Colorado le 1500

tt

River Li

166/2.68

212/3.43

247/3.72

Grand Canyon Canyonlands 75/3.87 Grand Wash Cliffs Wash Grand

27/4.41

1000 75/5.97

Distance from Gulf of California (km) Gulf of California from Distance

8/5.5 Needles

500 Intern. Boundary Intern. Figure 4. Long-term rates on longitudinal profi Figure ogy and canyon names. Vertical exaggeration (V.E.) ~500×. Yellow—Tertiary sedimentary rocks; green—Mesozoic sedimentary rocks; sedimentary rocks; green—Mesozoic sedimentary rocks; Yellow—Tertiary ~500×. exaggeration (V.E.) Vertical ogy and canyon names. than 1 Ma) incisi Compiled long-term (greater igneous and metamorphic rocks. pink—Precambrian blue—Paleozoic sedimentary rocks; to proportional are Arrows publication. published; purple arrows—this arrows—previously rates (including new data points); red strath elevation of dated terraces. Numbers indicate mean rate and date t represent magnitude; horizontal bars within arrows (i.e., rate/date = 126/1.5 for the new Hite date). Vector length: 1 km; elevation change = 100 m/Ma. Vector the new Hite date). (i.e., rate/date = 126/1.5 for Gulf of California River Colorado Lower 0 0

500

4000 3500 3000 2500 2000 1500 1000 Elevation in m in Elevation

Geosphere, October 2012 1031

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Lava Creek B Ash

Figure 5. Photograph of Bost- wick Park, near Black Canyon of the Gunnison, western Colo- rado (photo by L. Crossey). Strath is the base of the gravel pit. Approximately 10 m of fl u- vial gravel rest below the white 10 m band of Lava Creek B ash (Sandoval, 2007).

Strath

6 6 1.0 [26Al] = (3.33 +/– 0.18) [10Be] + (0.20 +/– 0.01) [26Al] = (3.86 +/– 0.56) [10Be] + (0.38 +/– 0.19) [26Al] = (5.90 +/– 0.48) [10Be] 5 5 0.8

4 4 /g) t/g) at/g) at a 0.6 6 6 6 10 10

3 3 ( Al] ( Al] Al] (10

6 0.4 26 26 2 [ [ 2 [ 2

Tabyago Canyon Peru Bench Hite 1.2 +/– 0.3 Ma 0.2 1 1.48 +/– 0.12 Ma 1 0.29 +/– 0.17 Ma two data points 0 0 0 0 0.5 1 1.5 2 0 0.20.4 0.6 0.8 1.0 1.2 1.4 0 0.05 0.1 0.15 0.2 0.25 [10Be] (106 at/g) [10Be] (106 at/g) [10Be] (106 at/g) 3 0.6 Figure 6. Isochron plots of 26Al/10Be [26Al] = (3.27 +/– 0.04) [10Be] + (0.10 +/– 0.03) [26Al] = (4.48 +/– 0.47) [10Be] –(0.03 +/– 0.12) data for determination of isochron 0.5 dates. Measured data are shown in light gray with one-sigma error 2 0.4 ellipses. Data linearized for regres- at/g) at/g) 6 6 sion are shown as darker ellipses, 0

(1 0.3 shifted to lower 10Be concentra- ] l A Al] (10 tions (see Balco and Rovey, 2008). 26 26 [ [ 1 0.2 Regression equations are shown Bullfrog Terrace Bostwick Park for each line, including errors in 1.5 +/– 0.13 Ma 0.1 0.87 +/– 0.22 Ma both slope and intercept. Errors in slope are calculated following 0 0 York (1966) or from measurement 0 0.2 0.4 0.6 0.8 1 0 0.02 0.04 0.06 0.08 0.10 0.12 uncertainty, whichever is greater. [10Be] (106 at/g) [10Be] (106 at/g)

1032 Geosphere, October 2012

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A B ~ 5 m NavajoNavajo MMountainountain

Figure 7. (A) Photograph of Bullfrog sampling site (photo by L. Crossey). View is to the south-southwest. Navajo Moun- tain is in the background. Inter- bedded gravel and fi ne-grained ~~77 m material are exposed below a protective calcrete soil hori- zon. (B) Photo taken near Hite ~ 7 m Crossing in a roadcut above the Dirty Devil River. Gravel is composed mostly of Mesozoic sedimentary rocks, moderately imbricated to the right in photo (photo by S. Blessing).

No postburial production may seem surprising Comparing the Upper Colorado and rivers evolved with similar or different control- for a shallow deposit. However, short burial Green Rivers ling parameters? Incision of the Colorado and time reduces the likelihood of measureable Green rivers is marked by extensive erosion, postburial nuclides. Since this terrace rests The upper Colorado River has been a fl uvial leaving a sparse record to explore these param- 107 m above the confl uence of the Dirty Devil system for at least 11 million years (e.g., Aslan eters. In the following sections, we compare and the Colorado River, the imprecise date et al., 2010), but the evidence for the early what is known of the Green and upper Colorado implies a relatively high rate of incision of Green River is less well known (see Geologic rivers to resolve the fi rst-order controls on the 300–900 m/Ma (Table 1). Background section). For instance, have the development of these systems.

1800 Battlement Mesa 1600 Grand Mesa

Figure 8. Plot of age versus height above the 1400 river for samples near Rifl e, Colorado, for four incision rate markers and the modern river. All locations except Grand Mesa are 1200 along a 50 km stretch in which the river drops ~130 m. Grand Mesa is an important 1000 ~ 170 m/ Ma regional reference ~50 km downstream from westernmost Morrisania Mesa. Heights of 800 terrace straths that are currently undated

are shown as red lines. The data for this plot (m) river Height above 600 Flatiron Mesa are listed in Table 1 for samples from Battle- ment, Grand, Grass, and Morrisania mesas. 400 Current data show apparent semi-steady High Mesa long-term average rates of incision in this region but need improved chronology. 200 Grass Mesa Morissania Mesa Jolly Mesa 0 Modern River 024681012 Age (Ma)

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Burial Dating Results—Morrisania Mesa locally bury Colorado River gravel deposits Thus, the incision rate at Morrisania Mesa is (Stover, 1993). Substantial oil and gas drill- poorly constrained; the average incision rate Existing incision rate data from western ing activity has led to numerous drill holes that is 214 m/Ma, but the uncertainty of incision Colorado show rates of ~150 m/Ma from both pierce these high abandoned terraces and alluvial rate ranges from 127 to 671 m/Ma (Table 1). 640 ka and 10 Ma time markers (Darling et al., fans. Morrisania Mesa is one alluvial fan com- A future research goal beyond this paper in 2009; Aslan et al., 2008, 2010). To fi ll in the gap plex on the north fl ank of Battlement Mesa. This this and other reaches is to establish variability in these timescales, we collected a cosmogenic site provided ideal shielding for a simple burial of incision rates through time in this and other burial sample from near Rifl e, Colorado (Fig. 1). date from an amalgamation of quartz-rich drill- reaches of the Colorado River system. All avail- At Rifl e, an extensive series of alluvial fan rem- hole cuttings 94 m above the river. Our sample able incision rate data in this area (Fig. 8) show nants are preserved along the northern fl ank of contained fragments of Colorado River gravel a semi-linear array suggesting a steady incision Battlement Mesa (Fig. 1). These deposits repre- from a well-shielded depth of 110 m. rate of 170 m/Ma from the 10 Ma basalt fl ows sent ancient alluvial fan and/or pediment com- Despite the ideal sample setting, this sample on Grand Mesa to the younger cosmogenically plexes that are comprised of locally sourced, yielded an imprecise burial age of 440 ka ± dated Colorado River deposits. Although the coarse colluvium and debris-fl ow deposits that 300 ka (Table 1) due to a low 26Al/27Al ratio. data are indistinguishable from constant steady state, the data are sparse enough that diverse incision histories would also be consistent

109°57′0″ W 109°56′0″ W 109°55′0″ W 109°54′0″ W 109°53′0″ W 109°52′0″ W with the data. Several other terraces buried by N alluvial fans exist in this region. Berlin et al. Explanation (2008) dated sediments beneath Grass Mesa at Qcf - Quaternary colluvial/alluvial fill 96 a height of 227 m above the Colorado River and Qdf - Debris flow/alluvial fan gravel 2 reported a date of 1.77 +0.71/–0.51 Ma, which Qt10 - Quat. terrace X (X m above river) 94 also yields an incision rate between 92 and 180 Qt15 95 39°50′0″ N m/Ma. More chronology may reveal incision Qt22 93 rate changes after 10 Ma; however, the sim- Qt30 plest scenario, weighted with other rates from Qt45 western Colorado, is semi-steady incision along Qt60 this stretch of the Colorado River at ~150 m/Ma Qt70 (Willis and Biek, 2001; Aslan et al., 2008; Dar- 39°49′0″ N Qt80 ling et al., 2009; Aslan et al., 2010; Table 1). 92 Qt90 Qt145 Green River Burial Dates

91 Qt145 Desolation Canyon represents a knickzone 39°48′0″ N on the Green River (Figs. 1 and 2) as it cuts 85 through the Tavaputs Plateau, which separates 90 the Uinta Basin from the Canyonlands region. 86 Near the upstream end of Desolation Canyon is Tabyago Canyon, which contains a large 84 88 ′ ″ 87 89 39°47 0 N entrenched Green River meander with a thin but laterally continuous gravel deposit (Fig. 9) that Miles upstream is overlain by locally derived colluvial material. from Green River, UT The strath surface is cut into the shale and thin 83 Qt60 fi ne-sandstone beds of the Green River Forma-

39°46′0″ N tion ~60 m above the present day level of the Green River. Recent erosion in an ephemeral Tabyago Canyon sample site 82 tributary cut bank exposed an outcrop of river gravel (Fig. 10). Further excavation by hand allowed us to sample clasts just above the strath Qcf surface. Burial depth of the sample was only 39°45′0″ N 81 Qdf ~4 m below the surface, and the upper 0.5 m 80 of this terrace consisted of reworked locally Qcf derived slope wash and colluvial material. The colluvial wedge of the deposits is deeper nearby 79 0 1 2 and suggests the sample site was deeper in the 76 kilometers 39°44′0″ N past. Approximately 3.5 m of gravel with pri- mary sedimentary structures is preserved in Figure 9. Map of Pliocene–Pleistocene terraces throughout Desolation Canyon. Sample the deposit, and thus the majority of the gravel location for new burial date for Tabyago Canyon is shown. Heights to terraces were mea- is not reworked. AMS results for four clasts sured with a laser range fi nder. Abandoned meanders and point bar deposition have punctu- (Fig. 6) yielded an isochron burial date of 1.48 ± ated canyon incision. 0.12 Ma for this terrace. Concentrations for one

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sample are very high, indicating slow erosion A prior to burial. The intercept indicates some postburial production, as expected. From these data, we estimate an average incision rate of 41 ± 3 m/Ma (Fig. 4). Peru Bench, located in the Green River Basin near Green River, Wyoming, represents a fl ight of Green River terraces that are up to 180 m above the river (Figs. 1 and 11; Hansen, 1986). These gravels are deposited on siltstone depos- its of the Green River Formation. The sampled terrace on Peru Bench is 120 m above the river and was sampled in a gravel pit (Fig. 12). The pit exposed ~3 m of imbricated sandy pebble- to cobble-sized gravel overlain by a ~1 m thick calcic soil with stage III carbonate development. Clast types include quartzite from the Protero- zoic Uinta Mountain Group, granite from the Wind River Mountains, and sparse black chert strath typical of far-traveled Green River gravels. The sample depth was 4 m. The 26Al/10Be ratios from four clasts lie on an isochron with no B outliers, indicating a common burial history C for all of the clasts (Fig. 6). Uncertainty in 26Al concentrations leads to a higher uncertainty in

this isochron fi t than for the Tabyago Canyon strath sample. Postburial production at Peru Bench was signifi cant, due to the very shallow burial depth. The isochron analysis indicates a date of 1.2 ± 0.3 Ma (Peru Bench; Table 1 and Fig. 1). This terrace date yields an average incision rate of 100 +33/–20 m/Ma (Fig. 4). This rate is compatible with an incision rate of 90–115 m/Ma from a Lava Creek B ash site on a Green River terrace in western Browns Park (Munroe et al., 2005; Counts, 2005), but it is faster than a terrace in the Green River Basin, Wyoming, that is 52–67 m/Ma over the past 640 ka based on Lava Creek B ash reported by Izett and Wil- cox (1982). The Lava Creek B sites of Izett and Wilcox (1982) are not affected by faulting that may have been active in Browns Park, and the relationships between ash, straths, and river are Figure 10. Collage of photos from Tabyago Canyon sample from Desolation Canyon. more obvious; thus it is a more robust measure (A) Deposits in a meander bend in ephemeral stream that was excavated. (B) Pit excavated of incision. Therefore, our data and that of Izett for burial samples; dated cobbles were taken from the bottom of the hole, ~4 m below the and Wilcox (1982) show the incision rate of the surface of the terrace. (C) Photo of the cut bank; excavation of the pit is started in lower upper third of the Green River to range from 50 center (photos by Ryan Crow). to 100 m/Ma.

DISCUSSION AND IMPLICATIONS et al., 2002a; Polyak et al., 2008; Karlstrom et al., ternary rates and post–3–4 Ma rates are similar 2008), whereas rates upstream of Lees Ferry and suggest semi-steady incision over the past Grand and Glen Canyon appear to be ~110–130 m/Ma (Fig. 4; Wolkow- 3–4 Ma. Because these data average similar Compilation Results insky and Granger, 2004; this paper). Although timescales as our new estimates of incision rate some of these data from eastern Grand Canyon above Lees Ferry (Fig. 4), we suggest the data Long-Term Incision Patterns are measured over timescales of 105 ka (e.g., reveal a robust pattern of higher average incision The long-term incision rates of the Colorado Pederson et al., 2002a), and may be subject to rates below the knickpoint. River appear to exhibit spatial differences in short-term variations in incision and/or aggra- Global climate change in the Pleistocene incision rates across the Lees Ferry knickpoint. dation driven by climate cycles, other data are (since ca. 2 Ma) is marked by an increase in cli- Incision rates in eastern Grand Canyon are on averaged over 2–3 Ma (e.g., Polyak et al., 2008). matic variability as recorded in the magnitude the order of ~170–230 m/Ma (Fig. 4; Pederson Karlstrom et al. (2008) showed that both Qua- and frequency of polar ice variation recorded

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in δ18O (e.g., Lisiecki and Raymo, 2005). How and whether these cycles are expressed in the effi ciency of fl uvial systems on the Colorado Plateau is not clear. Globally, the onset of Plio- Pleistocene climates has been suggested to have increased erosional effi ciency due to increased Green River climate variability since ca. 2–4 Ma (e.g., Zhang et al., 2001), although this conclusion has been strongly challenged recently (Willenbring and von Blanckenburg, 2010). Chapin (2008) sug- Peru Bench gests that the onset of monsoonal variability in late Miocene time beginning ca. 6 Ma (see fi g. 3, Chapin, 2008) could have contributed to an increase in exhumation rates. Sediment derived from the in the Salton Trough has an average accumulation rate of 2–3 mm/a from Pleistocene–Holocene data and an average of 1.9–2.3 mm/a throughout 5.3 Ma of deposition (Dorsey, 2010 and refer- ences therein). These data do not show a bulk Sample Location difference in deposition rate in the delta (but may still average across short-term variations in rate). From these studies, it is probable that the effect of climate on erosion rate was either an increase or negligible change in erosion rate on the Colorado Plateau in the past 2–6 Ma. Here, we consider whether a climate change can result in a false positive test for transient incision. If we consider the infl uence of chang- ing erosional effi ciency through time, it is possible that differences in the timescale over which the data average incision rate can affect the pattern of incision rates. The time frame of Highway 372 our data at Bullfrog (~1.5 Ma) and that from Polyak et al. (2008), ~3 Ma, bracket the begin- ning of the Pleistocene. If the onset of Pleisto- N cene climate was associated with more effi cient erosion, then average measures of incision that span this transition would be artifi cially lower 2 km than those measured within the Quaternary. This would only enhance the spatial difference Figure 11. Location map of sample taken on Green River terrace on Peru Bench along the between rates measured across the Lees Ferry Green River, north of the town of Green River, Wyoming. knickpoint. In fact, only for a decrease in ero- sional effi ciency, and a corresponding reduction in incision rates, would climatic modulation of erosion rate lead to a spurious, false-positive lated luminescence (OSL) dating of terrace fi ll The surface exposure dates in the literature result. Thus, although we cannot at present rule (Table 1; Fig. 13A; Davis et al., 2001; Hanks (e.g., Garvin et al., 2005) inherently date fi nal out the possibility that the observed differences et al., 2001; Garvin et al., 2005; Cragun, 2007; deposition (if zero erosion) and not the bedrock in erosion rate across the Lees Ferry knickpoint Cook et al., 2009; Hanks et al., 2011). The sur- strath. If future data can show roughly continual, are an artifact of climate change, we fi nd this face exposure dates may be subject to bias that slow deposition from 1.5 to 0.5 Ma in the Bull- to be unlikely. The simpler interpretation is that arises from the history of erosion and deposition frog terrace, then the surface dates on high eleva- high rates of incision downstream of the knick- on the surface, including transient eolian cover tion terraces may be accurate. However, bedrock point refl ect sustained, upstream migration of and/or denudation of the surface. To reconcile incision rate estimates would not be affected. this feature (cf. Cook et al., 2009). the contradictory dates for the Bullfrog terrace, The incision rates of Glen Canyon are plot- we infer that the previously published exposure ted as age versus height above the river in Fig- Short-Term Incision Patterns date of the deposit underestimates the deposi- ure 13B. The bedrock incision rate is estimated Previous studies of incision in the Glen Can- tional age of the Bullfrog gravels and the age of to be within the gray bar defi ned by the older yon region have suggested incision rates as high the bedrock strath. Further, the Bullfrog burial burial dates and the younger exposure and burial as 500 m/Ma based on surface exposure dating date reported here is consistent with the data dates from terraces at most ~100 m above the of clasts on terrace treads and optically stimu- from Bluff in Wolkowinsky and Granger (2004). river. The data in Figure 13B include terraces

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Overall, the data are compatible with integra- tion of the Colorado River through the Grand Canyon region ca. 5–6 Ma (e.g., Karlstrom et al., 2008), where topographic relief devel- oped locally in response to a signifi cant drop in local base level between the elevated Plateau and extending Basin and Range. Incision likely Gravel depth: 3.8 m proceeded upstream as a transient wave of inci- sion, increasing relief in the region. When this incisional wave reached the Paleozoic–Meso- zoic contact near but downstream of Lees Ferry, it engendered a quick local drop in base level above Lees Ferry and resulted in rapid incision upstream over the past few hundred thousand years in Glen Canyon (Cook et al., 2009; cf. Garvin et al., 2005). It is unclear if this wave of incision has reached the area of Bluff, Utah, where Wolkowinsky and Granger (2004) fi nd slightly lower long-term average rates of inci- sion than Bullfrog (~110 m/Ma at Bluff).

Strath surface Insight on Early Development of the Green River

Our new incision rates derived from burial Figure 12. Photograph of Peru Bench sample location in a gravel pit. Strath is exposed along dating of fl uvial deposits along the Green River the bottom of the photo. Gravel is 3.8 m deep (photo by A. Aslan). provide new insight into the history of the Green River. Integration of the Green River across the Canyon of Lodore must have taken place within Glen Canyon (Davis et al., 2001; Hanks 110 m above the river. These dates imply con- between the end of Browns Park deposition et al., 2001; Garvin et al., 2005), at Lees Ferry sistent high incision rates within Glen Canyon <8.25 Ma and prior to terrace gravel deposition (Cragun, 2007; Hidy et al., 2010) and along from three independent techniques (cosmogenic on Peru Bench at (1.2 ± 0.3 Ma) and Tabyago Trachyte Creek (Fig. 1; Cook et al., 2009). Four surface and burial; OSL), and they bracket the Canyon (1.5 ± 0.13 Ma). Higher and older high elevation dates are outside of the gray bar. rate change in time and space. undated terraces along many portions of the These dates are the surface exposure dates of We note that the recent numerical study of Green River system suggest that our terrace date Bullfrog (Davis et al., 2001) and other high sur- Cook et al. (2009) suggests that the knick- places a minimum constraint on the time of pos- faces in Garvin et al. (2005) and Hanks et al. point at Lees Ferry refl ects the interaction of a tulated drainage integration and development of (2001). Readers will note the roughly linear transient knickpoint and a dipping contrast in a south-fl owing Green River across the Uinta trend the surface exposure dates have produced lithologic strength between the Kaibab Lime- Mountains to >1.5 Ma. Thus, we differ from the in past work, and that the oldest of these pro- stone and the softer Mesozoic rocks above interpretation of Hansen (1986) that the Green duce a maximum age that does not vary with the knickpoint. This hypothesis is compatible River fl owed east away from the location of the elevation (“4103,” Navajo and Bullfrog surface with the incision rate acceleration in Figure town of Green River as recently as 640 ka. dates). Previous paragraphs discuss possible 13B through Glen Canyon (Hanks et al., 2001; explanations for divergence of burial dates and Garvin et al., 2005), which may be affecting the Comparison of the Colorado and surface dates, and the more likely implication Fremont River (Marchetti et al., 2005; Repka Green River Systems in our estimation is that surface dates are min- et al., 1997) and Trachyte Creek (Cook et al., imum age estimates in older terraces. The maxi- 2009), since it predicts a recent (~250–500 ka) The most prominent feature of the profi le of mum ages achieved in the three highest terraces acceleration of incision throughout the Glen the upper Colorado River system (Fig. 2) is that imply a limit in the exposure ages, leaving the Canyon as shown in Figure 13B. Since burial, the Colorado River maintains a steeper gradient data uninterpretable, and that a general posi- surface and OSL dating seem to more or less than the Green River above their confl uence. In tive trend in high terraces matching lower ones agree in young, low-lying deposits (<~100 m many rivers, channel steepness is inversely pro- is coincidental. The data sets in Fig 13B are above the river), and the transition in rate portional to discharge (Osterkamp, 1978) and, summarized in the long profi le of Figure 13A, between Bullfrog and Hite (and numerous other thus, canonical explanations for a “graded” except the small Trachyte Creek for simplifi ca- young dates) provides an average of slow inci- profi le (e.g., Mackin, 1948) attribute down- tion. Cragun (2007) used OSL dating of terraces sion (~60 m/Ma) for most of the past 1.5 Ma, stream decreases in gradient as adjustments to at Lees Ferry with straths at most 41 m above with an acceleration of incision rate (to ~400 downstream increases in discharge. To assess the river, and obtained incision rates that varied m/Ma) within the past ~250–500 ka. These data whether the steeper gradient of the upper Colo- ~300 m/Ma (summarized in Table 1). Trachyte and analysis represent this acceleration in time, rado relative to the Green River may refl ect Creek terraces are cosmogenic surface dates and especially space, much more effectively differences in discharge, we compare USGS from Cook et al. (2009) with the highest terrace than previous estimates. records for historic discharges (U.S. Geological

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Figure 13. (A) Incision con- Incision rate straints from eastern Grand A 100 m/Ma Terrace height 1900 Canyon and Glen Canyon from and age (Ma) Cave height and Karlstrom et al. (2008), Hanks age (Ma) et al. (2001), Garvin et al. (2005), Cragun (2007), and this 1700 River Mile 0 Bullfrog Lees Ferry volume on the long profi le of Mtn. Navajo Hite

the Colorado River near Lees Bridge Canyon Ferry. Long-term rates are red (previous publication) or San Juan River purple (this paper). Short-term 1500 rates are orange (previous pub- lication) or yellow (this paper). 1.3 Previous incision rates that underestimated terrace age 1300 are transparent (i.e., orange Elevation (m) >0.5 1.5 arrows with a gray outline at 0.30 Colorado River Bullfrog and Navajo Mountain [“Cha surface” of Hanks et al., 1100 2001]). (B) Plot of age versus ract height above the river for data 0.14 Cata between Lees Ferry, Arizona, ~0.12 Canyon and Bluff, Utah, through Glen Canyon. All data are in Table 1 900 under Cook et al. (2009), VE = ~380 Cragun (2007), Garvin et al. 1400 1500 1600 1700 1800 1900 (2005), Hanks (2001), Hidy Distance from Gulf of California (km) et al. (2010), Wolkowinsky and Granger (2004), and Davis et al. B (2001). Burial dates are larger “4103” surface 250 text. Note that the Trachyte Navajo Mountain Creek samples of Cook et al. (2009) are not on the long pro- 200 Bullfrogog fi le fi gure because they repeat Bullfrog already apparent age patterns and overcomplicate the fi gure. 150

Error bars are one standard Height (m) ~60 m/Ma deviation about the mean for HiteHite published analytical uncer- 100 tainty. The four highest eleva- Bridge Cyn tion surface dates of Garvin 50 et al. (2005) and Hanks et al. ~400 m/Ma (2001) are excluded from the Lees Ferry gray bar of likely bedrock inci- sion rates through time. Gray 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 shaded area is a best guess of Age (Ma) progression of the incising river through time. Approximate fast and slow incision rates are from linear regressions of either all the data younger and lower than Hite in the plot, or from the trio of burial dates at Bluff, Bullfrog, and Hite, respectively.

Survey, 2001) along both rivers. Data were discharge are minimum estimates of natural tions between discharge and slope (Mackin, averaged over several years from the same fl ow patterns. These discharge records never- 1948; Osterkamp, 1978). years of record for both systems whenever pos- the less show that the upper Colorado River Three possible explanations for why the sible to avoid annual variation in storm tracks consistently carries greater discharge than the Colorado River is steeper than the Green River and hydrograph shape. We concentrated on pre- Green River per unit drainage basin area (Fig. are (1) uplift of the Colorado River segment, dam data (Table 2) in order to avoid substantial 14). Thus, if the relative pattern in discharge (2) more resistant underlying bedrock along removal of fl ow via dams and irrigation sys- data is relevant over millennial and million- the Colorado, or (3) a substantial topographic tems. Since records are not complete and minor year timescales, a steeper upper Colorado River step along the western Rockies at the onset of anthropogenic surface water alteration began gradient relative to the Green River would be incision. If rock strength is the primary con- before the earliest records, specifi c values of inconsistent with generally accepted connec- trol on fl uvial evolution in this system, then

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TABLE 2. COMPILATION OF U.S. GEOLOGICAL SURVEY DISCHARGE DATA USED TO DERIVE THE DISCHARGE COMPARISON GRAPH TO COMPARE RELATIVE PATTERNS OF DISCHARGE BETWEEN THE GREEN AND COLORADO RIVERS Peak Q Area Peak Q Area Peak log Log Location (cfs) (mi2) (m3/s) (km2) (Q) (area) Record Green River discharge data Warren Bridge, Wyoming 2665 468 76 1212 1.88 3.08 1932–1935 Daniel, Wyoming 4050 932 115 2414 2.06 3.38 1914–1917 La Barge, Wyoming 7830 3910 222 10,127 2.35 4.01 1947–1950 Green River, Wyoming 14,838 7670 420 19,865 2.62 4.30 1915–1918 Flaming Gorge, Wyoming 11,798 14,900 334 38,591 2.52 4.59 1924–1927 Linwood, Wyoming 10,904 18,300 309 47,397 2.49 4.68 1929–1932 Greendale, Utah 14,125 19,350 400 50,116 2.60 4.70 1951–1954 Jensen, Utah 31,300 29,660 887 76,819 2.95 4.89 1904–1906 Ouray, Utah 30,600 35,500 867 91,945 2.94 4.96 1948–1951 Green River, Utah 42,250 44,850 1197 116,161 3.08 5.07 1914–1917 Colorado River discharge data Baker Gulch, Colorado 511 63.9 14 166 1.16 2.22 1953–1957 Grand Lake, Colorado 1138 102 32 264 1.51 2.42 1914–1917 Granby, Colorado 2675 323 76 837 1.88 2.92 1908–1911 Hot Sulphur Springs, Colorado 5640 825 160 2137 2.20 3.33 1914–1917 Kremmling, Colorado 12,028 2382 341 6169 2.53 3.79 1914–1917 Glenwood Springs, Colorado 21,425 4558 607 11,805 2.78 4.07 1914–1917 Palisade, Colorado 35,500 8738 1006 22,631 3.00 4.35 1914–1917 Fruita, Colorado 47,700 17,100 1352 44,289 3.13 4.65 1914–1917 Cisco, Utah 56,550 24,100 1603 62,419 3.20 4.80 1914–1917

the generally weaker rocks of the Green River 104 Figure 14. Historical discharge would allow more rapid incision than the Colo- of the Green and Colorado rado. However, measured incision rates on the rivers compiled from U.S. Geo- Colorado River are higher than on the Green 103 logical Survey records. The River (Figs. 3 and 4). Thus, rock type may not be Green River has systemati- Green River the sole control on the long profi le. The topogra- 102 Colorado River cally less discharge per drain- phy that existed at the onset of integration of the age area than the Colorado upper Colorado around 11 million years ago is River in these records, and we

available from limited data sets, but these paleo- (m3/s) Discharge 101 suggest the relative discharge topographic reconstructions support relatively distances could have been similar throughout Pleistocene horizontal-planar topography across the western 100 Rockies and Colorado Plateau (Pederson et al., 102 103 104 105 climates. Dates of record are 2002b; Karlstrom et al., 2012). Thus, we infer tabulated in Table 2. that the Colorado River channel is likely being steepened relative to the Green River by recent or ongoing epeirogeny in the Colorado Rockies OSL dates of terraces leads to measurement of Crow, Fran Lazear, Bruce Coriell, Richard Elliott, that is further supported by numerous data sets rapid incision rates corroborated by all three Tyler Doane, Robert Jacobsen, Alexander Kerney, in Karlstrom et al. (2012) and subsequent papers techniques, which followed long-term slower Anna Kutkiewicz, Kira Olsen, and Anna Phelps. Laboratory assistance from Tom Clifton and Greg from those authors. incision rates. Chmiel at PRIME is also greatly appreciated. We (2) The observations that the Colorado River thank Thomas C. Hanks and an anonymous reviewer CONCLUSIONS is steeper, has higher discharge, and higher inci- for their constructive and thoughtful comments on sion rates than the Green River may be well the manuscript as well as Kelin Whipple for further guidance. Our new burial ages from fl uvial deposits explained by uplift of the Colorado Rockies rela- along the Colorado and Green rivers, in con- tive to the Colorado Plateau in the past 10 Ma. REFERENCES CITED junction with existing constraints on incision (3) The new Green River data brackets inte- rates during the late Cenozoic, lead us to the fol- gration across the Uinta Mountains between 8.5 Anthony, D.M., and Granger, D.E., 2007, An empirical stream power formulation for knickpoint retreat in lowing conclusions. and >1.5 Ma, and further research is needed to Appalachian Plateau fl uviokarst: Amsterdam, Journal (1) The combined data sets of incision rates elucidate integration timing. of Hydrology, v. 343, no. 3–4, p. 117–126, doi:10.1016 around Grand and Glen Canyons support a /j.jhydrol.2007.06.013. ACKNOWLEDGMENTS Aslan, A., and Kirkham, R., 2007, Origin of the upper Colo- transient incision model for the Lees Ferry rado River system: The view from western Colorado: knickpoint. The data imply that the shallow- Geological Society of America Abstracts with Pro- dipping lithologic contrast at the top of the Funding for this project came from the National grams, v. 39, no. 6, p. 194. Science Foundation Continental Dynamics program Aslan, A., Karlstrom, K., Hood, W., Cole, R.D., Oesleby, T., Kaibab Formation may have split the migrating grant (EAR-0607808) to University of New Mexico, Betton, C., Sandoval, M., Darling, A., Kelley, S., Hud- knickpoint, leaving the Lees Ferry knickpoint and a seed grant from PRIME Lab, Purdue Univer- son, A., Kaproth, B., Schoepfer, S., Benage, M., and behind and separately excavating Glen Canyon. sity (EAR-0851981). Sample BP (Bostwick Park) Landman, R., 2008, River incision histories of the Black was supported by grant EAR-0844151 to DG. EK Canyon of the Gunnison and Unaweep Canyon: Interplay This split may have led to the sudden incision between late Cenozoic tectonism, climate change, and acknowledges support from the Alexander von Hum- rate increase in the latter half of the Pleisto- drainage integration in the western Rocky Mountains, in boldt Foundation during preparation of this manu- Raynolds, R.G., ed., Roaming the Rocky Mountains and cene through Glen Canyon. Our comparison script. We would like to extend thanks to many who Environs: Geological Field Trips: Geological Society of of cosmogenic surface, cosmogenic burial, and helped in the fi eld, including Shelby Blessing, Ryan America Field Guide 10, p. 175–202.

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