Geomorphic Constraints on the Age of the Western Grand Canyon

Home , Esplanade Sandstone, Hermit Formation, Shivwits Plateau

Research Paper THEMED ISSUE: CRevolution 2: Origin and Evolution of the Colorado River System II

GEOSPHERE Geomorphic constraints on the age of the western Grand Canyon Andrew Darling and Kelin Whipple Arizona State University, School of Earth and Space Exploration, PO Box 871404, Tempe, Arizona 85287-1404, USA GEOSPHERE; v. 11, no. 4

doi:10.1130/GES01131.1 ABSTRACT phic evidence of the Tertiary cutting and infilling of smaller paleocanyons that is incompatible with the 12 figures; 3 tables; 2 supplemental files Hypotheses for the age of the western Grand Canyon (WGC) range from younger than 6 Ma to older formation and maintenance of a deep canyon in than 70 Ma. We study the relationships among topography, geology, and available erosion rates in space and the WGC area prior to ca. 19 Ma. Furthermore, the CORRESPONDENCE: [email protected] time to place constraints on plausible canyon incision histories. Evidence suggests that lateral retreat of the Muddy Creek Formation of the Grand Wash trough Shivwits Plateau escarpment left a lithologically controlled bench on the Sanup Plateau, but the Hualapai (GWT) has long been interpreted to prohibit the ex- CITATION: Darling, A., and Whipple, K., 2015, Plateau is beveled indiscriminately across rock types of the Paleozoic stratigraphic section. A period of ac- istence of a large river at the terminus of the WGC Geomorphic constraints on the age of the western Grand Canyon: Geosphere, v. 11, no. 4, p. 958–976, celerated base-level fall in the Tertiary is implicated by the canyon incised into the beveled Hualapai Plateau (Longwell, 1946; Lucchitta, 1966; Pederson, 2008) doi:10.1130/GES01131.1. surface, consistent with higher erosion rates observed in canyons than on the surrounding plateau. Streams and the Muddy Creek constraint has been further draining the Hualapai Plateau preserve relict headwater segments that were equilibrated with a slower strengthened by detrital zircon analysis of the rocks Received 1 October 2014 base-level-fall rate before canyon incision. These relict segments are now separated from the Grand Canyon of Grand Wash trough at the mouth of Grand Can- Revision received 9 February 2015 by knickpoints indicative of a transient landscape. Relief production since the beveling of the Hualapai Pla- yon (Crossey et al., 2015; Ingersoll et al., 2013). Be- Accepted 29 April 2015 Published online 10 June 2015 teau is ~1000 m in the WGC. Comparison of hillslope and channel morphologies between the Grand Wash yond sorting out the local geologic history, resolv- Cliffs and the WGC provides a test to distinguish hypothesized ages of canyon incision. The data strongly ing this debate presents an opportunity for refining suggest that carving of the WGC is younger than relief production due to slip on the Grand Wash fault ca. our understanding of both thermochronologic and 18–12 Ma. Thus the geomorphic data are only fully consistent with the late Tertiary, transient incision model geomorphic records of river incision and relief pro- of canyon incision beginning at integration after 6 Ma. duction. In this paper we analyze constraints on the timing of the incision of the WGC from landscape morphology and existing incision rate estimates. INTRODUCTION western Grand Canyon (WGC) by 70 Ma (Wernicke, Classic interpretation of a young WGC is consis- 2011; Flowers and Farley, 2012), or perhaps since tent with the steep-walled, narrow, high-relief inner Motivation ca. 17 Ma (Polyak et al., 2008; Young, 2008), rein- gorge morphology that is immediately apparent on vigorating the debate over estimates of the canyon topographic maps (Fig. 1) and in the field (Fig. 2). The age of the Colorado River drainage system age. The hypothesized 17 Ma precursor WGC cut The range of circumstances that could produce and and timing of Grand Canyon incision have been de- by a moderate-sized pre–Colorado River drain- maintain such a landscape, however, has not been bated since the pioneering studies of John Wesley age (Polyak et al., 2008; Young, 2008) is disputed thoroughly explored. Moreover, given the broad Powell (1875). Although Powell interpreted the Col- (Karlstrom et al., 2008; Pearthree et al., 2008; Ped- attention this controversy has garnered, and given orado River as antecedent to Laramide structures erson et al., 2008; Karlstrom et al., 2014; Crossey the apparent challenge of the “old canyon” evi- and the uplift of the Colorado Plateau, since the et al., 2015). The idea of WGC formation by 70 Ma dence to this classical interpretation of a “youthful” turn of the twentieth century most geologists have is even more controversial (Karlstrom et al., 2013; landscape (e.g., Davis, 1901), an independent quan- interpreted a mid- to late Tertiary age of integration Lucchitta, 2013). Wernicke (2011) and Flowers and titative assessment of the geomorphic constraints of the superimposed Colorado River and carving of Farley (2012) argued on the basis of thermochrono- on the antiquity of the WGC is needed. Three main the Grand Canyon (Blackwelder, 1934; Davis, 1901; logic data that the WGC had been cut to within a few hypotheses for the age of the WGC (older than 70 Longwell, 1946; Lucchitta, 1972; Young and Bren- hundred meters of its current depth by 70 Ma and Ma, younger than 17 Ma, and largely younger than nan, 1974), and much of the evidence points to a maintained at the surface (not buried) since then. 6 Ma) are the focus of debate, and each must be ca. 6 Ma river integration event (House et al., 2008), The interpretation of the thermochronologic data is consistent with the known paleocanyons preserved contemporaneous with a significant fraction of not simple, however, and has been challenged (Fox on the Hualapai Plateau (Young, 1979, 2008; Young canyon incision (Karlstrom et al., 2008, 2007; Ped- and Shuster, 2014; Karlstrom et al., 2014). In addi- and Crow, 2014). Each hypothesis constitutes a set For permission to copy, contact Copyright erson et al., 2002, 2006). Some evidence, however, tion, Young and Crow (2014) reiterated, clarified, of testable predictions for both landscape mor- Permissions, GSA, or [email protected]. has been suggested to support the carving of the and augmented geologic and qualitative geomorphology and the spatiotemporal pattern of erosion.

GEOSPHERE | Volume 11 | Number 4 Darling and Whipple | Western Grand Canyon geomorphology Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/4/958/3336433/958.pdf 958 by guest on 23 September 2021 Research Paper

114 W Figure 1. Western Grand Canyon study area, defined as down- Elevation Path and ow direction stream of Hurricane fault and upstream of Grand Wash trough. 2460 m Elevation colored over hillshade; analyzed tributary rivers are of Paleocene-Eocene shown in blue. Major faults, major tributary knickpoints, early ¯ channels Tertiary paleochannels, and topographic and geologic section 345 m Knickpoints lines are indicated. Box shows extent of Figure 11. Area of Figure 11 Knickpoints not projected We analyze details of landscape morphology and Major Fault available constraints on erosion rates to test the vi- ability of each hypothesis on geomorphic grounds. ash Cli s Landscape morphology alone is not diagnostic Squaw Hidden of the cause or mode of canyon formation. Nar- and W row, steep-walled canyons incised into low-relief Gr plateau surfaces can form in response to either ash trough Andrus Pigeon Parashant (1) an acceleration in main-stem river incision (as

and W implied in the 17 Ma and 6 Ma models) or (2) ap- Gr proximately steady incision into a subhorizontal Shivwits Plateau stratigraphic succession with weak, easily eroded rocks or sediments overlying a notably stronger, Pearce erosionally resistant package of rocks (which could Burnt be consistent with the 70 Ma model). In the latter Twin Springs case, the low-relief erosion surface surrounding BatCave Dry ault the canyon forms as weak rocks are eroded along San Surprise up PlateauTincanebits the contact between weak and strong rocks (the 36 N 36 N surrounding bench postdates rather than predates canyon incision) and canyon formation need not and Wash F Colorado ault Gr reflect an increase in the rate of relative base-level

270L ane F fall (Fig. 3). Both scenarios involve relief produc- Cave Twin Point tion resulting from more rapid erosion within the River Separation R Hurric canyon than on the surrounding low-relief bench; GrapevineWGC Quartermaster 258 L SeparationW Separation in one case the erosion rate within the canyon in- Je creases in response to base-level fall, in the other Jackson case the erosion rate in the surrounding landscape tion 220R decreases in response to the formation of a litho- Fig. 8Horse Flat Separation Hill Cross-secA’ Spencer Canyon 231R 248L 225R logically controlled erosional bench. Consequently, line A- Clay Tank Gneiss the cause and timing of canyon formation must Hualapai Plateau be considered separately. It is fortunate that, for either mode of canyon formation (accelerated inci- Gr ion line Bridge 234L and Wash Fa ct sion versus incision into stronger rocks), there are Meriwhitica 8 eed Cross-se Fig. Hindu CanyonHindu Diamond clear differences in the expected evolution of both B-B’ Milkw canyon side walls and tributary valley profiles that

ult will be diagnostic of whether incision of the main- Canyon Spencer Blue Mountain Milkweed Spencer Peach stem valley was early (e.g. 70 Ma) and persisted to Springs L recent times, or whether the landscape has under- 10015 0 km Peach Springs gone a significant period of reduced rate of base- level fall, or even stability, after canyon formation 114 W (as required by the 70 Ma hypothesis).

Approach and Scope

We do not address the incision history of all of the Grand Canyon, but rather focus on the WGC west of the Hurricane fault (Fig. 1). In our study area we determine (1) if the WGC is more consistent with an increase in incision rate or simply a lithologic control on landscape morphology; (2) how much relief production is recorded by landscape morphology; and (3) the range of plausible incision ages. The first two goals encompass determining the cause of canyon formation and the amount of relief production involved. Both are accomplished through an analysis of two-dimensional landform morphology (topographic cross sections, analysis of modern tributary river profiles, and reconstruction of pre-incision river profiles) and a qualitative evaluation of the correlation between rock strength and landscape morphology. As shown here, details of tributary canyon morphology can be used to differentiate between models allowing sustained river incision to present (e.g., the 17 Ma and 6 Ma models) and models requiring negligible incision following initial canyon cutting (the 70 Ma model). Constraining the age of the canyon is in- dependently accomplished through (1) a quantitative comparison between canyon walls in the WGC and the escarpment along the Grand Wash fault (GWF; most active 18–12 Ma), and (2) a compilation of incision rates over a range of time scales in tributaries and on the surrounding plateau. Given the common climate, lithology, and base level, our comparative morphological analysis can readily gauge the relative timing of incision of the WGC and its tributaries and relief production related to slip on the GWF.

Background and Methods of River Profile Analysis

We employ river profile analysis and evaluate the relations between landscape morphology and geology to achieve our goals. United States Geo- logical Survey (USGS) 30 m resolution digital ele- Figure 2. Photographs taken from Twin Point overlook. A) View to the southeast, showing the Shivwitz Plateau escarpment above the Sanup Plateau (Photo credit: Rich Rudow). B) View to the south, showing the Sanup Plateau in the foreground and the Hualapai vation models (DEMs) are used for all topographic Plateau in the background. Surprise and Spencer canyons are prominent recesses in the plateaux (Photo credit: Rich Rudow). analyses. Pioneered and advanced by Hack (1957,

Figure 3. (A) Canyon formation (local-relief increase, Dz) associated with exposure of a subhorizontal layer of stronger rock (gray) under steady 2.5 main-stem incision (100 m/m.y.). (Stream power model simulation: ABSteady main-stem incision Accelerated main-stem incision blue—initial profile for steady-state 2.0 incision in weak rocks; green—inter- mediate time steps; magenta—final 1.5 Initia Initia time step of simulation). Ec—can- l l yon erosion rate; E —surrounding E weaker rock weaker rock b h Eh bench erosion rate; E — headwater Final Final h 1.0 catchments erosion rate; t—time. (B) Following an increase in main-stem elevation (km ) Eb ∆z Eb stronger rock Ec stronger rock incision. Model illustrates tributary 0.5 ∆z response to incision of main stem ∆z = (E - E )∆t ∆z = (E - E )∆t E (right edge). (C and D) The resulting c b c b c 0 erosion rate patterns. In both scenar- 300 ios canyon relief is set by the differ- erosion rate in penultimate time step Ec ence between E and E , times the c b 200 C D time since exposure of the harder rock layer or acceleration of incision Eh Ec Eh ( t). Note that the slight steepening 100 D Eb just above the main knickpoint in B Eb 0 and associated spike in erosion rate erosion rate (m/Ma) 0 10 20 30 40 0 10 20 30 40 50 in D reflect the fact that in the simulation the acceleration of incision pre- distance (km) distance (km) cedes exposure of the stronger rock. The main knickpoint is lithologic in nature in both cases.

1975), analysis of river profiles can provide signif- break knickpoints that separate reaches of distinct sion rates, hard rocks, low erosivity climates (few icant insight into the history of relative base-level steepness but similar concavity, or (2) channels large floods), and coarse bed material (e.g., Kirby fall in a region. As the methods and foundational with smooth profiles but generally high concavity and Whipple, 2012). Convex-up knickpoints are conceptual background are not as well known as (q > 0.6). Segmented channel profiles marked by thus associated with either a temporal increase in more standard analyses of river incision rates from slope-break knickpoints are expected to form in re- the rate of relative base-level fall or a downstream dated terraces, mean catchment hillslope gradi- sponse to temporal changes in river incision rate or increase in rock strength, as might be caused by ents, or topographic relief, we include here a brief spatial patterns in rock uplift rate, substrate proper- incision into subhorizontal stratigraphy with higher primer tailored to aspects of river profile evolution ties, or climatic conditions (e.g., Kirby and Whipple, average strength in lower rock layers. Concave-up that have diagnostic power. (For more detailed re- 2012; Whittaker, 2012; Lague, 2014). Because the knickpoints, conversely, are associated with either views of river profile evolution patterns and meth- concavity index of discrete segments varies little a decrease in the rate of relative base-level fall or a ods of profile analysis, see Wobus et al., 2006; Kirby and channel steepness and concavity determined downstream decrease in rock strength. Slope-break and Whipple, 2012; Whittaker, 2012; Lague, 2014.) from river profiles are strongly correlated, a nor- knickpoints associated with temporal changes in

River channels tend toward smooth concave-up malized channel steepness index, ksn, is used to the rate of base-level fall are expected to occur river profiles that are well described by Flint’s Law quantify the relative steepness of channels (e.g., at nearly constant elevations in a catchment sub- (Flint, 1974; Hack, 1957): Wobus et al., 2006): jected to uniform relative base-level fall (Niemann et al., 2001; Wobus et al., 2006). Conversely, knick-

−θ −θref Sk= sA , (1) Sk= snA , (2) points associated with rock strength controls are expected to coincide with lithologic contacts.

where S is local channel gradient, A is upstream where qref is a reference concavity often assumed We include a complementary analysis useful area (a proxy for water and sediment discharge), to be ~0.5 (here we use 0.45 for computing re- for visualizing perturbations to the long profile

ks is the channel steepness index, and q is the con- gional maps of ksn patterns, in keeping with most known as the integral method (Fig. 4C; Harkins et

cavity index (usually between 0.4 and 0.6) (Whip- published estimates of ksn). Typically, as here, the al., 2007; Perron and Royden, 2013). The integration

ple and Tucker, 2002). The most common deviations assigned qref represents local determinations of of equation two yields:

from this expected equilibrium form are either (1) the concavity of well-adjusted channel segments. x channels broken into segments marked by slope- High values of k are associated with rapid inci- zx = k A x ′ −θref dx ′ ≡ k χ x , (3) sn ( ) sn ∫ ( ) sn ( ) 0

Je Canyon 2000 2500

A Projection C x Reconstructed pr 1800 0.45 anchor oles z(x) k A(x )− dx k (x) = sn ∫ ′ ′ ≡ sn χ 2000

erosion Elevation (m) Projec 1600 0 ted Proles projection 3 1500 3 1 ref 1400 1 Muav, Temple Butte and θ = 0.4 (m) Elevation Redwall limestones 2 2 1200 slope = tion 1000 ksn = 6 .00+/- 0.83 k sn

incision 1000 500 800 relief produc Height =1148 +/-20 m 600 0 20 15 10 5 0 (lake-induced level) 400 χ(x) Colorado River (314m) 200 1 knickpoint 2 lithologically steepened 0 3 oversteepened drawdown 60 50 40 30 20 10 Distance (km) Drainage Area (m2) 110² 10⁴10⁶ 10⁸ 1 B 2

0.1 Figure 4. River profile reconstruction method illustrated for Jeff Canyon (see Fig. 1 for

location). (A) Channel profile and reconstructed pre-incision channel profile projec- Slope

tions, with uncertainty bounds. Note that qref (see text) is 0.4, 0.45, or 0.5 (see Table 2). Regression of Flint’s Law 3 (B) Log slope–log area diagram illustrating the dramatic slope-break knickpoint and -θ the regression used to characterize the relict upstream channel segment. Note the S=ksA single slope-break knickpoint, and that the channel remains steep to the confluence.

See text for variables. (C) Elevation versus c; slope of the line is ksn (see text). The highest k is closely associated with thick limestone packages. Normalized Regression 1 0.01 sn

ksn = 6.00 +/- 0.83 ks = 14.2 θref = 0.4 θ = 0.47

0.001

where c(x) is determined directly from numerical by incision rate, climate, and rock strength. Litho- (e.g., the 70 Ma model), even in the presence of integration of drainage area data. Any segments logic effects could be overprinted on this pattern, variable rock strength. along the channel profile that are well described potentially inducing segmented profiles below the Figure 5 also illustrates how reconstruction of

by qref will be straight lines on a plot of elevation canyon rim with higher ksn through stronger rock pre-incision river profiles (profiles prior to either

versus c (a chi plot; Fig. 4C), with slope equal to ksn. layers. Substantial reduction or cessation of inci- main-stem incision into harder rocks or at an accel- Figure 5 illustrates how canyon side walls sion (scenario 2) triggers a progressive relaxation erated rate) can be used to quantify the amount of and tributary channel profiles can be expected to of initially steep canyon walls and tributaries even relief production, and, if erosional lowering of the evolve as a function of different base-level histories as slope-break knickpoints defining the canyon rim surrounding landscape can be quantified, the total following the onset of canyon incision, assuming continue to migrate upstream. A sustained period amount of main-stem river incision during the pe- for simplicity no strong lithologic control on chan- of postincision base-level stability (or significantly riod of relief production. The method involved was nel steepness below the canyon rim (the canyon reduced base-level-fall rate) is expected to mani- used successfully by Schoenbohm et al. (2004), rim may be either lithologically controlled or reflect fest in tributary channel profiles as a distinct flat- Harkins et al. (2007), and DiBiase et al. (2015), and an acceleration in main-stem incision rate in either tening of downstream reaches (below concave-up is illustrated in Figure 4 for the example of Jeff case [Fig. 3]; canyon morphology is not diagnostic knickpoints) that can be anticipated to be most Canyon on the south rim of the WGC (location in of the underlying cause of canyon formation): (1) pronounced in larger tributaries (see Gasparini and Fig. 1). In our analysis, profile reconstruction and ongoing, roughly steady incision (plausibly consis- Whipple, 2014), to be mimicked in the morphology projection differ by an estimate of erosional low- tent with both 17 Ma and 6 Ma models), and (2) of surrounding hillslopes, and to not coincide with ering of the channel profile upstream of the slope- early incision followed by a substantial reduction lithologic layering. Thus scenarios involving re- break knickpoint over the period of interest (Fig. 4). or even cessation of incision (consistent with the 70 cently active, quasi-steady incision can be readily Projection begins with locating the major slope- Ma model). Ongoing incision (scenario 1) maintains distinguished from those with base-level stability break knickpoints on the long profile. Upstream steep hillslopes and channels, with morphology set over 106–108 yr time scales after canyon formation of many knickpoints, subtly oversteepened zones

Figure 5. Schematic illustration of landscape evolution associated with canyon A formation. Upper panel (scenario 1) shows landform evolution during active (assumed steady for simplicity) incision. Initial inter uve Convex-up knickpoints mark the canyon Initial channel rim. Lower panel (scenario 2) shows 1 evolution after cessation of incision; 1 1 concave-up knickpoints form as topography relaxes and advance upstream at a constant vertical rate (greater hor- 1 convex knickpoint izontal distances on larger streams). River profiles are in black, canyon side 2 concave knickpoint walls and interfluves are in gray. Later time steps are shown in thicker lines (each panel shows 4 time steps). Initial condition is shown as dashed gray line B (interfluve) and dotted black line (channel). First time step in lower panel is the e final time step from the upper panel. For -incision rat Paleo clarity, only initial and final time steps nal inte are shown for the interfluve, channel, 1 1 and reconstructed channel projection r A 1 associated with the low-relief erosion uve from surface. Arrows show net observed re- nal prole from lief production. Total incision is larger as this includes the slow lowering of A Incision the surrounding low-relief landscape Relief 2 (distance between final dashed channel Production 2 reconstruction and the initial channel profile shown as a dotted line).

truncate well-graded upstream stretches and are readily recognized on profiles and slope-area dia- grams (e.g., Fig. 4). Such oversteepened reaches TABLE 1. SUMMARY OF TOPOGRAPHIC ANALYSIS OF WESTERN GRAND CANYON TRIBUTARY STREAMS are thought to result from stream flow accelera- Related to tion and increased erosion rate that propagates k Stream name Hualapai Projection represents Rim RM k sn s θ (θ = 0.45) upstream, sometimes tens of kilometers, over time Plateau (e.g., Haviv et al., 2006) and are not representative surface? of pre-incision river profiles. The upstream limit 220R no High concavity/ks North220 560.00.7327.1 of these oversteepened reaches is selected as the 225R no High concavity/ks North225 236.00.5390.9 anchor point for profile projections. The average 231R no High concavity/ks North231 58000000.01.4 95.2 channel steepness and concavity of well-graded 234L yes Esplanade, Hualapai Plateau South234 56.0 0.53 15.8 sections upstream of the anchor point are found 248L yes Esplanade, Hualapai Plateau South248.5 14.2 0.45 11.9 by regression following the methods outlined in 258L yesInsufﬁcient S-A data South258 Wobus et al. (2006) and Kirby and Whipple (2012) 270L yes Esplanade, Hualapai Plateau South270 7.00.4210.3 Andrus no Shivwits North199 59.5 0.54 12.5 (see Table 1). Slope-area data are smoothed using a Bat Cave no Esplanade, Hualapai Plateau North267 26.2 0.47 17.2 250 m window along the profile and a 40 ft (12.192 Blue Mountain no High ksn South226 10.4 0.35 53.9 m) contour interval consistent with USGS eleva- Bridge yesInsufﬁcient S-A data South235 tion source data. Assuming that drainage area and BurntnoShivwits North260 7.50.437.7 channel network geometry have remained invari- Cave yes Esplanade, Hualapai Plateau South275 17.1 0.46 12.4 ant over the time period of interest, and that up- Clay Tank yes Esplanade, Hualapai Plateau South250 5.60.3617.4

stream reaches are reflective of channel form prior Dry no High concavity/ks North265 514.00.5681.3 to canyon formation, the best-fit channel steepness Gneiss yesInsufficient S-A data South236 and concavity upstream of anchor points are used Grapevine WGCnoInsufficient S-A data South281 Hidden no Shivwits, GWTNorth 2863.6 0.34 15.7 in Equation 1 to model the existing upstream pro- Hindu Diamond yesPoor S-A relationshipSouth 226 file and project the downstream profile as a func- Horse Flat yes Esplanade, Hualapai Plateau South253 45.0 0.49 25.7 tion of drainage area. The projection from modern Jacksonyes Esplanade, Hualapai Plateau South257 3.00.369.6 stream data is used as a baseline to which incision Jeff yes Esplanade, Hualapai Plateau South260 16.3 0.47 8.4 estimates can be added to allow reconstruction of Meriwhitica yes Esplanade, Hualapai Plateau South246 2.20.3211.0 the paleostream profile (Fig. 4). Milkweed Spenceryes Esplanade, Hualapai Plateau South246 12.9 0.40 24.1 Uncertainty in the profile reconstruction in- ParashantnoShivwits North199 2.10.3610.3 cludes restoration of the profile based on incision Peach Springs yesPoor S-A relationshipSouth 226 Peach SpringsLyes Insufficient S-A data South226 rate estimates and the projection uncertainty. Pearce no GWTNorth 2811820.00.6859.6 Projection uncertainties are: (1) any unrecognized Pigeon no GWTNorth 28698100.0 0.96 94.3 modification of channel slope above the knick- Quartermasteryes Esplanade, Hualapai Plateau South260 119.00.5720.9 point such that the upper channel segment is not SeparationnoInsufficient S-A data North240 reflective of pre-incision conditions, (2) possible Separation Wno High concavity/ks North240 12200.01.1063.3 changes in drainage area or network geometry, Separation RnoShivwits North240 14.1 0.38 6.2 (3) uncertainty in the channel concavity, and (4) Spenceryes Esplanade, Hualapai Plateau South246 31.5 0.50 11.6 uncertainty in the channel steepness index. Error SquawnoGWT North286 274.00.5555.8 sources 1 and 2 cannot be accurately quantified Surprise no Shivwits North249 161.00.6510.0 Tincanebits no High concavity/k North264 3220.0 0.80 106.0 and may be responsible for outliers. Error sources s Twin Springs–Surprise no Insufficient S-A data North249 3 and 4 are constrained by regression analyses and used to represent the range of uncertainties Note: WGC—Western Grand Canyon; GWT—Grand Wash trough; Esplanade—Esplanade surface; Shivwits— Shivwits Plateau surface; S-A—slope-area; RM—river mile; ks—channel steepness index; ksn—normalized channel in profile reconstructions (Fig. 4). Anchor points steepness index; Θ—concavity index; L—left; R—right; W—west. are at the downstream end of the slope-area data used in the regression. Projection uncertainty is

quantified by fixingq ref close to the regional mean

and using the associated channel steepness un- tion, incision estimates from the projection are aggradation. Because neither the geology nor the certainty. Particular streams require slightly differ- minimum estimates of erosion. We use pre-dam thermochronology supports reentrenchment of a ent concavity index (0.4, 0.45, or 0.5; Table 2), de- Colorado River surface elevations (USGS 1:24,000 previously buried canyon for the WGC, we do not termined by comparing the projection and actual topographic scale maps) when calculating heights consider this scenario further except in discussion profile upstream of the anchor point. If the maxi- for tributaries that enter the Lake Mead reservoir. of the partially reexhumed Laramide-age paleo- mum and minimum projections are not below and canyons on the eastern Hualapai Plateau (Elston

above the headwater, respectively, a different qref and Young, 1991; Young and Brennan, 1974; Young may be used within the uncertainty of the regional RIVER CANYON FORMATION: and Crow, 2014; Young and McKee, 1978). The third

mean and ksn (see Table 2 for qref and ksn of each LITHOLOGY OR BASE LEVEL scenario has been discussed in terms of the evolu- projection). The projection is calculated from the tion of Grand Canyon and river knickpoints (Cook mean and 2σ standard deviation (95% confidence The WGC is a deep, narrow, steep-walled gorge et al., 2009; Pederson and Tressler, 2012), and can

interval) of the channel steepness index, ksn. The inset into a broad low-relief plateau with tributaries produce landforms identical to those in the other projection and reconstruction terminate at the that head on the low-relief surface but are deeply scenarios, although cases with steadily falling ver- confluence with the Colorado River. The differ- incised below abrupt slope-break knickpoints near sus stable base-level conditions following initial ence in elevation between the Colorado River at the main canyon (Figs. 1 and 3–6). This morphology canyon cutting would have different expressions in this point and the projected elevation is reported implies 3 possibilities: (1) recent acceleration of in- tributary channel profiles and spatial incision rate as the height of the projection, an estimate of re- cision, (2) reentrenchment of a previously buried, patterns (Figs. 3 and 5). lief production. The reconstructed height (as op- ancient canyon, or (3) stripping of weak rocks to Hillslope profiles in the Grand Canyon and sur- posed to the projected height) above the river is form a low-relief surface cut on a hard, lithologically rounding Colorado Plateau are replete with classic the incision of the river. The incision magnitude controlled bench surrounding the canyon. The first examples of lithologic control on landform mor- is the projected height plus the restored incision two scenarios could produce identical morpholo- phology with alternating strong cliff-forming and of the upper tributary segment since initiation of gies and would have to be distinguished based on weak slope-forming rock layers (Selby, 1993). The canyon cutting (Fig. 4). Without the reconstruc- geologic evidence for base-level rise and regional topographic expression of different rock layers

TABLE 2. SUMMARY OF TRIBUTARY PROJECTIONS, WESTERN GRAND CANYON Incision magnitude

Analyzed Θref using assumed rate 6 m/m.y.: 6 m/m.y.: Anchor 0.5 0.45 0.4 Relief production 19 Ma 6 Ma Projected Elevation elevation Projection

(m above ksn ksn ksn Projected (m above height 1 0.9 0.8 Stream name Rim sea level) Easting Northing (mm ) (±) (m ) (±) (m ) (±) Θref sea level) Maximum Minimum (m) Maximum Minimum (±) 11436 Milkweed Spencer South 1494 251939 3944233 48.6 1.9 24.1 0.7 11.40 0.30 0.45 1295 1262 1254 948 954 942 9 1062 984 Spencer South 1443 268587 3945967 27.8 3.8 11.6 1.5 5.11 0.94 0.40 1356 1374 1340 1009 1027 993 26 1123 1045 248L South 1500 249329 3966343 24.7 2.2 11.5 0.5 5.99 0.26 0.40 1374 1378 1370 1031 1035 1027 61145 1067 Quartermaster South 1512 241767 3979567 38.4 1.1 20.9 1.1 10.10 0.84 0.45 1369 1377 1362 1056 1063 1048 11 1170 1092 Clay Tank South 1496 246477 3965557 44.4 1.7 17.4 1.0 8.47 0.42 0.40 1391 1396 1386 1050 1054 10457 1164 1086 Meriwhitica South 1551 244229 3956323 27.3 2.2 11.0 1.6 5.90 1.20 0.40 1423 1093 1059 1075 1093 1058 27 1189 1111 Horse Flat South 1531 243387 3969457 52.3 1.1 25.7 0.3 11.00 0.95 0.50 1432 1437 1428 1105 1110 1101 7 12191141 270L South 1481 244109 3982903 20.3 1.3 10.3 0.4 5.01 1.12 0.45 1397 1401 1394 1105 1109 1102 6 1219 1141 Jackson South 1539 244827 3973147 18.7 1.9 9.6 0.7 3.94 3.68 0.40 1453 1473 1429 1134 1155 1111 33 1248 1170 Cave Canyon South 1612 234267 3981877 23.8 2.7 12.4 1.2 5.77 2.83 0.45 1441 1457 1425 1158 1175 1142 25 1272 1194 Jeff South 1605 240537 3972787 14.3 2.8 8.4 1.3 6.00 1.25 0.40 1462 1481 1442 1148 1167 1128 29 1262 1184 234L South 1351 274889 3955573 31.1 2.4 15.8 0.9 7.99 1.61 0.45 1245 1251 1239 858 864 852 9 972 894 Bat Cave North 1350 247559 3999133 33.0 2.4 17.2 1.3 8.94 0.73 0.45 1236 1245 1227 935 944 923 15 1049 971

Note: ksn—normalized channel steepness index; Θref —reference concavity; 0.5, 0.45, and 0.4 are concavity indexes.

2500 A

2000

1500

1000 Elevation (m) Hualapai Plateau Peach Springs area Grand Wash Cli s 500 Shivwits Plateau (small streams) Shivwits Plateau (large streams) 0 0 5 10 15 20 25 30 Chi 2500 B

2000

1500

1000 Peach Springs wash

Elevation (m) Hualapai Plateau Peach Springs area Grand Wash Cli s Shivwits Plateau 500 (small streams) Shivwits Plateau (large streams)

0 0 5 10 15 20 25 30 Chi

Figure 6. (A) Chi (c; see text) versus elevation determined from drainage area data for exemplary channel profiles from each of five categories shown in B. All streams analyzed are included

in Supplemental File 2 (see footnote 2). Linear sections are associated with regions well represented by qref; the slope of the line is ksn (see text). All streams show two dominant regions of

ksn, one within the canyon (at low elevations) and one above the canyon. Smaller streams like 234L and Jeff exhibit anomalously high ksn reaches that are associated with thick sections of erosion-resistant limestone bedrock below the rim of the canyon. (B) Five categories of stream profiles are color coded in the compilation of profiles, with the exemplar streams in A shown in bold. Black—Hualapai Plateau; red—Peach Springs area of Hualapai Plateau; blue—Shivwits and Sanup Plateaus (larger north-side streams); purple—Shivwits and Sanup Plateaus (smaller north-side streams); green—Grand Wash Cliffs. (Supplemental File 2 [see footnote 2] contains chi, c, vs. z for all streams.)

114 W is a great aid in geologic mapping in the region. These familiar hillslope expressions of variable rock strength have been recently corroborated with Pk direct laboratory measurement of rock tensile and compressive strength and associated with varia- tions in river slope and channel width (Pederson and Tressler, 2012). It is ironic that the simple geology of only moderately deformed subhorizontal Tsb sedimentary rocks greatly complicates the geomorphology, making it difficult in much of the Grand Canyon to differentiate between lithologic and base-level controls on canyon morphology. In the 36 N A’ 36 N WGC the Sanup Plateau north of the Colorado appears to be continuous with the Esplanade surface because it generally coincides with the top of the resistant Esplanade Sandstone (Fig. 7). The Sanup Plateau appears to be a lithologically controlled bench associated with the erosional retreat of the Shivwits escarpment along the contact between Pep the easily eroded Hermit Formation shale and the Ph Pk resistant Esplanade Sandstone (Fig. 8; Lucchitta, 1966; Young and Crow, 2014). This configuration and the prevalence of cliff-forming limestones of B’ the Redwall, Temple Butte, and Muav Limestones

Pt and Proterozoic basement exposed in canyon walls are suggestive of, but not necessarily diagnostic of, Mr lithologic control of the narrow, steep-walled WGC. It is plausible that the morphology of the WGC re- A Dtb cords an accelerated river incision rate that is coin- cidental with incision below the level of the Espla- Cm nade Sandstone on the Sanup Plateau. Fortunately, Laramide-age deformation in the area of the WGC Cba lPMs created a setting that allows us to resolve this. Inspection of the geology, geomorphology, and Cenozoic stratigraphy of the Hualapai Plateau

Ct B

Figure 7. Geologic map of Hualapai Plateau and surrounding area by Billingsley et al. (2006; used and modified by permission). Cross-section lines in Figure 7 are shown. Map units: Ct—Tapeats Sandstone; Cba—Bright Angel Shale; Cm—Muav QTg Limestone; Dtb—Temple Butte Limestone; Mr—Redwall Lime- Tv stone; IPMs—lower Supai Group; Pep—Esplanade Sandstone; Ph—Hermit Formation; Pt—Toroweap Formation; Pc—Co- 10 5 010 km conino Sandstone; Pk—Kaibab Formation; QTg—Quaternary Ts1 and Tertiary local gravel; Ts1—Tertiary sediments, i.e., Music Mountain Formation, Buck and Doe Conglomerate; Tsb—Ter- 114 W tiary basalt; Tv—Tertiary volcanics. Red circles are knickpoint anchors, and black circles are unprojected knickpoints.

Figure 8. Geologic cross sections (A–A′ is from U.S. Geological Survey [USGS] map data [Billingsley and Wellmeyer, 2006]; B–B′ is modified from section B–B′ in Billingsley et al. [2006; used and modified by permission]). Modifications relate to projecting nearby outcrops of late Tertiary sediments and volcanics into the sections and illustrating the location, incision depth, and depth of fill associated with the Hindu Canyon paleochannel. Inset enhances vertical exaggeration (VE) of the angular unconformity. Units: Ct—Tapeats Sandstone; Cba—Bright Angel Shale; Cm—Muav Limestone; Dtb—Temple Angular unconformity Butte Limestone; Mr—Redwall Limestone; IPMs—lower Supai Group; Pep—Esplanade Sandstone; Ph—Hermit Formation; Pt—Toroweap Formation; Pc—Coconino Sandstone; Pk—Kaibab Formation; Ts1—Tertiary sediments (i.e., Music Mountain Formation, Buck and Doe Conglomerate); Tsb—Tertiary basalt; QTg—Quaternary and Tertiary local gravel. Q—Quaternary landslide deposits; Qs—Quaternary stream-channel deposits; Tv—Tertiary Volcanic rock; Ms—Mississippian Surprise Can- yon Fm.; Xu—Paleoproterozoic undifferentiated crystalline rocks; Xv—Paleoproterozoic volcanic rock. Enhanced VE A A’ 2500 Hualapai Plateau Sanup Plateau (Esplanade) Shivwits 2500 Plateau 2000 Erosion Surface Pk 2000 Tv Tg Tv QTg Tv QTg Tv QTgTv Tv QTg Tv QTg Tv Ms Pt Ms Qv Pep 1500 Dtb Mr Pep Ph 1500 Cm Dtb Mr Ms Pep Mr Ms Ms Elevation (m) Cba Cm Dtb Ms Dtb Ms Mr Mr 1000 Ct Cba Cm 1000 Cm Dtb Dtb Ct Cba Cba Cm Cm 500 Ct Cba Cba 500 Yg Ct 0 Ct Ct 0 0 5 10 15 20 25 30 35 40 45 Distance (km) B B’ Shivwits 2500 on 2500 Hualapai Plateau Sanup Plateau Plateau Pk

Separation Hill Pt 2000 Tv Dtb (Esplanade) 2000 Tv Dtb Tv Qv Tv Tv Tv Qv Tv Tv Mr MERIWHITICA MONOCLINE Ts1 Pep

Dtb Hindu Paleo-channel

ado River Ph Cm Ms Pc Pep 1500 Cba Mr Ms 1500 Ms Spencer Ca ny Ms Ms Color Ms Elevation (m) Ct Dtb 1000 Ct Ql Ql Mr 1000 Cm Dtb Qs Qs ? Cba Cm 500 ? 500 Xv ? Xv Xu ? Ct 0 Xu 0 0 5 10 15 20 Xgr 25 30 35 All structural trends and patterns are schematic VERTICAL EXAGGERATION X2 Distance (km)

on the south side of the Colorado River (Figs. 1 profiles (e.g., Johnson et al., 2009) that have the AMOUNT OF RIVER INCISION AND and 7) reveals that it is not a lithologically con- same steepness as segments in the Bright Angel RELIEF PRODUCTION trolled bench, but rather a product of a long period Shale on both sides of the canyon (Fig. 6). How- of erosion under a relatively stable base level. The ever, these remote observations require significant The amount of river incision is the relative critical observation is that the Hualapai Plateau is field testing. Additional complexities reflect young change in elevation of the main-stem river. Relief beveled indiscriminately across rock types rang- travertine dams in some tributaries (e.g., near the production associated with canyon cutting differs ing from Supai Group shales to Muav Limestone, outlet of Jeff and Quartermaster Canyons, and at from total river incision; it is the difference between consistent with arguments by Young and Brennan 800 m elevation in Meriwhitica canyon; Figs. 1, 4, river incision above a knickpoint and river incision (1974), Young and McKee (1978), and Young and and 6). Variable elevation ranges of the prominent below that knickpoint (Fig. 3). Relief production

Crow (2014) that this is an ancient and long-lived high ksn segments well expressed in Figure 6 are can be measured as the difference in elevation of a low-relief surface (Figs. 7 and 8). It is important inconsistent with temporal changes in the rate of projected stream and the modern stream (the pro- that the Hualapai Plateau is a low-relief erosional base-level fall following the initiation of canyon jected stream height; Table 2). A simple measure surface beveled to the same elevation as the Sa- cutting (Fig. 5) (e.g., Wobus et al., 2006; Kirby and of the vertical drop from canyon rim to river level nup Plateau; the two plateaus are part of a com- Whipple, 2012) and coincide with outcrop patterns will overestimate relief production because the rim mon surface incised by the Colorado River and its of the sequence of Paleozoic limestones. However, is higher in elevation than the upper streams in- tributaries, suggesting a common paleo–base-level the lack of similar lithologic control on profiles of cised into the low-relief plateau. Determining the control overprinted by lithologically controlled re- channels draining the Shivwits and Sanup Plateaus paleoelevation of the main-stem river involves two treat of the Shivwits Plateau escarpment to the suggests that canyon formation and the prominent steps. First, relief production is measured from north. Thus, we infer that the morphology of the slope-break knickpoints that define the canyon rim the elevation projected from relict upper channel WGC reflects a period of accelerated incision into a cannot be attributed to lithology. The surrounding segments above oversteepened zones down to the preexisting low-relief landscape (outlined in more low-relief surface (Hualapai and Sanup Plateaus) tributary confluence with the main-stem river (Fig. detail in the following). extends from the so-called Esplanade surface (the 5). Second, estimates of the lowering rate of the Tributary channel profiles have all the hallmarks top of this sandstone) to cut across the Muav, Tem- surrounding low-relief landscape can be multiplied of a disequilibrium landscape, i.e., deeply incised, ple Butte, and Redwall Limestones, and the shales by proposed or dated surface age to restore the 0 0 10 10

−1 −1 10 10

−2 −2 steep-walled canyons in the vicinity of the main and sandstones of the lower Supai Group and Es- lowering of relict channels over the time period of 10 220R 10 Andrus gradient gradient

−3 −3 10 10

= 0.4 θref −4 −4 stem that are marked by significant slope-break planade Sandstone (Figs. 7 and 8). Far from form- interest (Figs. 4 and 5). The sum of these provides 10 10 3 4 5 6 7 8 9 10 11 3 4 5 6 7 8 9 10 11 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 2 drainage area (m ) drainage area (m2) 0 0 10 10

−1 knickpoints where they cross the canyon rim (Figs. ing lithologically controlled benches, the resistant an estimate of total river incision, such as might be −1 10 10

−2 −2 Bat Cave 10 225R 10 gradien t gradient

−3 −3 1 and 5–7). Either an acceleration of incision rate Paleozoic limestones are beveled to the same level recorded by thermochronometers. 10 10

= 0.4 θ = 0.4 θref −4 ref −4 10 10 3 4 5 6 7 8 9 10 11 3 4 5 6 7 8 9 10 11 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 2 drainage area (m2) drainage area (m ) or a lithologically induced pattern of differential as the much weaker units of the lower Supai Group. Figure 9 summarizes the results of the profile 0 0 10 10

−1 −1 10 10 erosion could be the driver behind the transient Long-term stability of erosional base level at the el- projections, i.e., relief production. We analyzed 38 −2 231R −2 10 10 Blue Mountain gradient gradien t

−3 −3 10 10

θ = 0.4 ref θ = 0.45 evolution of landscape morphology implied by evation of the Hualapai Plateau (~1400 m elevation streams (slope area data available in Supplemen- −4 −4 ref 10 10 3 4 5 6 7 8 9 10 11 3 4 5 6 7 8 9 10 11 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 2 drainage area (m ) drainage area m( 2) 1 0 0 10 10 this topography (Fig. 3). In Figure 6, downstream at present) is required for such effective erosional tal File 1 ) to determine if projections to paleo–base

−1 −1 10 10

−2 −2 10 234L 10 Bridge gradient gradient of major knickpoints some tributaries show strong beveling of erosionally resistant cliff-forming lime- level could be made. We attempt profile projec-

−3 −3 10 10

= 0.45 θref θ = 0.45 −4 −4 ref 10 10 3 4 5 6 7 8 9 10 11 3 4 5 6 7 8 9 10 11 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 2 lithologic control of channel steepness, while oth- stones (e.g., Baldwin et al., 2003). These morpho- tions only where we have grounds to believe the drainage area (m ) drainage area (m2)

0 0 10 10

−1 −1 10 10 ers do not. Smaller tributaries and those lacking a logical observations are consistent with interpre- headwater reach of the profile reflects pre-incision

−2 −2 10 248L 10 Burnt gradient gradient

−3 −3 10 10 significant source of gravel in headwater reaches tation of landscape evolution based on the nature, channel morphology plausibly graded to paleo– θ = 0.45 −4 −4 ref 10 10 3 4 5 6 7 8 9 10 11 3 4 5 6 7 8 9 10 11 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 drainage area (m2) drainage area (m2)

0 0 10 10 appear to be detachment limited (e.g., Johnson distribution, provenance, and paleoflow directions base level. Thus channels that clearly express lith-

−1 −1 10 10

−2 −2 10 258L 10 Cave et al., 2009) and have segmented channel profiles of Tertiary sediments preserved on the Hualapai ologic controls above the slope-break knickpoints gradient gradient

−3 −3 10 10

θ = 0.45 0.9 θ = 0.45 ref −4 ref −4 10 10 3 4 5 6 7 8 9 10 11 3 4 5 6 7 8 9 10 11 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 with k > 400 m through the Paleozoic limestones Plateau (e.g., Elston and Young, 1991; Young and that define the canyon rim are excluded. Con- 2 drainage area (m2) drainage area (m ) sn 0 0 10 10

−1 −1 10 10 (Redwall, Temple Butte, and Muav Limestones), Brennan, 1974; Young and Crow, 2014; Young and versely, streams that flow on the Tertiary angular

−2 −2 10 270L 10 ClayTank gradient gradient 0.9 −3 −3 10 10 moderate k (~150–250 m ) through the Bright McKee, 1978). The morphological observations unconformity and/or Tertiary sedimentary deposits θ = 0.45 ref sn −4 −4 10 10 3 4 5 6 7 8 9 10 11 3 4 5 6 7 8 9 10 11 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 2 drainage area (m ) drainage area (m2) Angel Shale, and steepen somewhat where cutting summarized here, however, speak primarily to the are ideal (Figs. 7 and 8). Moreover, if these head- through basement rocks. Larger tributaries and cause of canyon formation (accelerated incision water stream segments preserve relict conditions 1Supplemental File 1. All stream slope-area data, those with significant headwater gravel sources dominating over lithologic controls), not to its tim- reflecting a stable base level for an extended pe- plotted and compiled for inspection. Please visit (such as most of the streams draining the Shivwits ing. However, concave-up knickpoints expected to riod they should all have similar slope-area rela- http://dx.doi.org/10.1130/GES01131.S1 or the full-text article on www.gsapubs.org to view Supplemental and Sanup Plateaus) appear to be transport lim- record any extended period of post-incision base- tionships. We identify 12 streams most suitable File 1. ited, showing no lithologic control on channel level stability are not seen in the WGC landscape. for analysis. In Table 1, these 12 representative

1300 In summary, although only 13 streams could be reliably projected and all but one rise on the 1200 Figure 9. Reconstruction of the Hualapai Plateau, this drainage-projection anal- height of the pre-incision surface 1100 above the Colorado River from ysis makes it clear that carving of the WGC into downstream projections of rel- the preserved older, low-relief landscape of the

tion (m) 1000 ict channel profiles upstream of Hualapai and Sanup Plateaus involved ~1050 m abrupt slope-break knickpoints 900 of relief production since incision rate increased oduc 234L that surround the western Grand Pr 800 Canyon, following the method during formation of the WGC. For comparison, illustrated in Figure 4 and show- we infer >700 m of incision from the base of the 700 ing uncertainties associated with Relief characterization of upstream Tertiary sediments (~1120 m elevation) exposed 600 channel form (concavity and in the axes of Tertiary paleocanyons in the Peach 280 270 260 250 240 230 220 steepness indices). Springs area. Considering the ~400 m thickness River Miles from Lees Ferry of the Tertiary sediments (Young and Crow, 2014) gives ~1100 m relief production, compared to our 1050 m for relief production. Previously noted Peach Springs area channels (234L) are modestly streams (red circles in Fig. 1) have a mean con- GWT, a now subareal region that was paleo–Lake inset into the paleosurface (see Fig. 6 and Supple- cavity of 0.44 ± 0.02 (1 standard error) and mean Hualapai until ca. 6 Ma (Faulds et al., 2001; Luc- mental File 22), suggesting that fluvial erosion in channel steepness of 27.5 ± 9.6 (1 standard error), a chitta, 1979), and given the certainty of drainage the vicinity of the paleocanyons took advantage of narrow range of channel profile morphology. Our network disruptions could not be projected with topographic lows or possibly slightly higher ero- analysis of surrounding streams yielded 1 stream, confidence. sion rates through the softer rock. informally called Bat Cave, that matches slope- The lithologic complications partly reflect the Flowers and Farley (2012) and Wernicke (2011) area metrics and may represent the same paleo- retreating Shivwits Plateau escarpment (Fig. 8), interpreted low-temperature thermochronometric surface but is not associated with independent which significantly complicates interpretation of data from river-level samples as requiring 70% to 2500

2000 geologic evidence of a stable base level. These 13 longitudinal profiles of drainages on the north 80% of canyon incision since 70 Ma. Although no Hualapai Plateau ) 1500 projectable streams yielded an estimate of relief side of the Colorado River. The hard (Kaibab and more than 70 m of incision is recorded directly by

1000 Elevation (m production in the WGC of 1050 ± 30 m (1 standard Toroweap Formations) over soft (Hermit Forma- the interaction of the river and basaltic lava flows 500

0 error) (Fig. 9). As expected, this is a minimum rel- tion) lithology of the Shivwits escarpment implies for the past 0.625 m.y. (Karlstrom et al., 2007) 0 5 10 15 20 25 30 Chi 2500 ative to the simple estimate of the elevation differ- that pre-incision channel profiles likely had litho- and <300 m is inferred from groundwater-table

2000

) ence between the Colorado River (~350 m) and the logically controlled knickpoints perched above the elevation inferred from speleothems since 3.87 1500

1000

Elevation (m lip of the Hualapai Plateau (>1400 m). base level of the Sanup and Hualapai Plateaus. Ma (Polyak et al., 2008), our result of ~1050 m of 500 The stream named 234L is located near the Projection of the 13 streams thought to represent relief production in the late Tertiary suggests that

0 0 5 10 15 20 25 30 Chi remnants of the Tertiary paleocanyons and projects the pre-incision topography is contingent on the the understanding and interpretation of thermo- 2500

2000 Sanup/Shivwits Plateaus lower than the other streams. Our criteria eliminate assumption that the Shivwits escarpment was chronometric data (Flowers and Farley, 2012) is in- ) 1500 the rest of the streams in the Peach Springs area already north of these streams when their head- complete, as suggested by Fox and Shuster (2014) 1000 Elevation (m

500 for projection due to a lack of reliable stretches of water reaches equilibrated to their current form. on the basis of models of thermal evolution and

0 0 5 10 15 20 25 30 slope-area data, but the knickpoint elevations and The Tertiary sediments and beveled unconformity kinetics of damage annealing and helium diffusion. Chi 2500 qualitative interpretation of these river profiles on the Hualapai Plateau support this assumption 2000 All Streams ) 1500 suggest that all the streams in this area are con- directly in the immediate area of most of these

1000 Elevation (m sistent with the relatively low projection of 234L streams. However, those streams that now exist Plateau Lowering Rate 500

0 (100–200 m lower than the rest of the Hualapai Pla- in the area between the modern Shivwits escarp- 0 5 10 15 20 25 30 Chi teau streams; Fig. 6). ment and the Tertiary sediments on the south rim We derive estimates of the lowering rate of Of the 26 other streams (Table 2), headwater are likely to have evolved from steep reaches the plateau from volcanic deposits (Wenrich et 2Supplemental File 2. All stream elevation-chi data, reaches of 10 were too short to allow reliable pro- draining off the retreating Shivwits escarpment. al., 1995; Young and Brennan, 1974) with incision plotted and compiled for inspection. Please visit jection. The remaining 16 either showed evidence Such streams would have complex histories not amounts estimated as the relief between tops of http://dx.doi.org/10.1130/GES01131.S2 or the full-text article on www.gsapubs.org to view Supplemental of lithologic complications (13 of 16) or were as- conducive to simple profile projection and recon- hills capped by dated basalts to closest channel File 2. sociated with drainages that traverse the modern struction. bottoms. Height measured from the tops of flows

to channel bottoms results in rates that are local here. In addition, the background erosion rate of the Morphological Analysis maxima. Ages range from 19 to 15 Ma, yielding plateau surface is the only direct constraint on the a lowering rate on the low-relief surface of 2–11 pre-canyon erosion rates in the region. At 6 m/m.y., Details of tributary river profile forms can be m/m.y., with a mean of ~6 m/m.y. (Table 3). The total Colorado River incision would be 36 m greater diagnostic, at least in general terms, of the history amount that incision estimates differ from relief than relief production if canyon incision began at 6 of base-level fall. Similar examples of this type of production requires an estimate of the time of on- Ma, and 100 m greater if canyon incision began at analysis were discussed in, for example, Whittaker set of incision (estimates in Table 3). 17 Ma. The latter estimate is a maximum given the (2012), Kirby and Whipple (2012), and Gallen et al. preserved thickness (typically <100 m) of aggrada- (2013). Comparison of all tributaries to the WGC tion that remains above low-relief modern stream (Figs. 4–7 and 9; Supplemental File 1 [see foot- Incision Magnitude versus Relief Production segments (i.e., above projected streams). Thus our note 1]), with expectation from theory, strongly estimate of total incision since the onset of canyon suggest that there is no indication of the morphol- The three competing hypotheses for the inci- formation ranges between 1050 and 1150 m, based ogy expected to result from a stable base level sion of the WGC suggest incision starting ca. 6 Ma on available measurements. maintained for millions to tens of millions of years or ca. 17 Ma, or virtually complete by 70 Ma. A suc- after canyon incision (see Fig. 5B). Tributary profiles cession of sedimentary deposits from early Tertiary are rather suggestive of relatively steady incision until ca. 19 Ma records net moderate aggradation AGE OF CANYON INCISION over the duration of canyon formation and per- (as much as ~400 m in infilled paleocanyons) on sisting to present (Fig. 5), with variable degrees of the Hualapai Plateau over that interval (Elston Here we develop geomorphologic constraints lithologic control on below-rim channel steepness and Young, 1991; Young, 1979, 2008; Young and on the time scale over which incision of the Colo- (Fig. 6), as recorded by lava flow remnants (Crow Brennan, 1974; Young and Crow, 2014; Young and rado River and its tributaries into the low-relief sur- et al., 2008) and speleothems (Polyak et al., 2008). McKee, 1978; Young and Spamer, 2001). The pla- face defined by the Hualapai and Sanup Plateaus The interpretation of the WGC tributary profile teau lowering rate estimate (~6 m/m.y.) is a mea- in the WGC occurred. We take two approaches to morphology is powerfully enforced by a morpho- sure of the average rate of lowering since the ca. this analysis. The first is based on a comparative logical comparison of the WGC landscape with the 19 Ma cessation of aggradation. It is important to analysis of landscape morphology in the WGC and landscape of the Grand Wash Cliffs (GWC). The his- acknowledge that slow headwaters erosion is not nearby GWT. The second is based on measure- tory of the GWF provides a well-known timing of re- zero, so total incision during canyon cutting will ments and estimates of erosion rates over a range lief production in a setting where both climate and exceed the relief production estimates presented of time scales within and around the WGC. rock strength are nearly identical to the WGC. Slip

TABLE 3. EROSION RATES SUMMARY FOR THE HUALAPAI PLATEAU SURFACE Sample Location or unit Age E1*E2* H1† Erosion rate Minimum or LatitudeLongitude Publication Comment (Ma) (m) (m/m.y.) maximum rate 14-B86 Grapevine Canyon 15.3 1640 1495 145 9.48 maximum35.9147113.9000Wenrich et al. (1995) Height above west side of hill UAKA 88-51 Grapevine Canyon near Buck and Doe Road 15.25 1655 1551 104 6.82 35.9150113.8908Wenrich et al. (1995) Height above east side of hill 8-B86 Separation Hill basalt 19 1495 1400 95 5.00 35.7867113.5972Wenrich et al. (1995) Height above surrounding plateau 12-B86 Grand Pipe Neck basalt 17.4 1420 1350 70 4.02 36.1133113.8860Wenrich et al. (1995) Neck is inset into a valley PED 27 -63 Peach Spring Tuff, Upper Milkweed Canyon 18.3 1536 1340 196 10.71 maximum35.6366113.7034Young and Brennan (1974) PED 27 -63 Plain Tank Flat 18.3 1430 1390 40 2.19 35.6697113.5680Young and Brennan (1974)

H2† (m) 14-B86 Grapevine Canyon 15.3 1640 1600 40 2.61 minimum 35.9147113.9000Wenrich et al. (1995) Height above south side of hill UAKA 88-51 Grapevine Canyon near Buck and Doe Road 15.25 1655 1600 55 3.61 35.9150113.8908Wenrich et al. (1995) Height above north side of hill 8-B86 Separation Hill basalt 19 1495 1440 55 2.89 35.7867113.5972Wenrich et al. (1995) Height above surrounding plateau Note: NAD83 (North American Datum of 1983) points mark area of incision points, in the area of original sample locations. Dating method for all samples was K/Ar. *E1—Sample or deposit elevation; E2—Landscape elevation (channel or interﬂ uve). †H1—Height above nearest stream (above any major knickpoints). H2—Height above nearest interﬂ uve.

Channel Steepness on the GWF is well documented to have begun ca. Area of Figure 11 ksn 18 Ma, with most of its 5.5 km of offset accumulated 0 - 50 by 12 Ma (Bohannon, 1984; Bohannon et al., 1993; 50 - 100 Karlstrom et al., 2010; Lucchitta, 1966, 1972, 1979; Quigley et al., 2010; Umhoefer et al., 2010). Ensu- 100 - 150 ing slip on the GWF was dramatically less, ~300 m, from 12 to 6 Ma. It is possible there has been lim- Squaw Hidden 150 - 200 ited post–6 Ma slip on the GWF. While the history 200 - 250 of the GWF is not fully known and is complicated, ault 250 + relief production is no older than 18 Ma and both Pigeon hillslope and channel morphologies may reflect re-

Wash F Parashant Andrus juvenation by younger episodes of slip and base- level fall associated with the ca. 6 Ma breaching of ash trough Lake Hualapai (Faulds et al., 2001; Lucchitta, 1979) Grand and the subsequent incision through the Hualapai and W Limestone and associated basin fill sediments. Gr We use topographic metrics to determine directly whether incision in the WGC is older, younger, Pearce Shivwits Plateau Burnt or similar in age to incision of the tributaries of the Bat Cave GWT in response to faulting on the GWF and base- Dry Twin Springs Surprise level fall associated with post ca. 6 Ma incision. Trib- San utaries to the GWT exhibit more subtle knickpoints up PlateauTincanebits at the edge of the Sanup Plateau, suggesting a period of slow incision or stable base level after initial relief production, but have nearly identical channel steepness and concavity to north-side tributaries 0.9 270L ault in the WGC (Figs. 6 and 7). The 150–250 m mean Cave Canyon steepness index (Fig. 10) is identical to channel seg- and Wash Cli s Quartermaster Separation R ane F ments not exhibiting lithologic control and strong Gr 258L GrapevineWGC oversteepening where cut on resistant Paleozoic SeparationW Separation Hurric limestones or basement rocks, suggesting a very Je Colorado River Jackson similar timing and rate of base-level fall, but under transport-limited conditions (e.g., Johnson et al., 220R Horse Flat 2009). Like channel steepness, hillslope gradients 248L Clay Tank are a function of erosion rate, rock strength, and 225R 231R Hualapai Plateau Gneiss

Figure 10. Normalized channel steepness index (k ) over gray- Grand Bridge 234L sn scale elevation map with hillshading. Red dots are profile pro- Meriwhitica Hindu Diamond jection anchor points; black dots are unprojected knickpoints. Wash Fault Major slope-break knickpoints (defined as largest visual topographic expression) on tributary rivers are indicated by cool to warm color transitions in channel steepness data. Tributar- Spencer Blue Mountain ies to the western Grand Canyon show similar mean channel Peach Springs L Milkweed Spencer steepness indices and exhibit the profile geometries illustrat- km ed in Figures 4 and 5. Streams draining the Grand Wash Cliffs 10 0 10 show more subdued knickpoints and smaller lithologic expression through the Paleozoic limestone reaches than ob- Peach Springs served elsewhere (see Fig. 4).

Figure 11. Slope map of the western Grand Canyon (WGC) Slope Angle and Grand Wash Cliffs (GWC) areas (30 m resolution, location 0 - 5 shown in Figs. 1 and 10). Although incised through the same rocks and in the same climate, the topographic expression of 5- 10 ¯ the GWCs and associated tributary canyons and side slopes is far more subdued than that of the WGC, indicating notably slower erosion rates, a longer period of topographic relaxation 10 - 15 ault since relief production (known to date to 18–12 Ma for the 15 - 20 GWC), or both. 20 - 30 Squa and Wash F w 30 - 40 Gr climate. Figure 11 shows a composite hillshade and slope map of the GWT and the WGC. In the WGC, 40 - 50 P igeon hillslopes are clearly steep from the rim of the 50 - 60 Hualapai and Sanup Plateaus down to the Colorado 60+ River, particularly where Paleozoic limestones and basement rocks crop out (Fig. 7). Canyon walls have not retreated laterally from rivers edge. However, the escarpment of the GWC has in places retreated 2–3 km from the trace of the GWF and is less steep Grand Wash trough and more subdued (Fig. 12). Average slopes of interfluvial ridges near 270L, Quartermaster, and Horse Flat canyons are 0.76, 0.44, and 0.49, respectively. Pearce Similar ridges along Pearce, Pigeon, and Squaw canyons of the GWT, show average hillslope gradients of 0.14, 0.21, and 0.17, respectively. The topo- Shivwits Plateau graphic comparisons of the WGC to the GWC show that the two regions have had significantly different base-level-fall histories. Minimal fault activity over 12 m.y. has resulted in several kilometers of retreat of the GWC and considerable relaxation of hillslope gradients. Morphologic analysis strongly suggests that relief production in the WGC is younger than Colorado along the GWC, where the bulk of relief production occurred from 18 to 12 Ma. The hypothesis that the

River narrow, steep-walled WGC is ≥70 m.y. old is unten- able. However, the controversy over the timing of formation of the WGC is over a difference of ~600 270L m of incision. Geologic and geomorphic constraints r demand ~1000 m of post–19 Ma incision, while ini- termaste tial thermochronometric interpretations allow for Quar no more than ~400 m of post–70 Ma incision (Flow- ers and Farley, 2012).

Hualapai Plateau Erosion Rate Constraints Horse at Kilometers 10 010 The hypotheses about the timing of WGC incision require different rates of tributary incision and

Figure 12. Comparative interfluvial topo- 2500 graphic profiles in the western Grand Can- yon (WGC) and along the Grand Wash Cliffs (GWC). Topographic section line locations 2000 Pe are shown in Figures 1 and 11. Profiles in Sq the WGC are shown in red and orange and Pi those along the GWC are in blue. Canyon abbreviations: Pe—Pearce; Sq—Squaw;

) 1500 Pi—Pigeon; QM—Quartermaster; HF— QM Horse Flat; 270L—270-mile. Red and orange on (m profiles are WGC interfluve profiles. Blue a HF and purple are Grand Wash Cliffs interfluve Elev 1000 profiles. Profiles are represented at actual elevations, but only shown above the Col- orado River in the WGC or above the trace 270L 500 Grand Wash Fault of the Grand Wash fault (GWF) along the GWC. Interfluves in the WGC show no indication of significant retreat or relaxation Colorado River from threshold slopes since canyon forma- 0 tion. Interfluves along the GWC document 0500010000 1500020000 25000 both significant retreat and significant -re Distance (m) laxation since relief production associated with normal faulting along the GWF.

cliff retreat in order to produce and maintain the the time of WGC basaltic volcanism in the gorge to ence in rates could indicate that part of the incision observed landscape morphology for the duration be consistent with the 70 Ma hypothesis. This rate of the WGC occurred between ca. 17 and ca. 6 Ma in of the different hypothesized incision histories. We is comparable with erosion on the flat plateau, but response to base level fall associated with slip on use several methods to determine average incision all independent measures of canyon incision rate the GWF (Polyak et al., 2008; Young, 2008; Young rates on the Colorado River in the WGC. Separa- are consistently and substantially higher than rates and Crow, 2014). tion Hill basalt (1500 m above river and 19 Ma, Fig. proposed in the hypothesis that a canyon of nearly 1; Wenrich et al., 1995) yields an average incision modern depth existed by 70 Ma. rate of 60 m/m.y. over the past 19 m.y. Quaternary The 17 Ma canyon hypothesis implies that aver- CONCLUSIONS basalt flows along the Colorado River overlie river age incision has occurred at ~65 m/m.y. (~1000 m/17 gravel yielding similar rates of ~70 m/m.y. (Karl- m.y.), implying incision slightly slower on average The primary observation that the Hualapai Pla- strom et al., 2008) over the past 0.625 m.y. Recently than estimates in the past 0.6 m.y. The 6 Ma canyon teau is a low-relief surface cut across numerous determined cosmogenic erosion rates in the WGC hypothesis, however, implies that ~1000 m of inci- rock-types is a strong indication that it reflects yield rates of 61 ± 18 m/m.y. (Nichols et al., 2011) sion has occurred at an average rate of 167 m/m.y. erosional beveling during a long period of base- averaged over the past ~10 k.y. Given the broad (~200 with correct significant figures) over the past 6 level stability. Paleo-base level of the region until at range of time scales of these data, it is possible that m.y. Although this appears to be a mismatch with in- least 17 Ma was therefore directly associated with there are significant rate fluctuations between data dependent constraints (faster than both the average this preserved landscape. The fluvial geomorphic points, but the available evidence implies that the since ca. 17 Ma and rates recorded in the past 0.6 data from headwater portions of Hualapai Plateau incision rate since ca. 0.6 Ma is roughly equal to the and 0.01 m.y.), the difference is consistent with ex- streams is interpreted to be consistent with pres- average incision rate since 19 Ma. pected evolution of the WGC following initiation of ervation of a relatively stable base level from 70 to The “old canyon” model suggests 70%–80% of incision. Integration of the Colorado River across the at least 17 Ma prior to incision of Grand Canyon. Colorado River incision in 70 m.y. (Flowers and Far- GWC would trigger a period of rapid incision that Channel projections primarily off the Hualapai Pla- ley, 2012). Using our estimate of incision magnitude would decrease with time as a knickpoint swept up- teau yield estimates of relief production magnitude of 1100 m, the 70 Ma model suggests at most 400 m stream (Pederson et al., 2002; Cook et al., 2009; Pel- near 1000 m throughout the WGC. of incision since 70 Ma. Basalt flows in the WGC are letier et al., 2009). Available constraints on incision We have tested three hypotheses for the age as much as ~70 m above the Colorado River, so the rate history allows a few million years over which of western Grand Canyon against geomorphic remaining 330 m of allowable incision in the model incision may have been faster than ~160 m/m.y. and constraints. Our data are most consistent with the imply an average incision rate of 4 m/m.y. prior to then slowed to ~70 m/m.y. Alternatively, this differ- 6 Ma model for the timing of canyon cutting. The

70 Ma model requires erosion at an improbably America Bulletin, v. 105, p. 501–520, doi:10.1130 /0016 -7606 Haviv, I., Enzel, Y., Whipple, K.X., Zilberman, E., Stone, J., Matmon, (1993)105<0501:SSATDO>2.3.CO;2. A., and Fifield, L.K., 2006, Amplified erosion above waterfalls low ~4 m/m.y. for tens of millions of years while Cook, K.L., Whipple, K.X., Heimsath, A.M., and Hanks, T.C., 2009, and oversteepened bedrock reaches: Journal of Geophysical supporting significant amounts of relief. The 17 Rapid incision of the Colorado River in Glen Canyon—In- Research, v. 111, F04004, doi:10.1029/2006JF000461. 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However, landscape mor- Crow, R., Karlstrom, K.E., McIntosh, W., Peters, L., and Dunbar, western Arizona, in Reheis, M.C., et al., eds., Late Cenozoic phology indicates that the erosion of the walls of N., 2008, History of Quaternary volcanism and lava dams drainage history of the southwestern Great Basin and lower 40 39 the WGC started considerably more recently than in western Grand Canyon based on lidar analysis, Ar/ Ar Colorado River region: Geologic and biotic perspectives: dating, and field studies: Implications for flow stratigraphy, Geological Society of America Special Paper 439, p. 335– the erosion of the GWC 18–12 m.y. ago. Therefore, timing of volcanic events, and lava dams: Geosphere, v. 4, 353, doi:10.1130/2008.2439(15). of the three hypotheses tested, data and analyses p. 183–206, doi:10.1130/GES00133.1. 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Wegmann, and one anonymous ica Bulletin, v. 127, p. 539–559, doi:10.1130/B31113.1. Research, v. 114, F02014, doi:10.1029/2007JF000862. reviewer for constructive reviews that greatly improved our analy Elston, D.P., and Young, R.A., 1991, Cretaceous-Eocene (Lar- Karlstrom, K.E., Crow, R.S., Peters, L., McIntosh, W., Raucci, J., sis, and the editor and guest editor for their efforts. We would amide) landscape development and Oligocene-Pliocene Crossey, L.J., Umhoefer, P., and Dunbar, N., 2007, 40Ar/39Ar also like to thank Rich Rudow for field assistance, photography, drainage reorganization of transition zone and Colorado and field studies of Quaternary basalts in Grand Canyon and digital enhancement of his photographs published here. Plateau, Arizona: Journal of Geophysical Research, v. 96, and model for carving Grand Canyon: Quantifying the in- And, we would like to thank the National Science Foundation no. 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