Carving Grand Canyon's Inner Gorge

Research Paper

GEOSPHERE Carving Grand Canyon’s inner gorge: A test of steady incision versus rapid knickzone migration GEOSPHERE, v. 14, no. 5 Ryan S. Crow1, Karl E. Karlstrom2, Laura J. Crossey2, Victor J. Polyak2, Yemane Asmerom2, and William C. McIntosh3 1U.S. Geological Survey, 2255 N. Gemini Drive, Flagstaff, Arizona 86001, USA https://doi.org/10.1130/GES01562.1 2Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA 3New Mexico Institute of Mining and Technology, Department of Earth and Environmental Science, Socorro, New Mexico 87801, USA 12 figures; 1 set of supplemental files

CORRESPONDENCE: rcrow@usgs.gov ABSTRACT INTRODUCTION

CITATION: Crow, R.S., Karlstrom, K.E., Crossey, A recent study posits that much of the 240-m-deep inner gorge of Grand Recent efforts to better understand the controls on river profile form and in- L.J., Polyak, V.J., Asmerom, Y., and McIntosh, W.C., 2018, Carving Grand Canyon’s inner gorge: A test Canyon was carved between 500 and 400 ka via passage of a migrating cision of the Colorado River have focused on the processes and timing of river of steady incision versus rapid knickzone migration: knickzone with incision rates of ~1600 m/Ma during that time period; this integration (Blackwelder, 1934; Longwell, 1936; Karlstrom et al., 2008; Polyak et Geosphere, v. 14, no. 5, p. 2140–2156, https://doi.org was based on dating of a ca. 500 ka travertine deposit perched on the rim al., 2008; Young, 2008; Cook et al., 2009; Pelletier, 2010; Hill and Polyak, 2014), /10.1130 /GES01562.1. of the inner gorge, near Hermit Rapid, and a ca. 400 ka travertine drape that the role of preexisting canyons (Flowers et al., 2008; Lee et al., 2011; Wernicke, extends to within 60 m of river level nearby. However, a new U/Th age of 2011; Karlstrom et al., 2014), the effects of tectonics (Pederson et al., 2002; Karl- Science Editor: Raymond M. Russo Associate Editor: Graham D.M. Andrews 517 ± 13 ka on the same travertine drape challenges this model of a migrat- strom et al., 2007; Crow et al., 2014), regional denudation (Karlstrom et al., ing knickzone and punctuated incision. The presence of ca. 500 ka traver- 2011; Pederson et al., 2013b), glacial cycles (Anders et al., 2005; Pederson et Received 23 May 2017 tine just 95 m above river level requires that most of the inner gorge was al., 2006), and substrate erodibility (Mackley, 2005; Cook et al., 2009; Peder- Revision received 11 December 2017 carved before that time. The resulting maximum bedrock incision rate of son and Tressler, 2012; Pederson et al., 2013a; Bursztyn et al., 2015). Many of Accepted 10 May 2018 Published online 26 July 2018 230 m/Ma is consistent with independent results from sites up and down- the models proposed by these studies suggest unique spatial and temporal stream and with models for semi-steady Quaternary bedrock incision and patterns of incision and can be tested by quantifying variations in incision dispels problems with the transient incision model. Downstream from the rates through time and space. Work to date has yielded incision constraints Hermit Rapid area, dikes present on both sides of the canyon have been throughout much of Grand Canyon (Pederson et al., 2002; Pederson et al., used to support the migrating knickzone model. We report a new 40Ar/39Ar 2006; Karlstrom et al., 2007; Pederson et al., 2013b, Crow et al., 2014; Abbott et age of 517 ± 16 ka on one of these dikes, but argue that they don’t neces- al., 2015); these constraints were calculated primarily by dating of material as- sarily gauge incision. sociated with perched gravel. The geochronology used to calculate these rates Field observations suggest that the discontinuous travertine deposits, comes primarily from 40Ar/39Ar dating of basalts and U/Th dating of travertine near Hermit Rapid, were deposited by springs that emanated from the Red- but also includes optically stimulated luminescence, U/Pb, cosmogenic burial, wall-Muav aquifer, mantled the Tonto Platform, and locally built downwards cosmogenic exposure, and 234U model ages. These data have been interpreted into the inner gorge and tributary canyons. The range of U/Th ages from by some to suggest either temporally steady but spatially variable incision as ca. 10–600 ka suggests these were long-lived spring systems. The travertine a result of spatially differential uplift (Karlstrom et al., 2007; Karlstrom et al., OLD G cements predominantly angular to subrounded locally derived clasts consis- 2008; Crow et al., 2014) or alternatively that a transient wave of incision passed tent with deposition on hillslopes and by tributaries. Well-rounded gravels through central Grand Canyon between 500 ka and 400 ka (Abbott et al., 2015; are exceedingly rare but have been used to suggest that the Colorado River Abbott et al., 2016). The former models don’t discount knickzone migration, but was at the rim of the inner gorge at ca. 500 ka. No exotic Colorado River suggest that it could only have occurred prior to 4 Ma in western Grand Can- OPEN ACCESS clasts, derived from the area outside of Grand Canyon, were observed by us. yon and prior to 650 ka in eastern Grand Canyon (Crow et al., 2014). In-place gravel from the main stem or tributaries (e.g., from paleo–Hermit The hypothesis of 500–400 ka knickzone migration is based on a recent Creek) within the travertine deposits can be reconciled with existing data, study that conducted U/Th dating on travertine deposits that occur at and be- if: (1) travertine was deposited at ca. 2 Ma, which is approximately when the low the rim of the inner gorge of Grand Canyon near Hermit Rapid (river mile steady incision model suggests the inner gorge began to incise; (2) a 500 ka [measured downstream from Lees Ferry (Stevens, 1983)] 95–96) (Abbott et al., lava dam in the Lava Falls Rapid area, 140 km downstream, backed water and 2015) (Fig. 1). The travertine occurs in a series of isolated deposits that rest This paper is published under the terms of the sediment up to the rim of the inner gorge in the Hermit area; or (3) regional primarily on a wide erosional bench in the Bright Angel Shale (Tonto Platform). CC‑BY-NC license. climate-driven aggradation took place at 500 ka. That bench is supported by the resistant Tapeats Sandstone and the underlying

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114° W 113° W 112° W

Water bodies and rivers Lake Powell 100 River miles 37° N Lees Ferryy Quaternary faults (USGS) fs Ve lif rm C r illion ere ivi R o R

Kanab Creek ddo Kaibab Upwarp a EchoEEc Clif rar c Surprise lolo hho o o Vaallllleyle Colorado River C lli lt fffs u a H f fs u p Kwaguntwaggunt r a r UinkaretUiU nknnkarkara eett e i MarbleMarbr Canyon c w a volcanicvovolclcanlcananiicc o n r e fieldfiieldele d o T f a u Havasu Creek l t fault Wheeler su Grand Wash Clif u Lake CCr 15159-mile59-9-m r Mead dikes ElElveslvev Little Chasm Co 36° N HermiHeHermrm t lo ra d areaararea withw intracanyon o

R lavalavav flowflfl s i v e

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Figure 1. Overview map showing Grand Canyon, Quaternary faults (U.S. Geological Survey and Arizona Geological Survey, 2010), river miles measured downstream from Lees Ferry (Stevens, 1983) and locations discussed in the text.

Proterozoic basement rocks, which define the top of the sharp 240-m-deep in- presented below) present on both sides of the river to a height of 400–450 m ner gorge of Grand Canyon. Their conclusion of recent knickzone migration above river level at river mile 159, the 159-mile dikes. was based on ca. 600–500 ka dates on rim travertines that mantle what they Crow et al. (2015a) suggested alternative explanations for the data includ- interpreted to be side-stream gravel (paleo–Hermit Creek and perhaps some ing: (1) that the rim gravel deposition could have taken place much earlier and main-stem gravel) on the Tonto Platform. In contrast, inner-canyon travertine that ca. 600–500 ka U/Th ages record secondary travertine infillings; or (2) that near the river level yielded a ca. 400 ka age. They concluded that incision rates the perched deposits, if due to side-stream aggradation, might have been a had increased to ~1600 m/Ma between 500 ka and 400 ka, then decreased to response to changes in local tributary base level associated with the formation <210 m/Ma over the past 400 ka due to passage of a transient wave of incision. of a lake behind a down-stream dam. Abbott et al. (2016) rejected the first hy- They also based their interpretation on ca. 500 ka basaltic dikes (40Ar/39Ar data pothesis based on textural observations and the fact that they considered the

dated travertine outcrops to be hydrologically isolated. They rejected the sec- gravel was deposited at 500 ka as suggested by Abbott et al. (2015) or poten- ond hypothesis based on a lack of lacustrine deposits. In this paper, we will in- tially earlier as suggested by us (Crow et al., 2015a). The lower travertine drape vestigate these and other alternative hypotheses in the light of new U/Th dates samples were collected to test the hypothesis that the Colorado River had al- from the same deposits and new geologic mapping. We conclude that the ages ready incised to its level (well into the inner gorge) by ca. 500 ka. obtained by Abbott et al. (2015) are reliable but that their interpretations of the In 2016 and 2017, we re-mapped all the travertine deposits. The goals were implication for Colorado River incision need revision. Our new geochronology to characterize each deposit, examine the internal bedding and travertine struc- and geologic mapping support semi-steady incision rather than passage of a tures in each, and characterize the rounding and lithology of clasts within the migrating knickzone. travertine. We did not repeat the rappel described by Abbott et al. (2015), but we were able to walk completely around the deposits (including P1) to characterize interbedded gravels. The result was a more detailed geologic map that recog- METHODS nized new deposits and refined the geometry of previously mapped deposits (Fig. 3). We have visited the 159-mile dikes repeatedly; in 2006, the attitudes During the summers of 2008 and 2015, we collected a suite of samples of dikes were measured, and the newly dated sample was collected in 2010. from the Hermit travertine deposits, including deposits on the Tonto Platform Travertine samples were slabbed on a rock saw with a dedicated blade, and and deposits that drape across the Tapeats Sandstone and extend into the in- the slabs were washed in deionized water for ~10 min in an ultrasonic bath. ner gorge (Figs. 2 and 3). Our sampling was focused on the largest travertine Discrete portions of the slabs were then subsampled with either a tungsten mound perched on the Tonto platform (P1; see Figs. 2 and 3), where Abbott et carbide dental bit or a diamond-impregnated microcore bit; in both cases, the al. (2015) obtained a 506 ± 33 ka U/Th date, and on a lower travertine drape that material from the surface of the slab was discarded. In the case of the micro-

extends from near the Tonto Platform into the inner gorge to within 60 m of the core bit, the core was etched with weak HNO3 before being crushed inside of river (Figs. 2 and 3; their I2 deposit or the Schist Camp drape), where Abbott plastic bags in an agate mortar and pestle; only a few of the resulting pieces

et al. (2015) obtained a 394 ± 32 ka U/Th date from the toe of the deposit. We were analyzed. Forty to 105 mg of material were then dissolved in HNO3 and collected both travertine infillings around gravel clasts and detrital travertine mixed with a 229Th-233U-236U spike. U and Th were separated using conventional clasts from these sites. Dating both infillings and detrital clasts from the upper anion exchange chromatography. U and Th isotopes were measured using a (P1) deposits would bracket the age of gravel deposition and test whether the Thermo Neptune multicollector–inductively coupled plasma mass spectrometer (MC-ICPMS) at the University of New Mexico; the instrument was optimized for U-series analytical work as described by Asmerom et al. (2006). 234U was Schist Camp travertine drape measured on a secondary electron multiplier (EM) with high abundance filter (I2 of Abbott et al., 2015) New age: 517 ± 13 ka @ 805 m asl or on the center Faraday cup, while the other isotopes of uranium were mea- Schist Camp sured on Faraday cups. Mass fractionation was monitored using the 238U/235U Perched travertine 394 ± 32 ka @ 771 m asl RM 96 ratio, while EM/Faraday gain was set using sample standard bracketing and mound (P1 of Abbott (Abbott et al., 2015) uranium standard NBL-112. A similar procedure was used for Th-isotope mea- Tr et al., 2015) avaveerttine Ca surements with 230Th measured in the EM or on the center Faraday cup, and nyon m 229Th and 232Th were measured in Faraday cups. An in-house 230Th standard was used to measure the EM/Faraday gain, and mass fractionation was corrected River - 710 m asl ~ 750 506 ± 33 ka @ 949 m asl using 238U/235U or 236U/233U. All analyses used new half-lives for 234U and 230Th (Abbott et al., 2015) from Cheng et al. (2013). Despite repeated field work at the 159-mile dikes site, unaltered material suit- able for 40Ar/39Ar dating was not discovered until the summer of 2010. On both sides of the river, the dikes typically have thick cooling rinds and show evidence for alteration, including the replacement of olivine phenocrysts by “iddingsite” Hermit Rapid rmit and green groundmass consistent with alteration to chlorite. Pristine material He C re with unaltered olivine phenocrysts and groundmass has only been found ~100 ek RM 95 m above river level on the north side of the river. The dated sample was collected from a piece of angular float tens of meters below an outcrop of the dike. The Figure 2. Annotated Google Earth image showing a perspective view of the locations of the angular morphology of the basalt piece and the lack of upstream sources pre- main travertine outcrops focused on in this study (outlined in white). The newly reported age of 517 ka on the I2 travertine drape requires that the canyon was carved to at least that level by clude significant transport distances to the site. It seems likely that the sample ca. 517 ka. asl—above sea level. came from a less altered outcrop of the dike inaccessible without rock climbing.

112°14'W 112°13'30"W 112°13'W

562*, 468*, 394 slate Cba Qt 246 QTa? ca Qt 3 Bedding, showing dip angle I2 Qcs mp Ct D 517 DD Qt Contact, certain Qa Qtr Contact, approximate Cba Ct Xu Normal fault, certain Normal fault, concealed Qt P4 P2 Normal fault, inferred Cba 500 Ct 491* 339* Color Qt ^ P3 DD 18 U-Th age in ka (Abbott et al., Qt ^ 497 D ado Ri Cba 489, 591 2015; this study), *low precision ver Qtr T1 Xu D Failed U-Th dating attempt Qt on ^ Qt ^ ^ ny (Abbott et al., 2015) 200,Cm 137, 64 Hermit Ra ^ Hypothesized spring locations Cm pid 36°6'N tine Ca water Dtb ^ Cba D I1193 er av 484 Qa Tributary alluvium Qtr D D 31 Tr 506 IPMs D Qa Qt 293* P1D Qt 11 Qcs Colorado River sediment T3 Qt Talus and landslide deposits Qtr Cba Ct Qtr Cm 53 Xu Qtr Travertine ^ 9, 17 11, 13 QTa Perched tributary alluvium Cba Qt 12 Qt IPMs Supai Group Mr Ms ^ Qt Qt Ms Surprise Canyon Formation Mr Mr IPMs it Creek Redwall Limestone

m QTa? Ct r Dtb Temple Butte Formation

He 45, 52 T2 Cm Muav Limestone 36° Cm 5'30"N Qt Cba Bright Angel QTa Qt Ct Tapeats Sandstone Mr Qt Xu Basement Mr IPMs Cm Cba t 000.25 .5 1 km

Figure 3. Geologic map of travertine deposits near Hermit Creek. Tonto Platform deposits (P1, P2, and P3), inner canyon drapes (I1and I2), and tributary deposits (T1, T2, and T3) are labeled using nomenclature of Abbott et al. (2015). Mapping modified from the published (Billingsley, 2000) and unpublished work of George Billingsley.

The 40Ar/39Ar sample was crushed in a disk mill. Phenocrysts were isolated reactive gases within an automated all metal extraction line using a combina- from groundmass by magnetic susceptibility and then handpicked to remove tion of various getters and a “cold finger” operated at −140 °C. Fish Canyon Tuff remaining phenocrysts and any visibly altered grains. The resulting ground- sanidine was used as a neutron flux monitor with the assigned age of 28.201 mass concentrate was treated in an ultrasonic bath for 40 min in 10% HCl and Ma (Kuiper et al., 2008). J-factors were determined by fusion of approximately

then rinsed multiple times in deionized H20. The treated groundmass concen- six flux monitor single crystals from equally spaced radial positions around trate was then loaded into 12-hole machined aluminum disks and irradiated the irradiation tray to a precision of 0.5% or better. Correction factors for in-

for 1 h in the U.S. Geological Survey (USGS) TRIGA reactor, near Denver, Colo- terfering reactions were determined from analysis of K-glass and CaF2 from rado. 40Ar/39Ar analysis using the step-heating age-spectrum method was con- relatively long irradiations, and values used here represent average values de- ducted at the New Mexico Geochronology Research Laboratory with a Mass termined from long-term monitoring of the reactors. The sample was analyzed

Analyzer 215-50 and a Synrad CO2 laser. Evolved sample gas was cleaned of along with a series of lava dam remnants from western Grand Canyon. See the

supplemental file from a manuscript on that topic (Crow et al., 2015b) for ad- Our infilling and detrital travertine samples from the higher perched mound ditional details about the 40Ar/39Ar methods used. Heights used in incision rate (P1) have yet to be analyzed. Figure 3 shows all dated travertines: six ages re- calculations were measured in the field using a Tru Pulse™ laser rangefinder ported by Abbott et al. (2015) for P1 and other platform deposits range from with an accuracy of ±1 m. 489 to 591 ka; ten ages from the inner canyon and tributary deposits range from 12 to 394 ka (two additional samples from the inner gorge gave ages of 468 ± 166 ka and 562 ± 177 ka but were not favored by Abbott et al. [2015] RESULTS because of their large errors); and seven samples from P1, P4, and I2 showed evidence for open-system behavior (Abbott et al., 2015) but are plausibly older U-Series Dating Results than the upper limit of U/Th dating (i.e., >650 ka).

Two U-series samples from the Schist Camp drape (I2) have been analyzed for U/Th age. The lowest sample (RC15-96-2; Fig. 4) was collected near Ar-Ar Dating Results the base of the deposit, deep in the inner gorge, and was dated three times

to assess reproducibility. Three aliquots (one powder and two pieces of the Step heating of the crystalline 159-mile dike sample with the Synrad CO2 la- same microcore) of the sample were successfully analyzed; when possible, ser yielded a well-defined plateau that includes six of the ten heating steps and fractions of the same Th solution were analyzed in the EM and the Faraday >75% of the 39Ar released (Fig. 5, Table S2 [footnote 1]). The resulting weighted cups. This resulted in five dates that all overlap at σ2 and give a weighted mean age of the steps in the plateau is 534 ± 16 ka with a mean square of mean age of 517 ± 13 ka (see Table S11). Results from an additional run with weighted deviates (MSWD) of 1.33. A reverse isochron including all the heating unsteady gain values and known software issues were discarded but would steps yields an age of 517 ± 16 ka with a 40Ar/36Ar intercept of 307 ± 3 (Fig. 5). have indicated an older age just outside of U/Th range. A stratigraphically Despite the higher than expected MSWD of 5.5, the isochron age is favored be- higher sample (RC15-96-3) from the top of the Schist Camp drape gave an cause the 40Ar/36Ar intercept is significantly greater than atmosphere indicating age of 246 ± 13 ka, which likely represents the last time this drape was ac- a trapped component not accounted for in the plateau age. tively depositing travertine. The older age from the base of the deposit and the sample’s height of 95 m above river level indicate a maximum bedrock incision rate of 229 m/Ma, assuming a depth to bedrock of 23.6 m below the Field Observations

DR Table 2. 40Ar/39Ar analytical data. modern river (Crow et al., 2014). Using the height of the drape’s toe (~60 m) ID Temp or Power 40 39 9 37 39 36 39 39 K/Ca 40 39 Age ±1 Ar/ ArAr Ar/ Ar Ar/ Ar ArK Ar* Ar

(°C or Watts) (x 10-3) (x 10-15 mol) (%) (%) (Ma) (Ma) RC10-159-02, Groundmass Concentrate, 108 mg, J=0.0001937±0.18%, D=1.001±0.001, NM-238C, Lab#=59878-01 instead of the sample height would result in a rate of 162 m/Ma, using the XA 3 49.64 5.349 158.2 1.35 0.095 6.73.1 1.17 0.11 Hermit Area XB 4 4.841 3.246 11.47 9.11 0.16 35.4 24.00.599 0.013 C62.844 2.650 5.128 21.70.19 54.3 73.90.538 0.008 D84.916 3.365 12.54 9.30 0.15 30.1 95.30.518 0.013 E912.83 7.285 38.68 0.830 0.070 15.5 97.20.698 0.086 same depth to bedrock value. Because the 517 ka age is on an infilling around F12 16.85 27.73 60.14 0.827 0.018 8.1 99.10.485 0.095 G15 30.45 46.20 113.7 0.212 0.0112.2 99.60.240.35 H20 64.65 45.51 223.5 0.077 0.0113.7 99.80.860.73 XI 30 121.3 49.76 369.3 0.049 0.010 13.4 99.95.9 1.3 XJ 50 150.5 56.80 478.8 0.055 0.009 9.1 100.05.0 1.1 colluvium and because no river gravel is present in or under the deposit, the Figure 3 shows results of new mapping of the travertine deposits. For clar- Integrated age ± 2 n=10 43.50.13K2O=0.80% 0.579 0.025 Plateau ± 2 steps C-H n=6 MSWD=1.33 33.0 0.17 ±0.16 75.70.534 0.016 40 36 Isochron±2 steps A-Jn=10 MSWD=5.47 Ar/ Ar= 307.1±3.40.517 0.016 resulting incision rate is interpreted as a maximum rate. ity, we maintain the nomenclature and numbering of Abbott et al. (2015) for Notes: x symbol preceding sample ID denotes analyses excluded from weighted mean plateau age calculations. i symbol preceding sample ID denotes analyses excluded from isochron age calculations. Isotopic ratios corrected for blank, radioactive decay, and mass discrimination, not corrected for interfering reactions. Errors quoted for individual analyses include analytical error only, without interfering reaction or J uncertainties.

Age calculations: Ages calculated relative to FC-2 Fish Canyon Tuff sanidine interlaboratory standard (28.201 Ma, Kuiper et al., 2008). Integrated age calculated by summing isotopic measurements of all steps. Integrated age error calculated by quadratically combining errors of isotopic measurements of all steps. Plateau age is inverse-variance-weighted mean of selected steps. Plateau age error is inverse-variance-weighted mean error (Taylor, 1982) times root MSWD where MSWD>1. Isochron ages and intercepts calculated using methods of York (1969). AB Decay constants and isotopic abundances after Steiger and Jager (1977). MSWD (Mean Square Weighted Deviation) values are calculated for n-1 degrees of freedom. Individual analysis errors reported at ±1 , weighted mean age errors reported at ±2 .

Sample preparation and irradiation: Groundmass seperates prepared using crushing, ultrasonic (typically in dilute HCl) cleaning, Franz magentic seperator, and hand-picking techniques. Samples were loaded into machined Al discs and irradiated in 8 separate batches RC10-159-02 was irradiated for 1 hour at the Denver Triga Reactor

Instrumentation: Mass Analyzer Products (MAP) 215-50 mass spectrometer on line with automated all-metal extraction system. Reactive gases removed by reaction with 2 SAES GP-50 getters, 1 operated Figure 4. Photographs showing: (A) the site where sample at ~450°C and 1 at 20°C. Gas also exposed to cold finger operated at -140°C and a W filament operated at ~2000°C. RC10-159-02 was step-heated with a Synard CO2 laser RC15-96-2 was collected at the base of the Schist Camp Analytical parameters: Analyses were blank corrected using the proceeding blanks Average 40, 39, 38, 37, and 36 blanks values were 4.3e-2,7.55e-4, 8.97e-5, 5.08e-4, and 1.96 e-4 Na, respectively for samples drape. Only colluvial material was found under or in the run along with RC10-159-02, sensitivity was 4.05e-17 moles/pA 39 Weight percent K2O calculated from Ar signal, sample weight, and instrument sensitivity. J-factors determined to a precision of ± 0.5% or better by CO2 laser-fusion of 6 single crystals from each of 6 radial positions around the irradiation tray. deposit suggesting that the Colorado River had already

Correction factors for interfering nuclear reactions were determined using K-glass and CaF2 and are as follows: NM-238 carved to a lower position before travertine deposition 39 37 ( Ar/ Ar)Ca = 0.0007 ± 2e-6 36 37 ( Ar/ Ar)Ca = 0.00028 ± 2e-5 38 39 occurred here. (B) En echelon dikes near river level at RM ( Ar/ Ar)K = 0.013 (40Ar/39Ar) = 0.01 ± 0.002 K 159. The edges of the dike are outlines in the dashed white line. The grip of the hammer is 18.5 cm long.

1Supplemental Files. Tables S1 and S2. Tables detail analytical results reported by this study. Please visit https://doi.org/10.1130/GES01562.S1 or the full-text article on www.gsapubs.org to view the Supple- mental Files.

RC10-159-02

0.0040 100 % 40

H Ar* 0.0035 G 20 A F 2.0 0 0.0030 J I 1.8 E 1.6 0.0025 D 1.4 A 0.534 ± 0.016 Ma Ar (MSWD = 1.33, p = 0.25)

40 1.2 B 0.0020 1.0 Ar /

Age (Ma ) E

36 C 0.8 0.0015 0.6 B 0.4 C D 0.0010 0.2 F

Apparent 0 0.0005 Age = 0.517 ± 0.016 Ma Ar/ Ar Int. = 307 ± 3 -0.2 MSWD = 5.5, P = 0.00 -0.4 G 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 0 10 20 30 40 50 60 70 80 90 100 39 39Ar /40Ar Cumulative % Ar Released

Figure 5. Reverse isochron and step-heated age spectrum for the 159-mile dike.

platform deposits that developed on the Tonto platform, inner gorge drapes deposited along those tributaries and has cement side-stream gravels. Modern that flowed into the steep inner gorge, and tributary deposits that are in mod- Hermit Creek water is only weakly carbonic (Crossey et al., 2006; Crossey et al., ern tributaries. However, in detail, this distinction breaks down for P4 and I1, 2009) and not associated with major travertine deposition. which are travertine drapes that flowed from the Tonto Platform into the inner The internal stratigraphy of the mounds was described by Abbott et al. gorge. Our mapping reveals that most if not all of the mounds are drapes that (2015) to involve predominately horizontal fluvial bedding and lenses of side- mantle the underlying bedrock and mimic the angle of the slopes that they stream gravel (see their figures 6 and 10). Our observations indicate both rest on (Figs. 6I–6L). In addition, all of them have conical shapes and slopes sloping internal stratification that parallels the top and base on the deposits that project up to what we postulate to have been spring sources at the base and subhorizontal layering. Travertine drapes, into which horizontal layering is of the Redwall-Muav aquifer (black stars in Fig. 3). The cliff retreat concept of often inset, demonstrate at least some paleotopography on a slope (Fig. 6N). Abbott et al. (2015) based on beheaded spring mounds seems to apply to all of A preponderance of angular to subrounded clasts within the deposits suggests them. These deposits are analogous to many other active and inactive spring both hillslope and tributary depositional environments (Abbott et al., 2015). and travertine systems in Grand Canyon such as the area from Kwagunt to the Abbott et al. (2015) reported isolated well-rounded clasts that were suggestive Little Colorado River (RM 56–61, river left), the Elves Chasm area (RM 115–117, of main-stem Colorado River gravel at a single location in the perched (P1) river left), along the Hurricane fault (RM 189–191, river left), near Quartermas- outcrop (see their figure 13 and Fig. 6N from this study). The outcrop is inac- ter Canyon (RM 260, river left), Travertine slot and Travertine bluff deposits (RM cessible and the lithology of the clasts could not be determined; perhaps a few 268–276, river right). In all of these cases, travertines formed in areas where hundred well-rounded clasts are present. A nearby out-of-place block (Fig. 6B) carbonic springs discharge from the base of the Redwall-Muav aquifer co- contains similarly well-rounded clasts that are locally derived. To our knowl- alesced on the Tonto Platform forming mounds there or draping farther into edge, these are the only locations in any of the Hermit area travertines where the canyon (Crossey and Karlstrom, 2012). Unlike the Hermit examples, some well-rounded gravels are present. Identification of exotic, far-traveled clasts, of these drapes extend downwards and cement unambiguous Colorado River foreign to Grand Canyon, would confirm the main-stem Colorado River prov- gravels above bedrock straths, and it is these occurrences that give the most enance. The rarity of well-rounded gravels suggests at most minor sediment reliable incision rates (e.g., Crow et al., 2014). Carbonic springs can also vent input from the Colorado River. into tributary side canyons and contribute to the tributary’s base flow. Havasu According to the Abbott et al. (2015) model, the travertine deposits were Creek and Travertine Grotto are examples where travertine is actively being synchronous with gravel deposition that occurred at ca. 600–500 ka by a paleo–

A T3 P2 I1 P3 P4 P1 E F G D I2

C B

~ 1 km B C D

Figure 6 (on this and following two pages). (A) Loca tions of photographs B through G. (B) Well-rounded to subrounded gravel in a loose travertine block. In total we are aware of two locations with well- rounded gravel, shown in B and K. (C–G) Note the angular and subrounded gravels in these perched travertine mounds consistent with deposition on hillslopes or by tributaries. The yellow bars in B, C, E, F, and G are all scaled to ~0.5 m.

E FG

H T3 P2 J I1 P3 P4 K P1 I

L I2

~1 km

~200 m Figure 6 (continued). (H) Locations of photographs I–L, which show major travertine outcrops draping over the Tonto Platform.

00 m

I KL

F G ~150 D m C ~200 m N

M T3 P2 I1 P3 P4 P1

I2 N

~1 km

well rounded Figure 6 (continued). (M) Locations gravel of Abbott of photograph N, which shows major et al., 2015 travertine drapes at the toe of P1 im- (their g. 13A) plying a lower base level at the time of travertine deposition.

Hermit Creek that joined the Colorado River at a height similar to that of the Tonto Platform today. In that model, paleo–Hermit Creek would have flowed slightly west of its current location to P1. This is consistent with newly mapped paleo–Hermit Creek deposits (labeled QTa on Fig. 3). From P1, the Abbott et al. Yumtheska (2015) model suggests that paleo–Hermit Creek would have flowed subparallel East vent to the Colorado River to P2 (Fig. 3). Although subrounded gravel is less common in P2, P3, or P4 than P1, Abbott et al. (2015) have documented subrounded clasts and sedimentary features (see their figure 11) that are plausibly con- The Basaltic Vent sistent with deposition by a tributary in P2. The tributary paleo–Hermit Creek Cork cinders geometry to link P1 to P2 is unlike all modern tributaries to the Colorado River 1015 m asl Yumtheska in Grand Canyon, which all join the river at high angles. Abbott et al. (2015) New age: 517 ± 16 ka West vent suggest they may have formed in a >1-km-long tributary fan that flanked the @ ~671 m asl paleo–Colorado River. Modern fans of similar dimensions exist but contain River level River - 571 m asl Colorado River deposits throughout, especially at their downstream end. dike outcrops Likely, Colorado River–derived material has only been found at the upstream- most P1 deposit. ~ 2 km

159-Mile Dikes Figure 7. Annotated Google Earth image showing a perspective view of the area around the 159- mile dikes. The dashed white lines show lineaments connecting vents and dike outcrops. Also Two dikes at river mile 159 intrude the Muav Limestone through the Supai shown is the sampling location for the new Ar-Ar age of 517 ka on one of the dikes. asl—above sea level. Group on both sides of the river to near heights of 400–450 m above river level (Fig. 7). In the lower Supai Group on the south side of the river, welded pyroclas- 90 tic tuff (Wenrich et al., 1997) is present along the strike of the downstream-most 50 dike. The dikes are roughly aligned but not coplanar with the Yumtheska vents 120 60 to the southeast and the Cork to the northeast. The Yumtheska West vent was 40 dated by the K-Ar method at 780 ± 15 ka, and the Cork was dated at 410 ± 70 ka (Wenrich et al., 1995); both of these pyroclastic cones were established at a 30 slightly higher stratigraphic level on the Esplanade Sandstone. This map pat- 150 30 20 tern and outcrop-scale field observations (Fig. 4B) suggest that the dikes were injected along en echelon fractures or preexisting joints. A similarly oriented 10 joint set is present both locally and throughout the region (Fig. 8), although

oblique joints are also present and well developed in the immediate vicinity of Blue = strike of joints in 15 km the dikes (Abbott et al., 2016). 1800radius around the 159-mile dikes Red = strike of 159-mile dike measurements DISCUSSION

Hermit Travertine Incision Constraints 210 330

In the Hermit Rapid area, the migrating knickzone hypothesis of Abbott et al. (2015) suggests that the Colorado River incised through bedrock from the level of the perched travertine mound on the Tonto Platform (P1) at ca. 500 ka 240 300 to at least the toe of the Schist Camp drape (I2) at ca. 400 ka. This elevation dif- 270 ference is ~180 m, which would result in an incision rate of ~1600 m/Ma (upper Figure 8. Rose diagrams showing a comparison between the strike of the 159- blue arrow in Fig. 9); however, this hypothesis is challenged by the 517 ± 13 ka mile dikes measured at river level and over 130 joints within the same area mea- age at the base of the I2 drape, within 95 m of river level. Our new data require sured using satellite imagery.

1,000 Tonto P1 980 Platform 506 ± 33 ka 960 Bright Angel Shale (Abbott et al., 2015) 940 920 el (m) Tapeats Sandstone 900

880 Inner Gorg Top: 246 ± 13 ka (this study) Figure 9. Schematic cross section showing the heights 860 I2 of dated deposits (stars) and the resulting incision e sea lev 840 e Base: 517± 13 ka (this study) rates (arrows). The blue arrows are those suggested by Abbott et al. (2015), and the green ones are those sug- 820 gested by this study.

800 1607 m/Ma 780 Precambrian basement 394 ± 32 ka (Abbott et al., 2015)

ation abov 760

740 Colorado <229 m/Ma Elev <210 m/Ma 720 River 700 680 <162 m/Ma

that approximately two-thirds of the inner gorge had been carved when the ca. These types of plots show the height and age of deposits interpreted to be 500 ka travertine at the foot of the perched P1 travertine mound (i.e., at the rim associated with past strath levels. Temporal variations in incision rate are of the inner gorge) was deposited. This provides new maximum incision rates noted by changes in the slope of the line connecting the past strath loca- of 162–229 m/Ma (green arrows on Fig. 9) that are consistent with rates of 160 tions. Figure 10 shows incision constraints mainly from Crow et al. (2014) m/Ma ~35 km (25 river miles) upstream from the site and rates of ~100 m/Ma and Abbott et al. (2015) and corresponding envelopes showing bedrock strath ~30 km (20 river miles) downstream at locations where unambiguous perched heights and ages consistent with each model. Note that for ages older than Colorado River gravel has been dated (Crow et al., 2014). The new age on the ca. 400 ka, the envelopes suggested by Abbott et al. (2015) and Crow et al. Schist Camp drape of 517 ± 13 ka and the 506 ± 33 ka age on the P1 deposit (2014) don’t overlap. Additionally, all of the bold incision points, including the overlap within error. Considering the 2 sigma errors, it is possible that the P1 seven incision points focused on in the reply (Crow et al., 2015a), plot below deposit is between 35 and 9 ka older than the Schist Camp drape (it is also pos- the envelope for the Abbott et al. (2015) migrating knickzone hypothesis; this sible that it is younger). This allows for the possibility of bedrock incision rates means that those data suggest less bedrock incision than predicted by their of ~5000 m/Ma between 539 ka and 504 ka or ~25,000 m/Ma between 539 ka model. This remains true when the 2 sigma analytical errors are included; so and 530 ka. Such bedrock incision rates over large river reaches are very high the imprecision of some rates is not relevant. globally and likely unrealistic geologically for the southwest United States. Abbott et al. (2015) pointed out that two U/Th ages reported by Crow et al. The new data on the timing of carving of the inner gorge at Hermit Rapid (2014) are at the upper limit of the U-series dating method. Because they are dispel several problematic aspects of the migrating knickzone hypothesis of close to secular equilibrium, even minor amounts of U or Th loss or gain could Abbott et al. (2015), chiefly the inconsistency with well-dated unambiguous produce large changes in the apparent age. However, careful examination of Colorado River gravels both up and downstream from the Hermit site. In a the analytical results, using the same methods as Abbott et al. (2015), indicates comment to their paper (Crow et al., 2015a), we focused on seven incision no evidence for open-system behavior as implied by Abbott et al. (2015, 2016); points that are inconsistent with the Abbott et al. (2015) migrating knickzone those two samples (plus a third of a similar age) fall along evolution curves hypothesis. In their reply, Abbott et al. (2016) refuted this claim focusing on on both 234U/238U versus 230Th/238U and 234U/238U versus 230Th/234U plots (Fig. 11). the accuracy of two dates, the precision of three dates, and they concluded Undetected open-system behavior is still possible but not indicated by the that the remaining two were consistent with their model. In Figure 10, we available data. Other studies have demonstrated that ages in this range can present a “strath-to-strath” plot similar to those shown by Crow et al. (2014). be precisely obtained. For example, Cheng et al. (2013) dated Chinese spele-

400

375 Incision constraints from Abbott et al., 2015 Strath heights and ages consistent with the Abbott et al., 2015, incision model 350 Incision constraints from Crow et al., 2014, for eastern Grand Canyon (EGC) 325

) Strath heights and ages consistent with the Crow et al., 2014, EGC incision model 300

(m incision constraints from Crow et al., 2014, for central Grand Canyon (CGC)

275 Strath heights and ages consistent with the Crow et al., 2014, CGC incision model Figure 10. Strath-to-strath plot showing the incision constraints used to establish the temporally 250 Select incision constraints for Karlstrom et al., 2007, for western Grand Canyon steady but spatially variable incision model of updated with new geochronology from Crow et al., 2015b Crow et al. (2014) in green, red, and orange for 225 New age of the Schist Camp drape from this study eastern, central, and western Grand Canyon, respectively. The similarly colored envelopes show 200 the strath heights and ages consistent with each model. No envelope is shown for western Grand 175 Canyon because of short-wavelength incision variations due to fault slip and folding. The inci- 150 sion constraints and envelope for the transient above river level incision model of Abbott et al. (2015) are shown 125 in blue. The incision constraints outlined in black, from upstream and downstream of the Hermit 100 and 159-mile site, are inconsistent with the Ab- bott et al. (2015) model; they would be below the

Height 75 bedrock strath predicted by that model at the 50 time when they formed.

-25 0200 400600 800100012001400160018002000 Age (ka)

othems back to 640 ka; δ18O values for those speleothems are consistent with from the Lava Falls and Surprise Valley areas, all based on the dating of un- insolation values, suggesting that their chronology was accurate. ambiguous river gravel deposits. How could the Colorado River have incised In summary, our new age on the Schist Camp drape (I2) indicates that to within 46 m of the modern river level at Elves Chasm (RM 116, upstream approximately two-thirds of the inner gorge was carved when the ca. 500 ka from the dikes) by ca. 650 ka (Crow et al., 2014), to within 42 m of the modern travertine at the base of the P1 deposit, on the rim of the inner gorge, was river level at RM 189 (downstream from the dikes) by ca. 830 ka (Crow et al,. deposited. This new age strongly suggests that the Abbott et al. (2015) conclu- 2015b), and be ~400 m above river level at RM 159 at ca. 520 ka? Alternatives sion of incision rates of ~1600 m/Ma between 500 and 400 ka at the Hermit site include: (1) the dikes vented into an existing canyon; (2) unrecognized differen- is incorrect. tial incision might be attributed to fault slip; or (3) the Muav Gorge may have been filled with aggraded gravel or lake beds at the time of dike emplacement. Historical observations indicate that the first hypothesis is at least plausi- 159-Mile Dikes Incision Constraints ble. For example, on the big island of Hawaii, dikes were injected across significant relief and volcanic craters during the 10 November 1973 eruption at In addition to the Hermit travertine deposits, Abbott et al. (2015) used Mauna Ulu without erupting at the crater bottom (Tilling et al., 1987). Abbott the 159-mile dikes as evidence for rapid incision of the Muav Gorge after ca. et al. (2015) suggested that this is a poor analogy because previous eruptions 500 ka. The carving of the Muav gorge during that time frame would require occurred at the crater bottom, because minor pooling occurred at the crater an incision rate of ~760 m/Ma based on our dating of the 517 ka 159-mile dike. bottom (it did not fill to the level of the dikes) and because the dikes crossing Such a high rate is inconsistent with independent incision rate calculations the craters were not coplanar. However, dike outcrops across the river and

2.8

2.6 20 ka

2.4 a k

100 RC15-96-3 2.2 U) 2 U/ a

200 ka ( 500 ka 1.8 600 ka 300 ka 650 k 1.6 400 kaRC15-96-2 RC10-PAL-2B 1.4 K03-116.5-2A RC08-EC-1A (3) 1.2 A 1 Figure 11. (A) 234U/238U versus 230Th/238U and 010.5 1.5223.5 (B) 234U/238U versus 230Th/234U evolution plots for ( Th/ U) Crow et al. (2014) analyses at the upper limit of U/Th dating and new analyses reported by this 5 study show no evidence for open-system behavior. Note the ellipses have an outline width that en- larges them slightly to increase visibility. 4.5

3.5

U) 3 ka 20 U / (

2.5 100 ka a RC15-96-3 a 500 k a 200 ka 2 600 k

300 ka 650 ka

1.5 400 ka RC15-96-2 RC10-PAL-2B K03-116.5-2A RC08-EC-1A (3) 1 B 0.5 00.5 11.5 ( Th/ U)

the locations of vents along the general strike of the 159-mile dikes indicate ary travertine infillings that significantly postdate gravel deposition. Second- a system of en echelon dikes that are also not coplanar (Fig. 7), and, instead, ary infilling of carbonate in travertine systems is common throughout Grand dikes are strongly controlled by regional joint sets (Fig. 8). In the second Canyon and often does not have clear textural indications (Crow et al., 2014). hypothesis, at least six sites downstream from the Toroweap fault record Other samples analyzed from this deposit suggest open-system behavior (Ab- less than 56 m of incision (neglecting the depth to bedrock below the river’s bott et al., 2015) but are plausibly outside of U/Th dating range (>650 ka) and surface) over a period greater than the age of the 159-mile dikes (Karlstrom would give 234U model ages of up to ca. 1.6 Ma, assuming a wide range in et al., 2007; Crow et al., 2015b). This suggests that any migrating knickzone initial 234U/238U activities; analysis of similar material in Grand Canyon indicates (e.g., in the Abbott et al. [2015] model) would have been stationary upstream initial 234U/238U activities between 1.1–8.9 (Pederson et al., 2002; Polyak et al., from the Hurricane fault from 830 to 520 ka, or that it was generated by slip 2008; Crow et al., 2014; Abbott et al., 2015). However given the evidence for on a fault. Slip-rate changes on the Toroweap fault, at RM 179, cannot have open-system behavior, this hypothesis is best tested by additional U/Th and been a major factor because the total Paleozoic separation on that fault is U/Pb work on both infillings and detrital travertine material from the deposit, only 177–193 m in Grand Canyon (McKee and Schenk, 1942; Huntoon, 1977; which should bracket the age of gravel deposition, assuming that unaltered Fenton et al., 2001), about half of the amplitude of the proposed knickzone of material can be found. Extrapolating back incision rates of 100–160 m/Ma sug- Abbott et al. (2015), and the post–500 ka slip is ~50 m (Karlstrom et al., 2007). gests that the deposit could be 1.6–2.6 Ma. The third hypothesis suggests that the dike could have intruded aggraded gravel or lake beds. Analogous dikes intruding gravels and cinders are present in the Lava Falls area (Hamblin, 1994). A complete lack of verifiable lake The P1 Deposit May Be Related to Local or Regional Aggradation deposits (Kaufman et al., 2002) casts some doubt on this hypothesis, but every study of Grand Canyon lava dams has concluded that the dams were A second possible reconciliation of all data might involve the deposits be- stable enough to at least impound Colorado River water in large lakes (Pow- ing related to side-stream aggradation (Crow et al., 2015a), possibly controlled ell, 1875; Hamblin, 1994; Fenton et al., 2004; Crow et al., 2015b). A general by a natural landslide or lava dams on the main stem or travertine accumula- lack of features indicating that runout lava flows interacted with river water tion on the side stream. Subrounded gravel present in P1 was likely deposited supports this. Based on the presence of monomictic basalt gravels overlying by paleo–Hermit Creek, and well-rounded gravels in the same deposit may most lava dam remnants, Crow et al. (2015b) concluded that most of Grand indicate isolated and exceedingly rare deposition by the main stem as well Canyon’s lava dams failed before completely filling with sediment, casting (Abbott et al., 2015). The origin of isolated sub-rounded gravel documented in some doubt on this model for dike intrusion. In summary, we reject the fault- P2 is less clear as the paleo–Hermit Creek geometry required to move tribu- slip hypothesis and remain ambivalent about whether or not the dikes could tary gravel to that location without mixing with Colorado River deposits seems have intruded sediments that had aggraded within the canyon behind a lava unlikely. Perhaps these gravels were deposited by a lower-order tributary or dam. Our favored interpretation is that the dikes vented into an existing can- reworked from older deposits. yon. No preserved basalt has been found on canyon walls in this area, but Building on the work of Hamblin (1994), Crow et al. (2015b) documented this is not unexpected given the extreme relief in the area if the erupted vol- ~17 different lava dams in Grand Canyon, downstream from the Hermit area. umes were small. Seven of those overlap with the 506 ± 33 ka age at the base of the perched (P1) travertine mound. These include: lower and upper Black Ledge, lower and upper Prospect, Buried Canyon, 183.4-mile, and the Toroweap dams. The heights Alterative Interpretations of the P1 Travertine Deposit of the original edifices are estimated from the highest remaining dam rem- nant. Although effort was made to focus on intracanyon remnants related to Although the presence of ca. 500 ka travertine on the Tonto Platform and damming, overlain by gravel, it is possible that some remnants are related to ~145 m lower in the inner gorge indicates that the ca. 500 ka date at the base of where lava cascaded into the canyon and not where it pooled at the canyon the perched (P1) travertine mound does not constrain bedrock incision, it may bottom. Conversely, relatively little of each dam remains; so it is also likely that inform other processes. Crow et al. (2015a) suggested two alternatives to the the highest parts of dam are no longer preserved. Figure 12 shows our best Abbott et al. (2015) model that could explain all the existing data. estimates for dam heights. It also shows the heights of landslide dams in the Surprise Valley area based on the identification of Colorado River gravel on top of landslide deposits (Robertson, 2015). Of these landslide and lava dams, The P1 Deposit May Be Much Older Than Originally Thought only the Toroweap and Prospect dams have estimated heights sufficient to in- undate water to the level of the perched (P1) travertine mound. The Prospect The first alternative suggested by Crow et al. (2015a) was that the ca. 500 ka Dam height is likely an overestimate because cascade remnants are likely con- U/Th ages reported by Abbott et al. (2015) at the Hermit site may be a second- voluted with dam remnants (Crow et al., 2015b). The height of the Toroweap

2000 L anab Creek K roweap fault olorado Rive Hurricane fault To Little C

1500 el (m)

Upper Prospect (maximum)

e sea lev Perched P1 tr 1000 avertine mound Toroweap

Buried Canyon Surprise Valley River Colorado Black Ledge

ation abov 500 Elev

200 180 160 140 120 100 80 60 40 20 0 Rivermiles downstream from Lees Ferry

Figure 12. Longitudinal Colorado River profile through much of Grand Canyon showing the best estimates of the sizes of water bodies that back up behind natural dams in Grand Canyon in the past ca. 1 Ma. The height of Grand Canyon’s lava dam is estimated from the erosional remnants of the dams (Crow et al., 2015b). Uncertainty in these height estimates stems from: (1) the difficulty in differentiating between basalt remnants that were parts of dams and those that mark where lava flows cascaded into the canyon and (2) the erosional nature of the remnants. Surprise Valley dam heights are based on the height of gravels on top of landslide deposits (Robertson, 2015). These estimates do not account for differential uplift since the formation of the dams, which could displace features by tens of meters relative to each other (Karlstrom et al., 2007; Crow et al., 2014).

dam, based on intracanyon remnants in the Lava Falls area (RM 18.5), is more Alternative scenarios involving aggradation to reconcile all of the data in- certain and very similar to the height of the perched (P1) travertine mound in clude regional climatically-driven aggradation and local side-stream aggrada- the Hermit area. Although this does not prove that perched side-stream gravel tion behind spring mounds. Climatically-driven aggradation of 50–60 m has deposition in the Hermit area was related to a lava dam, it is consistent with the been documented on the main stem at ca. 65 ka, 110 ka, and 320 ka (Anders hypothesis that some of the deposits could be related to base-level changes et al., 2005; Pederson et al., 2006). However, as of yet, to our knowledge, no fill and delta progradation into a transient standing body of water that was rapidly terraces have been documented in the southwestern United States at ca. 500 ka. filling with sediment at its head. The Toroweap dam would be the most likely culprit for creating that standing body of water. However, lacustrine deposits as proposed by Hamblin (1994) have yet to be found by us, Abbott et al. (2015), CONCLUSIONS or others (Kaufman et al., 2002). Crow et al. (2015b) also concluded that most of Grand Canyon’s lava dams likely failed in tens to hundreds of years, before Field observations and a new U/Th age of 517 ± 13 ka on the Schist Camp they completely filled with sediment. (I2) drape, in the Hermit Rapid area, cast serious doubt on the migrating

knickzone hypothesis, which suggested the inner gorge of Grand Canyon in Abbott, L.D., Lundstrom, C., and Traub, C., 2016, Rates of river incision and scarp retreat in eastern the Hermit area was carved rapidly between 500 and 400 ka (Abbott et al., and central Grand Canyon over the past half million years: Evidence for passage of a transient knickzone: REPLY: Geosphere, v. 12, no. 3, p. 1048–1053, https://doi .org /10 .1130 /GES01292 .1. 2015). Instead, our data require that approximately two thirds of the inner Anders, M.D., Pederson, J.L., Rittenour, T.M., Sharp, W.D., Gosse, J.C., Karlstrom, K.E., Crossey, gorge were carved (to within at least 95 m of modern river level) by ca. 500 L.J., Goble, R.J., Stockli, L., and Yang, G.A., 2005, Pleistocene geomorphology and geochro- ka. This is consistent with observations that show that both perched trav- nology of eastern Grand Canyon: Linkages of landscape components during climate changes: Quaternary Science Reviews, v. 24, no. 23–24, p. 2428–2448, https://doi .org /10 .1016 /j .quascirev ertine mounds (P1, at the rim of the inner gorge) and inner canyon drapes .2005.03 .015. were deposited at least in part on existing slopes by spring mounds with a Asmerom, Y., Polyak, V.J., Schwieters, J., and Bouman, C., 2006, Routine high-precision U-Th iso- strong pulse of travertine deposition at ca. 600−500 ka. Our new ages yield tope analyses for paleoclimate chronology: Geochimica et Cosmochimica Acta, v. 70, no. 18, p. A24, https://doi .org /10 .1016 /j .gca .2006 .06 .061. maximum incision rates of 162–229 m/Ma that are consistent with temporally Billingsley, G., 2000, Geologic map of the Grand Canyon 30’ × 60’ quadrangle, Coconino and Mo- steady but spatially variable incision models (Karlstrom et al., 2007; Karl- have Counties, northwestern Arizona: U.S. Geological Survey Geologic Investigations Series strom et al., 2008; Crow et al., 2014). I-2688, p. 15, scale 1:100,000. Blackwelder, E., 1934, Origin of the Colorado River: Geological Society of America Bulletin, v. 45, Subrounded gravel in the P1 deposit is consistent with deposition by pa- no. 1/3, p. 551–566, https://doi .org /10 .1130 /GSAB -45 -551. leo–Hermit Creek, and well-rounded gravels may indicate some deposition Bursztyn, N., Pederson, J.L., Tressler, C., Mackley, R.D., and Mitchell, K.J., 2015, Rock strength along by the main-stem Colorado River. Drapes within and at the base of travertine a fluvial transect of the Colorado Plateau—Quantifying a fundamental control on geomor- mounds also indicate hillslope topography at the time of deposition. Step phology: Earth and Planetary Science Letters, v. 429, p. 90–100, https://doi.org /10 .1016 /j .epsl .2015.07 .042. pools on travertine mounds draping the Tonto platform and partially filling pa- Cheng, H., Lawrence Edwards, R., Shen, C.-C., Polyak, V.J., Asmerom, Y., Woodhead, J., Hellstrom, leo–Hermit side canyon may have accumulated a mix of hillslope and tributary J., Wang, Y., Kong, X., Spötl, C., Wang, X., and Calvin Alexander, E., 2013, Improvements in and/or river deposits, some of which may have been reworked. These observa- 230Th dating, 230Th and 234U half-life values, and U-Th isotopic measurements by multi-col- lector inductively coupled plasma mass spectrometry: Earth and Planetary Science Letters, tions and the new age constraints can be reconciled in several ways. v. 371–372, p. 82–91, https://doi .org /10 .1016 /j .epsl .2013 .04 .006. 1. The tributary gravels of P1 are actually older than 600–500 ka, and the Cook, K.L., Whipple, K.X., Heimsath, A.M., and Hanks, T.C., 2009, Rapid incision of the Colorado U/Th dates are from secondary carbonate infillings. River in Glen Canyon—Insights from channel profiles, local incision rates, and modeling of lithologic controls: Earth Surface Processes and Landforms, v. 34, no. 7, p. 994–1010. 2. Main-stem aggradation could have occurred locally in response to delta Crossey, L.J., and Karlstrom, K.E., 2012, Travertines and travertine springs in eastern Grand Canyon: progradation within a lake controlled by a downstream dam, and the tributary What they tell us about groundwater, paleoclimate, and incision of Grand Canyon, in Timmons, deposits were graded to the higher lake and/or river level. J.M., and Karlstrom, K.E., Grand Canyon Geology: Two Billion Years of Earth’s History: Geo- 3. The main stem might have aggraded to the rim of the inner gorge at ca. logical Society of America Special Paper 489, p. 131–143, https://doi .org /10 .1130 /2012 .2489 (09). Crossey, L.J., Fischer, T.P., Patchett, P.J., Karlstrom, K.E., Hilton, D.R., Newell, D.L., Huntoon, P.W., 500 ka due to regional climatic fluctuations, and the tributary deposits may Reynolds, A.C., and de Leeuw, G.A.M., 2006, Dissected hydrologic system at the Grand Can- have been graded to this higher river level. yon: Interaction between deeply derived fluids and plateau aquifer waters in modern springs The first possibility is most likely in our opinion and is readily tested; it and travertine: Geology, v. 34, p. 25–28, https://doi .org /10 .1130 /G22057 .1. Crossey, L.J., Karlstrom, K.E., Springer, A.E., Newell, D., Hilton, D.R., and Fischer, T., 2009, Degas- could be confirmed if travertine from the perched deposits gave U/Pb ages of sing of mantle-derived CO2 and He from springs in the southern Colorado Plateau region— 1–2 Ma. Attention should be focused on dating not only infillings but also detri- Neotectonic connections and implications for groundwater systems: Geological Society of tal clasts to fully bracket the age of the reported tributary gravel deposition. The America Bulletin, v. 121, no. 7–8, p. 1034–1053, https://doi .org /10 .1130 /B26394 .1. Crow, R.S., Karlstrom, K.E., Darling, A., Crossey, L.J., Polyak, V.J., Granger, D., Asmerom, Y., and other alternate hypotheses might be supported by additional evidence for lake Schmandt, B., 2014, Steady incision of Grand Canyon at the million year timeframe: A case for deposits, documentation of fill-terrace sequences in analogous Grand Canyon mantle-driven differential uplift: Earth and Planetary Science Letters, v. 397, p. 159–173, https:// tributaries and in other southwestern river systems, and more detailed map- doi.org /10 .1016 /j .epsl .2014 .04 .020. ping and dating of the facies within the Hermit travertine deposits. Crow, R.S., Karlstrom, K.E., Crossey, L.J., Young, R.A., Ort, M.H., Asmerom, Y., Polyak, V.J., and Dar- ling, A., 2015a, Rates of river incision and scarp retreat in eastern and central Grand Canyon over the past half million years: Evidence for passage of a transient knickzone: COMMENT: Geosphere, v. 11, no. 6, p. 2130–2131, https://doi .org /10 .1130 /GES01243 .1. ACKNOWLEDGMENTS Crow, R.S., Karlstrom, K.E., McIntosh, W.C., Peters, L., Crossey, L.J., and Eyster, A., 2015b, A new model for Quaternary lava dams in Grand Canyon based on 40Ar/39Ar dating, basalt geochemis- This manuscript benefited from helpful reviews from Brian Wernicke, Kyle House, and an anony- try, and field mapping: Geosphere, v. 11, no. 5, p. 1305–1342, https://doi .org /10 .1130 /GES01128 .1. mous reviewer. 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