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., Pol yak, 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 © 2018 The Authors GEOSPHERE | Volume 14 | Number 5 Crow et al. | Carving Grand Canyon’s inner gorge Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/14/5/2140/4337541/2140.pdf 2140 by guest on 25 September 2021 Research Paper 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 HeHHermiermrm t lo ra d areaararea withw intracanyon o R lalavavav flowflfl s i v e r DiamondDiD a 0315 060 CCreek km 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 GEOSPHERE | Volume 14 | Number 5 Crow et al. | Carving Grand Canyon’s inner gorge Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/14/5/2140/4337541/2140.pdf 2141 by guest on 25 September 2021 Research Paper 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.