CRevolution 2: Origin and Evolution of the Colorado River System II themed issue Abandonment of Unaweep Canyon (1.4–0.8 Ma), western Colorado: Effects of stream capture and anomalously rapid Pleistocene river incision Andres Aslan1,*, William C. Hood2,*, Karl E. Karlstrom3,*, Eric Kirby4, Darryl E. Granger5,*, Shari Kelley6, Ryan Crow3,*, Magdalena S. Donahue3,*, Victor Polyak3,*, and Yemane Asmerom3,* 1Department of Physical and Environmental Sciences, Colorado Mesa University, Grand Junction, Colorado 81501, USA 2Grand Junction Geological Society, 515 Dove Court, Grand Junction, Colorado 81501, USA 3Department of Earth and Planetary Sciences, University of New Mexico, Northrop Hall 141, Albuquerque, New Mexico 87131, USA 4College of Earth, Ocean and Atmospheric Sciences, Oregon State University, 202D Wilkinson Hall, Corvallis, Oregon 97330, USA 5Department of Earth and Atmospheric Sciences, Purdue University, 550 Stadium Mall Drive, West Lafayette, Indiana 47907, USA 6New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, New Mexico 87801, USA ABSTRACT opment of signifi cant relief between adjacent through resistant Precambrian bedrock (Fig. 2). stream segments, which led to stream piracy. It has no major river at its base, and is currently Cosmogenic-burial and U-series dating, The response of rivers to the abandonment drained by two underfi t streams, East and West identifi cation of fl uvial terraces and lacus- of Unaweep Canyon illustrates how the Creeks, which drain the northeast and southwest trine deposits, and river profi le reconstruc- mode and tempo of long-term fl uvial incision ends of the canyon, respectively. Starting with tions show that capture of the Gunnison are punctuated by short-term geomorphic the Hayden Survey (Peale, 1877), geologists River by the Colorado River and abandon- events such as stream piracy. These short- recognized Unaweep as an abandoned canyon ment of Unaweep Canyon (Colorado, USA) term events can trigger signifi cant landscape that was once occupied by the Gunnison River occurred between 1.4 and 0.8 Ma. This event changes, but the effects are more localized (Peale, 1877; Cater, 1966, 1970), the Colorado led to a rapid pulse of incision unlike any relative to regional climatically or tectoni- River (Gannett, 1882), or both (Lohman, 1965, documented in the Rocky Mountains. Fol- cally driven events. 1981; Sinnock, 1981; Aslan et al., 2008a; Hood, lowing abandonment of Unaweep Canyon by 2011). Subsequent debate has focused on which the ancestral Gunnison River, a wave of inci- INTRODUCTION river or rivers cut the canyon, over what time sion propagated upvalley rapidly through period incision occurred, the timing and causes Mancos Shale at rates of ~90–440 km/m.y. Stream capture is a well-known process, but of abandonment, and the amount of fi ll in the The Gunnison River removed 400–500 km3 our understanding of its effects on rates and valley. There has also been debate over whether of erodible Mancos Shale and incised as deep magnitudes of fl uvial incision is hampered by Unaweep Canyon has both a Quaternary and as 360 m in 0.17–0.76 m.y. (incision rates of poor preservation of associated landforms and Paleozoic component to its history (Soreghan ~470–2250 m/m.y.). Prior to canyon aban- uncertainties involving the timing of capture et al., 2007). Recent drilling has resolved the donment, long-term (ca. 11–1 Ma) Gunnison events (Prince et al., 2011). Whereas tectonism thickness of fi ll to be at least 320 m locally River incision averaged ~100 m/m.y. and climate are known to drive landscape change (Soreghan et al., 2007; as predicted by Oesleby, The wave of incision also caused the sub- (e.g., Hoffman and Grotzinger, 1993; Harkins 1983, 2005a). This result demonstrates that sequent capture of the Bostwick–Shinn Park et al., 2007; Bonnet, 2009), the effects of auto- Unaweep is a partially fi lled bedrock canyon, at River by the ancestral Uncompahgre River cyclic processes such as stream capture have least 1 km deep, rivaling the Black Canyon of ca. 0.87–0.64 Ma, at a location ~70 km upval- received considerably less attention (Hasbargen the Gunnison and the inner gorge of the Grand ley from Unaweep Canyon. This event led to and Paola, 2000; Prince et al., 2011). Although Canyon in depth (Donahue et al., 2013). similarly rapid (to ~500 m/m.y.) but local- tectonic and climatic events set the stage for The purpose of this paper is to document the ized river incision. As regional river incision stream capture, the effects of stream piracy on context, timing, and geomorphic effects of the progressed, the juxtaposition of resistant spatial and temporal patterns of fl uvial erosion Unaweep Canyon abandonment. Specifi cally, Precambrian bedrock and erodible Mancos must be evaluated carefully in order to formulate we describe how canyon abandonment initi- Shale within watersheds favored the devel- accurate interpretations of landscape evolution. ated a wave of fl uvial incision that propagated Unaweep Canyon (Colorado, USA) is the upstream along the Gunnison River system, *Email: Aslan: aaslan@ coloradomesa .edu; Hood: most spectacular example of stream piracy triggered at least one additional stream capture whood@ bresnan .net; Karlstrom: kek1@ unm .edu; resulting in canyon abandonment in the upper event, and produced anomalously rapid short- Granger: dgranger@ purdue .edu; Crow: crow .ryan@ gmail .com; Donahue: magdalena .donahue@ gmail Colorado River system (Hunt, 1969, p. 78) term river incision rates. The rates and mag- .com; Polyak: polyak@ unm .edu; Asmerom: asmerom@ (Fig. 1). The present canyon is 40 km long, 5 km nitudes of landscape change brought about by unm .edu. wide, as much as 700 m deep, and is cut mostly this single event are compared with longer term Geosphere; June 2014; v. 10; no. 3; p. 428–446; doi:10.1130/GES00986.1; 18 fi gures; 3 tables; 3 supplemental fi les. Received 24 September 2013 ♦ Revision received 18 February 2014 ♦ Accepted 9 April 2014 ♦ Published online 13 May 2014 428 For permission to copy, contact [email protected] © 2014 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/10/3/428/3332562/428.pdf by guest on 25 September 2021 Abandonment of Unaweep Canyon 43° N 42° N Upper Colorado River Basin WY 41° N Uinta Mts UT CO Unaweep Cyn 40° N Sawatch Mts 39° N N Fig 2 38° N Fig 5 San Juan Mts 110° W 109° W 108° W 107° W 106° W 50 KM Figure 1. Map showing the location of the study area in Colorado (inset map) and the upper Colorado River basin region (30 m digital elevation model base) including locations of the Colorado and Green Rivers and Unaweep Canyon (Cyn). Locations of Figure 2 and 5 are also shown. Geosphere, June 2014 429 Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/10/3/428/3332562/428.pdf by guest on 25 September 2021 Aslan et al. 109.0°W 108.5°W 108.0°W 107.5°W Colorado Grand Junction Grand River Mesa 39.0°N Gunnison Fig. 3 Unaweep Canyon River Cactus Park Delta Uncompahgre Plateau Ancestral Black Canyon Gunnison Uncompahgre River 38.5°N Ancestral Bostwick Creek Pleistocene fill Mancos Shale 10 km Tertiary volcanics Pre-Mancos sedimentary formations Detrital zircon Post-Mancos sedimentary formations Precambrian igneous and metamorphic rocks sample Figure 2. Geologic map of study area showing locations of the modern Colorado, Gunnison, and Uncompahgre Rivers, and important areas including Unaweep Canyon, Cactus Park, Grand Mesa, and Black Canyon of the Gunnison. Location of Figure 3 is also shown. Modifi ed from Williams (1964). fl uvial incision governed by tectonic processes detrital zircon analysis at the University of Ari- races that postdate Unaweep Canyon abandon- that have operated over the past ~10 m.y. in the zona LaserChron Center to evaluate provenance ment were subsampled for U-series age dating upper Colorado River basin (Aslan et al., 2010; of ancient river drainages (Fig. 2; Supplemental (carried out at the University of New Mexico; Karlstrom et al., 2012). Table 11). Additional examination of Colorado Table 2, Supplemental Table 22). River and Gunnison River terraces that postdate Proterozoic Taylor Ranch and Vernal Mesa METHODS the Unaweep Canyon abandonment were also granite (Williams, 1964) were sampled near completed in the vicinity of Grand Junction. Unaweep Divide and Gateway, respectively, Field work included mapping of ancient river At key locations, clast counts (100–200 clasts for apatite fi ssion track (AFT) analysis to bet- gravels and lacustrine deposits in Cactus Park at each site) of representative gravel deposits ter constrain the long-term exhumation history and reconnaissance studies near Gateway. Two were used to characterize gravel compositions. of the area (Table 3; Fig. 3). AFT dates were shallow (<50 m deep) hollow-stem auger drill- Sparry calcite-cemented gravels acquired from determined following procedures outlined in holes were completed in Cactus Park to provide gravel pits of the oldest Colorado River ter- Kelley et al. (1992). Thermal history models core samples of lake beds and cuttings of buried were extracted from the age and track length Gunnison River gravels. Beige sandstone frag- 1Supplemental Table 1. U-Pb zircon geochrono- data using the HeFTy model of Ketcham (2005) ments from the lowermost sample from one of logic analyses of modern and ancient river samples the cores were dated by 26Al/10Be burial dat- by laser ablation–multicollector–inductively coupled 2Supplemental Table 2. U-series results for Colo- ing at the Purdue Rare Isotope Measurement plasma–mass spectrometry. If you are viewing the rado River terraces in the Grand Junction area. If you PDF of this paper or reading it offl ine, please visit are viewing the PDF of this paper or reading it offl ine, Labora tory (Purdue University) to determine http:// dx .doi .org /10 .1130 /GES00986 .S1 or the full- please visit http:// dx .doi .org /10 .1130 /GES00986 .S2 the timing of abandonment (Table 1).