CRevolution 2: Origin and Evolution of the Colorado River System II themed issue Soreghan et al. Geology of Unaweep Canyon and its role in the drainage evolution of the northern Colorado Plateau Gerilyn S. Soreghan1, Dustin E. Sweet2, Stuart N. Thomson3, Sara A. Kaplan1, Kristen R. Marra1, Greg Balco4, and Thaddeus M. Eccles1 1School of Geology and Geophysics, University of Oklahoma, 100 East Boyd Street, Norman, Oklahoma 73019, USA 2Department of Geosciences, Texas Tech University, 125 Science Building, Box 41053, Lubbock, Texas 79409, USA 3Department of Geosciences, University of Arizona, 1040 E. 4th Street, Tucson, Arizona 85721, USA 4Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, USA ABSTRACT Thermochronological data from Precam- teau and is the only major canyon in the Colo- brian basement within Unaweep Canyon rado River drainage not occupied by a river. It Unaweep Canyon (Colorado, USA) is a and Permian strata at the western mouth penetrates Mesozoic strata deep into Precam- large, Precambrian-cored gorge that bisects of the canyon indicate onset of incision in brian crystalline basement, but paradoxically the Uncompahgre Plateau of the northeastern latest Miocene time (ca. 6–5 Ma), at a time- hosts two underfi t drainages, East Creek and Colorado Plateau, but has no through-fl ow- averaged rate of ~210–275 m/m.y. Onset of West Creek, which fl ow in opposite directions ing axial stream; it is drained by two underfi t canyon occupation and rapid incision by the from a gentle divide within the canyon (Figs. creeks (East and West Creek) that head at a ancestral Gunnison River coincided with the 1 and 2). For this reason, Unaweep Canyon is divide within the canyon. The history of the timing of integration of the lower Colorado said to be the only canyon in the world with canyon and its role in drainage evolution of River system to the Gulf of California. The two mouths. the Colorado River system remain contro- synchroneity of this incision across the Colo- Originally described by the Hayden Sur- versial. New mapping of both bedrock and rado Plateau supports the inference of an vey of the late 1800s (Peale, 1877; Gannett, Quaternary units as well as analyses of Qua- ultimate tectonic or epeirorogenic driver for 1882), the origin of Unaweep Canyon remains ternary deposits in and near the canyon shed this widespread incision and ultimate drain- debated. All agree that the creeks currently light on its late Cenozoic history, and call into age integration. occupying the canyon are markedly underfi t. question whether the canyon was incised by Several aspects of this data set support Most have hypothesized that the canyon origi- a Cenozoic river, or merely exhumed by one. the previously published hypothesis that the nated from fl uvial erosion related to an ances- Gravels near the western mouth of Unaweep ancestral Gunnison River exhumed a paleo- tral Gunnison and/or Colorado River, which Canyon (Gateway, Colorado) exhibit a dis- valley. New mapping at the western mouth subsequently abandoned the canyon; but its tinctive intermediate volcanic provenance of the canyon documents a paleovalley fi lled odd geomorphology has aroused speculations recording the presence of an ancestral Gun- with Permian strata that leads into the mod- of a possible glacial infl uence. Soreghan et al. nison River; the youngest gravels are dated to ern Precambrian-hosted gorge of Unaweep (2007) proposed a late Paleozoic age for the 1.46 ± 0.33 Ma. Previously documented cor- Canyon. In addition, the ancestral Gun- Precambrian-hosted inner gorge of Unaweep ing within the canyon reveals a thick (locally nison River paralleled the Uncompahgre Canyon; Soreghan et al. (2008, 2014) fur- >330 m) fi ll that includes a lacustrine succes- Plateau before making a 90° turn to bisect ther suggested an initial (Paleozoic) glacial sion (~140 m thick), dated to 1.4–1.3 Ma, over- the structural axis in a manner that opposes origin followed by Cenozoic fl uvial exhuma- lain by stacked paleosols and a thick (~160 m) both the northwestward plunge of the uplift tion of the ancient valley. Both the Paleozoic conglomeratic unit emplaced between 1.3 Ma and the northeastward dip of its northern age and glacial origin remain controversial and the present, in addition to a basal unit of fl ank. The rate of incision of Unaweep Can- (Aslan et al., 2008, 2014; Hood, 2009; Hood possible late Paleozoic age. Lake formation yon exceeds regional time-averaged incision et al., 2009). refl ects catastrophic mass wasting in western rates, consistent with removal of sedimen- Unaweep Canyon would remain a geomor- Unaweep Canyon that blocked the ancestral tary fi ll rather than incision of crystalline phic oddity of primarily local curiosity were it Gunnison River, causing partial backfi lling basement. This hypothesis implies that very not positioned atop the Colorado Plateau and of the canyon, and forcing the river to seek ancient landforms can infl uence drainage connected to the history of the greater Colo- a lower elevation exit eastward by breach- evolution in even tectonically active land- rado River system, and possibly a deeper time ing the Mesozoic rim at the northeast end of scapes. history of the region. Resolving the geologic Cactus Park (Mesa County, Colorado). Ulti- history of Unaweep Canyon, including the tim- mately, the ancestral Gunnison River joined INTRODUCTION ing and processes of incision and abandonment the lower elevation Colorado River near of the canyon, contributes to our understand- Grand Junction by 1.3 Ma, incising the East Unaweep Canyon (western Colorado, USA) ing of the tectonic, geodynamic, and geomor- Creek of Unaweep Canyon during the over- bisects the northwestward-trending Uncom- phic forces driving Cenozoic integration of the spilling event. pahgre Plateau on the northern Colorado Pla- Colorado River system across the Colorado Geosphere; April 2015; v. 11; no. 2; p. 320–341; doi:10.1130/GES01112.1; 15 fi gures; 1 table; 1 plate; 2 supplemental tables. Received 2 August 2014 ♦ Revision received 26 November 2014 ♦ Accepted 3 February 2015 ♦ Published online 11 March 2015 320 For permissionGeosphere, to copy, contact April [email protected] 2015 © 2015 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/2/320/3334569/320.pdf by guest on 30 September 2021 Geologic history of Unaweep Canyon 109° 108° Colorado Uncompahgre Plateau UT Plateau CO FFS CLI • OOK B er iv ND VAL R RA LE G Y o d AZ a NM r er Grand Junction lo iv o R C o d ra lo GRAND 39° o C k Whitewater MESA e Unaweep e G r u Pure Oil #1 C n Divide st n Ea iso N n Creek R t YO iver s N Cactus e UNAWEEP CA WEST ELK Do W Park B lo l MOUNTAINS r Gateway a e U U c s N n k R COM c C i a v o nyo e m n LA SAL r PAH o p f GRE P ah th MOUNTAINS g e re Gunnison River L R A iv TE e AU r San Mig ue l R iv er D o l o r e s 38° R SAN JUAN iv e r MOUNTAINS Figure 1. Digital elevation model of the Uncompahgre Plateau and greater study region with key features labeled. Boxed region shows the approximate area of the geologic map in Plate 1. White stars (left—ca. 0.96 Ma gravels, right—ca. 1.2 Ma gravels; Carrara, 2001) indi- cate locations of ancient Colorado River gravels sampled in this study (see text and Supplemental Table 2 [see footnote 2]). The white line through the canyon from Gateway (west) to Whitewater (east) is the line of section shown in Figure 12. Plateau, a primary theme of this special issue. chrono logical data from units within and near GEOLOGIC SETTING Moreover, if the Cenozoic history of Unaweep western Unaweep Canyon to (1) document the Canyon proves to be one of exhumation, late Cenozoic occupation and abandonment of Colorado and Uncompahgre Plateaus rather than incision, it implies preservation Unaweep Canyon by a large tributary of the (through burial) of a paleolandform of exces- upper Colorado River system and identify that The Colorado and Uncompahgre Plateaus sive antiquity . Note that here we use the geo- tributary, and (2) assess the hypothesis that this form part of the greater Rocky Mountain oro- morphic defi nition of exhumation as referring late Cenozoic fl uvial history was one of rapid genic plateau, a large region of high elevation in to the exposure through erosion of a formerly exhumation of a pre-Cenozoic landform, rather the United States (McMillan et al., 2006). The buried landscape. Preservation of landscapes than primary incision. If valid, this scenario area has undergone multiple episodes of uplift, of great antiquity (e.g., Mesozoic, Paleozoic) implies a strong role for a form of historical including the Pennsylvanian to early Permian is well known from, e.g., cratonal regions of contingency in landscape evolution. These data rise of the Ancestral Rocky Mountains, which the Gondwanan continents (e.g., Twidale, bear on long-debated issues regarding the iden- included the Uncompahgre uplift, a precursor 1998, 2003), but less appreciated for tectoni- tity of the river or rivers that occupied Unaweep to the modern Uncompahgre Plateau (Kluth cally active regions, although debate continues Canyon, the cause and timing of river abandon- and Coney, 1981). The southwestern edge on the possible role of paleocanyons in shap- ment, the postabandonment history, the odd of the Uncompahgre Plateau coincides with ing the lower Colorado River system (Flowers orientation that crosscuts regional drainage pat- the southwestern margin of the late Paleozoic et al., 2008; Wernicke, 2011; Flowers and terns, and broader implications for identifying Uncompahgre uplift, defi ned by the Uncom- Farley , 2012, 2013).