Geology of Unaweep Canyon and Its Role in the Drainage Evolution of the Northern Colorado Plateau

Home , Dolores River, Gunnison River, Unaweep Canyon

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

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 a th MOUNTAINS hg e re Gunnison River L R A iv TE e AU r San Mig ue l R iv er D

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) indicate 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. chronological 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). the drivers of drainage integration across the pahgre fault zone, a buried reverse fault system In this paper we assemble unpublished Colorado Plateau. This work also highlights inferred largely from subsurface data (Frahme mapping, provenance, and paleocurrent data, the possible role of paleolandforms in shap- and Vaughn, 1983; White and Jacobson, 1983; and draw upon previously published sedi- ing landscape evolution of even tectonically Moore et al., 2008). In late Paleozoic time, mentologic, cosmogenic nuclide, and thermo- active regions. the Uncompahgre uplift formed the highland

Geosphere, April 2015 321

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/2/320/3334569/320.pdf by guest on 30 September 2021 Soreghan et al.

Quaternary strata A’ Whitewater Mesozoic strata Lower Permian PC basement

Uncompahgre UT Plateau Gunnison R. Colorado CO Plateau Cactus Park gravels ?

A AZ NM 38°50′

Fig. 3B Line of profile in Fig. 12 Unaweep Seep Core site Area

Gateway gravels

Gateway 38°40′ N A Gateway Area

Geologic map inset Dolores R. 0510km 109° W 108°50′ 108°40′ 108°30′

Figure 2. Digital elevation model of Unaweep Canyon area; inset shows generalized geologic map (Eccles, 2013); black square is area of main fi gure. Dashed white boxes (Gateway and Unaweep Seep map area) indicate mapping regions that focused on details of the Quater- nary deposits (Kaplan, 2006). See text for explanation of Cactus Park gravels (square), Gateway gravels (two squares, refl ecting two sites dated by Balco et al., 2013), and coring site (triangle) (modifi ed in part from Soreghan et al., 2007; Balco et al., 2013).

bordering the Paradox Basin, which accumu- exhibits as much as ~1000 m of structural relief. tion of the Colorado River drainage system (see lated thousands of meters of sediments. In the Moreover, the Colorado Plateau has under- overview in Karlstrom et al., 2012a). proximal part of the Paradox Basin, this thick gone epeirogenic uplift resulting in a modern The Colorado River and its upstream tribu- sedimentary succession includes the Permian average elevation of ~2.2 km (McQuarrie and taries, including the Gunnison, Uncompahgre, Cutler Formation, which onlaps the western Chase, 2000; Pederson et al., 2002). Details of Dolores, and San Miguel Rivers, drain the Colo- mouth of Unaweep Canyon (Gateway, Colo- the timing, amount, and mechanism of uplift of rado Plateau (Fig. 1). Evidence from the south- rado; Plate 1), and buries the Uncompahgre the Colorado Plateau remain debated, with esti- western Colorado Plateau indicates that drain- fault zone (Cater, 1970; Moore et al., 2008; mates of uplift timing ranging in age from Late ages there initially ﬂ owed to the northeast Soreghan et al., 2012). Cretaceous–Paleocene (Laramide) to late Ceno- following eastward tilting in early Cenozoic Marine strata present across the Colorado zoic (e.g., Hunt, 1956; Morgan and Swanberg, (Laramide) time, and that structural inversion Plateau, including the Uncompahgre Plateau, 1985; Spencer, 1996; McQuarrie and Chase, associated with Basin and Range extension document the ultimate return of the region 2000; Pederson et al., 2002,; Sahagian et al., caused a drainage reversal in Miocene time, to sea level by Cretaceous time (e.g., Hunt, 2002: Morgan, 2003; Karlstrom et al., 2007, with at least the lower Colorado River system 1969; Pederson et al., 2002). Tectonic activ- 2008, 2012b; Huntington et al., 2010; Liu and integrated and ﬂ owing to the Gulf of California ity then resumed during the Laramide orogeny Gurnis, 2010; van Wijk et al., 2010). Incised by ca. 6–5.3 Ma (e.g., McKee and McKee, 1972; (ca. 80–40 Ma). Across the Colorado Plateau landscapes characterized by deep canyons are Young and McKee, 1978; Young, 1982; Potoch- Laramide deformation is limited to a series of common on the Colorado Plateau and linked to nik, 1989, 2001; Spencer et al., 2001; Young and monoclines that include the Uncompahgre Pla- multiple events (Laramide through Neogene) Spamer, 2001; Pederson et al., 2002; Lucchitta , teau (Kelly, 1955; Williams, 1964; Bump and of plateau uplift, as well as possible roles of 2003; Dorsey et al., 2007, 2011; House et al., Davis, 2003; Davis and Bump, 2009), which geomorphology and climate in ultimate integra- 2005, 2008; Karlstrom et al., 2012b). Data

322 Geosphere, April 2015

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 zed version of Plate 1. Moenkopi Formation (Lower Triassic) Moenkopi Formation (Lower Morrison Formation (Upper Jurassic) Entrada Formation (Middle Jurassic) Kayenta Formation (Lower Jurassic) Triassic) Wingate Formation (Lower Jurassic to Upper Triassic) Chinle Formation (Upper Precambrian basement rocks (Early Proterozoic) Cutler Formation (Permian to Pennsylvanian) Coring sites Approximate Definite Strike and dip of beds Inferred Definite Concealed Concealed (1955a, 1955b). Cater ed from 5 This mapping project was funded jointly U D U D U D by the Colorado Geological Survey and U.S. Geological Survey through the National Cooperative Geologic Mapping Program under Jk Je Jm ^c = ^w ^m *Pc MAP SYMBOLS MAP Unconformity Unconformity Unconformity Unconformity Faults Contacts EDMAP agreement numbers G10AC00329, G11AC20217, and G12AC20266 agreement numbers G10AC00329, G11AC20217, EDMAP Pine Mountain (SE), and Gateway (SW) U.S. nt (see Fig. 5; discussed in text). If you are view- nt (see Fig. 5; discussed in text). If you are 38°37′30″ 38°00′ 108°22′30″ 108°22′30″ Triangle Mesa Triangle LIST OF MAP UNITS OF MAP LIST Escalante Forks See Eccles (2013) for description of map units Island Mesa Whitewater

Keith Creek Jacks Canyon

e Glade Park e (Foster and Snyder Flats

Casto Reservoir

Soreghan, 2013)

n U Unconsolidated Quaternary deposits, (Holocene to Pleistocene) undifferentiated deposits (Holocene) Talus Creek gravels (Holocene) West Palisade gravels (Holocene? to Pleistocene) Unaweep gravels (Pleistocene) Gateway gravels (Plio to Pleistocene) gravels (Quaternary?) Terrace Fish Creek (Eccles and Payne Wash Soreghan, 2013b) Pine Mountain 1955b) (Cater, (Eccles and Two V Basin Two Bieser Creek Gateway Colorado Soreghan, 2013a) Gateway, Gateway, (Cater, 1955a (Cater, Kaplan, 2006) Qt Index map of the Geologic Map Unaweep Area, showing extent of mapping, 7.5-minute Canyon quadrangles, and sources of geologic data. Green represents area of this map that modifies original mapping of Eccles (2013) shown in light yellow. Qu Qg 109°00′ 109°00′ Qpg Qug Qgg Qwcg 38°00′ 38°37′30″ N 108°47′30″ Massey #2 Qu ^w = Jk Massey #1 ^w

U ^c

D Qu 50′ ^c ^c 3 Qu ^w Jk Jk 12 Qt Qu Je ^w Qu Je Jm ^c 5 Qu Qt Jm Qu Jk Qu = Je = Je

U SCALE 1:48,000 1 Jk

D DATUM IS MEAN SEA LEVEL IS MEAN SEA DATUM 52′30″ Je Qpg ^w

U ^w Je Qug

D Jk ^w ^c CONTOUR INTERVALS 20 & 40 FEET INTERVALS CONTOUR Modified from Eccles (2013) Qu 15 see also Eccles, 2013). Geology of the Gateway and Pine Mountain quadrangles is modiﬁ gure; Jm 50′ Jm Kbc 1 = 12 0.9 0 0.9 1.8 2.7 3.6 Kilometers Qt Qug Qt 7 18

U Kbc

D ^c Qug 4,900 0 4,900 9,800 14,700 Feet Qpg

Qu Area of Figure 5 ^w Qt Jk 1 Qwcg

25 101234Miles Je *Pc = Qpg ^w ^c ine, please visit http:// dx .doi .org /10 .1130 /GES01112 .S1 or the full-text article on www.gsapubs.org to view the full-si the full-text article on www.gsapubs.org .S1 or /GES01112 .1130 /10 .org .doi dx ine, please visit http:// Qt Qt = Qpg

15 Jk 55′

U ^c Qgg

^c U Jk 2

18 Qt Jm ^w

15 Je ^c Qpg 2 = ^w

10 ^c Qwcg Jk Jk 9 Qpg =

7 Qu Qgg Qu *Pc

18 ^m *Pc

U 15 Je Qu 12 47′30″

7 quadrangles (see index map in ﬁ ^w ′ Qu 57′30″ Qu Qu *Pc Qu Qu ^w Qu

7 38°40′ ^w Jk Qg Qg ^w Jk

D ^c

U Je Jm

D Qu

U ^c Qu *Pc Qu Qu GEOLOGIC MAP OF THE WESTERN UNAWEEP CANYON AND GATEWAY AREA, MESA COUNTY, COLORADO COUNTY, AREA, MESA AND GATEWAY CANYON THE WESTERN UNAWEEP OF GEOLOGIC MAP Qu Qu 109°00′ 42′30″ White rectangular area indicates region where Permian Cutler Formation occurs within a paleovalley hosted in Precambrian baseme Formation occurs within a paleovalley hosted in Precambrian Permian Cutler where indicates region area White rectangular it ofﬂ reading or of this paper ing the PDF Plate 1. Geologic map of the western mouth of Unaweep Canyon. The map includes parts of the Two V Basin (NW), Fish Creek (NE), Basin (NW), Fish Creek V Two The map includes parts of the Plate 1. Geologic map of the western mouth Unaweep Canyon. Geological Survey 7.5 38°45′

Geosphere, April 2015 323

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/2/320/3334569/320.pdf by guest on 30 September 2021 Soreghan et al. c from the upper Colorado River system are less The prevailing ﬂ uvial hypothesis posits origi- i o z o well constrained, but indicate that regional nal canyon formation by the ancestral Gunnison e e g l n a

exhumation and associated major ﬂ uvial inci- River (Peale, 1877; Cater, 1966, 1970; Sin- a P r e e t sion accelerated ca. 11–6 Ma (e.g., Czapla and nock, 1981), the ancestral Colorado River, or a or older g a L Aslan, 2009; Aslan et al., 2010; Karlstrom et al., combined Colorado-Gunnison (Gannett, 1882; A a

2012b; Donahue et al., 2013); in Thomson et al. Stokes, 1948; Shoemaker, 1954; Cater, 1955a; t a d

(2012), the onset was constrained to ca. 6–5 Ma. Hunt, 1956; Lohman, 1961, 1981; Steven , 2002; s c n i t o i e Aslan et al., 2008, 2010, 2014; Hood, 2011; t n a g n i l Unaweep Canyon Hood et al., 2014). Some suggested that the a c m n i Dolores River ﬂ owed northeastward to carve o e l w a Unaweep Canyon perpendicularly bisects Unaweep Canyon (Peale, 1877; Hunt, 1956). o Steep inclinations ca. 1.3–0.5 Ma L P the northwest-southeast–trending Uncom- The eventual abandonment of the canyon by a pahgre Plateau, exposing Proterozoic crys- large river was attributed by some to neotectonic talline (igneous and metamorphic) basement warping of the Uncompahgre Plateau, although beneath a Mesozoic sedimentary carapace, and the prevailing idea posits stream piracy (Cater, a t

forms a 70-km-long wind gap extending from 1966; Lohman, 1961, 1965, 1981; Sinnock a d c White water, Colorado (elevation 1420 m) in 1978, 1981; Scott et al., 2001; Steven, 2002; i g d o l e

the northeast to Gateway, Colorado (elevation Oesleby, 1978, 1983; Aslan et al., 2005, 2008, s o s n e o r

1400 m) in the southwest (Figs. 1 and 2). The 2014). Speculations on the origin and evolution s s h c a

modern canyon is as deep as 1 km (>400 m in of Unaweep Divide have included river inci- o t Cosmogenic nuclide analysis e o N Cosmogenic nuclide analysis Steep inclinations 1.46–1.3 Ma the inner Precambrian gorge), and as wide as sion in response to hypothesized differential G 6 km (3 km in inner gorge; Lohman, 1981; Cole neotectonic uplift of the Uncompahgre Plateau and Young, 1983; Soreghan et al., 2007). Two (e.g., Cater, 1966; Lohman, 1965; Hunt, 1969; small creeks, East Creek and West Creek, cur- Scott et al., 2001), differential erosion (Sinnock, a t

rently occupy Unaweep Canyon, draining from 1981), and valley-ﬁ ll sedimentation (Oesleby, a d palynomorphs c i

the nearly imperceptible Unaweep Divide at 1978, 1983). c i h o p z

an elevation of 2148 m. Moreover, this divide Straightforward evidence for the former a o r e g l i t

is incongruously offset by ~20 km to the north- presence of a large river in Unaweep Canyon a a P r t s east from the axial crest (drainage divide) of remained elusive until recently. High terrace e t arboreal tree and aquatic plant pollen o i a None recovereded ashes; unidentifi Various L B Quaternary fungal spores, the Uncompahgre Plateau (Figs. 1 and 2). East remnants with fl uvial gravels are known from None recovered Cosmogenic nuclide analysis Not assessed 1.3 Ma Creek and West Creek fl ow through the canyon both the eastern (Cactus Park, Mesa County, in opposite directions to join, respectively, the Colorado; Lohman, 1961, 1965, 1981; Aslan Gunnison River near Whitewater, Colorado, et al., 2005, 2008, 2010, 2014) and western and the Dolores River near Gateway, Colorado (Gateway, Colorado; Cater, 1955a, 1966, 1970) (Figs. 1 and 2). mouths of the canyon (Fig. 2), suggesting that

Core recovered from within Unaweep Canyon a river connected these two locations, yet no t n

in 2004–2006 demonstrates that the canyon con- correlative terraces or deposits are exposed in e m e

tains a relatively thick ﬁ ll (~330 m; summarized Unaweep Canyon. Furthermore, although the s a in Soreghan et al., 2007) at least locally. Where eastern (Cactus Park) gravels have been long b n e a i c r

cored, this ﬁ ll consists of four stratigraphic inter- known, those in the Gateway region were men- n b a m vals (Table 1). On the basis of cosmogenic dating tioned only cursorily by Cater (1955a, 1966, n e a v c basement e o basement; intermediate volcanics basement TABLE 1. PROPERTIES OF MASSEY #1 CORE, UNAWEEP CANYON, COLORADO #1 CORE, UNAWEEP OF MASSEY 1. PROPERTIES TABLE r (Balco et al., 2013), the upper three units are of 1970), but never described or mapped until one r Mesozoic strata; Precambrian crystalline P P Mesozoic strata; Precambrian crystalline Pleistocene age. In Soreghan et al. (2007) a late of us (Kaplan, 2006) rediscovered and formally Mesozoic strata; Precambrian crystalline Paleozoic age was posited for the basal unit, but documented them (details herein). this hypothesis remains contested (Aslan et al., The odd geomorphology of Unaweep Can- 2008, 2014; Hood, 2009; Hood et al., 2009). (For yon inspired the Pleistocene glacial hypoth- additional details of methods and results of cor- esis. Lohman (1981) highlighted the prominent ing, see Marra, 2008.) U-shaped cross section of Unaweep Canyon, n

The anomalous course, size, and longitudi- and Cole and Young (1983) cited the many o i t e a t t i

nal proﬁ le of Unaweep Canyon, and recogni- apparent glacial features (e.g., inferred cirques, t e r c i p r

tion of the inability of East and West Creeks truncated spurs) of the Precambrian basement m e subaerial lake fi lling succession lake fi paleosols a t i n I to carve a canyon of such magnitude, has led gorge. The U-shaped cross section can now D to three primary hypotheses for its formation: be attributed to the presence of a thick fi ll that (1) late Cenozoic fl uvial incision, (2) late Ceno- obscures the true form of the basement surface, ) 5 m

zoic (Pleistocene) glacial incision, and (3) late whether V or U shaped. The cirques refer to the (

Paleozoic glacial incision and subsequent burial numerous amphitheater-shaped (steep head- Thickness followed by late Cenozoic ﬂ uvial exhumation of walls and stubby planform) tributaries of the the formerly buried landscape. See Hood (2011) inner gorge (Fig. 3). These are somewhat remi- r

for a comprehensive overview of the history of niscent of those found across the Colorado Pla- e w o Transitional 9 Relatively stable; stacked L Upper 164ows; Mass wasting; debris ﬂ Middle 143 Incomplete coarsening-upward thought on Unaweep Canyon. teau, commonly attributed to seepage erosion, a Unit

324 Geosphere, April 2015

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

Area shown in (C)

Figure 3. (A) Photo panorama showing part of the southwest- facing rim of Unaweep Canyon in the inner gorge. Note amphitheater-shaped tributaries carved in Precambrian basement; width of view is approximately 8 km. (B) Perspective image (from Google Earth) of amphitheater-shaped tributaries on the southeast-facing rim of Unaweep Canyon near the western mouth of the canyon; width of white box is approximately 750 m. (C) Ground-level photo looking west of the tributary noted in B; width of tributary is approximately 500 m. C Mesozoic

Precambrian

Mesozoic Precambrian

Precambrian

process well documented in valleys formed in gorge of Unaweep Canyon occur entirely in Pre- of the region (Richmond, 1962; Yeend, 1969; loose sediment and sedimentary rock (e.g., Laity cambrian igneous and metamorphic basement Sinnock, 1981), as is the Uncompahgre Plateau, and Malin, 1985; Howard and McLane, 1988; with no obvious competency contrasts (Plate 1) and no Quaternary glacial deposits have ever Howard et al., 1988; Baker, 1990; Schumm and no evidence for megafl oods or mass wasting been identifi ed within or proximal to the canyon et al., 1995; Abrams et al., 2009). Formation of of suffi cient scale. or on the plateau (Lohman, 1981; Scott et al., amphitheater shapes has also been observed in The Pleistocene glacial hypothesis has lan- 2001; Soreghan et al., 2007), effectively refuting stratifi ed basalt, attributed to megafl oods and guished owing to the relatively low elevation this hypothesis. mass wasting (Baker, 1990; O’Connor, 1993; of Unaweep Canyon. The highest point within In Soreghan et al. (2008, 2009a, 2009b, 2014), Lamb et al., 2006, 2007, 2008, 2014). It is odd Unaweep Canyon (Unaweep Divide, 2148 m) is it was proposed that Unaweep Canyon was that the amphitheater tributaries lining the inner well below most Pleistocene ice accumulations carved in the Permian–Pennsylvanian by upland

Geosphere, April 2015 325

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/2/320/3334569/320.pdf by guest on 30 September 2021 Soreghan et al.

glaciation when this region was at elevation atop the Dolores River, landslide deposits (Qls), and were analyzed using both the traditional and the ancient Uncompahgre uplift, and that this alluvium (Qal). the Gazzi-Dickinson methods. In the Gazzi- paleolandscape was exhumed by late Cenozoic We mapped the study area using 7.5 min Dickinson method, sand-sized grains and crys- fl uvial action associated with drainage evolu- U.S. Geological Survey quadrangle topographic tals within larger fragments are assigned to the tion across the northern Colorado Plateau of the maps as base maps in the fi eld, augmented with category of the grain or crystal, as opposed to upper Colorado River system. This hypothesis aerial photos and a handheld global positioning the category of the larger fragment as is done in has been criticized, and remains controversial system. Using cobble clast provenance data and the traditional method (Ingersoll et al., 1984). (for discussions and replies, see Soreghan et al., the map distribution of the Quaternary, we dis- For comparison of the (ancestral) Gunnison and 2007, 2008, 2009a, 2009b; Aslan et al., 2008; tinguished three units within what Cater (1955a) (ancestral) Colorado gravels in particular, we Hood, 2009; Hood et al., 2009). classifi ed as Quaternary fanglomerate (Qfg) in focused on identifi cation of the various types Given the continued controversy surround- the Gateway area, herein termed the Gateway of volcanic lithic fragments (vitric, felsitic, ing Unaweep Canyon, this paper aims in part to gravels, Palisade gravels, and West Creek grav- microlitic, lathwork; Supplemental Table 2 [see assess evidence that could bear on distinguish- els (Plate 1; Fig. 4), detailed in the following. footnote 2]) to distinguish mafi c (basaltic) from ing between (1) primary Cenozoic incision and For each of these units, we characterized the intermediate (andesitic) sources following Mar- (2) Cenozoic exhumation of a buried landform. sedimentology (grain size, rounding, texture, saglia (1993). If the canyon is entirely Cenozoic in age, then it bedding, thickness) and logged several vertical should contain strata of entirely Cenozoic age, exposures. Using similar techniques, we distin- Geochronologic and Thermochronologic should follow a planform course that generally guished two additional upper Cenozoic units in Analyses records migration down the structural plunges the Unaweep Seep area (Unaweep gravels and and dips of emerging Colorado Plateau uplifts, Quaternary talus; Plate 1). Where possible, clast We measured cosmogenic 26Al and 10Be on and should refl ect incision rates consistent with imbrications were measured to determine paleo- samples from both the lowest terrace of the time-averaged rates of comparable systems current directions. Gateway gravels in the Gateway map area, across the upper Colorado River drainage sys- and from several horizons of the cored interval tem. Alternatively, if the canyon refl ects in part Provenance Analysis (for details, see Balco et al., 2013). The Gate- exhumation of a preexisting landform, then we way gravels occur beneath a thick overburden might expect to detect evidence of burial of Cobble clast counts were conducted on the of younger gravels (Palisade gravels) at both the landform by pre-Mesozoic strata, a plan- mapped gravel deposits by point counting on of these sites. Within the core, samples were form that possibly ignores structural plunges outcrops using a 0.5 m × 0.5 m square grid and analyzed from several horizons representing and dips of uplifts, and incision rates that may identifying clasts at intersection points spaced inferred lacustrine, pedogenic, and colluvial exceed regional averages, refl ecting less resis- at ~8 cm intervals. Clasts >2 cm diameter were deposits (for details of sample preparation and tance to erosion of weak sedimentary fi ll rela- counted as framework grains (average n = laboratory analyses for cosmogenic-nuclide dat- tive to stronger crystalline bedrock (Sklar and 118), and distinguished as Precambrian crystal- ing, see Balco et al., 2013). Dietrich , 2001). line basement, sedimentary, or volcanic litho- Apatite fission track results have been logic types. obtained from several samples of the Precam- METHODS Additional provenance data were obtained brian basement in Unaweep Canyon, as well as from the sand-sized fraction of gravel deposits on Cutler Formation sediments in the Gateway Field Mapping in the mapped area, from sand recovered from area (Thomson et al., 2012). These data were subsurface strata (detailed in the following), collected to constrain more fully the longer term Field mapping to detail the Quaternary and from 1.2 to 0.96 Ma Colorado River ter- Mesozoic burial and Cenozoic erosion and inci- deposits includes ~30 km2 proximal to the west- races mapped (Carrara, 2001) northeast of the sion history of the canyon and the surrounding ern mouth of Unaweep Canyon: the Gateway Uncompahgre Plateau (Fig. 1; Supplemental Uncompahgre Plateau. Experimental details area, just west of the western canyon mouth, Tables 11 and 22). Several samples of uncon- and thermal history modeling were summarized and the Unaweep Seep area, just east (inside) solidated matrix (material <2 cm) were col- in Thomson et al. (2012). of the western canyon mouth (Fig. 2; Plate 1). lected and sieved to isolate (where possible) In addition, bedrock mapping was conducted of the medium to coarse sand fraction (cf. Critelli RESULTS the entire region surrounding Unaweep Canyon et al., 1997), and counts of 400–500 frame- (Plate 1; Eccles, 2013). Previous geologic map- work grains were performed on thin sections Precambrian–Permian Relations at the ping of the area encompassing Unaweep Can- stained for potassium feldspar (for additional Western Mouth of Unaweep Canyon yon consisted of two quadrangle maps (Cater, details see Kaplan, 2006; Marra, 2008). Grains 1955a, 1955b) capturing parts of the western The Permian Cutler Formation onlaps Pre- mouth and southwestern rim, and a 1:250,000- 1Supplemental Table 1. Location, Provenance, cambrian basement along the southwestern mar- scale compilation based on unpublished photo- and Paleocurrent Data for Gravels of Mapped Areas. gin of the Uncompahgre Plateau near the west- geologic mapping (Williams, 1964). Abundant If you are viewing the PDF of this paper or read- ern mouth of Unaweep Canyon. This onlap of ing it offl ine, please visit http:// dx .doi .org /10 .1130 Cenozoic (Quaternary) deposits occur adjacent /GES01112 .S2 or the full-text article on www the uppermost (exposed) Cutler Formation onto to the western mouth of Unaweep Canyon, .gsapubs .org to view Supplemental Table 1. Precambrian basement of the (paleo) Uncom- deposited unconformably on the Permian Cut- 2Supplemental Table 2. Provenance Data of Colo- pahgre uplift records Permian syndepositional ler Formation. Cater (1955a) subdivided these rado River Terrace Material and Gunnison Gravels. subsidence of the Uncompahgre highland, a If you are viewing the PDF of this paper or read- into the following units: a fanglomerate (Qfg) ing it offl ine, please visit http:// dx .doi .org /10 .1130 realization fi rst expressed by Cater (1970; see capping low ridges in the valley, gravel beds /GES01112 .S3 or the full-text article on www also Soreghan et al., 2012) on the basis of his (Qg) capping terraces along West Creek and .gsapubs .org to view Supplemental Table 2. original mapping (Cater, 1955a). Cater (1970,

326 Geosphere, April 2015

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

A B

Figure 4. Photos of gravel units. (A) Gateway gravels, character- C D ized by well-rounded clasts of large Precambrian and smaller intermediate volcanic and sedimentary clasts. (B) Close-up of an intermediate volcanic clast in the Gateway gravels. Pen for scale. (C) Contact between the Gateway gravels (hammer handle) and overlying Pali- sade gravels (hammer head). (D) Close-up of the Palisade gravels; note clast angularity E and the red color. Pen for scale. (E) Vertical cliff faces and hoodoos of the Palisade gravels. (F) West Creek gravels exposed along Highway 141 near Gate- way, Colorado. Note strath carved into the Cutler Forma- tion exposed at lower left. F

p. 68) stated, “After the highland attained its Cutler-Precambrian contact near the western tion, which marks the presence of a paleoval- maximum height and while the Cutler was mouth of Unaweep Canyon, the Cutler For- ley extending north-northwest toward modern being deposited, the highland began sinking—at mation buries ~520 m of paleorelief preserved Unaweep Canyon (Plate 1), ~90° offset (south least along its southwest fl ank.” on Precambrian basement (palinspastically and east) from the modern exit of West Creek, Our new mapping (Plate 1; Eccles, 2013) of restored for Laramide-age faulting; Soreghan and fi lled with undeformed and poorly con- the Cutler-Precambrian contact here extends et al., 2012). The new map of Eccles (2013) solidated Cutler Formation (Fig. 5). This paleo- and further clarifi es the remarkable onlap modifi es Cater’s (1955a) original map by valley is not inferred, but observable directly in contact fi rst depicted by Cater (1955a). At the extending the large reentrant of Cutler Forma- outcrop (Fig. 5).

Geosphere, April 2015 327

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/2/320/3334569/320.pdf by guest on 30 September 2021 Soreghan et al. y Precambrian basement Precambrian lled by Permian Cutler lled by Permian Cutler Precambrian basement Precambrian

Cutler Fm

Quaternary units Precambrian basement Precambrian Cutler Fm Cutler Precambrian basement Precambrian

Quaternary units Cutler Fm Cutler Cutler Fm Cutler B (in main canyon) Precambrian basement Precambrian Cutler Fm Cutler Cutler Fm Cutler Figure 5. (A) Photo panorama looking north-northwest up the paleovalley carved in Precambrian basement and ﬁ 5. (A) Photo panorama looking north-northwest up the paleovalley carved in Precambrian Figure Formation near the western mouth of Unaweep Canyon. Sloughed Quaternary deposits conceal the underlying Cutler Formation in man the western mouth of Unaweep Canyon. Sloughed Quaternary deposits conceal underlying Cutler Formation near areas. See Plate 1 for location of this paleovalley. (B) Detail of the Cutler Formation burying a knob of Precambrian basement. Formation burying a knob of Precambrian (B) Detail of the Cutler location of this paleovalley. See Plate 1 for areas. of photo in B of photo Approximate location Approximate basement A Precambrian

328 Geosphere, April 2015

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

Cenozoic Gravel Units in and Adjacent A B to Western Unaweep Canyon n = 117 n = 29 Within the Gateway area, we subdivided the Quaternary fanglomerate recognized by Cater (1955a) into three mappable units, informally named as follows: (1) the Gateway gravels, named for the town of Gateway, Colorado; (2) the Palisade gravels, named for the epony- mous erosional remnant towering above Gate- way; and (3) the West Creek gravels, named C for West Creek, the modern drainage (Plate 1; Fig. 4). Figure 6 shows the diagrammatic Mesozoic Strata stratigraphy of these three units. Within the Palisade gravels Unaweep Seep area of western Unaweep Can- West Creek gravels yon, we distinguished two additional deposits, Gateway gravels Modern referred to as the Unaweep gravels (Qug) and West Creek talus material (Qt) (Kaplan, 2006; Plate 1). Late Paleozoic Cutler Formation

Gateway Map Area Gateway gravels. The Gateway gravels occur Figure 6. (A) Paleocurrent directions of Gateway gravels (equal-area plot). (B) Paleocurrent as a fl ight of downward-stepping terraces, with directions of West Creek gravels (equal-area plot). (C) Schematic cross section showing terrace treads at ~1615, 1555, and 1514–1524 m lower West Creek valley and general stratigraphy of gravel units (no scale implied). (Qgg, Plate 1). Gravel thicknesses are 1.5– 13.0 m, with terraces confi ned to a narrow belt within ~1 km of the axis of West Creek. These Palisade gravels. The Palisade gravels con- to the northwest on the north and south sides, gravels unconformably overlie relatively planar stitute the most voluminous and widespread respectively, of West Creek. straths carved into the upper Paleozoic Cutler gravel unit in the Gateway map area (Qpg; Well-developed calcic soil profi les (Calcisols) Formation (Figs. 4 and 7). Plate 1), essentially including what Cater are present on the top of the Palisade gravels. A Clasts within the Gateway gravels are well (1955a) mapped as Quaternary fanglomerate. typical 2-m-thick profi le, located on top of the rounded, with a distinctive component of (inter- The Palisade gravels cap most of the low ridges Palisade gravels, consists of coatings on basal mediate) volcanic clasts; these gravels exhibit a in the study area (Plate 1; Figs. 4, 6, and 7) surfaces of large clasts, continuous coatings light gray color imparted by the abundance of and range in elevation from >1860 m near the enveloping sand grains, and indurated sheet- Precambrian granite clasts. The color contrasts Mesozoic escarpment to <1525 m near mod- like calcium carbonate, representing Stage IV with the red-tan of other Cenozoic gravels in ern West Creek. Despite poor cementation, the (>500 k.y.) development (cf. Machette, 1985; the area (Figs. 4 and 7) derived predominantly Palisade gravels locally form vertical cliffs as Retallack, 2001). from local redbeds. Clast counts of framework much as ~30 m thick and tall hoodoos (Fig. 4). West Creek gravels. The West Creek gravels grains average ~60% Precambrian basement, The Palisade gravels are stratigraphically above are within a narrow belt (<700 m wide) border- 30% sedimentary, and 10% volcanic clasts of the Gateway gravels (where present) or uncon- ing West Creek and are well exposed in cuts intermediate (andesitic) composition (Fig. 8). formably overlie straths cut in the Permian Cut- along Highway 141 (Qwcg; Plate 1). The depos- Point-count data of the sand-sized fraction also ler Formation (Figs. 6 and 7). The surface of the its locally reach ~30 m in thickness and underlie indicate abundant volcanic lithic fragments (Lv) Palisade gravels consists of a heavily dissected, a single terrace tread inset against older units, of intermediate (andesitic) composition (Fig. 9; yet discernible, terrace tread that slopes from including the Palisade gravels and the Permian Supplemental Table 1 [see footnote 1]). Depos- the Mesozoic escarpment toward West Creek Cutler Formation (Fig. 4F). Only one terrace is its of the Gateway gravels are clast supported (Fig. 7). evident (excluding the modern fl oodplain), and and generally massive, consisting primarily of The Palisade gravels are distinguished by it occurs at an elevation of ~1490–1550 m. well-rounded, poorly sorted, imbricated cobbles their relatively high percentage of sedimen- The West Creek gravels have a composi- to boulders (maximum clast size ~3.0 m) with tary clasts (Figs. 4 and 8). Point-count data on tion intermediate between the other two gravel minor sand matrix. Paleocurrent data measured the sand component indicate abundant quartz deposits, consisting of nearly subequal propor- from imbricated clasts in the Gateway gravels and sedimentary lithic fragments (see Kaplan, tions of Precambrian basement and sedimen- yield an average fl ow direction of ~200° (to the 2006; Supplemental Table 1 [see footnote 1]). tary clasts, with trace intermediate volcanic south-southwest; Fig. 6). The Pali sade gravels consist of a very poorly clasts (Fig. 8). Point counts of the sand-sized The lowest-mapped terrace of the Gateway sorted mix of angular to subangular pebbles fraction show that the matrix contains mostly gravels (elevation ~1514–1524 m) exhibits to boulders (maximum clast size ~4.0 m) with monocrystalline quartz and feldspar grains Stage IV calcic soil development (>500 k.y. locally signifi cant mud- to granule-sized matrix. (see Kaplan, 2006). Deposits of the West Creek development as defi ned by Machette, 1985; Deposits of the Palisade gravels are generally gravels are generally clast supported and display see also Mack, 1997; Retallack, 2001), with clast supported and massive to locally crudely crude stratifi cation consisting of poorly sorted, continuous coatings on clasts and a thick stratifi ed. Clast imbrication is rare, thus paleo- rounded to angular, imbricated pebbles to boul- (~30 cm) tabular accumulation of calcium car- current data are sparse. However, terrace treads ders (maximum clast ~3.0 m). Imbricated clasts bonate at the top. and straths slope ~4° to the southeast and 5° are abundant in the West Creek gravels and

Geosphere, April 2015 329

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/2/320/3334569/320.pdf by guest on 30 September 2021 Soreghan et al.

Palisade gravels Gateway gravels

Cutler Formation

Figure 7. Panoramas of the Gateway map area. (A) View looking west across the Gateway map area and lower West Creek Valley. High- way 141 and West Creek are visible across center left; note roadcuts of the West Creek gravels and the gentle sloping surface of the Palisade gravels. The subtle color change that occurs approximately half-way up hills is the contact between the Palisade gravels (reddish) and the underlying Cutler Formation (grayish). In the background, the Palisade, the erosional feature for which the Palisade gravels were named, towers above the area; buildings and vehicles in lower left for scale. (B) Cross-sectional view showing geometric relationship of Paleozoic Cutler Formation, Gateway gravels, and Palisade gravels. Note gray slope debris, derived from Gateway gravels, contrasting with red color here of the Cutler; Gateway gravels are approximately 3 m thick.

yield an average ﬂ ow direction of 198°, which Table 1). The Unaweep gravels consist gener- S Gateway gravels coincides with the axis of modern West Creek ally of a very poorly sorted mix of subangular to Palisade gravels 10 90 (Fig. 7). Sandy layers occur locally within the rounded pebbles to boulders with a locally sig- West Creek gravels coarser layers and commonly display decimeter- niﬁ cant sandy granule matrix. Approximately Unaweep gravels 20 80

scale cross-stratiﬁ cation. subequal proportions of sedimentary and Pre- 30 70 cambrian basement clasts are present (Fig. 8). Unaweep Seep Map Area Monocrystalline quartz and feldspar grains pre- 40 60 (Westernmost Unaweep Canyon) dominate within the sand fraction (Fig. 9). 50 50

Unaweep gravels. The Unaweep gravels Few vertical exposures of the Unaweep 60 40 and the talus material mapped in westernmost gravels exist, but roadcuts and a gravel-pit expo- 70 30 Unaweep Canyon exhibit similar compositions sure reveal a record of calcrete development on and sedimentological characteristics; however, top of these debris aprons (Fig. 10). A 2.5 m 80 20 we distinguish them primarily on the basis of section of a surfi cial exposure consists of alter- 90 10 slope characteristics. nating layers of conglomerate and granular sand pC V The Unaweep gravels form coalescing debris exhibiting calcifi cation, ranging from coatings 10 20 30 40 50 60 70 80 90 aprons that mantle the fl anks of Unaweep Can- on basal clast surfaces to veins and sheets in yon, forming the modern, highly vegetated sur- several discrete intervals separated by noncal- Figure 8. Clast composition data for the face (Qug; Plate 1; Fig. 10). Roadcuts indicate cifi ed intervals. The basal interval represents a gravels of the Gateway area. See text for that these debris aprons are thicker than 20 m and Stage I carbonate accumulation (discontinuous methods. Clast counts are from outcrop core data confi rm a local minimum of 330 m of clast coatings), whereas the middle and upper analysis (clasts >2 cm). S—Mesozoic sedi- fi ll within the canyon, including an uppermost intervals record two intervals of Stage IV devel- mentary, pC—Precambrian basement, V— unit of >160 m of mostly conglomerate essen- opment (clast coatings and platy sheets; cf. volcanics. For additional details (e.g., sam- tially correlative to the Unaweep gravels (see Machette, 1985; Retallack, 2001), similar to that pling locations) see Kaplan (2006).

330 Geosphere, April 2015

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

Q LvL In addition, this showed that the basal age of the ABlacustrine section in core was indistinguishable 10 90 10 90 from the isochron age for the Gateway gravels 20 80 20 80 in outcrop. 30 70 30 70 Comparison of apatite ﬁ ssion track results 40 60 40 60 from Precambrian basement from the top and near the base of Unaweep Canyon (Thomson 50 50 50 50 et al., 2012) shows contrasting thermal histo- 60 40 60 40 ries over the past ~25 m.y. A basement sample 70 30 70 30 from near the canyon rim (~2644 m) has long 80 20 80 20 track lengths indicative of slow erosion during 90 10 90 10 this time. In contrast, samples from the base of the canyon (down to 1445 m) have shorter track

P 10 20 30 40 50 60 70 80 90 K LvM 10 20 30 40 50 60 70 80 90 LvV lengths and slightly younger ages (ca. 20 Ma) K that are best reproduced by a thermal history with rapid cooling beginning ca. 6–5 Ma, C D 50 10 90 reﬂ ecting time of onset of canyon incision. This 40 20 80 estimate is more precise and somewhat younger 30 than previous estimates (11–6 Ma) for onset 30 70 20 of regional exhumation in the upper Colorado

40 60 Lv % of total 10 River drainage (Aslan et al., 2010; Karlstrom 50 50 0 et al., 2012b; Rosenberg et al., 2014).

60 40 vitric/Lv lathwork/Lv DISCUSSION 70 30 microlitic/Lv 80 20 Colorado sand Identifying the Ancestral River 90 10 Gunnison sand of Unaweep Canyon Gateway gravel matrix LvL10 20 30 40 50 60 70 80 90 LvV + LvM Debate continues on the identity of the river(s) that occupied Unaweep Canyon in Neogene Figure 9. (A–C) Framework mineralogy point-count data for the Gateway gravels (black; time; both the ancestral Colorado and ancestral location in Fig. 2), ancestral Gunnison gravels at Cactus Park (orange; location in Fig. 2), Gunnison have been suggested (Peale, 1877; and ancestral Colorado River gravels upstream of the Colorado-Gunnison confl uence (blue; Gannett, 1882; Stokes, 1948; Shoemaker, 1954; location in Fig. 1; sampled from the QT3 [ca. 0.96 Ma] and QT4 [ca. 1.2 Ma] terraces of Hunt, 1956; Cater, 1955a, 1966, 1970; Lohman, Carrara [2001]). Q—quartz, P—plagioclase, K—potassium feldspar, LvL—volcanic lithic 1961, 1981; Sinnock, 1981, 2002; Aslan et al., with lathwork fabric, LvM—volcanic lithic with microlitic fabric, LvV—volcanic lithic with 2008, 2010, 2014; Hood, 2011; Hood et al., vitric fabric (for defi nitions of volcanic lithic fabrics, see Marsaglia, 1993). All are indistin- 2014). The composition and provenance of guishable on the basis of quartz and feldspar alone. However, a clear distinction exists on the ancient fl uvial deposits provide one means the basis of the types of volcanic lithic fragments; the Gateway gravels show the strongest to distinguish these options. Although both the affi liation to the ancestral Gunnison gravels, rather than exhibiting a mixed signal, expected Colorado and Gunnison Rivers traverse sedi- if they were deposited by a combined (ancestral) Gunnison-Colorado river. The relative mentary, plutonic, and igneous bedrock types ratios of microlitic-vitric volcanic lithics versus lathwork volcanic lithics is consistent with a upstream of their confl uence, the compositions predominant mafi c volcanic (basaltic) source for the Colorado samples and a predominant of volcanic rocks in these drainages exhibit dis- intermediate volcanic (andesitic) source for the Cactus Park and Gateway (ancestral Gun- tinctive contrasts (Fig. 11). Specifi cally, modern nison) gravels (cf. Marsaglia, 1993). gravels of both the Gunnison River and Uncom- pahgre River consist predominantly of intermediate (andesitic) volcanic and shallow intrusive developed on the Palisades gravels of the Gate- upper conglomeratic interval in the borehole rocks that originated from the Tertiary vol- way area. In addition, all fan surfaces within range from 1.16 ± 0.25 to 0.76 ± 0.19 Ma; 2 canic provinces of the San Juan Mountains and Unaweep Canyon are heavily vegetated, refl ect- from the transitional interval are 1.37 ± 0.17 and West Elk Mountains (Cater, 1966, 1970; Aslan ing long-term (Holocene) stabilization (Fig. 10). 1.27 ± 0.23 Ma; and two from the lower lacus- et al., 2005, 2008; Fig. 1). Basalt clasts are rare trine unit are 1.69 ± 0.44 and 1.33 ± 0.18 Ma. in gravels of the Gunnison River (Lohman, Cosmogenic-Nuclide Burial Ages, The isochron age for the lowest Gateway grav- 1961; Cater, 1966). In contrast, Colorado River Thermochronology els is 1.46 ± 0.33 Ma. As discussed in detail in gravels upstream from the Colorado and Gun- Balco et al. (2013), fi tting a piecewise linear age nison confl uence contain basalt, derived from Seven 26Al-10Be burial ages from borehole model to these data under the assumption that the voluminous lava fl ows associated with the sediments as well as an isochron burial age (e.g., the lacustrine and conglomeratic units accumu- Grand Mesa and other upstream sources (Cater, Balco and Rovey, 2008) from the lowest terrace lated at constant rates yields a summary age esti- 1966; Aslan et al., 2005). Therefore, volcanic level of the Gateway gravels were reported in mate of 1.41 ± 0.19 and 1.34 ± 0.13 Ma for the and shallow (porphyritic) intrusive rocks of Balco et al. (2013). Three burial ages from the base and top, respectively, of the lacustrine unit. intermediate-felsic composition are rare in

Geosphere, April 2015 331

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/2/320/3334569/320.pdf by guest on 30 September 2021 Soreghan et al.

A Figure 10. Photos of Unaweep gravels. (A) Unaweep gravels Unaweep gravels exposed along Highway 141 in western Unaweep Canyon. At this location (east of Unaweep Pz Conglomerate Seep map area), the Unaweep gravels are overlying Precam- Precambrian brian basement and an inferred upper Paleozoic conglomerate (Soreghan et al., 2007); B C person left of center for scale. (B) Photo of gravel pit in western Unaweep Canyon exposing Unaweep gravels (modern surﬁ cial ﬁ ll of the canyon) with multiple prominent Calcisol horizons; heavy machinery at left for scale. (C) Photo looking west from near Unaweep Divide showing well-vegetated, stable fan surfaces; road at left for scale.

gravels of the Colorado River upstream of its 108° conﬂ uence with the Gunnison River. This contrast provides a viable basis for identifying the

river or rivers that formerly occupied Unaweep Quaternary Canyon by examining the provenance of the river deposits in and proximal to the canyon. 40° 40° The well-rounded-clast–supported and imbricated deposits mapped as the Gateway gravels Sedimentary rocks near the western mouth of Unaweep Canyon River of Tertiary age ado record a major river fl owing southwestward out olor C Glenwood of Unaweep Canyon toward the Dolores River. Springs These gravels contain a signifi cant contribution Igneous rocks, from intermediate volcanic clasts (Figs. 4, 8, mostly basalt of Neogene age and 9) that we interpret to indicate the presence Grand Grand Junction of the ancestral Gunnison River, or more likely, Mesa rk Fo rth combined Gunnison-Uncompahgre (hereafter Unaweep No Intermediate-felsic referred to as simply the ancestral Gunni- Canyon volcanics rocks of Un Paleogene age son River). G Dol c u o n m niso Gunnison Others have suggested that the ancestral river o nRiver r p e a s Montrose h of Unaweep Canyon was a combined Colorado- R g iv r er S e Sedimentary rocks Gunnison river (e.g., Aslan et al., 2008, 2010, a R n i v of Mesozoic age M e ig r 2014; Hood, 2011; Hood et al., 2014). In par- ue ticular, Hood (2011) cited the presence of minor l Riv 38° er 38° (~2%) red siltstone and sandstone clasts in terraces of the Gateway gravels (as much as 2.6% Sedimentary rocks in the higher terrace) near the western mouth of Paleozoic age of Unaweep Canyon to infer a combined Colo- rado-Gunnison river in Unaweep Canyon. Both 108° Aslan et al. (2014) and Hood et al. (2014) used 100 km Precambrian this argument to agree with the two-stage history of abandonment of Unaweep Canyon, fi rst Figure 11. Generalized bedrock lithology underlying the proximal Colorado River and Gun- by the ancestral Colorado River, and ultimately nison River drainage basins (Kaplan, 2006; modifi ed from Colorado state geologic map). by the ancestral Gunnison River, that was ini- Upper white dashed-line areas show the drainage basins for the upper Colorado River (top) tially proposed by Lohman (1961). Hood (2011) and Gunnison River (bottom).

332 Geosphere, April 2015

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

noted that the Colorado River terraces upstream ancestral Gunnison and ancestral Uncom pahgre ment highs in East Creek, West Creek, and at of the Colorado-Gunnison confl uence contain Rivers, but no gravels bearing an ancestral the coring site (Fig. 12). Figure 12 shows the ~8% red sedimentary clasts, and interpreted Colorado River provenance. In addition, Price profi le constrained with all known (outcrop and these clasts to derive from the Pennsylvanian et al. (2012) used detrital zircon data to infer a core) control points, and produces a slope that is Maroon Formation in the upper reaches of the Gunnison River source for gravels beneath the anomalously fl at in the western canyon. Regard- Colorado River drainage basin. However, there ca. 11 Ma basalt capping Grand Mesa, a gravel less, these relationships indicate that the Gunni- are abundant possible sources of red siltstone unit previously assumed to record the ancestral son River continued to occupy Cactus Park after and sandstone in the Mesozoic strata along the Colorado River (Czapla and Aslan, 2009). it had abandoned Unaweep Canyon. rims of Unaweep Canyon, and minor red silt and sandstone also occur within the Permian Cutler Relationship of the Gateway Gravels to Abandonment of Unaweep Canyon Formation near the mouth of Unaweep Canyon the Cactus Park Gravels (Soreghan et al., 2009c), where the Gateway Previous models for the abandonment of gravels crop out. Hood (2011) did not compare Gunnison River gravels are well known from Unaweep Canyon by the Gunnison River (or the red clasts from the gravels to red clasts from Cactus Park, a northwest-southeast–trending Gunnison-Colorado) differ in the timing of the local Mesozoic and Permian units; the presence inferred paleovalley of the Gunnison River event (ca. 1.4 Ma versus 0.8 Ma), and in the of red sedimentary clasts in distinguishing the located at the northeast end of Unaweep Canyon proposed mechanism, i.e., landslide blockage ancestral Gunnison and Colorado rivers remains near East Creek (Fig. 2; Lohman, 1961, 1965; (Marra, 2008; Balco et al., 2013) versus stream equivocal. Aslan et al., 2005, 2008, 2014; Hood et al., piracy from Cactus Park (Lohman 1961, 1965, The sand provenance data provide addi- 2014). The Cactus Park gravels are interpreted 1981; Aslan et al., 2014; Hood et al., 2014). Any tional support for a uniquely Gunnison River to represent a series of downstepping terraces at model for abandonment must honor the follow- provenance for the Gateway gravels. Figure 9 elevations ranging from ~1870 to 1950 m, inset ing observations: (1) establishment of a lake in illustrates sand framework composition from against the Jurassic Wingate Sandstone (Aslan western Unaweep Canyon (Table 1; Soreghan the (1) ancestral (1.2–0.96 Ma) Colorado River et al., 2005, 2008). Gravels with a clear Gun- et al., 2007; Marra, 2008) ca. 1.4 Ma that existed terraces upstream of the Gunnison River con- nison River (intermediate volcanic) provenance for a few millennia, evinced by the ages for the fl uence (Figs. 1 and 2; Supplemental Table 2 range in thickness from 3 to 5 m, are overlain by base and top of the lacustrine section within the [see footnote 2]), (2) Cactus Park (ancestral gravels derived from Mesozoic strata, and are core (Balco et al., 2013), and (2) the existence Gunnison gravels dated to ca. 0.8 ± 0.24 Ma; locally buried beneath inferred lake beds (Aslan of a Precambrian basement high (1890–1905 m Aslan et al. 2014), and (3) the Gateway gravels et al., 2005, 2008, 2014; Hood et al., 2014). above sea level; Eccles, 2013) just west of (ca. 1.46 ± 0.33 Ma; Balco et al., 2013). If the Cosmogenic burial ages of 0.8 ± 0.24 Ma for Cactus Park at an elevation that exceeds that Gateway gravels were deposited by a combined the youngest gravels (Aslan et al., 2014) below (1870 m; Aslan et al., 2014) of the 0.8 Ma Colorado-Gunnison river, then there should be a lake deposits suggest that these do not correlate Cactus Park gravels. discernible signal of the Colorado River, such as directly with the Gateway (ancestral Gunni- As noted in Soreghan et al. (2007) and the presence of mafi c volcanic lithic fragments son) gravels mapped near Gateway. Aslan et al. Marra (2008), the presence of lacustrine sedi- (Fig. 9), but there is not. The Gateway gravels, (2014) suggested that this date (0.8 ± 0.24 Ma) ments at the Unaweep coring sites (Massey #1 however, exhibit a provenance most consistent records a minimum date of canyon abandon- and Massey #2; Plate 1), indicates that a lake with an exclusive Gunnison River occupation. ment, and that the river that fl owed through occupied the western canyon. The lowest ter- This interpretation corroborates that of Aslan Unaweep Canyon continued to do so until that races of the Gateway gravels near the western et al. (2014), who used detrital zircon data from time. In this view, the ultimate abandonment at mouth of Unaweep Canyon archive the young- the Gateway gravels to infer a Gunnison River 0.8 Ma is marked by the occurrence of the Cac- est known record of the ancestral Gunnison and Uncompahgre River signal. tus Park lakebeds, which do not exhibit a Gun- River through Unaweep Canyon and yield These provenance data are most consistent nison provenance (Hood et al., 2014). We agree cosmogenic-nuclide burial ages of ca. 1.46 ± with a source from the Gunnison drainage sys- with Hood et al. (2014) that the cored lacustrine 0.33 Ma (Balco et al., 2013), indistinguishable tem upstream of the Unaweep Canyon region. strata in western Unaweep Canyon (Table 1) can- from ages obtained from the lacustrine (lower The youngest Gateway gravel deposit is ca. not correlate with the lacustrine strata of Cactus clayey-sandy) interval of the core. Furthermore, 1.46 ± 0.33 Ma, and there are no dates for the Park; they are separated by a Precambrian base- the provenance of the lake sediments, which oldest Gateway gravel deposit. Therefore, these ment high (1886–1905 m) in the canyon fl oor include a distinctive intermediate volcanic com- constraints exclude the occupation of Unaweep located just west of Cactus Park, which exceeds ponent (to 8%; Soreghan et al., 2007; Marra, Canyon by an ancestral Colorado River over at the elevation (1870 m; Aslan et al., 2014) of the 2008), link the Gateway gravels and the basal least the past ~1.5 m.y. Moreover, the hypothesis 0.8 Ma Cactus Park gravels. Figure 12 shows lake sediments to the same ancestral Gunnison that the ancestral Colorado River ever occupied the longitudinal profi le of the modern fl oor of source. Thus, the thick lacustrine unit present in Unaweep Canyon (suggested by Hood, 2011; Unaweep Canyon and the projected 1.4 Ma the core within the canyon records damming of Aslan et al., 2014; Hood et al., 2014) is prob- profi le of the ancestral Gunnison River through the ancestral Gunnison River near the western lematic since the only Cenozoic river deposits Unaweep Canyon; this profi le uses terrace lev- mouth of Unaweep Canyon (Figs. 12 and 13), older than 1.5 Ma are the older (higher eleva- els from the 1.46 Ma Gateway gravels, core data followed by the creation of a lake. tion) Gateway gravels that exhibit a Gunnison (from Soreghan et al., 2007; Marra, 2008; see Water supplied from the ancestral Gunni- provenance. Additionally, Aslan et al. (2008) Table 1), and new mapping (Eccles, 2013). Con- son River would have fi lled the available lake documented gravels of inferred ca. 7–2 Ma age necting the Cactus Park gravels to the Gateway volume (liberally ~135 km3, assuming 1.5 km on top of the Uncompahgre Plateau south of gravels to produce a profi le with a continuous depth, 3.5 km width, and 30 km length) in Unaweep Canyon that contain intermediate vol- gradient through Unaweep Canyon is untena ble, years to decades, assuming mean annual dis- canic clasts consistent with derivation from the owing to known points of Precambrian base- charge of the ancient Gunnison River was on

Geosphere, April 2015 333

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/2/320/3334569/320.pdf by guest on 30 September 2021 Soreghan et al.

Unaweep 2200 Divide ~Elevation of Mesozoic rim core at confluence of East Creek maximum height site and Cactus Park 2000 unknown eastward spill-overp elevation

? alluvial/colluvial ~0.8 Ma Cactus Park 1.4 Ma Gunnison profile 1800 Gateway lacustrine ? ? gravels (Aslan et al., gravels landslide ? ? 2008) dam Paleozoic? Elevation (m) (~1.4 Ma) depth to basement ? 1600 ? unknown depth to A basement ? A′ an 1400 Permi unknown 0 10203040506070 Distance along profile (km) Figure 12. Longitudinal profi le through Unaweep Canyon (line of section in Fig. 1), constructed using all known control points from outcrop and core. Thicker solid line segments denote three known elevations of Precambrian basement within Unaweep Canyon as follows: western mouth of the canyon, the coring site (representing a minimum-depth point), and the eastern canyon basement high. The dotted line segments and question marks denote uncertainty in depth of basement, owing to unknown amounts of sediment fi ll within the canyon. Also shown is the 1.4 Ma ancestral Gunnison River profi le. Note that gradients of this profi le must change considerably to account for known depths to basement. The coring site represents a minimum depth to basement; therefore, if basement is deeper, the slope on the (subsurface) basement surface from here westward through the dammed region could be negative. The eastern spill-over elevation is constrained by the elevation of Mesozoic strata at the confl uence of East Creek and Cactus Park (see text). The two dots depicting the Gateway gravels represent the locations used for cosmogenic-nuclide dating (Balco et al., 2013). See text for further discussion.

the same order as that of the modern (Qm = For Lake Unaweep to exist and receive sedi- ciently high (~1950–1975 m) to cause the water 73 m3/s; Pitlick et al., 1999). In contrast, the ment from the ancestral Gunnison River (early) to seek a lower exit toward the east, resulting in lacustrine sedimentation probably records a and Unaweep tributaries (later), for a few mil- the ancestral Gunnison River fl owing out what few millennia (Balco et al., 2013). These data lennia, we argue that the Gunnison River had is now East Creek. This interpretation implies suggest one of two options: (1) the Gunnison to abandon Unaweep Canyon between 1.4 and that the currently underfi t canyon now occu- River abandoned the canyon once water eleva- 1.3 Ma (Fig. 13). However, the presence of the pied by East Creek was carved by the ancestral tion rose suffi ciently (~1950–1975 m; Fig. 12) 0.8 Ma Gunnison gravels at 1870 m elevation Gunnison River upon abandonment of western to breach the lower elevation rim of Meso- (Aslan, et al., 2014) indicates that the Gunnison Unaweep Canyon. Once Lake Unaweep’s sur- zoic strata at the northeast end of the canyon River continued to occupy and incise Cactus face reached the eastern spillway elevation, lake after the damming event (and initial sediment Park. Geomorphic and limited outcrop data sug- levels would have lowered as incision occurred prograda tion); or (2) the Gunnison River delta gest that the lake-forming blockage occurred near along the modern East Creek canyon. Eventu- prograded completely to fi ll the lake, subse- the southwestern mouth of Unaweep Canyon ally, the lake level would have fallen below quently fl owed across the fi lled lake basin dur- (Figs. 2, 12, and 13), where the canyon narrows ~1890 m, at which point the basement sill just ing the time recorded by paleosol development markedly and is surrounded by steep cliffs. A west of Cactus Park would be above the Gun- in the core, and then over the top of the dam- roadcut here at ~1829 m elevation exposes a nison River in Cactus Park. Thereafter, Lake ming landslide without any incision and resul- deposit of relatively fresh, monolithologic base- Unaweep became effectively isolated from tant breaching of the dam for millennia before ment boulders derived from the surrounding Cactus Park and the local drainages were the abandoning the canyon as a result of stream steep canyon walls, but no lacustrine unit. The top only water and sediment inputs (shown as pur- capture from the east. The former option is of the lower lacustrine and overlying pedogenic ple in Fig. 14). This scenario is consistent with more tenable, for the following reasons: (1) the successions in the core (Table 1) also occurs at the upward decrease in lithic volcanic fragments lacustrine section exhibits incomplete progra- ~1829 m elevation, suggesting that river block- noted in the cored lacustrine interval (Soreghan dation in that the uppermost part is relatively age most likely occurred northeast of the roadcut et al., 2007), indicating that the Gunnison-fed fine grained (fine to lower medium) sand here, and that the elevation of the dam exceeded delta failed to prograde as far as the coring site (Marra, 2008), (2) the lacustrine sediments 1829 m. Subsequent landscape modifi cation before river abandonment occurred. exhibit a clear Gunnison provenance (volcanic within the canyon precludes determination of the This model contrasts with that of Aslan et al. lithic component) primarily in the lower part of maximum elevation of the proposed landslide, (2014) and Hood et al. (2014) in the (1) timing the core (Soreghan et al., 2007; Marra, 2008), but the mapping of Quaternary units (Kaplan, of abandonment of Unaweep Canyon (1.4 Ma (3) no evidence exists in the core for coarse 2006; Oesleby, 2005); illustrated in Aslan et al., versus 0.8 Ma), (2) identity of the river or rivers river gravels of volcanic provenance above the 2008) is consistent with a landslide-induced that occupied the canyon, and (3) age and man- lacustrine section, and (4) it seems unlikely blockage of the ancestral Gunnison River in this ner of incision of East Creek. To summarize, we that the landslide dam at the southwestern end general locality of western Unaweep Canyon. suggest that abandonment occurred ca. 1.4 Ma would remain intact for a protracted interval In this blockage-induced abandonment by the ancestral Gunnison River, as constrained while being overtopped by a large river. model, the landslide dam elevation was suffi - by the cosmogenic ages on the lake deposit and

334 Geosphere, April 2015

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

A 6–1.4 Ma B ~1.4 Ma Colorado River Colorado River

t hief cree et al. of(2014) A

downplunge spill slan k migration over point on R nis iv Lake un er Unaweep G G u parallel n dam n to UP R iso downdip migration of ive n Dolores River combined Uncompahgre & Dolores River r Gunnison rivers

C ~1.3 Ma D 0.8–0 Ma Co C lorado River olorado River

thief creek et al. of(2014) Aslan G un Precambrian basement East n (elevation1905-1890 m) is Creek o n R Lake G i u West v Unaweep n e n Creek r iso dam n R iver Dolores River Dolores River

Figure 13. Time slices on a modern digital elevation model of the region showing postulated drainage evolution of the Gunnison (Gunnison-Uncompahgre) and Colorado river systems over the past ~6 m.y. (A) Beginning ca. 6–5 Ma, major incision begins. The ancestral Colorado river migrates down the northwestward-plunging nose of the Uncompahgre Plateau, whereas the ancestral Uncompahgre River and Gunnison River generally migrate northeastward down the asymmetric dip of the Uncompahgre Plateau. The rivers, however, turn southwest to follow the anomalous course of Unaweep Canyon, guided in our view by a paleolow created by differential compaction of sedimentary fi ll of a paleovalley here. (B) Ca. 1.4 Ma, the ancestral Gunnison River becomes blocked by a landslide dam near the western mouth of Unaweep Canyon to create Lake Unaweep. The lake level ultimately backs up to a height suffi cient to overtop the elevation of the Mesozoic escarpment forming the eastern rim of the Cactus Park valley, and ultimately spills out toward lower elevations to the northeast. (Note that the lake shown here in Cactus Park is older than the Cactus Park lake documented in Aslan et al., 2005, 2008, 2014. The older “Lake Unaweep” inundated the Cactus Park region, but subsequent incision likely removed this older lacustrine record.) (C) Lake Unaweep persists until ca. 1.3 Ma, contained by a landslide dam in the west, and a Precambrian basement sill in the east. The ancestral Gunnison River continues to incise Cactus Park until ca. 0.8 Ma, exiting Cactus Park to the northeast, carving what is now known as East Creek. Continued incision through Cactus Park destroyed any remnants of the older Lake Unaweep lakebeds. (D) The ancestral Gunnison River ultimately aban- dons Cactus Park and East Creek, integrating with the Colorado River to form the modern confi guration; this fi nal step follows the stream piracy mechanism proposed by Aslan et al. (2014).

the provenance data on both the Gateway gravels carved during abandonment of Unaweep Can- was subject to relatively slow aggradation, and older gravels documented in the region. yon as the ancestral Gunnison River sought a evinced by the occurrence of stacked paleo- The data do not support continued occupation lower elevation exit eastward. sols in the interval above the lake deposits of Unaweep Canyon after 1.4 Ma by a large (172–164 m). The upper conglomeratic unit river entering from the east, because the 0.8 Ma Postabandonment History records an infl ux of sidewall canyon debris into gravels in Cactus Park occur below the eleva- the inner gorge, inferred to refl ect deposition tion of the Precambrian basement high within By ca. 1.3 Ma, the canyon fl oor formed a within shallow lacustrine and/or paludal, allu- the fl oor of Unaweep Canyon west of Cactus low-gradient surface at ~1829 m elevation near vial, and colluvial environments (Marra, 2008). Park (Fig. 12). We propose that East Creek was the western Unaweep Canyon coring site, and The age data within this upper conglomeratic

Geosphere, April 2015 335

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/2/320/3334569/320.pdf by guest on 30 September 2021 Soreghan et al.

Figure 14. (A) Panoramic aerial image of the A C o l o r a d Uncompahgre Plateau. White dashed rec- o R i v e r tangle shows the approximate area depicted N in C. Focus is on the northwestward-plunging nose of the Uncompahgre Plateau north of Unaweep Canyon. (B) Perspective view U n Gu looking along the spine of the Uncom pahgre c o n n m ison p Plateau toward the southeast, highlight- a R iv h er ing the extensive northeastward-flowing g r e drainage network, which reﬂ ects the dip- P l a ping ﬂ ank of the plateau that prevails in the t e a u region south of Unaweep Canyon. [These images were derived from combining digi- B tal elevation models (DEM) from the Shut- G u n U n i s tle Radar Topography Mission; created o n n R c o

and copyrighted by W. Bowen, 2014; see i v m N

e p http:// geogdata .csun .edu.) (C) Line tracing r a of modern drainage networks (with basins h g r outlined) of the northwestern Uncompah- e

gre Plateau (traced from standard DEM P l image). Drainage basins are color coded C o a l o r a t d e o R as follows: green drains to the Colorado a i v e u River, blue drains to the Gunnison River, r violet drains to Unaweep Canyon, orange drains to the Dolores River, yellow drains C 109 o W 108.5 o W to East Creek. See text for discussion.

Co lo ra N do Riv interval (Marra, 2008; Balco et al., 2013) indi- er cate that this unit accumulated during the mid- late Pleistocene (ca. 0.9 Ma), a time of marked

glacial-interglacial climate ﬂ uctuation and G u associated precipitation variation conducive to nn 39 o N ison R mass-wasting processes. Following deposition i v

of >160 m of the upper conglomeratic interval r documented at the coring site, the surface of Unaweep Canyon stabilized, recorded by the

k heavily vegetated fan surfaces and well-devel- e e r oped Calcisols (Fig. 10), as the slope progres- C t s sively decreased and the canyon walls were Ea increasingly buried in debris. Outside the western mouth of Unaweep Canyon , material shed from the retreating Meso- West Cr zoic cliffs accumulated by mass wasting and eek ephemeral streams. Eventually, these deposits, the Palisade gravels, buried the remnants of the o D 39.75 N o lo ancestral Gunnison River. Dissection of the sur- r e s face of the Palisade gravels (Fig. 7) then began R iv 10 km e as the Dolores River continued to lower base r level, forcing incision. In contrast, the surface of the Unaweep gravels inside the inner gorge of Unaweep Canyon remains minimally dissected (Fig. 10), with one or more late episodes canyon-interior landscape from major base- River that had long remained buried beneath the of aggradation and intervening stability, marked level–induced dissection common outside the Palisade gravels. This incision has not been con- by the multiple Calcisols, following the major western mouth of the canyon. tinuous, but rather included at least one period fan-building episode. Inside Unaweep Canyon, More recent incision near the western mouth of aggradation, as recorded by the higher terrace fl uvial base level is relatively fi xed by the Pre- of Unaweep Canyon by West Creek, coupled underlain by West Creek gravels found along cambrian bedrock fl oor at the western narrows with dissection of the Palisade gravel surface, the axis of the modern valley near Gateway canyon outlet, which somewhat isolates the exposed the remnants of the ancestral Gunnison (Fig. 6). The West Creek gravels are derived

336 Geosphere, April 2015

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

from the older deposits in the area, including rates on measurement interval (Finnegan et al., est age of the stratigraphic fi ll yields the mini- the Gateway gravels, the Palisade gravels, and 2014; Sadler and Jerolmack, 2014). mum age of the landform. This paleolandform material from Unaweep Canyon. Our inference that erosion in Unaweep Can- hypothesis is controversial (Aslan et al., 2005, Analogous to the Gateway area, alluvial and yon began in the latest Miocene is consistent 2008, 2014; Hood, 2009; Hood et al., 2009); colluvial aprons resulting from slope retreat ulti- with other work documenting probable late we propose, however, that it remains viable mately buried the remnants of the ancestral Gun- Miocene landscape evolution associated with because it explains aspects of the canyon that nison River in Unaweep Canyon. Furthermore, the integration of the lower Colorado River sys- are other wise perplexing. These aspects include the modern longitudinal profi le of Unaweep tem and inception of southwestward fl ow across the planform of Unaweep Canyon relative Canyon (Fig. 12), including enigmatic Unaweep the Colorado Plateau. Abundant data indicate to the structure and regional drainage network Divide, bears no relationship to the profi le of the that the lower Colorado River system was inte- of the Uncompahgre Plateau, together with the ancestral Gunnison River through the canyon. grated and fl owing to the Gulf of California by Cutler-Precambrian contact map relations, and Rather, Unaweep Divide formed as a result of ca. 6–5.3 Ma (e.g., McKee and McKee, 1972; the incision history. aggradation of coalescing debris aprons within Young and McKee, 1978; Young, 1982; Potoch- The modern planform course of Unaweep Unaweep Canyon after abandonment by the nik, 1989, 2001; Spencer et al., 2001; Young and Canyon remains problematic because it indi- ancestral Gunnison River, coupled with recent Spamer 2001; Pederson et al., 2002; Lucchitta, cates that the ancestral Gunnison River fl owed headward erosion of East and West Creeks from 2003; Dorsey et al., 2007, 2011; House et al., subparallel to the Uncompahgre Plateau before their respective base levels at the modern Gun- 2005, 2008; Karlstrom et al., 2012a, 2012b). turning to cleave through Precambrian base- nison and Dolores Rivers, respectively, into Data from the upper Colorado River system have ment perpendicular to the plateau axis in a man- the thick valley fi ll, with no need to appeal to been less well constrained, with regional uplift ner that ignores the regional drainage network differential neotectonic uplift (warping) to pro- and associated major fl uvial incision assumed (Fig. 14). Moreover, this crossing bisects the duce the divide (e.g., inferred by Lohman, 1961, to have begun accelerating ca. 11–6 Ma (e.g., structural crest of the Uncompahgre Plateau, 1981; Hunt, 1969; Cater, 1966; Sinnock, 1981; Aslan et al., 2010; Karlstrom et al., 2012b). The incising through the highest-elevation region Scott et al., 2001; Steven, 2002; as also noted by ca. 11 Ma date is based largely on the occur- (Figs. 1 and 14). Many rivers of the Colorado Oesleby, 1978). rence of river gravels below basal basalt fl ows River system are superposed atop Laramide dated to 10.8 ± 0.2 Ma on the Grand Mesa uplifts, likely refl ecting the positions of ances- Duration of Gunnison Occupation of (Kunk et al., 2002; Aslan et al., 2010) or similar tral stream courses that fl owed through easily Unaweep Canyon, and Implications for regions. Our new thermochronological results erodible strata in precursor lowlands (e.g., Hunt, Drainage Evolution and Drivers of Incision from Unaweep Canyon (Thomson et al., 2012), 1969; Dickinson, 2013). Dickinson (2013) high- Across the Colorado Plateau however, more precisely constrain major fl uvial lighted the example of the upper Colorado River incision in this region to beginning ca. 6–5 Ma, where it cuts through Precambrian basement Rosenberg et al. (2014) compiled incision approximating the values from the lower Colo- around the plunging nose of the northwestern rates for the Colorado River system and noted rado River system. This result supports the plau- Uncompahgre Plateau (Westwater Canyon; that most estimates of long-term incision rates sibility of an essentially synchronous onset of Figs. 1, 2, 13, and 14). Dickinson (2013) called in the upper Colorado River system are between rapid incision across the entire Colorado Plateau upon processes fi rst highlighted by Oberlander ~100 and 150 m/m.y. (cf. Larson et al., 1975; and, by extension, the inference of a regional (1965, 1985), i.e., that this superposition refl ects Kunk et al., 2002; Aslan et al., 2010). Aslan epeirogenic and/or isostatic driver for this rapid stream erosion into alternating layers of resis- et al. (2014) reported that, prior to abandon- incision, with secondary geomorphic (propaga- tant and erodible strata progressively exposed ment of Unaweep Canyon, long-term incision tion of headward erosion) or climatic drivers (cf. as the stream migrated down structural plunge, rates averaged 100 m/m.y. Similarly, long-term Karlstrom et al., 2012b; Rosenberg et al., 2014). ultimately forming a looping transit transverse incision rates reported for various parts of the to the structure, through basement-cored West- Colorado and Gunnison Rivers across the Colo- Late Cenozoic Incision, or Exhumation water Canyon. Figure 14 illustrates how the rado Plateau typically are between ~25 and 150 of Unaweep Canyon? drainage divide of the Uncompahgre Plateau m/m.y. (e.g., Aslan et al., 2008, 2011; Karlstrom trends northwestward south of Unaweep Can- et al., 2008; Cole, 2011; Donahue et al., 2013). The culmination of research over the last yon but curves sharply to the northeast north of Using new thermochronologic constraints for century, including data presented here, has the canyon. The Colorado River migrated from time of onset of incision ca. 6 Ma (Thomson delineated the broad strokes of the late Ceno- the northern margin of the northeast-trending et al., 2012), the elevation difference between zoic history of Unaweep Canyon: a large river drainage divide north of Unaweep Canyon, and the basement from the canyon rim and the (we posit the ancestral Gunnison alone) fl owed continued down plunge toward Westwater Can- borehole (~990 m) and the duration of inci- from northeast to southwest through the canyon, yon, as Dickinson (2013) detailed. The course of sion (4.7 m.y. using the abandonment age of beginning ca. 6–5 Ma (Thomson et al., 2012), the Colorado River follows the structural plunge 1.4–1.3 Ma) imply a time-averaged incision and persisting until canyon abandonment ca. of the base of the Dakota Sandstone around the rate for the ancestral Gunnison River through 1.4–1.3 Ma (Balco et al., 2013; this work), or ca. nose of the Uncompahgre Plateau (Fig. 15). In Unaweep Canyon of ~210–275 m/m.y. This 0.8 Ma by other estimates (Aslan et al., 2014; this model, the resistant layer was the Dakota rate is more than double most published long- Hood et al., 2014). Did this large river, however, Sandstone, which crops out discontinuously at term time-averaged rates for the upper Colorado incise a new canyon, or exhume and reoccupy a all elevations of the Uncompahgre Plateau. The River (Karlstrom et al., 2012b; Rosenberg et al., paleovalley? The latter hypothesis was proposed softer retreating layer was likely the Mancos 2014; Aslan et al., 2014). Short-term rates for (in Soreghan et al., 2007, 2008), in part on the Shale and the Mesa Verde Formation, as they this region can range much higher (e.g., Aslan basis of the occurrence of strata of inferred do not occur anywhere atop the Uncompahgre et al., 2014), but cannot be compared to long- late Paleozoic age in the basal interval of the Plateau, but form retreating fronts on the out- term rates owing to the dependence of incision Unaweep Canyon borehole (Table 1). The old- side loop of the modern Colorado River. Analo-

Geosphere, April 2015 337

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/2/320/3334569/320.pdf by guest on 30 September 2021 Soreghan et al.

Unaweep Canyon existed as a paleo valley in late Paleozoic time, and was subsequently ﬁ lled by Structural contour interval sediment of late-early Permian age (Soreghan ~330 m (1000 ft) et al., 2012) prior to deposition of the super- 6000 ~ 25 km jacent strata that cover the plateau and the greater region. Later, differential compaction between Col 3000 ora the 1 km compactible sedimentary ﬁ ll within the d o River N 7000 3000 4000 paleovalley and the noncompactible crystalline Precambrian basement should have resulted in a 5000 low valley in the overmass burying the ancient Uncompahgre uplift above the Precambrian- 8000 hosted gorge southwest of Cactus Park. The G u n ancient Gunnison-Uncompahgre gravels on top 2965 m.s.l. n is 9000 9727 f.s.l. on of the Uncompahgre Plateau south of Unaweep R iv er Canyon (Aslan et al., 2008) record the positions 4000 of the ancestral Gunnison and Uncompahgre 5000 100001 9000900 Rivers as they migrated northeastward down the

6000 U dipslope of the Uncompahgre Plateau. In this n co m interpretation, the topographic low created by p 7000 8000 a

h the differential compaction above the ancestral g

R gorge would have guided the combined Gunni-

i v 7000 5000 e r son-Uncompahgre river in its 90° turn westward to bisect the Uncompahgre Plateau, ignoring the prevailing northeastward dip slope. East 9000

6000 Creek east of Cactus Park was carved by the ancestral Gunnison River upon canyon aban- 7000 donment. The existence of the Cutler Forma- 8000 tion–ﬁ lled paleovalley at the western mouth of 9000 Unaweep Canyon corroborates the hypothesis of several hundred meters of preserved paleo-

9000 relief here (Soreghan et al., 2012). The rapid 8000 time-averaged incision rates documented from 6000 the thermochronological and geochronological data are consistent with the inference of erosion of a paleovalley fi lled by weak, more erodible 7000 8000 sedimentary strata, rather than primary incision through crystalline bedrock, owing to the relationship between fl uvial erosion and bedrock strength (Sklar and Dietrich , 2001). Figure 15. Structure contour map of the northwestern part of the Uncompahgre Plateau; Ultimately the ideas presented here imply rivers are in blue, Unaweep Canyon is in yellow, fault and fold features are in black, and preservation (through subsidence and burial), structure contours are in red. This map highlights the northwestward-plunging nose and and subsequent exhumation of a very ancient asymmetric northwestward-dipping fl ank of the Uncompahgre Plateau. Note that the struc- landscape, a phenomenon well documented in ture contours are in feet, as derived from the Williams (1964) map: m.s.l.—meters above sea cratonal regions (e.g., Gondwanan continents; level; f.s.l.—feet above sea level. Twidale, 1998, 2003), but generally dismissed for tectonically active landscapes, such as those of the western United States. Recent research on gously, south of Unaweep Canyon, the Gunni- plunge of the uplift, and the northeast dip of its the southern Colorado Plateau, however, has pos- son River (and Uncompahgre) fl owed parallel fl ank (Figs. 13, 14, and 15). The longitudinal ited the existence of paleocanyons (pre–6 Ma, to the spine of the Uncompahgre Plateau con- path of the Gunnison River where it parallels 70 Ma) infl uencing the evolution of the lower trolled by the northwest-trending drainage the axis of the Uncompahgre Plateau upstream Colorado River system and especially the iconic divide, and thus migrated northeastward along of Unaweep Canyon indicates that this part of Grand Canyon (Flowers et al., 2008; Wernicke, the top of the Dakota Sandstone, since that unit the river course was infl uenced by the northeast 2011; Flowers and Farley, 2012, 2013), although forms the youngest unit of the eastern dip slope structural dip of the plateau fl ank, but its perpen- this idea is debated (Dickinson, 2013; Karlstrom (Williams, 1964), transecting the dipping fl ank dicular path across the uplift does not follow the et al., 2013; Lee et al., 2013; Young and Crow, of the plateau. Through Unaweep Canyon, how- pattern of superposition outlined by Oberlander 2014). The example of Unaweep Canyon (and ever, the looping transect model breaks down; (1965, 1985) and Dickinson (2013). perhaps the Grand Canyon) highlights the poten- here, the Gunnison River made a nearly 90° turn The Paleozoic-age hypothesis can explain tial persistence, through burial and exhumation, to cut southwestward across the structural axis this odd map pattern. Consider the possibil- of paleolandforms in even noncratonal regions. in a manner that opposes both the northwest ity that the Precambrian-hosted inner gorge of Exploitation of paleolandforms by recent drain-

338 Geosphere, April 2015

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

ages may have played a greater role than com- drainage network, new mapping demonstrat- evolution of the Colorado Rockies: Evidence for Neo- gene uplift and drainage integration, in Morgan, L.A., monly appreciated in the landscape evolution of ing the existence of a paleovalley of substantial and Quane, S.L., eds., Through the generations: Geo- the western United States. relief at the western mouth of Unaweep Canyon, logic and anthropogenic fi eld excursions in the Rocky and the rapid long-term incision rates docu- Mountains from modern to ancient: Geological Soci- ety of America Field Guide 18, p. 21–54, doi: 10 .1130 CONCLUSIONS mented by the combined geochronologic and /2010 .0018 (02) . thermochrono logic data support the previously Aslan, A., Karlstrom, K., Kirby, E., Darling, A., and Kelley , 1. Composition and paleocurrent analyses of proposed hypothesis that the inner, Precam- S., 2011, Origin of the ancestral Colorado and Gun- nison rivers and post-10 Ma river incision rates in the Gateway gravels near Gateway, Colorado, brian-hosted gorge of Unaweep Canyon was western Colorado, in Beard, L.S., et al., eds., CRevo- comparisons with undisputed ancestral Gunni- initially carved in the late Paleozoic, and was lution 2—Origin and evolution of the Colorado River system, Workshop Abstracts: U.S. Geological Survey son River and Colorado River gravels, published simply exhumed in the late Cenozoic. Open-File Report 2011–1210, p. 22–27. provenance of older (10–2 Ma) gravels in the Aslan, A., Hood, W.C., Karlstrom, K.E., Kirby, E., Granger, region, and other considerations show that the ACKNOWLEDGMENTS D.E., Kelly, S., Crow, R., Donahue, M.S., Polyak, V., and Asmerson, Y., 2014, Abandonment of Unaweep ancestral Gunnison River, and not the ancestral Funding for this research was partially supported Canyon (1.4–0.8 Ma), western Colorado: Effects of stream capture and anomalously rapid Pleistocene river Colorado River, fl owed to the southwest through by grants from the National Science Foundation Unaweep Canyon prior to canyon abandonment incision: Geosphere, v. 10, doi: 10 .1130 /GES00986 .1 . (EAR-0230332 and EAR-0934259) and from the U.S. Baker, V.R., 1990, Spring sapping and valley network devel- at ~1.4 Ma. Geological Survey EDMAP program (G10AC00329, opment, in Higgins, C.G., and Coates, D.R., eds., 2. Previously documented coring within G11AC20217, G12AC20266), including in-kind sup- Groundwater geomorphology; The role of subsurface Unaweep Canyon confi rms the presence of port for fi eld mentorship and mapping from the Colo- water in earth-surface processes and landforms: Geo- rado Geological Survey (V. Matthews and D. Noe), logical Society of America Special Paper 252, p. 235– a thick (locally >330 m) fi ll that includes an and the Oklahoma Geological Survey (R. Keller, 265, doi: 10 .1130 /SPE252 -p235 . ~140 m lacustrine interval dated to ca. 1.3 Ma R. Standridge, and N. Suneson). Marra was partially Balco, G., and Rovey, C.W., II, 2008, An isochron method funded by a student grant from the DOSECC Con- for cosmogenic-nuclide dating of buried soils: Ameri- that exhibits, especially in its lower part, sands can Journal of Science, v. 308, p. 1083–1114, doi:10 of Gunnison River provenance. Lake forma- sortium (Drilling, Observation, Sampling of Earth’s .2475 /10 .2008 .02 . Continental Crust). We thank Himes Drilling (Grand tion resulted from catastrophic mass wasting in Balco, G., Soreghan, G.S., Sweet, D.E., Marra, K.M., and Junction, Colorado) for major in-kind support of drill- Bierman, P., 2013, Cosmogenic-nuclide burial ages the western narrows of Unaweep Canyon that ing operations, and J. Stowell of Mt. Sopris Instru- for Pleistocene sedimentary fi ll in Unaweep Canyon, blocked the ancestral Gunnison River, ultimately ments for well logging. Marra’s work on Unaweep Colorado, USA: Quaternary Geochronology, v. 18, resulting in abandonment of Unaweep Canyon core material benefi tted from Rock-Eval analyses p. 149–157, doi: 10 .1016 /j .quageo .2013 .02 .002 . Bump, A.P., and Davis, G.H., 2003, Late Cretaceous–early by the ancestral Gunnison River between 1.4 and provided by Humble Geochemical, tephra analyses by E. Wan (U.S. Geological Survey), and palynological Tertiary Laramide deformation of the northern Colo- 1.3 Ma, and carving of East Creek canyon as the rado Plateau, Utah and Colorado: Journal of Structural analyses by D. Willard and J. O’Keefe. University of Geology, v. 25, p. 421–440, doi: 10 .1016 /S0191 -8141 river sought lower elevation toward the Grand Oklahoma students D. Ambuehl, T. Foster, L. Keiser, (02)00033 -0 . Valley. The ancestral Gunnison River continued K. Patrick, V. Priegnitz, A. Shock, and A. Sweet pro- Carrara, P.E., 2001, Geologic map of the Clifton Quad- to occupy and incise Cactus Park even after the vided fi eld and laboratory support. We thank the rangle, Mesa County Colorado: U.S. Geological Sur- Powell family for their hospitality, the Massey Ranch vey Miscellaneous Field Studies Map MF-2359, scale abandonment of Unaweep Canyon. for permission to drill, and the many kind landowners 1:24,000. 3. Thermochronological data from Unaweep in the region for property access, including R. Beach, Cater, F.W., Jr., 1955a, Geology of the Gateway Quadrangle: Canyon indicate that the ancestral Gunnison P. Bristol, B. Chesnick, J. Lewis, R. Tipping, the U.S. Geological Survey Quadrangle Map GQ-55, scale 1:24,000. River began to occupy the Precambrian-hosted Larsens, and the Moores. We also thank M. Blum, Cater, F.W., Jr., 1955b, Geology of the Pine Mountain gorge of Unaweep Canyon in the latest Miocene C. Chase, and M. Soreghan for discussions of the geol- Quadrangle: U.S. Geological Survey Quadrangle Map ogy of Unaweep Canyon over the years, and W. Bowen GQ-60, scale 1:24,000.. (ca. 6–5 Ma). This date, coupled with abandon- (http://geogdata .csun .edu) for creating perspective digi- Cater, F.W., Jr., 1966, Age of the Uncompahgre Uplift and ment estimates (1.4–1.3 Ma) produces a time- tal elevation models for our use. Page contributions for Unaweep Canyon, west-central Colorado: U.S. Geo- averaged incision rate of ~210–275 m/m.y., this publication were covered by the Maxey Professor- logical Survey Professional Paper 550-C, p. C86–C92. more than double most long-term incision rates ship, University of Oklahoma. We greatly appreciate Cater, F.W., Jr., 1970, Geology of the Salt anticline region in the considerable time and effort invested in construc- southwestern Colorado: U.S. Geological Survey Pro- of the greater region. tive critiques of an earlier version of this manuscript by fessional Paper 637, 80 p. 4. Onset of canyon occupation and rapid Cole, R.D., 2011, Signifi cance of the Grand Mesa basalt Guest Editor S. Beard and two anonymous reviewers. fi eld in western Colorado for defi ning the early history incision by the ancestral Gunnison River coin- of the upper Colorado River, in Beard, L.S., et al., eds., cided with the timing of integration of the lower REFERENCES CITED CRevolution 2—Origin and Evolution of the Colorado Colorado River system to the Gulf of Califor- River System, Workshop Abstracts: U.S. Geological Abrams, D.M., Lobkovsky, A.E., Petroff, A.P., Straub, K.M., Survey Open-File Report 2011–1210, p. 55–61. nia. The synchroneity of this incision across McElroy, B., Mohrig, D.C., Kudrolli, A., and Rothman, Cole, R.D., and Young, R.G., 1983, Evidence for glacia- the Colorado Plateau supports the inference of D.H., 2009, Growth laws for channel networks incised tion in Unaweep Canyon, Mesa County, Colorado, in an ultimate tectonic or epeirogenic driver for by groundwater fl ow: Nature Geoscience, v. 2, p. 193– Averett , W.R., ed., Northern Paradox Basin–Uncom- 196, doi: 10 .1038 /ngeo432 . pahgre Uplift (Grand Junction Geological Society this widespread incision and ultimate drainage Aslan, A., Livaccari, R., Hood, W., Betton, C., and Garhart, Field Trip Guidebook): Grand Junction, Colorado, integration. A., 2005, Geological history of the Uncompahgre Pla- Grand Junction Geological Society, p. 73–80. teau and Unaweep Canyon: Rocky Mountain Section Critelli, S., Le Pera, E., and Ingersoll, R.V., 1997, The effects 5. Although the late Cenozoic history of Geological Society of America Field Trip Guidebook: of source lithology, transport, deposition and sampling Unaweep Canyon is one of fl uvial occupation Grand Junction, Colorado, Grand Junction Geological scale on the composition of southern California sand: by the ancestral Gunnison River, several per- Society, p. 1–46. Sedimentology, v. 44, p. 653–671, doi:10 .1046 /j .1365 Aslan, A., and 10 others, 2008, River incision histories of the -3091 .1997 .d01 -42 .x . plexing aspects of the canyon remain enigmatic, Black Canyon of the Gunnison and Unaweep Canyon: Czapla, D., and Aslan, A., 2009, Evidence of a Miocene and are reconciled by considering the possibility Interplay between late Cenozoic tectonism, climate Colorado River, western Colorado: Geological Society that the Cenozoic river exploited and ultimately change, and drainage integration in the western Rocky of America Abstracts with Programs, v. 41, no. 6, p. 39. Mountains, in Raynolds, R.G., ed., Roaming the Rocky Davis, G., and Bump, A., 2009, Structural geologic evolu- revealed a paleolandform. The anomalous Mountains and environs: Geological fi eld trips: Geo- tion of the Colorado Plateau, in Kay, S.M., et al., eds., course of the canyon across the highest-eleva- logical Society of America Field Guide 10, p. 175–202, Backbone of the Americas: Shallow subduction, pla- doi: 10 .1130 /2008 .fl d010 (09) . teau uplift, and ridge and terrane collision: Geologi- tion Precambrian surface of the Uncompahgre Aslan, A., Karlstrom, K.E., Crossey, L.J., Kelley, S., Cole, cal Society of America Memoir 204, p. 99–124, doi: 10 Plateau in a manner that ignores the regional R., Lazear, G., and Darling, A., 2010, Late Cenozoic .1130 /2009 .1204 (05) .

Geosphere, April 2015 339

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/2/320/3334569/320.pdf by guest on 30 September 2021 Soreghan et al.

Dickinson, W.R., 2013, Rejection of the lake spillover model Howard, A.D., Kochel, R.C., and Holt, H., 1988, Sapping Geological Society of America Bulletin, v. 119, for initial incision of the Grand Canyon, and discussion features of the Colorado Plateau: A comparative plan- p. 805–822, doi: 10 .1130 /B25986 .1 . of alternatives: Geosphere, v. 9, p. 1–20, doi:10 .1130 etary geology fi eld guide: National Aeronautics and Lamb, M.P., Dietrich, W.E., Aciego, S.M., DePaolo, S.M., /GES00839 .1 Space Administration Special Publication 491, 108 p. and Manga, M., 2008, Formation of Box Canyon, Donahue, M.S., Karlstrom, K.E., Aslan, A., Darling, A., Hunt, C.B., 1956, Cenozoic geology of the Colorado Pla- Idaho, by megafl ood: Implications for seepage erosion Granger, D., Wan, E., Dickinson, R.G., and Kirby, E., teau: U.S. Geological Survey Professional Paper 270, on Earth and Mars: Science, v. 320, p. 1067–1070, doi: 2013, Incision history of the Black Canyon of Gunni- 99 p. 10 .1126 /science.1156630 . son, Colorado, over the past ~1 Ma inferred from dat- Hunt, C.B., 1969, The Colorado River region and John Lamb, M.P., Mackey, B.H., and Farley, K.A., 2014, Amphi- ing of fl uvial gravel deposits: Geosphere, v. 9, p. 815– Wesley Powell, geologic history of the Colorado River: theater-headed canyons formed by megafl ooding at 826, doi:10 .1130 /GES00847 .1 . U.S. Geological Survey Professional Paper 669-C, Malad Gorge, Idaho: National Academy of Sciences Dorsey, R.J., Fluette, A., McDougall, K., Housen, B.A., p. C59–C130. Proceedings, v. 111, p. 57–62, doi:10 .1073 /pnas Janecke, S.U., Axen, G.J., and Shirvell, C.R., 2007, Huntington, K.W., Wernicke, B.P., and Eiler, J.M., 2010, .1312251111 . Chronology of Miocene–Pliocene deposits at Split Infl uence of climate change and uplift on Colorado Larson, E.E., Ozima, M., and Bradley, W.C., 1975, Late Mountain Gorge, southern California: Record of Plateau paleotemperatures from carbonate clumped Cenozoic basic volcanism in northwestern Colorado regional tectonics and Colorado River evolution: Geol- isotope thermometry: Tectonics, v. 29, TC3005, doi: 10 and its implications concerning tectonism and the ori- ogy, v. 35, p. 57–60, doi: 10 .1130 /G23139A .1 . .1029 /2009TC002449 . gin of the Colorado River System, in Curtis, B.F., ed., Dorsey, R.J., Housen, B.A., Janecke, S.U., Fanning, C.M., Ingersoll, R.V., Bullard, R.F., Ford, R.L., Grimm, J.P., Cenozoic history of the southern Rocky Mountains: and Spears, A.L.F., 2011, Stratigraphic record of basin Pickle, J.D., and Sares, S.W., 1984, The effect of grain Geological Society of America Memoir 144, p. 155– development within the San Andreas fault system: Late size on detrital modes: A test of the Gazzi-Dickinson 178, doi: 10 .1130 /MEM144 -p155 . Cenozoic Fish Creek–Vallecito basin, southern Cali- point counting method: Journal of Sedimentary Petrol- Lee, J.P., Stockli, D.F., Kelley, S.A., Pederson, J.L., Karl- fornia: Geological Society of America Bulletin, v. 123, ogy, v. 54, p. 103–116. strom, K.E., and Ehlers, T.A., 2013, New thermochrono- p. 771–793, doi: 10 .1130 /B30168 .1 . Kaplan, S.A., 2006, Revealing Unaweep Canyon: The late metric constraints on the Tertiary landscape evolution of Eccles, T.M., 2013, Structure of the southwestern Uncom pahgre Cenozoic exhumation history of Unaweep Canyon the central and eastern Grand Canyon, Arizona: Geo- Plateau (western Colorado) near Unaweep Canyon as recorded by gravels in Gateway, Colorado [M.S. sphere, v. 9, p. 216–228, doi: 10 .1130 /GES00842 [M.S. thesis]: Norman, University of Oklahoma, 97 p. thesis ]: Norman, University of Oklahoma, 52 p. Liu, L., and Gurnis, M., 2010, Dynamic subsidence and Finnegan, N.J., Schumer, R., and Finnegan, S., 2014, A sig- Karlstrom, K.E., Crow, R.S., Peters, L., McIntosh, W., uplift of the Colorado Plateau: Geology, v. 38, p. 663– nature of transience in bedrock river incision rates over Raucci, J., Crossey, L.J., Umhoefer, P., and Dunbar, 666, doi: 10 .1130 /G30624 .1 . timescales of 104–107 years: Nature, v. 505, p. 391– N., 2007, 40Ar/39Ar and fi eld studies of Quaternary Lohman, S.W., 1961, Abandonment of Unaweep Canyon, 394, doi: 10 .1038 /nature12913 . basalts in Grand Canyon and model for carving Grand Mesa County, Colorado, by capture of the Colorado Flowers, R.M., and Farley, K.A., 2012, Apatite 4He/3He and Canyon: Quantifying the interaction of river incision and Gunnison Rivers, in Geological Survey Research (U-Th)/He evidence for an ancient Grand Canyon: and normal faulting across the western edge of the 1961: U.S. Geological Survey Professional Paper 424, Science, v. 338, p. 1616–1619, doi: 10 .1126 /science Colorado Plateau: Geological Society of America Bul- p. B144–B146. .1229390 . letin, v. 119, p. 1283–1312, doi:10 .1130 /0016 -7606 Lohman, S.W., 1965, Geology and artesian water supply of Flowers, R.M., and Farley, K.A., 2013, Apatite 4He/3He and (2007)119 [1283 :AAFSOQ]2.0 .CO;2 . the Grand Junction area, Colorado: U.S. Geological (U-Th)/He evidence for an ancient Grand Canyon: Karlstrom, K.E., Crow, R., Crossey, L., Coblentz, D., and Survey Professional Paper 451, 149 p. Reply: Science, v. 340, no. 6129, p. 143, doi: 10 .1126 Van Wijk, J.W., 2008, Model for tectonically driven Lohman, S.W., 1981, Ancient drainage changes in and /science.1234203 . incision of the younger than 6 Ma Grand Canyon: south of Unaweep Canyon, southwestern Colorado, Flowers, R.M., Wernicke, B.P., and Farley, K.A., 2008, Geology, v. 36, p. 835–838, doi: 10 .1130 /G25032A .1 . in Epis, R.C., and Callender, J.F., eds., Western Slope Unroofi ng, incision, and uplift history of the south- Karlstrom, K.E., Beard, L.S., House, K., Young, R.A., Aslan, Colorado: New Mexico Geological Society 32nd Field western Colorado Plateau from apatite (U-Th/He) A., Billingsley, G., and Pederson, J., 2012a, Introduc- Conference Guidebook, p. 137–143. thermochronometry: Geological Society of America tion: CRevolution2: Origin and Evolution of the Colo- Lucchitta, I., 2003, History of the Grand Canyon and of the Bulletin, v. 120, p. 571–587, doi: 10 .1130 /B26231 .1 . rado River System II: Geosphere, v. 8, p. 1170–1176, Colorado River in Arizona, in Bues, S.S., and Morales, Frahme, C.W., and Vaughn, E.B., 1983, Paleozoic geology doi: 10 .1130 /GES00716 .1 . M., eds., Grand Canyon geology (second edition): New and seismic stratigraphy of the northern Uncompahgre Karlstrom, K.E., 23 others, and the CREST Working Group, York, Oxford University Press, p. 260–274. Front, Grant County, Utah, in Rocky Mountain fore- 2012b, Mantle-driven dynamic uplift of the Rocky Machette, M.N., 1985, Calcic soils of southwestern United land basins and uplifts: Rocky Mountain Association of Mountains and Colorado Plateau and its surface States, in Weide, D.L., ed., Soils and Quaternary geol- Geologists Field Conference Guidebook, p. 201–211. response: Toward a unifi ed hypothesis: Lithosphere, ogy of the southwestern United States: Geological Gannett, H., 1882, The Unaweep Cañon (Colorado): Popular v. 4, p. 3–22, doi: 10 .1130 /L150 .1 . Society of America Special Paper 203, p. 1–21, doi:10 Science Monthly, v. 20, p. 781–786. Karlstrom, K.E., and 10 others, 2013, Apatite 4He/3He and .1130 /SPE203 -p1 . Hood, W.C., 2009, An exhumed late Paleozoic Canyon in (U-Th)/He evidence for an ancient Grand Canyon: Mack, G.H., 1997, Continuing education manual on paleo- the Rocky Mountains: A discussion: Journal of Geol- Comment: Science, v. 340, no. 6129, p. 143, doi: 10 sols for sedimentologists: Geological Society of Amer- ogy, v. 117, p. 210–214, doi: 10 .1086 /595789 . .1126 /science.1233982 . ica Annual Meeting Short Course, 114 p. Hood, W.C., 2011, Unaweep Canyon—Which river ran Kelley, V.C., 1955, Monoclines of the Colorado Plateau: Marra, K.M., 2008, Late Cenozoic geomorphic and climatic through it?: Mountain Geologist, v. 48, p. 45–57. Geological Society of America Bulletin, v. 66, p. 789– evolution of the northeastern Colorado Plateau as Hood, W., Cole, R., and Aslan, A., 2009, Anomalous cold 804, doi: 10 .1130 /0016 -7606 (1955)66 [789 :MOTCP]2 recorded by Plio-Pleistocene sediment fi ll in Unaweep in the Pangaean tropics: Comment: Geology, v. 37, .0 .CO;2. Canyon, Colorado [M.S. thesis]: Norman, University p. e192, doi: 10 .1130 /G30035C .1 . Kluth, C.F., and Coney, P.J., 1981, Plate tectonics of the of Oklahoma, 160 p. Hood, W.C., Aslan, A., and Betton, C., 2014, Aftermath of Ancestral Rocky Mountains: Geology, v. 9, p. 10–15, Marsaglia, K.M., 1993, Basaltic island sand provenance, in a stream capture: Cactus Park lake spillover and the doi: 10 .1130 /0091 -7613 (1981)9 <10 :PTOTAR>2 .0 Johnsson, M.J., and Basu, A., eds., Processes control- origin of East Creek, Uncompahgre Plateau, western .CO;2. ling the compositions of clastic sediments: Geological Colorado: Geosphere, v. 10, doi: 10 .1130 /GES00970 Kunk, M.J., Budahn, J.R., Unruh, D.M., Stanley, J.O., Society of America Special Paper 284, p. 41–65, doi: House, P.K., Pearthree, P.A., Howard, K.A., Bell, J.W., Kirkham, R.M., Bryant, B., Scott, R.B., Lidke, D.J., and 10 .1130 /SPE284 -p41 . Perkins , M.E., Faulds, J.E., and Brock, A.L., 2005, Streufert, R.K., 2002, 40Ar/39Ar ages of late Cenozoic McKee, E.D., and McKee, E.H., 1972, Pliocene uplift of Birth of the lower Colorado River—Stratigraphic volcanic rocks within and around the Carbondale and the Grand Canyon region—Time of drainage adjust- and geomorphic evidence for its inception near the Eagle collapse centers, Colorado: Constraints on the ment: Geological Society of America Bulletin, v. 83, conjunction of Nevada, Arizona, and California, in timing of evaporite-related collapse and incision of the p. 1923–1932, doi: 10 .1130 /0016 -7606 (1972)83 [1923 Peder son, J.L., and Dehler, C.M., eds., Interior West- Colorado River, in Scott, R.B., et al., eds., Late Ceno- :PUOTGC]2 .0 .CO;2 . ern United States: Geological Society of America Field zoic evaporite tectonism and volcanism in west-central McMillan, M.E., Heller, P.L., and Wing, L.W., 2006, History Guide 6, p. 357–387, doi: 10 .1130 /2005 .fl d006 (17) . Colorado: Geological Society of America Special Paper and causes of post-Laramide relief in the Rocky Moun- House, P.K., Pearthree, P.A., and Perkins, M.E., 2008, 366, p. 213–234, doi: 10 .1130 /0 -8137 -2366 -3 .213 . tain orogenic plateau: Geological Society of America Stratigraphic evidence for the role of lake spillover in Laity, J.E., and Malin, M.C., 1985, Sapping processes and Bulletin, v. 118, p. 393–405, doi: 10 .1130 /B25712 .1 . the inception of the lower Colorado River in southern the development of theater-headed valley networks on McQuarrie, N., and Chase, C., 2000, Raising the Colorado Nevada and western Arizona, in Reheis, M.C., et al., the Colorado Plateau: Geological Society of America Plateau: Geology, v. 28, p. 91–94, doi:10 .1130 /0091 eds., Late Cenozoic drainage history of the southwest- Bulletin, v. 96, p. 203–217, doi: 10 .1130 /0016 -7606 -7613 (2000)028 <0091 :RTCP>2 .0 .CO;2 . ern Great Basin and lower Colorado River region: Geo- (1985)96 <203 :SPATDO>2 .0 .CO;2 . Moore, K.D., Soreghan, G.S., and Sweet, D.E., 2008, Strati- logic and biotic perspectives: Geological Society of Lamb, M.P., Howard, A.D., Johnson, J., Whipple, K.X., graphic and structural relations in the proximal Cutler America Special Paper 439, p. 335–353, doi:10 .1130 Dietrich, W.E., and Perron, J.T., 2006, Can springs cut Formation of the Paradox Basin: Implications for tim- /2008 .2439 (15) . canyons into rock?: Journal of Geophysical Research, ing of movement on the Uncompahgre front: Mountain Howard, A.D., and McLane, C.F., 1988, Erosion of cohe- v. 111, no. E07002, doi: 10 .1029 /2005JE002663 . Geologist, v. 45, p. 49–68. sionless sediment by groundwater seepage: Water Lamb, M.P., Howard, A.D., Dietrich, W.E., and Perron, J.T., Morgan, P., 2003, Colorado Plateau and southern Rocky Resources Research, v. 24, p. 1659–1674, doi: 10 .1029 2007, Formation of amphitheater-headed valleys by Mountains uplift and erosion, in Raynolds, R.G., and /WR024i010p01659 . waterfall erosion after large-scale slumping on Hawaii: Flores, R.M., eds., Cenozoic systems of the Rocky

340 Geosphere, April 2015

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

Mountain region: Denver, Colorado, Rocky Mountain in Smith, D.G., et al., eds., Strata and time: Probing the Spencer, J., 1996, Uplift of the Colorado Plateau due to Section SEPM (Society for Sedimentary Geology), gaps in our understanding: Geological Society of Lon- lithospheric attenuation during Laramide low-angle p. 1–31. don Special Publication 404, doi.org /10 .1144 /SP404 .7 subduction: Journal of Geophysical Research, v. 101, Morgan, P., and Swanberg, C.A., 1985, On the Cenozoic Sahagian, D., Proussevitch, A., and Carlson, W., 2002, Tim- p. 13,595–13,609, doi: 10 .1029 /96JB00818 . uplift and tectonic stability of the Colorado Plateau: ing of Colorado Plateau uplift: Initial constraints from Spencer, J.E., Peters, L., McIntosh, W.C., and Patchett, P.J., Journal of Geodynamics, v. 3, p. 39–63, doi: 10 .1016 vesicular basalt-derived paleoelevations: Geology, 2001, 40Ar/39Ar geochronology of the Hualapai Lime- /0264 -3707 (85)90021-3 . v. 30, p. 807–810, doi: 10 .1130 /0091 -7613 (2002)030 stone and Bouse Formation and implications for the Oberlander, T.M., 1965, The Zagros streams: Syracuse Uni- <0807 :TOCPUI>2 .0 .CO;2 . age of the lower Colorado River, in Young, R.A., and versity Geographical Series no. 1, 168 p. Schumm, S.A., Boyd, K.F., Wolff, C.G., and Spitz, W.J., Spamer, E.E., eds., Colorado River origin and evolu- Oberlander, T.M., 1985, Origin of drainage transverse to 1995, A ground-water sapping landscape in the Florida tion: Grand Canyon, Arizona, Grand Canyon Associa- structures in orogens, in Morisawa, M., and Hack, J.T., Panhandle: Geomorphology, v. 12, p. 281–297, doi: 10 tion, p. 89–92. eds., Tectonic geomorphology: Proceedings of the 15th .1016 /0169 -555X (95)00011 -S . Steven, T.A., 2002, Late Cenozoic tectonic and geomorphic Annual Binghamton Geomorphology Symposium: Scott, R.B., Harding, A.E., Hood, W.C., Cole, R.D., framework surrounding the evaporite dissolution area London, Allen & Unwin, Inc., p. 155–182. Livaccai, R.F., Johnson, J.B., Shroba, R.R., and Dick- in west-central Colorado, in Scott, R.B., et al., eds., O’Connor, J.E., 1993, Hydrology, hydraulics and geomor- erson, R.P., 2001, Geologic map of Colorado National Late Cenozoic evaporite tectonism and volcanism in phology of the Bonneville Flood: Geological Society of Monument and adjacent areas, Mesa County, Colo- west-central Colorado: Geological Society of America America Special Paper 274, 83 p., doi: 10 .1130 /SPE274 . rado: U.S. Geological Survey Geologic Investigations Special Paper 366, p. 15–30, doi: 10 .1130 /0 -8137 -2366 Oesleby, T.W., 1978, Uplift and deformation of the Series I-2740, 40 p. -3 .15 . Uncompahgre Plateau: Evidence from fi ll thickness Shoemaker, E.M., 1954, Structural features of southwestern Stokes, W.L., 1948, Geology of the Utah-Colorado salt- in Unaweep Canyon, west-central Colorado [M.S. Utah and adjacent parts of Colorado, New Mexico, and dome region, with emphasis on Gypsum Valley, Colo- thesis ]: Boulder, University of Colorado, 122 p. Arizona, in Coffi n, R.C., et al., eds., Uranium deposits rado: Utah Geological Survey Guidebook 3, 50 p. Oesleby, T.W., 1983, Geophysical measurement of valley- and general geology of southeastern Utah: Guidebook Thomson, S.N., Soreghan, G.S., and Eccles, T.M., 2012, fi ll thickness in Unaweep Canyon, west-central Colo- to the Geology of Utah number 9: Salt Lake City, Utah Elevated Cenozoic geothermal gradients and later rado, in Averett, W.R., ed., Northern Paradox Basin– Geological Society, p. 48–69. post–6 Ma incision of the Uncompahgre Plateau and Uncompahgre Uplift (Grand Junction Geological Sinnock, S., 1978, Geomorphology of the Uncompahgre Pla- Unaweep Canyon (western Colorado) revealed by low Society Field Trip Guidebook): Grand Junction, Colo- teau and Grand Valley, western Colorado [M.S. thesis]: temperature thermochronology: Geological Society of rado, Grand Junction Geological Society, p. 71. West Lafayette, Indiana, Purdue University, 201 p. America Abstracts with Programs, v. 44, no. 6, p. 18. Oesleby, T.W., 2005, Thick sediment fi ll in Unaweep Can- Sinnock, S., 1981, Pleistocene drainage changes in the Uncom- Twidale, C.R., 1998, Antiquity of landforms: an ‘extremely yon: Implications for the history of the Uncompahgre pahgre Plateau–Grand Valley region of western Colo- unlikely’ concept vindicated: Australian Journal uplift, western Colorado, in Geological History of the rado, including formation and abandonment of Unaweep of Earth Sciences, v. 45, p. 657–668, doi: 10 .1080 Uncompahgre Plateau and Unaweep Canyon, 2005 Canyon: A hypothesis, in Epis, R.C., and Callender, J.F., /08120099808728422 . Guidebook: Rocky Mountain Section of Geological eds., Western Slope Colorado: New Mexico Geological Twidale, C., 2003, The enigma of survival; problems posed Society of America, Mesa State College, 10 p. Society 32nd Field Conference Guidebook, p. 127–136. by very old paleosurfaces: Physical Geography, v. 24, Peale, A.C., 1877, Geological report on the Grand River dis- Sklar, L.S., and Dietrich, W.E., 2001, Sediment and rock- p. 26–60, doi: 10 .2747 /0272 -3646 .24 .1 .26 . trict: U.S. Geological and Geographical Survey of the strength controls on river incision into bedrock: Geol- Van der Plas, L., and Tobi, A.C., 1965, A chart for judging Territories, Ninth Annual Report, p. 31–102. ogy, v. 29, p. 1087–1090, doi: 10 .1130 /0091 -7613 the reliability of point counting results: American Jour- Pederson, J.L., Mackley, R.D., and Eddleman, J.L., 2002, (2001)029 <1087 :SARSCO>2.0 .CO;2 . nal of Science, v. 263, p. 87–90, doi: 10 .2475 /ajs .263 Colorado Plateau uplift and erosion evaluated using Soreghan, G.S., and Sweet, D.E., 2013, New views on late .1 .87 . GIS: GSA Today, v. 12, p. 4–10, doi:10 .1130 /1052 Paleozoic climate and tectonics in the Ancestral Rocky van Wijk, J.W., Baldridge, W.S., van Hunen, J., Goes, S., -5173 (2002)012<0004 :CPUAEE>2.0 .CO;2. Mountains, in Abbot, L.D., and Hancock, G.S., eds., Aster, R., Coblentz, D.D., Grand, S.P., and Ni, J., 2010, Pitlick, J., Van Steeter, M., Barkett, B., Cress, R., and Fran- Classic concepts and new directions: Exploring 125 Small-scale convection at the edge of the Colorado seen, M., 1999, Geomorphology and hydrology of years of GSA discoveries in the Rocky Mountain Plateau: Implications for topography, magmatism, and the Colorado and Gunnison Rivers and implications region: Geological Society of America Field Guide 33, evolution of Proterozoic lithosphere: Geology, v. 38, for habitats used by endangered fi shes: U.S. Fish and p. 295–330, doi: 10 .1130 /2013 .0033 (12) . p. 611–614, doi: 10 .1130 /G31031 .1 . Wildlife Service Recovery Program project 44-B, 58 p. Soreghan, G., Sweet, D., Marra, K., Eble, C., Soreghan, Wernicke, B., 2011, The California River and its role in Potochnik, A.R., 1989, Depositional style and tectonic impli- M., Elmore, R., Kaplan, S., and Blum, M., 2007, An carving Grand Canyon: Geological Society of America cations of the Mogollon Rim Formation (Eocene), east- exhumed late Paleozoic canyon in the Rocky Moun- Bulletin, v. 123, p. 1288–1316, doi: 10 .1130 /B30274 .1 . central Arizona, in Anderson, O.J., et al., eds., South- tains: Journal of Geology, v. 115, p. 473–481, doi: 10 White, M.A., and Jacobson, M.I., 1983, Structures associ- eastern Colorado Plateau: New Mexico Geological .1086 /518075 . ated with the southwest margin of the ancestral Uncom- Society 40th Field Conference Guidebook, p. 107–118. Soreghan, G.S., Soreghan, M.J., Poulsen, C.J., Young, R.A., pahgre Uplift, in Averett, W.R., ed., Northern Paradox Potochnik, A.R., 2001, Paleogeomorphic evolution of the Salt Eble, C.F., Sweet, D.E., and Davogustto, O.C., 2008, Basin–Uncompahgre Uplift (Grand Junction Geological River region: Implications for Cretaceous-Laramide Anomalous cold in the Pangaean tropics: Geology, Society Field Trip Guidebook): Grand Junction, Colo- inheritance for ancestral Colorado River drainage, in v. 36, p. 659–662. rado, Grand Junction Geological Society, p. 33–39. Young, R.A., and Spamer, E.E., eds., Colorado River Soreghan, G.S., Sweet, D.E., Marra, K.R., Eble, C.F., Williams, P.L., 1964, Geology, structure, and uranium origin and evolution: Grand Canyon, Arizona, Grand Soreghan, M.J., Elmore, R.D., Kaplan, S.A., and Blum, deposits of the Moab Quadrangle, Colorado and Utah: Canyon Association, p. 17–22. M.D., 2009a, An exhumed late Paleozoic canyon in the U.S. Geological Survey Miscellaneous Geologic Inves- Price, R., Karlstrom, K.E., Donahue, M., Aslan, A., and Rocky Mountains: A reply: Journal of Geology, v. 117, tigations Map I-360, scale 1:250,000. Pecha, M., 2012, Detrital zircon analysis of high ter- p. 215–220, doi: 10 .1086 /595788 . Yeend, W.E., 1969, Quaternary geology of the Grand and races of the early Colorado River system (Crooked Soreghan, G.S., Soreghan, M., Poulsen, C., Young, R., Eble, Battlement Mesa area, Colorado: U.S. Geological Sur- Ridge River, Grand Mesa, and upper Green River): C., Sweet, D., and Davogustto, O., 2009b, Anomalous vey Professional Paper 617, 47 p. Implications for Colorado Plateau drainage evolution: cold in the Pangaean tropics: Reply: Geology, v. 37, Young, R.A., 1982, Paleogeomorphic evidence for the Geological Society of America Abstracts with Pro- p. e193–e194, doi: 10 .1130 /G30148Y .1 . structural history of the Colorado Plateau margin in grams, v. 44, no. 6, p. 20. Soreghan, G.S., Soreghan, M.J., Sweet, D.E., and Moore, western Arizona, in Frost, E.G., and Martin, D.L., eds., Retallack, G.J., 2001, Soils of the past, an introduction to K.D., 2009c, Hot fan or cold outwash? Hypothesized Mesozoic–Cenozoic tectonic evolution of the Colorado paleopedology: Oxford, Blackwell Sciences Ltd., proglacial deposition in the upper Paleozoic Cutler For- River region, California, Arizona, and Nevada: San 404 p. mation, western tropical Pangea: Journal of Sedimentary Diego, California, Cordilleran Publishers, p. 29–39. Richmond, G.M., 1962, Quaternary stratigraphy of the La Research, v. 79, p. 495–522, doi:10 .2110 /jsr .2009 .055 . Young, R.A., and Crow, R., 2014, Paleogene Grand Canyon Sal Mountains, Utah: U.S. Geological Survey Profes- Soreghan, G.S., Keller, G.R., Gilbert, M.C., Chase, C.G., incompatible with Tertiary paleogeography and stra- sional Paper 324, 135 p. and Sweet, D., 2012, Load-induced subsidence of the tigraphy: Geosphere, v. 10, doi: 10 .1130 /GES00973 .1 Rosenberg, R., Kirby, E., Aslan, A., Karlstrom, K., Heizler, Ancestral Rocky Mountains recorded by preservation Young, R.A., and McKee, E.H., 1978, Early and middle M., and Ouimet, W., 2014, Late Miocene erosion and of Permian landscapes: Geosphere, v. 8, p. 654–668, Cenozoic drainage and erosion in west-central Ari- evolution of topography along the western slope of doi: 10 .1130 /GES00681 .1 . zona: Geological Society of America Bulletin, v. 89, the Colorado Rockies: Geosphere, v. 10, doi: 10 .1130 Soreghan, G.S., Sweet, D.E., and Heavens, N.G., 2014, p. 1745–1750, doi:10 .1130 /0016 -7606 (1978)89 /GES00989 .1 . Upland glaciation in tropical Pangaea: Geologic evi- <1745 :EAMCDA>2 .0 .CO;2 . Sadler, P.M., and Jerolmack, D.J., 2014, Scaling laws for dence and implications for late Paleozoic climate mod- Young, R.A., and Spamer, E.E., 2001, Colorado River origin aggradation, denudation, and progradation: The case eling: Journal of Geology, v. 122, p. 137–163, doi:10 and evolution: Grand Canyon, Arizona, Grand Canyon for time-scale invariance at sediment sources and sinks, .1086 /675255 . Association, 280 p.

Geosphere, April 2015 341

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/2/320/3334569/320.pdf by guest on 30 September 2021