Implications of the Miocene(?) Crooked Ridge River of Northern Arizona for the Evolution of the Colorado River and Grand Canyon

CRevolution 2: Origin and Evolution of the Colorado River System II themed issue

Implications of the Miocene(?) Crooked Ridge River of northern Arizona for the evolution of the Colorado River and Grand Canyon

Ivo Lucchitta1,*, Richard F. Holm2, and Baerbel K. Lucchitta3 1U.S. Geological Survey, Flagstaff, Arizona 86001, USA, and Museum of Northern Arizona, Flagstaff, Arizona 86001, USA 2Northern Arizona University, Flagstaff, Arizona 86011, USA 3U.S. Geological Survey, Flagstaff, Arizona 86001, USA

ABSTRACT epic journey of discovery on the Green and Col- river fl owed southward through Peach Springs orado Rivers. The debate has illuminated not Canyon (Fig. 1) until late Miocene time, when it The southwesterly course of the probably only the history of the Colorado River itself, but established its present course in western Grand pre–early Miocene and possibly Oligocene of rivers and canyons in general. Of especial sci- Canyon by some subterranean piping mecha- Crooked Ridge River can be traced continu- entifi c and popular interest is the question: How nism. The problem was not solved, however, ously for 48 km and discontinuously for 91 did this canyon, the Grand, come to be? because the widespread interior-basin deposits km in northern Arizona (United States). Views on the issue are in two main groups: would have blocked this course as well. Such The course is visible today in inverted relief. One holds that the river is old (possibly as old deposits are ubiquitous in the Basin and Range Pebbles in the river gravel came from at least as Eocene) and has always had approximately province, not just along the lower Colorado as far northeast as the San Juan Mountains its present confi guration; the other holds that River area. Furthermore, Young’s (1979, 1982) (Colorado). The river valley was carved out the river has achieved its present course only work showed that fl uvial deposits in Peach of easily eroded Jurassic and Cretaceous relatively recently (generally near the end of Springs Canyon were deposited by streams rocks whose debris overloaded the river with the Miocene) through one of several processes fl owing north, not south as proposed by Hunt. abundant detritus, probably steepening the of integration, including headward erosion and The stage was set for the notion that an ancient gradient. After the river became inactive, the stream capture, lake spillover, subterranean pip- upper river and a much younger lower one were regional drainage network was rearranged ing, and reactivation of pre-existing canyons, integrated after 5–6 Ma into a single river with three times, and the nearby Four Corners possibly of early Tertiary age. the present course. This idea was strongly infl u- region was lowered 1–2 km by erosion. The Most of the early workers from Powell on enced by the then-new discovery that the upper river provides constraints on the early evolu- (Powell, 1875; Dutton, 1882; Davis, 1901) Gulf of California, into which the Colorado tion of the Colorado River and Grand Can- were proponents of the fi rst notion, as was Hunt fl ows, had opened only in late Miocene to early yon. Continuation of this river into lakes in (1956, 1969), who painstakingly assembled a Pliocene time (Durham and Allison, 1960). Arizona or Utah is unlikely, as is integration host of information about ancient courses of the McKee et al. (1967) argued that the old upper through Grand Canyon by lake spillover. Colorado and San Juan Rivers on the Colorado river reached the east side of the Kaibab Plateau The downstream course of the river prob- Plateau. More recent work generally is part of (Fig. 1), which was seen as an insurmountable ably was across the Kaibab arch in a valley the second group. barrier, and then fl owed southeast along the roughly coincident with the present eastern Studies in the early to mid-1900s in the Basin present alignment of the Little Colorado River Grand Canyon. Beyond this point, the course and Range province along the present course of into the Rio Grande. In latest Miocene time this may have continued to the drainage basin of the lower Colorado River (Lee, 1908; Black- ancestral river was captured east of the Kai- the Sacramento River, or to the proto–Snake welder, 1934; Longwell, 1936, 1946) brought bab Plateau and diverted into its present course River drainage. Crooked Ridge River was about the transition from earlier views to the through Grand Canyon by a vigorous young beheaded by the developing San Juan River, later ones. These studies showed that areas now stream that propagated itself from the Gulf of which pirated its waters and probably was traversed by the Colorado are fi lled with middle California by headward erosion. However, the tributary to a proto–Colorado River, fl owing and late Miocene interior-basin deposits. This course into the Rio Grande was not supported by roughly along its present course west of the was later confi rmed in detail for the Pierce Ferry available data, so Lucchitta (1975, 1984, 1989, Monument upwarp. area at the mouth of Grand Canyon (Fig. 1) by 2013), making use of improved understanding Lucchitta (1966, 1967, 1972, 2013). The con- of the paleogeology and paleotopography of the INTRODUCTION clusion was that no Colorado River could have region, proposed instead that the ancient river existed in its present Basin and Range course had crossed the Kaibab Plateau in an arcuate Debate on how and when the Colorado River until after 5–6 Ma. strike valley controlled by the south-plunging and Grand Canyon (southwestern United States) The question then arose: What was the course part of the Kaibab dome. This valley followed the came into being as we know them today has of the mid-Miocene or earlier Colorado River present alignment of eastern Grand Canyon (see continued in the 140 years since J.W. Powell’s on the western Colorado Plateau for the 10+ Babenroth and Strahler, 1945), then continued million years when it did not fl ow into the Gulf northwest along regional strike, bypassing west- *Corresponding author: [email protected]. of California? Hunt (1969) suggested that the ern Grand Canyon and the lower Colorado River

Geosphere; December 2013; v. 9; no. 6; p. 1417–1433; doi:10.1130/GES00861.1; 14 ﬁ gures; 3 tables. Received 14 August 2012 ♦ Revision received 14 August 2013 ♦ Accepted 28 August 2013 ♦ Published online 11 October 2013

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Colorado River Uncompahgre Plateau Henry Mtns.

La Sal Mtns. Needle Mtns

San Juan Colorado River Abajo Mtns. Mtns. Marble Kaibab Plateau Canyon San Juan River Ute Mtns. Pierce Kaibito White Mesa Ferry Plateau Carrizo Mtns. Western /arch Grand Canyon Black Mesa Crooked Ridge

Eastern Little Colorado Grand Canyon River Hopi Buttes (area of "Hopi Lake")

Hualapai Peach Springs Plateau Canyon

Figure 1. Regional view of southwestern Colorado Plateau from the San Juan Mountains to the western Plateau edge near Pierce Ferry. Base map extracted from 10 m-resolution U.S. Geological Survey (USGS) National Elevation Dataset by J. Luke Blair, USGS. Labels by authors. Note: bar scale is accurate.

area. Subsequent capture and diversion by the history of the Colorado River and its integration in directions (generally north or northeast) other younger stream would have been as per McKee into the present course through Grand Canyon. than the present one (e.g., Scarborough, 2001; et al. (1967), but west of the Kaibab Plateau. Some have ignored the evidence for Muddy Potochnik, 2001; Hill and Ranney, 2008). These proposals contain two major novel- Creek (Miocene) interior-basin deposition at Three current concepts are of particular inter- ties: One is that the Colorado River in its pres- the mouth of Grand Canyon (e.g., Robert et al., est in the context of the present paper. The fi rst ent course through Grand Canyon is no older 2011), or have discounted it (Wernicke, 2011; is that an ancestral upper Colorado River emp- than 5–6 Ma; the other is that river systems are Flowers and Farley, 2012); others proposed that tied into “Hopi Lake” (in which was deposited not immutable, but are part of drainage net- this was never a constraint because some sort of the Pliocene and Miocene Bidahochi Forma- works that change with time through a quasi- canyon already existed in Muddy Creek time or tion) (Fig. 1) during the interval when the river Darwinian competition in response to exter- even earlier (Faulds et al., 2001; Wallace et al., did not fl ow in its present lower course west of nal circumstances such as tectonism and by 2005; Young, 2008; Wernicke, 2011; Flowers the Colorado Plateau. The second is that “Lake means such as headward erosion and capture. and Farley, 2012). Subterranean piping is a pop- Hopi” at one time drained northward along the Drainages with the steepest gradient survive ular theory (Hill et al., 2008; Pederson, 2008), as present alignment of Marble and perhaps Glen and expand their drainage area by capturing is lake spillover (Blackwelder, 1934; Meek and Canyons. The third is that the Colorado River the water of lower-gradient and therefore less Douglass, 2001; Scarborough, 2001; Spencer became integrated in its course through Grand aggressive drainages, which become inactive. and Pearthree, 2001). Another idea is that parts of Canyon and Basin and Range reaches by means There is no “beginning” and no “end” to most Grand Canyon are old and were occupied by the of cascading spillovers—Hopi Lake into Huala- rivers, only changes in the connections and Colorado River but were choked by debris dur- pai Lake (Pierce Ferry area; Fig. 1), Hualapai confi guration of the drainage network. ing interior-basin deposition to the west, so the Lake into Bouse Lakes (at the border between The last quarter century has seen an explosion river became inactive (Elston and Young, 1991), Arizona and California), and Bouse Lakes into of theories and ingenious ideas regarding the or were formerly occupied by rivers that fl owed the Gulf of California to the south. This is a top-

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down mechanism, as opposed to the bottom-up Observation and Science (EROS) Center’s long as the crow ﬂ ies, and 55 km long along its mechanism involved in headward erosion and seamless-data server (viewer.nationalmap.gov trace. An isolated remnant of river deposits with stream capture. /viewer). distinctive clasts similar to those on Crooked The data from Crooked Ridge River pro- Ridge is present near the northwest corner of vide useful new constraints on these concepts. PHYSICAL CHARACTERISTICS OF Black Mesa, ~43 km from the nearest exposures (Crooked Ridge River is a paleoriver whose CROOKED RIDGE on White Mesa and approximately on the same course is now visible through inverted relief as gradient and trend (Figs. 2 and 5). The discon- Crooked Ridge.) Scattered exposures of gravel Crooked Ridge extends continuously across tinuous river course can thus be traced for 91 in northern Arizona have long been known (e.g., the Kaibito Plateau of northern Arizona from km. The sinuous Crooked Ridge is an example Cooley et al., 1969; Hunt, 1969; R. Here ford, the eastern edge of White Mesa westward to of inverted relief that came about because depos- 1975, personal commun.), and in some cases The Gap, a large erosional gash carved into the its in the ﬂ oodplain of an ancient river were the gravel was thought to be derived from the Jurassic Navajo Sandstone at the Echo Cliffs protected by a 1–2 m cap of massive pedogenic San Juan Mountains in Colorado (Cooley et (Figs. 2, 3, and 4). Kaibito Plateau is an area stage V calcrete, whereas the bedrock outboard al., 1969; Hunt, 1969). However, integration of little organized drainage and abundant sand of the river deposits was not so protected and has of these observations into a coherent picture of dunes, excepting at the northern end, where can- been preferentially lowered by erosion (Figs. 3, the drainage system only became possible when yons extend southward from Glen Canyon into 4, 6, and 7). the ridge could be seen as a distinct topographic the plateau. The lack of drainage and the sand Remnants of the river deposits are now found entity with the advent of detailed topographic dunes have contributed to the preservation of the along about a quarter of the ridge’s length west- maps in the 1980s, and especially of satel- ancient Crooked Ridge. ward from White Mesa and about a third of the lite and digital elevation model (DEM) data. Ridges heading toward Crooked Ridge on length eastward from The Gap (Figs. 3, 5, and Such images presented here are composites the Kaibito Plateau, and gravel on the north side 6). The intervening part has either fragmentary of Landsat and shaded-relief DEM data with of White Mesa, suggest possible tributaries to river deposits or none. The capped parts of the 10–30 m resolution, obtained from the U.S. Crooked Ridge River. These features were not ridge rise as much as 110 m above the adjacent Geological Survey (USGS) Earth Resources examined during this study. The ridge is 48 km landscape, whereas the eroded bedrock parts

Monument Comb Ridge

Upwarp San Juan R. U T A H 37° N A R I Z O N A Lees Ferry Vermillion Cliffs Carrizo Mountains Comb Kaibito Plateau White Mesa Ridge Kaibab Plateau Marble Echo Cliffs CRCR 5 CRCR 2 Figure 2. Digital-elevation-model CRCR 3 CRCR 1 image showing geographic fea- CRCR 4 Platform CRCR 7 tures of the Kaibito Plateau– CRCR 6 Klethla Valley Black Mesa Black Mesa region in Arizona near Crooked Ridge. Blue dots Crooked Ridge show sample localities; red labels The Gap are sample numbers. 36° N Little Colorado Grand Canyon River

N Red Butte

Hopi Buttes 50 km 112° W 111° W 110° W

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Figure 3. Oblique view of Kaibito Pla- teau with Crooked Ridge in center, Mormon Ridges Gravel-covered area (dark) looking north. Approximate straight- line length along the ridge from east edge of White Mesa to The Gap is Gravel-covered 48 km. North side of old river valley area (dark) is clearly visible. Wildcat Peak is a monchiquite intrusive recently dated at 19.5 ± 0.10 Ma (Peters, 2011). No monchiquite clasts were found in the Preston Mesa gravel of Crooked Ridge River, even though the peak is less than 13 km from Crooked Ridge. Composite of Landsat TM and 10 m shaded-relief digital-elevation-model image from Echo Cliffs USGS National Map elevation dataset. Vertical exaggeration 5x.

Figure 4. Oblique view looking NE along Crooked Ridge from The Gap, carved into the upturned Echo Cliffs, toward White Mesa in the distance. Preston Mesa is to the right, and Mor- mon Ridges to the left in the middle distance. Straight-line distance to White Mesa near the right skyline is 48 km. Width of wind gap in Echo Cliffs in foreground is 3.4 km rim-to-rim. Composite of Landsat TM and 10 m shaded-relief digital-elevation model from USGS National Map elevation dataset. Vertical exaggeration 2x.

Mostly Navajo ss. bedrock Gravel 3.9:1000 * 10.4:1000 Wepo ss. Gravel 4.9:1000 6.6:1000

Figure 5. Longitudinal proﬁ le along Crooked Ridge River. Horizontal scale is distance along the ridge from The Gap, in meters. Vertical scale is elevation, in meters. Italics indicate bedrock into which the valley was cut. Roman lettering below indicates material forming ridge. Numbers in red indicate the gradient of the river (m/1000 m). *—location of gravel deposit on Black Mesa.

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Figure 6. Oblique view looking SW along Crooked Ridge to The Gap, 48 km away in a straight line. Ridge follows course of ancient river in inverted relief. Wider parts of ridge in foreground and at far end are mantled with river deposits; parts in between are primarily bedrock. Scarp cut into Mormon Ridges on right. Composite of Landsat TM and 10 m shaded-relief digital-elevation model from USGS National Map elevation dataset. Vertical exaggeration 5x.

Figure 7. Exposure of ﬂ uvial sediments on Crooked Ridge, 9 km NE of The Gap. The exposure consists primarily of sand, with subordinate mud and clay layers. Gravel is common near top of sand and within light- colored calcrete cap. Channeling is common. About 30 m of section are exposed at location of photograph.

are as much as 50–80 m above it. On satellite that parallels the trend of Crooked Ridge and served (and lowest) part of the ancient valley. images, the river deposits are as much as ~1000 truncates the south-southeast–trending Mor- The width of the upper and now-eroded part of m wide, though generally less. mon Ridges (Figs. 3, 4, and 6). The south edge the valley probably was much greater. Along most of the Kaibito Plateau reach, the of the valley is ill deﬁ ned, but probably was At The Gap (Figs. 2, 3, and 4), the valley bedrock for the Crooked Ridge River valley is along the northern end of bedrock prominences crosses the Echo Cliffs ridge, formed by the the relatively weak upper part of the Navajo such as Preston Mesa (Fig. 3) and the southern resistant Navajo Sandstone upturned along the Sandstone and the easily eroded Carmel For- part of White Mesa. This alignment parallels Echo Cliffs monocline. This large erosional gap mation at the base of the San Rafael Group. both Crooked Ridge and the northern edge of is visible today in cross section. The present These rocks allowed formation of a wide river the valley. If this alignment is taken as the south rim-to-rim width of the gap is 3.4 km and its valley. Evidence for this is visible on satellite edge, the valley may have been as much as depth is 290 m, as measured on satellite images images, where the north edge of the old valley 5–10 km wide over most of the Kaibito Plateau. and topographic maps. There are no other is marked by a south-facing 150-m-high scarp This width, however, is only that of the pre- breaks comparable in size to The Gap in the

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erosional escarpment of the Echo Cliffs for the sively lower in the section toward the west and LITHOLOGIC CHARACTERISTICS ~62 km from The Gap northward to the mouth downstream. AND PROVENANCE OF of the Paria River at Lees Ferry. This shows that The remnant of fl uvial deposits on Black TRANSPORTED MATERIAL no drainages comparable to Crooked Ridge River Mesa rests on the Wepo Formation in the crossed the escarpment in this section. South- middle of the erosionally resistant Upper Cre- A thickness of about 30 m of fl uvial mate- ward, the crest of the Echo Cliffs today is above taceous Mesaverde Group. The Wepo thus rial is exposed on Crooked Ridge. Good expo- or just below 1700 m (the lowest elevation seen formed the bottom of the valley, whose sides sures show that the bulk of the deposits consist on Crooked Ridge) for ~12 km from The Gap, but are not preserved, but must have been com- of sand containing stringers and interbeds of gradient would have brought the level of the river posed of Upper Cretaceous rocks above the gravel, mud, and clay. Gravel is more abundant to below 1700 m in this area. Therefore, the cliffs Wepo Formation. These strata, which are sub- near the top of several exposures. Channels <1 projected above the level of the river even then. horizontal here, include the Yale Point Sand- to ~10 m wide are common (Fig. 7), as is cross In these 12 km, there is just one smaller break in stone of the Mesaverde Formation (Page and bedding that shows southwest fl ow direction the cliffs, which we interpret as being formed by Repenning, 1958; Cooley et al., 1969), which (noted by us and by Hunt, 1969). The sand is a tributary to Crooked Ridge River. The conclu- is at most 190 m thick on Black Mesa. Inas- weakly indurated, fi ne to medium grained but sion is that Crooked Ridge River was the master much as the ancient valley was much deeper locally medium to coarse grained, and sub- stream for the entire Kaibito Plateau area. than 190 m on the Kaibito Plateau, it is likely rounded to subangular. A small percentage of that the valley walls on Black Mesa, which grains are fi ne, very well rounded and frosted, STRUCTURAL AND STRATIGRAPHIC were carved in resistant rocks, were also high, and probably derived from Mesozoic eolianites. FEATURES so would have been in Upper Cretaceous strata The scarcity of eolian sand suggests that the above the Yale Point, and possibly even in Oligocene Chuska erg of Cather et al. (2008) Between Black Mesa and White Mesa, overlying Tertiary strata. either was never present in the area traversed by Klethla Valley (Fig. 2) crosses the ancient drain- The sides of the old river valley between Black Crooked Ridge River, or was eroded by the time age at a low angle. This is a strike valley along Mesa and White Mesa, in the area now occupied the river was active. the south continuation of the Tsegi–Comb Ridge by Klethla Valley, were in the Mesaverde Group Composition, sampling methods, sampling system of monoclinal fl exures, a part of a major and the underlying Mancos Shale and Dakota locations, and inferred sources of gravel are structural trend that continues northeastward for Sandstone of Cretaceous age as well as the Mor- shown in Tables 1 and 2 and Figure 2. Clasts are ~250 km along Comb Ridge and separates the rison Formation and Entrada Sandstone of Juras- considered distinctive or non-distinctive accord- Monument Upwarp to the northwest from the sic age (Fig. 8). ing to level of confi dence in assessing prov- structurally low Black Mesa basin to the south- White Mesa is capped today by the Entrada enance; both locally derived and far-traveled or east (Figs. 2, 8, and 9). Sandstone and remnants of the Cretaceous exotic clasts are present. On Kaibito Plateau, Crooked Ridge crosses Dakota Sandstone (Cooley et al., 1969; F. Peter- Most locally derived clasts are quartz sand- several down-to-the-east monoclinal fl exures son, 2010, personal commun.). The reach stones from the upper Mesozoic formations (Fig. 8). West of the Echo Cliffs, the Kaibab between White Mesa and The Gap is rimmed that formed the valley sides on the Kaibito Plateau is a north- and south-plunging dome by the upper part of the Jurassic Navajo Sand- Plateau and Black Mesa, and upstream from (Figs. 1, 2, and 8) bounded on the east by the stone and the Carmel Formation, capped locally Black Mesa (Harshbarger et al., 1958; Page and East Kaibab monocline, which also has down- by small remnants of Entrada. In Crooked Ridge Repenning, 1958; Beaumont and Dixon, 1965; to-the-east displacement. The Colorado River in River time, Jurassic and Cretaceous rocks most Cooley et al., 1969). eastern Grand Canyon forms a great bend that likely rimmed all these parts of the valley (see Sandstone clasts showing eolian cross bed- follows strike of the strata around the south- below). In summary, the valley of Crooked ding and rounded and frosted quartz grains are plunging part of the dome. The effect of the Ridge River was carved mostly in easily eroded derived from the eolian parts of the Jurassic various monoclines is to expose strata succes- and clay-rich Upper Cretaceous rocks. Entrada Sandstone and the Cow Springs Sand-

SW NE Tsegi-Comb Ridge monoclinal flexures Kaibab Plateau Gravel Crooked Ridge gravel outcrop Marble Platform Echo Cliffs Kaibito Plateau White Mesa Klethla Black Mesa Valley

Paleozoic Cretaceous Moenkopi and Chinle Fms and San Rafael Group Glen Canyon Group and Morrison Fm. Dakota Fm. Figure 8. Sketch structural section across Kaibito Plateau and surrounding areas near the course of Crooked Ridge River. Small normal fault east of The Gap (in the Echo Cliffs) is not shown. River ﬂ owed on progressively lower stratigraphic units in downstream direction. For simplicity, “San Rafael Group” in this illustration includes Mor- rison Formation, and “Cretaceous” includes Dakota Sandstone. Width of view approximately 170 km. No scale.

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U n c o m p a h g r e U p w a r p erences therein). We have no isotopic ages for n c these clasts, but suspect that the ones incorpo- o m rated in the Crooked Ridge sediments are from p Uncompahgre a Plateau the Laramide-age centers (Carrizo, Ute, La La Sal h Mtns. g Plata) because these are along the northeasterly r * e trend of the river. Nevertheless, future determi- p r U p nations would be useful to sharpen interpreta- Henry Mtns. a ? w tions of paleodrainage relations. Emplacement w a p r * Abajo p depth of these Laramide and middle Tertiary U p w a r p Mtns. intrusions is uncertain. Published estimates are 38º N * w San Juan Mtns. 1.9–6 km below the surface at the time of intru- o l sion (Ross, 1998), and ~3 km (Jackson, 1998).

S a n J u a n M t n s.s. U p w a r p Erosion of Upper Cretaceous and possibly

n lower Tertiary strata unroofed these distinctive Elk Ridge Needle Mtns. e l porphyries in the Crooked Ridge River drainage m * a basin and fed pebbles into the river. u r u La Plata n * t All the distinctive clasts of metamorphic o Mtns. c M o n u m e n t rocks can be matched with Proterozoic rocks u Ute Mtns. r UT CO t mapped in the Needle Mountains (Figs. 1 and S AZ NM San Juan R. 9), where today they crop out at elevations as

high as 4000 m. The quartz metaconglomerate * S t r u c t u r a l l o w and metawacke are comparable with some of the lithologies in the Vallecito Conglomerate a Carrizo Mtns. s Monument Valley and the Irving Formation, and the fi ne quartz- e M e s a Volcanic field ofeldspathic gneiss matches the Twilight Gneiss (minettes) Black Mesa Defiance 50 km (Larson and Cross, 1956; Barker, 1969). Felsic Plateau lava fl ows, welded tuffs, andesites, latites, and k c hydrothermal mineral deposits are widespread a D e f i a n c e l 36º N B l a c k U p w a r p and abundant in the late Eocene to early Mio- cene San Juan volcanic fi eld (Lipman et al., 110º W 108ºW 1978). The distinctive clasts of these lithologies, Figure 9. Map of upper reaches of Crooked Ridge River. Thick dashed blue lines indicate as well as most if not all the non-distinctive inferred approximate courses of Crooked Ridge River and possible tributaries. Yellow lines clasts of the same type, are likely to have come with labels denote structurally high and low areas. Thin dashed white line bounds Monu- from this region. The pebbles of crystal tuff and ment Valley minette volcanic fi eld. White asterisks mark laccolithic mountains with hyp- altered rhyolite and andesite are not likely to be abyssal porphyry exposed. Yellow lettering denotes geologic features; black lettering geo- reworked from older gravel deposits because graphic features. UT—Utah; CO—Colorado; AZ—Arizona; NM—New Mexico. Source of the tuffs are friable and the altered rhyolites base image unknown. are soft. Pebbles of rhyolitic vitrophyre are not likely to be reworked from the Chinle and Mor- rison Formations because they are glassy and not devitrifi ed as are those from the Mesozoic stone; sandstone clasts without eolian features cm. They include many types of volcanic, hyp- formations (Cadigan, 1972; Thordarson et al., are from the Cretaceous Dakota and Mesaverde abyssal, plutonic, and metamorphic lithologies; 1972; Dodge, 1973). Notably absent from the units. Petrifi ed wood is from the Jurassic Mor- petrographic characteristics link them with spe- samples collected at Crooked Ridge and Black rison Formation, and limy sandstone contain- cifi c sources northeast of Crooked Ridge and Mesa are clasts of limestone similar to the Kai- ing abundant bivalve fossils is from the Creta- Black Mesa at least as far as the San Juan Moun- bab Formation on the Kaibab Plateau, red sand- ceous Dakota Sandstone. These clasts are found tains (Table 2). Minettes came from the late Oli- stone similar to that of Triassic rocks on the throughout the areas examined by us on Kaibito gocene to early Miocene dikes and diatremes Colorado Plateau, and monchiquite, a volcanic Plateau and White Mesa; we did not note areas of the Navajo volcanic fi eld, most likely from rock common in the Hopi Buttes to the south of higher concentrations. the Monument Valley section (Laughlin et al., (Figs. 1 and 2) and also present at Wildcat Peak, Typically, the clasts are subangular to sub- 1986, and references therein) (Fig. 9). Clasts of not far south of Crooked Ridge on the Kaibito rounded; maximum size generally is ~30 cm, granite, pegmatite, and perthitic microcline also Plateau (Fig. 3). but a few boulders, which probably rolled in probably came from intrusions in Monument from the valley sides, reach 100 cm in diameter. Valley, where these lithologies are especially DEPTH OF CROOKED RIDGE RIVER These clasts are important for determining what common as xenoliths. VALLEY WHEN THE RIVER WAS formations were present on the valley sides in The intermediate porphyries are strikingly ACTIVE Crooked Ridge time. similar to rocks reported in laccolithic centers of Selected exotic clasts are shown in Figure 10. the Colorado Plateau, including the Abajo, Car- It is useful to estimate the height of the val- The exotic clasts are subrounded to rounded, rizo, Henry, La Plata, La Sal, and Ute Mountains ley sides using the locally derived clasts in the and mostly 1–3 cm in diameter, but reach 10 (Fig. 9) (Friedman and Huffman, 1998, and ref- fl uvial sediments as described above. Because

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TABLE 1. COMPOSITION OF GRAVEL SAMPLES of structural complications, strata older than Statistical samples Grab samples Cretaceous were not exposed upstream from White Mesa, so any clasts from these strata were Lithology Percent Lithology Percent necessarily derived from exposures in the val- CR2-2 CR2-3 ley sides on the Kaibito Plateau, White Mesa, or Mesozoic sedimentary* 53 61 Quartzite 48.5 Quartzite 13 19 Chert 11.5 the monoclinal fl exure now occupied by Klethla Chert 12 5 Mesozoic sandstone†† 7.3 Valley (Fig. 8). Felsic volcanic† 5 7 Metaconglomerate§§ 5.2 The predominant sandstone pebbles and cob- Minette 9 2 Quartz 5.2 bles are similar in lithology to those described Granite 5 1 Granite 4.6 Intermediate volcanic§ 1 2 Metawacke 4.0 from Middle and Upper Jurassic to Upper Creta- Quartz 1 1 Microcline 3.4 ceous formations that crop out directly beneath Metaconglomerate# 0 1 Intermediate porphyry## 2.9 or near the gravel deposits of Crooked Ridge on Gneiss** 0 1 Felsic volcanic*** 2.9 the Kaibito Plateau and on the fl anks and top of Earthy hematite 1 0 Minette 2.3 Intermediate volcanic††† 1.1 Black Mesa (Harshbarger et al., 1958; Page and Earthy hematite§§§ 1.1 Repenning, 1958; Cooley et al., 1969). In particular, light gray, fi ne to coarse sand to granule Note: Statistical sample CR2-2: 100 contiguous clasts from the top surface of the deposit at the big quarry on Kaibito Road, White Mesa; c-axes = 15–72 mm. Statistical sample CR2-3: 100 contiguous clasts sandstones that contain feldspar and mica prob- from the bottom of a vertical face in the big quarry on Kaibito Road, White Mesa; c-axes = 15–83 mm. Grab ably were eroded from beds in the Mesaverde samples: 175 clasts picked up randomly (cherry picked) at seven sites, including 68 clasts from Black Mesa; c-axes = 10–200 mm. The Black Mesa clasts include quartzite, quartz, chert, granite, pegmatite, rhyolite, Group, and well-sorted fi ne sandstones with rhyolitic vitrophyre, and quartz metaconglomerate; the last two are “distinctive lithologies” in Table 2. rounded and frosted grains compare well with *Sandstone, conglomerate, siltstone, claystone. the Cow Springs Sandstone (Table 2). Other †Rhyolite, argillically altered rhyolite, vitrophyre, welded tuff, compacted tuff. §Andesite, latite. specimens are comparable to rocks in the San #Pebbles are quartz and quartzite. Rafael Group, Morrison Formation, and Creta- **Fine-grained, linear, quartzofeldspathic. ceous Dakota Sandstone. Petrifi ed wood is widely ††Only fossiliferous and iron-manganese–cemented pebbles. §§Pebbles are quartz and quartzite. reported in the Morrison Formation, and is also ##Hornblende and plagioclase phenocrysts crowded in light gray to tan aphanitic matrices; andesitic to known to occur in the Dakota Sandstone (Table 2). dacitic (dioritic to granodioritic). ***Rhyolite, vitrophyre. The Dakota Sandstone, which is ~60 m †††Andesite; propylitically altered andesite. thick in this area, is preserved at the foot of the §§§With calcite veins. monocline along the western scarp of Black Mesa, where it typically overlies the Juras- sic Morrison Formation (Fig. 11). On White

TABLE 2. INFERRED SOURCES OF PEBBLES AND COBBLES* Mesa, however, remnants of Dakota directly overlie the Cow Springs Sandstone because the Lithology NM SJV LAC NAV CH MO J-K Morrison has pinched out here (Beaumont and Distinctive lithologies Dixon, 1965; Cooley et al., 1969) (Figs. 11 and Compacted rhyolitic crystal tuff (1) 12). The Dakota at the foot of Black Mesa is Argillically altered rhyolite (3) 300 m below the level of Crooked Ridge River Rhyolitic vitrophyre (2) on White Mesa. Consequently, Dakota clasts Latite (2) Andesite (2) could not have entered the river sediments Propylitically altered andesite (1) from the Black Mesa area, but must have been Earthy hematite with calcite veins (3) derived from exposures on White Mesa and the Quartz metaconglomerate (10) PP Kaibito Plateau. This is confi rmed by remnants Metawacke (7) Fine quartzofeldspathic gneiss (1) P of the Dakota at the east edge of White Mesa, Hornblende-plagioclase porphyry (5) where the Dakota directly overlies the Jurassic Minette (15) Cow Springs Sandstone (Cooley et al., 1969). Sandstone (gray, tan, brown) (108) A good place to visualize the ancient river Conglomerate, siltstone, claystone (7) Tan sandstone with shells (12) valley is on the Kaibito Plateau between Pres- Petrifi ed wood L ton Mesa (Fig. 3) and two smaller mesas at the Non-distinctive lithologies south edge of Mormon Ridges. All these mesas are capped by remnants of the Entrada Sand- Quartz (11) L L L P L Quartzite (tan, gray, red, yellow) (117) PL stone. There are no structural features here to Chert (37) P P L P complicate the picture. Rhyolite (8) PL In this area, Crooked Ridge River fl owed at Felsic welded tuff (3) PL or near the top of the Jurassic Navajo Sandstone. Granite and pegmatite (red, tan) (13) L L L This unit is overlain by the Jurassic Carmel For- Fine hypidiomorphic-granular granite (gray) (1) P P L P Microcline (red, perthitic) (6) L L L mation and then the Jurassic Entrada Sandstone, which together rise to a level ~145 m above the Note: Numbers in parentheses are the number of specimens collected. NM—Needle Mountains; SJV—San Juan volcanic fi eld; LAC—Colorado Plateau laccoliths; NAV—Navajo volcanic fi eld; CH— river bed (Fig. 12). Chinle Formation; MO—Morrison Formation; J-K: Jurassic and Cretaceous strata. The Entrada is overlain by the Jurassic Cow *Symbols: —very likely; L—likely; P—possible. Springs Sandstone. Carmel, Entrada, and Cow

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Figure 10 (on this and following page).

Springs form the San Rafael Group, which is km/m.y. Even using a rate in the middle of this be younger than Crooked Ridge River and would overlain by remnants of the Cretaceous Dakota range (5 km/m.y.), and a very modest age of 10 have formed after the river became inactive. Sandstone. According to Peterson and Pipir- Ma for Crooked Ridge River, we fi nd that the ingos (1979), the thickness of the San Rafael Cretaceous rocks would have extended to well AGE OF CROOKED RIDGE RIVER Group at the north end of White Mesa is 270 northwest and west of the Crooked Ridge River m. Adding the ~60 m of the Dakota to the 270 valley at that time, so would have been exposed Crooked Ridge River is typical of rivers for m of the San Rafael Group brings the height on the valley sides. In this case, the sides of the which it is diffi cult, if not impossible, to deter- of the ancient valley sides to 330 m. However, valley would have been ~720 m high, and mine with reasonable certainty when and how this is a minimum fi gure because the probable the terrain surrounding the valley would have the river came into being. The best that can be Mesaverde clasts in the river deposits suggest been at an elevation of ~2600 m. The denuda- done is to estimate when the river might have that the Mancos and Mesaverde units were also tion implied by these fi gures since Crooked ceased to exist. This is because clasts (including present along the valley sides. Ridge time is consonant with the denudation in detrital zircons) of some specifi c age may date Cooley et al. (1969) gave a thickness of the Monument Upwarp area since late Miocene the deposit that contains the clasts but not the ~450 m for the Cretaceous rocks on the nearby time proposed by Hoffman et al. (2011) on the river itself, which may have existed long before fl ank of Black Mesa, which brings the prob- basis of apatite thermochronology. these clasts became available to the river. able total height of the valley sides to ~720 m Clearly, the valley of Crooked Ridge River The clast assemblage of Crooked Ridge (Fig. 12). These calculations are supported by would have been substantial, and the time required River includes a variety of rock types from the the observation that the retreating Mancos- to carve it considerable. Furthermore, the eleva- San Juan volcanic fi eld, which is of late Eocene Mesaverde scarp is directly above the outcrop tion of the area surrounding the river, probably to early Miocene age. The youngest detrital of the Dakota on all sides of Black Mesa. Fur- a regional plateau, would have been higher than zircons described by Price et al. (2012) are thermore, Holm (2001), Lucchitta (1975), and the surfaces discussed by Cooley (1962; Cooley 24–20 Ma, which fi ts into this age range. These Lucchitta and Jeanne (2001) calculated rates et al., 1969) and R. Hereford (2013, personal zircons clearly were derived from the San Juan of retreat for Mesozoic-rock scarps of 4–8 commun.). These lower surfaces therefore should volcanic fi eld.

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Figure 10 (continued). Photographs of exotic clasts from river gravels. (A) Fine lineated gneiss; (B) Granite; (C) Pegmatite, sheared face with chlorite; (D) Quartz metaconglomerate; (E) Quartzites of various colors; (F) Metawacke, rough face; (G) Three hypabyssal porphyries from laccolithic mountains; (H) Vesicular andesite (left) and propylitically altered andesite (right); (I) Latite; (J) Two rhyolites, brown and pink; (K) Two rhyolites, with argillitic altera- tion, amethysts circled; (L) Rhyolite vitrophyre (welded tuff); (M) Rhyolite welded tuff; (N) Compacted rhyolite tuff; (O) Earthy hematite with calcite veins; (P) Minette.

The clasts we sampled and the zircons sam- rocks for the clasts have been emplaced and The course of the river has no relation to the pled by Price et al. (2012) were all obtained exposed to erosion. present drainage network. Instead, three distinct from the fl uvial sediments that are exposed The massive calcrete that caps the deposits episodes of drainage arrangement have occurred today. These sediments are the last material can constrain the time of demise. Massive cal- since Crooked Ridge River time. The fi rst is the deposited by the river before its demise. There- cretes like this one often take several million development of the ancestral San Juan River fore, the demise (but not the birth) of the river years to form. U-Pb dating of the calcrete would drainage system along the south fl ank of the San could have happened at any time in the interval provide this age, but such a determination has Juan Mountains. This probably is what brought between 24 and 0 Ma because clasts can only be not been made yet. Therefore, we must resort to about the demise of Crooked Ridge River by incorporated into river deposits after the source indirect and imprecise methods. beheading it and pirating its waters (see below).

NW SE White Mesa Dakota ss. Level of Crooked Ridge Black Mesa Figure 11. Vertical offset between present-day location of Dakota 300 m Sandstone at base of Black Mesa and sediments of Crooked Ridge Dakota Sandstone River at east edge of White Mesa.

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R. Hereford (2011 and 2013, personal commun.) has suggested that Crooked Ridge River K Mancos and Mesa Verde 450 m was not a major river but a young local wash that obtained its exotic clasts by recycling them 720 m from a pediment or alluvial apron that extended Dakota ss over a large area south of the San Juan volcanic J Cow Springs fi eld. We do not believe this notion is tenable for 270 m the following reasons: J Carmel and Entrada 145 m (1) The Crooked Ridge deposits contain 24–20 Ma detrital zircon. The pediment depos- Thalweg of CRR its, proposed by Cather et al. (2003, 2008) as Navajo Sandstone part of the Chuska units, were deposited ca. Figure 12. Height of various stratigraphic boundaries above Crooked Ridge at the midpoint 35–33.5 Ma, and the youngest of the Chuska of the Crooked Ridge River (CRR) course on Kaibito Plateau. J—Jurassic, K—Cretaceous. units is 33.5–25.2 Ma, while the volcanic Cone- jos Formation (in the southern extension of the San Juan Mountains), allegedly the source of the clasts in the pediment, is ca. 35–30 Ma. There- The second is the development of Klethla Val- chiquite necks and lava fl ows in the Hopi Buttes, fore, deposition of all these units ended well ley (Figs. 2 and 8), which cuts across the course the volcanic edifi ce at the surface probably was before 24–20 Ma, the age of the detrital zircons of the ancient river. Klethla Valley is broad and much more extensive than the roots that are pre- in Crooked Ridge River, so the zircons, and pre- mature and typical of many such old valleys served today (Williams, 1936, his fi gure 4), thus sumably the fl uvial deposits that contain them, on the Colorado Plateau. This valley in turn providing abundant material to be eroded and could not be derived from the Chuska material. is being beheaded by canyons of the canyon- transported toward Crooked Ridge River. (2) The interpretation of a large pediment is cutting phase that are tributary to the Colorado No monchiquite clasts have been found in the based on a few exposures of sediments, all on and current San Juan Rivers. These three major gravel even though Wildcat Peak is less than 13 the Chuska Mountains, that are interpreted to rearrangements of the drainage network since km from Crooked Ridge. We cannot exclude be derived from the Conejos Formation, some Crooked Ridge time require an indeterminate, that the clasts are indeed present in the gravel but 200 km away. However, the Conejos Formation but surely substantial, time interval. were just not seen by us, in spite of our careful is mineralogically and petrologically different Another indirect method involves Wildcat examination. Nevertheless, we can reasonably from the “pediment” clasts in the Chuskas, so Peak (Fig. 3), a monchiquite intrusive that is infer that the Crooked Ridge River drainage the correlation does not hold. part of the Tuba volcanic fi eld (Akers et al., system became inactive before the intrusive was (3) The “pediment” deposits are nowhere pre- 1971). Wildcat Peak has the same lithology as emplaced or unroofed in earliest Miocene time. served outside the Chuska Mountains, they are volcanic rocks in the Hopi Buttes, which have Removal of ~1 km of strata from the area not documented, nor have they been shown to been dated at 6–8.5 Ma (Damon and Spencer, of Wildcat Peak is consonant with removal of have ever existed. 2001). Many of these volcanoes are composed 1–2 km of strata from the area of the lacco- (4) The Crooked Ridge clasts contain litholo- of tuff breccias capped by lava fl ows. Recently, lithic intrusives since they were fi rst unroofed. gies that would not survive recycling. Wildcat Peak itself has been dated at 19.5 ± Such deep erosion is in agreement with results (5) The extensive Crooked Ridge clast 0.10 Ma (Peters, 2011). obtained by other techniques (e.g., Hoffman et assemblage is composed of rocks exposed well The present-day exposure at Wildcat Peak al., 2011; Pederson et al., 2013), and presum- and extensively to the northeast and north of consists of the roots of a volcano that are made ably happened over a substantial time interval. Crooked Ridge. It is more logical and likely that up of a monchiquite and tuff-breccia neck and On the basis of these arguments, we believe this assemblage was derived directly from the a dike swarm of monchiquite. The neck reaches that the demise of Crooked Ridge River can plau- source areas rather than circuitously from an an elevation of ~2000 m. Delaney and Pollard sibly be assigned to the interval between 24 Ma, undocumented pediment deposit. (1981) suggested a minimum of 750 m of over- or possibly 20 Ma (zircon), and 19 Ma (Wildcat In conclusion, the age of Crooked Ridge River burden for Ship Rock, another eroded volcano Peak), or early Miocene. The river itself would is not known with any degree of precision. All in nearby New Mexico that is now preserved as be older. we know is that the demise of the river occurred a neck-and-dike complex. McGetchin and Sil- To arrive at a time when the river might have sometime between 24 and 0 Ma. There are no ver (1972) estimated 1000 m of overburden for been born we must add to the time of demise data to tell us when the river came into being. Moses Rock dike near Monument Valley. Adding the time needed to carve a valley many hundreds However, the circumstantial evidence discussed 1000 m to the top of Wildcat Peak would bring of meters deep, starting from an original posi- above leads us to think that the river is of the the volcano to an elevation of ~3000 m, which tion on top of Cretaceous (and possibly lower same age as the San Juan volcanic fi eld, i.e., late is in decent agreement with our estimate of the Cenozoic) strata. This must have taken a consid- Eocene to early Miocene, and that the demise elevation of the Crooked Ridge River valley erable time, but we have no constraints on just of the river occurred between ca. 24 and 19 Ma. sides. This is also well above the 1950–2000 m how long this time might have been. It is reason- elevation of the Crooked Ridge River thalweg able to postulate that the river was born while LONGITUDINAL PROFILE OF THE nearest Wildcat Peak. Therefore, one can infer the San Juan volcanic fi eld was active between RIVER substantial topographic relief between Wildcat late Eocene and early Miocene time because the Peak volcano and the valley of Crooked Ridge high elevation of these mountains would have The farthest upstream exposure of river River, which probably resulted in a slope toward started to provide abundant precipitation to feed deposits is on Black Mesa at 2240 m elevation; the valley. Furthermore, by analogy with mon- this and other rivers. the farthest downstream exposure is near The

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Gap at ~1700 m. Over the 91 km that Crooked ing on whether the gradient of Crooked Ridge of the rivers over tens of kilometers are 3.4 and Ridge River can be traced from Black Mesa to River was primary or not, on whether the river 3.3 m/km, respectively. The Gap, the river deposits drop 530 m, giv- was overloaded and braided, and on whether ing an average present-day gradient of 5.8 m/ the river was a substantial river or merely a EROSIONAL LOWERING km. The gradient of individual reaches varies local wash. Two suitable streams in the region considerably (Fig. 5), probably refl ecting dif- are a reach of the Animas River upstream from The relative scarcity of porphyry clasts in ferences in the rocks into which the valley was Durango, and the San Juan River upstream from Crooked Ridge gravel contrasts markedly with carved, structural features such as monoclines Pagosa Springs, both in southwest Colorado. their abundance in even the oldest terraces of and faults, and constrictions such as The Gap Data for these streams and for Crooked Ridge the Colorado River. We infer that the laccolithic in the river’s path. The relatively high gradi- River are in Table 3. intrusives were less exposed when Crooked ent can plausibly be ascribed to overloading The comparison shows that a steep primary Ridge River was active than today and that some by sediment derived from the easily eroded gradient for the overloaded Crooked Ridge were not exposed at all (Eckel et al., 1949). Jurassic and Cretaceous rocks that formed the River is not unreasonable, and indicates that Today, a few of these porphyries are exposed valley sides upstream from The Gap. Internal the overall channel and fl oodplain characteris- at altitudes as high as 4000 m, and many are at structures of the river sediments and the upward tics are in keeping with those of two substan- 3000–3500 m; in Crooked Ridge time, only the coarsening suggest a braided stream and support tial and overloaded rivers of the region. The highest would have been exposed, as implied the overloaded-stream interpretation. comparison, together with the exotic clasts, are by the scarcity of these clasts in the gravel. This An alternative explanation for the steep gra- also permissive with the proposition that the suggests a topographic surface in the region of dient of the deposits is post-depositional tilting ancient Crooked Ridge River was not a minor the intrusives in the 3500+ m range. Now, the of the channel due to crustal warping such as local wash, but a river of regional extent whose region in southwest Colorado and southeast a “bullseye” of isostatic unloading to the north discharge was at least comparable to those of Utah near the intrusives is at 1500–2000 m, sug- (Lazear et al., 2011; Pederson et al., 2013) or today’s Animas and upper San Juan Rivers. gesting that 1–2 km of strata has been removed mantle dynamics (Robert et al., 2011; Moucha It could be argued that comparison with the since Crooked Ridge time. et al., 2009). However, the inferred bullseye is two streams in Table 3 is not valid because in the Canyonlands country north to northwest these streams are near their headwaters. How- PERMISSIBLE AND IMPERMISSIBLE of Crooked Ridge River, so the river’s south- ever, proximity to headwaters is not necessar- PALEODRAINAGES westerly course is essentially tangential to the ily correlated with overloading and a braided uplift contours and should be little affected by channel. Some streams near the headwaters are The middle Miocene to lowermost Pliocene(?) this unloading. overloaded and braided, whereas others are not. Bidahochi Formation fi gures prominently in The mantle-dynamics studies suggest north- However, overloading always results in a steep hypotheses on the history and integration of the east tilting for nearly all the Miocene and gradient and a braided channel. The effect of Colorado River on the Colorado Plateau. The southwest tilting since then. Therefore, the overloading even far from headwater regions is formation was deposited in Bidahochi Basin, river would have been tilted fi rst northeast and well illustrated by two rivers in northeast Italy, roughly centered on the Hopi Buttes (Figs. 1 then southwest. It is currently not possible to the Tagliamento (46°3.562′N, 12°54.886′E), and 2). Origin of the formation is controversial. evaluate the net effect of these postulated tilting and the Piave (45°51.191′N, 12°10.303′E). Some (e.g., Spencer et al., 2008) proposed that events on the gradient of Crooked Ridge River. Both rivers emerge from the rugged Southern it was deposited in a deep “Hopi Lake,” possi- The erosional history of the Colorado Pla- Alps and fl ow southward on a coastal plain to bly large enough to extend into southern Utah. teau in the eastern Grand Canyon region shows their mouths in the Adriatic Sea. The Alps con- The lake would have been the sump of a large that erosion has progressed northeast with time, tribute great quantities of carbonate debris to the drainage system that crossed the Colorado Pla- mostly through cliff retreat at 4–8 km/m.y. (Luc- rivers, which are overloaded and highly braided teau and originated in the Snake River basin. chitta, 1975; Holm, 2001; Lucchitta and Jeanne, even on the coastal plain and at considerable These interpretations are based on the pres- 2001). As a result, Paleozoic rocks are exposed distances from the headwaters (60–65 km from ence of fossil fi sh of Snake River affi nities and near the south margin of the Colorado Plateau, where the Tagliamento emerges from the moun- adapted to large permanent aquatic habitats and whereas uppermost Mesozoic strata are exposed tains, 45 km in case of the Piave). The gradients swift-fl owing rivers. The lake would eventually in the inner part of the plateau to the northeast. This erosion would cause northeast tilting of the Kaibito Plateau region because of isostatic uplift TABLE 3. RIVER CHARACTERISTICS of the denuded region to the southwest, and may Characteristic Crooked Ridge River Animas River San Juan River have decreased the original gradient of Crooked Sinuosity* 1.15 1.17 1.22 Ridge River. In conclusion, the steep gradient of Meander amplitude (m) 110–380 105–240 65–440 the river deposits is plausibly explained by over- Floodplain width (m) 410–1000 150–241 145–230 loading, although tectonic adjustments can be Valley width (km) 5–10 on Kaibito Plateau 1.7–2.4 0.5–0.7 neither proved nor ruled out at this time. 3.4 at The Gap Average gradient (m/km) 5.8 5.47 8.25 Channel character Braided? Braided Meandering-braided COMPARISON WITH OTHER RIVERS Mean maximum daily discharge (m3/s) ? 85 45 Maximum daily fl ood (m3/s) ? 303† 130.7 It is useful to compare the channel character- Note: Animas River: 6.4-km-long reach upstream from Durango, Colorado. San Juan River: 13.1-km-long istics and gradient of Crooked Ridge River with reach upstream from Pagosa Springs, Colorado. those of present-day rivers that are overloaded *Ratio of the channel length to the down-valley distance. † and have substantial base fl ow. This has a bear- 99-year record.

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have spilled over into a drainage along the present eastern Grand Canyon, initiating a top-down cascade of spillovers that integrated the Colo- rado River in its present course through Grand Canyon and the lower Colorado region (e.g., Meek and Douglass, 2001; Scarborough, 2001). ? 37º Others (e.g., Ort et al., 1998; Dallege et al., 2001; Dickinson, 2011) argued that the formation was not deposited in one deep lake, but rather in shallow and ephemeral ones that at times probably were playas or marshes and were fi lled by fl uvial aggradation. ? Crooked Ridge River provides useful constraints on the Bidahochi Formation issue. The Crooked Ridge southwesterly course of the river from the San 36º Juan Mountains to the Kaibab upwarp makes it ? impossible for rivers such as an ancestral Colo- Grand Canyon rado to fl ow southward from western Colorado or eastern Utah into “Hopi Lake” or, conversely, northward from “Hopi Lake” (Fig. 13). As the current evaluations of tilting are speculative, we use the 1700 m altitude of Crooked 50 km Ridge River sediments near The Gap as a pre- Hopi Buttes 35º liminary constraint on possible continuations 112º 110º of Crooked Ridge River downstream, where no deposits are preserved (Fig. 13). Accordingly, Figure 13. Digital-elevation-model image showing terrain above (brown) and below (blue) the Bidahochi Basin is part of the excluded ter- 1700 m, the elevation of Crooked Ridge gravel near The Gap. Brown areas are excluded from rain, because the base of the Bidahochi Forma- possible continuation of river; blue areas are permissible continuations assuming no gradient tion is at 1750–1700 m (Love, 1989; Cather et for the river (boundary condition). Hopi Buttes are mostly excluded. Headwaters of Little al., 2008; Dickinson, 2011). Thus, it is unlikely Colorado River (off image) are excluded. Blue lines represent permissible river courses; red that Crooked Ridge River would have reached lines, impermissible. Blue lines with query denote possible ancestral Colorado and Little Colo- and fi lled the hypothetical Hopi Lake even in the rado Rivers, and possible continuation of rivers through and beyond eastern Grand Canyon. absence of any gradient between The Gap and the Hopi Buttes area. An alternative explanation is that Crooked Ridge River is younger than the basal Bidahochi the river would have been ~350 m above pres- elevation, to join Crooked Ridge River in the Formation because its deposits at The Gap are ent river grade. However, the present Colorado area of the present-day confl uence with the Lit- lower than those beds. Thus, the river could have River and its valley are much lower than they tle Colorado River. been graded to a fl uvial system that included the were in Crooked Ridge time because the Colo- A course westward is possible along the ca. 4–5 Ma upper Bidahochi fl uvial deposits rado and the bedrock underneath it were ~200 alignment of the present eastern Grand Can- and could have been of that age. We consider m higher just 525–600 k.y. ago (Lucchitta et yon (Figs. 1, 13, and 14). This potential route this possibility extremely unlikely in light of the al., 2000, 2001). This gives an incision rate of was proposed long ago on geologic grounds by arguments presented above that point to a much 380–330 m/m.y. Therefore, even at 1 Ma the Babenroth and Strahler (1945) and Lucchitta greater age for Crooked Ridge. Colorado River grade would have been at, or (1975, 1989) and, more recently, by Scarbor- Figure 13 shows that the only possible con- higher than, the elevation of Crooked Ridge ough (2001). Flowers et al. (2008), Lee et al. tinuation beyond The Gap is southward along River at The Gap. The incision is the product of (2013), and Flowers and Farley (2012) have the alignment of the Echo Cliffs, then west- vigorous post–5 Ma downcutting by the Colo- confi rmed these fi ndings using apatite thermo- ward to near the present-day confl uence of the rado River. Projecting the rate over 5 m.y. would chronology. Colorado and Little Colorado Rivers. From this bring river and bedrock elevation at Lees Ferry The greater width and complexity of eastern point, the river could fl ow either north along to 1600–1900 m above the present 940 m river Grand Canyon supports an older age than for the alignment of the present Marble Canyon, or grade. This would place them at ~1 km above other parts of the canyon. Most likely, the old west along the alignment of the present eastern the elevation of Crooked Ridge River at The course was in a broad valley that followed the Grand Canyon. Gap. Neither distant nor more local warping is curving strike of strata around the south-plung- If the ancient river fl owed north, possibly to likely to overcome such a topographic disparity. ing part of the Kaibab arch. Valley rims at the a hypothetical and undocumented “Glen Lake” We conclude that Crooked Ridge River was time were in Mesozoic rocks and the fl oor was (Hill and Ranney, 2008), its current average unlikely to have fl owed northward from the area incised some hundreds of meters below the top gradient would place it at an elevation of 920 of The Gap. On the other hand, our data do not of the Kaibab Limestone. m at Lees Ferry (Fig. 2), which is below even preclude an ancient river such as the ancestral Regarding the continuation of Crooked the present Colorado River elevation there. Colorado River fl owing southwestward along Ridge–Colorado River beyond the Kaibab Pla- If instead the gradient had been only 3 m/km, approximately its present course but at a higher teau, a course along western Grand Canyon

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Eastern Grand Canyon Kaibab Plateau

Marble Canyon

Little Colorado R. gorge Fig. 14. Oblique view to SW along Crooked Ridge (foreground) showing ridge heading for eastern Grand Canyon (background). Dis- tance from foreground to crest of Echo Cliffs Kaibab Plateau is approximately 100 km. Colorized shaded-relief digital elevation model from USGS National Map elevation dataset. Vertical exaggeration 5x. Crooked Ridge

and the Lake Mead area is precluded by wide- Ridge River already existed. Even had a spill- Mountains laccolith center (274 km) would spread and well-documented Miocene inte- over taken place, the resulting fl ow would have increase the percentage to 72%. rior-basin deposits at the mouth of the canyon been along the course of Crooked Ridge River Volcanism in the San Juan Mountains began (Longwell, 1936; Lucchitta, 1966, 1967, 1972, (and paleo–Colorado River?), not along some ca. 36 Ma with construction of numerous large 1989, 2013). We offer instead the hypothesis, new path. Consequently, a spillover could not andesitic stratovolcanoes scattered across the fi rst proposed by Lucchitta (1975), that the be the cause of integration of the Colorado nascent volcanic fi eld (Lipman et al., 1970); this river fl owed northwestward (Fig. 13) to some- River in western Grand Canyon and the adja- period of volcanism lasted 7–8 m.y. before large- where in the general vicinity of Hurricane and cent upper Lake Mead area. volume, caldera-related ignimbrite eruptions St. George, Utah. Possible continuations are greatly increased the size of the fi eld ca. 29–26 Ma unknown at this time and purely speculative. ORIGIN OF CROOKED RIDGE RIVER (Lipman, 2007). Post-caldera silicic magmatism A possible constraint is given by the fossil fi sh continued to ca. 19 Ma (Lipman, 2007). In the of Snake River affi nities that are found in the Evidence documented herein supports the Needle Mountains area (Figs. 1 and 9), the volca- Bidahochi strata (Spencer et al., 2008). These idea that the source of Crooked Ridge River nic edifi ces were built on top of the Laramide San fossil fi sh are found in the drainage basins of was northeast of the Kaibito Plateau and Black Juan uplift that was probably already more than 3 the Snake and Sacramento Rivers, suggesting Mesa. Salient data include: distinctive volcanic, km high (Larson and Cross, 1956). a connection with either of these areas. The hypabyssal, plutonic, and metamorphic pebbles Even today the San Juans are a high and large fi sh must have utilized the pre–Grand Canyon in the gravel deposit; northeast projection of the mountain complex that reaches altitudes of drainage network to reach the Bidahochi area. river’s course; southwest gradient and current more than 4300 m in the higher peaks. One of indicators; and late Oligocene to early Mio- these peaks is Mount Sneffels, which is entirely INTEGRATION BY SPILLOVER cene detrital zircon sand grains. In the northeast composed of the hypabyssal porphyry of a sub- direction, the San Juan Mountains are the most volcanic stock. Therefore, the bulk of a large It is diffi cult to reconcile the existence of a likely host of the headwaters (Fig. 9). volcano would have overlain the present sum- large Hopi Lake near (let alone across) the well- We can confidently identify pebbles of mit, bringing the altitude of the original volcano established Crooked Ridge River, which had cut minette and diverse lithologies of hornblende- to maybe 7000–8000 m. P.W. Lipman (2011, down to a lower elevation than that of the lake plagioclase porphyry (Figs. 8G and 8P) in the personal commun.) felt that such altitudes for and presumably had a well-developed regional gravel. This proves that, at a minimum, the the original volcanic complex are about right. drainage network that was tributary to it. On the river system crossed the Four Corners area A large and high mountain complex would other hand, shallow and ephemeral lakes and through the Navajo volcanic fi eld and passed certainly have given rise to many rivers fl owing marshes, possibly produced by aggradation of a near one or more laccolith centers (Figs. 1 and radially outward, as they do today. One or more stream fl owing through the area (Ort et al., 1998; 11). The straight-line distance from The Gap to of the rivers draining the southern and western Dallege et al., 2001; Dickinson, 2011) are com- the center of the San Juan Mountains is ~400 slopes of the complex most likely fl owed south patible with the Crooked Ridge River data. km, and the distance to the Carrizo Mountains or southwest; Crooked Ridge River would It is even more diffi cult to envision a sudden, (the nearest center) is ~225 km, so ~56% of the have been one of these streams and could have catastrophic, and canyon-forming spillover of course of the river can be confi dently identi- started collecting runoff as early as late Eocene the lake in an area where the valley of Crooked fi ed. The distances from The Gap to the Ute to early Oligocene.

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INTEGRATION WITH REGIONAL and capture could only have happened if the San (5) The southwesterly course of the river from DRAINAGE Juan were lower in elevation and more vigorous the San Juan Mountains to the Kaibab upwarp than the other streams, such as Crooked Ridge makes it impossible for other rivers such as an Erosion downstream from The Gap has River, issuing from the south slope of the San ancestral Colorado to have fl owed southward removed all traces of Crooked Ridge River Juan Mountains. This means that either the San across Crooked Ridge River into Hopi Lake, or such as deposits, terraces, and channels. How- Juan was the master stream of the region at the northward from the lake into western Colorado ever, inferences can be made about possible time, or that it was tributary to the master stream. or eastern Utah. river connections. We propose that the proto–San Juan River (6) The river could not have fl owed northward Aslan et al. (2011, 2012) reported Colorado came into being when the topographic surface along the present course of Marble Canyon. River gravel buried beneath 11 Ma basalt at of the region was still underlain by the upper (7) A southwest- or south-fl owing river (ances- 2935 m on Grand Mesa, southwestern Colo- part of the Mesozoic section or possibly even by tral Colorado?), topographically much higher rado. Price et al. (2012) considered this gravel early Tertiary rocks and was well above the pres- than the present Colorado River but lower than to be a deposit of the Gunnison River and not the ent topographic surface. The proto–San Juan Crooked Ridge River, could have joined Crooked Colorado River, but this difference is not impor- was likely a tributary to the proto–Colorado Ridge River somewhere near the present mouth tant for this paper. Aslan et al. (2011, 2012) also River, as was Crooked Ridge River. However, of the Little Colorado River. reported gravel of the ancestral Gunnison and with time the proto–San Juan River became the (8) A westward continuation of the river(s) Uncompahgre Rivers at 2500–3000 m on the master stream draining the southern San Juan along the alignment of the present eastern Grand Uncompahgre Plateau in the same region. They Mountains, whereas Crooked Ridge River was Canyon around the Kaibab arch is possible and proposed that the ancestral Colorado, Gunni- captured and abandoned downstream from the our favored alternative. son, and Uncompahgre Rivers joined near the capture point. (9) The demise of Crooked Ridge River was northwest end of the Uncompahgre Plateau and The San Juan River fl ows in a westerly caused by beheading and capture by the ances- then fl owed west onto the Colorado Plateau direction through a low region bounded on the tral San Juan River as this river expanded its ca. 11 Ma. The integration of these three rivers north by the Ute and La Plata Mountains, and drainage basin eastward along the south fl ank of would have preceded 11 Ma by some period of on the south by the Carrizo Mountains (Fig. 9). the San Juan Mountains. time, possibly measured in millions of years. The confl uence of the San Juan River with the (10) It is unlikely that a lake spillover would The conclusion is that at ca. 11 Ma the Colorado River is much upstream from where have integrated the Colorado River through Colorado River and its major tributaries in the Crooked Ridge River once joined the proto– western Grand Canyon because an older and Uncompahgre Plateau–Grand Mesa region were Colorado River in Marble Canyon. When the well-developed river system was already in at elevations 1.6 km or more above the present proto–San Juan River extended itself eastward place that did not fl ow through western Grand grade of the Colorado River, and fl owed onto the from the confl uence, it sequentially captured Canyon into the upper Lake Mead area. Colorado Plateau somewhere near the northwest all the older south- and southwest-fl owing (11) After the river became inactive, the Four end of the Uncompahgre Plateau. The further drainages coming off the San Juan Mountains, Corners region has been lowered erosionally course of the paleo–Colorado River across the including Crooked Ridge River. The San Juan by 1–2 km. Colorado Plateau has not been identifi ed, but one River thus became the local master stream, and ACKNOWLEDGMENTS possibility is that it was approximately along the the added water allowed it to cut down vigor- present southwesterly course on the west side ously. After capture, the downstream portion We thank Sue Beard, Richard Hereford, Fred of the Monument Upwarp (see also Cooley and of Crooked Ridge River was left high and dry, Peterson, and Marith Reheis of the USGS, as well as Davidson, 1963). The river would have been at a forming the erosional remnant we see today. two anonymous reviewers, for their helpful analyses. considerably higher elevation along this course Both Crooked Ridge River and the proto– Their perceptive and occasionally pithy comments have caused us to re-examine and strengthen our data than the present elevation, but lower overall than Colorado River in this area would be of Oligo- and conclusions with care, and to clarify our writing, the smaller and shorter Crooked Ridge River, cene to early Miocene age. which evidently was opaque in places. We also wish to which fl owed on the east side of the upwarp. thank Luke Blair and Trent Hare of the USGS for their Crooked Ridge River could have been a tribu- CONCLUSIONS indispensable help with the graphics. Finally, we must recognize gratefully those grand old masters, from tary to this paleo–Colorado River and would J.W. Powell to Edwin D. McKee, who constructed the have joined the master stream somewhere west (1) Crooked Ridge River was a substantial fi eld upon which our games are now played. or south of The Gap, possibly near the present stream of the region whose head was at least as confl uence with the Little Colorado River (Figs. far northeast as the San Juan Mountains. The REFERENCES CITED 2, 13, and 14). Both Crooked Ridge River and demise of the river probably occurred in early Akers, J.P., Shorty, J.C., and Stevens, P.R., 1971, Hydrology the paleo-Colorado River would have obtained Miocene time, between 24 and 19 Ma. 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Ross, M.L., 1998, Geology of the Tertiary intrusive centers tonism along the southwestern margin of the Colorado Pederson, J.L., 2008, The mystery of the pre–Grand Canyon of the La Sal Mountains, Utah: Infl uence of preexisting Plateau: Tectonophysics, v. 61, p. 25–47, doi:10.1016 Colorado River: Results from the Muddy Creek Forma- structural features on emplacement and morphology, in /0040-1951(79)90290-7. tion: GSA Today, v. 18, no. 3, p. 4–10, doi:10.1130 Friedman, J.D., and Huffman, A.C., Jr., coordinators, Young, R.A., 1982, Paleogeomorphic evidence for the struc- /GSAT01803A.1. Laccolith Complexes of Southeastern Utah: Time of tural history of the Colorado Plateau margin in western Pederson, J.L., Cragun, W.S., Hidy, AlJ., Rittenour, T.M., Emplacement and Tectonic Setting—Workshop Proceed- Arizona, in Frost. E.G., and Martin, D.L., eds., Meso- and Gosse, J.C., 2013, Colorado River chronostratigra- ings: U.S. Geological Survey Bulletin 2158, p. 61–83. zoic–Cenozoic Tectonic Evolution of the Colorado phy at Lee’s Ferry, Arizona, and the Colorado Plateau’s Scarborough, R., 2001, Neogene development of the Little River Region, California, Arizona, and Nevada: San bull’s-eye of incision: Geology, v. 41, no. 4, p. 427– Colorado River valley and eastern Grand Canyon: Field Diego, Cordilleran Publishers, p. 29–39. 430, doi:10.1130/G34-051.1 evidence for an overtopping hypothesis, in Young, R.A., Young, R.A., 2008, Pre–Colorado River drainage in west- Peters, L., 2011, 40Ar/39Ar geochronology results from the and Spamer, E.E., eds., Colorado River: Origin and Evo- ern Grand Canyon: Potential infl uence on Miocene Wildcat Peak volcanic dike: Socorro, New Mexico lution: Grand Canyon, Arizona, Grand Canyon Associa- stratigraphy in Grand Wash Trough, in Reheis, M.C., Geochronology Research Laboratory, New Mexico tion, p. 207–212. 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