Rejection of the Lake Spillover Model for Initial Incision of the Grand Canyon, and Discussion of Alternatives

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

Rejection of the lake spillover model for initial incision of the Grand Canyon, and discussion of alternatives

William R. Dickinson* Department of Geosciences, University of Arizona, Tucson, Arizona 85721-0077, USA

ABSTRACT INTRODUCTION borough (1989, p. 522) initially envisioned that Hopi Lake was drained when headward erosion One hypothesis for the origin of the Grand The reason the Grand Canyon was cut by the upstream through the Grand Canyon breached Canyon is that a broad Hopi Lake, of which Colorado River is disputed (Lucchitta, 1984, the Kaibab-Coconino barrier that dammed lakebeds of the Miocene Bidahochi Forma- 1989, 1990; Young and Spamer, 2001; Ran- the lake basin on the west, but later suggested tion are a vestigial record, ponded to a depth ney, 2005; Flowers et al., 2008; Pelletier, 2010; that the lake level might have overtopped the great enough near the Miocene-Pliocene Wernicke, 2011; Beard et al., 2011; Douglass, Kaibab-Coconino barrier near a locale where time boundary to spill over the topographic 2011; Karlstrom et al., 2011). This paper evalu- previous Laramide erosion had cut a drainage barrier of the Kaibab-Coconino Plateau to ates the lake spillover model, which holds that notch through the confi ning uplands (Scarbor- initiate incision of the Grand Canyon below (1) the Miocene Bidahochi Formation of north- ough, 2001, p. 212). Meek and Douglass (2001) the lake outlet. Bidahochi paleogeography eastern Arizona contains lakebeds that are a took the postulate of lake spillover a logical step indicates that Hopi Lake was a playa sys- vestigial record of a once-deep Hopi Lake, fi lled further by inferring that incision of the Grand tem that never achieved appreciable depth. with waters from the Colorado River in Utah, Canyon was triggered when Hopi Lake spilled Topographic relations in northern Arizona that ponded east of the Kaibab-Coconino Pla- across the elevated Kaibab-Coconino tract with- show that the maximum elevation of Bida- teau until it spilled over the crest of the plateau out benefi t of a pre-existing paleocanyon as a hochi lakebeds is not compatible with lake dam; and (2) lake spillover near the Miocene- guide for water fl ow. spillover through the Grand Canyon unless Pliocene time boundary initiated incision of the Studies of sedimentation along the course post-Bidahochi deformation or pre-Bida- Grand Canyon below the lake outlet. Bidahochi of the lower Colorado River downstream from hochi canyon-cutting altered the landscape paleogeography does not support the lake spill- the Grand Canyon (Fig. 1) have documented in ways unsupported by geologic evidence, over model without ancillary paleotopographic that the Colorado River did not fl ow through or the surface of Hopi Lake rose transiently hypotheses that are diffi cult to sustain (Dick- the Grand Canyon until near the Miocene- to elevations unrecorded by any sediment. inson, 2011). Consideration of the upstream Pliocene time boundary (Spencer and Patchett, The implications of erosional episodes morphology of the Colorado River drainage 1997; Faulds et al., 2001a; Spencer et al., 2001; affecting the Colorado Plateau, the tim- system (Fig. 1) suggests constraints for alternate Patchett and Spencer, 2001; House et al., 2005; ing of drainage reversal across the central hypotheses. Roskowski et al., 2010). The inception of water Colorado Plateau, the spatial pattern of the fl ow through the canyon initially formed a chain Colorado River drainage system, and the BACKGROUND of downstream lakes within which the lacus- analogous confi gurations of multiple river trine Bouse Formation was deposited along the canyons cut into Precambrian basement Blackwelder (1934) rejected pre-Laramide modern river course (Fig. 1). The best current within the river basin also challenge the antecedence for the course of the Colorado estimate for the entry of Colorado River water Hopi Lake spillover model. A viable alter- River through Laramide uplifts like the Kai- into desert basins below the Grand Canyon is nate scenario for incision of the Grand Can- bab uplift transected by the Grand Canyon. He ca. 4.9 Ma (Early Pliocene) based on geochemi- yon is the concept of an ancestral Miocene suggested instead that the “haphazard” course cal correlation of distal ashfall tuff within the Colorado River that transited the Kaibab of the river through multiple Laramide uplifts Bouse Formation with the 4.83 Ma Lawlor Tuff uplift on the site of the eastern Grand Can- originated from successive spillover of waters (Sarna-Wojcicki et al., 2011) erupted near San yon, but exited the Colorado Plateau into from lake basins on a semiarid Colorado Pla- Francisco Bay, California (Spencer et al., 2011a, an ancestral Virgin River drainage before teau of low relief, before incision of the Grand 2011b). That tephrochronology is compatible capture near the site of the present central Canyon promoted dissection of the plateau sur- both with the age (5.6 Ma) of a sub-Bouse tuff Grand Canyon by a stream working head- face upstream by headward erosion. Scarbor- (House et al., 2005) and with the age (6 Ma) of ward through the western Grand Canyon ough (1989) concluded that lakebeds within the the youngest dated tuff interbedded with lacus- from the Grand Wash Cliffs. Miocene Bidahochi Formation were deposited trine Hualapai Limestone, which was deposited within Hopi Lake (Williams, 1936), which in before the arrival of Colorado River water to his view covered ~30 × 103 km2 in northeastern interconnected desert basins north of the Bouse Arizona east of the Kaibab-Coconino Plateau lake chain (Fig. 1) but still downstream from *Email: [email protected] transected by the modern Grand Canyon. Scar- the mouth of the Grand Canyon (Spencer et al.,

Geosphere; February 2013; v. 9; no. 1; p. 1–20; doi:10.1130/GES00839.1; 10 ﬁ gures. Received 30 June 2012 ♦ Revision received 13 October 2012 ♦ Accepted 22 October 2012 ♦ Published online 13 December 2012

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Figure 1. The Colorado and 110° W 105° W Gila River drainages of the Colorado - Gila N drainage divides Colorado Plateau and adja- 0250 cent geologic provinces. The margin of Scale in km limit of the Colorado Plateau Colorado Plateau includes the faulted transition Great Divide Tb Bidahochi Formation Basin zone on the south and west, and (interior is marked by the south ﬂ ank G drainage) Tc Chuska Sandstone ID re of the Uinta Mountains on the R e i Colorado UT v n north, the White River mono- e Mineral Oligocene laccoliths r RS cline on the northeast, and the Belt WY edge of the Rio Grande rift on Laramide laccoliths CO the southeast. Hualapai paleo- SLC lake (HL), the composite chain 115° W 40° N r of Bouse paleolakes (BL) along 40° N e NV UT i v R D the course of the lower Colo- r en

e re rado River, and the restored (for v UT CO i Marysvale G San Andreas fault slip) position R o GJ volcanic d a of initial Colorado River del- field r o RMSJ l D San Juan taic deposits (Imperial Forma- o White MVW Belt o volcanic tion of the Salton Trough) after River C LS field Spencer and Patchett (1997), SM H Spencer et al. (2008a, 2008b, Ab 2011a), and Roskowski et al. Ccc SJ (2010). Mid-Cenozoic Reno– LP CO Vi UT U Marysvale–San Juan igneous NM AZ Pa C belt (RMSJ belt, in brown) NV HL CA n d Fig. 8 including Oligocene (Fig. 3) lac- r a C G y n LV coliths (Ab—Abajo; H—Henry; Tc Oligocene LS—La Sal) after Sullivan et al. Chuska L erg (1991) and Nelson and David- C son (1998). Laramide Colorado 35° N BL A 35° N Fig. 2 e Mineral Belt (in green) includ- V Tb d

Bristol e n ing Cretaceous–Paleocene (Fig. basin C a o r l o

G G 3) plateau laccoliths (C—Car- i l a rizo; LP—La Plata; SM—San PS o Miguel; U—Ute) after Chapin Sa i Mogollon- Ph R Datil (2012). Inferred extent of Oligo- volcanic Salton R i v e field cene Chuska erg after Dick- Sea r i l a inson et al. (2010) as modiﬁ ed SD G Y from Cather et al. (2008). Key marine Imperial AZ NM features in Nevada: Ccc—Cali- P O a Formation ente caldera complex; MVW– c c EP e i (restored) f a i Meadow Valley Wash. Rivers c n Gulf of (in blue): Do—Dolores; LC— California interior Mexico drainage Little Colorado; Sa—Salt; SJ— San Juan; Ve—Verde; Vi—Vir- gin. Cities: A—Albuquerque; D—Denver; EP—El Paso; GJ—Grand Junction; LV—Las Vegas; Pa—Page; Ph—Phoenix; PS—Palm Springs; RS—Rock Springs; SD—San Diego; SLC—Salt Lake City; Y—Yuma. States: AZ—Arizona; CA—California; CO—Colorado; ID—Idaho; NM—New Mexico; NV—Nevada; UT—Utah; WY—Wyoming.

2001). Post-Bouse aggradation of lower Colo- The Bouse Formation records rapid inunda- sodic integration of the lower Colorado River by rado River deposits culminated during the Plio- tion of desert basins along the Bouse lake chain. progressive “spilldown” from successive Bouse cene interval of 4.1–3.3 Ma (House et al., 2005), The abrupt arrival of Colorado River water lake basins (Spencer et al., 2008b; House et al., but older Pliocene Colorado River gravels are downstream from the Grand Canyon has been 2011) is unable, however, to deﬁ ne the mode interbedded with 4.7 Ma and 4.4 Ma basalts taken to support the model of Hopi Lake spill- of drainage evolution upstream from the Grand near the Grand Wash Cliffs farther north just over to initiate incision of the Grand Canyon Canyon that integrated upper and lower courses below the mouth of the Grand Canyon (Howard (Spencer and Pearthree, 2001, 2005; Spencer et of the Colorado River into a uniﬁ ed trunk stream and Bohannon, 2001). al., 2008a; House et al., 2008). Evidence for epi- passing through the Grand Canyon. The likeli-

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hood of spillover from Hopi Lake to initiate of a composite depositional system. Lower et al., 2001a; Lopez Pearce et al., 2011). The river fl ow through the Grand Canyon can be lacustrine and middle volcanic members are middle (volcanic) member (0–100 m thick), addressed by examining the sedimentary record shown jointly on Figure 2, and the upper fl uvial composed of mafi c lava, tuff, and volcaniclastic of the Bidahochi Formation, which contains member locally includes interbedded eolian sandstone, has yielded Late Miocene K-Ar and the only known lacustrine deposits thought to deposits (Love, 1989). Tuffs in the lower (lacus- 40Ar/39Ar ages of 8.5–6.0 Ma (Damon and Spen- record Hopi Lake sedimentation. trine) member (100–150 m thick) have yielded cer, 2001; Dallegge et al., 2001, 2003), leaving Middle Miocene 40Ar/39Ar ages of 15.5 Ma and a span of ~5 Myr undated isotopically between BIDAHOCHI PALEOGEOGRAPHY 13.7 Ma (Dallegge et al., 2001, 2003), postdat- the lower and middle members. A tuff near the ing the initiation of extensional faulting along base of the upper member has been correlated by The distribution of the Bidahochi Formation the Grand Wash Cliffs at the mouth of the Grand tephrochronology with the Blacktail Creek ash and modern elevations at its base are shown Canyon (Faulds et al., 2001b, 2010) but predat- with an 40Ar/39Ar age of 6.6 Ma (Dallegge et al., by Figure 2. The unit includes three lithologic ing deposition of the Hualapai Limestone adja- 2001, 2003). The isotopic age of ca. 6 Ma for the members (Fig. 3B), at least in part lateral facies cent to the cliffs (Spencer et al., 2001; Faulds base of the upper fl uvial member is compatible with a late Hemphillian mammalian fauna from the Bidahochi Formation dated at 7–5 Ma from the magnetostratigraphy of Lindsay et al. (1984) 109° 30′ W 1950 as recalibrated by Cande and Kent (1995). The 2195 2135 830 evolution of the Bidahochi depositional system KC 1 2075 from lacustrine to fl uvial environments predated 2195 elevations in meters 2012.5 of base of Colorado River fl ow through the Grand Canyon G Bidahochi Formation but coincided approximately with deposition of 1950 (from 216 intersections 1890 of basal contact with the youngest tuff (6 Ma) within the Hualapai topographic contour lines) Limestone (Fig. 3A). Volcaniclastic strata of the Bidahochi For- GS 1 2135 Gallup ′ 8 mation occur at multiple horizons intercalated 30 9 ′ 0 30 locally with lacustrine strata (Love, 1989; Ort

1 2195 2255 et al., 1998), and basal horizons of the fl uvial 8 ARIZONA 3 0 NEW MEXICO upper member intertongue with and grade lat- 2135 W erally into lacustrine successions of the lower PC member (Repenning et al., 1958) where the volcanic middle member is absent (Repenning S and Irwin, 1954). Lateral equivalence of hori- 1830 zons mapped within the lower and upper mem- 1890 e 1770 r bers is demonstrated by intertonguing of fl uvial l e 0 e v 3 v i 8 c R 0 o a 1 and lacustrine strata within a deltaic complex t 9 n i o 8 35° N t o a n 1 exposed along the fl anks of mesas west of Pueblo c 35° N t P u e r c Colorado Wash between Ganado and Grease- o Holbrook f u wood Springs (Fig. 2) near a topographic feature n b c a e s known locally as Sand Gap. In time, the upper r a 1 l 7 t a i 7 fl uvial member prograded across the lower and m o d e r n n 0 2135 L middle members, and also oversteps the older s t r e a m s ittle Co lor members up-paleofl ow to the north and east to ado Ri rest unconformably on pre-Tertiary strata along ver 2075 110° W CW the fringes of Bidahochi exposures (Fig. 2). Out- crops of the Bidahochi lacustrine facies nowhere B i d a h o c h i F o r m a t i o n 1770 North SJ reach elevations in excess of ~1950 m. 1830 30′ The age range of the upper member is not upper fluvial member 1890 2075 controlled by isotopic dating but the fl uvial suc- limit of lacustrine strata cession is 75–85 m thick in measured surface 1950 025sections and reaches thicknesses of 135–140 m lower lacustrine Q u a t e r n a r y and middle volcanic v o l c a n i c s scale in km in subsurface wellbores. Deposition of fl uvial members (undivided) 2012.5 109° W Bidahochi strata may have continued into Pliocene time after integration of lower and Figure 2. Paleogeographic map of the Miocene Bidahochi Formation, Arizona–New Mexico upper courses of the Colorado River through (see Figure 1 for regional location). Facies and elevations of the basal contact are updated the Grand Canyon (Fig. 3A), but basalt lava that from Repenning et al. (1958) and Love (1989) after Wilson et al. (1960), Hackman and locally mantles outcrops of the Bidahochi For- Olson (1977), Ulrich et al. (1984), Cather and McIntosh (1994), Reynolds (1989), Ort et al. mation has yielded an 40Ar/39Ar age of 2.4 Ma (1998), Dallegge et al. (2001, 2003), Gross et al. (2001), and NMBGMR (2003). Abbrevia- (McIntosh and Cather, 1994), precluding con- tions: CW—Carrizo Wash; G—Ganado; GS—Greasewood Springs; KC—Keams Canyon; tinuation of Bidahochi sedimentation into Pleisto- PCW—Pueblo Colorado Wash; S—Sanders; SJ—St. Johns. cene time.

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Bidahochi Paleoenvironments A B C lower Colorado River northeast Arizona and upper Colorado River Volcanological analysis of the middle vol- Ma and High Plateaus northwest New Mexico drainage system canic member forming the Hopi Buttes (White, 0 Quater- nary Colorado River S p r i n g e r v i l l e 1989, 1990, 1991; Ort et al., 1998) indicates that Bouse Lakes v o l c a n i c L o d o r e a n d Hopi Lake was never a deep body of water dur- Plio f i e l d W e s t w a t e r Grand C a n y o n s ing the Late Miocene time frame (8.5–6.0 Ma), cen Canyon

e volcanic field upper (fluvial) member when its level must ultimately have risen were

5 San Francisco ?? it eventually to achieve lake spillover to initiate m i d d l e incision of the Grand Canyon. Phreatomagmatic (v o l c a n i c) Browns eruptions gave rise to multiple diatremes (tephra- m e m b e r Park

Hualapai ﬁ lled volcanic necks) and maars (shallow cra- 10 Limestone Formation ters formed by explosive eruptions). Subaerial pre-Grand Canyon Grand Muddy Creek Fm l o w e r

volcanic field Mesa scoria cones formed locally within the maars. Mogollon Rim (l a c u s t r i n e) basalt B I D A H O C I Mormon volcanic field m e m b e r F O R M A T I N Hydrovolcanism stemmed from contamination field 15 of magma with waterlogged sub lacustrine sedi- Uinta Grand Wash Mountains ment, rather than from eruption into lake waters. Cliffs faults Water ponded within some maar craters, but Peach Springs b a s l t s (d e n u d a t i o n) (no denudation) maar rims were nowhere overtopped by lake 20 Tuff surfaces. Outﬂ ow pyroclastic aprons that spread Marysvale laterally from volcanic centers include dry base- volcanic

Pine San surge deposits and subaerial pyroclastic ﬂ ows Valley field laccolith Juan emplaced locally on muds with desiccation 30 eolian volcanic field (erg) Datil field cracks. At the time of the Late Miocene erup- volcanic Mogollon- Buck and Doe Conglomerate Hualapai CHUSKA tions, Hopi Lake within the area of Bidahochi Plateau fluvial SANDSTONE D.R. Fm sedimentation was a playa-like body, and per- ? VC haps ephemeral rather than perennial (White, 40 (hiatus) Uinta M o g o l l o n 1990; Ort et al., 1998; Dallegge et al., 2001,

Bishop Formation Abajo laccoliths ? Henry - La Sal - West Claron R i m 2003). A rise in the elevation of the base of the Water Conglomerate

Baca Bidahochi Formation in its westernmost expo- Fm F o r m a t i o n Formation Formation sures (Fig. 2) suggests that the lacustrine facies 50 Music formed within a local depression closed on the Fm River

? Ute- Green Mtn Colton west as well as on the east. Fm volcaniclastic La Plata- Formation Pine procursors of San Miguel The confi guration of the basal contact of the ? Hollow Mogollon - Datil laccoliths upper fl uvial member shows that fl uvial Bida- 60 Formation volcanic field North (vc) Horn hochi strata occupy paleovalleys of ancestral Canaan Formation Pueblo Colorado Wash, the Puerco River, and Peak Carrizo Wash tributary to the Little Colorado Formation 70 Carrizo laccoliths River (Fig. 2). The upper fl uvial member also Uinta spread, presumably by progressive aggradation, Basin over gently sloping (15 ± 3 m/km) pediment-

Cretaceous Paleocene E o c e n e Oligocene i o c e n M e like surfaces carved across interfl uves separat- S a n R a f a e l s w e l l Campanian Maas E M L Early M i d l e L Early Late a r l y E D i d l M e e f i a t e L n c e u p l i f t 80 K a i b a b u p l i f t ing those three paleovalleys. The thalwegs of the fl oors of fl uvial Bidahochi paleodrainages, Figure 3. Age relations of Bidahochi sedimentation (see column B) to key events affecting as defi ned by contours on the base of the Bida- the Colorado River drainage system (blue—water; yellow—sedimentary strata; pink— hochi fl uvial facies (Fig. 2), have slopes of igneous rocks). Timescale (Maas—Maastrichtian) after Walker and Geissman (2009) with ~0.008 (8 m/km). By contrast, the slopes of the scale change at 20 Ma. Inception of Laramide deformation (basal heavy lines with arrows) modern Pueblo Colorado and Carrizo valleys after Aschoff and Steel (2011), Cather (2004), and Tindall et al. (2010). Sources of data: are only 0.004–0.005 (4–5 m/km) in the area (A) Damon et al. (1996), Dickinson et al. (2012), Faulds et al. (2001a, 2001b, 2010), Holm of Bidahochi exposures, and the slope of the (2001b), Leighty (1998), Lopez Pearce et al. (2011), Pederson (2008), Reynolds et al. (1986), modern Puerco valley is even lower at ~0.002 Rowley et al. (1998), Spencer et al. (2001, 2011b), Young (1999, 2011), Young and Hart- (2 m/km), comparable to the slope of the mod- man (2011), and Young et al. (2011); (B) Cather and Chapin (1989), Dallegge et al. (2003), ern Little Colorado River valley between St. Damon et al. (1996), Damon and Spencer (2001), Dickinson et al. (2010), Elston (1989), Johns and Holbrook south of Bidahochi expo- McIntosh (1989), Potochnik (1989), Potochnik and Faulds (1998), Reynolds et al. (1986), sures (Fig. 2). The differences in paleovalley and Semken and McIntosh (1997); (C) Cole (2011), Cunningham et al. (1994), Dickinson and modern-valley slopes, coupled with the et al. (1986, 2012), Garcia et al. (2011), Hansen (1986), Kowallis et al. (2005), Lipman close coincidence of paleovalleys and modern (1989), Luft (1985), Nelson (1998), Nelson et al. (1992a, 1992b), and Sullivan (1998). In C, valleys in plan view, suggests that Bidahochi D.R.—Duchesne River. paleodrainages served as piedmont feeders to an ancestral Little Colorado River as fl uvial aggra-

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dation advanced over the Bidahochi lacustrine 150–300 m above the base of the Bidahochi western outcrops of the fl uvial (upper) member facies. Dissection of the Bidahochi Formation fl uvial facies. The elevation and confi guration and interpreted as lake-margin deposits (Gross and gradual reduction in valley slope can be of the basal contact suggest that Fence Lake et al., 2001). Strontium isotope ratios for the attributed to post-Miocene headward erosion streams were tributary to the ancestral Rio lacustrine member (Fig. 3B) closely match by Little Colorado tributaries as the elevation of Grande (Pazzaglia and Hawley, 2004), rather ratios for the modern San Juan River draining the Little Colorado River thalweg was progres- than to the ancestral Little Colorado River. Post- into the Colorado River from the east, but not sively lowered over time from an initial eleva- Miocene headward erosion by tributaries of the ratios for the modern Little Colorado River tion higher than the Bidahochi lacustrine facies Little Colorado River has transferred exposures closer to Bidahochi exposures. The signifi cance buried beneath the Bidahochi fl uvial facies. of the Fence Lake Formation into the Little of the isotopic difference between modern Colo rado drainage basin by shifting the conti- San Juan and modern Little Colorado water is Fence Lake and Quemado Formations nental divide to the east. uncertain with present information. Bidahochi The Quemado Formation was deposited by strontium isotope ratios overlap with but tend The alluvial Fence Lake and Quemado For- streams tributary, like Bidahochi paleodrain- to be somewhat lower than those of the Bouse mations of westernmost New Mexico east of St. ages, to the ancestral Little Colorado River but is Formation from the river-fed lake system that Johns (Fig. 2) have been regarded provisionally apparently younger than the Bidahochi Forma- formed along the California-Arizona border as lateral extensions of Bidahochi strata (Cather tion. Quemado strata are exposed in fi ve local immediately after the integrated Colorado River and McIntosh, 1994) but are excluded here from sub-basins or paleovalleys inset erosionally by fi rst fl owed through the Grand Canyon (Fig. 4). the Bidahochi depositional system for the fol- ~100 m into mesas capped by Upper Miocene Strontium isotope ratios from the pre–Grand lowing reasons. basalts (6.8–5.2 Ma) that are correlative with Canyon Hualapai Limestone deposited in local The Fence Lake Formation (McLellan et al., the fl uvial facies of the Bidahochi Formation, desert basins north of the Bouse paleolakes 1982) is equivalent in age (14.5–6.9 Ma) to and interfi nger locally with Pleistocene basalts (Fig. 1) are systematically higher than those the lacustrine and volcanic facies of the Bida- ≤1 Ma in age (Cather and McIntosh, 1994). from the modern Colorado River, Bouse carbon- hochi Formation (Lucas and Anderson, 1994) ates, and the Bidahochi Formation (Fig. 4). but its basal contact slopes to the east-north- Strontium Isotopes and Detrital Zircons Speculative coupling of Colorado River fl ow east (McIntosh and Cather, 1994), away from with Bidahochi sedimentation (Gross et al., the Bidahochi depocenter. The net gradient Initial 87Sr/86Sr ratios for carbonate materials 2001) is apparently denied by the ages of Bida- of the basal contact is ~0.003 (3 m/km) from in the Bidahochi Formation are comparable to hochi detrital zircons. Bidahochi detrital zircon Table Mountain, ~50 km south of St. Johns ratios in modern river waters on the Colorado populations closely resemble those in sands (Fig. 2), to the vicinity of Quemado, 75 km Plateau (Fig. 4). The Bidahochi samples include from the modern Little Colorado River but dif- distant in New Mexico. Near the Arizona–New ten of marl from the lower (lacustrine) member, fer substantially from detrital zircon popula- Mexico border east of St. Johns, the base of the three of molluscan fossils from the middle (vol- tions in modern sands from the Colorado River Fence Lake Formation is perched at elevations canic) member, and two of tufa collected from and its major upstream tributaries, including the

Figure 4. Initial 87Sr/86Sr ratios Bouse modern pre - Grand Canyon Formation Colorado Hualapai of carbonate materials (marl, (California - River 0.712 Limestone gastropods, tufa) from the Arizona) system (12 - 6 Ma) Bristol basin arm 0.7115 - 0.7195 Miocene Bidahochi Formation of Lake Blythe and Hualapai Limestone (right (Mojave Desert) Bidahochi columns) and segments of the N=4 Formation 0.711 N= Colorado River system (Fig. N= lower upper (16 - 6 Ma) 34 1): bivalves from tidal-ﬂ at and 21 Ge delta-front successions of the tributaries 86 marine SS Pliocene Colorado River delta 0.710 N= 87 assemblages LF SJ in the Salton Trough, mollusks Ga 10 from Neogene marine strata SJ Initial sr/ sr Lake Las Vegas

in the subsurface of the Yuma Lake Blythe Lake Mohave Basin beneath the modern 0.709 mouth of the Colorado River, V LC Pliocene Bouse Formation Miocene - Pliocene

(marl, barnacles, travertine or lower member western upper member seawater middle member tufa), and modern waters of 0.708 Yuma basin tributaries Salton Trough the Colorado River (denoted as lower and upper below and above the Grand Canyon) and selected tributaries. Dots are individual samples and N is the number of grouped samples falling within a plotted box. Labeled samples: Ga—Grand (upper Colorado) River (at Moab, Utah); Ge—Green River (at Green River, Utah); LC—Little Colorado River; LF—Lees Ferry; SJ—San Juan River; SS—Salton Sea; V—Virgin River. Western tributaries include the Escalante, Fre- mont, and Muddy Rivers. Data are from Goldstein and Jacobsen (1987), Spencer and Patchett (1997), Gross et al. (2001), Roskowski et al. (2010), Crossey et al. (2011), Lopez Pearce et al. (2011), and Spencer et al. (2011a). Uncertainty limits (2σ) for individual analyses are 1–3 × 10–5 (too narrow to plot at scale).

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Green, Grand (upper Colorado), and San Juan These relations jointly imply that spillover mantle tomography, suggests that the south- Rivers (Kimbrough et al., 2011). The Bida- of a Bidahochi-related Hopi Lake to initiate western rim of the Colorado Plateau has been hochi depositional system evidently received incision of the Grand Canyon is not a realistic uplifted by as much as 400 m over the past no sediment from a postulated ancestral Colo- hypothesis unless (1) post-Bidahochi tectonic or 6 Myr (Karlstrom et al., 2008). If the modern rado River fl owing southeast from Utah into isostatic deformation and associated erosion has elevation of the top of the Kaibab-Coconino Hopi Lake. substantially altered the morphology of north- Plateau is restored downward by 400 m, the ern Arizona to erect a Kaibab-Coconino topo- 6 Ma elevation of the plateau barrier damming Hopi Lake Spillover graphic barrier that did not exist in its present postulated Hopi Lake would stand near the form at Bidahochi time, (2) integration of the maximum elevation of the Bidahochi lacustrine The level of Hopi Lake could not have risen Colorado River through the Grand Canyon was facies. Spillover of even a shallow lake or an higher than ~2000 m without leaving a record of facilitated by the presence of an older paleo- array of ponded wetlands across such a subdued lacustrine transgression, which is not observed canyon that provided a pathway for water fl ow Kaibab-Coconino barrier would be feasible. within the fl uvial facies tract of the Bidaho- from lake spillover through an erosional notch The postulated tectonic welt continues, how- chi Formation (Fig. 2). Contours on the pres- incised into the Kaibab-Coconino Plateau at an ever, along the Mogollon Rim to the southeast of ent landscape (Fig. 5) show that a lake surface elevation lower than its crest, or (3) the water the Grand Canyon (Fig. 5) where present maxi- ≤2000 m could not have overtopped the Kaibab- surface of Hopi Lake rose at least transiently to mum elevations are approximately the same Coconino Plateau in its present confi guration. elevations at which no sedimentary record of (2200–2300 m) as on the Kaibab-Coconino Moreover, the Grand Canyon transects the high- lacustrine conditions has survived post–Grand Plateau where transected by the Grand Can- est rather than the lowest segment of residual Canyon erosion. None of these postulates is yon. Restoring the Mogollon Rim downward plateau uplands that extend northwest across attractive. to the same elevation in Late Miocene time as the Colorado River from the Mogollon Rim in the lacustrine facies of the Bidahochi Formation central Arizona. The lowest modern topographic Post-Bidahochi Deformation would confl ict, however, with paleogeographic saddle in pre-Pliocene strata forming the analysis implying a persistent regional slope to uplands lies due west of Bidahochi exposures Consideration of differential modern and the northeast from the Mogollon Rim toward beneath the post–6 Ma San Francisco volcanic Pliocene incision rates within the Grand Can- an ancestral Little Colorado River since Middle fi eld (Figs. 3A and 5). yon, coupled with geodynamic modeling from Miocene time (Holm, 2001a, 2001b).

112° W 111° W 110° W 109° W UTAH 37° N Paria . R Piute ARIZONA 0 Plateau Page Mesa S 00 a Kaibab 2 Plateau Lee’s Ferry n 2000 o d ra J lo C h i n l e u

o a C White nB Toroweap V a l l e y Mesa 20 Point 00 2 5 B l a c k a 0 2 s 0 0 0 i n M e s a 0

C D olo 2 ra 0 e do 00 Chinle R f . i 36° N a 36° N n Figure 5. Northeastern Arizona Cameron c Grand 2100 e 0 showing the extent of the Bida- ARIZONA Canyon 0 P Coconino NEWMEXICO 20 B i d a h o c h i hochi lacustrine facies in rela- rim Plateau l a L L a k e b e d s Ganado t i tion to the Grand Canyon and t e t a 050 l Gallup to contours of modern topog- e u raphy on pre-Pliocene strata scale in km San Francisco volcanic field (dashed beneath Pliocene– North Flagstaff Quaternary San Francisco and o ercer Springerville volcanic ﬁ elds). u v C o P i 35° N l o r R 35° N Winslow a d o

Pliocene - Quaternary Holbrook volcanic fields Mo g above 2000 m o R i ver l l 20 o 20 00 2000 n 00 St. Johns topographic counters R in meters 20 i m on subvolcanic strata 00 Show Low preserved extent Springerville of Miocene volcanic field Bidahochi lakebeds 34° N 111° W 34° N

6 Geosphere, February 2013

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Downward restoration of the Mogollon Rim Laramide Kaibab uplift, although by paleofl ow ridor developed along the lower Colorado River for Miocene time would allow potential pre– to the northeast rather than to the southwest. The west of the Colorado Plateau (Beard and Faulds, Grand Canyon drainage of the Bidahochi depo- uplift did not begin to grow until 75–80 Ma (Fig. 2011). Wernicke (2011) inferred that the Peach center toward the southwest across the present 3A) and was not overlapped by post-Laramide Springs paleovalley descended into a Paleogene Mogollon Rim (Potochnik, 2011) and into the strata of the Claron Formation (Dickinson et al., paleo–Grand Canyon to which it was tributary. ancestral Salt River (Fig. 1), thereby preclud- 2012) until ca. 55 Ma (Figs. 3A, 6). Most impor- That interpretation rested upon the estimate of ing a Bidahochi outlet through the Grand Can- tant for the evolution of Hopi Lake, his interpre- a net reversed gradient from backtilt of 0.8° yon. The Salt River paleocanyon was incised tations dictate that initial incision of the Grand (downward to the southwest or up-paleocurrent) to a depth of nearly 1500 m (Potochnik and Canyon predated all Bidahochi sedimentation by Young (2001) based on the subsurface con- Faulds, 1998; Potochnik, 2001) southwest of by ~50 Myr. fi guration of the Peach Springs paleovalley to Show Low (Fig. 5) by paleofl ow to the northeast Other thermochronological analyses (Kelley the southwest of Peach Springs (Fig. 6). Bed- during Laramide time. Detrital zircons from a et al., 2011) and geotectonic arguments (Karl- ding attitudes observed in the paleovalley fi ll sample of the Mogollon Rim Formation col- strom et al., 2012) do not support the existence near the head of Peach Springs Wash north of lected ~15 km due west of Show Low indicate of a pre-Neogene Grand Canyon, which four Peach Springs imply, however, that the amount that northward paleofl ow through the paleocan- key geological relationships argue against: of backtilt could have been greater northward yon continued at least into Early Miocene time (1) Headwater tributaries of the Verde River, from Peach Springs toward the Colorado River. (Potochnik et al., 2012). A cluster of the fi ve working headward into the Colorado Plateau Strata of the Buck and Doe Conglomerate and youngest detrital zircon grains that overlap in along the northern fringe of the Gila River overlying 20 Ma basalt exposed in road cuts at age at 1σ yield a weighted mean average age, drainage, reach to within <5 km of the rim of the the head of modern Peach Springs Wash near with both random and systematic errors taken western Grand Canyon and to within ~10 km of Peach Springs (Fig. 6) dip 2°–4° to the south. into account, of 18.4 ± 2.9 Ma (at 2σ), a time the Colorado River itself in the bottom of the Moreover, the present elevation (1200 m) of the frame immediately before lacustrine sedimenta- canyon (Fig. 1). It seems unlikely that a deep intersection of the Milkweed-Hindu Canyon tion was initiated in the Bidahochi Formation canyon could persist as such a narrow feature paleotributary with the master Peach Springs (Fig. 3). The cluster of young zircons was prob- for 55–75 Myr after its initial incision, yet other- paleovalley <10 km from the Colorado River ably derived from the Apache Leap Tuff erupted wise Verde headwaters could not approach the (Young, 2001; Young and Hartman, 2011) at 18.6 Ma (McIntosh and Ferguson, 1998) near Grand Canyon so closely if the latter originated argues against descent of the paleovalley into a the headwaters of the Salt River paleocanyon. in Cretaceous or Paleogene time. deep ancestral Grand Canyon. Paleogeographic Drainage reversal, with subsequent streamfl ow (2) A butte informally named “Separation reconstructions (Graf et al., 1987; Young, 2001; to the southwest through the paleocanyon, Canyon hill” (Young, 2011), perched on the Young and Hartman, 2011) suggest that the occurred during the interval 15–12 Ma within rim of the western Grand Canyon northwest of Peach Springs paleovalley fl owed parallel to the time frame of Bidahochi lacustrine sedimen- Peach Springs (Fig. 6), is composed of Oligo- the Hurricane fault zone northward from Peach tation (Potochnik and Faulds, 1998; Potochnik, cene Buck and Doe Conglomerate (Fig. 3A) Springs to cross the site of the modern Grand 2001). Drainage of a Bidahochi lake down the overlain by a cap of 19 Ma basalt. The butte is Canyon ~75 km north of Peach Springs (Fig. 6). Salt River paleocanyon after drainage reversal now completely surrounded by gorges of the Any suggested backtilt of the Peach Springs is disfavored, however, by the observation that Grand Canyon and its local tributary canyons paleovalley would carry its thalweg hundreds Bidahochi fl uvial paleovalleys were ancestral to over the positions of which the strata composing of meters above the rim of the modern Grand modern drainages tributary to the Little Colo- the butte would have projected laterally before Canyon where the Colorado River crosses the rado River rather than to the modern Salt River. canyon incision. The strata of Buck and Doe Hurricane fault. That conclusion is compatible Conglomerate exposed on the butte could not with the longstanding interpretation of Young Suggested Paleocanyons have reached their present position as part of the (1982, 1985, 1999; Graf et al., 1987) that the correlative gravel apron overlying the Huala- Paleogene paleodrainage system of the Huala- On the basis of thermochronometry, Wernicke pai Plateau (Fig. 6) at any time after the Grand pai Plateau south of Grand Canyon continued (2011) concluded that a deep (~1500 m) ancestral Canyon was incised, nor could the lava capping northward across the later site of an exclusively paleocanyon roughly the length and breadth of the butte have reached the site from its source Neogene Grand Canyon. the modern Grand Canyon was incised mainly vent ~8 km to the south after canyon incision (4) The Wernicke (2011) model of a Creta- in Campanian time (80–70 Ma) by reverse (Young, 2011). The relations at “Separation ceous California paleoriver fl owing northeast paleofl ow to the northeast along the course of Canyon hill” constrain the Grand Canyon to an through a paleo–Grand Canyon, supplanted the “California” River (or paleoriver). By his origin post–19 Ma. by a Paleogene Arizona paleoriver fl owing to hypothesis, the Colorado River fl owing to the (3) On the Hualapai Plateau south of the the southwest through the same canyon after southwest through the Grand Canyon played a western Grand Canyon, the Peach Springs and drainage reversal, is diffi cult to reconcile with major role in dissecting the interior of the Colo- Hindu-Milkweed paleovalleys (Fig. 6), with sedimentological and detrital zircon data for rado Plateau but was not an important factor in maximal paleorelief of 1225–1525 m, are par- foreland stratigraphic units in Utah (Davis et al., the excavation of the Grand Canyon. Wernicke tially backfi lled with gravelly Paleogene sedi- 2010; Dickinson et al., 2011, 2012). Detrital zir- (2011) postulated that drainage reversal of the ment (Fig. 3A) displaying clast imbrication cons from the Mesozoic Cordilleran magmatic California paleoriver by Paleogene time gave and other paleocurrent indicators recording arc in the Mojave region were transported in birth to the “Arizona” River (or paleoriver), a paleofl ow toward the northeast (Young, 1999, variable proportions longitudinally from south- precursor of the modern Colorado River in the 2008). The paleovalleys are now backtilted west to northeast along the keel of the Sevier Grand Canyon (Wernicke et al., 2012). His to the southwest at a gentle angle owing to foredeep beginning with the onset of Cretaceous interpretations revive the notion of antecedence post–16 Ma collapse of the Laramide Kingman marine sedimentation near the Early-Late Creta- for the course of the Grand Canyon across the uplift into the Colorado River extensional cor- ceous time boundary, and continuing into Paleo-

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Figure 6. Geologic relations of the Grand Canyon to the Sevier thrust front Tvn Neogene volcanics Kc Campanian strata Laramide Kaibab uplift bounded on the east by the East Tsu Oligocene - Miocene Ku pre - Campanian Upper edge of Basin and strata Cretaceous strata Kaibab (EKm) and Echo Cliffs Range Province (ECm) monoclines adapted Tvo Paleogene volcanics J Jurassic strata after Reynolds (1989), Billings- normal faults ley and Workman (2000), Sable Tse Eocene strata Tr Triassic strata and Hereford (2004), Biek et al. (2009), and Felger and Beard towns P (2010). Geology across the Tc Claron Formation Permian strata Markagunt (MP), Paunsagunt SP plateaus (italics) Canaan Peak - Pine (PP), and Kaiparowits (KoP) KT PP Precambrian to Hollow Formations Permian rocks plateaus north of the Grand Canyon after Dickinson et al. 050113° WTvo Tc 111° W (2012). Plateaus bordering the scale in km Pu Tc E Grand Canyon: CP—Coconino; N MP KT J HP—Hualapai; KaP—Kaibab; Tvo CC PP Ku SP—Shivwits; UP—Uinkaret. Tvn Major faults: Hf—Hurricane; Ku Tc Kc Kc Pf—Paunsagunt; Sf—Sevier.

m Ku Ku J KoP r Trend (bracketed line) of Mio- Kc K e

f E v

Ku P cene Crooked Ridge (CR) i J Ku paleochannel near Kaibito J R J f o S d after Lucchitta et al. (2011a, NV UT PP a Ka o r SG o l 2011b). Paleogene–Miocene 37° C UT Hindu-Milkweed (HM) and N P AZ Tr Peach Springs (PS) paleovalleys Tr m Pa K J L Tr H E (bracketed lines) on Hualapai f axis of Plateau after Young (2011). Dis- Kaibab J P tribution of “rim gravels” (Tse) uplift

NV AZ SP Ko on Coconino Plateau east of the E P C Peach Springs paleovalley sche- P m Tvn Tvn KaP matic after Hill and Ranney UP P CR (2008). Explanation for legend: Tr Neogene volcanics (Tvn)—San Francisco volcanic ﬁ eld and underlying Miocene volcanics; Tvn PP PP D PP C Oligocene–Miocene strata 36° N A N A G R N Tr (Tsu)—Buck and Doe Con- YO GC Ku PP P N Tse L i t t glomerate; Paleogene volcanics P l e (Tvo)—Oligocene (to Lower HP CP Tr C Miocene) Marysvale Volcanics Tr Tr Tse HM Tr and Pine Valley laccolith (near Tse Tse C o St. George); Eocene strata l Tsu P o (Tse)—Music Mountain and Tr r a West Water Formations; Maas- S Pe d P o trichtian–Paleocene strata (KT) Tvn R i of the Canaan Peak and Pine v e Tvn PP S r Hollow Formations (preserved Ki W 112° W F P Tr only along the axis of the Table Cliff syncline of the northwest Kaiparowits Plateau); Campanian strata (Kc)—Wahweap and Kaiparowits Formations; Permian strata (P)—Coconino, Toroweap, and Kaibab Formations; Precambrian to Permian rocks (PP)—Hermit Formation and older rocks including Proterozoic basement. Towns: C—Cameron; CC—Cedar City; E—Escalante; F—Flagstaff; GC—Grand Canyon village; Ka—Kanab; Ki—Kingman; Ko—Kaibito; L—Littleﬁ eld; Pa—Page; Pe—Peach Springs; Pu—Panguitch; S—Seligman; SG—St. George; W—Williams. States: AZ—Arizona; NV— Nevada; UT—Utah.

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gene time during Laramide structural breakup of COLORADO PLATEAU DENUDATION lower to middle Campanian Wahweap Forma- the foreland region until near the Early-Middle tion is overlain conformably by middle to upper Eocene time boundary. There is no indication Denudational episodes on the Colorado Pla- Campanian Kaiparowits Formation (ca. 75 Ma) that transit of arc-derived detrital zircons across teau have been associated with erosional inter- composed of ~1000 m of distal meander-belt and the Grand Canyon region was arrested near the vals that involved the stripping of sedimentary anastomosed fl uvial facies derived from the west Cretaceous-Tertiary time boundary as required cover from the pre-Laramide Mogollon high- (Lawton et al., 2003; Roberts, 2007; Lawton and by postulated drainage reversal of the California lands, the beveling of Laramide uplifts, and Bradford, 2011). The net Kaiparowits paleocur- and Arizona paleorivers. post-Laramide development of the Colorado rent vector is N70E (Dickinson et al., 2012), Of uncertain signifi cance for the specula- River drainage system. refl ecting ca. 75 Ma paleofl ow across the crest tive development of a paleo–Grand Canyon is a of the Kaibab uplift, which then had incipient gravel deposit associated with nonmarine lime- Mogollon Highlands structural relief as a growth fold but not enough stone displaying columnar internal structure sug- topographic relief to block sediment transport. gestive of lacustrine travertine or tufa exposed at The southwestern margin of the Colorado In the Table Cliff syncline, the Kaiparowits an elevation of 1795 m near Cape Solitude (Scar- Plateau, including the site of the modern Grand Formation is overlain unconformably with an borough, 2001, p. 209), the topographic point Canyon, was stripped of much of its pre-Creta- angularity of ~10° (Bowers, 1972) by the con- on the south rim of the Grand Canyon south of ceous Mesozoic cover by Upper Jurassic to Early glomeratic Canaan Peak Formation (80–140 m) the junction of the Colorado and Little Colo- Cretaceous erosion along the tilted northern fl ank of Maastrichtian to Early Paleocene age (Fig. 3) rado Rivers (Fig. 6). The gravel and limestone of the Mogollon highlands, which formed the deposited on a coarse proximal braidplain deposit, never dated or described in detail, stands rift shoulder of the Bisbee Basin in the exten- (Schmitt et al., 1991; Larsen et al., 2010) by ~1000 m above the fl oor of the Grand Canyon, sional border rift belt (Dickinson and Lawton, northeasterly paleofl ow (N55E) off the adjacent and has been suggested as possibly a vestige of 2001; Stern and Dickinson, 2010). A map of the Kaibab uplift (Dickinson et al., 2012). Continued the fl oor of a Laramide paleocanyon that incised paleogeology (Fig. 7) beneath Upper Cretaceous Laramide deformation produced a closed basin a notch through the Kaibab-Coconino Plateau to (Dakota-Mancos) marine strata that onlapped the of interior drainage in the Table Cliff syncline serve later as a Neogene spillway for Hopi Lake eroded fl ank of the Mogollon highlands refl ects where the Lower Paleocene to Lower Eocene (Scarborough 2001; Potochnik, 2011). How- removal of ~250 m of Jurassic strata from above Pine Hollow Formation (Fig. 3), which overlies ever, the deposit lies topographically only ~25 m the Grand Canyon during regional beveling of the Canaan Peak Formation with an angularity of above the lowest exposed base of the Bidahochi the tilted fl ank of the Mogollon highlands. The 5°–10°, is a cyclic succession of alluvial fan and Formation and ~150 m below the highest expo- deposition of Upper Cretaceous strata directly on playa lake deposits 80–120 m thick (Larsen et al., sures of the Bidahochi lacustrine facies. Its sub- Aztec Sandstone of the Lower Jurassic Glen Can- 2010). Centripetal Pine Hollow paleo current stratum could not readily have formed part of a yon Group (Stewart, 1980) at the Valley of Fire indicators record inward paleofl ow into the Table dam for Hopi Lake at any time during Bidahochi west of the Grand Canyon (Fig. 7) is compatible Cliff synclinal basin, with a weak resultant vector sedimentation. The suggestion that a Laramide with the paleogeologic relations shown. An esti- (R = 0.32) southward subparallel to the axis of paleocanyon developed along segments of the mated total of ~2500 m of Mesozoic strata above the syncline (Dickinson et al., 2012). Grand Canyon west of the Kaibab-Coconino the site of the Grand Canyon at the end of Cre- Following cessation of Laramide deforma- Plateau, and fl owed northward into Utah with- taceous time is in agreement with stratigraphic tion in Eocene time, the Claron Formation out crossing the Kaibab uplift (Hill and Ranney, assumptions made for thermochronological (Fig. 3) of lacustrine and associated fl uvial 2008), does not bear on the question of Hopi analy ses of denudation over time at the Grand deposits was deposited as an overlap succession Lake spillover. Canyon (Dumitru et al., 1994; Flowers et al., across the Kaibab uplift, East Kaibab mono- 2008; Kelley et al., 2011; Lee et al., 2011). The cline, and Table Cliff syncline (Fig. 6). Beneath Transient Lake Level thermochronology suggests two intervals of sig- the Claron overlap, only local remnants of the nifi cant additional denudation at the Grand Can- Kaiparowits Formation ≤25 m thick were pre- Speculation that Hopi Lake rose transiently yon, the fi rst Laramide and the second Neogene. served from Laramide erosion along the crest of to a level allowing Colorado River water to the Kaibab uplift on the present Paunsagunt and spill over the Kaibab-Coconino uplift without Laramide Uplifts Markagunt Plateaus (Bowers, 1991; Moore and leaving any discernible sedimentary record of Straub, 2001; Eaton et al., 2001). Exposures of the lake highstand is inherently impossible to The southern end of the Kaibab uplift near the Canaan Peak and Pine Hollow Formations address with any direct geological evidence. the Grand Canyon exposes no post-Paleozoic are confi ned to the Table Cliff syncline east of The same problem plagues evaluation of sug- strata (Fig. 6), and thus provides no informa- the Kaibab uplift (Fig. 6). These stratigraphic gested Miocene paleolakes located farther north tion about the timing of the Laramide defor- relationships across the East Kaibab monocline in Utah at stratigraphic levels since removed by mation that formed the uplift. Farther north, north of the Grand Canyon imply that Laramide erosion (Hunt, 1969; Hill et al., 2008, 2011). In however, Cretaceous–Paleogene strata of the deformation giving rise to the Kaibab uplift the absence of locally preserved geological evi- Table Cliff syncline adjacent to the East Kaibab began in Late Cretaceous time, reached some dence, the existence of either a deep Hopi Lake monocline, which bounds the uplift on the east, maximum rate in Paleocene time, and waned or analogous paleolakes farther north rests upon preserve a sedimentary record of the develop- during Eocene time. considerations of Colorado Plateau denudation ment of the uplift. Growth faults displacing Laramide removal of all or much of the and the geometry of the Colorado River and its units as young as the middle Campanian Wah- Mesozoic section once present above Paleozoic tributaries. A brief summary of plateau denuda- weap Formation (Tindall et al., 2010) along the strata forming the core of the uplift presumably tion and river evolution is indicated to address East Kaibab monocline show that deformation contributed to the evidence from thermochro- the issue of potential Neogene lakes within the was under way by 80–78 Ma (Fig. 3A). In the nology for an episode of Laramide denudation interior of the plateau. Table Cliff syncline east of the Kaibab uplift, at the Grand Canyon (Dumitru et al., 1994;

Geosphere, February 2013 9

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Figure 7. Sub–Dakota Sand- 112° W Kcm G R L E G E N D stone paleogeologic map of the Edge of R Basin and Range C Kcm - Kbc southern Colorado Plateau province UT CO illustrating the progressive 38° N Jmu Jmu Jml overlap of pre–Upper Cre- N M E taceous strata by the Dakota Jsr R Kbc C Jsr TrJgc Sandstone or laterally equiva- SJR SG lent to slightly younger beds of UT Trmc Pzs the basal Mancos Shale along AZ

the northeastern fl ank of the VoF R K C 108° W Mogollon highlands rift shoul- nd LV * r a G der of the Bisbee Basin. The o n Jsr Kbc C R n y limit of control is the southwest- C a Jml Jmu C ernmost extent of exposures TrJgc of Upper Cretaceous Dakota NV L C Sandstone and its equivalents R G AZ ? F (VoF—Valley of Fire). Plateau CA ? A stratigraphic units: Kcm— ? Jsr 35° N Cedar Mountain Formation; Trmc flank of towns and cities Kbc—Burro Mountain For- Rio Grande sub - Dakota SJ SW limit rift mation (Lower Cretaceous); stratigraphic of control Pzs contacts Pzs Jmu—upper Morrison Forma- S tion (Tithonian Upper Jurassic: AZ NM 100 km 112° W 108° W Fiftymile, Brushy Basin, and Westwater Canyon Members); Jml—lower Morrison Formation (Kimmeridgian Upper Jurassic: Tidwell, Salt Wash, and Recapture Mem- bers); Jsr—San Rafael Group (multiple Middle Jurassic formations and the basal Upper Jurassic Bluff Sandstone); TrJgc—Glen Canyon Group (Lower Jurassic and uppermost Upper Triassic including the Moenave Formation and the Rock Point–Church Rock interval at its base); Trmc—Triassic Moenkopi Formation (or Group) and Chinle Formation (or Group); Pzs—Paleozoic strata (multiple formations). Modifi ed from Potochnik (in Dickinson et al., 1989, their fi gure 9; 2011) after Cooley et al. (1969), Stewart and Carlson (1978), Reynolds (1989), Doelling and Davis (1989), NMBGMR (2003), Biek et al. (2009), Felger and Beard (2010), and multiple local surface geologic maps and subsurface well logs. Rivers (blue): CR—Colorado; GR—Green; LCR—Little Colorado; SJR—San Juan. States: AZ—Arizona; CA— California; CO—Colorado; NM—New Mexico; NV—Nevada; UT—Utah. Towns and cities: A—Albuquerque, C—Cuba, E—Escalante, F—Flagstaff, G—Gallup, K—Kayenta, LV—Las Vegas, M—Monticello, S—Socorro, SG—St. George, SJ—St. Johns.

Naeser et al., 2001; Flowers et al., 2008; Kelley the Oligocene Chuska Sandstone (Fig. 9). The The modern Little Colorado River fl ows at an et al., 2011; Lee et al., 2011). concave-upward confi guration of the base of elevation of 1475–1525 m only 25–50 km to the The effects of Laramide erosion are also the Bidahochi fl uvial facies suggests that Bida- southwest of Bidahochi exposures (Fig. 2), and shown by stratigraphic relations of Tertiary hochi streams draining toward the Bidahochi Bidahochi lakebeds are exposed at elevations as strata in northeastern Arizona east of the Grand lacustrine tract were working headward into the low as ~1750 m. Estimated post-Bidahochi ero- Canyon (Fig. 8). The Oligocene Chuska Sand- fl ank of the Chuska erg, which reached a maxi- sion has thus been ~500 m and net post-Bida- stone unconformably overlaps the East Defi ance mum thickness of >500 m (Cather et al., 2008). hochi denudation of sub-Tertiary strata has been monocline of Laramide age, and buries paleo- Areally restricted exposures of Chuska Sand- ~250 m in the vicinity of Bidahochi exposures. cuestas developed in Cretaceous strata along stone capping the present Chuska Mountains Those fi gures are ~50% of comparable fi gures the trend of the monocline during Eocene time are a surviving erosional remnant of the erg for erosion and net denudation between Chuska (Wright, 1956). The Miocene Bidahochi Forma- (Fig. 1). The uppermost preserved eolian strata and Bidahochi sedimentation. Approximately tion overlaps Permian through Cretaceous strata of the Chuska Sandstone reach an elevation of two-thirds of the net post-Laramide lowering of along the western fl ank of the Defi ance uplift ~3000 m at the crest of the Chuska Mountains the landscape in northeastern Arizona, and per- where beds dip regionally westward into the (Cather et al., 2008), implying that the local haps across much of the southern Colorado Pla- Black Mesa Basin, a downfold of Laramide age. landscape in the Bidahochi depocenter was teau (Cather, 2011), evidently occurred before Signifi cant Late Oligocene to Middle Miocene eroded to an estimated depth of 1000–1250 m post-Miocene river fl ow through the present denudation intervened, however, between the between Oligocene and Middle Miocene time. Grand Canyon. twin episodes of Chuska and Bidahochi sedi- Even after deposition of nearly 300 m of Bida- mentation (Cather et al., 2008; Cather, 2011). hochi strata, the Miocene ground surface over Crooked Ridge large areas of northeastern Arizona stood nearly Southern Plateau 1000 m below the Oligocene landscape atop The Crooked Ridge paleochannel located the Chuska erg. Analogous mid-Tertiary ero- not far east of the Grand Canyon and of prob- The surface of the pre-Tertiary substratum in sion farther west in the Grand Canyon region able intra-Miocene age (Lucchitta et al., 2011a, northeastern Arizona is ~500 m lower beneath is implied by thermochronology (Kelley et al., 2011b) underscores the regional scope of Mio- the Miocene Bidahochi Formation than beneath 2011; Lee et al., 2011). cene denudation on the southern Colorado

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U S 1 110° W9 J 109° W N 1 K Tr J J D Tc 36° 30' N E NAV F 12 BM Figure 8. Relations of Miocene I L Tc Se Bidahochi Formation (expo- YP A K Tr N C sures north of 35°N), Eocene– C h J u Oligocene Chuska Sandstone, E s A′ k

B L A C K 1 aM

and Mesozoic substrates (see 9 1

Fig. 1 for map location). Geol- S Tr U dC ogy after O’Sullivan and Beik- C Tr P Tc man (1963), Hackman and Olson M E S A LP o u K C U n (1977), and Ulrich et al. (1984). NAV 4 P SB ta See Figure 9 for proﬁ le A–A′. P i L I J n K J s Mesozoic formational units: ′ Tr 2 36° 0 N P F

K—Cretaceous (Mesaverde, T 1 V

A Tc Mancos, Dakota); J—Jurassic Tb N (Morrison, San Rafael Group, Tr AZ 264 SP Glen Canyon Group [includ- K T ing Rock Point interval]); Tb e

c 1 Tr—Triassic (Chinle, Moen- P 9 Tb A n 4 Z 2 e kopi); P—Permian (DeChelly, 64 a US Tb i n i Gd f Supai). Towns: C—Chinle; W l e Gd—Ganado; Gp—Gallup; J Tr c V6 D K K—Keams Canyon; L—Luka- A N t chukai; Sa—Sanders; Se— ono Gp P s Sanostee; T—Tohatchi; W— Tr a m Tb Tb 35° 30′ N Window Rock. Topographic P E I40

features (italics): BM—Beauti- US 1 ful Mountain; CdC—Canyon J 9 J 1 J de Chelly; LP—Lohali Point; Tr SB—Sonsela Buttes; SP— Salahkai Point; YP—Yale Point Sa J (the “points” are on the outer A Tr J Tr K rim of Black Mesa). States: AZ—

7 Tb US J Arizona; NM—New Mexico. 7 025 1 Z Tb 0 9 AZ J 4 NM A I 1 scale in km 110° W Tr Tb Tc K J Tr P

towns Bidahochi Chuska Cretaceous Jurassic Triassic Permian

Plateau. Its net thalweg slope of 0.006 (6 m/km) of southwestern Colorado. The inferred original exposure at The Gap, its existence during the toward the southwest over a preserved length extent of the Chuska erg (Fig. 1) would have Miocene would preclude delivery of Colorado of ~50 km implies fl ow into an ancestral Colo- blocked drainage off the San Juan Mountains River water to the Bidahochi depocenter. The rado River near the site of the present confl u- from reaching the Crooked Ridge paleochannel paleochannel lies athwart any logical pathway ence with the Little Colorado River (Fig. 6). The prior to erosional stripping of the erg from much leading from the upper Colorado River in Utah paleochannel elevation of ~1700 m at the south- of its original extent. toward Bidahochi lakebeds that reach higher western end of its exposures is within 50–100 m Detrital zircons as young as ca. 24 Ma elevations of 1850–1900 m (Fig. 2). of the base of the Bidahochi Formation, sug- extracted from fl uvial sediment in the Crooked gesting that development of the Crooked Ridge paleochannel (Price et al., 2012) confi rm Northern Plateau Ridge paleodrainage postdated dissection of that the Crooked Ridge stream was fl owing in the Chuska erg in northeastern Arizona. That Miocene time or later (Price et al., 2012). As the The history of erosion and denudation on the inference is supported by the presence of clasts Crooked River paleochannel descends from an northern Colorado Plateau contrasts strongly within the paleochannel derived from basement elevation of 1825 m near Kaibito on the north- with relations in northeastern Arizona. Near and volcanic rocks in the San Juan Mountains east (Fig. 6) to 1700 m at its southwesternmost Grand Junction, Colorado, at the confl uence

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A A´ Figure 9. Topographic proﬁ le 25 base of Chuska Sandstone 25 ′ A–A (see Fig. 8 for location) 24 24 of basal contacts of Miocene Bidahochi Formation and 23 23 Eocene–Oligocene Chuska Sand- stone from French Butte (on the 22 22 southwest) along the alignment n 21 tio 21 of ancestral Pueblo Colorado ~500 m a m s Wash (Fig. 2) past Ganado (Figs. r e 20 o i Mountains 20 F ac 2, 8) to the Sonsela Buttes (Fig. crest of Chuska

f Sonsela Buttes h i l 8) and the crest of the Chuska Elevation in m (x100) 19 b a o c ia 19 s e a h u v Mountains (on the northeast). l a o f i d f l c u s B 18 t rin i e s G a n a d o 18 Elevations after Figure 2, Wright French e f a c (1956), and Schmidt (1991). Ver- Butte 17 17 tical exaggeration 100 ×. 0 25 50 75 100 125 150 175 Distance in km

of the Colorado (Grand) and Gunnison Rivers , sion surface uniformly capping all uplifts and >50 km in diameter (Lipman, 1989) infl uenc- Gunnison River gravels (Price et al., 2012) basins simultaneously. The stratigraphy of the ing paleotopography. The San Juan and Dolores underlying Upper Miocene (9.5–11.0 Ma) Colorado Plateau involves alternating succes- Rivers, however, swing around or pass between basalt capping Grand Mesa (Fig. 10) stand at an sions of strata that are relatively resistant and Cretaceous–Paleocene laccolithic centers of elevation 1500+ m above modern river valleys nonresistant to erosion. The superposition of the Colorado Mineral Belt (Figs. 1, 10) just as (Aslan et al., 2011; Cole, 2011), showing that some streams across selected Laramide uplifts surely as the Colorado River and other tributar- the major erosional episode affecting the north- may stem from ancestral streams that once ies avoid Oligocene laccolithic centers farther ern plateau postdated eruption of the basalt. fl owed across extensive lowlands developed north on the central Colorado Plateau (Fig. 3C). Similarly late denudation of the Colorado Pla- within outcrop belts of less-resistant strata As no Paleocene volcanic edifi ces are likely to teau over much of Utah is implied by thermo- before downcutting lowered the stream courses have persisted for 30–40 Myr into mid-Tertiary chronology (Hoffman et al., 2011) indicating into more-resistant strata occupying the cores time, resistant fl atirons along the fl anks of local an onset of cooling after ca. 10 Ma through of the uplifts. For example, Hunt (1969, p. 108) igneous-cored uplifts were perhaps suffi cient removal of 1000–2000 m of overburden. The suggested that the incised meanders of the San to divert drainages away from the igneous cen- last 845 m of Neogene erosion on the central Juan River where it crosses the Monument ters. In effect, the laccoliths and accompanying Colorado Plateau has produced 640 m of iso- upwarp (Fig. 10) are inherited from meanders stocks punched holes through the plateau stra- static rock uplift (Pederson et al., 2002), imply- formed when the river was fl owing through tigraphy, thereby breaking the lateral continuity ing that surface elevations on the plateau were weak Mancos Shale, of Cretaceous age, at a of nonresistant stratigraphic horizons to prevent reduced by only 25% of the erosional denuda- much higher stratigraphic horizon than at pres- superposition of streams across the igneous- tion, allowing for persistence of a high-standing ent. Where preserved on the faces of high pla- cored uplifts. plateau as denudation proceeded. Geometric teaus rimming the central Colorado Plateau, patterns of the Colorado River and its tributaries the Mancos Shale is commonly 1200–1500 m Sequential Drainages may offer alternatives to Hopi Lake spillover for thick. If that thickness is projected across the the integration of upper and lower courses of the site of the Monument upwarp, the Mancos As late as Eocene time, plateau drainages Colorado River through the Grand Canyon. Shale could once have blanketed a broad span were still fl owing into Laramide basins (Fig. of the uplift to mask much of the structural 10). Paleofl ow in southeastern Utah, where the COLORADO RIVER DRAINAGE relief then hidden in more-resistant older strata. Colorado River now fl ows southward, was then northward into the Uinta Basin (Dickinson et al., The Colorado River and many of its major Laccolithic Centers 2012). There are only two fundamental mecha- tributaries transect multiple Laramide uplifts nisms by which a southward-fl owing Colo- of the Colorado Plateau and its margins, and Hunt (1956, p. 70–71; 1969, p. 103) noted rado River superimposed across the Laramide cross Laramide basins without notable diver- that neither the Colorado River nor any of its uplifts of Utah could have developed on the sion (Fig. 10). The maximum structural relief of major tributaries transect any of the laccolithic post-Laramide landscape: (1) headward erosion intra-plateau uplifts as inferred from structure igneous centers of the Colorado Plateau (Figs. 1, eating into the Colorado Plateau from the south contours is typically within the range of 1500– 10). Paleotopography above the laccolithic cen- to gradually capture drainages on the central 2000 m (Kelley, 1955; Hunt, 1956; Bump, ters evidently exerted strong control on the paths Colorado Plateau, or (2) progradation of head- 2004; Flowers et al., 2008). Given the Paleo- of plateau streams in the post-Laramide time water streams across the plateau from the north gene age of the structural uplifts and basins frame during which the modern Colorado River and east to progressively prolong the drainage (Dickinson et al., 1988), the Neogene Colorado drainage network evolved. Volcanic edifi ces system southward. Reversal of central plateau drainage network must refl ect superposition, may once have towered above the subvolcanic drainage was centered on Oligocene time, but but not necessarily from a unitary Tertiary ero- laccoliths and stocks, with volcaniclastic aprons there are no Oligocene strata preserved in the

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Laramide WY CO Figure 10. Geometric pattern of the Colo- uplift crests SLC Ui LIMIT OF rado River and its major tributaries in rela- LC V COLORADO tion to Laramide uplifts and basins on the Laramide PLATEAU Colorado Plateau after Kelley (1955), Hunt structural basins 40° (1956), Dickinson et al. (1988), Krantz (1989), UiB n e r N Laramide e e Bump (2004), Lawton (2008), and Flowers r v DC PiB sedimentary G et al. (2008). Selected rivers (see text for dis- Ri basins Pi cussion): CoR—Colorado; DoR—Dolores; CoR GuR—Gunnison; LCR—Little Colorado; Precambrian GJ river gorges WC San Juan SaR—Salt; SJR—San Juan; VeR—Verde; +GM SR E volcanic ViR—Virgin. Laccolith clusters (italics): UC field A—Abajo; Ca—Carrizo; H—Henry; LP— G Marysvale MM uR volcanic Un La Plata; LS—La Sal; SM—San Miguel; LS field HM U—Ute; E—West Elk. Laramide sedimen- H D SM tary basins (Paleogene strata): BaB—Baca; CC o A R PiB—Piceance; SJB—San Juan; UiB— r M e N v Uinta. Laramide uplifts: A—Apache; CC— i UT CO LP Kp R SJ Circle Cliffs; D—Deﬁ ance; DC—Douglas SG ViR R Co Creek Arch; Kb—Kaibab; MM—Miners U + o NV Mountain; M—Monument Upwarp; N— d a Ca r K Needles; SR—San Rafael Swell; Ui—Uinta; o l Un—Uncompahgre; Z—Zuni. Laramide o SJB Kb C structural basins (Cretaceous strata): BM— U Black Mesa; HM—Henry Mountains; Kp— GG BM Kaiparowits (see Fig. 6 for superimposed D Tertiary Table Cliff basin near northern LGG

end). Precambrian gorges: LC—Lodore G Z 35° Canyon; LGG—Lower Granite Gorge F 35° N L (Grand Canyon); UC—Unaweep Canyon CR AZ NM A N (paleo-river); UGG—Upper Granite Gorge V e R (Grand Canyon); WC—Westwater Canyon. BaB Selected topographic eminences: GM— Grand Mesa (+); NV—Navajo Mountain towns (×). Towns: A—Albuquerque; Co—Cor- A tez; F—Flagstaff; G—Gallup; GJ—Grand Oligocene R Sa Junction; K—Kayenta; Pi—Price; Ph— laccolith Ph Phoenix; SG—St. George; SLC—Salt Lake clusters Mogollon- City; V—Vernal. States: AZ—Arizona; N Datil CO—Colorado; NM—New Mexico; UT— volcanic Cretaceous 100 km field Utah; WY—Wyoming. laccolith clusters 112° W 110° W

interior of the plateau except for the dominantly topog raphy associated with Oligocene lacco- ual Laramide highlands of the Rocky Mountains eolian Chuska Sandstone. Both the Laramide lithic centers in Utah and the San Juan volcanic and on the west by the elevated Nevadaplano and the mid-Tertiary laccolithic centers of the fi eld in Colorado (Cather et al., 2008). The plateau (DeCelles, 2004) between the relict Colorado Plateau were emplaced by the end of existence of such a large erg implies an arid Sevier thrust belt and the Sierra Nevada. The Oligocene time, allowing the laccolithic struc- paleoclimate for the Oligocene Colorado Pla- Takla Makan dunefi eld of central Asia, although tures to guide either successive stream captures teau, suggesting that any through-going plateau twice the reconstructed size of the Chuska erg, upstream from the south or progressive elonga- streams were rare and of limited vigor, if present occupies a similar modern intramontane depres- tion of headwater drainages downstream across at all. There was evidently a time break between sion between the Kunlun and Tien Shan Moun- the plateau from the north. a previous regime of northward-flowing tains (Breed et al., 1979). Stream courses that Laramide streams and a subsequent Neogene issue from the enclosing high ranges dissipate Chuska Erg regime of southward-fl owing streams. In effect, into the Takla Makan dunefi eld as their waters an arid Oligocene interval wiped the drainage seep into the sand. Perhaps there was no pre- The Chuska erg (Fig. 1) occupied an area of slate clean on the central Colorado Plateau. Miocene Colorado River because no through- ~100 × 103 km2 on the southeastern Colorado Chuska sand accumulation occupied a broad going trunk stream traversed the Colorado Plateau, limited on the north by positive paleo- intramontane tract bounded on the east by resid- Plateau in any direction during Oligocene time.

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The Colorado River drainage system may have migration is arrested, and the stream is forced limb (Karlstrom et al., 2007). Before removal by initially been integrated across Utah by gradual to incise downward to form a steep canyon in erosion, the Triassic–Jurassic shaly strata could prolongation of streams issuing from highlands the resistant strata. The result of the process is once have buried most if not all of the structural surrounding the Colorado Plateau as desert con- a looping stream transit of the fold in a canyon relief of the Kaibab uplift. ditions within the interior of the plateau were that is convex down plunge in plan view with The concept of stratal superposition for the ameliorated during Miocene time. The pattern respect to the fold axis. The scenario can be evolution of the Grand Canyon is reinforced of paleofl ow on the Colorado Plateau north of repeated as stream erosion bites ever deeper by the similarly looping course of the western the Chuska erg is uncertain, however, because into alternating resistant and nonresistant stratal Grand Canyon through an anticlinal structure the only Oligocene strata preserved in the region intervals. into which the Colorado River has incised the surrounding the Uinta Basin are alluvial depos- The geologic histories of Lodore Canyon lower granite gorge (Fig. 10). The river and its of the Bishop Conglomerate that fl anked the (Hansen, 1986; Pederson and Hadden, 2005), canyon describe a prominent southward loop, core of the Uinta Mountains before deposition Westwater Canyon (Aslan et al., 2011), and the similar in geometry to the loop at the Kaibab of the Miocene Browns Park Formation within Grand Canyon differ in detail. The common uplift, in crossing a gentle anticline that trends an axial graben (Fig. 3C). plan-view geometry of the trunk river courses north-south between the Hurricane fault and the for each of the three uplift transits neverthe- mouth of the Grand Canyon (Fig. 6). Structural Laramide Uplift Transits less encourages speculation that the erosional relief on the anticlinal structure is ~750 m on the process described by Oberlander (1965, 1985) east and ~550 m on the west (Karlstrom et al., The Grand Canyon is not the only place was a fundamental constraint for the develop- 2007), comparable to the estimated ~800 m where the Colorado River has eroded into a ment of each of the three canyons. thickness of shaly Triassic–Jurassic strata as Laramide uplift to the depth of Precambrian projected speculatively over the western Grand basement. Farther north (Fig. 10), the upper Kaibab Uplift Canyon. The course of the Colorado River Colorado (Grand) and Green Rivers incise the through the Grand Canyon is thus partially Uncompahgre and Uinta uplifts, respectively, Babenroth and Strahler (1945) and Strahler adjusted to two separate anticlinal structures in to expose Precambrian rocks in Westwater and (1948) applied the logic of the later Oberlander the manner envisioned by Oberlander (1965, Lodore Canyons. The courses of the rivers fol- thesis to elucidate the looping course of the 1985) for the superposition of transverse drain- low the same pattern of defl ection for each of Colo rado River across the Kaibab uplift, evok- ages across structure. the three uplift transects including the Grand ing the Permian Kaibab Limestone, which forms Canyon, suggesting some commonality of ori- the stripped stratigraphic surface of the Kaibab- Crooked Ridge gin. In each case, upstream reaches of the rivers Coconino Plateau (Fig. 6), as the controlling fl ow directly toward the uplifts, the Kaibab in resistant layer for incision of the Grand Canyon. The location of the Crooked Ridge paleochan- the case of the Grand Canyon. The river courses They regarded the overlying Triassic–Juras- nel (Fig. 6) and the confi gurations of fl uvial then turn abruptly at uplift margins to fl ow for sic stratigraphic interval (Moenkopi, Chinle, Bidahochi paleochannels ancestral to tributaries 25–50 km subparallel to the trends of the uplifts, Moenave, and Kayenta Formations) as the non- of the Little Colorado River (Fig. 2) suggest that in each case in a direction down plunge to the resistant stratal cover that once blanketed the both systems fl owed into an ancestral Miocene confi guration of uplift crests. Final passages resistant core of the Kaibab uplift beneath more- Colorado River not far east of the Kaibab uplift of the rivers through canyons transecting the resistant Jurassic Navajo Sandstone. They envi- near the present confl uence of the Colorado uplifts then follow looping paths with curva- sioned north-facing cliffs of Navajo Sandstone, and Little Colorado Rivers. The elevation of tures that are convex down plunge with respect since removed by subsequent erosion, as the the downstream limit of the preserved Crooked to the plunges of uplift axes. upper tier of a retreating escarpment that curved Ridge paleochannel (~1700 m) and the eleva- This pattern of river fl ow across folded struc- (convex to the south) across the plunging Kai- tion of the downstream edge of the Bidahochi tures has been attributed by Oberlander (1965, bab uplift on the south side of an ancestral Colo- fl uvial facies (~1900 m) suggest that the termini 1985), from studies in the Zagros Mountains in rado River valley before the river incised into of the Crooked Ridge and Bidahochi fl uvial sys- Iran, to the transverse superposition of streams Kaibab Limestone. Chase (2001) has focused tems at an ancestral Colorado River were near across folds with an internal stratigraphy com- renewed attention on the manner in which the an elevation of 1625 m. That elevation is cal- posed of alternating resistant and nonresistant half-circular segment of the Colorado River in culated by projecting both paleofl ow systems stratigraphic intervals. When easily erod- the eastern Grand Canyon “sidehills” around the downstream at the net gradient (1.75 m/km) of ible strata are exposed at the surface, exten- nose of the Kaibab uplift with respect to struc- the modern Little Colorado River valley along sive lowland straths are developed across fold ture contours of the fold. the 250 km of its course across lowlands of crests. Only when a stream erodes downward The Babenroth-Strahler hypothesis of stratal northern Arizona upstream from the mouth of though the weak strata does it “fi nd” underly- superposition to explain the course of the east- Moenkopi Wash near Cameron (Fig. 6). The ing resistant strata previously hidden within the ern Grand Canyon through the Kaibab uplift calculation suggests that the Miocene Colorado core of the fold. When the downcutting stream and the upper granite gorge (Fig. 10) is broadly River had already breached the Kaibab Lime- encounters exposures of the resistant strata, it compatible with the known structural geometry stone on the Kaibab uplift, and was transecting migrates laterally down the plunge of the fold of the Kaibab uplift. The projected thickness of the Kaibab-Coconino Plateau in an ancestral at the uppermost horizon of the resistant strata. dominantly shaly Triassic–Jurassic strata once valley with its fl oor at ~1600 m near the top of A retreating escarpment of overlying strata present over the eastern Grand Canyon between the Redwall Limestone (Karlstrom et al., 2007). bounds the stream valley on its down-plunge Kaibab Limestone below and Navajo Sandstone That inference assumes that large areas of north- side. When the height of the retreating scarp is above is ~900 m. The structural relief of the eastern Arizona were denuded to levels within suffi cient to deliver more debris to the stream Kaibab uplift at the Colorado River is ~750 m less than ~500 m of the modern landscape by than the latter can transport, lateral stream on its western limb and ~1050 m on its eastern Middle Miocene time.

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Residual Mystery also developed south of Flagstaff (Fig. 6) in the Colorado River has yet been surely identifi ed. ancestral Gila River drainage below the nascent Any such stream would have continued north- Multiple arguments thus jointly disfavor Hopi Mogollon Rim (Blakey et al., 2011). west toward St. George (Fig. 6) in the drain- Lake spillover as the trigger for incision of the From model calculations, Pelletier (2010) age of the modern Virgin River (Figs. 1, 10). Grand Canyon through the Kaibab-Coconino concluded that a proto–western Grand Canyon The Virgin River now debouches from the Plateau, implying instead that an ancestral Colo- working headward from the foot of the Grand plateau into desert lowlands to the west where rado River with a large upstream drainage basin Wash Cliffs, which formed by normal fault- the Muddy Creek Formation (Fig. 3A) accu- but an unknown volume of discharge crossed the ing between 16 Ma and 9 Ma (Fig. 3A), could mulated during Miocene time in multiple sub- Kaibab uplift before the modern Grand Canyon by the end of Miocene time have captured the basins fringing the Hualapai paleolake system was cut. It remains a mystery where such a river pre–Grand Canyon Colorado River somewhere (Fig. 1) on the north. Precursors of the modern exited the plateau west of the Kaibab uplift. west of the Kaibab uplift but east of the Shivwits White River and Meadow Valley Wash, which There is conclusive evidence that no such Plateau at a point <150 km from the Grand rise in the Basin and Range province west of river passed through, or at some pre-canyon Wash Cliffs. Estimated rates of headward ero- the Colorado Plateau (Fig. 1), contributed sedi- erosional level above, the present mouth of the sion are not suffi cient to allow a knickpoint to ment to the Muddy Creek Formation, as did the Grand Canyon. The Upper Miocene Hualapai migrate upstream from the Grand Wash Cliffs paleo–Virgin River itself, but there is doubt that Limestone (Fig. 3), deposited within a lake sys- far enough to propagate through the Kaibab any Colorado River sediment is present in the tem that extended upstream along the course of uplift within the time frame (<6 Ma) available Muddy Creek Formation (Pederson, 2001a). the modern lower Colorado River as far as the for canyon evolution (Spencer and Pearthree, Muddy Creek sandstones contain volcaniclas- mouth of the Grand Canyon, displays strontium 2001; Pelletier, 2010). The greater breadth of tic detritus derived from the Caliente caldera isotope ratios elevated above those of any mod- the western Grand Canyon as compared to the complex (Fig. 1) of eastern Nevada (Pederson, ern or pre-modern waters of the Colorado River eastern Grand Canyon is interpreted by Pelletier 2008), and perhaps also debris from the Lower system (Fig. 4). Hualapai paleolake waters were (2010) to be a function of the time available for Miocene Pine Valley laccolith forming high apparently spring fed by local groundwater that lateral cliff retreat begun after a knickpoint had topography along the western fl ank of the Virgin emerged from Paleozoic limestones (Faulds migrated upstream, fi rst from the Grand Wash River drainage (Figs. 3A, 6). et al., 2001a; Hill and Ranney, 2008; Hill et al., Cliffs toward the capture point in the central U-Pb age data for detrital zircons in the 2008; Crossey et al., 2011; Lopez Pearce et al., Grand Canyon, and then past the capture point Muddy Creek Formation (Forrester, 2009; 2011). The detrital zircon populations in sand- into the eastern Grand Canyon. Swenberg and Hanson, 2010; Muntean and stones of the Hualapai Limestone depositional The role of groundwater sapping (Peder- Hanson, 2010; Hanson and Forrester, 2010; system are dominated by ca. 1400 Ma and ca. son, 2001b, 2008; Karlstrom et al., 2011) in Muntean, 2012) do not preclude the possibil- 1700 Ma grains derived from local sources the capture of the Colorado River in the Grand ity that Muddy Creek basin fi ll is an amalgam (Lopez Pearce et al., 2011). Wholly missing are Canyon is uncertain, but subterranean fl ow of of Colorado River sediment contaminated with the Grenville, Neoproterozoic, and Paleozoic groundwater through karstic features which Virgin River sediment and detritus from both subpopulations of detrital zircons that are prom- later collapsed may have played a major role in the Caliente caldera complex (Fig. 1) and other inent in all modern Colorado River sediments promoting incision of the eastern Grand Can- sources within the Basin and Range province. collected both upstream and downstream from yon through the Kaibab uplift (Hill and Ran- The age spectra of pre-Cenozoic detrital zircon the Grand Canyon (Kimbrough et al., 2011). ney, 2008; Hill et al., 2008). From a regional grains in Virgin River–Muddy Creek sands are perspective, the role of karstic erosion for either not qualitatively different from the age spec- Canyon Capture stream capture or knickpoint migration is a tra of modern Colorado River sand. The most local detail that does not alter the overall evalu- prominent nine pre-Neogene U-Pb age peaks on If an ancestral Colorado River crossed the ation of Grand Canyon timing. composited age-probability plots (age-distribu- Kaibab uplift but did not follow the route of No signifi cant sediment delivery to the Huala- tion curves) for 450 Virgin River–Muddy Creek the western Grand Canyon, the most logical pai Limestone depocenter at the mouth of the detrital zircons (Forrester, 2009) and 327 Colo- pathway for paleofl ow lies to the west-north- modern Grand Canyon would be expected rado delta detrital zircons (Kimbrough et al., west across the area of the northern Uinkaret before capture of the upper Colorado drainage 2011) are essentially indistinguishable (fi gures and Shivwits Plateaus north of the Grand Can- near the Miocene-Pliocene time boundary. In in Ma): 85–105, 165–175, 225–275, 350–475, yon (Fig. 6), as postulated by Lucchitta (1984, that respect, stream capture in the central Grand 535–675, 1025–1225, 1375–1500, 1625–1850, 1989) and presumed by Pelletier (2010). Surface Canyon would have abruptly initiated transit 2700–2825. elevations on the plateaus north of 8.2–6.2 Ma of water and sediment through the full length The principal contrast between Muddy Creek volcanic edifi ces near the Grand Canyon (Luc- of the Grand Canyon for the fi rst time near the and Colorado River detrital zircon populations chitta and Jeanne, 2001) are ≤1600 m, slightly Miocene-Pliocene time boundary just as surely is the consistent presence of Neogene grains less than the inferred elevation of the Miocene as Hopi Lake spillover might. Continued deep- (11–24 Ma) in the former, where they typically valley fl oor where the Colorado River crossed ening of the Grand Canyon would be expected form ∼10% of the grain populations (Forrester, the Kaibab uplift. Given the regional northeast- to proceed apace once Colorado River fl ow was 2009). In nine Muddy Creek samples from near erly dip of strata in the Grand Canyon region established through the full length of the canyon. the Overton Arm of Lake Mead near the mod- (Fig. 6), the inferred pre–Grand Canyon river ern Colorado River, ~10% of the detrital zir- valley crossing the Uinkaret and Shivwits Pla- Downstream Paradox cons are 18–21 Ma in age (Muntean, 2012), but teaus would have been a strike valley (Lucchitta, most of the other detrital zircons are compatible 1990; Lucchitta and Jeanne, 2001) with a south- The evolution of the Grand Canyon outlined with plateau derivation. Neogene detrital zircon east-to-northwest alignment. Subparallel strike above involves a paradox because no exit from grains (19–24 Ma) also form 6% of the total valleys of Late Oligocene–Early Miocene age the plateau for an ancestral Shivwits-crossing grain population in modern Virgin River sand

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(Forrester, 2009). The Neogene detrital zircons Stream capture at a point near the central Grand Geological Survey Miscellaneous Investigations Series derive mainly from the Caliente caldera com- Canyon could have diverted river water into Map I-2108, scale 1:24,000 with 15 p. text. Breed, C.S., Fryberger, S.G., Andrews, S., McCauley, C., plex (Fig. 1) or the Pine Valley laccolith (Fig. Bouse lakes downstream from the Grand Can- Lennartz, F., Gebel, D., and Horstman, K., 1979, 3A), with the latter source indicated for modern yon as abruptly as lake spillover could have. The Regional studies of sand seas using Landsat (ERTS) imagery, in McKee, E.D., ed., A Study of Global Sand Virgin River sand. An ancestral Colorado River point of exit of the pre-capture Colorado River Seas: U.S. Geological Survey Professional Paper 1052, emerging from the Shivwits and Uinkaret Pla- from the Colorado Plateau remains uncertain p. 305–397. teaus into a paleo–Virgin River drainage would but a paleo–Virgin River and the Muddy Creek Bump, A.P., 2004, Three-dimensional Laramide deformation of the Colorado Plateau: Competing stresses from have acquired a similar subpopulation of Neo- depositional basin are seemingly viable candi- the Sevier thrust belt and the fl at Farallon slab: Tecton- gene detrital zircons. Otherwise, headwaters of dates for the exit point. ics, v. 23, TC1008, 15 p. both the Colorado and Virgin Rivers tap essen- Cande, S.C., and Kent, D.V., 1995, Revised calibration ACKNOWLEDGMENTS of the geomagnetic polarity timescale for the Late tially the same plateau succession of Triassic Cretaceous and Cenozoic: Journal of Geophysi- through Cretaceous strata capable of yielding Comments by reviewer Ivo Lucchitta, an anony- cal Research, v. 100, p. 6093–6095, doi:10.1029 /94JB03098. analogous age spectra of detrital zircons. mous reviewer, and private reviewer Jon Spencer Cather, S.M., 2004, Laramide orogeny in central and north- Comparative detrital zircon ages from the improved parts of the text, and editorial suggestions ern New Mexico and southern Colorado, in Mack, Muddy Creek Formation and the Colorado by Richard Young improved my treatment of the G.H., and Giles, K.A., eds., The Geology of New Peach Springs paleovalley. 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