Age and Origin of the White Mesa Alluvium, Northeastern Arizona: Geosphere, V

Home , Bidahochi Formation, Inselberg, Narbona Pass, Roof Butte

Research Paper THEMED ISSUE: CRevolution 2: Origin and Evolution of the Colorado River System II

GEOSPHERE Reevaluation of the Crooked Ridge River—Early Pleistocene (ca. 2 Ma) age and origin of the White Mesa alluvium, northeastern GEOSPHERE; v. 12, no. 3 Arizona doi:10.1130/GES01124.1 Richard Hereford1, L. Sue Beard1, William R. Dickinson2, Karl E. Karlstrom3, Matthew T. Heizler4, Laura J. Crossey3, Lee Amoroso1, P. Kyle House1, 14 figures; 3 tables; 3 supplemental files and Mark Pecha5 1U.S. Geological Survey, 2255 N. Gemini Drive, Flagstaff, Arizona 86001, USA CORRESPONDENCE: rhereford@usgs.gov 2Department of Geosciences, University of Arizona, 1040 E. 4th Street, Tucson, Arizona 85721, USA 3Department of Earth and Planetary Sciences, University of New Mexico, 221 Yale Boulevard NE, Albuquerque, New Mexico 87106, USA 4New Mexico Geochronology Research Laboratory, New Mexico Bureau of Geology & Mineral Resources–New Mexico Institute of Mining & Technology, 801 Leroy Place, Socorro, New Mexico CITATION: Hereford, R. Beard, L.S., Dickinson, 87801, USA W.R., Karlstrom, K.E., Heizler, M.T., Crossey, L.J., 5Arizona Laserchron Center, Department of Geosciences, University of Arizona, 1040 E. 4th Street, Tucson, Arizona 85721, USA Amoroso, L., House, P.K., and Pecha, M., 2016, Re- evaluation of the Crooked Ridge River—Early Pleis- tocene (ca. 2 Ma) age and origin of the White Mesa alluvium, northeastern Arizona: Geosphere, v. 12, no. 3, p. 768–789, doi:10.1130/GES01124.1. ABSTRACT older than inset gravels that are interbedded with 1.2–0.8 Ma Bishop–Glass Mountain tuff. The new ca. 2 Ma age for the White Mesa alluvium refutes the Received 27 August 2014 Essential features of the previously named and described Miocene Crooked hypothesis of a large regional Miocene(?) Crooked Ridge paleoriver that pre- Revision received 12 November 2015 Ridge River in northeastern Arizona (USA) are reexamined using new geologic dated carving of the Grand Canyon. Instead, White Mesa paleodrainage was Accepted 22 February 2016 and geochronologic data. Previously it was proposed that Cenozoic alluvium the northernmost extension of the ancestral Little Colorado River drainage Published online 7 April 2016 at Crooked Ridge and southern White Mesa was pre–early Miocene, the prod- basin. This finding is important for understanding Colorado River evolution uct of a large, vigorous late Paleogene river draining the 35–23 Ma San Juan because it provides a datum for quantifying rapid post–2 Ma regional denuda- Mountains volcanic field of southwestern Colorado. The paleoriver probably tion of the Grand Canyon region. breeched the Kaibab uplift and was considered important in the early evolution of the Colorado River and Grand Canyon. In this paper, we reexamine the character and age of these Cenozoic deposits. The alluvial record originally INTRODUCTION used to propose the hypothetical paleoriver is best exposed on White Mesa, providing the informal name White Mesa alluvium. The alluvium is 20–50 m Cenozoic alluvium in a bedrock-bound paleovalley is perched on the Colo thick and is in the bedrock-bound White Mesa paleovalley system, which rado Plateau in northeastern Arizona (USA) only 60 km east of the Grand Can- comprises 5 tributary paleochannels. Gravel composition, detrital zircon data, yon (Fig. 1; Hereford et al., 2013). The paleovalley’s possible significance to and paleochannel orientation indicate that sediment originated mainly from carving of the Grand Canyon is emphasized by a southwest-descending slope local Cretaceous bedrock north, northeast, and south of White Mesa. Sedi- toward the Grand Canyon and lag gravels derived from a distant source. This mentologic and fossil evidence imply alluviation in a low-energy suspended alignment points 300–400 km northeast, directly toward a possible source in sediment fluvial system with abundant fine-grained overbank deposits, indi- the San Juan Mountains of southwestern Colorado, and lag gravels in the cating a local channel system rather than a vigorous braided river with dis- study area apparently support such an origin. Minor amounts of pebble to tant headwaters. The alluvium contains exotic gravel clasts of Proterozoic small cobble gravel in the alluvium are composed of Proterozoic basement basement and rare Oligocene volcanic clasts as well as Oligocene–Miocene and Oligocene volcanic rocks (Lucchitta et al., 2011, 2013, fig. 10, Tables 1 and 2) detrital sanidine related to multiple caldera eruptions of the San Juan Moun- resembling those in the San Juan Mountains volcanic field (Lipman, 1989). The tains and elsewhere. These exotic clasts and sanidine likely came from ancient paleovalley is the topographically highest and therefore oldest geomorphic rivers draining the San Juan Mountains. However, in this paper we show that feature in this erosional landscape, suggesting substantial antiquity. These the White Mesa alluvium is early Pleistocene (ca. 2 Ma) rather than pre–early characteristics, i.e., proximity and slope toward the Grand Canyon, exotic grav- Miocene. Combined 40Ar/39Ar dating of an interbedded tuff and detrital sani- els, and assumed antiquity, led workers to conclude that the paleovalley was dine ages show that the basal White Mesa alluvium was deposited at 1.993 ± formed by an ancient river, probably the combined ancestral San Juan and For permission to copy, contact Copyright 0.002 Ma, consistent with a detrital sanidine maximum depositional age of Animas Rivers, that drained the late Paleogene San Juan Mountains volcanic Permissions, GSA, or [email protected]. 2.02 ± 0.02 Ma. Geomorphic relations show that the White Mesa alluvium is field (Cooley, 1960; Hunt, 1969; Stokes, 1973; Lucchitta et al., 2011, 2013). This

GEOSPHERE | Volume 12 | Number 3 Hereford et al. | Reevaluation of the Crooked Ridge River Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/12/3/768/4092638/768.pdf 768 by guest on 26 September 2021 Research Paper

Abajo S a Mtns n J u a n . Straight Cli M t n s

Vallecito Res.

upwarp s Monument Juan Ri n ve A Sa Blu r el l ver ow Ri e P Navajo Mtn Monument 37°N Lak UT CO B’ Paria Valley AZ NM Navajo Lees Ferry C h i Navaj Lake

K Plt. K o Echo C Cree aibit r

aibab e n l Animas

e esa . k Carrizo e k o M r Skeleton e Cli Mtns iv R San NE Black Mesa (Table 1) s V o Plt. d y e l l a Figure 1. Study area in the eastern Grand a r h

o s Canyon region of the southwest Colorado l B la c a Chusk Juan

o W Plateau, northeastern Arizona. Mtn—

C Figure 2 o k t i mountain; Plt.—plateau; Res—reservoir. Cape b e Solitude Wash n M e s a a Basin pi in M

o M k D G n Blue tns ra e o 36° n o e Canyon d n . C o n A’ a ny M Blue ash ko W p Pt. Cameron i P

l ash

t W Black Pt. olacca sh P a B L W it t o l Oraibi it e d d Bidahochi C Je ol Formation or ado ~16–2.5 0 50 100 km

R ver iv Ma Ri e rco Flagsta r e

Pu Chambers 112° Winslow 110° 108° W 35°

conceptual paleoriver, called the Crooked Ridge River by Lucchitta et al. (2011, carving of the Grand Canyon (Lucchitta et al., 2011, 2013; Karlstrom et al., 2014, 2013), was thought to be involved in the early carving of the Grand Canyon; fig. 1 therein). The young age also disallows any relation between the alluvium overall the evolution of the canyon is a topic of debate (Karlstrom et al., 2012). and the Crooked Ridge paleoriver, which Lucchitta et al. (2013, p. 1427, 1430) This paper is a reassessment of the hypothetical foundations of the suggested was active during eruption of the 35–23 Ma San Juan Mountains Crooked Ridge paleoriver, i.e., its geomorphology, geology, depositional envi volcanic field. Moreover, proposed correlation (Cooley et al., 1969; Hereford ronment, sediment source, and age. These key elements of the paleoriver are et al., 2013) of the alluvium with the mid-Miocene to late Pliocene Bidahochi examined using new (2013 and later) and unpublished data sets. The data in- Formation (Dallegge et al., 2003; Dickinson, 2013) seems impossible. Despite clude topical field mapping (Fig. 2), stratigraphy, sedimentology, pedogenesis, the exotic lag gravels, the presence of the ancestral San Juan River in the White carbonate geochemistry, detrital zircon provenance, sanidine geochronology, Mesa–Crooked Ridge area after 2 Ma is unlikely based on the substantial eleva- and tephrochronology. Among our principal findings is that the alluvium and tion difference between the low-elevation mid-Pleistocene (Wolkowinsky and associated paleovalley system are younger than ca. 2 Ma and older than 1.2– Granger, 2004) San Juan River and the relatively high elevation study area. 0.8 Ma, based on 40Ar/39Ar dating of tuff and detrital sanidine and inset rela- This and other information reported herein motivate us to recommend aban- tions between dated geomorphic surfaces. donment of the term “Crooked Ridge River” as applied to early Pleistocene This young age at the base of the alluvium in the White Mesa–Crooked deposits in the study area. The age, geology, and geomorphology of these Ridge area poses insurmountable problems for earlier interpretations. The age deposits are substantially different from those attributed to the Crooked Ridge precludes any connection between the paleovalley and all except the youngest paleoriver by Lucchitta et al. (2011, 2013).

111°30′ EXPLANATION 111°15′ Ka 111°0′W

i White Mesa b it alluvium o Square Butte Dakota Sandstone Creek Je Entrada Sandstone a Je s WMZ-NUP Jn Navajo Je Sandstone Jn M e Measured R i d g e s B′ section Re PR-AZ-3* Plateau Detrital zircon d Highway Je quarry e and sanidine* M o r m o n Lake locality 36°30′N Figure 2. Topical geologic map of study FosslifFossiliferouserous W h i t area showing distribution of White Mesa s monoclin Shonto limestonelimestone gravel alluvium in White Mesa–Crooked Ridge area. Wildcat Peak is a prominent ero- g la PR-AZ-1 sional feature of volcanic origin composed PR-AZ-2 e Pr of monchiquite (Williams, 1936). Billing- Ridge Railroad Figure 5 eston quarry sley et al. (2012, and reference therein) reported an age of 19.1 ± 0.1 Ma for the Crooked southern dike. According to Lucchitta et al. (2013), the Crooked Ridge paleoriver Jn predated Wildcat Peak volcanic activity. A′ Mesa o h c E utcrop Wildcat o Jn esa Peak M and i C l Ridge n dike fault 19.1±0.1 Ma esto s Pr sh Hamblin 36°20′ PR-AZ-4* U Wa Crooked Re D Whit d The Gap barrierDo

e t to

Wash 0510 km

P H oin

ills shibi t A B Bega

The immediate study area extends 57 km northeast from The Gap, a wind alluvium, which is represented by the characteristic and abundant outcrops on gap in the Jurassic rocks of the Echo Cliffs monocline, to the north end of White Mesa. The ancient valley in which the alluvium was deposited is referred White Mesa encompassing all or parts of 12 7.5′ topographic quadrangle maps to as the early Pleistocene White Mesa paleovalley system within the larger (Fig. 2). Additional localities were studied on northeast Black Mesa and Blue White Mesa paleodrainage basin. Alluvium of similar age on the Moenkopi Point on the Moenkopi Plateau (Fig. 1). Deposits of the ca. 2 Ma alluvial system Plateau (Fig. 1) is within the paleodrainage basin and correlates with the White are preserved on Crooked Ridge and White Mesa. A narrow, sinuous ridge Mesa alluvium. capped by lag gravel connects the Crooked Ridge and White Mesa outcrops. Workers were previously concerned mainly with the paleogeomorphic im- This is the Crooked Ridge of modern topographic maps and the geomorphic plications of lag gravels in the White Mesa–Crooked Ridge area, and did not expression of the hypothetical Crooked Ridge paleoriver of Lucchitta et al. describe the characteristics, composition, or extent of the White Mesa allu- (2011, 2013). Examination by Lucchitta et al. (2011, 2013) of the Crooked Ridge vium (Cooley, 1960; Hunt, 1969; Stokes, 1973; Lucchitta et al., 2013). Cooley outcrop, lag gravels northeast of the ridge, and gravel quarries on southern et al. (1969) mapped (without the benefit of modern topographic maps) allu- White Mesa provided the interpretive basis of the Crooked Ridge paleoriver. vium on White Mesa and correlated it with the Bidahochi Formation based Billingsley et al. (2012) mapped the alluvium at Crooked Ridge and south- on similar topographic positions in the landscape. Hunt (1969) referred to lag ern White Mesa as “Pliocene? and Miocene? sedimentary deposits.” These gravels on White Mesa as the Kaibito gravels and related them to a Miocene deposits are informally referred to herein as the early Pleistocene White Mesa San Juan River. However, Cooley et al. (1969), Hunt (1969), and Stokes (1973)

evidently did not recognize the geologic and geomorphic connections among are coeval everywhere is unknown, although our dating of tuff associated White Mesa, Crooked Ridge, and The Gap, that were first reported by Lucchitta with the surfaces indicates that they are essentially contemporaneous in the et al. (2011). Cooley et al. (1969) considered Crooked Ridge geomorphically study area. younger than White Mesa and did not discuss The Gap. The White Mesa paleovalley system occupies the highest and oldest terrain in a region extending over 150 km from north of the Moenkopi Plateau and LANDSCAPE-SCALE GEOMORPHOLOGY OF northwest of Black Mesa to Navajo Mountain (Fig. 3A). A line of section drawn THE WHITE MESA PALEOVALLEY southwest of White Mesa (Fig. 3B) shows accordant surfaces at ~1800 m elevation on the Moenkopi Plateau, Middle Mesa, White Point, Crooked Ridge, The study area is a structurally intact, discontinuous remnant of an early and Paria Plateau east of Kaibab uplift. Cooley et al. (1969, plate 3) considered Pleistocene paleovalley system replete with alluvial fill and paleochannels these higher surfaces in the study area to be the L2A erosional surface; the that postdate initial carving of the nearby Grand Canyon. Northeastern Ari White Mesa alluvium underlies this surface. In the Cooley et al. (1969) model, zona in general and the study area in particular is a landscape of progres- the accordant surfaces and related deposits were parts of a regional landscape sively lower and younger low-relief erosional geomorphic surfaces of broad that existed mostly before the present canyons of the Colorado River system. extent. Earlier workers (cited in Cooley et al., 1969) recognized this pattern However, the ca. 2 Ma White Mesa alluvium and the 1.99 Ma Blue Point tuff and related it to multiple cycles of regional downcutting alternating with non- (Table 1; Fig. 1) on the Moenkopi Plateau demonstrate that the L2A surface is deposition or alluvial aggradation. These are accordant surfaces of similar younger than initial carving of the Grand Canyon. elevation forming mesas, plateaus, and pediments that are typically over- The main geomorphic features composing the base of the paleovalley lain by lag gravels or locally derived alluvium. Cooley et al. (1969, plate 3) from northeast to southwest are White Mesa, Crooked Ridge, composed of correlated these surfaces using their relative elevation; numerical geochro- the Crooked Ridge lag gravels and the Crooked Ridge outcrop of White Mesa nology was unavailable at that time (1955). Whether the accordant surfaces alluvium, and The Gap in the Echo Cliffs monocline (Fig. 2). White Mesa is a

A A A′ North Utah Arizona Northwest Southeast Navajo Meters Straight Cli s Mtn. 3000 Black Mesa Figure 3. (A) Topographic profile (A–A′, Cha White Mesa alluvium Begashibito Dinnebito Fig. 1) from Black Mesa northwest to 2500 pediment Moenkopi Wash ~500 ka Navajo Wash Wash Straight Cliffs. White Mesa topography Creek 2000 is inverted, and is the highest and oldest geomorphic feature in the area, rising well 1500 above dated late Pleistocene surfaces. V.E.—vertical exaggeration; Mtn.—moun- V.E. = 12× 1000 tain. (B) Profile (B–B′, Fig. 1) northwest from the Little Colorado River Valley at Glen 25 50 75 100 125 150 Canyon Black Point (Pt.) to the Colorado River at Kilometers Lake Powell Lees Ferry, Paria Plateau, and Buckskin Mountain. Topography of the Moenkopi Plateau is inverted. Profile shows accor- B B B′ dant regional geomorphic surfaces and Southwest Northeast Northwest deposits. Source of chronometric ages: Bishop–Glass Mtn. tu Crooked Ridge Johnson Point in Lucchitta et al. (2000) Blue Pt. tu 0.8−1.2 Ma Colorado Meters Preston River Buckskin and Garvin et al. (2005); Cha pediment 1.99 Ma Moenkopi Mormon Moenkopi White Mesa Lees Ferry Paria Mtn. age in Garvin et al. (2005); Black Point flow 2000 Plateau Wash Pt. Ridges Black Pt. Little Plateau in Hanson (2007); and Bishop Tuff–Glass ow Middle Echo Colorado Mesa Mountain volcanics at Blue Canyon in 870 ka River Cli s Valley Geib and Spurr (2002). Haines and Bowles 1500 (1976) reported 20 m of alluvium beneath Black Point flow in borehole.

Johnson Pt. 1000 ~500 ka V.E. = 39×

Kilometers 50 100 150 200 250

TABLE 1. KEY LOCALITIES MENTIONED IN THE TEXT Lat Long Elevation Number Site type and name (N)† (W) (m) Measured sections *1 Crooked Ridge, base: 2.02± 0.02 Ma 36.359759111.373881 1740 Crooked Ridge, top 36.388183111.342756 1768 2 Southern White Mesa, base 36.489864110.9921852017 Southern White Mesa, top36.486572110.9955562041 3 Northern White Mesa, base 36.585619110.9281862111 Northern White Mesa, top36.588546110.9291012124 Paleochannels 4 North channel 36.587537110.99905 2058 5 Northeast channel 136.593508110.9199762099 6 Northeast channel 236.524132110.9870102025 7 Northeast channel 336.489864110.9921852017 8 South channel36.476610110.9956151986 9 Narrow south channel 36.455732111.012838 2001 Limestones 10 White Mesa 136.476855111.044344 1977 11 Crooked Ridge 36.349029111.405144 1723 12 White Mesa 236.478205110.9983242023 Northeast Black Mesa 13 Lag surface 136.543334110.4958492234 14 Lag surface 236.528463110.5204112234 Quarries 15 Black Mesa Lake Powell Railroad 36.490432111.089346 1940 *16 Indian Route 21: 1.84 ± 0.05 Ma 36.490557111.049031 1980 Pleistocene tephra 17 Bishop–Glass Mountain tuff at Blue Canyon: 1.2–0.8 Ma 36.137124110.83907 1686 (Geib and Spurr, 2002) 18 Lava Creek B (analysis by M.E. Perkins, 2013, personal commun.) 36.588622110.9285792119 on White Mesa: 639 ± 2ka (Lanphere et al., 2002) 19 Blue Point tuff 1.99 ± 0.002Ma (M. Heizler, 2014, personal commun.)35.988647110.9970381817 *Detrital sanidine sample locality. †Datum World Geodetic System 1984.

19-km-long remnant of the paleovalley system, consisting of a northern and Begashibito headwaters adjoin the San Juan River drainage basin 39 km east southern portion, incised through the Dakota Sandstone (upper Cretaceous) of the northeast end of White Mesa. The mesa is 0.9 km above the junction of into the underlying Entrada Sandstone (Jurassic). Moenkopi Wash near the head of the Little Colorado River Gorge knickzone The topography of White Mesa and the Moenkopi Plateau is inverted in (Cook et al., 2009) that formed in resistant Paleozoic bedrock downstream of the sense that deposits once in the lowest parts of the landscape, an aban- Cameron, Arizona. doned ancient stream channel and valley, are now preserved on the highest Relative to the Colorado River in the Grand Canyon, White Mesa is 1.4 km terrain as drainage divides and on top of mesas and plateaus (Fig. 3). White above the junction of the Little Colorado and Colorado Rivers (Fig. 1) and is Mesa straddles the drainage divide between the Colorado and Little Colo- 1 km above the junction of the Colorado and San Juan Rivers. This inversion rado Rivers. The mesa drains to the Colorado River in Glen Canyon and is was accomplished by regional denudation after deposition of the White Mesa as much as 1 km above the base level at Lees Ferry, Arizona (Figs. 1 and alluvium ca. 2 Ma. Denudation is related to capture of the northern Little Colo- 3B). On the Little Colorado River side of the drainage divide, Begashibito rado River drainage basin by the San Juan and Colorado Rivers, whose drain- Wash (a tributary of Moenkopi Wash; Figs. 1 and 2) drains the mesa, and age divides have shifted south and east.

The 26-km-long sinuous Crooked Ridge connects the White Mesa paleo Crooked Ridge to the Mormon Ridges broadly resembles the profile of a val- valley with the Crooked Ridge outcrop (Fig. 2). The outcrop is 11 km long and ley (Fig. 4B). However, the resemblance is superficial because the valley-like both ridge and outcrop are above the surrounding topography with ~100 m of profile between the slope of Preston Mesa and the Mormon Ridges is below inverted relief. The Gap is the southwestern terminus of the paleovalley. Lag the 1790 m elevation of Crooked Ridge, showing that before topographic gravels at The Gap overlying Navajo Sandstone (Jurassic) are ~1 m thick with inversion the ancient valley bottom was higher and the valley narrower. pebbles and small cobbles of Cretaceous sandstone and exotic clasts of quartz- Subsequent erosion modified the cross-valley profile, making it difficult to ite, other Proterozoic metamorphics, and rare Proterozoic granite. Assuming estimate the width of the paleovalley. It is likely that the paleovalley north that the alluvium was 50 m thick (the maximum observed in the study area) at of Preston Mesa was no wider than the southern White Mesa paleovalley The Gap, the elevation at the top of the alluvium was 1740 m and the width at (Fig. 5). The maximum depth of the paleovalley from the summit of Preston The Gap was ~2.6 km (Fig. 4A), although the present width of The Gap is likely Mesa to the top of Crooked Ridge is 250 m. Although an older paleoriver increased by subsequent wind deflation and fluvial erosion. Just southwest of could have carved a deep canyon from Preston Mesa to and through The the study area, the Red Dot Hills compose a topographic and structural barrier Gap, no evidence of it was found in the present topography, and the White composed of the east-dipping resistant sandstone of the Shinarump Member Mesa alluvium is substantially younger than the hypothetical paleocanyon of of the Triassic Chinle Formation (Fig. 2). The barrier precluded a direct south- Lucchitta et al. (2013). westward path to the Grand Canyon, as indicated by the absence of beveled In the headwaters of Kaibito Creek (Fig. 5), the mostly uneroded margins bedrock surfaces; it is more likely that the paleovalley followed the north-south of the paleovalley are well preserved. The northwest-facing side of the paleo strike of the Echo Cliffs monocline. valley is particularly evident at the heads of the west to west-northwest–flow- The margins of the paleovalley in the Crooked Ridge area are not well de- ing tributaries of Kaibito Creek. Here, the Dakota and Entrada Sandstones fined. Nonetheless, Lucchitta et al. (2013) suggested that a paleovalley here form the steep bedrock margin of the paleovalley that rises 70–100 m above was 15 km wide and >1 km deep. The Mormon Ridges and Preston Mesa the top of the alluvium. On northernmost White Mesa, erosion related to bound Crooked Ridge on the north and south (Fig. 2). A topographic cross topographic inversion has removed the formerly elevated southeast-fac- section drawn from White Point over Preston Mesa and northwest across ing margin of the paleovalley. Square Butte (Fig. 2), however, remains as

A A A′ Meters South North Crest of t 2000 he The Gap Ech o Cli s 1900 White Mesa 1800 alluvium 50 m thick

Figure 4. (A) Topographic profile (A–A′, ?

1700 aleochannel Fig. 2) on crest of Echo Cliffs, an east-dip- P V.E. = 8× ping monocline, showing reconstructed 1600 width of paleochannel at The Gap. V.E.— Kilometers 2.5 5 7.5 10 12.5 15 vertical exaggeration. (B) Profile (B–B′, Fig. 2) showing gravel-capped Crooked Ridge and paleovalley between Preston Mesa and the Mormon Ridges. White B B B ′ Point is a pediment and L2A surface of South Preston Mesa North Cooley et al. (1969). U—upthrown; D— Meters Mormon downthrown. 2000 Ridges Crooked Ridge 1950 Preston Mesa 1900 White Point fault 1850 1800 D U V.E. = 18× 1750 Kilometers 5 10 15 20 25 30 35

North paleochannel NE paleochannel 1

Je 36°35′N

No Je rt her y n Whit e Mesa paleovalle

NE paleochannel 2

a ib M e s a Re i t o Figure 5. Topical geologic map of White d C Mesa area showing tributary paleochan- r margin Lake e ek Je + y + nels entering southern White Mesa paleo valley. Northeast (NE) paleochannel 3 and monoclin + south paleochannel contain exotic gravel paleovalle e clasts, whereas others have clasts com- ferred h i tJe EXPLANATION In Jn posed primarily of upper Cretaceous sand- White M W stone. Basal contact of Entrada Sandstone e White Mesa alluvium is not mapped. Contact of alluvium with Jn Southern bedrock is dashed where covered. 36°30′ esa paleovalle NE paleochannel 3 Dakota Sandstone e 5–6 Je Entrada Sandstone idg R km Jn Navajo Sandstone d oke Paleochannel o gravel y Cr + Inselberg lag Je

South paleochannel 024 km Je

111°5′W 111°0′ 110°55′

an isolated and elevated erosional remnant of the southeast-facing valley the alluvium. The gradient of the paleovalley was estimated from the slopes margin. On southern White Mesa, the northwest- and southeast-facing of the six possible combinations of the 4 control points; the median slope is valley margins are also missing due to erosion related to topographic in- 0.0067, or ~0.007. The gradient profile connecting the four points shows no version. The alluvium onlaps Entrada Sandstone at the northwest side of substantial knickzones or deviations from the median gradient. The study southern White Mesa paleovalley, which is near the inferred margin of the area was likely affected by isostatic rebound related to late Miocene to Plio- paleovalley (Fig. 5). Three inselbergs with steep slopes of Entrada Sandstone cene regional denudation that continues to the present (Hoffman, 2009; capped by ledge-forming Dakota Sandstone are evidence of a former valley Pederson et al., 2013; Lazear et al., 2013). In the Monument upwarp area that was roughly parallel with the southeast face of northern White Mesa (Fig. 1), denudation is 1–2 km. The study area is on the southern flanks of (Table 1, site 6). the uplifted area. The paleovalley gradient is useful for estimating the elevation of the The San Juan River almost certainly did not cross the high-elevation terrain headwaters of the paleodrainage north and northeast of White Mesa that of the White Mesa–Crooked Ridge area after ca. 2 Ma. The San Juan at Bluff, were removed by post–2 Ma denudation. The gradient was determined from Utah (Fig. 1), was in its present course 140 m above the active channel (ele- 4 control points placed from The Gap to northeast White Mesa (Fig. 2), a vation 1320 m) at 1.36 (+0.2/–0.15) Ma (Wolkowinsky and Granger, 2004). This distance of 57 km. The control points are at or within 2–3 m of the base of elevation is substantially below the northeast terminus of the paleovalley at

northeast White Mesa. Here the top of the alluvium is 2124 m (Table 1, site 3), coarse-grained to pebbly pale yellowish sandstone derived from the Dakota 800 m above the San Juan River locality and well above all intervening topog- Sandstone and pale red clasts of Navajo Sandstone. Rare metamorphic, ig- raphy, including most of Skeleton Mesa and all of Monument Valley (Fig. 1). neous, and volcanic clasts of presumed San Juan Mountains origin are also The minimum height above the San Juan River increases to ~1.8 km by pro- present in the basal gravel at Crooked Ridge and on southern White Mesa jecting the linear slope of the paleovalley along a northeast trend. Whether the (Table 1; Lucchitta et al., 2011, 2013). incision was 800 m or 1.8 km, the estimated mid-Pleistocene incision rate at The lateral accretion clay-sand unit at Crooked Ridge consists of poorly Bluff (110 m/m.y.; Wolkowinsky and Granger, 2004) is too low to accommodate sorted sand displaying lateral accretion surfaces, channel forms, and over- this much downcutting in ~2 m.y. bank fines (Fig. 6). Lateral accretion surfaces are prominent sigmoidal-shaped features in the cross-stratified sands. The lateral accretion unit overlies and appears to crosscut the interbedded clay unit (Fig. 7B), although several beds GEOLOGY OF THE WHITE MESA ALLUVIUM in the lateral accretion unit resemble those of the interbedded clay and very fine grained sand unit, suggesting that they could interfinger. No evidence of Here we describe and interpret the stratigraphy and depositional environ- weathering, strong soil development, or other indicators of a hiatus are pres- ment of the alluvium, the source of gravel in the alluvium, tributary paleo ent at the contact, although a thin (10 cm thick), soft carbonate horizon under- channels of the White Mesa paleovalley that point toward sediment sources, lies the contact. The lateral accretion unit has not been found on White Mesa. and the paleohydrology of the alluvium based on carbonate geochemistry. New Throughout the area, the interbedded clay and very fine grained sand topical geologic mapping identifies the early Pleistocene White Mesa alluvium unit shows multiple beds of poorly sorted sand overlain by silty clays with and documents its presence on White Mesa and Crooked Ridge (Fig. 2). The subhorizontal (or horizontal) stratification extending continuously across most informative and abundant outcrops are on White Mesa (Fig. 5), which is the outcrop (Fig. 7C). The sands are clayey and have distinctive pale grayish considered the area most characteristic of the alluvium. On White Mesa and shades of green, red, and yellow with subtle mottling and a sharp contact most of Crooked Ridge, the alluvium is generally not well exposed and is diffi- with underlying beds. Mottling and low chroma values likely result from re- cult to measure and describe without motorized mechanical excavation, which duced iron minerals (gleization) related to accumulations of organic matter is precluded by logistics and cultural considerations. Outcrops at most local- and locally shallow water tables in near-floodplain sediment (Kraus, 1997). ities are blanketed by younger eolian sand, slope weathering obscures strati Clay beds overlie the sand beds along a contact that is typically gradational, fica tion, and even where well exposed, sedimentary structures in the sands suggesting that they are fining-upward couplets or cycles. The couplets proba- are rare to absent possibly due to bioturbation or diagenetic alteration. These bly represent individual episodes of channel and floodplain aggradation. Clays conditions preclude meaningful paleocurrent analysis. The stratigraphic sec- are light olive-gray (5Y 5/2) with a pale greenish cast resembling the color of tions described and illustrated here are among the few in which the alluvium the Mancos Shale (upper Cretaceous), which was the likely source of the clays. is adequately exposed (Fig. 6). These fine-grained sand and clay deposits are the defining characteristic of the White Mesa alluvium. The continuity of stratification, multiple fining-upward Stratigraphy and Depositional Environment cycles, and poorly sorted sands suggest deposition in a multistory, relatively low energy suspended sediment aggradational channel system within a bed- The White Mesa alluvium ranges in thickness from 20 to >50 m, and is typi rock-bound paleovalley (Miall, 2010). cally 30–35 m thick. On White Mesa, the alluvium overlies the Navajo and En- Fossil evidence also supports the interpretation of a low-energy fluvial trada Sandstones. Downstream of the Red Lake monocline to The Gap (Fig. 2), environment. Thin, essentially contemporaneous marker beds of light colored the alluvium overlies Navajo Sandstone. The bulk of the White Mesa alluvium platy limestone containing small, 1–5-mm-diameter thin-walled gastropods is very poorly sorted clayey sand and interbedded clay; gravel beds are sub- are present 2–3 m above the base of the alluvium at widely spaced localities

Petrocalcic layer ordinate. Four stratigraphic units are present: discontinuous basal gravel, a on Crooked Ridge and southern White Mesa (Table 1, sites 10 and 12). The At the Crooked Ridge outcrop, the youngest unit is a 2–4m thick petrocalcic layer that lateral accretion clay-sand unit present only at Crooked Ridge, an interbedded limestones are 5–15 cm thick, of limited area (<10 ha), and are intercalated forms a prominent white ledge above the slope-forming alluvium (Figs. 6, 7b). This distinctive clay and very fine grained sand unit, and a prominent ledge-forming petro with clay beds of the interbedded clay and very fine grained sand unit. At the layer rests with apparent disconformity on the White Mesa alluvium. The layer is not present on

White Mesa perhaps because of higher elevation and somewhat wetter and cooler climate or calcic soil at the top of the alluvium found only at Crooked Ridge (Fig. 6; Sup- southern White Mesa outcrop, two limestone beds are present separated by 1 because the carbonate was removed by erosion. The degree of calcic soil development provides a plemental File 1 ). The interbedded clay and very fine grained sand unit is pres- 2 m of clay. The specimens are too poorly preserved for identification beyond minimum age of the White Mesa alluvium. ent throughout the area and is typical of the alluvium. family level (C. Powell II, 2014, personal commun.), which is Gastropoda Fam- The basal gravel is 0–9 m thick. At Crooked Ridge (Fig. 7A) and where ily Lymnaeidae. This taxon has a fossil record going back to the Mesozoic, so 1Supplemental File 1. Petrocalcic layer. Please visit http://dx .doi .org /10 .1130/GES01124 .S1 or the full- present on White Mesa, the gravel has an immature appearance consist- it cannot date the alluvium. However, the ecology is well known; the taxon text article on www.gsapubs.org to view Supplemen- ing mainly of subangular to subrounded clasts of sandstone supported in a lives in ponds, lakes, or slow-moving streams. A lake or marsh-like environ- tal File 1. coarse sand matrix. The clasts are pebbles to small cobbles of poorly sorted ment is favored based on the fine-grained platy character of the limestone.

37 km 11 km Crooked Ridge Southern White Mesa Northern White Mesa Clay Silty to sandy clay Very ne sand NE paleochannel 3 NE paleochannel 1 Medium to coarse sand Sandy gravel EXPLANATION k k k k DZ Detrital zircon and sanidine, DS Lateral Lateral accretion surface 30 accretion Interbedded clay Calcareous horizon, 10-20 cm thick and very ne- clay- ? k Petrocalcic layer sand grained sand unit DZ unit Carbonate nodules Clay, dense ? Clay, interbedded silt and sand Sand, very ne to medium, poorly sorted, 20 clay drapes, lateral accretion surfaces

ers Sand, very ne to ne, poorly sorted, massive

Met Sand, medium to coarse, cross strati ed

Gravel, granule to cobble, sandy

Entrada Sandstone Basal (Jurassic) gravel

DZ, DS 0 Navajo Sandstone Figure 6. Stratigraphic correlation of White Mesa alluvium, Crooked Ridge to northern White Mesa (Fig. 2). Green (Jurassic) is petrocalcic layer. NE—northeast paleochannel.

This fossil evidence and the sedimentologic characteristics of the alluvium do rived from the upper gravel; this exaggerates the amount of gravel in the allu not support and are inconsistent with the vigorous braided stream model of vium. Gravel beds as thick as several meters are present within the interbed- the Crooked Ridge paleoriver (Lucchitta et al., 2011, 2013). ded clay and very fine grained sand unit, but are discontinuous and typically not traceable beyond several hundred meters. Gravel in the alluvium generally Gravel Beds has a matrix-supported fabric. Regardless of stratigraphic position, the gravels appear texturally immature with abundant coarse sand matrix and numerous Gravel beds are subordinate to the beds of fine-grained sediment in the subangular to subrounded clasts (Fig. 7A). Their lateral discontinuity and im- White Mesa alluvium. A gravel bed 1–2 m thick is typically present at the top of mature texture suggest that the gravel beds were not deposited by a large and the alluvium. Hillslopes below this gravel are typically covered with clasts de- energetic fluvial system.

B Figure 7. Units of White Mesa alluvium at Crooked Ridge and southern White Mesa paleovalley (Fig. 2). (A) Basal gravel units at Crooked Ridge (Table 1, site 1). Gravel is A 9 m thick and consists of two subunits dis- tinguished by sand content and clast size. Pale reddish cobbles and small boulders are local bedrock of Navajo Sandstone; pale yellowish clasts are Cretaceous sand- 0 1 2 3 4 Meters A Coarse sand interbedded with sandy stone. Rare, exotic subrounded pebbles and small cobbles composed of quartz- granule to small cobble gravel ite; other metamorphics, igneous, and B Granule to small boulder sandy gravel volcanic rock (Lucchitta et al., 2011, 2013) are present. (B) Crooked Ridge outcrop showing main stratigraphic units of White B 3 Mesa alluvium above basal gravel and contacts between units (Table 1, site 1). CH 2 LA Stratigraphic units: 1—interbedded clay and very fine grained sand unit (dashed FF line); 2—lateral accretion clay sand unit; 3—petrocalcic layer (solid line). Bedforms: LA—lateral accretion; CH—channel; FF— 1 overbank fines. Note lack of channeling in the interbedded clay and sand very 0 20 meters fine grained unit. LA corresponds to area in figures 4 and 7 of Lucchitta et al. (2011, 2013). (C) Exposure of interbedded clay and very fine grained sand unit, southern White Mesa paleovalley (lat 36.50634, long –111.05634; World Geodetic System 1984). Numbers refer to four coarse to fine couplets, each bound by sharp contacts C (solid line) consisting of very poorly sorted fine-grained sand overlain gradationally (dashed line) by dense to fissile clay. Basal 4 coarse sand and gravel unit is present; note continuity of stratification.

10 meters 3

2 1

Where gravel is present, whether at the base, middle, or upper portions of the boulder trains. Some of the Dakota Sandstone boulders contain Gyrphaea the alluvium, pebbles, cobbles, and large boulders derived from Cretaceous newberryi derived from an oyster-rich bed present regionally at the top of the sandstone predominate. For the most part, these clasts resemble the Dakota Dakota Sandstone and in the basal 15 m of the overlying 210-m-thick Mancos Sandstone as described by Repenning and Page (1956). They are very pale Shale (Repenning and Page, 1956; Nations et al., 1995). Preserved outcrops of orange (10YR 7/2) to pale yellowish-orange (10 YR 8/6) sandstone with fine- to the Mancos Shale on White Mesa are too thin and discontinuous to map; how medium-grained clear and stained quartz grains, and common black accessory much of the shale was present on top of this portion of White Mesa is specu- minerals. Iron-rich concretions are abundant in Dakota Sandstone outcrops; lative. Hillslopes of shale above the paleovalley were present, and they proba- they occur in the gravels as irregularly shaped pebbles with rounded corners. bly had the characteristic dendritic drainage pattern of the Mancos Shale. The The Mesaverde Group (upper Cretaceous), 210 m above the top of the Dakota height of these hills was probably less than the thickness of the Mancos Shale, Sandstone (Nations et al., 1995), has two ledge-forming sandstones (Toreva because sandstone boulders of Toreva and Yale Point formations are absent in Formation and Yale Point Sandstone) that are possible sources of gravel in the the boulder trains, which suggests that these sandstone formations were not White Mesa alluvium. Clasts of Mesaverde Group sandstones have not been present on White Mesa. found in gravels of the White Mesa alluvium, although Lucchitta et al. (2013) Exotic clasts of quartzite, other metamorphic rocks, and volcanics in gravel suggested that they may be present. Bedrock older than the Navajo Sandstone beds of the White Mesa alluvium typically have distinctive shapes and sur- was likely not exposed along the upstream course of the paleovalley, as in- face features (Fig. 8). The clasts generally have multiple facets, unsmoothed ferred from the projected gradient of the alluvium and 1:250,000-scale structure reentrants, jagged cracks, rough uneven surfaces, and a variety of odd and contour maps (O’Sullivan and Beikman, 1963; Haynes et al., 1972; Hackman and asymmetric shapes. As much as 90% of pebbles and small cobbles in the Wyant, 1973; Haynes and Hackman, 1978; Ulrich et al., 1984; Dillinger, 1990). White Mesa alluvium display these features along with distinctly nonellip- In the White Mesa area, boulder trains several hundred meters long are soidal shapes. In contrast, approximately one-third of such clasts from late aligned with bedrock margins of the paleovalley. Well-exposed boulder align- Pleistocene deposits of the San Juan River near and upstream of Bluff, Utah, ments occur in the west- to west-northwest–flowing headwater drainages of have rounded facets and somewhat irregular shapes. Most San Juan River Kaibito Creek below the Dakota Sandstone rim rock (Fig. 5). Boulders of Dakota clasts appear approximately ellipsoidal, which is the usual shape of clastic Sandstone and less commonly Entrada Sandstone to 1–2 m on an edge form sedimentary particles (Cui and Komar, 1984). Peterson (1979) described fac-

f r, s, fm r, s, fm Figure 8. Randomly chosen gravel clasts of s, fm White Mesa alluvium and late Pleistocene r, s, fm gravels of San Juan River. (A) Pebbles and small cobbles from northeast paleochan- s, fm nel 3. (B) Cobbles from Pleistocene gravel f f of San Juan River in Four Corners area (Fig. 1). Labels: r—rough and pitted surface; s, fm f s—asymmetric nonellipsoidal shape; f— rounded facet; fm—two or more rounded s, fm facets. Surfaces features and shape of gravel clasts in White Mesa alluvium differ f f substantially from those of San Juan River.

r, s, fm r, s, fm 50 millimeters

A B

eted cobbles and boulders and related them to weathering and breakage on vium and bedrock. Deposition of the alluvium began in these paleochannels; an exposed outcrop followed by subsequent rounding of the facets in an ac- sediment eventually filled and then overtopped the paleochannels, resulting tive channel. In gravels of the White Mesa alluvium, clasts with these irregular in widespread deposition over the adjoining low-relief bedrock surface. At shapes, secondary roundings, and roughened surfaces are evidence of short Crooked Ridge, the alluvium occupies and overtops a steep-walled paleo- transport distance and minimal reworking in the depositional system. channel 15–20 m deep and several hundred meters wide incised into Navajo Sandstone. Clast composition of gravel differs among the White Mesa paleochannels, Tributary Paleochannels reflecting subbasins of the paleodrainage. Clasts in the basal gravel of the north paleochannel (Table 1; Fig. 5), which is 220 m wide, are almost entirely angular We describe here the composition of gravel clasts in tributary paleo to subangular Cretaceous sandstone and rare quartzite (a single quartzite clast channels present on White Mesa from which the source of the gravel and was found); no volcanics are present. Gravel beds in northeast paleochannel 1 the direction of the source from White Mesa are inferred. Five paleochannels (Fig. 6), which has a preserved width of >500 m within the estimated 5-km-wide are exposed around and just below the rim of the mesa; these converge to northern White Mesa paleovalley, contain abundant Cretaceous sandstone form the 5–6-km-wide southern White Mesa paleovalley (Fig. 5; Table 1 sites clasts and quartzite cobbles; one small cobble of irregularly shaped porphyry of 4–8). Three of the paleochannels combine on southern White Mesa from the unknown origin was found. Composition of clasts in these two paleochannels northeast (clockwise from 1 to 3), another from the north, and another from suggests a source terrain of mainly Cretaceous strata. Northeast paleochannel 2 the south. The defining characteristic of the paleochannels is the shape of the lacks alluvium along the southeast-facing rim of White Mesa, but three insel- contact between bedrock and alluvium. In cross section, the contact forms bergs in the area define the margins of a former channel (Fig. 5). the perimeter of a spatially restricted flat-floored channel-like feature with Gravel at the top of the alluvium of northeast paleochannel 3 (Fig. 6) con- subvertical walls incised into bedrock of the Navajo, Entrada, or Dakota Sand- tains abundant clasts derived from Cretaceous sandstones, minor amounts stones. Paleochannels range from 10 to 40 m deep and 150 to >500 m wide of quartzite, other metamorphic clasts, and a variety of volcanic clasts, along (Fig. 9). Above the upper margin of the paleochannels, the contact is on a with dark colored coarsely crystalline petrified wood. The exotic volcanic spatially extensive, low relief slightly undulating erosional surface; this con- clasts in northeast paleochannel 3 include several varieties identical to those figuration accounts for essentially all of the mapped contact between allu- illustrated by Lucchitta et al. (2013). In addition, this paleochannel and south paleochannel contain distinctive rounded pebbles of carbonate cemented feldspathic sandstone that is also present at both quarries. A thin sandstone Southern White Mesa paleovalley bed petrographically similar to the feldspathic sandstone clasts is present on Dakota Sandstone Black Mesa (Fig. 1; Table 1, sites 13 and 14). Northeast paleochannel 3 evidently 150 m drained a terrain composed largely of Cretaceous strata that in turn was appar- Entrada ently overlain by late Paleogene to Neogene sedimentary deposits containing Sandstone detritus composed of feldspathic sandstone, quartzite, other metamorphics, White Mesa alluvium and volcanics originally derived from the San Juan Mountains and elsewhere. The south paleochannel on the southern rim of White Mesa (Fig. 9) has two gravel beds containing rare volcanic clasts similar to those described at Crooked Ridge and the White Mesa quarries by Lucchitta et al. (2013), along with the usual Cretaceous sandstone and quartzite clasts. At 150 m, this paleo channel is the narrowest of the 5, suggesting it had a relatively short reach. The south paleochannel was probably not a minor reentrant in White Mesa paleovalley. Although relatively narrow, preserved deposits in the paleo channel extend 550 m south-southeast of the exposed subvertical walls of the paleochannel, where they are truncated by the southeast-facing escarpment of White Mesa. Moreover, flow in the south paleochannel was most probably not to the south, as this would require that White Mesa paleovalley had two out- Figure 9. View to north of south paleochannel exposed along rim of White Mesa where paleo- lets, one to the south and another much larger outlet draining to the southwest channel entered southern White Mesa paleovalley at skyline (Table 1, site 8; Fig. 5). Subverti toward Crooked Ridge and The Gap. Two outlets require a drainage divide to cal channel walls are 150 m apart. Alluvium overtops paleochannel margin spreading over Dakota Sandstone and Mancos Shale. Exotic volcanic and metamorphic clasts are present in split the southern White Mesa paleovalley, but no evidence of a divide is pres- two gravel beds. ent in the flat low-relief paleovalley.

Paleohydrology 6

The paleohydrology of the White Mesa alluvium is inferred from the pre- 4 viously described fossiliferous limestones using carbon and oxygen isotopic Colorado River data. These data are compared in Figure 10 with groundwater carbonates of 2 the Bidahochi Formation, Cape Solitude on the east rim of the Grand Canyon

(Fig. 1), and the modern Colorado River. The ponded conditions indicated by PDB 0

the ecology of the gastropods reflect times of groundwater saturation on the δ C Bidahochi Formation floodplain of the White Mesa alluvial system. The Bidahochi carbonates dif- –2 fer from the carbonates in the White Mesa alluvium; this seems reasonable Cape Solitude based on detrital zircon data (described herein). The Cape Solitude carbonate –4 White Mesa developed within the uppermost Permian Kaibab Limestone on top of the rim of the canyon. Scarborough (2001) interpreted the origin of the carbonate as –6 groundwater rather than pedogenic. The isotopic chemistry of the limestone Crooked Ridge at Crooked Ridge and White Mesa is essentially identical with that of the car- –8 bonates at Cape Solitude. Perhaps most important, the White Mesa carbonates –16 –14 –12 –10 –8 –6 differ substantially from the carbonate geochemistry of the Colorado River. Re- δOPDB charge of the Colorado River is from high-elevation snowmelt in the southern Rocky Mountains. This implies that groundwater from runoff during deposi- Figure 10. Carbonate geochemistry of fossiliferous limestone in White Mesa alluvium at Crooked Ridge and southern White Mesa tion of the White Mesa alluvium had a distinct, relatively low elevation source (Fig. 2; Table 1, sites 10 and 11) compared with Colorado River water, that differed from snowmelt runoff of the Rocky Mountains. carbonates of Bidahochi Formation, and groundwater carbonate in bedrock at Cape Solitude (Fig. 1). PDB—Peedee belemnite. For a more detailed version of the figure with O and C data for WMA and DETRITAL ZIRCON PROVENANCE Cape Solitude, please visit http://dx.doi.org/10.1130/GES01124.S2 or the full-text article on www.gsapubs.org. Here we use detrital zircon data to explore the provenance of early Pleisto cene and Neogene sedimentary deposits in the study region (Supplemental File 22). Zircon data from the White Mesa alluvium, the fluvial member of both Bidahochi samples were collected near Chambers, Arizona (Fig. 1) in the the Bidahochi Formation, and the largely eolian Chuska Sandstone (Oligo Rio Puerco paleovalley (Dickinson, 2013), and the modern Rio Puerco enters cene) were analyzed and compared with zircons of the modern San Juan the Little Colorado River upstream of the Little Colorado sampling sites near and Little Colorado Rivers (Fig. 1). The samples were analyzed using stan- Winslow and Cameron (Kimbrough et al., 2015). Five Bidahochi Formation dard techniques (described online at AZ Laserchron Publication Tools, www grains (4% of the total) are younger than 84 Ma, whereas the modern Little .laserchron.org). Colorado contains no grains that young. This difference evidently did not Five detrital zircon samples of the White Mesa alluvium were collected at markedly influence K-S statistics, thereby showing the imprecision of K-S four localities (Fig. 2); four were from the base and one was from the upper analysis for subtle details of grain populations. one-third of the alluvium (Fig. 6). Different samples of White Mesa alluvium The detrital zircon age population of the Oligocene Chuska Sandstone 95 420 165 WMZ-NUP : 93 total grains 3 grains (3%) omitted 1160 yield variable detrital zircon populations that do not correlate closely with each (Dickinson et al., 2010) differs significantly by K-S analysis (P = 0) from those 615 other (Table 2; Fig. 11). Nevertheless, 14 of 20 sample pairs (70%) yield P (prob- of both Miocene or Pliocene and modern assemblages of the southern Colo- 25 ability) values from Kolmogorov-Smirnov (K-S) analysis of >0.05, implying rado Plateau (Table 3), which suggest to us that Chuska sources made only a with >95% confidence that zircon in each sample pair could have been selected subordinate contribution to White Mesa alluvium. The only grains younger

RP-AZ-3 : 96 total grains 5 grains (5%) omitted at random from the same parent population. than 75 Ma in any Chuska Sandstone samples are 6 grains dating to 28–26 Ma The provenances of young sedimentary assemblages of the southern (weighted mean 27 ± 1 Ma at 2s) from uppermost Chuska at Roof Butte in the

170 95 225 Colorado Plateau (Table 3; Fig. 12) are not closely related (P = 0.000–0.007). northern Chuska Mountains (Fig. 1). The grains represent 6% of the detrital 1430 1700 1065 Notably, the provenance of the White Mesa alluvium is not closely related zircons at Roof Butte, but only 1.5% of the total Chuska detrital zircon popula- 0 250 500 750 1000 1250 1500 1750 2000 (P = 0.007) to the Bidahochi Formation and modern Little Colorado River. An tion. A cluster of 4 detrital zircons in the White Mesa alluvium date to 29–27 Ma exception is the close pairing of detrital zircon populations in the modern (weighted mean 28 ± 2 Ma at 2s); however, they occur jointly with 9 younger 2Supplemental File 2. NRP Plot. Please visit http://dx .doi.org /10 .1130/GES01124 .S3 or the full-text article Little Colorado River with late Miocene (ca. 6 Ma or younger) fluvial strata detrital zircons and an older cluster of 5 detrital zircons at 36–31 Ma (weighted on www.gsapubs.org to view Supplemental File 2. of the Bidahochi Formation (P = 0.69). This relationship is expected because mean 33 ± 3 Ma at 2s).

TABLE 2. PROBABILITY VALUES FROM STATISTICAL KOLMOGOROV-SMIRNOFF ANALYSIS OF U-Pb AGES FOR DETRITAL ZIRCON POPULATIONS IN FIVE SANDSTONE SAMPLES FROM THE WHITE MESA ALLUVIUM WMZ-NUP RP-AZ-3 RP-AZ-2 RP-AZ-1 RP-AZ-4 P values from K-S analysis with errors in the cumulative distribution function WMZ-NUP – 0.0850.073 0.004 0.075 RP-AZ-3 0.085 – 0.1050.181 0.135 RP-AZ-2 0.073 0.105 –0.003 0.020 RP-AZ-1 0.004 0.181 0.003–0.937 RP-AZ-4 0.075 0.135 0.020 0.937 – P values from K-S analysis without errors in the cumulative distribution function WMZ-NUP – 0.0800.060 0.0020.047 RP-AZ-3 0.080 – 0.1100.070 0.084 RP-AZ-2 0.060 0.110 –0.003 0.020 RP-AZ-1 0.002 0.070 0.003–0.766 RP-AZ-4 0.047 0.084 0.020 0.766 – Note: K-S—Kolmogorov-Smirnoff; P—probability. See text Figure 11. Samples are listed from upstream to downstream except that RP-AZ-1 and RP-AZ-2 are in the same stratigraphic section. Where P > 0.05 (bold numbers) there is <95% conﬁdence that 2 zircon age populations were not derived by random grain selection from the same parent population. Dash indicates 1.0.

95 420 165 WMZ-NUP : 93 total grains 3 grains (3%) omitted 1160 615

RP-AZ-3 : 96 total grains 5 grains (5%) omitted

Figure 11. Normative probability plots of 170 95 225 U-Pb ages for detrital zircon populations 1430 1700 1065 in five samples of White Mesa alluvium (Fig. 2) arranged from upstream (top) to downstream (bottom). Prominent age 170 95 peaks are labeled to the nearest 5 m.y. and minor grains older than 2000 Ma (5% of 25 RP-AZ-2 : 101 total grains 2 grains (2%) omitted total) in U-Pb age are omitted from plots. 260 430 1700 N—number of samples; n—total number of detrital zircon grains. (For data, see

630 1075 Supplemental File 2.)

25 95 220 1720 RP-AZ-1 : 100 total grains 7 grains (7%) omitted

1040

25 220 165 RP-AZ-4 : 92 total grains 410 9 grains (9%) omitted) 1710

1015 625 1440

0250 500750 1000 1250 1500 1750 2000 Ma

TABLE 3. PROBABILITY VALUES FROM STATISTICAL KOLMOGOROV-SMIRNOFF ANALYSIS OF U-Pb AGES FOR DETRITAL ZIRCON POPULATIONS IN SAND AND SANDSTONE SAMPLES San Juan WMAChuska BidahochiLittle Colorado P values from K-S analysis with errors in the cumulative distribution function San Juan –0.005 0 0.075 0 WM-CR 0.005 –00.0410.036 Chuska 00 –0 0 Bidahochi 0.075 0.0410 – 0.686 Little Colorado0 0.03600.686 – P values from K-S analysis without errors in the cumulative distribution function San Juan –0.002 00.036 0 WM-CR 0.002 –00.0320.027 Chuska 00 –0 0 Bidahochi 0.036 0.0320 – 0.531 Little Colorado0 0.02700.531 – Note: K-S—Kolmogorov-Smirnoff; P—probability. See text Figure 12. Samples are from 5 Cenozoic depositional suites of northern Arizona and southern Utah, including the White Mesa alluvium (WMA); where P > 0.05 (bold numbers) there is <95% conﬁdence that 2 zircon age populations were not derived by random grain selection from the same parent population. WM-CR—White Mesa–Crooked Ridge. Dash indicates 1.0.

65 160 modern San Juan River : N=2 ; n=175 6 grains (5%) omitted from plot 1720 225

455 1430

25 95 170 White Mesa alluvium : N=5 ; n=481 27 grains (5%) omitted from plot 1715 220 425

1060 1445 615

1695 Figure 12. Normative probability plots of U-Pb ages for detrital zircon populations

25 160 Chuska Sandstone : N=4 ; n=402 in two samples of modern rivers and three 11 grains (3%) omitted from plot 1420 samples of Oligocene–Miocene sedimentary strata of southern Colorado Plateau (Fig. 1). Data for San Juan and Little Colo

30 rado Rivers and Bidahochi Formation are 165 225 in Kimbrough et al. (2015); Chuska Sand- stone data are in Dickinson et al. (2014). Bidahochi Formation : N=2 ; n=117 3 grains (3%) omitted from plot Explanation as in Figure 11 (4% of grains 1675 older than 2000 Ma omitted from plots). 1425 N—number of samples; n—total number

220 of detrital zircon grains. Note the lack of 35–23 Ma grains from San Juan Moun- tains volcanic field in San Juan River plot.

85 modern Little Colorado River : N=2 : n=123 6 grains (5%) omitted from plot 170

1075 1700 585 1435

0250 500750 1000 1250 1500 1750 2000 Ma

K-S congruence for the White Mesa alluvium and the modern San Juan age of the White Mesa alluvium using dated tuff and detrital sanidine. The mid- River (Table 3) indicates that the two populations are statistically different (P = Pleistocene minimum age of the alluvium is inferred from an inset geomorphic 0.005). Notable in the modern San Juan is the lack of any grains younger than relation between the White Mesa paleovalley and a younger, lower surface 35 Ma derived from the San Juan Mountains volcanic field. Several other dif- associated with a tuff of known age. The geochronology of Oligocene–Mio- ferences are also interesting: a prominent San Juan River age peak ca. 65 Ma cene reworked ash-fall in the ca. 2 Ma White Mesa alluvium was investigated that is nearly absent from the White Mesa alluvium curve; a prominent age and linked to several calderas of similar age. peak ca. 95 Ma that is much reduced for the San Juan River curve; a composite 175–145 Ma age peak on the San Juan River curve (crest at 160 Ma) that is rep- Maximum Early Pleistocene Age of the White Mesa Alluvium resented by only a narrow 170 Ma peak on the alluvium curve; and a Grenville peak (ca. 1060 Ma) in the alluvium more prominent than and displaced in age Three independent dates establish the maximum early Pleistocene age from San Juan River Grenville peaks (Fig. 11). of the alluvium. One date is from tuff interbedded with basal White Mesa Age peaks reflecting southwest Laurentian basement are present in all alluvium on the Moenkopi Plateau and the others are detrital sanidine in the southern plateau Cenozoic assemblages (Fig. 12). Yavapai (1.8–1.7 Ga) and basal alluvium at Crooked Ridge and southern White Mesa. The tuff, herein Mazatzal (1.7–1.6 Ga) province rocks are represented by the 1720–1675 Ma referred to as the Blue Point tuff, is 10–15 cm thick, friable, and 2 m above the zircon, and 1445–1425 Ma grains are from younger anorogenic plutons intru- base of an ~3-m-thick gravel that underlies 17 m of well-stratified alluvium sive into both provinces. These grains are not diagnostic of provenance, as (Figs. 1 and 3B; Table 1, site 19). Although Blue Point is 50 km south of south- Colorado Plateau basement was exposed during the Cenozoic in the Mogollon ernmost White Mesa, the grain size, color, and bedding characteristics of the highlands to the south and parts of the Colorado Rockies to the north, and alluvium on Blue Point resemble the White Mesa alluvium. Geomorphically, reworking of those detrital zircons from Phanerozoic strata of the Colorado the deposits on the Moenkopi Plateau are inverted, and the landscape po- Plateau is likely. Oligocene (30–25 Ma) age peaks in the Neogene and Quater- sition of the deposits is similar to the White Mesa paleovalley (both were nary sedimentary formations may derive initially from the volcanic fields of the mapped as the same geomorphic surface by Cooley et al., 1969). Based on San Juan Mountains to the northeast or secondarily from the Mogollon-Datil these sedimentologic and geomorphic characteristics, the alluvium on Blue field to the southeast; however, low precision of the detrital zircon data do not Point is interpreted to correlate with the White Mesa alluvium. Individual allow quantitative distinction between these volcanic fields and perhaps those sanidine dates in the tuff range from 2.7 to 1.9 Ma. Of 49 fused grains, 41 of the Basin and Range. have normally distributed dates with a mean of 1.993 ± 0.002 Ma (Fig. 13A; Detrital zircon U-Pb ages loosely constrain the maximum age of the Supplemental File 33). This date is considered the eruption age of the Blue White Mesa alluvium to no older than early to mid-Miocene, i.e., after ca. Point tuff and a direct age of the basal White Mesa alluvium on the Moen- 20–15 Ma. The youngest single detrital zircon grain from the White Mesa kopi Plateau. alluvium has an age of 15 ± 4 Ma (see Supplemental File 2). The young- Detrital sanidine was extracted from bulk samples previously collected for est 4 grains that overlap in age at 1s have a weighted mean age of 19 ± detrital zircon analysis at Crooked Ridge and the Highway 21 quarry in south- 2 at 2s (95% confidence); the youngest 12 grains that overlap in age at s2 ern White Mesa paleovalley, which are 37 km apart (Figs. 2 and 6; Table 1, sites (where 2s ≤ 5 Ma) have a weighted mean age of 23 ± 1 at 2s. Because no 2 and 16). Approximately 100 grains from each sample were dated; the individ- strata can contain detrital zircon younger than its depositional age, these ual dates range from ca. 1.179 to 1700 Ma. During the mineral separation pro- relations indicate robustly that the White Mesa alluvium is no older than cess, recovery of volcanic sanidine from plutonic or metamorphic K-feldspar

40Ar39Ar data for sample MP6. ca. 25 Ma, probably no older than ca. 20 Ma, and possibly younger than ca. is maximized by choosing optically clear K-feldspars, which enhances identi-

40 39 38 39 37 39 36 39 39 40 ID Ar/ Ar Ar/ Ar Ar/ Ar Ar/ Ar ArK K/Ca Ar* Age ±1 (x 10-3) (x 10-15 mol) (%) (Ma) (Ma) 15 Ma. From this and considering the age of the two youngest grains (15 fication of age populations that may closely approximate depositional ages 60 4.4910.01430.00544.701 1.19994.769.02.016 0.009 22 3.4130.01490.00221.032 0.263231.4 91.0 2.0200.028 31 3.3950.01850.00320.91340.302 160.392.02.031 0.024 x053.388 0.0198 -0.0048 0.7685 0.271 -93.32.054 0.028 and 19 Ma), detrital zircon data restricts the age of the alluvium to younger (i.e., Heizler et al., 2013). Optically clear crystals older than ca. 500 Ma are most x104.851 0.0164 0.0098 5.6290.295 52.3 65.7 2.0730.027 x1810.78 0.0187 0.0056 25.650.329 91.3 29.7 2.0830.033 x174.451 0.0103 0.0322 4.2260.236 15.8 72.0 2.0830.033 than 20–15 Ma. This is in contrast to the pre–early Miocene assignment of likely basement grains (microcline and orthoclase) rather than sanidine; how- x113.494 0.0151 0.0125 0.9752 0.59141.091.82.084 0.013 x164.149 0.0181 0.0076 3.1830.709 66.9 77.3 2.0860.012 x526.899 0.0167 0.0046 12.081.229 111.148.22.164 0.011 x037.938 0.0173 0.0040 12.570.487 127.953.22.746 0.021 Lucchitta et al. (2011, 2013). ever, the majority of grains younger than 250 Ma are probably sanidine. These Mean age ± 1 n=41 MSWD=1.171.993 0.002

Notes: Mesozoic grains are likely recycled from bedrock units such as the Chinle For- Isotopic ratios corrected for blank, radioactive decay, and mass discrimination, not corrected for interfering reactions. Errors quoted for individual analyses include analytical error only, without interfering reaction or J uncertainties. Mean age is weighted mean age of Taylor (1982). Mean age error is weighted error mation and Entrada and Dakota Sandstones. of the mean (Taylor, 1982), multiplied by the root of the MSWD where MSWD>1, and also incorporates uncertainty in J factors and irradiation correction uncertainties. Isotopic abundances after Steiger and Jäger (1977). 40 39 x symbol preceding sample ID denotes analyses excluded from mean age calculations. TUFF AND DETRITAL SANIDINE Ar/ Ar GEOCHRONOLOGY The basal coarse sand and gravel unit was sampled at both localities. Sam- Ages calculated relative to FC-2 Fish Canyon Tuff sanidine interlaboratory standard at 28.201 Ma Decay Constant (LambdaK (total)) = 5.543e-10/a Correction factors: 39 37 ( Ar/ Ar)Ca = 0.00066 ± 1e-05 ple contamination by loose sediment was eliminated by horizontal and vertical 36 37 ( Ar/ Ar)Ca = 0.000264 ± 1e-06 38 39 ( Ar/ Ar)K = 0.013 40 39 40 39 2 ( Ar/ Ar)K = 0.0076 ± 0.0001 Dating detrital sanidine grains using the Ar/ Ar method offers an impor excavation with a hand shovel, forming fresh exposures of 0.5–1 m . Crooked tant complement to detrital zircon geochronologies, primarily because Ridge exposes an essentially complete section of the alluvium (Figs. 6, 7A, 3Supplemental File 3. 40Ar/39Ar data. Please visit http:// 40 39 dx.doi .org /10 .1130/GES01124 .S4 or the full-text article Ar/ Ar sanidine ages are very precise, typically having analytical errors of and 7B), and the sample was collected <1 m above the underlying Navajo on www.gsapubs.org to view Supplemental File 3. tens of thousands of years for Cenozoic grains. We define the early Pleistocene Sandstone. Only the basal gravel unit, however, is present at the Highway 21

150 250

A B a 100 150 K/C 50 50 60 20 40 N 20 10 Figure 13. Tuff and detrital sanidine 40Ar/39Ar geochronology. (A) Blue Point 0 0 tuff sanidine defines ash deposition at Blue Point tu sanidine 1.993 ± 0.002 Ma (n—number). Minor 1.993±0.002 Ma scatters of older dates are likely inherited MSWD = 1.17, n = 41 grains, included during postdepositional uff Detrital sanidine youngest grains T reworking at source, during eruption, or Highway quarry (PR-AZ-3) both. Youngest detrital sanidine grains 1.84 ± 0.05 Ma eap or from Crooked Ridge (PR-AZ-4) correspond MSWD = 0.78, n = 2 in age to Blue Point tuff, whereas Highway Crooked Ridge (PR-AZ-4) 21 quarry (PR-AZ-3) has two grains that define an apparent younger depositional age

2.02 ± 0.02 Ma each Springs MSWD = 0.31, n = 2 Apache L P of 1.84 Ma. (B) Detrital sanidine and zircon y spectra, 20–0 Ma. Many sanidine grains define multiple age populations; most are between 2.0 and 1.8 Ma, 10 and 9 Ma, and 18.7 Ma. Only a single zircon ca. 16 Ma is found in this age range. (C) Detrital sani- 1.7 1.9 2.1 2.3 2.5 2.7 0 2 4 6 8 10 12 14 16 18 20 dine and zircon spectra, 40–20 Ma. Dozens 250 150 of sanidine grains in this age range have many distinct age peaks compared with C 150 D 100 K/Ca minor zircon age peaks. (D) Detrital sani 50 50 dine and zircon data, 32–27 Ma. Multiple Relative probabilit 150 high-precision discrete age peaks for sani 100 100 dine data contrast with lower precision N zircon data that do not distinguish specific 50 50 sources. Note the distinct peak similar to 0 0 Fish Canyon sanidine at 28.2 Ma, volumetri- cally one of the largest eruptions of the San Fish EXPLANATION Juan Mountains volcanic field. N = num- Canyon Sani– Highway quarryb (PR-AZ-3) ber of grains. K/Ca is determined from K- Tuff dine Crooked Ridge (PR-AZ-4) derived 39Ar and Ca-derived 37Ar; MSWD— Zircon Highway quarry (PR-AZ-3) mean square of weighted deviates.

20 22 24 26 28 30 32 34 36 38 40 27 28 29 30 31 32 Age (Ma)

quarry, although complete stratigraphic sections are present within 1–1.5 km. indicating minimal transport to substantial abrasion during transport in the The lithology and elevation of the basal gravel at the quarry are similar to White Mesa fluvial system. Frosting of relatively soft sanidine may indicate those of nearby complete stratigraphic sections. The quarry sample was col- eolian transport. lected 3.1 m above the underlying Entrada Sandstone. Dated detrital grains in Both samples yielded at least 3 detrital grains that are younger than 2.7 Ma. the two samples varied from euhedral to highly rounded and frosted, probably Two from each set yield dates of 1.84 ± 0.05 and 2.02 ± 0.02 Ma (Fig. 13A)

at the Highway 21 quarry and Crooked Ridge, respectively. These maximum Oligocene–Miocene volcanic sanidines are present as detritus in the ca. 2 Ma depositional ages are statistically different; the younger age is from slightly alluvium and its paleodrainage. Details of the detrital sanidine and zircon dis- higher in the stratigraphic section, which possibly relates to sedimentation tributions are plotted at various time spans in Figures 13B–13D to demonstrate rate. The eruptive source of the tuff and detrital sanidine is unknown; however, temporal distinctions. Figure 13B is the sanidine age distribution between 20 they are younger than the regionally extensive Huckleberry Ridge eruption, and 0 Ma, which reveals a rich record of Miocene–Pleistocene volcanic sources with a tightly clustered age of 2.1 Ma (Ellis et al., 2012; Singer et al., 2014). represented by one detrital zircon grain in that age range. For example, possi- Our conclusion from the combined direct dating of an interbedded tuff and ble sources for the 18.7 Ma population are the Apache Leap Tuff or the Peach dated detrital sanidine is that the basal White Mesa alluvium was deposited Spring Tuff located near Phoenix and Kingman, Arizona, respectively (cf. Fer- at 1.993 ± 0.002 Ma (i.e., ca. 2 Ma), consistent with a detrital grain maximum guson et al., 2013). The 40–20 Ma spectra (Fig. 13C) show that ~100 sanidine depositional age of 2.02 ± 0.02. Slightly higher parts of the alluvium are grains are in this age range. A dense clustering of ~38 grains is between 31 and younger than 1.84 ± 0.05. 27 Ma. The ages are not precise enough to unequivocally distinguish between individual San Juan or Mogollon-Datil caldera eruptions that are both possi- Minimum Age of the White Mesa Alluvium ble source regions, but mean values strongly indicate a San Juan Mountains source. The most prominent peak in both samples is 28.20 ± 0.03 Ma, which A mid-Pleistocene minimum age of the alluvium and paleovalley was es- is clearly sanidine from the Fish Canyon Tuff. This age is nearly identical to tablished using inset geomorphic relations between the Bishop Tuff–Glass the Bloodgood Canyon Tuff of the Mogollon-Datil field (McIntosh et al., 1992). Mountain tuff at Blue Canyon (Figs. 1 and 3B; Table 1, site 17; Geib and Spurr, However, the K/Ca ratio of ~70 is diagnostic of the Fish Canyon Tuff, whereas 2002) and the Lava Creek B ash on northeast White Mesa (Table 1, site 18). the ratio for Bloodgood Canyon Tuff sanidine is ~20. The K/Ca ratio demon- Northeast of Blue Point and inset 130 m below the base of the 1.99 Ma tuff, the strates a powerful fingerprinting aspect of detrital sanidine geochronology 0.8–1.2 Ma Bishop Tuff–Glass Mountain tuff at Blue Canyon is interbedded with that can link detrital grains directly to caldera sources. 5–7 m of gravel that is 120 m above Moenkopi Wash. This gravel and related The terrain of the ca. 2 Ma White Mesa paleodrainage, as previously doc- surface forms the L2B erosional surface mapped by Cooley et al. (1969). They umented, was Cretaceous strata overlain by younger deposits and lag sur- mapped the surface downstream along Moenkopi Wash and upstream along faces containing gravel clasts composed of rare Oligocene volcanics of San Begashibito Wash to the drainage divide between the Little Colorado and Colo- Juan Mountains origin. However, detrital sanidine geochronology indicates rado Rivers, where the L2B surface is inset ~100 m below the southeast-facing that multiple age Oligocene–Miocene ash-fall deposits were also present in side of White Mesa (Fig. 2). This inset demonstrates that the Bishop Tuff–Glass the ca. 2 Ma White Mesa paleodrainage (Figs. 13B, 13C). Our interpretation Mountain tuff postdate the erosion and inversion of the east side of the mesa. is that over time sanidine, as ash fall and alluvial detritus, was deposited in Thus, the White Mesa alluvium is older than the Bishop Tuff–Glass Mountain one or more fluvial systems that substantially predated the White Mesa paleo tuff. On the northeast end of White Mesa near sample locality WMZ-NUP, the drainage. These earlier fluvial systems apparently contained Oligocene vol ca. 640 ka Lava Creek B ash disconformably overlies the White Mesa alluvium canic clasts comingled with essentially contemporaneous ash fall and detrital in a shallow swale-like feature on an erosional slope crosscutting the alluvium. sanidine from the San Juan volcanic field. It is unclear how these fluvial sys- This disconformable relationship and young age of the Lava Creek B ash are tems carried San Juan Mountains detritus across the present Chinle Valley and consistent with the inset geomorphic association between the Bishop Tuff– much of northeastern Arizona (Fig. 1). A lag gravel older than the White Mesa Glass Mountain tuff and the alluvium. alluvium on Black Mesa (Table 1, sites 13 and 14) was related to the Crooked Ridge paleoriver by Lucchitta et al. (2011, 2013), whereas Cooley et al. (1969) Detrital Sanidine Provenance considered the lag gravel to be an even older erosion surface. The record is fragmentary, and additional data are needed, but these lag gravels and asso- Sanidine is a common product of Cenozoic caldera eruptions related to the ciated carbonate-cemented feldspathic sandstone described herein could be Oligocene ignimbrite flare-up of the western United States (Lipman, 2007) and remnants of Oligocene–Miocene detritus that was later reworked into the ca. to younger Neogene and Quaternary eruptions. The caldera eruption timing is 2 Ma White Mesa alluvium. well known for the San Juan Mountains (Lipman, 2007; Lipman and McIntosh, 2008), Mogollon-Datil (McIntosh et al., 1992), and Marysvale volcanic fields (Best et al., 2013). Because of the precise age of single sanidine crystals and the DISCUSSION equally precise measure of caldera volcanism, it is possible to link individual detrital grains to contemporaneous caldera eruptions. The detrital sanidine ca. 2 Ma age of the White Mesa alluvium provides Based on the 2 previously described tuff samples, the detrital sanidine evidence for a much younger erosional history than was previously envisioned geochronology of the White Mesa alluvium indicates that reworked 40–20 Ma for this region (Hunt, 1969; Lucchitta et al., 2011, 2013). Moreover, the area of

regional denudation was possibly quite large, as suggested by paleodrainage These washes, including Moenkopi, could have extended northeast beyond reconstruction (Fig. 14). Reconstruction is possible by upslope extension of the present northeast-facing rim of Black Mesa. Beheaded valleys prominently the several paleochannels and downslope extension of the White Mesa paleo exposed on the rim of Black Mesa are evidence that the washes extended far- valley toward the Little Colorado River valley. Upslope extension of the paleo ther northeast (Schmidt, 1989). valley system into its ancient drainage basin is based on the elevation of the Five paleochannels converged on White Mesa paleovalley from the north, Dakota Sandstone as shown on the 6 previously cited 1:250,000-scale structure northeast, and south (Figs. 5 and 14). The north paleochannel, dominated contour maps. by clasts composed of Cretaceous sandstone, may have extended roughly Downstream of The Gap, blocked to the southwest by the Red Dot Hills north into the Navajo Mountain area, where a divide, perhaps near Navajo (Fig. 2), the paleodrainage turned south to south-southeast, following the Mountain, likely separated the paleochannel from the Colorado River. North- strike of the Echo Cliffs monocline 32 km to the end of the structure. Here it east paleochannel 1, apparently the largest of the five, and also dominated by probably joined the ancestral Moenkopi Wash near the base of the ca. 2 Ma clasts of Cretaceous sandstone and lacking exotic volcanic clasts, may have White Mesa alluvium present at the northwest end of the Moenkopi Plateau at extended far to the northeast along the 0.007 gradient of the paleovalley, prob- an elevation of ~1520 m, ~200 m above the wash (Fig. 1). The paleovalley likely ably ending at a drainage divide south of the San Juan River. A portion of continued 18 km south before joining the Little Colorado River at a structural the paleovalley system represented by northeast paleochannel 3 (perhaps in low near Cameron. Upstream of Cameron, the ancestral Little Colorado pos- combination with northeast paleochannel 2) probably extended east-northeast

sibly joined with ancient high-level versions of Dinnebito and Oraibi washes. of White Mesa where it intersected the Organ Rock monocline. Following a ? 0 50 100

upwarp km Straight t

Clis onumen

M ? San Juan Rive r

Mo . Canyon Navajo Mtn. num en nep 2 & 3 t UT CO Glen divide ? V mono Figure 14. Possible course and extent of 37° N a e e Skeleton lle y San AZ NM White Mesa paleodrainage (dashed blue nep 1 Ridg Drainag Juan lines). Shaded pattern is inferred former Comb zo np Mtns. rri extent of eroded Cretaceous terrain based Me Ca . ? piedmont on published previously cited structure u sa River White Mesa Mtns contour maps. Paleochannels: np and

Echo Platea sp—north and south paleochannel; nep Shonto . Chinle 1—northeast paleochannel 1; nep 2 and olorado mono C 3—northeast paleochannel 2 and 3. Head- Clis Rock ? waters region in Navajo Mountain area, Organ B l a c k sp M e s a Chus Monument Valley, and parts of Chinle Val- sh Va The Gap Wa ley removed during regional denudation ll

e ka beginning after 2 Ma. San Juan Moun-

y Moenkopi ash tains piedmont inferred from Cather et al. W Mtns ash (2008). Distribution of Bidahochi Forma- W . tion is mostly from Dickinson (2013). 36°

An Dinnebito ce Cameron Oraibi st r a l i L hoch Form i a at t io t id n le B C o lac l ustr o ine ra d volcan u o via R. ic l

112° 110° W

strike valley along the monocline, possibly at the Dakota Sandstone–Mancos CONCLUSIONS Shale contact, the paleovalley could have extended farther northeast over the present Monument Valley to a drainage divide with the San Juan River. Older The ca. 2 Ma White Mesa paleovalley, previously termed the Crooked Ridge lag gravels and deposits containing exotic volcanic clasts were present near River of late Eocene to early Miocene age (Lucchitta et al., 2011, 2013), extends the paleochannel course on Black Mesa. These ancient gravelliferous deposits 57 km northeast from The Gap to White Mesa in northeastern Arizona (Figs. may have been remnants of the Oligocene San Juan Mountains pediment, 1 and 2). Fine-grained alluvium, 20–50 m thick, is exposed at two localities, a belt of detrital and volcaniclastic deposits around the San Juan Mountains Crooked Ridge and White Mesa (Figs. 5 and 7). The alluvium is best exposed (Cather et al., 2008). We cannot exclude, however, that the exotics were origi- on White Mesa, which provides its name. The alluvium is informally referred nally deposited by a pre–2 Ma San Juan–like paleoriver. South paleochannel, to as the White Mesa alluvium, the geomorphic setting of which is the White the narrowest and probably shortest paleochannel (Fig. 9), drained the Black Mesa paleovalley within the White Mesa paleodrainage. Mesa area, which was a Cretaceous terrain overlain by Cenozoic lag gravel and The age of the alluvium and paleovalley has heretofore been poorly con- deposits containing late Paleogene to early Neogene volcanic clasts. strained. Detrital zircon suggests a maximum age of 20–15 Ma. This estimated The volume of Cretaceous strata removed south of the San Juan River and age was substantially reduced by detection and dating of detrital sanidine 40 39 southeast of the Colorado River is large. Although the former boundary of and tuff at the base of the alluvium. Applying the Ar/ Ar method to sani- the missing terrain is subject to substantial uncertainties, the boundary is con- dine from the base of the alluvium at three widely spaced locations yields an strained by the Colorado River on the northwest and the San Juan River to age of ca. 2 Ma, the age of the basal alluvium (Fig. 13A). Its minimum age is the north (Fig. 14). The elevations of structure contours on the eroded basal mid-Pleistocene, based on inset geomorphic relations between the Bishop Tuff and Glass Mountain tuff (0.8–1.2 Ma) at Blue Canyon (Figs. 1 and 3B) and the Cretaceous section within the former boundary are mostly well above pres- White Mesa alluvium. These ages confirm that the White Mesa alluvium and ent topography, except in the Black Mesa structural basin. Thus, the area of paleovalley are not related to the conceptual Crooked Ridge River of Lucchitta Cretaceous strata eroded after deposition of the White Mesa alluvium proba- et al. (2011, 2013). In addition, the young age and high landscape position of the bly exceeds 10,000 km2. Assuming a thickness of ~300 m (Dakota Sandstone paleodrainage document rapid denudation starting after 2 Ma that probably plus Mancos Shale), the volume of eroded Cretaceous rock was more than continues to the present. More than 10,000 km2 of the ancestral northern Little 3000 km3; the volume increases substantially if the ~465-m-thick upper Creta- Colorado River drainage basin was removed (Fig. 14). ceous Mesaverde Group was eroded. The total volume of sediment removed Fossil and sedimentologic evidence indicate that deposition of the White to reach the present elevation of the master river is even larger, as a thick sec- Mesa alluvium was in a relatively low energy suspended sediment aggradation of bedrock underlying the Dakota Sandstone was also eroded. tional channel system. This is consistent with the immature texture and lateral The divide separating the White Mesa paleodrainage from the San Juan discontinuity of gravel and the predominance of very fine grained sand and and Colorado Rivers was probably the edge of an abandoned, possibly deep clay that are characteristic of the alluvium (Fig. 7). A small, thin-walled gastro- canyon with steep sides (Fig. 14). Most of the paleocanyon is missing, as sug- pod (Gastropoda Family Lymnaeidae) present in limestone beds at Crooked gested by low elevations along both rivers. However, the Straight Cliffs, only Ridge and southern White Mesa inhabits slow-moving streams and marsh-like 6 km north of the Colorado River in Glen Canyon, is likely a remnant of the environments. Therefore, the alluvium was likely not deposited in a large, vig- right side (facing downstream) of the paleocanyon (Figs. 1 and 3A). The left orous braided river; rather, the deposits resemble those of washes currently side was probably between the Colorado River and the flanks of Navajo Moun- draining Black Mesa (Fig. 1) in grain size and stratification. tain. The top of the Straight Cliffs (Navajo Point) is 1 km above the 500 ka Cha The White Mesa alluvium was derived largely from upper Cretaceous Da- pediment (Garvin et al., 2005) and 1.25 km above the Colorado River. However, kota Sandstone and Mancos Shale. Gravel clasts in the alluvium are primarily evidence of the Colorado and San Juan Rivers on top of or adjacent to the Dakota Sandstone. Detrital zircon data suggest that sand-size sediment was Straight Cliffs such as alluvium and beveled surfaces (Hackman and Wyant, also derived from Cretaceous rocks (Figs. 11 and 12). Clay beds in the allu- 1973) is apparently lacking, precluding calculation of incision rates because vium have the distinctive color and platy character of the Mancos Shale. Exotic the depth of incision is poorly constrained. Rates to 830 m/m.y. were estimated gravel in the alluvium is composed of granite, quartzite, other metamorphics, for incision of Glen Canyon (Cook et al., 2009), although this rate may be in- and rare to absent volcanics originally derived from the San Juan Mountains flated by minimum ages derived from cosmogenic exposure dating (Darling of southwestern Colorado. Surface texture and shape of these clasts indicate et al., 2012). Nevertheless, the ca. 2 Ma age of the high-elevation White Mesa that most were reworked (Fig. 8), probably from older surficial deposits and paleodrainage suggests incision was rapid especially in areas of soft Mesozoic lag surfaces that overlaid Cretaceous bedrock in the White Mesa paleodrain- bedrock (Cook et al., 2009; Darling et al., 2013; Karlstrom et al., 2014). This rapid age. This source of far-traveled detritus was present during deposition of the incision coincided with voluminous bedrock erosion totaling several thousand alluvium in the ca. 2 Ma White Mesa paleodrainage, which was bound by the cubic kilometers of Cretaceous strata only in the White Mesa paleodrainage. San Juan and Colorado Rivers to the north and northwest, respectively.

The alluvium detrital sanidine geochronology confirms that multiple age National Science Foundation grants EAR-1119629 and EAR-1348007 from the Tectonics Program Oligocene–Miocene ash-fall deposits from the San Juan Mountains and else- (to Karlstrom). In particular we thank the Navajo Nation Minerals Department and Hopi Nation Cul- tural Preservation Office for permission to conduct geological research on Navajo Nation and Hopi where were preserved in what became the early Pleistocene White Mesa Nation territories. Anyone wishing to work on Navajo or Hopi Tribal Lands must obtain a permit. paleodrainage (Figs. 13C, 13D). These specifically include the voluminous 28.2 Ma Fish Canyon Tuff and other tuffs of the San Juan Mountains. This com- REFERENCES CITED ingling of fluvial and ash-fall volcanic detritus suggests that a fluvial system Best, M.G., Christiansen, E.H., and Grommé, C.S., 2013, Introduction: The 36–18 Ma southern much older than 2 Ma flowed across northeastern Arizona from the San Juan Great Basin, USA, ignimbrite province and flareup: Swarms of subduction-related supervol- Mountains volcanic field. These older deposits are cryptic, but at least one canoes: Geosphere, v. 9, p. 260–274, doi:10.1130 /GES00870 .1. fluvial system was abandoned after integration of the Colorado River system Billingsley, G.H., Stoffer, P.W., and Priest, S.S., 2012, Geologic map of the Tuba City 30’ × 60’ quad- at 6–5 Ma (Spencer et al., 2001; House et al., 2008). Regional denudation after rangle, Coconino County, northern Arizona: U.S. Geological Survey Scientific Investigations Map 3227, scale 1:50,000. 2 Ma removed the White Mesa paleodrainage north, northwest, and northeast Cather, S.M., Connell, S.D., Chamberlin, R.M., McIntosh, W.C., Jones, G.E., Potochnik, A.R., of White Mesa along with substantial evidence of prior fluvial systems. The Lucas, S.G., and Johnson, P.S., 2008, The Chuska erg: Paleogeomorphic and paleoclimatic only direct evidence of an older system is possibly lag gravel and thin feld implications of an Oligocene sand sea on the Colorado Plateau: Geological Society of Amer- ica Bulletin, v. 120, p. 13–33, doi:10.1130/B26081.1. spathic sandstone of poorly known provenance on high-level beveled surfaces Cook, K.L., Whipple, K.X., Heimsath, A.M., and Hanks, T.C., 2009, Rapid incision of the Colorado on top of Black Mesa (Fig. 1; Cooley et al., 1969). River in Glen Canyon—Insights from channel profiles, local incision rates, and modeling of The characteristics of the White Mesa alluvium and paleovalley described lithologic controls: Earth Surface Processes and Landforms, v. 34, p. 994–1010, doi:10.1002 /esp.1790. and interpreted herein are unlike those proposed for the Crooked Ridge River Cooley, M.E., 1960, Analysis of gravel in Glen-San Juan Canyon region, Utah and Arizona: Ari- of Lucchitta et al. (2011, 2013) or those of the Miocene paleoriver posited by zona Geological Society Digest, v. 3, p. 19–30. Cooley et al. (1969), Hunt (1969), and Stokes (1973). Specifically, the alluvium Cooley, M.E., Harshbarger, J.W., Akers, J.P., and Hardt, W.F., 1969, Regional hydrogeology of the Navajo and Hopi Indian reservations, Arizona, New Mexico, and Utah: U.S. Geological Survey and paleovalley are not contemporaneous with eruption of the 35–23 Ma San Professional Paper 521-A, 61 p. Juan Mountains volcanic field; rather, they are early Pleistocene (ca. 2 Ma). Cui, B., and Komar, P.D., 1984, Size measures and the ellipsoidal form of clastic sediment parti- Sedimentological and fossil evidence support a low-energy depositional en- cles: Journal of Sedimentary Petrology, v. 54, p. 783–797, doi:10 .1306 /212F84F9 -2B24 -11D7 -8648000102C1865D. vironment, not a vigorous braided river. Alluvium was mostly derived from Dallegge, T.A., Ort, M.H., and McIntosh, W.C., 2003, Mio-Pliocene chronostratigraphy, basin mor- a local source of Cretaceous bedrock that supplied sand-size sediment and phology and paleodrainage relations derived from the Bidahochi Formation, Hopi and Na- gravel composed of abundant Cretaceous sandstone. Instead of a primary San vajo Nations, northeastern Arizona: Mountain Geologist, v. 40, p. 55–82. Juan Mountains origin, exotic grains of detrital sanidine and pebble to small Darling, A.L., Karlstrom, K.E., Granger, D.E., Aslan, A., Kirby, E., Ouimet, W.B., Lazear, G.D., Coblentz, D.D., and Cole, R.D., 2012, New incision rates along the Colorado River system cobble volcanic clasts were reworked into the alluvium from the remnants of based on cosmogenic burial dating of terraces: Implications for regional controls on Quater- an inadequately identified, substantially older fluvial system. Groundwater- nary incision: Geosphere, v. 8, p. 1020–1041, doi:10.1130 /GES00724 .1. carbonate chemistry of the saturated floodplain indicates low-elevation runoff Dickinson, W.R., 2013, Rejection of the lake spillover model for initial incision of the Grand Can- yon, and discussion of alternatives: Geosphere, v. 9, p. 1–20, doi:10.1130 /GES00839 .1. rather than the snowmelt runoff of an imposing mountain range. Dickinson, W.R., Cathers, S.M., and Gehrels, G.H., 2010, Detrital zircon evidence for derivation We recommend that the term “Crooked Ridge River” (Lucchitta et al., 2011, of arkosic sand in the eolian Narbona Pass Member of the Eocene–Oligocene Chuska Sand- 2013) not be applied to the preserved sedimentary and geomorphic record of stone from Precambrian basement rocks in central Arizona, in Fassett, James E., et al., eds., Geology of the Four Corners country: New Mexico Geological Society Fall Field Conference early Pleistocene deposits on Crooked Ridge, White Mesa, and the Moenkopi Guidebook 61, p. 125–134. Plateau. The White Mesa alluvial system, as we interpret it, records the fluvial Dickinson, W.R., Karlstrom, K.E., Hanson, A.D., Gehrels, G.E., Pecha, M., Cather, S.M., and Kim- activity of a former tributary of the Little Colorado River that was comparable brough, D.L., 2014, Detrital zircon U-Pb evidence precludes paleo-Colorado sediment in the exposed Muddy Creek Formation of the Virgin River depression: Geosphere, v. 10, p. 1123– to ancient versions of washes currently draining Black Mesa. Dating of detrital 1138, doi:10.1130/GES01097.1. sanidine, not available previously, complements detrital zircon work, thereby Dillinger, J.K., 1990, Geologic and structure contour maps of the Gallup 30’ × 60’ quadrangle, transforming our understanding of the White Mesa alluvium and post–2 Ma McKinley County, New Mexico: U.S. Geological Survey Miscellaneous Investigations Series Map I-2009, scale 1:100,000. regional denudation of northeastern Arizona. Ellis, B.S., Mark, B.F., Pritchard, C.J., and Wolff, J.A., 2012, Temporal dissection of the Huckleberry Ridge Tuff using 40Ar/39Ar dating technique: Quaternary Geochronology, v. 9, p. 34–41, doi: 10.1016/j.quageo.2012.01.006. Ferguson, C.A., McIntosh, W.C., and Miller, C.F., 2013, Silver Creek caldera—The tectonically dis- ACKNOWLEDGMENTS membered source of the Peach Springs Tuff: Geology, v. 41, p. 3–6, doi:10.1130 /G33551 .1. Coauthor W.R. Dickinson died unexpectedly shortly before completion of this paper. His contribu- Garvin, C.D., Hanks, T.C., Finkel, R.C., and Heimsath, A.M., 2005, Episodic incision of the Colorado tions to this research were numerous and invaluable, and we sorely miss his cogent, pithy com- River in Glen Canyon, Utah: Earth Surface Processes and Landforms, v. 30, p. 973–984, doi: munications. Two anonymous reviewers and Keith A. Howard provided critical reviews that sub- 10.1002/esp.1257. stantially improved the content and readability of the manuscript. John Vogel helped us conduct Geib, P., and Spurr, K., 2002, Prehistory of the northern Kayenta Anasazi region: Archaeological a high-resolution global positioning system survey of the Blue Point area. Steven Semken took us excavations along the Navajo Mountain road: Window Rock, Arizona, Navajo Nation Archae- on several informative field trips in and around the study area. Analytical support was partly from ology Department Report 02–48, p. v.4.11–v.4.13.

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