U-Pb Ages of Detrital Zircons in Jurassic Eolian and Associated

Home , Entrada (Fringe), Glen Canyon Group, Group (stratigraphy), Navajo Sandstone, Nugget Sandstone, Redonda Formation

U-Pb ages of detrital zircons in Jurassic eolian and associated sandstones of the Colorado Plateau: Evidence for transcontinental dispersal and intraregional recycling of sediment

William R. Dickinson† George E. Gehrels Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA

ABSTRACT dispersed widely across southwest Laurentia from southern Arizona, and seven samples of by a transcontinental paleoriver system and fl uvial and marine sandstones closely associated U-Pb ages for 1655 individual detrital zir- paleowinds, which deposited extensive Juras- with erg deposits of the Glen Canyon and San con grains in 18 samples of eolian and asso- sic ergs, durable zircon grains were recycled Rafael Groups. We present here 1655 reliable ciated marine and fl uvial sandstones of the by multiple intraregional depositional sys- U-Pb ages (concordant or nearly so) for indi- Glen Canyon and San Rafael Groups from tems. Lower Jurassic fl uvial sand is locally vidual zircon grains from 18 samples. the Colorado Plateau and contiguous areas composed, however, of detritus derived from An evaluation of the enlarged database con- shed light on patterns of Jurassic sediment the nearby Cordilleran magmatic arc assem- fi rms transcontinental dispersal of sand to Juras- dispersal within Laurentia. Most detrital zir- blage and its Precambrian basement. sic ergs of the Colorado Plateau. Admixtures con grains in Jurassic eolianites were derived of grains derived from basement or magmatic ultimately from basement provinces older Keywords: Colorado Plateau, detrital zircon, arcs in southwestern North America are present, than 285 Ma in eastern and central Lauren- eolianite, Glen Canyon Group, provenance, San along with recycled eolian sand, in associated tia, rather than from rock assemblages of the Rafael Group. fl uvial units, and these combinations occur as nearby Cordilleran margin. The most promi- well in selected eolian units, including some nent peaks of constituent age populations INTRODUCTION either intercalated with arc volcanics or depos- at 420 Ma, 615 Ma, 1055 Ma, and 1160 Ma ited by westerly winds. Recycling of eolian refl ect derivation from Paleozoic, Neopro- Dickinson and Gehrels (2003) showed that sand by intraregional depositional systems can terozoic, and Grenvillian sources within typical Jurassic eolianites (eolian sandstones) be attributed to the durability of zircon grains in the Appalachian orogen or its sedimentary of Colorado Plateau ergs (sand seas) contain sedimentary environments. Nevertheless, docu- cover. Sediment was transported to a posi- age populations of detrital zircons derived from mentation that voluminous sand of the central tion upwind to the north of the Colorado essentially all Precambrian and Paleozoic grani- and eastern Laurentian provenance dominated Plateau by a transcontinental paleoriver sys- toid basement provinces of central and eastern Jurassic eolian deposits on the Colorado Plateau tem with headwaters in the central to south- Laurentia (Fig. 1), though a few Mesozoic zir- through a period of >40 m.y. indicates the per- ern Appalachian region, but subordinate con grains were derived from the nearby Cordil- sistence of an integrated system for transconti- non-Appalachian detritus was contributed leran orogen. Sand was transported across the nental sediment dispersal. by both northern and southern tributaries continent by a Jurassic paleoriver, which had Descriptions of sample localities (see Sup- during sediment transit across the continent. its headwaters in the Appalachian province, to plementary Text 1) and U-Pb age data for all Subordinate detrital zircons younger than fl uvial or deltaic plains lying north of the Colo- samples discussed in this paper are included 285 Ma in selected Middle to Upper Juras- rado Plateau. From temporary sediment storage in accompanying data repositories.1 The tabu- sic eolianites were derived from the Perm- there, sand was then blown southward to grow- lated analytical data (Supplementary Text 2) ian-Triassic East Mexico and the Mesozoic ing Colorado Plateau ergs by paleowinds well are supplemented by a concordia diagram for Cordilleran magmatic arcs. Lower Jurassic known from eolian cross-bedding. each sample and superimposed graphs of age- fl uvial sandstones typically contain a mixture Our preliminary study was restricted to just bin histograms and probability-density plots of detrital zircons redistributed from eolian three samples of Jurassic eolianite from a single (age-distribution curves) for grains in each sand and derived from the East Mexico arc, stratigraphic section near the center of the Colo- which lay up-current to the southeast. Zir- rado Plateau, and it was inadequate to gauge the 1GSA Data Repository item 2008251, ST1 (in cons in marine Curtis sandstone were largely possible variability of detrital zircons in eolian- Word) is a listing of sample localities, ST2 (in csv) reworked from underlying Entrada eolianite, ites of various ages exposed across the length and is U-Pb analytical data for all samples, ST3 (pdf) with minor contributions from the Jurassic breadth of the Colorado Plateau. For this study, is concordia diagrams and combined age-bin histo- backarc igneous assemblage of the Great we expanded analytical coverage to include grams and age-distribution curves (probability-density plots) for each sample, and ST4 (in Word) is a Basin. Once mature quartzose detritus was seven more samples of Lower to Upper Jurassic table of ages and references for Figure 9; available 3 2 eolianites distributed over >175 × 10 km of the at http://www.geosociety.org/pubs/ft2008.htm or by †E-mail: [email protected]. Colorado Plateau, correlative intra-arc eolianite request to [email protected].

GSA Bulletin; March/April 2009; v. 121; no. 3/4; p. 408–433; doi: 10.1130/B26406.1; 15 ﬁ gures; 3 tables; Data Repository item 2008251.

C al Ages of Belts in Ga 100° W edo nia Archean Cross-Hatched n ( 0.3 1000 km 6 - 0.4 8) 8) .4 - 0 6 .3 North (0 n itia nu In G r e e n l a n d 60° N

Da vis St rait clo sed

la A n a C 60° Slave Nain (>2.5) (>2.5) Ketilidian N Wopmay (1.8 - 1.9) (1.8 - 2.3)

) H u d s o n 5 . 2 > ( B a y e n a o s R d ) u .9 -H 1 - s - n 8 ra . e T (1 n r ) a 3 . e S u p e r i o r (> 2.5) 1 H -

0 . - Can 1 USA ( g n i 40° N m o an y e ) ok 5 W 8 en . 40° P (~1 e N l mid- l i Central Plains continent v (1.6 - 1.8) (1.35 - 1.5) n Appalachian e (0.36 - 0.76) Colorado r Plateau Amarillo - Wichita G (~0.525)

Yavapai - Mazatzal Continental ? ? re (1.6 - 1.8) lle tu slope vi ) Ouachita gins u USA en .3 ? ig S Gr - 1 orogen W Mex .0 ? ? Cordilleran (1 accretion ? and batholiths (<0.25) Suwanee Yúcatan - (0.54 - 0.68) Baja California Campeche (0.54 - 0.58) 20° N [0.40 - 0.43 in south] 20° N Gulf of Cenozoic California Chiapas overthrust closed Gulf of Mexico of East Mexico closed Antillean Oaxaquia magmatic arc arc 120° W (1.0 - 1.25) (0.23 - 0.29) [0.44 - 0.48 in south] 80° W

Figure 1. Location of Colorado Plateau in relation to Precambrian and Phanerozoic age belts of North Amer- ica with Davis Strait and Gulfs of California and Mexico closed (for pre–mid-Jurassic time), adapted after Ham et al. (1964), Hoffman (1988, 1989, 1990), Hatcher et al. (1989), Viele and Thomas (1989), Reed (1993), Ortega-Gutierrez et al. (1995), Van Schmus et al. (1996), Atekwana (1996), Steiner and Walker (1996), Torres et al. (1999), Lopez et al. (2001), Dickinson and Lawton (2001a), Iriondo et al. (2004), Talavera-Mendoza et al. (2005), and Barth and Wooden (2006). Abbreviations: Ala—Alaska; Can—Canada; Mex—Mexico.

Geological Society of America Bulletin, March/April 2009 409 Dickinson and Gehrels sample falling within the ranges of 0–4000 Ma, (operating at a wavelength of 193 nm) using a Interpreted ages are based on 206Pb/238U for 0–800 Ma, and 800–2400 Ma (Supplementary spot diameter of 35 µm. The ablated material grains younger than 1000 Ma and 206Pb/207Pb for Text 3). Preliminary U-Pb age data for three was carried in helium into the plasma source grains older than 1000 Ma. The division point of the eolianite samples (Jwnw—Wingate; of a GVI Isoprobe, which was equipped with for each sample is at a slightly different age near Jnnw—Navajo; Jenw—Entrada) from North a fl ight tube of suffi cient width that U, Th, 1000 Ma to avoid splitting up age clusters of Wash in Utah (Dickinson and Gehrels, 2003) and Pb isotopes could be measured simultane- grains. In any case, age uncertainties are inher- are superseded by data of improved precision ously. All measurements were made in static ently greatest near 1000 Ma (Gehrels, 2000). and accuracy obtained during the present study mode, using Faraday detectors with 10e11 Analyses that were >30% discordant (by (Gehrels et al., 2008). Preliminary data for other ohm resistors for 238U, 232Th, 208Pb, and 206Pb, comparison of 206Pb/238U and 206Pb/207Pb ages) samples were presented by Amar and Bren- a Faraday detector with a 10e12 ohm resis- or >5% reverse discordant were not consid- neman (2005), Brenneman and Amar (2005), tor for 207Pb, and an ion-counting channel for ered further (average of 92 grain ages retained Hurd and Schmidt (2005), Schmidt et al. (2005), 204Pb. Ion yields were ~1.0 mv per ppm. Each per sample). The resulting interpreted ages are Amar et al. (2006), Brenneman et al. (2006), analysis consisted of one 12-second integration shown for individual samples on superimposed and Dickinson et al. (2007). Comparative data on peaks with the laser off (for backgrounds), relative age-probability plots (from Ludwig, for the Navajo Sandstone in Zion National Park twelve 1-second integrations with the laser fi r- 2003) and age-bin histograms. For the latter, were reported by Rahl et al. (2003). ing, and a 30-second delay to purge the previ- best estimates of ages are assigned arbitrarily to ous sample and prepare for the next analysis. age bins of 20 m.y. each, starting at 0 Ma. The ANALYTICAL METHODS The ablation pit was ~12 µm in depth. age-probability plots incorporate each age and For each analysis, the errors in determining its uncertainty (for measurement error only) as Samples of sandstone were collected in plas- 206Pb/238U and 206Pb/204Pb resulted in a mea- a normal distribution and sum all ages from a tic buckets of fi ve-gallon capacity as 22–24 kg surement error of ~1%–2% (at 2σ level) in sample into a single curve. The resulting age- of rock chips <10 cm in largest dimension from the 206Pb/238U age. The errors in measurement probability plots derived from the probability selected areas or horizons of the outcrops at sam- of 206Pb/207Pb and 206Pb/204Pb also resulted in density function were made from an in-house pling localities. Zircon crystals were extracted ~1%–2% (at 2σ level) uncertainty in age for Excel program (available from www.geo.ari- from samples by traditional methods of crushing grains older than 1000 Ma, but errors were sub- zona.edu/alc) that normalizes each curve accord- and grinding, followed by separation with a Wil- stantially larger for younger grains due to the ing to the number of constituent analyses, such fl ey table, heavy liquids, and a Frantz magnetic low intensity of the 207Pb signal. For most analy- that each curve contains the same area, and then separator. Sample processing was designed to ses, the crossover in precision of 206Pb/238U and stacks the probability curves. retain all zircon grains in the fi nal heavy mineral 206Pb/207Pb ages occurs at ca. 1000 Ma. There is a temptation to regard the age-bin fraction. A large split of these grains (generally Common Pb correction was accomplished by histograms as raw data and the age-probability 1000–2000) was incorporated into a 1 in. epoxy using the measured 204Pb and assuming an ini- plots as derivative curves, but the reverse is the mount together with fragments of standard Sri tial Pb composition from Stacey and Kramers case. Age uncertainties for some individual Lanka zircon and SRM 610 trace-element glass. (1975) with uncertainties of 1.0 for 206Pb/204Pb grains are larger than the widths of the age bins The mounts were sanded down to a depth of ~20 and 0.3 for 207Pb/204Pb. Our measurement of for the histograms, which are accordingly selec- µm, polished, imaged, and cleaned prior to iso- 204Pb was unaffected by the presence of 204Hg tive plots of the analytical data. By contrast, topic analysis. because backgrounds were measured on peaks the age-probability plots take all age uncertain- For analysis by laser ablation, target zircons (thereby subtracting any background 204Hg and ties into account (Vermeesch, 2004) and are are preferably >35 µm (>0.035 mm) in diameter, 204Pb), and because very little Hg was present in accordingly here termed age-distribution curves or at least as coarse as very fi ne sand. Given the the argon gas (background 204Hg = ~300 counts because they display graphically all the analyti- high specifi c gravity of zircon (4.65), as com- per second). cal data. The value of the age-bin histograms is pared to quartz (2.65), hydraulically equivalent Interelement fractionation of Pb/U is gener- to provide a graphic impression of the numbers zircon is expected to be approximately one sand ally ~20%, whereas fractionation of Pb isotopes of grains associated with age peaks on the age- grade fi ner than accompanying quartz grains is generally ~2%. In-run analysis of fragments distribution curves and thereby allow age peaks, (Komar, 2007). Accordingly, we preferentially of a large Sri Lanka zircon crystal (generally no matter how sharp, associated with only one sought samples composed of medium quartzose every fi fth measurement) with known age of or two grain ages to be discounted relative to sand because detrital zircon grains might be too 564 ± 4 Ma (2σ error) was used to correct for age peaks, even if broad, associated with mul- small to occur in abundance with coarse quartz this fractionation. The uncertainty resulting tiple grain ages. Age-bin histograms are omitted sand, but we were also able to date zircon grains from the calibration correction was generally from age-distribution curves composited from from the two-thirds of our samples composed of 1%–2% (2σ) for both 206Pb/238U and 206Pb/207Pb multiple related samples. fi ne quartzose sand. ages. Concentrations of U and Th were cali- brated relative to the Sri Lanka zircon standard Statistical Comparisons Age Determination and SRM 610 trace-element glass, which contains ~460 ppm of each element. Although age populations of detrital zircons in U-Pb geochronology of ~100 individual zir- different samples can be compared by inspection con grains per sample was conducted by laser- Data Presentation of their respective age-distribution curves and ablation–multicollector inductively coupled age-bin histograms, it is diffi cult to gauge visu- plasma–mass spectrometry (LA-MC-ICP-MS) Full analytical data are reported in Supple- ally the degree of similarity or dissimilarity of at the Arizona LaserChron Center (Gehrels et mentary Text 2 (ST2; see footnote 1). Age any two age populations. We have found it useful al., 2006). The analyses involve ablation of zir- uncertainties (at 1σ) for individual grains in to apply Kolmogoroff-Smirnoff (K-S) statistics con with a New Wave DUV193 Excimer laser the data table include only measurement errors. (Press et al., 1986, p. 472–474) to intersample

410 Geological Society of America Bulletin, March/April 2009 U-Pb ages of detrital zircons in Colorado Plateau eolianites comparisons using an in-house Excel program If the permissive extent of Jurassic ergs is Glen Canyon Group (available from www.geo.arizona.edu/alc). The taken into account, exposures of eolianite con- K-S test mathematically compares two age dis- tiguous with outcrops on the Colorado Plateau Over the central Colorado Plateau, a distinc- tributions, taking the uncertainties of grain ages may represent just a relict fraction of eolian tive formational triad (eolian Wingate Sand- into account, to determine whether there is a sand that summed to a total volume approach- stone, fl uvial Kayenta Formation, eolian Navajo statistically signifi cant difference between the ing 250 × 103 km3 across southwest Laurentia Sandstone) is typical of the Glen Canyon Group two. The K-S test determines a P value, which as a whole. As the Jurassic ergs mantled sub- (Fig. 3). To the north, the fl uvial Kayenta Forma- assesses the probability that differences between stratum continuously throughout their extent, a tion (Miall, 1988) pinches out in the subsurface two age-distribution curves could be due to ran- superregional system for sediment dispersal was of the Uinta Basin (Fig. 2A), and the unbroken dom choice of grains during analysis. Where P > required to deliver the voluminous sand to the eolian succession farther north, along the south 0.05, one cannot be 95% confi dent that two age erg accumulations from outside the erg bound- fl ank of the Uinta Mountains, is equivalent to populations were not selected randomly from aries. Because the eolian sand is ~85% quartz the Nugget Sandstone of Wyoming (Johnson the same parent population. Where this criterion (see following), and quartz forms only a third or and Johnson, 1991). To the southwest (Fig. 2A), is met, we conclude that the two age populations less of typical granitic rocks by volume, perhaps eolian Wingate Sandstone intertongues with compared are statistically indistinguishable for 250 to 650 × 103 km3 of granitoid bedrock was fl uvial deposits of the Moenave Formation purposes of assessing provenance relations. eroded to produce the eolian sand in Colorado (Blakey, 1989, 1994; Peterson, 1994), and the Plateau and related Jurassic eolianites. Navajo Sandstone is termed the Aztec Sand- STRATIGRAPHIC CONTEXT The Glen Canyon Group is underlain by stone in exposures southwest of the Colorado Upper Triassic fl uvial strata of the Chinle For- Plateau (Stewart, 1980). The Springdale Sand- Jurassic erg accumulations of the Colorado mation or Group, and the San Rafael Group is stone Member (Fig. 3) was long regarded as the Plateau form much of the Glen Canyon (Fig. 2A) overlain by Upper Jurassic fl uvial strata of the uppermost member of the Moenave Formation and San Rafael (Fig. 2B) Groups, each of which Morrison Formation (Fig. 3). Glen Canyon and (Harshbarger et al., 1957; Blakey, 1989), but it contains erg deposits that were laterally continu- San Rafael ergs developed during progressive is now widely regarded as a lowermost member ous over >250 × 103 km2 of the Colorado Plateau northward drift of Laurentia across the global of the Kayenta Formation (Riggs and Blakey, and adjacent areas before Cenozoic erosion. The desert belt lying within paleolatitudes of 17°– 1993; Lucas et al., 2005; Lucas and Tanner, eolian deposits are assigned to multiple forma- 28° (Parrish and Peterson, 1988; Dickinson, 2006; Kirkland and Milner, 2006; Tanner and tions separated by thinner intervals of noneo- 2005). Underlying Chinle fl oodplains were Lucas, 2007). lian strata (Fig. 3). Lateral age relationships are deposited at lower paleolatitudes in the sub- inferred from sparse fossil localities dispersed tropical belt before transit of Laurentia through Basal Contact widely in the dominantly nonmarine strata. the desert belt (Dubiel, 1994), and overlying The eolian Wingate Sandstone is underlain Although exact calculations are impossible, the Morrison fl oodplains were deposited at higher by a red-bed interval, dominantly siltstone, net volume of eolian sand in the Jurassic ergs paleolatitudes within the global belt of westerly that was named the Rock Point Member of the exceeds 100 × 103 km3. winds (Turner and Peterson, 2004). Wingate Formation in Arizona (Harshbarger et That volume is a minimum because (1) an Paleowinds inferred from the orientation of al., 1957) and the Church Rock Member of the outlier (Fig. 2A) of the Glen Canyon Group eolian cross-bedding in the Jurassic erg depos- Chinle Formation in Utah (Stewart, 1957), with in the Great Basin to the west of the Colo- its blew southward (in modern coordinates) an arbitrary change in stratigraphic nomencla- rado Plateau suggests that much of the ground for the Glen Canyon Group (Fig. 2A) and for ture near the Arizona-Utah border (Stewart et between the Colorado Plateau and the Cordil- most of the San Rafael Group (Fig. 2B). The al., 1972; Blakey and Gubitosa, 1983; Dubiel, leran magmatic arc may once have been cov- observed paleowind directions are compatible 1989). The same stratigraphic interval in south- ered by Lower Jurassic eolian sand (Stewart, with prevailing wind patterns inferred from western Colorado forms the upper member of 1980); (2) the present limit of the San Rafael reconstructions of Jurassic paleogeography the Dolores Formation (Dubiel, 1989; Lucas et Group east of the Colorado Plateau (Fig. 2B) is and paleoclimate (Parrish and Peterson, 1988; al., 1997; Lucas and Heckert, 2005), and litho- erosional, and Middle Jurassic eolian sand may Golonka et al., 1994), but the plateau paleo- logically similar strata farther north have been have extended across an indeterminate width wind vectors (Fig. 2) are not dependent upon termed red siltstone, ocher siltstone, and upper of the Great Plains in the continental interior theoretical retrodictions. members of the Chinle Formation (Stewart et (Kocurek and Dott, 1983); and (3) quartzose The paleowind directions imply derivation of al., 1972; Dubiel, 1992). To resolve the nomen- dune sand intercalated with arc volcanics along the bulk of the eolian sand in the Jurassic ergs clatural dichotomy, the Rock Point–Church the inland fl ank of the Cordilleran magmatic from proximate sources lying north of the Colo- Rock interval has been termed the Rock Point arc in southeastern California and southern rado Plateau. Paleowinds that blew toward the Formation (Lucas and Hunt, 1992; Lucas and Arizona (Fig. 2B) occurs beyond a belt from east-northeast during deposition of the youngest Heckert, 2005), but that usage is not universal. which all correlative strata have been removed (Upper Jurassic) San Rafael erg deposits (Bluff The Rock Point Formation is widely exposed by erosion, and Jurassic ergs may once have Sandstone) of the southern Colorado Plateau eastward beyond the limit of Glen Canyon erg been continuous southward from the Colorado (Fig. 2B) imply either internal redistribution of deposition (Fig. 2A), and it locally interfi ngers Plateau until eolian sand transport was arrested erg sand or additional sources of sand, or both. upwind toward the northwest with the overlying by arc paleotopography (Busby-Spera, 1988). The persistence of eolian environments into Wingate Sandstone (Harshbarger et al., 1957). The inferred original extents of the Glen Can- Late Jurassic time on the southern Colorado The contact between the Chinle Group (or yon and San Rafael ergs are both comparable Plateau may refl ect a more southerly paleolati- Formation) and the Glen Canyon Group is to the extent (~500 × 103 km2) of the largest tude as Laurentia drifted north, or the formation commonly placed at the base of the Wingate modern erg, the Rub’ al Khali in the Empty of a rain shadow in the lee of rising Cordilleran Sandstone where desiccation mud cracks in Quarter of the Arabian Peninsula. highlands, or both effects. the tops of the underlying Rock Point Member

Geological Society of America Bulletin, March/April 2009 411

Dickinson and Gehrels

X K

T O 35° N of NM TX NM limits systems key fluvial depositional 105° W 105° W (only Rock Point-Church (only downwind assemblage downwind interval exposed Rock Sandstone) Entrada below extent of ergs eolian eolian paleowinds eolian fluvial paleocurrents fluvial Black Ledge samples cities Kayenta Moenave + Plateau lorado–New Mexico) is adapted after lorado–New Mexico) is adapted after Colorado A rmost Upper Jurassic San Rafael Group Jurassic San Rafael Group rmost Upper N 40°

Peterson (1972). Cities: A—Albuquerque; A—Albuquerque; Peterson (1972). Cities:

C W Middle Jurassic onlap or overlap G .) (Sundance-Entrada) + D xed to numerical sample numbers. D—sample DOL; N—North AZ NM

t +

W e

g 31 ° 110° W

g 0 erg

u 1 1

N not afﬁ x CP + 1 N overlap (Dakota) 10

? mid-Cretaceous + 3 ID WY ? +

t s

u r h t b e l t

v page Continued on following e

S 37 +

d n a l r e t n i h

n i

ID UT

l of edge

i erosional

r exposures

u P

O b

U S

t + W a 30

NV UT r t

° s

5 P O

1 R

C AZ CA

1 T U

O Dickinson (2004), retroarc after are (1993, 1996). Positions of magmatic arc Luttrell after are uvial paleocurrents

L O V N

I N r c erg Aztec erg outlier Currie V i ca N t A a C basin gm marine

backarc a m

R V an

O N l er i l r d

W o

° C 0 2 1 scale in km scale N 120° W

R A

O 0 100 200 300 C 35° N 40° N A Figure 2. Distribution and facies of (A) uppermost Triassic to Lower Jurassic Glen Canyon Group and (B) Middle Jurassic to lowe Jurassic Glen Canyon Group to Lower Triassic 2. Distribution and facies of (A) uppermost Figure preﬁ to sample localities (crosses): the Colorado Plateau (shaded outline) in relation within and near Arizona–Utah–Co Corners (conjunction of Four (Bluff) erg near Jenw). Subsurface limit of post-Todilto Jnnw, samples (Jwnw, Wash Peterson (1988b), and ﬂ after Lupe (1983). Paleowinds are Luning-Fencemaker thrust belt is after Wyld (2002), backarc magmatism is after Dickinson (2006), and Utah-Idaho trough is after Dickinson (2006), and Utah-Idaho trough magmatism is after (2002), backarc Wyld thrust belt is after Luning-Fencemaker F—Flagstaff; G—Grand Junction; L—Las Vegas; P—Phoenix; S—Salt Lake City. ( P—Phoenix; S—Salt Lake City. Vegas; F—Flagstaff; G—Grand Junction; L—Las

412 Geological Society of America Bulletin, March/April 2009 U-Pb ages of detrital zircons in Colorado Plateau eolianites TX OK N 35° NM TX NM 105° W 105° W eolian ergs (and related) CO NM extent of Entrada Jurassic backarc plutons lateral transition from Bluff paleowinds Bluff paleowinds Page-Entrada slickrock (erg) to earthy samples cities limit of Curtis marine facies of Entrada Sandstone salina above Entrada erg limit of Todilto nonmarine Todilto limit of * + transgression over Entrada erg N 40° A CO WY 54 + Plateau 24 Colorado + G AZ NM AZ 15 16 + + 110° W 110° N + overlap (Dakota) + 45 arc-flank 43 ). dune sands + mid-Cretaceous + 2 12 + S continued P ID UT edge of Figure 2 ( Figure erosional AZ UT exposures * * * * * * trough * NV UT * * * Utah - Idaho * (170 - 160 Ma) CA AZ CA * * L * 115° W 115° * * *

* V N * (165-145 Ma) A r c * C a * backarc magmatism Luning- c thrust belt i * * Fencemaker * t * a (175 - 165 Ma) * *

* m g

* a m * n r a l e i l r d o

basin C backarc ergs limits of subordinate CA NV CA N Moab Tongue Page Temple Cap scale in km limit of post-Todilto Bluff erg Bluff limit of post-Todilto 40° N 0 100 200 35° N 120° W B

Geological Society of America Bulletin, March/April 2009 413 Dickinson and Gehrels

n 150 m 100 m 50 m 0

o Bluff

t scale

s F area

d Sandstone Entrada

n Morrison —Temple Cap Sandstone; —Temple Sandstone

a Formation

s CP24 CP54 Gallup-Grants (strafal onlap) —Nevada; UT—Utah.

n k Point–Church Rock interval k Point–Church

r (noneolian) (erg eolianite) Lithologic Key

a CO NM marine) deposits eolian sandstone marine (and local

m marine sandstone fine-grained (shaly) nonmarine-marginal TF F nonmarine - marginal E D Group Group Jurassic UT CO UT AZ UT c Glen Canyon and San Rafael Groups (Colo- c Glen Canyon and San Rafael Groups C lacustrine salina hic correlations (dashed lines) are inferred from from inferred (dashed lines) are hic correlations B E Bluff Chinle Navajo Wingate Entrada Kayenta RP/CR Morrison San Juan Fm or Gp Formation Sandstone Sandstone River area Formation Sandstone

Sandstone

CP15

DOL T U A

N fluvial sandstone

n l i

s o e

s y

p f

a n

p a

U R Jurassic

C BL J Map Key MT D area Chinle Navajo RP/CR Entrada Kayenta Wingate Morrison Fm or Gp Sandstone Formation Formation Sandstone Sandstone fluvial strata CP16 Moab-Arches CP31 base of post-erg

r r f e o e

an en l w w

o o G

L L CF fluvial strata top of pre-erg CP3 (off map) CP45 C area Navajo Chinle Kayenta RP/CR Carmel Entrada Page Ss Wingate Formation Morrison Fm or Gp Sandstone Formation Formation Formation Hanksville Sandstone Sandstone CP1 Jwnw Summerville Jenw CP43 Jnnw

e f s f

o f a o b o o area B Page Chinle Navajo Carmel Kayenta Entrada Morrison Moenave

Formation Fm or Gp Formation Formation Formation CP10 Sandstone

Sandstone Sandstone Lake Powell CP12

a l

r RS

e e

v s e o

l a

p p

b o a a o

r t t b t SSM s

o A

k ("silty Carmel

a Navajo facies") Chinle Kayenta (-Aztec) Zion area Moenave Formation Formation Formation

D CP30 Fm or Gp Sandstone CP37 (Cretaceous) Figure 3. Regional stratigraphic relations (thicknesses approximate) of sampled erg eolianites and associated strata Jurassi (thicknesses approximate) 3. Regional stratigraphic relations Figure analysis. Chronostratigrap detrital zircon stratigraphic positions of samples collected for rado Plateau). Solid dots represent RP/CR—Roc Tongue; BL—“Black Ledge” sandstone; CF—Curtis Formation; MT—Moab Stratigraphic abbreviations: limited fossil control. TC of Kayenta Formation); (of Moenave Formation or (Rock Point Formation); RS—Romana Sandstone; SSM—Springdale Sandstone Member AZ—Arizona; CO—Colorado; NM—New Mexico; NV States: A–F. right shows locations of columns Formation. Map key at upper TF—Todilto

414 Geological Society of America Bulletin, March/April 2009 U-Pb ages of detrital zircons in Colorado Plateau eolianites

(Dubiel, 1989), Church Rock Member (Smith sedimentation spread northward from the Moe- leran magmatic arc was apparently required to et al., 1963), and Dolores Formation (Blodgett, nave facies tract during Kayenta deposition. control the Navajo erg depocenter and to carry 1984) are fi lled with Wingate eolian sand that An unconformity (“J-sub-K”) has been the thick fl ank of the erg below sea level (Allen infi ltrated downward into open cracks. This postulated locally at the base of the Kayenta et al., 2000). contact has been termed the J-0 unconformity Formation (Riggs and Blakey, 1993), but the (Pipiringos and O’Sullivan, 1978) at the base of Wingate-Kayenta contact is widely reported Glen Canyon–San Rafael Transition Colorado Plateau Jurassic strata, but the sedi- as gradational or even intertonguing over wide mentology of the contact suggests progradation areas (Doelling, 2003; Morris et al., 2003). Pres- The Navajo Sandstone (Glen Canyon Group) of eolian sand dunes across an unconsolidated ervation of the Wingate Sandstone as a sheet- is overlain over most of the Colorado Plateau substratum of mud-cracked red beds with no like body, 60–150 m thick over 100 × 103 km2 by the Carmel Formation (San Rafael Group), signifi cant hiatus in deposition and no erosion (aspect ratio ~3500:1), implies that any sub- which is composed of fi ner-grained strata of the substratum. Moreover, the Triassic-Juras- Kayenta erosion of the erg complex was limited. that record eastward transgression of marine, sic boundary (Fig. 3) lies within the Wingate Aggradation of the Moenave fl uvial succession marginal-marine, and associated coastal-plain Sandstone (Lockley et al., 2004) and the later- adjacent to the Wingate erg may have elevated deposits over the erg complex (Fig. 3). In south- ally equivalent Moenave Formation (Lucas et the land surface enough to allow streamfl ow central Utah, however, tongues of the Carmel al., 2005; Lucas and Tanner, 2007; Tanner and to overtop the erg and spread fl uvial Kayenta Formation interfi nger eastward with eolianite of Lucas, 2007). deposition across its undulating surface. Satu- the Page Sandstone, which locally overlies the From a sedimentological perspective, the ration of dune sand by lateral percolation of Navajo Sandstone (Figs. 2B and 3). The Navajo- best placement of the Chinle–Glen Canyon groundwater from the Moenave fl uvial environ- Page contact has been termed the J-2 unconfor- contact is at the base of the Moenave Forma- ment may have prevented defl ation of dune sand mity (Pipiringos and O’Sullivan, 1978), beneath tion or the Rock Point–Church Rock interval, and thereby aided preservation of the Wingate which subjacent sandstone is polygonally frac- whichever is present locally (Fig. 3). Both units erg accumulation beneath the aggrading Kay- tured. Polygonal disruption of Navajo dune are largely fl uvial red beds deposited by ephem- enta fl uvial system. sand below the contact was initially interpreted eral streams, but they contain intercalated eolian as jointing in consolidated rock (Peterson and and lacustrine strata and refl ect similar origins Thickness Relations Pipiringos, 1979), but Kocurek and Hunter in wadi-like arid environments. Both rest on the The regional parallelism of the upper and (1986) showed that the polygonal fracturing is Owl Rock Member (or Formation) of the Chinle lower contacts of the eolian Wingate Sandstone the record of desiccation cracking on an exposed Formation (or Group), which includes palustrine and the overlying fl uvial Kayenta Formation is evaporite-encrusted surface that formed within carbonate beds refl ecting deposition on water- striking but unexpected, for there is no evident an evolving erg during an episode of defl ation logged fl oodplains (Tanner, 2000). The record sedimentological reason why the surface of to the water table. The J-2 surface is a diastem of Chinle pedogenesis refl ects increasingly an erg accumulation should have the regional marking a hiatus of uncertain duration, but it arid environments during Chinle sedimentation slope of a fl uvial system. The explanation for is not a regional unconformity (Anderson and (Tanner, 2000, 2003), and Owl Rock deposition the consistent Wingate-Kayenta thicknesses Lucas, 1994). represents the fi nal phase of Chinle sedimenta- may lie in the relationship of the Wingate erg We accordingly view the Page erg as a rejuve- tion before transit of Laurentia into the desert to the substratum formed by Chinle fl oodplains nation of the Navajo erg, but we did not sample belt. Accordingly, we regard the “Black Ledge” that sloped to the west-northwest in a direction the thin local erg of the Temple Cap Sandstone sandstone (8–14 m thick) at the base of the subparallel to paleofl ow in the Kayenta fl uvial (Peterson and Pipiringos, 1979), which occupies Rock Point–Church Rock interval in central system. If Chinle rivers had an average slope of an analogous stratigraphic position farther west Utah (Figs. 2A and 3) as a basal sandstone of 6 × 10−4 (Heller et al., 2003), relief on the subja- but nowhere occurs in the same stratigraphic the Glen Canyon Group, and we sampled it for cent Chinle fl oodplain across the span of Wing- succession as the Page Sandstone (Figs. 2B detrital zircons for comparison with those from ate deposition would have been ~250 m, or two and 3). Polygonally fractured horizons marking other fl uvial strata of the group. to three times the mean thickness of the Wingate superbounding surfaces of temporary defl ation erg. This relation implies that the Wingate erg analogous to the basal J-2 surface are present Kayenta Interval formed only a veneer of dune sand covering a at multiple horizons within the Page Sandstone During Early Jurassic time, the Kayenta fl uvially constructed ramp, and that overtopping (Havholm et al., 1993; Havholm and Kocurek, fl uvial system spread northward over >100 × of the Wingate erg by the Kayenta fl uvial sys- 1994) and refl ect repetitive fl ooding of the Page 103 km2 of the Wingate erg lying to the northeast tem allowed streamfl ow to resume in the same erg during intermittent but progressive Carmel of the Moenave fl uvial tract. Readvance of Glen direction and at the same gradient as before erg transgression, which eventually overtopped the Canyon dune fi elds subsequently prograded construction. erg (Fig. 3). The most prominent superbounding the Navajo-Aztec erg back southward over the By contrast, the Navajo Sandstone thickens surfaces correlate with tongues of Carmel For- Kayenta facies tract (Figs. 2A and 3). Despite systematically westward from <100 m in west- mation that interfi nger with different members its wide extent, the Kayenta Formation averages ern Colorado to >300 m in southwestern Utah of the Page Sandstone (Blakey et al., 1996). only 60 m (range 40–90 m) in thickness (aspect (Fig. 3), where the fl ank of the Navajo erg was ratio >5000:1). Consistent paleocurrent indica- buried beneath marine to marginal-marine Mid- San Rafael Group tors document fl uvial transport of Kayenta sand dle Jurassic strata. This onlap of the thickest part westward to northwestward in a regionally inte- of the erg by strandline sedimentation implies The San Rafael Group can be divided into grated pattern (Luttrell, 1993, 1996). The direc- that the westward increase in net erg thickness lower and upper stratal assemblages; the contact tion of fl uvial sand transport (Fig. 2A) implies does not refl ect westward increase in the eleva- between them occurs at the base of the marine lateral expansion, rather than progradation, of tion of the erg surface. Dynamic backarc subsi- Curtis Formation in Utah (the J-3 unconformity fl uvial deposition over the Wingate erg, as fl uvial dence (Mitrovica et al., 1989) behind the Cordil- of Pipiringos and O’Sullivan, 1978) and the

Geological Society of America Bulletin, March/April 2009 415 Dickinson and Gehrels broadly correlative Todilto Formation deposited Eastward from the central Colorado Plateau, (Anderson and Lucas, 1994, 1996; Kirkland et in a nonmarine salina farther to the southeast beyond the fl exural infl uence of the sediment al., 1995). The Todilto Formation includes a thin (Fig. 3), or at the top of the Entrada Sandstone load in the Utah-Idaho trough, the Entrada Sand- lower limestone member, generally <5 m thick, across an intervening belt where neither of those stone forms a thin sediment blanket, <25 m thick, that is present throughout Todilto exposures, two distinctive units is present (Fig. 2B). Thick on the High Plains (Lucas, 2004) over a lobate and an upper gypsum member confi ned to the eolianites occur in both assemblages and form area of at least 150 × 103 km2, extending as far as interior of the salina, where a brine pool per- the widespread Entrada erg and the younger the Oklahoma panhandle (Fig. 2B). From paleo- sisted after evaporation had reduced its extent. (post-Todilto) and more restricted Bluff erg wind directions (Fig. 2B), the blanket Entrada The lateral continuity of the thin Todilto Forma- (Fig. 2B). Marine sandstone of the Curtis Forma- erg lying east of the central Colorado Plateau tion above the Entrada Sandstone shows that the tion was also sampled to compare its zircon age was not a downwind fore-erg accumulation, but upper surface of the Entrada erg drowned by the spectrum to that of underlying eolian sandstone. a crosswind fl ank of the main Entrada erg. Thin lacustrine salina was approximately level within Entrada Sandstone oversteps an eastern wedge an elevation range of <50 m. Lower Assemblage edge of Navajo Sandstone to rest directly on The Carmel Formation at the base of the San underlying Kayenta Formation across a narrow Upper Assemblage Rafael Group grades and thins eastward from facies tract in western Colorado. The blanket The upper San Rafael Group forms a sheet- largely marine strata (subtidal) on the west to phase of the Entrada erg also spreads still far- like body of sediment only 50–100 m thick over thinner marginal-marine (intertidal) and nonma- ther east over massive red siltstone of the Rock most of its distribution, although thicknesses rine (supratidal) red beds on the east (Blakey, Point–Church Rock interval and correlative >100 m are reached along the western edge of 1994). To the southeast, thin distal tongues lacustrine strata of the Redonda Formation on the Colorado Plateau toward the Utah-Idaho of Carmel lithology (Fig. 3) are commonly the High Plains (Hester and Lucas, 2001). There trough (Fig. 3). Different stratigraphic nomen- regarded as basal members of the overlying is no evidence, however, for fl uvial incision or clature is used for the upper assemblage by Entrada Sandstone. The Entrada Sandstone marine planation below the Entrada eolian inter- different workers and in different states, and is an internally heterogeneous assemblage of val, nor any discernible paleosol below it. controversy also surrounds the placement of cross-bedded dune sand, fl at-bedded eolian sand the stratigraphic base of the overlying Morrison sheets, interdune sabkha deposits, and erg-mar- Curtis-Todilto Event Formation. Resolution of nomenclature issues is gin red beds (Kocurek, 1981; Carr-Crabaugh and Recent ammonite collections document the beyond the scope of this paper, but a brief dis- Kocurek, 1998). Polygonally fractured super- Oxfordian (basal Late Jurassic) age of the Cur- cussion of their nature is necessary because we bounding surfaces commonly separate dune tis Formation (Wilcox and Currie, 2006), which sampled eolianite within the upper assemblage successions from sabkha intervals and attest to is composed of glauconitic marine sandstone to test for possible differences in age popula- temporary episodes of defl ation at times of ris- 40–60 m thick (Fig. 3) that transgressed over tions of detrital zircons in Entrada and post- ing water tables during evolution of the Entrada Entrada Sandstone in central Utah from the Entrada ergs for which prevailing winds blew erg. Across Utah, Entrada eolianites (“slickrock Utah-Idaho trough to the west and tapered to a from the northeast or north and from the west or facies”) grade westward (Fig. 2B) to an “earthy feather edge on the southeast (Figs. 2B). Curtis southwest, respectively (Fig. 2B). facies” of fi ner-grained strata, including water- marine strata grade upward and eastward into In central Utah, red beds of tidal-fl at and sab- laid deposits of coastal sabkhas and tidal fl ats fi ner-grained tidal-fl at and sabkha deposits of kha origin (Petersen and Pack, 1982; Caputo developed along a dune-fringed paleoshoreline the overlying Summerville Formation, and the and Pryor, 1991; Peterson, 1994) form the Sum- (Mariño and Morris, 1996). eolian Moab Tongue locally present at the top merville Formation (Fig. 3), which conform- The lower San Rafael Group (Carmel- of the Entrada Sandstone (Figs. 2B and 3) is a ably overlies the marine Curtis Formation and Entrada) thickens from <100 m on the east- nonmarine equivalent of the Curtis Formation the laterally equivalent Moab Tongue of eolian ern Colorado Plateau to almost 500 m along (Crabaugh and Kocurek, 1993; Peterson, 1994; origin. To the southwest, the Summerville For- its western fringe near the Utah-Idaho trough Doelling, 2003). Flooding of the Entrada erg mation grades laterally to a sandier and more (Fig. 2B), within which ~1500 m of correlative was punctuated by local scour associated with onshore facies (Peterson, 1988a, 1994; Blakey, Middle Jurassic marine strata overlie the Nug- ravinement along a desert coast and buried 1989) termed the Romana Sandstone (Fig. 3). get Sandstone (Glen Canyon Group). Several relict dune topography (Eschner and Kocurek, To the southeast, strata that are homotaxial and tectonic interpretations have been offered for 1988; Peterson, 1994), but there was no pro- lithologically similar to Summerville Formation development of the Utah-Idaho trough, but none longed hiatus or paleosol development beneath extend to areas where neither the Curtis Forma- is yet fully satisfactory. These include: (1) a the Curtis Formation. tion nor the Moab Tongue is present. A change retroarc foredeep (Bjerrum and Dorsey, 1995), Farther inland to the southeast, the broadly in nomenclature adopted by some workers stems but the basin keel lies 500 km from the nearest coeval (Anderson and Lucas, 1994) but lacus- from the lateral continuity of the limestone known coeval retroarc thrust system (Fig. 2B); trine Todilto Formation, which may be slightly member of the Todilto Formation in New Mex- (2) a backbulge basin (DeCelles and Currie, older than the Curtis Formation (Peterson, ico with the Pony Express Limestone Member at 1996), but the stratal thickness in the basin 1994), was deposited in an evaporative salina the base of the Wanakah Formation in Colorado, seems excessive for such an origin; and (3) a occupying >100 × 103 km2 (Fig. 2B). The beyond the extent of Todilto gypsum. This cor- backarc rift basin (Dickinson, 2006), but postu- Todilto salina may have been fl ooded initially relation has led to treatment of the Todilto inter- lated bounding faults are buried in the subsur- by marine waters related to the Curtis transgres- val as a member of the Wanakah Formation, and face and diffi cult to evaluate. In any case, west- sion, but it was then maintained by infl uxes of to the assignment of red beds of tidal-fl at or sab- ward thickening of the tapering wedge of lower freshwater from the south and by seepage of kha origin overlying the Todilto interval to the San Rafael strata across Utah can be attributed seawater through coastal sand barriers like the Wanakah Formation rather than the Summer- to the fl exural effect of the thick sediment load eolian Moab Tongue lying northwest of the ville Formation (Condon and Peterson, 1986; within the Utah-Idaho trough. salina toward the marine Curtis facies tract Condon and Huffman, 1988; O’Sullivan, 2003).

416 Geological Society of America Bulletin, March/April 2009 U-Pb ages of detrital zircons in Colorado Plateau eolianites

For Figure 3, we follow others (Anderson and Mesa Member of the Wanakah Formation, rounded to rounded aggregates of well-sorted Lucas, 1992, 1997) who prefer the name Sum- after a unit exposed to the south and traceable grains that plot as a compact elongate cluster merville Formation and retain the formational still farther southeast into the lower part of the in QmFLt space (Fig. 4), and they are quartz- status of the Todilto interval. “sandstone at Mesita” exposed near the Rio rich subarkose in the classifi cation of McBride In southeastern Utah, the eolian Bluff Sand- Grande rift (Condon and Peterson, 1986; Con- (1963). The two samples of marine sandstone stone, which is younger than the Moab Tongue don and Huffman, 1988; Condon, 1989a). The are well-sorted, fi ne-grained sandstones com- (Fig. 3) at the top of the Entrada Sandstone upper part of the “sandstone at Mesita” is cross- posed of subrounded to subangular grains that farther north (O’Sullivan, 1980), overlies fi ner- bedded, as is upper Bluff Sandstone at the type plot close to the eolianite samples in QmFLt grained strata assigned alternately (see previ- locality (Condon, 1989b). space (Fig. 4), with no statistically signifi cant ous) to the Summerville or the Wanakah For- Sedimentologically, the upward transition difference in QFL composition when inherent mation. The Bluff Sandstone, identical to the from horizontally bedded to cross-bedded sand- counting error is taken into account (Van der Junction Creek Sandstone of Colorado (Lucas stone in the type Bluff Sandstone and the “sand- Plas and Tobi, 1965), but they contain mica and Heckert, 2005), was originally described as stone at Mesita” is the record of dune prograda- fl akes (Table 1C) that are absent from the eolian- the basal member of the overlying and domi- tion over eolian sand sheets without a signifi cant ites. The fi ve fl uvial samples (Table 1B) are also nantly fl uvial Morrison Formation (Gregory, break in eolian sedimentation. Accordingly, we fi ne- to medium-grained sandstones, but they are 1938). The Bluff Sandstone was later shifted to regard the entire Bluff Sandstone as part of the only moderately to poorly sorted aggregates of the San Rafael Group on the basis of its lithol- San Rafael Group and follow interpretations of subangular to subrounded grains, and they are ogy and recognition that a vestige of the basal others (Anderson and Lucas, 1996; Lucas and less quartzose than the eolian and marine sand- Salt Wash Member of the Morrison Formation Heckert, 2003; Lucas, 2004) in regarding the stone samples (Fig. 4), though still subarkose, is present within the overlying and dominantly Horse Mesa and Mesita sand bodies as local with one exception. Some but not all fl uvial fi ner-grained Recapture Member (Craig et al., phases of the Bluff erg. Local stratal discor- samples contain detrital mica fl akes in variable 1955; Craig and Shawe, 1975; Lupe, 1983). dance between the lower, horizontally bedded abundances, which are probably dictated by A thin interval of nonfl uvial and even-bedded and the upper, cross-bedded facies of the “sand- local sedimentology rather than provenance. tidal-fl at and sabkha deposits resembling Sum- stone at Mesita” (Condon, 1989b) can be attrib- Table 2 compares mean detrital modes for merville or Wanakah Formation in lithology is uted to syndepositional fl owage of evaporites generic groups of samples. Eolianite samples are present within the type Recapture Member (of in the underlying Todilto Formation (Anderson tabulated both stratigraphically (columns 1–2) Gregory, 1938) between the Bluff Sandstone and Lucas, 1992). Our Bluff samples come from and areally (columns 3–4). The tabulations show and the Salt Wash Member of the Morrison For- the cross-bedded upper facies in both the type no statistical distinction among QmFLt values mation (Anderson and Lucas, 1997). locality and near Mesita, where the sandstone for the eolian sandstones no matter how they In recent years, some have reassigned the body we sampled was mapped as Bluff Sand- are grouped, nor any distinction in QmFLt val- upper part of the Bluff Sandstone to the Mor- stone (Moench and Schlee, 1967) before the ues between eolian and marine sandstones. All rison Formation and have postulated an uncon- stratigraphic disputes arose. samples of fl uvial sandstone are less quartzose formity (the J-5 unconformity of Pipiringos and and more feldspathic, with the “Black Ledge” O’Sullivan, 1978) between horizontally bedded SANDSTONE PETROFACIES sample (column 7) being more lithic as well. sandstone forming the lower part of type Bluff P/F ratios are different for Glen Canyon Sandstone and cross-bedded sandstone form- Table 1 and Figure 4 indicate numerically and and San Rafael eolianites and for eolianites ing the upper part of type Bluff Sandstone. This graphically the detrital modes of all samples. from the eastern and western Colorado Plateau interpretation has led to designation of the lower The 11 eolianite samples (Table 1A) are fi ne- to (Table 2). The contrasts in ratio are similar part of the type Bluff Sandstone as the Horse medium-grained sandstones composed of sub- because all Glen Canyon samples derive from

TABLE 1. DETRITAL MODES OF DETRITAL ZIRCON SAMPLES FROM SANDSTONES OF THE GLEN CANYON AND SAN RAFAEL GROUPS

A. Eolian samples B. Fluvial samples C. Marine samples Grains Jwnw Jnnw Jenw CP2 CP3 CP12 CP15 CP16 CP24 CP30 CP54 DOL CP1 CP10 CP31 CP37 CP43 CP45

Qm 80 83 84 88 88 87 87 85 82 84 83 77 74 72 65 71 82 83 Qp 2 2 1 2 2 2 5 1 2 2 1 2 1 1 4 2 4 4 Q 82 85 85 90 90 89 92 86 84 86 84 79 75 73 69 73 86 87

P 2 2 2 4 2 3 3 6 7 3 5 6 7 10 6 4 6 6 K 12 9 9 4 5 5 3 4 7 6 7 9 9 12 10 19 4 4 F 14 11 11 8 7 8 6 10 14 9 12 15 16 22 16 23 10 10

Lvm 2 2 2 1 1 1 1 1 1 3 2 2 4 3 4 2 1 1 Lsm 2 2 2 1 2 2 1 3 1 2 2 4 5 2 11 2 3 2 L 4 4 4 2 3 3 2 4 2 5 4 6 9 5 15 4 4 3

Lt 6 6 5 4 5 5 7 5 4 7 5 8 10 6 19 6 8 7 M – – – – – – – – – – – 1% 4% – 3% – 4% 3% Notes: See Supplementary Text 1 (ST1; see text footnote 1) for sample localities. Modes are based on point counts of 400 QFL framework grains per sample (plus extra framework points for mica). Monocrystalline grains: Qm—quartz; P—plagioclase; K—K-feldspar; F—total feldspar (P + K). Polycrystalline grains: Qp—polycrystalline quartz (dominantly chert), Lvm—volcanic and metavolcanic lithic fragments; Lsm—sedimentary and metasedimentary lithic fragments; L—total labile lithic fragments (Lvm + Lsm), Lt—total lithic fragments (L + Qp). Q—total quartzose grains (Qm + Qp), M—mica flakes (% of total framework with QFL grains summed to 100%, exclusive of mica).

Geological Society of America Bulletin, March/April 2009 417 Dickinson and Gehrels

Composite age-distribution curves for eastern Qm and western plateau samples are compared in Figure 6. All major age peaks for grains older eolianites quartz marine Curtis Fm. arenite San Rafael Gp. + (San Rafael Gp.) than 285 Ma are closely comparable on the two plots, and the P value from K-S analysis Glen Canyon Gp. fluvial Kayenta Fm. (Glen Canyon Gp.) for composite populations of grains older than 90 90 1 0 285 Ma in the two sample sets (n = 574 west- 0 1 ern; n = 316 eastern) is 0.98, where 1.0 would represent identity. The contrasting abundances + of grains younger than 285 Ma in eolianite sam- + sublitharenite ples from the eastern and western parts of the 80 80 plateau (Fig. 6) stem from variable additions of 2 0 0 arc-derived grains to otherwise uniform eolian 2 subarkose sand. Six western plateau samples contain only three grains younger than 285 Ma in age (0.5%), whereas four eastern plateau samples contain lithic subarkose 7%–25% (mean 13% or 49 total grains). 70 70 3 We ﬁ rst address the origins of the dominant 0 0 Springdale "Black Ledge" 3 grains older than 285 Ma in the eolianite samples Sandstone sandstone and then turn to the grains younger than 285 Ma Member of Cordilleran derivation (~5% overall). arkose litharenite to F 60% Qm to LT Non-Cordilleran Grains

Figure 4. Partial QmFLt plot (note scale) of sandstone samples (Table 1). Glen We evaluated the apparent similarity of het- Canyon samples include Page Sandstone (CP12) immediately overlying Navajo erogeneous age populations of grains older Sandstone, and San Rafael samples include correlative Mount Wrightson than 285 Ma in 10 Colorado Plateau eolianite Forma tion (CP3) from southern Arizona. Numerical values are percentages. samples (Fig. 5) by applying the K-S test to all 45 pairs of samples. The K-S test is a strin- gent approach to sample comparison because the west, whereas most San Rafael samples EOLIANITE DETRITAL ZIRCONS it is sensitive to proportions of grains of dif- derive from the east (Figs. 2 and 3). P/F ratios ferent ages as well as to the spread of ages. P for the marine and most fl uvial samples (col- Visual inspection of the superimposed age- values <0.05, indicating statistical differences, umns 6–7) resemble the higher eolianite values bin histograms and age-distribution curves of can thus derive from comparison of sets of (San Rafael and eastern plateau), but the P/F Figure 5 shows that heterogeneous age popula- samples containing age populations derived ratio for the Springdale sample (column 8) is as tions of detrital zircons in all eolianite samples from the same sources but in differing propor- low as the lowest eolianite values (Glen Can- from the Colorado Plateau are broadly similar. tions. P values for 75% of the sample pairs are yon and western plateau). The variations in P/F The salient difference is in the variable content >0.05, indicating that we cannot be 95% confi - ratio correlate with variations in the age popu- of grains younger than 285 Ma derived from dent that those pairs of grain populations were lations of detrital zircons, as developed next. Cordilleran igneous suites (Fig. 1). Arc-derived not selected at random from the same parent grains are paradoxically more abundant in eoli- population. All the samples have calculated P DETRITAL ZIRCON MORPHOLOGY anites from the eastern plateau (Figs. 5G–5J) values >0.05 when compared to half or more than from the western plateau (Figs. 5A–5F), of the other samples. We conclude that grains Zircon populations in all samples (eolian, even though the latter is geographically closer older than 285 Ma form a heterogeneous but fl uvial, marine) are dominantly rounded to to the Cordilleran orogen. coherent age spectrum in the plateau eolianite subrounded grains (although some are subangular), vary from subspherical to elongate in shape, and display varied colors. There is no TABLE 2. COMPARATIVE MEAN DETRITAL MO DES (KEY PARAMETERS) OF DETRITAL ZIRCON systematic correlation between shape, color, SAMPLES (TABLE 1) or angularity and U-Pb age. We conclude that Eolian Marine Fluvial sediment dispersal systems incorporated grains 1 2 3 4 5 6 7 8 Glen San Western Eastern “Black Springdale from age provinces that all included multiple Canyon Rafael Colorado Colorado Curtis Kayenta Ledge” Sandstone source rocks containing zircons of varying Group Group Plateau Plateau Formation Formation Sandstone Member character, and that the transport histories of (n = 5) (n = 6) (n = 6) (n = 5) (n = 2) (n = 3) (n = 1) (n = 1) Qm 84 ± 3 85 ± 2 84 ± 3 85 ± 2 84 74 ± 2 65 71 individual zircon grains were also variable. F 10 ± 2 10 ± 3 10 ± 2 10 ± 3 10 18 ± 3 16 23 Transport in stream bed load or as eolian sal- Lt 6 ± 1 5 ± 1 6 ± 1 5 ± 1 6 8 ± 2 19 6 tation blankets tends to abrade grains readily, M – – – – 3%–4% 2% ± 2% 3% – P/F 26 ± 10 46 ± 13 25 ± 9 52 ± 4 42 43 ± 2 38 17 whereas transport as suspended load in streams Note: See Table 1 for Qm, F, Lt, M; P/F = 100 × (plagioclase feldspar)/(total feldspar). Standard deviations (±) or sand storms allows transport over arbitrarily are for n samples where n > 2. Col. 1 includes Page Sandstone (CP12). Col. 2 includes Mt. Wrightson long distances with little grain abrasion. Formation (CP2). Cols. 3–4 were divided at 110°W longitude.

418 Geological Society of America Bulletin, March/April 2009 U-Pb ages of detrital zircons in Colorado Plateau eolianites I J H A G CP54 Bluff CP24 Entrada CP15 Bluff 1] for the same and additional 1] for alities (central map). Abscissas are Abscissas are alities (central map). CP16 Entrada CP12 Page 0 1000 2000 3000 4000 0 1000 20000 3000 1000 4000 2000 3000 4000 6 8 2 8 0 4 6 4 2 0 4 2 0 6 12 10 14 12 10 CO NM 0 1000 2000 3000 4000 0 1000 2000 3000 4000 6 4 2 6 4 2 0 0 AZ UT NV CP3 Nugget CP30 Aztec Jenw Entrada Jnnw Navajo Jwnw Wingate F B 0 1000 2000 3000 4000 0 1000 2000 3000 4000 6 8 5 3 4 2 0 1 4 2 0 10 E C D 0 1000 2000 3000 4000 0 1000 20000 3000 1000 4000 2000 3000 4000 0 8 6 6 2 4 2 0 4 4 8 6 0 2 10 10 Figure 5. Age-bin histograms and age-distribution curves (superimposed) for Colorado Plateau eolianites keyed to collecting loc Age-bin histograms and age-distribution curves (superimposed) for 5. Figure 3 [see text footnote Text numbers of grains (variable scales). See Supplementary U-Pb grain ages (0–4000 Ma), and ordinates are plots including concordia diagrams at larger scale. States: AZ—Arizona—CO—Colorado; NM—New Mexico; NV—Nevada; UT—Utah. scale. States: plots including concordia diagrams at larger

Geological Society of America Bulletin, March/April 2009 419 Dickinson and Gehrels

samples, and that variable P values reﬂ ect vagaries of sampling, including stratigraphic and areal selection of sample sites, choice of eastern plateau eolianites horizons collected on outcrop, and selection of grains in the laboratory for laser ablation. (CP15-CP16-CP24-CP54) Figure 7 is a composite plot of 890 grains N = 4; n = 365 older than 285 Ma in age in the eolianite samples (Fig. 5). Distinct and prominent age peaks include (ages rounded to nearest 5 Ma): (1) 420 Ma (Late Ordovician), with other minor Paleozoic peaks in the range of 290–390 Ma; (2) 615 Ma (Neoproterozoic); and (3) 1055 Ma (Grenvillian Mesoproterozoic), with a subordi-

Age probability nate Grenvillian peak at 1160 Ma. Paleoprotero- zoic and Archean grains defi ne fi ve less promi- western plateau eolianites nent but well-defi ned age peaks (Fig. 7). The distribution of granitoid basement prov- (CP3-CP12-CP30-Jwnw-Jnnw-Jenw) inces in North America (Fig. 1) provides a guide N = 6; n = 577 to the ultimate sources of the grains defi ning each age peak. Table 3 groups the net eolianite age populations into clusters around the age peaks and indicates the inferred dominant 0 500 1000 1500 2000 2500 3000 3500 sources for each age cluster. Approximately Age (Ma) two-thirds of the grains apparently derive from the Appalachian orogen and its Grenville fl ank Figure 6. Composite age-distribution curves for detrital zircon grains in Jurassic eolian- in southeast Laurentia, or its extensions into ite samples from the eastern (Figs. 5G–5J) and western (Fig. 5A–5F) Colorado Plateau the Mesoamerican region. No other basement (N—samples; n—grain ages). provinces in North America could provide the Paleozoic, Neoproterozoic, and Grenvillian grain populations in the observed abundances relative to grains of other ages.

Appalachian Provenance 420 Granitoid source rocks of Paleozoic, Neo- proterozoic, and Grenvillian age are present in elongate parallel domains along the Appala- chian orogenic belt and its extensions (Fig. 8). Basement rocks of the late Mesoproterozoic 1055 Grenville orogen were incorporated into the cratonal ﬂ ank of the Appalachian orogen. Neopro- Colorado Plateau eolianites (>285 Ma grains) terozoic granitic rocks are present both as rift N = 10; n = 890 plutons intruded into Grenville basement during the multiphase pre-Iapetan breakup of Rodinia 1160 within the interval 735–560 Ma (Tollo et al., 2004), and within peri-Gondwanan arc terranes Age probability Age 615 accreted to Laurentia during evolution of the Appalachian orogen. Neoproterozoic and Cam- brian grains are also present as detrital zircons in capping lower Paleozoic sedimentary strata of the accreted peri-Gondwanan assemblages (Murphy et al., 2004). Paleozoic plutons of

1855 2760

1675 diverse ages are present along the length of the 1465 Appalachian orogen, both intruded into native

2095 Laurentian basement of Grenville age and within Paleozoic assemblages accreted to the ﬂ ank of Laurentia during evolution of the orogen (Hib- 0 500 1000 1500 2000 2500 3000 bard et al., 2007). The tendency for populations Age (Ma) of detrital zircons to integrate age signals from Figure 7. Composite age-distribution curve for detrital zircon grains older than 285 Ma in diverse bedrock sources within the same prov- 10 eolianite samples from the Colorado Plateau (Fig. 5). enance is shown by the compound age peak for

420 Geological Society of America Bulletin, March/April 2009 U-Pb ages of detrital zircons in Colorado Plateau eolianites

grains of Paleozoic age in the eolianites (Fig. 7) TABLE 3. AGE POPULATIONS OF 890 NON-CORDILLERAN (≥285 MA) DETRITAL ZIRCON GRAINS IN COLORADO PLATEAU EOLIANITES without discrimination among pulses of Paleo- zoic magmatism along the Appalachian belt. Grain population descriptor from age peak Age peak Age range Number Percentage (inferred dominant source or sources) (Ma) (Ma) of grains of grains Reported U-Pb ages of granitoid rocks along (%) seven transects of the Appalachian belt (Fig. 8) Paleozoic (Appalachian sources) 420 285–504 137 15 are compiled in Figure 9, which shows that Neoproterozoic (Appalachian sources) 615 513–723 102 11 Unknown-indeterminate – 746–872 8 1 the four most prominent age peaks for detrital Grenville (Mesoproterozoic Grenville orogen) 1055 913–1295 352 40 zircons in Colorado Plateau Jurassic eolianites Anorogenic (Mesoproterozoic craton) 1465 1304–1532 75 8 match the distribution of known ages for gra- Yavapai-Mazatzal (southwest Laurentia) 1675 1546–1803 69 8 Paleoproterozoic (cratonal suture belts) 1855 1811–2013 49 6 nitic rocks along the Appalachian-Ouachita- Wopmay orogen (northwest Laurentia) 2095 2039–2326 18 2 Mesoamerican fl ank of Laurentia. Derivation Archean (Laurentian cratonic nucleus) 2760 2423–3196 80 9 of the Neoproterozoic grains dominantly from accreted peri-Gondwanan arc assemblages, rather than from pre-Iapetan rift assemblages, is inferred from the greater areal extent of the 40° 100° W 80° James N W former. The paucity of detrital zircon grains of Bay A Neoproterozoic age in Paleozoic strata of the B BC C Appalachian foreland basin (McLennan et al., G 2001; Eriksson et al., 2004; Becker et al., 2005, F 2006; Thomas and Becker, 2007) suggests that peri-Gondwanan terranes accreted along the 20° N distal fl ank of Laurentia were not uplifted to E D Cordilleran Mesozoicterranes ? become signifi cant sources of sediment until AvT rift highlands developed along the trend of the ? incipient Atlantic Ocean in Mesozoic time. CaT SuT S o u r c e r o c k b e l t s Coincidental mixing of Paleozoic, Neopro- 100° W terozoic, and Grenvillian zircon grains from Yúcatan- Florida native and accreted Campeche Paleozoic terranes disparate rock masses, including Gondwanan block 40° continents once adjacent to Laurentia, as an Oaxaquia accreted Neoproterozoic N peri-Gondwanan terranes alternative to postulated Appalachian origin, is disfavored for several reasons: (1) there are North late Mesoproterozoic Grenville orogen consistent proportions of key age populations of 500 km 20° N detrital zircons in plateau eolianites, all of which display separate peaks for Paleozoic and Neo- anorogenic plutons Ouachita limit of pre-Jurassic sediment Atlantic-Caribbean suture proterozoic age populations, and Grenvillian (1490–1340 Ma) cover over interior craton continental slope age peaks larger than either (Fig. 5); (2) a 520– Figure 8. Transects (heavy barred lines keyed by letter to Fig. 9) of the Paleozoic Appala- 510 Ma gap between ages of Neoproterozoic and chian-Ouachita-Mesoamerican orogen showing late Mesoproterozoic Laurentian and Oax- Paleozoic granitoids in the Appalachian-Ouach- aquian Grenville belts (Fig. 1), distribution of early Mesoproterozoic anorogenic plutons ita-Mesoamerican orogen is closely matched by (Anderson, 1983; Anderson and Morrison, 2005) in Grenville foreland after Anderson and a 515–505 Ma gap between age peaks for detri- Cullers (1999) and Barnes et al. (2002), and accreted Neoproterozoic terranes of the Appa- tal zircons in the eolianites; (3) derivation of zir- lachian belt (AvT—Avalon; CaT—Carolina; SuT—Suwannee) adapted after Hatcher et al. cons from cratons lying within Pangea, beyond (1989, 2007), Reed (1993), and Barr and Kerr (1997). Paleogeologic pre-Jurassic edge of the Hercynian Appalachian-Ouachita suture sedimentary cover over Laurentian craton is from Figure 10. Yucatan-Campeche block and between Laurentia and Gondwana, is unlikely Baja California Peninsula (BC) were restored after Dickinson and Lawton (2001a). because it would have required transport of detritus across remnant Paleozoic uplands along the relict Appalachian orogen and rift highlands that formed during initial phases of Mesozoic Birimian belt of West Africa is of similar age Grenville plutons (Moecher and Samson, 2006), Atlantic rifting; (4) derivation of Neoprotero- (Boher et al., 1992), yet 2200–1950 Ma detrital rather than especially voluminous Grenville zoic grains from Pan-African orogenic belts of zircons form only 2% of the grain population in sources. Recycling of Grenvillian grains from Africa or South America is unlikely because the plateau eolianites. Paleozoic strata of the Appalachian foreland nearest Pan-African sources on those continents basin (Fig. 10), in which detrital zircon grains of lay 1500 km and 3500 km, respectively, from Grenvillian Zircons Grenvillian provenance are abundant (McLen- the Hercynide Appalachian-Ouachita suture Zircon grains of Grenvillian age form 60% of nan et al., 2001; Eriksson et al., 2004; Becker et bounding Laurentia; and (5) the proximate edge the age suite of eolianite detrital zircons inferred al., 2005, 2006; Thomas and Becker, 2007), is of the Guiana craton, which lay directly south of to derive from the Appalachian orogen (Table 3), a clear possibility, as is recycling from Neopro- Laurentia before breakup of Pangea (Dickinson yet the Grenville province forms a relatively terozoic rift and miogeoclinal assemblages of and Lawton, 2001a), is dominated by the 2200– narrow belt along the proximal fl ank of the oro- Grenvillian provenance along the Appalachian 1950 Ma Maroni-Itacaiúnas belt (Tassinari et gen (Fig. 8). Domination of grains of Grenvil- belt (Cawood and Nemchin, 2001; Cawood et al., 2000; Chew et al., 2007), and the conjugate lian age may refl ect the high zircon fertility of al., 2007; Thomas and Becker, 2007). From our

Geological Society of America Bulletin, March/April 2009 421 Dickinson and Gehrels

ABCDEFGDZ Age Legend (Ma) 1500 1500 Central Ontario- Québec- Texas- Southern Oaxaca Labrador- Yúcatan Coahuila- New England Appalachians Appalachians Newfoundland Maritime Canada Figure 9. Comparison of U-Pb 1400 1400 age populations of detrital zir-

eolianite detrital zircons cons (DZ) in Jurassic eolianites of the Colorado Plateau 1300 1300 with U-Pb age spans of poten-

anorogenic tial granitic source rocks for enclaves within enclaves Grenville orogen detrital zircons along multiple 1200 1200 transects of the Appalachian- Ouachita-Mesoamerican orogen (Fig. 8), including early

1100 Mesoproterozoic anorogenic

plutons plutons and late Mesoprotero- Grenville synorogenic zoic synorogenic plutons and granulites of the Grenville oro- 1000 1000 gen, native Neoproterozoic pre- Iapetan (Rodinian) rift plutons along the Laurentian margin, M e s o p r o t e r o z o i c 900 900 Neoproterozoic plutons of peri- N e o p r o t e r o z o i c Gondwanan terranes accreted

null to Laurentia, and Paleozoic magmatic 800 800 plutons assigned to Penob- scot, Taconic, Acadian, and uncertain origin 12 grains (2%) of (2%) 12 grains Alleghanian orogenic phases after Hatcher (1989). Plotted 700 700 age ranges (U-Pb ages only) for bedrock sources are from Supplementary Text 4 (see text 600 600 footnote 1). Age spans (dark columns) and principal age pre-Iapetus Rodinian rift plutons rift pre-Iapetus Rodinian

accreted peri-Gondwanan plutons accreted peri-Gondwanan peaks (arrows) of detrital zir-

500 P e n o b s c o t 500 cons are depicted for 602 grains T a c o n i c with ages of 1295–285 Ma in 10 Jurassic eolianites of the Colo- T a c o n i c rado Plateau (Fig. 7; Table 3). 400 A c a d i a n 400 plutons Paleozoic syronogenic A c a d i a n A l l e g h a n i a n 300 300

P a l e o z o i c - M e s o z o i c b o u n d a r y

distant vantage point on the Colorado Plateau, rock that contains few, if any, zircon grains. beneath Paleozoic cover and similarly could not we cannot distinguish between bedrock and Neoproterozoic and lower Paleozoic strata of have contributed recycled Grenvillian zircon recycled sources of Grenvillian detritus lying the Western Cordillera are known to contain grains to Jurassic dispersal systems. Volumi- within the Appalachian orogen, but Jurassic abundant detrital zircon grains of Grenvillian nous sediment of partly Appalachian derivation paleogeographic implications are the same in derivation (Rainbird et al., 1992, 1997; Gross transported into the midcontinent by a major either case. et al., 2000; Stewart et al., 2001; Timmons et Pennsylvanian paleoriver that ﬂ owed parallel to Recycling of Grenvillian zircon grains from al., 2005; Mueller et al., 2007), but these strata the Appalachian chain (Archer and Greb, 1995) the Paleozoic platform cover of the continen- were buried beneath upper Paleozoic and lower has also been largely preserved from erosion tal interior is unlikely, both because the plat- Mesozoic cover until late Mesozoic Cordilleran even to the present time. form cover is still largely intact over most of orogenesis and were not exposed to erosion in Recycling of Grenvillian zircon grains into the expanse between the Rocky Mountains Lower to Middle Jurassic time. Neoproterozoic plateau eolianites from pre-Mesozoic strata and the Appalachian belt (Fig. 10) and because strata in the midcontinent containing Grenvil- is further disfavored by combined U-Pb and the platform cover is in large part carbonate lian detritus (Santos et al., 2002) are still buried (U-Th)/He dating of zircon grains of Grenvillian

422 Geological Society of America Bulletin, March/April 2009 U-Pb ages of detrital zircons in Colorado Plateau eolianites

Hudson Bay 110° Trans - Hudson 90° E a W (1.8 - 1.9) W r J ly a 50° B m a e y s N

e 50° J c u n r N a vi o ) s Superior Province r 3 s . P i c (>2.5) e -1 ll i 0 v . n 1 ( e r Canada G

s h USA o r e e in l l i e n r e o h s ic Paleozoic ass Penokean miogeocline Jur le (ca. 1.85) idd M n i s North a residual Cen b ARM (?) tral paleo (1.4 - 1.8) r d * (tru iver nk s n trea a m) l e r o f * * P Appalachian- a l * e o Ouachita z o Paleozoic * * i c Cordilleran accreted ? ? ? 30° and Mesozoic * USA * orogen N Mexico 30° O uac N hita Accreted sh Pan-African elf Yucatan - Campeche Oaxaquia e 110° 90° d 500 km (1.0 - 1.25) g W W e

Figure 10. Extent of preserved Jurassic eolianite facies tract, stippled after Figure 2, in relation to Paleozoic platform sediment cover (blank) of midcontinent North America and to potential surrounding pre-Mesozoic provenance terranes (ages of Precambrian provinces in Ga). Hypothetical transcontinental paleoriver system (barbed lines) is adapted after Dickinson and Gehrels (2003), with schematic of multiple alternate or complementary (tributary) paleoriver courses. Key erosional outliers (asterisks) of Jurassic eolianite south and west of Colorado Plateau are after Stewart (1980), Marzolf (1983), Bilodeau and Keith (1986), Busby-Spera et al. (1990), Riggs et al. (1993), and Lucas and Orchard (2007). ARM—Ancestral Rocky Mountains (Anasazi uplifts of Nesse, 2007). Dominant Jurassic paleowinds (dots with tails downwind) were recorded by cross-bedding in eolianites (after Marzolf, 1983; Peterson, 1988b; Fig. 2). Figure was adapted after Peterson (1972), Rankin (1989), Muehlberger (1992), Dickinson (2004), Kues and Giles (2004), Reed et al. (2005), and Figure 1.

age in the Navajo Sandstone of Zion National Paleoriver Course sand on riverine or deltaic ﬂ oodplains that were Park (Rahl et al., 2003), located midway between The prevalence of paleowinds blowing south- then systematically deﬂ ated to feed the eolian our Navajo (Jnnw) and correlative Aztec (CP30) ward in present coordinates across the Colorado depositional system of the Colorado Plateau. samples. The Grenvillian granitoid rocks, or Plateau (Figs. 2A–2B) shows that the proxi- Because no Jurassic strata are exposed across perhaps deeply buried metasedimentary rocks, mate source of the Jurassic ergs lay north of the the midcontinent region between the Rocky that yielded detrital zircon grains to the Zion plateau in a direction from which none of the Mountains and Triassic-Jurassic rift basins of sample were not unroofed until early Mesozoic zircon age populations of ultimate Appalachian the Appalachian belt, the detrital zircon record time (youngest He age only 35–45 m.y. older provenance could have been derived (Fig. 1). We in Mesozoic strata of the Colorado Plateau is the than depositional age). This “double-dating” infer, after Dickinson and Gehrels (2003) and only remaining evidence for the transcontinental result precludes posterosion sediment storage Rahl et al. (2003), that a transcontinental Juras- Jurassic paleoriver system. In the modern world, of basement-derived Grenvillian grains for any sic paleoriver system transported Appalachian however, rivers transiting cratons for 1500– appreciable length of time at shallow depths detritus toward Jurassic paleoshorelines north 2500 km from marginal or interior highlands before delivery to the Jurassic ergs. of the Colorado Plateau (Fig. 10) and deposited toward distant continental margins are common

Geological Society of America Bulletin, March/April 2009 423 Dickinson and Gehrels

(Amazon, Congo, Lena, Missouri-Mississippi, In general, however, the U-Pb age spans of anites (Table 3), that 55% of the eolian sand Ob, Saskatchewan, Yenisei), and the inferred Grenvillian plutons are similar along the entire was derived ultimately from Grenvillian and length of the Jurassic paleoriver feeding sand to length of the Grenville orogen fl anking Lauren- Paleozoic basement in the central and south- the erg system of the Colorado Plateau was no tia (Fig. 9), and do not provide a robust guide to ern Appalachians, an estimate of the net depth greater (~2000 km). The low-lying surfaces of regional provenance. Another factor that com- of erosion in the provenance can be made. The cratons are pathways for, not barriers to, sedi- pounds the diffi culty of sensing regional differ- calculation leaves aside the question of Neopro- ment dispersal. ences in sources within the Grenville orogen is terozoic detrital zircons derived from accreted The most attractive course for the trunk pale- the inherent imprecision of U-Pb ages across peri-Gondwanan terranes lying father to the oriver of the fl uvial system was from headwaters the Grenvillian age spectrum. For example, east, which contributed an additional 11% of the in the central to southern Appalachians, with the mean age uncertainty for ~265 individual detrital zircons to plateau eolianites (Table 3). tributaries of unknown number and location detrital zircon grains in plateau eolianites with A volume of quartzose Jurassic eolian sand of joining the paleoriver from both the north and best estimates of grain age in the range of 1200– Grenvillian-Paleozoic origin of 95 ± 40 km3 the south as it crossed the continent (Fig. 10). 1000 Ma is ±65 Ma. (embracing minimum and maximum estimates) The postulated course of the trunk stream requires erosion of 245 ± 100 km3 of granitoid- thereby followed a direct path along a belt of Provenance Erosion gneissoid bedrock to produce. The indicated net low inferred paleotopography between residual In evaluating central and southern Appala- depth of erosion into basement in the southern uplands of the Pennsylvanian Ancestral Rocky chian sources, whether juvenile or recycled, and central Appalachians is 4.5 ± 2.3 km if the Mountains to the southwest and the broadly ele- for sand in plateau eolianites, two principles of geographic extent of eroded basement between vated tract of the Canadian Shield forming the provenance analysis are relevant: the Blue Ridge thrust front and the western edge cratonic nucleus of Laurentia on the northeast (1) Potential source rocks still uneroded in the of accreted peri-Gondwanan terranes is taken to (Fig. 10). provenance region cannot have contributed to be 85–115 km (width) by 575 km (length). This A more northerly trunk stream rising in the any body of sediment in the past. This principle estimate does not allow for enhanced zircon fer- northern Appalachians is disfavored because (1) comes into play because the fi ll of the Appala- tility of Grenvillian plutons (which would reduce avoidance of signifi cant detritus from Archean chian foreland basin and its miogeoclinal sub- the required depth of erosion), nor for deriva- basement of the Superior province lying imme- stratum is still largely preserved, to thicknesses tion of some detritus from quartzose metasedi- diately to the west (Fig. 10) would then seem in the range of 2500–7500 km above basement mentary rocks (which would also decrease the diffi cult, yet Archean grains form <10% of the (Muehlberger, 1992), beneath the Allegheny- required depth of erosion), nor for erosional detrital zircons in plateau eolianites (Table 3); (2) Cumberland Plateau along the western fl ank removal of mafi c or other rocks yielding little the accreted peri-Gondwanan assemblages capa- of the orogen. Accordant ridges of the plateau or no zircon or quartz (which would increase ble of yielding Neoproterozoic grains lie pro- expose nearly undeformed Pennsylvanian and the inferred depth of erosion). Even without gressively farther east toward the north, beyond Permian strata that cap the miogeoclinal–fore- such adjustments, however, the fi gure derived wider Grenville and Paleozoic belts (Fig. 8), in a land succession and were not dissected until for depth of erosion seems unexceptional for position more diffi cult for potential headwaters incised by modern stream valleys during the exposure of plutonic and metamorphic rocks in to tap; and (3) Alleghanian (<340 Ma) plutons currently active post-Jurassic cycle of erosion. the core of a mature orogen, and it implies that are unknown north of New England (Fig. 9). Recycling of Grenvillian detritus from Appa- no recycling of detritus from outside the Appa- A more southerly trunk stream is disfavored lachian foreland successions in Jurassic time lachian belt is required to explain the abundance because (1) Grenville basement fl anking the could only have involved deformed strata form- of Grenvillian and Paleozoic detrital zircons in Ouachita suture belt west of the Appalachians ing the Valley and Ridge province, 25–65 km plateau eolianites. was largely buried beneath Paleozoic platform wide along the proximal fl ank of the foreland cover during Jurassic time (Figs. 8 and 10); basin adjacent to the Blue Ridge thrust front Pre-Grenvillian Grains (2) Grenville plutons younger than 1000 Ma in (Hatcher et al., 2007). None of the fi ve age groups of pre-Grenvil- age, which contributed a signifi cant fraction of (2) The volume of rock eroded to supply a lian grains in plateau eolianites forms more than Grenvillian grains to plateau eolianites (Fig. 7; body of sediment no longer exists in the prov- 10% of the net grain population (Table 3), and Table 3), are unknown along the Grenville belt enance. This principle comes into play because none is especially abundant in any of the eolian- where exposed sparingly west of the Appala- the Blue Ridge thrust sheet may once have ite samples (Fig. 5). In aggregate, however, they chians, except far away in southern Mexico extended farther westward, structurally above sum to approximately one-third of the detrital (Talavera-Mendoza et al., 2005); and (3) fl ysch deformed strata of the Valley and Ridge prov- zircon grains, and this amount could not have successions of the Ouachita orogen (Gleason et ince, before erosion of the gently dipping thrust been derived from the Appalachian region, al., 2007), from which some eolianite detritus sheet back to the present position of the Blue although some of the pre-Grenvillian grains may have been derived, contain detrital zircon Ridge thrust front. To the east, the complex could represent recycling of zircons from Appa- populations that (a) lack the Neoproterozoic Blue Ridge and Inner Piedmont belts, com- lachian rift or foreland successions (Cawood age peak near 615 Ma prominent for plateau posed of Grenville basement, overlying sedi- et al., 2007; Thomas and Becker, 2007), or eolianites, (b) have a prominent Cambrian age mentary cover, and both overthrust basement enclaves within the Grenville orogen (Fig. 9). peak at 533 Ma, probably refl ecting sediment and intrusive plutons of Paleozoic age, are now Tributary contributions to the transcontinental contributions from the Amarillo-Wichita prov- jointly 70–120 km wide west of accreted peri- paleoriver having its principal headwaters in the ince (Fig. 1), which are not present in plateau Gondwanan terranes (Hatcher et al., 2007), but Appalachian orogen are the most likely sources eolianites, and (c) display Grenvillian age peaks may perhaps have been 100–150 km wide dur- of the non-Appalachian detrital zircons. of 1125 Ma and 1245 Ma, which are somewhat ing Jurassic time. Archean grains could have reached the trunk different from the 1055 Ma and 1160 Ma Gren- If one postulates, based on the proportions of paleoriver from northern tributaries draining villian age peaks for plateau eolianites. detrital zircons of various ages in plateau eoli- the cratonic nucleus of Laurentia (Fig. 1). The

424 Geological Society of America Bulletin, March/April 2009 U-Pb ages of detrital zircons in Colorado Plateau eolianites

Wyoming province of Archean rocks immedi- grains in various of the four samples, range and Keith, 1986; Busby-Spera et al., 1990), our ately north of the Colorado Plateau (Fig. 1) is from 158 Ma to 261 Ma. The oldest known plu- sample was collected less than a kilometer along not a viable source because it was masked by tons of the Cordilleran magmatic arc along the strike from an ignimbrite body that yielded a sedimentary cover in Jurassic time, and it was western fringe of Laurentia are 245–235 Ma in markedly discordant U-Pb age, interpreted by also partly buried by the northern end of the the Mojave Desert region of southern Califor- modeling to refl ect an emplacement age of 171 Navajo-Nugget erg (Fig. 10). Most Paleopro- nia (Barth and Wooden, 2006), and older arc- ± 2 Ma (Riggs et al., 1993). A statistical age terozoic grains could similarly have entered the derived grains (older than 245 Ma) are inferred peak of 164 Ma (n = 5 grains) for detrital zircons dispersal system from northern sources, among to refl ect contributions from the Permian-Trias- in the sample suggests that the discordant U-Pb which the Penokean belt near the Great Lakes is sic East Mexico magmatic arc (284–232 Ma) of age for the ignimbrite is unreliable, even with the most proximal (Fig. 1). Zircon grains older Torres et al. (1999) along the Gondwanan mar- modeling corrections. The maximum deposi- than 2000 Ma that are ascribed provisionally gin of Pangea (Fig. 1). tional age implied by the detrital zircons invites (Table 3) to sources in northwestern Lauren- Eolianites of the Upper Jurassic Bluff erg correlation with the Entrada Sandstone (San tia (Dickinson and Gehrels, 2003) may have (Fig. 2B) were deposited by westerly winds that Rafael Group) of the Colorado Plateau (Fig. 3) reached the terminus of the transcontinental could have carried arc-derived grains directly and suggests that transport of eolian sand south- paleoriver system by longshore transport along from the Cordilleran orogen to the Colorado ward to the fl ank of the Cordilleran arc occurred the Jurassic paleoshoreline of the Cordilleran Plateau (Fig. 1). Notably, all four statistical age during multiple defl ationary episodes recorded margin (Figs. 1 and 10). peaks for arc-derived grains (n = 19) in the two by the evolution of the Entrada erg (Kocurek, Early Mesoproterozoic Yavapai-Mazatzal Bluff samples (Figs. 5H and 5J) are younger than 1981; Carr-Crabaugh and Kocurek, 1998). (1.8–1.6 Ma) detritus (Fig. 1; Table 3) was prob- 235 Ma, implying derivation from the Cordille- The ages of 25 arc-derived grains in the ably contributed to the transcontinental pale- ran rather than the East Mexico arc. Net grain Mount Wrightson sample include two spurious oriver by southern tributaries draining residual populations in the Bluff samples also imply ages (90 Ma; 140 Ma) that are younger than its highlands of the Ancestral Rocky Mountains extensive recycling of older eolian sand into the depositional age, which is older than 155 Ma lying immediately east of the Colorado Plateau. Bluff erg of restricted areal extent (Fig. 2B). based on its unconformable position below the The coeval Central Plains Paleoproterozoic belt Eolianites of the Middle Jurassic Entrada erg Bisbee Group (Dickinson and Lawton, 2001b). was masked by sedimentary cover in Jurassic were deposited by winds blowing southwest There are also two solitary outlier ages (251 Ma; time (Fig. 10). Contributions from anorogenic across the Colorado Plateau toward the Cordil- 269 Ma), but most (n = 21) of the arc-derived pre-Grenvillian granitic rocks (Table 3) intru- leran margin (Fig. 2B). Four of the six statisti- grains span the age range of 235–160 Ma, with sive into the Yavapai-Mazatzal belt, but present cal age peaks in the two Entrada samples from statistical peaks at 164 Ma (n = 5 grains), 178 Ma farther east as well, probably entered the sedi- the eastern Colorado Plateau (Figs. 5G and 5I) (n = 8 grains), and 219 Ma (n = 8 grains), which ment dispersal system from the same southern are older than 245 Ma, suggesting important are compatible with derivation of zircon grains tributaries that contributed Yavapai-Mazatzal contributions from the East Mexico arc. Of the from igneous rocks of the associated Cordille- detritus. The anorogenic plutons (Anderson 30 arc-derived grains in the two samples, more ran arc assemblage. The other 64 detrital zircons and Morrison, 2005) are widespread across the than half are older than 245 Ma, the apparent (72% of the population) span the same general continental interior (Fig. 8), reaching as far east age limit of the Cordilleran arc, and two-thirds age range as non-Cordilleran grains in Colorado as the Grenville front (Van Breemen and David- are within the nominal age bracket (older than Plateau eolianites, but the two age spectra dif- son, 1988), but most were buried beneath plat- 232 Ma) for the East Mexico arc. Given the pale- fer in detail (Fig. 11). Age peaks for Paleozoic, form cover in Jurassic time (Fig. 10). More east- owind direction, these relations imply that detri- Neoproterozoic, and Archean grains are similar erly analogues were incorporated as deformed tus was transported north from the East Mexico in both age and relative signifi cance, but the enclaves within the Grenville orogen (Rivers, arc where it was exposed in the headwaters of a Grenvillian age population is less prominent in 1997; Rivers and Corrigan, 2000), and some southern tributary to the transcontinental trunk the Mount Wrightson Formation, and peaks for 1300–1500 Ma grains in plateau eolianites may paleoriver (Fig. 10) and then blown southwest older Proterozoic grains are broader. A compari- have been derived from pre-Grenvillian sources into the Entrada erg from sites of temporary son of the two age-distribution curves (Fig. 11) incorporated into the Grenville orogen. storage on the interior plains. As the Cordilleran suggests that the content of zircons derived from arc extended into northeastern Mexico (Fig. 1), the Yavapai-Mazatzal belt intruded by bod- Arc-Derived Grains a limited volume of arc-derived Cordilleran ies of anorogenic granite was enhanced during detritus could have accompanied the arc-derived transit of eolian sand across Yavapai-Mazatzal Grains younger than 285 Ma in age could East Mexico detritus northward. basement lying south of the Colorado Plateau only have been derived from the Cordilleran (Fig. 1) but north of the sample site (Fig. 2B). region along the western fl ank of Laurentia Arc-Flank Eolianite Dilution of Appalachian-derived detritus with (Fig. 1). Many interpretations are possible for Local lenses of quartz-rich eolian sandstone more local detritus is inferred to have been the the three solitary grains younger than 285 Ma are intercalated with Jurassic volcanic rocks factor that reduced the relative proportion of in six eolianites from the western Colorado Pla- along the inland fl ank of the Cordilleran mag- Grenvillian grains. teau (Fig. 6), including analytical error, and they matic arc southwest of the Colorado Plateau are not discussed further. Four eolianites of the (Fig. 2B). To compare detrital zircons in arc- CURTIS DETRITAL ZIRCONS San Rafael Group on the eastern Colorado Pla- fl ank and Colorado Plateau eolianites, we col- teau (Fig. 6) contain a total of 49 grains that are lected a sample from an ~250-m-thick lens of Figure 12 displays age-bin histograms and younger than 285 Ma, but statistical analysis of quartzose eolianite in the Mount Wrightson For- age-distribution curves for samples of fl uvial their age distribution is challenging because half mation of southern Arizona (CP2 of Fig. 2B). sandstone from the Glen Canyon Group and occur in just one sample (Fig. 5I). Statistical age Although exact correlations of the unfossilif- marine sandstone from the San Rafael Group peaks (n = 10), each defi ned by three or more erous eolian strata are contentious (Bilodeau (Fig. 3) to compare with plots in the same

Geological Society of America Bulletin, March/April 2009 425 Dickinson and Gehrels

Colorado Plateau contain similar grain populations (Figs. 12D, 12E, and 12G), with compara- Mount Wrightson tive P values from K-S analysis of three sample Formation pairs in the range of 0.25–0.83. The “Black (>285 Ma grains) Ledge” sandstone (Figs. 3 and 12C), exposed N n = 1; = 64 locally in Utah (Fig. 2A), is of somewhat different petrofacies than the Kayenta Formation (Table 2), but contains a similar grain population yielding a P value from K-S analysis of 0.12 when paired with a composite Kayenta grain population derived from three samples. The Springdale Sandstone Member (Fig. 3) to the southwest (Fig. 2A) contains a distinctive grain population (Fig. 12F) dominated by arc-

Age probability Age derived grains (n = 40 or 42.5% of total grains) for which discussion is deferred until after con- sideration of the Kayenta Formation. Our assign- Colorado Plateau eolianites ment of the “Black Ledge” sandstone to the (>285 Ma grains) N = 10; n = 890 Glen Canyon Group (Fig. 3) rather than to the underlying Chinle Group or Formation is supported by a “Black Ledge” statistical age peak (seven grains) of 203 Ma, which is younger than the Chinle depositional age of Carnian-Norian (Brack et al., 2005).

Kayenta Fluvial Sands 0 500 1000 1500 2000 2500 3000 Age (Ma) A comparison of age-distribution curves for Kayenta fl uvial and Glen Canyon eolian sam- Figure 11. Age-distribution curves for detrital zircon grains older than 285 Ma in Mount ples (Fig. 14) indicates a close similarity in grain Wrightson Formation (CP2) and 10 Colorado Plateau eolianites (Fig. 7). populations, except that the fl uvial sandstones contain a signifi cant proportion of arc-derived grains younger than 285 Ma. The P value for format for eolianite samples (Fig. 5). Figure 13 fl er, 1964) of that region contains ignimbrites grains older than 285 Ma in the two composite compares composite grain populations in two produced by explosive eruptions that may have grain populations is 0.65, indicating that the pre- samples of marine sandstone from the Curtis spread zircon grains eastward into the Curtis arc grains in the Kayenta Formation and Glen Formation (Figs. 12A and 12B) and two sam- seaway. Abundant mica fl akes (Tables 1 and Canyon eolianites are statistically indistinguish- ples of eolian sandstone from the underlying 2) that are present in samples from the Curtis able. The overall Kayenta detrital zircon popu- Entrada Sandstone (Figs. 5E and 5G) at locali- Formation but not the Entrada Sandstone also lation can be interpreted as the result of mixing ties within 100 km of the southeastern limit of indicate some addition of non-Entrada detritus, arc detritus with eolian sand redistributed from the Curtis transgression (Fig. 2B). The marine perhaps transported by longshore currents along penecontemporaneous (or directly underlying) and eolian grain populations are closely simi- the southern margin of the Sundance seaway unconsolidated erg deposits. lar, but they are not identical. A P value of 0.13 that lay northeast of the Colorado Plateau. On Statistical age peaks (n = 8), each controlled from K-S analysis for grains older than 175 Ma balance, however, the bulk of Curtis sand was by three or more grains in various of the Kay- (= >205 Ma) in the two composite age popula- apparently reworked from underlying Entrada enta samples, fall within the range 288–231 Ma, tions indicates inability at the 95% confi dence Sandstone as the Curtis marine transgression suggesting derivation dominantly from the East level to be certain that the Curtis zircon popula- proceeded. Although we did not sample eolian Mexico arc (284–232 Ma) rather than the Cor- tion was not largely recycled from the Entrada sandstone of the Moab Tongue (Figs. 2B and dilleran arc (younger than 245 Ma). Consistent zircon population. On the other hand, a statisti- 3), there is no reason to suspect that its detrital paleocurrent trends toward the west and north- cal age peak at 165 Ma for six grains in Curtis zircons would not closely resemble the Curtis west for the Kayenta Formation are compatible sample CP45 (Fig. 12B) is younger than any of and Entrada zircon populations. with contributions from the East Mexico arc the six age peaks in the range of 260–200 Ma for lying to the southeast of the Colorado Plateau Entrada samples (Figs. 5G and 5I) collected both FLUVIAL ZIRCON PROVENANCE (Fig. 1). Zircon grains of Grenvillian age in the near and far from Curtis exposures (Fig. 2B). Kayenta samples might also have been derived The youngest zircon grains in the Curtis The most widespread fl uvial unit within the in part from source rocks lying to the south- Formation control a sharp spike on the age- Glen Canyon Group is the Kayenta Formation east, but no apparent sources for the Archean distribution curve at 165 Ma (Fig. 13) and (Fig. 3), which is more feldspathic than underly- and Paleoproterozoic grains are evident south- probably derive from the backarc igneous ing and overlying eolian strata (Tables 1 and 2). east of the Colorado Plateau (Fig. 1). Accord- province of northern Nevada and westernmost Three samples collected from the Kayenta ingly, redistribution of eolian sand is our pre- Utah (Fig. 2B). The Pony Trail Group (Muf- Formation at widely separated localities on the ferred interpretation for the origin of all the

426 Geological Society of America Bulletin, March/April 2009 U-Pb ages of detrital zircons in Colorado Plateau eolianites C D uvial sandstone uvial B CP31 Black Ledge DOL Kayenta DOL oup (A–B) and ﬂ (0–4000 Ma), and ordinates are (0–4000 Ma), and ordinates are concordia diagrams at larger scale. concordia diagrams at larger CP45 Curtis E 0 1000 2000 3000 4000 0 1000 2000 3000 4000 8 0 0 8 6 4 6 2 4 2 10 NM CO 0 1000 2000 3000 4000 4 0 6 2 UT AZ CP10 Kayenta NV 0 1000 2000 3000 4000 2 6 4 0 CP43 Curtis CP1 Kayenta CP37 Springdale A 0 1000 2000 3000 4000 6 4 0 2 F G 0 1000 2000 3000 4000 0 1000 2000 3000 4000 6 0 4 2 5 0 15 25 10 20 from the Glen Canyon Group (C–G) keyed to collection localities on the Colorado Plateau (central map). Abscissas are grain ages Abscissas are (C–G) keyed to collection localities on the Colorado Plateau (central map). the Glen Canyon Group from AZ—Arizona; CO—Colorado; NM—New Mexico; NV—Nevada; UT—Utah. States: numbers of grains (variable scales). See Supplementary Text 3 (see text footnote 1) for the same and additional plots including 3 (see text footnote 1) for Text numbers of grains (variable scales). See Supplementary Figure 12. Age-bin histograms and age-distribution curves (superimposed) for samples of marine sandstone from the San Rafael Gr samples of marine sandstone from Age-bin histograms and age-distribution curves (superimposed) for 12. Figure

Geological Society of America Bulletin, March/April 2009 427 Dickinson and Gehrels

Kayenta pre-arc grains. Lack of augmentation of Paleoproterozoic or early Mesoproterozoic zircon grains in the Kayenta Formation, as Curtis Formation marine compared to Glen Canyon eolianites (Fig. 14), (CP43 - CP45) argues against signifi cant contribution of detri- N n = 2; = 187 tal zircons to the Kayenta Formation from the Ancestral Rocky Mountains province lying immediately east of the Colorado Plateau. Ancestral Rockies bedrock (Figs. 1 and 8) is Yavapai-Mazatzal basement intruded by slightly younger anorogenic plutons. The proportions of arc-derived detritus and redistributed eolian sand in the Kayenta For- mation can be estimated in two ways. A mix- Age probability ing line from the mean detrital mode of Glen Canyon eolianites (Qt86-F10-L4 from Table 1), Entrada Sandstone eolian through the mean detrital mode of Kayenta (CP16 - CPJenw) sandstones (Qt76-F17-L7 from Table 1), to the mean detrital mode of fi rst-cycle Paleogene N = 2; n = 183 arkoses (Qt40-F45-L15) in southern California (Dickinson, 1995) implies an admixture of 79% redistributed eolian sand and 21% arc detritus in Kayenta fl uvial sandstone. The content of arc-derived zircon grains (n = 30 total or mean of 12%) in the Kayenta samples, as opposed to 0 500 1000 1500 2000 2500 3000 grains older than 285 Ma, implies a somewhat Age (Ma) higher proportion of redistributed eolian sand (88%), but mature eolianites may contain a Figure 13. Composite age-distribution curves for detrital zircon grains in marine Curtis signifi cantly higher proportion of zircon grains (Figs. 5E and 5G) and eolian Entrada (Figs. 11A and 11B) samples from Utah (N = samples; than juvenile arkosic detritus. In any case, the n = grain ages). two mixing calculations yield similar estimates of admixture (~15% ± 5% arc detritus). Kayenta paleofl ow across the Colorado Plateau from the east and southeast implies the existence of an Early Jurassic sand blanket available for rework- Kayenta Formation fluvial ing from areas where erosion has since removed (CP1-CP10-DOL) all traces of its former presence (Fig. 2A). The N = 3; n = 251 redistribution of eolian sand into the Kayenta Formation suggests widespread intraregional recycling of durable zircon grains of dominantly Appalachian origin once mature sand of Appalachian provenance had been spread across southwest Laurentia. The presence of detrital zircons apparently derived from the East Mexico arc in both fl u- Age probability vial sandstones of the Kayenta Formation (Glen Canyon Group) and eolian sandstones of the Glen Canyon Group eolian San Rafael Group invites comparison of the (CP3-CP30-Jnnw-Jwnw) two sample sets (Fig. 15). The only salient difference is the presence of 11 grains younger N = 4; n = 386 than 195 Ma in the composite San Rafael grain population. These young grains derived from the Cordilleran arc defi ne a sharp age spike on the San Rafael plot that is not present on the plot for older Kayenta sandstones (Fig. 15). The P 0 500 1000 1500 2000 2500 3000 3500 value from K-S comparison of grains older than Age (Ma) 195 Ma in the two populations is 0.28, indicat- Figure 14. Composite age-distribution curves for detrital zircon grains in samples of Glen ing a lack of statistical distinction. Higher P/F Canyon eolianites (Figs. 5B–5D, and 5F) and fl uvial Kayenta Formation (Figs. 11D, 11E, ratios in detrital modes (Table 2) appear to be a and 11G) of Glen Canyon Group (N = samples; n = grain ages). subtle but reliable guide to the presence of arc

428 Geological Society of America Bulletin, March/April 2009 U-Pb ages of detrital zircons in Colorado Plateau eolianites detritus in both San Rafael eolian and Kayenta fl uvial sandstones. San Rafael Group eolian Springdale Sandstone Member (eastern Colorado Plateau) (CP15-CP16-CP24-CP54) The Springdale Sandstone Member of the n = 4; N = 365 Moenave Formation has been correlated with the Kayenta Formation, but the contrasting Spring- dale suite of detrital zircons (Fig. 12F) makes the relationship equivocal because Springdale and Kayenta depositional systems tapped different provenances. The abundance of Springdale arc- derived grains, coupled with the absence of any Age probability Paleozoic–Neoproterozoic grains or any grains older than 1850 Ma, suggests derivation from a Kayenta Formation fluvial provenance in southwest Laurentia lying directly (Glen Canyon Group) south of Springdale exposures (Fig. 1). All arc- (CP1-CP10-DOL) derived Springdale detrital zircons (n = 40) are n = 3; N = 255 younger than 245 Ma in age, as expected for derivation from the Cordilleran magmatic arc, with the only age peaks controlled by three or more grains at 210 Ma, 217 Ma, and 235 Ma. By contrast, more than half the Kayenta arc-derived 0 500 1000 1500 2000 2500 3000 3500 grains are 281–247 Ma, indicative of derivation Age (Ma) from the older East Mexico magmatic arc. There is, however, an areal variation in Figure 15. Composite age-distribution curves for samples of Entrada-Bluff (San Rafael Kayenta zircon populations that suggests the Group) eolian sandstone (Fig. 5GJ) and Kayenta (Glen Canyon Group) fl uvial sandstones Springdale and Kayenta fl uvial systems may (Figs. 11D, 11E, and 11 G) containing arc-derived Cordilleran grains younger than 285 Ma have followed parallel courses fl owing from (n = samples; N = grain ages). ESE to WNW across a joint fl oodplain, with Springdale detritus derived from southwest of the provenance for Kayenta detritus. The most southwestern Kayenta sample (CP10) contains plateau eolianites are composed dominantly of ACKNOWLEDGMENTS arc-derived zircon grains (n = 7 or 9% of total detritus derived ultimately from bedrock sources grains) that may be exclusively of Cordilleran in eastern Laurentia rather than the Cordilleran Advice from Ronald C. Blakey, Brian S. Currie, Peter G. DeCelles, Spencer G. Lucas, and Fred Peter- derivation (younger than 241 Ma), with an age orogen or the Ancestral Rocky Mountains prov- son was indispensable for sample collection, and peak defi ned by three grains at 231 Ma. The ince; (2) the non-Cordilleran detritus was trans- Gerald Bryant kindly provided sample DOL. Permis- other two Kayenta samples (CP1, DOL) contain ported to a position upwind from the Colorado sion was granted by the Hopi Tribe through Arnold arc-derived zircon grains (n = 21), of which 75% Plateau by a transcontinental paleoriver system Taylor for collection of sample CP10, by Grand Staircase-Escalante National Monument through are older than 245 Ma (range 281–247 Ma), with having its headwaters in the Appalachian belt, Marietta Eaton for collection of sample CP37, and age peaks defi ned by 5–10 grains of 246 Ma, with contributions of non-Appalachian detritus by the Pueblo of Laguna through Josephine Cochran 251 Ma, 265 Ma, and 280 Ma (indicative of delivered to the trunk stream by both northern for collection of sample CP54. We also appreciate sources in the East Mexico arc). Kayenta streams and southern tributaries during transit of sediment the unfailing courtesy of resident members of the farthest toward the northeast on the compound across the continent; (3) subordinate detritus in Hopi Tribe and Laguna Pueblo. For the U-Pb ages of detrital zircons, we thank Joseph R. Amar, Erin fl oodplain evidently redistributed eolian sand, to the eolian sands was derived from the Permian- V. Brenneman, Owen V. Hurd, Jennifer L. McGraw, which East Mexico arc detritus was added from Triassic East Mexico magmatic arc as well as the Gregory R. Schmidt, Carl E. Anderson, Carla M. the southeast, whereas Kayenta streams toward Triassic-Jurassic Cordilleran magmatic arc; (4) Eichler, Linette C. Ancha, and Jaika Ojha (all then the southwest may have redistributed eolian arc-fl ank eolianite south of the Colorado Plateau undergraduate students in the Colorado Plateau Sem- inar supported by the National Science Foundation in sand to which Cordilleran arc detritus was added is composed of plateau eolian sand contaminated the Department of Geosciences at the University of from the south. Springdale streams still farther with contributions from intervening basement Arizona), who processed all samples under our gen- to the southwest mixed detritus from Yavapai- and the enclosing arc assemblage; (5) marine eral guidance in the Arizona LaserChron Center of Mazatzal basement of southwest Laurentia with Curtis sand was largely reworked from underly- the University of Arizona. Jerome Guynn developed arc-derived Cordilleran detritus without contri- ing Entrada eolian sand; (6) fl uvial Kayenta sand the algorithm for applying Kolmogorov-Smirnoff statistics to populations of detrital zircons. Alex Pul- butions from the East Mexico arc. is an admixture of redistributed eolian sand and len aided in sample processing, and Jim Abbott of younger arc detritus; (7) Springdale provenance in SciGraphics prepared the fi gures. Reviews and edi- SUMMARY AND CONCLUSIONS the Cordilleran arc and its basement was distinct torial comments by Brendan Murphy, Jan Golonka, from Kayenta provenance; and (8) the youngest R.H. Rainbird, and John Waldron improved both text and fi gures. Our research was supported by National U-Pb ages of detrital zircons in Jurassic ergs detrital zircons in the “Black Ledge” sandstone Science Foundation grants EAR-0341987 (for Colo- and associated deposystems of the Colorado confi rm its affi nity with the Glen Canyon Group rado Plateau detrital zircons) and EAR-0443387 (to Plateau support the following conclusions: (1) rather than the Chinle Formation or Group. the Arizona LaserChron Center).

Geological Society of America Bulletin, March/April 2009 429 Dickinson and Gehrels

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