U-Pb ages of detrital zircons in Jurassic eolian and ... · along with recycled eolian sand, in...

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408 ABSTRACT U-Pb ages for 1655 individual detrital zir- con grains in 18 samples of eolian and asso- ciated marine and fluvial sandstones of the Glen Canyon and San Rafael Groups from the Colorado Plateau and contiguous areas shed light on patterns of Jurassic sediment dispersal within Laurentia. Most detrital zir- con grains in Jurassic eolianites were derived ultimately from basement provinces older than 285 Ma in eastern and central Lauren- tia, rather than from rock assemblages of the nearby Cordilleran margin. The most promi- nent peaks of constituent age populations at 420 Ma, 615 Ma, 1055 Ma, and 1160 Ma reflect derivation from Paleozoic, Neopro- terozoic, and Grenvillian sources within the Appalachian orogen or its sedimentary cover. Sediment was transported to a posi- tion upwind to the north of the Colorado Plateau by a transcontinental paleoriver sys- tem with headwaters in the central to south- ern Appalachian region, but subordinate non-Appalachian detritus was contributed by both northern and southern tributaries during sediment transit across the continent. Subordinate detrital zircons younger than 285 Ma in selected Middle to Upper Juras- sic eolianites were derived from the Perm- ian-Triassic East Mexico and the Mesozoic Cordilleran magmatic arcs. Lower Jurassic fluvial sandstones typically contain a mixture of detrital zircons redistributed from eolian sand and derived from the East Mexico arc, which lay up-current to the southeast. Zir- cons in marine Curtis sandstone were largely reworked from underlying Entrada eolianite, with minor contributions from the Jurassic backarc igneous assemblage of the Great Basin. Once mature quartzose detritus was dispersed widely across southwest Laurentia by a transcontinental paleoriver system and paleowinds, which deposited extensive Juras- sic ergs, durable zircon grains were recycled by multiple intraregional depositional sys- tems. Lower Jurassic fluvial sand is locally composed, however, of detritus derived from the nearby Cordilleran magmatic arc assem- blage and its Precambrian basement. Keywords: Colorado Plateau, detrital zircon, eolianite, Glen Canyon Group, provenance, San Rafael Group. INTRODUCTION Dickinson and Gehrels (2003) showed that typical Jurassic eolianites (eolian sandstones) of Colorado Plateau ergs (sand seas) contain age populations of detrital zircons derived from essentially all Precambrian and Paleozoic grani- toid basement provinces of central and eastern Laurentia (Fig. 1), though a few Mesozoic zir- con grains were derived from the nearby Cordil- leran orogen. Sand was transported across the continent by a Jurassic paleoriver, which had its headwaters in the Appalachian province, to fluvial or deltaic plains lying north of the Colo- rado Plateau. From temporary sediment storage there, sand was then blown southward to grow- ing Colorado Plateau ergs by paleowinds well known from eolian cross-bedding. Our preliminary study was restricted to just three samples of Jurassic eolianite from a single stratigraphic section near the center of the Colo- rado Plateau, and it was inadequate to gauge the possible variability of detrital zircons in eolian- ites of various ages exposed across the length and breadth of the Colorado Plateau. For this study, we expanded analytical coverage to include seven more samples of Lower to Upper Jurassic eolianites distributed over >175 × 10 3 km 2 of the Colorado Plateau, correlative intra-arc eolianite from southern Arizona, and seven samples of fluvial and marine sandstones closely associated with erg deposits of the Glen Canyon and San Rafael Groups. We present here 1655 reliable U-Pb ages (concordant or nearly so) for indi- vidual zircon grains from 18 samples. An evaluation of the enlarged database con- firms transcontinental dispersal of sand to Juras- sic ergs of the Colorado Plateau. Admixtures of grains derived from basement or magmatic arcs in southwestern North America are present, along with recycled eolian sand, in associated fluvial units, and these combinations occur as well in selected eolian units, including some either intercalated with arc volcanics or depos- ited by westerly winds. Recycling of eolian sand by intraregional depositional systems can be attributed to the durability of zircon grains in sedimentary environments. Nevertheless, docu- mentation that voluminous sand of the central and eastern Laurentian provenance dominated Jurassic eolian deposits on the Colorado Plateau through a period of >40 m.y. indicates the per- sistence of an integrated system for transconti- nental sediment dispersal. Descriptions of sample localities (see Sup- plementary Text 1) and U-Pb age data for all samples discussed in this paper are included in accompanying data repositories. 1 The tabu- lated analytical data (Supplementary Text 2) are supplemented by a concordia diagram for each sample and superimposed graphs of age- bin histograms and probability-density plots (age-distribution curves) for grains in each For permission to copy, contact [email protected] © 2008 Geological Society of America GSA Bulletin; March/April 2009; v. 121; no. 3/4; p. 408–433; doi: 10.1130/B26406.1; 15 figures; 3 tables; Data Repository item 2008251. 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 E-mail: [email protected]. 1 GSA Data Repository item 2008251, ST1 (in Word) is a listing of sample localities, ST2 (in csv) is U-Pb analytical data for all samples, ST3 (pdf) is concordia diagrams and combined age-bin histo- grams and age-distribution curves (probability-den- sity plots) for each sample, and ST4 (in Word) is a table of ages and references for Figure 9; available at http://www.geosociety.org/pubs/ft2008.htm or by request to [email protected].

Transcript of U-Pb ages of detrital zircons in Jurassic eolian and ... · along with recycled eolian sand, in...

Page 1: U-Pb ages of detrital zircons in Jurassic eolian and ... · along with recycled eolian sand, in associated fl uvial units, and these combinations occur as well in selected eolian

408

ABSTRACT

U-Pb ages for 1655 individual detrital zir-con grains in 18 samples of eolian and asso-ciated marine and fl uvial sandstones of the Glen Canyon and San Rafael Groups from the Colorado Plateau and contiguous areas shed light on patterns of Jurassic sediment dispersal within Laurentia. Most detrital zir-con grains in Jurassic eolianites were derived ultimately from basement provinces older than 285 Ma in eastern and central Lauren-tia, rather than from rock assemblages of the nearby Cordilleran margin. The most promi-nent peaks of constituent age populations at 420 Ma, 615 Ma, 1055 Ma, and 1160 Ma refl ect derivation from Paleozoic, Neopro-terozoic, and Grenvillian sources within the Appalachian orogen or its sedimentary cover. Sediment was transported to a posi-tion upwind to the north of the Colorado Plateau by a transcontinental paleoriver sys-tem with headwaters in the central to south-ern Appalachian region, but subordinate non-Appalachian detritus was contributed by both northern and southern tributaries during sediment transit across the continent. Subordinate detrital zircons younger than 285 Ma in selected Middle to Upper Juras-sic eolianites were derived from the Perm-ian-Triassic East Mexico and the Mesozoic Cordilleran magmatic arcs. Lower Jurassic fl uvial sandstones typically contain a mixture of detrital zircons redistributed from eolian sand and derived from the East Mexico arc, which lay up-current to the southeast. Zir-cons in marine Curtis sandstone were largely reworked from underlying Entrada eolianite, with minor contributions from the Jurassic backarc igneous assemblage of the Great Basin. Once mature quartzose detritus was

dispersed widely across southwest Laurentia by a transcontinental paleoriver system and paleowinds, which deposited extensive Juras-sic ergs, durable zircon grains were recycled by multiple intraregional depositional sys-tems. Lower Jurassic fl uvial sand is locally composed, however, of detritus derived from the nearby Cordilleran magmatic arc assem-blage and its Precambrian basement.

Keywords: Colorado Plateau, detrital zircon, eolianite, Glen Canyon Group, provenance, San Rafael Group.

INTRODUCTION

Dickinson and Gehrels (2003) showed that typical Jurassic eolianites (eolian sandstones) of Colorado Plateau ergs (sand seas) contain age populations of detrital zircons derived from essentially all Precambrian and Paleozoic grani-toid basement provinces of central and eastern Laurentia (Fig. 1), though a few Mesozoic zir-con grains were derived from the nearby Cordil-leran orogen. Sand was transported across the continent by a Jurassic paleoriver, which had its headwaters in the Appalachian province, to fl uvial or deltaic plains lying north of the Colo-rado Plateau. From temporary sediment storage there, sand was then blown southward to grow-ing Colorado Plateau ergs by paleowinds well known from eolian cross-bedding.

Our preliminary study was restricted to just three samples of Jurassic eolianite from a single stratigraphic section near the center of the Colo-rado Plateau, and it was inadequate to gauge the possible variability of detrital zircons in eolian-ites of various ages exposed across the length and breadth of the Colorado Plateau. For this study, we expanded analytical coverage to include seven more samples of Lower to Upper Jurassic eolianites distributed over >175 × 103 km2 of the Colorado Plateau, correlative intra-arc eolianite

from southern Arizona, and seven samples of fl uvial and marine sandstones closely associated with erg deposits of the Glen Canyon and San Rafael Groups. We present here 1655 reliable U-Pb ages (concordant or nearly so) for indi-vidual zircon grains from 18 samples.

An evaluation of the enlarged database con-fi rms transcontinental dispersal of sand to Juras-sic ergs of the Colorado Plateau. Admixtures of grains derived from basement or magmatic arcs in southwestern North America are present, along with recycled eolian sand, in associated fl uvial units, and these combinations occur as well in selected eolian units, including some either intercalated with arc volcanics or depos-ited by westerly winds. Recycling of eolian sand by intraregional depositional systems can be attributed to the durability of zircon grains in sedimentary environments. Nevertheless, docu-mentation that voluminous sand of the central and eastern Laurentian provenance dominated Jurassic eolian deposits on the Colorado Plateau through a period of >40 m.y. indicates the per-sistence of an integrated system for transconti-nental sediment dispersal.

Descriptions of sample localities (see Sup-plementary Text 1) and U-Pb age data for all samples discussed in this paper are included in accompanying data repositories.1 The tabu-lated analytical data (Supplementary Text 2) are supplemented by a concordia diagram for each sample and superimposed graphs of age-bin histograms and probability-density plots (age-distribution curves) for grains in each

For permission to copy, contact [email protected]© 2008 Geological Society of America

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.

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. GehrelsDepartment of Geosciences, University of Arizona, Tucson, Arizona 85721, USA

†E-mail: [email protected].

1GSA Data Repository item 2008251, ST1 (in Word) is a listing of sample localities, ST2 (in csv) is U-Pb analytical data for all samples, ST3 (pdf) is concordia diagrams and combined age-bin histo-grams and age-distribution curves (probability-den-sity plots) for each sample, and ST4 (in Word) is a table of ages and references for Figure 9; available at http://www.geosociety.org/pubs/ft2008.htm or by request to [email protected].

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U-Pb ages of detrital zircons in Colorado Plateau eolianites

Geological Society of America Bulletin, March/April 2009 409

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Ages of Belts in GaArchean Cross-Hatched

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.

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sample falling within the ranges of 0–4000 Ma, 0–800 Ma, and 800–2400 Ma (Supplementary Text 3). Preliminary U-Pb age data for three of the eolianite samples (Jwnw—Wingate; Jnnw—Navajo; Jenw—Entrada) from North Wash in Utah (Dickinson and Gehrels, 2003) are superseded by data of improved precision and accuracy obtained during the present study (Gehrels et al., 2008). Preliminary data for other samples were presented by Amar and Bren-neman (2005), Brenneman and Amar (2005), Hurd and Schmidt (2005), Schmidt et al. (2005), Amar et al. (2006), Brenneman et al. (2006), and Dickinson et al. (2007). Comparative data for the Navajo Sandstone in Zion National Park were reported by Rahl et al. (2003).

ANALYTICAL METHODS

Samples of sandstone were collected in plas-tic buckets of fi ve-gallon capacity as 22–24 kg of rock chips <10 cm in largest dimension from selected areas or horizons of the outcrops at sam-pling localities. Zircon crystals were extracted from samples by traditional methods of crushing and grinding, followed by separation with a Wil-fl ey table, heavy liquids, and a Frantz magnetic separator. Sample processing was designed to retain all zircon grains in the fi nal heavy mineral fraction. A large split of these grains (generally 1000–2000) was incorporated into a 1 in. epoxy mount together with fragments of standard Sri Lanka zircon and SRM 610 trace-element glass. The mounts were sanded down to a depth of ~20 µm, polished, imaged, and cleaned prior to iso-topic analysis.

For analysis by laser ablation, target zircons are preferably >35 µm (>0.035 mm) in diameter, or at least as coarse as very fi ne sand. Given the high specifi c gravity of zircon (4.65), as com-pared to quartz (2.65), hydraulically equivalent zircon is expected to be approximately one sand grade fi ner than accompanying quartz grains (Komar, 2007). Accordingly, we preferentially sought samples composed of medium quartzose sand because detrital zircon grains might be too small to occur in abundance with coarse quartz sand, but we were also able to date zircon grains from the two-thirds of our samples composed of fi ne quartzose sand.

Age Determination

U-Pb geochronology of ~100 individual zir-con grains per sample was conducted by laser-ablation–multicollector inductively coupled plasma–mass spectrometry (LA-MC-ICP-MS) at the Arizona LaserChron Center (Gehrels et al., 2006). The analyses involve ablation of zir-con with a New Wave DUV193 Excimer laser

(operating at a wavelength of 193 nm) using a spot diameter of 35 µm. The ablated material was carried in helium into the plasma source of a GVI Isoprobe, which was equipped with a fl ight tube of suffi cient width that U, Th, and Pb isotopes could be measured simultane-ously. All measurements were made in static mode, using Faraday detectors with 10e11 ohm resistors for 238U, 232Th, 208Pb, and 206Pb, a Faraday detector with a 10e12 ohm resis-tor for 207Pb, and an ion-counting channel for 204Pb. Ion yields were ~1.0 mv per ppm. Each analysis consisted of one 12-second integration on peaks with the laser off (for backgrounds), twelve 1-second integrations with the laser fi r-ing, and a 30-second delay to purge the previ-ous sample and prepare for the next analysis. The ablation pit was ~12 µm in depth.

For each analysis, the errors in determining 206Pb/238U and 206Pb/204Pb resulted in a mea-surement error of ~1%–2% (at 2σ level) in the 206Pb/238U age. The errors in measurement of 206Pb/207Pb and 206Pb/204Pb also resulted in ~1%–2% (at 2σ level) uncertainty in age for grains older than 1000 Ma, but errors were sub-stantially larger for younger grains due to the low intensity of the 207Pb signal. For most analy-ses, the crossover in precision of 206Pb/238U and 206Pb/207Pb ages occurs at ca. 1000 Ma.

Common Pb correction was accomplished by using the measured 204Pb and assuming an ini-tial Pb composition from Stacey and Kramers (1975) with uncertainties of 1.0 for 206Pb/204Pb and 0.3 for 207Pb/204Pb. Our measurement of 204Pb was unaffected by the presence of 204Hg because backgrounds were measured on peaks (thereby subtracting any background 204Hg and 204Pb), and because very little Hg was present in the argon gas (background 204Hg = ~300 counts per second).

Interelement fractionation of Pb/U is gener-ally ~20%, whereas fractionation of Pb isotopes is generally ~2%. In-run analysis of fragments of a large Sri Lanka zircon crystal (generally every fi fth measurement) with known age of 564 ± 4 Ma (2σ error) was used to correct for this fractionation. The uncertainty resulting from the calibration correction was generally 1%–2% (2σ) for both 206Pb/238U and 206Pb/207Pb ages. Concentrations of U and Th were cali-brated relative to the Sri Lanka zircon standard and SRM 610 trace-element glass, which con-tains ~460 ppm of each element.

Data Presentation

Full analytical data are reported in Supple-mentary Text 2 (ST2; see footnote 1). Age uncertainties (at 1σ) for individual grains in the data table include only measurement errors.

Interpreted ages are based on 206Pb/238U for grains younger than 1000 Ma and 206Pb/207Pb for grains older than 1000 Ma. The division point for each sample is at a slightly different age near 1000 Ma to avoid splitting up age clusters of grains. In any case, age uncertainties are inher-ently greatest near 1000 Ma (Gehrels, 2000).

Analyses that were >30% discordant (by comparison of 206Pb/238U and 206Pb/207Pb ages) or >5% reverse discordant were not consid-ered further (average of 92 grain ages retained per sample). The resulting interpreted ages are shown for individual samples on superimposed relative age-probability plots (from Ludwig, 2003) and age-bin histograms. For the latter, best estimates of ages are assigned arbitrarily to age bins of 20 m.y. each, starting at 0 Ma. The age-probability plots incorporate each age and its uncertainty (for measurement error only) as a normal distribution and sum all ages from a sample into a single curve. The resulting age-probability plots derived from the probability density function were made from an in-house Excel program (available from www.geo.ari-zona.edu/alc) that normalizes each curve accord-ing to the number of constituent analyses, such that each curve contains the same area, and then stacks the probability curves.

There is a temptation to regard the age-bin histograms as raw data and the age-probability plots as derivative curves, but the reverse is the case. Age uncertainties for some individual grains are larger than the widths of the age bins for the histograms, which are accordingly selec-tive plots of the analytical data. By contrast, the age-probability plots take all age uncertain-ties into account (Vermeesch, 2004) and are accordingly here termed age-distribution curves because they display graphically all the analyti-cal data. The value of the age-bin histograms is to provide a graphic impression of the numbers of grains associated with age peaks on the age-distribution curves and thereby allow age peaks, no matter how sharp, associated with only one or two grain ages to be discounted relative to age peaks, even if broad, associated with mul-tiple grain ages. Age-bin histograms are omitted from age-distribution curves composited from multiple related samples.

Statistical Comparisons

Although age populations of detrital zircons in different samples can be compared by inspection of their respective age-distribution curves and age-bin histograms, it is diffi cult to gauge visu-ally the degree of similarity or dissimilarity of any two age populations. We have found it useful to apply Kolmogoroff-Smirnoff (K-S) statistics (Press et al., 1986, p. 472–474) to intersample

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Geological Society of America Bulletin, March/April 2009 411

comparisons using an in-house Excel program (available from www.geo.arizona.edu/alc). The K-S test mathematically compares two age dis-tributions, taking the uncertainties of grain ages into account, to determine whether there is a statistically signifi cant difference between the two. The K-S test determines a P value, which assesses the probability that differences between two age-distribution curves could be due to ran-dom choice of grains during analysis. Where P > 0.05, one cannot be 95% confi dent that two age populations were not selected randomly from the same parent population. Where this criterion is met, we conclude that the two age populations compared are statistically indistinguishable for purposes of assessing provenance relations.

STRATIGRAPHIC CONTEXT

Jurassic erg accumulations of the Colorado Plateau form much of the Glen Canyon (Fig. 2A) and San Rafael (Fig. 2B) Groups, each of which contains erg deposits that were laterally continu-ous over >250 × 103 km2 of the Colorado Plateau and adjacent areas before Cenozoic erosion. The eolian deposits are assigned to multiple forma-tions separated by thinner intervals of noneo-lian strata (Fig. 3). Lateral age relationships are inferred from sparse fossil localities dispersed widely in the dominantly nonmarine strata. Although exact calculations are impossible, the net volume of eolian sand in the Jurassic ergs exceeds 100 × 103 km3.

That volume is a minimum because (1) an outlier (Fig. 2A) of the Glen Canyon Group in the Great Basin to the west of the Colo-rado Plateau suggests that much of the ground between the Colorado Plateau and the Cordil-leran magmatic arc may once have been cov-ered by Lower Jurassic eolian sand (Stewart, 1980); (2) the present limit of the San Rafael Group east of the Colorado Plateau (Fig. 2B) is erosional, and Middle Jurassic eolian sand may have extended across an indeterminate width of the Great Plains in the continental interior (Kocurek and Dott, 1983); and (3) quartzose dune sand intercalated with arc volcanics along the inland fl ank of the Cordilleran magmatic arc in southeastern California and southern Arizona (Fig. 2B) occurs beyond a belt from which all correlative strata have been removed by erosion, and Jurassic ergs may once have been continuous southward from the Colorado Plateau until eolian sand transport was arrested by arc paleotopography (Busby-Spera, 1988). The inferred original extents of the Glen Can-yon and San Rafael ergs are both comparable to the extent (~500 × 103 km2) of the largest modern erg, the Rub’ al Khali in the Empty Quarter of the Arabian Peninsula.

If the permissive extent of Jurassic ergs is taken into account, exposures of eolianite con-tiguous with outcrops on the Colorado Plateau may represent just a relict fraction of eolian sand that summed to a total volume approach-ing 250 × 103 km3 across southwest Laurentia as a whole. As the Jurassic ergs mantled sub-stratum continuously throughout their extent, a superregional system for sediment dispersal was required to deliver the voluminous sand to the erg accumulations from outside the erg bound-aries. Because the eolian sand is ~85% quartz (see following), and quartz forms only a third or less of typical granitic rocks by volume, perhaps 250 to 650 × 103 km3 of granitoid bedrock was eroded to produce the eolian sand in Colorado Plateau and related Jurassic eolianites.

The Glen Canyon Group is underlain by Upper Triassic fl uvial strata of the Chinle For-mation or Group, and the San Rafael Group is overlain by Upper Jurassic fl uvial strata of the Morrison Formation (Fig. 3). Glen Canyon and San Rafael ergs developed during progressive northward drift of Laurentia across the global desert belt lying within paleolatitudes of 17°–28° (Parrish and Peterson, 1988; Dickinson, 2005). Underlying Chinle fl oodplains were deposited at lower paleolatitudes in the sub-tropical belt before transit of Laurentia through the desert belt (Dubiel, 1994), and overlying Morrison fl oodplains were deposited at higher paleolatitudes within the global belt of westerly winds (Turner and Peterson, 2004).

Paleowinds inferred from the orientation of eolian cross-bedding in the Jurassic erg depos-its blew southward (in modern coordinates) for the Glen Canyon Group (Fig. 2A) and for most of the San Rafael Group (Fig. 2B). The observed paleowind directions are compatible with prevailing wind patterns inferred from reconstructions of Jurassic paleogeography and paleoclimate (Parrish and Peterson, 1988; Golonka et al., 1994), but the plateau paleo-wind vectors (Fig. 2) are not dependent upon theoretical retrodictions.

The paleowind directions imply derivation of the bulk of the eolian sand in the Jurassic ergs from proximate sources lying north of the Colo-rado Plateau. Paleowinds that blew toward the east-northeast during deposition of the youngest (Upper Jurassic) San Rafael erg deposits (Bluff Sandstone) of the southern Colorado Plateau (Fig. 2B) imply either internal redistribution of erg sand or additional sources of sand, or both. The persistence of eolian environments into Late Jurassic time on the southern Colorado Plateau may refl ect a more southerly paleolati-tude as Laurentia drifted north, or the formation of a rain shadow in the lee of rising Cordilleran highlands, or both effects.

Glen Canyon Group

Over the central Colorado Plateau, a distinc-tive formational triad (eolian Wingate Sand-stone, fl uvial Kayenta Formation, eolian Navajo Sandstone) is typical of the Glen Canyon Group (Fig. 3). To the north, the fl uvial Kayenta Forma-tion (Miall, 1988) pinches out in the subsurface of the Uinta Basin (Fig. 2A), and the unbroken eolian succession farther north, along the south fl ank of the Uinta Mountains, is equivalent to the Nugget Sandstone of Wyoming (Johnson and Johnson, 1991). To the southwest (Fig. 2A), eolian Wingate Sandstone intertongues with fl uvial deposits of the Moenave Formation (Blakey, 1989, 1994; Peterson, 1994), and the Navajo Sandstone is termed the Aztec Sand-stone in exposures southwest of the Colorado Plateau (Stewart, 1980). The Springdale Sand-stone Member (Fig. 3) was long regarded as the uppermost member of the Moenave Formation (Harshbarger et al., 1957; Blakey, 1989), but it is now widely regarded as a lowermost member of the Kayenta Formation (Riggs and Blakey, 1993; Lucas et al., 2005; Lucas and Tanner, 2006; Kirkland and Milner, 2006; Tanner and Lucas, 2007).

Basal ContactThe eolian Wingate Sandstone is underlain

by a red-bed interval, dominantly siltstone, that was named the Rock Point Member of the Wingate Formation in Arizona (Harshbarger et al., 1957) and the Church Rock Member of the Chinle Formation in Utah (Stewart, 1957), with an arbitrary change in stratigraphic nomencla-ture near the Arizona-Utah border (Stewart et al., 1972; Blakey and Gubitosa, 1983; Dubiel, 1989). The same stratigraphic interval in south-western Colorado forms the upper member of the Dolores Formation (Dubiel, 1989; Lucas et al., 1997; Lucas and Heckert, 2005), and litho-logically similar strata farther north have been termed red siltstone, ocher siltstone, and upper members of the Chinle Formation (Stewart et al., 1972; Dubiel, 1992). To resolve the nomen-clatural dichotomy, the Rock Point–Church Rock interval has been termed the Rock Point Formation (Lucas and Hunt, 1992; Lucas and Heckert, 2005), but that usage is not universal. The Rock Point Formation is widely exposed eastward beyond the limit of Glen Canyon erg deposition (Fig. 2A), and it locally interfi ngers upwind toward the northwest with the overlying Wingate Sandstone (Harshbarger et al., 1957).

The contact between the Chinle Group (or Formation) and the Glen Canyon Group is commonly placed at the base of the Wingate Sandstone where desiccation mud cracks in the tops of the underlying Rock Point Member

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412 Geological Society of America Bulletin, March/April 2009

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aleo

curr

ents

exte

nt o

feo

lian

ergs

dow

nwin

d as

sem

blag

e(o

nly

Roc

k P

oint

-Chu

rch

Roc

k in

terv

al e

xpos

edbe

low

Ent

rada

San

dsto

ne)

depo

sitio

nal

limits

ofke

y flu

vial

syst

ems

back

arc

mar

ine

basi

n

eros

iona

led

ge o

fex

posu

res

mid

-Cre

tace

ous

(Dak

ota)

over

lap

Col

orad

oP

late

au

Nug

get

erg

Cur

rieer

gou

tlier

Mid

dle

Jura

ssic

(Sun

danc

e-E

ntra

da)

onla

p or

ove

rlap

S

3

G

31

D37

10

1 N

A

40°

N

Kay

enta

Moe

nave

Bla

ckLe

dge

L

30

P

Azt

ecer

g

010

020

030

0

scal

e in

km

A Fig

ure

2. D

istr

ibut

ion

and

faci

es o

f (A

) up

perm

ost

Tri

assi

c to

Low

er J

uras

sic

Gle

n C

anyo

n G

roup

and

(B

) M

iddl

e Ju

rass

ic t

o lo

wer

mos

t U

pper

Jur

assi

c Sa

n R

afae

l Gro

up

wit

hin

and

near

the

Col

orad

o P

late

au (s

hade

d ou

tlin

e) in

rel

atio

n to

sam

ple

loca

litie

s (cr

osse

s): p

refi x

CP

not a

ffi x

ed to

num

eric

al sa

mpl

e nu

mbe

rs. D

—sa

mpl

e D

OL

; N—

Nor

th

Was

h sa

mpl

es (

Jwnw

, Jnn

w, J

enw

). S

ubsu

rfac

e lim

it o

f po

st-T

odilt

o (B

luff

) er

g ne

ar F

our

Cor

ners

(co

njun

ctio

n of

Ari

zona

–Uta

h–C

olor

ado–

New

Mex

ico)

is

adap

ted

afte

r L

upe

(198

3). P

aleo

win

ds a

re a

fter

Pet

erso

n (1

988b

), a

nd fl

uvia

l pal

eocu

rren

ts a

re a

fter

Lut

trel

l (19

93, 1

996)

. Pos

itio

ns o

f m

agm

atic

arc

are

aft

er D

icki

nson

(20

04),

ret

roar

c L

unin

g-F

ence

mak

er th

rust

bel

t is

afte

r W

yld

(200

2), b

acka

rc m

agm

atis

m is

aft

er D

icki

nson

(200

6), a

nd U

tah-

Idah

o tr

ough

is a

fter

Pet

erso

n (1

972)

. Cit

ies:

A—

Alb

uque

rque

; F

—F

lags

taff

; G

—G

rand

Jun

ctio

n; L

—L

as V

egas

; P

—P

hoen

ix;

S—Sa

lt L

ake

Cit

y. (

Con

tinue

d on

fol

low

ing

page

.)

Page 6: U-Pb ages of detrital zircons in Jurassic eolian and ... · along with recycled eolian sand, in associated fl uvial units, and these combinations occur as well in selected eolian

U-Pb ages of detrital zircons in Colorado Plateau eolianites

Geological Society of America Bulletin, March/April 2009 413

+sa

mp

les

citi

es

*****

* ******* **

* *

**** ** ** *****

S

G

L

P

A

++

++

+

+

+

*

CA

NV

40°

N

N

110°

W

40°

N

105°

W

NV

U

T

35°

N12

0° W

115°

W

AZ

N

M

105°

WN

M

TX

35°

N

CA

A

Z

+

ID UT

WY

CO

OK

TX

CO

NM

UT

AZ

010

020

0

scal

e in

km

Pag

e-E

ntra

da p

aleo

win

ds

Blu

ff pa

leow

inds

Jura

ssic

bac

karc

plu

tons

Tem

ple

Cap

Pag

e

Moa

bTo

ngue

limit

of p

ost-

Todi

lto B

luff

erg

limits

of

subo

rdin

ate

ergs

limit

of T

odilt

o no

nmar

ine

salin

a ab

ove

Ent

rada

erg

limit

of C

urtis

mar

ine

tran

sgre

ssio

n ov

er E

ntra

da e

rg

late

ral t

rans

ition

from

slic

kroc

k (e

rg)

to e

arth

yfa

cie

s of

Ent

rada

San

dsto

ne

back

arc

mag

mat

ism

(165

-145

Ma)

Luni

ng-

Fen

cem

aker

thru

st b

elt

(175

- 1

65 M

a)

Uta

h -

Idah

otr

ough

(170

- 1

60 M

a)

eros

iona

led

ge o

fex

posu

res

mid

-Cre

tace

ous

(Dak

ota)

over

lap

arc-

flank

dune

san

ds

Col

orad

oP

late

au

C o r di l

l e r a

nm

a g ma t i c

a r c

45 43

16

N

12

24

2

15

NV

CA

ext

ent

of E

ntra

da(a

nd r

elat

ed)

eolia

n er

gs

+54

back

arc

basi

n

B

Fig

ure

2 (c

ontin

ued)

.

Page 7: U-Pb ages of detrital zircons in Jurassic eolian and ... · along with recycled eolian sand, in associated fl uvial units, and these combinations occur as well in selected eolian

Dickinson and Gehrels

414 Geological Society of America Bulletin, March/April 2009

A

B

C

DE

F

CP12

CP30

CP37

CP10

CP43

CP45 CP3

Jnnw

CP16

Jenw

CP1

Jwnw

CP31

DOL

CP15

CP24

Zio

n ar

ea

Dak

ota

Fm

(Cre

tace

ous)

Car

mel

For

mat

ion

Nav

ajo

(-A

ztec

)

San

dsto

ne

Kay

enta

For

mat

ion

("si

ltyfa

cies

")

Moe

nave

For

mat

ion

Chi

nle

Fm

or

Gp

Lake

Pow

ell

area

Mor

rison

For

mat

ion

Ent

rada

San

dsto

ne

Car

mel

For

mat

ion

Pag

eS

ands

tone

Nav

ajo

San

dsto

ne

Kay

enta

For

mat

ion

Moe

nave

For

mat

ion

Chi

nle

Fm

or

Gp

Han

ksvi

llear

ea

Mor

rison

For

mat

ion

Sum

mer

ville

For

mat

ion

En

tra

da

San

dsto

ne

Car

mel

For

mat

ion

Pag

e S

s (off

map

)

Nav

ajo

San

dsto

ne

Kay

enta

For

mat

ion

Win

gate

San

dsto

ne

RP

/CR

Chi

nle

Fm

or

Gp

Moa

b-A

rche

sar

ea

Mor

rison

For

mat

ion

Ent

rada

San

dsto

ne

Nav

ajo

San

dsto

ne

Kay

enta

For

mat

ion

Win

gate

San

dsto

ne

RP

/CR

Chi

nle

Fm

or

Gp

San

Jua

nR

iver

are

a

Mor

rison

For

mat

ion

Blu

ff

San

dsto

ne

Ent

rada

San

dsto

ne

Nav

ajo

San

dsto

ne

Kay

enta

For

mat

ion

Win

gate

San

dsto

ne

RP

/CR

Chi

nle

Fm

or

Gp

Gal

lup-

Gra

nts

area

Mor

rison

For

mat

ion

Blu

ffS

ands

tone

Ent

rada

San

dsto

ne

(str

afal

onl

ap)

Lith

olog

ic K

ey

mar

ine

sand

ston

e

nonm

arin

e -

mar

gina

lm

arin

esa

nds t

one

(non

eolia

n)

eolia

n sa

ndst

one

(erg

eol

iani

te)

fine-

grai

ned

(sha

ly)

nonm

arin

e-m

argi

nal

mar

ine

(and

loca

lm

arin

e) d

epos

itsflu

vial

san

dsto

ne lacu

strin

e sa

lina

top

of p

re-e

rgflu

vial

str

ata

SS

M

Jura

ssic

Gro

up

Gro

up

Jura

ssic

Raf

ael

Can

yon

San

Gle

n

TC ba

seto

p

top

strataloverla

p

RS

base of of of

of

CF

base

of p

ost-

erg

fluvi

al s

trat

a

UT

AZ

NVUT

CO

NM

UT

C

O

A

CD E F

B

scal

e

150

m

100

m

50 m

0

Upp

er

Jura

ssic

base

of

Low

er

Low

er

BL

Map

Key

TF

MT

CP54

Fig

ure

3. R

egio

nal s

trat

igra

phic

rel

atio

ns (

thic

knes

ses

appr

oxim

ate)

of

sam

pled

erg

eol

iani

tes

and

asso

ciat

ed s

trat

a of

Jur

assi

c G

len

Can

yon

and

San

Raf

ael G

roup

s (C

olo-

rado

Pla

teau

). S

olid

dot

s re

pres

ent

stra

tigr

aphi

c po

siti

ons

of s

ampl

es c

olle

cted

for

det

rita

l zir

con

anal

ysis

. Chr

onos

trat

igra

phic

cor

rela

tion

s (d

ashe

d lin

es)

are

infe

rred

fro

m

limit

ed f

ossi

l con

trol

. Str

atig

raph

ic a

bbre

viat

ions

: B

L—

“Bla

ck L

edge

” sa

ndst

one;

CF

—C

urti

s F

orm

atio

n; M

T—

Moa

b To

ngue

; R

P/C

R—

Roc

k P

oint

–Chu

rch

Roc

k in

terv

al

(Roc

k P

oint

For

mat

ion)

; R

S—R

oman

a Sa

ndst

one;

SSM

—Sp

ring

dale

San

dsto

ne M

embe

r (o

f M

oena

ve F

orm

atio

n or

of

Kay

enta

For

mat

ion)

; T

C—

Tem

ple

Cap

San

dsto

ne;

TF

—To

dilt

o F

orm

atio

n. M

ap k

ey a

t up

per

righ

t sh

ows

loca

tion

s of

col

umns

A–F

. Sta

tes:

AZ

—A

rizo

na;

CO

—C

olor

ado;

NM

—N

ew M

exic

o; N

V—

Nev

ada;

UT

—U

tah.

Page 8: U-Pb ages of detrital zircons in Jurassic eolian and ... · along with recycled eolian sand, in associated fl uvial units, and these combinations occur as well in selected eolian

U-Pb ages of detrital zircons in Colorado Plateau eolianites

Geological Society of America Bulletin, March/April 2009 415

(Dubiel, 1989), Church Rock Member (Smith et al., 1963), and Dolores Formation (Blodgett, 1984) are fi lled with Wingate eolian sand that infi ltrated downward into open cracks. This contact has been termed the J-0 unconformity (Pipiringos and O’Sullivan, 1978) at the base of Colorado Plateau Jurassic strata, but the sedi-mentology of the contact suggests progradation of eolian sand dunes across an unconsolidated substratum of mud-cracked red beds with no signifi cant hiatus in deposition and no erosion of the substratum. Moreover, the Triassic-Juras-sic boundary (Fig. 3) lies within the Wingate Sandstone (Lockley et al., 2004) and the later-ally equivalent Moenave Formation (Lucas et al., 2005; Lucas and Tanner, 2007; Tanner and Lucas, 2007).

From a sedimentological perspective, the best placement of the Chinle–Glen Canyon contact is at the base of the Moenave Forma-tion or the Rock Point–Church Rock interval, whichever is present locally (Fig. 3). Both units are largely fl uvial red beds deposited by ephem-eral streams, but they contain intercalated eolian and lacustrine strata and refl ect similar origins in wadi-like arid environments. Both rest on the Owl Rock Member (or Formation) of the Chinle Formation (or Group), which includes palustrine carbonate beds refl ecting deposition on water-logged fl oodplains (Tanner, 2000). The record of Chinle pedogenesis refl ects increasingly arid environments during Chinle sedimentation (Tanner, 2000, 2003), and Owl Rock deposition represents the fi nal phase of Chinle sedimenta-tion before transit of Laurentia into the desert belt. Accordingly, we regard the “Black Ledge” sandstone (8–14 m thick) at the base of the Rock Point–Church Rock interval in central Utah (Figs. 2A and 3) as a basal sandstone of the Glen Canyon Group, and we sampled it for detrital zircons for comparison with those from other fl uvial strata of the group.

Kayenta IntervalDuring Early Jurassic time, the Kayenta

fl uvial system spread northward over >100 × 103 km2 of the Wingate erg lying to the northeast of the Moenave fl uvial tract. Readvance of Glen Canyon dune fi elds subsequently prograded the Navajo-Aztec erg back southward over the Kayenta facies tract (Figs. 2A and 3). Despite its wide extent, the Kayenta Formation averages only 60 m (range 40–90 m) in thickness (aspect ratio >5000:1). Consistent paleocurrent indica-tors document fl uvial transport of Kayenta sand westward to northwestward in a regionally inte-grated pattern (Luttrell, 1993, 1996). The direc-tion of fl uvial sand transport (Fig. 2A) implies lateral expansion, rather than progradation, of fl uvial deposition over the Wingate erg, as fl uvial

sedimentation spread northward from the Moe-nave facies tract during Kayenta deposition.

An unconformity (“J-sub-K”) has been postulated locally at the base of the Kayenta Formation (Riggs and Blakey, 1993), but the Wingate-Kayenta contact is widely reported as gradational or even intertonguing over wide areas (Doelling, 2003; Morris et al., 2003). Pres-ervation of the Wingate Sandstone as a sheet-like body, 60–150 m thick over 100 × 103 km2 (aspect ratio ~3500:1), implies that any sub-Kayenta erosion of the erg complex was limited. Aggradation of the Moenave fl uvial succession adjacent to the Wingate erg may have elevated the land surface enough to allow streamfl ow to overtop the erg and spread fl uvial Kayenta deposition across its undulating surface. Satu-ration of dune sand by lateral percolation of groundwater from the Moenave fl uvial environ-ment may have prevented defl ation of dune sand and thereby aided preservation of the Wingate erg accumulation beneath the aggrading Kay-enta fl uvial system.

Thickness RelationsThe regional parallelism of the upper and

lower contacts of the eolian Wingate Sandstone and the overlying fl uvial Kayenta Formation is striking but unexpected, for there is no evident sedimentological reason why the surface of an erg accumulation should have the regional slope of a fl uvial system. The explanation for the consistent Wingate-Kayenta thicknesses may lie in the relationship of the Wingate erg to the substratum formed by Chinle fl oodplains that sloped to the west-northwest in a direction subparallel to paleofl ow in the Kayenta fl uvial system. If Chinle rivers had an average slope of 6 × 10−4 (Heller et al., 2003), relief on the subja-cent Chinle fl oodplain across the span of Wing-ate deposition would have been ~250 m, or two to three times the mean thickness of the Wingate erg. This relation implies that the Wingate erg formed only a veneer of dune sand covering a fl uvially constructed ramp, and that overtopping of the Wingate erg by the Kayenta fl uvial sys-tem allowed streamfl ow to resume in the same direction and at the same gradient as before erg construction.

By contrast, the Navajo Sandstone thickens systematically westward from <100 m in west-ern Colorado to >300 m in southwestern Utah (Fig. 3), where the fl ank of the Navajo erg was buried beneath marine to marginal-marine Mid-dle Jurassic strata. This onlap of the thickest part of the erg by strandline sedimentation implies that the westward increase in net erg thickness does not refl ect westward increase in the eleva-tion of the erg surface. Dynamic backarc subsi-dence (Mitrovica et al., 1989) behind the Cordil-

leran magmatic arc was apparently required to control the Navajo erg depocenter and to carry the thick fl ank of the erg below sea level (Allen et al., 2000).

Glen Canyon–San Rafael Transition

The Navajo Sandstone (Glen Canyon Group) is overlain over most of the Colorado Plateau by the Carmel Formation (San Rafael Group), which is composed of fi ner-grained strata that record eastward transgression of marine, marginal-marine, and associated coastal-plain deposits over the erg complex (Fig. 3). In south-central Utah, however, tongues of the Carmel Formation interfi nger eastward with eolianite of the Page Sandstone, which locally overlies the Navajo Sandstone (Figs. 2B and 3). The Navajo-Page contact has been termed the J-2 unconfor-mity (Pipiringos and O’Sullivan, 1978), beneath which subjacent sandstone is polygonally frac-tured. Polygonal disruption of Navajo dune sand below the contact was initially interpreted as jointing in consolidated rock (Peterson and Pipiringos, 1979), but Kocurek and Hunter (1986) showed that the polygonal fracturing is the record of desiccation cracking on an exposed evaporite-encrusted surface that formed within an evolving erg during an episode of defl ation to the water table. The J-2 surface is a diastem marking a hiatus of uncertain duration, but it is not a regional unconformity (Anderson and Lucas, 1994).

We accordingly view the Page erg as a rejuve-nation of the Navajo erg, but we did not sample the thin local erg of the Temple Cap Sandstone (Peterson and Pipiringos, 1979), which occupies an analogous stratigraphic position farther west but nowhere occurs in the same stratigraphic succession as the Page Sandstone (Figs. 2B and 3). Polygonally fractured horizons marking superbounding surfaces of temporary defl ation analogous to the basal J-2 surface are present at multiple horizons within the Page Sandstone (Havholm et al., 1993; Havholm and Kocurek, 1994) and refl ect repetitive fl ooding of the Page erg during intermittent but progressive Carmel transgression, which eventually overtopped the erg (Fig. 3). The most prominent superbounding surfaces correlate with tongues of Carmel For-mation that interfi nger with different members of the Page Sandstone (Blakey et al., 1996).

San Rafael Group

The San Rafael Group can be divided into lower and upper stratal assemblages; the contact between them occurs at the base of the marine Curtis Formation in Utah (the J-3 unconformity of Pipiringos and O’Sullivan, 1978) and the

Page 9: U-Pb ages of detrital zircons in Jurassic eolian and ... · along with recycled eolian sand, in associated fl uvial units, and these combinations occur as well in selected eolian

Dickinson and Gehrels

416 Geological Society of America Bulletin, March/April 2009

broadly correlative Todilto Formation deposited in a nonmarine salina farther to the southeast (Fig. 3), or at the top of the Entrada Sandstone across an intervening belt where neither of those two distinctive units is present (Fig. 2B). Thick eolianites occur in both assemblages and form the widespread Entrada erg and the younger (post-Todilto) and more restricted Bluff erg (Fig. 2B). Marine sandstone of the Curtis Forma-tion was also sampled to compare its zircon age spectrum to that of underlying eolian sandstone.

Lower AssemblageThe Carmel Formation at the base of the San

Rafael Group grades and thins eastward from largely marine strata (subtidal) on the west to thinner marginal-marine (intertidal) and nonma-rine (supratidal) red beds on the east (Blakey, 1994). To the southeast, thin distal tongues of Carmel lithology (Fig. 3) are commonly regarded as basal members of the overlying Entrada Sandstone. The Entrada Sandstone is an internally heterogeneous assemblage of cross-bedded dune sand, fl at-bedded eolian sand sheets, interdune sabkha deposits, and erg-mar-gin red beds (Kocurek, 1981; Carr-Crabaugh and Kocurek, 1998). Polygonally fractured super-bounding surfaces commonly separate dune successions from sabkha intervals and attest to temporary episodes of defl ation at times of ris-ing water tables during evolution of the Entrada erg. Across Utah, Entrada eolianites (“slickrock facies”) grade westward (Fig. 2B) to an “earthy facies” of fi ner-grained strata, including water-laid deposits of coastal sabkhas and tidal fl ats developed along a dune-fringed paleoshoreline (Mariño and Morris, 1996).

The lower San Rafael Group (Carmel-Entrada) thickens from <100 m on the east-ern Colorado Plateau to almost 500 m along its western fringe near the Utah-Idaho trough (Fig. 2B), within which ~1500 m of correlative Middle Jurassic marine strata overlie the Nug-get Sandstone (Glen Canyon Group). Several tectonic interpretations have been offered for development of the Utah-Idaho trough, but none is yet fully satisfactory. These include: (1) a retroarc foredeep (Bjerrum and Dorsey, 1995), but the basin keel lies 500 km from the nearest known coeval retroarc thrust system (Fig. 2B); (2) a backbulge basin (DeCelles and Currie, 1996), but the stratal thickness in the basin seems excessive for such an origin; and (3) a backarc rift basin (Dickinson, 2006), but postu-lated bounding faults are buried in the subsur-face and diffi cult to evaluate. In any case, west-ward thickening of the tapering wedge of lower San Rafael strata across Utah can be attributed to the fl exural effect of the thick sediment load within the Utah-Idaho trough.

Eastward from the central Colorado Plateau, beyond the fl exural infl uence of the sediment load in the Utah-Idaho trough, the Entrada Sand-stone forms a thin sediment blanket, <25 m thick, on the High Plains (Lucas, 2004) over a lobate area of at least 150 × 103 km2, extending as far as the Oklahoma panhandle (Fig. 2B). From paleo-wind directions (Fig. 2B), the blanket Entrada erg lying east of the central Colorado Plateau was not a downwind fore-erg accumulation, but a crosswind fl ank of the main Entrada erg. Thin Entrada Sandstone oversteps an eastern wedge edge of Navajo Sandstone to rest directly on underlying Kayenta Formation across a narrow facies tract in western Colorado. The blanket phase of the Entrada erg also spreads still far-ther east over massive red siltstone of the Rock Point–Church Rock interval and correlative lacustrine strata of the Redonda Formation on the High Plains (Hester and Lucas, 2001). There is no evidence, however, for fl uvial incision or marine planation below the Entrada eolian inter-val, nor any discernible paleosol below it.

Curtis-Todilto EventRecent ammonite collections document the

Oxfordian (basal Late Jurassic) age of the Cur-tis Formation (Wilcox and Currie, 2006), which is composed of glauconitic marine sandstone 40–60 m thick (Fig. 3) that transgressed over Entrada Sandstone in central Utah from the Utah-Idaho trough to the west and tapered to a feather edge on the southeast (Figs. 2B). Curtis marine strata grade upward and eastward into fi ner-grained tidal-fl at and sabkha deposits of the overlying Summerville Formation, and the eolian Moab Tongue locally present at the top of the Entrada Sandstone (Figs. 2B and 3) is a nonmarine equivalent of the Curtis Formation (Crabaugh and Kocurek, 1993; Peterson, 1994; Doelling, 2003). Flooding of the Entrada erg was punctuated by local scour associated with ravinement along a desert coast and buried relict dune topography (Eschner and Kocurek, 1988; Peterson, 1994), but there was no pro-longed hiatus or paleosol development beneath the Curtis Formation.

Farther inland to the southeast, the broadly coeval (Anderson and Lucas, 1994) but lacus-trine Todilto Formation, which may be slightly older than the Curtis Formation (Peterson, 1994), was deposited in an evaporative salina occupying >100 × 103 km2 (Fig. 2B). The Todilto salina may have been fl ooded initially by marine waters related to the Curtis transgres-sion, but it was then maintained by infl uxes of freshwater from the south and by seepage of seawater through coastal sand barriers like the eolian Moab Tongue lying northwest of the salina toward the marine Curtis facies tract

(Anderson and Lucas, 1994, 1996; Kirkland et al., 1995). The Todilto Formation includes a thin lower limestone member, generally <5 m thick, that is present throughout Todilto exposures, and an upper gypsum member confi ned to the interior of the salina, where a brine pool per-sisted after evaporation had reduced its extent. The lateral continuity of the thin Todilto Forma-tion above the Entrada Sandstone shows that the upper surface of the Entrada erg drowned by the lacustrine salina was approximately level within an elevation range of <50 m.

Upper AssemblageThe upper San Rafael Group forms a sheet-

like body of sediment only 50–100 m thick over most of its distribution, although thicknesses >100 m are reached along the western edge of the Colorado Plateau toward the Utah-Idaho trough (Fig. 3). Different stratigraphic nomen-clature is used for the upper assemblage by different workers and in different states, and controversy also surrounds the placement of the stratigraphic base of the overlying Morrison Formation. Resolution of nomenclature issues is beyond the scope of this paper, but a brief dis-cussion of their nature is necessary because we sampled eolianite within the upper assemblage to test for possible differences in age popula-tions of detrital zircons in Entrada and post-Entrada ergs for which prevailing winds blew from the northeast or north and from the west or southwest, respectively (Fig. 2B).

In central Utah, red beds of tidal-fl at and sab-kha origin (Petersen and Pack, 1982; Caputo and Pryor, 1991; Peterson, 1994) form the Sum-merville Formation (Fig. 3), which conform-ably overlies the marine Curtis Formation and the laterally equivalent Moab Tongue of eolian origin. To the southwest, the Summerville For-mation grades laterally to a sandier and more onshore facies (Peterson, 1988a, 1994; Blakey, 1989) termed the Romana Sandstone (Fig. 3). To the southeast, strata that are homotaxial and lithologically similar to Summerville Formation extend to areas where neither the Curtis Forma-tion nor the Moab Tongue is present. A change in nomenclature adopted by some workers stems from the lateral continuity of the limestone member of the Todilto Formation in New Mex-ico with the Pony Express Limestone Member at the base of the Wanakah Formation in Colorado, beyond the extent of Todilto gypsum. This cor-relation has led to treatment of the Todilto inter-val as a member of the Wanakah Formation, and to the assignment of red beds of tidal-fl at or sab-kha origin overlying the Todilto interval to the Wanakah Formation rather than the Summer-ville Formation (Condon and Peterson, 1986; Condon and Huffman, 1988; O’Sullivan, 2003).

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U-Pb ages of detrital zircons in Colorado Plateau eolianites

Geological Society of America Bulletin, March/April 2009 417

For Figure 3, we follow others (Anderson and Lucas, 1992, 1997) who prefer the name Sum-merville Formation and retain the formational status of the Todilto interval.

In southeastern Utah, the eolian Bluff Sand-stone, which is younger than the Moab Tongue (Fig. 3) at the top of the Entrada Sandstone farther north (O’Sullivan, 1980), overlies fi ner-grained strata assigned alternately (see previ-ous) to the Summerville or the Wanakah For-mation. The Bluff Sandstone, identical to the Junction Creek Sandstone of Colorado (Lucas and Heckert, 2005), was originally described as the basal member of the overlying and domi-nantly fl uvial Morrison Formation (Gregory, 1938). The Bluff Sandstone was later shifted to the San Rafael Group on the basis of its lithol-ogy and recognition that a vestige of the basal Salt Wash Member of the Morrison Formation is present within the overlying and dominantly fi ner-grained Recapture Member (Craig et al., 1955; Craig and Shawe, 1975; Lupe, 1983). A thin interval of nonfl uvial and even-bedded tidal-fl at and sabkha deposits resembling Sum-merville or Wanakah Formation in lithology is present within the type Recapture Member (of Gregory, 1938) between the Bluff Sandstone and the Salt Wash Member of the Morrison For-mation (Anderson and Lucas, 1997).

In recent years, some have reassigned the upper part of the Bluff Sandstone to the Mor-rison Formation and have postulated an uncon-formity (the J-5 unconformity of Pipiringos and O’Sullivan, 1978) between horizontally bedded sandstone forming the lower part of type Bluff Sandstone and cross-bedded sandstone form-ing the upper part of type Bluff Sandstone. This interpretation has led to designation of the lower part of the type Bluff Sandstone as the Horse

Mesa Member of the Wanakah Formation, after a unit exposed to the south and traceable still farther southeast into the lower part of the “sandstone at Mesita” exposed near the Rio Grande rift (Condon and Peterson, 1986; Con-don and Huffman, 1988; Condon, 1989a). The upper part of the “sandstone at Mesita” is cross-bedded, as is upper Bluff Sandstone at the type locality (Condon, 1989b).

Sedimentologically, the upward transition from horizontally bedded to cross-bedded sand-stone in the type Bluff Sandstone and the “sand-stone at Mesita” is the record of dune prograda-tion over eolian sand sheets without a signifi cant break in eolian sedimentation. Accordingly, we regard the entire Bluff Sandstone as part of the San Rafael Group and follow interpretations of others (Anderson and Lucas, 1996; Lucas and Heckert, 2003; Lucas, 2004) in regarding the Horse Mesa and Mesita sand bodies as local phases of the Bluff erg. Local stratal discor-dance between the lower, horizontally bedded and the upper, cross-bedded facies of the “sand-stone at Mesita” (Condon, 1989b) can be attrib-uted to syndepositional fl owage of evaporites in the underlying Todilto Formation (Anderson and Lucas, 1992). Our Bluff samples come from the cross-bedded upper facies in both the type locality and near Mesita, where the sandstone body we sampled was mapped as Bluff Sand-stone (Moench and Schlee, 1967) before the stratigraphic disputes arose.

SANDSTONE PETROFACIES

Table 1 and Figure 4 indicate numerically and graphically the detrital modes of all samples. The 11 eolianite samples (Table 1A) are fi ne- to medium-grained sandstones composed of sub-

rounded to rounded aggregates of well-sorted grains that plot as a compact elongate cluster in QmFLt space (Fig. 4), and they are quartz-rich subarkose in the classifi cation of McBride (1963). The two samples of marine sandstone are well-sorted, fi ne-grained sandstones com-posed of subrounded to subangular grains that plot close to the eolianite samples in QmFLt space (Fig. 4), with no statistically signifi cant difference in QFL composition when inherent counting error is taken into account (Van der Plas and Tobi, 1965), but they contain mica fl akes (Table 1C) that are absent from the eolian-ites. The fi ve fl uvial samples (Table 1B) are also fi ne- to medium-grained sandstones, but they are only moderately to poorly sorted aggregates of subangular to subrounded grains, and they are less quartzose than the eolian and marine sand-stone samples (Fig. 4), though still subarkose, with one exception. Some but not all fl uvial samples contain detrital mica fl akes in variable abundances, which are probably dictated by local sedimentology rather than provenance.

Table 2 compares mean detrital modes for generic groups of samples. Eolianite samples are tabulated both stratigraphically (columns 1–2) and areally (columns 3–4). The tabulations show no statistical distinction among QmFLt values for the eolian sandstones no matter how they are grouped, nor any distinction in QmFLt val-ues between eolian and marine sandstones. All samples of fl uvial sandstone are less quartzose and more feldspathic, with the “Black Ledge” sample (column 7) being more lithic as well.

P/F ratios are different for Glen Canyon and San Rafael eolianites and for eolianites from the eastern and western Colorado Plateau (Table 2). The contrasts in ratio are similar 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

laivulF .B selpmas nailoE .A 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).

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Dickinson and Gehrels

418 Geological Society of America Bulletin, March/April 2009

the west, whereas most San Rafael samples derive from the east (Figs. 2 and 3). P/F ratios for the marine and most fl uvial samples (col-umns 6–7) resemble the higher eolianite values (San Rafael and eastern plateau), but the P/F ratio for the Springdale sample (column 8) is as low as the lowest eolianite values (Glen Can-yon and western plateau). The variations in P/F ratio correlate with variations in the age popu-lations of detrital zircons, as developed next.

DETRITAL ZIRCON MORPHOLOGY

Zircon populations in all samples (eolian, fl uvial, marine) are dominantly rounded to subrounded grains (although some are suban-gular), vary from subspherical to elongate in shape, and display varied colors. There is no systematic correlation between shape, color, or angularity and U-Pb age. We conclude that sediment dispersal systems incorporated grains from age provinces that all included multiple source rocks containing zircons of varying character, and that the transport histories of individual zircon grains were also variable. Transport in stream bed load or as eolian sal-tation blankets tends to abrade grains readily, whereas transport as suspended load in streams or sand storms allows transport over arbitrarily long distances with little grain abrasion.

EOLIANITE DETRITAL ZIRCONS

Visual inspection of the superimposed age-bin histograms and age-distribution curves of Figure 5 shows that heterogeneous age popula-tions of detrital zircons in all eolianite samples from the Colorado Plateau are broadly similar. The salient difference is in the variable content of grains younger than 285 Ma derived from Cordilleran igneous suites (Fig. 1). Arc-derived grains are paradoxically more abundant in eoli-anites from the eastern plateau (Figs. 5G–5J) than from the western plateau (Figs. 5A–5F), even though the latter is geographically closer to the Cordilleran orogen.

Composite age-distribution curves for eastern and western plateau samples are compared in Figure 6. All major age peaks for grains older than 285 Ma are closely comparable on the two plots, and the P value from K-S analysis for composite populations of grains older than 285 Ma in the two sample sets (n = 574 west-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-ples from the eastern and western parts of the plateau (Fig. 6) stem from variable additions of arc-derived grains to otherwise uniform eolian sand. Six western plateau samples contain only three grains younger than 285 Ma in age (0.5%), whereas four eastern plateau samples contain 7%–25% (mean 13% or 49 total grains).

We fi rst address the origins of the dominant grains older than 285 Ma in the eolianite samples and then turn to the grains younger than 285 Ma of Cordilleran derivation (~5% overall).

Non-Cordilleran Grains

We evaluated the apparent similarity of het-erogeneous age populations of grains older than 285 Ma in 10 Colorado Plateau eolianite 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 it is sensitive to proportions of grains of dif-ferent ages as well as to the spread of ages. P values <0.05, indicating statistical differences, can thus derive from comparison of sets of samples containing age populations derived from the same sources but in differing propor-tions. P values for 75% of the sample pairs are >0.05, indicating that we cannot be 95% confi -dent that those pairs of grain populations were not selected at random from the same parent population. All the samples have calculated P values >0.05 when compared to half or more of the other samples. We conclude that grains older than 285 Ma form a heterogeneous but coherent age spectrum in the plateau eolianite

to F to LTQm

Qm

60%

90

80

70

10

20

30

90

10

80

20

70

30

+

++

San Rafael Gp.

Glen Canyon Gp.

marine Curtis Fm.(San Rafael Gp.)

fluvial Kayenta Fm.(Glen Canyon Gp.)

eolianites

SpringdaleSandstone

Member

"Black Ledge"sandstone

quartzarenite

sublitharenite

subarkose

lithicsubarkose

litharenitearkose

Figure 4. Partial QmFLt plot (note scale) of sandstone samples (Table 1). Glen Canyon samples include Page Sandstone (CP12) immediately overlying Navajo Sandstone, and San Rafael samples include correlative Mount Wrightson Forma tion (CP3) from southern Arizona. Numerical values are percentages.

TABLE 2. COMPARATIVE MEAN DETRITAL MO DES (KEY PARAMETERS) OF DETRITAL ZIRCON SAMPLES (TABLE 1)

laivulF eniraM nailoE 1 2 3 4 5 6 7 8 Glen

Canyon Group (n = 5)

San Rafael Group (n = 6)

Western Colorado Plateau (n = 6)

Eastern Colorado Plateau (n = 5)

Curtis

Formation(n = 2)

Kayenta Formation

(n = 3)

“Black Ledge”

Sandstone (n = 1)

Springdale Sandstone Member (n = 1)

Qm 84 ± 3 85 ± 2 84 ± 3 85 ± 2 84 74 ± 2 65 71 F 10 ± 2 10 ± 3 10 ± 2 10 ± 3 10 18 ± 3 16 23 Lt 6 ± 1 5 ± 1 6 ± 1 5 ± 1 6 8 ± 2 19 6 M – – – – 3%–4% 2% ± 2% 3% – P/F 26 ± 10 46 ± 13 25 ± 9 52 ± 4 42 43 ± 2 38 17 Note: See Table 1 for Qm, F, Lt, M; P/F = 100 × (plagioclase feldspar)/(total feldspar). Standard deviations (±) are for n samples where n > 2. Col. 1 includes Page Sandstone (CP12). Col. 2 includes Mt. Wrightson Formation (CP2). Cols. 3–4 were divided at 110°W longitude.

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U-Pb ages of detrital zircons in Colorado Plateau eolianites

Geological Society of America Bulletin, March/April 2009 419

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Dickinson and Gehrels

420 Geological Society of America Bulletin, March/April 2009

samples, and that variable P values refl ect vagaries of sampling, including stratigraphic and areal selection of sample sites, choice of horizons collected on outcrop, and selection of grains in the laboratory for laser ablation.

Figure 7 is a composite plot of 890 grains older than 285 Ma in age in the eolianite sam-ples (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-nate Grenvillian peak at 1160 Ma. Paleoprotero-zoic and Archean grains defi ne fi ve less promi-nent but well-defi ned age peaks (Fig. 7).

The distribution of granitoid basement prov-inces in North America (Fig. 1) provides a guide to the ultimate sources of the grains defi ning each age peak. Table 3 groups the net eolian-ite age populations into clusters around the age peaks and indicates the inferred dominant sources for each age cluster. Approximately two-thirds of the grains apparently derive from the Appalachian orogen and its Grenville fl ank in southeast Laurentia, or its extensions into the Mesoamerican region. No other basement provinces in North America could provide the Paleozoic, Neoproterozoic, and Grenvillian grain populations in the observed abundances relative to grains of other ages.

Appalachian ProvenanceGranitoid 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 Grenville orogen were incorporated into the cra-tonal fl ank of the Appalachian orogen. Neopro-terozoic granitic rocks are present both as rift plutons intruded into Grenville basement during the multiphase pre-Iapetan breakup of Rodinia within the interval 735–560 Ma (Tollo et al., 2004), and within peri-Gondwanan arc terranes 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 diverse ages are present along the length of the Appalachian orogen, both intruded into native Laurentian basement of Grenville age and within Paleozoic assemblages accreted to the fl ank of Laurentia during evolution of the orogen (Hib-bard et al., 2007). The tendency for populations of detrital zircons to integrate age signals from diverse bedrock sources within the same prov-enance is shown by the compound age peak for

eastern plateau eolianites

(CP15-CP16-CP24-CP54)

N = 4; n = 365

western plateau eolianites

(CP3-CP12-CP30-Jwnw-Jnnw-Jenw)

N = 6; n = 577

0 500 1000 1500 2000 2500 3000 3500Age (Ma)

Age

pro

babi

lity

Figure 6. Composite age-distribution curves for detrital zircon grains in Jurassic eolian-ite samples from the eastern (Figs. 5G–5J) and western (Fig. 5A–5F) Colorado Plateau (N—samples; n—grain ages).

Age

pro

babi

lity

Colorado Plateau eolianites(>285 Ma grains)

N = 10; n = 890

0 500 1000 1500 2000 2500 3000Age (Ma)

420

615

1055

1160

2760

1465

1675 18

55

2095

Figure 7. Composite age-distribution curve for detrital zircon grains older than 285 Ma in 10 eolianite samples from the Colorado Plateau (Fig. 5).

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U-Pb ages of detrital zircons in Colorado Plateau eolianites

Geological Society of America Bulletin, March/April 2009 421

grains of Paleozoic age in the eolianites (Fig. 7) without discrimination among pulses of Paleo-zoic magmatism along the Appalachian belt.

Reported U-Pb ages of granitoid rocks along seven transects of the Appalachian belt (Fig. 8) are compiled in Figure 9, which shows that the four most prominent age peaks for detrital zircons in Colorado Plateau Jurassic eolianites match the distribution of known ages for gra-nitic rocks along the Appalachian-Ouachita-Mesoamerican fl ank of Laurentia. Derivation 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 former. The paucity of detrital zircon grains of Neoproterozoic age in Paleozoic strata of the Appalachian foreland basin (McLennan et al., 2001; Eriksson et al., 2004; Becker et al., 2005, 2006; Thomas and Becker, 2007) suggests that peri-Gondwanan terranes accreted along the distal fl ank of Laurentia were not uplifted to become signifi cant sources of sediment until rift highlands developed along the trend of the incipient Atlantic Ocean in Mesozoic time.

Coincidental mixing of Paleozoic, Neopro-terozoic, and Grenvillian zircon grains from disparate rock masses, including Gondwanan continents once adjacent to Laurentia, as an alternative to postulated Appalachian origin, is disfavored for several reasons: (1) there are consistent proportions of key age populations of detrital zircons in plateau eolianites, all of which display separate peaks for Paleozoic and Neo-proterozoic age populations, and Grenvillian age peaks larger than either (Fig. 5); (2) a 520–510 Ma gap between ages of Neoproterozoic and Paleozoic granitoids in the Appalachian-Ouach-ita-Mesoamerican orogen is closely matched by a 515–505 Ma gap between age peaks for detri-tal zircons in the eolianites; (3) derivation of zir-cons from cratons lying within Pangea, beyond the Hercynian Appalachian-Ouachita suture between Laurentia and Gondwana, is unlikely 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 Atlantic rifting; (4) derivation of Neoprotero-zoic grains from Pan-African orogenic belts of Africa or South America is unlikely because the nearest Pan-African sources on those continents lay 1500 km and 3500 km, respectively, from the Hercynide Appalachian-Ouachita suture bounding Laurentia; and (5) the proximate edge of the Guiana craton, which lay directly south of Laurentia before breakup of Pangea (Dickinson and Lawton, 2001a), is dominated by the 2200–1950 Ma Maroni-Itacaiúnas belt (Tassinari et al., 2000; Chew et al., 2007), and the conjugate

Birimian belt of West Africa is of similar age (Boher et al., 1992), yet 2200–1950 Ma detrital zircons form only 2% of the grain population in plateau eolianites.

Grenvillian ZirconsZircon grains of Grenvillian age form 60% of

the age suite of eolianite detrital zircons inferred to derive from the Appalachian orogen (Table 3), yet the Grenville province forms a relatively narrow belt along the proximal fl ank of the oro-gen (Fig. 8). Domination of grains of Grenvil-lian age may refl ect the high zircon fertility of

Grenville plutons (Moecher and Samson, 2006), rather than especially voluminous Grenville sources. Recycling of Grenvillian grains from Paleozoic strata of the Appalachian foreland basin (Fig. 10), in which detrital zircon grains of Grenvillian provenance are abundant (McLen-nan et al., 2001; Eriksson et al., 2004; Becker et al., 2005, 2006; Thomas and Becker, 2007), is a clear possibility, as is recycling from Neopro-terozoic rift and miogeoclinal assemblages of Grenvillian provenance along the Appalachian belt (Cawood and Nemchin, 2001; Cawood et al., 2007; Thomas and Becker, 2007). From our

100°W

20° N

100° W 80°W

40°N

40°N

20°N

anorogenic plutons(1490–1340 Ma)

Ouachitasuture

limit of pre-Jurassic sedimentcover over interior craton

Atlantic-Caribbeancontinental slope

S o u r c e r o c k b e l t s

native and accretedPaleozoic terranes

accreted Neoproterozoicperi-Gondwanan terranes

late MesoproterozoicGrenville orogen

JamesBay

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ran

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rran

esG

F

E D

CB

A

SuT CaT

AvT

BC

500 km

Oaxaquia

North

FloridaYúcatan-Campeche

block

?

?

TABLE 3. AGE POPULATIONS OF 890 NON-CORDILLERAN (≥285 MA) DETRITAL ZIRCON GRAINS IN COLORADO PLATEAU EOLIANITES

Grain population descriptor from age peak (inferred dominant source or sources)

Age peak (Ma)

Age range (Ma)

Numberof grains

Percentageof grains

(%) 51 731 405–582 024 )secruos naihcalappA( ciozoelaP

Neoproterozoic (Appalachian sources) 615 513–723 102 11 1 8 278–647 – etanimretedni-nwonknU

Grenville (Mesoproterozoic Grenville orogen) 1055 913–1295 352 40 Anorogenic (Mesoproterozoic craton) 1465 1304–1532 75 8 Yavapai-Mazatzal (southwest Laurentia) 1675 1546–1803 69 8 Paleoproterozoic (cratonal suture belts) 1855 1811–2013 49 6 Wopmay orogen (northwest Laurentia) 2095 2039–2326 18 2 Archean (Laurentian cratonic nucleus) 2760 2423–3196 80 9

Figure 8. Transects (heavy barred lines keyed by letter to Fig. 9) of the Paleozoic Appala-chian-Ouachita-Mesoamerican orogen showing late Mesoproterozoic Laurentian and Oax-aquian Grenville belts (Fig. 1), distribution of early Mesoproterozoic anorogenic plutons (Anderson, 1983; Anderson and Morrison, 2005) in Grenville foreland after Anderson and Cullers (1999) and Barnes et al. (2002), and accreted Neoproterozoic terranes of the Appa-lachian belt (AvT—Avalon; CaT—Carolina; SuT—Suwannee) adapted after Hatcher et al. (1989, 2007), Reed (1993), and Barr and Kerr (1997). Paleogeologic pre-Jurassic edge of sedimentary cover over Laurentian craton is from Figure 10. Yucatan-Campeche block and Baja California Peninsula (BC) were restored after Dickinson and Lawton (2001a).

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Dickinson and Gehrels

422 Geological Society of America Bulletin, March/April 2009

distant vantage point on the Colorado Plateau, we cannot distinguish between bedrock and recycled sources of Grenvillian detritus lying within the Appalachian orogen, but Jurassic paleogeographic implications are the same in either case.

Recycling of Grenvillian zircon grains from the Paleozoic platform cover of the continen-tal interior is unlikely, both because the plat-form cover is still largely intact over most of the expanse between the Rocky Mountains and the Appalachian belt (Fig. 10) and because the platform cover is in large part carbonate

rock that contains few, if any, zircon grains. Neoproterozoic and lower Paleozoic strata of the Western Cordillera are known to contain abundant detrital zircon grains of Grenvillian derivation (Rainbird et al., 1992, 1997; Gross et al., 2000; Stewart et al., 2001; Timmons et al., 2005; Mueller et al., 2007), but these strata were buried beneath upper Paleozoic and lower Mesozoic cover until late Mesozoic Cordilleran orogenesis and were not exposed to erosion in Lower to Middle Jurassic time. Neoproterozoic strata in the midcontinent containing Grenvil-lian detritus (Santos et al., 2002) are still buried

beneath Paleozoic cover and similarly could not have contributed recycled Grenvillian zircon grains to Jurassic dispersal systems. Volumi-nous sediment of partly Appalachian derivation transported into the midcontinent by a major Pennsylvanian paleoriver that fl owed parallel to the Appalachian chain (Archer and Greb, 1995) has also been largely preserved from erosion even to the present time.

Recycling of Grenvillian zircon grains into plateau eolianites from pre-Mesozoic strata is further disfavored by combined U-Pb and (U-Th)/He dating of zircon grains of Grenvillian

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Figure 9. Comparison of U-Pb age populations of detrital zir-cons (DZ) in Jurassic eolian-ites of the Colorado Plateau with U-Pb age spans of poten-tial granitic source rocks for detrital zircons along multiple transects of the Appalachian-Ouachita-Mesoamerican oro-gen (Fig. 8), including early Mesoproterozoic anorogenic plutons and late Mesoprotero-zoic synorogenic plutons and granulites of the Grenville oro-gen, native Neoproterozoic pre-Iapetan (Rodinian) rift plutons along the Laurentian margin, Neoproterozoic plutons of peri-Gondwanan terranes accreted to Laurentia, and Paleozoic plutons assigned to Penob-scot, Taconic, Acadian, and Alleghanian orogenic phases after Hatcher (1989). Plotted age ranges (U-Pb ages only) for bedrock sources are from Supplementary Text 4 (see text footnote 1). Age spans (dark columns) and principal age peaks (arrows) of detrital zir-cons are depicted for 602 grains with ages of 1295–285 Ma in 10 Jurassic eolianites of the Colo-rado Plateau (Fig. 7; Table 3).

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U-Pb ages of detrital zircons in Colorado Plateau eolianites

Geological Society of America Bulletin, March/April 2009 423

age in the Navajo Sandstone of Zion National Park (Rahl et al., 2003), located midway between our Navajo (Jnnw) and correlative Aztec (CP30) samples. The Grenvillian granitoid rocks, or perhaps deeply buried metasedimentary rocks, that yielded detrital zircon grains to the Zion sample were not unroofed until early Mesozoic time (youngest He age only 35–45 m.y. older than depositional age). This “double-dating” result precludes posterosion sediment storage of basement-derived Grenvillian grains for any appreciable length of time at shallow depths before delivery to the Jurassic ergs.

Paleoriver CourseThe prevalence of paleowinds blowing south-

ward in present coordinates across the Colorado Plateau (Figs. 2A–2B) shows that the proxi-mate source of the Jurassic ergs lay north of the plateau in a direction from which none of the zircon age populations of ultimate Appalachian provenance could have been derived (Fig. 1). We infer, after Dickinson and Gehrels (2003) and Rahl et al. (2003), that a transcontinental Juras-sic paleoriver system transported Appalachian detritus toward Jurassic paleoshorelines north of the Colorado Plateau (Fig. 10) and deposited

sand on riverine or deltaic fl oodplains that were then systematically defl ated to feed the eolian depositional system of the Colorado Plateau.

Because no Jurassic strata are exposed across the midcontinent region between the Rocky Mountains and Triassic-Jurassic rift basins of the Appalachian belt, the detrital zircon record in Mesozoic strata of the Colorado Plateau is the only remaining evidence for the transcontinental Jurassic paleoriver system. In the modern world, however, rivers transiting cratons for 1500–2500 km from marginal or interior highlands toward distant continental margins are common

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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.

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424 Geological Society of America Bulletin, March/April 2009

(Amazon, Congo, Lena, Missouri-Mississippi, Ob, Saskatchewan, Yenisei), and the inferred length of the Jurassic paleoriver feeding sand to the erg system of the Colorado Plateau was no greater (~2000 km). The low-lying surfaces of cratons are pathways for, not barriers to, sedi-ment dispersal.

The most attractive course for the trunk pale-oriver of the fl uvial system was from headwaters in the central to southern Appalachians, with tributaries of unknown number and location joining the paleoriver from both the north and the south as it crossed the continent (Fig. 10). The postulated course of the trunk stream thereby followed a direct path along a belt of low inferred paleotopography between residual uplands of the Pennsylvanian Ancestral Rocky Mountains to the southwest and the broadly ele-vated tract of the Canadian Shield forming the cratonic nucleus of Laurentia on the northeast (Fig. 10).

A more northerly trunk stream rising in the northern Appalachians is disfavored because (1) avoidance of signifi cant detritus from Archean basement of the Superior province lying imme-diately to the west (Fig. 10) would then seem diffi cult, yet Archean grains form <10% of the detrital zircons in plateau eolianites (Table 3); (2) the accreted peri-Gondwanan assemblages capa-ble of yielding Neoproterozoic grains lie pro-gressively farther east toward the north, beyond wider Grenville and Paleozoic belts (Fig. 8), in a position more diffi cult for potential headwaters to tap; and (3) Alleghanian (<340 Ma) plutons are unknown north of New England (Fig. 9).

A more southerly trunk stream is disfavored because (1) Grenville basement fl anking the Ouachita suture belt west of the Appalachians was largely buried beneath Paleozoic platform cover during Jurassic time (Figs. 8 and 10); (2) Grenville plutons younger than 1000 Ma in age, which contributed a signifi cant fraction of Grenvillian grains to plateau eolianites (Fig. 7; Table 3), are unknown along the Grenville belt where exposed sparingly west of the Appala-chians, except far away in southern Mexico (Talavera-Mendoza et al., 2005); and (3) fl ysch successions of the Ouachita orogen (Gleason et al., 2007), from which some eolianite detritus may have been derived, contain detrital zircon populations that (a) lack the Neoproterozoic age peak near 615 Ma prominent for plateau eolianites, (b) have a prominent Cambrian age peak at 533 Ma, probably refl ecting sediment contributions from the Amarillo-Wichita prov-ince (Fig. 1), which are not present in plateau eolianites, and (c) display Grenvillian age peaks of 1125 Ma and 1245 Ma, which are somewhat different from the 1055 Ma and 1160 Ma Gren-villian age peaks for plateau eolianites.

In general, however, the U-Pb age spans of Grenvillian plutons are similar along the entire length of the Grenville orogen fl anking Lauren-tia (Fig. 9), and do not provide a robust guide to regional provenance. Another factor that com-pounds the diffi culty of sensing regional differ-ences in sources within the Grenville orogen is the inherent imprecision of U-Pb ages across the Grenvillian age spectrum. For example, the mean age uncertainty for ~265 individual detrital zircon grains in plateau eolianites with best estimates of grain age in the range of 1200–1000 Ma is ±65 Ma.

Provenance ErosionIn evaluating central and southern Appala-

chian sources, whether juvenile or recycled, for sand in plateau eolianites, two principles of provenance analysis are relevant:

(1) Potential source rocks still uneroded in the provenance region cannot have contributed to any body of sediment in the past. This principle comes into play because the fi ll of the Appala-chian foreland basin and its miogeoclinal sub-stratum is still largely preserved, to thicknesses in the range of 2500–7500 km above basement (Muehlberger, 1992), beneath the Allegheny-Cumberland Plateau along the western fl ank of the orogen. Accordant ridges of the plateau expose nearly undeformed Pennsylvanian and Permian strata that cap the miogeoclinal–fore-land succession and were not dissected until incised by modern stream valleys during the currently active post-Jurassic cycle of erosion. Recycling of Grenvillian detritus from Appa-lachian foreland successions in Jurassic time could only have involved deformed strata form-ing the Valley and Ridge province, 25–65 km wide along the proximal fl ank of the foreland basin adjacent to the Blue Ridge thrust front (Hatcher et al., 2007).

(2) The volume of rock eroded to supply a body of sediment no longer exists in the prov-enance. This principle comes into play because the Blue Ridge thrust sheet may once have extended farther westward, structurally above deformed strata of the Valley and Ridge prov-ince, before erosion of the gently dipping thrust sheet back to the present position of the Blue Ridge thrust front. To the east, the complex Blue Ridge and Inner Piedmont belts, com-posed of Grenville basement, overlying sedi-mentary cover, and both overthrust basement and intrusive plutons of Paleozoic age, are now jointly 70–120 km wide west of accreted peri-Gondwanan terranes (Hatcher et al., 2007), but may perhaps have been 100–150 km wide dur-ing Jurassic time.

If one postulates, based on the proportions of detrital zircons of various ages in plateau eoli-

anites (Table 3), that 55% of the eolian sand was derived ultimately from Grenvillian and Paleozoic basement in the central and south-ern Appalachians, an estimate of the net depth of erosion in the provenance can be made. The calculation leaves aside the question of Neopro-terozoic detrital zircons derived from accreted peri-Gondwanan terranes lying father to the east, which contributed an additional 11% of the detrital zircons to plateau eolianites (Table 3). A volume of quartzose Jurassic eolian sand of Grenvillian-Paleozoic origin of 95 ± 40 km3 (embracing minimum and maximum estimates) requires erosion of 245 ± 100 km3 of granitoid-gneissoid bedrock to produce. The indicated net depth of erosion into basement in the southern and central Appalachians is 4.5 ± 2.3 km if the geographic extent of eroded basement between the Blue Ridge thrust front and the western edge of accreted peri-Gondwanan terranes is taken to be 85–115 km (width) by 575 km (length). This estimate does not allow for enhanced zircon fer-tility of Grenvillian plutons (which would reduce the required depth of erosion), nor for deriva-tion of some detritus from quartzose metasedi-mentary rocks (which would also decrease the required depth of erosion), nor for erosional removal of mafi c or other rocks yielding little or no zircon or quartz (which would increase the inferred depth of erosion). Even without such adjustments, however, the fi gure derived for depth of erosion seems unexceptional for exposure of plutonic and metamorphic rocks in the core of a mature orogen, and it implies that no recycling of detritus from outside the Appa-lachian belt is required to explain the abundance of Grenvillian and Paleozoic detrital zircons in plateau eolianites.

Pre-Grenvillian GrainsNone of the fi ve age groups of pre-Grenvil-

lian grains in plateau eolianites forms more than 10% of the net grain population (Table 3), and none is especially abundant in any of the eolian-ite samples (Fig. 5). In aggregate, however, they sum to approximately one-third of the detrital zircon grains, and this amount could not have been derived from the Appalachian region, although some of the pre-Grenvillian grains could represent recycling of zircons from Appa-lachian rift or foreland successions (Cawood et al., 2007; Thomas and Becker, 2007), or enclaves within the Grenville orogen (Fig. 9). Tributary contributions to the transcontinental paleoriver having its principal headwaters in the Appalachian orogen are the most likely sources of the non-Appalachian detrital zircons.

Archean grains could have reached the trunk paleoriver from northern tributaries draining the cratonic nucleus of Laurentia (Fig. 1). The

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U-Pb ages of detrital zircons in Colorado Plateau eolianites

Geological Society of America Bulletin, March/April 2009 425

Wyoming province of Archean rocks immedi-ately north of the Colorado Plateau (Fig. 1) is not a viable source because it was masked by sedimentary cover in Jurassic time, and it was also partly buried by the northern end of the Navajo-Nugget erg (Fig. 10). Most Paleopro-terozoic grains could similarly have entered the dispersal system from northern sources, among which the Penokean belt near the Great Lakes is the most proximal (Fig. 1). Zircon grains older than 2000 Ma that are ascribed provisionally (Table 3) to sources in northwestern Lauren-tia (Dickinson and Gehrels, 2003) may have reached the terminus of the transcontinental paleoriver system by longshore transport along the Jurassic paleoshoreline of the Cordilleran margin (Figs. 1 and 10).

Early Mesoproterozoic Yavapai-Mazatzal (1.8–1.6 Ma) detritus (Fig. 1; Table 3) was prob-ably contributed to the transcontinental pale-oriver by southern tributaries draining residual highlands of the Ancestral Rocky Mountains lying immediately east of the Colorado Plateau. The coeval Central Plains Paleoproterozoic belt was masked by sedimentary cover in Jurassic time (Fig. 10). Contributions from anorogenic pre-Grenvillian granitic rocks (Table 3) intru-sive into the Yavapai-Mazatzal belt, but present farther east as well, probably entered the sedi-ment dispersal system from the same southern tributaries that contributed Yavapai-Mazatzal detritus. The anorogenic plutons (Anderson and Morrison, 2005) are widespread across the continental interior (Fig. 8), reaching as far east as the Grenville front (Van Breemen and David-son, 1988), but most were buried beneath plat-form cover in Jurassic time (Fig. 10). More east-erly analogues were incorporated as deformed enclaves within the Grenville orogen (Rivers, 1997; Rivers and Corrigan, 2000), and some 1300–1500 Ma grains in plateau eolianites may have been derived from pre-Grenvillian sources incorporated into the Grenville orogen.

Arc-Derived Grains

Grains younger than 285 Ma in age could only have been derived from the Cordilleran region along the western fl ank of Laurentia (Fig. 1). Many interpretations are possible for the three solitary grains younger than 285 Ma in six eolianites from the western Colorado Pla-teau (Fig. 6), including analytical error, and they are not discussed further. Four eolianites of the San Rafael Group on the eastern Colorado Pla-teau (Fig. 6) contain a total of 49 grains that are younger than 285 Ma, but statistical analysis of their age distribution is challenging because half occur in just one sample (Fig. 5I). Statistical age peaks (n = 10), each defi ned by three or more

grains in various of the four samples, range from 158 Ma to 261 Ma. The oldest known plu-tons of the Cordilleran magmatic arc along the western fringe of Laurentia are 245–235 Ma in the Mojave Desert region of southern Califor-nia (Barth and Wooden, 2006), and older arc-derived grains (older than 245 Ma) are inferred to refl ect contributions from the Permian-Trias-sic East Mexico magmatic arc (284–232 Ma) of Torres et al. (1999) along the Gondwanan mar-gin of Pangea (Fig. 1).

Eolianites of the Upper Jurassic Bluff erg (Fig. 2B) were deposited by westerly winds that could have carried arc-derived grains directly from the Cordilleran orogen to the Colorado Plateau (Fig. 1). Notably, all four statistical age peaks for arc-derived grains (n = 19) in the two Bluff samples (Figs. 5H and 5J) are younger than 235 Ma, implying derivation from the Cordille-ran rather than the East Mexico arc. Net grain populations in the Bluff samples also imply extensive recycling of older eolian sand into the Bluff erg of restricted areal extent (Fig. 2B).

Eolianites of the Middle Jurassic Entrada erg were deposited by winds blowing southwest across the Colorado Plateau toward the Cordil-leran margin (Fig. 2B). Four of the six statisti-cal age peaks in the two Entrada samples from the eastern Colorado Plateau (Figs. 5G and 5I) are older than 245 Ma, suggesting important contributions from the East Mexico arc. Of the 30 arc-derived grains in the two samples, more than half are older than 245 Ma, the apparent age limit of the Cordilleran arc, and two-thirds are within the nominal age bracket (older than 232 Ma) for the East Mexico arc. Given the pale-owind direction, these relations imply that detri-tus was transported north from the East Mexico arc where it was exposed in the headwaters of a southern tributary to the transcontinental trunk paleoriver (Fig. 10) and then blown southwest into the Entrada erg from sites of temporary storage on the interior plains. As the Cordilleran arc extended into northeastern Mexico (Fig. 1), a limited volume of arc-derived Cordilleran detritus could have accompanied the arc-derived East Mexico detritus northward.

Arc-Flank EolianiteLocal lenses of quartz-rich eolian sandstone

are intercalated with Jurassic volcanic rocks along the inland fl ank of the Cordilleran mag-matic arc southwest of the Colorado Plateau (Fig. 2B). To compare detrital zircons in arc-fl ank and Colorado Plateau eolianites, we col-lected a sample from an ~250-m-thick lens of quartzose eolianite in the Mount Wrightson For-mation of southern Arizona (CP2 of Fig. 2B). Although exact correlations of the unfossilif-erous eolian strata are contentious (Bilodeau

and Keith, 1986; Busby-Spera et al., 1990), our sample was collected less than a kilometer along strike from an ignimbrite body that yielded a markedly discordant U-Pb age, interpreted by modeling to refl ect an emplacement age of 171 ± 2 Ma (Riggs et al., 1993). A statistical age peak of 164 Ma (n = 5 grains) for detrital zircons in the sample suggests that the discordant U-Pb age for the ignimbrite is unreliable, even with modeling corrections. The maximum deposi-tional age implied by the detrital zircons invites correlation with the Entrada Sandstone (San Rafael Group) of the Colorado Plateau (Fig. 3) and suggests that transport of eolian sand south-ward to the fl ank of the Cordilleran arc occurred during multiple defl ationary episodes recorded by the evolution of the Entrada erg (Kocurek, 1981; Carr-Crabaugh and Kocurek, 1998).

The ages of 25 arc-derived grains in the Mount Wrightson sample include two spurious ages (90 Ma; 140 Ma) that are younger than its depositional age, which is older than 155 Ma based on its unconformable position below the Bisbee Group (Dickinson and Lawton, 2001b). There are also two solitary outlier ages (251 Ma; 269 Ma), but most (n = 21) of the arc-derived grains span the age range of 235–160 Ma, with statistical peaks at 164 Ma (n = 5 grains), 178 Ma (n = 8 grains), and 219 Ma (n = 8 grains), which are compatible with derivation of zircon grains from igneous rocks of the associated Cordille-ran arc assemblage. The other 64 detrital zircons (72% of the population) span the same general age range as non-Cordilleran grains in Colorado Plateau eolianites, but the two age spectra dif-fer in detail (Fig. 11). Age peaks for Paleozoic, Neoproterozoic, and Archean grains are similar in both age and relative signifi cance, but the Grenvillian age population is less prominent in the Mount Wrightson Formation, and peaks for older Proterozoic grains are broader. A compari-son of the two age-distribution curves (Fig. 11) suggests that the content of zircons derived from the Yavapai-Mazatzal belt intruded by bod-ies of anorogenic granite was enhanced during transit of eolian sand across Yavapai-Mazatzal basement lying south of the Colorado Plateau (Fig. 1) but north of the sample site (Fig. 2B). Dilution of Appalachian-derived detritus with more local detritus is inferred to have been the factor that reduced the relative proportion of Grenvillian grains.

CURTIS DETRITAL ZIRCONS

Figure 12 displays age-bin histograms and age-distribution curves for samples of fl uvial sandstone from the Glen Canyon Group and marine sandstone from the San Rafael Group (Fig. 3) to compare with plots in the same

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426 Geological Society of America Bulletin, March/April 2009

format for eolianite samples (Fig. 5). Figure 13 compares composite grain populations in two samples of marine sandstone from the Curtis Formation (Figs. 12A and 12B) and two sam-ples of eolian sandstone from the underlying Entrada Sandstone (Figs. 5E and 5G) at locali-ties within 100 km of the southeastern limit of the Curtis transgression (Fig. 2B). The marine and eolian grain populations are closely simi-lar, but they are not identical. A P value of 0.13 from K-S analysis for grains older than 175 Ma (= >205 Ma) in the two composite age popula-tions indicates inability at the 95% confi dence level to be certain that the Curtis zircon popula-tion was not largely recycled from the Entrada zircon population. On the other hand, a statisti-cal age peak at 165 Ma for six grains in Curtis sample CP45 (Fig. 12B) is younger than any of the six age peaks in the range of 260–200 Ma for Entrada samples (Figs. 5G and 5I) collected both near and far from Curtis exposures (Fig. 2B).

The youngest zircon grains in the Curtis Formation control a sharp spike on the age-distribution curve at 165 Ma (Fig. 13) and probably derive from the backarc igneous province of northern Nevada and westernmost Utah (Fig. 2B). The Pony Trail Group (Muf-

fl er, 1964) of that region contains ignimbrites produced by explosive eruptions that may have spread zircon grains eastward into the Curtis seaway. Abundant mica fl akes (Tables 1 and 2) that are present in samples from the Curtis Formation but not the Entrada Sandstone also indicate some addition of non-Entrada detritus, perhaps transported by longshore currents along the southern margin of the Sundance seaway that lay northeast of the Colorado Plateau. On balance, however, the bulk of Curtis sand was apparently reworked from underlying Entrada Sandstone as the Curtis marine transgression proceeded. Although we did not sample eolian sandstone of the Moab Tongue (Figs. 2B and 3), there is no reason to suspect that its detrital zircons would not closely resemble the Curtis and Entrada zircon populations.

FLUVIAL ZIRCON PROVENANCE

The most widespread fl uvial unit within the Glen Canyon Group is the Kayenta Formation (Fig. 3), which is more feldspathic than underly-ing and overlying eolian strata (Tables 1 and 2). Three samples collected from the Kayenta Forma tion at widely separated localities on the

Colorado Plateau contain similar grain popula-tions (Figs. 12D, 12E, and 12G), with compara-tive P values from K-S analysis of three sample pairs in the range of 0.25–0.83. The “Black Ledge” sandstone (Figs. 3 and 12C), exposed locally in Utah (Fig. 2A), is of somewhat dif-ferent petrofacies than the Kayenta Formation (Table 2), but contains a similar grain popula-tion 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-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-ment of the “Black Ledge” sandstone to the Glen Canyon Group (Fig. 3) rather than to the underlying Chinle Group or Formation is sup-ported 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

A comparison of age-distribution curves for Kayenta fl uvial and Glen Canyon eolian sam-ples (Fig. 14) indicates a close similarity in grain populations, except that the fl uvial sandstones contain a signifi cant proportion of arc-derived grains younger than 285 Ma. The P value for grains older than 285 Ma in the two composite grain populations is 0.65, indicating that the pre-arc grains in the Kayenta Formation and Glen Canyon eolianites are statistically indistinguish-able. The overall Kayenta detrital zircon popu-lation can be interpreted as the result of mixing arc detritus with eolian sand redistributed from penecontemporaneous (or directly underlying) unconsolidated erg deposits.

Statistical age peaks (n = 8), each controlled by three or more grains in various of the Kay-enta samples, fall within the range 288–231 Ma, suggesting derivation dominantly from the East Mexico arc (284–232 Ma) rather than the Cor-dilleran arc (younger than 245 Ma). Consistent paleocurrent trends toward the west and north-west for the Kayenta Formation are compatible with contributions from the East Mexico arc lying to the southeast of the Colorado Plateau (Fig. 1). Zircon grains of Grenvillian age in the Kayenta samples might also have been derived in part from source rocks lying to the south-east, but no apparent sources for the Archean and Paleoproterozoic grains are evident south-east of the Colorado Plateau (Fig. 1). Accord-ingly, redistribution of eolian sand is our pre-ferred interpretation for the origin of all the

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Figure 11. Age-distribution curves for detrital zircon grains older than 285 Ma in Mount Wrightson Formation (CP2) and 10 Colorado Plateau eolianites (Fig. 7).

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Dickinson and Gehrels

428 Geological Society of America Bulletin, March/April 2009

Kayenta pre-arc grains. Lack of augmentation of Paleoproterozoic or early Mesoproterozoic zircon grains in the Kayenta Formation, as compared to Glen Canyon eolianites (Fig. 14), argues against signifi cant contribution of detri-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-ing line from the mean detrital mode of Glen Canyon eolianites (Qt86-F10-L4 from Table 1), through the mean detrital mode of Kayenta sandstones (Qt76-F17-L7 from Table 1), to the mean detrital mode of fi rst-cycle Paleogene 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 grains older than 285 Ma, implies a somewhat higher proportion of redistributed eolian sand (88%), but mature eolianites may contain a signifi cantly higher proportion of zircon grains than juvenile arkosic detritus. In any case, the 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-ing from areas where erosion has since removed all traces of its former presence (Fig. 2A). The redistribution of eolian sand into the Kayenta Formation suggests widespread intraregional recycling of durable zircon grains of domi-nantly 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-vial sandstones of the Kayenta Formation (Glen Canyon Group) and eolian sandstones of the San Rafael Group invites comparison of the two sample sets (Fig. 15). The only salient dif-ference is the presence of 11 grains younger 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 value from K-S comparison of grains older than 195 Ma in the two populations is 0.28, indicat-ing a lack of statistical distinction. Higher P/F ratios in detrital modes (Table 2) appear to be a subtle but reliable guide to the presence of arc

0 500 1000 1500 2000 2500 3000 3500Age (Ma)

Age

pro

babi

lity

Kayenta Formation fluvial

(CP1-CP10-DOL)

N = 3; n = 251

Glen Canyon Group eolian

(CP3-CP30-Jnnw-Jwnw)

N = 4; n = 386

Age (Ma)0 500 1000 1500 2000 2500 3000

Age

pro

babi

lity

Curtis Formation marine(CP43 - CP45)N = 2; n = 187

Entrada Sandstone eolian(CP16 - CPJenw)

N = 2; n = 183

Figure 13. Composite age-distribution curves for detrital zircon grains in marine Curtis (Figs. 5E and 5G) and eolian Entrada (Figs. 11A and 11B) samples from Utah (N = samples; n = grain ages).

Figure 14. Composite age-distribution curves for detrital zircon grains in samples of Glen Canyon eolianites (Figs. 5B–5D, and 5F) and fl uvial Kayenta Formation (Figs. 11D, 11E, and 11G) of Glen Canyon Group (N = samples; n = grain ages).

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U-Pb ages of detrital zircons in Colorado Plateau eolianites

Geological Society of America Bulletin, March/April 2009 429

detritus in both San Rafael eolian and Kayenta fl uvial sandstones.

Springdale Sandstone Member

The Springdale Sandstone Member of the 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 Paleozoic–Neoproterozoic grains or any grains older than 1850 Ma, suggests derivation from a provenance in southwest Laurentia lying directly south of Springdale exposures (Fig. 1). All arc-derived Springdale detrital zircons (n = 40) are 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 grains are 281–247 Ma, indicative of derivation from the older East Mexico magmatic arc.

There is, however, an areal variation in Kayenta zircon populations that suggests the Springdale and Kayenta fl uvial systems may have followed parallel courses fl owing from 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 arc-derived zircon grains (n = 7 or 9% of total grains) that may be exclusively of Cordilleran derivation (younger than 241 Ma), with an age peak defi ned by three grains at 231 Ma. The other two Kayenta samples (CP1, DOL) contain arc-derived zircon grains (n = 21), of which 75% are older than 245 Ma (range 281–247 Ma), with age peaks defi ned by 5–10 grains of 246 Ma, 251 Ma, 265 Ma, and 280 Ma (indicative of sources in the East Mexico arc). Kayenta streams farthest toward the northeast on the compound fl oodplain evidently redistributed eolian sand, to which East Mexico arc detritus was added from the southeast, whereas Kayenta streams toward the southwest may have redistributed eolian sand to which Cordilleran arc detritus was added from the south. Springdale streams still farther to the southwest mixed detritus from Yavapai-Mazatzal basement of southwest Laurentia with arc-derived Cordilleran detritus without contri-butions from the East Mexico arc.

SUMMARY AND CONCLUSIONS

U-Pb ages of detrital zircons in Jurassic ergs and associated deposystems of the Colorado Plateau support the following conclusions: (1)

plateau eolianites are composed dominantly of detritus derived ultimately from bedrock sources in eastern Laurentia rather than the Cordilleran orogen or the Ancestral Rocky Mountains prov-ince; (2) the non-Cordilleran detritus was trans-ported to a position upwind from the Colorado Plateau by a transcontinental paleoriver system having its headwaters in the Appalachian belt, with contributions of non-Appalachian detritus delivered to the trunk stream by both northern and southern tributaries during transit of sediment across the continent; (3) subordinate detritus in the eolian sands was derived from the Permian-Triassic East Mexico magmatic arc as well as the Triassic-Jurassic Cordilleran magmatic arc; (4) arc-fl ank eolianite south of the Colorado Plateau is composed of plateau eolian sand contaminated with contributions from intervening basement and the enclosing arc assemblage; (5) marine Curtis sand was largely reworked from underly-ing Entrada eolian sand; (6) fl uvial Kayenta sand is an admixture of redistributed eolian sand and younger arc detritus; (7) Springdale provenance in the Cordilleran arc and its basement was distinct from Kayenta provenance; and (8) the youngest detrital zircons in the “Black Ledge” sandstone confi rm its affi nity with the Glen Canyon Group rather than the Chinle Formation or Group.

ACKNOWLEDGMENTS

Advice from Ronald C. Blakey, Brian S. Currie, Peter G. DeCelles, Spencer G. Lucas, and Fred Peter-son was indispensable for sample collection, and Gerald Bryant kindly provided sample DOL. Permis-sion was granted by the Hopi Tribe through Arnold Taylor for collection of sample CP10, by Grand Staircase-Escalante National Monument through Marietta Eaton for collection of sample CP37, and by the Pueblo of Laguna through Josephine Cochran for collection of sample CP54. We also appreciate the unfailing courtesy of resident members of the Hopi Tribe and Laguna Pueblo. For the U-Pb ages of detrital zircons, we thank Joseph R. Amar, Erin V. Brenneman, Owen V. Hurd, Jennifer L. McGraw, Gregory R. Schmidt, Carl E. Anderson, Carla M. Eichler, Linette C. Ancha, and Jaika Ojha (all then undergraduate students in the Colorado Plateau Sem-inar supported by the National Science Foundation in the Department of Geosciences at the University of Arizona), who processed all samples under our gen-eral guidance in the Arizona LaserChron Center of the University of Arizona. Jerome Guynn developed the algorithm for applying Kolmogorov-Smirnoff statistics to populations of detrital zircons. Alex Pul-len aided in sample processing, and Jim Abbott of SciGraphics prepared the fi gures. Reviews and edi-torial comments by Brendan Murphy, Jan Golonka, R.H. Rainbird, and John Waldron improved both text and fi gures. Our research was supported by National Science Foundation grants EAR-0341987 (for Colo-rado Plateau detrital zircons) and EAR-0443387 (to the Arizona LaserChron Center).

0 500 1000 1500 2000 2500 3000 3500Age (Ma)

Age

pro

babi

lity

San Rafael Group eolian(eastern Colorado Plateau)

(CP15-CP16-CP24-CP54)

n = 4; N = 365

n = 3; N = 255

(CP1-CP10-DOL)

(Glen Canyon Group)Kayenta Formation fluvial

Figure 15. Composite age-distribution curves for samples of Entrada-Bluff (San Rafael Group) eolian sandstone (Fig. 5GJ) and Kayenta (Glen Canyon Group) fl uvial sandstones (Figs. 11D, 11E, and 11 G) containing arc-derived Cordilleran grains younger than 285 Ma (n = samples; N = grain ages).

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430 Geological Society of America Bulletin, March/April 2009

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