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

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U-Pb ages of detrital zircons in Jurassic eolian and associated sandstones of the Colorado Plateau: Evidence for transcontinental dispersal and intraregional recycling of sediment William R. Dickinson† George E. Gehrels Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA ABSTRACT dispersed widely across southwest Laurentia from southern Arizona, and seven samples of by a transcontinental paleoriver system and fl uvial and marine sandstones closely associated U-Pb ages for 1655 individual detrital zir- paleowinds, which deposited extensive Juras- with erg deposits of the Glen Canyon and San con grains in 18 samples of eolian and asso- sic ergs, durable zircon grains were recycled Rafael Groups. We present here 1655 reliable ciated marine and fl uvial sandstones of the by multiple intraregional depositional sys- U-Pb ages (concordant or nearly so) for indi- Glen Canyon and San Rafael Groups from tems. Lower Jurassic fl uvial sand is locally vidual zircon grains from 18 samples. the Colorado Plateau and contiguous areas composed, however, of detritus derived from An evaluation of the enlarged database con- shed light on patterns of Jurassic sediment the nearby Cordilleran magmatic arc assem- fi rms transcontinental dispersal of sand to Juras- dispersal within Laurentia. Most detrital zir- blage and its Precambrian basement. sic ergs of the Colorado Plateau. Admixtures con grains in Jurassic eolianites were derived of grains derived from basement or magmatic ultimately from basement provinces older Keywords: Colorado Plateau, detrital zircon, arcs in southwestern North America are present, than 285 Ma in eastern and central Lauren- eolianite, Glen Canyon Group, provenance, San along with recycled eolian sand, in associated tia, rather than from rock assemblages of the Rafael Group. fl uvial units, and these combinations occur as nearby Cordilleran margin. The most promi- well in selected eolian units, including some nent peaks of constituent age populations INTRODUCTION either intercalated with arc volcanics or depos- at 420 Ma, 615 Ma, 1055 Ma, and 1160 Ma ited by westerly winds. Recycling of eolian refl ect derivation from Paleozoic, Neopro- Dickinson and Gehrels (2003) showed that sand by intraregional depositional systems can terozoic, and Grenvillian sources within typical Jurassic eolianites (eolian sandstones) be attributed to the durability of zircon grains in the Appalachian orogen or its sedimentary of Colorado Plateau ergs (sand seas) contain sedimentary environments. Nevertheless, docu- cover. Sediment was transported to a posi- age populations of detrital zircons derived from mentation that voluminous sand of the central tion upwind to the north of the Colorado essentially all Precambrian and Paleozoic grani- and eastern Laurentian provenance dominated Plateau by a transcontinental paleoriver sys- toid basement provinces of central and eastern Jurassic eolian deposits on the Colorado Plateau tem with headwaters in the central to south- Laurentia (Fig. 1), though a few Mesozoic zir- through a period of >40 m.y. indicates the per- ern Appalachian region, but subordinate con grains were derived from the nearby Cordil- sistence of an integrated system for transconti- non-Appalachian detritus was contributed leran orogen. Sand was transported across the nental sediment dispersal. by both northern and southern tributaries continent by a Jurassic paleoriver, which had Descriptions of sample localities (see Sup- during sediment transit across the continent. its headwaters in the Appalachian province, to plementary Text 1) and U-Pb age data for all Subordinate detrital zircons younger than fl uvial or deltaic plains lying north of the Colo- samples discussed in this paper are included 285 Ma in selected Middle to Upper Juras- rado Plateau. From temporary sediment storage in accompanying data repositories.1 The tabu- sic eolianites were derived from the Perm- there, sand was then blown southward to grow- lated analytical data (Supplementary Text 2) ian-Triassic East Mexico and the Mesozoic ing Colorado Plateau ergs by paleowinds well are supplemented by a concordia diagram for Cordilleran magmatic arcs. Lower Jurassic known from eolian cross-bedding. each sample and superimposed graphs of age- fl uvial sandstones typically contain a mixture Our preliminary study was restricted to just bin histograms and probability-density plots of detrital zircons redistributed from eolian three samples of Jurassic eolianite from a single (age-distribution curves) for grains in each sand and derived from the East Mexico arc, stratigraphic section near the center of the Colo- which lay up-current to the southeast. Zir- rado Plateau, and it was inadequate to gauge the 1GSA Data Repository item 2008251, ST1 (in cons in marine Curtis sandstone were largely possible variability of detrital zircons in eolian- Word) is a listing of sample localities, ST2 (in csv) reworked from underlying Entrada eolianite, ites of various ages exposed across the length and is U-Pb analytical data for all samples, ST3 (pdf) with minor contributions from the Jurassic breadth of the Colorado Plateau. For this study, is concordia diagrams and combined age-bin histo- backarc igneous assemblage of the Great we expanded analytical coverage to include grams and age-distribution curves (probability-den- sity plots) for each sample, and ST4 (in Word) is a Basin. Once mature quartzose detritus was seven more samples of Lower to Upper Jurassic table of ages and references for Figure 9; available 3 2 eolianites distributed over >175 × 10 km of the at http://www.geosociety.org/pubs/ft2008.htm or by †E-mail: [email protected]. Colorado Plateau, correlative intra-arc eolianite request to [email protected]. GSA Bulletin; March/April 2009; v. 121; no. 3/4; p. 408–433; doi: 10.1130/B26406.1; 15 fi gures; 3 tables; Data Repository item 2008251. 408 For permission to copy, contact [email protected] © 2008 Geological Society of America U-Pb ages of detrital zircons in Colorado Plateau eolianites C al Ages of Belts in Ga 100° W edo nia Archean Cross-Hatched n ( 0.3 1000 km 6 - 0.4 8) 8) .4 - 0 6 .3 North (0 n itia nu In G r e e n l a n d 60° N Da vis St rait clo sed la A n a C 60° Slave Nain (>2.5) (>2.5) Ketilidian N Wopmay (1.8 - 1.9) (1.8 - 2.3) ) H u d s o n 5 . 2 > ( B a y e n a o s R d ) u .9 -H 1 - s - n 8 ra . e T (1 n r ) a 3 . e S u p e r i o r (> 2.5) 1 H - 0 . - Can 1 USA ( g n i 40° N m o an y e ) ok 5 W 8 en . 40° P (~1 e N l mid- l i Central Plains continent v (1.6 - 1.8) (1.35 - 1.5) n Appalachian e (0.36 - 0.76) Colorado r Plateau Amarillo - Wichita G (~0.525) Yavapai - Mazatzal Continental ? ? re (1.6 - 1.8) lle tu slope vi ) Ouachita gins u USA en .3 ? ig S Gr - 1 orogen W Mex .0 ? ? Cordilleran (1 accretion ? and batholiths (<0.25) Suwanee Yúcatan - (0.54 - 0.68) Baja California Campeche (0.54 - 0.58) 20° N [0.40 - 0.43 in south] 20° N Gulf of Cenozoic California Chiapas overthrust closed Gulf of Mexico of East Mexico closed Antillean Oaxaquia magmatic arc arc 120° W (1.0 - 1.25) (0.23 - 0.29) [0.44 - 0.48 in south] 80° W Figure 1. Location of Colorado Plateau in relation to Precambrian and Phanerozoic age belts of North Amer- ica with Davis Strait and Gulfs of California and Mexico closed (for pre–mid-Jurassic time), adapted after Ham et al. (1964), Hoffman (1988, 1989, 1990), Hatcher et al. (1989), Viele and Thomas (1989), Reed (1993), Ortega-Gutierrez et al. (1995), Van Schmus et al. (1996), Atekwana (1996), Steiner and Walker (1996), Torres et al. (1999), Lopez et al. (2001), Dickinson and Lawton (2001a), Iriondo et al. (2004), Talavera-Mendoza et al. (2005), and Barth and Wooden (2006). Abbreviations: Ala—Alaska; Can—Canada; Mex—Mexico. Geological Society of America Bulletin, March/April 2009 409 Dickinson and Gehrels sample falling within the ranges of 0–4000 Ma, (operating at a wavelength of 193 nm) using a Interpreted ages are based on 206Pb/238U for 0–800 Ma, and 800–2400 Ma (Supplementary spot diameter of 35 µm. The ablated material grains younger than 1000 Ma and 206Pb/207Pb for Text 3). Preliminary U-Pb age data for three was carried in helium into the plasma source grains older than 1000 Ma. The division point of the eolianite samples (Jwnw—Wingate; of a GVI Isoprobe, which was equipped with for each sample is at a slightly different age near Jnnw—Navajo; Jenw—Entrada) from North a fl ight tube of suffi cient width that U, Th, 1000 Ma to avoid splitting up age clusters of Wash in Utah (Dickinson and Gehrels, 2003) and Pb isotopes could be measured simultane- grains. In any case, age uncertainties are inher- are superseded by data of improved precision ously. All measurements were made in static ently greatest near 1000 Ma (Gehrels, 2000). and accuracy obtained during the present study mode, using Faraday detectors with 10e11 Analyses that were >30% discordant (by (Gehrels et al., 2008). Preliminary data for other ohm resistors for 238U, 232Th, 208Pb, and 206Pb, comparison of 206Pb/238U and 206Pb/207Pb ages) samples were presented by Amar and Bren- a Faraday detector with a 10e12 ohm resis- or >5% reverse discordant were not consid- neman (2005), Brenneman and Amar (2005), tor for 207Pb, and an ion-counting channel for ered further (average of 92 grain ages retained Hurd and Schmidt (2005), Schmidt et al. (2005), 204Pb. Ion yields were ~1.0 mv per ppm. Each per sample). The resulting interpreted ages are Amar et al.
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