ALTERNATE APPRAISALS OF YOUNGEST U-PB GRAIN AGES IN DETRITAL ZIRCON POPULATIONS OF

MESOZOIC STRATA ON THE COLORADO PLATEAU

William R. Dickinson and George E. GehrelsDepartment of Geosciences, University of Arizona, Tucson, AZ 85721,

ABSTRACT (SLIGHTLY REVISED FROM PUBLISHED VERSION)U-Pb ages for youngest detrital zircon grains in sedimentary strata constrain maximum

depoages of stratigraphic units. We have tested alternate estimates of youngest valid grain age in detrital zircon populations against a database of 5365 U-Pb zircon ages in 58 samples of

Mesozoic sandstone from the Colorado Plateau and contiguous areas for which depoages can be inferred independently within 4-14 myr by correlation of stratigraphic stages with either the

GTS 2004 timescale for Jurassic-Cretaceous or a provisional Triassic timescale superseding GTS 2004. Approximately 100 U-Pb ages were determined for individual zircon grains in each

sample by LA-ICP-MS using a beam diameter of 35 microns, but ages >20% discordant or with age uncertainties >10% were rejected (average of 92.5 grain ages retained per sample). The very youngest grains present in each case may not have been selected for laser ablation,

but we tested five measures of youngest dated grains for consistency with depoage: (1) youngest single grain age (206Pb/238U) including 1σ analytical uncertainty; (2) weighted

mean average (with 2σ uncertainty) of youngest cluster of two or more grain ages overlapping at 1σ uncertainty (to provide minimal reproducibility); (3) youngest graphical age peak

controlled by two or more grain ages on a probability density plot (age distribution curve); (4) youngest calculated age peak controlled by three or more grain ages that overlap at 2σ; (5) TuffZirc age (with uncertainties) for youngest cluster of coherent grain ages selected by the 2002 algorithm of Ludwig and Mundil. Youngest single grain ages overlap depoages in 40%

of samples, but are older than depoages in 53% of samples and younger than depoages (apparently spurious) in only 7% of samples. Despite the inherent risk of relying upon single data points, youngest single dated zircon grains apparently provide a satisfactory measure of youngest grain age in 93% of samples provided overlap with depoage is acceptable (or even

desirable). Using alternate measures for youngest grain age involving multiple grains reduces overlap with depoage to 9%-19% of samples and poses age discrepancies (grains too young) in only two samples (3.5%), and in them for only selected multi-grain measures. The multi-grain

measures for youngest age are thus more conservative and robust. A choice between using youngest single grain age and some multi-grain measure of youngest grain age may depend from case to case on the purpose of a youngest-age analysis, but comparison of results from

the different approaches to estimating youngest grain age provides insights that no one approach alone can yield.

THE OPPORTUNITY1. The youngest detrital zircons in a sandstone should provide a fail-safe measure of the

maximum age of deposition, and this constraint should be useful as one irrefutable measure of age for Precambrian and other unfossiliferous strata

2. Our database of U-Pb ages (LA-ICP-MS) for 5365 individual detrital zircon grains in 58 samples of Mesozoic sandstone from the Colorado Plateau for which depoages are known independently to within 10±5 myr provides a sensitive test of that approach

THE METHODOLOGYBecause of inevitable analytical error and the inherent complexity of U-Pb isotopic systematics, selecting a valid age for the youngest detrital zircon grain in a sample population of nearly 100 grains (average 92.5 concordant or nearly concordant grain ages per sample in our case) is not a trivial exercise – accordingly we use five alternate measures of youngest grain age to test against inferred depoage:

1) YGA: youngest single grain age (±1σ), which is the youngest possible measure of youngest DZ in a sample but lacks any vestige of internal reproducibility2) YGP: youngest graphical age peak controlled by two or more grain ages (thus providing minimal reproducibility) on the sample age-distribution curve (probability-density plot)3) YGC: weighted average mean age (with 2σ uncertainty) of youngest cluster of two or more grains with age uncertainties overlapping at 1σ (another way of providing minimal reproducibility for estimating youngest age)4) YCP: youngest calculated age peak controlled by three or more grains with age uncertainties that overlap at 2σ (from the in-house DZ age peak algorithm of the Arizona LaserChron Center providing more robust reproducibility)5) TZA: median age of youngest coherent cluster of potentially cogenetic grains from the TuffZirc algorithm of Ludwig and Mundil (2002 Goldschmidt Conference Abstracts, p. A463) to provide a conservative and statistically robust measure of youngest grain age (plus-minus uncertainties unequal)

DEPOAGE TIMESCALEFor inferring depoages, the GTS 2004 timescale is used for Jurassic and Cretaceous time (thumbnail version

reproduced below), but post-2004 data render the GTS 2004 Triassic timescale obsolete (especially for Lower and Middle Triassic time), so provisional “long Carnian” (option A) and “long Norian” (option B) versions of

the Triassic timescale are used for estimating depoages of Triassic strata (see chart below for derivation of alternate

“long Carnian” and “long Norian” versions)

RECENT TRIASSIC TIMESCALE ADJUSTMENTSdownward revision of base of Jurassic accepted after Lucas and Tanner 2007double-headed arrows indicate stated age uncertainties of stage boundaries

[asterisks denote Ar/Ar age of mid-Carnian tuff (Rogers et al., 1993) compatible with “long Carnian” option but not with “long Norian” option and U/Pb age of upper Carnian tuff (Furin et al.,

2006) compatible with “long Norian” option but not with “long Carnian” option]

KEY TRIASSIC TIMESCALE REFERENCES

Brack, P., Rieber, H., Nicora, A., and Mundil, R., 2005, The global boundary stratotype section and point (GSSP) of the Ladinian Stage (Middle Triassic) at Bagolino (southern Alps, northern Italy) and its implications for the Triassic time scale: Episodes, v. 28, p. 233-244.

Furin, S., Preto, N., Rigo, M., Roghi, G., Gianolia, P., Crowley, J.L., and Bowring, S.A., 2006, High-precision U-Pb zircon age from the Triassic of Italy: Implications for the Triassic time scale and the Carnian origin of calcareous nannoplankton and dinosaurs: Geology, v. 34, p. 1009-1012.

Gradstein, F., Ogg, J., and Smith, A., 2004, A geologic time scale 2004: Cambridge, Cambridge University Press, 589 p.

Lucas, S.G., and Tanner, L.H., 2007, The nonmarine Triassic–Jurassic boundary in the Newark Supergroup of eastern North America: Earth-Science Reviews, v. 84, p. 1-20.

Muttoni, G., Kent, D.V., Olsen, P.E., Di Stefano, P., Lowrie, W., Bernasconi, S.M., and Hernández, F.M., 2004, Tethyan magnetostratigraphy from Pizzo Mondello (Sicily) and correlation to the Late Triassic Newark astrochronological polarity time scale: Geological Society of America Bulletin, v. 116, p. 1043-1058.

Rogers, R.R., Swisher, C.C. III, Sereno, P.C., Monetta, A.M., Forster, C.A., and Martinez, R.N., 1993, The Ishigualasto tetrapod assemblage (Late Triassic, Argentina) and 40Ar/39Ar dating of dinosaur origins (argon dating of dinosaur remains in Argentina): Science, v. 260, p. 794-797.

Graphical Representations of Depoage vs Youngest DZ: Colorado Plateau–High Plains Mesozoic Strata

(see table below for data plotted)

Table of Inferred Depoages (DA-DA’) and Youngest DZ (Detrital Zircon) Ages(for CPDZ samples from Colorado Plateau and High Plains listed in order of decreasing age or from west to east where estimated depoages are identical)

[DA=estimated depoage from nominal duration of stratigraphic interval; DA’= estimated depoage from span of stratigraphic interval including 1σ uncertainties of stage limits]

see left hand column of poster for definition of YGA-YGP-YGC-YCP-TZA(numbers of grains in parentheses for multi-grain measures)

Arc-Derived GrainsExcept for samples in which youngest DZ grains exceed depoage by ~100+ Ma, youngest DZ grains were derived from either the Permian-Triassic East Mexico

arc (284-232 Ma) or the Mesozoic Cordilleran arc (<245 Ma), or both. Plots below are age-distribution curves for arc-derived grains (nominal 297.5 Ma age limit on abscissa) in strata of various ages (N=number of samples; n=number of arc-derived grains), and the net population (bottom plot) in 49 samples (n=610) containing arc-derived grains (net population biased by preponderance of grains from the pre-Cretaceous sample sets, but note that abundant Upper Cretaceous Menefee grains, n=75 total, include multiple Triassic, Jurassic, and Cretaceous age peaks reflecting an integrated arc source); note appearance of progressively younger age peaks in strata of progressively younger depoage; ratios of Triassic-

Jurassic-Cretaceous sample subsets (and total grains therein):For N, Triassic:Jurassic:Cretaceous (depoage) = 17:22:10 or ~ 2:2:1

For n, Triassic:Jurassic:Cretaceous (depoage) = 249:228:133 or ~2:2:1