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

1
Graphical Representations of Depoage vs Youngest
DZ Colorado PlateauHigh Plains Mesozoic
Strata (see table below for data plotted)
DEPOAGE TIMESCALE For 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)
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. Gehrels Department 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 gt20 discordant or with age
uncertainties gt10 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 1s analytical
uncertainty (2) weighted mean average (with 2s
uncertainty) of youngest cluster of two or more
grain ages overlapping at 1s 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 2s (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.
Arc-Derived Grains Except 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 (lt245 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 (Nnumber of
samples nnumber of arc-derived grains), and the
net population (bottom plot) in 49 samples
(n610) 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, n75
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,
TriassicJurassicCretaceous (depoage) 172210
or 221 For n, TriassicJurassicCretaceous
(depoage) 249228133 or 221
RECENT TRIASSIC TIMESCALE ADJUSTMENTS downward
revision of base of Jurassic accepted after Lucas
and Tanner 2007 double-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

• THE OPPORTUNITY
• 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 105 myr provides a sensitive test of that
  approach

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. Gradstei
n, 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 TriassicJurassic 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.
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) DAestimated
depoage from nominal duration of stratigraphic
interval DA estimated depoage from span of
stratigraphic interval including 1s 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)
THE METHODOLOGY Because 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
(1s), which is the youngest possible
measure of youngest DZ in a sample but lacks any
vestige of internal reproducibility 2) 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 2s uncertainty) of
youngest cluster of two or more grains with age
uncertainties overlapping at 1s (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 2s (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)