Constraints on the Age and Provenance of the Chugach 

Accretionary Complex from Detrital Zircons in the Sitka 

Graywacke near Sitka, Alaska

By Peter J. Haeussler, George E. Gehrels1, and Susan M. Karl

Abstract

The Sitka Graywacke is the westernmost and youngest unit 

of the Chugach accretionary complex in southeastern Alaska. 

Using laser-ablation inductively coupled plasma mass spectroscopy, we obtained 492 detrital-zircon ages on seven typical 

samples of Sitka Graywacke turbidites, which were collected in 

a transect across much of the unit near Sitka, Alaska. Individual 

grains range in age from 66 to 1,802 m.y. The youngest peak ages 

on relative-probability plots of the western four samples (74, 72, 

74, and 74 m.y., from west to east) are distinctly younger than the 

youngest peak ages of the eastern three samples (105, 103, and 97 

m.y., from west to east). These youngest peak ages set maximum 

depositional ages for each sample. We suggest that these peak ages 

are not significantly older (within ∼5 m.y.) than the depositional 

age of the Sitka Graywacke because the deposits accumulated 

in a trench along a convergent margin, where magmatic sources 

likely continuously introduced juvenile zircons. The differences 

in the youngest cluster of detrital-zircon ages between the eastern 

and western sample localities is likely due to both a change in 

provenance and a fault. The similarity of the youngest peak ages in 

the Sitka Graywacke to fossil ages in the Valdez Group, in Prince 

William Sound, implies that the western part of the Sitka Graywacke is correlative with the Valdez Group, as previously inferred. 

However, the eastern part of the Sitka Graywacke has youngest 

detrital-zircon ages older than fossil ages in the Valdez Group and 

younger than fossil ages in the McHugh Complex, which in southcentral Alaska is the oldest part of the accretionary complex. The 

age distribution of zircons in the older, eastern sequence suggests 

sources along the British Columbia margin. The detrital-zircon 

ages in the younger, western sequence are similar to igneous ages 

from south-central Alaska to southern British Columbia. Rightlateral strike slip on various fault systems inboard of the Sitka 

Graywacke implies that it lay to the south when it was deposited 

and offscraped. Thus, although source areas as far north as the St. 

Elias Mountains and south-central Alaska are possible, they were 

most likely in coastal and interior British Columbia.

Introduction

Graywacke turbidites are notoriously poor in fossils, and 

so dating turbidite sequences can be a great challenge. The Sitka 

Graywacke, which lies along the west coast of southeastern 

Alaska, is one such turbidite sequence (Berg and Hinckley, 1963; 

Loney and others, 1963; Decker and others, 1979; Decker, 1980). 

The unit is part of a Mesozoic through early Tertiary accretionary complex rimming southern Alaska (fig. 1; Plafker and others, 

1977) that is commonly referred to as the Chugach terrane (for 

example, Plafker and others, 1994). Various workers (Nilsen and 

Moore, 1979; Nilsen and Zuffa, 1982; Plafker and others, 1994) 

have proposed that these accretionary-complex turbidites were 

deposited by northward-flowing trench-parallel turbidity currents 

derived from erosion of the Coast Mountains of southeastern 

Alaska and British Columbia.

Regional correlations between the Sitka Graywacke and 

other units of the Chugach accretionary complex are hindered by 

the absence of fossils in the turbidites. Thus far, only two fossils have been identified from the Sitka Graywacke. Reifenstuhl 

(1986) obtained a gastropod with a nondiagnostic age ranging 

from Silurian to Eocene. Also, a small limestone pod, with sharp 

contacts, in graywacke (at the location of our sample 65, fig. 2) 

contained a poorly preserved cone-shaped nasselarian radiolarian 

of possible Jurassic age (C.D. Blome, written commun., 1994).

Detrital-zircon ages can provide a maximum depositional 

age because the host sediment must have been laid down after the 

youngest detrital-zircon grain crystallized. In this study, we use the 

youngest ages from sandstone of the Sitka Greywacke to constrain 

depositional ages, subdivide the unit, and interpret the provenance 

of the detrital-zircon grains on the basis of a comparison with the 

ages of potential igneous source rocks along the northern Cordilleran margin.

Methods

We collected seven samples in an east-west-trending transect across much of the Sitka Graywacke near Sitka, Alaska 

(fig. 2; table 1). All samples were collected from normally 1

Department of Geosciences, University of Arizonia, Tucson.

Studies by the U.S. Geological Survey in Alaska, 2004

U.S. Geological Survey Professional Paper 1709–F
 Constraints on the Age and Provenance of the Chugach Accretionary Complex, Alaska

graded sandy turbidite beds, 5 to 20 cm thick, with sharp 

basal contacts. The sampled beds were typical of the beds at 

each sample locality. Detrital zircons were extracted by using 

standard mineral-separation techniques.

Zircons were mounted in epoxy and polished to expose 

the interior of the grains. Isotopic analyses were performed 

by using a Micromass IsoProbe multicollector inductively 

coupled plasma mass spectrometer (MC–ICPMS) with a 

laser-ablation system at the geochronology laboratory of the 

University of Arizona, Tuscon. Laser-beam diameter was 35 

μm, yielding ablation pits ∼15 μm deep. Interelement fractionation was monitored by analyzing fragments of a large 

concordant zircon with a known (isotope-dilution thermalionization mass-spectrographic, or ID–TIMS) age of 564±4 

m.y. (2σ error). This standard was analyzed once for every 

five unknowns. Grains were selected randomly from all sizes 

and morphologies present, avoiding grains with fractures or 

inclusions. The ablated material was carried in Ar gas into the 

plasma source of the mass spectrometer, which was equipped 

with a flight tube of sufficient width that U, Th, and Pb isotopes were analyzed simultaneously. All measurements were 

made in static mode, using Faraday detectors for 238U, 232Th, 

and 208–206Pb and an ion-counting channel for 204Pb. Ion yields 

were 0.5 mV/ppm. Each analysis consisted of 1 20-s integration on peaks with the laser off (for backgrounds), 20 1-s 

integrations with the laser firing, and a 30-s delay to purge the 

previous sample and prepare for the next analysis.

Common Pb correction was made by using the measured 204Pb content and assuming an initial Pb composition 

from Stacey and Kramers (1975) (with uncertainties of ±1.0 

for 206Pb/204Pb ratio and ±0.3 for 207Pb/204Pb ratio). Our 204Pb 

measurement is unaffected by the presence of 204Hg because 

backgrounds are measured on peaks (thereby subtracting any 

background 204Hg and 204Pb) and only trace Hg was present in 

the Ar gas.

Interelement fractionation of Pb/U is generally <20 

percent, whereas fractionation of Pb isotopes is generally <5 

percent. In-run analysis, generally every fifth measurement, of 

fragments of a large zircon crystal standard (Sri Lanka) with 

a known age of 564±4 m.y. (2σ error; G.E. Gehrels, unpub. 

data, 2004) was used to correct for this fractionation. The 

uncertainty resulting from the calibration correction (together 

with the uncertainty from decay constants and common Pb 

composition) is generally ±3 percent (2σ) for 206Pb/238U ages 

and for 206Pb/207Pb ages >1.2 b.y. Fractionation also increases 

with depth into the laser pit. The accepted isotopic ratios were 

accordingly determined by least-squares projection through 

the measured values back to the initial determination. The 

complete measured isotopic ratios and ages are listed in table 

2. Errors from measuring 206Pb/238U, 206Pb/207Pb, and 206Pb/204Pb 

Chichagof Island

Study Area

Prince

William

Sound

Talkeetna

Mountains

Saint Elias

Mountains

Coast Mountains

60oN56oN64oN138oW144oW132oW68oN

52oN

48oN

126oW120oW

Queen Charlotte

Islands

Figure 1. Gulf of Alaska area, showing location of study area and tectonostratigraphic terranes of Alaska, Yukon Territory, and 

British Columbia (from Nokleberg and others, 1998). The Sitka Graywacke, the Valdez Group, and the McHugh Complex are all parts 

of the Chugach accretionary complex. Terranes: CG, Chugach; ST, Stikine; YT, Yukon-Tanana. Faults: BR, Border Ranges; CS, Chatham 

Straight; DE, Denali; TI, Tintina.
Results and Discussion 

ratios are reported at the 1σ level. Additional errors that affect 

all ages include uncertainties from (1) U decay constants, (2) 

the composition of common Pb (assumed to be ±1.0 for the 206Pb/204Pb ratio and ±0.3 for the 207Pb/204Pb ratio), and (3) calibration correction. These systematic errors add an additional 

2-percent (1σ) uncertainty to 206Pb/238U ages and to 206Pb/207Pb 

ages >1.2 b.y. A total of 69 to 75 ages were determined for 

each of the seven samples.

Age interpretations for the analyses are based largely 

on 206Pb/238U ages. 206Pb/207Pb ages for these relatively young 

zircons are much less reliable given the low 207Pb content. As 

a result, discordance on an individual-grain basis cannot be 

assessed for grains younger than 1.2 b.y. Thus, 206Pb/207Pb ages 

were used for a total of four grains older than 1.2 b.y.

The U-Pb ages are plotted as histograms with a normalized relative-probability distribution in figure 3 (Ludwig, 

2003), where the relative height of peaks corresponds to the 

significance of that age population.

Results and Discussion

We interpret the youngest cluster of detrital-zircon ages 

as setting a limiting depositional age for that sample. The 

youngest cluster is used, rather than the youngest individual 

grain, because isotopic disturbance by Pb loss can yield 

detrital-zircon ages that are significantly younger than the 

crystallization age but still analytically concordant. Because 

each zircon in a group of grains would almost certainly not 

lose exactly enough Pb to yield similar ages, we interpret the 

peak age of the youngest cluster (determined from the relativeprobability plots in fig. 3) as a robust indicator of the age of 

the youngest igneous source, accordingly setting a maximum 

depositional age for the host graywacke.

The detrital-zircon ages indicate that the possible Jurassic 

nasselarian from the outcrop at sample locality 65, mentioned 

previously, is likely younger than Jurassic. The youngest peak 

in ages at this locality is much younger—74 m.y. This detritalzircon age helps constrain the loose fossil age.

The youngest peak ages of the western four samples (74, 

72, 74, and 74 m.y., from west to east) are distinctly younger 

Sample Field No. USGS map sheet Latitude(° N.) Longitude(° W.)

Youngest 

detrital-zircon

 age peak (Ma)

62 92PH062 Port Alexander D–5 56.93681 135.43308 74

63 92PH063 Port Alexander D–5 56.95279 135.39284 72

64 92PH064 Port Alexander D–5 56.97973 135.37554 74

65 92PH065 Port Alexander D–4 56.99984 135.32176 74

66 92PH066 Sitka A–4 57.01497 135.27063 105

67 92PH067 Sitka A–4 57.03072 135.23172 103

69 92PH069 Port Alexander D–4 56.99345 135.15739 97

Table 1. Summary data for samples of Sitka Graywacke sandstone analyzed for detrital zircons

[Latitude and longitude from NAD27 datum; youngest detrital-zircon age peaks from relative-probability plots in figure 3. USGS, U.S. Geological Survey]

Teg

Ks

Qu

Teg

Trsv

KJkk

Teg

Jd

Ks

57°05'

56°50'

135°30' 135°05'

62

63

64

65

66 67

69

0 5 10 kilometers

Sitka # Sitka Sound

Figure 2. Study area near Sitka, Alaska, showing regional 

geology simplified from Karl and others (in press). Squares, 

location of detrital zircon samples. Samples 62 and 65 are on small 

islands hidden beneath symbol. Irregular dashed line, inferred 

thrust fault (teeth on upper plate) between samples 65 and 66 that 

separates older, eastern samples from younger, western samples. 

Units: Jd, Jurassic diorite; Kjkk and Trsv, Jurassic through 

Cretaceous and Triassic parts of the Kelp Bay Group, respectively; 

Ks, Cretaceous Sitka Graywacke; Qu, Quaternary sedimentary 

rocks, undivided; Teg, Eocene granitic rocks. From U.S. Geological 

Survey Sitka A–4 and Port Alexander D–5 1:63,360-scale maps.
 Constraints on the Age and Provenance of the Chugach Accretionary Complex, Alaska NUMBER OF DETRITAL ZIRCONS

RELATIVE PROBABILITY, IN PERCENT

NUMBER OF DETRITAL ZIRCONS

RELATIVE PROBABILITY, IN PERCENT

206Pb/238U age,

IN MILLIONS OF YEARS

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Sample 62

N=70

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Sample 62 A N=70

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Sample 63

N=69

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Sample 63 B N=69

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Sample 64

N=69

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Sample 64 C N=69

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Sample 65

N=70

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Sample 65 D N=70

206Pb/238U AGE,

IN MILLIONS

OF YEARS

206Pb/207Pb AGE,

IN MILLIONS

OF YEARS

Figure 3. Detrital-zircon histograms (gray bars) and relative-probability plots (solid curve) for samples 62 (A), 63 (B), 64 (C), 65 (D), 66 

(E), 67 (F), and 69 (G). Left plot shows complete age range from 0 to 1,850 m.y., with a 50-m.y. bar width; right plot shows detailed area 

from 50 to 250 m.y., with a 5-m.y. bar width. All plots calculated with Isoplot software (Ludwig, 2003).
Results and Discussion 

than the youngest peak ages of the eastern three samples 

(105, 103, and 97 m.y., from west to east; figs. 2, 3; tables 1, 

2). Interpretation of these limiting ages depends on the time 

interval between crystallization and deposition of the zircons, 

which would largely be influenced by the presence or absence 

of igneous sources, the rate of exhumation of source terrains, 

and the geometry and efficiency of depositional systems.

We suggest that the youngest peak ages are close to the 

depositional age of the Sitka Graywacke. A subduction zone 

with associated arc magmatism was active along the western 

margin of North America during Late Cretaceous through 

early Tertiary time (for example, Miller, 1994; Moll-Stalcup 

and others, 1994; Plafker and others, 1994; Breitsprecher and 

Mortensen, 2004a, b), and so numerous sources of juvenile 

zircons were present. Also occurring at that time was rapid 

uplift of the Coast Mountains of southeastern Alaska and British 

Columbia (for example, Hollister, 1982; Crawford and others, 

2000; McClelland and Mattinson, 2000), which would have provided additional sources of young zircons. Moreover, because 

the Chugach terrane turbidites are considered to have been 

deposited by northward-flowing currents in the paleotrench 

(Nilsen and Moore, 1979; Nilsen and Zuffa, 1982; Plafker and 

others, 1994), little time intervened between deposition and 

accretion of the trench sediment. Taking these considerations 

together, we infer little time, possibly ≤5 m.y, between crystallization and deposition of the detrital zircons. Thus, these detritalzircon limiting ages would indicate a range in the maximum age 

of these Sitka Graywacke samples from ∼72 to ∼105 m.y.

0

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40

45

0 500 1000 1500

Sample 66

N=70

0

2

4

6

8

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Sample 66 E N=70

NUMBER OF DETRITAL ZIRCONS

RELATIVE PROBABILITY, IN PERCENT

NUMBER OF DETRITAL ZIRCONS

0

RELATIVE PROBABILITY, IN PERCENT

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Sample 67

N=75

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Sample 67 F N=75

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40

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Sample 69

N=70

0

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Sample 69

G N=70

206Pb/238U age,

IN MILLIONS OF YEARS

206Pb/238U AGE,

IN MILLIONS

OF YEARS

206Pb/207Pb AGE,

IN MILLIONS

OF YEARS

Figure 3.—Continued.
 Constraints on the Age and Provenance of the Chugach Accretionary Complex, Alaska

The difference in the youngest peak ages between the 

eastern three and western four sample localities may be due to 

both a change in provenance and a fault. Histograms and relative-probability plots of detrital-zircon ages from the eastern 

and western samples differ (fig. 3), and so the two population 

groups are here considered separately. Because the two groups 

appear to be internally similar, we combined the data for the 

western four and eastern three samples (fig. 4). Because the 

patterns of the combined histograms and relative-probability 

plots differ for the two groups, the provenance must have 

changed. Is the difference between the two areas due only 

to a change in provenance? If so, then the entire sampling 

area would consist of a single stratigraphic section more than 

15 km thick. Such a great thickness of a single panel in an 

accretionary complex would be extremely unusual. The overall 

westward-younging pattern of the youngest detrital-zircon ages 

is consistent with the standard model of accretionary prisms, 

which has trenchward-younging, fault-bounded panels of 

accreted sediment. Moreover, intraformational thrust faults are 

nearly ubiquitous in Chugach terrane turbidites (for example, 

Sample and Moore, 1987). Numerous faults occur within the 

Sitka Graywacke, most of which bound thrust panels and others of which are later-stage right-lateral strike-slip faults (Davis 

and others, 1998; Karl and others, in press). No previous fault 

was mapped between the western and eastern localities, and 

no obvious sedimentologic differences exist. We infer a fault, 

probably a thrust fault, in an uncertain location between the 

eastern three and western four sample localities (fig. 2).

The detrital-zircon limiting ages for the Sitka Graywacke 

allow comparison and correlation to other parts of the Chugach 

accretionary complex in southern Alaska. The Valdez Group 

comprises the largest area of the Chugach terrane in southcentral Alaska (fig. 1). The western four samples of Sitka 

Graywacke have youngest peak ages of late Campanian (fig. 

6) on the time scale of Gradstein and others (2005). Assuming 

that the detrital zircons were deposited within several million 

years of crystallization, they indicate that the western part of 

the Sitka Graywacke is correlative with the Valdez Group (fig. 

6; Grant and Higgins, 1910; Tysdal and Plafker, 1978). All 

reliable fossil ages from the Valdez Group are Maastrichtian 

(65.5–70.6 m.y.; Tysdal and Plafker, 1978; Nelson and others, 1985), although two samples have questionable age calls 

0

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Samples 66, 67, 69

N=215

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Samples 66, 67, 69

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Samples 62, 63, 64, 65

N=278

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Samples 62, 63, 64, 65

N=278

A

B

206Pb/238U age,

IN MILLIONS OF YEARS

206Pb/238U AGE,

IN MILLIONS

OF YEARS

206Pb/207Pb AGE,

IN MILLIONS

OF YEARS

NUMBER OF DETRITAL ZIRCONS

RELATIVE PROBABILITY, IN PERCENT

NUMBER OF DETRITAL ZIRCONS

RELATIVE PROBABILITY, IN PERCENT

Figure 4. Detrital-zircon histograms and relative-probability plots for eastern three samples (66, 67, 69) (A) and western four samples 

(62, 63, 64, 65) (B). Same symbols as in figure 3.
Results and Discussion 

that are Campanian-Maastrichtian (65.5–83.5 m.y.; Tysdal and 

Plafker, 1978; Winkler, 1992). If less than 5 m.y. intervened 

between crystallization and deposition of the western Sitka 

Graywacke samples, they would be Maastrichtian.

The younger part of the Sitka Graywacke also appears 

to be correlative with the upper part of the Yakutat Group of 

Tarr and Butler (1909). The Yakutat Group consists broadly 

of an eastern, or “lower,” melange facies and a western, or 

“upper,” flysch facies, both of which are considered correlative 

with the Chugach terrane (Plafker and others, 1977, 1994). 

The Yakutat Group is in the eastern part of the Yakutat terrane 

(fig. 1), which is presently accreting onto the southern Alaska 

margin (Plafker and others, 1978, 1994). The Yakutat terrane likely lay south of the Sitka Graywacke, off the coast of 

southern British Columbia, when it was offscraped (Plafker 

and others, 1994). Thus, on a large scale, the Yakutat Group 

was the now-dismembered part of the accretionary complex 

that lay to the south of the Sitka Graywacke. One Campanian fossil was obtained in outcrop from the upper part of the 

Yakutat Group (Jones and Clark, 1973), and Campanian and 

Maastrichtian foraminifers were recovered from an offshore 

borehole that penetrated the Yakutat Group (W.V. Sliter, in 

Rau and others, 1983). Flysch of the upper part of the Yakutat 

Group is more quartzofeldspathic and has a higher proportion 

of plutonic-rock fragments relative to the Sitka Graywacke 

and the Valdez Group (Nilsen, 1984). Nonetheless, the similarity of the youngest peak ages for the western four samples 

of Sitka Graywacke to the fossil ages for the Yakutat Group 

indicate that the rocks are approximately coeval, as previously 

considered (for example, Plafker and others, 1994), although 

they differ compositionally.

The Sitka Graywacke is at least ∼30 m.y. older than the 

older part of the Chugach accretionary complex in southeastern 

Alaska (fig. 6). The Kelp Bay Group of Berg and Hinkley (1963), 

a melange containing fossils as young as Valanginian (Plafker and 

others, 1976; Decker, 1980), lies in fault contact with the Sitka 

Graywacke (see units KJkk and Trsv, fig. 2). The oldest detritalzircon limiting age for the Sitka Graywacke is 105 m.y. (sample 

66), and so a substantial age gap of ∼30 m.y. exists on the time 

scale of Gradstein and others (2005). Of course, this age gap may 

be smaller if younger fossils are present in the Kelp Bay Group, or 

if older samples are present in the Sitka Graywacke.

The youngest peak ages for the eastern three samples of 

Sitka Graywacke (fig. 2) corresponds to the youngest fossil 

ages for melange of the McHugh Complex in south-central 

Alaska (fig. 6; see Plafker and others, 1994, for an overview 

of ages), which are Albian (99.6–112.0 m.y.) or Cenomanian 

(99.6–93.5 m.y.; Winkler and others, 1980). The McHugh 

Complex has been correlated with the Kelp Bay Group 

(Plafker and others, 1977, 1994; Decker, 1980). The oldest two 

samples of Sitka Graywacke are Albian (99.6–112.0 m.y.), and 

the easternmost sample (69) is Cenomanian (93.5–99.6 m.y.). 

Radiolarians from one small stratigraphically and structurally 

enigmatic area in Prince William Sound are dated at Coniacian through Turonian (89.3–83.5 m.y.; Haeussler and Nelson, 

1993; Nelson and others, 1999), between the ages of the 

Valdez Group and the McHugh Complex. Thus, assuming that 

the detrital-zircon limiting ages are close to the depositional 

age of the Sitka Graywacke, the age range of the unit is at 

least ∼26–32 m.y.2, significantly longer than the age range of 

reliably dated turbidites of the Valdez Group. Considering all 

these turbidite sequences together suggests relatively continuous turbidite sedimentation between Albian and Maastrichtian 

time; however, the volume of sediment deposited varies, and 

clearly a large pulse was deposited in Maastrichtian time (for 

example, Plafker and others, 1994).

The distribution of detrital-zircon ages for the Sitka 

Graywacke corresponds to that of magmatic ages from southcentral Alaska to British Columbia. The eastern three samples 

contain the largest number of zircons dated at 100–110 m.y. 

(particularly samples 66 and 67) and at 145–165 m.y. (figs. 

3, 4). Histograms derived from a compilation of igneous-rock 

ages from British Columbia (Breitsprecher and Mortenson, 

2004a) and Yukon Territory (Breitsprecher and Mortenson, 

2004b) are shown in figure 5. Numerous plutons in the Coast 

Mountains of British Columbia and southeastern Alaska dated 

at 90–110 m.y. may be the source of these zircons. In southcentral Alaska, little magmatism has been reported. Numerous 

sources are possible, both near and far, for the older ages of 

145–165 m.y. that may reflect (1) the Stikine terrane magmatic arc, which lies to the east and south of the study area 

(fig. 2; Van der Heyden, 1992); (2) plutons on the outer Queen 

Charlotte Islands in British Columbia (Butler and others, in 

press), (3) Middle to Late Jurassic plutons in southern British 

Columbia (for example, van der Heyden, 1992); (4) intrusions 

in the Saint Elias Mountains (Dodds and Campbell, 1988); or 

(5) younger phases of the Talkeetna and Chitina plutonic arcs 

in south-central Alaska (Plafker and others, 1989; Roeske and 

others, 2003; Trop and others, 2005). These detrital-zircon 

ages do not uniquely define the source terrain.

The western four samples have ages that are consistent 

with igneous sources from south-central Alaska to southern 

British Columbia. All four samples contain zircons dated at 

∼74 m.y. Plutons at the lower end of this age range occur in 

the western Alaska Range. Voluminous volcanism occurred in 

the Talkeetna Mountains of south-central Alaska between 80 

and 70 m.y. Detrital zircons from the Cretaceous Matanuska 

Formation of south-central Alaska have a significant peak 

in ages of 75–80 m.y. (J.M. Trop, written commun., 2005). 

Numerous intrusions dated at 57–71 m.y. occur in the region 

between Juneau and Haines (Gehrels, 2000). A significant 

peak in detrital-zircon ages from 90–100 m.y. corresponds to a 

period of voluminous magmatism in southeastern Alaska and 

British Columbia (fig. 5), and so this peak age seems to reflect 

2

The younger age comes from 100 m.y. (the youngest peak age for the eastern three samples, at loc. 69) minus 74 m.y. (the oldest peak age for the western four samples, at locs. 62, 64, and 65) equals 26 m.y., whereas the older age 

comes from 104 m.y. (the oldest peak age for the eastern three samples, at loc. 

66) minus 72 m.y. (the youngest peak age for the western four samples, at loc. 

63) equals 32 m.y. 
 Constraints on the Age and Provenance of the Chugach Accretionary Complex, Alaska

0

500

1000

1500

2000

2500

3000

0 500 1000 1500

AGE, IN MILLIONS OF YEARS

NUMBER OF DETERMINATIONS

0

100

200

300

400

500

600

700

50 100 150 200 250

A B

Figure 5. Histograms of 9,318 isotopic-age determinations from British Columbia and Yukon Territory, from compilations by 

Breitsprecher and Mortensen (2004a, b). A, All data. B, Data from 50 to 250 m.y., with a 5-m.y. bar width as in figures 3 and 4.

indeterminate sources from this region. Moreover, the relatively small number of zircons dated at 80–90 m.y. is also consistent with the absence of plutons in that interval in southern 

southeastern Alaska and British Columbia. Interestingly, the 

only >1-b.y. zircons were obtained from the western, young 

group of samples: 1,686±6, 1,693±8, 1,813±32, and 1,407±96 

m.y. (206Pb/207Pb ages). These old ages may reflect sources in 

the northern Cordilleran miogeocline (Gehrels, 2000) or in 

the Yukon-Tanana terrane of southeastern Alaska and British 

Columbia (Kapp and Gehrels, 1998; Gehrels, 2001).

In conclusion, the detrital zircons in the samples of Sitka 

Graywacke do not uniquely identify a source region. Instead, 

possible source regions cover a vast area from south-central 

Alaska to southern British Columbia. However, all considerations of the regional tectonics of southeastern Alaska involve 

some amount of right-lateral strike slip eastward of the Sitka 

Graywacke. The Border Ranges Fault bounds the east side of 

the Chugach terrane, and speculations on the amount of right 

slip range from hundreds of kilometers to more than 1,000 km 

(Smart and others, 1996; Roeske and others, 2003). The slip 

must have occurred before 50 m.y., when a pluton intruded 

across the fault on Chichagof Island (Johnson and Karl, 1985, 

L.W. Snee, written commun., 1997). The Chatham Strait Fault 

has ∼150 km of right slip (see discussion in Hudson and others, 1982). The Tintina Fault in British Columbia has ∼420 km 

of right slip (Gabrielse, 1985) that likely occurred in early Tertiary time (Jackson and Mortensen, 2000). The eastern Denali 

Fault has ∼370 km of right-lateral offset (Lowey, 1998). Thus, 

significant margin-parallel right slip clearly occurred in the 

interval between deposition of the Sitka Graywacke and ∼50 

m.y., after which major activity ceased on all these fault systems 

except the Chatham Strait Fault. Regardless of the uncertainties 

as to how much slip occurred on different faults at particular 

times, the Sitka Graywacke was situated south of its present 

position. Therefore, zircon sources in coastal and central to 

Paleocene

Upper

Lower

Selandian

Danian

Maastrichtian

Campanian

Santonian

Coniacian

Turonian

Cenomanian

Albian

Aptian

Barremian

Hauterivian

Valanginian

Berriasian

Cretaceous Tertiary Period

SeriesepochStageageAge(m.y.)

58.7±0.2

61.7±0.2

65.5±0.3

70.6±0.6

83.5±0.2

85.8±0.7

89.3±1.0

93.5±0.8

99.6±0.9

112.0±1.0

125.0±1.0

130.0±1.5

136.4±2.0

140.2±3.0

145.5±4.0

Valdez Group

Upper part of the

Yakutat Group

PWSSSA

McHugh Complex

Youngest detrital-zircon 

peak ages from samples 

of Sitka Graywacke

Kelp Bay Group

Figure 6. Age range for the Valdez Group (Tysdal and Plafker, 

1978, Winkler, 1992; Plafker and others, 1994), younger part of 

the Kelp Bay Group (Plafker and others, 1976, Decker, 1980), and 

the McHugh Complex (Winkler, 1992; Plafker and others, 1994), 

fossiliferous rocks of central Prince William Sound (PWSSSA 

[Prince William Sound Special Study Area]; Nelson and others, 

1999), “upper” flysch facies of the Yakutat Group (Jones and 

Clark, 1973, W.V. Sliter, in Rau and others, 1983), and detrital-zircon 

youngest peak ages for the Sitka Graywacke (this study). Time 

scale from Gradstein and others (2005).

southern interior British Columbia seem most likely. Although 

some ages are consistent with source regions in the St. Elias 

Mountains and in south-central Alaska, they are not demanded.
References Cited 

Acknowledgments

Discussions with Jeff Trop and Dwight Bradley aided 

our interpretation of the provenance data; their reviews of the 

manuscript were also greatly appreciated, as were comments by 

Maria Luisa Crawford and Rich Goldfarb. Thanks also to Alex 

Pullen for his assistance in processing the U-Pb geochronologic 

samples. The U-Pb geochronology was supported by National 

Science Foundation grants EAR–9526263 and EAR–0309885.

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Table 2

 
Table 2. Analytical data on detrital zircons

[All analyses by laser-ablation inductively coupled plasma mass spectroscopy (ICPMS)]

1 174 1,518 45 0.01036 0.5 0.06271 6.1 22.77 6.1 0.08 66.4 0.3 61.8 2 167 1,277 29 0.01097 1.6 0.07066 6.7 21.41 6.6 0.23 70.4 1.1 69.3 3 106 5,285 32 0.03082 1.6 0.20209 5.1 21.03 4.8 0.31 195.7 3.1 186.9 4 205 3,040 49 0.01328 1.5 0.08855 3.7 20.67 3.4 0.40 85.0 1.3 86.2 5 116 1,426 24 0.03218 1.0 0.25335 3.6 17.51 3.5 0.27 204.2 2.0 229.3 6 182 1,575 50 0.01736 1.7 0.11639 2.7 20.56 2.1 0.62 110.9 1.9 111.8 7 624 5,499 40 0.01156 0.6 0.07674 2.2 20.77 2.1 0.28 74.1 0.5 75.1 8 58 10,610 14 0.26190 1.2 3.73401 1.4 9.67 0.7 0.88 1,499.6 20.2 1,578.7 9 310 1,947 13 0.01388 1.8 0.09780 4.0 19.58 3.5 0.45 88.9 1.6 94.7 

10 582 3,858 25 0.01246 2.0 0.09618 2.6 17.87 1.7 0.76 79.8 1.6 93.2 11 48 626 32 0.01187 2.1 0.05028 4.5 32.55 4.0 0.46 76.1 1.6 49.8 12 115 2,152 48 0.01271 0.7 0.08244 6.3 21.25 6.2 0.11 81.4 0.6 80.4 13 575 5,485 23 0.01105 0.6 0.07384 2.6 20.64 2.6 0.23 70.9 0.4 72.3 14 55 618 24 0.01126 1.8 0.04703 25.7 33.01 25.7 0.07 72.2 1.3 46.7 15 240 1,712 21 0.01544 1.1 0.10393 3.4 20.49 3.2 0.31 98.8 1.1 100.4 16 64 610 65 0.01195 2.7 0.05233 7.7 31.49 7.1 0.36 76.6 2.1 51.8 17 916 7,112 53 0.01207 1.0 0.08152 1.4 20.42 1.1 0.66 77.4 0.7 79.6 18 272 3,812 21 0.01434 1.6 0.08823 2.7 22.41 2.2 0.59 91.8 1.5 85.9 19 342 4,617 12 0.01512 0.7 0.10298 2.8 20.24 2.7 0.26 96.7 0.7 99.5 20 79 1,714 101 0.02309 2.9 0.14268 6.2 22.32 5.5 0.46 147.2 4.3 135.4 21 140 1,036 120 0.01220 1.4 0.08865 6.7 18.98 6.6 0.21 78.2 1.1 86.2 22 220 2,789 49 0.01404 1.1 0.09104 3.7 21.26 3.5 0.30 89.9 1.0 88.5 23 514 6,619 22 0.01091 1.2 0.07533 3.6 19.96 3.4 0.35 69.9 0.9 73.7 24 89 12,945 17 0.27611 1.1 3.95064 1.4 9.64 0.9 0.79 1,571.8 19.3 1,624.1 25 410 3,986 10 0.01359 9.1 0.09969 10.2 18.79 4.7 0.89 87.0 7.9 96.5 26 378 3,213 74 0.01387 2.7 0.10538 4.0 18.15 2.9 0.68 88.8 2.4 101.7 27 184 2,040 48 0.01233 0.8 0.08421 6.9 20.20 6.8 0.12 79.0 0.7 82.1 28 161 3,923 28 0.02912 1.2 0.20582 3.2 19.51 2.9 0.39 185.0 2.3 190.0 29 174 1,501 27 0.01535 1.0 0.12429 6.9 17.03 6.8 0.15 98.2 1.0 119.0 30 515 3,856 35 0.01111 1.0 0.07611 2.3 20.13 2.1 0.43 71.2 0.7 74.5 31 756 3,662 13 0.01062 2.3 0.07743 3.6 18.91 2.7 0.64 68.1 1.6 75.7 32 80 853 58 0.01213 3.1 0.07814 13.7 21.41 13.3 0.23 77.8 2.4 76.4 33 131 1,278 39 0.01219 1.4 0.08371 8.0 20.08 7.9 0.18 78.1 1.1 81.6 34 83 1,114 47 0.01191 1.9 0.10958 10.2 14.99 10.0 0.19 76.3 1.5 105.6 35 37 498 26 0.01153 1.4 0.06054 16.7 26.26 16.7 0.08 73.9 1.0 59.7 36 92 64,668 54 0.01061 2.1 0.10404 26.4 14.06 26.3 0.08 68.0 1.4 100.5 37 124 1,304 38 0.01492 1.6 0.12926 10.1 15.92 10.0 0.15 95.5 1.5 123.4 38 175 546 48 0.01430 1.9 0.10767 6.3 18.32 6.0 0.31 91.5 1.8 103.8 39 77 410 62 0.01124 3.2 0.06565 7.8 23.62 7.2 0.41 72.1 2.3 64.6 40 63 513 37 0.01082 0.7 0.04868 15.1 30.66 15.1 0.05 69.4 0.5 48.3 41 146 5,387 142 0.01342 2.4 0.10595 6.1 17.47 5.6 0.39 86.0 2.1 102.3 42 102 1,368 45 0.01311 1.7 0.09386 11.7 19.26 11.6 0.15 84.0 1.5 91.1 43 238 951 39 0.01135 0.6 0.09435 5.6 16.59 5.6 0.11 72.8 0.5 91.5 44 177 1,516 24 0.01164 0.7 0.07871 5.9 20.39 5.9 0.11 74.6 0.5 76.9 45 147 1,171 29 0.01183 0.8 0.07098 7.0 22.99 6.9 0.12 75.8 0.6 69.6 46 349 2,642 16 0.01548 0.5 0.11711 3.8 18.22 3.8 0.13 99.0 0.5 112.4 

Sample 62

206Pb/208Pb

ratio

±

(pct)

205Pb/235U

ratio

±

(pct)

±

(pct)

±

(m.y.)

Analysis

U

content

(ppm)

U/Th

ratio

206Pb/204Pb

 ratio

206Pb/207Pb

ratio

Error

correlation

206Pb/238U

age

(m.y.)

207Pb/235U

age

(m.y.)
47 244 8,934 65 0.06705 1.4 0.51139 1.5 18.08 0.7 0.90 418.4 5.9 419.4 

48 344 3,790 23 0.01685 1.2 0.12062 2.3 19.26 2.0 0.52 107.7 1.3 115.6 49 315 3,063 32 0.01545 0.9 0.10319 3.5 20.65 3.3 0.27 98.9 0.9 99.7 50 135 1,401 25 0.01501 0.7 0.08852 6.1 23.39 6.0 0.11 96.1 0.7 86.1 51 133 1,689 34 0.01554 1.7 0.11988 10.1 17.87 10.0 0.17 99.4 1.7 115.0 152 119 1,630 32 0.02311 1.8 0.15298 4.7 20.83 4.4 0.38 147.3 2.6 144.5 53 108 728 63 0.01455 3.0 0.11625 8.1 17.25 7.5 0.37 93.1 2.8 111.7 54 45 689 41 0.01399 3.4 0.12501 17.2 15.43 16.8 0.20 89.6 3.1 119.6 255 1,139 4,694 29 0.01146 0.9 0.07642 1.8 20.67 1.5 0.52 73.4 0.7 74.8 56 453 2,568 20 0.01071 0.6 0.06663 2.5 22.16 2.4 0.24 68.7 0.4 65.5 57 160 899 35 0.01525 0.9 0.10584 4.2 19.87 4.1 0.21 97.6 0.8 102.2 58 267 2,332 43 0.01237 0.8 0.08554 3.5 19.94 3.5 0.22 79.3 0.6 83.3 59 186 1,509 41 0.01330 3.2 0.08058 6.1 22.76 5.1 0.53 85.2 2.8 78.7 60 199 896 51 0.01226 2.0 0.06797 5.3 24.87 4.9 0.37 78.6 1.6 66.8 61 187 1,285 46 0.01390 2.3 0.10999 8.1 17.42 7.7 0.29 89.0 2.1 106.0 62 319 4,204 20 0.01743 3.0 0.12210 4.2 19.69 2.9 0.71 111.4 3.3 117.0 63 421 2,707 40 0.01114 0.6 0.07046 2.7 21.81 2.6 0.24 71.4 0.5 69.1 64 324 2,778 33 0.01456 0.7 0.09647 2.5 20.81 2.4 0.28 93.2 0.7 93.5 65 180 1,060 26 0.01052 0.7 0.07889 6.3 18.39 6.3 0.12 67.5 0.5 77.1 66 337 1,929 20 0.01432 0.7 0.10075 3.2 19.59 3.2 0.20 91.6 0.6 97.5 67 220 970 41 0.01173 1.3 0.09427 6.2 17.16 6.0 0.21 75.2 1.0 91.5 68 164 1,344 40 0.01514 2.0 0.10335 6.6 20.19 6.2 0.31 96.8 2.0 99.9 69 437 1,811 –2 0.01085 1.0 0.06058 5.4 24.69 5.3 0.19 69.6 0.7 59.7 70 252 2,273 23 0.01512 1.0 0.10903 4.7 19.13 4.6 0.21 96.8 0.9 105.1 

1 59 13,918 31 0.02270 0.8 0.15078 9.0 20.76 8.9 0.09 144.7 1.2 142.6 12 160 5,137 28 0.01111 1.0 0.07275 5.1 21.05 5.0 0.19 71.2 0.7 71.3 3 238 3,161 149 0.01107 0.7 0.06868 2.7 22.21 2.6 0.25 70.9 0.5 67.4 4 22 573 43 0.01313 1.6 0.07440 25.5 24.34 25.5 0.06 84.1 1.3 72.9 15 79 5,831 36 0.06016 0.7 0.47700 2.5 17.39 2.4 0.28 376.6 2.7 396.0 16 25 1,964 20 0.01481 0.9 0.07898 19.1 25.85 19.1 0.05 94.7 0.9 77.2 17 37 83,222 9 0.32258 0.9 4.92856 3.7 9.02 3.6 0.24 1,802.3 18.2 1,807.2 168 72 1,342 56 0.01240 1.4 0.11052 16.5 15.46 16.4 0.09 79.4 1.1 106.4 19 173 7,344 27 0.01107 0.8 0.07715 7.2 19.79 7.2 0.10 71.0 0.5 75.5 

10 139 3,192 66 0.01263 1.4 0.08372 4.2 20.81 3.9 0.33 80.9 1.1 81.6 12 48 5,639 –149 0.02344 1.7 0.15879 5.6 20.35 5.3 0.31 149.4 2.6 149.6 13 104 23,808 37 0.01282 0.6 0.10712 17.4 16.50 17.4 0.04 82.1 0.5 103.3 114 471 10,746 25 0.01079 0.7 0.07227 1.9 20.59 1.8 0.35 69.2 0.5 70.9 15 119 28,627 31 0.01178 1.0 0.09242 14.5 17.57 14.4 0.07 75.5 0.7 89.8 116 131 12,296 31 0.01130 1.0 0.07432 7.6 20.97 7.5 0.13 72.5 0.7 72.8 17 181 3,725 29 0.03062 0.5 0.20932 2.7 20.17 2.6 0.19 194.4 1.0 193.0 18 48 5,001 103 0.01248 1.7 0.12078 18.2 14.25 18.1 0.09 80.0 1.4 115.8 219 112 1,125 58 0.01337 1.8 0.07625 3.8 24.18 3.4 0.47 85.6 1.5 74.6 20 483 5,095 22 0.01113 0.6 0.07776 3.4 19.73 3.3 0.17 71.3 0.4 76.0 21 214 3,919 139 0.01438 1.1 0.12694 3.6 15.62 3.4 0.31 92.0 1.0 121.3 22 134 1,873 37 0.01101 0.9 0.07754 5.3 19.58 5.2 0.17 70.6 0.6 75.8 

Sample 63

206Pb/208Pb

ratio

±

(pct)

205Pb/235U

ratio

±

(pct)

±

(pct)

±

(m.y.)

(m

Analysis

U

content

(ppm)

U/Th

ratio

206Pb/204Pb

 ratio

206Pb/207Pb

ratio

Error

correlation

206Pb/238U

age

(m.y.)

207Pb/235U

age

(m.y.)

Table 2. Analytical data on detrital zircons.—Continued[All analyses by laser-ablation inductively coupled plasma mass spectroscopy (ICPMS)]
Table 2. Analytical data on detrital zircons.—Continued

[All analyses by laser-ablation inductively coupled plasma mass spectroscopy (ICPMS)]

23 177 8,126 29 0.01192 0.7 0.07867 6.9 20.89 6.8 0.10 76.4 0.5 76.9 24 51 4,372 25 0.01652 1.5 0.11365 11.2 20.04 11.1 0.13 105.6 1.6 109.3 25 294 7,446 47 0.01150 2.9 0.07741 4.3 20.49 3.2 0.66 73.7 2.1 75.7 26 263 8,458 72 0.01466 1.2 0.09323 2.7 21.68 2.4 0.44 93.8 1.1 90.5 27 33 1,099 51 0.02267 1.9 0.16385 19.7 19.07 19.7 0.10 144.5 2.8 154.1 28 111 104,314 33 0.01125 1.3 0.08231 35.5 18.85 35.5 0.04 72.1 1.0 80.3 29 78 3,743 29 0.05339 0.7 0.40188 2.6 18.32 2.5 0.26 335.3 2.3 343.0 30 83 2,866 25 0.01235 1.0 0.08878 10.7 19.19 10.7 0.10 79.1 0.8 86.4 31 78 1,113 36 0.01086 1.1 0.06350 7.6 23.58 7.5 0.14 69.6 0.8 62.5 32 105 16,364 29 0.01155 0.9 0.07634 8.3 20.87 8.3 0.11 74.0 0.7 74.7 33 135 1,304 –4 0.01208 1.9 0.05835 7.7 28.54 7.5 0.24 77.4 1.5 57.6 34 144 15,149 18 0.05467 1.1 0.40752 2.0 18.50 1.7 0.53 343.1 3.7 347.1 35 90 1,311 43 0.01139 1.0 0.05916 8.0 26.55 8.0 0.12 73.0 0.7 58.4 36 165 7,645 16 0.01221 0.8 0.08735 5.5 19.27 5.4 0.15 78.2 0.6 85.0 37 69 778 55 0.01411 1.0 0.06097 7.0 31.91 6.9 0.15 90.3 0.9 60.1 38 604 3,928 34 0.01150 0.7 0.07040 2.9 22.52 2.8 0.26 73.7 0.6 69.1 39 285 4,345 12 0.02867 2.1 0.21587 3.3 18.31 2.6 0.62 182.2 3.8 198.5 40 121 3,609 42 0.01198 0.9 0.14225 3.9 11.61 3.8 0.22 76.8 0.7 135.0 41 83 2,687 27 0.01118 0.6 0.07305 6.3 21.10 6.3 0.09 71.6 0.4 71.6 42 40 3,714 27 0.02385 2.1 0.14782 18.1 22.24 18.0 0.12 151.9 3.2 140.0 43 172 4,776 102 0.01479 2.1 0.14146 5.5 14.42 5.1 0.38 94.7 2.0 134.3 44 82 1,327 63 0.01846 2.0 0.10719 6.4 23.74 6.1 0.31 117.9 2.3 103.4 45 473 7,172 33 0.01129 0.5 0.07470 2.2 20.84 2.2 0.23 72.4 0.4 73.1 46 58 2,165 26 0.01164 1.3 0.06051 12.4 26.52 12.3 0.10 74.6 0.9 59.7 47 90 7,619 31 0.01207 1.1 0.08245 10.8 20.19 10.7 0.10 77.4 0.9 80.4 48 50 2,388 22 0.01154 1.1 0.07187 14.8 22.13 14.8 0.08 73.9 0.8 70.5 49 123 1,114 49 0.01061 0.9 0.07244 6.9 20.19 6.9 0.13 68.0 0.6 71.0 50 66 5,583 27 0.03129 1.0 0.19217 5.0 22.45 4.9 0.19 198.6 1.9 178.5 51 90 5,745 18 0.06100 1.5 0.44518 2.4 18.89 1.9 0.62 381.7 5.8 373.9 52 108 1,193 23 0.01128 0.9 0.05842 7.5 26.63 7.4 0.12 72.3 0.6 57.7 53 120 49,185 42 0.06153 0.8 0.47921 4.7 17.70 4.7 0.16 384.9 3.0 397.5 54 190 837 17 0.01181 1.2 0.08156 5.9 19.96 5.8 0.20 75.7 0.9 79.6 55 38 1,119 23 0.01232 1.8 0.08867 35.1 19.16 35.1 0.05 78.9 1.4 86.3 56 155 18,802 29 0.03050 0.6 0.20881 2.8 20.14 2.8 0.21 193.7 1.2 192.6 57 109 896 42 0.01132 1.4 0.06251 4.2 24.96 4.0 0.34 72.5 1.0 61.6 58 42 2,803 41 0.02469 2.4 0.15078 7.3 22.58 6.9 0.33 157.2 3.9 142.6 59 25 399 29 0.01263 1.9 0.05939 30.3 29.32 30.2 0.06 80.9 1.5 58.6 60 125 28,732 19 0.02979 0.8 0.21519 4.1 19.09 4.0 0.19 189.2 1.5 197.9 61 140 3,849 25 0.01190 1.0 0.07418 6.2 22.11 6.1 0.17 76.2 0.8 72.7 62 56 2,725 30 0.02864 1.1 0.18994 7.4 20.79 7.3 0.15 182.0 2.0 176.6 63 135 6,664 46 0.01332 1.4 0.09034 6.3 20.34 6.1 0.22 85.3 1.2 87.8 64 50 1,876 73 0.01107 1.6 0.07028 12.9 21.73 12.8 0.12 71.0 1.1 69.0 65 83 1,582 42 0.01147 1.1 0.06950 6.8 22.75 6.7 0.16 73.5 0.8 68.2 66 79 3,213 34 0.02296 1.1 0.15040 3.9 21.05 3.8 0.28 146.3 1.6 142.3 67 94 9,235 34 0.05254 0.8 0.39639 2.6 18.28 2.5 0.29 330.1 2.6 339.0 68 227 8,155 27 0.01156 0.6 0.07625 6.2 20.90 6.2 0.10 74.1 0.5 74.6 69 72 1,239 20 0.05653 1.6 0.37073 5.1 21.03 4.8 0.32 354.5 5.9 320.2 70 66 1,449 41 0.01104 2.0 0.06147 12.7 24.77 12.6 0.16 70.8 1.4 60.6 

206Pb/208Pb

ratio

±

(pct)

205Pb/235U

ratio

±

(pct)

±

(pct)

±

(m.y.)

(

Analysis

U

content

(ppm)

U/Th

ratio

206Pb/204Pb

 ratio

206Pb/207Pb

ratio

Error

correlation

206Pb/238U

age

(m.y.)

207Pb/235U

age

(m.y.)
Table 2. Analytical data on detrital zircons.—Continued

[All analyses by laser-ablation inductively coupled plasma mass spectroscopy (ICPMS)]

1 107 753 38 0.01186 1.3 0.10486 9.6 15.60 9.5 0.14 76.0 1.0 101.3 102 82 1,669 22 0.01441 1.4 0.08538 8.5 23.27 8.4 0.16 92.2 1.3 83.2 73 218 3,857 39 0.01150 0.7 0.06840 4.3 23.17 4.2 0.16 73.7 0.5 67.2 34 124 2,154 11 0.01384 1.2 0.09876 5.6 19.32 5.5 0.21 88.6 1.1 95.6 55 121 1,671 41 0.01121 1.0 0.06446 7.2 23.98 7.1 0.15 71.9 0.8 63.4 47 188 2,703 25 0.01480 1.4 0.08951 7.0 22.80 6.8 0.21 94.7 1.4 87.0 68 191 1,847 43 0.01212 1.0 0.07342 3.8 22.76 3.6 0.26 77.7 0.8 71.9 29 381 5,374 21 0.01188 0.7 0.08194 3.1 19.98 3.0 0.23 76.1 0.5 80.0 2

10 281 6,578 18 0.01159 1.0 0.07854 4.0 20.34 3.9 0.24 74.3 0.7 76.8 311 204 1,007 31 0.01172 1.3 0.06537 4.9 24.73 4.7 0.26 75.1 0.9 64.3 312 316 388 84 0.01178 3.0 0.08824 6.8 18.41 6.0 0.45 75.5 2.3 85.9 613 101 5,068 19 0.05417 0.7 0.42835 1.7 17.44 1.6 0.41 340.1 2.5 362.0 714 260 4,869 24 0.01409 0.9 0.09759 3.1 19.91 2.9 0.29 90.2 0.8 94.6 315 170 1,545 39 0.01402 0.8 0.09165 4.0 21.09 3.9 0.19 89.7 0.7 89.0 316 139 1,697 272 0.01685 0.6 0.11463 2.2 20.26 2.1 0.27 107.7 0.7 110.2 217 77 1,930 22 0.01619 3.7 0.09738 12.3 22.93 11.8 0.30 103.6 3.8 94.4 1218 165 2,164 22 0.01458 0.9 0.09668 3.6 20.79 3.4 0.26 93.3 0.9 93.7 319 249 2,812 32 0.01157 2.6 0.07574 5.1 21.06 4.4 0.51 74.2 1.9 74.1 320 672 16,092 38 0.01238 1.4 0.08429 2.3 20.25 1.8 0.63 79.3 1.1 82.2 121 198 5,005 27 0.01471 0.9 0.10503 4.5 19.31 4.4 0.20 94.2 0.9 101.4 422 282 5,660 15 0.01079 0.6 0.07704 5.7 19.32 5.7 0.11 69.2 0.4 75.4 423 80 1,490 42 0.01564 2.8 0.10123 10.9 21.30 10.6 0.25 100.0 2.8 97.9 1124 66 320 116 0.01256 1.5 0.08840 7.9 19.60 7.7 0.19 80.5 1.2 86.0 725 172 1,308 63 0.01193 1.0 0.07652 3.9 21.49 3.8 0.25 76.4 0.7 74.9 326 250 3,314 51 0.01203 0.8 0.07588 3.7 21.86 3.6 0.22 77.1 0.6 74.3 227 111 11,325 31 0.06624 0.6 0.52233 1.6 17.49 1.5 0.36 413.4 2.4 426.7 828 513 4,975 29 0.01172 0.8 0.08892 2.7 18.18 2.6 0.31 75.1 0.6 86.5 229 174 1,128 66 0.01146 2.5 0.07198 8.4 21.95 8.0 0.29 73.5 1.8 70.6 630 258 3,501 41 0.01231 1.4 0.07956 3.3 21.33 3.0 0.43 78.9 1.1 77.7 231 182 2,851 36 0.01114 0.8 0.07597 7.5 20.22 7.5 0.11 71.4 0.6 74.3 532 267 721 84 0.01280 1.3 0.07264 2.3 24.29 1.9 0.56 82.0 1.0 71.2 133 383 5,018 23 0.01473 1.0 0.10927 3.7 18.59 3.6 0.27 94.3 0.9 105.3 434 519 5,740 14 0.01111 2.0 0.07778 2.8 19.69 2.0 0.71 71.2 1.4 76.1 235 125 1,486 28 0.01784 3.9 0.12643 6.9 19.45 5.7 0.56 114.0 4.4 120.9 836 202 3,799 35 0.01321 0.8 0.09898 5.1 18.40 5.0 0.15 84.6 0.7 95.8 537 171 1,317 38 0.01492 0.7 0.14103 5.1 14.59 5.0 0.13 95.5 0.6 134.0 738 333 1,783 33 0.01134 1.0 0.07609 5.6 20.54 5.5 0.18 72.7 0.7 74.5 439 261 6,149 27 0.01151 1.3 0.08247 5.0 19.25 4.8 0.25 73.8 0.9 80.5 440 61 1,182 27 0.01435 1.6 0.07682 10.4 25.75 10.3 0.15 91.8 1.5 75.2 841 63 6,499 20 0.05982 0.7 0.44407 2.5 18.57 2.4 0.28 374.5 2.6 373.1 1142 64 1,397 24 0.01415 5.6 0.10351 16.5 18.85 15.6 0.34 90.6 5.1 100.0 1743 30 1,061 76 0.01614 3.0 0.09616 12.9 23.15 12.6 0.23 103.2 3.1 93.2 1244 367 1,745 71 0.01188 0.8 0.11124 3.9 14.73 3.8 0.21 76.1 0.6 107.1 445 460 1,983 89 0.01197 1.1 0.10668 3.1 15.48 2.9 0.34 76.7 0.8 102.9 346 133 1,769 28 0.01117 0.9 0.06173 9.7 24.95 9.7 0.09 71.6 0.7 60.8 647 507 7,605 35 0.01138 0.7 0.07780 1.7 20.18 1.5 0.43 73.0 0.5 76.1 1

Sample 64

206Pb/208Pb

ratio

±

(pct)

205Pb/235U

ratio

±

(pct)

±

(pct)

±

(m.y.)

±

(m.y

Analysis

U

content

(ppm)

U/Th

ratio

206Pb/204Pb

 ratio

206Pb/207Pb

ratio

Error

correlation

206Pb/238U

age

(m.y.)

207Pb/235U

age

(m.y.)
Table 2. Analytical data on detrital zircons.—Continued

[All analyses by laser-ablation inductively coupled plasma mass spectroscopy (ICPMS)]

48 144 1,684 41 0.01219 5.7 0.09822 7.4 17.11 4.8 0.77 78.1 4.4 95.1 749 302 4,056 16 0.01147 0.8 0.07815 4.1 20.23 4.0 0.18 73.5 0.6 76.4 350 502 6,319 19 0.01138 0.7 0.07697 2.0 20.39 1.9 0.34 73.0 0.5 75.3 151 120 1,781 110 0.01806 4.2 0.14720 7.8 16.91 6.6 0.54 115.4 4.9 139.4 1152 155 1,032 43 0.01232 2.0 0.08832 5.9 19.23 5.5 0.34 78.9 1.6 85.9 553 620 5,940 49 0.01239 0.9 0.08836 1.7 19.34 1.5 0.52 79.4 0.7 86.0 154 108 6,800 27 0.05529 2.2 0.40141 2.8 18.99 1.7 0.80 346.9 8.0 342.7 1155 179 897 40 0.01275 0.8 0.07716 6.8 22.79 6.8 0.12 81.7 0.7 75.5 556 121 1,675 30 0.01206 0.9 0.07180 7.1 23.17 7.1 0.13 77.3 0.7 70.4 557 106 2,235 33 0.01665 1.5 0.11979 6.9 19.17 6.7 0.22 106.5 1.6 114.9 858 62 1,056 33 0.01095 2.1 0.05000 16.7 30.20 16.6 0.13 70.2 1.5 49.5 859 295 7,277 28 0.01377 0.8 0.09199 2.2 20.63 2.0 0.39 88.1 0.7 89.4 260 728 2,344 79 0.01136 1.0 0.07874 1.9 19.89 1.6 0.54 72.8 0.7 77.0 161 113 289 37 0.01589 3.9 0.12301 7.2 17.81 6.1 0.54 101.6 3.9 117.8 862 32 829 –9 0.01544 1.0 0.09741 4.6 21.86 4.5 0.21 98.8 0.9 94.4 463 124 1,680 51 0.01203 2.5 0.07329 10.2 22.64 9.9 0.25 77.1 1.9 71.8 764 58 980 144 0.01507 3.0 0.10731 30.7 19.37 30.6 0.10 96.5 2.9 103.5 3265 178 14,786 23 0.07129 0.9 0.56185 1.2 17.50 0.8 0.76 443.9 4.0 452.7 666 201 2,001 22 0.01316 4.2 0.10535 6.1 17.23 4.4 0.69 84.3 3.6 101.7 667 113 1,571 116 0.01716 3.1 0.12758 7.7 18.55 7.1 0.40 109.7 3.4 121.9 1068 128 2,117 34 0.01630 1.6 0.10415 9.9 21.58 9.7 0.16 104.2 1.7 100.6 1069 65 1,367 72 0.01598 1.3 0.10477 15.7 21.03 15.7 0.08 102.2 1.3 101.2 1670 340 3,209 38 0.01143 0.7 0.08013 2.7 19.66 2.6 0.27 73.2 0.5 78.3 2

1 351 4,928 454 0.01542 0.5 0.16388 3.8 12.98 3.8 0.12 98.7 0.5 154.1 62 428 6,085 822 0.01512 0.3 0.10041 3.5 20.77 3.5 0.09 96.8 0.3 97.2 33 262 7,204 148 0.01539 0.5 0.10055 3.4 21.11 3.4 0.15 98.5 0.5 97.3 34 17 1,514 1,297 0.01092 0.7 0.06172 1.9 24.40 1.7 0.39 70.0 0.5 60.8 15 40 749 131 0.01198 1.0 0.10992 5.8 15.03 5.7 0.17 76.8 0.8 105.9 66 60 6,396 51 0.01634 0.6 0.10965 3.0 20.54 2.9 0.19 104.5 0.6 105.6 37 15 1,298 91 0.01920 0.9 0.11448 8.1 23.12 8.1 0.11 122.6 1.1 110.1 98 20 2,186 618 0.01563 0.7 0.08967 2.3 24.04 2.2 0.31 100.0 0.7 87.2 29 90 6,466 37 0.01326 0.9 0.09029 8.6 20.25 8.6 0.10 84.9 0.7 87.8 7

10 118 2,122 673 0.01404 0.4 0.08698 2.3 22.25 2.3 0.18 89.9 0.4 84.7 211 94 3,016 65 0.01432 0.4 0.09308 2.1 21.21 2.0 0.20 91.7 0.4 90.4 112 23 2,534 811 0.01110 0.5 0.08416 3.4 18.19 3.3 0.16 71.2 0.4 82.0 213 101 2,768 43 0.01177 0.5 0.06960 12.4 23.32 12.4 0.04 75.4 0.4 68.3 814 99 4,467 35 0.01509 0.8 0.10817 2.7 19.23 2.5 0.30 96.5 0.8 104.3 215 74 3,807 60 0.01191 1.8 0.06255 15.4 26.26 15.3 0.12 76.4 1.4 61.6 9

0 25 3,560 645 0.01121 1.4 0.07280 7.6 21.24 7.4 0.19 71.9 1.0 71.3 5

17 152 4,936 1,878 0.01138 1.2 0.09541 5.7 16.45 5.5 0.22 73.0 0.9 92.5 518 95 3,877 409 0.01227 0.8 0.08354 1.9 20.25 1.7 0.43 78.6 0.6 81.5 119 100 3,015 186 0.01773 1.8 0.11738 3.4 20.83 2.9 0.54 113.3 2.1 112.7 420 140 3,559 255 0.01233 1.1 0.10911 4.4 15.58 4.2 0.25 79.0 0.9 105.1 421 131 6,891 979 0.01334 2.0 0.08362 35.0 21.99 34.9 0.06 85.4 1.7 81.5 2922 80 15,088 1,008 0.01158 1.0 0.09213 6.0 17.33 5.9 0.16 74.2 0.7 89.5 5

Sample 65

206Pb/208Pb

ratio

±

(pct)

205Pb/235U

ratio

±

(pct)

±

(pct)

±

(m.y.)

±

(m

Analysis

U

content

(ppm)

U/Th

ratio

206Pb/204Pb

 ratio

206Pb/207Pb

ratio

Error

correlation

206Pb/238U

age

(m.y.)

207Pb/235U

age

(m.y.)
Table 2. Analytical data on detrital zircons.—Continued

[All analyses by laser-ablation inductively coupled plasma mass spectroscopy (ICPMS)]

23 75 15,568 571 0.02322 0.7 0.16247 3.0 19.70 2.9 0.23 148.0 1.0 152.9 4.9 24 174 4,918 1,316 0.01420 0.9 0.08991 12.0 21.77 12.0 0.08 90.9 0.8 87.4 10.9 25 49 14,056 749 0.01454 0.7 0.09044 4.9 22.16 4.8 0.14 93.0 0.6 87.9 4.5 26 64 7,449 283 0.02294 1.0 0.15267 13.2 20.72 13.1 0.07 146.2 1.4 144.3 20.2 27 491 5,047 819 0.01469 0.6 0.10814 2.3 18.73 2.2 0.25 94.0 0.5 104.3 2.5 28 173 7,064 599 0.02460 1.1 0.20774 6.6 16.33 6.5 0.16 156.7 1.7 191.7 13.7 29 86 13,376 538 0.01500 1.0 0.10303 26.6 20.08 26.6 0.04 96.0 0.9 99.6 27.5 30 195 8,857 2,247 0.01295 1.5 0.10464 16.7 17.07 16.6 0.09 83.0 1.2 101.1 17.6 31 83 5,270 578 0.02238 1.0 0.14596 7.2 21.14 7.1 0.14 142.7 1.5 138.3 10.6 32 252 2,185 487 0.01441 0.8 0.08657 28.4 22.95 28.3 0.03 92.2 0.7 84.3 24.6 33 86 328,263 323 0.01515 0.4 0.11246 2.5 18.57 2.5 0.16 96.9 0.4 108.2 2.9 34 134 359,267 2,303 0.01135 0.7 0.07815 5.7 20.02 5.7 0.12 72.7 0.5 76.4 4.5 35 67 57,499 459 0.02283 1.8 0.21820 7.4 14.43 7.2 0.25 145.5 2.7 200.4 16.2 36 250 6,817 263 0.01425 0.9 0.09991 4.4 19.66 4.3 0.21 91.2 0.9 96.7 4.5 37 156 3,704 418 0.01532 0.8 0.10791 25.5 19.57 25.5 0.03 98.0 0.8 104.1 27.6 38 200 5,147 1,450 0.01122 0.6 0.06267 10.1 24.69 10.1 0.06 71.9 0.4 61.7 6.4 39 144 12,849 2,775 0.01846 1.1 0.13672 4.6 18.62 4.4 0.24 117.9 1.3 130.1 6.3 40 112 6,683 1,007 0.02289 0.5 0.15432 36.8 20.46 36.8 0.01 145.9 0.8 145.7 56.1 41 189 4,857 319 0.01491 1.3 0.11339 6.2 18.13 6.0 0.21 95.4 1.2 109.1 7.1 42 144 8,257 867 0.01588 0.7 0.11072 16.8 19.77 16.8 0.04 101.6 0.8 106.6 18.7 43 84 27,516 523 0.02301 0.6 0.17372 6.4 18.26 6.4 0.10 146.6 0.9 162.6 11.3 44 80 59,155 477 0.01458 0.5 0.11505 3.0 17.47 2.9 0.18 93.3 0.5 110.6 3.4 45 95 3,013 1,230 0.01444 0.7 0.09387 64.5 21.21 64.5 0.01 92.4 0.6 91.1 59.7 46 96 13,330 560 0.01402 0.6 0.09031 89.8 21.41 89.8 0.01 89.8 0.6 87.8 79.2 47 149 84,074 865 0.01441 0.3 0.10482 36.2 18.95 36.2 0.01 92.2 0.3 101.2 37.8 48 120 5,575 200 0.02786 0.9 0.19307 16.5 19.90 16.5 0.06 177.2 1.6 179.2 31.9 49 140 7,197 1,113 0.01351 0.5 0.09292 110.1 20.05 110.1 0.00 86.5 0.4 90.2 98.9 50 83 56,074 792 0.01154 0.4 0.08626 1.9 18.44 1.8 0.19 74.0 0.3 84.0 1.6 51 63 68,167 980 0.01132 0.8 0.08583 3.9 18.18 3.9 0.20 72.6 0.6 83.6 3.4 52 136 13,640 3,058 0.01550 0.7 0.10353 6.4 20.64 6.4 0.11 99.1 0.7 100.0 6.7 53 127 9,398 784 0.01475 0.7 0.09983 15.1 20.37 15.1 0.05 94.4 0.7 96.6 15.2 54 176 10,779 228 0.03180 0.9 0.28310 18.3 15.49 18.3 0.05 201.8 1.9 253.1 51.3 55 236 13,057 318 0.03206 1.1 0.23295 2.8 18.97 2.6 0.38 203.4 2.2 212.6 6.7 56 72 37,851 489 0.02344 0.5 0.14014 10.0 23.07 10.0 0.05 149.4 0.8 133.2 14.1 57 87 91,888 386 0.01176 0.7 0.08861 6.7 18.30 6.7 0.10 75.4 0.5 86.2 6.0 58 134 10,571 1,512 0.01451 0.5 0.09262 16.0 21.60 16.0 0.03 92.8 0.5 89.9 14.9 59 152 3,420 628 0.01225 0.7 0.07430 29.8 22.73 29.8 0.02 78.5 0.6 72.8 22.2 60 29 8,657 223 0.02196 1.3 0.16703 6.9 18.13 6.8 0.19 140.1 1.8 156.8 11.7 61 123 26,665 1,002 0.02195 1.4 0.15348 7.6 19.72 7.5 0.19 139.9 2.0 145.0 11.8 62 169 7,583 471 0.01080 0.9 0.09186 10.4 16.22 10.3 0.08 69.3 0.6 89.2 9.6 63 75 35,515 847 0.01650 1.1 0.10980 9.8 20.72 9.7 0.11 105.5 1.2 105.8 10.8 64 54 6,482 418 0.01147 0.6 0.07191 18.7 22.00 18.6 0.03 73.5 0.5 70.5 13.5 65 84 30,396 222 0.02982 0.7 0.21322 1.6 19.28 1.4 0.44 189.4 1.3 196.3 3.4 66 116 7,549 85 0.22531 1.2 2.76964 10.1 11.22 10.0 0.12 1,309.9 17.4 1,347.4 250.1 67 75 17,178 940 0.01452 0.3 0.10509 5.0 19.05 5.0 0.06 92.9 0.3 101.5 5.3 68 159 14,525 2,852 0.01267 0.5 0.09377 8.6 18.63 8.6 0.05 81.2 0.4 91.0 8.2 69 79 38,454 438 0.01176 1.4 0.09035 10.6 17.94 10.5 0.13 75.4 1.1 87.8 9.7 70 162 6,862 1,664 0.01099 1.1 0.08471 18.5 17.90 18.4 0.06 70.5 0.8 82.6 15.8 

206Pb/208Pb

ratio

±

(pct)

205Pb/235U

ratio

±

(pct)

±

(pct)

±

(m.y.)

±

(m.y

Analysis

U

content

(ppm)

U/Th

ratio

206Pb/204Pb

 ratio

206Pb/207Pb

ratio

Error

correlation

206Pb/238U

age

(m.y.)

207Pb/235U

age

(m.y.)
1 136 1,409 33 0.03093 1.6 0.19120 5.4 22.31 5.2 0.29 196.4 3.1 177.7 10

2 49 1,124 38 0.01673 1.5 0.08353 34.7 27.62 34.7 0.04 107.0 1.6 81.5 293 57 1,371 24 0.02531 0.9 0.17375 11.2 20.09 11.1 0.08 161.2 1.5 162.7 194 124 1,621 58 0.01765 36.2 0.11735 39.0 20.74 14.4 0.93 112.8 41.1 112.7 455 43 838 26 0.02652 1.7 0.19644 22.4 18.62 22.4 0.07 168.8 2.9 182.1 436 68 571 19 0.01961 1.4 0.15948 13.6 16.96 13.5 0.10 125.2 1.7 150.2 217 176 3,182 22 0.02363 2.3 0.15470 4.0 21.06 3.3 0.57 150.5 3.5 146.1 68 102 2,019 24 0.02484 2.2 0.16502 8.7 20.75 8.4 0.25 158.2 3.5 155.1 149 97 1,765 30 0.01687 1.0 0.10689 13.4 21.76 13.3 0.08 107.8 1.1 103.1 14

10 309 2,455 20 0.01627 1.5 0.11017 4.2 20.36 3.9 0.35 104.1 1.5 106.1 411 67 1,057 23 0.01970 2.5 0.11622 9.1 23.37 8.8 0.28 125.7 3.2 111.6 1012 72 1,088 57 0.01660 2.8 0.09292 9.9 24.63 9.5 0.29 106.1 3.0 90.2 913 67 1,142 66 0.02296 3.2 0.13832 8.7 22.89 8.1 0.37 146.3 4.7 131.5 1214 191 1,602 19 0.01609 1.6 0.10667 4.0 20.80 3.7 0.39 102.9 1.6 102.9 415 84 1,304 32 0.01717 1.9 0.10284 10.0 23.02 9.8 0.19 109.7 2.1 99.4 1016 200 1,651 20 0.01618 2.2 0.10037 4.3 22.23 3.7 0.50 103.5 2.3 97.1 417 55 1,226 17 0.02506 1.1 0.13134 11.5 26.31 11.5 0.10 159.6 1.8 125.3 1518 199 2,367 25 0.01629 1.8 0.10275 4.1 21.86 3.7 0.44 104.2 1.9 99.3 419 135 2,082 32 0.01660 0.9 0.11173 6.7 20.49 6.6 0.14 106.1 1.0 107.6 720 83 2,412 22 0.01802 2.2 0.13285 14.8 18.71 14.6 0.15 115.2 2.5 126.7 1921 87 2,277 32 0.02389 2.2 0.17692 11.1 18.62 10.9 0.20 152.2 3.4 165.4 1922 82 1,378 27 0.02507 2.0 0.16606 10.2 20.81 10.0 0.19 159.6 3.1 156.0 1723 74 2,119 19 0.02667 1.6 0.22040 9.6 16.69 9.5 0.17 169.7 2.8 202.2 2124 118 5,593 21 0.05711 1.4 0.42880 2.5 18.36 2.1 0.57 358.0 5.2 362.3 1025 108 1,542 22 0.01939 1.2 0.14609 10.2 18.30 10.1 0.12 123.8 1.5 138.5 1526 62 1,229 19 0.02405 1.9 0.15134 10.4 21.92 10.2 0.19 153.2 3.0 143.1 1527 105 1,818 20 0.01638 1.7 0.08838 16.4 25.56 16.3 0.11 104.7 1.8 86.0 1428 53 1,574 25 0.02400 1.1 0.16165 12.4 20.47 12.4 0.09 152.9 1.7 152.1 2029 41 1,277 21 0.02514 1.1 0.12795 8.9 27.09 8.8 0.12 160.1 1.8 122.3 1130 77 1,569 40 0.02551 1.7 0.16608 6.1 21.18 5.8 0.28 162.4 2.8 156.0 1031 181 943 19 0.01578 2.0 0.09816 9.1 22.16 8.8 0.23 100.9 2.1 95.1 932 124 1,511 26 0.01618 1.2 0.11573 7.4 19.27 7.3 0.17 103.4 1.3 111.2 833 157 2,551 18 0.01652 1.5 0.10246 6.7 22.23 6.5 0.22 105.6 1.6 99.0 634 87 1,780 31 0.01806 2.6 0.11037 14.2 22.56 13.9 0.19 115.4 3.1 106.3 1535 57 1,352 15 0.02418 1.6 0.14425 15.7 23.11 15.6 0.10 154.0 2.5 136.8 2236 35 2,015 30 0.02549 2.1 0.15932 25.5 22.06 25.4 0.08 162.3 3.5 150.1 4037 156 4,133 28 0.02444 1.6 0.15304 5.2 22.02 5.0 0.31 155.7 2.6 144.6 838 67 3,413 18 0.02427 1.2 0.15077 10.1 22.19 10.0 0.12 154.6 1.8 142.6 1539 692 18,764 14 0.03556 2.3 0.27225 2.6 18.01 1.3 0.87 225.3 5.2 244.5 740 73 2,193 22 0.03373 4.2 0.19837 9.5 23.45 8.5 0.44 213.9 9.1 183.7 1841 95 1,589 62 0.03730 5.1 0.30566 13.0 16.83 12.0 0.39 236.1 12.2 270.8 3942 71 1,869 24 0.02470 1.8 0.12768 13.5 26.67 13.4 0.13 157.3 2.8 122.0 1743 39 1,109 33 0.02519 1.8 0.15179 16.3 22.88 16.2 0.11 160.4 3.0 143.5 2444 57 1,392 16 0.02364 1.8 0.13432 12.7 24.27 12.6 0.14 150.6 2.7 128.0 1745 80 1,886 23 0.01693 1.5 0.11483 9.2 20.33 9.1 0.16 108.2 1.6 110.4 1046 59 1,023 30 0.01930 2.8 0.13833 13.0 19.23 12.7 0.22 123.2 3.5 131.6 18

Sample 66

206Pb/208Pb

ratio

±

(pct)

205Pb/235U

ratio

±

(pct)

±

(pct)

±

(m.y.)

±

(m.y

Analysis

U

content

(ppm)

U/Th

ratio

206Pb/204Pb

 ratio

206Pb/207Pb

ratio

Error

correlation

206Pb/238U

age

(m.y.)

207Pb/235U

age

(m.y.)

Table 2. Analytical data on detrital zircons.—Continued[All analyses by laser-ablation inductively coupled plasma mass spectroscopy (ICPMS)]
Table 2. Analytical data on detrital zircons.—Continued

[All analyses by laser-ablation inductively coupled plasma mass spectroscopy (ICPMS)]

47 89 646 51 0.02134 1.5 0.16572 6.8 17.76 6.6 0.22 136.1 2.0 155.7 1148 68 1,578 32 0.02341 1.7 0.14529 4.8 22.21 4.5 0.35 149.2 2.5 137.7 749 83 2,033 75 0.01885 2.3 0.10998 16.2 23.64 16.1 0.14 120.4 2.8 105.9 1850 104 3,325 42 0.02513 1.1 0.16130 6.3 21.48 6.2 0.18 160.0 1.8 151.8 1051 78 1,201 32 0.02468 1.0 0.18417 7.5 18.48 7.4 0.13 157.2 1.5 171.6 1352 62 1,838 22 0.02624 2.4 0.20637 10.6 17.53 10.4 0.22 167.0 4.0 190.5 2253 161 2,911 21 0.02518 2.0 0.16737 4.5 20.74 4.0 0.44 160.3 3.2 157.1 754 108 1,742 56 0.01740 2.7 0.11020 8.0 21.77 7.5 0.34 111.2 3.1 106.1 855 97 1,993 24 0.01630 1.7 0.10475 6.4 21.46 6.1 0.27 104.3 1.8 101.2 656 111 2,437 23 0.01608 2.5 0.11789 12.1 18.81 11.9 0.21 102.9 2.6 113.2 1457 90 1,562 63 0.01706 2.1 0.13934 12.6 16.88 12.4 0.17 109.1 2.3 132.5 1758 37 301 33 0.01702 3.5 0.08843 20.8 26.54 20.5 0.17 108.8 3.8 86.0 1859 47 1,181 33 0.02343 1.3 0.13513 12.0 23.90 11.9 0.11 149.3 2.0 128.7 1660 670 10,910 12 0.01594 1.5 0.10848 2.6 20.27 2.2 0.56 102.0 1.5 104.6 261 72 846 35 0.01640 1.7 0.10256 18.9 22.05 18.8 0.09 104.9 1.7 99.1 1962 57 1,547 43 0.02472 2.8 0.18774 13.9 18.16 13.6 0.20 157.4 4.5 174.7 2663 57 3,359 55 0.05664 4.2 0.43978 6.2 17.76 4.6 0.68 355.2 15.2 370.1 2764 144 2,468 33 0.01649 1.9 0.12716 7.5 17.88 7.3 0.25 105.5 2.0 121.5 965 51 1,631 120 0.02400 5.6 0.15258 6.3 21.69 3.0 0.88 152.9 8.6 144.2 966 60 1,327 30 0.02368 3.4 0.13297 15.8 24.56 15.4 0.22 150.9 5.2 126.8 2167 73 1,711 29 0.02584 1.2 0.21168 13.7 16.83 13.6 0.09 164.5 2.0 195.0 2968 88 1,397 23 0.01622 2.3 0.10255 15.5 21.81 15.3 0.15 103.7 2.4 99.1 1669 67 925 57 0.01816 1.4 0.11657 28.0 21.49 28.0 0.05 116.0 1.6 112.0 3270 60 1,405 42 0.01491 3.5 0.09236 48.0 22.26 47.9 0.07 95.4 3.4 89.7 44

1 351 9,857 332 0.01581 0.9 0.09892 2.2 22.04 1.9 0.43 101.1 0.9 95.8 22 428 12,170 413 0.02329 0.8 0.22182 3.6 14.48 3.5 0.23 148.4 1.2 203.4 83 262 14,409 69 0.02645 0.3 0.23517 6.2 15.51 6.2 0.05 168.3 0.6 214.5 144 17 3,028 696 0.01675 0.9 0.09069 9.9 25.47 9.8 0.09 107.1 0.9 88.1 95 40 1,499 60 0.02261 1.3 0.12452 14.5 25.03 14.5 0.09 144.1 1.9 119.2 186 60 1,279 47 0.01658 1.4 0.07857 30.7 29.10 30.7 0.05 106.0 1.5 76.8 247 15 2,597 46 0.03599 1.5 0.25001 10.4 19.85 10.3 0.15 227.9 3.6 226.6 268 20 4,372 351 0.02615 0.8 0.16989 4.5 21.22 4.4 0.18 166.4 1.4 159.3 79 90 12,933 26 0.01808 0.7 0.12414 4.7 20.08 4.7 0.14 115.5 0.8 118.8 5

10 118 4,243 527 0.01697 1.0 0.09861 7.6 23.73 7.5 0.14 108.5 1.1 95.5 711 94 6,032 33 0.02677 0.8 0.18569 3.3 19.88 3.2 0.23 170.3 1.3 172.9 612 23 5,068 361 0.02373 1.2 0.15052 3.4 21.74 3.1 0.36 151.2 1.9 142.4 513 101 5,537 19 0.02451 1.1 0.16819 5.8 20.09 5.7 0.18 156.1 1.7 157.8 914 99 8,935 30 0.01683 0.5 0.11587 2.4 20.03 2.3 0.22 107.6 0.6 111.3 215 74 7,615 44

0.01552

1.0 0.09205 21.3 23.25 21.3 0.05 99.3 1.0 89.4 19

16 25 7,121 420

0.01640

1.1 0.09951 12.7 22.73 12.7 0.09 104.9 1.1 96.3 12

17 152 9,871 1,178 0.01731 0.6 0.16349 2.2 14.60 2.2 0.26 110.6 0.7 153.8 318 95 7,753 140 0.03429 0.7 0.23412 1.3 20.19 1.1 0.53 217.3 1.5 213.6 319 100 6,031 199 0.01585 1.0 0.09940 21.8 21.98 21.8 0.05 101.4 1.0 96.2 2120 140 712 181 0.01661 2.0 0.07504 69.0 30.51 68.9 0.03 106.2 2.1 73.5 5121 131 13,782 524 0.02388 0.6 0.16830 1.9 19.57 1.8 0.30 152.2 0.9 157.9 3

Sample 67

206Pb/208Pb

ratio

±

(pct)

205Pb/235U

ratio

±

(pct)

±

(pct)

±

(m.y.)

±

(m

Analysis

U

content

(ppm)

U/Th

ratio

206Pb/204Pb

 ratio

206Pb/207Pb

ratio

Error

correlation

206Pb/238U

age

(m.y.)

207Pb/235U

age

(m.y.)
Table 2. Analytical data on detrital zircons.—Continued

[All analyses by laser-ablation inductively coupled plasma mass spectroscopy (ICPMS)]

22 80 30,177 310 0.03622 0.8 0.27452 1.5 18.19 1.3 0.52 229.4 1.9 246.3 423 75 31,137 761 0.01661 0.5 0.12607 4.9 18.17 4.9 0.09 106.2 0.5 120.6 624 174 9,836 1,071 0.01671 0.7 0.10260 18.1 22.46 18.1 0.04 106.8 0.7 99.2 1825 49 14,056 613 0.01702 1.0 0.11321 10.3 20.73 10.2 0.10 108.8 1.1 108.9 1126 64 7,449 353 0.01757 0.8 0.10713 35.3 22.61 35.3 0.02 112.3 0.9 103.3 3727 491 5,047 699 0.01651 0.7 0.10751 4.2 21.18 4.1 0.17 105.6 0.7 103.7 428 173 7,064 874 0.01613 0.8 0.10658 3.7 20.87 3.6 0.22 103.2 0.9 102.8 429 86 13,376 473 0.01640 1.6 0.13475 6.1 16.78 5.9 0.26 104.9 1.7 128.4 830 195 8,857 1,709 0.01638 0.9 0.11190 13.8 20.18 13.7 0.06 104.7 0.9 107.7 1531 83 5,270 772 0.01608 0.8 0.09930 6.6 22.33 6.6 0.12 102.8 0.8 96.1 632 252 2,185 276 0.02455 1.5 0.14990 55.8 22.58 55.8 0.03 156.4 2.4 141.8 8133 86 328,263 273 0.01727 0.6 0.12217 6.4 19.49 6.3 0.10 110.4 0.7 117.0 734 134 359,267 1,017 0.02486 0.7 0.18770 4.8 18.26 4.8 0.14 158.3 1.1 174.7 935 67 57,499 642 0.01571 0.8 0.11247 3.8 19.26 3.7 0.20 100.5 0.8 108.2 436 250 6,817 141 0.02578 1.0 0.17314 19.1 20.53 19.0 0.05 164.1 1.6 162.1 3337 156 3,704 375 0.01649 0.3 0.11106 4.8 20.48 4.8 0.06 105.5 0.3 106.9 538 200 5,147 600 0.02637 1.3 0.18161 32.3 20.02 32.3 0.04 167.8 2.2 169.4 5739 144 12,849 2,884 0.01717 0.8 0.12969 6.4 18.26 6.3 0.12 109.7 0.9 123.8 840 112 6,683 1,391 0.01599 0.4 0.11096 13.1 19.87 13.1 0.03 102.3 0.4 106.8 1441 189 4,857 289 0.01596 0.5 0.10463 27.6 21.03 27.6 0.02 102.0 0.5 101.0 2842 144 8,257 829 0.01610 0.5 0.14396 4.6 15.42 4.6 0.11 102.9 0.5 136.6 643 84 27,516 717 0.01624 0.6 0.11178 5.0 20.03 4.9 0.11 103.8 0.6 107.6 544 80 59,155 270 0.02505 1.5 0.19207 2.3 17.99 1.7 0.65 159.5 2.4 178.4 445 95 3,013 496 0.03504 0.8 0.22701 38.0 21.28 38.0 0.02 222.0 1.8 207.7 8447 149 3,109 395 0.03086 1.7 0.18757 4.2 22.69 3.9 0.41 196.0 3.4 174.6 848 120 5,575 325 0.01664 0.7 0.15150 5.0 15.15 5.0 0.15 106.4 0.8 143.2 749 140 7,197 930 0.01576 1.0 0.10425 4.9 20.84 4.8 0.20 100.8 1.0 100.7 550 83 56,074 579 0.01541 0.8 0.13274 8.4 16.00 8.3 0.10 98.6 0.8 126.6 1151 63 68,167 589 0.01842 1.8 0.15256 6.0 16.65 5.7 0.30 117.7 2.1 144.2 952 136 13,640 1,307 0.03566 1.0 0.24874 5.4 19.77 5.3 0.19 225.9 2.3 225.6 1353 127 9,398 450 0.02522 0.7 0.17441 16.3 19.94 16.3 0.04 160.6 1.1 163.2 2854 176 10,779 433 0.01634 0.8 0.12017 3.0 18.75 2.9 0.27 104.5 0.9 115.2 355 236 13,057 598 0.01659 0.9 0.11286 9.8 20.27 9.7 0.10 106.1 1.0 108.6 1156 72 37,851 651 0.01722 0.4 0.11072 2.7 21.44 2.7 0.14 110.0 0.4 106.6 357 87 91,888 281 0.01582 0.6 0.10011 5.7 21.80 5.6 0.10 101.2 0.6 96.9 558 134 10,571 1,342 0.01604 0.7 0.11428 5.1 19.35 5.0 0.14 102.6 0.7 109.9 559 152 3,420 313 0.02427 0.7 0.17284 5.3 19.36 5.2 0.14 154.6 1.1 161.9 960 29 8,657 296 0.01627 0.7 0.09600 6.3 23.36 6.3 0.10 104.0 0.7 93.1 661 123 26,665 885 0.02446 0.6 0.16215 3.0 20.80 2.9 0.19 155.8 0.9 152.6 462 169 7,583 302 0.01661 0.4 0.11163 2.5 20.51 2.4 0.18 106.2 0.5 107.5 263 75 35,515 559 0.02469 0.5 0.16793 0.7 20.27 0.5 0.69 157.2 0.8 157.6 164 54 6,482 195 0.02436 0.9 0.16369 5.3 20.52 5.3 0.16 155.2 1.3 153.9 865 84 30,396 400 0.01623 0.7 0.10086 8.0 22.18 8.0 0.09 103.8 0.7 97.6 866 116 7,549 1,033 0.01718 0.7 0.11840 2.2 20.00 2.1 0.33 109.8 0.8 113.6 267 75 17,178 840 0.01607 0.4 0.11113 3.3 19.93 3.3 0.11 102.7 0.4 107.0 368 159 14,525 2,129 0.01680 0.5 0.11023 2.8 21.02 2.8 0.16 107.4 0.5 106.2 369 79 38,454 208 0.02458 1.0 0.17568 4.2 19.29 4.1 0.24 156.6 1.6 164.3 770 162 6,862 1,124 0.01613 0.5 0.10965 7.4 20.29 7.4 0.07 103.2 0.5 105.6 871 351 4,928 340 0.01622 0.6 0.09962 25.9 22.45 25.9 0.02 103.7 0.6 96.4 2572 428 6,085 588 0.01721 1.2 0.16509 7.4 14.38 7.3 0.16 110.0 1.3 155.1 12

206Pb/208Pb

ratio

±

(pct)

205Pb/235U

ratio

±

(pct)

±

(pct)

±

(m.y.)

±

(m.y

Analysis

U

content

(ppm)

U/Th

ratio

206Pb/204Pb

 ratio

206Pb/207Pb

ratio

Error

correlation

206Pb/238U

age

(m.y.)

207Pb/235U

age

(m.y.)
Table 2. Analytical data on detrital zircons.—Continued

[All analyses by laser-ablation inductively coupled plasma mass spectroscopy (ICPMS)]

73 262 7,204 79 0.02433 0.7 0.15004 3.7 22.36 3.7 0.18 155.0 1.0 142.0 574 17 4,542 515 0.02390 0.5 0.15570 4.5 21.17 4.4 0.12 152.3 0.8 146.9 775 40 2,248 65 0.02192 0.7 0.14052 24.4 21.51 24.4 0.03 139.8 1.0 133.5 34

1 25 842 40 0.02287 0.9 0.10330 26.1 30.52 26.1 0.03 145.7 1.3 99.8 272 45 1,015 43 0.01553 1.3 0.07209 32.3 29.70 32.3 0.04 99.3 1.3 70.7 233 103 1,379 33 0.01518 0.9 0.07907 7.5 26.47 7.5 0.13 97.1 0.9 77.3 64 53 3,941 61 0.06115 0.8 0.43786 3.6 19.26 3.5 0.22 382.6 3.2 368.7 165 81 1,903 64 0.02382 0.8 0.13803 4.5 23.80 4.4 0.18 151.8 1.2 131.3 66 179 2,316 47 0.01400 1.2 0.08890 4.9 21.71 4.8 0.24 89.6 1.1 86.5 47 201 5,794 43 0.02591 1.0 0.17838 2.3 20.03 2.1 0.43 164.9 1.6 166.7 48 100 4,728 42 0.04168 1.0 0.27805 2.9 20.67 2.7 0.34 263.2 2.6 249.1 89 47 1,924 57 0.01835 2.9 0.10152 38.6 24.92 38.5 0.07 117.2 3.4 98.2 39

10 59 1,659 20 0.02317 1.2 0.13700 10.1 23.32 10.0 0.12 147.7 1.8 130.4 1311 73 1,770 55 0.02781 3.9 0.17364 12.8 22.08 12.2 0.30 176.8 7.0 162.6 2212 61 2,065 59 0.02236 0.8 0.12505 12.6 24.66 12.5 0.06 142.6 1.1 119.6 1513 89 4,392 19 0.02252 0.9 0.14038 10.7 22.12 10.7 0.09 143.6 1.3 133.4 1514 71 677 36 0.01614 1.9 0.09680 11.7 22.99 11.5 0.17 103.2 2.0 93.8 1115 37 1,265 25 0.02261 1.3 0.13465 12.0 23.15 11.9 0.11 144.1 1.9 128.3 1616 119 2,020 53 0.01362 0.7 0.07704 4.5 24.37 4.5 0.16 87.2 0.6 75.4 317 249 5,577 35 0.01468 0.5 0.09890 4.5 20.46 4.5 0.10 93.9 0.4 95.8 418 137 2,654 20 0.01422 0.7 0.08627 5.4 22.73 5.3 0.13 91.1 0.6 84.0 419 66 3,275 24 0.02275 1.2 0.11896 12.9 26.36 12.9 0.09 145.0 1.7 114.1 1520 200 6,985 25 0.01496 0.8 0.09813 4.9 21.02 4.8 0.17 95.7 0.8 95.0 421 40 1,358 31 0.02152 1.6 0.11897 21.0 24.95 20.9 0.07 137.3 2.2 114.1 2522 22 561 250 0.02662 2.6 0.26297 31.4 13.96 31.2 0.08 169.4 4.5 237.1 8023 43 1,243 18 0.02280 1.4 0.12379 12.9 25.39 12.8 0.11 145.3 2.0 118.5 1624 104 4,837 65 0.01529 1.1 0.11228 11.1 18.78 11.1 0.09 97.8 1.0 108.0 1225 63 996 76 0.01696 3.3 0.11500 16.3 20.34 15.9 0.21 108.4 3.6 110.5 1826 132 6,975 29 0.01511 1.1 0.09885 7.9 21.07 7.8 0.14 96.7 1.1 95.7 727 73 2,150 26 0.02288 1.4 0.14034 6.2 22.48 6.0 0.23 145.8 2.1 133.3 828 74 3,609 17 0.02071 0.9 0.14631 3.9 19.51 3.8 0.23 132.1 1.2 138.7 529 68 1,088 24 0.01433 1.0 0.07724 15.8 25.58 15.8 0.06 91.7 0.9 75.5 1230 48 1,170 21 0.02267 1.0 0.10972 9.7 28.48 9.7 0.10 144.5 1.4 105.7 1031 41 648 68 0.01462 1.6 0.05866 16.9 34.36 16.9 0.09 93.5 1.5 57.9 1032 1,341 45,289 10 0.03200 1.0 0.23019 1.2 19.17 0.6 0.87 203.1 2.1 210.4 233 242 3,240 52 0.01526 1.4 0.08797 4.4 23.92 4.1 0.32 97.6 1.4 85.6 334 114 2,683 23 0.01494 0.9 0.09948 8.7 20.70 8.7 0.10 95.6 0.8 96.3 835 61 1,836 38 0.02312 2.5 0.11138 9.5 28.62 9.1 0.27 147.3 3.8 107.2 1036 120 4,992 26 0.03340 0.8 0.21836 2.6 21.09 2.5 0.31 211.8 1.8 200.5 537 173 3,576 19 0.01559 0.9 0.09863 4.2 21.80 4.1 0.20 99.8 0.8 95.5 438 124 2,362 29 0.01629 2.3 0.09755 5.4 23.03 4.8 0.43 104.2 2.4 94.5 539 65 1,851 21 0.02442 2.0 0.14806 8.7 22.74 8.5 0.23 155.5 3.2 140.2 1340 60 1,462 46 0.02416 3.0 0.15412 10.8 21.61 10.4 0.28 153.9 4.6 145.5 1641 134 4,246 15 0.02381 0.9 0.15401 4.2 21.32 4.1 0.22 151.7 1.4 145.5 642 70 1,949 35 0.02641 2.2 0.17493 8.9 20.81 8.6 0.25 168.0 3.7 163.7 15

Sample 69

206Pb/208Pb

ratio

±

(pct)

205Pb/235U

ratio

±

(pct)

±

(pct)

±

(m.y.)

±

(m

Analysis

U

content

(ppm)

U/Th

ratio

206Pb/204Pb

 ratio

206Pb/207Pb

ratio

Error

correlation

206Pb/238U

age

(m.y.)

207Pb/235U

age

(m.y.)
Table 2. Analytical data on detrital zircons.

[All analyses by laser-ablation inductively coupled plasma mass spectroscopy (ICPMS)]

43 68 2,025 28 0.02839 3.2 0.16739 8.3 23.39 7.7 0.38 180.5 5.8 157.1 14.0 44 137 3,031 22 0.02255 0.9 0.14658 3.6 21.22 3.4 0.25 143.8 1.3 138.9 5.3 45 79 2,484 54 0.03677 0.5 0.24964 6.3 20.31 6.2 0.08 232.8 1.1 226.3 15.7 46 112 2,152 24 0.02173 0.6 0.13592 7.4 22.04 7.4 0.08 138.6 0.9 129.4 10.2 47 83 2,526 44 0.02592 1.0 0.17981 3.5 19.88 3.4 0.28 165.0 1.7 167.9 6.4 48 305 6,703 14 0.02118 1.3 0.14224 2.7 20.53 2.3 0.49 135.1 1.8 135.0 3.9 49 229 5,653 40 0.03472 0.6 0.25251 1.8 18.96 1.7 0.35 220.0 1.4 228.6 4.6 50 133 3,874 73 0.02510 2.6 0.17077 5.2 20.27 4.5 0.50 159.8 4.2 160.1 9.0 51 267 3,995 52 0.01966 3.7 0.13103 5.0 20.69 3.4 0.74 125.5 4.7 125.0 6.6 52 68 1,714 79 0.02576 1.5 0.19141 8.3 18.56 8.2 0.18 164.0 2.5 177.8 16.0 53 471 5,846 47 0.01612 1.5 0.10415 1.9 21.34 1.3 0.76 103.1 1.5 100.6 2.0 54 137 2,823 71 0.01984 2.0 0.13186 5.2 20.75 4.8 0.38 126.7 2.5 125.8 6.9 55 179 4,260 38 0.02394 1.4 0.15326 4.5 21.53 4.3 0.30 152.5 2.1 144.8 7.0 56 295 6,890 22 0.02318 0.8 0.15826 2.0 20.20 1.9 0.39 147.7 1.2 149.2 3.2 57 69 2,487 25 0.02776 2.6 0.19558 8.2 19.57 7.8 0.32 176.5 4.6 181.4 16.2 58 69 1,265 30 0.02553 2.2 0.13430 4.6 26.21 4.1 0.46 162.5 3.5 128.0 6.3 59 105 3,750 53 0.02343 1.9 0.16386 4.8 19.71 4.4 0.39 149.3 2.8 154.1 7.9 60 194 5,028 14 0.03107 0.9 0.21431 2.0 19.99 1.8 0.46 197.3 1.8 197.2 4.3 61 77 1,922 31 0.02295 1.6 0.14263 5.8 22.19 5.6 0.28 146.3 2.4 135.4 8.4 62 745 5,856 43 0.02696 1.5 0.18534 2.2 20.06 1.6 0.68 171.5 2.6 172.6 4.1 63 106 1,852 101 0.02113 2.6 0.13920 14.1 20.93 13.8 0.19 134.8 3.6 132.3 19.7 64 90 955 70 0.02381 1.3 0.14263 5.6 23.01 5.5 0.22 151.7 1.9 135.4 8.1 65 198 2,938 55 0.01595 1.5 0.10277 4.2 21.40 3.9 0.36 102.0 1.6 99.3 4.4 66 77 1,443 61 0.02278 1.0 0.11866 10.6 26.46 10.5 0.09 145.2 1.5 113.9 12.7 67 67 1,261 56 0.02467 4.5 0.15255 9.5 22.30 8.4 0.47 157.1 7.2 144.2 14.6 68 86 1,120 28 0.01566 1.4 0.08348 9.0 25.86 8.9 0.15 100.1 1.4 81.4 7.6 69 163 2,549 21 0.02237 0.9 0.16881 6.4 18.28 6.3 0.14 142.6 1.3 158.4 10.9 70 135 2,362 28 0.01571 0.7 0.11183 7.7 19.37 7.7 0.09 100.5 0.7 107.6 8.7 

206Pb/208Pb

ratio

±

(pct)

205Pb/235U

ratio

±

(pct)

±

(pct)

±

(m.y.)

±

(m.

Analysis

U

content

(ppm)

U/Th

ratio

206Pb/204Pb

 ratio

206Pb/207Pb

ratio

Error

correlation

206Pb/238U

age

(m.y.)

207Pb/235U

age

(m.y.)