EVALUATION OF STRATIGRAPHIC CONTINUITY BETWEEN THE...
Transcript of EVALUATION OF STRATIGRAPHIC CONTINUITY BETWEEN THE...
PIR 2008-1C 25
INTRODUCTIONThe Colville Basin in northern Alaska is a large east-trending foreland basin that developed north of the Brooks
Range fold and thrust belt in the Early Cretaceous (Mull, 1985; Bird and Molenaar, 1992; fi gs. 1–2). The southern margin of the basin is generally regarded as gas-prone, although it remains underexplored relative to the rest of the North Slope (Houseknecht and Bird, 2006). Seismic data are poor throughout this belt (Molenaar, 1988), leading to uncertainty regarding regional stratigraphic relationships.
The purpose of this paper is to examine the poorly understood stratigraphic relationship between the Fortress Mountain and Nanushuk2 Formations—two of the most prospective siliciclastic units exposed in the inner foot-hills. Many previous workers have interpreted the two units as discrete depositional cycles, often separated by a “tongue” of mudstone-dominated Torok Formation (Mull, 1985; Moore and others, 1994; fi g. 3). However, in the fi eld the relationship between the two formations is unclear and the two distinct belts of resistant exposures are never observed in close spatial association (fi g. 2). To date, it has been diffi cult to prove the two units did not overlap in time, leading some workers to entertain the hypothesis that the uppermost Fortress Mountain Formation graded northward into the lower Nanushuk Formation (Kelley, 1990). In addition to ambiguous fi eld relationships, available biostratigraphic constraints have largely proven insuffi cient to address this question. This study takes an alternate approach, testing the stratigraphic continuity of the two units on the basis of their provenance character-istics—specifi cally, the novel application of detrital zircon geochronology. Preliminary results strongly suggest the two units represent different depositional sequences.
1Alaska Division of Geological & Geophysical Surveys, 3354 College Rd., Fairbanks, Alaska 99709-3707Email for Marwan A. Wartes: [email protected]
2The Nanushuk has historically been ascribed Group status (Detterman, 1956). However, in a comprehensive revision and simplifi cation of Brookian stratigraphic nomenclature, Mull and others (2003) suggested the Nanushuk be demoted to formation rank.
EVALUATION OF STRATIGRAPHIC CONTINUITY BETWEEN THE FORTRESS MOUNTAIN AND NANUSHUK FORMATIONS IN THE CENTRAL BROOKS RANGE FOOTHILLS—ARE THEY PARTLY
CORRELATIVE?by
Marwan A. Wartes1
Fig. 2PK & IG
EK
Miles0
0 50 100 200
30 60 120
Kilometers
Figure 1. Regional location map of northern Alaska. Initials refer to localities outside of the study area that are mentioned in the text (PK & IG = Poko Mountain and Igloo Mountain; EK = Ekakevik Mountain).
26 PIR 2008-1C Preliminary results, fi eld investigations, Brooks Range foothils & North Slope, Alaska
Figure 2. Generalized geologic map of the east-central foothills (modifi ed slightly from C.G. Mull, unpublished mapping). Numbered locations refer to samples in this study (see table 1).
Okpikruak Formation(Neocomian)
Ko
Etivluk Group(Permian - Jurassic)
JPu
Lisburne Group( Pennsylvanian)Mississippian -
IPMI
Endicott Group(L )ower Mississippian - Devonian
MDu
Kayak Shale(Lower Mississippian)
Mk
Basalt sills and dikesba
Colville Group(Upper Cretaceous)
Kc
Nanushuk Formation(middle Albian - Cenomanian)
Kn
Torok Formation(Aptian - Albian)
Kt
Fortress Mountain Formation(Barremian(?) - early Albian)
Kf
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IPMIIPMI
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ba
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MDu MDu
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Kcrevi
Rkayiy
A
reviRreldnahC
reviR
kupkiski
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reviR
agalu
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reviR
kuvutkan
A
JPu
IPMI
JPu
JPu JPu
Ko
JPu
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Tuktu Escarpment
Kfm-Kto contact
?
Kt
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(3)(4)
(5)
CastleMountain
GunsightMountain
Desolation CreekFault zone
“HoaryMarmot”Mountain
“ThumbprintSyncline” “Rumpled Rug Folds”
FortressMtn.
10
Stratigraphic continuity between the Fortress Mountain and Nanushuk Formations 27
EXISTING CONSTRAINTSMap relationships
Early reconnaissance mapping recognized that at a regional scale progressively younger Brookian units typi-cally crop out from south to north (Patton and Tailleur, 1964). This general observation supports the interpretation that the southerly belt of Fortress Mountain Formation is older than the more northerly Nanushuk Formation trend. However, the top of the Fortress Mountain Formation is almost never exposed. Moreover, the two units are never observed together, but are instead separated by a poorly exposed, low-relief succession of fi ner-grained Torok Formation (fi gs. 2 and 4). The broad zone of Torok Formation exhibits locally complex deformation with variable fold vergence (Peapples and others, 2007). The northerly part of the Torok belt is generally interpreted as a structurally thickened triangle zone beneath a south-vergent roof thrust near the base of the Nanushuk Forma-tion (Moore and others, 2004; Wallace and others, 2006). The uplift and subsequent erosion of post-Torok rocks has obscured the relationship between the Fortress Mountain Formation and proximal reaches of the Nanushuk Formation (fi g. 4).
Upper
Low
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taceous
140
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FortressMtn.
Nanushuk Fm.
Fm.
Figure 3. Chronostratigraphy of middle Cretaceous units in northern Alaska showing revised stratigraphic nomen-clature (modifi ed from Mull and others, 2003). cs = cob-blestone sandstone (informal unit); ms = manganiferous shale unit (informal).
Roof thrust zone
Poorly exposed andcomplexly folded Torok Fm
Ge
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CASTLEMOUNTAIN GUNSIGHT
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?
??
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upper Fortress Mountain FormationFan delta - Alluvial fan/braided fluvial
lower Fortress Mountain FormationDeep water sediment gravity flows (slope)
upper Nanushuk FormationDominantly nonmarine (fluvial)
lower Nanushuk FormationDominantly marine shoreface to deltaic
Fortress Mountain Formation Nanushuk Formation
SW NE
Kfmu
Kfml
Knu
Knl
Kfmu
Kfml
Knu
Knl
Figure 4. Schematic cross section illustrating the geography of the Fortress Mountain and Nanushuk trends and the stratigraphic uncertainty of their relationship. Note the relative location of samples in this study.
28 PIR 2008-1C Preliminary results, fi eld investigations, Brooks Range foothils & North Slope, Alaska
Stratigraphic RelationshipsThe Fortress Mountain Formation includes a variety of facies ranging from deep marine basin fl oor up through
alluvial fan environments (Wartes, 2008, this volume; Houseknecht and others, 2007; Crowder, 1989; Molenaar and others, 1988; Hunter and Fox, 1976). The type section of the Fortress Mountain Formation consists of a series of discontinuous exposures in the Castle Mountain area (fi g. 2; Patton and Tailleur, 1964). This ~3,000–m-thick section is considerably thicker than any other known outcrop of the formation and is interpreted to grade from basin fl oor(?) to slope facies up through fan delta, alluvial fan, and braided fl uvial facies (Molenaar and others, 1988; Crowder, 1987).
The Nanushuk Formation records deposition in marine shelf, deltaic, and fl uvial environments (Ahlbrandt, 1979; Huffman, 1985; LePain and others, 2008). Seismic data clearly indicate that Nanushuk topsets transition basinward (to the northeast) into clinoforms of the Torok Formation (Molenaar, 1988; Houseknecht and Schenk, 2001). In the east-central foothills, where the samples for this study were collected, the Nanushuk is approximately 1,600 m thick (Patton and Tailleur, 1964) and is subdivided into a marine lower part and a dominantly nonmarine upper part (fi g. 3; Mull and others, 2003).
Regionally, the Fortress Mountain formation rapidly fi nes northward and locally grades into rocks assigned to the lower Torok Formation (Patton and Tailleur, 1964; Molenaar and others, 1988; fi g. 3). However, at Castle Mountain the upper 350 m of section includes the most proximal nonmarine facies interpreted as alluvial fan and braided fl uvial. The northerly equivalent of this nonmarine succession is not known and it remains possible that it graded northward into the marine facies of the lower Nanushuk. The uncertain relationship between the two units is schematically illustrated in fi gure 4.
Sandstone CompositionIn hand sample, sandstones of the Nanushuk Formation are often lighter colored and appear less lithic rich
than darker greenish-gray-weathering Fortress Mountain Formation sandstones. This qualitative observation is borne out by regional petrographic data that indicate that Nanushuk samples from the central and eastern foothills average about 58 percent total quartz (Bartsch-Winkler and Huffman, 1988), whereas Fortress Mountain samples average only 32 percent quartz in the east-central outcrop belt (Molenaar and others, 1988). An additional notable difference documented in these studies is captured in the metamorphic lithics, which average 15 percent for the Nanushuk samples as compared to only 1 percent for Fortress Mountain samples. This stark difference was also noted by Johnsson and Sokol (2000), who documented as much as 30 percent metamorphic lithic fragments in the easternmost exposures of the Nanushuk Formation.
BiostratigraphyThe Fortress Mountain Formation is generally ascribed an Aptian(?) to early Albian age, whereas the Na-
nushuk Formation is regarded as middle Albian to Cenomanian (fi g. 3). These assignments largely refl ect detailed megafossil work done by the USGS in the 1950s and 1960s, most notably Imlay (1961). The shallow marine Nanushuk Formation has generally yielded a more diverse fauna and the biostratigraphic range can be considered more robust than the Fortress Mountain Formation. The majority of the biostratigraphic control for the Fortress Mountain Formation came from the lower third of the unit (Patton and Tailleur, 1964), leaving the upper limit of the formation in question.
Bivalves identifi ed as similar to Inoceramus anglicus and Inoceramus altifl uminis were reported from both the Fortress Mountain and Nanushuk Formations by Imlay (1961). In Alberta and British Columbia, these species were generally regarded as no older than middle Albian (McLearn, 1945). On the basis of the entire fossil assemblage from the Fortress Mountain Formation, particularly ammonite specimens, Imlay (1961) favored an interpretation that these Inoceramids must have ranged down into the early Albian in northern Alaska. More recently, Kelley (1990) reported additional specimens of I. anglicus from the Fortress Mountain Formation at Hoary Marmot Mountain (informal name; fi g. 2). He interpreted an age no older than middle Albian and concluded the exposure was likely coeval with the lower part of the Nanushuk Formation.
Palynological and foraminiferal data generally offer limited biostratigraphic resolution for any of the Fortress, Nanushuk, or Torok Formations, and typically yield a signifi cant number of “Aptian–Albian” or “Lower Cretaceous undifferentiated” age assignments (unpublished DGGS micropaleontology reports). However, there is a general observation that can be gleaned from the subset of samples that do preserve abundant age-diagnostic microfos-sils: the Fortress Mountain Formation samples do not appear to produce ages younger than Aptian to early Albian and the Nanushuk Formation in the central foothills does not yield ages older than middle to late Albian (written commun., M. Mickey, 2006; LePain and others, 2008).
Stratigraphic continuity between the Fortress Mountain and Nanushuk Formations 29
SampleNumber Map # Description Latitude / Longitude
05ASD053 (1)Gunsight MountainTop of Nanushuk Fm. N 68.71775 W 151.83504
05WKW082 (2)Siksikpuk RiverBase of Nanushuk Fm. N 68.72206 W 151.88918
02MAW009 (3) Castle Mountain summitTop of Fortress Mountain Fm.
N 68.56649 W 152.58198
02MAW020 (4) Castle Mountain baseupper Fortress Mountain Fm.
N 68.56365 W 152.60785
LOCATION
Table 1. Sample location information; note map number that refers to sample notation in fi gure 4.
NEW DATAAs outlined in the preceding sections, existing constraints have proven inadequate to defi nitively assess the
stratigraphic relationship between the Fortress Mountain and Nanushuk Formations. The following new data attempts to address the question by focusing on the provenance of the two units, specifi cally the U-Pb ages of detrital zircons.
Detrital Zircon GeochronologyZircon is a common heavy mineral in most siliciclastic systems and is highly resistant to weathering and dia-
genesis (Fedo and others, 2003). Recent advances in analytical technology have allowed for the rapid acquisition of a large number of U-Pb ages from individual detrital zircons (Gehrels and others, 2006). The addition of this geochronologic component is revolutionizing studies of sedimentary provenance, particularly regional paleogeo-graphic and tectonic studies (Gillis and others, 2005; Dickinson and Gehrels, 2003).
Sample Selection and MethodsIn order to assess the population of detrital zircon ages from each formation, samples were selected from some
of the thicker, more continuous exposures in the foothills. In the case of the Fortress Mountain Formation, three samples were selected from the type section at Castle Mountain, one from the lower and two from the upper part of the formation (fi gs. 2 and 4; table 1). This section is particularly appropriate as it likely contains the stratigraphi-cally highest Fortress Mountain Formation exposed anywhere in the foothills. Two Nanushuk Formation samples were analyzed from well-exposed bluffs where the Siksikpuk River cuts across the Tuktu escarpment area; one sample is from the very base of the formation, the other from upper Nanushuk on Gunsight Mountain (fi gs. 2 and 4; table 1).
Zircons were separated at the Apatite-to-Zircon, Inc. laboratory using heavy liquid and magnetic separation techniques. Grains were mounted in epoxy and polished to expose grain interiors for analysis. One-hundred grains were selected at random and dated for each sample, a number commonly sought by most workers and one that reduces the probability of missing any given provenance components to <5 percent (Dodson and others, 1988). Uranium, thorium, and lead isotope data were collected at the Washington State University Geoanalytical Labora-tory on a ThermoFinnigan Element2 magnetic sector double-focusing ICP–MS (inductively coupled plasma–mass spectrometer). Each grain underwent laser ablation using a New Wave Research UP-213 system. See Chang and others (2006) for additional details regarding laboratory procedures such as standards and error corrections.
30 PIR 2008-1C Preliminary results, fi eld investigations, Brooks Range foothils & North Slope, Alaska
Detrital zircon populationsAppendix C-1 includes isotopic ratio and age data for 500 individual U-Pb zircon analyses. Only one grain from
sample 02MAW020 produced strongly discordant data and was rejected. For ages up to 1000 Ma, the 206Pb/238U age is selected as the most precise; those greater than 1000 Ma are taken from the 206Pb/207Pb ages (see Gehrels and others, 2006). The age distribution data for each of the fi ve samples are plotted in a histogram format and cumulative probability curves (fi gs. 5–9). Histograms illustrate the number of ages that fall into each 50 Ma bin. Cumulative probability curves, which are a common method for displaying detrital zircon data, represent the normalized sum of probability distributions for all analyses from a sample. Age peaks are only considered statistically signifi cant if defi ned by several analyses. All plots were created using Isoplot/Ex v. 3.32 (Ludwig, 2005).
DISCUSSIONThe detrital zircon data reveal statistically signifi cant differences between the two formations. This is best
illustrated by comparing selected groups of grain ages (table 2, fi gs. 10 and 11). All three Fortress Mountain For-mation samples include abundant Permian grains (10–19 percent), whereas the Nanushuk samples yielded only one Permian grain between both samples. Typically the parent lithology of individual zircons is diffi cult to assess. However, in the case of the Permian age spike, U-Pb zircon ages from igneous clasts within the Fortress Mountain Formation (Wartes and others, 2006) indicate the detrital population observed in this study was derived from an unknown tonalitic–dacitic igneous complex.
Late Neoproterozoic to Cambrian ages (500–665 Ma) form a prominent peak in both Nanushuk samples (aver-age 23 percent) but are much less abundant, on average, in the Fortress Mountain samples (7 percent). Nanushuk samples also contain a discrete pulse of early Neoproterozoic ages (850–950 Ma) that is much less common in the Fortress Mountain samples. Similarly, Mesoproterozoic ages (~1000–1300 Ma) form prominent peaks in both Nanushuk samples (average 20 percent), while the Fortress Mountain samples include a very weak record of this provenance (average 5 percent). Finally, although the Nanushuk Formation samples contain signifi cantly larger proportion of Precambrian grains, the Fortress Mountain samples average almost three times more early Proterozoic grain ages (Orosirian period 1800–2050 Ma).
If the Fortress Mountain and Nanushuk Formations were in part correlative, it seems probable that the up-permost Fortress Mountain sample (02MAW009) would have been most similar to the basal Nanushuk sample (05WKW082). In fact, these two samples produced some of the starkest contrasts in several groupings of detrital grain ages (table 2; fi gs. 10 and 11). The two formations clearly have a different provenance for zircons and the
Table 2. Comparison of signifi cant detrital-age groupings highlighting noteworthy differences between the Nanushuk and the Fortress Mountain Formations. Age groupings are listed as numbers of ages within each “bin”, although since ~100 grains were dated for each sample, these numbers are effectively percentages as well.
Sample No. 251-299 Maa 500-665 Mab 850-950 Mac 1.00-1.35 Gad 1.80-2.05 Gae > 542 Ma
05ASD053 1 14 12 24 6 7605WKW082 0 32 14 16 4 82
AVERAGE 0.5% 23% 13% 20% 5% 79%
02MAW009 19 2 5 4 10 4002MAW020 18 14 3 7 13 5802MAW006 10 5 3 4 20 54
AVERAGE 16% 7% 4% 5% 14% 51%
dGlobally significant orogenic episode in the Mesoproterozoic (e.g. McLelland and others, 1996)eOrosirian period of the Paleoproteroic, based on timescale of Ogg (2004)
aPermian period, based on timescale of Ogg (2004)bGlobally significant orogenic episode spanning the late Neoproterozoic - Cambrian (e.g. Murphy and Nance, 1991)cPart of the early Neoproterozoic
SIGNIFICANT GROUPINGS OF DETRITAL AGES
NANUSHUK FORMATION
FORTRESS MOUNTAIN FORMATION
Stratigraphic continuity between the Fortress Mountain and Nanushuk Formations 31
02468
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32 PIR 2008-1C Preliminary results, fi eld investigations, Brooks Range foothils & North Slope, Alaska
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Stratigraphic continuity between the Fortress Mountain and Nanushuk Formations 33
Permian ages(~251 - 299 Ma)
late Neoproterozoic - Cambrian “Pan-African” ages(~500 - 665 Ma)
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02MAW006Lower Kfm
Figure 10. Histograms (20 Ma bins) depicting a subset of grain age popu-lations (0–700 Ma). Samples are stacked in interpreted stratigraphic order. Highlighted groupings illustrate the signifi cant differences between Fortress Mountain and Nanushuk Formation samples (see also table 2). “Pan-African” refers to a globally signifi cant orogenic episode (Murphy and Nance, 1991).
balance of these results favors an interpretation that the two units do not represent parts of a single clastic wedge.
Very little U-Pb zircon geochronologic data exists for the Brooks Range and precise correlations between these detrital ages and specifi c units in the fold-and-thrust belt is premature and beyond the scope of this paper. Instead, the more salient, fi rst order result from this study is that the Fortress Mountain and Nanushuk Formations were clearly tapping unique and dif-ferent source areas and represent discrete phases in the unroofi ng sequence. This agrees with existing conventional petrographic point-count data noted earlier, which indicated clear differences in framework grain composition between the two formations (Bartsch-Winkler and Huffman, 1988; Molenaar and others, 1988). However, in some respects, the zircon data provide a more robust and quantitative comparison of the two units. It has long been recognized that sandstone mineralogy can be strongly infl uenced by transport distance (Pettijohn, 1957), particularly the labile components (Mack, 1978) that are abundant in both units. Given the geographic separation of the Fortress Moun-tain and Nanushuk outcrop belts, it remains possible that the observed differences were simply a function of transport biases. In contrast, durable zircons are unlikely to undergo signifi cant weathering or fractionation along a depositional profi le (Morton and Smale, 1991).
Ultimately, these provenance differences have stratigraphic implications and suggest the youngest exposed Fortress Mountain Formation is not correlative with the basal Nanushuk Formation. This interpretation is consistent with most depictions of Creta-ceous stratigraphy, which show a tongue of the Torok Formation separating the sandstone-rich Fortress Mountain and Nanushuk sequences (Mull and others, 2003; fi gs. 3 and 12). The upper-most Fortress Mountain Formation at Castle Mountain records alluvial fan and braided fl uvial deposition (Molenaar and others, 1988; Crowder, 1987). Thus, if a succession of Torok mudstones originally overlay this interval, it would represent a transgressive sequence (fi g. 12b). This hypothesis is diffi cult to evaluate due
34 PIR 2008-1C Preliminary results, fi eld investigations, Brooks Range foothils & North Slope, Alaska
Mesoproterozoic “Grenville” ages(~1000 -1350 Ma)
Paleoproterozoic Orosirian Period ages(~1800 - 2050 Ma)
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Nu
mb
er
0
2
4
6
8
700 950 1200 1450 1700 1950 2200
Age (Ma)
Nu
mb
er
Nan
ush
uk
Fo
rmati
on
Fo
rtre
ss
Mo
un
tain
Fo
rmati
on
05ASD053Top of Kn
05WKW082Base of Kn
02MAW009Top of Kfm
02MAW020Upper Kfm
02MAW006Lower Kfm
Figure 11. Histograms (50 Ma bins) depicting a subset of grain age populations (700–2250 Ma). Samples are stacked in interpreted stratigraphic order. Highlighted groupings illustrate the signifi cant differences between Fortress Mountain and Nanushuk Formation samples (see also table 2). “Grenville” refers to a globally sig-nifi cant orogenic episode (McLelland and others, 1996); Orosirian is a Mesoproterozoic period (Ogg, 2004).
to the lack of exposed formation tops in Fortress Moun-tain exposures (fi g. 12a; see also fi gure 4a of Bird and Molenaar, 1992). However, a few regional observations have cited evidence for an upward deepening at the top of the Fortress Mountain Formation. For example, in the Ekakevik Mountain area (fi g. 1), Hunter and Fox (1976) noted that the formation generally fi nes upward and, where best exposed, the top of the formation was characterized by transgressive shallow marine facies overlying nonmarine conglomerate. Farther west, be-tween Poko and Igloo mountains (fi g. 1), Mull (1985) inferred a gradational contact between the Fortress Mountain Formation and overlying fi ne-grained facies of the Torok Formation.
A major fl ood back of Torok slope facies over nonma-rine–marginal marine Fortress Mountain Formation was recently recognized in the Siksikpuk River area (labeled as Kfm–Kto contact on fi g. 2; Houseknecht and others, 2007; Peapples and others, 2007). However, this section is here interpreted as relatively low in the composite Fortress Mountain succession (Patton and Tailleur, 1964), suggesting that the transgressive event is intra-Fortress Mountain, rather than marking the regional top of the formation. It is also possible there is regional variation in the position of the top of the Fortress Mountain Forma-tion and that fl ooding of the Torok Formation across the Fortress Mountain Formation was diachronous locally, varying according to paleotopography across the complex orogenic wedge (Wartes and Swenson, 2005). Similarly complex relationships are known from the better-studied Upper Cretaceous foreland basin strata of the Western Interior Seaway (Krystinik and DeJarnett, 1995).
Existing data do not unequivocally rule out a scenario whereby now-eroded sections of the Fortress Mountain Formation once existed above Castle Mountain and formed part of a unifi ed prograding clastic wedge that
Stratigraphic continuity between the Fortress Mountain and Nanushuk Formations 35
Non
-dep
ositi
on(?
)
upper Kingak Shaleequivalent
UpperTorok Fm.
OkpikruakFormation
?
Allochthonous(?)
LowerTorok Fm.
Valanginian
Berriasian
Barremian
Ma
upper Kingak Shaleequivalent
UpperTorok Fm.
Hauterivian
140
120
110
Ne
oco
mia
n
Lo
we
r
Cre
taceo
us
Cenomanian
Aptian
Up
pe
r
Albian
100
130
OkpikruakFormation
?
Proximal
SSW NNE
Distal
AG
E
SER
IES
STA
GE
Allochthonous(?)
Pebble Shale equivalent
LowerTorok Fm.
Eroded
Valanginian
Berriasian
Barremian
Ma
Hauterivian
140
120
110
Ne
oco
mia
n
Lo
we
r
Cre
tac
eo
us
Cenomanian
Aptian
Up
pe
r
Albian
100
130
Valanginian
Berriasian
Barremian
Ma
Hauterivian
140
120
110
Ne
oco
mia
n
Lo
we
r
Cre
tac
eo
us
Cenomanian
Aptian
Up
pe
r
Albian
100
130
?
A
B
C
Non
-dep
ositi
on(?
)
upper Kingak Shaleequivalent
UpperTorok Fm.
OkpikruakFormation
?
Allochthonous(?)
LowerTorok Fm.
NanushukFormation
Formation
FortressMountain
FortressMountain
NanushukFormation
Formation
NanushukFormation
Formation
FortressMountain
Pebble Shale equivalent
Pebble Shale equivalent
Figure 12. Chronostratigraphic diagrams illustrating relationships between the Fortress Mountain, Nanushuk, and Torok Formations discussed in the text. Approximate sample positions shown by red dots. (A) Depiction of the observed spatial context of Fortress Mountain and Nanushuk exposures, highlighting the ambiguity of proposed stratigraphic correlations between the two units. This diagram contrasts with the common schematic representation of Early Cretaceous stratigraphy (Mull and others, 2003; fi g. 3) and emphasizes that the top of the Fortress Mountain Formation is a Holocene erosion surface and that the Nanushuk Formation cannot be demonstrated to overlie the Fortress Mountain Forma-tion based on fi eld evidence alone. On the basis of the available constraints discussed in the text, this fi gure interprets the exposed Fortress Mountain Formation as entirely older than the Nanushuk Formation in this area. (B & C) Portrayal of two possible strati-graphic scenarios as they would have appeared at the close of Nanushuk deposition: (B) proposes a retrogradational tongue of Torok separated the Fortress Mountain and Nanushuk Formations (model favored in this study), while (C) illustrates a model where the Fortress Mountain and Nanushuk Formations represent a continuous northward-prograding clastic wedge (see also fi gure 25.3c in Molenaar, 1988).
graded northward into the lower Nanushuk deltas (fig. 12c; see also figure 25.3c in Molenaar, 1988). However, if such a section existed, then a substantial upsection change in provenance would be required within the uppermost proximal Fortress Mountain For-mation. Based on available data (including that presented here), it seems more probable that the Fortress Mountain and Nanushuk Formations are regionally separated by an intervening tongue of the Torok Formation (fi g. 12b), possibly recording a basin-scale change in the balance between sediment sup-ply and newly created accommodation.
The Torok Formation can be generi-cally subdivided into lower and upper parts, roughly corresponding to proximal Fortress Mountain and Nanushuk Formations, re-spectively (Mull and others, 2003; fi g. 3). If a tongue of Torok separates these two progradational sequences (fi g. 12b)—an in-terpretation that is favored here—then it may prove useful to consider a tripartite genetic subdivision of the formation (fi g. 13). It is diffi cult to evaluate this proposed subdivi-sion in the outcrop belt where distinctions between lower, middle, and upper parts of the formation are hampered by: (1) inadequate biostratigraphic resolution, (2) non-unique, fi ne-grained facies, (3) discontinuous expo-sures, and (4) complex structure (Patton and Tailleur, 1964; Mull, 1985; Molenaar and others, 1988; Houseknecht and Schenk, 2007; Houseknecht and others, 2007). Regional seismic and well data indicate that an older succession of Brookian rocks exists beneath the Nanushuk-related Torok clinoforms (Molenaar, 1988). In the northern foothills, Houseknecht and Schenk (2001) document that this older Torok “foredeep wedge” was derived from the west and traced individual Torok clinoforms into Nanushuk topsets. However, this wedge continues to thicken dramatically southward, probably including southerly sourced Fortress Mountain-related lower(?) Torok facies (Cole and others, 1997; Molenaar, 1988). Regardless, in light of the strong differences in detrital zircon ages within the proximal Fortress Mountain and Nanushuk Formations, it seems likely that the affi nity of isolated Torok samples could be reliably assessed with the technique outlined in this study.
The new provenance data presented here confi rm that the Fortress Mountain and
36 PIR 2008-1C Preliminary results, fi eld investigations, Brooks Range foothils & North Slope, Alaska
Nanushuk Formations were clearly derived from different sources. Provided the two units both record unroofi ng of the nearby ancestral Brooks Range, it follows that the exposed Fortress Mountain Formation represents an entirely older depositional system than the Nanushuk topsets (as shown in fi g. 12a). However, it remains possible that the two units are partly coeval and that the detrital zircon data refl ects two different major sediment dispersal systems—transverse, Brooks Range-derived Fortress Mountain Formation and axially fed Nanushuk facies tapping a different source region to the west. Even in better-studied foreland basins, identifying the relative roles of axial versus transverse systems is diffi cult (Clevis and others, 2004). Farther north, in central NPRA (fi g. 1), seismi-cally mapped shelf margins indicate the Nanushuk–Torok depositional system is fed principally out of the west (Molenaar, 1988; Houseknecht and Schenk, 2001). However, the origin of Nanushuk topsets in the area sampled in this study were most likely sourced from transverse, north-fl owing rivers as indicated by paleocurrent data as well as contrasting deltaic style and provenance relative to the demonstrably western-sourced axial deltaic system (Huffman and others, 1988; Molenaar, 1988; LePain and others, in press).
The focus of this paper has been the stratigraphic implications of a modest new detrital zircon data set. Future papers will examine the potential sources of various zircon age populations and the broader tectonic implications of these data. However, it is worth noting that the marked change in provenance outlined in this study occurred at approximately the early to middle Albian boundary, a time characterized by signifi cant tectonic events in the Brooks Range hinterland (Vogl, 2002). In other words, the provenance changes may refl ect discrete tectonically-driven exhumation and orogen-wide reorganization of drainages.
CONCLUSIONSNew detrital zircon age data indicate the Fortress Mountain and Nanushuk Formations in the central Brooks
Range foothills were sourced from unique and different provenances. Signifi cantly different populations of grain ages suggest the Fortress Mountain and Nanushuk depositional systems were not part of a unifi ed north–northeast-erly prograding clastic wedge. Instead, the youngest exposed Fortress Mountain Formation is here interpreted to be entirely older than the Nanushuk Formation. Based on available data, it is inferred that the Fortress Mountain and Nanushuk Formations are separated by a transgressive tongue of the Torok Formation, although more data are needed to defi nitively reject alternate hypotheses. In particular, additional subsurface information will be required to document stratigraphic relationships that are presently ambiguous.
Figure 13. Schematic cross section of the Colville Basin (modifi ed from Mull, 1985, and Houseknecht and Schenk, 2001). The middle Torok is speculated to be a retrogradational package separating the Fortress Mountain and Nanushuk Forma-tions.
Kfm = Fortress Mountain Formation
Tecto
nic
Wedge
Kfm
Paleozoic &
seicafdesnednocnisabdevratS
Umiat
SSouthernFoothills
NorthernFoothills
Coastal Plain
~100 km
U. Cretaceous strata
N
Kto = Torok Formation (informal subdivision: Ktol=lower; Ktom=middle; Ktou=upper)
Ktol
Kto
m Ktou
Transgressive flooding surface (hypothesized)
Nanushuk FormationAllochthonous
Mesozoic Rocks
?
Stratigraphic continuity between the Fortress Mountain and Nanushuk Formations 37
ACKNOWLEDGMENTSThe impetus for this study came from a number of outcrop discussions with A. Carroll, who, unbiased by any
North Slope geological dogma, posed a number of insightful questions regarding established Brookian stratigraphic relationships, including, “Are you sure the Fortress Mountain and Nanushuk Formations aren’t, in part, correla-tive?” The ideas presented here also benefi ted from numerous discussions with D. LePain, R. Gillis, T. Moore, P. Decker, G. Mull, R. Reifenstuhl, R. Swenson, W. Wallace, D. Houseknecht, and P. O’Sullivan. A version of fi gure 13 was kindly provided by D. LePain. Funding for part of the original work in the Castle Mountain area was generously supplied to the University of Wisconsin by Anadarko Petroleum Corporation. DGGS studies in the foothills are supported by industry contributions from Anadarko, FEX, BG Alaska, Chevron, Petro-Canada, Pioneer Natural Resources, ENI, Shell, and ConocoPhillips. Funding for DGGS geologic mapping in the Siksikpuk River area was provided by the state of Alaska and the U.S. Geological Survey National Cooperative Geologic Mapping Program (award numbers 04HQAG0061 and 05HQAG0025). This paper benefi tted from constructive review comments by Bob Gillis.
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Appendix C-1 179
APPENDIX C-1
ANALYTICAL DATA AND CALCULATED AGES
180 PIR 2008-1 Preliminary results, fi eld investigations, Brooks Range Foothils & North Slope, Alaska
Ana
lysi
s #
207 P
b / 23
5 U±2
206 P
b / 23
8 U±2
207 P
b / 20
6 Pb
±220
7 Pb
/ 235 U
±220
6 Pb
/ 238 U
±220
7 Pb
/ 206 P
b±2
Bes
t Age
±285
234-
370.
0920
0.00
360.
0139
0.00
030.
0479
0.00
1989
.35
3.49
89.2
51.
7792
.16
3.68
89.2
51.
7785
234-
330.
0888
0.00
740.
0135
0.00
060.
0476
0.00
4186
.39
7.18
86.6
33.
5479
.50
6.82
89.6
33.
5485
234-
340.
2220
0.01
030.
0320
0.00
070.
0503
0.00
2420
3.57
9.45
202.
934.
4821
1.03
10.1
020
2.93
4.48
8523
4-28
0.31
920.
0300
0.04
450.
0014
0.05
210.
0050
281.
2626
.40
280.
478.
6728
7.70
27.7
028
0.47
8.67
8523
4-18
0.39
000.
0099
0.05
340.
0007
0.05
290.
0014
334.
398.
5033
5.59
4.67
326.
088.
5833
5.59
4.67
8523
4-32
0.39
510.
0198
0.05
360.
0009
0.05
340.
0027
338.
1216
.91
336.
785.
9634
7.44
17.7
533
6.78
5.96
8523
4-11
0.40
460.
0202
0.05
460.
0015
0.05
370.
0028
345.
0117
.25
342.
779.
3636
0.11
18.9
734
2.77
9.36
8523
4-35
0.40
370.
0174
0.05
470.
0018
0.05
350.
0024
344.
3414
.87
343.
5811
.42
349.
7315
.38
343.
5811
.42
8523
4-45
0.47
310.
0104
0.06
280.
0010
0.05
460.
0013
393.
368.
6739
2.88
5.94
396.
359.
1239
2.88
5.94
8523
4-21
0.49
640.
0123
0.06
520.
0012
0.05
520.
0015
409.
2910
.18
407.
267.
2342
0.84
11.0
340
7.26
7.23
8523
4-93
0.51
390.
0409
0.06
680.
0022
0.05
580.
0046
421.
0933
.54
417.
1113
.52
442.
8136
.56
417.
1113
.52
8523
4-79
0.51
480.
0404
0.06
700.
0020
0.05
580.
0044
421.
6433
.12
417.
8212
.65
442.
5834
.97
417.
8212
.65
8523
4-7
0.50
980.
0139
0.06
710.
0011
0.05
510.
0015
418.
2811
.39
418.
727.
0741
5.95
11.6
641
8.72
7.07
8523
4-2
0.51
740.
0247
0.06
770.
0021
0.05
540.
0028
423.
3920
.20
422.
5213
.36
427.
8621
.31
422.
5213
.36
8523
4-92
0.52
710.
0143
0.06
880.
0013
0.05
550.
0016
429.
8911
.69
429.
178.
3743
3.88
12.5
542
9.17
8.37
8523
4-30
0.62
700.
0204
0.07
930.
0011
0.05
730.
0019
494.
2516
.11
492.
027.
0350
4.61
17.0
949
2.02
7.03
8523
4-84
0.64
660.
0528
0.08
160.
0026
0.05
750.
0048
506.
4041
.34
505.
7416
.10
509.
4942
.87
505.
7416
.10
8523
4-31
0.64
860.
0222
0.08
210.
0019
0.05
730.
0021
507.
6017
.34
508.
4012
.08
504.
0718
.12
508.
4012
.08
8523
4-22
0.65
400.
0251
0.08
220.
0013
0.05
770.
0023
510.
9219
.64
508.
978.
3351
9.79
20.4
150
8.97
8.33
8523
4-55
0.65
240.
0322
0.08
300.
0018
0.05
700.
0029
509.
9625
.18
513.
9410
.99
492.
1724
.94
513.
9410
.99
8523
4-10
00.
6816
0.02
520.
0846
0.00
190.
0584
0.00
2252
7.72
19.4
952
3.79
11.9
954
4.89
20.6
452
3.79
11.9
985
234-
230.
6835
0.04
470.
0847
0.00
210.
0585
0.00
3952
8.90
34.5
652
4.37
13.1
254
8.25
36.8
152
4.37
13.1
285
234-
620.
6791
0.03
790.
0848
0.00
250.
0581
0.00
3352
6.26
29.3
452
4.74
15.2
053
2.72
30.5
952
4.74
15.2
085
234-
130.
6866
0.08
210.
0854
0.00
520.
0583
0.00
7353
0.73
63.4
952
8.11
32.4
254
2.14
67.6
352
8.11
32.4
285
234-
900.
7131
0.02
830.
0886
0.00
170.
0584
0.00
2454
6.61
21.6
854
7.35
10.4
754
3.59
22.1
354
7.35
10.4
785
234-
740.
7212
0.04
970.
0909
0.00
430.
0575
0.00
3355
1.37
38.0
256
0.84
26.5
051
2.54
29.8
156
0.84
26.5
085
234-
80.
7892
0.01
970.
0952
0.00
190.
0601
0.00
1659
0.74
14.7
458
6.23
11.7
460
8.22
16.1
458
6.23
11.7
485
234-
510.
8547
0.02
910.
1016
0.00
190.
0610
0.00
2162
7.22
21.3
562
3.70
11.4
464
0.11
22.1
462
3.70
11.4
485
234-
40.
8614
0.02
630.
1029
0.00
240.
0607
0.00
2063
0.91
19.2
463
1.57
14.7
162
8.59
20.4
363
1.57
14.7
185
234-
490.
8956
0.02
290.
1052
0.00
170.
0617
0.00
1764
9.38
16.6
264
5.06
10.4
166
4.52
17.8
564
5.06
10.4
1
Cor
rect
ed Is
otop
ic R
atio
sA
ges
(Ma)
Sam
ple
05A
SD05
3 (u
pper
Nan
ushu
k Fo
rmat
ion)
App
endi
x C
-1. A
naly
tical
dat
a an
d ca
lcul
ated
age
s ge
nera
ted
from
lase
r ab
latio
n in
duct
ivel
y co
uple
d pl
asm
a m
ass
spec
trom
etry
. Bol
d ag
es a
re c
onco
rdan
t; bl
ue a
ges
indi
cate
min
or P
b lo
ss
(plo
ts ju
st b
elow
the
conc
ordi
a cu
rve,
equ
ival
ent t
o <
2 pe
rcen
t of t
he to
tal P
b); a
ges i
n re
d in
dica
te e
vide
nce
for U
loss
or P
b ga
in (p
lots
abo
ve th
e co
ncor
dia
curv
e); a
ges i
n gr
een
exhi
bit
sign
ifi ca
nt P
b lo
ss (p
lots
bel
ow th
e co
ncor
dia
curv
e, e
quiv
alen
t to
>2
perc
ent o
f the
tota
l Pb)
; rej
ecte
d ag
es in
dica
ted
by st
rike
thro
ugh.
Isot
ope
ratio
s are
cor
rect
ed fo
r Pb/
U fr
actio
natio
n by
co
mpa
riso
n w
ith st
anda
rd zi
rcon
s with
an
age
of 5
64 ±
4 M
a (2σ)
and
109
9 ±
8 M
a (2σ)
. Rep
orte
d er
rors
incl
ude
mea
sure
men
t and
frac
tiona
tion
erro
rs. D
ecay
con
stan
ts: 23
5 U=
9.84
85x1
0-10 ;
238 U
=1.
5512
5x10
-10 ;
238 U
/235 U
=13
7.88
; see
Cha
ng a
nd o
ther
s (20
06) f
or d
etai
led
labo
rato
ry p
roce
dure
s.
Appendix C-1 181
Ana
lysi
s #
207 P
b / 23
5 U±2
206 P
b / 23
8 U±2
207 P
b / 20
6 Pb
±220
7 Pb
/ 235 U
±220
6 Pb
/ 238 U
±220
7 Pb
/ 206 P
b±2
Bes
t Age
± M
a (2
)85
234-
701.
0422
0.03
490.
1190
0.00
360.
0635
0.00
2372
5.00
24.2
672
4.76
21.8
272
5.86
25.9
972
4.76
21.8
285
234-
591.
1666
0.02
650.
1305
0.00
210.
0648
0.00
1578
5.07
17.8
379
0.78
12.9
976
8.97
17.7
479
0.78
12.9
985
234-
691.
2842
0.11
840.
1391
0.01
130.
0670
0.00
6583
8.71
77.3
083
9.53
68.0
183
6.64
81.1
883
9.53
68.0
185
234-
531.
3158
0.05
830.
1421
0.00
340.
0672
0.00
3285
2.67
37.8
185
6.56
20.5
184
2.82
39.6
885
6.56
20.5
185
234-
291.
3488
0.04
450.
1423
0.00
280.
0687
0.00
2486
7.03
28.6
185
7.72
16.6
889
1.04
31.6
885
7.72
16.6
885
234-
541.
3593
0.06
010.
1440
0.00
500.
0685
0.00
3387
1.55
38.5
286
7.14
29.9
788
2.80
42.3
386
7.14
29.9
785
234-
121.
4018
0.04
590.
1477
0.00
390.
0688
0.00
2588
9.71
29.1
588
8.15
23.3
189
3.90
32.2
688
8.15
23.3
185
234-
161.
4271
0.03
100.
1489
0.00
200.
0695
0.00
1690
0.36
19.5
989
4.74
12.2
391
4.19
20.4
089
4.74
12.2
385
234-
721.
4407
0.02
550.
1497
0.00
200.
0698
0.00
1390
6.00
16.0
189
9.43
12.2
792
2.09
16.6
389
9.43
12.2
785
234-
911.
4696
0.04
820.
1510
0.00
320.
0706
0.00
2491
7.95
30.1
190
6.78
19.3
394
4.98
32.1
790
6.78
19.3
385
234-
461.
4637
0.02
530.
1522
0.00
220.
0697
0.00
1291
5.54
15.8
491
3.54
13.4
492
0.41
16.1
291
3.54
13.4
485
234-
881.
4946
0.07
750.
1545
0.00
440.
0702
0.00
3692
8.18
48.1
092
6.12
26.0
793
3.23
47.4
692
6.12
26.0
785
234-
421.
5543
0.05
710.
1580
0.00
300.
0714
0.00
2895
2.20
34.9
894
5.65
18.0
296
7.48
37.8
294
5.65
18.0
285
234-
611.
5548
0.03
870.
1585
0.00
340.
0711
0.00
2095
2.39
23.7
294
8.66
20.0
496
1.23
26.9
694
8.66
20.0
485
234-
571.
6969
0.04
620.
1705
0.00
290.
0722
0.00
2010
07.3
527
.42
1014
.70
17.2
699
1.52
28.0
699
1.52
28.0
685
234-
641.
6970
0.04
110.
1685
0.00
340.
0731
0.00
2010
07.3
924
.41
1003
.71
20.0
810
15.5
127
.44
1015
.51
27.4
485
234-
821.
6705
0.08
320.
1647
0.00
520.
0736
0.00
3899
7.37
49.6
698
2.91
30.9
610
29.3
252
.51
1029
.32
52.5
185
234-
631.
7990
0.06
300.
1772
0.00
550.
0736
0.00
2710
45.0
936
.60
1051
.70
32.7
110
31.3
438
.28
1031
.34
38.2
885
234-
651.
7576
0.06
090.
1710
0.00
540.
0746
0.00
2710
29.9
835
.71
1017
.63
31.8
510
56.4
438
.86
1056
.44
38.8
685
234-
581.
8709
0.04
950.
1804
0.00
370.
0752
0.00
2110
70.8
328
.34
1069
.01
21.6
610
74.6
829
.45
1074
.68
29.4
585
234-
761.
9541
0.06
470.
1882
0.00
480.
0753
0.00
2510
99.8
436
.44
1111
.38
28.4
710
77.1
936
.44
1077
.19
36.4
485
234-
681.
8176
0.05
220.
1750
0.00
430.
0753
0.00
2310
51.8
130
.19
1039
.53
25.3
010
77.4
232
.25
1077
.42
32.2
585
234-
981.
8361
0.05
100.
1755
0.00
460.
0759
0.00
2310
58.4
829
.40
1042
.22
27.4
510
92.2
233
.24
1092
.22
33.2
485
234-
961.
8995
0.03
830.
1805
0.00
350.
0763
0.00
1710
80.9
121
.77
1069
.71
20.5
311
03.5
924
.72
1103
.59
24.7
285
234-
782.
1439
0.06
800.
2010
0.00
510.
0774
0.00
2611
63.0
936
.90
1180
.79
29.8
411
30.3
737
.41
1130
.37
37.4
185
234-
521.
9567
0.08
130.
1826
0.00
710.
0777
0.00
3611
00.7
745
.72
1080
.89
42.0
411
40.2
552
.13
1140
.25
52.1
385
234-
892.
0296
0.06
860.
1892
0.00
310.
0778
0.00
2811
25.4
938
.05
1117
.29
18.0
611
41.4
040
.58
1141
.40
40.5
885
234-
812.
1849
0.05
690.
1971
0.00
420.
0804
0.00
2311
76.2
430
.61
1159
.71
24.5
012
06.8
635
.17
1206
.86
35.1
785
234-
832.
3725
0.04
860.
2095
0.00
380.
0821
0.00
1812
34.3
625
.30
1226
.18
22.1
312
48.6
726
.75
1248
.67
26.7
585
234-
192.
3642
0.05
720.
2087
0.00
420.
0821
0.00
2112
31.8
629
.80
1222
.07
24.7
312
49.0
531
.99
1249
.05
31.9
985
234-
392.
5211
0.05
540.
2169
0.00
310.
0843
0.00
1912
78.1
428
.10
1265
.27
18.0
012
99.8
629
.95
1299
.86
29.9
585
234-
252.
5231
0.07
200.
2164
0.00
380.
0846
0.00
2512
78.7
036
.48
1262
.82
21.9
313
05.5
039
.07
1305
.50
39.0
785
234-
502.
5870
0.06
280.
2213
0.00
340.
0848
0.00
2112
96.9
631
.50
1288
.77
19.8
913
10.5
432
.93
1310
.54
32.9
385
234-
62.
7596
0.08
850.
2327
0.00
600.
0860
0.00
3013
44.6
743
.14
1348
.65
34.8
413
38.3
546
.24
1338
.35
46.2
485
234-
772.
7996
0.07
970.
2357
0.00
460.
0861
0.00
2413
55.4
438
.56
1364
.55
26.7
113
41.2
537
.96
1341
.25
37.9
685
234-
102.
7950
0.13
600.
2324
0.00
860.
0872
0.00
4613
54.2
065
.89
1347
.14
49.7
013
65.5
572
.06
1365
.55
72.0
6
Age
s (M
a)
App
endi
x C
-1 c
ontin
ued
-- S
ampl
e 05
AS
D05
3
Cor
rect
ed Is
otop
ic R
atio
s
182 PIR 2008-1 Preliminary results, fi eld investigations, Brooks Range Foothils & North Slope, Alaska
Ana
lysi
s #
207 P
b / 23
5 U±2
206 P
b / 23
8 U±2
207 P
b / 20
6 Pb
±220
7 Pb
/ 235 U
±220
6 Pb
/ 238 U
±220
7 Pb
/ 206 P
b±2
Bes
t Age
± M
a (2
)85
234-
562.
7724
0.05
110.
2299
0.00
340.
0875
0.00
1713
48.1
424
.85
1333
.89
19.9
113
70.8
527
.02
1370
.85
27.0
285
234-
152.
7889
0.12
110.
2258
0.00
900.
0896
0.00
3713
52.5
758
.73
1312
.28
52.3
214
16.9
757
.99
1416
.97
57.9
985
234-
993.
0638
0.11
250.
2477
0.00
670.
0897
0.00
3514
23.6
952
.28
1426
.65
38.6
014
19.4
156
.01
1419
.41
56.0
185
234-
52.
9422
0.07
960.
2374
0.00
660.
0899
0.00
2913
92.8
437
.68
1373
.29
38.4
414
23.0
445
.58
1423
.04
45.5
885
234-
13.
1257
0.14
690.
2504
0.00
760.
0906
0.00
4314
39.0
467
.61
1440
.32
43.8
514
37.4
268
.07
1437
.42
68.0
785
234-
383.
1488
0.05
170.
2504
0.00
320.
0912
0.00
1514
44.7
123
.72
1440
.32
18.1
714
51.2
624
.03
1451
.26
24.0
385
234-
853.
1900
0.07
070.
2533
0.00
490.
0913
0.00
2114
54.7
432
.26
1455
.39
28.3
614
53.8
233
.85
1453
.82
33.8
585
234-
803.
4716
0.09
780.
2724
0.00
740.
0924
0.00
2815
20.7
842
.86
1553
.01
42.1
614
76.3
644
.88
1476
.36
44.8
885
234-
93.
1953
0.05
650.
2494
0.00
440.
0929
0.00
1814
56.0
325
.74
1435
.34
25.5
814
86.4
729
.55
1486
.47
29.5
585
234-
243.
3209
0.10
290.
2587
0.00
700.
0931
0.00
2914
85.9
746
.05
1483
.31
40.3
514
90.0
245
.53
1490
.02
45.5
385
234-
673.
5184
0.13
330.
2694
0.00
910.
0947
0.00
3815
31.3
658
.00
1537
.79
52.1
115
22.6
061
.30
1522
.60
61.3
085
234-
973.
5781
0.10
420.
2734
0.00
560.
0949
0.00
2815
44.6
844
.98
1558
.25
31.9
915
26.2
245
.53
1526
.22
45.5
385
234-
713.
7208
0.15
430.
2778
0.00
570.
0971
0.00
4215
75.8
565
.34
1580
.32
32.3
115
69.9
467
.15
1569
.94
67.1
585
234-
863.
5918
0.06
510.
2676
0.00
410.
0974
0.00
1915
47.7
228
.07
1528
.37
23.4
315
74.2
530
.20
1574
.25
30.2
085
234-
873.
6447
0.08
360.
2701
0.00
520.
0979
0.00
2515
59.3
635
.75
1541
.27
29.5
315
84.0
940
.85
1584
.09
40.8
585
234-
953.
7106
0.11
280.
2744
0.00
630.
0981
0.00
3015
73.6
647
.85
1563
.16
36.0
815
87.7
249
.06
1587
.72
49.0
685
234-
413.
9316
0.06
590.
2890
0.00
420.
0987
0.00
1816
20.2
127
.17
1636
.44
23.7
215
99.2
728
.40
1599
.27
28.4
085
234-
944.
0812
0.09
440.
2959
0.00
650.
1000
0.00
2616
50.5
538
.17
1671
.00
36.4
716
24.7
641
.34
1624
.76
41.3
485
234-
264.
0183
0.09
680.
2862
0.00
610.
1018
0.00
2716
37.9
139
.47
1622
.34
34.8
516
58.0
643
.96
1658
.06
43.9
685
234-
404.
2691
0.10
910.
3011
0.00
600.
1028
0.00
2716
87.4
243
.12
1696
.98
33.5
716
75.5
744
.04
1675
.57
44.0
485
234-
144.
4528
0.12
160.
3113
0.00
770.
1038
0.00
2917
22.2
247
.03
1746
.89
43.1
816
92.4
746
.86
1692
.47
46.8
685
234-
364.
2478
0.13
520.
2961
0.00
740.
1041
0.00
3416
83.3
153
.56
1671
.84
41.8
116
97.7
355
.78
1697
.73
55.7
885
234-
734.
3037
0.07
450.
2961
0.00
460.
1054
0.00
1916
94.0
729
.34
1671
.77
26.0
417
21.8
030
.84
1721
.80
30.8
485
234-
174.
3858
0.07
210.
3006
0.00
430.
1058
0.00
1817
09.6
628
.09
1694
.36
24.3
917
28.4
828
.72
1728
.48
28.7
285
234-
444.
8601
0.11
380.
3205
0.00
550.
1100
0.00
2817
95.3
642
.05
1792
.14
30.9
817
99.2
445
.19
1799
.24
45.1
985
234-
485.
3383
0.11
360.
3393
0.00
590.
1141
0.00
2518
75.0
139
.89
1883
.34
32.4
818
65.8
840
.05
1865
.88
40.0
585
234-
435.
6514
0.14
510.
3547
0.00
720.
1156
0.00
3219
23.9
849
.40
1957
.10
40.0
018
88.5
851
.63
1888
.58
51.6
385
234-
275.
7355
0.13
460.
3538
0.00
700.
1176
0.00
2919
36.7
345
.45
1952
.70
38.8
619
19.8
247
.26
1919
.82
47.2
685
234-
205.
8321
0.11
500.
3578
0.00
610.
1182
0.00
2519
51.1
938
.47
1971
.75
33.6
219
29.5
540
.36
1929
.55
40.3
685
234-
665.
7032
0.11
240.
3495
0.00
730.
1184
0.00
2719
31.8
538
.07
1932
.06
40.4
219
31.6
144
.76
1931
.61
44.7
685
234-
311
.479
70.
3466
0.49
420.
0146
0.16
850.
0046
2562
.93
77.3
825
88.6
976
.68
2542
.73
68.5
925
42.7
368
.59
8523
4-60
13.3
156
0.39
820.
5265
0.01
400.
1834
0.00
6027
02.2
980
.81
2726
.54
72.4
026
84.2
987
.08
2684
.29
87.0
885
234-
7513
.377
10.
3025
0.50
820.
0105
0.19
090.
0043
2706
.64
61.2
126
49.0
054
.62
2750
.02
62.1
827
50.0
262
.18
8523
4-47
20.9
269
0.49
530.
6393
0.01
440.
2374
0.00
6031
35.2
174
.21
3186
.33
71.8
931
02.7
479
.06
3102
.74
79.0
6
App
endi
x C
-1 c
ontin
ued
-- S
ampl
e 05
AS
D05
3
Cor
rect
ed Is
otop
ic R
atio
sA
ges
(Ma)
Appendix C-1 183
Ana
lysi
s #
207 P
b / 23
5 U±2
206 P
b / 23
8 U±2
207 P
b / 20
6 Pb
±220
7 Pb
/ 235 U
±220
6 Pb
/ 238 U
±220
7 Pb
/ 206 P
b±2
Bes
t Age
±285
233-
70.
2225
0.00
670.
0321
0.00
060.
0503
0.00
1520
4.00
6.15
203.
613.
7520
8.74
6.17
203.
613.
7585
233-
60.
4054
0.03
400.
0548
0.00
110.
0537
0.00
4534
5.55
28.9
434
3.94
6.97
356.
6030
.08
343.
946.
9785
233-
630.
4057
0.01
400.
0549
0.00
090.
0536
0.00
1934
5.80
11.9
534
4.45
5.73
354.
9212
.70
344.
455.
7385
233-
700.
4156
0.01
220.
0562
0.00
080.
0537
0.00
1635
2.91
10.3
535
2.39
5.08
356.
4510
.77
352.
395.
0885
233-
100
0.41
650.
0161
0.05
640.
0009
0.05
350.
0021
353.
5413
.65
353.
895.
8635
1.10
13.8
835
3.89
5.86
8523
3-15
0.42
000.
0109
0.05
660.
0008
0.05
380.
0014
356.
079.
2735
4.91
5.27
363.
629.
5535
4.91
5.27
8523
3-4
0.43
840.
0230
0.05
870.
0009
0.05
410.
0029
369.
1219
.40
367.
915.
5437
6.43
20.2
436
7.91
5.54
8523
3-51
0.44
120.
0171
0.05
900.
0011
0.05
430.
0022
371.
1214
.36
369.
366.
5538
2.31
15.1
236
9.36
6.55
8523
3-36
0.47
090.
0158
0.06
240.
0015
0.05
480.
0020
391.
8013
.14
390.
029.
3340
2.37
14.5
439
0.02
9.33
8523
3-44
0.48
850.
0319
0.06
420.
0016
0.05
520.
0037
403.
8626
.39
401.
0710
.18
419.
6928
.00
401.
0710
.18
8523
3-9
0.49
080.
0162
0.06
460.
0010
0.05
510.
0019
405.
4913
.42
403.
786.
0041
5.50
14.1
540
3.78
6.00
8523
3-98
0.59
660.
0172
0.07
610.
0013
0.05
690.
0016
475.
0513
.74
472.
608.
1948
6.98
13.2
947
2.60
8.19
8523
3-61
0.62
620.
0353
0.07
900.
0018
0.05
750.
0034
493.
7427
.87
489.
9511
.01
511.
4730
.23
489.
9511
.01
8523
3-39
0.62
780.
0376
0.07
980.
0025
0.05
710.
0035
494.
7129
.61
494.
6815
.26
494.
7729
.90
494.
6815
.26
8523
3-12
0.66
060.
0201
0.08
260.
0016
0.05
800.
0018
514.
9615
.68
511.
499.
8353
0.17
16.6
351
1.49
9.83
8523
3-74
0.66
550.
0241
0.08
320.
0013
0.05
800.
0022
517.
9718
.77
515.
158.
0553
0.40
19.8
451
5.15
8.05
8523
3-54
0.67
170.
0142
0.08
430.
0012
0.05
780.
0013
521.
7611
.03
521.
567.
4052
2.61
11.6
752
1.56
7.40
8523
3-56
0.69
460.
0160
0.08
640.
0012
0.05
830.
0014
535.
5712
.36
534.
207.
4254
1.31
12.6
053
4.20
7.42
8523
3-81
0.74
110.
0121
0.09
120.
0011
0.05
890.
0010
563.
079.
1656
2.63
6.51
564.
969.
6256
2.63
6.51
8523
3-38
0.74
330.
0232
0.09
140.
0017
0.05
900.
0019
564.
3117
.58
563.
9710
.35
565.
5717
.80
563.
9710
.35
8523
3-42
0.75
050.
0323
0.09
170.
0014
0.05
940.
0026
568.
4924
.47
565.
428.
6258
0.94
25.8
756
5.42
8.62
8523
3-58
0.74
740.
0195
0.09
190.
0012
0.05
900.
0016
566.
7414
.80
566.
517.
4456
7.70
15.4
056
6.51
7.44
8523
3-84
0.75
300.
0340
0.09
200.
0019
0.05
940.
0028
569.
9525
.76
567.
4211
.58
580.
2227
.38
567.
4211
.58
8523
3-17
0.76
380.
0264
0.09
340.
0018
0.05
930.
0021
576.
1819
.91
575.
7710
.80
577.
9320
.63
575.
7710
.80
8523
3-90
0.76
380.
0243
0.09
390.
0017
0.05
900.
0019
576.
2118
.30
578.
8010
.55
566.
1818
.39
578.
8010
.55
8523
3-27
0.78
710.
0424
0.09
540.
0039
0.05
980.
0033
589.
5231
.76
587.
4723
.78
597.
3832
.94
587.
4723
.78
8523
3-62
0.78
930.
0241
0.09
650.
0019
0.05
930.
0019
590.
7518
.07
593.
6911
.49
579.
6118
.69
593.
6911
.49
8523
3-48
0.80
330.
0297
0.09
710.
0014
0.06
000.
0023
598.
6622
.16
597.
558.
5660
2.84
22.9
059
7.55
8.56
8523
3-57
0.80
390.
0371
0.09
730.
0027
0.05
990.
0028
599.
0427
.63
598.
3516
.55
601.
5828
.22
598.
3516
.55
8523
3-92
0.83
470.
0257
0.09
960.
0017
0.06
080.
0020
616.
2018
.98
612.
2810
.19
630.
4920
.72
612.
2810
.19
8523
3-67
0.83
220.
0219
0.10
050.
0019
0.06
010.
0017
614.
8316
.18
617.
3611
.39
605.
4716
.76
617.
3611
.39
8523
3-45
0.84
470.
0225
0.10
080.
0015
0.06
080.
0016
621.
7116
.57
619.
148.
9563
1.03
16.5
361
9.14
8.95
8523
3-93
0.85
290.
0338
0.10
150.
0016
0.06
090.
0025
626.
2524
.81
623.
269.
6663
7.13
25.7
062
3.26
9.66
App
endi
x C
-1 c
ontin
ued
Sam
ple
05W
KW
082A
(ba
sal N
anus
huk
Form
atio
n)C
orre
cted
Isot
opic
Rat
ios
Age
s (M
a)
184 PIR 2008-1 Preliminary results, fi eld investigations, Brooks Range Foothils & North Slope, Alaska
Ana
lysi
s #
207 P
b / 23
5 U±2
206 P
b / 23
8 U±2
207 P
b / 20
6 Pb
±220
7 Pb
/ 235 U
±220
6 Pb
/ 238 U
±220
7 Pb
/ 206 P
b±2
Bes
t Age
± M
a (2
)85
233-
190.
8431
0.02
790.
1018
0.00
230.
0600
0.00
1962
0.87
20.5
562
5.19
13.8
260
5.16
19.3
662
5.19
13.8
285
233-
290.
8704
0.04
570.
1022
0.00
310.
0617
0.00
3463
5.79
33.3
662
7.58
18.9
166
5.28
36.1
962
7.58
18.9
185
233-
720.
8546
0.03
000.
1028
0.00
160.
0603
0.00
2262
7.17
21.9
863
0.84
9.94
613.
8622
.04
630.
849.
9485
233-
160.
8748
0.02
430.
1033
0.00
240.
0614
0.00
1863
8.14
17.7
563
3.76
14.8
265
3.61
19.3
563
3.76
14.8
285
233-
530.
8787
0.03
370.
1037
0.00
180.
0615
0.00
2564
0.28
24.5
763
5.87
11.0
765
5.98
26.2
963
5.87
11.0
785
233-
370.
8815
0.01
750.
1040
0.00
170.
0615
0.00
1464
1.81
12.7
663
8.05
10.3
865
5.06
14.5
663
8.05
10.3
885
233-
730.
8907
0.02
420.
1048
0.00
160.
0616
0.00
1764
6.73
17.5
464
2.62
9.51
661.
2418
.20
642.
629.
5185
233-
880.
8826
0.02
480.
1059
0.00
230.
0605
0.00
1764
2.40
18.0
464
8.82
14.2
061
9.89
17.8
964
8.82
14.2
085
233-
780.
9012
0.03
180.
1069
0.00
350.
0611
0.00
2165
2.35
23.0
465
4.63
21.5
664
4.46
22.5
065
4.63
21.5
685
233-
960.
9091
0.02
480.
1069
0.00
170.
0617
0.00
1765
6.57
17.8
965
4.66
10.3
966
2.99
18.5
365
4.66
10.3
985
233-
860.
9125
0.04
470.
1069
0.00
470.
0619
0.00
3465
8.41
32.2
365
4.96
28.7
167
0.47
36.3
565
4.96
28.7
185
233-
80.
9102
0.04
580.
1082
0.00
210.
0610
0.00
3265
7.14
33.0
966
2.34
12.9
863
9.50
33.0
866
2.34
12.9
885
233-
830.
9222
0.01
830.
1087
0.00
170.
0615
0.00
1366
3.54
13.1
566
5.41
10.5
065
7.20
13.8
966
5.41
10.5
085
233-
411.
0342
0.02
420.
1175
0.00
170.
0638
0.00
1572
1.03
16.9
071
6.29
10.5
473
5.70
17.6
271
6.29
10.5
485
233-
651.
3298
0.02
960.
1419
0.00
280.
0680
0.00
1885
8.80
19.1
485
5.25
16.7
086
8.07
22.5
385
5.25
16.7
085
233-
821.
3710
0.02
650.
1445
0.00
200.
0688
0.00
1487
6.59
16.9
687
0.11
12.2
289
3.02
18.2
387
0.11
12.2
285
233-
231.
3633
0.05
050.
1449
0.00
460.
0683
0.00
2787
3.30
32.3
487
2.21
27.4
587
6.16
35.2
087
2.21
27.4
585
233-
891.
3796
0.04
080.
1456
0.00
350.
0687
0.00
2188
0.27
26.0
687
6.39
21.0
289
0.05
27.7
587
6.39
21.0
285
233-
851.
4142
0.04
360.
1469
0.00
420.
0698
0.00
2389
4.92
27.6
188
3.63
25.3
192
2.93
29.8
788
3.63
25.3
185
233-
261.
4072
0.03
180.
1473
0.00
260.
0693
0.00
1689
1.98
20.1
888
5.79
15.8
390
7.29
21.4
088
5.79
15.8
385
233-
661.
3985
0.03
900.
1476
0.00
270.
0687
0.00
2088
8.28
24.7
788
7.35
16.4
989
0.85
26.2
788
7.35
16.4
985
233-
401.
4730
0.02
800.
1526
0.00
190.
0700
0.00
1491
9.37
17.4
691
5.30
11.4
892
9.18
18.0
491
5.30
11.4
885
233-
691.
4644
0.05
430.
1549
0.00
390.
0686
0.00
2791
5.84
33.9
492
8.13
23.4
288
6.12
34.6
592
8.13
23.4
285
233-
871.
5092
0.02
670.
1569
0.00
210.
0697
0.00
1293
4.10
16.5
493
9.70
12.6
292
0.91
15.8
093
9.70
12.6
285
233-
711.
5472
0.05
680.
1576
0.00
370.
0712
0.00
2794
9.38
34.8
594
3.51
21.9
796
2.98
36.0
594
3.51
21.9
785
233-
341.
5414
0.06
320.
1587
0.00
410.
0705
0.00
2994
7.08
38.8
394
9.31
24.4
594
2.00
39.4
094
9.31
24.4
585
233-
431.
5727
0.05
200.
1588
0.00
270.
0718
0.00
2595
9.50
31.7
595
0.19
16.2
398
0.84
33.6
895
0.19
16.2
385
233-
491.
5803
0.03
200.
1615
0.00
270.
0710
0.00
1696
2.50
19.4
796
5.11
16.1
295
6.50
21.6
396
5.11
16.1
285
233-
101.
6071
0.05
710.
1629
0.00
270.
0715
0.00
2697
3.00
34.5
797
3.03
16.1
197
3.13
35.4
697
3.03
16.1
185
233-
281.
7236
0.03
340.
1729
0.00
300.
0723
0.00
1510
17.3
819
.70
1028
.24
17.6
699
4.19
20.6
699
4.19
20.6
685
233-
31.
7002
0.04
770.
1684
0.00
400.
0732
0.00
2110
08.6
228
.32
1003
.41
23.5
310
20.0
928
.89
1020
.09
28.8
985
233-
251.
8540
0.02
970.
1811
0.00
270.
0742
0.00
1310
64.8
517
.06
1073
.25
16.1
510
47.6
718
.24
1047
.67
18.2
485
233-
641.
9404
0.04
360.
1833
0.00
250.
0768
0.00
1910
95.1
524
.60
1085
.23
14.8
611
14.9
627
.20
1114
.96
27.2
085
233-
681.
9488
0.03
220.
1841
0.00
220.
0768
0.00
1310
98.0
418
.12
1089
.47
13.1
011
15.0
419
.23
1115
.04
19.2
3
App
endi
x C
-1 c
ontin
ued
-- S
ampl
e 05
WK
W08
2A
Cor
rect
ed Is
otop
ic R
atio
sA
ges
(Ma)
Appendix C-1 185
Ana
lysi
s #
207 P
b / 23
5 U±2
206 P
b / 23
8 U±2
207 P
b / 20
6 Pb
±220
7 Pb
/ 235 U
±220
6 Pb
/ 238 U
±220
7 Pb
/ 206 P
b±2
Bes
t Age
± M
a (2
)85
233-
182.
0527
0.06
690.
1936
0.00
310.
0769
0.00
2611
33.2
036
.92
1140
.85
18.2
111
18.7
037
.61
1118
.70
37.6
185
233-
112.
0317
0.03
730.
1892
0.00
260.
0779
0.00
1511
26.1
920
.70
1116
.81
15.6
111
44.4
921
.97
1144
.49
21.9
785
233-
22.
0778
0.03
850.
1927
0.00
220.
0782
0.00
1511
41.5
021
.12
1136
.10
12.8
811
51.8
922
.45
1151
.89
22.4
585
233-
242.
2057
0.04
030.
2028
0.00
210.
0789
0.00
1511
82.8
421
.62
1190
.17
12.0
111
69.5
122
.26
1169
.51
22.2
685
233-
752.
1402
0.03
650.
1955
0.00
270.
0794
0.00
1511
61.9
019
.82
1151
.03
15.7
811
82.3
322
.86
1182
.33
22.8
685
233-
992.
3186
0.10
030.
2056
0.00
520.
0818
0.00
3812
18.0
152
.70
1205
.24
30.4
712
40.8
557
.02
1240
.85
57.0
285
233-
762.
3805
0.06
120.
2099
0.00
480.
0823
0.00
2412
36.7
531
.78
1228
.03
28.2
312
52.0
636
.55
1252
.06
36.5
585
233-
552.
3651
0.04
870.
2083
0.00
310.
0824
0.00
1712
32.1
225
.38
1219
.50
17.8
912
54.2
325
.82
1254
.23
25.8
285
233-
52.
4721
0.05
160.
2134
0.00
400.
0840
0.00
2012
63.9
226
.37
1246
.71
23.4
712
93.4
530
.09
1293
.45
30.0
985
233-
212.
6236
0.08
210.
2226
0.00
340.
0855
0.00
2813
07.2
640
.90
1295
.59
19.9
713
26.4
142
.71
1326
.41
42.7
185
233-
132.
8291
0.12
420.
2352
0.00
910.
0873
0.00
4113
63.2
859
.86
1361
.44
52.6
413
66.2
764
.61
1366
.27
64.6
185
233-
312.
8249
0.07
650.
2328
0.00
470.
0880
0.00
2513
62.1
636
.87
1349
.28
27.1
513
82.5
639
.86
1382
.56
39.8
685
233-
952.
9163
0.06
610.
2371
0.00
340.
0892
0.00
2113
86.1
431
.42
1371
.44
19.4
314
08.9
633
.77
1408
.96
33.7
785
233-
602.
9351
0.05
050.
2385
0.00
360.
0892
0.00
1713
91.0
223
.93
1379
.07
21.0
214
09.3
426
.31
1409
.34
26.3
185
233-
523.
0833
0.05
570.
2477
0.00
390.
0903
0.00
1514
28.5
525
.79
1426
.72
22.2
514
31.3
923
.70
1431
.39
23.7
085
233-
793.
2462
0.04
780.
2571
0.00
240.
0916
0.00
1414
68.2
621
.62
1474
.97
13.8
814
58.5
522
.46
1458
.55
22.4
685
233-
913.
4485
0.08
670.
2679
0.00
550.
0934
0.00
2515
15.5
238
.11
1530
.25
31.4
514
95.2
539
.74
1495
.25
39.7
485
233-
773.
3585
0.07
040.
2591
0.00
510.
0940
0.00
2214
94.7
631
.34
1485
.45
29.5
015
08.1
035
.82
1508
.10
35.8
285
233-
973.
5785
0.06
500.
2736
0.00
390.
0949
0.00
1815
44.7
728
.05
1559
.24
22.4
115
25.1
929
.07
1525
.19
29.0
785
233-
593.
8354
0.06
230.
2858
0.00
450.
0973
0.00
1916
00.2
025
.99
1620
.31
25.3
915
73.9
130
.27
1573
.91
30.2
785
233-
143.
8416
0.18
530.
2801
0.01
410.
0995
0.00
5416
01.5
177
.27
1591
.82
79.9
016
14.3
887
.55
1614
.38
87.5
585
233-
203.
9057
0.08
090.
2845
0.00
540.
0995
0.00
2216
14.8
533
.47
1614
.21
30.9
216
15.6
835
.29
1615
.68
35.2
985
233-
463.
8416
0.10
430.
2785
0.00
580.
1000
0.00
2916
01.5
243
.48
1583
.94
32.9
416
24.7
646
.51
1624
.76
46.5
185
233-
13.
9172
0.06
480.
2831
0.00
350.
1004
0.00
1816
17.2
426
.74
1606
.72
19.9
416
31.0
529
.12
1631
.05
29.1
285
233-
473.
9157
0.11
030.
2807
0.00
570.
1012
0.00
3016
16.9
345
.53
1595
.13
32.6
016
45.4
348
.23
1645
.43
48.2
385
233-
224.
1150
0.05
980.
2939
0.00
380.
1016
0.00
1716
57.2
824
.08
1660
.97
21.4
416
52.6
827
.27
1652
.68
27.2
785
233-
803.
9940
0.06
080.
2848
0.00
360.
1017
0.00
1416
32.9
824
.87
1615
.29
20.6
916
55.9
623
.09
1655
.96
23.0
985
233-
504.
2460
0.06
170.
3002
0.00
350.
1026
0.00
1616
82.9
624
.44
1692
.11
19.7
816
71.7
225
.99
1671
.72
25.9
985
233-
304.
5967
0.11
400.
3074
0.00
710.
1084
0.00
2817
48.6
643
.37
1728
.12
40.0
117
73.2
645
.03
1773
.26
45.0
385
233-
944.
8977
0.11
870.
3269
0.00
750.
1087
0.00
2918
01.8
643
.69
1823
.36
41.7
617
77.3
147
.28
1777
.31
47.2
885
233-
325.
0110
0.11
120.
3219
0.00
650.
1129
0.00
2718
21.1
840
.42
1798
.82
36.2
618
46.7
744
.26
1846
.77
44.2
685
233-
355.
2035
0.09
590.
3326
0.00
600.
1135
0.00
2218
53.2
034
.14
1851
.19
33.1
718
55.4
135
.98
1855
.41
35.9
885
233-
335.
0362
0.09
930.
3218
0.00
580.
1135
0.00
2518
25.4
336
.01
1798
.59
32.5
418
56.1
641
.06
1856
.16
41.0
6
App
endi
x C
-1 c
ontin
ued
-- S
ampl
e 05
WK
W08
2A
Cor
rect
ed Is
otop
ic R
atio
sA
ges
(Ma)
186 PIR 2008-1 Preliminary results, fi eld investigations, Brooks Range Foothils & North Slope, Alaska
Ana
lysi
s #
207 P
b / 23
5 U±2
206 P
b / 23
8 U±2
207 P
b / 20
6 Pb
±220
7 Pb
/ 235 U
±220
6 Pb
/ 238 U
±220
7 Pb
/ 206 P
b±2
Bes
t Age
±248
502-
110.
2246
0.01
570.
0405
0.00
060.
0000
0.00
000.
000.
000.
000.
000.
000.
000.
000.
0048
502-
760.
1206
0.01
010.
0182
0.00
090.
0482
0.00
4111
5.65
9.70
116.
055.
9010
7.42
9.21
116.
055.
9048
502-
830.
1255
0.00
390.
0188
0.00
040.
0485
0.00
1512
0.00
3.71
119.
832.
2512
4.21
3.83
119.
832.
2548
502-
370.
1292
0.00
940.
0193
0.00
040.
0485
0.00
3612
3.39
8.94
123.
332.
3412
4.74
9.20
123.
332.
3448
502-
330.
1403
0.00
440.
0209
0.00
030.
0487
0.00
1513
3.33
4.14
133.
242.
1613
5.04
4.24
133.
242.
1648
502-
580.
1888
0.04
200.
0272
0.00
290.
0503
0.01
2317
5.62
39.1
117
3.21
18.1
720
8.36
51.0
317
3.21
18.1
748
502-
940.
2268
0.01
880.
0326
0.00
070.
0504
0.00
4320
7.58
17.2
220
7.01
4.53
214.
1618
.23
207.
014.
5348
502-
130.
2399
0.01
170.
0343
0.00
110.
0507
0.00
2621
8.30
10.6
221
7.35
6.79
228.
4211
.59
217.
356.
7948
502-
340.
2643
0.00
770.
0376
0.00
050.
0510
0.00
1523
8.14
6.97
237.
753.
3424
2.16
7.05
237.
753.
3448
502-
90.
2747
0.01
610.
0389
0.00
070.
0513
0.00
3124
6.47
14.4
424
5.71
4.73
253.
4515
.21
245.
714.
7348
502-
180.
2758
0.01
350.
0390
0.00
060.
0512
0.00
2624
7.35
12.0
824
6.85
3.69
251.
5412
.60
246.
853.
6948
502-
220.
2747
0.01
650.
0392
0.00
130.
0508
0.00
3224
6.42
14.8
324
7.95
7.95
232.
0114
.52
247.
957.
9548
502-
190.
2750
0.03
290.
0394
0.00
170.
0507
0.00
6324
6.71
29.5
024
8.81
10.9
322
7.05
28.2
624
8.81
10.9
348
502-
30.
2824
0.01
350.
0398
0.00
060.
0514
0.00
2525
2.57
12.0
425
1.83
4.08
259.
4012
.82
251.
834.
0848
502-
20.
2867
0.01
210.
0404
0.00
080.
0515
0.00
2325
5.94
10.8
325
5.05
5.15
264.
0511
.61
255.
055.
1548
502-
60.
2883
0.00
760.
0406
0.00
050.
0515
0.00
1425
7.25
6.76
256.
753.
1326
1.76
7.11
256.
753.
1348
502-
150.
2939
0.02
440.
0412
0.00
120.
0517
0.00
4426
1.63
21.7
526
0.28
7.87
273.
7423
.33
260.
287.
8748
502-
630.
2914
0.03
000.
0412
0.00
110.
0513
0.00
5425
9.66
26.7
126
0.49
6.92
252.
3026
.54
260.
496.
9248
502-
520.
2960
0.03
240.
0415
0.00
120.
0518
0.00
5826
3.28
28.7
926
2.08
7.27
274.
8130
.76
262.
087.
2748
502-
480.
2965
0.02
470.
0415
0.00
110.
0518
0.00
4426
3.65
22.0
026
2.33
7.24
275.
6523
.66
262.
337.
2448
502-
810.
2968
0.01
040.
0416
0.00
100.
0518
0.00
1926
3.93
9.20
262.
806.
4627
4.81
10.2
326
2.80
6.46
4850
2-44
0.29
920.
0287
0.04
190.
0009
0.05
170.
0051
265.
7725
.54
264.
865.
7227
3.90
26.8
326
4.86
5.72
4850
2-25
0.30
260.
0393
0.04
230.
0013
0.05
190.
0069
268.
4734
.87
266.
838.
2628
2.90
37.5
126
6.83
8.26
4850
2-56
0.30
280.
0240
0.04
230.
0014
0.05
200.
0043
268.
6021
.29
266.
908.
8028
3.66
23.6
326
6.90
8.80
4850
2-26
0.30
660.
0620
0.04
270.
0016
0.05
210.
0107
271.
5754
.92
269.
5610
.17
289.
1559
.22
269.
5610
.17
4850
2-16
0.30
470.
0123
0.04
280.
0007
0.05
160.
0021
270.
0510
.88
270.
424.
3326
6.80
11.0
127
0.42
4.33
4850
2-35
0.31
040.
0139
0.04
340.
0006
0.05
190.
0024
274.
5112
.31
273.
814.
0528
0.61
12.9
127
3.81
4.05
4850
2-87
0.31
150.
0099
0.04
360.
0004
0.05
180.
0017
275.
388.
7927
5.41
2.62
275.
278.
9727
5.41
2.62
4850
2-17
0.31
780.
0440
0.04
450.
0019
0.05
180.
0074
280.
1938
.77
280.
4011
.93
278.
4739
.77
280.
4011
.93
4850
2-64
0.32
320.
0153
0.04
520.
0011
0.05
190.
0025
284.
3413
.43
285.
006.
9327
9.16
13.6
528
5.00
6.93
4850
2-66
0.32
820.
0229
0.04
550.
0011
0.05
230.
0038
288.
2120
.10
286.
907.
0329
8.77
21.4
428
6.90
7.03
4850
2-21
0.33
020.
0425
0.04
560.
0018
0.05
250.
0070
289.
7537
.25
287.
4711
.38
308.
3041
.05
287.
4711
.38
4850
2-8
0.35
200.
0127
0.04
850.
0006
0.05
260.
0019
306.
2111
.07
305.
613.
9431
0.75
11.5
030
5.61
3.94
4850
2-95
0.36
020.
0058
0.04
950.
0006
0.05
270.
0009
312.
385.
0531
1.66
3.53
317.
925.
3731
1.66
3.53
4850
2-86
0.36
690.
0258
0.05
020.
0011
0.05
300.
0039
317.
3522
.33
315.
777.
1032
8.90
23.9
531
5.77
7.10
App
endi
x C
-1 c
ontin
ued
Sam
ple
02M
AW
009
(upp
erm
ost F
ortr
ess
Mou
ntai
n Fo
rmat
ion)
Cor
rect
ed Is
otop
ic R
atio
sA
ges
(Ma)
Appendix C-1 187
Ana
lysi
s #
207 P
b / 23
5 U±2
206 P
b / 23
8 U±2
207 P
b / 20
6 Pb
±220
7 Pb
/ 235 U
±220
6 Pb
/ 238 U
±220
7 Pb
/ 206 P
b±2
Bes
t Age
± M
a (2
)48
502-
390.
3748
0.00
770.
0515
0.00
050.
0528
0.00
1232
3.20
6.62
323.
543.
4132
0.82
7.02
323.
543.
4148
502-
550.
3892
0.02
360.
0528
0.00
140.
0535
0.00
3433
3.81
20.2
833
1.70
9.07
348.
4322
.46
331.
709.
0748
502-
970.
3903
0.05
620.
0537
0.00
330.
0527
0.00
7833
4.57
48.1
733
7.36
20.7
331
5.09
46.7
933
7.36
20.7
348
502-
620.
4019
0.03
260.
0542
0.00
200.
0538
0.00
4534
2.99
27.8
634
0.08
12.4
636
3.01
30.6
534
0.08
12.4
648
502-
670.
4213
0.01
690.
0573
0.00
140.
0533
0.00
2335
6.96
14.2
835
9.18
8.69
343.
4014
.54
359.
188.
6948
502-
690.
4558
0.02
200.
0605
0.00
140.
0546
0.00
2738
1.32
18.3
837
8.97
8.45
395.
6619
.88
378.
978.
4548
502-
420.
4637
0.01
460.
0617
0.00
070.
0545
0.00
1738
6.81
12.1
938
5.77
4.08
393.
3012
.62
385.
774.
0848
502-
410.
4813
0.01
890.
0642
0.00
100.
0544
0.00
2239
8.99
15.6
840
0.85
6.19
388.
2616
.05
400.
856.
1948
502-
740.
4950
0.02
780.
0647
0.00
200.
0555
0.00
3440
8.29
22.9
540
4.38
12.7
643
0.68
26.3
040
4.38
12.7
648
502-
70.
4964
0.01
760.
0651
0.00
140.
0553
0.00
2040
9.25
14.5
540
6.66
8.84
423.
8915
.61
406.
668.
8448
502-
310.
5148
0.02
000.
0673
0.00
100.
0555
0.00
2242
1.64
16.3
642
0.01
5.99
430.
6817
.03
420.
015.
9948
502-
730.
5318
0.02
770.
0697
0.00
130.
0553
0.00
3043
3.03
22.5
643
4.63
8.08
424.
7322
.74
434.
638.
0848
502-
650.
5445
0.03
880.
0707
0.00
280.
0559
0.00
4244
1.38
31.4
344
0.09
17.3
644
8.91
33.6
744
0.09
17.3
648
502-
820.
5493
0.02
260.
0711
0.00
120.
0561
0.00
2444
4.51
18.2
944
2.51
7.26
455.
0219
.36
442.
517.
2648
502-
430.
5511
0.02
390.
0713
0.00
110.
0561
0.00
2544
5.75
19.3
344
4.03
6.62
454.
7920
.38
444.
036.
6248
502-
960.
5559
0.03
060.
0717
0.00
130.
0562
0.00
3244
8.83
24.7
444
6.53
8.08
460.
5126
.29
446.
538.
0848
502-
140.
5602
0.02
320.
0721
0.00
150.
0564
0.00
2445
1.65
18.6
744
8.74
9.58
466.
5419
.98
448.
749.
5848
502-
320.
5665
0.01
470.
0730
0.00
080.
0563
0.00
1545
5.72
11.8
645
4.31
5.07
462.
9512
.45
454.
315.
0748
502-
270.
5832
0.01
970.
0751
0.00
130.
0564
0.00
1946
6.51
15.7
246
6.58
7.93
466.
3116
.07
466.
587.
9348
502-
450.
5920
0.01
680.
0755
0.00
150.
0569
0.00
1747
2.16
13.3
846
9.15
9.16
487.
0614
.51
469.
159.
1648
502-
930.
6107
0.02
220.
0777
0.00
100.
0570
0.00
2148
3.99
17.5
848
2.17
6.23
492.
7818
.57
482.
176.
2348
502-
750.
6129
0.01
990.
0778
0.00
130.
0572
0.00
1948
5.41
15.7
648
2.86
7.97
497.
6716
.87
482.
867.
9748
502-
720.
6122
0.01
710.
0779
0.00
090.
0570
0.00
1648
4.92
13.5
248
3.38
5.33
492.
4014
.20
483.
385.
3348
502-
980.
6277
0.02
130.
0793
0.00
140.
0574
0.00
2049
4.65
16.7
949
1.71
8.63
508.
4217
.98
491.
718.
6348
502-
910.
6361
0.02
780.
0803
0.00
150.
0574
0.00
2549
9.90
21.8
649
7.97
9.38
508.
8022
.52
497.
979.
3848
502-
510.
7650
0.07
710.
0927
0.00
230.
0599
0.00
6157
6.89
58.1
257
1.58
13.8
959
8.53
61.3
457
1.58
13.8
948
502-
10.
8221
0.02
750.
0983
0.00
190.
0607
0.00
2160
9.21
20.3
960
4.29
11.7
662
7.44
21.5
160
4.29
11.7
648
502-
681.
1222
0.03
330.
1248
0.00
210.
0652
0.00
2076
4.04
22.6
875
8.41
12.7
278
0.72
23.7
775
8.41
12.7
248
502-
291.
2781
0.03
890.
1385
0.00
270.
0669
0.00
2083
6.02
25.4
483
6.03
16.2
783
6.11
25.0
883
6.03
16.2
748
502-
701.
4159
0.04
080.
1470
0.00
330.
0699
0.00
2189
5.62
25.8
488
4.20
20.0
892
4.07
27.9
688
4.20
20.0
848
502-
100
1.48
100.
0315
0.15
250.
0022
0.07
050.
0016
922.
6619
.60
914.
7713
.47
941.
7021
.48
914.
7713
.47
4850
2-71
1.50
120.
0510
0.15
720.
0028
0.06
930.
0025
930.
8731
.60
940.
9616
.92
906.
9832
.56
940.
9616
.92
4850
2-57
1.55
290.
0668
0.15
770.
0036
0.07
140.
0031
951.
6640
.91
944.
2121
.27
968.
8642
.62
944.
2121
.27
4850
2-20
1.55
690.
0477
0.15
930.
0036
0.07
090.
0023
953.
2329
.18
952.
9021
.39
954.
0631
.36
952.
9021
.39
4850
2-80
2.01
260.
0563
0.18
770.
0027
0.07
780.
0023
1119
.78
31.3
411
08.9
116
.17
1140
.86
33.3
711
40.8
633
.37
4850
2-49
2.03
860.
0629
0.18
770.
0050
0.07
880.
0026
1128
.51
34.8
011
08.9
129
.41
1166
.61
38.7
711
66.6
138
.77
Cor
rect
ed Is
otop
ic R
atio
sA
ges
(Ma)
App
endi
x C
-1 c
ontin
ued
-- S
ampl
e 02
MA
W00
9
188 PIR 2008-1 Preliminary results, fi eld investigations, Brooks Range Foothils & North Slope, Alaska
Ana
lysi
s #
207 P
b / 23
5 U±2
206 P
b / 23
8 U±2
207 P
b / 20
6 Pb
±220
7 Pb
/ 235 U
±220
6 Pb
/ 238 U
±220
7 Pb
/ 206 P
b±2
Bes
t Age
± M
a (2
)48
502-
52.
2718
0.05
780.
2030
0.00
300.
0812
0.00
2112
03.5
830
.64
1191
.36
17.3
812
25.5
531
.17
1225
.55
31.1
748
502-
842.
5447
0.05
700.
2185
0.00
270.
0845
0.00
2012
84.9
128
.80
1273
.88
15.5
313
03.5
630
.12
1303
.56
30.1
248
502-
902.
9043
0.07
350.
2365
0.00
330.
0891
0.00
2413
83.0
435
.02
1368
.27
18.8
414
05.7
938
.01
1405
.79
38.0
148
502-
463.
0699
0.11
080.
2413
0.00
690.
0923
0.00
3714
25.2
051
.45
1393
.50
39.5
614
73.0
158
.71
1473
.01
58.7
148
502-
473.
3507
0.05
220.
2586
0.00
230.
0940
0.00
1614
92.9
623
.24
1482
.42
13.2
115
08.1
024
.94
1508
.10
24.9
448
502-
543.
6430
0.06
260.
2767
0.00
380.
0955
0.00
1815
58.9
726
.77
1574
.74
21.8
515
37.6
728
.89
1537
.67
28.8
948
502-
363.
5448
0.04
810.
2668
0.00
280.
0964
0.00
1315
37.2
820
.88
1524
.35
15.9
215
55.2
121
.62
1555
.21
21.6
248
502-
923.
6982
0.08
960.
2765
0.00
460.
0970
0.00
2515
70.9
738
.08
1573
.81
26.3
215
67.0
839
.61
1567
.08
39.6
148
502-
793.
9885
0.06
030.
2874
0.00
380.
1007
0.00
1616
31.8
624
.68
1628
.31
21.8
016
36.3
925
.77
1636
.39
25.7
748
502-
403.
9827
0.07
570.
2845
0.00
340.
1015
0.00
2116
30.6
830
.98
1613
.85
19.3
616
52.6
033
.77
1652
.60
33.7
748
502-
504.
0582
0.10
950.
2882
0.00
660.
1021
0.00
2916
45.9
544
.42
1632
.78
37.1
916
62.7
946
.72
1662
.79
46.7
248
502-
534.
2542
0.05
840.
3006
0.00
390.
1026
0.00
1416
84.5
523
.11
1694
.30
22.1
816
72.3
322
.66
1672
.33
22.6
648
502-
774.
9632
0.09
580.
3268
0.00
410.
1101
0.00
2218
13.0
835
.01
1822
.96
22.7
618
01.6
435
.86
1801
.64
35.8
648
502-
614.
6768
0.22
510.
3080
0.00
630.
1101
0.00
5217
63.1
084
.88
1730
.76
35.5
118
01.6
885
.23
1801
.68
85.2
348
502-
894.
9786
0.10
870.
3277
0.00
490.
1102
0.00
2518
15.6
939
.64
1827
.26
27.0
718
02.3
541
.70
1802
.35
41.7
048
502-
244.
8253
0.07
470.
3147
0.00
490.
1112
0.00
1817
89.3
227
.71
1764
.03
27.5
718
19.0
030
.20
1819
.00
30.2
048
502-
105.
0801
0.10
180.
3272
0.00
420.
1126
0.00
2318
32.7
936
.71
1824
.82
23.5
418
41.8
537
.88
1841
.85
37.8
848
502-
605.
1463
0.08
290.
3270
0.00
390.
1142
0.00
1918
43.7
929
.72
1823
.58
21.8
018
66.8
030
.63
1866
.80
30.6
348
502-
385.
2663
0.10
280.
3307
0.00
430.
1155
0.00
2418
63.4
136
.37
1841
.59
24.0
518
87.9
739
.40
1887
.97
39.4
048
502-
125.
8684
0.10
380.
3494
0.00
470.
1218
0.00
2219
56.5
734
.59
1931
.69
25.7
219
82.9
936
.59
1982
.99
36.5
948
502-
305.
9316
0.13
610.
3513
0.00
470.
1225
0.00
2919
65.8
745
.12
1940
.99
26.0
119
92.3
047
.66
1992
.30
47.6
648
502-
786.
0442
0.16
470.
3544
0.00
670.
1237
0.00
3519
82.2
454
.02
1955
.62
36.9
420
10.2
356
.63
2010
.23
56.6
348
502-
996.
6795
0.09
740.
3765
0.00
570.
1287
0.00
2220
69.9
130
.19
2059
.94
31.2
420
79.9
335
.59
2079
.93
35.5
948
502-
287.
5484
0.14
010.
3988
0.00
610.
1373
0.00
2721
78.7
540
.43
2163
.61
33.1
021
93.2
042
.51
2193
.20
42.5
148
502-
597.
7740
0.14
600.
4071
0.00
720.
1385
0.00
2822
05.2
041
.41
2201
.78
38.7
022
08.3
744
.59
2208
.37
44.5
948
502-
858.
1958
0.20
500.
4111
0.00
580.
1446
0.00
3622
52.8
856
.35
2219
.98
31.5
522
82.8
757
.63
2282
.87
57.6
348
502-
888.
5974
0.25
860.
4210
0.00
900.
1481
0.00
4622
96.2
969
.08
2265
.11
48.2
423
24.0
771
.60
2324
.07
71.6
048
502-
239.
1388
0.20
320.
4452
0.00
860.
1489
0.00
3623
52.0
152
.29
2373
.71
46.0
023
33.3
256
.39
2333
.32
56.3
948
502-
411
.170
20.
1585
0.48
550.
0065
0.16
690.
0024
2537
.43
36.0
025
51.2
233
.94
2526
.59
36.8
225
26.5
936
.82
App
endi
x C
-1 c
ontin
ued
-- S
ampl
e 02
MA
W00
9
Cor
rect
ed Is
otop
ic R
atio
sA
ges
(Ma)
Appendix C-1 189
Ana
lysi
s #
207 P
b / 23
5 U±2
206 P
b / 23
8 U±2
207 P
b / 20
6 Pb
±220
7 Pb
/ 235 U
±220
6 Pb
/ 238 U
±220
7 Pb
/ 206 P
b±2
Bes
t Age
±248
506-
360.
1378
0.00
600.
0206
0.00
030.
0486
0.00
2113
1.10
5.70
131.
301.
9012
7.49
5.63
131.
301.
9048
506-
600.
1823
0.02
080.
0266
0.00
080.
0497
0.00
5817
0.02
19.3
616
9.37
4.83
178.
9920
.86
169.
374.
8348
506-
840.
2242
0.01
860.
0323
0.00
080.
0504
0.00
4320
5.43
17.0
520
4.66
4.94
214.
2318
.25
204.
664.
9448
506-
230.
2730
0.02
390.
0389
0.00
140.
0509
0.00
4624
5.07
21.4
924
5.88
8.94
237.
7321
.37
245.
888.
9448
506-
730.
2884
0.01
510.
0406
0.00
070.
0515
0.00
2725
7.27
13.4
525
6.77
4.22
262.
6014
.00
256.
774.
2248
506-
620.
2896
0.01
450.
0408
0.00
060.
0515
0.00
2625
8.28
12.9
125
7.79
3.67
263.
3713
.42
257.
793.
6748
506-
210.
2921
0.02
510.
0412
0.00
270.
0514
0.00
4526
0.20
22.3
226
0.46
17.0
825
7.80
22.4
726
0.46
17.0
848
506-
70.
2941
0.01
300.
0413
0.00
060.
0516
0.00
2326
1.79
11.5
526
1.11
3.62
267.
9412
.14
261.
113.
6248
506-
30.
2979
0.01
080.
0418
0.00
070.
0517
0.00
1926
4.77
9.64
264.
144.
3027
0.39
9.97
264.
144.
3048
506-
930.
2985
0.01
460.
0419
0.00
080.
0517
0.00
2626
5.25
12.9
926
4.36
5.26
273.
5913
.89
264.
365.
2648
506-
190.
3052
0.02
030.
0428
0.00
070.
0518
0.00
3527
0.49
17.9
726
9.94
4.29
275.
8018
.41
269.
944.
2948
506-
550.
3067
0.02
110.
0429
0.00
080.
0519
0.00
3627
1.65
18.6
527
0.67
5.33
280.
1519
.66
270.
675.
3348
506-
310.
3094
0.01
310.
0432
0.00
100.
0520
0.00
2327
3.75
11.5
627
2.48
6.20
284.
7312
.43
272.
486.
2048
506-
920.
3131
0.01
620.
0436
0.00
110.
0520
0.00
2727
6.60
14.2
927
5.31
7.16
287.
5515
.11
275.
317.
1648
506-
300.
3143
0.02
140.
0437
0.00
150.
0522
0.00
3727
7.50
18.9
127
5.70
9.28
292.
7420
.90
275.
709.
2848
506-
290.
3111
0.01
640.
0439
0.00
150.
0514
0.00
2827
4.99
14.4
827
7.05
9.40
257.
8713
.83
277.
059.
4048
506-
750.
3221
0.03
870.
0446
0.00
160.
0524
0.00
6528
3.55
34.0
528
1.42
10.2
430
1.28
37.3
828
1.42
10.2
448
506-
110.
3235
0.01
030.
0449
0.00
110.
0522
0.00
1928
4.57
9.07
283.
256.
8229
5.41
10.6
028
3.25
6.82
4850
6-18
0.32
780.
0207
0.04
530.
0017
0.05
250.
0035
287.
9118
.17
285.
8010
.70
305.
4820
.46
285.
8010
.70
4850
6-12
0.32
480.
0162
0.04
550.
0007
0.05
180.
0027
285.
6114
.26
286.
594.
4527
7.56
14.2
728
6.59
4.45
4850
6-51
0.33
880.
0281
0.04
730.
0013
0.05
190.
0044
296.
2924
.58
298.
138.
3328
1.91
23.9
029
8.13
8.33
4850
6-66
0.34
540.
0188
0.04
740.
0020
0.05
280.
0030
301.
2616
.42
298.
5412
.71
322.
3418
.59
298.
5412
.71
4850
6-25
0.35
020.
0248
0.04
820.
0010
0.05
270.
0038
304.
8721
.61
303.
606.
4031
4.64
22.8
230
3.60
6.40
4850
6-49
0.35
120.
0142
0.04
840.
0008
0.05
260.
0021
305.
6312
.35
304.
855.
2231
1.58
12.6
530
4.85
5.22
4850
6-81
0.37
240.
0197
0.05
130.
0015
0.05
270.
0029
321.
4516
.99
322.
499.
5731
4.10
17.2
032
2.49
9.57
4850
6-95
0.37
370.
0207
0.05
150.
0009
0.05
270.
0030
322.
4317
.85
323.
615.
8531
4.10
18.0
932
3.61
5.85
4850
6-40
0.38
010.
0157
0.05
190.
0007
0.05
310.
0022
327.
1013
.51
326.
364.
1333
2.41
13.9
832
6.36
4.13
4850
6-87
0.38
230.
0125
0.05
220.
0007
0.05
310.
0018
328.
7410
.72
327.
834.
1533
5.16
11.3
932
7.83
4.15
4850
6-45
0.40
620.
0353
0.05
520.
0022
0.05
340.
0047
346.
1230
.07
346.
3013
.63
345.
0030
.25
346.
3013
.63
4850
6-5
0.42
790.
0220
0.05
730.
0015
0.05
410.
0028
361.
6918
.62
359.
369.
1437
6.66
19.7
735
9.36
9.14
4850
6-38
0.43
610.
0214
0.05
850.
0009
0.05
410.
0027
367.
5318
.02
366.
355.
5537
5.21
18.8
336
6.35
5.55
4850
6-94
0.44
420.
0245
0.05
920.
0013
0.05
440.
0031
373.
1920
.56
371.
018.
0638
6.66
22.3
737
1.01
8.06
4850
6-44
0.44
360.
0201
0.05
930.
0010
0.05
430.
0025
372.
7616
.91
371.
146.
0538
2.84
17.7
837
1.14
6.05
4850
6-2
0.48
980.
0458
0.06
510.
0015
0.05
460.
0052
404.
8137
.88
406.
499.
0939
5.66
37.7
240
6.49
9.09
4850
6-48
0.49
610.
0225
0.06
580.
0016
0.05
470.
0026
409.
0418
.55
411.
0010
.27
397.
9518
.73
411.
0010
.27
App
endi
x C
-1 c
ontin
ued
Sam
ple
02M
AW
020
(upp
er-m
iddl
e Fo
rtre
ss M
ount
ain
Form
atio
n)C
orre
cted
Isot
opic
Rat
ios
Age
s (M
a)
190 PIR 2008-1 Preliminary results, fi eld investigations, Brooks Range Foothils & North Slope, Alaska
Ana
lysi
s #
207 P
b / 23
5 U±2
206 P
b / 23
8 U±2
207 P
b / 20
6 Pb
±220
7 Pb
/ 235 U
±220
6 Pb
/ 238 U
±220
7 Pb
/ 206 P
b±2
Bes
t Age
± M
a (2
)48
506-
420.
5290
0.02
770.
0686
0.00
190.
0559
0.00
3043
1.15
22.5
642
7.56
11.6
045
0.36
24.3
442
7.56
11.6
048
506-
680.
5427
0.03
240.
0704
0.00
140.
0559
0.00
3444
0.21
26.2
443
8.37
8.54
449.
9127
.74
438.
378.
5448
506-
10.
5617
0.01
210.
0728
0.00
070.
0560
0.00
1245
2.61
9.75
452.
744.
3645
2.19
10.0
245
2.74
4.36
4850
6-35
0.58
690.
0241
0.07
510.
0011
0.05
670.
0024
468.
8619
.27
466.
946.
6047
8.29
20.4
246
6.94
6.60
4850
6-99
0.63
540.
0374
0.08
010.
0015
0.05
760.
0035
499.
4529
.42
496.
439.
5551
3.23
31.1
549
6.43
9.55
4850
6-9
0.64
690.
0223
0.08
120.
0016
0.05
780.
0021
506.
5917
.50
503.
389.
8652
1.01
19.0
150
3.38
9.86
4850
6-50
0.69
650.
0259
0.08
750.
0020
0.05
770.
0023
536.
6919
.99
540.
7812
.65
519.
3320
.64
540.
7812
.65
4850
6-63
0.73
750.
0291
0.09
150.
0018
0.05
840.
0024
560.
9422
.11
564.
5311
.33
546.
3422
.53
564.
5311
.33
4850
6-8
0.75
860.
0173
0.09
240.
0015
0.05
950.
0014
573.
1813
.10
569.
679.
4758
7.23
13.7
356
9.67
9.47
4850
6-65
0.77
020.
0349
0.09
350.
0015
0.05
970.
0028
579.
8726
.26
576.
179.
2759
4.41
27.8
457
6.17
9.27
4850
6-56
0.77
250.
0253
0.09
400.
0010
0.05
960.
0020
581.
1819
.04
579.
166.
0458
9.07
19.8
257
9.16
6.04
4850
6-52
0.79
990.
0252
0.09
630.
0016
0.06
020.
0020
596.
7618
.80
592.
879.
9261
1.57
20.0
959
2.87
9.92
4850
6-41
0.81
530.
0176
0.09
810.
0010
0.06
030.
0013
605.
4213
.08
603.
116.
0961
4.09
13.6
260
3.11
6.09
4850
6-88
0.81
960.
0236
0.09
830.
0013
0.06
050.
0018
607.
8217
.48
604.
557.
7461
9.96
18.6
760
4.55
7.74
4850
6-47
0.82
140.
0327
0.09
960.
0031
0.05
980.
0025
608.
8524
.27
612.
1919
.11
596.
4725
.42
612.
1919
.11
4850
6-58
0.82
820.
1232
0.10
050.
0038
0.05
980.
0091
612.
6291
.16
617.
4223
.24
595.
0990
.27
617.
4223
.24
4850
6-13
0.87
390.
0226
0.10
360.
0010
0.06
120.
0016
637.
7116
.52
635.
306.
1864
6.36
17.1
063
5.30
6.18
4850
6-37
0.87
740.
0235
0.10
370.
0014
0.06
140.
0017
639.
6017
.15
635.
908.
5265
2.77
17.7
363
5.90
8.52
4850
6-77
0.91
680.
0249
0.10
740.
0016
0.06
190.
0017
660.
6517
.96
657.
759.
9267
0.47
18.7
565
7.75
9.92
4850
6-79
0.98
630.
0722
0.11
230.
0054
0.06
370.
0048
696.
8351
.01
686.
3633
.22
730.
6755
.63
686.
3633
.22
4850
6-43
1.09
440.
0220
0.12
270.
0018
0.06
470.
0014
750.
6315
.08
746.
3511
.22
763.
4716
.14
746.
3511
.22
4850
6-54
1.11
200.
0168
0.12
430.
0013
0.06
490.
0011
759.
1611
.50
755.
187.
8777
0.95
12.7
475
5.18
7.87
4850
6-6
1.11
550.
0465
0.12
620.
0028
0.06
410.
0028
760.
8031
.70
766.
1216
.83
745.
1632
.44
766.
1216
.83
4850
6-69
1.22
880.
0594
0.13
350.
0038
0.06
680.
0033
813.
8039
.31
807.
8723
.01
830.
1540
.72
807.
8723
.01
4850
6-46
1.45
510.
0563
0.15
050.
0024
0.07
010.
0028
911.
9735
.29
903.
8114
.20
931.
7836
.99
903.
8114
.20
4850
6-27
1.53
340.
0458
0.15
620.
0041
0.07
120.
0022
943.
8528
.18
935.
4224
.71
963.
6729
.61
935.
4224
.71
4850
6-32
1.55
250.
0364
0.15
910.
0026
0.07
080.
0017
951.
4722
.31
951.
6215
.38
951.
4623
.18
951.
6215
.38
4850
6-15
1.60
430.
0273
0.16
150.
0020
0.07
210.
0013
971.
8916
.53
964.
9811
.79
987.
5917
.92
964.
9811
.79
4850
6-28
1.61
040.
0933
0.16
310.
0048
0.07
160.
0044
974.
2556
.46
973.
8628
.52
975.
1159
.23
973.
8628
.52
4850
6-26
1.63
490.
0439
0.16
350.
0022
0.07
250.
0020
983.
7626
.44
976.
2012
.95
1000
.75
27.6
510
00.7
527
.65
4850
6-78
1.83
290.
0530
0.17
930.
0041
0.07
410.
0023
1057
.33
30.5
610
63.1
224
.26
1045
.34
31.8
010
45.3
431
.80
4850
6-83
1.96
730.
0410
0.18
760.
0029
0.07
610.
0016
1104
.39
22.9
911
08.2
217
.14
1096
.76
23.6
810
96.7
623
.68
4850
6-39
1.94
760.
0504
0.18
430.
0019
0.07
660.
0020
1097
.63
28.4
010
90.4
111
.07
1111
.98
29.5
411
11.9
829
.54
4850
6-14
2.04
260.
0623
0.19
190.
0031
0.07
720.
0025
1129
.82
34.4
611
31.4
818
.37
1126
.63
36.0
411
26.6
336
.04
4850
6-76
2.19
170.
0306
0.19
920.
0019
0.07
980.
0012
1178
.40
16.4
411
70.9
711
.30
1192
.02
18.0
111
92.0
218
.01
4850
6-82
2.75
960.
0535
0.22
920.
0032
0.08
730.
0019
1344
.70
26.0
713
30.4
418
.77
1367
.42
29.1
013
67.4
229
.10
App
endi
x C
-1 c
ontin
ued
-- S
ampl
e 02
MA
W02
0
Cor
rect
ed Is
otop
ic R
atio
sA
ges
(Ma)
Appendix C-1 191
Ana
lysi
s #
207 P
b / 23
5 U±2
206 P
b / 23
8 U±2
207 P
b / 20
6 Pb
±220
7 Pb
/ 235 U
±220
6 Pb
/ 238 U
±220
7 Pb
/ 206 P
b±2
Bes
t Age
± M
a (2
)48
506-
202.
9801
0.07
210.
2399
0.00
360.
0901
0.00
2314
02.5
633
.95
1386
.29
20.6
514
27.3
835
.85
1427
.38
35.8
548
506-
853.
0314
0.04
600.
2433
0.00
320.
0904
0.00
1514
15.5
521
.50
1403
.77
18.5
614
33.2
223
.84
1433
.22
23.8
448
506-
103.
1564
0.12
940.
2520
0.01
040.
0908
0.00
3914
46.5
659
.28
1448
.92
59.8
014
43.0
662
.50
1443
.06
62.5
048
506-
343.
6017
0.05
640.
2732
0.00
360.
0956
0.00
1515
49.9
024
.25
1557
.06
20.5
815
40.1
824
.90
1540
.18
24.9
048
506-
593.
5738
0.08
730.
2652
0.00
570.
0977
0.00
1915
43.7
237
.69
1516
.25
32.8
515
81.6
131
.02
1581
.61
31.0
248
506-
803.
7864
0.08
780.
2767
0.00
460.
0992
0.00
2315
89.8
636
.88
1574
.93
26.4
116
09.6
537
.48
1609
.65
37.4
848
506-
743.
8319
0.06
110.
2784
0.00
330.
0998
0.00
1715
99.4
825
.49
1583
.45
18.8
116
20.6
427
.45
1620
.64
27.4
548
506-
644.
2236
0.13
820.
3062
0.00
930.
1000
0.00
3316
78.6
354
.92
1722
.13
52.2
016
24.6
453
.49
1624
.64
53.4
948
506-
334.
0276
0.05
710.
2880
0.00
330.
1014
0.00
1516
39.7
823
.26
1631
.56
18.6
916
50.3
124
.75
1650
.31
24.7
548
506-
674.
0071
0.11
920.
2866
0.00
440.
1014
0.00
3116
35.6
348
.65
1624
.27
25.0
916
50.3
949
.67
1650
.39
49.6
748
506-
163.
9999
0.08
150.
2839
0.00
480.
1022
0.00
2216
34.1
833
.30
1611
.07
27.0
816
64.1
236
.21
1664
.12
36.2
148
506-
864.
2897
0.06
780.
3003
0.00
270.
1036
0.00
1716
91.3
926
.74
1692
.91
15.3
516
89.4
528
.16
1689
.45
28.1
648
506-
704.
2633
0.11
580.
2977
0.00
460.
1039
0.00
2716
86.3
045
.78
1679
.67
26.1
216
94.4
944
.75
1694
.49
44.7
548
506-
964.
5704
0.04
730.
3073
0.00
330.
1079
0.00
1317
43.8
918
.05
1727
.30
18.7
317
63.7
621
.20
1763
.76
21.2
048
506-
974.
6795
0.04
670.
3120
0.00
260.
1088
0.00
1217
63.5
817
.60
1750
.66
14.5
317
78.8
318
.89
1778
.83
18.8
948
506-
714.
7561
0.11
910.
3133
0.00
400.
1101
0.00
2817
77.1
944
.51
1757
.16
22.2
618
00.7
745
.91
1800
.77
45.9
148
506-
984.
8540
0.05
850.
3180
0.00
280.
1107
0.00
1417
94.3
121
.62
1780
.15
15.6
618
10.6
822
.17
1810
.68
22.1
748
506-
244.
9630
0.07
680.
3251
0.00
460.
1107
0.00
1918
13.0
428
.05
1814
.79
25.7
918
10.8
430
.66
1810
.84
30.6
648
506-
724.
8294
0.09
350.
3164
0.00
360.
1107
0.00
2217
90.0
434
.67
1772
.09
19.9
518
10.9
936
.36
1810
.99
36.3
648
506-
224.
8339
0.14
000.
3154
0.00
740.
1112
0.00
3317
90.8
151
.85
1767
.28
41.2
018
18.7
353
.99
1818
.73
53.9
948
506-
895.
0485
0.08
120.
3243
0.00
480.
1129
0.00
1918
27.4
929
.41
1810
.64
27.0
518
46.7
031
.55
1846
.70
31.5
548
506-
100
5.21
580.
0558
0.33
460.
0032
0.11
310.
0013
1855
.21
19.8
318
60.4
217
.58
1849
.29
21.2
618
49.2
921
.26
4850
6-53
5.37
840.
1552
0.33
420.
0054
0.11
670.
0035
1881
.42
54.3
018
58.4
929
.76
1906
.95
57.3
519
06.9
557
.35
4850
6-57
5.61
150.
1018
0.34
300.
0032
0.11
870.
0022
1917
.87
34.8
019
00.9
717
.97
1936
.21
36.1
619
36.2
136
.16
4850
6-91
6.05
890.
2154
0.36
580.
0122
0.12
010.
0045
1984
.35
70.5
420
09.6
466
.88
1958
.05
73.1
919
58.0
573
.19
4850
6-61
5.96
160.
1869
0.35
120.
0101
0.12
310.
0042
1970
.26
61.7
819
40.1
955
.56
2001
.99
68.9
720
01.9
968
.97
4850
6-17
6.48
480.
2056
0.37
440.
0095
0.12
560.
0041
2043
.83
64.8
020
50.2
151
.89
2037
.47
67.2
920
37.4
767
.29
4850
6-4
10.3
511
0.23
010.
4637
0.00
810.
1619
0.00
3824
66.6
954
.82
2455
.97
43.0
624
75.6
258
.73
2475
.62
58.7
348
506-
9011
.042
40.
2667
0.47
880.
0079
0.16
730.
0043
2526
.71
61.0
225
21.9
841
.47
2530
.65
65.5
825
30.6
565
.58
Cor
rect
ed Is
otop
ic R
atio
sA
ges
(Ma)
App
endi
x C
-1 c
ontin
ued
-- S
ampl
e 02
MA
W02
0
192 PIR 2008-1 Preliminary results, fi eld investigations, Brooks Range Foothils & North Slope, Alaska
Ana
lysi
s #
207 P
b / 23
5 U±2
206 P
b / 23
8 U±2
207 P
b / 20
6 Pb
±220
7 Pb
/ 235 U
±220
6 Pb
/ 238 U
±220
7 Pb
/ 206 P
b±2
Bes
t Age
±248
509-
360.
1575
0.02
170.
0232
0.00
120.
0493
0.00
7114
8.55
20.4
814
7.76
7.77
161.
5923
.14
147.
767.
7748
509-
390.
1622
0.02
060.
0239
0.00
080.
0492
0.00
6315
2.59
19.3
615
2.18
4.84
158.
6920
.38
152.
184.
8448
509-
890.
1798
0.01
330.
0263
0.00
090.
0496
0.00
4016
7.92
12.3
816
7.28
5.80
177.
3114
.21
167.
285.
8048
509-
250.
2001
0.02
890.
0290
0.00
090.
0500
0.00
7318
5.23
26.7
518
4.56
5.99
193.
7128
.43
184.
565.
9948
509-
610.
2048
0.01
980.
0295
0.00
200.
0504
0.00
5118
9.21
18.2
518
7.38
12.4
321
1.49
21.5
818
7.38
12.4
348
509-
170.
2176
0.01
420.
0313
0.00
140.
0504
0.00
3719
9.90
13.0
419
8.73
8.72
213.
6215
.49
198.
738.
7248
509-
680.
2265
0.00
850.
0327
0.00
050.
0502
0.00
1920
7.32
7.79
207.
673.
3120
3.09
7.78
207.
673.
3148
509-
780.
2585
0.03
650.
0367
0.00
140.
0511
0.00
7423
3.48
33.0
023
2.09
8.63
247.
3535
.96
232.
098.
6348
509-
910.
2689
0.01
570.
0382
0.00
090.
0510
0.00
3124
1.80
14.0
924
1.98
5.74
240.
0214
.45
241.
985.
7448
509-
200.
2778
0.01
610.
0390
0.00
170.
0516
0.00
3224
8.89
14.4
324
6.92
10.7
126
7.33
16.6
524
6.92
10.7
148
509-
600.
3131
0.02
020.
0439
0.00
160.
0518
0.00
3327
6.61
17.8
327
6.75
9.93
275.
3417
.81
276.
759.
9348
509-
430.
3124
0.01
990.
0439
0.00
120.
0516
0.00
3427
6.04
17.5
727
7.07
7.35
267.
1817
.59
277.
077.
3548
509-
120.
3209
0.01
400.
0447
0.00
070.
0521
0.00
2428
2.63
12.3
328
1.73
4.71
289.
9213
.14
281.
734.
7148
509-
950.
3227
0.01
710.
0449
0.00
090.
0522
0.00
2928
4.01
15.0
728
2.91
5.94
292.
5916
.05
282.
915.
9448
509-
230.
3259
0.01
480.
0452
0.00
090.
0522
0.00
2528
6.44
13.0
428
5.27
5.43
295.
7213
.99
285.
275.
4348
509-
630.
3247
0.01
120.
0454
0.00
110.
0519
0.00
1828
5.49
9.84
286.
087.
0728
0.46
9.94
286.
087.
0748
509-
470.
3264
0.01
440.
0456
0.00
140.
0520
0.00
2328
6.80
12.6
828
7.16
8.56
283.
2812
.72
287.
168.
5648
509-
60.
3330
0.01
390.
0465
0.00
110.
0519
0.00
2229
1.84
12.1
529
3.10
6.72
281.
5211
.84
293.
106.
7248
509-
540.
3433
0.03
320.
0475
0.00
210.
0524
0.00
5329
9.69
29.0
029
9.17
13.4
930
4.03
30.8
329
9.17
13.4
948
509-
490.
3466
0.02
840.
0476
0.00
170.
0528
0.00
4530
2.16
24.7
729
9.76
10.6
332
1.04
27.5
429
9.76
10.6
348
509-
840.
3590
0.01
440.
0493
0.00
080.
0528
0.00
2231
1.44
12.5
231
0.19
5.23
320.
5913
.46
310.
195.
2348
509-
700.
3967
0.00
900.
0540
0.00
100.
0533
0.00
1333
9.22
7.69
338.
956.
4734
0.88
8.04
338.
956.
4748
509-
730.
3995
0.01
450.
0540
0.00
130.
0536
0.00
2134
1.26
12.3
533
9.26
8.19
354.
7713
.58
339.
268.
1948
509-
240.
4680
0.01
660.
0621
0.00
110.
0547
0.00
1938
9.83
13.8
138
8.17
6.80
399.
4014
.22
388.
176.
8048
509-
570.
4821
0.04
170.
0629
0.00
340.
0556
0.00
4839
9.52
34.5
339
3.49
21.4
343
4.42
37.4
939
3.49
21.4
348
509-
40.
4841
0.01
620.
0639
0.00
100.
0549
0.00
1840
0.90
13.3
839
9.35
6.33
409.
7013
.61
399.
356.
3348
509-
660.
5124
0.01
710.
0667
0.00
200.
0557
0.00
2042
0.09
14.0
341
6.27
12.4
544
0.90
15.9
141
6.27
12.4
548
509-
30.
5104
0.01
370.
0669
0.00
070.
0553
0.00
1541
8.72
11.2
541
7.53
4.48
425.
1111
.79
417.
534.
4848
509-
960.
5108
0.01
630.
0672
0.00
100.
0551
0.00
1841
8.98
13.3
341
9.22
6.09
417.
1013
.77
419.
226.
0948
509-
500.
5153
0.02
610.
0677
0.00
250.
0552
0.00
2742
1.98
21.3
642
2.10
15.6
142
1.22
20.8
242
2.10
15.6
148
509-
70.
5198
0.03
350.
0678
0.00
350.
0556
0.00
3742
5.02
27.4
142
2.89
22.0
143
6.40
28.9
842
2.89
22.0
148
509-
580.
5287
0.04
590.
0688
0.00
350.
0557
0.00
5043
0.93
37.4
342
9.04
21.7
744
0.52
39.5
542
9.04
21.7
748
509-
450.
5451
0.02
630.
0701
0.00
290.
0564
0.00
2944
1.80
21.3
043
6.55
18.0
346
9.06
23.8
643
6.55
18.0
348
509-
260.
5403
0.05
460.
0702
0.00
570.
0558
0.00
5843
8.60
44.3
643
7.35
35.6
044
4.72
46.2
743
7.35
35.6
048
509-
20.
5558
0.02
850.
0714
0.00
200.
0564
0.00
3044
8.78
23.0
544
4.70
12.6
546
9.51
25.0
744
4.70
12.6
5
Cor
rect
ed Is
otop
ic R
atio
sA
ges
(Ma)
App
endi
x C
-1 c
ontin
ued
Sam
ple
02M
AW
006
(low
er F
ortr
ess
Mou
ntai
n Fo
rmat
ion)
Appendix C-1 193
Ana
lysi
s #
207 P
b / 23
5 U±2
206 P
b / 23
8 U±2
207 P
b / 20
6 Pb
±220
7 Pb
/ 235 U
±220
6 Pb
/ 238 U
±220
7 Pb
/ 206 P
b±2
Bes
t Age
± M
a (2
)48
509-
100.
5576
0.02
480.
0724
0.00
150.
0559
0.00
2644
9.99
20.0
445
0.56
9.08
446.
9321
.03
450.
569.
0848
509-
650.
5752
0.02
250.
0736
0.00
210.
0566
0.00
2346
1.38
18.0
645
8.06
13.3
147
7.45
19.4
545
8.06
13.3
148
509-
800.
5778
0.01
640.
0741
0.00
110.
0565
0.00
1646
3.04
13.1
446
0.82
7.06
473.
9413
.77
460.
827.
0648
509-
190.
5768
0.01
960.
0744
0.00
140.
0562
0.00
2046
2.38
15.6
946
2.68
8.96
460.
5116
.44
462.
688.
9648
509-
990.
5821
0.02
080.
0746
0.00
160.
0566
0.00
2046
5.84
16.6
646
3.52
10.0
447
7.22
16.7
446
3.52
10.0
448
509-
870.
5879
0.02
730.
0747
0.00
260.
0571
0.00
2746
9.53
21.7
746
4.25
16.3
049
5.30
23.6
146
4.25
16.3
048
509-
310.
5936
0.02
030.
0765
0.00
180.
0562
0.00
2047
3.16
16.1
647
5.42
11.1
946
1.81
16.2
447
5.42
11.1
948
509-
790.
6129
0.02
990.
0785
0.00
310.
0566
0.00
2848
5.40
23.6
748
6.93
19.4
047
7.68
23.8
148
6.93
19.4
048
509-
460.
6229
0.03
360.
0798
0.00
150.
0566
0.00
3149
1.69
26.5
449
5.01
9.20
476.
0726
.44
495.
019.
2048
509-
710.
6410
0.03
290.
0810
0.00
270.
0574
0.00
3150
2.90
25.8
150
2.05
16.8
850
6.67
27.5
750
2.05
16.8
848
509-
560.
6559
0.04
410.
0836
0.00
410.
0569
0.00
3851
2.07
34.4
651
7.34
25.3
848
8.36
32.5
551
7.34
25.3
848
509-
900.
7186
0.02
370.
0883
0.00
190.
0590
0.00
1954
9.82
18.1
254
5.41
11.4
456
8.01
18.0
854
5.41
11.4
448
509-
130.
7763
0.02
920.
0955
0.00
170.
0590
0.00
2358
3.40
21.9
658
7.85
10.6
056
5.95
21.8
558
7.85
10.6
048
509-
640.
8991
0.02
910.
1050
0.00
290.
0621
0.00
2265
1.25
21.0
664
3.49
17.8
467
8.10
24.0
764
3.49
17.8
448
509-
831.
0565
0.11
460.
1183
0.00
520.
0648
0.00
7373
2.09
79.4
572
0.82
31.7
576
6.68
86.9
972
0.82
31.7
548
509-
411.
2096
0.03
840.
1321
0.00
170.
0664
0.00
2280
5.01
25.5
479
9.67
10.5
781
9.63
26.6
379
9.67
10.5
748
509-
421.
2155
0.04
380.
1322
0.00
240.
0667
0.00
2580
7.73
29.1
080
0.49
14.2
382
7.64
31.1
680
0.49
14.2
348
509-
551.
3338
0.09
890.
1430
0.00
820.
0676
0.00
4986
0.53
63.7
886
1.82
49.3
085
7.09
62.4
286
1.82
49.3
048
509-
821.
4073
0.03
700.
1475
0.00
240.
0692
0.00
1989
2.03
23.4
588
6.87
14.4
990
4.69
25.2
488
6.87
14.4
948
509-
921.
5499
0.05
430.
1616
0.00
410.
0696
0.00
2595
0.46
33.2
996
5.67
24.7
991
5.30
33.4
296
5.67
24.7
948
509-
401.
8951
0.05
110.
1796
0.00
380.
0765
0.00
2410
79.3
629
.09
1064
.56
22.7
811
09.2
834
.27
1109
.28
34.2
748
509-
162.
1436
0.06
070.
1940
0.00
500.
0801
0.00
2411
62.9
832
.91
1142
.89
29.2
512
00.5
636
.65
1200
.56
36.6
548
509-
482.
3177
0.03
360.
2094
0.00
230.
0803
0.00
1212
17.7
117
.63
1225
.81
13.4
212
03.2
317
.90
1203
.23
17.9
048
509-
972.
5908
0.05
930.
2246
0.00
450.
0837
0.00
2112
98.0
329
.72
1306
.37
26.2
112
84.6
032
.29
1284
.60
32.2
948
509-
943.
0974
0.06
890.
2473
0.00
510.
0909
0.00
2114
32.0
431
.83
1424
.33
29.5
114
43.4
833
.22
1443
.48
33.2
248
509-
593.
3994
0.13
130.
2649
0.00
960.
0931
0.00
3915
04.2
558
.10
1515
.00
55.1
514
89.4
163
.08
1489
.41
63.0
848
509-
813.
3380
0.07
200.
2563
0.00
400.
0945
0.00
2214
89.9
932
.13
1470
.92
23.2
415
17.1
434
.89
1517
.14
34.8
948
509-
113.
7943
0.10
800.
2866
0.00
780.
0960
0.00
2915
91.5
445
.28
1624
.76
44.3
815
48.0
046
.81
1548
.00
46.8
148
509-
853.
8283
0.08
640.
2787
0.00
290.
0996
0.00
2315
98.7
236
.09
1584
.70
16.4
916
17.0
937
.89
1617
.09
37.8
948
509-
444.
1071
0.08
960.
2898
0.00
300.
1028
0.00
2316
55.7
136
.14
1640
.58
17.2
416
74.8
837
.26
1674
.88
37.2
648
509-
514.
1867
0.09
870.
2947
0.00
640.
1030
0.00
2416
71.4
339
.39
1665
.12
36.3
716
79.2
338
.62
1679
.23
38.6
248
509-
694.
4646
0.07
090.
3043
0.00
430.
1064
0.00
1817
24.4
127
.38
1712
.35
24.4
417
38.9
329
.74
1738
.93
29.7
448
509-
304.
6855
0.12
250.
3171
0.00
770.
1072
0.00
2717
64.6
546
.13
1775
.48
43.3
617
51.7
143
.64
1751
.71
43.6
448
509-
724.
6662
0.10
040.
3133
0.00
530.
1080
0.00
2417
61.2
037
.88
1756
.96
29.6
717
65.8
438
.98
1765
.84
38.9
848
509-
214.
8185
0.06
820.
3224
0.00
400.
1084
0.00
1717
88.1
325
.31
1801
.65
22.4
617
72.3
128
.32
1772
.31
28.3
248
509-
224.
5415
0.13
130.
3026
0.00
670.
1088
0.00
3117
38.6
150
.27
1704
.28
37.9
617
80.0
949
.96
1780
.09
49.9
6
App
endi
x C
-1 c
ontin
ued
-- S
ampl
e 02
MA
W00
6
Cor
rect
ed Is
otop
ic R
atio
sA
ges
(Ma)
194 PIR 2008-1 Preliminary results, fi eld investigations, Brooks Range Foothils & North Slope, Alaska
Ana
lysi
s #
207 P
b / 23
5 U±2
206 P
b / 23
8 U±2
207 P
b / 20
6 Pb
±220
7 Pb
/ 235 U
±220
6 Pb
/ 238 U
±220
7 Pb
/ 206 P
b±2
Bes
t Age
± M
a (2
)48
509-
294.
8188
0.06
190.
3199
0.00
270.
1092
0.00
1417
88.1
922
.98
1789
.29
15.1
517
86.8
022
.50
1786
.80
22.5
048
509-
935.
0996
0.13
080.
3355
0.00
840.
1102
0.00
2818
36.0
347
.10
1864
.83
46.7
718
03.4
445
.73
1803
.44
45.7
348
509-
764.
9707
0.09
890.
3257
0.00
610.
1107
0.00
2118
14.3
436
.09
1817
.56
33.7
718
10.5
334
.94
1810
.53
34.9
448
509-
864.
8599
0.07
540.
3164
0.00
450.
1114
0.00
1917
95.3
427
.84
1772
.12
25.2
218
22.3
230
.87
1822
.32
30.8
748
509-
625.
2287
0.11
650.
3393
0.00
720.
1117
0.00
2318
57.3
141
.40
1883
.46
39.7
318
28.0
437
.95
1828
.04
37.9
548
509-
355.
1806
0.10
750.
3303
0.00
510.
1138
0.00
2418
49.4
338
.36
1839
.72
28.6
018
60.2
440
.02
1860
.24
40.0
248
509-
385.
0389
0.11
230.
3210
0.00
610.
1138
0.00
2718
25.8
940
.68
1794
.53
33.8
518
61.7
144
.72
1861
.71
44.7
248
509-
675.
2666
0.12
880.
3344
0.00
680.
1142
0.00
2818
63.4
745
.59
1859
.63
37.9
118
67.6
446
.58
1867
.64
46.5
848
509-
155.
3160
0.09
820.
3320
0.00
440.
1161
0.00
2318
71.4
434
.57
1848
.14
24.5
918
97.2
837
.23
1897
.28
37.2
348
509-
536.
0951
0.19
400.
3711
0.00
840.
1191
0.00
4019
89.5
563
.31
2034
.57
46.2
919
42.9
464
.93
1942
.94
64.9
348
509-
95.
5672
0.15
250.
3385
0.00
600.
1193
0.00
3419
11.0
352
.35
1879
.55
33.2
019
45.2
355
.31
1945
.23
55.3
148
509-
345.
6310
0.14
500.
3394
0.00
700.
1203
0.00
3319
20.8
649
.46
1883
.77
39.0
219
61.0
053
.38
1961
.00
53.3
848
509-
756.
3113
0.16
100.
3758
0.00
850.
1218
0.00
2920
20.0
351
.55
2056
.54
46.4
419
82.8
047
.37
1982
.80
47.3
748
509-
885.
9387
0.09
780.
3521
0.00
420.
1223
0.00
2219
66.9
132
.38
1944
.55
23.3
119
90.4
335
.08
1990
.43
35.0
848
509-
325.
9575
0.19
510.
3519
0.00
760.
1228
0.00
4119
69.6
664
.51
1943
.57
41.8
619
97.0
767
.04
1997
.07
67.0
448
509-
100
5.92
950.
1714
0.34
710.
0083
0.12
390.
0038
1965
.57
56.8
219
20.8
746
.01
2012
.86
62.0
620
12.8
662
.06
4850
9-1
6.40
020.
0803
0.37
270.
0033
0.12
460.
0017
2032
.30
25.4
920
41.8
817
.95
2022
.55
27.0
120
22.5
527
.01
4850
9-5
6.17
230.
1297
0.35
660.
0064
0.12
550.
0029
2000
.53
42.0
419
66.1
835
.11
2036
.10
47.6
420
36.1
047
.64
4850
9-77
6.32
360.
0896
0.36
390.
0040
0.12
600.
0019
2021
.73
28.6
520
00.8
321
.82
2043
.04
31.1
720
43.0
431
.17
4850
9-37
7.29
940.
2882
0.39
940.
0148
0.13
260.
0056
2148
.74
84.8
421
66.3
880
.21
2132
.13
89.8
221
32.1
389
.82
4850
9-14
7.04
840.
1105
0.38
410.
0055
0.13
310.
0022
2117
.55
33.1
820
95.2
430
.24
2139
.21
34.8
621
39.2
134
.86
4850
9-8
8.71
160.
1988
0.42
280.
0092
0.14
950.
0037
2308
.30
52.6
822
73.2
149
.21
2339
.67
57.8
823
39.6
757
.88
4850
9-98
8.85
520.
3647
0.42
840.
0178
0.14
990.
0067
2323
.19
95.6
722
98.5
295
.25
2345
.16
104.
5223
45.1
610
4.52
4850
9-74
10.0
693
0.27
000.
4605
0.01
130.
1586
0.00
4324
41.1
665
.46
2441
.87
59.8
724
40.4
566
.91
2440
.45
66.9
148
509-
1812
.068
30.
1983
0.49
950.
0069
0.17
520.
0030
2609
.73
42.8
826
11.6
036
.12
2608
.19
44.9
826
08.1
944
.98
4850
9-27
11.9
622
0.35
940.
4922
0.01
140.
1763
0.00
6526
01.4
578
.16
2580
.00
60.0
026
18.1
096
.05
2618
.10
96.0
548
509-
2812
.431
20.
1686
0.49
820.
0067
0.18
100.
0027
2637
.54
35.7
726
06.0
034
.80
2661
.71
39.5
126
61.7
139
.51
4850
9-52
13.7
393
0.31
700.
5421
0.01
180.
1838
0.00
4327
31.9
163
.03
2792
.17
60.7
226
87.5
763
.23
2687
.57
63.2
348
509-
3313
.985
10.
3551
0.53
760.
0130
0.18
870.
0048
2748
.70
69.7
927
73.3
666
.82
2730
.52
70.0
927
30.5
270
.09
App
endi
x C
-1 c
ontin
ued
-- S
ampl
e 02
MA
W00
6
Cor
rect
ed Is
otop
ic R
atio
sA
ges
(Ma)