Ar/39Ar geochronology of the Eocene Green River Formation ...bsinger/Publications/2003... · fauna were not directly caused by the ini-tiation of maximum greenhouse conditions. Keywords:

For permission to copy, contact [email protected] 2003 Geological Society of America 549

GSA Bulletin; May 2003; v. 115; no. 5; p. 549–565; 9 figures; 3 tables; Data Repository item 2003064.

40Ar/39Ar geochronology of the Eocene Green RiverFormation, Wyoming

M. Elliot SmithBrad Singer†

Alan CarrollDepartment of Geology and Geophysics, University of Wisconsin, 1215 West Dayton Street, Madison, Wisconsin 53706, USA

ABSTRACT

The deposits of Eocene Lake Gosiute thatconstitute the Green River Formation ofWyoming contain numerous tuff beds thatrepresent isochronous, correlatable strati-graphic markers. Tuff beds selected for40Ar/39Ar analysis occur within laminatedmudstone, are matrix supported, and lackevidence of reworking. These tuffs contain2%–15% euhedral phenocrysts of quartz,plagioclase, sanidine, biotite, and minoramphibole, pyroxene, and zircon, encasedin a matrix of altered glassy ash. Air abra-sion and handpicking under refractive-index oils were required to obtain clean,unaltered phenocrysts of sanidine. 40Ar/39Arage determinations from single-crystal and,1 mg multigrain aliquots of sanidine andbiotite allowed the identification and exclu-sion of xenocrystic contamination. Laser-fusion experiments on phenocrysts from theRife, Firehole, C Bed, Grey, Main, Sixth,and Analcite tuff beds from the Tipton,Wilkins Peak, and Laney Members yieldedweighted-mean ages (62s analytical uncer-tainties) of 51.25 6 0.31 Ma, 50.70 6 0.14Ma, 50.56 6 0.26 Ma, 50.39 6 0.13 Ma,49.96 6 0.08 Ma, 49.70 6 0.10 Ma, and48.94 6 0.12 Ma, respectively. Ages for san-idine and biotite from the Main tuff are in-distinguishable when presumably xenocrys-tic contaminants are excluded from the agecalculation. Moreover, the 40Ar/39Ar agesare consistent with the stratigraphic orderof the tuff beds and with provenance in theAbsaroka and Challis volcanic fields.

Our 40Ar/39Ar-based age model indicatesthat sediment accumulated three timesmore rapidly (327 6 86 mm/yr) during the

†Corresponding author e-mail: [email protected].

evaporative Wilkins Peak phase than thefreshwater to saline Tipton (88 6 34 mm/yr) and Laney (104 6 18 mm/yr) phases.The much lower accumulation rates for theTipton and Laney Members are permissiveof an annual origin for ,1-mm-thick lam-inae and precessional forcing of 1–3-m-thick depositional cycles in these units.However, previously described cycles in theWilkins Peak Member have average dura-tions that are significantly shorter than the19–23 k.y. precessional modes.

The Green River Formation encompassesan ;5 m.y. period between ca. 53.5 and48.5 Ma, spanning magnetic chrons 24nthrough 21r. The Green River Formationwas therefore deposited during the warmestperiod of the Cenozoic corresponding to theearly Eocene climatic optimum as definedby the global marine O isotope record. De-position of bedded evaporites (trona) of theWilkins Peak Member began at ca. 51 Ma,or ;2 m.y. after the onset of the highestinferred temperatures of the climatic opti-mum. The Bridgerian–Wasatchian faunalturnover occurred subsequently duringWilkins Peak time, at ca. 50.6 Ma. Thus,our 40Ar/39Ar ages strongly suggest thatWilkins Peak evaporite deposition and theturnover from Wasatchian to Bridgerianfauna were not directly caused by the ini-tiation of maximum greenhouse conditions.

Keywords: Green River Formation, sedi-mentation rates, lacustrine, 40Ar/39Ar,Wasatchian-Bridgerian, Early Eocene Cli-mate Optimum.

INTRODUCTION

Lake deposits are commonly exploited forthe long-lived and relatively continuous re-cords they provide of Earth’s surface process-

es. Much of this work has focused on pastclimates because of the sensitivity of lakes toclimate change and because of the potentialfor their deposits to preserve terrestrial proxyrecords of paleoclimate. Climatic forcing ofsedimentation has been inferred to operate ona variety of time scales, ranging from annualto orbital (e.g., Bradley, 1929; Van Houten,1962; Olsen, 1986; Fischer and Roberts,1991). Many of these studies link sedimentaryfacies successions to interpreted variations inclimatic humidity, on the basis of the expec-tation that wetter conditions should producelarger lakes. The translation of sediment-thickness patterns into time therefore repre-sents a crucial step in elucidating past rates ofclimate change.

Owing to their common occurrence withinorogenic plateaus, tectonically formed lakebasins are also well suited to deciphering re-gional interactions between climate and risingorogenic topography. Numerous studies haveshown that lacustrine sedimentation representsa complex geomorphic response to regionaltectonic, paleoclimatic, and, in some cases,magmatic influences (e.g., Surdam and Stan-ley, 1980; Sullivan, 1985; Carroll and Bohacs,1999). Understanding the rates and timing ofsediment accumulation in such basins is es-sential to deciphering the tectonic vs. climaticinfluences on lake evolution. In addition, la-custrine and associated alluvial strata alsocontain some the most complete archives ofthe rates of biological evolution on the con-tinents (e.g., Krishtalka et al., 1987; Lucas,1998; Alroy et al., 2000).

The Green River Formation of Wyoming,Utah, and Colorado (Fig. 1) is one of the best-known and most diverse lacustrine systems inthe world (e.g., Bradley, 1929; Eugster andHardie, 1975; Roehler, 1993). As such, it rep-resents an ideal body of lacustrine strata in

550 Geological Society of America Bulletin, May 2003

SMITH et al.

Figure 1. Location of Eocene lacustrine basins, Precambrian–cored uplifts, and possiblevolcanic source regions in the northern Rocky Mountains. Modified from Love and Chris-tiansen (1985), Roehler (1992b), and Snider and Moye (1989). Modeled 500 mbar windsare from Sewall et al. (2000).

which to develop a high-resolution geochro-nology. We have obtained laser-fusion 40Ar/39Ar ages from seven tuff beds in the GreenRiver Formation. These ages set limits onrates of sediment accumulation, periodicitiesof micro- to mesoscale depositional cyclicity,the timing of faunal changes, and the mag-netostratigraphy of the Green River Forma-tion. Moreover, by tying the Green River For-mation to the geomagnetic polarity time scale(GPTS; Cande and Kent, 1992, 1995; Berg-gren et al., 1995), the new 40Ar/39Ar ages al-low for correlation to other Laramide basins,to Eocene volcanism in western North Amer-ica, and to the global marine O isotope record.

THE GREEN RIVER FORMATION

The Green River Formation consists of Eo-cene lacustrine strata deposited in the GreaterGreen River Basin (5 Green River, Great Di-vide, Washakie, and Sand Wash Basins), Fos-sil, Uinta, and Piceance Creek Basins (Fig. 1).The formation comprises a broad range of sed-imentary facies that were deposited in fresh

water through hypersaline lake environments(Bradley, 1964; Eugster and Hardie, 1975;Smoot, 1983; Surdam and Stanley, 1979;Roehler, 1993). Carroll and Bohacs (1999) sub-divided these deposits into fluvio-lacustrine,fluctuating profundal, and evaporative faciesassociations on the basis of interpretation ofsedimentary facies, stratigraphic packaging,fossil occurrence, and organic geochemistry(see also Bohacs et al., 2000; Carroll and Bo-hacs, 2001). The Green River Formation in-terfingers with time-equivalent alluvial faciesof the Wasatch Formation. In the GreaterGreen River Basin, the Green River Formationis bounded at its base by the main body of theWasatch Formation and is overlain by theBridger and Washakie Formations (Fig. 2;Roehler, 1992a, 1992b; McCarroll et al., 1996;Evanoff et al., 1998). Lacustrine facies are di-vided into four major members: the LumanTongue and Tipton, Wilkins Peak, and LaneyMembers (Fig. 2; Roehler, 1992b). These unitsdocument the geographic and chemical evo-lution of Lake Gosiute from freshwater (Lu-man, Scheggs Bed–Tipton), to saline lake

(Rife Bed–Tipton), to hypersaline playa-lake(Wilkins Peak), then back to saline (LaCledeBed–Laney) and eventually freshwater lake(Hart Cabin–Laney) (Fig. 2; Roehler, 1993;Carroll and Bohacs, 1999; Rhodes et al.,2002).

The Green River Formation and time-equivalent alluvial strata in the Greater GreenRiver Basin contain one of the most completefaunal records of the late Wasatchian throughearly Bridgerian NALMA (North Americanland mammal age) period (e.g., Krishtalka etal., 1987; Zonneveld et al., 2000). This tran-sition is marked by the first appearances ofseveral primate and perissodactyl (e.g., horsesand rhinoceroses) taxa, high North Americanmammalian diversity, and greenhouse faunasand contains 1 of 18 major turnovers in theNorth American Cenozoic mammalian record(Stucky, 1990; Alroy, 1999; Zonneveld et al.,2000). NALMA biostratigraphy has been formany years the principal tool for inter- andintrabasinal correlation in western NorthAmerica (e.g., Wood et al., 1941; Krishtalkaet al., 1987; Lucas, 1998) and can be roughlycorrelated with Eocene faunas in Europe andAsia (Lucas, 1998). Consequently, improve-ments in geochronology have the potential toset limits on the rates and paleogeographicpatterns of Cenozoic faunal radiation and ex-tinction in both a regional and global context.Until now, the timing of the Wasatchian–Bridgerian period has been poorly determinedowing to limited and imprecise radioisotopicage control (Fig. 3; Evernden et al., 1964;Mauger, 1977; O’Neill, 1980). This study pro-vides the first laser-fusion 40Ar/39Ar radioiso-topic ages for this period and allows for ac-curate temporal comparison of lake type andfaunal changes with the global marine recordof the early Eocene.

PREVIOUS GEOCHRONOLOGICSTUDIES

It has been recognized at least since 1970that the many silicic tephra layers within theGreen River Formation might facilitate a pre-cise radioisotopic chronology for the accu-mulation of the sedimentary deposits. The pi-oneering work of Mauger (1977) undertookK-Ar dating of biotite separated from tuff bedsin the Greater Green River and Uinta Basins.The K-Ar ages discussed here have been con-verted by using the decay constants of Steigerand Jager (1977) following Dalrymple (1979).Mauger’s (1977) K-Ar ages between 50.9 62.3 Ma and 46.4 6 1.9 Ma provided the firstradioisotopic constraints on the timing of theupper Green River Formation in the Greater

Geological Society of America Bulletin, May 2003 551

40Ar/39Ar GEOCHRONOLOGY OF THE EOCENE GREEN RIVER FORMATION, WYOMING

Figure 2. Generalized cross section of Eocene strata in the Greater Green River Basin along line A–A9 in Figure 1, showing thestratigraphic position of the Green River Formation and dated tuff beds. Modified from Culbertson (1961) and Roehler (1992b).

TABLE 1. CHARACTERISTICS OF TUFF BEDS

Tuff Appearance Setting† Thickness Texture % Matrix Phenocrysts Alteration

Analcite White to orange with 1cm-diameter blueanalcime crystals atbase

lcm 15 cm Matrix supported; phenocrystsconcentrated at base

.95 Sanidine, biotite, plagioclase,quartz; minor amphibole,zircon

Analcite

Sixth White with hematitestaining at base

lcm 17 cm Matrix supported; phenocrystsconcentrated at base

90–95 Plagioclase, biotite, quartz;minor sanidine, pyroxene,zircon

Analcite

Main White—lowest of threecorrelatable 5–10cm subbeds

lcm 25 cm Matrix supported; possessesgraded 1–20 mm ‘‘fallout’’beds

90–95 Sanidine, biotite, plagioclase,quartz; minor amphibole,pyroxene, zircon

Analcite

Grey Mottled gray to white sdm 4–6 cm Matrix supported 85–90 Plagioclase, biotite, sanidine,quartz

K-feldsparand analcite

C Bed Gray to white lcm 8 cm Matrix supported 90–95 Sanidine, biotite, plagioclase,quartz; minor amphibole,pyroxene, zircon


Firehole Orange to yellow lcm 15 cm Matrix supported .95 Plagioclase, quartz; minorsanidine


Rife White to gray lcm 2 cm Matrix supported; phenocrystsconcentrated at base

80–85 Biotite, plagioclase, quartz Analcite

†Abbreviations: lcm—laminated calcareous mudstone, sdm—silty dolomitic mudstone.

Green River and Uinta Basins (Fig. 3). How-ever, the need for large bulk samples (;400mg) severely limited the resolution of thisstudy because of the inability to resolve po-tential alteration effects or the incorporation ofdetrital or xenocrystic components.

The only previous 40Ar/39Ar investigation ofGreen River Formation tuff beds was byO’Neill (1980), who performed incremental-heating experiments on biotite from tuff bedsin the Greater Green River and Uinta Basins.O’Neill’s (1980) five- to eight-step experi-

ments also utilized large bulk samples of;400 mg. Individual plateau ages for theSand Butte, Sixth, Main, and Wavy tuffsranged from 50.1 6 2.2 Ma to 45.5 6 2.2 Ma,with mean ages from replicate experiments of49.8 6 0.8 Ma and 46.6 6 1.0 Ma for the


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Figure 4. Field photographs and photomicrographs (crossed nicols) of tuff beds. Abbre-viations: san—sanidine; plag—plagioclase; and qtz—quartz.

Main and Wavy tuffs, respectively (Fig. 3).The neutron fluence monitor mineral used inthese experiments was an 811 Ma biotite(OSU DY-8c–71) provisionally calibrated to a519.5 Ma age for the MMhb standard (Sneeand Sutter, 1982), making it difficult to com-pare O’Neill’s (1980) results with data ob-tained by using modern intercalibrated stan-dards (e.g., Renne et al., 1998). Moreover, thelarge 2s uncertainties of .1 m.y. for the pla-teau ages represented little improvement uponMauger’s (1977) results.

Recent single-crystal laser-fusion 40Ar/39Ardating has defined the timing of deposition ofBridger Formation strata (Evanoff et al., 1998)that overlie the Green River Formation in thewestern Greater Green River Basin (Figs. 2and 3). For the purposes of intercomparison,all 40Ar/39Ar ages in this paper (unless other-wise noted) are reported (or recalculated) rel-ative to the standard ages of Renne et al.(1998), including intercalibration uncertain-ties, at the 2s precision level. Murphey et al.(1999) reported 40Ar/39Ar ages for sanidine, bi-otite, and plagioclase from the Church Butte(base of middle Bridger B), Henry’s Fork(base of upper Bridger C), and unnamed (baseof Bridger E) tuffs of 48.65 6 0.30 Ma, 47.606 0.37 Ma, and 46.83 6 0.90 Ma, respec-tively. Prothero (1996b) reported an 40Ar/39Arage of 47.56 6 0.14 Ma for Henry’s Fork tuff,which, though indistinguishable from the agereported by Murphey et al. (1999), is muchmore precise. These age determinations con-firm existing bio- and magnetostratigraphy(Flynn, 1986; Prothero and Swisher, 1992;McCarroll et al., 1996; Prothero, 1996a) andhelp to define the upper age limit of the GreenRiver Formation (Figs. 2 and 3). They alsotemporally quantify the distal record of Eo-cene alkali-calcic silicic volcanism in westernNorth America (Fig. 1) and provide a detailedpicture of post–Green River Formation basindevelopment that is complementary to our re-sults (Fig. 3).

GEOLOGIC SETTING OF TUFF BEDS

Lacustrine facies of the Green River For-mation are interbedded with numerous indi-vidual water-laid tuff horizons that range from1 mm to several meters thick; thinner bedsgreatly outnumber thicker beds. The tuff bedsrepresent widespread isochronous markers andhave been extensively used for stratigraphiccorrelation (e.g., Culbertson, 1961; Bradley,1964; Smoot, 1983; Roehler, 1992b). Gener-ally light gray to orange in outcrop, tuff bedsare fine grained and typically contain ,20%phenocrysts of quartz, biotite, sanidine, and

plagioclase and minor hornblende, pyroxene,and zircon (Table 1). The percentage of crys-talline to matrix material within any specifictuff horizon is independent of its thickness.Many of the beds possess angular, matrix-supported phenocrysts that exhibit a gradationto smaller, less abundant crystals from thebase upward (Fig. 4). Because these tuff bedsare generally found interbedded with profun-dal mudstone (Fig. 4, Table 1), we interpretthem to be primary ash-fall deposits. Re-worked tuff beds have similar compositionsbut also contain lithic fragments and carbo-naceous material and are characterized bysorting and cross-stratification indicative ofsubaqueous, tractive reworking (cf. Konigerand Stollhofen, 2001). Reworked beds are typ-ically found in association with coarser-grained facies indicative of higher-energy sed-imentary environments and were avoided inthis study.

Most of the original glassy matrix of thetuff beds was altered by saline and alkaline

lake water. Studies of alteration in modern andancient saline and alkaline lake deposits showthat volcanic glass is initially converted to ze-olites such as clinoptilolite and mordenite be-fore being altered to analcime (Hay, 1966;Goodwin and Surdam, 1967; Sheppard andGude, 1969). Analcime is then converted toK-feldspar under ultrasaline conditions whenthe Na1/K1 activity ratio is sufficiently low(Hay, 1966; Goodwin, 1973; Desborough,1975). Our observations confirm Surdam andParker’s (1972) conclusion that tuff beds de-posited during the most saline and alkalinephase of Lake Gosiute generally exhibit K-feldspar alteration, whereas those in less salinefacies contain only zeolite and analcime alter-ation. Additionally, in Green River Formationtuff beds we recognized the significant lateralalteration variations described by previousworkers (Hay, 1966; Iijima and Hay, 1968;Ratterman and Surdam, 1981; Buchheim andEugster, 1998). In particular, basin-center de-posits typically exhibit more alteration, sug-


SMITH et al.

Figure 5. Microprobe transects and backscattered-electron images of volcanic grains. A:Main tuff, sanidine. B: Grey tuff, magmatically zoned sanidine with authigenic K-feldsparovergrowth. C: Grey tuff, sanidine with authigenic K-feldspar rim, showing the effect ofprogressive air abrasion.

gesting that they were exposed to increasinglysaline and alkaline fluids as the lake shrank.

40Ar/39Ar METHODS

Tuff beds selected for this study are fromlocations exhibiting the least degree of alter-ation we observed. Sanidine and biotite crys-tals 75–500 (mm in diameter were separatedfrom seven tuffs by using a combination ofcrushing, acid leaching, heavy-liquid separa-tion, and air abrasion (Fig. 5; Krogh, 1982).Sanidine crystals (;0.001 mg each) werehandpicked under refractive-index oils, thencleaned ultrasonically in cold 10% HF for 10min, rinsed in deionized water, and packagedfor irradiation. Euhedral, optically fresh, 250–500 mm diameter biotite crystals were sepa-rated prior to acid leaching and handpicked forirradiation.

These crystals were loaded into 1.6 cm di-ameter aluminum discs together with flux mon-

itors and irradiated for either 75 (UW-06) or 50(UW-11) h at the Oregon State University Trigareactor where they received fast-neutron dosesof ;7.5–5 3 1018 cm–2, respectively. Correc-tions for undesirable nucleogenic reactions on40K and 40Ca are as follows: [40Ar/39Ar]K 50.00086; [36Ar/37Ar]Ca 5 0.000264; and [39Ar/37Ar]Ca 5 0.000673.

The 40Ar/39Ar method requires the age of asample to be calculated relative to mineralstandards that have been previously dated.Sanidine from the Taylor Creek rhyolite (TCs;Duffield and Dalrymple, 1990) was used tomonitor the experiments. Its age is 28.34 60.28 Ma relative to 98.79 6 0.96 Ma for theprimary standard GA-1550 biotite (McDou-gall and Roksandic, 1974; Renne et al., 1998),which is intercalibrated to 28.02 6 0.28 Mafor the sanidine from the Fish Canyon Tuff(FCs) and 523.1 6 4.6 Ma for hornblendefrom the McClure Mountain syenite (MMhb;Sampson and Alexander, 1987). We recognize

that the age of the GA-1550 primary standardremains controversial (e.g., Lanphere and Dal-rymple, 2000; Lanphere and Baadsgaard,2001), to some extent owing to uncertaintiesin the 40K decay constant (Min et al., 2000;Schmitz and Bowring, 2001). The decision touse the values just given is based, in part, onimproved agreement between 40Ar/39Ar andastronomical ages for the last several reversalsof the geomagnetic field (Renne et al., 1994)and on a U-Pb monazite intercalibration ofFCs at 27.98 6 0.15 Ma (Villeneuve et al.,2000). As an internal check on accuracy, wemade several laser-fusion measurements ofGA-1550 biotite, FCs, MMhb, and U.S. Geo-logical Survey standard biotite SB-3, with re-sults listed in Table DR1.1

Methods used in the extraction and analysisof argon follow Singer and Brown (2002).Mass discrimination was monitored by usingan on-line air pipette and varied between1.0025 6 0.0010 and 1.0035 6 0.0010 peramu during the 15-month analytical period.Inverse-variance weighted-mean ages andstandard deviations were calculated accordingto Taylor (1982) by using the ArArCALCsoftware of Koppers (2002). Where meansquared weighted deviation (MSWD) valueswere .1, the uncertainties were multiplied bythe square root of the MSWD. Precision es-timates for monitor positions, based on 14–54isotopic measurements of standard grains sur-rounding sample positions, reveal a gradientof up to 0.5% per cm in neutron fluence acrossthe discs. By using an inverse-distance-weighted spatial-interpolation algorithm, theuncertainty in J was estimated at between0.12% and 0.24%, which was propagated intothe final weighted-mean ages for each sample(Fig. DR1 [see text footnote 1]). To test forexcess argon, isochrons were regressed by us-ing the method of York (1969). Ages were cal-culated with the decay constants of Steigerand Jager (1977) and are reported with 62suncertainties.

Uncertainties are reported in three ways:first, as internal analytical uncertainties reflect-ing only precision on peak signals, systemblanks, mass discrimination, and neutron flu-ence; second, as analytical and intercalibrationuncertainties that relate secondary standards

1GSA Data Repository item 2003064, a tableshowing intercalibration of 40Ar/39Ar standards per-formed at the UW-Madison, a diagram illustratingthe distribution of measured neutron fluence (J)across the irradiation discs, a table containing thefull 40Ar/39Ar data set, and a table giving the recal-ibrated magnetic chron boundaries, is available onthe Web at http://www.geosociety.org/pubs/ft2003.htm. Requests may be sent also to [email protected].



TABLE 2. SUMMARY OF 40Ar/39Ar LASER-FUSION EXPERIMENTAL RESULTS FOR SEVEN TUFF BEDS

Sample Position n Isochron analysis† Average Apparent ages†

number (latitudeSUMS 40Ar/36Ari Isochron age‡

K/Ca 6 2sMSWD Weighted-mean‡ 62s§ 62s#

longitude)(N-2) (Ma) 6 2s age (Ma) 6 2s

Analcite tuff 41821901.380N 19 of 19 0.85 281.2 6 48.9 49.00 6 0.21 Sanidine 0.78 48.94 6 0.12 0.18 0.85SB-1 108840904.680W 779 6 340Sixth tuff 41832931.880N 22 of 26 0.76 294.1 6 9.7 49.72 6 0.14 Biotite 0.73 49.70 6 0.10 0.17 0.86TR-5 109828955.950W 884 6145

8 of 25 0.58 290.8 6 16.1 49.98 6 1.20 Sanidine 0.55 49.79 6 1.04 1.05 1.342778 6 529

Main tuff 41832931.880N 30 of 31 0.77 293.4 6 6.4 49.98 6 0.09 Sanidine 0.65 49.96 6 0.08 0.16 0.86TR-1 109828955.950W 97.5 6 11.1

16 of 30 1.05 296.1 6 8.1 50.00 6 0.20 Biotite 0.99 50.01 6 0.15 0.21 0.8718962 6 3066

Grey tuff 41839920.620N 18 of 18 0.73 285.7 6 11.3 50.55 6 0.21 Sanidine 0.67 50.39 6 0.13 0.19 0.87WN-1 109817915.160W 190 6 130C Bed tuff 41820942.360N 15 of 20 0.49 299.8 6 7.5 50.35 6 0.45 Biotite 0.55 50.56 6 0.26 0.30 0.90FC-3 109825937.850W 531 6 86Firehole tuff 41821900.960N 14 of 14 1.05 295.5 6 1.7 50.70 6 0.15 Sanidine 0.97 50.70 6 0.14 0.20 0.88FC-2 109822960.000W 190 6 130Rife tuff 41857947.230N 13 of 24 0.67 292.6 6 11.4 51.41 6 0.68 Biotite 0.63 51.25 6 0.31 0.34 0.93BT-18 109815908.660W 701 6 115

†Ages relative to 28.34 Ma for Taylor Creek rhyolite (TCs) (Renne et al., 1998).‡Analytical uncertainties.§Analytical and intercalibration uncertainties.#Fully propagated uncertainties.

with the GA-1550 primary standard; and third,as fully propagated uncertainties that incorpo-rate analytical and intercalibration uncertain-ty in addition to uncertainty associated withisotope-dilution K-Ar measurements of theprimary standard and rather large, but under-appreciated uncertainty in the 40K decay con-stants (Table 2; Karner and Renne, 1998;Schmitz and Bowring, 2001). Internal analyt-ical uncertainties are appropriate when assess-ing age differences among samples that ref-erence the same standard, whereasintercalibration uncertainties permit directcomparisons with ages or time scales calcu-lated relative to other 40Ar/39Ar standard ages.Direct comparison of our 40Ar/39Ar ages to U-Pb ages (e.g., Schmitz and Bowring, 2001)and other independent isotopic chronometersnecessitates the use of fully propagateduncertainties.

40Ar/39Ar RESULTS

Ages for the Green River Formation weredetermined on the basis of 207 single andmulticrystal laser-fusion 40Ar/39Ar experimentson phenocrysts from seven tuff beds (Fig. 6;Table 2; complete analytical data in TableDR2 [see text footnote 1]). Samples sites lat-erally span a total of 90 km and 410 m ofvertical stratigraphy in the Greater Green Riv-er Basin (Figs. 1 and 2; Table 2). Fusions of1–10 crystals each were performed in order toobtain adequate signal sizes from grains com-monly smaller than 1 mg. In some instances,analyses of one- to three-grain aliquots of bi-otite gave ages that skew the cumulative prob-

ability diagram toward older ages. Because oftheir euhedral shape and position at the baseof apparently unreworked ash-fall beds, a de-trital origin for these biotite crystals is unlike-ly. Instead, we interpret these outliers to resultfrom the inclusion of xenocrystic biotite aris-ing from either the incorporation of oldergrains from the preeruptive volcanic edifice orfrom the incomplete degassing of crystals de-rived from basement rocks fragmented duringeruption. As a result, biotite analyses havebeen excluded from isochron and weighted-mean calculations when their inclusion resultsin MSWD values .1 (Deino and Potts, 1990;Ton-That et al., 2001). A lack of excess argonis suggested by isochron analyses; however,the spread of data points along many of theisochrons is limited owing to high contents ofradiogenic argon, generally .80%–90% (Fig.6). Thus, we take the weighted-mean apparentages as our most precise estimates of timeelapsed since eruption and deposition of thesetuff beds. Because our emphasis is initially ondetermining the duration of lacustrine depo-sition in the Greater Green River Basin,weighted-mean apparent ages and isochronsare discussed in the following sections with2s analytical uncertainties that are appropriatefor intercomparison of each dated tuff.

Rife Tuff

The Rife tuff (sample BT-18) was collectedfrom the Rife Bed of the Tipton Member atthe Boar’s Tusk section of Pietras et al. (2003),;45 km north of Rock Springs, Wyoming(Table 1). It is located within the Rife Bed of

the Tipton Member, 2 m below the contactwith the Wilkins Peak Member. Because theRife Bed is much thinner (only 4 m) at theBoar’s Tusk section as compared with othermore basinward measured sections, it is dif-ficult to be precise about the stratigraphic po-sition of the Rife tuff. However, the Rife tuffwas chosen for dating because it containsabundant coarse-grained biotite, whereas othertuffs collected from the Tipton Member haveas yet failed to produce datable phenocrysts.

Twenty-four experiments on three-grain al-iquots of biotite allowed the identification andexclusion of outliers from the population. Thecumulative probability plot (Fig. 6A) for theRife tuff shows a bimodal age distribution. Al-though both populations (ca. 51.25 and ca.53.0 Ma) fit the stratigraphic order of ages ofother tuffs (Fig. 6), we prefer the minimumage as the best estimate of time elapsed sincedeposition of the Rife bed. Because the tuff isinterbedded with laminated calcareous mud-stones and shows no indications of reworking,a xenocrystic and not detrital interpretation forthe older population is favored. Because eachfusion incorporated three grains, we suggestthat the older apparent ages represent mixingof one or more xenocrystic grains with juve-nile biotite grains. Thus, the actual age of thexenocrystic material likely exceeds the olderpeak of probability density shown in Figure6A. When these outliers are omitted from theisochron and weighted-mean calculations, 13of a total of 24 fusions define an isochron of51.41 6 0.68 Ma with an atmospheric 40Ar/36Ar intercept of 292.6 6 11.4. These 13 bi-otite analyses yielded a weighted-mean age of


SMITH et al.

Figure 6. Cumulative probabilityand inverse isochron diagrams il-lustrating 40Ar/39Ar experimentalresults. Analyses excluded from agecalculations shown as open sym-bols. In all cases the weighted-meanage is the preferred depositionalage. Gray shading indicates 2s en-velope of uncertainty for each sam-ple. Note that the tuff samples arearranged in stratigraphic order andthat the determined ages are con-sistent with this order. MSWD—mean square of weighted deviates.



TABLE 3. SUMMARY OF AVERAGE MAJOR ELEMENT COMPOSITIONS OF PHENOCRYSTS

Oxide Sixth Main Oxide Analcite Sixth Main Grey C bed Firehole

Biotiten 5 26

Biotiten 5 10

Sanidinen 5 137

Sanidinen 5 19

Plagn 5 4

Sanadinen 5 18

Plagn 5 142

Auth.K-sparn 5 14

SanidineCore

n 5 44

Plagn 5 13

Sanadinen 5 15

Plagn 5 8

Sanadinen 5 11

Plagn 5 4

SiO2 34.4 33.1 SiO2 65.2 65.2 69.0 64.3 66.6 63.6 62.6 61.1 63.8 60.3 63.0 57.9Al2O3 13.6 13.7 Al2O3 19.0 19.1 19.9 18.6 19.4 17.6 18.7 25.0 18.8 24.7 19.1 26.0FeO 20.7 25.6 FeO 0.12 BDL BDL BDL BDL BDL 0.08 0.16 BDL BDL BDL 0.14MgO 9.97 6.38 CaO 0.28 0.09 BDL 0.10 BDL BDL 0.13 5.75 BDL 6.03 0.17 8.07MnO 0.13 0.47 Na2O 6.14 1.74 12.2 2.62 11.9 0.23 2.44 8.05 1.02 7.83 3.28 6.70CaO 0.01 0.02 K2O 7.69 14.5 0.03 12.7 0.04 15.8 12.5 0.86 15.6 0.55 11.4 0.36Na2O 0.49 0.31 BaO BDL 0.23 BDL 0.18 BDL BDL 1.22 BDL 0.70 BDL 2.03 BDLK2O 7.12 7.41 Total 98.5 100.8 101.3 98.6 97.9 97.3 97.7 100.9 100.0 99.5 99.1 99.2TiO2 4.96 4.05BaO 1.13 0.53 Anx 1 1 27 29 1 39Total 93.0 91.7 Aby 54 15 100 24 100 1 23 68 9 68 30 59

Orz 45 84 76 99 77 5 91 3 69 2

Note: Wavelength-dispersive measurements were made with a Cameca SX51 electron microprobe using a 10 nA, 15 keV, 5 mm defocused beam, with 10 s peak and10 s background counting times, and natural and synthetic mineral standards. Compilation includes 465 individual electron-microprobe analyses. Abbreviations: n—numberof analyses, BDL—below detection limit.

51.25 6 0.31 Ma (MSWD 5 0.63) (Fig. 6A,Table 2).

Firehole Tuff

The Firehole tuff (sample FC-2), or Firsttuff (Culbertson, 1961) was collected from thelower part of the Wilkins Peak Member of theGreen River Formation at Firehole Canyon(Table 1). It is stratigraphically ;60 m abovethe base of the Wilkins Peak Member (bed264, Plate 1 of Roehler, 1992a) and containschemically homogeneous, unaltered Or69 san-idine and An39 andesine phenocrysts (Table 3).Six single-crystal laser fusions taken togetherwith eight multicrystal measurements of san-idine form a well-defined Gaussian peak in acumulative probability diagram without evi-dence of xenocrystic contamination (Fig. 6B).Collectively, these analyses define an isochronof 50.70 6 0.15 Ma with an 40Ar/36Ar inter-cept of 295.5 6 1.7, consistent with an at-mospheric trapped component. The weightedmean of these analyses yields a preferred ageof 50.70 6 0.14 Ma (MSWD 5 0.97) for theFirehole tuff (Fig. 6B).

C Bed Tuff

The C Bed tuff (sample FC-3) was collected;5 m below the base of the ‘‘C’’ sandstone-mudstone unit (;bed 302, Plate 1 of Roehler,1992a; base of BC6 oil shale of Culbertson,1998) from the lower Wilkins Peak Memberat Firehole Canyon, Wyoming (Table 1).Chemically homogeneous, unaltered Or91 san-idine and An29 andesine phenocrysts (Table 3)are encased within a matrix of authigenic K-feldspar and analcime, necessitating the use ofair abrasion to eliminate overgrowths.

Because multigrain fusions of sanidine

crystals separated from the C Bed tuff wereindicative of xenocrystic or detrital contami-nation, 40Ar/39Ar experiments were undertakenon three-grain aliquots of biotite crystals.Twenty experiments allowed the identificationand exclusion of outliers from the population.The cumulative probability plot (Fig. 6C) forthe C Bed tuff is skewed toward older agesthat are interpreted to be xenocrystic in originand were omitted from weighted-mean andisochron calculations as already described.Fifteen of a total of 20 three-crystal fusionsdefine an isochron of 50.35 6 0.45 Ma withan atmospheric 40Ar/36Ar intercept of 299.8 67.5 and a preferred weighted-mean age of50.56 6 0.26 Ma (MSWD 5 0.55) (Fig. 6C,Table 2).

Grey Tuff

The Grey tuff sample WN-1 was collectedfrom the middle of the Wilkins Peak Memberat a section 5 km north of Rock Springs, Wy-oming (Figs. 1, 2, and 4A; Tables 1 and 2),and is stratigraphically ;9 m above the ‘‘D’’sandstone-mudstone unit (;bed 321, Plate 1of Roehler, 1992a; base of DE2 oil shale ofCulbertson, 1998). Unaltered, euhedral, Or75

sanidine phenocryst cores with magmatic Bazonation and andesine An27 phenocrysts areencased in Or99 K-feldspar resulting from thealteration of the original glassy ash matrix(Fig. 5B, Table 3). Plagioclase and sanidinecompositions form a near-equilibrium tie lineon a ternary-feldspar diagram, suggesting thecoexistence of these phases in the preeruptivemagma. Backscattered-electron images illus-trate the effectiveness of the air-abrasion tech-nique at removing the K-feldspar overgrowthsfrom phenocryst cores (Fig. 5C).

Eighteen multigrain aliquots form a single,

well-defined Gaussian peak in a cumulativeprobability plot (Fig. 6D). The 50.55 6 0.21Ma isochron defined by these experiments hasan 40Ar/36Ar intercept of 285.7 6 11.3. Theweighted mean of the 18 analyses is 50.39 60.13 Ma (MSWD 5 0.67) and gives the pre-ferred age for the Grey tuff (Fig. 6D, Table2).

Main Tuff

Sample TR-1 was collected from the upperpart of the Wilkins Peak Member at Toll GateRock (Figs. 1 and 2; Tables 1 and 2). TheMain or Third tuff of Culbertson (1961) is thelowermost of three .10-cm-thick tuff bedsand numerous smaller tuff beds in an 8-m-thick span 6–14 m below the ‘‘I’’ sandstone-mudstone unit (bed 392, Plate 1 of Roehler,1992a). Phenocrysts separated from the low-ermost unit are chemically homogeneous, un-altered Or76 sanidine and presumably alteredAb100 albite crystals (Fig. 5A, Table 3). As theglassy ash matrix of the Main tuff has beenaltered to fine-grained analcime, we speculatethat the albitization of plagioclase phenocrystsreflects authigenic processes.

Sanidine from the lowermost unit of theMain tuff yielded 30 (of 31) multigrain fu-sions that define an isochron of 49.98 6 0.09Ma with an 40Ar/36Ar intercept of 293.4 6 6.4.One analysis that was excluded from the iso-chron and weighted-mean calculations gave anapparent age of 54.87 6 1.18 Ma, indicatingprobable xenocrystic contamination. The 30included analyses form a well-defined Gauss-ian distribution in a cumulative probabilityplot (Fig. 6F). The weighted mean of thesemeasurements, 49.96 6 0.08 Ma (MSWD 50.78), is the preferred age for this tuff bed(Fig. 6F, Table 2).


SMITH et al.

40Ar/39Ar experiments were also undertakenon biotite crystals from the lowermost unit ofthe Main tuff. Thirty-one experiments allowedthe identification and exclusion of outliersfrom the population as already described. Thecumulative probability plot (Fig. 6E) for theMain tuff is skewed toward older ages, whichare interpreted to be xenocrystic in origin, andwere omitted from weighted-mean and iso-chron calculations as already described. Fif-teen of a total of 31 three-crystal fusions de-fine an isochron of 50.00 6 0.20 Ma with anatmospheric 40Ar/36Ar intercept of 296.1 68.1. Biotite analyses yielded a weighted-meanage of 50.01 6 0.15 Ma (MSWD 5 0.99),which is virtually identical to the more precisesanidine age of 49.96 6 0.08 Ma (Fig. 6F,Table 2) for the Main tuff.

Sixth Tuff

Sample TR-5 was collected from the tran-sitional interval between the Wilkins Peak andLaney Members at Toll Gate Rock ;1 kmwest of Green River, Wyoming (Figs. 1 and2; Tables 1 and 2). The Sixth tuff of Culbert-son (1961) is located ;32 m above the ‘‘I’’sandstone-mudstone unit (bed 423, Plate 1 ofRoehler, 1992a). Microprobe analyses of phe-nocrysts reveal chemically homogeneous, un-altered Or84 sanidine phenocrysts and presum-ably altered Ab100 albite crystals (Table 3).Similar authigenic albitization of plagioclasewas documented by Surdam and Parker(1972) to be associated with analcime and K-feldspar alteration in Green River Formationtuff beds.

Sanidine from the Sixth tuff proved to beheterogeneous in age, which limited the num-ber of grains that could be grouped with con-fidence to define the juvenile eruptive age.Only 8 of 25 multigrain analyses, each com-prising three crystals, gave stratigraphicallyreasonable ages and yielded an impreciseweighted-mean age of 49.79 6 1.04 Ma (Ta-ble 2).

40Ar/39Ar experiments were also performedon euhedral biotite crystals separated from thebasal 1 cm of the Sixth tuff. Twenty-six ex-periments allowed the identification and ex-clusion of outliers from the population. Thecumulative probability plot (Fig. 6G) for theSixth tuff is skewed toward older ages, whichare interpreted to be xenocrystic in origin, andwere omitted from the weighted-mean andisochron calculations as already described.Twenty-one of a total of 26 three-crystal fu-sions define an isochron of 49.72 6 0.14 Mawith an atmospheric 40Ar/36Ar intercept of294.1 6 9.7. Biotite analyses yielded a pre-

ferred weighted-mean age of 49.70 6 0.10 Ma(MSWD 5 0.73) (Fig. 6G, Table 2).

Analcite Tuff

Sample SB-1 is from the upper part of theLaClede Bed of the Laney Member on thenorthwest face of Sand Butte, eastern GreaterGreen River Basin (Fig. 1, Table 2). The Anal-cite tuff occurs 7 m above the Buff MarkerBed (bed 433, Plate 2 of Roehler, 1992a) andis correlative across the eastern Greater GreenRiver Basin (Ratterman and Surdam, 1981).Microprobe transects of sanidine grains revealhomogeneous Or45 compositions and show noevidence for chemical alteration (Table 3).

Sanidine crystals from the Analcite tuff arehighly radiogenic; their mean 40Ar* values of97.8% limit the spread along the isochron,which is 49.00 6 0.31 Ma. The 31 multigrainanalyses define a Gaussian peak on a cumu-lative probability plot (Fig. 6H) and show noevidence for xenocrystic contamination. Takentogether, these 31 analyses yield a weighted-mean age of 48.94 6 0.12 Ma (MSWD 50.78) (Fig. 6H, Table 2).

Summary of Analytical Data

The weighted-mean ages determined fromthe seven tuff beds are between 51.25 6 0.31and 48.94 6 0.12 Ma and are consistent withtheir stratigraphic positions (Figs. 2 and 6).We therefore interpret these ages to representthe depositional ages of the sediments inwhich they occur. The K-Ar and 40Ar/39Arages of Mauger (1977) and O’Neill (1980) forthe end of Green River Formation sedimen-tation are 3–4 m.y. younger than those pre-sented here (Fig. 3), suggesting that some bi-otite grains that had undergone 40Ar loss dueto diagenetic alteration may have been incor-porated into their analyses. We avoided thisproblem by analyzing aliquots of handpickedeuhedral biotite crystals that were ;100,000times smaller than those analyzed by Mauger(1977) or O’Neill (1980). The ages for theAnalcite, Sixth, Main, Grey, Firehole, andRife tuffs were the most precise, with internal2s uncertainties of 680–140 k.y. Xenocrysticor detrital contamination was clearly identifiedin experiments on biotite from the Sixth,Main, C Bed, and Rife tuffs, and these anal-yses were excluded from the age calculationsfor these beds. None of the mineral separatesanalyzed showed definitive evidence of excessargon (Fig. 6).

Despite evidence from other Tertiary ash-flow tuffs that biotite gives older ages thansanidine (e.g., Lipman, 2000), we are encour-

aged by the fact that biotite analyses for theMain tuff yielded an age that is indistinguish-able from its sanidine age when potential xen-ocrystic or detrital contamination was re-moved from the calculation (Fig. 6, C and D).Moreover, biotite ages for the Sixth, C Bed,and Rife tuffs are in agreement with theirstratigraphic position relative to the otherages.

DISCUSSION

Accumulation Rates of the Green RiverFormation

Previous estimates of Green River Forma-tion accumulation rates were based on mea-suring the thicknesses of laminae ,1 mmthick in organic-rich calcimicrites (oil shales),under the assumption that these laminae rep-resent varves (Bradley, 1929; Crowley et al.,1986; Ripepe et al., 1991). Ripepe et al.(1991) reported the average lamination thick-ness in the Tipton and Laney Members to be96 mm and 107 mm, respectively. However,using laminae thickness to estimate accumu-lation rates is problematic (cf. Dott, 1983), es-pecially because evidence for depositional hi-atuses and periods of erosion occursthroughout the Green River Formation(Smoot, 1983; Surdam and Stanley, 1979;Roehler, 1993; Bohacs et al., 2000). In addi-tion, much of the formation, particularly theevaporative Wilkins Peak Member, is com-posed of nonlaminated facies for which varvecounting cannot be applied to estimate accu-mulation rate.

Our 40Ar/39Ar results permit the first directmeasurement of accumulation rates in theGreen River Formation that do not rely onlamina counts. For a reference section near thesouthern Green River Basin depocenter(Roehler, 1992a, Plate 1), we calculate accu-mulation rates of 88 6 34 and 104 6 18 mm/yr for the Tipton and Laney Members, respec-tively. These rates are consistent with anannual origin for the laminae, as previouslyproposed (Fig. 7; Bradley, 1929; Crowley etal., 1986; Ripepe et al., 1991). However, be-cause of the evidence for depositional hiatusesthroughout the Tipton and Laney Members,our data cannot completely exclude faster truerates for sediment accumulation. Dependingon how much time is ‘‘missing’’ from thestratigraphic record, the laminae might in ac-tuality be seasonal rather than annual, and theindividual lake cycles might be shorter thancalculated.

A very different accumulation rate storyemerges from the Wilkins Peak Member. For



Figure 7. Chart showing average sediment-accumulation rates for the Green River For-mation based on reference section in the southern Greater Green River Basin (see Roehler,1992a, Plate 1). Lake-type patterns as in Figure 3. Note the abrupt increase in accumu-lation rate corresponding to the Wilkins Peak Member.

the same southern Green River Basin refer-ence section (Roehler, 1992a, Plate 1), we cal-culate a net average rate of 327 6 85 mm/yr,or approximately three times as fast as for theTipton or Laney Members. A part of this dif-ference might be attributable to differingamounts of compaction, as the Wilkins PeakMember contains a lower proportion of pro-fundal lake facies compared to the Tipton andLaney Members (Bradley, 1964; Eugster andHardie, 1975; Surdam and Stanley, 1979;Smoot, 1983; Roehler, 1993). Reed and Oertel(1978) used the degree of preferred orientationof clay particles to estimate compaction in theTipton and Wilkins Peak Members and con-cluded that thinly laminated, organic-rich mi-crite had undergone up to twice as much com-paction as massive micrite. However, theactual difference in gross interval compactionof these units should be less, because eachunit contains both lithologies. Furthermore, la-cunae appear to occur most frequently in theWilkins Peak Member (Smoot, 1983; Roehler,1993; Bohacs et al., 2000), suggesting that thecalculated accumulation rate more seriouslyunderestimates the true accumulation rates ofthe Wilkins Peak than is the case for the Tip-ton or Laney Members.

Our observation of faster accumulationrates in evaporative facies may seem counter-intuitive, but it is in accord with U-series dis-equilibrium dating results from Quaternaryplaya-lake evaporites. Ku et al. (1998) foundthat accumulation rates in a core from DeathValley were much higher for salt-pan evapo-

rite facies than for lake mud, and Bobst et al.(2001) noted that accumulation rates duringdry phases of the Salar de Atacama were upto three times as high as during wet periods.Net accumulation rates in these cases rangebetween ;0.5 and 1.8 mm/yr, which is up tofive to six times faster than deposition of theWilkins Peak Member. However, higher ratesof Wilkins Peak Member evaporite accumula-tion may have occurred at its subsurface de-pocenter (to the west of the reference section).Our observation of faster rates in lake-marginal facies supports that of Buchheim(1994), who noted that rates of carbonateprecipitation in the adjacent Fossil Basinwere faster near the lake shore than at its cen-ter. In contrast, studies of clastic-sediment–dominated lake systems have documented ba-sin-center rates that are much faster than rateson the basin margin (e.g., Ensley and Verosub,1982; Elston et al., 1994), presumably reflect-ing sediment bypass.

A tectonically derived increase in potentialaccommodation during deposition of the mid-dle part of the Wilkins Peak Member may ex-plain the higher rates of sediment accumula-tion during this period (cf. Carroll andBohacs, 1999). Uplift of surrounding rangesand concurrent basin subsidence may haveaugmented erosion rates adjacent to the basinwhile simultaneously increasing accommoda-tion space. Syndepositional relationships be-tween lacustrine strata and faults along thenorthern (Steidtmann and Middleton, 1991;Pietras et al., 2003), southern (Bradley, 1964;

Roehler, 1993), and western (DeCelles, 1994)margins of the Greater Green River Basin in-dicate active Eocene uplift of basin-boundingstructures. Furthermore, isopach trends (Sul-livan, 1985; Beck et al., 1988; Roehler, 1993)indicate increased tectonically driven differen-tial subsidence in the eastern Greater GreenRiver Basin during middle Wilkins Peak de-position. The increase in sediment-accumulation rate shown in Figure 7 maytherefore coincide with a pulse of Laramidetectonism at ca. 50.5 Ma.

Our new ages support the notion that 1–3m lake expansion-contraction cycles in the La-ney Member may record precessional forcing(Surdam and Stanley, 1979; Rhodes et al.,2002). However, age data indicate that indi-vidual lake expansion-contraction episodes re-corded in the evaporative Wilkins Peak Mem-ber did not occur in response to 21 k.y.precessional forcing as proposed by Roehler(1993). Roehler (1991, 1993) documented 77correlative lithologic cycles, each defined bya basal organic-rich calcimicrite (oil shale)bed, that represent episodes of lake expansionand contraction. On the basis of an estimated1.6 m.y. duration for the Wilkins Peak, Roeh-ler (1993) thus calculated an average periodof 21,779 k.y. In contrast, our 40Ar/39Ar agesgive an average duration of the 69 oil shalecycles that occur between the Firehole andSixth tuffs of 14.5 6 1.7 k.y. In addition, re-cent detailed outcrop and drill-core observa-tions (Smith et al., 2001) indicate that WilkinsPeak expansion-contraction cycles are morenumerous than indicated by Roehler (1993).Our age data therefore exclude 19–23 k.y. pre-cessional periods as the cause of these cycles.

Calibration and Correlation of the EocenePaleontologic Record

Before we can discuss the broader impli-cations of these new ages, they must be con-sidered at intercalibration uncertainties (Table2; Renne et al., 1998) and relative to a com-parable global time scale. Use of the most re-cent geomagnetic polarity time scale (GPTS)(Cande and Kent, 1992, 1995; Berggren et al.,1995) is inappropriate because it was con-structed with radioisotopic calibration pointsthat are not compatible with intercalibrated40Ar/39Ar standard values (Renne et al., 1998),as noted by Obradovich and Hicks (1999). Wehave addressed this problem by reevaluatingthe Eocene GPTS using calibration points cal-culated relative to the standard values of Ren-ne et al. (1998). This approach shifts the agesof the boundaries defining chrons 19–24 from10.36% to 11.08% (Fig. 8, Table DR3 [see


SMITH et al.

Figure 8. Chart showing recalibration of the early and middle Eocene GPTS (Cande andKent, 1992, 1995; Berggren et al., 1995). Cande and Kent’s (1992) scaled seafloor-anomalymodel was fitted by using a cubic spline curve to nine calibration points. We have recal-ibrated the Eocene GPTS by replacing two of the original tie points: the ages of 65.0 Mafor the Cretaceous/Paleocene (K/T) boundary and 55.0 Ma for Paleocene/Eocene (P/E)boundary. Adopting the Renne et al. (1998) intercalibration values, the age of K/T bound-ary shifts 0.77% from 65.0 Ma to 65.5 Ma (Obradovich and Hicks, 1999). Recalculatingthe 55.0 Ma age used for the P/E boundary is more problematic, however, because of theuncertain stratigraphic relationship of the dated no. 70/-17 ash unit to the P/E (NP9/NP10)nannofossil boundary (Berggren and Aubrey, 1996, 1998), unconformities (Aubrey, 1995,1998), and the transition from C24r to C25n (Townsend, 1985; Ali and Hailwood, 1998)within Deep Sea Drilling Project core 550 (cf. Wing et al., 2000). Moreover, as a furthercomplication to any estimates, disagreement between the 54.51 6 0.10 Ma (Swisher andKnox, 1991; Berggren and Aubry, 1998) and 55.07 6 0.32 Ma (Wing et al., 1991; Berggrenet al., 1992) 40Ar/39Ar ages for the no. 70/-17 ash used by Cande and Kent (1992, 1995) toestimate the age of the P/E boundary. Because of these problems with the P/E calibrationpoint, we have replaced it with a stratigraphically higher 53.09 6 0.34 Ma 40Ar/39Ar age(Wing et al., 1991) for a tuff bed that is at the base of chron 24n.1 in lower Eocene BighornBasin sedimentary rocks of the Willwood Formation (Fig. 1; Clyde et al., 1994; Tauxe etal., 1994; Flynn and Tauxe, 1998). After integrating these two revised calibration points,the early and middle Eocene GPTS is fully compatible with 40Ar/39Ar ages relative to theintercalibration standards of Renne et al. (1998).

text footnote 1]). We anticipate that similarmodification of the Cande and Kent (1992,1995) time scale for the early Eocene and oth-er time periods will further improve the globalGPTS. It must be noted that the precision inthe age of any individual chron boundary islimited by the intercalibration uncertainty,typically 60.2–0.5 m.y., of the calibrationpoints used to interpolate it. In addition, thestratigraphic position of the calibration pointsand the spline-curve calculation used to inter-polate between calibration points introduceadditional uncertainty. For comparison withisotopic chronometers other than 40Ar/39Ar, thefully propagated uncertainties for early Eo-

cene chron boundaries are approximately 61m.y.

The Wasatchian/Bridgerian NALMA bound-ary is biostratigraphically assigned to the allu-vial facies equivalent to the lower middle Wil-kins Peak Member (Morris, 1954; McGrew andRoehler, 1960; Gazin, 1965; Krishtalka et al.,1987; Honey, 1988; Zonneveld et al., 2000). Ifa position between the Firehole and Grey tuffsis assumed for the Wasatchian/Bridgerianboundary, its age can be interpolated on thebasis of new 40Ar/39Ar age determinations to be50.55 6 0.43 Ma. Likewise, the age of theGardnerbuttean/Blacksforkian NALMA sub-stage boundary, located in alluvial strata equiv-

alent to the upper fifth of the Wilkins PeakMember (Roehler, 1989; Zonneveld et al.,2000), can be interpolated to be 49.83 6 0.39Ma if a position between the Main and Sixthtuffs is assumed. Note that the current limitingfactor on the precision of faunal boundary agesin the Greater Green River Basin is biostrati-graphic, not radioisotopic in origin. Future bio-stratigraphy and correlation of alluvial strata tothe Green River Formation therefore have thepotential to further improve the age resolutionof these boundaries.

Biostratigraphic correlation of radiosotopi-cally and magnetostratigraphically definedBighorn Basin sedimentary rocks to the Great-er Green River Basin rocks extends the agemodel presented here by ;2 m.y. (Fig. 3). TheLysitean/Lostcabinian (Wa6/Wa7) NALMAsubstage boundary in the Bighorn Basin, lo-cated near the top of the Willwood Formation,was 40Ar/39Ar dated by Wing et al. (1991). In-cremental heating of 100 mg of sanidine froman ash-fall bentonite bed yielded a plateau ageof 53.09 6 0.34 Ma (Figs. 1 and 3). Wing etal.’s (1991) age was also used in this study torecalibrate the GPTS based on its position atthe base of chron 24n.1 (Fig. 8) within themagnetostratigraphy of Tauxe et al. (1994)and Clyde et al. (1994). Further, the magne-tostratigraphy of Tauxe et al. (1994) andClyde et al. (1994) limits the Graybullian/Lys-itean (Wa5/Wa6) NALMA substage boundaryto the upper third of chron C24n.3, dated atca. 53.5 Ma according to the revised GPTS(Figs. 3 and 8; Table 3).

The age constraints for the Graybullian/Lysitean (Wa5-Wa6) and Lysitean/Lostcabi-nian (Wa6-Wa7) substage boundaries, whichcoincide with the main body of the WasatchFormation–Luman Tongue and the LumanTongue–Niland Tongue lithostratigraphic tran-sitions (Gazin, 1965; McGrew and Roehler,1960; Holroyd and Smith, 2000; Anemone,2001), can also be applied to the GreaterGreen River Basin. Taken together with the40Ar/39Ar ages presented here, deposition ofthe Green River Formation is shown to havebeen initiated with the Luman Tongue at ca.53.5 Ma and to have continued for ;5 m.y.(Fig. 3).

Magnetostratigraphy

Clyde et al. (1997) presented magnetostra-tigraphy of a section on the western edge ofthe Greater Green River Basin encompassingthe main body of the Wasatch Formation, theTipton and Wilkins Peak Members, the Cathe-dral Bluffs Tongue, and the Laney Member(Roehler, 1989). Clyde et al. (2001) added



magnetostratigraphy from another section nearthe northeastern edge of the Greater GreenRiver Basin encompassing the upper Cathe-dral Bluffs Tongue, the Laney Member, andthe lower Bridger Formation. These sectionsdocument two normal-polarity zones withinthe main body of the Wasatch Formation andthe Cathedral Bluffs Tongue (Fig. 3), fromwhich two alternative correlations to theGPTS of Cande and Kent (1995) were pro-posed: Correlations ‘‘1’’ and ‘‘2’’ of Clyde etal. (1997) assign the interval of reversed po-larity located between the two normal-polarityzones to chron C23r and C22r, respectively. Ineither case, Green River Formation sedimen-tation is posited to have occurred for a max-imum of ;3 m.y.

The new age model presented here, how-ever, suggests that neither correlation pro-posed by Clyde et al. (1997) is correct. Ourage model shows that the Green River For-mation spans ;5 m.y. from ca. 53.5 Ma to48.5 Ma, encompassing both C23r and C22r(Fig. 3). We suggest that the section studiedby Clyde et al. (1997) did not record C23nwithin the Tipton Member, owing to weak nat-ural remanent magnetic (NRM) intensities(,1 mA/M) and erratic demagnetization be-havior. Similarly, Sheriff and Shive (1982) re-ported weak NRM intensities and inconsistentdemagnetization in Wilkins Peak Memberrocks. Thus, weak or overprinted magnetiza-tion may be responsible for the absence of achron C23n recording in the Tipton Member.

Correlation of the Green River FormationTuffs with Eocene Volcanism in WesternNorth America

Crystal-bearing ash-fall deposits such as theone produced from the eruption of Mount St.Helens on May 18, 1980, can occur in mea-surable quantities more than 400 km down-wind of an eruption (Sparks et al., 1997). Pos-sible sources of Green River Formation tuffbeds lie within the Absaroka volcanic prov-ince, Challis Volcanic Group, and numeroussmaller volcanic fields in Montana that rangein age from 54 Ma to 47 Ma (Marvin et al.,1980; Isoplatov et al., 1996; Figs. 1 and 3).The distribution of fallout from Plinian vol-canic plumes is predominantly controlled byeruptive magnitude and the strength and di-rection of stratospheric winds (Schmincke andvan den Bogaard, 1991; Sparks et al., 1997).Paleotopographically calibrated early Eoceneclimate models (Sewall et al., 2000) show agenerally southeast-directed 500 mbar windfield and are therefore permissive of any ofthe three just-mentioned volcanic regions as

sources for Green River Formation tuffs in(Fig. 1).

The nearest and therefore most likely sourc-es of Green River Formation tuffs lie withinthe Absaroka volcanic province, which coversa 25,000 km2 area of northwest Wyoming andsouthwest Montana 100–200 km northwest ofthe Greater Green River Basin (Smedes andProstka, 1972; Sundell, 1993, Fig. 1). Recentmapping and 40Ar/39Ar dating of several intru-sive and extrusive rocks document that theAbsaroka volcanic province consists of anumber of individual volcanic centers thatwere active between 54 and 43 Ma; youngereruptions occurred from volcanic centers inthe southwestern part of the province (Harlanet al., 1996; Feeley et al., 1999; Hiza, 1999,Figs. 1 and 3). Average modal compositionsof plagioclase and sanidine phenocrysts fromAbsaroka volcanic province ash-flow tuffsrange from An28 to An39 and from Or64 to Or77

(Hiza, 1999), overlapping those observed inGreen River Formation phenocrysts (Table 3).However, correlation based on phenocrystchemistry to specific distal Green River For-mation tuff beds is uncertain because of lim-ited distal and proximal phenocryst data.

The Challis Volcanic Group of central Ida-ho and southwest Montana, 400–500 kmwest-northwest the Greater Green River Basin(Fig. 1), may have also contributed distal ash-fall deposits to the Greater Green River Basinduring the early and middle Eocene. The de-positional area of the Challis Volcanic Groupis of comparable size to the Absaroka volcanicprovince; the Challis volcanoes exhibited ex-plosive cauldron-forming eruptions between49.5 Ma and 45.7 Ma (Fig. 3; Snider andMoye, 1989; McGonigle and Dalrymple,1996) and deposited extensive pyroclasticfalls and flows into the Eocene basins ofsouthwest Montana and east-central Idahoduring the same period (Fig. 3; Janecke andSnee, 1993; Palmer and Shawkey, 1997). Cur-rent 40Ar/39Ar ages, however, limit possiblecorrelations between dated Challis VolcanicGroup eruptions and distal Greater Green Riv-er Basin tuff beds to the uppermost GreenRiver Formation and overlying Bridger andWashakie Formations (Fig. 3).

Correlation with the Global OxygenIsotope Record

The Green River Formation was depositedbetween ca. 53.5 Ma and 48.5 Ma and there-fore spans the entire early Eocene climatic op-timum (EECO) defined by global marine Oisotope records from benthic foraminifera(Figs. 3, 6, and 9; Shackleton, 1986; Zachos

et al., 2001). The EECO represents the mostrecent period of protracted greenhouse cli-mate, characterized by elevated marine andterrestrial temperatures between ca. 54 and 49Ma. In addition, the O isotope composition ofearly Eocene planktonic foraminifera indicatethe lowest latitudinal oceanic temperature (Za-chos et al., 1994). This interpretation is con-sistent with terrestrial flora and fauna in theGreen River region that suggest a frost-freehigh-latitude environment (Markwick, 1994;Greenwood and Wing, 1995). Mean annualtemperatures (MAT) in the Rocky Mountainregion inferred from paleofloral data variedbetween 15 and 25 8C, and mean annual pre-cipitation (MAP) ranged from 75 to 150 cm/yr (Bradley, 1929; Axelrod, 1968; Wilf et al.,1998). These values are supported by recentestimates from the Green River Formation andassociated alluvial strata (Wilf, 2000).

Counter to suggestions by Roehler (1993)and Matthews and Perlmutter (1994) that theWilkins Peak phase of the Green River For-mation reflects a maximum in continental tem-perature and aridity, our new 40Ar/39Ar agesindicate that this most evaporative phase oflacustrine deposition in the Greater GreenRiver Basin occurred during the latest half ofthe EECO, when deep-ocean temperatureswere relatively constant or possibly cooling(Fig. 9; Shackleton, 1986; Zachos et al.,2001). Leaf-margin MAT and MAP estima-tions from the Greater Green River Basin mir-ror the long-term global marine trend, butthese data are currently limited to the fresh-water to saline Luman, Tipton, and Laneyphases of Lake Gosiute and do not sample thehypersaline Wilkins Peak phase (Fig. 9; Wilf,2000). Although the terrestrial and marineproxy records are based on limited data sets,there does not appear to be any striking shiftin climate regionally or globally that mightcorrespond to the abrupt increase in sediment-accumulation rate and the shift to stronglyevaporative conditions in Lake Gosiute duringWilkins Peak time. A novel alternative expla-nation for these changes was proposed by Pie-tras et al. (2003), who suggested that reneweduplift of the Wind River Range may havehelped to divert rivers away from the basin,thereby reducing runoff into the lake. Our pre-liminary conclusion is that regional tectonicprocesses may have been at least as importantas global climate trends in controlling the de-position of these evaporative facies.

CONCLUSIONS

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from seven tuff beds in the Green River For-mation set limits on interpretations of thetiming and duration of deposition of ;410 mof lacustrine mudstone, sandstone, and car-bonate rocks. The Green River Formationthus represents an unparalleled archive ofquantifiable information about how largelakes evolve. Specifically, we offer the fol-lowing conclusions:

1. Laser-fusion 40Ar/39Ar experiments onsingle-crystal or ,0.01 mg multicrystal sam-ples yielded weighted-mean ages of 51.25 60.31 Ma (Rife tuff), 50.70 6 0.14 Ma (Fire-hole tuff), 50.56 6 0.26 Ma (C Bed tuff),50.39 6 0.13 Ma (Grey tuff), 49.96 6 0.08Ma (Main tuff), 49.70 6 0.10 Ma (Sixth tuff),and 48.94 6 0.12 Ma (Analcite tuff) in theGreen River Formation.

2. Sediment-accumulation rates were high-est (327 6 85 mm/yr) during the most evap-orative phase (Wilkins Peak Member) of LakeGosiute and lowest (88 6 34 mm/yr and 1046 18 mm/yr) during the deposition of lessevaporative, organic-rich facies (Tipton andLaney Members). Accumulation rates are con-sistent with an annual origin for the ,1-mm-thick laminae in the Tipton and Laney Mem-bers. Increased accumulation rates during theWilkins Peak Member may reflect increaseduplift and corresponding basin subsidencefrom ca. 51 to 50 Ma.

3. In contrast to Clyde et al. (2001), whoassigned the Green River Formation to an ;3m.y. interval between C23r and C22r on thebasis of correlating magnetostratigraphy to thegeomagnetic polarity time scale, 40Ar/39Arages indicate that it was deposited over a pe-riod of ;5 m.y. spanning magnetic chrons24n through 21r. We suggest that Clyde et al.’s(1997, 2001) magnetostratigraphic sections donot record chron 23n within the Tipton Mem-ber. Accordingly, the major faunal turnoverrepresented by the Bridgerian/Wasatchianboundary occurred after C23n at 50.55 6 0.43Ma.

4. Tuff beds of the Green River Formationwere most likely derived from the nearby Ab-saroka volcanic province, although prove-nance of some of the Green River Formationtuff beds in the Challis Volcanic Group or oth-er Eocene volcanic fields remains apossibility.

5. The Green River Formation was depos-ited between ca. 53.5 Ma and 48.5 Ma andtherefore spans the entire early Eocene cli-matic optimum (EECO) defined by global ma-rine O isotope records. The most evaporativephase of the Green River Formation, i.e., theWilkins Peak Member, coincides with the lasthalf of this period of high global temperatures.

ACKNOWLEDGMENTS

We thank Jeff Pietras, Meredith Rhodes, NickDelebo, and Rachel Smith for assistance in the field,and Monica Relle and Brian Jicha for their help inthe Rare Gas Geochronology Laboratory. We alsothank Anthony Koppers for guidance with his pro-gram ArArCALC, used to regress, calculate, and an-alyze the Ar data, and John Fournelle for sharinghis expertise with the electron probe. We appreciatethoughtful reviews by D.R. Prothero and M. Ville-neuve, and are gratful to associate editor P.R. Rennefor helpful comments and suggestions. We are in-debted to Steve Binney and Jim Darrough at OregonState University Radiation Center for facilitating theirradiations. Supported by National Science Foun-dation grants EAR-9972851 (to Singer) and ATM-0081852 (to Singer and Carroll), Geological Societyof America grant 6766–00, a grant-in-aid from Sig-ma Xi, and by Conoco Inc. and Texaco Inc.

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Printed in the USA

Appendix DR-1. UW-Madison standard intercomparison

Standard n UW-MadisonAge (Ma) ± 2σ§ MSWD Accepted

Age (Ma) ± 2σ††

GA1550 7* 98.69 ± 0.57# 0.48 [98.79 ± 0.54]§§

MMhb 8* 522.39 ± 2.98** 17.7 523.1 ± 2.6##

SB-3 7† 165.72 ± 0.85** 5.21 [162.9 ± 1.8]***FCs 17* 28.02 ± 0.14# 0.81 28.02 ± 0.16†††

*single crystal laser fusions †multigrain fusions §intercalibration uncertainties relative to 28.34 ± 0.16 Ma for Taylor Creek Rhyolitesanidine (Duffield and Dalrymple, 1990; Renne et al., 1998) #weighted mean age **total fusion age, reported when MSWD >2 ††intercalibration uncertainties relative to 98.79 ± 0.54 Ma for GA1550 biotite (Renne etal., 1998) §§primary standard biotite (isotope dilution, McDougall and Roksandic, 1974; Renne etal., 1998) ##McClure Mountain syenite hornblende (Sampson and Alexander, 1987; Renne et al.,1998) ***USGS standard biotite (Lanphere and Dalrymple, 2000); note that the value reportedhere for SB-3 (relative to 28.34 ± 0.16 for TCs) is 1.73% older than the age reported byLanphere and Dalrymple (2000); it is, however, consistent with the GA1550 age adoptedby Renne et al. (1998), which is 1.85% older than the age determined by Lanphere andDalrymple (2000) †††Fish Canyon tuff sanidine (Renne et al., 1998) Note: Values shown in brackets represent K-Ar ages for primary standards

Appendix DR-2: Positions of samples and standards inirradiations UW-06 and UW-11 with J values

Appendix DR-3: Complete 40Ar/39Ar results40Ar/39Ar 37Ar/39Ar 36Ar/39Ar %40Ar* K/CaSample

Experimentxtal

#40Ar*

(10-16 mol)Apparent Age ±

2σ MaRife tuff BT-18

biotite J = 0.014710 ± 0.16% µ = 1.0025UW11F2A 3 2.982 0.00038 0.003426 14.53 65.90 1131.1 51.43 ± 1.13

*UW11F2B 3 2.621 0.00058 0.002043 9.64 76.79 744.27 52.64 ± 0.53UW11F2C 3 2.556 0.00026 0.001947 6.13 77.31 1627.9 51.69 ± 1.50UW11F2D 3 2.516 0.00036 0.001874 9.34 77.80 1198.1 51.21 ± 0.70

*UW11F2E 3 2.553 0.00044 0.001789 8.43 79.12 968.54 52.83 ± 1.45UW11F2F 3 2.488 0.00047 0.001880 16.00 77.48 919.93 50.45 ± 1.10

*UW11F2G 3 2.183 0.00036 0.000422 9.63 94.08 1207.1 53.69 ± 0.89*UW11F2H 3 2.647 0.00045 0.002054 10.74 76.90 953.18 53.23 ± 1.01*UW11F2I 3 2.607 0.00047 0.001889 11.25 78.41 914.30 53.45 ± 1.14UW11F2J 3 2.922 0.00046 0.003290 12.72 66.57 935.29 50.89 ± 1.06UW11F2K 3 2.974 0.00080 0.003516 11.49 64.91 538.46 50.52 ± 1.60

*UW11F2L 3 2.329 0.00038 0.001047 9.30 86.51 1129.2 52.69 ± 0.94*UW11F2M 3 2.903 0.00071 0.002688 8.31 72.48 605.97 54.99 ± 1.97UW11F2N 3 2.376 0.00062 0.001457 9.38 81.69 693.69 50.79 ± 1.42UW11F2O 3 3.112 0.00072 0.003959 7.72 62.27 598.68 50.71 ± 2.02UW11F2P 3 2.546 0.00071 0.001866 6.41 78.16 602.50 52.05 ± 2.19UW11F2Q 3 2.359 0.00033 0.001301 17.33 83.51 1311.8 51.54 ± 0.50UW11F2R 3 2.398 0.00074 0.001542 8.28 80.80 581.33 50.69 ± 1.57

*UW11F2S 3 2.577 0.00077 0.001866 9.54 78.43 562.00 52.86 ± 0.87*UW11F2T 3 2.987 0.00038 0.003257 11.30 67.62 1142.0 52.81 ± 1.11*UW11F2U 3 2.234 0.00034 0.000683 15.72 90.76 1282.3 53.01 ± 0.53UW11F2V 3 3.605 0.00060 0.005500 6.41 54.79 711.96 51.68 ± 1.73

*UW11F2W 3 2.364 0.00030 0.001040 7.28 86.81 1425.3 53.66 ± 1.13UW11F2X 3 2.409 0.00035 0.001440 5.72 82.15 1245.0 51.77 ± 1.51

Isochron age ± 2σ 51.41 ± 0.68 Total fusion age ± 2σ 52.10 ± 0.25Weighted mean age ± 2σ 51.25 ± 0.31

Firehole tuff FC-2sanidine J = 0.014629 ± 0.14% µ = 1.0025UW11G12A 3 7.716 0.00014 0.019616 5.53 24.82 3049.7 49.85 ± 8.40UW11G12B 12 6.734 0.00010 0.016055 22.72 29.48 4192.6 51.65 ± 2.21UW11G12C 6 9.625 0.00010 0.025633 15.76 21.26 4304.8 53.20 ± 4.34UW11G12D 6 5.042 0.00008 0.010362 10.20 39.17 5376.7 51.38 ± 2.24UW11G12E 6 18.077 0.00024 0.054615 34.85 10.70 1827.9 50.32 ± 2.61UW11G12F 6 5.270 0.00011 0.011430 8.62 35.83 3892.5 49.15 ± 1.81UW11G12G 5 2.043 0.00010 0.000372 4.25 94.40 4444.6 50.20 ± 0.59UW11G12H 15 2.020 0.00011 0.000230 15.77 96.40 3748.4 50.66 ± 0.49UW11G12I 5 2.091 0.00015 0.000461 7.33 93.27 2869.7 50.74 ± 0.49UW11G12J 5 2.120 0.00016 0.000459 3.82 93.39 2639.3 51.50 ± 1.00UW11G12K 15 2.054 0.00010 0.000352 15.32 94.72 4495.2 50.63 ± 0.43UW11G12L 15 2.041 0.00012 0.000309 19.77 95.30 3682.2 50.61 ± 0.37UW11G12M 15 2.028 0.00011 0.000265 18.60 95.90 4032.3 50.60 ± 0.29UW11G12N 20 2.076 0.00013 0.000405 21.47 94.01 3414.5 50.79 ± 0.19


C Bed tuff BT-18biotite J = 0.014660 ± 0.16% µ = 1.0025UW11F4A 3 2.311 0.00015 0.001252 5.47 83.79 2903.1 50.50 ± 0.52UW11F4B 3 2.538 0.00067 0.002004 8.15 76.48 643.57 50.61 ± 1.27

*UW11F4C 3 2.621 0.00028 0.001946 5.83 77.89 1539.7 53.21 ± 1.38UW11F4D 3 2.619 0.00104 0.002239 7.36 74.56 411.72 50.91 ± 0.72UW11F4E 3 2.379 0.00060 0.001559 7.71 80.44 719.17 49.93 ± 0.72


Experimentxtal

#40Ar*


2σ MaC Bed tuff cont...

UW11F4F 3 2.443 0.00042 0.001644 5.51 79.92 1018.9 50.90 ± 1.32UW11F4G 3 2.701 0.00030 0.002538 7.75 72.06 1429.2 50.76 ± 1.74UW11F4H 3 4.944 0.00083 0.010075 12.74 39.69 518.63 51.17 ± 1.67

*UW11F4I 3 2.398 0.00031 0.001016 4.28 87.29 1372.1 54.53 ± 1.04UW11F4J 3 2.489 0.00022 0.001822 5.53 78.19 1973.0 50.76 ± 0.95UW11F4K 3 2.768 0.00065 0.002768 6.47 70.28 661.35 50.72 ± 0.88UW11F4L 3 2.435 0.00053 0.001617 6.96 80.19 805.97 50.91 ± 1.53

*UW11F4M 3 2.609 0.00024 0.001767 10.00 79.80 1816.8 54.24 ± 1.30UW11F4N 3 2.451 0.00015 0.001752 8.17 78.69 2828.6 50.31 ± 0.65UW11F4O 3 2.364 0.00043 0.001441 5.97 81.80 992.72 50.44 ± 0.78UW11F4P 3 2.636 0.00019 0.002198 6.64 75.18 2250.7 51.66 ± 2.19UW11F4Q 3 2.530 0.00043 0.001956 4.67 76.98 1011.7 50.79 ± 1.77

*UW11F4R 3 2.542 0.00037 0.001671 4.83 80.40 1165.5 53.26 ± 1.82*UW11F4S 3 2.627 0.00006 0.001234 4.07 85.95 7556.1 58.76 ± 0.53UW11F4T 3 2.348 0.00065 0.001279 3.61 83.71 664.35 51.26 ± 2.11


Grey tuff WN-1sanidine J = 0.02222 ± 0.24% µ = 1.0035UW0612A 10 2.022 0.00257 0.000223 16.40 96.40 167.62 50.64 ± 0.59UW0612B 20 2.074 0.00293 0.000415 34.45 93.77 146.80 50.53 ± 0.63UW0612C 10 2.048 0.01507 0.000322 13.21 95.08 28.53 50.60 ± 0.44UW0612D 10 2.036 0.00233 0.000310 15.34 95.17 184.91 50.36 ± 0.53UW0612E 10 2.046 0.00264 0.000292 11.22 95.46 163.13 50.74 ± 0.65UW0612F 10 2.013 0.00239 0.000195 12.04 96.80 180.13 50.64 ± 0.30UW0612G 10 2.020 0.00259 0.000236 14.59 96.21 165.89 50.50 ± 0.80UW0612H 10 2.147 0.00329 0.000654 10.31 90.69 130.59 50.58 ± 1.13UW0612I 10 2.053 0.00259 0.000322 9.61 95.03 166.10 50.70 ± 0.90UW0612J 10 1.995 0.00259 0.000200 13.08 96.69 165.88 50.13 ± 0.56

sanidine J = 0.01459 ± 0.20% µ = 1.0025UW11G11A 10 2.058 0.00010 0.000388 9.79 94.20 4218.5 50.31 ± 0.67UW11G11B 10 2.343 0.00020 0.001351 18.73 82.76 2131.2 50.34 ± 0.35UW11G11C 10 2.109 0.00024 0.000559 15.40 91.95 1798.4 50.34 ± 0.36UW11G11D 10 2.063 0.00011 0.000417 18.89 93.80 3942.9 50.23 ± 0.27UW11G11E 10 2.084 0.00028 0.000470 13.38 93.12 1562.1 50.38 ± 0.22UW11G11F 10 2.173 0.00021 0.000785 19.73 89.11 2055.5 50.27 ± 0.19UW11G11G 10 2.115 0.00018 0.000555 15.58 92.02 2455.4 50.51 ± 0.31UW11G11H 10 2.280 0.00029 0.001140 20.70 85.02 1457.9 50.32 ± 0.49


Main tuff TR-1sanidine J = 0.02228 ± 0.24% µ = 1.0035UW069A 10 1.946 0.00373 0.000049 32.78 99.14 115.27 49.86 ± 0.24UW069B 10 1.951 0.00390 0.000065 24.84 98.90 110.24 49.85 ± 0.44UW069C 10 1.950 0.00357 0.000049 64.41 99.14 120.53 49.95 ± 0.22UW069D 10 1.954 0.00358 0.000054 63.75 99.07 120.07 50.01 ± 0.23UW069E 10 1.962 0.00366 0.000076 89.97 98.74 117.40 50.05 ± 0.30UW069F 10 1.945 0.00354 0.000043 33.28 99.23 121.53 49.87 ± 0.37UW069G 10 1.966 0.00373 0.000085 38.65 98.61 115.38 50.10 ± 0.42UW069H 10 1.987 0.00372 0.000175 99.57 97.29 115.65 49.94 ± 0.28UW069I 10 1.974 0.00524 0.000135 23.31 97.88 82.00 49.92 ± 0.32UW069J 10 1.977 0.00524 0.000135 23.35 97.88 82.00 50.01 ± 0.32UW069K 10 1.953 0.00480 0.000032 25.02 99.41 89.50 50.17 ± 0.33UW069L 10 1.993 0.00582 0.000191 21.30 97.07 73.93 49.98 ± 0.67


Experimentxtal

#40Ar*


2σ MaMain tuff (sanidine) cont...

UW069M 10 1.948 0.00466 0.000044 23.90 99.22 92.32 49.94 ± 0.45UW069N 10 1.961 0.00490 0.000069 22.46 98.85 87.73 50.08 ± 0.45UW069O 10 2.161 0.00516 0.000756 25.27 89.57 83.33 50.02 ± 0.43UW069P 10 1.964 0.00486 0.000049 29.26 99.16 88.46 50.32 ± 0.55UW069Q 10 1.958 0.00491 0.000083 29.82 98.63 87.62 49.90 ± 0.30UW069R 10 1.958 0.00489 0.000051 9.49 99.13 87.99 50.15 ± 0.57UW069S 10 1.955 0.00517 0.000054 27.94 99.08 83.14 50.05 ± 0.46UW069T 10 1.971 0.00488 0.000114 34.63 98.19 88.11 50.01 ± 0.37UW069U 10 1.957 0.00496 0.000040 25.77 99.28 86.67 50.20 ± 0.25

sanidine J = 0.014517 ± 0.12% µ = 1.0025*UW11G6A 10 2.935 0.00008 0.002718 10.97 72.48 5353.9 54.87 ± 1.18UW11G6B 10 4.115 0.00008 0.007426 12.87 46.56 5328.8 49.50 ± 1.93UW11G6C 10 2.359 0.00012 0.001281 7.74 83.76 3597.6 51.01 ± 1.12UW11G6D 10 2.118 0.00006 0.000581 6.27 91.68 6697.5 50.16 ± 0.64UW11G6E 10 2.174 0.00007 0.000803 28.66 88.88 6080.0 49.92 ± 0.19UW11G6F 10 1.975 0.00009 0.000124 37.52 97.90 4859.0 49.93 ± 0.10UW11G6G 10 1.987 0.00007 0.000180 25.71 97.09 6114.6 49.83 ± 0.18UW11G6H 10 2.065 0.00008 0.000420 25.60 93.76 5116.9 50.02 ± 0.18UW11G6I 10 2.078 0.00012 0.000487 40.67 92.86 3494.4 49.85 ± 0.16UW11G6J 10 2.071 0.00008 0.000439 53.47 93.50 5641.2 50.01 ± 0.17


Main tuff TR-1bbiotite J = 0.014517 ± 0.12% µ = 1.0025

*UW11G6bA 3 2.019 0.00001 0.000166 6.74 97.34 78213 50.74 ± 0.97*UW11G6bB 3 2.106 0.00002 0.000387 5.29 94.35 20892 51.31 ± 1.15UW11G6bC 3 2.005 0.00000 0.000477 2.43 92.74 327094 48.05 ± 2.33UW11G6bD 3 2.138 0.00003 0.000669 4.89 90.54 15936 49.99 ± 1.03UW11G6bE 3 5.675 0.00003 0.012525 17.48 34.69 14956 50.84 ± 2.49UW11G6bF 3 2.513 0.00005 0.002041 3.34 75.81 8503.6 49.21 ± 1.15

*UW11G6bG 3 2.306 0.00002 0.001132 29.65 85.30 26336 50.79 ± 0.35*UW11G6bH 3 2.059 0.00001 0.000337 32.99 94.93 37218 50.48 ± 0.53UW11G6bI 3 2.050 0.00001 0.000405 14.38 93.94 29751 49.74 ± 0.66UW11G6bJ 3 2.083 0.00002 0.000489 18.43 92.84 19527 49.95 ± 0.49

*UW11G6bK 3 2.042 0.00001 0.000279 27.37 95.74 54740 50.48 ± 0.23*UW11G6bL 3 2.066 0.00002 0.000326 12.44 95.12 25457 50.74 ± 0.48*UW11G6bM 3 2.075 0.00001 0.000343 6.72 94.89 33503 50.84 ± 0.54*UW11G6bN 3 2.417 0.00001 0.001533 22.90 81.07 59608 50.60 ± 0.32UW11G6bO 3 2.121 0.00001 0.000590 22.43 91.56 58057 50.15 ± 0.27

*UW11G6bP 3 2.054 0.00003 0.000244 14.09 96.26 16013 51.06 ± 0.36UW11G6bQ 3 2.019 0.00001 0.000324 11.06 95.02 51079 49.57 ± 0.54UW11G6bR 3 2.104 0.00001 0.000575 11.36 91.71 37217 49.83 ± 0.46

*UW11G6bS 3 2.044 0.00001 0.000209 16.73 96.75 55867 51.06 ± 0.46UW11G6bT 3 2.143 0.00002 0.000696 12.76 90.19 21085 49.92 ± 0.60

*UW11G6bU 3 2.044 0.00002 0.000222 10.68 96.57 23618 50.96 ± 0.63UW11G6bV 3 2.091 0.00002 0.000506 19.20 92.63 23824 50.02 ± 0.50UW11G6bW 3 2.063 0.00001 0.000389 11.00 94.21 39885 50.20 ± 0.48UW11G6bX 3 2.330 0.00002 0.001342 14.62 82.78 21105 49.83 ± 0.56

*UW11G6bY 3 2.099 0.00001 0.000429 12.65 93.74 34418 50.81 ± 0.41*UW11G6bZ 3 2.044 0.00003 0.000219 17.12 96.60 16895 51.00 ± 0.42


Experimentxtal

#40Ar*


2σ MaMain tuff (biotite) cont...

*UW11G6bAA 3 2.196 0.00003 0.000413 22.17 94.23 17148 53.39 ± 0.43UW11G6bBB 3 2.148 0.00002 0.000710 10.00 90.01 21194 49.94 ± 0.93UW11G6bCC 3 2.102 0.00001 0.000519 19.00 92.49 36768 50.20 ± 0.45

*UW11G6bDD 3 2.260 0.00001 0.000948 15.64 87.40 40088 51.00 ± 0.60


6th tuff TR-5sanidine J = 0.014660 ± 0.18% µ = 1.0025

*UW11F9A 3 2.612 0.00003 0.000470 3.15 94.50 13112 64.14 ± 1.71UW11F9B 3 2.348 0.00021 0.001559 1.18 80.19 2014.1 49.12 ± 5.35

*UW11F9C 3 8.198 0.00067 0.005996 3.65 78.33 641.45 162.31 ± 4.61*UW11F9D 3 5.055 0.00007 0.006971 0.89 59.16 6603.7 77.41 ± 12.48*UW11F9E 3 3.065 0.00011 0.001312 1.91 87.20 3883.6 69.33 ± 3.28*UW11F9F 3 4.580 0.00010 0.008006 6.11 48.25 4277.3 57.52 ± 5.00UW11F9G 3 3.626 0.00003 0.007329 0.55 40.14 16657 38.09 ± 23.86

*UW11F9H 3 226.013 0.00209 0.680165 4.30 11.07 205.67 563.72 ± 199.51*UW11F9I 3 6.057 0.00008 0.011029 4.72 46.12 5268.7 72.41 ± 7.26UW11F9J 3 3.674 0.00009 0.006424 1.95 48.20 4937.2 46.24 ± 8.06

*UW11F9K 3 4.713 0.00008 0.006181 5.39 61.15 5661.0 74.66 ± 2.87*UW11F9L 3 2.641 0.00022 0.000634 2.53 92.73 1998.8 63.63 ± 0.79*UW11F9M 3 3.028 0.00008 0.001865 3.08 81.64 5062.2 64.22 ± 1.76*UW11F9N 3 3.079 0.00004 0.001089 3.36 89.40 10413 71.37 ± 1.84*UW11F9O 3 3.039 0.00005 0.000516 2.10 94.82 8653.7 74.64 ± 4.38UW11F9P 3 7.254 0.00018 0.018332 3.45 25.26 2434.4 47.82 ± 7.59

*UW11F9Q 9 6.224 0.00013 0.013609 10.95 35.31 3420.1 57.20 ± 0.86UW11F9R 3 2.005 0.00012 0.000487 1.42 92.59 3641.2 48.43 ± 3.06UW11F9S 3 2.338 0.00012 0.001361 2.85 82.60 3470.7 50.36 ± 1.28

*UW11F9T 3 2.601 0.00004 0.000347 5.11 95.88 11262 64.79 ± 1.05*UW11F9U 3 2.465 0.00014 0.000914 2.63 88.86 3152.1 57.01 ± 1.08UW11F9V 3 2.018 0.00016 0.000380 1.85 94.20 2630.7 49.59 ± 3.30UW11F9W 3 1.953 0.00011 0.000237 1.30 96.18 3974.8 49.00 ± 4.11

*UW11F9X 3 2.168 0.00019 0.000175 3.08 97.40 2300.4 55.01 ± 1.02*UW11F9Y 12 10.275 0.00009 0.025539 21.51 26.51 5038.2 70.63 ± 3.19


6th tuff TR-5bbiotite J = 0.014660 ± 0.16% µ = 1.0025UW11F10A 3 1.976 0.00001 0.000232 20.41 96.29 33378 49.64 ± 0.28UW11F10B 3 2.067 0.00000 0.000568 14.45 91.65 266802 49.43 ± 0.42UW11F10C 3 2.037 0.00002 0.000431 15.49 93.51 17878 49.69 ± 0.53UW11F10D 3 1.996 0.00001 0.000263 8.13 95.87 38949 49.90 ± 0.56UW11F10E 3 2.048 0.00001 0.000500 16.29 92.56 30993 49.45 ± 0.43

*UW11F10F 3 2.097 0.00002 0.000587 20.86 91.50 18318 50.06 ± 0.38UW11F10G 3 2.003 0.00001 0.000304 17.71 95.28 32463 49.78 ± 0.51UW11F10H 3 1.993 0.00003 0.000261 12.38 95.89 16160 49.86 ± 0.55UW11F10I 3 2.270 0.00003 0.001252 22.33 83.50 14653 49.44 ± 0.91

*UW11F10J 3 2.143 0.00001 0.000685 10.91 90.34 48214 50.49 ± 0.58UW11F10K 3 1.993 0.00004 0.000269 16.29 95.78 10496 49.80 ± 0.40

*UW11F10L 3 1.981 0.00000 0.000153 10.08 97.48 566802 50.36 ± 0.74UW11F10M 3 2.204 0.00001 0.001035 12.03 85.91 52325 49.39 ± 0.70UW11F10N 3 1.952 0.00002 0.000134 18.42 97.74 19976 49.76 ± 0.27UW11F10O 3 1.971 0.00049 0.000207 27.52 96.66 878.18 49.70 ± 0.26UW11F10P 3 1.994 0.00002 0.000305 20.58 95.25 26486 49.55 ± 0.27

*UW11F10Q 3 2.038 0.00002 0.000411 22.51 93.82 22699 49.89 ± 0.23UW11F10R 3 2.210 0.00002 0.001000 42.55 86.43 17420 49.83 ± 0.30


Experimentxtal

#40Ar*


2σ Ma6th tuff (biotite) cont.

UW11F10S 3 2.000 0.00001 0.000279 22.68 95.64 32272 49.90 ± 0.38*UW11F10T 3 1.966 0.00001 0.000137 25.77 97.70 48494 50.11 ± 0.23UW11F10U 3 2.034 0.00001 0.000424 21.41 93.60 82611 49.66 ± 0.25UW11F10V 3 2.001 0.00001 0.000317 41.84 95.08 39290 49.63 ± 0.13UW11F10W 3 2.053 0.00001 0.000492 36.74 92.69 38171 49.64 ± 0.27

*UW11F10X 3 2.034 0.00001 0.000350 23.97 94.69 46534 50.25 ± 0.53UW11F10Y 3 2.490 0.00001 0.001994 25.94 76.15 38894 49.47 ± 0.59UW11F10Z 3 2.139 0.00001 0.000770 19.60 89.15 35256 49.74 ± 0.34


Analcite tuff SB-1sanidine J = 0.022235 ± 0.24% µ = 1.0035UW0610A 10 1.959 0.02026 0.000223 44.21 96.59 21.22 49.02 ± 0.44UW0610B 10 1.926 0.01585 0.000117 30.68 98.14 27.13 48.95 ± 0.36UW0610C 10 1.918 0.01789 0.000080 24.47 98.71 24.04 49.04 ± 0.62UW0610D 10 1.905 0.01568 0.000060 30.70 99.00 27.43 48.85 ± 0.39UW0610E 10 1.964 0.01873 0.000209 44.21 96.80 22.96 49.25 ± 0.49UW0610F 10 1.954 0.02542 0.000233 55.10 96.45 16.92 48.82 ± 0.43UW0610G 10 1.928 0.01696 0.000128 48.98 97.97 25.35 48.94 ± 0.26UW0610H 10 1.922 0.01826 0.000073 51.84 98.82 23.55 49.19 ± 0.38UW0610I 10 1.943 0.02063 0.000128 37.46 98.01 20.85 49.34 ± 0.47UW0610J 10 1.948 0.02321 0.000186 22.14 97.15 18.53 49.01 ± 0.49

sanidine J = 0.014549 ± 0.21% µ = 1.0025UW11G10A 5 1.918 0.00026 0.000058 10.20 98.87 1636.1 49.09 ± 0.41UW11G10B 5 1.913 0.00035 0.000149 5.64 97.45 1222.5 48.28 ± 1.30UW11G10C 10 1.929 0.00029 0.000118 26.15 97.95 1501.2 48.92 ± 0.16UW11G10D 10 1.915 0.00030 0.000095 24.15 98.29 1418.4 48.73 ± 0.30UW11G10E 10 1.939 0.00032 0.000148 24.35 97.50 1331.0 48.94 ± 0.19UW11G10F 10 1.917 0.00036 0.000070 17.81 98.68 1210.9 48.99 ± 0.38UW11G10G 10 1.934 0.00083 0.000145 27.32 97.55 518.43 48.85 ± 0.26UW11G10H 10 1.927 0.00029 0.000088 22.51 98.42 1458.0 49.11 ± 0.29UW11G10I 15 1.961 0.00034 0.000239 32.68 96.17 1270.2 48.83 ± 0.19


* indicates analyses that have been excluded from age calculations Note: All ages calculated relative to 28.34 Ma for the Taylor Creek rhyolite sanidine (Renne at al., 1998); usingthe decay constants of Steiger and Jäger (1977); uncertainties reported at 2σ analytical precision. Corrected for37Ar and 39Ar decay, half lives of 35.2 days and 269 years, respectively.

References cited in Data Repository:Duffield, W. A., and G. B. Dalrymple, 1990, The Taylor Creek Rhyolite of New Mexico: A rapidly emplaced field

of lava domes and flows: Bulletin of Volcanology, v. 52, p. 475-487.Lanphere, M.A., and Dalrymple, G.B., 2000, First-principles calibration of 38Ar tracers: Implications for ages of

40Ar/39Ar fluence monitors: US Geological Survey Professional Paper 1621.McDougall, I., and Roksandic, Z., 1974, Total fusion 40Ar/39Ar ages using HIFAR reactor: Geological Society of

Australia Journal, v. 21, p. 81-89.Renne, P.R., Swisher, C. C., Deino, A. L., Karner, D. B., Owens, T. L., and DePaolo, D.J., 1998, Intercalibration

of standards, absolute ages and uncertainties in 40Ar/39Ar dating. Chemical Geology, v. 145, p. 117-152.Sampson, S.D., and Alexander, E.C.Jr., 1987, Calibration of the interlaboratory 40Ar/39Ar dating standard, MMhb-

1: Chemical Geology, v. 66, p. 27-34.Steiger, R.H., and Jäger, E., 1977, Subcommission on geochronology: convention on the use of decay constants

in geo- and cosmochronology, Earth and Planetary Science Letters, v. 36, p. 359-362.

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