Eocene Green River Formation, Western United States

Home , Bridger Formation, Geology of Wyoming, Green River Formation, Notharctus, Pass Peak Formation, Tatman Formation

Synoptic reconstruction of a major ancient lake system: Eocene Green River Formation, western United States

M. Elliot Smith* Alan R. Carroll Brad S. Singer Department of Geology and Geophysics, University of Wisconsin, 1215 West Dayton Street, Madison, Wisconsin 53706, USA

ABSTRACT Members. Sediment accumulation patterns than being confi ned to a single episode of arid thus refl ect basin-center–focused accumula- climate. Evaporative terminal sinks were Numerous 40Ar/39Ar experiments on sani- tion rates when the basin was underfi lled, initially located in the Greater Green River dine and biotite from 22 ash beds and 3 and supply-limited accumulation when the and Piceance Creek Basins (51.3–48.9 Ma), volcaniclastic sand beds from the Greater basin was balanced fi lled to overfi lled. Sedi- then gradually migrated southward to the Green River, Piceance Creek, and Uinta ment accumulation in the Uinta Basin, at Uinta Basin (47.1–45.2 Ma). This history is Basins of Wyoming, Colorado, and Utah Indian Canyon, Utah, was relatively con- likely related to progressive southward con- constrain ~8 m.y. of the Eocene Epoch. Mul- stant at ~150 mm/k.y. during deposition of struction of the Absaroka Volcanic Prov- tiple analyses were conducted per sample over 5 m.y. of both evaporative and fl uctuat- ince, which constituted a major topographic using laser fusion and incremental heating ing profundal facies, which likely refl ects the and thermal anomaly that contributed to a techniques to differentiate inheritance, 40Ar basin-margin position of the measured sec- regional north to south hydrologic gradient. loss, and 39Ar recoil. When considered in tion. The most rapid sediment accumulation The Greater Green River and Piceance Creek conjunction with existing radioisotopic ages for the entire system (>1 m/k.y.) occurred Basins were eventually fi lled from north and lithostratigraphy, biostratigraphy, and between 49.0 and 47.5 Ma, when volcani- to south with Absaroka-derived detritus at magnetostratigraphy, these new age deter- clastic materials from the Absaroka and/or sedimentation rates 1–2 orders of magnitude minations facilitate temporal correlation of Challis volcanic fi elds entered the Green greater than the underlying lake deposits. linked Eocene lake basins in the Laramide River Formation lakes from the north. Rocky Mountain region at a signifi cantly Our new ages combined with existing Keywords: Ar-Ar, Absaroka, Uinta Basin, increased level of precision. To compare our paleomagnetic and biostratigraphic control Piceance Creek Basin, land-mammal ages, lake results to the geomagnetic polarity time scale permit the fi rst detailed synoptic comparison type, Laramide, Eocene, Green River Formation and the regional volcanic record, the ages of lacustrine depositional environments in of Eocene magnetic anomalies C24 through all the Green River Formation basins. Cou- INTRODUCTION C20 were recalibrated using seven 40Ar/39Ar pled with previously published paleocurrent ages. Overall, the ages obtained for this study observations, our detailed correlations show Large lakes are widely recognized for their are consistent with the isochroneity of North that relatively freshwater lakes commonly importance as economic resources and as American land-mammal ages throughout the drained into more saline downstream lakes. archives of faunal, fl oral, and climatic evolution study area, and provide precise radioisotopic The overall character of Eocene lake depos- (e.g., Bradley, 1929; Franczyk et al., 1989; Wilf, constraints on several important biostrati- its was therefore governed in part by the 2000) but are less well understood with regard to graphic boundaries. geomorphic evolution of drainage patterns the geomorphic evolution of the landscapes sur- Applying these new ages, average sedi- in the surrounding Laramide landscape. rounding them (cf. Surdam and Stanley, 1980; ment accumulation rates in the Greater Freshwater (overfi lled) lakes were initially Pietras et al., 2003a; Carroll et al., 2006). The Green River Basin, Wyoming, were approx- dominant (53.5–52.0 Ma), possibly related Eocene Green River Formation (Hayden, 1869) imately three times faster at the center of the to high erosion rates of remnant Cretaceous of Wyoming, Colorado, and Utah represents one basin versus its ramp-like northern margin strata on adjacent uplifts. Expansion of bal- of the best-documented ancient lake systems and during deposition of the underfi lled Wilkins anced-fi ll lakes fi rst occurred in all Green has long been a type example for understanding Peak Member. In contrast, sediment accu- River Formation basins at 52.0–51.3 Ma and lacustrine depositional systems (Bradley, 1929; mulation occurred faster at the edge of again between 49.6 and 48.5 Ma. Evapora- Eugster and Surdam, 1973; Carroll and Bohacs, the basin during deposition of the bal- tive (underfi lled) lakes occurred in various 1999). Since Marsh (1871) speculated that the anced fi lled to overfi lled Tipton and Laney basins between 51.3 and 45.1 Ma, coincident Green River Formation lakes were hydrologi- with the end of the early Eocene climatic cally connected, numerous authors have pro- optima and subsequent onset of global cool- posed temporal correlations of its strata across *Present address: Department of Geology, So- noma State University, 1801 East Cotati Avenue, ing defi ned from marine record. However, the Uinta uplift (e.g., Bradley, 1931; Roehler, Rohnert Park, California 94928, USA, e-mail: evaporite intervals in the different depocen- 1974; Surdam and Stanley, 1980). However, [email protected] ters were deposited at different times rather due to the absence of intervening strata between

GSA Bulletin; January/February 2008; v. 120; no. 1/2; p. 54–84; doi: 10.1130/B26073.1; 14 ﬁ gures; 5 tables; Data Repository Item 2007211.

Figure 1. Map showing the locations of Eocene basins and basin-bounding uplifts. Compiled from Ross et al. (1955), Grose (1972), Witkind and Grose (1972), Bond and Wood (1978), Stewart and Carlson (1978), Tweto (1979), Love and Christiansen (1985), Constenius (1996), and Mitchell (1998). Eocene stratal thicknesses are from Robinson (1972).

Geological Society of America Bulletin, January/February 2008 55 Smith et al. depocenters (Fig. 1), lateral facies changes, and which tend to be more susceptible to chemi- series of basement uplifts of central Colorado limited radioisotopic age control, temporal cor- cal weathering than sanidine (Renne, 2000; and southeastern Wyoming (Fig. 1). These relation has been insuffi ciently precise to address Smith et al., 2006). Age determinations were basins typically contain 1–2 km packages of more specifi c questions concerning basin evo- further limited in accuracy by the unavoid- coarse-grained alluvial strata (Dickinson et al., lution. This paper provides an 40Ar/39Ar–based able inclusion of inherited or altered biotite 1988). Extensional basins are strike-elongate age framework that allows for the fi rst detailed grains in the large samples analyzed (cf. Smith grabens and half grabens that overlie the former delineation on upstream-downstream relation- et al., 2006), and limited in their precision by Cordilleran fold and thrust belt, are commonly ships between the sequences of lakes that occu- lower resolution mass spectrometry. Recently, bounded by normal faults that reactivate Cordil- pied the Green River Formation basins and pro- 40Ar/39Ar geochronologic studies of tuff beds leran thrust faults, and often contain thick (2– vides a fundamental measurement of lacustrine using smaller samples and more sensitive mass 5 km) but areally restricted packages of alluvial sediment accumulation rates over ~8 m.y. of the spectrometry have begun to signifi cantly refi ne and lacustrine strata (Constenius, 1996). Eocene Epoch. the timing of the Green River Formation and Volcanism occurred over broad areas of the Until recently, mammalian biostratigraphy related alluvial strata (Wing et al., 1991; Smith northwestern United States during the Eocene was the only available method for determin- et al., 2003, 2004, 2006). This study integrates and provided both fallout tuffs and volcani- ing the relative age of strata in the terrestrial 25 new age determinations with detailed facies clastic sediment to the Green River Formation basins that contain the Green River Formation and geochemical analyses to construct the lake basins (Fig. 1; Surdam and Stanley, 1980; (Wood et al., 1941; Lillegraven, 1993; Robinson most comprehensive and highly resolved chro- Fritz and Harrison, 1985; Armstrong and Ward, et al., 2004). However, mammalian fossils are nostratigraphic model available for any major 1991). Major volcanic centers include the Absa- typically preserved in adjacent alluvial depos- pre-Quaternary lake system. roka Volcanic Province, Challis volcanic fi eld, its that can be diffi cult to correlate with lake and Lowland Creek Volcanics; minor fi elds are deposits using the physical characteristics of GEOLOGIC SETTING scattered throughout the region (Fig. 1). The the strata (cf. Clyde et al., 2004; Smith et al., stratigraphic and time-stratigraphic constraints 2004). The temporal resolution of mammalian The Green River Formation of Wyoming, for Eocene volcanic fi elds in Wyoming, Mon- biostratigraphy is also fundamentally limited Colorado, and Utah was deposited in a series of tana, and Idaho are summarized in Figure 5. by the richness of collections at particular sites. continental basins that occupy a broken foreland Moreover, phenomena such as diachronous fi rst province to the east of the Cordilleran fold and NONMARINE SEDIMENTARY FACIES and last occurrences of taxa caused by tapho- thrust belt. These basins are separated from one nomic biases (Smith and Holroyd, 2003) and another by chains of anticlinal basement-cored Lake Types geographic or climatic heterogeneity (Gunnell uplifts that collectively comprise the Laramide and Bartels, 2001) can only be assessed using orogeny, and were variably active from the Representing a broad range of lacustrine geochronology that is independent of mamma- Cretaceous through Eocene (Fig. 1; Beck et facies, several thick lenses of the Green River lian biostratigraphy. al., 1988; Dickinson et al., 1988). The forma- Formation occupy two principle basins: the Paleomagnetic polarity records have also been tion has a maximum thickness of nearly 2 km Greater Green River Basin and the Uinta– used to establish the timing of Eocene terrestrial and spans much of the early and middle Eocene Piceance Creek Basin, which are separated from strata and have the potential to provide precise Epoch (Figs. 2 and 3). The Green River Forma- one another by the east-west–trending, anticli- relative age control (e.g., Tauxe et al., 1994; tion basins are part of a suite of basins that have nal Uinta uplift (Fig. 1; Bradley, 1964; Johnson, Clyde et al., 2001). However, paleomagnetic been differentiated based on their structural set- 1985; Roehler, 1992a). The names Lake Gosiute records are inherently binary and can be ham- ting and strata into four principle types: pon- (King, 1878, p. 446) and Lake Uinta (Bradley, pered by changing sedimentation rates, lacuna, ded, perimeter, axial, and extensional (Dickin- 1931) were assigned to the lakes that existed in poor remanence acquisition, and magnetic over- son et al., 1988; Constenius, 1996; Fig. 4; see the northern (Greater Green River) and southern printing. Comparisons between magnetic polar- GSA Data Repository Table DR11). Ponded (Uinta–Piceance Creek) basins, respectively. ity records from continental strata and the geo- basins, for which the Green River Formation Each of these lakes varied greatly in their chem- magnetic polarity time scale (Cande and Kent, basins are the type example, are bounded by istries and areal extents during the course of 1992, 1995; Ogg and Smith, 2004) have proven basement-involved uplifts and contain evidence Green River Formation deposition (Fig. 2A). problematic because higher numbers of polarity for internal drainage during at least portion of Distinctive assemblages of lithologies and reversals are often preserved in terrestrial sedi- their history. They typically contain thick pack- fossils within the Green River Formation allow ments than are recorded by ocean-fl oor mag- ages (3–5 km) of alluvial and lacustrine strata for its subdivision into three principle lacustrine netic anomalies (Elston et al., 1994). Neverthe- (Dickinson et al., 1988; Baars et al., 1988). facies associations: fl uvial lacustrine, fl uctuating less, paleomagnetic stratigraphy provides a vital Perimeter basins occur on the east edge of the profundal, and evaporative (Carroll and Bohacs, component of terrestrial geochronology but also broken foreland province and contain alluvial 1999). These associations are the basis for inter- requires adequate independent calibration. strata with east-directed paleocurrent indica- preting lake type, which refl ects the long-term Several K-Ar and early 40Ar/39Ar geochro- tors that indicate external drainage (Dickinson balance between potential accommodation and nology efforts were undertaken in the Green et al., 1988). Axial basins are small, elongate, water plus sediment fi ll (Carroll and Bohacs, River Formation and related Eocene continental intermontane basins that formed amid the main 1999; Bohacs et al., 2000; Carroll and Bohacs, strata prior to ca. 1990 using phenocrysts from 2001). Carroll and Bohacs (1999) defi ned the ash beds (Evernden et al., 1964; Mauger, 1977; facies associations and lake type interpretations O’Neill, 1980). However, due to the lower sen- 1GSA Data Repository Item 2007211, containing for the Green River Formation in the Greater full documentation of 40Ar/39Ar geochronology and sitivity of older instruments and diffi culty in supporting references for stratigraphic synthesis, is Green River Basin. In this study we have acquiring suffi cient quantities of sanidine, these available at www.geosociety.org/pubs/ft2007.htm. extended these interpretations to Green River studies focused on large aliquots of biotite, Requests may also be sent to [email protected]. Formation strata in the Uinta, Piceance Creek,

56 Geological Society of America Bulletin, January/February 2008 Synoptic reconstruction of the Eocene Green River Formation X’ Tp Tp Twb Tga Tgf Tw Tf Tf Twb Tga Tgf Tw Sa Basin K Fossil K Basin Fossil Sa 1 Twnf Twnf ? CP Tgw TB ? R CP B B Bridger Basin TB G R Tw G 6 6 L CB CB L S M M Evaporative Interbedded evaporites ed to construct the cross section are section are ed to construct the cross 100 km S F Tw F Tb Ar/ Ar dated tuff bed 2 LC Correlation 40 39 stratigraphy and chronostratigraphy of stratigraphy and chronostratigraphy and Uinta Basins along cross-section X- and Uinta Basins along cross-section scale = 100x vert exag. Tgt LC HF Tb ion line was chosen in order to intersect area to intersect area ion line was chosen in order i d d i Tgl HF SC SM Tgw Greater Green River Basin 3 Uplift Rock Springs b Tgt An An Twa A A Twa Tglu Tgt Tglu SB SB Fluvial-lacustrine Fluctuating Profundal Tgl 4 Twc Twn Tgl b Twc Twn Sand Wash Basin coal molluscs stromatolites fish 5 Uplift Uinta Alluvial Volcaniclastic Feldspatholithic Y Tgc Td Tu Tgp Tu Y Tgp 6 Tgc Tgg Td Tgg ). ? Piceance Creek Basin ? ? ? Arch Tg Douglas Creek 7 Tc Tc Tc B B W Tc C W C Tu Tu Tgu Tgu Continued on following page Continued on following Tgu 8 Tgtr Fa O P O Fa P h h Tgsl Tgsl Tgtr St St Uinta Basin m m Tgs Tg Tgs 9 Tg Tcr Tg Tg Tcr Tc Tc Tc Wasatch Plateau Wasatch Plateau Chronostratigraphy X Lithostratigraphy 0 21r 22r 23r 20n 21n 22n 23n 24n

500

2000 1500 1000

Meters 51 52 53 54 50 49 48 47 43 46 45 44 ) a M ( e g A Figure 2. Lithostratigraphic and time stratigraphic cross sections of Eocene strata in the Greater Green River, Piceance Creek, River, Green sections of Eocene strata in the Greater 2. Lithostratigraphic and time stratigraphic cross Figure and dated tuff beds. Cross-sect (see Fig. 1) showing the stratigraphic position of facies associations, structural features, X’ segments us numbered for The stratigraphic references Plateau region. Wasatch Formation in the Fossil Basin and River the Green of thickest sediment accumulation, sites of bedded evaporites, and principle sills. Inset columns with white background depict of thickest sediment accumulation, sites bedded evaporites, and principle sills. Inset columns with white background DR7 (see footnote 1). ( Table in A B

Geological Society of America Bulletin, January/February 2008 57 Smith et al.

Formations and Members and Fossil Basins (Fig. 1). Although a wide vari- REGIONAL LITHOSTRATIGRAPHY ety of characteristics has been utilized to defi ne Tg _Green River Formation: lacustrine facies associations ( Horsfi eld et al., Greater Green River Basin Tgl _Laney Member b _Buff Marker bed 1994; Bohacs et al., 2000), several key features Tgw _Wilkins Peak Member outlined here and in Table 1 provide the stron- The Greater Green River Basin consists i _“I” clastic marker bed d _“D” clastic marker bed gest evidence for these associations. Evapora- of four subbasins (Bridger, Great Divide, Tgt _Tipton Member tive facies are best recognized via the presence Washakie, and Sand Wash) that are partitioned Tglu _Luman Tongue Tga _Angelo Member of bedded evaporites and absence of fi sh fossils, from one another by the north-south–trending Tgf _Fossil Butte Member and are interpreted to represent the deposits of Rock Springs uplift and several smaller east- Tgp _Parachute Creek Member hypersaline lakes within underfi lled basins in west–trending structures (Fig. 1; Love et al., h _Horse Bench sandstone m _Mahogany zone which water rarely rose above the level of the 1961). Each subbasin contains a unique suc- Tga _Anvil Points Member downstream sill. Fluvial-lacustrine facies pre- cession of strata, but all record a long-term Tgg _Garden Gulch Member Tgc _Cow Ridge Member serve abundant mollusc fossils and occasional evolution from open to closed and return to Tgsl _sandstone & limestone fi sh fossils, and are interpreted to have been open hydrologic conditions during deposition facies deposited from freshwater lakes in overfi lled of the Green River Formation (Fig. 2A; Roe- Tgs _saline facies Tgu _upper member basins where water consistently spilled over the hler, 1993; Carroll and Bohacs, 1999). The Tgtr _transitional interval downstream sill. Fluctuating profundal facies Luman, Tipton, and Wilkins Peak Members Tb _Bridger Formation are typically composed of laminated, organic- record a progression from fl uvial-lacustrine Tbtb _Turtle Bluff Member rich carbonate mudstones intercalated with to fl uctuating profundal through evaporative Tbt _Twin Buttes Member thin desiccation horizons, and are interpreted facies. The evaporative Wilkins Peak Mem- Tbb _Blacks Fork Member to represent the deposits of brackish to saline ber is primarily restricted to the Bridger sub- Twa _Washakie Formation lakes that occupied balanced-fi lled basins where basin and contains bedded evaporites, pre- Twka _Adobe Town Member Twkk _Kinney Rim Member water oscillated near the sill level. Assignments dominantly trona, shortite, and halite (Fahey, of facies association and lake type in most cases 1962; Pietras et al., 2003a), and is laterally Tw _Wasatch Formation correspond to previously identifi ed stratal units equivalent to alluvial deposits of the Cathe- Twc _Cathedral Bluffs Tongue Twnf _New Fork Tongue and are primarily employed to help standardize dral Bluffs Member of the Wasatch Forma- Twn _Niland Tongue terminology between the basins. tion in adjacent subbasins (Sullivan, 1985; Twm _Main Body Twb _Bullpen Member Roehler, 1992a). The fl uctuating profundal to Alluvial Facies fl uvial-lacustrine LaClede bed of the Laney Tf _Fowkes Formation Member overlies the Wilkins Peak Member Tp _Pass Peak Formation Tcr _Crazy Hollow Formation Alluvial strata surround and interfi nger with and records an expansion of lake facies into Tu _Uinta Formation the Green River Formation (Fig. 2A; Roehler, all of the subbasins of the Greater Green River Tc _Colton Formation Td _Debeque Formation 1992a). For this study, alluvial deposits have Basin (Fig. 2A; Surdam and Stanley, 1979; been subdivided into three broad facies asso- Roehler, 1992a). Volcaniclastic alluvium of ciations according to their mode of deposition: the Sand Butte bed of the Laney Member, Tuff beds deltaic, alluvial plain, and alluvial fan. Deltaic Bridger Formation, and lower Washakie For- Greater Green River Basin deposits signify infl uxes of water and sediment mation replace lacustrine strata from north to SC _Sage Creek Mtn. pumice from rivers and typically consist of well-sorted, south in a time-transgressive fashion (Stanley Sa _Sage tuff TB _Tabernacle Butte tuff coarsening- and shallowing-upward packages and Surdam, 1978; McCarroll et al., 1996a; HF _Henrys Fork tuff of sandstone and siltstone that exhibit progra- Evanoff et al., 1998). LC _Leavitt Creek tuff Co _Continental tuff dational stratal geometries (Fig. 2A). Alluvial CB _Church Buttes tuff plain facies are typically composed of mud, silt, Fossil Basin An _Antelope sand bed and sand deposits that are often channelized and SB _Sand Butte tuff A _Analcite tuff pedogenically altered, refl ecting the avulsion The smaller, wedge-top Fossil Basin occupies 6 _Sixth tuff of streams over broad, exposed plains (Brauna- the fold and thrust belt to the west of the Greater L _Layered tuff M _Main tuff gel and Stanley, 1977; Roehler, 1993). Alluvial Green River Basin (Fig. 1; Oriel and Tracey, G _Grey tuff fan deposits typically consist of moderately 1970; Lamerson, 1982; DeCelles and Currie, B _Boar tuff to poorly sorted sand- to boulder-sized clasts 1996; Chandler, 2006). The fl uvial-lacustrine F _Firehole tuff R _Rife tuff derived from and deposited adjacent to basin- to fl uctuating profundal Fossil Butte Member K _K-spar tuff bounding uplifts. They signify the downstream overlies the alluvial Wasatch Formation and is S _Scheggs tuff termini of short, steep drainage networks and overlain by the evaporative Angelo Member Uinta-Piceance Creek Basin record the denudation of the uplifts from which (Fig. 2A). The alluvial Bullpen Member of the St _Strawberry tuff the sediments were eroded (Crews and Ethridge, Wasatch Formation overlies the Angelo Mem- O _Oily tuff P _Portly tuff 1993; Carroll et al., 2006). Alluvial deposits as ber (Fig. 2A; Oriel and Tracey, 1970; Buch- Fa _Fat tuff a whole have also been differentiated into two heim, 1994; Buchheim and Eugster, 1998). In Bl _Blind Canyon tuff W _Wavy tuff petrographic types: (1) locally derived feld- the Fowkes and Woodruff Basins to the west of C _Curly tuff spathic, sublithic, and quartz arenite sandstones; the Fossil Basin, the Bullpen Member is uncon- Y _Yellow tuff and (2) volcanic-lithic deposits delivered from formably overlain by volcaniclastic alluvial contemporaneous volcanic fi elds (Fig. 2A; Sur- strata of the Fowkes Formation (Figs. 1 and 2A; Figure 2. (continued) dam and Stanley, 1980; Dickinson et al., 1986). Oriel and Tracey, 1970; Nelson, 1973).

58 Geological Society of America Bulletin, January/February 2008

Synoptic reconstruction of the Eocene Green River Formation

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on for Bonanza site: from 28 dicot species, Bonanza site: from on for

e t i r o p a v e s Uinta Formation (O.A. Peterson 10 ctonic, volcanic, and paleoﬂ stratigraphic references for North American North for stratigraphic references Site 213 Site 738 Site 690 Site 865 Site 757 Site 865 cold he intercalibration values of Renne et al. (1998) he intercalibration Nuttallides oral-based precipitation estimates and sites are estimates and sites are oral-based precipitation Cibicidoides , and P3 illustrate composite magnetostratigraphy LMl _Laney LMw _Wilkins Peak Paleofloral sites Bo _Bonanza LM _Little Mountain NT _Niland Tongue La _Latham Sd _Sourdough ) = 6.35. 2 Estimated Mean Annual 0 50 100 150 200 Co Sa G 6 Precipitation (cm/yr; Wilf, 2000) K B L LC TB R S F M Alluvial Volcaniclastic Evaporative SC Fluctuating Profundal Fluvial- Lacustrine HF CB Willwood ash Bighorn Basin (base of C24n.1n) P3 A E D C B Facies Associations Tbt

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a k w T i Tw d Tgt Twn Tglu Tgl Twkk 1 b 2 3 Twc Greater Green River Basin P2 P2 La Sd SB NT LMw LMl A This Study Smith et al. (2006) Smith et al. (2004) Smith et al. (2003) Y Ar/ Ar ages An 40 39 Tu Tga Tgg Td Bo P1b Tgp Tgc C A m B1 B2 Tg Tgu Tu Tc h Tgtr Tc Tgsl Tgs P1a O P Fa Bl W St Uinta-Piceance Creek Basin C et al., 2001): %Leptophyll = 0. Wilf Museum of Natural History locations 323 and 1732; see ora excavated by K. Johnson (Denver C20r C21n C21r C22n C22r C23n C23r C24n C24r GPTS C20n Ui-1 Ui-2 Br-2 Br-0 Br-3 Wa-4 Wa-6 Wa-7 Wa-5 Br-1b Br-1a analytical and intercalibration uncertainties. GPTS—geomagnetic polarity time scale, recalibrated from Cande and Kent (1992, 1 from uncertainties. GPTS—geomagnetic polarity time scale, recalibrated analytical and intercalibration σ

Lost- Early Twin- Gray- bullian forkian

Blacks- buttean Earliest buttean Lysitean cabinian

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51 42 43 44 45 46 47 48 49 50 52 53 54 ) a M ( e g A Figure 3. Composite age model for the Green River Formation and associated strata in the Greater Green River and Uinta–Piceance River Green Formation and associated strata in the Greater River the Green 3. Composite age model for Figure complete list of bio A 2. Lithostratigraphic symbols as in Figure and magnetostratigraphy. radioisotopic ages, biostratigraphy, All radioisotopic ages have been normalized to t DR1 (see footnote 1) (cf. Robinson et al., 2004). Table land-mammal ages is in shown with 2 and are 4 and 5) (cf. Renne et al., 1998; Smith 2003). Columns labeled P1a, P1b, P2 Tables values (see Fig. 9; intercalibration to divisions of the A, B1, B2, and C adjacent to Uinta Basin column refer 1981; Clyde et al., 1997, 2001). Letters P3—Jerskey, for the western and eastern Uinta, Washakie, and Bridger Basins, respectively (P1a—Prothero, 1996; P1b—Prothero, 1996; P2—Stuck 1996; P1b—Prothero, (P1a—Prothero, Basins, respectively and Bridger Washakie, the western and eastern Uinta, for 1895, p. 72–74; cf. Prothero, 1996). Numerical subdivisions of the Adobe Town Member of the Washakie Formation in the Washakie Washakie Formation in the Washakie of the Member Town Adobe 1996). Numerical subdivisions of the 1895, p. 72–74; cf. Prothero, 1909; cf. Evanoff et al., 1998). Regional te Beds (Matthew, to Bridger Basin column refer letters adjacent to Bridger Uppercase All paleoﬂ to the standard ages of Renne et al. (1998). all recalibrated benthic forams are global paleoclimatologic data from from Wilf (2000), except the Bonanza site in the Uinta Basin (P. Wilf and K. Johnson, 2006, personal commun.). Sample informati Wilf (2000), except the Bonanza site in Uinta Basin (P. Wilf from 894 specimens, in the Bonanza ﬂ phyll = 0.223, %Microphyll = 0.539, %Notophyll = 0.187, %Mesophyll = 0.039; mean ln (leaf area, mm = 0.539, %Notophyll 0.187, %Mesophyll 0.039; mean ln (leaf area, phyll = 0.223, %Microphyll

Geological Society of America Bulletin, January/February 2008 59 Smith et al.

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Td T Tv

C24r T 1-4 55 GGRB U-PCB Wyoming Colorado New Mexico Formations and Members K/Ar ages Ponded Basins Axial/PerimeterBasins Extensional Basins Love (1964) Black (1969) Twi _Willwood Formation Tw _Wasatch Formation Tf _Fowkes Formation Tt _Tatman Formation Th _Hanna Formation Tsp _Sheep Pass Formation Pekarek et al. (1974) Twr _Wind River Formation Tc _Coalmont Formation Tcs _“Conglomerate, Solomon et al. (1979) Twrl _Lysite Member Ts _South Park Formation sandsone, and shale unit” 40Ar/ 39 Ar ages Twrc _Lost Cabin Member Te _Echo Park Formation Tcl _Cherty Limestone Fm. Ta _Aycross Formation Tv _Vallejo Formation Tr _Renova Formation M’Gonigle and Dalrymple (1996) Twb _Wagon Bed Formation Td _Denver Formation Tsc _Sage Creek Formation Vandenburg et al. (1998) Twbg _Green and Brown Member Thu _HuerfanoFormation Td b _Dell beds Janecke et al. (1999) Twbh _Hendry Ranch Member Tfa _Farasita Conglomerate Smith et al. (2004) Tc _Claron Formation Tsj _San Jose Formation Smith et al. (2007) Absaroka Volcanic Province units in NW Wind River Basin and western Bighorn Basin are shown in Fig. 5 Formations and Members in Greater Green River, Piceance Creek , and Uinta Basins as in Fig. 2.

Figure 4. Age model for the Green River Formation and strata in adjacent Eocene basins. Ages for biostratigraphic and magnetostratigraphic boundaries as in Figure 2. References for individual basins are included in Table DR1 (see footnote 1). We use (and cite) the terminology of Dickinson et al. (1988) and Constenius (1996). All 40Ar/39Ar ages are shown with 2σ intercalibration uncertainties relative to the standard values of Renne et al. (1998). GGRB–Greater Green River Basin; U-PCB—Uinta–Piceance Creek Basin.

Piceance Creek Basin for alluvial deposits underlying and interfi n- evaporites, predominantly nacholite and halite gering with the Green River Formation in these (Bradley, 1931; Dyni, 1981). As in the Greater Green River Basin, lacus- basins (cf. Powell, 1876; Bradley, 1964; Rob- Lacustrine strata overlying the eastern fl ank trine strata in the Piceance Creek Basin record inson et al., 2004). of the Douglas Creek arch have been collectively a progression from open to closed and return In the center of the Piceance Creek Basin, referred to the Douglas Creek Member (Bradley, to open hydrologic conditions (Fig. 2A). the mollusc-bearing Cow Ridge Member of 1931; Donnell, 1961a). However, several fea- Alluvial deposits in the Piceance Creek and the Green River Formation (Johnson, 1984) tures of these deposits suggest genetic ties with Uinta Basins are physically separated by the overlies the alluvial DeBeque Formation (Don- their lateral equivalents in the basin center. At Douglas Creek arch, and from strata in the nell, 1961b; Kihm, 1984) and is overlain by Douglas Pass, which overlies the Douglas Creek Greater Green River Basin by the Uinta uplift the Garden Gulch Member of the Green River arch, lacustrine strata equivalent to the Cow and Axial Basin arch (Figs. 1 and 2). To more Formation, which lacks molluscs (Bradley, Ridge and Garden Gulch Members contain gas- clearly differentiate between these strata, we 1931; Johnson, 1985). Above the Garden Gulch tropods at their base but none above, mirroring have adopted the names DeBeque Formation Member, the evaporative lower Parachute Creek the shift to fl uctuating profundal facies observed (Piceance Creek Basin) and Colton Formation Member (Donnell, 1961a; Trudell et al., 1974) is in the basin center (Moncure and Surdam, 1980; (Uinta Basin) in place of Wasatch Formation intercalated with both bedded and disseminated Johnson et al., 1988). Strata equivalent to the

60 Geological Society of America Bulletin, January/February 2008 Synoptic reconstruction of the Eocene Green River Formation Ar ages Ar 39 Ar/ 40 Ar ages are from Hiza (1999) from ages are Ar am does not imply internally 39 Ar/ 40 iclastic Eocene strata in the Absaroka Absaroka iclastic Eocene strata in the /Ar), and (l) House et al. (2002). All /Ar), and (l) House et al. (2002). as in Figure as in Figure 2. (1995), (f) and Moye (1989), (g) Snider Janecke et al. ed from Smedes and Prostka (1972). A complete list of lithostratigraphic, biostratigraphic, and A (1972). Smedes and Prostka ed from cant interstratal lacunae are undoubtedly present, particularly within more proximal volcanic deposits. proximal particularly within more undoubtedly present, cant interstratal lacunae are intercalibration uncertainties relative to the standard values of Renne et al. (1998). Note that the chronostratigraphic diagr to the standard values of Renne et al. (1998). Note that chronostratigraphic uncertainties relative intercalibration σ (1997), (h) Janecke and Snee (1993), (i) M’Gonigle and Dalrymple (1996), (j) O’Neill et al. (2004), (k) Wallace et al. (1992, K (1997), (h) Janecke and Snee (1993), (i) Wallace M’Gonigle and Dalrymple (1996), (j) O’Neill et al. (2004), (k) except: (a) Feeley and Cosca (2003), (b) and Elliott (1997), (d) Harlan et al. (1996), (c) Wilson Ispolatov (1997), (e) Snider Figure 5. Map showing major eruptive centers and lithostratigraphic and time-stratigraphic cross section of volcanic and volcan eruptive centers and lithostratigraphic time-stratigraphic cross 5. Map showing major Figure 29 stratigraphic studies. Mapping modiﬁ compiled from Province, Volcanic DR8 (see footnote portrayed at the same scale 1). Note that strata are Table sites is in numbered for radioisotopic references are shown with 2 are uniform rates of accumulation, and signiﬁ

Geological Society of America Bulletin, January/February 2008 61 Smith et al.

TABLE 1. CRITERIA FOR CLASSIFICATION OF GREEN RIVER FORMATION LAKE TYPE limestone facies (McGookey, 1960; Sheliga, Basin type Facies Typical facies Stratigraphy Fauna Hydrologic association interpretation 1980). Overlying the Green River Formation Overfilled Fluvial- Sandstone, coal, Dominantly Molluscs Freshwater in the eastern half of the region is the alluvial lacustrine massive to laminated progradational common; open lake Crazy Hollow Formation (Weiss and Warner, mudstone, coquina occasional 2001), whereas the volcaniclastic alluvial Gold- limestone fish en’s Ranch Formation (Muessig, 1951; Doel- Balanced Fluctuating Organic-rich Mixed Fish, Fluctuating filled profundal laminated mudstone, aggradational and ostracodes salinity, ling, 1972) is the uppermost Eocene unit in the stromatolites, oolites progradational intermittently western half. Farther to the southwest along the open and fold and thrust belt, Eocene alluvial and lacus- closed lake trine strata are referred to as the Claron Forma- Underfilled Evaporative Na-rich evaporites, Aggradational Fauna absent Hypersaline tion (Figs. 1 and 4; Goldstrand, 1994). mudcracked closed lake mudstone and siltstone, thin organic- GEOCHRONOLOGY rich laminated mudstone beds 40Ar/ 39Ar Methodology lower Parachute Creek Member on the Douglas et al. (1990) and Remy (1992) but recommend The geochronology of Green River Forma- Creek arch contain evaporite casts and abundant future adoption of nongenetic formal stratal tion and associated strata was accomplished exposure horizons, consistent with the evapora- designations for these units. At the base of the via 40Ar/39Ar dating of volcanic phenocrysts tive facies in the basin center (Moncure and Sur- Eocene succession, nearly 1000 m of lacustrine preserved in ash beds and volcaniclastic sand- dam, 1980; Cole, 1985). deposits occupy the subsurface depocenter of the stone beds (cf. Smith, 2007). The majority of The upper part of the Parachute Creek Mem- basin (Ryder et al., 1976; Fouch, 1981) and are dated units are preserved within laminated to ber contains the Mahogany zone (Bradley, equivalent to mollusc-bearing fl uvial-lacustrine fi nely bedded lacustrine facies (Table DR2 [see 1931; Cashion, 1967), a 20–60 m interval pre- facies exposed at the basin margins (Bradley, footnote 1]; cf. Smith et al., 2003). The origins dominantly composed of laminated organic-rich 1931; Cashion, 1967; Remy, 1992). This interval of the names of sampled units are explained in micrite that extends across the Douglas Creek is also correlative to the upper part of the allu- Table DR2. Samples were collected from the arch into both the Uinta and Piceance Creek vial Colton Formation (Spieker, 1946; Cashion, base of tuff beds, which are often subtly graded, Basins (Fig. 2A; Cashion and Donnell, 1972; 1967; Pusca, 2003). These basal fl uvial-lacus- in order to maximize phenocryst grain size and Remy, 1992). Although it represents the broad- trine strata are overlain by ~200 m of fl uctuating limit contamination by admixed detrital grains. est expansion of Lake Uinta, the Mahogany zone profundal strata assigned to the transitional inter- Minerals for dating were obtained using the in the Piceance Creek Basin contains evaporites val (Remy, 1992; Pusca, 2003). The evaporative separation techniques outlined in Smith et al. (Trudell et al., 1973; Dyni, 1981) and largely upper member overlies the transitional interval (2003, 2006) and were irradiated together with lacks fi sh fossils in the Uinta Basin (Cashion, and contains the Mahogany zone at its base fl ux monitors at the Oregon State University 1967; Remy, 1992). In addition, a tuff bed within (Bradley, 1931; Cashion, 1967; Remy, 1992). Triga reactor (details of J value calculations are the Mahogany zone in the Piceance Creek Basin In the eastern Uinta Basin, the upper member found in Data Repository Fig. DR1). Ar isotopic exhibits K-spar alteration of its formerly glassy interfi ngers with and is overlain by the alluvial compositions were determined using a CO2 laser ash matrix, denoting deposition in alkaline Uinta Formation (Douglass, 1914; Cashion, to fuse or incrementally heat sanidine or biotite lake water with an elevated solute concentra- 1967). However, in the western Uinta Basin, the crystals following the procedures detailed in tion (Surdam and Parker, 1972; Mason, 1983). Green River Formation above the upper mem- Smith et al. (2003, 2006). We have therefore categorized the facies of the ber thickens to >500 m. Two additional units are Ages were determined on the basis of 2234 Mahogany zone as evaporative rather than fl uc- preserved as a result: the evaporative, evaporite- analyses of phenocrysts from 25 tuffaceous tuating profundal. Above the Mahogany zone in bearing saline facies, and the overlying fl uctuat- samples (Figs. 6–8; Tables 2 and DR3). Age the Piceance Creek Basin, the Parachute Creek ing profundal to fl uvial-lacustrine sandstone and plateaus are here defi ned as three or more con- Member is composed of fl uctuating profundal limestone facies above (Dane, 1955; Dyni et al., tiguous, concordant steps containing at least facies and interfi ngers with the volcaniclastic 1985; Weiss et al., 1990). These units are equiva- 50% of the total 39Ar released. Heating steps deltaic and alluvial Uinta Formation (Trudell et lent to the lower part of the alluvial Uinta Forma- were considered concordant if the mean squared al., 1970; Hail, 1987). tion in the eastern Uinta Basin, and are overlain weighted deviation (MSWD) resulting from by the upper Uinta Formation (Fig. 2A; Dane, their inclusion was less than the Students-T dis- Uinta Basin 1955; Prothero, 1996). tribution limit for the number of included steps (Koppers, 2002). When MSWD exceeded 1, The Green River Formation achieves its great- High Plateaus of Utah analytical errors were multiplied by the square est thickness in the Uinta Basin and records an root of the MSWD (cf. York, 1969; Koppers, open to closed to open hydrologic trajectory Extending southward from the Uinta Basin 2002). Plateau ages are the weighted mean of similar to that observed in the other two major into central Utah along the margin of the Sevier included steps, whereas integrated (total fusion) basins (Fig. 2A). Unfortunately, stratigraphic fold and thrust belt are isolated exposures of ages combine the Ar released during all heat- terminology in the Uinta Basin is beset by infor- Green River Formation (Figs. 1 and 2; Spieker, ing steps. Inverse-variance weighted mean ages mal and overlapping designations, due in part 1949; Doelling, 1972). These strata overlie the and uncertainties were calculated from both to limited surface exposure (cf. Remy, 1992). alluvial Colton Formation and consist of evapo- fusion and plateau ages according to Taylor In the interest of consistency, we have adopted rative to fl uvial-lacustrine strata that are loosely (1982) using Isoplot 3.00 (Ludwig, 2003). An the informal stratigraphic designations of Weiss correlated to the saline facies and sandstone and arbitrary outlier exclusion criteria adapted from

62 Geological Society of America Bulletin, January/February 2008 Synoptic reconstruction of the Eocene Green River Formation

46 48 50 52 54 56 u

Weighted Mean Age t 48.94 ± 0.29 Ma u MSWD = 0.38 B

S Sanidine Biotite fusion single crystal plateau age

f f plateau age multicrystal plateau age Weighted Mean Age u

50.61 ± 0.23 Ma r

MSWD = 0.80 a

o 42 44 46 48 50 52

B Discordant spectra

Weighted Mean Age t

45.14 ± 0.10 Ma y

l MSWD = 0.93 i

e Weighted Mean Age l

50.83 ± 0.13 Ma h

MSWD = 0.75 e

u Weighted Mean Age t

45.58 ± 0.14 Ma t MSWD = 0.17 r

Single crystal fusions

y Weighted Mean Age

i 51.77± 0.87 Ma

l i MSWD = 0.13

a Discordant spectra

u Weighted Mean Age

t e 46.34 ± 0.13 Ma

g MSWD = 2.10

l Combined fusions

h u Weighted Mean Age 51.90 ± 0.09 Ma c

MSWD = 0.34 f u Multicrystal fusions

C Weighted Mean Age

51.90± 0.09 Ma Discordant spectra n MSWD = 0.44 o

C Weighted Mean Age

47.04 ± 0.18 Ma n

i MSWD = 0.10 l

t Discordant spectra Weighted Mean Age

g 47.94 ± 0.17 Ma f

MSWD = 0.41 t

W Weighted Mean Age 48.37 ± 0.23 Ma MSWD = 0.51

Weighted Mean Age u

51.66 ± 0.09 Ma r

MSWD = 0.44 a

- Discordant spectra f

Weighted Mean Age u 49.02 ± 0.30 Ma C MSWD = 0.68

46 48 50 52 54 56 42 44 46 48 50 52 Age (Ma) Age (Ma)

Geological Society of America Bulletin, January/February 2008 63 Smith et al.

42 44 46 48 50 52 42 44 46 48 50 52

u Weighted Mean Age n

Weighted Mean Age i

B 48.11 ± 0.08 Ma a 47.17 ± 0.08 Ma t

n MSWD = 0.78 l MSWD = 0.69

44 46 48 50 52 54 56

Weighted Mean Age Weighted Mean Age 47.70 ± 0.12 Ma

f MSWD = 0.99 48.15 ± 0.08 Ma f

MSWD = 1.04 t

multi-crystal g

Wf single crystal

u e Weighted Mean Age

Weighted Mean Age e

m r 51.74 ± 0.09 Ma u 48.62 ± 0.28 Ma

C MSWD = 0.54

MSWD = 0.28 t

u Weighted Mean Age t 48.76 ± 0.09 Ma s

MSWD = 1.19 t

44 46 48 50 52 54 56 Age (Ma)

Weighted Mean Age 48.66 ± 0.28 Ma

MSWD = 0.29 f

l Figure 6 (on this and previous page). Cumulative prob-

a t 40 39 n ability diagrams summarizing Ar/ Ar results from

Discordant spectra t sanidine, plagioclase, and biotite in 16 tuff beds. Biotite

o symbols (diamonds) represent weighted mean plateau

Cf Weighted Mean Age ages. MSWD—mean square of weighted deviates. 48.64 ± 0.47 Ma MSWD = 1.30

42 44 46 48 50 52 Age (Ma)

64 Geological Society of America Bulletin, January/February 2008 Synoptic reconstruction of the Eocene Green River Formation

Strawberry tuff 112 Ma

Single crystal fusions

l Weighted Mean Age

b 43.29 ± 2.12 Ma

a MSWD = 0.10

u Combined 6-8 crystal fusions

m Weighted Mean Age Weighted Mean Age u 44.00 ± 0.92 Ma

C MSWD = 0.14 44.17 ± 1.02 Ma MSWD = 0.51

30 40 50 60 70 80

236 Ma Yellow tuff 148 Ma

b Combined

a Weighted Mean Age

o 51.24 ± 0.52 Ma

r 3 crystal fusions MSWD = 0.40 P Weighted Mean Age

e 51.39 ± 1.70 Ma

i MSWD = 0.23

m 20 crystal fusions 112 Ma u Weighted Mean Age

C 51.23 ± 1.10 Ma MSWD = 4.18

30 40 50 60 70 80 Apparent Age (Ma) Figure 7. Cumulative probability diagrams showing sanidine and biotite ages obtained from the Yellow and Strawberry tuff beds. Feldspars from these beds exhibit marked contamination by older grains. Note that multiple single-crystal analyses isolate a young, presumably juvenile magmatic population from both ash beds when older xenocrysts are excluded from the age calculation. MSWD—mean square of weighted deviates.

y Antelope sandstone bed

l i Weighted Mean Age

a 48.70 ± 0.19 Ma

r MSWD = 0.35

ulative 40 50 60 70 80

Cum

0 500 1000 1500 2000 Apparent Age (Ma) Figure 8. Cumulative probability diagram showing analyses of detrital ortho- clase and sanidine from a sandstone bed near the base of the Sand Butte bed of the Laney Member. MSWD—mean square of weighted deviates.

Geological Society of America Bulletin, January/February 2008 65 Smith et al.

TABLE 2. SUMMARY OF 40Ar/39Ar EXPERIMENTAL RESULTS FOR 29 ASH BEDS Basin Isochron analysis† Apparent ages† 40 36 σ σ†† Name (Sample) Location SUMS Ar/ Ari Isochron age Average MSWD Weighted mean age ±2 ±2 Formation, Symbol (N-2) ± 2σ (Ma) ± 2σ§ K/Ca ± 2σ (Ma) ± 2σ§ Mineral Analysis type n Greater Green River Basin Sage Creek Mt. pumice (SCM) N41°7′56.5″ W110°8′11.7″ Tb, SC Sanidine SF 35 of 35 0.74 312.3 ± 89.8 47.16 ± 0.11 74 ± 4 0.72 47.17 ± 0.08 MI (2 of 2) 10 of 10 0.22 444.2 ± 292.1 47.10 ± 0.19 59 ± 18 0.47 47.17 ± 0.13 SF + MI 45 of 45 0.63 336.9 ± 84.9 47.14 ± 0.10 72 ± 4 0.69 47.17 ± 0.08‡ ±0.16 ±0.81 Tabernacle Butte tuff (TaB) N42°26′0.6″ W109°22′31.4″ Tb, TB Sanidine SF 30 of 30 0.76 302.8 ± 33.6 48.10 ± 0.10 63 ± 3 0.81 48.11 ± 0.08 MI (2 of 2) 10 of 10 0.34 735.8 ± 1064 47.91 ± 0.36 55 ± 11 0.59 48.10 ± 0.12 SF + MI 40 of 40 0.76 306.7 ± 32.8 48.09 ± 0.09 61 ± 4 0.78 48.11 ± 0.08‡ ±0.16 ±0.83 Henrys Fork tuff (HeF) N41°7′25.3″ W110°9′27.7″ Tb, HF Sanidine SF 56 of 56 1.10 302 ± 60 48.14 ± 0.09 60 ± 2 1.08 48.15 ± 0.08 MI (3 of 3) 15 of 15 1.08 451 ± 207 48.02 ± 0.18 51 ± 7 1.39 48.16 ± 0.11 SF + MF 71 of 71 1.12 331 ± 60 48.13 ± 0.33 58 ± 2 1.04 48.15 ± 0.08‡ ±0.18 ±0.83 Leavitt Creek tuff (LeC) N41°14′13.1″ W110°12′41.8″ Tb, LC Sanidine SF 12 of 14 0.30 312.6 ± 38.3 48.55 ± 0.60 54 ± 14 0.28 48.62 ± 0.28‡ ±0.31 ±0.88 Church Buttes tuff (ChB) N41°28′34.5″ W110°8′04.3″ Tb, CB Sanidine SF 34 of 38 1.17 266 ± 40 48.74 ± 0.12 51 ± 9 1.17 48.72 ± 0.09 MI (2 of 2) 8 of 8 0.24 631 ± 474 48.47 ± 0.51 47 ± 4 0.73 48.88 ± 0.15 SF + MI 42 of 46 1.19 279 ± 43 48.77 ± 0.11 50 ± 7 1.30 48.76 ± 0.09‡ ±0.16 ±0.84 Continental Peak tuff (CP-1) N42°16′06.2″ W108°43′7.5″ Tb, CP Biotite SI (4 of 5) 20 of 26 0.53 312.6 ± 38.3 48.80 ± 1.12 35 ± 9 1.70 49.12 ± 1.08 MI (3 of 6) 15 of 30 0.73 274.0 ± 19.3 48.76 ± 0.49 40 ± 16 0.69 48.54 ± 0.54 SI + MI 35 of 56 0.75 287.0 ± 17.4 48.77 ± 0.46 36 ± 8 1.30 48.64 ± 0.47 Sanidine MF 12 of 16 0.27 359.5 ± 276.2 48.51 ± 0.64 37 ± 8 0.29 48.66 ± 0.28‡ ±0.31 ±0.88 Antelope sand bed (AC-3) N41°23′45.9″ W108°30′54.2″ Tgl, An K-feldspar SF 62 of 82 0.35 292.8 ± 32.7 48.70 ± 0.21 1188 ± 260 0.35 48.70 ± 0.19‡ ±0.23 ±0.86 Sand Butte tuff (SB-3) N41°20′43.7″ W108°40′13.7″ Tgl, SB Feldspar MF 9 of 9 0.42 290.3 ± 18.5 49.05 ± 0.45 0.1 ± 0.1 0.38 48.94 ± 0.29‡ ±0.32 ±0.89 Analcite tuff (SB-1) N41°21′1.4″ W108°40′4.7″ Tgl, A Sanidine SF 21 of 21 0.82 331.5 ± 87.9 48.61 ± 0.84 27 ± 3 0.82 48.57 ± 0.73 MF‡‡ 19 of 19 0.79 281.9 ± 52.3 49.00 ± 0.22 23 ± 2 0.71 48.95 ± 0.12 SF + MF 40 of 40 0.79 297.5 ± 42.7 48.95 ± 0.19 25 ± 2 0.75 48.95 ± 0.12‡ ±0.18 ±0.85 Sixth tuff (TR-5) N41°32′31.1″ W109°28′52.9″ Tgw, 6 Sanidine SF 37 of 40 0.09 450 ± 614 49.21 ± 1.48 108 ± 15 0.10 49.68 ± 0.59 MF 11 of 30 0.51 292.8 ± 14.6 49.71 ± 0.61 2360 ± 1300 0.47 49.68 ± 0.59 SF + MF 48 of 70 0.18 293.1 ± 14.4 48.69 ± 0.43 109 ± 14 0.18 49.68 ± 0.42 Biotite SI + MI (21 of 26)§§ 119 of 142 0.53 299.9 ± 6.5 49.61 ± 0.11 274 ± 46 1.00 49.62 ± 0.10‡ ±0.17 ±0.86 Layered tuff (TR-6) N41°32′33.6″ W109°28′55.6″ Tgw, L Sanidine MF§§ 64 of 73 0.32 315 ± 31 49.75 ± 0.12 79 ± 2 0.42 49.79 ± 0.09‡ ±0.17 ±0.86 Main tuff (TR-1) N41°32′28.1″ W109°28′52.0″ Tgw, M Sanidine MF‡‡ 30 of 31 0.77 293.4 ± 6.4 49.98 ± 0.09 98 ± 11 0.65 49.96 ± 0.08‡ ±0.16 ±0.86 Boar tuff (BT-14) N41°57′48.6″ W109°15′9.8″ Tgw, B Sanidine MF 10 of 14 0.66 179 ± 105 51.09 ± 0.41 3007 ± 1400 0.80 50.61 ± 0.23‡ ±0.27 ±0.90 Grey tuff (WN-1) N41°39′24.3″ W109°17′18.8″ Tgw, G Sanidine MF‡‡ 18 of 18 0.73 285.7 ± 11.3 50.55 ± 0.21 190 ± 130 0.67 50.39 ± 0.13‡ ±0.19 ±0.87 Firehole tuff (FC-2) N41°21′0.7″ W109°22′59.9″ Tgw, F Sanidine MF 41 of 41 0.77 295.3 ± 2.3 50.83 ± 0.13 71 ± 3 0.94 50.83 ± 0.13‡ ±0.19 ±0.88 Rife tuff (BT-18) N41°57′47.2″ W109°15′8.7″ Tgt, R Biotite MF + SF‡‡ 15 of 38 0.71 290.1 ± 10.7 51.58 ± 0.62 867 ± 240 0.68 51.30 ± 0.30‡ ±0.33 ±0.93 MI (4 of 8) 16 of 29 0.61 285.5 ± 14.9 53.35 ± 0.58 20 ± 8 0.92 53.15 ± 0.53 Scheggs tuff (WP-3) N41°31′9.9″ W109°19′29.8″ Tgt, S Sanidine SF 21 of 21 0.13 356 ± 585 51.52 ± 1.80 109 ± 19 0.13 51.77 ± 0.87 MF 62 of 62 0.30 562 ± 319 51.57 ± 0.29 81 ± 7 0.42 51.90 ± 0.09 SF + MF 83 of 83 0.27 508 ± 248 51.63 ± 0.24 86 ± 7 0.34 51.90 ± 0.09‡ ±0.17 ±0.89 (Continued)

66 Geological Society of America Bulletin, January/February 2008 Synoptic reconstruction of the Eocene Green River Formation

Piceance Creek Basin Yellow tuff (WR-1) N40°1′1.9″ W108°6′53.2″ Tgp, Y Sanidine MF 23 of 53 0.37 331.8 ± 82.5 50.03 ± 0.74 73 ± 12 0.40 51.24 ± 0.52‡ ±0.54 ±1.02

Uinta Basin Strawberry tuff (SR-1) N40°9′54.0″ W110°33′5.6″ Tgsl, St Sanidine MF 2 of 43 85 ± 10 0.51 44.17 ± 1.02 SF 20 of 25 0.09 –775 ± 9887 50.44 ± 10.87 63 ± 47 0.10 43.29 ± 2.12 SF + MF 22 of 68 0.15 383 ± 1248 43.77 ± 1.61 64 ± 40 0.14 44.00 ± 0.92‡ ±0.93 ±1.19 Biotite MI (IA; 5 of 12) 26 of 61 13 ± 3 1.80 41.30 ± 1.20 Oily tuff (IC-6) N40°2′56.8″ W110°31′42.1″ Tgs, O Biotite SI (9 of 10) 43 of 49 0.91 280.4 ± 11.9 45.18 ± 0.11 100 ± 29 0.93 45.14 ± 0.10‡ ±0.16 ±0.78 Portly tuff (IC-5) N39°58′47.6″ W110°37′16.3″ Tgs, P Biotite SI (11 of 11) 55 of 55 0.19 259.9 ± 29.5 45.81 ± 0.26 168 ± 45 0.13 45.67 ± 0.26 MI (5 of 5) 29 of 29 0.92 280.9 ± 19.9 45.61 ± 0.17 76 ± 31 0.19 45.55 ± 0.16 SI + MI 84 of 84 0.45 275.4 ± 16.5 45.66 ± 0.14 141 ± 33 0.17 45.58 ± 0.14‡ ±0.19 ±0.79 Fat tuff (IC-2) N39°58′46.5″ W110°37′6.9″ Tgs, Fa Biotite SI (4 of 13) 19 of 65 1.13 300.1 ± 12.9 46.34 ± 0.10 90 ± 36 2.10 46.34 ± 0.13‡ ±0.18 ±0.80 Blind Canyon tuff (SW-1) N39°50′41.4″ W110°11′11.8″ Tgu, Bl Biotite SI + MI (3 of 13) 13 of 73 0.95 256.7 ± 32.1 47.18 ± 0.20 418 ± 1100 0.10 47.04 ± 0.18‡ ±0.22 ±0.83 Wavy tuff (GC-2b) N39°50′59.3″ W110°15′17.5″ Tgu, W Biotite MF 7 of 9 1.44 281.6 ± 26.6 48.83 ± 0.71 27 ± 5 1.45 48.47 ± 0.18 SF 50 of 54 1.33 293.4 ± 14.9 48.56 ± 0.24 65 ± 15 1.31 48.53 ± 0.17 SI (4 of 18) 23 of 107 0.73 281.1 ± 16.8 48.49 ± 0.27 31 ± 12 0.51 48.37 ± 0.23‡ ±0.27 ±0.86 Curly tuff (GC-5b) N39°50′33.8″ W110°15′3.1″ Tgu, C Biotite SI (4 of 10) 11 of 26 0.89 194.5 ± 59.4 49.51 ± 0.37 548 ± 200 0.68 49.02 ± 0.30‡ ±0.33 ±0.89

Wind River Basin White Lignitic tuff (WB-1) N42° 42′ 54.3″ W108° 11′ 11.8″ Twb Sanidine SF 11 of 11 0.09 500 ± 255 46.70 ± 5.13 30 ± 4 0.10 47.97 ± 1.31 MF 24 of 25 1.24 510 ± 305 47.33 ± 0.37 3.9 ± 0.4 1.42 47.70 ± 0.13 SF + MF 35 of 36 0.85 507 ± 264 47.33 ± 0.33 3.9 ± 0.4 0.99 47.70 ± 0.12‡ ±0.18 ±0.83 Halfway Draw tuff (HD-1) N42° 51′ 52.9″ W108° 17′ 57.1″ Twr Sanidine SF 51 of 54 0.54 300 ± 7 51.73 ± 0.11 113 ± 7 0.57 51.75 ± 0.10 MI (6 of 6) 19 of 19 0.36 530 ± 411 51.32 ± 0.61 97 ± 13 0.38 51.72 ± 0.16 SF + MI 70 of 73 0.53 301 ± 7 51.73 ± 0.10 109 ± 6 0.54 51.74 ± 0.09‡ ±0.17 ±0.89 Note: Summary of 2234 individual analyses. Twi—Willwood Formation; Twr—Wind River Formation; Twb—Wagon Bed Formation; Tb—Bridger Formation; Tgl—Laney Member–Green River Formation; Tgw—Wilkins Peak Member–Green River Formation; Tgt—Tipton Member–Green River Formation; Tgf—Fossil Butte Member-Green River Formation; Tf—Fowkes Formation; Tgsl—sandstone and limestone member–Green River Formation; Tgs—saline member–Green River Formation; Tgp—Parachute Creek Member–Green River Formation. Only experiments yielding 100% concordant age spectra are included in calculation of weighted means of plateau ages. MI—multicrystal laser incremental heating experiments; SI—single crystal laser incremental heating experiments; MF—multicrystal laser fusion experiments; SF—single crystal laser fusion experiments; IA—integrated ages. Concordant experiments include all incremental heating steps. MSWD—mean square weighted deviate. Corrections for undesirable 40 40 40 39 36 37 39 37 nucleogenic reactions on K and Ca are as follows: [ Ar/ Ar]K = 0.00086; [ Ar/ Ar]Ca = 0.000264; [ Ar/ Ar]Ca = 0.000673. More than 1000 measurements of sanidine from the Taylor Creek rhyolite (TCs) were used to monitor the experiments. Its age is 28.34 ± 0.28 Ma relative to the GA-1550 biotite primary standard (98.79 ± 0.96 Ma; Renne et al., 1998). Mass discrimination was monitored using an on-line air pipette and varied between 1.0020 ± 0.0010 and 1.0065 ± 0.0010 per amu, during the analytical periods. K/Ca reﬂ ects measured 37Ar and 39Ar, and is a derived atomic ratio. The large uncertainty in K/Ca for some samples is an artifact of the imprecision of the 37Ar measurement for samples analyzed after a signiﬁ cant proportion of the nucleogenically derived 37Ar had decayed away. Latitude and longitude referenced to NAD27 datum. †Ages relative to 28.34 Ma for TCs (Renne et al., 1998). ‡Preferred age. §Analytical uncertainty. #Analytical and intercalibration uncertainty for preferred age. ††Fully propagated uncertainty for preferred age. ‡‡From Smith et al. (2003). §§From Smith et al. (2006).

Geological Society of America Bulletin, January/February 2008 67 Smith et al.

Deino and Potts (1990) was applied, in which these measurements are relatively imprecise to Proterozoic (Fig. 8). However, 62 analyses apparent plateau and fusion ages were excluded due to lower signal to blank ratios (i.e., >10% yield Eocene apparent ages that have a Gauss- if they contributed to a MSWD >1.5, thereby uncertainty), single fusions of Analcite tuff and ian distribution (Fig. 8), suggesting deriva- eliminating only obvious outliers from the age Scheggs tuff sanidine produced no obvious outli- tion from a common eruption or set of simi- calculation (Smith et al., 2006). Isochrons were ers and yield weighted mean ages that are indis- larly timed eruptions. We take the weighted regressed using the method of York (1969) in tinguishable from those of multicrystal analyses mean age of this younger component (48.70 order to test for excess argon, and in virtually all (Figs. 6 and DR4). Sanidine from the Sixth tuff ± 0.19 Ma) to indicate the age of this volca- cases exhibit atmospheric intercepts (Fig. DR2; proved to be more problematic (cf. Smith et nism and the maximum age of sand deposition see footnote 1). al., 2003). New single-crystal fusions (n = 37) (cf. Deino and Potts, 1990). Similarly, 19 of of Sixth tuff sanidine yield a weighted mean of 25 analyses of sanidine from the Sage tuff, a 40Ar/39Ar Results 48.68 ± 0.59 Ma that is consistent with the more volcaniclastic sand bed in the Bulldog Hollow precise age of 49.62 ± 0.10 Ma acquired via the Member of the Fowkes Formation, gave inter- Sanidine incremental heating of individual biotite, which nally consistent apparent ages with a weighted Fusions and incremental heatings were per- remains its preferred age (Smith et al., 2006). mean of 47.94 ± 0.17 Ma (Fig. 6). All 35 anal- formed on single crystals and small multicrystal However, three single-crystal fusions gave yses of >0.5-mm-diameter sanidine from the aliquots of sanidine from 18 of the dated sam- apparent ages that are signifi cantly overesti- Sage Creek Mountain pumice within a sand ples (Figs. 6 and DR3). Single-crystal fusions mated (>70 Ma; Fig. DR4). Accordingly, when bed near the base of the Turtle Bluff Member of sanidine from six samples yielded precise Sixth tuff sanidine were analyzed as 5 crystal of the Bridger Formation gave precise, consis- results due to their large size (>250 μm), showed aliquots (cf. Smith et al., 2003), only 11 of 30 tent apparent ages that have a weighted mean no distinct outliers, and gave weighted mean fusions yielded stratigraphically reasonable of 47.17 ± 0.08 Ma (Fig. 6), suggesting their ages of 51.74 ± 0.09 Ma (Halfway Draw tuff), apparent ages, whereas 19 gave distinctly older derivation from a single eruption or closely 47.70 ± 0.12 Ma (White Lignitic tuff), 48.76 ± ages (Fig. DR4). We interpret these older outli- timed set of eruptions. 0.09 Ma (Church Buttes tuff), 48.62 ± 0.28 Ma ers to refl ect the admixture of a small proportion (Leavitt Creek tuff), 48.15 ± 0.08 Ma (Henrys (<10%) of xenocrystic or detrital grains with a Biotite Fork tuff), and 48.11 ± 0.08 Ma (Tabernacle larger number of juvenile magmatic sanidine. Several of the tuff beds sampled from the Butte tuff) (Fig. 6; Table 2). However, sanidine Sanidine from the Strawberry and Yellow tuff Uinta Basin lack sanidine, which necessitated from several ash beds were too small (<180 μm beds exhibited the largest amount of age scatter, the use of biotite. Multiple laser incremen- in diameter) to achieve an adequate signal to which limits the precision of the age determina- tal heatings of hand-picked euhedral biotite blank ratio from single crystals, necessitating tions for these beds (Fig. 7). More than 10% of crystals were conducted to assess the potential the use of multicrystal aliquots to achieve use- single-crystal fusions of Strawberry tuff sanidine for alteration-derived discordance and inaccu- fully high precision (i.e., <5% uncertainty). In yielded distinctly older ages, and accordingly, racy. In addition, electron probe microanalysis most cases, multicrystal analyses yield Gauss- >90% of 6- to 8-crystal analyses gave anoma- was performed on biotite crystals to gauge ian apparent age distributions with few outliers lously old apparent ages. Similarly, only 50% of the presence or absence of alteration phases. and give stratigraphically consistent weighted 3-crystal fusions of sanidine from the Yellow tuff (Table DR4 and Figs. DR5 and DR6; see mean ages of 51.90 ± 0.09 Ma (Scheggs tuff), gave stratigraphically reasonable apparent ages, footnote 1) (cf. Smith et al., 2006). Age spec- 50.61 ± 0.23 Ma (Boar tuff), 48.94 ± 0.29 Ma and only 20% of fusions of 20-crystal aliquots tra produced from biotite from Green River (Sand Butte tuff), 48.66 ± 0.28 Ma (Continental could be included in the weighted mean calcula- Formation tuff beds are of two distinct types: tuff), and 51.66 ± 0.09 Ma (K-spar tuff) (Fig. 6; tion. We infer that the age scatter observed for (1) concordant and reproducible, and (2) dis- Table 2). Note that new analyses of sanidine both samples can be attributed to the inclusion cordant and indicative of alteration-related from the Firehole and Analcite tuff beds have of 10%–20% inherited grains. 40Ar loss and 39Ar recoil during irradiation. resulted in a slight revision of the preferred ages Excluding the Strawberry and Yellow tuffs, Discordant age spectra were observed from for these beds (Table 2). Incremental heating only 11 of 296 (3.7%) of sanidine analyses were biotite from several tuffs (Fig. DR5), and cor- experiments performed on multicrystal sanidine excluded from age calculations (Table 2; Fig. 6). relate to the presence of intergrown alteration aliquots from several tuffs all yield internally Based on the apparent absence of fl uvial rework- phases (Table DR4; Fig. DR6) and integrated concordant plateau ages consistent with fusion ing of most sampled tuffs (Table DR2; see foot- age scatter toward both older and younger ages, suggesting that 40Ar* loss due to alteration note 1) and largely unimodal sanidine age dis- apparent ages (Fig. DR7). We consequently is insignifi cant (Table 2; Fig. DR3). tributions (Fig. 6; Table DR2), we interpret their take the weighted mean of concordant biotite Although concerns have been raised about weighted mean ages to represent the best esti- experiments as the best estimate for the erup- the ability to distinguish xenocrysts when mul- mate of their age of eruption and deposition. tive age of the ash beds from which sanidine tiple sanidine crystals are analyzed (cf. Machlus was unavailable (Fig. 6; Table 2), but caution et al., 2004; Smith et al., 2006), single-crystal Detrital Feldspar that biotite populations yielding predominantly fusion results suggest that contamination pres- Laser fusion measurements were performed discordant spectra (i.e., the Curly, Wavy, Blind ent in Green River Formation ash beds is com- on single detrital feldspar grains from three Canyon, and Fat tuffs) are less reliable than posed of signifi cantly older grains that can be volcaniclastic sand beds and yielded apparent those yielding predominantly concordant pla- readily distinguished and excluded even when ages indicative of rapid transport and deposi- teaus, such as the Portly and Oily tuffs. using small multicrystal aliquots. To assess the tion of erupted materials with little admixture distribution and magnitude of potential contam- of detrital grains. Of 82 analyses of K-feldspar Analytical Results and Uncertainties ination of multicrystal aliquots, single-crystal from the Antelope sand bed near the base of fusions of sanidine from the Analcite, Scheggs, the Sand Butte bed of the Laney Member, 20 Because our initial emphasis is on delimit- and Sixth tuffs were conducted. Although gave apparent ages that range from Paleocene ing the timing of lacustrine deposition in the

68 Geological Society of America Bulletin, January/February 2008 Synoptic reconstruction of the Eocene Green River Formation

42 s bio- Myton t

n Pocket Absaroka Volcanic Province i zone U3 o

U3 U3 p

Ui-3 U3 C20n n

i U3? t 43 MV U3 a

Owl l

Trans- a

Creek San c St Pecos Greater Green River Basin Diego d Texas e

44 z U2 California i

U2 Washakie i Ui-2 l

Basin C20r t

U2 d O U2 n 45 a

U2 d

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9 ) Indian Mountain B3 B3 Ptarmigan 9

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t M TB BP

n ( 48 HF B3

B3 e Uinta Basin

e B3 K

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49 A CB B B2 e

Br-1b U1 Biostratigraphic zone B1b n B1b North C22r a

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Reversed polarity Mountain m

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e Marine fossils Pass l Bighorn Basin C23r a P10; CP12b-13b c Wa-7 (Walsh et al., 1996) S S McCullough e Central m 52 W7 i Dated units not in Fig. 2: Peaks T

t BP _Blue Point Marker ash W7 i Little Wi r

a (Hiza, 1999) Muddy W6 W6 C24n l Wa-6 o

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(Prothero and Emry, 1996) i

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MV _Mission Valley ash a

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Wi_Willwood ash G Figure 9. Integrated early and middle Eocene paleomagnetic chronostratigraphy for western North America. All 40Ar/39Ar ages are shown with 2σ intercalibration uncertainties relative to the standard values of Renne et al. (1998).

Greater Green River Basin and Uinta–Piceance Biotite phenocryst compositions provide an but has not been analyzed for biotite composi- Creek Basin, 40Ar/39Ar ages are reported with 2σ additional test of ash bed correlations (Des- tion (Fig. 10; Table 2). analytical uncertainties relative to the standard borough et al., 1973; Yen and Goodwin, 1976; ages of Renne et al. (1998). Table 2 also reports Mauger, 1977). One potential correlation con- AGE MODEL FOR THE GREEN RIVER intercalibration and fully propagated uncertain- nects the Henrys Fork, Tabernacle Butte, and FORMATION ties for each unit (Karner and Renne, 1998; Wavy tuffs, all of which have overlapping Renne et al., 1998). Intercalibration uncertain- 40Ar/39Ar ages (Table 2). The FeO/MgO and Calibration of North American Land- ties should be considered when making com- TiO2 compositions of biotite from the Hen- Mammal Ages parisons to 40Ar/39Ar ages obtained using other rys Fork and Wavy tuffs are similar (Fig. 10; standard minerals or to the geomagnetic polar- Table DR4); however, biotite from the Taber- The 40Ar/39Ar ages presented here add to ity time scale (Fig. 9; Cande and Kent, 1992, nacle Butte tuff was not analyzed. At a broader a radioisotopic data set (Wing et al., 1991; 1995; Ogg and Smith, 2004). Fully propagated spatial scale, these three ashes may correlate Smith et al., 2003, 2004, 2006) that provides uncertainties reﬂ ect uncertainty in the 40K decay to the more proximal Blue Point Marker tuff temporal calibration for a large suite of exist- constant and K/Ar age of the GA-1550 primary in the southern Absaroka Volcanic Province, ing lithostratigraphy, biostratigraphy, and mag- biotite standard, and are required for compari- which was 40Ar/39Ar dated by Hiza (1999) as netostratigraphy (Fig. 3). Overall, the ages sons with isotopic chronometers other than 40K 48.10 ± 0.17 Ma (Fig. 5). Another potential determined for this study are entirely consis- decay, such as U-Pb (Karner and Renne, 1998; correlation connects the Continental, Church tent with isochroneity of the North American Min et al., 2000). Buttes, and Curly tuffs, which have overlap- land-mammal ages throughout the study area ping 40Ar/39Ar ages and biotite with similar (Wood et al., 1941; Robinson et al., 2004). The

ASH BED CORRELATIONS FeO/MgO and TiO2 compositions. However, direct temporal implications of these ages for the ages of the Continental and Curly tuffs also the late Wasatchian, Bridgerian, and Uintan Several possible correlations between ash overlap with the age of the Leavitt Creek tuff, land-mammal ages are summarized in Figure 3 beds are suggested by 40Ar/39Ar ages (Fig. 2B). which is ~100 m above the Church Buttes tuff and detailed in Table 3.

Geological Society of America Bulletin, January/February 2008 69 Smith et al.

Implications for Paleomagnetic Records 5.5 and the Geomagnetic Polarity Time Scale 5.3 New 40Ar/39Ar ages allow for the calibration of early and middle Eocene paleomagnetic polarity records, which have been obtained at 5.1 seven sites in basins containing the Green River Formation (Fig. 9; Jerskey, 1981; Flynn, 1986; 4.9 Prothero and Swisher, 1992; McCarroll et al., 1996a; Stucky et al., 1996; Clyde et al., 1997, Tuffs 2001). These studies have typically focused 4.7 on alluvial strata where mammalian fossils are ) AV2-61

% preserved, but in several cases have reported the t AV2-59 w Mauger (1977) polarity of lacustrine strata. Previous efforts to ( 4.5 2 T-13 Curly correlate magnetic polarity records to the geo- O i AV1-41 n=5 Wavy Wavy magnetic polarity time scale have been ham- T 4.3 n=7 n=4 40 39 Desborough pered by a lack of common Ar/ Ar standard et al. (1973) values (cf. Renne et al., 1998; Smith et al., 2003) and uncertain time-stratigraphic relationships 4.1 Henrys Fork between basins (cf. Wing et al., 2000; Smith Wavy et al., 2003; Machlus et al., 2004). However, 3.9 Church Buttes when viewed in total (Fig. 9; Table 4), the cur- rent data set resolves many previous uncertain- Continental Desborough et al. (1973) ties and permits the recalibration, consistent 3.7 Curly with the standard ages of Renne et al. (1998), of the ages of chrons C24r through C20n. Our 3.5 provisional calibration (Table 5) was calculated 1.0 1.5 2.0 2.5 3.0 using seven 40Ar/39Ar ages for ash beds found FeO/MgO within paleomagnetically characterized strata or their correlative equivalents (Fig. 9; Table 4). Figure 10. FeO/MgO versus TiO2 plot of electron microprobe point analyses on biotite from The new magnetic reversal ages (Table 5) are select ash beds illustrating several likely correlations. Values are shown as weight percent. Bio- consistent with magnetostratigraphy at 19 loca- tite compositions for Henrys Fork and Church Buttes tuff beds are from Smith (2007). Lower tions, and with the presence of marine index TiO2 values of Desborough et al. (1973) and Mauger (1977) for biotite from the Wavy and taxa (P10, CP12b-13b) associated with C21n Curly tuff beds may refl ect the improved detection and interference correction capabilities of within the Ardath Shale; the shale underlies the Cameca SX51 electron microprobe and software used versus those utilized in the 1970s. alluvial strata near San Diego, California, that contain Ui-1 faunas (Berggren et al., 1995; Walsh et al., 1996). The most signifi cant result- equivalent to the lower half of the Green River determinations from the Eocene of the northern ing change involves a shift in the age of chron Formation in the Piceance Creek Basin, has Rocky Mountains, all recalibrated to the stan- C22, which becomes ~1 m.y. younger than indi- yielded Graybullian (Wa-5) through Gardner- dard ages of Renne et al. (1998). For a more cated by either Cande and Kent (1992, 1995) buttean (Br-0–Br-1a) faunas (Kihm, 1984; Froe- thorough discussion of the biochronology of or Ogg and Smith (2004). Another interesting hlich and Froehlich, 2002). Strata equivalent to Laramide basins, see Table DR1 and excellent feature is the presence of several brief polarity the lower part of the Green River Formation in reviews by Krishtalka et al. (1987), Lillegraven intervals that do not appear in seafl oor magnetic the Greater Green River Basin have produced a (1993), and Robinson et al. (2004). anomaly records (Cande and Kent, 1992). If not similar faunal succession (Fig. 3; Holroyd and the result of overprinting, such features may Smith, 2000; Zonneveld et al., 2000). In strata RATES OF DEPOSITION refl ect short-term weakenings or reversals of the where fossil collections are absent, such as the Earth’s magnetic fi eld similar to those observed Battle Spring Formation and Colton Formation Radioisotopic ages permit a direct numeri- in the Pliocene–Pleistocene record (Langereis et (Spieker, 1946; Love, 1970), temporal assign- cal calibration of average sediment accumula- al., 1997; Singer et al., 2004). ments are typically based on bulk lithologic tion rates for Eocene strata in several Laramide correlations (i.e., the deposits overlie Paleo- basins. Due to the presence of exposure hori- Extrapolation of Age Model to Strata Not cene deposits and are therefore probably early zons throughout Green River Formation and Directly Constrained by Tuff Beds Eocene in age). its distinctly heterolithic character at the meter Radioisotopic ages, mammalian fossils, and scale, a high potential exists for differential sedi- Ash beds have not been identifi ed in the magnetostratigraphy also provide varying lev- mentation rates for different facies. Therefore, lowest portions of the Green River Formation els of temporal resolution for Eocene strata in rates cited here represent long-term averages below the Sheggs and Yellow tuffs (Fig. 2), and the basins and volcanic fi elds that surround the rather than instantaneous sedimentation rates. therefore mammalian biostratigraphy remains Green River Formation basins (Figs. 4 and 5). Additional uncertainties arise due to the effects the most useful age constraint. For example, Table DR5 (see footnote 1) provides a compre- of differential compaction and the uncertainties the alluvial DeBeque Formation, which is hensive list of currently available 40Ar/39Ar age inherent to the 40Ar/ 39Ar method. Geochronology

70 Geological Society of America Bulletin, January/February 2008 Synoptic reconstruction of the Eocene Green River Formation Biostratigraphy references references Biostratigraphy Krishtalka and Stucky (1986); Doi (1990); Honey (1990); Gunnell Gunnell (1990); Honey (1990); Doi (1986); Stucky and Krishtalka McCarroll et al. (1996a, 1996b); Stucky et al. (1996) (1996) et al. Stucky 1996b); (1996a, al. et McCarroll Evanoff et al. (1998); Murphey (2001) and Bartels (1999); Froehlich and Froehlich (2002) (2002) Froehlich and Froehlich (1999); Bartels and Savage et al. (1972); West (1973); Stucky (1984); Honey (1988); (1988); Honey (1984); Stucky West (1973); (1972); al. et Savage et al. Zonneveld and Smith (2000); Holroyd et al. (2000); Anemone (2000, 2003); Smithand Holroyd (2003); Gunnell et al. (2004) West (1970, 1973); West and Dawson (1973); Stucky (1984); Honey Honey (1984); Stucky (1973); Dawson Westand 1973); (1970, West (1988); Gunnell and Bartels(1994); Gunnell and Yarborough (2000); (2001) al. et Clyde 2003); (2000, al. et Zonneveld (2001) Murphey (1999) al. (1992) al. (1992) (2001) Matthew (1909); Gazin (1976); West (1976); Covert et al. (1998); (1998); al. et Covert West (1976); (1976); Gazin (1909); Matthew McGrew (1959);West and Atkins(1970) Kay (1957); Gazin (1959); MacGinitie (1969); Izett et al. (1985); (1985); al. et Izett (1969); MacGinitie (1959); Gazin (1957); Kay (1973) Nelson (1970); Tracey and Oriel West and Huchison (1981); Evanoff et al. (1994); Murphey (2001) (2001) Murphey (1994); al. et Evanoff (1981); Huchison and West Matthew (1909); Gazin (1976); West (1976); Evanoff et al. (1998); (1998); al. et Evanoff West (1976); (1976); Gazin (1909); Matthew Sinclair and Granger (1911); Van Houten (1964); Emry (1975) Emry (1964); Houten Van (1911); Granger and Sinclair et al. Rasmussen (1996); Prothero 1957); (1934, Kay (1895); Osborn Rohrer and Gazin (1965); Wing et al. (1991); Gingerich and Clyde Clyde and Gingerich (1991); al. et Wing (1965); Gazin and Rohrer Ambrose et al. (1997); Froehlich and Breithaupt (1998) (1998) Breithaupt and Froehlich et al. (1997); Ambrose 1965); (1962, Gazin (1960); and Roehler McGrew (1954); Morris Clyde et al. (2001); Gunnell and Bartels(2001) McGrew and Roehler (1960); Gazin (1962,1965); Simnacher (1970); Sinclair and Granger (1911); Love (1970) (1970) Love (1911); Granger and Sinclair Kihm (1984); Krishtalka and Stucky (1986) (1986) Stucky and Krishtalka Kihm (1984); Eaton (1980, 1982, 1985); Bown (1982); Sundell et al. (1984) (1984) al. et Sundell (1982); Bown 1985); 1982, (1980, Eaton Walton (1992); Prothero and Emry (1996) (1996) Emry and Prothero (1992); Walton Ui-1 Ui-1 Ui-3 Prothero and (1996); Emry Walsh et al. (1996) (zone) (zone) strata strata strata strata strata transition: transition: Br-1a strata strata Br-1a overlies Br-2 overlies et Gunnell (1985); Torres (1983); Gingerich and Torres (1982); Bown Biostratigraphy Biostratigraphy Wa-6 to Wa-7 to Wa-6 underlies Br-3 Br-3 underlies (1978); Turnbull (1975); Dawson and West (1973b); Roehler underlies Wa-7 underlies to Wa-7, Br-0, and laterally equivalent equivalent laterally ON NORTH AMERICAN LAND-MAMMAL AGES (2004) (2004) Smith et al. this study this study Br-2 this study, this study, et al. (2003) et al. (2003) Emry (1996) (1996) Emry Emry (1996) (1996) Emry references references (2003, 2006) Feeley and Feeley Cosca (2003) (2003) Cosca Radioisotopic Radioisotopic † ) σ (± 2 50.83 ± 0.19 50.83 ± 0.16 47.17 this study Ui-1 48.62 ± 0.31 48.62 48.74 ± 0.16 ± 0.16 48.74 49.62 ± 0.17 ± 0.17 49.62 46.55 ± 0.15 ± 0.15 46.55 and Prothero ± 0.15 48.37 48.76 ± 0.19 48.76 oldest oldest youngest youngest youngest youngest oldest oldest Ash bed ± 0.49 43.07 and Prothero Curly tuff Curly 0.33 ± 49.02 study this Br-2 overlies Sage ash ash Sage ± 0.22 47.94 this study Br-3 Dated unit Age Tuff below Tuff below TABLE 3. RADIOISOTOPIC CONSTRAINTS Yellow tuff Yellow ± 0.54 51.24 this study Wa-7 overlies pumice bed bed pumice Groundmass Groundmass and Sixth tuffs tuffs Sixth and from lava flows flows from lava Henrys Fork tuff tuff Fork Henrys 0.16 ± 48.15 this study Br-3 Church Buttes & Buttes Church White Lignitic tuff tuff Lignitic White 0.18 ± 47.70 study this Ui-2 underlies Leavitt Creek tuffs tuffs Creek Leavitt Alamo Creek basalt basalt Alamo Creek Grey, Main, Layered, Layered, Main, Grey, Sage Creek Mountain Mountain Sage Creek Member Bridger Formation Bridger Formation Member Bluffs Turtle tuff Butte Tabernacle E) (Bridger ± 0.16 48.11 Basal Upper Member this study Br-3 Member–Fowkes Member–Fowkes Formation Wilkins Peak Member Firehole, Boar, Laney Member Laney Bridger Formation tuff Analcite 0.18 ± 48.94 Smith study, this Parachute and (UB) (PCB) Creek Member Devils Graveyard Graveyard Devils Formation Bridger Formation Formation Bridger tuff Continental 0.31 ± 48.66 study this Br-1b overlies Bridger Formation (top of Bridger C) Formation–Unit 3 Mission Valley Valley Mission Creek Member Member Creek Tipton Member Member Tipton tuff Scheggs 0.17 ± 51.90 this study and overlies (Bridger B) B) (Bridger Trout Peak Trachyandesite Formation Formation Blue Point Marker Blue Marker Point ash Blue Point ± 0.17 48.10 Hiza (1999) underlies Br-3, Analytical and intercalibrationuncertainties relative to 28.34 Ma for TCs (Renneet al., 1998). † Basin Formation or or Basin Formation Wind River River Wind Bed Wagon River Greater Green Green Greater Greater Green River Greater Green River Uinta–Piceance Fowkes Bulldog Hollow Hollow Bulldog Fowkes River Creek Fossil Fossil Member Butte Fossil tuff K-spar Green Greater 0.17 ± 51.66 study this Wa-7 Trans-Pecos, Trans-Pecos, Texas Bighorn Bighorn Formation Willwood ash Willwood 0.19 ± 52.59 al. Smith et River River Greater Green Green Greater River Uinta Member Upper tuff Blind Canyon 0.22 ± 47.04 this study Ui-2 underlies Absaroka Volcanic Volcanic Absaroka Wind River Wind River Formation Wind River tuff Draw Halfway 0.17 ± 51.74 study this Wa-7 Piceance Creek Creek Piceance Parachute Basal Green Greater Greater Green Green Greater River Absaroka Volcanic Province Province Province California California San Diego,

Geological Society of America Bulletin, January/February 2008 71 Smith et al.

TABLE 4. RADIOISOTOPIC CONSTRAINTS ON THE EARLY AND MIDDLE EOCENE GEOMAGNETIC POLARITY TIME SCALE Dated unit Formation or Member Age ± 2σ† Radioisotopic references Magneto- Magnetostratigraphy references (Ma) stratigraphy Willwood ash Willwood Formation 52.59 ± 0.19 Wing et al. (1991); C24n (base of Clyde et al. (1994); Smith et al. (2004) lowest subchron) Tauxe et al. (1994) Layered tuff Wilkins Peak Member 49.79 ± 0.17 Smith et al. (2006) C23n-C22r Clyde et al. (1997, 2001) transition Sixth tuff Wilkins Peak Member 49.62 ± 0.17 this study; Machlus et al. C23n-C22r Clyde et al. (1997, 2001) (2004); Smith et al. (2006) transition Continental tuff Bridger Formation 48.66 ± 0.31 this study C22n Clyde et al., (2001) Blue Point overlies Aycross Formation 48.10 ± 0.22 Hiza (1999) base of C21r Eaton (1982); Lee and Shive (1983); Marker ash and Trout Peak Trachyandesite Sundell et al. (1984); Flynn (1986) Montenari ash ? 45.94 ± 0.14 Berggren et al. (1995) C21n Berggren et al. (1995) Mission Valley Mission Valley Formation 43.07 ± 0.49 Prothero and Emry (1996) C20n Walsh et al. (1996) ash †Analytical and intercalibration uncertainties relative to 28.34 Ma for TCs (Renne et al., 1998).

derived accumulation rate uncertainty for strata doubling of stratal thickness and accumulation TABLE 5. AGES FOR PALEOMAGNETIC between tuff beds (expressed at the 2σ level) rate is likely. CHRONS 24N THROUGH 19R ranges from 9% to >40% and is highest when Chron Age (Ma) Age (Ma) Age (Ma) Absaroka Volcanic Province (base) Cande and Ogg and This study short time intervals are considered. Kent (1995) Smith (2004) C24n.3n 53.35 53.81 53.44 Greater Green River Basin Average accumulation rates in the Absaroka C24n.2r 52.90 53.29 52.89 Volcanic Province were in several cases an order C24n.2n 52.80 53.17 52.75 C24n.1r 52.76 53.12 52.71 During deposition of the evaporative Wilkins of magnitude faster than temporally equivalent C24n.1n 52.66 53.00 52.59 Peak Member, net average sediment accumula- portions of the Green River Formation (Fig. 5). C23r 52.36 52.65 52.23 tion rates in the Greater Green River Basin were At Trout Peak, Wyoming, for example, paleo- C23n.2n 51.74 51.90 50.50 approximately three times faster at the basin magnetic polarity stratigraphy (Shive and Pruss, C23n.1r 51.05 51.06 50.04 40 39 C23n.1n 50.95 50.93 49.88 center (200 mm/k.y.) versus the basin margin 1977) and several Ar/ Ar ages ranging from C22r 50.78 50.73 49.71 (60 mm/k.y.). In contrast, higher rates occurred 48.8 to 48.4 Ma (Feeley and Cosca, 2003) indi- C22n 49.71 49.43 49.07 at the basin margin (180 mm/k.y.) versus the cate that >1 km of volcaniclastic sediment and C21r 49.04 48.60 48.30 basin center (110 mm/k.y.) during deposition lava fl ows were deposited during C22, which is C21n 47.91 47.24 46.52 C20r 46.26 45.35 45.71 of the fl uctuating profundal Rife bed of the Tip- <1 m.y. in duration (Table 5). The resulting aver- C20n 43.79 42.77 43.16 ton Member (Fig. 11). The most rapid sediment age accumulation rate of >1000 mm/k.y. for this C19r 42.54 41.59 42.12 Note: The intercalibration uncertainties for the accumulation (>1000 mm/k.y.) occurred when section is an order of magnitude faster than the 40 39 volcaniclastic materials entered the basin from average sedimentation rate for the temporally Ar/ Ar ages used as calibration points for the geomagnetic polarity timescale range from ±0.2 the north and propagated out over fi ne-grained equivalent Laney Member in the Washakie sub- to ±0.5 m.y. Due to additional uncertainty Laney Member lake strata, which had accu- basin of the Greater Green River Basin. Notably, resulting from interpolation between these mulated an order of magnitude more slowly the bulk of materials preserved in the Absaroka calibration points, these uncertainties represent the minimum age uncertainty for magnetic chron (120 mm/k.y., Fig. 11). Volcanic Province are volcaniclastic rather than boundaries ages that are interpolated between primary volcanic deposits, and volcanic centers them. Comparisons of these ages to other Uinta Basin are typically identifi ed based on the presence chronometers (such as U-Pb) would require the of intrusive rocks (Fig. 5; Smedes and Prostka, use of fully propagated uncertainties (Renne et al., 1998), which range from ±0.8 to ±1.0 m.y. Sediment accumulation at Indian Canyon, 1972; Sundell, 1993). Consequently, the accu- Utah, which occupies a basin-margin position mulation rates cited above refl ect the accumula- within the Uinta Basin (Dane, 1955; Dyni et al., tion of strata in the areas between volcanic cen- 1985), was relatively constant at ~150 mm/k.y. ters, whereas individual stratocones may have and freshwater Lake Kinneret, which overfl ows over a 7 m.y. interval during which both evapo- accumulated at signifi cantly faster rates. into the hypersaline Dead Sea (Eugster and Har- rative and fl uctuating profundal facies were die, 1978). A few studies have documented sim- deposited. The lowest average accumulation DISCUSSION ilar upstream and downstream hydrologic rela- rate for the section (100 mm/k.y.) occurred tionships in Pleistocene lake systems (Benson et during the deposition of the evaporative saline Synoptic Lake Type Evolution al., 1990), but interbasin relationships for pre- facies, which is consistent with low accumula- Quaternary lake systems are diffi cult to evaluate tion rates observed at the basin margin of the An examination of modern lakes reveals that due to the limitations of nonmarine geochronol- Greater Green River Basin during Wilkins Peak regional drainage relationships can lead to sig- ogy (Surdam and Stanley, 1980). Member deposition. A lack of subsurface strati- nifi cant differences in the hydrologic budgets The age model presented here allows for the graphic information disallows a precise calcu- for adjacent lakes occupying similar climates. highest resolution reconstruction yet available lation of sediment accumulation rates in the Examples include freshwater Utah Lake, which for the Green River Formation lake system and depocenter for the saline facies interval, but a overfl ows into the hypersaline Great Salt Lake, permits for the fi rst time direct upstream and

72 Geological Society of America Bulletin, January/February 2008 Synoptic reconstruction of the Eocene Green River Formation

downstream comparisons of strata in separate Greater Green River Basin basins at the member scale and even bed scale

)

m (Figs. 2B). Lake type observations (Table 1) ( Laney

s 400 Wilkins

have been combined with paleocurrent indicators e Peak

k Tipton

and provenance data (cf. Table DR6) in order to c

i Basin margin

reconstruct the paleohydrologic conﬁ guration of T north central

0 Bridger Basin 400 n

the Laramide landscape for eight discrete time o

a slices (Fig. 12). The resulting maps are intended l 300 u

to be synoptic views of the Green River Forma- u

i 200 tion lakes and the hydrologic links between them a

g and were selected to illustrate the principle lake k

no strata s

i 100 r

Rate (mm/k.y.)

b type conﬁ gurations. As with any such recon- t

A struction, uncertainties incurred from stratal cor- 0 relation and geochronology inevitably introduce some time averaging of facies and paleoenviron- 1200 Bridger

ments. However, time averaging in this case is p

limited to ~±100 k.y., which represents an order i

of magnitude improvement in resolution relative ) 800

(

to previous reconstructions (Surdam and Stan- s Laney

ley, 1980; Grande, 1984; Dickinson et al., 1988; n

i Wilkins Lillegraven and Ostresh, 1988). h 400

T Peak 53.0 Ma Tipton At the onset of lacustrine deposition, two Basin center major extrabasin catchments drained into the 0 southern 1100 mm/k.y. Bridger Basin 400 n

Green River Formation basins and fed several o

a small freshwater lakes (Cow Ridge and Luman l 300 u

Members; Fig. 12A). One stream entered into u

c the northwest Greater Green River Basin and 200 c

the other ﬂ owed into the southern Uinta Basin. g

Rate (mm/k.y.) 100 r

Pinyon type quartzite clasts likely derived from e

A southwest Montana (Krause, 1985; Janecke et 0 al., 2000) in the Pass Peak Formation (Dorr et 54 52 50 48 46 44 al., 1977) attest to a signiﬁ cant catchment area Age (Ma) 2 (~100,000 km ) for the northern stream (Fig. 1). Uinta Basin Arkosic sandstone derived from uplifts in south- 800 ern Colorado in the Colton Formation and out- ward-directed paleocurrent directions in the )

m sandstone and limestone facies

( Basin margin

South and Echo Park, Denver, Raton, and San s

s Indian Canyon section

e 400 saline facies Juan Basins (Fig. 1; Table DR1) constrain the n

i potential catchment area of the stream feeding h upper member 2 T the southern Uinta Basin to ~50,000 km (Dick- Mahogany

zone n inson et al., 1986). Fluvial-lacustrine facies in 0 o

a all of the major basins imply the existence of l 200 u

an outlet stream somewhere, but its location u

c 100 c

remains unknown. Alluvial fan deposits limit A

possible outlet locations to the northeast margin g

0 Rate (mm/k.y.) of the Greater Green River Basin and southwest e

v margin of the Uinta Basin (Fig. 12A; Sklenar 54 52 50 48 46 44 A and Anderson, 1985). Any stream exiting to the Age (Ma) northeast would likely have joined with north- Lake Type directed streams that ﬂ owed from the Bighorn, Underfilled Balanced fill Overfilled and/or Alluvial Wind River, and Shirley Basins into the Pow- der River Basin (Fig. 1; Seeland, 1978, 1992, 1998). Any south-directed outlet streams would Figure 11. Cumulative thicknesses and average accumulation rates for have likely drained into the Claron Formation Eocene strata between 40Ar/39Ar dated units at basin-margin and basin-cen- basins of southwest Utah (Fig. 1), which con- ter sites in the Greater Green River Basin and at a basin-margin site in the tain poorly dated alluvial and lacustrine Eocene Uinta Basin. Thicknesses for Bridger Basin are from Roehler (1992b) and strata (Fig. 4; Goldstrand, 1994). Volcanism at Evanoff et al. (1998); thicknesses for Indian Canyon section were provided the time appears to have been conﬁ ned to the by J.R. Dyni (2005, personal commun.).

Geological Society of America Bulletin, January/February 2008 73 Smith et al.

AC21 B 20

1 2 3

4 ?

8 7 5 ? 15 16 6 ? 11 ? ? 9

12 10 13 14 ? 53.0 Ma 51.5 Ma 50.5 Ma DFE

18 19

49.5 Ma 48.7 Ma 48.0 Ma G H Eocene Facies strata absent Lacustrine Evaporative Bedded evaporites Fluctuating Profundal strata absent strata absent Fluvial-Lacustrine Alluvial Non-volcanic 100 km Volcaniclastic

Pre-Cretaceous

? coal Paleodrainage molluscs intrabasinal stromatolites strata absent extrabasinal fish (dashed arrows indicate where inferred from lake type observations) 46.5 Ma 45.0 Ma Figure 12. Annotated synoptic maps showing paleohydrologic conﬁ guration of the Green River Formation lakes at eight discrete times between 53 and 45 Ma. Locations of bedded evaporites are from Bradley and Eugster (1969), Dyni (1974), and Dyni et al. (1985). Time slices were selected to highlight major hydrologic conﬁ gurations of the Green River Formation lake system. Paleocurrent and provenance information are summarized from a large number of sources, referenced in Table DR6 (see footnote 1) to the numbers shown. Note that knowledge of the continuity of lacustrine deposition in the central Greater Green River Basin is limited by the absence of Eocene strata atop the Rock Springs uplift. See Figure 1 for detailed geographic reference.

74 Geological Society of America Bulletin, January/February 2008 Synoptic reconstruction of the Eocene Green River Formation

Lowland Creek Field and several centers in the Gros Ventre, Wind River, and Granite Mountains downstream basins during such expansions may northwest Absaroka Volcanic Province (Fig. 5; uplifts (Love, 1970; Lageson, 1987; Steidtmann explain, for example, the dolomitic stromato- Chadwick, 1969; Ispolatov, 1997; Hiza, 1999; and Middleton, 1991) likely diverted the north- lite horizons in the otherwise alluvial Cathedral Feeley et al., 2002). ern catchment away from the Greater Green Bluffs Member of the Wasatch Formation in the River Basin, causing it to become underfi lled Washakie subbasin (Roehler, 1992b), and short- 51.5 Ma (Pietras et al., 2003a). Pinyon type quartzite term salinity increases interpreted by Keighley Two broad, moderately saline lakes occupied gravels characteristic of the northern catchment et al. (2003) in the otherwise overfi lled transi- the Greater Green River and Piceance Creek were diverted into the Bighorn and Wind River tional interval in the Uinta Basin. Basins at this time and continued to be fed by the Basins, where deposition of fl uvial-lacustrine to Regionally, all of the perimeter and axial basins two aforementioned paleodrainages (Fig. 12B). alluvial facies prevailed (Krause, 1985; Seeland, east of long ~107°W stopped accumu lating sedi- Deposition in the Uinta Basin continued to be 1998). Coarse alluvial fan deposits, crosscutting ments at this time and lack further middle and late fl uvial lacustrine, whereas fl uctuating profun- fault relations, and pronounced differential sub- Eocene accumulations (Fig. 4). This widespread dal facies were deposited in the Greater Green sidence all imply concurrent uplift of the Uinta regional unconformity may refl ect the formation River Basin and Piceance Creek Basin (Tipton Mountains and Axial Basin arch (Fig. 1; Trudell of a regional drainage divide (Love, 1960), which and Garden Gulch Members), implying a lack et al., 1970; Ryder et al., 1976; Roehler, 1993), encouraged the continuation of internal drainage of consistent outfl ow (Fig. 12B). A freshwa- which became hydrologic barriers between Lake in the ponded basins to the west. Volcanoes con- ter lake also occupied the Bighorn Basin at Gosiute and Lake Uinta. As a consequence, tinued to erupt in the Lowland Creek and north- this time (Fig. 4, Tatman Formation; Rohrer smaller interbasin sills such as the Rock Springs western Absaroka Volcanic Province (Ispolatov, and Smith, 1969), but it is uncertain whether it uplift and Douglas Creek arch became impor- 1997; Hiza, 1999), and several volcanic centers drained to the north or south. Regional expan- tant controls on local deposition. Alluvial and in the north-central Absaroka Mountains (Cran- sion of lake facies was postulated by Carroll et fl uvial-lacustrine facies in the Washakie, Sand dall, Independence, Sunlight) erupted for the al. (2006) to refl ect a decrease in sediment deliv- Wash, Great Divide, and Uinta Basins refl ect fi rst time (Fig. 5; Harlan et al., 1996; Hiza, 1999; ery from basin-bounding uplifts subsequent to overfl ow of water and dissolved solutes into the Feeley and Cosca, 2003). the removal of easily eroded Cretaceous cover evaporative sinks that occupied Bridger subba- from their crests. Volcanism continued at this sin and Piceance Creek Basin (Fig. 12C). The 49.5 Ma time to be confi ned to the northwest Absaroka amount of evaporation responsible for solute The Green River Formation lakes were and Lowland Creek fi elds (Fig. 5), either of concentrations high enough for evaporite depo- increasingly infl uenced by northern volcanism which may have contributed distal ash beds to sition in these downstream basins was therefore at this time. In the northern Absarokas, the Sun- the Green River Formation lake basins. not only the consequence of evaporation at the light, Crandall, and Independence volcanoes depositional site, but also the integrated evapo- grew into large edifi ces, represented by the 50.5 Ma ration from all upstream subbasins. When lake proximal Wapiti Formation and Trout Peak Tra- The Bridger subbasin of the Greater Green level in downstream subbasins rose above the chyandesite and distal Aycross Formation to the River Basin and the Piceance Creek Basin had interbasin sills, solutes from neighboring sub- south (Fig. 5; Nelson and Pierce, 1968; Feeley both become terminal sinks in which evaporites basins could no longer be fl ushed downstream, and Cosca, 2003). In addition, the Slough Creek were deposited by this time (Wilkins Peak and causing solute concentrations in upstream tuff, the fi rst of several large ash-fl ow deposits, Parachute Creek Members; Fig. 12C). Concur- basins to rise (Fig. 13). Integrations of the col- was erupted from the northern Absaroka Vol- rent thrusting along the southern margins of the lective solute loads for all of the upstream and canic Province (Fig. 5; Hickenlooper and Gut- mann, 1982; Hiza, 1999). At roughly the same time, volcanism commenced in the Challis vol- A canic fi eld, with several major caldera-forming b1 b2 b1 b 2 eruptions of thick ash-fl ow tuffs (Moye et al., 1988). Several southeast-directed paleovalleys t3 t3 and extensional basins within the fold and thrust belt to the east of the Idaho Batholith in south-

e west Montana and eastern Idaho were actively

m t2 i T being ﬁ lled by extrusive Challis volcanic rocks

t2 (Janecke and Snee, 1993; M’Gonigle and Dal- rymple, 1996). Nonvolcanic strata of the Sheep

t1 Pass Formation fi ll similar basins to the south t 1 in Nevada (Figs. 1 and 4; Winfrey, 1960; Sol- Fresh Saline Hypersaline Less More omon et al., 1979). The Bighorn, Wind River, Fresh Saline Hypersaline Salinity Surface Water Gros Ventre, and Greater Green River Basins were all receiving a signifi cant infl ux of volca- Figure 13. Conceptual model illustrating the effects of an increase in available surface nic detritus from the Absaroka and potentially water on the salinity and distribution of lake waters in upstream and downstream basins Challis volcanic fi elds at this time (Fig. 5; Mac-

(modiﬁ ed from Kelts, 1988). During t1, available surface water is low, and basins b1 and Ginitie, 1974; Stucky, 1982; Torres, 1985). Vol-

b2 are occupied by freshwater and hypersaline lakes, respectively. An increase in avail- caniclastic sediment initially entered into the

able surface water (t2) causes the lake in basin b2 to rise and amalgamate with the lake in northwest corner of the Greater Green River

basin b2, resulting in one broad saline lake. Further increases in available surface water Basin and rapidly propagated southward to

during t3 result in a rise and freshening of the combined lake in b1 and b2. the central Bridger subbasin (Fig. 12D; West,

Geological Society of America Bulletin, January/February 2008 75 Smith et al.

1973; Surdam and Stanley, 1980). High rates of that the Greater Green River Basin likely over- Hiza, 1999), and became active in the Rattle- sediment accumulation (Fig. 11), the 48.70 Ma fl owed into the Piceance Creek Basin (Roehler, snake Hills area (Fig. 5; Van Houten, 1964), mean age for detrital feldspar in the Sand Butte 1992b). Lake Uinta expanded greatly in extent volcaniclastic input to the Greater Green River bed (Fig. 8), and andesite cobbles in the lower (the Mahogany zone of the upper Parachute and Piceance Creek Basins appears to have Bridger Formation (Kistner, 1973) point to a Creek Member and upper member), becoming ceased ca. 47.1 Ma (Hail, 1992; Stucky et al., major input of freshly erupted volcaniclastics. one vast saline lake that covered both the Uinta 1996; Murphey, 2001) and was replaced by Increased infl ux of water and volcaniclastic and Piceance Creek Basins and overlapped the deposition of locally derived arkosic alluvium. sediment caused Lake Gosiute to expand in Douglas Creek arch (Fig. 12E). The organic- This change indicates that volcanic areas to the areal extent and overfl ow occasionally into the rich Mahogany zone marks a shift from evapo- north were no longer part of the catchment of Piceance Creek Basin (cf. Surdam and Stanley, rative to fl uctuating profundal facies in the the Green River Formation basins. We specu- 1980), which is indicated by the change from Piceance Creek Basin, whereas in the Uinta late that doming caused by a thermal anomaly evaporative to fl uctuating profundal facies Basin it records a shift from fl uctuating pro- associated with the southern Absaroka Volcanic (lower LaClede bed of the Laney Member) in fundal to evaporative facies (Fig. 2B). Province and Rattlesnake Hills Volcanics (e.g., the former. This new input may have caused the Pierce and Morgan, 1992) may have uplifted Piceance Creek Basin lake to expand and merge 48.0 Ma the northern Greater Green River Basin and with the lake in the Uinta Basin, thereby causing By this time, volcaniclastic input from the promoted the rerouting of Absarokan detritus balanced-fi ll conditions (transitional interval) to north had fi lled much of the accommodation in and water away from the area. be propagated upstream (i.e., Fig. 13). However, the Greater Green River Basin with volcanicla- the continued deposition of evaporites in the stic alluvial plain strata (Bridger and Washakie 45.0 Ma Piceance Creek Basin at the time (Fig. 2) attests Formations) and displaced lacustrine deposition The last stages of Green River Formation to the existence of a west to east hydrologic downstream toward the Uinta Basin (Fig. 12F; deposition are the most sparsely documented. gradient across the Douglas Creek arch, at least Stucky et al., 1996; Evanoff et al., 1998; Buch- Surviving strata indicate that a lake occupy- during intervals of evaporite deposition. heim et al., 2000). Sediment not deposited in ing the Uinta Basin had become freshwater the Greater Green River Basin spilled into the by this time (sandstone and limestone facies; 48.7 Ma Piceance Creek Basin, building a thick deltaic Fig. 12H). South-directed paleocurrents in Major volcanism in the Absaroka and Chal- package on its northern margin (Uinta Forma- the overlying Uinta Formation (Anderson and lis fi elds continued with the growth of volca- tion; Hail, 1992). Volcaniclastic material con- Picard, 1974) indicate that overfl ow was prob- nic centers and several eruptions of ash-fl ow tinued to fi ll extensional basins to the west of ably directed into basins toward the southwest. tuff (Moye et al., 1988; Hiza, 1999) and had an the Greater Green River Basin and in southwest These basins contain the alluvial and lacustrine increasing infl uence on deposition in the Green Montana. Evaporite deposition occurred for the Claron Formation, which also exhibits south- River Formation basins (Fig. 12E). The locus fi rst time in the Uinta Basin (upper member), directed paleocurrent indicators (Goldstrand, of volcanism in the Absarokas had migrated marking a reversal of the hydrologic gradi- 1994). The volcaniclastic Golden’s Ranch southward, resulting in shorter transport dis- ent across the Douglas Creek arch (Fig. 12F). Formation on the east side of the Plateau Prov- tances for detritus delivered to the basins to the Volcanoes in the Absaroka Volcanic Province ince (Muessig, 1951) attests to a proximal but south (Fig. 5). Southwest-directed paleocurrent continued to propagate southward, abandoning unidentifi ed volcanic fi eld that may have been indicators adjacent to the Wind River uplift in northern centers and fi lling nearby basins with an important source for the thick, coarse tuff alluvial deposits in the northwest Greater Green proximal volcaniclastic sediment (Fig. 5). At beds in saline facies and sandstone and lime- River Basin (upper Bridger Formation) indicate the same time, Challis volcanism was nearing stone facies of the western Uinta Basin. that volcaniclastic detritus was transported its peak of activity and buried signifi cant local directly across its crest (Groll and Steidtmann, paleo topography in east-central Idaho and south- SEDIMENT ACCUMULATION RATES 1987). Deposition of volcaniclastic sediment west Montana. had propagated from north to south across the All of the main lacustrine depocenters of the Greater Green River Basin, replacing lacus- 46.5 Ma Green River Formation occupy sites of high dif- trine facies (Laney Member) with alluvial A lack of preserved strata from this time in ferential subsidence (Figs. 1 and 2; Dyni, 1969; facies (Bridger Formation and Washakie For- much of the study area limits paleogeographic Roehler, 1993). In detail, however, average sed- mation) and leaving Lake Gosiute restricted certainty, but a few remnant deposits yield imentation rate patterns differ signifi cantly for to its southeast corner (Roehler, 1993). To important clues (Fig. 12G). An evaporative lake closed versus open intervals, with rapid accu- the west of the Fossil Basin, the Fowkes and occupied the western Uinta Basin and extended mulation occurring in the basin center during Woodruff extensional grabens (Fig. 1) became into the plateau province (saline facies; She- underfi lled intervals and near the basin margin active and may have acted as conduits funnel- liga, 1980; Dyni et al., 1985), whereas allu- during overfi lled intervals (Fig. 11). This pattern ing volcaniclastic sediment (Fowkes Forma- vial sedimentation was predominant in the refl ects several fundamental differences between tion) from the north into the southwest Greater eastern Uinta Basin (Uinta Formation; Dane, open versus closed basins in terms of produc- Green River Basin (Figs. 2 and 12E; Oriel 1955; Prothero, 1996) and southwest (Turtle tion, delivery, and preservation of sediment. and Tracey, 1970; Nelson, 1973). Although Bluffs Member of the Bridger Formation; High accumulation rates in the basin center coarse sediment was deposited in these basins, Evanoff et al., 1998; Murphey, 2001) and during evaporite deposition are consistent with their lack of lacustrine strata implies that an southeast Greater Green River Basin (Adobe a closed basin in which all solutes are retained unknown fraction of that sediment was trans- Town Member of the Washakie Formation; and lake level fl uctuated signifi cantly below ported through them. Molluscan faunas and McCarroll et al., 1996a; Stucky et al., 1996). sill level. Although solute delivery rates may other fl uvial-lacustrine indicators in the upper Although volcanism continued in the Absaroka be relatively low due to low levels of infl ow, LaClede bed of the Laney Member indicate and Challis fi elds (Snider and Moye, 1989; very rapid net accumulation rates have been

76 Geological Society of America Bulletin, January/February 2008 Synoptic reconstruction of the Eocene Green River Formation

obtained in modern and Quaternary arid closed stream into the Bighorn Basin, which previously downstream subbasins (Bradley and Eugster, basins (Ku et al., 1998; Bobst et al., 2001). fed into the northwest Greater Green River 1969). This phenomenon has been observed in Sedimentation in underfi lled basins tends to Basin (Krause, 1985; Seeland, 1998), closely the modern Great Salt Lake following construc- be focused into the basin center during peri- coincided with southwest-vergent thrusting tion of a causeway (Eugster and Hardie, 1978), ods of low lake level when solutes are the most along the south edge of the Gros Ventre, Wind and in Holocene deposits of Lake Bogoria in concentrated and evaporite beds are deposited. River, and Granite Mountains uplift trend (Love, Kenya (Renaut and Tiercelin, 1994). Expan- In both the Wilkins Peak Member and saline 1970; Lageson, 1987; Steidtmann and Middle- sion of underfi lled downstream lakes above facies of the Green River Formation, for exam- ton, 1991). This diversion rerouted rivers drain- their upstream sills can lead to more saline ple, both alluvial and evaporite beds thicken ing the east slopes of the Idaho Batholith and conditions in upstream basins (Fig. 13; Kelts, basinward (Culbertson, 1966; Smoot, 1983; portions of southwest Montana (Janecke et al., 1988), as occurred in the Washakie Basin dur- Dyni et al., 1985). 2000), shut off delivery of Pinyon type quartzite ing the initiation of Laney Member deposi- In contrast, solutes are periodically to con- conglomerates into the northwest Bridger sub- tion (Fig. 12D), and in the Uinta Basin during tinuously fl ushed out of overfi lled and balanced- basin (Dorr et al., 1977), and hydrologically deposition of the Mahogany zone (Fig. 12E). fi lled basins and may never reach concentrations isolated the Greater Green River Basin, leading Counterintuitively, a shift toward more saline required to precipitate evaporites. Although the to an underfi lled Lake Gosiute (Pietras et al., conditions in an upstream basin may refl ect absolute rate of solute delivery is likely greater 2003a). The subsequent reintroduction of this an increase rather than decrease in climatic during periods of lake expansion due to a corre- or a similar northern drainage ~1.5 m.y. later, humidity (cf. Kelts, 1988). sponding increase in the hydrologic infl ow, these ca. 49.5 Ma, was an important factor in shifting Upstream subbasins (e.g., the Sand Wash solutes bypass the basin and are carried down- the Greater Green River Basin from an under- subbasin during Wilkins Peak time) that have stream. As a result, the most rapid sedimentation fi lled to balanced-fi ll state via overfl ow into the high rates of clastic infl ux may undergo alluvial rates occur at basin margins due to their prox- Piceance Creek Basin (Fig. 12; Surdam and rather then lacustrine deposition, even if subsi- imity to sediment sources, whereas basin-center Stanley, 1980). Although evidence is incomplete dence is rapid. In a long-term sense, such basins areas are sediment starved and characterized by due to poor preservation of younger strata, this served as upstream sediment traps that pre- deposition of condensed intervals. Examples of northern drainage and its volcaniclastic sedi- vented clastic material from inundating down- basinward thinning of strata include the Farson ments appear again to have been diverted away stream basins but allowed dissolved solutes to and Sand Butte beds of the Tipton and Laney from the Green River Formation lake system pass through. Thus, while absolute subsidence Members in the Greater Green River Basin ca. 47 Ma (Fig. 12G), and possibly contributed in particular subbasins may play an important (Fig. 2; Stanley and Surdam, 1978; Roehler, to a concurrent pulse of evaporite deposition role in determining the location of evaporative 1992a) and the Garden Gulch Member in the in the downstream Uinta Basin (saline facies). sinks, the paleogeography of sediment distribu- Piceance Creek Member (Johnson, 1985). Evidence includes the upsection replacement of tion is an equally important factor. Yearly accumulation rates in most balanced- volcaniclastic detritus by locally derived arkosic The Douglas Creek arch, which formed fi ll facies (~100 mm/k.y.) are comparable to alluvium in the Bridger, Washakie, and Uinta the main sill between the Uinta and Piceance the average lamination thicknesses observed in Formations (Fig. 2) and geochronologic support Creek Basins, preserves the most complete these units (Bradley, 1929; Crowley et al., 1986; for ongoing volcanism in the Absaroka Volcanic record available of an interbasin sill (Fig. 2; Ripepe et al., 1991) and therefore permissive Province (Fig. 5). Bradley, 1931; Cole, 1985). For ~4 m.y. (ca. of an annual origin for laminae. However, this 53–49 Ma), the dominant hydrologic polarity coincidence does not necessarily imply that all Downstream Sills over the Douglas Creek arch was from west laminae are annual. Laminae between two cor- to east into the downstream Piceance Creek related ash beds in the Fossil Basin have been For any given lake basin, the elevation of its Basin, but was reversed following deposition observed to increase in number by 134% from downstream sill is a fundamental control on effec- of the Mahogany zone (ca. 49.0–48.5 Ma). This the basin center to the basin margin (Church tive accommodation and can directly determine east to west polarity was subsequently retained and Buchheim, 2002), making the assumption whether a basin is open or closed (Carroll and until the demise of Green River Formation of an annual origin for laminae (Bradley, 1929; Bohacs, 1999). Because uplifts surrounded most deposition ca. 44 Ma. In detail, however, we Ripepe et al., 1991) strongly suspect. Buchheim margins of the ponded basins (Dickinson et al., suspect that the lake type status of the Eocene and Eugster (1998) proposed that infl ow of cal- 1988), the elevation of low-lying areas connect- deposits overlying the Douglas Creek arch is cium-rich stream waters into calcium-under- ing the major basins played a key role in deter- more complex than can be represented at the saturated but carbonate-rich lake waters may mining the hydrologic confi guration of the Green scale of Figure 2, and may have changed state explain higher nearshore carbonate sedimenta- River Formation lake system. Low points in the repeatedly during different phases of individual tion rates and lamination counts. Owl Creek, Granite Mountains, Wind River, and lacustrine expansion-contraction cycles in the Uinta uplifts functioned at varying times as sills Piceance Creek Basin (e.g., Fig. 13). Expanded ROLE OF INTERBASIN AND and barriers between lake basins and interbasin phases of the Piceance Creek Basin lake may INTRABASIN SILLS drainage networks (Fig. 12). During periods of have transformed the Douglas Creek arch from lower lake level, intrabasin arches such as the a sill to an underwater saddle (Keighley et al., Upstream Sills Douglas Creek arch and Rock Springs uplift 2003). Nevertheless, below the Mahogany played similar roles (Fig. 12), making the dif- zone, evaporites are restricted to the Piceance Initial closure of the Greater Green River ferentiation of intrabasin versus interbasin struc- Creek Basin (Fig. 2), indicating that it was Basin occurred at the inception of Wilkins Peak tures somewhat subjective. consistently downstream during periods of low Member deposition in response to diversion of Interbasin sills promote the partitioning lake level. More detailed stratigraphic work is major inlet stream due to uplift of an upstream of clastic sediments in overfi lled upstream needed in order to understand the detailed his- drainage barrier. The northward diversion of this basins from chemical sediments in underfi lled tory of the arch.

Geological Society of America Bulletin, January/February 2008 77 Smith et al.

REGIONAL MAGMATIC INFLUENCE It is also uncertain whether an increase in the specifi c basin is not necessarily a direct recorder average altitude of upstream volcanic areas of increased evaporation or decreased precipita- A marked increase in volcanic activity in the may have caused an increase in the annual pre- tion, but is actually a complex function of the northwest U.S. beginning ca. 50 Ma had a fun- cipitation in these areas (e.g., Wotling et al., precipitation-to-evaporation ratio and the geom- damental effect on the character of deposition 2000), and thereby caused downstream basins etry of surface fl ow over the entire catchment in Laramide basins (Fig. 12). Construction of to shift toward more overfi lled states. of a closed basin (Fig. 13; Eugster and Hardie, major volcanic edifi ces in the Absaroka, Chal- A potentially important factor that remains 1978). During the period ca. 51–45 Ma, regional lis, and other volcanic fi elds likely affected the relatively unexplored is the infl uence of short- deposition of evaporative facies in various Green Green River Formation lakes in several ways: term barriers to upstream catchments created by River Formation basins occurred nearly continu- (1) increased rainfall in upstream catchments extrusion of lava fl ows (Hamblin, 1994) and the ously (Figs. 2 and 12), at the limits of our tempo- due to volcanic topography, (2) regional doming effects of volcanic sector collapses on upstream ral resolution. This period corresponds broadly (Pierce and Morgan, 1992), (3) an increase in catchments in volcanic fi elds (Malone, 1995). to the end of the early Eocene climatic optima sediment supply, and (4) short-term interruption Studies of modern and ancient landslide dams (ca. 53–49 Ma) and onset of subsequent global of drainages by avalanches, volcanic collapse, in both volcanic and nonvolcanic catchments cooling, based on δ18O in marine benthic fora- and lava fl ows (Grant et al., 2003). have concentrated on the downstream effect of minifera (Fig. 2; Zachos et al., 2001). The addition of volcaniclastic detritus and their failure (e.g., fl ooding and debris fl ows; In contrast to the relatively dry conditions water derived from the Absaroka and per- Saula et al., 2002; Schneider et al., 2004), but implied by long-term evaporite deposition, haps the Challis volcanic fi elds overwhelmed virtually no information exists concerning post- Eocene fl ora preserved at Little Mountain in previously closed basins, causing them to landslide but pre-failure effects on downstream the southern Green River Basin have produced progressively fi ll with water, then sediment sediment and water budgets. Because lakes can precipitation estimates of ~80 cm/yr and higher. (Fig. 12; Surdam and Stanley, 1980). Volca- be highly sensitive to changes in water and sed- The Little Mountain fl ora is a composite of niclastic detritus had propagated downstream iment input, we suspect that ancient landslides several closely spaced collection sites that are to the eastern Uinta Basin prior to shutting in upstream drainage basins may be a highly ~10 km north of the Uinta Mountain front. Leaf off abruptly throughout the Green River For- under appreciated infl uence on the fi ne-scale fossils occur only within laminated lacustrine mation basins ca. 47.2 Ma. Subsequently, the character of their deposits. facies in a mixed alluvial and lacustrine inter- region reverted back to a similar confi gura- val that is equivalent to the upper Wilkins Peak tion of areally restricted evaporative lakes, as PALEOCLIMATIC SIGNIFICANCE and Laney Members (P. Wilf, 2006, personal existed previously (Fig. 12F). At a fi ner tempo- OF GREEN RIVER FORMATION commun.). Comparison of fl oral composition to ral scale, it remains unclear what the effect, if EVAPORITES living relatives (MacGinitie, 1969; Leopold and any, that individual eruptions had on sedimen- MacGinitie, 1972), leaf margin physiognomy tation, but it can be imagined that short-term Evaporite minerals contained within the Green (Wolfe et al., 1998), and leaf area physiognomy pulses of sediment delivery to the Green River River Formation have long been interpreted as (Wilf, 2000) of the Little Mountain fl ora and Formation lakes may have occurred following evidence for periods of relatively arid climate other Green River Formation fl oras all indicate explosive eruptions, as easily eroded fallout (Bradley, 1929; Bradley and Eugster, 1969; Roe- that Eocene climates in the region were humid ash was fl ushed from the landscape. The uni- hler, 1993). This interpretation is consistent with subtropical (Fig. 3; cf. Wilf, 2000). modal detrital sanidine age distribution in the the restricted occurrence of most large, modern The apparent confl ict between the occurrence Sage Creek pumice bed and Sage tuff (Fig. 6) hypersaline lakes in areas with mean annual of evaporite intervals and paleobotantical evi- suggests a lack of mixing of recently erupted precipitation of <40 cm/yr (Herdendorf, 1984; dence for moderately wet conditions cannot be material with sediment from older eruptions. Fig. 14). However, the presence of evaporites in a simply resolved by invoking long-term climate

120 Figure 14. Comparison of mean annual precipitation and evapora- Lake Rukwa Tanzania tion rates for modern lakes (Her- Mar Chiquita 1:1 dendorf, 1984) with mean annual Goose Lake Argentina precipitation estimates from leaf

r Oregon, California

/ 80 Average: Little Mountain fossils (see Fig. 2) from the interval m Expansion–fresher, laminites,

( of evaporite deposition in the Green leaf preservation at basin margin

o River Formation (Wilf, 2000). The

a Wilkins Peak cycle-inducing

t ﬂ ora from which this estimate is

p precipitation variance? i derived is actually a composite of

e Lake Tuz r two sites, one from the upper Wilkins

P) 40 Contraction–more saline, evaporites, Hypersaline lakes basin margin alluvial and paludal facies Peak near the Main tuff, and the Great Salt Lake Dead Sea Saline lakes other from the lower LaClede Bed Uvs Nuur of the Laney Member (P. Wilf, 2006, Bagrash Paleoprecipitation Lake Eyre Nur estimate (Wilf, 2000) personal commun.). Saline lakes are Lop Nur included to add additional context, 0 0 20406080100 based on their implication of higher Evaporation (cm/yr) relative evaporation.

78 Geological Society of America Bulletin, January/February 2008 Synoptic reconstruction of the Eocene Green River Formation

I.S., Rose, P.R., Ryder, R.T., Waechter, N.B., and change, because the temporal framework pre- account for lacustrine expansion-contraction Woodward, L.A., 1988, Basins of the Rocky Moun- sented in this study shows that deposition of the cycles observed in the Green River Formation. tain region, in Sloss, L.L., ed., The geology of North Little Mountain fl ora was broadly synchronous 4. Drainage diversions and reintegrations America: Boulder, Colorado, Geological Society of America, v. D-2, p. 198–220. with bedded evaporite deposition in the Piceance played a key role in determining the hydrologic Beck, R.A., Vondra, C.F. Filkins, J.E., and Olander, J.D., Creek Basin. One potential explanation for this balance of the Green River Formation lake sys- 1988, Syntectonic sedimentation and Laramide basement thrusting, Cordilleran foreland; timing of defor- apparent discordance is that the Little Mountain tem, and are attributed to specifi c episodes of mation, in Schmidt, C.J., and Perry, W.J., Jr., eds., fl ora refl ects local conditions near the Uinta crustal deformation and volcanism. Interaction of the Rocky Mountain foreland and the Mountain front, rather than regional climate. 5. Beginning ca. 49.5 Ma and continuing Cordilleran thrust belt: Geological Society of America Memoir 171, p. 465–487. Large location variations in precipitation are until ca. 47 Ma, water and volcaniclastic sedi- Benson, L.V., Currey, D.R., Dorn, R.I., Lajoie, K.R., Oviatt, well known in modern orogenic landscapes, but ment arrived diachronously into the Greater C.G., Robinson, S.W., Smith, G.I., and Stine, S., 1990, this idea is diffi cult to test based solely on the Green River, Piceance Creek, and Uinta Basins, Chronology of expansion and contraction of four Great Basin lake systems during the past 35,000 years: Pal- fl oral collections. It also may confl ict with other which each progressively freshened then fi lled aeogeography, Palaeoclimatology, Palaeoecology, v. 78, studies suggesting that moisture was imported with sediment from north to south. p. 241–286, doi: 10.1016/0031-0182(90)90217-U. Berggren, W.A., Kent, D.V., Swisher, C.C., III, and Aubry, to the Uinta Mountains from the south or south- 6. Lacustrine sediment accumulation M.-P., 1995, A revised Cenozoic geochronology and east (Fricke, 2003; Sewall and Sloan, 2006). occurred most rapidly in basin centers during chronostratigraphy, in Berggren, W.A., et al., eds., Alternatively, the association of leaf fossils underfi lled periods, and most rapidly along the Geochronology, time scales, and global stratigraphic correlation: SEPM (Society for Sedimentary Geology) with laminated facies may indicate that pres- basin margins during period of overfi lled condi- Special Publication 54, p. 129–212. ervation of fl ora was limited to relatively short tions. Volcaniclastic sandstones that overlie the Black, C.C., 1969, Fossil vertebrates from the late Eocene (<100 k.y.), climatically induced phases of lake Laney Member (49.6–48.4 Ma) in the Greater and Oligocene, Badwater Creek area, Wyoming, and some regional correlations, in Barlow, J.A., ed., Sym- expansion (Pietras et al., 2003b), whereas evapo- Green River Basin exhibit sediment accumula- posium on Tertiary rocks of Wyoming: Wyoming rite deposition occurred only during intervening tion rates that are an order of magnitude faster Geological Association 21st Annual Field Conference Guidebook, p. 43–47. dry periods (Burnside and Culbertson, 1979; (1000 mm/k.y.) than typical balanced-fi ll lacus- Bobst, A.L., Lowenstein, T.K., Jordan, J.E., Godfrey, L.V., Roehler, 1992b; Dyni, 1996). Further research trine deposition (100 mm/k.y.). Ku, T.-L., and Luo, S., 2001, A 106 ka paleoclimate is needed to help resolve these issues. record from drill core of the Salmar de Atacama, northern Chile: Palaeogeography, Palaeoclimatology, ACKNOWLEDGMENTS Palaeoecology, v. 173, p. 21–42, doi: 10.1016/S0031- CONCLUSIONS 0182(01)00308-X. This contribution represents a portion of Smith’s Bohacs, K.M., Carroll, A.R., Neal, J.E., and Mankiewicz, P.J., 2000, Lake-basin type, source potential, and hydrocar- More than 2000 40Ar/ 39Ar experiments on doctoral dissertation at the University of Wisconsin– Madison. R.N. Smith, J.J. Scott, L.J. Freimund, bon character: An integrated sequence-stratigraphic- sanidine and biotite from 22 fallout tuffs and geochemical framework, in Gierlowski-Kordesch, G. Nelson, and S.A. Hynek assisted with fi eld work E.H., and Kelts, K.R., eds., Lake basins through space 3 volcaniclastic sandstone beds allow for the and sample collection. J.H. Fournelle, B.R. Jicha, and time: American Association of Petroleum Geolo- paleogeographic reconstruction of the central X. Zhang, K. Min, and L.M. Chetel contributed to data gists Studies in Geology 46, p. 3–34. sector of the Eocene Rocky Mountains. Fur- analysis and interpretation. Discussions with H.P. Bu- Bond, J.G., and Wood, C.H., compilers, 1978, Geologic map chheim, P. Wilf, W.C. Clyde, J.J. Scott, K.M. Bohacs, of Idaho: Idaho Department of Lands, Bureau of Mines thermore, the chronologic framework presented J.C. Knox, J.A. Simo, B. Tikoff, J.R. Dyni, and and Geology, scale 1:500,000. here should serve as the starting point for a broad P.C. Murphey contributed to the development of this Bown, T.M., 1982, Geology, paleontology, and correlation suite of related tectonic, paleoclimatic, mag- manuscript. We are grateful to W.R. Dickinson for of Eocene volcaniclastic rocks, southeast Absaroka commenting on an early version, and to E.H. Chis- range, Hot Springs County, Wyoming: U.S. Geological matic, biotic, and paleogeomorphic studies. The Survey Professional Paper 1201-A, 75 p. following conclusions can be made concerning tiansen, P.W. Layer, and M.T. Heizler for their careful Bradley, W.H., 1929, The varves and climate of the Green reviews. Funding was provided by National Science River epoch: U.S. Geological Survey Professional the relative timing and evolution of Lake Gos- Foundation grants EAR-0230123, EAR-0114055, and Paper 158-E, 110 p. iute (Greater Green River Basin) and Lake Uinta EAR-0516760, the Bailey Distinguished Graduate Bradley, W.H., 1931, Origin and microfossils of the oil shale (Uinta and Piceance Creek Basins). Fellowship, Conoco-Phillips, Chevron-Texaco, and of the Green River Formation of Colorado and Utah: U.S. Geological Survey Professional Paper 168, 58 p. 1. All available chronostratigraphic evidence the Donors to the Petroleum Research Fund of the American Chemical Society. 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