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Sedimentology, detrital zircon geochronology, and stable isotope geochemistry of the lower Eocene strata in the Wind River Basin, central Wyoming

Majie Fan, Peter G. DeCelles, George E. Gehrels, David L. Dettman, Jay Quade and S. Lynn Peyton

Geological Society of America Bulletin 2011;123;979-996 doi: 10.1130/B30235.1

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Sedimentology, detrital zircon geochronology, and stable isotope geochemistry of the lower Eocene strata in the Wind River Basin, central Wyoming

Majie Fan†, Peter G. DeCelles, George E. Gehrels, David L. Dettman, Jay Quade, and S. Lynn Peyton Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA

ABSTRACT upsection. Rapid source terrane unroofi ng Snyder, 1978; Bird, 1988; DeCelles, 2004). The is also suggested by the upsection changes deformation partitioned what had been a conti- Shallow subduction of the Farallon plate in detrital zircon U-Pb ages. Detrital zircons nental-scale foreland basin that developed east of beneath the western United States has been in the upper Wind River Formation show the Cordilleran thrust belt, ~1000–1500 km in- commonly accepted as the tectonic cause for age distributions similar to those of modern land from the subduction zone. Modern regional the Laramide deformation during Late Cre- sands derived from the Wind River Range, elevation is ~1.5 km with the range summits at taceous through Eocene time. However, it re- with up to ~20% of zircons derived from the >4 km. Similar basement-involved uplifts in the mains unclear how shallow subduction would Archean basement rocks in the Wind River foreland of a major Cordilleran-style orogenic produce the individual Laramide structures. Range, indicating that the range was largely system are present in the Sierras Pampeanas in Critical information about the timing of in- exhumed by ca. 53–51 Ma. The rise of these South America, where fl at-slab subduction is individual Laramide uplifts, their paleoeleva- ranges by 51 Ma formed a confi ned valley in timately associated with the region of intrafore- tions at the time of uplift, and the temporal the northwestern part of the basin, and pro- land basement deformation (Jordan et al., 1983; relationships among Laramide uplifts have moted development of a meandering fl uvial Wagner et al., 2005). Although shallow sub- yet to be documented at regional scale to ad- system in the center of the basin. The modern duction of the Farallon plate underneath North dress the question and evaluate competing paleodrainage confi guration was essentially America is the commonly accepted tectonic tectonic models. The Wind River Basin in established by early Eocene time. cause for the Laramide deformation (e.g., Dick- central Wyoming is fi lled with sedimentary Carbon isotope data from paleosols and inson and Snyder, 1978; Bird, 1988; Saleeby, strata that record changes of paleogeog raphy modern soil carbonate show that the soil CO2 2003; Sigloch et al., 2008; Liu et al., 2010), it and paleoelevation during Laramide defor- respiration rate during the early Eocene was remains unclear how shallow subduction would mation. We conducted a multidisciplinary higher than at present, from which a more produce the individual Laramide structures and study of the sedimentology, detrital zircon humid Eocene paleoclimate is inferred. At- the extent to which Laramide deformation may geochronology, and stable isotopic geochem- mosphere pCO2 estimated from paleosol be viewed as an eastward propagation of the istry of the lower Eocene Indian Meadows carbon isotope values decreased from 2050 ± greater Cordilleran strain front (Erslev, 1993; and Wind River formations in the northwest- 450 ppmV to 900 ± 450 ppmV in the early DeCelles, 2004). Critical information about ern corner of the Wind River Basin in order Eocene, consistent with results from previ- the timing of individual Laramide uplifts, their to improve understanding of the timing and ous studies. Oxygen isotope data from paleo- paleoelevations at the time of uplift, and the tem- process of basin evolution, source terrane un- sol and fl uvial cement carbonates show that poral relationships among Laramide uplifts have roofi ng, and changes in paleoelevation and the paleoelevation of the Wind River Basin yet to be documented at regional scale to address paleoclimate. was comparable to that of the modern Great such questions. Depositional environments changed from Plains (~500 m above sea level), and that local Many previous structural, sedimentological, alluvial fans during deposition of the In- relief between the Washakie and Wind River thermochronological, and paleoelevation stud- dian Meadows Formation to low-sinuosity ranges and the basin fl oor was 2.3 ± 0.8 km. ies have shown that deformation and uplift in the braided river systems during deposition of Up to 1 km of post-Laramide regional net up- Laramide region is highly variable in time and the Wind River Formation. Paleocurrent lift is required to form the present landscape space (e.g., Love, 1939; Keefer, 1957; Soister, directions changed from southwestward to in central Wyoming. 1968; Seeland, 1978; Gries, 1983; Winterfeld mainly eastward through time. Conglomer- and Conard, 1983; Flemings, 1987; Dickinson ate and sandstone compositions suggest that INTRODUCTION et al., 1988; DeCelles et al., 1991; Gregory and the Washakie and/or western Owl Creek Chase 1992; Cerveny and Steidtmann, 1993; ranges to the north of the basin experienced The eastern portion of the Cordilleran oro- Crews and Ethridge, 1993; Omar et al., 1994; rapid unroofi ng ca. 55.5–54.5 Ma, producing genic belt in the western interior USA consists of Strecker, 1996; Hoy and Ridgway, 1997; Wolfe a trend of predominantly Mesozoic clasts the Laramide structural province, a region of Pre- et al., 1998; Dettman and Lohmann, 2000; Crow- giving way to Precambrian basement clasts cambrian basement-involved uplifts, and inter- ley et al., 2002; Fricke, 2003; Peyton and Rein- vening sedimentary basins that developed during ers, 2007; Fan and Dettman, 2009). Tectonic †E-mail: [email protected] Late Cretaceous–Eocene time (Dickinson and models explaining deformation during shallow

GSA Bulletin; May/June 2011; v. 123; no. 5/6; p. 979–996; doi: 10.1130/B30235.1; 12 ﬁ gures; 2 tables; Data Repository item 2011013.

For permission to copy, contact [email protected] 979 © 2011 Geological Society of America Downloaded from gsabulletin.gsapubs.org on July 11, 2011 Fan et al. slab subduction include basal shear traction Figure 1. (A) General map of the United States showing location of study area relative to the (Bird, 1988), thrust and back thrust connected major tectonic features and source terranes of detrital zircons (modifi ed from Dickinson and to a master detachment in lower crust, which is Gehrels, 2009). Black area stands for the Wind River Basin. (B) General map of Wyoming possibly linked to the Sevier thrust belt (Erslev, showing the age of Precambrian basement surrounding the Wind River Basin (Frost et al., 1993), lateral injection by mid-crustal fl ow from 2000; Kirkwood, 2000). Gray lines represent the Wind River and its tributaries. Stars rep- the overthickened Sevier orogenic hinterland resent locations of modern river sand for detrital zircon U-Pb age analysis: (a) East Fork; (McQuarrie and Chase, 2001), lithospheric buck- (b) Little Popo Agie River. (C) Geological map of northwestern Wind River Basin. Squares ling in response to horizontal endload (Tikoff represent locations of measured sections (modifi ed from Love and Christiansen, 1985). and Maxson, 2001), and isostatic rebound by removing the ecologized Shatsky conjugate plateau on the Farallon plate (Liu et al., 2010). Post- Laramide modifi cation of the lithosphere in the tiansen, 1985; Flemings, 1987). Among them, (Wa1–Wa3, 55.5–54.5 Ma), and the presence of Laramide region may have played a vital role in only Courdin and Hubert (1969) and Flemings Heptodon and Lambdotherium places the Wind shaping the modern landscape. Mechanisms of (1987) presented modal petrographic analysis River Formation in the late Wasatchian (Wa6– modifi cation include: (1) thermal uplift due to of Late Cretaceous and Paleocene strata in the Wa7, ca. 53–51 Ma) (Clyde et al., 1997; Robin son asthenospheric upwelling by removing the sub- western and southern basin. Flat-lying, lower et al., 2004; Smith et al., 2008). The youngest ducted slab or thickened mantle lithosphere be- Eocene, Wind River Formation occupies the U-Pb ages of detrital zircons collected from neath the western USA (Dickinson and Snyder, center of the basin, with the greatest thick- fl uvial sandstones in this study cluster at 62.3 ± 1978; Humphreys, 1995; Sonder and Jones, ness along the east-northeastern basin margin 1.3 Ma, which is older than the mammalian age 1999); (2) subcontinental-scale dynamic sub- (Keefer, 1965). In the northwestern corner of and thus provides no additional constraint on sidence by induced asthenospheric counterfl ow the basin, the slightly older Indian Meadows depositional age. The contact between the In- above the subducted slab (McMillan et al., 2002; Formation was tilted and displaced on the top dian Meadows Formation and the Wind River Heller et al., 2003; McMillan et al., 2006); (3) re- of the Wind River Formation by several north- Formation is complex but appears conformable gional uplift caused by isostatic rebound of litho- west-striking thrust faults (Love, 1939; Keefer, in the East Fork area (Winterfeld and Conard, sphere due to climate-driven erosion (Pelletier, 1957; Soister, 1968; Seeland, 1978; Winterfeld 1983; this study). 2009); and/or (4) thermal upwelling associated and Conard, 1983; Flemings, 1987). A moder- with the initiation of the Rio Grande Rift (Heller ately angular unconformity between the Indian Tectonic Setting et al., 2003; McMillan et al., 2006). Quantitative Meadows Formation and underlying Mesozoic data on paleoelevation, source-terrane unroofi ng rocks has been observed along the southwest- The Wyoming craton includes granite and exhumation, climate, and paleogeography ern fl ank of the Washakie Range (Winterfeld gneisses and minor amounts of metavolcanic- within a precise chronological context are re- and Conard, 1983; this study). Our study fo- metasedimentary rocks older than 2.6–2.7 Ga quired to evaluate these competing hypotheses. cuses on the northwestern corner of the Wind (Frost and Frost, 1993; Frost et al., 2000). Paleo- In this paper we report results of a multidisci- River Basin (Dubois–East Fork area, Fremont proterozoic metavolcanic and metasedimen- plinary study of the sedimentology, sandstone County), where downcutting by the Wind River tary rocks intruded by numerous plutons were petrography, detrital geochronology, and stable exposed ~700 m of the Indian Meadows Forma- accreted onto the Wyoming craton along the isotope geochemistry of the early Eocene basin tion and Wind River Formation along the north Cheyenne belt at ca. 1.86 Ga (Frost and Frost, fi ll in the Wind River Basin, central Wyoming. bank, which is the thickest outcrop of the lower 1993). Rifting of the western margin of Lauren- From these data we reconstruct the paleogeog- Eocene strata in the basin. tia during Neoproterozoic–early Paleozoic time raphy, paleoclimate, source-terrane exhumation, The ages of the Wind River and Indian Mead- accommodated the siliciclastic sedimentary tectonic setting, and basin evolution. ows formations are constrained by North Amer- and metasedimentary rocks deposited under ica land mammal ages (GSA Data Repository predominantly marine environments (Levy and REGIONAL GEOLOGY Fig. DR11) (Love, 1939; Keefer, 1957; Winter- Christie-Blick, 1991). During Paleozoic–early feld and Conard, 1983), whose boundaries in the Mesozoic time, the location of modern western Stratigraphy and Age Control early Eocene have been revised based on cor- USA was in the Cordilleran miogeocline, and a relation and calibration with radioisotope ages thick succession of carbonate rocks and subor- The Wind River Basin in central Wyoming is and magnetostratigraphy in the Green River dinate siliciclastic rocks was deposited in a pas- one of the many Laramide intermontane basins Basin (Clyde et al., 1997; Robinson et al., 2004; sive margin setting (Devlin and Bond, 1988). formed to the east of the Sevier thrust belt. The Smith et al., 2008). The presence of Canitius, From Late Jurassic on, the backarc crustal short- basin is surrounded by reverse fault-bounded, Hyopsodus, and Diacodexis places the Indian ening and thickening caused by the subduction basement-involved uplifts, including the Wind Meadows Formation in the early Wasatchian of the Farallon plate produced the Cordillera River Range on the southwest, the Washakie thrust belt, and a thick succession of marine Range, Owl Creek Mountains, and Bighorn and marginal marine sediments was deposited 1 Mountains on the north, the Casper arch on the GSA Data Repository item 2011013, Figure in Cordillera foreland when the Western Interior DR1: Chronostratigraphy of the early Eocene Wind east, and the Granite Mountains on the south River Basin; Figure DR2: Photos of zircon grains; Seaway inundation was terminated by the Late (Fig. 1). Strata of Late Cretaceous–Eocene Figure DR3: U-Pb concordia diagrams for detrital Cretaceous time (e.g., DeCelles, 2004). During age are exposed along the basin margin, and zircon samples; Table DR1: Modal petrographic ca. 80–40 Ma, Laramide deformation tectoni- have been studied extensively in the past cen- point-counting parameters; Table DR2: Modal cally partitioned the foreland basin in Wyo- petrographic data; Table DR3: Zircon U-Pb data; tury (Keefer, 1965; Soister, 1968; Courdin and Table DR4: Stable isotope data, is available at http:// ming, Montana, and Colorado, and produced the Hubert, 1969; Seeland, 1978; Phillips, 1983; www.geosociety.org/pubs/ft2011.htm or by request basement-involved uplifts and intervening Winterfeld and Conard, 1983; Love and Chris- to [email protected]. basins such as the Wind River Basin.

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Wyoming-Hearne- A Rae (>2.5 Ga) 100°W Superior (>2.5 Ga)

Canada (1.8–1.9 Ga) (1.8–1.9 Cordilleran Trans-Hudson USA accretion and batholiths (<0.25 Ga) 40°N 40°N WY Yavapai-Mazatzal mid-continent Grenville (1.0–1.3 Ga) (1.6–1.8 Ga) (1.3–1.5 Ga)

Sevier thrust belt Amarillo-Wichita (ca. 0.525 Ga)

Appalachian (0.33–0.76 Ga) ? USA ? Quachita orogen Mexico ? ? N

1000 km Suwanee (0.54–0.68 Ga) Yucatan-Campeche(0.54–0.58 Ga) East Mexico 120°W magmatic arc 80°W 20°N (0.23–0.29 Ga) 20°N

Bea rtoot Washakie Range B h Mtns MT CC

Bighorn Mtns (2.8–2.9 Ga) Absaroka volcanic field (51–46 Ma) Bighorn River Washakie Range

Owl Creek Mtns (2.8–2.95 Ga) a Wind River 2DB Wind River Basin Granite Mtns (3.0–3.3 Ga) Dubois b 1DB Wind River Range ID (2.5–2.8 Ga) 43.5°N 3DB 4DB

UT Sevier thrust front thrust Sevier

WY Wind River Range

Uinta Mountains 100 km Quarternary Post-early Eocene deposites 10 km Wind River Formation India Meadows Formation Mesozoic rocks Paleozoic rocks 109°W Precambrian basement

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Four schools of thought exist on the tectonic Basement volcanic A and metamorphic rock history of the Wind River Basin during Laramide vf m vc deformation. Keefer (1965) suggested that the f c Mesozoic

Silt sandstone basin experienced three stages of subsidence Clay Sand Gravel Pebble Cobble in response to three uplift events in surround- Boulder Mesozoic–Paleozoic carbonate ing mountains, with the climax of deformation Paleosol nodule Chert Quartzite happening to the Casper Arch, southern Bighorn Covered Mountains, and Owl Creek Mountains during Planar cross-stratification Siltstone contains cobble the early Eocene. In contrast, Flemings (1987) Stratigraphic correlation concluded that the maximum rate of uplift and Trough cross-stratification Erosional contact deformation was during Maastrichtian time and Normal grading Reverse fault contact was caused by the thrusting of the Wind River Clast imbrication and lamination Detrital zircon sample Sandstone petrology sample Range and Granite Mountains. Moreover, of N regional scale, Gries (1983) suggested that the n Paleocurrent direction and number (n) of measurements (imbricated E W clasts unless otherwise noted; tr—trough cross-stratification) NW-SE–oriented uplifts were generally ear- S lier than the E-W–oriented uplifts, and Brown (1988) and Erslev (1993) argued progressive Figure 2 (on this and following page). (A) Stratigraphic symbols used SW to NE arch development. in the stratigraphic sections in (B) and (C). (B) Measured sections of the Indian Meadows Formation in the northwestern Wind River SEDIMENTOLOGY Basin. (C) Measured sections of the Wind River Formation in the northwestern Wind River Basin. (B) and (C) include lithofacies, Depositional environments are interpreted paleocurrent directions, clast composition, and stratigraphic cor- on sedimentological analysis basis of four relations (dotted lines). Section locations are marked in Figure 1C. measured sections, totaling ~750 m in thick- Lithofacies descriptions are summarized in Table 1. Abbreviations: ness (Fig. 2). Paleocurrent directions are based c—coarse; f—fi ne; m—medium; vc—very coarse; vf—very fi ne. on 211 measurements of imbricated clasts (10 per station) and trough cross-stratifi cation (method I of DeCelles et al., 1983). The Eocene conglomerate (Gcm and Gmm) (Fig. 3B). Units ates. Siltstone of lithofacies Fm (~2 m) is sandy sedimentary rocks in the basin are described by of lithofacies Gch, Gct, and Gcm (~5 m thick) and contains fl oating granules. Paleosol carbon- Love (1939), Keefer (1957), Seeland (1978), are lenticular, overlie basal scour surfaces, and ate nodules are developed in most siltstone in- Soister (1968), Winterfeld and Conard (1983), exhibit crude, upward-fi ning trends. In gen- tervals, and organic-rich layers up to 1 m thick and Flemings (1987), but lithofacies have not eral, conglomerate grades rapidly upward into are preserved above some of the carbonate ac- been described in detail. Lithofacies that were sandstone. Beds of lithofacies Sm, Sh, and St cumulation horizons. In some instances, several documented in the fi eld are typical of fl uvial de- are composed of two or more, ~2- to 4-m-thick paleosols are stacked atop one another. posits and are well understood in terms of depo- sandstone packages separated from each other sitional processes (summaries in Miall, 1978, by scour surfaces. Plane-parallel laminations of Interpretation 1996). Therefore, we provide a summary table a few millimeters thick are present only in the The assemblage of lithofacies in the Indian of the most common lithofacies and physical upper ~0.5 m of many packages. The mudstone Meadows Formation is characteristic of allu- processes in this study, and we focus on inter- and siltstone beds (Fm) usually contain paleo- vial fan depositional systems (Nemec and Steel, preting lithofacies assemblages and the corre- sol carbonate nodules. 1984; DeCelles et al., 1991; Stanistreet and sponding depositional systems (Table 1). The middle 100 m of the Indian Meadows McCarthy, 1993; Miall, 1996). The lower inter- Formation (7–110 m of 4DB and top of 2DB) val was deposited in the proximal alluvial fan by Indian Meadows Formation: is dominated by pebble-cobble conglomerate gravel-bed braided channels and debris fl ows. Stream-Dominated Alluvial Fan Association composed of lithofacies Gcm, Gcmi, Gct, and The thin bodies of conglomerate containing Gcp. Two- to 4-m-thick beds are stacked to form sedimentary structures were deposited as longi- Description lenticular bodies up to 30 m thick, which extend tudinal bar deposits in small fl ashy, highly un- The Indian Meadows Formation consists of several kilometers laterally (Fig. 3C). These steady braided streams (Hein and Walker, 1977; interlayered gray and yellow conglomerate, kilometer-scale lithosomes have erosional bases Miall, 1978). The sandstones and siltstones were sandstone, and brick-red siltstone with paleo- and generally fi ne upward. Siltstone lithofacies deposited on the tops or fl anks of bars during sols, totaling 315 m in thickness (Fig. 2B). Fm forms beds 1–2 m thick between these con- waning fl ow or in slackwater ponds, such as The Formation rests on an erosional uncon- glomeratic bodies; although these units are mas- abandoned channels (Rust, 1972; Miall, 1978). formity on the upper Cretaceous Cody shale or sive, they lack distinctive paleosol features. The unstratifi ed, poorly sorted, matrix-supported the Wind River Formation in our studied area The upper 110 m of the Indian Meadows conglomerates represent viscous debris fl ows (Fig. 3A). The lower 100 m of the Formation Formation (110–215 m of 4DB) is composed of triggered by catastrophic events (Nemec and (11–110 m of section 3DB) is divided into intercalated lenticular pebble-cobble conglomer- Steel, 1984; Schultz, 1984; DeCelles et al., 1991; two 40- to 50-m-thick intervals of intercalated ate (Gct, Gcm, Gmm, Gcmi, and Gcp), medium- Stanistreet and McCarthy, 1993). The poorly pebble-cobble conglomerate (Gch, Gct, Gcm, to coarse-grained sandstone (Sm, Sh, and St) sorted, clast-supported conglomerates represent and Gcmi), medium-grained sandstone (Sh, (Fig. 3D), siltstone (Fm), and paleosol (P). The high-concentration fl oods that were not in equi- Sm, and St), siltstone (Fm), and paleosol (P), sandstones contain fl oating granules and pebbles librium with typical regime bedforms (Pierson, sandwiching a 15 m interval of cobble-boulder and generally cap the fi ning-upward conglomer- 1981; DeCelles et al., 1991).

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4DB 2DB 210 300 Gcm BC200 10 Gcm,Fm Gcmi, Gcm Gmh,Gct Gcp P 280 Gmm,Fm 180 Gcm,Gch P Gmm,P Gcm Sm,Fm Gmm 260 Gcm Sm 160 P P Fm,Sm Gcm,P P 240 Gch,Gct Gcp 140 Gcm,Gct Gcp P Gmm 220 Sm 1DB Gch P Fm 120 120 Sm,P 10 Gcm,Sm Gcm,Fm,P Fm,P Gmm Gch,Gchi Gct,Gcp Gch,Gct,Sh,St 22tr 200 P 12tr 100 Gch,St 100 Sm,Sr,Sh Gcm,Gct Sm Gch,St Gcp Sm,Fm 18tr Gct, Gch P 180 Sh,Fr,P Gmm,Sm P 80 Fm, Sm 80 Gmm,Sm Sh,St Gcp,Gch Sm,P P Gch,Gct Gcmi,Gcp 160 Gct Gch,Gct,Gcp 60 60 10 P Fm P Gct, Gcmi St P Gct,Gch,Gcp 140 Fm,P Gch,Gct Gch,sh Gcmi,Sm 40 Sm,P 40 Gcm,Gcp Gch,Gcm 10 P 20tr P 3DB St,Sr 120 Gch,Gct Sm 105 Gcmi,Gcp Gct,St 9tr 20 P Gcm,Gcmi 20 10 St,Sm Gcmi,Gct Gcp,Gcmi Gcm,Fm,P 10 Sh Gch,Fh,Fm,P 100 10 P Gct,Gcm,Sh 100 Gmm,Sm,Fm,P Gct,Gcp 10 Gch Sh,Fm Gct, Gmmi 0 Fm,P Sm,Fm 0 cs gpc b Thickness (m) sand Gcm,Sm 10 cs sand gpc b Gmm,Fm,P 80 P Gcm,Fm Gcm,Gchi,Gct 10 Gcm 80 Gct Gch,Gcm Sm,Fr,Fm,P St,Sh Gct,Gch Gch,Fm Sm 60 Fm Gcm,Sm,Fm,P 60 Gcm,Gct,Fm,P Gcm Sm,P 10 Gch,St Gmm Sm,Fr,P 40 10 40 Gcmi,Sh Fm Sm,Fr P P Sm,Sh,Fr 10 20 Sm,Sh Gchi Gcm,Fm 20 Gch,Gct St Gcm,Gct K P Shale Gcmi,Sh,Sm Fm 0 cs c 10 sand gp b 0 Gcmi cs sand gpc b

Figure 2 (continued).

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TABLE 1. LITHOFACIES AND INTERPRETATIONS USED IN THIS STUDY* Interpretation Lithofacies The assemblage of lithofacies in the Wind edoc noitpircseD noitaterpretnI River Formation is characteristic of low-sin- Massive, matrix-supported pebble to boulder Deposition by cohesive mud-matrix debris Gmm conglomerate, poorly sorted, disorganized, uosity, gravel-bed braided river depositional fl ows unstratifi ed systems (Hein and Walker, 1977; Miall, 1978, Pebble to boulder conglomerate, poorly sorted, clast- Deposition from sheet fl oods and clast-rich Gcm 1996). The conglomerates represent braided supported, unstratifi ed debris fl ows Pebble to boulder conglomerate, moderately sorted, Deposition by traction currents in unsteady channel deposits, and the sandstones were de- Gcmi clast-supported, stratifi ed, imbricated fl uvial fl ows posited on the tops or fl anks of bars or in aban- Deposition by large gravelly ripples under Pebble to cobble conglomerate, well sorted, clast- doned channels. The stacked lenticular thin beds Gct traction fl ows in relatively deep, stable supported, trough cross-stratifi ed of horizontally stratifi ed and cross-stratifi ed fl uvial channels Pebble to cobble conglomerate, well sorted, clast- sandstone are interpreted as crevasse-splay de- Deposition from shallow traction currents in Gchi supported, horizontally stratifi ed, imbricated (long longitudinal bars and gravel sheets posits (Miall, 1978, 1996). The lack of fl oating axis transverse to paleofl ow) pebbles in siltstones suggests the lack of sandy Pebble to cobble conglomerate, well sorted, clast- Deposition from shallow traction currents in Gch supported, horizontally stratifi ed longitudinal bars and gravel sheets mudfl ows (Pierson, 1981). Thick paleosol car- Deposition by large straight-crested gravelly bonate horizons suggest that the fl oodplain of Pebble to cobble conglomerate, well sorted, clast- Gcp ripples under traction fl ows in shallow supported, planar cross-stratifi ed the braided rivers was vegetated and the paleofl uvial channels sols are well developed when the mean annual Massive very coarse- to medium-grained sandstone, Sm Deposition from small gravity fl ow sometimes contains gravel precipitation was high, and the climate strongly Migration of small 2D and 3D ripples under seasonal (Retallack, 2005). Fine- to medium-grained sandstone with small, Sr weak unidirectional fl ows in shallow asymmetric ripples The paleocurrent direction varies between channels Migration of large 3D ripples (dunes) under eastward and southwestward, but the most Medium- to coarse-grained sandstone with trough St moderately powerful unidirectional fl ows in prominent direction was eastward (Fig. 2C). cross-stratifi cation large channels This suggests that the sediments were mainly Upper plane bed conditions, or fl ash fl oods Fine- to medium-grained sandstone, horizontally derived from the Wind River Range to south- Sh under unidirectional fl ows, either strong or stratifi ed very shallow west, with local input from the Washakie Range Deposition from suspension or weak traction Fr Siltstone, horizontally laminated, or small ripples and/or western Owl Creek Mountains to north current in overbank area (Figs. 1B and 1C). Deposition from suspension in ponds and Fm Massive siltstone, sometimes bioturbated fl oodplain Overall, the depositional environment in the P Massive siltstone with carbonate nodules Calcic paleosol northwestern Wind River Basin evolved from al- *Modifi ed from Miall (1978) and DeCelles et al. (1991). luvial fan into low-sinuosity fl uvial, with paleocurrent directions changing from southwestward to mostly eastward during 55.5–51 Ma. This evolution suggests that surface slope between the The thick conglomerate beds in the middle that the sediments were shed from the Washakie upland sediment source terranes and the basin de- interval of the Indian Meadows Formation are Range and/or western Owl Creek Mountains by creased through time (Stanistreet and McCarthy, typical of shallow gravel-bed braided river chan- transverse rivers (Figs. 1B and 1C). 1993; Miall, 1996). nels, with cross-stratifi cation interpreted as bar accretion sets, and the upward-fi ning trends re- Wind River Formation: SANDSTONE PETROGRAPHY fl ecting a progressive decrease in fl ow strength Braided River Association AND PROVENANCE (Hein and Walker, 1977; Miall, 1996). The ab- sence of debris fl ow facies and extensive lateral Description Methods and Description distribution of this interval indicates the depo- The Wind River Formation contains inter- sitional environment was an extensive gravelly calated, lenticular, gray pebble-cobble conglom- Standard petrographic thin sections were braidplain (Rust and Gibling, 1990). The upper erates (Gcm, Gcmi, Gcp, Gct, and Gch) with made from 16 medium-grained sandstone sam- interval was deposited in a stream-dominated al- basal scour surfaces (Fig. 3E), sandstones (Sh, ples that were collected while measuring strati- luvial fan environment, with the sandstone and Sr, St, and Sm), purple-red siltstones (Fr and graphic sections. The thin sections were stained siltstone containing fl oating pebbles deposited in Fm), and stacked calcisols (P), totaling ~260 m in for potassium and calcium feldspar and point- sandy mudfl ows that spilled out of channels onto thickness (Figs. 2C and 3F). The lithofacies are counted (450 counts per slide) using a modi- the preexisting ground surface as sheet-fl ood de- in many ways identical to those of the upper In- fi ed Gazzi-Dickinson method, in which crystals posits (e.g., Pierson, 1981; DeCelles et al., 1991). dian Meadows Formation discussed already, and larger than silt-sized within lithic fragments are The paleosols have distinct massive pedogenic therefore only the differences will be described. counted as monocrystalline grains (Ingersoll nodular carbonate intervals, and are classifi ed as Floating pebbles do not occur in the sandstones et al., 1984). The point-counting parameters are calcisol (Mack et al., 1993). Well-developed cal- and siltstones. Many thin lenticular sandstone listed in Table DR1 (see footnote 1), and modal cisols are typical of fl oodplain deposits and rep- beds (0.5–2 m thick) of lithofacies Sh and Sr are data are given in Table DR2 (see footnote 1). The resent relatively low sedimentation rates (Kraus, encased in the siltstone intervals. Locally, two to modal sandstone analyses are augmented by 11 1999). Overall, the depositional environment three sandstone beds are stacked on top of each clast-counts (at least 100 clasts per station). changed from debris fl ows–dominated alluvial other, and display an upward-coarsening textural Monocrystalline quartz is the primary con- fans into stream-dominated alluvial fans. trend. Calcisols are developed in most siltstone stituent of all samples. Feldspar mainly consists Paleocurrent data show consistently south- intervals and are often stacked as three to four of potassium feldspar and plagioclase, with a westward fl ow directions (Fig. 2B), suggesting paleosols of ~1 m thick (Fig. 3F). K/P ratio that varies between 0.6 and 4.7. Lithic

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Interpretation AB The sandstone modal petrographic data show SW Indian Meadows Formation that the lower Eocene sandstones in the northwestern Wind River Basin were derived from a recycled orogenic provenance (Fig. 4). Poten- Wind River Formation 30 mm tial source terranes in Wyoming include: Cre- 20 m taceous sandstone, siltstone, and chert-pebble conglomerate; Jurassic and Triassic eolian sandstone and marine limestone; Jurassic– upper Permian glauconitic siltstone, sandstone, and limestone; Permian–upper Cambrian lime- CDstone, dolostone, and sandstone; middle-upper Cambrian micaceous shale; Cambrian coarse- grained quartzite; and Archean granite gneiss (Love and Christiansen, 1985). 2 mm Lithic grains of carbonate, sandstone, mudstone, and phyllite were probably derived from the Phanerozoic sedimentary cover strata exposed along the northern and southwestern fl anks of the Wind River Basin. The large amount of well-rounded monocrystalline quartz grains in the Indian Meadows Formation suggests the sediment was recycled from underly- E F ing quartzose sedimentary units, mostly from Jurassic and Triassic eolianite. The occurrence of glauconite suggests that nearby Meso- zoic sedimentary rocks were major sediment sources. The abundant feldspar grains in the Wind River Formation were derived from Pre- 30 m cambrian basement rocks. 1 m The red sandstone clasts in the conglomerates are of recycled Jurassic and Triassic eolian and fl uvial sandstones, and the limestone and dolostone clasts were derived from the Phanerozoic marine carbonate units. The carbonate clasts Figure 3. Photographs of northwestern Wind River Basin outcrops. (A) Angular uncon formity with brachiopod fossils are of Mississippian between the Wind River Formation (below, horizontal) and the Indian Meadows Forma- and Pennsylvanian age (Love and Christiansen, tion (moderately dipping to northeast). (B) Disorganized conglomerate of lithofacies Gcm in 1985). The granite clasts were derived from the the lower Indian Meadows Formation. (C) Imbricated conglomerate of lithofacies Gcmi in the Precambrian basement rocks, which are now ex- middle Indian Meadows Formation. (D) Coarse sandstone containing fl oating pebbles (Sm) in posed in the Laramide ranges. the upper Indian Meadows Formation. (E) Cross-stratifi ed conglomerate of lithofacies Gct Overall, the Eocene sandstones and conglom- in the Wind River Formation. (F) Overview of the Wind River Formation. erates of the northwestern Wind River Basin exhibit a simple unroofi ng sequence, with initial source terranes dominated by Mesozoic grain populations are dominated by micritic sandstones that we studied are dominated by and upper Paleozoic rocks giving way to lower carbonate. Volcanic grains constitute <5% of calcite cements (Fig. 5). Paleozoic and Precambrian basement rocks total lithic grains. Other lithic grains include Clasts of dolostone, limestone, and sand- upsection. phyllite, siltstone, and quartzite derived from stone dominate the conglomerates in the Indian sedimentary and metasedimentary strata. Bio- Meadows Formation. Carbonate clasts with DETRITAL ZIRCON U-Pb tite, muscovite, pyroxene, olivine, glauconite, brachiopod fossils are abundant in the lower GEOCHRONOLOGY and chlorite are the most common accessory interval of the Indian Meadows Formation. The minerals. The Indian Meadows and Wind River sandstone clasts are quartzose and have distinct Samples and Methods formations sandstones have average modal red color. Granite gneiss clasts fi rst appear in the compositions of Qm:F:Lt = 57:10:33, Qt:F:L = middle interval of the Indian Meadows Forma- Six samples of medium- to coarse-grained 77:11:13; and Qm:F:Lt = 41:26:33, Qt:F:L = tion, and the content increases upsection, reach- sandstone from the Indian Meadows Formation 56:26:18, respectively. The Indian Meadows ing ~20% at the top of the Indian Meadows (3DB69, 4DB4.5, 4DB191, and 2DB273) and Formation contains greater amounts of quartz, Formation. Granite (~45%) and carbonate Wind River Formation (2DB2 and 2DB257), and lesser amounts of feldspar than the Wind (~40%) clasts dominate the conglomerates in one sample of a granite cobble (2DB2cobble), River Formation (Fig. 4). All of the Eocene the Wind River Formation. and two samples of modern river sands (East

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Qm Qt zoic age constitute the largest population on age-probability diagrams. Sample 2DB2cobble has a mean U-Pb zircon crystallization age of 2.61 Ga (Fig. 7). A single zircon grain in sample CB CB RO LP produced an age of 47.7 ± 2.1 Ma, but the RO other age groups are similar to those in the Eo- MA MA cene samples. The nine detrital zircon grains F Lt F L with concordant ages in sample EF cluster at Qp Wind River Formation n = 8 mean 2.7–3.2 Ga. The interpretation of detrital zircon Indian Meadows Formation n = 6 meanQm ages in the western USA is relatively well understood (Armstrong and Ward, 1993; Gehrels et al., 1995; Roback and Walker, 1995; Stewart et al., 2001; Torres et al., 1999; Gehrels, 2000; Gehrels et al., 2000; DeGraaff-Surpless et al., 2002; Dickinson and Gehrels, 2003; Link et al., 2005; Dickinson and Gehrels, 2009). Here, we Lv Ls P K summarize the sources of specifi c age groups in Table 2. The major source terranes are presented Figure 4. Ternary diagrams showing modal-framework grain in Figure 1A. compositions of sandstones from lower Eocene strata of the northwestern Wind River Basin. Provenance fi elds after Dickinson and Interpretation Suczek (1979). RO—recycled orogen; CB—continental block; MA—magmatic arc. Framework components are explained in GSA The euhedral morphology of zircon grains Data Repository Table DR1 (see footnote 1), and data are listed in suggests sample EF was derived from weath- Table DR2 (see footnote 1). ered granite in the Washakie Range and/or the Owl Creek Mountains. The zircon U-Pb ages are consistent with the crystallization age of the Fork [EF], Little Popo Agie River [LP]) were and precision are shown in Table DR3 (see granodiorite gneiss in west Owl Creek Mountains collected and processed by standard methods footnote 1). Nine of the 19 zircon grains from (ca. 2.8 Ga) (Kirkwood, 2000). The youngest zir- for separating zircons (Gehrels et al., 2000). sample EF show concordant ages. There is no con in sample LP is ca. 48 Ma; this grain was The Little Popo Agie River and East Fork rise in systematic correlation between shape, color, an- most likely derived from the Absaroka volcanic Precambrian crystalline basement of the Wind gularity, and U-Pb age for zircons in the Eocene fi eld (Armstrong and Ward, 1993). Approxi- River Range and Washakie Range, respectively sandstones and sample LP. mately 20% of the detrital zircons analyzed in (Fig. 1B). The former cuts an ~2-km-thick Paleo- sample LP are in the 2.5 to 2.7 Ga age range, zoic and Mesozoic sedimentary section on the Results which is consistent with zircon U-Pb ages of northeastern fl ank of the Wind River Range, the granitic plutons in the Wind River Range whereas no Paleozoic and Mesozoic sedimen- The youngest detrital zircons recovered from (2.55–2.67 Ga) (Frost et al., 2000). The anhedral tary strata are exposed along the East Fork. the Eocene basin fi ll are in the 60 to 80 Ma morphology and small grain size suggest most Zircon grains in sample EF are euhedral, and range, which is older than the depositional age of the zircon grains in sample LP were recycled, ~200 μm long, whereas those in sample LP are derived from North American land mammal presumably from Mesozoic sandstones. This is similar to those from Eocene sandstones, sub- ages. Ages of pre–80 Ma zircons generally fall supported by the fact that Mesozoic sandstones hedral to anhedral, and mostly 50–100 μm in within the following intervals: 80–220 Ma, 230– in western USA have similar age spectra, includ- diameter (Fig. DR2 [see footnote 1]). 290 Ma, 330–760 Ma, 1.0–1.3 Ga, 1.3–1.5 Ga, ing: (1) zircons from the Cordilleran magmatic About 100 individual zircon grains were 1.6–1.8 Ga, and 2.5–3.0 Ga. Zircons of Protero- arc in California and Nevada (80–220 Ma) and analyzed for the early Eocene sandstone samples and sample LP. All 19 zircons ex- tracted from sample EF and 23 zircons from A Plagioclase B Calcite Glauconite sample 2DB2cobble were analyzed. U-Pb geochronol ogy of zircons was conducted by laser- ablation–multi-collector–inductively coupled plasma–mass spectrometry (LA-MC-ICP-MS) at the University of Arizona LaserChron Cen- ter. Details of analytical procedures and data Weathered K-feldspar processes are described in Gehrels et al. (2008). The ages presented are 206Pb/238U ages for grains <1000 Ma and 206Pb/207Pb ages for grains with Plagioclase 250 μm 206Pb/238U ages >1000 Ma (age-probability plots 250 μm in Fig. 6; concordia diagrams in Fig. DR3 [see footnote 1]). Analyses that yielded isotopic data Figure 5. Photographs of petrographic thin sections of sandstone from the Wind River For- of acceptable discordance, in-run fractionation, mation (in cross-polarized light).

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2.61 Ga East Fork n = 9

2400 2600 2800 3000 3200 3400

Little Popo Agie River n = 113

2DB257 n = 108

2DB2 n = 88 Wind River Formation

4DB191 n = 97

2DB273 n = 89

Normalized age probability 4DB4.5 n = 94 Indian Meadows Formation

3DB69 n = 114

0 500 1000 1500 2000 2500 3000 3500 Age (Ma)

Figure 6. U-Pb age-probability diagrams of detrital zircons for samples in this study. The curves represent a sum of the probability distributions of all analyses from a sample, normalized so that the areas beneath the probability curves are equal for all samples. Thick dashed line represents 2.61 Ga, which is marked as the reference of Archean zircons from the Wind River Range (see Fig. 7). Sources of different zircon age peaks in this study are summarized in Table 2.

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0.56 et al., 2001) when the Washakie and/or western Owl Creek ranges were unroofed. 0.54 The detrital zircon age spectra in the Wind 2700 River Formation show sediment shed from the 0.52 Wind River Range dominated the basinal sedi- Figure 7. U/Pb concordia dia- ment in 53–51 Ma. The ca. 2.61 Ga zircon age 2600 grams for zircons from a gran- 0.50 from the granite cobble from the lower Wind ite cobble in the base of section U 238 River Formation matches the age of basement 2DB. The age ﬁ ts the crystalli- 0.48 2500

Pb/ granite in the Wind River Range (Frost et al., zation age of the granite in the 206 2000). Abundant granite clasts and Archean Wind River Range. MSWD— 0.46 zircons (ca. 2.6 Ga) in the lower Wind River For- mean square of weighted deviates. mation (sample 2DB2) suggest that Wind River 0.44 Mean age = 2606 ± 24 Ma MSWD = 4.4 Range basement rocks were a major source by n = 23, 68.3% confidence early Eocene time. The detrital zircon age spec- 0.42 10 11 12 13 trum changes into the shape of modern sand 207Pb/235U sample LP at the top of the Wind River For- mation (sample 2DB257), with predominant Yavapai-Mazatzal ages, a large component of Mesozoic ages indicating derivation from the Permian–Triassic magmatic arc sources in east- (2.7–3.1 Ga) are older than zircons from the Cordillera magmatic arc, and ~20% Archean ern Mexico (230–280 Ma); (2) recycled zircons Wind River Range (ca. 2.6 Ga). ages with sources in the Wind River Range from the Devonian–Mississippian Antler oro- The evolution of the detrital zircon age spec- basement rocks. genic belt in Nevada and Idaho (330–450 Ma); tra in the Indian Meadows Formation shows a In summary, detrital zircon geochronology (3) zircons derived from the Appalachian oro- rapid source terrane unroofi ng in 55.4–54.5 Ma of lower Eocene sandstones in the Wind River genic belt (360–760 Ma), and Grenville terrane (Fig. 6). The detrital age spectrum of the lower Basin shows: (1) the source terrane of the Indian (1.0–1.3 Ga) in the eastern USA; and (4) recy- part of the formation (3DB69 and 4DB4.5) is Meadows Formation (ca. 55.5–54.5 Ma) was cled zircons from the Yavapai-Mazatzal orogen similar to that of the Jurassic eolianite in Utah most likely the Washakie Range and/or Owl (1.6–1.8 Ga) (Roback and Walker, 1995; Dick- and Wyoming, with abundant Grenville (ca. 1.0– Creek Mountains, consistent with southwest ward inson and Gehrels, 2003, 2009). 1.3 Ga) and lesser amounts of Yavapai-Mazatzal paleofl ow directions. Sediment derived from Detrital zircon geochronology of the modern (1.6–1.8 Ga) age grains (Dickinson and Gehrels, these sources records an unroofi ng sequence river sands sheds light on the Eocene detrital 2003, 2009). The proportion of grains derived from upper Mesozoic sandstone to lower Paleo- zircon age spectra in the following aspects: (1) zir- from the Cordilleran magmatic arc increases in zoic strata; (2) grains derived from the Wind con grains recycled from Mesozoic sandstones the middle formation (sample 2DB273). This River Range began to enter the depositional sys- with mostly Grenville and Yavapai-Mazatzal change corresponds to the inferred increase of tem by ca. 53–51 Ma; and (3) the basement of ages, dominate the zircon population in fl uvial fl uvial discharge in the alluvial fan association. the Wind River Range was eroded and exposed basinal sediments; (2) basement exposures The age spectrum of the upper formation (sam- to its present extent by 51 Ma. along the modern basin margin contribute only ple 4DB191) is dominated by grains of Yavapai- ~20% of the total zircon grains to basinal sedi- Mazatzal age, refl ecting a recycled source in STABLE ISOTOPE GEOCHEMISTRY ment; and (3) zircons from the Washakie Range Paleozoic strata (Gehrels et al., 1995; Stewart Methods

TABLE 2. MAJOR U-Pb AGE POPULATIONS OF DETRITAL ZIRCONS IN MODERN The basic approach of this study and the ana- RIVER SAND AND EARLY EOCENE SEDIMENT IN THE WIND RIVER BASIN lytical methods for carbonate isotope analysis Minimum Maximum age age follow that of DeCelles et al. (2007). Modern Population (Ma) (Ma) Source regions soil carbonate formed under cobble clasts from Challis and Absaroka South-central Idaho, northwest Wyoming, southeast Montana 42 60 at least 50 cm depth in active soil horizons was Volcanic Supergroup (Armstrong and Ward, 1993) collected. Paleosol carbonate nodules were col- Cordilleran magmatic arc Central and southern California, Nevada, south-central Idaho (mainly Sierra Nevada 80 220 lected from the middle of paleosol beds at least (Armstrong and Ward, 1993) and Idaho batholiths) 50 cm below the paleosurface (Fig. 8). We also Permian–Triassic 230 290 Eastern Mexico (Torres et al., 1999) δ13 magmatic arc analyzed the C values of eight dominant Southern and eastern United States; recycled from plant species in the study area collected during Appalachian orogenic belt 330 760 Mesozoic to Paleozoic miogeocline (Dickinson and the growing season. Organic matter δ13C was Gehrels, 2003, 2009) Recycled from Neoproterozoic to Paleozoic miogeocline measured using a Costech EA combustion unit Grenville orogeny 950 1300 (Dickinson and Gehrels, 2003, 2009) connected to a Finnigan Delta Plus XL mass Recycled from Cordilleran miogeocline (Dickinson and Midcontinent magmatism 1304 1532 spectrometer via a Conﬂ o III split interface. The Gehrels, 2003, 2009) Recycled from Cordilleran miogeocline (Dickinson and precision is better than ±0.1% (1σ). All the iso- Yavapai-Mazatzal province 1600 1800 Gehrels, 2003, 2009) tope values of carbonate and organic matter are Mainly from exposed Archean craton; also recycled through Cordilleran miogeocline (Frost et al., 2000; Kirkwood, reported relative to PeeDee belemnite (PDB), Archean Wyoming craton 2500 3200 2000; Dickinson and Gehrels, 2003; Frost and Fanning, and that of water relative to standard mean 2006; Dickinson and Gehrels, 2009) ocean water (VSMOW).

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δ13 A to –0.3‰. The C values of secondary veins B and ﬂ uvial cements vary between –9.3‰ and Organic-rich A horizon –5.8‰ and –7.0‰ and –3.2‰, respectively. The δ13C values of eight species of modern plants are –24.5‰ ± 1.0‰, consistent with the 2 mm values obtained from Holocene soil organic matter by Amundson et al. (1996).

Paleosol carbonate Evaluation of Diagenesis

200 mm Several lines of evidence argue that early Eocene paleosol carbonate nodules and fl uvial Figure 8. Field photographs and images of thin sections of the carbonate nodules in the early carbonate cements in the Wind River Basin are Eocene Wind River Basin: (A) paleosol in the Wind River Formation; (B) micritic paleosol not diagenetically reset, and therefore that the carbonate nodule with microsparite. isotopic results are primary and thus can be used to reconstruct paleoelevation and paleoclimate. 0 First, carbonate petrography study shows the Evaporation paleosol carbonates are dominantly micritic –2 Marine (Fig. 8). Recrystallization or cement infi lling is not observed in thin sections. Calcite cement –4 constitutes up to ~50% of the sandstone, consistent with the high initial porosity of weakly –6 compacted sands (Fig. 5). Such high percent- ages characterize early diagenetic calcite, since –8 early cementation occurs prior to signifi cant compaction (Quade and Roe, 1999, and refer- O (PDB) –10 18

δ ences therein; Sanyal et al., 2005). Second, Paleosols the δ18O values of a few sparry calcite veins in –12 Secondary vein Marine limestone and coral fossil paleosol nodules are lower than the values of Modern soil –14 Fluvial cement micrite in the paleosol nodules (Fig. 9). This Corrected paleosols suggests that the calcite veins probably formed

–16 Low soil respiration rate by fl uids at temperatures higher than at the Eo- cene soil surface. If paleosol nodule micrite –18 underwent diagenetic resetting during the for- –15 –10 –5 0 5 10 mation of secondary sparry veins, the isotopic 13 values of micrite would be similar to the values δ C (PDB) of secondary veins. Third, the δ18O and δ13C Figure 9. Results of the stable isotope analyses in this study. See values of recycled marine limestone clasts and text for an explanation of the correction of paleosol isotope values. fossil corals in conglomerates immediately ad- PDB—PeeDee belemnite. jacent to paleosols and sandstones are similar to those from unaltered Mesozoic–Paleozoic marine carbonate (Veizer et al., 1999) but sig- Results generally lower than those of paleosol nodules, nifi cantly higher than the values from paleosol ranging from –14.1‰ to –9.4‰. The δ18O val- and fl uvial carbonate cement. The δ18O and δ13C The δ18O values of all marine and nonmarine ues of modern soil carbonates vary between values of soil nodules, secondary veins, fl uvial carbonate in the Wind River Basin range from –10.3‰ and –6.3‰. cement, and marine limestone are distinct from –17.6% to –1.5‰ (Table DR4 [see footnote The δ13C values of all marine and nonmarine each other, and form clusters in the δ18O versus 1]; Fig. 9). Recycled marine limestone clasts carbonate in the Wind River Basin range from δ13C diagram (Fig. 9). Diagenesis would tend to and coral fossils have the highest δ18O values, –9.5‰ to +4.4‰ (Table DR4 [see footnote homogenize the isotopic values of marine lime- –4.9‰ to –1.5‰, whereas early Eocene fl uvial 1]; Fig. 9). Recycled marine limestone clasts stones, micrites, and secondary spars. The large cements have the lowest values, from –17.6‰ and coral fossils have the highest δ13C values, variation of fl uvial cement δ18O values may be to –11.5‰. The δ18O values of paleosols and of –4.9‰ to –1.5‰. Early Eocene paleosols due to contamination during sampling by ma- isotope values of fl uvial cements do not show have δ13C values between –9.5‰ and –5.6‰, rine limestone detritus, which have relatively systematic change through the section (Fig. 10). with the highest values from the middle and positive values (Figs. 9 and 10). Early Eocene paleosol micrites have δ18O val- lower Indian Meadows Formation (Fig. 10). ues between –9.8‰ and –8.5‰, display little These paleosol δ13C values are similar to the Oxygen Isotopes and Paleoaltimetry variability, and are similar to the δ18O values of values of early Eocene paleosols in the Big- early Eocene paleosol nodules in the Bighorn horn Basin (Koch et al., 1995). The δ13C val- The δ18O value of ancient local meteoric Basin (Koch et al., 1995). The δ18O values of ues of modern soil carbonates are generally water can be calculated from paleosol δ18O sec ondary veins in the paleosol nodules are higher than paleosols, and range from –4.5‰ values and constrained temperature. Pedogenic

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600 m

500

400

? Wind River Formation (53–51 Ma) 300

200

100

Feldspar grains

Indian Meadows Formation (55.5–54.5 Ma) Granite clasts Paleosol Paleosol 0 Archean zircons Fluvial cement Fluvial cement sand 0 1020304050–20 –15 –10 –5 –10 –8 –6 –4 –2 Percentage (%) δ18O (PDB) δ13C (PDB)

Figure 10. Simpliﬁ ed stratigraphy, carbon and oxygen isotope values of paleosol and ﬂ uvial cement, percentage of granite clasts, Archean zircons, and feldspar through section. Vertically ruled areas are depositional lacunae. PDB—PeeDee belemnite.

calcite is thought to form in isotopic equilib- lower seawater δ18O value in the early Eocene in the Laramide province likely received abun- rium with soil water during the warm season (Zachos et al., 1994). dant summer precipitation from the Gulf of Mex- (Quade et al., 1989; Cerling and Quade, 1993; The δ18O value of groundwater can be calcu- ico and small amounts of winter precipitation Quade et al., 2007; Breeker et al., 2009). The lated from fl uvial cement δ18O values and con- from the Arctic Ocean (Bryson and Hare, 1974; temperature of the warm season is usually be- strained temperature. Early diagenetic cements Dutton et al., 2005). Today, the elevation along a tween the mean annual temperature and maxi- in fl uvial sandstone formed in equilibrium with west-east transect through the Rocky Mountains, mum summer temperature. Soil water derives shallow groundwater (Quade and Roe, 1999; Laramide province, and Great Plains decreases from rainfall that may undergo evaporative Mack et al., 2000). The deposition temperature from 3–4 km to 0.5 km, which strongly infl u- enrichment in δ18O, especially in arid settings. of the early cements should be close to mean ences the δ18O value of modern precipitation and We assume the summer temperature in Wyo- annual temperature since it formed near the sur- the local isotopic lapse rate (Fig. 11). By early ming in early Eocene was ~30 °C based on an face (18.6 ± 2.4 °C). The calculated δ18O value Eocene time, crustal shortening in the Sevier had early middle Eocene temperature reconstruc- of groundwater is –16.6‰ ± 0.5‰ (VSMOW), caused signifi cant crustal thickening, and formed tion in France, which has a similar mean an- using the most negative δ18O value of fl uvial an elevated hinterland plateau to the west of the nual temperature as Wyoming (Andreasson cement (–17.6‰, PDB). This value (–16.6‰) Laramide province (DeCelles, 2004). Therefore and Schmitz, 1996). The δ18O values of early is comparable with the unweighted mean δ18O the paleotopography of Wyoming in early Eo- Eocene precipitation can then be estimated by value of modern precipitation in the south fl ank cene time was probably similar to present, with using soil nodule formation temperatures of of the Wind River Range (–15.2‰, Pinedale, the main vapor source from the ancestral Gulf of 18.6–30 °C and standard calcite fractionation Wyoming, elevation of 2.4 km) after adding 1‰ Mexico. This justifi es the comparison of the δ18O factors (Kim and O’Neil, 1997). The calculated for the lower early Eocene seawater δ18O value values of the early Eocene precipitation with the δ18O values of early Eocene precipitation in the (Zachos et al., 1994) (http://www.uaa.alaska.edu/ modern isotopic patterns. Bighorn and Wind River basins are –6.8‰ ± enri/usnip). The large difference between the Warmer global temperature in early Eocene 1.9‰ (VSMOW), which are consistent with the δ18O values of groundwater and local precipi- time could affect the relationship between the values derived from the δ18O values of mammal tation in the early Eocene Wind River Basin δ18O value of precipitation and local tempera- tooth enamel in the early Eocene Bighorn Basin suggests the groundwater was not sourced in lo- ture. Therefore, we compare the δ18O values of and Powder River Basin (Fricke, 2003) and the cal basin precipitation, but rather from the sur- early Eocene precipitation in the Wind River values derived from freshwater bivalve fossils rounding highlands. Basin with modern summer precipitation iso- in the lower Eocene in the Powder River Basin At present, rainfall in the high elevations of topic gradient in the region (Fig. 11). The δ18O (Fan and Dettman, 2009). For comparison with the Rocky Mountains is dominated by win- values of early Eocene precipitation (–5.8‰ ± modern water, 1‰ is added to the δ18O value ter precipitation from the Pacifi c Ocean and 1.9‰ after correction) in the Wind River and of early Eocene precipitation to account for the the Arctic Ocean. The low elevation regions Bighorn basins are similar to the values of the

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Eocene paleosol, Wind River Basin (55–51 Ma) CO2 owing to low soil respiration rates in the Eocene paleosol, Bighorn Basin (54–53 Ma) local semiarid climate (Fig. 9). δ13 4 Eocene fluvial cement, Wind River Basin (55–51 Ma) –20 The C values of early Eocene paleosols Modern summer mean precipitation (–7.0‰ ± 2.0‰) in the Wind River Basin are Modern unweighted mean annual precipitation –18 signiﬁ cantly lower than the values of modern soil carbonate (Fig. 9), which suggests a wetter 3 –16 climate during the Eocene. This is supported by several other lines of evidence: (1) calcisols are –14 well developed in the ﬂ oodplain of braided rivers in early Eocene Wind River Basin. The thick (>50 cm) reddish argillic horizons that typify (km)

–12 2 Wind River Formation paleosols point to a relatively moist setting; and (2) abundant leaf fossils –10 of mixed deciduous and evergreen plants in the O (SMOW) Wind River Formation show high mean annual 18 Elevation 1 –8 δ temperature up to 21 °C and paleoprecipitation up to 2000 mm/yr (Hickey and Hodges, 1975; –6 Wilf et al., 1998). However, the δ13C values of early Eocene –4 0 Rocky Mountains Laramide Province Great Plains paleo sols are higher than the values of soil carbonate formed in wet settings today, in the pres- –2 ence of pure C biomass. This can be explained 120 115 110 105 100 95 90 3 by the presence of C4 plants or higher pCO2 Longitude (°°W) during early Eocene time. There is no robust

evidence for the existence of C4 plants in early Figure 11. Oxygen isotope data of early Eocene paleosol carbonate in the Wind River Basin Eocene (Cerling and Quade, 1993). This justi- compared to the oxygen isotopic gradient of modern precipitation along an elevation tran- ﬁ es the use of δ13C values of paleosols to esti- sect of Rocky Mountains–Laramide province–the Great Plains. The light-gray curve is the mate ancient atmospheric pCO2 levels based on modern elevation proﬁ le along 43°N. Isotope data of Eocene paleosol carbonate in the Big- a diffusion-based model developed by Cerling horn Basin are from Koch et al. (1995). Latitudes of sampling site for modern precipitation (1992), under the assumption of elevated soil δ18 O values range between 40°N and 46°N. Modern precipitation isotope data are com- respiration rates expected in moist soil-forming piled from the data set of the United States Network for Isotopes in Precipitation (USNIP) conditions. δ13C values of soil carbonate are de- (www.uaa.alaska.edu/enri/usnip); Simpkins (1995); Harvey and Welker (2000); Harvey termined by the δ13C value of soil CO , which is (2001); Harvey (2005); Gammons et al. (2006). SMOW—standard mean ocean water. 2 a mixture of soil-respired CO2 and atmospheric δ13 CO2. Therefore, the C value of soil CO2 is a δ13 function of the concentration (pCO2) and C

modern summer precipitation at the same lati- elevation estimates derived in previous paleo- value of CO2 in the atmosphere, and the con- δ13 tude of the Great Plains (~–5.5‰, Mead, Ne- botanical studies for the early Eocene (Gregory centration and C value of soil-respired CO2. δ13 braska, elevation of 352 m) (Harvey, 2001), but and Chase, 1992; Wolfe et al., 1998). The C value of ancient soil CO2 can be calcu- signiﬁ cantly higher than the values of modern lated from the measured δ13C value of paleosols,

summer precipitation in the Laramide province Carbon Isotopes, Paleoclimate, and pCO2 using the temperature-dependent fractionation (–12.6‰, Butte, Montana, elevation of 1653 m) equation (Romanek et al., 1992). The concen- δ13 (Gammons et al., 2006), suggesting the paleo- The C value of soil carbonate is determined tration of soil-respired CO2 is a function of soil

elevation of the early Eocene Wind River Basin by the relative proportion of C3 to C4 plants in respiration rate and soil depth. Carbonate nod- and Bighorn Basin was low (~500 masl). The moist climate, and by the extent of local plant ules were collected at least ~50 cm below the low basinal elevation of the Wind River Basin cover in arid climate where soil respiration rates paleosurface (the top of the paleosol proﬁ le). In was stable in early Eocene because there is no are low (Quade et al., 1989; Cerling and Quade, our reconstruction we assume: (1) soil respira- systematic variation of the δ18O value of paleo- 1993; Breeker et al., 2009). Modern climate in tion rates ≥4 mmol/m2/hr (Cerling, 1992; Ekart δ13 sols through the stratigraphic section (Fig. 10). the Wind River Basin is semiarid, with the mean et al., 1999); (2) the C value of CO2 in the The difference (9.8‰ ± 2.0‰) between the δ18O annual precipitation of ~200 mm. Our analy- atmosphere is assumed to be near pre-industrial values of early Eocene river water and basinal ses show that the most common grasses and values of –6.5‰, and the values of plant-derived rainfall suggests that the elevation difference be- the dominant plant, sagebrush, in the sparsely CO2 to be –24.5‰; and (3) an exponential CO2 δ13 tween the surrounding Laramide ranges and ba- vegetated basin are C3, with the C values of production function (Quade et al., 2007). sin ﬂ oor was 3.4 ± 0.7 km, based on the isotopic –24.5‰ ± 1.0‰. Soil carbonate formed in equi- Using the average value of the three most δ13 lapse rate of –2.9‰/km derived from modern librium with CO2 respired from pure C3 plants positive C values of paleosols in the lower δ13 precipitation in the USA (Dutton et al., 2005). will have a C value of –12‰ to –10‰ (Cer- Indian Meadows Formation, atmospheric pCO2 Such an elevation difference is higher than the ling and Quade, 1993). The relatively high δ13C at ca. 55 Ma is estimated to have been 2050 ± modern difference of 1.5–2.5 km. The high values of modern soil carbonate in the Wind 450 ppmV (based on 1σ of the δ13C value of the estimated elevations of the Laramide ranges River Basin (–2.4‰ ± 2.1‰) clearly suggests paleo sol carbonate). Using the average value of δ13 (~4 km) from our study are consistent with the mixing of atmospheric CO2 with plant-derived the more negative C values in the upper Indian

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Washakie Owl Creek Cenozoic sedimentary rock Range Mountains Mesozoic siliciclastic rock Sevier A B C Sevier Wind River Sevier Sevier Basin Paleozoic carbonate rock NW W ind River Range Precambrian basement W ashakie Range Washakie Range W ashakie Range

Wind River Range ? Wind River Range Wind River Range ? ? Stre ? Debri am-dominated alluvial fans s-flow–domi B all raided rivers uv ia 1 km l fans nated

Figure 12. Paleogeographic sketch maps of the northwestern Wind River Basin in early Eocene: (A) 55.2–55.0 Ma; (B) 55.0–54.5 Ma; and (C) 51.0–53.0 Ma.

Meadows and Wind River formations, atmo- the north of the region was high during the early Wind River Range had been exposed by ca. 53 Ma spheric pCO2 from 54 to 51 Ma is estimated to Eocene (Fig. 12A), which is also supported by (Fig. 10). Low-sinuosity rivers that developed have been 900 ± 450 ppmV. The calculated high stable isotope paleoaltimetry (~4 km). upstream could have merged into a large east- pCO2 values and declining early Eocene trend, Deposition in the upper Indian Meadows fl owing meandering river in the basin (Soister, based on paleosols in central Wyo ming, are Formation was dominated by shallow braided 1968; Seeland, 1978). Therefore, a paleodrain- consistent with previous estimates of Ekart et al. rivers on alluvial fans (Fig. 12B). Southward age pattern similar to the present one was devel- (1999) using the same method, and by Pearson fl ow delivered late Paleozoic clasts from the oped by late early Eocene time in the northwest and Palmer (2000) using boron isotope ratios of Washakie Range into the basin, and the Pre- Wind River Basin. The local relief based on planktonic foraminifer shells. cambrian basement core in the Washakie Range oxy gen isotope ratios was 2.3 ± 0.8 km, which is was gradually exposed. The change from debris also comparable to modern relief in the region, REGIONAL PALEOGEOGRAPHY fl ows to braided rivers suggests the slope be- but with lower basin fl oor. tween the mountain range and basin fl oor de- Our reconstruction of the paleogeography creased (Stani street and McCarthy, 1993). Such IMPLICATIONS FOR TECTONICS in the northwestern Wind River Basin during a change of slope may have been caused by the the early Eocene is divided into three temporal rapid erosion of the mountain range and accu- Rapid Late Paleocene–Early Eocene Uplift stages constrained by the relative stratigraphic mulation of sediment in wet climate. of Basement-Involved Ranges position of particular lithofacies associations, Braided rivers fl owing east dominate the the paleoelevations of the surrounding ranges, sedimentary environment of the Wind River Both the Wind River and Washakie ranges the change in sedimentary provenance re- Formation (Fig. 12C). We argue that the change had an elevation contrast of 2.3 ± 0.8 km rela- corded by detrital zircon age spectra, and ab- of paleofl ow direction was caused by the devel- tive to the basin by late Paleocene–early Eo- solute ages based on fossil assemblages in the opment of a northeastward paleoslope along the cene time, and the basin fl oor stood at ~0.5 km basin (Fig. 12). Wind River Range during or after the deposition (asl). Rapid late Paleocene–early Eocene tec- The lower Indian Meadows Formation was of the Indian Meadows Formation, and before tonic uplift and growth of surface topography deposited unconformably on upper Cretaceous deposition of the Wind River Formation. Paleo- throughout western Wyoming are supported by Cody shale in the East Fork area. The deposi- slopes along both the Wind River and Washakie evidence for synorogenic sedimentation, refl ec- tional hiatus formed at least partly by folding ranges formed a confi ned valley during the tion seismic records, and thermochronology. in front of the Washakie Range (Winterfeld and early Eocene, and low-sinuosity rivers were Alluvial fan conglomerates of late Paleocene– Conard, 1983). Deposition evolved from small developed in the narrow basin when discharge early Eocene age, indicating intense unroof- braided rivers into debris fl ows on proximal and sediment yield were high. The dominant ing of Laramide source terranes, also occurred alluvial fans along the range front. Southward- northeastward paleocurrent direction and gran- along the east fl ank of the Beartooth Range, fl owing debris fl ows delivered mainly Meso- ite cobbles derived from the Wind River Range east of the Bighorn Range, north of the Uinta zoic and Paleozoic detritus. The gravitational suggest that the range became the major source Mountains, and on the south fl ank of the Wind force associated with steep slope is important terrane of sediment in the northwest part of the River Range (Gries, 1983; DeCelles et al., 1991; to the initiation of debris fl ows (Stanistreet and basin from ca. 53 to 51 Ma. The abundance of Crews and Ethridge, 1993; Hoy and Ridgway, McCarthy , 1993; Coussot and Meunier, 1996). basement granite clasts, feldspar, and Archean 1997). Seismic and borehole records from the Therefore, we infer that the Washakie Range to zircons suggests that the basement core of the south fl ank of the Wind River Range show that

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Precambrian rocks are in thrust contact above et al., 2010), (2) late Cenozoic isostatic rebound unroofi ng at 55.5–54.5 Ma, and the Wind River lower Eocene sedimentary rocks (Gries, 1983, of lithosphere due to climate-controlled or en- Range to the southwest became the dominant and references therein). Thermochronological hanced sediment erosion (Pelletier, 2009), and sediment source terrane by ca. 53–51 Ma. Rapid studies of the Beartooth Range and Wind River (3) late Cenozoic mantle thermal uplift caused tectonic uplift of the two mountains in the late Range also show that of the mountains exhumed by the thermal upwelling associated with the Paleocene–early Eocene formed a confi ned val- 4–8 km in late Paleocene–early Eocene time initiation of the Rio Grande Rift (Heller et al., ley, and a paleodrainage similar to the present (Omar et al., 1994; Peyton and Reiners, 2007). 2003; McMillan et al., 2006). with rivers fl owing from both ranges was formed This exhumation amount is signifi cantly higher Southward migration of post-Laramide mag- in the northwestern Wind River Basin by 51 Ma. than the thickness of Phanerozoic sedimentary matism in the northern Basin and Range is ar- The sedimentary environment evolved from al- cover on the Precambrian cores. gued as the evidence for southward removal of luvial fan into low-sinuosity, gravel-bed braided Uplift of the Wind River Range may have Farallon slab or mantle lithosphere (Humphreys , rivers, and paleocurrents changed from south- started in Late Cretaceous as a paleosol devel- 1995; Sonder and Jones, 1999). The resulting westward into mostly eastward. oped at the base of the Paleocene deposits on asthenospheric upwelling could have caused (2) Detrital zircon grains equivalent to the the east fl ank of the Wind River Range (Gries, regional uplift. Post-Laramide magmatism oc- depositional age are absent in early Eocene 1983). Uplift during Late Cretaceous would curred in northwestern Wyoming and Idaho at sediment. Recycled detrital zircon grains de- be consistent with data from numerous other ca. 50 Ma (Armstrong and Ward, 1993), which rived from the Grenville and Yavapai-Mazatzal Laramide uplifts and basins. For example, the predicts the timing of regional uplift in the orogens are present in Mesozoic and Paleozoic Hoback Basin to the west of the Washakie and Laramide province in Wyoming. However, sedimentary rocks exposed in the Laramide Wind River ranges contains thick uppermost there is no robust evidence suggesting high ba- ranges, and dominate the zircon populations of Cretaceous and Paleocene sediments with sinal ele vation before early Eocene (Wolfe et al., both modern river sand and early Eocene sand- abundant conglomerates (Love, 1973); and the 1998; Sjostrom et al., 2006; this study). There- stones in the Wind River Basin. Presently ex- upper Cretaceous–lower Paleocene Beaver- fore it is unlikely that the removal of Faral lon posed crystalline Archean basement in the Wind head Conglomerate and Maastrichtian Sphinx slab caused the post-early Eocene regional ele- River Range contributes ~20% of the total zir- Conglomerate in southwestern Montana indi- va tion gain in the Laramide foreland. con grains to basinal sediments. Zircons of dif- cate uplifting of the Blacktail-Snowcrest and Thermo-geochronological and sedimento- ferent ages from Precambrian basement rocks Madison-Gravelly arches, respectively (Haley, logical data suggest an episode of Miocene ex- exposed in Laramide ranges are a potentially 1986; DeCelles et al., 1987). It is not clear humation in the Laramide ranges, which could useful tracer of sedimentary provenance. how such evidence of Late Cretaceous tec- have caused regional surface uplift (Cerveny (3) A more humid climate in the early Eocene tonism of the Laramide ranges was expressed and Steidtmann, 1993; Omar et al., 1994; Wind River Basin is inferred from the high soil in terms of paleoelevations, but the abundance Strecker, 1996; Crowley et al., 2002; Peyton CO2 respiration rate compared to today. Calci- of coarse-grained alluvial fan deposits contain- and Reiners, 2007). A recent reconstruc- sols were extensively developed in the fl ood- ing proximal sediment-gravity fl ows, as well as tion of post-Laramide basin fi ll also shows plain deposits. Atmosphere pCO2 estimated provenance studies, indicate substantial topo- that more than 1 km of incision occurred in from paleosol δ13C values decrease from 2050 ± graphic relief (Graham et al., 1986; DeCelles the Laramide basins and the western Great 450 ppmV to 900 ± 450 ppmV during the early et al., 1991). Large-scale tectonic exhumation Plains since late Miocene time (McMillan Eocene, consistent with previous reconstruc- and unroofi ng of the Laramide ranges in western et al., 2006). Therefore, the Miocene uplift tions of pCO2 for this period. Wyoming during late Paleocene–early Eocene due to climate-enhanced erosion and/or mantle (4) The elevation of the Wind River Basin time could have been enhanced by the relatively dynamic uplift most likely formed the present was comparable with the modern Great Plains, humid paleoclimate. Studies in the Himalaya landscape in central Wyoming (Heller et al., on the order of ~500 m (asl), and the local relief and Washington Cascades have shown that high 2003; McMillan et al., 2006). However, more between the Washakie Range and western Owl precipitation is coupled with rapid erosion and paleoelevation data covering Cenozoic time Creek Mountains, Wind River Range, and Wind sediment denudation rate (Reiners et al., 2003; are required to evaluate the amount of regional River Basin was 2.3 ± 0.8 km during the early Thiede et al., 2005). uplift during the late Cenozoic. Our research Eocene. Post-Laramide regional uplift of ~1 km does not preclude dynamic uplift between is required to form the present landscape in Post-Early Eocene Regional Uplift early Eocene and late Miocene time. west-central Wyoming, which was most likely caused by regional Miocene–Pliocene uplift and Currently the Laramide basins in Wyoming CONCLUSIONS exhumation. have an average elevation of ~1.5 km, roughly ACKNOWLEDGMENTS 1 km higher than their elevations during the This multidisciplinary study of the lower early Eocene, based on oxygen isotope ratios of Eocene strata in the northwestern corner of the This research was supported by the Robert J. basinal precipitation recorded in paleosol nod- Wind River Basin improved our understand- Weimer Fund from (SEPM) Society for Sedimen- ules. Therefore, the Laramide basin fl oors in ing of the timing and process of basin evolu- tary Geology, a Donald L. Smith award from Rocky Mountain Section of the Society for Sedimentary Wyoming must have reached their present ele- tion and source terrane unroofi ng, changes in Geology, a ChevronTexaco scholarship, ExxonMobil vation after early Eocene time. paleoelevation of basin fl oor and surround- grant, and National Science Foundation grant EAR- Three mechanisms are proposed to explain ing Laramide uplifts, and paleoclimate during 0732436 (to the Arizona LaserChron Center). We the surface uplift of Laramide basins after early Laramide deformation. thank Tamara Goldin and Dan Ross for assistance in Eocene: (1) removal of Farallon slab or thick- (1) Petrographic, detrital zircon U-Pb geo- the fi eld, Joel Saylor and Facundo Fuentes for help with petrographic work, and Antoine Vernon for pick- ened mantle lithosphere in western USA in mid- chronology, and paleocurrent analyses show the ing zircons. Reviews and editorial comments by Eric dle Cenozoic time (Dickinson and Snyder, 1978; Washakie Range and western Owl Creek Moun- Erslev, Kenneth Ridgway, Robert H. Rainbird, and Humphreys, 1995; Sonder and Jones, 1999; Liu tains to the north of the basin experienced rapid Brendan Murphy improved both text and fi gures.

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