Paleogeographic isolation of the to Sevier hinterland, east-central Nevada: Insights from U-Pb and (U-Th)/He detrital zircon ages of hinterland strata

P. Druschke1,†, A.D. Hanson1, M.L. Wells1, G.E. Gehrels2, and D. Stockli3 1Department of Geoscience, University of Nevada, Las Vegas, Nevada 89154, USA 2Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA 3Department of , University of Kansas, Lawrence, Kansas 66045, USA

ABSTRACT cover. (ca. 135 Ma) cool- The and Paleogene Sevier ing ages are potentially coeval with shorten- hinterland of central Nevada, located west of The Late Cretaceous to Paleogene Sevier ing along the central Nevada fold-and-thrust the foreland fold-and-thrust belt, has previously hinterland of east-central Nevada is widely belt, although ca. 80 Ma cooling ages within been interpreted as a region of high-elevation regarded as an orogenic plateau that has the Sheep Pass Formation are coeval with and low topographic relief, characterized by since undergone topographic collapse. New hinterland midcrustal extension. Together, broad, open folding (Armstrong, 1968, 1972; U-Pb detrital zircon age data consisting of these new data provide support for previous Gans and Miller, 1983; Miller and Gans, 1989; 1296 analyses from the Lower Cretaceous interpretations that the Sevier hinterland DeCelles, 2004). However, Upper Cretaceous Newark Canyon Formation and the Upper represents an ancient high-elevation oro- to Lower Eocene strata of east-central Nevada Cretaceous to Eocene Sheep Pass Formation genic plateau, and that the latest Cretaceous and adjacent Utah are widely interpreted as ex- indicate that detrital zircon locally marks a transition from contraction tensional basin deposits (Winfrey, 1958, 1960; populations recycled from local Paleozoic to extension. Kellogg, 1964; Vandervoort and Schmitt, 1990; strata are dominant. Subordinate Fouch et al., 1991; Potter et al., 1995; Camilleri, zircon populations are derived mainly from INTRODUCTION 1996; Dubiel et al., 1996), and surface-breaking local backarc volcanic centers of Late Juras- normal faults of latest Cretaceous age have sic and Early Cretaceous age, while ca. 38– The Late Cretaceous to Paleogene hinter- been documented within the type section of the 36 Ma detrital zircon age peaks record the land of the Sevier retroarc fold-and-thrust belt Sheep Pass Formation (Druschke et al., 2009a). local onset of Eocene volcanism. Sevier is widely interpreted as an ancient orogenic Furthermore, the presence of megabreccia hinterland deposits of east-central Nevada plateau similar to the modern Andean Puna- and boulder-bearing conglomerates within the lack signifi cant Triassic, , and Altiplano (Coney and Harms, 1984; Jordan and Sheep Pass Formation (Kellogg, 1964; Vander- Late Cretaceous populations common in ter- Alonso, 1987; Allmendinger, 1992; Jones et al., voort and Schmitt, 1990; Druschke et al., 2009a, ranes of western Nevada and the Sierra Ne- 1998; House et al., 2001; DeCelles, 2004). 2009b) indicates areas of locally high relief. In vada magmatic arc. These data suggest that Outcrops of Early Cretaceous to Eocene sedi- our interpretation, Late Cretaceous to Eocene long-term evolution of the Sevier Plateau in- mentary strata scattered across east-central Ne- basins of the Sevier hinterland are analogous to volved geographic isolation through a combi- vada record a transition from Early Cretaceous modern extensional basin systems documented nation of high relief and rugged topography contraction (Allmendinger, 1992; Taylor et al., within the hinterlands of the modern Puna- related to Early Cretaceous shortening, and 2000; DeCelles, 2004) to Late Cretaceous– Altiplano and Tibetan Plateaus (Dalmayrac and continued isolation through development Paleogene extension within the Sevier hinter- Molnar, 1981; Molnar and Chen, 1983; All- of latest Cretaceous to Eocene internally land (Vandervoort and Schmitt, 1990; Camilleri, mendinger et al., 1997; Kapp et al., 2008). drained, extensional basins. 1996; Druschke et al., 2009a, 2009b). Although Previous studies of the Grouse Creek–Raft The (U-Th)/He zircon ages obtained from numerous studies within the Sevier foreland River–Albion, Ruby–East Humboldt, and the Sheep Pass Formation record late Paleo- fold-and-thrust belt and Snake Range metamorphic core complexes zoic, Early Cretaceous, and Late Creta- have established a pattern of sediment accu- (Fig. 1) have documented latest Cretaceous ceous cooling through 180 °C. Preservation mulation and evolving provenance in response and Paleogene midcrustal extension within of late Paleozoic (ca. 265 Ma) cooling ages to changes in the kinematics of the contrac- the Sevier hinter land. Peak Barrovian meta- indicates that much of the Upper Paleozoic tional wedge (e.g., Wiltschko and Dorr, 1983; morphism occurred within hinterland core section within east-central Nevada that con- Allmendinger, 1992; DeCelles, 1994, 2004; complexes during the Late Cretaceous ca. 100– tributed detritus to the Sheep Pass basin DeCelles and Currie, 1996; DeCelles et al., 75 Ma and is interpreted to represent maximum was un affected by deep thrust burial, or by 1995; Lawton et al., 1997; Horton et al., 2004), crustal thickening (Miller et al., 1988; Miller burial beneath thick Mesozoic sedimentary the tectonic and paleogeographic implications and Gans, 1989; Wells, 1997; Lewis et al., of coeval sedimentary deposits within the Sevier 1999; McGrew et al., 2000; Sullivan and Snoke, †E-mail: [email protected] hinterland are poorly understood. 2007; Wells and Hoisch, 2008). Following peak

GSA Bulletin; May/June 2011; v. 123; no. 5/6; p. 1141–1160; doi: 10.1130/B30029.1; 10 fi gures; Data Repository item 2011056.

For permission to copy, contact [email protected] 1141 © 2011 Geological Society of America Druschke et al.

Scale (km) EXPLANATION Figure 1. Map of the western 0200 400 area of predominantly ID cover U.S. Cordillera showing the 42°N OR GRA Cretaceous-Paleogene location of major Paleozoic to Klamath Sevier/Laramide Mesozoic tectonic elements. CA NV foreland basin Cretaceous to Paleogene Box corresponds to area of Sevier fold-thrust belt Figure 2. GA—the - Cretaceous to Paleogene Triassic/Jurassic Triassic Golconda allochthon, RH metamorphic core complex backarc strata GA RMA—the to Mis- Cretaceous sissippian Roberts Mountain batholith RMA allochthon, CNTB—the to Jurassic SR volcanic strata Cretaceous central Nevada Great Valley

CNTB Triassic to Jurassic fold-and-thrust belt, GRA— backarc basin strata the Grouse Creek–Raft River– Sevier fold/thrust belt Paleozoic to early Albion core complex, RH—the 37°N Mesozoic accreted terranes Ruby–East Humboldt core Sevier/Laramide foreland Permian-Triassic complex, SR—the Snake Range Pacific Ocean Golconda allochthon Devonian to core complex. Figure is modifi ed Coast Ranges Roberts Mountain from Smith and Gehrels (1994); allochthon Transcontinen Wyld (2002); Wells and Hoisch N Paleozoic carbonate ramp/shelf (2008); and Dickinson (2008). tal arch thrust fault, dashed 120°W 114°W where inferred or covered

metamorphism, an estimated 14 km of mid- of the Sheep Pass Formation and underlying shelf system to the east (Burchfi el et al., 1992; crustal extensional thinning occurred within the Mississippian strata is used to constrain the Poole et al., 1992; Dickinson, 2000). Grouse Creek–Raft River–Albion and Ruby– shallow crustal thermal history of east-central Subsequent slab rollback within the fring- East Humboldt core complexes beginning in the Nevada. Together, these data offer new insight ing arc system led to initiation of the Antler Late Cretaceous (ca. 75–67 Ma), based on Bar- into the tectonic and paleogeographic evolu- backarc fold-and-thrust belt in latest Devonian rovian metamorphic mineral assemblages and tion of the Sevier hinterland and provide an time (Burchfi el et al., 1992; Dickinson, 2000, thermochronometry (Wells et al., 1990; Hodges analog for long-term processes affecting mod- 2006). During the Antler orogeny, deep-marine and Walker, 1992; Camilleri and Chamberlain, ern orogenic hinterland regions. deposits of the Roberts Mountain allochthon, 1997; Wells et al., 1998; Harris et al., 2007; Wells derived from arc terranes to the west and sedi- and Hoisch, 2008). Although clear evidence for GEOLOGIC BACKGROUND ment shed from the Laurentian craton to the Late Cretaceous midcrustal extension has not east, were thrust up to 200 km eastward onto the been documented in the Snake Range core com- Pre-Mesozoic Framework adjacent carbonate shelf (Roberts et al., 1958; plex, a post–75 Ma lowering of metamorphic Speed and Sleep, 1982; Poole et al., 1992). A gradients and sparse U-Pb monazite ages may Pre-Mesozoic geotectonic terranes of the thick succession of latest Devonian to early indicate Late Cretaceous to early Paleo gene tec- strongly influenced Mississippian clastic sediments, derived from tonic unroofi ng (Lewis et al., 1999). By middle the provenance of Sevier hinterland deposits, the Roberts Mountain allochthon, was de- to late Eocene time (42–36 Ma), all three core given that reworked Paleozoic lithologies are posited in the Antler foreland basin (Roberts complexes experienced signifi cant extension a dominant constituent of Cretaceous to Paleo- et al., 1958; Speed and Sleep, 1982; Poole et al., followed by voluminous magmatism (Sullivan gene silici clastic deposits within east-central 1992; Miller et al., 1992). Intermittent backarc and Snoke, 2007). Nevada (Nolan et al., 1956; Winfrey, 1958, contraction following the initial Antler orogenic This study presents new U-Pb detrital zir- 1960; Fouch, 1979; Vandervoort, 1987). Ini- pulse controlled deposition of Middle Mississip- con geochronology from the Lower Creta- tiation of rifting along the western margin of pian to Upper Permian mixed clastic-carbonate ceous Newark Canyon Formation and the in the Late Precambrian resulted strata in eastern Nevada (Miller et al., 1992; Upper Cretaceous to Eocene Sheep Pass For- in the deposition of to Lower Trexler et al., 2004; Dickinson, 2000, 2006). mation of east-central Nevada (Fig. 2). Given siliciclastic strata derived from the Late Paleozoic backarc contraction culmi- confl icting tectonic and paleogeographic in- adjacent craton, followed by development of a nated in the Permian to Early Triassic Sonoma terpretations for the Cretaceous to Eocene carbonate-dominated passive margin that per- orogeny, during which Cambrian to Permian Sevier hinterland, these new data are used to sisted from mid-Cambrian to Devonian time in deep-marine and volcaniclastic backarc strata test long-term provenance patterns, to provide eastern Nevada (Stewart and Poole, 1974; Poole comprising the Golconda allochthon were thrust depositional age constraints for units previ- et al., 1992). Subduction initiated outboard of up to 50 km eastward over portions of the older ously lacking absolute age control, and to the western Laurentian margin during the Late Antler fold-and-thrust belt (Oldow, 1984; Miller test previous lithostratigraphic correlations to , and western Nevada et al., 1992; Dickinson, 2000, 2006). In total, of Sevier hinterland strata. In addition, new transitioned to a backarc basin setting bordered pre-Mesozoic tectonism resulted in the deposi- (U-Th)/He detrital zircon thermochronometry by island arcs to the west and a carbonate ramp/ tion of a 13–15-km-thick, mixed-sedimentary

1142 Geological Society of America Bulletin, May/June 2011 Detrital zircon provenance within the Sevier hinterland

116°W 115°W Nevada

Map area

Valley Newark Valley

Diamond Mountains

NW Steptoe

39.5°N KC Eureka Figure 2. General geologic map of east-central Nevada, modi- fi ed from Stewart and Carlson

(1977). NW—Newark Canyon ake Valley L type section of the Newark Can- yon Formation, DW—Duck- water Mountain section of the DW Ely Sheep Pass Formation, SP— EB

Sheep Pass Canyon type section White Pine Range of the Sheep Pass Formation, SC/LS

EB—Elderberry Canyon sec- eek Range tion of the Sheep Pass Forma- Creek Range ll Cr tion, SC/LS—Sawmill Canyon and Lowry Spring sections 39°N Sche of the Sheep Pass Formation, KC—Kinsey Canyon type sec- Egan Range tion of the Kinsey Canyon For- mation, MW—Murphy Wash

Snake Range section. SP MW

Pancake Range alley

River V Railroad Valley Scale (km)

N Grant Range White 01020

Quaternary Late Cretaceous to Eocene Mississippian to Permian strata town deposits Sheep Pass Formation (includes sparse Triassic strata)

Neogene sedimentary Early Cretaceous Newark Neoproterozoic to Devonian fault deposits Canyon Formation strata

Paleogene to Neogene Jurassic to Cretaceous sample locality volcanic strata plutonic rocks SP

succession of Neoproterozoic to Early Triassic fold-and-thrust belt in western Nevada, during a series of transtensional basins that received age, which presently dominates the geology of which time Triassic to Jurassic backarc basin sedi ment from the coeval arc to the west, as well east-central Nevada (Stewart and Poole, 1974; strata were thrust eastward over the Golconda as from highlands to the east dominated by Paleo- Poole et al., 1992; Miller et al., 1992). allochthon (Oldow, 1984; Wyld, 2001; Wyld, zoic strata (Martin et al., 2010). By Aptian time 2002; DeCelles, 2004). Contraction along the (ca. 125–112 Ma), the locus of retroarc contrac- Mesozoic to Cenozoic Tectonic Framework Luning-Fencemaker fold-and-thrust belt was tion had migrated eastward to the Sevier foreland followed by a period of widespread Middle to fold-and-thrust belt of western Utah (DeCelles Following the Sonoma orogeny, Middle to backarc volcanism (165–145 Ma) et al., 1995), while several hundred kilometers backarc extension and thermal (Smith et al., 1993; du Bray, 2007). to the west in the Sevier hinter land, the central subsidence resulted in the establishment of a A period of westward arc migration followed Nevada fold-and-thrust belt experienced co- backarc marine basin in western Nevada and in the earliest Cretaceous, potentially represent- eval contraction as recorded by deposition of the deposition of >6 km of Triassic to Early ing a lull in retroarc contraction (Armstrong and the Albian-Aptian Newark Canyon Formation Jurassic volcaniclastic marine strata (Speed, Ward, 1991; DeCelles, 2004; Dickinson, 2006). (Vandervoort and Schmitt, 1990; Allmendinger, 1978; Wyld, 2000). Closure of the backarc sea- In northwestern Nevada, the terrigenous King 1992; Taylor et al., 2000). Deformation along the way occurred following establishment of the Lear Formation was deposited during Barremian central Nevada fold-and-thrust belt had ceased Luning-Fencemaker retroarc time (ca. 123–125 Ma; Quinn et al., 1997) within by the mid- to Late Cretaceous, as documented

Geological Society of America Bulletin, May/June 2011 1143 Druschke et al. by the emplacement of undeformed plutons Sevier hinterland is limited to middle Eocene similarly composed of reworked local Permian (ca. 100–85 Ma) that cut earlier compressional folding of the White Sage Formation (Potter to Ordovician sedimentary units (Vandervoort, structures (Taylor et al., 2000). et al., 1995), although no distinction was made 1987). Paleocurrents within the Newark Can- The Late Cretaceous to early Paleogene in this study between folding associated with yon Formation indicate transport was primar- marks an increase in the rate of shortening shortening deformation and folding associated ily to the south and east (Vandervoort, 1987). across the foreland of the Sevier orogen as re- with crustal extension. The tectonic setting of the Newark Canyon corded by coeval shortening in the thin-skinned Eastward-propagating contraction and syn- Formation is interpreted as a series of piggy- fold-and-thrust belt, and -involved tectonic sedimentation along the Sevier foreland back basins due to the interbedding of coarse defor ma tion in the Laramide foreland to the east fold-and-thrust belt and basin system of central braided fl uvial and lacustrine deposits, and the (DeCelles, 2004). Onset of Laramide deforma- Utah continued into the early Eocene (DeCelles, presence of east-vergent folds (Vandervoort, tion coincided with a cessation of volcanism 1994, 2004; Lawton et al., 1997). However, by 1987; Vandervoort and Schmitt, 1990). In the within the Sierra Nevada magmatic arc and the middle Eocene (ca. 49 Ma), contraction Fish Creek Range, the Newark Canyon Forma- rapid eastward migration of the magmatic front, within the Sevier foreland had ceased, as re- tion is overlain in part by megabreccia com- brought on by shallowing dip of the subducting corded by a change from shortening to exten- posed of Upper Paleozoic lithologies, and by Farallon plate (Dickinson and Snyder, 1978). sion in the fold-and-thrust belt locally over a lacustrine containing Maastrichtian Late Cretaceous (U-Th)/He apatite cooling ages time interval as short as 1–2 m.y. (Constenius, to fossils that suggest correlation to and paleodrainage profi les preserved in deeply 1996). Asymmetrical foundering of the Farallon the Sheep Pass Formation type section (Fouch incised canyons of the Sierra Nevada have been slab initiated southward-propagating extension et al., 1979; Vandervoort and Schmitt, 1990). interpreted as antecedent river systems similar and magmatism within the Sevier hinterland to modern western Andean drainages, suggest- of the Pacifi c Northwest in the early Eocene Sheep Pass Formation ing that a ≥3-km-high plateau lay to the east (Humphreys, 1995), which was partially co- The Sheep Pass Formation forms a series (House et al., 2001). Similar paleoelevation eval with westward extensional collapse of the of isolated exposures located throughout east- estimates of 3–5 km for the Late Cretaceous to Sevier fold-and-thrust belt (Constenius, 1996; central Nevada, over an area of >15,000 km2 Paleogene Sevier hinterland have been based DeCelles, 2004). Southward-progressing, syn- (Fouch et al., 1991) (Fig. 2). The Sheep Pass on comparison to the modern Puna-Altiplano extensional magmatism is documented in Formation was originally designated to describe and Tibetan Plateaus (Coney and Harms, 1984; northeastern Nevada beginning in the middle sections of nontuffaceous fl uvial, alluvial, and Dilek and Moores, 1999; DeCelles, 2004). Eocene ca. 43–41 Ma (Armstrong and Ward, lacustrine strata located in the Pancake, Grant, Throughout much of the Sevier hinterland, the 1991; Brooks et al., 1995), and it subsequently and Egan Ranges. The type section is at Sheep Late Cretaceous (ca. 80–75 Ma) marks a period affected east-central Nevada beginning in the Pass Canyon in the southern Egan Range (Win- of widespread intrusion of peraluminous gra- late Eocene ca. 38–35 Ma (Gans et al., 1989; frey, 1958, 1960) (Fig. 3). Six members (A–F) nitic plutons at midcrustal levels (Miller and Axen et al., 1993; Gans et al., 2001; Druschke are recognized within the Sheep Pass Formation Bradfi sh, 1980; Lee et al., 1986; du Bray, 2007), et al., 2009b). type section (Winfrey, 1958, 1960; Fouch, 1979). which overlap temporally and spatially with Sparse Maastrichtian detrital zircons (68– midcrustal extension in the Raft River–Grouse Stratigraphy of the Cretaceous–Eocene 70 Ma) analyzed within the basal conglomeratic Creek–Albion and Ruby–East Humboldt core Hinterland member (A), and a 66.1 ± 5.4 Ma U-Pb carbonate complexes (Wells et al., 1990; Hodges and age from the lower fossiliferous lacustrine lime- Walker; 1992; Camilleri and Chamberlain, Newark Canyon Formation stone member (B) indicate a Maastrichtian age 1997; Harris et al., 2007; Wells and Hoisch, The Newark Canyon Formation crops out for basal members of the Sheep Pass Formation 2008). Midcrustal extension has been attrib- as scattered exposures of fl uvial and lacustrine in the type section (Druschke et al., 2009a). A uted to gravitational spreading of overthickened deposits that extend from the Piñon Range and Maastrichtian to late Paleocene age (70–55 Ma) Sevier hinterland crust toward the low-elevation Cortez Mountains of north-central Nevada to the had been previously assigned to members B–C foreland (Hodges and Walker, 1992; Jones et al., Diamond Mountains, Fish Creek, and Pancake in the type section on the basis of biostratig- 1998; DeCelles, 2004), or lithospheric delami- Ranges of east-central Nevada (Fig. 2). Within raphy, whereas fossils within Member E of nation and uplift coupled with thermal weaken- the Diamond Mountains type section, the New- the type section indicate a middle Eocene age ing of the middle crust during the transition to ark Canyon Formation is ~500 m thick and is (Bridgerian, 50.5–45.4 Ma) (Fouch, 1979; Good, fl at-slab subduction (Wells and Hoisch, 2008). composed of alternating beds of conglomer- 1987; Swain, 1999). No major unconformities Thermochronometry of midcrustal rocks in ate, sandstone, siltstone, and limestone (Nolan have been documented within the Sheep Pass the Ruby–East Humboldt Range suggests early et al., 1956) (Fig. 3). An Early Cretaceous age Formation type section (Winfrey, 1958, 1960; Paleogene cooling from 63 to 49 Ma (Paleocene (Aptian-Albian ca. 122–112 Ma) is assigned on Kellogg, 1964; Fouch, 1979), although the sharp to middle Eocene) (McGrew and Snee, 1994; the basis of (Nolan et al., 1956; transition from Upper Paleocene Member C to McGrew et al., 2000), and from 57 to 46 Ma Smith and Ketner, 1976; Swain, 1999). Middle Eocene(?) Member D may represent a (Paleocene to middle Eocene) within the north- Within east-central Nevada, the Newark signifi cant hiatus (Druschke, 2008). The Sheep ern Snake Range (Lee and Sutter, 1991; Lewis Canyon Formation is unconformable atop Up- Pass Formation is unconformably overlain by et al., 1999). To the northeast of the Snake per Paleozoic strata, although in north-central the Garrett Ranch Group, a >500-m-thick suc- Range core complex, the Paleocene(?) to Lower Nevada, it lies in part on Upper Jurassic vol- cession of Upper Eocene to volcanic Eocene White Sage Formation of west-central canic strata (Smith and Ketner, 1976). Modal tuff, welded tuff, basalt and andesite fl ows, and Utah is interpreted to have been deposited analyses of sandstone within the Newark volcaniclastic sediment (Winfrey, 1958; Hose within an extensional basin setting (Potter et al., Canyon Formation indicate a recycled orogen et al., 1976; Druschke et al., 2009b). 1995; Dubiel et al., 1996). Direct evidence for provenance with no identifi able arc-sourced The Sheep Pass Formation unconformably post–Late Cretaceous contraction within the detritus; conglomerate clast populations are overlies sedimentary strata of Devonian to

1144 Geological Society of America Bulletin, May/June 2011 Detrital zircon provenance within the Sevier hinterland

Newark Canyon Sheep Pass Canyon Duckwater Mountain Height in section (m) Paleogene? h

megabreccia c

U not measured n 500 06NW1 Stone Cabin a

06SP22 Fm.? R 1200 t 200 t

07NW2 U p e 400 n u r s o r Stinking Stone Cabin Fm. o u 06SP21 y a r o

Spring late Eocene

n 100

e 1100 G G 05DW1 a c Conglomerate n 300 late Eocene a C t o i e U t r k 0 r

a 1000

C Member F a y m 06MR19 E Base l w

200 r r e a not exposed o E Sheep Pass Fm. D N F 900 r

100 e grain size b mssgrcb Eocene m

800 e early to middle 0 U M U? Permian Carbon 700 Kinsey Canyon

Ridge Formation 05SP18 C ? o r e o n b 05SP14 z 600 o m a ? i t e m grain size a M

mssgrcb a m l r 500 200 a o e Tuff K F n e s B c . s o 400 r e e a l

100 m b a P

Sawmill Canyon/ F late Eocene

P 05KC01 m p e n y 300 e o Lowry Spring M 0 e e

? y h Base s n S n s i not exposed a

n 05SP20 K C

e 200 v A grain size 06SP29 r O

e mssgrcb l b

a 100 m o Late

mega- e c M r f not breccia Cretaceous

f Explanation a late Eocene 0 u measured h U T

C 06SP30 tuff and U Mississippian Scotty Wash Sandstone tuffaceous sandstone 400 n and Chainman Shale sediments o i t grain size limestone/

a clay-rich e dolomitic t mssgrcb a m limestone l

300 e

r limestone n o t o e c e F l o carbonaceous silty or tuffaceous

d Murphy Wash E s

200 d i siltstone sandstone s m a

05SM2 P conglomerate covered interval 100 p fault e

04LC4 e

h 100 07SR1 Needles Range Group 0 S U paleosol U 05SP18 sample location Permian 0 and number Oligocene Arcturus Formation Base not exposed U unconformity

grain size grain size fault in section mssgrcb mssgrcb unnamed conglomerate Figure 3. Stratigraphic columns for Cretaceous to Eocene hinterland deposits within east-central Nevada, including the Newark Canyon Formation type section (after Nolan et al., 1956; Vandervoort, 1987), Sheep Pass Formation type section (after Winfrey, 1958, 1960; Fouch, 1979), and the Sawmill Canyon section of the “tuffaceous” Sheep Pass Formation. Additional age control for the Sheep Pass Formation type section is from Good (1987) and Druschke et al. (2009a, 2009b). Grain size abbreviations: m—mud; ss—sandstone; gr—gravel; cb—cobble.

Permian age throughout much of its outcrop Fouch, 1979). However, sparse clasts of the the presence of megabreccia (Kellogg, 1964). area. Within the type section, the Sheep Pass Ordovician Eureka Quartzite have been docu- Megabreccia and lacustrine limestone deposits Formation overlies mixed siliciclastic-carbonate mented in Member A of the Sheep Pass Forma- of Maastrichtian to Paleocene age also crop out units of Mississippian to age tion type section (Druschke, 2008). in the Fish Creek Range (Fig. 2). Vandervoort (Kellogg, 1963, 1964). Conglomerate clast An extensional half-graben basin setting has and Schmitt (1990) interpreted these deposits populations are dominated by Upper Paleozoic been hypothesized for the Sheep Pass Formation as representing extensional basin fi ll follow- carbonate lithologies, and it has been reported based on the dominance of lacustrine strata and ing a transition from contraction to extension. that no clasts older than Devonian age are dis- general westward thinning of the Sheep Pass More recently, megabreccia deposition and slip cernible (Winfrey, 1958, 1960; Kellogg, 1964; Formation (Winfrey, 1958, 1960), as well as on on a series of surface-breaking, west-dipping,

Geological Society of America Bulletin, May/June 2011 1145 Druschke et al. syndepositional normal faults within the Sheep Although the Sheep Pass Formation, as origi- have been identifi ed and are interpreted to have Pass Formation type section have been shown to nally defi ned by Winfrey (1958, 1960), lacks an been derived from the Snake Range to the east be Maastrichtian in age (Druschke et al., 2009a). obvious volcaniclastic component, the defi ni- (Drewes, 1967; Gans et al., 1989). Paleocur- Deposition of the Sheep Pass Formation within tion was later expanded to include lacustrine rent measurements from Eocene conglomer- the type section is interpreted to have been con- and fl uvial deposits within the central Egan ates within the Schell Creek and central Egan trolled by up to 3 km of latest Cretaceous to Range that are in part tuffaceous (Brokaw, Ranges reveal a dominantly westward transport Paleocene, down-to-the-northwest stratigraphic 1967; Hose et al., 1976). Elderberry Canyon, direction, supporting derivation from the east throw along the Ninemile fault, which is pres- located in the central Egan Range immediately (Druschke et al., 2009b). The presence of Pros- ently a low-angle normal fault exposed in the south of Ely, Nevada (Fig. 2), contains ~120 m pect Mountain Quartzite and Jurassic granitoid southern Egan Range to the southeast of the of conglomerate and lacustrine limestone that clasts suggests up to 7 km of unroofi ng within Sheep Pass Formation type section (Druschke thicken to the south. This interval is non vol- the Snake Range by the late Eocene (38–35 Ma) et al., 2009a). Paleocurrent measurements and cani clastic in its lower part, but it grades up- (Gans et al., 1989). Middle Eocene lacustrine depositional environment interpretations indi- ward into increasingly tuffaceous lacustrine limestone of the Sheep Pass Formation immedi- cate that Sheep Pass Formation conglomerates deposits (Fouch, 1979). A mammalian fossil ately east of Ely, Nevada (Good, 1987), is over- and sandstones were deposited in alluvial fans assemblage in the lower, non vol cani clastic lain by a series of synextensional tuff units that having sediment sources to the east (Druschke portion of the Elderberry Canyon section es- bracket a period of rapid late Eocene extension et al., 2009b). tablishes a middle Eocene depositional age between 37.56 ± 0.03 Ma and 36.68 ± 0.04 Ma (Bridgerian 50.5–45.4 Ma), indicating a po- (40Ar/39Ar sanidine) (Gans et al., 2001). Eocene Volcanism and “Tuffaceous” tential age overlap with the upper members of Sheep Pass Formation the Sheep Pass Formation in the type section METHODS Throughout much of its outcrop area, the (Fouch, 1979; Good, 1987; Emry, 1990). Based Sheep Pass Formation is unconformably over- on the similarity of depositional facies and pos- U-Pb Zircon Geochronology lain by ash-fl ow tuff units that mark the local on- sible age overlap, Fouch (1979) correlated the set of late Eocene volcanism (Fouch, 1979). In Elderberry Canyon section to the Sheep Pass In total, 15 samples were selected from fi ve the southern Egan Range, the Sheep Pass Forma- Formation as “type 2” (tuffaceous), although stratigraphic sections of Early Cretaceous to late tion is unconformably overlain by >500 m of the tuffaceous inter beds were not dated. Eocene age within the Sevier hinterland of east- volcanic Garrett Ranch Group, with 10° of an- Fouch (1979) also expanded the defi nition of central Nevada (Fig. 3), and we report here 1296 gular discordance between these units (Kellogg, “tuffaceous” Sheep Pass Formation deposits to U-Pb detrital zircon age analyses. One sample 1964). In Sheep Pass Canyon, the basal member include conglomerate and lacustrine limestone was obtained from the Mississippian Scotty of the Garrett Ranch Group is a >150-m-thick of the Kinsey Canyon Formation of Young Wash Sandstone in Sheep Pass Canyon, which conglomerate, designated the Stinking Spring (1960) that are exposed in the central Schell is overlain by the Sheep Pass Formation type Conglomerate (Kellogg, 1964). The Stinking Creek Range, fl uvial conglomerate exposed in section. Clasts of the Scotty Wash Sandstone Spring Conglomerate is in turn overlain by an Murphy Wash of the southern Snake Range, and are a common constituent of conglomerates in ash-fl ow tuff unit that has produced a 40Ar/39Ar scattered exposures of tuffaceous lacustrine and the Sheep Pass Formation. Two to 5 kg of mate- sanidine age of 35.43 ± 0.11 Ma (Druschke fl uvial strata in the Grant and Pancake Ranges rial were collected per sample, depending on the et al., 2009b). Paleocurrent measurements in- (Fig. 2). Each of these sections overlies Upper textural and compositional maturity of the sand- dicate that the Stinking Spring Conglomerate Paleozoic strata, and they are in turn overlain stone, which ranged from well-sorted quartz formed a series of west-draining alluvial fans by late Eocene to Oligocene tuff units. In the arenite to poorly sorted litharenite. similar to the underlying Sheep Pass Formation Grant and Pancake Ranges, tuffaceous Sheep Zircon separates were processed by crushing (Druschke et al., 2009b); potentially in response Pass Formation strata are overlain by welded and Wifl ey table gravity separation, followed to Eocene reactivation of the Ninemile fault. tuff of the 35.3 ± 0.8 Ma (40Ar/39Ar sanidine) by standard heavy liquid and magnetic separa- The Stinking Spring Conglomerate is domi- Stone Cabin Formation (Radke, 1992), and in tion. Zircon grains ranged from 60 to 300 µm nantly a carbonate-clast conglomerate similar to the Schell Creek Range, the Kinsey Canyon (c-axis) in length, although 100 µm was typical the basal Member A of the Sheep Pass Forma- section is overlain by the 35.39 ± 0.08 Ma (Fig. 4A). For each sample, a large fraction of the tion, but it contains a more diverse clast popu- (40Ar/39Ar sanidine) Kalamazoo Tuff (Druschke recovered zircons was mounted in epoxy resin lation. Clasts of local Ordovician to Devonian et al., 2009b). In Elderberry and Sawmill Can- and polished. Typically 100 zircons were ana- formations are relatively abundant within the yons of the central Egan Range, the Sheep Pass lyzed per sample, with the beam centered on the Stinking Spring Conglomerate, as are clasts de- Formation is overlain by the 36.17 ± 0.08 Ma core of grains to avoid metamorphic overgrowth rived from the underlying Sheep Pass Formation (40Ar/39Ar sanidine) Charcoal Ovens Tuff or alteration. Fractured grains were generally (Kellogg, 1964). Clasts are dominantly cobble- (Druschke et al., 2009b). avoided due to possible Pb loss from leaching or sized, though boulders up to 2 m in diameter Conglomerate clast populations within tuffa- alteration along fractures; cathodoluminescence are present (Druschke, 2008). Approximately ceous Sheep Pass Formation sections are domi- (CL) images of the sample mounts were cre- 20 km to the south of Sheep Pass Canyon, the nated by Upper Paleozoic lithologies, but they ated to evaluate potential fractures and inherited basal portion of the Garrett Ranch Group con- contain a greater abundance of clasts derived cores for each grain analyzed. In general, 10%– sists of megabreccia and block-slide deposits from Lower Paleozoic units as compared to the 15% of the analyses per sample displayed large derived from Pennsylvanian limestone and Mis- Sheep Pass Formation type section (Druschke, uncertainty and/or unacceptable discordance, sissippian sandstone (Kellogg, 1964). These 2008). Within Eocene conglomeratic sections and these results were discarded. Analyses in megabreccia deposits are overlain by an ash- of the Schell Creek Range, Neoproterozoic to which 206Pb/238U or 206Pb/207Pb changed sig- fl ow tuff that has produced a 40Ar/39Ar sanidine Early Cambrian Prospect Mountain Quartz- nifi cantly during data acquisition were also dis- age of 35.52 ± 0.08 Ma (Druschke et al., 2009b). ite and distinctive Late Jurassic granitic clasts carded, because these changes generally result

1146 Geological Society of America Bulletin, May/June 2011 Detrital zircon provenance within the Sevier hinterland

227.59 µm 246.2 98.95 µm µm 85.21 µm

A 100 µm BC100 µm 100 µm

Figure 4. (A) A plane light image of detrital zircon separates from sample 06SP29, illustrating the dominant well-abraded zircon population and subordinate euhedral to subhedral zircon population typical of the Sheep Pass Formation type sec- tion. (B) A plane light image of an abraded, subrounded grain (from 06SP29). Grains displaying morphology similar to that in B generally reveal Precambrian crystallization ages. (C) Plane light image of a typical euhedral grain (from 06SP29); euhedral grains within the Sheep Pass Formation type section typically reveal Mesozoic crystallization ages. Grains B and C were sampled for (U-Th)/He dating.

from ablation across an age boundary, fracture, generally 1.0%–1.5% at 2σ level. For samples Zircons were handpicked and selected based or inclusion. containing signifi cant population clusters of on minimum dimensions of 70 µm across a/b Analyses were performed at the University of euhedral, potentially tuff-sourced zircons, the axes, between 80 and 200 µm along the c axis, Arizona LaserChron Center with a Micromass TuffZirc (Ludwig and Mundil, 2002) age ex- and on the lack of visible fractures and minimal Isoprobe multicollector–inductively coupled tractor program was used to evaluate the prob- inclusions. Zircons were also selected based on plasma–mass spectrometer (ICP-MS) equipped ability of a single source for age clusters within two general morphologies: subrounded zircons with a New Wave DUV 193 nm Excimer laser 90% statistical confi dence. Asymmetrical that typically reveal Precambrian crystallization ablation system. Laser beam diameter was TuffZirc errors are reported as standard uncer- ages (Fig. 4B), and euhedral to subhedral zir- 35 µm with an output energy of 32 mJ (at 22 kV) tainty following conversion after the method of cons that typically reveal Mesozoic U-Pb crys- and a pulse of 8 Hz. An in-house zircon standard Ludwig and Mundil (2002). TuffZirc ages rep- tallization ages (Fig. 4C). Roughly two thirds of with a concordant thermal ionization mass spec- resent a reworked volcaniclastic/epiclastic com- the zircons selected were subrounded. All analy- trometry (TIMS) age of 563.5 ± 3.2 Ma (Gehrels ponent within mixed-sourced sedimentary strata ses were carried out on single grains. et al., 2008) was analyzed once after every fi ve and indicate the maximum age of deposition. All (U-Th)/He age determinations were unknowns. In addition, U and Th concentrations Additional data tables used for the construction carried out at the University of Kansas using were monitored by analyzing the National Insti- of concordia diagrams, probability plots, and labora tory procedures described in Biswas et al. tute of Standards 610 glass standard. TuffZirc ages are presented in the GSA Data (2007). Selected zircons were wrapped in Pt Age probability plots in this study were con- Repository (item DR1).1 foil, heated for 10 min at ~1300 °C, and reheated structed using the 206Pb/238U age for zircons until >99% of the He was extracted. All ages younger than 1 Ga, and the 206Pb/207Pb age for (U-Th)/He Zircon Thermochronology were calculated using standard α-ejection cor- grains older than 1 Ga, with cutoffs adjusted for rections using morphometric analyses (Farley each sample so as not to split clusters near 1 Ga. Zircon is a commonly used (U-Th)/He ther- et al., 1996; Reiners, 2005). After laser heating, For Paleoproterozoic and grains, ages mochronometer that is characterized by a He zircons were unwrapped from Pt foil and dis- with >20% discordance or >5% reverse dis- closure temperature of ~180–200 °C, assum- solved using HF-HNO3 and HCl pressure vessel cordance were considered unreliable and were ing a cooling rate of 10 °C/m.y. (e.g., Reiners digestion procedures. U and Th concentrations discarded. Because discordance is not a reli- et al., 2002) and a partial retention zone span- were determined by isotope dilution ICP-MS able indicator of the robustness of young ages ning a temperature range from ~120 to 180 °C analysis. Uncertainties (2σ) of single-grain ages due to the large uncertainty in measurement of (e.g., Stockli, 2005; Wolfe and Stockli, 2008). refl ect the reproducibility of replicate analyses 207Pb, we instead used clustering as a measure For this study, 52 zircons from the Sheep Pass of laboratory standard samples (Farley et al., of the signifi cance of young ages. In general, a Formation, Scotty Wash Sandstone, and Stink- 2001) and are ~8% (2σ) for zircon He ages. All population is considered statistically robust if ing Spring Conglomerate were selected for single-grain zircon (U-Th)/He data tables are three or more different grains within one sample (U-Th)/He dating. Zircons were selected from presented in the GSA Data Repository (item yield overlapping 206Pb/238U or 206Pb/207Pb ages. the remaining unmounted fractions of samples DR2; see footnote 1). This is because most sources of complexity in that had previously undergone U-Pb dating but zircons (e.g., Pb loss, inheritance, metamorphic were not performed on zircons that had been RESULTS recystallization, etc.) will scatter analyses from previously dated. their true 206Pb/238U age. U-Pb Detrital Zircon Geochronology Individual ages are reported at 1σ level, 1GSA Data Repository item 2011056, U-Pb geochronologic analyses and Sheep Pass detrital whereas ages that are calculated from clusters zircon (U-Th)/He data, is available at http:// Results of the U-Pb detrital zircon dat- of grains are reported at 2σ level. Age clusters www.geosociety.org/pubs/ft2011.htm or by request ing yielded crystallization ages ranging from reported include all systematic errors, which are to [email protected]. Archean to Late Cretaceous, with a signifi cant

Geological Society of America Bulletin, May/June 2011 1147 Druschke et al. component of Eocene zircons from samples Sheep Pass Formation Type Section Stinking Spring Conglomerate collected from the “tuffaceous” Sheep Pass In total, 5 samples were collected from Two samples were obtained from the middle Formation at several localities. The detrital zir- the Maastrichtian to middle Eocene Sheep (06SP21) and uppermost (06SP22) portions of con age distribution for each sample and sig- Pass Formation type section in the southern the Stinking Spring Conglomerate within Sheep nifi cant age peak determinations are displayed Egan Range (Fig. 2), including two samples Pass Canyon. Similar to Sheep Pass Formation as a series of probability density plots (Fig. 5). (06SP29 and 06SP20) from the middle and Member A, samples were not collected at the upper portions of conglomeratic Member A, base of the Stinking Spring Conglomerate due Scotty Wash Sandstone two samples (05SP14 and 05SP18) from the to a lack of sandy matrix within the conglomer- One sample was collected from the Missis- respective lower and middle portions of the ate and a lack of sandstone lenses or interbeds. sippian Scotty Wash Sandstone in Sheep Pass fl uvial sandstone–dominated Member C, and Sample 06SP21 was obtained from a poorly Canyon, directly below the basal contact with one sample (06MR19) from sandy interbeds sorted, dominantly medium-grained sandstone the Sheep Pass Formation (Fig. 3). This sample within Member E (Fig. 3). Detrital zircon sep- lens. Sample 06SP22 was collected from near was analyzed to provide direct comparison with arates reveal a population dominated through- the top of the member from a thick, medium- detrital zircon age distributions within the Sheep out the Sheep Pass Formation type section by grained sandstone bed containing detrital biotite Pass Formation type section, due to the prevalence yellow to dull-white, abraded, rounded to sub- and sanidine indicative of a tuffaceous compo- of Scotty Wash Sandstone clasts in conglomer- rounded zircons. A smaller population of clear, nent. Zircon separates from 06SP21 consisted ates of the Sheep Pass Formation. Within Sheep blocky to prismatic, euhedral to subhedral zir- of largely pale-yellow to dull-white, rounded Pass Canyon, the Scotty Wash Sandstone consists cons is also discernible, but it constitutes only to subrounded zircons, with a small component of a well-sorted, ripple-marked and cross-strati- ~10%–15% of the zircon population. Euhedral of clear, elongate, euhedral zircons. Separates fi ed quartz arenite deposited in a shallow-marine to subhedral zircons are most abundant within from 06SP22 were similar, but euhedral zircons setting during the Mississippian to early Pennsyl- Member A, generally decreasing in abundance comprised roughly 50% of the population. Re- vanian (Kellogg, 1963). Detrital zircon separates up section. sults from these samples were combined into a from the Scotty Wash Sandstone are dominantly Samples from Member A were collected single age probability plot (Fig. 5). The domi- pale-yellow to dull-white and subrounded to from coarse- to medium-grained, poorly sorted nant age populations within the Stinking Spring well rounded. A small percentage of blocky, sub- litharenites within sandstone lenses of the pre- Conglomerate are 1.65 Ga, 1.15 Ga, 1.1 Ga, hedral grains was observed. Results of U-Pb age dominantly conglomeratic member. The low- 422 Ma, and 37.5 Ma. A TuffZirc age extraction analyses indicate dominant age peaks at 1.82 Ga, ermost interval of Member A was not sampled of Eocene zircons within sample 06SP22 indi- 1.49 Ga, and 1.11 Ga. Smaller peaks occur at due to a lack of channel sands or sandy matrix cates a single tuff source with an eruptive age of 2.53 Ga, 1.65 Ga, and 426 Ma (Fig. 5). within the debris-fl ow–dominated base of the 37.7 ± 0.6 Ma (Fig. 6B). section. Results from Member A reveal zircon Newark Canyon Formation crystallization ages ranging from Archean to “Tuffaceous” Sheep Pass Formation Two samples were collected from the Newark Late Cretaceous. Although the two youngest In total, 5 samples were collected from widely Canyon Formation type section, from the Upper zircons (two single analyses of 68 and 70 Ma) separated sections of tuffaceous lacustrine and Conglomerate Member and the Upper Carbona- were obtained from the uppermost portion fl uvial strata previously correlated to the Sheep ceous Member (Fig. 3). Sample 07NW2 from of Member A (06SP20) (Druschke et al., Pass Formation (Fouch, 1979). Sample 05DW1 the Upper Conglomerate Member yielded a mix 2009a), overall similarity of the age peaks was collected north of Duckwater Mountain in of pale-yellow to clear subrounded to rounded and physical proximity of the samples allow the northern Pancake Range (Fig. 2). The lower zircons, with a nearly equal proportion of clear, for the combination of the two analyses into portion of this section consists of coarse fanglom- prismatic, euhedral zircons. Results of U-Pb a single probability plot (Fig. 5). Major age erate containing boulders of Devonian and Mis- isotopic analyses yield zircons ranging from peaks for Member A include: 1.67 Ga, 1.1 Ga, sissippian lithologies up to 2 m in diameter; the Archean to Early Cretaceous in age, with the 424 Ma, 110 Ma, and 103 Ma. Smaller peaks section fi nes upward into coarse sandstone and most signifi cant age peaks at 1.85 Ga, 1.17 Ga, occur at 2.78 Ga, 2.36 Ga, 1.88 Ga, 1.38 Ga, conglomerate interfi ngering with marginal lacus- and 121 Ma. Smaller peaks are recorded at and 363 Ma. trine strata (Fig. 3). Sample 05DW1 was col- 1.42 Ga, 1.25 Ga, 976 Ma, 449 Ma, 437 Ma, Results from samples collected from the lected from a coarse, poorly sorted sandstone bed and 129 Ma (Fig. 5). TuffZirc age extraction lower and middle portion of Member C were ~20 m above the base of the section. The zircon computations (Ludwig and Mundil, 2002) per- similarly combined into a single age probability separates reveal nearly equal proportions of pale- formed on the Cretaceous zircon component plot (Fig. 5). The dominant age peaks for Mem- yellow to dull-white, rounded to subrounded zir- from sample 07NW2 suggests a single tuff ber C include 1.91 Ma, 1.63 Ga, 1.5 Ga, 1.2 Ga, cons, and clear, elongate, euhedral zircons with source with an age of 120.7 ± 3.2 Ma (Fig. 6A). 1.05 Ga, and 155 Ma. Minor age peaks include abundant accessory barite. Results of U-Pb age Sample 06NW1 was collected from a thin 3.12 Ga, 2.88 Ga, 2.67 Ga, 650 Ma, 423 Ma, analyses indicate major peaks at 36 Ma, 1.86 Ga, bed of poorly sorted sandstone within the mud- and 186 Ma. and 1.91 Ga. Smaller peaks occur at 2.73 Ga, stone-dominated Upper Carbonaceous Mem- The fi nal sample analyzed from the Sheep 2.08 Ga, 1.02 Ga, and 112 Ma (Fig. 5). A TuffZirc ber (Fig. 3). Separates yielded only relatively Pass Formation type section was obtained from age extraction of the Eocene age component sug- large (>100 µm) clear, prismatic, euhedral zir- medium-grained, well-sorted and quartz-rich gests a single tuffaceous source with an eruptive cons. U-Pb age results and petrography from sandstone interbeds within the base of the lacus- age of 35.7 ± 0.7 Ma (Fig. 6C). 06NW1 indicate that this sample is a waterlain trine limestone–dominated Member E. Results Sample 04LC4 was collected at Lowry Spring tuff rather than sandstone as indicated in previ- indicate major age peaks at 1.85 Ga, 1.75 Ga, section in the central Egan Range (Fig. 2). The ous stratigraphic sections (Nolan et al., 1956; 1.48 Ga, 1.18 Ga, 1.06 Ga, and 112 Ma. Minor Lowry Spring section is contiguous with the Vandervoort, 1987), with a U-Pb concordia age peaks occur at 2.77 Ga, 1.94 Ga, 1.65 Ga, and Elderberry Canyon section 6 km to the north, of 116.1 ± 1.6 Ma (Aptian) (Fig. 5). 1.0 Ga (Fig. 5). which was designated as the type locality for

1148 Geological Society of America Bulletin, May/June 2011 Detrital zircon provenance within the Sevier hinterland

35 1.11 06SP30 n = 95 121 07NW2 n = 95 0.025 Newark Canyon 160 30 1.07

426 Relative probability Relative probability 30 Formation tuff 25 0.023 (Upper Carbonaceous 25 Member) 140 U 20 8 0.021 23 20 / 116.1 ± 1.6 Ma Pb 6 n = 46 0

15 2 0.019 120 1.48 15 1.17 Number Number 10 1.65 1.08 0.017 926 10 412 1.82 1.85 2.53 437 1.25 5 5 449 100 129 976 1.42 0.015 0 0 06NW1 0 500 1000 1500 2000 2500 3000 0 500 1000 1500 2000 2500 3000 0.013 Age (Ma) Age (Ma) 0.00 0.04 0.08 0.12 0.16 0.20 0.24 Scotty Wash Sandstone Newark Canyon Formation 207Pb/235U (Upper Conglomerate Member) 16 25 35 155 n = 203 05SP20 and 06SP29 combined n = 166 05SP14 and 05SP18 combined 112 06MR19 n = 95 110 14 Relative probability Relative probability Relative probability 1.75 424 30 20 1.10 12 103 25 1.63 10 15 20 1.20 1.02 1.06 1.85 1.67 8 1.18 15 1.48

10 Number

Number 1.50 6 Number 1.80 10 423 1.05 1.91 1.65 1.94 4 1.00 363 1.88 2.68 5 2.78 650 2.05 2.77 2.36 5 186 2 0 0 0 0 500 1000 1500 2000 2500 3000 0 500 1000 1500 2000 2500 3000 0 500 1000 1500 2000 2500 3000 Age (Ma) Age (Ma) Age (Ma) Sheep Pass Formation Member A Sheep Pass Formation Member C Sheep Pass Formation Member E

40 18 45 37.5 06SP21 and 06SP22 combined n = 154 154 04LC4 n = 95 05SM2 n = 84 16 40 36 Relative probability 35 Relative probability Relative probability 1.11 1.68 14 30 35 12 30 25 1.11 10 25 20 1.30 1.77 8 20 422 1.15 420 Number Number 15 Number 6 15 118 953 1.83 10 1.65 4 2.88 377 1.03 1.35 2.76 10 1.65 1.74 1.11 2.73 2 1.86 5 538 2.00 5 1.02 1.45 2.69 0 0 0 0 500 1000 1500 2000 2500 3000 0 500 1000 1500 2000 2500 3000 0 500 1000 1500 2000 2500 3000 Age (Ma) Age (Ma) Age (Ma) Stinking Spring Conglomerate Lowry Spring Sawmill Canyon

40 36 05KC1 n = 94 70 07SR1 n = 77 36 05DW1 n = 92 80 35

32 Relative probability Relative probability Relative probability 60 30 60 50 25 40 20 40 30 Number 15 Number Number 20 10 1.91 20 1.86 2.73 10 5 112 1.02 2.09 2.67 1.08 1.41 1.02 1.17 0 0 0 0 500 1000 1500 2000 2500 3000 0 500 1000 1500 2000 2500 3000 0 500 1000 1500 2000 2500 3000 Age (Ma) Age (Ma) Age (Ma) Kinsey Canyon Murphy Wash Duckwater Mountain

Figure 5. Probability density plots of U-Pb detrital zircon data from the complete suite of Sevier hinterland deposits reported in this study, and the Mississippian Scotty Wash Sandstone (06SP30). The histogram and scale at left indicate the number of single-grain U-Pb analyses corresponding to the probability curve. Precambrian peaks of 1 billion or older are given in Ga. Sample 06NW1 at upper right is a concordia diagram of a tuff within the Upper Carbonaceous Member of the Newark Canyon Formation. A U-Pb zircon age of 116.1 ± 1.6 Ma was determined. Error ellipses are 1σ.

Geological Society of America Bulletin, May/June 2011 1149 Druschke et al.

07NW2 Newark Canyon Formation 06SP22 Stinking Spring Conglomerate 134 48 A box heights are 2σ B box heights are 2σ 130 126 44 122 40 118 Age (Ma) 114 Age (Ma) 36 110 106 32 TuffZirc Age = 120.7 ± 3.2 Ma TuffZirc Age = 37.7 ± 0.6 Ma 102 n = 18 n = 19 98 28

05DW1 Duckwater Mountain 05SM2 Sawmill Canyon 44 CDbox heights are 2σ box heights are 2σ 39 42

40 37 38

36 Age (Ma) 35 Age (Ma) 34

33 32 TuffZirc Age = 35.7 ± 0.7 Ma 30 TuffZirc Age = 36.8 ± 1.1 Ma n = 22 n = 27 31 28

05KC1 Kinsey Canyon 07SR1 Murphy Wash 46 38 E box heights are 2σ F box heights are 2σ 36 42 34

38 32 30 Age (Ma) Age (Ma) 34 28

26 30 TuffZirc Age = 35.8 ± 0.5 Ma 24 TuffZirc Age = 31.9 ± 0.6 Ma n = 54 n = 31 26 22

Figure 6. TuffZirc age extraction analyses (Ludwig and Mundil, 2002) for samples containing a tuffaceous compo- nent. (A) Sample 07NW2 from the Newark Canyon Formation type section; (B) sample 06SP22 from the upper- most Stinking Spring Conglomerate in Sheep Pass Canyon; (C) sample 05DW1 from Duckwater Mountain in the Pancake Range; (D) sample 05SM2 from Sawmill Canyon in the central Egan Range; (E) sample 05KC1 from the Kinsey Canyon in the Schell Creek Range; and (F) sample 07SR1 from Murphy Wash in the southern Snake Range. the “tuffaceous” Sheep Pass Formation (Fouch, mill Canyon, but the two sections are separated 1.3 Ga, and 1.11 Ga. Smaller peaks occur at 1979). The Lowry Spring section consists of by a normal fault with an unknown amount of 2.88 Ga, 2.76 Ga, 1.83 Ga, 953 Ma, and 420 Ma ~50 m of conglomerate and coarse- to medium- displacement (Brokaw, 1967). Sample 04LC4 (Fig. 5). A single zircon with an age of 154 Ma grained sandstone deposited unconformably was collected from a medium-grained, well- was analyzed. on the Permian Arcturus Formation (Brokaw, sorted, quartz-rich sandstone. Detrital zircon Sample 05SM2 was collected from Sawmill 1967). Conglomerate clasts consist dominantly separates consist of pale-yellow to dull-white, Canyon immediately to the south of Lowry of Upper Paleozoic limestone and siliciclastic subrounded to well-rounded zircons, with a very Spring. The sample was collected from a strata, with no volcaniclastic component dis- small component of subhedral zircons. Results coarse-grained, poorly sorted litharenite within cernible. Lowry Spring lies 1 km north of Saw- reveal major populations at 1.77 Ga, 1.68 Ga, the lower portion of the section (Fig. 3). Detrital

1150 Geological Society of America Bulletin, May/June 2011 Detrital zircon provenance within the Sevier hinterland biotite within the sandstone and small well- cene), with minor peaks at 1.17 Ga and 1.02 Ga. shallower depths under conditions of higher rounded clasts of basalt within the dominantly A TuffZirc age extraction of the Oligocene crustal heat fl ow; above-average geothermal Paleozoic-clast conglomerates at Sawmill Can- zircon population suggests a single tuff source gradients are suggested by metamorphism that yon indicate a volcaniclastic component. This with an eruptive age of 31.9 ± 0.6 Ma (Fig. 6F). accompanied Late Cretaceous intrusions within volcaniclastic component increases up section, the Snake Range core complex at relatively shal- with thick beds of tuffaceous sandstone domi- (U-Th)/He Detrital Zircon low crustal levels (Miller et al., 1988; Miller and nating the upper portion of the section below Thermochronology Gans, 1989). Results from (U-Th)/He detrital the Charcoal Ovens Tuff. Detrital zircons from zircon dating of the Scotty Wash Sandstone de- 05SM2 consist of roughly 70% clear, elongate, Twelve zircons were analyzed from the fi ne a broad peak at 265 Ma (Permian), with a euhedral zircons, with the remainder consisting Scotty Wash Sandstone in Sheep Pass Canyon single outlier at 680 Ma (Fig. 7A). of pale-yellow to dull-white, rounded to sub- (06SP30) in order to constrain the thermal his- In total, 26 zircons from the Sheep Pass For- rounded zircons. Results indicate a major age tory for the Upper Paleozoic section serving as mation type section were analyzed from three peak at 36 Ma, with smaller peaks at 1.86 Ga, basement for the Sheep Pass basin. The (U-Th)/ samples corresponding to Member A (06SP29), 1.65 Ga, and 1.11 Ga (Fig. 5). A TuffZirc age He ages represent zircon cooling through Member C (05SP18), and Member E (06MR19) extraction indicates a single tuff source with an 180 °C, equivalent to approximate 6 km burial (Fig. 7B). In addition, 9 zircons from the over- age of 36.8 ± 1.1 Ma (Fig. 6D). depths under normal crustal geothermal gradi- lying Stinking Spring Conglomerate (06SP21) Sample 05KC1 was collected from the Kin- ents. Results are also compatible with somewhat were analyzed (Fig. 7C). Dominant (U-Th)/He sey Canyon section in the northern Schell Creek Range (Fig. 2). This is the type section of the Kinsey Canyon Formation of Young (1960), and 4 A 265 Scotty Wash n = 12 it was later correlated to the Sheep Pass Forma- Sandstone tion (Fouch, 1979). The Kinsey Canyon section 3 consists of ~120 m of dominantly carbonaceous and tuffaceous siltstone deposited within a shal- low lacustrine setting (Young, 1960; Fouch, 2 1979). The contact between the Kinsey Canyon Number section and underlying strata is not exposed, but it is unconformably overlain by the late 1 Relative probability Eocene Kalamazoo Tuff. Sample 05KC1 was collected from a coarse, litharenitic sandstone 0 at the base of the section (Fig. 3); separates con- 100 200 300 400 500 600 700 800 900 sist predominantly of clear, prismatic, euhedral Age (Ma) zircons with ~10% of the population consisting 6 B 80 n = 26 of pale-yellow to dull-white, rounded to sub- Figure 7. Probability density Sheep Pass Fm. 5 rounded detrital zircons. Results of U-Pb dating plots of (U-Th)-He detrital zir- Members A–E indicate a dominant population at 36 Ma, with con data from the: (A) Missis- 313 4 minor peaks at 1.41 Ga and 1.08 Ga (Fig. 5). sippian Scotty Wash Sandstone, A TuffZirc age extraction suggests a single tuff (B) the Sheep Pass Formation 3 source with an age of 35.8 ± 0.5 Ma (Fig. 6E). type section, and (C) the Stink- Sample 07SR1 was collected from the Mur- Number 135 220 ing Spring Conglomerate. The 2 phy Wash section of the southern Snake Range histogram and scale at left de- Relative probability (Fig. 2). Murphy Wash is the easternmost sec- pict the number of single-grain 1 tion correlated to the Sheep Pass Formation (U-Th)/He age analyses corre- (Fouch, 1979); it lies in close proximity to the 0 sponding to probability curve 0 100 200 300 400 500 600 700 800 90010001100 1200 northern Snake Range core complex. The Mur- peaks. Age (Ma) phy Wash section consists of 40 m of fl uvial to alluvial conglomerate and sandstone deposited 4 unconformably on the Mississippian Chainman C 40 Stinking Spring n = 9 Formation (Fig. 3). The Murphy Wash section Conglomerate is unconformably overlain by a series of ash- 3 fl ow tuffs and welded tuffs correlated to the Oligocene Needles Range Group; the basal tuff 2 of the sequence has produced a 40Ar/39Ar age of 31.07 ± 0.07 Ma (Oligocene) (Miller et al., Number 1999). Sample 07SR1 was collected near the 1 top of the section, and zircon separates consist Relative probability mainly of clear, prismatic, euhedral zircons, with <10% consisting of pale-yellow to dull- 0 0 100 200 300 400 500 600 700 800 90010001100 1200 white rounded to subrounded zircons. The dominant U-Pb age population is 32 Ma (Oligo- Age (Ma)

Geological Society of America Bulletin, May/June 2011 1151 Druschke et al. age peaks occur at 304 Ma, 135 Ma, and 106 Ma, craton to the east is possible based on compo- contains peaks of 426 and 412 Ma (Silurian), with the largest age peak at 80 Ma. A subset of sitional maturity and some west-directed paleo- which were typically obtained from moder- three euhedral zircons (out of nine total ) from current indicators (Trexler et al., 1995). ately abraded, subhedral zircons that suggest the Stinking Spring Conglomerate defi nes a The Scotty Wash Sandstone also contains a only relatively minor transport and rework- 40 Ma cooling peak. signifi cant Mesoproterozoic peak at 1.48 Ga, ing. Similar Late Ordovician to Silurian peaks Paleoproterozoic peaks at 1.65 Ga and 1.82 Ga, are present in the Newark Canyon Formation DISCUSSION and an Archean peak at 2.52 Ga. Mesoprotero- (449 and 437 Ma), and a 424 Ma peak repre- zoic and older zircon populations are common sents the largest peak within Member A of the Zircon Provenance constituents of western Laurentian Paleozoic Sheep Pass Formation type section (n = 22, or strata and were ultimately derived from Ar- 13%). Subordinate ca. 420 Ma peaks occur in Precambrian Zircon Source Interpretation chean cratons of the Laurentian interior, such as members C and E of the Sheep Pass Formation Detrital zircon U-Pb age populations of the Superior and Wyoming Provinces, as well as type section, the Stinking Spring Conglomer- Sevier hinterland strata are largely dominated from orogenic belts such as the Trans-Hudson ate, and the Lowry Spring section. Relatively by Precambrian peaks. The abundance of Pre- (1.85–1.80 Ga), Yavapai (1.80–1.72 Ga), and minor populations of Devonian zircons are cambrian zircon grains is consistent with the Mazatzal (1.72–1.65 Ga) orogens, and the mid- also present in Sheep Pass Formation Member predominance of recycled Upper Paleozoic continental Granite-Rhyolite province (1.5– A (363 Ma, n = 5) and in the Stinking Spring detritus within conglomerates and sandstone 1.3 Ga) (Gehrels and Dickinson, 1995; Gehrels Conglomerate (377 Ma, n = 6). of the Newark Canyon and Sheep Pass Forma- et al., 1995; Rainbird et al., 1997; Stewart et al., Lower Paleozoic population ages of 420– tions (Nolan et al., 1956; Winfrey, 1958, 1960; 2001; Gehrels et al., 2000). Detrital zircon age 350 Ma are recognized within Triassic strata of Fouch, 1979). The Mississippian Scotty Wash peaks of 1.43, 1.60, and 1.80 Ga have been re- eastern Nevada (Gehrels and Dickinson, 1995), Sandstone forms a portion of the basement to corded in the Ordovician Vinnini Formation of although major Silurian peaks are not recognized the Sheep Pass Formation type section, and de- the Roberts Mountain allochthon, and peaks of within the Roberts Mountain allochthon. Promi- trital zircon age determinations provide a direct 2.30–2.80 Ga similarly derived from the Rob- nent detrital zircon age peaks of 410–445 Ma comparison between the Sheep Pass Formation erts Mountain allochthon are common within are, however, recognized within Devonian to and Upper Paleozoic source strata. The most the Antler overlap sequence (Gehrels et al., Triassic strata of Alaska and British Columbia abundant U-Pb age population of the Missis- 2000). Within the Newark Canyon Formation, (Ross et al., 1997; Gehrels and Ross, 1998; sippian Scotty Wash Sandstone is defi ned by a peaks of 1.42 Ga and 1.85 Ga represent signifi - Gehrels et al., 1999). A compilation of Paleo- 1.1 Ga peak that is part of a broad population cant populations. A compilation of Precambrian zoic detrital zircon U-Pb ages from the Sheep of grains ranging from 900 Ma to zircon ages from the Sheep Pass Formation (Fig. Pass Formation and “tuffaceous” Sheep Pass 1.2 Ga (n = 53, or 55% of the total detrital zir- 8A) similarly reveals major peaks at 1.51 Ga, Formation indicates that the principal Paleo- con population). Similar Grenville age peaks 1.66 Ga, 1.86 Ga, 2.73 Ga, and 2.87 Ga. Detrital zoic age peaks are Silurian (423 and 442 Ma) make up the major Precambrian populations of zircons within Sevier hinterland strata of Meso- (Fig. 9B). Silurian detrital zircons were likely the Newark Canyon Formation (1.08–1.25 Ga, proterozoic to Archean age are typically sub- derived from Lower Paleozoic volcanic arc ter- n = 33, or 35%) and the Sheep Pass Formation rounded to well rounded (Fig. 4), suggesting ranes such as the Klamath Mountains, where type section (30%). Grenvillian age popula- long-range transport and multiple episodes of Silurian volcanism is documented (Metcalf et al., tions are also signifi cant within the Stinking recycling, with degree of roundness typically 2000), and were subsequently incorporated Spring Conglomerate and the Lowry Spring increasing with the age of the zircons. into backarc basin strata of the Roberts Moun- section. Smaller populations of Grenvillian age A trend toward older Precambrian zircons tain alloch thon. The existence of a Silurian age zircons are found in the Duckwater Mountain, relative to Grenvillian grains is seen within late peak in Antler foreland basin strata represented Sawmill Canyon, Kinsey Canyon, and Murphy Eocene hinterland strata, as demonstrated by by the Mississippian Scotty Wash Sandstone, Wash sections. U-Pb age peaks of the Sawmill Canyon–Lowry and presence in both the Newark Canyon and Grenvillian detrital zircon populations are Spring, Duckwater Mountain, and Kinsey Can- Sheep Pass Formations suggest that this may be ultimately derived from the 1.0–1.2 Ga Gren- yon sections. This trend correlates with a greater a more important age peak for Upper Paleozoic ville orogen of eastern Laurentia and were proportion of Cambrian to Devonian clasts strata in Nevada than previously recognized. transported to the western margin of Lauren- within conglomeratic beds comprising late tia during the Neoproterozoic to Cambrian via Eocene sections, as compared to dominantly Mesozoic Zircon Source Interpretation cross-continental fl uvial systems (Rainbird late Paleozoic clasts within the Sheep Pass For- Sevier hinterland strata contain signifi cant et al., 1992, 1997; Stewart et al., 2001). Gren- mation type section (Druschke, 2008). The Pre- populations of Mesozoic zircons (Fig. 8C), villian zircons within Sevier hinterland deposits cambrian U-Pb age peaks within the Stinking despite the fact that previously published con- are commonly subrounded to rounded (Fig. 4), Spring Conglomerate are similar to those of the glomerate clast counts and sandstone petrog- suggesting long-range transport and multiple Sheep Pass Formation type section, consistent raphy of the Newark Canyon Formation (Nolan episodes of reworking. Cambrian to Ordovician with the abundance of clasts within the Stinking et al., 1956; Vandervoort, 1987) and the Sheep strata within the Roberts Mountain allochthon Spring Conglomerate derived from recycling of Pass Formation type section (Winfrey, 1958, contain signifi cant populations of zircons with the underlying Sheep Pass Formation. 1960; Kellogg, 1964; Fouch, 1979) indicate Grenville affi nity (Smith and Gehrels, 1994; the lack of a discernible volcaniclastic compo- Gehrels and Dickinson, 1995; Gehrels et al., Paleozoic Zircon Source Interpretation nent. The Newark Canyon Formation contains 2000). The Scotty Wash Sandstone is part of the Paleozoic detrital zircons are a major com- both volcanic-derived zircons (TuffZirc age of Antler foreland basin, which received siliciclas- ponent of Sevier hinterland strata and are 120.6 ± 3.2 Ma) and a previously unrecognized tic sediment shed from the Roberts Mountain nearly as numerous as Mesozoic detrital zir- waterlain tuff (116.1 ± 1.6 Ma) within the upper allochthon, although additional input from the cons (Fig. 8B). The Scotty Wash Sandstone portion of the type section. Despite evidence for

1152 Geological Society of America Bulletin, May/June 2011 Detrital zircon provenance within the Sevier hinterland

A Precambrian B Paleozoic

14 n = 71 100 1.05 1.66 n = 629 442 12 423 80 10

60 1.86 8 1.51

Number 6 40 1.90 2.73 478 4 492 Figure 8. Probability density 611 365 377 539 plots for Precambrian, Paleo- 20 2.87 2 303 337 zoic, Mesozoic, and Eocene 766 2.35 3.12 zircon populations compiled 0 0 from the Sheep Pass Formation 500 1000 1500 2000 2500 3000 3500 4000 250 300 350 400 450 500 550 600 type section (samples 06SP29, Age (Ma) Age (Ma) 05SP20, 05SP18, 05SP14, C Mesozoic D Eocene 06MR19), Stinking Spring Con- 18 45 n = 80 n = 192 111 154 36 glomerate (06SP21 and 06SP22), 16 40 37.5 and “tuffaceous” Sheep Pass 14 35 Formation (04LC4, 05DW1, 30 05SM2, 05KC1). 12 10 25

104 20

8 Number Number Number 15 6 69

4 186 10 194 2 5

0 0 50 100 150 200 250 300 30 34 38 42 46 50 Age (Ma) Age (Ma)

an Early Cretaceous volcaniclastic component population, which defi nes a peak at 118 Ma distribution is also apparent from a compila- in the Newark Canyon Formation type section, (n = 7), and two additional grains with ages of tion of Mesozoic grains within the Sheep Pass signifi cant Jurassic or Triassic detrital zircon 92.1 ± 3.2 Ma and 97.7 ± 2.8 Ma. Formation type section and “tuffaceous” Sheep contributions derived from older igneous ter- Sections of the “tuffaceous” Sheep Pass For- Pass Formation, with major peaks at 154 Ma ranes of western Nevada are absent. mation typically contain no major Mesozoic and 111 Ma (Fig. 9C). This age peak correla- The Sheep Pass Formation type section con- detrital zircon populations, although relatively tion, and the generally euhedral to subhedral tains signifi cant populations of Mesozoic zir- small populations or individual grains of Meso- nature of Mesozoic detrital zircons in Sevier cons (n = 62, or 13%), the age population of zoic age are common and worth noting. The hinterland deposits (Fig. 4C) suggest local which changes markedly up section. Nineteen Lowry Spring section yielded only one Meso- derivation. While few localities of Cretaceous Mesozoic grains were recovered from Member zoic grain, which was of Late Jurassic age or Jurassic volcanic strata are known in east- A (11% of the total) with peaks at 103 (n = 8) (154 ± 1.5 Ma), while the overlying Sawmill central Nevada, erosion during the Cretaceous and 110 Ma (n = 4). Two Maastrichtian grains Canyon section yielded a total of three mid- to and early Paleogene may have removed extru- (with ages of 67.8 ± 1 Ma and 70 ± 1.3 Ma) Late Cretaceous grains (99.8 ± 1.1 Ma, 82.9 ± sive volcanic strata linked to currently exposed were obtained from the uppermost sample 2.0 Ma, and 69.8 ± 2.3 Ma). Grains from the intrusions (du Bray, 2007). Late Cretaceous within Member A (06SP20). The remaining Duckwater Mountain section produced a small plutons of 95–65 Ma are relatively common in Mesozoic grains within Member A range in age Cretaceous peak at 112 Ma, while the Kinsey eastern Nevada (du Bray, 2007), but the lack from Early Cretaceous to Late Triassic, without Canyon section yielded only two Mesozoic of corresponding age peaks in Sevier hinter- defi ning a robust population. Member C con- grains with ages of 81.3 ± 2.1 Ma and 238.7 ± land strata suggests that intrusions during this tains the largest component of Mesozoic grains 5.2 Ma. The Murphy Wash section of the south- period were dominantly deep-seated and were with 35 grains (23%), but it contains relatively ern Snake Range yielded a single Late Creta- unroofed following the Eocene. few Cretaceous grains. The major Mesozoic ceous zircon with an age of 94.9 ± 3.5 Ma. While reworking of tuff present in the age peak within Member C is Late Jurassic at A compilation of geochronologic data from Newark Canyon Formation type section is an 155 Ma, with a minor Early Jurassic popula- Mesozoic intrusions in north-central and east- obvious source for Early Cretaceous detrital tion at 186 Ma. Member E contains the smallest central Nevada indicates a bimodal distribution zircons, the Sheep Pass Formation type sec- percentage of Mesozoic grains (n = 7 or 8%), of Late Jurassic (145–175 Ma) and mid- to tion lacks identifi ed tuffaceous horizons or and these grains fall within a single 112 Ma age Late Cretaceous (65–110 Ma) ages produced vol canic conglomerate clasts. Our examina- peak. The overlying Stinking Spring Conglom- by major intrusive pulses within the Sevier tion of conglomerates within Member A of the erate also contains a relatively small Mesozoic hinterland (du Bray, 2007). A bimodal age Sheep Pass Formation reveals the presence of

Geological Society of America Bulletin, May/June 2011 1153 Druschke et al.

227

1.12 Triassic Lovelock/Luning assemblages 1.06 n = 84 271 1.01 1.42 1.68

1.83 Cambrian-Devonian miogeoclinal reference 1.41 1.13 2.07 n = 108 202 355 259 1.82 northern Sierra Nevada terranes n = 56

1.83 Roberts Mountain allochthon 1.03 1.72 2.68 n = 205 1.85 Golconda allochthon 345 1.91 1.77 2.50 2.66 n = 81 121

129 1.17 437 Newark Canyon Formation type section 449 1.08 976 1.25 1.42 1.85 n = 95 1.11 426 1.07 Mississippian Scotty Wash Sandstone 412 926 1.48 1.82 2.53 n = 97 111 103 Sheep Pass Formation type section 420 1.06 n = 466 440 1.67 1.88 2.68

0 200 400 600 800 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 Age (Ma) Age (Ga)

Figure 9. Normalized age probability density plots for detrital zircon data, including, from the bottom up, the Sheep Pass Formation type section, Mississippian Scotty Wash Sandstone, and the Upper Conglomeratic Mem- ber of the Newark Canyon Formation type section. These are plotted against previously published U-Pb detrital zircon ages for the Golconda allochthon (Riley et al., 2000), Roberts Mountain allochthon (Gehrels et al., 2000), Upper Paleozoic to Jurassic terranes of the northern Sierra Nevada (Spurlin et al., 2000), Cambrian to Devonian miogeoclinal reference for Nevada (Gehrels and Dickinson, 1995), and the Upper Triassic Lovelock/Luning as- semblages of western Nevada (Manuszak et al., 2000). coarse-grained to pebbly volcaniclastic lith- rived from the Newark Canyon Formation. At Paleogene Zircon Source Interpretation arenite clasts, a lithology that matches none of present, however, all known exposures of the Sections of the “tuffaceous” Sheep Pass For- the Paleo zoic units presently exposed in east- Newark Canyon Formation are located >50 km mation are distinct from the Sheep Pass Forma- central Nevada. We speculate that volcaniclas- west of Sheep Pass Canyon, and paleocurrent tion in that they contain reworked pumice and tic sandstone clasts within conglomerates of the measurements of latest Cretaceous to Eocene detrital biotite and feldspar. Detrital zircon Sheep Pass Formation type section represent strata of the Egan and Schell Creek Ranges separates contain euhedral, distinctly elongated Lower Cretaceous strata that were present lo- overwhelmingly indicate a westward direction zircons that range in age from 35 to 38 Ma, and cally during the latest Cretaceous–Paleocene of transport (Druschke et al., 2009b). The pres- that are interpreted as having originated from but were subsequently removed by erosion. ence of clasts derived from Mesozoic volcani- volcanic sources. A compilation of Eocene de- Given the Albian-Aptian biostratigraphic age clastic strata in the Sheep Pass Formation type trital zircon ages indicates a peak in magmatic assignment for various sections of the Newark section, and clasts of the Sheep Pass Formation activity in east-central Nevada at 36 Ma (Fig. Canyon Formation (Nolan et al., 1956; Fouch present in the overlying Stinking Spring Con- 8D). The TuffZirc age determination of 37.7 ± et al., 1979), similar Early Cretaceous detrital glomerate demonstrate, however, that intervals 0.6 Ma from the upper portion of the Stinking zircon populations present in the Sheep Pass of hinterland deposition were separated by Spring Conglomerate invalidates the “tuffa- Formation type section may have been de- widespread erosion and reworking. ceous” Sheep Pass Formation correlation of

1154 Geological Society of America Bulletin, May/June 2011 Detrital zircon provenance within the Sevier hinterland

Fouch (1979), indicating instead that the Duck- type section (n = 5) occurs at 80 Ma and does Early Cretaceous (Vandervoort and Schmitt, water Mountain, Sawmill Canyon, and Kinsey not correspond to a U-Pb crystallization age 1990). The central Nevada fold-and-thrust belt Canyon sections are younger than or coeval with peak. Given the west-fl owing paleocurrent in- largely involved east-vergent deformation of the deposition of the basal member of the Gar- dicators within the Sheep Pass Formation type Cambrian–Pennsylvanian strata (Speed et al., rett Ranch Group, which unconformably over- section (Druschke et al., 2009b), this Late Creta- 1988; Allmendinger, 1992; Taylor et al., 2000), lies the Sheep Pass Formation type section. The ceous cooling age peak is interpreted as an indi- with the Newark Canyon Formation deposited as TuffZirc age of 31.9 ± 0.6 Ma obtained from the cation of early unroofi ng within the Snake Range a series of wedge-top depocenters (Vandervoort Murphy Wash section indicates that the section core complex to the east. The principal cooling and Schmitt, 1990). Lack of signifi cant zircon is Oligocene. age peak of the Stinking Spring Conglomerate populations with a clear link to lower Meso- A small number of zircons range in age is 40 Ma (Fig. 7C), although the fact that zir- zoic volcaniclastic sources of western Nevada from 39 to 49 Ma; however, none of the sec- cons with Eocene cooling ages are euhedral suggests that local Paleozoic strata involved tions analyzed contains robust populations may indicate a genetic link with ca. 38 Ma tuff- in fold-and-thrust belt deformation served as with Paleogene ages older than 38 Ma. Vol- sourced U-Pb crystallization ages. the principal sediment source for the Newark canic strata ranging in age from 43 to 39 Ma Canyon Formation. This observation suggests have been documented in northeastern Nevada Tectonic Setting and Paleogeography that the mid-Jurassic Luning-Fencemaker belt and adjacent Utah (Brooks et al., 1995). Ages (Oldow, 1984; Wyld et al., 2001; Wyld, 2002) of 39–40 Ma have also been obtained from vol- Distributions of detrital zircon populations was not a major contributor of sediment to the canic strata and granitic dikes of the Kern and within the Newark Canyon Formation and Newark Canyon Formation, contrary to previ- Deep Creek Ranges northeast of the study area Sheep Pass Formation suggest that sediment ous paleogeographic reconstructions (DeCelles, (Gans et al., 1989) and from tuffs overlying the was derived primarily from local sources, with 2004). Geographic isolation of the Newark White Sage Formation of west-central Utah little evidence of long-range Cretaceous to Canyon Formation basin system from fl uvial (Dubiel et al., 1996). Paleogene transport that would have contrib- systems draining the Sierra Nevada and early uted late Paleozoic, early Mesozoic, and Late Mesozoic terranes of western Nevada was likely (U-Th)/He Zircon Age Interpretation Cretaceous zircon grains that are not present the result of intervening topographic lows repre- Results of (U-Th)/He detrital zircon dating in signifi cant quantities in Cretaceous–Eocene sented by the King Lear Formation basin system of the Mississippian Scotty Wash Sandstone strata of central Nevada. Detrital zircon stud- of western Nevada (Martin et al., 2010), and by (Fig. 7A) indicate predominately Permian ies of the volcaniclastic Pine Nut, Luning, and topographic highs within the central Nevada (ca. 265 Ma) cooling ages, derived largely Lovelock assemblages of western Nevada show fold-and-thrust belt (Fig. 10A). from dark-yellow, subrounded to well-rounded Triassic populations with ages ranging from 218 Extension and deposition of the Sheep Pass zircons that typically yield Precambrian U-Pb to 243 Ma (Manuszak et al., 2000), and Upper Formation type section was initiated during crystallization ages. Permian or older cool- Paleozoic to Lower Mesozoic terranes of the Campanian to Maastrichtian time (Vandervoort ing ages are also preserved in the Sheep Pass northern Sierra Nevada have yielded detrital and Schmitt, 1990; Druschke et al., 2009a), fol- Formation and Stinking Spring Conglomer- zircons ranging from 370 to 185 Ma (Permian lowing the onset of the Late Cretaceous amag- ate. These new data corroborate interpretations to Early Jurassic) (Spurlin et al., 2000) (Fig. 9). matic gap (Dickinson and Snyder, 1978) and based on previous conodont alteration studies, Geochronological studies within the Sierra the initiation of hinterland midcrustal extension which suggest that neither deep thrust burial nor Nevada magmatic arc indicate that pluton em- (Wells et al., 1990; Hodges and Walker, 1992; accumulation of a thick Mesozoic sedimentary placement occurred over protracted intervals Camilleri and Chamberlain, 1997; McGrew succession has affected large regions of Upper during the Triassic to Jurassic (206–155 Ma) et al., 2000; Wells and Hoisch, 2008). Strati- Paleozoic strata in east-central Nevada (Gans and Cretaceous (125–88 Ma) (Evernden and graphic patterns and paleofl ow indicators et al., 1990), because this would have resulted in Kistler, 1970; Stern et al., 1981; Bateman, 1983; within the Sheep Pass Formation type section Mesozoic U-Th/He cooling ages. The preserva- Saleeby et al., 1989), and volcaniclastic rocks indicate that proximal highlands lay to the east tion of late Paleozoic cooling ages also supports within the Cretaceous Great Valley forearc (Winfrey, 1958, 1960; Kellogg, 1964; Fouch, the hypothesis that following Early Cretaceous basin similarly display a wide range of Triassic 1979; Druschke et al., 2009a, 2009b) (Fig. contraction along the central Nevada fold-and- to Late Cretaceous zircon populations refl ecting 10B). The presence of Albian-Aptian detrital thrust belt, eastward propagation of the Sevier sediment derivation primarily from the Sierra zircons and reworked volcaniclastic sandstone foreland fold-and-thrust belt occurred without Nevada magmatic arc (DeGraaff-Surpless et al., clasts in the Sheep Pass Formation suggests that the development of major surface-breaking 2002). Lack of signifi cant populations of Trias- Early Cretaceous strata, in part coeval with the thrust faults in east-central Nevada (Armstrong, sic, Early to Middle Jurassic, and Late Creta- Newark Canyon Formation, were once exten- 1972; Gans and Miller, 1983; Gans et al., 1989; ceous zircons within Sevier hinterland deposits sive in the Sevier hinterland and were subse- Miller and Gans, 1989). of east-central Nevada suggests geographic iso- quently removed through erosion. However, the Mesozoic cooling ages preserved in the lation from early Mesozoic terranes of western preservation of Paleozoic cooling ages within Sheep Pass Formation type section (Fig. 7B) Nevada and the Sierra Nevada magmatic arc. Upper Paleozoic strata of the Sevier hinterland reveal three distinct peaks at 135 Ma, 106 Ma, Geographic isolation is also refl ected in late indicates that the thickness of Mesozoic strata and 80 Ma. Although 106 Ma cooling ages over- Eocene Sevier hinterland strata of east-central was <4 km. lap with U-Pb crystallization peaks and are thus Nevada by a lack of middle Eocene (43–39 Ma) The lack of detrital zircon populations younger indistinguishable, a 135 Ma crystallization peak zircons derived from the northern Nevada vol- than 103 Ma within the Sheep Pass Formation is unrepresented and may indicate exhumation canic fi eld (Brooks et al., 1995). implies that topography was suffi cient during related to the Early Cretaceous central Nevada The deposition of the Newark Canyon For- the Late Cretaceous and Paleogene to isolate fold-and-thrust belt. Similarly, the largest cool- mation has been linked with motion along depocenters within the Sevier hinterland from ing age peak within the Sheep Pass Formation the central Nevada fold-and-thrust belt in the the high-standing Sierra Nevada arc (Fig. 10B).

Geological Society of America Bulletin, May/June 2011 1155 Druschke et al.

Scale (km) t 110°W 105°W A l B u 45°N a 0 250 500 f I

N

S

M ID

NV UT KLB GRA ? WY ID NV d CO n d a n l la e e R r r WY o 40°N o O f f B r e T B d A e i B i RH C F T m T v L R N ra N e O a S C S L C ? r/ 40°N i e A ie C v r ? SPB e r WSB S a Formation basins B T N area of Newark? Canyon F e L ? v ? SR a

d

a AZ NM m a UT CO g

m

a AZ NM t 35°N i c M a og r o N c llo n H 35°N N ig hla nd M Maastrichtian to Scale (km) cC oy 0 250 500 Barremian to Albian b early Eocene (71–55 Ma) as (127–112 Ma) 115°W in 110°W McCoy basin 110°W 105°W

LEGEND zone of Late thrust fault transform Cretaceous to Paleogene Late Cretaceous to Paleogene Cretaceous Cretaceous teeth on upper fault Sevier fold-thrust belt metamorphic core complex magmatic arc midcrustal extension plate normal fault K-Paleogene Sevier/ Late Cretaceous to Eocene Zone of Late Cretaceous generalized inactive state Laramide foreland basin Laramide uplifts peraluminous granite intrusions paleocurrent direction thrust fault boundary

Figure 10. (A) Schematic reconstruction of elements of the Early Cretaceous (Barremian–Albian) Sevier orogen (modifi ed from DeCelles, 2004). Deposition of Newark Canyon Formation–correlative units within the Sevier hinterland likely occurred within localized, discrete subbasins (Vandervoort and Schmitt, 1990). MSNI—Mojave-Snow Lake-Nevada-Idaho dextral transform fault (after Wyld and Wright, 2001); KLB—King Lear basin system (location and paleocurrent data after Martin et al., 2010); LFTB—Luning-Fencemaker fold-and- thrust belt, which was inactive following the middle Jurassic (after Wyld, 2002); CNTB—central Nevada fold-and-thrust belt. (B) Sche- matic reconstruction of the Sevier orogen during latest Cretaceous to early Eocene time, including locations of developing core complexes (adapted from DeCelles, 2004), and paleocurrent data for the Sheep Pass basin from Druschke et al. (2009b). During the middle to late Eocene, renewed extension led to the establishment of more widely distributed extensional basins that partially overlapped elements of the Maastrichtian to middle Eocene Sheep Pass basin. GRA—Grouse Creek–Raft River–Albion metamorphic core complex, RH—Ruby–East Humboldt core complex, SR—Snake Range core complex, SPB—Sheep Pass basin system, WSB—White Sage basin. Location of the zone of Late Cretaceous peraluminous granite intrusions is after Miller and Bradfi sh (1980). This zone generally corresponds to areas that expe- rienced Late Cretaceous to Paleogene midcrustal extension. K—Cretaceous.

West-fl owing Late Cretaceous paleodrainages voort et al., 1995). Similar conditions may have (Drewes, 1967; Gans et al., 1989) points to ex- potentially extended well into the interior of the been widespread within the latest Cretaceous posure of progressively older strata during the Sierra Nevada (House et al., 2001), and a lack and Paleogene Sevier hinterland of east-central middle to late Eocene. The late Eocene (ca. 35– of Late Cretaceous zircons within the Sheep Nevada. Width and confi guration of the Sevier 38 Ma) marks a period of widespread extension Pass Formation suggests that the majority of plateau varied substantially along the length of and volcanism in east-central Nevada (Gans arc-related detritus was shed west into the Great the orogen, allowing post–Early Cretaceous di- et al., 1989; Armstrong and Ward, 1991; Axen Valley forearc basin. Peripheral antecedent river rect drainage connections between the magmatic et al., 1993; Gans et al., 2001; Druschke et al., systems and interior extensional basins exhibiting arc and the Sevier foreland basin system to exist 2009b); however, the existence of Late Creta- internal drainage patterns are features common elsewhere to the north or south, as in the case of ceous to middle Eocene extensional basin de- within the modern Tibetan, Turkish, and Iranian the McCoy basin (Fig. 10A) (Barth et al., 2004). posits lacking coeval volcanic-sourced zircons plateau systems (Dilek and Moores, 1999), and The presence of abundant clasts derived from suggests that the initiation of upper-crustal ex- internal drainage was initiated within portions Ordovician to Neoproterozoic lithologies within tension signifi cantly predated volcanism within of the Puna-Altiplano in the (Vander- sections of “tuffaceous” Sheep Pass Formation the Sevier hinterland. In this interpretation, the

1156 Geological Society of America Bulletin, May/June 2011 Detrital zircon provenance within the Sevier hinterland

7 km of structural unroofi ng interpreted by Gans CONCLUSIONS isolation of the Sevier hinterland during the Late et al. (1989) from the presence of Neoprotero- Cretaceous to Paleogene from the high-standing zoic quartzite and Jurassic granitic clasts within Approximately 1300 U-Pb detrital zircon Sierra Nevada to the west was likely due to the late Eocene conglomerates of the Schell Creek analyses of strata within east-central Nevada combination of (1) antecedent river systems on Range may represent the additive effects of Late record evolving tectonics and paleogeography the periphery of the plateau to the west that trans- Cretaceous to early Eocene extension within the throughout the transition from Early Cretaceous ported arc-derived detritus primarily to the Great hinterland upon which middle to late Eocene ex- contraction to latest Cretaceous through Eocene Valley forearc basin, (2) internal drainage within tension was superimposed. extension in the Sevier hinterland. Analyses the interior plateau following a Late Cretaceous Paleocurrent measurements within latest Cre- from the Mississippian Scotty Wash Sandstone transition from regional shortening to extension, taceous to Eocene sections of the Schell Creek reveals age peaks at 426 and 412 Ma, 1.1 Ga, and (3) locally rugged topography within the and Egan Ranges (Druschke et al., 2009b) in- 1.48 Ga, 1.65 Ga, 1.82 Ga, and 2.52 Ga, re- plateau interior recorded by widespread coarse dicate that sediment was derived from the east. fl ecting derivation primarily from the Roberts alluvial-fan deposits and megabreccia. These observations suggest that the Snake Mountain allochthon. Detrital zircon analyses The ca. 25–30 m.y. gap between the deposi- Range formed a long-lived highland and po- of the Newark Canyon Formation type section tion of Late Cretaceous–Paleocene extensional tential drainage divide, as proposed by Chris- and Sheep Pass Formation type section reveal deposits of the Sheep Pass Formation type tiansen et al. (1992). Previous studies have that the majority of the zircons present were section, and more widespread middle to late suggested that the Grouse Creek–Raft River– derived from recycling of Upper Paleozoic Eocene sedimentary deposits suggests that there Albion core complex may represent a major strata, which are found throughout east-central were two distinct extensional events within the ramp anticline related to the Sublett synclino- Nevada, and contain Silurian, Grenvillian (1.0– Sevier hinterland. Detrital zircon analyses of the rium, and that topographic uplift occurred as a 1.3 Ga), and late Mesoproterozoic to Archean Stinking Spring Conglomerate and the “tuffa- result of hanging-wall displacement over the (1.45–2.62 Ga) age peaks similar to those in the ceous” Sheep Pass Formation of Fouch (1979) ramp during the Jurassic to mid-Cretaceous Scotty Wash Sandstone. reveal an up-section increase of late Eocene (Wells, 1997). The Confusion Range synclino- The Newark Canyon Formation type section volcanic zircons, defi ning a peak in magmatic rium (Hose, 1977) of east-central Utah lies contains a previously unrecognized Aptian vol- activity at 36 Ma in east-central Nevada. Typi- along strike with the Sublett synclinorium and is caniclastic component as revealed by a 120.6 ± cally, thick intervals of coarse conglomeratic located east of the Snake Range core complex. 3.2 Ma zircon U-Pb TuffZirc age within the Upper strata lacking a tuffaceous component form the The Snake Range core complex may therefore Conglomerate Member, and a 116.1 ± 1.6 Ma base of “tuffaceous” Sheep Pass Formation sec- represent a southern continuation of this anti- zircon U-Pb concordia age from a waterlain tuff tions. This pattern suggests that extension pre- clinal ramp structure, which potentially contrib- within the Upper Carbonaceous Member. These ceded late Eocene magmatism in east-central uted to its high relief relative to Late Cretaceous new data indicate that the Newark Canyon For- Nevada and potentially initiated as early as the to Eocene depocenters of east-central Nevada. mation type section is Aptian or older. Absence middle Eocene (Bridgerian, ca. 50.5–45.4 Ma) Extensional basin deposits of latest Creta- of signifi cant Jurassic or Triassic detrital zircons based on biostratigraphic age correlations. ceous to early Eocene age have been identifi ed suggests that the Newark Canyon basin system The overlap of late Eocene TuffZirc ages in within the Fish Creek Mountains, Grant Range, was isolated from terranes to the west by a topo- the Stinking Spring Formation with sections Egan Range, and adjacent subsurface of Rail- graphic high within the central Nevada fold- of the “tuffaceous” Sheep Pass Formation in- road and White River Valleys of east-central and-thrust belt. Early Cretaceous volcaniclastic validates the correlation of Fouch (1979) and Nevada (Winfrey, 1958, 1960; Kellogg, 1964; detritus in the Newark Canyon Formation type indicates that late Eocene volcaniclastic strata Fouch, 1979; Vandervoort and Schmitt, 1990; section may have been deposited as air fall from variously assigned to the Sheep Pass Formation Fouch et al., 1991; Druschke et al., 2009a, the coeval arc to the west, but it was more likely and basal Garrett Ranch Group are coeval. 2009b), and the vicinity of Gold Hill in western the product of local volcanic sources within the The (U-Th)/He detrital zircon thermochro- Utah (Potter et al., 1995; Dubiel et al., 1996). Sevier hinterland, today represented by scattered nometry of zircons from the Scotty Wash Sand- Contrary to reconstructions that imply that the occurrences of coeval plutonic rocks. stone, Sheep Pass Formation type section, and Sheep Pass Formation represents a single large The Sheep Pass Formation type section con- Stinking Spring Conglomerate reveals late lake basin (Winfrey, 1958, 1960), distribution tains a relatively minor Mesozoic detrital zircon Paleo zoic cooling ages from local Upper Paleo- of megabreccia and coarse alluvial deposits of component comprising roughly 15% of the zir- zoic strata. These data corroborate earlier inter- variable Late Cretaceous to Paleocene age over cons analyzed, and it displays a distinctly bi- pretations based on conodont alteration studies a wide area of east-central Nevada suggest a modal distribution of Early Cretaceous (111 Ma) that Upper Paleozoic strata were not buried number of discrete sedimentary basins. Middle and Late Jurassic (154 Ma) ages. This pattern under a thick Mesozoic section (Gans et al., to late Eocene extensional deposits are gener- resembles the bimodal age distribution of intru- 1990). Cretaceous cooling ages of 80 Ma (Cam- ally more abundant within east-central Nevada sions within east-central and north-central Ne- panian) are preserved within the Sheep Pass For- than latest Cretaceous to early Eocene deposits, vada (du Bray, 2007); the lack of older Mesozoic mation type section, and no crystallization ages potentially due to greater extensional fragmen- populations and Late Cretaceous populations correspond to this Late Cretaceous cooling age tation of the Sevier hinterland, or due to preser- within the Sheep Pass Formation indicates con- peak. This cooling age peak, and paleocurrent vational bias of younger deposits. In many cases tinued geographic isolation of the Sevier hinter- data from the Sheep Pass Formation indicating (i.e., the Sheep Pass Formation type section), land from source areas in western Nevada and the west-directed transport (Druschke et al., 2009b) middle to late Eocene deposits unconformably Sierra Nevada. Mesozoic detrital zircons within suggest that 5–6 km of unroofi ng occurred in overlie older Sevier hinterland strata, which the Sheep Pass Formation were likely derived the vicinity of the incipient Snake Range core suggests that reactivation of extensional fault from widespread Early Cretaceous volcaniclastic complex between Campanian cooling through systems controlled long-lived (latest Cretaceous strata and hinterland volcanic centers that were 180 °C, and Maastrichtian redeposition in the to late Eocene) depocenters. subsequently removed by erosion. Geographic Sheep Pass Formation.

Geological Society of America Bulletin, May/June 2011 1157 Druschke et al.

ACKNOWLEDGMENTS Burchfi el, B.C., Cowan, D.S., and Davis, G.A., 1992, Tec- California and adjacent Oregon: Geosphere, v. 4, tonic overview of the Cordilleran orogen in the western p. 329–353, doi: 10.1130/GES00105.1. This study was supported by the National Sci- United States, in Burchfi el, B.C., Lipman, P.W., and Dickinson, W.R., and Snyder, W.S., 1978, Plate tectonics of Zoback, M.L., eds., The Cordilleran Orogen; Conter- the , in Matthews, V., ed., Laramide ence Foundation under EAR-0610103 (Hanson and minous U.S.: Boulder, Colorado, Geological Society Folding Associated with Block Faulting in the Western Wells), EAR-0443387 and EAR-0732436 provided of America, The Geology of , v. G-3, United States: Geological Society of America Memoir support for the Arizona LaserChron Center, and EAR- p. 407–479. 151, p. 355–366. 0414817 (Stockli) provided support for the University Camilleri, P.A., 1996, Evidence for Late Cretaceous–early Dilek, Y., and Moores, E.M., 1999, A Tibetan model for the of Kansas (U-Th)/He Laboratory. Additional support Tertiary(?) extension in the Pequop Mountains, Ne- early Tertiary western United States: Journal of was provided by student grants (to Druschke) from vada: Implications for the nature of the early Tertiary the Geological Society of London, v. 156, p. 929–941, the Nevada Petroleum Society, the Geological So- unconformity, in Taylor, W.J., and Langrock, H., eds., doi: 10.1144/gsjgs.156.5.0929. ciety of America, Rocky Mountain Section Society Cenozoic Structure and Stratigraphy of Central Ne- Drewes, H., 1967, Geology of the Connors Pass Quadrangle, vada: Nevada Petroleum Society 1996 Field Confer- Schell Creek Range, East-Central Nevada: U.S. Geo- for Sedimentary Geology, and American Association ence Guidebook: Reno, Nevada Petroleum Society, logical Survey Professional Paper 557, 93 p. of Petroleum Geologists (AAPG) Grants-in-Aid. We p. 19–28. Druschke, P., 2008, Sedimentology and tectonic setting of thank reviewers Phyllis Camilleri and Amy Weis logel, Camilleri, P.A., and Chamberlain, K.R., 1997, Mesozoic the Late Cretaceous to Eocene Sheep Pass Formation and Associate Editor Robert Rainbird for helpful tectonics and metamorphism in the Pequop Moun- in the southern Egan Range, in Trexler, J.H., Jr., ed., comments and suggestions. Also, special thanks go to tains and Wood Hills region, northeast Nevada: Im- Nevada Petroleum Society 2008 Field Trip Guidebook: Victor Valencia for aid in data reduction and TuffZirc plications for the architecture and evolution of the Reno, Nevada, Nevada Petroleum Society, 41 p. age determinations. 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