Late Cenozoic Paleogeographic Evolution of Northeastern Nevada: Evidence from the Sedimentary Basins

Late Cenozoic paleogeographic evolution of northeastern Nevada: Evidence from the sedimentary basins

Alan R. Wallace* U.S. Geological Survey, MS 176, Mackay School of Earth Sciences and Engineering, University of Nevada, Reno, Nevada 89557, USA Michael E. Perkins* Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112, USA Robert J. Fleck* U.S. Geological Survey, 345 Middleﬁ eld Road, Menlo Park, California 94025, USA

ABSTRACT lier faults are more pronounced east of the hot-spring deposits formed at and near the Tuscarora Mountains, possibly refl ecting a paleosurface in the Chimney, Ivanhoe, and Field and geochronologic studies of Neo- hanging-wall infl uence related to uplift of the Carlin basins as those basins were forming. gene sedimentary basins in northeastern Ruby Mountains-East Humboldt core com- The Neogene geologic and landscape evolu- Nevada document the paleogeographic and plex on the east side of the Elko basin. The tion had variable effects on all of these depos- geologic evolution of this region and the later faults are concentrated along the north- its, including uplift, weathering, supergene effects on major mineral deposits. The broad northwest–trending northern Nevada rift enrichment, erosion, and burial, depending area that includes the four middle Miocene west of the Tuscarora Mountains. The area on the events at any particular deposit. As basins studied—Chimney, Ivanhoe, Car- west of the rift contains major tilted horsts such, this study documents the importance of lin, and Elko, from west to east—was an and alluvium-fi lled grabens, and differential evaluating post-mineralization processes at upland that underwent prolonged middle extension between this more highly extended both regional and local scales when exploring Tertiary exposure and moderate erosion. region and the less extended area to the east for or evaluating the diverse mineral deposits All four basins began to retain sediments produced the intervening east-northeast– in this area and other parts of the Basin and at ca. 16 Ma. Eruption of volcanic fl ows in striking faults. Range region. the Chimney and Ivanhoe basins produced The Humboldt River drainage system short-lived (ca. 2 Ma), lacustrine-dominated formed as the four basins became integrated Keywords: sedimentary basins, tectonics, geo- basins before the dams failed and the streams after ca. 9.8 Ma. Flow was into northwestern morphology, Nevada, Miocene, Pliocene, gold, drained to the southwest. In contrast, early, Nevada, the site of active normal faulting and Humboldt River. high-angle, normal faulting induced fl uvial graben formation. This faulting lowered the to lacustrine sedimentation in the Carlin base level of the river and induced substantial INTRODUCTION and Elko basins, and volcanic fl ows further erosion in upstream regions. Erosion pref- blocked drainage in the Carlin basin until erentially removed the poorly consolidated Any physiographic map of northern Nevada the basin drained at ca. 14.5 Ma. The Elko Miocene sediments, progressively reexposed clearly shows numerous elongate mountain basin, with continued synsedimentary fault- the pre-middle Miocene highlands, and trans- ranges separated by sedimentary basins, per- ing, retained sediments until ca. 9.8 Ma and ported the sediments to downstream basins. fect examples of the Basin and Range physio- then drained west into the Carlin basin. Sedi- Thus, some ranges in the upstream region graphic province. Quaternary alluvium derived ment buildup in all basins progressively bur- are exhumed older highlands rather than from the ranges blankets many of the basins, ied existing highlands and created a subdued newly formed horsts. In addition, the drain- and Pliocene and Miocene sediments comprise landscape. age system evolution indicates that northern part of the underlying Tertiary sedimentary Relatively minor post-sedimentation exten- Nevada has become progressively lower than sequence (Stewart and Carlson, 1978; Effi moff sion produced early north-northwest–strik- central Nevada since the middle Miocene. and Pinezich, 1981; Gordon and Heller, 1993; ing normal faults with variable amounts Mineral belts with large Eocene gold Hess, 2004; Wallace, 2005). Historically, the for- of offset, and later east-northeast–striking deposits are exposed in uplands and con- mation of these basin-range pairs has been attrib- normal faults with up to several kilometers cealed beneath Neogene basin units in the uted to crustal extension and faulting that began of vertical and left-lateral offset. The ear- study area. Also, numerous epithermal in the middle Miocene and has continued to the

*[email protected]; [email protected]; ﬂ [email protected]

Geosphere; February 2008; v. 4; no. 1; p. 36–74; doi: 10.1130/GES00114.1; 14 ﬁ gures; 2 tables.

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present. Until recently, however, little work be relicts of early Tertiary uplands (Haynes, extends across north-central Nevada from the has focused on the actual ages of the horsts 2003; Wallace, 2005). In addition, some fault- Santa Rosa Range east to the western base of and grabens. ing events did not produce major uplift (Gordon the Ruby Mountains (Fig. 1). The primary goal Recent fi eld and thermochronologic studies and Heller, 1993; Wallace, 1993, 2005; Colgan of the study was to use the facies relations and have demonstrated that the ages of basin-range et al., 2008). ages of the sedimentary units to defi ne the evo- pairs in this area are diverse. The Ruby Moun- These relations indicate that the history of lution of the late Cenozoic paleogeography and tains (Fig. 1) experienced major uplift at ca. 15– “basin-range” tectonics is not straightforward. the relative effects of faulting, uplift, sedimen- 14 Ma (Colgan and Metcalf, 2006), other ranges To begin to understand the late Cenozoic his- tation, and erosion during long-term landscape did not begin to form until ca. 10 Ma (Wallace, tory of the area, this study has examined the evolution. While this paper does not attempt to 1991, 2005; Colgan et al., 2004, 2006), and Neogene geology, with a focus on the sedimen- explain the broader Basin and Range region, it some ranges, such as the Adobe Range, may tary basins, along a 200-km-long transect that does provide process-oriented information that may be applicable to the region, as a whole, and extensional terrains, in general. The study area includes world-class, Paleo- gene and older mineral deposits in the basement rocks and Miocene epithermal deposits in or adjacent to the Miocene basins (Fig. 1). Consequently, northern Nevada is the third-largest producer of gold in the world. An important goal of the study was to determine the effects of late Cenozoic landscape evolution on these mineral deposits, with implications for the formation and modifi cation of the deposits and deposit- to regional-scale mineral exploration and assessment. The study area includes four middle and late Miocene sedimentary basins. From west to east, they include what are referred to in this paper as the Chimney, Ivanhoe, Carlin, and Elko basins, which were actively receiving sediments between ca. 16.5 and 9.8 Ma (Figs. 1 and 2). All of the Miocene strata in each basin were examined on at least a reconnaissance basis, and appropriate samples of the sedimentary and coeval volcanic rocks were collected and dated to provide a time-stratigraphic framework. In addition, the Neogene sedimentary units in Pine and Independence Valleys (Fig. 1) were studied and dated on a reconnaissance basis. This paper describes the geology and histories of the basins from the early Miocene to the present, including their paleogeographic setting, sedimentation, faulting, erosion, and any coeval volcanism. Later sections of the paper discuss the similarities and differences between the basins, leading to a synthesis of the late Ceno- zoic evolution of the study area as a whole, its Figure 1. Locations of the Miocene Elko, Carlin, Ivanhoe, Chimney, and adjacent sedimen- effect on mineral deposits, and implications for tary basins, shown with the light blue color, and major geographic features in northeastern the more regional paleogeographic evolution. Nevada. Areas with the “v” pattern are underlain by middle Miocene rhyolitic and basaltic Each major section begins with a short sum- volcanic rocks; JR—Jarbidge Rhyolite; NNR—northern Nevada rift; SR-C—Santa Rosa– mary of the details presented in the ensuing Calico volcanic fi eld. The dotted yellow lines indicate the major late Eocene gold-deposit descriptions. trends; BM-E—Battle Mountain–Eureka; C—Carlin; G—Getchell; J—Jerritt Canyon. In all of the basins, the two principal compo- The solid red circles are the locations of middle Miocene epithermal systems that were active nents of the sediments are air-fall ash and pum- at the same time as the sedimentary basins; other epithermal systems beyond the limits ice that were derived from distal to, less com- of this study area are not shown. Abbreviated geographic locations: B—Beowawe; EH— monly, local eruptions, and materials that were Elko Hills; LT—Lone Tree mine; MC—Mule Canyon; MM—Marys Mountain; P—Preble eroded from pre-Miocene bedrock exposures in mine; PH—Peko Hills; PV—Paradise Valley; R—Rain mine; TC—Twin Creeks mine. R- uplands that surrounded the basins. The bed- EH—Ruby Mountains–East Humboldt Range detachment and high-angle faults. Inset map rock-derived materials are generically referred shows the location of the study area (gray); W—Winnemucca; PF—Pine Forest Range. to as epiclastic sediments to highlight their more

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paleosurface at that location (Fig. 1; Theodore and Blake, 1975). Middle Tertiary extension affected much of northern Nevada, although the amounts varied from minimal (Colgan et al., 2008) to at least 50 percent (Muntean et al., 2001). In most places, the age of extension can only be constrained to between the late Eocene and middle Miocene (Smith and Ketner, 1976; Smith and Howard, 1977; Wallace, 1993, 2003c). At Mule Canyon (Fig. 1), tilting took place between ca. 34 and 16 Ma (John et al., 2003), and, at Marys Moun- tain near Carlin (Fig. 1), tilting occurred both before and after the emplacement of a 25 Ma welded tuff and before the deposition of 15.3 Ma rhyolite fl ows (Henry and Faulds, 1999). In the southern Tuscarora Mountains, more deeply formed plutonic rocks were exposed by 16.5 Ma and shed clasts into the western part of the Carlin basin. Supergene alunite dates from gold deposits along the Carlin and Getchell trends indicate that there was enough uplift and erosion to erode and weather the deposits between 30 and 18 Ma (Hof- stra et al., 1999; Cline et al., 2005). As extension relaxed the crust, over-thick- Figure 2. Duration and primary lithologies (air-fall and epiclastic) of Miocene sedimentary ened parts of the crust became more buoyant. basins in north-central Nevada, shown from west to east. Also portrayed are signifi cant This allowed deep-seated metamorphic rocks to volcanic eruptions (“v”) that took place in the basins. The broad green line represents the ascend to shallower levels in the Ruby Moun- duration and general location of the northern Nevada rift. The two, best-exposed parts of tains and East Humboldt Range and produced a the Elko basin (Huntington Creek, Wells area) are presented and represent the age and lith- major, west-northwest–dipping detachment fault ologic variability of the basin as a whole. Parallel ash-rich and epiclastic bars indicate that along the west fl anks of those ranges. Displace- both lithologies were being deposited at the same time in different parts of the basin. Basin ment along this fault may have been more than abbreviations: CaB—Carlin basin; ChB—Chimney basin; EB—Elko basin; IB—Ivanhoe 50 km (Howard, 2003). The thermal, metamor- basin; IV—Independence Valley. Age data are provided in Table 1. phic, and igneous history of this complex dates back to before the Eocene, and major uplift related to high-angle faulting peaked between local derivation in comparison to the air-fall shallow lakes covered some lowlands, and 14 and 15 Ma (Dokka et al., 1986; Snoke et materials. Air-fall deposits that landed on the basin-fi lling sediments fl anked the uplands, al., 1997; Howard, 2003; Colgan and Metcalf, uplands were redistributed during erosion and including the southern Tuscarora Mountains, 2006). Uplift has continued into the Holocene stream transport and were intermixed with epi- Adobe Range, East Humboldt Range, and Ruby (Wesnousky and Willoughby, 2003). clastic materials at the depositional sites. Both Mountains (Haynes, 2003). Late Eocene fl ows Much more subdued extension stretched the sediment components were deposited in fl uvial and ash-fl ow tuffs were erupted onto and across crust by ~10% between the middle and late Mio- and lacustrine environments, and the distinc- this still-subdued landscape, in places fi lling cene (Muntean et al., 2001), although some areas tions between air-fall and bedrock-derived sedi- east- and west-draining paleovalleys (Henry just to the south were extended more than 100% ments in the strata are described in the text. and Ressel, 2000; Henry, 2008). Virtually all of (Colgan et al., 2008). This extension generally this igneous activity had ceased by ca. 38 Ma in was to the west, and pre-Miocene basement REGIONAL GEOLOGIC SETTING northeastern Nevada (Haynes, 2003). structures undoubtedly affected the apparent Major gold deposits, some of which formed local extension direction. For instance, north- A lithologically and structurally complex near coeval late Eocene igneous centers, are northwest–striking dikes along the northern assemblage of pre-Tertiary accreted terranes, hosted by Paleozoic rocks and comprise the Nevada rift (Fig. 1; see ensuing section) suggest sedimentary sequences, and plutons, a discon- northwest-trending Carlin and Battle Mountain- a west-southwest extension direction (Zoback et tinuous cap of Paleogene sedimentary and vol- Eureka mineral-deposit trends, the north-trend- al., 1994), whereas reconstructed offsets for a canic rocks, and Tertiary plutons underlie the ing Jerritt Canyon trend, and the north-north- large number of Miocene fault blocks in north- Neogene and Quaternary units along the tran- east–trending Getchell trend (Fig. 1; Cline et eastern Nevada indicate a generally northwest sect. Most of the pre-Neogene history does not al., 2005). The tops of the central Carlin trend but locally highly variable extension direction pertain to the Neogene story, but some Paleo- deposits were ~500–1500 m below the late (Muntean et al., 2001). gene events and features did lead into and (or) Eocene paleosurface (Haynes, 2003; Ressel Extension and the ascent of the Yellowstone affect local to regional Neogene processes. and Henry, 2006), and the tops of the slightly mantle plume into the crust along the Oregon- In the Eocene, moderate extension produced younger porphyry-related systems near Battle Nevada border induced widespread bimodal vol- modest-relief uplands and lowlands. Broad, Mountain were a few hundred meters below the canism starting at ca. 16.5 Ma (Christiansen et al.,

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2002). In northern Nevada, some of the early eastern Oregon between ca. 16.5 and 17 Ma, and likely was related to regional extension. In the mafi c magmas ascended and erupted along gen- it diverged to the east-northeast beneath the Snake northern Shoshone Range and at the latitude of erally north-northwest–striking crustal breaks, River Plain (the Yellowstone hot spot; Pierce et al., Midas, offset along north-northwest–striking such as the northern Nevada rift and related 2002) and to the west-northwest beneath the High faults along and near the locus of volcanism pro- mafi c dike complexes (John et al., 2000; Pierce Lava Plains of south-central Oregon (Christiansen duced a central downdropped zone. This zone is et al., 2002; Ponce and Glen, 2002). Partial et al., 2002). Early volcanism produced wide- 14–19 km wide in the northern Shoshone Range melting of the crust, coupled with magma mix- spread fl ood basalts, followed by felsic calderas, and ~30 km wide near Midas. Displacement ing, and fractional crystallization, created more domes, and fl ows. along the faults generally was about a kilometer, widespread rhyolitic volcanism (Coats, 1987; In north-central Nevada, bimodal volca- with little to no fault-related tilting of the Mio- John et al., 2000; Brueseke and Hart, 2007). The nism between ca. 16.8 and 14 Ma produced cene volcanic units. Overall, the total amount of heat from this magmatism, coupled with con- local to extensive volcanic fi elds composed of extension across the structural zone was no more duits provided by the faulting and water from mafi c fl ows to felsic domes, fl ows, and tuffs. than a few kilometers (John et al., 2000). Rift- the wet climate and lakes, generated a number The greatest amount of volcanic activity was related volcanism produced more than 1 km of of small to very large, epithermal gold + sil- focused along the north-northwest–trending volcanic rocks in 300,000 yr in the southwestern ver ± mercury deposits. Most of these deposits northern Nevada rift and the related Santa Rosa- Sheep Creek Range (John et al., 2000) and at formed near the interface between volcanic cen- Calico volcanic fi eld to the northwest (Fig. 1). least that much over 500,000 yr in the central ters and lacustrine basins (Fig. 1; John, 2001; Domes and fl ows of the Jarbidge rhyolite were Snowstorm Mountains (Wallace, 1993; Leavitt Wallace et al., 2004a, 2004b). erupted across the northeastern tier of the area et al., 2004; this study). With as much as 1 km of Extension continued after 10 Ma and led to as the mantle plume migrated to the east-north- synvolcanic downdropping, the relief of the vol- much of the fault-bounded horst-and-graben east, in some areas producing extensive and canic assemblage produced along the rift may physiography of northeastern Nevada (Stewart, thick masses of coarsely porphyritic rhyolite have been slight. Specifi c details on the relation 1998). Uplift rates may have been greater in with a minor amount of underlying basalt. Vol- between volcanism, faulting, and sedimentation the late Neogene than in the Quaternary (Col- canic activity occurred at the same time that the in the Snowstorm Mountains and the adjacent gan et al., 2004; Personius and Mahan, 2005). sedimentary basins described in this paper were Chimney and Ivanhoe basins are presented in Total extension across northern Nevada was forming and expanding, and, in many areas, vol- later sections of this paper. not great, and the formation of the horst-graben canism and sedimentation overlapped in both pairs was more vertical than in parts of southern time and space. Jarbidge Rhyolite and western Nevada, where the horsts are highly tilted fault blocks that formed during signifi cant Northern Nevada Rift The Jarbidge rhyolite loosely includes extension (Anderson, 1971; Proffett, 1977). coarsely porphyritic fl ows and domes, with The north-northwest–trending northern Nevada some related tuffaceous units (Coats, 1987). The Miocene Climate rift extends from east-central Nevada to near the type area is in the Jarbidge area of northeastern Oregon border (Fig. 1), a distance of ~500 km. Nevada (Fig. 1), and similar rhyolites are scat- As noted by Smith (1994), climate can have The rift originally was identifi ed by a pro- tered throughout northeasternmost Nevada and equal or greater infl uences on basin sedimen- nounced positive aeromagnetic anomaly related as far east as the Utah border. Because of the tation than do other geologic processes and to a narrow (~7 km wide), deep-seated middle highly viscous nature of the magmas and mode events. Precipitation causes erosion, creates Miocene mafi c dike complex that was intruded of emplacement, fl ow across the landscape streams that carry sediments, and fi lls confi ned along a preexisting crustal structure (Zoback probably was not great, and the distribution of basins with lakes. Modern annual precipitation and Thompson, 1978; Hildenbrand et al., 2000). the rhyolite likely refl ects the distribution of in the region is less, often much less, than 25 cm Intense bimodal volcanism occurred along the the eruptive centers. Miocene basin sediments (10 in), and a sage-grassland fl ora dominates rift and adjacent areas from ca. 16.0 to 14.9 Ma locally overlie, underlie, or encase rhyolite the landscape. In contrast, studies of middle (John et al., 2000; Leavitt et al., 2004). In gen- fl ows. North of the Independence Mountains, Miocene fl oras by Axelrod (1956) indicate that eral, thick mafi c fl ow sequences formed early, the rhyolite overlies 16.6 Ma basaltic andes- ~50–64 cm (20–25 in) of annual precipitation in followed in many, but not all, areas by the erup- ite fl ows (Rahl et al., 2002), and some nearby northeastern Nevada supported a mixed conifer tion of more viscous dacite to rhyolite fl ows and rhyolite domes were erupted at ca. 15.8–16 Ma and oak woodland fl ora, and pollen and other domes. The very similar and likely related Santa (Coats, 1987; C. Henry, 2004, written com- data indicate that pines were abundant and sage Rosa–Calico volcanic fi eld is just west of the mun.). Jarbidge-like fl ows and dikes just south- and grasslands were relatively minor (Davis and magnetic anomaly near the Oregon-Nevada bor- west of Wells in the East Humboldt Range were Moutoux, 1998; Retallack, 2001). As such, the der. This fi eld was active from ca. 16.7 to 14 Ma emplaced between 14.8 and 13.4 Ma (Snoke et middle Miocene basin sedimentation described (Brueseke and Hart, 2007), and volcanic units al., 1997), and some data suggest that rhyolitic in this paper was infl uenced by much greater related to both systems overlap in the area of the volcanism at Jarbidge, the type area, occurred at amounts of rainfall and different vegetative Chimney basin. Volcanism along the northern ca. 14 Ma (Bernt, 1998). cover, resulting in different runoff responses Nevada rift was more extensive, especially to Rhyolite fl ows were erupted at ca. 15.3 Ma than the predominantly sheet-fl ood runoff found the east, than the aeromagnetic anomaly, indi- west and south of Carlin (Fig. 1) to form the Pali- in the current semi-arid, sage-grassland setting. cating that the deeper crustal processes related sade Canyon rhyolite. These volcanic units have to magma genesis were not confi ned to just the an uncertain relation to both the northern Nevada MIOCENE VOLCANISM mafi c dike complex. rift and Jarbidge systems, although their mineral- Normal faulting along the rift occurred dur- ogies and chemistries suggest an origin similar to Initial, widespread, bimodal volcanism related ing the entire period of volcanism. Some of rhyolites in those systems. This unit is described to the Yellowstone mantle plume began in south- this faulting was related to the “rift,” and some in more detail in the Carlin basin section.

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Post-Basin Volcanic Units cal procedures). Sample 049-21E from the Car- 15 km. They extend south into the Midas dis- lin basin (Table 1) was dated using incremental- trict, where they have been dated at 15.7 Ma Extensive sheets of Miocene rhyolite and step heating on plagioclase. Tephra correlations (Table 1; Leavitt et al., 2004), and to the north basalt fl ows underlie much of the Owyhee Pla- were determined using the chemical and petro- to within 4 km of 16.1–15.5 Ma strata related to teau in northernmost Nevada and are related to logic methods described in Perkins et al. (1998). the Chimney basin. the Yellowstone mantle plume along the Snake Some of these samples produced more than one At Snowstorm Mountain, the sedimentary River Plain to the north (Fig. 1; Wood and Cle- possible correlation age—some strong, some units overlie and are interbedded with 16.1 Ma mens, 2002). These volcanic units were erupted weak, and some very disparate. Because almost basaltic andesite fl ow units and underlie 15.5– between ca. 15.4 and 8 Ma, and they conceal all tephra samples were collected either as pairs 15.7 Ma rhyolite fl ow units. The sedimentary virtually all underlying Miocene and older of tephra beds or as multiple samples from an units are as much as 100 m thick. Small areas of units, including the northern end of the north- unfaulted stratigraphic sequence, the correlation possibly related pyroclastic materials are inter- ern Nevada rift and any basin sediments that age that had the strongest chemical correlation bedded with basaltic andesite fl ows southwest may be present. These volcanic units are largely and that was consistent with other dates (tephra of Snowstorm Mountain. Sediments of this age unfaulted and retain their original low to hori- or isotopic) in that pair or specifi c section was are absent only in the west-southwestern part zontal dips. chosen. Some samples were dated using both of the volcanic fi eld, and the overall fi eld rela- isotopic and tephra correlation methods (cf. tions suggest that the sedimentary rocks may GEOCHRONOLOGY Wallace et al., 2007a), but most samples were be continuous beneath younger volcanic rocks dated using only one method. throughout much of the fi eld (Wallace, 1993). The geochronology used for this study Offset along predominantly north-northwest– included both new and previously published SEDIMENTARY ROCKS IN THE striking normal faults occurred during volcanism data obtained by several methods. The new data SNOWSTORM MOUNTAINS in the Snowstorm Mountains. As exposed in were obtained from 40Ar/39Ar dates on mineral both the Snowstorm Mountain and Castle Ridge separates from ash beds and volcanic rocks or, A large, bimodal, volcanic fi eld related to the areas, faulting offset and tilted the 15.7 Ma and in the case of aphanitic mafi c volcanic rocks, on northern Nevada rift formed in the Snowstorm older volcanic and sedimentary units prior to the whole rocks or groundmass separates, and from Mountains between the Chimney and Ivanhoe eruption of 15.5 Ma and younger rhyolite fl ows, tephra correlations of volcanic ash beds within basins (Fig. 3; Wallace, 1993). Middle Miocene some of which fi lled a broad, fault-controlled the sedimentary units. Previously published data sedimentary rocks in the area are composed of basin between Castle Ridge and Snowstorm included tephra correlations and 40Ar/39Ar and sediments that were deposited between 16.1 Mountain. The younger units are relatively conventional K-Ar dates, as well as four fi ssion- and 15.5 Ma during rift-related volcanism, as unfaulted and have very modest dips, and a few track dates on detrital zircons collected in the well as post-rift sediments that underlie 10 Ma ca. 15.1 Ma fl ows near Snowstorm Mountain Snake Mountains near Wells (Thorman et al., basalt fl ows along the northeastern margin of contain directional fl ow-related folds that indi- 2003). Table 1 provides all of the dates and cor- the Snowstorm Mountains. The strata in this cate fl ow westward away from the central part relation ages cited in this paper, and Appendix 1 area are described fi rst because of their relation of the volcanic center (Wallace, 1993). gives brief descriptions of unpublished samples to those in the adjacent Chimney and Ivanhoe The younger sedimentary rocks on the north- collected and dated for this study. Several fi g- basins, which are described next. eastern side of the Snowstorm Mountains overlie ures in the text show the approximate locations The older sedimentary units include leuco- the 15.5 Ma Little Humboldt rhyolite, and they where samples were collected, along with the cratic air-fall deposits and poorly consolidated, underlie and interfi nger with 9.8 Ma basalt fl ows date or date range. In addition, Table 2, intro- tuffaceous, sedimentary units. The strata are of the Big Island Formation (Wallace, 1993). A duced later in the paper, summarizes published generally fi ne grained and thickly to thinly broader study of the mixed sedimentary and and unpublished K-Ar and 40Ar/39Ar dates on bedded. The air-fall deposits are pumiceous, basaltic Big Island Formation in northernmost supergene alunite from several gold deposits in unwelded to poorly welded, and locally contain Nevada showed that sedimentation closely pre- the study area. All 40Ar/39Ar dates in the tables, abundant accretionary lapilli. Most of the sedi- ceded and coincided with the eruption of the including the dates used for the tephra corre- mentary materials are either primary or locally basalt fl ows (Coats, 1985). On the northeastern lations, were calculated or recalculated using reworked air-fall deposits or epiclastic sedi- side of the Snowstorm Mountains, the fi ne- to 28.02 Ma for the Fish Canyon Tuff sanidine ments derived from nearby volcanic fl ow units. medium-grained sedimentary units are weakly standard (Renne et al., 1998). Cross bedding and graded bedding of some to moderately consolidated. Most of the mate- Samples of bedded air-fall tuff in the Mio- ash-rich beds indicate some fl uvial reworking, rials are air-fall tuff and reworked tuffaceous cene strata were collected to ensure the fresh- but nonvolcanic, epiclastic material is absent. material, although some pebble-rich layers con- est and least contaminated material. As much Clinoptilolite and diagenetic chert replace some tain rhyolitic clasts. Clast imbrications and com- as possible, horizons with minimal evidence of of the thin-bedded sedimentary units, similar to positions, as well as cross-bedding directions, reworking were collected parallel to bedding alteration of units in the Chimney basin to the indicate that these clasts were derived from the to avoid mixed populations of tephras and any west, as described later in this paper. Snowstorm Mountains area to the south. Addi- vertical differences in the tephra compositions. The older sedimentary units are most exten- tional post-Little Humboldt rhyolite sedimentary Samples of volcanic fl ow units were collected sively exposed along Castle Ridge and at Snow- units are widespread throughout the Snowstorm from outcrops of unaltered rocks. storm Mountain itself (Fig. 3; Wallace, 1993). Mountains area, but their correlation to those in The 40Ar/39Ar dates were obtained by laser- The Castle Ridge units overlie and locally inter- the Big Island Formation can only be inferred fusion analyses of multiple sanidine crystals fi nger with 16.1 Ma basaltic andesite fl ow units from their general stratigraphic position. from the same sample, and dates and uncertain- and underlie 15.5–15.7 Ma rhyolite fl ow units These younger sedimentary and basaltic units ties are the weighted means of multiple analyses (Table 1). The units reach a thickness of ~100 m in the Snowstorm Mountains area are not faulted, (see Fleck et al. (1998) for details of the analyti- and are continuous along strike for more than and they overlie faulted and tilted volcanic and

40 Geosphere, February 2008

Relation to basin sediments sediments basin

– Below Theodore et al. (2006) (2006) al. et Theodore – Below – Below Theodore et al. (2006) (2006) al. et Theodore – Below (Ma) Tephra Tephra corr. age age corr. 0.99 0.99 0.34 0.34 ±

± Ar age Ar age – 15.4 ± 0.8 Within Perkins et al. (1998) (1998) al. et 0.8 Within Perkins ± – 15.4 – ± 0.25 15.41 Within (1998) al. et Perkins – ± 0.1 14.7 contact Lacustrine-fluvial (ChB-1) study This – ± 0.25 15.31 Within (1998) al. et Perkins 39 (Ma) Ar/ 9.8 ± 2.5* – units all Above et al. (1990) Wallace 15.6 ± 0.1 15.6 ± 0.1 – Below (2002) and Wrucke John 15.27 ± 07 ± 07 15.27 – Within (1998) al. et Fleck 15.22 15.24 16.07 ± 0.2 ± 0.2 16.07 – estimated age Below; (2004) al. et Leavitt 16.11 ± 0.1 ± 0.1 16.11 – Within (2003) Wallace 15.1 ± 0.04 0.04 ± 15.1 – estimated age Above; (1993) Wallace 15.1 ± 0.04 ± 0.04 15.1 – Above (2002) and Wrucke John 15.19 ± 0.1 15.19 – Within (1998) al. Fleck et 15.2 ± 0.04 ± 0.04 15.2 – Within (2003) and Wrucke John 14.95 ± 0.2 ± 0.2 14.95 – Above (2002) Wrucke and John 16.06 ± 0.2 16.06 – Below et al. (2004) Leavitt 16.06 ± 0.2 ± 0.2 16.06 – Below (2004) al. et Leavitt 40 22.1 ± 0.08* 0.08* ± 22.1 – Below (1994) McKee and Wallace 15.35 ± 0.11 ± 0.11 15.35 – Within (1998) al. et Fleck 15.45 ± 0.04 ± 0.04 15.45 – above and Interfingered (ChB-2) study This 15.69 ± 15.69 0.12 – Above et al. (2004) Leavitt 15.80 ± 15.80 0.09 – Within et al. (2004) Leavitt 14.67 ± 0.08 ± 0.08 14.67 – Within (1998) al. et Fleck 14.73 ± 0.05 – Within Fleck et al. (1998) 15.45 ± 0.04 ± 0.04 15.45 – within Above, (ChB-1) study This 15.69 ± 15.69 0.12 – Above et al. (2004) Leavitt 14.77 ± 0.05 ± 0.05 14.77 – Within (1998) al. et Fleck 16.11 ± 0.04 ± 0.04 16.11 – Below (ChB-4) study This 15.18 ± 0.05 ± 0.05 15.18 – Above (2003) Wallace 15.33 ± 0.08 15.33 – above Within, (2003) Wallace 15.36 ± 0.08 – Within Fleck et al. (1998) 15.85 ± 0.22 ± 0.22 15.85 ± 0.08 16.33 – – Interfingered (ChB-3) study This Below (ChB-5) study This 15.70 ± 15.70 0.09 – Within et al. (2004) Leavitt 15.28 ± 0.05 ± 0.05 15.28 – above Within, (1998) al. et Fleck 15.36 ± 0.05 ± 0.05 15.36 – Within (2003) Wallace 14.17 ± 0.08 ± 0.08 14.17 – at top Fluvial (2005) al. et Breit 15.36 ± 0.06 ± 0.06 15.36 – Within (2003) Wallace 15.43 ± 0.05 ± 0.05 15.43 – Within (2003) Wallace

e d u t 10.6 50.1 i

31.83 03.42 22.39 04.11 27.46 27.65 31.56 59.13 45.77 45.48 46.38 27.55 33.95 45.55 33.48 31.56 57.47 59.13 06.24 45.55 27.43 19.88 11.09 36.38 37.77 25.39 31.56 25.29 45.41 27.72 20.44 51.52 46.38 45.41 29.59 10.60 35.26 40.47 ° ° g ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° n o 117 116 L 117 117 116 116 117 116 116 116 116 116 116 116 116 116 116 116 116 116 117 116 117 117 117

0 3 4 5 4 9 2 8 6 8 8 0 6 4 1 2 1 1 5 6 2 7 e 2 6 5 8 3 2 5 9 2 9 1 6 6 6 8 6 4 4 5 1 3 6 d ...... u 4 .64 116 3 7 3 3 3 5 4 3 5 3 5 4 5 6 6 3 9 2 3 0 4

t ° i 1 1 1 1 1 2 0 0 0 0 0 4 0 0 2 2 2 2 1 5 2 5 14.08 116 14.08 04.87 116 04.87 15.98 116 15.98 26.51 116 26.51 116 27.45 15.98 116 15.98 11.44 116 11.44 05.02 116 05.02 14.58 116 14.58 14.08 116 14.08 116 14.58 05.47 116 05.47 07.73 116 07.73 06.70 116 06.70 t ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° a 1 1 1 1 1 1 1 1 1 1 1 1 0 1 1 1 1 1 0 1 1 0 41 L 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 41 41 41 46 41 41 41 41 41 41 41 41 41 41

e e . . . . s s s s s s s

g g

s s s s d d d d d d d n l l l l l l l n n a

e e e e

o e e e e e e e a i a s e i i i i i i i

R R R R t h r t k

t R R F F F F F F F

a r a s

k k k k e e e e e r

c t t s o s s s s s a k k h a a a a a a a t t e e e e e o o o o e e i i i i i i i t o s s r t a n a a a a a e e e s s l e e e e h h h h k

n n n n n n n u

i r r r r e e n d d d d d , d h l e e a a C n n n n r i i i i i i t t e e e e e e e r r o

FOR MIDDLE MIOCENE SEDIMENTARY AND RELATED UNITS, NORTHEASTERN NEVADA t e a r C C C C a a a a E E s W W

r n s S R R R R R R R M M M M M M C C i o v v v C v

e I I I I d w w w w TABLE 1. ISOTOPIC, TEPHRA CORRELATION, AND FISSION-TRACK GEOCHRONOLOGIC DATA l a a a a a a a w N p p n t t t t t t t o o o o W o l l l l e e T e l l l l n n n n n n n i i i i e e G a a a a a a a G h h W W W W S S S S S S S S S

) e

i s l

W a t e e e t l o t t t w i i i N

a y ( o

s r s s

e s e h t

f e t e e e r

e i w w s a

t l m p w w w

d d d i f e o o b k l

f t y l l o s o

o o o n n n i f f f f f f f f f f f t l l w f l f l e o f f f f f f f f f f f y u

f f

d f d a a a t f

y e o

k u u u u u u u u u u u h

l f o e e n

r t t t t t t t t t t t f r t t e e e e y c c c h c

i i u

t e t t i t i i a r l l l l l l l l l l l a a a a i i i i t t l t t t C h l l l l l l l l l l l

o l s s l l l l l l e r r r c e r

r r t s e a a a a a a a a a

a a g i o e e o o o

c d h h a h h a I a a y i f f f f f f f f f f f i l v l f

r d y c y y y - - - l ------l d s d s s r p p p p f a e r r r t r r r r r r r r g o e h h a h h r i e i i i n a e i i i i i i i a a i i n e e e e u

t i A R T Little Humboldt rhyolite rhyolite Humboldt Little Northeast 41°27.45 T R B A R K Rhyolite dome dome Rhyolite Range Creek Sheep 40°51.37 Craig rhyolite (west) Ivanhoe A A T A A B A Olivine basalt flow flow basalt Olivine Range Creek Sheep 40°45.68 R B A A Craig rhyolite (east) (east) rhyolite Craig Fields Renia Santa D T Little Humboldt rhyolite rhyolite Humboldt Little Northwest T B A A W R A Craig rhyolite (east) Ivanhoe V A V C n U Ivanhoe basin (Fig.5) Snowstorm Mountains (Fig. 3) (Fig. Mountains Snowstorm Chimney Basin (Fig. 3) (Fig. Basin Chimney

Geosphere, February 2008 41

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Relation to basin sediments sediments basin

– Below This study (CB-13) study This – Below (Ma) Tephra Tephra corr. age age corr. 0.03 ±

Ar age Ar age – 15.26 Within This study (CB-12) (CB-12) study This Within – 15.26 – 15.28 Within Perkins et al. (1998) (1998) al. et Perkins Within – 15.28 – 14.68 Within This study (CB-4) (CB-4) study This Within – 14.68 – 14 Within (1998) al. et Perkins – 9.91 Within (1998) al. et Perkins – <15.28 Within This study (CB-6) (CB-6) study This Within – <15.28 – 15.9 Within (1994) unpubl. S. Williams, – 15.5 ca. Within et al. (2007a) Wallace – ca. 15.1 Within This study (CB-7) (CB-7) study This Within 15.1 – ca. – ca. 15.5 Within Wallace et al. (2007a) (2007a) al. et Wallace Within 15.5 – ca. – 14.8 Within This study (CB-9) (CB-9) study This Within – 14.8 – ca. 14.6 Within This study (CB-11) study This Within 14.6 – ca. – ca. 15.1 Within This study (CB-7) (CB-7) study This Within 15.1 – ca. – 11.92 Within Perkins et al. (1998) (1998) al. et Perkins Within – 11.92 – 12.17 Within Perkins et al. (1998) (1998) al. et Perkins Within – 12.17 – 14.1 ca. Within study (CB-8) This – ca. 14.6 Within This study (CB-10) study This Within 14.6 – ca. – >15.28 Within This study (CB-6) (CB-6) study This Within – >15.28 – 15.18 Within This study (CB-3) (CB-3) study This Within – 15.18 – 12.12 Within Perkins et al. (1998) (1998) al. et Perkins Within – 12.12 – 16.0 ca. Within et al. (2007a) Wallace – 15.27 Ma Within This study (CB-1) (CB-1) study This Within Ma – 15.27 – 15.18 Within This study (CB-2) (CB-2) study This Within – 15.18 – 15.2 Within S. Williams, unpubl. (1994) (1994) unpubl. Williams, S. Within – 15.2 – 12.03 Within Perkins et al. (1998) (1998) al. et Perkins Within – 12.03 – ca. 15.8 Within Wallace et al. (2007a) (2007a) (2007a) al. al. et Wallace et Within 15.8 – ca. Wallace – Within – 16 – 15.84 15.94 Within (2007a) al. et Wallace Within (2007a) al. et Wallace 39 (Ma) Ar/ 15.4 ± 1.0* 1.0* ± 15.4 – Below (1970) Armstrong 15.3 ± 0.03 0.03 ± 15.3 – Within et al. (2007a) Wallace 15.11 ± 0.7 15.11 – Within (EB-1) study This 16.37 ± 0.1 16.37 – Within et al. (2007a) Wallace 40 15.26 15.26 14.82 ± 0.06 ± 0.06 14.82 14.68 Within (CB-5) study This 15.35 ± 0.06 ± 0.06 15.35 15.31 Within study this (1998); al. et Perkins 15.17 ± 0.03 ± 0.03 15.17 15.3 ca. Within (2007a) al. et Wallace 15.35 ± 0.06 ± 0.06 15.35 15.31 Within study this (1998); al. et Perkins 15.56 ± 0.05 ± 0.05 15.56 – Within (2007a) al. et Wallace 15.32 ± 0.04 – Within Henry and Faulds(1999) 16.16 ± 0.03 ± 0.03 16.16 – Within et al. (2007a) Wallace 16.03 ± 0.05 ± 0.05 16.03 – ± 0.08 15.92 15.7 Within Within et al. (2007a) Wallace et al. (2007a) Wallace ± 0.05 14.67 14.78 Within study this (1998); al. et Perkins 16.27 ± 0.04 ± 0.04 16.27 16.0 ca. Within (2007a) al. et Wallace 15.76 ± 0.02 ± 0.02 15.76 15.8 ca. ± 0.25 16.30 Within 16.02 et al. (2007a) Wallace Within (2007a) et al. Wallace 15.75 ± 0.03 ± 0.03 15.75 15.8 ca. Within et al. (2007a) Wallace 15.73 ± 0.06 ± 0.06 15.73 16.1–15.5 Within (2007a) al. et Wallace (continued)

e d u t 0.52 0.52 16.1 0.52 0.52 01.1 01.1 7.81 7.81 i

03.55 14.77 13.86 10.84 03.55 56.42 55.24 11.20 02.44 05.68 11.83 17.26 08.93 55.24 08.93 56.42 05.71 05.66 05.13 17.26 15.31 00.93 05.72 05.66 05.66 05.33 05.24 05.14 15.31 05.31 08.06 03.37 ° ° ° ° ° ° 42.730 42.730 42.730 42.730 42.730 42.730 42.730 42.730 31.155 31.155 42.730 42.730 42.730 42.730 42.730 42.730 42.730 42.730 42.730 42.730 42.730 g ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° n o TABLE 1. 116 116 L 116 116 116 116 116 116 116 116 115 115 115 115 115 115 115 115 115 115 115

4 8 8 8 8 8 8 8 8 8 8 8 0 8 9 4 3 8 9 e 4 7 7 7 7 7 7 7 7 7 7 3 3 3 8 3 2 8 8 d ...... 3 8 8 8 8 8 8 8 8 8 8 ...... u 2 8 2 1 1 4 4 4

t 1 4 4 4 42.7 116 42.7 4 4 4 4 4 4 4 i 4 5 5 5 5 4 4 4 39.95 116 39.95 116 38.41 07.00 116 07.00 46.03 115 46.03 51.71 116 51.71 49.82 116 49.82 36.92 116 36.92 37.34 116 37.34 46.16 115 46.16 116 48.19 51.81 116 51.81 116 51.39 116 51.35 50.78 116 50.78 48.19 116 48.19 116 54.94 49.86 116 49.86 48.30 116 48.30 50.60 116 50.60 116 51.81 116 51.39 115 46.16 51.61 116 51.61 116 50.78 33.66 116 33.66 46.03 115 46.03 50.62 116 50.62 ° t 0 3 3 3 3 3 3 3 3 3 3 ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° ° a 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 40 L 4 4 4 4 4 4 4 4 41 41 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 4 4 4 4 4 4 4 4 4 4 4

y k k k k k k k k k k e e n e

e e e e e e e e e e l l l l g g o l

i e e e e e e e e e e

a a a t t t t n n t t t r r r r r r r r r r a t t r r r s s s s l t t t a s s s a a s s l V C C C C C C C C C C

i e e e

n a a a t t a a R R

o e n e n n n n n e n n n n e s e e s e H w w w e e l

c c c e e n o o o o o o o o o o h h h e e h h h h h l i t - t t t t t - t t t t - t t t o t t t t t b b a r r g g g h g g g g h g g g h k P u u u u u u W W r o o t t t

o o n n n n n n n n n n e o o o r r r o o o e i i i i i i i i i i d d d t t t t t t t t t t N N o o o P S S S n S S S n A A n n n n n n n n n n

e N N N e . . u u u u u u u u u u

G S S H H H H H H H H H H N

h h h h h h h h h h h h h h

c c c c c c c c c c c c c c i i i i i i i i i i i i i i e r r r r r r r r r r r r r r ------p h h h h h h h h h h h h h h y t s s s s s s s s s s s s s s

a a a a a a a a a a a a a a k e

/ / / / / / / / / / / / / / t c i a a a a a a a a a a a a a a a a a a a a a a a a a a o s r r r r r r r r r r r r r r r r r r r r r r r r r r r e

h h h h h h h h h h h h h h h h h h h h h h h h h h r d p p p p p p p p p p p p p p p p p p p p p p p p p p o n e e e e e e e e e e e e e e e e e e e e e e e e e e

t i T T T T T T T T Palisade Canyon rhyolite rhyolite Canyon Palisade West T T T T T T Palisade Canyon rhyolite rhyolite Canyon Palisade Southwest Tephra/mixed epiclastic-air-fall epiclastic-air-fall Tephra/mixed rhyolite Canyon Palisade West 40°48.78 Southwest Tephra/mixed epiclastic-air-fall epiclastic-air-fall Tephra/mixed SW basin Elko A T Tephra/mixed epiclastic-air-fall epiclastic-air-fall Tephra/mixed epiclastic-air-fall Tephra/mixed North-central North-central T Tephra/mixed epiclastic-air-fall epiclastic-air-fall Tephra/mixed Northeast Tephra/mixed epiclastic-air-fall epiclastic-air-fall Tephra/mixed West 40°48.78 Tephra/mixed epiclastic-air-fall epiclastic-air-fall Tephra/mixed epiclastic Tephra/basal West-central Northwest Tephra/mixed epiclastic-air-fall epiclastic-air-fall Tephra/mixed SW basin Elko Tephra/mixed epiclastic-air-fall epiclastic-air-fall Tephra/mixed Northeast T T Tephra/mixed epiclastic-air-fall epiclastic-air-fall Tephra/mixed epiclastic-air-fall Tephra/mixed North-central epiclastic-air-fall Tephra/mixed North-central Northeast Tephra/mixed epiclastic-air-fall epiclastic-air-fall Tephra/mixed North-central T T T T T T T Tephra/mixed epiclastic-air-fall epiclastic-air-fall Tephra/mixed epiclastic-air-fall Tephra/mixed North-central North-central Tephra/mixed epiclastic-air-fall epiclastic-air-fall Tephra/mixed Northeast Tephra/mixed epiclastic-air-fall epiclastic-air-fall Tephra/mixed West-central Tephra/mixed epiclastic-air-fall epiclastic-air-fall Tephra/mixed North-central T n Carlin basin(Fig. 7) U Elko Basin (Fig. 10) (Fig. Elko Basin

42 Geosphere, February 2008

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sedimentary units related to the older assemblage. Although these Big Island-related units are somewhat younger than those in other basins described in this paper, they do indicate that 10 Ma streams drained northeastward from a modest highland that had been faulted by that time.

Reference Reference CHIMNEY BASIN

Chimney basin is generally west of the Snowstorm Mountains, east of Paradise Valley,

and north of the Osgood Mountains and Hot Springs Range in eastern Humboldt County and westernmost Elko County (Fig. 3). Widespread Miocene sedimentary units are centered on

Chimney Reservoir. The most extensive exposures are north of and along the Little Humboldt Relation to isted after “This study” refer to samples described in River. Pliocene and younger alluvial sediments basin sediments sediments basin largely conceal the Miocene units south of the river. On the basis of gravity, depth-to-basement data (Ponce, 2004), the aggregate thickness of

– Below This study (CB-13) study This – Below the Miocene units and any underlying Tertiary (Ma) Tephra Tephra corr. age age corr. volcanic rocks is much less than 1 km throughout the entire basin. Geochronologic data for the basin are provided in Table 1. 0.03

Ar age In general, the Chimney basin was active – 14.3 Within This study (IV-2) (IV-2) study This – 14.3 Within – ca. 15.5 Within This study (IV-5) (IV-5) study 15.5 Within This – ca. – ca. 15.9 Within S. Williams, unpub. data (1994) (1994) data unpub. Williams, 15.9 Within S. – ca. – 14.8 Within This study (IV-3) (IV-3) study This – 14.8 Within – ca. 15.9 Within This study (IV-4) (IV-4) study 15.9 Within This – ca. – 15.28 Within This study (IV-6) (IV-6) (CB-1) study study – 15.28 This Within – 15.27 This Within – 14.2 Within This study (IV-1) (IV-1) study This – 14.2 Within ± 39 (Ma) from ca. 16.3 Ma until 14.2 Ma (Fig. 2). The Ar/ 15.4 ± 1.0* 1.0* ± 15.4 – Below (1970) Armstrong 14.8 ± 0.5* 0.5* ± 14.8 – Below (1997) al. et Snoke 13.4 ± 0.5* 0.5* ± 13.4 – Below (1997) al. et Snoke 10.95 ± 0.9 ± 0.9 10.95 unit) (middle Within (2003) al. et Thorman 10.48 ± 0.1 – Within (upper unit) This study (EB-2) (EB-2) study This unit) (upper – Within 0.1 ± 10.48 40 15.26 basin was a broad lowland, and coeval volcanic activity blocked westward streamﬂ ow to

e (continued)

d form a shallow, ephemeral lake. The basin in u t i

11.83 08.06 11.20 59.16 its early stages extended to the southeast across 58.830 58.830 54.351 54.351 58.734 58.734 55.653 55.653 58.145 58.145 58.090 58.090 54.235 54.235 02.381 02.381 51.395 51.395 01.486 01.486 g ° ° ° ° ° ° ° ° ° ° ° ° ° ° n

o the northern Nevada rift and connected with the L 114 114 115 115

TABLE 1. Ivanhoe basin near Midas. Continued volcanism

along the fringes of the basin limited expansion

1 4 1 e 5 1 6 9 . d 1 5 4 of basin sedimentation, and late-stage uplift in . . 7 . u

t .312 115 .314 115 5 5 0 0 i 36.92 116 36.92 37.34 116 37.34 ° ° ° t 0 0 10.857 115 10.857 12.005 115 12.005 10.049 115 10.049 00.759 115 00.759 1 33.661 116 33.661 10.881 115 10.881 ° ° the Snowstorm Mountains produced ﬂ uvial sed- ° ° 1 ° ° ° ° ° ° ° a 1 1 4 1 40 40 L 40 40 4 4 4 41 41 41 41 40 41 imentation in the eastern part of the basin. Most fault activity and uplift, however, took place after

) ) ) ) 6 6 6

6 sedimentation, and uplift of two major horsts

2 2 2 2 ) )

2 2 2 ) k k 2

n just to the south did not affect the basin. Despite e e . . n i V V V V e e s s s o i r r N N N

n n N t a

a temperate, moist climate, no sediments were t t s s

, , , C C l l a , t t t B l l

c M M u u u e e p p h h

u e o c t c c t deposited after 14.2 Ma, and the basin presum- l c e e r r

u m m

d d d W W l l k k d

o o a a a a a f f a B a a a ably began to drain externally at that time. r N N ( o o o o o C C n n o

r r r ( ( e r ( ( (

h S S (

S S n t

h h l . . n n n t t e u r r r a u u S S o r G e e e t o o t t t Stratigraphy S n s s s S S e e e e C W W W The majority of the exposed sedimentary rocks in the Chimney basin are composed of ﬁ ne-grained, evenly bedded, tuffaceous sedi-

ments and somewhat coarser unwelded, pumi-

e ceous, air-fall deposits. The ﬁ ner grained sedi- t i l

o mentary layers generally are thinly bedded, y h r (continued)

whereas the coarser, more pumiceous beds are

n Ar and tephra correlation dates were calculated using the 28.02 Ma Fish Canyon tuff standard. standard. tuff Canyon Ma Fish 28.02 the using calculated were dates correlation tephra and Ar o 39

thicker and structureless (Fig. 4A); bedding y e n p Ar/ a

is usually planar. The sediments are almost y 40 e t h C

s n

k e

o a entirely air fall in origin; epiclastic materials are e e c

t t t d l a a a a a a a a i i l o s l l r r r r r r r r a Some samples pertain to more than one basin description in the text and are shown more than once for clarity. Sample numbers l numbers Sample clarity. for once than more shown are and text the in description basin one than more to pertain samples Some r a d o o

h h h h h h h h s rare, except adjacent to the Snowstorm Moun- f i r y y l - n p p p p p p p p r o h h a a i e e e e e e e e

t tains to the east, where clasts of Snowstorm- i T P T T T T T T Palisade Canyon rhyolite rhyolite Canyon Palisade Southwest S R T R 4) ave. (zircon, age Fission-track A Note: n U Independence Valley (Fig. 10) Elko Basin (Fig. 10) 10) (Fig. Basin Elko *Potassium-argon date. *Potassium-argon Pine Valley (Fig. 7) (Fig. Pine Valley Appendix 1. Appendix derived volcanic rocks are scattered through the otherwise ash-rich sediments. Evidence of

Geosphere, February 2008 43

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TABLE 2. SUPERGENE ALUNITE DATES FROM LATE EOCENE GOLD DEPOSITS, NORTHEASTERN NEVADA Gold trend General location Age Method Reference (Ma) Battle Mountain Lone Tree mine 8.86 ± 0.5 40Ar/39Ar A. Hofstra, L. Snee (unpubl. data, 2005) Carlin Gold Quarry area 6.7 ± 0.2 K-Ar P. Vikre (unpubl. data, 2005) Carlin Gold Quarry area 6.8 ± 0.2 K-Ar P. Vikre (unpubl. data, 2005) Carlin Post mine 8.6 ± 0.2 K-Ar Arehart et al. (1992) Carlin Post mine 9.5 ± 0.2 K-Ar Arehart et al. (1992) Carlin Genesis mine 11.0* K-Ar Heitt (1992) Carlin Rain mine 12.6 ± 0.5 K-Ar Williams (1992) Carlin Rain mine 18.5 ± 0.8 K-Ar Williams (1992) Carlin Beast mine 18.7 ± 1.2 40Ar/39Ar Ressel et al. (2000) Carlin Rain mine 18.8 ± 0.2 K-Ar Arehart et al. (1992) Carlin Mike deposit 19.7 ± 0.5 K-Ar Teal and Branham (1997) Carlin Rain mine 18.1 0.2 K-Ar Arehart and O’Neil (1993) Carlin Rain mine 20 ± .2 K-Ar Arehart et al. (1992) Carlin Rain mine 20.7 ± 1.6 K-Ar Williams (1992) Carlin Rain mine 22.3 ± 0.9 K-Ar Williams (1992) Carlin Gold Quarry mine 25.9 ± 0.6 K-Ar Arehart et al. (1992) Carlin Gold Quarry mine 27.4 ± 0.7 K-Ar Heitt (1992) Carlin Gold Quarry mine 27.7 ± 0.7 K-Ar Heitt (1992) Carlin Gold Quarry mine 27.9* K-Ar Arehart and O'Neil (1993) Carlin Gold Quarry mine 28 ± 0.7 K-Ar Heitt (1992) Carlin Gold Quarry mine 28.9* K-Ar Arehart and O'Neil (1993) Carlin Gold Quarry mine 30 ± 1.2 K-Ar Heitt (1992) Getchell Preble mine 11.3* K-Ar Arehart and O'Neil (1993) Getchell Twin Creeks mine 14.4* K-Ar Arehart and O'Neil (1993) Getchell Preble mine 23* K-Ar Arehart and O'Neil (1993) Note: Gold trends are shown on Figure 1. 40Ar/39Ar dates recalculated to 28.02 Ma Fish Canyon tuff standard. *Error not published.

fl uvial reworking, such as graded bedding, cross from that area across the sedimentary units. In the east-central part of the basin, just west of bedding, and channel features, is also uncom- Therefore, the ash-rich sediments in the western the Snowstorm Mountains and south of the Little mon, except in the area between the Snowstorm and northern parts of the basin apparently were Humboldt River, a thick sequence of epiclas- Mountains and Chimney Reservoir. Mud cracks deposited between ca. 16.1 and 15.5 Ma and tic sediments conformably overlies the ash-rich and siliceous sinter deposits with reed and leaf permissively as early as 16.3 Ma in the center of lacustrine sediments (Figs. 4C and 4D). The con- fragments are common in sedimentary rocks in the basin at Chimney Reservoir. tact between the two facies is sharp, and the base the northern and western parts of the basin and In the eastern and northeastern parts of the of the sequence above fi ne-grained ash beds con- indicate periodic subaerial exposure. Diagenetic basin, the ash-rich sedimentary rocks both under- tains abundant volcanic cobbles. A tephra layer, zeolites and Magadi-type chert replaced many lie and interfi nger with fl ow units of the Little 2 m beneath the contact, produced a tephra cor- of the fi ne-grained, ash-rich sediments (Shep- Humboldt rhyolite sequence. Farther east, toward relation age of 14.7 Ma (Table 1). Near the Snow- pard and Gude, 1983). the Little Humboldt ranch and at Rodear Flat storm Mountains, the epiclastic strata include At Chimney Reservoir, ash-rich sediments (Fig. 3), ash-rich strata overlie basaltic andesite alternating cobble-rich and sand-pebble beds. The were deposited on a 16.3 Ma rhyolite fl ow fl ow units that, in the Midas area to the south- coarser beds contain cross bedding, graded bed- sequence. This relation provides a maximum, east, were dated at 16.1 Ma (Leavitt et al., 2004), ding, and channel cut-and-fi ll structures. Clasts but not necessarily the initial, age for sedimen- and the strata interfi nger with and underlie the in all parts of the fl uvial section are composed of tation. In the western and northwestern parts of 15.5 Ma Little Humboldt rhyolite. Near the Little volcanic lithologies exposed in the Snowstorm the basin, the ash-rich sedimentary units inter- Humboldt ranch, the earliest rhyolite fl ow con- Mountains directly to the east. fi nger with 15.9 Ma andesite fl ow units, which, tains a thick phreatic breccia at its basal contact To the west, this sequence becomes fi ner in turn, overlie 16.1 Ma rhyolite tuffs. These with the underlying strata, and several tens of grained and is composed of planar-bedded, volcanic units are part of the Santa Rosa–Calico meters of additional sedimentary units overlie structureless sandstones and thin, pebble-rich volcanic fi eld, which is extensive to the north the brecciated fl ow unit; these sediments, in turn, beds with weak cross bedding and graded bed- and northwest (Brueseke and Hart, 2007). Near underlie a second, unbrecciated rhyolite eruptive ding. This fi ner grained sequence projects west Martin Creek (Fig. 3), sediments are absent at unit (Fig. 4B). The sedimentary units may extend across the incised Little Humboldt River valley the contact between 5 Ma basalt fl ows (Hart for an unknown distance to the northeast beneath to the top of the sedimentary section exposed et al., 1984) and the 16.1 Ma tuffs; therefore, the upper eruptive unit, and the Rodear Flat strata on the north shore of Chimney Reservoir. That the margin of the sedimentary basin was south likely correlate with the fi ne-grained sediments section grades upward from thinly bedded, ash- of that point. To the north and northeast, the exposed just to the south at the north end of Castle rich bedded sediments into the epiclastic strata regionally extensive 15.5 Ma Little Humboldt Ridge (see the previous section on the Snowstorm derived from the Snowstorm Mountains. This rhyolite sequence directly overlies the 15.9 Ma Mountains). In all of these northeastern expo- section, albeit fi ner grained and more grada- andesite units with no intervening sedimentary sures, the sediments are ash rich, fi ne grained, and tional, mimics the transition from ash-rich to units (Wallace, 1993; W. Walker, 2001, unpub- altered to zeolites and chert, similar to those in the coarser epiclastic facies near the Snowstorm lished mapping), but the rhyolite extends south main part of the basin. Mountains.

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Figure 3. Map of the Chimney basin and Snowstorm Mountains areas, showing the original known and inferred extents of the sedimentary basin, middle Miocene volcanic fi elds, dates of various sedimentary and volcanic units, and post-sedimentation normal faults and synforms. Areas that do not have an overlay color are underlain by pre-Miocene basement rocks (especially in the Santa Rosa and Hot Springs Ranges and Osgood Mountains), post-middle Miocene sedimentary cover (Paradise, Eden, and Kelly Creek Valleys), and post-sedimentation volcanic cover (Owhyee Plateau region). Some volcanic units along the northern Nevada rift and in the Santa Rosa–Calico volcanic fi elds were erupted after sedimentation ended, but they are included within the volcanic fi elds regardless of relative age. See Table 1 for geochronologic information. The modern digital elevation map is used as the base.

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Figure 4. Photographs of middle Miocene sedimentary and volcanic units in the Chimney basin. A: Fine-grained, thin-bedded, ash-rich lacustrine sedimentary strata overlying a thick ash bed at the base of the exposure. Photo was taken in the western part of the basin. B: Phreatic, vitric megabreccia in base of basal fl ow of the 15.5 Ma Little Humboldt rhyolite above lacustrine sedimentary units (blocky hill), possibly indicating eruption onto wet sediments or water. The overlying, younger fl ow unit (left background) overlies both the breccia and the sedimentary units but does not have a basal breccia. Photo was taken at the Little Humboldt ranch along the Little Humboldt River (Fig. 3). C: Conformable contact (dashed white line) between the lacustrine and fl uvial facies; a tephra sample from 2 m below the contact produced a 14.7 Ma correlation age. Photo taken between Chimney Reservoir and the Snowstorm Mountains (right far background). D: Closer view of contact between lacustrine (below, light colored) and conformably overlying fl uvial sediments (tan). The cobbles concentrated along the contact zone were derived from volcanic units exposed in the Snowstorm Mountains to the east (right).

At the spillway for Chimney Reservoir (Fig. 3), Miocene units are poorly to not exposed Creeks mine (Fig. 3), Miocene units above the sand- and pebble-rich clastic units overlie the south of the Little Humboldt River. This area Paleozoic basement include basal colluvial and 16.3 Ma rhyolite fl ow unit, and angular clasts includes the Hot Springs Range, the Osgood regolith deposits and overlying epiclastic sand derived from the rhyolite are abundant at the basal Mountains, and the intervening Eden and and gravel deposits derived from the nearby contact. Additional fl uvial sediments are exposed Kelly Creek Valleys. Strata exposed between Dry Hills; a 14.2 Ma, air-fall ash is interbedded in low hills along the Little Humboldt River down- the Dry Hills (Fig. 3) and the Little Humboldt with these units (Breit et al., 2005). Ash-rich stream from the reservoir. All of these deposits are River are composed of fi ne-grained, poorly basin strata extend a few kilometers south into isolated from other fl uvial deposits; they could be exposed sediments. Both epiclastic and ash- Eden Valley between the Osgood Mountains related to the Miocene epiclastic units described rich sedimentary units are exposed near the and Hot Springs Range, but most of that area above or are much younger deposits related to Snowstorm Mountains in the southeastern part has an extensive cover of Pliocene and younger the late Miocene to Pliocene development of the of the basin, but the section thins and laps onto alluvial sediments that masks the southern Little Humboldt River. Paleozoic basement to the south. At the Twin extent of the Miocene units.

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Faulting deposited at a very low angle, now dip ~20° to on 16.1 Ma basaltic andesite fl ow units until the west, and project east to just above the crest ca. 15.7 Ma. Numerous small, normal faults offset the of the range (Hotz and Willden, 1964). As such, With the exception of the late epiclastic sedi- Miocene strata throughout the northern half of the range crest was just below the early Miocene ments in the southeastern part of the basin, the the Chimney basin. Alluvial cover and very poor paleosurface, and the Osgood Mountains were generally planar, laterally continuous bedding in exposures conceal any possible faults in the not a highland at 22 Ma. The second block is the ash-rich units indicates deposition in a low- southern part of the basin. Dips on all Miocene the Dry Hills, which is bounded by the Getchell energy, lacustrine environment, and mud cracks sedimentary units generally are less than 15°, fault to the west and a relatively minor, north- and sinter deposits point to episodic subaerial and many units are nearly horizontal. The basal striking fault to the east. This fault, as noted exposure. The general absence of epiclastic sediments dip as gently as strata higher in the above, does cut the Miocene strata to the north. material or evidence of fl uvial reworking until section, indicating little or no synsedimentary, West-dipping, 22 Ma andesite fl ows (Wallace late in the basin history suggests that the high- fault-related tilting. and McKee, 1994), identical to those along the lands that constrained the margins of this lacus- Faults strike predominantly to the north to west side of the Osgood Mountains, are pres- trine environment had low relief. The shallow north-northeast, although faults of all orienta- ent in the Dry Hills and may be the downfaulted lacustrine connection of the Chimney and Ivan- tions are present in the basin. Offsets range remnants of a broader volcanic fi eld. hoe basins across the northern Nevada rift in the from a few tens of meters to ~100 m. Several Eden Valley separates the northeast-trending Snowstorm Mountains demonstrates that rift- down-to-the-east normal faults strike northerly Osgood Mountains and the north-trending Hot related volcanic and fault activity did not create through the middle of the basin, including the Springs Range, which converge to the south. any substantial positive or negative relief. Chimney Reservoir area. The most obvious of East-dipping, early Miocene andesite fl ows on The lacustrine environment was shallow, these faults forms the east side of the Dry Hills the east side of the Hot Springs Range mirror became ephemeral, and eventually transitioned (Fig. 3), extends north to near the dam at Chim- those along the west side of the Osgood Moun- into a fl uvial environment. The sinter depos- ney Reservoir, and continues to the north. Juxta- tains (Jones, 1997). Gravity data in Eden Val- its and mud cracks indicate periodic subaerial posed pre-sedimentation volcanic units and the ley indicate that the thickness of Pliocene and exposure during the lacustrine stage, and early, basal parts of the Miocene section indicate offset Miocene units above Paleozoic bedrock is gen- but not later, fl ows of the Little Humboldt of only ~100 m. Along the southern projection erally less than 1 km, consistent with the val- rhyolite were erupted onto wet or submerged of this fault at the Twin Creeks mine, a scarp ley being a shallow synform between the two sediments. The upsection increase in diagenetic along the fault appears to have controlled the opposite-tilted ranges. This synform is narrow minerals indicative of more saline and alkaline deposition of Pliocene alluvial deposits (Breit et at the south end of the ranges, and it broadens conditions may refl ect a progressively more al., 2005). A series of both down-to-the-east and to the north as the ranges diverge. North of the evaporative lacustrine environment (Sheppard down-to-the west normal faults parallel the west Little Humboldt River (north of the limits of the and Gude, 1983; R.A. Sheppard, 1992, personal side of the Snowstorm Mountains, but displace- ranges), all Miocene strata dip very gently or are commun.), and the transition from lacustrine ment along each fault was less than 100 m. fl at lying, and abundant small normal faults with to fl uvial environments at and east of Chimney Along the west side of the basin, east of Para- widely varying strikes obscure the synform, if Reservoir point to possible external drainage dise Valley (Fig. 3), down-to-the-west normal present. Farther north, west of Whiskey Springs starting at ca. 14.7 Ma. faults with individual offsets of less than 100 m (Fig. 3), any semblance of a synform in 10 Ma The evolution of the western part of the offset the basin strata and underlying Miocene and older volcanic units is absent. As with the Snowstorm Mountains volcanic fi eld signifi - volcanic rocks and tilted them modestly to the Osgood Mountains, the Eden Valley synform cantly affected the eastern margin of the basin. east. At Martin Creek, a series of west-dip- appears to die out into and is not a noticeable At Snowstorm Mountain (Fig. 3), the eruption ping normal faults tilted 16.1 Ma volcanic units structural element in the Chimney basin. of 15.7 Ma rhyolite fl ows ended sedimentation ~20°, but produced only minor offset of overly- in that area, and post–15.5 Ma, pre–15.1 Ma ing, nearly horizontal 5 Ma basalt fl ows. To the Paleogeography and Sedimentation faulting tilted the older units to the west (Wal- south, these faults tilted the Hot Springs Range lace, 1993). However, this faulting did not gen- to the east. To the west, the bulk of the uplift Deposition of ash-rich, waterlain sediments erate epiclastic sediments, which did not appear of the Santa Rosa Range and formation of the in the Chimney basin began sometime after in the basin until ca. 14.7 Ma; the sudden infl ux east-dipping Paradise Valley half graben took 16.3 Ma and was actively taking place from of coarse epiclastic sediments at that time indi- place between ca. 10 Ma and 5 Ma (Colgan et 16.1 Ma to ca. 14.7 Ma. Deposition of sands cates some uplift in the Snowstorm Mountain al., 2004). The Martin Creek faults continue and coarser materials in the southeastern part area, although ca. 15.1 Ma fl ow units at Snow- northwest and bisect the Santa Rosa Range, and of the basin occurred from 14.7 to 14.2 Ma. storm Mountain dip only ~5° to the west. Faults the southern part of the range is now structurally At limits of the exposed basin sediments, the along the west side of the range may have been lower than the northern half. sedimentary units interfi nger with and pinch active at this time, although there is no direct evi- The Osgood Mountains are composed of two, out against coeval volcanic fl ow units along the dence of such activity. Farther south, at the Twin west-dipping blocks. The largest is the main western, northwestern, and northern margins of Creeks mine, sediments were shed from the Dry north-northeast–trending, west-tilted range, the basin, and the volcanic activity likely limited Hills at ca. 14.2 Ma (Breit et al., 2005), but the with a major range-front fault along its east side the extent of the basin in those areas. The early relation of fl uvial sedimentation there and that (Fig. 3). The north-striking Getchell normal sedimentary environment may have extended west of Snowstorm Mountain is unknown due fault truncates the northeast end of this block to the northeast, but eruption of the 15.5 Ma to the lack of intervening exposures. (Hotz and Willden, 1964). This fault is not evi- Little Humboldt rhyolite created a new, more On the basis of the southward thinning of the dent in the main part of the Chimney basin to the proximal basin margin in that direction. In the basin strata onto Paleozoic rocks, the southern north. Early Miocene (ca. 22 Ma) andesite fl ow eastern part of the basin and in all Snowstorm margin of the basin at the time of sedimentation units on the west side of the range likely were Mountain exposures, sediments were deposited was most likely a low Paleozoic-cored upland or

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bedrock sill. Overall, the available stratigraphic and structural data indicate that the Osgood Mountains and Hot Springs Range formed after Miocene sedimentation but did not signifi cantly affect the Chimney basin, despite its proximity and the projection of major structures toward the basin. Uplift possibly started near the end of sedimentation, producing 14.4 Ma supergene alunite at the Twin Creeks mine (Arehart and O’Neil, 1993) and the nearby 14.2 Ma clastic sediments. Alternatively, the major uplift took place between ca. 10 and 5 Ma, based upon evidence of that uplift age in the Santa Rosa Range and the tilting constraints near Martin Creek. Sedimentary and fault-scarp data (Breit et al., 2005; Wesnousky et al., 2005) indicate that uplift continued into the Pliocene and Quaternary. Why and when sedimentation ended in the Chimney basin are unclear. The climate was moist and temperate, the depositional environment was becoming increasingly subaerial and fl uvial in the eastern part of the basin, and major rhyolite eruptions were signifi cantly disrupting the northern and eastern parts of the basin. The absence of younger sediments argues against infl owing streams, which would have carried sediments, simply evaporating when they reached the basin. This absence of younger sediments in this topographically low area, despite the presence of uplands to the east, strongly suggests that the confi ning volcanic or topographic dams along the southwestern part of the basin were breached and the basin began to drain externally. Unfortunately, uplift of the Osgood Mountains and Hot Springs Range and Pliocene and younger alluvial sedimentation in Eden and Kelly Creek Valleys have largely obliterated the middle Miocene record in those areas.

IVANHOE BASIN

The Ivanhoe basin is located in southwestern Elko County and northernmost Lander and Eureka Counties (Fig. 5), stretching east to west from the Tuscarora Mountains to the Sheep Creek Range, and south to north from Boulder Valley into the Snowstorm Mountains. Basin- related sedimentary units are well exposed in the Ivanhoe mining district and areas to the east, south, and west, as well as in the Midas district in the southeastern Snowstorm Mountains. Post-sedimentation faulting and Pliocene and younger sediments conceal basin-related units Figure 5. Map of the Ivanhoe basin area, showing the original known and inferred extents of along the Midas trough and along Rock Creek the sedimentary basin, middle Miocene volcanic units, dates of various sedimentary and vol- southwest of Ivanhoe. canic units, major post-sedimentation normal faults, and the late Eocene Carlin gold trend. In general, lacustrine and late, low-energy, Uncolored areas include areas of pre-Miocene basement rocks in the Tuscarora Mountains ﬂ uvial sedimentation in the Ivanhoe basin took and post-middle Miocene sedimentary cover in Boulder and Kelly Creek Valleys; the extent place from before 16.1 Ma to after 14.7 Ma of volcanic units beneath sediments in Boulder Valley is based upon exposures of identical (Fig. 2) as volcanism related to the northern volcanic units in the cliffs on either side of the valley. See Table 1 for geochronologic infor- Nevada rift blocked streams ﬂ owing westward mation. The modern digital elevation map is used as the base.

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from the Tuscarora Mountains. Relief was low, the central part of the Ivanhoe basin and the June eroded from exposed gold deposits along the the lake level rose, and sediments progressively Bell rhyolite in the Midas area (Fig. 5). Neither Carlin trend (Theodore et al., 1998, 2006). Air- blanketed the entire area. Most of the sediments rhyolite has been dated, but surface exposures fall materials are common in the lower half of were air-fall ash and pumice, and fi ne-grained and drilling data indicate that they are inter- the section, decrease in abundance upsection, epiclastic materials were carried into the southern bedded with or underlie the oldest sedimentary and are absent in the upper quarter of the sec- part of the basin from the Tuscarora Mountains units in both areas and thus were erupted just tion, which was deposited after 14.7 Ma and is only late in the basin history. Sedimentation likely before or during early sedimentation (Wallace, composed of epiclastic sand and silt. The total ended when volcanic dams were breached and the 1993; Goldstrand and Schmidt, 2000; Wallace, sedimentary thickness exceeds 500 m. basin drained externally. Minor faulting occurred 2003c). The Rock Creek rhyolite is exposed The large Goldstrike mine along the northern only during late sedimentation in the northern part over a large area and, based on exposed thick- Carlin gold trend is in the southeastern part of of the basin. Major east-northeast–striking faults nesses, likely had relief on the order of several the basin adjacent to the Tuscarora Mountains formed much later, perhaps after 10 Ma. hundred meters, greater than the thickness of the (Fig. 5). There, Miocene sedimentary units sediments to the east. include a relatively thin basal conglomerate and Stratigraphy The lower part of the Willow Creek Reser- overlying, thin-bedded, ash-rich strata (Bettles voir section thins to the south into the Ivanhoe and Lauha, 1996). These sediments largely The oldest sediments in the basin were depos- mining district (Figs. 5, 6A, and 6B), where overlie 15.2 Ma rhyolite fl ow units (Table 1; ited shortly before 16.1 Ma in the Willow Creek only ~50 m of thin-bedded, ash-rich strata lie Theodore et al., 2007). The rhyolite fl ow units Reservoir area (Fig. 5; Table 1; Perkins et al., between the widespread 15.4 Ma vitric tuff and extend northwest across the northern end of 1998; Wallace, 2003c). Sedimentation in that the Paleozoic basement rocks. Units above the Boulder Valley and north into the Santa Renia area continued until after 15.4 Ma, deposit- vitric tuff in this area include thin- to thick-bed- Fields area, where they underlie the middle ing more than 200 m of sediments above late ded, air-fall ash and epiclastic sand beds; locally Miocene sedimentary units (Theodore et al., Eocene and Paleozoic units. The basal Miocene abundant pebble conglomerate and debris-fl ow 1998, 2006). sediments are composed of relatively minor units contain clasts derived from Paleozoic The Craig rhyolite, a large, thick assemblage of epiclastic sand and silt and moderate amounts rocks that cropped out a few kilometers to the rhyolite fl ows and domes, was erupted between of reworked ash. The majority of the overlying east (Wallace, 2003b). the Santa Renia Fields area and the Ivanhoe units are composed of thinly planar-bedded, air- Rhyolite and andesite fl ow units and domes district between 15.4 and 15.2 Ma and likely fall ash and pumice with local to pronounced were emplaced between ca. 15.5 and 15.2 Ma is related to the Boulder Valley rhyolites. In the soft-sediment deformation textures. One of throughout the Ivanhoe district, and the volca- Ivanhoe district, the fl ow units overlie 15.4 Ma these ash beds was dated at 16.1 Ma (Table 1; nic units are interbedded with the sedimentary and older sedimentary units, and younger strata Wallace, 2003a). Thin conglomerate beds with units. Several large, rhyolite porphyry domes overlie the fl ows. In the Santa Renia Fields area, clasts derived from the Tuscarora Mountains were erupted at 15.2 Ma between Ivanhoe and 15.2 Ma sediments were deposited on the rhyo- to the northeast locally are interbedded with Willow Creek Reservoir, and they overlie most lite (Theodore et al., 1998), and clasts derived the ash-rich units (Henry and Boden, 1999; of the sedimentary units in that part of the dis- from the rhyolite are abundant in overlying, oth- Wallace, 2003a), and some beds are a mixture trict (Fig. 6A). Therefore, the sedimentary units erwise fi ne-grained epiclastic sediments along of reworked small pumice and epiclastic sand, in the Ivanhoe district include only the upper Antelope Creek south of the rhyolite exposures indicating periodic infl uxes of non-ash material. part of the section exposed at the reservoir, (Fig. 6D). Strata in the Ivanhoe area are con- The ash-rich section grades up into thin-bedded and they were deposited between ca. 15.5 and tinuous to the south, along the west side of the mudstones, thick-bedded sandstone, and a sub- 15.2 Ma (Wallace, 2003c). Craig rhyolite exposures, and they connect in aerial, 15.4 Ma vitric tuff that is a widespread The volcanic fl ow units in the district were the Antelope Creek area with the sedimentary marker unit throughout the northern part of the emplaced subaerially, although the distal ends units that extend west from the Santa Renia basin. Very thin-bedded, air-fall ash beds with of a few andesite fl ow units have hyaloclastic Fields area. locally pronounced, soft-sediment deforma- breccias. Sinter deposits are common through- The strata along Antelope Creek extend west tion, as well as some fi ne-grained epiclastic out most of the pre– and post–15.4 Ma sedimen- beyond Rock Creek and southwest to nearly the sediments, were deposited above the vitric tuff. tary units (Fig. 6C; Wallace, 2003c), and they topographic rim above Boulder Valley (Fig. 5). Faulting truncated the top of the section at Wil- also indicate periodic, subaerial exposure dur- Many of the sedimentary units just east of Rock low Creek Reservoir, but a poorly exposed, ing sedimentation. The locus of sinter activity Creek contain thin, extensive surface travertine thick section of fi ne-grained, ash-rich sediments shifted to the east with time, refl ecting changes deposits and pervasive carbonate cement in overlies the vitric tuff southeast of the reservoir in the ground-water table and paleotopography underlying strata. In the southwestern Sheep (Wallace, 2003a). produced by progressive volcanic activity, sedi- Creek Range, 15 Ma basalt fl ows conformably The Willow Creek Reservoir section extends mentation, and minor late synsedimentary fault- overlie a thin sequence of fi ne-grained, epiclas- discontinuously ~15 km north and northeast, ing (Wallace, 2003c). tic sedimentary units (Fig. 6E), which contain where it overlies both Eocene volcanic rocks and In the Santa Renia Fields area east and an ash bed dated at 15.2 Ma (Table 1; John and Paleozoic rocks. Closer to the Tuscarora Moun- southeast of Ivanhoe (Fig. 5), the oldest sedi- Wrucke, 2002, 2003). However, with the excep- tains (Fig. 5), the lower part of the section thins to ments were deposited shortly before 15.4 Ma, tion of two, very small exposures, sedimentary ~20 m above Eocene volcanic rocks and underlies and sedimentation continued to after 14.7 Ma units are absent at the extensive contact between the 15.4 Ma vitric tuff, which is the youngest pre- (Fleck et al., 1998). The sedimentary units are 15.6 Ma dacite fl ows and overlying 15.1 Ma served unit in that area (Henry and Boden, 1999). composed of a mixture of thin-bedded, air- rhyolite fl ows in the westernmost Sheep Creek Coeval volcanic activity produced rhyolite fall ash and pumice deposits and fi ne-grained, Range (John and Wrucke, 2002). fl ow units and domes west of Willow Creek Res- epiclastic sediments derived from the Tusca- Sedimentary units related to the basin are ervoir. These include the Rock Creek rhyolite in rora Mountains to the east, including materials exposed in the Midas district (Figs. 5 and 6F),

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Figure 6. Photographs of middle Miocene sedimentary and volcanic units in the Ivanhoe basin. A: Area between Willow Creek Reservoir and the Ivanhoe district, looking east. RR—Rimrock mercury mine; a—andesite fl ow unit; vt—15.4 Ma vitric tuff unit, both of which were erupted subaerially. Light-colored, ash-rich strata below the andesite were deposited subaqueously, as were strata between the andesite and the vitric tuff. Dark capping units in distance are 15 Ma rhyolite porphyry fl ow units and domes, which dip ~10° less than the underlying units, indicating tilting between ca. 15.4 and 15 Ma. The Rimrock mercury deposit formed in silica replacement bodies in lacustrine sedimentary units that overlie the vitric tuff unit. B: Thin-bedded, ash-rich lacustrine sedimentary units exposed in the north wall of the open pit of the Hollister gold mine, Ivanhoe district. These beds are stratigraphically between the andesite and the vitric tuff unit (see Fig. 6A). Draping near the base of the exposure is above the irregular top of an andesite fl ow unit that is exposed just below the photo. Note the progressive upward change to planar bedding. The variegated colors are due to hydrothermal and supergene alteration. C: Sinter and silicifi ed lacustrine sedimentary units (light, massive unit in lower part of photo), overlain by unsilicifi ed lacustrine sediments (middle of photo) and a capping 15.4 Ma rhyolite fl ow unit. The silicifi ed horizon is widespread throughout the Ivanhoe district and formed prior to the deposition of the 15.4 Ma vitric tuff. Photo was taken 2 km east of the Hollister gold mine. D: Tan, pebble-rich sandstone bodies along Antelope Creek south of the Ivanhoe district. The pebbles are subangular to angular and were derived from the ca 15 Ma rhyolite exposures in the left background, although the main transport direction for the sand component was from right to left. Arrows point to soil horizons between sand bodies. E: Tan—epiclastic sedimentary units overlain by a 15 Ma basalt fl ow in the southwestern part of the Ivanhoe basin just southwest of the confl uence of Rock and Antelope Creeks (Fig. 5). The basalt fl ow was emplaced subaerially. F: Midas district, looking to the north-northwest. Unwelded tuffs and lacustrine sedimentary units (light tan in foreground) are overlain by 15.7 Ma red rhyolite fl ows at the skyline. A wide, north-northwest–striking dike fed the fl ows and intruded the tuffs and sedimentary units in that area. The mine workings are along veins that fi lled north-northwest–striking faults in the tuffs and sediments; the veins formed at ca. 15.4 Ma (Leavitt et al., 2004).

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where they have been called the Esmeralda forma- At Midas, north-northwest–striking faults toward Antelope Creek, where the sedimentary tion (Rott, 1931; Goldstrand and Schmidt, 2000). began to develop by ca. 15.8 Ma, during early units are horizontal and largely unfaulted (Theo- The units include fi ne-grained, thin-bedded, ash- sedimentation, and thus may have affected dore et al., 1998; Wallace, 2003b). rich strata and sand- and pebble-rich conglomer- sedimentation (Goldstrand and Schmidt, 2000; Similarly, several closely spaced, east-north- ate units. Clast compositions and imbrications Leavitt et al., 2004). Faults in the Midas area and east–striking faults form the steep escarpment in the latter units indicate transport from Paleo- the Snowstorm Mountains area to the north and between the southern Sheep Creek Range and zoic exposures to the northeast (Wallace, 1993; northwest served as conduits for 15.8 Ma rhyo- Boulder Valley (Fig. 5). Volcanic and sedimen- Goldstrand and Schmidt, 2000). The sedimentary lite dikes (Fig. 6F) and cut volcanic and sedi- tary units near the escarpment dip modestly to units interfi nger with pyroclastic and 16.1 Ma mentary units as young as 15.5 Ma. Just west the north, decreasing to horizontal along Ante- mafi c fl ow units and underlie extensive 15.7 Ma of Midas, rhyolite fl ows erupted shortly after lope Creek. As such, Antelope Creek generally and younger rhyolite fl ows (Wallace, 1993; Gold- 15.5 Ma are nearly fl at lying and unconformably follows the axis of an east-northeast–trend- strand and Schmidt, 2000; Leavitt et al., 2004). overlie tilted older fl ow units (Wallace, 1993). A ing synform, and Rock Creek is an antecedent A thin tuff at the base of the sedimentary section few kilometers northeast of Midas, modest east- stream that incised into the northern and south- was deposited at 15.7 Ma (Leavitt et al., 2004). erly dips on 15.3 Ma fl ow units indicate contin- ern limbs of the synform. These faults die out to About 6 km northeast of Midas, beds of air-fall ued faulting in that area. the east-northeast and are not present along the ash and epiclastic sediments are exposed between Along the northern Carlin trend, near the west side of the Tuscarora Mountains. 15.7 and 15.3 Ma rhyolite fl ow units (Leavitt et Goldstrike mine (Fig. 5), generally north-strik- al., 2004). Although late Cenozoic alluvial sedi- ing, east-dipping normal faults have as much Sedimentation and Paleogeography ments cover most of the area between Midas as 150 m of displacement. These faults are in and Willow Creek Reservoir, isolated exposures part related to a west-dipping half graben on Sediments in the Ivanhoe basin were depos- of Miocene strata in the intervening area, as the west side of the Tuscarora Mountains. Early ited in shallow lacustrine, low-relief subaerial, well as the similar ages of the sediments, indi- sediments were deposited during the formation and low-energy fl uvial environments. In the cate that the strata in the two areas are related. of the graben, but later sediments were depos- areas where the basement-sediment contact is Also, as described earlier, the sedimentary units ited on top of the faults (Leonardson and Rahn, visible, post-Eocene, pre-middle Miocene sedi- in the Midas district are continuous with 16.1 to 1996). These sediments overlie 15.2 Ma rhyolite mentary units are not present. In the Ivanhoe 15.5 Ma strata exposed in the Snowstorm Moun- fl ow units; therefore, faulting likely occurred area, the top of the Paleozoic basement is com- tains immediately to the northwest. shortly after that time. posed of a variably thick regolith (Bartlett et al., The largest faults in the Ivanhoe basin 1991) that formed during exposure and weath- Faulting are along the Midas trough and the bound- ering prior to ca. 16 Ma sedimentation. In the ary between the southern Sheep Creek Range eastern part of the Ivanhoe district, a late Eocene A minor amount of faulting took place in the and Boulder Valley (Fig. 5). Movement along pluton (the Hatter stock) had been exposed by Ivanhoe basin near the end of middle Miocene these east-northeast–striking faults produced the ca. 15.2 Ma inception of sedimentation in sedimentation, but most of the faulting occurred as much as 1 km of vertical offset and several that area (Wallace, 2003b). Sediments older than much later. In addition, late Eocene volcanic kilometers of left-lateral displacement (Wal- ca. 15.5 Ma occur only in the northern half of rocks near Willow Creek Reservoir dip as much lace, 1991, 1993; John et al., 2000). These the basin at Willow Creek Reservoir and in the as 30° more steeply than overlying Miocene faults formed after sedimentation. Zoback and Midas area; the sediments in the southern half of sediments, indicating a post–38 Ma, pre–16 Ma Thompson (1978) estimated that faulting took the basin were deposited after ca. 15.3 Ma and period of fault-related tilting (Wallace, 2003c). place between ca. 10 and 6 Ma, although faults are laterally continuous with the upper part of Throughout most of the Ivanhoe basin, evi- along the Midas trough cut and modestly tilted the northern section. dence of synsedimentary faulting, such as 4.4 Ma basalt fl ows (Hart et al., 1984; Wallace, Basin sedimentation began shortly before decreasing stratal dips upsection and overlap of 1993). Fault scarps indicate continued offset 16.1 Ma in the Willow Creek Reservoir area, faults by younger strata, was not observed. In into the Quaternary (Wallace, 1993; Wesnousky which, as far back as the late Eocene, was a west- the Ivanhoe district, lateral migration of hydro- et al., 2005). draining Eocene paleovalley (Wallace, 2003a; thermal fl uids produced widespread, stratiform, Several of these faults along the Midas trough Henry, 2008). On the basis of the differences in silica-replacement zones in strata that directly extend from the western side of the Sheep Creek stratal thickness, the initial depocenter in the area underlie the 15.4 Ma vitric tuff unit, and silici- Range and Snowstorm Mountains east to the of the reservoir was ~200 m lower than areas to fi cation took place shortly before the deposition Willow Creek Reservoir area. Volcanic and sed- the south and possibly the north. Clast lithologies of the latter unit (Wallace, 2003c). Post–15.4 Ma imentary units on the north and south sides of and imbrication show that infl ow into the early faults modestly cut and tilted the silicifi ed hori- the fault-controlled trough dip north and south, basin was from the Tuscarora Mountains to the zon, but the lateral wide extent of the that hori- respectively, with dips decreasing away from the northeast and east. The epiclastic materials are zon indicates that the ground-water table was trough. To the east-northeast, offset along indi- generally fi ne grained and uncommon, in con- not hydraulically segmented by faults until after vidual faults diminishes, and displacement is trast to the abundant, coarse sediments that were 15.4 Ma. South of Willow Creek Reservoir, the distributed across numerous faults spread over being carried into the Carlin basin on the east side dips of the 15.4 Ma vitric tuff and enclosing tuff a broad area (Wallace, 2003a, 2003b). In and of the range at the same time (see description of units are, on average, 10° greater than those of around the Ivanhoe district, displacement along the Carlin basin). Air-fall materials comprise the overlying 15.2 Ma rhyolite fl ows (Fig. 6A). As individual faults usually is less than 100 m and bulk of the sediments deposited up to 15.3 Ma. such, some faulting in the Ivanhoe area took more commonly is less than 50 m, and the faults By ca. 15.3 Ma, lacustrine and low-energy fl uvial place during the later stages of sedimentation die out near Willow Creek Reservoir. Fault- environments had expanded to cover most of the and volcanism in the area, but it did not produce related offset and southerly tilts are greatest in adjacent, lower lying areas, and sedimentation signifi cant offset of the units. the northern part of the district and decrease continued primarily in the southern half of the

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basin. It was only after ca. 15.3 Ma that signifi - such as at Scraper Springs (Fig. 5), which is east tains, Swales Mountain, the Adobe Range, and cant amounts of fi ne-grained, epiclastic sediments of the rift, and the earliest basin sediments were the Piñon Range (Fig. 7). were carried into the basin, primarily in west- deposited directly on late Eocene volcanic units. In general, sedimentation in the Carlin basin fl owing, low-energy streams in the southern part At both Midas and Scraper Springs, the erup- began at ca. 16.5 Ma and continued until after of the basin. tion of extensive rhyolite fl ow units starting at 14.6 Ma (Fig. 2). Faulting within a remnant Coeval volcanic fl ows and domes erupted 15.7 Ma ended further sedimentation. Farther Eocene upland produced the early basin, and along the entire margin of and within the Ivan- west, in the southern Snowstorm Mountains and coarse fl uvial sediments were carried into the hoe basin strongly affected the timing and dis- northern Sheep Creek Range, the eruption of new lowland and southward toward Pine Valley. tribution of sedimentation. The eruption of the large rhyolite domes and extensive fl ows contin- Fault-related subsidence and damming and the Rock Creek and June Bell rhyolites west of the ued to create a basin margin until ca. 15.2 Ma. later 15.3 Ma eruption of the Palisade Canyon Willow Creek Reservoir area (Fig. 5) just prior The locus of sedimentation apparently shifted rhyolite at the south end of the basin produced a to sedimentation may have blocked westward to the south with time, but the reasons why lake that expanded over much of the basin and streamfl ow and, combined with the modest post–15.2 Ma sediments were not deposited across low divides in surrounding uplands, con- upland area to the south, produced the early in the northern half of the basin are unknown. necting with adjacent basins. The lake largely lacustrine environment centered on the reser- Given the moist, temperate climate, water drained at ca. 15.2 Ma, and fl uvial sediments voir area. That early depocenter fi lled, and the undoubtedly continued to fl ow into the northern then blanketed much of the basin, although this lacustrine environment expanded, reaching the area from the Tuscarora Mountains. At the same transition did not occur in the western part of highest point in the Ivanhoe district to the south time, epiclastic sediments were being carried the basin until ca. 14.6 Ma. Except for the initial by ca. 15.6 Ma and the Midas area to the west westward from this highland into the low-relief, basin-related faulting, all of the faulting within by ca. 15.7 Ma. southern part of the basin. It is possible that all the basin and adjacent highlands took place after Rhyolite to andesite fl ows were erupted in post–15.2 Ma sediments in the northern part of sedimentation. Late Miocene and younger ero- the Ivanhoe area between ca. 15.6 and 15.3 Ma. the basin were removed by later erosion, but the sion has preferentially removed large quantities These eruptions were largely subaerial despite total effi ciency of such erosion over a broad area of Miocene sediments and exhumed the pre- nearby coeval lacustrine sedimentation (Wal- seems improbable. The only possible outlet to sedimentation paleotopography. lace, 2003c), but deposition of lacustrine sedi- the northern part of the basin would have been ments on top of the fl ow units indicates that to the west, and the cessation of volcanism in the Stratigraphy the fl ows had a low relief and the lake was able western part of the basin at 15.2 Ma may have to expand over them after eruption. This cycle allowed streams to breach those dams and exit Air-fall ash and more locally derived epiclas- between volcanism and lacustrine sedimenta- the basin. Unfortunately, that area is along the tic sediments comprise the sedimentary units in tion continued until the eruption of large rhyo- Midas trough, which was signifi cantly modifi ed the Carlin Basin, which can be divided into four lite domes at 15.1 Ma between Willow Creek by later faulting and covered by Pliocene and broad units. These include three interfi ngering, Reservoir and Ivanhoe. The eruption of the Quaternary sediments. but increasingly younger, lower units—a basal, extensive Craig and Boulder Valley rhyolites at The fl oor of the southern half of the basin coarse epiclastic unit, a mixed epiclastic and ash- ca. 15.2 Ma may have created a volcanic upland was topographically higher, although modestly rich unit, and a fi ne-grained, ash-rich unit—and that focused streamfl ow into the southwestern so, than the northern part, and the absence of an upper, sandy to conglomeratic epiclastic unit part of the basin. sediments older than ca. 15.4 Ma indicates that (Wallace, 2005). Each unit, as described below, A thick sequence of basalt to dacite fl ows was rainfall, runoff, and any air-fall or epiclastic sed- is extremely variable. erupted in the western and southwestern Sheep iments left the area and were not retained. As The basement rocks include discontinuous, Creek Range between ca. 16.1 and 15.6 Ma noted above, the 15.2 Ma eruption of rhyolites late Eocene volcanic units that overlie various (John et al., 2000; John and Wrucke, 2002; in and along the western side of Boulder Valley Paleozoic sedimentary rocks. Eocene volcanic Leavitt et al., 2004). A thin sequence of 15.2 Ma may have blocked more southward streamfl ow rocks are widespread in the Swales Mountain sediments between those fl ows and overlying and diverted it to the west to merge with the area to the north and at Marys Mountain to 15 Ma basalt fl ows pinches out to the west, basin that was expanding from the north. the west (Evans and Ketner, 1971; Henry and and the older basalt-dacite fl ow sequence likely The post–15.4 Ma, westward-transported, Faulds, 1999). Only the Swales Mountain vol- defi ned the southwestern margin of the basin. An epiclastic sediments did not reach the south- canic rocks extend beneath Miocene strata in extensive fi eld of 15 Ma rhyolite domes forms western margin of the basin. Related streamfl ow the basin, and a few Eocene rhyolite fl ow units the southwestern and western margins of the may have either ponded and evaporated before are present beneath the eastern part of the basin. basin. These domes were erupted onto 15.8 Ma reaching that area (although there is no evi- Where Miocene and Eocene units are in depo- dacite fl ow units to the south (John and Wrucke, dence of evaporative conditions), or it may have sitional contact, the older rocks dip 15° to 20° 2002) and 15.5 Ma rhyolite fl ows to the north drained south out of the basin, perhaps in the more steeply than the younger strata. (Wallace, 1993; Leavitt et al., 2004). Sedimen- vicinity of the modern Rock Creek canyon. Rhyolite to andesite fl ows were erupted dur- tary units are absent at volcanic contacts in both ing Miocene basin sedimentation. At the south- areas, suggesting that eruption of these volca- CARLIN BASIN ern and southwestern margins of the basin, erup- nic rocks produced the western boundary of the tion of the 15.3 Ma Palisade Canyon rhyolite basin. The Miocene Carlin sedimentary basin was produced an immense pile of fl ows that is now In the Midas area and adjacent parts of the centered on an area in southwestern Elko County exposed in an arcuate belt from Marys Mountain Snowstorm Mountains, the sediments were that includes the town of Carlin. The basin southeast to the northwestern end of the Piñon deposited on 16.1 Ma basaltic andesite fl ow units extended over a broad area bounded by what Range (Fig. 7). On the basis of thickness, num- along the axis of the northern Nevada rift. These now are, in clockwise order from the southwest, ber of fl ows, and fl ow features, the main eruptive basaltic andesite units are absent east of the rift, Marys Mountain, the southern Tuscarora Moun- centers were just north of Palisade (Fig. 7), and

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~15.4 Ma

15.9 - 15.2 Ma

14.8 Ma

15.3 Ma

Figure 7. Map of the Carlin basin area, showing the original known and inferred extents of the sedimentary basin, the middle Miocene andesite and Palisade Canyon rhyolite ﬂ ow units, and dates of various sedimentary and volcanic units. Uncolored areas include areas of pre-Miocene basement rocks in the surrounding ranges. See Table 1 for geochronologic information. The modern digital elevation map is used as the base.

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the fl ow package generally thins in all directions indicate southerly streamfl ow, and clast litholo- is the basal sedimentary unit in the eastern and away from that area. Near Emigrant Pass, the gies are identical to those in Swales Mountain southwestern parts of the basin (Fig. 7), as well rhyolite underlies 14.8 Ma and younger strata and, to a lesser extent, the Adobe Range, similar as on topographic highs in the northern part of and overlies a very thin sequence of Miocene to clasts in modern Susie Creek. A distinct, kilo- the area that were not initially covered by ear- sedimentary units. Between Emigrant Pass and meter-wide horizon of conglomerate extends lier sediments. The top of the ash-rich unit was Palisade (Fig. 7), as well as on the west side of south-southwest toward the north-central part of deposited at ca. 15.5 Ma in the northern part the Piñon Range, the rhyolite directly overlies the basin from the southeastern base of Swales of the basin, at ca. 15.2 Ma in the southeastern Eocene and older rocks. At the north end of Pine Mountain before thickening into a 200-m–thick part, and after 14.8 Ma in the southwestern and Valley, the rhyolite overlies fl uvial Miocene section of alternating conglomerate and fi ner westernmost parts (Table 1). units and underlies fi ne-grained strata that were grained beds, possibly representing a wide The mixed epiclastic and ash-rich unit is deposited at ca. 15.3 Ma. In a small area north channel that fed a depocenter. Basal Miocene exposed throughout much of the northern part of Carlin, the rhyolite is exposed beneath some sediments in the northern part of Pine Valley to of the basin and is composed of alternating beds basin strata, and rocks in these exposures likely the south (Fig. 7) are epiclastic and underlie the typical of both the ash-rich and the basal epi- connect beneath sedimentary cover to the main Palisade Canyon rhyolite (see the later section clastic units (Fig. 8D). This unit was deposited rhyolite bodies to the southwest (Fig. 7). on Pine Valley). between ca. 16.3 and 15.7 Ma (Table 1). Given Thin andesite fl ows were erupted along Most epiclastic beds contain at least some the southerly streamfl ow indicated by clast the eastern margin of and in the southwestern reworked air-fall material, indicating deposition imbrications and source lithologies in this and part of the basin (Figs. 7 and 8A). The eastern of the pyroclastic materials on the source areas the basal epiclastic unit, the strata likely are fl ow units now extend over the low crest of the and simultaneous transport and deposition of present beneath younger units in the central and southern Adobe Range into the southwestern both types of sediments. In the northwestern part southern parts of the basin. The proportions of margin of the Elko basin (Fig. 7). Flow units in of the basin, as well as in the Camp Creek area both sediment types vary considerably both later- both areas are at the contact between the ash- to the northeast (Fig. 7), some thick sand bodies ally and vertically through the section, refl ecting rich unit and the upper epiclastic unit; the top of are composed partially to largely of reworked, the dynamic interplay between the fl uvial and the ash-rich unit in the eastern part of the basin small, mafi c to intermediate-composition pum- lacustrine environments. The fi rst appearance of was dated at 15.2 Ma. Petrographically identical ice clasts with locally abundant small chips of laterally persistent, thinly bedded, ash-rich beds andesite fl ow units in the Peko Hills, east of the Paleozoic rocks. In places, dominantly epiclas- defi nes the base of the mixed unit; the top of the Adobe Range, were dated at 15.1 Ma (Table 1; tic sand grades upward over several meters into unit is gradational into the ash-rich unit and is see the Elko basin descriptions). In the south- pumice-rich material, possibly the result of raft- defi ned somewhat arbitrarily where epiclastic western exposures, north of Carlin, andesite ing of the more buoyant pumice. The ages and sediments are only a minor component of the overlies the rhyolite with a few meters of inter- compositions of these coarser air-fall deposits whole. Southeast of Cottonwood Creek (Fig. 7), vening ash-rich strata, and magnetic data indi- are similar to those of volcanic rocks that were debris fl ows in the lower epiclastic unit interfi n- cate that the fl ow units extend a few kilometers erupted along the northern Nevada rift to the ger southward into the mixed unit and indicate a to the southwest beneath the sedimentary cover west (John et al., 2000). southward change from purely fl uvial to mixed (Plume, 1995). The ash-rich unit consists of thin- to thick- depositional environments. Combined epiclas- The basal epiclastic unit is exposed in the bedded, air-fall ash, locally abundant diatomite tic and pumiceous beds, similar to those in the northwestern, western, and northeastern parts at the top of the unit, and thin, uncommon beds lower fl uvial unit, are common, but the beds are of the basin (Wallace, 2005); it is concealed, if of limestone and chert. Epiclastic materials are much thinner, and some beds are composed of present, in the central and southern parts of the extremely rare. Beds or packages of beds can primary air-fall pumice. basin. In general, the unit includes thick sand be traced laterally for several kilometers. Most The upper epiclastic unit was deposited beds, debris fl ows, and braided stream deposits textures indicate deposition in a lacustrine envi- beginning at ca. 15.5 to 15.1 Ma, depending typical of the medial to distal parts of an alluvial ronment, and wind ripple marks, eolian cross on location; the upper age is unknown. Clast fan (Fig. 8B; Nilsen, 1982; Einsele, 1992). Thin- bedding, mud cracks, hot-spring sinter depos- lithologies are similar to those in the lower epi- bedded, ash-rich sediments that might indicate its, and algal mats indicate periodic subaerial clastic unit and indicate similar source areas and lacustrine conditions are absent except in small, exposure. Soft-sediment deformation is rare but transport directions. Clasts derived from the isolated areas. The basal epiclastic unit is older can be pronounced locally, such as along the Adobe Range, which contributed only minor than 16.3 Ma tephras at the base of the overlying west side of the Adobe Range (Fig. 8C). The amounts of fi ne-grained, epiclastic sediments to mixed epiclastic and ash-rich unit, but the age of ash was derived from distant eruptions (Perkins the lower epiclastic unit, are very common in initial sedimentation is unknown. and Nash, 2002), and coarser air-fall materials, the upper unit in the eastern part of the basin. In the northwestern part of the basin, clast such as those found in the basal fl uvial unit, are All sediments in this unit become fi ner grained compositions and imbrications indicate source absent. Almost all diatoms are planktonic (pre- toward the center of the basin. areas in the Tuscarora Mountains and south- dominantly Aulacoseira granulata), with minor The upper unit is composed of alternating western Swales Mountain and southeastward benthic diatoms near the bases of the diato- sand and gravel beds. The sand generally lacks streamfl ow toward the north-central part of the mite sections, indicating fresh, deeper water. sedimentary structures and probably represents basin (Fig. 7). The unit thickens between Cot- However, diagenetic formation of zeolites and overbank deposits that, on the basis of abundant tonwood Creek and the north end of Schroeder Magadi-type chert in the basal ash-rich sedi- rhyzoliths and some burrows, were bioturbated. Mountain (Fig. 7) and may have been deposited ments in the middle of the basin indicate early Weak soil horizons, identifi ed by their lighter in a southeast-draining paleovalley. alkaline, perhaps evaporative conditions (Shep- color, increased carbonate cement, and upward In the northeastern part of the basin, epiclastic pard and Gude, 1983; Wallace, 2005). termination of burrows and rhyzoliths, are com- sedimentary units include siltstone, pebble-rich The ash-rich unit was deposited throughout mon in some exposures. The unit typically is red- sandstone, and conglomerate. Clast imbrications much of southern two-thirds of the basin. This dish and commonly calcareous; early hematite

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Figure 8. Photographs of middle Miocene sedimentary units in the Carlin basin. A: Thin andesite fl ow units underlain by ca. 15.1 Ma, ash-rich strata (white); sediments of the upper epiclastic unit (not shown) overlie the andesite. Faulting and tilting occurred after basin sedimentation. Photo taken looking north along the west side of the Adobe Range. B: Sand, pebbles, and cobbles in braided stream and channel deposits of the basal epiclastic member; hammer for scale. Photo taken east of the Cottonwood Gulch area in the northwestern part of the basin. C: Soft-sediment deformation in the ash-rich unit along the west side of the Adobe Range. The deformed bed is ~1 m thick. D: Alternating fl uvial, epiclastic sand beds and lacustrine ash-rich beds of the mixed epiclastic and ash-rich unit. The thick ash bed being sampled was deposited at 16.3 Ma. The underlying sand bed fi lls a 2-m–deep channel just to the right of the photo. Photo taken along upper Susie Creek in the northeastern part of the basin. E: Contact between the ash-rich unit (white) and the upper epiclastic unit (tan) along Interstate 80 west of Carlin. A tephra in the ash-rich unit near where the photo was taken produced a 14.7 Ma correlation age. The contact is conformable and, where exposed, sharp. F: The upper epiclastic unit (f) and a local zone of the underlying ash-rich unit (a, arrow), both of which overlie weathered and oxidized Paleozoic sedimentary rocks and the Gold Quarry gold deposit in the left side of the photo. The dashed white line delineates the sediment-bedrock contact. Photo taken looking north in the Gold Quarry mine on the west side of the Carlin basin.

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coats sand grains, and later calcite cement fi lls 15.2 Ma mixed unit overlies the basal unit and mixed units in the Carlin basin and contain both the remaining pore spaces. is composed primarily of fi ne-grained epiclas- epiclastic sediments and air-fall ash and dark The contact between the upper fl uvial and tic sediments, some pebble conglomerates, and pumice similar to that in Carlin basin (Lovejoy, ash-rich units is conformable throughout most locally thick, air-fall ash and pumice deposits. 1959). These strata extend north into the main of the basin (Fig. 8E). In the eastern half of the On the east side of Schroeder Mountain, north- part of Independence Valley, as described in a basin, the contact is consistently just above a lat- east of the Gold Quarry mine (Fig. 7), a distinct later section. erally persistent, ca. 15.2 Ma diatomite zone in lacustrine unit is absent, and massive epiclas- At the south end of the Adobe Range, andes- the upper part of the ash-rich unit. In the center tic sand beds in its place are extremely ash rich. ite fl ow units and the ash-rich and upper epi- of the basin, the contact zone is only a few cen- These beds resemble the upper epiclastic unit but clastic units extend across the low crest of the timeters thick and represents a sudden infl ux of contain the abundant ash typical of the ash-rich range and connect with similar units on the east epiclastic sediments. Soil horizons in sandstone unit, suggesting a mixed but homogenized envi- side of the range, which is part of the Elko basin. beds above the contact indicate a change from ronment. These units overlie 14.8 Ma tephras. On Two tephra beds stratigraphically beneath the lacustrine to terrestrial environments. the basis of dates and fi eld relations in this and the andesite on the east side were deposited at 15.3 Elsewhere, the contact zone varies in time and Welches Canyon areas, both ash-rich and epiclas- and 15.1 Ma, similar to the age of the ash-rich character. East of Cottonwood Creek (Fig. 7), tic sediments were being deposited in the western unit in the southeastern part of the Carlin basin. the top of the ash-rich unit is ca. 15.5 Ma, older part of the basin at the same time that the upper Thus, a low-energy, presumably horizontal than at other locations in the basin. As such, epiclastic unit was being deposited in the eastern sedimentary environment extended across what part of the ash-rich sequence may have been half of the basin. is now the modern crest of the Adobe Range, eroded prior to deposition of the upper epiclas- Broad areas along the west side of the Adobe and the andesite lavas were able to fl ow over tic unit, or the shift in depositional environment Range and the east side of Marys Mountain the entire area. Epiclastic sediments, includ- began earlier there than in other places. In the remained exposed until covered by the ash-rich ing coarse debris fl ows, on the east side of the southeastern part of the basin, the contact zone unit at ca. 15.3 Ma. The resulting sedimentary range, were derived from rocks exposed nearby, immediately above the 15.2 Ma diatomite hori- cover in those areas is much thinner than in the indicating a highland just north of the interbasin zon spans several tens of meters, with alternat- rest of the basin. As such, these areas were topo- connection. These epiclastic deposits overlie the ing ash-rich beds and coarse conglomerates and graphically higher during early sedimentation than ash-rich unit and thus formed at approximately debris fl ows derived from the southernmost the central part of the basin. The basal epiclastic the same time that coarse conglomerates were Adobe Range. This high-energy environment unit was deposited along the western base of the deposited at the base of the upper epiclastic unit was restricted to this area and thus must have Adobe Range bench, and lacustrine sediments in the nearby Carlin basin. resulted from events, possibly a series of local- overlie both the bench and the older epiclastic Ash-rich and epiclastic sediments are exposed ized storms that were centered on and affected strata. The western edge of the bench in part coin- at and west of Emigrant Pass, tentatively con- only the southernmost Adobe Range. cides with a major post-sedimentation fault (Wal- necting with similar sediments in the south- Fluvial Miocene sediments are exposed in lace et al., 2007a); this fault may have been active western part of the Carlin basin. Some of these the upper Maggie Creek area north-northeast before or during sedimentation, or, alternatively, mixed sediments underlie the 15.3 Ma Palisade of Cottonwood Creek. These nearly horizontal the bench may have been a pediment formed dur- Canyon rhyolite. The base of the section locally sediments, although poorly exposed, resemble ing pre-middle Miocene erosion. is a conglomerate, which underlies ash-rich the upper epiclastic unit. Gravity data indicate On the east side of Marys Mountain, 14.8 Ma units. Ash-rich sediments are exposed as far that the depth to basement in the Maggie Creek and younger ash-rich and upper epiclastic sedi- west as Bobs Flat (Fig. 7; Peters, 2003), and the area is as much as 1700 m (Ponce and Morin, ments were deposited on the Palisade Canyon depositional environment may have extended 2000). Given the thickness of and sedimento- rhyolite and, to the north, Paleozoic rocks. At the beyond the present Marys Mountain highland logical relations in the Miocene sedimentary Gold Quarry mine, some ash-rich strata overlie and connected with a Miocene depocenter on sections near Cottonwood Creek just to the the Paleozoic basement and gold deposit, but the west side of the Cortez Range (Stewart and east, much of that 1700 m must be pre-Miocene the upper epiclastic unit directly overlies many Carlson, 1978). units. The juxtaposition of upper fl uvial unit in parts of basement rocks (Fig. 8F). Surface expo- the valley against the base of the lower epiclas- sures east of Emigrant Pass and drilling data Sedimentation, Paleogeography, and tic unit just to the east indicates a west-dipping from south of Gold Quarry indicate that the Faulting normal fault along the east side of the Maggie base of the Miocene section thins to the south- Creek valley. west onto the area underlain by the rhyolite. A late Eocene topographic high extended In the far western part of the basin, between The upper epiclastic unit gradationally overlies north-northwest from the northern Piñon Range, Welches Canyon and the Gold Quarry mine the ash-rich unit, extends out into the middle of through the future site of the Carlin basin, and (Fig. 7), all four units are present, although the the basin northwest of Carlin, and conceals any along the Tuscarora Mountains (Fig. 9; Haynes, ash-rich unit is much thinner. The overall form underlying relations. 2003). The Swales Mountain area was on the of this sedimentary package and the along- northeast fl ank of the paleohigh, and the Marys strike contact relations with basement rocks to Connections with Other Basins Mountain area was on the southwest fl ank. The the south and north suggest that this area was a Adobe Range also was a modest topographic moderately deep, east-northeast–trending paleo- Strata in the Carlin basin extend past mod- high that extended northeast from the other top- valley. The basal epiclastic unit is much thinner ern ranges and connect with strata in adjacent ographic high, creating a “V”-shaped highland. and fi ner grained than it is elsewhere, although basins. In the Camp Creek area to the north- Late Eocene volcanic rocks overlie the paleo- conglomerates with clasts derived from nearby northeast on the east side of Swales Mountain surface in at least the Marys Mountain and gold deposits comprise some of the basal (Fig. 7), 15.5 to 15.3 Ma sediments are similar Swales Mountain areas. Their absence in other beds (Norby and Orobona, 2002). The 14.8 to to and continuous with the basal epiclastic and areas could be due to nondeposition or erosion.

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sediments could indicate that the area remained relatively high until that time, and that the sediments produced during earlier erosion were transported out of the area. The deposition of the coarse, fl uvial sediments of the basal epiclastic unit in at least the western and northern parts of the basin indicates the creation of relief and a depocenter. The thickest parts of the basal epiclastic unit are just east of the Tuscarora Mountains, which might suggest that uplift of the Tuscarora Mountains initiated early fl uvial sedimentation. However, low-energy lacustrine sedimentation was taking place in the Ivanhoe basin, on the western side of the Tusca- rora Mountains area, at the same time that this much higher energy fl uvial sedimentation was taking place in the western part of the Carlin basin. Uplift of the highland should have shed coarse epiclastic sediments in both directions, not just one. Therefore, fault-related downdropping of the east side of the Tuscarora Mountains highland shortly before 16.3 Ma, possibly along the Tusca- rora fault (Evans, 1980; Ressel and Henry, 2006), most likely created the early basin into which the sediments were deposited. A comparison of the areas where early fl u- vial sediments were deposited and areas where sediments were not deposited until later provides some idea of where possible controlling faults, as well as topographically higher areas, may have been located (Fig. 9). As noted above, the Tuscarora fault likely provided a structural control to the northwestern part of the basin. Thickness distributions of the basal epiclastic unit suggest the presence of an elongate upland at and north of Schroder Mountain during early sedimentation. To the north, the basal epiclastic unit laps onto the southwest, south, and east Figure 9. Paleogeographic map of the Carlin basin area during its early stages of formation. sides of Swales Mountain. On the basis of angu- Shown are late Eocene highlands (estimated from page-size fi gure in Haynes, 2003), possible lar unconformities between the Eocene volcanic early-basin normal faults, and the distribution of the basal fl uvial unit of the Humboldt rocks and overlying Miocene units, that block Formation, which was the fi rst Miocene unit to be deposited in the basin. Shown also is the was tilted to the west prior to sedimentation, approximately outline of the Carlin gold trend (yellow lines), from which numerous Oligo- possibly by a fault on its east side (Evans and cene to early Miocene supergene alunite dates were obtained. Ketner, 1971). However, only ash-rich sandstones are present along the eastern fl ank of Swales Mountain, indicating little relief. The late Eocene gold deposits in the Tuscarora of fanning of stratal dips in the latter units, indi- Early sedimentation apparently was concen- Mountains, at Gold Quarry, and in the north- cate that this faulting ceased before the onset of trated in the central part of the basin, fl owed to ern Piñon Range (Rain deposit), all of which basin sedimentation (Henry and Faulds, 1999; the south toward Pine Valley, and did not begin formed within the paleohighs, were deposited at Wallace, 2005). Supergene alunite ages of to cover fl anking areas to the east and west until least 800 m beneath the paleosurface (Hickey et 30.0–18.5 Ma from the gold deposits (Table 2) ca. 15.3 Ma. As noted earlier, facies and age al., 2003; Ressel and Henry, 2006). indicate that protracted middle Tertiary erosion relations suggest that the west side of the Adobe A series of generally north-striking faults cut of the highlands induced supergene weathering Range bench may have been fault controlled just and tilted the late Eocene volcanic and sedimen- of the deposits. prior to or during early sedimentation (Fig. 9). tary units in the Marys Mountain and Swales The tops of the gold deposits were exposed To the southwest, a major north-striking fault Mountain areas, as well as a 26 Ma tuff in the by the middle Miocene, and clasts from the offset the 15.3 Ma Palisade Canyon rhyolite Marys Mountain area (Henry and Faulds, 1999; deposits were shed into the basal Carlin basin along the Humboldt River north of Palisade Wallace, 2005). The angular unconformities sediments. Therefore, at least 800 m of erosion (Fig. 9) and extends north into the Carlin basin between the volcanic units and overlying Mio- took place in the highlands prior to middle Mio- east of Marys Mountain. Although not exposed, cene basin strata, as well as the apparent lack cene sedimentation. The lack of pre–16.3 Ma the facies relations imply that it may have been

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active during early sedimentation, and it conjec- part of the basin may have been somewhat iso- basin-sediment contacts and subsequent prefer- turally may have controlled the locations of the lated from the main part of the basin until after ential erosion of the Miocene sediments relative eruptive centers for the rhyolite, which are in the 14.8 Ma, at which time the upper fl uvial unit to the resistant basement since the late Miocene same area. covered the entire basin. (see later section; Wallace, 2005). At ca. 16.3 Ma, a lacustrine environment As the sediments gradually fi lled the original Some sediment-basement contacts, such as began to develop in the Carlin basin. This lowland, the Carlin basin expanded and con- those on the east side of the Tuscarora Mountains environment interfi ngered with and gradually nected with the adjacent Elko and Independence and west side of Schroeder Mountain, are faulted. expanded over the early fl uvial environment to basins and areas to the southwest. In addition, Schroeder Mountain (Figs. 7 and 9) was a low form the mixed and then ash-rich units. The ori- sediments progressively buried the fl anks of upland that diverted early stream fl ow and, at its gin of this lake is unclear but could be related adjacent highlands. By the end of sedimenta- north end, remained exposed throughout basin to continued modest fault-related collapse of the tion, the Carlin basin was much larger than the sedimentation. West-dipping, post-sedimentation basin. By ca. 15.5 Ma, a shallow, low-energy original depocenter, and the areal extents of the normal faults along the west side of Schroeder lacustrine sedimentary environment covered upland sources were much smaller. Mountain extend to the north, in part creating the much of the basin, and it gradually inundated Numerous normal faults cut and tilted the major gravity low along Maggie Creek (Ponce the broad benches on the fl anks of the Adobe Miocene sedimentary units after deposition and Morin, 2000). The sediment-basement con- Range and Marys Mountain. The rise in the of the upper fl uvial unit. Strata in the western tact along the east side of Schroeder Mountain is level of sedimentation indicates that the base two-thirds of the basin dip to the east, and those depositional, but the sediments in that immediate level of the depositional surface rose and that in the eastern third dip to the west. Stratal dips area dip west into and ramp onto the basement the rate of sediment accumulation outpaced that through the basin generally are less than 20°, block. This most likely indicates an east-dipping of any continued fault-related basin subsidence. and the total amount of fault-related extension normal fault within the block. With an early, south-fl owing stream system, likely was not great. Generally north- to north- the 15.3 Ma eruptions of the Palisade Canyon east-striking normal faults caused the tilting, and ELKO BASIN rhyolite at the south end of the basin could have a fault-controlled synform separates the regions further dammed outfl ow from the basin and, of opposing dip. These normal faults are present The Elko basin is an extensive, elongate basin because the resulting volcanic pile was thick, throughout the basin, and those with the great- located between the Ruby Mountains and East raised the base level of the basin. est amount of offset (up to 1 km) are within the Humboldt Range to the east and the Adobe At ca. 15.2 Ma, the lake that covered most of basin, not along the margins. Major west-dip- Range and Piñon Range to the west. Its southern the basin drained, and fl uvial sediments derived ping normal faults within the Carlin basin con- end is south of Jiggs along Huntington Creek, primarily from the east, north, and northwest tinue to the north and south into Swales Moun- and it extends far to the north along the Marys were deposited across much of the basin fl oor tain and the northern Piñon Range, respectively River Valley (Fig. 10). Small islands of pre- to form the upper epiclastic unit. The general (Evans and Ketner, 1971; Smith and Ketner, Miocene bedrock are exposed in the Elko and sharpness of the contact between the lacustrine 1978). At Swales Mountain, the normal faults Peko Hills and Cedar Ridge, and a paleoknoll of and fl uvial units indicates that the change in offset and tilted both the basement and the sedi- Jarbidge rhyolite is exposed at Black Butte. depositional environment happened quickly. mentary units. However, the faults do not appear In general, the area of the Elko basin was in Given that the ensuing fl uvial sedimentation in to have accentuated the relief of the highland. the hanging wall of the Tertiary, west-dipping the basin was directed to the south, failure of the Assuming that the lacustrine environment at Ruby–East Humboldt detachment and high- volcanic dam at the south end of the basin was ca. 15.2 Ma was horizontal and laterally con- angle fault systems. Any sediments produced the probable cause for lake drainage. tinuous, the Emigrant Pass and southern Adobe during detachment-related uplift are largely The coarseness of the sediments in the upper Range areas have been uplifted only ~300 and absent in or near the Elko basin, and the area fl uvial unit could indicate an increase in relief 240 m, respectively, relative to the same hori- either drained externally or, like the Carlin basin, in the source areas relative to the Carlin basin. zons in the south-central part of the basin. was not a lowland. The oldest post-Eocene However, no faults that can be attributed to uplift Similarly, north-striking normal faults in both sediments were deposited just before 15.4 Ma, at that time have been identifi ed along the fl anks Marys Mountain (Henry and Faulds, 1999) approximately the same time that high-angle of or within the highlands. The failure of the and the southern Adobe Range tilted the sedi- faulting along the range front began; the rate dam may have lowered the base level and per- ment-basement contacts, and faults in the Marys of uplift peaked between ca. 15 and 14 Ma and haps increased the stream gradients in the basin, Mountain area offset Miocene sediments that then continued at a slower rate. During early allowing the transgression of coarser sediments extended west of Emigrant Pass. Thus, post- sedimentation, alluvial fans drained eastward across the basin. sedimentation normal faulting and tilting was from the structurally passive Adobe and Piñon On the western side of the basin, a mixed fl u- equally common in both the Carlin sedimentary Ranges into the developing half-graben basin, vial and lacustrine environment persisted until basin and the fl anking highlands, and the inter- and a lacustrine environment began to form in after 14.8 Ma, and, in the southwestern part face between the basin and ranges in most cases the southern half of the basin at ca. 15.4 Ma. The of the basin, sedimentation above some of the is a tilted depositional contact and not a fault alluvial sediments partially buried paleohills of rhyolite fl ows did not even begin until 14.8 Ma. contact. As such, the elevational differences in the Elko and Peko Hills and Cedar Ridge, and These western relations could indicate a mod- the 15.2 Ma lacustrine strata described above are uplift of the Ruby Mountains and East Humboldt est amount of westward tilting of the basin and products of tilting within the modern highland Range progressively shed coarse materials into concentration of lacustrine sedimentation in that areas and not faulting along their margins. The the eastern part of the basin. Sedimentation con- area. However, based on source lithologies for apparent steepness of the fl anks of the modern tinued until ca. 9.8 Ma, when the basin began to the clasts, streams that deposited the conform- topographic highs, as well as lower elevation of drain externally, likely between the Adobe and ably overlying upper epiclastic unit in that area the Carlin topographic basin itself, were largely Piñon Ranges, and connected with the Carlin fl owed to the east. More likely, the western produced by a combination of this tilting of the basin. This drainage was blocked temporarily in

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the ca. 15.2 Ma basal part of the section contain blocks of Paleozoic rocks up to several meters in dimension. Erosional windows through the Mio- cene units reveal that the sediments were deposited on a gently east-dipping paleosurface on the east side of the range. Underlying units include various Paleozoic, Mesozoic, and Eocene units (Figs. 11A and 11B), and the Eocene units dip more steeply than the overlying Miocene strata. Clast lithologies in the sediments match Paleo- zoic and Eocene source rocks directly to the west in the range, and the grain size decreases away from the range. The very gentle eastward dip of these beds may be primary, indicating relatively little, if any, post-sedimentation tilting along the east side of the Adobe Range. The Peko Hills are surrounded and overlapped by similar but somewhat fi ner grained, epiclastic sediments (Ketner and Evans, 1988), and the sedimentary package on the east side of the Adobe Range projects directly into these deposits (Figs. 10 and 11B). This laterally continuous sequence is preserved across the Devil’s Gate area just north of the Peko Hills, where the epiclastic sediments extend over steeper dipping Eocene volcanic units and continue eastward toward the Twin Buttes area (see later description). At the south end of the Peko Hills, an andesite fl ow was erupted during sedimentation at 15.1 Ma (Table 1). Similarly, the Adobe Range sedimentary package projects east to the Elko Hills (Fig. 10), which are partially surrounded by and locally overlain by fi ner grained, epiclastic sediments (Smith and Ketner, 1978; Coats, 1987; Ketner, 1990). As such, the Peko and Elko Hills appear to have been middle Miocene, basement-cored hills that progressively were buried by the east-fl owing alluvial fan systems. East-dipping Miocene basin sediments are exposed extensively in the southern part of the basin, especially along Huntington Creek and Figure 10. Map of the Elko basin area and the Independence Valley. The map shows the other areas between the Piñon Range and the Ruby original known and inferred extents of the middle Miocene and Pliocene sedimentary units, Mountains (Fig. 10; Smith and Ketner, 1976; dates of various sedimentary and volcanic units, and the Ruby–East Humboldt detachment Smith and Howard, 1977). In the Huntington fault system. Uncolored areas include areas of pre-Miocene rocks and, in some areas to the Creek area, sedimentation began shortly before north, the Jarbidge Rhyolite (see Fig. 1). Areas east and south of the Ruby Mountains and 15.4 Ma and continued to after 9.9 Ma (Table 1; East Humboldt Range are outside of the study area and include both pre-Miocene basement Perkins et al., 1998). The strata, with a thickness units and Quaternary sediments but only trivial amounts of Miocene units. The gravity, of more than 560 m, were deposited in both fl u- depth-to-basement contours were created from gravity data and maps in Ponce (2004). See vial and lacustrine environments. Conglomerates Table 1 for geochronologic information. The modern digital elevation map is used as the comprise the basal sediments; due north of Cedar base. Ridge, coarse conglomerates derived from the northern Piñon Range fi ll an east-trending paleovalley (Fig. 11D). With the exception of these the middle Pliocene, forming a shallow lake that Stratigraphy basal conglomerates, ash-rich units generally are covered much of the Elko basin before draining more common in the lower half of the section, at ca. 2 Ma. Late Miocene and younger erosion On the west side of the Elko basin, Miocene and beds are thin bedded and laterally continuous. and removal of enormous amounts of Miocene strata are composed predominantly of conglom- The sediments have increasingly more epiclastic sediments from the Elko basin have reexposed erate and clast-rich sandstone that resemble materials upsection (Smith and Ketner, 1976), and much of the pre-middle Miocene paleotopogra- alluvial-fan deposits. At the southeastern end of they include sandstone, pebble-rich sandstone, phy along the western side of the basin. the Adobe Range, local debris-fl ow deposits in and conglomerate. Clasts were derived from

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Figure 11. Photographs of middle Miocene sedimentary units in the Elko basin. A: Basal Miocene, epiclastic sand and conglomerate beds on the west side of the Adobe Range. Photo taken looking north along the western outskirts of Elko. The beds overlie more steeply dipping volcanic units of the late Eocene Indian Wells Formation (light-colored, below). The gentle eastward dip of the Miocene units may be nearly primary. B: Peko Hills (behind the green fl oodplain of the North Fork Humboldt River) and fl uvial Miocene sediments (tan, foreground) on the eastern dip slope of the Adobe Range. The fl uvial sediments continue across the river valley, underlie the low bench to the left (north) of the Peko Hills, and depositionally overlie Paleozoic rocks in the Peko Hills, indicating that the hills were paleohighs during sedimentation. The snowcapped peaks in the background are the Ruby Mountains. Late Cenozoic erosion of the Miocene strata during downcutting of the river has progressively exposed the Peko Hills. C: Conglomerate in an east-trending paleovalley north of Cedar Ridge (Fig. 10). The clasts were derived from Paleozoic units exposed in the Piñon Range to the west. When the channel fi lled, fi ner grained epiclastic sand and pebbles formed a continuous cover over the channel fi ll and adjacent bedrock areas. D: Pebble-rich fl uvial sediments in the Huntington Creek area in the southern part of the Elko basin. All of the clasts are crystalline metamorphic and igneous rocks derived from the Ruby Mountains to the east. These fl uvial beds overlie ash-rich lacustrine units along Huntington Creek. Arrow points to 6-cm pocket knife for scale. E: Ash-rich, thin-bedded lacustrine sediments in a railroad cut near Wells. Green beds to the right have been altered to chert and zeolites. An unaltered tephra from this sequence was dated at 10.5 Ma. Hammer in right-center of photo (arrow) provides scale. F: Horizontal Pliocene (ca. 2.1 Ma) lacustrine units (light, P) cover lowlands cut into gently west-dipping fl uvial strata of the middle Miocene Humboldt Formation (M, tan in middle distance). Photo taken looking east-northeast at the Secret Pass area between the East Humboldt Range (left horizon) and the Ruby Mountains (to right of photo).

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both the Piñon Range to the west and the Ruby the Miocene sedimentation (Good et al., 1995; 2 km of that depth as “upper Tertiary” sediments Mountains to the east, and the infl ux of Ruby- Snoke et al., 1997; Thorman et al., 2003). (Effi moff and Pinezich, 1981), but that would derived gneissic clasts beginning at 14–14.5 Ma Basin sediments in the Marys River area include overlying Pliocene and Quaternary records the progressive uplift and incision of the north of the Humboldt River are predominantly units, whose thicknesses are unknown, as well Ruby Mountains (Fig. 11E; Sharp, 1939; Smith fi ne-grained, epiclastic sands and silts with vari- as the Miocene strata. In contrast, the depth to and Ketner, 1976). Cedar Ridge, a low, Paleo- able amounts of conglomerate and primary and basement in the narrow zone between the Adobe zoic-cored knob within the basin east of the reworked air-fall ash. At Black Butte (Fig. 10), Range and the Elko and Peko Hills to the east Piñon Range, contributed clasts to the adjacent these sediments largely underlie but locally is generally just a few hundred meters. In those basal conglomerates and then was partially cov- overlie the Jarbidge Rhyolite, which forms the areas, full exposures of the Miocene sediments ered by ash-rich sediments (Smith and Ketner, resistant butte. Clasts were derived from the are 100 m or less thick and the underlying Elko 1976, 1978), a burial history similar to that in Snake Mountains to the east, and epiclastic sedi- Formation and Eocene volcanic rocks, where the Peko and Elko Hills. ments grade northwest into more thin-bedded, present, comprise the remainder of the basin fi ll Sharp (1939) studied the Miocene sediments ash-rich sediments (Smith et al., 1990). Just to above the Paleozoic basement. along the Ruby Mountains and the East Hum- the southwest of Twin Buttes (Fig. 10), andesite boldt Range between Huntington Creek and fl ow units identical to the 15.1 Ma fl ows in the Structure and Faulting the Wells area. Overall, he found that the sec- Peko Hills overlie beds of reworked air-fall ash tion grades upward from primarily lacustrine and small-pebble conglomerate derived from The western side of the Elko basin is largely to entirely fl uvial, and that the majority of the the west. This area is 10 km due east of Devils unfaulted and is composed of Miocene strata strata are composed of fl uvial sandstone to mud- Gate and “downstream” from the alluvial fan on that overlie a gently east-dipping paleosur- stone. West of Secret Pass (Fig. 10), some of the the east side of the northern Adobe Range, as face cut into older rocks. These Miocene units fl uvial units contain turbidites, indicating local described earlier; as such, these epiclastic sedi- extend east to and across outboard pre-Mio- subaqueous deposition, but most of the epiclas- ments may be a distal part of that fan. cene paleohighs. The steeper dips on underly- tic units were deposited in streams and alluvial Much of the central part of the Elko basin is ing Eocene volcanic and sedimentary units in fans derived from the Ruby Mountains and East directly underlain by ash-rich to epiclastic sedi- all of these areas indicate a poorly constrained, Humboldt Range to the east. The units dip gen- ments that, on the basis of limited tephra data, pre-Miocene period of tectonism that may have tly to the east except immediately adjacent to are middle Pliocene in age (Fig. 11F; Reheis et produced the outboard horsts prior to Miocene the range front, where they dip steeply east and al., 2003; Wesnousky and Willoughby, 2003; sedimentation. are in fault contact with moderately west-dip- M.E. Perkins, 2007, unpubl. data). These gener- Faults along the west side of the Elko basin ping normal faults related to uplift of the ranges ally horizontal sediments contain fl uvial strata are relatively minor and formed after sedimen- (Sharp, 1939; Snoke et al., 1997). near the Ruby Mountains and fi ne-grained tation. A west-dipping fault along the northern The Miocene sediments in the northeast- lacustrine units in the middle of the basin, and end of Cedar Ridge had ~700 m of post-sedi- ern East Humboldt Range and southern Snake they were deposited during a hiatus in episodic mentation offset, based on offset tephras, but the Range near Wells (Fig. 10) record a protracted late Miocene and younger erosion in the upper sediment-basement contacts along the main part period of sedimentation in the hanging wall of Humboldt River basin (Wallace, 2005). One of Cedar Ridge are depositional (Smith and Ket- the gently west-dipping Marys River fault sys- drill hole east of the Elko Hills indicates a thick- ner, 1978; M.E. Perkins, 2007, unpubl. data). An tem, which is exposed on the west sides of the ness of ~275 m. Similar Pliocene sediments are east-dipping fault within the type section of the two ranges and extends beneath the Elko basin exposed south and southwest of Elko (Smith and Humboldt Formation along Huntington Creek (Effi moff and Pinezich, 1981; Robison, 1983; Ketner, 1976). The Pliocene sediments are very had ~200 m of displacement. West-dipping nor- Mueller and Snoke, 1993). Most of the epiclas- similar to the Miocene sediments, including the mal faults along the southeast fl ank of the Adobe tic sediments in the lower two-thirds of the sec- distribution of the sedimentary facies, suggest- Range and the southern Elko Hills juxtaposed tion were deposited in low-energy, fl uvial and ing deposition in a similar topographic setting. Paleozoic basement and the basal Miocene sedi- lacustrine environments, but thin to thick zones Despite this blanket of Pliocene sediments, oil ments, and offsets likely were 200 m or less. of coarse conglomerate indicate periodic higher drilling, seismic, and gravity data provide some In contrast, the west- to northwest-dipping energy sedimentation. Abundant air-fall ash in clues about the basal morphology of the Tertiary Ruby–East Humboldt detachment fault and the upper third of the sequence was deposited in basin at depth. One complication with using the west-dipping normal faults are present along a lacustrine to mudfl at environment (Fig. 11E). gravity data is that Miocene sediments are not the east side of the Elko basin from Wells to The exposed Miocene section in the southern the only basin fi ll in most areas, as shown in oil south of Jiggs (Fig. 10). Various isotopic, ther- Snake Mountains is more than 1000 m thick drilling records (Hess, 2004), and the depth-to- mal, and barometric data show signifi cant cool- (Thorman et al., 2003), and seismic data in the basement data refl ect the thickness of all Ceno- ing in the Oligocene and early Miocene, which basin to the west suggest that the Miocene sedi- zoic units, not just the Miocene sediments. The some workers attributed to detachment-related ments there may be ~3000 m thick (Robison, Eocene Elko Formation alone is consistently at uplift of those ranges (see summary in Howard, 1983). Fission-track dates on detrital zircon in least several hundred meters thick in and near 2003). Fission-track uplift data indicate that the middle of the Snake Mountains sequence the Elko basin (Haynes, 2003). uplift and eastward tilting of the southern Ruby indicate a depositional age of less than 11 Ma The gravity data as a whole show that the top Mountains, related to high-angle normal fault- (Thorman et al., 2003), and the upper, ash-rich of the pre-Tertiary basement is generally deep- ing along the west side of the range, peaked at section was deposited at ca. 10.3 Ma (Table 1). est near the Ruby Mountains and East Humboldt ca. 15–14 Ma; this event may have affected the Fluvial to lacustrine sediments underlie 13.4 to Range, reaching a depth of more than 3 km northern part of the Ruby Mountains as well 15.1 Ma rhyolite fl ow units in the East Hum- south of Jiggs and more than 6 km just north- (Colgan and Metcalf, 2006). Stratigraphic and boldt Range and Snake Mountains, but these west of Wells (Fig. 10; Ponce, 2004). Seismic fi ssion-track data indicate that more than 4 km may be Eocene to Oligocene and unrelated to data across the basin southwest of Wells identify of Paleozoic sediments were eroded from the

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Ruby Mountains during this stage of uplift lithologies, where exposed in this area, are along the west sides of the modern Ruby Moun- (Smith and Ketner, 1976; Colgan and Metcalf, monotonous, and detecting offset in them would tains and East Humboldt Range (Colgan and 2006). The high-angle faults along the west be extremely diffi cult. Metcalf, 2006) cut the upland, created an east- side of the Ruby Mountains continue south to In contrast to the Carlin basin, signifi cant dipping half graben, and induced early alluvial- about Bald Mountain (Fig. 10), at which point intra-basin faults have not been identifi ed in fan sedimentation in the new basin. the polarity of the intermontane basin reverses. the Elko basin, although small-offset faults are Alternatively, the cooling data record uplift The Newark basin, south of the Elko basin, dips evident. In large part, this may be due to the and the formation of an east-dipping, hanging- to the west into range-bounding faults along the poor exposures of Miocene sediments and to wall basin above the detachment fault, but the east side of the Diamond Mountains, and the the extensive cover of fl at-lying Pliocene sedi- basin drained externally. In this case, all sedi- much shallower, intervening part of the basin at ments. However, the consistent fl atness and lack ments generated were very effi ciently carried the polarity reversal likely contains a structural of offset of the younger sediments over a very out of the basin and produced the same non- transfer zone (Wallace et al., 2007b). wide area indicates that any intra-basin faults sedimentary result. As with the fi rst scenario, In the vicinity of Wells, the west-dipping that may be present were active before Pliocene a relict older highland remained in the pas- Marys River fault controlled the western sides sedimentation. Locally chaotic to opposing sive Adobe and Piñon Ranges, with supergene of the southern Snake Mountains and northern dips in Miocene units east of Huntington Creek weathering in the latter area. In this scenario, East Humboldt Range. The complex tectonic (Smith and Howard, 1977) may indicate some the middle Miocene, high-angle faulting accen- and sedimentary features in this area indicate concealed faults near the range front, but offset tuated a half-graben setting and closed the once- the formation of an eastward-deepening middle apparently was minimal given the overall lateral open basin (or reduced its gradient), possibly Miocene and younger basin in the hanging wall continuity of that section. along the accommodation zone at the south end. of the fault system (Effi moff and Pinezich, 1981; Neither scenario contradicts the presence of an Robison, 1983; Mueller and Snoke, 1993; Thor- Paleogeography and Basin Evolution Eocene upland east of an Eocene Elko basin, man et al., 2003). Post-sedimentation (in this area, as described by Haynes (2003). In both cases, after ca. 10.3 Ma) faulting has signifi cantly modi- Similar to the other basins studied, late Eocene however, erosion did not appreciably remove fi ed the basin margin, which may have extended a to middle Miocene units are very rare in or near late Eocene sedimentary and volcanic units that few kilometers east of Wells (Mueller and Snoke, the Elko basin. Although thermochronologic were deposited and still remain in or near many 1993), and Pleistocene and younger faults have data have been interpreted to indicate middle parts of the Elko basin. offset terrace deposits along the entire range front Tertiary, detachment-related uplift of the Ruby Regardless of earlier scenarios, the combined (Dohrenwend et al., 1996). Mountains and East Humboldt Range, no sedi- stratigraphic, structural, and depth-to-basement Farther north, the sediment-basement contact ments of that age that might record that uplift are data indicate that the middle Miocene Elko along the west side of the Snake Range is both present in the basin except in a small area just basin was an east-dipping half graben. The west depositional and fault controlled (Coats, 1987; south of Wells (Snoke et al., 1997). Middle Ter- side was structurally passive during sedimen- Smith et al., 1990; Thorman et al., 2003). The tiary sediments may be present beneath younger tation, and ca. 15.3 Ma, east-dipping alluvial range front east of Black Butte forms an arcuate cover in the deep, middle parts of the basin, but fans extended out across the basin, partially topographic embayment (Fig. 10), and Miocene these are not evident on seismic lines or in drill- burying outboard, pre-Miocene paleohorsts. At sediments exposed throughout the embayment hole records. Sedimentary basins in the hang- ca. 15.4 Ma, outfl ow was blocked, possibly at dip moderately east into the range front. Late ing walls of detachment systems are common the south end, and an early lake formed along Cenozoic faults are closer to but outboard from (cf. Wagner and Johnson, 2006); therefore, the at least the southern axis of the basin. Clast the range front west of Wells, and they extend apparent absence of such a depocenter in this lithologies record the 15 to 14 Ma rapid uplift far out into the basin west of the Black Butte area is notable. Although the present study did and erosion of the Ruby Mountains to the east embayment (Dohrenwend et al., 1996). The not focus on this issue, the character of the pre- (Smith and Ketner, 1976; Colgan and Metcalf, basin deepens west of these faults (Ponce 2004), middle Miocene paleogeography is relevant to 2006) as fl uvial sediments inundated the lacus- with a shallowly buried pediment between the later sedimentation. trine setting. faults and the Snake Mountains. Two scenarios are possible. First, the area The eastern part of the basin probably con- The Wells fault is a west-striking transfer zone that now includes the Elko basin and the Ruby tinued to deepen structurally during sedimenta- that has been projected west from the southern Mountains and East Humboldt Ranges formed tion, as refl ected in the protracted sedimentation Snake Mountains toward the northern end of the a broad upland surface that connected to the in that area between 15.4 and 9.9 Ma (Table 1) Adobe Range (Thorman and Ketner, 1979). It west with the remnant uplands of the Adobe and and the thicker section along the east side of has been cited as the northern limit of detach- Piñon Ranges and the Carlin basin that were the basin shown by gravity, seismic, and drill- ment-related faulting in the Ruby Mountains described earlier. Supergene alunite dates of 22– ing data (see analogous descriptions in Rosend- and East Humboldt Range (Mueller et al., 1999; 18 Ma from the northern Piñon Range (Table 2; ahl, 1987; Chapin and Cather, 1994; Faulds and Howard, 2003). If detachment-related faulting Williams, 1992), indicate that area was exposed Varga, 1998). However, the near-original dips was active during Miocene sedimentation, then and weathering prior to middle Miocene basin of the alluvial fans along the Adobe and Piñon this fault should have been active as well. How- formation. As such, the cooling recorded in the Ranges (Fig. 11A), as well as consistent dips in ever, depth-to-basement data indicate that the thermochronologic data may refl ect some mid- the full stratigraphic section near Huntington mapped fault projects directly into the center of dle Tertiary uplift, but without the formation of Creek (Smith and Ketner, 1976), do not support a the deepest part of the basin, rather than form- a hanging-wall basin, and sediments generated progressive eastward tilting and deepening of the ing its northern fl ank as might be expected, if during erosion of the upland were transported basin. In addition, the ability of the western, non- it was a signifi cant depocenter control. Pliocene far beyond the area, as was postulated earlier tectonic side of the basin to connect with the Car- sediments cover much of the projected trace of for the Carlin basin. In this scenario, the mid- lin basin and Independence Valley at ca. 15.2 Ma the fault through the Elko basin; the Miocene dle Miocene formation of the high-angle faults indicates that the rate of early sedimentation in

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the basin as a whole outpaced the rate of struc- thicken modestly to the east and likely were epiclastic units in the eastern part of the valley tural subsidence, allowing the basin to fi ll up. deposited in an existing topographic basin are concentrated near the Piñon Range range Miocene sedimentary units are exposed dis- between the two ranges (Regnier, 1960; Smith front, whereas those in the western part extend continuously between the northwestern part of and Ketner, 1976; Gordon and Heller, 1993; far out into the basin; lacustrine deposits are the basin and the Independence Valley. Imme- Muntean et al., 2001; Hess, 2004). In the north- closer to the Piñon Range. Gordon and Heller diately north of this area, the middle Miocene ern end of the valley, the lower part of the section (1993) interpreted these relations to indicate that Jarbidge Rhyolite is widely exposed, and its is a mixture of fl uvial sand, mud, and conglom- faulting along the west side of the Piñon Range present southern extent generally coincides with erate. Identifi able clast lithologies on the eastern occurred during sedimentation, although the the northern limit of middle Miocene sedimen- side of the basin include Paleozoic sedimentary rate of sedimentation generally kept pace with tary units (Fig. 10). The poorly dated rhyolite and Eocene igneous lithologies exposed in the that of subsidence. The Miocene strata dip ~10° overlies most of the sediments at Black Butte Piñon Range and, permissively, southern Adobe more steeply than the overlying Hay Ranch For- and underlies them in the northeastern Indepen- Range; clast imbrications indicate predomi- mation, which indicates that faulting occurred dence Valley (see the ensuing section). These nantly westward fl ow from the Piñon Range, but or began before Pliocene sedimentation. extensive, thick, and roughly coeval rhyolite also some southward fl ow from the area of the Seismic data show a west-dipping, range- fi elds may have blocked the expansion of the Carlin basin. On the western side of the basin, bounding fault that controls the east side of the basin or external drainage to the north. clast lithologies include Jurassic volcanic rocks valley (Gordon and Heller, 1993), and post-Hay The youngest dated sediments are 9.9 Ma at exposed in the Cortez Range to the west (Smith Ranch offset along northwest-dipping normal Huntington Creek and ca. 10.5 Ma near Wells, and Ketner, 1976). In general, the sediments faults created several intra-basin half grabens. but younger sediments were not deposited until are fi ner grained and more ash rich upsection. However, the Hay Ranch in places is nearly hor- the formation of the relatively short-lived middle High in the exposed section, a meter of silicifi ed izontal, so that the faulting has only minimally Pliocene lake. However, the Ruby Mountains breccia composed of the Palisade Canyon rhyo- tilted the basin sediments since the early Pleis- and East Humboldt Range were notable adja- lite resembles a very distal, autobrecciated lava tocene. These northwest-dipping faults may be cent highlands that must have shed detritus into fl ow or fl ow-end rubble bed (also noted in Raine more gravitational than tectonic (Muntean et the basin. The absence of sediments deposited Ranch Formation section 3 of Regnier, 1960). al., 2001). As such, the Piñon Range appears after ca. 9.9 Ma indicates that the basin began One tephra sample collected ~20 m above the to have attained much of its altitude before the to drain externally at that time, but the outlet or rhyolite breccia unit produced a 15.3 Ma corre- middle Miocene than later. This supports Smith outlets for the basin are unknown. Signifi cant lation age (Table 1; Fig. 7), which also is the age and Ketner’s (1978) conclusion that the range amounts of Miocene sediments were eroded and of the rhyolite (Table 1). Therefore, the majority was present before Miocene sedimentation, as removed from the basin before the Pliocene lake of the fl uvial sediments in northern Pine Val- well as supergene alunite dates at the Rain mine event (see later section; Wallace, 2005); there- ley were deposited prior to 15.3 Ma, and their that indicate late Oligocene to early Miocene fore, the original outlet actually may have been lithologies indicate the presence of highlands to exposure and weathering (Table 2). topographically higher than the present base both the west and east. The northern half of Independence Valley lies level. Early formed strath terraces are paired Previous workers (Regnier, 1960; Smith between the Independence Range and Double around the Humboldt River, which fl ows south- and Ketner, 1976; Gordon and Heller, 1993) Mountain, and the southern half is between the west out of the basin at Carlin Canyon, suggest- concluded that the basin was not tectonically Swales–Lone Mountain highland (the southern ing that the west-draining river began to form active during middle Miocene sedimentation. Independence Mountains) and the Adobe Range at that time. As with the Carlin basin to the north, sedimen- (Fig. 10). The North Fork Humboldt River begins tary units older than perhaps 16 Ma (based on at the north end of the valley and drains eastward PINE AND INDEPENDENCE VALLEYS the 15.3 Ma date near the top of the section) through a broad gap between the Adobe Range are absent. Eocene and older units on the east and Double Mountain. Depth to basement typi- Pine and Independence Valleys were examined side of the Cortez Range dip moderately to the cally is less than 1 km, with some small areas only in a reconnaissance fashion. In the middle east, beneath the basin and against the Piñon deeper than 3 km (Ponce, 2004); how much of Miocene, these basins connected with adjacent Range. The angular unconformity between the this basin fi ll is composed of Eocene volcanic basins and thus merit brief descriptions. older and the middle Miocene units indicates and sedimentary units is unknown. Pine Valley is due south of the Carlin basin pre-middle Miocene tilting, presumably related Miocene sediments are poorly exposed and lies between the Piñon Range to the east to a west-dipping normal fault along the west throughout much of the valley. Highway road- and the Cortez Range to the west (Figs. 1 and side of the Piñon Range. Dips of the Miocene cuts in the middle of the basin southeast of the 7). Except at the very north end of the valley, sediments do not consistently decrease upsec- Independence Mountains expose epiclastic to Miocene sediments in Pine Valley are concealed tion; therefore, the tilting event either occurred ash-rich, fi ne-grained sandstone and siltstone beneath the Pliocene and Pleistocene Hay Ranch prior to or during the very early stages of basin with no coarser material; these sediments were Formation but are known to be present based on sedimentation. If it was before, then the absence deposited in low-energy, fl uvial to lacustrine seismic and drilling data (Gordon and Heller, of older sediments suggests that the area drained environments. Dips in this area are nearly hori- 1993; Hess, 2004). The Miocene sediments externally until ca. 16 Ma. zontal. On the western side of the valley, the fi ne- include the Carlin Formation and the upper part The Pliocene and Pleistocene Hay Ranch grained sediments were deposited unconform- of the Raine Ranch Formation as described by Formation overlies the Miocene sediments ably on more steeply dipping, Eocene volcanic Regnier (1960) and reclassifi ed as the middle along a very low-angle unconformity (Regnier, rocks. Tephra correlations in this area indicate Miocene Humboldt Formation by Smith and 1960; Gordon and Heller, 1993). The distribu- ages of 14.2–14.8 Ma (Fig. 10; Table 1). Closer Ketner (1976). tion and thicknesses of the two units are very to the center of the basin, a small, Paleozoic- On the basis of fi eld, seismic, and drill-hole similar, although the Hay Ranch Formation is and Eocene-cored, intra-basin fault block dips data, the Miocene sediments in Pine Valley much fi ner grained and ash rich overall. Coarse moderately to the east, and one tephra near the

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base of the Miocene section correlated with a have been relatively lower relief highlands dur- transect was minor except for the somewhat ca. 15.9 Ma tephra. ing sedimentation that experienced intra-high- more extended region west of the northern The sedimentary units exposed in the middle land faulting after sedimentation, followed by Nevada rift and south of Chimney basin. of the basin are continuous southward to simi- preferential late Miocene and younger erosion lar 15.8 to 15.3 Ma units in the Camp Creek of the Miocene strata. Pre-Sedimentation History and Blue Basin areas east of Swales and Lone Mountains (Figs. 7 and 10), which, in turn, REGIONAL PALEOGEOGRAPHIC One striking aspect of the entire 200-km- extend south into the Carlin basin (see the Car- EVOLUTION long transect is the general absence of middle lin basin descriptions). Strata along the eastern Tertiary sedimentary deposits and only rare vol- fl ank of the Swales–Lone Mountain highland The features and histories of the Chimney, canic units. Small areas of Oligocene tuffs are dip modestly to the east and are largely deposi- Ivanhoe, Carlin, and Elko sedimentary basins exposed in the western Carlin basin (Henry and tional on Eocene and older rocks that comprise provide evidence of the settings and events Faulds, 1999), and sedimentary units that may the highland (Lovejoy, 1959; Evans and Ketner, before, during, and after middle Miocene basin be as young as Oligocene are preserved just 1971; Ketner, 1998). Strata along the western formation. Although each basin evolved in south of Wells beneath middle Miocene volca- side of the Adobe Range dip to the west, and response to events in and around that particular nic rocks (Snoke et al., 1997). Middle Tertiary sediment-basement contacts are entirely deposi- basin, the combined histories create a picture units are similarly absent north of the transect tional (Ketner and Ross, 1990). The dip reversal of geologic and landscape evolution that has in northernmost Nevada (Coats, 1987; Rahl et takes place several kilometers outboard of the broader geologic and mineral-deposit implica- al., 2002; Brueseke and Hart, 2007) and south- Swales–Lone Mountain range front. Juxtaposi- tions for this and adjacent areas of the Great western Idaho (Ekren et al., 1981). In contrast, tion of stratigraphically lower strata on the east Basin. Figure 12 diagrammatically shows four widespread middle Tertiary volcanic rocks and, side and higher strata on the west side indicates stages of the geologic and paleogeographic evo- locally, sedimentary units are present south of that a steeply west-dipping, intrabasin, normal lution of the study area, starting with the setting the transect, as well as to the west in the Pine fault created the dip reversal. This fault may just prior to middle Miocene sedimentation, and Forest Range and adjacent areas (Stewart and connect with a similar fault in the Carlin basin; followed by three stages from the middle Mio- Carlson, 1978; Colgan et al., 2006, 2008; Fig. 1). its projection to the north is uncertain. cene to the present. The middle Tertiary climate was generally tem- The northern part of the basin, east of the In summary, the area east of the Tuscarora perate and moist (Axelrod, 1956; Zachos et al., Independence Mountains, has limited expo- Mountains was a broad upland prior to the mid- 2001), and erosion and runoff undoubtedly took sures. In the center of the basin, the units include dle Miocene that experienced modest middle place along the transect. Although sediments nearly horizontal, epiclastic sandstone and silt- Tertiary faulting but generally was exposed and produced during this period could have been stone beds, and beds in the northwestern part of weathering during that time. Streams drained deposited and then entirely removed by erosion the valley, adjacent to the Independence Moun- out of the area. At the same time, the broad just prior to 16 Ma, the more likely scenario is tains, dip modestly to the east (Henry, 2008). area west of the Tuscarora Mountains was a that the area was an upland and streams drained The Miocene sediments were deposited on somewhat lower area with streams that carried out of the area, carrying the sediments with and around Jarbidge Rhyolite fl ow units along sediments westward and out of the area. In the them. the western base of Double Mountain. East of middle Miocene, high-angle faulting segmented Middle Tertiary extension tilted all pre-mid- the valley, Miocene sediments are nearly con- the broad upland east of the Tuscarora Moun- dle Miocene units along the transect. However, tinuously exposed along the broad North Fork tains, created the early Carlin and Elko basins, with the exception of steeply dipping 33.5 Ma Humboldt River valley between Independence and trapped sediments in those areas. Volcanic tuffs at Mule Canyon (Fig. 1, John and Wrucke, Valley and the Devils Gate area in the Elko activity also dammed some outfl owing streams 2003), the difference between Eocene and Mio- basin (Fig. 10). As described earlier, those strata to accentuate sediment retention. Extensive vol- cene stratal dips generally is 20° or less, indi- overlie tilted, late Eocene volcanic rocks. canic activity related to the northern Nevada rift cating relatively small amounts of extension. Overall, Miocene sediments in the Indepen- blocked westward streamfl ow out of the Ivanhoe In most places, the age of tilting is not known. dence Valley were deposited in low-energy and Chimney, creating those sedimentary basins. Some deformation took place between ca. 36 fl uvial and epiclastic-rich lacustrine environ- Except for the Elko basin, dams failed within a and 31 Ma on the east side of the Piñon Range ments. As with other basins, the formation of couple of million years and streams resumed (Palmer et al., 1991), and pre–16.5 Ma tilting late Cenozoic strath terraces removed some their external fl ow, initiating a proto-Humboldt took place both before and after the eruption Miocene sediments from the valley. The limited River drainage system. The Elko basin contin- of 25 Ma tuffs at Marys Mountain (Henry and amounts of coarse fl uvial sediments, even close ued to retain sediments until ca. 9.8 Ma, when it Faulds, 1999), similar to the range of supergene to the source areas, suggests only low relief at began to drain to the west and integrate with the alunite dates along the Carlin trend that may the time of sedimentation. Swales Mountain did other streams. The integrated stream system has refl ect uplift events (see below). South of the shed coarse material into the Carlin basin; there- progressively removed large volumes of Mio- transect, middle Tertiary extension varied from fore, it was a highland at the time of sedimenta- cene sediments from these basins and carried minimal to locally 50% (Muntean et al., 2001; tion. Some post-sedimentation faulting occurred them to downstream reaches of the Humboldt Colgan et al., 2008). within the basin, but not enough to create major River in northwestern Nevada. This erosion has Middle Tertiary weathering and erosion offsets or steep dips, and the sediment-base- exhumed pre-middle Miocene basement that affected much of the study area. Pre-middle ment contacts along the basin margins remain was buried during sedimentation. High-angle Miocene regoliths formed in some areas, and depositional and not fault controlled. As with faulting affected all areas to some degree after materials derived from the regoliths were shed the Marys Mountain area on the west side of the sedimentation, with early minor faulting and into Miocene basins during early sedimentation. Carlin basin, the adjacent, high-relief Indepen- more signifi cant offsets later in the Miocene. Erosion exposed late Eocene plutons in the Tus- dence, Lone, and Swales Mountains areas may Overall Neogene crustal extension along the carora Mountains, eastern Ivanhoe district, and

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@ ~15.6 Ma S na

F ke

pendence Mtns. ChB volcanic 10 mi dam (early) Range (16 km) Inde volcanic dam

carora Mtns. IV Swales ea g IB Mtn. Tus n 41˚00’ Adobe R. ar 41˚00’ volcanicv e aini dam ((later) C B ge EB upland basin lly dr

Rang nant rna dt Ran ? em xte ing-wall R ? or e hang volc. on Olig. calderas dam Piñon s.-E. Humbol tn fault PV dam

Ruby M 40˚00’ 40˚00’

117˚00’ 116˚00’ 115˚00’ 117˚00’ 116˚00’ 115˚00’ 42˚00’ 42˚00’ (C) ca. 14-9 Ma (D) ca. 9 Ma - present

Snowstorm Mtns.

Sheep 41˚00’ Crk. R. 41˚00’

e g an

ez R ort C

40˚00’ 40˚00’

Pliocene to Quaternary lake Streamflow direction

Miocene sedimentary basin Normal fault Miocene volcanic areas Miocene epithermal deposit Upland Late Eocene gold belt

Figure 12. Diagrammatic geologic and paleogeography history of northeastern Nevada, including uplands (medium brown), areas of basin sedimentation (blue), Miocene volcanic fi elds (“v” pattern), major normal faults (bar and ball), and stream fl ow directions (blue arrows); lighter brown areas are uplands that may or may not have been present or were very subdued. Shown also are late Eocene gold trends (gold dotted lines) and middle Miocene epithermal deposits (red dots) from Figure 1. The area shown is the same as in Figure 1, where other geographic locations are presented; county boundaries (dashed lines) can be used as references. Time intervals include: (A) pre- middle Miocene, prior to the inception of middle Miocene basin sedimentation; (B) ca. 16 Ma, when the four basins described in this paper (Chimney (ChB), Ivanhoe (IB), Carlin (CB), and Elko (EB) basins and Independence Valley (IV)) began to form and fi ll with sediments; (C) ca. 14–9 Ma, the period when the western basins drained externally to form the early Humboldt River system, with continued sedimentation in the Elko basin (EB); and (D) ca. 9 Ma to the present, by which time the Elko basin had integrated into the Humboldt River system, with temporary Pliocene sedimentation in the Elko basin and late Pliocene and Pleistocene sedimentation in Pine Valley (PV) and downstream parts of the Humboldt River (latter two from Reheis, 1999a). The pre-middle Miocene uplands are simplifi ed from Haynes (2003) and include only those related to this study. Uplands had subdued topography, on the basis of only minor amounts of coarse fl uvial material except near fault-controlled range fronts. Note the contraction in the exposed extents of some uplands after partial burial by middle Mio- cene sediments (B to C) and their somewhat increased extent during late Cenozoic erosion of Miocene strata (C to D).

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southwestern Adobe Range, and middle Mio- cene sediments were deposited on and contain clasts derived from those exposed plutons. The Carlin trend gold deposits formed at least 800 m beneath a paleosurface in the late Eocene (Haynes, 2003; Hickey et al., 2003). Supergene alunite dates of 30–18.4 Ma from some of the Carlin trend gold deposits (Table 2; Fig. 13) record progressive erosion and weathering of that surface; they also may refl ect the age(s) of regolith formation. Some of the gold deposits were exposed by the middle Miocene, contributed sediments to the Carlin and Ivanhoe basins, and were in part covered by Miocene sediments. A single, supergene alunite date of 23 Ma from the Preble gold deposit along the southern part of the Getchell trend (Table 2; Fig. 13) refl ects weathering in that area, but the paleogeographic relation of that site to the Chimney basin to the north is unknown. Despite the erosion and weathering, late Eocene volcanic rocks were locally exposed in some of the highlands and all of the Miocene basins. Although these units were not uniformly deposited across the region (Ressel and Henry, 2006; Henry, 2008), their presence demonstrates that some of the late Eocene paleosurface was not signifi cantly eroded in the middle Ter- tiary. At Willow Creek Reservoir in the Ivanhoe basin, both Eocene and middle Miocene units were deposited in a west-draining paleovalleys, despite an intervening period of tilting (Wal- lace, 2003a), and the tilting event may have only minimally affected the landscape. In the Chimney and Ivanhoe basins, the pre- sedimentation topography was subdued and drained generally to the west. These two, lacustrine-dominated basins connected across the northern Nevada rift in the Snowstorm Moun- tains, which, shortly before sedimentation, was the site of mafi c volcanism and concurrent, fault-related subsidence. The lack of sedimentary thickening in the rift zone and the ability Figure 13. Supergene alunite dates and geomorphic events in northeastern Nevada. The of the lacustrine environment to extend across vertical blue bars indicate the sedimentary lifespans of the basins, simplifi ed from Fig- that area shortly after volcanism ceased demon- ure 2. Basin abbreviations: CaB—Carlin basin; ChB—Chimney basin; EB—Elko basin; strate that the eruption rate was similar to the IB—Ivanhoe basin; IV—Independence Valley. Supergene alunite dates for the Carlin and subsidence rate, leaving a fairly level landscape Getchell gold trends are shown as squares (open, Rain deposit in the northern Piñon Range; just prior to sedimentation. This contrasts with a solid, all other deposits); data are provided in Table 2. The major geomorphic periods and proposal that middle Miocene thermal bulging processes are shown along the right margin. created a kilometer-high upland in the area of the rift (Pierce et al., 2002). To the north, the absence of northern Nevada- nifi cantly controlled the distribution of volcanic 9), with streams draining to the east and west derived zircons in middle Miocene basins along fl ow units (Brueseke and Hart, 2007). (Haynes, 2003; Henry, 2008). As described the Oregon-Idaho border led Beranek et al. (2006) All of these relations indicate that much of earlier, the absence of sediments in the hang- to propose a drainage divide, and, thus, a paleo- the transect area was a moderately eroding, ing-wall region of the Ruby Mountains–East topographic high, somewhere near the Oregon- nonaggradational, topographic upland in the Humboldt Range detachment zone may indi- Idaho-Nevada border (Fig. 1). Paleotopography middle Tertiary (Fig. 12A). This feature may cate either an externally draining hanging-wall around the Santa Rosa–Calico volcanic fi eld, have originated in the late Eocene, when a broad basin or no basin at all. There, most of the sur- northwest of the Chimney basin, apparently was paleohigh extended roughly south from the Tus- face uplift may have been related to high-angle more pronounced, and uplands and valleys sig- carora area toward Marys Mountain (Figs. 1 and faulting that took place during sedimentation

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between 14 and 15 Ma (Howard, 2003; Colgan to adjacent basins. Failure of the dam caused the 100 m of displacement. The low stratal tilts and Metcalf, 2006). Thus, despite the evidence lake to drain and fl uvial sedimentation to blan- (often nearly horizontal) and minor fault dis- of middle Tertiary faulting along the transect, ket most of the basin fl oor, although a shallow placements demonstrate relatively little exten- that faulting apparently did not have a signifi - lacustrine environment persisted along the west- sion in this region. Muntean et al. (2001) esti- cant paleotopographic effect, such as the forma- ern side of the basin until shortly after 14.6 Ma. mated that the total amount of Miocene and tion of uplifts or basins. The origin and history of the Independence younger extension in north-central Nevada did Valley in part mimics that of the Carlin basin, not exceed 10%, which is consistent with these Formation of the Basins and the basin eventually fi lled and connected fi eld observations. with both the Carlin and Elko basins through East of the Tuscarora Mountains, faults are All of the basins began to retain sediments low gaps in the surrounding highlands. How- abundant in the Carlin basin and adjacent high- at nearly the same time: Chimney basin, 16.3– ever, the lack of coarse epiclastic materials and lands, and some have 1 km or more of displace- 16.1 Ma; Ivanhoe basin, slightly before 16.1 Ma; apparent depositional contacts with bedrock ment. However, most of the Miocene strata dip Carlin basin, ca. 16.5 Ma; Independence Valley, along the basin margins suggest a subdued sur- less than 20°, indicating relatively little exten- shortly before 15.9 Ma; and Elko basin, slightly rounding topography at the time of sedimenta- sion. Similarly, despite post-middle Miocene before 15.4 Ma (Table 1; Figs. 2 and 12). How- tion. The Independence Mountains on the west and also Pliocene faulting along the east side of ever, the origins of the basins differ. The fl oors side of the basin now rise more than a kilometer Pine Valley, all Neogene strata in that basin dip of the Chimney and Ivanhoe basins drained to above the valley fl oor, but there is no sedimen- gently or are nearly horizontal. Just to the east, the west just prior to sedimentation, and vol- tological evidence for such a large upland in the the Elko basin is in the immediate hanging wall canic eruptions in the downstream parts of the middle Miocene. of the Ruby Mountains–East Humboldt Range basins dammed the streams to create the lakes horst, and some faults with a few hundred (Fig. 12B). Ongoing eruptions along the mar- Post-Sedimentation History meters of offset cut basin sediments along the gins of both basins continued to confi ne infl ow- southeastern Adobe Range and the Huntington ing streams and caused the lakes to grow and Both faulting and erosion played variably Creek area. Overall, the dips of exposed Mio- expand across the landscape. Synsedimentary signifi cant post-sedimentation roles in the evo- cene strata are gentle, in places primary. There- faulting was relatively minor during the short lution of the transect. Sedimentation ended in fore, the area east of the Tuscarora Mountains lifespans of these two basins. However, contin- the Chimney, Ivanhoe, and Carlin basins and is much more faulted than areas to the west, but ued faulting along the northern Nevada rift in Independence Valley between ca. 15 Ma and the amount of extension was not signifi cant. the Snowstorm Mountains produced offsets of 14.6 Ma, and the youngest sediments in the The younger faulting event appears to be several hundred meters and, along with coeval Elko basin were deposited shortly after 9.9 Ma related to greater amounts of extension west rhyolitic volcanism, ended sedimentation in that (Table 1; Fig. 2). In all four basins, almost all of of the northern Nevada rift area and much less area. Both basins began to drain externally when the youngest sediments were deposited in fl uvial to the east. Extension west of the rift created the volcanic dams were breached. or mixed fl uvial and lacustrine environments tilted horsts with broad, intervening valleys that In contrast, high-angle faulting along the (Fig. 2). Highlands partially to completely sur- formed sometime after ca. 10 Ma (Zoback and west side of the Ruby Mountains and East rounded each basin; the climate, although grad- Thompson, 1978; Wallace, 1991; Colgan et al., Humboldt Range controlled the formation of ually drying, remained somewhat temperate 2004, 2006). Along the transect, these include the middle Miocene Elko basin (Fig. 12B). The and moist; and the precipitation caused erosion the Osgood Mountains and the Hot Springs basin evolved as an east-dipping half graben, and runoff. Therefore, the absence of younger Range and intervening Eden and Kelly Creek with the sedimentation rate slightly outpacing sediments indicates that the streams were able Valleys on the southwestern margin of the the subsidence rate. As tectonism diminished, to fl ow out of the basins, carrying the newly Chimney basin (Stewart, 1998). These structural sediments continued to be shed from the horsts formed sediments with them and likely initiat- features do not extend north into the main part into the eastern part of the basin, and streams ing erosion of older sediments. of the basin, indicating a change from signifi - cut through the western margin of the basin to cant to minimal extension northward across that connect with those in the Carlin basin. Faulting area. No transform or accommodation zone is Both faulting and volcanic dams produced The amount and style of post-sedimentation apparent between the more faulted domain to the Carlin basin and perhaps part of Pine Val- faulting varied along the transect, and the age the southwest and the nearly intact domain just ley (Fig. 12B). Early faulting lowered the core in most places is unknown. Along much of the to the northeast. of the existing upland to form the south-trending transect, high-angle normal faults cut the high- As described above, faulting east of the basin. This induced early fl uvial and debris-fl ow lands and basins equally, and the faults form northern Nevada rift was locally abundant but sedimentation in the western and northern parts two general populations—an earlier, north- to did not produce much extension. A transition of the basin, and additional faulting in the east- north-northwest–striking set and a younger, zone between the more extended region to the central part of the basin directed streamfl ow to east-northeast–striking set—although small west and the minimally extended area to the east the south toward Pine Valley. Thus, although faults of all orientations are present along the occurs in the area of the rift (Wallace, 1991). the Tuscarora Mountains were a divide between transect. Overall, the early faults are much more This zone contains a series of east-northeast– the Carlin and Ivanhoe basins, faulting was common east of the Tuscarora Mountains than striking, oblique-slip faults, such as those along restricted to the east side of the highland, creat- to the west. Conversely, the younger faults are the Midas trough and the north and south sides ing very different early sedimentary processes almost exclusively present west of the Tuscarora of Boulder Valley, all of which affected the Ivan- on either side. Eruption of the Palisade Can- Mountains (Figs. 12C and 12D). hoe basin and cut the earlier north- to northwest- yon rhyolite further blocked southerly fl ow out West of the Tuscarora Mountains, the early, striking faults (Fig. 12D). These faults have up of the basin and created a deepening lake that north- to north-northwest–striking faults in the to 1 km of normal displacement and as much gradually expanded to the north and connected Chimney and Ivanhoe basins have less than as several kilometers of left-lateral movement,

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refl ecting northwest-directed extension (Zoback 400 m (Wallace, 2005). Terraces are not present As described earlier, Miocene sediments and Thompson, 1978; Wallace, 1991). They ter- along the Humboldt River downstream of about nearly buried the Adobe Range and the Peko minate abruptly to the west at the margin of the Beowawe (Fig. 1). Erosion has preferentially and Elko Hills in the Elko basin. As the ero- more extended terrain described above, and the removed the poorly consolidated Miocene and sion progressively removed the Miocene and boundary likely is structurally controlled by west- Pliocene sedimentary units relative to the harder Pliocene sediments, this previously buried base- dipping, normal to oblique-slip faults along the bedrock units. In doing so, it has focused the ment was gradually exposed (Fig. 11B). Similar west side of the Sheep Creek Range and part of more modern drainage systems (and erosion) reexposure of basement rocks is evident in all the Snowstorm Mountains (cf. Wallace, 1993). into the areas that originally were the sites of of the other basins as well, such as in the west- The east-northeast–striking faults diminish middle Miocene drainages and sedimentation. ern and southwestern parts of the Carlin basin. in magnitude to the east, splay into numerous In some places, such as along Susie Creek in This combination of the original middle Mio- smaller offset faults, and die out before reaching the Carlin basin, the distribution of modern and cene paleogeography, relatively minor, post- the Tuscarora Mountains (Peters, 2003; Wallace, Miocene stream deposits shows that the mod- sedimentation faulting, and signifi cant, later 2003a, 2003b). Given the northwest-oriented ern stream follows the same route as the middle preferential erosion of the Neogene sediments internal extension within the transition zone, Miocene stream. has produced what appear to be structural horsts late Cenozoic, north-northwest–striking, strike- Field relations and limited age control indi- and grabens but which instead are slightly modi- slip faults documented along the west side of cate that erosion began after the early fault- fi ed but largely intact middle Miocene or older the Tuscarora Mountains may record the differ- ing that tilted the Miocene sedimentary rocks ranges and basins. The major exception is the ential offset between the transition zone and the but prior to the middle Pliocene. Rock Creek, Independence Range, west of Independence less extended terrain to the east (Wallace, 1991; which fl ows west and then cuts south through Valley, which appears to have formed after mid- Leonardson and Rahn, 1996). All of these faults the western Ivanhoe basin (Figs. 5 and 12), is an dle Miocene sedimentation. may have begun to form at ca. 10 Ma (Zoback antecedent stream that incised canyons during and Thompson, 1978), similar to the uplift age of the formation of the Midas trough and Boulder Drainage Evolution and Paleoelevations the Santa Rosa Range to the west (Fig. 1; Colgan Valley fault escarpments and thus is older than Previous studies have provided data regarding et al., 2004, 2006). Movement continued into the the younger faulting event. Similarly, 12.6 to Cenozoic paleoelevations in the Great Basin, Pliocene and Quaternary. Along the Midas trough 6.7 Ma supergene alunite dates from the Car- including parts of this study area (Sonder and and in Paradise Valley, faulting modestly tilted 4 lin trend (Table 2; Fig. 13) indicate that super- Jones, 1999; Wolfe et al., 1997; Horton et al., to 5 Ma mafi c fl ow units, and Quaternary fault gene processes had begun by the middle to late 2004). Wolfe et al. (1997) suggested that cen- scarps are present along the Midas trough (Wal- Miocene. In the Elko basin, as much as 150 m tral Nevada was 1–1.5 km higher in the middle lace, 1993) and the eastern end of the Boulder of downcutting took place before the middle Miocene than it is present day, and Pierce et Valley (Wesnousky et al., 2005). Pliocene. At that time, the bedrock sill at Carlin al. (2002) proposed a regional, kilometer-high Except for the region west of the northern Canyon (Fig. 7) slowed incision of the Hum- bulge related to the Yellowstone mantle plume. Nevada rift, extension in the transect area, both boldt River, and lacustrine and fl uvial sediments While nothing in the present study can quanti- during and after sedimentation, was not sig- were deposited on top of incised topography in tatively determine paleoelevations, water fl ows nifi cant. This supports Muntean et al.’s (2001) upstream areas (Figs. 11F and 12D). Strath ter- downhill, and the evolution of the drainage sys- conclusion that Miocene and younger extension race formation continued when the Pliocene lake tems provides evidence of what areas were rela- was ~10%. Dips on Miocene sedimentary units drained at ca. 2 Ma, dissecting both the Pliocene tively higher or lower and when. typically are 20° or less, and the faults likely and Miocene strata. Along the western base of In the middle Tertiary, the study area was were more vertical than listric. The area west of the East Humboldt Range and Ruby Mountains, topographically higher than surrounding areas. the rift is more extended, although by how much Quaternary faulting has offset some of the older Based on evidence presented in the basin is unknown. This area is due north of areas strath terraces (Sharp, 1939; Wesnousky and descriptions, streams in the west fl owed west; south of Battle Mountain that were extended Willoughby, 2003). streams in the central area fl owed to the west, by more than 100% after the middle Miocene The strath terraces in the Elko and Carlin basins south, and east; and streams in the area of the (Colgan et al., 2008). As such, and as pointed have a regionally consistent pattern centered ulti- Elko basin likely fl owed to the south and, possi- out by those authors, the rift zone (be it the pre- mately along the Humboldt River, and they thus bly, east and north (Figs. 12A and 14). As such, Miocene crustal structure or the deep mafi c dike formed in response to a drop in the base level of the greater Carlin-Tuscarora area was a paleo- complex) appears to have exerted a signifi cant the river downstream of Beowawe (Fig. 1; Wal- topographic upland, and other areas were rela- control on the amount and location of extension lace, 2005). Because the Chimney and Ivanhoe tively lower. In addition, a drainage divide was in north-central Nevada. basins also drain into the Humboldt River, the present along the Nevada-Idaho border (Beranek far-fi eld, base-level drop likely produced the ero- et al., 2006). These patterns generally contin- Erosion sion and strath terraces in those areas as well. ued into the middle Miocene (Figs. 12B and Late Cenozoic erosion affected much of the Therefore, material eroded from the upstream 14), although the structural development of the transect, and it was most pronounced in the parts of the Humboldt River drainage system has Carlin basin and Pine Valley induced southerly areas of the Elko and Carlin basins. Down- been transported to and deposited in intermontane streamfl ow in those areas. Drainage in the three stepping, erosional strath terraces are abundant valleys farther downstream (Wallace, 2005). As western basins ultimately merged and produced in those basins and, to a lesser degree, in the a result, Quaternary sediments, which are ubiq- a westerly fl ow system. The Elko basin began to Ivanhoe and Chimney basins. In the Carlin and uitous in most intermontane basins in the region drain to the west after ca. 9 Ma and merged with Elko basins, the progressive formation of as (Stewart and Carlson, 1978), are largely absent these drainage systems (Fig. 12D). many as a dozen strath terraces paired across in the upper half of the Humboldt River drain- The strath terrace patterns indicate that all both major and minor drainages has lowered the age system, where Miocene sedimentary units are streams in the region had integrated into the Humboldt River drainage system by as much as exposed over broad areas. west-fl owing Humboldt River system by the

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late Miocene. Pine Valley, which drained to the what was a series of 16–12 Ma lacustrine and the latitude of Austin and Eureka (Fig. 14), was south in the middle Miocene and drained inter- fl uvial sedimentary basins (Fig. 14; Colgan et higher than the Humboldt River by at least the nally in the Pliocene and Pleistocene, eventu- al., 2008). Similarly, Pliocene and Pleistocene Pliocene. Fault-controlled grabens in the lower ally was captured by the Humboldt River dur- lakes and streams in the region between cen- reaches of the Humboldt River, roughly down- ing downcutting and began to drain to the north tral Nevada and the Humboldt River became stream from Beowawe (Fig. 1), received the (Fig. 14; Gordon and Heller, 1993; Reheis, integrated into an overall drainage system that sediments eroded from upstream areas. 1999a, 1999b; Wallace, 2005). In the Pliocene, fl owed north to the river and then to the west The Yellowstone mantle plume migrated the Reese River began to fl ow north to the Hum- (Fig. 13; Reheis et al., 2002). All of these northeast-northeastward from the area of the McDer- boldt River from south of Austin, traversing draining streams indicate that central Nevada, at mitt eruptive center west of the Oregon-Idaho border at ca. 16 Ma to the Twin Falls eruptive center north of the Utah-Nevada border by ca. 10.5 Ma (Fig. 14; Perkins and Nash, 2002; Pierce et al., 2002). This migration path was due north of the four basins described in this paper, and it progressed eastward at the same time that sedimentation was taking place in the basins. As evidenced by the bimodal volcanism along the northern Nevada rift, middle Miocene plume- related crustal heating extended south into central Nevada. Pierce et al. (2002) proposed a broad thermal bulge related to the plume head, with a rain shadow on its east (leeward) side and an elevational decline in its wake (Fig. 14). Within the present study area, the entire western half of the transect has drained to the west since at least the middle Miocene (Fig. 14), and the Ivanhoe and Chimney basins connected across the northern Nevada rift, which was the proposed early axis of the bulge (Pierce et al., 2002). These relations do not support an early thermal bulge in that area, and the widespread streams and lakes in the basins to the east similarly do not support a signifi cant rain shadow (Pierce et al., 2002). However, even without a bulge, the progressive change to an entirely westerly fl ow direction for all four of the basins by the time the plume was east of the study area (Fig. 14) may in part refl ect a progressive decrease in elevation as the plume-related halo of crustal heating migrated to the east. At the same time, normal faulting and horst-basin development in northwestern Nevada after ca. 10 Ma (Colgan et al., 2004, 2006) also contributed to the observed decrease in the base level of the Humboldt River drainage system. Collapse of central Nevada after the middle Miocene is not supported by: (1) the progression from middle Miocene, southward-and-westward fl ow to late Miocene and younger, westward fl ow along the entire Humboldt River; (2) Figure 14. Map showing middle Miocene (blue arrows) and late Miocene and younger (red the reversal of fl ow in Pine Valley from south to arrows) streamfl ow directions in northeastern Nevada. Miocene streamfl ow directions are north; and (3) the integration of middle Miocene based on data presented in this paper. Pliocene and younger directions are based on data in basins into the north-fl owing Reese River in the this paper and in Reheis et al. (2002). Note the streamfl ow reversals in Pine Valley (PV) and Pliocene. Instead, and perhaps overly simplisti- the southern Elko basin (EB). Also shown are the general locations of the Chimney (ChB), cally, the pattern suggests that northern Nevada Ivanhoe (IB), and Carlin (CB) basins. The semicircular tan areas are the major eruptive has decreased in elevation relative to central centers that formed along the east-northeast–migrating track of the Yellowstone mantle Nevada, and that the westward fl ow of the Hum- plume (from Perkins and Nash, 2002). The thick dashed green line is the middle Miocene boldt River between Wells and Winnemucca axis of a proposed thermal bulge related to the mantle plume, with wetter conditions to the since the late Miocene refl ects both that relative west and drier conditions to the east (from Pierce et al., 2002). topographic change and a base-level decrease in

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northwestern Nevada (Wallace, 2005). If cen- et al., 2003). Miocene sediments likely did not deposits, must consider both the environment dur- tral Nevada did decrease in elevation, and if the cover the Rain deposit in the northern Piñon ing mineralization and the events that followed downstream tectonic activity and possible man- Range, but various stages of faulting, weather- mineralization. tle plume wake did produce the drainage-pattern ing, and erosion affected the deposit as it sat in a The late Miocene and younger transport of changes in northern Nevada, then the effects of topographically high position for several tens of sediments eroded from northeastern Nevada the latter two exceeded the rate of decline of millions of years. to downstream basins in northwestern Nevada central Nevada. The middle Miocene epithermal depos- has covered areas that previously were exposed, Horton et al. (2004) used isotopic data from its formed at or near the paleosurface during including old pediments along range fronts and Miocene sediments in northern Pine Valley to the formation of the sedimentary basins. The any Miocene sedimentary units. Some of these suggest an elevation decrease of 1–1.5 km to resulting deposits, such as those in the Ivan- buried bedrock areas contain economic mineral the west or southwest of the area, and thus a hoe district, included sinter deposits at the deposits, including the Twin Creeks gold deposit reduced orographic effect on rainfall in the Pine paleosurface, stratiform replacement bodies in and drilled resources northwest of the Lone Tree Valley area, between ca. 15 and 1 Ma. If central underlying sedimentary units, and veins along mine west of Battle Mountain (Fig. 1). Similar to Nevada remained high during this period, then faults in deeper volcanic and pre-Tertiary units the mineral deposits much farther upstream, the the elevation decrease must have been to the (Bartlett et al., 1991; Wallace, 2003c). The epi- geologic history at any one location affected the west. Indeed, the progressive decrease in the thermal deposits owe their origins to a combi- amount of pre-sedimentation erosion, weather- downstream base level of the Humboldt River, nation of volcanism, which provided the heat; ing, enrichment, and faulting, not to mention the as shown by the strath terraces and downstream faulting, which created deep conduits for fl uid paleotopography and thus the amount of later basin fi lling, indicates that the proposed eleva- circulation and served as important ore hosts; burial in the downstream areas. tion decrease may have been in northwestern and the lakes, which contributed abundant water Exploration for mineral deposits of all types Nevada. and also produced sedimentary aquifers that, in in the region, as well as the use and economics places, were silicifi ed and mineralized (John of various mineral processing methods needed IMPLICATIONS FOR MINERAL et al., 2003; Wallace, 2003c; Vikre, 2007). As to extract the metals, must consider the post- DEPOSITS shown on Figure 1, many epithermal systems mineralization events that affected the depos- formed where these three components over- its. This pertains to the Eocene Carlin-type North-central Nevada contains numerous lapped. Depending on the combination of events and Miocene epithermal deposits described in large, late Eocene, Carlin-type and middle Mio- at any place and time, the deposits formed both this paper, as well as the many other types of cene, epithermal mineral deposits (John, 2001; within and along the margins of the presently mineral deposits of various ages that formed in Cline et al., 2005), and the geologic history of exposed basins. Thus, exploration for epither- the region (Wallace et al., 2004b). As shown in the area described in this paper has had a major mal deposits in this region requires a knowledge this paper, no one landscape evolution model impact on the formation, modifi cation, and con- of the middle Miocene paleogeography, fault- applies to the region as a whole, and regional cealment of those deposits (Cline et al., 2005; ing, volcanism, and hydrothermal processes to subdomains that combine time, faulting, ero- Wallace, 2005). The Eocene deposits were target areas where optimal conditions existed at sion, and sedimentation must be developed to faulted, exposed, and weathered in the middle that time. The absence of even one component evaluate specifi c areas. For example, the major Tertiary. Some of the deposits were buried could have reduced or eliminated the chance of porphyry copper deposits at Yerington in west- beneath sediments in the middle Miocene, and forming an epithermal mineral deposit. ern Nevada, Ely in eastern Nevada, and Cop- many of the deposits or mineral districts were Because the epithermal deposits formed near per Canyon near Battle Mountain formed in the faulted, partially exhumed, and weathered for a the paleosurface, destruction of that surface could Jurassic, Cretaceous, and Eocene, respectively. second time after the middle Miocene. have eliminated parts or all of a mineral deposit Each has been subjected to different post- The weathering, erosion, and related super- (Wallace et al., 2004b). As shown in this study, mineralization histories that have affected their gene processes had an important infl uence on post-middle Miocene erosion and faulting along present morphologies and degrees of oxidation the degree of oxidation of sulfi des and carbon the eastern half of the transect has removed or and preservation (Proffett, 1977; Dilles and (thus infl uencing the mineral processing meth- modifi ed many parts of the middle Miocene Gans, 1995; Seedorff et al., 1996; Theodore, ods needed) and the amount of the original paleosurface. In contrast, much of the paleosur- 2000). The post-mineralization histories of even deposit that was preserved. The Gold Quarry face and many of the related epithermal systems small areas, such as the one that includes the deposit was subjected to all three stages of the have been preserved in the western half of the Gold Quarry and Mike gold deposits described landscape evolution described above, leading to transect, such as in the Ivanhoe district and the above, can be suffi ciently varied to produce dif- a complex geologic setting and degree of oxi- Chimney basin. At the Hollister gold deposit in the ferent degrees of weathering, oxidation, and dation. Some deposits, such as the combined Ivanhoe district, the water table has remained high amount of preservation. Paleozoic, Mesozoic, and Eocene gold- and base and supergene oxidation extends only 30–150 m metal-rich deposits at the Mike property west beneath the present surface (Bartlett et al., 1991). ACKNOWLEDGMENTS of Gold Quarry, were subjected to the middle The tops of some epithermal systems, such as at Chris Henry (Nevada Bureau of Mines and Geol- Tertiary and middle Miocene processes but less Midas, were domed and eroded after mineraliza- ogy) provided a 40Ar/39Ar date on a Chimney basin so the later events, leaving a partially modifi ed tion, leaving only the underlying fault-controlled rhyolite (sample ChB-5) and, along with Joe Colgan, but entirely concealed orebody (Norby and Oro- veins (although it might be argued that those veins Keith Howard, and David John, stimulating discus- bona, 2002). There, pre-middle Miocene super- are extremely high grade and the loss of the top sions on regional geochronology and paleogeography. gene weathering redistributed copper and zinc, of the system may not have been economically Steve Williams obtained tephra correlation ages from three samples in the Carlin basin and southern Indepen- and the zinc was concentrated at the boundary signifi cant). Therefore, exploration for epither- dence Valley, as cited in Table 1. Unpublished super- between oxidized and primary reduced ores; the mal deposits along the transect, as well as in other gene alunite dates in Table 2 from the Gold Quarry Miocene cover preserved that interface (Bawden areas in the region that may contain epithermal and Lone Tree mine areas came courtesy of Peter

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Vikre and of Al Hofstra and Larry Snee, respectively. CB-2: Upper part of 4-m–thick, fi ne-grained, lami- Elko Basin Bill Walker shared unpublished mapping data from nated, silver-gray, ash-rich bed in old road cut west the northern Chimney basin. Don Shamp and Chuck of the Gold Quarry mine, Eureka County. Tephra cor- EB-1: Andesite fl ow unit within epiclastic fl uvial Holdsworth provided mineral separates and analyses relation age only. sediments of the Humboldt Formation, southwest for new 40Ar/39Ar dates in the U.S. Geological Survey CB-3: Lower part of 4-m–thick, fi ne-grained, lami- fl ank of the Peko Hills, Elko County. Flow contains geochronology laboratory in Menlo Park, California. nated, silver-gray, ash-rich bed in old road cut west of 1-cm plagioclase phenocrysts and is very similar to Reviews and comments by Jack Stewart, Paul Link, the Gold Quarry mine, Eureka County. Tephra cor- undated fl ow units exposed in the Carlin basin and and Al Hofstra signifi cantly improved the presenta- relation age only. west of Twin Buttes between the North Fork Hum- tion of the data and interpretations in this paper. CB-4: Several-meter–thick, fi ne-grained, lami- boldt River and Marys River (Fig. 10). 40Ar/39Ar date nated, silver-gray, ash-rich bed in roadcut along east- on plagioclase. APPENDIX 1. SAMPLE DESCRIPTIONS bound I-80 east of Emigrant Pass, Eureka County. EB-2: Silver-gray, laminated, fi ne-grained ash Tephra correlates to the Obliterator Tuff (see Perkins bed. Unit is the upper member of the Sedimentary Table 1 includes numerous samples collected and and Nash, 2002). and volcanic rocks of Threemile Spring unit of Thor- dated for the present study. These are listed as “This CB-5: Several-meter–thick, fi ne-grained, lami- man et al. (2003), which is part of the Humboldt study” in the Reference column of that table, and the nated, silver-gray, ash-rich bed in direct depositional Formation (Sharp, 1939; Mueller and Snoke, 1993). sample number in parentheses refers to brief loca- contact with the Palisade Canyon rhyolite south of I- Sample collected from cuts along railroad just west 80 between Emigrant Pass and Carlin, Eureka County. of Wells, Elko County. 40Ar/39Ar date on plagioclase. tion and sample descriptions of those samples in this 40 39 Appendix. Not included in this Appendix are descrip- Ar/ Ar date on sanidine; tephra is the same unit as tions of dated samples that have been published else- CB-4 and correlates to the Obliterator Tuff (see Per- Independence Valley where or were collected and dated by other workers kins and Nash, 2002). CB-6: Silver-gray tephra bed, less than 1 m thick, and used by permission. The latitude and longitude IV-1: 627-6 Silver-gray to off-white, fi ne-grained, for each sample are in Table 1. All samples are of within planar-bedded, epiclastic siltstone strata. Units are near the contact of the Humboldt Formation poorly lithifi ed ash bed, 50–70 cm thick, enclosed fresh rocks that are free of diagenetic or hydrothermal within thick- to thinly bedded, ash and epiclastic alteration minerals. with underlying Paleozoic rocks. Excavated subcrop sample collected west-northwest of the Hunter exit siltstone units. Excavated from roadcut along NV along I-80 on the south side of the Adobe Range, Elko 226. Bedding nearly horizontal. About 3 m strati- Chimney Basin County. Tephra correlation age only. graphically above sample IV-2. Tephra correlation CB-7: Silver-gray, structureless to weakly lami- age only. ChB-1: One-meter–thick, poorly consolidated, sil- nated tephra bed, 2 m thick, within off-white ash-rich IV-2: 627-5 Silver-gray, fi ne-grained, moderately ver-gray, fi ne-grained, unreworked tephra bed (unit lacustrine and mixed lacustrine and epiclastic fl uvial lithifi ed ash bed, ~1.5 m thick, enclosed within thick- Tsl of Wallace, 1993). Variably reworked to unre- units. Units are near the contact of the Humboldt to thinly bedded, ash and epiclastic siltstone units. worked, leucocratic ash-rich lacustrine strata enclose Formation with underlying Paleozoic rocks. Outcrop Bedding nearly horizontal. Collected from borrow pit the tephra bed. Located ~2 m below the contact sample collected just north of the Hunter exit along just south of NV 226. Tephra correlation age only. between these lacustrine units and overlying fl uvial I-80 on the south side of the Adobe Range, Elko IV-3: 627-4 Thin (20 cm), silver-gray, fi ne- units. Exposure is on the hillslope on the north side County. Tephra correlation age only. grained, poorly lithifi ed ash bed enclosed within of Twentyone Creek, Humboldt County. Tephra cor- CB-8: Silver-gray, laminated, moderately well thick- to thinly bedded, ash and epiclastic siltstone relation age only. indurated, 20-cm–thick tephra bed within other ash- units. Bed dips gently to east. Excavated from road- ChB-2: Flow unit of the Little Humboldt rhyolite rich lacustrine strata. About 1 m above sample CB- cut along NV 226. Stratigraphically the lowest of IV- (Wallace, 1993). Sample also listed in the Snowstorm 9. Collected from outcrop in the uppermost Welches 1, 2, and 3, and upsection from Eocene volcanic tuff Mountains section of Table 1. Flow-folded, devitri- Canyon drainage basin, Eureka County. Tephra cor- units. Tephra correlation age only. fi ed, reddish-brown rhyolite with sanidine, plagio- relation age only. IV-4: 627-7 Coarse, silver-gray ash bed; thin, clase, and lesser quartz and iron oxide phenocrysts. CB-9: Silver-gray, laminated, moderately well planar bedding. Enclosed within structureless, tan Sample collected from outcrop southeast of the Little indurated, 20-cm–thick tephra bed within other ash- epiclastic to pumiceous sandstone beds, with bur- Humboldt Ranch (Fig. 3), Elko County. 40Ar/39Ar date rich lacustrine strata. About 1 m below sample CB- rows extending from overlying sandstone into ash on sanidine. 9. Collected from outcrop in the uppermost Welches bed, and from ash bed down into sandstone. Sample ChB-3: Massive, nearly aphyric andesite fl ow unit Canyon drainage basin, Eureka County. Tephra cor- selected was evenly bedded and not near burrows. enclosed in tuffaceous sedimentary units along Spring relation age only. Collected from roadcut along NV 226 ~1.5 km west Creek north of the Little Humboldt River, Humboldt CB-10: Silver-gray, laminated 1-m–thick tephra of intersection with NV 225. Ash bed is stratigraphi- County. 40Ar/39Ar whole-rock date. bed within tan, fi ne-grained epiclastic siltstone beds. cally above Eocene volcanic bedrock, and Humboldt ChB-4: Unwelded, phenocryst-rich, rhyolitic Bed is exposed in the overfl ow excavation at a reser- Formation dips more steeply here than in western ash-fl ow tuff with abundant quartz, sanidine, and voir northeast of the Gold Quarry mine, Elko County. part of basin. Tephra correlation age only. plagioclase phenocrysts, with lesser hornblende and Bed is ~1.5 m stratigraphically above sample CB-11. IV-5: Unbedded to bedded, moderately consoli- biotite. Exposed along Spring Creek north of sample Tephra correlation age only. dated, silver-gray ash bed, ~1.5 m thick, within ash- CB-3 and directly underlies that unit. 40Ar/39Ar date CB-11: Silver-gray, laminated 1-m–thick tephra rich epiclastic siltstone and sandstone beds. Sample on sanidine. bed within tan, fi ne-grained epiclastic siltstone beds. collected from outcrop on hillside south of Camp ChB-5: Flow-folded and -banded, moderately Bed is exposed in the overfl ow excavation at a reser- Creek, Elko County. Tephra correlation age only. phenocrystic rhyolite fl ow unit exposed at Chimney voir northeast of the Gold Quarry mine, Elko County. IV-6: Very thinly bedded and laminated, silver- Dam along the Little Humboldt River, Humboldt Tephra correlation age only. gray ash bed, ~4 m thick, within epiclastic sand- County. Underlies tuffaceous sedimentary units of CB-12: Silver-gray, laminated tephra bed in road- stone beds on hillside south of Camp Creek. Base Chimney basin; base not exposed but likely overlies cut south of Emigrant Pass and southwest of Carlin. of bed fi lls paleodepression or channel; upper part 22 Ma andesite fl ow units to south. 40Ar/39Ar date on Intermixed with tan siltstone and white tuffaceous of bed partially incised prior to deposition of overly- sanidine. beds. Sedimentary units overlie east-dipping, late ing sandstone bed. Some evidence of reworking and Eocene volcanic units (Henry and Faulds, 1999). Cor- sorting in middle third of bed; sample collected from Carlin Basin relative with Virgin Valley 12 tephra (Perkins et al., thinly laminated lower part of bed. Stratigraphically 1998). Tephra correlation only. ~50 m above sample IV-5. Tephra correlation age only. These two tephra beds (IV-5, 6; 15.5–15.3 Ma) CB-1: Three-meter–thick, silver-gray, planar-bed- CB-13: Palisade Canyon rhyolite. Massive fl ow unit directly above basal fl ow unit, which was deu- are in the same sequence from which Van Houton ded, fi ne-grained ash bed. Outcrop exposure a few (1956) collected vertebrate fossil remains. Identifi ca- hundred meters east of NV 278 between Cole and terically altered and not suitable for dating. Rhyolite package overlies late Eocene Indian Wells Forma- tion of those fossils led to an age assignment of late Ferdelford Creeks, Eureka County. Stratigraphically Miocene to early Pliocene; at the time, the Miocene- above distal breccia related to the Palisade Canyon tion. Collected from base of cliff along railroad and Humboldt River north of Palisade. 40Ar/39Ar date on Pliocene boundary was 10 Ma. Thus, the vertebrate rhyolite (unit B-3, Section 3 of Regnier, 1960). Tephra fossils are middle Miocene in age. correlation age only. sanidine.

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