Unit is present in all counties. Some counties divided the alluvium into younger and older units, and some did not. For those that did not, or used other generalized terms for Quaternary rocks, the unit Qal has been used for the general undivided alluvium. Additionally, when polygons have been edited and changed to alluvium, Qal was used as the general value; hence it now is present in all counties. Qya-Younger alluvium: Map unit is used in Churchill, Elko, Esmeralda, Eureka, Humboldt, Lander, and Lincoln Counties where geologic information suggests better-defined younger versus older alluvium. It is mostly interchangeable with Qal, except that it implies some specifically younger Quaternary deposits.
Map unit used in all counties for recent lake beds, playas, and flood plains. Polygons from the 1978 State map unit Qp were added where no playa was shown on the county maps.
The Jungo terrane, also called the Lovelock assemblage or Fencemaker allochthon (Oldow, Satterfield, and Silberling, 1993), consists of complexly deformed, thick basinal, turbiditic, fine-grained, terrigenous clastic rocks, mainly Norian, but also as young as Pliensbachian (Late Triassic and Early Jurassic) age. It crops out in southern Washoe, Churchill, Humboldt, and Pershing Counties. These rocks represent the basinal facies component of the Auld Lang Syne Group (Burke and Silberling, 1973; Lupe and Silberling, 1985). The Jungo terrane has no known basement and is structurally detached from coeval shelf facies (Silberling, Jones, and others, 1992). It is locally overlain unconformably by Middle or Upper Jurassic peritidal sedimentary rocks (Jcg) intruded by a gabbroic igneous assemblage (Silberling, 1991). Rocks included with the Jungo terrane were originally mapped as the Grass Valley Formation of the Auld Lang Syne Group in Humboldt and Pershing Counties; some rocks were mapped as the Happy Creek Volcanic “series” (now the Happy Creek Volcanic Complex) in Humboldt County, the Nightingale sequence in southern Washoe County, the Osobb Formation of the Auld Lang Syne Group in Churchill County, and the Winnemucca and Raspberry Formations of the Auld Lang Syne Group (Compton, 1960) in the Santa Rosa Range in Humboldt County.
Tuffaceous and other young Tertiary sedimentary rocks. Most of these rocks are sedimentary with a strong volcanic component - a few are tuffaceous with a strong sedimentary component. This unit includes rocks originally mapped as the High Rock sequence in Washoe County; the Horse Camp Formation in northern Nye County; the Esmeralda Formation in Mineral and Esmeralda Counties; older lake beds in Lincoln County; the Belted Range Tuff; the Indian Trail Formation (now abandoned); Timber Mountain, Paintbrush, and Crater Flat Tuffs; Wahmonie and Salyer Formations in southern Nye County; the Siebert Tuff in Esmeralda County; the Muddy Creek Formation in Clark County; and the Thousand Creek and Virgin Valley “beds” in Humboldt County; and other unnamed units. It corresponds to units Ts3 and Tts from the 1978 State map. It is present in all counties.
Unit consists mostly of older alluvium and alluvial fans. It also includes various stream deposits, gravel, fanglomerates, and older gravels. It is not very consistent in description from county to county. This is used in all counties except Clark.
Granodiorite, granite, and related rocks make up the largest group of granitic intrusions exposed in Nevada. They are present in every county, and are especially abundant in west-central Nevada in an arcuate belt along the border with California extending north and eastward towards Idaho.
The Golconda terrane is composed of deformed and imbricated thrust slices of upper Paleozoic rocks including deep-marine, pelagic and turbiditic, carbonate, terrigenous clastic and volcaniclastic rocks, radiolarian chert and argillite, and pillow basalt (Silberling, Jones, and others, 1992). While the terrane is characterized by a great diversity of rock types, all rocks are strongly deformed with an east-vergent fabric, a distinguishing characteristic of this terrane (Brueckner and Snyder, 1985; Jones, 1991a; Miller, Kanter, and others, 1982; Murchey, 1990; Stewart, Murchey, and others, 1986). It crops out in a long sinuous belt, up to 100 mi wide in places. Southwest of Mina, the belt trends east from the California border to just north of Tonopah, and then bends north-south to the west of Longitude 117° to about 50 mi north of Winnemucca, where it bends again, sharply to the east-north of Tuscarora with significant exposures eastward and to the northern border of the State. Outcrops of the Golconda terrane are present in Mineral, Esmeralda, northern Nye, Churchill, Elko, Humboldt, Lander, and Pershing Counties. It includes some rocks originally mapped as Banner and Nelson Formations in Elko County; rocks originally mapped as the Excelsior Formation in Mineral and Esmeralda Counties, later assigned to the Black Dyke and Mina Formations by Speed (1977b); the original Havallah and Pumpernickel Formations (Muller, Ferguson, and Roberts, 1951; Roberts, 1964; Silberling and Roberts, 1962), later revised to structural sequences (Murchey, 1990; Stewart, MacMillan, and others, 1977; Stewart, Murchey, and others, 1986; Theodore, 1991; 1994) in Elko, Humboldt, Lander, and Pershing Counties; the Inskip Formation in Pershing County; the Mitchell Creek Formation in Elko County; the Pablo Formation in northern Nye County; and the Schoonover Formation (see unit GChr) in Elko County. In all of the places where rocks of the Golconda terrane were originally believed to form a stratigraphic sequence, detailed mapping and biostratigraphic analysis with radiolarians and conodonts has demonstrated that it is characterized by complex imbrications of rocks ranging from mid-Permian through latest Devonian age (Holdsworth, 1986; Jones, 1991b; Miller, Holdsworth, and others, 1984; Murchey, 1990; Stewart, MacMillan, and others, 1977). In Pershing County, the Golconda terrane is unconformably overlain by Triassic volcanic rocks of the Koipato Group (TRkv) which form the stratigraphic base to the Humboldt assemblage (TRc, JTRs). In Mineral and Esmeralda Counties, it is unconformably overlain by the Gold Range assemblage (JTRgor) of mainly nonmarine, terrigenous clastic, and volcanogenic Upper Triassic and younger rocks. Elsewhere in northern and southwestern Nevada, it is structurally overlain by Mesozoic accreted terranes. Across the length of its exposure from the Independence Mountains north of Elko to the Candelaria region south of Mina, the base of the Golconda terrane has a remarkably consistent structural emplacement relationship with adjacent rocks. It commonly lies on a low-angle structure above Permian and Pennsylvanian rocks of the Siliciclastic overlap assemblage. In places where these rocks are missing, it is faulted directly onto either the nearby lower Paleozoic Basin assemblage, the Nolan belt rocks, or the Harmony Formation of the Dutch Flat terrane. The type locality of this regional feature, the Golconda thrust is well exposed along Interstate Highway 80 at Edna Mountain near the town of Golconda (Ferguson, Roberts, and Muller, 1952), and in the open pits of mines near Battle Mountain (Theodore, T., oral commun., 2006). In southwestern Nevada, the lower Lower Triassic rocks of the Candelaria Formation overlie Permian and Pennsylvanian Siliciclastic overlap assemblage rocks, and the Golconda terrane is exposed nearby, but not observable directly on top of the Candelaria because of younger cover rocks. Elsewhere, there is no youngest age constraint for the age of emplacement. In several places, notably in the Osgood Mountains and the Toiyabe Range, it is also bounded by large, steeply dipping, mélange-like shear zones against older rocks of the Nolan belt. Stratigraphic and structural studies within the terrane have locally identified lithostratigraphic groupings (Erickson and Marsh, 1974a, b; Jones, 1991a; Murchey, 1990), but only the Home Ranch subterrane can presently be distinguished on a regional scale (GChr). Interpretations of the size and character of the late Paleozoic basin where these rocks formed and the nature of its Late Permian or Early Triassic accretion are as varied as the lithologic and structural characteristics of the terrane itself (see references above).
Rhyolitic flows, domes, plugs, breccias, quartz latite, rhyodacite, quartz porphyry dikes, and other shallow intrusive rocks. This unit includes rocks mapped as the Cañon Rhyolite on the Washoe North map, the Jarbidge Rhyolite and phenorhyolitic and phenodacitic flows and domes on the Elko County map, and other unnamed units. It has a distribution similar to Tt3, with exposures in the northern and southern parts of the State, but only crops out in a few places in the central region. It corresponds to unit Tr3 on the 1978 State map, and also includes a few rocks mapped as Trt on the 1978 State map. This unit is exposed in every county except White Pine.
Unit is present in Clark, Humboldt, Lincoln, Churchill, Washoe, and Pershing Counties. There may be sand dunes in other counties that are not distinguished.
Generally poorly age constrained. This unit includes rocks originally mapped as the Pyramid sequence in Washoe County, the Mizpah Trachyte in Nye County, the Malpais Basalt, Rabbit Spring Formation, and Mira Basalt in Esmeralda County, and many other poorly dated unnamed basaltic and andesitic rocks around the State. It corresponds to unit Tba on the 1978 State map.
Unit consists of the Star Peak Group which lies depositionally on the volcanic and volcaniclastic rocks of the Koipato Group (TRkv). Map unit includes rocks mapped as Cane Spring, Natchez Pass, Prida, Augusta Mountain, Congress Canyon, Fossil Hill, Favret, Dixie Valley, and Tobin Formations, including Smelser Pass, Panther Canyon, and Home Station Members of the Augusta Mountain Formation. Basaltic flows and volcanic breccias (TRvm) are present in the Humboldt and northern Stillwater Ranges within the Smelser Pass Member of the Augusta Mountain Formation. The Star Peak Group includes carbonate platform deposits and grades westward into slope and basin paleogeographic environments. Complex stratigraphic patterns of carbonate and terrigenous rocks in the lower part of the group result from localized relative uplift. Widespread diagenetic secondary dolomitization of calcareous rocks complicates the stratigraphic patterns (Nichols and Silberling, 1977b). There is a major unconformity within the Star Peak Group underneath the Panther Canyon Member, which is late Ladinian (late Middle Triassic) in age. The Panther Canyon Member rests in places directly on the noncarbonate rocks of either the Koipato Group (TRkv) or the Golconda terrane (GC), and elsewhere on varying thicknesses of secondary dolomite that replaces Star Peak Group carbonate rocks. The Star Peak Group crops out in Churchill, Humboldt, Lander, and mostly Pershing Counties. Abundant fossil data from the Star Peak Group indicates this unit is latest Early (Spathian) to middle Late (Carnian) Triassic in age (Nichols and Silberling, 1977b).
Andesite, rhyolite, tuff, and generally siliceous volcaniclastic rocks make up the Koipato Group, which lies unconformably below the Humboldt assemblage. The Koipato Group consists of altered porphyritic andesite flows and flow breccia of the Limerick Greenstone, altered felsite and coarse-grained tuffaceous sedimentary rocks of the Rochester Rhyolite, and quartz-rich ash-flow tuff and tuffaceous sedimentary rocks of the Weaver Rhyolite. It is present in Churchill, Humboldt, Lander, and mostly Pershing Counties where it unconformably overlies deformed rocks of the Golconda terrane (GC). The upper part of the Koipato contains late Early Triassic (Spathian) fossils (Silberling, 1973; Wallace, Tatlock, and others, 1969). It is depositionally overlain by the Star Peak Group (TRc), a sequence of carbonate platform deposits at the base of the Humboldt assemblage. Radiometric dates from the 1970s (McKee and Burke, 1972) suggest a Middle to Early Triassic age.
Poorly dated felsic intrusions described as granitic rocks, granite porphyry, granodiorite, quartz monzonite, and many undivided plutonic rocks are included here. They crop out in every county except Elko and northern Washoe.
Unit is used for pre-Lake Lahontan deposits, weakly consolidated gravel and sand, older gravels, pediment gravels, and gravel deposits. It includes all units designated as QToa on the 1978 State map. This unit is used in all counties.
Includes the Valmy Formation in Eureka, Humboldt, Lander, and Pershing Counties; Devonian to Upper Cambrian mudstone, shale, chert, siltstone, and gray quartzite in Elko County (Leslie, Isaacson, and others, 1991); Devonian to Ordovician slate, chert, limestone, and sandstone in Mineral County; Devonian to Upper Cambrian rocks in Eureka County (Finney, Perry, and others, 1993); some rocks originally mapped as the Palmetto Formation in Esmeralda County (Albers and Stewart, 1972; Ferguson and Cathcart, 1954); and the Sonoma Range Formation (Ferguson, Muller, and Roberts, 1951) in the Sonoma Range in Humboldt County (later included with the Valmy Formation). The distinctions between these rocks and rocks of the Slope assemblage (DOts) are (1) a more complex and varied history of deformation; (2) less well-defined internal stratigraphic characteristics, which may be a function of structural complexity; (3) fewer shale, siltstone, and sandstone interbeds; (4) less carbonate; and (5) in the Roberts Mountains at least, the Ordovician rocks of this unit are older than the Slope assemblage Ordovician rocks. Like unit DOts, no basement is preserved with these rocks, making it difficult to determine where they were originally laid down, and how far they have been transported. This unit includes Devonian, Silurian, Ordovician, and uppermost Cambrian rocks imbricately faulted and folded together. In a few places, Silurian rocks are defined regionally and broken out separately (Ss), but for the most part they are included in this unit. Likewise, significant exposures of Devonian rocks have been included in unit MDst, but many more are not differentiated from this unit. A great variety of depositional settings are present in ocean basins, and this diversity is represented in these rocks (Watkins and Browne, 1989). While these rocks share a common deformation history indicative of east-directed transport from folding and thrusting along regional structures in different areas of Nevada, these rocks have been subject to additional distinct tectonic events during the Mesozoic and the Paleozoic resulting in significant spatial variability in the structure of these rocks (Evans and Theodore, 1978; Oldow, 1984b).
Rocks of the Grass Valley, Osobb, and Dun Glen Formations, and their unnamed overlying rocks elsewhere known as the Winnemucca Formation, exposed in Pershing, Churchill, Lander, and Humboldt Counties, characterize this unit. These rocks are depositional on top of the Star Peak Group carbonate and detrital rocks (TRc). Crossbedding, lode casts, and other depositional features indicate uniform northwest-trending current directions. The lithology and depositional characteristics of these rocks suggest shallow marine conditions on and around a westerly prograding delta (Silberling and Wallace, 1969). Fossils from these rocks range in age from Late Triassic (Norian) to Early Jurassic (Toarcian) (Silberling and Wallace, 1969).
Includes units mapped as the High Rock sequence on the Washoe North map; the Timber Mountain, Paintbrush, Crater Flat, and Belted Range Tuffs, and Indian Trail Formation (now abandoned) on the Nye South map; the Thirsty Canyon Tuff on the Nye South and Esmeralda maps; and other unnamed units. Locally it includes tuffaceous sedimentary rocks interstratified with tuffs. It is present in the northernmost part and southernmost parts of the State, and is not exposed in the central region. It corresponds to unit Tt3 on the 1978 State map, although a few rocks also mapped as Trt on the 1978 State map also are included. It is present in Clark, Churchill, Washoe, Nye, Lincoln, Lyon, Douglas, Carson, Esmeralda, Elko, Humboldt, Pershing, and Mineral Counties.
Intrusive rocks crop out in the East Range and Humboldt Range in Pershing County associated with the Koipato Group volcanic rocks (TRkv). They intrude upper Paleozoic rocks of the Golconda terrane (GC) and rocks of the Koipato Group. Limited older evidence suggests that these rocks may be Triassic (Silberling and Wallace, 1967; Wallace, Silberling, and others, 1969; Wallace, Tatlock, and Silberling, 1960; Wallace, Tatlock, and others, 1969), but new data (du Bray and Crafford, 2007) suggests that most of the intrusive rocks mapped as Triassic in the East and Humboldt Ranges are Cretaceous or younger. In northern Esmeralda County between the Royston Hills and the Monte Cristo Range small exposures of Upper Triassic plutons are inferred to relate to the Lee Vining intrusive epoch in eastern California (Stewart, 1980). These rocks also intrude into the Golconda terrane (GC).
Andesite flows and breccias and other related rocks of intermediate composition such as dacite, rhyodacite, quartz latite, and biotite-hornblende porphyries. This unit includes units mapped as the South Willow Formation on the Washoe North map, the Milltown Andesite on the Nye South and Esmeralda County maps, the Mizpah Trachyte on the Nye North map, and other units. It corresponds to unit Ta2 on the 1978 State map. It crops out in all counties except Clark, Eureka, Lyon, Douglas, and Carson.
A large complex of gabbroic rocks forms a series of related intrusions in the northern parts of the Stillwater Range and Clan Alpine Mountains of Churchill County and in the West Humboldt Range of Pershing County (Willden and Speed, 1974). It also may extend into the Trinity Range and Shawave Mountains in western Churchill County (Greene, Stewart, and others, 1991). The complex contains highly differentiated facies near the periphery of the body and more homogeneous gabbro in the interior. Layered rocks near the margins include picrite, olivine gabbro, hornblende gabbro, and anorthosite. The homogeneous rocks consist largely of feldspathic hornblende gabbro and analcite gabbro. The complex is interpreted to be part of a continental Jurassic volcanic arc that is the northern continuation of a Jurassic continental margin arc that extended from the Sonora Desert region in the south to northern California in the north (Dilek and Moores, 1995; Zientek, Sidder, and Zierenberg, 2004). Biotites from several places in the gabbro have been dated by K/Ar and range from 140 to 170 Ma.
Welded and nonwelded silicic ash flow tuffs. Aside from alluvium, this unit covers more of Nevada than any other rock, with over 4,000 polygons representing it on this map. It is principally exposed in the central regions of the State. It locally includes thin units of air fall tuff and sedimentary rocks. It includes rocks mapped on the Washoe South, Lyon, Douglas, and Carson Counties maps as the Hartford Hill Rhyolite Tuff (now abandoned); on the Nye South map as the tuff of White Blotch Spring, the tuffs of Antelope Springs, and the tuff of Monotony Valley; in Lander County it is mapped as the Bates Mountain Tuff, Caetano Tuff, Edwards Creek Tuff, New Pass Tuff, tuff of Hall Creek, and the tuff of McCoy Mine; in Lander and Pershing Counties it is the Fish Creek Mountains Tuff; on both of the Nye County maps it is the Fraction Tuff; it also includes the Pancake Summit Tuff, Northumberland Tuff, Shingle Pass Tuff, some outcrops of Darrough Felsite shown to be Tertiary (other outcrops have been shown to be Mesozoic or Paleozoic), tuffs of Moores station, tuffs of Peavine Canyon, tuffs of the Pancake caldera complex, the Stone Cabin Formation, tuff of Saulsbury Wash, tuff of Kiln Canyon, the Tonopah Formation, tuffs of Hannapah, tuff of Bald Mountain, the Needles Range Formation, and the Calloway Well Formation on the Nye North map; in Esmeralda County it is the Kendall Tuff and latite; and in northern Nye and Lander Counties it is the Toiyabe Quartz Latite (now abandoned), and other unnamed units. It corresponds to unit Tt2 on the 1978 State map. It crops out in every county except Clark.
Includes some rocks mapped as the Kate Peak and Alta Formations on the Washoe South map; Wahmonie and Salyer Formations on the Nye South map; Gilbert Andesite on the Esmeralda map; pyroxene, hornblende phenoandesite, and phenodacite on the Elko map; and other unnamed units. It corresponds to the unit Ta3 on the 1978 State map. It is present everywhere except Eureka and White Pine Counties.
The Dutch Flat terrane is the Late Devonian Harmony Formation. It consists of coarse-graded feldspathic sandstone and siltstone with rare quartzose turbiditic limestone interbeds that have yielded sparse, reworked Late Devonian and post-Ordovician conodonts and conodont fragments (Jones, 1997a; Ketner, Crafford, and others, 2005). The age of the Harmony has never been well constrained. It was originally interpreted as Mississippian(?) because of its position unconformably beneath Pennsylvanian conglomerate at Battle Mountain (Ferguson, Roberts, and Muller, 1952; Roberts, 1951). Cambrian fossils were later found in close proximity to the unusual feldspathic sandstone and became the most commonly assumed age (Hotz and Willden, 1964), although the Cambrian fossils have since been recognized to be part of a structurally disrupted upper Paleozoic section (Jones, 1991b; Jones, Wrucke, and others, 1978; McCollum and McCollum, 1991). Ordovician microfossils from the Harmony Formation in the Sonoma Range (Madden-McGuire, Hutter, and Suczek, 1991) turned out to be unreliable as well. In 1994, a single Late Devonian Palmatolepis sp. conodont was recovered from a calcareous turbidite interbedded with the feldspathic sandstone in the Hot Springs Range (Jones, 1997a), and has remained the most convincing lower-age constraint thus far. Subsequent post-Ordovician conodont fragments also recovered from the Hot Springs Range have confirmed that the unit is clearly post-Ordovician in age (Ketner, Crafford, and others, 2005). The Dutch Flat terrane crops out in Humboldt, Lander, and Pershing Counties. In the Hot Springs Range, it is structurally bounded to the northwest by the Golconda terrane and on the southeast by unit DCs of the Basin assemblage. In the Osgood Mountains, it has been structurally dismembered into mélange blocks that are part of an upper Paleozoic matrix of argillite and shale associated with the Golconda terrane (Jones, 1991b). In the Sonoma and East Ranges, much of it is mélange-like in character and has additionally been folded and faulted with Triassic and Ordovician rocks (Silberling, 1975). At Battle Mountain (Doebrich, 1994; Theodore, Murchey, and others, 1994), it is interpreted as faulted over adjacent rocks of the Basin assemblage (DCs), and is also unconformably overlain by the Pennsylvanian rocks of the Siliciclastic overlap assemblage, providing a critical constraint on the timing of its accretion to adjacent rocks. Because it is structurally bounded everywhere, its stratigraphic relation to other units in Nevada remains uncertain, although it has lithologic features in common with rocks of the Golconda terrane and the lower Paleozoic Basin assemblage (Ketner, Crafford, and others, 2005). In places it has west vergent folding throughout (Jones, 1993; Stahl, 1987), while in other places the formation is characterized by east vergent folding (Evans and Theodore, 1978). Interpretations of the origin of the rocks of the Harmony Formation and its tectonic history (Gehrels, Dickinson, and others, 2000; Ketner, Crafford, and others, 2005; Smith and Gehrels, 1994) have yet to fully explain its significant role in the mid-Paleozoic tectonism that affected Nevada. Its varied structural characteristics and enigmatic lithology suggest that this terrane is far traveled and has had a complex history of interaction with other Paleozoic rocks in Nevada.
Limestone, basalt, chert, and volcaniclastic rocks. The Home Ranch subterrane of the Golconda terrane shares similar structural characteristics with the rest of the Golconda terrane, but it has more specific age and lithologic features. It is restricted to Mississippian age (generally Early) and consists of shallow-water fossiliferous limestone, black chert, basalt, and volcaniclastic rocks. Olistostromal debris flows of basalt and limestone, indicative of steep paleotopography, are a distinguishing characteristic (Jones, 1991a). The depositional setting for this subterrane can be interpreted as a seamount. It includes rocks in Elko County mapped as the Banner and Nelson Formations, at least parts of the Inskip Formation in the East Range in Pershing County, the Goughs Canyon Formation in the Osgood Mountains, similar rocks in the Hot Springs Range in Humboldt County, and likely includes Mississippian limestone in the San Antonio Mountains in northern Nye County. To what extent these rocks have a history distinct from other rocks of the Golconda terrane is unclear. They are present structurally in a position outboard or west of most other exposures of the Golconda terrane, and are separated in the northern part of the State from other exposures of the terrane by the Nolan belt.
The Sand Springs terrane is a highly deformed, thick, mainly basinal volcanogenic assemblage of rocks at least partly of early Mesozoic age and possibly having affinities with rocks of the Black Rock-Jackson terrane (Silberling, 1991). The presumably oldest Mesozoic rocks are volcanogenic and carbonate turbidites interbedded with mudstone which grade upward into interbedded basinal carbonates and volcanogenic rocks containing Late Triassic faunas (Oldow, 1984a). Elsewhere, interbedded carbonate, volcanic, and volcanogenic rocks are assigned an Early to Middle Jurassic age and represent relatively shallow-marine to subaerial deposition (Oldow, 1984a). Although structural relations in the Sand Springs terrane are locally complicated by later Cenozoic deformation, the rocks appear to have been involved in major northwest-southeast shortening between the Early Jurassic and Late Cretaceous (80 Ma) (Oldow, 1984a). The rocks of the Sand Springs terrane crop out in southern Washoe, Pershing, Churchill, Mineral, and northern Nye Counties.
Unit is mixed on the Washoe North map with basalt, tuff, diatomite, and tuffaceous sediments. It includes the units mapped as Qls from the 1978 State map. It is present in Churchill, Washoe, Nye, Esmeralda, Elko, Eureka, Humboldt, Lander, Lincoln, Mineral, and Pershing Counties.
This composite terrane includes Mississippian to Middle Triassic oceanic-basin and island-arc rocks in isolated exposures in northwesternmost Nevada originally assigned to the Black Rock terrane, and Upper Triassic to Middle Jurassic volcanogenic and volcanic rocks of the Jackson terrane in the same region. These rocks crop out in southern Washoe, Humboldt, and Pershing Counties. Parts of the Black Rock terrane can be interpreted as the base of the Jackson terrane, but they are generally structurally juxtaposed throughout the region (Jones, 1990; Russell, 1984; Silberling, Jones, and others, 1987; Wyld, 1990). Rocks of this terrane have affinities with correlative rocks in the Eastern Klamath and Northern Sierra terranes in California (Silberling, Jones, and others, 1992).
Shale, thin-bedded limestone, phyllite, hornfels, quartzite, chert, and siltstone are typical of this Cambrian unit which exhibits regional metamorphism suggesting significant burial depths have heated and recrystallized many of these rocks. This unit includes rocks mapped informally as the Bull Run Dolomite and Edgemont Formation in northern Elko County by Ehman (1985); the Crane Canyon sequence in the Toiyabe Range; some regions mapped as Dunderberg Shale; and the Swarbrick Formation in northern Nye County, the Emigrant Formation in southern Nye and Esmeralda Counties, the Mule Spring Limestone in Esmeralda County, the Preble Formation in Humboldt and Pershing Counties (Madden-McGuire, 1991), the Paradise Valley Chert in Humboldt County, and the Schwin Formation (Gilluly and Gates, 1965) in the Shoshone Range in Lander County. In most exposures this unit lies transitionally above the Cambrian-Precambrian quartzite unit CZq. In places this unit is transitional into OCtd. This unit is also in structural contact with DCs, DOts, OCc, OCtd, CZq, the Golconda terrane (GC), and the Dutch Flat terrane (DF). In the Osgood Mountains (Boskie and Schweickert, 2001; Crafford and Grauch, 2002; Madden-McGuire and Marsh, 1991), the Bull Run Mountains (Ehman, 1985), the Toiyabe Range (Means, 1962), and the Miller Mountain area (Oldow, 1984b) these rocks exhibit complex polyphase deformation with a strong west-vergent component. At Edna Mountain near Golconda in Humboldt County, these rocks are unconformably overlain by both Pacl and PIPacl of the Siliciclastic overlap assemblage.
Tertiary mafic intrusive rocks are widely scattered across Nevada north of Clark County. They include rocks mapped as dacite and rhyodacite, diorite, quartz latite, and numerous undivided intrusive rocks on the county maps.
Concentrated in two areas of the State; common in the west-central part of the State along the California border in Mineral, Esmeralda, Lyon, Douglas, and Carson Counties. There is another more widely scattered group in eastern and central Nevada in Elko, Eureka, and White Pine Counties. Scattered occurrences also are present in Humboldt, Churchill, Lander, and Pershing Counties. Compositions are mainly granitic, granodiorite, and quartz monzonite.
Tertiary rhyolitic intrusive rocks also are present in every county of Nevada. They include many rocks mapped as rhyolite or rhyolite porphyry, rhyolite intrusive rocks, rhyolite plugs or flows, microgranite dikes, and many other undifferentiated intrusive rocks.
Amygdaloidal, nonporphyritic, massive flows and breccia, tuff, and tuffaceous argillite are interbedded with limestones in the Smelser Pass Member of the Augusta Mountain Formation in the Star Peak Group Triassic sedimentary rocks (TRc) in Pershing County (Nichols and Silberling, 1977b). They are well dated by abundant fossils from the surrounding rocks and range from lower Upper Triassic (Carnian) to lower Middle Triassic (Anisian) (Silberling and Wallace, 1969). They are not divided out on the 1978 State map from the surrounding Triassic carbonate unit TRc.
These dike rocks of unknown composition are mapped in the Shawave Mountains in Pershing County, the Osgood Mountains and Edna Mountain in Humboldt County, and just outside of Eureka.
The Boyer Ranch Formation in the Clan Alpine and Stillwater Ranges in Pershing and Churchill Counties consists of a basal conglomerate overlain by partly silicified limestone that is overlain by quartz sandstone. In places it rests unconformably over Upper Triassic or younger rocks (Speed and Jones, 1969) of the Jungo terrane (JO), constraining its maximum age, and elsewhere it is faulted over Late Triassic and Early Jurassic rocks (Speed and Jones, 1969). The occurrence of conglomerate-bearing clasts of underlying units at the base of the formation supports the interpretation of unconformable basal contacts even though the unit is strongly folded (Speed and Jones, 1969). It is overlain by volcanic rocks that are comagmatic with the adjacent Middle Jurassic gabbro. In the Pamlico-Lodi (WPL) and Luning-Berlin (WLB) assemblages of the Walker Lake terrane and the Gold Range assemblage (JTRgor), a coarse clastic and shallow marine unit of Jurassic age has been mapped as the Dunlap Formation (Stewart and Carlson, 1978). It lies unconformably over both Permian and Triassic rocks (Oldow, 1981), and disconformably over other Triassic and Lower Jurassic rocks (Oldow and Bartel, 1987). Some of the rocks mapped as Dunlap likely belong in unit Jcg, however, it is not consistently defined on the Nye, Mineral, and Esmeralda County maps, and in many places rocks originally mapped as Dunlap have turned out to be a variety of other units. The Dunlap Formation therefore has not been separated from the other Mesozoic rocks on this map at this time, but it may belong with a more regional Jcg unit that defines an important mid-Mesozoic tectonic constraint.
Quartz monzonite to Quartz diorite intrusions crop out in west-central Nevada in the Singatse Range in Lyon County, the Gillis Range in Mineral County, the Toquima Range on the Nye/Lander County boundary, in northern Nevada at Buffalo Mountain in Humboldt County, and in the East Range in Pershing County.
Tertiary felsic intrusive rocks are widely scattered in every county across the State. They are generally described as granitic rocks, granodiorite, monzonite, quartz monzonite, alaskitic granite, quartz diorite, dacite, and rhyodacite in the places where they are shown separately on county maps.
Includes rocks mapped as the rhyolite of Big Sand Springs Valley on the Nye North map, the Sandstorm Formation in Esmeralda County, rhyolite flow domes in the Sheep Creek Range in Lander County, and other units. It corresponds to unit Tr2 on the 1978 State map. It is present in Nye, Lincoln, Churchill, Esmeralda, Eureka, Mineral, Elko, Humboldt, and Lander Counties.