Geologic units in Eureka county, Nevada

Additional scientific data in this geographic area

Older alluvium and alluvial fan deposits (Pleistocene and Pliocene) at surface, covers 30 % of this area

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.

Alluvium, undifferentiated (Holocene and Pleistocene) at surface, covers 20 % of this area

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.

Tuffaceous limestone, siltstone, sandstone, and conglomerate (Holocene to Pliocene) at surface, covers 5 % of this area

Present in Esmeralda, Elko, Mineral, Lyon, Douglas, Carson, and Eureka Counties and corresponds to unit QTs on the 1978 State map.

Slope Assemblage - Calcareous shale, siltstone, chert, quartzite, and greenstone (Devonian to Ordovician) at surface, covers 5 % of this area

Calcareous shale, siltstone, sandstone, chert, quartzite, and greenstone in the Vinini Formation in Lander, Eureka, Elko, and northern Nye Counties, and the Clipper Canyon Group in the northern Toquima Range are the core rocks of unit DOts. Difficulties in identifying distinct paleogeographic settings within Ordovician slope facies rocks are discussed in Finney and Perry (1991) and Finney and others (1993). On a regional scale, the distinction between this unit and rocks traditionally mapped as the Valmy Formation (DCs) is the preponderance of shale and siltstone of cratonal derivation that is present in the Vinini rocks but less common in the Valmy rocks. Both rock units contain bedded chert, massive quartzite, and greenstone (Finney and Perry, 1991) in many places. Many lower Paleozoic rocks grouped here likely formed in a basinal rather than slope setting, but the presence of more common siliciclastic horizons of shale, siltstone, and sandstone distinguish them as a regional grouping from the lower Paleozoic Basin assemblage rocks. Whether this is a function of distinct paleogeographic settings of coeval units as interpreted by early workers, or is actually an age distinction of older (Valmy) versus younger (Vinini) Ordovician rocks, as suggested more recently for at least the Roberts Mountains (Finney, Perry, and others, 1993), remains to be determined on a regional scale. Originally thought to be primarily Ordovician, studies and biostratigraphic data have demonstrated that this unit consists of tightly imbricated Devonian, Silurian, and Ordovician rocks (Coles and Snyder, 1985; Noble and Finney, 1999). The distinction between units DOts and DCs as currently mapped on a regional scale is ambiguous in many places. Identifying the numerous occurrences of Devonian and Silurian rocks that are embedded within this unit on a regional scale would significantly enhance our understanding of the complex structural history of these rocks. These rocks are everywhere in structural contact with other Paleozoic rocks including units IPMcl, Pacl, Dc, MDst, DSt, DSc, and Dcd. Stratigraphic correlation has been made between rocks of the Vinini Formation and the Carbonate shelf sequence in Nevada (Finney and Perry, 1991) on the basis of occurrence of quartzite that is coeval with the shelf unit Ocq. While this does suggest a connection between the Ordovician rocks of this composite unit and North America, the quartzite was deposited along a 1,000-mile length of the margin (Ketner, 1986) and thus does not constrain the rocks of unit DOts to deposition along a specific section of the margin. These rocks are unconformably overlain sporadically by units Pacl and PIPacl, and post-Paleozoic cover rocks.

Younger tuffaceous sedimentary rocks (Pliocene and Miocene) at surface, covers 4 % of this area

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.

Intermediate silicic ash flow tuff (lower Miocene and Oligocene) at surface, covers 4 % of this area

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.

Playa, lake bed, and flood plain deposits (Holocene and Pleistocene) at surface, covers 3 % of this area

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.

Older andesite and intermediate flows and breccias (lower Oligocene to middle Eocene) at surface, covers 3 % of this area

Unit includes andesite or dacite flows, flow breccias, and hypabyssal rocks in Lander County, andesitic to latitic flows, pyroclastic rocks, and phenoandesitic and phenolatitic flows in Elko County, and other undifferentiated volcanic rocks in other counties. Present in Humboldt, northern Nye, Churchill, Elko, Eureka, Lander, and White Pine Counties. It corresponds to the 1978 State map unit Ta1.

Carbonate Shelf Sequence - Limestone and minor dolomite (Upper and Middle Devonian) at surface, covers 3 % of this area

Includes generally cliff-forming, thin- to thick-bedded limestone. These rocks are mainly shallow water subtidal, intertidal, and supratidal deposits formed on a broad inner carbonate shelf (Stewart, 1980). The Devils Gate Limestone and Guilmette Formation in northern Nevada are the principal units, and the Sultan Limestone is included from the southern part of the State. Unit is overlain (usually disconformably) by the Pilot Shale of unit MDcl except in southernmost Nevada where it is overlain by Mississippian carbonate (Mc). It depositionally overlies Middle and Lower Devonian unit Dcd. In a few places, such as southern Nevada and parts of Eureka County, regional mapping did not distinguish the Upper and Middle Devonian section from the Lower Devonian section, and all of the Devonian is included in unit Dc. Rocks mapped as the Simonson Dolomite would fit into this depositional sequence (sequences 9 and 10 of Cook and Corboy, 2004), but they are not differentiated from the underlying dolomites in White Pine or Elko Counties, so they are all included in unit Dcd here, not unit Dc. This unit crops out in Clark, Elko, Eureka, Lander, Lincoln, Nye, and White Pine Counties.

Rhyolite flows, tuffs, and volcaniclastic rocks (Upper Jurassic) at surface, covers 2 % of this area

Rhyolite flows, felsic ash-flow tuffs and volcaniclastic rocks of the Pony Trail Group (Muffler, 1964) are the only recognized Jurassic felsic volcanic rocks in Nevada, cropping out in northern Eureka County in the Cortez Mountains area. They are dated as Jurassic by a radiometric date from 1972 (Smith and Ketner, 1976). The Pony Trail Group is made up of (in ascending order) the volcaniclastic Big Pole Formation; a silicic ash-flow tuff unit, the Sod House Tuff; and the boldly outcropping Frenchie Creek Rhyolite made of tuffs, volcaniclastic horizons and flows (Smith and Ketner, 1976). While some of these rocks likely are Jurassic, rocks mapped on the Elko County map as the Frenchie Creek Rhyolite exposed in the Elko Hills northeast of Elko have been shown to be Tertiary and renamed (Ketner, 1990) so it is possible that parts of the section included in the Frenchie Creek Rhyolite are not Jurassic. This unit corresponds to unit Jv on the 1978 State map.

Basin Assemblage - Shale, chert, quartzite, greenstone, and limestone (Devonian to Upper Cambrian) at surface, covers 2 % of this area

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).

Older gravels (Pleistocene and Pliocene) at surface, covers 2 % of this area

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.

Andesite and basalt flows (Miocene and Oligocene) at surface, covers 2 % of this area

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.

Older felsic phaneritic intrusive rocks (Jurassic) at surface, covers 2 % of this area

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.

Carbonate Shelf Sequence - Dolomite, sandstone, and limestone (Middle and Lower Devonian) at surface, covers 2 % of this area

Crops out over the same area as unit Dc. Its primary formations are the Sevy Dolomite and the Nevada Formation (now abandoned). In White Pine County, there may be some undivided Guilmette Formation included with unit Dcd. Also included are the Lower Devonian Tor and McMonnigal Limestones in northern Nye County. The Simonson Dolomite is also included here as it is not differentiated in White Pine and Elko Counties. These rocks correspond to depositional sequences 6, 7, and 8 of Cook and Corboy (2004). Unit Dcd is overlain by unit Dc and is depositional on DSc. In White Pine County and most of Elko County, unit DSc is not broken out from unit SOc, hence the Devonian dolomite sequence appears to rest directly on SOc.

Foreland Basin Assemblage - Shale, siltstone, sandstone, and conglomerate (Middle Pennsylvanian to Lower Mississippian) at surface, covers 2 % of this area

Unit crops out across all of eastern Nevada, generally east of 116 west longitude, and somewhat farther west in the southern half of the State. It includes rocks mapped as the Chainman Shale in Elko, northern Nye, and Lincoln Counties; the Diamond Peak Formation in northern Nye, Elko, Eureka, and White Pine Counties; the Scotty Wash Quartzite in Lincoln County; the upper part of the Eleana Formation in Nye County; and undivided sedimentary rocks in Eureka and Lincoln Counties. Clastic and carbonate rocks mapped in Elko County, including undivided Moleen and Tomera Formations (the Tomera Formation includes Middle Pennsylvanian rocks) are also grouped here. Most of these rocks are Upper Mississippian and Lower Pennsylvanian in age, but unit IPMcl also includes Lower Mississippian rocks, overlapping with unit MDcl where they have not been clearly distinguished. In places the Chainman Shale is time transgressive into the Diamond Peak Formation, and in other places they represent different coeval facies, based on limited biostratigraphic data. Where possible, younger siliciclastic rocks have been separated from the older sequence that includes the Pilot Shale and Joana Limestone because of significant differences in the character of the rocks. Unit IPMcl is overlain conformably or disconformably in the eastern part of its exposure by carbonate rocks of units PIPc and (or) IPMbc. In the northern and western parts of its exposure it is overlain unconformably by Permian and Upper Pennsylvanian clastic rocks of the Siliciclastic overlap assemblage (Pacl or PIPacl). Assignment of siliciclastic Pennsylvanian units to either unit IPMcl or the unconformably overlying PIPacl is challenging unless biostratigraphic data are available and outcrop observations reveal the presence of the unconformity such as in Carlin Canyon (Dott, 1955). Unit IPMcl lies either conformably or disconformably above unit MDcl.

Carbonate Shelf Sequence - Dolomite (Lower Devonian and Silurian) at surface, covers 1 % of this area

Unit corresponds with sequence 5 of Cook and Corboy (2004) and includes the Laketown and Lone Mountain Dolomites and equivalent unnamed rocks. In White Pine County these rocks are grouped with the underlying unit SOc, but otherwise are mapped in Elko, Eureka, Nye, Lincoln, and Clark Counties. Disconformities and discontinuities are commonplace along both upper and lower contacts (Langenheim and Larson, 1973). Unit DSc is depositionally overlain by unit Dcd, except where those rocks are grouped with unit Dc. In general, unit DSc overlies unit SOc. In Clark County and parts of Elko County, unit SOc is not differentiated from unit OCc, and therefore DSc lies directly on OCc. In the Sulphur Spring Range, DSc depositionally overlies unit DSt, and in the Roberts Mountains it grades laterally and vertically down into unit DSt. The Lone Mountain Dolomite has been shown to be both primary and secondary dolomite (Nichols and Silberling, 1977a). Therefore the boundaries mapped between unit DSc and both underlying DSt and overlying Dcd are not primary depositional features in all cases, especially in the Roberts Mountains.

Carbonate Shelf Sequence - Limestone, dolomite, and quartzite (Middle Ordovician to Upper Cambrian) at surface, covers 1 % of this area

Carbonate platform rocks are present in Nye, Lincoln, Elko, Eureka, Lander, White Pine, Esmeralda, and Clark Counties. This unit is primarily Ordovician in age but does include Upper Cambrian rocks at the base (Page, Lundstrom, and others, 2005). The Pogonip Group, including the Antelope Valley Limestone is the most common name used. In Clark County it also includes the Ely Springs Dolomite, and includes the Eureka Quartzite in White Pine and Clark Counties. Unit OCc corresponds to depositional sequence 2 of Cook and Corboy (2004). Where Ocq is mapped separately, it overlies OCc. Otherwise OCc is depositional under SOc, or in southern Nye and Clark Counties, it is overlain directly by DSc where SOc is not differentiated. Unit OCc depositionally overlies unit Cc.

Younger rhyolitic flows and shallow intrusive rocks (Miocene) at surface, covers 1 % of this area

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.

Older rhyolitic flows and shallow intrusive rocks (lower Oligocene to middle Eocene) at surface, covers 1.0 % of this area

Includes rhyolitic lava of Portuguese Mountain in northern Nye County, rhyodacite in Elko Hills in Elko County, and other unnamed units. The rhyodacite in Elko Hills was shown on the 1978 State map as unit Tr1, and on the Elko County map as the Jurassic Frenchie Creek Rhyolite. It was subsequently renamed the Rhyodacite of Elko Mountain (Ketner, 1990) when the late Eocene radiometric age of approximately 39.5 Ma was obtained. It corresponds to unit Tr1 on the 1978 State map. This unit is present in northern Nye, Elko, Eureka, and White Pine Counties.

Carbonate Shelf Sequence - Quartzite (Middle Ordovician) at surface, covers 0.7 % of this area

Because of its importance as a stratigraphic marker horizon, the Eureka Quartzite is depicted on this map wherever it is mapped separately from the Ordovician carbonate shelf rocks. It represents depositional sequence 3 of Cook and Corboy (2004). It is not differentiated from the rest of the Ordovician (OCc) in White Pine or Clark Counties, but is shown in Elko, Eureka, Nye, and Lincoln Counties. Rocks mapped as the Ely Springs Dolomite are included with the Eureka Quartzite in southern Nye County. The Eureka Quartzite depositionally overlies the Pogonip Group (OCc), and is overlain by either the Hanson Creek Formation or the Ely Springs Dolomite (SOc).

Siliciclastic Overlap Assemblage - Sandstone, siltstone, limestone, conglomerate, and carbonaceous limestone (Permian) at surface, covers 0.6 % of this area

Unit is mapped in Elko, Mineral, Humboldt, Lander, Eureka, White Pine, northern Nye, and Esmeralda Counties. Included in this unit are the Carbon Ridge Formation in Eureka and White Pine Counties, parts of the Carlin sequence of Coats (1987), the sandstone and siltstone of Horse Mountain in Elko County, the Edna Mountain Formation in Humboldt and Elko Counties, the Garden Valley Formation in Eureka County, and the Diablo Formation in northern Nye, Mineral, and Esmeralda Counties. In the Candelaria area south of Mina, unit Pacl rests unconformably on deformed Upper Cambrian through Devonian Basin assemblage (DCs) and is overlain by the Lower Triassic Candelaria Formation (TRcl). In the Toiyabe Range, it lies unconformably on deformed Cambrian through Ordovician rocks of the Nolan belt (OCtd). In the Simpson Park Mountains and the Sulphur Spring Range, it rests unconformably on Ordovician and Devonian Slope assemblage rocks (DOts). In the Diamond Mountains it rests unconformably on the Ely Limestone (IPMbc). In the Eureka area, Pacl unconformably overlies the Ely Limestone (IPMbc) and the Diamond Peak Formation (IPMcl) and is unconformably overlain by Cretaceous conglomerate (Kcg). Near Golconda it unconformably overlies PIPacl. In the Adobe Range in Elko County it overlies Foreland basin assemblage rocks (IPMcl, MDcl) and PIPacl, and in the Snake Mountains and HD Range of northeast Nevada it lies unconformably on lower Paleozoic Slope and Basin assemblage rocks (DCs, Ss, DOts) and on older Siliciclastic overlap assemblage rocks (PIPacl).

Siliciclastic Overlap Assemblage - Conglomerate, sandstone, siltstone, and limestone (Permian to Middle Pennsylvanian) at surface, covers 0.6 % of this area

Unit represents rocks that are stratigraphic sequences that include both Lower Permian and Pennsylvanian rocks, and also sections that have not been broken out regionally into younger and older Permian and Pennsylvanian units. The Antler sequence (Roberts, 1964) rocks are present in Humboldt and Lander Counties and include the Antler Peak Limestone, the Highway Limestone, the Battle Formation or Battle Conglomerate, and the Etchart Limestone. The Brock Canyon Formation of Permian or Pennsylvanian age is in the Cortez Mountains in Eureka County and the siliciclastic and carbonate Strathearn Formation is exposed in Elko County (Theodore, Moring, and others, 2003). Scattered remnants of conglomerate, sandstone, siltstone, and limestone in Nye County, and unnamed limestone and dolomite in Elko County are also included. In the northern Hot Creek Range in Nye County, PIPacl is faulted with lower Paleozoic Carbonate shelf sequence rocks. Additionally, Early Triassic fossils in the area have caused reassignment of some of the rocks to the Candelaria Formation (TRcl). In the Pancake Range, PIPacl lies on the Ely Limestone (IPMbc). In the Toquima Range, the Pennsylvanian Wildcat Peak Formation lies unconformably on Slope assemblage rocks (DOts). In the Monitor Range and in Lander County, this unit lies unconformably on the lower Paleozoic Basin assemblage rocks (DCs). At Battle Mountain the Antler sequence lies unconformably on both the Harmony Formation, which is the Dutch Flat terrane (DF), and the Valmy Formation of Basin assemblage unit DCs. At Edna Mountain near Golconda and in the Osgood Mountains it lies unconformably on Cambrian and Late Proterozoic quartzite (CZq) and Cambrian phyllite and shale (Ctd) of the Nolan belt, as well as on units of the Basin and Slope assemblages (DCs, DOts). In the Cortez Mountains of northern Eureka County, it lies unconformably on Basin and Slope assemblage rocks (DCs, DOts). In the Adobe Range and the Sulphur Spring Range, it lies unconformably on Pennsylvanian and Mississippian Foreland basin rocks (IPMcl) (Trexler, Cashman, and others, 2004). In northern Elko County in the Bull Run and Copper Mountains, it lies unconformably on strongly deformed Ordovician to Cambrian rocks of the Nolan belt (OCtd). In the Snake Mountains and the HD Range, the Pennsylvanian Quilici Formation lies unconformably on the Basin and Slope assemblages (DCs, DOts, Ss) and is unconformably overlain by the Permian Siliciclastic overlap assemblage rocks (Pacl). In far northeastern Nevada, upper Paleozoic rocks around Contact are very poorly known, but are similar to the Siliciclastic overlap assemblage rocks recognized in the HD range, and are thus included in this group.

Siltstone, shale, conglomerate, and limestone (Cretaceous) at surface, covers 0.6 % of this area

Includes detrital deposits of continental origin, and locally derived fluvial and lacustrine clastic rocks, some interbedded with siltstone and freshwater limestone. Outcrops are concentrated in three separate areas of the State. In each place, limited biostratigraphic data indicate these rocks are Cretaceous. The King Lear Formation in the Jackson Mountains in Humboldt County lies unconformably on Triassic and older rocks of the Black Rock-Jackson terrane. The Newark Canyon Formation crops out mostly in Eureka and White Pine Counties but extends into Elko and Nye Counties as well, and rests unconformably on Ordovician to Permian rocks. In places it is difficult to distinguish Upper Devonian, Pennsylvanian, and Permian clastic rocks also derived from the nearby underlying bedrock from the Newark Canyon Formation, and some confusion still exists. The Willow Tank Formation in Clark County lies unconformably on Jurassic rocks and is overlain by what was mapped as the Baseline Sandstone and Overton Fanglomerate (now referred to as the Overton Conglomerate Member of the Baseline Sandstone), all of Cretaceous age.

Slope Assemblage - Platy limestone, dolomite, and chert (Lower Devonian to Silurian) at surface, covers 0.5 % of this area

Platy limestone, dolomite and chert are characteristic of the auriferous Roberts Mountains Formation in Nye, Elko, Eureka, and Lander Counties and of the Masket Shale and Gatecliff Formation in northern Nye County. This unit lies with depositional contact over the Hanson Creek Formation of unit SOc of the Carbonate shelf sequence (unit OCc in southern Nevada), and is also structurally imbricated with Carbonate shelf sequence rocks (OCc) and other Slope and Basin assemblages rocks (units DCs, DOts, MDst) across its area of exposure. In the Carlin area, rocks assigned to the Popovich Formation and the informal Bootstrap Limestone (Berger and Theodore, 2005; Jory, 2002) are also included. In the Monitor Range, the Roberts Mountains, and the Sulphur Spring Range, unit DSt is mapped as stratigraphically overlain by unit DSc. To what extent this “overlying” dolomite is truly a stratigraphic unit as opposed to an alteration product of this unit (Nichols and Silberling, 1977a) is unclear. A stratigraphic contact with unit MDst in the Carlin area is possible based on recent mapping (Berger and Theodore, 2005; Theodore, Moring, and others, 2003).

Basalt (Miocene) at surface, covers 0.3 % of this area

Basalt flows, plugs and dikes, some olivine basalt, and andesite and latitic rocks. This unit corresponds with unit Tb on the 1978 State map. It is present on the Washoe North, Washoe South, Lincoln, Clark, Elko, Eureka, Humboldt, Nye South, and Lander County maps.

Carbonate Shelf Sequence - Dolomite, limestone, and shale (Cambrian) at surface, covers 0.3 % of this area

Occurs in southern and eastern Nevada. The Bonanza King and Carrara Formations are the primary formations in southern Nye County; the Dunderberg Shale in northern Nye and Lincoln Counties; the Hamburg Dolomite in Eureka County; the Nopah Formation in southern Nye and Esmeralda Counties; the Patterson Pass and Pioche Shales, the Chisholm and Highland Peak Formations, and the Lyndon Limestone in Lincoln County; the Pole Canyon Limestone and the Lincoln Peak and Windfall Formations in northern Nye County; and undifferentiated limestone and dolomite in Lincoln, Clark, White Pine, Eureka, northern Nye, and Elko Counties. This unit is conformably overlain by the Ordovician shelf rocks (OCc), and is depositional on the underlying Proterozoic-Cambrian quartzite of CZq.

Foreland Basin Assemblage - Siltstone, limestone, shale, and sandstone (Lower Mississippian and Upper Devonian) at surface, covers 0.3 % of this area

Unit crops out across all of eastern Nevada, generally east of 116 west longitude. It includes rocks mapped primarily as Pilot Shale, Joana Limestone, Chainman Shale, and their equivalents. This also includes the Tripon Pass Limestone in Elko County, the Cockalorum Wash Formation (now abandoned) in northern Nye County, the Mercury and Bristol Pass Limestones in Lincoln County, and some of the rocks mapped as Monte Cristo Limestone in Clark County. While it may be desirable to separate out the different lithologies, they are not well enough differentiated at this regional map scale. The Chainman, Joana and Pilot are grouped in White Pine County, and the Joana and Pilot are grouped in Elko County. The Pilot Shale lies depositionally (both conformably and disconformably) on Upper Devonian carbonate rocks (Dc) and signals a major change in the depositional setting across most of the carbonate platform which has long been attributed to the onset of deformation attributed to the Antler orogeny. The Pilot Shale is generally recognized as carbonaceous shale, overlain by the cliff-forming Joana Limestone. Siliciclastic quartz-bearing grit, chert, quartz sand, and siltstone in a calcareous matrix become increasingly common as the section turns into the Chainman Shale and other equivalent siliciclastic rocks. The sequence is interrupted by disconformities in a number of places, but structural disruption and poor age control hinder determination of the nature of the contacts between the distinct lithologies. Unit MDcl is overlain by unit IPMcl, but there are places where the age and distinction between the units is poorly constrained. In southernmost Nevada, in Clark and southeastern White Pine Counties, Devonian carbonate is overlain by Mississippian carbonate (Mc) with little or no intervening Pilot Shale equivalent and few overlying siliciclastic rocks with western-derived source material. North and west of the area of exposure of unit MDcl, fault-bounded slivers of Lower Mississippian and Upper Devonian platform margin and slope facies rocks with siliciclastic horizons have been grouped into unit MDst and separated from unit MDcl.

Slope Assemblage - Shale, graywacke, siltstone, chert, conglomerate, and limestone (Lower Mississippian and Devonian) at surface, covers 0.2 % of this area

Carbonaceous shale, black chert and argillite, graywacke, chert-pebble conglomerate, and detrital limestone are the primary lithologies described from all of the rocks assigned to this unit, representing a mixed slope and basinal facies. On other maps these rocks have been included in a variety of units including the foreland basin and Devonian siliceous and transitional rocks. Mapping and new biostratigraphic data gathered in the last 30 years have shown that many of these rocks mapped only as Devonian also contain Early Mississippian fossils, thus making it difficult to distinguish them from known lithologically similar Lower Mississippian rocks. Although this unit is everywhere structurally bounded by faults, a stratigraphic link to older Slope assemblage rocks is possible. These rocks are imbricated with units DCs, IPMcl, OCc, Ocq, DSt, Dc, and MDcl. Whether there is a definable continuous Early Mississippian through Devonian sequence within this unit is unknown, but is suggested in the Carlin-Piñon Range (Smith and Ketner, 1978). The Slaven Chert first described in the Shoshone Range (Gilluly and Gates, 1965) is black chert with carbonaceous shale beds 4–10 feet thick, limy brown-weathering sandstone as much as four ft thick with coarse fragments of chert, shale, greenstone, limestone, graywacke, feldspathic siltstone, and brown-weathering limestone 2–20 ft thick, and contains Late Devonian radiolarians (Boundy-Sanders, Sandberg, and others, 1999). The Mississippian Waterpipe Canyon Formation is a similar formation with basal medium-grained graywacke with interlayered black, carbonaceous shale; chert-pebble conglomerate; and bedded chert grading upward into sandstone layers with black, well-rounded quartz and a black, pyritic, phosphate- and barite-bearing, argillaceous matrix interlayered with black, platy, quartz siltstone and fine-grained graywacke interbeds. It contains Early Mississippian radiolarians (Peters, Armstrong, and others, 2003). In the HD Range in northeastern Elko County, an undated, light-gray weathering, brittle, black shale, structurally underlies the other thrust sheets and was referred to as the Chainman Shale by Riva (1970), but is included here in unit MDst. In the Windermere Hills a fissile black argillite with sporadic interbeds of quartz-chert arenite is poorly exposed with variable dips suggesting a complex structure (Oversby, 1972). In the Cockalorum Wash quadrangle along the Eureka-Nye County boundary, a pale yellow-brown, organic-detrital limestone contains quartz and chert grains locally interbedded with and succeeded upward by light-colored siliceous mudstone, claystone, and siltstone. The basal limestone contains mixed Mississippian and Devonian faunas; a thin chert from a higher zone has Osagean radiolarians (Hose, 1983). In the northern Adobe Range, this unit is recognized as dark siliceous rocks consisting of shale, argillite, and bedded chert. They are faulted and folded with sparse collections of Kinderhookian and Famennian radiolarians and conodonts (Ketner and Ross, 1990). The Webb Formation in the Carlin-Piñon Range is a gray siliceous mudstone with black to gray, tan-weathering, dense limestone in lenses near the top (Smith and Ketner, 1978). The argillite of Lee Canyon is a black siliceous argillite with a little black chert and very little conglomerate and sandstone near the top (Smith and Ketner, 1978). In the Sulphur Spring Range, the Bruffey sequence (Carlisle and Nelson, 1990) is a black chert pebble to boulder conglomerate and well-bedded gritty limestone, chert and limestone conglomerate, gray limy shale, and minor sandstone. Smith and Ketner (1978) describe the same rocks as gray limestone, sandy limestone, chert, and chert-pebble conglomerate. The Woodruff Formation from the same area is described by Carlisle and Nelson (1990) as a gray fissile shale, dolomitic siltstone, and black and brown bedded chert. Smith and Ketner (1978) describe the Woodruff as dark gray to black siliceous mudstone and chert, with lesser amounts of shale, siltstone, dolomitic siltstone, dolomite, and limestone. In the Shoshone Range, pale-red to pale-brown weathering, platy, silty dolomite interbedded with black chert in the basal 50 ft of rocks referred to as the Pilot Shale by both Gilluly and Gates (1965) and Wrucke (1974) is included here. In the southern Independence Range, this unit consists of fine-grained limestone, bedded chert, shale, conglomerate, and prominent ledges of limy sandstone with Famennian and Frasnian (Late Devonian) conodonts (Ketner, 1998). In Welches Canyon northwest of Carlin, this unit is gray to black limestone, fine grained, and thin to thick bedded with common sand- and silt-size clasts of quartz and chert grains. It also contains pebbles and cobbles of chert, and interlayered chert and siliceous shale as much as 50 feet thick (Evans, 1974). In the Snake Mountains, the unit is dark carbonaceous limestone apparently overlain by a light-gray, siliceous platy siltstone. Other outcrops that belong with unit MDst, but are not mapped separately on a regional scale from Slope or Basin assemblage units DCs and DOts include the Pinecone sequence in the Toquima Range (Coles and Snyder, 1985), and gold-bearing chert (Theodore, T., oral commun., 2006) mapped informally as the “Rodeo Creek Formation” (Peters, 1997b) in the Carlin area.

Carbonate Shelf Sequence - Dolomite, limestone, and shale (Lower Silurian to Middle Ordovician) at surface, covers 0.2 % of this area

Ely Springs Dolomite and Hanson Creek Formation are the main formations included in this unit. Many of the rocks in this unit are not assigned to a formation. A large section of the carbonate platform from Early Devonian through latest Ordovician time is represented by dolomitic rocks. They commonly look similar, have poor biostratigraphic control, and thus are not always well differentiated on the county maps. Additionally, not all of the dolomite is primary, and thus boundaries between secondary dolomite and other rock units have been misinterpreted as primary stratigraphic boundaries, further confusing the stratigraphy of the lower Paleozoic shelf (Nichols and Silberling, 1977a). Rocks in this unit correspond to sequence 4 of Cook and Corboy (2004). This unit includes rocks deposited immediately above the Eureka Quartzite, but disconformably below the Lone Mountain and Laketown Dolomites, hence it includes the Silurian and uppermost Ordovician. Rocks included in unit SOc that are mapped as the Hanson Creek Formation are depositionally overlain by the Roberts Mountains Formation of unit DSt in the northern and western part of the exposure area. The SOc rocks mapped as Hanson Creek Formation are difficult to distinguish from units DSt and DOts, and should more appropriately be included in unit DOts, but inconsistent mapping makes this difficult. In general unit SOc is not differentiated from unit OCc in Clark County, and thus unit DSc lies directly on unit OCc. In Lincoln and Nye Counties unit SOc lies directly on the Eureka Quartzite (Ocq) and is overlain by the Laketown Dolomite (DSc). In southern Nye County, rocks mapped as Ely Springs Formation are grouped with the Eureka Quartzite as unit Ocq. In White Pine and eastern Elko Counties, the Eureka Quartzite is not mapped separately, and unit SOc therefore lies directly on unit OCc, which includes the quartzite. Also in White Pine and eastern Elko Counties, unit DSc is not differentiated from unit SOc, so SOc is overlain directly by unit Dcd. In the northern and western areas of exposure where unit SOc is mapped as Hanson Creek Formation it is overlain depositionally by unit DSt of the Slope assemblage.

Basin Assemblage - Feldspathic sandstone, siltstone, shale, and chert (Silurian) at surface, covers 0.2 % of this area

In the HD Range in northeastern Elko County, the Noh Formation was described by Riva (1970) and consists of a basal, dark-gray chert and light-gray shale, light-brown weathering, siliceous and tuffaceous siltstone and shale, and tan- and light-brown-weathering, thin-bedded siltstone, sandstone, and minor shale. It contains a large and diagnostic Wenlockian (Early Silurian) graptolite fauna, and is partly coeval with the base of the Roberts Mountains Formation (DSt) which also has a conspicuous basal chert ledge. The similar age Elder Sandstone in Lander and Eureka Counties was named for moderately cemented sandstones exposed in the Shoshone Range (Gilluly and Gates, 1965). It is primarily a fine-grained, silty sandstone, sandy siliceous and tuffaceous shale, and thin, platy, light brown chert. Much of the sandstone and siltstone is notably feldspathic, including abundant angular fragments of potassium feldspar, and has reportedly interbedded rhyolite in places (Theodore, T., oral commun., 2006). It is grouped with unit DCs or DOts in many places. Its unusual lithologic characteristics warrant a separate grouping where it can be separated from these units (Noble, Finney, and Cluer, 2000). Zircon studies have suggested that the feldspathic source material for these rocks was not located adjacent to the Nevada part of the continental margin, but is derived from a source either farther to the north or in Mexico (Gehrels, Dickinson, and others, 2000). Likewise, tuffaceous source material for the shale described in the Noh Formation is not known from the Nevada continental margin of this time. Like most other rocks of the Slope and Basin assemblages, unit Ss is everywhere in structural contact with other Paleozoic rocks. It is structurally imbricated with units DCs, DOts, and MDst. Whether these rocks have traveled a significant distance either toward or along the margin as discrete tectonic blocks or as sediment transported in offshore turbidity systems is not known, but no basement is preserved with them, and they are unconformably overlain by the Pennsylvanian and younger Siliciclastic overlap assemblage.

Carbonate Shelf Sequence - Bioclastic limestone (Pennsylvanian and Upper Mississippian) at surface, covers 0.2 % of this area

Mostly Lower Pennsylvanian limestone is present in Nye, Elko, Eureka, White Pine, Lincoln, and Clark Counties, and is most commonly referred to as the Ely Limestone. A ledgy gray limestone mapped as the Moleen Formation is included here. It is not mapped separately from unit PIPc in most of White Pine County, southeastern Elko County, southern Lincoln, and western Clark Counties. Throughout most of the area of exposure unit lies conformably or disconformably beneath unit PIPc and depositionally above unit IPMcl. In southern Nye County this unit includes the Tippipah Limestone, and in Clark County it includes the Callville Limestone. In a north-south trending belt starting at the north end of the Pancake Range in Nye County and continuing north up through the Diamond Mountains along the Eureka-White Pine County border, Lower Pennsylvanian limestone is overlain unconformably by clastic rocks of the Siliciclastic overlap assemblage (Pacl, PIPacl). North of the Diamond Mountains, where Lower Pennsylvanian carbonate is not recognized separately, the coeval facies are grouped with unit IPMcl. Unit IPMbc is primarily Pennsylvanian, but in places contains Late Mississippian fossils as well.

Older silicic ash flow tuffs (lower Oligocene to middle Eocene) at surface, covers 0.1 % of this area

Welded and nonwelded silicic ash flow tuffs, locally includes thin units of air fall tuff and sedimentary rock. This unit corresponds with the 1978 State map unit Tt1. These rocks are present in northern Nye, Elko, Eureka, and White Pine Counties.

Mixed breccias including volcanic, thrust, jasperoid, and landslide megabreccia (Tertiary to Jurassic) at surface, covers < 0.1 % of this area

Breccias of various origins are scattered across Clark, Nye, Lincoln, Elko, Eureka, Lander, and White Pine Counties. Most are interpreted to be Tertiary in age, but have tectonic, volcanic, and metamorphic origins, and include jasperoids, brecciated tuffs, exotic slide blocks, landslide deposits, megabreccia, thrust breccia, and debris beds.

Landslide deposits, colluvium, and talus (Holocene to Pliocene) at surface, covers < 0.1 % of this area

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.

Felsic phaneritic intrusive rocks (Miocene to Eocene) at surface, covers < 0.1 % of this area

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.

Conglomerate, lacustrine, and tuffaceous sedimentary rocks (lower Oligocene to Upper Cretaceous (?)) at surface, covers < 0.1 % of this area

Includes the Sheep Pass Formation and equivalents in northern Nye, Lincoln, Elko, Eureka, Lander, and White Pine Counties. In most places the Sheep Pass Formation is Paleocene or Eocene (Fouch, Hanley, and Forester, 1979), although rocks from the Carlin-Piñon Range area that contain Late Cretaceous fossils have been included in the Sheep Pass Formation (Smith and Ketner, 1976, 1978). It corresponds to unit Ts1 on the 1978 State map.

Felsic phaneritic intrusive rocks (Miocene (?) to Jurassic (?)) at surface, covers < 0.1 % of this area

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.

Intermediate rhyolitic flows and shallow intrusive rocks (lower Miocene and Oligocene) at surface, covers < 0.1 % of this area

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.

Crossbedded quartzite, siltstone, and phyllite (Lower Cambrian and latest Proterozoic) at surface, covers < 0.1 % of this area

These lowermost Cambrian to Precambrian strata are scattered over much of central and eastern Nevada and form the base of the Phanerozoic part of the continental margin stratigraphic section. They include the Campito, Deep Spring, Harkless, and Poleta Formations, and the Reed Dolomite in Esmeralda County; the Gold Hill Formation in northern Nye County; unnamed quartzite and shale in White Pine County; the Osgood Mountain quartzite in Humboldt County; the Prospect Mountain Quartzite in northern Nye, Lincoln, Eureka, and Elko Counties; unnamed quartzite and shale in Lander and Clark counties; and the Stirling Quartzite, Wood Canyon Formation, and Zabriskie Quartzite in southern Nye County. In a number of places, these rocks are depositional on Late Proterozoic unit Zqs. In southernmost Clark County, CZq is lying unconformably directly on Early Proterozoic gneiss (Xm). In the east-central part of Nevada, CZq is overlain depositionally by Cambrian carbonate (Cc) of the Carbonate shelf sequence. In the Nolan belt, these rocks are depositionally overlain by unit Ctd. In the Osgood Mountains in Humboldt County, Permian and Pennsylvanian rocks of the Siliciclastic overlap assemblage (PIPacl, Pacl) rest unconformably directly on the Osgood Mountain Quartzite.

Mafic phaneritic intrusive rocks (Miocene (?) to Jurassic (?)) at surface, covers < 0.1 % of this area

Poorly dated mafic intrusions are concentrated in two regions of Nevada, northwestern and west-central to southwestern parts of the State. They crop out in northern Nye, Mineral, Esmeralda, Eureka, Humboldt, and Lander Counties, and include rocks described on the county maps as dioritic to andesitic rocks, diorite and related rocks, and granodiorite.

Rhyolitic intrusive rocks with aphanitic groundmass (Miocene to middle Eocene) at surface, covers < 0.1 % of this area

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.

Hot spring travertine, sinter, and tufa (Holocene to Pliocene) at surface, covers < 0.1 % of this area

Calcareous and siliceous sinter and tufa deposits that are present in Washoe, Nye, Elko, Eureka, and Lander Counties.

Older tuffaceous sedimentary rocks (lower Miocene and Oligocene) at surface, covers < 0.1 % of this area

Locally includes minor amounts of tuff. It includes rocks mapped as the Titus Canyon Formation on the Nye South map, the Gilmore Gulch Formation on the Nye North map, lacustrine limestone in Lincoln County, and other unnamed units. This unit corresponds to unit Ts2 on the 1978 State map. It is present in Nye, Lincoln, Elko, and Lander Counties.

Intermediate andesite and intermediate flows and breccias (lower Miocene and Oligocene) at surface, covers < 0.1 % of this area

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.

Felsic phaneritic intrusive rocks (Cretaceous) at surface, covers < 0.1 % of this area

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.

Dikes (Cretaceous) at surface, covers < 0.1 % of this area

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.

Mafic phaneritic intrusive rocks (Miocene to middle Eocene) at surface, covers < 0.1 % of this area

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.