Geologic units in Clark county, Nevada

Additional scientific data in this geographic area

Alluvium, undifferentiated (Holocene and Pleistocene) at surface, covers 49 % 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.

Carbonate Shelf Sequence - Limestone, dolomite, siltstone, sandstone, and shale (Lower Permian and Pennsylvanian) at surface, covers 7 % of this area

Present in Elko, White Pine, Lincoln, and Clark Counties. This unit represents mostly Upper Pennsylvanian and Lower Permian rocks that have not otherwise been separated into units Psc or IPMbc. Unit includes unnamed Pennsylvanian and Lower Permian limestone and sandstone beds in Lincoln County, the Bird Spring Formation in Clark County, the Riepe Spring and Ely Limestones (undivided) in White Pine County, and limestone and dolomite rocks not otherwise assigned in Elko County. This unit lies depositionally below unit Psc and above the Ely Limestone (IPMbc) where it is mapped separately. Where unit IPMbc is not mapped separately in southern Nevada, the unit lies directly on Mississippian carbonate (Mc) and in White Pine County it rests on undivided Chainman and Pilot Shales and Joana Limestone (shown as either unit IPMcl or MDcl).

Younger tuffaceous sedimentary rocks (Pliocene and Miocene) at surface, covers 6 % 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.

Proterozoic basement rocks - Gneiss and schist (Early Proterozoic) at surface, covers 5 % of this area

Exposed mostly in Clark and Lincoln Counties, with two small outliers in southern Nye County.

Andesite and basalt flows (Miocene and Oligocene) at surface, covers 4 % 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.

Carbonate Shelf Sequence - Dolomite, limestone, and shale (Cambrian) at surface, covers 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.

Carbonate Shelf Sequence - Limestone, dolomite, and quartzite (Middle Ordovician to Upper Cambrian) at surface, covers 2 % 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.

Felsic phaneritic intrusive rocks (Miocene to Eocene) at surface, covers 2 % 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.

Playa, lake bed, and flood plain deposits (Holocene and Pleistocene) at surface, covers 2 % 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.

Carbonate Shelf Sequence - Limestone and minor dolomite (Upper and Middle Devonian) at surface, covers 2 % 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.

Carbonate Shelf Sequence - Limestone (Mississippian) at surface, covers 2 % of this area

This unit is present in southern Nye, Lincoln, and Clark Counties. Unit includes the Monte Cristo Limestone, and Lower Mississippian rocks referred to as the Joana, Mercury, Bristol Pass, and Rogers Spring Limestones. It generally lies depositionally above Devonian carbonate rocks and beneath Pennsylvanian carbonate and clastic rocks. In the Meadow Valley Mountains in southern Lincoln County it is also shown sitting on a thin horizon of Pilot Shale and overlain by a thin Mississippian clastic unit assigned to unit IPMcl.

Conglomerate and clastic rocks (Tertiary (?) and Cretaceous (?)) at surface, covers 2 % of this area

These conglomeratic, tuffaceous, and other clastic rocks are not well enough constrained to be assigned either a Tertiary (unit TKs1) or Cretaceous (unit Kcg) age, so they are grouped as TKcg. Like the Cretaceous clastic unit Kcg, these rocks sit unconformably on many different age rocks. Included in this unit are units previously mapped as “Older clastic rocks” in Lincoln County; conglomerate, clastic rocks, and tuff in northern Nye County; the Gale Hills Formation in Clark County; and the Pansy Lee Conglomerate in the Krum Hills and Jackson Mountains in Humboldt County.

Younger andesite and intermediate flows and breccias (Miocene) at surface, covers 2 % of this area

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.

Cratonal Sequence - Marine siltstone, limestone, and conglomerate (Middle (?) and Lower Triassic) at surface, covers 1 % of this area

Unit consists of marine deposits of siltstone, sandstone, claystone, mudstone, limestone, and conglomerate (Stewart and Carlson, 1978). It includes rocks assigned to the Moenkopi and Thaynes Formations and related unnamed rocks in northern Nevada (Stewart, 1980). It crops out in the eastern part of the State in Elko, White Pine, Lincoln, and Clark Counties.

Carbonate Shelf Sequence - Cherty limestone, dolomite, shale, and sandstone (Middle to Lower Permian) at surface, covers 1 % of this area

These Permian rocks include cherty limestone, dolomite, shale, sandstone, bioclastic limestone, and phosphatic limestone exposed in Elko, White Pine, Lincoln, and Clark counties. This unit includes rocks mapped as the Phosphoria Formation; the Gerster Limestone, Plympton Formation, Kaibab Limestone, and Grandeur Formation of the Park City Group; the Park City Group undivided; the Toroweap Formation; and the Coconino Sandstone. Unit Pc is disconformably overlain by Triassic unit TRmt in scattered places in eastern and southern Nevada. It depositionally overlies unit Psc. It matches closely with unit Pc of Stewart and Carlson (1978).

Undivided and Metamorphosed Carbonate Shelf Sequence Rocks - Dolomite and limestone (Middle Devonian to Upper Cambrian) at surface, covers 1 % of this area

Lower Paleozoic dolomite and limestone present in southeastern Lincoln and Clark Counties are grouped together into unit DCc. The lower Paleozoic section is too thin to map regionally as individual units, and the structure is too complex in these rocks to accurately portray the individual units at this scale. In part of Clark County, these rocks are referred to as the Goodsprings Dolomite. In the Mormon Mountains of Lincoln County, these rocks are overlain by Mississippian carbonate (Mc). In Clark County in the Spring Mountains, they are overlain by the Devonian Sultan Limestone (Dc).

Cratonal Sequence - Eolian crossbedded sandstone (Jurassic) at surface, covers 1 % of this area

Consists of the Aztec Sandstone. Unit is a friable fine- to medium-grained sandstone with conspicuous large scale cross-strata (Stewart and Carlson, 1978). It is considered eolian. Its age is wholly Jurassic and does not include Triassic rocks as indicated on the 1978 State map (Stewart, 1980). The Aztec is the westward continuation of the Navajo Sandstone of the Colorado Plateau. It crops out only in southern Nevada in Clark and Lincoln Counties.

Tuffaceous sedimentary rocks (middle Miocene to upper Oligocene) at surface, covers 1 % of this area

Consists of the Horse Spring Formation in Clark and southern Nye Counties. This unit corresponds to unit Ths from the 1978 State map, and likely represents a composite of units Ts3 and Ts2. It is poorly known and may include rocks of other ages including Cretaceous.

Proterozoic basement rocks - Felsic phaneritic intrusive rocks (Middle Proterozoic) at surface, covers 1 % of this area

This porphyritic rapakivi granite is present only in Clark County where it intrudes Proterozoic gneiss and schist (Xm).

Carbonate Shelf Sequence - Siltstone, sandstone, limestone, and dolomite (Lower Permian, Leonardian and Wolfcampian) at surface, covers 1.0 % of this area

This largely siliciclastic unit of siltstone, sandstone, limestone, and dolomite crops out in Elko, White Pine, Lincoln, and Clark Counties. It includes rocks originally mapped as the Arcturus Formation, Rib Hill Sandstone, undivided Kaibab Limestone, Toroweap Formation, and Coconino Sandstone in Clark County; and the Pequop Formation and red beds in Lincoln County. Unit Psc represents a strong influx of clastic material over the carbonate shelf during the Early Permian, presumably derived primarily from the craton to the east. It is depositionally overlain by unit Pc and lies conformably above unit PIPc. At its western and northern edges it can be difficult to distinguish from Permian clastic rocks of the Siliciclastic overlap assemblage (units Pacl and PIPacl). It follows closely with unit Psc of Stewart and Carlson (1978).

Carbonate Shelf Sequence - Dolomite (Lower Devonian and Silurian) at surface, covers 0.8 % 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 - Bioclastic limestone (Pennsylvanian and Upper Mississippian) at surface, covers 0.6 % 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.

Crossbedded quartzite, siltstone, and phyllite (Lower Cambrian and latest Proterozoic) at surface, covers 0.6 % 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.

Older gravels (Pleistocene and Pliocene) at surface, covers 0.5 % 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.

Cratonal Sequence - Continentally derived siltstone and clay (Lower Jurassic and Upper Triassic) at surface, covers 0.4 % of this area

These continental deposits include variegated bentonitic claystone, siltstone, and clayey sandstone, ledge-forming sandstone, and red siltstone (Stewart and Carlson, 1978). The lower part of this unit is equivalent to the Upper Triassic Chinle Formation and the upper part corresponds to the Moenave and Kayenta Formations which are now considered Lower Jurassic (Stewart, 1980). It crops out in Elko, Lincoln, and Clark Counties.

Younger silicic ash flow tuffs (Miocene) at surface, covers 0.3 % of this area

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.

Siltstone, shale, conglomerate, and limestone (Cretaceous) at surface, covers 0.3 % 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.

Younger rhyolitic flows and shallow intrusive rocks (Miocene) at surface, covers 0.3 % 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.

Foreland Basin Assemblage - Siltstone, limestone, shale, and sandstone (Lower Mississippian and Upper Devonian) at surface, covers 0.2 % 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.

Quartzite, siltstone, conglomerate, limestone, and dolomite (Late Proterozoic) at surface, covers 0.1 % of this area

Limestone, quartzite, dolomite, siltstone, conglomerate, and metamorphic rocks crop out in the southeastern, east-central, and northeastern regions of the State as part of Zqs. It forms the Proterozoic base of the continental margin stratigraphic section. This unit includes the Johnnie Formation in southern Nye and Lincoln Counties, schist in Elko County, the McCoy Creek Group metamorphic rocks in Elko and White Pine Counties, and the Wyman Formation in Esmeralda and southern Nye Counties. This rock is overlain by CZq. Its base is not exposed.

Basalt (Miocene) at surface, covers 0.1 % 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.

Sand dunes (Holocene and Pleistocene) at surface, covers < 0.1 % of this area

Unit is present in Clark, Humboldt, Lincoln, Churchill, Washoe, and Pershing Counties. There may be sand dunes in other counties that are not distinguished.

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.

Carbonate Shelf Sequence - Dolomite, limestone, and shale (Lower Silurian to Middle Ordovician) at surface, covers < 0.1 % 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.

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.

Basalt flows (Holocene to Pliocene) at surface, covers < 0.1 % of this area

Olivine basalt and basaltic and andesitic rocks. This unit is present in Clark, Elko, Mineral, Esmeralda, Humboldt, Lincoln, Lyon, Douglas, Carson, Nye, Washoe, and Lander Counties. It corresponds to the 1978 State map unit QTb.

Carbonate Shelf Sequence - Dolomite, sandstone, and limestone (Middle and Lower Devonian) at surface, covers < 0.1 % 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.

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.

Quaternary surficial deposits, undivided (Quaternary) at surface, covers < 0.1 % of this area

Unconsolidated to strongly consolidated alluvial and eolian deposits. This unit includes: coarse, poorly sorted alluvial fan and terrace deposits on middle and upper piedmonts and along large drainages; sand, silt and clay on alluvial plains and playas; and wind-blown sand deposits. (0-2 Ma)

Early Proterozoic metamorphic rocks (Early Proterozoic) at surface, covers < 0.1 % of this area

Undivided metasedimentary, metavolcanic, and gneissic rocks. (1600-1800 Ma)

Carbonate Shelf Sequence - Quartzite (Middle Ordovician) at surface, covers < 0.1 % 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).

Intermediate silicic ash flow tuff (lower Miocene and Oligocene) at surface, covers < 0.1 % 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.

Quaternary alluvium and marine deposits (Pleistocene to Holocene) at surface, covers < 0.1 % of this area

Alluvium, lake, playa, and terrace deposits; unconsolidated and semi-consolidated. Mostly nonmarine, but includes marine deposits near the coast.

Foreland Basin Assemblage - Shale, siltstone, sandstone, and conglomerate (Middle Pennsylvanian to Lower Mississippian) at surface, covers < 0.1 % 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.

Holocene river alluvium (Holocene) at surface, covers < 0.1 % of this area

Unconsolidated to weakly consolidated sand and gravel in river channels and sand, silt, and clay on floodplains. Also includes young terrace deposits fringing floodplains. (0-10 ka)

Pliocene to middle Miocene deposits (Middle Miocene to Pliocene) at surface, covers < 0.1 % of this area

Moderately to strongly consolidated conglomerate and sandstone deposited in basins during and after late Tertiary faulting. Includes lesser amounts of mudstone, siltstone, limestone, and gypsum. These deposits are generally light gray or tan. They commonly form high rounded hills and ridges in modern basins, and locally form prominent bluffs. Deposits of this unit are widely exposed in the dissected basins of southeastern and central Arizona. (2-16 Ma)

Permian to Pennsylvanian sedimentary rocks (Pennsylvanian to Permian) at surface, covers < 0.1 % of this area

Interbedded sandstone, shale, and limestone usually characterized by ledgy outcrops. Orange to reddish sandstone forms cliffs near Sedona. This unit includes Supai Group and Hermit Shale in northern Arizona and Naco Group in southern Arizona. It was deposited in coastal-plain to shallow-marine settings during time of variable and changing sea level. Rocks of this map unit in southern Arizona may be in part equivalent to Permian rocks of map unit P in central and northern Arizona. (280-310 Ma)

Early Pleistocene to latest Pliocene surficial deposits (Late Pliocene to Early Pleistocene) at surface, covers < 0.1 % of this area

Coarse relict alluvial fan deposits that form rounded ridges or flat, isolated surfaces that are moderately to deeply incised by streams. These deposits are generally topographically high and have undergone substantial erosion. Deposits are moderately to strongly consolidated, and commonly contain coarser grained sediment than younger deposits in the same area. (0.75-3 Ma)

Moenkopi Formation (Early and Middle(?) Triassic) at surface, covers < 0.1 % of this area

Dark red sandstone and mudstone; includes gypsum beds in northwestern Arizona; deposited on a low-relief coastal plain. (230-245 Ma)

Holocene surficial deposits (Holocene) at surface, covers < 0.1 % of this area

Unconsolidated deposits associated with modern fluvial systems. This unit consists primarily of fine-grained, well-sorted sediment on alluvial plains, but also includes gravelly channel, terrace, and alluvial fan deposits on middle and upper piedmonts. (0-10 ka)

Late to middle Miocene basaltic rocks (Middle to Late Miocene) at surface, covers < 0.1 % of this area

Mostly dark, mesa-forming basalt deposited as lava flows. Rocks of this unit are widely exposed south of Camp Verde (Hickey Formation basalts), in the Mohon Mountains north of Bagdad, "The Mesa" east of Parker, and at other scattered locations in western Arizona. Rocks of this unit were not tilted by middle-Tertiary normal faulting except in a narrow belt from north of Phoenix to the northwest corner of the state. (8-16 Ma)

Middle Miocene to Oligocene volcanic rocks (Oligocene to Middle Miocene) at surface, covers < 0.1 % of this area

Lava, tuff, fine-grained intrusive rock, and diverse pyroclastic rocks. These compositionally variable volcanic rocks include basalt, andesite, dacite, and rhyolite. Thick felsic volcanic sequences form prominent cliffs and range fronts in the Black (Mohave County), Superstition, Kofa, Eagletail, Galiuro, and Chiricahua Mountains. This unit includes regionally extensive ash-flow tuffs, such as the Peach Springs tuff of northwestern Arizona and the Apache Leap tuff east of Phoenix. Most volcanic rocks are 20-30 Ma in southeastern Arizona and 15 to 25 Ma in central and western Arizona, but this unit includes some late Eocene rocks near the New Mexico border in east-central Arizona. (11-38 Ma)

Middle Proterozoic granitic rocks (Middle Proterozoic) at surface, covers < 0.1 % of this area

Mostly porphyritic biotite granite with large microcline phenocrysts, with local fine-grained border phases and aplite. Associated pegmatite and quartz veins are rare. This unit forms large plutons, including the Oracle Granite, Ruin Granite, granite in the Pinnacle Peak - Carefree area northeast of Phoenix, and several bodies west of Prescott. (1400-1450 Ma)

Early Pleistocene to late Miocene basin deposits (Late Pliocene to Early Pleistocene) at surface, covers < 0.1 % of this area

Poorly sorted, variably consolidated gravel and sand that range widely in age. These sediments are generally light gray or tan. This unit is generally mapped in areas of deep late Cenozoic stream incision and landscape degradation where thin Quaternary deposits (map units Qy, Qm, Qo) discontinuously blanket older deposits (map units Tsy or Tsm) and the two cannot be differentiated at the scale of this map. (0.75-10 Ma)

Chinle Formation (Late Triassic) at surface, covers < 0.1 % of this area

Colorful mudstone, such as in the Painted Desert, and less abundant lenses of sandstone and conglomerate, deposited by a large river system. This unit typically is eroded into badlands topography and contains clays that are prone to shrinking and swelling. (210-230 Ma)

Older alluvium and alluvial fan deposits (Pleistocene and Pliocene) at surface, covers < 0.1 % 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.