Geologic units in Humboldt county, Nevada

Additional scientific data in this geographic area

Alluvium, undifferentiated (Holocene and Pleistocene) at surface, covers 22 % 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.

Older alluvium and alluvial fan deposits (Pleistocene and Pliocene) at surface, covers 14 % 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.

Playa, lake bed, and flood plain deposits (Holocene and Pleistocene) at surface, covers 11 % 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.

Younger silicic ash flow tuffs (Miocene) at surface, covers 8 % 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.

Younger rhyolitic flows and shallow intrusive rocks (Miocene) at surface, covers 8 % 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.

Andesite and basalt flows (Miocene and Oligocene) at surface, covers 5 % 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.

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.

Jungo Terrane - Turbiditic, fine-grained, terrigenous clastic rocks (Middle Jurassic to Upper Triassic) at surface, covers 4 % of this area

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.

Basalt, gravel, and tuffaceous sedimentary rocks (Miocene) at surface, covers 3 % of this area

Basalt flows, cinder and lava cones, gravel, and tuffaceous sedimentary rocks mostly in Elko and some in Humboldt Counties. This unit includes the Banbury Formation (Stewart and Carlson, 1978) and the Big Island Formation in Elko County and other unnamed units. It corresponds to unit Tbg from the 1978 State map.

Felsic phaneritic intrusive rocks (Cretaceous) at surface, covers 3 % 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.

Black Rock-Jackson Terrane - Basinal, island arc, carbonate, and volcanogenic rocks (Middle Jurassic to Mississippian) at surface, covers 2 % of this area

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).

Basalt (Miocene) at surface, covers 2 % 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.

Basalt flows (Holocene to Pliocene) at surface, covers 2 % 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.

Intermediate rhyolitic flows and shallow intrusive rocks (lower Miocene and Oligocene) at surface, covers 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.

Older gravels (Pleistocene and Pliocene) at surface, covers 1 % 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.

Golconda Terrane - Basinal, volcanogenic, terrigenous clastic, and minor carbonate rocks (Permian to Upper Devonian) at surface, covers 1 % of this area

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).

Intermediate andesite and intermediate flows and breccias (lower Miocene and Oligocene) at surface, covers 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.

Landslide deposits, colluvium, and talus (Holocene to Pliocene) at surface, covers 0.8 % 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.

Basin Assemblage - Shale, chert, quartzite, greenstone, and limestone (Devonian to Upper Cambrian) at surface, covers 0.8 % 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).

Dutch Flat Terrane - Feldspathic sandstone, shale, and turbiditic limestone (Upper Devonian) at surface, covers 0.7 % of this area

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.

Younger andesite and intermediate flows and breccias (Miocene) at surface, covers 0.7 % 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.

Felsic phaneritic intrusive rocks (Miocene (?) to Jurassic (?)) at surface, covers 0.6 % 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 silicic ash flow tuff (lower Miocene and Oligocene) at surface, covers 0.5 % 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.

Nolan Belt - Phyllite, schist, shale, thin-bedded limestone, chert, and siltstone (Cambrian) at surface, covers 0.4 % of this area

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.

Sand dunes (Holocene and Pleistocene) at surface, covers 0.3 % 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.

Mafic phaneritic intrusive rocks (Miocene (?) to Jurassic (?)) at surface, covers 0.3 % 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.

Crossbedded quartzite, siltstone, and phyllite (Lower Cambrian and latest Proterozoic) at surface, covers 0.3 % 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.

Siliciclastic Overlap Assemblage - Conglomerate, sandstone, siltstone, and limestone (Permian to Middle Pennsylvanian) at surface, covers 0.2 % 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.

Basalt, tuff, and breccia (lower Miocene and Oligocene) at surface, covers 0.2 % of this area

Basalt flows, basaltic tuff, tuff breccia, and andesitic rocks in Elko and Humboldt Counties. These rocks correspond to unit Tob on the 1978 State map.

Siltstone, shale, conglomerate, and limestone (Cretaceous) at surface, covers 0.1 % 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.

Older felsic phaneritic intrusive rocks (Jurassic) at surface, covers 0.1 % 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.

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.

Older andesite and intermediate flows and breccias (lower Oligocene to middle Eocene) at surface, covers < 0.1 % 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.

Golconda Terrane - Home Ranch subterrane - Limestone, basalt, chert, and volcaniclastic rocks (Mississippian) at surface, covers < 0.1 % of this area

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.

Glacial moraines (Holocene and Pleistocene) at surface, covers < 0.1 % of this area

Sediments are present in southern Washoe, northern Nye, Esmeralda, Elko, Humboldt, White Pine, and Lander Counties in high mountain ranges.

Slope Assemblage - Calcareous shale, siltstone, chert, quartzite, and greenstone (Devonian to Ordovician) at surface, covers < 0.1 % 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.

Phaneritic intrusive rocks (Jurassic) at surface, covers < 0.1 % of this area

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.

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.

Conglomerate and clastic rocks (Tertiary (?) and Cretaceous (?)) at surface, covers < 0.1 % 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.

Humboldt Assemblage - Shale, siltstone, sandstone, and minor carbonate (Lower Jurassic to Upper Triassic) at surface, covers < 0.1 % of this area

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).

Siliciclastic Overlap Assemblage - Sandstone, siltstone, limestone, conglomerate, and carbonaceous limestone (Permian) at surface, covers < 0.1 % 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).

Nolan Belt - Shale, chert, phyllite, quartzite, and limestone (Ordovician to Cambrian) at surface, covers < 0.1 % of this area

Rocks included in this unit have been mapped as the Broad Canyon Formation and Crane Canyon sequence in Lander County (Means, 1962), the Palmetto Formation in Esmeralda and Nye Counties (Ferguson and Cathcart, 1954), the Van Duzer Limestone in northern Elko County (Coats, 1971; Coats, Howard, and Greene, 1984; Decker, 1962; Ehman, 1985), and many other unnamed and locally named rocks. These rocks are strongly deformed, although the nature of the deformation is variable across the belt (Oldow, 1984b) and not well understood regionally. This unit is usually shown both in fault contact with adjacent units Ctd and CZq and gradational with them. Unit Pacl of the Siliciclastic overlap assemblage is shown unconformably deposited on this unit in Wall Canyon of the Toiyabe Range in Nye County. In a few cases, this unit is a stratigraphic continuation from Ctd, but in most places it represents undifferentiated rocks of both Ordovician and Cambrian age that overlap with Ctd, or whose age is poorly constrained.

Andesite, rhyolite, tuff, and volcaniclastic rocks (Middle and Lower Triassic) at surface, covers < 0.1 % of this area

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.

Humboldt Assemblage - Limestone, dolomite, shale, sandstone, and conglomerate (middle Upper to upper Lower Triassic (Carnian to Spathian)) at surface, covers < 0.1 % of this area

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).

Ultramafic rocks and serpentine (Triassic or upper Paleozoic) at surface, covers < 0.1 % of this area

Ultramafic rocks are present in very small belts or lenses in a few places across the State. In the Candelaria Hills along the Mineral-Esmeralda County boundary, they crop out in a thrust complex that overlies the Candelaria Formation (TRcl). At Willow Spring, at the southern end of the Toquima Range south of Manhattan, serpentine is exposed again adjacent to the Candelaria Formation and deformed lower Paleozoic rocks (OCtd). A few small outcrops also are present on the east side of the Toquima Range near Belmont adjacent to lower Paleozoic rocks. In the Toiyabe Range in Nye County, scattered outcrops of serpentine form a narrow north-south trending belt adjacent to the Golconda terrane (GC), deformed lower Paleozoic rocks (OCtd, Ctd), and the Siliciclastic overlap assemblage. An early Triassic conodont was recovered near the serpentine near Marysville Canyon (Poole and Wardlaw, 1978), although the Candelaria Formation does not show on the map in this area. All of these exposures of ultramafic rocks are in a similar relative tectonic position above deformed lower Paleozoic rocks and the Siliciclastic overlap assemblage, and below the structurally overlying Golconda terrane. A narrow belt of serpentine and gabbro is exposed at the northern edge of the Golconda terrane in the Hot Springs Range in Humboldt County. In this case, the ultramafic rock is structurally above the Golconda terrane, and beneath the overlying Mesozoic Jungo terrane (JO).

Olivine basalt (Miocene to Pliocene) at surface, covers < 0.1 % of this area

Thin, commonly open-textured (diktytaxitic), subophitic to intergranular olivine basalt flows, intercalated with and grades laterally through palagonite breccia and tuff into tuffaceous sedimentary rocks (unit Ts). In places includes flows of platy olivine andesite or basaltic andesite. Several potassium-argon ages ranging from about 4 to 7 Ma indicate unit is mostly of early Pliocene and late Miocene age. Includes Shumuray Ranch Basalt and Antelope Flat Basalt of Kittleman and others (1965), Grassy Mountain Basalt of Corcoran and others (1962), Drinkwater Basalt of Bowen and others (1963), basalt formerly assigned to Danforth Formation by Piper and others (1939) (see Walker, 1979), Hayes Butte Basalt of Hampton (1964), Pliocene and upper Miocene basalt flows capping and interstratified with the Madras (or Deschutes) Formation, and basalt flows interstratified in the Dalles Formation of Newcomb (1966; 1969)

Tuffaceous sedimentary rocks and tuff (Miocene to Pliocene) at surface, covers < 0.1 % of this area

Semiconsolidated to well-consolidated mostly lacustrine tuffaceous sandstone, siltstone, mudstone, concretionary claystone, conglomerate, pumicite, diatomite, air-fall and water-deposited vitric ash, palagonitic tuff and tuff breccia, and fluvial sandstone and conglomerate. Palagonitic tuff and breccia grade laterally into altered and unaltered basalt flows of unit Tob. In places includes layers of fluvial conglomerate and, in parts of the Deschutes-Umatilla Plateau, extensive deposits of fanglomerate composed mostly of Miocene basalt debris and silt. Also includes thin, welded and nonwelded ash-flow tuffs. Vertebrate and plant fossils indicate rocks of unit are mostly of Clarendonian and Hemphillian (late Miocene and Pliocene) age. Potassium-argon ages on interbedded basalt flows and ash-flow tuffs range from about 4 to 10 Ma. Includes the Drewsey Formation of Shotwell and others (1963); sedimentary parts of the Rattlesnake Formation of Brown and Thayer (1966); an interstratified ash-flow tuff has been radiometrically dated by potassium-argon methods at about 6.6 Ma (see Fiebelkorn and others, 1983); Bully Creek Formation of Kittleman and others (1967); Dalles Formation of Newcomb (1966, 1969); Shutler Formation of Hodge (1932), McKay beds of Hogenson (1964) and Newcomb (1966) (see also Shotwell, 1956); Kern Basin Formation of Corcoran and others (1962); Rome beds of Baldwin (1976); parts of the (now obsolete) Danforth Formation of Piper and others (1939), Idaho Group of Malde and Powers (1962), Thousand Creek Beds of Merriam (1910); the Madras (or Deschutes) Formation, the "Simtustus formation" of Smith (1984), and the Yonna Formation (Newcomb, 1958). In areas west of Cascade crest, includes the Sandy River Mudstone and the Troutdale Formation of Trimble (1963) and the lower Pliocene Helvetia Formation of Schlicker and Deacon (1967)

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.

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

Sand, gravel, and silt forming flood plains and filling channels of present streams. In places includes talus and slope wash. Locally includes soils containing abundant organic material, and thin peat beds

Basalt and andesite (Miocene) at surface, covers < 0.1 % of this area

Lava flows and breccia of aphyric and plagioclase porphyritic basalt and aphyric andesite; locally includes flow breccia, peperite, some palagonite tuff and breccia, and minor silicic ash-flow tuff and interbeds of tuffaceous sedimentary rocks. In Basin and Range and Owyhee Upland provinces unit grades upward into more silicic, andesitic, and quartz latitic flows and flow breccia, as well as some interbedded tuffs and ash-flow tuffs; also in this region includes aphyric and highly porphyritic, plagioclase-rich basalt. Interfingers with and grades laterally into units Tit and Tts. Commonly contains montmorillonite clays, zeolites, calcite, and secondary silica minerals as alteration products on fractures and in pore spaces. Age, mostly middle Miocene, but includes some rocks of early Miocene age based on vertebrate fossils from related sedimentary units and on potassium-argon ages that range from about 13 Ma to about 19 Ma; most isotopic ages are about 13 to 16 Ma. Includes Steens Basalt (Steens Mountain Basalt of Fuller, 1931) Owyhee Basalt of Corcoran and others (1962) and Kittleman and others (1967), Hunter Creek Basalt and "unnamed igneous complex" of Kittleman and others (1965, 1967), and flows of Prineville chemical type (Uppuluri, 1974; Swanson and others, 1979), which previously were considered part of the Columbia River Basalt Group (Swanson, 1969a)

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.

Intrusive rocks (Jurassic to Cretaceous) at surface, covers < 0.1 % of this area

Hornblende and biotite quartz diorite (tonalite), trondhjemite, granodiorite, and small amounts of norite, in batholithic masses and large dikelike bodies. Includes Bald Mountain Tonalite and Anthony Lake Granodiorite of Taubeneck (1957), tonalite and trondhjemite of Wallowa batholith and Cornucopia stock (Taubeneck, 1964; Nolf, 1966), quartz diorite intrusion in the Snake River area (Morrison, 1963), quartz diorite and minor other intrusive rocks in the Caviness quadrangle (Wolff, 1965), quartz diorite northeast of John Day and southeast of Ironside Mountain (Thayer and Brown, 1964), quartz diorite in the Sparta and Durkee quadrangles (Prostka, 1962; 1967), and granodiorite and related rocks of the Pueblo Mountains (Roback and others, 1987). Rubidium-strontium and potassium-argon ages indicate an age range from about 94 to 160 Ma (Taubeneck, 1963; Thayer and Brown, 1964; Armstrong and others, 1976)

Rhyolite and dacite (Miocene to Pliocene) at surface, covers < 0.1 % of this area

Ash-flow tuff, lava flows, pumice-lapilli tuff, coarse pumicite, flow breccia, and domal complexes of rhyolitic, rhyodacitic, and dacitic composition; in places includes peralkaline rhyolite and some andesite and andesite breccia. Locally porphyritic with phenocrysts of alkali feldspar, plagioclase, and minor augite, ferro-hedenbergite, hornblende, hypersthene, or biotite. Commonly flow banded; locally glassy. Many of the ash--flow tuffs exhibit flow features and only obscure vitro-clastic textures. In places includes interlayers of silicic volcaniclastic rocks and tuffaceous sedimentary rocks. Includes rhyolite at Owyhee Dam, Jump Creek Rhyolite, and Littlefield Rhyolite, all of Kittleman and others (1965); Dooley Rhyolite Breccia of Gilluly (1937), radiometrically dated at 14.7 ñ 0.4 Ma by potassium-argon methods (Fiebelkorn and others, 1983); resurgent domal masses in McDermitt caldera area; and extensive unnamed flows and ash-flow tuffs in the central and southern part of the Owyhee Upland. Also includes isolated masses of dacitic and rhyodacitic flows, breccia, and ash-flow tuff along eastern slope of Cascade Range that are lapped by flows and sediments of the Madras (or Deschutes) Formation. Potassium-argon ages on rocks in unit from southeast Oregon range from about 13 to 16 Ma; lenses of interbedded tuffaceous sedimentary rocks locally contain a Miocene (Barstovian) vertebrate fauna

Welded tuffs and tuffaceous sedimentary rocks (Middle to Late Miocene) at surface, covers < 0.1 % of this area

Partly to densely welded vitric and vitric-crystal tuff of soda-rhyolitic, rhyolitic, and rhyodacitic composition that interfingers with and grades laterally into unit Tit. Includes some nonwelded ash-flow tuff and tuffaceous sedimentary rocks. Potassium-argon ages range from about 13 to 16 Ma. In Harney and Malheur Counties, it commonly overlies unit Tmb. Includes Dinner Creek Welded Tuff of Haddock (1965; 1967) and middle and upper Miocene ash-flow tuffs of Rytuba and others (1982; 1983a, b), widely exposed in the Trout Creek Mountains and adjacent areas, erupted from the McDermitt caldera complex, west and southwest of McDermitt, Nevada-Oregon, the White Horse caldera, northwest of McDermitt, and several other vent areas

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

Clay, silt, sand, and some evaporites

Olivine basalt (Miocene to Pliocene) at surface, covers < 0.1 % of this area

Thin, commonly open-textured (diktytaxitic), subophitic to intergranular olivine basalt flows, intercalated with and grades laterally through palagonite breccia and tuff into tuffaceous sedimentary rocks (unit Ts). In places includes flows of platy olivine andesite or basaltic andesite. Several potassium-argon ages ranging from about 4 to 7 Ma indicate unit is mostly of early Pliocene and late Miocene age. Includes Shumuray Ranch Basalt and Antelope Flat Basalt of Kittleman and others (1965), Grassy Mountain Basalt of Corcoran and others (1962), Drinkwater Basalt of Bowen and others (1963), basalt formerly assigned to Danforth Formation by Piper and others (1939) (see Walker, 1979), Hayes Butte Basalt of Hampton (1964), Pliocene and upper Miocene basalt flows capping and interstratified with the Madras (or Deschutes) Formation, and basalt flows interstratified in the Dalles Formation of Newcomb (1966; 1969)

Dune sand (Holocene) at surface, covers < 0.1 % of this area

Large areas of windblown sand composed of rock-forming minerals, mostly feldspar and small amounts of quartz, and, in southeastern Oregon, also pumice

Basalt (Pliocene and Miocene). (Pliocene and Miocene) at surface, covers < 0.1 % of this area

Flows and cinder cones of olivine tholeiite basalt, and shallow basalt intrusives (~15-3 Ma); includes basalt in Owyhee County and southwest of Twin Falls, basalt of Weiser (basalt to andesite), basalt of Cuddy Mountain (alkali basalt and picro-basalt) north of Cambridge, basalt in Mount Bennett Hills north of Gooding, andesite at Square Mountain near Magic Reservoir, and Cub River diabase sill east of Preston. Includes gabbro at depth in cross section D-D’-D”. (Quaternary to Eocene Continental Volcanic and Intrusive Rocks).

Terrace, pediment, and lag gravels (Pleistocene to Holocene) at surface, covers < 0.1 % of this area

Unconsolidated deposits of gravel, cobbles, and boulders intermixed and locally interlayered with clay, silt, and sand. Mostly on terraces and pediments above present flood plains. Includes older alluvium of Smith and others (1982) in the Klamath Mountains and both high- and low-level terraces along Oregon coast. Includes dissected alluvial fan deposits northeast of Lebanon, and Linn and Leffler Gravels of Allison and Felts (1956)