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Geologic units in Crook county, Oregon

Clastic rocks and andesite flows (lower Oligocene?, Eocene, and Paleocene?) (Paleocene to Early Oligocene) at surface, covers 27 % of this area

Mostly andesitic lava flows, domes, breccia, and small intrusive masses and lesser basaltic to rhyolitic rocks; interlayered saprolite, bedded volcaniclastic and epiclastic mudstone, claystone, siltstone, sandstone, conglomerate, and mudflow (lahar) deposits. Mostly consists of Clarno Formation of central Oregon and unnamed rocks of Basin and Range Province in south-central Oregon. Fossil plants and vertebrates in these rocks are Eocene in age. Andesite and basalt lava flows are typically slightly altered; most glass is devitrified and altered to clay minerals, zeolites, and secondary feldspar. Reliable K-Ar ages of rocks from unit range from about 54 Ma to about 37 Ma (Evernden and James, 1964; Fiebelkorn and others, 1983). A number of K-Ar ages on rocks shown on source maps as part of the Clarno Formation are in the range of about 36 to 19 Ma (Fiebelkorn and others, 1983). Although these rocks are lithologically similar to, but generally less altered than, rocks of the Clarno Formation, they are coeval with the John Day Formation. Most of these enigmatic rocks of Oligocene and early Miocene age are included in unit Tas. At base of unit in Blue Mountains Province, locally includes quartzose and feldspathic sandstone, siltstone, and shale largely of granitic or metamorphic provenance; fossil plants in these sedimentary rocks indicate an early Eocene or Paleocene age

Lithology: andesite; basalt; rhyolite

Picture Gorge Basalt (middle and lower Miocene) (Early to Middle Miocene) at surface, covers 18 % of this area

Flows of aphyric and plagioclase porphyritic flood basalt. Potassium-argon ages mostly 15.0 to 16.4 Ma (Watkins and Baksi, 1974; Fiebelkorn and others, 1983)

Lithology: basalt

John Day Formation of east-central Oregon (lower Miocene, Oligocene, and uppermost Eocene?) (Late Eocene to Early Miocene) at surface, covers 7 % of this area

Predominantly tuffaceous facies of Clarno Formation (lower Oligocene? and Eocene) (Eocene to Early Oligocene) at surface, covers 6 % of this area

Mapped separately by Swanson (1969a) in the Ochoco and Maury Mountains of the Blue Mountains Province

Silicic ash-flow tuff (lower Pliocene and upper Miocene) (Late Miocene to Early Pliocene) at surface, covers 6 % of this area

Ash-flow tuff and associated pumiceous air-fall tuff mostly of rhyolitic and rhyodacitic composition; includes minor tuffaceous sedimentary rocks. Grades laterally through less-densely welded tuff to nonwelded ash-flow tuff and interlayered tuffaceous sediments of unit Ts. Potassium-argon ages (Greene and others, 1972; Parker and Armstrong, 1972; Walker, 1979; McKee and others, 1976) on several different ash-flow tuffs included in unit range from about 4 to 10 Ma; although most ages are in the range of 6 to 9 Ma. Includes the Devine Canyon, Prater Creek, and Rattlesnake Ash-flow Tuffs (Walker, 1979), originally considered a part of the (now obsolete) Danforth Formation of Piper and others (1939), and the Rattlesnake Formation and the volcanic and fluvial deposits (undivided) and marginal facies of the Columbia River Group (undivided) of Brown and Thayer (1966). Also includes the welded soda--rhyolite tuff breccia of Dickinson and Vigrass (1965) in the Suplee-Izee area, the upper Miocene or lower Pliocene welded tuff of Prostka (1962; 1967) in the Baker area. "Welded ash-flow tuff" of Swanson (1969a) and the Pliocene Peyerl Tuff (Hampton, 1964) west of Fort Rock Valley, dated at about 4.5 Ma (McKee and others, 1976)

Lithology: rhyolite; dacite

Alluvial deposits (Holocene) (Holocene) at surface, covers 5 % 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

Lithology: sand; gravel; silt; peat

Tuffaceous sedimentary rocks, tuffs, pumicites, and silicic flows (Miocene) (Miocene) at surface, covers 3 % of this area

Moderately well indurated lacustrine and fluvial (flood-plain) deposits of tuff, pumicite, palagonite tuff, and lesser siltstone, arkosic sandstone, and pebble and cobble conglomerate. Locally contains some lignite beds. Former glass in silicic vitroclastic debris commonly crystallized and altered to secondary silica minerals, alkali feldspar, zeolites, and clay minerals. Contains some welded and nonwelded ash-flow tuffs, and minor rhyolite flows. Widespread and abundant vertebrate fossils and minor plant fossils indicate that most of unit is of middle Miocene (Barstovian) age; parts of unit between Goose Lake and Warner Valley may include rocks of early Miocene age. Locally interlayered with and locally overlies basalt and andesite flows of unit Tmb. Overlies and locally interfingers with Picture Gorge Basalt (Thayer and Brown, 1966) and with Miocene basalt south of Prineville. Includes Mascall Formation of Merriam (1901), Sucker (Succor) Creek Formation of Corcoran and others (1962) and Kittleman and others (1967), Drip Spring Formation of Kittleman and others (1965, 1967), Trout Creek Formation of Smith (1926), and "rocks of Miocene age" of Malde and Powers (1962) in the southern Owyhee Upland province. In southeast Oregon, some of these rocks represent caldera and moat-fill deposits

Olivine basalt (Pliocene and Miocene) (Miocene to Pliocene) at surface, covers 3 % 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)

Lithology: basalt; andesite

Basalt (Pleistocene and Pliocene) (Pliocene to Pleistocene) at surface, covers 3 % of this area

Thin flows and minor flow breccia of open--textured (diktytaxitic) olivine basalt in southeastern part of map area. Locally contains thin interbeds of sedimentary rocks. Grades laterally through palagonite tuff and breccia into sedimentary rocks (unit QTs)

Lithology: basalt; tuff

Fanglomerate (Holocene? and Pleistocene) (Pleistocene to Holocene) at surface, covers 3 % of this area

Poorly sorted and poorly stratified alluvial fan debris, slope wash, colluvium, and talus; composed mostly of silt and fragments of basalt, basaltic andesite, and andesite. In places includes small areas of pediment gravels and colluvium

Tuffaceous sedimentary rocks and tuff (Pliocene and Miocene) (Miocene to Pliocene) at surface, covers 3 % 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)

Rhyolite and dacite domes and flows and small hypabyssal intrusive bodies (Miocene to upper Eocene?) (Late Eocene to Miocene) at surface, covers 3 % of this area

Mostly light-gray to red, dense, flow-banded, nonporphyritic and porphyritic rhyolite and dacite in nested domes, small intrusive bodies, and related flows. Includes some near-vent breccias, pumice-lapilli tuffs, and coarse pumicites. Commonly associated with mercury mineralization. Includes several small hypabyssal intrusions of diorite, granodiorite, and quartz monzonite exposed in Paisley Hills of Lake County (Muntzert, 1969; Muntzert and Field, 1968). In many places represents vents for lava flows and tuff of unit Tsf

Lithology: rhyolite; dacite; tuff

Landslide and debris-flow deposits (Holocene and Pleistocene) (Pleistocene to Holocene) at surface, covers 2 % of this area

Unstratified mixtures of fragments of adjacent bedrock. Locally includes slope wash and colluvium. Largest slides and debris flows occur where thick sections of basalt and andesite flows overlie clayey tuffaceous rocks. May include some deposits of late Pliocene age

Lithology: landslide

Lacustrine and fluvial sedimentary rocks (Pleistocene) (Pleistocene) at surface, covers 2 % of this area

Unconsolidated to semiconsolidated lacustrine clay, silt, sand, and gravel; in places includes mudflow and fluvial deposits and discontinuous layers of peat. Includes older alluvium and related deposits of Piper (1942), Willamette Silt (Allison, 1953; Wells and Peck, 1961), alluvial silt, sand, and gravel that form terrace deposits of Wells and others (1983), and Gresham and Estacada Formations of Trimble (1963). Includes deltaic gravel and sand and gravel bars, in pluvial lake basins in southeastern part of map area. In Rome Basin, includes discontinuous layers of poorly consolidated conglomerate characterized by well-rounded, commonly polished pebbles of chert and pebbles and cobbles of quartzite. In places contains mollusks or vertebrate fossils indicating Pleistocene age; mostly deposits of late Pleistocene age, but locally includes some deposits of early Holocene age. Includes Touchet Beds of Flint (1938), deposits of valley terraces of Newcomb (1965), and, in southeast Oregon, basin-filling deposits that incorporate Mazama ash deposits (Qma, Qmp) in the youngest layers

Lithology: clay or mud; silt; sand; gravel

Basalt and andesite (Miocene) (Miocene) at surface, covers 2 % 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)

Basalt and basaltic andesite (Holocene and Pleistocene) (Pleistocene to Holocene) at surface, covers 1 % of this area

Thin flows of aphyric and porphyritic basalt and basaltic andesite, and open-textured (dikytaxitic), generally nonporphyritic, subophitic olivine basalt that commonly is highly feldspathic. Also includes some dissected intracanyon flows of porphyritic basalt and related vent complexes. Pressure ridges and tumuli on upper surfaces well preserved. Occurs principally along crest of Cascade Range; also in areas near and east of Newberry volcano, along southeast margin of Harney Basin, and in Rome Basin. Older than Mazama ash deposits (Qma, Qmp; approximately 6,800 yr old; 14C)

Lithology: basalt

Mafic and intermediate vent rocks (Pliocene? and Miocene) (Miocene to Pliocene) at surface, covers 0.7 % of this area

Basaltic and andesitic agglomerate, breccia, scoria, cinders, flows, and intrusive masses forming lava cones and small shields

Lithology: basalt; andesite

Sedimentary rocks, partly metamorphosed (Paleozoic) (Paleozoic) at surface, covers 0.6 % of this area

Well-bedded limestone, fossiliferous cherty limestone, calcareous and carbonaceous sandstone, chert grit, argillite, and some conglomerate. In places foliated and metamorphosed. Includes fault slivers of Devonian rocks (Kleweno and Jeffords, 1961), Coffee Creek Formation of Mississippian age, Spotted Ridge Formation of Pennsylvanian age (Merriam and Berthiaume, 1943; Mamay and Read, 1956), and Paleozoic sedimentary and metamorphosed sedimentary and volcanic rocks, including sericite schist, amphibolite, and hornblende-garnet schist of Brown and Thayer (1966)

Lithology: limestone; sandstone; chert

Basalt (upper and middle Miocene) (Middle to Late Miocene) at surface, covers 0.6 % of this area

Basalt flows, flow breccia, and basaltic peperite; minor andesite flows; some interbeds of tuff and tuffaceous sedimentary rocks. Basalt is aphyric to moderately porphyritic with phenocrysts of plagioclase and olivine and exhibits both subophitic and diktytaxitic textures. Includes Picture Rock Basalt of Hampton (1964), radiometrically dated by potassium-argon methods as middle(?) and late Miocene in age (see Fiebelkorn and others, 1983), flows of Deer Butte Formation of Kittleman and others (1967), and extensive unnamed flow sequences in the Basin-Range and Owyhee Upland Provinces of southern Lake, Harney, and Malheur Counties that are younger than Steens Basalt, dated at about 15 Ma (Baksi and others, 1967) and the Owyhee Basalt, dated at about 14 Ma (Bottomley and York, 1976; see also Fiebelkorn and others, 1983), and older than 7 or 8 Ma. Partly coeval with the Saddle Mountains Basalt of the Columbia River Basalt Group (Swanson and others, 1979)

Rhyolitic tuff, tuffaceous sedimentary rocks, and lava flows (lower Miocene, Oligocene, and uppermost Eocene?) (Late Eocene to Early Miocene) at surface, covers 0.5 % of this area

Rhyolitic to dacitic varicolored bedded tuff, lapilli tuff, and fine- to medium-grained tuffaceous sedimentary rocks with interstratified welded and nonwelded ash-flow tuff and interbedded basalt and andesite flows. Also includes minor rhyolite and dacite flows and domes. Glass in tuff and tuffaceous sedimentary rocks is commonly altered to zeolites, clay minerals, and small amounts of opal, chalcedony, orthoclase, and calcite. Fossil plants and vertebrates indicate an Oligocene and Miocene age. Locally a late Hemingfordian age indicated by mammalian fauna (Woodburn and Robinson, 1977). May include some rocks of middle Miocene age in the area west and northwest of Lakeview. Potassium-argon ages on rocks from unit range from about 36 Ma (Swanson and Robinson, 1968) to about 20 Ma. Includes Pike Creek Formation of Walker and Repenning (1965), originally identified as Pike Creek Volcanic Series by Fuller (1931), and unnamed volcanic and volcaniclastic rocks of southern Lake County, some of which have been correlated with the Miocene and Oligocene Cedarville Formation of northeastern California

Lithology: tuff; basalt; andesite

Mafic vent deposits (Pleistocene, Pliocene, and Miocene?) (Miocene to Pleistocene) at surface, covers 0.5 % of this area

Mostly in small stratovolcanoes or shield volcanoes and lava cones of basalt and andesite. Includes agglomerate, breccia, scoria, cinders, ash, restricted flows, and small basaltic intrusive bodies. Transitional into pyroclastic rocks of cinder cones (QTp). May also include rocks of late Miocene(?) age

Lithology: basalt; andesite

Andesite and dacite and sedimentary rocks (Miocene? and Oligocene) (Oligocene to Miocene) at surface, covers 0.4 % of this area

Lava flows, breccia, volcaniclastic and epiclastic rocks mostly of andesitic and dacitic composition; includes minor amounts of altered basaltic rocks. Joint surfaces and cavities commonly lined with hematite or montmorillonite clay, secondary silica minerals, zeolites, celadonite, or calcite. Andesite and dacite typically have plagioclase, hornblende, and clinopyroxene phenocrysts; some flows aphyric. Platy flow-jointing common. Age, mostly Oligocene; may include some rocks of early Miocene age. As shown, may include some rocks older than Oligocene, correlative with upper parts of unit Tea. One potassium-argon age of about 28 Ma on porphyritic hornblende andesite from Sheep Creek, southwest corner of Union County, indicates in part coeval with unit Tsf

Lithology: andesite; dacite; clastic

Sedimentary and volcanic rocks (Upper? Triassic) (Late Triassic) at surface, covers 0.4 % of this area

Undifferentiated marine sedimentary rocks and volcanic rocks, locally slightly to moderately metamorphosed, of Late(?) Triassic age, exposed principally in Hells Canyon of Snake River, locally in tributary canyons of Imnaha River, and in several areas marginal to the Wallowa Mountains

Columbia River Basalt Group and related flows (Miocene) (Miocene) at surface, covers 0.4 % of this area

Subaerial basalt and minor andesite lava flows and flow breccia; submarine palagonitic tuff and pillow complexes of the Columbia River Basalt Group (Swanson and others, 1979); locally includes invasive basalt flows. Flows locally grade laterally into subaqueous pillow-palagonite complexes and bedded palagonitic tuff and breccia. In places includes tuffaceous sedimentary interbeds. Joints commonly coated with nontronite and other clayey alteration products. Occurs principally in the Willamette Valley from Salem north to the Columbia River, and in the northern Coast Range. Unit includes correlative Cape Foulweather and Depoe Bay Basalts in the Coast Range (Snavely and others, 1973, 1976a, 1976b; Swanson and others, 1979; Wells and others, 1983). In Eastern Oregon, occurs principally in Deschutes-Umatilla Plateau and in the Blue Mountains. K-Ar ages range from about 6 to about 16.5 Ma (McKee and others, 1977; Swanson and others, 1979; Sutter, 1978; Lux, 1982). Locally separated into Tcs, Tcw, Tcg, Tcp, and Tci

Lithology: basalt; andesite

Open Water (Holocene) at surface, covers 0.3 % of this area

water

Lithology: water

Sedimentary and volcanic rocks, partly metamorphosed (Triassic and Permian) (Permian to Triassic) at surface, covers 0.3 % of this area

Complexly folded, locally highly foliated and recrystallized undifferentiated sedimentary and volcanic rocks that in places are lithologically similar to Jurassic and Triassic rocks in the Aldrich Mountains of the Blue Mountains province and in other places resemble Elkhorn Ridge Argillite, Clover Creek Greenstone, and Burnt River Schist (Gilluly, 1937). Age probably mostly Late Permian to Late Triassic, but, as shown, may include some Early Jurassic rocks

Mafic vent and intrusive rocks (Eocene?) (Eocene) at surface, covers 0.2 % of this area

Mostly plugs, dikes, and irregular intrusive bodies of basaltic andesite and porphyritic hornblende or pyroxene andesite. Represents some of vents for unit Tca and possibly for unit Tas

Lithology: andesite; basalt

Sedimentary rocks (Pleistocene and Pliocene) (Pliocene to Pleistocene) at surface, covers 0.2 % of this area

Semiconsolidated lacustrine and fluvial ashy and palagonitic sedimentary rocks, mostly tuffaceous sandstone and siltstone; locally contains abundant palagonitized basaltic debris and some pebble conglomerate. Includes alluvial gravel and mudflow deposits of Walters Hill and Springwater Formations (Trimble, 1963). In places, grades laterally through palagonite tuff and breccia into basalt flows

Sedimentary rocks (Cretaceous) (Cretaceous) at surface, covers 0.1 % of this area

Marine graywacke, subgraywacke, conglomerate, and shale. Pebbles and cobbles in conglomerate are well rounded volcanic and metavolcanic rocks, low-grade metasedimentary rocks, quartzite, chert, and minor silicic and intermediate plutonic rocks. Shales are gray to black and are fissile to blocky. Sandstones commonly display graded bedding; conglomerate beds are commonly thick and poorly bedded. Shales, near Mitchell, have yielded latest (Early Cretaceous (Albian) fossils; some earliest Late Cretaceous (Cenomanian) fossils occur in beds southeast of Mitchell (D.L. Jones, oral Commun., 1972). Includes Hudspeth and Gable Creek Formations (OR049), Bernard Formation (OR028), and Cretaceous sedimentary rocks (OR008)

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

Black and gray mudstone, shale, siltstone, graywacke, andesitic to dacitic water-laid tuff, porcelaneous tuff, and minor interlayers and lenses of limestone and fine-grained sediments metamorphosed to phyllite or slate. Locally includes some felsite, andesite and basalt flows, breccia, and agglomerate. Marine invertebrate fauna indicates age range from Early Jurassic (Hettangian) to early Late Jurassic (Oxfordian). In northeast Oregon, includes Graylock Formation, Mowich Group, and Shaw Member (of Snowshoe Formation) of Dickinson and Vigrass (1965); Keller Creek Shale of Brown and Thayer (1966); Weberg, Warm Springs, Snowshoe, Trowbridge, and Lonesome Formations of Lupher (1941); the Coon Hollow Formation of Morrison (1964); and unnamed Jurassic rocks near Juniper Mountain in northern Malheur County (Wagner and others, 1963)

Lithology: shale; siltstone; graywacke

Pyroclastic ejecta of basaltic and andesitic cinder cones (Holocene, Pleistocene, Pliocene, and Miocene?) (Miocene to Holocene) at surface, covers < 0.1 % of this area

Mostly unconsolidated, oxidized, fine to coarse, scoriaceous cinders, bombs, and agglutinate deposited in subaerial environment

Lithology: basalt; andesite

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

Unconsolidated to poorly consolidated, poorly sorted gravels and bouldery soil above modern stream channels. In Cascade Range, clasts mostly basalt and andesite. Includes some glacial outwash deposits. In Eastern Oregon, commonly cemented by caliche

Silicic vent rocks (Pliocene, Miocene, Oligocene, and Eocene?) (Eocene to Pliocene) at surface, covers < 0.1 % of this area

Plugs and domal complexes of rhyolitic, rhyodacitic, and dacitic composition; includes related near-vent flows, flow breccia, and deposits of obsidian, perlite, and pumice. Locally includes resurgent domes related to caldera complexes. In southeast Oregon many domal complexes younger than 11 Ma exhibit a well-defined southeast to northwest age progression (Walker, 1974; MacLeod and others, 1976) from about 11 Ma to less than 1 Ma

Lithology: rhyolite; latite; dacite

Welded tuffs and tuffaceous sedimentary rocks (upper? and middle Miocene) (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

Lithology: dacite; rhyolite

Subaqueous pyroclastic ejecta of basaltic cinder cones (lower Pliocene? and Miocene?) (Miocene to Early Pliocene) at surface, covers < 0.1 % of this area

Deposits of bombs, breccia, and mafic to intermediate tuff; occurs as palagonitic tuff and breccia cones, rings, and ridges. In places interbedded with lacustrine sedimentary rocks

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

Clay, silt, sand, and some evaporites

Pyroclastic ejecta of basaltic cinder cones (lower Pliocene? and Miocene?) (Miocene to Early Pliocene) at surface, covers < 0.1 % of this area

Mostly unconsolidated, oxidized, fine to course, scoriaceous cinders, bombs, and agglutinate deposited in subaerial environment

Lithology: basalt; andesite

Rhyolite and dacite (Pliocene? and Miocene) (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 +/- 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

Lithology: rhyolite; dacite; andesite

Terrace, pediment, and lag gravels (Holocene and Pleistocene) (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)

Sedimentary rocks, partly metamorphosed (Triassic and Paleozoic) (Paleozoic to Triassic) at surface, covers < 0.1 % of this area

Poorly bedded argillite, chert, phyllite, phyllitic quartzite, calc-phyllite, impure limestone, and marble. In places rocks are strongly foliated. Sparse fossils (Fusilina, corals, and crinoids) indicate that the unit includes rocks of Leonardian, Ochoan, and Late Triassic age (OR084). Includes Elkhorn Ridge Argillite (OR035), Mesozoic and Paleozoic sedimentary rocks of Brown and Thayer (OR008), and the Permian Coyote Butte Formation (OR085). In Baker County includes "sedimentary and volcanic rocks" (MzPza) of Brooks and others (OR039) and metamorphosed sedimentary and minor volcaniclastic rocks containing mineral assemblages indicative of quartz-albite-muscovite-chlorite subfacies and quartz-albite-epidote-biotite subfacies of the greenschist facies. In Jefferson and Wasco Counties north of Prineville, includes "phyllite and sedimentary rocks " of Swanson (OR031). Includes part of the Burnt River Schist (OR035; OR081) and volcaniclastic facies of several metavolcanic units of Permian and Late Triassic age. Not on State map (OR001) in area of La Grande 100K quadrangle, butmapped in OR291 as Elkhorn Ridge Argillite (Triassic Permian, Pennsylvannian, and Devonian?)

Basalt and basaltic andesite (Pleistocene and Pliocene) (Pliocene to Pleistocene) at surface, covers < 0.1 % of this area

Flows, flow breccia, and pyroclastic deposits. Flows are aphanitic to finely crystalline, commonly diktytaxitic, and aphyric to porphyritic. Textures are mostly intergranular grading to intersertal; some andesite flows are finely trachytic and a few basalt flows are subophitic. Phenocrysts, mostly unaltered, include bytownite and labradorite, olivine, calcic augite, and hypersthene. Flows and breccia form shields, lava cones, and valley fill; in places greatly dissected and modified by fluvial erosion. Includes Boring Lava of Trimble (1963) and Hampton (1972) and Battle Ax Basalts of Thayer (1936). Potassium-argon ages from this unit range from about 1.2 to 3.9 Ma; in places difficult to distinguish from youngest flows of unit Trb

Lithology: basalt; andesite

Ridge-capping basalt and basaltic andesite (Pliocene and upper Miocene) (Late Miocene to Pliocene) at surface, covers < 0.1 % of this area

Flows and flow breccia of basaltic andesite and lesser diktytaxitic to intergranular olivine basalt. Includes some dense, aphyric flows, commonly with either cryptocrystalline or pilotaxitic to trachytic texture, and porphyritic flows with phenocrysts and glomerocrysts of olivine, hypersthene, and labradorite. A few flows contain both hypersthene and calcic augite phenocrysts. Olivine mostly fresh or slightly altered to iddingsite in flows high in section; flows low in section show some alteration to clays (nontronite and saponite), secondary silica minerals, and calcite; pinkish-brown glass in some flows unaltered. Locally includes some andesite and dacite. Some flows of this unit are lithologically similar to flow rocks of the High Cascade volcanic sequence and some are more like flows that in the past have been mapped as part of the Sardine Formation (Peck and others, 1964) and Elk Lake Formation of McBirney and others (1974) and Sutter (1978). Potassium-argon ages of rocks from this unit range from about 4 to 8 or 9 Ma. Includes some rocks formerly mapped as Rhododendron Formation by Peck and others (1964)

Mafic vent complexes (Pleistocene, Pliocene, and Miocene?) (Late Miocene to Pleistocene) at surface, covers < 0.1 % of this area

Plugs, dikes, and related near-vent flows, breccia, cinders, and agglutinate of basalt, basaltic andesite, and andesite; commonly in the form of either little-modified lava cones or partly eroded piles of reddish, iron-stained thin flows and fragmental ejecta cut by mafic intrusions. May also include rocks of late Miocene(?) age

Lithology: basalt; andesite

Youngest basalt and basaltic andesite (Holocene) (Holocene) at surface, covers < 0.1 % of this area

Little-modified flows and associated breccia of basaltic andesite and some basalt in both central part of Cascade Range and on slopes of Newberry Volcano. Relations to Mazama pumice deposits indicate most of these rocks are less than 6,800 yr old (14C); isotopic ages on flows range from about 1,000 to 6,000 yr B.P. (14C)

Lithology: basalt; andesite