Flows of dark-gray to black, aphyric tholeiitic basalt, including both high- and low-Mg chemical types (Swanson and others, 1979). Potassium-argon ages mostly in the range of 15 to 17 Ma (Lux, 1982; Watkins and Baksi, 1974; Fiebelkorn and others, 1983)
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)
Unconsolidated, poorly sorted silt, sand, and gravel. Includes lacustrine deposits west of Columbia River Gorge (Trimble, 1963). Mostly in northern Morrow and Umatilla Counties where unit represents deposits of swollen late Pleistocene Columbia River (Hogenson, 1964)
Flows of gray to dark-gray, medium-grained, commonly plagioclase porphyritic basalt of Frenchman Springs petrochemical type (Wright and others, 1973). Generally exhibits blocky to platy jointing. Potassium-argon ages mostly about 15 Ma (Lux, 1982; Fiebelkorn and others, 1983)
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
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)
John Day Formation of east-central Oregon (lower Miocene, Oligocene, and uppermost Eocene?)
Petrographically diverse flows of basalt erupted between about 13.5 and 6 Ma (McKee and others, 1977; Swanson and others, 1979)
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
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
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
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)
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
Mostly unconsolidated silt, sand, and gravel valley fill with some clay; includes low-level terrace, marsh, peat, artificial fill, and glacial deposits locally.
Dark-gray to black, dense aphanitic basalt flows; commonly columnar jointed, less commonly irregularly and platy jointed; some flows vesicular, grading to scoriaceous; includes minor pillow lava, palagonite beds, and interbedded soil profiles and sedimentary beds; contains diatomite beds locally. Maximum thickness in south-central Washington may be in excess of 10,000 feet; much thinner in western Washington, where flows are mostly associated with marine sedimentary rocks. Includes acidic and intermediate volcanic rocks in northern Cascade Mountains.