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Geologic units in Douglas county, Oregon

[Additional scientific data in this geographic area]

Glacial deposits (Pleistocene) (Pleistocene) at surface, covers 2 % of this area
Unsorted bouldery gravel, sand, and rock flour in ground, terminal, and lateral moraines. Locally partly sorted
Lithology: gravel; moraine
Ultramafic and related rocks of ophiolite sequences (Jurassic) (Paleozoic(?), Triassic(?), and Jurassic) at surface, covers 1 % of this area
Predominantly harzburgite and dunite with both cumulate and tectonite fabrics. Locally altered to serpentinite. Includes gabbroic rocks and sheeted diabasic dike complexes. Comprises Josephine ophiolite of Harper (1980), ophiolites of Onion Mountain, Sexton Mountain, Pearsoll Peak, Rogue River, and Riddle areas (Smith and others, 1982) and Coast Range ophiolite and serpentinite melange of M.C. Blake, Jr. and A.S. Jayko (unpublished data, 1985). In southwest Oregon, locally includes small bodies of early Mesozoic or late Paleozoic serpentinized and sheared ultramafic rocks, mostly in shear zones. Locally, volcanic and sedimentary rocks shown separately
Lithology: peridotite; serpentinite; gabbro; diabase
Basalt and basaltic andesite (Holocene and Pleistocene) (Pleistocene to Holocene) at surface, covers < 0.1 % 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
Basaltic and andesitic rocks (upper and middle Miocene) (Middle to Late Miocene) at surface, covers 2 % of this area
Lava flows and flow breccia of hypersthene and olivine andesite, basaltic andesite containing plagioclase and pyroxene phenocrysts, and basalt; many flows contain phenocrysts of both hypersthene and augite. Includes interbedded volcaniclastic and epiclastic rocks mostly of andesitic composition, but partly of dacitic or rhyodacitic composition. Includes really restricted flows of silicic andesite or dacite. Upper part of unit mostly unaltered, although olivine crystals are locally altered to clay minerals. Lower parts commonly altered; secondary minerals include nontronite and saponite, chalcedony, calcite, and zeolites. Older parts of this unit locally are propylitically altered adjacent to larger intrusions. Erupted mostly from widespread, northwest- and north-trending dikes and dike swarms and related plugs and lava cones. Potassium-argon ages range from about 10 Ma to about 17 Ma. Much of this unit was previously assigned to the Sardine Formation (Peck and others, 1964), although the type locality of the Sardine Formation ("Sardine Series" as mapped by Thayer, 1939) may be older. Includes Elk Lake Formation (White, 1980a, 1980b), part of the Rhododendron Formation (Trimble, 1963; Wise, 1969), and andesite of Nohorn Creek of Hammond and others (1982)
Lithology: andesite; basalt; dacite
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
Youngest basalt and basaltic andesite (Holocene) (Holocene) at surface, covers 0.2 % 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
Andesite (Holocene and Pleistocene) (Quaternary) at surface, covers < 0.1 % of this area
Forms major stratovolcanoes dominantly of aphyric to porphyritic basaltic andesite and andesite; phenocrysts are principally pyroxene, olivine, plagioclase, and, rarely, hornblende. Locally includes dacite and minor basalt
Lithology: basalt; andesite; dacite
Basalt and andesite intrusions (Pliocene, Miocene, and Oligocene?) (Oligocene(?) to Pliocene) at surface, covers 0.4 % of this area
Sills, plugs and dikes of basaltic andesite, basalt, and andesite. Mostly represents feeders, exposed by erosion, for flows and flow breccias of units Tba and Trb. Includes a few dikes of hornblende and plagioclase porphyritic andesite, commonly altered, and aphyric basaltic andesite that probably were feeders for parts of unit Tub
Lithology: basalt; andesite
Flows and clastic rocks, undifferentiated (Miocene) (Miocene) at surface, covers < 0.1 % of this area
Chiefly basaltic andesite and andesite lava flows and flow breccia containing plagioclase and pyroxene (hypersthene and augite) phenocrysts, mudflows (lahars), and volcanic conglomerates; locally includes some dacite flows. Includes lesser, coarse- to fine-grained epiclastic volcanic sedimentary rocks and ash-flow and air-fall tuffs. Partly equivalent in age to unit Tba and may be partly coeval with younger parts of unit Tstb. Locally altered adjacent to larger intrusions. The oldest radiometrically dated rocks assigned to this unit are about 17 Ma (Sutter, 1978); in part lapped by flows questionably assigned to unit Tba, radiometrically dated at about 10 Ma, and unconformably overlain by flows of unit Trb. Includes some of rocks formerly mapped as Sardine Formation and some mapped as Rhododendron Formation
Lithology: basalt; andesite; pyroclastic; clastic
Marine sandstone and siltstone (middle Eocene) (Middle Eocene) at surface, covers 2 % of this area
Thin- to thick-bedded, crossbedded, well-sorted, fine- to medium-grain sandstone, siltstone, and mudstone; characterized by sparse fine white mica; shallow marine depositional setting at least partly of deltaic origin. Contains foraminiferal and molluscan faunas of early middle Eocene age. Included by Diller (1898) in the upper part of the Umpqua Formation, by Baldwin (1974) and Ryberg (1984) in the Flournoy Formation of the Umpqua Group, and by Molenaar (1985) in Camas Valley and the White Tail Ridge Members of Baldwin (1974) of the Umpqua Formation
Lithology: sandstone; siltstone; mudstone
Sedimentary and volcaniclastic rocks (Tertiary) at surface, covers 5 % of this area
Lapilli tuff, mudflow deposits (lahars), flow breccia, and volcanic conglomerate, mostly of basaltic to dacitic composition; rare iron-stained palagonitic tuff and breccia of basaltic and andesitic composition; and ash-flow, air-fall, and water-laid tuff of dacitic to rhyolitic composition. The palagonite tuff and breccia grade laterally into peperite and into lava flows of basalt and basaltic andesite
Lithology: sedimentary rock; pyroclastic; tuff
Undifferentiated tuffaceous sedimentary rocks, tuffs, and basalt (Miocene and Oligocene) (Oligocene to Miocene) at surface, covers 5 % of this area
Heterogeneous assemblage of continental, largely volcanogenic deposits of basalt and basaltic andesite, including flows and breccia, complexly interstratified with epiclastic and volcaniclastic deposits of basaltic to rhyodacitic composition. Includes extensive rhyodacitic to andesitic ash-flow and air-fall tuffs, abundant lapilli tuff and tuff breccia, andesitic to dacitic mudflow (lahar) deposits, poorly bedded to well-bedded, fine- to coarse-grained tuffaceous sedimentary rocks, and volcanic conglomerate. Originally included in Little Butte Volcanic Series (Peck and others, 1964); includes Mehama Volcanics and Breitenbush Tuffs or Series of Thayer (1933, 1936, 1939), Breitenbush Formation of Hammond and others (1982), Mehama Formation of Eubanks (1960), and Molalla Formation of Miller and Orr (1984a). In Columbia River Gorge, includes Miocene and older rocks previously assigned to the Skamania Volcanic Series (Trimble, 1963), or to the Eagle Creek Formation (Waters, 1973). Lower parts of unit exhibit low-grade metamorphism with primary constituents altered to clay minerals, calcite, zeolites (stilbite, laumontite, heulandite), and secondary silica minerals. In contact aureoles adjacent to stocks and larger dikes of granitic and dioritic composition or in areas of andesitic dike swarms, both wallrocks and intrusions are pervasively propylitized; locally rocks also have been subjected to potassic alteration. Epiclastic part of assemblage locally contains fossil plants assigned to the Angoonian Stage (Wolfe, 1981) or of Oligocene age. A regionally extensive biotite-quartz rhyodacite ash-flow tuff, the ash-flow tuff of Bond Creek of Smith and others (1982), is exposed in southern part of Western Cascade Range near and at base of unit. A K-Ar age of 34.9 Ma was determined on biotite from the tuff (Smith, 1980). Ash-flow tuffs, higher in the section and in the same area, have been radiometrically dated at 22 to 32 Ma by potassium-argon methods (J.G. Smith, unpublished data; Evernden and James, 1964; Fiebelkorn and others, 1983). In the central part of the Western Cascade Range, the unit has yielded a number of K-Ar ages in the range of about 32 to 19 Ma (Verplanck, 1985, p. 53-54). A fission-track age of 23.8 +/- 1.4 Ma was obtained on a red, crystal-rich ash-flow tuff (J.A. Vance, oral communication, 1983) collected at an elevation of about 3,000 ft on U.S. Highway 20 west-southwest of Echo Mountain. Most ages from basalt and basaltic andesite lava flows are in the range of about 35 to 18 Ma. Locally intruded by small stocks of granitoid rocks and by dikes, sills, plugs, and invasive flows of basaltic andesite and basalt; in many places, the intrusions are indistinguishable from poorly exposed interbedded lava flows; K-Ar ages on several of the mafic intrusions or invasive flows are about 27 to 31 Ma. In places subdivided into Tus, Tut, and Tub
Lithology: basalt; volcanic breccia (agglomerate); dacite; andesite; ash-flow tuff; clastic
Siletz River Volcanics and related rocks (middle and lower Eocene and Paleocene) (Paleocene to Middle Eocene) at surface, covers 4 % of this area
Aphanitic to porphyritic, vesicular pillow flows, tuff-breccias, massive lava flows and sills of tholeiitic and alkalic basalt. Upper part of sequence contains numerous interbeds of basaltic siltstone and sandstone, basaltic tuff, and locally derived basalt conglomerate. Rocks of unit pervasively zeolitized and veined with calcite. Most of these rocks are of marine origin and have been interpreted as oceanic crust and seamounts (Snavely and others, 1968). Foraminiferal assemblages referred to the Ulatisian and Penutian Stages (Snavely and others, 1969); K-Ar ages range from 50.7 +/- 3.1 to 58.1 +/- 1.5 Ma (Duncan, 1982); includes the lower part of the Roseburg Formation of Baldwin (1974), which has yielded K-Ar ages as old as 62 Ma
Lithology: tholeiite; alkaline basalt; volcanic breccia (agglomerate); sandstone; siltstone; conglomerate
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
Basalt and basaltic andesite (Pleistocene and Pliocene) (Pliocene to Pleistocene) at surface, covers 1.0 % 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
Fisher and Eugene Formations and correlative rocks (Oligocene and upper Eocene) (Late Eocene to Oligocene) at surface, covers 4 % of this area
Thin to moderately thick bedded, coarse- to fine-grained arkosic and micaceous sandstone and siltstone, locally highly pumiceous, of the marine Eugene Formation; and coeval and older andesitic lapilli tuff, breccia, water-laid and air-fall silicic ash of the continental Fisher and Colestin Formations; upper parts of the Fisher Formation apparently lap onto and interfinger with the Eugene Formation. Megafauna in the Eugene Formation were assigned an Oligocene age by Vokes and others (1951) and foraminifers have been assigned to the upper part of the lower Refugian Stage (McDougall, 1980), or of late Eocene age. Basalt lava flows in the Fisher Formation have yielded isotopic ages as old as 40 Ma (Lux, 1982), and south of the latitude of Cottage Grove the Fisher is overlain by a welded tuff in unit Tu dated at about 35 Ma. North of Eugene, rocks of this unit are overlain unconformably by continental volcanogenic rocks of unit Tu, including an ash-flow tuff with a K-Ar age of 30.9 +/- 0.4 Ma
Lithology: sandstone; siltstone; tuff; volcanic breccia (agglomerate)
Marine siltstone, sandstone, and conglomerate (lower Eocene) (Early Eocene) at surface, covers 4 % of this area
Cobble and pebble conglomerate, pebbly sandstone, lithic sandstone, siltstone, and mudstone; massive to thin bedded; shelf and slope depositional setting. Contains foraminiferal faunas referred to the Penutian Stage of early Eocene age. Included by Diller (1898) in the Umpqua Formation; Baldwin (1974) and Ryberg (1984) included it in the Lookingglass Formation of the Umpqua Group of Baldwin; may be partly a shelf and slope facies of the sedimentary rocks of the Roseburg Formation of Baldwin (1974) according to Molenaar (1985) and Heller and Ryberg (1983)
Lithology: siltstone; sandstone; mudstone; conglomerate
Basaltic lava flows (Oligocene to Miocene) at surface, covers 7 % of this area
Basaltic and basaltic andesite lava flows and breccia; grades laterally into rare bedded palagonitic tuff and breccia
Lithology: basalt; volcanic breccia (agglomerate)
Basaltic andesite and basalt (Holocene? and Pleistocene) (Quaternary) at surface, covers 2 % of this area
Flows and flow breccia dominantly of basaltic andesite containing plagioclase, olivine, and pyroxene phenocrysts and olivine-bearing basalt representing part of the volcanic sequence of the High Cascade Range (Thayer, 1937). Unit mostly forms small shield volcanoes, gentle-sided lava cones, and, in places, intracanyon flows
Lithology: basalt; andesite
Tuffaceous siltstone and sandstone (upper and middle Eocene) (Middle Eocene to Late Eocene) at surface, covers 1 % of this area
Thick- to thin-bedded marine tuffaceous mudstone, siltstone, and sandstone; fine to coarse grained. Contains calcareous concretions and, in places, is carbonaceous and micaceous. Includes the Nestucca Formation, which contains a foraminiferal assemblage assigned to the upper Narizian and lowermost Refugian Stages (Snavely and others, 1969; McKeel, 1980); the Spencer Formation, which contains Narizian Stage foraminifers; the Keasey Formation, which contains upper Narizian and lower Refugian Stage foraminifers (McDougall, 1975, 1980); the Coaledo and Bateman Formations of Baldwin (1974); upper Eocene sandstone of Bela (1981); and the Sager Creek formation (informal name) of Niem and Niem (1985)
Lithology: mudstone; siltstone; sandstone
Open Water (Holocene) at surface, covers 0.5 % of this area
water
Lithology: water
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)
Lithology: gravel; terrace; clay or mud; silt; sand
Yamhill Formation and related rocks (upper and middle Eocene) (Middle Eocene to Late Eocene) at surface, covers 5 % of this area
Massive to thin-bedded concretionary marine siltstone and thin interbeds of arkosic, glauconitic, and basaltic sandstone; locally contains interlayered basalt lava flows and lapilli tuff. Foraminiferal assemblages in siltstone referred to the Ulatisian and lower Narizian Stages (Snavely and others, 1969; McKeel, 1980) Includes the Elkton Formation of Baldwin (1974; also see Beaulieu and Hughes, 1975), which consists of thin-bedded siltstone and minor sandstone interbeds
Lithology: siltstone; sandstone; basalt; 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
Volcanic rocks (Jurassic) (Late Jurassic) at surface, covers 3 % of this area
Lava flows, flow breccia, and agglomerate dominantly of plagioclase, pyroxene, and hornblende porphyritic and aphyric andesite. Includes flow rocks that range in composition from basalt to rhyolite as well as some interlayered tuff and tuffaceous sedimentary rocks. Commonly metamorphosed to greenschist facies; locally foliated, schistose or gneissic. Includes the Rogue Formation and volcanic rocks commonly assigned to the Galice Formation (Wells and Walker, 1953; Wells and Peck, 1961). Considered to be accreted island-arc terrane
Lithology: andesite; basalt; felsic volcanic rock; mudstone; chert
Mafic vent complexes (Miocene) (Miocene) at surface, covers 0.2 % of this area
Intrusive plugs and dike swarms and related near-vent flows, breccias, cinders, and agglutinate of basaltic andesite, basalt, and andesite; commonly in the form of eroded piles of red, iron-stained thin flows, cinders, and agglutinate cut by mafic intrusions
Lithology: basalt; andesite; volcanic breccia (agglomerate)
Sedimentary rocks of Dothan Formation and related rocks (Lower Cretaceous and Upper Jurassic) (Late Jurassic to Early Cretaceous) at surface, covers 4 % of this area
Sandstone, conglomerate, graywacke, rhythmically banded chert lenses. Includes western Dothan and Otter Point Formations of M.C. Blake, Jr. and AS. Jayko (unpublished data, 1985) in Curry and southern Coos Counties
Lithology: graywacke; mudstone; conglomerate; chert
Tuff (Tertiary) at surface, covers 2 % of this area
Welded to unwelded, mostly vitric crystal and vitric ash-flow tuff of several ages. Glass in tuff locally altered to clay, zeolites, and secondary silica minerals
Lithology: ash-flow tuff
Granite and diorite (Jurassic and Triassic) (Triassic and Jurassic) at surface, covers < 0.1 % of this area
Felsic to intermediate, granitoid intrusive rocks. Includes Jurassic muscovite granodiorite, hornblende gabbro, tonalite, and quartz diorite of southwest Oregon (Smith and others, 1982)
Lithology: tonalite; quartz diorite; granodiorite; gabbro; diorite
Mazama pumice deposits (Holocene) (Holocene) at surface, covers 0.6 % of this area
Primary and reworked air-fall rhyodacite pumice related to climactic eruptions of Mount Mazama about 6,845 yr B.P.(14C). Mapped only where it extensively covers older units
Lithology: rhyodacite; dacite; andesite
Silicic vent complexes (Pliocene, Miocene, and upper Oligocene) (Miocene) at surface, covers 1 % of this area
Large, rhyolitic to dacitic vent areas in the Cascade Range that commonly include multiple intrusions and much associated silicic eruptive breccia and erosional debris and some flows
Lithology: rhyolite; dacite
Otter Point Formation of Dott (1971) and related rocks (Upper Jurassic) (Late Jurassic) at surface, covers 0.7 % of this area
Highly sheared graywacke, mudstone, siltstone, and shale with lenses and pods of sheared greenstone, limestone, chert, blueschist, and serpentine. Identified as melange by some investigators
Lithology: graywacke; mudstone; conglomerate; greenstone; limestone; chert; blueschist; serpentinite
Hypabyssal intrusive rocks (Miocene and Miocene?) (Miocene) at surface, covers 0.6 % of this area
Hypabyssal, medium-grained, hornblende diorite and quartz diorite in small stocks and large dikes; includes intrusions of medium- to fine-grained gabbro and plugs and small stocks of medium-grained, holocrystalline, olivine andesite. Also includes medium-grained, commonly porphyritic biotite quartz monzonite and leucocratic granodiorite. Many of these intrusive bodies are moderately to intensely propylitized, as are wallrocks they intrude; locally, along shears, the rocks also are sericitized. Potassium-argon ages on several of these shallow intrusions range from about 8 Ma to about 22 Ma (Wise, 1969; Bikerman, 1970; Sutter, 1978; Power and others, 1981a, b; Fiebelkorn and others, 1983)
Lithology: diorite; quartz diorite; gabbro; andesite; quartz monzonite; granodiorite
Myrtle Group (Lower Cretaceous and Upper Jurassic) (Late Jurassic to Early Cretaceous) at surface, covers 2 % of this area
Conglomerate, sandstone, siltstone, and limestone. Locally fossiliferous. As shown, includes Riddle and Days Creek Formations (Imlay and others, 1959; Jones, 1969)
Lithology: siltstone; sandstone; conglomerate; limestone
Sedimentary rocks of Dothan Formation and related rocks (Lower Cretaceous and Upper Jurassic) (Late Jurassic to Late Cretaceous) at surface, covers 2 % of this area
Sandstone, conglomerate, graywacke, rhythmically banded chert lenses. Includes western Dothan and Otter Point Formations of M.C. Blake, Jr. and AS. Jayko (unpublished data, 1985) in Curry and southern Coos Counties
Lithology: mudstone; sandstone; conglomerate; limestone; blueschist; eclogite
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)
Lithology: mafic volcanic rock
Tyee Formation (middle Eocene) (Middle Eocene) at surface, covers 20 % of this area
Very thick sequence of rhymically bedded, medium- to fine-grained micaceous, feldspathic, lithic, or arkosic marine sandstone and micaceous carbonaceous siltstone; contains minor interbeds of dacite tuff in upper part. Foraminiferal fauna are referred to the Ulatisian Stage (Snavely and others, 1964). Groove and flute casts indicate deposition by north-flowing turbidity currents (Snavely and others, 1964), but probable provenance of unit is southwest Idaho (Heller and others, 1985)
Lithology: sandstone; siltstone; tuff
Andesite (Pleistocene and Pliocene) (Pliocene to Pleistocene) at surface, covers < 0.1 % of this area
Flows and flow breccia in the High Cascade Province composed dominantly of aphyric to porphyritic basaltic andesite and andesite. Mostly represents remnants of moderately to deeply eroded stratovolcanoes. Phenocrysts are mostly plagioclase, olivine, clinopyroxene, and lesser hypersthene and hornblende
Lithology: andesite
Alluvial deposits (Holocene) (Holocene) at surface, covers 3 % 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
Marine sandstone, siltstone, and mudstone (lower Eocene and Paleocene?) (Paleocene to Early Eocene) at surface, covers 5 % of this area
Rhythmically interbedded sandstone, siltstone, and mudstone with minor conglomerate; deposited in deep-sea fan depositional setting on submarine basalts of the Siletz River Volcanics. Contains foraminiferal faunas referred to the Penutian Stage of early Eocene age and locally contains assemblages of probable Paleocene age (McKeel and Lipps, 1975; P.D. Snavely, Jr. and David Bukry, written communication, 1980). Included by Diller (1898) in the Umpqua Formation; Baldwin (1974) and Ryberg (1984) mostly mapped unit as sedimentary rocks of the Roseburg Formation of the Umpqua Group; according to Heller and Ryberg (1983) and Molenaar (1985), may be partly correlative with the Lookingglass Formation of Baldwin (1974). Includes lower Eocene-Paleocene turbidite sedimentary rocks exposed at Five Mile Point, about 11 km north of Bandon, that are considered by Snavely and others (1980) to represent allochtonous terrane
Lithology: sandstone; siltstone; mudstone; conglomerate
Ultramafic and related rocks of ophiolite sequences; Basaltic volcanic and sedimentary rocks (Jurassic) (Jurassic) at surface, covers < 0.1 % of this area
Basalt flows, flow breccia, agglomerate, pillow basalt and pillow breccia, and lesser shale, chert, siltstone, and mudstone of ophiolitic complexes
Lithology: basalt; mudstone; chert
Volcanic rocks of the Dothan Formation and related rocks (Lower Cretaceous and Upper Jurassic) (Late Jurassic to Early Cretaceous) at surface, covers 0.2 % of this area
Basaltic pillow lavas, volcanic breccia, and silicified basalt lava flows
Lithology: basalt
Granitic rocks (Cretaceous and Jurassic) (Late Jurassic and Early Cretaceous) at surface, covers 3 % of this area
Mostly tonalite and quartz diorite but including lesser amounts of other granitoid rocks. Potassium-argon ages determined on hornblende indicates plutons range in age from 143 to 166 Ma (Hotz, 1971)
Lithology: quartz diorite; diorite; tonalite; gabbro
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
Tillamook Volcanics (upper and middle Eocene) (Middle Eocene to Late Eocene) at surface, covers < 0.1 % of this area
Subaerial basaltic flows and breccia and submarine basaltic breccia, pillow lavas, lapilli and augite-rich tuff with interbeds of basaltic sandstone, siltstone, and conglomerate. Includes some basaltic andesite and, near the top of the sequence, some dacite. Potassium-argon ages on middle and lower parts of sequence range from about 43 to 46 Ma (Magill and others, 1981): one potassium-argon age from dacite near top of sequence is about 40 Ma (see Wells and others, 1983)
Lithology: basalt; volcanic breccia (agglomerate); andesite; dacite; sandstone; siltstone; conglomerate
Mafic intrusions (Oligocene) (Oligocene) at surface, covers < 0.1 % of this area
Sheets, sills, and dikes of massive granophyric ferrogabbro; some bodies strongly differentiated and include pegmatitic gabbro, ferrogranophyre, and granophyre (MacLeod, 1981). Plagioclase and amphibole from unit have yielded K-Ar ages of about 30 Ma (Snavely and others, 1976a)
Lithology: gabbro; granitoid
May Creek Schist (Paleozoic) (Paleozoic(?) to Jurassic) at surface, covers 0.2 % of this area
Layered amphibolite, schist, gneiss, and quartzite. Protolith considered to be of Paleozoic age
Lithology: amphibolite; mica schist; quartz-feldspar schist; quartzite; calc-silicate schist
Dune sand (Holocene) (Holocene) at surface, covers 0.2 % 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
Lithology: sand
Sedimentary rocks (Jurassic) (Late Jurassic ) at surface, covers 3 % 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 Klamath Mountains of southwest Oregon, includes Galice Formation (Wells and Peck, 1961) and unnamed, hornblende- and (or) pyroxene-bearing clastic rocks of Jurassic age (Smith and others, 1982)
Lithology: mudstone; graywacke; tuff; limestone; felsic volcanic rock; andesite; basalt; phyllite; slate
Mazama ash-flow deposits (Holocene) (Holocene) at surface, covers 1 % of this area
Rhyodacitic to andesitic ash-flow deposits related to climactic eruptions of Mount Mazama about 6,845 yr B.P. (14C) (Bacon, 1983)
Lithology: dacite; rhyodacite; andesite

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