Geologic units in Lincoln county, Oregon

Tyee Formation (Middle Eocene) at surface, covers 57 % 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)

Siletz River Volcanics and related rocks (Paleocene to Middle Eocene) at surface, covers 13 % 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 ñ 3.1 to 58.1 ñ 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

Yamhill Formation and related rocks (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

Yaquina Formation (Late Oligocene to Early Miocene) at surface, covers 5 % of this area

Thick- to thin-bedded sandstone, conglomerate, and tuffaceous siltstone of deltaic origin; locally contains thin coal and ash beds. Conglomerate contains abundant clasts of pumice and dacitic volcanic rocks. In places includes thick lenses of marine tuffaceous siltstone and fine-grained sandstone. Foraminifers in formation assigned to the Zemorrian and lower part of the Saucesian Stages of Kleinpell (1938) and molluscan fauna to the lower Blakeley Stage of Weaver and others (1944)

Terrace, pediment, and lag gravels (Pleistocene to Holocene) at surface, covers 4 % 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)

Alsea Formation (Late Eocene to Oligocene) at surface, covers 4 % of this area

Massive to thick-bedded tuffaceous marine siltstone and fine-grained sandstone; locally concretionary. Foraminiferal assemblages assigned to the Zemorrian and upper Refugian Stages (Kleinpell, 1938; Rau, 1975) and molluscan fauna assigned (Snavely and others, 1976a) to the Lincoln and lower Blakeley Stages of Weaver and others (1944)

Tuffaceous siltstone and sandstone (Middle Eocene to Late Eocene) at surface, covers 3 % 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)

Mafic intrusions (Oligocene) at surface, covers 3 % 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)

Marine sedimentary rocks (Early Miocene to Middle Miocene) at surface, covers 2 % of this area

Fine- to medium-grained Marine siltstone and sandstone that commonly contains tuff beds. Includes the Astoria Formation, which is mostly micaceous and carbonaceous sandstone, and the middle Miocene Gnat Creek Formation of Niem and Niem (1985), which overlies Frenchmen Springs Member of the Wanapum Basalt east of Astoria. The Astoria Formation locally contains calcareous concretions and sulfide nodules; foraminifers in formation are assigned to the Saucesian and Relizian Stages (Kleinpell, 1938; Rau, 1981) and molluscan fossils to the Newportian Stage of Addicott (1976, 1981). Also includes Nye Mudstone, which is massive to poorly bedded siltstone and mudstone; foraminiferal assemblages assigned to the Saucesian Stage (Kleinpell, 1938; Rau, 1981) and molluscan fauna to Pillarian(?) Stage (Armentrout, 1981)

Porphyritic basalt (Late Eocene) at surface, covers 2 % of this area

Subaerial lava flows and breccia of porphyritic basalt, minor basaltic andesite, and rare dacite. Includes basalt of Cascade Head (Wells and others, 1983), Yachats Basalt (Snavely and others, 1976c) and Goble Volcanic Series (Warren and others, 1945). Also includes camptonitic extrusive rocks (tuff breccia, lapilli tuff, and minor pillow flows) interbedded in Nestucca Formation

Alluvial deposits (Holocene) at surface, covers 0.5 % 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

Lacustrine and fluvial sedimentary rocks (Pleistocene) at surface, covers 0.4 % 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

Columbia River Basalt Group and related flows (Miocene) at surface, covers 0.2 % 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

Alkalic intrusive rocks (Eocene to Oligocene) at surface, covers 0.1 % of this area

Sills, dikes, stocks, and irregular intrusions of porphyritic or aphanitic camptonite, shonkinite, and. nepheline syenite or phonolite. Potassium-argon ages of 32 to 35 Ma obtained on camptonite and nepheline syenite (Snavely and others, 1976c; Fiebelkorn and others, 1983)

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

Intrusive gabbroic rocks (Eocene to Oligocene) at surface, covers < 0.1 % of this area

Sills and dikes of basalt, diabase, gabbro, and granophyric gabbro; locally albitized and zeolitized (Snavely and others, 1976a, b)

Mafic and intermediate intrusive rocks (Miocene) at surface, covers < 0.1 % of this area

Dikes, plugs, and sills of basalt, diabase, gabbro, and lesser andesite that fed many of the Miocene basalt and andesite flows in unit Tc. Some intrusions are rootless and are invasive into sedimentary sequences; includes related breccia and peperite. Includes Depoe Bay and Cape Foulweather dikes, sills, and plugs in the Coast Range (Snavely and others, 1976a, b; Wells and others, 1983)