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Tillamook Volcanics (upper and middle Eocene)

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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)
State Oregon
Name Tillamook Volcanics (upper and middle Eocene)
Geologic age Middle Eocene to Late Eocene
Primary rock type basalt
Secondary rock type volcanic breccia (agglomerate)
Other rock types andesite; dacite; sandstone; siltstone; conglomerate
Lithologic constituents
Major
Igneous > Volcanic > Mafic-volcanic > Basalt (Flow)
Igneous > Volcanic > Mafic-volcanic > Basalt (Volcaniclastic, volcanic breccia)
Minor
Igneous > Volcanic > Mafic-volcanic > Basalt (Pyroclastic, tuff)
Igneous > Volcanic > Mafic-volcanic > Basaltic-andesite (Volcaniclastic, volcanic breccia)
Igneous > Volcanic > Mafic-volcanic > Basaltic-andesite (Flow)
Incidental
Igneous > Volcanic > Mafic-volcanic > Basaltic-andesite (Pyroclastic, tuff)
Igneous > Volcanic > Felsic-volcanic > Dacite (Flow)
Igneous > Volcanic > Felsic-volcanic > Dacite (Volcaniclastic, volcanic breccia)
Igneous > Volcanic > Felsic-volcanic > Dacite (Pyroclastic, tuff)
Sedimentary > Clastic > Sandstone (Bed)
Sedimentary > Clastic > Siltstone (Bed)
Sedimentary > Clastic > Conglomerate (Bed)
Map references
Walker, G.W. and MacLeod, N.S., 1991, Geologic map of Oregon: U.S. Geological Survey, scale 1:500,000.
Unit references
Walker, G.W. and MacLeod, N.S., 1991, Geologic map of Oregon: U.S. Geological Survey, scale 1:500,000.
Magill, J., Cox, A., and Duncan, R., 1981, Ttillamook volcanic series-further evidence for tectonic rotation of the Oregon Coast Range: Journal of Geophysical Research, v. 86, p. 2953-2970.
Wells, R.E., Niem, A.R., MacLeod, N.S., Snavely P.D., Jr., and Niem, W.A., 1983, Preliminary geologic map of the west half of the Vancouver (Washington-Oregon) 1ø by 2ø quadrangle: U.S. Geological Survey Open-File Report 83-591, scale 1:250,000.
Counties Clackamas - Clatsop - Columbia - Douglas - Grant - Multnomah - Tillamook - Washington - Yamhill