Miocene volcanic rocks

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.
State Washington
Name Miocene volcanic rocks
Geologic age Middle Miocene
Lithologic constituents
Major
Igneous > Volcanic > Mafic-volcanic > Basalt (Flow, Pyroclastic-tuff, Dike or sill, Volcaniclastic-volcanic breccia)
Incidental
Sedimentary > Clastic > Siltstone (Bed, Tuffaceous)Tuffaceous or volcanic.
Sedimentary > Clastic > Sandstone (Bed, Tuffaceous)Tuffaceous or volcanic.
Igneous > Volcanic > Mafic-volcanic > Andesite (Flow, Pyroclastic-tuff, Volcaniclastic-volcanic breccia)
Sedimentary > Clastic > Conglomerate (Bed, Tuffaceous)Tuffaceous or volcanic.
Igneous > Volcanic > Felsic-volcanic > Dacite (Pyroclastic-tuff, Volcaniclastic-volcanic breccia)
Igneous > Volcanic > Felsic-volcanic > Rhyolite (Pyroclastic-tuff, Volcaniclastic-volcanic breccia)
Comments Yakima basalt used by some workers interchangably with Columbia River Basalts, by others as specific formation or subgroup within the Columbia River Basalt Group. Entire unit consists of basalt flows with minor exceptions: outcrop of andesite and basalt flows in southwestern Washington near Silver Lake and outcrops of basalt, dacite, andesite, and rhyolite breccia and tuff (including Hannegan volcanics and Skagit volcanics) with volcanic and tuffaceous conglomerate and sandstone in northwestern Washington near Mt. Baker. Any sedimentary interbeds included in this unit, especially in eastern Washington, probably of, or correlative to, the Ellensburg Formation (see WAMPLc and WAOLc) or the Latah Formation (see WAMc); in western Washington, may be of, or correlative to, the Astoria Formation (see WAMPL and WAM).
Stratigraphic units Columbia River Basalt Group: Columbia River Basalts, Yakima basalt, Saddle Mountains Basalt (in part), Grande Ronde Basalt, Wanapum Basalt, Imnaha Basalt; Hannegan volcanics; Skagit (Mt. Raum) volcanics
References

Huntting, M.T., Bennett, W.A.G., Livingston, V.E.Jr., and Moen, W.S., 1961, Geologic Map of Washington: Washington Division of Mines and Geology, scale 1:500,000.

Schuster, J.E., Gulick, C.W., Reidel, S.P., Fecht, K.R., Zurenko, S., 1997, Geologic Map of Washington--Southeast Quadrant: Washington Division of Geology and Earth Resources Geologic Map GM-45, scale 1:250,000.

Walsh, T.J., Korosec, M.A., Phillips, W.M., Logan, R.T., Schasse, H.W., 1987, Geologic Map of Washington-Southwest Quadrant: Washington Division of Geology and Earth Resources, Geologic Map GM-34, scale 1:250,000.

Snavely, P.D. and Wells, R.E., 1984, Tertiary Volcanic and Intrusive Rocks on the Oregon and Washington Continental Shelf: U. S. Geological Survey Open-File Report 84-282, 17 p.

Warren, W.C., 1941, Relation of the Yakima Basalt to the Keechelus Andesite Series: The Journal of Geology, v. 49, p. 795-814.

Snavely, P.D., Macleod, N.S., Wagner, H.C., 1973, Miocene Tholeiitic Basalts of Coastal Oregon and Washington and Their Relations to Coeval Basalts of the Columbia Plateau: Geological Society of America Bulletin, v. 84, no. 2, p. 387-424.

Stoffel, K.L., Joseph, N.L., Waggoner, S.Z., Gulick, C.W., Korosec, M.A., Bunning, B.B., 1991, Geologic Map of Washington--Northeast Quadrant: Washington Division of Geology and Earth Resources Geologic Map, GM-39, scale 1:250,000.

Misch, P., 1977, Bedrock Geology of the North Cascades, in Brown, E. H. and Ellis, R. C. (eds.), Geological Excursions in the Pacific Northwest, Geological Society of America 1977 Annual Meeting, Seattle: Western Washington University, p. 1-62.

Tabor, R.W., Waitt, R.B.,Jr., Frizzell, V.A.,Jr., Swanson, D.A., Byerly, G.R., Bentley, R.D., 1982, Geologic Map of the Wanatchee 1:100,000 Quadrangle, Central Washington: U. S. Geological Survey Miscellaneous Investigations Series Map I-1311, scale 1:100,000.

Dragovich, J.D., Logan, R.L., Schasse, H.W., Walsh, T.J., Lingley, W.S., Jr., Norman, D.K., Gerstel, W.J., Lapen, T.J., Schuster, J.E., Meyers, K.D., 2002, Geologic Map of Washington-Northwest Quadrant: Washington Division of Geology and Earth Resources Geologic Map GM-50, scale 1:250,000.

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Counties Adams - Asotin - Benton - Chelan - Clark - Columbia - Cowlitz - Douglas - Ferry - Franklin - Garfield - Grant - Grays Harbor - Kittitas - Klickitat - Lewis - Lincoln - Okanogan - Pacific - Skamania - Spokane - Stevens - Wahkiakum - Walla Walla - Whatcom - Whitman - Yakima