Andesitic and basaltic rocks on Steens Mountain (ORTbas;0)

State	Oregon
Name	Andesitic and basaltic rocks on Steens Mountain
Geologic age	Miocene
Original map label	Tbas
Comments	Two dacite flows, a rhyolite dome, and a basalt flow were dated at 22.4 Ma (upper flow), 23.6+0.7 Ma (middle flow), and 22.1+0.7 Ma (OR184). Steens Mountain Volcanics (OR184; formerly OR062 and OR064) consists of hornblende- and pyroxene-hornblende-bearing andesite flows, flow breccia and pyroclastic rocks. One of the andesite flows was dated at 19+1.1 Ma (OR184). Basaltic andesite at the top of Steens Mountain Volcanics was dated at 23+1.3 Ma (OR180).
Primary rock type	basalt
Secondary rock type	andesite
Other rock types
Lithologic constituents	Major Igneous > Volcanic > Mafic-volcanic > Andesite (Flow) Minor Igneous > Volcanic > Mafic-volcanic > Andesite (Pyroclastic) Igneous > Volcanic > Mafic-volcanic > Basalt (Flow) Igneous > Volcanic > Felsic-volcanic > Rhyolite (Dome) Igneous > Volcanic > Felsic-volcanic > Dacite (Flow) Igneous > Volcanic > Mafic-volcanic > Basaltic-andesite (Flow)
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. Walker, G.W. and Repenning, C.A., 1965, Geologic map of Adel quadrangle, Lake, Harney, and Malheur Counties, Oregon: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-446, scale 1:250,000. [http://pubs.er.usgs.gov/publication/i446] Walker, G.W., 1977, Geologic map of Oregon east of the 121st meridian: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-902, scale 1:500,000. [http://pubs.er.usgs.gov/publication/i902] Fuller, R.E., 1931, the geomorphology and volcanic sequence of Steens Mountain in southeastern Oregon: University of Washington Publication in Geology, v. 3, no. 1, 130 p. Williams, Howell and Compton, R.R., 1953, Quicksilver deposits of Steens Mountain and Pueblo Mountains, southeast Oregon: U.S. Geological Survey Bulletin 995-B, p. B19-77. [http://pubs.er.usgs.gov/publication/b995B] Hart, W.K. and Carlson, R.W., 1985, Distribution and geopchronology of Steens Mountain type basalts from the northwestern Great Basin: Isochron/West, no. 43, p. 5-10. Minor, S.A., Rytuba, J.J., Vander Meulen, D.B., Grubensky, M.J., and Tegtmeyer, K.J., 1987, Geologic map of the Wildhorse Lake quadrangle, Harney County, Oregon: U.S. Geological Survey Miscellaneous Field Studies Map MF-1915, scale 1:24,000. [http://pubs.er.usgs.gov/publication/mf1915] Dalrymple, G.B., 1979, Critical tables for conversion of K-Ar ages from old to new constants: Geology, v. 7, no. 11, p. 558-560. Minor, S.A., Rytuba, J.J., Grubensky, M.J., Vander Meulen, D.B., Goeldner, C.A., and Tegtmeyer, K.J., 1987, Geologic map of the High Steens and Little Blitzen Gorge Wilderness Study Areas, Harney County, Oregon: U.S. Geological Surevy Miscellaneous Field Studies Map MF-1876, scale 1:24,000. Minor, S.A., Rytuba, J.J., Goeldner, C.A., and Tegtmeyer, K.J., 1987, Geologic map of the Alvord Hot Spring quadrangle, Harney County, Oregon: U.S. Geological Survey Miscellaneous Field Studies Map MF-1916, scale 1:24,000. [http://pubs.er.usgs.gov/publication/mf1916] Minor, S.A., Rytuba, J.J., Vander Meulen, D.B., Grubensky, M.J., and Tegtmeyer, K.J., 1987e, Geologic map of the Wildhorse Lake quadrangle, Harney County, Oregon: U.S. Geological Survey Miscellaneous Field Studies Map MF-1915, scale 1:24,000. [http://pubs.er.usgs.gov/publication/mf1915]
Geographic coverage	Harney