Andesite and dacite and sedimentary rocks

Lava flows, breccia, volcaniclastic and epiclastic rocks mostly of andesitic and dacitic composition; includes minor amounts of altered basaltic rocks. Joint surfaces and cavities commonly lined with hematite or montmorillonite clay, secondary silica minerals, zeolites, celadonite, or calcite. Andesite and dacite typically have plagioclase, hornblende, and clinopyroxene phenocrysts; some flows aphyric. Platy flow-jointing common. Age, mostly Oligocene; may include some rocks of early Miocene age. As shown, may include some rocks older than Oligocene, correlative with upper parts of unit Tea. One potassium-argon age of about 28 Ma on porphyritic hornblende andesite from Sheep Creek, southwest corner of Union County, indicates in part coeval with unit Tsf
State Oregon
Name Andesite and dacite and sedimentary rocks
Geologic age Oligocene to Miocene
Lithologic constituents
Major
Igneous > Volcanic > Felsic-volcanic > Dacite (Flow, Volcaniclastic-volcanic breccia)
Igneous > Volcanic > Mafic-volcanic > Andesite (Flow, Volcaniclastic-volcanic breccia)Also contains dacite and sedimentary rocks
Incidental
Igneous > Volcanic > Mafic-volcanic > Basalt (Flow)
Comments Includes Alvord Creek Formation (OR064; formerly Alvord Creek Beds of Fuller (OR062). Contains multi-colored interstratified silicic to intermediate tuff, volcaniclastic sandstone and siltstone, and minor conglomerate and tuff breccia. Tuff and tuffaceous rocks are commonly silicified and argillically altered. Sedimentary rocks contain plant fossils, but age assignment is based on stratigraphic relations with overlying Pike Creek Formation (unit Tsf). Basalt in the Pike Creek was dated at 21.9 Ma (OR010; recalculated using constants of OR185). In La Grande 30' x 60' quadrangle (OR291), Tas is reinterpreted as part of the Oligocene and lower Miocene Tower Mountain volcanic field (see description under unit Tsfj)
Stratigraphic units Alvord Creek Formation (OR064; formerly Alvord Creek Beds; 062)
References

Walker, G.W. and MacLeod, N.S., 1991, Geologic map of Oregon: U.S. Geological Survey, scale 1:500,000.

Greene, R.C., Walker, G.W., and Corcoran, R.E., 1972, Geologic map of the Burns quadrangle, Oregon: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-680, scale 1:250,000.

https://pubs.er.usgs.gov/publication/i680

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.

https://pubs.er.usgs.gov/publication/i446

Brown, C.E. and Thayer, T.P., 1966, Geologic map of the Canyon City quadrangle, northeastern Oregon: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-447, scale 1:250,000.

https://pubs.er.usgs.gov/publication/i447

Brooks, H.C., McIntyre, J.R., and Walker, G.W., 1976, Geology of the Oregon part of the Baker 1o by 2o quadrangle: Oregon Department of Geology and Mineral Industries Geologic Map Series GMS-7, scale 1:250,000.

Walker, G.W., 1973, Reconnaissance geologic map of the Pendleton quadrangle, Oregon and Washington: U.S. Geological Survey Miscellaneus Geologic Investigations Map I-727, scale 1:250,000.

https://pubs.er.usgs.gov/publication/i727

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.

https://pubs.er.usgs.gov/publication/i902

Wolff, E.N., 1965, The geology of the northern half of the Caviness quadrangle, Oregon: Eugene, University of Oregon, Ph.D. dissertation, 214 p.

Evans, J.G., 1989, Geologic map of the Desolation Butte quadrangle, Grant and Umatilla Counties, Oregon: U.S. Geological Survey Geologic Quadrangle GQ-1654, scale 1:62,500.

https://pubs.er.usgs.gov/publication/gq1654

Ferns, M.L., Madin, I.P., and Taubeneck, W.H., 2001, Reconnaissance geologic map of the La Grande 30' x 60' quadrangle, Baker, Grant, Umatilla, and Union Counties, Oregon: Oregon Department of Geology and Mineral Industries Reconnaissance Map Series RMS-1, 52 p., scale 1:100,000.

Ferns, M.L., 1999, Geologic map of the Fly Valley quadrangle, Union County, Oregon: Oregon Department of Geology and Mineral Industries Geological Map Series GMS-113, scale 1:24,000.

Ferns, M.L. and Taubeneck, W.H., 1994, Geology and mineral resources of the Limber Jim Creek quadrangle, Union County, Oregon: Oregon Department of Geology and Mineral Industries Geological Map Series GMS-82, scale 1:24,000.

NGMDB product
Counties Baker - Crook - Grant - Harney - Lake - Malheur - Umatilla - Union