Undifferentiated tuffaceous sedimentary rocks, tuffs, and basalt

Heterogeneous assemblage of continental, largely volcanogenic deposits of basalt and basaltic andesite, including flows and breccia, complexly interstratified with epiclastic and volcaniclastic deposits of basaltic to rhyodacitic composition. Includes extensive rhyodacitic to andesitic ash-flow and air-fall tuffs, abundant lapilli tuff and tuff breccia, andesitic to dacitic mudflow (lahar) deposits, poorly bedded to well-bedded, fine- to coarse-grained tuffaceous sedimentary rocks, and volcanic conglomerate. Originally included in Little Butte Volcanic Series (Peck and others, 1964); includes Mehama Volcanics and Breitenbush Tuffs or Series of Thayer (1933, 1936, 1939), Breitenbush Formation of Hammond and others (1982), Mehama Formation of Eubanks (1960), and Molalla Formation of Miller and Orr (1984a). In Columbia River Gorge, includes Miocene and older rocks previously assigned to the Skamania Volcanic Series (Trimble, 1963), or to the Eagle Creek Formation (Waters, 1973). Lower parts of unit exhibit low-grade metamorphism with primary constituents altered to clay minerals, calcite, zeolites (stilbite, laumontite, heulandite), and secondary silica minerals. In contact aureoles adjacent to stocks and larger dikes of granitic and dioritic composition or in areas of andesitic dike swarms, both wallrocks and intrusions are pervasively propylitized; locally rocks also have been subjected to potassic alteration. Epiclastic part of assemblage locally contains fossil plants assigned to the Angoonian Stage (Wolfe, 1981) or of Oligocene age. A regionally extensive biotite-quartz rhyodacite ash-flow tuff, the ash-flow tuff of Bond Creek of Smith and others (1982), is exposed in southern part of Western Cascade Range near and at base of unit. A K-Ar age of 34.9 Ma was determined on biotite from the tuff (Smith, 1980). Ash-flow tuffs, higher in the section and in the same area, have been radiometrically dated at 22 to 32 Ma by potassium-argon methods (J.G. Smith, unpublished data; Evernden and James, 1964; Fiebelkorn and others, 1983). In the central part of the Western Cascade Range, the unit has yielded a number of K-Ar ages in the range of about 32 to 19 Ma (Verplanck, 1985, p. 53-54). A fission-track age of 23.8 ñ 1.4 Ma was obtained on a red, crystal-rich ash-flow tuff (J.A. Vance, oral communication, 1983) collected at an elevation of about 3,000 ft on U.S. Highway 20 west-southwest of Echo Mountain. Most ages from basalt and basaltic andesite lava flows are in the range of about 35 to 18 Ma. Locally intruded by small stocks of granitoid rocks and by dikes, sills, plugs, and invasive flows of basaltic andesite and basalt; in many places, the intrusions are indistinguishable from poorly exposed interbedded lava flows; K-Ar ages on several of the mafic intrusions or invasive flows are about 27 to 31 Ma. In places subdivided into Tus, Tut, and Tub
State Oregon
Name Undifferentiated tuffaceous sedimentary rocks, tuffs, and basalt
Geologic age Oligocene to Miocene
Lithologic constituents
Igneous > Volcanic > Mafic-volcanic > Basaltic-andesite (Flow, Volcaniclastic-volcanic breccia)
Igneous > Volcanic > Mafic-volcanic > Basalt (Flow, Volcaniclastic-volcanic breccia)
Igneous > Volcanic > Felsic-volcanic > Dacite (Pyroclastic-air fall, Pyroclastic-ash flow, Volcaniclastic-lahar)
Sedimentary > Clastic > Mixed-clastic (Bed)
Igneous > Volcanic > Mafic-volcanic > Andesite (Pyroclastic-air fall, Pyroclastic-ash flow, Volcaniclastic-lahar)
Sedimentary > Clastic > Conglomerate (Bed)

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

Peck, D.L., Griggs, A.B., Schlicker, H.G., Well, F.G., and Dole, H.M., 1964, Geology of the central and northern parts of the Western Cascade Range in Oregon: U.S. Geolotgical Survey Professional Paper 449, 56 p.

Thayer, T.P., 1933, Structural relations of central Willamette Valley to Cascade Mountains: Pan American Geologist, v. 59, p. 317.

Thayer, T.P., 1936, Structure of the North Santiam River section of the cascade Mountains in Oregon: Journal of Geology, v. 44, p. 701-716.

Thayer, T.P., 1939, Geology of the Salem Hills and the North Santiam River basin, Oregon: Oregon Department of Geology and Mineral Industries Bulletin 15, 40 p.

Hammond, P.E., Geyer, K.M., and Anderson, J.L., 1982, Preliminary geologic map and cross sections of the upper Clackamas and North Santiam Rivers area, northern Oregon Cascade Range: Portland State University Department of Earth Sciences, Portland, Oregon, scale 1:62,500.

Eubanks, W., 1960, Fossil woods of the Thomas Creek area, Linn County, Oregon: The Ore Bin, v. 22, p. 65-69.

Miller, P.R., and Orr, W.N., 1984a, Geologic map of the Wilhoit quadrangle, Oregon: Oregon Department of Geologyand Mineral Industries Geologic Map Series GMS032, scale 1:24,000.

Trimble, D.E., 1963, geology of Portland, Oregon, and adjacent areas: U.S. Geological Survey Bulletin 1119, 110 p.


Waters, A.C., 1973, The Columbia River Gorge-basalt stratigraphy, ancient lava dams, and landslide dams, in Geologic field trips in northern Oregon and southern Washington: Oregon Department of Geology and Mineral Industries Bulletin 77, p. 135-154.

Wolfe, J.A., 1981, A chronologic framework for Cenozoic megafossil floras of northwestern North America and its relation to marine geochronology, in Armentrout, J.M., ed., Pacific Northwest Cenozoic Biostratigraphy: Geological Society of America Special Paper 184, p. 39-47.

Smith, J.G., Page, N.J, Johnson, M.G., Moring, B.C., and Gray, F., 1982, Preliminary geologic map of the Medford 1ø by 2ø quadrangle, Oregon and California: U.S. Geological Survey Open-File Report 82-955, scale 1:250,000.


Smith, J.G., 1980, An important lower Oligocene welded-tuff marker bed in the western Cascade Range of southern Oregon: Geological Society of America Abstracts with Programs, v. 12, p. 153.

Evernden, J. F., and James, G.T., 1964, Potassium-argon dates and the Tertiary floras of North America: American Journal of Science, v. 262, p. 945-974.

Fiebelkorn, R.B., Walker, G.W., Macleod, N.S., McKee, E.H., and Smith, J.G., 1983, Index to K-Ar determinations for the State of Oregon: Isochron/West, no. 37, p. 3-60.

Verplanck, E.P., 1985, Temporal variations in volume and geochemistry of volcanism in the Western Cascades, Oregon: Corvallis, Oregon, Oregon State University, M.S. Thesis, 115 p.

NGMDB product
Counties Clackamas - Douglas - Jackson - Lane - Linn - Marion