Ridge-capping basalt and basaltic andesite

Flows and flow breccia of basaltic andesite and lesser diktytaxitic to intergranular olivine basalt. Includes some dense, aphyric flows, commonly with either cryptocrystalline or pilotaxitic to trachytic texture, and porphyritic flows with phenocrysts and glomerocrysts of olivine, hypersthene, and labradorite. A few flows contain both hypersthene and calcic augite phenocrysts. Olivine mostly fresh or slightly altered to iddingsite in flows high in section; flows low in section show some alteration to clays (nontronite and saponite), secondary silica minerals, and calcite; pinkish-brown glass in some flows unaltered. Locally includes some andesite and dacite. Some flows of this unit are lithologically similar to flow rocks of the High Cascade volcanic sequence and some are more like flows that in the past have been mapped as part of the Sardine Formation (Peck and others, 1964) and Elk Lake Formation of McBirney and others (1974) and Sutter (1978). Potassium-argon ages of rocks from this unit range from about 4 to 8 or 9 Ma. Includes some rocks formerly mapped as Rhododendron Formation by Peck and others (1964)
State Oregon
Name Ridge-capping basalt and basaltic andesite
Geologic age Late Miocene to Pliocene
Lithologic constituents
Igneous > Volcanic > Mafic-volcanic > Basalt (Flow)
Igneous > Volcanic > Mafic-volcanic > Basaltic-andesite (Flow)
Stratigraphic units Rhododendron Formation

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.

McBirney, A.R., Sutter, J.F., Naslund, H.R., Sutton, K.G., and White, C.M., 1974, Episodic volcanism in the central Oregon Cascade Range: Geology, v. 2, p. 585-589.

Sutter, J.F., 1978, K-Ar ages of Cenozoic volcanic rocks from the Oregon Cascades west of 121ΓΈ30': Isochron/West, no. 21, p. 15-21.

NGMDB product
Counties Baker - Clackamas - Crook - Douglas - Harney - Hood River - Jackson - Jefferson - Klamath - Lane - Linn - Marion - Wasco - Washington - Yamhill