Tillamook Volcanics

Subaerial basaltic flows and breccia and submarine basaltic breccia, pillow lavas, lapilli and augite-rich tuff with interbeds of basaltic sandstone, siltstone, and conglomerate. Includes some basaltic andesite and, near the top of the sequence, some dacite. Potassium-argon ages on middle and lower parts of sequence range from about 43 to 46 Ma (Magill and others, 1981): one potassium-argon age from dacite near top of sequence is about 40 Ma (see Wells and others, 1983)
State Oregon
Name Tillamook Volcanics
Geologic age Middle Eocene to Late Eocene
Lithologic constituents
Igneous > Volcanic > Mafic-volcanic > Basalt (Flow, Pyroclastic-tuff, Volcaniclastic-volcanic breccia)
Sedimentary > Clastic > Sandstone (Bed)
Sedimentary > Clastic > Siltstone (Bed)
Sedimentary > Clastic > Conglomerate (Bed)
Igneous > Volcanic > Mafic-volcanic > Basaltic-andesite (Flow, Pyroclastic-tuff, Volcaniclastic-volcanic breccia)
Igneous > Volcanic > Felsic-volcanic > Dacite (Flow, Pyroclastic-tuff, Volcaniclastic-volcanic breccia)

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

Magill, J., Cox, A., and Duncan, R., 1981, Ttillamook volcanic series-further evidence for tectonic rotation of the Oregon Coast Range: Journal of Geophysical Research, v. 86, p. 2953-2970.

Wells, R.E., Niem, A.R., MacLeod, N.S., Snavely P.D., Jr., and Niem, W.A., 1983, Preliminary geologic map of the west half of the Vancouver (Washington-Oregon) 1ø by 2ø quadrangle: U.S. Geological Survey Open-File Report 83-591, scale 1:250,000.


NGMDB product
Counties Clackamas - Clatsop - Columbia - Douglas - Grant - Multnomah - Tillamook - Washington - Yamhill