Oligocene volcanic rocks

Andesite and rhyodacite flows, tuff, and tuffaceous sandstones of northern Lincoln County and southwestern Stevens County.
State Washington
Name Oligocene volcanic rocks
Geologic age Oligocene
Lithologic constituents
Igneous > Volcanic > Mafic-volcanic > Andesite (Flow, Pyroclastic-tuff)
Igneous > Volcanic > Felsic-volcanic > Dacite (Flow, Pyroclastic-tuff)
Igneous > Volcanic (Volcaniclastic-volcanic breccia)
Igneous > Volcanic > Felsic-volcanic > Rhyolite (Flow, Pyroclastic-tuff)
Sedimentary > Clastic > Mudstone > Shale (Bed, Carbonaceous)
Sedimentary > Clastic > Sandstone (Tuffaceous)
Sedimentary > Clastic > Conglomerate (Bed)
Sedimentary > Clastic > Sandstone > Arkose (Bed)
Comments Mapped as andesite and dacite by Stoffel and others, 1991; mapped as primarily rhyodacite flows by Becraft and Weis, 1963. Note that Stoffel and others, 1991, report that while the term "Gerome Andesite" was once widely used (first coined in 1920 by Stevens), it has since been abandoned.
Stratigraphic units Gerome andesite

Huntting, M.T., Bennett, W.A.G., Livingston, V.E.Jr., and Moen, W.S., 1961, Geologic Map of Washington: Washington Division of Mines and Geology, scale 1:500,000.

Weaver, C.E., 1920, The Mineral Resources of Stevens County: Washington Geological Society Bulletin No. 20, 350 p.

Stoffel, K.L., Joseph, N.L., Waggoner, S.Z., Gulick, C.W., Korosec, M.A., Bunning, B.B., 1991, Geologic Map of Washington--Northeast Quadrant: Washington Division of Geology and Earth Resources Geologic Map, GM-39, scale 1:250,000.

Waggoner, S.Z., 1990a, Geologic Map of the Coulee Dam 1:100,000 Quadrangle, Washington: Washington Division of Geology and Earth Resources Open File Report 90-15, scale 1:100,000.

Becraft, G.E. and Weis, P.L., 1963, Geology and Mineral Deposits of the Turtle Lake Quadrangle, Washington: U. S. Geological Survey Bulletin 1131, 73 p., 2 pls.

NGMDB product
Counties Ferry - Lincoln - Skagit - Snohomish - Stevens