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Lower Tertiary volcanic rocks, undivided

Predominantly andesite flows and flow breccia; includes basalt flows, minor rhyolitic rocks, and some sedimentary rocks.
State Washington
Name Lower Tertiary volcanic rocks, undivided
Geologic age Eocene
Comments More recent maps show that this unit is predominantly basalt and rhyolite with subordinate andesite and dacite (including Silver Pass volcanic) (Luedke, 1998, Tabor and others, 1988, 1993, Dragovich and others, 2002). Barlow Pass volcanics mostly altered to "dense mat" of chlorite, epidote, calcite, and sericite (Tabor and others, 1988, 1993).
Primary rock type basalt
Secondary rock type rhyolite
Other rock types andesite; dacite; sandstone; conglomerate; shale; argillite
Lithologic constituents
Major
Igneous > Volcanic > Mafic-volcanic > Basalt (Flow)
Igneous > Volcanic > Felsic-volcanic > Rhyolite (Flow)
Minor
Igneous > Volcanic > Mafic-volcanic > Basalt (Volcaniclastic, volcanic breccia)Possibly only incidental in rank (estimated rank based on rank of basalt, regardless of form, being major).
Igneous > Volcanic > Felsic-volcanic > Rhyolite (Pyroclastic, tuff)Possibly only incidental in rank (estimated rank based proportion of rhyolite, regardless of form, being major).
Igneous > Volcanic > Mafic-volcanic > Andesite (Flow)
Igneous > Volcanic > Felsic-volcanic > Dacite (Flow)
Incidental
Igneous > Volcanic > Mafic-volcanic > Basalt (Pyroclastic, tuff)Possibly minor in rank (estimated rank based on rank of basalt, regardless of form, being major).
Igneous > Volcanic > Mafic-volcanic > Andesite (Volcaniclastic, volcanic breccia)
Igneous > Volcanic > Mafic-volcanic > Andesite (Pyroclastic, tuff)
Igneous > Volcanic > Felsic-volcanic > Dacite (Volcaniclastic, volcanic breccia)
Igneous > Volcanic > Felsic-volcanic > Dacite (Pyroclastic, tuff)
Sedimentary > Clastic > Sandstone (Bed)Tuffaceous and feldspathic.
Sedimentary > Clastic > Conglomerate (Bed)Tuffaceous and feldspathic.
Sedimentary > Clastic > Mudstone > Shale (Bed)
Metamorphic > Argillite
Map references
Huntting, M. T., Bennett, W. A. G., Livingston, V. E. Jr., Moen, W. S., 1961, Geologic Map of Washington: Washington Division of Mines and Geology, scale 1:500,000.
Unit references
Huntting, M. T., Bennett, W. A. G., Livingston, V. E. Jr., Moen, W. S., 1961, Geologic Map of Washington: Washington Division of Mines and Geology, scale 1:500,000.
Luedke, Robert G., 1998, Maps showing distribution, composition, and age of Early and Middle Cenozoic volcanic centers in Oregon and Washington: U.S. Geological Survey Miscellaneous Investigations Series Map I-2291-D, scale 1:1,000,000.
Tabor, R. W., Frizzell, V. A., Jr., Booth, D. B., Waitt, R. B., Whetten, J. T., Zartman, R. E., 1993, Geologic Map of the Skykomish River 30- by 60-Minute Quadrangle, Washington: U. S. Geological Survey Miscellaneous Investigations Series Map I-1963, scale 1:100,000.
Dragovich, J. D., Logan, R. L., Schasse, H. W., Walsh, T. J., Lingley, W. S., Jr., Norman, D. K., Gerstel, W. J., Lapen, T. J., Schuster, J. E., Meyers, K. D., 2002, Geologic Map of Washington-Northwest Quadrant: Washington Division of Geology and Earth Resources Geologic Map GM-50, scale 1:250,000.
Tabor, R. W., Booth, D. B., Vance, J. A., Ford, A. B., Ort, M. H., 1988, Preliminary geologic map of the Sauk River 30 by 60 minute quadrangle, Washington: U. S. Geological Survey Open File Report 88-692, 51 p., 1 pl.
Counties King - Kittitas - Snohomish