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Siletz River Volcanics and related rocks (middle and lower Eocene and Paleocene)

Aphanitic to porphyritic, vesicular pillow flows, tuff-breccias, massive lava flows and sills of tholeiitic and alkalic basalt. Upper part of sequence contains numerous interbeds of basaltic siltstone and sandstone, basaltic tuff, and locally derived basalt conglomerate. Rocks of unit pervasively zeolitized and veined with calcite. Most of these rocks are of marine origin and have been interpreted as oceanic crust and seamounts (Snavely and others, 1968). Foraminiferal assemblages referred to the Ulatisian and Penutian Stages (Snavely and others, 1969); K-Ar ages range from 50.7 +/- 3.1 to 58.1 +/- 1.5 Ma (Duncan, 1982); includes the lower part of the Roseburg Formation of Baldwin (1974), which has yielded K-Ar ages as old as 62 Ma
StateOregon
NameSiletz River Volcanics and related rocks (middle and lower Eocene and Paleocene)
Geologic agePaleocene to Middle Eocene
Original map labelTsr
Primary rock typetholeiite
Secondary rock typealkaline basalt
Other rock typesvolcanic breccia (agglomerate); sandstone; siltstone; conglomerate
Lithologic constituents
Major
Igneous > Volcanic > Mafic-volcanic > Basalt (Volcaniclastic, volcanic breccia)
Igneous > Volcanic > Mafic-volcanic > Basalt (Flow)
Minor
Sedimentary > Clastic > Conglomerate (Bed)
Igneous > Hypabyssal > Mafic-hypabyssal > Hypabyssal-basalt (Dike or sill)
Incidental
Sedimentary > Clastic > Siltstone (Bed)
Sedimentary > Clastic > Sandstone (Bed)
Map references
Walker, G.W. and MacLeod, N.S., 1991, Geologic map of Oregon: U.S. Geological Survey, scale 1:500,000.
Unit references
Walker, G.W. and MacLeod, N.S., 1991, Geologic map of Oregon: U.S. Geological Survey, scale 1:500,000.
Snavely, P.D., MacLeod, N.S., and Wagner, H.C., 1973, Miocene tholeiitic basalts of coastal Oregon and Washington and their relations to coeval basalts of the Columbia Plateau: Geological Society of America Bulletin, v. 84, p. 387-424.
Snavely, P.D., MacLeod, N.S., and Wagner, H.C., 1968, Tholeiitic and alkalic basalts of the Eocene Siletz River Volcanics, Oregon: american Journal of Science, v. 266, p. 454-481.
Duncan, R.A., 1982, A captured island chain in the coast Range of Oregon and Washington: Journal of Geophysical Research, v. 87, p. 10,827-10,837.
Baldwin, E.M., 1974, Eocene stratigraphy of southwestern Oregon: Oregon Department of Geology and Mineral Industries Bulletin 83, 40 p.
Geographic coverageBenton - Coos - Douglas - Lane - Lincoln - Polk - Tillamook - Washington - Yamhill

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