Siletz River Volcanics and related rocks (middle and lower Eocene and Paleocene) (ORTsr;0)

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

State	Oregon
Name	Siletz River Volcanics and related rocks (middle and lower Eocene and Paleocene)
Geologic age	Paleocene to Middle Eocene
Original map label	Tsr
Primary rock type	tholeiite
Secondary rock type	alkaline basalt
Other rock types	volcanic 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 coverage	Benton - Coos - Douglas - Lane - Lincoln - Polk - Tillamook - Washington - Yamhill