Yamhill Formation and related rocks (upper and middle Eocene) (ORTy;0)

Massive to thin-bedded concretionary marine siltstone and thin interbeds of arkosic, glauconitic, and basaltic sandstone; locally contains interlayered basalt lava flows and lapilli tuff. Foraminiferal assemblages in siltstone referred to the Ulatisian and lower Narizian Stages (Snavely and others, 1969; McKeel, 1980) Includes the Elkton Formation of Baldwin (1974; also see Beaulieu and Hughes, 1975), which consists of thin-bedded siltstone and minor sandstone interbeds

State	Oregon
Name	Yamhill Formation and related rocks (upper and middle Eocene)
Geologic age	Middle Eocene to Late Eocene
Original map label	Ty
Primary rock type	siltstone
Secondary rock type	sandstone
Other rock types	basalt; tuff
Lithologic constituents	Major Sedimentary > Clastic > Siltstone (Bed) Minor Sedimentary > Clastic > Sandstone (Bed) Incidental Igneous > Volcanic > Mafic-volcanic > Basalt (Pyroclastic, tuff) Igneous > Volcanic > Mafic-volcanic > Basalt (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., Jr., McLeod, N.S., and Rau, w.w., 1969, Geology of the Newport area, Oregon: The Ore Bin, v. 31, p. 25-71. McKeel, D.R., 1980, Micropaleontological study of five wells, western Willamette Valley, Oregon: Oregon Department of Geology and Mineral Industries Open-File Report 0-80-1, 21 p. Baldwin, E.M., 1974, Eocene stratigraphy of southwestern Oregon: Oregon Department of Geology and Mineral Industries Bulletin 83, 40 p. Beaulieu, J.D., and Hughes, P.W., 1975, Environmental geology of western Coos and Douglas Counties, Oregon: Oregon Department of Geology and Mineral Industries Bulletin 87, 148 p.
Geographic coverage	Clatsop - Coos - Douglas - Lincoln - Polk - Tillamook - Washington - Yamhill