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Fisher and Eugene Formations and correlative rocks (Oligocene and upper Eocene)

Thin to moderately thick bedded, coarse- to fine-grained arkosic and micaceous sandstone and siltstone, locally highly pumiceous, of the marine Eugene Formation; and coeval and older andesitic lapilli tuff, breccia, water-laid and air-fall silicic ash of the continental Fisher and Colestin Formations; upper parts of the Fisher Formation apparently lap onto and interfinger with the Eugene Formation. Megafauna in the Eugene Formation were assigned an Oligocene age by Vokes and others (1951) and foraminifers have been assigned to the upper part of the lower Refugian Stage (McDougall, 1980), or of late Eocene age. Basalt lava flows in the Fisher Formation have yielded isotopic ages as old as 40 Ma (Lux, 1982), and south of the latitude of Cottage Grove the Fisher is overlain by a welded tuff in unit Tu dated at about 35 Ma. North of Eugene, rocks of this unit are overlain unconformably by continental volcanogenic rocks of unit Tu, including an ash-flow tuff with a K-Ar age of 30.9 +/- 0.4 Ma
State Oregon
Name Fisher and Eugene Formations and correlative rocks (Oligocene and upper Eocene)
Geologic age Late Eocene to Oligocene
Primary rock type sandstone
Secondary rock type siltstone
Other rock types tuff; volcanic breccia (agglomerate)
Lithologic constituents
Major
Sedimentary > Clastic > Sandstone (Bed)
Sedimentary > Clastic > Siltstone (Bed)
Minor
Igneous > Volcanic > Felsic-volcanic (Pyroclastic, air fall)
Incidental
Igneous > Volcanic > Mafic-volcanic > Andesite (Pyroclastic)
Igneous > Volcanic > Mafic-volcanic > Andesite (Volcaniclastic)
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.
Counties Douglas - Lane - Linn