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Siletz River Volcanics and related rocks (ORTsr;0)

Geologic Formation in OR

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Lithology

Siltstone

Sandstone

Siltstone

Other (2,505 mi²)

Geologic Age

Paleocene – Middle Eocene (66 Ma – 37.8 Ma ago)

Geologic Unit Label

Tsr

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 ñ 3.1 to 58.1 ñ 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

— USGS

Taxonomy

Classified As

Geologic Formation