Siletz River Volcanics

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Exposed pillow lava in the Northern range.

The Siletz River Volcanics, located in the Oregon Coast Range, United States, are a sequence of basaltic pillow lavas that make up part of Siletzia. The basaltic pillow lavas originally came from submarine volcanoes that existed during the Eocene.

The Paleocene to Eocene volcanics consist of volcanism flows and sills of tholeitic to alkalic basalts with associated tuff-breccia, siltstone and sandstone. The flows are vesiculated with zeolite filled amygdules.

The volcanics originated as oceanic crust and seamounts. Potassium argon dating gives an age of 50.7 ± 3.1 to 58.1 ± 1.5 Ma.[1]

The sequence has been divided into a lower pillowed tholeiitic unit and an upper porphyritic alkali basalt unit.[2]

The volcanics occur in the following counties of western Oregon: Benton, Coos, Douglas, Lane, Lincoln, Polk, Tillamook, Washington and Yamhill.[1]


  1. ^ a b "Siletz River Volcanics and related rocks". USGS Mineral Resources On-Line Spatial Data. U.S. Geological Survey. 
  2. ^ Snavely, Parke D.; MacLeod, Norman S.; Wagner, Holly C. (June 1968). "Tholeiitic and alkalic basalts of the Eocene Siletz River Volcanics, Oregon Coast Range". American Journal of Science. 266 (6): 454–481. doi:10.2475/ajs.266.6.454.