Siletz River Volcanics

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.[1][2] The basaltic pillow lavas originally came from submarine volcanoes that existed during the Eocene.

Siletz River Volcanics
Stratigraphic range: Selandian-Ypresian (Tiffanian-Bridgerian)
59.6–47.6 Ma
Exposed pillow lava in the Northern range
TypeFormation
UnderliesYamhill Formation
Lithology
PrimarySills of tholeitic to alkalic basalts
OtherTuff-breccia, siltstone, sandstone
Location
Coordinates44°54′N 123°24′W / 44.9°N 123.4°W / 44.9; -123.4
Approximate paleocoordinates47°00′N 107°30′W / 47.0°N 107.5°W / 47.0; -107.5
RegionBenton, Coos, Douglas, Lane, Lincoln, Polk, Tillamook, Washington & Yamhill Counties, Oregon
Country United States
Type section
Named forSiletz River
Siletz River Volcanics is located in the United States
Siletz River Volcanics
Siletz River Volcanics (the United States)
Siletz River Volcanics is located in Oregon
Siletz River Volcanics
Siletz River Volcanics (Oregon)

Description

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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 ages of 58.1 ± 1.5 to 50.7 ± 3.1 Ma; Selandian to Ypresian.[2]

The sequence has been divided into a lower pillowed tholeiitic unit and an upper porphyritic alkali basalt unit.[3]

The volcanics occur in the following counties of western Oregon: Benton, Coos, Douglas, Lane, Lincoln, Polk, Tillamook, Washington and Yamhill.[2]

Fossil content

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The sedimentary beds at the Ellendale Basalt and Portland Cement Company Quarries, interbeds in the upper part of the Siletz River volcanics, have provided fossils of the archaeogastropods Pleurotomaria (Entemnotrochus) baldwini, P. (E.) schencki and P. (E.) siletzensis.[4]

See also

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References

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  1. ^ Siletz River Volcanics at Fossilworks.org
  2. ^ a b c "Siletz River Volcanics and related rocks". USGS Mineral Resources On-Line Spatial Data. U.S. Geological Survey.
  3. ^ Snavely et al., 1968
  4. ^ Hickman, 1976

Bibliography

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Further reading

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  • Baldwin, E.M., 1974, Eocene stratigraphy of southwestern Oregon: Oregon Department of Geology and Mineral Industries Bulletin 83, 40 p.
  • 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
  • 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