Siletz River Volcanics

Siletz River Volcanics
Stratigraphic range: Selandian-Ypresian (Tiffanian-Bridgerian) 59.6–47.6  Ma PreꞒ Ꞓ O S D C P T J K Pg N
Exposed pillow lava in the Northern range
Type	Formation
Underlies	Yamhill Formation
Lithology
Primary	Sills of tholeitic to alkalic basalts
Other	Tuff breccia, siltstone, sandstone
Location
Coordinates	44°54′N123°24′W / 44.9°N 123.4°W / 44.9; -123.4
Approximate paleocoordinates	47°00′N107°30′W / 47.0°N 107.5°W / 47.0; -107.5
Region	Benton, Coos, Douglas, Lane, Lincoln, Polk, Tillamook, Washington & Yamhill Counties, Oregon
Country	United States
Type section
Named for	Siletz River
Siletz River Volcanics (the United States) Show map of the United States Siletz River Volcanics (Oregon) Show map of Oregon

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.

Description

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

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

• Coldwater Beds of British Columbia
• Golden Valley Formation of North Dakota
• Hanna Formation of Wyoming
• Indian Meadows Formation of Wyoming
• Margaret Formation of Ellesmere Island, Northwest Territories and Nunavut
• Nanjemoy Formation of Virginia, Maryland and District of Columbia
• Paskapoo Formation of Alberta
• Wasatch Formation of Colorado, Idaho, Montana, New Mexico, Wyoming and Utah
• Washakie Formation of Colorado and Wyoming
• Willwood Formation of Wyoming

References

1. Siletz River Volcanics at Fossilworks.org
2. "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

• Hickman, C.S (1976), "Pleurotomaria (Archaeogastropoda) in the Eocene of the northeastern Pacific: a review of Cenozoic biogeography and ecology of the genus", Journal of Paleontology , 50: 1090–1102
• Snavely, Parke D.; MacLeod, Norman S.; Wagner, Holly C. (1968), "Tholeiitic and alkalic basalts of the Eocene Siletz River Volcanics, Oregon Coast Range", American Journal of Science , 266 (6): 454–481, Bibcode:1968AmJS..266..454S, doi:10.2475/ajs.266.6.454

Further reading

• 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