# Siletz River Volcanics Canonical page: https://lithomap.app/resources/geologic-formations/united-states/siletz-river-volcanics Last reviewed: September 17, 2026 Page purpose: Comprehensive United States geologic formation reference for human readers, search systems, and language models. Major region: Pacific Coast Regional setting: Central and northern Oregon Coast Range Age: Paleocene to early Eocene Age detail: Largely about 56–49 million years ago within the broader Siletzia volcanic province Depositional setting: Large submarine volcanic province with local emergent centers Rock types: Basalt, Pillow basalt, Tuff breccia, Lapilli tuff, Volcaniclastic sediment States in recognized usage: Oregon ## Summary The Siletz River Volcanics are a thick stack of oceanic basalt, pillow lava, and fragmental volcanic rock that forms much of the basement beneath the Oregon Coast Range. ## Distribution The unit is exposed in the central Oregon Coast Range and continues widely in the subsurface. Deep river canyons reveal volcanic sections that may be thousands of feet thick, while younger marine sediment covers broad areas. Correlations become more complicated toward northern Oregon, where younger volcanic units were once grouped with the Siletz River rocks. Modern maps separate those packages using age, chemistry, magnetic polarity, and stratigraphic position. ## Geological history Fluid basalt erupted underwater and formed rounded pillow structures, breccia, and tuff; massive flows and intrusions developed around volcanic centers. Marine siltstone and sandstone accumulated between some eruptive episodes. The volcanic plateau was later attached to North America, uplifted, and eroded. Circulating fluids altered volcanic glass and filled fractures and pillow cavities with secondary minerals. ## Fossil record The basalt itself is generally unfossiliferous. Interbedded marine sediment can preserve microfossils, and fossils in overlying strata help constrain when eruption and accretion occurred. Fossil scarcity is meaningful: repeated lava and coarse volcanic debris created harsh depositional settings, while only quieter sedimentary intervals offered conditions favorable for preservation. ## Minerals and geological resources Plagioclase, pyroxene, magnetite, and altered volcanic glass dominate the primary rock. Cavities and fractures may contain calcite, quartz, pyrite, smectite, and zeolites including analcime, stilbite, heulandite, natrolite, and laumontite. Zeolite assemblages vary from site to site and record alteration rather than a continuous mineral zone. Roadcuts, quarries, and river canyons require permission and careful hazard assessment. ## Field context - Pillow structures are easiest to interpret in intact outcrop, not from isolated rounded float blocks. - Coast Range slopes are commonly wet, vegetated, and prone to falling rock or debris movement. - Check local maps because related Eocene basalts have been revised and subdivided. ## Connected field guides - Basalt (rock; Named rock type) — https://lithomap.app/resources/rock-identification/basalt - Breccia (rock; Named rock type) — https://lithomap.app/resources/rock-identification/breccia - Calcite (mineral; Mineral discussed in this formation) — https://lithomap.app/resources/mineral-identification/calcite - Magnetite (mineral; Mineral discussed in this formation) — https://lithomap.app/resources/mineral-identification/magnetite - Pyrite (mineral; Mineral discussed in this formation) — https://lithomap.app/resources/mineral-identification/pyrite - Quartz (mineral; Mineral discussed in this formation) — https://lithomap.app/resources/mineral-identification/quartz Mapped location and formation context support an identification but cannot establish one. Compare several physical properties and use laboratory analysis when the material remains ambiguous. ## Connected state geology guides - Oregon geology — https://lithomap.app/resources/geology/united-states/oregon ## Authoritative sources - Geologic history of Siletzia — U.S. Geological Survey: https://www.usgs.gov/publications/geologic-history-siletzia-a-large-igneous-province-oregon-and-washington-coast-range - Zeolites in Siletz River pillow lavas — U.S. Geological Survey: https://www.usgs.gov/publications/zeolites-eocene-basaltic-pillow-lavas-siletz-river-volcanics-central-coast-range - Oregon Coast Range groundwater geology — U.S. Geological Survey: https://pubs.usgs.gov/wri/1976/0090/report.pdf ## Formation photograph - Image: https://lithomap.app/_delivery/formations/siletz-river-volcanics.webp - Description: Dark volcanic rocks exposed in the Oregon Coast Range - Creator: M.O. Stevens - License: Public domain — https://creativecommons.org/publicdomain/mark/1.0/ - Source record: https://commons.wikimedia.org/wiki/File:OregonCoastRangeRocks.JPG Formation-level information does not prove that a fossil or mineral occurs at a specific coordinate. It also does not establish ownership, access, collecting permission, present conditions, or specimen identity.