Siletz River Volcanics
Encyclopedia
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
ic pillow lava
Pillow lava
Pillow lavas are lavas that contain characteristic pillow-shaped structures that are attributed to the extrusion of the lava under water, or subaqueous extrusion. Pillow lavas in volcanic rock are characterized by thick sequences of discontinuous pillow-shaped masses, commonly up to one metre in...
s that make up part of the Oregon Coast Range
Oregon Coast Range
The Oregon Coast Range, often called simply the Coast Range and sometimes the Pacific Coast Range, is a mountain range, in the Pacific Coast Ranges physiographic region, in the U.S. state of Oregon along the Pacific Ocean...
, United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
. The basaltic pillow lavas originally came from submarine volcano
Submarine volcano
Submarine volcanoes are underwater fissures in the Earth's surface from which magma can erupt. They are estimated to account for 75% of annual magma output. The vast majority are located near areas of tectonic plate movement, known as ocean ridges...
es that existed during the Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...
.
The Paleocene
Paleocene
The Paleocene or Palaeocene, the "early recent", is a geologic epoch that lasted from about . It is the first epoch of the Palaeogene Period in the modern Cenozoic Era...
to Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...
volcanics consist of volcanism
Volcanism
Volcanism is the phenomenon connected with volcanoes and volcanic activity. It includes all phenomena resulting from and causing magma within the crust or mantle of a planet to rise through the crust and form volcanic rocks on the surface....
flows and sills
Sill (geology)
In geology, a sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock. The term sill is synonymous with concordant intrusive sheet...
of tholeitic
Tholeiite
The tholeiitic magma series is one of two main magma series in igneous rocks, the other magma series being the calc–alkaline. A magma series is a series of compositions that describes the evolution of a mafic magma, which is high in magnesium and iron and produces basalt or gabbro, as it...
to alkalic basalt
Alkali basalt
Alkali basalt is a fine-grained, dark-coloured, volcanic rock characterized by phenocrysts of olivine, titanium-rich augite, plagioclase feldspar and iron oxides. For similar SiO2 concentrations, alkali basalts have a higher content of the alkalis, Na2O and K2O, than other basalt types such as...
s with associated tuff
Tuff
Tuff is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption. Tuff is sometimes called tufa, particularly when used as construction material, although tufa also refers to a quite different rock. Rock that contains greater than 50% tuff is considered...
-breccia
Breccia
Breccia is a rock composed of broken fragments of minerals or rock cemented together by a fine-grained matrix, that can be either similar to or different from the composition of the fragments....
, siltstone
Siltstone
Siltstone is a sedimentary rock which has a grain size in the silt range, finer than sandstone and coarser than claystones.- Description :As its name implies, it is primarily composed of silt sized particles, defined as grains 1/16 - 1/256 mm or 4 to 8 on the Krumbein phi scale...
and sandstone
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
. The flows are vesiculated
Vesicular texture
Vesicular texture is a volcanic rock texture characterised by a rock being pitted with many cavities at its surface and inside. The texture is often found in extrusive aphanitic, or glassy, igneous rock...
with zeolite
Zeolite
Zeolites are microporous, aluminosilicate minerals commonly used as commercial adsorbents. The term zeolite was originally coined in 1756 by Swedish mineralogist Axel Fredrik Cronstedt, who observed that upon rapidly heating the material stilbite, it produced large amounts of steam from water that...
filled amygdules.
The volcanics originated as oceanic crust
Oceanic crust
Oceanic crust is the part of Earth's lithosphere that surfaces in the ocean basins. Oceanic crust is primarily composed of mafic rocks, or sima, which is rich in iron and magnesium...
and seamounts. Potassium argon dating gives an age of 50.7 ± 3.1 to 58.1 ± 1.5 Ma.
The sequence has been divided into a lower pillowed tholeiitic unit and an upper porphyritic
Porphyritic
Porphyritic is an adjective used in geology, specifically for igneous rocks, for a rock that has a distinct difference in the size of the crystals, with at least one group of crystals obviously larger than another group...
alkali basalt unit.
The volcanics occur in the following counties of western Oregon
Oregon
Oregon is a state in the Pacific Northwest region of the United States. It is located on the Pacific coast, with Washington to the north, California to the south, Nevada on the southeast and Idaho to the east. The Columbia and Snake rivers delineate much of Oregon's northern and eastern...
: Benton
Benton County, Oregon
-National protected areas:*Siuslaw National Forest *William L. Finley National Wildlife Refuge-Demographics:As of the census of 2000, there were 78,153 people, 30,145 households, and 18,237 families residing in the county. The population density was 116 people per square mile . There were 31,980...
, Coos
Coos County, Oregon
-National protected areas:*Bandon Marsh National Wildlife Refuge*Oregon Islands National Wildlife Refuge *Siskiyou National Forest *Siuslaw National Forest - Incorporated cities:- Unincorporated communities and CDPs:-See also:...
, Douglas
Douglas County, Oregon
-National protected areas:* Crater Lake National Park * Rogue River-Siskiyou National Forest * Siuslaw National Forest * Umpqua National Forest * Willamette National Forest -Adjacent counties:* Lane County, Oregon -...
, Lane
Lane County, Oregon
-National protected areas:*Oregon Islands National Wildlife Refuge *Siuslaw National Forest *Umpqua National Forest *Willamette National Forest -Government:...
, Lincoln
Lincoln County, Oregon
-National protected areas:*Oregon Islands National Wildlife Refuge *Siletz Bay National Wildlife Refuge*Siuslaw National Forest -Demographics:...
, Polk
Polk County, Oregon
Polk County is a county located in the U.S. state of Oregon. The county is named for James Knox Polk, the 11th president of the United States. In 2010, its population was 75,403. The seat of the county is Dallas....
, Tillamook
Tillamook County, Oregon
Tillamook County is a county located in the U.S. state of Oregon. The county is named for the Tillamook, a Native American tribe who were living in the area in the early 19th century at the time of European American settlement. In 2010, the county's population was 25,250...
, Washington
Washington County, Oregon
- Major highways :* Interstate 5* Interstate 205* U.S. Route 26* Oregon Route 6* Oregon Route 8* Oregon Route 10* Oregon Route 47* Oregon Route 99W* Oregon Route 210* Oregon Route 217* Oregon Route 219-Demographics:...
and Yamhill
Yamhill County, Oregon
-National protected areas:*Siuslaw National Forest *Tualatin River National Wildlife Refuge -Demographics:As of the census of 2000, there were 84,992 people, 28,732 households, and 21,376 families residing in the county. The population density was 119 people per square mile . There were 30,270...
.