The Rattlesnake Formation is a

Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

to late

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58geologic formation A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics ( lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exp ...

found along the John Day River Valley of

Oregon Oregon ( , ) is a U.S. state, state in the Pacific Northwest region of the United States. It is a part of the Western U.S., with the Columbia River delineating much of Oregon's northern boundary with Washington (state), Washington, while t ...

, in the

Western United States The Western United States (also called the American West, the Western States, the Far West, the Western territories, and the West) is List of regions of the United States, census regions United States Census Bureau. As American settlement i ...

Description

The formation is described in ''Geologic Formations of Eastern Oregon'' (1972) as follows:

The unit is composed of up to 700 feet of fanglomerate and finer terrestrial sediments and a 40-foot thick
ignimbrite Ignimbrite is a type of volcanic rock, consisting of hardened tuff. Ignimbrites form from the deposits of pyroclastic flows, which are a hot suspension of particles and gases flowing rapidly from a volcano, driven by being denser than the surrou ...
unit which crops out in the middle of the section. The gravel is well rounded and consists of pebbles of
basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...
,
chert Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a prec ...
,
siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility. Although its permeabil ...
,
diorite Diorite ( ) is an intrusive rock, intrusive igneous rock formed by the slow cooling underground of magma (molten rock) that has a moderate content of silica and a relatively low content of alkali metals. It is Intermediate composition, inter ...
,
rhyolite Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture (geology), texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained matri ...
, and chert set in a medium-grained matrix of poorly indurated volcanic sandstone. The ignimbrite displays zonation typical of welded ashflow
tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock co ...
s and is a prominent ridge former.

Age

The ignimbrite was

radiometrically dated Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed. The method compares t ...

by the Potassium–argon method at 6.4 million years by Evernden and James (1964).

Fossil content

Middle and late Pliocene

mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

s have been recovered from beneath the ignimbrite. Fossils found here include '' Amebelodon sp.'', the extinct peccary '' Mylohyus longirostris'', the extinct North American rhino '' Teleoceras fossiger'', '' Indarctos oregonensis'', '' Pliohippus spectans'', '' Machairodus sp.'', '' Hemiauchenia vera'', an extinct species of fox known as '' Vulpes stenognathus'' and the earliest record of modern-day

beavers Beavers (genus ''Castor'') are large, semiaquatic rodents of the Northern Hemisphere. There are two existing species: the North American beaver (''Castor canadensis'') and the Eurasian beaver (''C. fiber''). Beavers are the second-large ...

. In the lower fanglomerate member of the Rattlesnake Formation, remains of '' Pekania occulta'' were found.Pekania occulta type locality
at

Fossilworks Fossilworks was a portal which provides query, download, and analysis tools to facilitate access to the Paleobiology Database, a large relational database assembled by hundreds of paleontologists from around the world. History Fossilworks was cr ...

.org

References

Bibliography

* {{cite journal , last1=Samuels , first1=J. X. , first2=J. , last2=Cavin , year=2013 , title=The Earliest Known Fisher (Mustelidae), a New Species From the Rattlesnake Formation of Oregon , journal=

Journal of Vertebrate Paleontology The ''Journal of Vertebrate Paleontology'' is a bimonthly peer-reviewed scientific journal that was established in 1980 by Jiri Zidek (University of Oklahoma). It covers all aspects of vertebrate paleontology, including vertebrate origins, evolu ...

, volume=33 , issue=2 , pages=448–454, doi=10.1080/02724634.2013.722155 , bibcode=2013JVPal..33..448S Geologic formations of Oregon Miocene Series of North America Pliocene Series of North America Neogene geology of Oregon Hemphillian Sandstone formations of the United States Siltstone formations of the United States Tuff formations of the United States Miocene volcanism Alluvial deposits Paleontology in Oregon Formations