Rattlesnake Formation

Rattlesnake Formation
Stratigraphic range: Miocene-Late Pliocene
Rattlesnake Formation near Picture Gorge.jpg
The caprock in this photo (near Picture Gorge) is the ignimbrite layer
TypeSedimentary and igneous
OverliesMascall Formation, Columbia River Basalt Group
AreaJohn Day Valley
Thickness700 feet (210 m)
Lithology
PrimaryFanglomerate, siltstone
OtherTuff (ignimbrite)
Location
Coordinates44°30′N 119°36′W / 44.5°N 119.6°W / 44.5; -119.6Coordinates: 44°30′N 119°36′W / 44.5°N 119.6°W / 44.5; -119.6
Approximate paleocoordinates44°30′N 116°54′W / 44.5°N 116.9°W / 44.5; -116.9
RegionOregon
Country United States
Extenteastern Oregon
Type section
Named forRattlesnake Creek
Named byJ. C. Merriam
Year defined1901
Rattlesnake Formation is located in the United States
Rattlesnake Formation
Rattlesnake Formation (the United States)
Rattlesnake Formation is located in Oregon
Rattlesnake Formation
Rattlesnake Formation (Oregon)
John Day Fossil Beds map

The Rattlesnake Formation is a Miocene to late Pliocene geologic formation found along the John Day River Valley of Oregon, in the Western United States.

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 unit which crops out in the middle of the section. The gravel is well rounded and consists of pebbles of basalt, chert, siltstone, diorite, rhyolite, and chert set in a medium-grained matrix of poorly indurated volcanic sandstone. The ignimbrite displays zonation typical of welded ashflow tuffs and is a prominent ridge former.

Age

The ignimbrite was radiometrically dated by the Potassium–argon method at 6.4 million years by Evernden and James (1964).

Fossil content

Restoration of the animals of the Rattlesnake Formation

Middle and late Pliocene mammals 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.

In the lower fanglomerate member of the Rattlesnake Formation, remains of Pekania occulta were found.


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