Rattlesnake Formation
Stratigraphic range: Miocene-Late Pliocene
The caprock in this photo (near Picture Gorge) is the ignimbrite layer
Type	Sedimentary and igneous
Overlies	Mascall Formation, Columbia River Basalt Group
Area	John Day Valley
Thickness	700 feet (210 m)
Lithology
Primary	Fanglomerate, siltstone
Other	Tuff (ignimbrite)
Location
Coordinates	44°30′N 119°36′W / 44.5°N 119.6°W / 44.5; -119.6
Approximate paleocoordinates	44°30′N 116°54′W / 44.5°N 116.9°W / 44.5; -116.9
Region	Oregon
Country	United States
Extent	eastern Oregon
Type section
Named for	Rattlesnake Creek
Named by	J. C. Merriam
Year defined	1901[1]
Rattlesnake Formation (the United States) Show map of the United States Rattlesnake Formation (Oregon) Show map of Oregon

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.^[2]

Age

The ignimbrite was radiometrically dated by the Potassium–argon method at 6.4 million years by Evernden and James (1964).^[3]

Fossil content

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.^[4][5][6]

In the lower fanglomerate member of the Rattlesnake Formation, remains of Pekania occulta were found.^[7][8]