Rattlesnake Formation


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 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.

Fossils

Middle and late Pliocene mammals have been recovered from beneath the ignimbrite.Fossils include ambeledon,Mylohyus,teleoceras,Indarctos,pliohippus and possibly Machairodus