Rattlesnake Formation

Rattlesnake Formation
Stratigraphic range: Miocene to 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
Other tuff (ignimbrite)
Location
Extent eastern Oregon
Type section
Named for Rattlesnake Creek
Named by J. C. Merriam (1901)[1]

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]

Fossils

Middle and late Pliocene mammals have been recovered from beneath the ignimbrite.

References

  1. ↑ Merriam, J. C., 1901. A contribution to the geology of the John Day Basin, Oregon: Univ. Calif., Dept. Geol. Sci. Bull., v. 2, p. 269-314.
  2. ↑ Geologic Formations of Eastern Oregon (East of longitude 121°30'), 1972. John D. Beaulieu. Bulletin 73. Oregon Department of Geology and Mineral Industries.
  3. ↑ Evernden, J. F., and James, G. T., 1964. Potassium-argon dates of the Tertiary floras of North America: Am. Jour. Sci., v. 262, p. 945-974.


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