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Mascall Formation
Stratigraphic range: Hemingfordian-Barstovian
~20.4–13.6Ma
Mascall Formation near Picture Gorge.jpg
Outcrop near Picture Gorge
Type Sedimentary
Underlies Rattlesnake Formation
Overlies John Day Formation
Area John Day Valley, Fox Basin
Thickness 2,000 feet (610 m)
Lithology
Primary Sandstone, conglomerate
Location
Extent eastern Oregon
Type section
Named for Mascall Ranch (south of Dayville)
Named by J. C. Merriam
Year defined 1901
NPS john-day-fossil-beds-geologic-map
John Day Fossil Beds map

The Mascall Formation is a Miocene 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 Mascall Formation consists of a maximum of 2,000 feet of fluvial sandstone, ash, light colored water-laid tuff, and well-rounded conglomerate. The conglomerate is increasingly coarse-grained towards the east and is composed of clasts of chert, metavolcanics, and rhyolite. Within the Mascall Formation is a widespread ignimbrite unit which consists of 97 to 99 percent glass shards and minor amounts of anorthoclase, quartz, magnetite, zircon, and clinopyroxene. That 10 percent of the anorthoclase consists of orthoclase is considered to be a unique feature of the unit by Davenport (1970)."

Age

The ignimbrite was radiometrically dated at 13 million years. Parts of the Mascall are interfingered with the Columbia River Basalt Group.

Fossils

Barstovian vertebrates have been recovered from the Mascall.

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