Mascall Formation

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

Mascall Formation
Stratigraphic range: Hemingfordian-Barstovian ~20.4–13.6 Ma PreꞒ Ꞓ O S D C P T J K Pg N
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[1]

John Day Fossil Beds map

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. 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."^[2][3]

Age

The ignimbrite was radiometrically dated at 13 million years.^[2] Parts of the Mascall are interfingered with the Columbia River Basalt Group.

Fossils

Barstovian vertebrates have been recovered from the Mascall.