The Big Island Formation is named for a mesa called the Big Island in the Jarbidge Quadrangle, and has a type section on the west wall of Jarbidge River canyon in Sec. 28, T. 11 N., R. 9 E. in southeastern Owyhee County, southwestern Idaho (Coats, 1985).

The Big Island Formation is Late Miocene in age. K-Ar ages within the volcanic portion of the unit are from 8.2 +/- 0.6 to 10.6 +/- 1.0 Ma (Mark and others, 1975). The Big Island commonly overlies the Cougar Point Tuff and is overlain by Quaternary deposits.

Much of the Big Island Formation is composed of relatively fresh, low alkali olivine basalt, up to 200 feet in thickness. The basalt is vesicular with porphyritic plagioclase laths and clots, and olivine as much as 3 mm in diameter in a groundmass of ophitic augite and olivine (Coats, 1985). Magnetite, illmenite and apatite are present as accessories in most samples. Feeder dikes and basalt buildups that represent either shield volcanoes or small cinder cones are locally present, as in the Owyhee Desert area (Coats, 1985).

The basalt in many places makes up the upper of three members exposed within the Big Island Formation. Below the basalt is about 20 feet of rhyolitic tuff, which is in turn underlain by 300 to 600 feet of unconsolidated gravel and boulder gravel with clasts from less than an inch to about 1 foot in diameter that are derived from the underlying Cougar Point Tuff or Paleozoic units (Coats, 1985).

The Big Island Formation ranges in thickness from about 200 to 900 feet across northwestern Nevada.

The Big Island Formation is exposed in a broad swath in northwestern Elko County from the Jarbidge Quadrangle on the east through the Hat Peak and Wilson Reservoir Quadrangles, and the quadrangles to the west that comprise the Owyhee basin area.

The Big Island Formation represents a Miocene basalt plain related to the volcanic extrusions of the Snake River Plain. The northwestern portion of the evaluation area is the southern extension of the Snake River Plain into northern Nevada.