The Owyhee Plain Domain is a relatively flat to gently undulating tract about 40 miles on a side, along the Owyhee River drainage in the northwestern corner of the evaluation area. This domain contains a broad and essentially undisturbed expanse of the Late Miocene Big Island Formation. The Big Island Formation is composed of 8.2-10.6 Ma basalt flows, cinder cones, and dikes which were erupted along the southern extension of the Snake River Plain. These basalts are essentially unfaulted and must have been erupted after major Basin and Range extension. Discontinuous patches of older Miocene rhyolites and ignimbrites and Oligocene ignimbrites are exposed beneath the Big Island basalt flows. These older rocks also show almost no high-angle faulting. Miocene rhyolites in the northeastern portion of the domain however, are cut by several sub-parallel north-northwest-trending and nearly east-west-trending high-angle normal faults with minor displacement. The Owyhee Plain Domain then is characterized by relatively few high-angle normal faults and no exposure of low-angle faults or large-scale folds.

A small faulted exposure of the Permian-Pennsylvanian Havallah Formation is present in the southwestern portion of the Domain east of Milligan Creek. This exposure suggests that allochthonous rocks related to the Golconda thrust may be present beneath the nearly undeformed Oligocene through Quaternary cover.

The only well penetration in the area is the Exxon 1 Fourmile Butte-Federal in Sec. 8, T. 43 N., R. 49 E. This well apparently penetrated 14,500 feet of volcanics and thin interbedded volcaniclastic sands in the southern portion of the domain.