The Sheep Pass Formation was named for exposures of lacustrine sediments along Sheep Pass Canyon in the Egan Range. The type section is within Sec. 7 and 12, T. 10 N., R. 62 E. (Winfrey, 1960).

The Sheep Pass Formation is considered Middle Eocene in age (Winfrey, 1960; Brokaw and Shawe, 1965). The Sheep Pass has been correlated with similar lacustrine sediments exposed in the Antelope and northern Schell Creek Ranges where they were called the Kinsey Canyon Formation by Young (1960). At its type locality the Sheep Pass Formation unconformably overlies the Ely Limestone and is disconformably overlain by the 34 Ma Oligocene Garrett Ranch Volcanics (Murray and Bortz, 1967).

Winfrey (1960) divided the Sheep Pass Formation into six members, which are described in ascending order. The basal Member A is a calcareous cemented conglomerate-breccia about 650 feet thick at the type section and thins rapidly both to the north and south. The cobbles and boulders of Pennsylvanian and Permian limestones and Mississippian sandstones are angular near the base of the unit, and subrounded near the top. No clasts older than Mississippian have been found in the unit suggesting the Mississippian was the oldest portion of the exposed erosional surface at this locality during the Eocene-Oligocene.

The overlying Member B is composed of a lower portion of brown to buff mudstone and reddish brown arenaceous limestone; a middle section of brownish gray, shaly to silty, lithographic limestones containing gastropod and ostracod coquina beds, and minor interbedded brown mudstones and gray to black shales; and an upper portion of gray to dark-gray, platy to thin-bedded, fine-grained, fetid, ostracod and gastropod-bearing limestones which contain tarry residues along fractures and partings.

Member C is about 640 feet of light brown, calcareous and limonite-cemented, fine to medium-grained, quartz-rich sandstones interbedded with massive 1 to 3 foot thick, bluish, fine to medium-grained, friable sandstones, and lesser thin brown ostracod and pelecypod-bearing shale and siltstone, gray arenaceous limestone, and pebble conglomerate.

Member D is about 730 feet of white, siliceous, very fine grained, calcareous siltstone with rare gastropods. It is gradationally overlain by about 90 feet of chert-bearing limestones of Member E. Member F is the uppermost member of the Sheep Pass and is about 150 feet of poorly exposed, red, green, and gray claystone and calcareous siltstone. It is locally removed along an erosional unconformity at the top of the Sheep Pass Formation.

The Kinsey Canyon section is composed of 100 to 200 feet of basal conglomerate composed of angular fragments of Pennsylvanian and Permian limestones up to an inch in diameter that pinches out both to the north and south. Overlying the conglomerate is about 300 feet of white, thin-bedded to platy, ostracodal limestones, calcareous mudstone and siltstone, and subordinate laminated sandstone. Tuffaceous beds at the top of the unit are probably part of the overlying Kalamazoo Volcanics. The Kinsey Canyon was correlated with the Sheep Pass by Young (1960) and is here considered an eastern section of the Sheep Pass Formation.

Small patches of Early Paleocene lacustrine sediments in the central Schell Creek Range are composed of about 50 feet of dark greenish-gray shale and mudstone, and tuffaceous sandstone and chert pebble-conglomerate beds (Drewes, 1967). This local exposure may be equivalent to the Sheep Pass Formation.

In the Grant Range all six members of the Sheep Pass Formation are exposed with a total thickness of about 500 feet, while in the Pancake Range only the lower two members are present with a total thickness of about 220 feet (Winfrey, 1960 as measured from cross-section).

The Sheep Pass Formation is about 3,000 feet thick at its type locality in the Egan Range and thins radially outward (Winfrey, 1960). Kellogg (1964) measured 3,325 feet of Sheep Pass Formation in the southern Egan Range where it pinches out entirely a few miles to the south at Shingle Pass. Scott (1965) measured 400 to 700 feet of section in various localities in the Grant Range.

The Sheep Pass Formation is exposed in the Pancake, Grant and Egan, Quinn Canyon, Antelope, Schell Creek and Golden Gate Ranges, and discontinuously in the subsurface of Railroad and White River Valleys. The Sheep Pass basin covers an area of between 1,100 and 1,800 square miles (Winfrey, 1960; Scott, 1965).

Winfrey (1960) suggested that the Sheep Pass Formation was deposited in a series of small lacustrine basins that may or may not have been interconnected. The continuity of various members over large distances suggests that much of the Sheep Pass was originally continuous, with some local high topography in the Egan Range (Kleinhampl and Ziony, 1985; Kellogg, 1964). The volcanic tuffs overlying the Sheep Pass are continuous and anomalously thick, reflecting a regional topographic low in the Grant Range area during the Eocene. The Sheep Pass predates volcanic activity since tuffaceous debris are absent from the formation. It reflects the initial stages of extension responsible for Basin and Range topography.

Gas chromatographic comparison of Sheep Pass extractable organic matter, with oil from the Eagle Springs Oil Field in Railroad Valley suggests that the Sheep Pass Formation is the most probable source of oil for that field. Geochemical analysis done during this evaluation indicates that much of the formation is organic lean and that the organic facies of the Sheep Pass is areally restricted.