The Loray Formation was named for exposures at the head of Loray Wash in a Southern Pacific Railroad cut in Sec. 28, T. 38 N., R. 68 E. on the southwest side of Montello Valley in northeastern Elko County (Steele, 1960).

The Loray is considered Permian (Late Leonardian to Early Guadalupian) in age (Bissell, 1964). The Loray conformably overlies the Pequop and underlies the Kaibab Formations (Steele, 1960).

At the type section on the southwestern flank of Montello Valley, the Loray Formation is composed of yellow-tan gypsiferous silts and thin bioclastic limestones (Steele, 1960). In the Medicine Range, the formation is thin-bedded to thinly laminated silty dolomite and siltstone, with sporadic beds of shelly and algal limestone in the lower half of the formation where it intertongues with and overlies the Pequop Formation (Collinson, 1968). Ostracodes and mollusks are sparse within the formation.

At Dead Horse Wash (Sec. 2, T. 15 N., R. 62 E.) in the central Egan Range near Ely, over 1,000 feet of the Loray are exposed and have been described by Bissell (1964). The lower 262 feet are interbedded red, brown, orange-yellow, and gray siltstones, shaly fine-grained sandstones, claystones and shale, silty argillaceous micrite to skeletal limestone, and a small amount of collapse breccia. Overlying this are about 420 feet of thin-bedded to platy and medium-bedded, silty and argillaceous, micritic to skeletal limestones with pelecypods and gastropods, calcisiltites, and earthy shaly friable quartz sandstones. Bissell (1964) reports minor interbeds of encrinal and lithoclastic limestones saturated with dead hydrocarbons. The next 145 feet of the Loray consists of interbedded collapse limestone breccia and soft red-brown shaly sandstones, calcisiltites, and platy mollusk-bearing limestones. This is overlain by 93 feet of gypsum and anhydrite interbedded with minor claystone. The uppermost 111 feet are platy brown-weathering skeletal limestones with gastropods, pelecypods, and nautiloids, and thin silty fine-grained sandstones (Bissell, 1964). This section is typical of several exposed within the Egan Range.

In the central Butte Mountains near Moorman Ranch, the Loray Formation is dominantly thinly bedded, very fine to fine-grained, yellowish-orange weathering siltstone and sandstone interbedded with yellowish-gray to olive, very fine-grained, silty limestone and dolomite. The upper part of the formation contains several beds of light gray to brown gypsum and anhydrite, evaporitic dolomite, gray collapse or solution breccias, and abundant soft sediment slump features (Steele, 1960; Bissell, 1964). Bissell (1964) reported relatively abundant gastropods, pelecypods, and scaphopods in the grayish platy limestones. The section along the eastern flank of the Butte Mountains is similar with red, gray, yellow, and orange shale, calcisiltite, dolomite, argillaceous limestone, dolomitic and calcareous fine-grained quartz sandstone, collapse limestone breccia and claystone (Bissell, 1964). The limestones are fetid and contain abundant gastropods and pelecypods.

In the Maverick Springs Range, the Loray Formation consists of several hundred feet of interbedded lavender to gray argillaceous micritic limestone, calcisiltite, claystone, and fine-grained calcareous sandstone with limestone collapse breccia and a basal 4 foot thick granule to pebble conglomerate (Bissell, 1964). The limestones contain dead oil staining, and are strongly fetid.

In the Cherry Creek Range east of Corral Canyon, the basal 130 feet of the Loray are pink-gray to orange-brown interbedded thin to medium-bedded, dolomitic sandstone, sandy dolomite, evaporitic aphanitic dolomite, and cherty dolomitic limestone overlain by about 30 feet of algal micritic limestone (Bissell, 1964). Overlying this is about 425 feet of interbedded red, brown, orange-gray and tan calcisiltite, shale, claystone, collapse limestone breccia, dolomite, argillaceous mollusk-bearing limestone, and peletoid micrite. Next are 550 feet of interbedded red and gray, orange-gray and red weathering evaporitic dolomite, calcareous and dolomitic siltstone, silty sandstone, laminated argillaceous limestone and collapse breccia. The uppermost 332 feet of the Loray are interbedded porous sandy limestone, calcareous dolomite, dolomitic limestone, calcareous and dolomitic siltstone and shale (Bissell, 1964).

In the southern Pequop Mountains near Brush Creek, the Loray is about 1,000 feet of interbedded gray, yellow, and red, laminated and fetid, argillaceous limestone, dolomite, calcareous-dolomitic siltstones and sandstones (Bissell, 1964). In the Spruce Mountain Quadrangle, the Loray Formation is a sequence of interbedded platy, tan-weathering siltstone and silty limestone which overlie the Pequop and are overlain by the Kaibab Formation (Hope, 1972). The thickness of the formation was not measured here.

In the Dolly Varden Mountains about 550 feet of Loray Formation are exposed. The formation is varicolored dolomitic and calcareous as well as evaporitic siltstones, friable to carbonate-cemented quartz sandstones, dolomites, and argillaceous limestones (Bissell, 1964).

In the southern Schell Creek Range in T. 12 N., R. 65 E., nearly 1,000 feet of Loray were described by Bissell (1964). The basal 200 feet are varicolored shale, claystone, argillaceous platy limestone with gastropods, and collapse breccia, and are overlain by about 207 feet of interbedded yellow and brown claystones, red-gray argillaceous mollusk-bearing limestones, and red-gray to tan collapse breccia. The uppermost 495 feet are composed of interbedded claystone, shale, gypsiferous sandstone, siltstone, fetid limestone, and collapse breccia (Bissell, 1964).

The Loray is 1,937 feet thick in the central Butte Mountains (Bissell, 1964; replaces Steele's (1960) thickness of 2,475 feet), and 1,357 feet in the northern Butte Mountains (Bissell, 1964). The Loray was measured in several ranges by Bissell (1964). These thicknesses include 1,007 to 1,031 feet thick in the central Egan Range, 476 feet in the Maverick Springs Range, 439 feet near Ruby Wash north of the Maverick Springs Range, 1,007 feet in the southern Pequop Mountains, 550 feet in the Dolly Varden Mountains and 902 feet in the southern Schell Creek Range (Bissell, 1964).

The Loray is exposed in the Butte, Pequop and Dolly Varden Mountains, Ferguson Mountain, Egan, Medicine, Maverick Springs, Cherry Creek and southern Schell Creek Ranges, and the Spruce Mountain Quadrangle.

The Loray is considered a very shallow restricted marine to brackish water unit which formed as part of a regressive cycle of sedimentation within the Butte-Deep Creek basin or trough (Stevens, 1965). The Loray was deposited in shallow lakes, ponds and bays where hypersaline gypsiferous lithologies and finely laminated dolomites could form (Collinson, 1968).