The Pilot Shale was named by Spencer (1917) for Pilot Knob in the western portion of the Ely Quadrangle, approximately 35 miles northeast of the Nye-White Pine County boundary. Because of the lithologic variability of the unit, we follow the designation Pilot Formation as suggested by Willden and Kistler (1979).

The Pilot Formation is Late Devonian to Early Mississippian (Kinderhookian). The lower contact of the Pilot Formation is diachronous. In the Eureka district the Pilot Formation underlies the Joana Limestone and overlies the Devils Gate Limestone. In the southern Diamond Mountains and western Pancake Range, and at Lone Mountain, the Pilot is overlain by the Chainman Formation (Nolan and others, 1956; Poole and Sandberg, 1977). The Pilot overlies the Devonian Guilmette and underlies the Joana Limestone in the Silver Island and northern Egan Ranges (Poole and Sandberg, 1977).

The Pilot Formation is generally a platy, brownish-grey to black, pink to yellowish-brown weathering, silty to sandy, calcareous and phosphatic shale interbedded with olive-gray, dolomitic siltstone. Where well exposed, the lower portion of the formation is more calcareous with thin beds of pinkish shaly limestone along with platy calcareous shales. The upper portion is yellowish-brown to grey weathering, black, platy, calcareous shale (Nolan and others, 1956; Merriam, 1963).

In the northern Shoshone Range, the Pilot Formation is exposed within the Gold Acres window in T.28 N., R. 47 E. (Gilluly and Gates, 1965), where both the top and bottom of the formation lie along faults. The Pilot is a dark gray to pinkish, yellowish or purplish brown weathering limy shale with a few thin interbedded limestones. Conodonts collected from the Pilot here are Lower Devonian in age (Gilluly and Gates, 1965). This exposure is the farthest west outcrop of the Pilot Formation within northeastern Nevada.

In the southern Cortez Mountains, two small thrusted outcrops of Pilot Formation are present near the Cortez Mine. Elsewhere in the range the Pilot is covered by small chips of shale and limestone (Gilluly and Masursky, 1965). In the Cortez Mountains, the Pilot is a highly folded, dark-gray, shaly limestone which weathers to shades of pink and yellow, and contains thin interbeds of dark-gray calcareous sandstones and black chert beds up to 1 inch in thickness. Echinoderm and questionable syringoporoid coral debris have been found in the limestones.

In the Diamond Mountains, the Pilot Formation is a medium-gray to dark-gray or brownish calcareous shale with a darker gray shale near the top of the formation (Larson and Riva, 1963). The shale weathers to a pale-reddish or yellowish brown, or to a light-gray.

In the Ruby Mountains, the Pilot Formation is mainly dark gray to black, thin-bedded and platy, tan to yellowish-gray weathering shaley and sandy limestone which is gradationally overlain by the Joana Limestone (Willden and Kistler, 1979). Thin black, light brown-gray weathering carbonaceous shales are interlayered with the limestones, and are particularly abundant near the top of the formation.

Near the Buck Mountain-Bald Mountain area, the Pilot Formation is a medium to dark gray to yellowish gray siltstone, with a few 3 to 6 inch thick beds of brown-gray crinoidal limestone which becomes more abundant upward in the unit (Rigby, 1960). Near Moody Peak in the Pancake Range, the Pilot is a yellowish siltstone and silty limestone (Kleinhampl and Ziony, 1985).

In the northern Reveille Range, the Pilot is composed of chert, silicified dark gray laminate to thick-bedded shale and minor dark-gray to yellowish and brownish-gray weathering laminated to splintery fissile shale and gray laminated siltstone (Ekren, Rogers, and Dixon, 1973). In the White Pine Range the Pilot is composed of thin-bedded and platy, yellow-brown to black, pinkish-weathering shale, with brown to black calcareous shale and siltstone, and yellow, sandy, platy limestone in the lower portion of the unit (Gaal, 1958).

In the southern Cherry Creek Range and southern Butte Mountains, the Pilot interval is represented by poorly exposed, yellow to orange weathering, platy siltstone and shale with light gray weathering limestone forming about 3 percent of the formation (Fritz, 1968; Douglass, 1960). In the northern Cherry Creek Range, the Pilot is dark gray-brown weathering calcareous siltstone, and thin-bedded and platy, cross-bedded fine-grained light brown sandstone (Coats, 1985). Shaly brown-gray limestone, with shale ripups, are abundant near the top of the formation.

In the northern Egan Range, the Pilot Formation can be broken into a lower brownish, red, and yellow weathering shaly limestone with minor intercalated resistant, thin-bedded, shaly limestones, and an upper unit composed of less calcareous darker and finer-grained shale which weathers to black flakes in the soil (Woodward, 1962). In the southern Egan Range, Kellogg (1963) divided the West Range Limestone, now the Pilot Formation, into a lower unit of thin-bedded argillaceous limestone with intercalated shale, overlain by an upper unit of quartzose sandstone overlain with platy limestone and interbedded calcareous mudstone. In the central Schell Creek Range, the Pilot is composed of gray and brown, thin-bedded and silty limestone and calcareous siltstone (Conway, 1965).

In the Snake Range, the Pilot Formation is composed of olive to dark-gray, calcareous, laminated siltstone, and both calcareous and non-calcareous shales with interbedded medium to dark gray to pinkish weathering silty limestone, and limestone in beds 2 inches to 2 feet thick that are more abundant in the upper portion of the unit (Whitebread, 1969). The limestones contain both chert granules and scattered lenses of chert, and may compose 30 to 50 percent of the unit. In the central Schell Creek Range, the Pilot is poorly exposed, tan to black, pink-weathering, platy siltstone and shale which have been tectonically segmented by low-angle normal faults (Young, 1960).

In the Spruce Mountain area in southern Elko County, Hope (1972) mapped one small exposure of Pilot Formation which is a light gray, platy, calcisiltite and interbedded olive-green platy shale. Thorman (1962) reported about 500 feet of Pilot Formation in the Pequop Mountains. The formation is composed of medium-bedded to platy, argillaceous, gray, pinkish-gray to olive weathering limestone, with interbedded medium brown-gray shale which weathers to a reddish-brown.

In the western part of the Red Hills, the Pilot is divided into three members (Bartel, 1968). The lower limestone and shale is composed of 276 feet of fine-grained, medium gray, fossiliferous limestone with interbedded, dark brown, orange-tan weathering thin-bedded and fissile, locally silty shale. The middle silty shale member is 196 feet of thin-bedded and platy, light-brown, yellowish-brown weathering silty shale. The upper black shale member is about 280 feet of dark gray or black non-calcareous, carbonaceous, brittle shale (Bartel, 1968). Brachiopods and worm burrows were reported in the lower limestone member.

In the Kern Mountains, the Pilot Formation is highly deformed and poorly exposed. The lower portion of the unit is composed of dark brown to purplish-black, calcareous siltstone with a few lenses of dark brown fossiliferous limestone (Nelson, 1959). The upper portion of the unit is alternating ten foot thick beds of flaggy, dark bluish-gray limestone, and brown-weathering quartzose sandstone, with interbedded black fissile shale (Nelson, 1959).

Tschanz and Pampeyan (1970) report that the Pilot Formation in northern Lincoln County contains little shale and is primarily a thin-bedded or platy, silty limestone which weathers a yellowish-orange or gray, and often contains thin-bedded, gray, cherty limestone and fine-grained sandstone in the lower half of the formation. At Dutch John Mountain, the Pilot is a calcareous platy siltstone and silty limestone. The upper 85 feet are thin-bedded platy calcareous shale, and the lower 300 feet are fossiliferous yellowish-orange weathering platy fossiliferous limestone (Tschanz and Pampeyan, 1970). In the Seaman Range, the Pilot Formation is a yellow-orange calcareous siltstone or silty limestone.

Because the Pilot Formation is the "grease" for much of the low-angle thrust and normal fault dislocations in northeastern Nevada, its thickness is highly variable. Its absence, for instance, near Lund and Duckwater (Langenheim, 1956a, 1956b) and across portions of Lincoln County (Tschanz and Pampeyan, 1970), have been attributed to pre-Early Mississippian erosion, but are at least in part the result of tectonic thinning along low-angle normal faults. Less than 10 feet of Pilot Formation are present in the northern Grant, White Pine, Horse, and Pancake Ranges (Kleinhampl and Ziony, 1985).

The Pilot Formation is 315 to 425 feet in the Diamond Mountains (Roberts and others, 1967) and 100 to 125 feet thick near Moody Peak in the Pancake Range (Kleinhampl and Ziony, 1985). It is 300 to 400 feet thick in the Shoshone Range, where both the base and top have been faulted out (Gilluly and Gates, 1965; Stewart and McKee, 1977), and is 300 feet thick in the southern Cortez Range where contacts are thrust faults (Gilluly and Masursky, 1965). 645 feet of Pilot are present in the northern Reveille Range (Ekren, Rogers, and Dixon, 1973), 400 to 600 feet in the southern Ruby Mountains near Overland Pass (Rigby, 1960; Hose and Blake, 1976) and about 300 feet along the crest and eastern flank of the Ruby Mountains (Willden and Kistler, 1979).

In the Buck Mountain-Bald Mountain area, the Pilot Formation is 400 to 500 feet thick (Rigby, 1963). It is 75 to 100 feet thick in the southern White Pine Range (Hose and Blake, 1976) and about 270 feet in the northwestern White Pine Range (Gaal, 1958), 400 to 515 feet thick in the northern Egan Range and about 200 feet near Lund (Playford, 1961), about 125 feet thick south of Sunnyside (Kellogg, 1963) and less than 50 feet thick just north of Shingle Pass (Tschanz and Pampeyan, 1970; Woodward, 1962). The Pilot is 50 to 150 feet thick in the southern Schell Creek Range (Tschanz and Pampeyan, 1970; Hose and Blake, 1976) from 0 to 600 feet in the north-central Schell Creeks (Conway, 1965) and about 700 feet in the northern Schell Creek Range (Dechert, 1967). It is 615 feet thick in the southern Cherry Creek Range (Fritz, 1968), about 500 feet in the Pequop Mountains (Thorman, 1962), 400 to 800 feet in the southern Snake Range (Whitebread, 1969), 756 feet in the western Red Hills (Bartel, 1968) and 950 feet in the southern Red Hills (Hose and Blake, 1976), 400 to 540 feet in the Kern Mountains (Nelson, 1959), 170 feet in the West Range (Tschanz and Pampeyan, 1970), 90 feet in the Seaman Range (Tschanz and Pampeyan, 1970), 140 feet in the Golden Gate Range (Tschanz and Pampeyan, 1970), 385 feet on Dutch John Mountain (Merriam, 1940), and 425 feet in the southern Silver Island Range (Blue, 1960).

The Pilot is exposed throughout the evaluation area in the Gold Acres window of the eastern Shoshone Range, southern Cortez Mountains, Lone Mountain, southern Diamond Mountains, Pancake Range, southern Ruby Mountains, Hot Creek, Reveille, Antelope, White Pine, Egan, Cherry Creek and southern Schell Creek Ranges, Red Hills, Butte and Kern Mountains, southern Snake, West, northern Pahroc, Seaman and Golden Gate Ranges, Dutch John Mountain, southern Silver Island and northern Leppy Ranges, Spruce Mountain Quadrangle and Pequop Mountains.

The diachronous boundary of the Pilot Formation, although not demonstrably an unconformity, most probably represents the inception of erosion of the Roberts Mountains allochthon. The Pilot is considered the initial Mississippian influx of terriginous sediments into the Antler foreland basin. This early portion of the Antler foreland is referred to by Poole and Sandberg (1977) as the "Pilot protoflysch basin", and formed after Late Devonian initiation of the Antler or Roberts Mountains thrusting.

The deposition of the Pilot Formation represented the first major interruption in continuous carbonate deposition over the region from Middle Ordovician to Late Devonian time. The details of sedimentation, particularly water depth, are controversial. Poole and Sandberg (1977) suggest that these calcareous phosphatic siltstones and shales were slowly deposited in a starved basin under slightly anoxic conditions. Wilson and Laule (1979) as well as Nolan and others (1956) suggest that the Pilot Formation is shallow-marine in origin and in part represents brackish-water deposition as indicated by linguloid brachiopods, plant fragments, and rippled siltstones. The observations of Western Cordillera geologists throughout the evaluation area suggest that much of the Pilot Formation is shallow-marine and brackish in origin.

Forty-six (46) surface geochemical samples were collected from the Pilot Formation during this evaluation. These samples show the Pilot to be mature to over-mature at the surfaces, with an average TOC of 0.99 percent. The Pilot Formation is a good potential source rock within a narrow fairway. This fairway may represent the deep-water portion of the formation, deposited under local anoxic conditions or at least conditions hostile to a burrowing infauna. A discussion of the geochemical analysis of the Pilot is provided in the Geochemical and Geothermal Data Volume, and are contoured on Overlays IX and X.