The Roberts Mountains Formation was designated by Merriam (1940) for the section between the Hanson Creek Formation and Lone Mountain Dolomite, between the north and south forks of Pete Hanson Creek on the west side of the Roberts Mountains.

The Roberts Mountains Formation is an equivalent of the Masket Shale of Kay and Crawford (1964). It is Middle Silurian (Niagaran) to possibly Late Devonian in age (Roberts and others, 1967). Graptolites of Middle Silurian (Llandovery to lower Wenlock) age have been recovered from the formation in the Toquima Range (McKee, 1976b). Mullens (1980) speculates that the Roberts Mountains is also equivalent to the Chellis Limestone, Storff Formation, and the Van Duzer Limestone in the Bull Run Mountains (Decker, 1962), as well as the lithologically similar Noh Formation in the HD Range (Riva, 1970) of northern Elko County.

The Roberts Mountains Formation commonly consists of medium to dark grey, micritic, platy and shaly, wackestone and lime mudstone. Lesser amounts of thick-bedded, graded and nongraded skeletal packestone and grainstone of allodapic origin are 30-40 centimeters thick and constitute less than 5 percent of the unit (Merriam and others, 1975). The lower half of the formation is commonly a laminated to slabby argillitic limestone with the upper half dolomitic limestone, dolomite, and calcareous mudstones. The lower portion commonly contains thin chert nodules, stringers, and lenses one-half to six inches thick with the base marked by a massive bed of black chert (Nolan and others, 1956; Merriam, 1960). The formation is dolomitic at Lone Mountain, in the Mahogany Hills and Pinon Range (Merriam, 1960; Smith and Ketner, 1975).

Mullens (1980) has broken the Roberts Mountains Formation into three units in ascending order. The chert unit is composed of half chert and half limestone in beds up to 50 cm thick. The black and brown cherts form 25 to 100 percent of this lower unit which is as much as 50 meters thick. The overlying laminated limestone unit is platy, medium to coarse-grained, black to dark-gray, silty and carbonaceous limestone in lenticular beds, with thin interbedded black and brown chert as layers and nodules. The limestones weather pink, red and reddish-brown or brown and are locally dolomitic and cherty (Mullens, 1980). The interbedded laminated limestone and coarse grained limestone unit contains gray to black carbonaceous and silty laminated limestone and clastic and pelletal fossiliferous coarse grained limestone beds commonly 3 to 12 meters thick. Included in this unit locally, are portions of the Rabbit Hill, Windmill, Wenban, and McMonnigal Formations which according to Mullens (1980) have been broken out and treated inconsistently in the literature.

In the Toquima, Toiyabe, and Shoshone Ranges, the Roberts Mountains Formation consists of dark-grey to black carbon-rich (enough to "smear the fingers"), thin-bedded, platy and silty limestones which weather into pink, light brown and light gray chips and plates. Graptolites and corals are abundant within the limestones particularly within graded bioclastic medium-bedded limestones (Kay and Crawford, 1964; Gilluly and Gates, 1965; McKee, 1976; Stewart and McKee, 1977). Faults are often present on the top and/or bottom of the formation and beds are locally overturned as in the Goat window area of the Shoshone Range. In the Shoshone Range, the dolomitic upper portion of the Roberts Mountains Formation and Lone Mountain Dolomite are not present, which may be the result of faulting or an unconformity (Gilluly and Gates, 1965). In the Toquima Range, yellowish-orange, shaly and platy, argillitic limestone and siliceous argillite form the rounded hills which expose the Roberts Mountains Formation (Kleinhampl and Ziony, 1985). Thick-bedded bioclastic coquina beds are also present in the unit in the Toquima, Monitor, and Hot Creek Ranges, where many outcrops are too small to depict on our map.

In the Monitor Range, the Roberts Mountains Formation is a graptolitic, platy to flaggy limestone which forms broken slopes that are reddish to yellowish-brown in color. Some beds contain quartz silt or sand and chert lamellae and lenses up to several inches thick that are erratically distributed throughout the unit (Kleinhampl and Ziony, 1985). In the southern Monitor Range the base of the formation is a low-angle fault and the upper part of the formation has been removed by erosion (Kleinhampl and Ziony, 1985). Wise (1977) described the Roberts Mountains interval in the Dobbin Summit area as about 60 percent fine-grained, laminated, argillaceous and silty, gray dolomites which weather to a pinkish-gray, brown, and yellow gray, and about 40 percent graded, medium-grained, pelletal dark, gray packstone and wackestone concentrated in the upper and lower portions of the formation. Fossiliferous, massive, gray lime wackestone breccias are present in the lower third and at the top of the formation (Wise, 1977). Limestone and dolomite clasts in these breccias are commonly .5 to 3 inches in diameter and up to 7 inches long. Fossils within the formation include crinoids, bryozoans, brachiopods, trilobites, gastropods, horn and colonial corals.

In the northern portion of the Reveille Range the Roberts Mountains interval is a medium to dark-gray dolomite (Ekren, Rogers, and Dixon, 1973). The dolomite is a medium-grained, thin to thick-bedded and finely banded rock with abundant pelmatazoan debris and nodular to lenticular chert partially or wholly replaced by dolomite in the upper 40 and lower 100 feet of the formation (Ekren, Rogers and Dixon, 1973).

In the Hot Creek Range, the Roberts Mountains Formation is composed of dolomite rather than limestone. Near Tybo, the upper two-thirds of the unit are medium to dark gray, fine to medium-grained, laminated to thick-bedded dolomite. The lower third of the unit is dark-gray, laminated, fine-grained slightly phosphatic dolomite with lenticular layers of brown-weathering silty dolomite and black chert with biohermal layers up to 20 feet thick, and a basal 10 to 20 foot thick zone of chert nodules and lenses (Quinlivan and Rogers, 1974). In Hot Creek Canyon, the base of the formation is a 7 to 10 foot thick light-gray chert overlain by alternating, medium-gray, massive, laminated, and very fine-grained dolomite, and coarser grained ledge-forming bioclastic limestone and dolomite (Kleinhampl and Ziony, 1985). The bioclastic beds have abundant corals, brachiopods, bryozoa, crinoids, and various bioherms up to 3 feet long and about 1 foot thick (Kleinhampl and Ziony, 1985).

A distinctive thin-bedded black chert zone occurs within the basal 10 to 30 feet of the formation in several localities. Where absent, (Cortez area and northern Shoshone Range, Clear Creek area of the Monitor Range) the base of the formation is marked by the transition from massive limestones below to platy limestones above. Stewart and McKee (1977) suggest this contact may be a significant unconformity, at least in Lander County. As evidence, they point to a significant time gap at the base of the Roberts Mountains Formation as suggested by paleontological data, the presence of the formation on rocks of various ages in differing areas, and the apparent absence of Eureka Quartzite and Hanson Creek Formation in southern Lander County as a result of erosional removal below the formation.

In the Roberts Mountains, the lower portion of the formation is a dark-gray, platy, shaly to silty limestone with coarser grained crinoidal limestones up to 2 feet in thickness. The upper 800 feet is a more massive, light grey to brown, slightly dolomitic coral and brachiopod-rich limestone (Merriam, 1960). The upper contact with the Lone Mountain is a gradational change to light gray dolomite. Where both formations are dolomitic it is difficult to distinguish between the two.

In the Sulphur Spring Range, the Roberts Mountains Formation lies between the Hanson Creek Formation and Lone Mountain Dolomite (Coats, 1985). It is a thin-bedded and platy, silty gray limestone to calcareous siltstone with minor medium to coarse-grained clastic limestone.

Johnson (1959) mapped the Roberts Mountains Formation in the northern Simpson Park Mountains as thin-bedded and platy, gray to brown, silty limestone; 1,255 feet of the limestone is Silurian in age and 400 feet are Devonian, according to Berry and Murphy (1975).

In the Cortez and Tuscarora Mountains, the Roberts Mountains Formation is composed of dark gray to black, laminated, silty limestone which is platy, very fine to medium-grained, and locally shows a small amount of dolomite near the top of the unit (Gilluly and Masursky, 1965; Roberts and others, 1967). As much as 3 percent organic carbon has been described in the Roberts Mountains Formation in the Cortez Mountains (Gilluly and Masursky, 1965). The top of the Roberts Mountains Formation here is defined at the base of the lowest bioclastic limestone bed of the Wenban Limestone, which is in gradational contact (Stewart and McKee, 1977).

In the Bootstrap window area of the Tuscarora Mountains, Evans and Mullens (1976) have described the Roberts Mountains Formation as silty, dolomitic, laminated, dark brown to black limestone with black chert beds 1 to 2 inches thick, and coarse-grained limestone with scoured, graded, conglomeratic, and bioclastic beds. Fossils in the unit include corals, brachiopods, algae, and stromatoporoids. The laminated silty limestone and coarse grained limestone are in a ratio of 4:1 (Evans and Mullens, 1976). In the Bootstrap area, the unit is overlain by several hundred feet of the Rabbit Hill Limestone (Coats, 1985). The Roberts Mountains Formation has been mapped in the Rodeo Creek (Evans, 1974a), and Welches Canyon (Evans, 1974b) Quadrangles within the Tuscarora Mountains.

In the southern Independence Mountains, the Roberts Mountains Formation has been mapped in the Schroeder Mountain (Evans and Cress, 1972), and Swales Mountain (Evans and Ketner, 1971) Quadrangles. At Swales Mountain it is platy, black and gray, silty, carbonaceous limestone with bedding varying from an inch to several feet, and silt content from 15 to 40 percent. The Roberts Mountains is exposed here and in the Carlin and Maggie Creek areas as windows through the Roberts Mountains thrust (Evans and Ketner, 1971). At Schroeder Mountain, the Roberts Mountains Formation is composed of platy to slabby, dark-gray to black, argillaceous and dolomitic, silty micrite with thin iron-stained chert lenses, and minor amounts of interbedded, massive, medium-grained peloidal sparite (Cress, 1972).

At Lone Mountain in the Independence Mountains, the Roberts Mountains Formation is a platy, thin to medium-bedded, carbonaceous, blue-gray limestone which is conformably overlain by the Nevada Formation (Ketner, 1975b). In the Wheeler Mountain area of the northern Independence Mountains as well as to the north, the Roberts Mountains includes rocks originally mapped by Kerr (1962) as the Roberts Mountains Formation and the overlying Taylor Canyon Formation (Coats, 1985). The lower 108 feet is light-gray calcisiltite and calcilutite which is dolomitized near the base and is overlain by about 65 feet of alternating thin-bedded black chert and gray calcisiltite. The upper 1,150 feet are calcareous argillaceous quartz siltite (Coats, 1985).

In the Carlin-Pinon Range, the Roberts Mountains Formation is exposed in a small thrust slice less than a quarter of a mile across (Smith and Ketner, 1975). This allochthonous section is composed of platy, silty and shaly, very dark brown to black laminated dolomite, and dark gray to black, dolomitic marl which are rich in graptolites and organic material.

In the Pequop Mountains, the Roberts Mountains Formation is present in a fault slice as 595 feet of non-fossiliferous light-gray, cherty limestone with about 45 feet of argillaceous non-resistant dolomite at the top of the formation (Thorman, 1970). The Roberts Mountains is conformably overlain by the Lone Mountain Dolomite.

In the Snake Mountains, the formation is platy and silty, light to dark gray and brown limestone interbedded with platy calcareous and silty shale. These carbonaceous limestones weather to an olive or light gray. The basal portion of the unit is about 30 feet of thin-bedded black chert interlaminated with dark-gray micritic limestone which overlies the Hanson Creek Formation (Gardner, 1968; Peterson, 1968).

The Roberts Mountains Formation has a variable thickness as a result of exposure and structural complications along thrust faults. It varies from l,650 feet in the Antelope Range (Merriam, 1963), 610 feet at Clear Creek in the Monitor Range (Greene, 1953) and 260 to 280 feet in the Dobbin Summit area of the Monitor Range (Wise, 1977), 740 feet at Lone Mountain (Merriam and Anderson, 1942), 1,900 feet in the Roberts Mountains (Merriam and Anderson, 1942), about 1,000 feet in the Cortez Mountains (Gilluly and Masursky, 1965), 1655 feet in the Simpson Park Mountains (Coats, 1985), 1,334 feet in the Independence Mountains near Wheeler Mountain (Kerr, 1962), 2,700 feet at Swales Mountain in the southern Independence Mountains (Evans and Ketner, 1971; Coats, 1985), 1,640 feet at Schroeder Mountain (Cress, 1972), 350 to 700 feet in the northern Toquima Range (McKee, 1976), 1,000 feet in the Toiyabe Range (Stewart and McKee, 1977), 555 feet in the Reveille Range (Ekren, Rogers, and Dixon, 1973), about 300 to 540 feet in the Hot Creek Canyon and Tybo areas of the Hot Creek Range (Quinlivan and Rogers, 1974; Kleinhampl and Ziony, 1985), 600 to 1,000 feet in faulted and locally overturned sections in the northern Shoshone Range (Gilluly and Gates, 1965), 1,700 feet in the Lynn window area of the Tuscarora Mountains (Roen, 1961), 200 to 500 feet in the Carlin-Pinon Range (Smith and Ketner, 1975), 640 feet in the Pequop Mountains (Thorman, 1970), and 800 to 1,500 feet in the northern and southern portions of the Snake Mountains (Gardner, 1968; Peterson, 1968). There is in general, a progressive northward thickening in the Roberts Mountains Formation.

The Roberts Mountains Formation is exposed within the Roberts, Cortez, Simpson Park, Independence and Tuscarora Mountains, Mahogany Hills, Lone Mountain, Carlin-Pinon, Monitor, Sulphur Spring, Fish Creek, Toquima, Toiyabe, Reveille, Hot Creek, and Shoshone Ranges, and Pequop and Snake Mountains. The Roberts Mountains Formation has not been found east of the Hot Creek Range.

The Roberts Mountains Formation was formed in a quiet water basinal setting along the outer shelf. Thin even laminations in these limestones indicate very slow deposition from suspension below storm and wave base. The preservation of these laminae indicate little disturbance by bioturbation or bottom currents (Matti and McKee, 1977).

Matti and McKee (1977) suggest the Roberts Mountains was deposited under very low oxygen to completely anoxic conditions as a result of restricted circulation and depths of deposition on the order of 250 meters, at or below the oceanic oxygen minimum zone. The formation may well have been deposited in a series of gently silled basins similar to modern borderland basins off the coast of western North America. Mullens (1980) feels that the abundant pyrite in the formation suggests reducing depositional conditions but that water depths were not more than 100 meters and certainly in most cases a few meters to tens of meters.

Coarse-grained, skeletal limestones and dolomites containing crinoids, corals, and brachiopods are interpreted as gravity flow deposits formed by grain-flow type mechanisms. These beds occasionally show grading and some evidence of reverse grading. Mullens (1980) interpreted the coarse-grained limestones and dolomites as a reefal facies with laminated limestone and dolomite representing fore-reef sediments, and cherts as inorganically precipitated calcite, possibly derived from ash, and later replaced by chert. Mullens (1980) has drawn some excellent facies distribution diagrams for the Roberts Mountains Formation.

The chert-bearing carbonates towards the base of the Roberts Mountains Formation have been interpreted as exotic restricted marine sediments which were deposited in local topographic depressions on the shelf (Matti and McKee, 1977). Lithologic variety within the Roberts Mountains has been ascribed to a complex and undulating paleosurface which represented the shelf upon which this formation was deposited.

Abrupt limestone to dolomite transitions within the Roberts Mountains and associated units represents abrupt topographic and bathymetric changes on the shelf margin (Matti and McKee, 1977). Winterer and Murphy (1960) suggested these rapid transitions represented a reef and lagoonal environment for the dolomitic rocks, separated from deeper fore-reef limestones. Mullens (1980) has suggested the northeast-southwest-trending Cortez-Uinta arch has influenced the depositional pattern and facies distribution within the Roberts Mountains Formation.

The anoxic environment under which the Roberts Mountains Formation formed makes it an attractive possibility as a source rock. Geochemical sampling and analysis during the course of this evaluation indicate TOC which varies from 0.05 to 5.67 percent, with an average of about 0.66 for 17 surface samples. Maturity data indicate that the Roberts Mountains Formation is thermally overmature to mature at the surface. The results of the geochemical analysis are discussed in the Geochemical and Geothermal Data Volume and are displayed on Overlays IX and X.

The Roberts Mountains Formation is also the host for gold ores in the Shoshone (Gold Acres), Tuscarora (Carlin, Bootstrap), and Cortez Ranges (Gilluly and Gates, 1965: Roen, 1961; Erickson and others, 1966).