The Guilmette Formation is named for exposures along Guilmette Gulch along the western flank of Deep Creek Mountains in Utah. The type section is in T. 9 S., R. 18 W. (Nolan, 1935).

The Guilmette Formation contains a Stringocephalus fauna which is Middle and Upper Devonian (Givetian through Fammenian) in age, and correlates with both the Nevada Formation (Bay State Dolomite) and Devils Gate Limestone (Hose, 1966). The Guilmette gradationally overlies the Simonson Dolomite with a contact marked by resistant medium to thick-bedded limestones above alternating layers of laminated dolomite and limestone below. It is generally overlain by the Pilot Formation, and locally where the Pilot is missing, as in the Grant Range, by the Joana Limestone.

Regionally, the Guilmette is a lithologically consistent unit which is characteristically an even-bedded, dark-gray to grayish-black, sublithographic limestone which weathers an olive to medium gray and is often cut by small white veins of calcite (Hose and Blake, 1976). These dark cliffy limestones are both thin and massively bedded, commonly in beds 1 to 5 feet in thickness, and occasionally reveal wavy and graded bedding. Lenticular zones of well-cemented intraformational solution-type limestone breccias up to 300 feet thick are present in several sections of the Guilmette (Kleinhampl and Ziony, 1985).

As much as 30 percent of many Guilmette sections are composed of alternating layers of medium to dark-gray brownish weathering dolomite, most common in the lower and upper portions of the unit. More than 50 percent of the section in the southern Egan Range is dolomite, and limestone is more prevalent in sections to the west and north of the southern Egan Range (Kleinhampl and Ziony, 1985). Kellogg (1960) reports a variation from about 1 percent dolomite in the north and about 56 percent dolomite in the south of his map area in the southern Egan Range.

Much of the upper Guilmette is biostromal with corals and stromatoporoids of either the Stromatopora or Amphipora types (Hose and Blake, 1976). Well-rounded, fine-grained quartz sandstone beds are locally present in the upper few hundred feet of the formation in the White Pine, Grant, Horse, and Seaman Ranges (Kleinhampl and Ziony, 1985; Lumsden, 1964; Tschanz and Pampeyan, 1970). The lower hundred feet of the unit are often a yellowish-weathering, thin-bedded, argillaceous to silty limestone and dolomite.

Along the northwest flank of the Ruby Mountains, graphitic dolomitic marble, originally termed the Snell Creek Marble, is correlated with the Guilmette by Howard (1971). The marble contains muscovite, tremolite, hornblende, idocrase and hematite, and calcite megacrysts which may represent recrystallized crinoid fragments.

In the Pancake Range and Buck Mountain-Bald Mountain area, about two-thirds of the Guilmette are exposed (Dressen, 1969; Rigby, 1963). The Guilmette here is composed of unfossiliferous, dense laminated dolomite and dolomitic limestone, interbedded with gray to dark brown, fine-grained limestone which is commonly in beds about 6 inches in thickness, but is more massive in the upper portion of the formation.

In the Grant and White Pine Ranges, the Guilmette is composed of a basal 200 foot section of interlayered thin-bedded fossiliferous (Cladopora and Stromatoporoidea) limestone, and thin-bedded, gray to brown dolomite; overlain by about 500 feet of blue-gray, dolomitic limestone and yellowish weathering nodular limestone; 300 feet of brecciated dark bluish-black limestone; and about 1,000 feet of dark-brown silty limestone and dolomite, interbedded with 2 to 4 foot thick beds of siltstone and fine-grained sandstone (Lumsden, 1964; Moores and others, 1968).

In the southern Cherry Creek Range, the Guilmette is composed of three members (Fritz, 1968). The lower third of the formation is a light gray, thick-bedded, dense, medium-grained limestone with some slightly thinner and darker limestones in the lower 200 feet. The middle member is composed of roughly equal amounts of brown calcareous shale and siltstone, and medium to thick-bedded, medium-gray limestone and brown dolomite. The upper third of the Guilmette is composed of thin to medium-bedded, dark gray to light gray weathering limestone, and medium-bedded, grey-brown dolomite with minor siltstone and shale. The lower 130 feet of the upper member is evenly bedded, dark gray, nodular limestone (Fritz, 1968). The upper contact between the Guilmette and the Pilot Formation is a bedding plane fault.

Woodward (1962) described four members in the Guilmette in the northern Egan Range. The lowest member, about 230 feet thick, is composed of a basal red, yellow, and gray, argillaceous and platy limestone overlain by interbedded dark-gray, dense limestone, and medium to dark-gray, finely laminated dolomite. The second member is composed of about 610 feet of light-gray weathering, massive, fine-grained limestone which forms steep cliffs. The third member is 960 feet of platy limestone with red, yellow and gray, sandy, silty and argillaceous intercalations up to 6 feet thick, and an upper portion composed of roughly equal amounts of dense to fine-grained, silty and argillaceous limestone, and brown and dark gray, finely laminated to coarse-grained dolomite in beds a few inches to 2 feet thick. The upper member is composed of about 410 feet of dense, black, dark gray weathering, gastropod-bearing limestone and a few dark-gray, medium to coarse-grained dolomite interbeds, overlain by about 90 feet of dense, tan, nodular limestone with yellow argillaceous and silty partings and abundant brachiopods and stromatoporoids, in turn overlain by the uppermost 50 feet of the formation which are fine-grained, medium gray limestone beds less than 1 foot thick.

Woodward (1962) notes that in contrast to the calcareous facies containing about 18 percent dolomite in the southern portion of his map area, the northern portion of the area exposes a dolomitic facies of Guilmette which is over 50 percent dolomite. This facies occurs along a flat fault which appears to have cut out the lower 200 feet of the formation. In this section, the Guilmette has a basal 100 feet section of interbedded, fine-grained, dark-gray to black limestone, and light gray to brown or black, medium-grained dolomite; overlain with 140 feet of massive, fine-grained, cliff forming, light to dark-gray limestone; 120 feet of medium-grained, brownish to gray dolomite; 150 feet of thin-bedded, black, platy dolomite with yellowish argillaceous partings in beds 2 to 6 inches thick which grades upward into 485 feet of finely laminated, brown and gray dolomite containing brachiopods and stromatoporoids in 2 to 3 inch beds; and 75 feet of blue-gray, nodular limestone with yellow argillaceous partings at the top of the formation. Woodward (1962) postulates that juxtaposition of such divergent facies may at least in part be due to structural dislocation.

Similar rapid Guilmette facies changes have been reported by Kellogg (1963) who documented a facies change from 1 percent to over 56 percent dolomite over approximately 30 miles in the southern Egan Range. Kellogg (1963) broke the Guilmette into two members. The lower member is massive calcareous siltstones and yellow clayey limestone, and dolomite, which he suggested may be equivalent to the Meister Member of the Devils Gate Limestone as defined by Nolan and others (1956). The upper member is composed of interbedded, fine-grained, medium-gray limestone, and gray to brown, medium-grained dolomite. Facies changes here appear to be depositional, and dolomites are probably secondary replacement of limestone.

In the central Schell Creek and Snake Ranges, the Guilmette is composed of several hundred feet of basal light to dark gray, fine to medium-grained, medium-bedded, cliff and ledge forming limestone, overlain by thin-bedded limestone with yellowish argillaceous partings interbedded with massive to thinly bedded, brown-weathering dolomite (Young, 1963; Whitebread, 1969). In the northern Schell Creek Range, Drewes (1967) recognized four members in the Guilmette. The lower member is a massive, cliff-forming gray limestone; overlain by a member of alternating dark brown, coarse-grained dolomite, and thin-bedded shaly limestone; overlain by a member of silty gray limestone and calcareous siltstone with a few thin interbeds of dolomite; and an uppermost member of limestone, with lesser amounts of sandstone and quartzite and limestone pebble conglomerate unconformably overlain by the Pilot Formation (Drewes, 1967).

In the western portion of the Red Hills, Bartel (1968) divided the Guilmette into four members. The lower member is 253 feet of massive, thick-bedded, medium to coarse-grained, light brown limestone, with 10 feet of laminated unfossiliferous dolomite in the middle of the unit. Overlying this is interbedded and alternating, massive, light to dark gray weathering, fine to coarse-grained, thin-bedded, locally fossiliferous limestone, and brown, laminated, silty and unfossiliferous dolomite which is locally fetid (Bartel, 1968). The next member is 665 feet of dark and light brown dolomite, and alternating light gray dolomite, with overlying thin-bedded, medium-gray, highly fossiliferous dolomite. The basal part of the member contains abundant, dark blue and black chert nodules, and abundant poorly preserved "spaghetti beds", gastropods, and brachiopods. The highest member is arenaceous, medium to dark brown weathering, mottled dolomite with reddish, fine to medium grained, dolomitic sandstone in the upper 3 feet (Bartel, 1968).

In the Kern Mountains the Guilmette is massive, dark blue-gray limestone, alternating with thin-bedded limestone which is overlain by a member of dark blue-gray limestone interbedded with dark brown and gray dolomite beds (Nelson, 1959).

In northern Lincoln County, the Guilmette is either represented by a light gray limestone facies, or a dolomite facies consisting of alternating light and dark gray "spaghetti" dolomite (Tschanz and Pampeyan, 1970). The limestone facies is present in the southern Egan Range and Dutch John Mountain area and grades southward into the dolomite facies in the Bristol, Fairview, and West Ranges. The base of the formation is always a thin-bedded, grayish-yellow, laminated, silty dolomite about 50 to 70 feet in thickness that locally contains several massive dark-gray limestone beds. These basal beds are overlain by several hundred feet of gray, cliff-forming limestone and dolomite and massive sedimentary reef breccias up to 100 feet thick composed primarily of large broken stromatoporoid heads (Tschanz and Pampeyan, 1970). A yellowish-brown crossbedded sandstone or quartzite is present at the top of the Guilmette in the Bristol and Seaman Ranges.

In the Windermere Hills, the Guilmette is represented by dark gray to black, light gray weathering, argillaceous and bituminous, calcilutite and calcisiltite, interbedded with dark-gray, calcareous dolomite in beds from a few feet to more than 10 feet in thickness (Oversby, 1972).

In the southern Goshute Range at Ferguson Mountain, Berge (1960) described the Guilmette as dark-gray to black, crystalline, cliff forming limestone. To the south in the White Horse Mountain area, the Guilmette is a light to dark blue-gray, thinly to thickly bedded, dense, medium-grained cliff-forming dolomite and limestone (Messin, 1973). The dolomite weathers to a light brown color and is locally silty. A 5 foot thick light gray quartzite is present near the middle of the section. The upper surface is brecciated and probably represents a low-angle normal fault, rather than a thrust as suggested by Misch (1957).

In the Spruce Mountain Quadrangle the Guilmette is thick-bedded, medium-gray, fine-grained limestone which overlies the Simonson Dolomite. The upper surface of the Guilmette is a low angle fault surface similar to that in the Goshute Range (Hope, 1972).

In the Pilot Range, the Guilmette is gray to black, fine to medium grained limestone which conformably overlies the Simonson Formation and is overlain by the Chainman Formation (O'Neill, 1968). The section appears to be thinned several hundred feet by faulting.

In the Wood Hills and Pequop Mountains, the Guilmette occurs in several tectonic slices with varying stratigraphic sequences and thicknesses that conformably overly the Simonson Dolomite, and are unconformably overlain by the Joana Limestone (Thorman, 1970). In general, the unit is platy to massive limestone with a few local interbeds of dolomite (Thorman, 1962). In complete sections, the Guilmette was divided into five informal units in ascending order. The lower unit is about 460 feet of thin-bedded, argillaceous, yellow-brown limestone. Unit 2 is about 420 feet of thick-bedded, gray, cliff-forming limestone in beds 6 to 8 feet thick (Thorman, 1962). Unit 3 is about 690 feet of gray, thick-bedded limestone with thin-bedded gray limestone that contains red-brown silty limestone in the upper 40 feet. Unit 4 is massive limestone, 180 feet thick, and is overlain by the uppermost unit 5 which is massive gray limestone in beds 6 to 10 feet thick, with the lower 30 feet composed of nodular, thin-bedded argillaceous limestone (Thorman, 1962).

The Guilmette varies from about 770 feet in the Pancake Range (Dressen, 1969) to 950 feet in the Buck Mountain-Bald Mountain area (Rigby, 1963), 1,050 to 1,175 feet in the Red Hills (Hose and Blake, 1976), about 200 feet in the Ruby Range (Howard, 1971), 1,400 feet in the southern Cherry Creek Range (Fritz, 1968) and 500 feet in an apparently faulted section in the central Cherry Creek Range (Hose and Blake, 1976), 1,900 feet in the Horse Range (Ptacek, 1962), 1,200 to 1,600 feet in the central Schell Creek Range (Conway, 1965; Young, 1963), and 2,500 feet in the Ely District (Hose and Blake, 1976).

In the Grant Range, the Guilmette varies from 1,850 feet in the north to 2,000 feet in the south (Moores and others, 1968; Hyde and Huttrer, 1970). The Guilmette is 2,100 to 2,200 feet thick in the northern and central Egan Range (Playford, 1961; Woodward, 1962), and about 2,410 feet in the southern Egan Range (Kellogg, 1963), about 300 feet in the Windermere Hills (Oversby, 1969), 1,609 feet in the western Red Hills (Bartel, 1968), 600 to 1,500 feet in highly faulted sections in the Kern Mountains (Nelson, 1959), and southern Snake Range (Whitebread, 1969), 2,750 feet thick on Dutch John Mountain and 3500 feet in the Fairview and West Ranges in northern Lincoln County, 1,000 to 1,300 feet in the southern Goshute Range (Messin, 1973), 1,500 feet in the Spruce Mountain Quadrangle (Hope, 1972), 300 and 1,840 feet in various fault slices in the Wood Hills and Pequop Mountains (Thorman, 1970), 570 feet in the southern Pilot Range (O'Neill, 1968) as opposed to 2,160 feet in the northern Pilot Range (Blue, 1960) and 2,229 feet in the southern Silver Island Range (Blue, 1960).

The Guilmette is exposed in the Pancake Range, Buck Mountain-Bald Mountain area, Ruby Mountains, Egan, Cherry Creek, Grant, White Pine, Quinn Canyon, Horse, Schell Creek, Snake, Fairview, Seaman, northern Pahroc, Golden Gate and Bristol Ranges, Red Hills, Kern Mountains, Limestone Hills, southern Silver Island, Goshute and western Pilot Ranges, Spruce Mountain Quadrangle, Wood Hills and Pequop Range.

The Guilmette Formation represents shallow inner-shelf subtidal, intertidal and supratidal shelf deposition with algal mat-rich restricted lagoonal facies, and local stromataporoid bank-type carbonate build ups (Niebuhr, 1980).