The Simonson Dolomite was named for exposures along Simonson Canyon in T. 9 S., R. 18 W., in the western Deep Creek Mountains of north central Utah (Nolan, 1935).

The Simonson Dolomite is Middle Devonian (Eifelian and Givetian) in age based upon fauna of the Stringocephalus zone. It correlates with the Woodpecker Limestone and Bay State Dolomite Members of the Nevada Formation in the Eureka area (Hose and Blake, 1976). West of Railroad Valley, in northern Nye County, a designation of Nevada Formation is used, while the same general interval is designated the Simonson Dolomite to the east (Kleinhampl and Ziony, 1985). The Simonson gradationally overlies the sandy upper portion of the Sevy Dolomite and is gradationally overlain by the Guilmette Formation. Although both upper and lower contacts are conformable, local unconformities of varying magnitudes are present within the Simonson according to Osmond (1954).

The Simonson is separated from the underlying Sevy Dolomite over a gradational contact zone about 15 feet thick which is generally a contact between dense, medium to light gray, dolomite below and pale-brown to yellowish gray, sugary and massive dolomite of the Simonson above (Hose and Blake, 1976). Overall the unit is evenly and well-bedded, fine to coarse-grained, laminated dolomite which is striped with alternating light and dark gray to brown and dark-brown mottled dolomite. Beds are 1 to 2 feet in thickness and commonly contain abundant coralline "spaghetti beds". Bitumen forms as much as 3 to 5 percent of the darker dolomites according to Osmond (1954). Soft sediment deformation and intraformational conglomerates are locally present in the Simonson, as is a poorly developed quartz sand zone in the basal portion of the unit (Kellogg, 1963).

Osmond (1954) divided the Simonson into four persistent regional members. The basal "coarse member" consists of sugary textured, friable, pale yellowish-brown, cliff-forming dolomite which weathers light brown to olive gray. The overlying "lower alternating member" contains dense, whitish-gray, aphanitic dolomite alternating with somewhat coarser, medium to dark gray or brown, finely laminated dolomite that weathers olive gray. Bioherms composed of Stromatopora and Amphipora are abundant in the lower alternating member (Osmond, 1954), as are local intraformational conglomerates. The overlying unit is the resistant "brown cliff member" which consists of about 50 feet of finely crystalline, dark to medium gray dolomite which weathers olive-gray to light yellowish-brown, and contains abundant fossils. Tschanz and Pampeyan (1970) report that this member is a massive and fetid dolomite biostrome of stromatoporoids, bryozoans and corals, 70 to 180 feet thick in Lincoln County. The youngest Simonson unit is the "upper alternating member" that is once again alternating light gray dolomite, and coarse, medium to dark laminated dolomite, with the addition of fine-grained medium-gray limestone lenses. Sedimentary breccias are also locally common in the uppermost member. These members are recognizable in exposures across the entire study area, however not all members are present everywhere (Kleinhampl and Ziony, 1985; Tschanz and Pampeyan, 1970).

In the northern Pancake Range, the Simonson is characterized by alternating, unfossiliferous, fine to medium-grained brown dolomite, and finer grained laminated gray dolomite, which average about 10 feet and 4 feet thick respectively (Dreessen, 1969). Beds are folded and fragmented and contain intraformational conglomerate. The base of the Simonson contains about 130 feet of olive to pinkish gray, vitreous, cross-bedded quartzite. In the southern Pancake Range, the Simonson is alternating light and dark-colored, thin to medium-bedded, fine to medium-grained mottled dolomite, with contorted laminae and intraformational conglomerate (Scott, 1969).

In the Buck Mountain-Bald Mountain area, the Simonson can be divided into a lower 340 feet of light to medium gray, very coarse, massive dolomite, overlain by an upper 350 feet of interbedded light gray, aphanitic dolomite, and medium to dark gray or gray-brown, medium-crystalline, wavy laminated and mottled dolomite (Rigby, 1960).

In the Grant-White Pine Range area, Moores and others (1968) described the Simonson as containing a lower coarse-grained dolomite which is succeeded by alternating, fine to medium grained, dark gray and medium-gray, finely laminated dolomite. In the southern Grant Range, the Simonson is massive, coarse-grained, light gray to tan dolomite overlain by alternating medium to thick-bedded, brown and gray, laminated, fine to coarse crystalline dolomite with minor amounts of thin-bedded limestone in the upper portion of the formation (Cebull, 1967). Cladopora are abundant in the Simonson.

In the Cherry Creek and Egan Ranges, three members have been described within the Simonson (Kellogg, 1960; Fritz, 1968; Woodward, 1962). The lower member is composed of massive, light gray, medium to coarse-grained, sugary dolomite in beds about 2 feet in thickness. The resistant middle member is intercalated, finely laminated, light gray and light brown, dolomite with a few light tan to chocolate brown weathering, mottled dolomite in beds from 4 inches to 2 feet in thickness. The upper member is a cliff forming, fine to medium-grained, dark grey to brown weathering dolomite in 2 inch to 2 foot beds. Interbedded 10 to 15 foot thick gray limestones and intraformational dolomite breccias are present in the upper few feet of the formation but comprise less than 2 percent of the total thickness (Woodward, 1962; Fritz, 1968).

In the northern Schell Creek Range, the Simonson is alternating light and dark gray and brownish-gray, medium to thick-bedded, fine to medium-grained dolomite (Dechert, 1967). Lamination and mottling is common as are coralline "spaghetti" beds and brachiopods. A few thin interbeds of medium gray to blue-gray limestone are present in the middle of the formation (Dechert, 1967). Young (1960) recognized all four of Osmonds' (1964) members in the Simonson in the northern Schell Creek Range. The lower coarse member is 160 feet of light gray, medium to coarse-crystalline dolomite in beds one-half to one foot thick. Overlying this is about 395 feet of the lower alternating member composed of striped brown and gray dolomite with slump structures and slide breccias in the lower 50 feet (Young, 1960). The brown cliff member is about 40 feet of massive, brown-gray, fossiliferous dolomite and is overlain by 255 feet of massive brown and gray dolomite of the upper alternating member (Young, 1960). In the central Schell Creek Range, the Simonson is composed of alternating medium to thick-bedded, finely laminated, medium to coarsely crystalline dolomite with aphanitic dolomite near the base of the unit (Conway, 1965).

In the western portion of the Red Hills west of the Kern Mountains, the basal coarse member of the Simonson is about 107 feet of arenaceous and unfossiliferous, orange to yellow weathering dolomite with a few thin beds of gray dolomite (Bartel, 1968). The lower alternating member is 314 feet of very fine-grained, arenaceous, light gray weathering dolomite and alternating dark brown, argillaceous to arenaceous laminated dolomite with abundant brachiopods and "spaghetti" beds. This is overlain by about 210 feet of dark brown, laminated and mottled, arenaceous dolomite with abundant spaghetti beds (Bartel, 1968). The upper alternating member is composed of 374 feet of alternating, gray and brown, poorly laminated, coarse to fine grained dolomite with some silty laminated limestone in the upper 60 feet. In addition to Thamnopora and Cladopora spaghetti beds, brachiopods, and crinoids are common in the formation (Bartel, 1968).

In the Kern Mountains, the Simonson is medium to dark-gray, laminated and locally mottled dolomite which weathers to a purplish-brown or gray (Nelson, 1959). The lower member is light gray, finely laminated dolomite and alternating dark gray, finely laminated dolomite in 10 to 20 foot thick beds. Cladopora are abundant in the lower member, and a few interbeds of finely-laminated mottled black dolomite are present (Nelson, 1959). The upper member of the Simonson is Cladopora-bearing dark gray dolomite which passes upward into thin gray limestones with stromatoporoids (Nelson, 1959).

In the southern Snake Range, the Simonson is alternating light and dark grey to brown, very fine to medium-grained dolomite in beds 1 to 6 feet in thickness (Whitebread, 1969). The dolomites are faintly laminated and often crowded with stromatoporoid heads, and occasionally show wavy algal laminations. Very thin-bedded aphanitic limestone beds and lenses as much as 15 feet thick grade into dolomite along strike. The darker dolomites are reported by Whitebread (1969) to have a strongly fetid odor in the Snake Range.

In the Spruce Mountain Quadrangle, the Simonson is laminated, medium to dark gray, very fine-grained dolomite with thin beds of interbedded quartz sand near the base, and interbedded limestone in the upper 200 feet (Hope, 1972).

Elsewhere in the Pequop Range, the Simonson was mapped by Thorman (1970) in several fault slices which differ slightly lithologically. In general, the Simonson is a laminated, alternating light and dark gray dolomite with a basal sandy crossbedded dolomite from a few feet to 200 feet in thickness. This basal unit is not present in several fault slices, and the upper half of the Simonson contains interbedded limestone in several exposures. The Simonson rests on the Laketown or Lone Mountain Dolomite and is conformably overlain by the Guilmette Formation. Thorman (1970) has correlated this interval with the lower portion of his Ordovician-Devonian Marble unit in the adjacent Wood Hills. In the Wood Hills the Simonson is about 70 feet of medium-grained, layered, reddish weathering, quartzose dolomitic marble (Thorman, 1970).

In the southern Pilot Range, O'Neill (1968) mapped a faulted section of Simonson Dolomite which he divided into informal mappable members. The lower member is composed of about 75 feet of alternating light and dark gray, medium-grained dolomite in beds which average 2 feet in thickness. The upper member is about 290 feet of gray, finely laminated, calcareous dolomite with local red argillaceous partings and fossil hash beds.

In the Leppy Peak area north of Wendover, the Simonson has been divided into two members by Schaeffer and Anderson (1960). The lower member is 336 feet of alternating light and gray, fine-grained dolomite, with the upper member of 934 feet of alternating light and dark gray, interbedded limestone and dolomite similar to those exposed in the Pilot Range. In the Goshute and Toana Ranges and southern Snake Mountains the Simonson is typically medium to dark gray laminated dolomite with abundant "spaghetti (Amphipora) beds" and brachiopods (Bissell, 1964; Coats, 1985).

In the Windermere Hills, R. Hope considered the unit originally mapped as the Nevada Formation, as the Simonson Dolomite (Coats, 1985). The Simonson is gray, medium-grained, calcareous dolomite which is commonly laminated and mottled with alternating light and dark gray dolomite in 1 to 10 foot intervals in the upper 120 feet of the unit. Finely laminated, aphanitic dolomite beds about 1 foot in thickness are also present in the upper potion of the formation. Abundant soft sediment structures, common regionally in the Simonson, are present, as is a 6 foot thick white orthoquartzite about 200 feet above the base of the unit (Coats, 1985).

The Simonson has a thickness estimated at about 730 feet in the northern Pancake Range (Dreessen,1969) and about 900 feet in the southern Pancake Range (Scott, 1969), and 690 feet in the Buck Mountain - Bald Mountain area (Rigby, 1960). Hose and Blake (1976) estimated 600 to 700 feet in the Cherry Creek Range and Fritz (1968) measured 1,310 feet in the southern Cherry Creek Range. Moores and others (1968) estimated 800 to 1,000 feet of Simonson in the Grant-White Pine Range area, while Cebull (1967) measured 2,000 feet in the southern Grant Range in a section which may be complicated by faulting. Ptacek (1962) measured 1,260 feet of Simonson in the Horse Range, Woodward (1964) measured 650 and 1,300 feet in differing structural plates in the northern Egan Range, and Playford (1961) and Kellogg (1960) measured about 950 and 1061 to 1,168 feet respectively in the southern Egan Range.

Thickness varies from 850 to 1,200 feet in the Ely area (Hose and Blake, 1976), about 1,005 feet in the western Red Hills (Bartel, 1968), 900 to 1,000 feet in the Kern Mountains (Nelson, 1959), 600 to 700 feet in the Schell Creek Range according to Drewes (1967), 800 feet according to Dechert (1967) and 850 feet according to Young (1960) and about 1,200 feet in the central Schell Creek Range (Conway, 1965). Whitebread (1969) reported a thickness of 575 feet in the southern Snake Range, and it is over 1,000 feet thick in the Limestone Hills (Tschanz and Pampeyan, 1970).

In Elko County thicknesses vary from 500 feet in the Spruce Mountain Quadrangle (Hope, 1972), 365 feet in the southern Pilot Range (O'Neill, 1968), 1,270 feet in the Leppy Range (Schaeffer and Anderson, 1960), 760 feet in the Goshute Range (Bissell, 1964), 970 to 1,170 feet in the Pequop Range-Wood Hills (Thorman, 1970), 1,700 feet in the Windermere Hills (Coats, 1985). Variations in thickness are the result of contact placement, depositional variation, and structural complication along flat faults.

The Simonson is exposed in the Buck Mountain - Bald Mountain area and Red Hills, Pancake, Cherry Creek, Egan, Grant, White Pine, Horse, Schell Creek, Snake, Seaman, and Golden Gate Ranges, Limestone Hills, Spruce Mountain quadrangle, Pequop Mountains, Wood Hills, Goshute and Toana Ranges, Windermere Hills, southern Pilot Range, Leppy Range, southern Snake and Kern Mountains.

The Simonson Dolomite represents shallow subtidal to intertidal and locally supratidal dolomite deposited along the broad Devonian shelf platform (Poole and others, 1977).

The Simonson Dolomite commonly shows intracrystalline porosity as well as pervasive fracturing. Both of these factors are playing a role in the Grant Canyon Field within Railroad Valley which produces from the Simonson Dolomite. The Blackburn Field in Pine Valley is also producing from Devonian carbonates at the Simonson level, underscoring the importance of the Simonson as a producing and potential reservoir facies.