AN INTEGRATED PETROLEUM EVALUATION OF NORTHEASTERN NEVADA
Type Section Information
The Sevy was named by Nolan (1935) for exposures along Sevy Canyon in T. 9 S., R. 18 W., within the Gold Hill Mining District of the Deep Creek Range in west-central Utah. Although one of the most extensive and distinctive units in the Great Basin, its nature has been muddied by giving the unit a different stratigraphic name in nearly every other range.
The Sevy Dolomite is considered to be Early Devonian (Helderbergian and Oriskany) in age, on the basis of sparse faunal evidence. The Sevy, Beacon Peak, and McColley Canyon all have common lower boundaries (Osmond, 1962). The top of the Sevy Dolomite is the approximate top of the Oxyoke Canyon Sandstone Member of the Nevada Formation in the Diamond Mountains (Nolan and others, 1956; Hose and Blake, 1976). The contact of the Sevy Dolomite with the underlying Fish Haven-Laketown Dolomite sequence is gradational but well marked with the lighter, finer grained, and less massive Sevy above. The basal contact is an inconspicuous but regional unconformity of differing magnitude (Osmond, 1954). The Sevy commonly grades into the overlying Simonson Dolomite (Coats, 1985).
The Sevy is generally a finely crystalline to microcrystalline, dense, tan, gray, or light pink, unfossiliferous dolomite which weathers a mouse gray to yellowish gray, and breaks with conchoidal fracture (Osmond, 1962; Hose and Blake, 1976; Kleinhampl and Ziony, 1985). Osmond (1962) divided the Sevy into three regional units with a basal dolomite member up to 1,300 feet thick, a middle cherty argillaceous member, and an upper sandy member.
Although most of the Sevy is massive and homogenous dolomite, thin beds of medium to coarse-grained, well sorted and rounded quartz sandstone, and sandy dolomite are common in the upper 100 to 200 feet. This is the regionally persistent "Sevy sand" of Osmond (1956) and is locally underlain with orangeish and yellowish-gray weathering, argillaceous and cherty dolomites with abundant chert bands and nodules. Sandstone beds range from 6 inches to 6 feet in thickness, and are commonly finely laminated (Osmond, 1962). The lower part of the Sevy is often somewhat darker and coarser grained dolomite, as in the Quinn Canyon Range (Kleinhampl and Ziony, 1985). Sedimentary breccias, cross-bedding, and various forms of soft sediment deformation are also common within the Sevy.
In the Diamond Mountains and Pinon Range, the Sevy is composed of a basal coarse quartz sandstone with a dolomitic matrix; a thick finely crystalline, yellowish gray to brown, dense, laminated to structureless middle dolomite with scattered lenses of intraformational conglomerate; and an upper unit of light gray to bluish dolomite with 1 inch to 10 foot thick interbeds of low-angle cross-bedded, or horizontally laminated quartz sandstone with pelecypods, brachiopods, and vertical burrows (Kendall, 1975).
In the Mahogany Hills, a tongue of Sevy unconformably overlies the Lone Mountain Dolomite and conformably underlies the McColley Canyon Formation (Kendall, 1975). It is composed of finely crystalline, yellowish gray to brown, laminated to structureless dolomite with a few thin interbeds of intraformational conglomerate. In the Sulphur Spring Range, the Sevy is composed of a lower unit of structureless to faintly laminated, yellowish-gray, brachiopod-bearing quartzose dolomite in beds from 6 inches to 3 feet thick, and an upper unit of olive-gray, dolomitic mudstone and slightly quartzose, low-angle cross-bedded dolomitic wackestone (Kendall, 1975).
In the Pancake Range, the Sevy is a dense, light gray, very fine-grained, laminated dolomite (Dreessen, 1969). Beds are from 4 to 15 inches in thickness and break into angular sharp-edged blocks which weather a pinkish color. The upper 100 feet of the formation become progressively more sandy as the upper contact with the Simonson Dolomite is approached. Although generally unfossiliferous, small biohermal structures containing brachiopods, bryozoans and crinoids were found in the upper 150 feet of the Sevy in the Pancake Range (Dreessen, 1969). In the southern Pancake Range, Scott (1969) divided the unit into upper and lower units. The lower 200 feet are fine-grained, laminated, pink and yellow to light-gray dolomite, and the upper 400 feet are massive, well-sorted, medium-grained, vitreous, pink-white, cross-bedded quartzite and gray dolomite.
In the Buck Mountain-Bald Mountain area south of the Ruby Mountains, the Sevy can be divided into two mappable members (Rigby, 1960). The lower 1,080 feet is light-gray, medium to coarsely crystalline dolomite which is indistinctly bedded. The upper portion is about 280 feet of very-fine grained, white, pinkish weathering dolomite in 2 to 3 foot thick beds (Rigby, 1960).
In the Grant, White Pine, Cherry Creek, Egan, and Snake Ranges the Sevy is remarkably consistent lithologically. It is made up of light-gray, sub-lithographic, unfossiliferous dolomite with some graded bedding and crossbedding obvious in blocky weathered fragments and blocks (Moores and others, 1968; Fritz, 1968; Cebull, 1967; Kellogg, 1963; Woodward, 1962; Whitebread, 1969). Bedding is commonly 1 to 2 feet in thickness. Thin white quartzite and sandy dolomite to dolomitic sandstone beds are present in the upper 40 to 150 feet of the Sevy. In the southern Grant Range, the Sevy is a shattered, thin to thick-bedded and massive, light blue-gray, coarsely crystalline dolomite with light yellowish-orange, porous quartz sandstone a few tens of feet thick in the uppermost portion of the unit (Cebull, 1967). In the Egan Range near Lund, Playford (1961) divided the unit into a lower member of light gray, medium crystalline, thin-bedded dolomite, and an upper member of sandy, gray dolomite and medium-grained, well-sorted and well rounded, dolomitic and siliceous quartz sandstone.
In the central and northern Schell Creek Range, the Sevy is aphanitic and fine-grained, dense, brown-gray, light gray weathering, faintly laminated dolomite that is distinctly layered in beds 1 to 3 feet thick (Conway, 1965; Dechert, 1967). The top of the unit contains some fine to coarse-grained quartz sands which form brown streaks in the dolomite. In the Red Hills, it is aphanitic and dense, conchoidally fractured, olive-brown unfossiliferous dolomite with about 14 feet of light gray sandy dolomite about 45 feet from the top of the unit (Bartel, 1968).
In the Limestone Hills and southern Schell Creek Range, the Sevy is a dense, tan, gray or pinkish dolomite which has conchoidal fracture. The dolomite is faintly laminated and well-bedded in layers from 6 inches to about 2 feet in thickness. A thin persistent brown weathering sandstone or quartzite, from 2 to 85 feet thick, is present at the top of the Sevy nearly everywhere in Lincoln County (Tschanz and Pampeyan, 1970).
In the Pequop Range, the Sevy Dolomite is fine to medium-grained, faintly laminated, medium-bedded dolomite which is very difficult to distinguish from the underlying Laketown Dolomite (Thorman, 1970). In the southern Wood Hills, the Sevy is a massive, bench-forming, light gray, very fine-grained, massive dolomite with an upper member of light gray, fine-grained, sandy, cross-laminated dolomite (Coats, 1985). Blue-gray dolomitic limestone about 200 feet thick is present several hundred feet below the upper sandy member. The upper sandy unit of the Sevy grades upward into the overlying Simonson Formation (Coats, 1985).
In the Kern Mountains and southern Deep Creek Range, the Sevy is a pale olive, light gray weathering, dense, aphanitic dolomite with conchoidal fracture. Brown-weathering quartz sand laminae and lenses define the bedding in the dolomite. The Sevy rests disconformably on the Laketown Dolomite (Nelson, 1959).
The Sevy is 256 feet thick in the Mahogany Hills (Kendall, 1975), 195 to 400 feet in the Sulphur Spring Range (Kendall, 1975), 620 to 940 feet thick in the Diamond Mountains (Kendall, 1975), 894 feet in the Pancake Range (Dreessen, 1969), about 600 feet in the southern Pancake Range (Scott, 1969), and about 850 feet thick near Coffin Mountain in the Pinon Range (Kendall, 1975). It is estimated to be about 400 feet thick in the southern Ruby Mountains (Hose and Blake, 1976) and about 1,360 feet thick to the south in the Buck Mountain-Bald Mountain area (Rigby, 1960). The Sevy is 450 to 650 feet in the Ely area (Hose and Blake, 1976), 500 to 535 feet in the central and southern Grant Range (Hyde, 1963; Cebull, 1967), 700 to 1,100 feet in the southern Grant-White Pine Range area (Moores and others, 1968; Osmond, 1962), 855 to 1,100 feet thick in the Horse Range (Ptacek, 1962; Lumsden, 1964), and 425 feet in the southern Cherry Creek Range (Fritz, 1960).
In the northern Egan Range, Woodward (1964) reported about 900 feet of Sevy in one thrust plate and 425 feet in a higher thrust and Playford (1961) reported about 890 feet near Lund. The Sevy is 456 feet thick in the western Red Hills (Bartel, 1968), 500 feet in the Kern Mountains (Nelson, 1959), 800 feet in the southern Snake Range (Whitebread, 1969), 560 feet in the northern Schell Creek Range (Dechert, 1967) and 800 to 1000 feet in the central Schell Creek Range (Conway, 1965), 1200 feet in the northern Quinn Canyon Range (Osmond, 1962), 1300 feet in the Limestone Hills of northern Lincoln County (Tschanz and Pampeyan, 1970), 485 feet in the Goshute and Toana Ranges (Bissell, 1964; Coats, 1985), 85 to 375 feet in faulted sections in the Pequop Range (Thorman, 1970; Osmond, 1962), and about 300 feet in the southern Wood Hills (Coats, 1985).
Osmond (1957) found that the Sevy Dolomite is recognizable over 100,000 square miles as far east as the Confusion Range in Utah, as far west as Eureka, Nevada, and as far south as the Inyo Mountains of California. In the evaluation area the Sevy Dolomite is present in the Mahogany Hills, Diamond and Ruby Mountains, Sulphur Spring, Pinon, Pancake, Grant, Quinn Canyon, White Pine, Horse, Egan, southern Schell Creek, Golden Gate, Snake, Deep Creek, Pequop, Goshute and Toana Ranges, Kern Mountains, Wood and Red Hills, and Limestone Hills. Sevy is retained for units in the eastern part of Nye County as far west as the Pancake Range, with correlative units to the west commonly being variously assigned to the Nevada Group formations such as the Beacon Peak Dolomite and Oxyoke Canyon Sandstone.
The Sevy Dolomite was deposited on extensive mud flats as a restricted lagoonal shelf or sabkha facies, and is time equivalent with both the Lone Mountain Dolomite and the Bartine Member of the McColley Canyon Formation. Quartz sands within the formation may have been carried to the edges of the mud flats by streams and later blown across the periodically emergent flats to be entrapped in shallow pools or create lenses and beds of sand (Osmond, 1962). These sands are equivalent to the Oxyoke sandstone of Nolan and others (1956).
Kendall (1975) felt that the laminations, birds-eye and fenestral structures, intraformational conglomerates, scarcity of organic remains or evidence of activity, and apparent early dolomitization, indicated an intertidal to supratidal environment for the dolomites of the Sevy. The upper quartz sandstones represent intertidal deposits in high-energy beach or bar environments with sands deposited by either tidal or wind-driven currents (Kendall, 1975).
The possibility of both intracrystalline and fracture enhanced porosity and permeability is strong in the Sevy Dolomite, particularly in the coarse crystalline and sandy facies.