AN INTEGRATED PETROLEUM EVALUATION OF NORTHEASTERN NEVADA
Type Section Information
The Pequop Formation was named for a type section 1.5 miles north of the Jasper Railroad tunnel in the central Pequop Mountains, in Sec. 3, T. 33 N., R. 65 E. (Steele, 1960).
The Pequop Formation is considered Middle Permian (lower Leonardian to lower Guadalupian) in age (Steele, 1960). It overlies the Ferguson Mountain Formation and is overlain by the Loray Formation. In regional mapping in Elko County, Coats (1985) combined the Pequop with the overlying Loray Formation, and also included the Arcturus Formation in the southeastern portion of the county. This grouping is also followed for this evaluation.
At the type section in the central Pequop Mountains, the Pequop Formation is composed of purplish-gray, irregularly bedded, platy, silty limestone which is interbedded with fusulinid coquinas (Steele, 1960). The basal member of the formation is about 8 to 130 feet of reddish-tan siltstone. In the Wood Hills-Pequop Mountains area, the Pequop is composed of thin-bedded to platy, silty, medium-gray fusulinid-bearing limestones which are interbedded with a few thin beds of reddish-brown sandstone and quartzite (Thorman, 1962).
Near the town sites of Riepetown and Ruth along the western flank of the central Egan Range, the Pequop Formation has been described and broken into many informal units by Bissell (1964). The Pequop here is interbedded red-gray, gray-brown and brown dolomitic limestone, calcareous and silty and sandy dolomite, micritic and skeletal limestone which are locally algal and pelletoidal, and dolomitic-calcareous and siliceous orthoquartzites and sandstones. A small amount of claystone and siltstone are also interbedded in the upper several hundred feet of the formation. Bryozoans, echinoid spines, small gastropods, fusulinids, crinoid fragments are relatively abundant in the Pequop (Bissell, 1964).
At Moorman Ranch in the Butte Mountains, Steele (1960) divided the Pequop Formation into upper and lower members. The lower member is about 1,750 feet of grayish-orange to yellowish-brown, aphanitic to fine-grained, thin to medium-bedded limestone with thin interbeds of dark yellow-orange, fine-grained siltstone and sandstone. The upper member is about 1,050 feet of olive-gray to yellowish-gray, aphanitic to fine-grained, thin to thick-bedded fusulinid limestones with interbedded light yellow, very fine-grained quartz sandstone (Steele, 1960). About 10 miles to the south at Robbers Roost, Bissell (1964) described 5 units within a thick section of the Pequop Formation. The basal member is 924 feet of massive olive brown micrite, sandy pelletoid micrite, and sandy skeletal limestone. The overlying member is 546 feet of gray-brown, sandy to cherty and skeletal limestone and is overlain by about 566 feet of interbedded olive-gray to brown, sandy and cherty, skeletal micrite, and calcareous fine-grained sandstone with echinoid spines, crinoid fragments and fusulinids. Above this is about 390 feet of interbedded dolomite, calcareous and sandy dolomite, dolomitic limestone and sandstone, and micritic to skeletal limestone. The uppermost 512 feet are alternating light brown, gray, and red-gray micritic limestone, skeletal limestone, argillaceous and silty algal limestone and minor calcareous dolomite (Bissell, 1964).
In the Maverick Springs Range, the Pequop was divided into 3 members by Bissell (1964). The lower member is about 1,330 feet of thin-bedded to platy argillaceous to silty limestone, claystone, calcisiltite, calcareous sandstone, algal micrite, and collapse breccia which weather to shades of lavender or orange. This is overlain by 1,722 feet of olive-gray to brown argillaceous micrite, sandy skeletal limestone and algal limestone. The upper member is 1,361 feet of argillaceous limestone and interbedded calcisiltite with abundant pelecypods and gastropods and also weathers to a lavender or orange-gray (Bissell, 1964).
In the Medicine Range, the Pequop is composed of several hundred feet of dark gray, massive, ledge-forming, fusulinid-bearing limestone, overlain by over 1,000 feet of slope-forming brown fine-grained calcareous siltstone and silty limestone, and an upper interval several hundred feet thick composed of dark-gray fine-grained limestone interbedded with gray-brown to yellow calcareous siltstone (Collinson, 1966, 1968). Gastropods and pelecypods are abundant throughout the unit, as are bryozoans, algal plates, brachiopods, and echinoderm fragments in the lower 440 feet of the unit (Collinson, 1968).
In the Cherry Creek Range, the Pequop is divided into three members (Bissell, 1964). The lower member is 663 feet of thin to thick-bedded and massive, brown to red-gray micritic or skeletal limestone, argillaceous to sandy limestone, and cherty limestone which weather to a lavender or orange-brown color. The middle 1,095 feet is composed of olive-gray, pink, and brown aphanitic dolomite, sandy and calcareous dolomite, orthoquartzite, and thick bedded to massive algal limestone. The upper member is 497 feet of gray to blue-gray or brown micritic limestone, sandy and calcareous dolomite, dolomitic limestone, and skeletal limestone with mollusks, algae, fusulinids, and crinoids (Bissell, 1964).
In the Dolly Varden Mountains, the Pequop Formation has been divided into 3 members (Bissell, 1964). The lowermost informal member is 1,265 feet of gray, brown and lavender micrite, sandy and skeletal as well as detrital limestone, and minor interbedded slightly calcareous orthoquartzites and dolomitic limestone. The middle member is 1,105 feet of aphanitic to medium crystalline silty, sandy and calcareous gray, pink, and brown dolomite with minor interbedded dolomitic and calcareous orthoquartzites, dolomitic and sandy limestones. The upper member is 1,445 feet of interbedded micritic limestone, skeletal limestone, sandy and dolomitic limestone, dolomitic and calcareous orthoquartzite and minor calcareous dolomite (Bissell, 1964).
In the Kingsley Range, about 500 feet of Leonardian rocks overlying the Ferguson Mountain Formation were assigned to the Arcturus Formation by Buckley (1967) and are here considered as part of the Pequop Formation. The unit consists of fine to medium-grained, olive-brown, white, purple or red-brown weathering sandstone.
In the Spruce Mountain area, the Pequop was divided by Hope (1972) into two informal members. The lower member is about 1,200 feet of medium to thick-bedded, medium-gray, fine-grained limestone with locally abundant chert nodules, which is interbedded with platy brown siltstone. The upper member is over 4,000 feet of massive, thick-bedded, medium to fine-grained crinoidal limestone with gastropods and rare fusulinids (Hope, 1972).
In the southern Schell Creek Range, the Pequop Formation is composed of several hundred feet of interbedded sandy skeletal limestone, micrite, and argillaceous limestone with well-preserved algae, echinoid spines, pelecypods and gastropods along with fusulinids (Bissell, 1964).
In the Ferguson Mountain area near the southern Goshute Range, the Pequop was described by Berge (1960) as several hundred feet of interbedded gray, silty, fine to medium-grained limestone, and platy, yellow-brown weathering, medium-grained, calcareous quartzite.
In the Leppy Range, the Pequop Formation is incompletely exposed where it disconformably overlies the Ferguson Mountain Formation (Bissell, 1964). The Pequop is composed of a few hundred feet of interbedded light gray-brown to olive-gray algal and gastropod micrite, dolomitic skeletal limestone, sandy limestone, dolomite and orthoquartzite.
In the northern Leach Mountains, the Pequop is massive, medium gray, medium-grained limestones with bryozoans, algae, crinoids, brachiopods and gastropods, interbedded with platy, fine-grained, light gray, calcareous siltstone, and gray thin-bedded silty limestone and thick-bedded light gray chert (LeCompte, 1978). Thin interbeds of intraformational limestone conglomerate are scattered throughout the formation. Limestones form about 90 percent of the formation, siltstones about 5 to 10 percent and cherts about 5 percent (LeCompte, 1978). In the southern Leach Mountains the Pequop is composed of thin to medium-bedded, fine-grained, gray to yellowish, silty and sandy limestone with gastropods, crinoids, bryozoans, brachiopods, and pelecypods, cherty silty and sandy limestone, thin to medium-bedded, yellowish-brown dolomitic limestone, thin-bedded, gray calcareous quartz-arenite, and thin to medium-bedded black chert (Martindale, 1981).
The Pequop Formation is 3,087 feet thick at the type locality in the central Pequop Range (Robinson, 1961), 2,800 feet near Moorman Ranch in the Butte Mountains (Steele, 1960) and 3,695 feet at the southern terminus of the Butte Mountains (Bissell, 1964), 2,400 and 3,087 feet in the Medicine Range (Collinson, 1968; Robinson, 1961), 4,413 feet in the Maverick Springs Range (Bissell, 1964), 3,815 feet in the Dolly Varden Mountains (Bissell, 1964), 2,355 feet in the Cherry Creek Range (Bissell, 1964), 500 feet in the Kingsley Range (Buckley, 1967), about 5,200 feet in the Spruce Mountain Quadrangle (Hope, 1972), 4,264 feet in the northern Leach Mountains (LeCompte, 1978) and 840 feet in the southern Leach Mountains (Martindale, 1981), 682 feet at Ferguson Mountain (Berge, 1960), 723 feet in the southern Schell Creek Range (Bissell, 1964), and is 547 feet thick in the Leppy Range (Bissell, 1964). Variations in thickness are probably primarily a result of poor exposures of repetition of lithologies, and lack of persistent marker beds which make structurally repeated and attenuated sections difficult to detect (Collinson, 1968).
The Pequop is present in the East Humboldt, Medicine, Maverick Springs, central Egan, Cherry Creek, Pequop, Kingsley, and southern Schell Creek Ranges, Butte, Leach, and Dolly Varden Mountains, Spruce Mountain Quadrangle, Carbon Ridge, Ferguson Mountain, and in undivided Pennsylvanian and Permian sections in the Pilot, Leppy, and southern Silver Island Ranges.
Abundant algae, bryozoans and gastropods suggest that the Pequop Formation represents shallow marine deposition, probably in water less than 100 meters in depth (Bissell, 1964; Collinson, 1968; Mercantel, 1975). The upper part of the formation, which locally contains gypsiferous units, suggests shallow marine to brackish hypersaline conditions probably in bays, lagoons or estuaries related to deltaic deposition (Collinson, 1968). Intraformational conglomerate and rip-ups are probably the result of episodic storms which disturbed the upper portion of the unit (Mercantel, 1975). The Pequop represents a large percentage of the total thickness of Permian sediments filling the Butte-Deep Creek Trough.