AN  INTEGRATED PETROLEUM  EVALUATION OF NORTHEASTERN  NEVADA


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PHOSPHORIA FORMATION

Type Section Information

The Phosphoria was named by Richards and Mansfield (1912), for exposures along Phosphoria Gulch near Montpelier, in Bear Lake County, Idaho.

Geologic Age

The Phosphoria is Permian (upper Leonardian to lower Guadalupian) in age. In northeastern Elko County, the Phosphoria both grades into and is overlain by the Gerster Formation. In several cases, the two formations have been mapped together (Coats, 1985). The Rex Chert Member of the Phosphoria Formation is also equivalent to the Murdock Mountain Formation (Wardlaw and others, 1979).

General Lithology

The Phosphoria Formation has been divided into three members which have regional extent, the Mead Peak Phosphatic Shale Member, the Rex Chert, and unnamed dolomite in ascending order (McKelvey and others, 1959).

In the Currie Hills area, Nelson (1956) mapped the Phosphoria as a unit overlying the Kaibab and underlying the Triassic Dinwoody Formation. The lowest unit is about 90 feet of of shale and siltstone with yellow-brown to black chert and black oolitic phosphate rock which represent the Mead Peak Phosphatic Shale Member. The overlying 1,100 feet are assigned to the Rex Chert Member. The basal 100 feet of this member are composed of light-gray, fine to medium-grained, massive limestone-pebble conglomerate, and are overlain by about 100 feet of massive gray limestone with argillaceous interbeds, and red medium-grained quartzite. The remainder of the unit is composed of fine to medium-grained, brown-gray limestone with sparse pelecypods and bryozoans, brown to white chert, and minor interbedded shale. Above the Rex Chert Member is about 500 feet of light to medium-gray, medium-grained, massive, highly fossiliferous (brachiopods) limestone with lenses of white chert. This unit is probably equivalent to the unnamed dolomite member. Coats (1985) has regarded this entire sequence as equivalent to the Plympton and Gerster Formations and has mapped it as a part of the Park City Group.

In the Snake Mountains, the Phosphoria is composed of two units (Gardner, 1968). The lower unit is composed of interbedded and poorly exposed, dark-gray to brown phosphatic shale, calcareous and phosphatic siltstone, and finely crystalline carbonaceous limestone. The upper unit is composed of thin to thick-bedded, light to dark gray chert, and lesser amounts of phosphatic brown-gray shale, calcareous and phosphatic siltstone, and micritic limestone (Gardner, 1968). Sponge spicules, bryozoa and brachiopods are present in the limestones.

In the southern Pequop Mountains, the Phosphoria Formation was divided into a lower chert and phosphate member, a middle member, and an upper productid limestone member by Snelson (1955). The lower member is about 175 feet of black and brown chert interbedded with black mudstone, and minor bluish-black oolitic phosphate rock. The middle member is about 950 feet thick and is composed of a basal gray, fine to medium-grained limestone about 50 feet thick, overlain by about 90 feet of orange-brown to buff chert-pebble conglomerate with a calcareous matrix. The remaining 810 feet are gray to brown fine to medium-grained organic limestone and argillaceous limestone with brown chert nodules and layers (Snelson, 1955). The upper limestone member is about 400 feet of gray to brown, fine to medium-grained, very fossiliferous limestone.

In the Montello Canyon area of the Leach Mountains, T.M. Cheney (in Coats, 1985) described a thick Phosphoria section, the lower 980 feet of which are probably the Murdock Mountain Formation of Wardlaw and others (1979) and are therefore not described here. The lowermost 200 feet of the formation are interbedded limestones and dolomites which Coats (1985) suggests are probably the Grandeur Formation and are therefore described with the Grandeur.

The section considered the Phosphoria in the Leach Mountains, consists of about 650 feet of black, locally cherty shale, and interbedded chert and massive light-gray dolomite with a 1 to 10 foot thick bed of conglomerate composed of green chert, quartzite and other rock fragments. Phosphorite beds, 3 to 10 feet thick, are present at the base and in the middle of the unit, and the lower 80 feet of the shale are mostly phosphatic (T.M. Cheny in Coats, 1985). This unit is the Meade Peak Phosphatic Shale Member which overlies the Grandeur Formation and underlies the Murdock Mountain Formation in the Leach Mountains (Wardlaw and others, 1979).

In the southern Leach Range, Martindale (1981) reported about 90 feet of the Mead Peak tongue of the Phosphoria Formation. It is composed of interbedded oolitic and peloidal phosphorite, thin to medium-bedded, light gray, calcareous and phosphatic siltstone, medium-bedded phosphatic limestone with gastropods, pelecypods, ostracods, ammonoids, brachiopods, scaphopods, algae and sponge spicules, thin-bedded gray phosphatic siltstone, and calcareous siltstone and sandstone.

In the Snake Mountains, the Phosphoria is composed of interbedded phosphatic brownish-black shale, gray fine-grained limestone, brown and black bedded chert, and fine-grained brown platy siltstone which correlate with the Mead Peak and Rex Chert Members (Gardner, 1968; Coats, 1985).

The Phosphoria Formation is also present in the northwestern Wild Horse and western Elk Mountains Quadrangles where it is folded, dark brown to gray or black shale and siltstone with minor amounts of bedded chert (Coats, 1985). Exposures are poor and may be along low angle faults, and no estimate of thickness was made.

Average Thickness

The Phosphoria Formation is 1,690 feet thick in the Currie Hills area (Nelson, 1956), 1,525 feet in the southern Pequop Mountains (Snelson, 1955), 650 feet in the Montello Canyon area of the Leach Mountains (T.M. Cheney in Coats, 1985), about 90 feet in the southern Leach Mountains (Martindale, 1981), and 700 to 800 feet in the Snake Mountains with 1,100 feet in the northern Snake Mountians (Bezzerides, 1967; Gardner, 1968).

Areal Distribution

The Phosphoria Formation has been reported within the Currie Hills, southern Pequop, Snake and Leach Mountains, and Wild Horse and Elk Mountains Quadrangles.

Depositional Setting

The Phosphoria Formation represents shallow subtidal to intertidal deposition (Martindale, 1981). The phosphorites at least in part represent intertidal oolitic shoals similar to those now present along the Great Bahama Bank.


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Last modified: 09/12/06