AN INTEGRATED PETROLEUM EVALUATION OF NORTHEASTERN NEVADA
Type Section Information
Nolan (1935) named the Gerster for Gerster Gulch in the western Deep Creek Range, Utah. As originally defined in T. 7 S., R. 19 W., the Gerster contained rocks now mapped as the Kaibab Formation (Steele, 1960).
The Gerster is Permian (Guadalupian) in age as determined from brachiopod fauna. At its type locality, it overlays the upper portion of the Oquirrh Formation also known as the Loray Formation of Steele (1960). The Gerster forms the upper unit of the Park City Group throughout its regional extent (Wardlaw and others, 1979).
The Gerster Formation is generally composed of light-brown-gray, resistant, coarse to very coarse-grained limestone, and interbedded thicker units of less resistant, yellowish-gray, argillaceous limestone, containing finely laminated lime mud and terriginous quartz grains (Hose and Blake, 1976). The ledge forming limestones often contain abundant fragments of brachiopods, mollusks, crinoids, bryozoans and echinoid spines. Scattered dark rusty brown weathering chert nodules are present throughout the unit. As the Gerster is traced to the north and northeast into the Leach Mountains and across the Utah state line, it appears to grade laterally into the shales and cherts of the Phosphoria Formation (Steele, 1960; Bissell, 1964).
In the Butte Mountains, the Gerster is over 1,600 feet of gray, pink, red, and brown interbedded skeletal, micritic, and skeletal-detrital limestones which are commonly encrinal and contain abundant bryozoans, brachiopods, and sponges (Douglass, 1960; Bissell, 1964). Sides (1966) also noted possible algae within the silty and sandy limestone of the Gerster, and reported thin-bedded, light-gray chert about 200 feet below the top of the formation and several thin chert-pebble conglomerate beds in the middle and upper part of the formation.
In the southern Pequop Mountains, the Gerster Formation is composed of light gray, thin to thick-bedded limestone with gray and brown, nodular and banded chert throughout the unit, and a few thin beds of fine-grained chert-granule conglomerate (Yochelson and Frazer, 1973). Between Indian and Burnt Creeks in the southern Pequop Mountains, Bissell (1964) divided the Gerster into an upper and lower portion. The lower portion is composed of about 190 feet of thin to thick-bedded and massive, red to yellow-gray, skeletal limestone, sandy micritic limestone, and slightly cherty limestone. The upper 225 feet are laminated and thin to medium-bedded, yellow-gray to orange-brown, sandy limestone, encrinal and brachiopod-rich skeletal limestone, and argillaceous bryozoan-bearing limestone (Bissell, 1964).
In the northern Cherry Creek Range and at Phalen Butte north of Currie the Gerster is composed of over 500 feet of thin to thick-bedded and massive, cherty and sandy limestone, brachiopod, bryozoan and crinoid-rich limestone and minor calcareous sandstone (Bissell, 1964).
In the Medicine Range, Collinson (1968) described upper and lower portions of the Gerster. The upper portion is massive, light-gray skeletal limestone, interbedded with less resistant thin-bedded, sandy limestone and siltstone, and brown weathering chert nodules which form 10 to 60 percent of the unit. The lower portion of the Gerster is slope forming calcareous siltstone and interbedded dolomitic limestone. In the Medicine Range the Gerster lies conformably on the Plympton Formation and is unconformably overlain by the Triassic Thaynes Formation. Bissell (1964) reported brachiopods, bryozoans, and crinoids in the skeletal-detrital limestones in the Medicine Range.
The Gerster Formation in the Leach Mountains is composed of an upper portion of alternating gray, silty, medium to thick-bedded, fossiliferous limestone underlain by lenses and nodules of brown chert beds which compose as much as 50 percent of some sections (Martindale, 1981). The fossils in these limestones include brachiopods, crinoids, bryozoans, algae, and scaphopods (Martindale, 1981). Dark gray-brown dolomitic and phosphatic mudstones with horizontal algal laminations are present in the middle portion of the unit.
Although Bissell (1964) reported a thickness of 4,190 feet for the Gerster in the Medicine Range, the section is repeated several times along faults and is only about 1,200 feet thick according to Collinson (1968). It is 140 to 376 feet thick in the southern Pequop Mountains according to Yochelson and Frazer (1973), and Bissell (1964) measured about 415 feet in the southern Pequop Range in the Burnt Creek-Indian Creek area. It is about 1,638 feet in the central Butte Mountains (Sides, 1966), 1,685 feet in the Leach Range (Martindale, 1981), 585 feet in the Cherry Creek Range (T. 27 N., R. 63 E., Bissell, 1964), and about 742 feet at Phalen Butte northwest of Currie (Bissell, 1964).
The Gerster has been found in the Snake (formerly Burnt Canyon), Pequop, northern Butte, and Ferguson Springs Mountains, northern Cherry Creek, Maverick Springs, and Medicine Ranges, northwest of Currie at Phalen Butte, just north of the Kern Mountains, northern Schell Creek and Antelope Ranges, and Leach Mountains.
The abundance of fenestrate bryozoans, brachiopods, sponges and crinoids attest to the warm shallow marine nature of the Gerster Formation which represents shallow shelf and shallow subtidal deposition, probably in low-energy tidal flat and shallow carbonate bank environments (Sides, 1966; Wardlaw, 1975; Martindale, 1981).