AN INTEGRATED PETROLEUM EVALUATION OF NORTHEASTERN NEVADA
Type Section Information
The Oquirrh Formation was named for exposures near South Mountain in the Oquirrh Mountains, west of Salt Lake City Utah (Gilluly, 1932).
In northeastern Elko County, a sequence of poorly exposed and highly faulted, lithologically variable units have been dated as Late Mississippian to Early Permian (Chesterian to Wolfcampian) in age. These rocks are tentatively assigned here to the Oquirrh Formation although they may locally include units which could be correlated with the Strathearn, Sunflower, Chainman and Diamond Peak Formations and the Ely Limestone. The Oquirrh is essentially an eastern sequence, dominantly exposed in western Utah, which is equivalent to these formations in northeastern Nevada. An assignment of these rocks to the Oquirrh is considered reasonable based upon the age, lithology, and general sequence of these rocks. Coats (1985) felt that "any attempt to divide this unit into named formations at the present time is inadvisable". The Hammond Canyon, Poorman Peak, and Storff Formations, and the Chellis Limestone, included by Coats (1985) in this unit, have been mapped by other workers as individual formations and are retained here as an individual map unit.
The sequence of rocks tentatively assigned to the Oquirrh Formation in northeastern Elko County is commonly poorly exposed and often cut by numerous low angle faults. Sequence is often difficult to determine, and in some cases, sections are probably para-autochthonous and allochthonous along thrust faults (Coats, 1985).
In the northwestern portion of the Marys River Basin Northwest Quadrangle, along Major Gulch, the entire section appears to be overturned and may be cut by numerous bedding plane faults (Coats, 1985). Stratigraphically, the lowest 500 feet of the unit are bedded, dark brown to black chert and are overlain by several thousand feet of shale, and medium-gray, light-gray weathering limestone (calcisiltite and calcarenite) in beds as much as 20 feet thick. Conodonts from the limestone show that the section straddles the Mississippian/Pennsylvanian boundary (Coats, 1985).
In the Marys River Basin Northeast and Southeast Quadrangles, three distinct facies can be mapped in the unit. These are a limestone and shale facies (Late Mississippian to Late Pennyslvanian), a calcareous siltstone facies (Upper Pennsylvanian to Lower Permian) and a quartz-arenite facies. These three facies appear to this writer to be rather similar to the "three-fold lithologic subdivision" which Roberts and others (1965) suggest are regionally present in the Oquirrh Formation. These regional facies consist of a lower unit mainly composed of limestone and sandstone, a middle part composed of sandstone and subordinate limestone, and an upper part composed of sandstone and lesser limestone, chert, and shale. In the Marys River Quadrangles, the limestone-shale facies has the widest distribution and represents the greatest time span (Coats, 1985). The limestone is micritic to calcarenitic, and locally quartzose in beds a few inches to 6 feet in thickness with sparse brachiopods, fusulinids and conodonts. The overlying facies is dominantly massive to thin-bedded, light-gray calcareous siltstone which may be structurally interleaved with the limestone-shale facies. The upper quartz-arenite facies is composed of white to maroon-weathering, fine to medium-grained, massive quartz-arenite or orthoquartzite. Thin siltstone beds and 1 foot beds of conglomerate with angular to rounded dark-gray quartzite and chert in a light-gray quartzite matrix are also present (Coats, 1985).
In the O'Neill Pass area of the Snake Mountains, Bezzerides (1967) mapped a unit which he tentatively assigned to the Oquirrh Formation. The formation here consists of several hundred feet of massive to platy, locally silicified, gray limestone which is interbedded with tan to brown, red and yellowish weathering mudstone. This section may represent the lower member of the Oquirrh Formation.
In the HD Range, Riva (1970) mapped nearly 2,000 feet of Lower Permian (Wolfcampian) section near Blanchard Mountain which he assigned to the Miller Canyon Section. This Permian section is cut by numerous high-angle faults and is intruded by the Contact pluton, and was divided into 4 informal units (Riva, 1970). The basal unit is about 600 feet of thin to medium-bedded, light to dark-gray or black sandstone, silicified siltstone and shale and thin limestone beds which weathers light brown to dark gray as a unit. Overlying this, is about 210 feet of unit 2 which is composed of dark gray limestone that is sandy and cross-bedded near the base and regularly bedded and less sandy near the top. The limestone contains crinoids, tabulate corals and fusulinids (Riva, 1970). Unit 3 is about 750 feet of fine-grained, medium to thick-bedded, reddish-weathering quartzite and lesser gray limestone, with alternating black and white quartzite with several thick limestone beds in the lower 200 feet of the formation. The uppermost unit is thin-bedded and brittle, black siltstone which is about 310 feet thick (Riva, 1970).
Gibbons (1973) reported a similar thickness of Permian rocks in the Contact mining district, which he divided into an upper and lower sequence. The lower sequence is about 1,570 feet of dark gray argillite overlain by medium-grained quartzite, massive blue-gray silty limestone, and several units composed of interbedded chert, limestone, and fine-grained quartzite. The upper sequence is about 600 feet of blue-gray argillaceous shale overlain by 300 feet of coarse-grained quartzite (Gibbons, 1973).
The Oquirrh Formation is about 4,300 feet thick in a faulted and probably repeated section in Major Gulch in the northwestern portion of the Marys River Basin Northwest Quadrangle (Coats, 1985), 900 feet in the O'Neill Pass area of the Snake Mountains, and 1,870 to 2,200 feet in the HD Range (Riva, 1970; Gibbons, 1973).
Late Mississippian to early Permian rocks considered part of the Oquirrh Formation are exposed in the northeastern portion of Elko County within the Mount Velma, Marys River Basin Northwest, Northeast and Southeast Quadrangles, the O'Neil Pass area of the Snake Mountains, and the northern HD Range near Blanchard Mountain. Depositional Setting
The Oquirrh Formation represents a shallow to moderate depth marine outer shelf and slope assemblage which was deposited in a basin with rapid sea level fluctuations and periodic subsidence. Density currents periodically carried clastic debris down the slope and into the quieter water setting where carbonate sediments were being deposited (Rich, 1971). The Oquirrh in northeastern Nevada was deposited along the western margin of the northern lobe of the basin which extended west from Salt Lake City. The Oquirrh grades westward into the interbedded clastics and limestones of the Strathearn, Moleen and Tomera Formations, and southward into the Ely and Riepe Spring Limestones.