AN INTEGRATED PETROLEUM EVALUATION OF NORTHEASTERN NEVADA
Type Section Information
The Copenhagen Formation has its type section on the west side of Martin Ridge, 1 mile southeast of the union of Ryegrass and Copenhagen Canyons in the Monitor Range (Merriam, 1960).
The Copenhagen Formation is upper Middle Ordovician in age. It is a restricted depositional facies and is a time equivalent of the lower portion of the Eureka Quartzite (Merriam, 1960). The lower 25 feet of the Eureka Quartzite in the Tuscarora Quadrangle of the Tuscarora Mountains have been shown to be equivalent to the Copenhagen Formation (Ross and Shawe, 1972). The Caesar Canyon Limestone and unnamed limestone of Kay and Crawford (1964) are equivalents of the Copenhagen in the Toquima Range (McKee, 1976).
In general the Copenhagen Formation is a yellow-brown weathering, silty and arenaceous, aphanitic to fine-grained limestone with argillaceous partings and abundant gray to black carbonaceous shale in the upper portion, and basal quartzitic sandstone with prominent brachiopod and trilobite hash beds (Kleinhampl and Ziony, 1985). Merriam (1960) broke the formation into members A, B, and C. The basal member A is a fine-grained, gray, quartzitic sandstone 25 feet in thickness. Member B is 200 feet of yellowish-brown, medium to fine-grained limestone with argillaceous and silty interbeds. The upper member C is a dark-gray, calcareous siltstone and very fine-grained calcareous sandstone, with dark grey to black, carbonaceous shale interbeds. The upper contact of the Copenhagen is gradational with the Eureka Quartzite (Merriam, 1960).
In the Copenhagen Canyon area of the Monitor Range, Bortz (1959) described three members of the Copenhagen Formation. The lower quartz arenite member is about 20 feet of pinkish-brown weathering, light gray, very fine-grained, well rounded and well sorted quartz arenite with vugular porosity. The middle member is about 500 feet of argillaceous, clastic and crystalline, fossiliferous, gray to brown limestone. Fossil hash makes up as much as 30 percent of some sections (Bortz, 1959). The upper argillaceous member is composed of about 100 feet of alternating calcareous, thin-bedded, gray to brown shale, and thin to thick-bedded, dark gray to black, calcareous mudstone. In the Clear Creek area of the Monitor Range, the basal 30 feet of the Copenhagen is composed of very fine-grained, light gray to brown, finely laminated and burrowed, angular to well-rounded, well sorted, quartz arenite with calcite cement. This is overlain by a unit of interbedded quartz arenite, silty, gray, flaggy and highly fossiliferous lime wackestone and grainstones and brown-weathering burrowed and cross laminated calcareous siltstones. Fossils included crinoids, gastropods, horn corals, brachiopods, and bryozoans (Wise, 1977).
In the southern portion of the Mount Callaghan area of the northern Toiyabe Range, 125 feet of the Copenhagen Formation lies between the Antelope Valley and Roberts Mountains Formations. The unit consists of a basal dark-grey limestone and light-grey limestone with abundant fine to medium-grained quartz grains, and an upper member of poorly exposed limestone (Stewart and McKee, 1977).
In the Petes Canyon area of the Toquima Range, the Copenhagen is represented by about 100 feet of cherty limestone and dark siltstone containing graptolites and conodonts similar to those found in the upper portions of the Copenhagen Formation at its type section (Stewart and McKee, 1977). Thin lenticular phosphatic sandstones as much as 3 inches thick, containing pebbles, sand grains, trilobite and graptolite fragments, are present in the lower portion of the Copenhagen (McKee, 1976).
In the Hot Creek Range near Tybo, the Copenhagen is about 75 feet thick with an upper 25 foot thick layer of calcareous siltstone that is locally a shell coquina, and a lower 50 feet of sandy dark brownish-gray weathering fine-grained limestone (Quinlivan and Rogers, 1974; Kleinhampl and Ziony, 1985).
The Copenhagen Formation varies in thickness from 20 to 75 feet in the Hot Creek Range near Tybo, to 100 feet in the Petes Canyon area of the Toquima Range (Stewart and McKee, 1977), 125 feet in the Mount Callaghan area of the northern Toiyabe Range (Stewart and McKee, 1977), 350 feet at Martin Ridge, 120 to 130 feet in the Clear Creek, and about 600 feet in the Copenhagen Canyon areas, in the northern Monitor Range (Bortz, 1959; Merriam, 1960; Wise, 1977), to about 431 feet in the northern Antelope Range (Kleinhampl and Ziony, 1985). This variation in thickness is primarily the result of a different placement of the basal contact by various workers, and a covered upper contact. Cook (1966), however, suggested that the upper contact of the formation may be an erosional surface.
The Copenhagen Formation has been mapped in exposures within the Antelope, Monitor, Toiyabe, northern Toquima, and Hot Creek Ranges.
The presence of gastropods, corals, brachiopods, crinoids and bryozoans, as well as burrowed and cross-laminated siltstones and sandstones that are locally phosphatic, suggest that the Copenhagen Formation represents subtidal to intertidal deposition. The argillaceous facies of the Copenhagen Formation may represent a deeper water facies.