AN  INTEGRATED PETROLEUM  EVALUATION OF NORTHEASTERN  NEVADA


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SUNFLOWER FORMATION

Type Section Information

The Sunflower Formation is named for exposures at Sunflower Reservoir in Sec. 6, T. 45 N., R. 55 E., in the southwestern portion of the Rowland Quadrangle in northern Elko County (Coash, 1967).

Geologic Age

Fusulinids and brachiopods suggest an Upper Pennsylvanian (Virgilian) age (Bushnell, 1967), while corals in the Mount Velma Quadrangle suggest the upper portion of the formation may be Lower Permian (Wolfcampian) in age (Coash, 1967). The Sunflower has been correlated with the Antler Peak and Strathearn Formations (Bushnell, 1967). The basal conglomerates of the Sunflower Formation apparently unconformably overlie the Tennessee Mountain Formation in the Rowland and Mount Velma Quadrangles (Coash, 1967), and locally near Cornwall Mountain in the northern Mt. Velma Quadrangle, rest unconformably on western assemblage cherts tentatively assigned to the Valmy Formation (Coash, 1967).

Coats (1985) considered the Sunflower Formation equivalent to the Strathearn (Missourian to Wolfcampian) and has included it with the Strathearn in mapping. This practice is followed here.

General Lithology

The Sunflower Formation is shattered and brecciated at the type section where it contains two members. The basal 325 to 800 feet is massive, reddish-brown to light gray or yellow, angular to rounded chert-pebble conglomerate with a few thin beds of massive quartzite and friable brown sandstone (Bushnell, 1967). These conglomerates grade into the upper member which is composed of gray, cherty, sandy fossiliferous limestone 296 to 530 feet thick. Cherts in the upper member are thin-bedded, black and olive-green lenses, beds and nodules. Fossil fragments include corals, brachiopods, bryozoans, fusulinids and crinoids. A local lens of thin-bedded, yellowish-gray to brown very fine-grained argillaceous sandstone overlies the limestone (Bushnell, 1967).

In the Mount Velma Quadrangle, three members have been mapped by Coash (1977). The lower conglomerate member is a gray to brown, reddish weathering, massive and structureless chert pebble conglomerate with thin interbeds of friable sandstone to quartzite particularly common near the base. The conglomerates are commonly well rounded and sorted, with pebbles and cobbles of sandstone and quartzite and chert. The sandstones are also well sorted and rounded and are composed of approximately 95 percent quartz (Coash, 1967). One filled channel up to 2 feet wide and a few inches deep is present within the conglomerates. The conglomerate member is commonly about 100 feet but may be as much as 1,985 feet thick (Coash, 1967).

The middle limestone member conformably overlies the conglomerate and is composed of clastic to crystalline, gray, yellow weathering limestone, fusulinids, corals, bryozoans and brachiopods, and crinoids (Coash, 1967). The middle limestone member is 296 to 530 feet thick. The upper sandstone member is composed of fine-grained, micaceous sandstone and siltstone with thin beds of silty limestone. A basal bed of volcanic ash contains pelecypods, bryozoa and brachiopod molds. Possible plant stems are present in at least one sandstone bed (Coash, 1967). The upper member is 1,460 to 2,000 feet thick.

In the southeastern Mountain City, northwestern Marys River Northwest, and Osino Quadrangles, the Sunflower Formation is composed of several hundred feet of massive limestone with a thrusted base and top (Coats, 1985).

Average Thickness

The Sunflower Formation is from 1,850 to 4,440 feet thick in the Mount Velma (Coash, 1967) and 621 to 1,330 feet in the southern Rowland Quadrangles (Bushnell, 1967).

Areal Distribution

The Sunflower Formation occurs in the Rowland Quadrangle in the Sunflower Flat area, in the southeastern portion of the Mountain City Quadrangle, and may be equivalent to limestones exposed in the Copper Mountains and along the western flank of the Snake Mountains (Bushnell, 1967). Massive limestone correlated with the Sunflower Formation has also been mapped in the Marys River Basin Northwest Quadrangle, and in the Osino Quadrangle north of Elko (Coats, 1985).

Depositional Setting

The abundant abraded shallow marine fossil fragments in the limestones and clastic basal portion of the formation, channel cut in the conglomerate and presence of possible plant fragments suggest deposition in a shallow marine environment very near shore with minor current activity.


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Last modified: 09/12/06