AN INTEGRATED PETROLEUM EVALUATION OF NORTHEASTERN NEVADA
Type Section Information
Nolan and others (1956) proposed the name Windfall Formation for the rocks between the Dunderberg and Pogonip Group as exposed along Windfall Canyon, north of the old Windfall Mine near Eureka.
The Windfall Formation is middle and late Late Cambrian (Franconian and Trempealeauan) in age. In the northern Antelope and Hot Creek Ranges, the Windfall apparently extends upward into the Early Ordovician (Tremadocian) according to Quinlivan and Rogers (1974) and Kleinhampl and Ziony (1985).
The Windfall commonly overlies the Dunderberg shale along a sharp conformable contact and upward into a conformable and gradational contact with the lower Pogonip Group. It is approximately equivalent to the Whipple Cave Formation in the Egan and Schell Creek Ranges (Kellogg, 1963) and the Notch Peak Limestone in the Snake and Cherry Creek Ranges (Whitebread, 1969; Adair, 1961).
Western Cordillera recommends that the name Notch Peak Limestone or Formation, and Whipple Cave Formation be dropped in favor of the Windfall Formation. This would only require reassignment of rocks designated as Notch Peak by Whitebread (1969) in the Snake Range and in the Kingsley Mountains by Mathias (1959), and designated Whipple Cave Formation in the southern Egan and Schell Creek Ranges (Kellogg, 1963).
Nolan and others (1956) divided the massive and shaley limestones of the Windfall Formation into two members. The upper Catlin Member was described from exposures near the Catlin shaft of the Croesus Mine in New York Canyon, while the lower Bullwhacker Member is named for exposures in the vicinity of the Bullwhacker Mine.
The Catlin Member consists of alternating massive and thinly bedded, silty and sandy, gray to blue-gray, fine-grained, trilobite-rich limestones, and thin-bedded to platy, blue-gray, laminated, sandy and silty limestone. Dark and light gray, banded chert layers are present in the lower half of the member.
The underlying Bullwhacker Member is made up of thin-bedded, platy or shaly, light to dark gray limestones that often weathers a yellowish-tan color. Grey chert nodules, trilobite and brachiopod fragments are locally abundant in the member (Roberts and others, 1967).
In the southern Ruby Mountains, Hose and Blake (1976) have broken out about 500 feet of rocks they consider to be the Windfall Formation. Unfortunately they do not describe the specific locality, or the lithology or metamorphism of the formation, except to state that stromatolite-bearing limestones near the top of the section are lenticular and considerably thinner than in corresponding sections, and that the limestones are locally interbedded with dolomites. Willden and Kistler (1979) described the Windfall in the Jiggs Quadrangle as containing two members which they correlated with the Catlin and Bullwhacker members of the Eureka District. Millikan (1979) also broke the Windfall into the Catlin and Bullwhacker Members in the southern Ruby Mountains within the Sherman Mountain Quadrangle. The lower member is approximately 350 to 500 feet of gray limestones in beds from a few inches to 1 foot in thickness, with thin beds of recrystallized chert. The upper member is about 500 feet of alternating light brown, shaly and silty limestone and light-gray limestone in beds up to 2 inches in thickness (Willden and Kistler, 1979; Millikan, 1979). Most workers in the Ruby Mountains have not been able to effectively break out formations within the Cambrian section because of extensive metamorphism and recrystallization during deformation (Howard and others, 1979).
In the Buck Mountain-Bald Mountain area to the south of the Ruby Mountains two members equivalent to the Catlin and Bullwhacker Members have been described by Rigby (1960). The lower member is composed of 450 to 500 feet of interbedded black limestone and calcareous olive shale in two to six-inch thick units. The upper member is brown to black, argillaceous limestone interbedded with lesser flaky gray shale (Rigby, 1960).
In the White Pine and Horse Ranges, the Windfall contains a basal 300 to 500 foot thick unit of thin-bedded, shaley, lavender limestone overlain by about 800 feet of bluish to dark gray, medium-grained limestone with nodular chert (Lumsden, 1964). Above this is about 500 feet of chocolate brown, medium to coarse-grained, mottled and cherty dolomite which locally contains algal beds, and an upper unit of gray limestone with interbedded chert layers 0.5 to 2 inches thick giving a striped appearance to the rock (Lumsden, 1964).
In the Grant Range, the Windfall Formation is represented by four members. The upper 300 to 500 feet are composed of thin-bedded, blue-gray limestone and interbedded shale. The middle section is composed of about 800 feet of dark-gray cherty limestone, underlain with about 500 feet of medium-grained, chocolate brown, mottled, cherty dolomite. The lower portion is 250 to 400 feet of grey to yellowish weathering limestone, and interbedded dark nodular cherts (Moores and others, 1968).
In the Cherry Creek Range, the lower several hundred feet of the Windfall Formation is gray to olive-gray, thin-bedded to platy limestone, and interbedded black laminated chert (Hose and Blake, 1976). The basal 25 feet of the formation are a distinctive light gray weathering limestone in 1 foot thick beds (Fritz, 1968). The upper 1,000 feet of the formation are light to medium gray, stromatolite-bearing limestone, and minor interbedded medium brown dolomite, both of which contain abundant gray, white and brown chert nodules (Adair, 1961; Fritz, 1968). Overall, the formation weathers to a light gray, yellow and light orange. This sequence of rocks was originally designated as the Notch Peak Formation by Adair (1961) and as Windfall Formation plus Ordovician Formation A by Fritz (1960). It is here considered Windfall Formation as designated by Hose and Blake (1976).
In the Kingsley Range, Buckley (1967) described the Windfall Formation as overlying the Dunderberg Shale. It is composed of a lower member composed of 1,200 feet of thin-bedded, dense, fine-grained and dark gray limestone with thin chert laminae and reddish argillaceous partings. This is overlain by an upper portion about 950 feet thick which is composed of thick-bedded to massive, gray limestone with irregular stringers and nodules of chert (Buckley, 1967; Coats, 1985).
In the northern Schell Creek Range, the Windfall is about 650 to 1,100 feet of thin-bedded and argillaceous, aphanitic to finely clastic, gray limestone with dark gray and green chert in the lower part (Young, 1960; Dechert, 1967). Well preserved trilobites are abundant in the formation. In the southern Deep Creek Range, rocks correlated with the Windfall are composed of flaggy, thin-bedded, dark gray trilobite-bearing limestone with thin beds and lenses of brown weathering chert (Nelson, 1959).
In the Elk Mountains, Mathias (1959) described nearly 1,000 feet of section overlying the Dunderberg Shale which he correlated with the Windfall Formation. The lower member is about 150 feet thick with a basal massive, fine-grained, blue limestone containing shale partings in beds 2 to 3 feet in thickness, overlain by thin-bedded, flaggy, gray limestone interbedded with tan shale beds, and light to dark gray or white chert layers up to an inch thick (Mathias, 1959; Fifer, 1960). The upper member is about 90 feet of yellow to gray, massive, fine grained quartzite, overlain with about 390 feet of bright orange and red weathering, flaggy siltstone and limestone with brachiopods and trilobites, and 280 feet of very thin bedded and laminated, light to medium-gray, siliceous, platy limestone (Mathias, 1959; Coats, 1985).
The Windfall Formation varies in thickness from 500 feet in the Antelope Range to 650 feet in the Eureka area (Roberts and others, 1967), 500 to 1,000 feet in the southern Ruby Mountains (Hose and Blake, 1976; Willden and Kistler, 1979) and 800 to 850 feet in the Bald Mountain area at the southern tip of the Ruby Mountains (Rigby, 1960), about 640 to 1,140 feet in the Grant Range (Cebull, 1967; Kleinhampl and Ziony, 1985), 1,846 to 2,392 feet in the northern Horse and southern White Pine Ranges (Lumsden, 1964), about 1,114 feet in Egan and Schell Creek Ranges (Young, 1960; Fritz, 1960) and about 650 feet in the northern Schell Creek Range (Dechert, 1967), about 650 feet in the southern Deep Creek Range (Nelson, 1959), 2,150 feet in the Kingsley Range (Buckley, 1967), 940 feet in the Elk Mountains (Mathias, 1959) and 370 feet to the south in the Jarbidge Quadrangle (Fifer, 1960).
The Windfall Formation is exposed in the Eureka area, Ruby Mountains, northern Antelope, Monitor, Pancake, Hot Creek (Hales Limestone of Quinlivan and Rogers, 1974), Quinn Canyon, northern Egan, Schell Creek, southern White Pine, Grant, Horse, southern Cherry Creek, southern Deep Creek, and Kingsley Ranges, and Elk Mountains.
The depositional setting of the Windfall Formation is poorly understood. The abundance of trilobites, algal stromatolites, and trilobites, suggest the thin-bedded, cherty limestones and dolomites, and shales of the Windfall Formation represent shallow marine shelf sediments. The siltstones and quartzites in the Windfall may represent beach and barrier-bar lithologies.