AN INTEGRATED PETROLEUM EVALUATION OF NORTHEASTERN NEVADA
Type Section Information
The Dinwoody Formation was originally named for exposures along Dinwoody Canyon on the northeastern slope of the Wind River Mountains, near Dubois, Wyoming (Blackwelder, 1918). Newell and Kummel (1942) redefined the Dinwoody at the type locality as the dominantly silty strata between the Phosphoria Formation and the top of the resistant siltstone that was originally in the middle of the Dinwoody as defined by Blackwelder (1918).
The Dinwoody Formation is Early Triassic (Sycthian) in age. It is overlain conformably in many sections by the Thaynes Formation and often overlies the Phosphoria Formation.
In northeastern Nevada, the Dinwoody Formation is dominantly fine-grained gray-brown massive to well bedded siltstone and minor amounts of conglomerate.
In the Snake Mountains, Gardner (1968) described the Dinwoody Formation as over 2,000 feet of massive, calcareous, greenish to brown or gray, siltstone and silty gray to olive shale with pyrite and carbonaceous matter, and thin interbeds of fine-grained, dark gray limestone with pelmatazoan columnals overlain by the Thaynes Formation. Bezerrides (1967) divided the Dinwoody into upper and lower members in the O'Neill Pass area of the Snake Mountains. The lower member is composed of 660 feet of conglomerate in lenticular beds one inch to 5 feet thick and as much as 50 feet thick. Clasts are well rounded to angular, chert, limestone, and siltstone that vary in size from pebbles to boulders up to 7 feet long and 4 feet wide. The conglomerates are all poorly sorted with local graded and scoured beds (Bezzerides, 1967). Siltstone beds up to 2 inches thick are interbedded with the conglomerate, and are locally rippled and cross-bedded and contain worm burrows. The upper member of the Dinwoody Formation is poorly exposed, massive to well bedded, laminated and crossbedded, gray siltstone which weathers to a gray-brown. Blocks of phosphorite and chert up to 12 feet in diameter locally float in the siltstone, and were probably derived from the underlying Phosphoria Formation (Bezzerides, 1967). Thin, black to blue-gray micritic limestone beds up to 10 feet in thickness are also present in the upper member, and grade into the overlying Thaynes Formation. The upper member is about 1,740 feet thick in the Snake Mountains (Bezzerides, 1967).
Small masses of cobble conglomerate mapped regionally as "marine conglomerate" by Coats (1985) in the Marys River Northeast and Wild Horse Quadrangles, and just south of the Jarbidge Wilderness are included here with the Dinwoody Formation. They are probably partially equivalent to the lower member mapped by Bezzerides (1967) in the O'Neill Pass area of the Snake Mountains. These rocks rest unconformably on upper Paleozoic sediments or western assemblage units. Thicknesses have not measured.
In the southern Pequop Range, the Dinwoody Formation is composed of a few hundred feet of olive-gray shale and interbedded red-brown weathering, chocolate-brown limestone (Snelson, 1955). In the Currie area, the Dinwoody is poorly exposed greenish-gray to olive, calcareous siltstone and shale with a 10 to 50 foot thick unit of dark-gray, medium grained, medium-bedded reddish-weathering fossiliferous limestone about 50 feet above the base of the formation.
In the Windermere Hills, the Dinwoody conformably overlies the Permian Carlin Canyon Formation and is composed of green silty argillite in beds 4 to 6 inches thick interbedded with dark brown-gray, platy argillaceous calcisiltite and gray to green-brown silty shale (Oversby, 1972).
Early Triassic rocks exposed in the Mount Velma Quadrangle (Coash, 1967) and East Humboldt Range (Snelson, 1957) are tentatively assigned to the Dinwoody Formation. In the Mount Velma Quadrangle, the Triassic is a folded and faulted sequence composed of pale green, light yellow-tan weathering, massive siltstone (Coash, 1967). A few thin beds of water lain tuff and limestone also appear to be interbedded with the sediments. The thickness of the sequence is estimated to be between several hundred and several thousand feet (Coats, 1985). In the southern portion of the East Humboldt Range, the Early Triassic rocks are composed of gray-brown limestone with intercalated shales, overlain by poorly exposed shale, siltstone and limestone (Snelson, 1957).
Early Triassic rocks exposed in the northeastern corner of Elko County along Long Canyon in the Dairy Valley Quadrangle are also tentatively assigned to the Dinwoody Formation in this report. R.A. Hope (in Coats, 1985) divided the section into an upper portion of tan and orange-weathering, platy siltstone and crosbedded, sandy and pure, gray to brown, orange weathering limestone, and a lower portion of platy, dark-gray, light gray-weathering laminated silty limestone with pelecypods. Our observations in this area concur with Hope with the addition of several thin brown-gray to green shaley interbeds near the base of the formation which were also noted by Clark (1957). Fossils of the Meekoceras zone were found in limestones about 412 feet above the underlying Permian rocks by Clark (1957).
Undifferentiated Triassic sediments mapped by H.F. Bonham (in Coats, 1985) in the Montello Canyon area of the Leach Range may include the Thaynes Formation, but are assigned tenatively here to the Dinwoody Formation. The section consists of over 2,000 feet of interbedded gray and green shale, and brown shaly limestone in the lower portion overlain by massive, well-bedded gray to brown-gray-weathering limestone with thin interbeds of shale. This section overlies the Permian Gerster Formation.
The Dinwoody Formation is several hundred feet thick in the East Humboldt Range (Snelson, 1957), about 2,400 feet in the Snake Mountains (Bezzerides, 1967; Gardner, 1968), 2,200 feet in the Montello Canyon area of the Leach Range (Clark, 1957), 400 feet in the southern Pequop Mountains (Snelson, 1955), 300 feet in the Windermere Hills (Oversby, 1972), 560 feet in the Currie area (Nelson, 1956), and several hundred to several thousand feet in a faulted and folded section in the Mount Velma Quadrangle (Coats, 1985).
The Dinwoody is exposed within the Marys River Northeast and Wild Horse Quadrangles, just south of the Jarbidge Wilderness, Snake Mountains, Leach Range, Mount Velma Quadrangle, southern East Humboldt Range, southern Pequop Mountains, Windermere Hills, and Currie area.
The Dinwoody represents a complex of depositional environments that deposited continental red-beds, and quiet water shallow-shelf sediments on and bordering a broad and generally shallow marine shelf (Collinson and Hasenmueller, 1977). The presence of water lain tuff in some sections indicates local Triassic volcanism.