AN INTEGRATED PETROLEUM EVALUATION OF NORTHEASTERN NEVADA
PLATY SILTSTONE, LIMESTONE, SHALE AND CHERT UNIT
Type Section Information
This unit is a general map unit which includes unnamed transitional facies lithologies in several areas. It has no designated type section.
Lithologies within this transitional unit are Upper and Middle Devonian in age (Gardner, 1968; Lovejoy, 1959; Ketner, 1975b; Coats, 1985). It is Upper Devonian in the Independence Range (Lovejoy, 1959), Middle Devonian in the Snake Mountains based upon corals and conodonts, and early Late Devonian in the Sulphur Spring Range based upon conodonts (Smith and Ketner, 1975). This unit is a partial age equivalent of the Woodruff Formation, as well as the Nevada Formation and overlying Devils Gate Formation (Smith and Ketner, 1975).
In the Sulphur Spring and Pinon Ranges, the transitional unit is exposed in fault bounded and locally overturned exposures (Smith and Ketner, 1975). A structural sequence includes about 50 feet near the base of gray quartz and chert-rich sandy limestone and calcareous sandstone. This is overlain by about 50 feet of dark gray, brown and black chert, and conglomerate and sedimentary breccia composed of chert pebbles and blocks. This section is overlain by an indeterminate thickness of gray, thin to thick-bedded clastic, cross-bedded lenticular limestone with abundant black chert pods and fracture fillings (Smith and Ketner, 1975). About 10 feet of angular gray limestone-clast conglomerate is also present in the unit. The uppermost unit is exposed along the western flank of the Sulphur Spring Range and is gray to black flaggy limestone interbedded with sandy limestone with well rounded chert and quartz fragments and grains. Its thickness is also indeterminate (Smith and Ketner, 1975).
In the Lone Mountain area of the Independence Mountains, the unit is exposed in two fault slices, one containing limestone, calcareous siltstone and shale, and the other containing limestone (Lovejoy, 1959). Ketner (1975b) summarized the unit as consisting of limestone, argillaceous limestone, and graded sequences of quartz-rich sandy limestone, siltstone, shale, and bedded black chert. Soft, brown to black, calcareous shale is the dominant lithology over large areas.
In the Snake Mountains, the unit is exposed along the western flank of the northern portion of the range. It is composed of dark gray-brown carbonaceous shale and fine-grained carbonaceous gray limestone, overlain with platy, light gray and pinkish, siliceous siltstone (Gardner, 1968).
This unit occurs in faulted and locally overturned sections in the Sulphur Spring Range and Independence Mountains where at least 200 to 500 feet of section are present. The unit is about 300 feet thick in the western Snake Mountains (Gardner, 1968).
This transitional unit occurs in the Lone Mountain area of the Independence and Snake Mountains, and in the Sulphur Spring and Carlin-Pinon Ranges.
The depositional setting of this unnamed unit is very poorly understood. In general, the entire sequence of lithologies appear to represent outer shelf to upper slope deposition. The carbonaceous nature of the shales and cherts, and presence of graded siltstone and limestone, suggest relatively deep marine deposition, at least in part controlled by turbidity currents.