Discussion

AN INTEGRATED PETROLEUM EVALUATION OF NORTHEASTERN NEVADA

REGIONAL PALEOGEOGRAPHY

DISCUSSION

Structural complexities, both compressional and extensional in nature, create considerable difficulties in understanding stratigraphic relationships which are already complicated by locally deposited and discontinuously exposed stratigraphic units within structurally separated ranges. Relatively rapid facies changes both in Paleozoic and Tertiary units, a lack of Mesozoic or Paleozoic well penetrations, and poor exposure due to erosional removal or Tertiary sedimentary and volcanic cover also insure minimal stratigraphic control for paleogeographic reconstructions. Many contact relationships between lithologic units are difficult if not impossible to distinguish as being structural or stratigraphic, and many units are of such local extent that their appearance or disappearance is often enigmatic.

This is particularly true where facies have been displaced laterally along the Roberts Mountains thrust zone or in the eastern portion of the area where stratigraphic sections have been attenuated along low angle detachment-type normal faults. Facies changes in Paleozoic units within White Pine County, for instance, are commonly quite gradual with fairly uniform units both in thickness and lithology, until the Diamond Mountains- Pancake Range area where units change radically both in thickness and lithology. Hose and Blake (1976) suggested this change may represent a persistent narrow zone of facies change within the miogeocline, or conversely may be the result of shortening within a low-angle normal fault zone.

There is also some difficulty in sorting out the effects of arches and paleogeographic highs from the effects of various structural events. A major, generally east-west-trending positive element or axis, appears to have intermittently affected facies distribution and thickness from the Ordovician through the Early Permian. This positive was called the Cortez-Uinta arch by Roberts and others (1965) who suggest it is present throughout northeastern Nevada from the Uinta Mountains in western Utah to the Cortez Mountains in Nevada.

This axis probably includes local arches and highs named by various workers in various areas including the Ordovician Tooele Arch of Webb (1958), the Pennsylvanian Northeast Nevada High of Steele (1960), and the east-southeast high proposed for the Permian Park City Group in the Pequop Mountains by Yochelson and Frazer (1973). Several other named and unnamed elements are also included in the arch. This arch has been used to explain the source of clastic debris, rapid facies changes and changes in thickness, and absence or thinning of several stratigraphic units.

Although it appears clear that the Cortez-Uinta Arch and local highs associated with it have affected sedimentation to some degree, it also seems clear that the importance of structural control of facies changes, particularly thinning along low-angle normal faults, has been neglected and confused with paleogeographic elements. A few workers (Misch, 1960; Mercantel, 1975; Yochelson and Frazer, 1973; and others) have made note of structural complications in facies reconstruction. The most notable changes in stratigraphic units occur in the eastern and northeastern portion of the study area where units are abruptly thinned or removed.

Yochelson and Fraser (1973) note the thinning of 2,500 feet of Upper Pennsylvanian and Lower Permian strata to only 300 feet from the southern to northern portion of the Pequop Mountains. Mercantel (1975) has pointed out that the Permian section thins from 5,500 to 3,000 feet from Ferguson Mountain and southern Pequop Mountains to the northern tip of the Schell Creek Range and that nearly 7,000 feet of section is missing in this area. After incorrectly pointing out that "Pennsylvanian and Permian sediments are basically unaffected" by low-angle faulting, Steele (1960) made note of the variation in thickness of the Diamond Peak Formation from about 3,000 feet in the Diamond Range to less than 20 feet in the Goshute Range. This thin section in the Goshute Range was examined by Western Cordillera geologists and is the result of attenuation along a low-angle fault. These observations suggest that thinning along arches is probably dominantly the result of structural omission along low-angle normal faults.

Also removed along an "unconformity" from the southern Egan Range to the northern Schell Creek Range is the Pennsylvanian through Mesozoic section. The Pennsylvanian-Permian Ely Limestone, Riepetown Formation, Arcturus Formation, Park City Group, and possibly the lower part of the Triassic were removed prior to deposition of the Eocene Sheep Pass Formation which contains clasts from rocks as old as the Cambrian Prospect Mountain Quartzite in the Schell Creek Range (Young, 1960; Drewes, 1967; Hose and Blake, 1976). This unconformity is related to the Butte Structural Trough which formed to the west of an arched area in the Snake, Schell Creek, and northern Egan Ranges possibly as a result of isostatic adjustment to massive stratigraphic attenuation along low-angle normal faults there (Hose and Blake, 1976). The trough extends from the westward dipping flank of the Cherry Creek and Egan Ranges to the western flank of the Ruby Mountains, with an axis through the Maverick Springs Range, Butte Mountains, southwest margin of Radar Ridge, and across the Egan Range and Schell Creek Range where it dies out to the south in the Fortification Range (Hose and Blake, 1976).

Structural complication of facies patterns has been and continues to be overlooked by most stratigraphers. This writer stresses the importance of understanding structural complication of facies by omission, repetition and segmentation along thrust faults and low and high-angle normal faults. This understanding must be applied to this, or any other, paleogeographic discussion or "reconstruction". Both unconformities and arches and basins must be re-evaluated with a possible structural connection in mind.

The following section is a brief discussion of the generalized paleogeography of the study area. Appropriate time slices are discussed from the Precambrian up-section through the Cenozoic. Paleozoic rocks through the Upper Devonian are discussed in terms of platform, outer shelf-upper slope, and subduction related rocks now within the Antler allochthon. Mississippian through Permian rocks are discussed in terms of the Antler foreland basin and successor basins. Mesozoic and Cenozoic rocks are discussed in terms of local marine and and fluvio-lacustrine basins, with the exception of subduction related rocks now within the Golconda allochthon.