Abstract					2
Introduction					2
Geography					2
Access						2
Climate						2
Vegetation					2
Human Activities				3
Previous Work/Present Study			4
Acknowledgements				4
Geologic Setting				4
Stratigraphy					5
Introduction					5
Map Unit Descriptions				5
Structural Geology				21
Introduction					21
Deformation and Intrustion of Lower Plate	21
Jurassic					21
Cretaceous					21
Faulting History				21
Introduction					21
Upper Plate Faults				22
Fault System on West Side of Range		22
John's Wash Fault System			22
Murphy Wash Fault System			22
Decathon Fault System and Related Faults	22
Anything Else???				23
The SSRD					23
Minvera Lower Plate Fault System		23
Western Range Front Fault			23
Summary						23

Abstract

Introduction

Geography

The geology of the southern Snake Range, including the Minerva Canyon and Wheeler Peak quadrangles, was previously mapped by Whitebread (1969) at a scale of 1:48,000. Whitebread mapped the area as consisting of two major structural plates, the "upper" and "lower" plates, separated by the southern Snake Range d残ollement. Part of this was mapped at a scale of 1:24,000 by the Stanford Geological Survey during the summer field season of 1993 using new 7.5 minute topographic base maps now available for the region. The area mapped included all of the Wheeler Peak and Minerva Canyon quadrangles (Figure 00) which constitute 2 of 6 quads which contain the new Great Basin National Park.

The Minerva Canyon Quadrangle contains Paleozoic rock units ranging in age from the Cambrian to the Carboniferous, as well as Tertiary volcanic rocks and conglomerates, and various Quaternary deposits. The Wheeler Peak quadrangle contains units ranging from the Precambrian to the Ordovician, and also contains Tertiary conglomerates. In addition, the Wheeler Peak quadrangle contains the Jurassic Snake Creek/ Williams Canyon pluton. Depositional setting for the units ranging from the Cambrian to the Carboniferous correspond to the eastern miogeoclinal setting as outlined by Stewart (1980). In general, units in the study area become older in a north to south, and west to east fashion ranging from Precambrian units in the far northwest of the study area to Tertiary conglomerates in the south.

The topographic expression of the area is variable though it shows a general increase in relief and elevation from south to north. The southwestern portion of the quadrangle is dominated by a north-south trending ridge of drastic cliffs, while the southeastern portion of the area is characterized more by hills and low ridges. The cliffs of the southwest eventually whip eastward to join with Highland Ridge and continue northward to become such landmarks as Baker Peak and the aptly named Pyramid Peak. Eventually, this series of ever heightening peaks culminates in Wheeler Peak, the highest point in the area, from which the surrounding ranges and valleys may be easily observed.

Access

In addition to roads, there are many trails in the vicinity of Wheeler Peak which are clearly marked and well maintained. Unfortunately, trails decrease to the south until there is a paucity of any sort of path. A trail was broken along Highland Ridge at some point in the past and is shown on most maps, but in reality, it has long since degenerated to a glorified deer path. Many parts of the Minerva Canyon quadrangle are, in fact, nothing more than trackless swaths of mountain mahogany and sage reserved for the hiker with tough clothing and tougher skin. Higher elevations tend to have less vegetation and are thus more manageable, but are often guarded by formidable cliffs.

Climate

This rainfall gradient is reflected in the vegetation... (Go Elizabeth!)

Vegetation

Ever denser stands of juniper and pinon pine clad the upper alluvial fans and foothills of the southern Snake Range, representing the first woodland community above the sage and grassland of lower elevations. The height and density of these trees, together with the lack of relief of their habitat make it annoyingly difficult to locate oneself in the field. Overgrazing and increased fire control have led to an expansion of the juniper community over the years which is notable when using older air photos. During September, pinon cones ripen. They should be picked when fat but still green, oozing with sap. Placed in a dark, dry, safe place, they dry and open, yielding one of the key raw ingredients of that delectable dish- pasta al pesto- but good luck finding fresh basil in Ely!

Mountain streams provide water that supports riparian communities in the drainages of the southern Snake Range. Appearing enticingly shady from the hot ridge tops, they will no doubt lure an eager hiker into their dense and impenetrable thickets of wild roses, and more rarely, raspberries, in addition to young aspen, alder, chokecherry, willow, and cottonwood. Although luxuriously vegetated, some of these drainages prove to be dry.

Higher elevations, above 8,000' bring some relief temperature wise from the hot and scratchy lowlands. Mixed conifer forests of ponderosa, white, and Douglas fir are found on north facing slopes, but the drier, mountain mahogany covered south-facing slopes present a new set of challenges. Woe to the geologist who innocently begins a descent from higher elevations across such a slope. Mountain mahogany, particularly young and stubby mountain mahogany, can deter one's progress as nothing else can. Its inflexible branches make passage impossible except by desperate and violent destruction with a sledge hammer. Its elongate, fuzzy flowers consist of a delicate yellow fur covered with tiny hooks that, when inside the back of one's shirt, feels like crushed fiberglass insulation. If these bushes are old and high enough, one can generally make one's way in a backbreaking, stooped position.

In ascending the range, cool alpine climates at elevations greater than 9,000' host the engelwood spruce and limber pine community. Aspen forests in valleys over 8,000' and in moist upper elevation slopes with their stark white trunks and quaking leaves ("quaking groves") provide welcome respite, shade, and comfort for afternoon naps. Their white trunks boast a myriad of carvings documenting the past 50 years of shepherding, ranching, hunting, and geologic mapping. Of particular note are the many carvings incised by the notorious Whitebread from 1955 to 1968 throughout Decathon Canyon.

Approaching tree line, bristle cone pine stands represent the last plant life, clinging for survival on the rocky crests and ridges of the range. Bristle cone pines, a primary Great Basin National Park attraction, are also present at much lower elevations (7-8,000') on north facing slopes, most commonly underlain by dolomite. These bristle cone pines do not, however, share the ancient histories and longevities of their counterparts at higher elevations. The southern Snake Range hosts many spectacular stands of bristle cone pines, only a small part of which are touted to the public by the National Park. The most spectacular of these forests covers the gently sloping region above 11,000' between Mt. Washington and Lincoln Peak, extending south of Lincoln Peak down Highland Ridge into the Minerva quadrangle. Four-wheel drive roads to access mining claims criss-cross the bristle cone pine forest between Mt. Washington and Lincoln Peak.

Above tree line, one can finally breathe clean, cool, thin air, even on scorching valley days. Here the alpine community, including primrose, sticky polenium and phlox, granes, and sedges, grows in rocky conditions too harsh for trees. Growing on the rocky quartzite slopes are two types of flowering plants, one of which smells distinctly of skunk and the other simply foul.

Human Activities

The Great Basin was not a particularly hospitable region. Although the paleoclimate was certainly less arid than today, life could have been more easily lived in many other places. Based on the archaeological record, it took until 8,000 B.C. before population pressure (or war, or a socio-cultural revolution, or random chance) caused humans to adopt this region for any significant amount of time. The first group to do so has been termed the Anasazi, reflecting their linguistic heritage. From the archaeological record, it seems that these were a sedentary people who employed a horticultural subsistence pattern later exemplified by the cliff dwelling Anasazi of Mesa Verde.

Over time, roughly 8,600 years, the Anasazi drifted laterally to areas farther east, gradually being replaced by the Fremont, Chewaucanian, and Lovelock cultures in a medley of shifting territories and tribal movements only vaguely documented. It appears that those cultures which replaced the Anasazi group and the Anasazi themselves, were responding to gradual climactic changes, moving into areas which suited their previously set subsistence patterns. The Fremont were horticulturally based like the Anasazi. The Chewaucanian and Fremont cultures were adapted to lacustrine survival. The Lovelock even produced stuffed waterfowl decoys to assist in their foodgetting. Eventually however, the climate took a relatively drastic turn towards the more arid.

Sometime around the seventh century (A.D.), the present Basin and Range was more or less abandoned by the previous cultures, having become semi-desert. The Anasazi had long since moved toward present day Colorado and New Mexico, the Fremont just north of that, the Chewaucanian to the north-west, and the Lovelock into the rainshadow of the Sierras. This left a gap that came to be filled by a Numic speaking people. Based on distinct new styles of basketry, pottery, architecture and rock art, and glotochronological evidence, it seems that this new culture came from the most northern part of the Utaztekan group which was located in present day Death Valley. These people were adapted to desert survival and managed to remain in the area through the monumental drought of the twelfth and thirteenth centuries which caused the abandonment of settlements throughout the Southwest. It was not until European intrusion that a drastic cultural change occurred again in the region.

Two Spanish friars, Dominguez and Escalante, were the first known visitors of European descent to visit the area in 1776. Approximately fifty years later the famous explorer Jedediah Smith traversed the area. Twenty years after Smith's travels, John Charles Fremont of the U.S. Army encircled the area and named it the Great Basin, being the first person to recognize that no rivers within the area drain to the sea.

Among the first permanent white settlers was Howard Egan, an Irish immigrant, who came west with Brigham Young in 1845. Egan befriended the Shoshones and so was able to lead wagon trains across Nevada to California during the 1849 and subsequent Gold Rush years. In 1858, the town of Baker was established as a Mormon outpost. By 1860 the Pony Express was carrying mail along a route to the north by the Deep Creek Range.

In the mid-1860's, Ab Lehman established a ranch on Strawberry Creek and later moved six miles south and opened Lehman Cave. The discovery of gold at Osceola in 1872 began a long history of mining in the southern Snake Range and surrounding areas. The Osceola Ditch was constructed in the 1870s to carry water to the Osceola mine which produced between one and six million dollars of gold ore. Mining is no longer a major activity in the region but mining for gold, mostly as placer deposits in older Quaternary alluvium continues in the general vicinity of Osceola and the northwest flank of the southern Snake Range. Many mining claims are still maintained and have helped to define the present "indefinite park boundary" of the Great Basin National Park. By the 1880s, mining was waning and livestock ranching growing. The lumber industry became active in the area and partially in response to the unregulated nature of this activity Teddy Roosevelt established the Nevada National Forest, which included the Snake Range, in 1909. By the 1910s, Basque immigrants had become prominent sheepherders, establishing large sheep herds in the region.

In 1922 President Warren Harding proclaimed the Lehman Caves a national monument. On October 27, 1986 Great Basin National Park was established as a token of President Reagans "environmental policy". The designation of national park status has been greeted with mixed emotions due to the no-grazing laws generally associated with national parks. However, because of the past use of the lands for cattle ranching, a compromise was struck allowing the continuation of grazing for an indefinite period of time. It is expected that grazing will be phased out as it is not considered to be a tourist attraction by park officials. Plans devised by park officials to make the park more attractive are underway, yet as a current resident of Baker noted, "Sure, the area is nice, but it isn't ever going to be Yosemite".

Previous Work/Present Study

Acknowledgements

This Open-File results from the 1993 collaboration of the U.S. Geologic Survey, the U.S. National Park Service, Stanford University, and University of California at Santa Barbara . We gratefully acknowledge the research funding from the U.S. National Park Service through an Interagency Agreement with the U.S. Geologic Survey, and collaborators to provide geologic mapping for the Great Basin National Park, Nevada. Janet L. Brown of the U.S. Geologic Survey formulated the collaborative Interagency Agreement and the effort to complete and publish these geologic maps. We owe thanks to Albert J. Hendricks, Superintendent, Great Basin National Park, Vidal Davila, Acting Chief Ranger, and Kurt S. Pfaff for their expertise and support.

We are inimitably indebted to Lisl Gans, a woman who was quietly transformed into camp manager, nanny, range rover pilot, spirit lifter and thrift store queen in the course of one short summer.We are also grateful to Pip the Wonderdog for his boundless enthusiasm in providing convenient wake-up calls in the form of drooling kibble-kisses and howling forays into bovine territory. We also thank the Vehicular Maintenance Department of Stanford University for their understanding concerning the sheared shocks, displaced fender, dented panelling, broken door handle, missing mirror, and broken windshield of the fuel efficient vehicles they so conscientiously maintain. Next, we feel compelled to recognize the administration of the recently formed Great Basin National Park for their exemplary efforts to reduce government spending and our National Debt by drastically reducing the number of trails and thus saving on trail maintenance. Finally, and most importantly, we would like to acknowledge our moms: Maria, Marcia, Margaret, Marcella, andLinda. And of course our dads: Julio, Robert, Bob, Frank, and Paul.

Geologic Setting

The dominant structural feature of the Minerva Canyon quadrangle and adjacent Wheeler Peak quadrangle is a regionally extensive detachment fault known as the southern Snake Range d残ollement (SSRD) first named and described by Misch (1960). On average this fault dips 10｡-15｡ eastward, but like many low-angle detachment faults in the Basin and Range Province, it is warped by gentle map-scale folds or corrugations plunging approximately parallel to the inferred movement direction along this fault (in this case, approximately 102｡,6｡; see below). The upper plate of the SSRD exposes a severely attenuated, normal fault-bounded mosaic of various non-metamorphosed to slightly metamorphosed sedimentary rocks representing parts of the Paleozoic miogeoclinal carbonate sequence and portions of an overlying Tertiary sequence of conglomerate and lesser sandstone, marl and tuffaceous marl. The lower plate of the SSRD exposes a thick sequence of mildly deformed latest Precambrian to Middle Cambrian age units. Two plutons of Late Jurassic, and Cretaceous age intrude these strata (Plate IB, map).

The position of the Snake Range in the hinterland of the Cretaceous-aged Sevier orogenic belt (Fig. 2) led earlier workers to relate the low-angle "d残ollement faulting" in the southern Snake Range to thin-skinned thrust faulting further east (Misch, 1960; Miller, 1966; Hose and Danes, 1973; Hintze, 1978), and this interpretation was adopted by Whitebread (1969) in his more detailed mapping of the area. However, as discussed in more detail below, cross-cutting relationships with upper plate normal faults have constrained the age of the SSRD, a low angle normal fault, to Tertiary in age.

Stratigraphy

Introduction

The oldest part of the stratigraphic section in the study area is represented by rocks from the upper two units of Precambrian McCoy Creek Group - the Osceola Argillite and the underlying quartzites of the McCoy Creek Group unit 1. These units form part of an impressively thick section believed to be late Precambrian in age but they have not been dated with any certainty. Generally the McCoy Creek Group strata are considered as part of the broader Windemere Group believed to be 800-600 Ma old (Stewart, 1992). Because this represents such a long time span prior to the middle Cambrian and younger miogeoclinal deposits, the McCoy Creek Group may represent an older, unrelated episode of rifting and subsidence along the western margin of North America (Stewart, 1992).

The McCoy Creek Group is conformably and gradationally overlain by the Prospect Mountain Quartzite, a 1.5 km thick unit of clastic rocks assigned to the Early Cambrian, although it lacks fossils. The first Cambrian fossils occur in the overlying Pioche Shale which has been well dated and studied by stratigraphers and biostratigraphers. Trilobite fossils indicate the Pioche Shale is upper lower Cambrian in age (Stewart, 1974).

Several systems of normal fault cut Paleozoic and Tertiary rock unit sequences in the Minerva Canyon quadrangle, effectively reducing the amount of complete sections in the area. Where complete section thicknesses could not be calculated in the quadrangle, we have calculated minimun thicknesses and cited unit thicknesses estimated by Whitebread (1969) for the old Wheeler Peak and Garrison 15" quadrangles. Rock unit descriptions build upon Whitebread (1969) and on descriptions of units in reports from adjacent quadrangles (e.g. McGrew & Miller, Kious Spring- Garrison quadrangle USGS Open File Report - submitted; Miller and others, Windy Peak quadrangle USGS Open File Report - submitted; Gans and others, The Little Horse Canyon quadrangle NBM Open File Report - submitted).

Despite the amount of faulting that has occurred in this region, the rocks in the Minerva Canyon quadrangle have remained relatively undeformed. Even the units in the lower plate of the southern Snake Range d残ollement, which have undergone variable ductile deformation and metamorphism in the Northern Snake Range and in the adjacent Kious Springs and Garrison quadrangles, are mostly intact and only mildly deformed and metamorphosed in the Minerva Canyon and Wheeler Peak quadrangles.

Map Unit Descriptions

pCMc Precambrian McCoy Creek Group

pCm1 Late Precambrian McCoy Greek Group Unit 1 (quartzite)

pCmo Late Precambrian McCoy Creek Group Osceola Argillite

Cpm EARLY CAMBRIAN PROSPECT MOUNTAIN QUARTZITE

Cpi EARLY CAMBRIAN PIOCHE SHALE

Cpc CAMBRIAN POLE CANYON LIMESTONE

A member:

B member:

C member:

D member:

E member:

The E member of the Pole Canyon Limestone consists of two types of limestone: a fine grained, platey and silty slope-forming limestone and a very fine to fine grained, cliff-forming limestone. The lower 23 - 33 m of the E member crops out as a dark grey slope-former with tan or pink silt abundant along partings. Near the lower contact, the E member is well-bedded, and platy fracture is common. The bedding planes are mottled, with tan or pink silt along the partings. The mottled appearance of the silt beds is the result of bioturbation or burrowing. Fenestral fabric is common in the E member limestone. Higher in the section is a massive, medium grey cliff-former. Lenses of oolitic limestone are common in the massive member, and lenses of rip up clasts are present. Small Girvinella (5-10 mm in diameter) are also present in the massive section of the E member. The upper section of the E member is a poorly-exposed, dark grey slope-former.

Clp CAMBRIAN LINCOLN PEAK FORMATION

Cjw CAMBRIAN JOHNS WASH LIMESTONE

Ccs CAMBRIAN CORSET SPRING SHALE

Where better exposed, distinctive units may be seen within the Corset Spring Shale. According to Whitebread (1969), the upper portion of the formation is characterized by units of shale interbedded with limestone units. The shale is fissile and occurs in layers approximately 2 mm thick, and contain scattered discoidal nodules of medium-grey aphanitic limestone 3-8 cm in diameter. The limestone units in the upper portion of the Corset Spring are described by Whitebread as medium-grey, and coarsely clastic with beds 6-16 cm thick. The bedding planes are uneven and punctuated by layers of trilobite and echinoderm fragments.

The middle units of the Corset Spring Shale are, according to Whitebread, entirely light-olive-grey shales. These contain lenses of coarsely bioclastic limestone which average .8-3 cm thick and are between 9 and 72 cm long. Also in the middle units Whitebread describes a single 18 cm bed of intraformational conglomerate with rounded flat limestone pebbles in a clastic limestone matrix.

The lower portion of the Corset Spring Shale, by Whitebread's description, is once again interbedded units of limestone and shale. The shale has no peculiar characteristics in this portion of the section. The limestone units are described as being medium-grey to dark-grey and weathering light-grey. the bedding is uneven and between 1.5 and 6 cm thick. The limestone are locally nodular due to coalescence of greyish-orange-weathering shaly partings, and some beds are crowded with trilobite fragments.

OCn ORDOVICIAN-CAMBRIAN NOTCH PEAK FORMATION

The Ordovician-Cambrian Notch Peak Limestone is best exposed on a north-south trending ridge in the Minerva Canyon Quadrangle. A partial section of 770 m was measured on this ridge. The Ordovician-Cambrian Notch Peak Limestone is a resistant, medium-grey to dark grey, cliff-forming limestone. The limestone is in general a very fine to fine grained limestone, and chert nodules and stringers subparallel to bedding are its most characteristic feature. Chert nodules (2.5-7.5 cm thick) parallel to bedding are very common lower in section, with individual chert layers ranging up to 30 cm in thickness. There are medium-dark grey, nonfossiliferous, massive sections common in the middle of the Notch Peak that weather in a blocky manner. The float from the massive and chert-rich sections is commonly made up of equant cobbles with sharp fracture surfaces. Scattered, localized dolomitization has occurred in the massive sections of the Notch Peak unit. Well-bedded limestone sections (bedding 2.5-30 cm thick) defined by tan or pink silty interbeds (5-15 mm thick) are common higher in section. Platy fracture is common in these well-bedded sections, with evidence for bioturbation or burrowing evident along exposed, silty parting surfaces. Although chert is common in the Notch Peak limestone, it seldom occurs where there is silty interbedding. Sections of the unit containing chert have bedding thicknesses ranging from 15 cm to 1.2 m thick. Gastropods, trilobite debris, inarticulate brachiopods, stromatolites, and fossil hash are common higher in the section. The contact with the overlying Fillmore Limestone occurs at the sharp break in slope from the cliff- to ledge-forming Notch Peak Limestone to the slope-forming Fillmore Limestone.

Op ORDOVICIAN POGONIP GROUP

Opf ORDOVICIAN FILLMORE LIMESTONE of POGONIP GROUP

Opw ORDOVICIAN WAHWAH LIMESTONE of the POGONIP GROUP

Opj ORDOVICIAN JUAB LIMESTONE of the POGONIP GROUP

Opk ORDOVICIAN KANOSH SHALE of the POGONIP GROUP

Opl ORDOVICIAN LEHMAN LIMESTONE of the POGONIP GROUP

Oe ORDOVICIAN EUREKA QUARTZITE

The Ordovician Eureka Quartzite weathers to a rust color and is usually highly jointed and fractured. Because of its resistance to erosion, quartzite rubble and large blocks are common downslope of its exposures. Bedding is generally difficult to discern because of its purity and the presence of joint and fracture planes, except near the top of the unit where increasing carbonate content accentuates bedding within the unit. The Ordovician Eureka Quartzite is an orthoquartzite composed of well rounded and well sorted, fine to medium grained quartz. The unit is distinctively lacking in fossils, although vertical burrows can locally be seen.

OS ORDOVICIAN AND SILURIAN FISH HAVEN AND LAKETOWN DOLOMITES, UNDIFFERENTIATED

Dse DEVONIAN SEVY DOLOMITE

The Devonian Sevy Dolomite consists of distinctive white-weathering, light grey, thin to medium-bedded, very fine-grained, nonfossiliferous, generally well laminated dolomite. Quartz sand occurs in small stringers and lenses and as isolated grains "floating" in a dolomite matrix. These sand occurrences are more abundant near the top of the unit where quartz sand beds are up to several tens of cm to 1/2 m in thickness. The Devonian Sevy Dolomite is also noted for its lack of fossils. These features are diagnostic of the Devonian Sevy Dolomite.

Although there is no complete section of the Devonian Sevy Dolomite in the Minerva Canyon quadrangle, its lower and upper contacts were studied at separate locations. Its gradational contact with the underlying Ordovician-Silurian undifferentiated dolomite section can be seen just east of hill 8395T in the southeast portion of Johns Wash. The dark, coarse-grained, cherty, dolomitic beds of the Ordovician-Silurian become less common as the contact with the overlying Devonian Sevy Dolomite is approached. Where the Devonian Sevy Dolomite is best exposed, its upper contact with the overlying Devonian Simonson Dolomite can be clearly seen. The transition is marked by an obvious change from the light grey, fine grained texture of the Devonian Sevy Dolomite to interbedded dark brown layers and the sugary texture characteristic of the Devonian Simonson Dolomite. In some places the transition appears to be an unconformity or disconformity where large cracks or fissures in the underlying Devonian Sevy Dolomite are filled with layered sandy to conglomeratic dolomite that is similar in lithology to the overlying Devonian Simonson Dolomite.

In the Minerva Canyon quadrangle the Devonian Sevy Dolomite has a minimum thickness of 213 m Whitebread (1969) cites an estimated thickness of 244 m for this unit in the old Wheeler Peak and Garrison 15'' quadrangle.

Ds DEVONIAN SIMONSON DOLOMITE

The Devonian Simonson Dolomite is predominantly a ledgy slope former. It consists of light to dark brown, thin to medium bedded, laminated, microcrystalline to coarsely crystalline dolomite. Its basal part is thicker bedded and uniformly light brown. Fossils present in the unit include gastropods, crinoids, brachiopods, and stromatoperoids.

At the above location, the transition from the underlying light-colored Devonian Sevy Dolomite to the Devonian Simonson Dolomite is quite clear and has been described in the previous section. The stratigraphic contact with the overlying Devonian Guilmette is everywhere a fault contact in the Minerva Canyon quadrangle. Whitebread (1969) reports an estimated thickness of 575 ft. (175 m) of Devonian Simonson Dolomite which was measured west of Big Springs Ranch, 3 miles south of the Garrison quadrangle. In the Minerva Canyon quadrangle, the minimum thickness estimated for the Devonian Simonson Dolomite is about 168 m.

Dg DEVONIAN GUILMETTE FORMATION

MDp MISSISSIPPIAN DEVONIAN PILOT SHALE

The Mississippian Devonian Pilot Shale is a pink weathering grey to yellow calcareous shale with thin limestone interbeds and occasional chert stringers. It is usually evidenced by shaley float.

The thickness of the Mississippian Devonian Pilot Shale appears to be variable in the map area although this is probably related to faulting of the sequence. Whitebread (1969) has estimated its thickness to be between 122-244 m. We have estimated an apparent thickness of only 91 m in the Minerva Canyon quadrangle.

Mj MISSISSIPPIAN JOANA LIMESTONE

Mc MISSISSIPPIAN CHAINMAN SHALE

Although there is no complete section of Mississippian Chainman Shale in the Minerva Canyon quadrangle, it is at least 366 m where it is best exposed. Whitebread (1969) has estimated the thickness of the Mississippian Chainman in the old Wheeler Peak and Garrison 15" quadrangle to be 305 - 610 m.

Pe PENNSYLVANIAN ELY LIMESTONE

Tco OLDER TERTIARY CONGLOMERATE

The upper division of the conglomerate is pebbly with occasional cobbles. It is a clast supported conglomerate and is poorly sorted. The clasts are subangular to subrounded and occur within a sand and silt matrix. A few beds of sandstone up to 5 cm thick are interbedded with the conglomerate. The lower division of the conglomerate is very similar to the upper division but it is not as well bedded. A few granite clasts were found in the outcrop; these were several inches in diameter. No imbrication was discernable from the clasts.

Two clast counts were done on the conglomerate at this outcrop. One hundred clasts were counted in each location. The results are presented in Table 1.

Tnr TERTIARY NEEDLES RANGE FORMATION

Several divisions are visible in the Needles Range Formation beginning with a grey basal surge layer about 60 to 90 cm thick, followed by a pink ash flow that is about 1.5 to 3 m thick. Overlying these two is ~60 cm of conglomerate with a volcanic matrix. Imbricated pebbles in both conglomerate intervals indicate south-directed paleocurrents. Above the second conglomerate is another laminated, grey, ~1 m thick basal surge layer of crystal-rich tuffaceous sediments, overlain by pink to red ashflow tuff that forms the conspicuous red to red-brown vertically jointed cliffs which make up most of the mapped the Needles Range Formation. Portions of the area mapped as Needles Range by Whitebread (1969) in the lower portions of Murphy Wash area we have reinterpreted as landslide deposits based on the hummocky topography and random orientation of compaction foliation.

The Needles Range tuff is rhyolitic to rhyodacitic in composition and is generally moderately welded. The basal surge layer is rich in hornblende and biotite phenocrysts and the main overlying pink-colored ashflow tuff is exclusively biotite bearing. The tuff also contains plagioclase feldspar, quartz and lesser sanidine phenocrysts. The biotite books are up to half a centimeter in diameter. Generally, the phenocryst to matrix ratio is constant throughout the tuff and is estimated to be about 35% phenocryst and 65% matrix.

The source of the various cooling units that comprises the Needles Range Formation is the Indian Peak caldera complex, which lies south of the Minerva Canyon quadrangle. The Needles Range Formation has been dated at 33-27 Ma (Best and others, 1989). Its maximum exposed thickness is approximately 110 m but its top is erosional.

QTc QUATERNARY OR TERTIARY CONGLOMERATE

The conglomerate appears to contain clasts of all Paleozoic units down to at least the Pogonip group. Positively identified clast types include those derived from the Ordovician Eureka Quartzite, the Ordovician Fillmore Limestone, the Ordovician Kanosh Limestone, and few Ordovician - Silurian undifferentiated Dolomites. Most noticeable are the car size Eureka Quartzite boulders that litter the landscape at the northern exposures of the conglomerate. The conglomerate also contains abundant clasts of the Needles Range Formation in its more southerly exposures which indicates that exposure of the Needles Range Formation was once much more widespread than its current extent. Because these clasts were not seen in float further to the north, however, it is presumed that exposures of the Needles Range Formation did not extend much further up the present valley than their present day location. Based on these relationships we conclude that the conglomerate on the eastern side of the Minerva Canyon quadrangle is younger than the Needles Range Formation. The conglomerate predates Quaternary older alluvium, but it is unclear how much older it is relative to this alluvium.

The thickness of the Quaternary or Tertiary conglomerate is estimated to be about 153 -183 m from the relief of the unit.

Qol OLDER QUATERNARY ALLUVIUM

Qls QUATERNARY LANDSLIDE DEPOSITS

Qg QUATERNARY GLACIAL MORAINE

Qal QUATERNARY ALLUVIUM

Intrusive Rocks

Jgr JURASSIC GRANITE

Accessory minerals in the eastern portion of the pluton include biotite, epidote, titanite, magnetite, and allanite, whereas the central and western portions locally include garnet, ilmenite, and monazite primarily in aplite/pegmatite dikes and in felsic border phase rocks; apatite and zircon are ubiquitous throughout (Lee and Van Loenen, 1971). Within 10-20 m of the country rock contact, muscovite is often present together with or in place of biotite. West of Pyramid Peak in the Wheeler Peak quadrangle, extensive areas of the intrusion have been altered to a pyrite-muscovite quartz griesen, labeled Jgra on Plate 1A, associated with abundant quartz veins. These 0.05-5-meter-thick quartz veins strike approximately N60E, are steeply dipping, and have steeply plunging slickensides (Fig.9). They often contain the mineral assemblage muscovite-galena-wolframite (Smith, 1976). A hornblende-bearing, fine-grained granodiorite dike was mapped east of Pyramid Peak, and garnet-bearing aplite dikes are often found cutting the pluton near its contact with the country rock. Occasionally, somewhat more porphyritic equivalents of the pluton are found as dikes and in some parts of the pluton itself. In these, quartz, biotite, and plagioclase occur as phenocryst phases.

The Snake Creek/Williams Canyon pluton has a sharp intrusive contact with the several units that surround it, including the Precambrian McCoy Creek Group Quartzite1 and the Osceola Argillite, and the Cambrian Prospect Mountain Quartzite, Pioche Shale, and Pole Canyon Limestone. Contact effects include the growth of muscovite, chlorite, biotite, and aluminum silicate minerals within argillaceous layers and calc-silicate minerals within calcareous layers. Pervasive retrogression of shists and slates prevents conclusive identification of the aluminum silicate porhyroblasts that grew in the aureole of the pluton, but euhedral outlines allow identification of the pseudomorphs as staurolite. Staurolite porphyroblasts increase in size and quality towards the pluton. Grey, elongate, prismatic grains with square cross-sections were tentatively identified in the field as andalusite, and other retrograded aluminum silicate porphyroblasts may include chloritoid and/or cordierite. Skarn assemblages in calc-silicate rocks include epidote, garnet, diopside, and actinolite. Metamorphic grade in the contact aureole rises from greenschist to amphibolite facies over horizontal distances of less than 1.5 km (McGrew and Miller, 1993). In addition to the observed growth of certain minerals in the aureole of the granite, a penetrative cleavage is developed in pelitic units adjacent to the pluton. Systematic observation of the relationship of cleavage development to growth of contact metamorphic minerals indicate the two are coeval and that deformation was synchonous with granite emplacement (Miller, et al., 1988). This cleavage is discussed in more detail elsewhere in the text.

Kgr Cretaceous Granite

MUSCOVITE BEARING QUARTZ RHYOLITE PORPHYRY

HORNBLENDE MICRODIORITE

Structural Geology

Introduction

The most important structural feature of the study area is an extensive low angle fault or fault system first termed the Snake Range decollement by Misch (1957) which places younger rocks on older. Work by Misch and Easton (1954), Misch (1957), and Hazzard and Turner (1957) indicated that large areas of Nevada were underlain by this feature which they believed to represent a shearing off zone for Mesozoic thrust faults of the Sevier belt (Fig. 1). Subsequent work suggested a Tertiary age for this and similar features in Nevada (Armstrong, 1972). In the southern Snake Range, involvment of Tertiary strata in faulting and the fact that the decollement cuts an Oligocene pluton provides solid data supporting a Cenozoic age for this fault (McGrew, 1993). The decollement is a shallowly southeast dipping (10｡13｡) fault which separates the dichotomous structural history of upper and lower plate rocks. The lower plate of the decollement exposes a thick, stratified sequence of late Proterozoic to Early Cambrian quartzites and schists overlain by a thick succession of Middle Cambrian carbonates. This succession is generally structurally intact, however penetrative deformation and metamorphism adjacent to plutons and along the eastern margin of the range are cut by a mylonitic fabric inferred to be related to the deeper structural levels of the decollement (McGrew, 1993).The upper plate rocks above the decollment are cut by normal faults that attenuate or thin a section of a dominantly carbonate miogeoclinal strata overlain unconformably by Tertiary conglomerates and volcanic rocks.

Metamorphism and penetrative deformation in the study area is spatially and temporally associated with plutonic intrusion of the lower plate rocks. There are two intrusive bodies in the study area, the Jurassic Snake Creek-Williams Canyon pluton and the Cretaceous Pole Canyon-Can Young pluton. These have both been dated by a variety of isotopic geochronologic techniques including Rb-Sr, 87Sr/86Sr, and Uranium-Thorium-Lead meathods which give excellent agreement suggest a date of 155 +/- 4 Ma for the Snake Creek-Williams Canyon, and 79.1 +/- .5 Ma for the Pole Canyon-Can Young pluton (Lee et al., 1986).

The lower plate Precambrian and early Cambrian quartzites have locally been intesely deformed and recrystalized adjacent to the plutons. However, a majority of the quartzites retain relict sedimenatry features including a banded and little deformed cross-bedding. Two deformational fabrics may be observed in this succession. The first (S1) is an eastward dipping, penetrative cleavage, gently inclined with respect to bedding. The second (S2) is a westward dipping crenulation fabric with variable bedding to cleavage angle (Figure 8). The S1 cleavage seems to have slightly pre-dated or broadly coincided with the Jurassic pluton emplacement. Miller et al. (1988) suggests on the basis of regional relationships that the S2 cleavage is Cretaceous in the southern Snake Range, constrained between 160 Ma and 36 Ma (McGrew, 1993).

Aside from the decollement, the study area contains five faults or fault systems of major importance: The Johns Wash fault system; a series of westward dipping faults in the extreme southwest portion of the study area, possibly related to the Johns Wash fault system; the Murphy Wash fault; the Decathon fault; and a small system of east dipping faults in the west central area of the Minerva Canyon quadrangle. Only the latter two cut rocks of the lower plate of the decollement. The actual relation of these lower plate faults to the decollement is ambiguous.

The Johns Wash Fault system, as named on Whitebreads' map (1969), derives its name from the valley in which it occurs. It extends from the southern portion of the study area to the mid central portion of the Minerva Canyon quadrangle where it is cut by the Murphy Wash fault. It is a north-south trending, steeply west dipping fault system which offsets the Guilmette Formation or the Pilot Shale against the Ordovician Silurian undifferentiated dolomites.

A similar stratigraphic relationship and fault geometry may be seen among the west dipping faults in the southwestern portion of the study area. To the south of the Minerva Canyon quadrangle border, these faults trend toward the Murphy Wash Fault and may be bent into it and/or cut by it. Thus, if this is the case,it is possible that the Johns Wash fault system and the westward dipping faults on the southwestern border of the study area are intimately related and part of the same system.

The Murphy Wash fault is best exposed along the eastern side of Johns Wash, trends north-south, and dips approximately 40 degrees to the east. Dip slip motion generally drops Ordovician Cambrian Notch Peak on Cambrian Johns Wash or Cambrian Lincoln Peak to the north, though Pennsylvanian units are displaced against Cambrian ones to the south in conjunction with the Johns Wash fault. Also to the south, the Murphy Wash fault was projected by Whitebread (1969) in close proximity to the Needles Range Tuft. This area has been remapped as landslide deposits and thus the Murphy Wash fault may not be constrained by the tuft. The fault is assumed to be cut by the decollement not far below the surface based on cross cutting relations to the north.

The Decathon Fault is north-south trending and east dipping. It trends along the east side of Decathon Valley and generally places the Ordovician Pogonip on the Cambrian Notch Peak. Its age relation to the other faults discused here is unclear. The Decathon fault, like the other upper plate faults is cut by the decollement.

Other than the above faults, there is only one other fault system of apparent importance. These faults occur in the lower plate near the Tungsten Queen Mine on the west side of the range. These faults strike slightly east of north, and dip to the east at low angles (10｡-15｡). A cross cutting relationship with the decollment seems unlikely, though not impossible, and what these faults do at depth is unclear.

Deformation and Intrusion of Lower Plate

Jurassic

Evidence obtained from this study in the Wheeler Peak quadrangle neither bolsters or negates the proposal that the first cleavage is synchronous with the emplacement of the Jurassic pluton. East-dipping cleavages were found with as much regularity in the vicinity of the pluton as they were a significant distance from it. Additionally, at some points near the pluton, the east-dipping cleavage was obscured and only the later west-dipping cleavage was observed. Figure 8a is a stereonet diagram of the cleavages; bedding attitudes were utilized in order to display the attitudes of cleavage before beds rotated. Each cleavage reading was rotated back to horizontal using a bedding reading taken at the same outcrop, thereby increasing the accuracy of the restoration. Figures 8b and 8c are diagrams of the calculated and measured intersection lineations between cleavages and bedding (both unrotated). The earlier cleavage produces a NNW trending, subhorizontal intersection lineation, while the younger produces a more scattered range of intersection lineations varying in trend from N80W to N50W. The large scatter could be due to a variety of causes, among them heterogeneous strain, disruption of bedding by the intrusion of the pluton, and post cleavage development folding.

Unfortunately, the small amount of data collated makes it impossible to draw any firm conclusions about the timing of the cleavage in this quadrangle. Future thin section work will reveal more firmly the nature of the cleavage in this area and their cross-cutting relations. Until that study, we can postulate that results in adjacent quadrangles are applicable. Thin section studies of the Jurassic Willard Creek pluton in the Hogum and Windy Peak quadrangles indicate that contact metamorphic porphyroblasts described in more detail below grew synchronously with and overprint the S1 cleavage. Adjacent to the pluton itself, hornfels textures locally obliterate the cleavage (Miller et al., 1988). In the same report, intersection lineations between bedding and east-dipping cleavages were relatively well clustered in a NNW trend, indicating a regionally uniform strain.

In the Wheeler Peak quadrangle, silty and shaley units such as the Precambrian Osceolla Argillite of the McCoy Creek group, the Cambrian Pioche Shale, and shaley/silty interbeds in the Cambrian Prospect Mountain Quartzite exhibit spotted hornfels textures as one approaches the Snake Creek-Williams Creek pluton. Muscovite, andalusite, and altered pseudomorphs of staurolite occur in the contact aureole of the pluton. These metamorphic porphyroblasts increase in size and abundance with structural depth and proximity to pluton. They are ubiquitously retrograded to white mica, chlorite, and chlorite and white mica. Some retrograded porphyroblasts preserve dark euhedral habit and have been identified as staurolite. Other, more indistinct, spots and blobs have not been identified with confidence, but may represent retrograded cordierite, chloritoid, or staurolite. Immediately adjacent to the pluton margins, we have tentatively identified 3 new andalusite localities (Plate 1). If this field identification is correct, it could support intrusion of the pluton at depths shallower than the AlSiO3 triple point (3.8Kb--Holdaway, 1971). Estimated stratigraphic depth using unit thicknesses calculated in this study is approximately 8,000m, which is compatible with this estimate

Cretaceous

Faulting History

Upper Plate Faults

Fault System on West Side of Range

The faults in question are subparallel, but appear to be continuous in map pattern with much more gently dipping faults. The steeply-dipping faults are westward-dipping, with bedding in adjacent fault blocks dipping steeply into the fault planes (Plate IB). More competent units, such as the Ordovician-Silurian Fish Haven and Laketown Dolomites (OS), retain the best record of the total faultblock rotation (e.g. Miller and Gans, 1983) associated with these faults. Assuming original horizontality of bedding prior to faulting, these faults must have been eastward dipping at the time of formation and during early motion on the fault planes. The subhorizontal fault associated with a klippe of Devonian Guilmette Limestone that overlies the Ordovician Fillmore Limestone differs in orientation but, based on bedding to fault angles, is also east-dipping. This gently-dipping fault plane is believed to be continuous with the steeply westward-dipping faults to the west (Fig. 6). The faults discussed above continue southward into the Red Ledges Quadrangle, a portion of which was mapped as part of this study. Here, these faults also appear to rotate to more gentle angles and may curve to become a klippe of Ordovician Eureka Quartzite and OS which overlies the Ordovician Fillmore Limestone, much like the klippe of Devonian Guilmette in the north. These relations suggest the possibility that these faults may be folded in an anticlinal fashion.

This antiform can also be inferred or defined by bedding orientations in the Ordovician-Cambrian Notch Peak Limestone (Fig.6 and Plate IB). The axial trace of the antiform is poorly defined but appears to trend north-south and plunge 5｡-10｡ south. The mechanism of its formation, assuming no compressional events, may be related to normal drag along younger north-south-trending normal faults. The two major faults that may be responsible for its formation and the bending of older faults are the east-dipping Murphy Wash fault on the east and possibly a range-bounding fault on the west (Fig. 10). Although field study revealed no conclusive evidence for the existence of the range-bounding fault, the north-south trending linearity of the range front on the west (Plates 1A and 1B) suggests a major westward-dipping fault.

John's Wash Fault System

The John's Wash faults generaly place upper Paleozoic rock units (eg. Guillmette Formation, Pilot Shale, Joanna Limestone, Chainman Shaleand Ely Limestone) agains Ordovician-Silurian and Devonian dolomites. The dipslip component of displacements calculated for the John's Wash and related faults one kilometer south of the spring in John's Wash give an estimated total displacement of about 2575 meters. The individual displacements for each fault going from west to east are 790, 1220, 400, 165 meters. This gives the John's Wash Fault system a minimum of about 2225 meters stratigraphic displacement.

The John's Wash fault system probably represents the oldest system of upper plate faults. To the north, the fault system is cut by the Murphy Wash fault. Therefore, the John's Wash fault system either predates or alternatively could be antithetic and thus synchronous with the Murphy Wash fault. It is more likely that it former is true. The youngest unit that the John's Wash fault system cuts is the Ely Limestone, which give the fault system an upper age limit of Pennsylvanian. The Tertiary conglomerate (Tco) which occurs on both the east and west side of the canyon appears not to be disturbed by any faults in the system. These conglomerates predate the overlying Needles Range Formation to the west. Therefore, the John's Wash Fault system must predate the Needles Range Formation ashflow tuff which has been dated at 27-33 Ma by Best and others (1989b).

Murphy Wash Fault

In the southern portion of the Wheeler Peak quadrangle and the northeast corner of the Minerva Canyon quadrangle and the the Murphy Wash Fault displaces Ordovician-Cambrian Notch Peak Limestone to the east down against the Cambrian Lincoln Peak Limestone. Here, it has a minimum stratigraphic displacement of 850 m (Fig. 5). In the southern portion of the Minerva Canyon quadrangle the Murphy Wash Fault down drops Devonian through Pennsylvanian rocks to the east against the Ordovician Cambrian Notch Peak Limestone, appearing to have a stratigraphic displacement of about 3600m (from reported unit thicknesses). This apparently large stratigraphic displacement along the Murphy Wash Fault compared to its displacement to the north can be explained by the fact that earlier faulting and offset along the parallel Johns Wash Fault system predated motion on the Murphy Wash Fault (Fig. 6). There is at least 1960m of stratigraphic displacement along the Johns Wash Fault which when taken into consideration results in 1640m, at most, displacement on the Murphy Wash Fault which is significantly less than appears from map relations (Fig. 6 and Plate II).

The age of the Murphy Wash Fault and its relation to other fault systems in the Minerva Canyon and Wheeler Peak quadrangles has been one of our main concerns in the study of these two regions. Previously, Whitebread (1969) had mapped the Murphy Wash Fault system in such a way that it cut the Tertiary Needles Range Formation, not only implying that the Murphy Wash Fault was Tertiary and postr Needles Range in age but also that the southern Snake Range d残ollement must ahve moved in post Needles Range time, since the SSRD truncates the Murphy Wash Fault (Armstrong, 1972). Further study by us in this area, however, does not conclusively demonstrate that the Murphy Wash Fault system cuts the Tertiary Needles Range Formation. Remapping of the region of Whitebread's (1969) map shows a more restricted area of exposure of the Needles Range Formation, as we have remapped the western portion of this unit as Quaternary landside deposits composed entirely of the Needles Range Formation (Plate IB).

As mapped, the trend of the Murphy Wash Fault no longer coincides with western edge of exposure of the Needles Range Formation as mapped by Whitebread, opening the posibility that the Murphy Wash Fault does not cut the Needle Range Formation. This new interpretation no longer permits for the unequivocal conclusion that the southern part of the Snake Range d残ollement, which cuts the Murphy Wash Fault, moved in post Needles Range time.

Decathon Fault System and Related Faults

A Quaternary landslide southwest of Granite Peak, first mapped by Whitebread(1969), consists of debris from the overhanging Eureka cliffs and has obscured the trace of the fault locally. However, it is likely that the fault continues south of this area of cover and is represented along strike by the fault that juxtaposes the Eureka Quartzite above the Juab Limestone. Stratigraphic displacement on the fault is approximately 950 m as measured in Figure 5.

Disagreement with mapping by Whitebread (1969) occurs concerning the high angle, east and west dipping faults that are found east of, and which generally parallel the Decathon Fault. Based on the occurrence of two isolated fault blocks of Eureka Quartzite found southwest and northwest of Granite Peak, Whitebread mapped two faults with short traces. Utilizing a more detailed stratigraphic subdivision of the Pogonip Group, however, it is apparent that these faults extend further south and are interconnected, contemporaneous faults which anastamose, but generally downdrop the section to the west. These faults also serve to increase the apparent thickness of the Lehman Formation to the southwest of Granite Peak by successively offsetting the units down to the west.

The curved fault that cross cuts the Decathon Fault in the south, hereby referred to as the QTc boundary fault, is a high angle, east-dipping normal fault that displaces Quaternary or Tertiary Conglomerate on Ordovician Silurian Fishhaven and Laketown Dolomites, undifferentiated, and Devonian Sevy Dolomite. Whitebread (1969) had mapped this contact as depositional, but field work this summer provided significant evidence that it is a fault contact. First, the contact cuts across topography in a manner that suggests that is a fault rather than a depositional contact. Additionally, slickenslided surfaces on the adjacent bedrock were observed and several such fault planes were measured. Dips ranged from 47｡ to 84｡. With the exception of observations at these particular sites, the contact is generally not well exposed, and exposures of the conglomerate immediately adjacent to the fault are nonexistent. Similarly, offset on the QTc fault is difficult to determine. By examining the displacements of the related fault strands, we roughly calculated that the stratigraphic offset on the system of faults was approximately 900 m

The timing of the QTc boundary fault is somewhat misleading. The cross-cutting relationship of the fault to the Quaternary or Tertiary conglomerate would suggest that motion on the fault postdates the deposition of the conglomerate. However, with more thought, it appears likely that deposition of the conglomerate may be the result of motion on the fault and consequent uplift of bedrock. Faulting could result in a buttress unconformity between bedrock and conglomerate derived from that uplifted bedrock. The presence of slickenslided surfaces also indicates that some degree of motion along the fault could have occurred after deposition of the conglomerate. The complete picture thus appears to be more likely that motion along the fault initiated before the conglomerate was deposited, provided the relief necessary for the deposition, and may have continued after deposition of the conglomerate. The lack of any apparent tilting of the conglomerate in its exposures further along the valley suggest that this later motion was probably not great.

It is possible that the QTc fault is an extension of the Decathon Fault. The only evidence for this is the approximate parallelism of the two faults. However, more convincingly, the disparity between the steepness of the two faults and the possible angle or cross-cutting relationship suggest that the QTc fault is a younger fault which cuts the Decathon Fault.

The SSRD

The significance of the southern Snake Range decollement and its relation to the more conspicuous decollement in the northern Sanke Range remains controversial. In the southern part of the the southern Sanke Range, the decollement occurs well up in the Middle Cambrian part of the section, following either the top of the Pole Canyon Limestone or even the basal part the Lincoln Peak Formation along the southwestern flank of the range. In an east to west transect south of Mount Washington, it appears that the decollemnt cuts down section very gently to the east, from the basal part of the Lincoln Peak Formation on the west to down into Unit D or C of the Pole Canyon Limestone on the east. Bedding to fault cutoff angles in the westernmost exposures may be as much as 10-15｡ with respect to both lower plate (Middle Cambrian) and upper plate Notch Peak Limestone, but in general, it appears to closely parallel the lower plate bedding. In the southern part of the southern Snake Range, the lower plate is entirely unmetamorphosed except in the vicinity of Jurassic and Cretaceous granites and lacks any significant strain. Oolites, declicate sedimentary structures, and fossils are well preserved in the Middle Cambrian rocks of the lower plate. To the north and east near the mouth of Snake Creek in the Kious Springs quadrangle the SSRD appears to cut down section to near the top of the Pioche Shale. In this area, there appear to be an increase in both the Mesozoic metamorphic grade (well developed Mesozoic cleavages) as well as in the development of the inferred Tertiary fabrics. From here northward, only lower plate rocks are exposed until Sacrament Pass, where apparently younger (~east-west trending) normal faults have downdropped the SRD such that only upper plate(?) Tertiary rocks are exposed. In addition, lower plate Precambrian and Cambrian units on the northwest flank of the southern Snake Range (Windy Peak quadrangle) are involved in a series of east-dipping normal faults whose displacement diminishes southward in the southern Snake Range. Thus, the continuity and correlation of the SSRD to the NSRD is obscured and complicated in the Sacramento Pass area, and we cannot be certain that they are in fact the same fault (see Miller et al., 1983). Nevertheless, our current prejudice is that these once were the same fault or same fault system and that the differences in lower plate stratigraphy, metamorphic grade, and degree of Tertiary deformation between the northern and southern Snake Rgae were once gradational. The gradual decrease in the lower plate ductile deformation as well as the overall lessening in the apparent metamorphic break between upper and lower plate can perhaps best be explained as the consequence of a single fault whose displacement decreases gradually to the south. However, there remain some important questions regarding the SSRD:

1. What is the total amount of displacement on the decollement in the southern Snake Range? The total amount of stratigraphic omission across the SSRD is relatively minor as it icludes rocks as young as the lower part of the Lincoln Peak Fm in the lower plate and commonly has either upper LIncoln Peak Fm. or Notch Peak Formation immediately above the fault - an ommission of only a few 100s of meters, with no discernable break in metamorphic grade. Depending on what bedding to fault angle is assumed over its entire length, the allowable displacement might be as little as 1 km or many 10's of kilometers. We believe it likely that the SSRD as exposed in the southwestern part of the range is near its western limit and this area has only a few kilometers or less displacement, but that the displacemnet increases easward due to the combined slip on the upper plate faults, such that by the eastern flank of the range the displacement is as much as 8-22km (McGrew, 1993). To the north, the western terminus of the SRD must shift gradually westward and the cumulative slip in the SRD must gradually increase to perhaps 20-30 km.

2. What is the age(s) of displacement on the SSRD? This remains one of the most intractable questions as there are relatively few places where dateable strata or intrusions permit tight brackets to be placed on the timing of slip of either upper plate faults or of the SRD itself. Existing constraints include:
   • The ~ 36 Ma Young Canyon Granite is cut and locally mylonitized by the SSRD indicating at least some slip in Oligocene or younger time.

   • Conglomerates that underlie ~30 Ma (and perhpas 36 Ma) tuffs inthe Murphy Wash area contain clasts as old as the Notch Peak Formation that were derived from the north (Wheeler Peak area (?) indicating that significant upper plate faulting and extensional unroofing of these older formations must date back to at least this time. Since the only obvious faulting that could have unroofed these formations is restricted to the upper plate, this implies that some movement on the SSRD may be Eocene or younger.

   • Conglomerates in the vicinity of Majors Place in the Schell Creek Range - directly west of the Willard Creek Region - contain clasts as old as Middle Cambrian, including abundant clasts of Eureka, Notch Peak Formation and coarse grained 2 mica granite that appears identical to the Willard Creek Granite, and coarse grained Pole Canyon Marble. These conglomerates underlie the 35.7 Ma Charcoal Ovens Tuff and given that they include lithologies now only found in the lower plate, implies that movement on the western part of the SSRD had ceased by the Oligocene.

   • Samples from the lower plate on the east side of the southern Snake Range as well as from the northern Snake Range and from boulders in synorogenic tilted conglomerates (Sac Pass Section) yield ~15-20 Ma apatite fission track ages and track lengths indicative of very rapid cooling. These data virtually require that the latest movement on the SRD at least on the eastern flank of the Snake Range to be mid Miocene.

   All of this data taken together imply a rather protracted history of extensional deformation in the vicinity of the Snake Range, beginning sometime in the Eocene or even older and continuing into the mid Miocene. It is likely that this history has been fundamentally episodic, with perhaps an older phase in the early Tertiary and another in the mid Miocene. The data also imply that the SSRD itself is a composite structure of different ages in different places and may never have all been active ast a single time despite its apparent geometric or map continuity.

3. A final question concerns the initial dip of the SRD. Gans and Miller (1983) argued that it initially had to have been subhorizontal based on the fact that it followed the same stratigraphic inbterval over long distances combined with the fact that the nature of the Tertiary unconformity throughout east-central Nevada implied that prior to extensional faulting, the Paleozoic section was nearly flat lying and intact. However, the Tertiary unconformity is not well exposed in the vicinity of the northern and southern Snake Range, so it is appropriate to question whether the Paleozoic stratigraphy - in particular the lower plate stratigraphy- was flat lying at the time of the development of the SRD. If we undo the Tertiary slip on the SRD, the lower plate units probably restore to beneath Snake Valley or beneath the western part (i.e east-dipping limb) of the Confusion synclinorium. Depending on what intital dip is assumed for the lower plate strata, the SSRD may have dipped anywhere from 0 to 60｡ to the east. Some amount of initial eastward dip seems probable and would help explain the fact the amount of Tertiary(?) ductile deformation asociated with the SRD increases towards the east as would be expected for progressively deeper levels of such a major normal fault.

Minerva Lower Plate Fault System

As these are minor faults of limited extent, it is unclear what their 3-D geometry is at depth and whether they cut or are cut by the SSRD. However, projection of these faults and the decollement suggest the possibility that they may cut the decollement at some point. This possibility is in no way born out by map relations though. To both the north and south structural relationships involving the lower plate faults are obscured by Quaternary deposits. In all, the role of these faults in the overall structure of the range is obscure and likely very minor, yet they appear to be spatially related to an unusual westward bending of units along the western range front.

Western Range Front Fault

Constraints on the timing of latest fault motion and uplift of the Southern Snake Range as a mountain range came from the eastern flanks of the southern Snake Range and the fission track dating of apatite.

The trace of the southern Snake Range Decollement exposed on the eastern flank of the range cuts Paleozoic strata and unconformably overlies a sequence of Tertiary conglomerate at least 2 km thick. Conglomerate beds are strongly tilted between 20 - 45｡ to the west by this faulting. In the location of Big Wash, conglomerates contain scattered clasts of Needles Range Formation tuff. Thus their deposition is post-Needles Range, and the age of their tilting is also post Needles Range.

Apatite fission track dating, a technique providing information on the temperature-time history of the rocks in the 120｡C-60｡C temperature range, can be used to date formation of topography by faulting. A preliminary data set of apatite fission ages for the southern Snake Range suggests cooling and uplift between 20-15 Ma. In addition, fission track ages of apatite separated by granitic boulders derived from the Snake Creek-Williams creek pluton in tilted Tertiary conglomerate sequences along the eastern flank of the range are 15-18 Ma, supporting a Miocene age for significant erosion ( and inferred uplift) of the range.

Summary

The Johns Wash fault system is most likely the oldest upper plate fault system represented in the area. Cutting it is the Murphy Wash fault. The Johns Wash fault system clearly predates pre-Needles Range Conglomerate. It is likely, based on relative age relations, that Needles Range Formation also postdates motion along faults along the southwest flank of the range, which are inferred to have bottomed or merged with the SSRD while they moved. Thus there is substantial evidence in the Minerva Canyon quadrangle for faulting (involving rocks at least as old as Ordovician Cambrian Notch Peak Limestone) that pre-dates conglomerates that are earlier than the Needles Range Formation (pre- 27-33 Ma).

The Murphy Wash fault cuts both of the above systems of faults and merges with or is cut by the SSRD in a fashion that could be explained as either synchronous faulting or Murphy Wash Fault motion predating SSRD motion. It is unclear based on map relations, however, whether or not the Murphy Wash fault cuts the Needles Range tuffs. These map relations leave open the possibility that the Murphy Wash fault could predate Needles Range tuffs and underlying conglomerate accumulated in a N-S trending, pre-existing valley bound on one side by this fault.

To the east, cross-cutting relationships exist between the Decathon fault , the SSRD, the QTc boundary fault, and the Tertiary Needles Range Formation. The Decathon fault appears to resemble the Murphy Wash fault in that it merges with or is cut by the SSRD, thus its motion predates or is synchronous with motion along the SSRD. It is relatively certain that the Decathon fault predates the QTc boundary fault. The presence of Tertiary Needles Range Formation clasts, dated at 33-27 MA (Best and others, 1989) in the Quaternary or Terttiary conglomerate prove that at least some motion along the QTc boundary fault occurred after deposition of the Needles Range . This would suggest that the QTc boundary fault, and perhaps the SSRD were active also before 33-27 MA. This evidence, in consort with studies in the Kious Spring quadrangle, to the east of the Wheeler Peak quadrangle, which clearly demonstrate that motion along the SSRD postdated the Needles Range Formation, suggests that the SSRD may have been active at different times in different areas spanning a range of time prior to pre-Needles Range conglomerate to post-Needles Range. the available evidence also suggests that youngest motion is best documented further east along the trace of the fault, with youngest motion along easternmost trace in the Kious Spring quadrangle.

The youngest fault in the map area is the postulated Basin and Range range front fault on the western side of the southern Snake Range. Although the inferred fault is nowhere exposed, the extreme linear nature of the western range flank and the steep topography on this side of the range provide indirect evidence for the presence of a relatively recent fault that may be in part responsible for the present uplift of the area. Fission track dating suggests this uplift of the range is between 15 and 20 Ma (Miller et al, 1993). A set of small west-dipping lower plate faults on the west side of the range may be subsidiary faults related to this range front fault.