i

\ University of Nevada

Reno

(jreeology of Paleozoic Basinal Rocks in the Northern Fox Range; Washoe County, Nevada

A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geology.

by

Mark F. Thiesse

V i

August 1988 11

MINES LIBRARY

The thesis of Mark F. Thiesse is approved: 2H03

Thesis Advisor

Department Chairman

Dean, Graduate School

University of Nevada Reno

August 1988 iii

ACKNOWLEDGEMENTS

I would like to thank Dr. R .A. Schweickert for his help, encouragement and patience throughout this project. Dr. Bruce Wardlaw provided the conodont identification which suggested the Carboniferous (?) age of the metamorphic rocks. Other thanks go to Andy Markos and Bob Strobel for help with the thin sections and interesting discussions. And finally, thanks to Carol for typing and for funding this project. iv

ABSTRACT

The Fox Range in Northwestern Nevada is a typical Basin and Range fault block mountain range which has been tilted about 30° to the east, accelerating the erosion of Tertiary volcanic rocks along the steep western flanks, and exposing

Paleozoic metamorphic rocks.

The Paleozoic rocks consist of a very fine grained, siliceous mudstone which was regionally metamorphosed from mid-greenschist to mid-amphibolite facies producing argillite, schist, limestone, quartzite, and gneiss. These Paleozoic rocks exhibit three generations of structures. Di created the schistose and gneissic layering common throughout the range, along with abundant tight to isoclinal folds. D2 structures are dominated by two large, map-scale antiforms and occasional smaller, open to tight, east to northeast trending folds and assorted lineations. D3 structures are limited to a few north-south trending, open to gentle folds which affect all pre-Tertiary lithologies.

These rocks were previously correlated with Upper

Triassic to Lower Jurassic mudstones common in northern

Nevada. Lithologic, metamorphic, structural and fossil data suggest this correlation is not valid. Instead, these rocks are Carboniferous, and most probably are correlative with clastic rocks of the upper Paleozoic Golconda allochthon. V

TABLE OF CONTENTS

Page

Introduction ...... 1

Geographic Setting ...... 5

Geologic Setting ...... 9

Rock U n i t s ...... 12

Structure ...... 39

Geologic History ...... 74

Recommendations for Further W o r k ...... 85

References 87 1

INTRODUCTION

The Fox Range of north central Washoe County contains some of the westernmost exposures of metamorphosed sedimentary rocks in Nevada. Several thousand feet (1000 m) of metasedimentary rocks are exposed in the range and are composed primarily of argillite, phyllite, schist, marble, quartzite and scarce gneiss.

These metasedimentary rocks resemble pelitic rocks that are common throughout central and western Nevada, and are separated by an extensive tract of Tertiary volcanic rocks from the Klamath Mountains and northern Sierra Nevada. The pelitic rocks in central Nevada have been studied exten­ sively by numerous authors who have worked out the depositional and tectonic history in some detail. The rocks in far western Nevada, however, have been less extensively studied and those in the Fox Range have only been studied in reconnaissance by a few workers.

Because of the lack of research and the potential significance of these pelitic rocks for understanding the regional tectonic framework, a study of the metamorphic rocks in the Fox Range was undertaken to provide information about the stratigraphy, structural history, and metamorphism.

Scope of Investigation

This project primarily involved detailed mapping and structural analysis of the metasedimentary rocks. Particular 2 attention was focused on style, orientation, and overprinting of folds and other structures.

Thin sections were prepared of the metasedimentary rocks and the surrounding granitic and volcanic rocks to aid in lithologic indentification and to identify possible sources and depositional environments of the original sedimentary rocks. In addition, attempts were made to extract conodonts to date the metasedimentary rocks.

Method of Investigation

Approximately 30 square miles (78 km2 ) of the northern

Fox Range were mapped at a scale of 1:24,000 during the Fall of 1983, and the orientation and style of cleavage, other foliations, lineations, and folds were noted in detail. Since the best exposures of the metasedimentary rocks are in deep narrow canyons along the range front, all of the canyons were walked and a majority of the ridges were also examined to map this area accurately.

The areas with the best exposure and the greatest structural detail were re-examined in an attempt to measure sections and to determine specific generations of observed structures. The attitudes of folds were plotted on equal area stereonets to aid in the recognition of different generations of folding.

Three base maps were used; only one, the Smith Canyon

7-1/2 minute quadrangle, was readily available. The other two maps, the Pyramid Lake 1 NE Nevada and the Peephole 3 SW Nevada sheets are older 7-1/2 minute T-maps which were used as preliminary maps to construct 15 minute series maps. These maps had to be specially ordered from the U.S.G.S. in Menlo

Park, California. Air photos covering the area were obtained

from the Eros Data Center in Sioux Falls, South Dakota and were used to trace large scale features and to assist in mapping lithologic contacts.

Previous Investigations

The first mention of this area was by Hague and Emmons in

1877 during the 40th Parallel Survey under the heading of the

Lake Range. They provided a general description of the rock

types and the physiography of the area north of Pyramid Lake,

and suggested the metasedimentary rocks were Jurassic in age because of their similarity with other rocks to the east that

overlie Jurassic limestones.

The first mention of the Fox Range was by Hill (1915),

who described the mining districts in the area. Overton

(1947) also used the name Fox Range while describing the

location of the Cottonwood mining district. Both of these

authors briefly described the rock types and mineralogy

present in the mining districts.

Bonham (1969) discussed the Fox Range in general,

together with structure, lithology, and mining history, and

mapped the area at a scale of 1:250,000 in his report on

Washoe and Storey Counties. He informally named the

metasedimentary rocks the Nightingale sequence because they 4 resembled rocks in the Nightingale Range about 50 miles (81 km) southeast of the Fox Range. Bonham suggested that these rocks are probably correlative with the upper part of the

Winnemucca Sequence of Silberling and Roberts (1962), and are uppermost Triassic and Lower Jurassic in age.

Dixon (1977) studied the structural, stratigraphic, and economic relationships between the metasedimentary and volcanic rocks located in Wild Horse Canyon and the immediate vicinity, about 4 miles (6.5 km) south of the present study area. He agreed with Bonham’s (1969) correlation with the

Winnemucca Sequence of Silberling and Roberts (1962). 5

GEOGRAPHIC SETTING

Location and Accessibility

The Fox Range is located in central Washoe county (Fig.

1) with its southern end about 7 miles (11 km) north of

Pyramid Lake. The range is about 30 miles (48 km) in length and is up to ten miles (16 km) wide at its widest point.

The range is oriented approximately North-South, being slightly convex to the west, and is bounded to the west by the

Smoke Creek Desert and to the east by the smaller San Emidio

Desert (Fig. 2), which is a southern arm of the Black Rock

Desert to the northeast of Figure 2.

The mapped area encompasses approximately 30 square miles

(78 km2 )in the northern part of the range. The west side of the range is reached by traveling to Gerlach along State

Highway 34, and then taking State Highway 81 about one mile northwest to the Gerlach landfill road. The landfill road intersects the Western Pacific Railroad tracks about 4.5 miles

(7.25 km) to the south. The road that parallels the tracks skirts the northern tip of the range, curves around to the southwest and follows the western flank of the range.

Access to the eastern portion of the mapped area is much easier. State Highway 34 is taken as far north as the Empire

Hay Ranch road, a dirt road about 7 miles (11.3 km) south of

Empire and 13 miles (21 km) south of Gerlach. This road intersects a powerline road after four miles (6.5 km).

Following the powerline road west across the San Emidio 6 7 8

Desert, the eastern flank of the Fox Range is about 6 miles

(10 km) farther.

This entire range is criss-crossed by old .jeep trails and

roads, so it is wise to use the previously mentioned

topographic maps as a guide. Few of the dirt roads are maintained, so caution is advised when traveling the area.

Physiography

The Fox Range, typical of mountain blocks in the Basin

and Range province, is a moderately rugged fault-block

mountain surrounded by relatively low playas. The range is dissected by intermittent streams which carried water

throughout 1983 because of the record amounts of precipitation during the winter of 1982-83. These small streams have cut deep narrow canyons that provide good exposures of the metamorphic rocks. Vertical cliffs over 200 feet (61 m) high were observed in many of the canyons, making it difficult to study these exposures thoroughly.

The highest part of the range, the Pah Rum peak area,

just south of the central part of the range, reaches a maximum height of 7600 feet (2320 m). The base of the range has an elevation of approximately 4000 feet (1220 m). The highest elevation in the study area is about 6800 feet (2043 m) and is located near the southern boundary. 9

GEOLOGIC SETTING

Tertiary to Recent volcanic and minor sedimentary rocks, and Quaternary lacustrine and alluvial deposits are the predominant rock types in northwestern Nevada. The volcanic rocks include basalt, andesite and rhyolite flows and dikes.

The sediments consist of occasional interbeds of shale, sandstone and rare conglomerates mainly derived from the volcanic rocks.

Scarce pre-Tertiary outcrops of Jurassic-Cretaceous plutonic, Triassic-Jurassic pelitic and metamorphosed

Permo-Triassic volcanic and sedimentary rocks are exposed in many of the nearby ranges, especially east and northeast of the Fox Range.

The Permo-Triassic rocks, the Happy Creek volcanics

(Willden, 1963; Russell,1984) are the least abundant of the pre-Tertiary rocks and are exposed around the edges of the

Black Rock Desert northeast of the Fox Range. These volcanic rocks consist generally of pyroxene andesite with minor basalt and dacite and occur as flows, breccias, and tuffs. Minor sedimentary rocks including limestone, chert, breccia, conglomerate and sandstone are associated with the volcanic rocks. Speed (1977a) suggested that the Happy Creek unit represents the main volcanic body of an intra-oceanic island arc, whose activity ceased during Late Permian time.

Upper Triassic to Lower Jurassic pelitic rocks are the most common pre-Tertiary rocks in northwestern Nevada. These 10 fine-grained silica-rich clastic rocks with associated carbonates have been extensively studied by numerous workers

(i.e.: Silberling and Roberts, (1962); Muller and Ferguson,

(1936); Compton, (1960); Willden, (1963); Speed, (1977, 1978a,

1978b); et. a l .) in north central Nevada and numerous formational names have arisen. Burke and Silberling (1973) created the Auld Lang Syne Group to collect all of these similar formations into one unit.

The Auld Lang Syne Group (ALSG) includes two depositional provinces, a basinal and a shelf sequence. The basinal sequence consists of pelitic rocks with minor sandstone, conglomerate and limestone which are thought to be distal turbidite and hemipelagic deposits with minor carbonate bank deposits (Stewart, 1980; Speed, 1978 a & b).

The shelf sequence is composed primarily of shale, mudstone, siltstone and occasional sandstone, with scattered, but relatively common carbonates. These rocks are thought to be shallow marine deposits, possibly associated with a deltaic environment (Stewart, 1980; Speed, 1978 a & b). Bonham (1969) correlated the metamorphosed sedimentary rocks of the Fox

Range with the Winnemucca Formation, part of the shelf province.

Jurassic-Cretaceous plutonic rocks are relatively widespread throughout northwestern Nevada. They usually form stocks, but a few batholiths occur in the region. The granitic rocks range in composition from granite to gabbro, the most common rock type being an equigranular to porphyritic 11 granodiorite. Radiometric dates of these plutonic rocks are predominantly Cretaceous, with fewer Jurassic and several

Triassic ages (Stewart, 1980; Carlson et . al., 1975; and

Evernden and Kistler, 1970), suggesting these plutons are associated with the Sierra Nevada batholith. 12

ROCK UNITS

Introduction

The oldest rocks in the Fox Range are a thick sequence of pelitic rocks with minor sandstones and carbonates, informally named the Nightingale sequence by Bonham (1969). These pelitic rocks have experienced extensive low to medium-grade metamorphism and deformation, and consist in decreasing abundance of argillite, schist, marble, phyllite, quartzite, gneiss and rare amphibolite. The base of the Nightingale sequence is not exposed, and the metamorphic rocks are unconformably overlain by Tertiary volcanic rocks, although this relationship is not clearly exposed. The lithology and structure of the Nightingale sequence, the major topics of this report, will be discussed in greater detail than the other rock types.

Several large and numerous small Mesozoic granodiorite bodies have intruded and contact metamorphosed the pelitic rocks. Tertiary volcanic flows, informally named the Pyramid

Sequence by Bonham (1969), and consisting of intermediate to mafic basalt, andesite, dacite, and volcanic breccia, cap the range. Minor deposits of Quaternary alluvium are located within the range along the major drainages, with more extensive fan and slope deposits along the flanks grading to lacustrine deposits out in the playas. 13

Fox Range Sequence (PzF)

A thick accumulation of fine-grained pelitic rocks is exposed in the northern Fox Range. These rocks were informally named the Nightingale Sequence by Bonham (1969), however, due to age and lithologic discrepancies, which are discussed later, the metamorphic rocks of the Fox Range will be informally referred to as the Fox Range sequence throughout this report.

The predominant rock type is a quartz-rich organic mudstone which has been regionally metamorphosed to argillite, phyllite, schist and gneiss. Scattered lenses and rare beds of sandstone have been metamorphosed to quartzite. Small lenses and pods of sandy carbonate rocks consisting primarily of blue-gray recrystallized limestone or marble with abundant hornfels and tactite are located throughout the range. A single large outcrop of white to light gray, coarsely crystalline marble is located along the northeastern edge of the mapped area (Plate I).

The exposed section in the Fox Range is probably quite thick. Bonham (1969) estimated a thickness of several thousand feet (1000 m), and in the nearby Nightingale Range,

Rai (1968) estimated a total thickness of nearly 10,000 feet

(3050 m). These estimates are of little value however, t because of the lack of basal exposure and because layering is, in most areas, a structural surface and probably not bedding.

Quartz-Rich Organic Mudstone (PzFa, PzFs, PzFg). The most commonly exposed rock of the Fox Range sequence is a 14 black, dark brown and dark gray mudstone containing occasional silty horizons. The mudstone is predominantly homogeneous with occasional silty horizons. Sand and carbonate lenses and pods are scattered throughout the range. Layering is highly variable ranging from massive, to possible original thin to thick bedding (?), to a variety of structural layering.

Varying degrees of regional metamorphism have altered these mudstones creating rocks ranging from argillite to gneiss.

Argillite (PzFa). Argillite is the most common product of metamorphism, and occurs throughout the range, especially along the uplifted western side. The argillite is dense, well indurated, dark brown to black and very fine grained.

Layering grades from massive, to thin, possibly original (?) bedding to schistose and gneissic structural layering. The most common layering is a relatively thinly layered sequence of harder, fine grained argillite (6 inches - 2 feet, .15 -

.75 m) with softer, silty interbeds (2-6 inches, 5-15 cm).

This layering resembles original bedding, but the absence of depositional features and the complex structure of the area prevents this conclusion. Multiple fractures and joint sets intersect the layering, producing widespread blocky, rubble strewn talus slopes. Folds in the argillite are relatively common but due to the disrupted nature of the outcrops, orientations are usually difficult to obtain and are of questionable reliability.

Local, light to medium gray shaly horizons have a better developed spaced cleavage which grades to slaty cleavage. 15

Exposures of slate are rare and will be considered a minor variation of the argillite for this discussion.

The argillite commonly exhibits a phyllitic sheen on fresh surfaces, but the continuous cleavage necessary for phyllite is only rarely observed. The phyllite is included with the argillite on the map.

Hand samples of the argillite are rather featureless, homogeneous, very fine-grained and slightly to non-calcareous. In thin section, the argillite is granoblastic with abundant (up to 60%) very fine (.05 - .02 mm), well rounded, moderately to well sorted detrital quartz grains. Carbonaceous material is common (up to 40%) and occurs both in bands and dispersed throughout the rock. The rest of the rock is composed mainly of metamorphic biotite and muscovite (5-10%). The micas are generally xenoblastic with occasional subhedral crystals, ranging from .05 to .02 mm in size. Pyrite, plagioclase, chlorite and calcite are also present in minor amounts.

The argillite grades into phyllite locally and is commonly found in narrow, less than 10 feet (3 m) thick, patchy zones among the argillite. The outcrops are incoherent piles of rubble due to the ease of weathering of the phyllite. The rubble is often lath shaped, caused by intersection of the continuous (slaty) cleavage and the spaced cleavage. Measurements of folds, cleavages and lineations were difficult and relatively unreliable due to the poor outcrops. Shinier surfaces, silkier texture, larger and more 16 common muscovite crystals, a light gray color and most importantly, a continuous cleavage, distinguish the phyllite from the argillite.

The mineralogy of the phyllite is very similar to that of the argillite, with detrital quartz and carbonaceous material commonly comprising up to 80% of the rock. The main difference is the increase in size and amount of muscovite and the decrease in the biotite. Muscovite ranges from 10-20% of the rock with crystals as large as 1 mm but averaging closer to 0.5 mm. They are usually well developed but are often xenoblastic. Small (< .2 mm), poorly developed crystals of biotite compose 5% or less of the phyllite. Less than 5% of plagioclase, chlorite, calcite and opaques were also observed.

Two large map scale folds, located near the northern and southern boundaries of the study area contain more highly metamorphosed rocks in their cores. The northern fold, the better exposed of the two, has argillite along its flanks grading to schist and gneiss closer to the core. The core of the southern fold exposes patchy outcrops of schist.

Quartz mica schist (PzFs). Schist is quite common in the core of the northern fold, comprising up to 60% of the rocks in the area. The schist ranges from light gray to dark gray-brown, and varies from soft and friable to hard and well indurated. The schistose texture is well defined by oriented muscovite and biotite flakes. Closer to the hinge of the fold the quartz and feldspar are more apparent and they increase in 17

grain size leading to the development of gneiss. The

schistose foliation is often highly folded and crenulated with

very irregular orientations and styles of folds. Efforts to

measure these folds proved fruitless. Occasionally hingelines

of these small folds produce a lineation, the only lineation

observed in the schists.

The mineralogy of the schist is very similar to the

argillite, with well rounded, very fine (.025 - .1 mm)

detrital quartz grains and amorphous carbon usually composing

more than 50% of the rock. Biotite is more common than

muscovite although in hand samples the muscovite is more

apparent because of its lighter color. The micas formed

during metamorphism, mainly from the recrystallization of

clays but also from the breakdown of occasional detrital

feldspars. The micas are located in bands formed by alignment

of individual flakes and elongated masses. The single flakes

and groups of flakes are rarely folded and crenulated.

The biotite, usually forming anhedral crystals, makes up

5 to 30% of the schist and ranges in length from .05 to .8 mm. In one thin section, biotite grades from bright green around the edge of the slide to red and then to orange at the center, reflecting a change from ferric (Fe+ 3 ) iron to ferrous

(Fe+2) iron. Muscovite is less abundant (0-25%) and usually smaller (.05 - .4 mm) and more poorly formed. Plagioclase feldspar is relatively common (5-20%). It is composed predominantly of euhedral to subhedral well formed white to cream metamorphic grains, with occasional partially 18 decomposed, pink-cream, anhedral, detrital feldspar grains.

Grain size of the plagioclase is dependent on the degree of metamorphism, with increasing grain size from the schist to the gneiss (.05 to 1.0 mm). Accessory minerals (< 5%) include calcite, chlorite, andalusite, actinolite, tourmaline and garnet.

In the interior part of the northern fold, the schist grades into gneiss. This transition is marked by the decrease in abundance of micas, especially muscovite, and an increase in abundance of coarser-grained quartz and feldspar.

Gneiss (PzFg). The gneiss, or more precisely, the paragneiss, crops out along one drainage near the northwestern boundary of the area in the core of a map scale F2 fold (Plate

1 ) .

Subvertical cliffs up to 500 feet (150 m) high provide excellent exposures of the gneiss but also limit accessibility. The rocks are dark gray to black and gray-brown, with distinctive white and light gray quartzo- feldspathic banding. The banding is commonly highly folded, often ptygmatically, creating a crumpled appearance.

Occasional granodiorite and pegmatite dikes intrude the gneiss further complicating and distorting the outcrops.

Fine-grained (.05 - .5 mm), detrital quartz grains are the major component of the gneiss, making up 20-50% of the rock. Anhedral metamorphic biotite and muscovite are relatively common, 10-20% and less than 10% respectively. The grain size of the biotite (.1 - .6 mm) and muscovite (.05 - .3 19 mm) in the gneiss is similar to that in the schist.

Metamorphic feldspar (predominantly plagioclase) is

increasingly common near the core of the fold, composing

10-30% of the gneiss with the largest crystals nearly 15 mm across. Hornblende and diopside are more prominent (5-10%).

Garnet, andalusite and amorphous carbon are still present, although in lesser amounts (< 5%).

These rocks, the argillite, phyllite, schist and gneiss, although texturally different, have a very similar mineralogy, suggesting a similar protolith, a quartz-rich carbonaceous mudstone.

Limestone (PzFm). Carbonates are found throughout the range most commonly as small pods and thin horizons (< 10-20 ft. thick, 5 m) interbedded with the siliceous mudstone.

These recrystallized limestones are light to dark gray, with white mottling, poorly to well indurated and have a silty or sucrosic texture. Most of the exposures of limestone have a white and gray banded appearance caused by thin (< 2’, 0.5 m), alternating clean, calcite rich and silty, argillaceous layers. This banding provides an excellent record of folding. No megascopic fossils were found in the limestone.

The regional metamorphism which altered the mudstones also affected the limestones but not enough to classify them as marbles. However, coarsely crystalline marble, skarn and tactite in a few small exposures and one large exposure, have been more extensively metamorphosed by the later Cretaceous intrusions. The best exposed and largest outcrop of marble is located near the northeastern contact of the Fox Range sequence and the large granodiorite body. This has been labeled PzFm on the map. This outcrop occurs for about 1/2 mile (.75 km) along the contact and is about 150-200 feet (50 m) thick. The marble is light to medium gray, and is massive in most parts, but has occasional thin, silty to argillaceous layers. These interbeds reveal excellent examples of several generations of folds.

Both the recrystallized limestones and the coarse marbles are relatively resistant, and are less fractured and jointed than the argillite, forming cliffs and bedrock canyon floors.

The sucrosic texture of limestone is readily observed in thin section. Blocky to equant, anhedral calcite crystals of varying size (0.1 to 8 mm) form a mosaic texture and compose

80% of the limestone. Detrital quartz, the most common accessory mineral, forms very fine, well rounded grains that make up 5 - 10% of the rock. Other minerals, all less than 5% include anhedral plagioclase, muscovite, diopside, garnet

(possibly grossular) and amorphous carbon.

Numerous small (< 100 m2 ) bodies of calcsilicate hornfels and tactite have been produced by further contact metamorphism, primarily metasomatic, caused by intrusion of the granodiorite. These rocks grade from fine-grained, dense, well-indurated calcsilicate hornfels to soft, friable, coarser grained skarns and commonly occur along the country rock - intrusive contact. The finer grained rocks are predominantly 21

light gray to white with occasional tan and rose colors. The

coarser rocks are darker and more red brown. The outcrops are

massive with no remnant bedding or structure observed. The

rocks are predominantly granoblastic with a sucrosic texture.

Thin sections of the calcsilicate hornfels and tactite

contain numerous very fine grained (0.05 - 0.2 mm) minerals

including calcite (30%), quartz (20%), diopside (20%), cummingtonite-tremolite (10%), graphite (trace), garnet

(trace), and opaque-rich zones.

Sandstone (PzFq). Thin sandstone lenses and layers occur throughout the range interbedded with the mudstone. They make up a relatively small percentage of the Fox Range sequence.

Several thick (100 ft, 30 m) beds of sandstone crop out in the area, especially in Smith Canyon (Plate I), but only a few are large enough to be shown on the map. The outcrops are massive with a blocky appearance due to fractures and joints. The sandstones are light colored, predominantly quartzose, well sorted and very fine to fine grained.

These sandstones have been regionally metamorphosed to quartzites throughout the range. The quartzites have retained the very fine grained homogeneous nature of the sandstones.

Layering in the quartzites is rare and is reserved to occasional iron rich zones which produce a light red-brown banding. Occasional diffuse quartz veins cut through the quartzites with a wide variety of orientations. No remnant bedding lamination or primary sedimentary structures were observed. 22

Thin sections of the the quartzites reveal very fine

(0.01 - 0.10 mm) well sorted detrital quartz grains making up

the bulk of these rocks. The quartz grains are granoblastic

and show no preferred orientation or strain features.

Anhedral plagioclase (15%) and microcline (trace - 5%)

feldspars, metamorphic muscovite (5-10%) and biotite (10-20%)

micas, and opaque minerals, including organic matter, are the main accessories. Other minerals found in trace amounts (<

5%) include chlorite, garnet, calcite, and zircons.

Amphibolite (PzFam). A small outcrop (less than 50 ft2,

5.5 m2 ) of amphibolite occurs near the contact of the granodiorite, along the north limb of the large northern F2 antiform (Plate I). The amphibolite is black to dark gray,

fine grained, hard and dense. Layering is nonexistent except

for local plagioclase-rich laminations.

Thin sections reveal a mineralogy composed predominantly

of xenoblastic to idioblastic hornblende and plagioclase which compose 70% of these rocks. The hornblende grains are small

(0.01 - .07 mm), equant to elongated crystals with slightly

larger (0.04 - .1 mm) equant feldspar grains. Other minerals

include quartz (10%), biotite (15%), and garnet (15%).

This amphibolite most likely formed from contact metamorphism of a mafic dike or sill which intruded the sedimentary rocks prior to the regional metamorphism.

Metamorphic Grade. The metamorphic rocks of the Fox

Range vary in metamorphic grade from the mid-greenschist facies to the low to possibly mid-amphibolite facies. The 23

lowest grade rocks, the argillite and associated phyllite,

quartzite, and limestone, occur throughout the range. The

higher grade schist and gneiss are exposed only in the cores

of the large northern and southern antiforms. The coarsely

crystalline marble, formed by later contact metamorphism, is

located along the contact of the Cretaceous granodiorite in

the northeast corner of the area.

The presence of abundant muscovite, biotite and organic

matter along with occasional chlorite places the argillite and

other lower grade rocks in the mid-greenschist facies (quartz

+ biotite + muscovite + chlorite +/- plagioclase). The

recovery of several conodont fragments (discussed in the

following section) from limestones in Smith Canyon provides an

accurate means of estimating host rock temperatures, using the

color alteration index (Epstein, et al, 1977). Briefly, this method matches the color of the recovered fragments with a

standard color chart determining the index and its associated maximum temperature range. The fragments recovered in the Fox

Range produced an index of 5.5 - 6.0 indicating a maximum host

rock temperature of 400-500° Celsius. This temperature range

falls in the upper greenschist-low amphibolite facies for

regional metamorphism, and is consistent with mineral assemblages in Smith Canyon.

The schist and gneiss formed during the same phase of deformation as the argillite but experienced a higher grade of metamorphism due to their structurally lower position. These higher grade rocks are exposed beneath the argillite near the 24

cores of the large F2 antiforms. Biotite and muscovite are

common in the schist but chlorite and organic matter are less

abundant. An increase of plagioclase and the appearance of

andalusite and garnet suggest an increase in metamorphic grade

to upper greenschist or low amphibolite facies (quartz +

biotite + plagioclase +/- muscovite +/- chlorite +/-

andalusite +/- garnet).

The gneiss, which crops out only in the core of the

northern antiform, has experienced an even higher grade of

metamorphism to mid or possibly slightly higher amphibolite

facies. The virtual disappearance of muscovite and chlorite

and the appearance of metamorphic feldspars, hornblende, and

the increase in grain size confirm this increase in

metamorphic grade (quartz + plagioclase + hornblende + diopside +/- andalusite +/- garnet +/- biotite +/- muscovite).

The coarsely crystalline marble, skarn, and hornfels all

formed by later contact metamorphism. These rocks all occur along contacts with or near the Cretaceous granodiorite.

These rocks represent the hornblende - hornfels facies of contact metamorphism (calcite + quartz +/- plagioclase +/- diopside +/- garnet).

Fossils, Age and Correlation. No megascopic fossils were found during this investigation. However, several conodont

fragments were recovered from argillaceous limestone of the

Fox Range sequence. Fifteen samples of the limestone were dissolved in glacial acetic acid in an attempt to separate 25 conodonts. The resulting residues were analyzed by Dr. Bruce

Wardlaw at the Natural Science Museum in Washington, DC. In samples from location A, Plate I, he found four fragments of a gnathoid platform element (Fig. 3) which is common in the

Mississippian, Pennsylvanian and Lower Permian. This element

is very rare in the lower part of the Upper Permian and does not occur in rocks of other ages. He speculatively identified the fragments as part of Rachistognathus which is common to nearshore environments of the latest Mississippian and earliest Pennsylvanian epochs.

Approximately twenty more samples of the limestone were taken from the vicinity of the previous find (Location A on

Plate 1), and throughout the range in an effort to duplicate this result. These samples were processed as above and sent to Dr. Wardlaw for analysis. No additional conodonts were recovered.

Only one previous fossil find has been recorded in the region. Tatlock (1966) reported a lowermost Jurassic fossil from the metasedimentary rocks in the northeastern part of the

Nightingale Range of Pershing County, aproximately 50 miles

(80 km) southeast of the Fox Range.

The age discrepancy between the fossils in the Fox and

Nightingale Ranges suggests that either the metamorphosed sedimentary rocks of the Fox and Nightingale Ranges are not the same age and are not correlative or that one of the fossil ages is inaccurate. Dr. Bruce Wardlaw (personal communication, 1985) is certain that although the conodont Fig. 3 One of four conodont fragments recovered from limestones in the Fox Range. See text for discussion. fragments may not be Rachistognathus. they certainly are part

of an element from a Carboniferous or Lower Permian conodont.

The failure to find additional conodonts during the second

sampling is somewhat distressing but the initial recovery of

the four related fragments gives credibility to this find.

Tatlock (1966) seemed confident of his fossil identification and age. Therefore, it seems the metamorphic rocks of the Fox

Range and the Nightingale Range are not correlative.

Although the metamorphic rocks of the Auld Lang Syne

Group (ALSG) are generally lithologically similar to those of the Fox Range and consist of fine-grained pelitic rocks with minor amounts of coarser sandstone and detrital carbonate, differences in fossil ages make correlation unreasonable.

Therefore, the metamorphic rocks of the Fox Range will be considered mid-Carboniferous in age in this paper.

Carboniferous rocks lithologically similar to the metamorphic rocks of the Fox Range are rare in Nevada and throughout the western Cordillera. However, possible correlatives include the Inskip Formation in the East Range, the Peale Formation in the northern Sierra, and the Bragdon and Baird Formations of the eastern Klamath Mountains (Fig.

4) .

The Inskip Formation is a little-studied sequence in the

East Range approximately 80 miles (130 km) due east of the study area in Pershing County. It was divided into two units by Johnson (1977). The lower unit consists of sandstone, conglomerate, sandy siltstone and schist. The upper unit 28 29 contains quartzite, sandstone, sandy siltstone and thin limestone beds. This upper unit has been regionally metamorphosed to phyllite and amphibolite. The occurrence of the schist in the lower part, with phyllite, quartzite and amphibolite in the upper part, suggest a possible lithologic correlation. The age of the Inskip is not certain.

Silberling and Roberts (1962) assigned an age of Mississippian

(?) based on poorly preserved corals taken from limestone.

Stewart (1980) stated that, based on conodont data, the Inskip is Mississippian or Devonian or might contain rocks of both ages.

The other possibly correlative units, the Peale, Bragdon and Baird Formations, are dubious at best. The Peale

Formation is located approximately 80 miles (130 km) west of the Fox Range in the northern Sierra Nevada, east of the

Melones fault zone. The Peale consists of upper and lower members. The lower member is composed of latite or rhyolite.

The upper member contains abundant vari-colored chert, with thin shale interbeds and scattered sandstone, siltstone and rare limestone. The upper unit also contains local volcaniclastic rocks (D’Allura, et. al ., 1977). The age of the upper member of the Peale ranges from Mississippian to

Early Pennsylvanian (Schweickert,et.al.,1984). The presence of the abundant chert and local volcaniclastic rocks in the

Peale Formation makes correlation improbable.

In the same general area, near Lake Tahoe, Harwood and

Fisher (1984) have new paleontological data which suggests that metamorphic rocks which were previously thought to be

Jurassic, may in fact be Ordovician to early Mississippian age. This thick (4000 m) sequence of quartz-rich pelitic turbidites was previously correlated with the Jurassic Sailor

Canyon Formation.

Harwood and Fisher (1984) divide this sequence into four units which consist of, in ascending order; (1) a dark gray

✓ pelite-quartzite turbidite, (2) thin-bedded white orthoquartzite which they dated as post Ordovician Paleozoic age, (3) quartz-rich conglomerate, and (4) interbedded quartzite, limestone, calc-silicate rock, black pelitic hornfels and pods of marble, which were dated as late Devonian to early Mississippian.

Their brief report was summarized by suggesting that if these rocks prove to be late Devonian to early Mississippian, the rocks are probably westward transported sediments from the

Antler highland.

The Bragdon and overlying Baird Formations are located in the eastern Klamath mountains in northern California approximately 200 miles (300 km) westnorthwest of the Fox

Range. The Bragdon consists predominantly of argillite with lesser amounts of sandstone, conglomerate, and volcaniclastic rocks. The argillite is commonly silty and tuffaceous with occasional plant and fossil debris and numerous sedimentary structures (Watkins, 1979). The Bragdon conformably overlies the Middle Devonian Kennett formation and contains Upper

Mississippian fossils near its top. 31

The Baird Formation conformably overlies the Bragdon and contains argillite, bedded chert, minor limestone, siltstone,

fine sandstone and volcaniclastic mudstone (Watkins, 1979).

The Baird ranges from Late Mississippian through the

Pennsylvanian and possibly into the Early Permian in age.

Although these rocks are roughly age equivalent, the presence of volcaniclastic rocks, chert, and abundant fossils make correlation with the rocks of the Fox Range doubtful.

The most likely correlation for the metamorphic rocks of the Fox Range is with the Inskip Formation and with the metamorphic rocks near Lake Tahoe discussed by Harwood and

Fisher (1984). This correlation, however, is tentative due to the lack of firm age dates from both of these formations.

Origin. The protolith of the argillite, quartz rich mudstone, was deposited on the floor of a relatively deep marine basin located far enough from shore to limit deposition to very fine-grained clastic sediments. Occasional turbidites of coarser sand and detrital carbonate, probably derived from nearshore or shelf regions, account for the sparse sandstone/quartzite and limestone interbeds in the Fox Range sequence.

The source of the thick quartzose mudstones can only be speculated upon because of the lack of data about these rocks. However, two reasonable possibilities can be suggested based on what is known about the paleogeography of the

Carboniferous period.

The sediments may have been derived from highlands 32 created by the Late Devonian and Early Mississippian Antler orogeny to the east. Thrusting produced by a collision between an encroaching island arc and the continental shelf, emplaced deep water siliceous and volcanic rocks (western assemblage rocks) of Cambrian to Devonian age above coeval carbonate and shelf rocks, forming the Antler highlands

(Stewart, 1980; Speed, 1977; Dickinson e t .al.,1983). These highlands shed clastic sediments (Antler "flysch”) to the east and west. These clastic rocks are mainly preserved to the east (e.g. the Pilot and Chainman Shales, the Joana limestone, and the Diamond Peak Formation). However, scattered outcrops west of the Antler highlands occur in eastern Pershing and

Humboldt Counties (Stewart, 1980), and consist of both clastic and volcanic rocks (e.g. Schoonover Formation and Havallah sequence) with rare clastic units (e.g. Inskip formation).

The mudstone of the Fox Range may be another of these clastic units, as suggested by the tentative correlation with the

Inskip Formation and the metamorphic rocks of Harwood and

Fisher (1984 ) .

The other possible source for the siliceous mudstone was the volcanic arc that collided with the continent during the

Antler orogeny. The major problem with the volcanic arc as a source is the absence of volcanic detritus in these basinal rocks.

In summary, the quartz rich mudstone of the Fox Range was probably deposited on a relatively deep basin floor with occasional turbidite deposition of coarser sand and detrital 33

carbonate. The exact source of the quartzose mud is unknown.

The source may have been either from the Antler highlands,

formed following the Late Devonian to Early Mississippian

collision between an arc and the continental shelf, or less

likely, from the island arc itself.

Granodiorite (Mzgr)

The intrusive rocks in the study area vary somewhat in

composition but can be broadly classified as granodiorite.

There are also many aplite and granite dikes and sills

scattered throughout the area.

When observed in hand sample or at outcrop scale, the

granodiorite varies in color from white to pink to copper

colored. Quartz and plagioclase are rather equigranular and coarse, with phenocrysts as large as one cm. Close to contacts with the country rocks the granodiorite becomes finer grained and darker due to an increase of biotite and hornblende. Outcrops exhibit the characteristic rounded, rough-weathered surface of granitic rocks. The granodiorite

is massive with abundant joints and fractures which help promote weathering. Some of the intrusions, most notably the

large northern body, exhibit a poorly developed foliation formed by alignment of biotite and hornblende along the contact with the metasedimentary rocks, suggesting forceful emplacement (Bonham 1969).

In thin section, the granodiorite consists of quartz

(10-30%), plagiclase feldspar (40-65%), alkali feldspar (5- 34

25%), hornblende (< 10%), and a trace of sericite (all amounts

are visual estimates).

The plagioclase feldspars are zoned and exhibit growth

twinning. By use of the Michel-Levy method, a composition of

Amo to Ams was determined, indicating the plagioclase is

oligoclase. The K-feldspar is mainly microcline as indicated

by the distinctive cross-hatched twinning. Occasional quartz

intergrowths occur in both potassium and plagioclase feldspars

creating a granophyric texture. This feature was formed when

temperatures, pressures, and chemical compositions allowed for crystallization of all three minerals at the same time and place (M.J. Hibbard, personal communication).

The granodiorite bodies vary in size from many square miles (tens of km2 ) as along the northern boundary, to outcrops too small to map at this scale. These intrusions have contact metamorphosed the clastic rocks and probably are responsible for several ore deposits. The area around the

Cottonwood Mining District (Plate 1) has been saturated with dikes, sills, and stocks and has many mines associated with these intrusions. From the distribution of surface exposures of the intrusions, it appears that one large intrusive body may be located just below the surface in the vicinity of the mines.

The granodiorite of the Fox Range has not been dated, but similar granitic rocks in surrounding ranges have yielded Late

Cretaceous ages. Bonham (1969) dated a sample of granodiorite from the Granite Range, about 10 miles (16 km) north of the Fox Range. He reported a K/Ar biotite age of 88.8 +/-2.6

M.Y. Similar plutons to the south and west have yielded late

Cretaceous ages (Carlson, et. a l ., 1975; Evernden and Kistler,

1970; and Silberman, et. al., 1975).

Tertiary volcanic rocks (Tv)

Tertiary volcanic rocks, which probably covered the entire range at one time, now occur mainly in the central and eastern parts of the range.

The volcanic rocks in the study area vary in composition and include dacite and andesite flows, volcanic breccia, and vesicular basalt. The dacite and andesite flows are the oldest of the volcanic rocks, as indicated by their low stratigraphic position and by dacite and andesite clasts found in the younger volcanic breccia. The andesite is usually dark gray, black and dark red-brown. The groundmass is aphanitic with large, up to 7.5 cm, phenocrysts of brown to black hornblende and smaller phenocrysts of plagioclase feldspar.

Plagioclase phenocrysts and microlites make up the bulk of the rock (40-60%). Hornblende (20-40%), augite, opaques

(magnetite?), glass, and clays (all < 10%) compose the remainder. All amounts are visual estimates. The dacite varies widely in color from brown and gray to green, pink and red-brown. Plagioclase feldspar is the predominant mineral composing about 50-70% of the rock as both phenocrysts and microlites. Other minerals include hornblende phenocrysts

(10-20%), quartz (10%), biotite and opaques (< 10%). 36

Small, scattered outcrops of volcanic breccia contain clasts of the andesite and dacite but lack clasts of basalt, which suggests the basalt is younger. The matrix of the breccia is a light gray, gray-brown and buff colored, slightly to moderately calcareous clay. The clasts range in size from a few millimeters to boulders a couple of meters across. The clasts are composed mainly of angular andesite fragments with the remainder dacite fragments and other unidentified clasts.

The youngest of the volcanic rocks, a black to dark red-brown vesicular basalt, occurs in the higher elevations overlying the other volcanic rocks. The basalt is more extensive than the other volcanic rocks, and usually forms blocky, rubble strewn talus slopes with little vegetation. In thin section, the basalt is composed mainly of plagioclase phenocrysts and microlites (30-50%). Other minerals include augite, olivine, opaques, and some alteration minerals

(limonite and calcite), each composing less than 10% of the rock.

Bonham (1969) informally grouped the volcanic rocks into the Pyramid sequence and correlated them with such units as the Bonta Formation of Plumas County, California and the

Relief Peak formation of the central Sierra Nevada. He dated these rocks as middle Miocene to earliest Pliocene based on potassium-argon and paleobotanical ages.

Quaternary Deposits (Q1 and Qal)

Quaternary deposits include playa deposits formed by Lake 37

Lahontan during the late Pleistocene, and alluvial and eolian

deposits. During the late Pleistocene, glacial Lake Lahontan

covered a great deal of northwestern Nevada including the

Black Rock, Smoke Creek and San Emidio deserts as well as

Pyramid and winnemucca lakes. Clays and silts were deposited

in the lake with sands, gravels and some tufa deposits along

the shorelines. Many wave cut benches provide an excellent

datum for the most recent fault activity.

Deposition of playa sediments began in the Smoke Creek

and San Emidio basins as Lake Lahontan dried up. These

deposits consist of evaporites, clay, silt and very fine

grained sand that have been carried into these basins by

fluvial or eolian means. Both basins contained some moisture

throughout 1983-1984 due to the record amounts of precipitation, mainly snow, in the region. The very fine grained sediments coarsen rapidly toward the range front. The two units, the lake and playa deposits are grouped together on the map as Q 1 .

Alluvial and eolian deposits (Qal) are found throughout the range. Alluvial deposits consisting of a mixture of rock types and sediment sizes occur in all the canyons and on out to the desert floors. The western margin of the range is more highly dissected and irregular than the eastern margin due to the steeper range front and the recent fault activity. Eolian deposits of sand and silt are scattered along the entire western side of the range near the desert edge, occasionally exhibiting crossbedding as in figure 5.

39

STRUCTURE

The Fox Range is an east-tilted fault block bounded to

the west and northwest by a Basin and Range high angle fault.

The tilting has accelerated erosion of the Tertiary volcanic

rocks, especially along the uplifted west side, uncovering the

older, structurally complex metamorphosed pelitic rocks. The

structure of the metamorphic rocks is difficult to decipher

due to the lack of a recognizable stratigraphy, complicated by

multiple generations of folds, and later (?) intrusive

activity. Structures of three deformational events (Di - D3)

have been identified in the metamorphic rocks by overprinting

relationships, different tectonic styles, fold orientations,

and stereoplots.

Do Structures

Original bedding and depositional features of the

protolith mudstone were not positively identified during this

study. It is possible that the argillite layering could be,

in part, original bedding but this seems suspect due to the

intensive, penetrative deformation.

Di Structures

The first phase of deformation is depicted by abundant tight to isoclinal folds in all of the Paleozoic lithologies, but these structures are most pronounced in the schist and gneiss. The Fi folds are asymmetric, disharmonic, usually 40 plunging, inclined, parallel folds with slightly thickened hinges. Rare intrafolial folds and rootless limbs of folds were observed in the marble and in the schist (Fig. 6 and 7).

Poles to Fi axial surfaces and fold hingelines were plotted on Wulff nets (Fig. 8 and 9). Although these plots show a rough girdle distribution due to later refolding, a rough average trend of WNW - ESE was obtained for the axial surfaces of the Fi folds. The Fi hinges have a wide range of orientations with no discernible pattern. The Fi folds were only observed at outcrop scale.

The initial phase of tectonic deformation (Di) affected all of the Paleozoic rocks in the Fox Range but apparently not with equal intensity. The structurally deeper, more ductile rocks were folded more tightly, transposing the original (So) layering and forming the Si gneissic and schistose layering.

The argillite layering was also created by Di, but due to its structurally higher position, the argillite responded in a more brittle fashion.

The schist and gneiss contain most of the isoclinal folds but the large outcrop of coarsely crystalline marble along the northeastern boundary also provides an example of transposition of the So layering (Fig. 10). Isoclinal folds and outcrops exhibiting remnant transposed layering are limited in extent to areas where the large northern and southern F2 antiforms (Plate I) have exposed the structurally lower, more deformed rocks. The less deformed, structurally higher argillite, quartzite and marble on the limbs of the F2 Fig. 6 Pods of argillite "floating” in coarsely crystalline marble. Layering in marble is Si.

Fig. 7 Intrafolial fold of argillite located in the quartz-mica schist. Poles to Axial Surfaces, F, Folds N=50 + 44

Fig. 10 An Fi similar, tight, fold in the crystalline marble. The dark gray So (?) layering has been transposed into Si layering parallel to the hammer handle.

46

antiforms exhibit only rare, tight, disharmonic folding, and

no observed transposition of layering. The transition between

the schistose and argillite layering is not clearly exposed,

but must be fairly abrupt, as indicated by exposures of schist

and argillite within 350 feet (110 m) of each other.

Figure 11 is a photomicrograph of the metamorphic mica in

the schist, folded by an isoclinal fold. This relationship

seems to indicate that another deformational event caused

isoclinal folding after the creation of the mica schistosity,

but no other data supports this idea. It is probable,

however, that Di deformation was relatively long lived and

actually created several generations of isoclinal folds and

transposition of layering. This would permit the isoclinal

folding of the recently created metamorphic mica as shown in

the photomicrograph. No other Si foliations or Li lineations

were observed.

D2 Structures

The structures produced by the second phase of tectonic

deformation are dominated by the large map scale antiforms

located near the northern and southern boundaries of the study

area (Plate I). Other D2 structures include outcrop-scale

close to open folds and L2 lineations.

The F2 folds are recognized by their close to open, parallel, harmonic profiles and their relatively consistent axial plane strike of northeast with vertical to steep northerly dips. These orientations were obtained by plotting 47

field measurements of axial planes and hingelines of the F2

folds on Wulff nets. The plots of poles to axial surface and

hingeline orientations (Fig. 12 and 13) show much less scatter

than the Fi plots due to only a single generation of later,

gentle refolding. The hingeline plot exhibits a very

consistant strike direction of northeast-southwest but the

degree of plunge varies considerably due to the later

refolding.

The F2 folds are the most widespread, affecting all of

the metamorphic rocks. However, several dikes which intrude

the metamorphic rocks are not folded by the F2 folds. These

folds are both mesoscopic and macroscopic in scale and

commonly have a south to southeast vergence. The F2 folds

were rarely observed refolding Fi folds. Figure 14 shows a

very tight Fi fold of a clean, calcareous layer in marble

which has been refolded by the later F2 fold.

Several types of foliations and associated lineations

formed during F2 folding were observed in the argillite. In

one example (Fig. 15b) the Si layering in argillite has been

folded by an open, parallel F2 fold. A faint S2 convergent,

fanning axial surface cleavage formed approximately perpendicular to Si layering. The intersection of these two

foliations forms an L2 lineation oriented parallel to the axis of the Brunton compass. Another argillite outcrop (Fig. 16) exhibits S2 crenulation cleavage formed by minute F2 folding of the nearly vertical Si layering. The Si layer is cut at right angles by the subhorizontal S2 crenulation cleavage.

49

Hingeline Orientations, F~ Folds N = 12

Fig. 13 50

Fig. 14 An outcrop of argillaceous marble located along the eastern flank of the northern antiform. The arrow (near the bottom left corner of left photo) shows location of fold on right. A white, calcareous layer shows a tight Fi fold, refolded by an F 2 . This parallel close F2 fold has an axial surface orientation of N40°E, 72°NW. The F2 fold verges to the southeast. 51

Fig. 15a A relatively large, poorley defined, asymmetric, harmonic, close F2 fold in the argillite. The S2 orientation is N70°E dipping 28°NW. The light brown to white, cross cutting aplite dike is about 1 foot (30 cm) wide.

Fig. 15b An open, parallel, harmonic F2 fold in argillite south of the southern antiform. S2 is N7 5° E, 34° SE dip. An L2 lineation formed by the intersection of Si layering and a poorly developed S2 convergent fanning axial surface cleavage parallels the axis of the Brunton compass.

53

Fig. 17 An F2 fold of the Si schistose layering. The S2 axial surface is parallel to the hammer handle. An L2 lineation formed by the intersection of the Si surface and the S2 axial planar cleavage is best observed above the hammer. 54

Fig. 18 A large F2 fold in the argillite, located about 1/2 mile (800 m) north of the southern antiform on west side of range. S2 has an orientation of approximately N35°E dipping 30° to the southeast. The bushes are about 10 feet (3 m) tall. 55

An example of an F2 fold of schist is shown in Figure

17. An S2 axial planar cleavage intersects the Si schistose

layering forming an L2 intersection lineation, best observed

just above the hammer head. Figure 18 shows a large F2 fold

in the argillite along the southwest side of the range.

D3 Structures

The third generation of structures is limited to several

open to gentle, relatively large scale folds (Fig. 19) which affect all of the lithologies including dikes in the gneiss.

The Fa folds are parallel with constant layer thickness. They vary from upright to steeply inclined, gently to steeply plunging and trend roughly north-south (Fig. 20). Due to the large scale and gentle profile of these folds, hinge orientations were difficult to obtain directly. Many orientations were obtained by measuring the limbs of the fold and then using a stereonet to locate the orientation of the hinge. No Da foliations or lineations were observed.

Summary

The three episodes of deformation are categorized by the lithologies affected, the orientation, style and degree of folding and by the grade of metamorphism observed in the rocks of the Fox Range.

The first phase of deformation created the schist and gneiss by penetrative, isoclinal folding of the structurally lower, more ductile, protolith mudstone. This intense 56

Fig. 19 A gentle, nearly vertical F3 fold in recrystallized limestone near the northeastern edge of the study area. mmnnmn

57

Poles to Axial Surfaces, F3 Folds • N=9

Hingeline Orientations, F3 Folds o N=3

Fig. 20 deformation reached the low or possibly mid-amphibolite metamorphic grade. The structurally higher mudstone experienced only rare isoclinal folding and its accompanying penetrative deformation. Instead, these rocks reacted in a more brittle manner creating more gentle folds and the well defined structural layering common in the argillites. These rocks reached mid-greenschist facies.

The second phase of deformation affected all of the

Paleozoic rocks in the Fox Range but to a much lesser degree.

The principal features of this episode are the large, map scale antiforms located along the northern and southern boundaries of the study area (Plate I). These antiforms expose the structurally lower schist and gneiss. The D2 event did not produce any further noticeable metamorphism.

The third phase of deformation was by far the mildest event, producing only scattered gentle folds. This episode affected all of the Paleozoic rocks along with aplite and granite dikes which intrude the schist and gneiss. The large granitic bodies in the study area do not exhibit any signs of folding, but the northern body shows a faint foliation along its contact with the Paleozoic rocks (Bonham, 1969), suggesting forceful emplacement.

The ages of the deformational events and the structures they produced are difficult to determine with any degree of confidence due to the lack of definite age dates from any of the rocks. The absence of a viable stratigraphic sequence contributes to the difficulty in dating the structures. 59

The Di event occurred sometime between the Carboniferous period when deposition took place and Late Cretaceous time when the granitic rocks were emplaced. This earliest, penetrative, intense event is most likely related to initial movement and shuffling of the mudstone in an accretionary wedge. The second phase of deformation (D2 ) possibly relates to the actual emplacement of this wedge on the North American continent (Sonoma orogeny).

D3 structures are the youngest of all and affect the

Paleozoic rocks as well as the aplite and granite dikes which intrude the schist and gneiss. The Tertiary volcanic rocks which cover the Paleozoic rocks are not affected by any of these deformations. The D3 age, therefore, must fall between the Sonoma orogeny and Miocene time. The two most likely events which may have caused the D3 deformation are the Late

Jurassic Nevadan orogeny or the forceful emplacement of the large, northern, Late Cretaceous granodiorite.

Faults

Recognition of faulting in the Fox Range is difficult due to the lack of a detailed stratigraphy and the relatively poor exposures. The faults that appear on the map were identified by fault scarps, abrupt topographic variations, brecciated fault zones or by a combination of these features. These faults are thought to be late Tertiary in age.

The Basin and Range normal fault which runs along the 60 western side of the range is especially obvious near the southwestern edge of the study area where a 150 to 200 foot

(50 m) scarp is exposed (Fig. 21). The fault in this area has displaced Tertiary volcanic rocks from an elevation around

6400 feet near the center of the range to an elevation of 4900 feet just west of the fault, suggesting about 1500 feet (460 m) of net slip in the last 15 million years. The range has rotated along normal faults producing dips of approximately

30° to the east in the volcanic flows throughout the range

(Bonham,1969 ) .

The exposed fault scarp reveals that the fault is actually a fault zone which is composed of two or possibly three separate faults each offset about 10 feet (3m) in an en echelon pattern. The orientations of these faults range from

N20°E, 47°W to N22°W, 56°W, suggesting the fault extends into the Smoke Creek Desert to the north and south (Fig. 21).

At one location along the scarp, a 15-20 foot (5 m) high exposure of slickensides is preserved with highly polished pseudotachylyte developed on the fault surface (Fig. 22).

Striations on the slicks show only dip slip movement. Beneath the slickensides and the pseudotachylyte is a section of silicified fault breccia about four to five feet (1.5 m) thick and beneath that is a two foot (.75 m) section of fault gouge which has mostly weathered out where visible. A 50 foot (15 m) adit has been driven into the breccia and gouge zone along the strike of the fault near its southern end in Adit Canyon

(Fig. 23). The adit appears to be in gossan and skarn its 61

Fig. 21 Fault scarp located near southwestern edge of study area. Upper photo looking southeast with large granodiorite body in background. Adit canyon (Fig. 23) located at right edge of base of scarp. Bottom photo shows fault surface trending northwest toward the Smoke Creek Desert.

Fig. 23 Left photo shows adit (arrow) driven along strike into the fault zone. The adit lies in gossan and skarn. Right photo shows slickensides with highly polished pseudo- tachylyte. entire length.

Two other faults are located in the Cottonwood Basin area

(Plate 1) at the Modoc and Silver Fox Mines, and were discussed by Bonham (1969). The faults trend NE-SW with southeasterly dips of 30° to 70°. Both faults consist of a one to six foot (0-2m) zone of fault gouge and breccia which has been intruded by later quartz veins.

Bonham (1969) showed several other faults in the study area on his Washoe and Storey County map but efforts to locate these faults proved unsuccessful and they were not included on

Plate I .

The faults are late Pliocene or Pleistocene (Bonham,

1969) Basin and Range normal faults. In the Fox Range, these normal faults affect the mid-Miocene to lower Pliocene volcanic rocks but do not affect the late Pleistocene Lake

Lahontan shorelines.

Several air photo lineations are shown on Plate I by crosses with question marks on each end. These features may be faults, but no surface evidence was found.

Structural Interpretation

Before a detailed structural interpretation of the metamorphic rocks can be proposed, accurate ages of the structures and a means of interpreting the stratigraphy of these rather homogeneous rocks must be obtained. This report, therefore, should be considered an initial, reconnaissance study of these structurally complex metamorphic rocks. 65

I'urther detailed, structural analysis should provide the data

necessary to interpret the multiple generations of

deformation. It may be possible, however, to present some

tentative structural interpretations by comparing the

structures in the Fox Range with the structures of the

probably correlative, upper Paleozoic Golconda allochthon

(Table 1).

The structures of three different areas of the Golconda

allochthon are summarized below and will be compared with the

Fox Range structures discussed in this paper. The lithology

of the Golconda allochthon differs somewhat from that of the

Fox Range sequence. The main difference is the abundance of

chert throughout the Golconda allochthon and its absence in

the Fox Range. In addition, the Golconda allochthon contains minor amounts of greenstone, pillow basalt, tuff and

volcaniclastic rocks near its base. The remainder of the allochthonous rocks consist of argillite with minor amounts of

silty to limy turbidites and sandstones, which are similar to

the rocks of the Fox Range sequence.

Snyder and Brueckner (1983, and Brueckner and Snyder,

1986), described four phases or generations of structures in

the Havallah sequence, about 90 miles (145 km) east of the Fox

Range in the Sonoma and Tobin Ranges in north central Nevada

(Fig. 24). These upper Paleozoic rocks of the Golconda allochthon consist of chert and interbedded argillite with minor sandstone, limestone and pillow lava.

The first two phases of deformation, Do and Di , were TABLE 1 STRUCTURAL COMPARISON

Fox Range (This report)

Phase 1: Do - No original depositional features observed.

Phase 2: Di structures include tight to isoclinal Fi folds, hinge surfaces trend WNW-ESE, hinges have no definite pattern, south or southeast vergent, asymmetric plunging, inclined, parallel folds. Rare intrafolial folds and rootless limbs. Created Si layering throughout the range (argillite, schistose and gneissic). Affected only Paleozoic rocks.

Phase 3: Large map scale D2 folds, close to open, hinge surfaces trend E-NE, dip steeply to N, S-SE vergence, hinge line orientation of NE-SW with variable plunge, parallel harmonic profiles, rare refolding of Fi folds. Several lineations including axial surface cleavage, intersection lineations and crenulation cleavage. Affects only Paleozoic rocks.

Phase 4: Open to gentle map scale Da folds, trend N-S, upright, involve all pre-Tertiary lithologies. 65b

T A B L E 1 (cont)

Havallah .sequence, Northcentral Nevada, Tobin & Sonoma Ranges Snyder & Brueckner, 1983; and Brueckner & Snyder, 1985.

Phase 1: Do , soft sediment deformation, microstylolites, Monroe structures, slump folds, and dilation breccias. Di , early tectonics and east-west extension, formed slaty cleavage, high angle fractures, rare rootless isoclinal folds.

Phase 2: D2 , several generations of thrusts and folds related to deformation in an accretionary wedge. F2 A folds are tight to isoclinal, harmonic, ductile appearing, are offset by small thrusts, north plunging hinges. F2 b folds are disharmonic, open, north plunging hinges, and affect small thrust faults.

Phase 3: D3, related to obduction of accretionary wedge, Sonoma orogeny. Large scale thrusts and folds, parallel, open, disharmonic, and east verging.

Phase 4: D« , possibly related to Mesozoic events. Relatively minor, small scale, open buckle folds, west verging, north plunging hinges, occasional strong axial surface cleavage. 65c

T A B L E 1 (c o n t )

Golconda—Allochthon, Antler Peak Quadrangle, Northcentral Nevada Miller, et. a l , 1982

Phase 1: Di structures include rare axial planar cleavage and intersection lineations, occasional boudinage and isoclinal to open folds. Fi folds are subdivided by the two sub-units of the Pumpernickle Formation. F2 folds of sub-unit 1 exhibit N-S trending, subhorizontal hinges, open profiles, surfaces trend N-S to N20°E with moderate to steep dip. East vergent. Sub-unit 2 Fi folds vary considerably with different lithologies, open to isoclinal, trend N-N10°E and dip west, hinges trend N-S with a gentle north plunge. Small east directed thrusts.

Phase 2: Only local D2 kink style box folds, axis trend N and S, and are subhorizontal, planes trend north-south and dip both east and west. Rare F2 folds have planes which trend E-W with steep southerly dips, no hinge orientations, open, small east verging thrusts. All folding occurred prior to emplacement of Golconda allochthon in submarine conditions. 65d

T A B L E 1 {c o n t )

Schoonover sequence. Northern Nevada Miller, et. a l ., 1984

Phase 1. Only one generation of folds observed, related to emplacement of Golconda allochthon upon North American continent. Variable folding styles due to variable lithology. Generally tight to isoclinal, asymmetric, disharmonic to parallel or concentric, east verging, hinges trend N30°-40°E, plunge shallowly NE and SW. Fold axes trend N30-40°E, and dip 60—90° to NW or SE. Fi folds are refolded in a few places by younger coaxial Fi folds. Abundant SE thrusts post dating folds. Di deformation occurred during Permian to Early Triassic time. 66 grouped together because of the lack of data separating the

two phases. Structures include soft sediment folds, formation

of slaty cleavage and microstylolite development. Rare,

rootless isoclinal folds may have been formed during Di.

D2 features are abundant and include at least two

generations of tight, east vergent folds with variable

geometry due to later refolding. This phase of deformation

was discussed extensively by Brueckner and Snyder (1985).

Besides the two generations of folding, the authors also

discussed several generations of small scale thrusts which

affect the earlier Do and Di structures and in some cases the

early D2 structures. Other D2 structures related to high

fluid pressures during deformation include solution cleavage,

clastic intrusions and dilation breccia.

Phase three structures (D3 ) include large, parallel,

open, east-verging folds which refold the tight F2 folds.

Other D3 structures are major thrusts, including the Golconda

thrust.

D4 structures are relatively rare and consist mainly of small scale, open, concentric folds with a western vergence.

Snyder and Brueckner (1983, Brueckner and Snyder, 1985) interpreted the first two phases of deformation to have been related to sedimentary loading, diagenesis and early tectonics with some east-west extension. The D2 deformation is thought to represent a long period of stacking and shuffling of basinal sediments in an accretionary wedge. The authors suggested D3 deformation was related to the actual obduction Fig. 24. Location of areas discussed in text. 68

of the accretionary wedge onto the North American continent

during the Sonoma orogeny. The fourth phase of deformation

was tentatively related to Mesozoic events.

An earlier study by Miller et. al. (1982; Table 1)

discussed the structure of the Golconda allochthon in the

Antler Peak quadrangle (Battle Mountain) in north central

Nevada, about 100 miles (160 km) east of the Fox Range (Fig.

24). Miller et. al.’s study concerned the lower Upper

Pennsylvanian to upper Lower Permian Pumpernickel Formation, and the Mississipian Havallah Formation which lies

structurally above the Pumpernickel Formation. The

Pumpernickel Formation was divided into two subunits. Subunit one is a medium to thick bedded chert with silicified carbonates, immediately above the Golconda thrust. Subunit two, which overlies subunit one, consists of a thin to medium bedded chert and argillite sequence and a thick platy siltstone. The Havallah Formation also consists of two members. The lower Jory member, which lies above subunit two of the Pumpernickel Formation, is a chert-conglomerate-sandstone unit, and has been dated as

Mississippian. The overlying, undated upper Trenton member consists of dark chert and shale.

Two phases of folding in these rocks were described by

Miller et . a l . (1982). The first phase folds are parallel and disharmonic with curvilinear fold axes and curviplanar axial surfaces, suggesting later refolding. The folds vary from isoclinal to open and often die out into slip along shaly horizons. Occasional boudinage, disrupted layering and

floating fold hinges are also present. Only rarely are axial

planar cleavage and intersection lineations present. The Fi

folds are common throughout the area.

They further subdivided the Fi folds by comparing the two subunits of the Pumpernickel Formation. The silicified carbonates of subunit one exhibit N-S trending, subhorizontal east vergent folds. Axial surfaces trend generally N-S to

N20°E and dip to the west at moderate to steep angles. Fi folds in subunit two vary considerably in style, ranging from open to isoclinal, with associated boudinage, floating fold hinges and small scale thrusts. Axial surfaces trend N to

N10° E and dip to the west. Slip directions of the small thrusts indicate west to east movement.

The second phase folds (F2) are only locally present.

These kink-style box folds trend N-S, are subhorizontal, with axial surfaces dipping both east and west. Another rare set of F2 folds refolds Fi folds, and are open folds with an E-W orientation, have hinge surfaces that dip steeply to the south and are occasionally truncated by small-displacement, east- verging thrusts.

Miller et. al. (1982) suggested that the deformation of the Pumpernickel and Havallah Formations took place during a single progressive event. They suggested that the folding took place in a submarine environment, while the Golconda allochthon was being formed, prior to the actual emplacement of the allochthon onto the North American continent. 70

A third example of the structural style of the Golconda

allochthon is from the Independence Mountains, about 200 miles

(320 km) ENE of the Fox Range in northern Nevada (Fig. 24).

Miller and others (1984) described the uppermost Devonian to

Lower Permian Schoonover sequence and suggested that it

correlates with the Havallah sequence. The Schoonover

sequence consists of an Upper Devonian to Lower Mississippian

chert unit with interbedded volcanic and volcaniclastic rocks,

and overlying turbidite sequences of pebbly mudstone,

sandstone, conglomerate and shale. The remainder of the

sequence is composed mainly of chert and argillite, with

occasional quartzose turbidites, some conglomerates, and

increasingly common limestone turbidites towards the top of

the section (all Pennsylvanian and Permian age). Miller and

others (1984) suggested the Schoonover sequence had a dual source, the volcaniclastic rocks shed from an island arc in

the west and the clastic rocks derived from the Roberts

Mountain allochton (Antler highlands) to the east.

Only one generation of folds was described in the

Schoonover sequence (Table 1), although several styles of folding occur due to the variation in lithology. Generally, the folds are tight to isoclinal and verge to the southeast.

Fold hingelines trend N30°-40°E and plunge shallowly to the NE and SW. Numerous southeast-directed thrusts occur. Miller et. al. (1984) believed the deformation is entirely post Early

Permian and pre-Jurassic. They suggested the Schoonover sequence was deformed prior to emplacement on the North 71

American continental shelf, in a back arc setting, during

Permian to Early Triassic time.

These three examples of structural sequences in rocks of the Golconda allochthon (the Havallah and Pumpernickel

Formations and the Schoonover sequence), which have broadly similar ages and lithologies as the metamorphic rocks of the

Fox Range, exhibit varying degrees of structural similarity with the Fox Range. The closest similarity is with rocks of the Havallah Formation described by Snyder and Brueckner

(1983, and Brueckner and Snyder, 1985). The two main phases of deformation in the Fox Range, phases two and three (Table

1), show an especially good comparison between the style, orientation and degree of folding. The remaining phases show little similarity.

The structures in the Fox Range are not as similar to structures described in both papers by Miller et. al. (1982 and 1984). However, Miller et. al. (1982 and 1984) discussed early isoclinal folds and occasional floating hinges, structures which occur in the Fi Fox Range sequence. Also east to southeast thrusts and southeast verging folds are common in both the Antler Peak area and the Independence

Mountains. Southeast verging F2 folds, as noted earlier, are important in the Fox Range.

If broad correlations between the Fox Range sequence and the Golconda allochthon are valid, the large scale, southeast vergent F2 folds in the Fox Range sequence may have formed during the emplacement of the Golconda allochthon, as 72

suggested by Snyder and Brueckner (1983, and Brueckner and

Snyder, 1985) for the third phase of structures in the Havallah sequence.

By analogy with the Havallah sequence, the Fi folds and

the schistose and gneissic layering (Si ) in the Fox Range

sequence, probably developed during the initial movement and

stacking of the allochthonous wedge prior to being obducted.

The fourth phase of deformation (D3 this study, D4 Snyder and

Brueckner) was most likely related to the next major event

following the Sonoma orogeny, probably the Mesozoic Nevadan

orogeny, or possibly the emplacement of the Late Cretaceous

batholiths.

These correlations and deformation scenarios are very

speculative due to the lack of firm age constraints on the

structures and the rocks themselves, but if these rocks are of

Carboniferous age, these correlations seem plausible. Only

further research can provide the answers.

A brief comment about variation in the degree of deformation in the western cordillera was made by Schweickert and Snyder (1981) in an article postulating a correlation between the Calaveras complex in the Sierra Nevada and the

Golconda allochthon. They discussed the polyphase deformation in Golconda rocks described by MacMillan (1972) and by Snyder and Brueckner (unpublished data), and noted schistose rocks of the Inskip and Leach Formations. They observed that these rocks are "similar to those in terrains like the Calaveras and

Shoo Fly Complexes" in California. They also suggested "that the grade of metamorphism and degree of ductile deformation

increased westward within the Golconda allochthon, toward the volcanic arc", (Schweickert and Snyder, 1981, page 191). The inetamorphic rocks of the Fox Range, which are 100 miles (160 km) farther west than other exposures of Golconda rocks and exhibit more ductile deformation (as indicated by the presence of schist and gneiss) may support this hypothesis. 74

GEOLOGIC HISTORY

The fine grained siliceous mud, which is the predominant

protolith of the metamorphic rocks of the Fox Range, was

deposited in a basinal environment west of the Antler

highlands during Carboniferous time. The highlands were

created when lower Paleozoic oceanic basinal rocks (e.g.

Harmony, Valmy, and Vinini formations) were obducted eastward

up onto coeval North American shelf rocks during the Late

Devonian to Early Mississippian Antler orogeny (Silberling and

Roberts, 1962; Stewart, 1980; Miller et. al., 1984; and many

others). These highlands were the major source of clastic

sediments for the western cordilleran margin through the end

of the Paleozoic period (Silberling and Roberts, 1962;

Stewart, 1980). Thick accumulations of these sediments were

preserved to the east of the highlands (e.g. the Pilot and

Chainman Shales, the Diamond Peak Formation, etc.) but only

small scattered outcrops of clastic rocks (e.g. Schoonover,

Havallah and Fox Range sequences) have been preserved to the west.

The Antler orogeny changed the western margin of the

North American continent from a passive, Atlantic type margin

(Dickinson, 1976), which had existed since latest Precambrian time (Stewart, 1980) to a Japanese-type configuration which

Dickinson (1976) described as a "complicated pattern of marginal seas and intra-oceanic islands lying offshore".

The actual configuration of the continental margin during 75 mid and late Paleozoic time is still controversial. Two general models seem to be discussed more than others.

The first model, most recently discussed by Miller et. al. (1984), has an eastward subducting oceanic plate dipping under an offshore island arc and the North American plate

(Figure 25). A change in plate motions or a change in the rate of subduction during Late Devonian to Early Mississippian time caused the arc to close the relatively small back or inner arc basin and obduct the lower Paleozoic basinal rocks up onto the coeval North American shelf rocks forming the

Antler highlands. This compression was later relaxed, allowing for back arc spreading and possibly some strike slip faulting, creating another back arc oceanic basin into which the pelagic sediments of the Fox Range and the Havallah sequence were deposited.

The second model (Figure 26), most recently discussed by

Schweickert and Snyder (1981), Dickinson et. al. (1983),

Snyder and Brueckner (1983), Miller et. al. (1984), and

Brueckner and Snyder (1985), suggests that westward subduction of oceanic crust which lay between the continental shelf and an island arc allowed the arc to encroach from the west. As the arc approached, it scraped or bulldozed sediments off the descending plate, piling the sediments in front of the arc

(Fig. 26,a). When the arc collided with the continent these allochthonous rocks were obducted above the autochthonous rocks creating the Antler highlands (b). Two possible scenarios follow this collision; the first has subduction 76

MODEL 1

• LOWER PALEOZOIC

EARLY MISSISSIPPI (AntlerOrogeny)

LATE MISSISSIPPI To PERMIAN C

LATE PERMIAN To EARLY TRIASSIC (Sonoma Orogeny)

Fig. 25. "Cartoon" of Model 1, tectonics during Paleozoic Era. R.M.A., Roberts Mountain allochthon; R.M.T., Roberts Mountain thrust; G.A., Golconda allochthon; G.T., Golconda thrust; from Miller et. a l ., 1984. Discussion in text. No scale. 77

feno MODEL 2 14 WEST EAST LOWER PALEOZOIC

EARLY MISSISSIPPI (Antler Orogeny)

LATE M IS S IS S IPPI To PERMIAN

LATE PERMIAN To EARLY TRIASSIC

Fig. 26. "Cartoon" of Model 2 tectonics during the Paleozoic Era. Abbreviations are as in Fig. 25. From Miller et. al. 1983, and Snyder and Brueckner 1983. No scale. Discussion in text. 78

stepping westward beyond the arc allowing the arc to cool and contract forming a marginal basin or what Speed (1977a) called

a successor basin (c). The second scenario has the arc being

rifted away leaving a basin behind (d). This basin was the

site of deposition for the upper Paleozoic pelagic rocks of

the Fox Range and the Havallah sequence.

et. al. ( 1984) and Stewart (1980) have provided

further discussion of these models and many more related

references.

The fine-grained siliceous muds of the Fox Range probably were deposited in this back arc or successor basin as relatively distal, pelagic or hemipelagic sediments very similar in origin and setting as the basinal sediments of the upper Paleozoic Golconda allochthon (e.g. Havallah sequence) described by Speed (1977a) and by Schweickert and Snyder

(1981). The coarser sands and detrital carbonates are probably turbidite deposits that originated from the Antler

Highlands to the east (Speed 1977a; Dickinson 1983).

Deposition continued in this basin until the Late Permian and

Early Triassic Sonoma orogeny. This orogeny is thought to have been a repeat of the events that caused the earlier

Antler orogeny.

The first model suggests that another change in plate motions or rate of subduction in late Permian time caused the arc to drift to the east, closing the back arc basin, and thrusting the allochthonous basinal rocks (Golconda allochthon) above the coeval autochthonous rocks of the North American continent (Miller et. al. , 1984; Fig. 25.d).

Compression eased again, allowing for backarc extension and

possible strike slip faulting, creating a basin similar to the

earlier post-Antler oceanic basin.

The second model (refs, as above) suggests a similar

scenario for the Sonoma orogeny as for the earlier Antler

orogeny (Fig. 26). Again an island arc approached from the

west due to westward subduction of oceanic crust between the

continent and the second arc (e). This second arc may be the

same arc which was rifted away following the Antler orogeny

(f). Basinal sediments (including those in the Fox Range)

were scraped off the oceanic crust and thrust above the coeval

autochthonous rocks as the second arc collided with the

continent. Subduction then stepped westward allowing the arc and the surrounding region to cool and contract forming a

second marginal or successor basin.

Deposition in this post-Early Triassic basin continued throughout the Triassic and most of the Jurassic periods, resulting in a thick sequence of Mesozoic pelitic rocks named the Auld Lang Syne Group (Burke and Silberling, 1973). These extensively studied rocks are widespread in north central and western Nevada (Speed, 1978 a and b; Burke and Silberling,

1973; Silberling and Roberts, 1962; etc.) and were previously thought to be correlative with the metamorphic rocks in the

Fox Range (Bonham, 1969; Speed, 1978a; Stewart, 1980).

Deposition in this basin tapered off and ended around Late

Jurassic time when the brief but intense Nevadan orogeny took 80

place (Schweickert and Cowan, 1975; Speed, 1978 a and b; Dickinson, 1976 ) .

The Nevadan orogeny is not well understood, but is

thought to have involved an arc-continent collision

(Schweickert and Cowan, 1975 and Schweickert et. al. , 1984)

which severely deformed Mesozoic rocks located in the Sierra

Nevada Mountains. Large amounts of mid-Mesozoic thrusting and

deformation, possibly related to the Nevadan orogeny, have

been documented in Paleozoic rocks in Nevada (Stewart, 1980;

Snyder and Brueckner, 1983; Brueckner and Snyder, 1985; Miller

et. a l ., 1984). The F3 folds in the Fox Range may have been

formed during this orogeny.

Sometime prior to the Nevadan orogeny, in the late

Triassic, Japanese-style subduction ended and Andean-style

subduction of the Farallon plate began (Stewart, 1978;

Dickinson, 1976; Schweickert and Cowan, 1975). Dickinson

(1976, p g . 1269) described an Andean-style margin as "where an

active trench lies at the edge of a continent and the volcanic

chain of the magmatic arc stands within the continental block". The magmatic arc produced the abundant granitic batholiths located in and around the Sierra Nevada Mountains, that range from Late Triassic to Late Cretaceous in age

(Stewart, 1980; Evernden and Kistler, 1970; Carlson et. al.,

1975). The granodiorite stocks in the Fox Range and surrounding ranges are related to these batholiths (Bonham,

1969) .

Andean-style tectonics persisted through Cretaceous and 81

most of Tertiary time causing abundant deformation and

orogenic activity in eastern Nevada and western Utah (the

Sevier orogeny - Latest Jurassic through Latest Cretaceous

time), and in the Rocky Mountain region (the Laramide orogeny,

Paleocene through Mid to Late Eocene time), but did not affect

western Nevada except for intrusive activity and related local

uplift (Dickinson, 1976; Stewart, 1980 and 1978).

During Late Miocene and Oligocene time, the volcanic activity which was present in Idaho, Montana, and Wyoming

slowly migrated southwestward, ending in a northwesterly trend along the western margin of Nevada, north through the Cascade region into Canada (Stewart, 1980; Dickinson, 1976; Nilsen and

McKee, 1979). Some fruther deformation of Nevadan deformed rocks also occurred during this time. The cause is not well understood but is thought to have been related to normal faulting (Stewart, 1980).

Beginning about 20 m.y.a., Andean-type tectonics began to diminish due to the complete subduction of the Farallon plate beneath the North American plate. Consumption of the Farallon plate placed the Pacific plate against the North American plate in a transform margin, beginning in southern California and spreading north and south as the remainder of the Farallon plate was subducted (Stewart, 1978; Dickinson, 1976). This juxtaposition created the San Andreas right lateral fault system and initiated the northwest-southeast Basin and Range extension which continues to the present (Dickinson, 1976;

Stewart, 1978 and 1980). 82

Shortly after the end of subduction of the Farallon plate

the composition of volcanic rocks erupted in the Great Basin

began to change from andesitic to basaltic (Stewart, 1978 and

1980; McKee, 1971) with widespread flows occurring in Idaho,

Washington, Oregon and parts of Nevada. The Tertiary volcanic

rocks of the Fox Range are related to these basaltic flows and are some of the oldest, erupted about 15 m.y.a. (Bonham,

1969). The Tertiary volcanic rocks most likely covered the metamorphic rocks of the Fox Range in an angular unconformity but this relationship is not clearly exposed in the range.

The originally flat lying volcanic rocks now dip about

30° to the east giving a good approximation to the amount of tilt this fault block range has experienced. The eastward tilt of the Fox Range was most likely produced by movement along listric normal faults on the east side of the San Emidio desert near the Lake Range (Bonham, 1969; this report). These normal faults which are steep at the surface probably flatten at depth allowing the entire range to rotate during displacement (Stewart, 1978). The block containing the Smoke

Creek desert tilted towards the east, rotating along the fault on the west side of the Fox Range (Bonham, 1969; Stewart, 1978 and 1980). These rotations produced the steep, abrupt western flank and more gentle and gradual eastern side of the Fox

Range (Fig. 27 ) .

This uplift has allowed erosion of the volcanic rocks, uncovering the older Cretaceous granitic rocks and Paleozoic metamorphic rocks. All three of these rock types have i. 7 Shmtc rwn f osbecos eto through section cross apossible of drawing Schematic 27.Fig. W EST o scale. No ae o einl is n h ae. icsin n text. in Discussion area. the in dips regional on Based h FxRne ra hwn BsnadRne faulting. Range and Basin showing area Range Fox the \C OX RANGE X FO

EAST

84 provided sediment for the Recent alluvial, eolian and playa deposits in and around the range. 85

RECOMMENDATIONS FOR FURTHER WORK

The discovery of Paleozoic basinal rocks in the Fox Range raises many questions about the tectonic development of westernmost Nevada. Further research on these structurally complex metamorphic rocks should provide insights on the evolution of the continental margin in this region.

The first priority should be obtaining an accurate date for the Fox Range sequence. Once this is accomplished, it may be possible to develop the stratigraphy of the basinal rocks and to date closely the generations of structures. The most promising method for dating these rocks is using conodonts, which was successful in this study, or possibly radiolarians.

Miller et. al. (1984) used microfossils to date and interpret the Schoonover Sequence in the Independence Mountains. That study could be used as a model for investigating the Fox Range sequence.

Once these rocks are dated, a detailed structural analysis including dating of the structures, determining their sequence of development, and ascertaining the number of structural generations, should disclose the age of metamorphism of the various metamorphic rock.

Dating the dikes and other intrusive bodies would also provide valuable information; however, these dates may have been reset by later intrusive activity.

Other possibilites for further study include the well exposed normal fault near the southwestern edge of the study 86 area and other, less well defined, faults and linear trends throughout the range. The metamorphic transition between the gneiss, schist, and argillite also needs further study.

The wide variety of structural, stratigraphic, and metamorphic problems, the relatively good access, and it’s close proximity to Reno make the Fox Range an area that merits additional study. REFERENCES

Bonham, H.F., 1969, Geology and mineral deposits of Washoe and Storey Counties, Nevada, with a section on Industrial rocks and mineral deposits by Keith G. Papke: Nevada Bureau of Mines Bull. 70, 140 p.

Brueckner, H.K., and Snyder, W.S., 1985, Structure of the Havallah sequence, Golconda allochthon, Nevada: Evidence for prolonged evolution in an accretionary prism: Geol. Soc. American Bull., v. 96, p. 1113-1130.

Burke, D.B., and Silberling, N.J., 1973, The Auld Lang Syne Group, of Late Triassic and Jurrassic (?) age, North- central Nevada: U.S. Geol. Bull. 1394-E, p. E1-E14.

Carlson, J.E., Laird, D.W., Peterson, J.A., Schilling, J.H., Silberman, M.L., and Stewart, J.H., 1975, Preliminary map showing distribution and isotopic ages of Mesozoic and Cenozoic intrusive rocks in Nevada: U.S. Geol. Survey Open file Rept. 75-499, scale 1:1,000,000.

Coney, P.J., 1980, Cordilleran metamorphic core complexes: An overview, in Crittenden, M.D., Jr., Coney, P.J., and Davis, G.K., eds., Cordilleran metamorphic core complexes: Geol. Soc. America Mem. 153, p. 7-31.

Coney, P.J., Jones, D.L., and Monger, J.W.H., 1980, Cordilleran suspect terranes: Nature, v. 288, p. 329-333.

D ’Allura, J.H., Moores, E.M., and Robinson, L., 1977, Paleozoic rocks of the northern Sierra Nevada: Their structural and paleographic implications, in Stewart, J.H., Stevens, C.H., and Fritsche, A.E., eds., Paleozoic paleography of the western United States: Soc. Econ. Paleontologists and Mineralogists, Pacific Sec., Pacific Coast Paleogeography Symposium 1, p. 395-408.

Dickinson, W.R., 1976, Sedimentary basins developed during evolution of Mesozoic - Cenozoic arc-trench system in western North America: Can. Journal of Earth Science, V. 13, p. 1268-1287.

Dickinson, W.R., Harbaugh, D.W., Sailer, A.H., Heller, P.L., and Snyder, W.S., 1983, Detrital modes of upper Paleozoic sandstones derived from Antler orogen in Nevada: Implications for nature of Antler Orogeny: American Journal of Science, v. 283, p. 481-509.

Dixon, James, 1977, Geology of the Wild Horse Canyon area, Fox Range, Washoe County, Nevada: University of Nevada-Reno, Masters Thesis, 82 p.

Dunn, D.L., 1970, Conodont zonation near the Mississippian- Pennsylvanian boundary in western United States; Geol. Soc. American Bull., v. 81, p. 2959-2974.

Epstein, A.G., Epstein, J.B., and Harris, L.D., 1977, Conodont color alteration - an index to organic metamorphism: U.S. Geol. Survey Prof. Paper 995, 27 p.

Evernden, J.F., and Kistler, R.W., 1970, Chronology of emplacement of Mesozoic batholithic complexes in California and western Nevada: U.S. Geol. Survey Prof. Paper 623, 67 p.

Harwood, D.S., and Fisher, G.R., 1984, Paleozoic rocks along the Sierran crest west of Lake Tahoe, California: Geol. Soc. America Abs. with Prog., v. 16, p.531.

Hauge, A., and Emmons, S.F., 1877, Descriptive geology: U.S. Geol. Explor. 40th Parallel (King), v. 2.

Hill, J.M., 1915, Some mining districts in northeastern California and northwestern Nevada: U.S. Geol. Survey Bull. 594.

Johnson, M.G., 1977, Geology and mineral deposits of Pershing County: Nevada Bureau of Mines and Geology Bull. 89, 115 P-

MacMillan, J.R., 1972, Late Paleozoic and Mesozoic tectonic events in west-central Nevada: Northwestern Univ., Chicago, IL, Ph.D. thesis, 217 p.

McKee, E.H., 1971, Tertiary igneous chronology of the Great Basin of western United States - Implications for tectonic models: Geol. Soc. America Bull., v. 82, p. 3497-3502.

Miller, E.L., Kanter, L.R., Larue, D.K., Turner, R.J., Murchey B . , and Jones, D.L., 1982, Structural fabric of the Paleozoic Golconda allochthon, Antler Peak quadrangle, Nevada: Progressive deformation of an oceanic sedimentary assemblage: Journal Geophysical Research, v. 87, p. 3795-3804.

Miller, E.L., Holdsworth, B.K., Whiteford, W.B., and Rodgers, D., 1984, Stratigraphy and structure of the Schoonover sequence, northeastern Nevada: Implications for Paleozoic plate-margin tectonics: Geol. Soc. America Bull., v. 95, p. 1063-1076.

Nilsen, T.H., and McKee, E.H., 1979, Paleogene Paleogeography of the Western United States, in Armentrout, J.M., et. al., eds., Cenozoic paleogeography of the western United States: Pac. Coast Paleogeography Symp. #3, p. 257-276.

Overton, T.D., 1947, Mineral resources of Douglas, Ormsby, and Washoe Counties (Nevada): Nevada Univ. Bull., v. 41, no. 9, Geol. and Min. ser. 46. Poole, F.G., and Sandberg, C.A., 1977, Mississippian paleography and tectonics of western United States, in Stewart, J.H., Stevens, C.H., and Fritsche, A.E., eds., Paleozoic paleography of the western United States: Soc. Econ. Paleontologists and Mineralogists, Pacific Sec., Pacific Coast Paleography Symposium 1, p. 67-85.

Ragan, D.M., 1968, Structural Geology: John Wiley and Sons, Inc., New York, 208 p.

Rai, V.N., 1968, Geology of a portion of the Nightingale and Truckee Ranges, Washoe and Pershing Counties, Nevada: University of Nevada-Reno, Masters Thesis, 45 p.

Russell, B.J., 1984, Mesozoic Geology of the Jackson Mountains, Northwestern Nevada: Geol. Soc. America Bull., v. 95, p. 313-328.

Ross, C.A., and Ross, J.R.P., 1983, Late Paleozoic accreted terranes of western North America, in Stevens, C.H., ed., Pre-Jurassic rocks in western North American suspect terranes: Soc. Econ. Paleontologists and Mineralogists, Pacific Section, p. 7-22.

Schweickert, R.A., and Snyder, W.S., 1981, Paleozoic plate tectonics of the Sierra Nevada and adjacent regions, in Ernst, W.G., ed., The geotectonic development of California, Rubey v. 1: Englewood Cliffs, New Jersey, Prentice-Hall, Inc., p. 182-202.

Schweickert, R.A., Harwood, D.S., Girty, G.H., and Hanson, R.E., 1984, Tectonic Developement of the Northern Sierra Terrane: An Accreted Late Paleozoic Island Arc and it’s Basement, in Lintz, J.,Jr., ed., Western Geologic Excurions, v. 4: Geol. Soc. America 1984 Annual Meeting, Reno, Nevada, p. 1-65.

Silberling, N.J., and Roberts, R.J., 1962, Pre-Tertiary stratigraphy and structure of Northwestern Nevada: Geol. Soc. America Spec. Paper 72, 58 p.

Silberman, M.L., Bonham, H.F., and Osborne, D.H., 1975, New K-Ar ages of volcanic and plutonic rocks and ore deposits in western Nevada: Isochron/West, no. 13, p. 13-21.

Snyder, W.S., and Brueckner, H.K., 1983, Tectonic evolution of the Golconda allochthon, Nevada: Problems and perspectives, in Stevens, C.H., ed., Pre-Jurassic rocks in western North America suspect terranes: Soc. Econ. Paleontologists and Mineralogists Pacific Section, Spec. Pub., p. 103-123.

Speed, R.C., 1977, Island-arc and other paleogeographic teranes of late Paleozoic age in the western Great Basin, in Stewart, J.H., Stevens, C.H., and Fritsche, A.E., eds., Paleozoic paleogeography of the western United States: Soc. Econ. Paleontologists and Mineralogists, Pacific Sec., Pacific Coast Paleogeography Symposium 1, p. 349-362.

______, 1978a, Basinal terrane of the early Mesozoic marine province of the western Great Basin, in Howell, D.G., and McDougall, K.A., eds., Mesozoic Paelogeography of the western United States: Soc. Econ. Paleontologists and Mineralogists, Pacific Sec., Pacific Coast Paleogeography Symposium 2, p. 237-252.

______, 1978b, Paleogeographic and plate tectonic evolution of the early Mesozoic marine province of the western Great Basin, in Howell, D.G., and McDougall, K.A., eds., Mesozoic Paleogeography of the western United States: Soc. Econ. Paleontologists and Mineralogists, Pacific Sec., Pacific Coast Paleogeography Symposium 2, p. 253-270.

Stewart, J.H., 1978, Basin and Range structure in western North America - a review, in Smith, R.B., and Eaton, G.P., eds., Cenozoic tectonics and regional geophysics of the western Cordillera: Geol. Soc. America Mem. 152, p. 1-31.

______, 1980, Geology of Nevada: Nevada Bureau of Mines and Geology Spec. Pub. no. 4, 136p.

Stewart, J.H., Murchey, B., Jones, D.L., and Wardlaw, B.R., 1986, Paleontologic evidence for complex tectonic interlayering of Mississippian to Permian deep-water rocks of the Golconda allochthon in Tobin Range, north-central Nevada: Geol. Soc. American Bull., v. 97, p. 1122-1132.

Tatlock,D.B.,1966,Geology of western Pershing County, in Guidebook for field trip excursions in northern Nevada: Geol. Soc. America Cordillera Sec. meeting, Reno, Nevada, Apr. 7-9, 1966, p. E1-E5.

Watkins, R., 1979, Carboniferous rocks of the eastern Klamath Mountains, California: U.S. Geol. Survery Prof. Paper 1110-cc, p. cc33-cc36.

Willden, R., 1963, General geology of the Jackson Mountains, Humboldt County, Nevada: U.S. Geol. Survey Bull. 1141-D, p .D1-D65.