U.S. Department of the Interior

U.S. Geological Survey 

The Nature and Timing of Tectonism in the Western

Facies Terrane of Nevada and California—An

Outline of Evidence and Interpretations Derived

From Geologic Maps of Key Areas

U.S. Geological Survey Professional Paper 1592
The Nature and Timing of Tectonism in the Wester

Facies Terrane of Nevada and California—An 

Outline of Evidence and Interpretations Derived 

From Geologic Maps of Key Areas

By Keith B. Ketner

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1592

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1998

M A R C H 3, 18 4 9

U.S. DEPARTMENT OF THE 
For sale by U.S. Geological Survey, Information Services

Box 25286, Federal Center

Denver, CO 80225

Any use of trade, product, or firm names in this publication is for descriptive purposes only and

does not imply endorsement by the U.S. Government

Library of Congress Cataloging-in-Publication Data

U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Mark Schaefer, Acting Director

Ketner, Keith Brindley, 1921–

The nature and timing of tectonism in the western facies terrane of Nevada and California : 

an outline of evidence and interpretations derived from geologic maps of key areas / by 

Keith B. Ketner.

p. cm. — (U.S. Geological Survey professional paper ; 1592)

Includes bibliographical references.

1. Geology, Structural—Nevada. 2. Geology, Structural—California. I. Title. 

II. Series.

QE627.5.N3K47 1998

551.8′09793—dc21 97-44447

 CIP
III

CONTENTS

Abstract........................................................................................................................... 1

Introduction ................................................................................................................... 1

Definitions of Terms ...................................................................................................... 2

Significant Features of Maps in Key Areas of Nevada and California ......................... 3

Rowland Area ........................................................................................................ 3

Bull Run Mountains ............................................................................................... 5

Mountain City Area................................................................................................ 6

Northern Independence Mountains ....................................................................... 7

Southern Independence Mountains ....................................................................... 8

Mount Ichabod Area............................................................................................... 8

Northern Adobe Range and Peko Hills .................................................................. 8

Elko Hills................................................................................................................ 9

Piñon Range............................................................................................................ 9

Cortez Mountains ................................................................................................... 10

Northern Shoshone Range...................................................................................... 10

Edna Mountain ....................................................................................................... 11

Osgood Mountains and Hot Springs Range ........................................................... 11

Battle Mountain ...................................................................................................... 12

Sonoma Range........................................................................................................ 12

East Range .............................................................................................................. 12

Southern Toyabe Range.......................................................................................... 14

Emigrant Pass and Railroad Pass............................................................................ 14

Monte Cristo Range and Candelaria Hills.............................................................. 15

Convict Lake........................................................................................................... 15

Pilot Knob Valley and El Paso Mountains ............................................................. 15

Conclusions .................................................................................................................... 16

References Cited............................................................................................................. 17

FIGURE

1. Index map showing localities discussed in text ......................................................................................................... 3
IV CONTENTS

TABLES

1. Locations of exposures showing Ordovician western facies strata conformably overlying

thick sequences of miogeoclinal units ....................................................................................................................... 4

2. Locations of exposures showing Upper Devonian and Mississippian strata of the overlap

assemblage disconformably overlying western facies units ...................................................................................... 4

3. Locations of exposures of Middle Pennsylvanian to Permian strata of the overlap assemblage

disconformably or unconformably overlying older parts of the overlap assemblage, western

facies units, and miogeoclinal strata underlying the western facies assemblage....................................................... 5
The Nature and Timing of Tectonism in the Western 

Facies Terrane of Nevada and California—An Outline of 

Evidence and Interpretations Derived From 

Geologic Maps of Key Areas

By Keith B. Ketner

ABSTRACT

Along the outer miogeocline of Nevada and southern

California, lower Paleozoic siliceous sediments and basalt

flows, known collectively as the western facies rocks or the

western assemblage, accumulated in deep water concordantly on a thick substrate of Proterozoic to Upper Cambrian

shallow-water to deep-water terrigenous sediments. In the

mid-Frasnian of Late Devonian time, the western facies terrane emerged and was deeply eroded. Structural evidence

that this event, the Antler orogeny, involved intense folding

and thrust faulting is notably scarce. Starting in late Frasnian, siliceous sediments and intercalated basalt flows accumulated disconformably on deeply depressed parts of the

western assemblage forming the lower strata of the Havallah

and equivalent sequences of the overlap assemblage. In Middle Pennsylvanian the entire stack was moderately

deformed, elevated, and again deeply eroded. Starting in

later Middle Pennsylvanian, the western assemblage and

overlying units subsided and marine sediments accumulated

through the Permian over broad areas. Beginning in the

Jurassic and ending in the Eocene, the entire region was

strongly folded and thrust faulted. Almost all of the intense

folding and thrust faulting of Proterozoic to Permian strata

dates from the Jurassic to Eocene interval. If this tectonic

history is valid, then the genesis of the Antler orogeny is

reduced to a question of vertical tectonics. The presence of

basaltic flows at all levels in the Paleozoic column suggests

a persistently tensional stress regime both before and after

the Antler orogeny. Pre-Jurassic contraction involving subduction and collision with island arcs is not indicated or

required, but the ultimate origin of Paleozoic tectonism

remains unclear.

INTRODUCTION

Detailed geologic maps are one of the most important

tools available to geologists for determining tectonic history.

This report is essentially a critique of selected maps (f

most of which have been published since 1958 whe

rently accepted concepts of the tectonic history of the 

were first formulated by R.J. Roberts and others. The pu

of this report is to highlight the most significant stratig

and structural relations displayed on these geologic 

stressing the significance of the maps individually an

lectively for the tectonic history of the region.

The age ranges of several stratigraphic units sho

many of the maps are no longer valid, and the ma

unlikely to be revised in the near future. Accumulating 

ontological data has led to improved stratigraphic co

and therefore made possible an improved understand

the tectonic history. A principal aim of this report is to 

the revised ages of stratigraphic units more widely kno

Existing maps represent tremendous physical and 

tal effort by geologists over the last four or five decade

unfortunately, the tectonic interpretations in many 

seem to have been driven more by ideology than b

observed structural relations. As a result, interpretatio

commonly at variance with the map evidence as noted 

following pages.

This report necessarily demands much from the r

For maximum benefit from the discussions that follo

reader should examine the principal geologic maps to 

reference is made. Brief summaries of pertinent aspe

stratigraphy and structure are presented, but to dup

herein parts of the many geologic maps to which refere

made, or to describe in detail the stratigraphy and struc

each map area, would be impractical. Simplified sketc

the maps would be useless; the message is in the detai

Tables 1–3 give locations of some of the more c

exposures.

Acknowledgments. This paper benefitted greatly 

reviews by N.J. Silberling and Hugh McLean. My gr

debt is to the paleontologists, mostly of the U.S. Geol

Survey, who have provided me with reports on the a

fossils from critical stratigraphic units in critical areas w
2 TECTONISM IN THE WESTERN FACIES TERRANE OF NEVADA AND CALIFORNIA

in turn, permitted the correct dating of key strata and

improved structural interpretation. I thank especially Reuben

J. Ross, Jr., Anita G. Harris, John E. Repetski, Charles A.

Sandberg, Bruce R. Wardlaw, W.B.N. Berry, John Huddle

(deceased), Benita Murchey, Robert G. Stamm, Mackenzie

Gordon, Jr., John Pojeta, Jr., Bruce Runnegar, Norman J. Silberling, and Raymond C. Douglass.

DEFINITIONS OF TERMS

Outer miogeocline. The outer, western part of the Cordilleran miogeocline; also termed outer continental margin

(Turner and others, 1989).

Inner miogeocline. The continental shelf and upper part of

the continental slope.

Western assemblage, western facies rocks. Siliceous sedimentary rocks and intercalated basaltic strata deposited

in relatively deep water along the outer miogeocline;

commonly refers to rocks of Ordovician to Devonian

age.

Western facies terrane. Area extending from northeastern

Nevada southwestward to southern California that is

occupied by the western assemblage.

Eastern assemblage, eastern facies rocks. Shallow-water

carbonate rocks and lesser quartzite and shale of the

Cordilleran inner miogeocline; the term commonly refers to rocks of Cambrian to Devonian age.

Overlap assemblage. Sedimentary and volcanic strata of

Late Devonian to Permian age deposited on the western

facies assemblage following tectonism. The term commonly refers to Middle Pennsylvanian to Permian units

deposited after Middle Pennsylvanian tectonism, but I

include in this category also Upper Devonian to Lower

Pennsylvanian units deposited after Late Devonian tectonism. The Havallah and Schoonover sequences embrace the entire range of Upper Devonian to Permian

units.

Foreland basin assemblage. Upper Devonian to Permian

sedimentary strata deposited eastward beyond the eastern limit of the western assemblage. Most of the sediments were derived from uplifted western assemblage

rocks. I do not use the term in a genetic sense and do not

imply, as some do, that the basin of deposition was

formed as a result of tectonic loading of the adjacent

area by thrust plates.

Valmy Formation, Vinini Formation, Palmetto Formation. Contemporaneous Ordovician deep-water, dominantly siliceous sedimentary rocks and intercalated

basaltic rocks deposited in the outer miogeocline. As

used in this report, all three formations span the Ordovician, but the Valmy probably was deposited in deeper

water than the Vinini and Palmetto. The temporal

equivalence of the Valmy and Vinini has been questioned based on relations in the type area of the Vinini

(Emsbo and others, 1993), but as used in most reports,

including this one, the two formations are regarded as

correlative.

Havallah sequence, Schoonover sequence. Cont

ous Upper Devonian to Permian, dominantl

sedimentary rocks and intercalated basaltic 

posited on the western facies assemblage foll

tonism. Each includes a Middle Pennsylva

igraphic break.

Antler orogeny. Late Devonian to Mississippian 

during which western facies rocks were ele

partially eroded, resulting in the accumulatio

siliceous, detrital deposits in the foreland bas

on depressed parts of the western assemblag

of earliest emergence of marine sedimen

equated with the age of the oldest dated oro

ments in the foreland basin—the lower beds 

Shale. This age is mid-Frasnian, early Late 

As used here, the term Antler orogeny signifi

gional uplift along the outer miogeocline; i

imply folding and thrust faulting. Ironicall

tonic event in the vicinity of Antler Peak th

brated as the type occurrence of the La

ian–Early Mississippian Antler orogeny is p

Middle Pennsylvanian age.

Sonoma disturbance. A Late Permian or Early T

formational event best represented by unco

relations between Triassic and Permian rock

Mountain, Nevada. The widely used term 

orogeny overstates the importance of this loc

Thrust fault. A contractional low-angle fault; i

tional nature indicated by older-over-younge

sition, associated strong folding an

convincingly, by juxtaposition of rocks of c

contemporaneous facies.

Roberts Mountains thrust. Originally, a thrust f

Roberts Mountains that juxtaposed contem

western- and eastern-facies rocks. The term 

used in other areas where the same kind of jux

has taken place. Originally this fault in th

Mountains area was regarded as of post-Pe

(Nolan and others, 1956), but currently it is 

garded as associated with the mid-Paleoz

orogeny following the usage of Roberts 

(1958). However, in many places thrust faul

tapose contrasting facies are demonstrably of 

age or their ages are indeterminate. The ter

Mountains thrust should be restricted to th

Mountains area; its depiction on countless 

single, continuous line extending from Idaho 

nia is totally unjustified.

Roberts Mountains allochthon. Deep-water, silic

imentary and volcanic rocks of early Paleoz

the upper plate of the Roberts Mountains t

term should be restricted to the Roberts Moun

Golconda thrust. A thrust fault that carried deep

iceous and volcanic rocks of middle to late 

age, the Havallah sequence, over partly cont

ous, relatively shallow water rocks near 

Summit on Edna Mountain. Commonly assu

linked with the Late Permian or Early Triass

disturbance, but that has not been proven; i

much younger. The term Golconda thrust sh

stricted to the type area.
SIGNIFICANT FEATURES OF MAPS IN KEY AREAS OF NEVADA AND CALIFORNIA 

OREGON IDAHO

NEVADA

CALIFORNIA

118° 116°

40°

38°

42°

El Paso Mts

Mojave

Bishop

Tonopah

Winnemucca

Elko

Mountain City

Bull Run Mts Mt Ichabod

N Independence Mts

Osgood Mts Sonoma Range

East Range

S Independence Mts

Battle Mtn

Shoshone Range

Cortez Mts

China Mtn

Edna Mtn

Pinon Range

Roberts Mts

Fish Creek Mts

S Toyabe Range

Convict

 Lake

Candelaria Monte Cristo Range Hills

Emigrant Pass

Railroad Pass

Pilot Knob

Valley

0

0 50

50 100 MILES

100 KILOMETERS

N Adobe Range

Rowland

Tyrone Gap

Eureka

Alamo

N Toquima Range

ESMERALDA

COUNTY

36°

Bearpaw Mtn

Copper Mts

Jarbridge Mts Pennsylvania Hill

Hot Springs Range Peko Hills

Golconda allochthon. Deep-water, siliceous sedimentary

and volcanic mafic rocks in the upper plate of the Golconda thrust. The term should be restricted to the Golconda area.

Greenstone. Altered submarine basaltic flows or shallow

sills. Reaction with sea water typically has converted

calcic feldspars partially or completely to albite, and

pyroxene to sodic amphibole and chlorite. Tabular layers of greenstone intercalated with sedimentary rocks

are a common component of both the western and overlap assemblages. The presence of greenstone in rocks

of all ages between Proterozoic and Early Pennsylvanian could be interpreted to indicate a generally persistent tensional stress regime.

Unconformity, disconformity. As used here, unconformity

implies angular discordance; disconformity implies

that beds above and below are parallel even though

erosion may have cut down to lower stratigraphic levels in some areas than in others. In some published

descriptions the term unconformity has been used

where disconformity is more appropriate. The distinction is important because of the different tectonic

implications.

SIGNIFICANT FEATURES OF MAPS IN 

KEY AREAS OF NEVADA AND 

CALIFORNIA

Following are brief commentaries on maps of key areas

of the western facies terrane. In this context, a key area is one

that includes clear evidence relating to the timing or nature

of tectonism. This report is, as the title states, an outline of

work in progress. I hope that it will revive interest and serve

as a guide to important localities where additional productive

detailed studies can be made. I have examined every locality

discussed, some of them many times over a period of 40

years. The interpretations presented in this report are based

on a foundation laid by the best work of my predecessors and

colleagues, new data not available to the original authors,

and a broad background of stratigraphic and structural studies in the region.

ROWLAND AREA

The Rowland area includes exposures in the canyon of

Bruneau River, on Bearpaw Mountain, and in the Copper

Mountains. Two results of recent mapping are outstanding:

(1) the Valmy Formation, commonly regarded as the quintessential component of the Roberts Mountains allochthon, is

part of a thick miogeoclinal assemblage and is autochthonous with respect to underlying Proterozoic and Cambrian

strata; (2) Mississippian beds are disconformable on Middle

Ordovician strata.

The Rowland area was originally mapped by 

Bushnell (1967) in 1952–54 as part of the Rowlan

minute quadrangle. The geologic relations as he interp

them are shown with few changes on the Elko C

(Coats, 1987) and the Nevada State geologic maps (St

and Carlson, 1978). Recently part of the area was mapp

a scale of 1:24,000 (Ketner, Repetski, and others, 1995

the dating of stratigraphic units was much improved by 

of 23 conodont collections. The following discussi

based mainly on the map by Ketner, Repetski, and o

partly on that of Bushnell, and partly on unpublished 

ping by Ketner.

Pre-Tertiary strata form a well-exposed, steeply 

ping sequence that ranges from Proterozoic to Permi

age. The sequence includes Ordovician deep-water dep

generally assigned to the Roberts Mountains allochtho

in the Rowland area these rocks are not allochthonous

Figure 1. Index map showing localities discussed in text.
4 TECTONISM IN THE WESTERN FACIES TERRANE OF NEVADA AND CALIFORNIA

contact between the Valmy Formation and underlying strata

is exposed at Trail Gulch and near the confluence of Meadow

Creek and Bruneau River. In these two areas the predominantly limestone Tennessee Mountain Formation of Cambrian and Ordovician age grades upward into Ordovician

beds of the Valmy Formation over a stratigraphic 

several hundred feet by upward-increasing prop

bedded chert, siltstone, and volcanic rock.

Strata of late Early Mississippian age disco

overlie Middle Ordovician quartzite of the Valmy F

Table 1. Locations of exposures showing Ordovician western facies strata conformably overlying thick sequences of miogeo

Stratigraphic relations Location (fig. 1)

UTM East

UTM North

(meters)

Referenc

Valmy Formation on Tennessee Mountain 

Formation.

Bearpaw Mountain, near 

Trail Gulch.

612440

4639550

Ketner, Repetski, 

others, 1995.

Valmy Formation on Tennessee Mountain 

Formation.

Rowland, near mouth of 

Meadow Creek.

609850

4640420

Ketner, Repetski, 

others, 1995.

Valmy Formation on Van Duzer Formation 

(Tennessee Mountain Formation).

Pennsylvania Hill, near 

head of Lime Creek.

577100

4622090

Ehman, 1985.

Valmy Formation on OrdovicianCambrian(?) meta-limestone sequence.East Range, near Rawhide 

Spring.

427140

4493620

Whitebread, 1994

Palmetto Formation on Cambrian sequence 

that includes Prospect Mountain Quartzite.

Southern Toyabe Range, 

along Summit Creek.

476900

4313900

Ferguson and Cat

1954.

Palmetto Formation on Emigrant Formation Emigrant Pass, near the 

“guzzler.”

422200

4200000

Robinson and oth

1976.

Palmetto Formation on Emigrant Formation Railroad Pass ------------- 456430

4157280

Albers and Stewa

Table 2. Locations of exposures showing Upper Devonian and Mississippian strata of the overlap assemblage disconformabl

western facies units.

Stratigraphic relations Location (fig. 1)

UTM East

UTM North

(meters)

Referenc

Osagean unit on Ordovician Valmy 

Formation.

Rowland, near Bruneau Mine --- 608840

4642550

Ketner, Repetski, 

ers, 1995.

Osagean unit on Ordovician Valmy 

Formation.

Northern Independence Mountains, near Schoonover Creek.598460

4595645

Miller and others

Frasnian and Famennian units on 

Middle Silurian.

Southern Independence Mountains, near Basco Field.581070

4550580

Ketner, 1974, and 

Middle Frasnian unit on Middle 

Silurian and Ordovician.

Northern Adobe Range, near 

Badger Spring.

611070

4561760

Ketner and Ross, 

Osagean beds of the Inskip Formation 

on Ordovician Valmy Formation.

East Range, Inskip Canyon ------ 424600

4496140

Whitebread, 1994

Kinderhookian Robbers Mountain 

Formation on Middle Devonian unit.

Pilot Knob Valley, south of 

Randsburg Road.

486090

3932525

Carr and others, 1

in press.
SIGNIFICANT FEATURES OF MAPS IN KEY AREAS OF NEVADA AND CALIFORNIA 

This disconformable contact is exposed in the canyon of Bruneau River about 3,000 ft (914 m) southwest of the townsite

of Rowland. The Mississippian beds are composed of conglomerate, sandstone, phosphatic siltstone, limestone, and

greenstone. Above the Mississippian section, with obscure

contact relations, are siliceous and carbonate beds of Pennsylvanian and Permian age. Faults nearly parallel to bedding

are probably present in the Pennsylvanian and Permian part

of the Paleozoic sequence based on anomalous age relations

determined by conodont collections. The disconformable

contact between Ordovician and Mississippian units demonstrates that the Antler orogeny of latest Devonian to Early

Mississippian age here consisted of uplift of deep-water

deposits and exposure to erosion. There is no evidence here

of strong folding and intense thrust faulting during the 120-

m.y. interval between Middle Ordovician and late Early Mississippian time.

BULL RUN MOUNTAINS

The Bull Run Mountains display three important geologic relations: (1) a continuous stratigraphic sequence from

Proterozoic strata through the Ordovician Valmy Formation

including a good exposure of the sedimentary base of the

Valmy; (2) exposures of the contact between shallow-

Cambrian strata and overlying Cambrian deep-water s

(3) a good example of Upper Pennsylvanian strata lyi

Cambrian rocks with a moderately angular unconform

contact.

Part of the Bull Run Mountains was mapped orig

in the early 1950’s as a Ph. D. dissertation by Robert De

and the map was published in Nevada Bureau of Mines 

letin 60 (Decker, 1962). Kenneth D. Ehman subsequ

mapped the northern part of the range as a Ph. D. disser

and Theodore Clark mapped the southern part as a Ma

thesis (Ehman, 1985; Clark, 1985). Their combined ma

scale of 1:24,000 includes unpublished mapping of Te

deposits in the Bull Run Basin by University of Calif

Professor Daniel Axelrod. Stratigraphic and structural 

cepts were later modified by Ketner, Ehman, and o

(1993).

Certain stratigraphic relations in the Bull Run M

tains have been complicated by metamorphism or by 

ing, and require clarification. The Aura Formation and 

Duzer Limestone of Decker (1962) and of Ehman (198

probably parts of a single stratigraphic sequence equiv

to the Cambrian to Ordovician Tennessee Mountain Fo

tion. Lithic differences between the two parts apparent

due to different degrees of thermal alteration, as sugg

Table 3. Locations of exposures of Middle Pennsylvanian to Permian strata of the overlap assemblage disconformably or unconfor

overlying older parts of the overlap assemblage, western facies units, and miogeoclinal strata underlying the western facies assembl

Stratigraphic relations Location (fig. 1) Reference

Pennsylvanian unconformable on Cambrian dolomite Bull Run Mountains, at Porter Peak Ehman, 1985.

Permian unconformable on Ordovician Valmy Formation 

and underlying Cambrian carbonate.

Northern Independence Mountains, 

near mouth of North Fork Canyon.

Miller and other

1984.

Pennsylvanian to Permian disconformable on Late Devonian unit, all within the Schoonover sequence.Northern Independence Mountains, 

near Rocky Bluff.

Miller and other

1984.

Middle to Upper Permian unit unconformable on Ordovician Valmy Formation and Silurian chert.Mount Ichabod, near Dorsey Canyon Ketner and othe

1993.

Permian unconformable on Ordovician Valmy Formation 

and Silurian chert.

Adobe Range, near Garamendi Mine Ketner and Ross

1990.

Pennsylvanian unconformable on Cambrian quartzite 

unit.

Osgood Mountains, near upper Hogshead Canyon.Hotz and Willde

1964.

Pennsylvanian unconformable on Ordovician Valmy 

Formation.

Battle Mountain, near Cottonwood 

Creek.

Roberts, 1964.

Pennsylvanian disconformable on Ordovician Valmy 

Formation.

Northern Shoshone Range, near upper 

Horse Canyon.

Gilluly and Gate

1965.

Pennsylvanian unconformable on Ordovician Valmy 

Formation.

Northern Toquima Range, at Wildcat 

Peak.

McKee, 1976.
6 TECTONISM IN THE WESTERN FACIES TERRANE OF NEVADA AND CALIFORNIA

by relations of similar units in the Copper Mountains area.

There, in correlative rocks, lithic features of the Tennessee

Mountain Formation that characterize the Aura are due to

contact alteration and they prevail close to large granitic

intrusives, whereas features that characterize the Van Duzer

in the same stratigraphic unit prevail at a greater distance

from the intrusives. The lithic features characteristic of each

unit therefore appear to be due to different degrees of contact

metamorphism.

The contact between the Cambrian Bull Run Dolomite

of Ehman (1985) and the overlying Aura–Van Duzer

sequence separates shallow-water deposits below from deepwater deposits above. Ehman originally interpreted the contact as a fault, but our joint examination of the contact in

1992 convinced both of us that it is probably depositional. In

the Bluejacket area where exposures are relatively free from

colluvium, bedding attitudes in the two units are concordant,

and beds of dolomite, similar lithically to the Bull Run Dolomite, are present in the Aura–Van Duzer sequence a few feet

up section from the contact. The average dip of beds just

above the contact and the average just below the contact as

recorded by Ehman (1985) are virtually identical. Recognition of the depositional nature of this contact relates the

Aura–Van Duzer sequence to the underlying Cambrian shelf

strata. The Cambrian rocks below the contact are shallowwater deposits; the Aura–Van Duzer strata are deep-water

deposits. In the Copper Mountains the relations are analogous. There, the Bull Run Dolomite is absent, and shallowwater deposits assignable to the Cambrian Pioche Shale are

overlain gradationally by deep-water Cambrian strata of the

Tennessee Mountain Formation.

The contact between the Aura–Van Duzer sequence and

the Ordovician Valmy Formation is exposed in the Owyhee

quadrangle (Coats, 1971) at the northern margin of the Bull

Run Mountains. This contact was interpreted without supporting evidence as a thrust fault by Coats. However, along

the divide between Van Duzer Creek and Breakneck Creek

just north of the peak of Pennsylvania Hill in Section 1, the

contact zone is well exposed where it crosses a ridge, and it

consists of interbedded limestone, chert, sandstone, and

greenstone. Phosphatic beds composed mainly of Caryocaris

skeletons indicate an Early Ordovician age for the transitional beds. The Aura–Van Duzer sequence clearly grades

upward into the Valmy. Thus, the Valmy Formation, here as

in the Rowland area, is in its normal stratigraphic position in

the Proterozoic-Cambrian-Ordovician stratigraphic sequence of the outer miogeocline. The transition from shallowto deep-water deposition in the middle part of the Cambrian

was probably due to a relative rise in sea level because it took

place over a large part of the continental margin.

In a small area of the Bull Run Mountains, at Porter

Peak, marine fusulinid-bearing limestone beds of Late Pennsylvanian age overlie Middle Cambrian rocks with a moderately discordant unconformable contact. The unconformity

represents erosion of many thousands of feet of deep-water

strata, moderate deformation, and resumption of 

in a shallower marine environment.

MOUNTAIN CITY AREA

The Mountain City area, including the vici

Rio Tinto Mine, and Merritt Mountain, is notable 

play of the disconformable relation between t

Ordovician and Mississippian and good exposures 

sippian volcanic rocks.

The area around Mountain City was mapped 

R. Coats and his associates. The following discuss

to the Owyhee 15-minute quadrangle just west of 

City, published at a scale of 1:48,000 (Coats, 

southwest quarter of the Mountain City 15-minut

gle, released as an Open-File Report (Coats, 1968

of 1:20,000; and the southeast quarter of the Mou

quadrangle, released as an Open-File Report at 

scale (Coats and others, 1984).

Part of the northeast quarter of the Mountain 

rangle and adjacent areas to the east were mappe

(1987). The scale of Little’s map varies widely fro

age of 1:14,300 (depending on whether latitude or 

ticks or the bar scale is used to calculate it), and th

published without topographic contours or other g

features that could have made it much more usefu

Most of the Owyhee quadrangle is covered b

deposits, and by older sedimentary rocks too metam

to be identifiable as to formation or age. Howe

southeast corner of the quadrangle, Coats showed 

Duzer” Limestone, misdated as “Devonian(?),” o

the Ordovician Valmy Formation with a thrust fau

In an interpretive diagram in the margin of the map

fault is labeled Roberts Mountains thrust. The Va

now known to be stratigraphically equivalent to th

see Mountain Formation and to be Late Cambrian 

Ordovician in age (Ehman, 1985; Ketner, Ehman, 

1993). There is therefore no necessity for a thrust 

the contact. In fact the contact is concordant and de

as indicated by relations on nearby Pennsylvania 

Bull Run Mountains.

In the southeastern part of the Owyhee quad

the adjacent southwestern part of the Mountain Cit

gle, Mississippian strata overlie the Ordovician V

mation. The Mississippian sequence was divided 

stratigraphic units by Nolan (1932) and Coats (1

type section of the lowest unit, the Grossman Fo

in the Owyhee quadrangle near the Idaho-Nev m

The Grossman is undated, but the Banner Forma

overlies the Grossman is Mississippian (Osagean 

mecian) in age. The Grossman is a coarse conglom

posed mainly of quartzite boulders derived 

underlying Valmy Formation.
SIGNIFICANT FEATURES OF MAPS IN KEY AREAS OF NEVADA AND CALIFORNIA 

Published statements indicating that the contact

between Ordovician and Mississippian units in the Mountain

City area is unconformable (implying angular discordance)

are not supported by evidence. Coats (1969, p. A22) stated

that the Grossman lies unconformably on the Valmy Formation and that, elsewhere, the Banner lies with marked angularity on the Valmy (1969, p. A24; 1987, p. 30). These

statements were repeated by Little (1987, p. 5).

No discordant attitudes above and below the contact

that might justify such statements are shown on maps by

Coats and Little in the Owyhee and Mountain City quadrangles, nor could I find any. In fact, the sparse field evidence

in those quadrangles indicates that the strike and dip of strata

above and below the Ordovician-Mississippian contact are

approximately parallel.

NORTHERN INDEPENDENCE MOUNTAINS

Late Devonian to Early Mississippian deformation in

the northern Independence Mountains consisted solely of

uplift, whereas Pennsylvanian deformation involved folding

or tilting of strata.

Mapping in the northern Independence Mountains by

Churkin and Kay (1967) at a scale of 1:62,500 and by Miller

and others (1984) at a scale of 1:35,000 describes rocks that

the authors assigned to the Roberts Mountains and Golconda

allochthons and the overlap assemblage. In this area, the

stratigraphic term Schoonover sequence is applied to strata

equivalent to the Havallah sequence; the two terms are virtually synonymous. 

According to Miller and others (1984), the Schoonover

sequence, a highly faulted packet of Devonian to Permian

rocks, was emplaced over an autochthon consisting of lower

Paleozoic western facies rocks and of Mississippian to Permian rocks assignable to the overlap assemblage.

Stratigraphic relations in three areas are especially significant: (1) Schoonover Creek in the southwestern part of

the map area of Miller and others, (2) the mouth of North

Fork Canyon in the eastern part of the map area, and (3)

Rocky Bluff in the north-central part of the map area.

At Schoonover Creek, basal conglomerate of the Mississippian overlap assemblage depositionally overlies interbedded quartzite and chert of the Ordovician Valmy

Formation. This contact was described by Churkin and Kay

(1967), without citing evidence, as an angular unconformity

and by Miller and others (1984) simply as an unconformity.

The contact is well exposed in several outcrops and is concordant in each of them. In some places the conglomerate

overlies bedded chert and in others it overlies quartzite, but

the concordant relations suggest erosional relief on the surface of the Ordovician rocks rather than angular discordance

due to deformation. The concordance of beds above and

below the contact indicates again an absence of discernible

deformation between Middle Ordovician and Mississippian.

In the lower North Fork Canyon area, Permian 

glomerate overlies a Cambrian and Lower Ordovician 

bonate sequence with a sedimentary contact that ranges 

concordant to very discordant. The concordant re

between Mississippian strata of the overlap assemblag

the Ordovician strata of the Roberts Mountains allochth

Schoonover Creek on the west side of the range con

with discordant relations between Cambrian and Per

strata in North Fork Canyon on the east side of the range

deformation of the Cambrian strata prior to Permian t

fore must be younger than Late Mississippian and is lik

Middle Pennsylvanian age as in areas described i

present report and elsewhere (Ketner, 1977). There is 

responding discontinuity within the Schoonover sequ

Middle Pennsylvanian to Permian strata lie disconform

on Upper Mississippian and older beds.

The northern Independence Mountains and n

areas supply significant information on the times of in

faulting and of tectonic emplacement of the Schoo

sequence. At Rocky Bluff and over a large area to the n

west of Rocky Bluff, volcanic rocks of Eocene age (Eh

1985) unconformably overlie the Schoonover sequence 

angular discordance. Structure contouring of the base 

Eocene strata, as it was mapped by Miller and others (1

indicates that the base slopes gently to the northwest at 

5°, whereas beds and bedding-parallel faults in 

Schoonover sequence dip to the northwest at an avera

about 50°. The relations at Rocky Bluff indicate s

deformation within the allochthon in earliest Eocene or 

to Eocene. They do not necessarily cap the age of emp

ment of the allochthon because the Eocene rocks ar

known to overlap the contact between the Schoonove

the autochthon.

Miller and others (1984) cited the presence of Ju

plutons in the nearby Bull Run Mountains to support 

Jurassic age for the emplacement of the Schoo

sequence in the northern Independence Mountains

according to Ehman (1985), those intrusives do not p

allochthon and their presence is irrelevant to the age 

emplacement. In the Mount Ichabod area, 15 miles e

the Independence Mountains, an allochthon compos

rocks identical to much of the Schoonover sequence ov

rocks of Early Triassic age with a low-angle fault co

(Ketner, Murchey, and others, 1993, 1995). All that c

said presently concerning the emplacement of 

Schoonover allochthon is that it took place later than 

Triassic based on relations at Mount Ichabod.

The question has been raised as to whethe

Schoonover sequence was emplaced in an extens

regime (Ehman, 1985, p. 150). Structural relations ne

mouth of North Fork Canyon and in the Mount Ichabod 

just east of the Independence Mountains supply per

evidence. In both areas, partially correlative upper Pale

strata of contrasting facies have been juxtaposed by 

angle faulting. This juxtaposition must be the resu
8 TECTONISM IN THE WESTERN FACIES TERRANE OF NEVADA AND CALIFORNIA

contractional faulting, but superposed extensional faulting

cannot be ruled out.

SOUTHERN INDEPENDENCE MOUNTAINS

The southern Independence Mountains display some

unfaulted segments of stratigraphy typical of the western

assemblage and good evidence that the basal beds of the

overlap assemblage are as old as mid-Frasnian.

Parts of the Blue Basin and Singletree Creek quadrangles were mapped at a scale of about 1:21,400 by Lovejoy

(1959) as a Ph. D. dissertation, and the entire Blue Basin

quadrangle was subsequently mapped at 1:24,000 by Ketner

(1974). Maps of the entire area have been revised and ages of

stratigraphic units improved by a large number of graptolite

and conodont collections (Ketner, in press). Lower, Middle,

and Upper Ordovician and Lower and Middle Silurian strata

of the western assemblage are present as are Upper Devonian

units of the overlap assemblage. The unit designated as Oc in

the Blue Basin quadrangle (Ketner, 1974) was originally

thought to be solely of Ordovician age. However, upper parts

of that unit are probably of Early Silurian age. No Lower Silurian fossils were found in the southern Independence Mountains, but uppermost Ordovician and Lower Silurian

graptolites are present in similar beds of the neighboring

Adobe Range.

An important stratigraphic discontinuity separates the

upper Frasnian and Famennian strata from the underlying

Middle Silurian beds in the southern Independence Mountains. The presence of this discontinuity representing the

Antler orogeny is based on the absence of Upper Silurian to

lower Frasnian beds and also on the lithic and faunal content

of the Upper Devonian strata. These beds are composed of

conglomerate, sandstone, bedded chert, bedded barite, limestone, and greenstone. The Famennian beds contain clasts

lithically identical to certain distinctive beds in the Lower

Ordovician western assemblage rocks. Some of those clasts

are composed of Nuia, an alga restricted to the Cambrian and

Early Ordovician. The Famennian strata also contain

reworked conodonts of earliest Ordovician age (C.A. Sandberg, oral commun., 1976).

Frasnian beds of the southern Independence Mountains

are among the oldest known strata of the overlap assemblage,

and together with the Frasnian age of the oldest known foreland basin deposits, indicate a very short interval between the

earliest elevation of the western assemblage and subsequent

depression of part of this assemblage below sea level.

MOUNT ICHABOD AREA

The Mount Ichabod area displays good evidence that

rocks commonly assigned to the Roberts Mountains and

Golconda allochthons were tectonically emplaced 

Triassic time and most likely in a contractional en

The Mount Ichabod area was originally m

1952–53 as part of the Mount Velma 15-minute q

by J.R. Coash (1967), and his stratigraphic and 

interpretations were incorporated with little chan

geologic maps of Elko County (Coats, 1987) an

(Stewart and Carlson, 1978). Unfortunately, owing 

paleontological data, Paleozoic and Triassic roc

correctly distinguished on any of these maps. Inte

herein are based on new mapping at a scale of 1:24

ner, Murchey, and others, 1993, 1996). The Paleo

were dated by means of conodonts. The Triassic r

dated by use of conodonts and also through lit

blance to strata in the northern Adobe Range cont

assic conodonts and ammonites (Ketner and R

C.A. Sandberg, oral commun., 1996).

The Mount Ichabod area displays two major 

features: a low-angle fault that emplaced Paleo

above Lower Triassic strata, and a pair of high-an

faults bounding Mount Ichabod. Mount Ichabod 

that brings into view the Paleozoic strata unde

Lower Triassic sequence.

Rocks structurally overlying the Triassic sequ

prise Ordovician and Silurian western assembl

Mississippian to Permian rocks comprising the H

Schoonover sequence, and conglomeratic Pennsyl

Permian rocks of the overlap assemblage.

The Mount Ichabod horst is composed o

assemblage Ordovician rocks and unconformably 

Permian strata of the overlap assemblage. The p

Ordovician western facies rocks and strata of t

assemblage lying both above and below th

sequence indicates tectonic thickening and rep

strata and therefore a compressional, rather than a 

stress regime.

The time and manner of emplacement of th

sequence at Mount Ichabod are pertinent to the 

manner of emplacement of its equivalent, the S

Formation, in the Independence Mountains only 15 

km) distant to the west.

NORTHERN ADOBE RANGE AN

PEKO HILLS

These areas afford evidence that western a

rocks commonly assigned to the Roberts Mounta

thon were emplaced after Early Triassic time by 

directed compressional stress. Stratigraphic relat

Adobe Range indicate that the Antler orogeny invo

and erosion but provide no support for strong 

thrust faulting of Antler age.

The northern Adobe Range (Ketner and R

and Peko Hills (Ketner and Evans, 1988) were 
SIGNIFICANT FEATURES OF MAPS IN KEY AREAS OF NEVADA AND CALIFORNIA 

intervals between 1967 and 1988 at a scale of 1:24,000, and

the stratigraphic units were well dated by more than 125 fossil collections, mainly of conodonts and graptolites. There

were no earlier maps of these areas. Subsurface stratigraphy

near the northern Adobe Range was revealed by an exploratory well (Diamond Shamrock Kimbark Fed. 1-28, T. 37

N.; R. 56 E.) that reached shelf and upper slope strata as old

as Ordovician.

In the northern Adobe Range, relatively autochthonous

strata of Mississippian and older ages were overridden by

Paleozoic and Triassic rocks, and then the entire stack was

deformed into southeast-verging folds and overridden by

thrust plates composed of strata ranging in age from Ordovician to Triassic.

Ordovician and Silurian strata in the allochthonous

plates are western facies rocks conventionally assignable to

the Roberts Mountains allochthon, and the thrust faults on

which they rode conventionally could, collectively, be

termed the Roberts Mountains thrust because they stacked

rocks of shelf and basin facies.

Siliceous, deep-water, allochthonous rocks of Devonian

age are commonly assigned to the western assemblage. However, such rocks in the Adobe Range should be assigned to

the overlap assemblage because they overlie Ordovician and

Silurian beds disconformably. In the central part of the map

area, 10 conodont collections from Devonian beds indicate

unequivocally a Frasnian age. Five collections indicate a

Middle Devonian age, but of those five, two contain conodonts of diverse ages, indicating that some or all of the specimens were reworked. The most thoroughly studied area is

the east bank of Badger Creek from 500 to 1,500 feet

(152–457 m) north of Badger Spring. Here each of six conodont collections from limestone beds yielded a Frasnian

age. Radiolarians from chert beds associated with the six

limestone beds indicated a Late Devonian age. The Frasnian

beds lie directly on thin remnants of Middle Silurian siltstone

and on Ordovician beds. Pending further detailed collecting,

I conclude that the Devonian beds are all of Frasnian age but

that some contain reworked conodonts of earlier ages.

Paleozoic and Triassic rocks of the northern Adobe

Range were warped on a vertical axis to form an orocline and

then capped by Eocene sedimentary and volcanic rocks.

Finally, part of the area was domed resulting in dips of

Eocene strata as much as 45°.

The Peko Hills lie about 15 miles (19.2 km) southeast

of the northern Adobe Range. Formations ranging from Late

Devonian to Triassic in age were folded to form a tight

southeast-verging syncline that was overridden by thrust

plates of Mississippian to Permian rocks.

The Adobe Range and Peko Hills illustrate the contractional history of the area exceptionally well: thrust faulting

and large-scale, southeast-verging folding followed by overthrusting with telescoping of contrasting facies. The two

areas also illustrate the post-Paleozoic age of principal contractional structures in the region.

ELKO HILLS

The Elko Hills reveal evidence that primarily Mes

contractional deformation persisted at least locally int

Eocene.

The Elko Hills were mapped intermittently bet

1968 and 1988, and the map was published at a sca

1:24,000 (Ketner, 1990). Information from Solomon 

others (1979) and Solomon and Moore (1982) was inco

ated in the published map.

On the north and northeastern flanks of Elko Mou

klippen of Mississippian and Pennsylvanian rocks l

folded Eocene and older strata with low-angle fault con

In nearby areas the folded Eocene strata are overlain u

formably by undeformed upper Eocene strata (Ketne

Alpha, 1988). The map scale folds in the Eocene strat

the distinctly older-on-younger relation are interpret

indicate compressional stress.

PIÑON RANGE

The stratigraphy and structure of the Piñon Rang

uncommonly complex and are a source of ongoing co

versy. However, the stratigraphic and structural relatio

the Carlin Canyon area are unequivocal: the area was su

to deformation in Pennsylvanian and again after mid

mian time.

A geologic map of the core of the Piñon Range by 

ner and Smith (1963) was published at a scale of 1:12

and later a map of the entire range and part of the Su

Spring Range by Smith and Ketner (1978) was publish

a scale of 1:62,500. The adjacent Elko Hills were mapp

1:24,000 by Ketner (1990). Although most of the forma

in the Piñon Range are fairly well dated and the expo

moderately good, stratigraphic relations in the critical D

nian to Mississippian interval were not clearly understo

first and they remain unclear. Upper Devonian sili

strata and some Mississippian strata probably shou

assigned to the overlap assemblage rather than, respect

to the western and foreland basin assemblages. The pro

presence of Jurassic to Eocene contractional faults and 

tiary extensional faults was not adequately recognized 

ing the original mapping or in subsequent studies.

Evidence for Middle Pennsylvanian tectonism i

played widely in the Piñon Range and especially in the 

lin Canyon area, where the Upper Pennsylvanian and L

Permian Strathearn Formation lies with angular discord

on deformed Mississippian and Lower Pennsylvanian s

The discordant contact has been interpreted as a fault 

sma and Speed, 1990). Faults, both large and small, are 

uitous in the region, and the contact therefore may inde

faulted in places. However, exposures of Middle Penns

nian to Permian strata are common in the region, an

sedimentary nature of their lower contacts has been w
10 TECTONISM IN THE WESTERN FACIES TERRANE OF NEVADA AND CALIFORNIA

observed (Dott, 1955; Smith and Ketner, 1978; and Ketner,

1977).

Johnson and Pendergast (1981) argued that in the Piñon

Range the Roberts Mountains allochthon normally lies on the

Mississippian Chainman Shale and is overlapped by the Mississippian and Pennsylvanian Diamond Peak Formation.

Carpenter and others (1993, 1994) presented a detailed

account of stratigraphic and structural features of the Piñon

Range in which the Ordovician to Devonian deep-water siliceous rocks and the Lower Mississippian Webb Formation

are lumped together and included in the Roberts Mountains

allochthon. Otherwise, their model is similar to that of

Johnson and Pendergast. Jansma and Speed (1993) offered

the hypothesis that the Mississippian Chainman Shale had

been thrust over older foreland basin strata including the

Kinderhookian Webb Formation by the encroaching Roberts

Mountains allochthon. Their figure 2 of an area only 0.4 mi2

(1 km2) in extent depicts the principal field evidence on

which they based their hypothesis.

All of the above-named authors would adjust the

emplacement of rocks assigned to the Roberts Mountains

allochthon from earliest Mississippian to mid-Mississippian.

This is a step in the right direction but more steps are called

for. The Piñon Range could hardly have escaped the tectonism that affected nearby ranges. Most of the structure is

probably of Jurassic to Eocene age with a probable overprint

of extensional faults.

I believe that in the original mapping, and subsequent

studies, certain basic assumptions were made that, in retrospect, are probably erroneous. First, it was assumed that

upper, Famennian, beds of the Woodruff Formation are part

of the western assemblage, and second, that all Mississippian

strata are part of the foreland basin assemblage. It now seems

more in accord with the regional picture to assign the Famennian beds and some of the Mississippian beds to the overlap

assemblage. In particular, this applies to the Devonian and

Mississippian units between Trout and Willow Creeks. They,

and the underlying Ordovician rocks of the western assemblage, are probably equally allochthonous.

The Piñon Range should be remapped at 1:24,000 scale;

the Devonian and Mississippian strata should be more finely

subdivided and accurately dated. Stratigraphy and structure

of the range should be interpreted with due regard to that of

the neighboring Cortez Mountains, Adobe Range, and Elko

Hills where profound Jurassic to Eocene contractional structures are more clearly evident.

CORTEZ MOUNTAINS

The northern Cortez Mountains display good evidence

of strong deformation in the Jurassic to Early Cretaceous

interval.

The Cortez 15-minute quadrangle mapped by Gilluly

and Masursky (1965) reveals much about the style and

intensity of deformation in the range but little abo

of deformation. However, the nearby Frenchie 

minute quadrangle (Muffler, 1964) and mapping 

and Ketner (1978) in the adjacent Pine Valley q

supply reliable evidence for the principal 

deformation.

In the Frenchie Creek quadrangle, the Upper(

sedimentary beds of the Big Pole Formation in its m

sure and type locality strike northeast and dip stee

westward. The northeast-trending strike of the 

Formation was unaffected by the discordant intr

large granitic stock of Cretaceous age, except dir

cent to the contact, indicating that the steep dips 

the Big Pole probably were not caused by igneous 

The Cortez Mountains are part of a tilted Basin 

fault block. If the 10°–20° east tilt of the range, d

from the dips of Cretaceous, Miocene, and you

were to be restored to horizontal, the dip of bedd

Big Pole Formation would become steeper, 

approaching vertical, and in places would be 

(Ketner and Alpha, 1992). The Lower and Upper C

Newark Canyon Formation in the vicinity of the 

Formation is nearly undeformed (Smith and Ketn

and its beds would be horizontal if Tertiary tilt 

subtracted.

Regional structural trends suggest that the B

enclosed in the post-Early Triassic Adobe R

cline—a notably contractional feature (Ketner a

1974). In any event, the evidence clearly indica

deformation of Late Jurassic to Early Cretaceous 

NORTHERN SHOSHONE RANG

The northern Shoshone Range is important 

its well-displayed evidence for Pennsylvanian and 

deformation and, considering its location in the h

the Roberts Mountains allochthon, a remarkable 

evidence for Antler-age deformation.

The Mount Lewis and Crescent Valley qu

located principally in the northern Shoshone Ra

mapped by Gilluly and Gates (1965) and their 

assistants, including myself, at intervals between 

1959. The map was published at a scale of 1:48

range, composed mainly of lower Paleozoic, silice

water deposits of the western assemblage, is extre

plex structurally. The tremendous effort that went 

ping this extensive, rugged area resulted in greatly 

understanding of the stratigraphy and style of defo

the western facies rocks and could have resulted in 

standing of the true age of deformation if the evi

been more logically interpreted.

Gilluly and Gates divided structures in th

Shoshone Range into two packages separated by t

Canyon fault—a west-dipping low-angle fault o
SIGNIFICANT FEATURES OF MAPS IN KEY AREAS OF NEVADA AND CALIFORNIA 

slope of Mount Lewis. Structures below the Whisky Canyon

fault were arbitrarily attributed to the Antler orogeny of Late

Devonian to Early Mississippian age; those above the fault

were arbitrarily attributed to an event termed the Lewis orogeny. The Lewis orogeny was considered to be of Mesozoic

age because strata assigned to the Triassic China Mountain

Formation were involved. Those strata are still undated by

fossils, but the Pennsylvanian and Permian Antler Peak

Limestone is present and is highly deformed. The following

quotation (p. 98) reveals the weakness of the reasoning by

which the tectonism was divided into two distinct orogenies

instead of a single orogeny of Mesozoic age.

Although it seems more than likely that all the structures have been somewhat modified by the post-China Mountain(?) Lewis orogeny, we consider

the structures below the Whisky Canyon fault to be mainly products of the

Antler orogeny; they are treated as wholly of this epoch. The structures

associated with the Whisky Canyon fault and those at higher tectonic levels

are described under the heading “The Lewis orogeny.”

The two phrases that I have italicized are especially significant because they indicate that the assignment of the

structures to two distinct orogenic events was based on inference rather than on hard evidence. The following quotation

(p. 125) indicates Gilluly’s assessment of the intensity of

deformation that the authors assigned to the Lewis orogeny.

A glance at the map of plate 1 shows how much broken by faulting is the

rock mass above the Whisky Canyon fault. Along its eastern side the mass

is a jumble of much faulted blocks of Battle Conglomerate, Antler Peak

Limestone, Havallah and China Mountain(?) Formations. These are still

further confused by a whole series of fault slices grouped***as the Pipe

Canyon fault group.

The style and intensity of deformation above and below

the Whisky Canyon fault are identical, as the map patterns

indicate, and the distinction between Antler-age structures

below and Mesozoic structures above has no basis in evidence or reason. If the intensity of post-Permian deformation

above the Whisky Canyon fault is subtracted from that

below the fault, little is left that can be ascribed to the midPaleozoic Antler orogeny.

The northern Shoshone Range provides evidence that

contractional structures and the juxtaposition of contrasting

facies customarily assigned to the Antler orogeny are actually much younger than that event. Just how much younger

depends on the age assigned to the youngest deformed strata.

The youngest well-dated formation involved in the thrust

faulting is the Upper Pennsylvanian and Lower Permian

Antler Peak Limestone. If the China Mountain(?) Formation

was correctly identified, then the deformation in the

Shoshone Range is younger than Early Triassic, and in any

case it is younger than mid-Permian.

EDNA MOUNTAIN

Edna Mountain is the type area of the Golconda thrust,

defined as the thrust that regionally emplaced deep-water

facies of upper Paleozoic rocks over contemporaneous

shallow-water facies. The traditionally assumed 

between the Golconda thrust and the latest Permian or 

Triassic Sonoma disturbance, while theoretically possib

not supported by any evidence here or elsewhere

Golconda thrust could be much younger than the So

disturbance.

The Edna Mountain area was first mapped by Ferg

and others (1952) and later by Erickson and Marsh (1

b). The Golconda allochthon, here consisting of Penns

nian and Permian deep-water, siliceous components, 

lies correlative overlap assemblage strata of shallower 

facies on the Golconda thrust. The structure is clear fro

field relations but the timing of the thrust is not. The yo

est beds in both the upper and lower plates are mid-Per

in age. The oldest units overlapping the thrust are Te

deposits. Based on the local evidence, the thrust could 

old as Late Permian or as young as early Tertiary.

OSGOOD MOUNTAINS AND

HOT SPRINGS RANGE

The Osgood Mountains and the Hot Springs Rang

more notable for stratigraphic and structural puzzles th

solutions to problems. However, one stratigraphic relat

certain: shallow-water to subaerial Lower Cambrian qu

itic strata are in gradational sedimentary contact with ov

ing Middle to Upper Cambrian and Ordovician deep-

carbonate deposits (Hotz and Willden, 1964). This imp

observation confirms the interpretation of correspondin

less clearly exposed stratigraphic relations in the Bul

Mountains and contributes to an understanding of 

Paleozoic stratigraphy of the East Range.

The Osgood Mountains also clearly display M

Pennsylvanian conglomerate lying with a sedimentary 

tact on Lower Cambrian quartzite where rocks of the we

assemblage have been eroded away completely.

The Osgood Mountains and part of the adjacen

Springs Range were mapped by Hotz and Willden (19

a scale of 1:62,500. The utility of the map for tectonic 

pretation is limited by uncertainty over the ages of the 

dise Valley Chert and the overlying very thick ar

Harmony Formation. Stratigraphic units bearing 

names are confined to the Osgood Mountains and r

within 50 miles (80 km) of the Osgood Mountains.

Both the Paradise Valley and Harmony were ass

an age of Late Cambrian based on trilobites from lime

lenses at several locations. All of the trilobite-bearing 

stone bodies appear to me to be debris flows. They lac

lateral continuity normally exhibited by marine lime

strata and are typically composed of large, rounded to 

lar loosely cemented fragments. If the limestone bodie

correctly interpreted to be debris flows, the contained 

bites may or may not correctly date the two formatio

which the debris flows occur. The history of dating o
12 TECTONISM IN THE WESTERN FACIES TERRANE OF NEVADA AND CALIFORNIA

Scott Canyon Formation in the Battle Mountain area is pertinent: it was dated as Cambrian on the basis of trilobites from

debris flows until Devonian radiolarians were discovered in

the chert beds.

Arkosic boulders apparently derived from the Harmony

Formation are present in Mississippian units at widely separated localities: the northern Independence Mountains

(Miller and others, 1984), the Adobe Range (Ketner and

Ross, 1990), the Piñon Range (unpublished data), The East

Range (unpublished data), and the Fish Creek Mountains

(unpublished data). Therefore neither the Paradise Valley

Chert nor the Harmony Formation can be younger than Early

Mississippian. The trilobites preclude an age older than Late

Cambrian. Within the Cambrian-Mississippian interval they

are quite possibly of Silurian age based on their lithic composition. Commonly, Lower Silurian chert at several localities in Nevada is red or green, and some unusually thick beds

are pure white (Ketner, 1991a, b). In its type locality, the Paradise Valley Chert exhibits these features. Middle Silurian

strata in several localities in northern Nevada are composed

of somewhat arkosic sandstone or siltstone that is conspicuously rich in flakes of muscovite, suggesting possible correlation with the arkosic Harmony Formation.

The tectonic interpretation of stratigraphic and tectonic

relations in the Osgood Mounains and Hot Springs Range

must await a resolution of the problem of the ages of the Paradise Valley Chert and Harmony Formation, but the gradational contact linking the Lower Cambrian shelf quartzite to

overlying Cambrian and Ordovician deep-water carbonate

units is a secure relation of regional significance.

BATTLE MOUNTAIN

Structural relations in the Battle Mountain area remain

uncertain owing mainly to uncertainties in the ages of the

Harmony and Scott Canyon Formations. However, the area

contains a good example of an angular unconformity below

Middle Pennsylvanian strata.

Two versions of the Antler Peak 15-minute quadrangle,

mapped by R.J. Roberts, were published at a scale of

1:62,500 (Roberts, 1951, 1964). Parts of the same area, published at 1:24,000 scale by J.L. Doebrich (1992, 1994) and by

T.G. Theodore (1991a, b), show greater detail and revised

formation ages but do not significantly change original structural interpretations outlined by Roberts. Some formations in

the area have been difficult to date, and consequently, their

stated ages have changed drastically and are still undergoing

revision. The Harmony Formation has evolved from Mississippian(?) (Roberts, 1951) to Cambrian (Roberts, 1964). The

Scott Canyon Formation has evolved from Mississippian(?)

(Roberts, 1951) to Cambrian (Roberts, 1964) to at least partly

Devonian (Doebrich, 1994).

As mapped by both Roberts and Doebrich, the Harmony

Formation is separated from the Valmy Formation by the

Dewitt thrust fault. Along most of its length, the 

covered, but on hill 6835 it is fairly well exposed 

be interpreted as sedimentary. Whatever the nat

contact between the Valmy and the Harmony, 

lapped unconformably by the Middle Pennsylvan

Conglomerate. The angular unconformity testif

tonism between Ordovician and Middle Pennsylv

the nature and exact time of the tectonic event w

interval are indeterminate from local evidence. Th

tion could be the same age as the Late Devonia

Mississippian Antler orogeny or, more likely, an 

of widespread Middle Pennsylvanian tectonism do

in this report and elsewhere (references in Ketner

is ironic that the type area of the Antler orogeny d

clear evidence of Late Devonian to Mississippian 

SONOMA RANGE

The basal conglomeratic beds and substr

Havallah sequence are exposed in Clearwater C

Grand Trunk Canyon (Ferguson and others, 1951; 

1975). Silberling’s map and report are very inform

the lithic composition and locations of map units, b

tological data are notably scarce. According to S

mapping, the Havallah is a continuous, concordan

composed of three stratigraphic units—a lower be

unit underlain by a thrust fault, a middle clastic u

upper greenstone unit, all indicated to be of Permi

In my view the bedded chert unit, which c

black and distinctive white chert, correlates lithic

similar chert sequence in northeastern Nevada that 

as Late Ordovician and Early Silurian by stratigra

tion and by contained graptolites (Ketner and R

Ketner, 1991b). Based on conodonts from sample

stone that I collected near Clear Creek, the lower 

clastic unit is of Famennian age and the upper pa

mecian (B.R. Wardlaw, written commun., 1978). 

the greenstone unit to be of Mississippian age be

intercalated with, and is overlain by, sedimentary 

ilar to the upper part of the clastic unit. If my int

of the stratigraphy is correct, the area was broadl

probably in Late Devonian, was eroded down to 

Silurian, and then was submerged in latest Devo

disconformity below the Famennian beds represen

ler orogeny and indicates that that event, here as 

consisted of uplift and erosion without strong defo

EAST RANGE

The East Range is one of the most important 

tectonic information in Nevada. It supplies unequ

dence that, for 120 million years between Middle O

and late Early Mississippian, (1) the area was una
SIGNIFICANT FEATURES OF MAPS IN KEY AREAS OF NEVADA AND CALIFORNIA 

folding and (2) almost all the large-scale folding now evident

in the area is younger than Triassic. It also supplies evidence

on the sedimentary substrate of the Havallah sequence.

The East Range was first mapped by Ferguson and others (1951). Although the geologic map of the Dun Glen 15-

minute quadrangle by D.H. Whitebread was released in 1978

as a U.S. Geological Survey Open-File Report, it remained

unnoticed and the tectonic information that can be derived

from it has had no effect on widely accepted tectonic concepts. This map now is published in full color as Miscellaneous Investigations Map I–2409 at a scale of 1:48,000

(Whitebread, 1994). It is the product of tremendous effort

and attention to detail, but new information requires revision

of the ages of certain units and therefore revision of the structure as it was perceived by Whitebread.

In the Willow Creek area a thick unnamed sequence of

metamorphosed limestone, siltstone, and greenstone is overlain with a gradational, concordant contact by the Ordovician Valmy Formation. For whatever now-unknown reason,

the lower part of this sequence was originally thought to be

of Mesozoic age, and since the Valmy Formation overlies it,

the contact between them had to be regarded as a thrust fault,

and was originally mapped as such by Ferguson and others

(1951), and also by Whitebread (1978). On the basis of lithic

correlation with dated units in other ranges and a conodont

collection from the Rawhide Canyon area, the upper part of

the sequence underlying the Valmy Formation can now be

quite certainly dated as of Late Cambrian(?) and Early

Ordovician age. The base of this sequence is not exposed in

the East Range, but the base of the equivalent sequence in the

Osgood Mountains is well exposed. There, the lower part of

the sequence is in sedimentary contact with Lower Cambrian

quartzitic rocks of the miogeocline.

Because the Ordovician Valmy Formation is in normal

stratigraphic sequence above the unnamed Cambrian(?) and

Ordovician sequence in the East Range, there is no structural

requirement for an important thrust fault separating the two

formations. Moreover, a sector of the contact where it

crosses a ridge northwest of Rawhide Spring appears to be

concordant and gradational. This is not to say that the contact

is unfaulted along its entire length. Bedding attitudes suggest

the presence of a fault of some kind in one segment of the

contact, but the concept of the contact as a major thrust fault

is no longer valid.

The Valmy Formation is composed of a lower interval

of greenstone, argillite, siltstone, chert, and minor limestone, and an upper member consisting mainly of quartzite.

The lower interval is dated by means of conodonts as Early

Ordovician. The quartzite member, based on its stratigraphic position and conodont dating elsewhere, is Middle

Ordovician.

The quartzite member of the Valmy Formation is overlain by the Inskip Formation, a sequence of arkosic sandstone, conglomerate, siltstone, limestone, and volcanic rock

at least 9,000 ft (2,743 m) thick. The contact with the Valmy

Formation is a disconformity, as indicated by concor

of bedding attitudes on both sides and the presence i

Inskip of conglomerate composed of pebbles derived 

the Valmy. It was mapped as a disconformity by Ferg

and others (1951) and by Whitebread. Silberling and Ro

showed the contact as a fault on an interpretive sketch 

(1962, fig. 4) but described it in the text as “***for the 

part steeply faulted,” implying that they regarded part 

contact as sedimentary.

The contact between the Valmy and Inskip Forma

is remarkably concordant. The strike of beds above 

below the contact is essentially parallel along the 12-

long (19.2 km) contact within and to the south of the 

Glen quadrangle. The average dip of the quartzite mem

the Valmy based on nearly 100 measurements is within 

the average dip of the Inskip, based on more than 200 

surements. Although the Inskip is partially mylonize

mylonite foliation is clearly parallel to original strat

were relatively unaffected by shearing. These include b

tic flows, limestone beds, and conglomerate deposits.

At Hot Spring Canyon, 5 miles (8 km) south of the 

Glen quadrangle (Muller and others, 1951), the basal 

glomerate beds of the Inskip are composed in part of 

cobbles of quartzite apparently derived from quartzite 

of the concordantly underlying Valmy Formation. Here 

the Valmy Formation lies concordantly on a lime

sequence. The limestone sequence was originally desig

as Triassic, but based on lithic correlation with dated 

in the Dun Glen quadrangle, it is more likely to be Late 

brian or Early Ordovician in age.

The age of lower beds in the Inskip Formation was 

inally thought to be Devonian(?) and Mississippian(?) o

basis of a coral collection and preliminary evaluation o

odont collections from thin limestone bodies in the 

part of the formation (Whitebread, 1978). The coral c

tion is now regarded as early Late Mississippian by 

Sando (1993) and reevaluation of the conodont collec

by Anita Harris (revised unpub. report, 1980) indicates 

Early Mississippian age. The corals appear to be redepo

and the conodonts certainly may have been redeposite

the age of the lower, but not lowest, part of the Inskip 

be regarded as mid-Mississippian or possibly younger. 

odonts collected from the uppermost exposed beds in 

formable sequence with the Mississippian part o

formation are Morrowan, Lower Pennsylvanian (B.R. W

law, oral commun., 1997).

North of Rockhill Canyon in the northern part o

Dun Glen quadrangle and separated from the Inskip 

fault, Whitebread showed Triassic rocks in normal co

with rocks he designated as the Havallah sequence. 

odonts collected by Whitebread and fusulinids report

Silberling and Roberts (1962) indicate a Permian age fo

exposure of the Havallah. The collections contain frag

tary conodonts apparently reworked from older units. T

is no apparent reason other than its age for separatin
14 TECTONISM IN THE WESTERN FACIES TERRANE OF NEVADA AND CALIFORNIA

exposure from the Inskip, and in my opinion, it should be

regarded as the uppermost segment of that formation. If that

interpretation is valid, then the age range of the Inskip is

essentially the same as that of the Havallah sequence, and the

two units logically should bear the same name.

The disconformable contact between the Middle Ordovician Valmy and mid-Mississippian beds of the Inskip

records little if any deformation between Middle Ordovician

and mid-Mississippian—an interval of at least 120 million

years. This hiatus represents (1) regional uplift probably in

Late Devonian, (2) erosion of Devonian, Silurian, and Upper

Ordovician strata assumed to have been present, then (3) submergence in mid-Mississippian and accumulation of more

than 9,000 ft (2,743 m) of sedimentary and volcanic strata in

a deep marine environment.

At the map scale, the Triassic rocks in the East Range

are no less deformed than the lower and middle Paleozoic

units. If the degree of large-scale deformation of the Triassic

rocks is subtracted from that of the Paleozoic rocks, little or

none is left to ascribe to older tectonic events. The concept

that the Antler orogeny of middle Paleozoic age involved

strong folding and thrust faulting is supported by neither

stratigraphic relations nor map-scale structural relations in

the East Range.

SOUTHERN TOYABE RANGE

The east slope of the southern Toyabe Range is important for several good exposures of shallow-water shelf strata

of Early Cambrian age overlain with a sedimentary concordant contact by deep-water western facies rocks of Late

Cambrian to Ordovician age.

Reconnaissance mapping by Ferguson and Cathcart

(1954) at a scale of 1:125,000 is still the only published

source of critical stratigraphic and structural data. The grand

exposures in the Summit Creek and Wisconsin Creek area

show a sequence of Lower Cambrian rocks, mainly of

coarse- to medium-grained, crossbedded quartzite, grading

upward into fine-grained siltstone, shale, chert, limestone,

and greenstone. The fine-grained strata resemble the Upper

Cambrian to Ordovician Emigrant Pass and Palmetto Formations of Esmeralda County. Cambrian conodonts were

extracted from limestone beds in the upper part of the

sequence along Summit Creek, and Early and Middle Ordovician graptolites at unspecified locations in the range were

reported by Ferguson and Cathcart.

A belt of coarsely detrital rocks of the Mississippian and

(or) Permian Diablo Formation overlies the lower Paleozoic

sequence. The base of the Diablo is shown as a normal fault

and the original nature of the contact is unknown. However,

the parallel orientation of outcrop belts and the generally

similar attitude of beds in the entire sequence as mapped by

Ferguson and Cathcart provide no support for the concept of

intense tectonism in the middle Paleozoic.

Here, as in parts of northern and southwester

Ordovician western facies rocks are autochtho

respect to underlying Cambrian shallow-water 

Deposition of sediments of the western assembla

tively deep water reflects sea-level change or verti

ics, and their occurrence in sequence with older

water deposits does not require eastward tectonic 

The frequent depiction of the Roberts Mountain

this latitude is unsupportable by stratigraphic or 

evidence.

EMIGRANT PASS AND RAILROAD 

Geologic relations at Emigrant Pass and Rai

in Esmeralda County are significant because the

clear evidence that the Palmetto Formation, lithica

and temporally equivalent to the Ordovician Valm

tion of northern and central Nevada, is autochtho

respect to the underlying miogeoclinal sequence.

The Cambrian Emigrant Formation, which 

ally underlies the Palmetto, is part of a concordan

of Proterozoic and Cambrian formations assigna

Cordilleran miogeocline. The Emigrant is a d

deposit, but it is underlain by strata that include 

shallow water units containing an abundant shelly 

The geologic map of Esmeralda County (A

Stewart, 1972) and the Rhyolite Ridge quadrangle 

and others, 1976), in which Emigrant Pass is loc

the contact between the Palmetto Formation and 

grant Formation as a thrust fault. Moreover, the i

of countless publications on aspects of Nevada geo

the Roberts Mountains thrust extending through 

County. Apparently this reflects the assumption th

ician western facies rocks, by whatever name and 

found, must be allochthonous. Actually, the conta

the Emigrant Pass and the Palmetto is concordant 

tional in several good exposures near Emigrant 

road Pass, and elsewhere in the county. This was r

by John H. Stewart (1980), who stated (p. 40) in re

the Esmeralda County area: “***the Palmetto is 

in conformable contact with the underlying Cam

grant Formation.” A sample collected from the t

beds between the two formations at Emigrant Pa

basal Ordovician conodonts (J.R. Repetski, oral 

1997).

Recently, microfossils collected at Railroad 

lower levels of the transition zone between the Em

Palmetto include proconodonts and euconodonts th

latest Cambrian ages, whereas slightly higher 

indicate an Early Ordovician age (John Repets

commun., 1993). The lithic similarity of the upper 

Emigrant and the lower part of the Palmetto, the co

of bedding above and below the contact, and the 

the Cambrian-Ordovician contact determined by 

within a lithically homogeneous sequence togeth
SIGNIFICANT FEATURES OF MAPS IN KEY AREAS OF NEVADA AND CALIFORNIA 

that the Palmetto Formation is autochthonous with respect to

underlying Proterozoic and Cambrian units.

MONTE CRISTO RANGE AND 

CANDELARIA HILLS

At a locality in the Monte Cristo Range, 5 miles (8 km)

north of Coaldale, the contact between a Mississippian dolomite and conglomerate unit and the underlying Ordovician

Palmetto Formation is definitely an angular unconformity

and therefore constitutes an exception to one of the main

themes of the present report.

The Monte Cristo Range was mapped in reconnaissance by Ferguson and others (1953). Their map, published

at a scale of 1:125,000, remains the principal source of geologic relations in that area. Ferguson and others assigned the

dolomite and conglomerate interval to the Permian Diablo

Formation. The formation is now known to be Mississippian,

based on corals collected there by F.G. Poole and identified

by Sando (1993). The distinctly unconformable contact

between Mississippian rocks and underlying Palmetto Formation constitutes evidence of local tectonism between

Ordovician and Early Mississippian. The area deserves

detailed study to determine the nature and exact age of this

exceptional occurrence of pre-Mississippian tectonism.

The Candelaria Hills, a short distance northwest of the

Monte Cristo locality, were mapped originally by Ferguson

and others (1953, 1954), who included undated Mississippian dolomite and limestone in the Permian Diablo Formation. Page (1959), in a detailed map of the Candelaria mining

district, included the Mississippian strata, still undated, in

the Ordovician Palmetto Formation. The correct, Mississippian, age of the dolomite and limestone unit was determined

between 1972 and 1976 by Sando (1993) based on corals

collected by F.G. Poole. Most of the deformation in the area

appears to be post-Mississippian and pre-Permian, but without detailed mapping done in the light of the new paleontological data, the relations among the Permian, Mississippian,

and Ordovician strata remain unclear.

CONVICT LAKE

The geologic map of the Convict Lake area in the

Mount Morrison 15-minute quadrangle (Rinehart and Ross,

1964) together with new conodont-based dates of stratigraphic units reveals a west-facing Cambrian to Devonian

stratigraphic sequence similar in some respects to the lower

Paleozoic sequence in adjacent parts of Nevada.

The Buzztail Spring Formation and the overlying

Mount Aggie Formation constitute a thick sequence of thinto thick-bedded metamorphosed limestone, sandstone,

shale, chert, and greenstone. The presence of bedded chert

and greenstone and the lack of shelly fauna suggest a

relatively deep water environment of deposition, and 

units are therefore assignable to the western or transi

assemblage.

A collection of conodonts from the middle part o

Mount Aggie Formation on the northwest side of Co

Lake indicates a stratigraphic position of these beds ne

Cambrian-Ordovician boundary (J.E. Repetski, oral 

mun., 1994); therefore most of the Buzztail Spring–M

Aggie sequence below the conodont collection is of 

brian age and possibly older. Overlying the Mount A

Formation with a gradational contact is the Convict 

Formation of Ordovician age as determined by a wel

served graptolite fauna (Rinehart and Ross, 1964; 

Stevens, written commun., 1995). Disconformably a

the Convict Lake Formation is a series of quartzitic 

originally assigned by Rinehart and Ross to the Mount 

rison Formation and Sevehah Cliffs sequence. This qu

itic sequence is now known to be of Devonian age, bas

conodont fragments from the upper beds identified by 

Harris and J.E. Repetski (oral commun., 1994) and co

tions by Stevens and Greene (1994, 1995).

The Devonian beds are overlain in the Convict 

area and in other pendants of the Sierras, by the Brigh

Formation, a sequence of chert, argillite, and sandstone 

taining Mississippian conodonts (C.H. Stevens, written 

mun., 1995). The contact between the two units is gen

concordant according to Stevens. The stratigraphic rela

indicate two intervals of erosion—one between Ordov

and Devonian and another between Devonian and Miss

pian—but do not indicate any significant folding bet

Ordovician and Mississippian.

The Buzztail Spring Formation and lower part o

Mount Aggie Formation are lithically similar to, and c

ative with, the Emigrant Formation of Esmeralda Co

Nevada. The upper part of the Mount Aggie Formatio

the Convict Lake Formation are similar to, and corre

with, the Palmetto Formation. None of the strata near 

vict Lake has been dated as Silurian, and beds of that ag

presumed to have been eroded prior to deposition o

Devonian quartzitic strata.

The Convict Lake area displays good evidence of 

tinuous deposition through the Upper Cambrian–L

Ordovician interval and for periods of erosion or nonde

tion. It displays no evidence for intense mid-Pale

deformation.

PILOT KNOB VALLEY AND

EL PASO MOUNTAINS

Pilot Knob Valley and the El Paso Mountain

important because they provide exposures of Lower M

sippian strata lying disconformably on western facies, 

water, siliceous deposits correlative with western f

deposits in Nevada.
16 TECTONISM IN THE WESTERN FACIES TERRANE OF NEVADA AND CALIFORNIA

Parts of Pilot Knob Valley (Carr and others, 1992, pl. 1)

and the northern El Paso Mountains (Carr and others, in

press) were mapped at a scale of 1:24,000.

In Pilot Knob Valley the lower Lower Mississippian

Robbers Mountain Formation, composed of a basal conglomerate and overlying meta-argillite, disconformably

overlies sandy marble and quartzite of Middle Devonian age

according to Carr and others (1992). They described the contact of the Robbers Mountain Formation with underlying

units in general as “***either a slight angular unconformity

or a major disconformity regionally having significant erosional relief, because the formation rests on different Devonian units in the El Paso Mountains than it does in Pilot Knob

Valley” (p. 14).

In the El Paso Mountains, near Iron Canyon, a sandstone, shale, and conglomerate sequence of undated Mississippian(?) and dated Pennsylvanian ages overlies a western

facies map unit of Cambrian and Ordovician age along a normal fault contact about 4 miles (6.4 km) long according to

Carr and others (in press). Attitudes and outcrop patterns do

not seem to require the contact to be a fault along its entire

length. Near the Apache Mine, beds above and below the

contact are parallel, and some beds above the contact contain

clasts apparently derived from the underlying unit. I interpret

the contact near the Apache Mine to be a probable disconformity although it may be faulted elsewhere.

The slightly angular or disconformable contact separating Devonian and Ordovician strata from overlying Mississippian strata in Pilot Knob Valley and the El Paso

Mountains supplies evidence for a period of erosion prior to

deposition of the Mississippian strata but no support for the

concept of strong deformation in the Devonian-Mississippian interval.

CONCLUSIONS

Verifiable stratigraphic and structural relations and reasonable interpretations based on those relations cited in this

report lead to the following general conclusions.

1. Lower Paleozoic sediments of the western assemblage were deposited along the continental margin, not

directly on oceanic crust but concordantly on a thick substrate of terrigenous sediments. The substrate includes Proterozoic sandstones, Lower Cambrian shallow-water quartzite and shale, and Middle to Upper Cambrian detrital limy

turbidites.

2. In mid-Frasnian time the deep-water deposits of the

western assemblage emerged and were eroded as deeply as

the Lower Ordovician. The Frasnian age of emergence is

determined by the age of the oldest siliceous detrital sediments derived from the western assemblage—the lower beds

of the Pilot Shale in the foreland basin. Structural evidence

that this tectonic event, the Antler orogeny, involved intense

folding and thrust faulting is notably scarce.

3. Immediately after uplift, part of the elevat

assemblage terrane began to subside below sea 

became the site of deposition of the overlap assem

oldest deposits, identified in the southern Ind

Mountains and the Adobe Range, are of Frasnian 

were deposited concordantly on Silurian and O

strata. Elsewhere, deposits of Famennian and Mi

ages lie generally concordantly on eroded remna

western assemblage. Such deposits comprise the l

of the Havallah and Schoonover sequences and 

Formation. Sediments composing the overlap as

and foreland basin deposits, were derived from a

western assemblage that remained elevated. Basa

stone is a significant component of the overlap ass

all stratigraphic levels.

4. In Middle Pennsylvanian time the western 

rane and overlying Upper Devonian to Lower Pen

strata of the overlap assemblage were elevated, m

deformed, and deeply eroded. Whether the de

involved folding or tilting is uncertain in most 

there is no evidence of thrust faulting. Subseque

ence resulted in detrital deposits of Middle Pennsy

Permian age lying directly on Frasnian to Lower P

nian sequences of the overlap assemblage, Ord

Devonian sequences of the western assemblag

Cambrian substrate of the western assemblage. Mi

sylvanian tectonism in Nevada was a small part o

tal-scale tectonism, including the Ouachita oro

formation of the ancestral Rocky Mountains. Mi

sylvanian to Permian deposits of the western fac

are a small part of the regional blanket that cover

Nevada and includes sequences such as the Oquir

and the Wood River of Idaho.

5. Beginning in the Jurassic and ending in th

the entire region was strongly folded and thrust fau

the exceptions of moderate folding or tilting in t

Pennsylvanian, and Tertiary extension, defor

almost all areas of the western assemblage date

Jurassic to Eocene interval—a conclusion reach

Dover (1980) with respect to the western facies 

Idaho and by Ketner (1986; and with Noll, 1987) w

to the western facies terrane in northern Mexico.

If the above conclusions are valid, then the 

the Antler orogeny is reduced to a question of verti

ics. And if the abundance of basaltic greenstone in

with sediments throughout the Proterozoic to Pen

interval indicates tensional stress, then the Antle

and Pennsylvanian deformation must have taken p

generally extensional conditions. Subduction an

with island arcs are not indicated or required, but th

of origin remains. Any hypothesis of origin of 

orogeny must deal with a remarkable coincidence i

space: the Alamo megabreccia said to have been c

meteor impact in or near western Nevada (Warme 

berg, 1996) was formed in mid-Frasnian time at 
REFERENCES CITED 

same time as the onset of the Antler orogeny. Similar megabreccias were formed at the same time in Alberta (Cook and

others, 1972). Were the Alamo and other megabreccias

effects of the Antler orogeny and not the result of impact? If

there actually was a meteor impact in the mid-Frasnian, did

it trigger the Antler orogeny? In any event, the old model for

the tectonic development of the western facies terrane is outdated and should be replaced by a new model that accords

with field relations—Late Devonian to Early Pennsylvanian

uplifts and depressions without contraction; middle Pennsylvanian deformation and uplift followed by regional depression; intense Jurassic to Eocene folding and thrusting;

Tertiary and Quaternary extension.

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Manuscript approved for publication October 1, 1997

Published in the Central Region, Denver, Colorado

Graphics by Dick Walker

Photocomposition by L.M. Carter

Edited by L.M. Carter

On-line version by Joan G. Nadeau