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Paleozoic and Mesozoic Rocks of Mount Ichabod 

and Dorsey Canyon, Elko County, Nevada 

Evidence for Post-Early Triassic Emplacement of 

the Roberts Mountains and Golconda Allochthons

By KEITH B. KETNER, BENITA L. MURCHEY, ROBERT G. STAMM, and 

BRUCE R. WARDLAW

EVOLUTION OF SEDIMENTARY BASINS EASTERN GREAT BASIN 

HARRY E. COOK and CHRISTOPHER J. POTTER, Project Coordinators

U.S. GEOLOGICAL SURVEY BULLETIN 1988-D

A multidisciplinary approach to research studies of sedimentary 

rocks and their constituents and the evolution of 

sedimentary basins, both ancient and modern

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1993
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

For sale by

USGS Map Distribution

Box 25286, Building 810

Denver 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

Paleozoic and Mesozoic rocks of Mount Ichabod and Dorsey Canyon, Elko County, Nevada : evidence for post-early Triassic emplacement of the Roberts Mountains and Golconda Allochthons / by Keith B. Ketner ... [et al.]

p. cm. (Evolution of sedimentary basins Eastern Great Basin : ch. D.) (U.S. Geological Survey bulletin : 1988-D)

"A multidisciplinary approach to research studies of sedimentary basins, both ancient and modern."

Includes bibliographical references.

Supt.ofDocs.no.: I28.3:1988D

1. Geology, Stratigraphic Paleozoic. 2. Geology, Stratigraphic Mesozoic. 3. Geology Nevada Elko 

County. 4. Paleontology Nevada Elko County. I. Ketner, Keith Brindley, 1921- . II. Series. III. Series: U.S. 

Geological Survey bulletin : 1988-D. 

QE75.B9 1988-D 

[QE654]

557.3 s dc20 93-17176 

[551.7'2'0979316] ' CIP
CONTENTS

Abstract................................................................................................................................. Dl

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

Descriptions and Ages of Map Units.................................................................................... 3

Roberts Mountains Allochthon................................................................................... 3

Valmy Formation.................................................................................................. 3

Silurian Strata....................................................................................................... 3

Overlap Assemblage................................................................................................... 6

Golconda Allochthon.................................................................................................. 7

Havallah Sequence................................................................................................ 7

Dolly Creek Sequence ................................................................................................ 8

Tertiary and Quaternary Rocks................................................................................... 10

Principal Structural Features................................................................................................. 10

Post-Early Triassic Contraction.................................................................................. 10

Paleogene Extension................................................................................................... 10

Neogene Extension..................................................................................................... 10

Discussion............................................................................................................................. 11

References Cited................................................................................................................... 11

FIGURES

1,2. Maps showing geology of:

1. Bruneau River region.......................................................................................................................................... D2

2. Mount Ichabod area............................................................................................................................................ 4

3. Diagrammatic cross section showing structure of Mount Ichabod area..................................................................... 6

4. Map showing geology of Dorsey Canyon area........................................................................................................... 6

TABLE

1. Age range of fossil collections in and near the Mount Ichabod and Dorsey Canyon map areas............................... D8

III

Paleozoic and Mesozoic Rocks of

Mount Ichabod and Dorsey Canyon,

Elko County, Nevada Evidence for

Post-Early Triassic Emplacement of the

Roberts Mountains and

Golconda Allochthons

By KEITH B. KETNER, BENITA L. MURCHEY, ROBERT G. STAMM, and BRUCE R. WARDLAW

ABSTRACT

In the Mount Ichabod-Dorsey Canyon area of northeastern Nevada the Roberts Mountains allochthon, the 

overlap assemblage, and the Golconda allochthon are tectonically interlayered with Lower Triassic rocks. The 

Roberts Mountains allochthon consists of the Ordovician 

Valmy Formation, composed of quartzite, bedded chert, 

shale, and argillite, and unnamed Silurian siltstone and 

chert. The overlap assemblage, a Pennsylvanian and Permian sequence of conglomerate, sandstone, siltstone, limestone, and bedded chert, overlies the Roberts Mountains 

allochthon unconformably, and parts of it are in thrust 

fault contact with other structural plates. The Golconda 

allochthon is composed of the Mississippian to Permian 

Havallah sequence, a rootless, internally faulted and 

folded mass of siltstone, argillite, bedded chert, and redeposited limestone. Lower Triassic rocks, here termed the 

Dolly Creek sequence, are deep-water deposits of limy 

shale and redeposited argillaceous limestone. The significance of the Mount Ichabod-Dorsey Canyon area is in the 

unequivocal evidence, afforded by the tectonic interlayering, that both the Golconda and Roberts Mountains allochthons were emplaced after Early Triassic time.

INTRODUCTION

The Bruneau River region (fig. 1), of which Mount 

Ichabod and Dorsey Canyon are parts, is a tract of hills 

and canyons in northeastern Nevada just south of the 

Idaho State line between the Jarbidge Mountains on the 

east and the Independence and Bull Run Mountains on the 

west. A large part of the region was first studied from

1952 to 1954 as doctoral dissertations by K.O. Bushnell 

and J.R. Coash, and their maps and reports were later published as bulletins of the Nevada Bureau of Mines 

(Bushnell, 1967; Coash, 1967). Revised versions of the 

geology as described in these bulletins were incorporated 

into the geologic maps of Elko County (Coats, 1987) and 

Nevada (Stewart and Carlson, 1978). Most of the sedimentary rocks in the region are deep-water deposits 

almost devoid of identifiable macrofaunas, and, as a consequence, the age ranges of most of the stratigraphic units 

are difficult to determine and some are still uncertain. 

Complex structural relations reflect contractional and 

extensional tectonic events and multiple igneous intrusive 

events. Consequently, although both the published maps of 

Coash and Bushnell and the later revisions are useful, they 

should be regarded as preliminary and used with great 

caution. Recent studies of parts of the Bruneau River 

region (Little, 1987; Schwarz and others, 1991) resulted in 

improved concepts but leave many important questions 

unanswered. The present study of the Bruneau River 

region was begun in 1988 as a long-term effort to definitively date the stratigraphic units, determine their depositional environments, and provide an improved 

interpretation of the complex structural relations. A geologic map of the Mount Ichabod area at a scale of 

1:24,000 (Ketner, 1992) and this report on the Mount 

Ichabod-Dorsey Canyon area are the first results of that 

effort.

The early Paleozoic Roberts Mountains allochthon, 

the overlap assemblage, and the late Paleozoic Golconda 

allochthon all are present in the Mount Ichabod-Dorsey 

Canyon area. As commonly understood, the Roberts 

Mountains allochthon is a tectonic assemblage composed 

of lower Paleozoic, deep-water, mainly siliceous, sedimentary rocks and volcanic rocks and is underlain by the
D2 EVOLUTION OF SEDIMENTARY BASINS EASTERN GREAT BASIN

115°30'

60009 771004 71015

60010 S *.71013 !

41°30' -

10 MILES

15 KILOMETERS
PALEOZOIC AND MESOZOIC ROCKS OF MOUNT ICHABOD AND DORSEY CANYON, NEVADA D3

EXPLANATION

Volcanic and sedimentary rocks, undivided (Quaternary and 

Tertiary) Jarbidge Rhyolite, and unnamed lava flows, tuff, 

ignimbrite, alluvium, colluvium, lake deposits

Intrusive rocks (Cretaceous) Quartz monzonite stock

Intrusive rocks (Jurassic) Granodiorite and quartz monzonite stock

Havallah sequence (Permian, Pennsylvanian, and Mississippian) 

Limy siltstone, fine-grained sandstone, bedded chert, argillite, and 

limestone; principal component of Golconda allochthon

Sedimentary and volcanic rocks (Permian to Protcrozoic) 

Contact

72063 Fossil locality, numbers refer to table 1

Figure 1 (above and facing page). Geology of the Bruneau 

River region, northeastern Nevada. Locations of map areas of 

figures 2 and 4 are also shown.

Roberts Mountains thrust fault. The allochthon generally 

is considered to be linked with the Antler orogeny and to 

have been emplaced in latest Devonian to earliest 

Mississippian time (Roberts and others, 1958; Johnson and 

Pendergast, 1981).

Upper Paleozoic sequences that disconformably or 

unconformably overlie the Roberts Mountains allochthon 

are conventionally termed the overlap assemblage (Roberts and others, 1958). In the Mount Ichabod-Dorsey 

Canyon area this assemblage is of Late Pennsylvanian and 

Permian age and consists, from the base upward, of conglomerate, limestone, siltstone and sandstone, and bedded 

chert. In some other areas of the region, the lowermost 

beds are of Mississippian age, and the depositional order 

of lithic units differs somewhat from that in the Mount 

Ichabod and Dorsey Canyon areas.

As commonly understood the Golconda allochthon is 

a tectonic assemblage of upper Paleozoic, deep-water, 

mainly siliceous, sedimentary rocks and volcanic rocks 

and is underlain by the Golconda thrust fault. The allochthon is generally considered to be linked with the Sonoma 

orogeny and to have been emplaced in Late Permian or 

Early Triassic time (Silberling and Roberts, 1962).

In the Mount Ichabod area, the Roberts Mountains 

and Golconda allochthons and slices of the overlap assemblage are tectonically interlayered with the Lower Triassic 

Dolly Creek sequence (figs. 2, 3). In this report we 

describe the strata comprising the allochthons and the Triassic rocks and present evidence that the Paleozoic rocks 

were emplaced simultaneously by contractional faulting in 

post-Early Triassic time.

Fossil collections are described in table 1, on pages 8 

and 9.

Acknowledgments. Robert R. Coats and his assistants collected all paleontological samples numbered 

above 58000 (Coats, 1986). These were examined and

dated by the following former members of the U.S. 

Geological Survey: R.C. Douglass, Mackenzie Gordon, 

Jr., W.H. Mass, J.W. Huddle, and E.L. Yochelson. Three 

additional collections were examined and dated by C.A. 

Sandberg of the U.S. Geological Survey. Suggestions by 

Edward A. Johnson and Norman J. Silberling, both of the 

U.S. Geological Survey, greatly improved the content and 

style of this report.

DESCRIPTIONS AND AGES OF 

MAP UNITS

ROBERTS MOUNTAINS ALLOCHTHON 

VALMY FORMATION

Exposed parts of the Valmy Formation in the Mount 

Ichabod-Dorsey Canyon area are composed of orthoquartzite, bedded chert, argillite, and shale/ The quartzite 

is composed almost entirely of silica-cemented mediumto coarse-grained quartz sand. Beds are commonly 3-9 ft 

(1-3 m) thick and lack such sedimentary structures as 

grading, sole marks, or cross stratification that might 

reveal some aspect of depositional conditions. Chert beds 

are 1 in.-l ft (2.5-30 cm) thick and tend to be dark gray 

or black. Shale and argillite, interbedded with quartzite 

beds, are black and commonly are silty and sandy.

Middle Ordovician conodonts were recovered from 

the Valmy Formation about 1.5 mi (2.4 km) northwest of 

the Dorsey Canyon map area (fig. 1). Fossils were not 

found in the parts of the Valmy exposed within the Mount 

Ichabod and Dorsey Canyon map areas, and the identity of 

the formation in these areas is based entirely on lithic similarities with Valmy strata of known age elsewhere. The 

identification is considered secure, however, because in no 

other sequence are pure quartzite, bedded chert, and shale 

or argillite so closely associated. The exposed part of the 

Valmy at Mount Ichabod is probably several hundred feet 

thick, but the base and top of the formation are not 

exposed and internal faulting prevents an accurate 

measurement of its thickness. In the Rowland area, 20 mi 

(32 km) north of Mount Ichabod, where the Valmy is better exposed, the Valmy is at least 5,000 ft (1,524 m) thick.

SILURIAN STRATA

Micaceous, feldspathic siltstone and light-colored 

bedded chert are associated with the Valmy Formation in 

both the Mount Ichabod and Dorsey Canyon areas. The 

siltstone displays graded bedding, abundant trace fossils 

on bedding surfaces, and weathers to tones of greenish 

gray and tan. The chert is generally very light colored, in 

contrast to the dark grays and black of chert in the Valmy.
D4 EVOLUTION OF SEDIMENTARY BASINS EASTERN GREAT BASIN

115° 32'30"
PALEOZOIC AND MESOZOIC ROCKS OF MOUNT ICHABOD AND DORSEY CANYON, NEVADA D5

CORRELATION OF MAP UNITS

Mt. Ichabod area

Dorsey 

Canyon

QTu

Dolly 

Creek

QTu

Mason 

Creek

QTu

Mt. Ichabo 

horst

QTu

 QUATERNARY

> TERTIARY

 fide

PPo

Roberts 

Mountain 

illochtho

SOu

Golco

s 

n

\-fid Rd Lower Triassic TRIASSIC

nda allochthon

PMh

i»

a

PPo

I PC

Fault

PPo

Fault

Roberts Roberts 

/fountains Mountains 

llochthon allochthon

Su

Fault

Ov Ov

 Lower Permian

Upper to Lower 

Pennsylvanian

Upper 

Mississippian

 PERMIAN 

 PENNSYLVANIAN 

 MISSISSIPPIAN

 SILURIAN 

 ORDOVICIAN

DESCRIPTION OF MAP UNITS

Volcanic and sedimentary rocks, undivided (Quaternary and 

Tertiary) Jarbidge Rhyolite and unnamed lava flows, tuff, 

ignimbrite, alluvium, colluvium, lake deposits

Dolly Creek sequence (Lower Triassic) Principally thinly 

laminated deep-water deposit of limy, argillaceous siltstone, limy 

sandstone, and silty, argillaceous limestone. Also locally includes 

a conglomeratic facies (Tide)

Heterogeneous conglomerate (Permian) Principally boulder 

conglomerate; lesser amounts of sandstone, siltstone, and bedded 

chert; boulders derived from Valmy Formation, overlap 

assemblage, and Permian phosphatic rocks; presence of Permian 

phosphatic clasts indicates that unit is younger than the overlap 

assemblage

Overlap assemblage (Permian and Upper Pennsylvanian) 

Complete sequence consists, in order, of basal conglomerate, 

limestone, sandstone and siltstone, and phosphatic bedded chert

Havallah sequence (Permian, Pennsylvanian, and Mississippian) 

Limy siltstone, fine-grained sandstone, bedded chert, argillite, 

and limestone; principal component of Golconda allochthon

Su Siltstone and chert undivided (Silurian) Upper micaceous 

feldspathic siltstone unit of Middle(?) Silurian age and 

lower light-colored bedded chert unit of Early(?) Silurian age

Valmy Formation (Ordovician) Thick-bedded medium- to 

coarse-grained quartzite, bedded chert, black shale, argillite

Contact

 7000 Structure contour Numbers are feet above sea level

Section corner

- High-angle fault U, upthrown side; D, downthrown side; 

 dotted where concealed

 ^^* Thrust fault Sawteeth on upper plate

,25 Strike of beds showing degree of dip

12715* Fossil locality Number refers to table

Figure 2 (above and facing page). Geology of the Mount Ichabod area. The Havallah sequence, representing the Golconda allochthon, 

and rocks of the Roberts Mountains allochthon overlie Lower Triassic rocks on a thrust fault of low relief. Mount Ichabod is a horst that 

displays strata underlying the Triassic rocks. Inset map shows structure contours of the thrust fault between Paleozoic and underlying 

Triassic rocks. Geologic and structure contour maps modified from Ketner (1992).
D6 EVOLUTION OF SEDIMENTARY BASINS EASTERN GREAT BASIN

Mount 

Ichabod

Dolly 

Creek

Figure 3. Diagrammatic cross section showing the structure of 

the Mount Ichabod area as inferred from the geologic relations 

shown in figure 2 and from regional relations. Strati graphic units 

and their letter symbols same as those for figure 2. Not to scale.

Both the chert and the siltstone units are assigned a 

Silurian age on the basis of lithic resemblance to paleontologically dated strata of Early and Middle Silurian age in 

the Adobe Range, 30 mi (48 km) to the south (Ketner and 

Ross, 1990), and elsewhere (Ketner, 1991). The correlation is considered quite reliable because no other abundantly micaceous, feldspathic siltstone associated with 

light-colored, nonphosphatic chert is known in the region. 

The thickness of the Silurian strata is unknown because 

neither the top nor the base is exposed and because of 

complex internal faulting.

115° 45' 115° 45'30"

T42N 

T41 N

40° 

27' 

30"

t

QTu

Unconformity 

13551

-I

1 KILOMETER

OVERLAP ASSEMBLAGE

Conventionally, upper Paleozoic sequences that nonconformably overlie the Roberts Mountains allochthon 

collectively are called the overlap assemblage. The overlap assemblage can range in age from Early Mississippian 

to Permian, but commonly, as in the Dorsey CanyonMount Ichabod area, it ranges from Late Pennsylvanian 

into Permian.

In both the Mount Ichabod and Dorsey Canyon areas 

(figs. 2, 4), strata of the Roberts Mountains allochthon are 

overlain unconformably by an Upper Pennsylvanian and 

Permian sequence that consists, from base to top, of conglomerate, limestone, sandstone and siltstone, and bedded 

chert. In both areas, however, the rocks are disrupted by 

low- and high-angle faults, and the original succession 

must be pieced together from fragments. On Mount Ichabod, the unconformity, basal conglomerate, and limestone 

are well exposed, but the siltstone and sandstone strata are 

poorly represented and the chert unit is missing; near 

Mason Creek, the limestone and chert units are missing 

and the conglomerate is superposed tectonically on the 

siltstone-sandstone unit. At Dorsey Canyon, the unconformity is exposed and the conglomerate, siltstone and

.. 

D

_LT ° 

61 107

EXPLANATION

Volcanic and sedimentary rocks, undivided (Quaternary and 

Tertiary) Jarbidge Rhyolite, and unnamed lava flows, tuff, 

ignimbrite, alluvium, colluvium, lake deposits

Overlap assemblage (Permian and Upper Pennsylvanian) 

Complete sequence consists, in order, of basal conglomerate, 

limestone, sandstone and siltstone, and phosphatic bedded chert

Unnamed strata (Silurian) and Valmy Formation (Ordovician) 

undivided Micaceous, feldspathic siltstone of Middle(?) Silurian 

age, bedded chert of Early(?) Silurian age; quartzite, bedded chert, 

shale, and argillite of Ordovician age

Contact

Section corner

High-angle fault U, upthrown side; D, downthrown side; dotted 

where concealed

Strike of beds showing degree of dip 

Fossil locality Number refers to table 1

Figure 4. Geology of Dorsey Canyon area. The Roberts Mountains allochthon is overlain unconformably by strata of the overlap 

assemblage displaying, in the western exposure, the basal unconformity and, in the eastern exposure, the gradational contact between 

the siltstone and sandstone unit and the overlying bedded chert unit.
PALEOZOIC AND MESOZOIC ROCKS OF MOUNT ICHABOD AND DORSEY CANYON, NEVADA D7

sandstone, and chert units are well represented, but the 

limestone unit is absent. Clasts in the conglomerate are 

composed of quartzite and chert, both evidently derived 

from components of the underlying Roberts Mountains 

allochthon. The siltstone and sandstone unit is composed 

mainly of quartz and chert grains.

In the northwestern corner of the Mount Ichabod 

map area (fig. 2) is a small outcrop of heterogeneous 

conglomerate (unit PC) whose contact with underlying 

units is not exposed. It is unique in its lithic composition 

and totally unlike any other stratigraphic unit in the Bruneau River region. Principal components are boulder conglomerate, sandstone, siltstone, and bedded chert. 

Boulders were derived from phosphatic Permian rocks, 

from the Pennsylvanian part of the overlap assemblage, 

and from the Valmy Formation. The bedded chert yielded 

radiolarians whose age range is Pennsylvanian to Permian 

(Wolfcampian), but the presence of boulders rich in phosphate suggests that the formation is younger than Wolfcampian. Richly phosphatic strata in the region are 

commonly of Leonardian or Guadalupian age. The unit 

does not resemble any known Triassic rocks. We interpret the paleontological and lithic data to indicate a Permian, rather than Triassic, age for the unit, and we regard 

the unit as an allochthonous facies of the overlap 

assemblage.

In spite of the lack of an exposed limestone unit at 

Dorsey Canyon, the overlap assemblage is more complete 

there than elsewhere in the Mount Ichabod-Dorsey 

Canyon area. Moreover, both the depositional basal contact and the gradational transition from the siltstone and 

sandstone unit to the chert unit are exposed. The exposure 

of a gradational transition from siltstone and sandstone to 

overlying chert is important because it solves a stratigraphic problem of long standing. Coats and Gordon 

(1972) discussed two stratigraphic units in the Dorsey 

Canyon area: a siltstone and sandstone unit termed the 

Edna Mountain Formation that they believed was allochthonous, and a chert unit termed the Phosphoria Formation 

that they believed was autochthonous. Actually, these 

units are in stratigraphic continuity and constitute the 

upper two stratigraphic divisions of the overlap assemblage near Dorsey Canyon.

The overlap assemblage in the Mount IchabodDorsey Canyon area is Late Pennsylvanian and Permian in 

age. Fusulinids from limestone on Mount Ichabod are Late 

Pennsylvanian (R.C. Douglass, oral commun., 1988), and 

brachiopods and molluscs from the siltstone-sandstone 

unit are Permian (Coats and Gordon, 1972; Coats, 1986).

The presence of abundant meandering trace fossils on 

bedding surfaces of the siltstone-sandstone unit could indicate a depositional environment of relatively deep water, 

but some strata of this unit also contain a sporadic macrofauna of large whole brachiopods and molluscs in a silty 

or sandy matrix that indicates a well-oxygenated and probably relatively shallow water environment. However the 

paleontological evidence is interpreted, the overlap

assemblage includes basal conglomerate composed of 

large clasts identical in composition to unconformably 

underlying strata that indicates an environment above 

wave base. This sequence is in contrast to the Havallah 

sequence in which beds approximately contemporaneous 

with the basal conglomerate of the overlap assemblage 

consist of bedded chert and fine-grained detrital rocks that 

commonly are indicative of a deeper water environment.

The overlap assemblage is estimated to be several 

hundred feet thick in the study area, but an accurate measurement is impossible because of the presence of lowand high-angle faults.

GOLCONDA ALLOCHTHON

In this report, the tectonic term "Golconda allochthon" is used to describe a tract of rootless, complexly 

deformed, siliceous, dominantly deep water, sedimentary 

rocks of late Paleozoic (Mississippian to Permian) age. 

Upper Devonian strata in the allochthon elsewhere in 

northern Nevada have been reported (Miller and others, 

1984). Volcanic rocks are not important constituents of the 

allochthon in the Mount Ichabod area as they are elsewhere. "Havallah sequence" is a stratigraphic term used to 

describe the principal component of the allochthon; however, slices of the overlap assemblage in low-angle fault 

contact within or above the Havallah also can be regarded 

as part of the allochthon. Parts of the Havallah and the 

overlap assemblage are lithically identical, and in some 

places it is impossible to determine whether isolated outcrops should be assigned to the Havallah or to the overlap 

assemblage.

HAVALLAH SEQUENCE

In the vicinity of Mount Ichabod, the Havallah 

sequence is composed mainly of yellow-weathering siltstone and fine-grained sandstone, but dark-gray to black 

bedded chert and argillite are important constituents. Only 

a few very thin lenses of limestone were observed in the 

vicinity of Mount Ichabod, although most of the siliceous 

detrital beds are somewhat calcareous. In contrast, thick 

limestone turbidites and debris flows are prominent constituents in more northerly outcrops of the Havallah 

sequence. Typically the siltstone and fine-grained sandstone are composed mainly of angular clasts of quartz and 

lesser proportions of detrital calcite and dolomite rhombs. 

Detrital chert, microcline, plagioclase, muscovite, siliceous 

spicules, and phosphate are sporadically present in small 

amounts.

Many beds classed as chert in outcrop are actually 

detrital rocks as seen in thin sections, consisting almost 

entirely of siliceous spicules of mainly fine sand size. 

Minor constituents of these rocks are quartz sand, dolomite rhombs, clastic calcite, clastic phosphate, and
D8 EVOLUTION OF SEDIMENTARY BASINS EASTERN GREAT BASIN

Table 1. Age range of fossil collections in and near the Mount Ichabod and Dorsey Canyon map areas, Elko County, 

Nevada.

[C, conodont; R, radiolarian; B, brachiopod; M, mollusc; F, fusulinid; G, gastropod, see acknowledgments for discussion of specialists]

Sample

number

Kind

of fossil Age range of collection Specialist

VALMY FORMATION, DORSEY CANYON VICINITY (FIG. 1)

70139 C Middle Ordovician Huddle

OVERLAP ASSEMBLAGE, MOUNT ICHABOD (FIG. 2)

12722

12771

12770

12931

71076

F

C

R

R

C

Late Pennsylvanian

Early Middle Pennsylvanian (in clast)

Pennsylvanian to Permian (Wolfcampian)

Pennsylvanian to Permian (Wolfcampian)

Pennsylvanian to Permian

Douglass

Wardlaw

Murchey

Murchey

Huddle

OVERLAP ASSEMBLAGE, DORSEY CANYON (FIG. 4)

13551

61107

62013

62037

62038

62302

BM

BM

BM

BM

BM

BM

Permian

Permian

Permian

Permian

Permian

Permian

Gordon

Gordon

Gordon

Gordon

Gordon

Gordon

HAVALLAH SEQUENCE, MOUNT ICHABOD MAP AREA (FIG. 2)

12715

12716

12717

12718

12739

12750

12751

12752

12760

12762

12764

12805

12810

12918

12924

12991

C

R

C

R

R

R

R

R

R

C

C

R

R

R

R

R

Early Pennsylvanian

Middle Pennsylvanian

Late Middle or Early Pennsylvanian

Middle Pennsylvanian

Pennsylvanian to Permian (Wolfcampian)

Pennsylvanian to Permian (Wolfcampian)

Pennsylvanian to Permian (Wolfcampian)

Permian (Leonardian)

Permian (Leonardian)

Mississippian (Chesterian)

Mississippian

Pennsylvanian to Permian (Wolfcampian)

Pennsylvanian to Permian (Wolfcampian)

Early Permian (probably Leonardian

Middle Pennsylvanian

Pennsylvanian to Permian (Wolfcampian)

Sandberg

Murchey

Sandberg

Murchey

Murchey

Murchey

Murchey

Murchey

Murchey

Wardlaw

Wardlaw

Murchey

Murchey

Murchey

Murchey

Murchey

DOLLY CREEK SEQUENCE, MOUNT ICHABOD (FIG. 2)

12723

12755

12756

C

C

C

Latest Permian (presumably in clasts)

Earliest Triassic (Griesbachian)

Early Middle Pennsylvanian (in clast)

Sandberg

Wardlaw

Wardlaw

phosphatic pellets. Such beds are commonly of Permian 

age. Chert beds without detrital components and characterized by scattered radiolaria are possibly of hydrothermal 

origin (Ketner, 1991) and are more commonly of Mississippian or Pennsylvanian age.

The limestone lenses, very thin and sparse in the 

Mount Ichabod area, consist, in part, of brachiopod, pelecypod, and crinoidal debris. Other exposures of Havallah 

limestone in the Bruneau River region are extensive debris 

flows consisting of bioclastic debris and a significant proportion of radiolarian-bearing chert clasts, apparently 

derived ultimately from the Roberts Mountains allochthon. 

The source of the detrital carbonate component could have 

been the shelf area bordering the western margin of the 

Antler orogen, as suggested by Sando (1992).

The age range of the Havallah sequence in the vicinity of Mount Ichabod, based on numerous collections of 

conodonts and radiolarians, is late Early Mississippian to 

mid-Permian. The stratigraphic thickness of the sequence 

is impossible to determine because of structural complexity and poor exposures.

DOLLY CREEK SEQUENCE

The Dolly Creek sequence is composed of two distinct facies of detrital rocks: a relatively fine grained 

facies derived from a distant source to the east and a relatively coarse grained debris-flow facies apparently derived 

from nearby exposures of the Valmy Formation and the
PALEOZOIC AND MESOZOIC ROCKS OF MOUNT ICHABOD AND DORSEY CANYON, NEVADA D9

Table 1. Age range of fossil collections in and near the Mount Ichabod and Dorsey Canyon map areas, Elko County, 

Nevada Continued.

Sample 

number

Kind 

of fossil Age range of collection Specialist

HAVALLAH SEQUENCE, NEAR THE MOUNT ICHABOD MAP AREA (FIG. 1)

12953

12968

12972

13011

13014

13015

13016

13027

13028

58056

60009

60010

60011

61029

62301

69113

69114

69117

69118

69119

71004

71011

71013

71015

71021

71022

71024

71027

71070

71097

72035

72037

72040

72043

72063

72064

72150

72151

R

R

R

C

C

C

C

C

C

C

F

F

C

G

C

C

C

C

C

C

C

B

B

C

C

C

C

C

C

C

C

C

C

C

F

C

C

C

Pennsylvanian to Permain (Wolfcampian)

Early Permian (Leonardian)

Mississippian (probably Late)

Late Mississippian (late Chesterian)

Latest Meramecian to early late Chesterian

Mississippian (early late Chesterian)

Pennsylvanian (Atokan)

Late Mississippian (early late Chesterian)

Mississippian (late Osagean to Chesterian)

Late Mississippian.

Middle Pennsylvanian (Desmoinesian)

Middle Pennsylvanian

Middle Pennsylvanian to Permian

Latest Pennsylvanian to Permian

Late Mississippian

Early Pennsylvanian

Early Pennsylvanian

Late Mississippian

Late Mississippian

Late Mississippian

Early Pennsylvanian

Mississippian

Mississippian.

Pennsylvanian to Permian

Late Mississippian to Early Pennsylvanian(?)

Late Mississippian

Late Mississippian

Pennsylvanian to Permian

Early Pennsylvanian

Late Mississippian

Early Pennsylvanian

Early Pennsylvanian

Early Pennsylvanian

Early Pennsylvanian

Late Pennsylvanian

Early Pennsylvanian

Early Pennsylvanian to Permian

Late Mississippian

Murchey

Murchey

Murchey

Stamm

Stamm

Stamm

Stamm

Stamm

Stamm

Hass

Douglass

Douglass

Huddle

Yochelson

Huddle

Huddle

Huddle

Huddle

Huddle

Huddle

Huddle

Gordon

Gordon

Huddle

Huddle

Huddle

Huddle

Huddle

Huddle

Huddle

Huddle

Huddle

Huddle

Huddle

Douglass

Wardlaw

Huddle

Huddle

overlap assemblage to the west. The coarse-grained facies 

merges laterally with strata of the fine-grained facies in 

the Mount Ichabod area, and alternating beds of the two 

facies are exposed in an area of 0.04 mi2 (0.1 km2) in one 

of the tributaries of Dolly Creek. The easterly derived 

fine-grained facies is also exposed in the Adobe Range 30 

mi (48 km) to the south (Ketner and Ross, 1990), but 

there easterly derived coarse debris flows are present in 

the fine-grained facies and westerly derived debris is 

absent.

The fine-grained facies is composed primarily of limy 

siltstone and silty limestone. Limy sandstone is a minor 

constituent. The facies displays thin lamination, graded

bedding, and, rarely, sole marks. Most of the clasts that 

are coarse enough to be identifiable are composed of 

debris of brachiopods, corals, pelmatozoans, and other 

shelly organisms. Many clasts are composed of spiculitic, 

phosphatic chert, phosphorite, and fine-grained quartz 

sandstone. Most of the clasts observed in the field and in 

thin section suggest a shallow-water provenance terrane. 

Reworked Permian conodonts and the spiculitic and phosphatic clasts suggest that the provenance terrane was at 

least partly of Permian age. In the Adobe Range, debris 

flows in the fine-grained turbidite facies include fossiliferous boulders, whose lithic features and fauna clearly indicate that they were derived from the Gerster Formation.
DIG EVOLUTION OF SEDIMENTARY BASINS EASTERN GREAT BASIN

The Gerster is a Permian shallow-water shelf deposit of 

the Nevada-Utah border area.

The coarse debris-flow facies near Mount Ichabod is 

composed of structureless masses and eastward-thinning 

beds of conglomerate. The structureless masses include 

very large clasts of Valmy quartzite, one of which is at 

least 10 ft (3 m) in diameter. One large limestone clast 

contains, in addition to a shelly macrofauna, conodonts of 

Middle Pennsylvanian age. Its size, age, and lithic composition indicate that it was derived from the local overlap 

assemblage.

Conodonts collected from the fine-grained turbidite 

facies where it is interlayered with the debris-flow facies 

indicate an earliest Early Triassic (Griesbachian) age for 

both the conglomerate and the lower beds in the finegrained turbidite sequence. Because the Triassic rocks are 

detrital, it is possible that the conodonts were reworked. 

Ammonites from the same formation exposed in the 

Adobe Range indicate a middle Early Triassic age, and 

conodonts from large clasts in a similar unit in the northern Snake Mountains, 30 mi (48 km) to the east of Mount 

Ichabod, indicate a latest Early Triassic age. The age 

range of the Dolly Creek sequence in the Mount Ichabod 

area therefore could span the entire Early Triassic.

The depositional base and top of the Dolly Creek 

sequence are not exposed, but the formation must be at 

least 6,000 ft (1,829 m) thick, assuming there is no repetition of beds along concealed faults. It is unlikely that the 

exposed part of the unit is isoclinally folded because only 

one small fold nose and no large ones were observed. 

Moreover, the distinctive conglomerate facies is not 

repeated. The homoclinal dip of the sequence, however, 

suggests that the exposed part could be the limb of a very 

large fold.

TERTIARY AND QUATERNARY DEPOSITS

Paleogene ignimbrite, alluvium, and lake deposits are 

widely exposed in the region and these commonly dip 

about 25° to the northwest as a result of rotation on extensional faults. Locally, 4-5 mi (6-8 km) northwest of 

Mount Ichabod, Paleogene alluvium has been tilted about 

65° to the west. Neogene rocks include the ubiquitous 

middle Miocene Jarbidge Rhyolite, slightly older and 

younger volcanic strata, and alluvium and lake deposits. 

The Neogene deposits have been only slightly tilted by 

basin and range faulting. Quaternary deposits include alluvium along streams, colluvium, and landslides.

PRINCIPAL STRUCTURAL FEATURES

The Mount Ichabod area displays two principal structural styles: low-angle contractional faulting and highangle extensional faulting.

POST-EARLY TRIASSIC CONTRACTION

The emplacement of the Roberts Mountains and Golconda allochthons on Triassic rocks is the most visible 

display of low-angle, older-over-younger, contractionai 

faulting in the area, but the presence of Ordovician rocks 

in the Mount Ichabod horst shows that parts of the Roberts 

Mountains allochthon and of the overlap assemblage 

underlie as well as overlie the Triassic sequence (fig. 3). 

The imbrication resulted in thickening of the stratigraphic 

section, a certain indicator of contraction.

Two partly contemporaneous facies of upper Paleozoic sequences, originally more widely separated, were 

telescoped together and imbricated in the Mount Ichabod 

area by low-angle faulting: the relatively fine grained, 

more outboard Havallah sequence and the relatively coarse 

grained, more inboard overlap assemblage. This, too, rep^ 

resents contractional faulting. Extensional faulting would 

have moved the contemporaneous facies farther apart than 

they were originally. ;

Strong, large-scale folding is not clearly displayed in 

the Mount Ichabod-Dorsey Canyon area. Restoration of 

the prevalent northwest dip of the older Tertiary strata tp 

horizontal, however, increases the amount of dip of the 

Triassic strata to 60° or more to the east, suggesting that 

the Triassic strata had been strongly folded on a very large 

scale, another indication of contractional strain.

PALEOGENE EXTENSION

Evidence from the northern part of the study area, 

10-20 mi (16-32 km) north of Mount Ichabod (fig. 1), 

suggests that early Tertiary extension was an important 

tectonic event in at least part of the Bruneau River regionThere, metamorphosed Proterozoic and Cambrian quartzite 

intruded by large coarse-grained monzonitic stocks of Cretaceous age crop out extensively. The very coarse grained 

borders of these stocks indicate that the stocks cooled at 

great depth. Proterozoic and Cambrian strata, the oldest 

rocks exposed in the area, now are exposed at some of the 

highest elevations in the region. Moreover, Paleogene sedimentary and volcanic rocks have been rotated from the 

horizontal and dip toward their fault contact with the Proterozoic rocks at an angle of about 25°. These relations are 

typical of highly extended terranes and are interpreted to 

imply the presence of early Tertiary extensional low-angle 

faults in the northern half of the Bruneau River region, but 

no such faults have been certainly identified in the Mount 

Ichabod-Dorsey Canyon area.

NEOGENE EXTENSION

Late Tertiary high-angle extensional faults are definitely present in the Mount Ichabod-Dorsey Canyon area.
PALEOZOIC AND MESOZOIC ROCKS OF MOUNT ICHABOD AND DORSEY CANYON, NEVADA Dll

Among the most prominent are those flanking Mount 

Ichabod and Mason Mountain on the west. These are 

partly exposed, steeply west dipping, normal faults that 

cut the middle Miocene Jarbidge Rhyolite. Mount Ichabod 

is also bounded on the east by a high-angle, east-dipping 

normal fault. The bounding normal faults define Mount 

Ichabod as a horst that reveals the kind of strata underlying, and concealed by, the stratigraphic and structural 

units exposed in the adjacent grabens.

now comprising the Roberts Mountains allochthon and 

rapid erosion of Devonian, Silurian, and Upper Ordovician strata down to a uniform stratigraphic level, the 

Middle Ordovician. The emplacement of the Roberts 

Mountains allochthon at Mount Ichabod should not, 

therefore, be interpreted as reactivation of a hypothetical 

middle Paleozoic thrust fault. Evidence for middle 

Paleozoic folding or low-angle faulting has not been 

discovered in the Bruneau River region.

DISCUSSION

The Roberts Mountains allochthon is imbricated with 

the Dolly Creek sequence and therefore was emplaced 

after Early Triassic time, confirming the age of emplacement demonstrated earlier from evidence in the Adobe 

Range (Ketner and Ross, 1990). The Golconda allochthon, 

identified by its lithic composition and an abundance of 

paleontological data, is present in the Mount Ichabod area 

and by its structural position above the Dolly Creek 

sequence is proved to have been emplaced after Early Triassic time. The superposition of the allochthon on Lower 

Triassic rocks constitutes the most unambiguous evidence 

pertaining to the age of the Golconda thrust fault that 

exists anywhere. Although the surface on which both the 

Roberts Mountains allochthon (in part) and the Golconda 

allochthon lie is curved owing to original form or folding 

(fig. 2), the lack of a distinct break in the pattern of structure contours indicates that the allochthons probably lie on 

essentially the same fault surface. It would be an unlikely 

coincidence if they had happened to arrive at the same 

structural horizon at different times, and therefore we conclude that they probably were emplaced simultaneously. 

The most likely time of emplacement was during the Elko 

orogeny in Middle to Late Jurassic time (Thorman and 

others, 1991), but earlier or later times between the Middle Triassic and the Tertiary cannot be ruled out (Ketner 

and Alpha, 1992).

The question of whether emplacement of the Roberts Mountains allochthon in the Mount Ichabod-Dorsey 

Canyon area might represent reactivation of a middle 

Paleozoic Antler-age fault is answered in the negative 

by stratigraphic evidence from three areas: the northern 

part of the Bruneau River region (Ketner and others, 

1993a, b), the Mountain City area adjacent to it on the 

west (Coats, 1971; unpublished data), and the East 

Range of northwestern Nevada (Whitebread, 1978; 

unpublished data). In all three areas Lower Mississippian 

strata of the overlap assemblage lie disconformably on 

the quartzite member of the Valmy Formation. There is 

no structural evidence in these areas of strong folding or 

faulting between Ordovician and Mississippian time, but, 

rather, the evidence suggests widespread uplift of rocks

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Bushnell, K.O., 1967, Geology of the Rowland quadrangle, Elko 

County, Nevada: Nevada Bureau of Mines Bulletin 67, 38 p.

Coash, J.R., 1967, Geology of the Mount Velma quadrangle, Elko 

County, Nevada: Nevada Bureau of Mines Bulletin 68, 21 p.

Coats, R.R., 1971, Geologic map of the Owyhee quadrangle, Nevada-Idaho: U.S. Geological Survey Miscellaneous Geologic Investigations Map 1-665, scale 1:48,000.

 1986, Invertebrate and paleobotanical fossils collected in 

Elko County, Nevada: Nevada Bureau of Mines and Geology 

Open-File Report 86-1, 397 p.

1987, Geology of Elko County, Nevada: Nevada Bureau of

Mines and Geology Bulletin 101, 112 p.

Coats, R.R., and Gordon, Mackenzie, Jr., 1972, Tectonic implications of the presence of the Edna Mountain Formation in northern Elko County, Nevada: U.S. Geological Survey 

Professional Paper 800-C, p. C85-C94.

Johnson, J.G., and Pendergast, Anne, 1981, Timing and mode of 

emplacement of the Roberts Mountains allochthon, Antler 

orogeny: Geological Society of America Bulletin, v. 92, 

p. 648-658.

Ketner, K.B., 1991, Stratigraphy, sedimentology, and depositional 

conditions of lower Paleozoic western-facies rocks in northeastern Nevada, in Cooper, J.D., and Stevens, C.H., eds., 1991, 

Paleozoic paleogeography of the western United States-II: Pacific Section, Society of Economic Paleontologists and Mineralogists, v. 67, p. 735-746.

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Elko County, Nevada: U.S. Geological Survey Open-File Report 92-580, scale 1:24,000.

Ketner, K.B., and Alpha, A.G., 1992, Mesozoic and Tertiary rocks 

near Elko, Nevada Evidence for Jurassic to Eocene folding 

and low-angle faulting: U.S. Geological Survey Bulletin 

1988-C, 13 p.

Ketner, K.B., Ehman, K.D., Repetski, J.E., Stamm, R.G., and 

Wardlaw, B.R., 1993a, Paleozoic stratigraphy and tectonics in 

northernmost Nevada Implications for the nature of the Antler orogeny: Geological Society of America Abstracts with 

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1993b, Preliminary geological map of the Rowland-Bearpaw 

Mountain area, Elko County, Nevada: U.S. Geological Survey 

Open-File Report 93-220, scale 1:24,000.

Ketner, K.B., and Ross, R.J., Jr., 1990, Geologic map of the northern Adobe Range, Elko County, Nevada: U.S. Geological Survey Miscellaneous Investigations Map 1-2081, scale 1:24,000.
D12 EVOLUTION OF SEDIMENTARY BASINS EASTERN GREAT BASIN

Little, T.A., 1987, Stratigraphy and structure of metamorphosed 

upper Paleozoic rocks near Mountain City, Nevada: Geological Society of America Bulletin, v. 98, p. 1-17.

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: Geological Society of America Bulletin, v. 95, 

p.1063-1076.

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coral localities in Antler orogen, central Nevada: Geological 

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Programs, v. 24, no. 6, p. 60.

Schwarz, D.L., Snyder, W.S., and Hutter, T.J., 1991, Tectonostratigraphy of the Ordovician Tennessee Mountain Formation, Permian Sunflower Formation, and the Poorman Peak 

sequence, Wild Horse area, north-central Elko County,

Nevada, in Cooper, J.D., and Stevens, C.H., eds., 1991, Paleozoic paleogeography of the western United States-II: Pacific 

Section, Society of Economic Paleontologists and Mineralogists, v. 67, p. 357-369.

Silberling N.J., and Roberts, R.J., 1962, Pre-Tertiary stratigraphy 

and structure of northwestern Nevada: Geological Society of 

Nevada Special Paper 72, 53 p.

Stewart, J.H., and Carlson, I.E., 1978, Geologic map of Nevada: 

U.S. Geological Survey, scale 1:500,000.

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northeastern.Nevada, in Raines, G.L., Lisle, R.E., Schafer, 

R.W., and Wilkinson, W.H., eds., Geology and ore deposits of 

the Great Basin: Geological Society of Nevada Symposium, 

Proceedings, v. 1, p. 25-45.

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Published in the Central Region, Denver, Colorado 

Manuscript approved for publication January 28, 1993 

Graphics prepared by Mari L. Kauffmann and George Garcia 

Type composed by Shelly A. Fields 

Edited by Judith Stoeser

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Ketner and others PALEOZOIC AND MESOZOIC ROCKS OF MOUNT ICHABOD AND DORSEY CANYON, ELKO COUNTY, NEVADA