Isolated Carbonate Bodies Composed of 

Stacked Debris-Flow Deposits on a 

Fine-Grained Carbonate Lower Slope of 

Devonian Age, Antelope Peak, 

Elko County, Nevada

U.S. GEOLOGICAL SURVEY BULLETIN 1988-E

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Isolated Carbonate Bodies Composed of 

Stacked Debris-Flow Deposits on a 

Fine-Grained Carbonate Lower Slope of 

Devonian Age, Antelope Peak, 

Elko County, Nevada

By Peter M. Sheehan, John M. Pandolfi, and Keith B. Ketner

EVOLUTION OF SEDIMENTARY BASINS EASTERN GREAT BASIN 

Harry E. Cook and Christopher J. Potter, Project Coordinators

U.S. GEOLOGICAL SURVEY BULLETIN 1988-E

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 

Robert M. Hirsch, Acting 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

Sheehan, Peter M.

Isolated carbonate bodies composed of stacked debris-flow deposits on a fine-grained carbonate lower slope of Devonian age, 

Antelope Peak, Elko County, Nevada / by Peter M. Sheehan, John M. Pandolfi, and Keith B. Ketner.

p. cm. (U.S. Geological Survey bulletin ; 1988-E) 

(Evolution of sedimentary basins Eastern Great Basin ; ch. E)

Includes bibliographical references.

Supt.ofDocs.no.: I 19.3: 1988E

1. Rocks. Carbonate Nevada Elko County. 2. Geology, Stratigraphic Devonian. 3. Geology Nevada Elko County. 

I. Pandolfi, John M. II. Ketner, Keith Brindley, 1921- . III. Title. IV. Series. V. Series: Evolution of sedimentary 

basins Eastern Great Basin ; ch. E. 

QE75.B9 no. 1988-E 

[QE471.15.C3]

557.3 s dc20 93-3544 

[552'.58'0979316] CIP
CONTENTS

Abstract................................................................................................................................. El

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

Geologic Setting................................................................................................................... 2

Regional Geologic History................................................................................................... 3

The Carbonate Slope at Antelope Peak................................................................................ 4

Lower Part of the Roberts Mountains Formation....................................................... 4

Upper Part of the Roberts Mountains Formation....................................................... 5

Setting of the Roberts Mountains Formation.............................................................. 6

The Isolated Carbonate Bodies............................................................................................. 6

Origin of the Isolated Carbonate Bodies.............................................................................. 7

References Cited................................................................................................................... 9

FIGURES

1. Photographs showing carbonate body hosted in fine-grained, thin-bedded calcareous turbidites............................. E2

2. Map showing position of Silurian and Devonian carbonate-platform margin........................................................... 2

3. Sediment logs of slope deposits.................................................................................................................................. 4

4. Photographs showing calcareous slope turbidites....................................................................................................... 5

5. Sediment log of the largest carbonate body................................................................................................................ 7

6,7. Photographs showing:

6. Matrix-supported, disorganized conglomerates................................................................................................... 8

7. Large clasts from matrix-supported conglomerates ............................................................................................ 9

8. Schematic drawing showing reconstruction of the Devonian platform to basin transition........................................ 10

TABLE

1. Thickness of matrix-supported conglomerate beds and size of largest clasts in four intervals in the carbonate body 

described in figure 5.................................................................................................................................................... E7

Hi

Isolated Carbonate Bodies Composed of

Stacked Debris-Flow Deposits on a

Fine-Grained Carbonate Lower Slope of

Devonian Age, Antelope Peak,

Elko County, Nevada

By Peter M. Sheehan, 1 John M. Pandolfi,2 and Keith B. Ketner 3

ABSTRACT

The Roberts Mountains Formation at Antelope Peak in 

northeastern Nevada contains Lower Devonian isolated carbonate bodies in a predominantly thin-bedded carbonate-turbidi te sequence. The largest of five exposed bodies is 600 m 

wide and 47 m high. The turbidites that enclose the bodies 

were deposited on an outer carbonate apron near the margin 

of a basin. The carbonate bodies are composed of as many as 

150 discrete, matrix-supported conglomerate beds, each of 

which was deposited by a separate debris-flow event. Clasts 

in the conglomerate beds are as much as 90 cm in diameter 

and were derived from the shallow carbonate shelf. Clasts 

include angular carbonate blocks, colonial corals, and large 

segments of fused pelmatozoan stems. The debris-flow 

deposits are interpreted to have been derived from a point 

source on the shelf margin. Because numerous conglomerate 

beds were deposited at single locations, the debris flows may 

have been channeled down long canyons incised on the carbonate slope, then deposited as fanlike bodies at the mouths 

of the canyons.

INTRODUCTION

Lower Devonian resistant, isolated carbonate bodies 

are present near the transition between outermost carbonate slope and basin in the Antelope Peak area of northeastern Nevada (fig. 1). We interpret the 10-47-m-thick 

carbonate bodies to have an allochthonous origin and to be

Department of Geology, Milwaukee Public Museum, 800 W. Wells 

St., Milwaukee, Wisconsin 53233.

Australian Institute of Marine Science, P.M.B. No. 3, Townsville, 

M.C., Queensland 4810, Australia.

 ^U.S. Geological Survey, MS939, Federal Center, Denver, Colorado 

80225.

composed of stacks of numerous carbonate mud-supported 

sediment gravity-flow deposits.

Resedimented material deposited near carbonate shelf 

margins is commonly derived from line sources (Schlager 

and Chermak, 1979). Line sources produce slope deposits 

in broad aprons arrayed parallel to the shelf margin (Cook 

and others, 1972, 1983; Cook and Mullins, 1983; Mullins, 

1983; Mullins and others, 1984; Mullins and Cook, 1986). 

The aprons commonly are composed of grain flows, debris 

flows, turbidites and periplatform ooze. In contrast, noncarbonate slopes are characterized by point sources; fans 

of sediment radiate from canyons that are fed clastic material by rivers and longshore currents (Mutti and Ricci Lucchi, 1978; Normark, 1978). Point-source deposits similar 

to siliciclastic fans are uncommon in carbonate systems 

(Cook and Egbert, 1981; Cook, 1983; Ruiz-Ortiz, 1983; 

Wright and Wilson, 1984).

In a review and expansion of the carbonate apron 

model, Mullins and Cook (1986) recognized two types of 

aprons, the carbonate slope apron, developed along relatively gentle platform-margin slopes, and the base-of-slope 

carbonate apron, developed on relatively steep slopes. 

The isolated carbonate bodies discussed herein are on the 

outermost part of a base-of-slope carbonate apron. These 

bodies were built upward by repeated sediment gravityflow deposits derived from localized point sources on the 

shelf margin. The clasts comprising the bodies bypassed 

the upper carbonate slope and most of the carbonate apron 

before being deposited at the basin margin.

Acknowledgments. Research was supported by 

National Science Foundation grant EAR-7901013 to 

P.M. Sheehan. E.M. Schwartz and R.E. Johnson provided assistance in the field and laboratory. C.H. 

Thorman provided valuable suggestions, and we thank 

H.E. Cook for an extensive, critical review of the manuscript. Mr. F. Egbert and his family graciously provided 

access to their property.
E2 EVOLUTION OF SEDIMENTARY BASINS EASTERN GREAT BASIN

Figure 1. Carbonate body hosted in fine-grained, thin-bedded calcareous turbidites in the Antelope Peak area, northeastern Nevada. A detailed measured section of this 47-m-thick 

body is given in figure 5. A, Beds dip steeply away from viewer; 

stratigraphically lowest beds are in the foreground; uppermost 

beds are at the small peak. The resistant carbonate rocks that 

form the body are surrounded by slope-forming carbonate turbidites. B, Upper part of body (shown from a different viewpoint, top to left) composed predominantly of conglomerate and 

amalgamated Tab turbidite. Upper beds (left) dip more steeply 

away from body than do lower beds (right).

GEOLOGIC SETTING

Antelope Peak is in northeastern Nevada, 18 km 

north of the town of Wells (fig. 2). The sequence of rock 

strata discussed herein was deposited on a Silurian and 

Devonian carbonate slope. Thrust faults of Mesozoic age 

(Ketner, 1984; Smith and others, 1990) transported lower 

slope and basinal deposits into this region from the northwest. The lower slope deposits are dominated by finegrained carbonate turbidites and hemipelagic, periplatform 

ooze. South and east of Antelope Peak shallow-water carbonate shelf deposits are exposed (Johnson and others, 

1991; Sheehan and Boucot, 1991).

.OREGON LjDAHO 

NEVADA

CARBONATE-^:

PLATFORM ~

MARGIN 

Figure 2. Position of Silurian and Devonian carbonate-platform 

margin, Antelope Peak area, northeastern Nevada. Dashed where 

inferred.

The Antelope Peak area was first mapped by Peterson (1968), who did not record the carbonate bodies. A 

detailed map by Smith and others (1983) first recorded the 

carbonate bodies, which in the map explanation are 

referred to as possible reefs or debris flows.

Five structural plates are exposed in the Antelope 

Peak area (Smith and others, 1983). The plates contain 

rocks ranging from relatively autochthonous platform carbonate strata in the lowest plate to allochthonous slope 

and basinal deposits in the upper plates (Ketner, 1984; 

Smith and others, 1990). Structural plate 1 of Smith and 

others (1990) has been overridden by the other thrust 

plates.

The stratigraphically lowest rocks exposed in structural plate 1 are shallow-water Devonian dolomite and 

limestone assigned to the Simonson Dolomite and Guilmette Formation. These strata are younger than those discussed herein and were probably deposited at or very near 

a seaward-prograding rimmed shelf margin during the 

Middle and Late Devonian. Rocks of similar lithology are 

part of a prograding rimmed shelf margin sequence in the 

nearby Toano Range (Sheehan and Pandolfi, 1983, and
STACKED DEBRIS-FLOW DEPOSITS OF DEVONIAN AGE E3

our unpublished data). Structural plate 2 consists of thinly 

laminated limestone of probable Devonian age.

The carbonate bodies described herein are in structural plate 3, which comprises a sequence of Middle 

Ordovician to Devonian rocks (Smith and others, 1990). 

The Upper Ordovician and Lower Silurian strata of the 

plate were deposited on a carbonate ramp well beyond the 

margin of the carbonate platform (Carpenter and others, 

1985). In the late Llandoverian the carbonate ramp was 

disrupted by collapse of the platform margin along presumed listric growth faults (Johnson and Potter, 1975; 

Sheehan and Pandolfi, 1983; Carpenter and others, 1985; 

Sheehan, 1986). This episode of faulting ended the carbonate ramp and established a rimmed carbonate shelf 

margin upslope from the sedimentary rocks of structural 

plate 3. Subsequent deposition in this plate was in a deepwater slope environment. Ketner (1984) and Carpenter 

and others (1985) recognized an abrupt fades change in 

this structural plate between Ordovician through Lower 

Silurian rocks and Middle Silurian through Devonian 

rocks. The Ordovician to Lower Silurian interval is characterized by fine-grained, quiet-water, argillaceous carbonate rocks and a lack of gravity-flow deposits and slumps. 

The Middle Silurian through Devonian interval is characterized by carbonate turbidites and common rotational and 

translational slumping. Carpenter and others (1985) 

ascribed the facies change between the two groups of rock 

to be a result of the episode of regional downfaulting first 

described by Johnson and Potter (1975).

Rocks of structural plate 4 are basinal facies, including a variety of lower Paleozoic units referred to as western-fades rocks that include quartz sandstone, siltstone, 

argillite, limestone, and chert (Smith and others, 1990). 

Silurian and Devonian carbonate rocks in this interval are 

commonly thinly laminated and contain sporadic starved 

ripples. These strata were probably deposited by dilute 

turbidites. Structural plate 5 includes Devonian siliceous 

rocks of varied, mostly basinal, facies.

REGIONAL GEOLOGIC HISTORY

A rimmed carbonate platform margin (Read, 1982) of 

Silurian and Devonian age is preserved in central and 

northern Nevada. Resedimented carbonate strata deposited 

along the ocean-facing slope produced a line-sourced 

apron between the platform margin and the basin to the 

west (Cook and others, 1983; Hurst and Sheehan, 1985; 

Hurst and others, 1985; Mullins and Cook, 1986). The 

platform margin and slope are arrayed in a north-south 

band extending from southern Nevada to near Elko 

(Johnson and Potter, 1975; Matti and others, 1975; 

Nichols and Silberling, 1977; Cook and others, 1983; 

Hurst and Sheehan, 1985; Hurst and others, 1985; Johnson

and others, 1991; Sheehan and Boucot, 1991)(fig. 2). Near 

Elko, this band of rocks turns abruptly eastward and 

between Elko and Wendover, Utah, it trends east-west 

(Sheehan, 1986). Beyond Wendover the platform margin 

turns northward again into central Idaho (fig. 2). Two 

explanations of the eastward deflection of lithofacies patterns have been proposed, and we disagree as to which of 

the explanations is correct. Ketner prefers the view that 

the present northern margin of shallow-shelf deposition is 

a right-slip fault, or fault zone, that disrupted the original 

pattern of deposition (Thorman, 1970; Thorman and 

Ketner, 1979). In this model the north-trending margin 

between Wendover and central Idaho (fig. 2) was originally aligned with the north-trending margin between 

southern Nevada and Elko. Sheehan and Pandolfi prefer 

an alternative model in which the eastward deflection of 

the shelf margin is an original depositional pattern and the 

east-west margin marks the carbonate platform edge rather 

than a right-slip fault (Sheehan, 1979, 1986; Stevens, 

1981; Sheehan and Pandolfi, 1983; Miller and others, 

1991). Acceptance of one model or the other has little 

effect, however, on the conclusions presented herein.

The transition from a gently inclined carbonate ramp 

to a rimmed carbonate shelf margin in Nevada occurred in 

the Early Silurian (Johnson and Potter, 1975; Sheehan, 

1979, 1986; Sheehan and Pandolfi, 1983; Carpenter and 

others, 1985; Hurst and others, 1985). Sediments deposited on the slope during the remainder of the Silurian and 

the Early Devonian have been assigned to the regionally 

extensive Roberts Mountains Formation and a number of 

more local formations (Cook, 1983; Johnson and Murphy, 

1984; Hurst and Sheehan, 1985; Johnson and others, 1991; 

Sheehan and Boucot, 1991).

In central Nevada, autochthonous deposits comprising 

the Lone Mountain Dolomite at the edge of the carbonate 

shelf margin prograde basinward over the slope; however, 

the structural plate of this study was originally sufficiently 

seaward on the slope such that the prograding Lone 

Mountain Dolomite did not reach it. As a result, sedimentary rocks ranging in age from Middle Silurian well into 

Devonian are of lower slope facies in this plate.

The Silurian and Devonian carbonate platform margin 

in northern Nevada generally was a rimmed shelf with car- 

.bonate-sand shoals and possible patch reefs. Deposits on 

the upper slope are dominated by very fine grained, periplatform ooze, and slump features are common (Sheehan 

and Pandolfi, 1983; Hurst and Sheehan, 1985; Mullins and 

Cook, 1986). On the inner apron, coarse-grained sediments 

deposited in turbidity flows and some debris flows alternate 

with periplatform ooze (Sheehan and Pandolfi, 1983; Hurst 

and Sheehan, 1985; Mullins and Cook, 1986).

In general, coarse-grained sediments on inner aprons 

commonly bypass the upper slope by means of gravity 

transport along numerous parallel, line-source channels 

that dissect the fine-grained sediments of the upper slope
E4 EVOLUTION OF SEDIMENTARY BASINS EASTERN GREAT BASIN

(Mullins and Cook, 1986). Such channels have not been 

identified in the upper slope of Nevada, but coarse-grained 

inner apron sediments resemble material that must have 

bypassed the upper slope through channels.

Generally, in the outer apron the gravity-flow deposits are progressively finer grained down the slope. 

Between the outer apron and the basin, thin-bedded, very 

fine grained, platy limestone is composed of base-cutout 

carbonate turbidite alternating with periplatform carbonate 

ooze. The allochthonous carbonate strata that comprise the 

carbonate bodies described herein probably were deposited 

in such an intermediate environment. The depositional 

environment was assigned to the basin plain by Mullens 

and Cook (1986); however, we assign it to the distal margin of the apron because distal turbidites are dominant 

here, whereas true basinal deposits dominated by argillite 

are present in more basinal settings preserved in structurally higher plates 4 and 5.

Deposition of the carbonate bodies on the outermost 

apron in structural plate 3 was markedly different from the 

gravity-flow deposition that was common on the inner 

apron. Coarse sediments on the inner apron bypassed the 

fine-grained sediments of the upper slope and formed 

apron-shaped deposits. Each of the carbonate bodies was 

formed, however, by a series of gravity-flow deposits that 

accumulated at one place on the basin margin, having 

bypassed the fine-grained upper slope facies, the coarsegrained inner apron facies, and most of the fine-grained 

outer apron facies.

THE CARBONATE SLOPE AT 

ANTELOPE PEAK

LOWER PART OF THE 

ROBERTS MOUNTAINS FORMATION

The Roberts Mountains Formation in structural plate 

3 is about 450 m thick (Peterson, 1968). The stratigraphically lower part of the formation is composed of gray 

limestone sequences of thick amalgamated Tab Bouma 

sequences interbedded with intervals of complete TQ_Q turbidites and repetitive, thin, platy, calcareous T^ 

sequences (fig. 3A). Fossils collected from the lower part 

of the formation range in age from late Llandoverian to 

Early Devonian (Smith and others, 1990).

The Devonian beds in the upper part of the lower unit 

of the Roberts Mountains Formation are well exposed 

because the calcarenite of the Ta and T^ units is resistant 

to weathering. The turbidites consist of amalgamated Ta 

and Tab units interbedded with complete Ta_g sequences 

(figs. 3, 4A-Q. The Ta units are normally graded and

, c o c E 

oto o£

01 E. 01

c O c

1.5m

.Om

0.5m

Om

x x

Te 

Td 

TO 

Tb 

Ta

Typical turbidite 

cycle

/////// /I

 j_

Erosional 

base

Figure 3. Sediment logs of slope deposits that dominate the Roberts Mountains Formation in structural plate 3 of Smith and others 

(1990). Reddish-purple pelagic sedimentary rocks commonly cap 

the Te units, but, because they are less than 1 mm thick, they are not 

recorded in these logs. The carbonate bodies of the Antelope Peak 

area are enclosed within these platy weathering sedimentary strata. 

A, Lower part of formation. B, Upper part of formation.

contain some shells; commonly, they are normally graded 

echinoderm grainstone. Individual Ta and T^ calcarenite 

beds are lens shaped and laterally discontinuous. Individual beds commonly thin from several centimeters to 1 or 2 

cm over lateral distances of 10-20 cm.

Bases of Ta divisions fill scours in the underlying 

beds. Soft-sediment deformation features such as flame 

structures are common where a Ta unit rests on finegrained carbonate rocks (fig. 4Q. Similar load features
STACKED DEBRIS-FLOW DEPOSITS OF DEVONIAN AGE E5

Figure 4 (above and facing column). Calcareous slope turbidites, Antelope Peak area, northeastern Nevada. Scale is 15 

cm long. A, Lower part of Roberts Mountains Formation. Scale 

rests on a Ta unit. Note small-scale translational slump. B, Bouma Ta_c turbidite, lower part of Roberts Mountains Formation. 

Basal Ta unit about 3 cm thick is overlain by a 2-cm-thick, Tb 

unit that is locally scoured below the Tc unit. C, Lower part of 

the Roberts Mountains Formation. Dark interval near bottom of 

photograph is a 2-cm-thick Ta unit; note soft sediment deformation at base. The Ta unit is overlain by a 1.5-cm-thick Tb unit 

and a 1.5-cm-thick Tc unit, then repetitions of amalgamated T^ 

and Tc- units. D, Thin-bedded turbidites, upper part of the 

Roberts Mountains Formation. This outcrop is from below the 

largest carbonate body described in figure 5 and is typical of 

sedimentary rocks that enclose the bodies.

are common in carbonate turbidite sequences (Markello 

and Read, 1981; Cook and others, 1983; Hurst and 

Surlyk, 1983).

In situ pelagic sediments deposited between turbidity 

flow events consist of 1-mm-thick fine silt and clay strata. 

The pelagic strata are seldom more than 1 mm thick and 

are commonly distinguished by a bright-reddish-purple 

color (5R4/4). Very thin pelagic units between turbidite 

beds are consistent with a distal carbonate slope position 

in the Paleozoic, when little carbonate pelagic material 

was present. James and Mountjoy (1983) noted that during 

the Paleozoic there were few calcareous plankton and 

most fine-grained basinal carbonate sediment was derived 

from the shallow-water shelf and deposited seaward near 

the shelf margin.

UPPER PART OF THE 

ROBERTS MOUNTAINS FORMATION

The upper part of the Roberts Mountains Formation 

is very platy limestone, with thin Tab turbidites and a predominance of thin, parallel and wavy laminated, platy 

weathering Tc_g units (fig. 3B, 4D). Fossils from the 

upper part of the formation are of Early Devonian age 

(Smith and others, 1990). The carbonate bodies described 

herein are in strata assigned to the upper part of the 

Roberts Mountains Formation; corals in one body have 

been dated as Early Devonian in age (Smith and others, 

1990).

The upper part of the Roberts Mountains Formation 

is less well exposed than the lower part because thin,
E6 EVOLUTION OF SEDIMENTARY BASINS EASTERN GREAT BASIN

recessive weathering, platy limestone dominates the 

sequence. The tannish color of the upper part of the 

formation reflects noncarbonate argillaceous content. Most 

of the section is composed of Tc_e sequences that are 2-5 

mm thick (fig. 3). Amalgamated Ta and Tab sequences are 

present but are thinner and less common than in the lower 

part of the Roberts Mountains Formation (figs. 3,4D). 

Although some Ta units are as thick as 30 cm, most are 

only 1-2 cm thick and many are only a few millimeters 

thick. Bases of Ta units are scoured and commonly exhibit 

load structures. Well-rounded, normally graded bioclasts 

(commonly echinoderm grains) less than 1 mm in diameter characterize the Ta units. Rounded lithoclasts are less 

common than bioclasts. Bouma T^, units contain fine, 

rounded bioclasts or are pelleted. Reddish-purple hemipelagic and periplatform sediments are similar to those in the 

lower part of the formation. Tilted 3-10-m-thick blocks of 

platy, laminated sediment at several outcrops are probably 

the result of rotational slumping. Translational slides from 

5 cm to 1 m in thickness are present in less than 1 percent 

of the sequence (figs. 4A, Q. Sporadic translational slides 

can be traced laterally into thin debris-flow units, similarly 

to those described by Cook and Taylor (1977), Cook 

(1979) and Cook and others (1983).

The sediments, primary sedimentary structures, and 

slump structures in the upper part of the Roberts Mountains Formation are all characteristic of deposition on an 

outermost apron. Complete Bouma T^ turbidites, thin, 

amalgamated Tab units, and thick sequences of platy, calcareous Tjj.g and Tc_^ units are common on the outer 

apron of a carbonate slope (Walker, 1978; Mcllreath and 

James, 1979; Read, 1982; Cook and others, 1983; Mullins 

and others, 1984; Mullins and Cook, 1986). Small slumps 

such as are present in the study area are common in thinbedded outer slope sequences (Cook and Taylor, 1977; 

Mcllreath and James, 1979; Cook and others, 1983).

SETTING OF THE ROBERTS 

MOUNTAINS FORMATION

In the Antelope Peak area, upper slope deposits are 

not exposed; however, in the Toano Range, 60 km to the 

east, Silurian turbidites and conglomerates of inner apron 

sequences are overlain by prograding, fine-grained deposits of the upper slope (Sheehan and Pandolfi, 1983).

A variety of shallow-water lithic limestone clasts and 

clasts of solitary corals, colonial corals, pelmatozoans, and 

bryozoans are in the matrix-supported conglomerate of the 

carbonate bodies. Because the clasts were derived from 

the rimmed shelf margin, they provide an indication of the 

types of organisms inhabiting that shallow-water environment.

Strata of structural plate 3 are interpreted to have a 

deepening-upward pattern; the upper part of the lower unit

of the Roberts Mountains Formation is made up of coarser 

grained, thicker turbidites that are made up of more common amalgamated Ta and Tab sequences than in the upper 

unit of the formation.

The carbonate bodies were deposited in the outermost 

apron, adjacent to a deep basin. Regional patterns and the 

upward-deepening sequence of sediments in structural 

plate 3 (see fig. 8) indicate that progressively upslope 

from the carbonate bodies are (1) an outer apron made up 

of predominantly thin bedded, fine-grained carbonate turbidites, (2) an inner apron of conglomerate and turbidites 

that are thicker bedded and coarser grained than those 

lower on the slope, (3) an upper slope of fine-grained, 

hemipelagic and periplatform carbonate sediments in an 

area that was being bypassed by coarse-grained material, 

and (4) the shelf margin made up of shallow-water carbonate deposits from which the clasts in the carbonate 

mounds were being derived.

THE ISOLATED CARBONATE BODIES

Five carbonate bodies mapped by Smith and others 

(1990) range from about 120 m in width and 10 m in 

height to about 600 m in width and 47 m in height. The 

largest bodies are composed of as many as 150 matrixsupported conglomerate units alternating with less common, thick-bedded turbidite units. The upper beds of the 

carbonate bodies dip away from the center of the bodies, 

suggesting a moundlike shape. The carbonate bodies are 

encased in fine-grained, thin-bedded carbonate turbidites 

that weather to covered slopes (fig. 1). In addition, the 

carbonate bodies are at different stratigraphic horizons and 

therefore are not erosional remnants of a single layer.

The exposures do not permit the exact shape of the 

carbonate bodies to be determined. Upper beds dip away 

from the centers of the bodies, but the exposures of the 

bases of the bodies are inadequate to determine whether 

the bases are flat or concave.

The largest body is depicted graphically in figure 5. 

Individual conglomerate beds contain large, unsorted, 

matrix-supported clasts (figs. 5-7). In order of relative 

abundance, clasts are angular lithic limestone fragments 

(35 percent), broken bases of fused pelmatozoan stems (30 

percent), colonial corals (20 percent), bryozoans (5 percent), and solitary corals (5 percent). Lithoclasts are of a 

variety of shallow-water lithologies (figs. 6, 7) including 

carbonate mudstone, cryptalgal laminite, wackestone, and 

crossbedded grainstone. The massive morphology of many 

of the colonial corals is characteristic of high-energy environments.

The bases of the conglomerate beds within the isolated carbonate bodies are scoured into the underlying 

beds. Large clasts commonly project above the tops of the
STACKED DEBRIS-FLOW DEPOSITS OF DEVONIAN AGE E7

EXPLANATION

Turbidite sequence (Bouma cycle)

7ZZZZ2 I*

 ''*' '"-X Ta.Tab.Tb

 -s Matrix-supported conglomerate

Q Lithoclast

Colonial coral /

44m 

Pelmatozoan stem

vj/ Bryozoan 

§§^7 Rugose coral

Table 1. Thickness of matrix-supported conglomerate beds and 

size of largest clasts in four intervals in the carbonate body 

described in figure 5.

[Note that the thickness of conglomerate beds and the size of the largest 

clasts decrease upward in the body]

Figure 5. Sediment log of the largest carbonate body in the Antelope Peak area, northeastern Nevada. Photographs of this body 

are shown in figures 1 and 6. The sediment log was made beginning at the southeast corner of the carbonate body mapped by 

Smith and others (1990) in the SW'/4 sec. 29, T. 40 R, R. 62 E. 

The log of the entire body is shown on the left, and three parts of 

body are shown on the right.

conglomerate beds. A few conglomerates grade upward 

into Tab Bouma divisions. The conglomerate beds are progressively thinner bedded and finer grained upward (table 

1). A few conglomerate units contain imbricated clasts; 

however, directions of imbrication vary greatly from one 

unit to the next, and no consistent pattern of orientation 

was found.

In the lower parts of the bodies the turbidites between 

conglomerates are primarily grain supported, calcareous

Height above 

base of body 

(meters)

35^0 

25-30 

10-15 

0-5

Thickness of 

conglomerate bed 

(centimeters)

5-21 

5-36 

10-55 

10-90

Size of largest 

clast in interval 

(maximum dimension, 

centimeters)

11 

18

30 

90

graded Ta units and amalgamated Tat and T^ turbidites. 

Clasts in the Ta units are much smaller than those in the 

conglomerates. Turbidites in the middle and upper parts 

of the bodies include some Ta_e sequences (fig. 5) in addition to Ta and Tab units. Turbidites are more common in 

the upper parts of the bodies than near the bases. At the 

base of the measured section is a sequence of amalgamated Ta and Tab turbidites that have much larger clasts 

than are common in Ta units of the upper part of the formation.

The strata that enclose the bodies are poorly exposed, 

as is typical of the platy upper part of the Roberts Mountains Formation; however, 20 m below the largest body is 

a typical basin-margin sequence composed of very thin 

bedded, platy weathering, argillaceous-carbonate turbidites 

(figs. 4D, 5). Similar argillaceous-carbonate turbidites are 

lateral to, and overlying, the various bodies.

ORIGIN OF THE ISOLATED 

CARBONATE BODIES

The Lower Devonian carbonate bodies in the Antelope Peak area were built by repeated depositional events 

that brought coarse, shallow-water-derived clasts into very 

localized areas near the basin margin. Most of the conglomerate beds contain chaotic, matrix-supported clasts, 

scoured bases, and irregular tops with clasts projecting 

above the tops of the units. The conglomerates are interpreted to be debris-flow deposits. Some beds have imbricate clasts, which is consistent with grain interaction in a 

turbulent flow (Davies and Walker, 1974; Walker, 1975). 

In general, disorganized conglomerate beds that have 

Bouma Ta and Tab units at their tops are consistent with 

the presence of turbidity currents in the waning phase of a 

debris flow (Cook and others, 1972; Hampton, 1972; 

Krause and Oldershaw, 1979).

The allochthonous material making up the carbonate 

bodies is interpreted to have been derived from point
E8 EVOLUTION OF SEDIMENTARY BASINS EASTERN GREAT BASIN

Figure 6 (above and facing column). Matrix-supported, disorganized conglomerates from the largest body in the Antelope 

Peak area, northeastern Nevada. The conglomerates are interpreted to have been deposited by repeated debris flows. Sediment logs of the body are shown in figure 5. A, Angular to 

subangular margins of limestone clasts. Note that the three almost vertical clasts in lower right of photograph appear to fit together. Pelmatozoan stem bases are common in upper left of 

photograph. Photograph taken 2 m above base of body. Scale 4 

cm. B, Pelmatozoan stem bases (P), and subangular lithic clasts 

(L). Note centimeter-scale banding of lithic clast in left-center 

of photograph. Lower part of mound. Scale 15 cm. C, Angular 

limestone clasts (L), isolated pelmatozoan columnals (P), and 

spherical tabulate coral colonies (C). Middle part of body. Scale 

9 cm. D, Lithic limestone clasts of varied lithology including 

dark, blocky mudstone clasts (top of photograph), light-colored 

elongate mudstone (bottom), and tabular, possibly cryptalgal 

laminite (lower right). Spherical tabulate corals (C) are partially 

silicified at their margins. Photograph taken 6 m above base of 

section. Scale 15 cm.

sources as in siliciclastic submarine fans (Mutti and Ricci 

Lucchi, 1978; Normark, 1978; Walker, 1978). As many as 

150 separate debris flows were involved in the formation 

of the largest bodies. The debris-flow units are thinner and 

finer grained upward in the bodies, and turbidites are 

dominant near the tops of the bodies (fig. 5). These features may reflect deflection of debris flows laterally as the 

bodies grew in size.
STACKED DEBRIS-FLOW DEPOSITS OF DEVONIAN AGE E9

Figure 7. Large clasts from matrix-supported conglomerates in largest carbonate body in the Antelope Peak area (figs. 5, 6). Scale 

15 cm. A, Clast of colonial tabulate coral, 45-cm-diameter, from 3.0 m above the base of the body. B, Angular limestone clast, 60 cm 

in maximum diameter, from 2.6 m above base of body. Mudstone matrix contains many small lithic clasts such as those adjacent to the 

scale.

Debris-flow deposits are uncommon in this part of 

the basin margin except those in the carbonate bodies. Isolated debris-flow deposits also are present in the upper 

part of the Roberts Mountains Formation on Antelope 

Peak, but as noted previously they are thin and are 

composed of slope-derived lithoclasts rather than clasts 

generated in shallow water on the shelf. Thick debris-flow 

deposits are present in the lower part of the Roberts 

Mountains Formation in the Toano Range (Sheehan and 

Pandolfi, 1983), but they are discrete and laterally extensive units on the inner apron, a position much farther up 

the carbonate slope than the debris-flow deposits of 

Antelope Peak. Debris-flow deposits are also present in 

the Roberts Mountains Formation in central Nevada (Matti 

and others, 1975; Matti and McKee, 1977; Hurst and 

Sheehan, 1985; Hurst and others, 1985), but, again, these 

units are stratigraphically isolated, sheetlike sequences that 

do not form distinct, compact bodies. Most debris-flow 

deposits in central Nevada are associated with a 

prograding, accretionary rimmed-shelf margin in shallowing-upward sequences located high on the slope (Hurst 

and others, 1985). The debris-flow deposits of Antelope 

Peak are thus a unique feature on the Silurian to Devonian 

slope in the Great Basin.

It is not possible to exclude the possibility that the 

carbonate bodies were deposited as channel fillings 

because poor exposure does not allow the shape of the 

bases of the carbonate bodies to be adequately studied. 

However, because the upper beds dip away from the centers of the bodies, it is more likely that the carbonate bodies are mounds that were deposited at the mouths of 

channels rather than in channels. In addition, the coarse 

nature of the gravity-flow deposits makes it likely that 

flowage continued through the channels and that deposition occurred at the mouths of the canyons.

In the study area debris-flow conglomerate deposits 

are present only in carbonate bodies where they are 

stacked one upon another. They are not present as isolated, single, unstratified deposits. Emplacement of numerous flows at one downslope location requires derivation 

from a point source and would be likely to occur if the 

flows were confined by a canyon incised in the upper 

slope (fig. 8). Clasts may have been fed into such a canyon by longshore currents or storms moving parallel with 

the coast, although the angularity of most clasts makes 

extensive transport unlikely. Canyons are common on the 

upper parts of modern carbonate slopes (Schlager and 

Chermak, 1979; Mullins and others, 1984), but they produce the line-sourced sediments of the upper apron 

(Schlager and Chermak, 1979; Cook and others, 1983; 

Mullins and others, 1984; Mullins and Cook, 1986). In the 

Antelope Peak area, which was below the upper apron, a 

few larger, longer canyons may have been the route 

through which repeated debris flows from the carbonate 

shelf margin bypassed the inner slope and apron. Thus, 

we believe that the outer slope carbonate bodies on Antelope Peak were formed from point sources by repeated 

stacking of debris-flow deposits, probably at the mouths of 

canyons.

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E10 EVOLUTION OF SEDIMENTARY BASINS EASTERN GREAT BASIN

STACKS OF DEBRIS-FLOW 

DEPOSITS

Figure 8. Reconstruction of the Devonian platform to basin transition in the Antelope Peak area. The stacks of conglomerate beds 

form isolated carbonate bodies in the dominantly fine grained platy carbonate deposits of the outer apron. The conglomerates are interpreted to have been derived from the platform and transported to the apron-basin margin by debris flows. The conglomerates are not 

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E12 EVOLUTION OF SEDIMENTARY BASINS EASTERN GREAT BASIN

Thorman, C.H., and Ketner, K.B., 1979, West-northwest strike-slip 1978, Deep-water sandstone facies and ancient submarine

faults and other structures in allochthonous rocks in central and fans Models for exploration for stratigraphic traps:

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Published in the Central Region, Denver, Colorado 

Manuscript approved for publication February 5,1993 

Cartography by Norma Macs 

Photocomposition by Shelly A. Fields 

Edited by Judith Stoeser

irU.S. GOVERNMENT PRINTING OFFICE: 1993-774-049/66063
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