Clastic Dikes of Heart Mountain 

Fault Breccia, Northwestern Wyoming, 

and Their Significance
Clastic Dikes of Heart Mountain 

Fault Breccia, Northwestern Wyoming, 

and Their Significance

By WILLIAM G. PIERCE

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1133

UNITED STATES GOVERNMENT PRINTING OFFICE, _ WASHINGTON : 1979
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary

GEOLOGICAL SURVEY 

H. William Menard, Director

Library of Congress Cataloging in Publication Data

Pierce, William Gamewell, 1904-

Clastic dikes of Heart Mountain fault breccia, northwestern Wyoming,

and their significance.

(Geological Survey professional paper ; 1133)

Includes bibliographical references.

Supt.ofDocs.no.: 119.16:1133

1. Dikes (Geology)-Wyoming-Heart Mountain. 2. Breccia-WyomingHeart Mountain. I. Title. II. Series: United States. Geological 

Survey. Professional paper ; 1133.

QE611.5U6P53 551.8'8 79-607991

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C. 20402

Stock Number 024-001-03252-0
CONTENTS

Page Page

Abstract ________________________________ 1 Description of dikes at specific sites—Continued

Introduction ___________________________________ 1 Area 3, north of Republic Mountain ______________13

General features of the Heart Mountain fault____________ 3 Area 4, south of Pilot Creek __________________13

Clastic dikes of fault breccia_--_-__-________________ 3 Area 5, north of Jim Smith Peak_________________15

General features of clastic dikes in upper-plate Area 6, near White Mountain _ _____ ____ _______17

carbonate rocks_____________________________ 4 Area 7, west of Trout Creek __________________18

General features of clastic dikes in volcanic rocks ______ 4 Other sites of clastic dikes._____________________20

Description of dikes at specific sites _______________ 5 Breccia and brecciation produced by the Heart Mountain fault __20

Area 1, south of Silver Gate, Mont. _______________ 5 Mechanism of dike injection ____________________21

Calcibreccia dikes in Paleozoic rocks of Geologic significance of the clastic dikes __ _________ _23

the upper plate _________________________ 5 Summary __________________________________24

Calcibreccia dikes in the Wapiti Formation-______ 6 Acknowledgments _________________________——24

Area 2, southeast of Silver Gate, Mont.______________10 References cited _________________________24

ILLUSTRATIONS

Page

FIGURE 1. Map showing location of four phases of Heart Mountain fault and sites of clastic dikes of fault breccia _____- __ 1

2. Photograph of Calcibreccia dikes in the Three Forks Formation, area 1 ___________———__——— ——— ——— 4

3. Cross section in area 1 showing Calcibreccia dikes in blocks of the upper plate and in the Wapiti Formation _____ 5 

4-8. Photographs from area 1 showing:

4. Calcibreccia dike with three components _________________________________________ 6

5. Specimen of Calcibreccia dike containing carbonized wood__________________——__ ——— ——— ——— 7

6. Calcibreccia dike containing carbonized wood ___ _ ____ ____________ _ _ ________ _ _______ 8

7. Irregular borders between Calcibreccia dikes and enclosing dikelike body of the Wapiti Formation — _————— 9

8. Calcibreccia dike containing xenoliths of Precambrian rocks ___________________________— 9

9. Cross section in area 1 showing movement of fault breccia and basal part of the Wapiti Formation into dikes

and dikelike bodies ____________________________________________________10

10. Cross section in area 2 showing relation of Calcibreccia to Heart Mountain fault, surface of tectonic denudation,

and the Wapiti Formation______________________________________________________11

11-22. Photographs showing:

11. Polished specimen of plume of Wapiti Formation extending into Calcibreccia dike, area 2 ______ __ ___11

12. Upper end of Calcibreccia dike, area 2 _ ______ _______ _ _ _______ _ _ _____ _________12

13. Antiform of Calcibreccia, area 2 ______________________________________________—12

14. Calcibreccia dike intermixed on its borders with the Wapiti Formation, area 3 ________———____—_ —13

15. Heart Mountain fault breccia overlain by the Wapiti Formation, area 4 ___— —————————— ————— __14

16. Polished specimen of upper third of Heart Mountain fault breccia, area 4 _____________—_______14

17. Mound of Heart Mountain fault breccia, area 4 _______________________— ——— _————14

18. Polished specimen of Calcibreccia from mound, area 4 _________________________ — ____ —15

19. Polished specimen of Calcibreccia dike in lower part of the Wapiti Formation, area 4 __———___ —————15

20. Flat-lying Calcibreccia dike, area 4_ _ __ _____ _ ___ _ _ _ _______ _ _________ _ _ ___ _15

21. Polished specimen of Calcibreccia interlayered with basal part of the Wapiti Formation, area 5—————————16

22. Irregular layer of Calcibreccia in basal part of the Wapiti Formation, area 5 _________________—_ —16

23. Cross section in area 7 showing relation of a dike of the Wapiti Formation containing stringers of Calcibreccia to

wallrock of Madison Limestone and Wapiti Formation _______________________——————— ———18

24. Photograph of dike of volcanic breccia of the Wapiti Formation containing Calcibreccia in contact with fault block

of Madison Limestone, area 7 ________________________________________________19

25. Photographs showing polished specimens of Calcibreccia from dike of volcanic rock of the Wapiti Formation, area 7 —19

26. Cross section in area 3 showing degrees of deformation in brecciated limestone mass above Heart Mountain fault——22

27. Photograph showing polished specimen of brecciated limestone, area 3 ____________________—______————22

in
IV CONTENTS

TABLE

Page

TABLE 1. Location of and brief data on other calcibreccia dikes__ __ ______——- —————— ———— — ——— — —————— — — 20
CLASTIC DIKES OF HEART MOUNTAIN FAULT BRECCIA, 

NORTHWESTERN WYOMING, AND THEIR SIGNIFICANCE

By WILLIAM G. PIERCE

ABSTRACT

Structural features in northwestern Wyoming indicate that the 

Heart Mountain fault movement was an extremely rapid, cataclysmic event that created a large volume of carbonate fault breccia 

derived entirely from the lower part of the upper plate. After fault 

movement had ceased, much of the carbonate fault breccia, here 

called calcibreccia, lay loose on the resulting surface of tectonic denudation. Before this unconsolidated calcibreccia could be removed 

by erosion, it was buried beneath a cover of Tertiary volcanic rocks: 

the Wapiti Formation, composed of volcanic breccia, poorly sorted 

volcanic breccia mudflows, and lava flows, and clearly shown in 

many places by inter lensing and intermixing of the calcibreccia with 

basal volcanic rocks. As the weight of volcanic overburden increased, 

the unstable water-saturated calcibreccia became mobile and 

semifluid and was injected upward as dikes into the overlying volcanic rocks and to a lesser extent into rocks of the upper plate. In 

some places the lowermost part of the volcanic overburden appears to 

have flowed with the calcibreccia to form dikelike bodies of mixed 

volcanic rock and calcibreccia. One calcibreccia dike even contains 

carbonized wood, presumably incorporated into unconsolidated calcibreccia on the surface of tectonic denudation and covered by volcanic rocks before moving upward with the dike. Angular xenoliths 

of Precambrian rocks, enclosed in another calcibreccia dike and in an 

adjoining dikelike mass of volcanic rock as well, are believed to have 

been torn from the walls of a vent and incorporated into the basal 

part of the Wapiti Formation overlying the clastic carbonate rock on 

the fault surface. Subsequently, some of these xenoliths were incorporated into the calcibreccia during the process of dike intrusion.

Throughout the Heart Mountain fault area, the basal part of the 

upper-plate blocks or masses are brecciated, irrespective of the size of 

the blocks, more intensely at the base and in places extending 

upward for several tens of meters. North of Republic Mountain a 

small 25-m-high upper-plate mass, brecciated to some degree 

throughout, apparently moved some distance along the Heart 

Mountain fault as brecciated rock. Calcibreccia dikes intrude 

upward from the underlying 2 m of fault breccia into the lower part 

of the mass and also from its top into the overlying volcanic rocks; an 

earthquake-related mechanism most likely accounts for the observed 

features of this deformed body.

Calcibreccia dikes are more common within the bedding-plane 

phase of the Heart Mountain fault but also occur in its transgressive 

and former land-surface phases. Evidence that the Wapiti Formation 

almost immediately buried loose, unconsolidated fault breccia that 

was the source of the dike rock strongly suggests a rapid volcanic 

deposition over the area in which clastic dikes occur, which is at least 

75 km long.

Clastic dikes were injected into both the upper-plate and the volcariic rocks at about the same time, after movement on the Heart 

Mountain fault had ceased, and therefore do not indicate a fluidflotation mechanism for the Heart Mountain fault.

The difference between contacts of the clastic dikes with both indurated and unconsolidated country rock is useful in field mapping

at localities where it is difficult to distinguish between volcanic rocks 

of the Cathedral Cliffs and Lamar River Formations, and the Wapiti 

Formation. Thus, calcibreccia dikes in the Cathedral Cliffs and 

Lamar River Formations show a sharp contact because the country 

rock solidified prior to fault movement, whereas calcibreccia dikes in 

the Wapiti Formation in many instances show a transitional or 

semifluid contact because the country rock was still unconsolidated 

or semifluid at the time of dike injection.

INTRODUCTION

General features of the Heart Mountain fault have 

been described by Dake (1918), Hewett (1920), Bucher 

(1933, 1940), Pierce (1941, 1957,1960), Voight (1974), 

and Prostka (1978); detailed mapping of the fault and 

of the areal extent of the upper plate have been published (see fig. 1 for references). In this report attention 

will be focused upon clastic dikes of the Heart 

Mountain fault breccia, which furnish striking and 

unusual evidence on the brief lapse of time involved in 

the Heart Mountain fault emplacement process and 

provide some clues on the probable mechanism of both 

fault movement and dike injection.

Four major points will be developed in this report. 

(1) The calcibreccia dikes intruded two quite different 

kinds of country rock—Paleozoic carbonate and Tertiary volcanic rocks—but apparently were injected at 

essentially the same time and show a common mechanism, namely, lithostatic pressure due to burial by a 

rapidly accumulating cover of volcanic rocks, the 

Wapiti Formation. (2) The dikes are an aid in distinguishing fault-emplaced from nonfault-emplaced volcanic rocks. (3) Their wide distribution indicates that 

at least the lower part of the Wapiti Formation was 

rapidly deposited over a large area. (4) The dikes confirm an earlier conclusion (Pierce, 1968, 1973b) that 

the Heart Mountain fault movement was a cataclysmic 

event and cast further doubt on some of the mechanisms proposed for this fault movement.

In the course of their preparation of a geologic map of 

the Pilot Peak quadrangle, Wyoming, Pierce, Nelson, 

and Prostka (1973) differed as to whether the Heart 

Mountain fault movement had occurred during or after 

deposition of the Lamar River Formation, a name prol
CLASTIC DIKES OF HEART MOUNTAIN FAULT BRECCIA, NORTHWESTERN WYOMING

45' 30' 109° 15'

Miller

Mountain Cooke 

City

YellowstoneNational Park

1-816 

One

Mile 

Creek

WYOMING GQ-935

BEARTOOTH 

UPLIFT

Yellowstone

National

Park

EXPLANATION

land-surface phase

Heart Mountain fault block - Faults 

within block not shown

Trace of Heart Mountain fault - Hachures on 

upper plate or preserved surface; fault-breccia 

dikes also on hachured side

Heart Mountain break- away fault - Squares on 

depositional side. Dotted where removed by erosion

High-angle fault — u, up thrown side; D, down thrown side

Site of fault-breccia dike - Number refers to areas 1-7 

described in text Buffalo Bill 

Reservoir

45° 00'

44°30'

Additional site of fault-breccia dike - Letters A-H 

refer to table 1

10 15 20 25 KILOMETERS

10 15 MILES

FIGURE 1.—Map showing location of four phases of Heart Mountain fault and sites where clastic dikes of fault breccia are found. For geologic 

setting and detailed location of calcibreccia dikes in northwestern part of area, see Pierce, Nelson, and Prostka (1973). GQ numbers refer to 

Geologic Quadrangle Maps and I number to Miscellaneous Investigations Series Map published by U.S. Geological Survey at a scale of 

1 : 62,500.

posed by Smedes and Prostka (1972) for the lowest volcanic rocks of northeastern'Yellowstone National Park 

including all rocks of the Cathedral Cliffs Formation 

(Pierce, 1963a). These differences remained unresolved 

at the time of publication, and so areas with alternate 

interpretation were designated accordingly on the

map. Subsequent studies of the Heart Mountain fault, 

fault breccia, and clastic dikes (for example, areas 1-3 

and 4-5, fig. 1, which Prostka (1978, p. 428) interpreted as sites of fault-emplaced volcanic rocks) have 

shown that the volcanic rocks are depositional over the 

Heart Mountain fault surface of tectonic denudation.
CLASTIC DIKES OF FAULT BRECCIA

On their map, Pierce, Nelson, and Prostka designated 

the volcanic rocks in areas 1-3 (fig. 1) as the Lamar 

River and Cathedral Cliffs Formations, undivided; this 

assignment is unsatisfactory because the Cathedral 

Cliffs Formation is part of the upper plate and thus is 

older than the Heart Mountain fault, and the Lamar 

River Formation in its type area (Smedes and Prostka, 

1972) in the Abiathar Peak quadrangle (Prostka and 

others, 1975) also predates the faulting. In view of the 

difficulty in distinguishing between the Lamar River 

and Wapiti Formations east of the Heart Mountain 

breakaway fault, it is suggested that the Lamar River 

Formation be discontinued in the area of the Heart 

Mountain fault and that the Cathedral Cliffs Formation be used for the pre-Wapiti volcanic rocks.

GENERAL FEATURES OF THE HEART 

MOUNTAIN FAULT

The Heart Mountain fault is in northwestern Wyoming, between the northeast corner of Yellowstone National Park and the west flank of the Bighorn Basin. 

Near the close of the early Eocene Epocl , rocks in a 

1,300-km2 area between the breakaway and transgressive phases of the Heart Mountain fault (fig. 1) 

became detached along a bedding plane and moved 

southeast. During movement, the upper plate broke 

into more than 50 blocks, ranging from a few tens of 

meters to 8 km across, scattered over an area of 3,400 

km2 that stretched 105 km in a north west-southeast 

direction. Horizontal movement of the most southeasterly blocks was 50 km; to the northwest, the extent of 

movement diminished progressively toward the 

breakaway fault. The overall slope of the fault, which 

can be determined fairly closely, was less than 2°.

The upper plate of the Heart Mountain fault is composed of about 500 m of predominantly carbonate rocks 

of the Bighorn Dolomite (Ordovician), the Jefferson 

and Three Forks Formations (Devonian), and the 

Madison Group (Mississippian); and of from 0 to 600 m 

of volcanic rocks of the Cathedral Cliffs and Lamar 

River Formations (Eocene). The detachment horizon is 

underlain by 360 m of Cambrian shale and limestone of 

the Snowy Range, Pilgrim, and Gros Ventre Formations, and by the Flathead Sandstone, which rests unconformably on Precambrian granitic rocks.

The distribution of the Crandall Conglomerate, an 

unusual stream-channel deposit (Pierce, 1973a), indicates a preliminary southeastward movement in the 

northeastern part of the Heart Mountain fault mass 

that opened a deep rift about 1 km wide through the 

upper plate. Erosion then cut a channel as deep as 100 

m into the underlying Cambrian shale, in which 150 m 

or more of this coarse conglomerate was subsequently

deposited. This preliminary movement was followed in 

rapid succession by the Reef Creek detachment fault 

(Pierce, 1963b) and then by a main movement of the 

entire upper plate that left the lower part of the conglomerate in place but carried the upper part, along 

with rocks of the upper plate, roughly 24 km southeastward.

CLASTIC DIKES OF FAULT BRECCIA

The clastic dikes of carbonate fault breccia were first 

described by Pierce (1968) as limestone dikes at 6 general sites; since then, the number of sites has been 

increased to 15. These dikes are composed of finely 

crushed dolomite and limestone in a matrix of carbonate flour, firmly cemented by calcium carbonate to form 

a resistant rock that breaks with conchoidal fracture. 

For brevity the term "calcibreccia dike" will be used 

here in place of the longer "clastic calcium and magnesium carbonate breccia dike"; the term "calcibreccia" will be used at times to refer to the Heart 

Mountain fault breccia either as tectonic breccia or as 

dike rock. The source of carbonate fault breccia or calcibreccia was the clastic rock formed by movement of 

the upper plate of the Heart Mountain fault.

The Heart Mountain fault breccia is a cataclastic 

rock (cataclasite) composed of angular carbonate clasts 

in a microbreccia of smaller fragments without primary cohesion. The fault breccia is not mylonite, even 

in a broad sense, nor does it have fluxion structure; it is 

not only found on the fault surface beneath the large 

blocks of Paleozoic rock composing the upper plate, but 

also is irregularly distributed on the surface of tectonic 

denudation between these blocks. A few calcibreccia 

dikes are seen in Paleozoic rocks that form remnants of 

the upper plate, but most are found in the volcanic 

rocks deposited on the surface of tectonic denudation. 

The dikes are believed to have intruded both upperplate and volcanic rocks at essentially the same time, 

soon after deposition of the first postfault volcanic 

rocks.

The associated volcanic rocks in which clastic dikes 

occur, originally informally known as the early basic 

breccia of Hague (1899), were subsequently named the 

Wapiti Formation by Nelson and Pierce (1968). In the 

Pilot Peak quadrangle map (Pierce and others, 1973), 

the volcanic rocks containing clastic dikes were called 

the Lamar River and Cathedral Cliffs Formations, undivided. However, inasmuch as throughout a much 

larger area of the Heart Mountain fault the volcanic 

rocks that were deposited on the surface of tectonic 

denudation are unquestionably included in the Wapiti 

Formation, that designation will be followed here, es-
4 CLASTIC DIKES OF HEART MOUNTAIN FAULT BRECCIA, NORTHWESTERN WYOMING

pecially because the surface of tectonic denudation 

permits a precise time correlation between units.

Features of the Heart Mountain fault breccia clearly 

indicate that it is the source material for the calcibreccia dikes in both the upper-plate blocks of Paleozoic 

rocks and in the volcanic rocks. The lower parts of 

many blocks of the upper plate are intensely brecciated, and so the dike rock that they enclose was evidently derived from fault breccia. However, most of the 

volcanic rocks containing calcibreccia dikes are not 

from the upper plate, nor were they fault emplaced. 

They are postfaulting in age, as shown by: (1) the absence of clasts of volcanic rocks in the fault breccia, in 

contrast to the abundance of carbonate fault breccia; 

(2) the semifluid mixing of calcibreccia with volcanic 

rock (figs. 21, 22); and (3) their depositional relation 

against and over Heart Mountain fault masses of all 

sizes (figs. 3, 26). Consequently, the only possible 

source for the calcibreccia in the dikes in volcanic rocks 

also was the Heart Mountain fault breccia. Some 

geologic features indicating that the dikes in both the 

upper-plate and volcanic rocks were in fact derived 

from Heart Mountain fault breccia are: (1) the fault 

breccia and dike rock are lithologically similar except 

for minor additions of wallrock to the dikes, (2) the 

dikes are most abundant in the basal part of the rocks 

resting on the Heart Mountain fault, (3) many dikes 

extend downward to the Heart Mountain fault but 

none are below it, and (4) the dikes pinch out upward 

and exhibit an upward-branching trend.

GENERAL FEATURES OF CLASTIC DIKES IN UPPER-PLATE 

CARBONATE ROCKS

Calcibreccia dikes in blocks of Paleozoic rock in the 

upper plate of the Heart Mountain fault have been observed at four sites (areas 1, 3, and 6 and locality E, fig. 

1). These dikes were not formed by the movement of 

country rock along dike walls, because the adjoining 

rock is not brecciated and the dikes show abrupt 

changes in trend through the country rock. An example of this irregular trending is a 2- to 5-cm-wide calcibreccia dike in thin-bedded shale of the Three Forks 

Formation in area 1 (fig. 2).

Although numerous faults have been observed and 

mapped in Paleozoic rocks of the upper plate, none 

have been found that contain calcibreccia. The calcibreccia dikes observed in upper-plate Paleozoic rocks 

die out upward and can be traced downward to the 

Heart Mountain fault or, in most instances, to concealed areas within 1 m or so of the fault. Therefore, 

these dikes, which are composed of fault breccia, were 

evidently injected upward along fractures in the upper 

plate.

FIGURE 2.—Calcibreccia dike (cb) in steeply inclined thin-bedded 

shale of the Three Forks Formation in upper plate of the Heart 

Mountain fault in area 1, south of Silver Gate, Mont, (see fig. 3 for 

location). Dike is as much as 5 cm thick and pinches and swells as 

it alternately follows and cuts across bedding. End of hammer 

handle marks edge of dike.

GENERAL FEATURES OF CLASTIC DIKES 

IN VOLCANIC ROCKS

Calcibreccia dikes in volcanic rocks have been observed at 15 sites (areas 1-7 and localities A-H, fig. 1), 

nearly four times the number for those in the upper 

plate. This unequal distribution may be more apparent 

than real because (1) more of the Heart Mountain fault 

area is covered by the Wapiti Formation rather than by 

upper-plate rocks, and (2) calcibreccia dikes are more 

likely to be observed in the volcanic rocks because of 

the greater contrast between dikes and country rock.

The volcanic rocks in which calcibreccia dikes occur 

almost exclusively are in the lowermost part of the 

early or middle Eocene Wapiti Formation, which is 

composed of volcanic breccia and of poorly sorted andesitic volcaniclastic rocks and lava flows; the calcibreccia dikes are more commonly found in the volcanic 

breccia. Much of the trachyandesitic material in the 

breccia consists of volcanic detritus, either pyroclastic 

rock or fragments of preexisting solid volcanic rocks of
DESCRIPTION OF DIKES AT SPECIFIC SITES

the Wapiti Formation. The matrix of the breccia is fine 

grained and is composed dominantly of silt- to coarse 

sand-size detritus.

Although some calcibreccia dikes in the Wapiti Formation consist entirely of carbonate fault breccia, most 

contain scattered fragments of volcanic rock. A few 

dikes have a central core of calcibreccia with borders of 

volcanic dike rock (fig.6), or with borders of mixed calcibreccia and volcanic rock (fig. 8). One calcibreccia 

dike contains carbonized wood (fig. 5) and another, 

fragments of Precambrian granitic rock (fig. 8). Several 

dikes consist of volcanic breccia mixed with a minor to 

moderate amount of carbonate breccia; this breccia is 

referred to as a mixed breccia. The dikes usually range 

in thickness from a few centimeters to 0.5 m; commonly they pinch and swell and are discontinuous, but 

some have a fairly uniform thickness over short distances. The calcibreccia also occurs as irregular bodies, 

lenses, and pods in the volcanic rocks (figs. 17, 21, and 

22), and is irregularly distributed over or against small 

blocks of the upper plate (fig. 10); at one site (area 2, 

fig. 1), calcibreccia has flowed into a small antiform 

(fig. 13).

DESCRIPTION OF DIKES AT SPECIFIC SITES

Seven sites of dike occurrence to be discussed are 

numbered on the map (areas 1-7, fig. 1) : areas 1 to 3 

are in the Silver Gate-Cooke City, Mont., area; areas 4 

and 5 are 12 km and 14 km to the southeast, respectively; area 6 is near White Mountain, 32 km 

farther southeast; and area 7 is west of Trout Creek, 28 

km still farther to the southeast. Eight additional sites

of calcibreccia dikes are lettered (Iocs. A-H, fig. 1), and 

data on them are summarized in table 1.

Clastic dikes of fault breccia have not been described 

in detail in the geologic literature, so far as this author 

is aware, although fault gouge injected into cracks in 

the upper-plate rocks of the Muddy Mountain thrust, 

Nevada, have been noted (Stanley and Morse, 1974; 

Brock and Engelder, 1977). Their occurrence, however, 

in both upper-plate and volcanic rocks is most unusual 

if not unique. The exceptional circumstances associated with the dikes of Heart Mountain fault breccia 

and the collateral geologic information that they provide make it desirable to give a detailed account of a 

number of dikes, rather than merely a generalized description.

AREA 1, SOUTH OF SILVER GATE, MONTANA

The calcibreccia dikes in area 1, south of Silver Gate, 

Mont., are the most easily accessible and can be 

reached on foot by a 200-m climb up the steep 

mountain slope due south of the town. As indicated by 

figure 3, a variety of calcibreccia dikes can be seen 

there, both in upper-plate Paleozoic rocks and in the 

Eocene Wapiti Formation composed of volcanic rocks.

CALCIBRECCIA DIKES IN PALEOZOIC ROCKS OF THE UPPER

PLATE

On the far right of figure 3 is the previously mentioned calcibreccia dike in the Three Forks Formation, 

shown in figure 2. Left of center in figure 3 is a much 

smaller, highly deformed upper-plate block with calcibreccia dikes. There, a very small Heart Mountain

o o & o 0 O

o a <=> o

- p * - ° o " " *

*—— 0 °»°o ^« °

0 o o v Area of u ° o D c>

figures ^ C K/ 0 /^^X° * " ^> r:i/vf±^:_°^

ENE ^ a

0 C3 O

^ ° wsw

HMf Ob

_€gc.

50 METERS

Approximate scale

FIGURE 3.—Diagrammatic cross section of relations in area 1, south of Silver Gate, Mont., between calcibreccia dikes (cb) in Paleozoic rocks of 

the upper plate (Pz) and in volcanic rocks of the Wapiti Formation (Tw). Dots indicate flow banding adjacent to dikes and intermixing of 

calcibreccia with volcanic rock. HMf, Heart Mountain fault; td, surface of tectonic denudation; Ob, basal bed1 of Bighorn Dolomite; €gc, 

Grove Creek Limestone Member of the Snowy Range Formation. Faults in Paleozoic rocks do not extend into the Wapiti Formation. Not 

to scale.
6 CLASTIC DIKES OF HEAR! MOUNTAIN FAULT BRECCIA, NORTHWESTERN WYOMING

fault block of the Three Forks Formation, about 6 m 

across and 5 m high, contains several small irregular 

dikes of calcibreccia; one, about 1.5 m long, is in the 

west side of the block. The Wapiti Formation overlies 

this fault block, extends down its sides, and rests on the 

Heart Mountain fault surface of tectonic denudation 

adjacent to the block. In the east side of the block is one 

dike composed mainly of carbonate rock and some scattered fragments of shale of the Three Forks Formation, 

but the presence of volcanic rock fragments indicates 

that this dike was injected after the adjoining volcanic 

rocks were deposited. Evidence does not suggest that 

the dike material moved down, because that would be 

very unlikely, considering that the dike is composed 

almost entirely of fault breccia from beneath the block 

and from the adjoining surface of tectonic denudation. 

Rather, it appears that the calcibreccia in the dike 

moved up, and as it did so, a lateral flow occurred, both 

of underlying calcibreccia and of calcibreccia mixed 

with some volcanic rock from the adjoining area. Such 

general lateral flow of calcibreccia toward the dikes, as 

shown in figure 9, seems implicit as a means of acquiring the required volume of dike-filling material.

Layering is well developed in the calcibreccia parallel to the dike walls and is due to the flow of material 

through the dike. Although debris conceals a 1.5-m 

interval between the bottom of the dike and the Heart 

Mountain fault, the dike can be traced diagonally 

upward for 1.5 m to the edge of the fault block, where it 

terminates at the contact between the Three Forks 

Formation and a 1.5-m-wide dikelike body (not shown 

in fig. 3) that consists of mixed volcanic and carbonate 

breccia containing stringers and pod-shaped bodies of 

calcibreccia. Above and to the southwest, a carbonate 

dike in volcanic rocks of the Wapiti Formation penetrates this dikelike body for about 0.5 m before pinching out 1 m from the end of the calcibreccia dike intruding shale of the Three Forks Formation.

CALCIBRECCIA DIKES IN THE WAPITI FORMATION

The dikes in the Wapiti Formation indicated in 

figure 3 will be discussed beginning with_the one 

slightly to the right of the center of the figure. This one, 

shown in figure 4, is a lensing dike of calcibreccia and 

volcanic breccia about 25 m long, intruded into the 

Wapiti Formation. Rubble conceals a 7-m interval between the lowermost exposed part of the dike and the 

Heart Mountain fault. The dike strikes N. 30° E., has 

an arcuate shape, and is near vertical. A short distance 

from the bottom of the exposed portion of the dike is a 

detached block of Madison Limestone in the upper 

plate of the Heart Mountain fault (see figs. 3, 4). The 

Wapiti Formation, which covers the limestone block,

FIGURE 4.—Calcibreccia dike in area 1, south of Silver Gate, Mont., 

containing three components: calcibreccia (cb), volcanic breccia of 

the Wapiti Formation (vb), and mixed calcibreccia and volcanic 

breccia (mb) of the Wapiti Formation (Tw). The Madison Limestone (Mm) is part of detached block of upper plate lying on Heart 

Mountain fault 6 m below view, "x" indicates a 15-cm limestone 

clast.

contains a few limestone fragments as much as 30 m 

across that were presumably picked up and incorporated into the Wapiti as it was deposited over the block. 

The dike shown in figure 4 is made up of three components: calcibreccia, volcanic rocks of the Wapiti 

Formation, and a mixture of these two materials. In 

the central part of the dike is an irregular lenslike 

body of fine-grained calcibreccia containing scattered 

angular clasts of volcanic rock from less than 0.1 to 50 

cm or more in diameter. The bulk of the fragments, 

however, are 1 to 2 cm across. Where the volcanic 

fragments are 1 to 2 cm in size, they constitute no more 

than 10 percent of the calcibreccia; where they are 

smaller, they appear to form a lesser proportion of the 

calcibreccia. Within the calcibreccia, in the central
DESCRIPTION OF DIKES AT SPECIFIC SITES

part of the dike, is an irregular mass of breccia about 2 

m long composed of about equal proportions of carbonate and volcanic rock fragments as much as 30 cm 

across. This breccia body is elongated roughly parallel 

to the walls of the dike. At one place in the lower part 

of the calcibreccia is a limestone clast 15 cm across (fig. 

4). Volcanic breccia, much coarser than the calcibreccia, forms a zone 0.3 to 1 m wide on either side of the 

calcibreccia core and extends upward about 15 m above 

the core before pinching out. The volcanic breccia is 

intensely sheared and exhibits more or less vertically 

oriented, striated shear surfaces parallel 'o the dike 

walls. A specimen of the dike taken at th > boundary 

between the calcibreccia and volcanic breccia, showing 

a very narrow transitional contact 1 mm or so thick 

between the two rock types, indicates that the volcanic 

breccia was unconsolidated to some degree at the time 

the calcibreccia was injected. Within the specimen, 

elongate grains and fragments of volcanic rock in both 

the calcibreccia and volcanic breccia show a preferential lengthwise orientation parallel to the dike walls.

About 25 m above and to the southeast of this dike, a 

similar, less prominent, thinner body of calcibreccia 

occurs within a moderately distinct dike of volcanic 

breccia. This dike, indicated on figure 3 by an arrow to 

the area of figures 5 through 7, is about 120 m long, 

trends N. 55° E., and contains fragments of volcanic 

rock. Calcibreccia within the dike of volcanic breccia 

pinches and swells, is discontinuous, and is locally absent, ranging from less than a few centimeters to 30 cm 

in width. At one place in the dike are two thin (2-5 cm) 

tabular bodies of calcibreccia 0.5 m apart.

In the summer of 1973, while I was showing a group 

of visitors through this area, Charles Frye noticed a 

few small fragments of what appeared to be carbonized 

wood in one calcibreccia body in this dike; on further 

examination later in the summer, I identified definite 

particles of carbonized wood, some as much as 4 cm 

long (fig. 5), in calcibreccia in other parts of the dike. 

The wood occurs as numerous small pieces in a matrix 

of very fine grained carbonate breccia. Although fossil 

wood has been found in many places in the Wapiti 

Formation, the wood in this dike evidently was not 

secondarily derived from the volcanic rocks because it 

is entirely enclosed within calcibreccia and is free of 

associations to volcanic rock in the dike. Furthermore, 

the wood particles seem to have been incorporated into 

the calcibreccia after that breccia was formed because 

the wood is much too fragile to have withstood the deformation that produced the fault breccia. It is surprising that the wood has withstood the deformation accompanying its injection into the dike as well as it has, 

inasmuch as some of the wood, represented by very 

small specks in the calcibreccia, has thoroughly disinFIGURE 5=—Specimen of calcibreccia dike in area 1, south of Silver 

Gate, Mont., containing fragments of carbonized wood (W) and a 

few fragments of volcanic rock (V) in a matrix of carbonate breccia 

(cb). Photograph by Lowell Kohnitz.

tegrated. The surface of tectonic denudation, created 

by movement along the Heart Mountain fault, was exposed for perhaps only a few hours or days after the 

fault movement ceased before being buried by volcanic 

rocks of the Wapiti Formation (Pierce, 1968). During 

this extremely brief interval, the carbonized wood became a constituent of the Heart Mountain fault breccia 

presumably by aerial or aqueous transport from a short 

distance away. The conditions that permitted inclusion 

of this wood into the Heart Mountain fault breccia probably were extremely rare because carbonized wood 

has been found only in this one dike. However, similar 

fossil wood, albeit much silicified and presumably from 

the same source as that in the calcibreccia, has also 

been noted in volcanic breccia of the dike near its 

northeast end.

The body of calcibreccia with the carbonized wood 

occurs within the seemingly flow banded volcanic breccia of the Wapiti Formation that constitutes the main 

mass of the dike (fig. 6). The bodies of volcanic breccia 

in the dike that borders the calcibreccia are about 10 

cm wide, slightly darker than the adjoining wallrock, 

and contain a small amount of fine calcareous breccia. 

The core of calcibreccia at the southwest end of this 

dike lies about 25 m above the Heart Mountain fault, 

and the enclosing breccia of the Wapiti Formation 

pinches out a few meters higher up in the dike. At the 

northeast end of the dike, the discontinuous calcibreccia core can be traced down to within 3 m of the fault, 

below which it is concealed by talus. The contact between calcibreccia and volcanic breccia in the dike (fig. 

7) is extremely irregular rather than planar, and the 

fragments in the calcibreccia do not show a preferred 

orientation parallel to the contact. Very thin stringers 

and protuberances of volcanic breccia into the calcibreccia indicate that the volcanic rock was in a some-
CLASTIC DIKES OF HEART MOUNTAIN FAULT BRECCIA, NORTHWESTERN WYOMING

FIGURE 6.—Calcibreccia dike (cb) in area 1, south of Silver Gate, 

Mont., containing carbonized wood. The Wapiti Formation (Tw) 

seems to be flow banded (Twd) on both sides of calcibreccia dike.

what mobile state, that is, incompletely consolidated at 

the time of dike injection, and also that the fault 

movement which formed the calcibreccia preceded deposition of the Wapiti Formation.

One calcibreccia dike from a few to 45 cm wide (left, 

fig. 3), well exposed at the bottom and on the west side 

of a steep ravine, pinches and swells and is irregularly 

distributed within another, larger 1- to 2-m-wide dike 

of volcanic breccia of the Wapiti Formation that is 

sheared and has fairly well defined shear borders with 

the enclosing volcanic rock (fig. 8); the contact between 

the calcibreccia and intrusive volcanic breccia is gradational. The calcibreccia is predominantly carbonate 

rock but contains abundant fragments of volcanic rock. 

At the outer border of the calcibreccia is a 2-cm-wide 

zone of fine-grained rock composed of volcanic breccia 

intermixed with a small amount of calcibreccia. The 

contact of the mixed breccia with the calcibreccia and 

the outer contact with the volcanic breccia are both 

gradational through a thin zone.

In addition to this irregular but more or less continuous body of calcibreccia (fig. 8), other discontinuous 

stringers less than 1 cm to a few centimeters wide 

occur within the dike. The composite dike of volcanic 

breccia and calcibreccia is bordered by vertically 

sheared dikelike bodies of rock of the Wapiti Formation. The dikelike body on the south side (left, fig. 8) is 

about 3.5 m wide, with the outer half less sheared and 

its outer border less well defined than its inner border. 

The dikelike body of the Wapiti on the north side of the 

calcibreccia dike (right, fig. 8) is mostly covered, but 

the part that can be observed is similar to its counterpart on the south side of the dike.

An unusual feature of the dike shown in figure 8 is 

the occurrence of fragments of Precambrian granitic 

rock in the rocks that form the dike and in the dikelike

FIGURE 7.—Caption on facing page.
DESCRIPTION OF DIKES AT SPECIFIC SITES

FIGURE 7.—Polished specimens of calcibreccia dike (cb) and adjoining dikelike body of the Wapiti Formation (Twd) in area 1, south of 

Silver Gate, Mont. Very thin stringers (not shown here) and protuberances of Wapiti Formation into calcibreccia indicate that 

country rock was incompletely consolidated at time calcibreccia 

dike of Heart Mountain fault breccia was injected into it. Photographs by Lowell Kohnitz.

bodies of sheared Wapiti volcanic breccia that border 

it. The xenoliths in the dike are angular and range 

from 1 or 2 cm across in the calcibreccia to 1 m across in

FIGURE 8.—Calcibreccia dike (cb) in area 1, south of Silver Gate, 

Mont., within and part of a dike of the Wapiti Formation (Twd). 

Calcibreccia dike and enclosing volcanic breccia both contain 

xenoliths of Precambrian rock (p€). The bordering, slightly less 

dikelike bodies of the Wapiti Formation (Twdl) also contain 

xenoliths of Precambrian rocks. None of the dikes extend beneath 

Heart Mountain fault (3 m below view shown).

the adjacent dikelike bodies of Wapiti breccia. The 

xenoliths occur in the lowest exposed parts of the dike 

and adjacent breccia, about 3 m above the Heart 

Mountain fault, and are scattered upward for 15 m in 

the dike and in the Wapiti that borders it. The Precambrian fragments in the vertically sheared Wapiti 

breccia occur close to the dike.

Inasmuch as the calcibreccia originally was irregularly distributed between the base of the Wapiti Formation and the surface of tectonic denudation, the calcibreccia must have flowed laterally and upward to 

form the dike, as illustrated in figure 9. Vertical striations and shears in the rocks of the Wapiti Formation 

that enclose the calcibreccia, as well as the relation
10 CLASTIC DIKES OF HEART MOUNTAIN FAULT BRECCIA, NORTHWESTERN WYOMING

1 METER

Approximate scale

FIGURE 9.—Diagrammatic cross section of calcibreccia dike (cb) on left side of figure 3 in area 1, south of Silver Gate, Mont., showing its 

relation to the Heart Mountain fault carbonate breccia (fb), dike (Twd), dikelike (Twdl) bodies of Wapiti Formation, and bordering 

rocks of the Wapiti Formation (Tw). p€, angular xenoliths of Precambrian granitic rocks; Ob, basal bed of Bighorn Dolomite; -Ggc, 

Grove Creek Limestone Member of Snowy Range Formation; td, Heart Mountain fault surface of tectonic denudation. Arrows indicate 

direction of flow.

and mixing of the volcanic rock and calcibreccia, indicate that the Wapiti must have flowed with the 

carbonate fault breccia. The angularity of the granitic 

clasts, their wide range in size, and their lack of association with any sedimentary deposits seem to rule out 

any derivation by stream transport. A search was 

made for clasts of Precambrian rock in the basal part of 

the flat-lying Wapiti Formation but without success; 

however, about 100 m to the southwest, near the location of figure 4 shown on figure 3, crystalline clasts 

were found less than 1 m above the base but not higher 

up in the sequence. The basal volcanic rocks overlying 

a 25-m-high mass of deformed Devonian rocks in area 

3, north of Republic Mountain, also contain clasts of 

Precambrian rock. So far as is known, the Wapiti Formation never rests directly on Precambrian crystalline 

rocks, and Precambrian clasts have never been observed in the Heart Mountain fault breccia that is 

overlain by upper-plate rocks. Therefore, these clasts 

are interpreted to be xenoliths torn from the walls of a

vent and incorporated into the basal part of the volcanic rocks immediately after the Heart Mountain 

fault movement.

AREA 2, SOUTHEAST OF SILVER GATE, MONT.

In area 2, southeast of Silver Gate, Mont., most of the 

calcibreccia dikes and related features are exposed on 

the east side of Falls Creek, and these features are 

described first. Here, the upper part of the Grove Creek 

Limestone Member of the Snowy Range Formation is 

well exposed on the east side of a steep V-shaped valley 

and is overlain by 2 to 3 m of the basal bed of the 

Bighorn Dolomite. The top of this bed is the surface of 

tectonic denudation produced by movement on the 

Heart Mountain fault. Resting on this surface is (1) a 

small block of fault-transported Madison Limestone 

about 3 m high and (2) as much as 2/3 m of carbonate 

fault breccia and mixed carbonate and volcanic breccia 

that are buried by the Wapiti Formation. The volcanic
DESCRIPTION OF DIKES AT SPECIFIC SITES 11

cover contains numerous irregular to dikelike bodies of 

calcibreccia. These features are diagrammatically 

shown to the center and left in figure 10. The fault 

contact between the block of Madison Limestone and 

the underlying basal bed of the Bighorn Dolomite is 

well exposed, and no carbonate fault breccia or volcanic 

breccia occur there. This block, as well as numerous 

other blocks of the Madison Limestone resting on the 

fault surface, are considered to have toppled off the 

trailing edge of a larger mass and come to rest on the 

surface of tectonic denudation before the overlying volcanic rocks were deposited (Pierce, 1957).

Also in area 2, a yellow-weathering calcibreccia 

commonly containing a small amount of volcanic rock 

is irregularly distributed between the basal bed of the 

Bighorn Dolomite and the overlying Wapiti Formation; a thin zone of mixed carbonate and volcanic breccia at the base of the volcanic cover overlies the layer of 

calcibreccia. The contact between the Wapiti Formation and the calcibreccia dike shown in figure 11 suggests that the volcanic rocks were not fully consolidated at the time of contact with the calcibreccia. Apparently, a considerable amount of fault breccia lay 

loose on the fault surface in this area as a result of the 

cataclysmic fault movement because a layer of calci - 

breccia 1 to 2 cm thick is plastered irregularly onto the 

sides and top of the block of Madison Limestone, as 

shown in figure 10. The contact between the Madison 

and the calcibreccia is sharp and distinct, that between 

the calcibreccia and mixed breccia is less distinct, and 

that between the mixed breccia and the Wapiti Formation is least distinct.

FIGURE 11.—Polished specimen of calcibreccia dike in area 2, on east 

side of Falls Creek. Curving plume of calcibreccia (cb) extending 

into the Wapiti Formation (Tw) suggests semifluid mixing Photograph by Lowell Kohnitz.

Numerous irregular pods and dikelike masses as 

much as 1 m across, commonly tapering into short 

dikelike forms composed of light-tan- to yellowishweathering calcibreccia, occur in the basal 1 to 2 m of 

the Wapiti Formation in area 2. The central parts of

10 METERS

Approximate scale

FIGURE 10.—Diagrammatic cross section in area 2, southeast of Silver Gate, Mont., illustrating relation of calcibreccia to Heart Mountain 

fault (HMf), surface of tectonic denudation (td), and the Wapiti Formation (Tw). Mm, Heart Mountain fault block of Madison Limestone; 

cb, fault breccia (calcibreccia) on surface of tectonic denudation and in dikes and irregular bodies in the Wapiti Formation; mb, mixture 

of calcibreccia and volcanic breccia; Ob, basal bed of Bighorn Dolomite; -€gc, Grove Creek Limestone Member of the Snowy Range 

Formation. Not to scale.
12 CLASTIC DIKES OF HEART MOUNTAIN FAULT BRECCIA, NORTHWESTERN WYOMING

these bodies usually consist almost entirely of calcibreccia and a few volcanic fragments, although some 

pods contain numerous small lenses and inclusions of 

volcanic rock as much as a few centimeters across. The 

outer parts of the calcibreccia bodies generally contain 

more volcanic rock than the inner parts, and the adjoining wallrock contains some calcibreccia. The contacts between some calcibreccia masses and the enclosing Wapiti Formation are sharp and distinct, but other 

contacts are gradational. Not uncommonly, a single 

calcibreccia mass may display both kinds of contacts, A 

few meters higher up in the Wapiti but usually within 

the lower 10 m of the volcanic rocks, the calcibreccia 

occurs in dikelike bodies a few meters long and as 

much as 0.3 m across. These bodies commonly contain 

some volcanic rock derived from the enclosing Wapiti 

Formation, and the adjoining rocks also contain some 

calcibreccia (fig. 12).

An exceptionally well exposed body of calcibreccia in 

a cliff 15 m west of Falls Creek in area 2 (fig. 13), in the 

same mass of the Wapiti Formation as dikes on the 

east side of the creek (fig. 10), forms a unique feature 

about 2 m high, slightly resembling an anticlinal 

fold—not a true fold, but apparently related more to 

flowage than folding. For lack of a better term, this 

feature is here referred to as an antiform. The base of 

this body is exposed to within 1 m of the Heart 

Mountain fault and presumably extends downward to 

the fault; 2 m to the west (right, fig. 13), the Wapiti

FIGURE 12,—Upper end of calcibreccia dike (cb) 5 m above base of 

Wapiti Formation (Tw) and Heart Mountain fault, area 2. Calcibreccia contains a minor amount of volcanic rock derived from 

enclosing country rock, and adjoining country rock contains some 

carbonate breccia. The mixing of calcibreccia and volcanic breccia 

and the contacts between them indicate that the Wapiti Formation 

was emplaced in a mobile or semifluid state onto carbonate fault 

breccia, which is now incorporated into it as dikes and irregular 

masses.

FIGURE 13.—View southwest along trend of an antiform, or calcibreccia mass resembling an anticlinal fold, at base of the Wapiti 

Formation (Tw) in area 2, 15 m west of Falls Creek. Shearing 

surfaces (s) in inner part of the calcibreccia (cb) also have an anti - 

clinal form. Irregular masses of calcibreccia border flanks of antiform, and lozenge-shaped mass of calcibreccia lies in a shear plane 

along right flank of the antiform. To right, the Wapiti Formation 

rests on Heart Mountain fault surface of tectonic denudation (td) 

and on basal bed of Bighorn Dolomite (Ob). The Wapiti Formation 

near and adjoining calcibreccia is calcareous, a feature suggesting 

that some calcibreccia became mixed with it during deposition on 

the fault surface.

Formation can be seen to rest on the fault surface. The 

absence there of carbonate fault breccia is not surprising, however, inasmuch as the fault breccia was swept 

off the fault surface into the antiform. The calcibreccia 

on the flanks of the antiform is composed of angular 

carbonate fragments, less than 1 to a few millimeters 

across, bonded in a carbonate matrix containing scattered angular fragments of volcanic rock 2 to 8 cm 

across. Polished specimens of the calcibreccia do not 

show any distinct or layered orientation of fragments, 

although a few thin, wavy layers a few centimeters 

long of contrasting material suggest flowage. The volcanic breccia of the Wapiti Formation in which the 

antiform occurs is highly sheared and fractured on the 

flanks of and above the antiform; these features become less distinct and disappear farther out on the 

flanks. The Wapiti Formation above this body is 

inaccessible, but the lines of shear that can be observed 

from a distance are inclined at about 45° parallel to the 

inner shearing surfaces (fig. 13) and apparently die out 

upward in an inverted "V". The fact that the steeper 

side of the antiform lies on the southeast suggest a 

movement from northwest to southeast; the fault surface, on which the Wapiti Formation was deposited 

(that is, the surface of tectonic denudation), sloped to 

the southeast in the general direction of movement 

along the Heart Mountain fault.
DESCRIPTION OF DIKES AT SPECIFIC SITES 13

AREA 3, NORTH OF REPUBLIC MOUNTAIN

In area 3, on the north side of Republic Mountain, 

several calcibreccia dikes occur in the eastern part of a 

highly brecciated and fractured limestone mass that 

probably belongs to the Jefferson Formation (fig. 26). 

One dike, 8 m long and as much as 15 cm wide, extending to within about 1.5 m of the Heart Mountain fault, 

consists essentially of carbonate rock with some specks 

of asphalt; no volcanic rock was observed. A few meters 

east of this dike, an irregular but somewhat linear 

mass as much as 1 m across of calcibreccia of the same 

composition has sharp contact with the enclosing limestone, which is contorted, deformed, and brecciated. 

The fact that this deformation does not extend to any of 

the calcibreccia dikes or to their contacts with the 

limestone is convincing evidence that injection of the 

dikes occurred after the enclosing rock was deformed.

Overlying this brecciated limestone block are volcanic rocks of the Wapiti Formation, the lower 1 m or 

so of which contains a few scattered angular xenoliths 

of Precambrian rock. Along the contact and within the 

basal few meters, thin irregular streaks and discontinuous layers of calcibreccia occur; several short, thin 

calcibreccia dikes extend several meters upward from 

the contact into the Wapiti Formation (fig. 14). One 

small dike, less than 1 mm wide in its lower part but 

increasing upward in width to several centimeters,

25 mm 

I____I

FIGURE 14.—Polished specimen of calcibreccia dike in area 3, north 

of Republic Mountain. Mixture (mb) of calcibreccia (cb) and volcanic breccia of the Wapiti Formation (Tw) suggests that the 

Wapiti Formation was unconsolidated at the time the calcibreccia was intruded. Photograph by Lowell Kohnitz.

seems to indicate that the lower part of the dike may 

have been squeezed together after the calcibreccia was 

injected. Such an event may explain the fact that many 

calcibrecci dikes cannot be traced to their points of origin within the underlying Heart Mountain fault breccia.

AREA 4, SOUTH OF PILOT CREEK

Area 4 is about 1 km south of Pilot Creek, near the 

foot of a waterfall of a north-flowing stream that heads 

on the west side of Jim Smith Peak. The Grove Creek 

Limestone and the basal bed of the Bighorn Dolomite, 

which has the Heart Mountain fault surface of tectonic 

denudation at its top, are well exposed on the east side 

of the valley from the waterfall to a point 150 m north. 

The 2-m-thick basal bed of the Bighorn Dolomite is 

overlain either by carbonate fault breccia or by volcanic rocks of the Wapiti Formation. The volcanic 

rocks contain several dikes and irregular bodies of calcibreccia. At a site 300 m to the north, an upper-plate 

block of Madison Limestone 30 m long and 10 m high 

rests on 30 to 35 cm of fault breccia. A layer of calcibreccia 0.7 m thick (figs. 15, 16), exposed 100 m north 

of the waterfall, rests on the basal bed of the Bighorn 

Dolomite and is overlain in turn by the Wapiti Formation. No volcanic rock was observed at these sites in the 

fault breccia, which consists almost entirely of carbonate rock and a few very fine fragments of chert. A 

murky-brown submicroscopic matrix in the uppermost 

part of a polished specimen of the breccia (fig. 16) gives 

that part a darker color.

Also in area 4, 15 m north of the basal bed of the 

Bighorn Dolomite shown in figure 15, a mound of 

Heart Mountain fault breccia almost 2 m high rests on 

the basal bed of the Bighorn Dolomite and is adjoined 

and was overlain by the Wapiti Formation (fig. 17). A 

polished specimen that was collected near the top of 

this mound of breccia (fig. 18) is composed entirely of 

carbonate rock displaying a well-defined wavy to crinkled layering. A specimen from the lower part of the 

mound is similar, except that high-angle offsets of 1 to 

2 mm in the layering are more numerous than in the 

upper part. It is suggested that the crinkled, wavy 

layering with high-angle offsets may reflect strong 

earth tremors at the time the fault breccia was unconsolidated.

The Wapiti Formation, which overlies the Heart 

Mountain fault in area 4, contains several calcibreccia 

dikes, some near the fault and others higher up, even 

as much as 20 to 35 m above the fault. One dike lower 

down is distinctly layered parallel to the dike walls, 

and nonspherical fragments in the dike are similarly 

alined (fig. 19). The contact between the dike and the 

enclosing volcanic rock is irregularly convolute. Many
14 CLASTIC DIKES OF HEART MOUNTAIN FAULT BRECCIA, NORTHWESTERN WYOMING

FIGURE 15.—Heart Mountain fault breccia (calcibreccia) on fault in 

area 4, south of Pilot Creek; Ob, basal bed of the Bighorn Dolomite; 

fb, fault breccia. The Wapiti Formation (Tw) is cut by small dikes 

of carbonate fault breccia (cb). Head of hammer marks base of fault 

breccia.

of the dikes in this area are curving in plan and inclined from vertical to 45° or less, others are inclined at 

a very low angle, and a few are nearly horizontal. One 

that is 1 m thick and inclined at 35° has a fluted top 

and is 10 m above the Heart Mountain fault; it consists 

mostly of carbonate rock, probably dolomite, because it 

effervesces slowly on acid treatment and contains, in 

addition, some volcanic material. Another calcibreccia 

dike (fig. 20) is nearly horizontal and pinches and 

swells irregularly.

About 15 m further east and 35 m above the Heart 

Mountain fault is an irregular dikelike mass of calcibreccia 0.5 to 0.7 m thick and 7 m across, composed 

mostly of carbonate rock; its east border is steeply inclined to vertical, and the west margin is inclined

25mm

FIGURE 16.—Polished specimen of 

upper third of Heart Mountain fault 

breccia, showing its angular character. Top of specimen is at contact 

with the overlying Wapiti Formation. Photograph by Lowell Kohnitz.

FIGURE 17.—Mound of Heart Mountain fault breccia (fb) in area 4, 

south of Pilot Creek. The mound, nearly 2 m high, is composed 

entirely of carbonate breccia and rests on the basal bed of the 

Bighorn Dolomite (Ob).
DESCRIPTION OF DIKES AT SPECIFIC SITES 15

I

FIGURE 18.—Polished specimen of calcibreccia from near top of 

2-m-high mound in figure 17. Note unusual crinkled, wavy layering. Photograph by Lowell Kohnitz.

about 20°. As revealed in a polished specimen, a 1-cmthick layer of calcibreccia adjoining the volcanic country rock is finer grained than the rest of the calcibreccia in the dikelike mass but is not layered and shows no 

distinct orientation of fragments. The bloblike mass, 

which crops out 15 m or more east of the other calcibreccia bodies, could not be traced downward; it exhibits a sharp but highly irregular contact with the volcanic wallrock, which is darker than the rock enclosing 

the other calcibreccia bodies.

AREA 5, NORTH OF JIM SMITH PEAK

Area 5 is about 0.5 km east of area 4 and extends 

southeast for 1.5 km along the Heart Mountain fault to 

Jim Smith Creek, which crosses the fault northeast of 

Jim Smith Peak. In the northwestern part of this site, 

two large upper-plate blocks of Madison Limestone, 

each about 80 m long and 20 to 35 m high, as well as 

three smaller upper-plate blocks, one of Madison Limestone and the other two of the Three Forks and Jeffer2cm

FIGURE 19.—Polished specimen of calcibreccia dike (cb) in lower part 

of the Wapiti Formation (Tw) in area 4, south of Pilot Creek, showing irregularly convolute contact with wallrock and layering parallel to dike wall. Photograph by Lowell Kohnitz.

FIGURE 20.—Flat-lying calcibreccia dike (cb) in area 4, south of Pilot 

Creek. Dike pinches and swells irregularly and is about 20 m 

above base of the enclosing Wapiti Formation (Tw), which rests on 

the Heart Mountain fault surface of tectonic denudation.

son Formations, rest on the basal bed of the Bighorn 

Dolomite, which is of less than its normal thickness 

here. Underlying the large block of Madison Limestone 

farthest to the northwest is 5 to 8 cm of fault breccia; no 

volcanic rock fragments occur in the fault breccia or in 

the adjoining rocks. About 35 m southeast of this large 

block, a smaller block of Madison Limestone, about 5 m 

high and 8 m across, crops out in the middle of a steep
16 CEASTIC DIKES OF HEART MOUNTAIN FAUET BRECCIA, NORTHWESTERN WYOMING

ravine. Adhering to the top of the small block is an 

irregular layer of mixed calcibreccia and volcanic breccia 0.5 to 4 cm thick; resting on the breccia layer are 

volcanic rocks of the Wapiti Formation, the lowermost 

several centimeters of which contains lenses of mixed 

calcibreccia and volcanic breccia (fig. 21). The wavy to 

cuspate contact between the calcibreccia lenses and the 

volcanic rocks indicates that both rock types were unconsolidated at the time of intermixing.

Between these upper-plate limestone blocks just described and Jim Smith Creek, which heads just east of 

Jim Smith Peak, the Heart Mountain fault is well exposed along steep ravines. Along this segment of the 

fault the Wapiti Formation rests on the basal bed of the 

Bighorn Dolomite, which ranges in thickness from less 

than 1 m near Jim Smith Creek to 2 m in the area to 

the west. At the bottom of a ravine 50 m west of the 

creek, 1 m of the Heart Mountain fault breccia rests on 

this basal bed. This carbonate fault breccia, which has 

considerable volcanic rock mixed with it, including 

abundant angular and subangular volcanic fragments 

from 0.5 to 15 cm across, is overlain by Wapiti rocks 

showing no evidence of tectonic brecciation. At this 

same site, a small calcibreccia dike 15 cm thick extends 

upward from the fault breccia into the Wapiti Formation, and several other calcibreccia dikes of similar size 

occur nearby in the lowermost part of the volcanic 

rocks. In some places the base of the volcanic rocks has 

some carbonate rock mixed with it, and in others it 

does not. Where carbonate rock occurs, it is either intimately mixed with the volcanic rocks or forms irregular stringers or layers, such as the one shown in figure 

22.

Prostka (1978, p. 430-431) has suggested that in the 

area north of Jim Smith Peak (areas 4 and 5 of this 

report), rocks from the top part of Heart Mountain 

fault blocks have spilled forward over the leading edge 

of the blocks, owing to their momentum as the fault 

blocks were coming to rest; these features he called 

"low-angle ramp structures or spillovers." He cited as 

evidence steep dips to the southeast —in the direction 

of sliding—on the leading edges of volcanic rocks; 

low-angle ramp structures were found to occur only on 

the leading edges of volcanic fault masses, whereas the 

trailing edges and sides of blocks are steep. He concluded "*** that the ramp structures were not caused 

by simple collapse and slumping of steep slopes, but 

that the momentum of the moving volcanic masses was 

necessary to produce the spillovers." He believed that 

these volcanic rocks are fault emplaced, but to me the 

relation between the Heart Mountain fault breccia and 

the volcanic rocks resting on the Heart Mountain fault 

(figs. 15-17, 21, 22), as well as the occurrence of calcibreccia dikes, indicates that the volcanic rocks were deposited on the fault surface.

FIGURE 21.—Polished specimen of calcibreccia (cb) interlayered with 

basal part of the Wapiti Formation (Tw) in area 5, north of Jim 

Smith Peak. Inasmuch as calcibreccia was formed concurrently 

with the Heart Mountain fault movement, the volcanic rocks with 

which it is interlayered must be postfaulting. Wavy to cuspate 

interlayering indicates that the volcanic rock was unconsolidated 

at the time the calcibreccia was mixed with it. Photograph by 

Lowell Kohnitz.

FIGURE 22.—Irregular layer 2 to 5 cm thick of carbonate breccia or 

calcibreccia (cb) 1.5 m above base of the Wapiti Formation (Tw) in 

area 5, west of Jim Smith Creek. Volcanic breccia also has carbonate breccia intimately mixed with it.

Observations mentioned above in discussing areas 4 

and 5 negate the spillover concept and indicate that 

most of the volcanic rocks are depositionally emplaced 

in these areas. Several small fault blocks of Paleozoic 

carbonate rock north of Jim Smith Peak, shown on the 

geologic map of the Pilot Peak quadrangle (Pierce and 

others, 1973), rest on the Heart Mountain fault. Volcanic rocks overlie these blocks and extend downward 

to the fault surface, as shown in figures 3 and 10. Four 

exposures of fault breccia—one overlain by volcanic 

rock (fig. 15) and three overlain by Paleozoic rocks— 

consist entirely of carbonate fault breccia that contains
DESCRIPTION OF DIKES AT SPECIFIC SITES 17

no volcanic material. These occurrences of carbonate 

fault breccia without volcanic material are incompatible with Prostka's spillover concept and with his conclusion that all of this volcanic rock is fault emplaced 

as part of the Heart Mountain fault movement.

AREA 6, NEAR WHITE MOUNTAIN

In area 6, near White Mountain, calcibreccia dikes 

occur in upper-plate rocks on the southwest side of the 

mountain and at sites 1,900 and 2,100 m to the eastnortheast(Nelson and others, 1972, fig. 4). At the site 

1,900 m away, some fragments and disconnected pieces 

of igneous dike rock occur in a broken and faulted 

upper-plate block of the Bighorn Dolomite. One of the 

calcibreccia dikes at this locality passes through one of 

the igneous dike fragments, a feature indicating that 

the igneous rock is older than the clastic dike of Heart 

Mountain fault breccia and evidence compatible with 

the conclusion of Nelson, Pierce, Parsons, and Brophy 

(1972) that some of the igneous dikes at White 

Mountain are older than the Heart Mountain fault 

movement. Above and to the northeast of the block 

containing calcibreccia dikes in the Bighorn Dolomite, 

a well-exposed high-angle fault brings the Jefferson 

Formation into contact with the Bighorn Dolomite; 

however, no fault breccia occurs along this fault, nor 

has fault breccia similar to that in the calcibreccia 

dikes been found in the numerous other high-angle 

faults mapped at White Mountain. Furthermore, although calcibreccia dikes occur along fractures in the 

upper plate, the trends of many clastic carbonate dikes 

at White Mountain are too irregular for the enclosed 

carbonate material to have been fragmented by movement of one side of the dike relative to the other (Nelson and others, 1972).

At the site 200 m farther northeast, two clasts of 

dark-gray shale less than 1 m apart are enclosed in 

about the middle of a calcibreccia dike in dolomitic 

limestone, about 2 m thick and dipping 25° west: one is 

15 cm across and well rounded, the other about 13 cm 

long, 3 cm across, and well tapered at both ends. Although both clasts resemble and presumably were derived from the Three Forks Formation, it is not known 

whether they came from the fault breccia or from the 

wallrock. The dike itself pinches out about 7 m above 

the clasts.

The calcibreccia dikes on the southwest side of White 

Mountain are unusual in that they intrude a block of 

Paleozoic rock of the upper plate that is metamorphosed and strongly folded in contrast to other Heart 

Mountain fault blocks. This metamorphism ends abruptly downward at the Heart Mountain fault. 

Rounded fragments of carbonate rock in the fault breccia show that the metamorphism occurred before fault 

movement. Another unusual feature of these calcibreccia dikes is that they contain abundant fragments of 

igneous rock. These dikes can be traced down to a thick 

layer of fault breccia at the base of the upper plate that 

is similarly rich in igneous fragments derived from 

igneous dikes injected into the upper plate before it 

was emplaced by the Heart Mountain fault movement 

(Nelson and others, 1972). However, the absence of 

such fragments in the shale-containing dike 2,100 m 

east-northeast and in the closest body of fault breccia 

600 m northeast suggests that the source of the calcibreccia in each dike was fault breccia from within a 

very limited area. If, as seems likely, the carbonate 

dikes containing igneous fragments were derived from 

a small body of fault breccia at the base of the upperplate block that contains them, then these dikes must 

have have been injected after the upper-plate block 

was emplaced.

AREA 7, WEST OF TROUT CREEK

Area 7 lies on the west side of Trout Creek, at the 

south tip of a narrow strip of upper-plate Madison 

Limestone that is bordered and partly overlapped on 

the west by volcanic rocks of the Wapiti Formation, 

which were deposited on the Heart Mountain fault surface (the surface of tectonic denudation). The volcanic 

rocks in area 7 are for the most part flow breccia but 

include some lighter, hornblende-bearing polybreccia 

similar to the Cathedral Cliffs Formation. The vertical 

contact between the volcanic rocks and the upper-plate 

block of Madison Limestone is well exposed and accessible in the lower and upper parts of a steep face 100 m 

high but inaccessible in the intermediate part. At the 

lower exposure, lenses and stringers of calcibreccia 

occur in a dike about 3 m wide, somewhat variable and 

irregular in width, composed of volcanic rocks of the 

Wapiti Formation (fig. 23), similar to the dikes of volcanic rock containing calcibreccia described in the section "Area 1, South of Silver Gate, Montana." (See figs. 

4, 6, and 8.) The contact between this dike of volcanic 

rock and the Madison Limestone is sharp but irregular 

to wavy, and the adjoining limestone is not sheared or 

brecciated. On the other hand, the contact between volcanic breccia of the dike and the Wapiti Formation is 

indistinct because their compositions are much the 

same, the only differences being the vague, more or 

less horizontal flow lines in the Wapiti Formation, the 

presence of carbonate breccia in the dike, and the steep 

to vertical shears in the volcanic breccia of the dike 

(fig. 24).

The calcibreccia stringers in the dike are vertically 

alined and occur in a zone about 1 m wide adjacent to 

the limestone. Vertical lineations and striations in the
18 CLASTIC DIKES OF HEART MOUNTAIN FAULT BRECCIA, NORTHWESTERN WYOMING

WNW

0 30 CENTIMETERS

FIGURE 23.—Cross section in area 7, on west side of Trout Creek, showing relations of a dike of the Wapiti Formation (wd) enclosing irregular 

stringers of carbonate fault breccia (cb) to country rocks of the Wapiti Formation (Tw) and an upper-plate block of Madison Limestone 

(Mm). Scattered dots in dike of Wapiti Formation indicate some intermixed carbonate rock. Stippled area at bottom of left diagram 

indicates talus cover. Heart Mountain fault (HMf) and surface of tectonic denudation (td), which are obsured by talus, lie about 20 m 

below lowermost exposure.

dike of volcanic rock are also most pronounced in this 

zone. The part of this zone that could be observed and 

sampled lies in the lowermost 4 m of outcrop, above 

which it becomes inaccessible and apparently contains 

no calcibreccia because none could be seen from a distance. Most calcibreccia stringers are irregularly 

layered parallel to the dike walls (fig. 25A) and are

composed almost entirely of carbonate breccia, even 

near the contact with the enclosing volcanic breccia. 

One irregular calcibreccia pod (upper left of enlarged 

diagram, fig. 23), however, exhibits irregular wavy 

bands, and the irregular, nonparallel banding is horizontal to inclined rather than vertical (fig. 25B). The 

presence of carbonate rock in the volcanic matrix ad-
DESCRIPTION OF DIKES AT SPECIFIC SITES 19

FIGURE 24.—Dike of volcanic breccia of the Wapiti Formation containing calcibreccia (cb) in contact with edge of a Heart Mountain 

fault block of Madison Limestone in area 7, west of Trout Creek. 

Note sharp, irregular to wavy contact between the Wapiti Formation (Tw) and the Madison Limestone (Mm), and vertical shear (s) 

between irregular pod of calcibreccia to left (on which hammer 

rests) and calcibreccia stringer on right. For relation to general 

area, see figure 23. Photograph by Allan Krill.

joining these stringers indicates that botl _ the volcanic 

matrix and the calcibreccia were unconsoiidated at the 

time of injection.

The upper, accessible part of the contact between the 

Wapiti Formation and the Madison Limestone is about 

100 m above the lower part. Here, as in the lower part, 

a distinct lineation occurs in the volcanic rocks parallel 

to the vertical contact between them and the limestone

FIGURE 25.—Polished specimens of calcibreccia from a dike of volcanic rock in area 7, on west side of Trout Creek (see fig. 23 for 

locations of specimens within dike). A, Stringer of irregularly 

banded calcibreccia (cb) composed almost entirely of carbonate 

breccia, even along contact with volcanic rocks of the Wapiti Formation (Tw). Banding is alined with stringer wall. B, Irregular 

pod of calcibreccia. Very irregular, wavy layering is not alined 

vertically with wall of dike. Photographs by Lowell Kohnitz.
20 CLASTIC DIKES OF HEART MOUNTAIN FAULT BRECCIA, NORTHWESTERN WYOMING

(fig. 23) but does not seem to be a dikelike feature. The 

lineation forms a zone about 7 m wide that dies out 

irregularly westward in the volcanic rocks and upward 

near the top of the limestone (fig. 23). The lineation 

seems to be a flowage feature apparently formed as the 

volcanic breccia flowed along the face of the limestone 

block and, as it did so, cut horizontal grooves in the 

limestone. Near the middle of the zone is a lens of brecciated limestone, probably of Madison Limestone, 

about 0.7 m wide and 7 m high, with a small prong of 

predominantly volcanic rock extending out horizontally (fig. 23), which is exposed in a nearly vertical face 

so that its horizontal dimension is not measurable. 

Much of the limestone breccia is composed of limestone 

fragments 0.2 to 0.5 cm long, some cobble size, 8 to 15 

cm across, quite unlike the Heart Mountain fault breccia. No stringers of calcibreccia similar to those 100 m 

below occur in the volcanic rocks, nor are these rocks 

calcareous; the zone of lineation disappears about 25 m 

below the lens of brecciated limestone.

Several additional small masses of calcibreccia as 

much as 0.7 m across occur in a large area of loose 

volcanic landslide debris near Trout Creek, 2 km 

northeast of area 7; their source is unknown.

OTHER SITES OF CLASTIC DIKES

In addition to the seven sites of clastic dikes of the 

Heart Mountain fault breccia that have been described 

above (areas 1-7, fig. 1), eight additional sites (Iocs. 

A-H, fig. 1) are known, information on which is summarized in the accompanying table (table 1).

BRECCIA AND BRECCIATION PRODUCED 

BY THE HEART MOUNTAIN FAULT

The Heart Mountain fault movement caused extensive brecciation of rocks in the lower part of the upper 

plate, most intense at the base and extending upward 

into shattered carbonate rock for as much as several

TABLE 1.—Location of and brief data on other calcibreccia dikes

Locality Location Country rock Dike size Remarks

SE. of Miller Mountain, SWy4NW'/4 sec. 

23, T. 9 S., R. 14 E., in Cooke 

City quadrangle.

2 km due E. of Pilot Peak on north 

side of Fox Creek tributary, near 

foot of 65-m waterfall.

On north side of One Mile Creek, upstream from point where large 

fault block crosses creek and NE. 

of small block of Madison Limestone surrounded by Wapiti 

Formation.

On north side of One Mile Creek, 

0.8 km ENE. of locality C.

In bottom of valley, 0.5 km N. of 

One Mile Creek and 0.2 km ENE. of 

locality D.

On divide between North Fork Crandall Creek and Blacktail Creek.

In three areas: one near divide between Dead Indian Creek and west 

fork of Paint Creek, others 400 m 

and in ravine 600 m to SW.

On ridge between two forks of Paint 

Creek, 0.5 km SE. of an abandoned 

CCC camp.

Calcibreccia dike terminates downward in Heart 

Mountain fault breccia, 0.3-1 m thick, altered 

to yellow brown color; fault breccia has sharp, 

nontransitional contact with underlying 65 m 

of Bighorn Dolomite, but upper contact with the 

Wapiti Formation is irregular, with a gradational zone of mixed carbonate and volcanic breccia 

at top.

Dike begins at the Heart Mountain fault and curves 

irregularly upward; near the top a 3-m-long fork 

branches off the main dike and bluntly terminates 

1 m beyond the fork. About 200 m to NE. at the 

foot of a 50-m waterfall on Fox Creek, a calcibreccia dike 5—10 cm thick occurs near the base 

of volcanic rocks.

Dike trends N. 30° W., dips 55° NE. At short distance to W. are several small calcibreccia dikes, 

each about 15 cm wide and 1-2 m long.

Dike trends N. 10° E., dips 45° NW.

The calcibreccia in the limestone is entirely

carbonate rock, but that in volcanic rock contains 

some volcanic fragments. Lenses and stringers of 

calcibreccia occur along the very irregular but 

sharp contact between the limestone and volcanic 

rock. The volcanic rock contains a little hornblende and is calcareous in vicinity of the calcibreccia. The volcanic rock here appears to have 

been unconsolidated at the time it came in contact with the calcibreccia.

Dike striking S. 55° W. is at unusual height of

about 170 m above the Heart Mountain fault. Fault 

blocks of upper plate are exposed 300 m to S. and 

600 m to E.

Dikes are directly above the change in slope of the 

Heart Mountain fault from a transgressive to a 

former land-surface phase (fig. 1). Abrupt 

change in slope here may have favored accumulation of fault breccia that became the source 

of dike rock.

Height of dike above the Heart Mountain fault is 

probably less than 25 m. Enclosing volcanic 

rock dips 25°SSW. is fractured and striated, 

and displays some slickensides.

Wapiti Formation __ 0.15m wide, 

5 m high.

Volcanic rocks _0.6-1 m thick, 

20 m high.

Wapiti Formation __0.15 m wide, 

50 m long.

Some in broken and

deformed limestone, others in

the Wapiti Formation.

10 m long.

In limestone:

0.5-5 cm wide

or as irregularshaped bodies.

In volcanic

rock: irregular masses

and stringers

as wide as 0.3

Wapiti Formation __ 1 m wide and 15 m 

long; pieces 

scattered 150 m 

along strike.

do ___ _ Dike on divide:

0.15-0.3 m wide, 

10 m long, with 

scattered fragments for 100 m 

SW. Dike 400 m 

SW. represented 

by numerous 

fragments 30 cm 

or more wide.

Cathedral Cliffs 

Formation

0.3-0.5 m wide, 

15 m or more 

long.
MECHANISM OF DIKE INJECTION 21

tens of meters—a feature that has also been noted in 

the upper plate of the Muddy Mountain thrust 

(Longwell, 1922; Brock and Engelder, 1977). The 

abundant fault breccia remaining on the surface of tectonic denudation was part of the brecciation phenomenon associated with movement of the upper plate. The 

lower plate, on the other hand, has been thoroughly 

examined and nowhere shows any significant deformation that could be due to Heart Mountain faulting. This 

strange paradox is most probably related to the 

emplacement mechanism for the Heart Mountain 

fault.

A particularly informative example of brecciation of 

rocks in the upper plate is the limestone mass described in the section "Area 3, North of Republic 

Mountain," where calcibreccia dikes in its lower part 

and at the top have been described. This intensely 

brecciated block of limestone, which is composed 

mostly of Jefferson and a little Three Forks Formation 

at the top, lies on the Heart Mountain fault surface 

with the basal bed of the Bighorn Dolomite, which is 

dark gray here and has a petroliferous odor immediately below. The block of limestone was not only 

moved horizontally as a brecciated mass for some distance along the fault but also was displaced downward 

100 m because the Bighorn Dolomite that normally 

underlies it is absent. How this movement was accomplished is unknown, but the consequent deformation of 

this upper-plate block is well preserved and will be 

described further as an aid in understanding the faulting process.

The brecciated limestone block is about 25 m high 

and zoned roughly into three parts of about equal 

thickness and varying degrees of deformation (fig. 26). 

The lower zone consists at base of 2 m of carbonate 

fault breccia, finely comminuted and grading into 

about 6 m of overlying intensely brecciated limestone 

of the Jefferson Formation. Throughout this zone, some 

parts of the Jefferson are slightly less brecciated than 

others (fig. 27), and a crude horizontal lenticularity, 

resembling bedding, has been preserved. A few calcibreccia dikes occur in the lower zone, some of which 

have already been described in area 3. Near the top of 

this zone, vugs in less severely brecciated parts of the 

limestone block contain an asphalt residue, and some 

nearby breccia contains numerous blebs or specks of 

asphalt that give the rock a banded appearance. One 

indistinct calcibreccia dike in the lower zone also contains black asphalt specks.

The middle zone, which consists of about 10 m of several somewhat massive beds of bluish-gray limestone, 

is quite thoroughly brecciated but exhibits the outward 

form and appearance of the original sedimentary beds; 

on fresh fracture it has a petroliferous odor.

At the base of the upper zone, mostly unbrecciated, is 

a bed of limestone 2.5 thick containing many fine fractures, in places closely spaced and forming a crisscross 

pattern that gives a breccialike appearance, although 

deformation has not proceeded far enough to rotate the 

enclosed fragments. Above the basal bed in this zone 

are sheared and fractured rocks, apparently a mixture 

of rocks of the Jefferson and Three Forks Formations; 

the more massive limestone beds contain innumerable 

hairline fractures, and the thin-bedded limestone has 

been deformed by shearing and faulting.

MECHANISM OF DIKE INJECTION

An important clue to the origin of the clastic dikes of 

Heart Mountain fault breccia is provided by evidence 

that the volcanic country rock of the Wapiti Formation 

was unconsolidated at the time many of the dikes were 

intruded. Krill (1976) suggested a steam-injection 

model as a working hypothesis for the mechanism of 

dike injection, according to which the volcanic rocks 

were the source of heat. In this model, water in the 

calcibreccia was not vaporized immediately upon burial but only after burial beneath several volcanic flows 

(immediate vaporization is negated by the presence of 

dikes 20 m high passing through at least two distinct 

flows). Although this idea is interesting and imaginative, it does not seem to apply equally well at all sites, 

particularly those with dikes in both upper-plate and 

volcanic rocks.

Voight (1973a) concluded that the clastic dikes injected into upper-plate rocks are somewhat older than 

those in volcanic rocks but did not state the basis for 

his conclusion. No evidence has been found, so far as I 

am aware, that clastic dikes of Heart Mountain fault 

breccia intruded upper-plate rocks at a different time 

from rocks of the Wapiti Formation. The calcibreccia 

dikes in the Wapiti Formation were evidently injected 

very soon after movement on the Heart Mountain fault 

had ceased, and so also the dikes in the upper plate, as 

shown by (1) the calcibreccia dike in a small block of 

the Three Forks Formation in area 1 that contains volcanic fragments from the adjoining Wapiti Formation, 

evidence showing that it was emplaced after both deposition of the Wapiti and Heart Mountain faulting; and 

(2) the undeformed dikes in the highly brecciated limestone mass of the upper plate in area 3, north of Republic Mountain, which unequivocally shows that dike injection occurred after deformation of the enclosing rock 

and thus after faulting.

An alternative explanation for the calcibreccia dikes, 

proposed by Voight (1973b), is that dikes in the upper 

plate were injected as part of the Heart Mountain fault 

emplacement mechanism. Their presence was said to
22 CLASTIC DIKES OF HEART MOUNTAIN FAULT BRECCIA, NORTHWESTERN WYOMING

w

Wapiti Formation o

•3 O

Brecciated 

Jefferson 

Formation

cb

cb

Igneous sill x x x

Area of 

o figure

° 14 Precambrian {h_^— -^^

clasts *^^Three Forks and

Jefferson Formations. 

Hairline fractures in thicker \. Q 

limestone beds; shearing, faulting, \ 

and brecciation in thin-bedded limestone

Brecciated bluish-gray limestone of Jefferson Formation 

which retains appearance of original sedimentary beds; \ 

has petroliferous odor on fresh fracture ^

--------••-• \

Brecciated limestone of Jefferson Formation containing some calcibreccia dikes; \ ° 

transitional downward into Heart Mountain fault breccia at base \

•••X'HEA'RTMOUNTA'M'FAULT :.-. .^ cb

Basal bed of the 

Bighorn Dolomite

Grove Creek Limestone Member 

of Snowy Range Formation

METERS

5 -i

XVv^- x x___x_x x ^^_x x x •

FIGURE 26.—Diagrammatic cross section showing various degrees of deformation in brecciated limestone above Heart Mountain fault in 

area 3, north of Republic Mountain, cb, calcibreccia. Arrows show direction of movement on fault.

50 mm

I I I I I I I I

FIGURE 27.—Polished specimen of brecciated limestone from 10 m 

above Heart Mountain fault in area 3, north of Republic Mountain. 

Degree of brecciation is not uniform, and many fragments are not 

rotated. Photograph by Lowell Kohnitz.

"*** demand a fluid 'flotation' mechanism in the mechanics of the Heart Mountain rockslide [his term], 

and by analogy, of South Fork and Reef Creek 

rockslides." Several factors, however, indicate that this 

explanation is not correct. (1) The upper plate was 

under lateral tension rather than confining pressure, 

as shown by its having been broken up and pulled 

apart into numerous detached blocks during movement. In the absence of confining pressure, small 

upper-plate masses containing calcibreccia dikes such 

as those described here would simply have come apart 

and failed to retain the intrusions. (2) The dikes in the 

upper plate are not genetically related to the emplacement process because evidence indicates that they 

were injected some time after movement had ceased. 

(3) The dikes in volcanic rocks are younger than the 

Heart Mountain fault movement (Pierce, 1968; Voight, 

1974, fig. 9). If the dikes in upper-plate rocks are not 

the same age but older and part of the faulting process, 

then different mechanisms are required for the fault 

breccia dikes in one type of country rock and those in 

another type, although the dikes are similar in all 

other respects.
GEOLOGIC SIGNIFICANCE OF THE CLASTIC DIKES 23

Voight (1974) also cited the clastic dike of fault-zone 

material along the Muddy Mountain thrust in Nevada 

as evidence for the former existence of high fluid pressure along that fault. In a recent report, however, 

Brock and Engelder (1977) proposed that these clastic 

dikes are probably not related to high fluid pressure 

during faulting and from several lines of evidence concluded that the advance of the Muddy Mountain thrust 

could not have been aided by high pore pressure.

If the premise be accepted that the calcibreccia dikes 

intruded both the upper-plate and volcanic rocks of the 

Wapiti Formation in the Heart Mountain fault area at 

essentially the same time, then a common mechanism 

for their injection seems far more likely than separate 

mechanisms. The model proposed here, which would 

satisfy all known constraints imposed by the observed 

data, assumes that lithostatic pressure was imposed by 

a rapidly accumulating overburden of volcanic rock. 

The sequence of events associated with dike injection 

may be reconstructed as follows. Once the surface of 

tectonic denudation had been created by catastrophic 

movement on the Heart Mountain fault, watersaturated carbonate fault breccia, from 0 to 4 m or 

more thick, was irregularly distributed over the fault 

surface. As volcanic rock was poured out onto this land 

surface, some of the calcibreccia was mixed with the 

volcanic rocks, some was caught up as irregular pods 

and lenses in the lower part of the Wapiti Formation, 

and some remained as irregular tabular bodies at the 

base of the volcanic rocks. These irregular bodies were 

unconsolidated, and so an unstable condition was 

formed, with the more mobile fault breccia below and 

the less mobile volcanic rocks above. As deposition of 

volcanic rocks continued, the smallest upper-plate 

blocks were immediately buried, and the somewhat 

larger ones were covered soon after. Lithostatic pressure on the unconsolidated fault breccia at the base of 

the upper plate increased with the thickening volcanic 

cover until the cohesive strength of the overlying rock 

was exceeded, and the calcibreccia was injected upward 

as dikes along fractures or lines of weakness or lesser 

consolidation, regardless of whether the overlying 

rocks were upper-plate rocks or volcanic rocks. The degree of lithostatic pressure needed to initiate intrusion 

of a dike into the Wapiti Formation would have depended on the degree of consolidation and the character of the volcanic rock, as well as on the fluidity of the 

calcibreccia; in some cases perhaps only a few tens or 

hundreds of meters of overburden may have been sufficient.

GEOLOGIC SIGNIFICANCE 

OF THE CLASTIC DIKES

The calcibreccia dikes and related features affirm 

some earlier conclusions regarding the Heart

Mountain fault and furnish additional data on the 

events associated with the fault movement.

It has already been established that the Wapiti Formation was deposited on the surface of tectonic denudation soon after the fault movement ceased because 

the bedding-plane phase (fig. 1) of the fault has not 

been altered by erosion. This conclusion can now be 

extended to include the transgressive and former 

land-surface phases of the fault because calcibreccia 

dikes occur in these areas as well. The significance of 

the dikes is that the fault breccia from which they were 

derived was almost immediately buried by volcanic 

rocks, otherwise it would have been eroded away. 

Rapid deposition of at least the basal part of the Wapiti 

Formation is now considered to have extended over the 

entire 75-km-long area where calcibreccia dikes are 

known to occur.

Voight (1973a, p. 118-119) stated that "*** the presence of fault breccia-derived dikes injected within glide 

blocks seem to demand a fluid flotation mechanism for 

the Heart Mountain structure (Voight, 1973b)." His 

conclusion is based on the premise that the fault 

breccia-derived dikes (calcibreccia dikes of this paper) 

were injected as part of the Heart Mountain fault 

movement, but he presents no evidence for this assumption. Moreover, his mechanism of dike injection is 

invalid because the data presented here show that the 

clastic dikes were not injected as part of the fault 

movement but only after this movement had ceased 

and after basal rocks of the Wapiti Formation had been 

deposited. Likewise, the fault breccia-derived dikes are 

not indicative of the other fluid-pressure mechanisms 

that have been suggested: the high-fluid-pressure 

mechanism of Hubbert and Rubey (1959) or the 

volcanic-gas mechanism of Hughes (1970) and Prostka 

(1978, p. 435).

The calibreccia dikes are useful in conjunction with 

the surface of tectonic denudation for a stratigraphic 

correlation of the various volcanic rock formations. Because this surface was exposed for only a very brief 

interval, the rocks deposited on it must be of the same 

age. The problem that arises in using this surface for 

stratigraphic correlation of the volcanic rocks is to determine whether these rocks were deposited on the surface or were fault-emplaced during the Heart 

Mountain fault movement, that is, whether the volcanic rocks have a depositional or a fault contact with 

the surface of tectonic denudation. If these volcanic 

rocks were fault emplaced, they should follow the 

well-known pattern of other upper-plate rocks and 

form fault breccia composed of volcanic rock fragments. 

In this case, the basal part of the rock unit in question 

should also show tectonic brecciation, although opinions may differ as to whether a given volcanic breccia 

is of tectonic or other origin. However, the presence of
24 CLASTIC DIKES OF HEART MOUNTAIN FAULT BRECCIA, NORTHWESTERN WYOMING

calcibreccia bodies can help to determine whether the 

rock is either depositional or fault emplaced because, 

although clastic dikes occur in both types of deposits, 

these dikes may exhibit differences that reflect the origin of a given rock unit. Thus, those dikes in volcanic 

rocks deposited on the surface of tectonic denudation 

may have a mixture of carbonate and volcanic rock in 

the dike wall (figs. 4, 6-8), and the basal part of the 

volcanic rock may contain lenses and irregular bodies 

of calcibreccia (figs. 10, 21, 22).

SUMMARY

The salient points revealed by the clastic dikes are:

1. They are widely distributed within the area of the 

Heart Mountain fault. They are more common 

within the bedding-plane phase of the fault, but a 

few also occur in the transgressive and former 

land-surface phases of the fault.

2. They occur in the upper plate and in volcanic rocks 

that were deposited very soon after movement on 

the Heart Mountain fault had ceased.

3. They were intruded into the upper-plate and volcanic rocks at approximately the same time.

4. They confirm an earlier conclusion that the Heart 

Mountain fault movement was a cataclysmic 

event.

5. They were injected after movement on the Heart 

Mountain fault had ceased and therefore are not 

indicative of a fluid-flotation mechanism for the 

Heart Mountain fault, as suggested by Voight.

6. Injection of the dikes was caused by a rapidly increasing overburden of volcanic rocks that also 

rapidly increased the lithostatic pressure on the 

unstable fault breccia.

7. Wapiti volcanism began by rapid deposition of at 

least the lower part of the Wapiti over many 

square kilometers.

8. In some places where calcibreccia was abundant, 

deposition of the lower part of the Wapiti was so 

rapid that before the basal part was consolidated, 

the lithostatic pressure on it and the unstable calcibreccia became sufficient to cause both to flow 

upward as a dike.

9. Differences between calcibreccia dikes in indurated 

country rock and those in unconsolidated country 

rock can be used as an aid in distinguishing between volcanic rocks of the Cathedral Cliffs and 

Lamar River Formations that are fault emplaced 

by the Heart Mountain fault movement, and the 

Wapiti Formation that is depositional on the tectonic surface.

10. The high permeability of the extensively brecciated rocks in the lower part of the upper plate

and the separation of the upper plate into 

numerous blocks and pieces indicate that the 

Heart Mountain fault movement was not aided 

by high pore pressure of water, air, or volcanic 

gas. In my opinion, the extensive brecciation of 

rock above the fault surface, which is observed to 

occur immediately above a line of seismic discontinuity separating a lower 350-m-thick layer 

predominantly of shale from an upper 500-mr 

thick layer of carbonate rock, may well have 

been caused by a catastrophic earthquake of vertical acceleration approaching 1 g. However, it 

remains possible that heat generated by rapid 

movement of the upper plate produced feteam 

(Goguel, 1969), which helped to reduce friction 

on the fault.

ACKNOWLEDGMENTS

The author is indebted to R. A. Loney and W. H. 

Nelson for examining many thin sections of the calcibreccia. This report is based on field work performed 

over a period of several years, and I wish to thank D. H. 

Anspach, W. C. Barnes, J. H. Clark, K. R. Jayne, Allan 

Krill, C. C. McAneny, and J. I. Ziony, who served so 

capably and enthusiastically as my field assistants. 

The suggestions and comments of R. G. Schmidt and E. 

B. Ekren on an earlier draft of this paper are appreciated and have been most helpful.

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