Geometry and kinematics of the Heart Mountain detachment fault, northwestern Wyoming and Montana

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Content GEO M ETRY A N D KINEMATICS O F THE HEART MOUNTAIN DETACHM ENT FAULT
NO RTHW ESTERN W YOM ING A N D M O N TA N A
by
Thomas Armitage Hauge
A Dissertation Presented to the
FACULTY O F THE G RADUATE SC H O O L
UNIVERSITY O F SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
D O C TO R O F PHILOSOPHY
(Geological Sciences)
May 1983
UMI Number: DP28565
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UNIVERSITY OF SOUTHERN CALIFORNIA
T H E G R A D U A TE S C H O O L
U N IV E R S IT Y PARK
LOS A N G E LE S . C A L IF O R N IA 9 0 0 0 7
This dissertation, written by
Thomas Armitage Hauge
under the direction of M s..... Dissertation Com
mittee, and approved by all its members, has
been presented to and accepted by The Graduate
School, in partial fulfillm ent of requirements of
the degree of
D O C T O R O F P H I L O S O P H Y
Dean
Date S E E I_E _H B ER 3 J 9.82
DISSERTATION COMMITTEE
j t / r j n
^ ~ *"■ • • ■ ^ *' “" ” ■ * ’ * ■ ’ ‘" r*..........*.**i—
Chairman
ACKNO W LEDG M ENTS
I would like to thank Gregory A. Davis, Chairman of my
supervisory committee, for in it ia l suggestion of the project and for
assistance during the course of the study, including valuable
guidance during v is its to the fie ld area. William G. Pierce was
extremely helpful, providing encouragement, h o sp ita lity, and an
introduction to the fie ld area in the early phase of the study.
Greg Davis, Lawford Anderson, Don Barry, Bob Osborne, and Charlie
Sammis served on the supervisory committee. Charlie Sammis provided
consultation regarding mechanism considerations. Several friends
ably assisted in the fie ld , most notably Chris Fuller, Eric Johnson,
and Linda Thurn. Discussion of the geology of low-angle faulting
with Eric Frost, Jan-Claire P h illip s, Linda Thurn, and others were
most helpful and enjoyable. M y family of friends too numerous to
mention provided love and moral support —thanks to you a ll.
William G. Pierce and the Shoshone National Forest provided
access to aerial photographs and base maps. Thanks for friendship,
h o sp ita lity, and local lore are due to my friends Mary Kenevan of
Red Lodge, Robert and Edie Phillips of Thermopolis, and Tim Frey,
late of Cooke City.
M y thanks also go to Gloria Lee, who acted as typ ist and
indispensible log istica l consultant; to Gay Havens, who drafted the
graphics; and to Jim and Nancy Slosson, who provided encouragement
as well as space, time, and materials to work on the manuscript.
i i
Financial assistance was provided by Penrose Grants from the
Geological Society of America (1978, 1980) and by the Geological
Sciences Graduate Research Fund (1977, 1978, 1980, 1981),
University of Southern California.
TABLE O F CONTENTS
Page
ACKNOW LEDGMENTS............................... ii
LIST O F FIGURES..................................................................................... ix
LIST O F TABLES.................... xv
LIST O F PLATES..................................................... xvi
ABSTRACT.................... 1
INTRODUCTION.................................................................................................... 3
Purpose and Background................................................................ 3
Location and Physiography............................................................... 3
Regional Geologic Setting............................................................... 7
Method of Investigation................................................................... 11
Previous Work........................................................................................ 12
General S ta te m e n t............................................................. 12
Paleozoic Rocks and Heart Mountain Faulting............... 13
Tertiary Volcanic Rocks and Heart Mountain
Faulting................................... 21
Clastic Dikes and Heart Mountain Faulting 30
The Crandall Conglomerate and the Blacktail Fold... 31
The Mechanics of Heart Mountain Faulting........................... 35
STRATIGRAPHY.................................................................................................... 56
General Statement......................... 56
Paleozoic Rocks................................................................................... 57
General S tatem ent........................................ 57
Flathead Sandstone................................................................... 58
i v
Page
Gros Ventre Formation (Middle Cambrian)..................... 61
Pilgrim Limestone (Late Cambrian).................................... 62
Snowy Range Formation (Late Cambrian)........................... 62
Bighorn Dolomite (Late Ordovician).................................. 63
Jefferson Formation (Late Devonian)............................... 64
Three Forks Formation (Late Devonian-Early
Mi ssi ssi ppi an)................................. 65
Madison Limestone (Mi ssi ssi ppi an).................................... 65
Tertiary Rocks..................................................................................... 66
General Statement............................................. 66
Crandall Conglomerate.......................................................... 66
Volcanic Rocks, General Statement.................................... 67
Cathedral C liffs Formation.................................................. 69
Lamar River F o rm a tio n ..................................................... 70
Wapiti Formation.................................................................... 71
STRUCTURAL GEOLOGY........................................... 73
General Statement............................................................................. 73
The Heart Mountain Break-Away Fault.............. 73
General Statement ............ 73
North of Soda Butte Creek.............. 75
Interpretation............................................................ 77
South of Soda Butte Creek.................................................... 78
Interpretati on................................................................. 83
Upper Forks of Cache Creek.................................................. 84
Interpretati on......................................... 84
Headwaters of North Fork of Crandall Creek................. 87
v
Page
Interpretati on........................................... 87
Summary..................... . .......................... . .............. 87
The Heart Mountain Bedding Fault , ............................... 89
General Statement..................................................................... 89
West of Republic Mountain.................................................... 91
Interpretation.................................................................... 100
Republic Creek A r e a ............................................................... 105
Interpretation.................................................................... I l l
South of Colter Pass.............................................. I l l
Interpretati on............................................... 115
Northeast of Index Peak....................................... 116
Interpretati on.................................................................... 118
Fox C re e k ....,............................................................................... 118
Interpretati on............................................ 122
Pilot Creek............................................................ 122
Interp reta tion. .............................................................. 130
Jim Smith Creek.,........................................................................ 132
Interpretation.................................................................... 137
Southwest of B-4 Ranch.............................................. 138
Interpretation............................................. 141
Onemile Creek................................................................................. 141
Interpretation.................................................................... 142
Onemile Creek to Squaw Creek.................................................. 142
Interpretation................................................................... 148
Cow Creek........................................................... 149
vi
Page
Interpretation..................................... 152
West of Blacktail Creek........................................................ 152
Interpretation................................................................. 154
Hunter Peak. ................................................... 154
Interpretati on ........................................................ 156
East of Lodgepole Creek......................................................... 156
Interpretation................................................................. 159
Between Oliver Gulch and Corral Creek............................ 159
In te rp re ta tio n .. , . . ...................................................... 163
Cathedral C lif f s ............................................................ 163
Interpretati on...................................................... 171
East of Reef Creek................................................................ 171
Interpretati on....................................... 176
East of Painter Gulch ................................................ 176
Interpretation................................................................. 177
Sugarloaf Mountain.............................................................. .. 180
Interpretat i on................................................................. 186
Steamboat ......................................................................... 186
Interpretati on................................................................. 187
East of Dead Indian Creek........................ 192
Interpretation ...................... 193
Summary.......................................................................................... 193
Kinematics: Bedding Fault........................................... 193
Kinematics: Internal Deformation of Allochthon 196
Kinematics: Interpretati on.................. 199
vi i
Page
Involvement of Volcanic Rocks.................................... 199
The Heart Mountain Fault on Former Land Surface...................... 202
General Statement....................................................................... 202
Heart Mountain............................................................................. 203
Interpretation................................................................. 205
South End of Logan Mountain.................................................. 205
Interpretation................................................................. 210
CONCLUSIONS........................................................................................................ 211
Involvement of Volcanic Rocks; Lack of Tectonic
Denudation.......................................................................................... 211
Kinematics of Heart Mountain Faulting........................................ 213
Rate of Heart Mountain Faulting............................... 219
Revised Geologic History................................................................... 220
New Constraints on the Mechanics of Heart Mountain
Faulting................................................................................................ 223
Recommendations for Future Work....................................... 226
REFERENCES.......................................................................................................... 228
APPENDIX.............................................................................................................. 238
v i i i
LIST O F FIGURES
Figure Page
1. Generalized geologic map showing location of Heart
Mountain detachment fau lt terrane and distribution
of a l1ochthonous Paleozoic rocks and u n diffe ren ti-
ated volcanic rocks . ................................... 4
2. Tectonic context of the Heart Mountain detachment fau lt 8
3. Columnar section of the Paleozoic stratigraphy of the
Heart Mountain fau lt upper and lower plates................. 59
4. View to the east of Bald Ridge, showing early Paleozoic
s tra ti graphic section overlying Precambrian crysta l
line rocks........................................................................................ 60
5. Sketch map showing interpretations of Pierce ( - * - .) and
Prostka (— ) of the location of the northward
termination of the break-away f a u l t . . . ............................. 76
6. View to south of exposure of the break-away fa u lt south
of Soda Butte Creek......................................... 79
7. Stereographic projection of orientations of the break
away fa u lt (great circles) and striae (°) in the area
1 to 3 km south of Soda Butte Creek (Plate I Locality
A2)...................................................................................................... 81
8. View to north of break-away fa u lt zone as exposed about
2 km south of Soda Butte Creek ............................... 82
9. Sketch map showing part of break-away fa u lt (from Pierce,
et al 1973) and structure of volcanic rocks east of
break-away fa u lt (from Pierce et al , 1 973; E llio t t ,
1979; and this study ...................... 85
10. Stereographic projections of orientations of upper-plate
faults (great circles) and striae ( o ) south of Silver
Gate (Plate I Locality C)................ 93
11. Al1ochthonous Madison Limestone (Mm) overlain by
al 1 ochthonous volcanic rocks (Tv).................... 95
12. Stereographic projection of orientations of upper-plate
faults (great circles) and striae (o) , Falls Creek
area (Plate I Locality D) .............. 97
i x
Page
99
101
103
104
107
109
110
113
119
121
124
x
Diagrammatic cross-section of c l i f f exposures at the
northwest corner of Republic Mountain (Plate I
Local i ty E)................................................................ ..
Stereographic projections of orientations of upper-
plate fa u lts , stria e , and dikes, northwest corner of
Republic Mountain (Plate I Locality E)................................
Stereographic projection of orientations of all fa u lt
striae observed in volcanic rocks between the break
away fault and the northeast corner of Republic
Mountai n.............................................................................................
Diagrammatic cross-section of view to south from
historical marker about 0.4 km west of Cooke City on
Route U S 2 1 2 ................................ ..................................................
Stereographic projections of orientations of upper-plate
faults (great c irc le s ), dikes (great circle s, short
dashes), and striae (o), Republic Creek area (Plate
I Locality F)....................................................................................
View to east of south end of a l1ochthonous Paleozoic
rocks (M0) on the east side of Republic Creek.................
View to west of inaccessible apparent graben where
volcanic rock (Tv) is downfaulted between Paleozoic
rocks (Mn).........................................................................................
Stereographic projections of orientations of upper-plate
faults (great circles) and striae (o) and of bedding
fa u lt striae (&) south of Colter Pass..................................
Geologic map of north-facing slope south of Fox Creek
(PI ate I Locali ty K) ......................................................
Stereographic projection of orientations of faults
(great circles) and striae at Fox Creek (Plate I
Locality K) (o) = striae on faults in upper-plate
Paleozoic rocks; (U) = striae on faults along
contact between upper-plate Paleozoic and volcanic
rocks; (<$) = striae on faults in lower-plate
volcanic rocks.................................................................................
Geologic map and cross-sections of area north of Pilot
Creek (Plate I lo c a lity L )........................................................
Fi gure
Page
24. Stereographic projections of orientations of upper-plate
faults (great circ le s ), striae (o), and dikes (dashed
great c irc le s ), north side of Pilot Creek (Plate I
Locality L )....................................... 126
25. Exhumed fa u lt at north end of a l1ochthonous Paleozoic
rocks in cross-section A-A' of Fig. 23. View is
downslope to east.................. 128
26. View to east of area shown on cross-section B-8‘ of
Fig. 23................................................. 129
27. View to northwest toward Pilot Peak, showing alloch
thonous voocanic rocks (Tv) overlain by in situ(?)
volcanic rocks ( T t).......................................................... 131
28. View to west across Jim Smith Creek showing bedding
fa u lt, faults within upper-plate volcanic rocks, and
inclined truncated strata within volcanic rocks............. 134
29. Stereographic projections of orientations of fa u lts,
s tria e , and clastic dikes at Jim Smith Creek (Plate I
Local i ty M)........................................................................................ 135
30. Bedding fau lt at Jim Smith Creek................................................. 136
31. Stereographic projection of orientations of upper-plate
faults (great circles) and striae (o) and of bedding-
fa u lt striae (A), southwest of B-4 Ranch (Plate I
Locality N ) . . . .................... 140
32. Stereographic projection of orientations of upper-plate
faults (great circles) and striae (o), Onemile Creek
(Plate I Locality P)..................................................................... 143
33. Stereographic projections of orientations of upper-plate
fa u lts , stria e, and dikes and of lower-plate beds
Onemile Creek and Squaw Creek (Plate I Locality Q) 146
34. C liffs of volcanic sediments along north side of Squaw
Creek............................. 147
35. Stereographic projections of orientations of upper-plate
faults (great c irc le s ), striae (o), and dikes (dashed
great c irc le s ), Cow Creek (Plate I Locality R) 150
36. Dikes within a l1ochthonous Madison Limestone in area of
Cow Creek (Plate I Locality R ) . . . . . . .................................... 151
xi
Figure Page
37. Stereographic projections of orientations of upper-plate
faults (great c irc le s ), striae (o) , and dikes (dashed
great c irc le s ), west of Blacktail Creek (Plate I
Local ity S).................................................. ...................................... 153
38. Stereographic projections of orientations of upper-plate
faults (great c irc le s ), striae (o) , and dikes (dashed
great c irc le s ), Hunter Peak (Plate I, Locality T) 155
39. Stereographic projections of orientations of upper-plate
faults (great c irc le s ), striae (o), and dikes (dashed
great c irc le s ), east of Lodgepole Creek (Plate I
Locality U)....................................................................................... 157
40. Dikes within volcanic rocks cut by low-angle fa u lt,
offset unknown........................................................ 158
41. Network of carbonate clastic dikes witnin volcanic rocks
overlying the bedding fa u lt, west of Corral Creek
(Plate I Locality V)........................................................................ 161
42. Stereographic projections of orientations of upper-plate
faults (great c irc le s ), striae (o), and dikes (dashed
great c irc le s ), between Oliver Gulch and Corral Creek
(Plate I Locality V)..................................................................... 162
43. Al1ochthonous Madison Limestone (Mm) and undifferentiated
volcanic rocks (Tv) overlying lower-plate rocks (-60)
along bedding fa u lt between Oliver Gulch and Corral
Creek (Plate I Locality V)............... .......................................... 164
44. Geologic map of steep north-trending ravine at eastern
edge of graben (Plate I, Locality W ) at Cathedral
C lif f s ..................................................................................................... 166
45. Stereographic projections of orientations of upper-plate
faults (great c irc le s ), striae (o ), and dikes (dashed
great c irc le s ), Cathedral C liffs (Plate I, Locality W ) 168
46. View to east of fa u lt contact between a l1ochthonous
Madison Limestone and volcanic rocks......................................... 170
47. Igneous dikes in a l1ochthonous Paleozoic rocks (MO) and
in overlying volcanic rocks (Tv) at Cathedral C liffs
(Plate I Locality W)........................................................................ 172
48. Stereographic projections of orientations of upper-plate
faults (great c irc le s ), striae (o), and dikes (dashed
great c irc le s ), east of Reef Creek (Plate I Locality X) 174
xi i
Figure Page
49. Diagramattic geologic map of NNE-facing slope between
Reef Creek and Deadman Creek (Plate I Locality X ).... 175
50. Geologic map of parts of sections 7 and 8, T55N R105W,
east of Painter Gulch (Plate I Locality Y)..... 178
51. Stereographic projections of orientations of upper-plate
faults (great circ le s ), striae (o) , and poles to
primary s tra tific a tio n (flows, breccias) ( o) , east of
Painter Gulch (Plate I Locality Y)........................................ 179
52. Locality map for Sugarloaf Mountain (Plate I Locality Z) 181
53. Stereographic projections of orientations of upper-plate
faults (great c irc le s ), striae (o), in areas of Sugar-
loaf Mountain indexed on Fig. 52............................................ 182
54. View upslope to south of high-angle fa u lt steepening
downward, Sugarloaf Mountain (Fig. 52, area of Fig.
53d).............................................................. 185
55. Stereographic projections of orientations of upper-plate
dikes (dashed great circles) and poles to bedding ( o) ,
Steamboat (Plate I Locality AA) ................................... 188
56. View to west of Steamboat (Plate I Locality AA)................... 189
57. Stereographic projections of orientations of upper-plate
faults (great c irc le s ), striae (°), east side of
Steamboat (Plate I Locality AA).............. 190
58. Stereographic projections of orientations of upper-plate
faults (great c irc le s ), striae ( o ) s and dike (dashed
great c irc le ), east of Dead Indian Creek (Plate I
Locality BB).................... 194
59. Diagrammatic E-W cross-sections showing observed (A-A')
and inferred (B-B1, C-C1) geometries of the break
away f a u lt ......................................................................... 201
60. Stereographic projections of orientations of upper-plate
fa u lts , striae, and poles to bedding, Heart Mountain
(Plate I Locality C C )...................... 204
61. Geologic map and cross-section of an area east of Trout
Creek (Plate I Locality DD).............. 206
62. Views of area mapped in Fig. 61.................................. 207
xi i i
Figure Page
63. Stereographic projections of orientations of upper-plate
faults (great circ les ), and striae at south end of
Logan Mountain (Plate I Locality DD).................................. 209
64. Generalized geologic m ap showing distribution of upper-
and lower-plate exposures of Crandall Conglomerate... 214
65. S trike-slip fault zones in the upper plate of the Heart
Mountain detachment fa u lt......................................................... 215
xi v
TABLE L Summary of
patterns
Mountain
L IS T OF TABLES
interpretations of domi nant kinematic
for upper-plate rocks of the Heart
Fa u11 . . . . . . . . . . . . . . . . . . . . . ..
PI ate
I.
I I .
I I I .
LIST O F PLATES
Location
Geographic Reference and Locality Map, Heart Mountain M ap
Detachment Fault........................................................ Pocket
Generalized Geologic M ap of the Heart Mountain Fault M ap
Terrane........................................................................... Pocket
Generalized Geologic M ap Showing Trend of Striae M ap
Observed on the Heart Mountain Bedding Fault . Pocket
xv i
ABSTRACT
Previous studies of the Heart Mountain detachment fa u lt,
north-western Wyoming, have concluded that detachment and
displacement of Paleozoic sedimentary and Eocene volcanic rocks
occurred catastrophically, with the upper plate fragmenting into
several tens of separate a l1ochthonous blocks during gravity
slid in g . After fa u ltin g , newly extruded Eocene volcanic rocks were
thought to have blanketed the disrupted terrane, protecting
tectonically denuded areas of the bedding-plane detachment fa u lt
from subaerial erosional and sedimentary processes.
Field work for the present study indicates volcanic rocks
d ire c tly overlying the bedding-plane detachment fa u lt are everywhere
tectonica lly emplaced rather than in s it u . The base of the
a l1ochthonous volcanic rocks is commonly characterized by a striated
microbreccia layer a few mi 11imeters th ick, fa u lt breccia containing
clasts of Paleozoic sedimentary and Eocene volcanic rocks, or a zone
of shattered volcanic rock up to 10 m thick with swarms of clastic
dikes of fa u lt breccia. Tectonic deformation of a l1ochthonous
Tertiary volcanic rocks ranges from very slight (e.g. minor
rotation) to intense (steep rotation, common internval fa u ltin g ).
Thicknesses of a l1ochthonous volcanic rocks exceeding 650 m are
recognized. Masses of intervening and underlying a l1ochthonous
Paleozoic rocks up to 500 m thick display sim ila rly broad ranges in
degree and style of deformation. Crosscutting relationships between
dikes and faults require multiphase upper-plate fa u ltin g . Kinematic
data indicate dominantly southeastward translation accompanied by
extension (E-W-oriented in Paleozoic rocks, variably oriented in
volcanic rocks) and s trik e -s lip faulting of the upper plate.
A revised geologic history of Heart Mountain faulting is
proposed. The upper plate moved not as numerous separate detached
blocks but as a continuously spreading allochthon of both Paleozoic
sedimentary strata and Tertiary volcanic rocks. Tectonic
denudation, catastrophic fault-displacement rates, and catastrophic
post-tectonic volcanism are not indicated. Displacement rates along
the Heart Mountain fa u lt of one to five cm/yr are suggested by
lim ited age data.
Because of the low slope (<2°) and catastrophic displacement
rates indicated by previous workers, the Heart Mountain fa u lt has
exemplified the mechanical paradox of low-angle faulting and has
evoked several unusual hypothetical mechanisms. Previously
suggested hypothetical mechanisms are re-evaluated in lig h t of the
greatly reduced displacement rates and the continuous allochthon
indicated by the present study. Reduction of fric tio n along the
detachment fau lt by flu id pressure is more plausible for a
continuous allochthon than for numerous separately moving
a l1ochthonous masses. Reduced displacement rates make
earthquake-osci11ation hypotheses more viable. Abnormal flu id
pressures and earthquake oscillatio ns may both have fa c ilita te d
movement on the Heart Mountain fa u lt.
2
INTRODUCTION
Purpose and Background
The Heart Mountain detachment fa u lt of northwest Wyoming and
adjacent Montana has been viewed as a structural and mechanical
enigma for roughly 50 years. As fie ld mapping over the years
revealed its large areal extent, superb exposures, remarkable
preservation from erosion, and lack of la te r tectonic deformation,
the Heart Mountain fau lt became the North American showpiece of
low-angle gravity sliding and tectonic denudation. However, no
satisfactory explanation of the mechanics of Heart Mountain faulting
has emerged.
The present study was designed to answer two questions:
1) What were the kinematics of Heart Mountain faulting?
An understanding of the movement pattern of detached rock
might constrain mechanism hypotheses.
2) How much of the terrane of volcanic rocks overlying
the Heart Mountain fau lt was involved in Heart Mountain
faulting? The lite ra tu re to date reflects controversy
in this regard;
Location and Physiography
The Heart Mountain detachment fa u lt terrane covers a minimum
area of about 3400 km^ in northwestern Wyoming just east of
Yellowstone National Park (see Fig. 1). From the eastern boundary
of the Park near its north-eastern corner the terrane extends to the
3
FIGURE 1. Generalized geologic map showing location
of Heart Mountain detachment fa u lt terrane and
distribution of allochthonous Paleozoic rocks and
undifferentiated volcanic rocks. Cross-section is
diagrammatic. Modified from Pierce (1980) Fig 1.
! 1 0 000‘ i 09°43‘ i09°30’ 109*18'
N a t i o n a l Park
City
\ Republic
Mountain
W Y 0 M I N 6
MONTA NA
4 8* 00 -
WYOM I NG
Tv
Tv
Tv
a*
Tv
I 1 Tv
B E D D IN G -P L A N E
DETACHMENT
FA U LT-T— ■«;
Tv
E X P L A N A T I O N
FAULT ON
F O R M E R LA ND
SURFACE
Eocene volcanic rocks
Paleozoic rocks allochthonous
on Heart Mountain Fault
Tv
McCulloch
Paak
Heart Mountain Fault - hachures
on upper plate
B r e a k - away fault
B u ffa lo B ill
N o rth F o rk S*'°
20 25 K IL O M E T E R S
0 5 1 0 15 MILES
NW
T R A N S G R E S S I V E FA U LT
B E D D I N G - P L A N E DE TAC HM EN T FAULT
FAULT ON FORMER LAND SURFACE
B R E A K - AWAY FAU LT
5
southeast, occupying parts of the northeastern fo o th ills of the
northern Absaroka Mountains and part of the east-central Bighorn
Basin. Silver Gate, Montana and Cooke City, Montana are in the
northwestern corner of the terrane, and Cody, Wyoming lies near its
southeastern edge.
The eastern part of the fa u lt terrane lie s within the Bighorn
Basin, a structural and topographic basin of moderate re lie f.
Average elevation is about 1400 m; Heart Mountain, the dominant
topographic feature, rises to 2476 m. This desert area is sparsely
populated, supporting farming and ranching along the banks of the
Bighorn, Shoshone, Greybul1, and Clarks Fork Rivers and in areas
irrigated with water from the Buffalo B ill Reservoir. A network of
roads, unpaved except for major arteries, permits vehicular
access to most areas. The terrain is lo ca lly very rugged, with some
badlands. Sparse vegetation of sage and grass gives way at higher
elevations to conifer forest. Summers are hot and dry, and winters
are cold.
The central and western parts of the detachment terrane occupy
! the northeastern fo o th ills of the northern Absaroka Mountains, an
extremely rugged area of high r e lie f with peaks exceeding 3600 m in
elevation. Access by motor vehicle is via U.S. 14-20-16 from Cody
to the east entrance to Yellowstone National Park and via an unpaved
secondary road, State Route 296, and U.S. 212 connecting Silver
Gate, Cooke City, and Cody. The la tte r route in part follows the
broad, deep steep-walled glacial valley of the southeast-flowing
Clarks Fork of the Yellowstone River, which separates the Beartooth
Mountains to the north from the Absaroka Mountains to the south.
Excellent exposures of the bedding fa u lt phase of the Heart Mountain
fa u lt terrane lie along the south wall of the Clarks Fork valley and
along the walls of the trib u ta rie s to the Clarks Fork that drain the
Absaroka Mountains. West of Colter Pass, near Cooke City, the
terrane is exposed on the north and south sides of Soda Butte Creek.
Access to exposures along the Clarks Fork and along Soda Butte Creek
is by 1 to 2 hour hikes, usually through forest, involving ascents
of 1000 feet or more. The in te rio r of the northern Absaroka
Mountains, except for along Sunlight Creek and the Shoshone River,
lie s within the Worth Absaroka Wilderness, so no motor vehicles are
permitted. Access there is by pack t r a ils , many of which are
maintained by the Shoshone National Forest. The high r e lie f, with
steep valley walls often crested by aretes, prevents easy access
from one drainage to another within the Absarokas. "As one old
mountain sheep hunter expressed i t , 'In these parts its either up or
down the creek — not east or west, north or south'" (Bonney and
Bonney, 1977, p. 259). Summers are cool and may be rainy; snow can
fa ll any month of the year. Winters are very cold and snowy. The
fie ld season extends from June or July to September or October.
Regional Geologic Setting
The Heart Mountain fa u lt terrane lies within the Wyoming
province of the Rocky Mountain foreland (Prucha et a l., 1965), a
region typ ifie d by block u p lifts of Precambrian crysta llin e rocks
and Paleozoic shelf sediments (see Fig. 2). Less resistant Mesozoic
sediments have been eroded from the tops of the block u p lifts but
* 0 h t
* v 0
ndex map showing location of the Wyo
boundary. (Prucha et al 1965)
KILOMETERS 10 0
PRYOR
MONTANA
WYOMING
A 8S A R 0 K
POWDER
RIVER
BASIN
cooy
RIVE
v m ///A
LATE IGNEOUS CENOZOIC MESOZOIC PALEOZOIC PRECAMBRIAN
b. Generalized geologic map of northwestern
Wyoming and adjacent Montana. (Stearns 1978)
FIGURE 2. Tectonic context of the Heart Mountain
detachment fa u lt.
8
commonly crop out as hogbacks on the flanks of the ranges. Inter-
montaine structural and topographic basins preserve the entire
Paleozoic and Mesozoic continental shelf sequence covered by late
Cretaceous foreland basin deposits and latest Cretaceous and
Cenozoic b a s in -fill deposits. The present structural pattern
evolved during the Late Cretaceous to Late Eocene (70-40 m.y.a.)
Laramide Orogeny, the only Phanerozoic orogeny which immediately
affected the area of the Heart Mountain fa u lt terrane.
Post-Laramide broad regional u p lif t , local tectonic adjustments,
regional erosional events, and glaciation of up lifted areas shaped
the present physiography (Samuel son, 1974; Mackin, 1937).
The Laramide Orogeny probably occurred due to the transmission
of compressive tectonic stresses from the west across a convergent
plate boundary into the margin of the formerly stable craton (Sales,
1968; Lipman et a l , 1971; Burchfiel and Davis, 1975; Coney, 1976;
Dickenson and Snyder, 1978). The geometry of the Laramide
range-front fa u lt zones, which unlike those of the foreland fold and
thrust belt clearly involve crysta llin e basement, has been a matter
of controversy for many years, but recent seismic reflection
p ro filin g (Smithson, et al , 1978) suggests that some faults fla tte n
to re la tiv e ly low angles at depth (cf. Stearns, 1978). Throw
across the fa u lt zone between the Beartooth u p lif t and the Bighorn
Basin exceeds 9000 m (Prucha et al , 1965).
The Bighorn Basin is an asymmetrical structural depression
about 195 by 145 km in area with a N40W sinuous axial trend
positioned near the southwestern edge of the basin (Samuel son,
1974). W ei 1-developed asymmetrical anticlines 16 to 48 km in length
flank the basin, usually with the steeper limb facing the adjacent
range. Rattlesnake Mountain, which lies within the Heart Mountain
fa u lt terrane, is one such asymmetrical a n ticlin e . Its structure is
well exposed where the Shoshone river is superimposed, exposing the
faulted Precambrian core (Stearns et al , 1974). The Bighorn Basin
contains a sedimentary section consisting of 760 to 1000 m of
D aleozoic sediments, 1950 to 3600 m of Mesozoic sediments and
lo ca lly up to 4500 m of Tertiary b a s in -fill deposits (Samuel son,
1974).
The Absaroka volcanic fie ld covers a 23,000 km^ area including
the Absaroka and Owl Creek Mountains of Wyoming, the southwestern
parts of the Beartooth Mountains, and the Gallatin Range of Montana
(Parsons, 1974). The northeastern part of the Absaroka Mountains,
the area of interest regarding Heart Mountain fa u ltin g , consists of
Precambrian through Mississippian rocks, exposed at elevations
between roughly 1500 m and 2750 m above sea level, with up to 1500 m
of Eocene Absaroka volcanic rocks overlying sedimentary strata of
upper Cambrian through Mississippian age. Thus the northern
Absaroka volcanic rocks lie on a Laramide u p lif t that is about 2 km
stru c tu ra lly lower than the neighboring Beartooth u p lift to the
north but more than 8 km s tru ctu ra lly higher than the Bighorn Basin
to the east.
Heart Mountain fa u lt blocks are exposed in the Absaroka
Mountains south of the fa u lt zone separating the Beartooth Mountains
from the Absaroka Mountains, on a south-plunging asymmetric
anticline forming the eastern edge of the Absaroka Mountains at this
| latitu de (i.e . Bald Ridge), on the eastern edge of the Absaroka
I Mountains west and south of Rattlesnake Mountains, and in the
j Bighorn Basin (see Fig. 1 and Plate I).
| Method of Investigation
| Previous workers, most notably Bucher and Pierce, have
j described the occurrence of fa u lts , igneous dikes, and cla stic dikes
I in rocks overlying the Heart Mountain fa u lt. Reconnaissance studies
! by this author, begun in 1977, revealed exposures of rock overlying
i
i
i the bedding fau lt to be nearly continuous from the break-away fa u lt
i
I to the transgressive fa u lt, a stra ig h t-lin e distance of about 60 km.
I The degree and style of deformation of the rocks varies greatly, as
does th e ir a cce ssib ility. Exposures of the bedding fa u lt and fa u lt
j bounding rocks occur on the steep southwestern and southern sides of
the valleys of the Clarks Fork River and Soda Butte Creek,
| presenting a cross-sectional exposure of the detachment terrane.
I
! Exposed above the bedding fau lt in these areas are about a
i
i dozen large a l1ochthonous masses of Paleozoic rocks separated by
! areas where volcanic rocks d ire ctly overlie the bedding fa u lt. The
i
I
! present e ffo rt was designed to allow fie ld study of parts of all
i
! such areas of exposure of carbonate rocks and all intervening areas
I of volcanic rocks as well. D iffic u ltie s of access and steepness of
| slopes restricted the study of most areas to exposures along and
I immediately above the basal detachment fa u lt.
i
Geometric and kinematic data of several types were recorded,
including orientations of primary s tra tific a tio n , fa u lts , fa u lt
s tria e , igneous dikes and dike swarms, and clastic dikes and dike
swarms. Where the basal detachment fa u lt was observed i t was
examined for the presence of tectonic stria tions and breccia, and
specimens of fault-bounding rock were collected for laboratory
examination. Where appropriate, detailed geologic maps and
cross-sections of portions of the upper-plate terrane were prepared.
Stereographic projections summarize the geometric and kinematic
data.
W hen the fie ld investigation was terminated 27 areas had been
studied, including most of the accessible areas of good exposure of
the bedding fa u lt, two areas within the terrane of the fa u lt on the
former land surface, and four lo c a litie s along the northern 11 km of
exposure of the break-away fa u lt.
Previous Work
General Statement
Due to the large number of papers and abstracts addressing
aspects of the Heart Mountain fa u lt problem, th is section is sub
divided to present coherent summaries of several topics of
pa rticular interest. The f i r s t section describes the understandings
of Heart Mountain faulting that evolved from early (mostly pre-1960)
fie ld studies, which tended to focus on the Paleozoic rocks of the
fa u lt terrane. The second section outlines la te r (mostly post-1960)
studies that emphasize interpretations of the involvement of
volcanic rocks in Heart Mountain fa u ltin g . The th ird section is
12
devoted to cla stic dikes that occur along the basal detachment
fa u lt, and the fourth describes studies of the Crandall Conglomerate
and Blacktail fold. The last section is a summary of hypotheses
regarding the mechanics of Heart Mountain fa u ltin g . Plate I
provides geographical reference and Plate II is a map of generalized
geology.
Paleozoic Rocks and Heart Mountain Faulting
The Heart Mountain detachment fa u lt was f i r s t recognized in the
Bighorn Basin, where a l1ochthonous blocks of Paleozoic carbonate
rocks overlie b a s in -fill sediments. Earliest workers (Eldridge,
1884; Fisher, 1906) viewed Heart Mountain as a plug surrounded by a
cylindrical fa u lt and emplaced from below. Dake (1916) was f i r s t to
suggest that Heart Mountain is an erosional remnant of an
eastwardly-directed thrust sheet. Dake considered Logan and Sheep
Mountains, as well as smaller carbonate blocks on Dead Indian H ill
to the north and Carter Mountain to the south (see Dake, 1916,
figure on page 48) to be erosional remnants of a Heart (then spelled
"Hart") Mountain allochthon. He noted lo c a lly numerous normal
faults in the a l1ochthonous masses but attributed them to "settling
of the great block after thrusting had ceased" (p. 53). Dake (1916)
also described the "South Fork th ru s t", which he noted to be
stru ctu ra lly below the Heart Mountain "th rust". He suggested that
both "thrusts" might be parts of a decken (nappe) structure. Hewett
(1920) added the small carbonate masses of McCulloch Peaks to
Dakes's "Hart Mountain Overthrust" Sheet. Sinclair and Granger
(1912) had considered the p o s s ib ility that the limestone masses of
13
McCulloch Peaks were remnants of a thrust sheet, but seeing "no
evidence for such an extensive overthrust of the Paleozoic series on
the Eocene" (p. 65), they preferred a glacial transport mechanism
for the McCulloch Peaks klippe. Johnson (1934) added two small,
previously unknown bodies of a l1ochthonous limestone to the Heart
Mountain fa u lt allochthon, one south and one west of Pat O'Hara
Peak.
Lawrence and Sheets' (1934) detailed studies of the Logan
Mountain area revealed the a l1ochthonous block of Bighorn "lime
stone", Three Forks Shale, and Madison Limestone to be highly
shattered and cut by many small normal faults and a few thrust
fa u lts . They apparently considered Logan Mountain an erosional
remnant of a thrust sheet which was engulfed by Tertiary volcanic
rocks afte r erosion had lo ca lly exposed lower-plate Cretaceous
strata. Bucher (1935) noted the predominance of "tensional
fractures," as opposed to compressional features, within the "thrust
masses." He suggested that folds in Mesozoic strata along the South
Fork of the Shoshone River were caused by gravitational slumping
above a bedding-plane detachment (Dake's South Fork fa u lt)
roughly coeval with Heart Mountain fa u ltin g . He believed that the
folding and the Heart Mountain fa u lt were not due to "normal
orogenic processes" of compressional thrust fa u ltin g .
Sheets (1935) published a detailed structural map of the part
of Logan Mountain exposed along the west side of Trout Creek, where
he observed high-angle s trik e -s lip , normal, and reverse fa u lts , 1ow-
angle thrusts, and asymmetrical open folds, lo ca lly overturned
14
both to the northeast and west. This complex pattern of
deformation, associated with large apparent changes in elevation of
the unexposed "thrust" contact between Paleozoic and Cretaceous
strata, led Sheets to conclude that the deformation postdated
thrusting and occurred during intrusion, extrusion, and
syn-extrusive deformation of the nearby igneous rocks.
Stevens' (1938) study of the Sheep Mountain area and
reconnaissance of Carter Mountain, Meeteetse Mountain, and Heart
Mountain led him to conclude that the a l1ochthonous strata moved as
one or a few thrust sheets from the southwest to the northeast.
Thrust movement was thought to be slow, and extensive erosion of the
thrust sheet(s) occurred prior to extrusion of volcanic breccias.
Local severe deformation of a l1ochthonous carbonate and volcanic
rocks at Sheep Mountain was inferred to postdate fa u ltin g , having
occurred in response to extrusion of the volcanic breccias. Stevens
viewed the Heart Mountain thrust as the easternmost expression of
the foreland fold and thrust belt of western Wyoming.
W . G. Pierce, who began fie ld studies of the Heart Mountain
detachment terrane in 1936, published the f i r s t of his many papers
on the Heart Mountain detachment fa u lt in 1941. Pierce (1941)
reported the discovery of several previously unrecognized
a l1ochthonous blocks on Rattlesnake Mountain, on Pat O'Hara
Mountain, and in Sunlight Basin. Although he did not d ire c tly
address the question of whether emplacement occurred as individual
blocks or as a continuous thrust sheet, several references to the
"thrust sheet" and "eroded remnants of the thrust sheet" suggest
15
that he favored the la tte r iinterpretation. Pierce's work increased
Hewett's (1920) estimate of eastward displacement from 45 to 55 km.
Based on the observations of Love (1939) and Rouse (1940) of
possibly a l1ochthonous blocks of carbonate rock in Eocene
sedimentary rocks and southern Asbaroka volcanic rocks, Pierce also
suggests that the Heart Mountain detachment terrane might extend
southward into the Graybull River and Wind River drainages. In
addition, he described the South Fork thrust as a southeasterly-
directed Eocene bedding thrust predating the Heart Mountain
detachment. The South Fork detachment lies within Jurassic strata.
Bucher (1947) presented an alternative interpretation of the
South Fork fa u lt, including the apparently contradictory ideas of
northwest-vergent thrusting, and southeast-vergent folding, both due
to gravity sliding to the southeast. He did not view the carbonate
masses of Dead Indian Creek as part of the Heart Mountain detachment
terrane.
In an abstract, Pierce (1950) reported extension of the known
lim its of the faulted terrane to include areas west of Dead Indian
Creek where blocks broken by numerous faults lay in normal
stratigraphic sequence on Cambrian strata. He suggested that
a l1ochthonous Paleozoic carbonate rocks had slid southeastward down
the south limb of the Beartooth Mountain u p lif t .
Pierce (1957) provided a detailed development of the concept of
the Heart Mountain and South Fork faults as "decol1ements" or
"detachment thrusts". His mapping had extended the Heart Mountain
16
detachment terrane to include the area of the "bedding thrust" as
far to the northwest as P ilo t Peak (p. 591). Previous mapping of
th is area (Hague, 1899) had not recognized the presence of a low
angle fa u lt. Pierce interpreted much of the bedding-plane
detachment as having been tectonica lly denuded, and he envisioned
the tectonica lly denuded terrane as the source area for the fa u lt
blocks scattered over the "shear thrust" (la te r, 1960, his
"transgressive fa u lt") and the fa u lt on the former land surface.
Movement of the detached sheet was to the east-southeast, not to the
southeast from the Beartooth Mountain u p lif t .
Pierce (1960) announced the discovery of the "break-away"
fa u lt, the western boundary of the Heart Mountain detachment
terrane. He established a terminology for the four phases of the
Heart Mountain detachment fa u lt that has been in common usage
since: the break-away fa u lt, the bedding fa u lt, the transgressive
fa u lt, and the fa u lt on former land surface.
The Reef Creek fa u lt, f i r s t described in part by Pierce
(1958), was interpreted by him (1963b) as consisting of a bedding
fa u lt (detachment occurred near the base of the Madison) with a
transgressive fa u lt and a fa u lt on the land surface to the
southeast. It occurred within the area that was la te r affected by
Heart Mountain detachment fa u ltin g , so that Heart Mountain faulting
further scattered the blocks that had been transported during Reef
Creek fau lting.
That the Heart Mountain bedding fa u lt detachment characteris
t ic a lly occurs about 2 m above the base of the Bighorn Dolomite, and
17
not at the Cambrian-Ordovician contact, was f i r s t mentioned in
Pierce (1968). Pierce (op. c i t . ) also suggested that masses of
carbonate rock in landslides in the Carter Mountain area, southeast
of the South Fork of the Shoshone River, were derived from Heart
Mountain fa u lt blocks.
Publication of U.S. Geological Survey 15' quadrangle geologic
maps covering portions of the areas underlain by the Heart Mountain,
Reef Creek, and South Fork detachment fa u lt terranes began in 1965.
The Deep Lake Quadrangle (Pierce, 1965) and the Cody Quadrangle
(Pierce, 1966b) show relationships on parts of the bedding,
transgressive, and former land surface phases of the Heart Mountain
fa u lt. The Pat O'Hara Quadrangle (Pierce and Nelson, 1969) covers
most of the area of the South Fork decollement as well as portions
of the Heart Mountain fa u lt terrane. The Beartooth Butte Quadrangle
(Pierce and Nelson, 1971) covers the central portion of the
bedding-plane fa u lt, including part of the Reef Creek fa u lt
terrane, the igneous dike swarm at Cathedral C liffs , and the
association of lower-plate deposits of Crandall Conglomerate with
the Blacktail fold (discussed in Pierce and Nelson, 1973). A
1:125,000 geologic map of Yellowstone National Park (U.S. Geological
Survey, 1972) shows a small portion of the Heart Mountain fa u lt
terrane near the breakaway fa u lt.
In a summary paper, Pierce (1973) reviewed his overall
understanding of the geological history of Heart Mountain faulting
and introduced some new inte rpretatio ns. Pierce re-emphasized his
argument that the upper plate had moved for long distances as
18
separate blocks, c itin g as evidence the argument that the
a l1ochthonous blocks now distributed over the 2000 km^ area of the
land surface fa u lt had apparently been derived from a 650 km^ area
(approximately one-half of the area of the bedding-plane fa u lt
that is te cto n ica lly denuded). He also suggested that the absence
of erosion of the supposedly tectonically-denuded portion of the
bedding-plane fa u lt and the preservation of carbonate fault-breccia
on i t argue for movement of the upper plate as separated blocks.
This inference was considered a p a rticu la rly sig n ifica n t constraint
on proposed mechanism hypotheses.
Pierce (op. c i t . ) noted that a l1ochthonous blocks appear to be
most widely separated on the land-surface portion of the fa u lt and
least widely scattered in the area of the transgressive fa u lt. H e
explained this distrib u tio n by suggesting that the transgressive
fa u lt dipped northwestward and that southeastward movement of blocks
was retarded there. Separated blocks therefore presumably abutted
against one another at and northwest of the transgressive fa u lt.
Pierce also refined his argument regarding the overall slope of
the fa u lt surface. He pointed out that while absence of marine
deposits in the Eocene of the Bighorn Basin argue for the
land-surface portion of the fa u lt having been above sea level, the
flora of the fossil forests near the breakaway fa u lt suggested to
Dorf (1964) an elevation of no more than 900 m (3000 f t . ) above
sea-level. Thus a slope of less than 2° seemed to Pierce to be
indicated by the data. Pierce (1973) went on to review the
mechanism problem of Heart Mountain faulting (discussed herein in a
subsequent section). 19
Prostka (1978) observed that a l1ochthonous masses of Paleozoic
rocks range widely in degree of deformation, from v irtu a lly
undeformed to intensely folded or faulted. H e also noted that some
a l1ochthonous masses of Paleozoic rocks, such as those at Cow Creek
and in Sunlight basin, consist only of Devonian or Mississippian
strata. He interpreted occurrences of carbonate blocks emplaced on
volcanic rock, such as are found south of Dead Indian Creek (compare
Pierce, 1958) and in Pierce's (1963b) Reef Creek Fault terrane, as
overthrusts that occurred during Heart Mountain fa u ltin g . The
"s tru c tu ra lly complex" carbonate blocks, together with the deformed
volcanic masses, were cited by Prostka as evidence that other
detachments commonly occurred above the basal detachment fa u lt,
especially at the bases of the Mississippian and the Eocene, during
Heart Mountain fa u ltin g . Prostka suggested that the blocks in the
area of the transgressive fa u lt were probably once a single large
a l1ochthonous sheet only recently separated by erosion (compare
Pierce, 1973).
Pierce (1980) described the Heart Mountain break-away fa u lt in
unprecedented d e ta il, summarizing and evaluating several previously
published maps. He mentioned th a t, although "decollement or
detachment sheets in general" (p. 272) should have a break-away
fa u lt, the Heart Mountain break-away fa u lt seems to be the only such
fa u lt known. Because rocks in the hanging wall of the steeply
east-dipping fa u lt are interpreted as having been deposited against,
rather than faulted along i t , i t was thought to be, lik e the
presumably tectonically-denuded portions of the bedding plane fa u lt,
a "h a lf-fa u lt" (p. 276). 20
Pierce's (1980) mapping of the breakaway fa u lt north of Soda
Butte Creek d iffe rs from that of the U. S. Geological Survey (1972)
and Prostka (1978). It also d iffe rs from Pierce et a l . (1973) in
the area of the headwaters of Cache Creek, where Pierce (1980)
believes the break-away fa u lt flattens at depth within volcanic
rocks and then, presumably, steepens to jo in the basal detachment at
its usual horizon (cf. Prostka, 1978). Some hanging-wall flows of
Wapiti Formation are steeply inclined westward into the fa u lt in
th is area, despite th e ir supposed post-faulting age; Pierce explains
that "perhaps the Wapiti at this place slid in as the lower part of
the formation was being deposited" (p. 278).
Pierce (op. c i t . ) also mentions the occurrence of slickenside
striae plunging 25° south on Madison Limestone strata along the
break-away fa u lt ju s t south of Soda Butte Creek. These striae
suggested to Pierce that the in it ia l movement along the breakaway
"seems to have represented shear rather than extension" (p. 277),
a fte r which movement was directed to the southeast away from the
breakaway fa u lt. Pierce's paper (op. c i t . ) contains a structure
contour map of the configuration of the Heart Mountain fa u lt surface
throughout its known and inferred extent, and he discusses a
lower-plate high angle fa u lt near Silver Gate that has 60 m of throw
and is truncated by the detachment fa u lt.
Tertia r y Volcanic Rocks and Heart Mountain Faulting
Dake (1916) inferred a long period of erosion between Heart
Mountain faulting and deposition of Hague's (1899) early basic
21
breccia. Hewett (1920) thought Heart Mountain faulting occurred
a fte r extrusion of the early acid breccia of Hague (op. c i t . ) but
before extrusion of the early basic breccia. O n Logan and Sheep
Mountains, Hares (1933) observed faults offsetting both Tertiary
agglomerates and Paleozoic limestone. He interpreted these offsets
as secondary to and contemporaneous with the "main overthrust", thus
lending support to Hewett's (op. c i t . ) conclusions. Stevens (1938)
believed that the faulting of volcanic rocks at Sheep Mountain
postdated Heart Mountain fa u ltin g .
In his study of Absaroka Volcanic rocks, Rouse (1937) concluded
that Heart Mountain faulting must have occurred between the times of
extrusion of the early acid breccia and early basic breccia of Hague
(1899). He reported the occurrence of an a n ticlin e , t ilt e d beds,
and a thrust fa u lt in the early basic breccia of the South Fork of
the Shoshone River area. H e also reported the occurrence of
volcanic rock as far east as McCulloch Peaks. To illu s tr a te the
high re lie f of pre-volcanic topography he showed cross-sections of
the southwest side of the Clarks Fork Valley and the south side of
Soda Butte Creek; both of these areas were la te r shown by Pierce
(1957) to represent terranes that were tectonica lly disrupted during
Heart Mountain fa u ltin g .
Pierce's (1938) studies of the oldest Absaroka volcanic rocks
(Hague's 1988 "early basic breccia") along the west side of the
Bighorn basin suggested that they were in part intrusive into both
a l1ochthonous rocks of the Heart Mountain detachment and into
sediments underlying the a l1ochthonous rocks. They were, thus,
22
interpreted as younger than the "overthrust". Bucher (1940)
acknowledged some uncertainty regarding the time of fa u ltin g , but he
emphasized that faulting clearly predated the beginning of "violent"
volcanic a c tiv ity .
Hay (1954) b rie fly discussed the controversy regarding the
emplacement of Heart Mountain fa u lt blocks in the context of his
study of deformed bodies of andesitic tuff-breccia exposed in the
Greybull River Valley and South Fork of the Shoshone River valley.
Hay suggested that extrusive le n ticu la r sheets of tuff-breccia might
have become mobilized a fte r b u ria l, undergoing lateral injection
into adjacent d e trita l volcanic beds. He envisioned no tectonic
control over such events, inferring a postdepositional gravity
adjustment mechanism. These inferred events presumably postdated
and had no relation to Heart Mountain fa u ltin g . Pierce (1957)
believed that the early basic breccia had been deposited upon the
tectonica lly disrupted terrane after fa u ltin g , and that no volcanic
rocks were involved in Heart Mountain fa u ltin g .
Pierce (1958) reported the occurrence of a l1ochthonous blocks
of Madison limestone overlying "early acid breccia". Pierce
suggested that both the deposition of the early acid breccia and
subsequent fault-emplacement of the limestone blocks predated Heart
Mountain fa u ltin g , and that some of the early acid breccia and
a l1ochthonous Madison blocks were subsequently carried "piggyback
style" (p. 142) during the main movement along the Heart Mountain
fa u lt. Pierce's (1960) discovery of the break-away fa u lt, which
cuts a thick footwall section of volcanic rocks, further
23
demonstrated involvement of volcanic rocks in Heart Mountain
fa u ltin g .
Pierce (1963a) gave the formal name Cathedral C liffs Formation
to the rocks of the Clarks Fork area which Hague (1899) had
inform ally referred to as the "early acid breccia". He correlated
the Cathedral C liffs Formation with the early acid breccia in
northern Yellowstone Park. All masses of Cathedral C liffs formation
were interpreted as a l1ochthonous, having ridden piggyback during
Heart Mountain detachment faulting on blocks of Paleozoic carbonate
rocks. The blocks of Madison Limestone that overlie Cathedral
C liffs Formation at Cathedral C liffs were interpreted as
a l1ochthonous along a detachment fa u lt called the Reef Creek fa u lt
by Pierce (1963b). The Reef Creek fa u lt was thought to have
developed a fte r extrusion of the Cathedral C liffs Formation but
before Heart Mountain fa u ltin g . Pierce (1963a) also noted that in
places on the surface of tectonic denudation, volcanic rock adheres
to or is "frozen on" (p. 195) the underlying limestone, but that no
slickensides were observed.
Nelson and Pierce (1968) gave the name Wapiti Formation to
rocks referred to by Hague (1899) as "early basic breccia". They
note that in some places the Wapiti Formation "surrounds and buries"
(p. 48) a l1ochthonous carbonate blocks emplaced during Heart
Mountain fa u ltin g . Blocks of carbonate rock occurring in the lower
part of the Wapiti formation were interpreted as having been shed
from topographically high parts of carbonate fa u lt blocks, and
steeply and randomly tilt e d blocks of both the Wapiti Formation and
24
Eocene b a s in -fill sedimentary rocks (Willwood Formation) were
suggested to have been emplaced by small-scale gliding tectonics.
Brophy (1969) reported that the metamorphosed Paleozoic
carbonate rocks of White Mountain, recognized as a l1ochthonous by
Pierce (1957), are folded ptygmatically and cut by numerous thrust
fa u lts . Multiple sets of dikes occur, including metamorphosed
igneous dikes, unmetamorphosed igneous dikes, and carbonate cla stic
dikes. The entire mass rests on unmetamorphosed limestone,
suggesting to Brophy that the older igneous dikes and the
metamorphic event that altered them predated Heart Mountain
fa u ltin g . This metamorphism, of unknown age and in it ia l location,
was interpreted to be pre-Eocene unassociated with Eocene Absaroka
volcanism. Nelson (1969) reported that the spire of volcanic rock
at White Mountain, thought by Parsons (1939) to occupy the volcanic
vent that metamorphosed the adjacent marble and quartzite, is
actually steeply-dipping alluvia l facies Cathedral C liffs formation
and is therefore a l1ochthonous. Nelson, lik e Brophy, reported the
occurrence of two generations of igneous dikes, but Nelson viewed
the older, clea rly a l1ochthonous dikes as probably responsible for
the metamorphism.
Nelson et al (1972) believed that the unmetamorphosed igneous
dikes, which cut across the metamorphosed carbonate rocks and
igneous dikes, are related to or younger than the Wapiti volcanic
rocks and postdate movement on the basal detachment fa u lt.
Subvertical unmetamorphosed igneous dikes exposed at Cathedral
C liffs , six miles northwest of White Mountain, terminate downward at
25
the basal detachment fa u lt. The dikes at Cathedral C liffs were
inferred to postdate Heart Mountain faulting to be la te ra lly
intruded rather than tectonica lly truncated at the basal detachment
fa u lt. The unmetamorphosed dikes at White Mountain were thought to
be sim ila rly post-tectonic.
The s tra ti graphic framework of the Cathedral C liffs , Lamar
River, and Wapiti Formations, among others, is discussed by Smedes
and Prostka (1972). In 1973 the U.S. Geological Survey geologic map
of the 15' P ilo t Peak quadrangle (Pierce et a l . 1973) was published,
revealing disagreement among the authors regarding the relationships
between certain of the volcanic rocks and the Heart Mountain fa u lt.
In most places between the breakaway fa u lt and Jim Smith Creek the
volcanic rocks in contact with the Heart Mountain fa u lt were mapped
by Nelson and Prostka, in "semi-reconnaissance" fashion, as "Lamar
River and Cathedral C liffs Formations, undivided." Prostka believed
these rocks and associated rocks mapped as Lamar River Formation to
be in fa u lt contact with underlying sedimentary and adjacent
volcanic rocks. Pierce and Nelson believed these contacts to be
depositional.
Prostka's (Pierce et a l , 1973) interpretation was amplified by
the publication of the Abiathar Peak quadrangle (Prostka et a l .
1975), which shows an a l1ochthonous block of Lamar River formation
at the junction of the break-away and bedding-plane phases of the
Heart Mountain fa u lt. A note on the cross-section indicates that
th is a l1ochthonous block was interpreted as having slid into place
from the north-northwest and having been subsequently buried by
26
Wapiti volcanic rocks.
Prostka (1978) agreed in broad outline with the sequence of
events inferred by Pierce for Heart Mountain faulting but differed
from Pierce in many important de tails, especially with regard to the
volcanic rocks associated with Heart Mountain fa u ltin g . Prostka
(op. c it . ) attributed his previous (in Pierce et a l , 1973)
disagreements with Pierce and Nelson to the subtlety of cataclastic
effects in the a l1ochthonous volcanic rocks and to the gross
s im ila rity between pre-faulting (Lamar River Formation) and
post-faulting (Wapiti Formation) volcanic rocks. Prostka ide ntified
extensive (about 5 km across) a l1ochthonous masses of faulted
volcanic rocks at P ilo t Peak, Jim Smith Peak, and Squaw Peak,
nominantly southeastward dips in these volcanic rocks suggested to
Prostka that they had spilled forward o ff the tops of a l1ochthonous
blocks. At Republic Mountain and P ilot Peak dips of up to 50° were
recorded. Prostka reported the occurrence of "low-angle ramp
structures or spillovers" (p. 431) consisting of volcanic material
which, while s t i l l unconsolidated, had presumably spilled forward
o ff the tops of al 1ochthonous blocks as sliding ceased, landing on
the tectonica lly denuded surface. The north side of Jim Smith Peak
was cited as an example. The "pseudo depositional" (p. 437) nature
of the contact between the a l1ochthonous volcanic rocks and the
lower plate in part caused m isidentification of the contact as
depositional by e a rlie r workers.
Prostka (1978) also discussed the occurrence of igneous dikes
in Heart Mountain fa u lt blocks, suggesting, in disagreement with
27
Pierce (1963b) and in agreement with Voight (1974a) that igneous
dikes at Cathedral C liffs were sheared o ff at the basal detachment
fa u lt rather than being la te ra lly intruded. He also mentioned
a l1ochthonous dikes in volcanic rocks at White Mountain and along
the North Fork of Crandall Creek.
In the IJ.S. Geological Survey Cody 1° x 2° quadrangle, Pierce
(1978) revised parts of the mapping shown on e a rlie r 15' quadrangles
covering the Heart Mountains detachment terrane. Volcanic rocks on
Rattlesnake Mountain, Pat O'Hara Mountain, between Pat O'Hara
Mountain and Dead Indian Pass, and west of Dead Indian Creek (a ll on
Pierce and Nelson, 1968) were reinterpreted as a l1ochthonous Lamar
River and Cathedral C liffs Formations instead of as in situ Wapiti
Formation. In the area between the break-away fa u lt and Jim Smith
Creek, all volcanic rocks that had been mapped by Pierce et al
(1973) as undifferentiated Lamar River and Cathedral C liffs
Formation were redesignated as Wapiti Formation, though the
explanation states that the Wapiti Formation's "lower part may
include some fault-emplaced blocks of Lamar River and Cathedral
C liffs Formations... east of the Heart Mountain breakaway fa u lt."
E llio t t 's (1979) map of the southwest part of the Cooke City
quadrangle shows part of the area of contention between Pierce
(1978) and Pierce et a l . (1973). E llio t t 's mapping is in accord
with the la tte r. E llio t t also depicts Heart Mountain fa u lt
relationships occur north of Soda Butte Creek, ju s t east of the
inferred northern extent of the breakaway fa u lt.
Nelson et al (1980) published a geologic map of the North
28
Absaroka Wilderness Area, which includes large portions of the
breakaway, bedding-plane, and transgressive phases of the Heart
Mountain fa u lt. The mapping of volcanic rock in the areas of Pat
O'Hara Mountain, Rattlesnake Mountain, Dead Indian Pass, Dead Indian
Creek, and between the break-away fa u lt and Jim Smith Creek went
unchanged from that shown on the Pat O'Hara Mountain quadrangle
(Pierce and Nelson, 1968) and the Pilot Peak quadrangle (Pierce et
a l . 1973), so in these areas Nelson et a l . (1980) disagree with
Pierce (1978). Whereas Pierce et a l . (1973) and Pierce (1978, 1980)
mapped the southern terminus of the breakaway fa u lt as disappearing
beneath younger volcanic rocks, on Nelson et al (1980) the
break-away fa u lt is shown to be terminated by a northeast-striking
normal fa u lt. In his generalized geologic map of the breakaway
fa u lt and surrounding area Pierce (1980) mapped the volcanic
rocks between the breakaway fa u lt and Jim Smith Creek as Wapiti
Formation (post-tectonic) including "incompletely mapped,
te cto n ica lly transported masses of undivided Lamar River and
Cathedral C liffs Formations."
Thus, at the time of the beginning of the present study (1977),
some volcanic rocks (those labeled Tic on Plate II) were recognized
by all principal workers as a l1ochthonous and other volcanic rocks
(those labled Tw on Plate II) were recognized by all principal
workers as predominantly in s itu , having been deposited upon a
surface of tectonic denudation. In many areas (those of T v is jv2,
Tv3 , and Tv4 on Plate I I ) , however, no consensus had been reached
among previous workers regarding the volcanic rocks were
29
a l1ochthonous as in s it u . Although several new publications by
previous workers appeared during the course of the present study,
these disagreements went unresolved.
Clastic Pi kes and Hea rt Mountai n Faulti ng
Pierce (1968) f i r s t reported that cla stic dikes of carbonate
breccia with lesser amounts of volcanic rock occur within volcanic
rocks overlying the surface of inferred tectonic denudation.
Locally a layer up to 50 cm thick of carbonate breccia containing no
volcanic material was observed between the surface of tectonic
denudation and the volcanic rock containing carbonate dikes. The
carbonate breccia in cla stic dikes and along the bedding-plane fa u lt
was interpreted as fa u lt breccia which had lain irre g u la rly
distributed on the surface of tectonic denudation before overlying
volcanic rock was deposited. Subsequent injection of carbonate
breccia into overlying volcanic rock, possibly during earthquakes,
was envisioned. Pierce (1973) believed that th is inferred geologic
history indicates that Heart Mountain faulting occurred at a "very
rapid, probably cataclysmic" (p. 462) rate.
Clastic dikes were observed by Brophy (1969) and Nelson et al
(1972) in a l1ochthonous carbonate rocks at White Mountain. The
breccia along the basal detachment fa u lt and in the cla stic dikes at
White Mountain contains clasts of metamorphosed carbonate and
porphyritic igneous rock. The sources of the clasts were inferred
to be the superjacent metamorphosed carbonate rocks, igneous dikes,
and volcaniclastic sedimentary rocks of the White Mountain
a l1ochthon.
Voight (1974b) reported the occurrence of cla stic dikes in 30
a l1ochthonous carbonate rocks along the transgressive fa u lt.
Pierce (1979) described 15 sites where cla stic dikes of fa u lt
breccia occur in the Heart Mountain fa u lt terrane, including areas
in the transgressive fa u lt phase and former land-surface phase.
Clastic dikes were observed in upper-plate carbonate rocks, in
volcanic rocks interpreted as a l1ochthonous, and in volcanic rocks
interpreted as in situ on the bedding-plane detachment fa u lt
(Voight, 1973c; Pierce, 1979). The cla stic dikes were viewed as a
s ig n ifica n t constraint on the mechanism of Heart Mountain fa u ltin g .
Voight (1973a, b, 1974a) viewed them as evidence for abnormal flu id
pressure along the detachment fa u lt, while Pierce (1968, 1979)
believed th e ir occurrence supported an earthquake-oscil1ation
mechanism (discussed herein in a subsequent section).
The Crandal 1 Cong!omerate a _ n _ d the B 1 acktai 1 Fold
Pierce (1941) reported the occurrence of a body of
conglomerate, with clasts derived from Madison Limestone and younger
formations, in the v ic in ity of Dead Indian Creek. He interpreted i t
as having formed during Heart Mountain faulting and having been
subsequently overridden by the allochthon. Pierce (1950) mentions
three occurrences of conglomerate in the fa u lt terrane. Bucher
(1947) reported deposits of quartzite conglomerate at Sheep Mountain
and Bald Ridge.
Pierce (1957) gave the name "Crandall Conglomerate" to fourteen
deposits of coarse conglomerate exposed in the drainages of Squaw,
Crandall, Lodgepole, Sunlight, and Dead Indian Creeks. All deposits
consist of clasts of Madison Limestone and older formations.
31
Deposits in the northwest part of the area were thought to postdate
Heart Mountain fa u ltin g , but those in the southern part were
interpreted as syntectonic. The occurrence of a faulted a n ticlin e
in Cambrian strata beneath the conglomerate bodies in the Crandall
area was also noted.
In an abstract, Pierce (1958) reported the occurrence of
deposits of Crandall Conglomerate both upon brecciated thrust debris
and below a l1ochthonous Heart Mountain fa u lt blocks. The former
deposits were interpreted as postdating fa u lt movement and the
la tte r as predating fa u lt movement. To explain the apparent age
discrepancy two separate episodes of movement along the Heart
Mountain fa u lt were postulated. Pierce (1968) modified his e a rlie r
interpretations of the Crandall Conglomerate, suggesting that its
deposition predated movement on the Heart Mountain fa u lt.
Pierce and Nelson (1973) provide a detailed description of the
nature and occurrence of the Crandall Conglomerate and describe an
inferred sequence of events linking its origin to Heart Mountain
fa u ltin g . They believe that both lower- and upper-plate occurrences
of Crandall Conglomerate were derived from a single stream system.
The unique coarse-grained texture of the conglomerate was attributed
to a local condition of unusually high r e lie f. The faulted
an ticlin e in the Cambrian Pilgrim Limestone and adjacent shales that
occurs only beneath lower-plate deposits of the Crandall
Conglomerate was named the Blacktail Fold.
The unusual aspects of the texture and d is trib u tio n of the
Crandall Conglomerate led Pierce and Nelson (1973) to propose a
32
preliminary movement (cf. Pierce 1958) along the Heart Mountain
detachment fa u lt that involved only the northeastern one-third of
the future upper plate. This postulated preliminary movement was
thought to have produced a 1.6 km wide, 600 m deep r i f t in which a
channel up to 100m below the detachment horizon (down to the Pilgrim
Limestone) was quickly eroded. The Blacktail fold was thought to
have formed in response to the gravitational load of 600 m c l i f f s on
either side of the channel, with subsequent deposition of the
Crandall Conglomerate to levels lo c a lly exceeding 60 m above the
detachment horizon. The Crandall Conglomerate consists of clasts of
Paleozoic and Precambrian rocks; no clasts of volcanic rock have
been found. Thus, to accord with other fie ld relationships as
reported by Pierce and others in previous papers, the Crandall
Conglomerate must have formed before Cathedral C liffs volcanism of
either late early Eocene or early middle Eocene age.
Consequently, the following complex sequence of events was
inferred by Pierce and Nelson (1973) to be associated with Heart
Mountain fa u ltin g : preliminary movement on the Heart Mountain basal
detachment, with attendant formation of the Blacktail fold and
Crandall Conglomerate; deposition of the Cathedral C liffs Formation;
emplacement of the Reef Creek fa u lt blocks; second (main) movement
on the Heart Mountain detachment fa u lt, moving upper-plate deposits
of Crandall Conglomerate southeast toward the (future) Dead Indian
Creek area and producing the d istrib u tio n of a l 1ochthonous blocks
seen in part today; and fin a lly , deposition of the Wapiti formation,
which occurred before erosion and folding analogous to the Blacktail
33
fold could occur adjacent to the breakaway fa u lt. The kinematics
inferred by Pierce and Nelson are complicated by the occurrence of
one upper-plate body of Crandall Conglomerate southwest of the
lower-plate exposures.
Voight (1974b) offered a d iffe ren t interpretaton of the
formation of the Crandall Conglomerate and Blacktail Fold:
Perhaps the Crandall Conglomerate is not related at all to
the main movement of the Heart Mountain slab, but to the
unequal u p lif t of the Beartooth and Sunlight-Crandal1
blocks. D ifferentia l extension of the Paleozoic drape
fold sequence would occur, as described in detail by
[Stearns et a l , 1974]... and thus a gap (probably f ille d
by ductile clay shale) could form in the b r it t le Paleozoic
sequence a few kilometers distant from the Clarks Fork
block fa u lt. Subsequently exhumed by erosion, and rapidly
f ille d , the geologic relationships would resemble... the
stage prior to volcanism and Heart Mountain disruption.
(P. 121).
Voight, however, acknowledged that Pierce and Nelson's (1973)
hypothesis had not been disproven.
34
The Meehanics o£ Heart Mountain Faulting
Bucher, et al (1933) discussed several unsolved problems
regarding the Heart Mountain "thrust" of Dake (1916) and Hewett
(1920). A minimum transport distance of 45 km seemed indicated for
the easterly-di rected thrust sheet, yet no intensely compressed
folded mountain system could be found in the supposed source area.
The Absaroka volcanic terrane seemed to cover too small an area to
conceal such a source terrane. Bucher (1933) suggested that the
a l1ochthonous blocks were never part of a continuous thrust sheet,
but that they were individual a l1ochthonous masses that had been
"scattered much as they exist today by the horizontal component of
the force of a large volcanic explosion. The volcanic gases,
perhaps largely pent-up steam, exploded in such a way to shear o ff
large sheets of limestone from the highly micaceous Cambrian slabs
and to drive them eastward (or, perhaps better, southeastward) down
the sediment slope into the plain" (p. 239). Johnson's (1934) study
of the Shoshone Canyon area of the Heart Mountain detachment terrane
supported Dake's (1916) concept of the Heart Mountain "overthrust"
and discounted Bucher's (1933) volcanic explosion hypothesis.
Bucher's (1935) mapping of the South Fork thrust of Dake (1916)
caused Bucher to suggest "gravity as the moving force" (p. 69) for
the South Fork and Heart Mountain "thrusts". Bucher also described
the internal structure of Heart Mountain "thrust masses" as
dominated by "tensional fractures" and devoid of signs of
compression, further discounting the idea that they are "mere
erosional remnants of a larger uniform sheet."
35
Based upon his fie ld study of Sheep Mountain and his
reconnai ssance elsewhere, Stevens (1938) inferred nort'neastwa rd
movement of thrust sheets with large displacements that, to him,
precluded "radial landslide thrusting" lik e that of the Bearpaw
Mountains, Montana, on mechanical grounds. Stevens called upon the
"in e rtia mechanics" of underthrusting (Stevens, 1936), whereby the
lower plate is viewed as active and the upper plate as passive with
respect to applied stress, to explain displacements of thin thrust
sheets for distances of 65-80 km.
Bucher (1940) expanded on e a rlie r arguments (Bucher et a l ,
1933; Bucher, 1933) regarding the structural context of the Heart
Mountain fa u lt terrane, suggesting that the reversals of fold - and
fault-assymmetries in the Beartooth, Owl Creek, Pryor and Bighorn
Mountains and smaller u p lifts such as Rattlesnake Mountain were not
compatible with the large-scale overthrusting postulated by Dake,
Hewett, Johnson and Stevens. He reemphasized the dominance of
extensional features in rocks a l1ochthonous above the Heart Mountain
fa u lt and referred to minor structures in the lower plate that
suggest radial displacement from an area near Trout Peak, about 25
miles west-northwest of Cody Wyoming. Bucher also argued that the
amount of weathering and erosion necessary to have removed 99% of an
inferred continuous thrust sheet could not have occurred in the time
available. He concluded that the nature of the deformation that
produced the faulted terrane was not yet understood, but reiterated
that sim ilar patterns of radial transport had been observed in the
Bearpaw Mountains of Montana and in and around the Reis basin in
southern Germany. ^
Bucher (1947) elaborated on his previous arguments against the
idea of a once-continuous thrust sheet, emphasizing that the
wedge-shaped cross-sectional geometry of most displaced blocks, with
strata commonly truncated by the basal detachment fa u lt, is
incompatible with the concept of a continuous thrust sheet. He also
remarked that the absence of calcite or dolomite veins at the base
of the a l1ochthonous blocks, such as were observed along the Bannock
overthrust, argues for a rate of displacement several orders of
magnitude greater than is typical of thrust sheets. Modifying his
previous concepts, Bucher suggested that individual blocks, having
been sheared o ff the base of the Bighorn limestone, slid down
"temporarily steepening si opes. . .under the action of gravity,
probably aided by frequent earthquake shocks that preceeded the
outbreak of volcanic a c tiv ity " (1947, p. 196). Volcanic explosions
may have aided detachment.
Pierce (1950) viewed the Heart Mountain fa u lt allochthon as
having moved, "probably with the aid of gravity, southeastward down
the south limb of the Beartooth Mountain u p lif t" (p. 1493). H e
(Pierce, 1957) envisioned the South Fork thrust as a decollement or
"detachment thrust" sim ilar to the Heart Mountain fa u lt. The South
Fork thrust mass deformed prim arily by folding rather than fau lting
but th is difference in deformational style was attributed to the
mechanical contrast between the carbonate rocks of the Heart
Mountain fa u lt blocks and the predominantly shale section of the
South Fork thrust mass. "Gravitational sliding" (p. 591) was
envisioned as the force prim arily responsible for moving the
detached sheets. 37
Hubbert and Rubey (1959) mentioned the Heart Mountain fa u lt in
th e ir general discussion of the occurrence of “ overthrusting",
implying that the mechanical problems associated with implacement of
the Heart Mountain allochthon might be alleviated by th e ir (1959)
flu id pressure hypothesis. Pierce (1963a) envisioned seismic energy
as having contributed to emplacement of a l1ochthonous blocks:
Movement on such a low slope is due to gravity acting
concomittantly with a great number of severe
earthquake shocks that imparted an intense jig g lin g
motion to the detached blocks. Movement occcurred
during a very short time interval (p. 19).
Pierce (1963b) suggested that the Reef Creek, South Fork, and Heart
Mountain detachment fa u lt blocks may all have been moved down gently
sloping surfaces by the force of gravity in conjunction with the
shaking motion of many earthquakes. He further suggested that the
high-fluid-pressure hypothesis of Hubbert and Rubey (1959) for the
reduction of fric tio n across overthrust faults is not applicable to
the Heart Mountain or Reef Creek fa u lts . At the shallow depths of
these faults high flu id pressure would not be expected, and i f i t
did occur i t would be lost as the detached upper plate broke apart.
Davis (1965) cited the Heart Mountain detachment fa u lt as one
of several low-angle faults that he believed could not have
undergone floatation by high flu id pressure during displacement. H e
elaborated on and reinforced Pierce's statement that high flu id
pressures would be unlikely at the base of the Heart Mountain or
38
Reef Creek fa u lts , c itin g th e ir shallow depths of detachment and
discontinuous upper plates and emphasizing that th e ir extensional
natures preclude development of abnormal flu id pressures by tectonic
compressional stress. Davis also pointed out that the Heart
Mountain detachment occurs within dolomite, not within shales as is
favored by the Hubbert and Rubey (1959) model.
Rubey and Hubbert (1965) replied to Davis' statements regarding
the Heart Mountain detachment fau lt by questioning parts of Pierce's
interpretation of the Heart Mountain detachment fa u lt. They
suggested that i f Heart Mountain fau lting continued during
deposition of the early basic breccia,
the intrusive emplacement of some of the volcanic
b re c c ia ...; the large blocks of Paleozoic limestone
caught withn i t . . . ; and the evidence of lateral
movement of a mile or more by "underground mudflows"
within the breccia.. .could support a broadened
interpretation that some part of the fa u lt movement
was the result of re la tiv e ly local movement within
the volcanic breccia, and that another part was the
result of large-scale movements of an extensive
thrust sheet composed of limestone blocks plus
breccia (p. 470).
In his discussion of Davis (1965) and Rubey and Hubbert (1965),
Pierce (1966a) elaborated on several aspects of the Heart Mountain
fa u lt which he interpreted as precluding the presence of high flu id
39
pressure. He emphasized the idea of tectonic denudation of the
bedding plane fa u lt, arguing that tectonic denudation of the bedding
fa u lt was demanded by: (1) fie ld observations of the contact
between early basic brecica and the lower plate; (2) the scattered
d is trib u tio n of upper-plate carbonate blocks; and (3) the occurrence
of blocks of Madison limestone on the bedding fa u lt. He also
pointed out that no early basic breccia had been found in fa u lt
breccia along the basal detachment fa u lt. Pierce (1968) estimated
that the bedding-plane detachment probably covered an area roughly
20 by 55 km, and that about half of th is area, or 500 krn^, was
te cto n ica lly denuded when faulting ceased.
Goquel (1969) suggested that abnormal flu id pressures might be
generated beneath sliding sheets of rock at shallow depths (less
than 1 km) by either the dehydration of minerals such as gypsum or
serpentine or by fric tio n a l heating and vaporization of water. He
cited the Heart Mountain fa u lt and several other low-angle faults as
examples where such conditions may have obtained.
Hsu (1969), in a general discussion of the mechanical role of
the cohesive strength of rocks across a low angle fa u lt or landslide
surface, cited the Heart Mountain fa u lt as an example of the
thrusting of cohesionless blocks at catastrophic ve lo citie s. He
compared many details of the descriptions of Heart Mountain fa u lt
blocks of Pierce and Bucher to his observations of the Flims slide
of Grisons, Switzerland. He suggested that the detached slab may
have originated in an area of steeper (5 to 10°) slope and that
catastrophic sliding may have been rendered cohesionless by
40
development of a basal "a ir cushion" lik e that postulated by Shreve
(1966, 1968) for landslides like the Flims slide of Switzerland and
the Blackhawk slide of southern C alifornia.
Hughes (1970a) agreed with most aspects of Hsu's (1969) model
for Heart Mountain fa u ltin g but noted that Pierce's mapping did not
support Hsu's postulate of a steeper slope in the area of
detachment. Hughes suggested that injection of volcanic gas, rather
than a steeper slope, may have in itia te d fa u lt movement. Hughes
(1970b) believed that the mechanism problem of the Heart Mountain,
South Fork, and Reef Creek faults was greatly reduced by the idea
that the a l1ochthonous sheets were detached by a s i l l - l i k e injection
of pressurized volcanic gas and then floated downslope lik e
"hovercraft" (p. 107). Hughes agreed with Pierce that the detached
blocks had moved down gentle slopes (less than 3°) very sim ilar to
the present slope of the detachment fa u lt, and he sought to
understand why the blocks moved during Eocene time but not
subsequently. Hughes dismissed the fluid-pressure idea of Hubbert
and Rubey (1959) because the detachment environment seemed not to
favor the development of high flu id pressures of the type they
envisioned. He argued that i f earthquake o s cilla tio n s (Bucher,
1947; Pierce, 1963a, b) had produced the detachments and movements,
then detachment faults should be much more common in areas of
inferred earthquake a c tiv ity . Hughes f e lt the answer lay in the
association of detachment faulting with Absaroka volcanism.
To Hughes, a "hovercraft" mechanism best explained the
stratigraphic location of the Heart Mountain detachment, the lack of
a result of gravity would approach zero. With
repetitions of upward acceleration, the rocks above
the fa u lt would then be nearly unrestrained by
fr ic tio n and periodically would be free to move
la te ra lly on a very low slope (p. 121).
Furthermore, they stated that the breccia along the basal detachment
at White Mountain was unique among the more than t h ir t y places where
the detachment horizon had been studied, in that no fragments of
volcanic rock were found within breccia along the basal detachment
except at this lo c a lity . Pierce and Nelson interpreted the breccia
at White Mountain not as having been la te ra lly injected but as
simple fa u lt breccia formed by comminution of the metamorphosed
carbonate rock and pre-faulting igneous dikes of the upper plate at
White Mountain.
In his reply to Pierce and Nelson (1970), Hughes (1970c) argued
that reduction of overpressure by the opening up of lateral or
vertical fissures could greatly increase the volumetric supply of
volcanic gas, thereby allowing prolonged "hovercrafting" of blocks
along the detachment horizon. He suggested that large displacements
across the land surface could be accounted for by momentum of blocks
whose movement had begun while s t i l l supported by a layer of
volcanic gas. F in a lly, he argued that the structure, not the
composition, of breccia occurring along the basal detachment was the
crite rio n by which flu id iz a tio n of the breccia could be established.
Volcanic rock fragments need not be present. Thus Hughes f e lt that
the hovercraft mechanism remained viable.
In reply to Hughes (1970c), Nelson et a1. (1972) argued that
the composition and structure of the breccias along the Heart
Mountain basal detachment fa u lt are not compelling evidence in favor
of the hovercraft model. They further suggested that the Heart
Mountain allochthon moved as numerous separated blocks that were
probably too small to have maintained high-pressure volcanic gas
along the basal detachment, and that momentum alone seemed
inadequate to move fa u lt blocks up to 48 km across the Eocene and
land surface.
Hughes (1973) responded to Nelson et a l . (1972) by explaining
that the pre-faulting occurrence of "extrao rdina rily fla t- ly in g and
undeformed sediments" (p. 3109) was the unusual condition that
allowed the volcanic gas to be injected la te ra lly for great
distances, at the vesiculation depth, producing sequentially the
South Fork, Reef Creek, and Heart Mountain detachments. He
reiterated that volcanic material need not be present in fluidized
breccia, c itin g as an example breccias associated with the Karroo
s i l l of South Africa, where injection of steam apparently proceeded
igneous intrusion. The composition, texture, and fabric of the
breccia along the fa u lt, and especially the presence of quartz-and
c h lo r ite - f ille d vesicles, seemed to Hughes more compatible with a
fluid ize d breccia than with an ordinary fa u lt breccia. Suggesting
that movement of blocks across the land surface was a d if f ic u lt y for
any hypothesized mechanism, Hughes nominated the hovercraft
hypothesis as the "leading contender" (p. 3110) among m ultiple
working hypotheses.
43
Nelson et al (1973) disagreed. They acknowledged the potential
significance of vesicles in the basal fa u lt breccia, but after
"examination of many outcrops and many th in sections" (p. 3111), had
seen none. They c la rifie d th e ir concept of the nature of
upper-plate movement, envisioning upper-plate blocks that continued
to move for many kilometers a fte r becoming separated from the
detachment sheet. They saw as a major weakness of the hovercraft
hypothesis the necessity for d iffe re n t mechanisms to operate on the
bedding fa u lt and the fa u lt on the former land surface.
Kehle (1970) advanced the concept that the movement of gravity
slides, major thrusting plates, and large crustal plates is
accomodated by a basal zone of low viscosity material which
undergoes shear flow during translation of the upper plate.
Calculated shear stresses within these zones are much less than
stresses calculated by Hubbert and Rubey (1959) for fr ic tio n across
sole fa u lts , suggesting that flowage w ithin a low viscosity zone is
a more lik e ly operative mechanism in natural settings. In the
context of this model Kehle mentions the Heart Mountain detachment
fa u lt as an example of gravity slide characterized by complete
detachment in the updip direction from la te ra lly equivalent
autochthonous strata. No e x p lic it argument for the existence of a
basal 1ow-viscosity zone was presented.
In reviewing the several mechanisms proposed to explain
detachment and movement of Heart Mountain fa u lt blocks, Pierce
(1973) began by pointing out tha t, since a l1ochthonous blocks are
presently at rest on slopes of up to 10° (e.g. on the transgressive
44
fa u lt) but apparently had moved on slopes of less than 2°, some
factor in addition to gravity seemed necessary to explain detachment
and movement (cf. Hughes, 1970b). Pierce reviewed the discussion
surrounding the Hubbert and Rubey (1959) abnormal flu id pressure
model as applied to Heart Mountain fa u ltin g , adding to his previous
arguments the idea that movement along the Heart Mountain detachment
was apparently simultaneous across the entire area of the
bedding-plane fa u lt, while Hubbert and Rubey (1959) envisioned
dislocation as a linear front propagating across the sole fa u lt
through time. While continuing to argue against the involvement of
Wapiti volcanic rock in Heart Mountain fa u ltin g , Pierce did
acknowledge the p o s s ib ility of a relationship between Absaroka
volcanism and the mechanism of Heart Mountian fa u ltin g .
Pierce (1973) argued that Hsu's (1969) comparison between the
Heart Mountain fa u lt and the Flims landslide was ill-fou nde d,
pointing out the great textural differences between landslide
deposits and Heart Mountain fa u lt blocks: Heart Mountain
a l1ochthonous blocks, unlike landslide deposits, remain coherent
enough to be mapped as the original formations which comprise them.
Indeed, the a l1ochthonous character of many carbonate fa u lt blocks
went undetected for many years. Pierce also mentioned that the
Heart Mountain fa u lt terrane is exposed over an area many times
greater than that covered by any known landslide, and that no steep
source so slope such as that postulated by Hsu exists in or adjacent
to the Heart Mountain fa u lt terrane.
45
In countering Hughes' (1970b, c) hovercraft hypothesis, Pierce
added to his previous arguments the observation that igneous s ills
mapped in the Heart Mountain detachment terrane occur only within
Cambrian strata near the break-away fa u lt area. None occur at the
detachment horizon, where Hughes postulates the si 11-lik e injection
of volcanic gas.
In commenting on Kehle1s (1970) c itin g of the Heart Mountain
fa u lt as an example of sliding along a 1ow-viscosity zone, Pierce
(1973) argued that Heart Mountain detachment and sliding took place
along a single plane, not within a zone of simple shear. He argued
that the basal fa u lt breccia, the only possible 1ow-viscosity zone
along the detachment fa u lt, showed no evidence of flowage or
internal shear, and that shear flow would favor the Cambrian shales
rather than the Ordovician dolomite for a detachment horizon.
The remaining hypothesis, favored by Pierce, was the earthquake
o s c illa tio n model of Bucher (1947). Elaborating on his e a rlie r
arguments (Pierce, 1963a, b), Pierce (1973) suggested that
earthquake a c tiv ity (perhaps associated with Absaroka volcanism or
u p lif t of the Beartooth Massif) could impart a lateral component of
motion as well as a vertical acceleration greater than 1 g to the
upper plate. In support of this concept Pierce cited several
instances of high vertical (some greater than 1 g) and lateral
accelerations recorded during or inferred for the 1971 San Fernando,
California earthquake. H e argued that the detachment horizon lies
essentially at a seismic velocity d isco n tin u ity, allegedly
fa c ilita tin g detachment at this horizon. The earthquake o s c illa tio n
46
mechanism, though poorly understood, was viewed as equally
applicable to movement of blocks on the bedding-plane,
transgressive, and land surface phases of the Heart Mountain fa u lt
and to the Reef Creek and South Fork fau lts as well.
In an abstract, Voight (1972) outlined a new hypothetical
mechanism for Heart Mountain and Reef Creek fa u ltin g . He considered
"earthquake o s c illa tio n " hypotheses in tra c tib le because of:
"1) extreme assumptions required for large-motion
durations and/or displacement per o s c illa tio n ;
2) a "time-problem" which arises with "reasonable"
assumption of duration and frequency;
3) slig h t deformation engendered by slide blocks;
4) lack of deformation below the decol1ement" (p. 698).
In its stead he advanced a complex, abnormal-fluid-pressure model,
the "flu id wedge" hypothesis, which he described as follows:
"1) a confined water-saturated i n it ia l condition
existed along future decollement zones;
2) basal flu id pressure increased to a value overburden
pressure in zones favorable to pressure transmission
(the inferred mechanism involved multi-phase artesian
pressure transfer from magmatic bodies);
3) "flu id wedge" forces (>105 to n s /ft width) developed
at the rear edge, were s u ffic ie n t to induce "toe-
rupture" and provide in it ia l acceleration and
momentum;
47
4) "flu id wedge" thrust diminished rapidly with block
displacement, and continued movement and block
dispersion were due to momentum, g ra vity, and
boundary conditions;
5) movement on a water-saturated erosion surface was
fa c ilita te d by basal flu id pressures progressively
generated by the frontal portions of glide blocks"
(p. 698).
The next year, in two more abstracts, Voight revealed more of the
reasoning behind his assertion that abnormal flu id pressures
occurred during South Fork, Reef Creek, and Heart Mountain fa u ltin g .
Voight (1973a) contended that intrusive and extrusive bodies of
c la stic material constitute geologic proof that pore flu id pressure
equalled that total principal stress when they formed. Voight
(1973b, abst.) applied th is concept d ire c tly to the South Fork, Reef
Creek, and Heart Mountain fa u lts , arguing that the lack of
deformation beneath these decollements and the presence of cla stic
dikes of fa u lt breccia "demand" a flu id "flo ta tio n " mechanism for
the Heart Mountain fa u lt and, by analogy, for the South Fork and
Reef Creek faults as well. The proposed mechanism, an elaboration
of Voight (1972), involved:
1) multipie-horizon steam/water injectio n from
either a juvenile source within an active vent,
a "thermal contact" source arising from
intrusives in saturated sedimentary rocks, or
both;
48
2) steam/water s i l l intrusion by pneumatic/hydraulic u p lif t
and (lo c a lly ) fracture;
3) local development of steam/water dikes by pneumatic/
hydraulic fracture, and consequent
horizontally-directed "fluid-wedge" forces;
4) toe rupture in response to horizontal thrust and
pneumatic/hydraulic fracture mechanisms;
5) saturation of land surface by toe water vents;
6) behavior of upper plate as fle x ib le slid e r on
compressible multipi e-phase turbulent squeeze film
lubricant; lubricant escape velo citie s sonic, henc
"flo ta tio n times" for larger blocks of a few hours;
7) plate displacement due to gravity and (short-lived)
fluid-wedge impulse, with minimum i n it ia l plate
length, ca. 10 mi. on a 2° decollement, required
to overcome resistance of transgressive fa u lt ris e r;
8) movement over s lig h tly sloping (ca. 1°) land surface
re flects high basal flu id pressures induced by rapid
loading; maximum block velo citie s high, ca. 102 knots;
9) fragmentation and dispersion of glide blocks attributed
to squeeze film "shear", pneumatic/hydraulic fracture
and local fric tio n a l seizing" (p. 234).
Voight (1973c) reiterated the arguments of Voight (1973b) while
drawing an analogy between the Heart Mountain fa u lt and the
Turnagain Heights (Anchorage, Alaska) landslide, the la tte r being
the principal topic of th is 1973 paper.
49
Voight (1974a) is prim arily an abbreviated summary of the
history of debate regarding the Heart Mountain fa u lt mechanism
problem, including verbatum parts of Voight (1973b) and Voight
(1973c). New information revealed in th is paper included the
opinion that igneous dikes at Cathedral C liffs , thought by Pierce
(1963b) to have been la te ra lly intruded, were instead cut by the
Heart Mountain decollement and not la te ra lly intruded. Voight also
modified his interpretation of the mechanism of the South Fork
fa u ltin g , suggesting that the decollement may have occurred within a
thick (up to 60 m thick) layer of shale. Thus wide la titu d e in
mechanics of fau lting was considered possible.
F in ally, Voight (1974a) noted that the velocity of Heart
Mountain sliding was d i f f i c u l t to constrain. Order of magnitude
estimates by previous workers range from Kehle's (1970) 10-5 to 10-6
km/hr to Voight's (1973b) 102 km/hr.
A roadlog in a guidebook for a fie ld t r ip passing through the
Heart Mountain fa u lt terrane, edited by Voight (1974b), contains
several more observations regarding the Heart Mountain fa u lt,
including: the speculation that the Clarks Fork fa u lt, the
southwestern frontal fa u lt of the Beartooth u p lif t , might be the
northern boundary of the Heart Mountain detachment fa u lt (see Fig.
13 of Voight, 1974a); and the suggestion th a t, once the upper plate
of the Heart Mountain fa u lt had become detached and started moving,
fragmentation of the plate was "systematically related to escaping
f lu id , that small blocks would... not remain in motion long (except
on the land surface, where saturated alluvium was presumably
50
available for continuous pressurizaton), and that the whole event,
in the bedding plane and transgressive fa u lt areas, occurred in
merely a geologic instant --vs. on the order of an hour" (p. 116).
Voight (op. c i t . ) also acknowledges the com patibility with his own
ideas of the preliminary movement along the Heart Mountain fa u lt
inferred by Pierce and Nelson (1973). Voight noted that the
postulated preliminary movement would have involved an area
approximately equal to the minimum size necessary to "cause
in s ta b ility " (p. 122), given that cohesion and effective fr ic tio n
along the decollement surface had vanished. Voight's meaning here
is not clear, but in lig h t of Voight (1973b, c) th is apparently
refers to a simple mathematical model assuming slope of the
decollement, mass of the detached slab, and several mechanical and
geometrical properties of the rocks in the bedding and transgressive
fa u lt areas.
Referring to a cross-section of Cathedral C liffs in Pierce
(1963b), Voight (1974c) suggests that graben zones in a l1ochthonous
Heart Mountain fa u lt blocks were "instrumental in the dispersion of
the glide mass," (p. 406). Voight considered graben-like "plastic
wedges" to have provided the driving force for displacement of slide
masses at Turnagain Heights, Alaska (see Voight, 1973c), and
considered graben in Heart Mountain fa u lt blocks to be in part
analogous.
Stearns et al (1974) discuss the geometry of drape folds within
the Paleozoic sedimentary section overlying range-front faults at
Rattlesnake Mountain, west of Cody. They assert a geometric
51
requirement for the occurrence of a detachment near the contact
between Cambrian strata, which are greatly attenuated over the
basement fa u lt, and Ordovician strata, which show no attenuation.
At Rattlesnake Mountain they report such a detachment at the same
stratigraphic horizon as that of the Heart Mountain fa u lt (p. 23-24
and Fig. 7, p. 24). Stearns et al (1974) suggest that the Heart
Mountain detachment formed as "an integral part of the drape folds
on the southern edge of the Beartooth Mountains" (p. 24).
Stearns et al (op. c i t . ) also present experimental data
suggesting that the fric tio n a l resistance to sliding along a contact
between two b r it t le strata (e .g ., both dolomite) might be less than
that along a b rittle -d o lo m ite contact (e.g. 1 imestone-dolo.mite).
The horizon of the Heart Mountain detachment fa u lt, being the
s tra tig ra p h ic a lly lowest contact between two b r it t le strata, might
therefore be the mechanically preferred horizon of detachment during
drape folding.
In assessing proposed mechanisms of Heart Mountain fa u ltin g ,
Prostka (1978) emphasized the contrast in deformation across the
basal detachment fa u lt and the occurrence of Wapiti-age intrusive
centers (presumably volcanic vents) within the detachment area. He
favored a variation of Voight's fluid-wedge hypothesis, suggesting
that volcanic gas (largely steam) had acted as both basal cushion
and flu id wedge (sensu Voight). The volcanic gas presumably was
associated with formation of the dominantly vent-facies, primary
breccias of the post-tectonic Wapiti Formation of the detachment
area. His model, as he saw i t , was l i t t l e d iffe re n t from that
52
o rig in a lly advanced by Bucher (1933).
Voight and Pariseaun (1978), in the introduction to the volume
in which Prostka (1978) appears, suggest that the d is trib u tio n of
thick a l1ochthonous blocks over the wide area of the "incredible"
(p. 39) Heart Mountain terrane "undoubtedly speaks in the favor of a
rapid emplacement" (p. 39). The documentation by Prostka of
widespread involvement of volcanic rock in Heart Mountain faulting
is acknowledged, and the pressurized volcanic steam mechanism of
Prostka is praised by Voight and Pariseaun.
Pierce (1979) argued that the nature and d is trib u tio n of
c la stic dikes of carbonate fa u lt breccia in the Heart Mountain fa u lt
terrane favor the earthquake o sc illa tio n mechanism of Heart Mountain
fa u ltin g , being incompatible with any of the proposed abnormal flu id
pressure hypotheses. His argument hinges on the contention that
c la s tic dikes were injected into both a l1ochthonous blocks and
post-tectonic (Wapiti Formation) volcanic rock at the same time,
shortly a fte r movement on the Heart Mountain fa u lt ceased. As
evidence he cites examples of undeformed c la s tic dikes within
"contorted, deformed, and brecciated" (p. 13) Paleozoic rocks and of
c la s tic dikes injected into Wapiti volcanic rock while i t was
presumably unconsolidated. The occurrence of carbonate fa u lt
breccia beneath Wapiti volcanic rock was interpreted as demanding a
cataclysmic fau lting event immediately followed by Wapiti volcanism.
Clastic dikes were thought to have been injected due to rapidly
increasing overburdens of volcanic rocks. Because c la s tic dikes
occur in the bedding-plane, transgressive, and land-surface phases
53
of the decollement, the former presence of a cover of Wapiti
volcanic rocks on parts of the transgressive and land-surface phases
of the decollements was inferred. Because injection of cla s tic dikes
was thought by Pierce to postdate fa u ltin g , th e ir presence could not
demand a flu id - flo ta tio n mechanism for Heart Mountain fa u ltin g . For
this and for other reasons mentioned in previous publications,
Pierce (1979) and to this date of w riting favors the earthquake
o s c illa tio n hypothesis.
In abstracts, Straw and Schmidt (1981a, b) posed a
"phreatomagmatic-hydraulic hypothesis" to explain detachment and
movement of Heart Mountain fa u lt blocks. Hydraulic pressures
exceeding lith o s ta tic stress, in part due to magmatic a c tiv ity , are
envisioned at the base of the re la tiv e ly jo in t-fre e Bighorn
Dolomite. Earthquake motion aided in dislodging the plate.
Separation of a l1ochthonous blocks reduced the hydrostatic pressure
along the fa ilu re plane, causing the superheated water to flash to
steam, expand, and thereby provide continued buoyancy to aid
downslope movement. Post-tectonic injection of c la s tic dikes was
also envisioned as related to mobilization of flu id s by volcanic
explosi ons.
Melosh (1981) explains the occurrence of the Heart
Mountain bedding-plane detachment in the "apparently strongest unit"
of the sedimentary section as being readily explained by an
"acoustic lubrication mechanism" (p. 1046). Partial entrapment of a
strong acoustic wave along the dolomite-shale contact was
envisioned resulting in detachment at the level of the largest
54
wave-induced vertical stress fluctuations, about 1/4 wavelength
away from the contact. To accord with the observed roughly 10 m
distance between the detachment horizon and the dolomite-shale
contact, a 40 m wavelength wave (50 Hz) was envisioned. Once
detachment had occurred, catastrophic sliding would continue as
potential energy was transformed to acoustic energy, maintaining the
acoustic wave. Melosh does not e x p lic itly state the source of the
inferred acoustic wave, but he does mention the association of
detachment with nearby volcanic and, presumably, seismic events.
55
STRATIGRAPHY
General Statement
The Bighorn Basin contains a thick sedimentary section
consisting of 760 to 1000 m of Paleozoic sedimentary rock, 1950 to
3600 m of Mesozoic sedimentary rock, and up to 8500 meters of
Tertiary b a s in - fill deposits (Samuel son, 1974). This sedimentary
section, which lie s nonconformably upon Precambrian c ry s ta llin e
rock, can be conveniently subdivided into three groups on the basis
of lith o lo g y (compare Hewett, 1920).
The lowest of these groups, overlying c ry s ta llin e basement,
consists of 350 to 400 m of Cambrian sedimentary rock of which
roughly 80% is shale. The second group is composed of approximately
400 to 500 m of Ordovician, Devonian, and Mississippian strata,
about 90% of which are limestone or dolomite. These carbonate rocks
are the part of the stratigraphic section that is most resistant to
erosion. The th ird group, ranging in age form late Paleozoic to
early Tertiary, consists of up to approximately 6,600 m of shale,
sandstone, silts to n e , and minor evaporites and dolomite.
In the neighboring Absaroka Mountains and Beartooth Mountains
only the f i r s t two of these groups (Cambrian through Mississippian)
remain. As in the other mountain ranges surrounding the Bighorn
Basin, a ll formations overlying the Mississippian Madison Group
apparently were removed by erosion before the close of the Laramide
Orogeny. In the Absaroka Mountains, the stratigraphic sequence
56
consists of the Cambrian through Mississippian marine shelf
sediments overlain by Tertiary volcanic rocks and, lo c a lly ,
stream-channel deposits of probable Laramide (Eocene?) age. The
Beartooth Mountains retain only a few buttes and mesas of early
Paleozoic strata.
Heart Mountain fa u lt blocks, which were derived at least in
part from the area of the northeast Absaroka Mountains, now consist
only of Ordovician through Mississippian sedimentary rocks,
Eocene(?) stream-channel deposits, and Eocene volcanic rocks.
Except for these stream-channel deposits, rocks younger than the
Madison Group but older than the Eocene volcanic rocks took part in
Heart Mountain fau lting only inasmuch as they constitute the lower
plate of the transgressive and former land surface phases of the
fa u lt. They w ill receive no further consideration in the present
report.
Because they have been frequently mentioned in the context of
the mechanism problem of the Heart Mountain fa u lt, the strata
underlying the basal detachment fa u lt (the bedding-plane phase) w ill
be described. Thus the formations to be described below consist
only of those of Cambrian through Mississippian (in part) and Eocene
(in part) ages.
Paleozoic Rocks
General Statement
Lithologic descriptions of the Paleozoic rocks of the Heart
Mountain fa u lt terrane appear on U.S. Geological Survey geologic
57
maps (see References). M ills (1956) and Lovering (1929) present
valuable discussions of the Paleozoic stratigraphy of the area.
The Paleozoic section (Figs. 3, 4) comprises part of a
widespread epicratonic sequence of marine strata. Many of the
formations included in th is section (e.g. Flathead, G allatin, Gros
Ventre, Bighorn, Jefferson, Three Forks, Madison) are presently
recognized over broad areas of Montana, Wyoming, Idaho and northern
Utah. Profound unconformities in the section occur between the
Upper Cambrian Snowy Range Formation and the Upper Ordovician
Bighorn Dolomite, and between the Bighorn Dolomite and the Devonian
Jefferson Formation. These unconformities are the interregional
unconformities that separate the Sauk, Tippecanoe, and Kaskaskia
sequences of the cratonic in te rio r of North America (Sloss, 1963).
FIathead Sandstone (Middle Cambrian)
Peale (1893, 1896) assigned the name Flathead to the basal
Cambrian quartzite, shale, and limestone of the Threeforks
quadrangle of southwestern Montana. Hague (1899) extended this
usage to the Absaroka quadrangle of northwest Wyoming. Lovering
(1929) referred to the basal arenite of the New World (Cooke) mining
d is t r ic t north of Cooke C ity, Montana, as the Flathead quartzite but
included the overlying shale and limestone in the Gros Ventre
formation. The U.S. Geological Survey now uses the name Flathead
Sandstone to connote the basal Cambrian, 30 to 45 m thick sandstone
of northwest Wyoming and adjacent Montana.
58
s ' t ~ ~ T ~ | r
S T R A T I G R A P H 1C
H O R IZ O N OF H E A R T
M O U N T A I N B E D D I N G -
P L A N E D E T A C H M E N T
FA U L T ---------------------------------
30 m ( iooo‘)
PRECAM BRIAN
zzz
rn
Madison Limestone
Three Forks Formation
Jefferson Formation
Bighorn Dolomite
Grove Creek Member
Snowy Range Formation
Pilgrim Limestone
-— P a r k S h a / e
Gros Ventre Formation
M e a g h e r L i m e s t o n e
W o / s e y S h a l e
Flathead Sandstone
FIGURE 3. Columnar section of the Paleozoic stra tig ra p h y
of the Heart Mountain fa u lt upper and lower plates.
Formation names used in Montana but not in Wyoming appear
in i t a l i c s .
59
FIGURE 4. View to the east of Bald Ridge, showing early
Paleozoic s tra ti graphic section overlying Precambrian
c ry s ta llin e rocks. About 1000 m o f r e lie f shown. Photo
by Chris Fuller.
60
The Flathead Sandstone is generally a poorly sorted coarse
grained angular q u a rtz itic sand. It overlies Precambrian
c ry s ta llin e basement, commonly with a conglomeratic, more
feldspathic unit (M ills , 1956). Hard and ledge-forming at its base,
i t becomes softer and brown-speckled in its upper part (Pierce,
1966b).
Gros Ventre Formation (Middle Cambrian)
Blackwelder (1918) referred to the roughly 200' of shale and
limestone that overlies the Flathead quartzite in western Wyoming as
the Gros Ventre Formation. Hague (1899) had referred to correlative
strata in the Absaroka quadrangle as part of the Flathead formation,
but Lovering (1929) extended B1ackwelder' s terminology into the New
World (Cooke) mining d is t r ic t . The U.S. Geological Survey continues
to use the term Gros Ventre Formation for these strata in northwest
Wyoming, but in adjacent Montana three formations are recognized,
the Wolsey Shale, Meagher Limestone, and uppermost Park Shale. The
la tte r terminology stems from the work of Weed (1899) in the L it t le
Belt Mountains of central Montana, who included the Flathead,
Woolsey, Meagher, and Park as members of the Barker formation.
The Gros Ventre Formation consists of a basal 50 to 65 m of
thin-bedded green shale and minor sandstone (the Wolsey Shale); a
10-30 m thick thin-bedded nodular limestone with green shale
interbeds (the Meagher Limestone); and an uppermost 100 to 150 m of
thin bedded green shale and gray limestone (the Park Shale) (Pierce
and Nelson, 1971; E llio t t , 1979). The Meagher Limestone lo c a lly
61
crops out as gentle c l i f f s , but the underlying and overlying shale
units are usually concealed by vegetation and are susceptible to
landsliding. The Gros Ventre Formation is 160 to 250 m thick and
include beds of limestone flat-pebble conglomerate.
Pi 1 g r im Limestone (Late Cambrian)
The U.S. Geological Survey presently uses Weed's (1899) term
Pilgrim Limestone for the thick bedded to massive limestone, o o lit ic
limestone, and 1imestone-pebble conglomerate that overlies the upper
Gros Ventre Formation (i.e . the Park Shale). Local residents refer
to the Pilgrim Limestone as "The Reef", due to its typical exposure
as an impassible c l i f f up to 30 m high. "The Reef" is the most
continuously exposed marker bed within the lower plate of the Heart
Mountain detachment terrane. It is usually 30 to 33 m thick but in
the Cooke City Quadrangle is up to 75 m thick ( E llio t t , 1979).
Snowy Range Formation (Late Cambrian)
Peale (1893) o rig in a lly divided the Cambrian of the Threeforks
Quadrangle, southwestern Montana, into the Flathead and Gallatin
formations (see also Hague, 1899; Blackwelder, 1918; Lovering,
1929). Dorf and Lochman (1938), recognizing that "Upper Cambrian
formations recognized in central Montana [by Peal el cannot be
extended into the southern area [o f Montana]" (p. 275), defined
three new Upper Cambrian formations for southern Montana: the
Maurice, the Snowy Range, and the uppermost Grove Creek. The U.S.
Geological Survey continues to use the la tte r two names in the area
62
of the Heart Mountain detachment fa u lt: the Grove Creek was
referred to as a formation by Pierce (1965) but Pierce (1970) and
Pierce and Nelson (1971) include the Grove Creek as a member of the
Snowy Range Formation.
As described at its type section (Dorf and Lochman, 1938), the
Snowy Range Formation consists of: a lower member (about 30 m
thick) of thin sandstone, gray-green fis s ile shale, and a few beds
of 1imestone-pebble conglomerate; a middle member (about 50 m thick)
of intercalated gray-green f is s ile shale and "innumerable beds and
lenses of glauconitic limestone f la t pebble conglomerate" (p. 276);
and the uppermost Grove Creek member (about 10 m thick) , of
glauconitic limestone and conglomerate, white c ry s ta llin e limestone,
and intercalated yellow-green f is s ile shale, thin gray limestone,
and buff to orange-red, thin-bedded sandy dolomites. In the Heart
Mountain detachment fa u lt terrane the Snowy Range Formation is 100
to 130 m thick.
Bighorn Polom ite (Late Ordovician)
Parton (1904, 1906a) f i r s t used the name Bighorn Dolomite in
reference to 80 to 100 m of dolomite and limestone on the
eastern slope of the Bighorn Mountains. Blackwelder (1918), Darton
(1917) and Hewett and Lupton (1917) adopted the name for broader
areas of western Wyoming, and Lovering (1929) recognized the Bighorn
Dolomite in the New World (Cooke) mining d is t r ic t .
The Bighorn Dolomite consists of 100 to 150 m of white,
tan, or gray fin e ly c ry s ta llin e or sucrose dolomite and dolomite
63
limestone. This massive unit commonly stands in bold c l i f f s , in
contrast to the re la tiv e ly gentle slopes underlain by the Cambrian
shales. The lower 30-35 m is especially hard and massive, the upper
parts being generally thick-bedded but occasionally thin-bedded
(Darton, 1904).
Jefferson Formation (Late Devonian)
Peale (1893) described Devonian strata exposed in the
Threeforks quadrangle of southwestern Montana, naming the upper 75 rn
the Three Forks shales and the lower 210 m the Jefferson limestone.
For the Absaroka quadrangle Hague (1899) used the term Jefferson
Limestone to designate strata now included in the Bighorn Dolomite
and Jefferson Formation. Blackwelder (1918) distinguished the
Bighorn Dolomite, Jefferson Limestone, and Threeforks Limestone in
western Wyoming. Lovering (1929) recognized the Bighorn dolomite,
Jefferson Limestone, and Three Forks Formation in the New World
(Cooke) mining d is t r ic t . The names Jefferson Limestone, Jefferson
Dolomite, Three Forks Shale, and Three Forks Limestone were in
current use by the U.S. Geological Survey as of 1938 (Wilmarth,
1938). More recent U.S. Geological Survey publications (e.g.
Pierce, 1965) employ the terms Jefferson Formation and Three Forks
Formation in the area of the Heart Mountain detachment fa u lt.
The Jefferson Formation in the area of the Heart Mountain
detachment fa u lt consists of "fe tid brown dolomite and light-gra y
and tan limestone; uppermost part is mottled y e l1owish-orange
dolomite and y e l1owish-gray siltsto n e " (Pierce and Nelson, 1971).
It is medium to thick-bedded and ranges from 60 to 100 m thick.
64
Three Forks Format 1 on (Late Devoni an-Early Mlssi ssippi an)
The history of usage of th is formational name is discussed with
that of the Jefferson Formation, above. The Three Forks Formation
in the area of the Heart Mountain detachment fa u lt consists of
"yellow, greenish-gray, and dark-gray dolomitic s ilts to n e , black
f is s ile shale, and s ilt y dolomite" (Pierce and Nelson, 1971).
Thickness ranges from 10 to 30 m.
Madi son Limestone (Mi ssi ssi ppi an)
Peale (1893) named the Madison Limestone fo r the Madison Range
of the Threeforks quadrangle, Montana. Hague (1899), Blackwelder
(1918), and Lovering (1929) extended use of th is term to western
Wyoming and the Heart Mountain fa u lt terrane. The Madison Limestone
is conspicuously developed in Montana, Wyoming, Idaho, and northern
Utah. C o llie r and Cathcart (1922) viewed the Madison as a group
comprised of the Lodgepole Limestone and the overlying Mission
Canyon Limestone. Sloss and Hamblin (1942) assigned a type section
based on Peale's (1893) work.
The Madison Limestone is blue-gray, massive, and in part
dolomitic. The upper part is more massive than the lower part. An
abundant fauna including 79 species was described from the Madison
of Yellowstone National Park (G irty, 1896); pelmatozoan columnals
are especially abundant. The Madison Limestone is up to 300 m thick
in the area of the Heart Mountain detachment fa u lt, and up to 500 m
thick elsewhere.
65
Tertiary Rocks
General Statement
Tertiary rocks occur in both upper-plate and lower-plate
positions in the Heart Mountain fa u lt terrane. Formations which
constitute the lower plate in the transgressive and
former-1and-surfaces phases of the Heart Mountain fa u lt are not of
dire ct interest in the present study and w ill not be discussed
fu rth e r. The Crandal1 Conglomerate of Eocene(?) age occurs in
both lower-plate and upper-plate positions along the bedding and
transgressive phases of the Heart Mountain fa u lt. Three formations
of volcanic rock of the Eocene Absaroka Supergroup occur immediately
above the Heart Mountain fa u lt. These are the Cathedral C liffs
Formation, the Lamar River Fonnation, and the Wapiti Formation.
Volcanic rock te n ta tive ly correlated with the Wapiti Formation
occurs lo c a lly in the lower plate. The Crandall Conglomerate and
the three volcanic formations are described below.
Crandal1 Cong1omerate
The Crandall Conglomerate (Pierce, 1957; Pierce and Nelson,
1973) is a very coarse stream channel conglomerate known from only
the area of the Heart Mountain fa u lt. It is found in a lower-plate
position south of Squaw Creek and in the Crandall area where i t
ranges from 70 to IPO m th ic k . It occurs in an upper-plate position
in the Crandall area, along Bear Gulch in Sunlight Basin, and in the
drainage of Oead Indian Creek, where i t is up to 70 m th ick.
The Crandall Conglomerate is dominantly a boulder conglomerate
with most clasts composed of Ordovician, Devonian, and Mississippian
66
limestone and dolomite, the remainder being of Cambrian and
Precambrian rock. No clasts of volcanic rock have been found.
Clasts range in size from 2 m across to sand size with a greater
proporotion of pebble-size material in the upper part. The clasts
are generally well rounded.
The Crandall Conglomerate demonstrably predates some movement
on the Heart Mountain fa u lt and in a few places i t is overlain by
Cathedral C liffs Formation. It is clea rly younger than the Madison
Group. N o fo ssils have been found. Pierce (1957) found co n flic tin g
evidence as to the age of the Crandall Conglomerate and assigned i t
a tentative early Eocene(?) age (see Pierce, 1963a). Pierce and
Nelson (1973) on the basis of an inferred relationship between the
Crandall Conglomerate and Heart Mountain fa u ltin g , assigned i t an
Eocene age. Direct evidence only requires the Crandall Conglomerate
to be younger than the Madison Group but older than the Absaroka
Supergroup. Regional considerations suggest a Laramide age, so for
present purposes the age of the Crandall Conglomerate is considered
to be Eocene(?).
Volcanic Rocks, General Statement
Volcanic rocks of the Absaroka Mountains were f i r s t studied by
Iddings and Weed (1894), Peale (1896), Hague et al , (1896), Hague
(1899) and Hague et al , (1899). Parsons (1958) summarized la te r
study (largely his own) of Absaroka volcanic rocks, discussing th e ir
texture, fa b ric, modes of deposition, and associated intrusive
centers. Smedes and Prostka (1972) proposed a stratigraphic
framework for Absaroka volcanic rocks, defining three groups of
67
formations within the Absaroka Volcanic Supergroup. In th is context
the Lamar River and Cathedral C liffs Formations are uppermost
Washburn Group and the overlying Wapiti Formation fa lls within the
Sunlight Group.
According to Smedes and Prostka (op. c i t . ) , rocks of the
Washburn Group were erupted nearly coevally from several vent areas
located in the G allatin, northern Absaroka, and Washburn Ranges.
The darker-colored Lamar River Formation interfingers northeastward
in its lower part with the 1ighter-colored Cathedral C liffs
formation (p. C ll) , presumably having issued from d iffe re n t eruptive
centers. The Sunlight Group overlies the Washburn Group with
contacts that appears to be variably conformable, erosionally
unconformably, and lo c a lly in te rfin g e rin g (p. C14). Sunlight Group
vents occur southeast of Washburn Group vents, between Hurricane
Mesa and Sunlight Basin and up Sunlight creek.
Hue to broad overlapping ranges of composition and texture,
these volcanic formations are d i f f i c u l t to distinguish in the fie ld
in the absence of unambiguous stratigraphic relationships. As a
re s u lt, mapping of volcanic rocks by d iffe re n t workers in the Heart
Mountain fa u lt terrane has led to markedly d iffe re n t interpretations
(see Previous Work). All previous workers employing th is
terminology agree that: the Cathedral C liffs Formation predates
Heart Mountain fa u ltin g ; the Wapiti Formation postdates Heart
Mountain fa u ltin g ; and the Lamar River Formation for the most part
predates but may in part postdate Heart Mountain fa u ltin g . Within
the Heart Mountain fa u lt terrane, then, the questions of (1) whether
68
volcanic rock in a given area is a l1ochthonous and (2) to which
formation that volcanic rock should be assigned are often
inseparable. The present (1982) disagreement in the lite ra tu re
regarding the formational a ff in itie s of volcanic rock in the Heart
Mountain fa u lt terrane reflects disagreement as to the amount of
volcanic rock involved in Heart Mountain fa u ltin g (see Plate I I ) .
The Cathedral C liffs , Lamar River, and Wapiti Formations are
herein referred to simply as volcanic rocks, although they include
varying amounts of e p icla stic (sedimentary) rocks derived from
volcanic rocks. Epiclastic rocks are uncommon in the area of the
present study.
Cathedral Cli ffs Formation
The Cathedral C liffs Formation consists of about 300 to 500 m
(usually 160 to 300 m) of t u f f , volcanic sediment, l a p i l l i t u f f , and
breccia, usually grayish white to lavender, but occasionally darker
greenish brown. The mineral composition varies considerably but is
dominated by hornblende andesite and hornblende b io tite andesite.
Quartz-bearing rocks, dacite and perhaps rhyodacite, occur. Pierce
(1963a) provides several measured sections and detailed lith o lo g ic
descriptions from the type area. Smedes and Prostka (1972) describe
the Cathedral C liffs Formation in the Heart Mountain fa u lt terrane
as dominantly lig h t colored, fine-grained a llu v ia l facies. The
Cathedral C liffs Formation is of late early Eocene or early middle
Eocene age based upon paleontologic (Pierce, 1963a) and
potassium-argon (Smedes and Prostka, 1972) dating, the la tte r via an
inte rfing erin g relationship with the Lamar River Formation.
69
Lamar River Format i o n
The Lamar River Formation (Smedes and Prostka, 1972) consists
of "medium-brown andesitic lavas and volcaniclastic rocks and minor
mafic lava flo w s ...in north-central and northeastern Yellowstone
National Park and v ic in ity " (P. C19). Throughout most of it s extent
i t consists of"wel1-bedded coarse a llu v ia l facies volcanic
conglomerate, breccias, and tu ffs " (P. C19) which along the Lamar
River contain the fossil forests of Yellowstone National Park.
Smedes and Prostka (1972) state that the Lamar River Formation was
formerly included in the early basic breccia of Hague et a l . (1899),
it s s lig h tly 1ighter-colored lower part in northeastern Yellowstone
National Park being the Lamar River Formation. Pierce (1980, p.
27b) reports that the Lamar River Formation had been previously
known as early acid breccia.
A lluvial facies deposits of Lamar River Formation vary in
thickness from 160 to 730 m. Near Mount Washburn, a vent of the
Lamar River Formaton, the vent facies are up to 1060 m th ick.
Brecciated andesite intrusives up to 8 km across are
reported to occur lo ca lly in the lower part of the formation as
sheets, lenses, and irre gula r bodies (Smedes and Prostka, 1972). In
the northeast corner of the park, where the lower one-third of the
Lamar River Formation in te rfing ers wtih the Cathedral C liffs
Formation, intrusive breccias constitute up to 330 m of the 660 m
pre-Wapiti volcanic section (U. S. Geological Survey, 1972; Prostka
et al , 1975). The Lamar River Formation is of late Wasatchian to
early Bridgerian provincial age (late early Eocene to early middle
70
Eocene), based upon an inte rfing erin g relationship with the
Sepulcher Formation, which is dated by both fo ssils and one
potassium-argon date of 49.2 + 1.5 (Smedes and Prostka, 1972).
Wapiti Formation
The Wapiti Formation (Nelson and Pierce, 1968), as described in
its type-section along the North Fork of the Shoshone River, is
composed of volcanic breccia, sandstone, s ilts to n e , conglomerate,
and lava flows. The breccia occurs in somber shades of brown,
reddish brown, gray, and rarely greenish-gray and was interpreted as
mudflow deposits and, less commonly, stream deposits. The matrix of
the breccias consists of coarse sand-size m aterial, and larger
fragments up to several feet across consist of fine-grained
trachyandesitic volcanic rock, sometimes vesicular, with phenocrysts
of plagioclase, pyroxene, and lo c a lly o liv in e or hornblende. A
small proportion of the breccia is lig h t-co lo re d , hornblende-rich
and d a citic in composition.
Volcanic sandstone, silts to n e , and conglomerate of the Wapiti
Formation are interpreted as stream deposits. A few may be t u f f
beds. They are composed of medium-brown and gray volcanic detritus
including pyroclastic m aterial, fragments of lava flows, d e tritu s
derived from volcanic breccia deposits, quartz and quartzite, and
p e trifie d wood (Nelson and Pierce, 1968). Trachyandesitic and
dacitic lava flows occur sporadically. The formation ranges in
thickness from 500 to 1150 m.
71
Although equivalent to the early basic breccia in its type
section (Nelson and Pierce, 1968), the Wapiti Formation constitutes
only the upper one-third of what Hague et a l . (1899) called early
basic breccia, the lower two-thirds now being included in the Lamar
River Formation (Smedes and Prostka, 1972). In the Sunlight Creek
and Crandall Creek areas the Wapiti Formation may be up to 1500 m
th ic k , consisting e n tire ly of vent facies rocks with many dikes and
dike swarms. In its type section i t is dominantly a llu v ia l facies.
The most probable age of the Wapiti Formation is early middle
Eocene, based upon its stratigraphic position. It overlies the
Lamar River and Cathedral C liffs Formations and is overlain by the
Trout Peak Trachyandesite of middle Eocene age (one potassium-argon
date of 48.0 +1.3 my).
72
STRUCTURAL GEOLOGY
General Statement
The structural geology of the detachment terrane is described
in geographic order from the northwest (area of break-away fa u lt) to
the southeast (area of fa u lt on former land surface). Geometric and
kinematic features of the of upper-plate rocks of 27 areas are
described and portrayed as maps and sterographic projections.
Lower-plate structural features are also described. After each
descriptive section, the data are interpreted with regard to the
nature and degree of involvement of the upper-plate rocks in Heart
Mountain fa u ltin g .
The Heart Mountain Break-away Fault
General Statement
Pierce (1960) reported the discovery of a steeply east-dipping
fa u lt south of Soda Butte Creek that marks the western margin of the
Heart Mountain detachment fa u lt terrane. The bedding fa u lt's
westward termination at th is fa u lt and the steeply-dipping fa u lt's
downward termination at the bedding fa u lt demonstrate the rootless
nature of the Heart Mountain detachment. The high angle at which
these faults appear to meet and the concept of gravity sliding
envisioned for the Heart Mountain detachment led to the concept of
th is steeply dipping fa u lt as a "break-away" fa u lt (Pierce, 1960).
Pierce believed that the break-away fa u lt and the
adjacent bedding fa u lt had been te cto n ica lly denuded by Heart
73
Mountain fa u ltin g , and he mapped the volcanic rocks overlying these
fau lts as in situ ( i. e . , younger than Heart Mountain fa u ltin g ).
Subsequently Prostka et al (1975) mapped the basal 220 m of volcanic
rocks east of and adjacent to the break-away fa u lt as a l1ochthonous,
but the concept of a "break-away" fa u lt at least in part
te cto n ica lly denuded was retained.
Subsequent mapping, largely by Prostka and Nelson in the
in te rio r of the Absaroka Mountains, increased the lin e a r extent of
the break-away fa u lt from about 10 km (Pierce 1960) to about 37 km
(Pierce et al 1973). Despite the fact that the fau lts mapped as the
break-away fa u lt constitute a discontinous sequence of highly
dissim ilar features (Pierce, 1980), the term "break-away" fa u lt and
the connotation of tectonic denudation were extrapolated from the
exposure south of Soda Butte Creek to the entire extent of the
feature.
The present study included examination of the break-away fa u lt
in four general areas, each of which is discussed below.
Reconnaissance of the volcanic terrane east of the break-away fa u lt
is also discussed in th is section, as are current disagreements in
the lite ra tu re regarding mapping of the break-away fa u lt. Clearly,
the Heart Mountain detachment terrane has a western boundary marked
by a zone west of which detachment did not occur; but whether i t is
a "break-away" fa u lt in the sense that i t is a underwent tectonic
denudation, and whether i t was simultaneously active along its
entire extent as implied by Pierce (1973, 1980) remain open
questions. For convenience the term "break-away" is employed in the
74
following discussion, but the genetic connotations of the term
( sensu Pierce) are not assumed. The break-away fa u lt was o rgina lly
named the "Abiathar break-away fa u lt" (Pierce, 1960) a fte r nearby
Abiathar Peak, but Pierce (1980) renamed i t the "Heart Mountain
break-away fa u lt."
North of Soda Butte Creek
The break-away fa u lt is not exposed north of Soda Butte Creek,
and previous workers disagree as to its location in th is area
(Plate I, lo c a lity Al; see also Fig. 5). It is mapped by the
1J.S.G.S. (1972) and by Prostka (1978) as curving eastward into the
Cooke City quadrangle between Meridian Park and Sunset Peak. It is
not shown, however, on E l li o t t 's (1979) map of the southeastern part
of the Cooke City quadrangle. Pierce (1980) believes that the
breakaway fa u lt continues in a northerly direction rather than
curving to the east in th is area, based:
(1) on the occurrence of blocks of Paleozoic rocks
east of the inferred breakaway fa u lt [also shown
on E llio t t , 1979], which are believed to be part
of the upper plate of the Heart Mountain
detachment fa u lt; and (2) on d iffe re n t mapping
[Pierce's as opposed to that of U.S.G.S. (1972)
and Prostka (1978) and Prostka (1978)] of the
volcanic rock units there (p. 277).
A few days' reconnaissance by th is w riter on the north side of
Soda Butte Creek valley, in the headwaters area of Pebble Creek, and
75
Lake
, A bundance
COOKE CITY
QUAD
CUTOF F
MTN
QUAD
Wolverine
^ - 4 0
Pass
Faulted Paleozoic rocks
probably ailochthonous
on H e a rt Mountain fault
Miller
A Mtn.
'Wol veri ne
^.Pea k
Eastward
extent of
in situ
Pa Ieozoi c
rocks ^
Sunset
IPea k
Mineral
Mtn.
' ‘Meridian
Peak
Cooke City
Silver
Gate
Soda Butte
45 ° 00 ’
110 ° 0 0 ‘
2 km.
FIGURE 5. Sketch map showing interpretations of Pierce
( — • ) and Prostka ( — — — ) of the location
of the northward termination of the break-away fa u lt.
76
in the Wolverine Pass area (see Fig. 5) generally supports the
mapping of Pierce (1980). O n the north side of Soda Butte Creek
valley the location of the break-away is evident from the
juxtaposition of carbonate and volcanic rocks, despite the fa u lt
i t s e l f being concealed beneath a tree-covered landslide area. The
easterly dip of the break-away fa u lt is inferred to be about 50° in
th is area, based on the composition of the slopewash and landslide
debris. In the Pebble Creek drainage are continuous exposures of
unfaulted in situ Paleozoic rocks and there is no evidence of a
high-angle fa u lt in superjacent volcanic rocks in the area of the
divide to the east. At Wolverine Pass there appears to be an
easterly truncation of Paleozoic carbonate strata against volcanic
rocks to the east, but the contact (which appears to dip about 40°
E) is not exposed. O n either side of a saddle 0.8 km east of Sunset
Peak lie s a block of Paleozoic carbonate rock which may be a Heart
Mountain fa u lt block ( E l l i o t t , 1979; Pierce, 1980); i t is cut by
several no rth-striking normal fa u lts .
Interp retation. The normal faults o ffse ttin g Paleozoic strata
east of Sunset Peak are best explained by assuming that the
Paleozoic rocks are allochthonous on the Heart Mountain fa u lt.
Therefore the break-away fa u lt extends to the area west of the
allochthonous rocks but east of the in situ Paleozoic rocks in the
Pebble Creek drainage. Further resolution of the location of the
break-away fa u lt in th is area, i f possble, requires resolution of
the differences in the mapping by previous workers of the volcanic
rocks. Rocks east of the break-away fa u lt (as Pierce maps i t )
77
that are shown by Pierce (1980) to be Wapiti Formation are shown by
the U.S.G.S. (1972), Prostka (1978), and E llio t t (1979) as Lamar
River and Cathedral C liffs Formations. Pending resolution of these
differences by further fie ld study, Pierce's (1980) mapping of the
break-away fa u lt between Soda Butte Creek and Wolverine Pass is
te n ta tiv e ly accepted. Relationships north of Wolverine Pass have
not been assessed in the present study.
South of Soda Butte Creek
yhe best exposures of the break-away fa u lt are south from Soda
Butte Creek to the arete trending east from Abiathar Peak (Plate I,
lo c a lity A2). Exposures are excellent for about one mile along
strike and through a vertical r e lie f of over 600 m (Fig 6). The
junction of the breakaway and bedding fau lts is covered by slopewash
and vegetation; the most westerly exposure of the bedding-plane
fa u lt on the south side of Soda Butte Creek is roughly 500 m east
of the break-away fa u lt.
O n the south wall of Soda Butte valley, above the concealed
horizon of the bedding-plane fa u lt, the break-away fa u lt is inferred
from slopewash composition (volcanic rock as opposed to Bighorn
Dolomite) to dip eastward at an angle of about 70°. The dip
fla tte n s s lig h tly to 55°-64° where the break-away fa u lt, here well
exposed, cuts the Jefferson and Three Forks Formations and appears
to steepen again where i t cuts the Madison. Three hundred meters
farther south, in a small valley trib u ta ry to Soda Butte Creek,
volcanic rocks occur on both sides of the well-exposed break-away
fa u lt. Here easterly dips range from 44 to 64 degrees in
78
Break - away
fault
FIGURE 6. View to south of exposure of the break-away fau lt south of Soda Butte Creek.
K O
discontinuous exposures. In all these exposures the break-away
fa u lt strikes between N1°E and N15°E (Fig. 7). About 2.5 km south
of Soda Butte Creek, at the highest exposure accessible without
technical climbing equipment (elevation about 9600') the a ttitu d e of
the break-away fa u lt changes to N40W, 64°NE.
Good exposures of the breakaway fa u lt consistently reveal fa u lt
stria e , usually both subhorizontal and steeply oblique (Fig. 7).
inhere the fa u lt strikes between N1E and N15E the steeply or plunging
striae trend roughly due E; where the fa u lt strikes N40W the more
steeply plunging striae trend ESE.
About 2 km south of Soda Butte Creek continuous exposure across
the break-away fa u lt reveals a 8 m wide fa u lt zone separating
hanging-wall and foot-wall volcanic rocks (Fig. 8). The material in
the fa u lt zone is generally finer-grained and less resistant to
weathering than are the coarse-grained breccias in the hanging wall
and foot w all. The fa u lt zone material consists of two layers, a 5
m thick coarser-grained breccia zone adjacent to the hanging-wall
and a softer, finer-grained 3 m thick layer adjacent to the foot
w all. Striae occur on both sides of the fa u lt zone, where i t abuts
the hanging wall (e.g. 9°S2E, 64°N72E) and where i t abuts the foot
wall (e.g. 73°N71E).
About 1.3 km south of Soda Butte Creek, where Madison Limestone
occurs in the foot w all, the fa u lt zone is less well exposed but
appears to be about 6 m th ic k , bounded on the east by a shear zone
separating subhorizontally layered volcanic breccia of the hanging
wall from volcanic breccia within the fa u lt zone.
80
N
FIGURE 7. Stereographic projection of orie nta tions of
the break-away fa u lt (great c irc le s ) and stria e (o)
in the area 1 to 3 km south of Soda Butte Creek
(Plate I Lo cality A2).
81
ij V ' - l . / . f • i 7- ?*' •;
Fault
zone
FIGURE 8. View to north of break-away fa u lt zone as
exposed about 2 km south of Soda Butte Creek.
82
The volcanic breccias bounding the fa u lt at high elevation on
the flank of the arete appear from a distance to be subhorizontally
layered, but on a mesoscopic scale layering is d i f f i c u l t to discern.
Prirnary(?) layering in the hanging wall dips gently south and east.
A few poorly-defined, discontinuous fau lts were observed in the
hanging wall within several tens of meters of the break-away fa u lt,
but rocks of both the hanging wall and foot wall appear largely
unfaulted. Hanging wall rocks lo ca lly contain abundant cobble- to
boulder-size carbonate clasts and fragments of p e trifie d wood.
The carbonate rocks of the footwall are largely undeformed and
dip gently to the south. Minor faults occur near the breakaway
fa u lt and the Jefferson Formation is lo c a lly t ilt e d so that bedding
dips steeply east subparallel to the break-away fa u lt.
In te rp re ta tio n. The thickness of the fa u lt zone, stria e (both
subhorizontal and steeply plunging) on the hanging w all, and local
faulting within the hanging wall all suggest that the hanging wall
volcanic rocks are tr u ly a l1ochthonous and not deposited on a steep
denuded fa u lt scarp as suggested by Pierce (1960, 1980). Prostka et
a l. , (1975) and Prostka (1978) mapped the basal 200 m of the hanging
wall volcanic rocks as a l1ochthonous, but i t is here suggested that
at least the basal 480 m and probably the entire section of volcanic
rocks east of the break-away fa u lt in th is area is a l1ochthonous,
a lb e it apparently very l i t t l e deformed. Tectonic denudation of the
break-away fa u lt at th is lo c a lity is not indicated.
Striae indicate movement of rocks east of the break-away fa u lt
in two directions: to the east, producing d ip -s lip motion, and
83
(probably) to the south (down the inferred paleoslope of the
bedding-plane fa u lt) , producing s trik e -s lip striae on the break-away
fa u lt. The kinematics of the movement of the carbonate rocks that
presumably lay d ire c tly east of the break-away fa u lt before being
faulted away are not d ire c tly represented by the observed stria e .
Upper Forks of Cache Creek
A b rie f reconnaissance of th is area (Plate I, lo c a lity A3)
revealed geometric relationships in agreement with Pierce et al
(1973) and Pierce (1980, "Locality 3"). Volcanic strata dip about
30° southwest into a northeast-dipping (up to 80° --Pierce, op.
c i t . ) l i s t r i c fa u lt, with footwall volcanic strata subhorizontal.
Most of the 3 to 5 km distance from th is fa u lt to the "South of Soda
Butte Creek" exposures is underlain by younger volcanic rock or
alluvium, so the nature of any dire ct connection between these
fa u lts is unknown. No fa u lts were observed within volcanic rocks
west of the l i s t r i c fa u lt, but numerous fau lts were observed by
previous workers and by th is w rite r in volcanic rocks north of th is
1o c a lity (Fig. 9).
In te rp re ta tio n. The hanging wall strata appear to have been
rotated by movement along the l i s t r i c normal fa u lt. Tectonic
denudation is not indicated. The relationship between the l i s t r i c
fa u lt and the faults observed to the north is unknown. The l i s t r i c
fa u lt, lik e the fa u lt south of Soda Butte Creek, appears to be the
western boundary of the detachment terrane, but volcanic rocks
adjacent to the east are faulted and are probably a l1ochthonous
rather than in situ .
84
FIGURE 9. Sketch map showing part of break-away fault
(from Pierce, et al 1973) and structure of volcanic rocks
east of break-away fault (from Pierce et al , 1973;
E llio tt, 1979; and this study).
Cooke
/ w ^ Creek
t^V M O ^ fV °
Si Iver
Gate „
MO Mtn.
G €
4 5 ° 0 0 '
MO
MO
Tv
4 0
‘30
Abiathar
a Pea k
24
Tv
Republic
Pass x 20
2 km
4 4 ° 55 '
1 1 0 ° 0 0 *
86
E X P L A N A T I O N
0 Quaternary surficial deposits
Tv Tertiary volcanic rocks, undifferentiated
Q£
MO
Paleozoic rocks a Iloc ht honou s on
Heart Mountain fault
Lower plate Cambrian and Ordovician
rocks and Quaternary su rf icial deposits
^22 Strike and dip of beds
© H orizon tal beds
S - f o l d , showing plunge of axis
C Locality discussed in text
Contact (contacts between upper plate
carbonate and volcanic rocks are poorly
exposed and may be fa u lts )
Faults, dashed where approximately located,
dotted where inferred or concealed, queried
where location uncertain
= —_ . . Heart Mountain detachment f a u l t , sawteeth
on upper plate-, arrows show trends of striae
77— •• Heart M o un ta in break-away fault, hachures
on detachment side
Other faults
Headwaters of North Fork of Crandal1 Creek
The break-away fa u lt is mapped s im ila rly in th is area (Plate I,
lo c a lity A4) by Pierce et a l . (1973), Pierce (1980), and Nelson et
al . (1980). Reconnaissance by th is w rite r of the roughly 8 km linear
extent of the break-away fa u lt in and ju s t northwest of the North
Fork of Crandall Creek drainage revealed no exposures of a
high-angle fa u lt and no compelling reason to map a fa u lt through
th is area. The closest clea rly a l1ochthonous rocks occur 6 to 8 km
to the east and the volcanic rocks immediately to the east could be
assigned to either the undifferentiated Lamar River and Cathedral
C liffs Formation or to the Wapiti Formation.
The only suggestion of the occurrence of a fa u lt in the
volcanic rocks in th is area is a steeply east-dipping lineament on
the arete on the south side of the North Fork Crandall Creek
drainage. Although th is lineament is mapped by previous workers as
the break-away fa u lt, the volcanic strata on both sides appear
identical in composition, general stra tig ra p h y, and orientation.
In te rp re ta tion. The existence and location of the break-away
fa u lt in th is area cannot be substantiated by local observation. I f
i t exists in th is area, it s location and geometry are unknown.
Summary
In the course of the present study the break-away fa u lt was
visited in a number of areas spanning over half of its entire linea r
extent as previously mapped. In lig h t of the uncertainties involved
in distinguishing between the volcanic formations in th is region,
87
the marked differences between the features mapped as parts of the
break-away fa u lt, and the discontinuous nature of exposures of the
break-away fa u lt, i t is suggested that the nature of the western
boundary of the Heart Mountain fa u lt terrane remains poorly
understood. It must be emphasized that the excellent and re la tiv e ly
accessible exposures south of Soda Butte Creek should not be taken
as representative of the western boundary of the detachment terrane.
Pierce (1980) suggested that in areas within a few miles of Republic
Pass, the break-away fa u lt may step down in one or more L-shaped
steps until i t reaches the base of the Bighorn Dolomite and
terminates at the bedding plane phase of the Heart Mountain fa u lt
(p. 278). USG S mapping of areas up P ilot Creek, Crandall Creek, and
Sunlight Creek indicate that in these areas the allochthonous
carbonate blocks exposed closest to the mapped trace of the
break-away fa u lt are missing most or all of the Ordovician or
Devonian sections. These relationships suggest that from the area
of Republic Pass southward the break-away fa u lt is usually a series
of L-shaped steps ( sensu Pierce, op. c i t . ) rather than a single
steep surface lik e that exposed south of Silver Gate. The western
boundary of the detachment terrane may be a single steep fa u lt only
in the Soda Butte Creek area and west of the North Fork of Crandall
Creek.
Pierce (1980) character!*zed the break-away fa u lt as being
"...n o t a fau lt in the usual sense in which the rocks on
one side have moved re la tive to the rocks on the other
88
side. It might be referred to as a " h a lf- fa u lt" , for
only one side is a fa u lt surface; the other side is a
surface of deposition" (p. 276).
However, the present study indicates that hanging-wall rocks are in
fa u lt contact rather than depositional contact with the footwall
along the break-away fa u lt. As a re s u lt, relationships along the
break-away fa u lt do not require tectonic denudation. Instead, in
the areas of best exposure tectonic denudation appears very
u n like ly. Thus i t is herein suggested that the connotation of
tectonic denudation must be divorced from the term "break-away
fa u lt" i f the term is to be apropos of the western boundary of the
Heart Mountain fa u lt terrane.
The Heart Mountain Bedding Fault
General Statement
William G. Pierce was f i r s t to recognize the bedding-plane
phase of the Heart Mountain fa u lt (Pierce, 1950), and his mapping
along the south sides of the Clarks Fork River and Soda Butte Creek
extended i t to the area of P ilo t Peak (Pierce, 1957) and fin a lly to
the Abiathar Peak exposure of the break-away fa u lt (Pierce, 1960).
W hen w riting his early papers, Pierce (1941, 1957) recognized only
Ordivician Devonian, and Mississippian rocks as a l1ochthonous, but
la te r he reported the involvement of the early acid breccia in Heart
Mountain fau lting (Pierce 1958). Pierce (1963a, 1963b) discusses
these relationships in some d e ta il.
89
Prostka (in Pierce et a l . 1973; Prostka, 1978; and Nelson et
al , 1980) was f i r s t to suggest widespread involvement of volcanic
rocks in Heart Mountain fa u ltin g . P articu larly in the area between
Abiathar Peak and Jim Smith Peak, Prostka mapped rocks as
a l 1ochthonous undifferentiated Lamar river and Cathedral C liffs
Formation rocks that Pierce and Nelson (in Pierce et a l , 1973 and
Pierce, 1978) mapped as Wapiti Formation in depositional contact
with the lower plate. All these workers agreed that in some areas
(e.g. between Onemile Creek and Sugarloaf Mountain) the volcanic
rocks overlying the lower plate are Wapiti Formation and were
deposited upon a te cto n ica lly denuded fa u lt surface and have not
undergone subsequent fa u ltin g .
The present study included examination of portions of the
bedding fa u lt and the rocks that bound i t in areas along its entire
length of exposure, from the Silver Gate area to Dead Indian H ill.
Problems of access, exposure, and ruggedness of te rra in dictated the
sampling pattern. All areas where large blocks of a l1ochthonous
Paleozoic carbonate rocks overlie the bedding fa u lt received a
minimum of several days of study. Many intervening areas, where
volcanic rocks overlie the bedding fa u lt, were also given several
days of study. These la tte r areas received special attention to
structural data that might bear upon the question of whether the
volcanic rock is a l1ochthonous or in s itu . Kinematic data were
collected from all areas v is ite d .
The results of these studies are presented below in geographic
order from northwest (break-away fa u lt area) to southeast
90
(transgressive fa u lt area). Plate II is a generalized geologic map
showing details of previous mapping of volcanic rocks.
West of Republic Mountain
Eastward from the break-away fa u lt for about 5 km volcanic
rocks and occasional small carbonate blocks up to a few tens of
meters thick overlie the Heart Mountain bedding fa u lt. Pierce
(1979) describes in detail two lo c a litie s in th is area (his "South
of Silver Gate" and "Falls Creek" lo c a litie s ) , emphasizing the
cla s tic dikes of carbonate fa u lt breccia that occur within both
carbonate and volcanic rocks. All or part of th is area appears on
the geologic maps of Pierce et al (1973), Prostka et a l . (1975),
Pierce (1978), E l li o t t , (1979) and Nelson et al (1980).
Pierce (1978) maps these volcanic rocks as Wapiti Formation,
depositionally in place. Prostka et al (1975), E llio t t (1979), and
Nelson et al . (1980) map these volcanic rocks as a l1ochthonous Lamar
River Formation or Cathedral C liffs Formation. The coauthors of
Pierce et al (1973) disagreed as to whether these volcanic rocks are
a l1ochthonous.
Immediately east of the break-away fa u lt, on the south side of
Soda Butte Creek, the horizon of the bedding fa u lt is in most places
concealed beneath vegetation and slopewash. The f i r s t exposure of
the bedding fa u lt is about 500 m east of the break-away fa u lt in a
steep ravine that lie s within the Abiathar Peak quadrangle less than
200 m west of its eastern margin (Plate I, lo c a lity B). The fa u lt
horizon is well-exposed for only a meter or two beneath shrubs on
91
the west side of th is ravine, hut the base of the volcanic rocks is
cle a rly sheared at the contact with the lower-plate dolomite and for
at least 15 centimeters above the contact. Several slickensided
shear surfaces were observed with well-defined striae trending from
S71E to S77E. The volcanic rocks immediately overlying the shear
zone are concealed. Good exposures of apparently unfaulted volcanic
rocks occur 30 to 60 m above the shear zone.
About 1.2 km farther east, several blocks of Madison Limestone
up to 50 m thick and 150 m across overlie the bedding fa u lt (Plate
I, lo c a lity C). These a l1ochthonous carbonate blocks are overlain
and separated by volcanic rocks. Pierce (1979) describes the c la s tic
dikes of th is lo c a lity (his "South of Silver Gate" lo c a lity ) .
Examination of th is terrane revealed numerous fau lts within the
blocks of Madison Limestone (Fig. 10a) within the volcanic rocks
(Fig. 10b) and at contacts between the volcanic rocks and the
Madison Limestone (Fig. 10c). Striae on the bedding fa u lt at this
lo c a lity trend S32W and plunge 6°. They appear on a 4 m m thick
microbreccia lamina that overlies the lower plate Bighorn Dolomite.
A sim plified summary of a ll observed fa u lt striae appears as Figure
lOd.
Figure 11 shows the faulted, lo c a lly planar contact between the
upper-plate volcanic and carbonate rocks about 6 m above the bedding
fa u lt. A c la s tic dike (shown in Figures 3 and 4 of Pierce, 1979) in
the volcanic rock appears along the le f t edge of Figure 11.
Where the carbonate-volcanic contact is easily accessible near the
92
FIGURE 10. Stereographic projections of orientations of
upper-plate faults (great circles) and striae (o) south
of Silver Gate (Plate I Locality C).
a. Faults and striae in Madison Limestone.
b. Faults and striae in volcanic rocks.
(a ) = striae on the bedding fault.
c. Faults and striae along contacts between
volcanic rocks and Madison Limestone.
d. All observed fa u lt striae.
94
Fault
contact
Mm
Depos itiona
contact
U D
c _ n
FIGURE 11. Allochthonous
Faulted and depositional
Madison Limestone (Mm) overlain by allochthonous volcanic rocks (Tv),
contacts shown. View to west. Note clastic dike (cd). Plate I Locality C
clastic dike it is not demonstrably faulted, but higher, along the
planar contact that appears in the right center of Figure 11, the
contact is clearly a slickensided shear surface. This contact can
j
| be reached by a d iffic u lt climb about 40 m west of the area shown in
j Figure 11, and along the 15 m accessible portion of the faulted
i
| contact wel1-developed striae, trending S50E to S65E, were observed
| in four places.
i
i Stratification is not apparent in the volcanic rocks in this
i
area and access is d iffic u lt. N o attempt was made to trace observed
faults upward into the steep c liffs of volcanic rock.
| About 1.2 km farther east, where Falls Creek crosses the
| bedding fault, the volcanic rocks and a small carbonate block that
I
overlie the bedding fault are well exposed (Plate I, locality D).
Numerous faults were observed in the accessible basal 30 m of the
volcanic rocks (Fig. 12). Although the bedding fault is moderately
well exposed, no striae were observed on the rocks bounding i t .
Clastic dikes and bodies of fault breccia that occur in this area
are described by Pierce (1979).
Faults observed in the volcanic rocks included faults bounding
a phacoidal or lens-shaped 3-meter-long body of volcanic porphyry,
its long axis subhorizontal, which lies a few meters above the
i
! bedding fault within sheared (?) volcanic breccias. The faults
i
| bounding this phacoid could not be traced into the surrounding
i
j breccia. A nearly vertical fault zone , this one throughgoing,
i
j offsets the bedding plane fau lt, subjacent strata, and the
| overlying volcanic rocks. Thus the present study supports
r
FIGURE 12. Stereographic projection of orie nta tions of
upper-plate fa u lts (great c irc le s ) and s tria e ( o ) , Falls
Creek area (Plate I Locality D). ------ b = fa u lt
zone, with stria e (□), that offsets the bedding fa u lt.
97
E llio t t 's (1979) mapping of th is high-angle fa u lt rather than that
of Pierce et al (1973), who terminate th is fa u lt upward at the
i
i
I bedding fa u lt horizon. Striae in th is steeply-dipping NNE-striking
! fa u lt zone plunge very steeply (see Fig. 12).
Cross-cutting relationships were observed among two sets of
fa u lts within the volcanic rocks. In both cases gently
I
I southwest-dipping fau lts with gently west-plunging stria e predate
j
j high-angle faults with steeply north- or south-plunging s tria e ,
j A specimen of the volcanic rocks that d ire c tly overlie the
bedding fa u lt revealed moderately pervasive subhorizontal shearing
and local high-angle fa u ltin g in thin section. Clasts of undeformed
| plagioclase porphyry occur within th is sheared breccia zone. A
specimen of Bighorn Dolomite subjacent to the bedding fa u lt and
laminated along the fa u lt horizon with a 1 m m thick veneer of very
fine grained green material was examined in thin section, revealing
the presence in the green veneer of very small fragments of
plagioclase crystals within a very fine grained m atrix, suggesting
that the green veneer is fa u lt gouge derived at least in part from
volcanic rocks.
About 0.5 kilometers east of Locality D, at the northwest
corner of Republic Mountain (Plate I, lo c a lity E), Madison Limestone
!
I blocks a few tens of meters th ick overlie the bedding fa u lt. As
I
| shown diagrammatically in Figure 13, the volcanic rock overlying the
i
I
j shattered limestone consists of an overlying zone that stands in
near vertical c l i f f s and is inaccessible but appears less deformed
than the basal zone. Igneous intrusive bodies occur as subvertical
i 98
VIEW TO SOUTH
EXPLANATION
Allochthonous Madison
Li mestone
Autochthonous Cambrian
and Ordovician rocks
Mm
€0
Fault N80E 22NW,
striae 2i°N9E
Fault N76W 29NE,
striae 30°N9E
V V V
v \ \
V
Fault N 79 W 17 NE
\/
V V
Fau It N 34 W 63 SW
V V v
V
30 m
Mm,
shattered
Mm,
T v •, inaccessible
planar fabric
N 55 W 63NE v
: ^ N 70 E 36 NW
v v"
shattered
Heart Mountain bedding fault
^rN5E 89 N W
•€0 €0
(NOT TO SCALE)
FIGURE 13. Diagrammatic cross-section of c l i f f exposures at the northwest corner
of Republic Mountain (Plate I Locality E). Vertical exaggeration about 3x.
View to south.
J dikes that end at the bedding fault and as discontinuous s ills along
I
the bedding fault and within the lower plate. Upper -plate dikes
are unfaulted. Locally the top of the shattered limestone is
!
j sheared, but usually the contact is occupied by apparently unsheared
j carbonate breccia. A layer of carbonate breccia also underlies the
|
| base of the upper zone of volcanic rock, which is locally sheared.
| Figure 14 shows the orientation of faults, fault striae, igneous
!
dikes, and clastic dikes in this area.
Interpretation. The volcanic rocks examined between the
break-away fault and the northeast corner of Republic Mountain are
interpreted as allochthonous due to: the presence along the bedding
i fault of a discontinuous lamina of microbreccia containing volcanic
rock fragments; the presence of slickensides and striae on the upper
surface of the lamina; the presence of slickensides and striae along
the contact between volcanic rocks and subjacent allochthonous
carbonate rocks; the common presence of faults in the basal few
i
meters or few tens of meters of the volcanic rock; the occasional
steep dips of volcanic rock strata; and the presence of clastic
!
j dikes of carbonate breccia within the basal, about ten meters thick, j
! deformed zone of volcanic rocks. The clastic dikes are interpreted j
I !
| as having been intruded upward during faulting from the bedding j
fault and from the faulted contact between the allochthonous !
I carbonate rocks and overlying volcanic rocks. The contact between
j j
j upper-plate volcanic and Paleozoic rocks is rarely unfaulted.
i
|
i
j
i
I
! 100
FIGURE 14. Stereographic projections of orientations of
upper-plate faults, striae, and dikes, northwest corner of
Republic Mountain (Plate I Locality E).
101
a. Faults (great circles) and striae (o).
Clastic dikes
Igneous dikes
upper plate
lower plate
1
FIGURE 15. Stereographic projection of orientations of
all fa u lt striae observed in volcanic rocks between the
break-away fault and the northeast corner of Republic
Mountain, (o) = striae on upper-plate faults in
volcanic rocks; (a ) = striae on the bedding fa u lt where
i t is overlain by volcanic rocks.
104
Mm
MDtj
Tv
Shattered Carbonate , mostly Ob
HEART MOUNTAIN 8EDO ING- PLANE
DETACHMENT FAULT
■ € 0
I" s 150 m ( 500'
Tv
M m
MDtj
Ob
•CO
E X P L A N A T I O N
Tertiary volcanics
Madison Limestone
Three Forks and
Jefferson Formations
Bighorn Dolomite
Lower plate
— Marker bed
Contact
. — Fault
-M — h
Di ke
, . .- Fault intruded
by dike
Fault and striae orientations
@ N35E 27 S E » striae N80E,20°
(B) NI2W 78SW: Striae NI2W,0°
(C ) N 49 E 67 NW = Striae N 2 W,6 10
s N 12 E 65 NW in Tv
(D ) ~E-W 35 S
(D ~ E - W, 90°
0 N 39 E 44 N W
FIGURE 16. Diagrammitic cross-section of view to south from h isto rica l marker
about 0.4 km west of Cooke City on Route US 212. Intersection of faults D and [
shown in Fig. 19. Geometry of intersection of faults C and D is speculative.
Contacts and faults dashed where concealed or approximately located.
A summary of striae orientations for faults within volcanic
rocks between the break-away fault and the northwest corner of
Republic Mountain is shown as Figure 15. The broad range of slip
directions suggests movement (probably extensional , as will be
argued in the Conclusions section) in many directions during
emplacement.
Republic Creek Area
Allochthonous carbonate rocks are exposed on both sides of the
mouth of Republic Creek (Plate I I , locality F). The larger
exposure, on the west side of the creek, forms part of the northeast
corner of Republic Mountain. Dominating the southerly view from
Cooke City, i t is the firs t conspicuous carbonate block east of the
break-away fault on the south side of Soda Butte Creek. Its
east-facing side is heavily forested, but its north-facing side
presents excellent but very steep exposures. Access to and across
these steep slopes is quite d iffic u lt. The smaller exposure of
carbonate rock on the east side of Republic Creek is more easily
accessible but exposures are not as good. These areas appear on the
maps of Pierce et a l . (1973) and E llio tt (1979).
Figure 16 is a diagrammatic sketch of an east-west
cross-section of the northeast corner of Republic Mountain looking
south from the historical marker 1/4 mile west of Cooke City. It
shows faulting within the carbonate rocks, within the volcanic
rocks, and at the contact between volcanic and carbonate rocks.
Several igneous dikes are shown. Bedding orientations in the
105
carbonate rocks range from subhorizontal to gentle or moderate south
or southeast dips. Figure 9 shows attitudes of volcanic rocks south
and west of this area.
The orientations of faults and striae sets from within this
carbonate block appear on Fig. 17a. Although these orientations
range widely, striae on the major faults all trend roughly N15W or
S15E. One of the major faults extends upward from the carbonate
rocks into volcanic rocks where N36W-trending striae appear; a
nearby fault at the volcanic-carbonate contact to the east has
striae that plunge steeply north (Fig. 17b). Drag folds on several
faults indicate normal movement, in accord with the impression
gained from distant views.
Wherever the contact between the upper-plate carbonate and
volcanic rocks is well exposed i t is sheared. At the top of the
carbonate block east of Republic Creek the poorly exposed
carbonate-volcanic contact may be depositional, but the contact at
the south end of the block (Fig. 18) is clearly faulted, with
slickensides and striae evident. Orientations of faults and striae
observed at volcanic-carbonate contacts are plotted on Figure 17b.
In one area (see Figs. 16, 19) the volcanic rocks appear to
form a graben downdropped into the carbonates. The graben-bounding
faults, with attitudes estimated at E-W, 90° and E-W, 35°S, are
inaccessible. Several possible faults were observed in the volcanic
rocks overlying the western part of the carbonate block on the
northeast corner of Republic Mountain, but only two, with a moderate
northwest dips, were accessible (Fig. 17c).
106
FIGURE 17. Stereograph!c projections of orientations of
upper-plate faults (great circles), dikes (great circles,
short dashes), and striae (o), Republic Creek area
(Plate I Locality F).
a. Faults and striae in Paleozoic rocks.
b. Faults and striae along contacts between
upper-plate Paleozoic and volcanic rocks.
Long dashes are estimated attitudes of inac
cessible faults.
c. Faults and striae in volcanic rocks.
d. Upper-plate dikes and striae.
107
FIG URE 17
Tv
Tv
Tv
MO
FIGURE 18. View to east of south end of allochthonous Paleozoic
rocks (MO) on the east side of Republic Creek. Tv = volcanic rocks.
Fault A: N44W 22SW, striae 21°S36W
Fault B: N56W 52SW
M m
Tv
M m
Ta I us
FIGURE 19. View to west o f inaccessible apparent graben where
volcanic rock (Tv).is downfaulted between Paleozoic rocks (Mm).
110
Six igneous dikes were observed; five strike N10W + 25°, one
strikes E-W, and all dip steeply (Fig. 17d). The E-W striking dike
occurs along a fault but appears undeformed. Other dikes either cut
across faults, are truncated or offset by faults, or are intruded
along and sheared by faults.
Interpretation. The deformation patterns of the carbonate
blocks on both sides of the mouth of Republic Creek are complex, but
two dominant patterns appear. The major faults in the carbonate
rocks west of Republic Creek indicate north-northwest-oriented
oblique extension. The one east-west striking dike which was
observed east of the creek fits this pattern. The other five dikes,
in contrast, suggest east-west extension, as do the most westerly
carbonate-volcanic contact and several faults within the carbonate
rocks. Dikes were, at least in part, intruded synkinematically.
There is no clear evidence to suggest whether the inferred
north-south and east-west oriented extension events were coeval or
sequenti a l.
South of Colter Pass
A n unmaintained pack tra il (Bonney and Bonney, 1977) leads
east-southeast from Cooke City to excellent exposures of the bedding
fault east of Woody Creek (called Hayden Creek in Wyoming). Pierce
et al (1973), Prostka (1978), and E llio tt (1979) show Lamar
River-Cathedral Cliffs Formation overlying the bedding plane fault
in the west part of this area and Wapiti Formation overlying the
bedding plane fault to the east. Pierce (1978) includes all these
111
volcanic rocks in the autochthous Wapiti Formation, but Pierce
| (1980) allows for the possible occurrence of Lamar River-Cathedral
Cliffs volcanic rocks in this area.
For the present study examination of this area was restricted
I to outcrops within the steep slopes and c liffs where the
J bedding-plane fault is exposed. A clear lithologic change occurs at
! the contact mapped by Pierce et a l . (1973), Prostka (1978), and
|
I E llio tt (1979) between dark red and brown interlayered flows and
breccias (undifferentiated Lamar River and Cathedral Cliffs
Formations) west of the contact (Plate I, locality G) and bright
j
j red-matrixed polybreccias (Wapiti Formation) to the east (Plate I,
locality H). The former are commonly faulted and dip 20-30°
southeast while faults are rare in the la tte r, which dip gently to
the south. Exposures of the contact between the two lithologies
were not observed. For purposes of this discussion, the terms
"eastern volcanic rocks" and "western volcanic rocks" are used to
distinguish between features observed on either side of this
i
contact.
i The bedding fault is striated beneath both the eastern volcanic
I
rocks and the western volcanic rocks. In both areas, bedding-fault
j striae occur on the basal bed of the Bighorn Dolomite and on a black
j or dark green microbreccia lamina immediately overlying the basal I
I |
; bed of Bighorn Dolomite. Bedding-fault striae underlying the
( western volcanic rocks trend dominantly S80W; those beneath the '
j
I eastern volcanic terrane trend dominantly S45W (Fig. 20).
! i
j I
112
a. Western area [Lamar River and Cathedral
C liffs Formations of Pierce et al (1973),
Prostka (1978), and E llio tt (1979)]. Plate I
Locality G.
b. Eastern area [Wapiti Formation of
Pierce et al (1973), Prostka (1978),
and E llio tt (1979)]. Plate I Locality H ,
FIGURE 20. Stereographic projections of orientations of upper-plate faults
(great circles) and striae (o) and of bedding-fault striae (a ) south of
Colter Pass.
Specimens of rocks that bound the bedding fa u lt were collected
fo r petrographic examination. One specimen of lower-plate Bighorn
I
| Dolomite studied in thin section revealed three undeformed layers of
I
fine-sand sized quartz grains, each layer 1 to 2 m m th ick, occurring
within a few centimeters of the bedding fa u lt. The sand grains are
usually rounded but some are subangular, a few are embayed and a few
i
are s lig h tly offset along fractures. The presence of a 0.3 to 1.2 m
|
! th ick basal sand bed in the Bighorn Dolomite was noted in the Big
Horn Mountains by Darton (1906b) and in the Wind River Mountains by
M ille r (1930), but no such sand bed occurs in the area of the Heart
Mountain detachment fa u lt (M ills , 1956), except possibly fo r these
thin stringers. Thin section examination of specimens which include
the bedding fa u lt and adjacent upper- and lower-plate rocks revealed
a breccia zone at least 4 cm thick overlying lower-plate carbonate
rock. The breccia contains clasts of volcanic rock up to 2 cm
across and clasts of carbonate rock up to 5 m m across. The clasts
of carbonate rock, lik e the subjacent lower-plate carbonate rock,
contain quartz grains up to 0.4 m m across. In the fie ld a striated
shear surface was observed to form the top of th is breccia layer.
These specimens were collected from the area of the eastern volcanic
i
| rocks.
Faults within both the eastern and the western volcanic rocks
are dominantly northeast-striking and high-angle (Fig. 20). Striae
or fau lts in the western area present a diffuse pattern of west to
northeast trends, while striae or fau lts in the eastern area trend
southwest.
114
| Only one igneous dike was observed. It is v e r tic a l, strikes
| N20W, occurs within the eastern area, and is shown on E llio t t
i
(1979). Due to poor exposure, i t is not known whether the dike
extends across the contact in to the volcanic rocks of the western
! area or whether i t is truncated along th is contact. Clastic dikes
i
| of fa u lt breccia were observed in volcanic rocks in several places.
I
j In te rp re ta tio n . The volcanic rocks observed south of Colter
| Pass are believed to be a l1ochthonous, because: rocks east of the
| contact mapped by Pierce et al (1973), Prostka (1978), and E llio t t
i
i
! (1979) are in te rn a lly faulted, are sheared and brecciated along
i
j
j th e ir base, and rest upon a striated detachment fa u lt with a thin
I
microbreccia veneer containing clasts of volcanic rock; and rocks
west of the contact are in te rn a lly faulted, rotated, and also rest
upon a striated detachment fa u lt. Striae on the bedding fa u lt
indicate dominantly E-W tra nslatio n for rocks west of the contact
and dominantly NE-SW tra nslatio n for rocks east of the contact.
I
j Striae or fau lts above the bedding fa u lt suggest oblique
i
extension subparallel to translation for rocks east of the
contact, with more complex deformation west of the contact.
j
I E llio t t 's (1979) mapping of the contact, confirmed by the present
j
| study, suggests that the more complexly deformed rocks west of the
I
contact in part underlie the less-deformed rocks to the east. The
!
j lower structural position of the rocks to the west may explain th e ir
j more complex deformation: in other areas (e.g. between the
i
break-away fa u lt and Republic Mountain) in te n sity of deformation of
volcanic rock increases downward toward the bedding fa u lt, and
i 115
| south of Colter Pass the rocks west of the contact may have already
been deformed by movement along the bedding fa u lt when the rocks
east of the contact were downfaulted to th e ir present structural
1 evel .
Northeast of Index Peak
The steep c l i f f s near the base of the northeast corner of Index
i
| Peak v is ib le from several places along the highway between Sunlight
j
| Basin and Colter Pass, present a dramatic distant view of the planar
contact between volcanic rocks and the underlying Heart Mountain
bedding fa u lt. As in the area south of Colter Pass, the Middle
| Cambrian Park Shale (known in Wyoming as the upper part of the Gros
i
j Ventre Formation) is intruded by an 80 to 100 m thick Pal eocene
| trachyandesite porphyry dike ( E llio t t , 1979; Pierce et a l . 1973).
The exposure of the bedding fa u lt can be reached from the east by a
i
j steep climb across te rra in from the highway near the Index Creek or
from the west via the Woody (Hayden) Creek t r a il and cross-terrain
hi ki ng.
A b rie f reconnaissance of th is area (Plate I, lo c a lity J)
confirmed that the dramatic c l i f f exposure of the Heart Mountain
1
! Fault is too steep to allow study without technical climbing aids.
i
Interm ittent exposures of the bedding fa u lt can be found in the
heavily forested slopes northwest of the c l i f f exposure. The
bedding fa u lt horizon in th is area occurs within the Grove Creek
member of the Snowy Range Formation; no Ordovician dolomite is
present. The bedding fa u lt horizon is immediately overlain by a 1
mm-thick veneer of very fine-grained dark red m aterial. Striae were
observed in two places on the bedding fa u lt horizon; they trend S80W
|
| where the fa u lt orientation is N65W 14SW and S85W where the fa u lt
i
orientation is N50W 18SW. Moderately penetrative shearing of the
basal 60 cm of volcanic rocks was observed, and undeformed bedding
! was evident in the Grove Creek limestone 15 cm beneath the fa u lt
i
I horizon. In poor exposure a c la s tic dike, containing clasts of both
! volcanic and carbonate material , was observed roughly 1 meter above
i
j the bedding fa u lt.
j
| Specimens of rocks bounding the bedding fa u lt were collected
i
I fo r petrographic examination. Thin-section examination of uppermost
! lower-plate rocks revealed a 4 cm-thick layer of rounded to
j subangular fine quartz sand with carbonate m atrix, overlying
re crystallized carbonate. The quartz sand layer, with quazrtz
clasts up to 0.4 m m in apparent diameter and matrix constituting
about 40% of the area of view, is overlain by a 1-2 mm-thick veneer
composed of rounded to subangular quartz grains less than 0.1 m m in
apparent diameter in a submicroscopic matrix constituting about 90%
of the fie ld of view. No fragments of volcanic material were
i
observed in th is veneer. A th in section of the lowermost upper
| plate revealed, in ascending order:
j - a layer up to 5 m m thick containing 90% submicroscopic
j matrix and clasts of angular quartz {up to 0.1 m m
!
apparent diameter), clasts of carbonate (up to 0.3 m m
apparent diameter, sometimes containing rounded
117
quartz c la s ts ), and a few clasts of volcanic rock
with twinned a lb ite crystals;
- a layer up to 1.5 cm thick of volcanic rock fragments
from 2.0 to less than 0.1 m m apparent diameter, with
occasional clasts of carbonate up to 0.6 m m apparent
di ameter;
- a layer of volcanic rock, with discontinuous very
fine grained subhorizontal layers about 0.1 m m thick
along which shearing may have occurred.
In te rp re ta tio n . Mesoscopic and microscopic examination of
rocks bounding the bedding fa u lt in th is area indicate that the
volcanic rock is a l1ochthonous on the Heart Mountain fa u lt. Fault
breccia contains clasts of both lower-plate quartz and carbonate and
upper-plate volcanic rock. Shearing of the basal volcanic rocks and
formation of s tria tio n s on the bedding fa u lt occurred during
tectonic emplacement of the volcanic rocks.
Fox Creek
Al1ochthonous carbonate rocks are exposed on the steep
northeast-facing wall of the Clarks Fork canyon ju s t south of Fox
Creek (Plate I, lo c a lity K). Access to th is 1.1 km long exposure
can be gained via game t r a ils along the south bank of Fox Creek.
This a l1ochthonous block may be part of the larger carbonate mass
exposed 0.5 km to the south along P ilo t Creek. A sim plified
geologic map of th is area appears on Pierce et al (1973); a more
detailed map appears herein as Figure 21.
118
M D t j 5 4
, ' 3 >-
•• y . 35
'•> > / MDtj
Ob
-Gsr
l"= 3 0 0 m CiOOO’)
Tv
Jv 67
50 ir
Ob
68
Ob
MDtj
^ o f - V
• • • • l i
M m
Tv
E X P L A N A T I O N
Tertiary volcanic rocks, undifferentiated
Madison Limestone
Three Forks and Jefferson Formations,
undifferentiated
Big horn Dolomit e
Snowy Range Formation
Lower plate Cambrian and Ordovician rocks
and Quaternary s u r f ic ia l deposits
Contact, dashed where approximately located
Fault, showing direction of dipj dashed where
approximately located, dotted where concealed
or infe r r e d .
Heart Mountain detachment fault
Strike and dip of bedding
FIGURE 21. Geologic map of north-facing slope south of
Fox Creek (Plate I Locality K ).
M m
MDtj
Ob
■Csr
q -c
50
T -T
I 7^
119
| Bedding in th is faulted carbonate block generally dips 10 to
j
j 20° to the south, although southerly dips approach 60° at the
i
northwest and southeast ends of the exposure area. An unusual
lower-plate feature discovered during the present study occurs
beneath the Fox Creek carbonate allochthon: a well-exposed
channel-shaped body, about 300 m across and 60 m th ic k , of crudely
i
I
bedded volcanic breccias and flows. Rounded cobbles of limestone
I and fragments of green shale occur within the base of the
j lower-plate volcanic sequence, and p e trifie d wood occurs higher in
j the section.
| Also discovered during the present study was the only known
occurrence of Cambrian rocks above the bedding fa u lt. Just
northwest of the lower-plate volcanic mass, roughly 15 m of
stratigraphic thickness of Snowy Range Formation, including the
Grove Creek Member, overlies the bedding plane fa u lt, with the basal
units of the Bighorn Dolomite conformably overlying the upper-plate
Cambrian strata. High-angle normal faults cut th is a l 1ochthonous
sequence. |
Orientations of fa u lts and fa u lt striae appear as Figure 22. |
|
Faults within the carbonate block are generally n o rth -strikin g with ■
north-or south-trending, gently plunging s tria e . Faults occurring
i
j
at the volcanic-carbonate contact within the upper plate carbonate j
i
rocks, and within lower plate volcanic rocks are also shown. j
Lower-plate faults are not shown in Figure 21. !
A one-inch thick igneous dike occurs in the lower-plate ;
i
;
volcanic rocks. Several cla s tic dikes occur in rocks that overlie j
120 |
FIGURE 22. Stereographic projection of orientations of
fa u lts (great c irc le s ) and s tria e at Fox Creek (Plate I
L o c a lity K) (o) = stria e on fa u lts in upper-plate
Paleozoic rocks; (a) = stria e on fa u lts along contact
between upper-plate Paleozoic and volcanic rocks;
(0) = stria e on fa u lts in lower-plate volcanic rocks.
the bedding plane fa u lt: subvertical c la s tic dikes s trik in g N10W and
N8E occur withn the carbonate rocks, and, at the faulted contact
between carbonate and volcanic rocks at the southeast end of the
upper-plate carbonate block poorly exposed cla stic dikes appear
along the contact and within the adjacent carbonate and volcanic
rocks. Thin-section study of a specimen collected from a c la s tic
dike within the carbonate block revealed no clasts of volcanic
m a te ria l.
Inte rp re ta tio n . Geometric and kinematic data indicate a
clear-cut picture of s tr ik e - s lip movement on steeply-dipping
no rth -strikin g faults within the carbonate allochthon. The tectonic
i
contact at the southeast end of the carbonate allochthon suggests
that the adjacent volcanic rocks are fault-emplaced. Striae at th is
contact indicate down-dip s lip to the southwest and southeast.
The lower-plate body of volcanic rock is interpreted as an in
situ ch a n n e l-fill deposit that predates fa u ltin g . The nearby
upper-plate Cambrian strata were fault-emplaced from an unknown
source area.
Pi 1ot Creek
Carbonate rocks underlie the slopes on the north side of P ilo t j
!
Creek, from the mouth of the valley to about 5.5 km upstream. A I
smaller exposure of carbonate rocks lie s on the south side of P ilo t j
Creek (Pierce et a l ., 1973). Access is easily obtained via a pack j
t r a i l maintained by Shoshone National Forest and good exposures |
occur in several side canyons north of the creek. Most of the j
i carbonate terrane is heavily forested, however, providing poor
exposures. Relationships mapped during the present study are shown
on Figure 23.
In comparison with the carbonate rocks of the Republic Creek
and Fox Creek areas, the carbonate rocks north of P ilo t Creek are
very l i t t l e deformed. Only a few minor fa u lts , and no igneous
i
s
! dikes, were observed. The fa u lt orientations and striae
i
| orientations are presented in Figure 24a. This lack of apparent i
deformation is especially remarkable, as the Ordovician section
appears to be missing from the concealed base of the carbonate block
in most areas (Pierce et a l . 1973).
j A contact between the a l 1 ochthonous carbonate rocks and the
volcanic rocks to the north is well exposed in a narrow
south-trending canyon which issues into P ilo t Creek canyon 2.4 miles
due west of the P ilo t Creek t r a il head (Plate I, lo c a lity L). The
contact is a narrow fa u lt zone which dips steeply north and exhibits i
subhorizontal striae (Fig. 24 b) both on the exhumed footwal1 j
surface of Madison Limestone (Fig. 25) and s tru c tu ra lly higher where j
i
volcanic rocks occur in both the hanging wall and the footwal1 (see !
| , i
! Fig. 26). The volcanic rocks are well-bedded a llu v ia l facies i
i '
I
breccias, conglomerates, and sandstones. Hanging-wall volcanic j
| i
rocks dip 25 to 40° to the south, show minor internal faulting and J
are cut by several east-west-striking igneous dikes. A
noncalcareous c la s tic dike was observed in hanging-wall rocks about !
i
12 m from the volcanic-carbonate contact. j
FIGURE 23. Geologic map and cross-sections of area north
of Pilot Creek (Plate I lo c a lity L). From th is study and
Pierce et al (1973).
Tv i
26
S
r 0 0
-"x
28
Tv
2 5
80
Mm
MDtj L - 0 0
-300 m. to
Pilot Creek
A A*
Mm
Tv
MDtj
N
o o o m (looo’)
— h > i i—
~ Tv
| Mm~
MDtj
EXPLAN ATION
Tertiary dikes
Tertiary volcanic rocks (Lamar River. Cathedral
Cliffs, and Wapiti Formations, undifferentiated
Madison Limestone
Three Forks and Jefferson Formations,
und ifferentiated
Contact
Fault, showing direction of dip-, dashed where
approximately located, dotted where concealed
Strike and dip of beds
Approximate strike and dip of beds
FIGURE 23. Geologic m ap and cross-sections of area north of Pilot Creek
(Plate I Locality L). From this study and Pierce et al (1973).
1 2 5
FIGURE 24. Stereographic projections of orientations of
upper-plate faults (great circles), striae (o), and
dikes (dashed great circles), north side of Pilot Creek
(Plate I Locality L).
a. Faults and striae in Paleozoic rocks.
b. Faults and striae from fault zone at contact
between upper-plate Paleozoic and volcanic rocks.
( — b— ) = fault and striae orientations at
highest exposures, where volcanic rocks are in
both hanging and foot walls.
c. Faults and striae in volcanic rocks.
d. Dikes in volcanic rocks.
126
FIG U R E 24
FIGURE 25. Exhumed fa ult at north end of allochthonous
Paleozoic rocks in cross-section A-A' of Fig. 23. View
is downs!ope to east.
FIG URE 26. View to east of area shown on cross-section B-B' of Fig. 23
The volcanic strata are well exposed along the ridge crests
trending north from the carbonate-volcanic contact, where they
appear largely unfaulted. About 0.6 km north of the
carbonate-volcanic contact a throughgoing fault occurs within the
volcanic strata. The fault dips gently to the west-northwest and
j displays west-trending striae. Zones of complex faulting occur on
j both sides of the fault (Fig. 24c), and distant views to the
j north-northwest suggest that the complex deformation continues into
i the extremely rugged terrain approaching the summit of Pilot Peak
I
| (Fig. 27). Prostka (1978, Fig. 3) maps these volcanic rocks as
j
| a l1ochthonous. See also Nelson et al (1980), Figure 9.
| The orientations of igneous dikes in this terrane are plotted
j in Figure 24d; several other dikes striking roughly east-west and
dipping steeply north were observed.
Interpretation. The carbonate rocks exposed along Pilot Creek
I
I are al1ochthonous. Although the good exposures on the north side of
I the creek reveal only minor faulting, the heavily forested blocks !
|
i
south of the creek are discordantly tilte d (Pierce et a l ., 1973). I
! The volcanic strata observed north of the carbonate rocks are j
i
|
also clearly a l1ochthonous. South of the major east-west-striking j
i I
|
fault of Figures 24b, 25 and 26, volcanic rocks overlie the t
i i
| i
| carbonate rocks with depositional contact, but the volcanic strata j
| l
I north of the east-west-striking fault are locally intensely faulted j
and were tectonically emplaced against allochthonous carbonate j
i
rocks. The major faults exhibit east-west trending striae, both '
strike-slip and steeply oblique. Intrusion of igneous dikes appears j
!
130 j
Tv
FIGURE 27. View to northwest toward P ilo t Peak, showing
a l 1ochthonous volcanic rocks (Tv) overlain by in situ(?)
volcanic rocks (Tt). About 800 m of r e l i e f shown.
131
to have at least in part postdated faulting, since at least one dike
cuts across but is not offset by a fault in volcanic rocks. If
a l1ochthonous volcanic rocks extend to elevations of 10,400 ft at
Pilot Peak, as suggested by Figure 27 herein and Figure 9 of Nelson
et a l , 1980, the allochthonous volcanic rocks are 600 m thick in
this area (compare Pierce et al , 1973).
Jim Smith Creek
The earliest detailed geologic map of volcanic rocks in the Jim
Smith Creek area (Pierce et a l , 1973) shows a contact dipping gently
southward between Lamar River and Cathedral Cliffs Formations to the
north and the overlying Wapiti formation to the south. This contact
extends 10 km westward from Jim Smith Creek, nearly to the
break-away fault. More recent maps of Prostka (1978) and Nelson et
al (1980) maintain this interpretation, but Pierce (1978, 1980) maps
all these volcanic rocks as Wapiti Formation. In the present
discussion the volcanic rocks assigned by Pierce et a l . (1973) to
the Lamar River and Cathedral Cliffs formations are discussed as the
rocks of Jim Smith Creek locality (Plate I, locality M). The
volcanic rocks mapped by Pierce et a l . (1973) as Wapiti Formation
I
t
are discussed as the rocks South of B-4 Ranch locality (Plate I,
locality N) in the next section.
i
In the canyon of Jim Smith Creek, and along the steep c liffs j
extending roughly 300 meters to the northwest and 150 meters to the
i
southeast, excellent exposures of the bedding fault and overlying j
volcanic rocks occur (Pierce, 1979, discussed this area). Local j
132 j
primary layering in the volcanic rocks dips 30° to 50° to the south
and is truncated downward by gently to moderately dipping faults
within a few meters of the bedding plane fault (Fig. 28). In one
accessible exposure a sequence of volcanic strata oriented N35W 33SW
is truncated downward by a fault oriented N65E 39Nw. The fault, in
turn, is cut by an igneous dike (N27W 90°) which is cut by the
bedding-plane fault. Attitudes of faults and striae observed in the
volcanic rocks appear as Fig. 29a.
Several apparently noncalcareous clastic dikes occur on the
west side of Jim Smith Creek within the volcanics. Thin section
examination of a specimen of one of these dikes revealed many clasts
of volcanic material, but very few of carbonate material, within a
microbreccia matrix. The dike walls are commonly striated.
Geometric data for these features appear as Fig. 29b.
Striae commonly appear on the basal detachment fault as well.
Their orientations are plotted on Fig. 29a. Most of these occur on I
a 1 m m thick microbreccia lamina overlying the basal bed of the
i
Bighorn Dolomite; one set appeared on a shear surface withn volcanic |
i
material about 10 cm above the base of the volcanic rocks (Fig. 30). |
The basal bed of the Bighorn Dolomite immediately underlying
the bedding fault is deformed in this area. It is only about 80 cm
thick and contains two subhorizontal shatter zones. The upper i
i
i
shatter zone, up to 2 cm thick, occurs 6 to 13 cm below the bedding j
j
fa u lt, pinches and swells, and contains clasts of carbonate material j
up to 2 cm across in an orange-colored microbreccia matrix. It is
offset by numerous normal faults, some of which are lis tr ic (see
133
28. View to west across Jim Smith Creek showing
} fa u lt, fau lts within upper-plate volcanic rocks,
:linded truncated strata within volcanic rocks.
134
FIG URE 29. Stereographic projections of orientations
of faults, striae, and clastic dikes at Jim Smith
Creek (Plate I Locality M).
a. Faults (great circles) and striae (o) in
upper-plate volcanic rocks, (a) = striae along
bedding fault.
b. Clastic dikes (dashed great circles) and
striae (o) on their margins.
c. Lower-plate faults in Bighorn Dolomite
(long-dashed great circles) and in Grove Creek
m em ber of Snow y Range Formation (solid great
circles), (o) = striae.
FIGURE 30. Bedding fa u lt at Jim Smith Creek. Pen and pencil
in lower h a lf of picture lie parallel to two s tria tio n
directions on the bedding fa u lt. Pencil, in upper rig h t corner
of photo defines s tria tio n s on a surface w ithin volcanic rock
about 10 cm above the bedding fa u lt.
136
! Fig. 29c), that die out downward at a second subhorizontal shatter
j zone. The lower shatter zone, 28 cm beneath the bedding fa u lt, is
I
! sim ilar to the upper one but has a gray-colored matrix.
!
| Northwest of Jim Smith Creek Canyon numerous carbonate c la s tic
! dikes occur in the volcanic rocks immediately overlying the bedding
i
! plane fa u lt. Pierce (1979) describes these in d e ta il. In addition,
I
| a i m thick igneous dike was observed in the lower plate. It
j is oriented N34E 85Nw and is truncated upward at the bedding fa u lt.
| Specimens of rocks bounding the bedding fa u lt were collected
| for petrographic examination. Examination of sawn surfaces and thin
i
j sections revealed 1 mm-thick c la s tic dikes and 1 to 8 mm-thick
i
| microbreccia zones o ffse t by fau lts within the basal volcanic rock.
i
Similar c la s tic dike material occurs in lower-plate carbonate rocks
within a few inches of the bedding fa u lt. Breccia along the bedding
fa u lt contains clasts of plagioclase porphyry. In one lower-plate
specimen masses of carbonate and volcanic material 2 to 5 cm across
occur separated by zones of black microbreccia containing small
clasts of carbonate rock.
The 1 mm-thick dark-colored veneer which commonly occurs along
the bedding fa u lt on the lower plate was revealed in thin section to
be a microbreccia layer with clasts up to 0.2 m m across of volcanic
rock and carbonate rock. Lower-plate rocks contain many undeformed
fo ssils such as pelmatzoan columnals. No quartz grains were j
observed. j
In te rp re ta tio n . The volcanic rocks of th is area are clea rly
a l1ochthonous, as is indicated by steep dips, internal fa u ltin g ,
137
cla s tic dikes, striae on the bedding-plane fa u lt, truncation of
igneous dikes at the bedding plane fa u lt, and a mixture of carbonate
and volcanic material within the fa u lt breccia. The zone of
detachment includes not only the bedding-plane fa u lt but also
detachment zones within the d ire c tly underlying Bighorn Dolomite (as
: evidenced by shatter zones and normal fa u lts) and low-angle fau lts
i
j within the d ire c tly overlying volcanic rocks.
| Bedding fa u lt stria e , upper-plate fa u lt striae and striae on
i
i
I c la s tic dike walls indicate a kinematic picture dominated by NNW-SSE
I
directed translation and extension. A secondary pattern of
east-west extension and translation is suggested by bedding-plane
fa u lt striae and an igneous dike. Cross-cutting relationships
indicate that the former preceeded the la tte r .
j
i Southwest of B-4 Ranch
! This lo c a lity (Plate I, lo c a lity N) extends from the previously
described "Jim Smith Creek" lo c a lity southeastward along the steep
northeast-facing c l i f f s for a distance of about 1.5 km. All
previous workers (Pierce et a l ., 1973; Prostka, 1978; Pierce, 1978,
1979; Nelson et a l . 1980) mapped the volcanic rocks of th is area as
i
| Wapiti Formation, deposited on the te c to n ic a lly denuded bedding
| fa u lt surface.
| In the southeastern part of th is lo c a lity both the bedding
i
| fa u lt and the overlying volcanic rocks are well-exposed. The
i
I
volcanic rocks consist of crudely bedded breccia layers, in part
a llu v ia l, that are subhorizontal to gently southeast dipping. The
138
volcanic strata appear largely undeformed except for nearly vertical
i
shear zones spaced tens of meters apart along which the ill-d e fin e d
| s tra tific a tio n does not appear to be markedly o ffs e t. Shear (?)
| planes within these zones are rarely stria te d ; stria e plunging 32°
due east appear within one such zone.
Striae were observed in several places on the surface of the
j
| basal bed of Bighorn Dolomite that forms the top of the autochthon.
i
i In four places subhorizontal striae trending S50°E and plunging at
low angles were observed; in one of these places secondary striae
trending SHE were observed.
! These striae appear on the lower plate carbonate rock; the
i
immediately overlying material is eith er apparently undeformed
j volcanic rock or a green to dark gray breccia up to 1 cm thick.
! Microscopic examination of samples from the veneer of green to dark
i
| grey breccia revealed the presence of angular clasts of volcanic
! material up to 1 m m across and less common clasts of carbonate
material up to 0.7 m m across, in a matrix of carbonate m aterial. A
0-2 m m thick layer of carbonate breccia with no fragments of
volcanic rock lo c a lly underlies the green to dark grey breccia.
Within about 800 m of Jim Smith Creek, fa u lts are present
within the volcanic rocks, and th e ir apparent frequency increases
| toward Jim Smith Creek. Orientations of fau lts and fa u lt striae
| appear as Fig. 31. Faults in the volcanic rock within a few meters
I of the bedding-plane fa u lt commonly exhibit striae plunging gently
! northwest or southwest.
FIGURE 31. Stereographic projection of orientations of
upper-plate fa u lts (great c irc le s ) and stria e (o) and of
bedding-fault stria e ( ^ ) , southwest of B-4 Ranch
(Plate I Locality N).
No upper-plate dikes, either c la s tic or igneous, were observed
in th is terrane. One lower-plate igneous dike, 1.5 m thick and
oriented N1E82E, was observed; i t ends upward at the bedding plane
fa u lt horizon.
In te rp re ta tio n . The volcanic rocks at th is lo c a lity are
interpreted as al 1 ochthonous because of: the basal rnicrobreccia
layer that contains clasts of both carbonate and volcanic rock; the
local presence of fa u ltin g at the base of the volcanic sequence; and
the presence of stria e on the bedding fa u lt. The apparent lack of
deformtion of subhorizontally layered volcanic rocks suggests that
these rocks may be less far-traveled than the intensely deformed
rocks of the Jim Smith Creek lo c a lity . Kinematic data indicates
NW -SE displacement of the volcanic rocks south of the B-4 Ranch.
Onemi1e Creek
The Onemile Creek area (Plate I, lo c a lity P) is underlain by
carbonate and volcanic rocks with complex structural relationships.
The area is discussed in some detail by Pierce (1963b) and appears
on the geologic maps of Pierce et al (1973), Prostka (1978), Pierce
(1978), and Nelson et al (1980). Certain volcanic rocks mapped by
Pierce (1963b) as early basic breccia (now called Wapiti Formation) j
are mapped by la te r workers, including Pierce, as Lamar River or
j
Cathedral C liffs formations. The Reef Creek detachment fa u lt of j
Pierce (1963b) appears in the upper part of the carbonate section |
exposed in the drainage of Onemile Creek. !
The bedding fa u lt is not well exposed along Onemile Creek, but j
j
a 30 cm thick zone of carbonate breccia appears at the
141
j Cambrian-Ordovician contact about 2 m below the bedding fa u lt,
suggesting a secondary, stratigraphically-1ower detachment surface
(compare Jim Smith Creek area). N o striae were observed on the
bedding-plane fa u lt.
In the carbonate rocks overlying the bedding-plane fa u lt many
! fa u lts (see Fig. 32) and one throughgoing 2 meter-thick upper-plate
igneous dike (N85E 90°) were observed. Mineral lineations of
j ca lcite parallel to striae are common on fau lts of th is area.
Carbonate lith o lo g ie s juxtaposed across several upper-plate fau lts
contrast greatly, suggesting offset of several meters to several
tens of meters or more. Yet some of these faults strike toward but
i do not offset the igneous dike. One fa u lt emplaces Bighorn Dolomite
!
upon Jefferson or Three Forks Formation with a geometry indicating
reverse movement.
Interp reta tion. Two phases of faulting are represented, the
second characterized by re la tiv e ly small (<1 m) displacements that
postdate igneous intrusion and involve ENE-W SW extension (and
possible minor orthogonal shortening). The e a rlie r phase of
fa u ltin g predated igneous intrusion and probably generated the
i
| north-or northeast-plunging fa u lt stria e.
Onemi1e Creek to Squaw Creek
[ Between the mouths of these two creeks, a 3.5 km length of
northeast-facing steep slopes and c l i f f s exposes volcanic rocks
overlying the bedding-plane fa u lt (Plate I, lo c a lity 0). The
volcanic rocks of the southeastern 2.5 km of th is mountainside, and
the exposures extending roughly 1.1 km up the northwest side of
142
FIGURE 32. Stereographic projection of orientations of
upper-plate faults (great circles) and striae (o ),
Onemile Creek (Plate I Locality P). r = reverse fault.
| Squaw Creek Canyon, were examined for indications of tectonic
disruption. All previous workers (Pierce and Nelson, 1971; Pierce
| et al , 1973; Prostka, 1978; Pierce, 1978, 1979; and Nelson et al
| 1980) map these rocks as Wapiti Formation in depositional contact
j with an underlying te c to n ic a lly denuded bedding-plane fa u lt.
| Throughout th is area the volcanic rocks (more c o rre ctly,
|
| volcaniogenic sedimentary rocks) are well-bedded dominantly a llu via l
| facies breccias that dip very gently northwestward and weather into
i
hoodoos. Occasional dikes or more complex intrusive bodies appear,
| but faulting is cle a rly absent in the vast m ajority of exposures in
i
| th is area.
I
| Nonetheless, low-angle faults were observed near the base of
| the volcanic section at several lo c a litie s . Near the northwest end
I
!
of the area studied (from which the intersection of U S Route 212
and the Crandal1-Sunl ight Road bears N35E), a tectonic breccia is
| mesoscopically evident overlying the bedding fa u lt. Thin sections
| of several specimens from here and from areas up to 300 m to the
| southeast reveal clasts of volcanic rock up to 1.5 cm across and
| clasts of carbonate rock up to 6 m m across in a submicroscopic j
m atrix. Volcanic rock overlying the breccia is poorly exposed. j
About 200 m southeast, a low-angle fa u lt occurs about 60 cm above
i i
I the bedding plane fa u lt along roughly 60 m of outcrop. Striae were j
i ;
i ;
j observed on th is fa u lt at two places roughly 15 m apart, oriented 4°
i i
i N76W and 5° N86W. In thin section volcanic rock bounding th is fa u lt !
i j
| appears sheared along discrete, poorly developed planar zones which
|
| in places truncate plagioclase crysta ls. The fa u lt orientation is
144 I
N44W 9SW (see Fig 33) and beds of Grove Creek limestone nearby lie
at N58E 10NW. The bedding fa u lt in th is area is not well exposed.
| Another low-angle fa u lt zone near the base of the volcanic
i
section is exposed on the north side of Squaw Creek Canyon, in a
trib u ta ry ravine about 600 m up Squaw Creek from where the t r a il at
the top of the Pilgrim Limestone crosses Squaw Creek. (The head of
| th is t r a il is shown on the USG S Beartooth Butte 15' topographic
! quadrangle where the tr a il system crosses Squaw Creek at the foot of
| the north-facing slope near the base of the Cambrian section, but
| the part of the t r a il that crosses Squaw Creek at the top of the
1
i
| Pilgrim Limestone does not appear on the map.) In th is
|
i southward-draining ravine, the bedding fa u lt consists of a >2 m
thick (central part is concealed) layer of breccia that contains
clasts of both volcanic and carbonate rock. Thin sections of
j specimens from the basal meter of the fa u lt zone exhibit clasts of
!
j carbonate rock up to 5 m m across, widely varying textures (some are
! mosaics of rhombs, others contain quartz grains) and clasts of
volcanic rock (plagioclase porphyry). A subhorizontal striated
(2°S43E) shear zone is present cutting through the breccia, and the
top of the breccia is defined by a striated (9°N10W) fa u lt (N84W
9NE). Faulted volcanic rocks are exposed for less than a meter
i
[ above the fa u lt breccia zone. Excellent exposures of s tru c tu ra lly
|
i higher a llu v ia l volcanic breccias (N62E 9NW ) are present about 100
yards to the northeast (Fig. 34). The a llu v ia l strata are well
j bedded and appear unfaulted.
i
145 |
146
FIG URE 33. Stereographic projections of orientations
of upper-plate faults, striae, and dikes and of
lower-plate beds, between Onemile Creek and Squaw
Creek (Plate I Locality Q).
a. Faults (great circles) in volcanic rocks and
striae: (a) = striae on bedding fault; (o) =
striae on subhorizontal faults within 1 m of
base of volcanic allochthon; (o) = striae on
structurally higher faults.
b. Beds (great circles) of Snow y Range Formation
involved in lower-plate fold.
c. Upper-plate dikes (great circles), (o) =
striae on their margins.
FIGURE 34. C liffs of volcanic sediments along north side of
Squaw Creek. At the basal detachment fa u lt exposed about
50 m upstream, the base of the volcanic sequence is sheared
and overlies a tectonic breccia containing clasts of
carbonate and volcanic rock. View to north.
147
A fo ld , with 2.5 m amplitude and 5 m wavelength was observed in
the Snowy Range formation about 15 m down the ravine from th is
exposure of the bedding fa u lt. Bedding attitudes from the lower
plate fold (Fig. 33b) indicate a subhorizontal fold axis trending
about N60E. N o Bighorn Dolomite was observed in the lower-plate in
i
| th is area.
Igneous dikes occur within volcanic rock both upstream and
I downstream from the area of the bedding-fault exposure. The dikes
are subvertical and strike northwest, and some have striated margins
! (Fig. 33c). Northwest of the mouth of Squaw Creek Canyon the
j bedding fa u lt and superadjacent rocks are well exposed. Striae on
I
i
| the bedding plane fa u lt here trend northwest (Fig. 33a). Several
i
c la s tic dikes intruded into the volcanic rocks were observed. The
basal 30 m of volcanic rock is commonly porphyritic aphanitic bodies
(in tru sive rocks or flows) rather than breccias or a llu v ia l facies.
This basal zone is separated from the overlying a llu v ia l facies
volcanic rocks by inaccessible low-angle and high-angle fa u lts .
i
Inte rp re ta tio n . Despite the undisturbed appearance of the
bedded volcanic rocks exposed north and west of Squaw Creek, these
rocks are interpreted to be a l1ochthonous, being underlain by a zone
i
of fau lting and tectonic breccia. The kinematic patterns are
| d iffu se . Igneous dike orientations indicate NE-SW extension, while
i
I
fa u lt striae suggest N-S, NW-SE, and NNW-SSE extension, and
bedding-fault striae indicate NW -SE tra n sla tio n .
Cow Creek
Cow Creek flows southward into the North Fork of Crandall Creek
about 6 km west of the confluence of North Fork Crandal1 Creek and
Crandall Creek. Just north of the North Fork, Cow Creek crosses a
carbonate mass shown by Pierce and Nelson (1971) as a Heart Mountain
fa u lt block composed of Madison Limestone intruded by many vertical
northwest-striking igneous dikes (Plate I, lo c a lity R). This area
I is accessible via a pack t r a il and a network of cow paths whose
t
I
! tr a il head is 0.3 km north of the Sunlight Road bridge across
; Crandall Creek. South-facing c l i f f s bounding the North Fork Valley
j provide excellent exposures of the carbonate allochthon, but nearby
volcanic rocks are poorly exposed and no good exposures of the
bedding fa u lt were found.
In exposures of the carbonate mass, at elevations about 15 m
and more above the concealed bedding plane fa u lt horizon, many
fau lts and many igneous dikes are evident. The dikes commonly
strike northwestward; only a few orientations were measured (Fig.
35b). Dikes are very rarely cut by the many fa u lts observed, and
where faults cut dikes demonstrable offset (always with an apparent
i
I !
j normal component) is about 3 m or less. Dikes are usually tabular j
i
| subvertical bodies but occasional irre gula r shaped masses and sudden
I |
changes in orientation occur (Fig. 36). j
Orientations of observed faults and fa u lt striae are plotted as
Fig. 35a. A dominance of northerly strikes of fa u lts and easterly
| plunges of stria e is evident.
149
a. Faults and striae. b. Dikes. (*) represents 20-30 dikes.
FIGURE 35. Stereographic projections of orientations of upper-plate faults
(great circles), striae (o), and dikes (dashed great circles), C ow Creek
(Plate I Locality R).
FIGURE
in area
36. Dikes within al I ochtnonous Radi son Limestone
of Cow Creek (Plate I Locality R).
151
Interp reta tion. This a l 1ochthonous carbonate mass has
undergone east-west directed extension on no rth-striking normal
fa u lts . Igneous intru sio n , which probably postdated the major
extensional faulting event, involved E-W-to NE-SW-oriented extension
lo c a lly exceeding 20%.
West of B1acktail Creek
One-half to 1.1 km east of the Cow Creek carbonate mass a much
smaller carbonate mass crops out in steep south-facing c l i f f s (Plate
I, lo c a lity S). This carbonate mass, at about the same structural
level as the Cow Creek mass, contains re la tiv e ly few igneous dikes
but is more highly faulted. The bedding-plane fa u lt is poorly
exposed in one lo c a lity east of the carbonate mass, and the
surrounding volcanic rocks are largely covered by vegetation.
One portion of the carbonate mass is characterized by a series
of east-dipping normal fa u lts , ty p ic a lly l i s t r i c , truncating
west-dipping beds. Fault striae trend easterly. This simple
pattern is lo c a lly complicated by zones of northeast-striking
s trik e -s lip fa u lts . The fa u lt and striae pattern appears as Fig.
37a.
|
Attitudes of the three igneous dikes observed in th is carbonate j
mass are presented in Figure 37b. The two which strike j
i
north-northwest have s lig h tly sheared margins with striae sets |
plunging very gently north-northwest. The north-northeast strikin g
dike is intruded into a fa u lt zone. j
a. Faults and striae. b. Dikes and striae on dike margins.
FIGURE 37. Stereographic projections of orientations of upper-plate faults
(great circles), striae (o), and dikes (dashed great circles), west of
Blacktail Creek (Plate I Locality S).
Inte rp re ta tio n . The igneous dikes are l i t t l e deformed compared
to the country rock, indicating that dike intrusion and the NNW-SSE
oriented s trik e -s lip shearing of th e ir margins postdated the major
fau lting event. The e a rlie r period(s) of deformation involved
east-west extension and NNE-SSW s trik e -s lip motion. Cross-cutting
striae on one fa u lt indicate that NNE-trending striae predate
NNW-trending s trik e -s lip s tria e .
Hunter Peak
Although Hunter Peak (Plate I, lo c a lity T) is heavily forested,
bold c l i f f s of Bighorn Dolomite and Madison Limestone present
excellent exposures th a t, from views along the Sunlight Basin Road,
give no indication that the strata are a l1ochthonous. The bedding
fa u lt is exposed in at least one place beneath Hunter Peak, where
shattering and local brecciation of the Bighorn Dolomite are
apparent 20 m above the bedding fa u lt. No fa u lt striae were
observed.
Examination of outcrops of the Devonian and Mississippian
strata revealed the presence of several fa u lts , but the maximum
observed apparent normal offset of the subhorizontal strata was 2 m,
|
and the average about 30 cm. The fau lts commnly strike N-S ± 15°,
have steeply plunging d ip -s lip striae (Fig. 38a), and die out
down-dip. j
i
Many igneous dikes are exposed on the northeast-facing c l i f f s
of Hunter Peak. They usually occupy steep narrow re-entrants in the
c l i f f face and therefore are hidden from distant view. They strike j
154
^ \
1 M l
a. Faults and striae. b. Dikes and striae on dike margins.
FIG URE 38. Stereographic projections of orientations of upper-plate faults
(great circles), striae (o), and dikes (dashed great circles), Hunter Peak
(Plate I Locality T).
northwesterly, at shallow to moderate angles to the c l i f f face, and j
I
are usually subvertical (Fig. 38b). A few dikes are s lig h tly sheared
or have striated margins, and one is o ffse t about 1.5 m along a
subhorizontal striated transform fa u lt.
In te rp re ta tio n . The mass of carbonate rock at Hunter Peak is
a l 1ochthonous, despite its sparcity of internal deformation. It has
undergone minor extension, which both dikes and fa u lt striae
indicate to have been east-west to ENE-W SW oriented.
East of Lodgepole Creek
Volcanic rocks, mapped by Pierce and Nelson (1971) and Pierce
(1978) as Wapiti Formation in depositional contact with the bedding
fa u lt and by Prostka (1978) as "volcanics, undivided," are exposed
on moderately steep north-facing slopes between Lodgepole Creek and
Oliver Gulch (Plate I, lo c a lity U). Access to within a mile of
these exposures can be gained via the K-Z Ranch road by passenger
car. |
The upper plate is dominated by igneous dikes which lo c a lly j
i
I
constitute more than 50% of the outcrop area. Dikes are tabular, up j
to 10 m thick but usually about 3 m th ic k , and consistently strike j
N25W + 5° adn dip steeply southwest. Attitudes representative of j
I
over t h ir t y dikes observed appear as Fig. 39b. Most dikes appear j
j
unfaulted, but a gently southeast dipping fa u lt displays about 10 m
of apparent normal offset of several dikes (Fig. 40) . This fa u lt !
j
lie s roughly 30 m above the bedding fa u lt. !
The bedding fa u lt is lo c a lly well exposed but only one striated !
i
locale was observed, with s tria tio n s trending S41E. The Grove Creek j
156
\ l\
\'N 'i
a. Faults and striae. b. Dikes. (*) represents over 30 dikes.
FIG URE 39. Stereographic projections of orientations of upper-plate faults
(great circles), striae (o), and dikes (dashed great circles), east of
Lodgepole Creek (Plate I Locality U).
FIGURE 40. Dikes within volcanic rocks cut by low-angle fa u lt,
offset unknown. Dikes 1.5-2 m thick. East of Lodgepole Creek
(Plate I Locality U).
158
Limestone is continuously exposed in several areas, and no
lower-plate dikes were observed. The basal 6 to 15 meters of
upper-plate volcanic rock is talus covered, and the overlying c l i f f
| exposures commnly reveal networks of diffuse, inaccessible shear
|
| zones. Measured fault and striae attitudes appear in Fig. 39a.
| One clastic dike, striking N65W and containing carbonate
|
j material, was observed about 15 m above the bedding fault,
j Interpretation. The volcanic country rock of this terrane is
severly deformed, as indicated by a probable basal sheared zone up
to 15 m thick, mildly penetrative shearing evident in c l i f f faces,
| and a very high density of igneous dikes that reflect ENE oriented
| extension locally exceeding 50%. Fault information provides l i t t l e
! further insight, although a southeast translation direction is
indicated on the bedding fault.
Extension by igneous intrusion occurs only above the bedding
fault, requiring a l1ochthoneity of the superjacent volcanic country
t
j rock whether the dikes are laterally intruded or truncated by
faulting. N o direct evidence for a pre-intrusion phase of
deformation of the volcanic country rock was observed, but the
j i
j contrast in extent of deformation between the sheared country rock j
j |
| and the largely unfaulted dikes argues for a pre-intrusion phase of j
| faulting and for only minor post-or synintrusive faulting. Thus the j
! i
local field relationships lend no support to the idea that these j
1 !
volcanic rocks are depositionally in place. !
Between Oliver Gulch and Corral Creek
T ~ _ _ _ ^
! This area (Plate I, 1 Locality V), which contains the contact j
between the Cathedral Cliffs carbonate mass and the volcanic terrane
to the west, appears on a detailed map in Pierce (1963b, Plate I)
and in Pierce and Nelson (1971), both of which show the volcanic
rock to be Wapiti Formation (early basic breccia) in depositional
contact with the underlying masses of Madison Limestone and Bighorn
Dolomite and the subadjacent bedding fault. Prostka (1978) maps
these volcanic rocks as "volcanics, undivided."
In excellent exposures just west of the Cathedral Cliffs
upper-plate carbonate block, near-vertical c liffs of lower-plate
Grove Creek limestone and Bighorn dolomite are overlain by a 6 to 30
m thick layer iwth moderately steep rubbly slopes, above which stand
near-vertical c liffs of volcanic rock. The rubbly, moderatly steep
slopes are underlain by sheared volcanic rock penetrated by a
boxwork of carbonate clastic dikes (Fig. 41).
The base of the overlying c l i f f is commonly marked by a clastic
dike along a shear zone, and clastic dikes occasionally penetrate
into the cliff-forming volcanic rock (compare relationships j
|
northwest of Republic Mountain). At the base of the volcanic rocks, i
i
i
along the bedding fault, fault striae on a carbonate microbreccia j
!
layer plunge 2° S45E. Other striae on the bedding fault plunge j
!
j
0°S33E. Although shear zones in the volcanic rocks are commonly i
I
diffuse and inaccessible, several attitudes were measured and appear j
as Fig. 42a. |
i
The contact between the volcanic rocks and the upper-plate
carbonate rocks to the east is not well exposed, but its geometry,
!
inferred from rock distribution, is clearly a N-S-striking , !
160 |
BF
FIGURE 41. Network of carbonate c la s tic dikes w ithin volcanic
rocks overlying the bedding fa u lt, west of Corral Creek (Plate
I Locality V). View to west. BF = Heart Mountain bedding fa u lt.
a. Faults and striae (o) in volcanic
rocks, (q) = striae on margins of
clastic dikes; (a ) = striae on
bedding fault.
b. Faults and striae (o) in
Paleozoic rocks.
FIG URE 42. Stereographic projections of orientations of upper-plate faults
(great circles) and striae and of bedding-fault striae, between Oliver Gulch
and Corral Creek (Plate I Locality V).
west-dipping l i s t r i c fa u lt whose dip decreases downward from near !
vertical to approach parallelism with the bedding fa u lt (Fig. 43).
Fault-bounding carbonate and volcanic rocks are sheared, and clasticj
I
dikes of carbonate breccia appear above the shallow-dipping part of j
the contact within 15 m of the bedding fa u lt. j
i
Good exposures of the a l1ochthonous carbonate rocks within a
i
few hundred meters east of the volcanic rocks reveal a complex j
pattern of fa u ltin g represented in Figure 42b. Most faults dip at
moderate angles east or west, many being subparallel to the contact
with the volcanic rocks to the west. Most striae trend within 30°
of east or west. The dominant striae pattern is defined by W NW -ESE j
!
trends. j
i
In te rp re ta tio n . The intense shearing and faulting of the j
I
volcanic rocks indicates that they are a l1ochthonous and that many j
cla stic dikes w ithin th e ir base were intruded during th e ir j
emplacement. Distant views of these volcanic rock suggest moderate !
eastward dips of bedding that are truncated along faults within the j
i
volcanic rocks near the bedding fa u lt. The Madison Limestone block j
is also clea rly a l1ochthonous. The kinematic pattern suggested by j
faults and fa u lt striae in the a l1ochthonous rocks can be viewed as i
I
j
a composite of east-west extension in conjunction with synchronous
or sequential northwest-southwest extension and oblique s lip . j
Cathedral Cli ffs I
j
The tu rrets of Cathedral C liffs are perhaps the most imposing i
topographic feature on the south face of the Clarks Fork valley.
Mm
Tv
H eort Mountoi_n_____
chaotic
f aulti ng
network of
clastic dikes
shown in
Figure 41
FIGURE 43. A l1ochthonous Madison Limestone (Mm) and u n d iffe r
entiated volcanic rocks (Tv) overlying lower-plate rocks (€0)
along the bedding fa u lt between O liver Gulch and Corral Creek
(Plate I Locality V). View to south.
They form part of an a l1ochthonous carbonate block that extends fo r i
i
5 km along these steep north-facing slopes, with over 750 m of !
r e lie f, and extends another 5 km southward to the eastern slopes of j
Windy Mountain. j
The maps of Pierce (1963b) and Pierce and Nelson (1971) show j
some of the major fau lts and many of the igneous dikes that extend j
i
down to but not below the bedding fa u lt (see Pierce, 1963b, Plate |
I). Pierce (1963a, b) mapped several hundred feet of Cathedral
C liffs Formation (the type section) and several Reef Creek fa u lt
E
blocks of Madison Limestone as part of the allochthonous terrane, j
with most of the local volcanic rocks mapped as younger Wapiti
Formation (Pierce, 1978). Prostka (1978) maps all the volcanic
rocks overlying the Cathedral Cliffs-Windy Mountain carbonate block
i
as a l1ochthonous Lamar River-Cathedral C liffs Formation. During the j
present study a 1 km long east-central portion of the north-facing |
exposure (Plate I, lo c a lity W ) was examined and lo c a lly mapped in
d e ta il.
The geologic maps of Pierce (1963b) and Pierce and Nelson !
j
(1971) portray th is large carbonate block as a stratigraphic section j
that is largely undeformed except for (1) loss of part of the ,
Bighorn Dolomite at the bedding fa u lt, (2) two graben within which j
i
the Ordovician strata are absent, and (3) intrusion by many ;
i
no rth-striking dikes. The eastern margin of one of these graben was!
S
mapped in d e ta il; i t is exposed in the part of the c l i f f face that
bears S60E from the K-Z Ranch. The geologic map (Fig. 44) covers
an area of roughly 400 by 400 m with about 200 m of r e lie f. The
_ .165.
I « 6 0 m (2 0 0 )
30- 90
Tv
MO
Mm
Dj
Ob
QO
E XP L A N A T IO N
Quaternary surficial deposits
Tertiary dike
Tertiary volcanic rocks, undifferentiated
Paleozoic sedimentary rocks, undifferentiated
Madison Limestone
Jefferson Formation
Bighorn Dolomite
Lower plate Cambrian and Ordovician rocks
and Quaternary surficial deposits
47
34
Contact
Faults, dashed where approximately located
" Heart Mountain detachment fault
— — Upper-plate low-angle fault
' ■" Fault, showing direction of dip
—— Fault intruded by Tertiary dike
Fault, showing direction and plunge of striae
S trike and dip of beds
FIG U RE 44. Geologic m ap of steep north-trending ravine at eastern edge of graben (Plate I Locality W ) at Cathedral Cliffs.
attitudes of observed faults and striae in the graben area appear on
Figures 45a and b.
The geologic map indicates two periods of fa u lt a c tiv ity :
a fte r the f i r s t period of faulting dikes were intruded along fa u lts .
Subsequently some of these dikes were offset across other fa u lts .
The no rth-striking fa u lt in the center of the sketch map is the
eastern boundary of the graben. The orientations of striae on this
fa u lt are plotted on Fig. 45b. The fa u lt is inaccessible where i t
continues south beyond the carbonate terrane.
The east-striking fa u lt contact between Madison Limestone and
volcanic rocks east of the graben is shown on Fig. 46. From the
vantage point from which th is photomosaic was taken the fa u lt
appears to strike N85E with the dip overturning downward from 50SE
to 82NW. The fa u lt is inaccessible except at its highest area of
exposure, where strikes from N54E to N88E, with 50° to 69° SE dips,
were measured. South-southeast trending striae were observed (Fig.
45c).
Reconnaissance of the bedding fa u lt and adjacent rocks for
about 1.0 km to the east of the graben revealed numerous other
upper-plate faults (Fig. 46d). Orientations of upper-plate dikes
observed in and to the east of the graben are plotted in Figure 45e.
Several dikes east of the graben are bounded by discolored
(nearly white or very dark gray) shattered Bighorn Dolomite. One
such dike was traced down to within a few feet of the bedding fa u lt,
with no observed discoloration of lower-plate Bighorn Dolomite. The
pervasive shattering of the upper plate dolomite was not apparent
FIGURE 45. Stereographic projections of orientations of
upper-plate faults (great circles), striae (o), and
dikes (dashed great circles), Cathedral C liffs
(Plate I Locality W).
a. Faults and striae in Paleozoic rocks in
graben area.
b. Fault zone forming east margin of graben;
Paleozoic rocks in footwall, Paleozoic and
volcanic rocks in hanging wall.
c. Fault at contact between upper-plate
Paleozoic and volcanic rocks east of graben
(see Fig. 46).
d. Faults and striae in Paleozoic rocks east of
graben area.
e. Dikes, some with striated margins.
M ii\ W
' //j4'/vfy
i S 1 /1 / 'VV
k < h A
4 < : /f '-Vn
/ 1 IA a y i \
^ ' rA i J i\/
FIG U R E 45
FIGURE 46. View to east of fa u lt contact between allochthonous
Madison Limestone and volcanic rocks. Fault is exposed over
about 30 m of r e lie f .
170
within th is dike. Other dikes displayed sheared margins or internal
shear zones. Some of the dikes so well exposed in the western part
of Cathedral C liffs are shown on Fig. 47.
Interp reta tion. The upper-plate carbonate mass at Cathedral
C liffs is allochthonous. All observed contacts between volcanic
rocks and upper plate carbonate rocks are fa u lts , indicating that
the volcanic rocks are also allochthonous. None of the volcanic
rocks mapped as Wapiti Formation by Pierce (1963a, b; 1978) were
observed. Igneous dikes were intruded a fte r most of the upper-plate
fa u ltin g had occurred, but these dikes were lo c a lly involved in a
second phase of upper-plate fau lting and are te cto n ica lly truncated
at the bedding fa u lt.
The kinematics of the allochthonous carbonate blocks are
dominated by internal east-west oriented extension, as indicated by
fa u lt, fa u lt s tria e , and igneous dike orientations. The
graben-bounding fa u lt that appears on Figure 44, which has greater
demonstrable displacement than other observed fa u lts , re flects an
additional component of north-south oriented s trik e -s lip motion
(45b). The kinematics of the east-west s trik in g volcanic-carbonate
contact (Fig. 45c, 46) are unclear. The geometry of the contact
suggests dominantly s trik e -s lip motion, but the few observed striae
are steeply oblique (compare Pilot Creek fa u lt of Fig. 23, 24b).
East of Reef Creek
The volcanic rocks that overlie the bedding fa u lt between
Cathedral C liffs carbonate allochthon and the Sugarloaf Mountain
171
Tv
MO
BF
FIGURE 47. Igneous dikes in allochthonous Paleozoic rocks (MO)
and in overlying volcanic rocks (Tv) at Cathedral C liffs
(Plate I Locality W). View is to south of about 600 m of
r e lie f . BF = Heart Mountain bedding fa u lt. Contact between
M O and Tv is probably a south-dipping normal fa u lt (compare
Fig. 46).
172
carbonate mass to the east have been mapped by Pierce (1965), Pierce
and Nelson (1971), and Pierce (1978) as Wapiti formation
depositionally in place, whereas Prostka (1978) mapped these as
"volcanics, undivided." Good exposures of these volcanic rocks
occur in steep north-northeast facing slopes between Reef Creek and
Deadman Creek (Plate I, lo c a lity X). The bedding fa u lt is very
poorly exposed in the forested area downslope from the exposures of
volcanic rock.
The volcanic rocks consist of massive flows and flow breccias
up to several meters thick that dip 5 to 30° to the southwest.
Several northeast-striking dikes cut the volcanic sequence.
The strata are la te ra lly discontinuous with chaotic flow structures,
irre g u la r contacts and discontinuous intrusive (?) bodies. Several
northeast-striking dikes were observed (Fig. 48b).
Three throughgoing fa u lt zones were observed in the volcanic
rocks in th is area, although in most exposures fau lting was not
apparent in these possibly vent-facies rocks. The fau lts and stria e
observed are plotted in Figure 48a. A sketch map (Fig. 49) depicts
relationships in the area of these fa u lts. Southwest-dipping
volcanic strata are folded to gentle southerly dips in the hanging
wall of the westerly east-dipping fa u lt with steeply oblique s tria e .
Projected to the concealed bedding fa u lt about 10 meters below the
exposures, these volcanic units would be truncated along i t .
Displacement along the faults is not well constrained but probably
exceeds 10 m based upon a mismatch of units across the fa u lts .
a. Faults and striae b. Dikes
FIGURE 48. Stereographic projections of orientations of upper-plate faults
(great circles), striae (o), and dikes (dashed great circles), east of
Reef Creek (Plate I Locality X).
N
A
2\ /
Approx.
60m (200’)
_l
-€0
\
25
Tv
35^
E X P L A N A T I O N
Tv Tertiary volcanic rocks, u n d ifferen tia ted
-60 Lower p la te C a m b ria n and O rd o vician rocks
—. — — — — Heart Mountain bedding f a u l t , c o n c e a le d
+ *
58 62
S
Fault, showing dip of fa u lt and plu ng e of
striae (s), dashed where concealed
25
Dip and strike of beds
15
Approximate dip and strike of beds
FIGURE 49. Diagrammatic geologic map of NNE-facing slope
between Reef Creek and Deadman Creek (Plate I Locality X).
175
About 1 km southwest of these fa u lt exposures, on the
east side of Reef Creek, the bedding fa u lt is poorly exposed.
Ill-d e fin e d stria e on the base of the volcanic rocks in this area
plunge 1°S63E.
In te rp re ta tio n . Despite the apparent lack of tectonic
deformation of most exposures, the volcanic rocks in this area are
believed to be allochthonous. Three fa u lt zones display striae
indicating east-west extension of the volcanic terrane. Southwest
dips of the volcanic flows are unlikely to be in it ia l depositional
dips due to relationships depicted in Figure 49 and due to the
location of inferred (Smedes and Prostka, 1972) source areas to the
south of th is area.
East of Pai nter Gulch
Two large areas underlain by allochthonous carbonate rocks are
exposed in the Sunlight Creek drainage. One such area is exposed
along the lower fiv e miles of Sunlight Basin, and the other lies
roughly four miles farther upstream. Between these allochthonous
carbonate bodies, volcanic rocks, usually poorly exposed, overlie
the Heart Mountain bedding fa u lt. Nelson et al (1972) interpret
these volcanic rocks as Wapiti Formation, depositionally in place.
Pierce (1978) maps these volcanic rocks as dominantly Wapiti
Formation with lesser areas underlain by allochthonous
undifferentiated Lamar River and Cathedral C liffs Formations. S till
larger portions of the volcanic terrane are mapped by Nelson et a l .
(1980) and Prostka (1978) as allochthonous (at least in part)
undifferentiated Lamar River and Cathedral C liffs Formations.
For the present study a small area of well-exposed volcanic
rocks on the east side of Painter Gulch (Plate I, Locality Y) was
mapped in d e ta il. This area had been mapped by Nelson et a l . (1972)
and Pierce (1978) as Wapiti Formtion and by Prostka (1978) and
Nelson et a l. (1980) as undifferentiated Lamar River and Cathedral
C liffs Formations. The present mapping (Fig. 50) revealed an
inclined sequence of volcanic flows and breccias (Fig. 51a)
truncated downdip by a zone of fa u ltin g . A throughgoing l i s t r i c
fa u lt (Fig. 51b) separates the volcanic sequence from a dominantly
intrusive complex to the northwest. The fa u lt is exposed across 180
m and is inferred to extend beneath and across a total of 240 m of
r e lie f. Orientations of other faults observed in the volcanic
sequence are represented in Figure 51c.
Inte rp re ta tio n . The throughgoing fa u lt truncating the volcanic
sequence to the northwest is a dominantly d ip -s lip fa u lt, as
indicated by s tria tio n orientations (Fig. 51b). The dissim i1a rity
of units separated by th is fa u lt across more than 240 m of r e lie f
suggest more than 240 m of d ip -s lip (probably normal) movement along
the fa u lt. As the fa u lt is exposed within about 100 m v e rtic a lly of
the projected elevation of the Heart Mountain bedding fa u lt,
movement along the bedding fa u lt below the volcanic sequence is
indicated. The lower strata within the volcanic sequence are
probably truncated downdip by the bedding fa u lt. The direction of
movement of these allochthonous volcanic rocks is unknown, but
north-south oriented oblique extension is indicated by striae on
E X P L A N A T IO N
Quaternary surficial deposits
30 m
60
37
Tertiary dike
Tvi
40 82
54 J
i i I I w I J I i w n j , * u i ^ v w i u ^ !
and epiclastic volcanic rocks (Tv); massive
breccias and porphyritic intrusive rocks (Tvi)
30
26-1
0 € Cambrian and Ordovician lower plate rocks
20
Contact
Faults, dotted where concealed
Heart Mountain detachment fault
26
Fault, showing direction of dip
70
Strike and dip of beds
0€
Approximate strike and dip of beds
FIGURE 50. Geologic m ap of parts of sections 7 and 8, T55N R105W, east of
Painter Gulch (Plate I Locality Y). Compare^Pierce and Nelson (1971),
Nelson et al (1972, Fig. 3).
a.
FIG U R E 51. Stereographic projections of orientations
of upper-plate faults (great circles), striae (o),
and poles to primary stratification (flows, breccias
(o), east of Painter Gulch (Plate I Locality Y).
a. Poles to primary stratification of volcanic
rocks.
b. Throughgoing fault truncating stratification
of volcanic rocks, with striae.
c. Faults and striae within volcanic rocks.
faults within the volcanic sequence. The observed east-west
s trikin g vertical igneous dike is compatible with north-south
extension. The sequence of flows moved eastward with respect to the
la te ra lly adjacent intrusive complex to the northwest.
Sugarloaf Mountai n
Pierce's (1965) map (see Fig. 52) shows the Sugarloaf Mountain
carbonate block (Plate I, lo c a lity Z) to consist of the Ordovician
through Mississippian carbonate sequence cut by several high-angle
faults and abutted on the west along a north-trending contact
by early basic breccia (Wapiti Formation). The southeastern part of
the carbonate mass is separated from Sugarloaf Mountain per se by a
small mass of early basic breccia that extends to the concealed
bedding fa u lt.
Several areas of the Sugarloaf Mountain carbonate block were
visite d for the present study. The north-south trending western
margin of the block proved to be so heavily forested that the
contact with volcanic rock is nowhere exposed. The adjacent
carbonate rocks are also forested, and the few good exposures
revealed v ir tu a lly no kinimatic data. The southeast portion of the
block, including the small mass of early basic breccia, yielded
abundant kinematic data. It has been subdivided into domains based
on lith o lo g ic and kinematic differences. The domains are indexed on
Figure 52, and fa u lt and striae orientations are plotted on Figure
53.
The mass of volcanic rock that overlies a 0.2 km width of the
bedding fa u lt is adjoined to the northwest and southeast by
(M o d ifie d from
Pi erce, 1965 )
109 ° 3 0
4 4 ° 50*
Fig. 5 3a
Fig. 53 b
/Fi g.53 c
I ' 6 2 , 5 0 0
ugarl
^ .T ^ rF i g. 5 3 e
• /
EXPLANATION
Heart Mountain bedding fault, teeth on upper plate
Fault
Depositional contact
Contact, nature unknown
(faults within allochthonous carbonate and volcanic
rocks are not shown )
FIGURE 52. Locality map fo r Sugarloaf Mountain
(Plate I Locality Z).
181
FIGURE 53. Stereographic projections of orientations of
upper-plate fa u lts (great c irc le s ) and stria e (o) in
areas of Sugarloaf Mountain indexed on Fig. 52.
a. Faults and s tria e in Paleozoic rocks northwest
of graben.
b. Faults and s tria e in volcanic rocks of graben.
c. Faults and s tria e in Paleozoic rocks southeast
of graben.
d. Faults and stria e in Paleozoic rocks of
northeast-facing slopes at east end of mountain.
e. Faults and s tria e in Paleozoic rocks of
southeast-facing slopes at east end of mountain.
FIG U R E 53
carbonate rocks. Outcrops occur in discontinuous exposures 30 to
150 m above the bedding fa u lt. The carbonate rocks to the northwest
and within 0.3 km of the volcanic rocks are cut by a series of
faults with small (usually less than 3 m) normal displacement (Fig.
53a). Bedding in these rocks is subhorizontal. The
carbonate-volcanic contact is not exposed. The volcanic rocks,
which are nowhere wel1-exposed, also contain several fau lts (Fig.
53b), but the offset across these faults could not be determined.
Probable bedding in the volcanic rocks dips 40° southwest. The
carbonate rocks southeast of the volcanic rocks are highly deformed
by shattering and brecciation, with the several faults observed
plotted as Figure 53c. Bedding is obscure. Again, the
volcanic-carbonate contact is not exposed.
i
Farther southeast is an area of excellent exposures along steep ;
northeast-facing c l i f f s . Numerous well developed faults (Fig. 53d)
occur along these c l i f f s , with several tens of meters of apparent
normal displacement. These fa u lts , in some instances, f i r s t fla tte n j
and then steepen downward (Fig. 54). Bedding is consistently
subhorizontal but lo c a lly dips up to 25° to the southwest. The !
i
bedding fa u lt is concealed a few tens of meters below the exposures.
The southeast corner of the Sugarloaf carbonate mass includes
discontinuous exposures along and above the bedding fa u lt. The
consistently northwest-striking faults observed in th is area are j
represented in Figure 53e. Beds dip 20 to 30° southwest and are cut
by usually northeast-dipping faults with occasional bedding-plane !
fa u lts . I
184 i
FIGURE 54. View upslope to south of high-angle fa u lt
steepening downward, Sugarloaf Mountain (Fig. 52, area of
Fig. 53d). Person fo r scale. Photo by Linda Thurn.
185
N o igneous dikes were observed within the carbonate rocks of
the Sugarloaf Mountain carbonate mass. One vertical dike striking
N8E was observed in the poorly exposed volcanic rocks just west of
the carbonate mass.
Interpretation. The volcanic rocks are in fault contact
with the carbonate rocks to the northwest and southeast, forming a
graben within the al 1 ochthonous carbonate block. This is suggested
by:
1) the internal faulting of the volcanic rocks which includes
indications of east-west extension (Fig. 53b);
2) the pattern of southeast-dipping faults with east-plunging
slip directions (Fig. 53a) in adjacent carbonate rocks
(these faults may parallel the concealed graben-bounding
fault contact);
3) the highly cataclasized nature of the carbonate rocks
adjacent to the southeast.
The overall kinematic pattern of the Sugarloaf Mountain
allochthon is dominated by east-west oriented oblique slip. Striae
of Fig. 53d, in contrast, displays dominantly southeast to
south-southwest plunging slip vectors. In general, east-west
extension locally complicated by NNW -SSE slip is indicated.
Steamboat
The hill known as "Steamboat" lies about 0.8 km south of
Sunlight Creek between Elk and Dead Indian Creeks (Plate I, locality
AA). Geologic maps of the area appear in part in Pierce (1965) and
Pierce and Nelson (1968), where Steamboat is shown to consist of a
500' thickness of intensely faulted carbonate rocks overlying the
Heart Mountain bedding fault. Pierce (1957, Fig. 4) shows a
cross-section of Steamboat.
For the present study the eastern side of Steamboat was
examined. Bedding dips gently to moderately southward to
south-southwestward in almost all areas (Figure 55a). A
cross-section showing many of the major faults observed appears as
Figure 56. It shows south-dipping beds at the north end of
Steamboat truncated downward along a lis tric normal fault. In other
areas faults are steeper and beds are less rotated.
Fault and striae geometries are present in Figures 57a, b and c !
representing three separate geographic areas in order that no single
stereographic plot be too crowded. All striae are presented
together in Figure 57d.
Several igneous dikes were observed, commonly along faults and
with sheared margins. Dike orientations appear as Figure 55b.
Interpretation. Fault striae orientations observed at
Steamboat constitute a diffuse pattern that forms a broad band
trending NNE-SSW (Fig. 57d). N o simple pattern of slip directions
is evident, as striae were observed to plunge in all directions.
Bedding and dike orientations present a more consistent pattern.
Dominantly west-northwest striking igneous dikes together suggest
NNE-SSW extension. The dikes commonly display minor internal
faulting and sheared margins and are sometimes offset across faults, |
187 J
0°^
00
a. Poles to bedding.
b. Dikes.
FIG URE 55. Stereographic projections of orientations of upper-plate dikes
(dashed great circles) and poles to bedding (o), Steamboat (Plate I Locality AA).
f \ I
dding
Heort Moun
Pilgrim Limestone
FIGURE 56. View to west of Steamboat (Plate I Locality AA).
Line-drawing shows bedding (light lines), faults (heavy
lines), and dikes (hachured lines).
FIGURE 57. Stereographic projections of orientations of
upper-plate faults (great circles) and striae (o ), east
side of Steamboat (Plate I Locality AA).
a. Faults and striae, northern 1/3 of east side.
b. Faults and striae, central 1/3 of east side.
c. Faults and striae, southern 1/3 of east side
and south end.
d. Summary, all observed striae.
190
FIG U R E 57
1 9 1
indicating that they predate final fault movement. Steamboat as a
whole appears to have undergone extension in varied directions, with
NNE-SSW extension clearly dominanting.
East of Dead Indian Creek
Mapping by Pierce and Nelson (1968) shows a l1ochthonous blocks
of Paleozoic carbonate rocks on both sides of Elk Creek and Dead
Indian Creek. Prostka (1978) suggested that before erosion these
were probably a continuous a l1ochthonous block. Pierce (1973)
viewed these carbonate masses as formerly more widely separated,
believing that "scattered blocks that were moving along the bedding
fault were retarded [in their movement] as they started moving up
along the transgressive fault [which presumably sloped to the
northwest, opposite the inferred direction of movement]" p. 462.
Pierce and Nelson's (1968) mapping shows the basal detachment fault
cutting upward through the Ordovician and younger strata in Sections
16 and 17 of T55N, R105W (about 1.6 km ESE of Steamboat) and about
8.0 km to the south along Dead Indian Creek. This is the area
described by Pierce (1957) as the transition from the "bedding
thrust" to the "shear thrust" ("bedding fault" and "transgressive
fault" in current terminology —Pierce, 1960).
For the present study a small portion of this large carbonate
block, on the northwest-facing slopes of Sections 17 and 20, T55N,
R105W, was studied (Plate I, Area BB). The northeastern part of the
area is underlain by the transgressive fa u lt, and the southwestern
part is underlain by the easternmost edge of the bedding fault.
Orientations of faults and striae observed in these areas are
presented as Figure 58a and b. One vertical igneous dike striking
N80E was observed in the southern part of the area (Fig. 58d).
The basal detachment fault in this area, where it begins to cut
upward across bedding, is not a discrete slip surface, but rather is
a zone in which the Bighorn Dolomite is complexly and
discontinuously deformed by brecciation, closely-spaced jointing,
and faulting. N o wel1-developed faults occur in this zone, and no
striae were observed.
Interpretation. The kinematic pattern suggested by fault
striae for the areas overlying the bedding fault and transgressive
fault in this area are similar: both are diffuse and both indicate
NE-SW to ENE-W SW slip directions analagous to Steamboat H ill. The
striae orientations for the entire area are plotted together on
Figure 58c.
Summary
Two immediate goals of this study were to characterize the
kinematics of deformation of the rock overlying the Heart Mountain
bedding fault and to assess the involvement of volcanic rocks in
Heart Mountain faulting.
Ki nematics: Bedding Fault. Plate I I I summarizes the trends of
fault striae observed on the bedding fault. All observed striae
occur where volcanic rocks overlie the bedding fault; despite
careful searching in many areas, no bedding fault striae were found
where carbonate rocks overlie the bedding fault.
193
FIGURE 58. Stereographic projections of orientations of
upper-plate faults (great circles), striae (o), and dike
(dashed great c irc le ), east of Dead Indian Creek
(Plate I Locality BB).
a. Faults and striae, Paleozoic rocks overlying
transgressive fault.
b. Faults and striae, Paleozoic rocks overlying
bedding fault.
c. Summary, a ll observed striae.
d. Dike.
1 94
OO o
The striae reveal a pattern dominated by diffuse, generally
east-west trends from the break-away fault to northeast of Index
Creek (Plate I, locality J) and by northwest-southeast trends from
Jim Smith Creek to Reef Creek. Between Republic Creek and Onemile
Creek multiple trends were observed at several localities.
Wherever striae were observed along the bedding fault, the
overlying volcanic rocks were judged to be allochthonous due to:
faults observed within the volcanic terrane; shearing and shattering
of the base of the volcanic terrane; fault breccia containing
fragments of volcanic rocks and carbonate rocks observed
mesoscopically or in thin section; striae on the bedding fault;
rotated stratification; truncated stratification; or some
combination of these. The bedding fault striae are interpreted as
indicating the direction of late or final phase(s) of movement of
upper-plate rock. Multiphase movement in varying directions is
indicated at least locally. Clastic dikes were intruded during
faulting and their contacts are sometimes faults that do not appear
to offset the underlying Heart Mountain bedding fault.
i
Kinematics: Internal Deformation of Allochthon. Orientations
of faults, fault striae, rotated bedding, and igneous dikes
constitute upper plate kinimatic informtion. Igneous dikes, usually j
subvertical, are interpreted to indicate extension of the allochthon
perpendicular to the strike of the dikes. Rotated beds truncated
downdip by normal faults indicate a component of extension of the
upper plate perpendicular to the strike of the beds. The trend of
striae with a dip-slip component of motion gives the direction of
196
extension of the allochthon; striae trending subparallel to strike
give directions of nonextensional differential movement within the
allochthon (with respect to the subhorizontal basal detachment
surface).
Directions of extensional and nonextensional movement withni
the allochthon do not necessarily indicate the directions of motion
of the allochthon with respect to the autochthon. Trends of striae
on upper plate faults probably reflect significant components of the
direction of movement of the allochthon with respect to the
autochthon.
Several generalizations can be drawn from a comparison of the
kinematic data from the various localities (see Table 1 and the
Figures referenced therein):
- data from within volcanic rocks are less abundant
and more diffusely distributed than data from within
carbonate rocks;
- well-defined kinematic patterns arise from the kinematic
data from carbonate terranes:
i
- faults within carbonate rocks tend to be north- j
i
striking j
- localities are usually character!’zed by a
i
predominance of dip-slip striae (trending j
E-W), sometimes with less com m on strike- |
slip striae (trending N-S) are predominant
- where faults are variably oriented, fault
striae commonly indicate E-W oblique slip
197 '
PALEOZOIC CARBONATE ROCKS EOCENE VO LC ANIC ROCKS
Localities
NW to SE
CONTACT
DDF SS DS JLTi DS SS _LTi Ref. Fig.
B re a k -a w a y fa u lt
SW of S ilver Gate
S of S ilv e r Gate
Falls Creek
Northwestern
Republic Mt. ( I )
(D)
Republic Creek Area
2 0 a S of C olter Pass(W)
2 0 b S of Colter Pass (E)
NE of Index Peak
22 Fox C r e e k
2 4 P ilo t C re e k
SS
2 9 J im S m ith Creek
SW of B - 4 Ranch
O n e m ile C re e k 3 2
One m ile C re e k to
Squaw C re ek
3 3
3 5 Cow C re ek
West of
B la c k ta il Creek
3 7
3 8 Hunter Peak
East of
L o dg epo le Creek
3 9
Between Oliver Gulch
and Corral C re e k (— )
4 2
C a th e d r a l C lif f s 4 5
E a s t of
Reef Creek
4 8
East of
Pa i n t e r Gulch
(/)
51
S u g a r l o a f M t n . 5 3
5 5 , 5 7 S t e a m b o a t
East of Dead
In d ian Creek
5 8
H eart M o u n ta in 6 0
L o g a n M o u n ta in
6 3
BDF = Trends of bedding f a u l t s t r ia e
SS = D o m in a n t tren d of s t r i k e - s l i p u p p e r-p la te f a u l t s t r ia e
DS = D om ina nt tren d of d i p - s l i p u p p e r -p la t e f a u l t s t r i a e
JLTi = T re n d of pole to planar igneous dikes
Trend of s t r i a e a lo n g f a u l t ( s ) at c o n ta c t b e tw e e n upper-
= p |a t e c a r b o n a te and v o lc a n ic rock (ss= s t rik e slip)
Ref. Fig. = F ig u r e s from which i n t e r p r e t a t i o n s w e r e d r a w n
D = D i f f u s e p a tte rn of s t r ia e
( ) = S m a ll s a m p le
TABLE 1. Summary of in te rp re ta tio n s of dominant kinematic patterns fo r upper-plate rocks of
the Heart Mountain Fault. North at top of table so that plots have geographic significance.
- igneous dikes consistently indicate E-W extension
- s lip direction on faults at contacts between upper plate
volcanic and carbonate terranes are variously oriented.
The overall pattern is one of dominantly E-W extension of the
upper-plate carbonate blocks interrupted lo ca lly by zones of
N-S, or rarely E-W, strike s lip motion. The intervening volcanic
rocka are extended and otherwise faulted with a less systematic
pattern.
Kinematics: Interp reta tion. The a l1ochthonous carbonate
rocks, which for the most part remained at the same structural
levels during fa u ltin g , display more consistent movement patterns
than do the a l1ochthonous volcanic rocks. This may be due to the
volcanic terranes having been s tru c tu ra lly lowered greater distances
during extension of the upper plate, resulting in more complex
movement patterns. However, in te n sity of deformation does not seem
to be a consistent or re lia ble indication of amount of structural
lowering: there exist several areas of exposure of largely
undeformed carbonate blocks from which basal Ordovician or
Ordovician and Devonian strata are missing (P ilot Creek, Crandall
Creek, Sunlight Creek). Sim ilarly, a l1ochthonous volcanic strata
overlying the bedding fa u lt may have been s tru c tu ra lly lowered
during detachment fa u ltin g , without necessarily undergoing
mesoscopically apparent internal deformation.
Involvement of Volcanic Rocks. More than a dozen lo c a litie s
where the bedding fa u lt is overlain by volcanic rocks were examined
during the present study. Some of these lo c a litie s represent
bedding fau lt segments exceeding 1 km in length. The lo c a litie s
span the entire length of exposure of the bedding fa u lt, from the
break-away fa u lt to the Deadman Creek area and in Sunlight Basin.
In all lo c a litie s where good exposures were found the contact is
demonstrably tectonic. In a few areas poor exposures prevent
collection of conclusive information.
In these areas, as well as in some areas where the fa u lt
horizon is not exposed, the volcanic rocks were examined for a few
meters to several tens or hundreds of meters upward from the bedding
fa u lt. Occasionally the volcanic rock was lo c a lly undeformed, but
usually varying degrees of shattering, fa u ltin g , and rotation of
bedding occurs, with occasional swarms of cla s tic or igneous dikes.
Therefore, i t is concluded from direct observation that the
volcanic rocks overlying the bedding fa u lt for thicknesses of meters
to tens of meters or more are a l1ochthonous. From distant views,
and by comparison with the mapping of Pierce et al (1973) and Nelson
et al (1980), i t is concluded that thicknesses exceeding 700 m of
volcanic rocks are a l1ochthonous in the areas of P ilot Peak and
Squaw Peak. It is inferred that comparable thicknesses of volcanic
rocks may be a l1ochthonous thorughout the detachment terrane, but
th is inference has yet to be tested by fie ld mapping.
Arguments regarding the geometry of the break-away fa u lt (see
Break-away Fault, Summary, above) suggest that a l1ochthonous
carbonate strata reach lower positions by descending one or more
L-shaped steps to the bedding fa u lt horizon. Such a geometry would
allow allochthonous volcanic rocks to be s tru c tu ra lly lowered in a
A’
I09°45'
S of Soda
Bu 11 e
^ Creek
Republic
Mtn. Republic
Creek
Mm
MDtj
Ob
shattered OtK p€0
B B'
Cache
Creek
Pilot Creek
concealed
Mm
Mm
Mm MDtj
MDtj
Ob
p€0 MDtj
City
2 0 KM
Republic
Mountain 10 Ml
4 3 ° 0 0 '
\ p € 0
p€0
Ru»»tl
Pk.«r-
White
INDEX MAP
C '
Cathedral Cliffs
Crandall
Creek
concealed
Reef
Creek
Corral
Creek
Sugarloaf Lodgepole
Creek
Oliver
Gulch
Mm Mm Mm Mm
' M m '
Mm
MDtj MDtj MDtj MDtj
Mm Ob Ob Ob Ob
Symbols on Map
Eocene volcanic and intrusive rocks
Mm
MDtj
(on map)
Ob
p £ 0
Madison Limestone
Jefferson and Three Forks Formations
Bighorn Dolomite
Lower plate rocks of Precombrian,
Cambrian, and Ordovician ages
(Surficiol deposits not shown)
Heart Mountain fault,
barbs on upper plate
Break-away foult
Fault and depositional contacts
in upper plate of Heart Mountain
detachment fault
FIG UR E 59. Diagrammatic E -W cross-sections showing observed (A-A1) and inferred (B-B1, C-C1) geometries of the break-away fault. Cross-section C-C'
extends east of the break-away area to show the horst-and-graben structure that characterizes the upper plate. Graben-bounding faults are oblique-
normal, som e probably dominantly stike-slip. Cross-sections which are vertically exaggerated and not drawn to scale are based on this study and
reinterpretations of Pierce (1965), Pierce and Nelson (1971), and Pierce et al (1973).
m
sim ilar fashion and to remain lo ca lly in te rn a lly undeformed like
some analogous carbonate blocks. Some re la tiv e ly undeformed blocks
of a l1ochthonous volcanic rocks may have been i n i t i a l l y deposited in
re la tiv e ly low structural positions and therefore have not been
greatly lowered s tru c tu ra lly (see Figure 59).
The Heart Mountai n Fault on Former Land Surface
General Statement
The f i r s t recognition of the Heart Mountain fa u lt was at Heart
Mountain i t s e lf (Plate I, Locality CC), where early Paleozoic marine
strata rest upon Eocene sedimentary rocks (Eldridge, 1894).
Similar, older-over-younger relationships occur at Logan Mountain,
Sheep Mountain, and McCulloch Peaks. The history of the recognition
of these terranes as part of the same fau lting phenomenon is
presented in the "Previous Work" section.
At Heart Mountain no volcanic or igneous intrusive rocks have
been recognized. Volcanic rocks at Logan Mountain are shown by
Pierce and Nelson (1968) and by Pierce (1978) as Wapiti Formation,
depositionally in place, while Prostka (1978) and Nelson et al
(1980) show some of these same rocks as a l1ochthonous Lamar River
Formation. The volcanic rocks at Sheep Mountain are shown on all
published maps (Pierce and Nelson, 1969; Pierce, 1978; Prostka,
1978) as Wapiti Formation depositionally in place, although Hares
(1933) reports faulting of these volcanic rocks. Rouse (1937)
reported the presence at McCulloch Peak of "a small patch of loose,
angular volcanic fragments... that may indicate former extensions of
the volcanic series to th is peak" (p. 1294).
202
The present study included examinations of the a l1ochthonous
carbonate block at Heart Mountain and of the a l1ochthonous carbonate
blocks and overlying volcanic rocks at the south end of Logan
Mountain.
Heart Mountain
The Heart Mountain klippe is remarkably l i t t l e deformed.
Pierce (1966) correctly shows two major faults cutting the klippe;
no exposures of these fa u lt planes were found. The other faults
observed in the klippe are smal1-dispiacement (a few meters at most)
features, with the possible exception of a gently southwest-dipping
fa u lt near the base of the klippe within the Bighorn dolomite.
Striae and mull ions on a shear within a 30 cm thick breccia zone are
oriented S10E, 11°. Fault, s tria e , and bedding attitudes appear in
Figure 60.
The basal detachment fa u lt is poorly exposed on the north side
of Heart Mountain. Hand trenching revealed closely jointed Devonian
limestone overlying clayey s i l t , sand, pebbles, and clay of the
Eocene Willwood Formation. The orientation of the contact changes
from N60W 20SW to N25W 44SW within a few feet. The dominant
upper-plate jo in t set was oriented N-S, 35 to 55W. The exposure of
fria b le Willwood Formation was inadequate to assess the nature of
any deformational fab ric. Although faulted and crushed quartzite
pebbles in the Willwood Formation at the base of the Heart Mountain
allochthon were reported by Hares (1933) no such deformation was
observed during reconaissance fore th is study. A poorly developed
lineation of unknown origin and oriented N55W 20° appears on the
a. Faults (great circles) and striae (o)
(a ) = poorly-defined 1ineation on Heart
Mountain Fault.
b. Poles to bedding
FIG U R E 60. Stereographic projections of orientations of upper-plate faults,
striae, and poles to bedding, Heart Mountain (Plate I Locality CC).
base of the allochthon at one lo c a lity .
Interp reta tion. No clear-cut pattern is evident from upper
plate kinematic indicators. The diffuse pattern of fa u lt striae
orientation indicates oblique extension oriented dominantly NNE-SSW
to NW-SE, but due to poor exposure the major faults of the klippe
are not represented in these data. Northeast dips of beds suggest
NE-SW extension.
South End of Logan Mountai n
Generalized geologic mapping of Logan Mountain appears on
Pierce and Nelson (1968, 1969). At the south end of Logan Mountain
(sections 2, 3, 10, 11, T52N, R104W; Plate I Locality DD) in te rn a lly
faulted a l 1ochthonous Paleozoic rocks are shown as overlain by in
situ volcanic rocks of the Wapiti Formation. Nelson et al (1980)
and Prostka (1978) map these volcanic rocks as undifferentiated
Lamar River and Cathedral C liffs Formations.
The present mapping of the southwest corner of Logan Mountain
(Fig. 61) reveals a series of southeast-dipping normal faults that
offset and rotate both Paleozoic sedimentary and Tertiary volcanic
rocks. Orientations of faults and striae observed in the map area
of Figure 61 are plotted in Figure 63a. Figure 62 is a photographic
cross-sectional view of the mapped area. The Paleozoic rocks
exposed within 2 km to the east of the area shown in Figure 62 are
cut by many fa u lts , represented in Figures 63b and 63c. Figure 63c
presents geometric data for a major l i s t r i c normal fa u lt that is
exposed across more than 90 m of r e lie f. It juxtaposes Madison
Limestone against Jefferson and Three Forks Formation, and i t
205
7000
6000
Mm
27
67
3 0 0 M (1000')
33
1 3 \/
2 4 ,
67
\Mm
MDtj
30
Ob
I M T n
Qls
Twi
Kc
EXPLANATION
QUATERNARY Qls
EOCENE
Tv
Twi
CRETACEOUS
MISSISSIPPIAN
DEVONIA
ORDOVICI A
{
{
- {
■{
Kc
Mm
MDtj
Ob
24
Landslide deposits
Volcanic rocks
Willwood Formation
Cody Shale
Madison Limestone
Three Forks and Jefferson Formations
Bighorn Dolomite
Contact
Fault, showing direction of dip,
dashed where approximately located,
dotted where concealed or inferred
Heart Mountain fault on former land
A * surface, concealed. Orientation and
elevation shown in cross section
are not well constrained.
Strike and dip of inclined beds
Horizontal beds
References1 Pierce and Nelson (1 968,1969)
FIG U RE 61. Geologic m ap and cross-section of an area east]
of Trout Creek (Plate I Locality D D )
2 0 6
FIGURE 62. Views of area mapped in Fig. 61.
a. View to northeast across Trout Creek from
Highway US 14-20 west of Cody.
b. View to east showing detail of Fig. 62a
from closer vantage point. Faults truncating
Paleozoic and volcanic strata are shown.
207
FIGURE 62b.
208
FIG U R E 63. Stereographic projections of orientations
of upper-plate faults (great circles) and striae at
south end of Logan Mountain (Plate I Locality DD).
a. Faults and striae, area of Figs. 61 and 62.
(o) = striae on faults in Paleozoic rocks;
(□) = striae on faults in volcanic rocks;
(0) = striae on fault contact between Paleozoic
and volcanic rocks.
b. Faults and striae in Paleozoic rocks 1 to 2 k m
east of area of Figs. 61 and 62.
c. Major fault zone in Paleozoic rocks in area of
Fig. 63b.
appears to fla tte n into a bedding fa u lt near the top of the Bighorn
Dolomite. The volcanic rocks overlying these faulted sedimentary
rocks are concealed beneath vegetation and slopewash.
Interp reta tion. The major faults that cut both the volcanic
and sedimentary sequences merge with the Heart Mountain fa u lt on
former land surface, which is concealed a few meters to tens of
meters beneath present exposures. The volcanic rocks appear to have
overlain the Paleozoic sedimentary rocks before Heart Mountain
fa u lti ng.
Fault striae present a diffuse kinematic pattern marked by
NNE-SSW s tr ik - s lip motion and oblique extension. Elements of east-,
southeast-, and south-directed oblique extension also appear.
Northerly dips of carbonate and volcanic rocks indicate a component
of N-S extension in the area of Figure 60.
210
CONCLUSIONS
Involvement of Volcanic Rocks; Lack of Tectonic Denudation
Contrary to the mapping of previous workers, no volcanic rocks
are known to be in depositional contact with the lower plate in the
area of the Heart Mountain bedding-plane detachment fa u lt. In a ll
areas where volcanic rocks were observed in contact with the lower
plate they are demonstrably fault-emplaced. It is concluded th a t,
wherever upper-plate volcanic rocks are in contact with the lower
plate, the volcanic rocks are a l1ochthonous. The a l1ochthonous
nature of the volcanic rocks is also indicated by the contacts
between volcanic rocks and underlying upper-plate Paleozoic rocks.
In most areas observed these contacts are faults or faulted
unconformities, not depositional contacts as indicated by previous
workers. Brecciation, shattering, internal fa u ltin g , and rotation
of volcanic rocks were commonly observed.
Therefore, no direct evidence exists to support the contention
of e a rlie r workers that Heart Mountain fau lting was accompanied by
tectonic denudation of the bedding fa u lt. The absence of erosion
down across the supposed surface of tectonic denudation was
previously explained by very fast distension and displacement of
upper-plate blocks followed by rapid blanketing of a 1300+ km^ area
with volcanic rocks before erosion could occur. The observed
absence of erosion is more easily explained i f tectonic denudation
did not occur.
Two stream channels occur in the lower plate in the area of the
bedding fa u lt. The channel occupied by the Crandall Conglomerate
has been cited as evidence fo r tectonic denudation during a
preliminary phase of Heart Mountain fau lting (Pierce and Nelson,
1973). The previously unreported lower-plate channel at Fox Creek
could sim ila rly be cited as indicating tectonic denudation.
However, channel deposits in both areas are overlain te cto n ica lly by
a l1ochthonous Paleozoic rocks, and both channels are more simply
explained as predating Heart Mountain fa u ltin g . The Blacktail fold
probably formed coevally with formation of the Crandall Channel in
response to shortening across nearby Laramide basement fau lts (c f.
Voight, 1974b). It is concluded that tectonic denudation did not
occur during Heart Mountain fa u ltin g .
I f tectonic denudation of the bedding fa u lt did not occur,
continuity of the upper plate must have been maintained during
extension of the upper plate in the area of the bedding fa u lt.
Therefore gravity sliding of numerous isolated, detached blocks did
not occur in this area. Rather, spreading of a continuous rock mass
by (Paleozoic plus Tertiary components) normal fa u ltin g , local
brecciation, and lateral intrusion of igneous dikes is indicated.
Volcanic rocks must have been downfaulted from higher structural
levels to come to rest on the bedding fa u lt. Some volcanic rocks in
areas west of Hunter Peak were probably emplaced from areas to the
north where the Heart Mountain fa u lt has been removed by erosion.
During spreading, the volume of the allochthon increased as volcanic
rocks were episodically deposited upon the distending terrane and
were subsequently involved in upper-plate fa u ltin g ; the area of the
allochthon also grew with time as a consequence of lateral
spreadi ng.
Kinematics of Heart Mountain Faulting
Kinematic indicators suggest that movement of a l1ochthonous
rocks was dominantly to the southeast in the eastern part of the
bedding-plane detachment but was variably oriented in the western
part of the bedding-plane detachment. Striae on the bedding fa u lt
(Plate I I I ) suggest th is pattern for the fina l phases of movement of
volcanic rocks, and the Paleozoic rocks may have shared th is pattern
of movement. Previous workers inferred an overall southeastward
transport direction for a l1ochthonous Paleozoic rocks from:
a) the occurrence of a l1ochthonous Paleozoic rocks in
the Bighorn Basin and in the Shoshone River drainage,
southeast of th e ir supposed source area (the area of
the bedding fa u lt) (Pierce, 1957); and
b) the d is trib u tio n of the Crandall Conglomerate, since most
allochthonous masses of Crandall Conglomerate occur j
southeast of the lower-plate occurrences (Pierce and
Nelson, 1973).
However, one of the nine upper-plate exposures of Crandall j
j
Conglomerate occurs about 5 km southwest of the lower-plate |
|
exposures (Figure 64; Pierce, 1963a; Pierce and Nelson, 1973). j
Pierce and Nelson (1971) and Pierce (1978) show another exposure of
upper-plate Crandall Conglomerate west of the lower-plate exposures, j
213
E X P L A N A T IO N
Eocene volcanic rocks,
undi ff erenli ated
Crandall Conglomerate
Ordovician, Devonian
and Mississippian
rocks allochthonous on
the Heart Mountain
detachment fault
Heart Mountain
detachment fault
0
* . MO
C ra n a a ft
C a t h e d f
Russel
Steamboat
C r e t *
6
V MO
Dsad
Upper-plate fault
and depositional
contacts
FIG URE 64. Generalized geologic m ap showing distribution of upper- and lower-plate exposures of Crandall Conglomerate.
Adapted from Pierce (1963 b), Pierce and Nelson (1968, 1971, 1973), and Pierce et al (1973).
214
R #P“ b " c Pilot
M1n
10 Km
P#ok
O f m j t *
Z6 2 2
Co thtdraI
Cliffs
a. Lo cation and strikes of s trik e -s lip fa u lt zones in allochthonous
Paleo zoic rocks. See Table I and figures referenced therein.
Base map from Pierce ( 19s o ).
y = N 19 w
\N
N 8 E
*
J = N 3 4 E
b. Strikes of twelve strike-slip
fault zones in allochthonous
Paleozoic rocks. All but one
fall into two groups (mean
strikes N I9W and N 3 4 E ) ,
su gg esting formation as
conjugate shears due to a
horizontal maximum
compressive stress trending
N 8 E . Dominantly E - w
extension in dicated by
d i p - s l i p fa u lts in alloch
thonous Paleozoic rocks
suggests a horizontal E - w
trending axis of least
compressive stress.
FIGURE 65. S t r ik e - s lip f a u lt zones in the upper plate of the Heart
Mountain detachment f a u lt .
but this appears to be a drafting error. Pierce and Nelson (1973)
explained the southwestern occurrence of upper-plate Crandall
Conglomerate by postulating pre-faulting occurrences of Crandall
Conglomerate west of the westernmost allochthonous masses. It is
here suggested in light of the variable directions of movement
indicated by bedding-fault striae for volcanic rocks that the
distribution of Crandall Conglomerate indicates variable directions
of movement for upper-plate Paleozoic rocks. The Crandall
Conglomerate of the upper-plate would therefore have moved in part
to the southeast and in part to the southwest. If , as seems likely,
the volcanic rocks shared a southerly component of motion with the
Crandall Conglomerate and the Paleozoic rocks, then a source area to
the north, along the south flank of the Beartooth U plift, is
indicated for the allochthonous rocks now exposed along the Clarks
Fork River west of Hunter Peak. Pierce (1950) inferred a northerly
source area for allochthonous Paleozoic rocks of the Dead Indian
Creek area, but later (Pierce, 1957) rejected this idea; Prostka
(1978) suggested a northerly source area for allochthonous volcanic
rocks between the break-away fault and Republic Creek. Thus it
seems likely that variable (but generally southerly) movement of
allochthonous rocks in the western part of the bedding fault gave
way to less variable dominantly southeasterly movement of
allochthonous rocks in the eastern part of the bedding fault and on
the fault on the former land surface.
Movement along the Heart Mountain fault was accomodated by
discontinuous b rittle extension of a continuous upper-plate.
Kinematic data from Paleozoic rocks and from the break-away fault
indicate dominantly east-west oriented extension interspersed with
local zones of north-south strike-slip; i f this strike-slip motion
were dextral, an inferred aggregate translation to the southeast is
explained. Kinematic data from upper-plate volcanic rocks are
variably oriented, probably reflecting more complex deformtion due
to downfaulting from higher structural levels as well as to lateral
translation. The kinematics of fault contacts between allochthonous
Paleozoic and volcanic rocks, which probably accomodated a large
proportion of the extension of the Heart Mountain allochthon, are
not well known. If the kinematics of allochthonous carbonate rocks
are representative of the kinematics of the entire allochthon, then
the direction of extension of the allochthon (E-W) diverges by 30 to
60° from the dominant direction of translation of the allochthon
[ESE to SSE, the direction of the maximum principal quadratic
elongation (Hobbs, Means and Williams, 1976, p. 26)] the difference
being accomodated by N-S simple dextrel shear.
The geometric and kinematic data generated by the present study
can also be viewed in light of the suggestion by Stearns et al
(1974) that detachment along the Heart Mountain bedding fault firs t
occurred to accomodate flexural slip between strata drape-folded
across the basement faults at the southern edge of the Beartooth
Mountains. If displacement across the Clarks Fork fault was
accompanied by horizontal shortening, then resultant faulting in the
sedimentary section should be systematically oriented with respect
to the direction of maximum shortening. In the carbonate
allochthon, normal faults strike dominantly north (Table 1) and
strike-slip fault zones strike dominantly N19W and N34E (Fig. 65,
Table 1). This pattern could have resulted from horizontal
shortening across the Clarks Fork fault, i f shortening oriented
about N8E occurred along the N60W-striking fault zone.
This postulated Laramide compression across the Clarks Fork
fault would have been accomodated by b rittle failure and reverse
faulting of near-surface crystalline rocks and by flow and attendant
reverse faulting (e.g., the Blacktail Fold) in the Cambrian strata.
Within the b rittle overlying carbonate section, east-west extension
or high-angle faults, conjugate strike-slip fault zones, and
bedding-plane detachment faulting may have been the preferred
accomodational strain. Boundary conditions (structurally low areas
to the east and south, structurally higher areas to the north and
west, and a basal detachment) may have permitted stress release by
tensional fracture, east-west extension, dominantly dextral
strike-slip, and translation to the southeast before stresses
sufficient to produce east-west striking-reverse faults in the
carbonate section developed. Subsequent extension of Paleozoic
sedimentary and Tertiary volcanic rocks during Heart Mountain
faulting per se, although not driven by basement compression, would
have been similarly constrained to move generally to the southeast,
and upper plate normal and strike-slip faulting would have occurred
along zones of structural weakness whose orientations were inherited
from the previous episode of basement-controlled deformation.
Displacement across the Clarks Fork fault system could have
occurred any time during the Laramide Orogeny, before or during
Heart Mountain faulting. N o data are known to constrain this
timing. The geometry at depth of the Clark's Fork fault system is
unknown. Thus, the suggested relationship between Clark's Fork
faulting and the Heart Mountain fault remains speculative, but it
does provide an explanation for the orientations of faults in the
upper-plate of the detachment terrane.
The kinematics of extension of upper-plate Paleozoic rocks may,
therefore, in part be inherited from a phase of basement-driven
deformation that preceedeed Heart Mountain faulting. Alternatively,
the conjugate strike-slip fault zones and east-west extension may be
related to stresses generated internally within the allochthon
during gravity-driven spreading from the area of the Sunlight
volcano northward toward the south-dipping buttressing flank of the
Beartooth u p lift.
Rate of Heart Mountain Faulting
Tectonic denudation with no immediately subsequent erosion of
the bedding fault required previous workers to infer catastrophic
velocities for Heart Mountain faulting. The present study indicates
that tectonic denudation did not occur and so removes the associated
time constraint. I f , as is likely, volcanic rocks in the lower
plate at Fox Creek were deposited before Heart Mountain faulting,
and i f Heart Mountain faulting ceased at about the time of
transition from Wapiti to Trout Peak Trachyandesite volcanism, then
Heart Mountain faulting could have occurred over a period of
millions of years. Available K-Ar date (Smedes and Prostka, 1972)
indicate oldest Lamar River formation to be older than 48.0 + 1.3
m.y. and Trout Peak Trachyandesite to be older than 49.3 to 49.5 +
1.5 m.y.). Cross-cutting relationships at Cathedral Cliffs indicate
two phases of upper-plate faulting separated by an episode of dike
intrusion, thereby precluding a single catastrophic faulting event.
If the 50 km displacement indicated for McCulloch Peaks (the largest
inferred displacement in the Heart Mountain fault terrane) occurred
over a period of 1 to 5 m.y., slip rates along the Heart Mountain
fault of 1 to 5 cm/yr. are indicated.
Revised Geologic History
In light of the new information revealed by the present study,
the field relationships of Heart Mountain fault terrane are
reinterpreted to constitute a new hypothesis describing the sequence
of events that occurred during Heart Mountain faulting. (Compare
Pierce, 1973; Pierce and Nelson, 1973; Voight, 1974). The new
interpretative hypothesis is summarized as follows:
1) Before Heart Mountain faulting began, Laramide
tectonics had created the Beartooth u p lift, the
Absaroka u p lift, and the Bighorn Basin. Stream
erosion had incised deep, narrow valleys into the
Paleozoic section, locally, (e.g. in the areas of
future Crandall Creek and Fox Creek) exposing
Cambrian shale of the Snowy Range Formation.
Continuing compression across the Clarks Fork fault
during relative up lift of the Beartooth Massif was
220 !
accomodated by (1) local folding in the relatively
ductile Cambrian rocks (e.g. the Blacktail fold),
(2) extension along north-striking fractures and normal
faults in the relatively b rittle Ordovician, Devonian,
and Mississippian carbonate section, and (3) formation of
bedding-plane detachment faults (i.e . at the horizon
of the future Heart Mountain fault, and at
stratigraphically higher horizons as well). The Crandall
Conglomerate was deposited in parts of the stream channel
system south of the Clarks Fork fault.
2) Absaroka volcanic activity began, blanketing the area
of the northern Absaroka mountains (fillin g the
channel in Cambrian strata now exposed at the Fox
Creek locality and capping the deposits of Crandall
Conglomerate) and emplacing dikes (into, for instance,
the north-striking fractures in the Paleozoic carbonate
section).
3) As Absaroka volcanism continued, rocks above the bedding-
plane detachment fault at the top of the basal bed of the
Bighorn Dolomite ( i . e . , the horizon of the Heart
Mountain bedding fault) underwent components of
gradual east-west-oriented extension and localized
north-south-oriented dextral strike-slip, thereby
translating to the south and southeast and moving
across the transgressive fault. This was the
beginning of Heart Mountain faulting per se, in that
it occurred independent of faulting along the Clarks
Fork fault except inasmuch as 1) the uplift Beartooth
Massif prevented spreading to the northeast, and 2)
extension and displacement of the Heart Mountain
allochthon occurred along zones of weakness (the
bedding fault, upper-plate faults, and upper-plate
fractures) inherited from basement-involved
deformation. Extension of the upper-plate, along
normal faults and by lateral intrusion of dikes,
occurred over a protected period of time, probably
several million years. Volcanic rocks were
downfaulted from higher structural levels to
accomodate about 100% extension along the Heart
Mountain fault. Lateral continuity of the upper
plate in the bedding-fault area was maintained by
the periodic deposition of more volcanic rock upon
the extending terrane, mostly in structurally (and
topographically) low areas.
4) During faulting, Paleozoic rocks were apparently more
prone to brecciation than were volcanic rocks,
so the preponderance of fault gouge is carbonate
material. Clastic dikes were intruded into upper-
plate carbonate and volcanic rocks during faulting.
5) Either the spreading mass extended into the
Bighorn Basin as far east as McCulloch Peaks, or the
eastern margin of the spreading mass shed gravity slide
blocks downslope into the Bighorn Basin, producing
the klippen at Heart Mountain, McCulloch Peaks,
and probably others now eroded away.
New Constraints on the Mechanics of Faulting
The geologic history of Heart Mountain faulting as indicated by
the present study is different from previous understandings in two
ways that are fundamental to questions of the mechanics of Heart
Mountain faulting:
1) Except possibly in the areas of the the transgressive
fault and the fault on the former land surface, Heart
Mountain faulting occurred not by gravity sliding of
numerous detached blocks but rather by spreading of a
single continuous allochthonous mass;
2) The extremely high rates inferred by previous workers
for movement of the upper plate along the Heart
Mountain fault are not indicated. Faulting may have
occurred throughout the 1 m.y. to 4 m.y. (K-Ar dates of
Smedes and Prostka, 1972) duration of Lamar River-
Cathedral Cliffs and Wapiti volcanism, allowing
average displacement rates along the basal fault on
the order of 1.25 to 5 cm/year.
Previous to the present study, hypothetical fluid-pressure
mechanisms for reducing friction along the Heart Mountain fault met
with the problem that such pressures, even i f developed before
fragmentation of the upper-plate slab, could not be maintained for
very long after the slab had been broken into smaller allochthonous
blocks. Prostka (1978) suggested that some of the allochthonous
carbonate blocks "may be large pieces that broke off a much larger
sheet and were le ft behind on the bedding fault" (p. 429), but he
s t ill envisioned sliding of numerous individual detached blocks that
moved "at high velocities so that leakage of the [basal] fluid
cushion should not be a problem (p. 435). Voight (1973b) had
inferred high velocities of sliding (10^/hr) based upon similar
reasoning. Hughes' (1970a, b, c, 1973) hovercraft mechanism, and
Straw and Schmidt's (1981a, b) "phreatomagmatic-hydraulic"
hypothesis also assume or require high velocities of sliding.
The earthquake oscillation hypothesis as described by Pierce
(1973, 1979, 1980) is also constrained to geologically high
velocities because it is premised upon tectonic denudation of the
bedding fault with no syn-or post-tectonic erosion of the
tectonically denuded surface. Thus detachment, displacement, and
post tectonic volcanism must have occurred within a time period too
short to allow processes of subaerial erosion or deposition to act.
If a year of exposure of the surface of tectonic denudation is
allowed, and displacements across the land surface occurred coeval
with displacements on the bedding fault, then a displacement rate
averaging up to 50 km/yr (1.5 mm/sec) is indicated. As earthquake
activity would presumably not be continuous during the assumed
year-long period of faulting, actual rates of discrete fault
displacements indicated by this model would be much greater, and
simultaneous displacement along the entire base of each large block
would probably be required. The acoustic fluidization mechanism of
Melosh (1981), which is premised upon the idea of simultaneous
detachment along the entire bedding fault as the result of a seismic
energy wave, also seems to require high velocities of displacement
along the Heart Mountain fault to maintain floatation of the
allochthon with acoustic energy.
The new information developed during the present study, by
indicating that the upper plate overlying the bedding fault
remained laterally continuous during faulting, reduces the
d ifficulties associated with fluid pressure hypotheses. This in
effect supports the suggestion by Rubey and Hubbert (1965) that
Heart Mountain faulting occurred during rather than before
deposition of the early basic breccia and may have involved
"large-scale movements of an extensive ...sheet composed of
limestone blocks plus [early basic] breccia" (p. 470) and that,
therefore, fluid pressure could have reduced friction along the
Heart Mountain fault.
Because tectonic denudation of the bedding fault is no longer
indicated, new information developed during the present study also
reduces the rates of displacement inferred for the Heart Mountain
fault to geologically normal rates (i.e . on the order of 1 to 5
cm/yr), thereby no longer requiring simultaneous displacement along
large areas of the Heart Mountain fault. Thus the simultaneous
elimination of friction along large areas of the bedding fault is no
longer required, although the mechanical enigma associated with
low-angle faulting in general s till remains applicable to Heart
Mountain faulting.
A s a result, previously posed hypothetical mechanisms that can
be revised to accomodate displacement rates along the Heart Mountain
fault on the order of 1 to 5 cm/yr remain viable working hypotheses.
Such hypotheses would be modifications of the earthquake-osci1lation
hypotheses of Bucher and Pierce, the fluid pressure mechanism of
Rubey and Hubbert (1965), and the fluid wedge/plastic wedge concept
of Voight (1972, 1974c). Hypotheses no longer viable are those
requiring catastrophic velocities (e.g. Melosh's acoustic
fluidization hypothesis, and various aspects of Voight's Prostka's,
Hughes', and Straw and Schmidt's fluid pressure models).
Recommendations for Future Work
Detailed structural mapping of the volcanic terrane is needed
to determine the present thickness of the volcanic rocks involved in
Heart Mountain and the kinematics of their involvement. If Heart
Mountain faulting ceased when volcanic extrusives changed from
dominantly breccias to dominantly flows (at about Trout Peak
Trachyandesite time), then the greater amounts of volcanic gas
presumably associated with the breccias could be called upon to
fa c ilita te fault displacement.
A quantitative analysis of the mechanics of gravity spreading
(e.g. E llio tt, 1976) could be applied to the Heart Mountain fault
terrane. A mechanical analysis could be used to predict principal
stress orientations within the allochthon as a function of such
variables as thickness of the allochthon and configuration of the
basal detachment fault. Then perhaps kinematic data from the
upper-plate could be brought to bear upon the question of the
direction of dip of the basal fault surface, especially in the area
of the transgressive fault.
227
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APPENDIX
Summary of geometric data plotted on lower hemisphere equal-area
stereographic projections.
* = major feature
I = orientations from within or
along same feature (e.g. dike
or fault zone)
t 2 = sequence indicated by cross
cutting relationships; t^ earliest
dikes are igneous unless otherwise noted
faults are upper-plate unless otherwise noted
Figure 7, Break-away fa u lt, south of Soda Butte Creek
Fault Orientations Striae Orientations
N12E 55SE 55°S85E; 126N21E, 19°S2E
N2E 70SE
N12E 67E 64°N72E
N15W 67NE
N11E 71SE 15°N16E to 65°N59E
N12E 60SE 5°N15E, 9°S7W, 60°S78E
N21E 64SE 4°S19W, 14°S14W, 59°N74E
N15E 79SE 35°S7W
N 9W 69NE 11°S11W, 26°S3E
N-S 74E 9°S2E, 73°N71E
N-S 66E 2°N1E, 14°S6E
N20W 55NE 49°S74E
N18E 44SE 44°S73E
N40W 64NE 44°S68E, 1°N40W, 61°N78E
ure] , Bedding fault stri ae , 0.8 km east of break-away
of Soda Butte Creek
0° S71E
0°S77E
Figure 10, south of Silver Gate
a. Faults within Madison Limestone
Fault Orientations Striae Orientations
"N28W 31NE 306N45E
-N13W 34NE 34°N81E
~*N 5W 77SW 20°N10W
N 2W 75SW 68°N49W
-N19E 74NW 72°N40W
N30E 25SE
N49E 54SE 52°S62E
N57E 38SE 12°N74E
b. Faults within volcanic rocks
Fault Orientations Striae Orientations
N55W 25SW 206S74W
N15W 66NE 15°N8W, 64°N52E
N 9W 83SW 83°S81W
N-S 53W 53°W
N31E 40SE
N33E 83SE 14°N35E
N52E 80SE 17°N55E, 66°N76E
T N54E 48SE 16°N70E
LN60E 51SE 10°N68E
N58E 46SE 45°S15E
E-W 64N 58°N40W
Bedding Fault Striae
6°S32W
239 S
c. Faults at contacts between upper-plate limestone and volcanic
rocks
Fault Orientations Striae Orientations
(-N86W 72SW 72°S20W
N86W 84SW 75°S71W
N85W 72SW 51°S71W
N81W 76SW 49°S82W
N82W 54SW
"N62W 25NE 1°S65E
N24W 29NE 14°S50E
N30E 16SE 17°S50E
_N40E 18SE 21°S55E
N22W 46NE
Pniow 80NE 51°N3E
1 N 4W 75NE 63°N28E
N22E 44NW 44°N55W
Figure 12, Falls Creek
Fault Orientations
N50W 52SW
( t i)
N47W 34SW
*
N40W 35SW
N40W 76NE
*
N20W 77SW
(t 2)
N20W 77NE
N10W 56SW
N 6W 74SW
N 4W 74NE
N18E 85NW
N31E 71NW
★
N69E 49SE
( t i 1)
N85W 50NE
N54W 43NE
*
N83W 18SW
N 43W 62NE
N41W 66SW
N41W 89NE
N 15W 44SW
(^2 ‘ )
N45E 83SE
N54E 50SE
★
N66E 43SE
N70E 54SE
Fault '
Fault Orientations
NlOE 82NW
N18E 90°
N26E 78N W
Striae Orientations
22°N68W 36S48"E
26°S89W
30°S87W 16°N65W
68°N3W
38°S10E
70°N20E
0°N10W, 29°N32W, 40°N45W
48°N24W
72°N57E
42°N13E
50°N7E
41°S18W
47°N22W
43°N23E
5°S80W, 11°S62W
42°N16W, 60°N24E
58°N87W
58°N40W, 32°S42E
16°N32W, 29°N50W, 16°S2W
80°S4W
6°N72E
12°N77E
Fault cutting bedding fault
Striae Orientations
75°N21W
72°N16W
240
Figure 4, northwest corner of Republic Mountain
a. Faults and striae
Fault Orientations Striae Orientations
i
r 1
u
N79W 17NE
N76W 29NE 30°N9E
N80E 22NW 21°N9E
N69W 27SW
N34W 63SW
N 8W 81SW
N8E 21SE
N75W 20NE
N72W 54SM
N 8W 90°
N53E 64SE
N70E 4N W
b. Dikes
Clastic Dikes
N75W 20NE
N72W 54SW
N 8W 90
N53E 64SE
N70E 4N W
Igneousi Dikes
upper pi ate
N10W 72SW
N10W 90
1 ower pi ate
" W 89W
N85E 58SE
Figure 17, Republic Creek area
a. Faults and striae in Paleozoic rocks
Fault Orientations Striae Orientations
E-W 29N
N55W 26NE
N51W 60NE
N24W 54SW 26°N44W
N16W 78NE 75°N39E
N14W 85NE 9°N13W
* N12W 78SW 0°N12W
N 5W 38NE
* N1E 73NW 60°N31W
N7E 64SE 64°S71E
N25E 69NW
N26E 60SE 241
Fault Orientations Striae Orientations
N37E 34NW 0°N37E, 23°S73W, 26°S81W
* N49E 67NW 61°N1W
N52E 73SE 65°S86E
N64E 36SE 9°N76E, 3°S59W
N73E 50NW
b. Faults and striae at contacts between upper-plate volcanic
Paleozoic rocks
E-W 75°N
N 5W 38NE
N26E 82NW 71°N2E
N35E 27SE 21°N81E
E-W 90 “I inaccessible,
E-W 35°SJ approximate
N56W 52SW
N46W 46SW
N45W 22SW 22°S36W
c. Faults and striae within volcanic rocks
Fault Orientations Striae Orientations
N12E 65NW 58bN36W
N39E 44NW
Upper-Plate Dikes
N35W 73SW cuts across fault
E-W 75N intruded along fault
N13W 45SW
N 5W 90 sheared
N1E 75NW 60° N31W
-i ntruded along fault and later sheared?
N15E 90 sheared
Figure 20, south of Colter Pass
Fault Orientations Striae Orientations
N87W 17NE 8°N63E ( t i ) , 14°N35W ( t 2
N65W 68NE ol°N68E
N S W 56NE 27°N14E, 46N85E
f N3E 58NW 50°N46W
LN15E 70NW 59°N22W
N26E 62NW 61°N85W
N30E 60NW
Bedding Fault Striae
2°N41W
12°S84W
7°S79W
29°S78W
7°S76W
242 i
b. Faults and striae in volcanic rocks, eastern area
Fault Orientations Striae Orientations
N62W 73SW 73°S28W
N13W 45SW 1°S12E
N11W 63SW
N18E 84SE 12°S17W
N24E 65NW 29°S38W
N30E 84NW 0°N30E
N30E 90
N32E 82SE 61°S16W
N49E 62NW
N55E 71SE 69°S7E
Bedding Fault Striae
rT87W
2°S74W
7°S71W
1°S60W
5°S49W
4°S46W
4°S44W
6°S44W
8°S44W
4°S41W ( t x)
2°N77E ( t 2)
Figure 22, Fox Creek
Fault Orientations Striae Orientations
a. Faults in lower plate volcanic rocks
N83W 33SW 21°S50E
N76W 29SW 27°S42W
b. Faults at contacts between upper-plate volcanic and Paleozoic
rocks
N35W 43SW 42°S64W
N 7W 51SW 42°S42W
N30E 29NW
c. Faults within Paleozoic rocks
N75W 68SW
N30W 86NE
N28W 54NE 17°N15W
N21W 66SW 5°S18E
N18W 70SW 6°S15E, 29°N30W
N20W 68NE
N10W 54SW
243
Fault Orientations Striae Orientations
N 4W 65SW 0°N4W, 3°S3E
N 4W 71NE
N2E 50SE
13°N
N3E 76N W
N3E 64SE
5°S5W, 47°S19W
N 6E 58SE 0°N6E, 5°N9E, 20°S6E, 58°S84E
N22E 86S E
N26E 56N W
9°N23E
N27E 85NW 13°N26E
N29E 67NW 67°N60W
N30E 55NW
N53E 90
N54E 60NW
15°N19E
Figure 24, Pilot Creek
a. Faults and stri ae within Paleozoic rocks
Fault Orientations Striae Orientations
N81W 74SW
N77W 49SW
636S47E
N 68W 52SW 10°S61E
N18E 62NW
N34E 58N W
N44E 50N W
3°N14E
N65E 10SE 37°S2W, 24°S7E
Faults and striae and
volcanic rocks
contact between upper-plate Paleozoic and
Fault Orientations Striae Orientations
N89W 80NE 156N86W
N 88W 82NE 0°N88W
N87W 64NE 41°N62W
N 86W 72NE 5°N84W
N82W 57NE 28°N62W
N10W 73NE 7°S13E
N12E 67SE
N77E 52NW
N80E 50NW
11°S7W
N 88E 68N W 11°N83E, 8°N89W, 12°N88W
Upward continuation of
both sides of fault
same fau lt zone, with volcanic rocks on
Fault Orientations Striae Orientations
N84E 56N W 16dN84W
c. Faults and striae in volcanic rocks
Fault Orientations Striae Orientations
N 87W 67NE 66°N14E
*N82W 35NE 0°N82W, 2°N79W, 22°N48W
14°N77E, 6° due E
N62W 89NE 23°N61W, 58°N60W, 89°N28E
N56W 56NE 43°N86E
*N35E 26NW 15°N84W, 17°S83W
N37E 85NW 20°N35E
N42E 56 N W 12°N34E, 54°N26W, 52°N78W
N51E 84SE 51°S44W
N54E 70NW 44°S74W
N67E 74NW
N 74W 49SW 14°S62E
N 55W 70NE 16°N49W, 19°S62E
N 7W 70SW 65°S45W
N 1W 19SW 19°N89W
N10E 42SE 21°N34E, 35°N59E
N34E 67NW 66°N34W, 66°N75W, 40°S54W
*N50E 15N W 15°N40W
N54E 27SE 26°S18E, 28°S43E
N60E 45SE
N69E 42SE 42°S24E ( t x) , 31°S71E ( t 2)
d. Upper-plate dikes
Dike Orientations
N80W 80NE
N 55W 90
N 30W 85NE
N 26W 81NE
N 20W 90
N 11W 90
Figure 29, Jim Smith Creek
a. Faults and striae
Fault Orientations Striae Orientations
N70W 85NE
r N 60W 80NE 39°N51W, 48°N60W
.N84E 36N W 38°N5W
*N31W 2S W 0°N31W
*N25W 34SW 25°N67W
N10W 18SW
fN26E 32NW 25°N20W
N56E 44NW 44°N33W
N61E 42NW 42°N20W
N64E 39N W 39°N25W
N80E 70NW 6°N78E
245
Bedding Fault Striae
12°N77E
0°S80E
4°S11W
4°S10W
2°S20E
4°S20E
6°S21E
1°S22E
3°S23E
1°S25E
2°S25E
0°S30E
b. Clastic dikes and striae
Dike Orientations Striae Orientations
N83W 45NE 34°N42W
N63W 49SW 49°S29W, 43°S11E, 54°S20W
N20E 82NW 15°S22W
N43E 60SE 60°S49E
N46E 58SE 58°S37E
N57E 56SE 56°S38E
c. Lower plate faults in Grove Creek member of Snowy Range
Formation
Fault Orientations Striae Orientations
N 80W 37SW 36°S31W to 38°S6W
21°N5W
N12W 26NE (25°N69E)
d. Lower plate faults in Bighorn Dolomite offsetting shatter zone
Fault Orientations Striae Orientations
N70W 44NE
N62W 35 to 20NE
N60W 31NE
N50W 35NE
N37W 49NE
N34W 47 to 24NE
246
Figure 31, Faults and striae southwest of B4 Ranch
Fault Orientations Striae Orientations
E-W 62S 62°due S
N 66W 34SW 26° S71W, 32° due S
N 58W 85SW 22°N60W, 51°S52E
“ N 43W 74NE *7°S44E , 60°S72E, 71°N10E
N42W 67NE *3°S44E (to ) *58°S83E ( t o ) , 60°N4E to 67°N35E ( t i )
N 16W 20NE 11°S31E, 14°S16E, 37°N2W, 2°S36E, 11°S53E, 1°S9E
N 38W 32SW 8°N50W
N 30W 10SW 11°S51W
N 11W 39NE 19°S36E
N 10W 82S 61°N24W
N 6W 81SW 35°N12W
N10E 90 56°N10E
[~N13E 89NW 40°N12E
LN39E 87NW 63°N32E
N17E 87NW 59°N13E
N18E 11NW 9°N40W
N47E 74SE 73°S65E
N53E 51SE 49°S63E
N56E 51SE 36° due E
N70E 51NW 46°N12E
N72E 83NW 76°N42E
N75E 86S E 69°N38E
N 86E 26SE 25°S26E
Bedding Fault Stri ae
2°S11E
3°S17E
1°S44E
2°S46E
2°S50E
2°S52E
3°S55E
Figure 32, Onemile Creek
Fault Orientations Striae Orientations
N26W 85SW 72°N42W
*N18W 54NE 49°N39E
N10W 58NE 58°N84E cuts dike
N14E 57SE 51°N68E
? N14E 47SE 54°N67E
N18E 65SE 51°N54E
N21E 70NW 57°N14W (reverse faults)
N44E 68N W 0°N44E, 50°S72W
N46E 55SE 49°S82E
N61E 79NW 9°N58E, 45°N49E
*N77E 85SE 0°N77E
N78E 63SE 30°S61W
N82E 44NW 43°N9E
N87E 79SE 8°N88E 2'
Figure 33, Onemile Creek to Squaw Creek
a. Faults and striae
Fault Orientations Striae Orientations
N75W 72SW 29^N85W, poor
N31W 48NE 5°N26W
N20W 44SW
N4E 74SE 73°N76E
N 6E 34SE
N23E 70SE 4°N24E, 44°N43E
N55E 45NW 31°N22E, 15°N18E, 45°N35W
N73E 63NW 63°N20W
N74E 88N W 69°S80W
22°N6W
bedding fault zone, Squaw creek
'N84W 9NE 9°N10W
■ N 42W 34NE 21° due N, 2°S43S, 0°S50E
6°N86W , 4°N76W, 4°S42E
Bedding Fault Striae
1/26 N43W ”
b. Beds of Snowy Range Formation in lower-plate fold
Bedding Orientations
N31W 17NE
N52E 41SE
N60E 40SE
N63E 37NW
N70E 31NW
N79E 49NW
c. Upper-plate dikes
Dike Orientations Striae Orientations
N64W 75SW 75°S33W
N 58W 74NE 73°N53E
N52W 90
N 50W 90
N 28W 83S W
Figure 35, Cow Creek
a. Faults and striae
Fault Orientations Striae Orientations
N83W 75NE 5°S85E
*N55W 70NE 13°N51W cuts dike
*N54W 66N E 60°N75E, 55°N88E
N53W 68N E 16°S60E
248
K-l
Faul t Orientations Striae Orientations
"N 43W 67NE 61°N88E
»N 17W 66N E 65° due E
*N22W 55NE 55°N77E, 51°S79E, 37°S54E, 11°S30E
N20W 42NE 42°N66E
N16W 31SW 30°S52E
N12W 68N E 64°S69E, 68°N81E
N12W 69SW 64°S42W
* N6W 86N E 86°S77E
N11E 37SE 38°S75E
N 2W 63NE
N 2W 52NE 52°S87E
N 1W 50NE 47°N61E
N 8E 71SE 70°N83E
N49E 79N W 79°N46W ( t ^ , 63°N26E ( t 2)
N59E 67SE 57°S19W
N68E 58SE 59°S19E
b. Upper-plate dikes
Dike Orientations
N45W 90° (represents 20-30 dikes)
N40W 90°
N32W 90°
N14W 86SW , cuts fault
Figure 37, West of Blacktail Creek
a. Faults and striae
Fault Orientations Striae Orientations
N 5 9V J 59NE 7°S63E
N 58W 40NE 25°due E
N47W 42NE 38°N74E
N36W 25SW 5° due S
N 32W 67NE *57°S73E, 32°S48E, 17°S39E
N17W 56SW
*N9W 61NE 60°S84E
N 4W 62NE 43°N26E
N2E 52SE 38°N41E, 47°N59E, 52°S88E
N9E 52SE 17°N23E, 51°N86E
"N10E 89SE 7°S10W
N10E 69NW 31°S23W
N19E 70N W 33°S32W
N32E 55N W 5°S35W
N12E 75N W 26°S19W, 65°S47W
N14E 86S E 0°N14E
■ *
rN22E 16SE 2°N32E, 16°S56E
N22E 90 11°N22E
N22E 83NW 17°S24W
N27E 61SE 45°N60E
'N31E 82SE 0°N31E 249
Fault Orientations Striae Orientations
N33E 30NW 4°N24E ( t ^ , 23°N13W ( t 2)
N35E 50SE 28°N60E
N41E 63SE 8°S37W
PN43E 19W 19°N61W
LN17E 25NW 0°N17E, 5°S28W
N57E 79NW 25°N51E
b. Upper-plate dikes
Dike Orientations Striae Orientations
N22W 67NE 7°N19W
N16W 57NE 5°N13W
N27E 61SE
Figure 38, Hunter Peak
a. Faults and striae
Fault Orientations Striae Orientations
E-W, 89N
N 86W 33SW 9° due E, 41°N88W
N38W 65NE 33°S8E
N18W 63SW 65°N54E (cuts
N11W 64SW 62°S64W
"N 5W 60SW 63°N83W
N3E 58N W 59°S66W (cuts
56°S67W
N 4W 57SW 10°S75W
N3E 82SE 57°S73W
FN14E 64NW 69°N24E
LN22E 56N W 63°S89W
N16E 60NW 55°N85W
N22E 68N W 15°N7E
N27E 74N W 68°N67W
N60E 30SE
N60E 68S E
70°N78E
b. Upper-plate dikes
Dike Orientations Striae Orientations
N 45W 80NE sheared margin
r N44W 74SW
LN9W 90 76°N9W
N40W 90
N38W 72NE 53°S63E
N35W 90
f N27W 90
I N 6W 90
N 24W 90
N 23W 90
N22W 88S W 57°N22W
N15W 60SW 250
Dike Orientations Striae Orientations
N15W 86N E
N10W 90
N10W 65NE 65°N80E
N 8W 80SW
Figure 39, east of Lodgepole Creek
a. Faults and striae
Fault Orientations Striae Orientations
N71W 24NE
N69W 55SW 39°S35E
N66W 40NE 37°N54E
N30W 57NE
N28W 56SW 56°S62W
N5E 70N W
N20E 45NW 21°N2W
N29E 84NW 66°N15E
Bedding Fault Striae
0°S41E
b. Upper-plate dikes
Dike Orientations
N31W 60SW offset by fault
N30W 57NE sheared margin
N26W 56SW (represents approximately 15
di kes)
N25W 70SW
N21W 57SW
N20W 64SW
N15W 90
N10W 65SW
N5E 90 offset by fault
Figure 42, Between Oliver Gulch and Corral Creek
a. Faults and striae in volcanic rocks
Striae Orientations
455Nl9E
43°S86E !
12°N70W
69°N69E |
62°N61E
57°N21W striated |
clastic dike margin j
37°S75E j
251
Fault Orientations
E-W 47N
N80W 80NE
N 68W 59SW
N63W 67NE
* N45W 27SW
N20W 69NE
N12W 63NE
N 5W 75SW
N9E 72N W
N19E 37SE
Fault Orientations
N65E 56SE
Striae Orientations
Bedding Fault Striae
2°S45E, 0°N33W
b. Faults and striae in Paleozoic rocks
Fault Orientations Striae Orientations
*N70W 60SW 516S65W
N59W 82SW 7°N60W
N56W 46SW 45°S50W
r*N 38W 62SW 31°N57W
L.N 15W 74SW 67°N59W, 16°N20W
*N23W 56NE 26°S42E, 35°S51E
fN8W 30NE 21°S47E
LN33E 62SE 61°S75E
N6W 51SW 51° due W
N-S 29E ( t i )
29°S76E
N-S 55W ( t 2)
50°N58W ( t ' i ) 50°S58W
N3E 49NW 49°N79W
*N8E 64SE 63°S78W
"N16E 64SE 64°S68E
N17E 83SE 83°S73E
54°S71W
N31E 34SE 33°S77E
N49E 60SE 34°N71E
N50E 80SE 80°S66E
FN80E 64SE 53°S61E
LN89E 55SE 47°S41W
( t
Figure 45, Cathedral C liffs
a. Faults and striae in area of east side of graben
Fault Orientations Striae Orientations
N76W 56SW
N 75W 15NE
N62W 47SW
N 50W 68S W
N 33W 35SW
N67E 25SE
N 30W 11NE
N 29W 84SW
N20W 54SW
N 20W 26NE
N10W 71SW
N2E 67NW
N5E 20W
N 6E 67 N W
N 9W 74SW
18°S27E ( t i ) , 74°N52W ( t 2)
17°N50W
Fault Orientations Striae Orientations
N 1W 72SW
N3E 63NW
N12E 35SE
N15E 47SE 20°N35E, 28°N44E
N21E 75SE 33°N31E
N41E 84NW
N45E 44SE
N54E 84SE
N55E 30SE 30°S35E
N79E 49SE 32°S63E
N68E 43SE
N80E 44SE
N 14W 69SW 41°N33W, 54°N46W
N 8W 48NE 47°N63E
N3E 58SE 54°N63E
N1 IE 78N W 67°S42W
N12E 46SE 43°N73E
N42E 83SE 67°N59E
N58E 51SE 44°S72E
N78E 68S E 36°S86E
N85E 60SE 40°S68E
b. Fault zone forming east side of graben
Fault Orientations Striae Orientations
c.
N21W 87SW 24°N22W, 836S10W, 686S13E
N12W 76SW 54°N33W
N12W 84SW 49°N19W ( t i ) , 64°S1W ( t 2)
NlOW 70NE 43°S30E
N 9W 88N E 77°N1W
N 6W 89SW 70°S3E, 85°S7W, 40°N7W
N 5W 81SW 81°S70W
N 4W 84NE 69°N12E, 79°S36E
N 2W 78NE 38°S8W, 49°S21W
N7E 67SE 37°N26E, 61°N58E
N11E 78N W 68°S42W
N21E 86S E 68°S11W
•stri ki ng fault contact between Paleozoic and volcanic rocks
Fault Orientations Striae Orientations
N72E 51SE
N84E 59SE
N12W 54SW
N85E 50SE to 82NW (estimate from vantage point of photo
Fig. 46)
N 88E 69SE 67°S30E
N54E 56SE 56°S28E
253
d. Faults and striae east of the graben area
Fault Orientations Striae Orientations
N 71W 71SW 68°S51W ( t i ) , 54°S8lW ( t 2)
45°S77E, 49°S87E N 31W 54NE
N28W 81SW 20°N31W
N19W 47NE 43°S77E
N11W 85SW 85°S79W
N 9W 73NE 68°N39E
N S W 68S W 66°S56W
N 4W 86S W 82°N37W
N 2W 76NE 76°S85E
N 1W 76SW 72°N55W
N 2E 66S E 66°N87E
N4E 76SE 23°N10E, 42°N17E, 71°S44E
N7E 64NW 88°N74E
N7E 88S E 65°N83E
N7E 65SE 38°S89E
*N11E 38SE 60°N81E
N16E 62SE 68°N37W
f N19E 72NW 64°N79E
LN27E 69SE 21°N29E
N26E 83SE 30°N39E
N28E 70SE 77°N10W
N80E 77NW 56°S80E
e. Upper-plate dikes
Fault O rientations Striae Orientations
43°S77E
N29W 90
N27W 19SW
t N19W 47NE
N18W 90
N15W 90
N 12W 80NE
N10W 90
N S W 48NE
N 8W 80NE
N6W 90
N 6W 75NE
N 6W 25NE
t N 4W 86S W
N 3W 90
N4E 90
t N4E 76SE
N 7E 65SE
N9E 90
N12E 90
N13E 45SE
N18E 90
N22E 65SE
N23E 68SE
N26E 82SE
N34E 90
81°N36W
23°N10E, 42°N17E, 72°S44E
64°N83E
21°N29E
t = al so plotted as faults on fig . P6 .....? 5 ._ 4 _
Figure 48, east of Reef Creek
a. Faults and striae
Striae Orientations
606N64E
56°N58E
49°N25E
68°N76E see fig . Q 3
51°N71W
63°N82W
63°S84W
61°N50E
58°S67W
Dike Orientations
N20E 50SE
N45E 26SE (represents 6 dikes)
N60E 90
Figure 51, east of Painter Gulch
a. Volcanic strata
Fault Orientations Striae Orientations
N76W 30SW
N75W 37SW
N 70W 60SW
N 60W 33SW
N60W 37 S W
N 5W 12SW
N5E 17NW
N14E 16N W
N15E 12NW
N15E 16NW
b. Major fa u lt zone
Fault Orientations Striae Orientations
N5E 50SE 49°S65E
N17E 51SE
N40E 54SE 50°S20E
N46E 56SE 49°S82E
N55E 70SE
N59E 60SE 49°S80E
c. Other faults and striae
Fault Orientations Striae Orientations
N42W 74SW 58°S15E
N 40W 47SW 30°S8E
N 36W 40SW 31°S7W, 39°S34W
Fault Orientations
N20W 6O N E
*N12W 58NE
N 9W 64NE
N 7W 68N E
N6W 54SW
N 2W 64SW
N-S 55E
N3E 63NW
N 6E 69SE
N15E 64NW
Fault Orientations Striae Orientations
N 36W 82SW 82°S54W
N 31W 85SW 10°S31E
NlO W 8S W 6°N58W
N 7W 15SW 12°S32W
N39E 64SE 6°N42E, 29°S23W
N75E 67SE 51°S44W
Figure 53, Sugarloaf Mountain (see Fig. 52)
a. Faults and striae on Paleozoic rocks immediately northwest of
volcanic rocks
Fault Orientations Striae Orientations
N17E 84SE 70°due S
N17E 67SE 66°S86E
N40E 85SE 65°N51E, 75°N59E
N41E 74SE 60°N71E
N48E 62SE 58°S75E
N51E 57N W 56°N24W
N56E 86S E 64°N64E
N78E 77NW 76°N3E
b. Faults and striae in volcanic rocks
rault Orientations Striae Orientations
r N89W 88N E 6dN89W
N78E 88N W 22°S79W
. N66E 89NW 56°S68W
N 87W 40SW 22°S60E
N 82W 85SW 21°S80E
★
N79W 82SW 12°S77E
N 40W 40SW
N 38W 70NE 36°S54E
N7E 57SE 57°S85E
N 34W 57SW 43°S4W
f N29W 30NE 27°S86E
L N67E 82SE 33°N72E
N 7W 85SW 60°N15W
N 4W 58NE 55°N63E
N85E 90
c. Faults and striae in Paleozoic rocks immediately southeast of
volcanic rocks
N84W 78SW 67°S53E
N35W 55SW 55°S61W
N18E 87SE 21°N19E
N28E 72NW 69°S85W
N60E 70SE 25°N70E, 52°N87E, 70°S32E
N74E 78NW 78°N26W
N75E 83NW 81°N50W
256
d. Faults and stria e , Paleozoic rocks, northeast side of east end of
mountai n
Fault Orientations
------------ r r
N 68E
N86W
*N78W
N62W
N57W
N44W
N41W
N39W
*N24W
*TN22W
LN6W
N20W
fN5W
I N52E
N 3W
* N4E
N12E
N16E
* N29E
N30E
N35E
N38E
N50E
N63E
63SE
84SW
60SW
31SW
45SW
59SW
87SW
82SW
82SW
68N E
88N E
88N E
84SW
68N W
82SE
88S E
53NW
82SE
82NW
84NW
45NW
64SE
22SE
56SE
Striae Orientations
3 3 ° S ^ r i T j T , 45"0 'S'2'$r~[t 2)
57°S61E
84°S10W
35°S44E, 58°S11E
30°S46W
27°S29E
57°S22W
82°N66W
76°S6E
3°N24W
12°S23E
20°S6E
55°S24E
69°N21W
67°N17W, 64°N72W
70°N26E, 80°N83E
4°S4W
49°N47W
81°S53E
36°N23E, 79°S85W
55°N21E, 69°S45W
45°N46W
27°S24W
41°S83E
e. Faults and stria e, Paleozoic rocks, southeast side of east end of
mountai n
Fault Orientations
N 88W
N 86W
N71W
N68W
N57W
N 54W
N48W
N 44W
t f N 45W
3
LN39W
N42W
N40W
N27W
N 9W
N 2W
80SW
64NE
87SW
55NE
32NE
59SW
89SW
48NE
37NE
34NE
25SW
65SW
27SW
25SW
59NE
Striae Orientations
54^78W, 47 6S 77E
64°N11E
57°N65W
54°N40E
2°N54W
52°S76W
34°N49W
47°N65E, 37°S86E
32°N81E
31°N79E
24°S27W
55°N82W
20°N73W ( t i ) , 28°S63W ( t 2)
25 S86W
58°N73E
257 |
Figure 55, Steamboat
a. Bedding of Paleozoic rocks
Bedding Orientations
N87W 33SW
N82W 15SW
N 82W 50SW
N 81W 34SW
N 81W 6S W
N80W 37SW
N 74W 13NE
N73W 35NE
N72W 37SW
N 68W 35SW
N 68W 37SW
N55W 28SW
N50W 13SW
N50W 30SW
N 40W 20SW
N72E 36SE
N78E 28NW
b. Upper-plate dikes
Dike Orientations
N 86W 61SW
N85W 65SW
N85W 67SW
N78W 77NE
N74W 90
N70W 50SW
N67W 88N E
N60W 75SW
N 48W 90
N45W 55SW
N67E 29N W
Figure 57, Steamboat
a. Faults and striae, northern 1/3 of east face
Fault Orientations Striae Orientations
27*s2"0'E N 88W 27SW
N87W 33SW
? N86W 45NE
N82W 15SW
N 81W 34SW
N80W 70SW
35°S1W { t l ) 9 32°S17E ( t 2)
occupied by sheared dike
N79W 56N E
N78W 29NE
258|
Fault Orientations Striae Orientations
( t 2)
( t : )
N75W 82SW
N72W 58NE 71°S51E
N69W 33SW
N 68W 37SW 34°S25W, 34°S15W
? N67W 12SW 36°S29W
N55W 74NE
N37W 74SW 71°N68E, 45°S72E
? N15W 50NE 69°N86W
N 7W 59NE
N-S 63E 57°S76E
N29E 90
N35E 60NW 37°S60W
N51E 27N W
N 68E 60NW 60°N26W
? N60E 70N W
* N67E 28M W 22°N18E
N67E 42NW 34°N24E
N67E 86S E
N72E 36SE 80°N88E
N77E 48NW 34°S3W
39°N31E ( t 1), 46°N8E ( t 2), 37°N26W
N87E 84NW 69°N71E
b. Faults and striae, central 1/3 of east face
Fault Orientations Striae Orientations
i N 88W 86S W 66°S78E
N 88W 71SW 50°S64E
N 86W 21SW 22°S6E
N 86W 61SW 61°S6E
N85W 67SW 63°S27E sheared dike
m a rg i n
N82W 62SW 60°S16E
N 68W 60SW
N 81W 39SW 39°S4E
N80W 86S W
N 80W 87SW 87°S9W
N79W 70NE 68°N17W
N75W 78SW 70°S70W ( t 1), 73°S28E
N74W 33SW 29°S20E, 33°S34W cuts dike
N73W 63SW 62°S2E
N67W 88N E 53°S70E
N 66W 60SW 52°S19E
N64W 48SW 47°S11W dike margin
N60W 41SW 41°S24W cuts dike
N60W 66N E 63°N59E cuts dike
i* N59W 64NE 63°N11E
N 54W 84NE 73°S75E
N 49W 78NE 50°N35W
N 9W 81SW 55°N23W
N58E 72NW 67°N71W 259
Fault Orientations Striae Orientations
N58E 49SE 49S33°E
N65E 45NW
N73E 84SE 65°S59W
N78E 81SE 78°S35W
N79E 65SE 65°S5E
N81E 81SE 76°S59E
N84E 61SE 60°S26E cuts dike
N 86E 83SE 77°S63E
N 86E 78SE 78°S5E
c. Faults and stria e , southern 1/3 of east face, and south face
Fault Orientations Striae Orientations
N84W 36SW 37°S7W
N77W 64SW 63°S29W
N 72W 68S W 64°S14E
N70W 47SW 47°S18W
N70W 52SW 34°S39E, 17°S57E, 51°S11W (ti)
45°S57W ( t 2), 26°S49E ( t 2)
N70W 89NE 4°S71E, 25°S72E
N33W 78NE 62°N10W
N19W 66N E 10°S23E, 42°S42E, 60°S69E
N11E 62NW 61°S82W
N84E 53SE 53°S9E
N84W 75NE 84°N19W
N 75W 68N E 66°N40E
N70W 83NE 80°N70E
N 29W 45NE 45°N62E
N35E 78N W 32°S43W
N53E 80NW 13°S55W
35°N31E
N 63W 88N E 30°S64E
N 59W 55SW 48°S69W
N 21W 71SW 19°S14E, 32°S8E, 70°S70W ( t x)
0°S20W
N9E 63NW 63°N85W
N10E 71SE 44°S9E
N27E 84NW 12°N26E
N40E 89SE 10°N40E
N52E 89NW 11°N52E
N69E 80NW
Figure 58, Dead Indian Creek
a. Faults and stria e, north end of allochthon where underlain by
transgressive fa u lt
Fault Orientations
N 89W 46NE
N 81W 57NE
* N78W 58NE
Striae Orientations
35°N49E
31°N76E
56°N36E, 57°N7W
250
Fault Orientations
N 75W 42SW
★
N70W 65NE
★
N62W 38NE
*
N60W 39SW
- [
"N59W 87NE
L
.N22W 74NE
N52W 58NE
N 9W 67NE
N 9W 77NE
N 7W 46NE
N41E 44SE
N43E 75SE
N44E 87NW
N67E 87NW
N74E 81SE
★
N 88E 56N W
Striae Orientations
19°S53E
61°N55E
38°N12E
35°S63W, 15°N69E
85°N22W, 75°S76E
73°N51E
54°N71E
67°N78E
37°N1E
46°S89E
41°S77E
69°S1E, 15°S39W
52°N40E
24°S68W
67°S86E
56°N4W
b. Faults and striae, north end of allochthon where underlain by
bedding fault
Fault Orientations Striae Orientations
N 85W 69SW 43°S75W
N 81W 23SW 23°S4E
N67W 30NE
N 64W 78SW 75°S61W
N65W 48NE
N 58W 56NE
N 58W 78NE
N 56W 89 S W 80°S50E
N50W 70NE
N47W 22NE 22°N43E
N34W 44SW 33°N76W, 43°S72W
N29W 70SW 70°S61W
N20W 78SW 77°S48W
N 6W 67NE 62°N46E
*N-S 54E 48°N52E
N2E 75SE
N19E 74SE 63°N53E
N21E 79NW 78°N44W, 64°S44W
N30E 79NW 12°S33W
N34E 41SE 41°S56E
N35E 75NW 75°N55W
N37E 88S E 6°N37E
N40E 90 65°S40W
N52E 86N W 82°S81W
N62E 74NW 71°N60W
N81E 40SE 40°S8E
26°N15E
261
c. Upper-plate dike
Dike Orientations
N80E90
Figure 60, Heart Mountain
a. Faults and striae
Fault Orientations Striae Orientations
N 87W 77SW 73°S44W
N 87W 58SW
"N84W 72NE 72°N12E
N70E 78NW 76°N10E
N80E 86N W 86°N9W
N 74W 76SW 59°S50E
N60W 67 S W 65°S5W
P N 58W 42NE 42°N30E
LN36W 37NE 32°N20E
FN50W 61NE 59°N16E
LN13E 68 N W 59°N30W
*N41W 26SW 11°S19E
N 35W 30NE
N 25W 22SW
N 22W 77SW 74°N76W
N 16W 44SW
N 15W 68N E 67°S88E
N 12W 70SW 61°S28W
N10W 18SW 16°N66W
N 7W 77SW 70°46W
f N5E 60NW 59°S75W
L N27E 52NW 50°N42W, 50°N88W
N30E 42SE
N35E 58SE 57°S35E, 50°S12E
N35E 63NW
N40E 57N W 57°N50W
[ N39E 57SE 32°S16W
L N58E 56SE 52°S2E
N52E 86N W 79°S75W
N60E 54SE 54°S13E, 31°N85E
N65E 55SE 55°S25E
b. Bedding of Paleozoic rocks
Bedding Orientations
N64W 35NE
N56W 45NE
N54W 13NE
N 4B W 27NE
N43W 25NE
N42W 35NE
N40W 38NE
N35W 20NE 262
Bedding Orientations
N32W 26NE
N20W 27NE
N11W 34NE
N 9W 36NE
N 3W 25SW
N26W 20SW
Figure 63, South end of Logan Mountain
a. Faults and striae, area mapped in Fig. 61
In volcanic rocks
Fault Orientations Striae Orientations
N 86W 85SW 78°S70E, 72°S78W
N30W 77SW 55°S11E, 74°S22W, 76°S85W
N26W 59NE 53°S79E
N28E 58NW 41°N4W
N40E 50NW 11°N31E
N46E 75SE 62°S15W
N54E 33SE 31°S63E
N60E 70SE 28°N71E, 63°S73E
N70E 67NW 3° S72W
N73E 57SE 55°S8W
Along contact between upper-plate volcanic and Paleozoic rocks
N-S 45E 2°N2E
CO 3 Faults
In Paleozoic rocks
N49W 84NE 6°S49E, 56°S58E
N24E 82NW 29°N19E, 54°N12E, 5°S24W
N3E 85SE 12°S2W, 55°S4E
b. Faults and striae in Paleozoic rocks in area 0-2 km east of
area of Fig. 61
Fault Orientations Striae Orientations
N 86W 50NE 50°N12E
N73W 56NE 53°N8W
N67W 66S W 29°S54E
N66W 40NE 13°N50W, 40°N25E
N65W 82SW 79°S68W
N40W 46SW 30°S7E
N20W 40NE
N19W 68N E 52°N13E reverse fa u lt
N5E 50SE
N10E 71NW 58°S44W, 68°S70W
N10E 58SE 58°S71E
N19E 88S E 7°S18W
N23E 67SE 33°N39E
N28E 37SE 26°N68E, 37°S50E, 36°S82E
N33E 36NW
N33E 70SE 30°N45E 263
Fault Orientations Striae Orientations
N33E 10SE 9°S28E
N35E 89SE 15°S35W, 29°S35W
N37E 74SE 51°N58E
N46E 29SE 30°S41E
N51E 90 58°S51W
N56E 52N W 52°N34W
N60E 46NW 22°S82W, 36°N77W
N61E 79N W 20°N57E
N67E 87SE 81°S49W
N70E 81NW 39°N63E
N74E 44SE 7°N82E, 12°S62W
N75E 82SE 66°S57W
N77E 65SE 65°S25E
N81E 67 S E
c. Major fault in Paleozoic rocks
t Orientations Striae Orientations
N 17W 55NE 54°N62E, 47°S64E
N5E 70SE 58°N40E
N10E 65SE 13°N16E, 41°N35E
N12E 20SE 19°N80E, 19°S51E
N14E 37SE 36°N86E
N22E 35SE 0°N22E
N27E 30SE 26°N83E
N34E 64SE 3°N36E, 32°N51E
N36E 69SE 61 °N81E, 14°S30W, 15°N43E
N37E 35SE 35°S48E
N39E 71SE 13°N44E, 30°N50E, 47°N61E
N40E 67SE 12°S34W
i
264 !

P h . t ) .
G e
H&ll
PLATE IE
EXPLANATION
E o c e n e volcanic rocks,
undifferentiated
Ordovician, D evonian, a n d
Mississippian rocks
allochthonous o n th e
M O
Heart Mountain bedding
trend of m ost well-
developed striae, light
line show s trend(s) o f
Heart Mountain break-
depositional contacts in
upper plate of Heart
M ountain detachm ent
M O
PLATE m . GENERALIZED GEOLOGIC MAP SHOWING TREND OF STRIAE
OBSERVED ON THE HEART MOUNTAIN BEDDING FAULT. ALL
OBSERVED STRIAE UNDERLIE VOLCANIC ROCKS.

Asset Metadata

Creator Hauge, Thomas Armitage (author)

Core Title Geometry and kinematics of the Heart Mountain detachment fault, northwestern Wyoming and Montana

Contributor Digitized by ProQuest (provenance)

Degree Doctor of Philosophy

Degree Program Geological Sciences

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag Geology,OAI-PMH Harvest

Language English

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c29-355411

Unique identifier UC11220734

Identifier DP28565.pdf (filename),usctheses-c29-355411 (legacy record id)

Legacy Identifier DP28565.pdf

Dmrecord 355411

Document Type Dissertation

Rights Hauge, Thomas Armitage

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the au...

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus, Los Angeles, California 90089, USA

Linked assets

University of Southern California Dissertations and Theses