Western Michigan University ScholarWorks at WMU
Master's Theses Graduate College
4-1984
Structural Geology of the Northern Snowcrest Range, Beaverhead and Madison Counties, Montana
Mark Kenneth Sheedlo
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Recommended Citation Sheedlo, Mark Kenneth, "Structural Geology of the Northern Snowcrest Range, Beaverhead and Madison Counties, Montana" (1984). Master's Theses. 1571. https://scholarworks.wmich.edu/masters_theses/1571
This Masters Thesis-Open Access is brought to you for free and open access by the Graduate College at ScholarWorks at WMU. It has been accepted for inclusion in Master's Theses by an authorized administrator of ScholarWorks at WMU. For more information, please contact [email protected]. STRUCTURAL GEOLOGY OF THE NORTHERN SNOWCREST RANGE, BEAVERHEAD AND MADISON COUNTIES, MONTANA
by
Mark Kenneth Sheedlo
A Thesis Submitted to the Faculty of The Graduate College in partial fu lfillm e n t of the requirements for the Degree of Master of Science Department of Geology
Western Michigan University Kalamazoo, Michigan April 1984
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission STRUCTURAL GEOLOGY OF THE NORTHERN SNOWCREST RANGE, BEAVERHEAD AND MADISON COUNTIES, MONTANA
Mark Kenneth Sheedlo, M.S.
Western Michigan University, 1984
The Snowcrest Range was uplifted in Late Cretaceous and Early
•Tertiary time as a consequence of thrusting in Precambrian basement
and Phanerozoic cover rocks.
The Snowcrest-Greenhorn thrust system is associated with large-
scale overturned, eastward verging folds. Mean orientation of the
thrust system is N.40°E., 35°N.W.. The trend of the thrust system
and associated transverse faults appear to be controlled by earlier
structures. Mesoscopic and microscopic analyses indicate that meta-
morphic basement rocks accommodated congressional strain largely by
b rittle faulting and cataclastic shearing. Kinematic indicators
suggest that local stresses were directed east-southeastward.
Northeast-trending Neogene faults along the western flank of
the range follow the trend of the thrust system. A listric-normal
geometry for these faults is demonstrated by rotation of Tertiary
beds along the range front. The position and inferred geometry of
Neogene normal faults may, therefore, reflect localization of
basin-range extension along zones of Laramide weakness.
with permission of the copyright owner. Further reproduction prohibited without permission ACKNOWLEDGEMENTS
Sincere thanks are extended to my advisor, Dr. Christopher J.
Schmidt of Western Michigan University for his support, encourage
ment and critique throughout the course of this study. Thanks also
go to Dr. W. Thomas Straw (Western Michigan University) and Dr. James
S. Monroe (Central Michigan University) for providing suggestions and
constructive criticism during preparation of the manuscript. For
their helpful encouragement, I wish to thank Dr. William J. Perry
(United States Geological Survey) and Dr. John M. Garihan (Furman
University).
Special thanks go to Dr. Hugh W. Dresser (Montana College of
Mineral Science and Technology) for generously providing a reconnais
sance flig h t over the study area during the 82 fie ld season, and
Peter J. McQuade (Central Michigan University) for providing able
assistance during the 83 fie ld season.
For sharing in numerous discussions of mutual geologic problems,
I wish to thank my colleagues William G. Gierke (B ill, for short) and
Michael Werkema (Western Michigan University), and Susan Young
(University of Texas - Austin). Robert Havira (Western Michigan
University) gave willing ass'stance in the technical areas and with
photography. Nancy Likam typed the final manuscript.
i i
with permission of the copyright owner. Further reproduction prohibited without permission. Field work was supported by a grant from the Graduate College
of Western Michigan University, and by National Science Foundation
Grant EAR 7926380 to Schmidt and Garihan.
I would especially like to thank my parents, Ken and Dorothy,
for unfailing financial and moral support. My friends, Mary Lannon
and Kathleen Jalkut, deserve special recognition for putting up with
me during the final stages of completion of this project.
Mark Kenneth Sheedlo
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SHEEDLO, MARK KENNETH
STRUCTURAL GEOLOGY OF THE NORTHERN SNOWCREST RANGE, BEAVERHEAD AND MADISON COUNTIES, MONTANA
WESTERN MICHIGAN UNIVERSITY M.S. 1984
University Microfilms
International 300 N. Zeeb Road, Ann Arbor. MI 48106
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS
ACKNOWLEDGEMENTS ...... 11
LIST OF TABLES...... v iil
LIST OF FIGURES...... ix
LIST OF PLATES...... x iii
Chapter
I. INTRODUCTION ...... 1
Scope and Purpose of In v e s tig a tio n ...... 1
Methods of Investigation ...... 5
Previous and Contemporaneous Investigations ...... 6
II. GENERAL STRATIGRAPHY AND PRE-LARAMIDE TECTONIC SETTING ...... 8
General Stratigraphy ...... 8
Precambrian Units ...... 10
Paleozoic and Mesozoic Units ...... 11
Late Mesozoic and Cenozoic Units ...... 13
Pre-Laramide Tectonic Setting ...... 14
Precambrian (Pre-Beltian) Setting ...... 14
Precambrian (Beltian) Setting ...... 17
Paleozoic and Pre-Laramide Mesozoic Setting ...... 20
I I I . LARAMIDE DEFORMATION IN THE NORTHERN SNOWCREST RANGE . . 27
Regional Laramide Tectonic Setting ...... 27
i v
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter
Description of Faulting ...... 30
General Statement ...... 30
Snowcrest Thrust Z o n e ...... 32
Greenhorn Thrust Z o n e ...... 36
Minor F a u lts ...... 36
Description of Folding ...... 42
General Statement ...... 42
Snowcrest Anticline ...... 42
Spur Mountain Syncline ...... 43
Sliderock Mountain Anticline ...... 43
Jobe Creek Antic!ine-Syncline ...... 45
Tectonic Inheritance ...... 48
Snowcrest-Greenhorn Thrust System ...... 48
Northwest-Trending Faults ...... 50
Overall Structural Relationship ...... 51
Age of Deformation ...... 61
Timing of U p l i f t...... 61
Timing of Thrusting and Folding ...... 62
Sequence of Deformation ...... 62
Mechanical Behavior of Pre-Beltian Rocks ...... 64
Thrust Movement Patterns ...... 69
Stress Configuration ...... 72
v
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter
IV. POST-LARAMIDE DEFORMATION IN THE NORTHERN SNOWCREST RANGE...... 74
Regional Post-Laramide Tectonic Setting ...... 74
Description of Faulting ...... 76
Snowcrest-Greenhorn Range-Front Fault System .... 76
Northwest-Trending Normal Faults ...... 79
Description of Folding ...... 81
Ledford Creek Anticline ...... 81
Tectonic Inheritance ...... 82
Snowcrest-Greenhorn Range-Front Fault System .... 82
Northwest-Trending Normal Faults ...... 83
Overall Structural Relationship ...... 83
Age of Faulting ...... 87
Northwest-Trending Normal Faults ...... 88
Dynamics of Normal Faulting ...... 88
Fault Movement Patterns ...... 91
Northwest-Trending Faults ...... 91
Snowcrest-Greenhorn Range-Front Fault System .... 92
Stress Configurations ...... 94
Tectonic Controls on Sedimentation ...... 95
Sixmile Creek Formation ...... 95
Environment of Deposition ...... 96
Tectonic Significance of Sixmile Creek Sedimentation ...... 96
vi
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V. SUMMARY AND SYNTHESIS...... 100
Geologic History ...... 100
Conclusions ...... 104
APPENDIX
A. Stereographic Analysis of Folds ...... 106
BIBLIOGRAPHY ...... 12°
v ii
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF TABLES
TABLE
1. Generalized stratigraphic column of the northern Snowcrest Range ...... 9
2. Comparison of fold-thrust characteristics ...... 55
v iii
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF FIGURES
FIGURE
1. Photograph (looking south along the Upper Ruby Road) of the northern Snowcrest Range ...... 2
2. Index map showing the area covered by this project . . . 3
3. Generalized geologic map showing mountain ranges which are cored by pre-Beltian metamorphic rocks in southwestern Montana ...... 4
4. Aerial photograph (looking south) of the core of the northern Snowcrest Range ...... 12
5. A. Photograph (looking southwest) of Stonehouse Mountain, capped by Beaverhead Formation. B. Photograph (looking east) of recent land slide deposits along the western flank of the northern Snowcrest Range ...... 15
6. Generalized tectonic map showing the relationship between the Belt Basin, the LaHood Formation, and the zone of northwest-trending fa u lts ...... 19
7. A. Generalized geologic map showing the trend of the Snowcrest-Greenhorn thrust system. B. Iso- pachous map of the Cambro-Ordovician strata in southwestern Montana ...... 22
8. A. Generalized geologic map showing the trend of the Snowcrest-Greenhorn thrust system. B. Isopachous map of the Mississippian strata in southwestern Montana ...... 25
9. Schematic map showing important Laramide tectonic elements including major arches in southwestern Montana ...... 29
10. S-pole diagram of bedding for the east and west flanks of the Blacktail-Snowcrest uplift ...... 31
ix
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. FIGURE
11. A. Composite photograph (looking north) of the Snowcrest thrust zone. B. Interpretive sketch of the structural relationships along the Snowcrest thrust zone ......
12. A. Photograph (looking north) of the easternmost Snowcrest thrust segment. B. Interpretive sketch of the structural relationships associated with the easternmost Snowcrest thrust segment ......
13. A. Photograph (looking south) of the Snowcrest thrust zone near Olsen Peak in the central Snow crest Range. B. Interpretive sketch of the Snowcrest thrust zone ......
14. Photograph (looking northwest) of the eastern flank of the southern Greenhorn Range, and the northern Snowcrest Range ......
15. A. Aerial photograph (looking south) of the minor thrust associated with Sliderock Mountain anticline. B. Interpretive sketch of the structural relationships along the thrust ......
16. A. Photograph (looking north) at the Fawn Creek structure, sec. 24, T. 10 S., R. 4 W. B. Sketch showing the structural relationships between folding and faulting ......
17. A. Photograph (looking north) along the western flank of the northern Snowcrest Range. B. Interpretive sketch showing the local steep ening of dips in Cambrian units on the hanging- wall of the Snowcrest thrust zone ......
18. Stereoplots comparing the orientation of solution cleavage in the Big Snowy Group to: A) the orien tation of the Sliderock Mountain anticline, and B) the average orientation of the Snowcrest thrust zone (32 poles to cleavage contoured at 1%, 6%, 11%, and 17%) ......
19. A. Photograph (looking south) of parasitic folds in the Amsden Formation and Big Snowy Group exposed in sec. 23, T. 10 S ., R. 4 W. B. Sketch showing the relationship between parasitic folds and the Sliderock Mountain anticline ......
x
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. FIGURE
20. Schematic map of southwestern Montana showing the spatial relationship between dominant pre-Beltian structural trends, the Snowcrest-Greenhorn lineament, and the Snowcrest-Greenhorn thrust system ......
21. Two diagrammatic models representing contrasting geometric interpretations for the Blacktail- Snowcrest u p lift. A. Fold-thrust model (Berg, 1962). B. Upthrust model (after Prucha et a l., 1965) ......
22. Diagrammatic model showing the possible relation ship between observed foreland thrusting and deep crustal detachment ......
23. Generalized Pal eocene tectonic map showing struc tural and sedimentological relationships in the Blacktail-Snowcrest arch region ......
24. Generalized Paleocene structural sections A-A1 and B-B' across the Blacktail-Snowcrest arch. SG = Snowcrest-Greenhorn thrust system; G = Gravelly thrust system; R = Ruby thrust zone; FT = fold and thrust belt; B = synorogenic deposits (Beaverhead Formation); and P = Phanerozoic sedimentary rocks ......
25. A. Photograph (looking northeast) of well foliated pre-Belt amphibolites exposed in sec. 23, T. 11 S., R. 5 W. B. Photograph of well developed slickensides along foliation surface ......
26. Stereoplots showing the orientations of bedding and fo lia tio n : A) 75 poles to pre-Beltian meta- morphic fo lia tio n (contours at 1%, 4%, 9%, and 13%), and B) 116 poles to Paleozoic bedding (contours at 1%, 3%, 5%, 7%, and 13%) ......
27. A. Analysis of B for parasitic folding (Appendix A) with the Snowcrest thrust (42 measured and constructed fold hinges; contours at 3%, 8%, 13%, and 32%). B. Analysis of frac tures parallel to the Snowcrest thrust (30 slickensides; contours at 4%, 7%, 10%, and 13%) . . .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. FIGURE
28. Generalized post-Laramide tectonic map of south western Montana (upper right). Generalized regional tectonic map showing the relative positions of Basin-and-Range, Fold-and-Thrust belt, and Rocky Mountain foreland provinces ...... 75
29. Generalized post-Laramide tectonic map of extreme southwestern Montana ...... 77
30. Aerial photograph (looking north) of the Snow crest Range. The Snowcrest-Greenhorn range- front fault system is exposed along the western flank of the r a n g e ...... 78
31. Generalized block diagram showing the relation ship between post-Laramide normal faulting and low angle Laramide thrusting. T = Tertiary deposits, M = Mesozoic rocks, P = Paleozoic rocks, and A = pre-Beltian r o c k s ...... 84
32. Schematic diagram showing: A) pre-normal fault geometry, and B) "reverse drag" structure formed as a result of listric normal faulting ...... 86
33. Diagrammatic model showing the possible relation ship between extension due to broad arching and basin-range normal faulting ...... 90
34. A. Photograph (looking east) of massive c l i f f of conglomeratic mudstones of the Sixmile Creek Formation. B. Photograph (looking north) of scour-and-fill structure in conglomeratic sand stone overlying waterlaid cross-stratified ash tuffs of the Sixmile Creek Formation ...... 97
35. A. Photograph (looking east) of a distinctive eastward thickening wedge of travertine belong ing to the Sixmile Creek Formation. B. Inter pretive sketch of the relationship between Tertiary Sixmile Creek basin deposits and the Snowcrest-Greenhorn range-front fa u lt ...... 98
36. Diagrammatic model showing the structural evolution of the Blacktail-Snowcrest arch region (vertical exaggeration approximately 3X) 102
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF PLATES
PLATE
I. Geologic map of the northern Snowcrest Range, Beaverhead and Madison counties, Montana
II. Structure sections of the northern Snowcrest Range, Beaverhead and Madison counties, Montana
III. Tectonic map of the Snowcrest, Greenhorn, and Gravelly ranges
IV. Structure sections of the Snowcrest, Greenhorn, and Gravelly ranges
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER I
INTRODUCTION
Scope and Purpose of Investigation
The Snowcrest Range is one of several mountain ranges in south
western Montana which are cored by pre-Beltian metamorphic rocks
(Figure 3). Although the Snowcrest Range has long been recognized
as the thrust faulted, overturned limb of the major Blacktai1-Snow-
crest u p lift (Scholten, 1955), disagreement persists as to the precise
nature of deformation in the region (Scholten, 1955, 1967; Perry et
a l., 1981; Ruppel, 1981; and Schmidt and Garihan, 1983). Therefore,
the purpose of this investigation is to more clearly define the nature
and controls of deformation in and adjacent to the northern Snowcrest
* Range. The methods employed in this study include the following:
(1) detailed field mapping and analysis of folding and faulting in
the northern Snowcrest Range, (2) examination of the degree to which
observed fault trends reflect reactivation of previously developed
structural trends, and (3) interpretation of the regional signifi
cance of structures observed in the northern Snowcrest Range.
Approximately 155 square kilometers in T. 9, 10 and 11 S.,
R. 3, 4 and 5 W., Madison and Beaverhead Counties, Montana
(Figures 1 and 2) were mapped in detail. The study area generally
coincides with the northern half of the Snowcrest range.
1
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2
Figure 1. Photograph (looking south along the Upper Ruby Road) of the northern Snowcrest Range.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3
n ^ MADISONrr^~COUNTY ^n \ \ J
BEAVERHEAD COUNTY N ▲ 45° 00' C
112 00
80 Km
| | | | f § | Area mapped
| A V \ Area covered by tectonic compilation
Figure 2. Index map showing the area covered by this project.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4
Bozeman /V“/ Twin Tobacco'Roo Bridges ■
' \ / i Northern u / '7^ ' \ / \ / N. Virgini^^V) v' / s A Madison / X ^ . / I ' ' ' /\ ^ Ennis / r / \ ^ \ Greenhorn Ruby^ Range Gravel uthern Ra dison Range Snowcrest B lack tail Range Range / y West ^ ^ v //Yellowstone®h MONTANA TDAHO MONTANA pre-Beltian metamorphic rocks Figure 3. Generalized geologic map showing mountain ranges which are cored by pre-Beltian metamorphic rocks in south western Montana. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5 The northern and eastern margins of the range are accessible by the Upper Ruby and the Centennial Divide roads respectively. The western flank of the range is reached by a system of secondary roads along Ledford, Robb, and Blacktail Deer creeks. The inte rio r of the range is generally inaccessible to field vehicles. The northern Snowcrest Range was suggested as a study area by Dr. Christopher J. Schmidt as part of a regional investigation of north-trending, basement involved thrusts in southwestern Montana. Methods of Investigation Initial phases of the investigation involved the preparation of a geologic map. Approximately eight and one-half weeks were spent in the fie ld during the summer of 1982, and two weeks were spent in the area during the early fa ll of 1983. Geologic mapping was done on U.S.D.A. and U.S. Forest Service aerial photographs. Field data were then transferred to United States Geological Survey topographic base maps at a scale of 1:24,000 (Plate I). The study area covers portions of Home Park Ranch, Swamp Creek, Spur Mountain, Antone Peak, and Stonehouse Mountain 7 h° quadrangles; and portions of the Varney and Monument Ridge 15° quadrangles. A regional tectonic map covering approximately 2,375 square k ilo meters in portions of T. 7 to 13 S., R. 2 to 7 W., Beaverhead and Madison Counties, Montana, was compiled (Plate I I I ) . The compilation incorporates previous work by Vaughn (1948), Lemish (1948), Honkala (1949), Brasher (1950), Keenmon (1950), Klepper (1950), Gealy (1953), Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Mann (1954, 1960), Scholten (1955), Flannagan (1958), Hadley (1960, 1969a), Monroe (1976), and Tilford (1978). Structural analyses were made for several portions of the study area with emphasis on analysis of folding (Appendix A). Statistical evaluations of structural data were conducted following the methods outlined in Turner and Wiess (1963). The final phases of the investigation involved the construction and interpretation of structural profiles (Plates II and IV). Previous and Contemporaneous Investigations The earliest geologic work in the vicin ity of the Snowcrest Range was conducted in 1871, when the Hayden survey traveled north ward through the valley of the "Stinking Water" (Ruby) River. Hayden (1872) noted the presence of Tertiary sedimentary rocks including extensive hot-spring deposits along the western flank of the Upper Ruby Basin. During the period from 1916 to 1924, the United States Geological Survey conducted reconnaissance investigations as part of a regional evaluation of phosphatic o il shale potential. Work by Condit, Finch and Pardee (1927) included descriptions of Paleozoic stratigraphy in the Snowcrest Range. The fir s t published reconnaissance map of the region was pro duced by Klepper (1950). Portions of the area covered by the 1:24,000 scale map area overlap the area mapped by Klepper. The extreme north eastern portion of the Snowcrest Range was originally mapped in reconnaissance by Mann (1954). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 7 In contrast with the northern Snowcrest Range, adjoining areas have received comparatively more attention. The Greenhorn Range was in itia lly mapped by Hadley (1960, 1969a) and Berg (1979). A detailed structural analysis of pre-Beltian metamorphic rocks in the Greenhorn Range was completed by Tilford (1978). Geologic studies were con ducted in the Gravelly Range by Mann (1954, 1960), Vaughn (1948), and Lemish (1948). The southern Snowcrest Range was mapped by Honkala (1949), Brasher (1950), Keenmon (1950), Gealy (1953), and Flannagan (1958). The Ruby Range has been studied extensively by Heinrich (1960), Okuma (1971), Garihan (1973), Tysdal (1976, 1981), and Karasevich (1981). Tertiary geology of the Upper Ruby Basin adjacent to the study area was well documented by Monroe (1976, 1981). The tectonic his tory of the region has been considered in numerous studies (see, for example, Scholten, 1955*, 1957, 1960, 1967; Eardley, 1960, 1963; Perry et al_., 1981; and Schmidt and Garihan, 1979, 1983). Most recently, the southern Snowcrest Range has been the site of renewed interest on the part of the United States Geological Survey and private industry. Geological and geophysical investiga tions are being carried out in an attempt to re-evaluate the petroleum potential of the region (Kulik and Perry, 1982; and Perry et a l., 1981, 1983). Field investigations leading to this report were conducted in conjunction with sim ilar studies in the northern Madison Range by Werkema (1984) and Young (1984). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER I I GENERAL STRATIGRAPHY AND PRE-LARAMIDE TECTONIC SETTING General Stratigraphy The strati graphic record in the northern Snowcrest Range is repre sented by Precambrian (pre-Beltian) metamorphic rocks and a relatively complete section (approximately 3,000 m) of Paleozoic and Mesozoic rocks. Tertiary basin deposits flank the Snowcrest Range to the west and south. Regional stratigraphy in southwestern Montana has been studied by many workers (see, for example, Honkala, 1949; Moritz, 1951; Sloss and Moritz, 1951; Kummel, I960; McMannis, 1965; Maughn and Roberts, 1967; Suttner, 1969; Swanson, 1970; Peterson, 1981; and Schwartz et al_., 1983), and a continuing detailed study of the Mississippian- Pennsylvanian boundary is presently being conducted by the United States Geological Survey (Perry, 1983, personal communication). Complete stratigraphic descriptions of the region were given by Mann (1954) and Hadley (1980). Although such descriptions are not presented in this report, an overview of general stratigraphy is presented, and a generalized stratigraphic column is shown in Table 1. 8 with permission of the copyright owner. Further reproduction prohibited without permission. 9 < A lluvium 0-20rr TABLE 1 3 Glacial tHI o- Six Mile 610m CrockCroeft / >- _Eflttmolifln./ =fer^s5 IDEALIZED STRATIGRAPHIC COLUMN ce 510- < Ron ova Norlhem. Snowcrest Range H 1,525 CC Formation m U4 Montana Beaverhead 0- Formation 1,000 m Modified from Hodley,1980 ______ co Metamorphic Undivided Mission complex 1,000- Canyon oIx l Upper < 1,400 Formation t— Cretaceous m l i i Sandstone a: o- Kootenai Lodgepole S llts to n e Formation Formation Morrison <£ ! Formation =3 Ellis Three Forks Group Shalo or mudstone Thaynes Formation Formation Jefferson T ~ rr CO CO Woodside Formation < Formation Bighorn IQ- Llm ostono CC 43m . Dinwoody Dolomite H 34 - Formation Red Lion Formation 3 2 ^ - y ', ^ s Phosphoria Shaloy limestone CC 110- Formation Pilgrim £ I30m Formation z < Park D o lom ite Quadrant Formation Formation Meagher Bedded chert Formation z UJ Amsden s VVolsey .'o • a. Formation O '. Ca Formation Conglomeratic < Flathead sandstono zl Undivided CL Formation Big Snowy Undivided m 8 Conglomeratic CO Group pre-Beltian CO m u d sto n e strata Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 10 Precambrian Units The Snowcrest Range is cored by pre-Beltian metamorphic rocks and is similar to other ranges in the Rocky Mountain foreland province in southwestern Montana. The pre-Beltian metamorphic rocks exposed in the northern Snowcrest Range consist chiefly of quartzo-feldspathic gneisses, amphibolites and subordinate marbles, schists and quart- zites. Similar lithologies have been assigned to the Cherry Creek Group in the Greenhorn Range, approximately 10 km north of the present study area (Berg, 1979; Hadley, 1969a; & T ilford, 1978). However, recent work by Vitaliano et al_. (1979) and Erslev (1982, 1983) in southwestern Montana suggests that the designation of pre-Cherry Creek and Cherry Creek assemblages may require reevaluation. Therefore, pre- Bel tian rocks of the pre-Cherry Creek and Cherry Creek assemblages are undivided in the northern Snowcrest Range. Klepper (1950) fir s t recognized pre-Beltian metamorphic rocks along the western flank of the Snowcrest Range, near the Middle and East forks of B1acktai1 Deer Creek. Good exposures of these rocks are present in secs. 23 and 26, T. 11 S., R. 4 W., about 2 km north of the East Fork of B1acktai1 Deer Creek (Plate I). Other exposures are scattered along the western flank of the range, but good exposures may be observed in secs. 24 and 25, T. 9 S., R. 4 W., roughly 3 km southwest of Ruby Canyon (Plate I). Exposures of Precambrian Beltian sedimentary rocks (Middle Pro- terozoic) are not exposed within the Rocky Mountain foreland and, therefore, these rocks do not outcrop in the Snowcrest Range. Closest Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 11 exposures of Beltian sedimentary rocks occur in the fold-and-thrust belt in the northern Tobacco Root Mountains and in the south-central Highland Range, roughly 80 km north of the Snowcrest Range (McMannis, 1963, 1965). Paleozoic and Mesozoic Units Rocks of Early Paleozoic age are exposed only along the western, thrust-faulted flank of the Snowcrest Range. Klepper (1950) recog nized Cambro-Devonian rocks, but did not differentiate them on his reconnaissance map. Rocks ranging in age from the Middle Cambrian Flathead Sandstone to the Late Devonian Three-Forks Shale are typi cally poorly exposed, but are best exposed in secs'. 13 and 24, T. 11 S., R. 4 W., about 2 km west of Olsen Peak. Upper Paleozoic rocks are somewhat better exposed in the region. Rocks ranging in age from the Early Mississippian Madison Group to the Middle Pennsylvanian Quadrant Formation "hold up" the core of the Snowcrest Range (Figure 4). Complete sections of Mississippian Madison and Big Snowy Groups are obscured by thrusting along the western flank of the range. Mesozoic rocks rest comformably on Paleozoic rocks in the study area. The overturned section exposed along much of the eastern flank of the range consists of rocks of the Triassic Dinwoody Formation through the Early Cretaceous Kootenai Formation. Spectacular exposures of this overturned section occur in secs. 5 and 8, T. 11 S., R. 4 W., along the eastern slope of Hogback Mountain, and in secs. 19 and 30, T. 11 S., R. 4 W., along the eastern slope of Olsen Peak (Plate I). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 4 Aerial photograph (looking south) of the core of the northern Snowcrest Range. Major peaks are "held up" by Upper Paleozoic rocks. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 13 Upper Cretaceous rocks are typically poorly exposed along the eastern flank of the range. The best exposures of these rocks occur along the Upper Ruby River. Late Mesozoic and Cenozoic Units Synorogenic sedimentary deposits of probable Late Cretaceous to Early Tertiary age are extensively exposed along eastern slope of the Snowcrest Range (Klepper, 1950; and Gealy, 1953). These rocks belong to the Beaverhead Formation, named by Lowell and Klepper (1953) in south western Montana and adjacent Idaho. The Beaverhead Formation is exposed in the extreme southern portion of the study area near Stone- house Mountain (Figure 5A, Plate I), and probably belongs to the Clover Creek lithosome described by Ryder (1957). Basin deposits of Tertiary age are exposed along the length of the western flank of the Snowcrest Range. Monroe (1976) recognized these deposits as Early Miocene-Pliocene Sixmile Creek Formation in - the northwestern portion of the present study area (Plate I). Quaternary sediments exposed in the area were mapped as alluvium and undivided colluvial deposits. Spectacular recent landslide deposits were recognized by the writer in secs. 1, 12, and 25, T. 11 S., R. 4 W., in the central portion of the study area (Figure 5B, Plate I). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 14 Pre-Laramide Tectonic Setting Numerous structural and stratigraphic studies in southwestern Montana have discussed the influence that previously developed struc tural elements may have had on later deformation and sedimentation (see, for example, McMannis, 1965; Schmidt, 1975; Schmidt and Garihan, 1979, 1983, 1984; Schmidt and O'Neill, 1982; Perry et a]_., 1981; and Maughn et al_., 1983). Because precursory structural elements may have played an important role in later deformation and sedimentation, the pre-Laramide tectonic history of southwestern Montana w ill be out lined, and important pre-Laramide structural elements w ill be discussed. Precambrian (Pre-Beltian) Setting As previously mentioned, southwestern Montana is characterized 1 by a number of Precambrian cored mountain ranges. These ranges expose pre-Beltian rocks which have complex deformational and metamorphic histories (Heinrich, 1960; Garihan, 1973; ReidetaJ_., 1975; Spencer and Kozak, 1975; Vi tal iano £t al_., 1979; and Erslev, 1983). Generally speaking, pre-Beltian rocks exposed in the region can be divided into two groups based roughly on age and lithologic char acter. The fir s t group consists of metasedimentary and subordinate metaigneous rocks belonging to the Cherry Creek Group. Pre-Beltian rocks exposed in the northern Snowcrest Range include strata assigned to the Cherry Creek Group. Vitaliano et a^. (1979) have suggested that this sequence originated as a stratiform sequence of epiclastic, volcaniclastic and carbonate sedimentary deposits. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission Figure 5. A. Photograph (looking southwest) of Stonehouse Mountain, capped by Beaverhead Formation. B. Photograph (looking east) of recent landslide deposits along the western flank of the northern Snowcrest Range. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 16 In view of recent studies of pre-Beltian rocks in southwestern Montana, a marked divergence of opinion exists regarding the environ ment of Cherry Creek deposition. Fountain and Desmarias (1980) suggested that Cherry Creek protoliths represent subduction-related accretionary prism deposits. In contrast, Erslev (1983) suggests these rocks were originally representative of distal shelf deposits on a passive continental margin. Although radiometric dates for these rocks span a wide range, the latest thermal event registers with ages ranging between 1600 and 1700 m.y.b.p. (G ile tti, 1966). This age probably corresponds to thermal a ctivity associated with the Hudsonian orogeny (Condie, 1975) and places the pre-Beltian rocks of this region within the age range of the Churchill province as defined in the Canadian 3 .-Id, The second group of pre-Beltian metamorphic rocks is exposed farther to the east and has been referred to as the Snowy Block (Reid et al_., 1975). The Snowy Block is also characterized by metasedi- mentary and metaigneous assemblages, but these rocks are generally much older than the Cherry Creek rocks. Although radiometric dates for rocks in this region are variable, typical ages range roughly between 2500 and 3200 m.y.b.p. (G ile tti, 1966; and Condie, 1975). Therefore, the age of rocks in this terrane lies within the age range of the Wyoming province and correlates roughly with rocks of the Superior province as i t is defined in the Canadian shield. Because these two distinct groups of pre-Beltian rocks are of markedly different age, i t has been suggested that southwestern Montana may be the site of a major tectonic boundary or "suture zone" Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 17 between the Churchill and Wyoming (Superior) provinces (Reid et a 1., 1975; G ile tti, 1966; Fountain and Desmarias, 1980; and Erslev, 1982, 1983). Although the proposed "suture zone" has not been precisely defined, i t appears to trend approximately northeast-southwest. The idea of a major northeast-trending structural boundary compares favor ably with regional studies of pre-Beltian structural trends. Although pre-Beltian structural trends are diverse, the dominant regional trend appears to be northeast-southwest. Northeast-trending structural elements such as fold axes, fo lia tio n , major fracture patterns and mylonite zones a ll support the thesis that a major northeast-trending structural boundary may exist in southwestern Montana (Hadley, 1960, 1969a; Reid et al_., 1975; Spencer and Kozak, 1975; Fountain and Desmarias, 1980; Vitaliano et al_., 1979; and Erslev, 1982, 1983). The dominant northeast-trending structural grain is clearly apparent on a regional scale and, therefore, may have been significant in influencing the development of later structures in southwestern Montana. Precambrian (Beltian) Setting The most significant Late Precambrian tectonic feature in south western Montana is the Belt basin — a deep fault-controlled trough which formed in older Precambrian metamorphic rocks (Figure 6). This trough was fille d with sedimentary deposits of the Belt Supergroup between 800 and 1600 m.y.b.p. (Stewart, 1976). Many workers (see, for example, McMannis, 1963; Harris et al_., 1974; and Schmidt, 1975) have suggested that fault control along the southern margin of the Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 18 Belt basin is indicated by the northward thinning prism of coarse clastic sedimentary rocks (Lahood Formation) deposited there (Figure 6). I t has been suggested that tectonism leading to the development of the Belt basin was also responsible for the formation of a prominent northwest-trending fa u lt set exposed in the region (Schmidt and Garihan, 1979). Although these faults have been interpreted to have o riginally been extensional in nature (Schmidt and Garihan, 1979), other opinions have been forwarded (Ruppel, 1982). The suggestion is that extensional faulting led to the development of the Belt trough — an aulacogen transverse to the Proterozoic North American continental margin. Primary evidence that these faults in itia lly formed during Proterozoic times lies in their close association with Late Precam brian diabase dikes. The dikes appear to have been intruded parallel to most major northwest-trending faults in the region (Heinrich, 1960; Vitalliano et a]_., 1979; and Schmidt and Garihan, 1979, 1984). Two groups of diabase dikes reported by Wooden ert a]_. (1978) yield whole rock Rb-Sr dates of 1455-1430 m.y.b.p. and 1130-1160 m.y.b.p.. Additional age determinations for diabase dikes in the region yield dates ranging from approximately 2000 m.y.b.p. to 740 m.y.b.p. (Condie, 1975). The disparity in ages of diabase dikes may be explained by a multiple sequence of Late Precambrian riftin g events. Burchfiel and Davis (1975) indicated that there may have been two distinct riftin g events. They suggested that two continental terrace-wedge sequences -- the Late Precambrian Belt-Purcell Supergroup, and the latest Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 19 ROCKY MOUNTAIN.FORELAND " , I / ' \ / BELT o \ 07 o ($ c? LaHood .0° 0' jw ® /.Formatio]j^v O' <3'o .one MONTANA V basement'rifting ' WYOMING Isopach thickness / U t a n a ^ O . - I interval = 5,000 ft. IOAHO 0 80 \ M 0 N TANA 1 1 , 1 Km Figure 6. Generalized tectonic map showing the relationship between the Belt Basin, the LaHood Formation, and the zone of northwest-trending faults (modified from Schmidt and Garihan, 1984). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 20 Precambrian Windemere Group may indicate two periods of continental riftin g . This question of multiple riftin g events has yet to be resolved. Regardless of the details of the actual riftin g event(s), a prominent northwest-trending structural grain associated with Protero zoic riftin g is well established in southwestern Montana and may well have been a significant factor in the development of later structures in the region. Paleozoic and Pre-Laramide Mesozoic Setting Following the inferred Late Precambrian riftin g event(s), latest Precambrian time marked the establishment of a passive margin sequence. Consequently, two d istin ct tectonic regions developed: (1) the Cordilleran miogeocline, characterized by extensive "geosynclinal" sedimentation, and (2) the stable craton, characterized by shallow- water platform sedimentation. In southwestern Montana, these two tectonic regions were prominent features throughout Paleozoic and Early Mesozoic time (McMannis, 1965). As might be expected, they significantly influenced patterns of later deformation (Scholten, 1967, 1968). Early Paleozoic sedimentation in southwestern Montana began in Middle Cambrian time and continued into Ordovician time. Cambro- Ordovician marine sedimentary rocks exposed in the northern Snowcrest Range reflect deposition along the existing continental margin near the edge of the stable craton. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 21 Tectonic a ctivity is suggested by significant changes in thickness which occur in these rocks (Figure 7) along a narrow northeast-trend ing zone herein referred to as the Snowcrest-Greenhorn lineament (Maughn and Perry, 1982). Substantial thicknesses (300-500 m) of Cambro-Ordovician rocks are present along a narrow trough to the west of the Snowcrest-Greenhorn lineament, but most of these rocks are absent to the east of the lineament (McMannis, 1965). This marked change in thickness is suggestive of u p lift along the northeast- trending Snowcrest-Greenhorn lineament during Cambro-Ordovician to pre-Late Devonian time (McMannis, 1965; and Hadley, 1980). The ultimate cause of u p lift is unknown. In terms of regional tectonic evolution, the period between Cambrian and Late Devonian was dominated by the Antler orogeny and is marked by the developement of the Klamath-Sierran arc complex and attendant marginal das in between the arc and continent in north- central Nevada (Burchfiel and Davis, 1972, 1975). Deformation related to the partial closure of the Antler marginal basin in Devonian time was dominated by the emplacement of the east-directed Roberts Mountain Allochthon in Nevada and central Idaho (Burchfiel and Davis, 1975). In view of the regional tectonic setting, it seems likely that u p lift along the narrow Snowcrest-Greenhorn lineament may reflect reactivation of a zone of crustal weakness as a result of partial collapse of the Antler marginal basin. I f regional compressive stresses caused the eastward translation of the Roberts Mountain thrust onto the craton in central Idaho, then i t is possible that these stresses were transmitted eastward into the region of u p lift in Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 22 Butte ■ Bozeman Km 750 MONTANA\ 1,000 - WYOMING Isopach thickness interval = 250 ft 0 80 Km Figure 7. A. Generalized geologic map showing the trend of the Snowcrest-Greenhorn thrust system. B. Isopachous map of the Cambro-Ordovician strata in southwestern Montana (modified from McMannis, 1965). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 23 southwestern Montana. Two factors lend support to this idea. First, the Snowcrest-Greenhorn 1 ineament is less than 100 km from the inferred position of the Roberts Mountain thrust plate, so the spatial relation ship between these structures appears compatible. Secondly, as pre viously mentioned, prominent northeast-trending crustal flaws did exist in the pre-Beltian rocks of southwestern Montana and may have influenced Early Paleozoic deformation. Throughout much of Middle to Late Paleozoic time, the Snowcrest Range was located along the western margin of the North American craton. From Late Devonian through Pennsylvanian time the craton was the site of continued platform sedimentation along the eastern edge of the Antler marginal basin between the arc and continent. Platform sedimentation resulted largely in the deposition of carbonate sedimentary rocks which dominate the stratigraphic record of this period in southwestern Montana. The northeast-trending zone of Early Paleozoic u p lift along the Snowcrest-Greenhorn lineament appears to have influenced Mississippian- Pennsylvanian sedimentation and may suggest recurrent movement along this prominent zone of crustal weakness (Figure 8; Hadley, I960-, McMannis, 1965; and Maughn and Perry, 1982). Recurrent tectonism is best illustrated by the marked changes in thickness observed in the Late Mississippian Big Snowy Group exposed in the Snowcrest Range. Along the western flank of the range these rocks attain thicknesses in excess of 300 m (Brasher, 1950; Gealy, 1953; and Flannagan, 1958) and thin rapidly to the east of the range (Mann, 1954). Similar stratigraphic relationships exist in the Pennsylvanian Quadrant Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 24 Formation. Over 300 m of Quadrant Formation, which is locally exposed in the Snowcrest Range, thins to roughly 60 m to the southeast in the Centennial basin (Perry et al_., 1981). Although such stratigraphic evidence may indicate Laramide structural telescoping, it likely suggests significant renewed tectonic a ctivity along the Snowcrest- Greenhorn lineament during Mississippian-Pennsylvanian time. In con trast with the occurrence of shallow marine sedimentary rocks in south western Montana, Mississippian-Pennsylvanian rocks in north-central Nevada are characterized by deep marine and volcanic assemblages of Havallah sequence. These rock assemblages are interpreted to be indicative of back-arc spreading similar to present-day marginal basins in the western Pacific (Burchfiel and Davis, 1975). Perhaps the Late Paleozoic cratonic subsidence along the Snowcrest-Greenhorn lineament was related to contemporaneous crustal adjustments in the expanding marginal basin setting envisioned by Burchfiel and Davis (1975). Marine and non-marine Permian and Triassic rocks in southwestern Montana typically thicken to the southwest and reflect deposition related to orogenic a c tivity occurring to the west. This period is represented by the development of the Sonoma orogeny, which marked the final closing of the Antler marginal basin. By Early Mesozoic time, collision and accretion of the Klamath-Sierran arc complex to the craton strongly modified the western Cordillera of North America (Burchfiel and Davis, 1975). A major erosional event during Jurassic time is recorded in southwestern Montana. A broad north-trending arch developed near the present location of the Boulder batholith during deposition of the Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 25 Butte ■ Bozeman '-mq^t An X WYOMING Km 2 ,0 0 0 1,000 WYOMING 3 ,0 0 0 ; Isopach thickness interval = 250 ft, Km Figure 8. A. Generalized geologic map showing the trend of the Snowcrest-Greenhorn thrust system. B. Isopachous map of the Mississippian strata in southwestern Montana (modified from McMannis, 1965). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 26 Upper Jurassic E llis Group (McMannis, 1965; Suttner, 1969; and Meyers, 1981). The Late Jurassic-Early Cretaceous Morrison and Kootenai forma tions mark a change in tectonic setting and represent synorogenic deposition from source areas in the Nevadan orogen to the west (Suttner, 1969). Although stratigraphic and petrologic data suggest these rocks represent clastic wedge (retro-arc) deposits shed from predominantly western source areas, recent studies show that localized arching in southwestern Montana placed additional controls on Morrison- Kootenai sedimentation (Schwartz et al_., 1983). Rocks of the Lower Colorado Group appear to be retro-arc deposits related to orogenic a ctivity in central Idaho. Stratigraphic and paleocurrent evidence from the Lower Colorado-Blackleaf Formation also suggests localized arching in southwestern Montana (Schwartz et a_l_., 1983). The abundance of volcanic detritus in these rocks records an increase in volcanic a ctivity, corresponding to theeast ward sh ift of magmatism, and to the in itia l emplacement of the Idaho batholith (McMannis, 1965; and Scholten, 1968). This eastward sh ift in orogenic a ctivity culminated during Late Cretaceous to Eocene time, with the development o f the classic Laramide orogeny. Cretaceous through Holocene time represents the period of Lara mide and post-Laramide structural evolution in the northern Snowcrest Range. Consequently, Laramide and post-Laramide tectonic setting w ill be discussed individually in the chapters that follow. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER I I I LARAMIDE DEFORMATION IN THE NORTHERN SNOWCREST RANGE Regional Laramide Tectonic Setting The period of magmatism and deformation affecting the North American Cordillera during Late Cretaceous to Eocene time is typically referred to as the Laramide orogeny. During this orogenic episode andesitic magmatism related to arc-subduction shifted eastward into the craton in response to accelerated plate convergence (Coney, 1978). Consequently, a distin ct igneous "gap" developed in the formerly active Mesozoic igneous centers to the west (Burchfiel and Davis, 1975; Coney, 1978). Laramide deformation also shifted eastward and structural style changed from "thin skinned" fold-and-thrust belt tectonics in the Cordilleran miogeocline, to widespread basement u p lift in the craton (Burchfiel and Davis, 1975). The mechanism of basement u p lift in the Rocky Mountain foreland remains a matter of speculation. It is possible that the eastward sh ift in magmatism increased the d u c tility of the craton, particularly at lower levels in the crust (Burchfiel and Davis, 1975). The im pli cation is that compressive shortening in the thermally weakened craton ultimately gave rise to major Laramide arches in the Rocky Mountain foreland. An alternative hypothesis holds that arches may reflect large ramp anticlines above major foreland thrusts (Young et al., 1983). 27 with permission of the copyright owner. Further reproduction prohibited without permission. 28 Evidence of the eastward s h ift of magmatism and deformation is clearly expressed in southwestern Montana. Laramide magmatism resulted in extrusion of the Elkhorn Mountains Volcanic complex during Late Cretaceous time (Klepper et , 1957; and Klepper et aK > 1971). Lara mide deformation was characterized by a continuation of "thin skinned" deformation along the eastern margin of the Sevier fold-and-thrust belt (Woodward, 1981; and Schmidt and O'Neill, 1982), and by the development of several major fault-bounded basement uplifts in the Wyoming structural province or Rocky Mountain foreland. Basement upl-ift progressed along northwest-trending zones of weakness developed in the Late Precambrian (Schmidt and Hendrix, 1981), and along several broad north-trending arches (Scholten, 1967; and Schmidt and Garihan, 1983). Major Laramide arches in the foreland of southwestern Montana include the Tobacco Root, Madison-Gravelly, and B1acktai1-Snowcrest u p lifts (Figure 9). These uplifts were the probable source for syn- tectonic deposition of locally thick sequences of coarse conglom erates and sandstones. Sedimentary deposits of this type are well exposed along the southeastern flank of the B1acktai!-Snowcrest u p lift, where they comprise a portion of the Beaverhead Formation (Lowell and Klepper, 1953; Ryder, 1967; Ryder and Scholten, 1973; and Haley, 1983). Genetically similar sedimentary deposits mapped collectively as Sphinx Conglomerate are exposed along both flanks of the Madison-Gravelly u p lift (Beck, 1959; Hall, 1961; and Hadley, 1969a). with permission of the copyright owner. Further reproduction prohibited without permission 29 ,cACCO - ROOt ./AkCH \MONTANA pif Idaho 7 Boulder ^Batholith/ / Bath°lith»^M^ ,, ^ \? ^ < /* Tobacco Root \\i M&WOr j . ' 2 Batholith '' ' +,'4 , « # ' "iW,« ^ ^ V YA >* *> H A < <• A/ A A >1 ! ROCKY MOUNTAIN loneer * A FORELAND (jiatholit o, o ' • ;/ 0‘(^> H O V W N ■*. V^. 3 0/> jjj / / v. ,\ MONT ANA \ I- " WYOMING Pre Beltian Cretaceous Tertiary metamorphic rocks intrusive rocks Thrust faults Reverse faults Anticline-Syncl me Figure 9. Schematic map showing important Laramide tectonic elements including major arches in southwestern Montana. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 30 Laramide structures expressed in the northern Snowcrest Range are related to deformation along the overturned southeastern limb of the Blacktail-Snowcrest arch (Figure 9; Scholten, 1955, 1967). The arch plunges approximately 16°, S.35° W. (Figure 10). A major system of northeast-trending thrust faults (Snowcrest-Greenhorn thrust system) and associated eastward verging folds is exposed on the overturned east limb of the arch in the northern Snowcrest Range. This system appears to have been torn by northwest-trending faults which may have been controlled by older northwest-trending faults. Syntectonic deposits of the Beaverhead formation are widely ex posed along the southeastern flank of the Snowcrest Range and are locally exposed near Stonehouse Mountain, in the southern part of the study area (Figure 5A; Plates I and III) . Description of Faulting General Statement The Snowcrest-Greenhorn thrust system represents a major struc tural element in the Rocky Mountain foreland of southwestern Montana. Together with associated asymmetrical folds, i t comprises the over turned southeastern limb of the Blacktail-Snowcrest u p lift (Scholten, 1955). The trace of the thrust system extends for roughly 80 km from the northern Greenhorn Range southwestward, where i t is overlain by structures of the fold-and-thrust belt (Scholten, 1967; and Perry et al■, 1981). The Snowcrest-Greenhorn thrust system trends roughly N.40°E., and dips northwest at 20° to 45°. It can be divided into Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 31 N Figure 10. S-pole diagram of bedding for the east and west flanks of the Blacktail-Snowcrest uplift. Derived from field data by Klepper (1950) and Flannagan (1958). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 32 two major thrust zones, a western (Snowcrest thrust) zone, and an eastern (Greenhorn thrust) zone. These thrust zones form the eastern leading edge of the Snowcrest sheet and the Greenhorn sheet, respecti vely. Snowcrest Thrust Zone The Snowcrest thrust zone was fir s t recognized by Klepper (1950), and its northern extension in the Greenhorn Range was mapped by Hadley (1960). The Snowcrest thrust zone consists of two distinct segments with a total stra ti graphic throw estimated to exceed 2 km. The westernmost segment of the thrust zone generally places pre- Bel ti an metamorphic rocks and Middle Cambrian sedimentary rocks on Upper Mississippian sedimentary rocks. This thrust segment is best exposed in sec. 30, T. 9 S., R. 3 W., about 3 km south of Ruby Canyon (Figure 11) and in sec. 26, T. 11 S., R. 4 W., in the extreme southwestern portion of the study area, near the East Fork of Black- tail Deer Creek (Plate I). Three-point constructions yield dip estimates ranging between 20° and 35° to the northwest. Estimated minimum stratigraphic throw is approximately 1000 m. The easternmost segment of the Snowcrest thrust zone is best' exposed in sec. 1, T. 10 S., R. 4 W., roughly 2 km west of Snowcrest Mountain (Figure 12) and in sec. 24, T. 11 S., R. 4 W., approximately 3 km north of the East Fork of Blacktail Deer Creek (Figure 13, Plate I). The thrust places steeply dipping, overturned Mississi ppian Mission Canyon Formation over upright to overturned Upper Mississippian Big Snowy Group. Dip angles calculated for this with permission of the copyright owner. Further reproduction prohibited without permission. sketch sketch of the structural relationships along the thrust Snowcrest zone. Figure Figure 11. A. photograph (looking Composite north) of the thrust Snowcrest zone. B. Interpretive owner. Further reproduction prohibited without permission. CO t >it',ry'^ovw Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 35 ,|WtiCTWftVW'ptTOlW Figure 13. A. Photograph (looking south) of the Snowcrest thrust zone near Olsen Peak in the central Snow crest Range. B. Interpretive sketch of the Snowcrest thrust zone. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 36 segment of the thrust zone from map data, and measured in the fie ld range from 21° to 45° to the northwest. Estimated minimum strati - graphic throw is approximately 200 m. Greenhorn Thrust Zone The Greenhorn thrust zone was fir s t recognized by Mann (1954) along the eastern flank of the Greenhorn Range. The presence of a fault zone along the eastern flank of the Snowcrest Range was fir s t suggested by Gealy (1953), and the fault was later recognized by Hadley (1960, 1969a) about 4 km south of Ruby Canyon (Figure 14). Regional structural profiles (Plate IV) suggest that the thrust zone along the eastern flank of the Snowcrest Range is the southwestward projection of the Greenhorn thrust zone in the Greenhorn Range. Because of typically poor exposures of Cretaceous sedimentary rocks along the eastern flank of the Snowcrest Range, the Greenhorn thrust cannot be precisely located. The position of the thrust on the map (Plate I) is inferred on the basis of the marked omission of over turned Cretaceous beds along the eastern flank of the range. The thrust dips at approximately 30° to the northwest in the Greenhorn Range, and strati graphic throw is estimated at roughly 3000 m (Hadley, 1969a). Minor Faults Numerous minor faults are associated with deformation along the Snowcrest-Greenhorn thrust system. A minor thrust is exposed at the with permission of the copyright owner. Further reproduction prohibited without permission. CO Mii a. ...tf is inferred along the slopes in the foreground (far le ft). Range (right)Range and the (le northern Range Snowcrest ft). thrust Greenhorn The ;*&;• ;*&;• ' Figure Figure 14. (looking Photograph northwest) of the eastern flank of the southern Greenhorn Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 38 base of Sliderock Mountain, in secs. 24 and 25, T. 10 S., R. 4 W., near the head of Fawn Creek {Figure 15, Plate I). Here, the thrust strikes north and dips 40° to the west. The thrust is located in the hinge zone of the overturned Sliderock Mountain anticline, and duplicates rocks of the Upper Mississippian Big Snowy Group. Strati graphic throw on the thrust is estimated to be less than 100 m. Another minor thrust is located in secs. 18 and 19, T. 9 S., R. 3 W., in the northern portion of the study area. The thrust strikes roughly north and is inferred to dip 35° to the west. The fa u lt places the Permian Phosphoria Formation over the Triassic Dinwoody Formation, and is interpreted to have formed as a forelimb thrust along the eastern overturned flank of the Jobe Creek anti cline (Plates I and I I ). Two distinct high-angle reverse faults are expressed in the steeply dipping limbs of major folds in the study area. These faults strike approximately N.10°E., and are estimated to dip between 65° and 75° to the northwest. The westernmost reverse fault is located in secs. 22 and 27, T. 10 S., R. 4 W., (Plate I) and is associated with the Spur Mountain syncline (Gealy, 1953). Poor exposures along the fault prohibit accurate determination of the amount of slip , but stratigraphic throw is estimated to be less than 50 m. A spectacular exposure of the easternmost reverse fault exists along the north wall of the cirque near the head of Fawn Creek in sec. 24, T. 10 S., R. 4 W., (Gealy, 1953). The fault is generally oriented parallel to bedding in the overturned limb of Sliderock Mountain anticline. The with permission of the copyright owner. Further reproduction prohibited without permission. 39 J i f k i 1 '* ^ Up vMk. *9 J \ i t t i l a % * , 4 % M f e 5 » Figure 15. A. Aerial photograph (looking south) of the minor thrust associated with Sliderock Mountain anticline. B. Interpretive sketch of the structural relationships along the thrust. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 40 The fault places steeply dipping overturned Mississippian Big Snowy Group on steeply dipping overturned Pennsylvanian Amsden Formation, and stratigraphic throw is estimated to be less than 100 m. The fault appears to be localized along the overturned limb of a large para s itic fold in the Big Snowy Group and the Amsden Formation (Figure 16). Two steeply dipping northwest-trending transverse faults are exposed in the study area. The southernmost fa u lt is located in secs. 23 and 26, T. 11 S., R. 4 W., approximately 2 km west of Sunset Peak (Plate I). The fault strikes N.25°W. and places Cambrian Meagher Formation against Cambrian Pilgrim Formation. The orientation of bedding along the fault suggests that this struc ture may represent a minor transverse ramp in the Snowcrest thrust. The northernmost fa u lt is located along Robb Creek in sec. 12, T. 11 S., R. 4 W., (Plate I). The fault strikes N.25°W. and places Cambrian Meagher Formation against Pennsylvanian Amsden Formation. Although structural relationships have been obscured along the fault trace by Cenozoic normal faulting and Quaternary alluvial sedimen tation, the orientation of bedding along the fa u lt suggests that this structure is probably a tear fault in the Snowcrest thrust zone. Left separation along the fault is estimated to exceed 2 km. with permission of the copyright owner. Further reproduction prohibited without permission. 41 fMwMwWMwM':< W m i m i i ^ A $ 0 m m m m i w m m ^ m ^ fa a g fe £:• » ■ -5f',v.'.-ii.' It* !? .. •>.■•.•. Figure 16. A. Photograph (looking north) at the Fawn Creek structure, sec. 24, T. 10 S., R. 4 W. B. Sketch showing the structural relationships between fold ing and faulting. Note that the high angle reverse fault has formed along the overturned limb of a major parasitic fold in the Amsden Formation. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 42 Description of Folding General Statement Overturned, eastward-verging folds with wavelengths of up to 6 km are exposed in the northern Snowcrest Range. Major folds are ex pressed as hanging-wall anticlines and footwall synclines associated with the Snowcrest-Greenhorn thrust system. Fold hinges consistently plunge gently to the southwest and are typically oriented parallel to major thrusts. Fold limbs exhibit well developed parasitic folds, and hinge zones are typically highly fractured and faulted. Style of folding is predominantly concentric, but a similar style is locally indicated by marked thinning of incompetent beds, alongsteep fold limbs. Consequently, both flexural slip and flexural flow mechanisms were operative. Results of the Stereonet analysis of folding in the northern Snowcrest Range are presented in Appendix A. Major fold names appear on the Geologic map (Plate I). Snowcrest Anticline The hanging-wall anticline associated with the Snowcrest thrust zone is herein referred to as the Snowcrest anticline. The Snow crest anticline is locally exposed along the western flank of the range, and best exposures are in secs. 25 and 36, T. 9 S., R. 4 W., in the northern portion of the study area. Here, the anticline involves pre-Beltian metamorphic rocks, and Middle to Upper Cambrian sedimentary rocks. Fold limbs are typically gently to moderately with permission of the copyright owner. Further reproduction prohibited without permission 43 dipping, with an approximate interlimb angle of 110°. Local steepen ing and overturning of the eastern limb of the anticline near the Snow crest thrust zone may be observed in sec. 36, T. 9 S., R. 4 W., (Figure 17). The Snowcrest anticline is oriented subparallel to the trace of the Snowcrest thrust zone, and plunges approximately 15° to the southwest (Appendix A, p. 107). Spur Mountain S.yncline The Spur Mountain syncline (Gealy, 1953) is associated with the footwall of the Snowcrest thrust zone. The western, overturned limb of the syncline strikes N.30°E. and dips roughly 50° to the northwest. Spectacular exposures of this limb exist along the eastern flanks of Hogback Mountain and Olsen Peak (Plate I). The eastern, upright limb of the syncline strikes N.20°W., dips approximately 25° to the southwest, and forms extensive dip slopes in the central portion of the study area. The hinze zone of the Spur Mountain syncline is cut by a high-angle reverse fa u lt, parallel to its axial trace (Plate I). The hinge of the syncline trends parallel to the Snowcrest-Greenhorn thrust system, and plunges 20° to the northwest (Appendix A, p. 107). Sliderock Mountain Anticline The eastward verging overturned Sliderock Mountain anticline (Gealy, 1953) is probably the most striking structural feature in the northern Snowcrest Range (Plate I). The upright western limb of the fold was described in conjunction with Spur Mountain syncline. The eastern, overturned limb strikes N.20°E. and dips approximately with permission of the copyright owner. Further reproduction prohibited without permission. & n — ' - - , Range. Range. B. Interpretive sketch the showing local steepening of dips in Cambrian units on the hanging-wall of the thrust Snowcrest zone. Figure Figure 17. A. (looking Photograph north) along the western flank of the northern Snowcrest Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 45 45° to the northwest. The fold plunges to the southwest at roughly 24° (Appendix A, p. 108). The hingezone of Sliderock Mountain anti cline exhibits a minor low-angle thrust (p. 38, Plates I and II) and locally well developed axial planar solution cleavage (Figure 18). Both limbs of the anticline preserve widespread parasitic folds with wavelengths up to approximately 50 m (Figure 19). The anticline its e lf is interpreted to be the main hanging-wall structure of the Greenhorn thrust system. Jobe Creek Anticline-Syncline The Jobe Creek anticline-syncline is exposed in the extreme northern portion of the study area, and was fir s t recognized by Mann (1954). The overturned syncline is associated with the footwall of the Snowcrest thrust zone, and the overturned anticline is associated with the hanging-wall of the Greenhorn thrust zone. The eastern overturned limb of the Jobe Creek anticline strikes roughly N.10°W., dips 45° to the west, and plunges approximately 20° to the south- southwest (Appendix A, p. 108). The wavelength of the Jobe Creek fold set is small (less than 1 km) compared to the wavelength of the Spur Mountain-Sliderock Mountain fold set (greater than 4 km). Therefore, the Jobe Creek fold set is interpreted to be a major parasitic fold exposed along the eastern overturned limb of the Sliderock Mountain anticline. A minor thrust is associated with the overturned limb of the Jobe Creek anticline (p. 108, Plates I and II). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 46 24° S55°W PLANE B Figure 18. Stereoplots comparing the orientation of solution cleavage in the Big Snowy Group to: A) the orien tation of the Sliderock Mountain anticline, and B) the average orientation of the Snowcrest thrust zone (32 poles to cleavage contoured at 1%, 6%, 11%, and 17%). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission 47 vw$M!Mi&%&&mMM, Figure 19. A. Photograph (looking south) of parasitic folds in the Amsden Formation and Big Snowy Group exposed in sec. 23, T. 10 S., R. 4 W. B. Sketch showing the relationship between parasitic folds and the Sliderock Mountain anticline. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 48 Tectonic Inheritance Snowcrest-Greenhorn Thrust System The history of the northeast-trending Snowcrest-Greenhorn thrust system has been previously described by Perry et al_. (1981), Maughn and Perry (1982), and Schmidt and Garihan (1983). Pre-Beltian struc tural evidence, together with Paleozoic stratigraphic evidence suggest that the Snowcrest-Greenhorn thrust system originated along an existing zone of crustal weakness. Prominent northeast-trending structural elements such as foliation patterns and major fold axes in pre-Beltian rocks clearly indicate a prominent northeast-trending structural grain in the region (Figure 20; Berg, 1979; Vitaliano et al_., 1979; Spencer and Kozak, 1975; and Erslev, 1983). One major structural element, the Madison mylonite zone, is believed to extend for over 150 km in southwestern Montana (Erslev, 1982, 1983). This major zone of pre-Beltian shearing trends approximately N.40°E. and dips 45° to the northwest. Because this approximates the orientation of the Snowcrest-Greenhorn thrust system (that is , N.35°E., 30°-45° NW.), i t is not unreasonable to suggest that similar Precambrian structural elements may have controlled deformation in the Snowcrest Range. Paleozoic stratigraphic evidence (see p. 20-24) supports this hypothesis and may indicate recurrent crustal movement along the Snowcrest-Greenhorn lineament. The spatial relationship between pre-Beltian structural trends, the Paleozoic Snowcrest-Greenhorn lineament, and the Snowcrest-Green horn thrust system is shown in Figure 20. with permission of the copyright owner. Further reproduction prohibited without permission to Gneisses thrust system Metasediments Beartooth (G ile t ti, 1 9 6 6 ) trends Churchill- Wyoming Snowcrest- Greenhorn province boundary ?? a t in 1 Dominenf stru ctu ral J= rNn^ ® \P al Southern h. k^'Range Rang £/u\Madison Q $ Madi^nJ^^^Range Gravelly Rang Ranged Tobacco A 1983). Greenhorn thrustGreenhorn (modified system from Fountain and 1980; Desmarias, and Erslev, pre-Beltian structural trends, lineament, the Snowcrest-Greenhorn and the Snowcrest- Greenhorn Range Figure Figure 20. the of showing map spatial southwestern Montana Schematic relationship dominant between Blackt Highland Range Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 50 Northwest-Trending Faults The tectonic inheritance of the major northwest-trending fault set in southwestern Montana has been studied in detail (Reid, 1957; Garihan, 1973; Schmidt, 1975; and Schmidt and Garihan, 1979, 1984), and a Late Precambrian origin for these faults is well documented. However, faults of this set have not been previously identified in the Snowcrest Range. Three northwest-trending faults -- the Stone Creek, Sweetwater, and Blacktail faults, were described by Schmidt and Garihan (1979, 1983) in the Ruby and Blacktail ranges. These faults are also expressed along the western flank of the Snowcrest Range. Geologic map data (Plates I and III) suggest that the Sweet water and Blacktail faults may exist as tear faults along the Snow crest-Greenhorn thrust system. The Robb Creek tear (see p. 40) appears to be the southeasterly extension of the Sweetwater fa ult (Plate I). Similarly, the East Fork tear fa ult (located in secs. 4 and 5, T. 12 S., R. 5 W., approximately 4 km northwest of Sawtooth Mountain in the central Range), appears to be the southeasterly extension of the Blacktail fault (Plate III). The location and orientation of these tear faults strongly suggests recurrent Laramide faulting along Late Precambrian northwest-trending fault zones. Eastward trans lation of the Snowcrest-Greenhorn thrust system may have reactivated major northwest-trending faults developed in pre-Beltian rocks of the Blacktail-Snowcrest arch. Such recurrent fault movement would like ly form tear faults in the Snowcrest-Greenhorn thrust system (Plates I and II I) . Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 51 Additional evidence suggesting reactivation of major Late Pre cambrian northwest-trending fault zones is present in the Gravelly Range. The northwest-trending Wetzel Creek anticline (Vaughn, 1948; and Lemish, 1948) may represent a fold above a northwest-trending reverse fa ult in pre-Beltian rocks (Plates III and IV). Similar Laramide structural relationships have been described by Schmidt and Garihan (1983) in the Madison, Tobacco Root, and Ruby ranges. In the northern Gravelly Range, the trend of the Red H ill and Warm Springs thrust zones (Mann, 1954; and Hadley, 1960, 1969a) is approximately N.30°W., whereas the average trend of the Gravelly thrust zone system is approximately north-south (Plate III). This divergence of thrust trend may reflect structural control by north west-trending faults in basement rocks. These thrust splays ramp to the surface where northwest-trending faults in the pre-Beltian rocks have produced basement controlled anticlines (Plate IV, sections B-B' , C-C' , and E-E*). Similar situations showing the localization of foreland detachment thrust ramps above basement controlled anticlines have been previously described by Petersen (1983). Overall Structural Relationship Two different geometric interpretations have been presented to explain the observed deformation in the Snowcrest Range. Scholten (1967) and Ruppel et aj_. (1981) interpreted the Snowcrest-Greenhorn thrust system as an "upthrust" (Figure 21B) similar to basement involved structures found elsewhere in the Rocky Mountain foreland Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 52 3 :j . l x ± = L XA /nV/V.X I iI -— \ v. *\ . N / . I t X^ A > I — - / I — •/ I 1 \ -^'xP^' Ww-'Js x vo , [~ j s \ ' B/~s \ -^.^/ \ /. . ^ 4 / - 1 \ A ^ / i / Figure 21. Two diagrammatic models representing contrasting geo metric interpretations for the Blacktail-Snowcrest u p lift. A. Fold-thrust model (Berg, 1962). B. Upthrust model (after Prucha et ^1_., 1965). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 53 described by Prucha et ah (1965). More recently, Perry et a]_. (1981, 1983) and Schmidt and Garihan (1983) interpreted the Snowcrest-Green horn thrust system as a major low-angle foreland thrust (Figure 21A) similar to mountain-flank structures described by Berg (1962) and Gries (1981). The upthrust model as applied to deformation along the Snowcrest- Greenhorn thrust system appears untenable on several grounds. As pre sented by Prucha et al_. (1965), the model calls upon vertical u p lift to produce a convex-upward fault geometry, that dips steeply at depth and more gently near the surface. However, a convex-upward fault geometry cannot be demonstrated along the Snowcrest-Greenhorn thrust system, and no major steeply dipping reverse fa u lt zones were ob served. Although Gealy (1953) mapped the Snowcrest thrust as a steeply dipping feature, detailed field mapping by the writer indicates that the Precambrian involved western segment of the Snowcrest thrust zone maintains a dip of 20° to 35° along its entire exposed length in the northern and central Snowcrest Range (Plate I). However, observed dips along the fault do not completely rule out the possibility that only the gently dipping portion of the upthrust is exposed. Two other interpretations argue against the upthrust model. First, gravity data presented by Kulik and Perry (1982) is not con sistent with a steeply dipping upthrust geometry. Secondly, other important Late Cretaceous-Early Tertiary structures in the foreland and frontal fold-and-thrust belt of southwestern Montana are with permission of the copyright owner. Further reproduction prohibited without permission. 54 mutually compatible with east-west compressive forces (Schmidt and O'Neill, 1982; and Schmidt and Garihan, 1983). Inferred vertical uplift to produce an upthrust is difficult to explain in the presence of such forces. Two explanations may be offered for a compressional low-angle foreland thrust: the fold-thrust model (Berg, 1962) and the thrust- u p lift or ramp-flat model. Perry et ajh (1981, 1983) and Kulik and Perry (1982) presented aeromagnetic, gravity and well-log data to support the theory that a major low-angle mountain flank thrust system may be present beneath the Snowcrest and Greenhorn ranges. Detailed field mapping indicates that structural relationships (Table 3.1) along the Snowcrest-Greenhorn thrust system are clearly compatible with a fold-thrust geometry as previously suggested by Perry et aj_. (1981 , 1983) and Schmidt and Garihan (1983). There are, however, some d iffic u ltie s in applying the fold- thrust idea to these structures. As presented by Berg (1962), the fold-thrust model requires that major thrusts form along steeply overturned fold limbs, and i t implies that folding ultimately results in thrusting. However, interpretations based on deep crustal seismic reflection data across the Wind River u p lift (Smithson ert aj_., 1978) suggest that the Wind River thrust persists to a depth of at least 24 km, with an average dip of 30°-35°. Such evidence is inconsistent with the fold-thrust concept because the low-angle portion of major thrusts would be expected to extend no deeper than the base of the Precambrian-Phanerozoic contact on the footwall of such a structure (Figure 21A). with permission of the copyright owner. Further reproduction prohibited without permission 55 TABLE 2 Comparison of Fold-Thrust Characteristics A. General fold-thrust B. Observed fold-thrust characteristics presented characteristics for the by Berg (1962). Snowcrest-Greenhorn thrust system. 1. Multiple fault planes are com 1. Major thrusts within the Snow mon. Two major faults are crest-Greenhorn thrust zone typically present, one between are typically paired. One Precambrian and Paleozoic fault brings Precambrian rocks rocks, and another between against Upper Paleozoic rocks, Paleozoic, and Cretaceous the other fault brings Missis- rocks. sippian rocks against Upper Missi3sippian-Pennsylvanian rocks. The Greenhorn thrust zone is typically between Pal- eozoics and Cretaceous rocks. 2. Tault zones are comprised of 2. 'l'he Snowcrest-Greenhorn thrust strongly overturned Paleozoic system is comprised largely of and Mesozoic rocks . Inverted strongly overturned Paleozoic sections may be undisturbed, and Mesozoic sedimentary rocks. moderately folded and faulted, Inverted sections are moderate or highly deformed. ly folded and faulted in the westernmost part of the thrust system, and are relatively un deformed in the easternmost part of the thrust system. 3T T5Ip of tne major rauits is var j. bip or tne major taults is var iable . iable C20° - 45° ) . 4. Stratigraphic throw is variable. 4. Stratigraphic throw is varia ble (0 - 3,000 m ) . 5. A frontal thrust zone may be 5. The Gravelly thrust system may present. Frontal thrusts do represent a frontal thrust not involve Precambrian rocks. zone. This thrust zone does not involve Precambrian rocks. 6. Major transverse tear laults b. Tear faults are associated may be associated with thrust with the Snowcrest-Greenhorn zones. Good examples occur a- thrust system, and may be repre long major structures like the sented by the Stone Creek, Beartooth and Wind ' River Sweetwater, and Blacktail thrust systems. faults. 1. Thrusting is associated with a /. Tne BlacKtail-Snowcrest Arch may major hanging-wall anticline be interpreted as a major hang involving Precambrian rocks. ing-wall anticline associated with the Snowcrest-Greenhorn thrust system. 8. The major fold may exhibit min 8. The Ruby thrust is located on or Precambrian involved back- the gently dipping western limb limb thrusting. of the Blacktail-Snowcrest arch, and may represent a minor back- limb thrust. 9. Major normal faults are typi y. a major range tront tault system cally associated with major is closely associated with the fold-thrust structures. Snowcrest-Greenhorn thrust sys tem. 10. Subthrust Phanerozoic rocks are 10. No subthrust data is available. typically undeformed. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 56 Because the Snowcrest-Greenhorn thrust system is associated with zones of inherited weakness, the th ru st-u p lift model for mountain flank deformation appears to be more acceptable than the fold-thrust model. Although it is geometrically similar to the fold-thrust model, the thrust-uplift model implies that faulting was ultimately the cause of folding. Folding and thrusting may develop concurrently; that is , folds form and become progressively overturned with continued propagation along major thrust zones. Evidence supporting a direct relationship between folding and thrusting is clearly present in the Snowcrest Range. Minor thrusts are exposed in the hinge zones of the eastward verging Sliderock Mountain and Jobe Creek anticlines (see p. 36-38, Plate I). The Snowcrest thrust front near the East Fork of Blacktail Deer Creek (Plates I and I I I ) probably moved farther eastward than equivalent thrust fronts to the south. Consequently, this portion of the thrust front is associated with the strongly overturned western limb of the Spur Mountain syncline. To the south, the Snowcrest thrust front did not propagate as far eastward, and i t is associated with more open folds near Sawtooth Mountain (Plate II I ) . The sharp transition between strongly overturned folds and open folds may be a function of the position of the Snowcrest thrust front. According to the th ru st-u p lift model, the Blacktail-Snowcrest arch may represent a major ramp-anticline associated with the Snowcrest-Greenhorn thrust system. This relationship is analogous with permission of the copyright owner. Further reproduction prohibited without permission. 57 to fa ult-fo ld relationships originally described by Rich (1934) for the Appalachian fold-and-thrust belt. In this case, folds develop on the upper thrust plate in response to the underlying ramp geometry. Such a large-scale fault-fold relationship requires that the ramp region merge with a subhorizontal detachment surface deep within the crust (Figure 22). Although major detachments within basement rocks of the foreland have been discussed (Lowell, 1974, 1983; Petersen, 1983; and Young et a K , 1983), such structures are typically assoc iated with fold-and-thrust belt tectonics (Bally et al_., 1966; and Dahlstrom, 1970, 1977). I f a deep crustal detachment structure does exist in southwestern Montana, i t is like ly that i t formed along a zone of fundamental mechanical anisotropy. One such zone may have been the b rittle -d u c tile transition for those rocks during Late Cretaceous-Paleocene time (approximate depth of 10 to 20 km). The structural relationship between major existing northwest- trending faults and the Snowcrest-Greenhorn thrust system was pre viously discussed. Northwest-trending faults such as the Sweetwater, and Blacktail faults are thought to have been locally activated as tear faults along the Snowcrest-Greenhorn thrust system. The resultant thrust front exhibits an irregular "lobate" map trace (Figure 23). A geometrically similar interpretation involving major transverse tear faults was made by Berg (1983), for the frontal por tion of the Wind River thrust system. A summary of the Laramide structural relationships associated with the Blacktail-Snowcrest arch is shown schematically in Figures 23 and 24 (see also, Plates I and IV). with permission of the copyright owner. Further reproduction prohibited without permission. 58 \>/i 0 5 V\T< d u c tT ie lOKm \ / " / 1' ^ _ rv>jw Figure 22. Diagrammatic model showing the possible relationship between observed foreland thrusting and deep crustal detachment. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. . r r v r n i ; P zM z --- 1—J-- V K T ^ . - Pre-Beltian rocks Paleozoic—Mesozoic Beaverhead Fm, rocks U —«=:---- Thrust fault Left-reverse fault Anticline Figure 23. Generalized Paleocene tectonic map showing structural and sedimentological relationships in the Blacktail- Snowcrest arch region. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4 , 0 0 0 m 4 , 0 0 0 m ■ 4 ,0 0 0m - 4 , 0 0 0m -- -- 0 \ / \\ / »x / / - \ / / \> I.a. NS6 / \/,' / \ i A''. \ ' V V / - T V /s ',/ I BLACKTAIL-SNOWCREST ARCH BLACKTAIL-SNOWCREST ARCH thrust zone; = FT fold tion); and thrust belt;= and P sedimentary Phanerozoic rocks. B = synorogenic deposits Forma (Beaverhead arch. arch. thrust = Snowcrest-Greenhorn SG system; = Gravelly G thrust system; = R Ruby EXAGGERATION EXAGGERATION 3 x//V i ✓ / \ x1< / V ^ Figure Figure 24. structural Generalized Paleocene sections A-A‘ across B-B1 and the Blacktail-Snowcrest Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 61 Age of Deformation Although ages of the various stages of Laramide deformation along the Snowcrest-Greenhorn thrust system (that is, the timing of u p lift, thrusting and folding) are not known precisely, estimates of the age lim its of deformation can be made using available geological, geochemical, and paleontological evidence. Timing of U plift The age of in itia l th ru st-u p lift can be estimated by determin ing the age of deposition of detritus (Beaverhead Formation) shed from the Blacktail-Snowcrest arch. Palynological data indicate that the basal unit of the Beaverhead Formation is probably of late Early Cretaceous (Albion) age (Ryder and Ames, 1970). This lower age lim it is consistent with estimates presented by Suttner et a l. (1981) and Schwartz et aJL (1983). They present isopach, lithofacies, mineralogic, and paleodispersal data that indicate the presence of localized positive structural elements in the foreland of south western Montana by Early Cretaceous time. Palynological data yield upper age lim it estimates of Late Paleocene for the upper unit of the Beaverhead Formation (Ryder and Ames, 1970). There fore, the Blacktail-Snowcrest arch may have existed as a positive structural element from as early as late Early Cretaceous through the Late Paleocene. with permission of the copyright owner. Further reproduction prohibited without permission 62 Timing of Thrusting and Folding The lower age lim it on thrusting and folding is indicated by deformed rocks of the Cretaceous Frontier Formation. Paleontological evidence suggests an early Late Cretaceous (Cenomanian) age for the Frontier Formation (Hadley, 1980). The Beaverhead Formation is mildly deformed near Antone Peak in the central Snowcrest Range (Klepper, 1950), probably indicating the continuation of thrusting and folding through Paleocene time. Upper age lim its on thrusting and folding can be determined from geologic relationships between Tertiary volcanic rocks and the Snowcrest-Greenhorn thrust system in the Greenhorn Range. Deformed rocks of the Snowcrest-Greenhorn thrust system are overlain by felsic tuffs and olivine basalt flows. Basal felsic tuffs are tentatively assigned an Eocene age (Hadley, 1980), and the over- lying olivine basalts yield K-Ar age determinations (Marvin et a l., 1974), ranging from 33 to 34 m.y.b.p. (Early Oligocene). Therefore, the age of thrusting and folding is Late Cretaceous to Eocene. This age range is compatible with preliminary estimates for the Snowcrest-Greenhorn thrust system made by Schmidt and Garihan (1983). Sequence of Deformation Relative ages of the various stages of deformation clearly suggest that initial late-Early Cretaceous uplift of the Blacktail- Snowcrest arch culminated in thrusting and folding during Late with permission of the copyright owner. Further reproduction prohibited without permission. 63 Cretaceous to Eocene time. As discussed earlier, major overturned folds in the region formed concurrently with thrusting. Consequently, folds became progressively more overturned as the Snowcrest and Green horn thrust sheets moved eastward. The general sequence of thrusting for the region has been recently discussed by Schmidt and Garihan (1983). They observed that in individual north-trending thrust systems, displacement along thrusts becomes progressively smaller in an easterly direction, and associated folds become progressively more open. They attributed these relationships to a west-to-east progression of thrusting prin cipally by analogy to thrust imbrication sequences in fold-and- thrust belts. However, field observations and detailed structural profiles across the Snowcrest and Greenhorn ranges suggest that the observa tions made by Schmidt and Garihan (1983) cannot be conclusively demonstrated along the Snowcrest-Greenhorn thrust system. Total stratigraphic displacement along the westernmost (Snowcrest) thrust zone is estimated to be approximately 2 km (Plate I I , sections A-A' through G-G1). The easternmost (Greenhorn) thrust zone exhibits a total stratigraphic displacement of more than 3 km (Hadley, 1980). Further more, the interlimb angle of folds associated with thrusting is fa irly constant (50°-70°) across the thrust system and does not appear to become progressively more open in a west-to-east fashion. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 64 Although local observations do not completely support a west-to- east sequence of thrust development, regional structural relation ships across the Snowcrest and Gravelly ranges suggest a general west-to-east progression of thrusting and associated folding. Dis placement along the Snowcrest-Greenhorn thrust system is relatively large, with net slip estimates ranging between 4 and 8 km (Plate II, sections A-A1 through G-G'). In contrast, the Gravelly thrust system to the east yields net slip estimates of only 600 m (Mann, 1954). The significantly smaller displacement of the Gravelly thrust system may reflect a general west-to-east progression of deformation across the region. In summary, the sequence of Laramide deformation in the northern Snowcrest Range can be characterized by: (1) u p lift of the Blacktail Snowcrest arch possibly as early as late Early Cretaceous time, (2) development of progressive west-to-east thrusting and attendant folding by Late Cretaceous time, and (3) cessation of Laramide thrusting and folding during Eocene time. Mechanical Behavior of Pre-Beltian Rocks The mechanical Behavior of metamorphic basement rocks is sig nificant in any model of mountain flank thrusting -- upthrust, fold-thrust, or thrust uplift. A number of different behaviors have been described: B rittle faulting (Bruhn and Beck, 1981), closely spaced shearing and microfracturing (Wise, 1964), and localized folding of fo lia tio n (Schmidt and Garihan, 1983). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 65 Pre-Beltian metamorphic rocks exposed in the extreme northern and central portion of the detailed study area (Plate I) are character ized by weakly foliated quartzo-feldspathic gneisses and subordinate amphibolites. These rocks locally exhibit zones of intense cata- clastic shearing. Pre-Beltian metamorphic rocks exposed in secs. 29 and 32, T. 10 S., R. 4 W., roughly 2 km south of Ledford Creek canyon are intensely sheared and pervaded with mesoscopic and micro scopic fractures. Farther to the south, pre-Beltian lithologies are typified by quartzo-feldspathic gneisses and amphibolites (Figure 25A). These lithologies are strongly foliated parallel to compositional layer ing and show evidence of well-developed shear fracturing. Shear fractures are particularly distinctive along foliation planes. Well-developed slickensides may be observed along foliation surfaces of pre-Beltian rocks in secs. 23, 26 and 27, T. 11 S., R. 4 W., near the East Fork of Blacktail Deer Creek (Figure 25B). Chlorite mineralization is typically associated with shear fracture surfaces and may indicate hydrothermal alteration during Laramide deformation (Bruhn and Beck, 1981; and Schmidt and Garihan, 1983). The possibility that slickensided foliation surfaces repre sent evidence for Laramide flexural slip folding of pre-Beltian metamorphic rocks in this region has been previously discussed by Sheedlo (1983). Additional evidence supporting this hypothesis has been presented for other Precambrian involved up lifts in southwestern Montana by Schmidt and Garihan (1983), and Werkema Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. m m m i W m wo- >..-! >.">$&. fir e } '•■»/>*.■'. *>''••>} Figure 25. A. Photograph (looking northeast) of well foliated pre-Belt amphibolites exposed in sec. 23, T. 11 S., R. 5 W. B. Photograph of well developed slickensides along foliation surface. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 67 (1984). Recent analysis o f stru ctu ra l re la tio n sh ip s, however, suggests that s ig n ific a n t Laramide fo ld in g cannot be conclusively demonstrated in pre-Beltian rocks of the northern Snowcrest Range. First of a ll, the orientation of foliation in pre-Beltian metamor phic rocks is generally consistent with regional pre-Beltian struc tural trends in southwestern Montana. Therefore, the observed orien tation of these rocks does not necessarily require that appreciable fo ld in g took place during Laramide deformation. Secondly, and more importantly, a strongly overturned depositional contact between pre- Beltian metamorphic rocks and Paleozoic sedimentary rocks was not observed in the northern Snowcrest Range, and detailed structural sections across the range suggest that the presence of such a contact is unlikely (Plates I and II). The sim ilarity of overall orientation of pre-Beltian meta morphic foliation and Paleozoic sedimentary bedding (Figure 26) does not necessarily reflect folding of the entire section. Because pre- Beltian rocks do not appear to have been strongly folded during Laramide deformation, i t is u n like ly th a t slickensides on fo lia tio n surfaces indicate flexural slip folding. Alternatively, the coincidence of the orientations in Figure 26 may simply re fle c t Laramide e xp lo ita tio n o f the dominant northeast- trending pre-Beltian structural grain in the region. If this is the case, observed slickensides probably represent distributed fault- parallel shear along favorably oriented pre-Beltian foliation surfaces. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. A N B Figure 26. Stereoplots showing the orientations o f bedding and foliation: A) 75 poles to pre-Beltian meta morphic foliation (contours at 1%, 4%, 9%, and 13%), and B) 116 poles to Paleozoic bedding (contours at 1%, 3%, 5%, 7%, and 13%). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 69 In summary, u p lift of the Blacktail Snowcrest arch progressed by thrusting and fo ld in g . Pre-Beltian metamorphic rocks accommodated compressional strain largely by widescale b rittle faulting and cata- c la s tic shearing. Evidence o f Laramide deformation is p a rtic u la rly well developed along foliation in pre-Beltian rocks. Although pre- Beltian rocks were broadly folded during the in itia l stages of deformation, no significant overturning of these rocks can be con clusively demonstrated. Thrust Movement Patterns Because fold axes are generally subhorizontal and are oriented parallel to the strike of the Snowcrest-Greenhorn thrust system, movement along individual thrusts is inferred to be predominantly d ip -s lip . Slickenside data taken from pre-B eltian metamorphic rocks along the Snowcrest thrust zone (Figure 27) indicate pre dominantly dip-slip movement, with a minor component of right lateral strike-slip movement. Although the consistent southwesterly plunge of major eastward-verging folds in the Snowcrest Range (Plate I) is probably in part a reflection of initia l thrust- u p lift, it may also indicate a minor right lateral component to thrusting. Similarly, in the Greenhorn Range, the distinct south easterly plunge of the Baldy Mountain fold set (Tilford, 1978) may be the result of right lateral thrust-slip. I f movement along the Snowcrest-Greenhorn th ru st system is predominantly dip-slip (that is, if strike-slip movement is con sidered negligible), estimates of net slip may be determined from with permission of the copyright owner. Further reproduction prohibited without permission Slip Line Slip Line B Figure 27. A. Analysis of B for parasitic folding (Appendix A) with the Snowcrest thrust (42 measured and constructed fold hinges; contours at 3%, 8 %, 13%, and 32%). B. Analysis o f fra c tures parallel to the Snowcrest thrust (30 slickensides; contours at 4%, 7%, 10%, and 13%). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 71 total dip separation shown on structural profiles across the Snow crest-Greenhorn th ru st system (Plate I I ) . Net s lip along the Snow crest thrust zone was determined using structural profiles which show Cambrian rocks on the hanging-wall of the thrust. Sections A-A1 and D-D' yield an average net slip estimate of roughly 2 km. Estimates of net slip along the Greenhorn thrust zone cannot be precisely determined from field relationships in the northern Snowcrest Range. Minimum estimates of net slip shown on structural profiles A-A 1 to G-G1 across the Greenhorn th ru st zone range roughly between 2 and 3 km, but actual net s lip may exceed 6 km or more depending upon the amount o f th ru st overlap, and the attitude of the thrust zone at depth. Thus, combined estimates of dip separation based on structural profiles across the Snow crest-Greenhorn thrust system yield a total net slip ranging between 4 and 8 km. These estimates are consistent with previous estimates of dis placement made by Hadley (1980) and Schmidt and Garihan (1983). Hadley (1980) suggests a minimum stra tig ra p h ic displacement o f 3 km and a horizontal displacement exceeding 8 km fo r the Greenhorn th ru st zone. Schmidt and Garihan (1983) estimated net s lip across the Snowcrest-Greenhorn th ru st system to range roughly between 4 and 5 km. An estimate of east-west horizontal shortening related to fo ld in g and thrusting along the Snowcrest-Greenhorn th ru st system was made for structure sections B-B 1 (Plate II) and C-C' (Plate IV) across the Snowcrest and Gravelly ranges. Estimates of total east- Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 72 west horizontal crustal shortening across the Snowcrest-Greenhorn th ru st system using section B-B' (Plate I I ) range between 9 and 10 km. Similar estimates using section C-C' (Plate IV) across the Snow crest-Greenhorn and Gravelly th ru st systems range between 11 and 15 km. The latter estimate is equivalent to approximately 20% east- west horizontal crustal shortening of that portion of the foreland shown on the tectonic compilation (Plate III). These values are compatible with recent estimates made by Schmidt and Garihan (1983). They suggested approximately 11 km of east-west horizontal shortening across the Snowcrest-Greenhorn and Gravelly th ru st systems. Stress Configuration Fold orientations and fault plane slip data were used as kine matic indicators in determining Laramide stress configurations and th ru st movement patterns along the Snowcrest-Greenhorn th rust system. The geometry o f Laramide folding and fa u ltin g in the northern Snow crest Range appears to indicate horizontal compression directed east- southeast-west-northwest, and is generally consistent with regional east-west Laramide compression documented by many workers (see, fo r example, Beutner, 1977; Schmidt and Hendrix, 1981; and Schmidt and Garihan, 1983). The inferred stress configuration fo r Laramide deformation in the northern Snowcrest Range is shown in Figure 27. Maximum principal stress direction (o'-]) is horizontal, and trends roughly west-northwest. Intermediate principal stress direction (C^) is with permission of the copyright owner. Further reproduction prohibited without permission. subhorizontal, and trends roughly southwest-northeast, Minimum principal stress direction ( 0*3 ) is essentially vertical Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER IV POST-LARAMIDE DEFORMATION IN THE NORTHERN SNOWCREST RANGE Regional Post-Laramide Tectonic Setting In the western Cordillera of North America, the period between Middle Eocene and Early Miocene marked a s ig n ific a n t change in tectonic se ttin g . Laramide compressional tectonics in the Sevier fold-and- thrust belt and the Rocky Mountain foreland gave way to basin-range extensional tectonics. Igneous a c tiv ity changed from intermediate arc-subduction related to magmatism to bimodal volcanism (Chadwick, 1981). Patterns in sedimentation became more localized with deposition occurring in fault-bounded basins. Most workers agree that basin-range deformation "follows" older zones of crustal weak ness (see, for example, Stewart, 1971), and this relationship is borne out on both regional and local scales (Figure 28). Southwestern Montana marks the eastern lim it of the Basin-and- Range tectonic province (Figure 28). Here, as in much of the western Cordillera, basin-range extension resulted in the formation o f elongate block-faulted mountain ranges separated by allu viated basins. By Middle to Late Eocene time, alluvial sedimentation began in subsiding intermontane basins (Kuenzi and Fields, 1971). Bimodal volcanic activity began in the region at about the same time, and continued sporadically until Pleistocene time (Chadwick, 74 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission 75 _ Canada United States Km EM - Pro- Beltian Thrust taults normal faults metamorphic. rocks BASIN AND RANGE PROVINCE ROCKY MOUNTAIN FORELAND PROVINCE 150 • vS / ' Eastern limit of Eastern limit of Pre Beltian fold-and-thrust Basin-and-Range metamorphic rocks belt deformation normal faulting Figure 28. Generalized post-Laramide tectonic map of southwestern Montana (upper right). Generalized regional tectonic map showing the re la tiv e positions o f Basin-and-Range, Fold-and-Thrust belt, and Rocky Mountain foreland provinces. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 76 1981). Important structural and volcanic trends related to basin-range a c tiv ity in extreme southwestern Montana are summarized in Figure 29 (see also Plates I and III). Basin-range structures in the northern Snowcrest Range are p ri marily normal faults and associated gentle folds along the Snowcrest- Greenhorn range-front fa u lt system. In addition to these structures, existing northwest-trending faults were reactivated as normal faults. Basaltic and rhyolitic volcanic rocks of Tertiary age reported in the southern Snowcrest Range by Brasher (1950), Gealy (1953), and Flannagan (1958). Intermontane basin deposits of Middle Eocene- Pliocene age are exposed along the western flank of the Snowcrest Range and have been studied in d e ta il by Monroe (1976) in the Upper Ruby basin. Description of Faulting Snowcrest-Greenhorn Range-Front Fault System A major system o f range-front fa u lts extends fo r some 130 km along the western fla nk o f the Snowcrest and Greenhorn ranges in Madison and Beaverhead Counties, Montana (Figure 30). The fa u lt system trends north-northeastward and has u p lifte d the Snowcrest and Greenhorn ranges to the east, and downdropped the Upper Ruby and Sage Creek basins to the west. In comparison to other Cenozoic normal fa u lts in the region, the Snowcrest-Greenhorn range-frorit fa u lt system has received very little attention. Although Pardee (1950) conducted a regional study with permission of the copyright owner. Further reproduction prohibited without permission 77 V ' \ \ ' f * 's /*" ^ A _N 7 A . V✓N. Tobacco\ ^ \ /> \ Root ✓ I /, ^MfSjMONTANA r i C ^ Range / j- / ^ i ’yM / ^ / .«./1 / i Virginia > ^ AVy'', 1 a ^Tvi Beaverhead C ity fcj' 1 River.Basin Greenhor Rang Dillon Kang <© Gravelly River 1 Blackta Red'Rock Snowc Bas asm N Centennial Basin L— £ v \ s ' \ ^ •o - > p N-»TVr,v 4 1 p€a AV Pre-Beltian Pre-basin Post-Laramide Tertiary metamorphic Phanerozoic basin deposits volcanic rocks rocks rocks .1______X______X______X______X— Laramide tfcrust faults Post Laramide normal faults Figure 29. Generalized post-Laramide tectonic map o f extreme south western Montana. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 78 Figure 30. Aerial photograph (looking north) of the Snow crest Range. The Snowcrest-Greenhorn range- fro n t fa u lt system is exposed along the western flank of the range. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 79 o f Cenozoic normal fa u lts in southwestern Montana, no discussion o f Snowcrest-Greenhorn range-front fa u lt system was presented. Klepper (1950) and Gealy (1953) both suggested possible stru ctu ra l control on T e rtia ry basin sedimentation along the western flan k o f the Snowcrest Range, but detailed descriptions of range-front faulting were not presented. The existence of the Snowcrest-Greenhorn range-front fault system has been inferred on the basis of Tertiary basin map ping (Monroe, 1976) and gra vity studies (Burfeind, 1967). In the northern Snowcrest Range, the trend of the range-front fa u lt system varies between N.25°E. and N.40°E. Although the trace o f the fa u lt system is exposed in sec. 15 and 20, T. 10 S., R. 4 W., i t is ty p ic a lly obscured along much o f its length by Holocene pedi ment surfaces. Consequently, dips along the fault are not usually measurable in the field. Three-point constructions from map data y ie ld westward dips ranging from 40° to 80°, and stra tig ra p h ic separation along the main fa u lt ranges from 2,300 to 3,000 m. Middle to Late Cenozoic sedimentary rocks are downdropped against variably dipping Paleozoic and pre-Beltian metamorphic rocks. A secondary system o f synthetic normal fa u lts places gently dipping Upper Paleozoic rocks against steeply dipping Paleozoic and pre- Beltian metamorphic rocks (Plates I and I I ) . Northwest-Trending Normal Faults Major post-Laramide northwest-trending normal faults are present along the western flank of the Snowcrest Range. These with permission of the copyright owner. Further reproduction prohibited without permission 80 faults extend for over 40 km to the northwest, into the Ruby Range where they have been studied by many workers (see, for example, H einrich, 1960; Garihan, 1973; Schmidt and Garihan, 1979; Okuma, 1971; and Karasevich, 1981). The Stone Creek fault transects the Upper Ruby basin (Plate III). The Early Miocene-Pliocene Sixmile Creek Formation is downdropped against Late 01igocene-Early Miocene Renova Formation. S trati- graphic throw is probably less than 30 m (Monroe, 1976). Along the western flank of the northern Snowcrest Range, the fault is exposed in sec. 16, T. 10 S., R. 5 W., displacing rocks.of the Sixmile Creek Formation. Here, the fa u lt trends N.48°W., dips steeply to the northeast, and has a strati graphic throw of roughly 5 m. The Sweetwater fault zone also transects the Upper Ruby basin (Plate III). Along the western margin of the Upper Ruby basin near Sweetwater Canyon, approximately 225 m of post-Pliocene strati graphic throw can be demonstrated on the Sweetwater fa u lt zone (Peterson, 1974). In the central portion of the basin and along the western flank of the Snowcrest Range, a fault scarp associated with the Sweetwater fa u lt zone has been modified by Holocene pediment surfaces. The fault trends roughly N.40°W. and dips steeply to the northeast. Although the Blacktail fault is not expressed within the study area, it is a major range-bounding normal fault along the northern flank of the Blacktail Range (Plate III). It trends roughly N.50°W. and dips steeply to the northeast. Klepper (1950) suggested that Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 81 the Blacktail fault may exhibit as much as 2,000 ft. (610 m) of post- Laramide structu ra l r e lie f. The Olsen Peak fa u lt is located in sec. 19, T. 11 S ., R. 4 W., along the eastern flank of Olsen Peak (Plate I). Minor faults related to the main Olsen Peak fa u lt are exposed in sec. 13, T. 11 S ., R. 4 W. The main fault trends N.34°W. and dips roughly 80° to the southwest. It places the Permian Phosphoria Formation against the Triassic Woodside Formation, with an estimated strati graphic throw of roughly 80 m. Sense of movement along the fault is probably left-normal (Plate I), but precise estimates of movement patterns are unavailable. The location and attitude of the fault suggest it may be related to the major northwest-trending fault set in the region (Plate III). Description of Folding Although widespread folding is not typically associated with extensional basin-range tectonics, minor post-Laramide folds have been recognized in the study area. These features appear to be directly related to major range-front normal faults. Ledford Creek Anticline The most distinctive post-Laramide fold of this type in the study area is the Ledford Creek anticline, located along the north western fla n k o f the range (Plates I and I I I ) . T e rtia ry beds which comprise the eastern limb of the anticline, dip at angles of up to 25° into the Snowcrest-Greenhorn range-front fa u lt system. Additional fie ld data on the fo ld is scarce, but i t appears to have Reproduced with permission of the copyright owner. Further reproduction prohibited without permission 82 an upright, horizontal orientation, with gentle interlimb angles ranging between 145° and 160°. The hingeline of the anticline is not well defined, but i t appears to trend north-northeast, subparallel to the Snowcrest-Greenhorn range-front fa u lt system. Tectonic Inheritance Snowcrest-Greenhorn Range-Front Fault System The tectonic inheritance of the Snowcrest-Greenhorn range-front fa u lt system cannot be conclusively demonstrated as exposures along the fa u lt system are poor. However, available fie ld evidence suggests th a t th is normal fa u lt system may have developed along a previously established zone o f weakness. In map view, the Snowcrest-Greenhorn range-front fa u lt system follows the trace o f the Laramide Snowcrest- Greenhorn th ru st system very closely (Figure 29; Plates I and I I I ) . Because the Snowcrest-Greenhorn range-front fa u lt system dips at 40° to 80° to the west-northwest, and the Tertiary rocks dip into the fa u lt system angles o f up to 25°, the fa u lt must have a concave- upward or lis tric geometry. This trend and geometry suggests that the Snowcrest-Greenhorn range-front fault probably flattens with depth and may merge w ith the Laramide Snowcrest-Greenhorn th ru st system (Figure 31, Plates II and IV). Similar cases of inheritance of l is t r ic normal fa u lts by Mesozoic thrusts in the Basin-and-Range province of the western Cordillera have been discussed by Dahlstrom (1970), Allmendinger et ah (1983), and Anderson et a}_. (1983). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission 83 Northwest-Trending Normal Faults Evidence suggesting th at post-Laramide northwest-trending normal faults in the Ruby Range were also active during Late Cretaceous- Paleocene time and Precambrian time has been well documented (Garihan, 1973; and Schmidt and Garihan, 1979, 1983). Two of these fa u lts , the Stone Creek and Sweetwater fa u lts , are expressed in the northern Snowcrest Range. Their overall length (greater than 40 km) and trend suggest that these faults are major structures that have been recurrently active since Proterozoic time. Overall Structural Relationship The relationship between northwest-trending normal faults and the Snowcrest-Greenhorn range-front fa u lt system was discussed above. The Snowcrest-Greenhorn range-front fa u lt appears to have "backed- down" the previously developed foreland thrust zone (Figure 31). Because displacement is probably greatest along the Snowcrest- Greenhorn range-front fa u lt system, the dominant stru ctu ra l geometry is that of a half-graben. The eastern Ruby range-front fault is probably antithetic to the main Snowcrest-Greenhorn range-front fault (Figure 31). It is represented by a distinctive lineament of 4 m.y.b.p. (Pliocene) extrusive volcanic rocks along the southern Ruby, the B la ckta il Range, and the Sage Creek basin (Marvin et a l., 1974; Figure 29; and Plate III). Although field evidence is lim ited, northwest-trending normal faults appear to show pivotal movement, with maximum displacement Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 00 a u l t F fau l p a u l t zoic rocks and A = pre-Beltian rocks. and and low angle Laramide th rustin g. T = T e rtia ry deposits, = M Mesozoic rocks, P = Paleo Figure 31. Generalized block diagram showing the relationship between post-Laramide normal fa u ltin g o o T3 owner. Further reproduction prohibited without permission. 85 occurring near the margins of the basin. The inferred movement pattern along northwest-trending faults is clearly compatible with rotational range-front faulting about a north-trending axis in the Upper Ruby basin. Therefore, it is likely that significant rota tional fault movement along the Snowcrest-Greenhorn range-front fault system contributed to the reactivation of existing northwest- trending faults. Although northwest-trending faults such as the Stone Creek and Blacktail faults clearly form major range-front faults, the degree to which these faults effected the in itia l stages o f basin development is unknown. The relationship between minor folding in Tertiary basin rocks and range-front normal fa u ltin g in the Snowcrest Range has not been previously described. The Ledford Creek anticline is oriented sub- parallel and adjacent to the Snowcrest-Greenhorn range-front fault (Plates I and III). As previously mentioned, the eastern limb of the fold dips distinctly into the fault system at angles of up to 25°. Similar arching in the central part of the basin along the southern Snowcrest Range was reported by Gealy (1953). It appears likely that the gentle folding of Tertiary basin rocks is directly related to range-front normal faulting. Struc tures of this type have been referred to as "reverse drag" struc tures (Hamblin, 1965). In the s tr ic t sense, however, th is term is inco rre ct because these structures do not form by processes related to fault drag. Rather, the folds form by rotational normal faulting, as the hanging-wall block is downdropped along a curved fault surface (Figure 32). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 86 T^vTTr^TZTr- 7v / '/ \/^y> ^ ' / N i V \ / / \ / . N / / \ k ^ r K ^ T ^j i!- 1 ^ \" V- \^ /\/\ . < V. O l V r / ^ W V Figure 32. Schematic diagram showing: A) pre-normal fa u lt geometry, and B) "reverse drag" structure formed as a result of listric normal faulting (after Hamblin, 1965). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 87 Field evidence of actual down-dip flattening along the Snowcrest- Greenhorn range-front fa u lt system is scarce. Yet, simple geometric construction requires that the fault system must flatten at depth to accommodate the observed rotation of the Tertiary beds along the eastern margin of the basin. S im ilar inte rpre ta tio n s o f l is t r ic normal fa u lt geometry have been made along the Madison range-front fault based on seismic data, (Werkema and Young, 1983, personal communication; Schmidt et a l., 1984). Age of Faulting On the basis of stratigraphic and geophysical evidence, Monroe (1976) concluded that the Upper Ruby basin was probably in itia lly outlined during Middle to Late Eocene time. This age is compatible * with estimates made for similar intermontane basins (Kuenzi, 1966; Kuenzi and Fields, 1972; and Petkewich, 1972) and corresponds well with the cessation o f Laramide compressional tectonics roughly 40 m.y.b.p.. According to Monroe (1976), in it ia l development o f the Upper Ruby basin during Eocene time was probably fault-controlled. Rocks of the Sixmile Creek Formation are truncated by the Snowcrest-Greenhorn range-front fa u lt along the eastern flank o f the Upper Ruby basin. This episode o f normal fa u lt movement prob ably represents a Pliocene event. Therefore, the movement along the Snowcrest-Greenhorn range-front fa u lt system may range from as early as Middle Eocene (?) to Pliocene. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission 88 Northwest-Trending Normal Faults As with major range-front faults, the age of northwest-trending normal fa u lts can only be broadly bracketed. The Stone Creek fa u lt is believed to have undergone three periods of Tertiary movement (Monroe, 1976). During the e a rlie s t episode o f movement (th a t is , during Middle to Late Eocene (?) tim e), the Stone Creek fa u lt may have acted as a major range-bounding fault along the western margin o f the Upper Ruby basin (Monroe, 1976). The Stone Creek, Sweetwater and Blacktail faults all cut 4.2 m.y.b.p. basalt flows, and form distinctive scarps (Marvin et al_., 1974). This may be the most recent episode of faulting and probably represents a Late Pliocene or Pleistocene event. Therefore, the age of post-Laramide normal faulting along major northwest-trending zones may range from as early as Middle Eocene (?) to Pleistocene. Dynamics o f Normal Faulting The fa c t th a t Middle to Late Cenozoic time marked a major period of basin-range extensional tectonics in southwestern Mon tana has been well established (see, fo r example, Reynolds, 1982). Numerous stra tig ra p h ic studies (Kuenzi and Fields, 1971; Petkewich, 1972; and Monroe, 1976) have suggested that fa u lt- bounded intermontane basins may have begun to develop as early as Middle Eocene time, follow ing the fin a l stages o f Laramide compression. with permission of the copyright owner. Further reproduction prohibited without permission. 89 Three possible c o n tro llin g factors related to compression during the Laramide orogeny may have contributed to the in it ia l development of the Upper Ruby-Sage Creek basin along the crest of the Blacktail- Snowcrest u p lift during Eocene time. First, broad folding of rocks on the uplifted hanging-wall of the Snowcrest-Greenhorn thrust system may have contributed to exten sional faulting. Inasmuch as broad folding was probably largely associated with b rittle deformation, it seems likely that exten sional features ( jo in ts , fractures and fa u lts ) may have formed in the hinge zone as a re s u lt o f compressional buckling. The anticline on the hanging-wall of the Snowcrest-Greenhorn thrust system, whatever its origin, must have developed extension fractures along its outer surface. One such fracture or system of fractures may have helped localize later normal faulting (Figure 33). A similar mechanism has been suggested to explain the origin of range-front faulting associated with the Madison-Gravelly u p lift (Young, 1984). Second, the waning o f foreland thrusting was lik e ly accompanied by release of horizontal compressive stresses and associated stored elastic strain. This release may have caused the rocks in the u p lifte d hanging-wall o f the Snowcrest-Greenhorn th ru st system to dilate, locally forming extensional fractures normal to the former direction of maximum compression. If extensional fractures did form in th is manner, th e ir orie n tation would probably have been favorable fo r the development o f la te r extensional fa u ltin g . with permission of the copyright owner. Further reproduction prohibited without permission. 90 _ . - . ^ 7 L S I ~ / S V -I1--/-// C> V = - ' 7 ' ■ V' - a -- / * i./.TI ^ *7 ' ^ ■ i, i Vr,7 - v / v z ^7 / - - v / ' ' \ "-• / s. / /«- / Figure 33. Diagrammatic model showing the possible relationship between extension due to broad arching and basin-range normal fa u ltin g . Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 91 A s im ila r mechanism has been suggested to explain normal fa u lts behind ramp regions of the decollement-style Cumberland thrust in the Appalachians of Tennessee (Schmidt, 1982, personal communication). Finally, gravity-induced collapse of the Blacktail-Snowcrest up lif t may have ultimately contributed to the formation of the fault- bounded Upper Ruby-Sage Creek basin. The suggestion is that the major overhanging Precambrian thrust block associated with the Snow crest-Greenhorn thrust system was "held up" by its own internal strength. Consequently, the thrust block was susceptible to gravity stressing which may have resulted in the eventual collapse near the outer thrust lip. The close spatial relationship between foreland thrusts and basin-range normal faults strongly supports this basic thesis (Plates I and III). The localization of basin-range normal fa u lts as a re su lt o f collapse o f major compressive Laramide stru c tures has been well documented (see, fo r example, Berg, 1962; and Gries, 1983) and was discussed in detail by Sales (1983). Although this mechanism may have been, in part, responsible for localization of range-front faulting, the region has clearly undergone post- collapse basin-range extension. Fault Movement Patterns Northwest-Trending Faults No slickenside data is available along northwest-trending faults in the region. As previously mentioned, however, significant amounts of dip-slip movement can be demonstrated along portions of Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 92 the Stone Creek, Sweetwater, and B la cktail fa u lts . The Stone Creek fa u lt appears to e x h ib it le ft- la te r a l separation o f roughly 6 km along the western margin of the Upper Ruby basin (Plate III). How ever, since evidence o f such movement elsewhere along the Stone Creek fault is lacking, it is unlikely that significant left-lateral move ment actually occurred. Therefore, left-lateral "displacement" of the north-trending eastern Ruby range-front fault segment is probably only apparent separation. If, as suggested by Monroe (1976), the Stone Creek fault acted as a major basin-bounding fau lt, movement would lik e ly have had a large component o f d ip -s lip . Snowcrest-Greenhorn Range-Front Fault System No slickenside data is available along the Snowcrest-Greenhorn range-front fault system, but dip-slip movement along the fault is inferred by the existence of a horizontal fold hinge (Ledford Creek anticline) in Tertiary rocks subparallel and adjacent to the fault. If the inference of dip-slip movement along the fault system is valid, estimates of net slip may be determined from estimates of dip separation. However, any estimates of dip separation along the Snowcrest-Greenhorn range-front fa u lt system are s p e c ific a lly dependent upon two variables: (1) the to ta l thickness o f T e rtia ry sedimentary rocks in the basin, and (2) the thickness of Paleozoic rocks underlying Tertiary basin deposits. Estimates of sedimentary basin thickness have been made on the basis of gravity studies in the Upper Ruby basin. Burfeind (1967) suggested up to 2134 m of Tertiary f ill may be present. with permission of the copyright owner. Further reproduction prohibited without permission. 93 Monroe (1981) presented strati graphic evidence to indicate that the thickness of Tertiary rocks in the Upper. Ruby basin is at least 1120 m. Estimates o f the thickness o f Paleozoic rocks beneath the Upper Ruby basin are more problematical. Although Belt clasts represent a major component of the Beaverhead Formation derived from the fold- and-thrust belt, Mississippian (and younger) limestone cobble con glomerates represents the dominant c la s t lith o lo g y derived from the Blacktail-Snowcrest u p lift (Ryder, 1967). Therefore, it may be assumed that Mississippian rocks were widely exposed in the Black- t a il -Snowcrest region during Paleocene time. Clasts from Cambrian, Ordovician, and Devonian rocks are not ty p ic a lly present in the Beaverhead Formation and were probably not completely eroded from this region during Laramide time. It is suggested that local thick nesses (up to 650 m) of pre-Mississippi an rocks are likely present beneath Tertiary basin fill deposits. More precise estimates of the Paleozoic sedimentary thickness must await further detailed study o f provenance o f the Beaverhead Formation. Assuming the foregoing estimates are reasonable, and assuming the original interpretation of predominantly dip-slip movement is correct, estimates of net slip may be obtained from estimates of dip separation on reconstructed structural profiles across the northern Snowcrest Range. T e rtia ry basin depth values (1120 m and 2134 m), and the estimated locally underlying Paleozoic thickness value (650 m) were Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 94 used to calculate the range of net slip and structural relief values for the Snowcrest-Greenhorn range-front fault system. Structure sections B-B 1 and G-G' (Plate II), thus, yield net slip estimates ranging from 3290 to 5885 m. This corresponds to a maximum struc tural re lie f of 2590 m to 3445 m. Estimates based on geophysical evidence for the structurally similar Madison basin are compatible with estimates presented in this report. Schofield (1981) presented g ra vity data ind ica ting that stru ctu ra l r e lie f may range from about 5000 f t . to as much as 15,000 f t . (1524-4572 m) along the Madison range-front fa u lt. Werkema (1984) presented comparable estimates based on seismic data. He suggested up to approximately 13,000 f t . (3963 m) of structural re lie f may exist along the Madison range-front fa u lt. In addition to net slip and structural relief determinations, the amount of east-west crustal extension was estimated, again from reconstructed structural profiles across the northern Snowcrest Range. Structure sections B-B' and G-G 1 (Plate II) yield east-west crustal extension estimates ranging from 1,660 to 3,870 m. This corresponds to 2.5 to 6.0% east-west crustal extension for that portion of the foreland covered by the regional tectonic compilation (Plate III). Stress Configurations Some contemporary seismic evidence indicates that multiple stress systems may have been responsible for basin-range extension with permission of the copyright owner. Further reproduction prohibited without permission 95 in southwestern Montana. Fault plane solutions fo r the Disturbed B elt region o f western Montana generally indicate normal fa u lt mechanisms and tensional axes directed approximately east-west (Sbar, 1972; and Smith and Sbar, 1974). In contrast, Smith and Sbar (1974) show fa u lt plane solutions for the Yellowstone Hebgen Lake region of south western Montana fo r both normal and s trik e -s lip fa u lt mechanisms, which generally indicate north-south-trending tensional axes. Observed fa u lt movement patterns compare favorably with these in te rp re ta tio n s. The geometry o f the Snowcrest-Greenhorn range-front fault system is consistent with east-west extension about a north- ■trending horizontal rotational axis located along the center of the Upper Ruby basin. In a ddition , the geometry o f the northwest-trend- ing faults is consistent with north-south extension in the region. Thus, contemporary seismic evidence, together with normal fa u lt geometry and movement patterns, suggest that two d is tin c t post- Laramide stress systems may have been operative in southwestern Montana: one producing east-west extension, and another producing north-south extension. Tectonic Controls on Sedimentation Sixmile Creek Formation Tertiary rocks adjacent to the northern Snowcrest Range are part o f the Late Miocene-Pliocene Sixmile Creek Formation (Monroe, 1976). The Sixmile Creek Formation is typically characterized by coarse grained sedimentary deposits representing predominantly alluvial fan Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 96 deposition. The formation overlies rocks of the Renova Formation (Early Oligocene to Middle Miocene) w ith angular unconformity (Monroe, 1976), and has a total thickness estimated to exceed 2000 ft. (610 m). Lithologically, the Sixmile Creek Formation iri the Upper Ruby basin is typically characterized by tuffaceous conglomerates, shard-rich sandstones and subordinate s ilts to n e s , limestones and stream deposited volcanic ash (Monroe, 1976). Environment o f Deposition As suggested by Monroe (1976), the predominantly coarse-grained sequence and rapid facies changes in the Sixmile Creek Formation probably represent alluvial fan deposition. Massive unstratified conglomeratic mudstones (Figure 34A) are evidence for proximal fan deposition with debris flow acting as the dominant depositional process (B u ll, 1972). W ater-laid ash t u f f and shard-rich sandstones e x h ib it graded beds, sco u r-a nd -fi 11 structures, and trough cross stratification (Figure 34B) and are indicative of deposition in minor braided stream channels. Volumetrically minor travertine beds represent sporadic hot-spring activity along the margin of the basin (Figure 35). Tectonic Significance of Sixmile Creek Sedimentation The idea that alluvial fans typically represent orogenic deposits related to renewed or accelerated u p lift and erosion of source areas has been established by many workers (see, for example, Blissenbach, 1954; Beaty, 1963; and B u ll, 1972). I t is Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 97 gipsi? & ,£>. I . V V s > ' Figure 34. A. Photograph (looking east) o f massive c l i f f of conglomeratic mudstones of the Sixmile Creek Formation. B. Photograph (looking north) of scour- a n d -fill structure in conglomeratic sandstone overlying waterlaid cross-stratified ash tuffs of the Sixmile Creek Formation. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 98 e> Mbs o Ta 30 Figure 35. A. Photograph (looking east) of a distinctive eastward thickening wedge of travertine belong ing to the Sixmile Creek Formation. B. Inter pretive sketch of the relationship between Tertiary Sixmile Creek basin deposits and the Snowcrest-Greenhorn range-front fault. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 99 likely that Six Mile Creek alluvial fan deposits are indicative of deposition in response to movement on major normal fa u lts produced by basin-range extensional tectonics during Late Miocene-Pliocene time. In addition to alluvial fan deposits, travertine beds belonging to the Sixmile Creek Formation also appear to re fle c t fa u lt con trol on sedimentation. Travertine deposits are exposed only along the margins o f the Upper Ruby and B la ckta il Deer Creek basins (Brasher, 1950; Flannagan, 1958; and Monroe, 1976) and, thus, appear to be re s tric te d to range-front fa u lt zones. Brasher (1950) reports travertine "limestones" up to approximately 50 m in thickness along the western flank of the central Snowcrest Range. The location of travertine deposits on Brasher1s map corresponds well with the inferred position of the Snowcrest-Greenhorn range-front fault system. In the northern Snowcrest Range, additional evidence supporting range-front fault control on Tertiary sedimentation exists. One travertine bed is exceptionally well exposed along the north wall of Ledford Creek Canyon in sec. 20, T. 10 S., R. 4 W. This distinctly wedge-shaped travertine bed thickens markedly toward the eastern margin of the basin and attains a maximum thickness of about 20 m where i t abuts the Snowcrest-Greenhorn range-front fa u lt system. This relationship appears to indicate fault control on sedimentation, and may reflect "ponding" of hot-spring deposits along an active range-front fault during Late Miocene-Pliocene time (Figure 35). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER V SUMMARY AND SYNTHESIS Geologic History Local structural relationships together with regional struc tural and stratigraphic evidence clearly indicate that the northern Snowcrest Range has experienced long-term recurrent tectonic activity. The following is a summary of the tectonic history of the Black- ta il -Snowcrest u p lift: 1) Pre-Cherry Creek and Cherry Creek metamorphic assemblages experienced complex deformational and metamorphic h isto rie s. 2) The Madison mylonite zone (Erslev, 1982) and associated northeast-trending structures probably formed during Middle Protero- zoic time (1500 to 1800 m.y.b.p.). The development of a similar shear zone in the Snowcrest-Greenhorn region may have marked the establishment of the Snowcrest-Greenhorn lineament during this time. 3) Regional u p lift and erosion occurred in conjunction with Proterozoic continental riftin g , and deposition of the Belt Super group occurred to the north and west. Northwest-trending faults including the Stone Creek, Sweetwater and Blacktail faults were probably active between 800 and 1455 m.y.b.p. (Wooden et c f[., 1978). 4) Relatively stable cratonic platform sedimentation developed by Middle Cambrian time. Fluctuations in the sedimentation occurred 100 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 101 as a result of renewed tectonic activity (Antler orogeny) along the Snowcrest-Greenhorn lineament between late Middle Cambrian and Middle Devonian time. The area regained relative tectonic stability during deposition of the Late Devonian Jefferson Dolomite, and remained stable until Mississippian time. 5) Recurrent tectonic activity is marked by significant subsi dence along a narrow trough related to the Snowcrest-Greenhorn line ament during deposition of the Mississippian Madison and Big Snowy Groups, and the Pennsylvanian Quadrant Formation. Following th is period of subsidence, the Snowcrest region regained tectonic s ta b ility during Permian through T riassic time, and remained relatively stable until the onset of the Laramide orogeny. 6 ) The f i r s t evidence o f the Laramide orogeny in extreme southwestern Montana is offered by the in itia l development of the Blacktail-Snowcrest arch during late Early Cretaceous time (Figure 36A). Regional compression directed approximately east- west resulted in basement involved thrust-uplift along the Snow crest-Greenhorn lineament and along existing northwest-trending faults. U plift was attended by erosion and deposition of the syn- tectonic Beaverhead Formation (Figure 36B). 7) U plift of the Blacktail-Snowcrest arch continued with thrusting and folding along the Snowcrest-Greenhorn thrust system during the Late Cretaceous. Deformation progressed from west to east, and folds became progressively overturned with continued eastward propagation along low-angle thrusts. with permission of the copyright owner. Further reproduction prohibited without permission. 102 -o \ - - 4 Km EARLY CRETACEOUS -0 - 4 Km LATE CRETACEOUS - EARLY EOCENE ... C ~ ' ' " L"/ '' ' ', / - 1 i > i - 1 > / -4Km -\ /n" " I,"'- /v i ✓ MIDDLE EOCENE- LATE EOCENE -4Km MIOCENE - HOLOCENE Figure 36. Diagrammatic model showing the structural evolution of the Blacktail-Snowcrest arch region (vertical exaggeration approximately 3X). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 103 8 ) Folding and thrusting along the Snowcrest-Greenhorn thrust system culminated with the development o f a fro n ta l th ru st zone in the Gravelly Range during Late Paleocene time and Eocene time. This zone is characterized by detachment thrusts involving Phanerozoic rocks (Figure 36B). 9) Range-front normal fa u lts outlined the present western boundary o f the Snowcrest Range, possibly as early as Middle Eocene (? ), follow ing the fin a l stages o f Laramide compression (Figure 36C). Listric normal faults developed along existing low-angle thrust faults. Existing northwest-trending faults were also reactivated as normal fa u lts . The Upper Ruby Basin was i n it ia lly outlined and began fillin g with sediments (Figure 36C). 10) Regional u p lift and erosion occurred during Middle Miocene time, attended by renewed tectonic activity along normal faults (Figure 35D). Minor rhyolitic volcanism took place along the western margin of the Upper Ruby basin. 11) Sporadic tectonic activity in the Blacktail-Snowcrest arch region occurred from Pliocene to Pleistocene time. Extensive basal tic volcanism took place along the western margin of the Upper Ruby Basin, and in the Sage Creek Basin. The la s t s ig n ific a n t movement along the Snowcrest-Greenhorn range-front fa u lt system occurred at th is time. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 104 Conclusions Analysis o f local and regional Laramide stru ctu ra l elements associated with the Blacktail-Snowcrest arch indicate a thrust-uplift geometry fo r mountain fla nk deformation in the northern Snowcrest Range. The Laramide Snowcrest-Greenhorn th ru st system developed as a result of recurrent movement along a prominent northeast-trending zone of pre-Beltian crustal weakness. In addition, recurrent move ment occurred along major Late Precambrian northwest-trending fa u lts . The Stone Creek, Sweetwater, and B la ckta il fa u lts appear to have been locally activated as tear faults associated with the Snowcrest th ru s t zone. The analysis o f infe rre d subsurface geometry and local kine matic indicators suggest that deformation along the Snowcrest-Green horn thrust system was largely compressional. Maximum principal stress configurations in the northern Snowcrest Range indicate horizontal compression directed approximately east-southeast-west-northwest. These directions are generally consistent with regional east-west compression documented in adjacent parts o f the foreland (Schmidt and Garihan, 1983) and in the frontal portion of the,fold-and-thrust belt (Schmidt and O'Neill, 1982). Overturned folds and slickensides associated with the Snowcrest- Greenhorn th ru st system indicate oblique s lip w ith a major component of thrust sliD and a minor component of dextral slip. Locally well- developed zones of cataclasis and widescale shearing, particularly along foliation surfaces, indicate predominantly b rittle deformation Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 105 in pre-Beltian metamorphic rocks associated with the Snowcrest-Greenhorn th ru st system. Although local Laramide folding o f pre-B eltian meta morphic rocks, particularly those adjacent to the Phanerozoic contact has been documented in the foreland o f southwestern Montana (T ilfo rd , 1978; and Schmidt and Garihan, 1983), s ig n ific a n t fo ld ing o f pre- Bel tia n metamorphic rocks associated with the Snowcrest-Greenhorn th ru st system cannot be conclusively demonstrated in the northern Snowcrest Range. A major post-Laramide (Neogene) range-front fa u lt system closely follow s the trace of the Snowcrest th ru st zone and appears to repre sent a half-graben stru ctu ra l geometry. Because the Snowcrest- Greenhorn range-front fault system is inferred to flatten at depth, basin-range extension along low-angle thrusts is suggested. Exten- sional range-front faulting associated with the Blacktai1-Snowcrest u p lift developed follow ing the fin a l stages o f Laramide compression. The localization of these structures along the hanging-wall of the Snowcrest-Greenhorn th ru st system may be a ttrib u te d to broad crustal arching and compressional buckling, compression release, and/or gravity induced collapse of the Blacktail-Snowcrest uplift. Basin- range extension along the Snowcrest-Greenhorn range-front fa u lt sys tem was, in part, responsible for the reactivation of major northwest- trending normal faults in the region. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. APPENDIX A Stereographic Analysis o f Folds 106 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4 ° S 5 3 °W A. S-pole diagram fo r the Snowcrest a n tic lin e . B. S-pole diagram fo r the Spur Mountain syncline. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 108 2 4 ° S 55°W 2 0 ° S5°W S-nole diaqram fofo r r thethe Sliderock Mountain a n tic lin e . S-pole~ diagram for _ the . i . Jobe n _ l Creekr s fold I„ set. 1J ^ ^ 4- Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 109 N 13° S23°W Station MKS-82-85. S-pole diagrams for folds in the Phosphoria Formation. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. no A 6 ° S8°W N 0 SII°W, Station MKS-82-88. S-pole diagrams fo r folds in the Big Snowy Group and the Amsden Formation. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. m N 15° S I6°W Station MKS-82-88. S-pole diagrams fo r folds in the Big Snowy Group and the Amsden Formation. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. N _L S 2 4°W Station MKS-82-100. S-pole diagrams fo r folds in the Phosphoria Formation. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 113 14° S29°W N 6 ° S22°W Station MKS-82-100. S-pole diagrams fo r folds in the Phosphoria Formation. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 114 3° S 7 6 ° W N 13° S 6 2 ° W Station MKS-82-159. S-pole diagrams fo r folds in the Amsden Formation. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 6 ° S47°W N 21° S 58°W Station MKS-82-203. S-pole diagrams fo r folds in the Big Snowy Group and the Amsden Formation. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 116 12° S 2I°W N 14° S23°W Station MKS-82-203. S-pole diagrams fo r folds in the Big Snowy Group and the Amsden Formation. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 117 2 2 ° SII°W N 16° S I4°W Station MKS-82-203. S-pole diagrams fo r folds in the Big Snowy Group and the Amsden Formation. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 118 N _L Station MKS-82-226. S-pole diagrams fo r folds in the Big Snowy Group. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 119 N 2 2 ° S54°W Station MKS-82-226. S-pole diagrams fo r folds in the Big Snowy Group. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. BIBLIOGRAPHY Achuff, J. M. (1981). Structural analysis of faulting and folding in Small Horn Canyon, Beaverhead County, Montana. Montana Geological Society 1981 Field Conference Guidebook (pp. 239-243). Allmendinger, R. W., Sharp, J. W., Von Tish, D., Serpa, L., Brown, L ., Kaufman, S., & O live r, 0. (1983). Cenozoic and Mesozoic structure of the eastern Basin and Range province, Utah, from C0C0RP seismic-reflection data. Geology, 11, 532-536. Anderson, R. E., Zoback, M. L ., & Thompson, G. A. (1983). Im plica tions of selected subsurface data on the structural form and evolution of some basins in the northern Basin and Range province, Nevada and Utah. Geological Society o f America B u lle tin , 94, 1055-1072. Beaty, C. B. (1963). Origin of alluvial fans, White Mountains, California and Nevada. Annals of the Association of American Geographers, 53, 516-535. Beck, F. M. (1960). Geology o f the Sphinx Mountain area, Madison and Gallatin counties, Montana. In Billings Geological Society 11th Annual Field Conference Guidebook (pp. 129-134)“ B iflin g s , MT: Billings Geological Society. Berg, R. B. (1979). 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Geology of part of the Snowcrest Range, Beaverhead County, Montana. Unpublished master's th e sis, Univer- with of Michigan, Ann Arbor. Bruhn, R. L ., & Beck, S. L. (1981). Mechanics o f th ru st fa u ltin g in c ry s ta llin e basement, Sevier oroqenic b e lt, Utah. Geology, 9, 200-204. Bull, W. B. (1972). Recognition of alluvial fan deposits in the stratigraphic record. In Rigby, J. K., & Hamblin, W. K. (Eds.), Recognition o f Ancient sedimentary, environments. Society o f Economic Paleontologists and M ineralogists, (Spec. Pub. 16), 63-83. Burchfiel, B. C., & Davis, G. A. (1972). Structural framework and evolution of the southern part of the Cordilleran orogen. American Journal o f Science, 272, 97-118. Burchfiel, B. C., & Davis, G. A. (1975). Nature and controls of Cordilleran orogenesis, western United States: Extension of an e a rlie r synthesis. American Journal of Science, 275-A, 363-396. Burfeind, W. J. (1967). A gravity investigation of the Tobacco Root Mountains, Jefferson Basin, Boulder batholith, and adjacent areas o f southwestern Montana. Unpublished Ph.D. th e sis, Indiana University, Bloomington. Chadwick, R. A. (1981). Chronology and stru ctu ra l setting o f volcanism in southwest and central Montana. Montana Geological Society 1981 Field Conference Guidebook (pp. 301-310). Condie, K. (1975). The Wyoming province in the western United States. In B. F. Windley (Ed.), The Early History of the Earth. New York: Wiley & Sons. Condit, D. D,, Finch, E. H., & Pardee, J. T. (1928). Phosphate rock in the Three Forks Yellowstone Park region, Montana. United States Geological Survey B u lle tin 795, 147-209. Coney, P. J. (1978). Mesozoic-Cenozoic Cordilleran plate tectonics. In R. B. Smith & G. P. Eaton (Eds.), Cenozoic tectonics and regional geophysics of the western Cordillera (Memoir 152). Boulder, CO: Geological Society of America. Dahlstrom, C. D. A. (1970). 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Geology of the talc deposits of the central Ruby Range, Madison County, Montana^ Unpublished Ph.D. thesis, Pennsylvania State University, University Park. Garihan, J. M. (1979). Geology and structure of the central Ruby Range, Madison County, Montana. Geological Society o f America B u lle tin , 90, 695-788. Gealy, W. J. (1953). Geology o f the Antone Peak quadrangle, south western Montana. Unpublished Ph.D. thesis, Harvard University, Cambridge. G iletti, B. J. (1966). Isotopic ages from southwestern Montana. Journal o f Geophysical Research, 71, 4029-4036. with permission o, the o „pyrigtlt ow„ e, Further reproductjon ^ 123 G illuly, J. (1965). Volcanism, tectonism and plutonism in the west ern United States. Geological Society o f America Special Paper 80. Gries, R. (1981). Oil and gas prospecting beneath the Precambrian of foreland thrust plates in Rocky Mountains. American Associa tion of Petroleum Geologists Bulletin, 67, 1-28. Hadley, J. B. (1960). Geology of the northern part of the Gravelly Range, Madison County, Montana. B illin g s Geological Society 11th Annual Field Conference Guidebook (pp. 149-153). Hadley, J. B. (1969a). Geologic map of the Varney quadrangle Madison County, Montana. United States Geological Survey Map GQ-814. Hadley, J. B. (1980). Geology o f the Varney and Cameron quadran gles, Madison County, Montana. United States Geological Survey B u lle tin 1459. Haley, C. J. (1983). The sedimentology of a synorogenic deposit: The Beaverhead Formation o f Montana and Idaho. Geological Society o f America Abstracts with Programs, 15(6), 589. H a ll, W. B. (1961). Geology o f the Upper G a lla tin River basin, southwestern Montana. Unpublished Ph.D. th e sis, U niversity o f Wyoming, Laramie. Hamblin, W. K. (1965). Origin of "reverse drag" on the downthrown side o f normal fa u lts . Geological Society o f America B u lle tin , 76, 1145-1164. Harrison, J. E., Griggs, A. B., & Wells, J. D. (1974). 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Geology of the Blacktail-Snowcrest region o f Beaverhead County, Montana. Unpublished Ph.D. the sis, University of Michigan, Ann Arbor. Klepper, M. R. (1950). A geologic reconnaissance of parts of Beaverhead and Madison counties, Montana. United States Geological Survey Bulletin 969-C (pp. 55-85T Klepper, M. R., Weeks, R. A ., & Ruppel, E. T. (1957). Geology of the southern Elkhorn Mountains, Jefferson and Broadwater counties, Montana! (Prof. Paper 292). Washington, D.C.: United States Geological Survey. Kuenzi, W. D. (1966). Tertiary stratigraphy in the Jefferson River Basin, Montana. Unpublished Ph.D. th e sis, U niversity o f Montana, Missoula. Kuenzi, W. D., & Fields, R. W. (1971). Tertiary stratigraphy structure and geologic history, Jefferson Basin, Montana. Geological Society o f America B u lle tin , 82, 3373-3394. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission 125 Kulik, D. M., & Perry, W. J. (1982). Gravity modeling of the steep southeast limb of the Blacktail-Snowcrest u p lift, southwest Mon tana. Geological Society o f America Abstracts with Programs, 14 (6 ), 318: Kummel, B. (1960). The Triassic of southwestern Montana. Billings Geological Society 11th Annual Field Conference Guidebook (pp. 233- 238). Lemish, J. (1948). The geology o f the West Fork of the Madison River area, Montana. Unpublished master's th e sis, U niversity of Michigan, Ann Arbor. Lowell, J. D. (1974). Plate tectonics and foreland basement defor mation. Geology, 2, 275-278. Lowell, J. D. (1977). Underthrusting origin for thrust fold belts with application to the Idaho Wyoming belt. Wyoming Geological Association 29th Annual Field Conference (pp. 449-4557: Lowell, W. R., & Klepper, M. R. (1953). Beaverhead Formation, a Laramide deposit in Beaverhead County, Montana. Geological Society o f America B u lle tin , 64, 235-244. Mann, J. A. (1954). Geology o f part o f the Gravelly Range Montana. Yellowstone-Bighorn Research Association, (Contribution 190). Mann, J. A. (1960). Geology o f part o f the Gravelly Range, Montana. B illin g s Geological Society 11th Annual Field Conference Guide book (pp. 114-127). Maughn, E. K., & Perry, W. J. (1982). Paleozoic tectonism in south western Montana. Geological Society of America Abstracts with Programs, j[4(6), 341. Maughn, E. K ., & Roberts, A. E. (1967). Big Snowy and Amsden Groups and the Mississippian-Pennsylvanian Boundary in Montana. (Prof. Paper 555-B). Washington, D.C.: United States Geological Survey. Marvin, R. F., Wier, K. L., Mehnert, H. H., & M erritt, V. M. (1974). K-Ar ages of selected Tertiary igneous rocks in southwestern Montana. Isochron/West, 10, 17-20. Meyers, J. H. (1981). Carbonate sedimentation in the E llis Group (Jurassic) on the southern flank of Belt Island, southwestern Montana. Montana Geological Society 1981 Field Conference Guidebook (pp. 83-91). with permission of the copyright owner. Further reproduction prohibited without permission 126 McMannis, W. J. (1963). LaHood Formation — a coarse facies of the B elt Series in southwest Montana. Geological Society o f America Bulletin, 74> 407-436. McMannis, W. J. (1965). Resume of depositional and structural his to ry o f western Montana. American Association o f Petroleum Geologists B u lle tin , 49, 1801-1823. Monroe, J. S. (1976). Vertebrate paleontology, stratigraphy, and sedimentation of the Upper Ruby River Basin, Madison County, Montana. Unpublished Ph.D. th e sis, U niversity o f Montana, Missoula. Monroe, J. S. (1981). Late Oligocene-Early Miocene facies and lacustrine sedimentation, Upper RubyRiver Basin, southwestern Montana. Journal o f Sedimentary Petrology, 51, 939-951. Moritz, C. A. (1951). Triassic and Jurassic stratigraphy of south western Montana. American Association of Petroleum Geologists B u lle tin , 35, 1781-1814. Moritz, C. A. (1960). Summary of the Jurassic stratigraphy of southwestern Montana. B illin g s Geological Society 11th Annual Field Conference Guidebook (pp. 239-243). Okuma, A. F. (1971). Structure of the southwestern Ruby Range, near D illo n , Montana. Unpublished Ph.D. th e sis, Pennsylvania State U niversity, U niversity Park. Pardee, J. T. (1950). Late Cenozoic block fa u ltin g in western Montana. Geological Society of America Bulletin, 61, 359-406. Perry, W. J., Ryder, R. T., & Maughn, E. K. (1981). The southern part of the southwest Montana thrust belt: A preliminary evalua tion of structure, thermal maturation and petroleum potential. Montana Geological Society 1981 Field Conference Guidebook (pp. 261-273): Perry, W. J . , Wardlaw, B. R., Bostick, N. H., & Maughn, E. K. (1983). Structure, burial history, and petroleum potential of the frontal thrust belt and adjacent foreland, southwest Montana. American Association o f Petroleum Geologists B u lle tin , 67(5), 725-743. Petersen, F. A. (1983). Foreland detachment structures. In J. D. Lowell (Ed.), Rocky Mountain Foreland Basins and U plifts. Rocky Mountain Association of Geologists, 65-78. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 127 Peterson, J. C. (1974). Geology o f the Sweetwater Canyon area, and o rig in o f interbasin canyons, southwestern Montana. Unpublished master's th esis, Western Michigan U niversity, Kalamazoo. Petkewich, R. M. (1972). Tertiary geology and paleontology of the Beaverhead East area, southwestern Montana. Unpublished Ph.D. th e sis, U niversity o f Montana, Missoula. Prucha, J. J., Graham, J. A., & Nickelsen, R. P. (1965). Basement controlled deformation in Wyoming province of Rocky Mountains foreland. American Association o f Petroleum Geologists B u lle tin , 49(7), 966-992. Ragan, D. M. (1973). Structural geology: An introduction to geo m etrical techniques. New York: John Wiley & Sons, 1973. Reid, R. R., McMannis, W. J ., & Palmquist, J. D. (1975). Precam brian geology of the north Snowy Block, Beartooth Mountains, Montana. Geological Society o f America Special Paper 157. Reynolds, M. W. (1979). Character and extent of basin-range faulting western Montana and east-central Idaho. In G. Newman & H. Goode (Eds.), Rocky Mountain Association of Geologists and Utah Geological Association 1979 Basin and Range Symposium. B illin g s , Montana. Rich, J. L. (1934). Mechanics of low-angle overthrust faulting as illustrated by the Cumberland thrust block, Virginia, Kentucky, and Tennessee. American Association o f Petroleum Geologists ’ B u lle tin , 18, 1584-1596. Robinson, G. D. (1963). Geology of the Three-Forks quadrangle, southwestern Montana (Prof. Paper 370). Washington, D.C.: United States Geological Survey. Ruppel, E. T. (1982). Cenozoic b lo c k -u p lift in east-central Idaho and southwest Montana" (Prof. Paper 1224). Washington, D.C.: United States Geological Survey. Ruppel, E. T., Wallace, C. A., Schmidt, R. G., & Lopez, D. A. (1-981). Preliminary interpretation of the thrust belt in southwest and west-central Montana and east-central Idaho. Montana Geological Society 1981 Field Conference Guidebook (pp. 139-159). Ryder, R. T. (1967). Lithosomes in the Beaverhead Formation, Montana-Idaho: A preliminary report. Montana Geological Society 18th Annual Field Conference Guidebook (pp. 63-70). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission 128 Ryder, R. T., & Ames, H. T. (1970). Palynology and age of Beaver head Formation and their paleotectonic implications in the Lima region, Montana-Idaho. American Association of Petroleum Geologists B u lle tin , 54, 1155-1171. Ryder, R. T., & Scholten, R. (1973). Syntectonic conglomerates in southwestern Montana: Their nature, origin, and tectonic signifi cance. Geological Society o f America B u lle tin , 84, 773-796. Sales, J. K. (1968). Crustal mechanics of Cordilleran foreland deformation: A regional scale model approach. American Associa tio n o f Petroleum Geologists B u lle tin , 52, 2016-2044. Sales, J. K. (1983). Collapse of Rocky Mountain basement uplifts. In J. D. Lowell (Ed.), Rocky Mountain Foreland Basins and U plifts. Rocky Mountain Association of Geologists, 79-97. Sbar, M. L ., Barazangi, M., Dorman, J ., Scholz, C. H., & Smith, R. B. (1972). Tectonics of the Intermountain Seismic belt, western United States: Microearthquake seismicity and composite fault plane solutions. Geological Society of America Bulletin, 83, 13-20. Schmidt, C. J. (1975). An analysis of folding and faulting in the northern Tobacco Root Mountains, southwest Montana. Unpublished Ph.D. thesis, Indiana University, Bloomington. Schmidt, C. J., & Garihan, J. M. (1983). Laramide tectonic develop ment of the Rocky Mountain Foreland of southwestern Montana. In J. D. Lowell (Ed.), Rocky Mountain Foreland Basins and U plifts. Rocky Mountain Association of Geologists, 271-294. Schmidt, C. J ., & Garihan, J. M. (1979). A summary o f Laramide basement fa u ltin g in the Ruby, Tobacco Root, and Madison ranges and its possible relationship to Precambrian continental rifting. Geological Society o f America Abstracts w ith Programs, 11, 301. Schmidt, C. J ., & Garihan, J. M. (1984). Development o f the Rocky Mountain Foreland o f southwestern Montana. Unpublished manuscript. Schmidt, C. J., & Hendrix, T. E. (1981). Tectonic controls for thrust belt and Rocky Mountain Foreland structures on the north ern Tobacco Root Mountains — Jefferson Canyon area, south western Montana. Montana Geological Society 1981 Field Conference Guidebook (pp. 167-180). with permission of the copyright owner. Further reproduction prohibited without permission 129 Schmidt, C. J., & O'Neill, J. M. (1982). Structural evolution of the southwest Montana transverse zone. In W. Powers (Ed.), The western Cverthrust Belt from Alaska to Mexico. Billings, MT: Rocky Mountain Association of Geologists. Schmidt, C. J., Sheedlo, M. K., & Werkema, M. (1984). Control of range-boundary normal fa u lts by e a rlie r stru ctu re s, southwestern Montana. Geological Society o f America Abstracts w ith Programs. Schofield, J. D. (1981). Structure of the Centennial and Madison valleys based on gravitational interpretations. Montana Geolog ical Society 1981 Field Conference Guidebook (pp. 275-283). Scholten, R. (1957). Paleozoic evolution of the geosynclinal margin north of the Snake River Plain. Geological Society of America B u lle tin , 6 8 , 151-170. Scholten, R. (1967). S tructural framework and o il potential o f extreme southwestern Montana. Montana Geological Association 18th Annual Field Conference Guidebook (pp. 7-19). Scholten, R. (1968). Model for evolution of Rocky Mountains east o f Idaho B atholith. Tectonophysics, 6(2), 109-126. Scholten, R., Keenmon, K. A ., & Kupsch, W. 0. (1955). Geology of the Lima region, southwestern Montana and adjacent Idaho. Geological Society o f America B u lle tin , 6 6 , 345-403. Schwartz, R. (1972). Strati graphic and petrographic analysis of the Lower-Cretaceous Blackleaf Formation, southwestern Montana. Unpublished Ph.D. th e sis, Indiana U niversity, B1oomington. Schwartz, R., DeCelles, P. G., & Suttner, L. J. (1983). Tectonic control on Early Cretaceous foreland basin evolution and sedimen ta tio n in southwestern Montana. Geological Society o f America Abstracts with Programs, Jj[( 6 ), 682. Sheedlo, M. K. (1983). Laramide and Neogene fault styles in the central Snowcrest Range, southwestern Montana. Geological Society o f America Abstracts w ith Programs, 15(5), 297. Skipp, B., & Hait, M. H. (1977). Allochthons along the northeast margin of the Snake River Plain, Idaho. Wyoming Geological Association 29th Annual Field Conference Guidebook (pp. 499-515). Sloss, L. L., & Moritz, C. A. (1951). Paleozoic stratigraphy in southwestern Montana. American Association o f Petroleum Geologists B u lle tin , 31, 2135-2169. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission 130 Smith, R. B. (1978). Seismicity, crustal structure, and interplate tectonics of the interior of the western Cordillera. In FI. B. Smith & G. P. Eaton (Eds.), Cenozoic tectonics and regional geo physics of the western Cordillera (Memoir 152). Boulder, CO: Geological Society of America. Smith, R. B., & Sbar, M. L. (1974). Contemporary tectonics and seismicity of the western United States with emphasis on the Inter mountain Seismic B elt. Geological Society o f America B u lle tin , 85, 1205-1218. Smithson, S. B., Brewer, J., Kaufman, S., & Oliver,.J. (1978). Nature o f the Wind River th ru s t, Wyoming, from C0C0RP deep reflection data and from gravity data. Geology, 6, 648-652. Spencer, E. W. (1977). Introduction to the structure of the earth. New York: McGraw-Hill. Spencer, E. W. (1975). Precambrian evolution o f the Spanish Peaks area, Montana. Geological Society o f America B u lle tin , 8 6 , 785- 792. Stewart, J. H. (1978). Basin-range structure in western North America: A review. In R. B. Smith & G. P. Eaton (Eds.), Cenozoic tectonics and regional geophysics o f the western C ordillera (Memoir 152). Boulder, CO: Geological Society o f America. Stewart, J. H. (1972). Late Precambrian evolution of North America. Plate tectonic implications. Geology, 4^ 11-15. Stewart, J. H. (1972). Initial deposits in the Cordilleran geo- syncline: Evidence o f Late Precambrian (850 m.y.) continental separation. Geological Society o f America B u lle tin , 83, 1345- 1360. Suttner, L. J. (1969). Stratigraphic and petrographic analysis o f Upper Jurassic Lower Cretaceous Morrison and Kootenai formations, southwest Montana. American Association o f Petro leum Geologists B u lle tin , 53, 1391-1410. Suttner, L. J., Schwartz, R. K., & James, W. C. (1981). Late Mesozoic to Early Cenozoic foreland sedimentation in southwest Montana. Montana Geological Society 1981 Field Conference Guidebook (pp. 93-103). Swanson, S. E. (1970). Mineral resources in Permian rocks of southwest Montana. (Prof. Paper 313-E). Washington, D.C.: United States Geological Survey. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 131 Tysdal, R. G. (1976). Geologic Map of the northern part of the Ruby Range, Madison County, Montana (Misc. Geologic Investigations Map 1-951). Washington, D.C.: United States Geological Survey. Tysdal, R. G. (1981). Foreland deformation in the northern part o f the Ruby Range o f southwestern Montana. Montana Geological Society 1981 Field Conference Guidebook (pp. 215-224). Tysdal, R. G. (1970). Geology of the northern end of the Ruby Range southwestern Montana. Unpublished Ph.D. thesis, U niversity o f Montana, Missoula. Tilford, M. J. (1978). Structural analysis of the southern and western Greenhorn Range, Madison County, Montana, and magnetic beneficiation of Montana talc ores'. Unpublished master's thesis, . Indiana University, Bloomington. Turner, F. J., 8t Weiss, L. E. (1963). Structural analysis of meta- morphic tectonites. New York: McGraw-Hill. Vaughn, W. J. (1948). The geology o f the West Fork o f the Madison River area, Montana. Unpublished master's thesis, University of Michigan, Ann Arbor. V ita lia n o , C. J ., Cordua, W. S., Burger, H. R., Hanley, T. B., Hess, D. F., & Root, F. K. (1979). Geology and structure o f the southern part of the Tobacco Root Mountains, southwestern Montana: Map summary. Geological Society o f America B u lle tin , 90, 712-715. Werkema, M. (1984). Structural geology of the west-central Madison Range, southwestern Montana. Unpublished master's thesis, Western Michigan U nive rsity, Kalamazoo. Wise, D. U. (1964). M icrojo in tin g in basement, middle Rocky Moun tains o f Montana and Wyoming. Geological Society o f America Bulletin, 75, 287-306. Wooden, J. L., Vitaliano, C. J., Koehler, S. W., & Ragland, P. C. (1978). The Late Precambrian mafic dikes of the southern Tobacco Root Mountains, Montana: Geochemistry, Rb-Sr geochron ology and relationship to Belt tectonics. Canadian Journal of Earth Science, 15, 467-479. Woodward, L. A. (1981). Tectonic framework o f Disturbed Belt o f west central Montana. American Association o f Petroleum Geolo gists Bulletin, 65(2), 291-302. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 132 Young, S. W. (1984). Structural geology of the Jordan Creek area, northern Madison Range, Montana. Unpublished master's thesis, University of Texas, Austin. Young, S. W., Werkema, M., & Sheedlo, M. K. (1983). Mechanisms of rock deformation in basement involved thrusts of southwestern Montana. Geological Society o f America Abstracts w ith Programs, 15(6), 725. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. if GEOLOGIC MAP OF THE BEAVERHEAD AND MAD EX Qal >• oc < z z Alluvium < Three Forks Formation DC Ui z b- o < > 3 Ui a a Dj Colluvial deposits undivided Jefferson Dolomite > DC < h- oc Ts Cpi UJ t- Six Mile Creek Form ation Pilgrim Limestone Ku V) 3 Park Shale O Ui z O .Thermopolis Shale, Frontier Formation < < and overlying shales I— DC UJ CD Cm tt: s a Kootenai Formation •€ W o U i ' \ u> Wolsey Formation < Jm DC 3 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. E NORTHERN SNOWCREST RANGE ADISON COUNTIES. MONTANA EXPLAN ATI ON 25 45 -&■ C ontact Inclined Overturned Vertical Horizontal s Formation Dashed where approximately located Strike and dip of beds Dj Inclined Vertical High angle fault Dashed where approximately located Strike and dip of fo liatio n ) D olom ite Arrow show relative horizontal movement U, upthrown side; D, downthrown side. Cpi Normal fault Dashed where approximately located | Limestone dotted where concealed -I Thrust fault ICp Dashed where approximately located dotted where concealed Shale -H- A nticline Cm Showing trend and plunge of axis Limestone Syncli ne Showing trend and plunge of axis ------a ------ Formation Overturned anticline Showing trend and plunge of axis li ii i l l llil l ll lllllllll ll lllW lllll lllll ll fl l illll l|i»|il'li|||i Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. v ! \ ! pun.Mimummai GE If10 : j - 12 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. •€ W o CO CO Wolsey Formation < Jm QC ZD ~3 Morrison Formation -€f Flathead Quartzite I t z < IT CD Thaynes Formation 2 p€u «t O O CO UJ CO a; Pre-Beltian Metamorphic r < ■fcw a. undivided a: F- Woodside Formation I d Dinwoody Formation z < 2 cc PP UJ a. Phosphoria Formation z IPq < z < > Quadrant Formation >- CO IPa UJ a. Amsden Formation IPMu R. 5 W. / / Amsden - Big Snowy undivided Mbs Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. — ...... A ------— Ts Form ation Overturned anticline Showing trend and plunge of axis Overturned syncline d Quartzite Showing trend and plunge of axis '\A/' Trend and plunge of parasitic folds jtamorphic rocks i vided Q al Qc Qa! Qal SG ■€m . Qal IPMu Qc 62 ■Cpi Reproduced with permission of the copyright owner. Further reproduction prohibited without permission Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Mbs z < Big Snowy Group CL o. CO CO Mmc CO CO s Mission Canyon Limestone Lodgepole Limestone Qal Q c Qal Ts, Qc Qc 40 aKBBBBHensmBaniiis Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. s ' Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. PLATE I Reproduced with permission of the copyright owner Further reproductioTprohibited withoutpCTm'iTsto' Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. r • i y- /• - f / APPROXIMATE DECLINATION .ATE I \ \ Reproduced with permission of the copyright owner. Further reproduction prohibited without permission Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. STUDY AREA INDEX MAP SHOWING LOCATION OF AREA SCALE 1:24,000 o I MILE I 0 I KILOMETER ll-L/-; - • L™ -r^.:------= — - = 1 CONTOUR INTERVAL 40 FEET Geology by Mark K. Sheedlo, 1982-83, assisted by Peter J. McQuade Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Mi 10,0 0 0 ' - 6,0 00'- 4 , 0 0 0 ' - 2 ,00 0'- -2 ,000'- p « N Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. c°Pynght owner. Furthe] reProducti Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. A' B €m i€br — 3 ,0 0 0 m 10,0 0 0 - — 2 ,5 0 0 m 8 ,0 0 0 - Ku _ 2 ,0 0 0 m IPa Mbs 6 ,000- Mmc IPq Ku — 1 ,5 0 0 m IP a Mbs 4 , 0 0 0 - — 1,000m — 5 0 0 m 2,000'- Kk Tiw - 0 IPq IPa Mbs - - 500qi, Mmc -2,000'- ^Gw -1 ,0 0 0 m c ' D Reproduced with permission of the copyright owner. Further reproduction prohibited without permission...... ■ ■ ■ i i i m i h — m m i i ..... Illlllll lllll I III HIM Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. O Cbr Cm Cw Cpi Mbs Cm Mmc €pi Cm MDt OGbr, Mmc M D t Cm, Cpi IPa 'Cw Cm Mbs Mmc Cm cGvv Mmc Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. — 3 ,0 0 0 m Snowcrest M ountain — 2,5 0 0 m Mbs "fid "R w €w Mmc. ,Jm 2 ,0 0 0 m Mmc - 1 ,5 0 0 m Mbs Jm MDt Mmc "fi w. 0 0 0 m - 5 0 0 m IPa Kk Jm - 0 Ti d IPq IPa Mbs Mmc -1 ,0 0 0 m Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. B' - 3 , 0 0 0 m _ 2 ,5 0 0 m -2 ,0 0 0 m - 1 ,5 0 0 m - | , 0 0 0 m ' — 5 0 0 m - 0 —-5 0 0 m --1 ,0 0 0 m eproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 10.00 0 8,000 6,000 4,000 2,000 - 2,000 10,000 - 8,000 - Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ^■Cp -6w IPo Jin Mmc. Mbs Ku Mbs Mmc Mmc MDt IPo, Mmc -Cpi •6m €w •€f. MDt IPo Mbs ■fi w Mmc IPq IPo Mbs 0 € b r €m Hogback Mountain lid Kk IPq IPa Mbs 1i w Mmc I Umr I I 1 ______------Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. •Cm, sCw- r O € b f/ Mb °K V p € M m c > Cm , € p i Ml 0,000 - 8,000 fi_oanU Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Sliderock Mountain - Jm Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. -3 , OCX 2 ,5 0 0 ' 2,000 m Ku f|500m hOOOm ' 5 0 0 m P a Mbs "me Ml 0/ ■'iOOOi 3 »0 0 0 m Repro efi pem,,-ssi0„ ° r(h e c °p y rig h t owner Fnrth P 2,0° 0rr) ------• £ — « s - Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Mmc 6,000' H Mmc 4,000' - I •MDt ■€pi •Of, V 6 m p€ 2,000 •6m - 2,000 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Pr°hibite a Without Permh'ssion Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3t0 0 0 m Sunset Peak \' \-*• V KT £;2,500 m . • • y . * * t • . • * • •«• o. r. • 1 • v.v*• a.V•. ^*,»• f ° \. * v *■. • ^• 2 ,000 m 1,500m 1,000 m //■< ___ .. reproduction prohibited without permission. ! the copyright owner. Further Reproduced with permission of Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. IP a Mbs "E tv IPa luced wilh permission ° fthe C0Pyright 0Wner■ Further ^Production Prohibited without Permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. '.Soo 1,000 500 l~~ 0 M/n, Repro^ ^ Pem)jssio \ ,ss'on of(/)e COp Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. STRUCTURE SECTIONS OF THE NOR7 SCALE = 1:24,000 0 PLATE II f- *' Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ______— ——— —— — ■-■mimiw— Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1,000m IPq I Pa Mb Ku 5 0 0 m IPq Mbs Mbs 500m Jm Kk ,000m NORTHERN SNOWCREST RANGE )00 ______1^ MILE I KILOMETER Reproduced with permission of the copyright owner. Further reproduction prohibited without permission...... ' ...... ■ m in im i...... 11' ' i' I1 |I Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. h /( ^ 1 1 !\ ^ iS 1 %• T I y \ >, Reproduced with permission of the copyright owner. Further reproduction prohibited without permission Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. £ £ ^ » " ' ^ 7 - ^ v >rt' “/ »*^ f * ^^gy^g^'twU^'l. ‘i^vf % ~ i \ ^*rl-'^W .^eifii'^/\»*'-*-“ H 1 * i l ' * ’ T ^ sH,'*^ * ^ V *1'*‘ Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Compiled by Mark K. Sheedlo 5 KIL tz: SCALE EXPLANATION Contact MONTANA J L Normal fault, knob on downthrown side. Thrust fault, sawteeth on upper INDEX MAP ji plate. 12 Anticline showing trend and plunqe f" of axis, i 10 Syncline showing trend and plunge Modified from Vaughn(1948); Honkala(19', of axis. Brasher(l950); Klepper(l950); Keenmon(l9h 15 -sr~- Overturned anticline showing trend and plunge of axis. Mann(l954); Scholten et a I (1955)-, | .2 0 ~ f f S~ Overturned syncline showing trend Flannagan(l958); Hadley (I969); Tysdal(l9V and plunge of axis. 25 40 - 1- -4 - Strike and dip of bedding Monroe(l976)'i and Ti Iford (1978). 58 Strike and dip of foliation SEDIMENTARY ROCKS QT Quaternary and Tertiary deposits undivided Cretaceous-Tertiary synorogenic rocks Cretaceous rocks undivided APPROXIMATE DECLINATION 'A J Jurassic and Triassic rocks undivi ded 1 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 53^ 5 MILES 5 KILOMETERS / / < O' X 48); Honkala (1949); / '0 ); Keenmon(1950); / (1955); r 969); Tysdal (1970); (1978). v \ fiB Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. APPROXIMATE DECLINATION Jurassic and Triassic rocks undlvi ded Permian rocks undivided Pennsylvanian rocks undivided 45° 00, Mississippian rocks undivided CD Cambrian, Ordovician and Devonian rocks undivided Pre-Beltian metamorphic rocks undivided IGNEOUS ROCKS Tertiary volcanic rocks undivided T8 'IV TV : 30 Tv 30 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. •f:n 25 '(iD 59, 45°00' Tv 2 0 . 30 80 47, Reproduced with permission of the copyright owner. Further reproduction prohibited without permission Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 30 45 TV; 45 M &I5 85 50 KT.b T rf ,3b' 65, Tvf 30 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. PLATE III Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. 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