Investigation of a Triangle Zone Structure Between Augusta and Bowman’S Corners, Lewis and Clark County, Montana

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Graduate Student Theses, Dissertations, & Professional Papers Graduate School

2007

Investigation of a triangle zone structure between Augusta and Bowman’s Corners, Lewis and Clark County, Montana

Heather Marie Henry The University of Montana

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Recommended Citation Henry, Heather Marie, "Investigation of a triangle zone structure between Augusta and Bowman’s Corners, Lewis and Clark County, Montana" (2007). Graduate Student Theses, Dissertations, & Professional Papers. 597. https://scholarworks.umt.edu/etd/597

This Thesis is brought to you for free and open access by the Graduate School at ScholarWorks at University of Montana. It has been accepted for inclusion in Graduate Student Theses, Dissertations, & Professional Papers by an authorized administrator of ScholarWorks at University of Montana. For more information, please contact [email protected]. INVESTIGATION OF A TRIANGLE ZONE STRUCTURE

BETWEEN AUGUSTA AND BOWMAN’S CORNERS,

LEWIS AND CLARK COUNTY, MONTANA

Heather Marie Henry

B.S. Geology, California State University Dominguez Hills, Carson, California, 2005

Thesis

presented in partial fulfillment of the requirements for the degree of

Master of Sciences in Geosciences

The University of Montana Missoula, MT

Spring 2007

Approved by:

Dr. David A. Strobel, Dean Graduate School

Dr. James Sears, Chair Geosciences

Dr. Marc Henrdix Geosciences

Dr. Paul Wilson Geography

Henry, Heather, M.S., May 2007 Geosciences

Investigation of a triangle zone structure between Augusta and Bowman’s Corners, Lewis and Clark County, Montana

Chairperson: James Sears

Productive hydrocarbon reservoirs along Canada’s Rocky Mountain thrust belt are contained by a triangle zone, which is comprised of complex antiformal wedge structures that commonly form along the leading edges of foreland fold-thrust systems world-wide. Duplex imbrication within triangle zones create stacked structural traps for hydrocarbons driven toward the surface by the propagation of thrust belts. This geometry is well documented in a series of en echelon structures that stretch approximately 900 kilometers, from northeast British Columbia into southwest Alberta, along the western flank of the Alberta syncline, Canada. Along structural trend, the Rocky Mountain foothills triangle zone has not been well documented south of the Canadian-United States border, into Montana. Select parts of four 7.5-minute United States Geologic Survey quadrangles, in Lewis and Clark County, Montana, were mapped in detail to precisely locate structures associated with triangle zones. The region is dominated by thick packages of upper Cretaceous sediment, including the Saint Mary River Formation, Horsethief Formation, Two Medicine Formation, Virgelle Sandstone, and Telegraph Creek Formation of the Montana Group. These formations were subdivided based on laterally distinct lithologies, allowing for greater structural resolution of the area. Field observations and analysis of geologic maps and cross-sections show passive roof thrusting has raised the Augusta syncline above an east dipping thrust creating disharmonic folding parallel to the Rocky Mountain Front. This is analogous to the Alberta syncline, allowing for a correlation to be made between the Canadian triangle zone and west-central Montana. iii

ACKNOWLEDGMENTS

I would like to express my gratitude towards Dr. Jim Sears for his endless wisdom throughout this project. Without his help and encouragement I would probably still be staring at my cross sections, waiting for divine inspiration. Also, I would like to thank my committee members Dr. Marc Hendrix and Dr. Paul Wilson for their review of this document. The assistance of Meg Doolittle during the summer 2006 field season was greatly appreciated. I would like to thank Mick Bradway for numerous discussions regarding our adjacent study areas and his assistance in the field. Funding for this study came from the following sources: McDonough Scholarship through the Geology department at the University of Montana, the University of Montana Michael Lee Wilson Alumni Scholarship, the Tobacco Root Geologic Society Prospector Scholarship, and Geological Society of America grant number 8288-06. This work would not have been possible without the cooperation of the ranchers and landowners in the study area. Among these individuals, I want to express my gratitude towards Lance and Lennie Krone for opening their home to me during the field season. Their amazing hospitality will never be forgotten. Words cannot express the incredible support of my fabulous friends who made my stay in Missoula absolutely unforgettable. Last, but definitely not least, I would like to thank my parents, Patrick and Diana Henry, for unknowingly instilling in me a freakish love of rocks and encouraging me to have fun above all else. iv

TABLE OF CONTENTS

Page ABSTRACT ii

ACKNOWLEDGEMENTS iii

LIST OF FIGURES vi

LIST OF APPENDICES vii

LIST OF PLATES viii

1. INTRODUCTION 1

1.1 Introduction 1

1.2 Previous Work 2

1.3 Methodology 4

2. STRATIGRAPHY 6

2.1 Regional Stratigraphy 6

2.2 Middle Cambrian through Devonian Strata 6

2.3 Mississippian 7

2.4 Jurassic 8

2.5 Cretaceous System 8

2.5.1 Kootenai Formation 9

2.5.2 Colorado Group 9

2.5.3 Montana Group 11

Telegraph Creek Formation and Virgelle Sandstone 12

Two Medicine Formation 14

Sedimentary Member 15

Volcanic Member 17 v

Horsethief Formation 21

2.5.4 St. Mary River Formation 23

2.5.5 Volcanic Rock and Igneous Intrusives 24

Rhyolite sills and dikes in the volcanic member of the Two 24 Medicine Formation

Adel Mountain Volcanic Debris Flow 24

2.6 Quaternary System 25

3. STRUCTURAL ANALYSIS 27

3.1 Introduction 27

3.2 Descriptive and Kinematic Analysis 27

3.2.1 Division I 29

3.2.2 Division II 34

3.2.3 Division III 38

3.3 Displacement Anomaly 47

3.4 Triangle Zone 50

4. CONCLUSIONS 52

REFERENCES 53

LIST OF FIGURES

Figure Page

1.1.1 Canadian triangle zone cross-section modified from Price (1986). 2

1.1.2 Index map of the study area. 3

2.5.1 Stratigraphic correlations of the Montana Group across the study area. 11

2.5.2 Image of a typical Telegraph Creek Formation outcrop. 13

2.5.3 Image of a typical Virgelle Sandstone outcrop. 14

2.5.4 Image of the iron-rich caprock of the sedimentary member of the Two 17 Medicine Formation.

2.5.5 Image of the A unit of the volcanic member of the Two Medicine 19 Formation.

2.5.6 Image of the volcanic breccia characteristic of the D unit of the volcanic 20 member of the Two Medicine Formation.

2.5.7 Image of the lower member of the Horsethief Formation. 23

2.5.8 Image of the Adel Mountain Volcanic debris flow on the Flat Creek 25 syncline.

3.2.1 Structural divisions of the study area. 28

3.2.2 Division I from cross-sections A-A’ (3.2.2a) and B-B’ (3.2.2b). 30

3.2.3 Image of the deformed burrow cast found on the northern limb of the 33 Horsethief syncline.

3.2.4 Division II from cross-sections A-A’ (3.2.4a) and B-B’ (3.2.4b). 35

3.2.5 Division III from cross-sections A-A’ (3.2.5a) and B-B’ (3.2.5b). 40

3.2.6 Image of two imbricate, east-verging thrust faults within Division III. 42

3.2.7 Aerial image of the Auchard Creek anticline. 46

3.2.8 Image of the deformed burrow cast found on Gobbler’s Knob. 47

vii

LIST OF APPENDICES

Page APPENDIX A Measured stratigraphic columns. 58

A-1 Virgelle Sandstone 59

A-2 Sedimentary member of the Two Medicine Formation 61

A-3 Volcanic member of the Two Medicine Formation 71

A-4 Horsethief Formation 81

APPENDIX B Well-logs 84

B-1 List of pertinent drill-hole data. 85

B-2 Well-logs for the Krone 31-32, State 33-1, Soap Creek 13-31, 86 Milford Colony 1, and Soap Creek Cattle Co. A-1 drill-holes from the Montana Oil and Gas Commission (http://bogc.dnrc.state.mt.us/).

APPENDIX C Field measurements for select folds. 88

APPENDIX D Stereonet analyzes for select folds. 92

viii

LIST OF PLATES

PLATE 1 Geologic map of the area between Bowman’s Corners and Augusta, Lewis and Clark County, Montana.

PLATE 2 Cross-sections A-A’ and B-B’.

1. INTRODUCTION

1.1 Introduction

The eastern edge of the Canadian Rocky Mountains foreland is marked by structures having a triangular cross-section that parallel the Cordillera to the west. This

“triangle zone” is bounded to the east by an east-dipping upper detachment along the western limb of the Alberta syncline. West-dipping faults of the Rocky Mountain overthrust belt define the west edge of the triangle zone (Jones, 1982). The triangle zone has been thoroughly mapped in Canada, but its occurrence along structural strike farther south into central-western Montana has not been well documented. Triangle zone structures similar to those documented by Price (1986) in Canada (Fig. 1.1.1) have been reported by Sears et al. (2005) between Sun River Canyon and Choteau, Montana and along Montana State Highway 200 south of Augusta, Montana (Sears et al., 2002). In

Canada, the triangle zone is a productive hydrocarbon province. Documentation of its existence into Montana could provide exploration targets. Figure 1.1.2 shows the location of the study area with respect to the eastern limit of the fold and thrust belt and approximate location of the triangle zone extending from Canada into Montana.

The purpose of this study was to test the hypothesis that the triangle zone extends south, into central-western Montana, through geologic mapping, structural analysis, cross-section construction using oil and gas well log data in Lewis and Clark County.

The study area encompasses approximately 250 square kilometers south of Augusta within the Bowman’s Corners, Krone Ranch, Bowman’s Corners NW, and Gouchnour

Ranch 7.5-minute United States Geologic Survey (USGS) quadrangles. This region is 2 bounded to the south by Montana State Highway 200 and to the north by the Sun River.

United States Highway 287 runs north-south through the middle of the study area.

Figure 1.1.1 Canadian triangle zone at 49.75 North latitude (after Price 1986). Stratigraphic units: Kbl, Blairmore Group; Kag, Alberta Group; Kbr, Belly River Formation; Kbp, Bearpaw Formation; Ksm, St. Mary River Formation; KTw, Willow Creek Formation; Tph, Porcupine Hills Formation.

1.2 Previous Work

Viele (1960), Harris (1963) and Mudge et al. (1982) previously mapped parts of the study area at various scales. Schmidt (1972a, 1972b, 1972c, 1978) and Dolberg

(1986) mapped areas adjacent to the study area. Viele (1960) and Harris (1963) created cross-sections within the study area prior the establishment of the geometric rules for balancing sections set forth by Dalhstrom (1969), yielding foreland-verging thrust faults that, in cross-section, showed varying displacement along the fault plane. As a result, these cross-sections are not restorable. Wells drilled in the study area that post-date these previous studies allow for better subsurface control and structural interpretation.

Many studies addressed the stratigraphy of the region, including Cobban (1950,

1955), Cobban and Reeside (1952), Williams (1951), Bibler and Schmitt (1986), King

(1997), and Clapp and Deiss (1931). Schmidt and Zobovic (1962) interpreted the 3

Figure 1.1.2 Index map highlighting the study area (yellow box) and approximate location of triangle zone structures between the eastern boundary of the thrust belt and Alberta, Augusta, and Adel synclines. Modified from Constenius (1996). 4

relationship of the Adel Volcanics and Upper Cretaceous rocks south of the study area.

Harlan et al. (2005) complied a geochronological study of the Adel Volcanics and other

spatially associated igneous rocks.

1.3 Methodology

This study includes four 1:24,000 USGS quadrangles, drill-hole data from nine oil and gas wells in the region from Montana’s Department of Natural Resources, and aerial images provided by Montana’s Natural Resource Information System (NRIS) and Google

Earth™.

This study involved detailed geologic mapping and stratigraphic analysis and subdivision of the Cretaceous formations, including the type sections of Viele (1960) and

Harris (1963). Of the five Cretaceous formations in the area—in ascending order, the

Marias River Shale, the Telegraph Creek Formation, Virgelle Sandstone, Two Medicine

Formation, and Horsethief Formation—I subdivided the Two Medicine Formation into two members based on lithologic and depositional features. I conducted geologic mapping at a scale of 1:24,000 in order to obtain detailed surface control and establish structural style of deformation; to determine structural plunges and trends in various stratigraphic units; and to integrate surface and subsurface data. Interpretations based on these integrated data were augmented by constructing two balanced cross sections using standard geometric techniques.

Structural features and stratigraphic contacts were drawn directly on the USGS quadrangles with the aid of aerial images provided by NRIS. Field observations were 5

collected using standard field techniques as described by Compton (1985). Waypoints were obtained using a Garmin eTrex GPS unit for the locations of most field stations.

The final geologic map was digitized in ArcMap™ from the hand-drawn field maps. Scanned copies of the field maps were imported into ArcMap™ and georeferenced to shapefiles with the Montana State Plane North American Datum 1983.

Aerial images and topographic maps obtain from NRIS were projected beneath the

georeferenced maps in order to ensure accuracy during the digitization processes. Point

(field stations), line (folds, faults, and marker bed), and polygon (formations) shapefiles

were created in ArcCatalog™ and assigned the Montana State Plane North American

Datum 1983 spatial reference system. These shapefiles were then edited in ArcMap™

according to the locations provided by the georeferenced field maps.

Final copies of the measured stratigraphic sections and cross sections were

digitized from the hand-drafted copies using Abode Illustrator 10. All figures were also

digitized using Adobe Illustrator 10. 6

2. STRATIGRAPHY

2.1 Regional Stratigraphy

The chapter describes the main stratigraphic units of the study area and their gross structural behavior. Although the stratigraphy includes Middle Cambrian to Upper

Cretaceous rocks, exposures are limited to the Upper Cretaceous units. The subsurface locations of older rocks have been determined from well-log data and appear in cross- sections. Stratigraphic columns of measured sections and well-log data are presented in

Appendices A and B, respectively.

2.2 Middle Cambrian through Devonian Strata

There are no outcrops of this age range in the study area although the Three

Forks, Jefferson, and Maywood Formations of the Devonian were penetrated by the

Krone 31-32 well. West of the study area, Cambrian rocks are exposed in the Rocky

Mountain Front Range. On Steamboat Mountain, 22 kilometers west of the study area, the total thickness of Middle and Upper Cambrian rocks is 681 meters (Viele, 1960) and the Devonian is 365 meters. The Cambrian and Devonian strata are dominated by limestone and dolomite beds with lesser shale and mudstone units. The Jefferson and

Three Forks formations of the Devonian contain evaporite-solution breccias. These breccias are present in thin beds in the lower parts of the Jefferson Formation. The Three

Forks Formation is dominated by evaporite-solution breccias except for the upper unit of the formation. Formation thicknesses shown in cross-section were determined from well- 7 log data, where available, and measured stratigraphic sections from the region (Mudge,

1982; Viele, 1960).

In the eastern part of the study area, these formations are autochthonous and dip gently westward across the foreland basin. In the westernmost extent of the study area, the Devonian Formations are allochthonous rocks that have been transported northeastward on a bedding plane detachment between the Cretaceous Blackleaf and

Marias River Shale Formations. Regional dip was determined from well-log data and from cross-sections provided by Mudge (1982).

2.3 Mississippian

Early and Late Mississippian rocks do not crop out in the study area but were intersected by the Krone 31-32 and Soap Creek 13-31 wells. The entire Mississippian is known collectively as the Madison Group, which is subdivided into two formations, in ascending order, the Allan Mountain or Lodgepole Limestone and the Castle Reef

Dolomite or the Mission Canyon Limestone. These rocks are thick limestone and dolomite units with some shale in the lower member of the Allan Mountain Limestone

(Mudge, 1982). The Madison Group lies unconformably between the Devonian Three

Forks Formation and the Jurassic Sawtooth Formation. Regional dip and formation thickness was determined in the same manner as those discussed in section 2.2.

Subsurface behavior of these rocks is similar to that of the Cambrian and

Devonian rocks mentioned in section 2.2, where in the west they have been transported east along with the Devonian rocks and in the east are west-dipping autochthonous rocks.

Aerial exposures of these rocks and their structural behavior can be observed in Sun 8

River Canyon, 30 kilometers northwest of the study area, where they are thrust onto

Cretaceous rocks.

2.4 Jurassic

Middle and Late Jurassic rocks in the study area are present in the subsurface and were penetrated by the Milford Colony 1, Krone 31-32, and Soap Creek Cattle Co. A-1

wells. Rocks of Jurassic age can be divided, in ascending order, into the Ellis Group and

Morrison Formation. The Ellis Group is divided into the Sawtooth, Reirdon, and Swift

formations. The combined thickness of the Morrison and Swift Formations in the

Milford Colony 1 well is 152 meters. According to Mudge (1982), the entire Ellis Group

is approximately 274 meters thick. The Ellis Group is marine in origin and is composed

of siltstones, sandstones, and mudstones with some limestone. The Morrison Formation

is nonmarine in origin and is composed of siltstones and mudstones.

The Jurassic rocks overlie the Mississippian and Devonian rocks of the thrust

sheet in the west and the west-dipping autochthonous rocks in the east.

2.5 Cretaceous System

Wells in the vicinity of the study area penetrate Lower Cretaceous strata of the

Kootenai Formation and Lower Cretaceous Blackleaf Formation of the Colorado Group.

Upper Cretaceous rocks are exposed in the area and include the Marias River Shale of the

Colorado Group; the Telegraph Creek Formation, Virgelle Sandstone, Two Medicine

Formation, and Horsethief Formation of the Montana Group; the St. Mary River

Formation; Adel Mountain Volcanics; and igneous intrusive rocks. 9

2.5.1 Kootenai Formation

The Kootenai Formation is Early Cretaceous in age and lies unconformably above

the Jurassic Morrison Formation. According to the Milford Colony 1 well-log, the

Formation is approximately 236 meters thick in the study area. The Kootenai is

comprised of interbedded sandstones and shales, which are interpreted to represent

migratory stream channel deposits and adjoining floodplains (Viele, 1960). Structurally, the Kootenai has behaved in conjunct with the Devonian, Mississippian, and Jurassic rocks beneath it. In the western part of the study area, it rests on the same thrust sheet that brings up Devonian rocks. Across the study area, it is autochthonous, dipping gently westward across the foreland basin.

2.5.2 Colorado Group

The Colorado Group lies unconformably above the Kootenai Formation and

conformably below the Montana Group. The Colorado Group is divided into two

Formations, the Marias River Shale and the Blackleaf Formation. The Marias River

Shale lies unconformably above the Blackleaf Formation. The approximate thickness of

the Colorado Group in the region is 564 meters according to the well-log for MOSS-1

drill-hole.

The Blackleaf Formation does not outcrop in the study area but has been

penetrated by several wells in the area. The Blackleaf Formation has been divided into

three members: Flood, Taft Hill, and Vaughn, in ascending order. The Flood and Taft

Hill members are marine in origin, both consisting of fine-grained sandstone and 10 mudstone. The Vaughn member is nonmarine and dominated by gray to green mudstone and sandstone.

The Marias River Shale is exposed in outcrop and has been penetrated by the

Milford Colony 1, Soap Creek Cattle Co. A-1, Soap Creek 13-31, and Soap Creek Cattle

1 wells in the study area. The Formation is exposed in the hanging wall of the Auchard

Creek thrust, trending northwest-southeast from sec. 16, T. 18 N. R. 6 W. to sec. 3, T. 17

N. R. 5 W. Here it is poorly exposed and forms a covered hillslope beneath Virgelle and

Telegraph Creek Formations colluvium. Where exposed, the Marias River Shale is a dark grey to black, silty, fissile shale. It has been divided into four members: Kevin

Shale, Ferdig Shale, Cone Calcareous, and Floweree Shale, in ascending order (Mudge,

1982).

Due to the high shale content and associated internal weakness of this lithology, the Colorado Group was readily deformed under compressional stresses. Highly deformed Marias River Shale can be seen in the Pishkun Canal, 25 kilometers northwest of the study area, in a series of high angle, east-dipping thrust faults. The Blackleaf

Formation-Marias River Shale contact serves as a detachment surface throughout the study area. In the west, this surface is split by the thrust sheet that carried the Devonian through Cretaceous Blackleaf Formation rocks above their regional depth. This detachment surface provides the basal decollement from which a sequence of imbricate thrusts originate. These thrusts carry Marias River Shale, Telegraph Creek Formation, and Virgelle Sandstone and merge upward into the detachment surface of the roof thrust located along the Virgelle-Two Medicine Formation contact.

2.5.3 Montana Group

The outcrops of the study area are dominated by the formations of the Montana

Group: the Telegraph Creek Formation, Virgelle Sandstone, Two Medicine Formation, and Horsethief Formation. This group is comprised of mostly clastic rocks that form an eastward-thinning wedge. The Group can be divided into a lower regressive and an upper transgressive sequence that represents the fluctuations of the Cretaceous Interior

Seaway. The lower regressive sequence includes the Telegraph Creek Formation,

Virgelle Sandstone, and Two Medicine Formation. The upper transgressive sequence encompasses the Horsethief Formation (Clayton et al., 1982). As a whole, the Montana

Group is over 1,067 meters thick in the study area. Figure 2.5.1 is a schematic diagram of the correlations between the formations of the Montana Group and the overlying Adel

Mountain Volcanics and St. Mary River Formation.

Figure 2.5.1 Stratigraphic correlations of the Montana Group from southwest to northeast across the study area. Schematic diagram, not to scale.

Telegraph Creek Formation and Virgelle Sandstone The Telegraph Creek

Formation and Virgelle Sandstone crop out in the southwestern part of the study area,

predominately on the Krone Ranch USGS 7.5 minute quadrangle. In the study area, the

combined thickness of these formations total 78 and 195 meters according to the MOSS 1

and Soap Creek Cattle 1 well-logs, respectively. The Telegraph Creek Formation lies

conformably between the Virgelle Sandstone and Marias River Shale Formation.

The Telegraph Creek Formation is a transitional unit between the Marias River

Shale and Virgelle Sandstone, representing the second last regression of the Cretaceous

Interior Seaway (Mudge, 1982). In the study area, the Telegraph Creek Formation is poorly exposed and covered with soil and float from the overlying Virgelle Sandstone.

Isolated outcrops reveal buff, very fine to fine grained, lenticular, cross laminated sandstone with beds not greater than 15 centimeters thick. Viele (1960) classified this as a “calcareous, slightly feldspathic, quartz sandstone.” Figure 2.5.2 shows a typical outcrop of the Telegraph Creek Formation in the hanging wall of the Auchard Creek thrust fault. The formation consists of a coarsening upwards series of beds.

The Virgelle Sandstone is a massive, ridge-forming sandstone above the

Telegraph Creek Formation and below the first coal-bearing unit of the Two Medicine

Formation. The Virgelle Sandstone crops out in the footwall of the Auchard Creek thrust. The lower units are comprised of medium- to thick-bedded, lenticular sandstones that thicken upwards through the section. Middle and upper units are massive appearing, thick-bedded sandstones with sparse, interbedded, thinly bedded, carbonaceous shales.

The massive sandstone facies contain large sets of lenticular cross beds. The Virgelle

Sandstone is comprised of very fine- to medium-grained, well-sorted sandstones 13 comprised primarily of quartz and potassium feldspar in a calcite cement with lithic fragments, biotite, and magnetite as minor constituents. I classify these sandstones as slightly calcareous, subarkosic arenites. A distinguishing feature of the Virgelle

Sandstone is a dark-colored, iron-rich zone that caps massive, light-grey sandstone ledges

(Figure 2.5.3). The iron-rich zone is the uppermost unit of the formation and is a very hard, thin- to medium-bedded, slabby, slightly calcareous, quartz sandstone. This sandstone is fine- to medium-grained, poorly sorted, and rich in detrital magnetite and ilmenite. This zone weathers out nodular, calcareous concretions.

Figure 2.5.2 View northewest of Telegraph Creek Formation on the hanging wall of the Auchard Creek thrust fault. Jacob's staff for scale, bedding dips to the left (southwest).

The Telegraph Creek Formation and Virgelle Sandstone, as mentioned in section

2.5.2, occupy in imbricate thrusts along with the Marias River Shale. These wedge into the roof thrust of the triangle zone. 14

Figure 2.5.3 View looking north, at the Virgelle Sandstone in the footwall of the Auchard Creek thrust fault. Note the dark, rust-colored iron-rich cap-rock. Beds are dipping to the left (southwest).

Two Medicine Formation For the purpose of this study, the Two Medicine

Formation has been divided into two members: a lower sedimentary member and upper volcanic member (King, 1997). The sedimentary member of Two Medicine Formation is found throughout the study area as topographically low, badland areas and soil-covered, southeast-trending valleys. The widest belts of Two Medicine Formation occur north of the Auchard Creek thrust. The volcanic member of the Two Medicine Formation is found throughout the study area as topographically high, ridge-forming outcrops. The members will be discussed relatively briefly here. Bradway (2007) describes the lower sedimentary member in detail in the Sun River area 23 kilometers north of the study area in the vicinity of Sun River Canyon. Viele (1960) has subdivided and described in detail 15 the volcanic member in the vicinity of the Dearborn River in the southwest part of this study area. South of the study area, near Wolf Creek, MT, King (1997) performed a facies analysis on part of the volcanic member. As a whole, the Two Medicine

Formation can be described as nonmarine (Mudge, 1982).

In the Auchard Creek thrust zone, the sedimentary member is exposed in a sequence of west-dipping imbricate thrust sheets that wedge into the upper detachment surface of the triangle zone. Elsewhere in the study area, the Two Medicine Formation, as a whole, comprises the passive roof of the triangle zone and is exposed in a series of broad folds across the study area.

Sedimentary Member The sedimentary member lies conformably above the Virgelle

Sandstone and disconformably below the volcanic member of the Two Medicine

Formation. The sedimentary member can be divided into three units. The lower unit comprises black lignite and carbonaceous shale intercalated with thicker sequences of carbonaceous siltstone and thin beds of very fine-grained subarkosic arenites (Harris,

1963). These arenites are similar to the upper Virgelle Sandstone, with a massive appearance, buff to light-grey color, and an iron-rich caprock that is very hard, slabby, and ridge-forming. The beds of this caprock tend to be 1-2 centimeters thick while the iron-rich zone of the Virgelle tends to be 10-15 centimeters thick. In the study area, the lower unit is poorly exposed except for isolated outcrops of the iron-rich caprock (Figure

2.5.4).

The middle and upper units are comprised of highly erodable, sandy siltstone, fine-grained sandstone, limestone, and lesser amounts of shale. The sandstone is grey to grey-green in color, thinly bedded, and interbedded with thin shale beds. The sandstones 16 are massive in appearance, very fine- to fine-grained, with laminated crossbeds. As well, they are very silty and slightly calcareous dominated by quartz with lesser amounts of potassium feldspar, dark lithic fragments, chert, and biotite cemented by calcite. The uppermost become lean to medium volcanic sandstones with a higher abundance of dark lithic fragments, biotite and potassium feldspar than the lower units; grains of hornblende, augite, and magnetite are present. The limestone is generally 20 centimeters thick and nodular, weathering to red-brown and pink. The sandy siltstone is easily erodable, calcareous, and gray to gray-green, brown, maroon, and purple in color. The purple, sandy siltstone crops out on the northeastern flank of the Flat Creek syncline.

These units generally form soil covered, badland topography with isolated outcrops throughout the study area.

The upper unit of the sedimentary member grades laterally into the volcanic member of the Two Medicine, thickening from southwest to northeast at the expense of the volcanic member. Southwest of the Auchard Creek thrust fault, the sedimentary unit of the Two Medicine is 160-190 meters thick, increasing to 365 meters northeast of the

Auchard Creek thrust fault (Viele, 1960). Due to the extremely poor exposures of the sedimentary member within the study area, a complete measured section could not be measured. The measured stratigraphic section in Appendix A is compiled from data collected by Viele (1960). 17

Figure 2.5.4 View, looking east, at the iron-rich caprock of the sedimentary member of the Two Medicine Formation on the southwestern limb of the Auchard Creek anticline. Jacob’s staff for scale, beds are dipping towards the camera (southwest).

Volcanic Member Viele (1960) and Harris (1963) recognized the volcanic member of the Two Medicine Formation, in the study area, as the Hogan Formation, but later referred to it as the Big Skunk Formation (Viele and Harris, 1965). To the south of the study area, rocks of similar stratigraphic and sedimentary features were referred to as the volcanic member of the Two Medicine by Schmidt (1972a, 1972b, and 1972c) and King

(1997). Mudge (1982) reclassified the Big Skunk Formation within the study area as the volcanic member of the Two Medicine Formation. In order to maintain consistency throughout the region, this study will refer to these volcanic sedimentary rocks as the volcanic member of the Two Medicine Formation. The volcanic member of the Two

Medicine Formation lies disconformably above and intertongues with the sedimentary 18 member of the Two Medicine Formation. Locally, the volcanic member lies unconformably beneath the Adel Mountain Volcanics debris flow deposits. Between

U.S. Highway 287 and the Dry Creek anticline, the volcanic member intertongues with the Horsethief Formation. Northwest of the study area, the volcanic member lies conformably beneath the St. Mary River Formation (Viele, 1960).

The volcanic member as described by Viele (1960) and Harris (1963) is divided into five units. For the purpose of this study, these units were identified in the field but the volcanic member was not subdivided on the geologic map due to poor exposure. All five units were distinguished on the flanks of the High Basin syncline. Northeast of the

High Basin syncline, only the upper three units were readily identifiable. Due to the extensive work done previously on describing this member of the Two Medicine

Formation, key marker beds and distinguishing features of the volcanic member will be discussed here. Taking from Viele (1960) these units will be named A through E, A being the lowest unit.

The unit A is dominated by purple-maroon and bright green shales with cut and fill structures that have been filled by grey to black, volcanic-rich greywackes. These greywackes are massive and irregularly bedded, locally exhibiting normal grading.

Clasts within the greywackes are angular, very coarse sand to pebble size, and are composed of prophyritic andesite, silicified tuff, and lesser amounts of chlorite and quartz. Generally, the green shales are better indurated than the purple-maroon shales

(Figure 2.5.5). 19

Unit A grades into unit B which is identified by its numerous tuffs that are light

pink to maroon and bright green. The tuffs are massive to slabby in appearance and

weather brown, buff, and green.

Unit C is distinguished in the field by grey limestone beds in its lower section

containing secondary chalcedony and, in some localities, silicified wood fragments. This

unit is dominated by maroon and green sandy mudstones that are well indurated. The

upper section of unit C is identified by a fine- to coarse-grained sandstone containing

pink potassium feldspar grains.

Figure 2.5.5 Purple and green shales of the A unit of the volcanic member of the Two Medicine Formation in the High Basin syncline. Image is looking northwest, beds are dipping northeast.

Unit D is dominated by dark grey to black mudstones and greywackes with less

purple and maroon strata than the lower members. The sandstones in this unit are very 20 thin- to thick-bedded, massive to nodular, medium- to coarse-grained containing clasts of purple and green shale, and pink potassium feldspar grains. The limestones in this unit tend to weather black and are nodular. The uppermost section of unit D is comprised of a well indurated volcanic breccia ranging in thickness from tens of centimeters to over 2 meters thick. The breccia consists of subrounded, cobble-sized, black volcanic clasts containing abundant feldspar laths in a volcanic rich matrix containing feldspar laths as well (Figure 2.5.6). The matrix appears to have experienced thermal alteration where in contact with the clasts as there is a visible 2-4 millimeter thick rind surrounding the clasts. This indicates differential cooling between the matrix and clasts and that the clasts were most likely hot when deposited.

Figure 2.5.6 Close-up of the volcanic breccia characteristic of the D unit of the volcanic member of the Two Medicine Formation. Image taken on the northeastern limb of the High Basin syncline. Jacob's staff for scale.

Unit E begins at the contact between the upper breccia of unit D and the base of a bright green, feldspathic, volcanic, sandy siltstone that is locally overlain by a green crystalline tuff containing biotite crystals and feldspar laths. Unit E contains at least five tuffs that are light grey to white and is dominated by green and purple mudstones and argillites. The upper section contains tan and grey, thick-bedded sandstones that are fine- to coarse-grained with abundant quartz and feldspar grains with minor volcanic lithic and green and purple argillite clasts.

Horsethief Formation The Horsethief Formation crops out in the northeastern portion of the study area, northeast of U.S. Highway 287, and represents the western margin of the Cretaceous Interior Seaway during its final transgression (Bibler and

Schmitt, 1986). It has been interpreted that the deposition of the Horsethief Formation occurred along a barrier-island coastline (Bibler and Schmitt, 1986). Between the Dry

Creek anticline and U.S. Highway 287, the Horsethief Formation intertongues with the volcanic member of the Two Medicine Formation. This are is interpreted to be the westernmost limit of the shoreline of the Cretaceous Interior Seaway as the Horsethief

Formation does not crop out west of U.S. Highway 287. As well, east of U.S. Highway

287, the Horsethief Formation appears to thicken at the expense of the volcanic member of the Two Medicine Formation. The Horsethief Formation is conformably overlain by the St. Mary River Formation and lies conformably above the sedimentary member of the

Two Medicine Formation.

The Horsethief Formation is characterized by dark brown and tan sandstone units that are approximately 56.4 meters thick and tend to form ridges. The formation can be 22 divided into three members: lower, middle, and upper. The lower member is 35.5 meters thick and contains beds with abundant oyster shell fragments and whole shells. Units in this member are generally massive and cross-bedded with basal conglomeratic sections

(Figure 2.5.7). The basal conglomerates contain well-rounded, sub-spherical black volcanic clasts that contain abundant feldspar laths similar to those found in the volcanic conglomerate of unit D of the volcanic member of the Two Medicine Formation. As well, the basal conglomerates contain abundant oyster shell fragments and whole shells.

The middle member is 12.5 meters thick and is characterized by its lightness in color when compared to the other members. The units of this member tend to be light grey to light tan and are very fine- to fine-grained sandstones. The upper member is 8.4 meters thick and is comprised of brown to rust-brown, iron-rich sandstones. These units are massive and often ledge forming. The uppermost unit is magnetic and contains isolated silica filled burrows and sparse oyster shell fragments.

Where exposed, the Horsethief Formation was deformed in unison with the Two

Medicine Formation beneath it in that it comprises the passive roof of the triangle zone. 23

Figure 2.5.7 Lower member of the Horsethief Formation just east of U.S. Highway 287. Bedding is right side up and dipping northeast. Conglomeratic section contains black volcanic pebbles of similar composition to the volcanic breccia of the D unit of the volcanic member of the Two Medicine Formation.

2.5.4 St. Mary River Formation

The St. Mary River Formation is predominately exposed north of the Dry Creek anticline. Elsewhere, the formation is preserved atop the Horsethief Formation in synclinal folds. Within the study area, the St. Mary River Formation lies conformably over the Horsethief Formation north and northeast of U.S. Highway 287. South and southwest of U.S. Highway 287, the St. Mary River Formation does not crop out in the study area although it was mapped by Viele (1960) to the northwest of the study area.

Where mapped by Viele (1960), the St. Mary River Formation lies conformably over the volcanic member of the Two Medicine Formation. This change in stratigraphic contact is 24 due to the change in depositional environment of the underlying strata. As discussed in the previous subsection, the westernmost extent of the margin of the Cretaceous Interior

Seaway existed north and northeast of U.S. Highway 287, preventing the deposition of the Horsethief Formation southwest of the highway. The St. Mary River Formation was deposited during the final regression of the Cretaceous Interior Seaway. Generally, the formation forms low ridges and hills of badland topography. In the study area, the formation is easily erodable and only remnants its basal coquina and shell beds are exposed.

2.5.5 Volcanic Rocks and Igneous Intrusives

Rhyolite Sills and Dikes in the volcanic member of the Two Medicine Formation

The volcanic member of the Two Medicine formation contains rhyolitic sills and dikes.

They crop out as massive, light gray to pink rocks that exhibit jointing perpendicular to the bedding plane. In hand sample, quartz crystals are visible. According to Viele

(1960), the rock consists of “cryptocrystalline, siliceous glass or microcrystalline quartz” with phenocrysts of quartz and weathered oligoclase. One such dike is 1 meter thick and defines the southwestern ridge of the High Basin syncline and adjacent ridges to the southwest. A sill, approximately 5 meters thick, of this material is poorly exposed north of Black Rock Road in the Flat Creek syncline. These sills and dikes pre-date deformation in the area.

Adel Mountain Volcanic Debris Flow In section 22, T. 18 N., R. 5 W. and sections 17, 18, 19, 20, T. 18 N., R. 4 W., a volcanic debris flow unconformably overlies 25 the volcanic member of the Two Medicine Formation. The flow is approximately 10 meters thick and lacks internal, sedimentary structure (Figure 2.5.7). Clasts are boulder sized, rich in pyroxene crystals, very angular, have a very low sphericity, and are in a welded matrix. This flow was mapped by Mudge (1982) as the Adel Mountain Volcanics of Lyons (1944) in the study area. Similar volcanic debris flows were recognized south of the study area by Schmidt (1972) near Wolf Creek, Montana.

Figure 2.5.8 Adel Mountain Volcanic debris flow capping the southeastern part of the Flat Creek syncline. View to the southeast, jacob's staff for scale.

2.6 Quaternary System

Two lithologies comprise the Quaternary system: terrace gravels and alluvium.

Alden (1932) mapped terrace gravels in the region and deemed them early and middle 26

Pleistocene in age. These gravels are found in the vicinity of Flat Creek and are comprised of well-rounded pebbles and cobbles and poorly sorted sands. The pebbles and cobbles mostly are limestones and quartzites. Alluvium fills the valleys of Auchard and Flat creeks. 27

3. STRUCTURAL ANALYSIS

3.1 Introduction

This chapter describes and interprets the geologic structures within the study area.

The discussion has been divided into two sections, the first of which addresses the descriptive and kinematic analysis of the structures found in the study area. The second section compares the geologic structures that define a triangle zone with those found within the study area.

3.2 Descriptive and Kinematic Analysis

Geologic structures in the study area comprised folds and thrust faults; transform

and normal faults are not witnessed. The study has been divided geographically in order

to better isolate structural features for descriptive purposes. From northeast to southwest,

these divisions are as follows: from the northeastern boundary of the study area to Hogan

Creek (Division I); Hogan Creek to Flat Creek (Division II); and Flat Creek to the High

Basin syncline (Division III). Figure 3.2.1 is a structure map of the study area showing

the locations of these divisions. These divisions were created based on the prevailing

thrust system contained within the area. The cross-sections—A-A’ and B-B’—referred

to in this chapter are located in Plate 2. The cross-sections are oriented sub-parallel to

each other and perpendicular to the average trend of the geologic structures in the study

area. Cross-section B-B’ is located 4 kilometers northwest of cross-section A-A’. Strike

and dip data and stereonet analysis are located in Appendices C and D, respectively. 28

Figure 3.2.1 Three structural divisions of the study area. Division I is pink, Division II is green, and Division III is yellow.

3.2.1 Division I

Division I contains the area between the northeastern boundary of the study area and Hogan Creek. The distinguishing feature of this division is a system of westward- verging thrust faults. These faults are recognized by the westward displacement of the sedimentary member of the Two Medicine Formation onto the volcanic member of the

Two Medicine Formation and of the volcanic member of the Two Medicine Formation onto the Horsethief Formation (Figures 3.2.2a and 3.2.2b).

On cross-section A-A’, two west-verging thrust faults with a common basal detachment between the Virgelle Sandstone and Two Medicine Formation are recognized

(Figure 3.2.2a). From northeast to southwest, the first west-verging thrust fault displaces the sedimentary member of the Two Medicine Formation onto the volcanic member of the Two Medicine Formation. The hanging wall of this thrust is comprised of the sedimentary member of the Two Medicine Formation and the Horsethief Formation.

Recall that the sedimentary member of the Two Medicine Formation thickens eastward at the expense of the volcanic member. This lateral relationship is poorly understood, as is the relationship between the volcanic member of the Two Medicine Formation and the

Horsethief Formation. Inconsistencies arose between the geologic map (Plate 1) and cross-section A-A’ (Plate 2) when constant bed thicknesses were maintained for these formations within Division I. The geologic map does not recognize the presence of the volcanic member of the Two Medicine Formation northeast of this first thrust fault while in cross-section this member is present. As a result, the Horsethief Formation is not recognized in cross-section, but is present on the geologic map as a topographic high between the Augusta syncline and the first west-verging thrust fault. 30

Figure 3.2.2a Division I from cross-section A-A.' From right to left, the first west-verging thrust displaces the sedimentary member of the Two Medicine Formation on to the volcanic member of the Two Medicine Formation. The second west-verging thrust is wedged upward, creating a passive roof.

Figure 3.2.2b Division I from cross-section B-B.'

The hanging wall of the second west-verging fault is comprised of the volcanic and sedimentary members of the Two Medicine Formation; the Horsethief Formation was not mapped southwest of the previously discussed fault in the vicinity of cross- section A-A’. This sedimentary package was displaced relatively toward the hinterland as a passive roof thrust that was wedged upward by a system of east-verging, duplex thrust faults that will be further discussed in section 3.2.2.

Division I on cross-section B-B’ is comprised thrust faults with a basal detachment between the Virgelle Sandstone and Two Medicine Formation similar to those recognized on cross-section A-A’. From northeast to southwest, the first west- verging thrust fault displaces the volcanic member of the Two Medicine Formation onto the Horsethief Formation. The hanging wall of this thrust is comprised of the Two

Medicine Formation, Horsethief Formation, and St. Mary River Formation. The sedimentary member of the Two Medicine Formation does not crop out southwest of the

Augusta syncline as it pinches out along the fault. The hanging wall of the second west- verging thrust fault is comprised of the Two Medicine Formation and the Horsethief

Formation. This sedimentary package was displaced relatively toward the hinterland as a passive roof thrust. The roof thrust was wedged upward by the duplex thrust system of

Division II which will be discussed in section 3.2.2.

Again, the poorly understood relationships between the members of the Two

Medicine Formation and volcanic member of the Two Medicine Formation and

Horsethief Formation have resulted in inconsistencies between the geologic map (Plate 1) and cross-section B-B’. The spatial extents of the volcanic member of the Two Medicine

Formation and the Horsethief Formation, as expressed on the geologic map, are not 32

accurately represented on cross-section B-B’. The area comprising the surface

expression of the volcanic member of the Two Medicine Formation as interpreted in

cross-section is greater than what is observed in on the geologic map and vice versa for

the Horsethief Formation. This indicates that the Horsethief Formation may thicken

eastward at the expense of the volcanic member of the Two Medicine Formation.

Dead Man’s thrust fault, located along U.S. Highway 287 northwest of cross-

section B-B’, is another west-verging thrust fault. This fault differs from the previously

discussed thrust faults in that it displaced the Horsethief Formation onto the volcanic

member of the Two Medicine Formation. This displacement was most likely

accommodated by westward displacement of the Horsethief Formation along a bedding

plane slide.

Folds in Division I involve the Two Medicine Formation, Horsethief Formation,

and St. Mary River Formation. Northeast of the west-verging thrust faults, the Augusta

syncline, a syncline of regional scale, is observed. The northeastern limb of the Augusta

syncline dips gently southwest across the foreland basin while its southwestern limb dips

more steeply to the northeast. The axial surface of this syncline dips to the southwest. A

series subhorizontal, upright anticlines are located between the Augusta syncline and

westernmost west-verging thrust. These anticlines have an average trend of 170o, plunge

9o northwest, and axial planes that have average dips 81o southwest. These anticlines are

asymmetric with the northern limb having a steeper dip than the southern.

A deformed burrow cast of the shrimp, Camborygma, found on the northern limb of the Horsethief syncline allowed for a finite strain measurement (Figure 3.2.3). The burrow penetrated perpendicular to the bedding plane of the uppermost unit of the 33

Figure 3.2.3 Deformed burrow cast found on the northern limb of the Horsethief syncline. Bed strikes 204o and drips 26o.

Horsethief Formation. This bedding surface strikes 204o and dips 26o. The axis of finite maximum shortening is 1.754 cm long and located 109o counterclockwise from normal to

strike. The axis of finite minimum shortening is perpendicular to the axis of finite

maximum shortening and is 2.039 cm long. This burrow exhibits noncoaxial plane strain,

as the axes of finite shortening are rotated away from normal to strike. Percent

shortening on the axis of finite maximum shortening is 7.5%. This value is less than

those calculated by Bradway (2007) and the lack of multiple burrows does not provide

enough statistical certainty to make any statements regarding shortening due to 34 compression in this area with respect to Bradway’s study area to the north. A deformed burrow was also found in the sedimentary member of the Two Medicine Formation on

Gobbler’s Knob. This burrow will be discussed in section 3.2.3.

3.2.2 Division II

Division II is located between Hogan Creek and the northeastern limb of the Flat

Creek Syncline, just southwest of Flat Creek. It is characterized by antiformal, duplex thrust faults that propagated northeast (Figures 3.2.4a and 3.2.4b). The basal decollement is a bedding plane along the Blackleaf Formation-Marias River Shale contact. These thrust sheets primarily involved the Marias River Shale, Telegraph Creek Formation, and

Virgelle Sandstone. Adjacent to the northeastern limb of the Flat Creek Syncline, these thrusts sheets also involved the sedimentary member of the Two Medicine Formation.

Continuation of the west-verging thrust of Division I prevails across Division II as a roof thrust under which the antiformal duplex system is located.

Two fold systems are present in Division II due to the juxtaposition of these two thrust systems. Folds of the roof thrust sheet are concentric, trend 175o, plunge 7o north, and have an axial surfaces that dip 83o southwest. The geometry of these folds is directly related to the geometry of the thrusts sheets beneath them. Anticlines of the folded roof thrust have eroded along their axes to reveal fault-bend folds related to the duplex system. These folds could be traced on aerial images but their geometries could not be obtained in the field due to the good lack of outcrops. Folds of the passive roof have a greater amplitude and longer wavelength than those of the duplex system.

Figure 3.2.4a Division II as recognized on cross-section A-A'.

Figure 3.2.4b Division II as recognized on cross-section B-B'.

In cross-section A-A’, the antiformal duplex system is comprised of seven east- verging thrust faults. From northeast to southwest, the first five thrusts involved the

Marias River Shale, Telegraph Creek Formation, and Virgelle Sandstone. The next two thrusts involved the Marias River Shale, Telegraph Creek Formation, Virgelle Sandstone, and sedimentary member of the Two Medicine Formation. Of the duplex system, these 36

two are the only faults that break the surface, displacing the lower sedimentary member

of the Two Medicine Formation onto the upper sedimentary member of the Two

Medicine Formation, the Krone Ranch and the Flat Creek thrust faults.

The west-verging, passive roof thrust, as described in section 3.2.1, continues

across Division II and is expressed on the geologic map as a series of broad folds of the

upper plate. The anticlines are cored by the sedimentary member of the Two Medicine

Formation while the synclines are cored by the volcanic member of the Two Medicine

Formation. In this division, the fault plane of the passive roof climbed up-section to the

west to the contact between the sedimentary and volcanic members of the Two Medicine

Formation. As recognized on cross-section A-A’, this occurs southwest of the Milford

Colony 1 drill-hole, where the passive roof is intersected by the Krone Ranch thrust fault.

The Flat Creek syncline further supports the interpretation of the migration of the fault

plane up-section as it is a klippen of the passive roof with the fault plane located between

the sedimentary and volcanic members of the Two Medicine Formation.

Structures of Division II on cross-section B-B’ are similar to those recognized on

cross-section A-A’. On cross-section A-A’, the antiformal duplex system is comprised of

eight east-verging thrust faults. From northeast to southwest, the first six thrusts involved the Marias River Shale, Telegraph Creek Formation, and Virgelle Sandstone. The Krone

Ranch thrust fault involved the Marias River Shale, Telegraph Creek Formation, Virgelle

Sandstone, and sedimentary member of the Two Medicine Formation. Between cross- section A-A’ and B-B’, the fault plane of the Flat Creek thrust fault stepped up-section from the Blackleaf Formation-Marias River Shale contact to the Marias River Shale-

Telegraph Creek Formation contact. As a result, the Flat Creek thrust fault involved the 37

Telegraph Creek Formation, Virgelle Sandstone, and sedimentary member of the Two

Medicine Formation.

With respect to cross-section A-A’, the west-verging roof thrust is recognized to have behaved in the same manner in Division II of cross-section B-B’. Here it is expressed on the geologic map as a series of broad folds which involve the Two

Medicine Formation. As well, the fault plane of the roof thrust migrated up-section where the roof thrust was intersected by the Krone Ranch thrust fault. The Flat Creek syncline is also recognized in cross-section B-B’ as a klippen of the passive roof thrust with the fault plane between the sedimentary and volcanic members of the Two Medicine

Formation.

When comparing the two cross-sections various observations can be made with respect to Division II. First, the thrust faults associated with the duplex system maintained consistent geometries between A-A’ and B-B’ while the lengths of the individual thrust sheets increased to the northwest. This resulted in a difference in the amount of shortening associated with the duplex system between the two cross-sections.

Displacement within B-B’ is roughly 30 kilometers, or 66% shortening, which is greater than that of A-A’, roughly 22 kilometers or 59% shortening. This difference in displacement between the two cross-sections may be reduced by an alternate subsurface interpretation for cross-section B-B’. One such alternative could be the existence of a

Paleozoic cored thrust sheet below the duplex thrust system. At a regional scale,

Paleozoic cored thrust sheets penetrate the surface north of the study area and plunge into the subsurface as they approach the study area (Mudge, 1982). The existence of a thrust sheet would raise the detachment surface for the duplex thrust system above regional dip, 38 as interpreted in this study, thus reducing the amount of displacement exhibited by the system within cross-section B-B’, possibly making it comparable to the displacement represented in cross-section A-A’.

Second, well-log data from the Milford Colony 1 and Soap Creek Cattle Co. A-1 drill-holes indicate an increased depth to autochthonous rocks from southeast to northwest between the two cross-sections. Data obtained by folds associated with the passive roof indicate an average plunge of 6o to the northwest, supporting this increase in depth from A-A’ to B-B’. With respect to the geologic map, this increase in basin depth is supported by the strata involved in the surface expression of the passive roof. The strata is dominated by the sedimentary member of the Two Medicine Formation in the vicinity of A-A’ while strata in the vicinity of B-B’ is dominated by strata of the volcanic member of the Two Medicine Formation. This preservation of younger strata to the northwest is observed in B-B’ and indicates the northwestern plunge of structures in the area.

3.2.3 Division III

The final division lies between the southwestern limb of the Flat Creek syncline and the southwestern limb of the High Basin Syncline which is located northeast of the

Dearborn River (Figures 3.2.5a and 3.2.5b). Two additional thrust systems and the continuation of the roof thrust sheet described in Division I around found in this area.

The additional systems are grouped into a sole thrust system and an imbricate thrust system. 39

The sole thrust system is comprised of a single thrust sheet that raises strata of

Devonian age and younger above regional dip. In cross-section A-A’ this is supported by well-logs for Krone 31-32 and Soap Creek Cattle 13-31 drill-holes which revealed strata at depths higher than expected given regional dip and thickening due to igneous sills.

The Krone 31-32 drill-hole intersected Devonian strata at a shallower depth than expected given regional dip. The Soap Creek Cattle 13-32 intersected the Blackleaf

Formation and Marias River Shale at depths shallower than expected and indicated thickening of the Blackleaf Formation and Marias River Shale due to igneous sills. In cross-section B-B’, the well-log for the Soap Creek Cattle Co. A-1 indicates thickening of Jurassic strata and the Kootenai Formation due to igneous sills. This drill-hole also intersected Jurassic strata, the Kootenai Formation, and Blackleaf Formation at depths higher than expected given regional depth.

It is interpreted that this sole thrust propagated eastward along the bedding plane between the Blackleaf Formation and Marias River Shale as this plane also served as the detachment surface for the antiformal duplex system of Division II. The thrust sheet involved Devonian strata through the Cretaceous Two Medicine Formation. In the subsurface, the hanging wall cut-off is interpreted to be located beneath the southwestern limb of the Flat Creek syncline as the Milford Colony 1 drill-hole intersected

Mississippian strata through the Blackleaf Formation at the depth expected for regional dip.

Figure 3.2.5a Division III from cross-section A-A'.

Figure 3.2.5b Division III from cross-section B-B'. 41

The imbricate thrust system is located between the southwest limb of the Flat

Creek syncline and northeastern limb of the High Basin syncline. The detachment surface of this system migrated up-section from southwest to northeast from a bedding plane contact between the Blackleaf Formation and Marias River Shale, to the bedding plane contact between Marias River Shale and the Telegraph Creek Formation, to the bedding plane contact between the Virgelle Sandstone and Two Medicine Formation.

Strata involved in this thrust system involve a sedimentary package comprised of the

Marias River Shale, Telegraph Creek Formation, Virgelle Sandstone, and sedimentary member of the Two Medicine Formation. Between the Flat Creek syncline and Auchard

Creek, the imbricate thrust system wedges between the sole thrust system and the roof thrust.

The imbricate thrust system is recognized in cross-section A-A’ as a sequence of east-verging thrusts that wedged into the southwestern limb of Flat Creek Syncline

(Figure 3.2.5a). These thrusts will be discussed from southwest to northeast. The two southwestern most thrusts are interpreted to have a basal detachment surface at the

Blackleaf Formation-Marias River Shale contact as the Soap Creek 13-31 well-log indicates a straight stratigraphic sequence below the Marias River Shale and repetition of the Marias River Shale. The westernmost thrust involved the Marias River Shale through the Two Medicine Formation and displaced the Marias River Shale onto the Telegraph

Creek Formation. The next fault to the east, the Auchard Creek thrust fault, involved the

Telegraph Creek Formation and the Marias River Shale and displaced the Marias River

Shale onto the sedimentary member of the Two Medicine Formation (Figure 3.2.6). 42

Figure 3.2.6 Looking northeast from the northeastern limb of the High Basin syncline at two imbricate, east-verging thrust faults.

In cross-section B-B’, the imbricate thrust system is recognized as a sequence east-verging thrust faults that wedge into the Flat Creek syncline (Figure 3.2.5b). These thrusts will be discussed from southwest to northeast. The two westernmost thrusts are interpreted to share the same basal detachment surface at the Blackleaf Formation-Marias

River Shale contact, which is the same as the two westernmost thrusts in cross-section A-

A’. The westernmost thrust involved the Marias River Shale through the Two Medicine

Formation and displaced the lower Marias River Shale eastward onto the upper Marias

River Shale. The second thrust, the Auchard Creek thrust fault, involved the Marias

River Shale and displaced it eastward onto the sedimentary member of the Two Medicine

Formation. Between the Auchard Creek thrust fault and the Flat Creek syncline, a 43 sequence of seven, east-verging imbricate thrust sheets are wedged into the passive roof of Division I. These faults share a common basal detachment at the Marias River Shale-

Telegraph Creek Formation contact and involved the Telegraph Creek Formation,

Virgelle Sandstone, and sedimentary member of the Two Medicine Formation. Each of these seven faults displaces the Telegraph Creek Formation onto the sedimentary member of the Two Medicine Formation.

Several observations can be gleaned from the comparison of cross-sections A-A’ and B-B’ with respect to Division III and will be discussed from southwest to northeast.

The first observation is the strata involved with the Auchard Creek thrust fault. Cross- section A-A’ recognizes the Marias River Shale and Telegraph Creek Formation as being involved with the Auchard Creek thrust fault while cross-section B-B’ recognizes only the Marias River Shale. This observation is consistent with the geologic map in that from southeast to northwest, along trend the Telegraph Creek Formation is only locally exposed between A-A’ and B-B’. The Soap Creek 13-31 well-log provided detailed interpretations of the Upper Cretaceous strata and evidence for the involvement of the

Telegraph Creek Formation in displacement of the Auchard Creek thrust fault on cross- section A-A’. Cross-section B-B’ lacked such subsurface data southwest of the Auchard

Creek thrust and the surface geology did not support the involvement of the Telegraph

Creek Formation in displacement of the Auchard Creek thrust fault.

The second observation is that the imbricate thrust faults between the Auchard

Creek thrust fault and the Flat Creek syncline differ in quantity and amount of displacement per thrust sheet. In cross-section A-A’, eight faults are present with a combined displacement of 12 kilometers, or 49% shortening. In cross-section B-B’, 44 seven faults are present with a combined displacement of 15 kilometers, or 60% shortening. With respect to the seven faults recognized on B-B’, the eight faults on A-A’ exhibit less displacement per thrust sheet.

The third observation is that in both cross-sections, the imbricate thrust faults wedged into the passive roof. In this division and Division II, the westernmost surface expression of the passive roof is located at the Flat Creek syncline. As mentioned in section 3.2.2, the Flat Creek syncline is a klippen of the passive roof with a basal detachment between the sedimentary and volcanic members of the Two Medicine

Formation. It is interpreted that the imbricate thrust zone of Division III wedged into the roof thrust on the southwest limb of the Flat Creek syncline, raising the roof thrust upward.

The last observation is that the displacement of the sole thrust system and the displacement of the imbricate thrust system do not appear to be equivalent. The sole thrust propagated eastward from a location west of the study area and its total displacement is not known. Five kilometers of displacement, as measured from the base of the Blackleaf Formation, is exhibited by the sole thrust within Division III in both cross-sections. The imbricate thrust system exhibited 12 and 15 kilometers of displacement—cross-sections A-A’ and B-B’ respectively—within Division III. It is interpreted that this difference in displacement is due to the eastward migration of pre- existing, east-verging thrust faults that were subsequently involved with the eastward propagation of the sole thrust.

Folding in Division III can be grouped into two categories: spatial extensive synclines and fault-bend folds. Division III is bound to the northeast and southwest by 45 two spatially extensive, asymmetric synclines: Flat Creek syncline and High Basin syncline. Based on the Fleuty diagram, the Flat Creek syncline is a sub-horizontal, upright fold and the High Basin syncline is a gently plunging, steeply inclined fold. The

Flat Creek syncline trends 175o, plunges 12o southeast, and has an axial surface that dips

83o southwest. The High Basin syncline is essentially parallel to the Flat Creek syncline with a trend of 173o. The High Basin syncline plunges 7o southeast and has an axial surface that dips 78o southwest. Folding between the Flat Creek syncline and the High

Basin syncline is comprised of fault-bend folds of the imbricate thrust system. These folds are visible on aerial images but their exact geometries are unknown due to poor exposures. The Auchard Creek anticline is the only fold from which measurements were obtained (Figure 3.2.7). This fold trends 168o, plunges 2o southeast, and the axial surface dips 88o southwest. Based on the Fleuty diagram for describing folds, this anticline is described as a sub-horizontal, upright fold. 46

Figure 3.2.7 Aerial image obtained from NRIS of the Auchard Creek anticline.

A shrimp burrow cast, Camborygma, was located in this division within the sedimentary member of the Two Medicine Formation on Gobbler’s Knob (Figure 3.2.8).

The bed in which this burrow is located strikes 260o and dips 56o. The axis of finite maximum shortening is 1.482 cm long and normal to strike. The axis of finite minimum shortening is 1.526 cm long and perpendicular to the axis of finite maximum shortening.

This burrow appears to have not been deformed as the ratio of long-short axes is 0.97, yielding a 1.46% shortening. No conclusions regarding shortening in Division III can be drawn from this analysis as only one burrow was found. 47

Figure 3.2.8 Deformed burrow cast in the sedimentary member of the Two Medicine Formation on Gobbler's Knob. Bed strikes 259o and dips 56o.

3.3 Displacement Anomaly

As discussed in detail, cross-sections A-A’ and B-B’ are extremely similar in their structural geometries. A key difference between the two cross-sections is that the thrust systems, interpreted within Division I and II, of cross-section B-B’ represent greater eastward displacement than those interpreted within cross-section A-A’. Cross-section

A-A’, which is approximately parallel to and located southeast of cross-section B-B’, accommodated a total of 20 kilometers of horizontal displacement, or 48% shortening.

Cross-section B-B’ accommodated a total of 32 kilometers of horizontal displacement, or 48

62% shortening. This difference, 12 kilometers, in displacement is anomalously large considering that the distance between the cross-sections is only 4 kilometers. This section serves to postulate as to why this difference in total displacement exists.

Deformation within the region encompassing the study area occurred in response to the emplacement of the Lewis-Eldorado-Hoadley (LEH) thrust slab between 74 and 59

Ma (Sears, 2001). The LEH experienced clockwise rotation of approximately 30o about an Euler pole located near Helena, Montana (Price and Sears, 2000). As a result, horizontal displacement of the foreland fold and thrust belt increases northward, from less than 20 kilometers near Helena, Montana to 140 kilometers at the U.S.-Canada border (Price and Sears, 2000; Sears 2001). This is a change of 120 kilometers horizontal eastward displacement over a distance of 300 kilometers. This can also be expressed as an increase of 0.4 kilometers horizontal displacement eastward per kilometer traveled north. With this observation made, one would expect there to be a difference in horizontal displacement between the two cross-sections.

In an attempt to determine how much of the displacement between the two cross- sections can be explained by the emplacement of the LEH slab, the cross-section lines were treated as vectors where the direction is considered to be trend and magnitude the total horizontal displacement. These vectors were resolved into their corresponding x-

(east) and y-vectors (north). Cross-section A-A’ trends N 25o E with a displacement of

20 kilometers resulting in 8.4 kilometers horizontal displacement eastward and 18 kilometers horizontal displacement northward. Cross-section B-B’ trends N 19o E with a displacement of 32 kilometers resulting in 10.4 kilometers horizontal displacement eastward and 30 kilometers horizontal displacement northward. Consider a point along 49

the line of cross-section A-A’ as the Euler pole about which a plate rotated clockwise

with 0.4 kilometers of eastward horizontal displacement per kilometer traveled north, the

same conditions as the LEH slab. The distance between cross-section A-A’ and B-B’ as

measured due north from the pole located on line A-A’ is 10 kilometers. This results in

an expected eastward horizontal displacement of 4 kilometers for cross-section B-B’.

The expected eastward displacement for cross-section B-B’ is 6.4 kilometers less than the

actual eastward displacement. Thus, total shortening for B-B’ is larger than what can be

explained by the emplacement of the LEH thrust slab assuming constant displacement

along the LEH thrust slab.

Palinspastic restoration of the LEH thrust slab reveals the angle of clockwise

rotation about the Euler pole southeast of Helena, Montana is not constant along trend

resulting in varying displacement along trend (Sears, 2000). Between Helena and

Choteau, Montana, the angle of clockwise rotation increased northward from 30o to 37o

and is known as the Helena salient; this corresponds area to cross-sections F-F’, E-E’,

and D-D’ of Sears (2000). North of Choteau, this angle peaked at 40o and decreased to

25o northward, corresponding to cross-sections C-C’, B-B’, and A-A’ of Sears (2000).

The area encompassed by this study is located between cross-sections D-D’ and F-F’ of

Sears (2000). The increase in rotation between D-D’ and F-F’ resulted in a difference of displacement between the two cross-sections of 40 kilometers. Cross-section D-D’ is approximately 40 kilometers northwest of cross-section E-E’, resulting in approximately

1 kilometer of displacement per kilometer north.

If this new value for displacement per kilometer is applied to the area of this study, the expected eastward displacement for cross-section B-B’ would be 10 50

kilometers. This difference between the actual eastward displacement measured from

cross-section B-B’, 10.4 kilometers, and this expected value is small enough, 0.4

kilometers, to be negligible. Thus the difference in displacement between cross-sections

A-A’ and B-B’ can be accounted for by the increased amount of eastward displacement

associated with the Helena salient.

Recall from section 3.2.2, the alternative subsurface interpretation of cross-section

B-B’ that involves a Paleozoic cored thrust sheet in the subsurface of Division II. This

alternative was offered as a method by which the amount of shortening contained within

cross-section B-B’ at this location could be reduced so that the difference in displacement

between the two cross-sections would be reduced. From the above discussion, this

alternative interpretation is no longer required as the difference in shortening between the

two cross-sections can be explained by the Helena salient of the LEH thrust slab.

3.4 Triangle Zone

The structure created where two thrust systems with opposing senses of vergence interact is known as a triangle zone (Price, 1986; Couzens-Shultz, 2003; Erickson, 1995).

There are two end members of triangle zone structures: type I includes fold dominated structures bound by two thrusts of opposing vergence that originate off the same detachment surface; type II is a passive roof duplex (Couzens and Wiltschko, 1996).

A series of en echelon type II triangle zones occur north of the study area along the Canadian Rocky Mountain front, recall Figure 1.1.1 from chapter 1. There a tectonic wedge, comprised of imbricate thrust sheets, is bound above and below by thrust faults having opposite senses of vergence. This wedge propagated eastward along bedding- 51 parallel basal-detachment surfaces. Movement of the wedge eastward caused delamination of the foreland basin stratigraphic succession, forcing the strata above the basal detachment upwards resulting in the formation of a west-verging backthrust and raising of the western limb of the Alberta syncline (Price, 1986).

Structures similar to these are interpreted to exist within the study area. The system of east-verging duplex thrusts characteristic of Division II comprises the foreland propagating wedge which is bound above and below by thrust faults with opposing senses of vergence. The basal decollement of this wedge exists along the bedding plane between the Blackleaf Formation and Marias River Shale. The duplex system wedges into the bedding contact between the Virgelle Sandstone and Two Medicine Formation, raising the strata above this contact upwards creating the west-verging backthrust characteristic of Division I. These structures collectively create a triangle zone where

Divisions I and II coincide. 52

4. CONCLUSIONS

This study provides evidence for the existence of a triangle zone structure along the North American Rocky Mountain Front in northwestern Montana. Cross-sections A-

A’ and B-B’ show a great similarity to the triangle zone demonstrated in Figure 1.1.1.

Further, this interpretation is supported by the structures mapped by Bradway (2007), in which the triangle zone was mapped 23 kilometers north of this study area. The combination of this study and Bradway (2007) proves the continuation of the Canadian triangle zone south of the Canada-United States border into northwestern Montana and subsequently redefines the geometry of the foreland edge of the Rocky Mountain Front.

This conclusion is based upon detailed mapping of the study area and the subsurface projection of these features in collaboration with well-log data. An increased density of drill-hole data and seismic profiles across the area would aid in a more precise interpretation with respect to the detachment surfaces involved and the exact location of the triangle zone. As well, mapping of the area between this study and Bradway (2007) could provide further evidence for the existence of triangle zone structures.

Further stratigraphic and petrographic studies in this area would be recommended, as there appears to be a sedimentological relationship between the volcanic facies of the

Horsethief Formation and the upper part of the volcanic member of the Two Medicine

Formation. 53

REFERENCES

Alden, W.C., 1953, Physiography and glacial geology of western Montana and adjacent areas: U.S. Geological Survey Professional Paper 231, 200 p.

Bibler, C. J., and J. G. Schmitt, 1986, Barrier-Island coastline deposition and paleogeographic implications of the Upper Cretaceous Horsethief Formation, Northern Disturbed Belt, Montana: The Mountain Geologist, v. 23, no. 4, p. 113-127.

Bradway, M.D., 2007, Stratigraphy and structural geometry at the leading edge of the Montana thrust belt, east of Sun River Canyon, Lewis and Clark and Teton Counties, Montana: unpublished M.S. thesis, University of Montana, Missoula, x p.

Clapp, C.H., and Deiss, C.H., 1931, Correlation of Montana Algonkian formations: Geologic Society of America Bulletin, v. 42, p. 673-695.

Clayton, J., Mudge, M.R., Lubeck, Sr. C., and Daws, T.A., 1983, Hydrocarbon source rock evaluation of the disturbed belt, northwestern Montana, in Powers, R.B., ed., Geologic studies of the Cordilleran thrust belt -1982: Rocky Mountain Association of Geologists, v. 2, p. 817-830.

Cobban, W.A., 1950, Telegraph Creek Formation, Sweetgrass Arch, northcentral Montana, American Association of Petroleum Geologists, v. 34, p. 1899-1900.

______, 1955, Cretaceous rocks of northwestern Montana, in Billings Geological Society Guidebook, 6th Annual Field Conference, p. 107-119.

______, and Resside, J.B., 1952, Correlation of the Cretaceous formations of the western interior of the United States, Geologic Society of America Bulletin, v. 63, p. 1001-1043.

Compton, R.R., 1985, Geology in the field: John Wiley & Sons, New York, 398 p.

Constenius, K.N., 1996, Late Paleogene extensional collapse of the Cordilleran foreland fold and thrust belt, Geological Society of America Bulletin, v. 107, p. 16-20.

Couzens, B.A., and Wiltschko, D.V., 1996, The control of mechanical stratigraphy on the formation of triangle zones, Bulletin of Canadian Petroleum Geology, v. 44, p. 165-179.

Couzens-Shultz, B.A., Vendeville, B.C., and Wiltschko, D.V., 2003, Duplex style and triangle zone formation: insights from physical modeling, Journal of Structural Geology, 25, p. 1623-1644.

Dalhstrom, D.C.A., 1969, Balanced cross sections: Canadian Journal of Earth Sciences, v. 6, p. 743-757.

Dolberg, D., 1986, A duplex beneath a major overthrust plate in the Montana Disturbed Belt: Surface and subsurface data: unpublished M.S. thesis, University of Montana, 57 p.

Erickson, S.G., 1995, Mechanics of triangle zones and passive-roof duplexes: implications of finite-element models, Tectonophysics, 245, 1-11.

Harlan, S.S., Snee, L.W., Reynolds, M.W., Mehnert, H.H., Schmidt, R.G., Sheriff, S.D., and Irving, A.J., 2005, 40Ar/39Ar and K-Ar geochronology and tectonic significance of the Upper Cretaceous Adel Mountain Volcanics and spatially associated Tertiary igneous Rocks, U.S.G.S. Professional Paper 1696.

Harris, G.F., 3d., 1963, Geology of the Dry Creek area, Lewis and Clark County, Montana: unpublished M.S. thesis, University of Missouri, Columbia, 113 p.

Hoffman, J., Hower, J., and Aronson, J.L., 1976, Radiometric dating of timing of thrusting in the Disturbed Belt of Montana, Geology, v. 4, p. 16-20. 55

Jones, P.B., 1982, Oil and gas beneath east-dipping underthrust faults in the Alberta Foothills, in Powers, R.B., ed., Geologic studies of the Cordilleran thrust belt -1982: Rocky Mountain Association of Geologists, v. 2, p. 61-74.

Jones, P.B., 1996, Triangle zone geometry, terminology and kinematics, Bulletin of Canadian Petroleum Geology, v. 44, no. 2, p. 139-152.

King, J.T., 1997, Facies Analysis of the volcaniclastic Two Medicine Formation, Wolf Creek, Montana: unpublished M.S. thesis, University of Montana, Missoula, 54 p.

Lyons, J.B., 1944, Igneous rocks of the Northern Big Belt Range, Montana: Geological Society of America Bulletin, v. 55, no. 4, p. 445-472.

Mudge, M.R., 1972, Structural geology of the Sun River Canyon and adjacent areas, northwestern Montana, U.S. Geological Survey Professional Paper 663-B.

______, 1982, Structural geology of the northern disturbed belt, Montana, in Geological studies of the Cordilleran Thrust Belt, Rocky Mountain Association of Geologists, p. 91-122.

______, Earhart, R.L., Whipple, J.W., Harrison, J.E., 1982, Geologic and structure map of the Choteau 1x2 degree Quadrangle, western Montana: U.S. Geological Survey Map I-1300, scale 1:250,000.

Price, R.A., 1986, The southeastern Canadian Cordillera: Thrust faulting, tectonic wedging, and delamination of the lithosphere, Journal of Structural Geology, 8, p. 239- 254.

Schmidt, R.G., 1972a, Geologic map of the Wolf Creek quadrangle, Lewis and Clark County, Montana: U.S. Geological Survey Quadrangle Map GQ-974, scale 1:24,000. 56

______, 1972b, Geologic map of the Coburn Mountain quadrangle, Lewis and Clark County, Montana: U.S. Geological Survey Quadrangle Map GQ-975, scale 1:24,000.

______, 1972c, Geologic map of the Comb Rock quadrangle, Lewis and Clark County, Montana: U.S. Geological Survey Quadrangle Map GQ-976, scale 1:24,000.

______, 1978, Rocks and mineral resources of the Wolf Creek area, Lewis and Clark and Cascade counties, Montana: U.S. Geological Survey Bulletin 1441, 91 p.

Schmidt, R.G., and Zobovic, P., 1961, Coburn Mountain overthrust, Lewis and Clark County, Montana, U.S. Geological Survey Investigations Map I-379.

Sears, J.W., 2000, Rotational kinematics of the Rocky Mountain thrust belt of Northern Montana, in Montana/Alberta Thrust Belt and Adjacent Foreland, vol. 1, ed., R.A. Schalla and E.H. Johnson, Montana Geological Society, p. 143-149.

______, 2001, Emplacement and denudation history of the Lewis-Eldorado-Hoadley thrust slab in the northern Montana Cordillera, USA: Implications for steady state orogenic processes, American Journal of Science, v. 301, p. 359-373.

______, Braden, J., Edwards, J., Geraghty, E., Janiszweski, F., McInenly, M., McLean, J., Riley, K., Salmon, E., 2005, Rocky Mountain foothills triangle zone, Sun River, northwest Montana. Northwest Geology. 34, p. 45-70.

______, Hansen, William B., Ambrose, Rachele B., Burtis, Erik W., Hennes, Andrew M., Hofmann, Michael H., Laatsch, Nicholas A., and Pallister, Beau J., 2002, Montana’s Triangle Zone, abstract, Geological Society of America, Rocky Mountain section Annual Meeting.

______, and Price, R.A., 2000, A preliminary palinspastic map of the Mesoproterozoic Belt-Purcell Supergroup, Canada and USA: Implications for the tectonic setting and structural evolution of the Purcell anticlinorium and the Sullivan deposit, in The Geological Environment of the Sullivan Deposit, British Columbia, ed., J.W. Lydon, T. Hoy, J.F. Slack, and M.E. Knapp; Geological Association of Canada, Mineral Deposits Division, Special Publication No. 1, p. 61-81.

Viele, G.W., 1960, The geology of the Flat Creek area, Lewis and Clark County, Montana: unpublished Ph.D. dissertation, Utah University, 213 p.

Williams, E.P., 1951, St. Mary River Formation in Spring Coulee-Magrath area, Alberta, Canada, American Association of Petroleum Geologists Bulletin, v. 35, p. 885-898. 58

APPENDIX A: MEASURED STRATIGRAPHIC COLUMNS 59

80 81

APPENDIX B: WELL-LOGS 85

B-1: List of pertinent drill-hole data.

otal Depth 3300 7800 5480 6891 6882 7720 T ate Completed

D 5/16/1938 9/23/1962 6/28/2001 9/21/1955 3/7/1959 12/14/1982 ell Type

Dry Hole Dry Hole Dry Hole Dry Hole Dry Hole Dry Hole W

r osbacher perato as, Inc. orthern Natural as Producing

ompany ompany o. lying J Oil and Co roduction Hanlon Et al. al. Hanlon Et Shell Oil O C F G N G C M P Yandt, Carl Carl Yandt,

a L 47.33491768 47.33491768 47.27685678 47.26742268 47.31141249 47.32530726 47.43923735 Long Long -112.2073208 -112.2650025 -112.2650025 -112.2957636 -112.177724 -112.3258368 -112.2407497 API # 25-049- 05004-00-00 25-049- 07293-00-00 25-049- 21106-00-00 25-049- 05001-00-00 25-049- 05002-00-00 25-049- 21007-00-00 Well Name Durnin & 1 Procktor 31-32 Krone Soap Creek 13-31 Milford Colony 1 Soap Ck. Cattle Co. A- 1 STATE 33-1

B-2: Well-logs for the Krone 31-32, State 33-1, Soap Creek 13-31, Milford Colony 1, and Soap Creek Cattle Co. A-1 drill-holes from the Montana Board of Oil and Gas (http://bogc.dnrc.state.mt.us/).

Well Name Depth to top (feet) Formation Krone 31-32 4680 Kootenai Krone 31-32 5562 Swift Krone 31-32 5592 Madison Krone 31-32 6910 Bakken Krone 31-32 6938 Three Forks Formation Krone 31-32 7240 Nisku

STATE 33-1 2610 Eagle STATE 33-1 2696 Virgelle Sandstone STATE 33-1 2818 Telegraph Creek Formation STATE 33-1 3954 Blackleaf STATE 33-1 4130 Bow Island STATE 33-1 4335 Newcastle Sandstone STATE 33-1 4374 Skull Creek STATE 33-1 4468 Dakota STATE 33-1 4610 Lakota STATE 33-1 4626 Kootenai STATE 33-1 4652 Sunburst STATE 33-1 4736 Morrison STATE 33-1 5090 Rierdon STATE 33-1 5193 Bowes Member STATE 33-1 5290 Tampico Shale STATE 33-1 5308 Sun River Dolomite STATE 33-1 5640 Mission Canyon STATE 33-1 6164 Lodgepole STATE 33-1 6606 Three Forks Formation STATE 33-1 6648 Potlatch Anhydrite STATE 33-1 6806 Nisku STATE 33-1 6870 Duperow STATE 33-1 7334 Souris River Formation STATE 33-1 7610 Cambrian Undifferentiated

Soap Creek 13-31 790 Two Medicine Formation Soap Creek 13-31 1300 Eagle Soap Creek 13-31 1376 Virgelle Sandstone Soap Creek 13-31 1540 Telegraph Creek Formation Soap Creek 13-31 1728 Marias River Formation Soap Creek 13-31 2928 Telegraph Creek Formation Soap Creek 13-31 3062 Marias River Formation Soap Creek 13-31 3236 Tertiary Undifferentiated 87

Soap Creek 13-31 3472 Cretaceous Undifferentiated Soap Creek 13-31 3620 Tertiary Undifferentiated Soap Creek 13-31 4034 Cretaceous Undifferentiated Soap Creek 13-31 4264 Vaughn Member, Blackleaf Fm Soap Creek 13-31 4340 Tertiary Undifferentiated Soap Creek 13-31 4678 Cretaceous Undifferentiated Soap Creek 13-31 4928 Taft Hill Member, Blackleaf Fm Soap Creek 13-31 4957 Tertiary Undifferentiated Soap Creek 13-31 4994 Cretaceous Undifferentiated Soap Creek 13-31 5165 Taft Hill Member, Blackleaf Fm

Milford Colony 1 420 Livingston Group Milford Colony 1 3500 Two Medicine Formation Milford Colony 1 3680 Niobrara Milford Colony 1 4000 Carlile Shale Milford Colony 1 4440 Greenhorn Milford Colony 1 4520 Mowry Shale Milford Colony 1 4825 Newcastle Sandstone Milford Colony 1 5245 Skull Creek Milford Colony 1 5515 Kootenai Milford Colony 1 6290 Morrison Milford Colony 1 6585 Swift Milford Colony 1 6788 Sun River Dolomite

Soap Ck. Cattle Co. A-1 2900 Colorado Shale Soap Ck. Cattle Co. A-1 3565 Blackleaf Soap Ck. Cattle Co. A-1 4410 Sill, Igneous Soap Ck. Cattle Co. A-1 4650 Kootenai Soap Ck. Cattle Co. A-1 5174 Sill, Igneous Soap Ck. Cattle Co. A-1 5325 Kootenai Soap Ck. Cattle Co. A-1 5655 Sill, Igneous Soap Ck. Cattle Co. A-1 6474 Morrison Soap Ck. Cattle Co. A-1 6583 Sill, Igneous

APPENDIX C: FIELD MEASUREMENTS FOR SELECT FOLDS. 89

Auchard Creek anticline

Strike (RHR) Dip Trend Plunge 240.0 44.0 150.0 46.0 221.0 44.0 131.0 46.0 280.0 57.0 190.0 33.0 260.0 49.0 170.0 41.0 84.0 53.0 354.0 37.0

Flat Creek syncline

Strike (RHR) Dip Trend Plunge 101.0 31.0 11.0 59.0 76.0 28.0 346.0 62.0 89.0 28.0 359.0 62.0 113.0 17.0 23.0 73.0 72.0 24.0 342.0 66.0 102.0 22.0 12.0 68.0 104.0 13.0 14.0 77.0 104.0 14.0 14.0 76.0 110.0 19.0 20.0 71.0 95.0 29.0 5.0 61.0 96.0 34.0 6.0 56.0 96.0 14.0 6.0 76.0 135.0 10.0 45.0 80.0 95.0 31.0 5.0 59.0 94.0 25.0 4.0 65.0 103.0 19.0 13.0 71.0 89.0 28.0 359.0 62.0 72.0 24.0 342.0 66.0 102.0 22.0 12.0 68.0 113.0 17.0 23.0 73.0 76.0 28.0 346.0 62.0 101.0 31.0 11.0 59.0 311.0 4.0 221.0 86.0 235.0 3.0 145.0 87.0 255.0 12.0 165.0 78.0 250.0 19.0 160.0 71.0 208.0 3.0 118.0 87.0 273.0 24.0 183.0 66.0 229.0 8.0 139.0 82.0 290.0 30.0 200.0 60.0 100.0 9.0 10.0 81.0 217.0 7.0 127.0 83.0 250.0 4.0 160.0 86.0

High Basin syncline

Strike (RHR) Dip Trend Plunge 69.0 40.0 339.0 50.0 89.0 78.0 359.0 12.0 85.0 64.0 355.0 26.0 93.0 90.0 3.0 0.0 70.0 86.0 340.0 4.0 83.0 88.0 353.0 2.0 65.0 75.0 335.0 15.0 68.0 88.0 338.0 2.0 61.0 49.0 331.0 41.0 86.0 45.0 356.0 45.0 83.0 15.0 353.0 75.0 78.0 24.0 348.0 66.0 62.0 24.0 332.0 66.0 45.0 12.0 315.0 78.0 76.0 15.0 346.0 75.0 76.0 89.0 346.0 1.0 54.0 72.0 324.0 18.0 65.0 42.0 335.0 48.0 77.0 25.0 347.0 65.0 85.0 26.0 355.0 64.0 280.0 50.0 190.0 40.0 265.0 15.0 175.0 75.0 270.0 4.0 180.0 86.0 310.0 22.0 220.0 68.0 305.0 18.0 215.0 72.0 272.0 34.0 182.0 56.0 256.0 44.0 166.0 46.0 260.0 18.0 170.0 72.0 295.0 14.0 205.0 76.0 280.0 10.0 190.0 80.0 265.0 67.0 175.0 23.0 259.0 56.0 169.0 34.0 270.0 24.0 180.0 66.0 280.0 14.0 190.0 76.0 292.0 37.0 202.0 53.0 285.0 40.0 195.0 50.0 295.0 14.0 205.0 76.0 280.0 14.0 190.0 76.0 280.0 50.0 190.0 40.0

Horsethief syncline

Strike (RHR) Dip Trend Plunge 110.0 52.0 20.0 38.0 125.0 54.0 35.0 36.0 90.0 46.0 0.0 44.0 65.0 67.0 335.0 23.0 85.0 26.0 355.0 64.0 63.0 56.0 333.0 34.0 210.0 26.0 120.0 64.0 204.0 26.0 114.0 64.0 228.0 21.0 138.0 69.0

Triangle anticline

Strike (RHR) Dip Trend Plunge 208.0 31.0 118.0 59.0 301.0 55.0 211.0 35.0 250.0 34.0 160.0 56.0 239.0 35.0 149.0 55.0 226.0 29.0 136.0 61.0 250.0 34.0 160.0 56.0 221.0 29.0 131.0 61.0 236.0 25.0 146.0 65.0 110.0 52.0 20.0 38.0 125.0 54.0 35.0 36.0 90.0 46.0 0.0 44.0 65.0 67.0 335.0 23.0 85.0 26.0 355.0 64.0 63.0 56.0 333.0 34.0

APPENDIX D: STEREONET ANALYZES FOR SELECT FOLDS. 93

Auchard Creek anticline

Flat Creek syncline

High Basin syncline

Horsethief syncline

Triangle anticline

Plate 1. Geologic map of the area between Augusta and Bowman's Corners, Lewis and Clark County, Montana.

r D Ktms •

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Base map data source: Montana State Library, Montana Natural Resource Information System, Geographic Information (http://www.nris.mt.gov).

Spatial reference system: Montana State Plane North American Datum 1983, FIPS 2500. STUDY LOCATION

1:24,000

1 0.5 0 1 Miles

2.5 1.25 0 2.5 Kilometers Plate 2: Cross-sections A-A' and B-B'.

Cross-section A-A'

Division III Divison II Division I

Southwest Northeast t i c l n e u s t f a l y 1 u s t f a l e k a n r e k t h r e k 1 3 - r r o l n a s i n y c l e d C e k s y n c l i r d C A e k t h r A' d C r t C o a p C o r u c h a r t C l a S H i g h B i l f F A o n e 3 1 - 2 l a u g s t a y n c l i e u c h a r r F M A A K Ktmv Ktmv Ktms Ktmv Ktmv Ktms Ktms Ktmv Ktms Ktmv Ktmv Ktms Ktms Kv Ktms Ktms Ktms Ktms Ktms Kv Ktc Kv Kv + 1,000 m Kv Kv Ktc Ktc + 1,000 m Ktms Ktms Ktc Ktc Ktc Kv Kv Ktc Kv Kv Ktc Ktc Ktc Kv Kmr Kmr Kmr Ktc Kv Ktms Ktms Kv Kv Kmr Ktc Kv Ktc Kv Ktc Kv Ktc Ktc Kv Ktc Kv Kmr Ktc Ktc Kmr Ktc Kmr Kbl Kmr Kmr Kmr Kmr Kmr Kmr Kk-Jr

SEA LEVEL 0 m 0 m SEA LEVEL Kbl

Kbl Mm Kk-Jr

Kk-Jr D

- 1,000 m - 1,000 m D

Kbl

Cambrian undiff.

Kk-Jr

Mm - 2,000 m

D 1 kilometer

Cambrian undiff.

Cross-section B-B'

Division III Divison II Division I Legend Southwest A - 1 . o Northeast u s t f a l t i c l n e t l e C a u s t f a l e k t h r e k C St. Mary River Formation r r a n c h a s i n y c l e d C e k s y n c l i e k t h r r

B r B' o r s e t h i f y n c l o a p C o n e R r H S t C t C H i g h B K l a u c h a r l a e 3 - 1 F t F

A Horsethief Formation u g s t a y n c l i e t a Ktmv A Ktmv Kh S Ktmv Ktmv Ksm Ktmv Kh Kh Ktms Ksm Ktms Ktms Ktms Ktms Ktms Ktms Volcanic member of the Two Medicine Formation Kv Ktms Ktms Kh + 1,000 m Ktmv + 1,000 m Ktc Kv Ktmv Ktc Ktms Kv Kv Ktms Ktms Ktms Ktc Ktc Kv Sedimentary member of the Two Medicine Formation Ktc Kv Kv Ktms Kv Ktc Kv Kv Kv Kmr Ktc Ktc Ktc Ktc Ktc Kv Ktc Virgelle Sandstone Kmr Kmr Kv Kv Kv Ktc Kv Ktc Ktc Ktc Kv Kv Kmr Ktc Ktc Telegraph Creek Formation Kbl

SEA LEVEL 0 m Kmr Kmr Kmr Kmr Kbl 0 m SEA LEVEL Kv Ktc Marias River Shale Kmr Kmr Kmr Kmr Kk-J Blackleaf Formation Kk-J Kbl Mm

Kk-J Cretaceous Kootenai-Jurassic undifferentiated D

- 1,000 m Mm Mm - 1,000 m Mississippian Madison Group

D Devonian undifferentiated D

Kbl Cambrian undifferentiated

Cambrian undiff. Kk-J Igneous sill - 2,000 m Mm

D 1 kilometer

Cambrian undiff.