University of Montana ScholarWorks at University of Montana 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 Follow this and additional works at: https://scholarworks.umt.edu/etd Let us know how access to this document benefits ou.y 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 By 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 ii 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 vi 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.