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Structural Analysis of a Triangle Zone Culmination, Comb Rock, Lewis and Clark County, Montana Shane S

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Structural Analysis of a Triangle Zone Culmination, Comb Rock, Lewis and Clark County, Montana Shane S University of Montana ScholarWorks at University of Montana Graduate Student Theses, Dissertations, & Graduate School Professional Papers 2017 Structural Analysis of a Triangle Zone Culmination, Comb Rock, Lewis and Clark County, Montana Shane S. Fussell Let us know how access to this document benefits ouy . Follow this and additional works at: https://scholarworks.umt.edu/etd Part of the Geology Commons, and the Tectonics and Structure Commons Recommended Citation Fussell, Shane S., "Structural Analysis of a Triangle Zone Culmination, Comb Rock, Lewis and Clark County, Montana" (2017). Graduate Student Theses, Dissertations, & Professional Papers. 10946. https://scholarworks.umt.edu/etd/10946 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]. STRUCTURAL ANALYSIS OF A TRIANGLE ZONE CULMINATION, COMB ROCK, LEWIS AND CLARK COUNTY, MONTANA By SHANE STEPHEN FUSSELL B.S. Geological Sciences, University of Florida, Gainesville, FL, 2015 Thesis presented in partial fulfillment of the requirements for the degree of Master of Science in Geosciences The University of Montana Missoula, MT May 2017 Approved By: Scott Whittenburg, Dean of The Graduate School Graduate School James W. Sears, Chair Department of Geosciences James R. Staub Department of Geosciences Andrew Ware Department of Physics ii Fussell, Shane, M.S., May 2017 Geosciences Structural Analysis of a Triangle Zone Culmination: Comb Rock, Lewis and Clark County, Montana Chairperson: James Sears A triangle zone is a structure commonly found at the leading edge of a thrust belt, characterized by a triangular cross-section of thrust faults. In the Montana triangle zone, west-dipping thrust faults intersect east-dipping ones above a regional detachment fault. Most studies of triangle zones focus on the structure in a cross-sectional sense, but little work addresses variations along the trend. The Canadian Rocky Mountain triangle zone has long been a productive petroleum province, so understanding the southern extent of this zone, where it enters Montana, is of interest for economic purposes as well as geologic understanding. Previous studies have concluded that the triangle zone continues into Montana, and this study interprets the triangle zone an additional 25 km farther south. This area is, however, more complex than previously thought. The triangle zone in Montana undulates vertically in a system of culminations and depressions. The surface geology of the area was mapped in detail in the early 1960’s by the United States Geological Survey (USGS), but the subsurface interpretations were made before triangle zones were identified in the area. Multiple cross sections were created in this study to update these interpretations. Select areas from the USGS Comb Rock quadrangle in Lewis and Clark County, Montana, were studied in detail to locate, measure, and document a culmination of the triangle zone. Erosion of the culmination produced structural windows through the triangle zone, whereas the neighboring depressions preserve the overlying thrust plates. Strain accumulation in shale units, specifically the Marias River Shale, as well as thick igneous sills elevated the culmination. In addition, penetrative deformation styles and disharmonic folding were documented in three formations in the study area: the Telegraph Creek Formation, Marias River Shale, and Blackleaf Formation. This allowed for a new interpretation of triangle zone formation to be made that helps to explain the structurally thickened units that are commonly found near the apex of the blind thrust. This interpretation closely examined the geometries necessary to create a triangle zone and found a thrust angle discrepancy that can be accommodated by extra material being piled up on the thrust flat. iii ACKNOWLEDGEMENTS I would like to extend a huge thank you to my advisor Jim Sears. Without his insight and understanding, I would likely still be staring at maps hoping for divine inspiration. I would also like to thank Jim Staub for his guidance throughout not only the thesis process, but also my job search. I would like to thank Andrew Ware for his time as a committee member. Funding for this study came from the following sources: McDonough Scholarship through the Geoscience department at the University of Montana, and the Tobacco Root Geological Society. This work would not have been possible without the cooperation and hospitality of ranchers and landowners in the study area. Specifically, I would like to thank Charles and Susie Melaney of the Melaney Ranch for being so kind and open to my research. Also, I would like to thank Rocky, the ranch manager for the Dearborn Ranch, who allowed me onto the ranch despite his busy schedule. I would be remiss if I did not mention my family and friends who deserve thanks for their moral support throughout my entire college career. Specifically, I’d like to thank my parents, Steve and Allison Fussell, for instilling in me a deep love for the outdoors and always encouraging me to follow my heart. iv Table of Contents Page ABSTRACT ii ACKNOWLEDGEMENTS iii LIST OF FIGURES vi LIST OF PLATES ix 1. INTRODUCTION 1 2. METHODOLOGY 6 3. STRATIGRAPHY 8 4. STRUCTURAL ANALYSIS 11 4.1 Regional Triangle Zone Structure 12 4.2 Comb Rock Culmination 22 4.2.1 Comb Rock West 24 4.2.2 Comb Rock Central 26 4.2.3 Comb Rock East 27 4.2.4 Geometry of Comb Rock Culmination 30 5. DISCUSSION 34 6. CONCLUSION 40 7. REFERENCES 41 8. APPENDIX 46 8.1 Stratigraphy 47 8.2 Well Data 70 8.3 Stereonets 8.3.1 Plunging Syncline 1 76 8.3.2 Broken Syncline 81 v 8.3.3 Doubly Plunging Syncline 90 8.3.4 River Crossing Folds 93 vi LIST OF FIGURES Figure Page 1. Passive-roof duplex model 1 2. Map showing Euler pole of rotation from Montana to Alberta 2 3. Index Map showing study area 3 4. Thrust duplexing in Marias River Shale 5 5. Two Medicine Outcrop photo showing heterogeneity 8 6. Chevron folds in Blackleaf Formation 8 7. Virgelle outcrop photo showing massive, cross-bedded sandstone 8 8. Telegraph Creek Formation outcrop showing internal deformation 8 9. Marias River Shale imbrications near Sun River Canyon 9 10. Stratigraphic Column of study area 10 11. Henry (2007) cross section 11 12. Regional Geologic maps 1:100,000 and 1:24,000 scales combined 13 13. Close-up of Comb Rock Quadrangle showing differences in deformation 15 14. Map showing current geologic maps of the area with 18 some annotations to show study area and prominent features 15. Map showing the current geologic maps of the area with 19 colored overlay to show the different parts of the triangle zone system 16. Cross Section A-A’ showing internal geometry of the Craig 20 Anticlinorium vii 17. Cross Section D-D’ shows a composite section from the Eldorado 21 Thrust to the Passive Roof 18. Photos of Comb Rock looking east and south 22 19. Map showing the culmination study area with 24 highlighted subdivisions 20. Stereonet of Plunging Syncline 1 24 21. Satellite image of Plunging Syncline 1 26 22. Stereonet of Comb Rock Central 27 23. Stereonet of Broken Syncline 27 24. Satellite image of Broken Syncline 29 25. Comb Rock Cross Section inset of B-B’ 30 26. Cross Section C-C’ 32 27. Cross Section B-B’ 33 28. Cross Section of Canadian triangle zone 37 29. Schematic of Triangle Zone formation explaining mushwad material 39 30. Map showing well locations and proximity to study area 46 31. Leaf fossils in Blackleaf Formation 52 32. Chevron folds in Blackleaf Formation 53 33. Floweree Shale member of the Marias River Shale 54 34. Imbricated Bentonite beds in Marias River Shale 56 35. Small-scale folds in Telegraph Creek Formation 58 36. Ridge formed by Virgelle Sandstone 59 37. Lenticular cross beds within the Virgelle Sandstone 60 viii 38. Green sandstone within the Eastern Facies of the 63 Two Medicine Formation 39. Rounded gravel of the Eastern facies of the 64 Two Medicine Formation 40. Two Medicine Formation outcrop showing heterogeneity 65 41. View of the resistant Adel Mountain Volcanics 67 ix LIST OF PLATES Plate 1 – Cross Section A-A’ Plate 2 – Cross Section B-B’ Plate 3 – Cross Section C-C’ Plate 4 – Cross Section D-D’ 1 1. INTRODUCTION A triangle zone is a structure along the frontal edge of a thrust belt that is characterized by thrust sheets that were displaced in the same direction as overall thrust sheet emplacement to create a ‘duplex’ fault system, as well as an overlying, sub-regional ‘antithetic’ thrust sheet that was relatively displaced in the opposite direction (Jones, 1996). The area where these oppositely displaced thrust sheets meet creates a triangle in cross-section when combined with the detachment surface on the “floor” of the duplex. The antithetic fault forms when the thrust system becomes a 'passive-roof' duplex (Couzens and Wiltschko, 1996). Commonly, a shale unit at the base of the passive-roof allows the newly-formed tectonic wedge to split the rock layers and raise the roof thrust. Figure 1: Passive-roof duplex model (Banks and Warburton, 1986) Units in the passive-roof typically deform through folding as opposed to thrust faulting that is typical underneath (Fig. 1). Thrust faults do not propagate to the surface and begin to stack on top of one another underneath the roof. This lifts the roof upward and creates a prograding monocline on the limb of a ‘foreland syncline’. Foreland synclines occur in most areas that are hypothesized to have a triangle zone system present in the subsurface, such as the Alberta syncline in the foreland of the Canadian Rocky Mountains at the edge of the well-documented Canadian triangle zone (Price, 1981; Jones, 1982; Price, 1986; Sanderson and Spratt, 1992; 2 Skuce et al., 1992; Lawton et al., 1994; Jones, 1996; Skuce, 1996; Slotboom et al., 1996).
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