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Geology of Igneous Extrusive and Intrusive Rocks in the Sundance Area, Crook County, Wyoming Earl F

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Geology of Igneous Extrusive and Intrusive Rocks in the Sundance Area, Crook County, Wyoming Earl F University of North Dakota UND Scholarly Commons Theses and Dissertations Theses, Dissertations, and Senior Projects 1979 Geology of igneous extrusive and intrusive rocks in the Sundance area, Crook County, Wyoming Earl F. Fashbaugh University of North Dakota Follow this and additional works at: https://commons.und.edu/theses Part of the Geology Commons Recommended Citation Fashbaugh, Earl F., "Geology of igneous extrusive and intrusive rocks in the Sundance area, Crook County, Wyoming" (1979). Theses and Dissertations. 89. https://commons.und.edu/theses/89 This Thesis is brought to you for free and open access by the Theses, Dissertations, and Senior Projects at UND Scholarly Commons. It has been accepted for inclusion in Theses and Dissertations by an authorized administrator of UND Scholarly Commons. For more information, please contact [email protected]. GEOLOGY OF IGNEOUS EXTRUSIVE AND INTRUSIVE ROCKS IN THE SUNDANCE ;\REA, CROOK COUNTY, WYOMING by Earl F. Fashbaugh Bachelor of Arts in Geology, University of Minnesota, Duluth, 1975 Bachelor of Science in Earth Science, University of Minnesota, Duluth, 1975 A Thesis Submitted to the Graduate Faculty of the University of North Dakota in partial fulfillment of the requirements for the degree of Master of Science Grand Forks, North Dakota May 1979 This Thesis submitted by Earl F. Fashbaugh in partial fulfillment of the requirements for the Degree of Master of Science from the University of North Dakota is hereby approved by the Faculty Advisory 1 \ Committee under whom the work has been done. d-><t~- ~ (Chairman) t:22a c.' ~~- &_. n This Thesis meets the standards for appearance and conforms to the style and format requirements of the Graduate School of the University of North Dakota, arul is hereby approved. Dean of the Graduate School ii , r' Permission Title GEOLOGY OF IGNEOUS EXTRUSIVE AND INTRUSIVE ROCKS OF THE SUNDANCE AREA, CROOK COUNTY, WYOMING Department Geology Degree -- Master of Science --~-·-·-·-·----------------- In presenting this thesis in partial fulfillment of the requirements for a graduate degree from the University of North Dakota, I agree that the Library of this University shall make it freely available for inspection. I further agree that permission for extensive copying for scholarly purposes may be granted by the professor who supervised my thesis work or, in his absence, by the Chairman of the Department or the Dean of the Graduate School. It is understood that any copying or publication or other use of this thesis or part thereof for financial gain shall not be allowed without my written per­ mission. It is also understood that due recognition shall be given to me and to the University of North Dakota in any scholarly use which may be made of any material in my thesis. Date~ /f'; /77_'?__ iii TABLE OF CONTENTS LIST OF ILLUSTRATIONS vi LIST OF TABLES vii ACKNOWLEDGMENTS viii ABSTRACT ix INTRODUCTION 1 Location of the study area 1 Previous work ... ~ l Problem and approach. 5 General geology of the igneous bodies 6 Sundance Mountain 6 Sugarloaf Mountain 9 Bear Lodge Mountains 12 FIELD RELATIONSHIPS 15 Introduction. 15 Sundance Mountain and Sugarloaf Mountain 16 PETROGRAPHY AND CHEMISTRY. 22 Analytical Techniques 22 Sundance Mountain 22 Modal analyses 22 General petrography of the quartz latite 26 Chemistry and mineralogy of the quartz latite 27 Sugarloaf Mountain. • • . ... 34 Bear Lodge Mountains .•...•.•.. 35 Chemical composition of the foyaite 37 Occurrence of primary analcime 38 INTERPRETATION 42 The origin of Sundance Mountain and Sugarloaf Mountain 42 Field observations .....•.•.... 43 Petrologic evidence for an extrusive model 45 Sundance Mountain--a mixed cone .... 50 Relative ages of the foyaite and quartz latite 51 iv TABLE OF CONTENTS, continued CONCLUSIONS 52 APPENDICES . 55 APPENDIX A 56 APPENDIX B 59 APPENDIX C 80 APPENDIX D 82 REFERENCES 92 V LIST OF ILLUSTRATIONS Figure Page 1. Location map of study area 2 2. Monolithic breccia from SE side of Sundance Mountain 7 3. Layered sequence from Sundance Mountain •• • • • • # 8 4. Generalized measured sections from Sundance Mountain 10 5. Sugarloaf Mountain graded breccia . ~ . 11 6. Sundance Mountain breccia (SD-98) using plane polarized light 17 7. Contact between breccia and layered sequence 18 8. Pyroclastic parting lineations 20 9. Effects of quartz spike on (101) . peak intensity 24 10. Layering of quartz latlte using plane polarized light light (SD-168) • • . 28 11. Ordering of Si/Al in alkali feldspars 29 12. Zoned oligoclase crystals from SD-133 33 13. Invariant and univariant phase relationships as determined by Peters et al. (1966) . • • . • • . 39 14. Phase relationships for oscillatory zoning in Sundance Mountain oligoclase •.•••..••••• , . • 47 15. Possible relationships in system CaA1 si o -NaA1Si 0 - 2 2 8 3 8 KA1Si3 o8-Sio2 . , , . • • • . , . 49 Plate L Geologic Map of Sundance Area, Crook County, Wyoming in pocket 2. Geologic Map of Sundance Mountain . in pocket 3. Geologic Map of Sugarloaf Mountain in pocket 4. Geologic Map of Southeastern Bear Lodge Mountain in pocket vi LIST OF TABLES Table Page 1. Igneous rock and outcrop descriptions . • 59 2. Sedimentary rock and outcrop descriptions 82 3. Modal quartz determination using spiking techniques 23 4. Chemical composition of Sundance Mountain cryptocrystalline groundmass, sample SD-133b . • • . 31 5. Chemical analyses of zoned oligoclase from SD-133b 32 6. Chemical analyses and normative mineral percentages 37 vii I ACKNOWLEDGMENTS I appreciate the constructive criticism and advice given to me by members of my graduate faculty committee. Dr. Frank Karner offered useful advice concerning the organization of the thesis. He was in­ strumental in obtaining chemical data on my samples. Dr. Odin Christensen and Dr. Walter Moore provided encouragement and moral support for my efforts. Mr. Dave Brekke deserves recognition for his skill in servicing and maintaining analytical equipment at the University of North Dakota Geology Department. Mr. Stanley White rendered technical assistance in the preparation of polished thin sections. Don Halvorson provided information and ideas concerning the theor­ etical petrology of Devil's Tower rocks. Chemical data was obtained through use of facilities of the Uni­ versity of North Dakota Geology Department and the Grand Forks Energy Technology Center. I am especially grateful for the indefatigable love, understanding and moral support that my wife, Susan, has given me. Her field as­ sistance on Sundance Mountain was a tremendous aid in the preparation of field notes and the labeling of samples. viii ABSTRACT Two major igneous rock types occur in the Sundance, Wyoming area including foyaite of the Bear Lodge Mountain sill and quartz latite of Sundance Mountain and Sugarloaf Mountain. The igneous bodies were mapped on a scale of 1:5000 in an effort to determine petrogenetic relationships between the two rock types. Sundance Mountain and Sugarloaf Mountain are extrusive in origin. Quartz latite occurs as subparallel units of breccia, tuff, and massive flows without any clear cross-cutting relationships. Fragmental types (breccia and cuff) constitute 41 to 76 percent of the rocks, suggesting that Sundance Mountain is a mixed cone. ,The quartz latite has a pre­ dominantly cryptocrystalline groundmass consisting of alkali feldspar and quartz. Oligoclase occurs as microlites and as zoned phenocrysts. The phenocrysts have distinct, oscillatory zones with narrow ranges in composition, suggesting that the crystals were subject to sudden changes in pressure. Sugarloaf Mountain is interpreted to be a satellite volcano of Sundance Mountain. Rocks from both igneous bodies have similar texturest structures, and compositions. The Bear Lodge Mountain sill was passively emplaced along the con­ tact between the Pennsylvanian Minnelusa Formation and the Mississip­ pian Pahasapa Formation. Foyaite dikes in the area have similar tex­ tures and compositions, and were probably emplaced during the same ix intrusive eve,1t, Primary analcime is the only feldspathoid present in the foyaite. The quartz latite is interpreted as being the youngest of the two rock types, A maximum age for the quartz latite is Paleocene, based on the presumed age of plagiofoyaite(?) clasts found in Sugarloaf Mountain breccla, If the rocks are extrusive in origin the establishment of the present erosional character of the Black Hills region gives a younger age of post-early Oligocene. X INTRODUCTION Location of the study area Sundance, Wyoming is located in Crook County, approximately 40 km southwest of Spearfish, South Dakota and about 35 km southeast of Devil's Tower. The study area is in the northwestern part of the Black Hills region (fig. 1). A general geologic map was prepared on a scale of 1:10000 (Plate 1) which includes the following sections: 1, <I, 2, 3, 4, 10, 11, 12, 13, 14, 23, 24, 25, 26, (T, 52 N., R. 63 W,) and II the western halves of 6 and 7 (T. 52 N., R. 62 W.). Detailed maps were prepared on a scale of 1:5000 for the major igneous bodies, including Sundance Mountain (Plate 2; sections 23, 24, 25, and 26, T. 52 N., R. 63 W.), Sugarloaf Mountain (Plate 3; section 12, T. 52 N., R. 63 W. and section 7, T. 52 N., R. 62 W.), and a portion of the southern Bear Lodge Mountains (Plate 4; sections 2 and 3, T. 52 N., R. 63 W.). Previous work Darton (1905) mapped the Black Hills region on a scale of 1:125000. Sundance Mountain rocks were mapped as syenite porphyry and igneous rocks of the southeastern Bear Lodge Mountain as phonolite. Darton interpreted Sundance Mountain as being a "remnant of a laccolith 1 on a platform of Sundance Formation' • Brown (1952) attempted to model the igneous history of the Bear Lodge Mountains. He postulated the presence of an earlier trachyte 1 Fig. 1. Location map of the study area. 2 3 R.63W R.62W z I() i,-: N -.,. / ,,, ,, I 0 kU.ometen 0 .:I 1.0 kl! 0111• te re Key mm Pm/Po !Pm Lf2J Mp eoihnium sedimenlory roek• 4 magma which separated from a parent phonolitic magma in a ' 11 1accolith01 chamber.
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