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Magnetic Anomalies Over a Portion of the Granite Mountains, Central Wyoming Larry F

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Magnetic Anomalies Over a Portion of the Granite Mountains, Central Wyoming Larry F University of North Dakota UND Scholarly Commons Theses and Dissertations Theses, Dissertations, and Senior Projects 1979 Magnetic anomalies over a portion of the Granite Mountains, central Wyoming Larry F. Smyers University of North Dakota Follow this and additional works at: https://commons.und.edu/theses Part of the Geology Commons Recommended Citation Smyers, Larry F., "Magnetic anomalies over a portion of the Granite Mountains, central Wyoming" (1979). Theses and Dissertations. 281. https://commons.und.edu/theses/281 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]. MAGNETIC ANOMALIES OVER A PORTION OF THE GRANITE MOUNTAINS, CENTRAL WYOMING by Larry F. Smyers Bachelor of Seience, Pennsylvania State University, 1972 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 DEPTH TO llAS:em:N'I' MODELING .. .. .. .. .. • .. .. .. $ .. • .. .. • • .. • .. 55 Method Data Preparation Modeling INTERPRETATION. , • .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. ' .. .. .. 63 Interpretation of Anomalies Interpretation and Evaluation of Depth to llssement DISTRIBUTION OF PRECAMBRIAN ROCKS • 73 SUMMARY 77 CONCLUSIONS • 79 APPENDICES , 81 APPENDIX A. Foliation Data 82 APPENDIX B. Description and Data Input for Analysis of Anomaly Along Line A - A' , • • • , • • 84 APPENDIX C, Data Input and Magnetic Data for Program MAGDEJ.> 90 REFERENCES CITED, •••••••• , . .. ' .. .. ' . 99 V TABLE OF CONTENTS ILLUSTRATIONS vi LIST OF TABLES vii ACl(Nj:JWLEDGMENTS viii ABST~CT X INTRODUCTION • l General Statement Purpose of Investigation tocation and Accessibility of the Study Area ~ethod of Study rrevious Work GENr GEOLOGY • • • • , , , • • • • . • • • • • , , • • • • • • 9 ~egional Setting and Stratigraphy lopography and Drainage GEOL GY OF THE PRECAMBRIAN ROCKS • , •. , . , • , , , • • • • • • 15 ges of the Precambrian Rocks tructural Trends LITH LOGIC AND MAGNETIC CHARACTERISTICS OF THE PRECAMBRIAN ROCKS 26 neisses chiats )\mphibolites i;ranitic Rocks Diabase Dikes SED:Q;{ENTARY ROCKS .. 42 Geology of the Split Rock Formation AERO~GNETIC SURVEY •..••.•• . 44 Observed Aeromagnetic Anomalies Modeling of Anomalies Method iv !'ermission MAGNETIC ANOMALIES OVER A PORTION OF THE GRANITE Title MOUNTAINS, CENTRAL WYOMING Department Geo lo 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 lolho supervised my thesis vork or, in his absence, by the Chairman of the Department or the Dean of the Graduate School, It is understood that any copying or pub­ lication or other use of this thesis or part thereof for finan­ cial gain shall not be allowed without my written permission. It is also understood that due recognition shall be given to me and to the University of North Dakota in any scholarly use vhich may be made of any material in my thesis • . i'-' ;\ Signsture~•~'J_._·~-~-~~J~L]"_,_r~~-·i,_~_·~-~~.~·~-- Date -'--',11,F,='-"--~-,r-'-'~-'------A@1 ~ J /919 iii ILLUSTRATIONS Figure 1. Location of Study Area ••••••••• . 4 2. Sections Diagramming Inferred Structural Development and Stratigraphy Along Line N1 -S 1 12 3. Geologic Map of Study Area •••.• 17 4. Steroplots of Poles to Foliation Planes for Blocks A, B, and C •••••••••••• 22 5. Structural Subprovinces Within the Metamorphic Terrane of the Study Area •••••• 24 6. Magnetic Susceptibility of Sampled Precambrian Crystalline Rocks , •••• . .. 30 7. Modified IU,inigsberger Ratio (Q') in the Precambrian Rocks of Study Area 32 8. Directions of Remanent Magnetization for Sampled Precambrian Crystalline Rocks of Study Area 35 9. Aeromagnetic Map of Study Area ••••••• 46 10. Magnetic Susceptibility of Some Common Rocks 47 11. Obaerved and Computed Anomaly Along Line A - A' 53 12, Profiles Along Lines A, B, C, and D Showing Estimated Depths to Precambrian Basement , . 61 13. Effect of Half-Maximum Half-width (W/2) Estimates from Various Magnetic Anomalies •• , ••• 70 14. Distribution of Known and Inferred Precambrian Rocks in Study Area ••.••..•.••• 75 Plate 1. Sampling Precambrian Crystalline Rocks 7 2. Device Used to Orient Core Sample • 7 3. Physiography of Study Area 14 vi LIST OF TABLES Table 1. Estimated Depth VRS. Actual Depth to Precambrian Basement ....... .. .. .. .. 58 Vii ACJ.<NOWLEDGMJ!NTS The writer wishes to express his appreciation to the following who have made this study possible. Special thanks is given to Mr. Donald Laub for his suggesting the study, continued support, and guid­ ance throughout the project. The Phillips Uranium Corporation supplied the aeromagnetic data for the study area, Dr. Walter Moore served as chairman and advisor throughout the graduate program, and provided a valuable source of information on the technical aspects of the project. Drs. Lee Gerhard and Odin Christensen served as committee members, and made many helpful suggestions for technical refinement of the manu­ script. Dr. William Henry, of the Phillips Petroleum Company, ran the remanent magnetization measurements. Mrs. Lorraine Rose provided many helpful suggestions as to the preparation of the manuscript, and professionally typed the final copy. Fran Thomas has been a continued source of encouragement and moral support. Mark Whitney and Carol Brown have given assistance and friendship throughout the project. To these dear friends, thank you. Last, but not least, to my mother, brothers, and sister, who have given spirit to the project. viii DEDICATION In memory of my father. ix Al!STRACT Precambrian rocks exposed in the northern Pathfinder Reservoir area, Natrona County, Wyoming, are partially exhumed crystalline rocks of the northwestern portion of the Granite Mountains. These rocks com­ prise three major types: (1) the oldest metamorphic rocks, which may represent a metamorphosed sedimentary-volcanic terrane, exposed on Black Rock Mountain and in other small areas south and west of Black Rock Mountain; (2) granites, which intrude the metamorphic sequence, and are exposed on the Sweetwater Rocks and areas marginal to Pathfinder Reservoir; and (3} diabase dikes with northeastern trends that intrude both the metamorphic rocks and granites. Exposures of the Precambrian rocks rise above flat-lying Tertiary rocks and sediments that fill intervening basinal areas. Aeromagnetic data shows a nearly uniform magnetic field that is interrupted by a few isolated, but strong, anomalies, which are caused by magnetic contrasts within the basement complex. Studies of the magnetic characteristics of the exposed Precambrian rocks indicate that the magnetic susceptibilities of the amphibolites, pelitic schist, and diabase dikes are nearly twice that of the gneisses and granites. Partitioning of Fe and Ti oxides within the amphibolite-pelitic schist sequence, or depletion of magnetic minerals in the gneisses and gran­ ites may be responsible for some of the observed magnetic contrasts. Comparison of the magnetic susceptibility and remanent magneti­ zation measurements suggest that the main contribution to total X magnetization is magnetic susceptibility. Although remanent magnetiza­ tion does exist within the samples measured, modified Kliinigsberger ratios (Q') indicate that it contributes a negligible com:ponent to total magnetization, Strong magnetic anomalies northwest and south of Black Rock Mountain and interpreted to be caused by isolated bodies of "iron­ formation," since the constraints provided by the modeling analysis of these bodies, and rock associations conform well with that of "iron-formationsn found in other portions of the Granite Mountains and Precambrian terrane of the northern Rocky Mountains. A concealed rock mass, probably of mafic composition, is inferred to underlie one magnetic anomaly, Remaining portions of the study area, characterized by a nearly uniform magnetic field, are inferred to be underlain by the essentially non-magnetic gneisses snd granites. h A fault is inferred on the observed irregularities in the mag­ netic contours associated with the amphibolite-pelitic schist sequence - ) exposed on Black Rock Mountain. ' Reliability of depth estimates varied with the nature of the ' 1'' anomalies used in the analysis, Broad anomalies produced more accu­ :1 ' ~ 1.: ~ rate estimates in comparison to narrow anomalies. Since the modeling ~ technique used for depth estimates depended upon an accurate estimate of the half-maximum half-width parameter of the anomaly, narrow anom­ alies not only produced shallower than actual estimates, but were sus­ ceptible to greater error due to data spacing, xi INTRODUCTION General Statement In recent years aeromagnetic surveys have become one of the most popular tools for reconnaissance "basement" mapping. The results of the interpretation of aeromagnetic surveys are the probable distribution of magnetic material below the surface, influence of the possible geological structure, and most important, the determination of depth to the basement rock and thickness of the
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