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Report of Investigation 12 – Page 1 of 23 MICHIGAN DEPARTMENT of NATURAL Aeromagnetic and Geologic Profiles Report of Investigation 12 – Page 1 of 23 MICHIGAN DEPARTMENT OF NATURAL Aeromagnetic and Geologic Profiles............................15 RESOURCES CONCLUSIONS ..............................................................20 GEOLOGICAL SURVEY REFERENCES CITED ....................................................21 Report of Investigation 12 GEOLOGIC INTERPRETATION OF AEROMAGNETIC DATA IN WESTERN UPPER Illustrations PENINSULA OF MICHIGAN Figures by W. M. Meshref and W. J. Hinze Figure 1. Location of area. ......................................................3 Prepared in cooperation with Michigan State University Figure 2. Regional geology of western Upper Peninsula; division of study area. ......................................................3 1970 Figure 3. Comparison of magnetic trend elements between areas. .............................................................................15 Contents Figure 4. Distribution of magnetic anomalies within pre- Keweenawan (southern) area. .......................................16 PREFACE ......................................................................... 2 Figure 5. Distribution of anomalies within Keweenawan ABSTRACT....................................................................... 3 (northern) area. ..............................................................16 INTRODUCTION............................................................... 3 Figure 6. First explanation of development of structure along Keweenaw fault..............................................................17 Location ......................................................................... 3 Figure 7. Second explanation of development of structure General Geology ........................................................... 3 along Keweenaw fault. ...................................................17 Purpose ......................................................................... 4 Tables Techniques .................................................................... 4 Table 1. Generalized geologic column for Northern area........5 Previous Investigations.................................................. 4 Table 2. Generalized geologic column for Southern Area.......8 REGIONAL GEOLOGY .................................................... 5 Table 3. Magnetic susceptibility measurements......................9 Northern Area ................................................................ 6 Table 4. Summary of magnetic properties ..............................9 Stratigraphy..............................................................6 Lower Keweenawan .....................................................6 Middle Keweenawan ....................................................6 Upper Keweenawan .....................................................6 PREFACE Jacobsville Sandstone..................................................6 The authors wish to acknowledge the aid of students Structure ..................................................................6 and staff of the Department of Geology, Michigan State Folds ............................................................................7 University and members of the Geological Survey Faults............................................................................7 Division of the Michigan Department of Natural Southern Area ............................................................... 7 Resources for their assistance in this study. The Stratigraphy..............................................................7 interest, advice, and criticism of Robert C. Reed and Dr. Lower Precambrian ......................................................7 James W. Trow are particularly acknowledged. Middle Precambrian (Animikie).....................................7 Middle Precambrian Igneous Rocks.............................8 The Geological Survey Division of the Michigan Department of Natural Resources furnished magnetic Structure ........................................................................ 8 maps. The financial support provided by the author by MAGNETIC PROPERTIES OF ROCKS........................... 9 the United Arab Republic made this study possible. INTERPRETATION OF MAGNETIC DATA ................... 10 This publication is an outgrowth of a Ph.D. thesis completed by the senior author under the supervision of Aeromagnetic Maps..................................................... 10 the junior author. Northern Area ........................................................10 Southern Area........................................................11 Cairo, United Arab Wafik M. Meshref Lake Gogebic .............................................................11 Republic Assistant Professor Iron River-Crystal Falls...............................................12 Atomic Energy Depth Determinations.................................................. 13 Establishment Northern Area ........................................................14 East Lansing, Michigan William J. Hinze Southern Area........................................................14 December 1969 Professor Magnetic Trend Analysis ............................................. 14 Department of Geology Michigan State University Report of Investigation 12 – Page 2 of 23 The Porcupine Mountains are believed to be a lopolithic intrusion of rhyolite. Withdrawal from a magma chamber to the south formed the Iron River syncline. South of the Keweenaw fault, a linear magnetic source, the Middle Trap Range is interpreted as an up-faulted block of the South Trap Range lavas. South of the Barb Lake fault granitic rocks occupying the center of the Marenisco anticline are probably Early Precambrian. Magnetic strata south of the Marenisco anticline are interpreted as the Ironwood Iron-formation of the Tyler Slate series. INTRODUCTION The U.S. Geological Survey, in cooperation with the Figure 1. Location of area. Geological Survey Division of the Michigan Department of Natural Resources, is conducting a comprehensive restudy of the mineral-bearing districts of Michigan. As a part of this program, aeromagnetic surveys have been completed over much of the Northern Peninsula of Michigan. Geologic interpretation of portions of these surveys were published by Balsley and others (1949), Wier and others (1953), and Case and Gair (1965). The present investigation is based upon a portion of these aeromagnetic maps. Location The area of study covers about 5,000 square miles, including most of the western half of the Upper Peninsula of Michigan. It includes Ontonagon, Gogebic, Iron, and parts of Baraga and Houghton counties (fig. 1) south of 47° 00' N latitude, west of 88° 07' 30" longitude, and is bounded on the west and south by the Michigan- Wisconsin border. For the purpose of discussion the study area (fig. 2) is divided into a northern part and a southern part, based primarily on geologic considerations. The northern part includes rocks of Keweenawan age, as well as younger rocks. The southern part includes mostly pre- Figure 2. Regional geology of western Upper Peninsula; Keweenawan rocks (both Lower and Middle division of study area. Precambrian) and is further subdivided. The western portion covers mainly the area west and south of Lake Gogebic. The eastern portion includes the Iron River- ABSTRACT Crystal Falls district and the Amasa oval. The regional structure and lithology of Precambrian formations in parts of Ontonagon, Gogebic, Iron, Baraga, General Geology and Houghton counties is interpreted using available The geology of the study area is varied and highly geologic data and published aeromagnetic maps. complex. Though studied for nearly a century, the The position and attitude of the near-surface contact regional geology has not been completely defined between the North Trap Range (Portage Lake) lavas and primarily because of the covering of Paleozoic the Jacobsville Sandstone is uncertain west of longitude sediments, and glacial drift. Because these 89° 20' W. A linear magnetic anomaly south of the Precambrian rocks have varied magnetic properties, the Keweenaw fault may be the result of cross-faulting or magnetic method should provide an excellent tool for sliding of the southern portion of the lavas subsequently delineating the regional geology and structural patterns. buried by sandstone. The near-surface contact between Lower, Middle, and Upper Precambrian rocks underlie the lavas and the sandstone may dip south. most of the area. The Lower Precambrian forms the basement and consists of granite, gneisses, Report of Investigation 12 – Page 3 of 23 greenstones, and other igneous and metamorphic types. well as the nature of the tectonic forces involved in One or more periods of metamorphism and deformation developing these trends. occurred before the end of Early Precambrian time. 3. A second vertical derivative map of the total magnetic These basement rocks are separated from the overlying intensity was prepared for the central area to aid in metasedimentary rocks of Middle Precambrian age depth determinations and to isolate the boundaries of (Animikie) by a major unconformity. Widespread magnetic units. deformation and regional metamorphism occurred at the close of Middle Precambrian time. Prior to that time 4. Theoretical magnetic profiles were calculated for intrusive bodies of both Lower and Middle Precambrian assumed geologic bodies and compared with the age were emplaced. observed magnetic profiles. The Keweenawan (Upper Precambrian) rocks are In addition, the magnetic interpretation was
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