69-16,164 M E S H R E F , W afik M ., 1 9 3 7 A ERO M AG NETIC STUDY O F T H E REGIONAL GEOLOGY O F T H E W ESTERN H A LF O F TH E NORTHERN P E N IN SU L A O F MICHIGAN. M ic h ig a n S tate U n iv e r s ity , P h .D ., 1969 G e o lo g y U niversity Microfilms, Inc., A nn A rbor, M ichigan AEROMAGNETIC STUDY OP THE REGIONAL GEOLOGY OP THE WESTERN HALF OP THE NORTHERN PENINSULA OF MICHIGAN By Wafik M. Meshref A THESIS Submitted to Michigan State University In partial fulfillment of the requirements for the degree of DOCTOR OP PHILOSOPHY Department of Geology 1968 ABSTRACT AEROMAGNETIC STUDY OF THE REGIONAL GEOLOGY OF THE WESTERN HALF OF THE NORTHERN PENINSULA OF MICHIGAN By Wafik M. Meshref U.S. Geological Survey aeromagnetic maps of the western portion of the Northern Peninsula of Michigan from Wisconsin border to Keweenaw&n Bay and the Amasa Oval have been com­ piled to a common arbitrary magnetic base level and hori­ zontal scale. This study has shown conclusively that aeromagnetic interpretation is an extremely useful tool in determining the regional structure of a Precambrian terrain consisting of widely diverse magnetic formations. The complexity of magnetic rock properties restricted the Interpretation to a semi-quantitative approach based upon the integration of the surface geology, Bouguer gravity anomalies, and the results of analytical studies of the magnetic data. The near surface contact between the Northern Trap Range lavas and the Jacobsville sandstone Is believed to have a southerly dip along most of its extent west of longi­ tude 89° 20' W. This dip probably originates from cross- faulting and/or sliding of a block of Keweenawan lavas over the Jacobsville sandstone after the major thrust of the Northern Trap Range lavas along the Keweenawan fault. A Wafik M. Meshref time break in deposition of the Jacobsville sandstone is believed to have taken place during the period of tectonism associated with the Keweenawan fault. The structure in the Porcupine Mountains area and the origin of the Iron River syncline is interpreted to be the result of a lopolithic intrusion of rhyolite. The structure of the Middle Trap Range is interpreted as an upfaulted block of lava in form of a horst. These volcanics are burried beneath 1250 to 2500 feet of sandstone. This variation in depth to the Middle Range is believed to be due to several cross faults of considerable vertical displacement. The thickness of the Michigamme slate varies between 1500 and 4000 feet in different portions of the basin. However, a magnetic source underlying the Michigamme slate was recorded at a depth of about 7000 feet. This is the deepest magnetic source in the study area. The Wolf Lake granite outcropping south of the Barb Lake fault is believed to be of Lower Precambrian age as the granitic basement rocks in the center of the Marenisco anticline. The magnetic strata of the Marenisco Range, mapped to be of uncertain stratigraphic position, are interpreted as the Ironwood iron formation of the Tyler slate series on the south limb of the Marenisco anticline. A magnetic trend analysis indicated that the acting stress on the Pre-Keweenawan rocks was of a non-rotational Wafik M. Meshref nature and that the principal stress axis was northsouth. It also indicated that the acting stress during the Keweenawan time was of a rotational nature due to a shear couple which was shifting in time and space. ACKNOWLEDGMENTS The author wishes to express his sincere appreciation to Dr. William J. Hinze for his invaluable guidance and suggestions throughout the project. Special appreciation is also due to Dr. James W. Trow for his particular inter­ est in this study. The author is also indebted to Dr. Hugh F. Bennett, Dr. C. E. Prouty, and Dr. Harold B. Stonehouse for reviewing the manuscript and active participation in the guidance committee. The author is deeply indebted to the United Arab Republic Government for awarding me such a scholarship that enabled me to do this project. Special thanks is also expressed to Mr. Robert C. Reed of the Michigan Geological Survey for his helpful advise and criticism during the study. Appreciation is expressed to the Geological Survey Division of the Michigan Department of Conservation for providing a copy of the aeromagnetic maps used in this study. Thanks is also expressed to Michigan State University for the free use of the C.D.C. 3600 computer. Thanks is also due to Dr. Donald W. Merritt for his assistance in writing computer programs. ii TABLE OP CONTENTS Page ACKNOWLEDGMENTS ........................................ LIST OF T A B L E S ......................................... 11 v LIST OP F I G U R E S ............................................ vi LIST OP P L A T E S ........................................... vii Chapter I. II. III. INTRODUCTION .................................. 1 PHYSIOGRAPHY OP THE A R E A ..................... 8 GEOLOGY OF THE A R E A ............................... 12 G e n e r a l .........................................12 The Keweenawan A r e a ........................... 15 Stratigraphy .............................. 15 Lower Keweenawan Rocks ................. 15 Middle Keweenawan Rocks . . . . . 18 Upper Keweenawan R o c k s .............. ■ . 19 S t r u c t u r e ..................................... 21 F o l d s ..................................... 21 F a u l t s ..................................... 22 The Pre-Keweenawan A r e a ........................ 22 Stratigraphy .............................. 22 Lower Keweenawan Rocks . . . . . . 24 Middle Precambrlan Rocks "Anlmlkle" . 24 Middle Precambrlan Igneous Rocks . . 29 Structure . 29 IV. V. VI. PREVIOUS GEOPHYSICAL STUDIES ................. 31 AEROMAGNETIC M A P S ............................... 37 MAGNETIC PROPERTIES OF ROCKS ................. 39 Introduction .............................. Induced Magnetic Properties ............. Remanent Magnetic Properties ............. 39 4l 46 ill Chapter VII. VIII. Page METHODS OP INTERPRETATION ................. 50 Introduction .............................. Depth D e t e r m i n a t i o n s ....................... M e t h o d s ................................. The Half-Width Method ................. Smellie's Method ....................... Peters' Slope Method ................. Vacquier's Method .................... Results . . . . . . . . . . . Correlation of Estimated Depth Values with Bedrock G e o l o g y ................. G e n e r a l ................................. Keweenawan Rocks ....................... ................. Pre-Keweenawan Rocks Magnetic Trend Analysis .................... 50 51 51 51 52 55 58 60 I N T E R P R E T A T I O N .............................. 80 63 63 66 67 68 G e n e r a l .......................... 80 Aeromagnetic M a p s ........................... 81 Introduction .............................. 81 The Keweenawan A r e a .................... 82 Pre-Keweenawan A r e a .................... 87 Discussion of Profiles .................... 97 G e n e r a l ................................. 97 Profile A - A 1 ........................... 99 Profile B - B 1 ...............................102 Profile C - C ...............................108 Profile D - D ' .......................... 109 Profile E - E ' ...............................110 Profile F - F ' ...............................Ill Profile G - G ' ...............................112 Profile H - H ' ...............................113 Profiles I-I* and J - J ' .....................116 Profiles K-K' to N - N ’ .....................118 IX. S U M M A R Y ............................................ 121 BIBLIOGRAPHY ............................................ iv 126 LIST OP TABLES Table 1. 2. 3. *4. 5. Page Comparison of major subdivisions of Precambrian rocks of Northern Michigan with previously used terminology ................. 13 Generalized geologic column for the Keweenawan a r e a ............................................ 16 Generalized geologic column for Pre-Keweenawan rocks as distributed in different districts of the a r e a .................................. 23 Magnetic susceptibility measurements of Pre­ cambrlan rocks in the a r e a ................. *45 Summary of magnetic p r o p e r t i e s ................. *49 6 . Depth determinations 7. ........................... Magnetic susceptibilities determined by Vacquler's m e t h o d ........................... 8 . Correlation of depth determinations with bed­ rock g e o l o g y ................................. 9. 10. 61 6*4 65 Distribution of magnetic trend elements of different anomaly orders in the Keweenawan and the Pre-Keweenawan a r e a s ................. 71 Distribution of magnetic trend elements in the Keweenawan area, Pre-Keweenawan area, and the survey a r e a .................................. 73 v LIST OF FIGURES Figure Page 1. Index map of area of i nv estigation............. 2 2. Division of study area into two subareas according to regional geology ............. 7 Aeromagnetic flight lines flown by the University of Wisconsin .................... 34 4. Cutaway view of the sample holder 43 5. Half-width method of depth determination. 53 6. Peters* 57 7. Distribution of magnetic trend elements in (a) total survey area, (b) Keewanawan rocks, (c) Pre-Keewanawan r o c k s .............. 76 Distribution of magnetic trend elements within Pre-Keewanawan rocks according to order of magnetic anomalies ........................... 77 Distribution of magnetic trend elements within the Keewanawan rocks according to order of magnetic anomalies ........................... 78 Possible development of structural relations along the Keewanaw fault .................... 104 Possible development of structural relations along the Keewanaw fault .................... 105 3. 8. 9. 10. 11. ............. slope method of depth determination. LIST OF PLATES Plate I. II III IV. V. VI. VII. Page Aeromagnetic Map of the Western Half of the Northern Peninsula of Michigan, scale 1 :62,500 (I-a, I-b, I-c,. I-d, and I-e ) ......................................... In Pocket Magnetic Trend Analysis Map, scale 1 :62,500 (Il-a, Il-b, I I - c , Il-d, and ..................................... II-e) In Pocket Bedrock Geology Map, scale 1:62,500 ClII-a,. Ill-b, III-c, III—d, and III-e) ...................................... Aeromagnetic and Geologic Profiles I V - A : Profiles A-A' and B-B* I V - B : Profiles C-C' and D-D* I V - C : Profiles E-E* and F-F* I V - D : Profiles G - G 'and H-H* I V - E : Profiles I - I *, J-J', K-K', and L-L' I V - F : Profiles, M-M* and N-N* In Pocket In Pocket Composite: Aeromagnetic Map of the Western Half of the Northern Peninsula of Michigan, scale 1 : 2 50 ,0 00 ................. In Pocket Composite, Bedrock Geology Map, scale 1 :250,000 .............................. In Pocket Composite, Aeromagnetic and Geology Profiles, Horizontal scale 1:250,000 In Pocket vii . CHAPTER I INTRODUCTION The U.S. Geological Survey in cooperation with the Geological Survey Division of the Michigan Department of Conservation conducted a comprehensive restudy of the mineral-bearing districts of Michigan. As a part of this program aeromagnetic surveys have been conducted over the majority of the Northern Peninsula of Michigan. The geo­ logic interpretation of portions of these surveys have been published by Balsley and others (1953), and Case and others (1949), Wier and others (1965)* A portion of these aeromagnetic maps has been selected for geologic interpretation. The area chosen for this study covers about 5,000 square miles, including most of the western half of the Northern Peninsula of Michigan. It includes Ontonagon, Gogebic, of Baraga and Houghton C o u n t i e s . Iron, and parts The area shown in Figure 1 is located south of latitude 47° 00' N, west of longitude 88° 07' 30" W and is bounded on the west and south by the Michigan-Wisconsin border. The geology of the area of investigation is varied and highly complex, and although it has been studied for nearly a century, the regional geology has not been completely defined primarily 1 Fig.l. Index m a p of area o f i n v e s t i g a t i o n . 3 because of the surficial covering of sediments and Pleisto­ cene glacial drift. Due to the varied magnetic properties of the Precambrian rocks, the magnetic method should provide an excellent tool for delineating the regional geology and structural patterns of the area. Rocks of Lower, Middle (Animikle), and Upper (Kewee­ nawan) Precambrian age underlie most of the area. The Lower Precambrian rocks form the basement and consist of granite, gneisses, greenstones, and other metavolcanic rocks. One or more periods of metamorphism and deformation occurred before the end of Early Precambrian time. These basement rocks are separated from the overlying metasedimentary rocks of Middle Precambrian age (Animikle), by a major unconformity. Widespread deformation and regional metamorphism occurred at the close of Middle Precambrian time. Prior to that time intrusive bodies of both Lower and Middle Precambrian age were emplaced. The Keweenawan rocks are separated from the under­ lying Animikle rocks by another major unconformity. These rocks include the Keweenawan lava flows, conglomerates, sandstone, and shales. During Late Precambrian (Keweena­ wan) diabase dikes, which have a dominant westward trend, intruded the older rocks. These Precambrian rocks are unconformably overlain in the central part of the area by flatlying Jacobsville sandstone and generally are con­ cealed by glacial debris of Pleistocene age. The general purpose of this study Is to map the regional structure and llthology of the Precambrlan rocks In the area, using the aeromagnetic data as the main source of geophysical information. The specific objectives of this study are: 1. To map the Precambrlan rocks where information is sparse due to the lack of outcrops and/or the inaccessibility of the area. 2. To trace the location of the Keweenaw fault in the area. 3. To delineate the structure of the Keweenawan rocks in the area and the basement structure that underlies the Animikle se diments. The primary objective in magnetic interpretation is to draw inferences about the attitude, depth, configuration, and lithology of the subsurface structure. To accomplish this, information must be obtained about the regional geology and.the induced and remanent magnetization of the underlying basement complex. Therefore, geologic informa­ tion about the area of investigation was compiled on one map of the same scale as the aeromagnetic map of the area (1:62,500). The purpose of this map is to facilitate the correlation of the known geology with major magnetic anomalies and broad areas that have characteristic mag­ netic patterns. The character of these magnetic anamalies was used to interpret the geology of areas which have not 5 been previously mapped. Rock samples were collected representing most of the lithologies in the area for the purpose of determining their magnetic properties. Qualitative and quantitative magnetic interpreta­ tion was accomplished using the following interpretation tools: 1. Published depth determination techniques were applied to the study area in order to interpret adequately the geology and structure of the basement rocks. 2. A magnetic trend analysis was conducted to help define the major structural trends in the area under investigation, and correspondingly the direction as well as the nature of the tectonic forces Involved in developing these t r e n d s . 3. A second vertical derivative map of the total magnetic intensity was prepared for the central portion of the study area to aid in the depth determinations and to isolate the boundaries of magnetic units. Theoretical magnetic profiles were calculated for assumed geologic bodies and compared with the observed magnetic p r o f i l e s . In addition, the magnetic interpretation was checked against the regional gravity map of the study area. % 6 For the purpose of facilitating the discussion of the geology and the results of interpretation, the area of investigation is divided into two a r e a s . This division is based primarily on geologic considerations as shown in Figure 2. The first area includes rocks of the Keweenawan age, as well as younger rocks. The second area is charac­ terized by both Lower and Middle Precambrian rocks, i.e., Pre-Keweenawan rocks. This area constitutes the southern part of the study area and is further subdivided into an eastern and a western protion. The western protion covers mainly the area to the west and south of Lake Qogebic. The eastern protion includes the Iron River-Crystal Falls district and the Amasa Oval. Explanation U fVecainbrian t a L. and M. firccairibrian MMOWtm V •OOUEXTI Ifqlgot^c edim ?r>ts Fig. 2. Division of study a r e a into t wo s u b a r e a s according t o regional geology. ( A f t e r Gai r 19 5 6 ) CHAPTER II PHYSIOGRAPHY OP THE AREA A comprehensive description of the physiography of selected protions of the area is presented in U.S. Geologi cal Survey Monograph 52 (Van Hise and Leith, 1911) and in previous U.S. Geological Survey publications. The study area is a part of the highland topographic province of the Lake Superior region. This is contrasted with the remainder of the Northern Peninsula of Michigan from Marquette eastward to Sault S t e . Marie, which is con­ sidered to be a lowland nowhere rising more than 900 feet above sea level or 300 feet above Lake Superior. The relief of the highland area varies between 1000 and 1700 feet with some exceptions as in the Porcupine Mountains area, which lies in the northwest part of the study area. The mean elevation or the Porcupine Mountains is about 1500 feet, but in some areas it exceeds an elevation of 1950 feet above mean sea level. One of the major topo­ graphic features in the area is the depression occupied by Lake Gogebic. The highland forms a broad upland cut by valleys that lie 100 to 400 feet below the general ground level and It is diversified by monadnocks and other ridges. 9 The upland Is made up chiefly of the Lower Precambrlan rocks, that Include greenstones, granites, and other coarse grained rocks together with schists and gneisses, most of which are homogeneous over broad areas in their resistance to weathering and erosion. Where folding and faulting occurs, the Animikle schists, gneisses, and quartzites and the Keweenawan lavas usually present homogeneous resistance to weathering over narrow linear belts, thus, resulting in ridges and monadnocks. The Northern Trap R a n g e , composed mainly of Keweena­ wan lavas, extends across the northern and northwestern parts of the area. It is a northeastward trending homo- cllnal ridge, dipping to the northwest. It has rugged topography as a result of the differential erosion of the more resistant lavas and the less resistant surrounding sedimentary rocks. feet. The average local relief is about 300 A broad, relatively flat plain, occurs south of the Northern Trap Range. It has an average elevation of 1300 feet above mean sea level. the Jacobsville sandstone. This plain is covered with Uralitized basalt flows outcrop within this plain at Silver Mountain which is located in sections 1 and 12 of T^9N, R36W, and section 6 of T^9N, R35W, Houghton County. Limestone Mountain, an outlier of Ordovician limestone is located about 10 miles north of Silver Mountain. 10 The South Trap Range outcrops occasionally along the southern edge of the Jacobsville sandstone plain. East of Lake Qogeblc the South Trap Range dips to the northwest at about 15° 9 but it dips about 70-80° west of Lake Qogeblc. A narrow lowland area skirts the south shore of Lake Superior in the study area. This area is underlain by rocks of Upper Keweenawan age, mainly the Outer Con­ glomerate of the Copper Harbor group, Nonesuch shale and Freda sandstone of the Oronto group. The most important topographic feature in the south­ western part of the area is the Penokee—Qogeblc Range, which extends from west of Lake Qogeblc to the western border of the study area. The Penokee-Qogebic Range rises to elevation of 1500 to 1800 feet (Irving, 1880) which is 100 to 300 feet above the lower land to the south. Extending along the southern border of the Animikle rocks is a prominent ridge. This ridge is not continuous, but rather consist of a series of disconnected linear high­ lands, the crest of which In some parts of the district is formed by the Ironwood formation. In other parts of the district the highlands are formed by the granitic rocks of the Lower Precambrlan which occur as rather rugged hills. The topographic characteristics of the Lower Pre­ cambrian rocks In the extreme northeastern portion of the study area has been described In great detail by Van HIse and Bayley (1895) as follows: 11 North Complex— which constitutes the Huron Mountains, the southern part of which Is exposed In the northeastern part of the area. In this complex isolated remnants of schists form rugged hills, while granite and syenite form rounded knobs. South Complex— primarily consists of knobs as in the northern granite areas. The topography of the Crystal Falls district has been described by Clements and others (1899)* The topography is typical of the glaciated Lake Superior uplands. Low, swampy areas, alternate with undulating plains, knobs, and kettle terrains. In most areas the topography is controlled by the bedrock. The uplands are predominantly massive igneous and volcanic rocks or sillcious facies among the sedimentary rocks. The low swampy tracts, generally are underlain by slates and schists as in the Michigamme slate plain, west of the Crystal Falls area. The average relief is about 200 feet. Michigamme Mountain which reaches an elevation of 1600 feet is the highest topographic feature in this district. CHAPTER III GEOLOGY OP THE AREA General The geology of the Precambrlan terrain of the Northern Peninsula of Michigan has been described in early publications of the U.S. Geological Survey, and the Michi­ gan Geological Survey (Irving and Van Hise, 1892; Van Hlse and Bayley, 1897; Clements and Smyth, 1899; Van Hlse and Leith, 1911; Allen and Barrett, 1915; Barrett, Pardee, and Osgood, 1929; Leith, Lund and Leith, 1935; Martin, 1936; and many o t h e r s ). Unlike the situation with respect to postPrecambrian time, there exists no widely accepted reference framework of eras and periods to which Precambrian lithologic units and geologic events can be related. James (1958) introduced new formal names and summarized the stratigraphic nomenclature used by the U.S. Geological Survey for the Precambrian rocks that occur in this part of Northern Michigan. His nomenclature is shown with pre­ vious investigator's classification in Table 1. The three subdivisions of the Precambrlan as used by the U.S. Geological Survey are separated at most places by major unconformities. They are overlain in places by 12 TABLE 1.— Comparison of major subdivisions of Precambrian rocks of Northern Michigan with previously used terminology. Van Hise & Leith (1911) Leith, Lund & Leith (1935) Algonkian o ** Archean o u 24 names in the different districts as described by previous investigators. Lower Precambrian Rocks These are the oldest rocks in the area. They form the basement and consist of gneisses, granites, green­ stones and other metavolcanic rocks. gneisses predominate. Granites and granite One or more periods of metamorphism and deformation occurred before the end of Early Pre­ cambrian t i m e . Middle Precambrian Rocks "Anlmikle1* The Chocolay Group The Chocolay group and its correlatives as defined here are equivalent to the Lower Huronian of earlier reports. Prom Table 3 it is clear that it comprises two major units, a thick basal quartzite (the Sturgeon, Mesnard, and Sunday quartzites of the different localities), and an equally thick dolomite (the Randville, Kona, and Bad River dolomite). members. Both units contain some slaty The quartzite formation is made up of a basal conglomerate which is overlain by the rather coarse grained quartzite. The quartzite member is separated from the underlying "Archean" rocks by a major unconformity. The quartzite grades upward into the dolomite member. The aggregate thickness of the two units is typically about 25 4,000 feet, but in many places these strata are absent because of non-deposition or post-dolomite erosion. The Menominee Group The Menominee group and its correlatives as described here are equivalent to Middle Huronian of preceding reports. The type locality of this group consists of two formations, a basal clastic formation (Pelch formation and AJlbik and Palm quartzites) overlain by iron formation. These com­ monly are conformable or nearly so with the underlying Chocolay group. The base of this group is marked by a thin conglomerate, one to three feet thick. Where the formation is in contact w i th the "Archean," as in parts of the Gogebic area, the pebbles are granite gneiss and greenschist, but where the underlying formation is the dolomite member of the Chocolay group, the conglomerate also Include fragments of chert and limestone. The basal clastic forma­ tion ranges from 10 to 800 feet in thickness and from vitreous quartzite to graywacke and slate (or schist) in lit ho lo gy . This clastic formation grades upward into the iron formation which consists primarily of alternating thin layers of chert and iron minerals. Siderite, hematite, iron silicates, or magnetite are predominant. The Negaunee iron formation of the Marquette Range attains a maximum thickness of 2,000 feet, whereas the Ironwood 26 iron formation of the Penokee-Gogebic Range rarely exceeds 800 feet. The.Baraga Group The Baraga group and its correlatives comprise most of the strata referred to as Upper Huronian of previous reports. It is made up chiefly of graywacke, slate, and basic volcanic rocks, but conglomerate, quartzite, and iron formation are common particularly in the lower part. The principal stratigraphic unit is the Michigamme slate and its correlatives in the Penokee-Gogebic Range area. The exposed parts of the Michigamme slate and its correla­ tives consist of graywacke and slate in about equal p r o ­ portions. The graywackes are dark gray, massive, and fine to medium grained. The slates are light to dark gray and transitional in grain size into the graywackes with which they are interbedded. Bedding is commonly Indistinct or subordinate to slaty cleavage as the obvious structure. Most graywacke beds show no cleavage. The Michigamme slate and its correlatives and prob­ ably 5,000 to 10,000 feet of basic volcanic rock, now mainly greenstones (Hemlock formation and its correlatives) form several thick units in the Baraga group. Much of this rock shows agglomeratic or pillow structures and sub­ marine origin is probable. Pritts (1967) published a geologic map of the area around Lake Gogebic (Marenisco-Watersmeet) and presented 27 the geologic column on that map. This sequence of rocks includes at its base a metatuff and tuffaceous metagraywacke member which consist of minor quartzite, conglomer­ ate, and magnetic iron formation in the lower part. It also possibly includes pillow lavas, east of Cup Lake, dipping at 60° to the southeast. This is overlain by the Graywacke series developed near Banner Lake, the upper part of which includes magnetic iron ore especially south of the Barb Lake f a u l t . This is overlain by a metavol- canic and metasedimentary formation which crops out north of the Barb Lake fault. This could be separated into two units north of the fault, the metatuff and magnetic iron formation and the pillow lava and fragmental volcanic rocks. This is overlain by the uppermost member of this group which is labeled by Pritts as the graywacke slate near Paulding. The Paint River Group The Paint River group and its correlatives in p r e ­ vious reports, whi ch include productive iron formation, generally were considered part of the Michigamme slate (Leith, Lund, and Leith, 1935). The Iron River-Crystal Palls district is a deep tightly folded major synclinal structure incompletely bounded by Badwater greenstone. In the area immediately east of Crystal Falls, greenstone is absent and the strate of the district rest directly on 28 the Michigamme slate, which doubtless is the reason they were previously considered an extension of that unit. Recognition of the stratigraphic position of the Badwater greenstone forms the basis for the definition of the Paint River G r o u p . In the Iron River-Crystal Palls area this group Includes a basal sequence of siltstone and slates named the Dunn Creek slate, which ranges in thickness between 400 and 800 feet. Much of the Dunn Creek is graywacke and slate that are physically indistinguishable from the Michigamme slate. This is overlain by the Riverton iron formation which consist dominantly of interbedded chert and siderite. In -places the upper part of the formation is absent because of erosion prior to the deposition of the overlying Hia­ watha graywacke which is clastic in nature, mainly gray­ wacke with considerable Interbedded slate. This is over- lain by the Stambaugh formation which is an iron-rich rock that ranges from chlorite mudstone and slate to a laminated cherty siderite-magnetite rock. This is overlain by the Fortune Lake slates which consists primarily of slate and minor gra yw ac ke . The aggregate thickness of the group is at least 4,000 feet. 29 Middle Precambrian Igneous Rocks The Middle Precambrian igneous rocks in the area fall into two principal groups, metadiabase and metagabbro of Animikie or post-Animikie age, and granite and allied rocks that are younger than the metadiabase and metagabbro. The metadiabase and metagabbro were metamorphosed during the post-Animikie, 1955). Pre-Keweenawan interval (James, The existence of post-Animikie granite in Northern Michigan has been known almost since geologic work began more than a hundred years ago, but considerable debate has arisen as to its extent. absolute ages, The results to date, of work on suggest that the age of the Post-Animikie, Pre-Keweenawan epoch of diastrophism, metamorphism, and granite instusion is more than 1,400 million years, as compared with 1,100 million; years for the Keweenawan (Duluth Gabbro and related rocks). term Killarney to the post-Animikie, James (1955) used the Pre-Keweenawan granite rocks in Michigan, but its validity is uncertain. Structure The major structure in the eastern portion of the Pre-Keweenawan area is a large faulted syncline which widens and plunges to the southwest. continuous to Iron River. The north limb is The south limb of the syncline has been subjected to a greater degree of deformation. Secondary folds on this limb maintain a northwestward trend and generally plunge in that direction. Transverse 30 faulting has occurred subparallel to the secondary folding. The beds have been tightly folded producing a very irregu­ lar outcrop pattern. To the northeast of this area the major structure is the Amasa Oval which is a domal uplift. The long axis of the Amasa Oval plunges north-northwest and south-southeast. The major structures in the far eastern portion of the area and to the east of it are three Pre­ cambrian metasedlmentary synclinoriums; the Marquette syncline, the Pelch trough and the Qwinn trough. The first two generally trend east-west, while the third trends northwest-southeast. These are considered areas of extremely complex folding and faulting, probably the result of a period of a major orogeny. The major structural feature in the western portion of this area is the Penokee-Gogebic Range. steeply northward dipping monocline. It is a The most noticeable features are the great thrust fault at Wakefield and the Barb Lake fault which strikes east-west. With the exception of the general northward tilting, the folding and faulting of the Anlmlkle rocks took place prior to Keweenawan time and after the deposition and induration of the Middle Animikie Series. Gogebic is very complicated. The structure around Lake In this area the Animikie rocks are believed to be compressed Into a syncline with the axial plane striking northeast-southwest and dipping to the northwest. CHAPTER IV PREVIOUS GEOPHYSICAL STUDIES Balsley, James and Weir (19^9) published a geo­ physical report including some preliminary interpretations of an aeromagnetic survey of parts of Baraga, Iron, and Houghton Counties. They concluded that the correlation of the magnetic data with the known geology is good in most parts of the area. They also pointed out that the use of aeromagnetic data is one of the fastest and most reliable methods for outlining the areal distribution of the magnetic rock units as well as their structure. Campbell (1952) Investigated the vertical component of the magnetic field and the gravitational field In the Silver Mountain area. He considered the subsurface structure to be quite complex. Bacon and Wyble (1952) conducted a gravity investiga­ tion in the Iron River-Crystal Palls mining district of Michigan. The purpose of their study was to determine the merits of the gravity methods in Iron ore exploration. They concluded that there is some possibility that gravity methods may be able to differentiate between large Iron ore bodies and the iron formation in the Iron RiverCrystal Palls district. They stated that the regional 31 32 gravity work outlines quite clearly the major structural features of the Iron River-Crystal Falls synclinal basin. They also pointed out that a large anomaly occurring about fifteen miles west of Iron River may well be asso­ ciated with a structurally similar basin and consequently may be another potential Iron ore district. Thiel (1956) and Bacon (1957) Illustrated the rela­ tionship of Bouguer gravity anomalies to geology of the south shore of Lake Superior. Thiel correlated the "Mid- continent gravity high" with the gravity anomalies over the Keweenawan of Wisconsin. The regional gravity study by Bacon showed a similar high in the Northern Peninsula of Michigan. Bacon (i9 6 0 ) advanced the hypothesis that a major fault parallel to the Keweenaw fault exists in approximately the central poriton of the Jacobsville sandstone. Bacon (1966) on the basis of geophysical data explored more fully some of the structure in the area to the east and south of the Keweenaw fault. He suggested that a Middle Range of basalts lies beneath the Jacobsville sandstone, and that It may be either the northern limb of a shallow symmetrical syncline plunging to the west, with the South Range as the southern limb, or that the Middle Range lavas are a h o r s t . He also Indicated the possibility of another fault parallel to the Keweenaw fault In the graben area between the Middle Range and Northern Trap Range. 33 Wold (1966) flew 7,500 miles of magnetic traverses over the Lake Superior region. This survey consisted of 37 north-south oriented profiles, spaces at six mile inter­ vals (Figure 3). Some portions of these profiles cover the area of investigation. 3,000 Flight elevation was at feet above sea level as determined by a standard aneroid altimeter. A U.S. Navy P2V-5 (Neptune) aircraft, instrumented with a Wold (196*0 digital recording proton precession magnetometer system was used for the survey. He only correlated in general the results of his survey with the Keweenawan. lavas and elastics. Case and Weir (1965) in their aeromagnetic study of parts of Marquette, Dickinson, Baraga, Alger, and School­ craft Counties, Michigan, covered a small strip of the extreme eastern portion of the investigation area. They correlated the major magnetic anomalies and broad areas that have characteristic magnetic patterns with the geology as determined from published reports. They defined the following six district group of anomalies or anomaly patterns: 1. The iron formation in the Animikie series is shown by magnetic highs of large amplitude up to 27,000 gammas. 2. Westward-trending reversely magnetized diabase dikes of Keweenawan age are shown by prominent elongate magnetic lows of moderate to high amp li tu de . n* Fig.3 . A e r o m a g n e t i c flight lines flown by t h e U n i v e r s i t y of Wisconsin. ( Af t e r Wo l d, 1 9 6 6 ) 35 3. Mafic intrusions into Pre-Animikie basement rocks northwest of Ishpeming are the source of a group of large magnetic highs. The basement gneiss is characterized by a pattern of discontinuous highs and lows of low to moderate amplitude. 5. Intrusive greenstone in the basement gneiss causes, in some places, Isolated magnetic highs of moderate amplitude, but in* other places, it is apparently only weakly magnetic. 6. Where Precambrian rocks are covered by 2,000 feet of Lower Paleozoic sedimentary rocks, large magnetic highs and lows with relatively flat magnetic gradients predominate. Miller (1966) conducted a gravity investigation of Porcupine Mountains and adjacent area. He concluded that, in general, the gravity map correlates very well with the geology as mapped by White (1962). from gravity profiles observed over there is He further indicated the Keweenaw fault that a decrease in the throw of the fault from north­ east to southwest, varying from 6,000 feet north of Ewen, to 3,000 feet west of Lake Gogebic. He also indicated that the dip of the fault appears to be to the south, instead of to the north as previously postulated. Several reports have been published by the U.S. Geological Survey on limited portions of the area, 36 involving aeromagnetic studies and their geologic inter­ pretation; these include: 1. Qair and Weir (1956) in their study of geology of the Kiernan Quadrangel, Iron County, Michigan. 2. Bayley (1959) in his study of the geology of Lake Mary Quadrangle, Iron County, Michigan. 3. Weir (1967) studied the geology of Kelso Junction Quadrangle, Iron County, Michigan, based partly on the aeromagnetic map of the area. M. Prinz (1967) studied the Pre-Quaternary geology of part of the eastern Penokee-Gogebic Range, Michigan, utilizing results from ground magnetic surveying. CHAPTER V AEROMAGNETIC MAPS Aeromagnetic maps of the area of investigation were obtained from the U.S. Geological Survey through the Geological Survey Division of the Michigan Department of Conservation. The aeromagnetic maps are divided Into two g r o u p s . The first group that covers the area east of longitude 88" 30' W, west of longitude 88° 07* 30" W, and south of latitude 46° 25* N, was mapped at a scale of 1:31,680. The flight traverses for this area were flow in an eastwest direction because in this area most of the geologic units and structures, such as the Amasa Oval and the rocks of the Lower Precambrian of the South Marquette Range, strike approximately north-south. The second group of maps cover the rest of the study area. These maps were mapped at a scale of 1:62,500 and the flight traverses were flown in a north-south direc­ tion which is perpendicular to the general strike of the trend of the geologic structures In this part of the area. The contour Interval of these maps varies between 50 gammas, which is the basic unit, to 1000 gammas depend­ ing on the intensity of the magnetic field. 37 38 A constant flight elevation of 500 feet from the ground surface was maintained throughout the study area, although this might deviate in areas of rugged relief. Traverses were spaced at intervals of approximately onequarter of a mile. The flight path of the aircraft was recorded by a gyrostabilized continuous strip camera, and the elavation was continuously recorded by a radio­ altimeter. Magnetic measurements were made by an AN/ASQ-3A fluxgate magnetometer. Base lines were flown perpendicu­ lar to the flight traverses in two directions to obtain data to correct for diurnal magnetic variations. The aeromagnetic maps for the area were assembled and compiled together to a common arbitrary magentic base level, and one common scale of 1:62,500 (Plate I). Available geologic information about the area, also was compiled to this scale, in order to facilitate the correlation of the magnetic anomalies with the areas of known geology. CHAPTER VI MAGNETIC PROPERTIES OP ROCKS Introduction The purpose of this chapter is to discuss and sum­ marize the magnetic properties of the rock types in the area as a basis for the analysis of aeromagnetic data. The most important magnetic property of a rock is its magnetic susceptibility substance is magnetized. (K), the ease with which the Magnetic susceptibility is affected by many different factors but the major factor is the volume fraction of the magnetic minerals in the rock. Other factors such as field strength, state of magnetization, grain size, fabric of rock, temperature and pressure, may affect the magnetic susceptibility. However, they are of limited consequence in practical application. The magnetic susceptibilities of rocks exhibit a very wide range. Sediments, excluding iron formations, are relatively nonmagnetic and are considered to have zero magnetic susceptibility. The magnetic susceptibility of igneous rocks generally range between 100x 10*“^ e.g. s. units as represented by the lowest rank in the acidic group of rocks such as granite, and 10,000X10” ^ e.g.s. units for basic rocks. 39 The magnetic polarization (I) of a rock unit is determined not only by its magnetic susceptibility, but also by the strength of the inducing geomagnetic field (H). Thus, (1 ) I - K H where, H is the e a r t h ’s magnetic field which is 0.595 oersteds in the study area. In the last decades, it was discovered that the remanent magnetization (1^ ) , which may be present in rocks, greatly affects the magnetic properties of the rock. In such cases, the total magnetic polarization of the rock is given by: (2 ) where —^ 1^ is the induced magnetic polarization I j^ j Is the remanent magnetic polarization The ratio of remanent magnetization to the induced magnetization (KH), is given by Q, which Is known as the Konigsberger ratio. m Induced Magnetic Properties Rock samples representing most of the llthologles in the area were collected for magnetic susceptibility measurements. two In collecting these samples, the following conditions were taken inconsideration: a. Only unweathered samples were collected. b. Representative samples were collected from ferent outcrops of dif­ the same rock type and samples of each rock type in an outcrop were selected for analysis. Magnetic susceptibility measurements for 120 samples were conducted in the laboratory using MS— 3 magnetic susceptibility bridge, manufactured by the Geophysical Specialties Company. The calibration range of this instru­ ment extends from approximately 2X10” ^ to *40,000X10""^ e.g.s. units of volume magnetic susceptibility, which covers the ranges of susceptibility of the rocks encountered in the area. The upper limit can be extended to beyond 100,000 X10“ ^ e.g.s. units by simple techniques. Relative accuracy on a homogeneous sample is about one per cent. The absolute accuracy is somewhat lower, of the order of five to ten per cent. The MS-3 magnetic susceptibility bridge does not measure remanent magnetiza­ tion nor will measurements taken with the MS-3 be affected by the presence of remanent magnetization. 42 The principal of operation may be understood by reference to Figure 4, which shows a cutaway view of the sample holder. Three co-axial coils are spaced vertically along a cylindrical form. This form is machined from phenolic resin which has been selected for its high thermal and mechanical stability. Alternating current at an audio frequency flows through coils A and C in series in a direction such that the magnetic fields which they produce will be effectively cancelled at the position of coil B. From Figure 4 it may be seen that the sample holder opening passes through coil A and downward to Coil B. When a rock sample is introduced into the sample holder, the magnetic coupling between coils A and B will be increased, whereas the coupling between coils C and B will be relatively unaffected. The amount of the unbalance will depend upon the susceptibility (K) of the sample and can be measured by means of an alternating current bridge. An oscilloscope was used in place of the headphones as a means for detect­ ing the condition of balance. The amount of balance is measured in terms of (AR) which is converted to magnetic susceptibility either by a calibration curve when (AR) was greater than 500 ohms, in other cases for values of (AR) less than 500 ohms, the magnetic susceptibility was obtained by: K - 3.57 X AR X 10" 6 e.g.s. (3) (K) 43 Fig.4* C u t a w a y view of the sample holder. This quantity which has been computed is the apparent susceptibility. To obtain the true value of "K” these values were corrected for: a. Diameter correction: The calibration curve applies only to a solid sample of diameter I.I87 of an inch, and of length 3 inches or greater. Since a sample tube of 1.08 inch internal diameter was used, all the readings (AR) were multiplied by: i±±ml. (1 .08)2 b. .1.312 1,165 Air space correction: The reading corrected for diameter was multiplied by the correction factor (C), where c m True density of sample material Apparent sample density /jo } Instead of measuring the true sample density and the apparent density directly, a volumetric method was used for determining the ratio of the two quantities. Table *4 shows the range of magnetic susceptibilities as well as the average value for most of the Precambrian rocks in the area. The following conclusions can be drawn from this table: TABLE 4.— Magnetic susceptibility measurements of Precambrian rocks in the area. „ II. f Range of Volume Average Volume Keweenawan rocks sediments acidic flows basic flows basic intrusives acidic intrusives 18 5 4 6 2 11-48 143-1000 1220-1773 1881-9730 31-56 44 21-112 58 12 64-752 l4l 32 438 1561 5683 43 ■Keweenawan rocks Animikie rocks a. metasediments b. limonitic & hematitic iron ore c. metamorphosed iron formation d. stambaugh formation e. metabasic intrusives f. acid intrusives "Archean" rocks a. acid intrusives and gneiss b . greenstones 1 1 2 9 9 8 4230 10260 72-79 8-1000 11-86 10-57 4230 10260 76 121 40 37 46 1. Sediments (excluding iron ores), greenstones and acid intrusives have the lowest magnetic sus­ ceptibility values in the area. All sediments and greenstones have a susceptibility of less than 100X10""^ e.g.s. 2. units. Due to visible magnetite content in a hand specimen of granite the volume susceptibility value reached 1000X10 3. — 6 e.g.s. Llmonitic and hematitic iron ores and acidic lava flows have rather moderate values of magnetic susceptibility. 4. Basic flows and intrusives and metamorphosed iron formations have relatively high magnetic susceptibility. 5. The Stambaugh formation wh ich is mainly a magnetic slate has the highest value of magnetic susceptibility. 6. Metabasic intruxives show remarkably low magnetic susceptibility v a l u e s . Remanent Magnetic Properties The magnetic properties of the Keweenawan rocks of the Lake Superior region have been studied in more detail than the Pre-Keweenawan rocks due to the heterogeneity of the latter. Cox and Doell (I960) and Irving (1964) have compiled the results of these studies. 47 Dubois (1957) showed that cobbles of basalt of Keweenawan age have random magnetic orientation, and that they have acquired a stable magnetization. He also showed that the main cause of the magnetization is thermoremanent magnetization. By reconstructing the original dip of the Keweenawan volcanics, Dubois (1962) and Jahren (1965) gave an average inclination of +45° and a declination of approximately 285° for the remanent magnetic field of the Keweenawan lavas. These are the values used for quantitative inter­ pretation of magnetic profiles in this study. Graham (1953) has reported on the paleomagnetism of certain dikes from Baraga County, Michigan. These and similar dikes occur in the eastern part of the study area as an east-trending swarm. The dikes were found to have consistent magnetization with declination of 90° and steep upward dip of -87°. Graham explained the origin of the reverse magnetization of these dikes to be due to partial oxidation of the magnetite of magnetite-ilmenite inter­ growth to maghemite which has been magnetized in the demagnetizing field of the magnetite. Subsequent easier demagnetization of the residual magnetite on account of its larger grain size took place. Dubois (1962) correlated these dikes with the Logan sills of Canada due to their common computed pole positions. Due to the fact that the dikes intrude Animikle type rocks 48 and are overlain by the Jacobsville sandstone, he con­ sidered them to be of Keweenawan age. HInze, O'Hara, Secor, and Trow (1966) summarized the magnetic properties of the rocks of the Lake Superior region reported in literature including the studies of Mooney and Bleifuss Cox and Doell (1953)* Bath and Schwartz (I960), Jahren (1960-1963), Bath Irving (1964) and Case and Qair is given in Table 5. (I960), (1965). (1962), Their compilation The measured values shown In Table 4 agree well with the ranges of magnetic susceptibilities shown In Table 5Prom the previous discussions and by analysis of the aeromagnetic maps, the following generalizations appear valid for the rocks In the area. 1. High amplitude anomalies are expected to be associated with iron formations and basic i n tr usives. 2. All sediments of all ages should give the lowest magnetic intensity level In the area. 3. Keweenawan lava flows are expected to give high magnetic response but not as high as mag­ netic iron formations. 4. Acidic Intrusives and greenstones are to show some magnetic Irregularities. expected TABLE 5.— Summary of magnetic properties (after Hinze, O'Hara, Secor and Trow, 1966). Rock Type IV. III. Paleozoic sediments Negligible sediments basic flows basic intrusives acid intrusives and flows Negligible 10 ,000-1,000 9 ,000-2,000 3,000-100 Negligible 3 .0-1.0 2 .0-1.0 ------ 3 ,000-100 4,000-200 900,000-500 200-0 Generally Low Negligible Variable Variable Pre-Keweenawan Rocks: a. b. c. d. I. Negligible Keweenawan Rocks: a. b. c. d. II. Susceptibility KX10- c.g.s Konigsberger Ratio Q = IRM/KH H = 0.6 Oersted acid intrusives and gneisses metabasic intrusives and flows iron formations metasediments Undifferentiated Precambrian 2 .0-0.5 10 .0-0.0 CHAPTER VII METHODS OP INTERPRETATION Introduction Many methods of Interpreting magnetic data have been developed over the past thirty y e a r s . Some of these methods are applicable only under certain conditions. Others are "Rule of Thumb" methods that apply only where certain assumptions are known to hold true. In this study, selected depth determination tech­ niques were used in interpreting the aeromagnetic anomalies. These included the half-width method (Nettleton, 19^2), D. W. Smellie's method (1956), P e t e r s ’ slope method and V a c qu i er ’s method (19^9), (1951)* A second vertical derivative map of the total mag­ netic intensity was constructed for the central portion of the study area using the method of Henderson (I960). The purpose of the second vertical derivation map is to aid in determining depths by Vacquier's method and to assist in locating the contacts of magnetic rock units. A magnetic trend analysis also was carried out to define the major tectonic trends in the area. 50 51 Depth Determinations The depth to causative geological bodies is one of the most important parameters that must be determined in order to interpret adequately the geology and the struc­ ture of an area. A variety of magnetic depth determination techniques have been discussed in the geophysical litera­ ture. Each method has its own assumptions, as well as limitations. The half-width technique, D. W. Smellie's method and Peters' slope method were applied to most of the anomalies in the study area. Vacquier's method was applied to only a relatively few anomalies. All of the methods are based on the assumption that the magnetization of rocks is only induced, i.e., no remanent component is present. Therefore, the depth determinations may be subject to error because most of the basic intrusives and extrusives, and iron formations in the area have a strong remanent magnetic c om ponent. Methods The Half-Width Method The half-width method utilizes a magnetic profile, the principal profile, taken perpendicular to the strike of the anomaly at or near its maximum amplitude. The depth to the causative body is related to the "half-width" 52 of the profile curve at one-half the maximum amplitude. This distance is marked X2./2 on P:1Sure 5* In general X^yg must be multiplied by a constant factor depending upon the shape and length of the body and its depth extent. The factors for the Idealized bodies that were used to approximate the causative masses in the study area are as follows: Dipole Appro xi ma ti on s: Sphere Horizontal Cylinder 2.00 2.05 Pole Appro xi m at io ns : Narrow Vertical Dikes Vertical Cylinders 1.00 1.00 This method is based upon vertically induced magnetic polarization. This condition is not exactly met In the survey area, but is approximated due to the high magnetic Inclination (76°). Great care must be exercised in determining the zero level of the anomaly which will affect the maximum amplitude and accordingly the X.jy2 measurements. anomalies Also Interference from neighbouring will have a profound effect on the measured 2 distance. S m e l l i e r Method Henderson and Zeitz (19^8) studied the relationship between the total magnetic field Intensity anomalies and the point pole and line of poles sources. Theoretical 53 Zero level of a n o m a l y Fig.5. HaUf- width m e t h o d of depth determination. 54 profiles were examined for maxima and minima and it was established that the depth is a linear function of the half-maximum abscissa. They calculated factors for depth determinations based upon this parameter. Smellie (1956) modified this work and derived the total magnetic expressions for four simple sources: the point pole, line of poles, dipole, and line of dipoles. He also worked out theoretical curves for depth factors for all these cases. These curves were established for different geomagnetic latitude (I), for bodies parallel to the magnetic meridian and those at an angle (3) to it. In this regard, Smellie's method can be looked upon as the half-width method corrected for both magnetic latitude and orientation with respect to the magnetic meridian. Magnetic bodies of limited horizontal extent, but of great depth extent can be approximated with a point pole. However, narrow dikes which have great horizontal and vertical extent are approximated with a line of poles. The dipole approximations are used for spherical bodies and line of dipoles for bodies which approximate a hori­ zontal cylinder in shape. For the pole approximations the estimated depth is to the top of the body causing the anomaly, whereas for the dipole it is the depth to the center of the body. As a general rule, the Interpretation of anomalies characterized by nearly circular contours may be approximated using a pole or dipole, and elongated 55 anomalies can be interpreted using a line of poles or line of dipoles approximation. The dipole or line of dipoles approximation is used in those cases where the anomalies exhibit a definite negative magnetic anomaly associated with the positive anomalies. The applicability of these approximations depends to a large extent on the dimensions of the source. Sources which are wide, compared with their depth will give depths that are too great. This is also the case with complex sources consisting of several closely spaced anomalous bodies, whose effects merge to give a single anomaly. In cases where the isolation of anomalies is not complete, error is anticipated because of the difficulty in determin­ ing the relative maximum amplitude of the anomaly. Serious error also may be caused by making the wrong approximation to the shape of the source. Errors also may rise in original plotting or contouring of the data. Peters1 Slope Method L. J. Peters (19^9) has developed several methods of depth determinations. The most commonly used of his methods is the "slope" method. It relates the maximum gradient of the anomaly to the depth of the top of its source. 1. Peters' method is based mainly on two assumptions: The anomalous mass is in the shape of an infinitely long slab with vertical sides, extending infinitely downward. However, it 56 has been found that the sides can deviate from the vertical by 10 degrees without introducing serious errors in estimated depths. 2. The source of the anomaly is vertically polarized. Peters1 method is applied to the principal profile of the anomaly, Figure 6 illustrates this technique. The inflection point is located at the maximum slope of the anomaly profile. A tangent is drawn to the inflection point (Line A), and measures its slope and two tangents (C,D) are drawn to the anomaly curve which are parallel to B. The horizontal separation "S" is approximately related to the depth (Z) by the formula S - 1.6 Z (5) This relation only holds true, where Z and T (the width), are about the same magnitude, i.e., T/Z»l. When T = 0, S - 1.2 Z (6) When T = «*, S-2.0Z (7) Equation (5) is generally used in cases where "T" is neither zero nor infinity. The accuracy using equation (5) is as follows: (a) ±f 0 < T/Z £ 0.5 Poor results are obtained. (b) If 0.5 < T/Z < 1.1 Good results are obtained. (c) If 1.1 < T/Z Excellent results are obtained. 57 ,-Tangent of m a x i m u m Slope Inflection point Z 1 Fig. £ Peters* slope m ethod of depth determination. 58 Greater depth values are expected from Peters' method In cases where the sides of the rock mass are sloping downward and outward. Shallower values of depth are expected In cases where the sides are Inward-sloping or where It Is applied to anomaly slopes between two adjacent masses that overlap. Any Inaccuracy In contouring will have a great effect on the slope of the anomaly, as well as the Inflection point, which affect the results accordingly. In spite of the previously mentioned limitations, Peters' method utilizes data fairly close to the apex of the anomaly, thereby avoiding some of the influence of neighboring anomalies. Another advantage to it, is that it does not require definition of the zero-level of the anomaly. Vacquier's Method A second vertical derivative map for an area of about 3,700 square miles in the center of the study area was constructed from the total magnetic intensity map. The numerical analysis procedure of calculating derivatives developed by Henderson (i9 6 0 ) was used with a mesh inter­ val of one-half mile. The main purpose of the second derivative map was to carry out depth determinations using Vacquier's method. Also the derivative map was used as a guide for mapping lithologic contacts because 59 the zero curvature approximates the boundary of the geologic source. Vacquier's method is initiated by comparing the observed with the computed magnetic effect of idealized bodies, which are rectangular prisms with vertical sides extending infinitely downwards. The prisms are considered similar to large lithologic units in crystalline rock with polarization in the direction of the present earth's m ag ­ netic field. The susceptibility contrast constant for the magnetized bodies. (K) is assumed The models are measured in terms of depth of burial to the top of the prism. Dimensions are expressed in terms of "n x m " > where n is the side of the prism more nearly parallel to magnetic meridian and m is the side of the prism, p e r­ pendicular to n. Depth indices are determined by measuring the ho r i ­ zontal extent of the steepest gradient on the total m a g ­ netic intensity map and the second vertical derivative map. The same procedure is used to measure the depth indices of the prism model. The depth is estimated by dividing the depth indices of the observed anomaly by the depth indices from the prism model. The A and 0 indices were determined for most anomalies from the second vertical derivative and total intensity maps, respectively, because of their independence of the size of the prism model. 60 The magnetic susceptibility (K) of the anomalous prism was calculated by applying the relation K - ATm/ATc T • (8) where K is the magnetic susceptibility contrast AT is the maximum amplitude of the observed anomaly in gammas AT is the maximum amplitude of the prism model anomaly in g a m m a s , and T is the earth*s magnetic field intensity in gammas. Results The half-width method and Smellie's method were applied to most of the anomalies in the study area. Peters' slope method was applied only to those anomalies which approached two dimensionality. The location of all the anomalies used for depth determinations is shown in Plate II. The results obtained from the methods, the assumed source for the causative mass, and the bedrock geology of each anomaly are shown In Table 6. It Is clear from Table 6 that the depth estimates by the various methods are in a fairly good a g r e e me nt . The differences can be attributed to natural conditions which differ markedly from the assumptions on which the methods are based. 61 T A B PE 6 ■ — i'epth (lelermluat i o n s . * Anomaly Mo. (2) Hal f w ldth method (fe e t) (3) I , . I). lime 1 1 l e 1s method (fe e t) 1 '} 1 4 6 6 7 8 9 10 11 l,.1 11 1 'i 15 16 17 18 19 70 .'1 in 1 74 i r( 76 77 7H 79 30 •1 •v *3 ;4 36 i6 37 i8 39 1)0 '11 42 43 44 HO 46 **7 48 49 90 51 5? 53 >39 55 56 57 68 59 60 61 62 63 64 65 66 67 68 69 70 71 656 550 9 96 17 8 6 1600 1600 1 390 650 1 30 995 — 9 69 0 ,’7 30 1 390 3910 2 35 7 36 866 655 660 39o 996 3910 996 2766 7070 946 3780 665 4010 4170 7900 760 6 60 656 51 7 0 7 860 3070 7 35 665 390 560 2440 1075 1390 865 760 760 25 235 3280 445 2335 1920 656 550 1300 172 130 235 656 2 36 288 1390 2070 393 340 2 35 445 135 76 61 3 473 294 1 305 17 67 1709 i 4 86 323 553 737 3596 3798 2798 1777 3864 396 768 56 3 3«7 566 1 46 469 4605 471 7701 7167 498 3 395 661 4 7 89 4076 6 399 49 3 4/7 668 4733 7610 7716 1 65 591 311 635 2464 107 3 1249 989 763 670 40 273 3400 535 7353 1443 653 59 2 1416 225 150 261 8 33 303 416 1984 1975 472 493 493 524 493 166 (1) ( 4 ) P e t e r *a s 1o p e n e t hod ( feet ) -__ 1 2 81 — 197 8 1797 — — — 3740 3740 21 7 0 1 166 3110 -— — — 616 — 660 3700 660 2170 7170 470 7848 747 1097 4 0 1>4 7111 6 80 — — 4 7 f. 0 7917 79 17 ----------7190 1 340 1600 — 747 812 — — 2700 __ 7460 1 7 37 -500 1010 __ — __ __ __ 17 9 7 1403 _ — _ _ _ ---- (7) (€) V i v - q u l e r r r; method (feet ) — 1 — — — — — — — — — — — — — — __ — — — — -.'6^6 9 '? tt — — a i ii — — — — — — — — — __ — — — — — .— — — — — ... __ — __ 1*1 __ __ ---—— _ 097 befirot k source get. l o g y ■■tr, r u m e d p . p . , M) p.p. P.P. P.P. Wort h e r n ft ft .1 ct I ' u b r . v 11 I e ft tf 7'r a p fl ft sandstone M t» Ml c h i gami ne Mange fl 11 (M iddle It 11 Trap 11 It alain p.p. ti ■1 h.r. tr ft p . p . ft It p . p . , ft ) p . p . p (t II tt P . P . P. ti fl P.P .P . ft h .h .P . J f t C d b s v i 1 l e s a n d s t o n e O ’o u t h T r a p P . P . P. B adw atcr g re e n sto n e h. P . it «t P.P. If tt P.P. 11 tt P.P. tl m P . 1‘. tl t» P . P. fl ti L .P . iro n form ation t il v e r t o i P .P .P . .It s m b a u g h f o r m a t i o n P. P. 111 e h 1 g a mm e s l i d e P.P.P . tt tl P .P .P . tl fl P. P . II ft P .P .P . ft •1 P.P. II II P .P .P . 11 ft P .P .P . II It P.P .P . 1 1 e m 10 r k f o r m a t 1 on P. P . II II P .P. H i i ’ h i g amme a l a t e P .P. K a r t uik> J. n k r s 1 .jt.f P.P.P. 11 n tt P.P.P. M *) " P.P.I . B.tdwat e l g r e u n B t n i i u P.P. Heml0 0 k f o r m a l 3on P.P. tt 11 P.P. 11 11 P.P. K* i r t L i n u P u k e s l a t e P .P .P . Hem lock f o r m a t Io n P . P. II it P.P. II II P.P. It 11 P.P. It II P.P. 11 •1 P.P. II If P.P. II II P.P .P . 11 II P.P. M l c h i ga mi ne s l a t e P.P .P . ft It P.P. tl II P.P. II II P.P. II tl P.P. M e t a t u f f ami m e ta se d im e n ts P.P. tl 11 P.P. II 11 P.P. 11 II P.P. II 11 P.P. 11 II P.P. 11 11 p . r>. p . II II p .n .p . 11 11 p .p . 11 11 P.P. 11 II p . p . II II p.p. II II p.p. II II p . p . Range) If ft Range) 62 ijt'Jf-.I. rfjf'111o 1r 1 , O!, \ / r> jf. ii ■;1 1 V) 9 1 0 7'i ? l9 .'39 2 39 990 ? 39 1 3*10 1P i 39? L' .O -,9 1 0 ■i 1 7*4 ■|. 7*7 PL99 91 8 6'P , 970 fid 9 n ', 869 6 1, 1, 990 ■' 7 9 9 7 1 79 J (; Pi BP ‘ H.Jt i (, !; . ■9 19 9 1 9P 9 l aP 'Jo 96 97 Jp. 99 19 0 li,: i 1, i tiJi l no . no I" ! io 1:: i: ■1 h -.ail ■9 Pro •do 7 1 ... H i mp . •• 970 . i ’i 100 1.0 ■: -— 4i9 i -.1 1 1 1. p I. 1 . 1-‘1 1P‘ 1. t, 1. 7 ! .'9 1. I ia ) [b) fe ) 1.. i,. i,. j ] hc 11^0 BLS L. L. — — — -- - — — — — - — --- - /7liJ . — — - — — —— — — - — — — 7 37 47 M ; "■- —*“ - - ?^h h [t 3 — -- _ — — — - --— n i p o l e m a s s ^•;y 991 M odel h a l f - w f o r l d l h p o l e a n d S m tl i t i ii i i ii n n ii i i n Metatuff and metasedlmont al* J.'ieiitsvl lie sand tone a 1' a northern Trap Ban .lout h Trap Range Ironwood Iron formation -P. N orthern Trap Range a u PxB ?x6 Px8 Px6 Px6 Px8 e l l i e ' s a p p r o x i m a t i o n s , m e t h o d s w h i l e i t a r e I s from the ground surface to the center of the a p p r o x i m a t i o n s . •d . t n l n t d I'lmm la-tt-rs1 and Vacquier's methods i , e t i p. i d ’ t l i i 1 I ' - i i h i i t l " t ; Id h e i.f ph J e t P.P. p o i n t 1 .P.P. - L i n e II * 1• I* . La lit.tie 1 MotJr tl H 'nie 1 Modol litjriel 17?6 ii Iron formation Granitoid gneiss Graywacke near Bonner Lake La Ii. La ll. !i. ll. L. 1*. P. La i 1a I . l a. P. P. I.. 1,. Li. La I 1a I. a I, a li. La L. La I-. P. •-'-u8 1 g eo lo gy ii L* — Bj l ■; 'i ^ — - 1 J hit 7 h( *~ I -H) t i , B edrock Granitoid gneiss L. L. L* 11. L. I.* ]., Ira la. L. Ii. Ira P. — 6 *;* — — ,d’ tie 'ulue.-i 1 . ; . — — - - o h 1! . i n s a t l v e i- a.-numei] -Mi.' .>9 I r ' 3 : ho ‘, 7 i j* PB'J J Pup Hi valiu a obtained Iron iLc top ■f iIn.- ■■ l.i Cjuri'O (fc?f t > ] 1 'J0 0 1L ---- -... 1 ij'. l __ b iir moth'ifi i . 1*7 - — ---- ' 1 ‘, 1* ... -- p.. In P'-ptL I ■' s rietatuff’ and rietancdirnents • ' i 'i0 pod 31 1 I 306 1 P.d* P76 69? 99? HI ? ?09 1 P 39 ?p 1 380 030 39 9 ? 3 0 H7 906 H ?9hP ‘ .1 7 99 P 900 987 109 79? 99? 9 8H P I '18 .'9H 9 1 -;.-H 1 i 3 811 1 f PH 9 | 9 - 11 L9 1 1■‘i ; . t, < . ir 1 i f. * * ■1 9 1 1 ni l i ] TIP *'i i Poo ■ ;■ 1n . lo j 1, , , 1n . 1. 1i. ! 1 1 1 •• 1 + 1o . 1 l tfi (7) (C) ('■) V n o q IJ I r p o l e d' h i P . >! c s m uss. Indicate the depth from the 63 The depth determinations for anomalies number 123 to 129 were calculated using Vacquier's method alone. These anomalies are not sufficiently well-defined on the total magnetic intensity map, to utilize the other depth deter­ mination methods. However, they are well-defined on the second vertical derivative map and, therefore, could be analyzed by Vacquier's method. Table 7 shows the calculated volume magnetic suscep­ tibility of the causative source for the anomalies analyzed by Vacquier's Method assuming only Induced polarization. It is to be noted that the magnetic susceptibility values obtained from this method are quite high compared to the values obtained from laboratory determination or the previously published data. The reason for this is mainly attributed to the fact that in these calculations, the amplitude of the observed anomaly (ATm ) was assumed to be only due to induced magnetization. This is not true because most of the magnetic formations in the study area have a remanent magnetization component. Correlation of Estimated Depth Values with Bedrock Geology General It Is interesting and informative to correlate the depth determinations with the bedrock geology. This corre­ lation has been made for both the Keweenawan and the PreKeweenawan rocks and the results are summarized in Table 8. 64 TABLE 7.— Magnetic susceptibilities determined by Vacquier's method. Volume magnetic Susceptibility (K) in 10-6 e.g.s. units Model Used AT 5 2x6 2.75 200 1,200 26 2x6 2.75 4500 27,300 28 2x6 2.75 3250 14,500 29 2x6 2.75 450 2,700 33 2x8 2. 00 2100 17,500 64 2x6 2.75 4750 28,800 70 2x8 2.00 2250 18,800 71 2x6 2.75 1250 8,100 72 2x8 2.00 400 3,300 76 2x8 2.00 2500 20,800 77 2x8 2.00 560 4,700 78 2x8 2.00 1500 12,500 81 2x8 2.00 250 2,100 91 2x8 2.00 450 3,800 123 2x8 2.00 2250 13,700 124 2x8 2.00 800 6,700 125 2x6 2.75 750 4,500 126 2x8 2.75 750 4,500 126 2x8 2.00 300 2,500 127 2x6 2.75 350 2,100 128 2x6 2.75 200 1,200 129 2x8 2.00 200 1,700 Anomaly Number c AT m A value of 59>500 gammas was considered for T (Earth's magnetic field). TABLE 8.— Correlation of depth determinations with bedrock geology. Pole Approximation Depth Range in Feet Dipole Approximation Number of Anomalies ----------------------------PreKeweenawan Keweenawan Rocks Rocks No. % No. Number of Anomalies Depth Range -------------------------------in Feet PreKeweenawan Keweenawan Rocks Rocks S Mo. X No. % 12 31.6 36 59.0 0-1000 0 0.0 0 0.0 501-1000 6 42.1 17 27.3 1001-2000 0 0.0 4 19.0 1001-2000 6 15.8 8 13-2 2001-3000 0 o 9 42.9 2001-3000 2 5-3 0 0.0 3001-4000 2 100.00 3 14.3 3001-4000 1 2.6 0 0.0 4001-5000 0 0.0 4 19.0 4001-5000 0 0.0 0 0.0 5001-7000 0 0.0 0 0.0 5001-6000 1 2.6 0 0.0 > 7000 0 0.0 1 4.8 38 100.0 61 100.0 2 100.0 21 100.0 • TOTAL o 0- 500 66 It should be noted that the source of anomalies that are at a depth of less than 500 feet could be outcropping. This error in depth determinations may be attributed to errors in measuring anomaly characteristics or may originate In deviations from the 500 foot flight elevation, especially in areas of rough topography. Keweenawan Rocks It is evident from Table 8 that most of the Keweenawan rocks in the study area could be approximated by either a point pole or a line of poles. This suggests that such rocks are mainly in form of narrow dikes that' extend to a considerable depth or in form of thin sheet like bodies. The Keweenawan lavas of the Northern Trap Range occur at or near the surface along its entire length across the northern portion of the study area. However, they may reach a depth of 1^400 to 2500 feet at the southern border of the Iron River syncline. South of the Keweenaw fault, in the central and western portions of the study area, a narrow band of Keweenawan rocks of the Northern Trap Range appear to be covered by a thin wedge of sediments. The volcanics of Middle Trap Range are buried beneath 1250 to 2500 feet of Jacobsville sandstone. ever, they outcrop in the Silver Mountain area. How­ This variation in depth to the Middle Range is believed to be due to several cross faults with considerable vertical displacements. 67 The South Trap Range Is outcropping in the western portion of the study area (west of Lake Qogebic). East of Lake Qogebic it outcrops occasionally along its exten­ sion while in some other parts it is covered by 3*000 to 5,000 feet of Jacobsville sandstone. Pre-Keweenawan Rocks Table 8 shows that sources of anomalies within the Pre-Keweenawan rocks can be approximated either by poles or dipoles and that, although the majority of the depth estimates are shallow, the sources have a wide range of depths. The Michigamme slate contains more than one possible source of anomalies at different depth levels. Those causative masses that are near to the surface could be either a magnetiferrous slate member or outcropping basic intrusives. Another group of anomalies has a depth range between 1500 and 4000 feet. The source of these anomalies is believed to be due to basic intrusives and extrusives that lie at the base of the Michigamme slate. It is worth noting that the Michigamme slate includes the anomaly with the deepest source in the entire study area. This source reaches an approximate depth of 7000 feet. The metatuff and metavolcanic rocks that occur in the western portion of the area around Lake Gogebic are mostly outcropping or at few hundred feet from the surface. However, due to local structures these rocks produce 68 anomalies which originate at depths up to 1500 feet from the surface. The source of anomalies in the Hemlock formation and Badwater greenstone are either outcropping or lie very near to the surface. However, in the Hemlock formation some anomalous sources occur at a depth of 1400 to 2000 feet. The Fortune Lake slate seems to include a deep magnetic source that has a depth range between 2500 and 4500 feet. There are other scattered anomalies due to sources of limited extent such as those occurring over the Stambaugh formation, Riverton Iron formation, Ironwood Iron formation, graywacke near Banner Lake, and granitoid gneiss. The source rock In most of these cases Is either outcrop­ ping or very near to the s u r f a c e . Magnetic Trend Analysis The use of what has come to be known as "character” in magnetic maps, is a common quantitative approach to magnetic Interpretation. upon the wavelength, The term "character” is based amplitude, grouping of anomalies, and their magnetic trend pattern. It has been shown that trend patterns can be used to define magnetic provinces which reflect tectonic provinces (Affleck, 1963)* The study area was divided into two areas on the basis of regional geology; the Pre-Keweenawan and the Keweenawan areas. On the basis of magnetic properties of 69 rocks, depth determinations, and examination of the aeromagnetic maps, the magnetic anomalies in the study area were divided into the following three main groups, according to amplitude: la First order anomalies, with amplitudes greater than 2500 gammas, which are indicative of: a. Pre-Keweenawan basic intrusives and extrusives. b. 2. Pre-Keweenawan iron f o r m at io ns . Second order anomalies, with amplitudes between 500 and 2500 gammas, which are indicative of: a. Keweenawan acidic and basic extrusives. b. Near surface Pre-Keweenawan basic intrusives. 3. Third order anomalies, with amplitudes less than 500 gammas, which are indicative of: a. Westward trending, reversely magnetized diabase dikes of Keweenawan age. b. Keweenawan extrusives buried beneath Keweenawan sediments. c. Basement gneisses, intrusive greenstones and near surface, slightly magnetic rocks, of Pre-Keweenawan age. The rock types ascribed to each anomaly group is necessarily a generalization. Anomalies associated with specific rock types may fall within more than one group 70 due to varying depths, magnetization or volume of the source. In general, the anomalies align themselves along definite axes, forming '’trends.” The trends for the three groups of anomalies were traced out and marked according to the amplitude classification (Plate II). A simple and standard method of portraying the twodimensional magnetic trend patterns is to construct a frequency plot showing the number of elements lying in various direction ranges (Miller and Kahn, 1962). The study area was divided into squares of two miles on a side. The squares serve to define "elements" of the pattern. The number of elements is equal to the number of squares in which a particular trend occurs. The direc­ tions of the element was measured as an azimuth, clockwise from north, and each element contributed a separate measure­ ment of azimuth. An element that changed its azimuth by more than 5 degrees along its length was broken into two or more separate elements. The number of elements within each area and amplitude group, in each five degrees of azimuth, was tabulated and their frequency percentages calculated. These data are shown in Table 9. The fre­ quency percentage of the total number of elements in the entire survey area as well as in the Keweenawan and the Pre-Keweenawan areas for each five degrees of azimuth also was calculated. These data are shown in Table 10. TABLE 9.— Distribution of magnetic trend elements of different anomaly orders in the Keweenawan and the Pre-Keweenawan areas. Range of azimuth in degrees _______________Keweenawan Area____________________________________________Pre-Keweenawan Area 500-2500 gammas <500 gammas >2500 gammas 500-2500 gammas _ — . , .— ._ . Number of Frequency in Humber of Frequency in Humber of Frequency Humber of Frequency trends S trends 5 trends in % trends in % <500 gammas ■ ---Humber of Frequency in trends % 5 2 1.26 23 1.64 7 4.14 12 2.73 25 3.63 6- 10 2 0.32 12 0.86 2 1.13 1 0.23 10 1.45 11- 15 3 0.47 9 0.64 U 2.37 4 0.91 5 0.73 16- 20 6 0.95 17 1.21 2.37 2 0.46 8 1.16 21- 25 5 0.79 14 1.00 1 0.59 5 1.14 2 0.29 26- 30 9 1.42 13 0.93 r 2.96 5 1.14 2 0.29 31- 35 14 2.21 24 1.71 0.59 c 0.46 3 0.44 36- 40 16 2-52 20 1.43 - 1.13 3 0.63 41- 45 33 5.21 26 1.36 7 j 1 *1 3 6 1.36 6 0.83 46- 50 27 4.26 49 3.50 * 0.59 10 2.27 12 1.74 51- 55 40 6.31 50 3-57 ■fc 0.59 0 1.32 16 2.32 56- 60 43 6.78 67 4.79 1 0.59 22 5.00 27 3.92 61- 65 48 7.57 56 4.00 3 1.73 24 5.46 23 3.34 66- 70 60 9.46 72 5.14 2 1.18 37 5.-1 31 4 .50 71- 75 55 8.68 102 7.79 f) 2.96 35 -.96 43 6.97 76- 80 46 7.26 133 9.60 '4 2.37 22 5.00 24 3.43 81- 85 57 3.99 190 13.57 6 3o5 23 6.36 25 3.63 86- 90 97 15.30 277 19-79 21 12.43 53 13.13 34 12.19 0- 0.24 91- 95 18 2.84 45 3.21 4 2.37 5 1.14 27 3.92 9 6 - 10 0 17 2.68 54 3.86 2 2.18 20 4.55 40 5.81 101-105 4 0.63 28 2.00 14 8.28 13 4.09 33 4.79 106-110 6 0.95 10 0.70 11 6.51 17 3.86 25 3.63 111-115 8 1.26 9 0.64 4 2.37 14 3.19 32 4.64 1 16-120 3 0.47 20 0.14 e 4.73 6 1.36 18 2.61 — 10 0.70 8 4.73 6 1.36 15 2.18 0.3? 7 0.50 4 2.37 4 0.91 11 1.60 — 6 0.43 2 2.18 3 0.63 27 3.92 0.16 4 0.29 6 3.55 3 0.63 12 1.74 — 6 0.43 3 1.73 8 1.32 20 2.90 0.32 1 0.07 5 1.14 10 1.45 — 6 0.43 4 2.37 14 3.13 12 1.74 121-125 — 2 126-130 131-135 — 136-140 141-145 1 — 2 146-150 156-160 — ■ — — 161-165 1 0.16 15 1.07 4 2.37 7 1.59 23 3.34 166-170 2 0.16 9 0.64 7 4.14 6 1.36 6 0.88 171-175 1 0.32 8 0.57 7 4.14 9 2.05 6 0.38 0.16 5 0.36 2 2.37 10 1.45 176-180 — — TABLE 10.— Distribution of magnetic trend elements in the Keweenawan area, Pre-Keweenawan area, and the survey area. „ . .. Range of azimuth in degrees Keweenawan Area Pre-Keweenawan Area Total Survey Area ___________________________________________________________________________________________ '_____________ Number of trends Frequency in % Humber of trends Frequency in I Humber of trends Frequency in 5 31 1.52 44 3.39 75 2.25 6- 10 14 0.69 13 1.00 27 0.81 11- 15 12 0.59 13 1.00 25 Q.75 1 6 - 20 23 1.13 14 1.0? J ■ 1.11 21- 25 19 0.93 a 0.62 27 0.51 26- 30 22 1.08 12 0.93 34 . 1.02 38 1.87 6 0.46 44 1.32 36- H0 36 1.77 7 0.54 43 1.29 41- 45 59 2.90 15 1.16 74 2.22 46- 50 76 3.74 26 2.00 102 3.06 51- 55 90 4.43 24 1.85 114 3.42 56- 60 110 5.41 50 3.S5 160 4.30 61- 65 104 5.11 50 3.35 154 4.62 66- 70 132 6.49 70 5.39 202 6.06 71- 75 157 7.72 . 88 6.73 245 7.35 76- 80 179 8.80 50 3.85 229 6.87 CO 247 12.14 59 4.55 ' 306 9.18 86- 90 374 18.39 163 12.56 537 16.12 91- 95 63 3.10 36 2.77 .59 2.97 3.49 62 4.73 133 3.99 0- 31- 35 IT S CO 1 H 96-100 - 71 % -~4 U) 101-105 32 1.57 65 5.0 97 2.91 106-110 16 •79 53 9.03 69 2.07 111-115 17 .89 50 3-85 67 2.01 116-120 23 1.13 32 2.97 55 1.65 121-125 10 0.99 29 2.23 39 1.17 126-130 9 0.914 19 1.96 23 0.39 131-135 6 0.30 32 2.97 35 1.19 136-190 5 0.25 2 1.62 26 0.73 191-195 6 0.30 31 2.39 37 1.11 196-150 3 0.15 15 1.16 13 0.59 151-155 6 0.30 30 2.31 36 1.03 156-160 3 0.20 26 2.00 29 0.89 161-165 16 0.79 39 2.62 50 1.50 166-170 11 0.59 19 1.96 30 0.90 171-175 9 0.99 22 1.70 31 0.93 176-180 5 0.25 12 0.92 17 0.51 ]_ 75 Figure 7 shows the frequency distribution per five degrees of azimuth of the total survey area, the PreKeweenawan area, and the Keweenawan area. Figure 8 shows the frequency distribution per five degrees of azimuth of the magnetic trend patterns of the three amplitude groups within the Pre-Keweenawan area. Figure 9 shows the cor­ responding distribution within the Keweenawan area. It Is to be noted that the elements of magnetic trend patterns associated with the Keweenawan diabase dikes were tabluated with other elements of the Keweenawan area, though most of these dikes occur within the Pre-Keweenawan area. From Figures 7, 8 and 9 it is clear that there is a dominant east-west trend through the study area for all groups of anomalies. This east-west trend can be regarded either as a basic tectonic trend or as an overprinted pattern on previous ones. Figure 7— C shows two subsidiary i peaks on either side of the major east-west peak associated with the Pre-Keweenawan area. N 72° E and N 102° E planes. They occur roughly at and are comparable to two shear This fact suggests that the acting stress on these rocks at the time of their formation or right after was probably of a nonrotational nature (pure shear). Assuming that the rocks were ductile, as evidenced by folding, it can be concluded that the direction of the principal stress axis was north-south. Figure 9“ B shows that the subsldary peak at N 72° E is more developed than the other subsldary 76 16-r (A) Total n u m b e r of m a g n e t i c elements = 3 3 3 2 Total numb«r of m a g n e t i c trend elem ents = 2 0 3 4 frequency i n percentage tre n d Total n um ber of m a g n e t i c trend e le m e n ts e ( 2 9 8 160 Azimuth fn d e q r e e S F i g . 7. £ > i s t r i b u t i o n of m a g n e t i c t r e n d e l e m e n t s in (A) T o t a l s u r v e y a re a CB) K e e w a n a w a n r o c k s ( C ) P r,e - K e e w a n a v i o n rocKS . 77 12 - T h ird order m agnetic an om alies 8- Totol num ber of m a g n e t i c t r e n d elements* 6 8 9 4- 12Second o rd er magnetic a n o m a lie s 8- Total num ber of m a g n e t ic tre n d e l e m e n t s s 4 4 0 l i 16First o rd er m agnetic an om alies 12- Total number of m a g n e t i c trend elements » t69 3 20 40 60 80 140 too F ig .g . D is tr ib u tio n of m a g n e tic tre n d elements Azimuth in degrees within P r e - k e e w a n a w a n r o c k s according t o o r d e r o f m a g n e t i c a n o m a l i e s . 78 “Third order magnetic anomalies CA) "Total number of m agnetic trend e lement s* I<40O Second order magnetic anomalies (B) “Total number off magnetic trend elements 20 40 60 lO O =634 140 160 Atimutb in degrees Fig.9. Distribution of magnetic trend elements within the K e e w a n a w a n rocKs according to order of magnetic anomalies. 79 peak. This may suggest that the acting stress during the Keweenawan time was of a rotational nature due to a shear couple. (simple shear) Figure 7-B shows a gradual increase in frequency of magnetic elements that have an azimuth between 40 and 85 degrees that resulted in an asymmetry of the major east-west peak for the Keweenawan area. This may reflect a gradual shift in both time and space of the acting shear couple. CHAPTER VIII INTERPRETATION General Heiland (19^0) states that Most Interpretation of magnetic data Is of a quali­ tative nature. This is due to several factors, Cl) The magnetic methods lack depth control, (2) Most quantitative interpretation is indirect, (3) Magnetic properties of geologic formations, particu­ larly of igneous and metamorphic rocks, are subject to great horizontal and vertical variations and are dependent on the thermal and mechanical history, the effects of which are difficult to evaluate, and (4) Magnetism is the only physical rock property which is of a bipolar nature, and variability of polariza­ tion adds another unknown. Quantitative interpreta­ tion is further handicapped because the proportion of induced and remanent magnetism is rarely known. Although these conclusions are about 30 years old, they still hold true in general for an area with complex Precambrian geology such as the Northern Peninsula of Michigan. Assuming that the overlying sediments show no mag­ netic effect, the broad range of magnetic anomalies can be interpreted as reflecting changes in the composition of the igneous and metamorphic basement rocks. Geological features such as faults, extrusives, and Intrusives can frequently be identified by observing the shape and extent of the anomalies over a contact between rock units, 80 81 together with the known regional geology. Abrupt shifts of the magnetic contour pattern is often an indication of a fault or an unconformity. The ultimate purpose of magnetic interpretation is to deduce the geometry of magnetic bodies causing the anomalies. Unfortunately, an infinite number of sub­ surface distributions of magnetization can explain a set of magnetic field observations on the earth's surface. This holds true even if the field is known with perfect precision at every point on the surface. It follows that magnetic anomalies alone are not sufficient for uniquely determining the bodies or structures causing the anomalies. Therefore the Indirect method of interpretation utilizing theoretical anomalies, calculated from bodies of plausible shape and magnetic characteristics, plus the extrapolation and Interpolation of known geology was the principal approach used in this study. The geological interpretation of the aeromagnetic maps is presented in form of bedrock geology map (Plate III) and geological profiles (Plate IV). Aeromagnetic Maps Introduction It is highly desirable to initiate the Interpreta­ tion by discussing the aeromagnetic maps. The aeromagnetic maps depict a wide range of magnetic intensity which is characteristic to the Lake Superior region. Positive 82 anomalies range from few tens of gammas to about 8,000 gammas. Negative anomalies seldom reach a value of 1000 gammas. These anomalies are superimposed on the normal magnetic variation which averages about four gammas per mile over the study area, as computed from the U.S. Coast and Geodetic total intensity map (1955). The normal magnetic variation increases from south to north. The following discussion involves the qualitative interpreta­ tion of the aeromagnetic maps, based upon the previous division of the study area. The Keweenawan Area Several lineations within the Keweenawan area are immediately apparent from the aeromagnetic map (Plate I). These lineations are associated with the Keweenawan lavas of the Northern, Middle, and Southern Trap Ranges. In the northwestern portion of the area and extending across it, skirting the south shore of Lake Superior, is a major multipeak magnetic anomaly. This anomaly is composed of alternating positive and negative peaks maximum amplitude of 750 to 2000 gammas along its exten­ sion. that reach a It coincides with and is caused by the outcropping Keweenawan lavas of the Northern Trap Range. The width and amplitude of the anomaly are relatively uniform within the area, which suggests geologic formation. that the source Is a single The elongate shape of the anomaly suggests the trace of a dipping sheet-like mass. The 83 steep magnetic gradient of this anomaly and its narrowness is indicative of the outcrop of the source rock. This anomaly reaches its maximum breadth in the Porcupine Mountains area. The broadening could be explained by a local increase in the volume of the extrusives and or due to doming in the area. Another major elongate anomaly is shown in the central portion of the area. It extends from west of the Keweenawan Bay southwestward toward Lake Qogebic and to the west of it. The amplitude, ranging from 500 to 900 gammas and the discontinuous nature of this anomaly, reflect variations in depth to its source. This anomaly is interpreted to be due to buried Keweenawan lavas of the Middle Trap Range. The Middle Range only outcrops at Silver Mountain which is located in sections 1 and 12 of T*49N, R36W and section 6 of T*49N, R35W, Houghton County. The Middle Trap Range reaches its steepest magnetic gradient and highest amplitude at Silver Mountain. According to Roberts (19*40), this outcrop is composed of at least fourteen uralitized basalt flows. The flows strike N 20° E and dip 15° to the northwest. They are fine grained amygdaloidal extrusive rocks, and according to Lane (1909)> are typical Keweenawan basalts. The structure of this range, is interpreted to be the result of upfaulted blocks in the form of a hors t. Some magnetic patterns cut through the Middle Trap Range anomaly and are 84 believed to be associated with transverse faults that transect the Range. The difference in elevation of the Middle Range along its extension is attributed to these faults. The abrupt termination of the Middle Range anomaly at the northern end is interpreted to be the result of an east-west striking crossfault. The Bouguer gravity map of the area (Bacon, 1957) and the magnetic second vertical derivative map substantiate this conclusion. A negative magnetic anomaly to the south and east of the Middle Trap Range correlates with the Jacobsville sandstone which ter­ minates at the outcropping of the unconformity between the Jacobsville sandstone and the Pre-Keweenawan rocks. Along the western protion of T50 and 51N, R34W, a north-south striking fault is believed to have dropped the lavas to a considerable depth to the east of it. The contact between the Jacobsville sandstone and the Pre-Keweenawan rocks east of this fault can be traced out by the increased wavelength of the magnetic anomalies associated with the mapped Jacobsville sandstone. West of Lake Qogebic, at longitude 89° 50' W, the anomaly associated with the Middle Trap Range, Joins with a subparallel anomaly south of it. The southern anomaly is believed to be due to the South Trap Range which out­ crops occasionally at the southern edge of the Jacobsville sandstone. The outcrops of the South Trap Range east of the Junction point are associated with no definite magnetic 85 character in contrast to situation west of the Junction point. This can be explained by a difference in magnetic polarization of the lavas in these two localities due to a remanent component and/or due to differences in the dip of the lavas. To the east of the Junction point some lava outcrops were found to dip at an angle of 15° to the northwest, while to the west of the Junction point, out­ crops of lava dip Between the at 70 to 80° to the north. Northern Trap Range and the Middle Trap Range there is a relative magnetic low area which is characterized by. low magnetic gradients. This is believed to be associated with the basalt flows in the downthrown block buried beneath the Jacobsville sandstone. In T50N and to the east of R36W there is a positive magnetic anomaly that reaches an amplitude of 500 gammas which is probably due to an anticlinal flexure in the lavas. In T48N, R38 and 39W the magnetic pattern indicates a fault which may have brought some parts of the lava nearer to the surface. A well-defined magnetic low of roughly 1100 gammas magnitude exist over the Iron River syncline. Another magnetic minimum of much lower magnitude occurs over the Presque Isle syncline and extends to the northwest out of the study area. A positive anomaly of about 300 gammas magnitude surrounds the Iron River syncline anomaly from the west and north. This anomaly is correlated with a 86 rhyolite extrusive body of the Nonesuch shale. that roughly parallels the base In sections 5* 6> 7 and 8 of T^9N, R44W, this anomaly reaches a maximum amplitude of about 1500 gammas. The high magnitude anomaly may be due to remanent polarization of the rhyolite. Another positive magnetic anomaly of 1500 gamma amplitude and of large areal extent occurs east of the Iron River syncline and centers in T50N, R42W. This anomaly has been interpreted to be due to an upfaulted block of a previously folded anticlinal structure of the Keweenawan lavas that underlie the rhyolite and the Nonesuch shale. A negative magnetic anomaly in T*40N, R*42W, within the Northern Trap Range, by Wright (1909). correlates with a rhyolite body mapped Rhyolite is found in drill holes south and within the Iron River syncline and in the Porcupine Mountains area. Miller (1 9 6 6 ), on basis of quantitative interpretation of gravity profiles across the Porcupine Mountains area, concluded that the rhyolite is about 2000 to 3000 feet thick under the syncline, 15,000 feet thick in the Porcupine Mountains and extends for some distance to the south of the syncline. The structure in the Porcu­ pine Mountains area and the origin of the Iron River syncline could be the result of the lopolithic intrusion of rhyolite. In this case, basinlng has been contempor­ aneous with the intrusion, w ith the overlying sediments sagging downward while masses of rhyolite are being 87 withdrawn from the underlying magma reservoir. Billings (1959), states, In fact, some geologists consider this contemperaneous basining, an essential part of the definition of a lopolith. If a large, concordant sheet injected into flat sedimentary rocks were deformed into a basin, during some later orogenic period, these geologists would use the term sill rather than lopolith. Near the western end of the Northern Trap Range in the study area, there is an elongated negative anomaly of about 200 gamma amplitude centered at the border between T48N and T49N and extends across the area from the western border of R47W, eastward to the eastern border of R43W. This anomaly correlates with outcrops of fellstic conglomer­ ate . Pre-Keweenawan Area Lake Qogebic Area Strong magnetic highs are associated with the eastern end of the Qogebic Range. They are mainly due to the Tyler slate which includes the Ironwood iron formation as its lower most member. In sections 7, 8, and 9 of T47N, R45W, the anomaly reaches Its highest magnitude of about 6000 gammas. At this location An apparent thickening of the Ironwood iron formation ends at the center of T47N, R43W. The magnetic anomaly associated with the Tyler slate wraps around the nose of an eastward plunging anti­ cline, the Marenlsco anticline. West of Lake Qogebic near Marenisco, Michigan, the magnetic anomaly associated with 88 the upper members of the Tyler slate disappear and the magnetic high associated with the Ironwood Iron formation appears on the south limb of the Harenlsco anticline. This anomaly lies in the center of T46N and extends from the eastern border of R^3W to western border of R^5W, on the upthrown side of the fault and extends across the Michigan-Wisconsin border. To the south of the Ironwood iron formation anomaly, along the northern limb of the Marenisco anticline, there is a negative magnetic gradient of about 250 gammas per mile that extends southward for about four miles at the extreme western border of the study area and averages about one mile in width at the eastern end of the Qogebic Range. This negative, uniform magnetic gradient is inter­ preted to be due to greenstones and greenschlsts that under­ lie the Tyler slate. West of T.^N, R46W, there are some granite rocks that outcrop through the greenstones. These outcrops are not reflected in the magnetic map. The center of the Marenisco anticline, west of Lake Qogebic is occupied by a very large number of weak mag­ netic anomalies that strike about N 70° E. These anomalies are associated with granitic rocks mapped by Pritts (1965), namely granite and banded gneiss. The boundary mapped by Pritts between the banded gneiss and the granite is apparent from the magnetic pattern east of T47N, R ^ W . To 89 the west of that point the boundary is not apparent on the magnetic map and the two units are mapped as one unit. The complexity of the magnetic character of the Animikie rocks on the north limb of the Marenisco anti­ cline and east of T47N, R44W reflects a higher degree of deformation than the same rocks west of that point. According to Prinz (1967 )> to the west of that point, the period of major deformation of these rocks postdated the Keweenawan basalt flows, whereas to the east of it the deformation is Pre-Keweenawan. The Tyler slate anomaly on the south limb of the anticline is located in T46N and extends from western portion of R^IW to western portion of R^3W. To the south of the Tyler slate anomaly, a positive magnetic high that reaches about 1500 gammas in amplitude lies in the south­ east portion of T46N, R42W. This anomaly is related to the iron formation in the lower part of the metatuff and tuffaceouB metagraywacke that Pritts (1967) has mapped. Skirting the magnetic anomaly associated with this iron formation is a small negative anomaly that reaches a magnitude of 250 gammas in some places. This anomaly is correlated with the upper parts of the metatuff and tuffaceous metagraywacke, minor quartzite and clomgomerate. At the upper contact of this formation with the graywacke near Banner Lake, there is another narrow negative anomaly of higher magnitude that ranges between 350 and 500 g a m m a s . 90 There is a discontinuity in this anomaly at the border between T46N and T45N, R43W which may be attributed to Barb Lake fault. At the upper part of this formation, on the south side of the fault, there is a magnetic high that reaches an amplitude of 4000 gammas. This is cor­ related with the magnetic iron formation in the upper part of the graywacke member. This is bordered to the southeast by a magnetic low and then a magnetic high. The magnetic low ranges between 300 and 500 gammas and is believed to be associated with the pillow lava and frag­ mental volcanic rocks. The magnetic high which has an amplitude of 200 gammas is believed to be associated with the metatuff and magnetic iron formation. The discontin­ uity of these anomalies is also attributed to the Barb Lake fault which strikes east-west across the center of the Lake Gogebic area. It is clear from the aeromagnetic map of this area and from the geologic information that the principal struc­ ture to the north of the Barb Lake fault is a south-dipping monocline. North of the Barb Lake fault and in T46N and extending eastward from R40W and south of the upper con­ tact of the metatuff formation, there is a decreasing uniform magnetic gradient that averages about 500 gammas per mile. This uniform magnetic gradient is correlated with the Michlgamme slate. 91 Prom the aeromagnetlc map (Plate I-d), It is clear that the structure south of the Barb Lake fault is more complex and highly folded. Fritts (1965) reported that diamond drilling near Banner Lake indicated a synclinal flexure. The character and magnitude of the magnetic high associated with the metatuff helped in delineating the fold structures south of the Barb Lake fault. The folding of these rocks is believed to have taken place after their deposition over the eroslonal surface of the Wolf Lake granite. The Wolf Lake granite to the south of the Barb Lake fault is considered to be the oldest rock in the area, and occurs at the center of the anticlines and is asso­ ciated with weak negative magnetic anomalies. There are two more broad negative anomalies that occur south of the Barb Lake fault. The first occupies the center of T^5N and extends from the eastern border of R^3W to the center of RlJlW. The second occupies T44N and extends from the center of R^OW to the western border of R 38W. These two anomalies reach an amplitude of about 600 gammas and are correlated with the Michigamme slate. The Michigamme slate in this area is named the graywacke formation near Paulding by Fritts (1967). These magnetic lows lie along the axes of synclines in that area. 92 The Iron River-Crystal Palls Area The Iron River-Crystal Palls area is characterized by widely-contrasting magnetic anomalies. One of the most dominant features is a group of remarkably sharp, long negative anomalies which cut across all other mag­ netic anomalies. This group of anomalies occurs between latitude 46° 15' and 46° 45' N and east of longitude 85° 05' W, with greatest concentration in T^8N and T^9N. All of these anomalies are related to reversely magnetized diabase dikes. These dikes intrude strata as young as Michigamme slate, while other dikes intrude the Margenson Creek gneiss, which is mainly a granitic.rock, generally foliated and gneissic. The age of the Margenson Creek gneiss is believed to be Lower Precambrian. Some negative magnetic anomalies associated with the diabase dikes extend through the area overlain by the Jacobsville sand­ stone, and they may intrude the Keweenawan lavas of the South Trap Range. These dikes occur in T48N, R37W and western portion of R36W. The anomalies associated with the diabase dikes that occur to the west of the contact betw-een the Jacobsville sandstone and the Pre-Keweenawan rocks have a greater wavelength than those associated with the diabase dikes occurring east of this contact. This suggests that these dikes do not reach the bedrock surface and thus does not Intrude the Jacobsville sandstone. 93 The second dominant feature in this area is a group of large positive anomalies gamme slate. Plate I-c. that occur within the Michi­ There are two anomalies of this group in One occurs in T48N, R32W and the other one centers approximately on the south border of T*l8N, R3*4W. These anomalies are believed to be due to deep seated mafic rocks of unknown origin. type are shown on Plate I - e . and 47N, R33, 34, and 35W. by Balsley and others Other anomalies of this They are located in T45, 46, These anomalies were studied (1949) and are believed to be mainly due to dark magnetic slate of volcanic origin on crests of anticlinal folds. The fact that these anomalies become weaker and broader westward indicates the presence of a series of west plunging anticlines, on which the magnetic rock becomes progressively deeper. The third group of anomalies occurs in T42, 43N and extends from the western border of R32W to the eastern border of R36W. This group of moderately strong positive anomalies reach amplitudes of 2500 gammas. The major part of the Iron River basin is located within this area. Most of these anomalies are caused by a strongly magnetic slate known as the Stambaugh formation. This magnetic slate is exposed in some places in this area, notably on the hill on which the town of Stambaugh is located. The slate is composed of fine-grained, intergrown chert and siderite, with abundant tiny crystals of magnetite 94 throughout. This group of anomalies appears erratic in both trend and intensity. This indicates that the Stam­ baugh formation is contained in tightly folded synclines of highly variable trend and plunge, characteristic of the Iron River-Crystal Falls area. Another group of positive anomalies of moderate magnitude occur in T42N, R34, and 35W. The source of this group of anomalies lies within the Badwater greenstone. It is composed of flows, tuffs, and agglomerates. The Brule River anomaly (Balsley, 1949) which occurs to the south of the Randville dolomite is associated with another belt of greenstone, called the Brule River greenstone. This formation is exposed in a number of places in the southern part of Sections 20* 21 and 22, T42N, R35W. It is a massive metabasalt, locally agglomeratic and ellip­ soidal. Another dominant magnetic feature in this area is the strong positive anomaly bordering the Amasa Oval. This anomaly reaches a magnitude of 6000 gammas in several places. It is caused by the upper part of the Animikie Hemlock formation which is composed mainly of volcanic breccia and basaltic flows. On the eastern side of the Amasa Oval, the anomaly exhibits a north-south strike and also may be due in part to the Fence River formation. This formation is a fine-grained, magnetite-bearing quartzite. On the western and northern sides of the Amasa 95 Oval, the anomaly Is generally broader and discontinuous due to faulting. The Amasa formation which Is chiefly a martlte slate with layers of cherty-lron formation is non­ magnetic. This fact has been verified by Weir (1967) as a result of a ground magnetic survey that has been made along the southwestern protion of the inferred belt of the iron-bearing Amasa formation. The group of anomalies north and west of the Amasa Oval are interpreted to be due to the same volcanic breccia and basaltic flow member of the upper Hemlock formation that underlies the Michigamme slate. The positive anomalies that occur in the center of T^7, **6N, and the northeast portion of ^5N, R31, and 32W, bordering the two granitic intrusions are believed to have their source within the Hemlock formation. A large, broad negative magnetic anomaly in T^6N, R31W, extends southeastward along the eastern border of the Amasa Oval. This anomaly correlates with the non­ magnetic graywacke of the Michigamme slate. Within the Amasa Oval, the lower members of the Hemlock formation show two weaker magnetic zones that trend northwest. The source of these anomalies may be a magnetic volcanic rock within the Hemlock formation. Moderate to strong anomalies occur along the eastern border of the West Kiernan sill. Similar anomalies are associated with the metagabbro dikes in the area. 96 A broad, positive belt of magnetic anomalies extend­ ing northwest-southeast in T*43N, R31W is interpreted by Bayley (1959) to be due to the magnetite-bearing volcanic schist that underlies the Randville dolomite. A broad negative magnetic anomaly roughly parallels the inferred belt of Randville dolomite within the Amasa Oval. The magnetic anomaly associated with the Margenson Creek granite gneiss at the center of the Amasa Oval is not clearly defined as those associated with the two Margenson Creek granite gneiss outcrops to the northeast of the Oval. This can be explained by the fact that the magnetic contrast between the volcanic breccia and basaltic flows and the granitic rocks outside of the Oval is much higher than that between the granite and the peripheral dolomite within the Oval. The granitic rocks north of the Marquette Range cover a large area from the center of T48N to T50N* and from R33W to the eastern border of the study area. The granite is characterized by a pattern of discontinuous highs and lows of varying orientation and of low to moderate magnitude. It seems that the magnetic character of these granites is highly affected by the Marquette Range anomaly to the south of it and by the several dia­ base dikes that cut across i t . The northern contact of the granite which is an erosional surface is mapped primarily on basis of geologic information because there 97 is no sharp and definite magnetic contrast between the granite and the Michigamme slate north of it. The mag­ netic anomalies associated with the Michigamme slate north of this granite are of higher frequency and of more irregular strike than those associated with the Michigamme slate at the center of the basin. This reflects the thinning of the Michigamme slate in this area and that the magnetic character of the underlying granite shows through the overlying Michigamme slate. A large positive magnetic anomaly of 8,000 gamma amplitude occurs in the center of T48N, R31W. This anomaly is associated with the iron formation at western end of the Marquette Range. Another distinct positive anomaly in the northeast portion of T^9N, R3^W, correlates with the iron formation at Tyler mine. Another positive anomaly to the west of Tyler mine anomaly is believed to be due to an iron formation. Several small, positive anomalies which occur in the eastern portion of T49N, R33W, and T50N, R32W, are believed to be due to either local iron formations or small knobs of basic extrusives. Discussion of Profiles General In this study fourteen profiles (Plate IV, A-N) were chosen across the area for quantitative Interpreta­ tion. The geographic position of these profiles is shown 98 on Plate III. They are laid out as closely as possible to cross at right angles the major geological features in the area and are positioned in areas where some geological con­ trol is available in order to extrapolate from and inter­ polate between the known geology utilizing the magnetic data. The topography of the ground surface along the pro­ files was taken from the U.S. Geological Survey topographic maps. The general purpose of the quantitative magnetic interpretation was to determine the geologic relationship along the vertical cross-section. In particular, the quantitative interpretation was used to study the geologi­ cal relations along the Keweenaw fault and to determine the structure of the Keweenawan lavas of the Middle Trap Range below the Jacobsville sandstone. Many of the geologic bodies and structures of inter­ est in the area of investigation are horizontally linear and thus can be approximated by the two-dimensional form of analysis. A two-dimensional magnetic program outlined by Talwani, Worzel, and Landisman (1959) was used in these computations utilizing the Michigan State University C.D.C. 3600 computer. This program is based on the assumption that the boundary of the vertical cross-section of a twodimensional body can be approximated by a polygon. This approximation can be made as accurate as one wishes by increasing the number of sides of the polygon. The total 99 magnetic intensity due to the polygon can be obtained at any given point and there are no limits on the size or position of the body. Initially several trials were conducted to compute anomalies from bodies of assumed geometric configuration and magnetic polarization contrasts to fit the observed anomalies along three of the fourteen profiles. The assumed forms of these bodies were selected on the basis of available geologic information. All other available information concerning the parameters of the bodies such as depth, length, dip and magnetic polarization also were used In the calculations. Unfortunately, little success was achieved in matching theoretical anomalies with the observed magnetic profiles. The inability to match theoretical with observed anomalies is believed to stem from the complexity of magnetic rock properties. As a result, a qualitative approach was made to the Interpretation of the profiles utilizing the surface geology information, depth determinations, second vertical derivative magnetic map, Bouguer gravity map (Bacon, 1957). plus the results of one profile, H-H' (Plate IV-D) in which a reasonable match was obtained between the theoretical and observed anomalies. Profile A - A 1 The magnetic gradient at the northern end of Profile A-A' is related to the dip of the Keweenawan lavas into the 100 Lake Superior basin at depth and the thickening of the Keweenawan sediments. There Is a low magnitude positive anomaly of about 150 gamma amplitude that lies over the outcrop of Co'pper Harbor sandstone. This anomaly may be due to a conglomeritic member of the Copper Harbor sand­ stone which contains abundant basalt pebbles. To the immediate south of this anomaly there are two positive peaks separated by a large negative anomaly. The two positive peaks reach an amplitude of about 500 gammas and are related to the Keweenawan lavas of the Northern Trap Range. The negative anomaly reaches an amplitude of about 400 gammas and is associated with felsitic, concordant intrusion within the Northern Trap Range. A negative anomaly occurs to the immediate south of the positive anomaly associated with the Northern Trap Range lavas. This negative anomaly is believed to be attributed to loss of magnetism of the lavas near the Keweenaw fault or to the complications of remanent magnetization in the neigh­ borhood of the fault. Hemming (1965) in a typical cross section across the Keweenawan fault near Portage Lake mapped the Keweenawan fault at a distance of about one mile to the south of the positive anomaly associated with the Keweenawan lavas. Accordingly the Keweenawan fault throughout the study area has been mapped at the center of this negative anomaly at the southern margin of the Northern Trap Range. 101 The Jacobsville sandstone to the south of the Keweenaw fault yields a magnetic low. At the southern margin of the Jacobsville sandstone there Is a magnetic high of about 2400 gamma magnitude which is associated with the outcropping lavas of the South Trap Range. It is to be noted that although the width of the outcrop of the South Trap Range is less than that of the Northern Trap Range, the South Trap Range anomaly has a much higher amplitude. This may be attributed to the higher angle of dip along this profile and/or higher magnetic polarization of the South Trap Range lavas. To the immediate south of the South Trap Range anomaly there is a positive anomaly of 500 gamma amplitude asso­ ciated with the Ironwood iron formation. The anomaly associated with the greenstones that underlie the iron formation seems to be partially masked by the local strong regional effect resulting from the iron formation anomaly. The granite and banded gneiss at the center of the Marenisco anticline is associated with a magnetic low that includes narrow positive anomalies which have amplitudes generally less than 100 gammas. To the south of the granite there is a magnetic high of about 1500 gamma ampli­ tude associated with the iron formation on the south limb of the Marenisco anticline. The contact between the granite and this iron formation is considered to be an unconformity, as evidenced by the erosion of the greenstones 102 on the south limb of the Marenisco anticline. To the south, the Iron formation is overlain by the metatuff and tuffaceous metagraywacke which is associated with a magnetic low. Profile B - B 1 The prominant feature at the northern portion of profile B - B ’ is the positive magnetic anomaly associated with the rhyollte outcrop. This anomaly is of higher magnitude than that associated with the Northern Trap Range lavas. This may be the result of strong magnetic polarization of the rhyollte due to a remanent component. The negative anomaly associated with the felsite intrusion within the Northern Trap Range also occurs on this pro­ file. The negative anomaly immediately south of the posi­ tive anomaly associated with the Northern Trap Range lavas suggests a southerly dip of the contact between these lavas and the Jacobsville sandstone. There are two positive anomalies that occur within the area of the Jacobsville sandstone outcrop. The anomaly that lies to the immediate south of the Keweenaw fault has a steeper magnetic gradient than the one that lies further to the south. The latter is interpreted to be caused by a horst structure associated with the Middle Trap Range. The anomaly to the immediate south of the Keweenaw fault is interpreted to be due to a block of Keweenawan lavas which is separated from the main body of the 103 Northern Trap Range by faulting. This anomaly continues for about fifty miles across the study area and lies immediately south and parallel to the Interpreted posi­ tion of the Keweenaw fault. This suggests an association of the origin of this block with the thrusting of the Northern Trap Range along the Keweenaw fault. The author suggests that the origin of this block may be due to either one of two possibilities. One possibility stems from the assumption that faulting took place across the southern portion of the Northern Trap Range (Figure 10-B). Due to the thrust with rotation along the Keweenaw fault, the wedge of sediment north of the fault has been brought nearer to the surface with the Middle Trap Range lavas assuming a greater angle of dip in relation to the South Trap Range (Figure 10-C). Differential erosion followed and resulted in separation of the smaller block of lava from the main body of the Northern Trap Range (Figure 10-D). Still later a younger sandstone formation was deposited (Figure 10-E) leading to the present geologic structure. Another explanation of this anomaly is based upon the assumption that faulting took place along the contact of the sandstone and the lavas of the Northern Trap Range or parallel to it. As illustrated in Figure 11-C, a crossfault developed within the Northern Trap Range lavas approximately at right angles to the Keweenaw fault plane and sliding of the faulted block took place (Figure 11-D). 104 2 A) Southern portion o f Lo*c Superior bosin C)Thru*tlng with rotation B) Faulting D) Erosion F»9. 1 0 , Possible development of s t r u c t u r a l r e l a t i o n s along t h e Keewanaw fault 105 A) S o u t h e r n p o r t i o n o f Laka S u p a r i o r b a t i n B> Faulting J>) S l U l n j o f f a u l t o d blo ck C)Thru*ti«j *ith rotation Vounatr lanrfttona ■apqcxsaac C) Later dapocltlon F19 J l . P o tsib la development of s t r u c t u r a l re la tio n s a lo n g t h e K e e w a iia w f a u l t 106 The cross-faulting and/or sliding may have been facilitated by insufficient support of the Northern Trap Range lavas by the underlying sandstone. Later deposition of a younger sandstone formation followed, leading to the present geo­ logic structure (Figure 11-E). Results obtained from quantitative magnetic inter­ pretation substantiates the configuration along the Kewee­ naw fault shown in Figure 11-E. Also Miller (1966), as a result of his gravity investigation of the Porcupine Mountains area concluded that the southern margin of the lavas where they are in contact with the sandstone dips to the south rather than the north as previously postulated. He came to this conclusion as a result of quantitative interpretation of gravity profiles across the Keweenaw fault. A third possible origin of the positive anomaly to the immediate south of the Keweenaw fault is an intrusion into the sandstone that did not reach the surface. This possibility seems unlikely due to the persistance of this feature for a distance of about fifty miles. The anomaly associated with the South Trap Range reaches an amplitude of about 2000 gammas and is separated from the iron formation anomaly on the northern limb of the Marenisco anticline by a magnetic low. This low is associated with the upper member of the Tyler slate which is nonmagnetic. The anomaly associated with the greenstones 107 along this profile Is more pronounced than that shown on the previous profile and reaches an amplitude of 500 gammas. The positive magnetic anomaly associated with the Iron formation on the south limb of the Marenisco anticline reaches an amplitude of about 1000 gammas. To the south of the Marenisco anticline there Is a syncline that Includes rocks of the Upper Anlmlkle series. A graywacke slate formation occurs at the center of this syncline and is correlated with the Michigamme slate to the east. The graywacke slate formation is associated with a a magnetic low and overlies the metavolcanics and metasediments near Blair Lake which is slightly magnetic. The metavolcanics and metasediments are underlain by the graywacke near Banner Lake whlfch includes a magnetic iron formation in its upper p a r t s . The two anomalies associated with the metavolcanics and iron formation member of the graywacke at the northern limb of the syncline merge into one anomaly that reaches about 200 gamma magnitude. The low magnitude and breadth of this anomaly can be attributed to the small angle of dip of these formations. The gray­ wacke is underlain by the metatuff and tuffaceous metagraywacke which thickens along this profile due to repitition by faulting. At the extreme southern protlon of the profile the granitic basement rocks of Lower Precambrian age outcrop at the surface. 108 Profile C - C 1 The northern portion of Profile C - C ’, associated with the Keweenawan rocks, is similar to the corresponding por­ tion of profile B - B 1. However, the Northern Trap Range anomaly includes more magnetic lows. These lows are attributed to a number of felsite conglomerates of the Upper Middle Keweenawan Great Conglomerate and Outer Con­ glomerate. These conglomerates are estimated (Bulter and Burbank, 1929) to have maximum thickness of 2200 and 3500 feet respectively. Within the area of Pre-Keweenawan rocks the anomaly associated with the iron formation at the base of the Tyler slate on the south limb of the Marenisco anti­ cline is not present probably as result of faulting. Further to the south there is a large positive anomaly of about 3500 gamma magnitude associated with the iron forma­ tion in the upper series of the graywacke near Banner Lake. The anomaly over the metavolcanic and metasedimentary rocks divides into a magnetic low and a magnetic high. netic low lies to the immediate south The mag­ of the iron formation anomaly and is associated with the pillow lavas and frag­ mental volcanic rocks. The magnetic high reaches a mag­ nitude of about 500 gammas and is associated with the meta­ tuff and magnetic metasedimentary rocks. The granitic basement rocks outcrop at the extreme southern portion of this profile. 109 Profile D - D 1 Along Profile D-D' the Northern Trap Range anomaly reaches Its maximum amplitude of 2000 gammas and the mag­ netic gradient north of it reflects a thickening of the sediments over the center of the Iron River syncline. The steep gradient anomaly to the south of the Keweenaw fault and the anomaly associated with the Middle Trap Range lavas is shown on this profile as in previously discussed profiles. It Is to be noted that the South Trap Range does not outcrop along this profile and is assumed to underlie Lake Gogebic. There is a small positive magnetic anomaly of about 200 gamma magnitude associated with the Iron formation at the base of the metatuff and tuffaceous metagraywacke. Further to the south, an asymmetrical anomaly of about 600 gamma magnitude occurs along this profile. This is related to the combined effect of the outcropping metavolcanics and metasediments and the underlying iron formation in the upper part of the graywacke near Banner Lake. The steeper gradient of this anomaly on the northern side is attributed to the Barb Lake fault. The gentler slope of the southern side of this anomaly reflects the low angle of dip of the magnetic formations. There is anpther small positive anomaly of 200 gamma magnitude associated with the outcrop of the matavolcanics and metasediments at the southern limb of the syncline. At the southern portion of this 110 profile and within the granitic basement rocks, there are two positive anomalies associated with two outliers of the metavolcanics and metasediments. Profile E—E 1 The portion of the profile E - E ’ associated with the Keweenawan rocks is similar to the corresponding portion of the profile D-D'. However, the South Trap Range anomaly is present along this profile and reaches a magnitude of about 500 gammas. Immediately to the south of the South Trap Range volcanics, the metatuff and tuffaceous meta­ graywacke outcropB and is intruded by a younger granite, probably of late Animikie age. This younger granite out­ crops along this profile and in several other places south of Lake Gogebic and north of the Barb Lake fault. To the south of this granite there is a broad magnetic low asso­ ciated with the upper members of the metatuff and tuffaceous metagraywacke, the graywacke near Banner Lake, and the metavolcanic member of the younger r o c k s . There are four positive magnetic anomalies associated with the outcrops of the metatuff and magnetic iron formation on both Bides of the Barb Lake fault. The magnitude of these anomalies differ due to local thickening of this formation as a result of minor folding. The two magnetic lows that lie to the immediate south of the Barb Lake fault and at the southern portion of the syncline are associated with the graywacke slate formation. The central negative magnetic Ill anomaly Is due to local thinning of the metatuff and mag­ netic iron formation at that location. At the southern portion of this profile and within the granitic basement rocks, there is a brioad positive anomaly of about 250 gamma magnitude associated with an outlier of the meta­ volcanics and metasediments. Profile F - F * The magnetic gradient at the northern end of Profile F-F' reflects a doming of the buried Keweenawan lavas. This doming is mainly related to folding. noted that there It is to be is a sharp decrease in the magnetic gradient at the northern end of this profile which is attributed to faulting of the Keweenawan lava. The steep gradient positive anomaly to the immediate south of the Keweenaw fault reaches its maximum amplitude of about 850 gammas along this profile. The positive magnetic anomaly associated with the South Trap Range lavas reflect a thinning of these lavas due to erosion along its outcrop. South of the South Trap Range anomaly there is a sharp positive anomaly of about 600 gamma magnitude which is associated with the iron formation which occurs in the upper parts of the graywacke near Banner Lake. This anomaly is followed by a broader and stronger positive anomaly associated with the outcrop of the metavolcanics and metasediments. The Michigamme slate on the downthrown side of the Barb Lake 112 fault Is associated with a magnetic low. The granitic basement rocks are outcropping along the southern portion of the profile on the upthrown side of the Barb Lake fault. Several positive magnetic anomalies of varying amplitudes are associated with outliers of metavolcanics and metasediments In the granitic basement rocks. Profile Q - Q 1 The anomalies associated with the Keweenawan rocks along profile G - G 1 are generally similar to the correspond ing anomalies along profile F - F 1. However, the magnetic gradient at the northern portion of the profile reflects a thickening of the Keweenawan sediments along this pro­ file as result of the disappearance of the domal uplift encountered along the previous profile. It is to be noted that the South Trap Range is associated with a nega­ tive anomaly along its outcrop which reflects a change in the magnetic polarization of the South Trap Range lavas. The negative anomaly is emphasized by the positive anomaly immediately to the south of it and associated with the metavolcanics and metasediments. The gentler gradient on the south side of the positive anomaly is attributed to a lower angle of dip of the metavolcanics and metasedi­ ments and due to thickening of the Michigamme slate toward the center of the syncline. The granitic basement rocks outcrop immediately to the south of Barb Lake fault and are associated with a small positive magnetic anomaly. 113 South of the Barb Lake fault, a strong positive magnetic anomaly of 5500 gamma amplitude is associated with the out­ crop of the metavolcanics and metasediments on the northern limb of the syncline. The outcrop of the same formation on the south limb of the syncline is associated with another anomaly of much lower magnitude. This decrease in amplitude of the anomaly is attributed to thinning and gentler angle of dip of the metavolcanics and metasdeiments at the south limb of the syncline. Another positive magnetic anomaly at the southern end of this profile is related to outcropping or very near surface metavolcanics and metasediments. Profile H - H 1 The portion of Profile H-H' associated with the Keweenawan rocks has been interpreted quantitatively. Separate body configurations were utilized to approximate the Northern Trap Range, and the combined Middle and South Trap Ranges. The magnetic program utilized in the quantitative interpretation is based on calculating the combined induced and remanent field vector. Both the induced and remanent field vectors are resolved into orthogonal components. cally. These components are then added algebrai­ The sums specify the coordinates of the end point of the combined field vector. The combined angles of declination and inclination are then calculated. An inclination of +45°, a declination of 285°, and a total magnetic intensity of 0.0035^ e.g.s. units were used for the remanent magnetization vector of the Keweenawan volcanics. These are the approximate values given by Dubois (1962) and Jahren (1965) for the remanent magnetic polarization vector of the basic Keweenawan plutons and extrusives. These values are given for the remanent mag­ netic vector assuming that the lavas lie in a horizontal position. A structural correction of 50° dip was applied to the remanent vector applied for the Northern Trap Range body, and a correction of 20° dip was applied for the remanent vector used for the other body representing the Middle and South Trap Ranges. Consequently, the resultant magnetization vector for each body was calculated by the vector summation of the induced and remanent magnetization values. A magnetic inclination of + 71°, a magnetic declination of 0°, and a total field intensity of 59,500 gammas were the magnetic elements utilized for the induced vector in the calculations. The resultant magnetic vector used in the calculation of the magnetic effect for the Northern Trap Range body has a magnetic inclination of +74°, a magnetic declination of 1^3° > and a combined field intensity of 527 gammas. The resultant magnetic vector used for calculating the magnetic effect for the other body has a magnetic inclination of +63°, a magnetic declination of 127°, and a combined field intensity of 513 gammas. 115 The value of magnetic susceptibility used for the combined vector for both bodies was unity and the combined intensity was read, in gammas, into the computer in place of the total magnetic field. The units of the dimensions of the bodies, the flight elevation (500), and station spacing (200), were in feet. The assumption has been made in the magnetic program that the inclination and declination do not vary as the magnitude of the anomaly increases. This assumption allows the summations of the effects of indi­ vidual bodies along the profile. Repeated trials with reasonable configurations bring­ ing the Keweenaw fault to the surface greatly distorted the theoretical anomaly as compared to the anomaly. observed However, a reasonable match between the theoreti­ cal and the observed anomalies was achieved over this portion of the profile by assuming the configuration shown for the Northern Trap Range body in profiel H - H ’ (Plate IV-D). The steep gradient anomaly to the south of the Keweenawan fault is not present along this profile. A close correlation over the Middle Trap Range is obtained between the theroretical anomaly and the observed magnetic profile. The estimated depth from the quantitative inter­ pretation for the Middle Trap Range is about 2000 feet. This value is in close agreement with the value of 2 300 feet obtained from a borehole drilled at latitude 46° 31' N and longitude 89° 1 6 1 W through sandstone (Bacon, 1966). 116 This location is about 2.7 miles west of this profile. is evident from the theoretical profile It that no match for the negative anomaly associated with the outcrop portion of the South Trap Range has been achieved. This necessitates- the assumption of a different magnetic polarization for the South Trap Range which in turn casts suspicion on the configuration of the Middle Trap Range anomaly determined by the calculation. It Is significant to mention that the thickness of the Keweenawan lavas below the Jacobsville sandstone is not predictable from the magnetics. However, a minimum thickness of about 2000 feet is reasonable to assume. To the immediate south of the South Trap Range anomaly there Is an asymmetrical positive magnetic anomaly associated with the outcropping portion of the metavolcanics and metasediments. The gentle gradient on the south side of this anomaly is attributed to the thickening of the overlying non-magnetic Michigamme slate toward the center of the syncline. The granite outcrop to the south Is associated with a magnetic low. At the southern por­ tion of this profile another asymmetrical positive mag­ netic anomaly is associated with another metavolcanic outcrop to the south of the granitic basement rocks. Profiles I-I' and J - J 1 The profiles I - I ’ to N-N' are primarily aimed toward the discussion of the Keweenawan geology in the 117 northeastern portion of the study area. This Is Justified by the fact that the Precambrian geology to the southeast of these profiles has been discussed in detail by the U.S. Geological Survey. The results of these studies are published by Wier and others (1953)» Gair and others (1956), Bayley (1959)> and Wier (1967). Due to the general similarity between Profiles I -I ’ and J - J f, they are discussed together. It is to be noted that the Northern Trap Range in both profiles is associated with a multiple peak positive anomaly which decreases in magnitude due to the thinning of the lava flows along this portion of the study area. The southern most peak is of a smaller amplitude and separated from the main Northern Trap anomaly by a negative anomaly. This anomaly is related to outcrops of felsitic intrusions within the Northern Trap Range. There is a possibility that the smaller amplitude anomaly to the south of the negative may be due to the continuation of the block of volcanics along the south edge of the Keweenaw fault. It is to be noted that the Middle Trap Range is cut by three normal faults which bring the volcanics nearer to the surface. Along Profiel J-J' there is a peculiar negative anomaly associated with the Middle Trap Range. This negative anomaly is attributed to tilting of the faulted blocks of the Middle Trap Range. Also, it is to be noted that no South Trap Range anomaly is present along 118 the southern end of the Keweenawan portion of these pro­ files on Profile J-J' or the profiles to the northeast. This is attributed to the feathering edge of the South Trap Range lavas due to erosion. It is important to men­ tion that the thinning of the South Trap Range lavas shown on these profiles as well as the following ones is only a schematic representation. Along the southern portion of these profiles diabase dikes cut through the older formation. The northwestern most dike along these profiles is believed to cut through the erosional wedge of the South Trap Range, but does not continue through the overlying Jacobsville sandstone. This is evidenced by the broader negative anomalies associated with these dikes as compared to the dikes out­ cropping in the southeastern portion of these profiles. Profiles K-K * to N - N « Profiles K-K' to N-N* are discussed together because of their general similarity. The Northern Trap Range anomaly along these profiles range in amplitude from 250 to 1000 gammas which reflects local change in the volume of the lavas along these profiles. The steep gradient positive anomaly to the immediate south of the Keweenaw fault is not present along these profiles. It is to be noted that there is a very broad positive anomaly of low magnitude that lies over the Jacobsville sandstone out­ crop to the south of the Keweenaw fault. This anomaly is 119 attributed to an anticlinal flexure of the Keweenawan lavas at depth. The Middle Trap Range anomaly reaches its maximum amplitude of about 750 gammas and its steepest gradient along profile K - K ’ where it appears to be very near to the surface. This observation is substantiated by the fact that the Keweenawan volcanics of the Middle Trap Range outcrop nearby at Silver Mountain. The Middle Trap Range anomaly is not present along Profile N-N* because this profile passes north of an east-west cross­ fault along which the Middle Trap Range terminates. In all of these profiles there is no magnetic anomaly asso­ ciated with the erosional wedge of the South Trap Range lavas. Several diabase dikes along the southeastern portion of these profiles cut across Pre-Keweenawan formation. None of these dike anomalies cut across the erosional wedge of the Keweenawan volcanics. There is a positive magnetic anomaly of about 2500 gamma magnitude that occurs along the southeastern portion of the Profile K - K 1. This anomaly is related to an anticlinal flexure of the metavolcanics and metasediments that underlie the Mlchigamme slate. Several other positive anomalies of smaller amplitude along the southeastern portion of the profile are associated with local anticlinal flexures along the metavolcanics and the metasediments. It is worth noting that the portion of the magnetic profile overlying the Michigamme slate is leveling off in an easterly direction from profile K - K 1 to profile N - N ' . This is mainly attributed to the thinning of the metavolcanics and metasediments, possibly due to erosion before the deposition of the younger Michigamme slate. CHAPTER IX SUMMARY This study has shown conclusively that aeromagnetic interpretation is an extremely useful tool in determining the regional structure of a Precambrian terrain consisting of widely diverse magnetic formations. It is evident, however, that the complexity of the magnetic rock proper­ ties restricts the interpretation to a semi-quantitative approach based upon the integration of the surface geology, Bouguer gravity anomalies, and the results of analytical studies of the magnetic data. Analytical studies that have proven useful in the interpretation include magnetic depth determinations and trend analysis, second vertical derivative total magnetic intensity anomalies, and corre­ lation of theoretical magnetic anomalies with the observed magnetic data. The magnetic interpretation has confirmed some geo­ logical relations and suggested new valid solutions to the major structural problems of the western portion of the Northern Peninsula of Michigan. The near surface contact between the Keweenawan lavas of the Northern Trap Range and the Jacobsville sandstone has a southerly dip along most of its extent west of longitude 89° 20' W. 121 This dip 122 probably originates from cross-faulting and/or sliding of a block of Keweenawan lavas over the Jacobsville sand­ stone to the south after the major thrust of the Northern Trap Range lavas along the Keweenaw fault. This explains the previous difficulty in tracing the Keweenaw fault on the surface along this portion of the study area. A time break in deposition of the Jacobsville sand­ stone is suggested which results in two major series of sandstone. The lower series of this formation is believed to have been deposited before extrusion of the Northern Trap Range lavas and the upper series of sandstone after the major Keweenaw faulting. The author believes that this break in deposition of the sandstone during the period of tectonlsm associated with the Keweenaw fault is the main reason for the present ambiguity about the age of the Jacobsville sandstone. It is evident that the earlier sandstone series is not older than Middle Keweenawan age and that post-faulting Beries can be either of Late Keweenawan or Early Cambrian age, depending on the magni­ tude of the unconformity that separates the two series of sandstone. The structure in the Porcupine Mountains area and the origin of the Iron River syncline are interpreted to be the result of a lopolithic intrusion of rhyolite. The sagging of the overlying sediments is believed to be con­ temporaneous with the withdrawal of the rhyolite from the underlying magma reservoir. 1 ?? The structure of the Middle Trap Range is inter­ preted as an uplifted block of lava in the form of a horst. The volcanics of the Middle Trap Range generally are buried beneath 1250 and 2500 feet of Jacobsville sand­ stone. However, they outcrop in the Silver Mountain area. This variation in depth to the Middle Range is believed to be due to several cross-faults of considerable vertical displacement. The Keweenawan lavas in the graben between the Northern Trap Range and the northern portion of the Middle Trap Range exhibit an anticlinal flexure that extends parallel to the strike of the Middle Trap Range. A thinning of the Jacobsville sandstone in the central portion of Houghton County is indicated from quantitative Interpretation of the aeromagnetic map. This thinning is suggestive of another anticlinal flexure of the Keweenawan lava. The lavas of the Middle Trap Range are interpreted as younger members of the South Trap Range lavas that have been separated from the South Trap Range east of longitude 89° 5 0 ’ W. The older members of the South Trap Range lavas extend along the contact between the older Animikie rocks and younger Keweenawan rocks. The South Trap Range lavas occasionally outcrop, but In general these volcanics are buried beneath 3000 to 5000 feet of sandstone. 12*4 The Wolf Lake granite outcropping south of the Barb Lake fault and mapped by Pritts (1966) as of Late Animikie age is believed to be of Lower Precambrian age as the granitic basement rocks in the center of the Marenisco anticline. The basement rocks are believed to have been tilted to the north during early Precambrian time and were subjected to erosion, thus resulting in a greater angular unconformity between the basement and the overlying sedi­ ments . The northerly tilt of the basement rocks resulted in a thickening of the sediments toward the north. This fact is evident in the study area where younger Animikie rocks, metavolcanics, and metasediments exposed south and west of Watersmeet, unconformably overlie the granitic basement rocks, while the older Tyler slate unconformably overlies the granitic basement rocks outcropping near Marensico. The magnetic strata of the Marenisco Range, as mapped by Fritts (1966) to be of uncertain stratigraphic position, are interpreted as the Xronwood iron formation of the Tyler slate series on the south limb of the Marenisco anticline. The volcanic breccia and basaltic flows in the upper part of the Hemlock formation, which are associated with a group of strong positive anomalies surrounding the Amasa Oval, are believed to underlie the Michigamme slate basin across the study area. These rocks were mapped as meta­ volcanics and metasediments by Pritts (1966) at the western portion of the Michigamme slate basin. 125 The thickness of Michigamme slate varies between 1500 and J4000 feet In different portions of the basin. However, a magnetic source underlying the Michigamme slate was recorded at a depth of about 7000 feet. This is the deepest magnetic source in the study area. The magnetic trend analysis results suggest that the stress acting on the Pre-Keweenawan rocks was of a nonrotational nature, and that the principal stress axis was north-south. However, the stress acting on the Keweenawan rocks is believed to be of a rotational nature due to a shear couple which was shifting in time and space during Keweenawan tectonism. BIBLIOGRAPHY BIBLIOGRAPHY Affleck, J. 1963Magnetic anomaly trend and spacing patterns. Geophysics, v. 28, p. 379-395* Allen, R. C., and Barrett, L. P. 1915* Contributions to Precambrian geology of Northern Michigan and Wisconsin. Michigan Geol. and Biol. Survey Pub. 18, Geol. Ser. 15* Bacon, L. 0. 1954. Geophysical prospecting for iron ore in the Lake Superior region (abs.). Geophysics, v. 19, no. 3* P* 6 3 4* ______ . 1957* Relationship of gravity to geologic structure in Michigan's Upper Peninsula. Inst, on Lake Superior Geology, Houghton, Michigan, pp. 54-58. ______ . I960. Subsurface geologic structure in the Jacobsvllle-Gay area of the Keweenaw Peninsula as inter­ preted from geophysical data. Sixth Annual Inst, on Lake Superior Geol., Madison, Wisconsin. ______ . 1966. Geologic structure east and south of the Keweenaw fault on the basis of geophysical evidence. The Earth Beneath the Continents, Geophysical Mon. 10, A.G.U., pp. 42-55* ______ , and Wyble, D. 0. 1952. Gravity investigations in the Iron River-Crystal Falls area* mining districts of Michigan. A.I.M.E. Transactions, v. 193, pp. 9730 979* Balsley, J. R . , James, H. L, and Wier, K. L. 1949* Aeromagnetic survey of parts of Baraga, Iron and Houghton Counties, Michigan, with preliminary geologic Interpretation. U. S. Geol. Survey, Geophysical Inv. Preliminary report. Barrett, L. P., Pardee, F. G . , and Osgood, W. 1929. Geologic map of Iron County. Michigan Geol. Survey Division, Dept, of Conservation. Bath, G. C. 1962. Magnetic anomalies and magnetizations of the Biwabile Iron formation, Mesabi area, Minne­ sota. Geophysics, v. 27, no. 5, pp. 627-650. 127 128 ______ , and Schwartz, G. M. I960. Magnetic anomalies and magnetization of main Mesabl iron formation (abs.). Inst, on Lake Superior Geology, Madison, Wisconsin, p. 27. Bayley, R. W. 1959. Geology of the Lake Mary quadrangle, Iron County, Michigan. U.S. Geol. Survey Bull. 1077, 112 pp. Billing, M. P. 1959* Structural geology. N.J.: Prentice-Hall, Inc. Englewood Cliffs, Butler, B. S., and Burbank, W. S. 1929. The copper deposits of Michigan. U.S. Geol. Survey Prof. paper 1*44, 283 pp. Campbell-* R. E. 1952. Geophysical investigation of the Silver Mountain area, Houghton County, Michigan. Unpublished M.S. thesis, Michigan Technological University, Houghton. Case, J. E., and Gair, J. E. 1965. Aeromagnetic map of parts of Marquette, Dickenson, Baraga, Alger, and Schoolcraft Counties, Michigan, and its geologic interpretations. U.S. Geol. Survey, Geophys. Inv. Map G P 1+67. Clements, J. M., and Smyth, H. L . , Jr. 1890a. The Crystal Falls iron-bearing district of Michigan. U.S. Geol. Survey, 19th Annual report, p t . 3, pp. 45-63. _______ . 1899b. The Crystal Falls iron-bearing district of Michigan. U.S. Geol. Survey Mon. 3 6 , 512 pp. Cox, A., and Doell, R. R. i960. Review of paleomagnetism. G.S.A. Bull., 71. pp. 645-768. Dubois, P. M. 1957. Comparison of paleomagnetic results for selected rocks of Great Britain and North America. Adv. Physics, 6. ______ . 1959. Correlation of Keweenawan rocks of Lake Superior district by paleomagnetic methods. Proc. Geol. Assoc. Canada, 11, pp. 115-128. ______ . 1962. Paleomagnetism and correlation of Keweenawan rocks. Bull. Geol. Survey Canada, 71, 75 pp. Fritts, C. E. 1965. Stratigraphy, structure, and granitic rocks in the Marenisco-Watersmeet area, Michigan. Eleventh Annual Inst, on Lake Superior Geology, p. 15. 129 ______ . 1967Stratigraphy, structure, and metamorphism of Upper Animikie rocks in the Marenisco-Watersmeet area (abs.). 13th Annual Inst, on Lake Superior Geol., Lansing, Michigan. Gair, J. E., and Weir, K. L. 1956. Geology of the Kiernan quadrangle, Michigan. U.S. Geol. Survey Bull. 1044, 88 pp. Graham, J. W. 1953. Changes of ferromagnetic minerals and their bearing on ferromagnetic properties of rocks. J.G.R., v. 58, PP. 243-260. Grout, P. F., Gruner, J. W., Schwartz, G. M . , and Thiel, G. A. 1951. Precambrian stratigraphy of Minnesota: G.S.A. Bull., v. 62, pp. 1017-1078. Hamblin, W. K. 1958. The Cambrian sandstones of Northern Michigan. Michigan Geol. Survey Publication 51. 145 pp. ______ . 1961. Paleographic evolution of the Lake Superior region from late Keweenawan time to late Cambrian time. G.S.A. Bull., v. 72, pp. 1-18. Heiland, C. A. 1940. Geophysical exploration. Publishing Co., N.Y., and London. Hafner Hemming, L. 1965. Total field magnetic investigation of the Keweenaw fault contact. Unpublished report for the National Science Foundation undergraduate geo­ physical research, Geophysics Dept., Michigan Tech­ nological University, Houghton. Henderson, R. G. I960. A comprehensive system of auto­ matic computation in magnetic and gravity interpreta­ tion. Geophysics, v. 25, no. 3, pp. 569-585. ______ , and Zeitz, I. 1948. Analysis of total magnetic intensity anomalies produced by point and line sources. Geophysics, v. 13, pp. 428-236. Hinze, W. J., O'Hara, N. W., Trow, J. W . , and Secor, G. B. 1966. Aeromagnetic studies of eastern Lake Superior. The Earth Beneath the Continents. Geophysical Mon. 10, A.G.U., pp. 95-110. Hubbard, H. A. 1967. Keweenawan volcanic rocks near Ironwood, Michigan. Thirteenth Annual Inst, on Lake Superior Geology, pp. 20-21. 130 Irving, E. 1964. Paleomagnetism and its application to geological and geophysical problems. John Wiley and Sons, Inc., N.Y. Irving, R. D. 1880. 101-103. Geology of Wisconsin, v. 3, pp. 62-70, ______ . 1883. The copper-bearing rocks of Lake Superior. U.S. Geol. Survey Mon. 5. 464 pp. ______ , and Van Hise, C. R. 1892. The Penokee iron-bearing series of Michigan and Wisconsin. U.S. Geol. Survey Mon. 19* 534 pp. Jahren, C. E. i9 6 0. Magnetizations of iron formations and igneous rocks of northern Minnesota (abs.). Inst, on Lake Superior Geology, Madison, Wisconsin, p. 28. ______ . 1963. Magnetic susceptibility of bedded iron formations. Geophysics* v. 28, pp. 756-766. ______ . 1965. Magnetization of Keweenawan rocks near Duluth, Minnesota. Geophysics, v. 30, pp. 858- 8 74 . James, H. L. 1955Zones of regional metamorphism in the Precambrian of Northern Michigan. G.S.A Bull., v. 66, pp. 1455-1487. ______ . 1958. Stratigraphy of Pre-Keweenawan rocks in parts of Northern Michigan. U.S. Geol. Survey Prof. paper 3l4-c, pp. 27-44. ______ , Dutton, C. E., Pettijohn, F. J., and Weir, K. L. 1959. Geologic map of the Iron River-Crystal Falls district, Iron County, Michigan. U.S. Geol. Survey Mineral Inv. Field Studies Map MF-225Lane, A. C. 1909. The Keweenaw series of Michigan. Michigan Geological Survey Publication 6, series 4, pp. 628- 629. Leith, C. K., Lund, R. J., and Leith, A. 1935Precam­ brian rocks of Lake Superior region. U.S. Geol. Survey Prof. paper 184, 34 pp. Martin, H. M. (compiler). 1936. The centennial geological map of the Northern Penninsula of Michigan. Michigan Geol. Survey Div. Pub. 39> Geol. Ser. 33. Miller, R. L and Kahn, J. S. 1962. Statistical analysis in the geological sciences. John Wiley and Sons, Inc., N.Y. 469 pp. 131 Miller, W. R. 1966. A gravity investigation of the Porcu­ pine Mountains and adjacent area, Ontonagon and Oogebic Counties, Michigan. Unpublished M.S. thesis, Geol. Dept., Michigan State University. Mooney, H. M., and Bleifuss, R. 1953* Magnetic suscepti­ bility measurements in Minnesota, 2, Analysis of field results. Geophysics, v. 18, pp. 383-393. Nettleton, L. L. 19*12. Gravity and magnetic calculations. Geophysics, v. 7* no. 3» pp. 293-310. Peters, L. J. 1949. The direct approach to magnetic inter­ pretation and its practical application. Geophysics, v. 14, pp. 290-320. Prinz, W. C. 1967. Pre-quaternary geologic and magnetic map and sections of part of the eastern Gogebic iron range, Michigan. U.S. Geol. Survey. Miscellaneous geologic investigations, Map 1-497. Reed, R. 1966. Personal communication. The Geol. Survey Division of the Michigan Dept, of Conservation. Roberts, E. 1940. Geology of the Alston district. Unpublished M.S. thesis, California Institute of Technology, Pasadena, pp. 10-21. Smellie, D. W. 1956. Elementary approximation in aeromagnetic interpretation. Geophysics, v. 21, pp. 1021-1040. Taiwan!, M., Worzel, J. L, and Landisman, M. 1959. Rapid gravity computations for two-dimensional bodies with application to the Mendocino submarine fracture zone. J.G.R., v. 64, pp. 49-59. Thaden, R. E. 1950. The Porcupine Mountains "Red rock." Unpublished M.S. thesis, Geol. Dept. Michigan State University. Thiel, E. 1956. Correlation of gravity anomalies with the Keweenawan geology of Wisconsin and Minnesota. G.S.A. Bull. v. 67, pp. 1079-1100. Vacquier, U., Steenland, N . , Henderson, R. G., and Zeitz, I. 1951. Interpretation of aeromagnetic maps. G.S.A. Memoir, no. 4 7 . Van Hise, C. R . , and Bayley, W. S. 1895. Preliminary report on the Marquette iron-bearing district of Michigan. U.S. Geol. Survey 15th annual report, pp. 477-650. 132 ______ . 1897. The Marquette Iron-bearing district of Michigan. U.S. Geol. Survey, Mon. 26. ______ , and Leith, C. K. 1911. The geology of the Lake Superior region. U.S. Geol. Survey, Mon. 52. White, W. S. 1953* Stratigraphic sections in the vicinity of the White Pine copper mine. Geol. Survey Preliminary report, Open file, Lansing, Michigan. ______ . 195^. The White Pine copper deposits. Geol., v. 49, pp. 675-716. Econ. ______ . 1957. Regional structural setting of the Michigan native copper district. Inst, on Lake Superior Geology, Houghton, Michigan, pp. 3-16. ______ . i9 6 0. The Keweenawan lavas of Lake Superior, An example of flood basalts. Amer. Jour, of Science, 258-A, pp. 367-374. ______ . 1966a. Tectonics of the Keweenawan basin, western Lake Superior region. U.S. Geol. Survey Prof. paper 424-E, 23 pp. ______ , and Wright, J. C. 1962. Geologic maps showing outcrops of the Nonesuch shale from Calumet to Black River, Michigan. U.S. Geol. Survey, Open file report. Wier, K. L. 1967. Geology of the Kelso Junction quadrangle, Iron County, Michigan. U.S. Geol. Survey Bull. 1226, 47 pp. ______ , Balsley, J. R . , and Pratt, W. P. 1953. Aeromagnetic survey of part of Dickinson County, Michigan, with preliminary geologic interpretation. Geophysical Inv. Map GP-1 1 5 . Wold, R. J., and Ostenso, N. A. 1966. Aeromagnetic, gravity and sub-bottom profiling studies in western Lake Superior. The Earth Beneath the Continents, Geophysical Monograph 10, A.G.U., pp. 66-94. Wright, P. E. 1909. The intrusive rocks of Mount Bohemia, Michigan. Michigan Geol. Survey report, pp. 355-402. ______ , and Lane, A. C. 1909. Preliminary geological map of the Porcupine Mountains and vicinity. Michigan Geol. Survey report, p. 11. 9Q 50 46 ° 45* 25 I 5' IO 90* 00' ■ ■ - - r i" ■■■ ■■ 89*5! 40 l a k e R survey c S2 -O6 P£.9 * *630 90° 25 20 PLATE I a 25 P L A T E I-a 25 * 20 I7SO 20 -46 15 90 15 90~ 00‘ 7 / 46 5 5 50 45 PL A' 3J. 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