72-8718 KELLOGG, Richard Lynn, 1938AN AEROMAGNETIC INVESTIGATION OF THE SOUTHERN PENINSULA OF MICHIGAN. Michigan State University, Ph.D., 1971 Geology University Microfilms, A XEROX Company. Ann Arbor, Michigan AN AEROMAGNETIC INVESTIGATION OF THE SOUTHERN PENINS U L A OF M I C H I G A N By Richard L. Kellogg A THESIS Submitted to M ichigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Geology 1971 ABSTRACT AN AEROMAGNETIC INVESTIGATION OF THE SOUTHERN PENINSULA OF MICHIGAN By Richard L. Kellogg Only fragmentary direct information is available on the basement complex underlying the Phanerozoic sediments of the Michigan Basin because of the limited and poorly distributed basement drill tests. T o supplement this information, a regional aeromagnetic survey has been con­ ducted of the Southern Peninsula. Approximately 17,000 miles of total magnetic intensity data were recorded along north-south flight lines spaced at three mile intervals, and flown at 3,000 feet MSL. A basement configuration map prepared from magnetic depth estimates and basement drill tests confirms that the basement surface under the Southern Peninsula has the form of an oval depression reaching a maximum depth of app r o x i ­ mately 15,0u0 feet below sea level on the western shore of Saginaw Bay. A basement high underlies the Howell anticline in Livingston County and a roughly north-south striking regional basement trough plunges into the basin from the Richard L. Kellogg common boundary point of Indiana, Ohio and Michigan to the vicinity of 42°30'N. The map shows a broad basement platform striking northwest in the extreme southwest corner of the Peninsula. Interpretation of the residual aeromagnetic map in conjunction with geologic and other regional g e o p h y s i ­ cal data from the Souchern Peninsula and adjacent areas indicates that the basement of the Michigan Basin has had a complex geologic history. Four basement provinces are delineated on the basis of magnetic and gravity anomalies, isotope ages of samples obtained from basement drill holes, and extrapolation of known Precambrian geology from the margin of the Basin. The Penokean province can be traced from northern Michigan and Wisconsin into the northern portion of the Southern Peninsula by means of several regional magnetic a n o m a l i e s . Peninsula, In the Southern this province is characterized by east-southeast striking anomalies. Central and southwestern Michigan is underlain primarily by felsic rocks correlating wit h the Central province. Basement rocks in southeastern Michigan, which strike generally north-northeast are interpreted as metamorphosed intrusives and extrusives and mafic and felsic gneisses of the Grenville province. The western boundary of this province strikes south-southwest from Saginaw Bay to west of the Howell anticline and then due Richard L. Kellogg south to the Michigan-Ohio boundary. A Keweenawan rift zone characterized by mafic intrusives, extrusives and uplifted gneisses transects the Peninsula from the Grand Traverse Bay area to southeastern Michigan. Keweenawan igneous activity may also occur southwest of the Keweenawan rift zone where several local magnetic anomalies occur along northwest striking trends. These trends parallel the regional gravity anomaly pattern. The geology of the Keweenawan rift zone was in­ vestigated by matching observed magnetic and gravity anomalies with theoretical anomalies derived from models based on geological and geophysical interpretation of the Mid-Continent and Kapuskasing gravity anomalies. The observed anomalies can be derived from a variety of p o s s i ­ ble geological sources. An interpretation bas e d on a rift zone containing volcanics, gneisses and intrusives d i f ­ ferentially uplifted against granite, gneisses and metasediments of adjacent provinces is favored for the origin of the magnetic anomaly. The gravity anomaly may be explained by these lithologies plus associated m o d i ­ fication of deep crustal layers. PLEASE NOTE: Some Pages have i n d i s t i n c t prin t. Filmed as r e c e i v e d . UNIVERSITY MICROFILMS ACKNOWLEDGMENTS The author wishes to express his sincere a p p r e c i a ­ tion to Dr. William J. Hinze for his invaluable guidance, helpful suggestions and thorough review of all phases and aspects of this study. Special thanks are due to Drs. Hugh F. Bennett, James W. Trow, and James K. Fisher for their interest and helpful suggestions, and to Dr. C. E. Prouty and H. B. Stonehouse who read and criticized portions of the manuscript. Of the ma n y individuals who assisted in the data gathering and reduction phases, I am particularly indebted to Dr. Donald W. Merritt for his assistance in developing computer programs and reviewing portions of the manuscript,. This investigation was ma d e possible by grants of operating funds from the following organizations: Consumers Power Company, Humble Oil Company, Leonard Refineries, McClure Oil Company, Michigan Consolidated Gas Company, Miller Brothers, Mobile Oil Company, Musk e g o n Development Company, Shell Oil Company, Sun Oil Company, Union Oil C o m ­ pany, and the Michigan State University All-University Research Fund. TABLE OF CONTENTS Page A C K N O W L E D G E M E N T S .............................................. ii LIST OF T A B L E S ............................................. V LIST OF F I G U R E S .............................................. vi Chapter I. INTRODUCTION ...................................... 1 The Aeromagnetic Survey of the Southern Peninsula of Michigan ........................ Nature and Objectives ........................ Scope and O r g a n i z a t i o n ................. ... General Geology of the Southern Peninsula . . Previous Baseme n t Studies and Precambrian Geology ...................................... The Tectonic Framework of the Michigan Basin Basement Lithologies ........................ Paleozoic Features Assoc i a t e d with the Michigan B a s i n ........................... ... Development of the Michigan Basin . . .. 1 5 Influence of Basement Topography on the Paleozoic Sediments in the Midwestern United States ............................... II. 4 9 11 13 19 COLLECTION OF AEROMAGNETIC D A T A ................ 21 Instrumentation ............................... Flight Crew and A i r c r a f t ................... 22 Survey Procedures ............................... Accuracy of Navigation ........................ Meteorological Considerations ................. Summary of Field Operations ................. III. 1 1 3 4 COMPILATION AND REDUCTION OF MAGNETIC DATA . . Introduction ................................... Digitization of Navigational Fix Data . . . Digitization of Analog Records.................. Merger of Navigational Data and Digitized Magnetograms ................................... iii 21 24 29 31 32 34 34 36 37 38 Chapter Page Preparation of the Diurnally Corr e c t e d Data . Removal of the Earth's Normal M a g n e t i c Field . Preparation of the Corre c t e d Data for Machine Contouring ............................ Contouring of Aeroma g n e t i c Data .............. IV. INTERPRETATION OF A E R OMAGNETIC D A T A . . . . 39 40 41 44 47 Configuration of Basement Surface . . . . 47 Introduction ................................... 47 Selection of Depth D e t ermination Techniques. 50 Discussion of the Basem e n t Configuration M a p ................................................. 59 Sources of Anomalies ............................ 60 Characteristics of M a g n e t i c Anomalies in the Southern Peninsula of Michigan .............. 68 Basement Structure and Litho l o g y .............. 75 Introduction ................................... 75 Grenville Province ............................ 84 Central and Penokean Provinces .............. 88 Keweenawan Rift Zone and Related A c t i v i t y . 94 Interpretation of the M i d - M i c h i g a n Anomaly . 100 Mi d North American P a l e o - R i f t Systems . . 100 The East African Rift S y s t e m ................. 104 Quantitative Study of the Mid-Michigan A n o m a l y ......................................... 107 Relationship of the Keweenawan Rift Zone to the Grenville Province in Southeastern M i c h i g a n ......................................... 120 V. C O N C L U S I O N S ................................... “ BIBLIOGRAPHY ............................................. . 124 129 A P P E N D I X ....................................................... 138 iv LIST 'OF TABLES Table Page 1. Basement Drill Holes in the Southern Peninsula of M i c h i g a n .......................................... 12 2. The Earth's Normal Total M a g n e t i c Intensity in the Vicinity of the Southern Peninsula of M i c h i g a n .............................................. 43 3. Comparison of Magnet i c Depth Determinations with Basement Drill Depths... .......................... 4. Magnetic Depth Determination Results 5. Isotope Age Dates Merritt, 1969) . . . . 51 55 (Modified from Hinze and 78 6. Specific Gravity and Magnetic Susceptibility D a t a ................................................ 108 7. Parameters of Induced, Remanent and Combined Ma g n e t i c Polarization Vectors ................... 109 LIST OF FIGURES Basement Drill Holes, Southern Peninsula of M i c h i g a n .......................................... 5 Structure in the Vicinity of the Mich i g a n Basin 10 Regional Structure and Structure Contours on the Basement Complex of the M i c h i g a n Basin 16 ..................... 23 Flow Diagram Illustrating Sequence of Data Reduction Steps ............................... 35 Normal Geomagnetic Field over the Southern Peninsula of Michigan ................. 42 Representative Magnetogram . Basement Configuration Ma p of the Southern Peninsula of Michigan ........................ 58 Simulated Gravity Profiles (after Hinze and Merritt, 1 9 6 9 ) ................................... 62 Simulated Magnetic Profiles (after Hinze and Merritt, 1 9 6 9 ) ................................... 63 Residual Total Magnetic Intensity M a p of the Southern Peninsula of Michigan . 69 Residual Bouguer Gravity Anomaly Ma p of the Southern Peninsula of Mich i g a n .............. 73 Basement Province Ma p of the Southern Peninsula of Michigan ........................ 80 Double Fourier Series Residual Gravity Anomaly Map of the Southern Peninsula of Michigan (after Hinze and Merritt, 1969) .............. 82 Magnetic Trend Map, Southern Peninsula and V i c i n i t y .......................................... 83 vi Figure 15. 16. 17. 18. 19. Page Typical Cross Sections, East African and Mid-America Rift Systems ..................... 101 Observed and Computed Bouguer Gravity and Total Magnetic Intensity Anomaly Profiles across the Howell Anticline Area, Illustrat­ ing Basalt Trough Model ..................... Ill Observed and Computed Bouguer Gravity and Total Magnetic Intensity Anomaly Profiles across the Howell Anticline Area, Illustrat­ ing High Grade Metamorphics Model . . . . 115 Observed and Computed Bouguer Gravity and Total Magnetic Intensity Anomaly Profiles from St. Joseph County to Thunder Bay Area, Illustrating High Grade Metamorphics Model . 116 Observed and Computed Bouguer Gravity and Total Magnetic Intensity Anomaly Profiles across the Howell Anticline Area, Illustrat­ ing Basalt Trough and Deep Crustal Layer Model. ......................................... 118 A-l. Location of Profiles Selected A-2. Magnetic Anomaly Profiles 1 and 2 ............. 14 9 A-3. Magnetic Anomaly Profiles 3 and 5 ............. 150 A-4. Magnetic Anomaly Profiles 9, 9 U.c., and 10. A-5. Magnetic Anomaly Profiles 4, 7, and 8. A-6. Comparison of Fourier Spectra of Profiles 1 and 2 ............................................ 153 A-7. Comparison of Fourier Spectra of Profiles 2 and 3 ............................................ 154 A-8. Comparison of Fourier Spectra of Profiles 2 and 9 ............................................ 155 A-9. Comparison of Fourier Spectra of Profiles 9 and 5 ............................................ 156 A-10. Comparison of Fourier Spectra of Profiles 9 and 1 0 ............................................ 157 A - 11. Comparison of Fourier Spectra of Profile 9 Upward Continue?, and Profile 1 0 .................158 Vii for Analysis . . . . 148 . 151 . 152 . Figure Page A-12. Comparison of Fourier Spectra of Profiles 8 and 4 ........................................ 159 A-13. Comparison of Fourier Spectra of Profiles 7 and 8 .........................................160 A - l 4. Comparison of Fourier Spectra of Profiles 4 and 7 .........................................161 viii CHAPTER I INTRODUCTION The Aeromagnetic Survey of the Southern Peninsula of Michigan Nature and Objectives The Michigan Basin, centered in the Southern Peninsula of Michigan covers the basement with an est i ­ mated ma x i m u m 15,000 feet of Phanerozoic sediments. basement, which is the object of this study, The is defined as the first igneous or metamorphic rocks found under the unmetamorphosed sedimentary cover. The basement complex of the entire Midwe s t has been the subject of considerable recent interest not only for the purpose of determining the Precambrian geologic history, but also because of the increasing awareness of the role of the basement on the sedimentation and structure of the overlying Phanerozoic sediments. Exploration for oil and other natural resources has provided a weal t h of infor­ mation about the shallow formations of the Michigan Basin; however, the deeper sediments and the basement itself have largely remained untouched by the drill. Therefore, limited direct geologic information is available. 1 2 To augment the available geologic data, a number of regional geophysical studies have been undertaken. These studies indicate that the base m e n t in the Midwest is complex and contains man y interesting structural and lithologic features. Ma g n e t i c investigations conducted in adjacent states and Canada where Precambrian structures are exposed have indicated considerable magnetic relief that can be correlated wi t h these geologic features. Magnetic studies of the Southern Peninsula also show many interesting magnetic anomalies. However, magnetic coverage is limited to a reconnaissance vertical intensity survey with a station spacing of approximately six miles (Hinze, 1963) which is inadequate to define the magnetic anomalies for anything but the most regional of inter­ pretations. Thus, an aeromagnetic survey was conceived as an economical and rapid means of improving our k n o w ­ ledge of the regional configuration, lithology and ■ structure of the basement of the Mich i g a n Basin. Specifically, the objectives of this study are to: 1. determine the configuration of the basement surface and prepare a structure contour map of this surface from the magne t i c and well c o n ­ trol data; 2. delineate the structural trends and basement provinces of the Southern Peninsula and particularly the Grenville Front; 3 3. quantitatively investigate the possible sources of the major positive gravity and magnetic feature known as the Mid-Michigan gravity and magnetic anomaly crossing the State from Lake St. Clair to the Grand Traverse Bay region. Scope and Organization Continuous observations of the magnetic field were made along north-south traverses flown at three mile intervals. Magnetic observations were made with a proton precession magnetometer and navigation was by visual fixes to cultural features. A series of east-west flight lines provided a network into which all flight lines wer . tied. Nearly 22,000 line miles of data were recorded, of which 17,000 north-south line miles were finally incorporated into the aeromagnetic maps. Flight elevation for the survey was 3,000 feet above mean sea level or roughly 2,000 to 2,400 feet above the ground surface. The selection of the traverse separa­ tion was largely dictated by economic factors; however, the flight elevation was chosen to be consistent with aeromagnetic surveys previously flown in the upper M i d ­ west (Patenaude, 1964, Hinze et a l . , 1966). To facilitate the collection of data and to reduce access time to a minimum, the area of investigation was divided along latitude 44°00'N into two overlapping sec­ tions. The base of operations for the southern section 4 was East Lansing, and the base for the northern portion was located at Pellston, near the northern tip of the Southern Peninsula. Observed total intensity aeromagnetic data were correlated with navigational data and computer processed to produce a residual total intensity m ap of the Southern Peninsula. Specific information regarding the collection, reduction, presentation and methods of interpretation of the aeromagnetic data will be d iscussed in sections treating each of these subjects individually. General Geology of the Southern Peninsula Previous Basement Studies and Precambrian Geology Direct geologic investigation of the basement of the Southern Peninsula has been restricted because of the paucity of deep drill holes; however, the available basement tests information from (Figure 1) has been s up­ plemented by the extrapolation of geologic data from outcrops exposed on the Precambrian shield to the west, north and east of the Michigan Basin. In addition, numerous drill holes have p enetrated basement in nearby Wisconsin, northern Illinois, southern Ontario. Indiana and Ohio, and Several authors have recently studied the basement provinces in the vicinity of the Southern Peninsula. Lidiak et al. (1966) studied provincial 5 BEAVER ISLAND 86 ° 00 ' 8 3 ° 00* EMMET CHEBOYGAN PRESQUE ISLE + 45® 00’ 4»® 00' + ALPENA LEELANAU TRAVERSE KALKASKA CRAWFORD ROSCOMMON OSCODA OGEMAW ALCONA IOSCO ARENAC 4 4 ° 0 0 '4 - MASON + LAKE GLADWIN OSCEOLA OCEANA MECOSTA ISABELLA MIDLAND TUSCOLA SANILAC MONTCALM SAGINAW KENT MACOMBOAKLAND LIVINGSTON KALAMAZOO CALHOUN HILLSDALE JOSEPH MONROE LENAWEE * * V SB® 0 0 ' SCALE 0 10 20 30 MILES A basem ent o r ill h o le Figure 1 .— Basement Drill Holes, Southern Peninsula of Michigan. 6 boundaries on the basis of geochronology in the Midconti­ nent region. Following Hinze (1963), they show a belt of Keweenawan mafic igneous rocks transecting the Southern Peninsula. These rocks extend from the Grand Traverse Bay region to a point roughly 35 miles west of Lake St. Clair, where they are abruptly terminated by the western margin of the Grenville province. They suggest that "the supracrustal Keweenawan rocks terminate near Lake Erie, possibly as a result of erosion during uplift of the intracrustal Grenville rocks, or because of nondeposition of these rocks beyond southeastern Michigan" (Lidiak et a l . , 1966, pg. 5429). Muehlberger et al. (196 7) also reviewed the struc­ tural provinces underlying the Michigan Basin. between the Central province province (1.6-1.8 b.y.) (1.2-1.5 b.y,) The boundary and the Penokean was drawn just north of the northern tip of the Southern Peninsula. Most of the Southern' Peninsula was considered to be underlain by rocks of the Central province except for the transecting Keweenawan rift zone discussed above and the Grenville province in the eastern portion of the Thumb area. Hinze and Merritt (1969) show much the same picture as Muehlberger et a l . except that the boundary between the Penokean and Central provinces was drawn so as to roughly divide the Southern Peninsula along latitude 44°00'N. Hinze and Merritt also assigned lithologies to the basement 7 rocks, on the basis of geophysical anomalies, basement tests and regional geology. Cohee (1945) published a basement configuration map of the Southern Peninsula bas e d on four basement tests plus extrapolation from known geology. He depicted the basement as elliptical in plan with regularly spaced contours, and little or no expression of intrabasin structures. Rudman et a l . (1965) revised the basement surface map by taking into account subsequent basement tests. Again, the picture obtained was one of regularly spaced contours. Bayley and M u ehlberger essentially the same map as Rudman (196 8) show et a l .; however, their map shows a broadening of the basement depression into the southern extremity of Lake Michigan. Hinze and Merritt (1969), assuming major basement relief to be reflected in shallower horizons, prep a r e d the most detailed map of the basement surface to datfe. They utilized an Ordovician Trenton structure contour map and available well control and published magnetic depths in the preparation of their map. Their map is characterized by a maximum depth of 14,000 feet bel o w sea level in Bay, Midland, and Gladwin counties, a top o ­ graphic ridge associated wi t h the Howell anticline, and a general elongation of the basin to the southwest. In addition, they hypothesize a fault-line scarp to exist in the basement in Hillsdale, Calhoun, and Barry Counties, associated with the Albion-Scipio oilfield. 8 Many other p a p e r s , aimed primarily at studying the basement rocks in the upper Midw e s t have been published. Among the more important are aeromagnetic investigations of eastern Lake Superior, Lake Huron and Lake Michigan, by Hinze et al. O'Hara (1966), (1971) Secor et al. respectively. (1967) , and Hinze and Aeromagnetic studies of Wisconsin and Indiana have been undertaken by Patenaude (1966), and Henderson and Zietz (1958) respectively. A gravity survey of the eastern portion of the Northern Peninsula of Michigan has been completed by Oray Nwachukwu et al. (1971). (1965) have conducted shipborne magnetic work over Lake Huron, Georgian Bay, and adjacent areas of southwestern Ontario, Manitoulin Island, and eastern Michigan. Rudman (196 3) and McGinnis (1966) have studied the basement surface in Indiana and Illinois, utilizing the seismic method. However, respectively there are n o - p u b ­ lished accounts of the use of the seismic m e t h o d in studying the configuration of the basement surface in the Southern Peninsula. This can probably be attributed to the absence of a well defined acoustical boundary between the weathered basement surface and the overlying sandstones. Cambrian arkoses and No crustal refraction seismic investigations have been reported from the Southern Peninsula although considerable work has been undertaken in the Lake Superior region to the north ^nd wes t (Cohen and Meyer, 1966, 9 Smith et a l . ,1966/ Steinhart et a l . , 1966 and Mooney et a l . , 1970). Additional local gravity and magn e t i c surveys aimed primarily at investigating specific anomalous areas of the basement of the Mich i g a n Basin are reported by Brett (1964) (1960), Meyer and Shaw (196 3), Stevenson (196 4), Patenaude (1971). The Tectonic Framework of the Michigan Basin Midwestern United States is a stable area charac­ terized by prominent sedimentary basins and somewhat less well defined arches. The M i c h i g a n Basin is bordered on the north by the Precambrian shield complex of igneous and metamorphic rocks (Figure 2). The w e s t e r n boundary is the Wisconsin Arch, while to the southwest, the Michigan Basin is separated from the Illinois Basin by a d i s c o n ­ tinuous structural high, Kankakee Arch. generally referred to as the The Kankakee Arch joins the Findlay Arch near the Indiana-Ohio boundary. N o r t h of this junction lies the Indiana-Ohio Platf o r m generally assumed to be the northern extension of the bro a d Cincinnati Arch (Green, 1957). Woodward (1961) has identified an anticlinal structure lying about 60 miles east and subparallel with, Waverly Arch. the Findlay Ar c h w h i c h he named the 10 MAJOR STRUCTURAL TRENDS SOUTHERN PENINSULA AND VIC IN ITY ANTICLINE SYNCLINE FAULT GRENVILLE FRONT FAULT LAKE SUPERIOR SCALE 20 20 40 MILES O — o' CHATHAM SAG INDIANA IN D IA NA -OH ld PLATFORM Figure 2. — Structure in the Vicinity of the Michigan Basin. 11 The Algonquin Axis of southeastern Ontario appears, at first glance, to be a continuation of the Findlay Arch although the two features are not on trend and apparently are not related (Sanford and Quillian, 1958). Hamblin (1958) has identified an east-west trending highland in the Northern Peninsula of Michigan, on the basis of sedi­ ment dispersal studies. Basement Lithologies The lithology of the basement is important in defining basement provinces. Direct information on basement lithology in the Southern Peninsula is confined to fifteen poorly distributed basement tests (Figure 1) whose lithologies are described in Table Samples 1. obtained from these basement tests often lack a c o m p r e ­ hensive description; in addition, they show evidence of contamination from overlying sediments. However, the descriptions listed are compatible wit h descriptions of basement samples from surrounding areas (Rudman et a l ., 1965;Lidiak et a l . , 1966 and M u ehlberger et a l . , 1967). The majority of the tests are in the southeastern portion of the Southern Peninsula and have encountered both granite, granite gneiss and more mafic gneisses. The age and lithologic relationships for the basement test in Presque Isle County are uncertain. Bass (1968) believes the rocks encountered in this well are TABLE 1.— Basement Drill Holes in the Southern Peninsula of Michigan.^ (Modified from Hinze and Merritt, 1969) Location County Sec. Berrien Charlevoix Lenawee Livingston Monroe Monroe Monroe Presque Isle St. Clair St. Clair St. Clair Washtenaw Washtenaw Washtenaw Wayne 10 6 32 11 29 19 16 29 31 17 26 27 16 12 16 Twn. 6S 37N 8S 3N 5S 7S 7S 35N 4N 2N 5N IS IS 2S 4S Precambrian Surface Rnge. 17W 10W 5E 5E 10E 7E 6E 2E 15E 16E 16E 7E 7E 7E 9E Below Sea Level 3802 3988 3150 6179 2745 2926 2951 4902 3989 3436 4065 5185 5459 4852 3360 (Ft) Lithology Granite Gneiss2 Granite Granite & Granite Gneiss^ Intermediate Gneiss5 Granite Granite Granite Gneiss Quartzite & Greenstone Granulite Plagioclase-Quartzite Gneiss Biotite G n e i s s 4 Chlorite Schist^ Gneiss^ Gneiss^ Granite Gneiss ^Data from Michigan Geological Survey, except as noted. 2Yettaw (1967) . ^Summerson (1962). 4Lidiak et al. 5Laaksonen (1966) . (1971). 13 Keweenawan basalts subjected to alteration either subse­ quent to or in late Keweenawan t i m e . (Bradley, 19 71) Other workers (personal communication) have reported a much younger age for the greenstone based on K-Ar determinations. Paleozoic Features Associated with the Michigan Basin The Tectonic Map of Canada shows numerous folded areas and major faults at the northern edge of the Michigan Basin and similar features occur in northern Michigan and Wisconsin bordering the Basin. We can therefore presume that the basement of the Mich i g a n Basin is characterized by comparable features. Several structures have been identified within the Michigan Basin. In southeastern Michigan, there are three maj o r Paleozoic features trending northwestsoutheast, (Fisher, all of which may be associated with faulting 1969). The Howell anticline northwest across Livingston County. (Figure 2) trends The southwest flank of this structure shows evidence of fault control 1928, 1933), a flexure. although Ells (1969) (Newcombe has mapped this area as Regional considerations, based on the Trenton (Middle Ordovician) datum suggest that this anticlinal trend may continue northwesterly through the southeastern part of Clinton County before losing identity A recent b asement tej^t (Howard J. Messmore No. (Ells 1969). 1, Mobil 14 Oil Corp., 1970) located in Sec. 11, T3N, R5E, about six miles northeast of the axis of the Howell anticline co n ­ firmed the long held belief that this structure is reflected in the basement surface, map (Hinze and Merritt, as shown on a previous basement 1969). Bowling Green fault has been mapped in Ohio (Figure 2). This fault may extend into the Southern Peninsula where it is associated with the Lucas-Monroe monocline along the southern half of the Monroe-Lenawee County line. The Albion-Scipio oilfield which trends northwest across Hillsdale and Calhoun Counties is also considered to be fault controlled (Ells, 1962). Wells drilled into Ordovician Trenton and Black River formations show no vertical displacement; primarily strike slip thus, the fault is believed to be (Fisher, 1969). However, Merritt (196 8) has attributed a change in the regional gravity gradient occurring in the vicinity of the Albion - S c i p i o field to basement topographic relief. He further suggests that this topographic feature represents either a fault or fault-line scarp having several hundred feet of relief. Ells (1962) has pointed out that the dominant trend of other Paleozoic structures and faults t h r o u g h ­ out the Basin is northwest-southeast. Regional g e o p h y s i ­ cal data also confirm the northwest-southeast trend of these features south of roughly 43°30'. Thus the dominant 15 northwesterly intra-basin structural trend strongly suggests lines of weakness along which Paleozoic str u c ­ tural features later developed. Development of the Michigan Basin The Michigan Basin is a roughly circular str u c ­ tural basin (Figure 3). It includes the Southern Peninsula and eastern portion of the Northern Peninsula of Michigan, eastern Wisconsin, northeastern Illinois, northern Indiana, northwestern Ohio, and southwestern Ontario. The basin includes an area of 122,000 sguare miles, part of which is covered by Lakes Michigan, Huron and St. Clair, and is estimated to contain more than 14,000 feet of Phanerozoic sediments. The time of initial subsidence of the basin has been debated in the literature. (19 33) Pirtle (19 32) and Newcombe are of the opinion that the basin began to form in Keweenawan time. Subsequent folding took place parallel to the major axis of the downwarp which in turn parallels the direction of the Kankakee and W isconsin Arches. Lockett (1947) has suggested that a very early mountain chain extended from Ontario through western Ohio and that the reflection of this belt may be found in the Paleozoic sediments as the Cincinnati and Findlay Arches. During the Paleozoic era, the dominant structural move m e n t was thought to be subsidence of the intervening bas i n areas. 16 8 3' 90 * OUTCROP UTHOLOGY UPPER SCALE (>H«»«tT») | M IOOLE and KEWEENAW AN SEDIMENT KEWEENAW AN EXTRUSIVE M ETAV O LC AN IC ROCK 46* PRE-KEW ECNAW AN FEUSIC f t MAFIC ROCK -f- 45* 90* 63 ' Figure 3.— Regional Structure and Structure Contours on the Basement Complex of the Michigan Basin. 17 Green (1957) assigned a late Silurian time to the inception of subsidence in the Michigan Basin. He also invoked the idea of Basin subsidence rather than uplift of the structural“"features between basins to explain the origin of the positive features. Fisher (19 69) on the basis of isopach studies of the Ordovician sediments believes the Basin, as it is known today, was created during Ordovician time. Deposition of Paleozoic marine sediments in the Michigan Basin, to the best of our knowledge, did not begin until late Cambrian time when the sea transgressed from the south. Sand from the w e a t h e r e d Precambrian surface to the north and northwest formed a thick blanket sandstone over the area of the Michigan Basin. Cambrian deposits accumulated to an average thickness of about 2,600 feet of sandstone, dolomite and shale (Cohee, 1965). Two depositional centers separated by an intervening high are shown by Fisher (1969). Fisher correlates the high with the Mid-Michigan gravity high (Hinze, 1963) inferring that the feature responsible for the gravity anomaly was a structural or topographic high during Cambrian time. Sedimentation was continuous from late Cambrian through early Ordovician time. Clastic sediments were deposited around the margins of the basin. A profound unconformity separates lower O rdovician rocks from the overlying middle Ordovician rocks. Both the Kankakee 18 and Findlay Arches are slightly positive during the late Ordovician. Lower Silurian rocks consist largely of shales and carbonates. By Middle Silurian time a massive barrier reef had grown around the margin of the basin reaching a maximum thickness of over 900 feet in the northern part of the Southern Peninsula. The single greatest episode of sinking occurred during Salina (Late Silurian) time with the accumulation of over 2,800 feet of evaporites, carbonates, and shales with a depocenter near the southwest end of Saginaw Bay. Devonian rocks, which are about 3,500 feet thick, are largely dolomite, sandstone, salt and anhydrite in the lower part and limestone and shale in the upper part. Evidence that the Devonian depocenter shifts with time has been suggested by Prouty (1971). More than 2,100 feet of Mississippian sandstone and shale outcrop almost entirely within the Southern Peninsula of Michigan. Most of the folding in the central part of the basin is late Mississippian in age. About 750 feet of Pennsylvanian sandstone and shale occupy the central part of the basin. Overlying Pennsylvanian rocks in the western part of the central basin area are a few erosional remnants of the Jurassic period, only known Mesozoic rocks in Michigan. the Pleistocene 19 glacial deposits cover the bedrock surface ranging in thickness up to 1,000 feet. Influence of Basement Topography on the Paleozoic Sediments in the Midwestern United StatesThe role of the base m e n t in shaping the major structural and tectonic features in the M idwestern United States has already been considered (see also Figure 3). A more detailed examination is required if some relations between local basement scarps, ridges, sedimentary folds, and faults within these maj o r structural and tectonic divisions are to be established, and related to the Michigan Basin. A prominent semi-continuous east-west basement scarp extends from south-western Illinois into western West Virginia. This scarp, which underlies the Rough Creek and Kentucky River fault systems influenced pri m a r i ­ ly Cambrian and Ordovician sedimentation, but was also a zone of structural movemen t during late Paleozoic time and, in places, is still active. Apparent vertical di s ­ placements as great as 5,0 00 feet have been measured along this fault system (Summerson, 1962, Rudman et a l . , 1965). A north-south base m e n t ridge has been correlated with the LaSalle anticline in Illinois, and apparently controlled the structural development of the anticline during Paleozoic timej Again the influence of the ridge was felt mainly during early Paleozoic time. 20 Highly localized basement features are also known to influence Paleozoic sedimentation and structure. Workman, and Bell (1948) noted a correlation between early Paleozoic sedimentation and a small anticline in western Illinois. A sudden change in dip of basement rocks and overlying sediments occurs at the Mt. Carmel fault in west-central Indiana. Coons et al. (1967) list nine Paleozoic anticlinal structures along the trend of the Mid-Continent gravity high. They note that Paleozoic deformation appears to be nearly continuous along Precambrian faults which extend from Kansas to Lake Superior, Ells and are associated with the gravity high. (1962) has pointed out that the dominant trend of anticlinal structures and faults in the Michigan Basin is northwest-southeast. Hinze and Merritt (1969) noted that "the anomalies of the vertical magnetic intensity map and particularly the Bouguer gravity anomaly m a p show a marked northwesterly trend south of the 44°30'N latitude closely paralleling the trend of the intra-basin structures." Noting the correlation of the Howell anticline w i t h the Mid-Michigan anomaly in Livingston and adjacent counties, they concluded that the n o r t h ­ westerly intra-basin structural trend reflects lines of weakness perhaps associated with a rift zone interpreted as the source of this major anomaly. CHAPTER II COLLECTION OF AEROMAGNETIC DATA Instrumentation Continuous observations of the total intensity of the earth's magnetic field were made wit h an Elsec type 592J proton precession magnetometer manufactured by Littlemore Scientific Engineering C o . , Oxford England. The instrument measures the reciprocal of the frequency of precession of protons in the earth's field. The frequency of precession is directly proportional to the intensity of the earth's magnetic field. This reciprocal reading magnetometer system is provided wi t h analog recording facilities in the form of a chart recorder. As used in this survey, the instrument has a sensitivity of approximately 3 gammas. A sensing unit, contained in an aerodynamically stabilized housing is trailed about 100 feet behind the aircraft during flight operations. For a complete description of the principle of operation of the proton precession magnetometer, reader is referred to Hood the (1969). The output of the instrument is displayed in the form of reciprocal frequency counts on an 8 inch chart 21 22 recorder. These records, along wit h navigational informa­ tion, are the basic data of the survey. Figure 4 is a representative record from the survey. One m a gnetometer unit on this record equals 2.8 gammas. Recording p a r a ­ meters and airspeed (which was m a i n t a i n e d at 120 n.p.h.) were established so that a h o r i z o n t a l distance of one inch on the chart represents a distance of approximately one mile on the ground. Abo u t eight readings were obtained per mile giving semi-continuous coverage along the flight path. Variations in ground speed due to winds aloft caused the number of readings per mile to fluctuate, but never by more than two or three per mile. Flight Crew and Airc r a f t A three man crew, consi s t i n g of a pilot, navigator and magnetometer operator was utilized in survey ope r a ­ tions., The pilot was responsible for keeping the air­ craft directly over the intended traverse at the proper flight elevation. The navigator was responsible for plotting flight traverses, identifying and recording navigational data on the flight maps and supervising all field operations. Finally, the m a gnetometer operator was primarily responsible for the operation and m a i n t e ­ nance of the magnetometer system. In addition, he kept a log of all take-offs and landings and recorded all magnetometer malfunctions. CHART No. 630813 to to o> smoothing curve Figure 4.— Representative Magnetogram CO 24 A Cessna 182 single engine aircraft was selected for the survey. This aircraft has a five hour range and was operated at a constant airspeed of 120 m.p.h. The magnetometer system was installed behind the pilot in the place of a passenger seat. This resulted in minor modifications to the aircraft w h i c h required the approval of the Federal Aeronautical Administration. Survey Procedures The regional nature of this survey did not warrant close spacing of flight lines. One cons ;ration in choosing a flight line separation is the expected depth to basement. The depths to basement in the Southern Peninsula vary between approximately 3,500 and 15,000 feet below mean sea level. A commonly used rule of thumb in regional aeromagnetic studies holds that the flight line separation should not exceed the distance from the disturbing body to the magnetometer. This assumes a dipolar magnetic source giving rise to a circular anomaly; however, most anomalies have widths appreciably greater than their lengths. Therefore, a compromise traverse separation of three miles was selected taking into con­ sideration the objectives and the available financial resources. Each traverse was flown at a constant barometric altitude of 3,000 feet M S L or roughly 2,000 to 2,400 25 feet above the ground surface. This provides a fixed datum for basement depth determinations and is compatible with previous aeromagnetic surveys flown in the vicinity of the Southern Peninsula. A barom e t r i c altimeter c a l i ­ brated prior to each take off was considered adequate for elevation control. In selecting a flight line direction, siderations are necessary. two co n ­ The first is the desirability of maintaining the lines of flight at right angles to the strike of the disturbing bodies, because the mos t useful and informative picture of a magnetic anomaly is obtained when flight traverses are oriented in this manner. second consideration is the navigational control. A In this survey navigation was achieved p rimarily by visual observations of cultural features, p a r ticularly the road and section line network. Roads in the Southern Peninsula are laid out along a rectangular grid and generally" coincide with north-south and east-west section lines. In laying out the flight paths, it was therefore necessary to choose between a north-south and an east-west d i r e c ­ tion. Both were considered, but the north-south direc­ tion was finally chosen because flight lines oriented in this direction take maxim u m advantage of the general tendency of most magnetic anomalies in the Southern Peninsula to possess a mar k e d east-west component. / i 26 Navigation was accomplished by visual fixes to cultural features such as road intersections or railroad tracks. Occasionally, natural features such as lake shores were used. Flight lines were ruled on county highway maps prepared by the State of Michigan before any flights were attempted. Once in the air, the pilot was directed by the navigator to the preselected flight traverse above a road or section line. tional fix was approached As each navi g a ­ (generally a road or section line at right angles to the t r a v e r s e ) , the instrument operator was alerted by the navigator. At the precise moment wh e n the aircraft passed over the desired fix, the observer, on command from the navigator, triggered a fiducial marker located on the front panel of the chart recorder. The location of the navigational fix was then carefully marked on the map and a number was assigned to both the map location and the fiducial on the chart. In addition, the time at which the observa­ tion was made was recorded. Navigational fixes were established, on the average, every 5 miles. Another factor which must be considered in laying out the survey is time variations in the magnetic field. Accurate surveys are impossible unless time or "diurnal" variations are properly removed. Particularly important is the early detection of large amplitude time variations known as magnetic storms. Ordinarily it is necessary to 27 repeat traverses flown during these storms; therefore, a system of identifying these disturbances in advance of flight operations is desirable to eliminate the need for costly reflights. Arrangements were made with the Space Disturbances Laboratory of the ESSA Research Center at Boulder, Colorado to notify the flight crew of impend­ ing disturbances of the earth's magnetic field. It generally became the p r a c t i c e , towards the end of the survey, to call the Space Disturbances Lab prior to co m ­ mencing each day's operations to obtain the daily forecast. Only 6 hours of flight operations out of 250 total hours had to be repeated because flights were conducted during magnetic disturbances, and none after the initiation of regular calls to the Space Disturbances Laboratory. The problem of eliminating the smaller scale diurnal variations was achieved by flying a network of control lines east-west across the north-south traverses. Six control lines were flown averaging about 50 miles apart. An attempt was made to establish control lines in areas having low magnetic relief; however, this was not always possible, particularly in the northern portion of the Southern Peninsula where magnetic gradients are steep. In addition to the network of control lines and the daily call to the Space Disturbances Laboratory, a number of procedures were initiated to reduce or eliminate 28 the need for reflights and to insure that equipment was functioning properly. Prior to each takeoff, the sensing head or "bird" was removed from the aircraft and placed in a convenient location about 100 feet from the aircraft and as far as possible from extraneous metallic material. The instrument was then turned on and the signal monitored for a period of several minutes. By this procedure, it was possible to detect magnetic disturbances already in progress. The sensing head was located in the same place each day prior to beginning the test. An identical test was also conducted at the termination of each day's opera­ tions . A second test was also c onducted on a daily basis to provide a further check on diurnal conditions and to insure that the instrument was functioning properly. test traverse, A about five miles long, was selected in an area convenient to the base of operations and doubly flown. This traverse was located along a prominent road or highway, in an area having gentle magnetic relief, and was flown at the beginning and end of each day's flight operations. The data from this test, in addition to providing a check on the equipment and diurnal condi­ tions, were later used in the diurnal reduction process (Chapter I I I ) , and in evaluating the accuracy of n a v i ­ gation. 29 Accuracy of Navigation The accuracy of navigation in this survey is difficult to assess except in a qualitative manner. Because flight traverses were conducted over cultural features, particularly roads, navigational control and accuracy is best where these features are in greatest abundance. This condition is best satisfied south of latitude 44°00'N. However, it was usually possible to follow section lines in the northern portion of the area so that lack of road coverage was not a critical factor affecting navigation. Locally, within the northern portion of the survey, there were areas having a paucity of navigational features. occurs One such area, covering 2,500 square miles in Montmorency, Alpena, Oscoda and Alcona Counties in the northeastern portion of the Southern Peninsula. Both marked section lines and roads are infrequent in this area; thus, it became necessary to rely on a com b i ­ nation of dead reckoning and sighting on natural features such as the shores of lakes and ponds to establish the traverse position. The use of lake shores as nav i g a ­ tional fixes caused the actual flight paths to deviate from the intended straight line t r a v e r s e s . Unknown factors affecting this type of navigation are variations in airspeed and wind velocity between navigational fixes. 30 Navigational accuracy is dependent on the pilot's ability to keep the aircraft directly over the intended traverse. In addition, accuracy is critically dependent on the navigator's ability to determine the exact instance at which the aircraft passes directly above a navigational fix, such as a road intersection. Only then can the navigational data be properly correlated with the magnetic data. manner: Errors of this type were evaluated in the following a series of short test traverses were selected and doubly flown. These test traverses were previously discussed under survey procedures. Ratios of distances on the records were then compared for both flight direc­ tions. These distances agreed, on the average, to within one cycling of the magnetometer chart recorder. one cycling of the magnetometer During (about 3 s e c o n d s ) , and assuming an airspeed equivalent to ground speed the air­ craft will have moved about 600 feet. Therefore, assuming no errors caused by straying of the aircraft right or left of the intended traverse, the north-south accuracy is better than 600 feet. Accuracy of the magentic observations can also be evaluated in terms of chart recorder units from the same repeated flights over short test traverses. These errors include the inherent sensitivity error of 3 gammas of the instrument. Test results indicated errors of the order of three or six gammas in low gradient areas 31 and from six to twenty or more gammas in steep gradient areas. A serious problem which cannot be tolerated is the mislocation on the flight line maps of cultural features used as navigational fixes. A n improperly located road intersection may produce a "herringbone" pattern on the aeromagnetic map. true in high gradient areas. This is especially Errors from this source were eliminated in the following manner: the ratios of distances between pairs of points on the records and on the flight line maps were visually compared. Any points which deviated from a relatively constant ratio were then deleted from the data. In this manner, serious navigational errors were detected before data processing began. Meteorological Considerations Weather conditions strongly influenced the timing and execution of the field operations. days operations, Prior to each the local Flig h t Service Station of the FAA was consulted about the location of air masses, frontal systems, visibilities, ceilings and winds aloft. This information was then used to plan the day's flights. Areas having good visibility and w i t h i n easy reach of the base of operations were assigned first priority. 32 On many summer days, however, extensive fog and haze covered most of the Southern Peninsula, reducing visibility to four miles or less. These conditions c om­ monly preceded afternoon thunderstorms. Flight operations were severly limited during these periods, often having to be curtailed for several days or more. Turbulent conditions, due to rising pockets of w a r m air, were almost a daily occurrence on this survey. These pockets, or "thermals," usually reached the flight elevation of the aircraft about 10:00 AM and persisted until about 4:00 PM. Repeated flights over test strips indicated turbulent conditions were not a source of error in making the magnetic measurements. "Turbulent air prevents successful flights more from conditions of safety than from the appearance of noise on the record" (Reford and Sumner, .1965) . Summary of Fie l d Operations The aeromagnetic survey was flown between the dates of July 25, 196 8 and March 14, 1969. However, the majority of the survey was completed by October 1, 196 8. Flights conducted subsequent to this date replaced data flown during diurnally disturbed periods and data having questionable navigation. approximate area surveyed, miles, Listed below is the approximate number of traverse and days required to conduct the survey. 33 1. Area surveyed: 2. Traverse mileage: 3. 4. 41,000 square miles a. north-south traverse mileage: b. east-west control line mileage: c. access mileage: d. total mileage: 17,000 miles 4,000 miles 1,000 miles 22,000 miles Flight hours: a. north-south traverse time: b. east-west control line time: c. access time: d. total time: Days in field attempted) ' • 19 4 hours 45 hours 11 hours 250 hours (days on whi c h operations were 40 a. days on which useable data were collected: b. days on which no data were collected due to poor weather, breakdown of the magn e t o ­ meter, breakdown of the aircraft, periodic aircraft inspections: c. total days in field: 48 20 ahd 28 CHAPTER III COMPILATION AND REDUCTION OF MAGNETIC DATA Introduction Before the magnetic data can be prese n t e d in the form of a residual total intensity contour map for i n t e r ­ pretation, they must be corrected for diurnal variation and the effect of the earth's normal field. this survey was conceived, compile, reduce, ble, At the time the decision was made to and present the data, as much as p o s s i ­ through computer reduction. The decision to machine contour the data entailed the preparation of several computer programs. In addition to providing for the removal of diurnal and normal field variations, these programs merged navigational and analog data and prepared the magnetic data for input into a machine contouring package. The role of each of these programs is briefly discussed in the following section. The method of data reduction is explained through the use of Figure 5, a flow chart illustrating the steps involved. 34 35 Hand digitization of coordinates of naviga­ tional fixes in arbitrary units from USGS 1:250,000 base maps ''Magnetogram Machine digitization of analog magnetograms at 2mm intervals Basic Coord. Data ~ ~ T . PROGRAM COORDINATE: Converts arbitrary coordinates of each navigational fix into degrees latitude and longitude Inspection of digitized data for errors. Correction of \errors or redigitization l X Coordinate Output r PROGRAM MAGPLOT: Plots all navigational fixes on universal transverse mercator projection PROGRAM EDIT: Removes certain machine induced errors. Primary function is to output digitized data on tape. Plot Fixes Inspection of Edit output for / V mispunchoa, wrong jumps, errors/ \not detected above. / Inspection of plot for errors Corrected Coordinate Data PROGRAM MAGCOORD: Assigns latitude and longitude coordinate using coordinates of navigational fixes supplied from Program Coordinate data. Also converts frequency counts to gammas. Determination of diurnal corrections from drift curves (output on cards) Diurnal Data Inspection of Magcoord output for incorrect gamma or coordi-, nate values, and other errors / not detected above. / Magcoord PROGRAM MAGDRIFT Calculates and applies diurnal corrections at all data points as a linear function of distanco between control points. Inspection of Magdrift output for errors Correcte Magdrift PROGRAM G-FIELD Calculates and subtracts the earth’s normal field variation from the magnetic data PROGRAM PLOT Plotseach individual data point on 1:250,000 scale base map G-Field Final inspection of plat and all data for iorroro, corrections to/ \any of above programs./ I Inspection of G-Fiold data for errors t f G-Fiold PROGRAM MAPSIZE Blocked data into groups con­ taining approximately 2,000 data points. Mapsize data Figure 5.— Flow Diagram Illustrating Sequence of Data Reduction Steps. PROGRAM MAGCONTOUR Contouring of Aeromagnetic data, output contour map at 50 gamma interval and 1:250,000 scale 36 Digitization of Navigational Fi x Data Following a data gathering flight, navigational fix data were plotted on U . S. Geological Survey maps at a scale of 1:250,000. The coordinates of each navi­ gational fix and the beginning and end of each flight line were then measured from these maps in arbitrary units from a single arbitrarily chosen origin. data were keypunched for digital proce s s i n g These (Figure 5). The next step was to convert the coordinates measured above to degrees latitude and longitude. was accomplished with program COORDINATE. This The output of COORDINATE consisted of punc h e d cards containing the navigational fixes, and their coordinates in degrees of latitude and longitude, to five decimal places. To insure that each navigational fix was in its proper geographical location, i.e., that no errors were committed in the hand digitization process, the following procedure was adopted: 1. The COORDINATE data became the input to p r o ­ gram MAGPLOT which p l o t t e d all fixes on a Universal Transverse Mercator projection at the same scale as the U. S. G eological Survey base maps 2. (1:250,000). Errors in the COORDINATE data were detected by overlaying these computer plot t e d n a v i ­ gational fix maps on the base maps. 37 3. Misplaced navigational fixes detected by this technique were remeasured and steps (1) through (3) were repeated until all of the n a v i g a ­ tional fix data were in their proper ge o ­ graphical location. Digitization of Analog Records Another major data reduction step was the machine digitization of the analog magnetograms Prior to digitization, (see Figure 5). occasional erratic values caused by instrumental problems were eliminated by hand smoothing of the magnetograms. This process involved drawing a smooth line through the magnetic values addition, (Figure 4). In all navigational fixes were uniquely identified on the magnetograms. An X-Y machine digitizer was utilized for the digitization. The sequence of processing steps and procedures adopted to insure that the data were free of errors is outlined below. 1. Each traverse was machine digitized; deck being prepared for each line. a separate A header card contained pertinent information about the traverse. Data were digitized at 2mm intervals, or roughly the separation between readings on the magnetogram. Navigational fixes were i encoded in the data as they were encountered on the magnetograms. 38 2. The digitized data were then inspected for human or machine induced e r r o r s , such as misplaced alphabetic characters, dropped digits or other obvious mistakes. 3. Corrected data were then input into program EDIT (Figure 5). The function of this p r o ­ gram was to remove certain machine induced errors and to output the corrected data on tape. 4. The output from EDIT was inspected for further errors, and steps 3 and 4, and where necessary, 1 and 2 were repeated until all of the data were in prop e r form for further processing. Merger of Navigational Data and Digitized Magnetograms Until this point, were treated individually navigational and magnetic data (Figure 5). The function of pr ogram MAGCOORD is to merge the navigational data with the machine digitized magnetograms and to convert the frequency counts to gamma values. The input to this p r ogram is the output deck from program COORDINATE co n ­ sisting of navigational fixes, with their associated geographical coordinates in degrees latitude and longitude, and the output from p r og r a m EDIT, a tape containing the digitized data keyed to the navigational data. Parogram MAGCOORD assigned a latitude and longitude coordinate to 39 each data point on the EDIT tape utilizing the coordinates of navigational fixes supplied from the COORDINATE data. In addition, this program converted the frequency counts associated with each data point to gamma values. Thus, the output of program MAGCOORD is a tape containing the data points described in terms of their latitude, longitude and gamma values. Preparation of the Diurnally Corrected Data The object of the diurnal correction is to remove the time variations in the earth's magn e t i c field from the magnetic data. The sequence of steps developed to accomplish this goal are outlined below. 1. A north-south master control line was doubly flown along the approximate geographical center of the State. This line crossed all east-west control lines. 2. A drift curve was prep a r e d for this line by referring all values to a single point at the intersection of this master control line wi t h the southernmost eastwest control line. This was necessary to remove time variations in the magnetic field which occurred while the north-south control line was being flown. The test strip data recorded at the beginning and end of the day provided additional information about the rate of drift for the day. 40 3. Drift curves were then similarly prepared for each doubly flown east-west control line and all values were referenced to the common tie point wit h the north-south line. Finally, a small correction equal to the difference between adjusted values at the intersection of the east-west and north-south control lines was added to all navigational fixes on the east-west control line. This was necessary to bring the level of observation of these points into coincidence with the level observed at the master control point of the north-south control line. 4. Having established a control net, the values of all diurnally corrected navigational fixes including fixes at the beginning and end of each line together with their associated latitude and longitude values were obtained from the curves and coordinate information and keypunched (Figure 5). These data, and the output tape from program MAGCOORD were the input to prog r a m MAGDRIFT. This program calculated and applied the diurnal co r ­ rections at all data points. Corrections were applied as a linear function of distance between control points and between a control point and the beginning or end of a traverse. Removal of the Earth's Normal Magnetic Field Before a residual total magnetic intensity map can be prepared, it is necessary to remove the earth's 41 regional or normal field from the magnetic data. Over the surface of the Southern Peninsula of Michigan, the variation of the total intensity is roughly 1,300 gammas as determined from maps prep a r e d by the U. S. Naval Oceanographic Office. The normal gradient, which varies chiefly with latitude, is 4 gammas per mile. The earth's normal field was removed utilizing a program developed by Cain et al. Figure 5). The p r ogram uses (1968) (Program G-Field, 8 degrees of spherical harmonic expansions of the geomagnetic potential, treats only the main internal field. and The coefficients used in this program were determined by Daniels and Cain (1964). The earth's normal magnetic field was calcu­ lated for an elevation of 3,000 feet MSL and for September, 196 8. A contour map showing the normal geomagnetic field variations is presented in Figure 6. The regional geomagnetic field values are given in Table 2. Preparation of the Corrected Data for Machine Contouring Several programs were developed to group the data into blocks of 15 minutes latitude by 15 minutes longitude, each containing approximately 2,000 data points. Only every third data point was utilized because of restric­ tions imposed by the available computer facilities. This 42 4 6 o30* 4 5°3 0' +- 4 4 ° 30' +■ 43° 3 0 ' + 42°30* + 41° 30'- 86° 00 ' CONTOUR 8 5 °0 0 ’ INTERVAL: 8 4 °0 0 ' 2 5 0 GAMMAS 25 Figure 6.— Normal Geomagnetic Field over the Southern Peninsula of Michigan. TABLE 2.— The Earth's Normal Total Magnetic Intensity in the Vicinity of the Southern Peninsula of Michigan. Latitude Longitude Gamma Value Latitude Longitude Gamma Value 41° 30' 87° 86° 85° 84° 83° 00' 00' 00' 00' 00' 57,964 57,922 57,870 57,806 57,732 44° 30' 87° 86° 85° 84° 83° 00' 00' 00' 00' 00' 59,059 59,005 58,940 58,865 58,780 42° 30' 87° 86° 85° 84° 83° 00' 00' 00' 00' 00' 58,355 58,308 58,252 58,184 58,106 45° 30' 87° 86° 85° 84° 83° 00' 00' 00’ 00' 00' 59,371 59,313 59,245 59,167 59 ,078 43° 30' 87° 86° 85° 84° 83° 00' 00' 00' 00' 00' 58,720 58,669 58,609 58,537 58,455 46° 30' 86° 00' 85° 00' 84° 00' 59,595 59 ,523 59,442 u> 44 set of programs is combined on the flow chart (Figure 5) under program MAPSIZE. The output of MAPSIZE was recomposited and used to plot a 1:250,000 scale base map showing the location of all data points. This ma p was superposed on the U. S. Geological Survey base maps for a final check of the navigational data before machine contouring of the data. Contouring of A e romagnetic Data A machine contour package developed by California Computer Products Inc. was used to contour the aeromagnetic data. This computer package determines the contours and presents them in a form suitable for display on an X-Y plotter. The p r o g r a m is capable of generating a contour map from arbitrarily spaced data points. Contours can be annotated with labels, hachures and heavy lines as necessary. The surface to be contoured is specified by data, points arranged along flight lines. However, it is necessary to estimate the values at the mesh points of a rectangular array before contouring. The contour package then operates on the gridded data points to generate the contour map. Gridded data are gene r a t e d from flight line data in the following manner: A plane is passed through each data point, determined by the gradient from nearby data I l 45 points and the data point under consideration. This plane is derived as a least squares function of these v a l u e s , w e ighted according to their distance f r o m the data point. The plane passes through the data point and agrees as closely as possible wit h the selected data points surrounding it. The intersection of the plane with a vertical line passing through a desired grid point is an approximation of the magnetic field at that point. In this study, the weighted average of the intersections of the planes determined from the nearest 50 data points was used to approximate the field at the grid point. Grid points are evaluated sequentially, starting from one corner of the map. In this survey, mile. cells. the grid spacing is one-half The smallest rectangles of the grid are called These cells are in turn, smaller cells, called subcells. subdivided into even The purpose of this division is to remove angularity or "herringbone" from the magnetic data by increasing the number of int e r ­ mediate interpolation points. - Contours are generated from the i n terpolated function values at the mesh points of the subgrid. This interpolation, which preserves the gradient of the func­ tion across cell b o u n d a r i e s , is third order in X and Y. The contour maps were plotted on a 30 inch CALCOMP drum plotter. Because of the large scale and 46 dimensions of the final map (1:250/000), three runs were made and the results assembled to produce the final maps. CHA P T E R IV INTERPRETATION OF A E ROMAGNETIC DATA Configuration of Basement Surface Introduction The basement of the Michigan Basin is commonly depicted as an oval depression w i t h little or no modifying topographic relief. Unlike the Michi g a n Basin, the Illinois Basin, which resembles the Michigan Basin in area and maximum depth, is illustrated as having prominent topography that is reflected in the overlying lower Paleozoic sediments. With additional control information, localized basement topography of the Michigan Basin may be found to resemble that of the Illinois Basin. Depths obtained from aeromagnetic studies are a valuable supplement to basement drill holes in determining the configuration and topography of the basement surface. Only fifteen poorly distributed basement tests have been made in the Southern Peninsula (Figure 1 and Table 1). While the basement is estimated to attain a depth of over 14,000 feet below sea level, only three drill holes, all located in the southeastern corner of the state, intersect the basement surface at depths greater than 5,000 feet and 47 48 none are at depths greater than 6,20 0 feet. In addition, only two basement tests are located north of 43° 00' N. Basement depths obtained from wel l cuttings may be uncertain because of contamination of the samples by chips from overlying formations. In addition, it may be dif­ ficult to distinguish the lowermost sediments an arkosic sandstone) (commonly from the w e a t h e r e d surface of basement granitic rocks on the basis of cuttings 1967). (Yettaw, Thus, discretion is necessary in interpreting depths obtained from well cuttings. Both gravity and magnetic data are available for the Southern Peninsula of Michigan. Magnetic methods are preferred to gravity methods w h e n making depth d e t e r m i n a ­ tions primarily because magnetic anomalies originating from w ithin the basement are not distorted by anomalies from the overlying sediments as are gravity anomalies. Sediments are essentially n o n - m a g n e t i c , hence structure or facies changes w i thin them will not give rise to m a g ­ netic anomalies. However, these same variations will usually cause horizontal density changes and thus gravity anomalies. In addition, gravity anomalies are broader and less definitive than magnetic anomalies originating from the same sources. One of the factors controlling the amplitude and sharpness of the gravity or magnetic anomaly, the distance from the source, varies inversely one power faster for magnetic anomalies than for gravity anomalies. 49 Thus magnetic anomalies have a high e r resolving power than gravity anomalies. Finally, gravity anomalies may be more distorted by deep seated intra-crustal structural or lithologic variations than magnetic anomalies, which increases the problem of isolating gravity anomalies for depth determinations. Depth determinations by magnetic methods a r e , however, subject to a number of assumptions regarding the configuration and magnetic properties of the source. particular, the effect of remanent m a g netization is dif­ ficult to predict with certainty. tions, In Despite these limita­ an average error of less than 10 per cent can be obtained under favorable conditions b y a trained inter­ preter (Steenland, 1963). The p recision of magnetic depth determinations is strongly influenced by the anomalies selected for analysis. from a single source. Anomalies chosen should originate Thus, in so far as possible, anomalies free of effects from adjacent anomalies should be selected for analysis. Also, the direction of the line connecting the m a xim u m and m i n i m u m of the magnetic anomaly should be parallel to the declination. remanent magnetization may be suspected 1951). However, If not, (Vacquier et a l . , few anomalies are sufficiently isolated to permit the unrestricted application of this principle. Finally, anomalies with a promi n e n t m i n i m u m should be avoided; at the magnetic latitude of this survey, a 50 prominent minimum may be indicative of remanent p o l a r i z a ­ tion effects, a restricted depth extent or dip of the anomaly source. These factors will result in errors if the anomaly is interpreted according to standard theory. Selection of Depth Determination Techniques Numerous magnetic depth determination techniques have been developed and their advantages and disadvantages discussed in the literature Riddell, 1966). (Reford and Sumner, 196 4 and In general, methods bas e d on the higher amplitude portions of the anomaly curve and not on its total amplitude are less subject to error arising from overlapping anomalies and definition of the zero level of the anomaly. The magnetic depth determination methods developed by Vacquier et al. (1951), Bean (1966), and Peters (19 49) were applied to critically selected magnetic anomalies located near basement tests to "calibrate" the techniques and determine their usefulness in this study. methods, Peters' Of these half-slope and the straight slope tech­ nique of V a cquier et al. proved to be the mos t accurate in determining depths from magnetic anomalies adjacent to drill holes. Table 3 summarizes these results. While the average per cent error for these det e r ­ minations appears to be large, the control wells are located a minimum of 4.3 miles and range to a max i m u m of 12.2 miles from the site of the measurement. The TABLE 3.— Comparison of Magnetic Depth Determinations with Basement Drill Depths. Well Control County, Location Distance of Magnetic M e a s ­ urement From Control Well (Miles) Depth of Basement F r o m Co n ­ trol Well (Feet) Bean Depth/ % Error Vacquier/1.0 Depth/ % Error Peters Depth/ % Error 9.4 3,802 No Fit 2, 460/-55% 4 ,650/+22% 11.3 2,951 No Fit 3 ,780/+22% 3,900/+32% Monroe 29-5S-10E 4.3 2,745 5,900/ +115% 2 , 800/+2% 3,120/+12% Wayne 16-4S-9E 4.5 3,360 5,900/ +75% 2, 800/-20% 3,120/-8% 12.2 6,179 8,750/ +29% 5, 100/-17% 6 , 850/+ll% 23% 17% Berrien 10-6S-17W Monroe 16-7S-6E Livingston 11-3N- 5E I Average Error W/0 Regard to Sign: 52 correlation of the results possibly w o u l d improve con­ siderably if the magnetic depths could be compared with basement depths from hypothetical wells located at the site of the depth determination. Half-slope lengths obtained from P e t e r s 1 method we r e generally divided by a factor of 1.6 w h i c h applies when the width of the body is equal to twice the depth of burial (Peters, 1949). However, other factors were used when the dimensions of the anomaly suggested a markedly broader or narrower source. In the method of Vacquier et a l . the horizontal extent of the steepest gradient of the north flank of the total intensity curve is m e a s u r e d value is an approximate depth, of the body. In this method, (the G i n d e x ) . This uncorrected for the shape the shape of the body is expressed in terms of units of depth to its upper surface. To obtain the depth corrected for shape, the G index is divided by a factor between 1.0 and 1.3 for sources located at the magnetic latitude of this study. This factor is determined from theoretical magnetic anomalies of idealized prismatic sources. the north-south direction, 1.0. For bodies elongated in this factor is approximately For bodies elongated in the east-west direction, the factor is variable, reaching a m a x i m u m of 1.3 when the east-west extent is roughly 3 times the north-south. In this study, all G indices we r e divided by a constant 53 •factor of 1.0. This was justified because use of a variable index failed to improve depth estimates made adjacent to control wells over estimates made with a constant index. In addition, depth indices average close to 1.0 for many body shapes at the magnetic latitude of this survey. The selection of anomalies for depth determinations proceeded in the following manner. Detailed total intensity maps at a scale of one inch equals eight thousand feet and a contour interval of 20 gammas were used to select anomalies for analysis, discussed. on the basis of the criteria previously Depth estimates w; made from the original magnetograms to avoid pro;..er:js of interpolation in construction of the magnet': o rh. s maps and profiles. Only magnetograms which :trrc.i sooted the anomaly at roughly right angles to its s: ; analysis. Finally, with few excej. re used in the , only the north flanks of the anomalies were analyzed. The regional or normal field generally was not taken into account when making the depth d e t e r m i n a t i o n s . This was justified because the depth determination tech­ niques used in this study are applied to only a relatively short horizontal distance of the anomaly curve two m i l e s ) . (one or The effect of the regional gradient is generally negligible over this distance considering the other sources of error. This was substantiated by tests which indicate that depths obtained from regionally 54 corrected anomaly curves are not significantly different from depths obtained from u n corrected curves. Generally, values from Peters' were averaged and Vacquier's methods (Table 4) to arrive at the depth to be utilized in preparing the b a sement configuration ma p Occasionally, (Figure 7). unreasonable values obtained from one method were rejected. In addition, average values bas e d on wid e l y different depths we r e downgraded w h e n making the final interpretation. Although 56 depths are listed in Table 4, only 44 anomalies were analyzed. Thus, several anomalies have more than one depth estimate. Multiple depths obtained from a single anomaly are m a r k e d in Table 4. The number of anomalies suitable for making depth determinations is greater in the southern and northern thirds of the Peninsula. This reflects, in part, the greater depth to basement in the middle third, but also could reflect a change in the character of the anomalies in the central portion of the Basin. Thus, the reliability of the contours decreases in the central portion of the state. The construction of a detailed basement configura­ tion map was not justified because of limited data and the potential errors in making magnetic depth determinations. Regional contours were therefore drawn, permitting local errors of several hundred feet in some areas. TABLE 4.--Magnetic Depth Determination Results. County Depth from Straight Slope M e t h o d (Feet BMSL) Location Depth from Peters Half Slope M e t h o d (Feet BMSL) Averagi Depth Berrien 41° 50 'N 86° 20 'W 2,460 4 ,650 3 ,555 Cass 41° 4 7 'N 86° 11 'W 3 ,740 3 ,250 3 ,495 Cass 41° 52 'N 85° 49 •w 3,200 4 ,280 3 ,740 3 ,0 1 2 ' St. Joseph 41° 52 'N 85° 46 'W 2 ,725 3 ,300 St. Joseph 41° 50 'N 85° 35 'W 3,320 — 3 ,320 Branch 41° 59 'N 84° 5 3 'W 4,025 6 ,050 5 ,037 Lenawee 41°53 'N 8 3° 5 7 1W 3,780 '3 ,900 3 ,840 ■w 2 ,855 W ayne 42° 7 1N 83° 15 2,750 2 ,960 V/ayne 42° 6 1N 83°18 'W 2,800 3 ,120 2 ,960 Jackson 42°21 'N 84°21 'W 5,900 7 ,000 6 ,450 Calhoun 42° 13 ’N 84° 43 'W 11,500 9 ,500 10 ,500 Kalamazoo 42° 7' N 85° 35 'W 4,440 7 ,500 4 ,440 Kalamazoo 42° 21 'N 85° 42 'W 3,750 4 ,900 4 ,325 K a l a mazoo 42° 21 'N 85° 45 ■w 4,025 6 ,725 4 ,025 Van Buren 42° 20 'N 8 6 ° 10 'W 5,330 6 ,100 5 ,715 V an Buren 42° 2 3 'N 86° 6 "w 4,150 3 ,000 3 ,575 V a n Buren 42° 22 'N 86° 10 'W 3,500 3 ,150 3 ,325 Allegan 42° 30 'N 85° 5 3 •w 3,260 4 ,525 3 ,890 Barry 85° 8 "w 4,750 5 ,100 4 ,925 Eaton 'N 42° 36 'N 85° 4 '! w 7,800 8 ,800 8 ,300 Eaton 42° 35 'N 85° 1 '■w 4,400 5 ,600 5 ,000 42°28 ' Comments on De termination b y the Straight Slope M e t h o d Comments on D e termination by Peters Half Slope Meth o d Mul t i p l e Depths on One Anomaly in in TABLE 4.— Continued. County Location Depth from Straight Slope M e t h o d (Feet BMSL) Depth from Peters Half Slope M e t h o d (Feet BMSL) Aver a g e Depth Eaton 42° 3 8 1N 84° 53 1N 5 ,050 6 ,250 5,650a Eaton 42° 3 3 1N 84° 43 'W 6 ,050 6 ,800 6 ,425 Eaton 42° 34 'N 84° 40 1W 6 ,950 12,200 6,950 Livingston 42° 40'N 84°4'W 5,10 0 6 ,850 5,975 Oakland 42° 52 'N 83° 7 'W 6 ,050 5 ,500 5,775 Ionia 42° 46 1N 85°10 1W 5,300 6,850 6 ,075a Ionia 42° 4 7 'N 85°13 1W 6 ,450 8,700 7,575 Ionia 43°2 'N 85°18'W 14,300 14,800 Kent 43°00 1N 85°43 ’W 7, 800 Ottawa 42° 49 1N 86° 7 1W — 1 4 ,550a 7,800 Comments on D e termination by the S t raight Slope M e thod Comments on Determi n a t i o n b y Peters Half Slope Method 1 c d 5 ,100 6,100 5,600 Ottawa 43°2 'N 86° 1 1 1W 5,500 8,150 6,825 d Ottawa 42° 45 'N 85° 53 'W 7,100 — 7,100a d G r a tiot 4 3 ° 1 6 'N 84° 46 'W 18,800 — 1 8 , 800a d Gratiot 4 3 ° 1 6 'N 84° 43'W 14,700 — 14 , 7 0 0 a d Saginaw 43° 3 4 'N 84° 6 1W 15,000 — 15,000 d Saginaw 43°21'N 84° 3 1W 9 ,700 — 9,700 d Tuscola 43°20'N 83° 7 'W 9 ,150 8,100 8,625 Sanilac 5,650 6 ,500 6,075 Huron 43° 30 'N 82° 4 7 1W 43049 .N 8 3 ° 1 2 'W 10,200 14,400 12.300 Bay 43° 41'N 84° 3 1W 15,200 15,40 0 15.300 Bay 43° 47'N 84° 3 1W 13,900 13,900 13,900 Oceana 4 3° 30 'N 86°15'W 6 ,900 — 6,900 Mul t i p l e Depths on One Anomaly b O’ TABLE 4 .--Continued. County Location Depth from S traight Slope M e t h o d (Feet BMSL) 86° 2 'W Depth from Peters Half Slope Method (Feet BMSL) 6 ,600 Average Depth Comments on D e t e r m ination by the S t raight Slope M e t h o d Comments on Determination by Peters Half Slope Method Lake 44° 3 'N Ros common 44° 13 'N 84° 2 7 'W 11,550 A lcona 44° 33 'N 83°23 •W 7,550 9 ,950 8,750 A l cona 44° 44 'N 83° 48 'W 5,350 8,800 8,800 Benzie 44° 33 'N 4,9 50 8,000 6,475 d A l pena 45° 4 'N 86 0 4 1W 83° 51 1w 5,850 9 ,100 5,850 d O t sego 44° 58 'N 84° 42 'W 9 ,000 — 9 ,000 O t sego 44° 59 'N 84° 40 'W 7,050 10,700 8,875 Antrim 44° 57 'N 85° 2 3 'W 8, 700 14,000 8, 700 a A n trim 44° 56 'N 85° 20 'W 9 ,200 14,700 9 ,200 d Antrim 44° 57 'N 84° 5 3 'W 8,400 — 8,400 Em mett 45° 23 'N 85°1''w 4,900 6 ,350 5,625 4, 300 5,140 4,720 45° 33 'N 84° 50 E m mett 'w 10 ,500 — 8,550 ll,550a d e aDepth value w h i c h deviates m o r e than 10 per cent from value in d i c a t e d b y contours. ^This determination made on south flank of anomaly. cSouth Flank of anomaly analyzed only. Enable to accurately pick tangent to' lower port i o n of anomaly curve. 0 Straight slope part of anomaly curve slightly distorted. Multiple Depths on One Anomaly 58 8 6°0 0 ' 8 5 °0 0 ' 8 4 °0 0 ' 4 5° 30' 8 3 °0 0 ' 5000 ABASEMENT +N 4 5 °0 0 ' io o o o D R IL L H O L E • B A S E M E N T M A G N E T IC D E P T H © B A S E M E N T M A G N E TIC D E P TH W H IC H E X C E E D S C O N T O U R V A LU E BY 1 0 % OR M ORE ; 4 4 °0 0 ' 44 °0 0 ' -13000' -n&oo— J 1.1 ioooo.— 4 3 °0 0 ' 43 °0 0 + 9000 7000 4 2 ° 0 0 '/+ •C) -4 0 0 0 86 ° 00 ' 8 4 °0 0 8 5 °0 0 ' 0 10 8 3 °0 0 ' 20 SCALE Figure 7.— B A S E M E N T SURFACE C O N F IG U R A T IO N MAP OF TH E S O U T H E R N P E N I N S U L A OF M I C H I G A N Datum Sea Level Contour In te r v a l 1 0 0 0 feet 59 Discussion of the Basement Configuration Map The basement configuration map is p r e s e n t e d in Figure 7. Datum for the ma p is mean sea level and the contour interval is 1,000 feet. The smooth, regional nature of the contouring has downgraded the role of depths shown in Table 4 which are inconsistent wit h the great bulk of the data. Depth estimates which deviate from the contour values indicated in Figure 7 by more than 10 per cent, are marked on the Figure and in Table 4. However, these depths are included in the table for completeness. The map includes a fault in the south- central part of the state inferred from gravity data by Merritt (1968). The basement configuration map confirms that the basement surface of the Southern Peninsula of Michigan has the form of an oval depression reaching a m a x i m u m depth of approximately 15,000 feet below sea level on the western shore of Saginaw Bay. Following Hinze and Merr i t t (1969) , it shows a minor topographic depression in the northwest which plunges southeast into the Basin. A similar depres­ sion exists in the structure contour map of the Ordovician Trenton formation (Hinze and Merritt, 1969). A basement high is depicted in the southeast und e r ­ lying the Howell anticline in Livingston County. The areal extent of this feature is clearly open to question; however, 60 well control in Livingston and Washtenaw Counties and adjacent magnetic depths suggest a basement closure of perhaps 1,000 feet. To the w e s t of the Howell structure, a broad trough plunges to the north-northwest into the Basin. The existence of this feature is postulated solely on the basis of magnetic depth determinations as no basement wells are located in this area. shown by Bayley and Muehlberger A similar trough has been (196 8). The southwestern portion of the Southern Peninsula is characterized by a broad basement platform. This feature, which has not been discussed in connection wi t h previous basement maps strikes northwest and may indicate a n o r t h ­ ward broadening of the Kankakee Arch into the extreme southwest portion of Michigan. Eight depths from magnetic anomalies provide the control for this feature. Sources of Anomalies Geological and geophysical studies indicate that the Precambrian rocks of the upper Midwest are lithologically complex. Gravity and magnetic maps are part i c u ­ larly useful in deciphering this complex pattern where the Precambrian surface is bur i e d beneath Phanerozoic sediments. Basement lithologic variations are generally associated with density and magnetic susceptibility changes. Further, it can be shown that gravity and magnetic anomalies 61 originating from intra-basement lithologic and structural variations are an order of m agnitude greater than anomalies originating within the overlying sediments. trated in Figures 8 and 9 This is illus­ (after Hinze and Merritt, 1969) which depict cross sections of a h y pothetical basin, similar in gross characteristics to the Mich i g a n Basin. The anomaly sources include b a s e m e n t lithology and topography, structures w i t h i n the sediments, bedrock topography and the overall effect of the basin. presented are the theoretical gravity netic Also (Figure 8) and m a g ­ (Figure 9) anomalies for each of these groups of bodies and their combined effect. The conclusion reached from an examination of these diagrams is that the prominent gravity and magnetic anomalies are derived from intra­ basement lithologic variations. No t included in Figure 8 are the effect of lateral facies changes in the sediments and deep crustal or upper mantle density variations giving rise to long wavelength anomalies of a regional nature. In Figures 8 and 9 the gravity and magnetic effects of idealized subsurface bodies we r e computed. In inter­ preting gravity and magnetic maps where the subsurface structure is unknown, cedure, i.e., one attempts to reverse this p r o ­ to determine the configuration and lithology of the causative bodies from the observed anomaly maps and profiles. However, the process involves a double source of ambiguity because both the physical property contrasts 62 --5 -5 -10 15 -13 — --2 0 - 20 — -2 5 — BASIN 25 r BEDROCK TOPOGRAPHY SEDIMENTARY STR U C TU R E BASEMENT TOPOGRAPHY -5 — 25 CO 20- -J — 20 < CO — 15 5 ,0I — K> < 5— — 5 — 0 z < BASEMENT 15— >- LITHOLOGY — 15 § »- (Z o — 10 5— — 0 0— •5 — — -K )— -K) -15 — - 20 — 25 -2 5 — COMBINED 120 140 EFFECT 160 180 D IS T A N C E -M IL E S 4— &&025 A