A GRAVITY STIIBV OF THE GEOLOGY 0F NCRTIIEASTERN WISCONSIN _ “1853 for the Degree of 9h 9 1’37:::r‘:f7;‘:"§:‘75371-i 1 ,_ _ MICHIGAN STATE UNIVERSITY BARRY ALBIN CARLSON 51' I974 LIBRARY I“ m I Michibm State University Warm: \AAA.‘ I ML “I TAFV nICflOOK built}? a :j. to certify thatthe I fix . new“ thesis entitled A Gravity Study of the Geology of Northeastern Wisconsin presented by Barry Albin Carlson has been accepted towards fulfillment of the requirements for Ph.D. Geology degree in 5:.“ I R 5/ if c a" I 1.51m ’ Major professor 0-7639 n s max mun EII'IIIA i ‘ usmav unnzns I I m. In... ABSTRACT A GRAVITY STUDY OF THE GEOLOGY OF NORTHEASTERN WISCONSIN By Barry Albin Carlson IResults of a gravity survey conducted in northeastern Wisconsin were used to extend known Precambrian geology into a sparse outcrop area. During the survey, an area of 14500 square miles was covered by over 3500 gravity observations. The Bouguer Gravity Anomaly Map with the aid of a Double Ihnxrier'Series Regional Gravity Map indicates prominent nega- tiAna gravity anomalies underlying the granitic rocks of the Wolj7ZRiver Batholith and the Northeastern Wisconsin Complex. Thu; latter is much more extensive than suggested by outcrop studies, extending from the Pembine area across the entire study area to the western boundary of Forest County. The Dunbar Gneiss occupies the central portion of the Northeastern Wisconsin Complex anomaly in the Pembine area where units of the complex have been geologically defined. The Argonne Gravity Trend is a strong positive gravity anomaly which separates the two major granitic areas and is associated with little known mafic and ultramafic rocks. The p- A GRAVITY STUDY OF THE GEOLOGY OF NORTHEASTERN WISCONSIN By Barry Albin Carlson A THESIS Submitted to Michigan State University in partial fulfillment of the requirement for the degree of DOCTOR OF PHILOSOPHY Department of Geology 197“ Dedicated to my sons, Duncan and Eric. ii ACKNOWLEDGEMENTS An endeavor such as this cannot be accomplished without the aid of many people. Foremost among these is Dr. William J. Hinze, who provided direction and supportive encouragement throughout all phases of the study. The helpful suggestions of Drs. Harold S. Stonehouse, James W. Trow, Hugh F. Bennett, and Chilton E. Prouty are also greatly appreciated. This study would not have been possible without the encouragement of Dr. Meredith E. Ostrom of the Wisconsin Geo— logical and Natural History Survey, who was instrumental in providing financial assistance for the field work. I am indebted to Dr. Donald W. Merritt for his assistance in develop— ing computer programs. Gravity meters were provided by the U. S. Army Topo- graphical Command. The author would also like to acknowledge the kind hospitality and assistance extended on numerous occasions by many of the people of northeastern Wisconsin, with special thanks to Mr. Dan Skrupty of Crandon, Wisconsin for sharing his knowledge of the area. iii ACKNOWLEDGEMENTS An endeavor such as this cannot be accomplished without the aid of many people. Foremost among these is Dr. William J. Hinze, who provided direction and supportive encouragement throughout all phases of the study. The helpful suggestions of Drs. Harold S. Stonehouse, James W. Trow, Hugh F. Bennett, and Chilton E. Prouty are also greatly appreciated. This study would not have been possible without the encouragement of Dr. Meredith E. Ostrom of the Wisconsin Geo— logical and Natural History Survey, who was instrumental in providing financial assistance for the field work. I am indebted to Dr. Donald W. Merritt for his assistance in develop— ing computer programs. Gravity meters were provided by the U. S. Army Topo- graphical Command. The author would also like to acknowledge the kind hospitality and assistance extended on numerous occasions by many of the people of northeastern Wisconsin, with special thanks to Mr. Dan Skrupty of Crandon, Wisconsin for sharing his knowledge of the area. TABLE OF CONTENTS Chapter I. INTRODUCTION . . . . . . . . . . . . . . . Statement of the Problem . . . . . . . . . Objectives . . . . . . . . . . Organization of the Survey . . . . . . . . Previous Geophysical Studies . . . . . . . II. FIELD METHODS AND DATA REDUCTION . Gravity Survey and Instrumentation . Elevation Control. . . . Accuracy of Horizontal Coordinates . . . Reduction of Data. . . . . . . . . . . . Error Analysis . . . . . . . . . . . . . . III. SUPPLEMENTARY GEOPHYSICAL INFORMATION. . . . Rock Densities . . Residual Gravity Map— —Double Fourier Series Aeromagnetic Maps. . . IV. REGIONAL GEOLOGY . . General Statement. . Geology of the Pembine Area. . The Menominee Iron—Bearing District. The Florence Area. . . . . . . . . . . Area West of Florence Area . . . . Goodman-Long Lake Area . . Amberg—High Falls— Mountain—McCaslin Area . V. INTERPRETATION . . . . . Introduction Greenstone Belts . . Argonne Gravity Trend. . . . . . . Granitic Rocks . . . . . . . Menominee Trough— Florence Area Western Florence and Northern Forest Counties. iv — III. I_.. x... weiqsdo . . . Hi}?'n‘33(30§1T-L4I .I 1'. Luff-I ".:i-- IL?! 't”""-.:-'0JI. :I’F- . - E O I ." . xix-'4. '.-__.-) -..-‘: 3- .u..f.;-s.’ :72.-':f,';'. .. [:‘-:|.-.'.'-."m"u. '." ' r_.2_'.r_'.-;- . '-'-:‘-'*- .:"-'i"-‘ ' -.'1|.' - i-‘G-T - - ‘. . r l' u p Chapter Page Role of Double Fourier Series Method in Regional Residual Separation. . . . . . . . . 101 ADDENDUM. . . . . . . . . . . . . . . . . . . . 103 Deeper Anomaly Sources . . . . . 103 Relationship of Gravity Anomaly Trends and Elevation. . . . . . . . . . 105 VI. CONCLUSIONS . . . . . . . . . . . . . . . . . . . llO BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . 11A v .. -' _ .7 ' ‘ LIST OF TABLES r—m 'I" ":“‘-:,4§:-H'.v , _ 1. Density Data. 2. List of Geologic Names. . . . . . . . . . . . . 36 3. Mean Bouguer, Free-Air, and Elevation Values for Township Tiers . . . . . . . . . . . 106 LIST OF FIGURES Figure Page 1. Index Map . . . . . . . . . . . . . . . . . . . . . 3 2. Base Station Ties . . . . . . . . . . . . . . . . . 7 3. Distribution of Elevation Control . . . . . . . . . 10 4. Elevation Errors - Altimeter—Controlled Gravity Stations . . . . . . . . . . . . . . . . 12 5. Bouguer Gravity Anomaly Map of Northeastern Wisconsin. . . . . . . . . . . . . . . . . . 15 6. Double Fourier Series Residual Gravity Anomaly Map of Northeastern Wisconsin. . . . . . . . . 2M 7. Double Fourier Series Regional Gravity Anomaly Map of Northeastern Wisconsin. . . . . . . 25 8. Profile A — Bouguer, Regional, and Residual Curves . . . . . . . . . . . . . . . . . . . . . 27 9. Profile B — Bouguer, Regional, and Residual Curves . . . . . . . . . . . . . . . . . . . . . 28 10. Profile C - Bouguer, Regional, and Residual . Curves . . . . . . . . . . . . . . . . . . . . . 29 11. Index Map to Profiles . . . . . . . . . . . . . . . 30 12. Index Map to Geologic Study Areas . . . . . . . . . 35 13. Geology of Pembine Area . . . . . . . . . . . . . . 37 14. Geology of Menominee District—Florence Area . . . . AA 15. Geology of Northwestern Florence and Northern Forest Counties. . . . . . . . . . . . . . . . . 51 16. Known Outcrops of the Granitic Gneiss Complex in Goodman-Long Lake Area. . . . . . . . . . . . 5A vii aqua '.'-i . . qs! tsbut .E . . . . . . :— 3; 1 now-3.33 9.2258 L..'. - a .' .1." .'. '. . "“15 '10 I‘C-ii-J-ii’14"“‘ I -- c '4 -. I. -‘ 5L, ,. 'G! '.- N ‘ I II | s.- -‘ 3 n r I ' p ~. Figure l7. 18. 19. 20. 21. 22. Geology of Amberg-High Falls Area . Geology of McCaslin-High Falls-Mountain Area. Geological Model and Observed and Computed Gravity Anomaly Profile Along D — D' Geological Model and Observed and Computed Gravity Anomaly Profile Along E - E' Mean Bouguer Anomalies of Township Tiers. Derived Geologic Map of Northeastern Wisconsin. viii Page 55 56 77 89 . 108 . 113 II. TABLE OF PLATES Bouguer Gravity Anomaly Map of Northeastern Wisconsin Double Fourier Series Residual Gravity Anomaly Map of Northeastern Wisconsin ix In Pocket In Pocket CHAPTER I INTRODUCTION Statement of the Problem Precambrian igneous and metamorphic rocks, including gneisses, greenstones, and basic and acidic intrusives and extrusives, underlie the surface glacial deposits and Paleo- zoic sedimentary rocks of northeastern Wisconsin. The Pre— cambrian geology south of the Menominee Iron Range has not received intensive study because of scattered outcrops and relative lack of economic interest until the last few years. Geological studies of selected areas in northeastern Wisconsin by many investigators have shown that the Precambrian rocks are structurally and lithologically complex, with signi- ficant density contrasts present. A gravity study has there- fore been made to aid in lithologic identification and deter- mination of the regional geologic framework. Objectives The overall objective of this study is to determine the potential of the gravity method in establishing the regional geologic framework of a part of the Southern Province Pre- cambrian complex where there are limited outcrops. If success- ful, this could assist in using gravity results to extrapolate geologic trends into areas of Paleozoic cover. 1 Primary specific objectives are: 1. to extrapolate known geology into geologically unknown areas 2. to determine the regional geologic framework by synthesizing geological and geophysical data 3. to define the subsurface configuration of crit— ical lithologic units. Auxiliary objectives are: 1. to ascertain effectiveness of Fourier techniques in regional-residual separation for this type of geology 2. to help in the location of areas favorable for mineral exploration. Organization of the Survey Figure 1 shows the area of investigation. This specific area was chosen to include the published local geologic studies, and was made large enough to include an extensive area of limited outcrops that could be used for testing the mapping. Gravity observations were made at one-mile (1.6 kilometer) intervals along all available roads. The density of roads is extremely variable, from section-line roads in agricultural areas of Marinette and Oconto Counties to widely scattered roads in the forested areas to the west and north. Over 3500 stations were established in the study area of approximately AAOO square miles (ll,A00 square kilometers). Elevation con- trol was achieved by several different methods, dependend upon Florence Co. For." Co. Merino". Co. - / tangled / Co. I Ocon'ofi. IJ /./ I ‘Monpminoo Co. / C __._. ______ Sc 0 I o no 0 10 20 30 40 miles 50 ‘ kilometers 2 fit I INDEX MAP the availability of elevation survey data. Rock samples were collected during the course of the gravity survey for later measurements to determine rock densities. Previous Geophysical Studies Several aeromagnetic maps have been published of local areas in the eastern and northern portions of the study area. Patenaude (1966) has compiled an aeromagnetic map of Wisconsin, but with a flight-line spacing too widely separated for detail- ed geologic analysis. Mack (1957) made a gravity survey of Wisconsin based upon very widely spaced observations which was later incorporated in the compendium maps of northern Wisconsin by Dutton and Bradley (1970)- Crustal seismic studies have been made in the general area by Slichter (1951) and Steinhart, Mayer, and Woollard (1961). Changes in crustal thickness were inferred. Crustal magnetotelluric studies in northern Wisconsin were begun by Dowling (1970) and have been continued by' Bentley (1973). No heat-flow measurements are known to have been made in northeastern Wisconsin. The closest known measurement is 50 miles to the northwest at White Pine, Michigan (Roy, 1963). Many geophysical surveys of a local nature have been carried out by exploration companies, particularly in the last few years. Results have not been made public. CHAPTER II FIELD METHODS AND DATA REDUCTION Gravity Survey and Instrumentation LaCoste and Romberg Geodetic Gravimeters were used dur— ing the summers of 1967, 1968, and 1969 to take over 3500 gravity observations. Observations were made at one-mile (1.6 kilometer) intervals along all available roads. Ten base stations were used for the purpose of determin— ing instrumental drift by reoccupation of a base station before and after each day's field observations. More frequent re- occupations were made early in the survey, but it was discov- ered that the combinations of high instrumental stability and the application of earth tide corrections made any error intro- duced by drift almost negligible. For the 1967 data a com- parison was made between the early morning and late afternoon reoccupations. For seventy—four available days, a mean reoccupation without respect to sign of 0.018 mgal was found, with a standard deviation of 10.023 mgal. Thirty—three changes were positive, thirty-two changes were negative, and on nine days no change was observed. Reoccupation measurements for other years showed comparable agreement. A drift curve was constructed for each summer's data by fitting straight line segments to the daily base station 5 reoccupation means, after correction for tides. For the 1967 data, four line segments with negative drift account for the first nine weeks, reaching a cumulative drift of —0.57 mgal, or approximately 0.01 mgal per day.. One line segment of positive drift represents the last three weeks, with a total drift value of +0.11 mgal. No reoccupation mean is more than 0.02 mgal from its line segment. A drift correction was applied for the drift represented by the straight line segments, but not for the deviations from the lines, as it was believed that this is a random error. Drift characteristics for other years had similar behavior patterns. Base stations were tied to each other by triple—looping and by other ties made at convenient times during the survey (Figure 2). Base stations were tied to the international gravity network through University of Wisconsin base station GW3 (Behrendt and Woollard, 1961) by multiple ties. Gravity connections were also made with a previous survey in Michigan (Oray, 1971) and to a network of base stations in Wisconsin (Richard J. Wold, personal communication, 1969). Oray's and Wold's work had been tied to the international network, and all base station values for the present survey agree with their values within a tenth of a mgal after connections have been made. Elevation Control Approximately one—half of the gravity observations were made at points of known elevation, such as U. S. Geological Survey or U. S. Coast and Geodetic Survey bench marks, road I (Hi .1.. F. 111...... p. ....I Emit 3w:. 1|". J. a .un. u-ufir .. _ ..._ ...._ . intersections, section corners, lake elevations, or other specific elevations taken from U. 8. Geological Survey topo— graphic quadrangle maps. These stations have minimum accuracy of i 1 foot (0.3 meter). A total of 236 station elevations were determined by interpolation from U. S. Geological Survey maps which have a contour interval of either 5 or 10 feet. Stations were located where contour lines were at identifiable features, such as buildings or timber lines, so that they could be readily identi- fied. Recent U. S. Geological Survey maps have an accuracy, at contour lines, of i 10% of their contour interval according to a U. S. Geological Survey surveyor (Gerald Scroggins, personal communication, 1969). This error is increased by location uncertainty, so it is believed that these stations should have an accuracy of i 2 feet (0.6 meter) or the same as those gravity stations located at known elevations. This assumes that station locations have been carefully placed with respect to contour lines. An additional 91 station elevations were interpolated from U. S.Geologica1 Survey maps having a contour interval of 20 feet. Applying the same reasoning as above, and allowing for the fact that these maps are somewhat older, these stations have an esti- mated accuracy of i 5 feet (1.5 meters). Elevations for four stations were interpolated from U. S. Geological Survey maps having a contour interval of 50 feet. This was necessary because of poor road conditions making stations inaccessible at the time of altimeter surveying. The gravity values for these stations fit smoothly with contours drawn from nearby stations, so they were not discarded.. Their estimated accuracy is i 10 feet (3 meters). Elevations for the remaining stations were determined by barometric altimeter surveying with Wallace and Tiernan Survey— ing Altimeters. During 1967 a recording barograph was not avail- able, so two traveling instruments were used according to a triple-looping method where reoccupations were used to correct for changing conditions on days with relative atmospheric stabi- lity. Traverses were constructed of triple—looping sequences which connected points of known elevation, and then other station elevations were tied to these sequences. Operations were terminated whenever barometric conditions became unstable. This was possible be alternating gravimetric and barometric observa- tions depending upon atmospheric conditions. Coleman, Athel— stane, Dunbar, and Goodman quadrangles were completed during 1967 using these methods (Figure 3). During 1968 and 1969 a recording barograph was used as a base station, with the two traveling altimeters employed on loop traverses with frequent reoccupations as field checks on accuracy. The same precautions as above were taken with res- pect to barometric conditions. The remaining planimetric quad— rangles west of the 1967 area were covered in this way. Included here are Mountain, Langlade, White Lake, Lily, Wabeno, Thunder Mountain, Laona, Long Lake, and Alvin quadrangles, and the eastern portions of Antigo, Elcho, and Three Lakes quadrangles (Figure 3). 10 46°N I .\ ask V \V 3W 45"” a {6/// ////1 ////. /// Topographic maps :1 Planimetric maps Fig. 3 DISTRIBUTION OF ELEVATION CONTROL 11 A check on the accuracy of the two altimeter methods has been made possible by the recent topographic mapping of certain quadrangles within the area. The preliminary maps for Dunbar, Goodman, and Laona quadrangles from the Rolla, Missouri office of the U. S. Geological Survey were searched for road corners and other known elevations that correspond to gravity stations whose elevations were determined by altimeter surveying. Locations were used for comparison only if it could be deter— mined that the gravity station and the new elevation were made at essentially the same point. Results of the comparison are shown on Figure A. The most significant result is that no dis- crepancies were greater than 6 feet (1.8 meters). There does not appear to be any essential difference in the accuracy of the two different altimeter methods, and accuracy does not seem to be dependent upon altimeter traverse distance. From the above discussions, it can be seen that the survey area is divided into two areas of elevation accuracy, which are shown on Figure 3. The most reliable elevations have an accu- racy of i 2 feet (0.6 meter) and include the areas covered by topographic maps. Ninety—one elevations within this area were interpolated from 20—foot contour intervals and have a probable error of i 5 feet (1.5 meters). These are scattered, and would not have a systematic effect. The area covered by planimetric maps has a maximum error of i 6 feet (1.8 meters). fi- _._ . -— Baptists-tsivs1a .3nivevw;: -me:9§ so hfn’ a‘vfi-‘d '1‘!!!" n: ..' ‘_-;. ‘ 761.; r1233 L“L“]fl$. v - __ In W m' "‘7‘“ m W cal-“E“ m .1! «ram M has .WO m Me 1:01: ado-mo: new tor-ma targeted .a .U or" to o: bnoqasztoc Jedi enOISsvefe nwonfl media has msJuMIILH id benim1ndeb tier anol3svels eardw '5 .5 21%” r-’2 a-n" '52 ' :; stew scofir .noe- rifvswn 5i! 3°11 JGJLM 12 FFET O o O 2‘ o Q . C . O O .0 O C O :2 . C Q ~ . O. Q 0 .. .0 ..o C C. O . 12345437591011: ousnwcs mow KNOWN ELEVATION (MILES) " Dunbor a Goodman Ouondronglee (without Barooroph) °= Loono Quadrangle (wlth Borooroph) Fla. 4 ELEVATION ERRORS- ALTIMETER- CONTROLLED GRAVITY STATIONS 13 Accuracy of Horizontal Coordinates Latitude and longitude coordinates were determined for each station from U. S. Geological Survey maps. The combined uncertainty of map error and station location error is esti— mated to reach a maximum on the planimetric maps of i 0.05 minutes. Horizontal coordinates on all the maps were there— fore determined to the nearest 0.05 minute. Reduction of Data Observed readings were first corrected for the earth tidal effect using the published tables of the European Association of Exploration Geophysicists. They were then corrected for observed instrumental drift as described previously in the first section of this chapter. The drift corrected readings were next converted to mgal using the manufacturer's calibra- tion tables for the individual gravimeters, and then adjusted to the national gravity network (Behrendt and Woollard, 1961) using the gravity ties as previously discussed. This process has then produced the observed gravity values. The simple Bouguer gravity anomalies were computed using standard techniques as described by Dobrin (1960). The theo- retical gravity was determined for each station from the International Gravity Formula of 1930 and the Free-air cor— rection was calculated using the vertical gradient of gravity of 0.09A06 mgal per foot (0.3086 mgal per meter). The Geodetic Reference System of 1967 was not used so that the Bouguer gra— vity anomaly map would be consistent with published gravity maps from surrounding areas. The mass correction factor for 1A the material above sea level was computed using a.density of 2.67 gm/cc, which gives a correction factor of 0.03A07 mgal per foot (0.1117 mgal per meter). .Terrain corrections were not made because gravity observations were made in areas with— out marked local topographic relief. The observed gravity values were then adjusted for the above corrections to obtain the simple Bouguer gravity anomalies according to the following: Simple Bouguer Anomaly = observed gravity — sea level gravity (theoretical) + free—air correction - mass correction The Bouguer gravity anomaly map (Plate I), which shows the location of the observations sites, was machine contoured utilizing a computer program prepared by California Computer Products, Inc. A reduced version of this map is shown on Figure 5. The gravity and elevation data are available at the Wisconsin Geological and Natural History Survey in Madison, Wisconsin. Error Analysis The potential sources of error can be divided into those errors arising from the survey itself, and the more difficult to evaluate uncertainty due to choice of the mass correction factor. A mass correction factor based upon an assumed density may not be representative of the material between sea level and the gravity station. The magnitude of this error is a function of the amount of topographic relief, and is equal to 0.00128 MMMMM / ., 7% 29 €29 I; ask M\ LORENCE c_QLII'MmmsrrE c_‘\ > m 3% \ @figa O O O» LL11 ‘0 o lo 20 [ITIIIIIIII I l g ‘k I M! ens ' I _Jéfik’fi. 90-1 _ I—MENOMINEE co. % far BOUGUER ANOMALY GRAVITY MAP Fig 5 OF NORTHEASTERN WISCONSIN 16 mgal per foot (0.00A2 mgal per meter) for each 0.1 gm/cc den- sity error. As an example,_in an area having relief of 200 feet (61 meters) and underlain by_greenstone having an average density of 2.95_gm/cc, the Bouguer anomaly values will be 0.7 mgal too high at the higher elevations. The range in elevations for gravity stations in the study area is from 582 feet (177 meters) above sea level along Lake Michigan to 1856 feet (566 meters above sea level in the northwest corner of the thesis area. Therefore, for the same greenstone rocks, the anomalies in the higher area could be in error by A.6 mgal relative to the area along Lake Michigan. Glacial drift densities depart signi- ficantly from bedrock densities in the area, but drift thick- nesses probably do not exceed a few hundred feet and would cause anomaly distortion only in local areas. The uncertainty arising from the mass correction factor, therefore, is of significant magnitude, but is inherent in any gravity survey and cannot be reduced by survey procedures. It can, however, be compensated for during interpretation. The largest possible survey error is found for those sta- tions whose elevations were determined using altimeters. The maximum elevation error for this method is 6 feet (1.8 meters). This would give a maximum error from the combined Free-air and mass correction factor of i 0.36 mgal. In areas of topographic maps, an estimated uncertainty of 2 feet (0.6 meter) would give an error of i 0.12 mgal. Figure 3 shows the distribution of these two areas of different types of elevation control. Terrain corrections were not calculated, but it is _— £1; M» "-. " ¢ .arikfim m m Wins it tun-b snow-1579.19 n1: sans-t er‘i' .amiaaveie «mum 8. w 003 Leon TYII anal 1GP m.cl a1 news thaia an: at enoiinia tstvsma 101 BC) 3391 33%; ”I nsrifistfi 1x51 “this Sets: sax evade (euessm , . -- . -_-=~._.. - -.- .-_r;,-: "u lax-'9- z-ea ear-111v? '"s -.:.. u; -' .. I ‘ g. {:915'} ‘1 3.; 'lxtr' I ‘5'.“ .:'19'1n-15 ' I " H1 \1‘ .— l L 1 f ‘ .L I. l 1 l r w l7 estimated that the maximum error from irregular topography would not exceed 1 0.25 mgal. The accuracy of the determination of latitude controls the accuracy of the throretical gravity. The maximum esti- mated error of 0.05 minutes is equivalent to i 0.07 mgal in the latitude range of this survey. Normal reading scatter of the gravimeter after correction for drift, is estimated to contribute an error of i 0.04 mgal. An estimated error of i 0.01 mgal is possible from both the earth tide correction and the drift correction. A net maximum error of i 0.74 mgal is therefore possible for gravity stations whose elevations were determined by alti- meters. A net maximum error of i 0.50 mgal is possible for gravity stations with known elevations. CHAPTER III SUPPLEMENTARY GEOPHYSICAL INFORMATION Rock Densities Bouguer gravity anomaly values are the summation of the gravimetric effect of horizontal density variations, both in the nearsurface and in the deeper crustal and upper mantle layers. In northeastern Wisconsin only minor gravity effects of the order of magnitude of 1 mgal or less are attributed to bedrock topography, to the presence of Lower Paleozoic sedi- mentary rocks in the southeastern corner of the area, and to lithologic variations within the glacial overburden. Lower crustal and upper mantle lithologic changes may cause broad gravity effects, but the majority of gravity anomalies in the study area are caused by variations in the composition of Precambrian rocks, and these changes are related to density variations. Accessible rock samples were collected during the gravity survey in an attempt to gather information about significant density contrasts. Samples were collected as free from weather— ing effects as possible. In addition, published information (Woollard, 1962; Daly and other, 1966; Gibb, 1968; and others) concerning the densities of Precambrian rocks was utilized in 18 ..I. 19 the gravity interpretation. All rock densities were determined using the formula D = wl / (w1 - w2), where D is the density, w is the weight of l the sample in air, and W2 is the weight of the sample in water. Samples were soaked for thirty minutes and agitated to remove air bubbles. The density values measured fall between the dry and saturated densities, but porosity and permeability are low enough in igneous and metamorphic rocks that these values can be assumed to approximate closely the field densities. Gravitational effects of the geology of the areaare domi- nated by two assemblages of rocks with contrasting densities, reflecting different compositions. These are the greenstones and granitic rocks, including felsic gneiss. Their distribution is responsible for the major features of the Bouguer gravity anomaly map. A summary of density information is given in Table 1. Samples from rock units defined in published geologic studies are listed together with samples from rock outcrops located in unstudied areas. From this information it can be seen that the most important density contrast in the area is that between the granitic rocks, including felsic gneiss, and the more dense greenstones. This contrast varies between 0.2 and 0.3 gm/cc. 20 TABLE 1 Greenstones Quinnesec of Pembine area Goodman Park area Allen Creek (5 miles east of Alvin) Brule Creek (2 miles northwest of Alvin) Waupee of Mountain area Nashville Quad (just west of thesis area) Badwater Greenstone All greenstones Other Volcanics "Siliceous rock" of Jenkins (Pembine area) Andesitic fragmental volcanics (Pembine area) Hager Rhyolite (at Thunder Mountain) Waupee water-laid ash (High Falls area) Waupee rhyolite porphyry (Mountain area) Rhyolite Porphyry (Nashville uad) Quinnesec felsic volcanics (west of 12 Ft. Falls) Serpentinized basalt, (asbestos prospect, Pembine area) Granitic and Intermediate—Composition Rocks Marinette Quartz Diorite Newingham Granodiorite " " (Cain,l96u) Hoskin Lake Granite Twelvefoot Falls Quartz Diorite N I\.) l\) NNN DOM .97 .00 .89 .9h 06 97 -99 -75 ~95 .78 -75 .63 .65 -77 .80 308 .08 .01 .01 .02 .01 .08 .03 .06 FJHHZ DUI-JR) O\I\J U) km 4‘: r H H m 21 Amberg Grey Quartz Monzonite Athelstane Pink Quartz Monzonite (Amberg area) Athelstane Pink Quartz Monzonite (southern area) Granite north of Cauldron Falls (near sta. 7A0) Granite in Laona Quad (near sta. 1601) Granite 7 miles east of Mountain Peshtigo River Porphyry Otter Creek (Crandon, NE Quad) Wolf River Batholith (not all samples from study area Hager Feldspar Porphyry Belongia Granite Wolf River Quartz Monzonite Anorthosite Red River Porphyritic Quartz Monzonite Wiborgite Porphyry Waupaca Quartz Monzonite Monzonite-Trachyandesite Metasediments Quartzite (at Thunder Mountain) Michigamme Slate (Leney, 1966) Michigamme Slate (Klasner and Cannon, 197A) Gneissic Rocks Dunbar Gneiss felsic phase " " intermediate phase on Dunbar Gneiss gravity trend (Armstrong Creek) on Dunbar Gneiss gravity trend (Nelligan Pond) on Dunbar Gneiss gravity trend (Nelligan Pond) on Dunbar Gneiss gravity trend (Long Lake) on Dunbar Gneiss gravity trend (7 mi. west of Long Lake) on Dunbar Gneiss gravity trend (Monico Quad) Macauley Granite Gneiss MN N NM NW N NM NM MNNI’U NMNN .68 -99 .89 .01 .00 .02 .03 .00 .00 .00 .02 .03 .01 .00 NA .17 .Ol .04 .01 .03 .00 .00 .05 m ‘0 u: (Hz I-‘NNUO I—‘I—‘WW I—‘UJ NH L104: .124: I\.) I-‘WUO 22 Gneiss (Starks Quad, west of thesis area) Gneiss (Starks Quad, west of thesis area) 50% or more of samples are mafic Hornblende Gneiss (Laona Junction) Gneiss (A miles northeast of Argonne) Gneiss (A miles northeast of Argonne) 80% of sample is mafic xenolith Mafic Gneiss (north of Tipler) Miscellaneous Mafic Rocks Metagabbro Gabbro (Peshtigo River Resort, below Cauldron Falls) Gabbro (High Falls Reservoir, highly weathered) Gabbro (7 miles east of Mountain) Diorite (2 miles northeast of Peshtigo Resort) Diorite (7 miles east of Mountain) Diorite (southwest of Amberg) Mafic Dike (Goodman Park area) Mafic Dike (Laona Quad, SE) Mafic Dike (Nashville Quad, west of thesis area) Mafic xenolith in Athelstane Pink Quartz Monzonite Mafic xenolith (Athelstane Quad, near sta. 7A1) Amphibolite (Wolf River Bridge, SE of Pearson) 2.80 2.90 2-79 2.71 MM CDC!) com 3-03 2-99 .01 .01 .03 .00 .10 .03 .02 .03 .01 .07 .Ol .01 :42 crH H r4 H U1 N I-‘ I-‘ LDI—‘LUIU 23 Residual Gravity Map — Double Fourier Series Major geologic trends within the study area are readily apparent on the Bouguer gravity map. Examples would include the main outcrop area of the Quinnesec greenstone belt, the Dunbar Gneiss and its extensions, and the positive trend that extends across the study area through Wausaukee and Argonne. Anomalies which are less obvious were searched for by prepar- ing a double Fourier series residual gravity map. The method of James (1966) for irregularly spaced data was used to cal- culate the regional and residual maps shown in Figures 6 and 7. Reference axes were aligned east—west and north-south to accommodate the observed dominance of east-west directions on the Bouguer gravity map. Orientation of the reference axes can cause distortion of anomalies not parallel to the refer- ence axes (Whitten and Beckman, 1969). However, in the example given by Whitten and Beckman (1969) to illustrate this problem, a reference axis shift of 400 produced structural axis trend shift of only 1.50. An improvement in the total variability (sum of squares) from 94.37 to 95.44 percent was noted with the reference axis shift. It is concluded that distortion due to orientation of reference axes is not a serious problem for this study. Most of the smaller anomalies on the Bouguer gravity map have a width of three to six miles (4.8 to 9.7 kilometers). The double Fourier series technique was therefore used to isolate anomalies of this width that might be obscured by 24 $9,. MILES m 10 0 IO 20 IIIFILIIIII I j 75m 10 o 10 20 [1111111111 j l Kl lOM E‘IERS Florence Co. ' Forest Co. Forest Co. . JEN???" Ler_I_gIade Co. -‘-’=-' Menom'm 9.2;. .:.-:.-:-... ”ANNETTE a: I. lie I .............. ................... ...... ....... ....... eeeeeeeee eeeeeeeee ...... '''''''' ..... ...... ............................ "..I..I.'IIO.. lllllllllllll ........................................ :........"__”:::::::: ................... ...... ..................................... ...................... POSITIVE ANOMALY C] NEGATIVE ANOMALY Fig 6 FOURIER RESIDUAL MAP OF NORTHEASTERN WISCONSIN x- .1"' -. . _ - - - . . ' l - - e . . 'l . a . - . . . 25 ‘ c.I. : 5 legals N Was “'7' ,4- 0° -A VF, yr / 4° «\A-V‘R 4°“ FLORENCE" '3 o 5 IO S/Afi -co (4) mm . I . . -10 30/ O 8 lg“ LAKE IIIDIIIIIIJIJIIIL { w “WK 1NIAGARA km- N ENCE orence Co. 4) Forest Co. Marinette 0. RES > \@ ...Mgsgg fl> 'AMBERG R I \PEARSON Oconto Co. .10 \I \'so MOUNTAIN Lon Ia e Co. MenomInee Co. ‘10/‘/ O ' WA SAUKEE Fig. 7 FOURIER REGIONAL MAP 26 major features. Fundamental wavelengths of 85.35 miles (137.4 kilometers) in the north—south and_75.35 miles (121.3 kilometers) in the east-west direction were used. A fifth harmonic trend surface was calculated to map all anomalies larger than a wavelength of approximately eight miles (13 kilometers). This was removed from the Bouguer gravity map to give a residual gravity map delineating all anomalies hav— ing a width less than eight miles (13 kilometers). Anomalies having a width less than twice that of the data spacing are not adequately defined due to the Nyquist frequency effect. Gravity stations were spaced one mile (1.6 kilometers) apart where roads made this possible. Therefore, the double Fourier series residual gravity map displays anomalies having a width of two to eight miles (3.2 to 13 kilometers), with less defination at the lower end of the range in areas of fewer roads. An additional problem is spectral overlap between differ- ent anomalies (Odegard and Berg, 1965). Any anomaly contains a spectrum of wavelengths instead of just one. Distortion can therefore be caused by removing longer wavelengths components from shorter wavelength anomalies. A comparison of Bouguer gravity anomaly profiles with derived double Fourier series regional and residual profiles can be seen in Figures 8, 9, and 10. Location of these pro— files is shown in Figure 11. The regional and residual pro- files when summed should be equivalent to the Bouguer profile. Discrepancies of 1 or 2 mgals can be noted which probably 27 wm>m30 >._._>¢30 >._._><¢@ J\I/ ‘I'I'll f.\ :1-5 ...I:- ' I STVOW 29 mw>mDU >.—._><&0 J \I/ c I 3...; 3.3:. .43 ~u "- 30 5 O 5 I0 miles I mco=fiekilometers 4‘ Florenc B —\ _ A \ -5040 Q 0 ' W 0/ desgno Waueguke / '10 \f00 L A N 6 AD E . a oumauo I C —————————— A ‘10) 0 C O N ‘eMorInerI esmig MENO INEE NeopiP c,Suring B c L Fig.” INDEX MAP TO PROFILES 31 can be explained by the computer contouring program. Aeromagnetic Maps A regional aeromagnetic survey of Wisconsin (Patenaude, 1966) was made along range lines, with the exception of the extreme southwestern corner of the survey area where a flight line spacing of three miles was used. The survey was con- ducted at a barometric altitude of 3,000 feet (914 meters) above sea level. This constant altitude resulted in variation of flight elevation above the Precambrian surface of north— eastern Wisconsin of between 3,000 feet (914 meters) near Lake Michigan and 1,100 feet (335 meters in the northwestern portion of the survey area. Patenaude published this map of Wisconsin with a contour interval of 400 gammas. Dutton and Bradley (1970) published Patenaude's data for northern Wisconsin, nmluding all of the present study area, with a contour inter- val of 100 gammas. A more detailed survey of portions of Florence, Forest, and Marinette Counties was flown with a flight line spacing of 1/4 mile (0.4 kilometer) (King, et. a1, 1966). This area is underlain by portions of the Dunbar Gneiss and associated intrusives, by part of the Quinnesec Greenstone Belt, and by the Badwater Greenstone and metamorphosed sedimentary rocks of the Florence area. Dutton (1971a) republished the Florence area data at a more detailed scale. The survey was flown at an elevation of 500 feet above ground, and in this area Pre— cambrian rocks are either at the surface or covered by thin glacial sediments. .— [- n-- 15 -_-¢": "'"‘E"‘O" -.- h; " , lag-1153 .‘_I_I-:"l1;!.'F_I"'-J'!'T'_s _fanotpe': A I ‘l I, .-'- . - I.. i In... . _ .4 . _ -. — “I u . r . ."5 -~.- ' .‘él'uifi 9.3. ., I._ U f " .. -I. 3 .IL" ‘ _ T; 32 Aeromagnetic maps with an approximate flight line spac- ing of 1/4 mile (0.4 kilometer) have been published of adjoin— ing areas in Michigan between 870 45' and 890 W longitude (U. S. Geological Survey, 1967). All of these aeromagnetic maps covering portions of Michigan, together with the map of King et. a1. (1966), have been published as a colored regional map (Zietz and Kirby, 1971). These maps, together with the recently published Precambrian map of the Upper Peninsula of Michigan (Bodwell, 1972), were consulted for possible infer— ences that could be drawn from the geology of Michigan. An aeromagnetic survey of the northern two-thirds of Wisconsin with a flight line spacing of 1/2 mile (0.8 kilometer) is planned (Ostrom, 1973). CHAPTER IV REGIONAL GEOLOGY General Statement Precambrian igneous and metamorphic rocks underlie all of northeastern Wisconsin, with surficial glacial deposits cover- ing most areas and a thin wedge of Lower Paleozoic sedimentary rocks occurring in the southeastern corner of the study area. hmvious geological investigators have shown that these Pre- cmmrian rocks are both structurally and lithologically com— leg and that most of them are of Middle Precambrian age. Weenstones of the area have been assigned to the Lower Pre— cmmrian by some authors. The Upper Precambrian is apparently I'eDresented only by a few diabase dikes. TWO prominent greenstone belts of relatively high—density mammorphosed volcanic rocks have been intruded by lower-density @finitic rocks. These granitic rocks occur widely, and belong tothree different ages of acidic igneous activity. Felsic an intermediate gneissic rocks are common throughout the area. Mm Florence area is underlain by a thick sequence of metasedi- mentary rocks, and the McCaslin Range is made up of quartzite. GI‘eehstones and granitic and gneissic rocks are the most common rock types in northeastern Wisconsin, but a variety of other I 33 ‘ 34 lithologies are found. The following discussion summarized the geology of the pre— viously studied areas within the survey region, incorporating geological observations made during the gravity investigation. Figure 12 is an index map to local maps showing geology and Bouguer anomaly contours. Certain critical isolated outcrops will not be discussed until the section on interpretation of gravity data. Table 2, adapted from Medaris and Anderson (1973), shows the sequence of Precambrian rocks in northeast— ern Wisconsin. Geology of the Pembine Area The most extensive outcrop area of northeastern Wisconsin is centered about Pembine. Figure 13 shows the mapped geo- logic units and Bouguer anomaly values in the Pembine area. As a result of these good exposures which include both green— stones and granitic intrusives, investigations have been car— ried out by several geologists (Cain, 1963, 1964a, 1964b; Wadsworth, 1963; Banks and Cain, 1969; Banks and Rebello, 1969; Hall, 1971; Jenkins, 1973). The Quinnesec Formation has been considered by all workers to include the oldest rocks in the Pembine area, but its age has been variously interpreted as Lower Precambrian based on indirect evidence to the north (Bayley and others, 1966; DUtton, 1971a), or as late or post-Animikean (Middle Pre- cambrian) based on a zircon age date of 1905 (+30 to -10) m.y. fI’Om porphyritic meta-rhyolite located 5 miles (8 kilometers) northwest of Amberg (Banks and Rebello, 1969). Recent work % 35 5 O i IO Miles 0 8 16 Kilometers \ 3 F r ”ff/Q54 loémé ,6 >Long F L O E C E Lake ' C: B x? Auro iogard L L m J 2 we I06/ {(3% V N 30 T 10“ F O R E T dmar Pembin 4 Crandon ‘0 La 0 I I )1: N \/‘° ‘3 L A NG L A o E '0 Mountain i I \ 1°) M E N o I N E E Neopito A Pembine Area D Northwestern Florence - Northern Forest Counties B Menominee District E Goodmon- Long Lake Area c Florence Area F Amberg - High Falls Area G Mr: Caslin - High Falls - Mountain Area -Fiq. l2 INDEX MAP TO GEOLOGIC STUDY AREAS 36 mxoom oapwmaoflcmoao> ucm 0choao> ownsms mmflmco zmHSMOMZ coapmahom mpovczmm mpmam mesmecOHz mxoom candoao> ofimmz Loumscmm madam xmmno case coapmfinom :opH cepho>fim ompwwpcmhmmwficcs .Q3090 hw>flm unflmm wxoom cacaoao> ofimmz ommmcqfisa mxoom OHCMoHo> oamawm communaso mxoom cacaoao> oamamm Lonowmm wufihowm Nphmsa mwcfim .m.E oaoalomma manflmwom opHNpMMSG :Hmeooz mthwEOHmcoo casmem moh< :fimucsoz CdHhDEdOOLm QHUUHS .z.E OHmHIommH chu pwoao Lo on Hmzvm .CprhwocD mxoom oacwoao> mumficmEhoan hmpmHHH oflwawm mcoEmm Ohnnmmwpms opHLOHQ nupmso maawm uoom m>am39 mmfiwco pmncso muahoao NphdSG mupwcfihmz maficmnu mxmq :fixmom opHMOHwocmpw Emcwcfismz ouHc0nco= uphwsa xcfim wcmumawca< opacoucoz nphmso ammo mponE¢ maficmno mHHmm nmfim mufimwvcmznowpa can opacowcoz owfipnmmm maficowcoz Nphmzo xwmho mm: mpflcmzm hmwmm hhzcahom mewcaom memm mpHHoznm nmwwm opacoucoz uppmzo can maficmpo occwapcnom Lm>Hm Mao: mpa:dho afiwqoamm cpaaocpmm pw>Hm Mao: wnwnE.E oamauomma szHmmom ACHMpnwoca .m.E oamanomma .2.E onmalooza mdnflmmoa .cfimuhmocb .>.E oomalomza zanfiwmom .cfldpmeCD .z.s oomalom H mm< wopmm ofiQOpomH :o ummmm mwm< mm20040m0 n. of :33 29:2. II 23:0 220... so {Egc 22.8.5 5:55 3:33. ->._:noa + IT 32m 2:609:25 .l NOEL \ thm \ wZOhwzwwmo . :58 m3; , l-:.u ij-H'ivl u;- 53 in another drill hole approximately one and one—quarter miles (2.0 kilometers) southwest of the section 9 drill hole in the NEl/h of section 17. Goodman-Long Lake Area Figure 16 summarizes information on known outcrops in this area, and also shows Bouguer anomaly contours. Scattered out- crops of gneiss occur in this area. Rocks similar to the Dun— bar Gneiss are exposed at Nelligan Pond, and just west of Long Lake. The latter outcrop is shown by Dutton and Bradley (1970) to be a mafic gneiss, though samples were not examined by Dutton. Density samples collected by the writer at this out- crop are gneissic and are of granitic composition. Granitic rocks are shown by Dutton and Bradley (1970) as outcropping both north and south of the gneissic complex in a manner simi- lar to the western part of the Pembine area, where the Dunbar Gneiss is flanked by felsic intrusives. Amberg—High Falls-Mountain—McCaslin Area Figures 17 and 18 show the geology and Bouguer anomaly contours for this area. The Amberg Granite (Figure l?) origi- nally named by Cain (1963), has been divided into pink and grey varieties, with the grey variety finer—grained and some- What more mafic (Cain and Beckman, l96h). Both varieties show fOliation and recrystallization textures. Banks and Cain (1969) have delineated a granodiorite variety, which corresponds to part of the grey Amberg Granite of Cain and Bechman. The granitic mass is generally unaltered and undeformed, although the grey variety is commonly gneissic at the margins of the *1 I; 1’ xx. 11%“ . a, v or we» I \e Paleozoic Sedimentary Rocks Wolf River Batholith Belongia Granite Hager Rhyolite Hager Feldspar Porphyry Pontiac Monzonite High Falls Granite Amberg Quartz Monzonite Athelstane Quart: Monzonite GEOLGY 0F AMBERG — HIGH KMQ Mcceelln Quartzite Diorite Go here Waupee Volcanics Oulnneeee Formation in Felsic Volcanics IO Matte VOICOMCI 0 Eddzizizizid FALLS AREA 56 \ :3: Is \\ \\ \,\\,§\ \ \ - “ \ fl / ’ WW if V \\ \‘7. in. S " muu- \" k ‘ ------ ..v. §\‘--“‘" w {titanium \‘~‘-.\\\\\\* \ \ \\ \ \\ l: ccccccccccccccccccccccccc 5C3 ooooooooooooooooooo E 4: ooooooooooooooooo slII 2E3 ccccccccccccc eeeeeeeeeeeeeee ,,,,,,,,,,,,,,, ' ‘6- ©\/" ‘ \‘\\\\ [\A \ \ ¢ \§ \\‘K\ .‘t “.1\\ \\ \ s\\§i‘\\\\\““‘.‘ “3m ‘ n. s},\\..\“:n ,. \. \ “lily/hm“, r ‘ ‘ g \|. n u ' s / (&‘v%? 4 EW Y I a“ ‘e \> xxh \\\\'f. \\ \ V‘s \\\ ii “WW“ s) «$0 3 as ‘ 000000 .......... [4 57 granite body, and until radiometric dates were available was considered younger than granitic bodies to the north. Zircon ages (Banks and Cain, l969)_yield synchronous dates for the pink Amberg Granite and the Newingham Granodiorite of 1860 i 15 m.y. A Rubidium-Strontium whole rock isochron for both the pink Amberg Granite.and the Hoskin Lake Granite of 1810 i 50 m.y. is in good agreement (Van Schmus, 1973). Field mapping by Medaris, Van Schmus (1973) and others has found four distinct plutonsof the grey variety in the Amberg area whose individual size ranges from one by two miles (1.6 to 3.2 kilometers) to a width of 8 miles (12.9 kilometers). The pink variety is widely distributed both in the Amberg area and to the west and southwest through the Athelstane area all the way to the High Falls area. They have therefore proposed that the grey variety be called the Amberg Grey Quartz Monzonite and the pink variety be called the Athelstane Pink Quartz Monzonite. Preliminary isotopic data for the Amberg Grey Quartz Mon- zonite reported by Medaris, Van Schmus and others (1973) sug— gests a younger age in the range of 1640 to 1670 m.y. Field relations confirm the above relative age dates of these two units, with dikes of Amberg Grey Quartz Monzonite intruding Athelstane Pink Quartz Monzonite. This absolute age places the Amberg Grey Quartz Monzonite in the Central Wisconsin Com- plex of Van Schmus (1973), which according to him comprises the bulk of the Precambrian basement of Wisconsin and is part of a major structural belt extending from Arizona to Wisconsin. /.4 58 The Amberg Grey Quartz Monzonite is, however, the only defi— nitely dated representative of the complex within the study area. Mafic and felsic metavolcanic rocks occur in several places and have been identified as Quinnesec Formation. It is uncertain how these rocks would correlate with Jenkin's expanded classification of his restricted Quinnesec Formation and three other formations. Cain and Beckman (1964) identi— fied metabasalt and metarhyolite outcrops north and northwest of Amberg as being an east-west extension of Quinnesec outcrops in the Pembine area. Distribution of these units is after the compilation map of Medaris and Anderson (1973). An age date for the metarhyolite was discussed in the section on the Pem— bine area. Quinnesec-type outcrops have been mapped southwest of Amberg, and at an abandoned molybdenum mine 5 miles (8 kilo- meters) southwest of Wausaukee (Cain and Beckman, 1964). South— west of Amberg the outcrops are primarily metabasalt with amphibolite schist at the eastern margin (Cain and Beckman, 1964). At the old molybdenum mine schistose greenstone is found (Cain and Beckman, 1964), although the molybdenite occurs in quartz beins in the surrounding granite (Kirkemo and others, 1965). Kirkemo and others (1965) have interpreted the green- stone as a basalt dike. Two drill holes near Crivitz have penetrated greenstone (Dutton and Bradley, 1970). The most recent study of the Waupee greenstone belt(Figure 18) ..L 59 is by Lahr (1972), and the following discussion is after him. The Waupee Volcanics in the type area east of Mountain include metamorphosed volcanic flows, tuffs, agglomerates, and sedi- mentary rocks. Basalt is the dominant type of flow, with some andesite present. The Waupee rocks dip steeply to the north— west, and strike N. 450 E. Regional metamorphism is to the amphibolite facies. The basal Waupee member consists of mas— sive flows, volcaniclastic sedimentary rocks and minor agglo- merates. This is overlain by a middle sandstone member with subordinate flows and an upper thin—bedded tuff member. An island are setting is suggested by the calcalkaline nature of the volcanic rocks. The Macauley Gneiss (granodiorite to quartz monzonite) then cut the Waupee rocks, and this was followed by deformation and regional metamorphism to the amphi— bolite facies. The Hines Quartz Diorite was then intruded, with accompanying contact metamorphism (Medaris, Van Schmus, and Lahr, 1973). The Baldwin Conglomerate was deposited next, and according to Lahr, the Hager Granite was then intruded. This unit was originally named the Hager Rhyolite by Mancuso (1957) on the basis of field relations, although he pointed out that the coarse grain size and great thickness suggested a granite. Read and Weis (1962) suggested that the Hager might actually be intrusive. Contact metamorphism described by Lahr (1972) supports the intrusive origin of the Hager. Medaris, Anderson, and Myles (1973) on the basis of further mapping, have defined three separate bodies: the Hager Rhyolite, the Hager Syenite, and the Hager Feldspar Porphyry. Intrusion of ." -")I9f‘-": ywi; I'I . . . .,. .. . ._ . _ ‘ . .- ' .:- - I _. . .. 60 the Belongia Granite then took place southwest of the main Waupee mass at approximately the same time as the High Falls Granite was intruding to the northeast. The High Falls Granite is clearly intrusive into.Hager rocks (Mancuso, 1960). The Hager, Belongia, and High Falls bodies have been included by Medaris, Anderson, and Myles (1973) within their Wolf River Batholith, a rapakivi massif that was defined as the result of their field Work and the age dating of Van Schmus (1973). The High Falls Granite is a questionable member of the batholith, and isotopic dating will be necessary to resolve its age. Shear zones occur extensively within the High Falls Granite, but are not found in other units of the Wolf River Batholith, except for the Eau Claire mylonite zone, a major shear zone mapped by LaBerge (1973) along the western margin of the batho— lith. It extends for thirty miles (48 kilometers) along a strike of N3OOE to near Antigo, at a point nine miles (14.5 kilometers) west of the western boundary of the study area. Medaris, Anderson and Myles (1973) have assigned the High Falls Granite to the Wolf River Batholith on the basis of its spatial relation to the other bodies of the batholith and because it intrudes the McCaslin Quartzite. Also included in the Wolf River Batholith, and occurring within the study area, are the Peshtigo Monzonite and Trachyande— site, the Hay Creek Quartz Monzonite, and the Wolf River Granite and Quartz Monzonite. The Peshtigo Monzonite and Trachyandesite are more mafic than other units within the batholith, and the trachyandesite is a porphyritic equivalent of the monzonite. 61 The Hay Creek Quartz Monzonite is a small body between the High Falls Granite and the Hager Rhyolite. The Wolf River Granite and Quartz Monzonite occurs southwest of the Belongia Granite and extending beyond the southern boundary of the study area. Quartz monzonite predominates within this unit, with granite occurring close to the Belongia Granite. A total of twelve intrusive bodies belonging to the batho- lith have been distinguished over an area of 3600 square miles (9300 square kilometers), extending southwest from the area of High Falls Reservoir and out of the study area. Of the exposed area, 87% is made up of quartz monzonite. Also present are gra- nite, syenite, monzonite, rhyolite, and trachyandesite. Anortho- site occurs within the batholith and has been intruded by granite (Weis, 1973), and it is believed to underlie part of the south- western corner of the study area. Van Schmus (1973) has deter- mined an age range of 1450-1500 m.y. for the Wolf River Batholith, so it is the youngest of three major phases of acidic plutonic activity in northeastern Wisconsin. Two structural features are readily noticed within the batho— lith without further detailed studies. These are an ENE trend within the quartz monzonites south of the study area, and the possibility of a ring complex at the northeastern end of the batholith. The Belongia Granite, Hager Rhyolite, Hager Feldspar Porphyry, and the Peshtigo Monzonite would be included in the postulated ring complex. This is suggested by their arcuate pattern, porphyritic textures, high—level characteristics, and the structural attitude of metasedimentary rocks occurring within the igneous rocks. I!» II' liHW sdT " - --"i'- :1-‘~ -' .'1 .' _-..-:' '. '- ."-""-'d..'..e"a v."‘.'l-".}.'h". ' —- an .- , _ -; {1'5 hrrdjs'” . J bncvnd garbnsixs has 62 The McCaslin district was studied by Mancuso (1960), and the following discussion is after him. The McCaslin Range (Figure 18) is formed of the resistant McCaslin Quartzite, which reaches a maximum thickness of 5,000 feet (1520 meters) and extends twenty miles (32 kilometers) in an east—west direc— tion. This quartzite also forms Thunder Mountain, a shorter north-south ridge located roughly half-way between the east end of the McCaslin Range and the east end of the main Waupee out— crop area near Mountain. The quartzite is unconformable upon rocks that have been correlated with the Waupee volcanics. These rocks outcrop at several places north of the McCaslin Range, and also just east of Thunder Mountain. Over half of the pebbles in the basal conglomerate of the McCaslin Quartzite are quartzite. Other rock types present have been identified as Waupee volcanics, banded hematite iron formation, and red jasper. The quartzite pebbles are well rounded indicating a distant source; the non—quartzite pebbles are angular to sub- rounded. This relationship, together with current directions inferred from cross-bedding observations, suggests a source in Waupee or Waupee-like rocks north and west of the McCaslin Range. The McCaslin Quartzite dips to the south, and the quartzite beds at Thunder Mountain dip to the west. Mancuso correlated the Baldwin Conglomerate with the quartzite to set up the framework for a postulated broad syncline plunging westward. Only the eastern portion of the syncline remains, however, as granitic intrusives have cut off both the Baldwin Conglomerate and the McCaslin Quartzite to the west. A fault cuts the McCaslin Range -. - r".- . .1! - -. ' "" ' ' H ' . _ .' ..f - .-.'.'.1i‘!LIE$'? -":--' 63 in section u, T33N, R15E with a trend of N. 30° w. The east side has been displaced southward by a distance of 1/4 to 1/2 mile (0.4 to 0.8 kilometer). There is some evidence for a simi— lar fault near the east end of the range in section 28, T34N, R17E. A few mafic outcrops occur in the area of the High Falls Reservoir, which is east and northeast of Thunder Mountain. Most important of these is an extensive area of gabbro south of the Peshtigo River between Caldron Falls Reservoir and High Falls Reservoir (Myles, personal communication, 1972). Another gabbro outcrops three miles (4.8 kilometers) to the south at the bridge across High Falls Reservoir in association with the infor— mally-named Peshtigo River Porphyry (Read and Weis, 1962). A diorite outcrop occurs in section 31, T34N, R19E, two miles (3.2 kilometers) to the northeast of the main gabbro area (Myles, personal communication, 1972). esnsirib 5 id baswflduoa .‘. - . . r.- 12"“ ' ~-'-'; assist-1.. some —-t arc-fl. .J-£.~ l:-'.:-. .7...- 2.. .. r r. .5‘:;.:- . I. :l P.- “II“: .‘.._. I k U 1 "J "- r ' CHAPTER V INTERPRETATION Introduction Interpretation of gravity and geology patterns has been carried out using all available types of data, including the Bouguer anomaly map, published geological information, rock densities, the Fourier residual anomaly map, and various mag— netic maps. Refraction seismologic data has been utilized to- gether with broad gravity variations to investigate the lower crust and upper mantle. The two major greenstone belts and the three ages of grani- tic activity will be discussed first because they cause the dominant gravity anomalies in northeastern Wisconsin. Other rocks will then be treated, followed by interpretation of possi- ble causes of gravity variations from below the upper part of the crust. Extrapolation and interpolation of mapped geology has been based upon continuity of anomalies in terms of their critical characteristics. Quantitative calculations of two—dimensional model profiles were made in certain critical areas in an effort to better define geological relationships of the area. An attempt was also made to point out situations where gravity is not useful. 64 'v 9.1"" r- -.-. 65 Bouguer anomaly values range from —2 mgal just south of Alvin to -84 mgal approximately 9 miles (15 kilometers) south of Pearson (Plate I). In the southwest and southcentral por— tions of the map the gravity field is dominated by a regional low. A strong negative gravity anomaly also extends across the northern part of the map from Pembine and Dunbar to near the western boundary of Forest County. These two gravity low areas are separated by a belt of strong positive gravity anomalies extending across the entire study area from near Wuasaukee through Argonne. Other positive anomalies are prominent, especially along the entire northernmost portions of north- eastern Wisconsin, in the Pembine area, and in the Mountain- Crivitz areas. Directional trends on the Bouguer map are strongly east- west, with important northeast—southwest and northwest—southeast lineations. A few anomalies are circular. Greenstone Belts The existence of the Quinnesec and Waupee greenstone belts as separate bodies is supported by the Bouguer gravity map. They are separated by about 15 miles (24 kilometers) of quartz mon- zonite, and the gravity map does not show any connections between the two positive belts. Individual greenstone belts are shown in Figures 13 and 18, and the intervening area is shown in Figure 17. The Quinnesec belt has extensive outcrop areas, but the Waupee exposures in the Mountain area may be the exposed west- ern portion of a much larger greenstone belt. The term Quinn— esec belt will be used to include the Beecher, McAllister, and to anti: an”; 66 Pemene Formations in that a typical greenstone belt of the Superior province in Canada includes differentiated inter— mediate and felsic volcanic rocks (Wilson and others, 1965). The surface configuration of both greenstone belts as derived from gravity is similar in character to the areas of preserved early Precambrian lavas in the Superior province of Canada and in South Africa (Anhaeusser and others, 1969). The characteristic pattern of long and relatively narrow arcuate synformal structures with many shorter arcuate tongues extend- ing out into the surrounding granitic terrain is developed in both the Quinnesec and Waupee greenstone belts. The eastern end of the Quinnesec belt under the Paleozoic overlap is undefinable at the present time due to lack of suffi- ciently close gravity control in Michigan, but Oray's (1971) gravity mapping with a station spacing of approximately 5 miles (8 kilometers) does allow for an eastward extension of the Quinnesec greenstones under the Paleozoic sediments of Michigan. More detailed mapping would be necessary to identify the char— acteristic outline of this greenstone belt in Michigan. It is also possible that the Waupee greenstone belt could extend east under Lake Michigan from a point several miles south of Marinette. Details of these two greenstone belts will be dis— cussed in following paragraphs, the Quinnesec first because of its more extensive outcrops. The greenstone outcrops of the Goodman Park area (Figure 13) are part of an elongated arcuate tongue extending west— south—west from the main mass of the Quinnesec belt. Density 1 $3.1Heesw3 '_-."."! 'I‘Qii ' ::“E:=- a £1 wavi'uh 67 sampling of these outcrops yields density values very close to those of the main Quinnesec outcrop area (Table l). The posi— tive anomaly associated with the Goodman Park area outcrops lies astride the Marinette-Forest County line, and is the only break in the otherwise continuous positive anomaly extending from the Michigan border east of Wausaukee to the western boundary of the study area. Beyond the Goodman Park area a positive trend can be followed continuously to the southwest corner of Forest County, where it bifurcates into a narrow anomaly continuing westward to the margin of the map, and a southwestern extension which rapidly widens to become almost circular near Pearson (Plate I). Density samples collected at the Wolf River bridge 1.5 miles (2.5 kilometers) southeast of Pearson appear to be from an amphibolite. The circular nature of this anomaly sug- gests an original gabbro or diorite intrusive body. The Twelvefoot Falls Quartz Diorite (Figure 13) appears to be part of the positive anomaly associated with the Quinnesec Formation, and there are field indications that it may be alter- ed Quinnesec rocks (R. Jenkins, personal communication, 1973). This unit is highly variable, and density sampling was inade— quate to give a valid estimate of its average density. It is possible, however, to conclude from density sampling and gravity mapping that the Twelvefoot Falls Quartz Diorite is probably not a low density diapiric intrusive such as postulated for the Marinette Quartz Diorite to the north. The Fourier residual map (Figure 7 and Plate 11) shows a seas qouoéro nsmhncv an? as ir - -. ‘.; ~n‘. .1 c' ‘ -“25wuars::"3M ed: SbIfiJBS ='- ‘i = -' -. . '- ' = ..- .'.:#.n.: -‘.;."19 ire ed: hi ., i. - -- ' vrzgun' r'." '..I 68 continuous anomaly extending from the southeastern portion of the Pembine area westward all the way to the western boundary of the study area, including the Twelvefoot Falls Quartz Dio- rite, the Goodman Park area, and the above-mentioned extensions. The Bouguer and Fourier residual maps, together with outcrop information, indicate a long, narrow extension of the Quinnesec greenstone belt that stretches across the entire width of the study area. Density samples from the Quinnesec outcrop area southwest of Amberg (Figure 17) yield an average density of 2.75 gm/cc and appear to be from a metamorphosed diorite. Dutton and Bradley (1970) show no greenstone outcrops in this area, and so this is not interpreted as a tongue of the Quinnesec greenstone belt. This area has no apparent expression on the Bouguer map, although surrounded by the slightly lower density Amberg Grey Quartz Monzonite. The Fourier residual map shows that these outcrops occur just west of a west-northwest trending positive anomaly which could be a northern extension of the Wausaukee positive trend. The lack of close association with the gravity anomaly suggests that these outcrops represent either large xenoliths or an intrusive body which has been deeply eroded. An arcuate tongue of Quinnesec greenstone has been mapped geologically extending westward 8 miles (12.9 kilometers) from the main outcrop mass of the Pembine area, and is clearly shown by gravity (Figure 13). It separates the Hoskin Lake Granite on the north from the Newingham Granodiorite on the south, and is a good example of how the arcuate shape of a tongue of - "- . l ' ._ :7 seminal-1 no .-3.-:l-_' ' ". ..-;_- " Pc- '6' - " , - '.- l _-, l 69 greenstone is caused by granitic intrusives (Anhaeusser and others, 1969), Within the main Pembine outcrop area (Figure 13) a south— ward decrease of gravity values reflects both a broad regional trend across the entire study area and a southward transition from mafic to intermediate and felsic volcanic rocks. The meta- gabbro sills intrusive into the northern portions of the Quinnesec greenstones have the same densities as the metabasalts, and so are not defined as separate anomalies. The southward decrease in gravity values through the Pembine area is interrupted by a flattening and a 2 mgal positive anomaly in the area of Pemene felsic volcanics. This is not defined on the Fourier residual map (Plate II) because of map edge effects and also because it is weak and narrow compared to nearby fea— tures. A negative gravity anomaly would normally be expected under an area of felsic volcanics surrounded by more dense mafic and intermediate volcanics. The faults defined by Jenkins (Medaris and others, 1973) may explain this relationship if the Pemene felsic volcanics have been preserved here because of their location in a down—faulted basin. A great thickness of mafic volcanics could have escaped erosion under the felsic volcanics, thereby causing the positive anomaly. Jenkins (1973) has des— cribed the structure of this area as an east-trending asymmetric doubly plunging syncline, with vertical dips on the south limb and dips of 550 to the south on the north limb. An east-west trend is shown by the Bouguer gravity anomaly. ‘_ 'e _ T'I" LIL: “.1. L 1’1 " .'I'fl 69 greenstone is caused by granitic intrusives (Anhaeusser and others, 1969). Within the main Pembine outcrop area (Figure 13) a south— ward decrease of gravity values reflects both a broad regional trend across the entire study area and a southward transition from mafic to intermediate and felsic volcanic rocks. The meta— gabbro sills intrusive into the northern portions of the Quinnesec greenstones have the same densities as the metabasalts, and so are not defined as separate anomalies. The southward decrease in gravity values through the Pembine area is interrupted by a flattening and a 2 mgal positive anomaly in the area of Pemene felsic volcanics. This is not defined on the Fourier residual map (Plate 11) because of map edge effects and also because it is weak and narrow compared to nearby fea- tures. A negative gravity anomaly would normally be expected under an area of felsic volcanics surrounded by more dense mafic and intermediate volcanics. The faults defined by Jenkins (Medaris and others, 1973) may explain this relationship if the Pemene felsic volcanics have been preserved here because of their location in a down—faulted basin. A great thickness of mafic volcanics could have escaped erosion under the felsic volcanics, thereby causing the positive anomaly. Jenkins (1973) has des- cribed the structure of this area as an east—trending asymmetric doubly plunging syncline, with vertical dips on the south limb and dips of 550 to the south on the north limb. An east-west trend is shown by the Bouguer gravity anomaly. 1.; 70 Maximum Bouguer values in the Pembine area are reached in the northeastern part of the area. Shape and orientation of the maximum—value contours show a relationship to the previously discussed Quinnesec tongue extending between the Hoskin Lake Granite and the Newingham Granodiorite. The Fourier residual map (Plate II) shows this continuity by an east—west positive anomaly, suggesting structural preservation of a greater thick— ness of greenstone along this trend. A positive trend along the Michigan border north of the Hoskin Lake Granite is shown on both the Bouguer and Fourier residual maps. It has been produced primarily because of the density contrast between the Hoskin Lake Granite and the Quinne— sec greenstones with its intruded metagabbro sills. This posi- tive trend is continuous along a west—northwest trend through the Florence area (Figure 14 and Plate 11) following the known outcrop pattern of the Quinnesec greenstones. This trend con- tinues on both Bouguer and Fourier residual maps to the vici— nity of the Michigan border, where the trend becomes east—west through Alvin (Figure 15) and then curves slightly to the west—southwest before continuing westward to the margin of the study area. Greenstone was sampled at Allen Creek, 5 miles (8 kilometers) east of Alvin, and at Brule Creek 2 1/2 miles (4 kilometers) northwest of Alvin (Figure 15). These outcrops are located on negative Fourier residual anomalies, and are close to granite outcrops (Dutton and Bradley, 1970). They are north of the positive trend interpreted here as an extension of the Quinnesec greenstones, and probably represent intruded .:'-noses '71s :L'LF. 't lien-r- ..J': .' . _ . . - .....-...,,-_.~; ; -.-_: - ‘i-' ..' "e.-'_'-."1{ilbl.i — J I ~ " s . r ’I‘: i'lb' :..!:*. 1;. . r-_'l’ .. — "5. . '55.” ' -.‘ . . 71 remnants of a formerly wider greenstone belt. Greenstone also outcrops on the southeast side of the gravity maximum southeast of Alvin (Dutton and Bradley, 1970). Bouguer gravity values reach a maximum for the study area of -0.1 mgal at Gravity Station 3408, 3 miles (5 kilometers) southwest of Alvin. A magnetic high is located over the southeast portion of the gra- vity anomaly (Patenaude, 1966). Dutton inferred a greenstone belt west of the study area underlying parts of Price, Oneida and Vilas Counties based on a few scattered outcrops and linear magnetic anomalies suggest— ed by the work of Hotchkiss (1915), Hotchkiss and Bean (1929), and Patenaude (1966). The map outline of his inferred green— stone belt ends 3.5 miles (5.6 kilometers) west of the Forest County line (Figure 15), and the trend of its axis is aligned with the positive Fourier residual anomaly extending westward from the Alvin area. The continuous nature of the Fourier residual anomaly along this entire trend from north of Pembine through Alvin to the margin of the map is similar to the gravity pattern of the postulated westward extension of the Quinnesec greenstones through the Goodman Park area and beyond to the western edge of the study area. The extent of the Quinnesec greenstone belt is therefore similar to greenstone belts in the Superior Province of the Canadian Shield, with long, narrow extensions and arcuate tongues extending into surrounding granitic rocks. Mursky (1973) has noted the chemical similarities of the Wisconsin greenstones to the Archean volcanic assemblages of the Superior Province. 72 A Fourier positive residual anomaly extends southwestward from the Alvin positive anomaly towards the Argonne positive trend. Density samples were collected from intermediate to mafic gneisses on this anomaly at the edge of the map. It is common for non—felsic gneisses to occur around the margins of the Dunbar Gneiss and its extensions, and the location of the anomaly suggests that it is caused by these denser gneisses. No greenstone outcrops are known in the immediate area of this anomaly. The Waupee volcanic rocks of the Mountain area (Figure 18) are associated with a well-defined positive anomaly on both the Bouguer and Fourier maps. Gravitational delineation of the heavier Waupee rocks is due to proximity to the surrounding Wolf River Batholith and Northeastern Wisconsin Complex. Paleozoic sedimentary rocks have concealed Precambrian relation- ships to the southeast and east of the Waupee outcrop area, but anomaly patterns show that the Wolf River Batholith bounds the Waupee rocks all along their southern margin, and suggests that the Waupee greenstones may continue eastward under the Paleozoic cover. Two drill—holes have penetrated greenstones in the Crivitz area (Dutton and Bradley, 1970). Granite was found in two adjacent drill—holes, suggesting intimate greenstone—granite relationships similar to those found in the Quinnesec greenstone belt. An alternate hypothesis will be suggested in the follow— ing section. This postulated eastward continuation is shown by both the Bouguer and Fourier maps, with an east—west axis passing south .hwrwod ‘ " 3L —l4i'= 1A fiEiJ ’J'v'i. .. L '. Pvrtu-ruaéqi new. heflrn.‘ “ i! v -w. :ifix en sessions ottsm ' '" 1.:AH-ncn 151 Hemmer ' ssdnud a ’ I I 73 of Crivitz. This is especially well shown by the Fourier map due to its designed removal of features larger than eight miles (12.9 kilometers). For this reason the broader overall shape of the postulated Waupee greenstone belt south of Crivitz is not apparent on the Fourier residual map, but can readily be seen on the Bouguer map. The characteristic pattern of a greenstone belt, as discussed previously, is evident with its elongate extensions of greenstone and embayments of granitic intrusives. An eastward extension under Lake Michigan of postulated Waupee greenstones is suggested by a steep Bouguer gravity gradient south of Marinette and an east-west positive gravity trend through Peshtigo (Plate I). This is also supported by the Fourier map, although use of this map may be ambiguous close to its edges. The two stronger positive gravity trends within this positive anomaly, one south of Crivitz and the other through Peshtigo, suggest that these may be synclinal structural zones that have preserved thicker sections of mafic rocks. These trends, as in the Quinnesec greenstone belt, are east-west. The Crivitz trend is also a magnetic positive anomaly (Dutton and Bradley, 1970), but is aligned in an east-southeast direction. This disagreement in direction may be the result of the six—mile aeromagnetic flight line spacing. The positive anomaly south of Porterfield (Plate 1) appears to represent a short eastward extension of greenstone from the postulated greenstone belt, separated from greenstone to the qsm "uliflfiq and: 133151 esmnssjf 2 sud JC'i31 tin: it? ."-'.'I'. ".'3 ..I i—.: . ' 1.":1-113‘161 '_ :1? . '-.:-'. ',- :5: .' "'6', .-" 11:.“ '._—;|-"~:. F-‘JL’E‘W T'SJSIUJBOQ a“: 1,: 3.. car _s... - . Ews~" ar' n rqeasqqs so: 'S..-'."'L' "If“. [15 74 south by a narrow granitic intrusive embayment. It is on trend with the Crivitz magnetic anomaly. The southeastern boundary of the Waupee greenstone belt is not clearly defined and could extend southeastward under Lake Michigan. Gravity gradients are of considerably less magnitude than in the Crivitz area, however, suggesting a thinner section ' of greenstone combined with increasing depth of burial beneath Paleozoic rocks. A positive gravity anomaly in the Breed and Suring areas is separated from the Waupee greenstone belt by a negative anomaly varying in width from 3 to 6 miles (5 to 8 kilometers). This negative anomaly almost disappears between Mountain and Breed, however, indicating a possible connection between the Waupee greenstone belt and the Breed—Suring positive anomaly. The positive anomaly is almost rectangular, slightly longer in the north-south direction, and its southern portion is beyond the limit of the survey. There are two gravity maxima within this anomaly, one located east of Breed with a northeast—trending axis, and the other south of Suring and cut off by the map boundary. Both of these coincide with magnetic positive anomalies, with the Suring magnetic anomaly circular in shape (Dutton and Bradley, 1970). The close proximity of the Waupee volcanic rocks and the magnetic positive anomalies are suggestive of greenstones, but the overall shape of the anomaly and lack of gravity evidence for granitic embayments discourage this hypothesis. No out- crops or drill—holes are known on this anomaly, either within 75 the survey area or to the south. An alternative hypothesis will be suggested later relating this anomaly to the Wolf River Batholith. The Badwater Greenstone of the Florence area represents a small portion of a third greenstone belt, with most of it occurring in Michigan. The Wisconsin portion has not under- gone widespread granitic intrusion like the Quinnesec and Wau- pee greenstone belts, and so it cannot be analyzed in a simi— lar manner. It will be discussed later in the section dealing with the Florence area. Argonne Gravity Trend This gravity survey has defined a strong positive anomaly extending through Argonne (Plate I) which is associated with poorly known mafic and ultra—mafic rocks. Maximum values on this anomaly are reached near the western boundary of the study area, suggesting that it extends to the west beyond its length of 30 miles (48 kilometers) as defined by this survey. The anomaly is too wide to appear on the Fourier residual map, but is very obvious on both the Bouguer anomaly map and the Fourier regional map. A peridotite dike has been identified by drilling 4 miles (6.4 kilometers) north of Argonne on the crest of the anomaly. Recent exploration by several companies for sulfide mineraliza- tion has taken place in the Armstrong Creek area, which is on the east flank of this anomaly. Continuity of this trend is broken to the southeast by the Goodman Park extension of the Quinnesec greenstone belt. 76 A two—dimensional analysis was carried out along profile D (Figure 19) to investigate the subsurface configuration at this junction. The location of this profile is shown on Figure 11. Known outcrops have been used to constrain the interpretation. A first attempt at modeling assumed that the major feature is a basin—shaped greenstone belt, but it was not possible to approximate the anomaly shape while adhering to the distribution of rocks at the surface. Other shapes were tried, and the model in Figure 19 came closest to the observed Bouguer pro— file. Body 1 is a mafic body with greenstone outcrops, but may include units of the Argonne gravity trend. Body 2 con— sists of known felsic volcanic rocks. An intrusive mixture of felsic and mafic rocks is interpreted as Body 3. Body 4 is not represented by outcrops, but is modeled as an intermediate—com- position gneiss body such as is known to occur marginal to the Northeastern Wisconsin Complex in other areas. Body 5 repre— sents the Northeastern Wisconsin Complex of granitic rocks, and the Wolf River Batholith is present as Body 6. A positive anomaly of diminished amplitude extends east- ward through Wausaukee from this junction with the Quinnesec greenstone belt, and is a possible continuation of the Argonne Gravity Trend. Gabbro outcrops occur on this anomaly along the Peshtigo River 7 miles (11.3 kilometers) southwest of Athelstane (Figure 17). Other outcrops have been mapped as granitic rocks similar to those to the north and south, suggesting that a possible mafic source of this anomaly is below the surface. The gabbro outcrop area, however, does not produce a local anomaly 76 A two-dimensional analysis was carried out along profile D (Figure 19) to investigate the subsurface configuration at this junction. The location of this profile is shown on Figure 11. Known outcrops have been used to constrain the interpretation. A first attempt at modeling assumed that the major feature is a basin-shaped greenstone belt, but it was not possible to approximate the anomaly shape while adhering to the distribution of rocks at the surface. Other shapes were tried, and the model in Figure 19 came closest to the observed Bouguer pro- file. Body 1 is a mafic body with greenstone outcrops, but may include units of the Argonne gravity trend. Body 2 con- sists of known felsic volcanic rocks. An intrusive mixture of felsic and mafic rocks is interpreted as Body 3. Body 4 is not represented by outcrops, but is modeled as an intermediate-com- position gneiss body such as is known to occur marginal to the Northeastern Wisconsin Complex in other areas. Body 5 repre— sents the Northeastern Wisconsin Complex of granitic rocks, and the Wolf River Batholith is present as Body 6. A positive anomaly of diminished amplitude extends east— ward through Wausaukee from this junction with the Quinnesec greenstone belt, and is a possible continuation of the Argonne Gravity Trend. Gabbro outcrops occur on this anomaly along the Peshtigo River 7 miles (11.3 kilometers) southwest of Athelstane (Figure 17). Other outcrops have been mapped as granitic rocks similar to those to the north and south, suggesting that a possible mafic source of this anomaly is below the surface. The gabbro outcrop area, however, does not produce a local anomaly 0 airless 3!: I' . 1:94 svrfi eqcuoruc aneufl -I:'-. il"-" ' "II-I . I .- . 1'. '5'} " . ‘- ' ;-u-,r.-._--.+.:le i-‘Z'Il’i l“. ‘ 'llzf‘n . .' Pb 77 >._._>._._>JF_>mmmm0 OZ< Ammo—2 440.6040w0 .vm.N n .w.m mxoom ozoatxw new use 253:: 2.22 rm new ¢0.Nu .@.w Nm minatxw 22o... e can u on anm nxoom 333...... 22o... low: 9.0 0:05. not: -2- ohm u .o.m 33.2.0 :oEoonEoo - 3032535 100! w 9 W ND.N u .O.m lam- S 3.950 309.202 ...ch :Kw u .0.w 0.”. . . .m.«. _. .O.N. . . _mm. . . .°.n. _ . . m . . . .ng £—__O£«Om LO>_E :0; l €0~¢EO=X on- D D 78 as it should if it is surrounded by granitic rocks. An alternate explanation for the anomaly through Wausaukee would involve granitic rocks of a somewhat higher density. Density data (Table I) for the Athelstane Pink Quartz Monzonite can be divided into two areas, with significantly higher den- sities in a southern area located over the Wausaukee positive anomaly. The lower density contrast involved in this hypo— thesis correlates with the observed lower anomaly amplitude. Reconnaissance geologic field work by the students of Medaris at the University of Wisconsin (Medaris, personal communication, 1973) has not delineated this more dense facies of the Athel- stane Pink Quartz Monzonite. Further detailed petrologic work should be done. Inspection of the Fourier regional map (Figure 6) suggests that the Argonne Gravity Trend has a continuation to the south through the Crivitz area. It has been previously suggested in this study that the positive anomaly in the Crivitz area rep- resents an extension of the Waupee greenstone belt. An alter- nate hypothesis would extend the mafic and ultra—mafic rocks of the Argonne Gravity Trend through the Crivitz area and imply that the Waupee greenstone belt is not much larger than its out— crop area. This gravity survey is not sufficient by itself to choose between the two different interpretations of the Crivitz positive anomaly. It will be necessary to have aeromagnetic data to decide between a greenstone belt and a mafic intrusive. ,aflouu inUOTHT v Aron 79 Granitic Rocks Of the three ages of granitic activity in northeastern Wisconsin (Van Schmus, 1973), the youngest and oldest are represented by igneous complexes of batholithic proportions, while only scattered plutons belong to the intermediate-age phase of intrusive activity. The Northeastern Wisconsin Complex, the oldest of the granitic rocks, is well—defined both by geological mapping and by gravity in the Pembine area (Figure 13) where all but one of its various units have been differentiated. These units occur as the eastern portion of a major gravity minimum extend- ing westward through Dunbar, Goodman, Fence, and Long Lake (Plate 1) almost to the boundary of the study area. The Athelstane Pink Quartz Monzonite to the south is of the same approximate age according to radiometric dating, but is not as well defined gravimetrically and is separated from the main part of the Northeastern Wisconsin Complex by the Goodman Park extension of the Quinnesec Greenstone Belt and its Bouguer positive anomaly. The major negative gravity anomaly extending west through Dunbar and then into Forest County can be sub- divided into three connected closures. The entire anomaly will be referred to as the Dunbar Gneiss Gravity Trend, as this unit coincides with the dominant portion of the anomaly in the Pembine— Dunbar area. The separate closures along this trend will be termed the Dunbar-Goodman, Fence—Long Lake, and Forest County closures. It is uncertain whether the Dunbar Gneiss underlies the entire Dunbar—Goodman closure, as only a few granitic 1 beansaETQSfi 'ih edifice (Ins eiiflw .r-' l.-- -=-'-:.“.:"-= 80 outcrops are known in its western half (Dutton and Bradley, 1970). More extensive glacial drift cover in this area and lack of geologic investigations are probably both responsible for the lack of outcrop information. Continuity of the Dunbar- Goodman closure with a constant magnitude suggests continuation of the Dunbar Gneiss throughout the area of the closure. The Bouguer gravity pattern suggests that the Hoskin Lake Granite and Marinette Quartz Diorite together form a satellite body to the Dunbar Gneiss, extending northeast and east from the northeast corner of the Dunbar Gneiss. Both of these eastward extensions are isolated by the Fourier residual map. A similar granitic extension from the Dunbar Gneiss into surrounding greenstones to the south and southwest is suggested by the Bouguer anomaly pattern near Goodman. The Fourier resi- dual map shows a negative anomaly here, and a granitic outcrop occurs 3 miles (4.8 kilometers) south of Goodman (Dutton and Bradley, 1970). No other outcrops are known in the immediate area. An analogous northern extension into Quinnesec greenstones of the Florence area has been mapped by Dutton (1971a) using scattered granitic outcrops and aeromagnetic data (King, et. a1., 1966). Dutton assumed that the granite was underlain by low magnetic values, and the contact with the Quinnesec greenstones shown on the geology map is based on the magnetic pattern. He correlates this granite with the Hoskin Lake Granite, although it isn't as porphyritic as the Hoskin Lake Granite in its type 81 area. Regardless of whether this granite is part of the Hoskin Lake Granite, the gravity-pattern.demonstrates that it is found in a similar tectonic setting as a satellite.body to the Dunbar Gneiss. The four granitic extensions from the Dunbar Gneiss cor- relate with negative Fourier residual anomalies having similar sizes and magnitudes. The four anomalies have a magnitude of six to eight mgal. The negative anomaly associated with the Dunbar Gneiss is remarkably flat within the last closing contour line, considering the variable amounts of mafic constituents present in different phases of the gneiss. Four samples of the felsic coarse, por- phyroblastic phase have a mean density of 2.62:.01gm/cc whereas four samples of the intermediate migmatitic phase from north- west of Dunbar have a mean density of 2.72:.04gm/cc. The mig— matitic phase is known primarily from the area northwest of Dunbar, which is at the west end of the known outcrop extent of the Dunbar Gneiss. Smoothness of the central part of the Dunbar— Goodman closure suggests either thorough mixing of the two phases to avoid producing local anomalies, or that the migmatitic phase is insignificant in terms of its total mass. Two areas of greenstone have been mapped on the Dunbar— Goodman gravity closure, but they have no gravity expressions, suggesting they are thin erosional remnants. A half—mile (0.8 kilometer) wide metagabbro sill has been mapped extending east-northeast for over 6 miles (10 kilometers) through the east- ern portion of the Dunbar-Goodman closure. Detailed outcrop 82 information is not available, although Dutton and Bradley's (1970) outcrop map shows four outcrop locations. This body also has no gravity expression, which is surprising considering that several gravity stations are well located to identify any local anomaly. The age problem of this mafic intrusive body was mentioned previously, resulting from the demonstrated intrusion of the Hoskin Lake Granite and Marinette Quartz Dio— rite into the metagabbro sills to the north and the approximate age equivalence of the Dunbar Gneiss with the Hoskin Lake Gra— nite and Marinette Quartz Diorite. Lack of gravity expression by this body suggests either that it is very thin, unlike the metagabbro bodies to the north, or that mapped outcrops are actually metagabbro xenoliths of the same age as the sills to the north. The Fence—Long Lake closure (Figure 16) is the same size and magnitude as the Dunbar-Goodman closure, but much less geo- logic information is available. Two granitic outcrops are reported in the southeastern portion of the closure and two outcrops of gneissic rocks are known in the Long Lake area (Dutton and Bradley, 1970). As mentioned previously, these gneissic rocks have been termed mafic, although samples col- lected from the outcrop just west of Long Lake have densities in the felsic range (Table l). The samples are similar to the coarse porphyroblastic phase of the Dunbar Gneiss. An additional outcrop of felsic gneiss located 7 miles (11.3 kilometers) east of Long Lake was discovered during the gravity survey. North— west of this outcrop a granitic outcrop occurs on a northeast 83 nosing of the anomaly (Dutton and Bradley, 1970). Distribution of the few known outcrops on the Fence—Long Lake closure can be summarized as felsic gneiss outcrops located on an east-west axis and granitic outcrops associated with extensions from that axis. The Dunbar Gneiss also has an east-west axis. Central portions of both the Dunbar-Goodman and Fence-Long Lake gravity closures have little magnetic relief, but the saddle area between the two closures is characterized by an 8 mile (13 kilometers) wide area of many ovoid positive magnetic anomalies (King, et. al., 1966). No outcrops are known in this area. Magnetic character is similar to the Quinnesec Formation to the north, but the broad shape and low magnitude of the posi- tive gravity saddle argue against the area being underlain by greenstone. An extensive outcrop area at Nelligan Pond on the southwest flank of the Fence-Long Lake closure was discovered during the gravity survey and offers a possible clue to the cause of the positive saddle. Felsic gneiss outcrops similar to the coarse porphyroblastic phase of the Dunbar Gneiss are found together with mafic gneiss outcrops. Locally felsic intrusions into the mafic gneiss are found, and in places mafic xenoliths occur. The aeromagnetic map of King et. a1. (1966) does not include this area, but it is suggested that local con— centrations of mafic gneiss could cause positive magnetic anomalies and decrease the amplitude of the Dunbar Gneiss Gravity Trend between the Dunbar—Goodman and Fence-Long Lake closures. - '- '. C:".- :.:--:|'.:r.‘_i diiw 84 A gravity saddle similar in size and magnitude to the above area also separates the Fence-Long Lake and Forest County closures. Detailed magnetic maps are lacking for this area, however. The only exposures reported on the saddle are three granitic outcrops (Dutton and Bradley, 1970). The Forest County closure (Figure 16) is smaller than the other two closures on the Dunbar Gneiss Gravity Trend, and has not been as well defined due to lack of roads. Anomaly shape, amplitude, and orientation all suggest, however, a geologic continuation of felsic gneiss and associated granite. The only known bedrock on this closure is an outcrop area of gneiss 7 miles (11.3 kilometers) west of Long Lake. These rocks were sampled for density values (Table 1). Several previously unreported outcrops of gneiss occur along the western boundary of Forest County (Figure 16) at the intersection of the Forest County negative gravity closure and the positive Argonne Trend. These rocks are of intermediate composition. The location of this intersection of gravity trends along the margin of the study area makes it impossible to predict whether the Dunbar Gneiss Gravity Trend continues to the west. The Wolf River Batholith, youngest of the three phases of granitic activity, does not show the close correlations between geology and gravity that the Northeastern Wisconsin Complex does, possibly due to the less detailed geologic mapping that is has received to date, partial cover by the Paleozoic overlap, 85 and lack of outcrops west of the Mountain-McCaslin area. There are certain correlations, however, which suggest that gravity could aid in further study of this rapakivi massif. Medaris, Anderson, and Myles (1973) have compiled a geologic map of the known extent of the Wolf River Batholith. Dist- ribution of outcrops strongly suggests that the southwestern portion of the study area, which is lacking in known outcrops, is underlain by units of the Wolf River Batholith. This coin- cides with a broad gravity minimum having the lowest Bouguer gravity values of the study area. The Eau Claire River shear zone (LaBerge, 1973) is at or near the western margin of the Wolf River Batholith to the south. Gravity coverage of the exposed Eau Claire River shear zone is not available. It strikes N300E, and has been extended into the study area using topographic lineaments (LaBerge, 1972), passing just south of Pearson. This would be too far north for the gravity—indicated edge of the Wolf River Batholith, but a parallel topographic lineament extends along the steep gravity gradient north of Lily (LaBerge, 1972). The Wolf River Batho— lith anomaly does not continue to the north with the lineament but has a generally east—west boundary in the area east of Lily. The positive anomaly in the Breed-Suring area, along the southern margin of the study area (Plate I) has been discussed as a possible greenstone area. An alternate hypothesis would. relate it to known anorthosite bodies in the Wolf River Batho— lith south of the study area. A sample of one of these outcrops has yielded a density of 2.74 gm/cc. It is therefore suggested 86 that the Breed-Suring positive anomaly could indicate the presence of anorthosite. A sharp local positive anomaly occurs in the extreme southwestern corner of the study area. Patenaude (1966) found a ring-shaped magnetic anomaly in this area, with a strong negative central zone and an annular positive outer zone. The positive gravity anomaly of this study coincides with the northeastern portion of the positive magnetic zone. The gravity anomaly trends north—south whereas the lineation of the magnetic anomaly here is northwest-southeast. This disagreement may be partly explained by the 3 mile (4.8 kilometer) flight—line spacing of the magnetic survey. The magnetic anomaly becomes irregular south of the gravity study area, with small, sporadic anomalies in an area of anorthosite outcrops. Weis (Read and Weis, 1962) describes the anorthosite as locally containing magnetite. Patenaude has suggested three possibilities to explain the magnetic anomaly: a ring—dike, preservation of iron formation, and a carbonatite deposit. The existence of a mafic intrusive in the area near the margins of the broader magnetic anomaly suggests a local denser phase of the intrusive complex. The well—defined gravity anomaly at the margins of the study area, suggests that gravity mapping of the entire magnetic anomaly area to the west and south would be useful, particularly if combined with a more detailed magnetic survey. Units of the Wolf River Batholith do not occur on the east— west positive gravity anomaly passing through Wausaukee, "E é-J'I 0 9 .- ..' - - -ysn hsqsda—aniu a -- '.i-'-;- 1"Vf-H‘.1,'~\i 87 although granitic rocks of the other two age groups have been found. The High Falls Granite has not been dated radiomet— rically, but Medaris et. a1. (1973) have assigned it to the Wolf River Batholith based on lithology. The Bouguer gravity map supports this interpretation. Menominee Trough - Florence Area The Florence area and the Menominee Trough in Wisconsin (Figure 14), discussed together here because of their geologic continuity, are part of a larger province which includes adjoin— ing portions of Michigan. Because of this, an attempt at inter- pretation must consider available information from Michigan. It must also be kept in mind that the Fourier residual and regional maps become less reliable near their margins. The geology of much of this area contrasts greatly with previously discussed areas. The Niagara Fault appears to be a major structural feature separating crustal blocks of differing composition. This is supported by the regional contrast of Bouguer anomaly values which change from strongly negative south of the fault to relatively more positive values to the north. The only exception to this is in the Pembine area where the dense greenstones of Quinnesec Formation extend to the south, causing relatively positive values to also extend south— ward. Rocks which are not found in Wisconsin north of the Niagara Fault include granitic intrusives, felsic gneisses, and the Quinnesec Formation greenstones. Felsic gneisses and granites are found in Michigan, however, north of the Menominee Trough. . e.-.-l n .- I! g I" ‘ - I -e. -IJO ‘3 -—35 | H - \l _-’_.I- -.Pocqua o 88 Rocks which occur north of the Niagara Fault but not to the south include the iron formation and characteristic associated metasedimentary rocks of the Middle Precambrian. Major meta- gabbro bodies are located south of the fault, but only minor occurrences outcrop to the north. A two—dimensional modeling analysis was carried out along Profile E (Figure 20) to investigate the subsurface configura- tion. This profile extends from the Michigan border on the north to the negative anomaly of the Dunbar Gneiss, and its location is shown on Figure 11. It crosses the faulted Florence area, underlain by metavolcanic and metasedimentary rocks, and is located to include the effect of the metagabbro intrusions. The interpretation has been constrained by known geology. Fault blocks were named by Dutton (1971a) and will be dis- cussed from south to north. He has not estimated the amounts of movement on the faults, but Bayley and others (1966) have estimated displacements immediately to the east based on dia— mond drilling and field mapping. Fault offsets shown in Figure 20 are based on the work of Bayley. All three faults are up- thrown to the south. The two northern faults are assumed to be near—vertical based upon geologic control to the east, and this study suggests that the southern fault dips steeply to the north. The Dunbar Gneiss has been used as the standard of compari— son for all densities. A mean density for the Dunbar Gneiss of 2.67 gm/cc was determined. Granitic gneiss is believed to under— lie the rocks filling the Menominee Trough, as is characteris- tic of similar troughs in northern Michigan. Klasner and Cannon ansih “si 2 u Jul-F 89 _w I m 020:3. mlzuoma >._._._>mwmmo 024 JMQOZ 4._._>t>