_ .v-HVEIIUAN Thesis For i‘ho Degree of N1. 5. MICHIGAN STATE UNIVERSITY Alfons Buzas 1960 THESIS A: I P r. , . 4 : e i 3 3‘ . I V 1 '. ~ ‘ g '7- ; 1‘ r ' i 3' i ‘ ‘ fl 3 .. ‘4‘". i t» ‘ u. ‘0‘ 1. Ike". * LIBR A R Y Li Michigan State University i! ll ._._..~. —-——v--.-v— ,c—u— Michigan State University Eam Ldlltilllg, xmwhuqu APR 18 1960 THE INTERPRETATION OF THE AEROMAGNETICS OF SOUTHEASTERN MARQUETTE COUNTY, MICHIGAN b3! Alfons Buzas Submitted to the School of Science and Arts of Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Geology 1960 LP” _e2 13 'N B ‘ ' "I" 53% page Abstract ............................................ iii Introduction ........................................ 1 Location and Extent of Investigation ........... 1 Purpose of Investigation ....................... 1 Previous Investigation ......................... 1 Aeromagnetic Survey ................................. Field Work and Compilation ..................... Reliability of Data ............................ Geography ........................................... General Geology ..................................... Stratigraphy 00.0.0000.........OOOOOOOOO00...... O\O\U'lUl-I>UJ\N Lower Precambrain .........................< Middle Precambrian ........................ 6 Upper Precambrian ......................... 12 Paleozoics ................................ 12 Structure ...................................... 12 Results of Survey ................................... 13 Methods of Interpretation ... 16 Interpretation ...................................... 19 Correlation with Results of a Gravity Survey 2+1 Smmnazw and Recommendations ......................... 41 .Mflumowledgments ..................................... 45 References Cited cocococooncooooooooooooooooooooooooo 4‘6 Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure F igure Figure F1sure Figure F1Sure Figure ILLUSTRATIONS Location and General Geology of Study Area in Northern Michigan ............... N-S Profile Across Study Area ........... Spacing for the Second Derivative I‘CethOd 0.00.00.00.00......OOOODOOOOOOOOOO Spacing for Continuation Downward ....... Diagram Illustrating Relationship of Values Used in K. L. Cook's Formula ..... Profiles A1, A2 Profile Profile Profile Profile Profile Profile Profile Profile Profile Profile Profile Profile B1 82 33 C1 D1 D2 F2 F3 H1 T46N, R24‘v 0.0.0.000... T46N, R2441 ......OOOOOOOO... T46N, RQQW oooooooooo T46EJ, R2411 ......OOOOOOOOOOO T45‘46N, R25w coo-000000000. T45N, T44N, T43N, T43N, T43N, T43N, T43N. R24“! ......OOOQOOOOOOO R23N R27W R26!” ......OOOOOOOOOOO R27W R24W R24W T42-43N, R24w coo-0000...... Profiles G2, G5. Profiles G1, G3, G4 T45N, RQSW ........ T4511, RQSW oooooooooooo Gravity Map of Portion of Study Area, Frantti, 1956 ooooooooooooooooooooooocoo. Possible Distribution of Precambrian Rocks in Study Area ..................... ii 42 44 ABSTRACT The aeromagnetic survey of southeastern Marquette County was made by the Jones and Laughlin Steel Corporation as part of their iron ore exploration program. An attempt is made to correlate the anomalies with geologic features as determined from available information. Since Paleozoic sediments cover most of the study area, except for Pre- cambrian exposures at the western edge, any interpretation of rock type had to be projected from the west or inter- preted from the anomaly. The magnetic highs are produced by remnants of meta- morphosed basic rock in the granite./ In the northern part of the survey area, two of three linear highs correlate with basic rock outcrop. At the southern end a dis- continuous linear high coincides with a belt of basic schists. Areas of granite and known Huronian metasediments cannot be differentiated magnetically, except that the sediments are often bounded by highs as is the case in the Gwinn district. Two negative linear anomalies, which ex- tend across the area, are probably caused by reversely ipodarized Keweenawan dikes. A fault mapped in the Gwinn Etrea can be traced to the northeast by the break in magnetic C30ntours . 111 INTRODUCTION Location and Extent g§_;nvestigation The aeromagnetic survey studied in this investigation includes a roughly rectangular area 30 by 36 miles in the Upper Peninsula of Michigan. The area lies between 45°50' and 46°30' north latitude and 87°oo' and 87°50' west longi- tude and covers the southeastern townships of Marquette County and the northeastern townships of Dickinson County as shown in Figure 1. Ninety per cent of the study area is covered by Paleozoic sediments Which overlie the igneous and meta- morphic rocks which compose the Precambrian of the Lake Superior region. Purpose 9; Investigation The objectives of this investigation were to delineate the structure and lithology of the studied area by: (1) correlating anamalies with known geology and projecting into areas of no outcrop; (2) determining shape, size, and susceptibility by various geophysical techniques. Previous Ingestigatiog The part of Marquette County covered in this study has IPeceived little investigation because there are few rock cN—ltcrops. However, the Precambrian, which is exposed along the western edge of the area surveyed, has been extensively 1 LEGEND Poloozoics- Sandstones , Limestone: .Huronion - Meioeediments Archean - lntrueivee, Metovolconice ':‘ml)°"‘°r - L'ANBE STUDY AREA V.(7v>4l/>l/4 ...,- FiQure 1. Location and general Index Mop ...-o. ii I waif-'1! to €33 -_.~ Ruin; :.-. :1'i2ew'. geology of etudy one in northern Michigan Kr! studied since the discovery of iron ore in the Marquette range to the northwest over 100 years ago. The first comprehensive description of the geology of the Marquette district was written by g. g. Egg,fl;§§ and E. S, Bayley (1897) in their Monograph 28 and was expanded by Q, 3. y§Q_fl;§§ and Q. E. Lgith (1911) in Monograph 52 to include the Lake Superior region. Q. A. L§g§y_(1933, 1935) did considerable work in the Southern Complex and parti- cularly in the belt of schists and gneisses of the Palmer area and he also mapped the Felch trough in detail (1946). The structurally isolated Precambrian sediments of the Gwinn district were described by 5. Q. All§n_(1914). The geophysical work has also been limited to the western edge of the study area. The United States Geological Survey has made regional airborne magnetic and radio- activity surveys of the western part of Marquette County, overlapping slightly on the area covered by this study. The radioactivity surveys have been released but the aero- nmgnetics have not. The United States Geological Survey aeromagnetics results of Dickinson County are available. Recent ground geophysical work, other than dip needle, includes a few detailed magnetometer and gravity surveys in “the Gwinn area. These were conducted by commercial organi- zations and are not available. A regional gravity and magnetic survey of the central portion of the Upper Peninsula, 11h31uding a portion of the study area was conducted by E- g. Frantti (1956). AEROMAGNETIC SURVEY ' Field Work and Compilation The magnetic measurements of the study area were made for the Jones and Laughlin Steel Corporation by the Aero Service Corporation employing a Gulf flux—gate type magnet- ometer installed in an aircraft. The sensing instrument of the flux-gate magnetometer is installed in a bomb-shaped casing and towed below the aircraft. It makes use of high permeability, usually Mumetal cores, in which a substantial portion of the saturation value can be induced by the earth's magnetic field. When this field is superimposed upon a cyclic field induced by an alternating current in the coils around the core, the resultant field saturates it. The phase at which saturation is reached gives a measure of the earth's magnetic field. The area of investigation was covered by north-south traverses at one-quarter mile intervals and flown at an altitude of five hundred feet. The usual method of conduct- ing such a survey is to fly east-west base lines at either end of the area so that the readings can be adjusted to a common arbitrary datum. Each base line is flown consecutively in opposite directions to obtain corrections for instru- :mental drift and diurnal variations. Flight lines are plotted on aerial photos and are used :for flight control, while the actual flight paths are re- Cnarded by a gyrostabilized continuous strip film camera. 5 A continuous recording radio altimeter gives the altitude of the aircraft. The data was plotted on eight map sheets of 1/20,000 scale and contoured at an interval of 10 gammas. Overlays of these were made with a 50 gamma contour interval so that upon reduction the contours would be decernable. The over- lays were reduced photographically to a map with a scale of one mile to the inch, making the regional picture more obvious. Profiles of anomalous areas were plotted from the original data. Reliability 9; Data The values plotted along the flight lines on the original map sheets represent the relative total intensity of the earth's magnetic field at the position of the instru- ment along the flight path. Because of the density of read- ings, the resulting contour map should be an accurate picture of the magnetics in the area. However, the possi- bility of missing anomalies between traverses should be considered. Since the anomalies are elongate and usually trend east-west, to escape detection, they would have to be situated between the flight lines and less than 1/4 mile long. The probability of crossing such a feature randomly distributed over the survey area is given by the formula: P = 137-);- (W. B. Agocs, Geophysics, 1955) Ffliere P is the probability which cannot exceed unity, L is idle length of the anomaly and cannot exceed S which is the 6 line spacing. In this survey, anomalies of 1,000 feet randomly distributed would have a probability of .5 of being detected. Since the anomalies in this area are not randomly distributed but have a definite east-west orientation, the probability of their detection is much higher than .5. The diurnal and instrumental drift effects are mini- mized by adjusting the readings to the base lines which are referred to as the common arbitrary datum. In the adjust- ments it is assumed that the corrections are linear. GEOGRAPHY The area of this survey is covered by an extensive outwash sand plain which has very little relief. This fea- tureless surface is broken in the northwest by the hills of the Marquette range and in the west-central section by low knobs of granite which encircle the northwest and north side of the Gwinn synclinorium. Southeast of Gwinn there is an extensive swamp area. The region is drained by the Escanaba River system into Lake Michigan. Although several paved roads and secondary gravel roads and fire trails traverse the area, much of it is accessible only on foot. GENERAL GEOLOGY The area of this investigation lies along the southern nIargin of the Precambrian shield of North America. It is 7 almost entirely covered by flat-lying Paleozoic sandstones and limestones which thicken both to the east and southeast. Pleistocene glacial drift mantles most of the region. Igneous and metasedimentary rocks of Precambrian age are exposed only at the western edge of the study area with a few isolated outcrops in the north-central section. The Huorian metasediments are restricted to the Marquette syn- cline whose eastern limits extend to the northwest corner of the area, the Gwinn trough which lies in the west-central part, and the Felch trough which is in the southwest corner. These metasedimentary troughs, as shown in Figure 1, are separated by extensive bodies of gneisses and intrusive granites which are referred to as the Southern Complex. Stratigraphy Lower Precambrian The dominant rocks of the Southern Complex, which are the oldest in the area, are mostly granites and granite gneisses. Micaceous, amphibole and chlorite schists are found in several localities but these are subordinate. The schists have been locally intruded by the granites to form migmatites. Middle Precambrian Lower Huronian Lying unconformably upon the older rocks is the Mesnard (L0 quartzite which is the basal Huronian formation. It grades upward from a basal conglomerate through a quartzite to a slate. The equivalent of this in the Felch trough area is the Sturgeon formation which is a massive cherty quartzite. Overlying the quartzite is the Kona dolomite which is predominately a dolomite but interstratified with it are layers of slate, graywacke and quartzite. The Randville <3f the Felch district may be correlated with the Kbna. In ‘the Gwinn region the Lower Huronian is absent. Middle Huronian The Ajibik quartzite lies unconformably on the Lower Hirronian rocks. In the Marquette area it can be divided irrto two facies, a basal conglomerate which is subordinate 1H3 the quartzite. Slates and graywackies are inter- Stratified with the quartzite. In the Gwinn district the 138.5381 member rests on the Archean granite and is an arkose and arkose conglomerate. The Siamo slate overlies the Ajibik and it varies from a CHDarse grained graywacke approaching a quartzite to a Slate. At Gwinn a dark graphitic slate overlies the arkose and is equivalent to the Siamo.’ The Negaunee iron formation, which rests upon the 318-1310, consists of cherty siderite which has been altered to gruheritic, magnetitic, hematitic or limonitic, grunerite or magnetite schists and iron ore. The equivalent iron for- mation of the Gwinn area is a soft banded ferrugenous chart and 8 runeri te schist . oxowzhmnw s opnnosoawcoo .mponosoawsoo a opfiuppmsu sodscoow .soapwsnom Sosa woossoonw .mo>amsnpsa a meannefio> wasnmxnnao seasonsm .moumam ossnwfinoaz posoq soda: .sOHpsssou sons «Assam mopwam a mops .mopwam % mopfiupnnsw amazon modem impasse ossnwfinoaz poms: ossnwdnoaz nouns opasnsw hocsmaaam csn 111 an: wstHom snasoasm pmom hpdsaowsoosb moxac cascade osa>aao cozecoozom in: In: 11: mpaspomaca moCOpmoswm sedansoo moSOpmosaq undefi>aono mpamoaon Huaowaw mpamommu Heaowaw mucosficom Hmaomaw ozoOOpmaon .m.w.m.cham¢cdoz Scamm mopnzm .Hoow .noazassazw QCHN I opponents mBonBmHQ moth 92¢ .ZZHBw .MBBHDQM¢S mme 2H Mondem Moom _ mHnt 1O mpcosavom oasmoao> .ms>mH .mosOpmnoonw sapwzom mmpfisnpw snapsogdnq mopasmhw smaesmsan movasnsw snapsonan thELOMSOOSD mofisdoao> s mpsosduom madswxmasoa mopasmpw sasomag hpassogsoosb opauppnsv soowASpm ovauppssv cansmoz opasoaou oaad>cssm .ouasoaoc snow swasossm . .opnam oosoz sozoq It: 111 :11 mpassomcooc: .ouw Inmsoawsoo omoxng s omoxn< massagesv xaxafig .opwam .opmam osmam smfisoasm soapmsnom saga smoas> “woosswwozv sedansnom sonH .sOHmeLOM send ooczwmoz manna: I‘ll! mpwssommooc: G; 11 In the Felch trough the Middle Huronian consists of the Vulcan iron formation with alternating layers of iron oxides, slates and cherts. Hematite is the dominant con- stituent but magnetite is also present and locally may be more abundant than the hematite. Upper Huronian Unconformably overlying the Negaunee is the Goodrich formation which has a conglomerate at the base but is pre- dominantly a quartzite. There is an equivalent basal con- glomerate in the Gwinn area but it grades upward into a graywacke rather than a quartzite. In the eastern part of the Marquette syncline, the :remainder of the Upper Huronian is missing. In the western isection the Greenwood formation overlies the Goodrich and it CHInsists of two main phases, a ferruginous slate and chert and a grunerite magnetite schist. Overlying the Greenwood, the Clarksburg or the Good- 1310b, depending on the area, are the Michigamme slates. In 1flme Gwinn district they consist of a graywacke quartzite, Slimlcious black slates and arkoses which may or may not be feJ-"3."1.1gino1.is. In the Felch trough the Upper Huronian is called the Falch schist series and are slates and quartzite schists. Intrusive Rocks Intruded into all the Huronian formations are many basic«d1kes, sills, and granites, the latter of which are of Killarney age. Upper Precambrian Keweenawan Cutting all of the Lower Precambrian rocks are olivine diabase dikes which extend for great distances. They generally appear fresh and are usually a dark color. Paleozoics As mentioned previously, Paleozoic sediments cover most of the area. These are Cambrian and Ordivician sand- stones and limestones. Structure The major structures in or adjoining the area of in- 1Iestigation are three Precambrian metasedimentary syn- CXLinoriums, the Marquette syncline, the Gwinn trough, and tflle Felch trough. These generally trend east-west and are arweas of extremely complex folding and faulting, probably the result of a period of major orogeny. The Marquette syncline extends for approximately 40 mJLLes west of the City of Marquette and at its widest point 1&3 about 6 miles across. The beds are, generally, steeply dipping, but this varies considerably. Fracturing is ex- tensive, and in a few localities are there extensive faults. The Gwinn synclinorium is a spoon-shaped basin 4 miles long With a maximum width of 2 miles, with a maximum depth 13 of 1,200 feet, adjacent to the northeast limb, as determined from drilling. The trough terminates against a fault zone in the southeast and another fault trending diagonally through Section 28 has a horizontal displacement of not less than 700 feet, also determined from drill hole information. The Felch trough, which is about 1 to 2 miles wide, joins the Crystal Falls area to the west, but its eastern limits are uncertain. It is also a complexly folded syn- cline flanked by major faults and broken by both transverse and longitudinal faults. The dominant strike of the beds is east-west and they generally are steeply dipping. RESULTS OF SURVEY The maximum variation in total intensity of the sur- 1heyed area is 1,833 gammas (1,645 X - 3,4783 ), but on the Erverage is usually.less. This variation is considerably Eunaller than that found by the United States Geological Survey in other Lake Superior regions, namely Dickinson, Baraga, Iron, and Houghton Counties, where differences up to 53(300 gammas are common. The lower readings may be partially accounted for by the increasing distance between points of Observation and the source of the anomalies due to the thickening of the sediments to the east. However, the major cause must be due to the smaller difference between the EmSceptibilities of the rocks in this region. As can be seen from the north-south profiles in Figure 2, there is a regional gradient with the average total 14 .l 00...... 1000 TOOnN 1. Don. .I OOnN 100m. IOOON IOOON 32:50 33:2... .20... 3:2 e “I ill. n 0:3:— N a. 29:". _ 0. 32¢ O Location on Plate 2 Profilee , N—S FIG. 2 15 intensity decreasing from north to south by approximately 150 gammas. The regional total intensity gradient, as com- puted from the United States Coast and Geodedic Total Inten- sity Map of the United States (1955), would be approximately 130 gammas, accounting for most of this change. From the regional magnetics Plate 1, several line- ations are immediately apparent. In the northern part of the area, T46N and T47N, there is a negative anomaly which strikes approximately NBOE and is continuous from the east side of R25W to R23w where it broadens out to the average magnetic level. This negative anomaly transects a linear high and appears continuous through it, while the positive anomaly appears to terminate on either side of the low and is slightly offset. In T43N and extending from R23W to R27W is another Zlinear negative anomaly striking N80E, which becomes a lxroad area of lows with a few isolated highs toward the SCNithwest. A discontinuous positive anomaly in T43N, R27W fixxtends eastward to T43N, R25w where it terminates at the negative anomaly. The positive anomalies of the area have two definite trends. Those north of T44N have a strike of moo-70w and orua is continuous for approximately 20 miles. Those to the Scnlth have an east-west linear trend and are discontinuous. Most of the profiles across the anomalies are quite E’iymmetri cal. METHODS OF INTERPRETATION The geOIOgy of the area is such that the features causing the anomalies may, in most cases, be considered as two dimensional, the best approximation being a vertical sheet. Consequently, the methods of interpretation employed considered features of this shape. To obtain a regional picture, a total intensity con- tour map and a lineation map were constructed. 0n the line- ation map, Plate 2, the magnitude of the total intensity re- lief is represented by circles of varying size, indicating a certain range of values. Barbs on the-circles indicate the direction of elongation of the anomaly and its areal extent. This method of representation emphasizes linear trends of a <3ertain range of values more clearly than the contour map. In anomalous areas where detail was desired, profiles Vnere drawn, Figures 6-20, directly from the original map Eflleets. Their locations are shown on the lineation map, Platte 2. From the profiles depth determinations were made CNVer-the anomalies using D. W. Smellie's method (Geophysics, 1956) and L. J. Peters' method (Geophysigp, 1949). The MUflths of the two dimensional sources were approximated by E‘econd derivative profiles which were constructed using the formula: d2T = 20 - b1 - b3 (V. Vacquier'. 1951) dx2 r2 b, c bI Figure 3- (——r—-I—r—9 17 Spacing for second derivative method is illustrated in Figure 3, with c as a value at any point along the pro— file, and b1, b are values taken at a constant interval 3 from c. The spacing cb and ob usually used is that equal 1 to the distance from the instruient to the body causing the anomaly. For purposes of this study, both a 833' (10,000") and 416' (5,000") spacing were experimented with, the latter approximations giving a slightly smaller anomaly. Since the narrower spacing gives better control, it was used on the profiles. The width of the feature causing the anomaly was considered to be .6 of the distance between the crossover points. This value was obtained from V. Vacquier's (1951) ‘work with model studies and the model used was the 1 x 6 striking east-west. The method employed for the continuation downward has IDeen proposed by W. J. Hinze. For the two dimensional case the formula is: d = 3.6817 T - 1.34085 (a + b) q f-—-€D I 11,: Figure 4 18 Spacing for continuation downward where d is the value of T at the new elevation and a and b are values taken at a distance from T equal to the distance continued downward as shown in Figure 4. To estimate the magnetite content of the rocks causing the anomaly, the susceptibility was computed using the formula: V k: (K. L. Cook, 1950) zzo[%:|n_p. + (o. - o,)] Figure 5 'where V is vertical intensity, k is susceptibility, Z0 is the vertical component of the earth's field (58,0008), and Ho the horizontal component (14,2008), r1r3 the length to the edges of the feature causing the anomaly and <1), (133 the angle in radians to r1 and r3 as illustrated in Figure 5. The vertical intensity V is related to the total in- tensity T by the formula V = T sin I, where I is the angle 0f inclination. At high magnetic latitudes sin I approaches 1 and consequently T = V. In this study T was substituted for V in the formula. 19 INTERPRETATION A complete geologic interpretation of the aero— magnetic anomalies has not been possible because of the limited geologic and geophysical information available about the area. Correlation with geology is hampered by the Paleogoic sediments and glacial drift which covers most of the region. Many rocks of the Lake Superior district contain enough magnetite to cause magnetic anomalies and the magnitude of the anomalies varies directly with the quantity of magnetite present. The major anomalies in this district are confined mainly to Huronian rocks although some of the basic dikes, areas of greenstone, and metavolcanics are also magnetic. The variability of magnetic susceptibility poses an extremely difficult, if not impossible, task of predicting a particular rock type. In the following table the magnetic suscepti- bility and per cent magnetite of rocks common to the area are given. Magnetite Minimum Maximum Avepage % kx106 % kx106 z kx106 Granites 0.2 600 1.9 5,700 .9 . 2,700 Gabbros 0.9 3,000 3.9 12,000 2.4 ~7,200 Diorites 1.2 3,600 7.4 22,000 3.45 10,400 Basalts 2.3 6,900 8.6 26,000 4.76 14,300 Diabases 2.3 6,900 6.3 19,000 4.35 13,100 Table 2. Calculated Susceptibility of Rock Materials 10 O In the following discussion the capital letters (eg, AA') are used to designate particular anomalies and are marked on both Plate 1 and Plate 2. The location of the profiles, which are designated by a letter corresponding to the anomaly and a number (eg. A1, A2) are shown on Plate 2. T46N anomaly AA' is a continuous linear negative anomaly which extends from the western edge of the study area to the eastern edge. The profile A1 is sharp as shown in Figure 6, but to the west it broadens out as illustrated in profile A2. By computing the second derivative, the width of the feature was estimated to be approximately 630 feet. The broadening out of this anomaly indicates that the distance between it and the instrument is increasing. Con- sequently, the anomaly at A2 could be continued downward to equal A1. It was found that A2 had to be continued downward 3,333 feet to give intensities equal to A1. Because of the steep slope involved between these two profiles, it can be assumed that the change of anomaly shape is not caused by the thickening of the Paleozoic sediments alone. The fea- ture must dip to the east at a greater angle than can be attributed to the sedimentary overlap. Depth determinations calculated from profile A1 give a depth of 1,000 feet from the ground surface to the source causing the anomaly. This represents the distance to the magnetic pole, which is 1/12 the length of the entire body. from the end, rather than the depth to the top of the body. .I 00. O at... 2.. >228 2.23.30 ocooom «a o... $0 0. "I N 4 3.2.2... 3:; 4 2:21 com. 000. _< 0:35 A Al , A2 T46-47N , 824W Promos FIG. 6 10 to If a depth of 200 feet to the top of the body is assumed, the magnetic pole would be situated 800 feet from the end. Since this is 1/12 the length, the body must extend downward for 9,600 feet. This anomaly is caused by a negatively polarized riarrow body extending to great depth and length. Many JKeweenawan olivine diabase dikes have these characteristics. (3. L. Kipp, 51. R. Balsely, 31. 2. m, 1948.) T45-46N - Anomaly BB' strikes N60w and extends from tile western to the eastern edge of the study area. It is nuare or less continuous with a few isolated highs. The width of the feature causing the anomaly, esti- rnsited from second derivative profiles, is not constant. At Ixrpfile B1, Figure 7, the width is 1,025 feet, but the smecond derivative is quite irregular indicating that the adaomaly may be caused by more than one body. At B2, Figure 53, the width of the feature is 1,250 feet and may also be Ceuased by more than one body and at B3, Figure 9, the width is 840 feet. The percentage of magnetite calculated at B2 and B3 Was approximately 6 per cent. In these calculations, depths 1“3 the body from flight elevation had to be assumed. Since a few hundred feet north of B3 there is outcrop a depth of 500 feet was used and at B2 a depth of 600 feet was used. The computations indicate that the feature causing the anc>Ina1y is irregular in shape, extending to depth and pos:Esibly is made up of two or more parallel bodies. The 23 _ _ _ _ _ . d _ _ _ — _ 4 _ _ _ _ _ O .IOONI d O < 1.00.1 00¢. a o d a o o o .10 o o o o o o o o o o o O 4 loo. q com. 0 ) O a q a q q d a a d o loom s a q s s 4 a 4 1.00.». o OOON «So... m_e\ooEEoe o a 3.555 0 03.0230 vcooow Q 3.23:: .20... d 4 o CONN q d :0“— www. H a a OOVN o a d l T46 N, RZQW “6.7 Proflle BI a: on. a; \ooEEoo 0 9:32.30 be ooom «.0... 602 H ill. o o. o O O O O O O 0°0- O O Q 4 Q d s q sooom Q oomu ooEEoo 6 £23.... .23 ooou comm A FIG.8 Profile 82 T46N,R24W .100. O e...... o2. \3558 3.33.36 vacuum .00... mwo. H 3.559 G 3.2.2:. .20... COG. OOVN OOmN I OOQN . Profile 83 T46 N, R24W FIG. 9 26 relatively high percentage of magnetite would point to a basic rock, of basaltic composition, as the source. In Section 36, T47N, R26W, there are outcrops of a hornblende gneiss which strikes in the same direction as the anomaly. Studies of this gneiss by A. Sahakian (1959) led to the conclusion that the original rock was a basalt which later underwent several periods of metamorphism. T45-46N, R25W - Anomaly CC' consists of two parallel anomalies striking N60w which are 4 miles long. The northern anomaly is caused by a body 600 feet wide-which has a 6 per cent magnetite content and the southwest anomaly re- sults from a body 450 feet wide with a 7 per cent magnetite content. The anomalies are probably the result of small bodies of basic rock similar to that at BB'. At the northwest end of CC' is a greenstone outcrop in a predominately granite area. The difference of suscepti- bility of these two rock types could result in the anomaly observed. T44-45N - Anomaly DD' consists of a series of highs extending through R23W and R24W. The second derivative pro- file at D1, Figure 11, gives a 650 food width to the feature causing the anomaly and the computed content of magnetite was 6 per cent. The irregularity of the profile could be an indication of more than one body being present. The second derivative profile at D2, Figure 12, also appears to indi- cate more than one anomalous zone. DD' is probably a continuation of CC' and the cause of .I. 00. 1 CON 1. con 0 1 oo... o o «30.... 02‘ \o 06:30 252...}. neon-m .00... 600. ”I 6 £22... .28. 00¢. 000. 000. OOON OONN 1 1' 45-40", R 26* 01 Profile FIG. l0 no 9. 1 OD. IIOON «.3... 0.? \o 2.330 0 3.33:0 ocooom acct 000. nHHHHUIIIIII LIT (.1 d. 6 £22... .28. 000. 000. 000. OOON OONN T45" , 324W Promo 01 FIG. II 1 OO. «.8... Box-see; 0 est—Econ ocooo m .00.... 000. "NH com. 1. com. I. OOON l ODEEOO CONN l 4 2.2.2... .28. T44N , RZSW FIBJZ Profile 02 30 the anomaly is due to the difference in susceptibility between the acidic and basic rocks. Between anomalies BB' and CC', DD' is an area of low magnetic variation, where even small isolated highs are generally absent. In T45Ny R24w, R25W, outcrops of quartzite and slate are present indicating that there are meta- sedimentary rocks in the region. The extent of these sedi- ments is purely speculative, but they may extend throughout the area of low readings as shown in Figure 22. In T45N, R24w, there is a break in the magnetic con- tours which strikes N65E and continues southwest to the Gwinn area where a fault has been postulated by R. g. Allgp (1914) from drill hole information. Although the strike of the proposed fault is slightly different, there is little deubt that it caused the break in the magnetic contours. It is more extensive than previously mapped and strikes as shown in Figure 22. T43N - Anomaly EE' consists of a series of highs which strike N75W and extend from the western edge of the area to R25w where they terminate abruptly at the negative anomaly FF'. At profile E1, Figure 13, the width of the feature causing the anomaly is 550 feet and magnetite content is 7 per cent; while at E2, Figure 14, the estimated width was 750 feet with 6 per cent magnetite. This anomalous zone is similar to those in the northern part of the area and is probably caused by irregularly shaped bodies of basic rock similar in composition, incorporated in a predominantly 31 T.oo_ .l 00N 0 at... o_o\.oeeoo 2:33.00 scorer. . .00.... 000. .3550 ""HI 6 2.2:... .23 00¢. 000. 000. 000N 00NN l l T43N, R27w FIG.l3 Profile El 11 00. 100N .1 00m 0 «3!. 0:. \noEEoo 2:33.30 neooom ...k 000. H i1 o o o o . com... com..1 d d d. d ooou i oomu 1 38:30 4 £22... .28. l ooom 1 Fl6.l4 Profile £2 T43N,R26W granite area. This series of highs extends westward through to R29N of Dickinson County where greenstone outcrop is found directly on the anomaly. T43N -- FF' is a linear negative anomaly which strikes N75E and extends from the western to the eastern edge of the study area. The second derivative profiles, Figures 15-17, give widths varying from 600 to 800 feet, but there are irregularities in the profile which could indicate more than one anomalous zone. The strike of the anomaly coincides with the strike of the major faulting bordering the Felch Trough and since this anomaly terminates EE' abruptly, this feature may be related to the faulting. This anomaly could be caused by one of the negatively polarized Keweenawan diabase dikes which was intruded along a fault plane. The estimated widths are probably large, but in the Lake Superior region, diabase dikes over 100 feet across the strike have been mapped with neither contact evident. North of the negative anomaly FF' and extending to the anomalous zones C and D is an area where no magnetic patterns are evident. It is relatively low magnetically with a few isolated highs. To the west in T43N, R28W, there is granite outcrop and since there are no discontinuities indicating a change in lithology, it is assumed that granite is predomi- nant through this area. T43N — Anomaly HH' is the largest local magnetic 34 T 00. 0 00m. «000“. m.¢\0o...60o 02.02.00 0:025 00¢. 006600 Q 5.2.0.... .30.. .00... 000. "I 000. com. 000N T43 N , RZTW Profile Fl FlG.l5 F3 7) .l 00. 0 «000“. m.¢\0o§..00 03.03.00 acooom .00.... 000. "H .4111 00.5.50 OOON 6 £22... .20.. 00N. 00¢. 000. l roan, R24W FIGJS Profile F2 x 0 7) .l 00. «.00.... 0.¢\ 006.000 0 03.03.00 00000.0. .00... 000. "HI 11111 00N. 00¢. 000. :00. 36.50 000m 4 £22... .23 l T43N, R24W‘ F3 FIG.” Profile ‘3 k»! variation in the area with a difference of 1,700 gammas. It is an isolated high at the southeastern edge of the study area which extends westward as a series of smaller anomalies. The width estimated from the second derivative profile, Figure 18, is 1,100 feet and the magnetite content is 6 per cent. This series of highs extends westward into Dickinson County where the anomalies were found to be over a hornblende biotite schist which grades into a magnetic iron formation. Anomaly H may be the result of a similar rock type. T42N, R25W, R26W - The magnetic characteristics of this area are very broad anomalies which appear to have no linear trend and whose shape suggests a source which extends to great depth and covers a large area. These large anomalies seem to separate the linear highs of the west from the east. This could indicate large intrusive bodies of basic igneous rock. T45N, R25w - From the regional aeromagnetic map the presence of the Gwinn trough is not immediately apparent. It is an area of little magnetic relief and is surrounded by linear highs. The profiles in Figure 19 show small highs which are situated over the trough, while in Figure 20 there is no indication of the Gwinn trough from the profile. These characteristics may apply to other meta- sedimentary troughs in the immediate area and consequently the regions of small magnetic variation may have economic iron ore deposits and should not be overlooked. 38 .100N 1 00m «.00... 0.¢ \005500 0 00. .0300 00000 0 .00.; 000. ”H AV 2.000.... .30» 00¢. 000. 000. 000N 00NN 00¢N 000N 000N 000m 00Nm 00¢n T42-43N , R 24W FlGuIB Profile HI 100.1 100. .100N Too.. 0 N.00... 0.¢ \0 00. 0.00 03.03.00 000000 .00.. 000. "I 1.111 no 0.30.0. 00 0.0.00 3.0030. .03... NO 02.0.... 000. I. 000N .1 000. l 000N l r45" . stw 62,65 Profiles FIG. 4O .00... 000. "Ml .Jr 4?)! 4/; J 000... I 00 ....o... 000. 1 In no 202.. 0000... 000. ... 01 I»... .0 ....o... 0000 L 000.600 .022... .20» 1 .34], T45N, 825W Profilee GI , 63 , 64 FIG. 20 41 CORRELATION WITH RESULTS OF A GRAVITY SURVEY A regional gravity survey which included a portion of the study area as shown in Figure 21 was conducted by g. E. Frantti (1954). As indicated by the Banger gravity map, a regional high extends from the northwest to the southeast part of the area and has a magnitude of 20 milligals. Two miles south of the gravity high is a parallel linear magnetic high. Since the gravity high coincides with granite outcrop in T46N, R24w, Frantti interpreted the anomaly as resulting from a structural feature dividing the Lake Superior and LMichigan Basins. If this interpretation is correct, then the magnetic anomaly must be caused by basic rocks Which ‘border the structural feature. In T44N, R23W the gravity .and magnetic anomalies coincide. Both of these anomalies sire probably caused by basic igneous rock. Traverses of this survey were along roads and since nuach of the study area is not accessible by roads, the lxacation of the gravity anomalies may not be accurate. The srtation spacing would be too spares for good control. SUMMARY AND RECOMMENDATIONS The Precambrian rocks of southeast Marquette County Billie magnetic anomalies which reflect the geologic trends of tile: area. Compared to other surveyed districts of the Lake SLlperior region, the anomalies are moderate to small and are TDIVJbably caused by metamorphosed basic rock rather than iron I l \ I"//// /////K////// ‘ \ K O MARQUETTE LEGEND C.|. = ID Milligolt Magnetic Highs Scale t: 0 Mile: 6 Study Area E 3 T48N T47N \\ Q Q \~"‘V T45N \ ' ewmu \\ \ \ R \ W12- \ T44N \ — --2°-\L43~ \ _ h ~—~_ § T42N L R /30— \\l3‘2§“:\l\\\aggyl\\\ R24w [ stw | new: | [RZIW Fl8.2l Gravity map of portion of study one , Frantti I956 formation. In Figure 22 a hypothetical generalized geologic map of the Precambrian has been postulated from the magnetic and limited geologic data. The positive anomalies of the study area correlate with basic metamorphic rocks in several localities and arethought to be caused by these. An area of metasediments northeast of the Gwinn basin has been postulated because of its resemblance to the Gwinn basin and from the few metasedi- mentary outcrops in this region. From the break in the magnetic contours, the fault mapped in the Gwinn area can be extended to the northeast. Proposed granite areas have also been projected from areas of known granite outcrop because of magnetic similarity. The large body of basic intrusive in the southern part of the area is hypothesized :from the shape of the anomaly only. Also the negative eanomalies are thought to be reversely polarized dikes, but :in.this area there is no outcrop evidence. The regional gravity anomalies coincide with the Huagnetics, but a more detailed-gravity investigation would 1362 desirable to facilitate the interpretation. In regions of scattered outcrop, aeromagnetic maps <3cnild be of considerable assistance in delineating geologic I‘eatures. However, if working in an area of limited outcrop Sllch.as the one studied, more geophysical information would ‘36? desirable before an accurate interpretation could be InEude. R27W FIG. 22 44 Basic lntrusives Metasedimentary.metvolconic _ Huronian metasediments Gronites in port Archean and ’- 7 I\ \1 F°u|f _.s_o._.. 7 a was 44 Outcrops A Dikes '-:'°'- MARQUETTE ~ W Possible distribution of Precambrian in study area I'OCII ' Greenstone , basic schists , gneisses L' ’ ACKNOWLEDGMENTS The writer wishes to express his sincere thanks to Dr. W. J. Hinze for the original suggestion of this problem and for his untiring assistance in directing the study. Acknowledgment is made of the Jones and Laughlin Steel Corporation for making the data concerning the study available. Special thanks also go to Dr. J. Zinn, Dr. J. W. Trow, and to Mr. R. C. Reed of the Michigan Geological Survey for their valuable assistance throughout this study. Without the help of these men who gave so freely of their time and knowledge, this study would not have been possible.‘ 45 REFERENCES CITED Agocs, W. 3., 1955, Line Spacing Effect and Determination of Optimum Spacing Illustrated by Mormora Ontario Magnetic Anomaly: Geophysics, v. 20. Allen, R. 3., 1914, Correlation and Structure of the Pre- cambrian Formations of the Gwinn Iron-Bearing District of Michigan: Jour. Geology, v. 22. Allen, R. C., and Barrett; L. P., 1914, Evidence of the Middle Upper Huronian Unconformity in the Quartzite Hills of Little Lake, Michigan: Jour. Geology, v. 22. Bacon, L. 0., 1956, Relationship of Gravity to Geologic Structure in Michigan's Upper Peninsula: Institute on Lake Superior Geology. Balsley, J. R., Davis, E. J., Nelson, R. A., and Reinhardt, 1950, Airborne Radioactivity Survey of Parts of Marquette, Dickinson and Baraga Counties, Michigan: U. S. Geol. Survey. Balsley, J. R., Pratt, W. P., and Wier, K. L., 1948, Aero- magnetic Survey of Parts of Dickinson County, Michigan, with Preliminary Geologic Interpretation: U. S. Geol. Survey, Geophysical Investigations. , and , 1949, Aeromagnetic Survey of Parts of Baraga Iron and Houghton Counties, Michigan, with Preliminary Geologic Interpretation: U. S. Geol. Survey, Geophysical Investigations. Cook, K. L., 1950, Quantitative Interpretation of Magnetic Anomalies Over Veins: Geophysics, v. 15. Dickey, R. 4., 1936, The Granite-Sequence in the Southern Complex of Upper Michigan: Jour. Geology, v. 44. Eardley, A. J., 1951, Structural Geology of North America: Harper and Brothers, New Yerk. Frantti, G. E., 1956, Geophysical Investigations in the Central Portion of Michigan's Upper Peninsula: Transactions of the American Institute of Mining, Metallurgical and Petroleum Engineers, v. 205. Hinze, W. J., 1960, Professor, Department of Geology, Michigan State University: Personal communication. Lamey, C. A., 1931, Granite Intrusions in the Huronian Formations of Northern Michigan: Jour. Geology, v. 39. 47 1935, The Palmer Gneiss: Geol. Soc. America Bull., Mooney, H. M., and Bleifuss, R., 1953, Magnetic Suspecti- bility Measurements in Minnesota: Geophysics, v. 13. qufly, 1951, Airborne Magnetometer: Geophysics, v. 16. Mettleton, L. L., 1940, Geophysical Prospecting for Oil: McGraw-Hill, New York. Reed, R. C., 1960, Mine Appraisor, Michigan Geol. Survey: Personal communication. Sahakian, A., 1959, Unpublished M. S. Thesis, Michigan State University. Smellie, D. w., 1956, Approximations in Aeromagnetic Interpretation: Geophysics, v. 21. Vacquier, et a1, 1951, Interpretation of Aeromagnetic Maps: U. S. Geol. Survey, Memoir 47. Van Hise, C. R., and Bagley, W. S., 1897, The Marquette Iron-Bearing District of Michigan: U. S. Geol. Survey Man. 28. Van Hise, C. R., and Leith, C. K., 1911, The Geology of the Lake Superior Region: U. S. Geol. Survey Man. 52. Zinn, J., 1960, Professor, Dept. Geology, Michigan State University: Personal communication. r-r ' {Til-“l 9”“, bar. UnL. ' r W 2331 (i 7 a“? .uw W I 3.8 4,4 .1. A, . .2 104—3” "'111111111111111111111111“