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The results obtained from the above averaging techniques were compared with the results of numerical integration technique. In areas where contours were regular and smooth all the techniques gave nearly the same average value within the practical limits possible and differed from the standard average by no more than 1.5 percent. However, in areas where contours were complex the average values varied considerably, not only with each other. but also from standard values. The results obtained from the sixteen point average is the single exception. In the areas of complex basement topography errors as large as 66 percent were obtained. The sixteen point averaging method was accurate usually within 1.5 percent and the maximum error obtained was approximately “.2 percent in a complex area. A comparision of results obtained by different averaging techniques is shown in Table 1. Based on the above results the sixteen point technique was used to average the basement elevations for 1°x 1° quadrangles and also to obtain average Bouguer gravity anomalies from Bouguer gravity anomaly contours to check the values obtained from the U. S. Air Force. The difference between the Air Force average values and the average Bouguer gravity obtained from the contour map usually did not exceed one milligal. 10 The average values for 2°x 2° quadrangles were obtained by numerically averaging the values of the four 1°x 1° quadrangles involved. Basement Data The basement is normally defined as the crystalline igneous and metamorphic complex lying beneath a sedimentary sequence. However. in this study the use of the term is broadened to include the Precambrian complex outcropping or subcropping beneath the glacial drift and some Precambrian supracrustal rocks such as Keweenawan volcanics and elastics. This more general definition follows the usage of the term by Bayley and Muehlberger (1968) in preparation of the Basement Rock Map of the United States in the Precambrian craton basement province. The basement elevations were calculated by sixteen point averaging technique from basement elevation contours given by Bayley and Muehlberger (1968). Gravity Data Gravity anomalies. both Bouguer and Free-air. were calculated in normal manner (Heiskanen and Vening Meinesz. 1958) utilising the 1930 international gravity formula and a density of 2.67 gm/cc for the earth material between sea level and observation sites. No terrain correction has been used because of generally low relief in the area studied. 11 The number of gravity observations used in obtaining the average values varies from only a few to more than a thousand per 1°x 1° quadrangle. The quadr- angles with only a few observations are in general limited to those areas overlapping the Rocky Mountains and the Great Lakes. The average values of quadrangles having few observations will undoubtedly be in error. but their influence on the results of this study should be minimal as such areas are few in number. Residual Bouguer gravity anomalies reflect local geological effects and are obtained by subtracting the regional Bouguer gravity anomaly at the observation site from the observed Bouguer gravity anomaly. The meaning of regional and residual varies depending upon individual purposes. The division between regional and residual is dictated by the lateral extent of the effect. Strange and Woollard (196“) have discussed the limitations of several methods of separating residuals from regional anomalies on a continental basis for purposes of studying relationships between anomalies and geology. In this study the regional Bouguer gravity anomaly is represented by the mean curve obtained from the Bouguer gravity anomaly-surface elevation relationship. The difference between the average Bouguer gravity anomaly for 1°x 1° quadrangle areas and the regional Bouguer gravity anomaly. corresponding to the mean elevation for the quadrangle. gives the residual anomaly. Residual gravity anomaly for 12 2°x 2° area are obtained in a similar manner. The effect of the deficiencies of mass within basins due to the density contrast between low density sedimentary rocks of the basins and the enclosing basement rocks has been calculated for individual basins of the Midcontinent and applied to the Bouguer gravity anomaly resulting in a geologically corrected Bouguer gravity anomaly. The mean density of the sedimentary rocks of the basins is assumed to be 2.52 gm/cc resulting in a density contrst of 0.15 gm/cc with the enclosing rocks of 2.67 gm/cc density. The geological correction is the same as the gravitational effect of a horizontal slab of density 0.15 gm/cc and is calculated to be 0.0019 mgals per foot. This correction is added to the observed Bouguer gravity anomaly to obtain geologically corrected Bouguer gravity anomaly. Statistical Procedures Regression coefficients for least square lines of first order have been calculated. wherever possible. to indicate the linear relationships. This line is represented by an equation of the form Y = A + BK. where Y is the dependent variable. X is independent variable. A is the intercept and B is the regression coefficient. A and B are calculated in the following manner: ny - - 2x B = --E 3 where x = X-X. X = -—— . n is tx n number of data pairs and 13 y=Y-Y3and A=Y-B-i. whereY=£§1—. The confidence level for the least square line is established by calculating the critical values of t-distribution. using the equation. t = -§— . where S is the standard error of regression coefficient and is given by equation. 8 = -25 . where "s” is given by 21 2 2d 2 _ 2 _ 2 _ (2x%) 8 - n-2 and zdy -Zy zx . The correlation coefficient. ”r". between the two parameters “X" and “Y" is obtained by the relation r 3 EX! J (2x2) (U2) The correlation coefficient. "r". also may be calculated by using the equation. 2 r2 = t n-2 1+t /(n-2) The value of ”r" varies from +1 to -1. These extreme values indicate 100 percent correlation. either positive or negative. A zero value of ”r“ indicates no correlation. Intermediate values represent the degree of correlation. In calculating the regression coefficients 14 for this study. basement elevation has been considered the independent variable and Bouguer and Free-air gravity anomalies and surface elevation the dependent variables. GEOLOGY The Midcontinent area of the United States under investigation in this study generally falls within the central stable region which is bordered on the west by the Rocky Mountains and on the east and partially on the south by Paleozoic orogenic belts. Tectonically the north-central portion of the area lies within the southern extension of the Canadian Shield and in the south-central portion the Mississippi embayment overlaps the area of study. The southern extension of the Precambrian Canadian Shield in Minnesota. Wisconsin and Michigan consists primarily of felsic crystalline rocks with common east to northeast striking metasedimentary belts and minor mafic intrusives and extrusives. The age of these basement rocks vary from greater than 2.5 b.y. to 1.2 b.y. in a complicated geographic pattern. but in general the ages decrease to the south. Superimposed on and intruded into these rocks are Keweenawan extrusives and intrusives. primarily basic in composition and late Keweenawan clastics. Drill holes and geophysical data indicate that these Keweenawan supra-basement rocks extend beneath the Paleozoic sedimentary rocks from the southwest corner 15 16 of Lake Superior along a linear belt into Kansas and from eastern Lake Superior into the Southern Peninsula of Michigan. Isolated patches of these rocks are also indi- cated in Indiana. Illinois and Ohio. The basement of the central and major portion of the central stable region is largely composed of felsic rocks dated from 1.2 b.y. to 1.5 b.y.. the so called Central province. This province is bounded on the east and south by the Grenville province (0.9 to 1.1 b.y.). on the northwest by the Superior province (2.5 b.y.) and on the west primarily by the Penokean province (1.6 to 1.8 b.y.). In many areas of the central stable region evidence suggests overprinting of Central province ages on the Penokean province. Details on Midcontinent basement rock types and ages from limited exposures and drill holes penetrating the basement are given by Goldich. et al.. (1966a). Goldich. et al.. (1966b). Muehlberger. et al.. (1966). Lidiak. et al.. (1966) and Bayley and Muehlberger (1968). In the central stable region shallow water sedimentary rocks of variable thickness overly the Pre— cambrian basement reflecting numerous broad basins. domes and arches. The strata have only gentle dips and give evidence for ”... slow and prolonged vertical movements that created basins . arches and domes" OEardley.1951. p. 12). ”The arches and basins developed chiefly in the Paleozoic era. but later. during the Mesozoic and tertiary. 'vast amount of elastic sediments from the evolving 17 Cordilleran mountain systems were spread eastward over the Paleozoic strata as far as Lake Superior and beyond the Mississippi River“ (Eardley. 1951. p.12). Following the Appalachian orogeny. the Mississippi embayment deve- loped in the southern portion of the area. The Illinois. Michigan and Williston Basins are similar features which had their origin in early Paleozoic time. Subsequent to their formation they have undergone mild deformations as evidenced by numerous minor folds and faults and a few major anticlines: e.g.. the La Salle. Howell and Cedar Creek anticlines. The depth of the basement in the center of each of these basins is nearly 15000 feet below sea level. The central portion of the Midcontinent is underlain by the ill defined Forest City and Salina Basins which have maximum sedimentary rock thickness of about 4000 feet. These basins are separated by the Nemaha uplift of early Pennsylvanian age. The assymetrical Denver Basin and Powder River Basin which occur along the edge of the Front Range and Bighorn Uplifts respectively are of tertiary age. The Anadarko Basin which is asymmetrical trough bordering the Wichita Mountains to the south contains over 30.000 feet of Paleozoic sedi- ments which were deformed during the Wichita orogeny in late Mississippian and early Pennsylvanian time. The geological history and structural relationships of these basins can be found in reviews by Eardley (1951) and King (1969). CORRELATION OF GRAVITY ANOMALIES WITH BASEMENT ELEVATIONS Results of Investigation Midcontinent Area Bouguer Gravity Anomaly Relationships Bouguer Gravity Anomaly-Basement Elevation Relationship for 1°x 1° Quadrangles: The average Bouguer gravity anomaly for 1°x 1° quadrangle areas are plotted against average basement elevations for 1°x 1° quadrangle areas for the entire area of study (Figure 2). The points are widely scattered and no obvious relationships can be established between these two parameters for the area. Therefore. for the purposes of studying the rela- tionships in greater detail the area is divided into an eastern and western portions along 96°W meridian. West of the 96°W meridian the regional surface elevations increase rather regularly towards the Rocky Mountain front. In the eastern portion of the Midcontinent (Figure 3) the points are quite scattered. but considering basement elevations less than -1000 feet and neglecting the points falling in the area adjacent to the Appalachian Basin. an inverse linear relationship is observed between Bouguer gravity anomaly and basement elevation. This relationship can be represented by the equation. 18 19 20 000 -20- - d 1 _ q - _ _ a...» .u. x... n l 1 L WEIIJW::.mw W" a. “W m. LW .0 AU .4 ,o no no 9. .4 ,o .8 .0 .2 . . . ... a ... ... ... A. ... mamwe cw hamsosm hvd>mnw hoswsom Nooo oo INooo Itooo Imooo Imooo IHoooo IHNooo kaooo Ipmooo Basement elevation in feet Ipmooo Imoooo relationship (Midcontinent: 1°x 1°) Figure 2. Bouguer anomaly- basement elevation 20 20 00* -20' -60 I a) O -120 Bouguer gravity anomaly in mgals :. to o: Regression equation: BA = -46.60 -0.0034BE -1#O- (Points within the dotted line are omitted from calculations) -16OP -180’ -200” l J L J I L l I J l l I '220 I I I I I I I I I I I0 IO Ia I» 04 Id Id a) Os #' l0 C3 C3 0 «m (h -s :0 c: c: c> c> 0. <3 <3 c> c> c> c> c> c> c> c> c> C) c: c> c: c: c: c: c: c> <> <3 <3 <3 c: c> c> c> Io Basement elevation in feet Figure 3. Bouguer anomaly-basement elevation relation- ship (Eastern Midcontinent: 1°x 1°) 21 Nooo l. INooo l J Ikooo [Imooo Ilmooo LIpoooo IIHNooo Ilurooo lumooo Iumcoo l [Noooo r 20 00- _m 8 . . . . mamws cw Samsosm apw>unw noswsom -20' ~60 L- 0 h. - 100*‘ 120‘ -1000 160. -180- -200- -220 ion in feet Basement elevat Bouguer anomaly-basement elevation relation- ship (Western Midcontinent: 1°x 1°) Figure 0. 22 BA = -h6.60 - 0.003bBE. where BA is the Bouguer gravity anomaly in milligals and BE is the basement elevation in feet relative to the sea level. This notation will be used in all subsequent equations. This relationship is highly significant. having a confidence level of over 99.9 percent and a correlation coefficient of -0.62. In the areas where basement elevations are greater than sea level the Bouguer gravity anomalies are independent of the basement elevations. The western portion of the Midcontinent shows a wide scatter of points (Figure 0) so that no obvious relationship is observable. Bougger Gravity Anomaly-Basement Elevation Relationship for 2°x 2° Quadrangles: The average values of Bouguer gravity anomaly and basement elevation for 2°x 2° quadrangle areas are plotted in Figure 5. The effect of local geology on the Bouguer gravity anomaly is averaged out by using 2°x 2° quadrangles. as a result the points are less scattered than in case of 1°x 1° area. The wide scatter of points makes it impossible to discern a correlation although there is an obvious linear trend indicating an inverse relationship between Bouguer gravity anomaly and basement elevation for the areas with an average basement elevation between sea level and 9000 feet below sea level. Those points generally falling in the eastern part of the Midcontinent. show a highly significant relationship. with more than 99.9 percent 23 00‘ l I r I ' ' Equation of calculated line: . BA = -3h.0 -0.00}§BE____~ calculated line -2 I”° ~‘~f~~/ - -40 -6O -80_ ouguer gravity anomaly in mgals B I ..I. N O l -140.. Regression equation: BA = ~43.55 -0.0025BE .1 -160 I In J J I I I I I I I I N H H a) O\ F N O o a: c> <0 o: c: c: <3 :0 o. lo o. :o c: c> c: O O O 0 O O o 0 Basement elevation in feet Figure 5. Bouguer anomaly-basement elevation relationship (Midcontinent: 2°x 2°) Regression equations calculated for eastern Midcont. Bouguer gravity anomaly in mgals 24 00__ -' I -20 -#O— -60" ~80” I ...-b O O T I ...s N O T I ...; p- O -160 ~180 -20 00 000:- 0002- 0 06» 0009- 0009~ OOOWT Surface elevation in feet Figure 6. Bouguer anomaly-surface elevation relationship (Midcontinent: 1°x 1°) OOOL 25 confidence level and a -0.66 correlation coefficient. The least square line is given by the equation. BA = -43.55 - 0.0025BE. Bouguer gravity anomalies. in areas where basement elevations are greater than sea level are indepe- ndent of the basement elevations. The points obtained from the western portion. generally less than -60 mgals Bouguer gravity anomaly. shows a wide scatter. There is no single possible relationship for this area. Bougaer Gravity Anomaly-Surface Elevation Relationship for 1°x 1° Quadraagles: Figure 6 shows the relationship of Bouguer gravity anomalies to surface elevations for the entire Midcontinent. The relationship between Bouguer gravity anomaly and basement elevation are distorted to some degree by this relationship between Bouguer anomaly and surface elevation. The Bouguer gravity anomaly in general shows a continuous decrease with increase in surface elevation. The relationship between the two as shown in Figure 6 is determined by inspection. This relationship is nearly a straight line following the gravitational effect of an infinite horizontal slab of density 2.67 gm/cc. Thus. the gravity anomaly is approximately given by. Ag(mgals) = Zflrddh = 0.01276 x 2.67 anh(ft.) = 0.03#10h. Deviations from this line assuming isostatic equilibrium represent the effects of local geology. These effects cause the change in the slope of the line. 26 OO -20 -#O -60 guer gravity in mgals .L c> {o -120 ' i Bou -100 L -160 0009 Surface elevation in feet Figure 7. Bouguer anomaly-surface eleva ion relationship (Midcontinent: 2 x 2°) 27 increasing the slope at less than 1000 feet and decreasing it around 6000 feet. However. the latter effect may be due to the paucity of data and hence not valid. or it may be caused by the lateral effect of anomalies originating from the Rocky Mountains. Bouguer GravityAnomaly-Surface Elevation Relationship for 2°x 2° Quadrangles: This relationship is similar to the one obtained from 1°x 1° quadrangle averages. The points are less scattered due to averaging of the local geological effects and hence deviation from the straight line is less (Figure 7). Thus the relationship follows the effect of the slab more closely. The major difference between the 1°x 1° line and 2°x 2° line is in areas of surface elevation less than 1000 feet. Surface Elevation-Basement Elevation Relationship: Considering the entire Midcontinent area on 1°x 1° quadrangle basis no single obvious relationship between surface and basement elevations is observed (Figure 8). The points are widely scattered and the straight line relationship. exhibited by points in areas above sea level. is primarly in areas where basement elevation and surface are considered equivalent. However, the data tend to cluster into fields which exhibit an increasing surface elevation with decreasing basement elevation. Study of the relationship between surface elevation and basement elevation on 2°x 2° quadrangle 28 Ao_xo_ usesapsoouflsv nfigmsowvmaou soapm>eao PsosommnIcowpm>mHo oommnsm -2000 00 l-ZOOO 1JIOOO a-BOOO 1-10000 poem ca sowvm>eao psosommm --12000 4-1h000 ‘-16000 "-18000 .0 enamaa .-ZOOOO e . . 0‘. O O o "0 I I I'“. J-6000 l I oo oooH ooom coon coo: lee; u; uotieaete SOBJJRS 0000 ooow coon 29 flow xom «psoswpsoouazv mwsmsowpmaon sowpm>oao psosommnIsowam>oao oommnsm .m onswwm 2000 poem cw sowpo>oao psosommm OO .1 ”10000 -12000 CO 1 0 0 O 2 _ A . \ck: .28.... 1003 anvcudc ooom coon co: ass; up notienete OOBJJnS mmomno.o + 5N0 n mm.+o .psosavsoooaz snopmmo you codename scammonwom . P - — L Li ooom 30 average basis indicates the tendency of points to generally fall into two groups in areas where basement elevations are below sea level (Figure 9). One group which has the surface elevations in general greater than 1500 feet shows a wide scatter of points and lies entirely in the western part of the Midcontinent. The other group in general has surface elevations less than 1500 feet and the points form a coherent field. Both these groups exhibit a general increase in surface elevation with increase in basement elevation. A least square line drawn for the eastern area where elevations are less than 1500 feet shows a linear relationship between surface ele- vations and basement elevations and is represented by the equation SE = 927 + 0.03803E. where SE is the surface elevation in feet above sea level. This relationship is significant. having over 98 percent confidence and has a correlation coefficient of 0.07. The effect of this relationship between surface and basement elevations will be to cause an inve- rse correlation between Bouguer gravity anomaly and base- ment elevation. A Bouguer gravity anomaly-basement elevation relationship was calculated using relations in Figure 7 and Figure 9 and is given by equation BA = -3h.0 - 0.0016BE. This line is shown in Figure 5 and closely follows the least square line drawn showing the relationship of Bouguer gravity anomaly to the basement elevation in the eastern part of the Midcontinent. 31 Ao_xo_ .pcosflvsooowzv nwsmsowpmaou sowpm>oao psosommnImamsosm nwmIooum .oH ouswfim poom cw sofipm>oao psosommm 0 0 0 0 O 0 0 0 0 O 0 0 0 0 0 0 0 0 0 O O 0 O 0 0 0 O 0 0 O 0 0 0 0 0 0 O O O 2 .4 6 8 0 2 0 o O 2 l4 6 8 1 1. 1.. 1. 1 2 2 I4 2 . . . b P . . . - . .OWI . 4 . a a I . n T T q A i. ..d , . . u o: a ... I e c e L I to . . . on a ... . 00 I I e O o e 0 e I m 0« A . O. 0., t. a. 0+ I a. e“ C O O lea- K .. .. . ... . m T . ... .. . . . . l m 0 e o o B . . T. r scene.” 0 O O K 8 0 . I o o 10." Ta. one. u . . . . -0« am . . .. e n. I e [on T I 0.: b. p P _ _ P p b F . _ I on .Aoa xoa $503083: 50:05 Awnmcoflpmaou cowpmboao PsosommpIhHmsocm nHmIoonm Pmmh Cd GOHPd>GHm Pflofiomdm 0 0 0 0 0 0 0 0 0 0 0 0 O 0 0 0 0 0 0 O O O 0 O O O 0 O 0 O 0 0 0 0 0 0 0 0 O 2 .4 6 8 0 2 0 O 0 2 I4 6 8 1. I 1. 1 1 2 2 I; 2 . _ . . . . . . . . _ . . . . . 4 q n a _ . . : 1 000 00 e“ O I 32 0'. IO'I .HH onswwm on: u. Cal 6 a . e. 00.. n as 0«.. m A ...... OuI.A 00 m m 0H T: U 0« am 3 n. on 0.: 0m 33 .A m we. psosfipsooofls snowmozv nfizmsowpoaou soflwm>oao vsosommnIzamsoso uLMIoonm .«H ousmwm POO.“ CH COMPN>$H0 Pgmfiomdm 1-12000 ‘-1hOOO --16000 “-18000 1-20000 -22000 00 .-2000 0-0000 n-6000 --8000 --10000 #000 ‘2000 0:: r I I :o c: stefim up Atsmoue AitAsJB ate—001g 30 The calculated relationship generally falls within the 95 percent prediction limit of the observed relationships. Thus the observed general relationship between Bouguer anomalies and basement elevations in the eastern Midcont- inent can be explained by the surface elevation. In the western part of the area. west of 96°W longitude. the Bouguer gravity anomaly and basement elevations do not exhibit a. definable relationship. Free-air Gravity Anomaly Relationships Free-air gravity anomalies are best suited to studying the tectonic adjustments as they do not take into consideration the gravitational attractions due to the mass included between sea level datum and elevation of the observation site. The Free-air anomaly is measure of the mass of the subjacent earth. Free-air Gravipy Anomaly-Basement Elevation Relationship: The relationship between Free-air gravity anomalies and basement elevations are studied for 1°x 1° as well as 2°x 2° mean values. The values for 1°x 1° quadrangles are shown in Figure 10 for the entire Midcontinent and FiguresII and 12 show the relationships for the eastern and western portion respe- ctively. The points are so widely scattered in these figures that no relationship is possible. The Free-air gravity anomaly-basement elevation relationship for the entire area for 2°x 2° quadrangle 35 Aomxomipsosfipsoocwsv manmsoapmaou soapm>oao vcosommnIaaososm nwmIoonm pooh cw soflvm>oao psosommm .mH ousmnm o o o o o o 0 O o o O o o O O O o O 0 0 0 0 2 o o 2 I”. 6 8 1 1 2 — c - - - - fl 1 . . . 1 Oél r o o I ONI I L 00 000. e I I ON - — P F b _ o: stsBm up Atemoue AitAsaS ate-001g 36 A0« xo« .peocapeooeaz anopnamv nflnmsodpmaou sofipo>oao psosomeIhamsosm has oonm .sa onswam pooh 2H sofivm>oao Pcosommm n. no no no no no no no no no 0. o. .u no .0 no nu nu no no no no 9. O 0 2 .4 6 8 1 1 2 . . _ . . . 4 . n q _ q 0:... / Amsowpmasoamo sopm noppaso soon o>ms // . mesa 000000 0:» cusps: apauoav I ./, . . . mmmH00.0: «0.0H: u 4m -0«: // . . . . .sowpmsuo scammouwom 0 cm ate-easy stfism up Atemous AitAsJS 37 82000 00 A m x N .vsosfipcoovwz snowmozv awnmsowvmaon sowpm>mao msosommnIzamsosm uonooum poom :H sowpm>oao psosomom "-2000 -*-4000 - -6000 ‘ —8000 4 -10000 .ma museum -12000 Cal I O N I I c> o: stefim up Atemous ate-eeag I O N o: 38 mean values is shown in Figure 13. The widely scattered values ranging between :25 mgals indicate no apparent relationship. Study of the relationship in eastern port- ion (Figure 10) reveals a linear relationship for values where basement elevations are less than sea level. The Free-air anomaly decreases with increasing basement elevation and the relationship can be expressed in terms of the equation. FA 8 -11.82 - 0.0013BE where FA is the Free-air gravity anomaly in milligals. This relationship is probably significant as indicated by a confidence level of over 95 percent and a -O.4h correlation coeffi- cient. In areas where basement elevations are greater than sea level there is an indication of increasing Free-air gravity anomaly with increasing basement elevation. This increase in anomaly with surface eleva- tion may indicate that the area is under compensated. However. even if the area is in regional isostatic com- pensation the great depths of the compensating mass deficit for the mass excess at the surface may lead to positive Free-air anomalies. The western portion of the area shows more incoherency and no apparent relationship is exhibited (Figure 15). However. there is a general bias towards positive Free-air anomaly values indicating an over all under compensation of the area. 39 so .::espp:oouaz snepmemv gasesoppeaoe soape>oao psesmmenIhHesose neswsom Hesvaeom .wH ohswfim poem :p soppe>eao psoSoemm nu nu .0 nu nu O nu o. nu nu o. nu .u nu o 0. .0 nu 0. nu o. nu o. nu .u o 0. nu 0 n. no 0. nu as u. ,o 00 0 0 O 2 .4 6 8 1. 1. 1. 1. 1. 2 9. . . . . . . . . . . 4 L 7 M q T l— 4 m 4 a 1. OWIH a S cm.” I n 011m. q. . o . n .n. L 81m . a a n . is . a . a I oo 9 A m .. 8 m o m to I o: u m is B h r p P F - h P r b P _ ow fl 40 A N xom .psosppsooepzv npgmcowpeaon coppe>eae pseseeenIhHemose neswsom Hesepeom .aa museum poem :p sowpe>eao psosemem m m 0 0 o m w 0 O O 0 o 0 o 0 o 0 O 2 0 0 2 .4 6 8 1 1 a d J a u A w 8 T... p n B T. m I .4 ONIQM 8 J 9. m - . . 8 u. 4 K O m I I om T: u m 3 a I 8 u r F p a b O 3 40 A N on .pcocppsooepzv mpnmcowpeaeu soppe>eao pseSemenIzaemose nosmsom Hesupmom .NH ensmpm poem as soppe>eae psoSeeem 0 m m 0 0 O m 0 0 O o 0 o 0 0 O o 0 o 2 0 0 2 4 6 8 I 1. 92 . . . . . .osl _ A l . A o: 10 ° ‘i’ sIsSm up °mous AitAeJB Jenfinoq temptsaa O N o» .3 41 2000 x oN .psoSHpsoomnz snepeemv manmsoapeaon coape>eao psesmeenIhaesose. neswsom Heseaeom .ma ousMNm OO poem CH Coppe>eae pseEomem T -10000 -12000 4 -2000 ‘ -4000 a -6000 l -8000 Amsowpeasoaeo soup eopppso seen e>eg espa ooppou esp swapps epsponv mmmH00.0I m«.HHI n eae psoseemnIhHesose neswsom Hesepmom .afiensmnm poem an coppe>oao psesemem nu .0 nu nu nu nu nu nu 0 nu nu .0 nu .0 n. n. n. nu nu nu 9. 0 2 .4 6 8 1 1. _ . . . _ . _ fl _ _ 4 4 3| I O N I I O O O :f stefim up °moue Kernel? aenSnog eruptseu 44 20 'v j 1 f f T I l I 00 y. e _ ' w “'20 P . r ' . . . ' ... e ’ .4 ., '. e 5. . . ‘ 0. v‘ .D I . 0 9 e e . m -40 r' e e ' ’ v : I J... 1' '4 '3 ’ ' .. . ‘ ' w 0 . ~ 0 a" ..l . E -60 D e e ’ e 9 "I II ‘A :2: ° ° . ’ . ....I o 9 0 D >3 '80 h o o - '33 . ’ g -100 _ , ° I Legend ’ 3-120 .. . . . I ,H Michigan BaSln . . e.‘ 5 Illinois . ’ ," Williston ° £1,440 )- Anadarko p .d a a" — g Salina - 80-160 a Forest City . .I I s Powder River J .I ‘3 L Denver 3 -180 a .J _ _200 I J I 4 I I J J -° I L I I I I I I I I I I I a: m H H H H H 00 O\ 4-? N 0 0 <3 cm c3 c> c: c> c3 c3 c> c: c3 c: c: c3 c3 c3 c3 c> Io Basement elevation in feet Figure 20. Bouguer an (Basins: 1 x 1 8 mal y;basement elevation relationship 45 Basins General Relationships Bougaer Gravity Anomaly-Basement Elevation Relationships for 1°x 1° Quadraagles: Bouguer gravity anomaly and basement elevation relationships for 1°x 1° quadrangles within the basins widely distributed and no correlation is possible (Figure 20). No single correlation is indicated even within the individual basins except for the Michigan and Illinois Basins which exhibit an inverse relationship of Bouguer gravity anomaly to the basement elevation. However. the Williston Basin. exclu- ding the area bordering the Minnesota craston. and the Anadarko Basin except for the area bordering the Ozark and Nemaha uplifts tend to exhibit a possible inverse relationship between Bouguer gravity anomaly and basement (elevation. Bougaer Gravity Anomaly-Basement Elevation Relationship for 2°x 2° Quadrapgles: The relationship between Bouguer gravity anomaly and basement elevation for 2°x 2° quadrangles within the basins is indicated in Figure 21. The values are widely scattered. but they tend to fall in two general groups. One group consists of values corresponding to the Michigan. Illinois. Salina. Forest City and a part of the Williston Basins. This group shows an inverse relationship of Bouguer gravity anomalies to basement elevations and can be represented by a straight line given by the equation. OO -20 I -40 -60 ~80 anomaly in mgals -100 er gravity -120 Bougu -140 -160 46 [ 1 T I I Regression equation: BA = -SSe 21 "OeOOLIIBE 'Regression equation: BA = -145.46 -0.0088BE J I I I L I I I I I I O .1 .k an 0\ s— a: c> :0 o» c> <3 <3 <0 <3 CI :3 <3 <3 Io o: Io c> <3 <3 lo 0. c3 Basement elevation in feet Figure 21. Bouguer anomaly-basemeng elgvation relationship (Basins: 2 x 2 ) 47 Slevation relationship (Basins: 1 x 1 ) (Deeper point of Anadarko Basin has been neglected.) 7000 | I I T I 6000 - . .3 I 5000" -’ _ I. e v ,5 I 'H v _v ' A c . “II #000 n- 3 ’ ) D d :2 3 ‘ .9. -' 'p ' 0 e 3 8 3000 ° ° 3 '3 «P " 0 D e O D 0 g o “"I O O O 03 0 g 2000 T a . ... .. to 0 ° ° 3 ° ‘% 3 ° °ot> 0 3 . D 3 I I: {'15. 1000 " - ' . ’ f - . i e 9 e 9 'V y. 3 V V ' ' V"' t ' 0'. . . t V . e . 00 I I I I I I I I I I I .... H m ON 4-7 N O :0 <3 0» c> c: <3 10 «o c> <3 c> <3 <3 o: :0 <3 <3 :0 :0 o 0 Basement elevation in feet Figure 22. Surface elevation-basemsnt 48 5000 I 1* I r I P 4000 L- y -I 3 3 ) g‘ . c: 0H a 3000 .— 0 .. o -:-I *3 b o ‘3 . e '3 2000 - . - o O o .3 3 ‘5 00 I 011 o f * ' e . 0 V v ‘ e V 00 l l I l I I I I I I I H H (1) O\ I? N a: <3 <3 0. o lo o lo 0: c: CI 0 <3 <3 <3 Io o» o O> :0 Basement elevation in feet Figure 23. Surface elevation-basem relationship (Basins: 2 nt levation OO 48 5000 4000 3000 2000 Surface elevation in feet '0‘ <3 <3 as 'thin ip is -vely. vase of the wer Basins as ”crest City Basins. f‘basement elevations makes it impossible y-basement elevation 5 for the entire eastern gravity anomaly-surface 49 BA 2 -55.21 - 0.0041BE with a highly significant relationship. having over 99.9 percent confidence level and a correlation coefficient of -O.78. The second group is much more scattered and consists of the values falling within the Williston. Anadarko. Powder River and Denver Basins. The relation- ship between Bouguer gravity anomaly and basement elev- ation is represented by the equation. BA = -145.46 - 0.00888E and has a correlation coefficient of -O.60. but the significance of this relationship is doubtful having only 90 percent confidence level. The negative slope of the line clearly shows the inverse relationship. i.e.. Bouguer gravity anomaly decreasing with increasing basement elevations. Basement Elevation-Surface Elevation Relationships: The points relating basement to surface elevations for both 1°x 1° and 2°x 2° quadrangles within the basins are widely scattered and no relationship is observed as shown in Figures 22 and 23 respectively. The scatter is much more pronounced in the case of the Williston. Anadarko. Denver and Powder River Basins as compared to the Michigan. Illinois and Forest City Basins. The lack of correlation of basement elevations to surface elevations for the basins makes it impossible to develop a Bouguer gravity anomaly-basement elevation relationship as noted in Figure 5 for the entire eastern Midcontinent using the Bouguer gravity anomaly-surface 50 00 K N espeemv A « gpgmsoppeaon soapepoao psesemepIszso nNeIoonm .zN ouswpm poem an coppe>oao psesoeem --10000 ~12000 q -2000 . -4000 -a-6000 _.-8000 1:2- mmmaoo.o: NH.0I u c3 <3 <3 :3 :3 <3 c> <3 <3 :3 <3 c> <3 <3 <3 c> <3 c: <3 Basement elevation in feet Figure 25. Michigan Basin: 1°x 1° relationship 53 expressed by the equation. SE = 281 - 0.0626BE and has a correlation coefficient of -O.71. This relationship is definitely significant as indicated by a 99.9 percent confidence level . As a result of this inverse relation- ship between surface and basement elevations and the general Bouguer gravity anomaly-surface elevation relat- ionship. the effect of the surface elevation should be to cause a direct Bouguer anomaly-basement elevation rela- tionship. However. an inverse relationship is shown in Figure 25c. The Bouguer gravity anomaly-basement elevation relationship for the Michigan Basin (Figure 25c) is given by the equation. BA = -37.05 - 0.0017BE . This relationship is probably significant as reflected by over 95 percent confidence level and has a correlation coefficient of -0.51 . The Free-air gravity anomaly basement elevation relationship (Figure 25b) is also inverse. but the slope is greater than the Bouguer gravity anomaly-basement elevation relationship (Figure 25c). The least square line is given by the equation FA = ~24.28 - 0.0034BE with 99.9 percent confidence level and -O.76 correlation coefficient. Illinois Basin: Surface elevation. Bouguer gravity anomaly and Free-air gravity anomaly are plotted against basement elevation for 1°x 1° area of the Illinois Basin in a similar manner as the Michigan Basin in Figure 26. 54 #32000 I a I I I .5 Regression equation: 5 SE a 784.5 + 0.0292BE “31000 p _ W '7 ¥_ 8 . - Lin—7' V k V ' a "'—'vv 7 O I: g 00 l I I L L a: 20 I I .T’ I I - Regression equation 8 ' . ' FA: -15.08-0.00183E 5% a. 0000 _ b :32 I I: 0W4 E -20 20 g 00 - e u) s s ...: g-zo Ac '2 e E II-40” ' ' I — 5) Regression equation: 3 BA 2 -41.45 -0.0027BE ‘n I -60I IL '1 . l. .1 “’ H' a: O\ p- 53 <3 n3 <3 .0 <3 <3 c» <3 8 8 8 8 8 8 oI c> Basement elevation in feet Figure 26. Illinois Basin: 1°x 1° relationship 55 The surface elevation-basement elevation relationship is direct (Figure 26a), similar to the one obtained for Midcontinent area as shown in Figure 9. This straight line relationship is given by the equation. SE = 78#.5 + 0.0292BE with a correlation coefficient of 0.72. The relationship is definitely significant as indicated by over 99.9 percent confidence level. As a result of this relationship. the inverse Bouguer gravity anomaly-basement elevation relationship will be slightly increased by the surface elevation effect. The Bouguer gravity anomaly-basement elevation relationship is shown in Figure 26c. The least square line representing the inverse relationship. that is the decrease in Bouguer gravity anomaly with increase in basement elevation. is given by the equation. BA = -41.45 - 0.0027BE and has 99 percent confidence level and a correlation coefficient of -0.57 . The Free-air gravity anomaly-basement elevation relationship (Figure 26b) is similar to the Bouguer gravity anomaly basement elevation relationship, but with smaller lepe. This relationship is represented by the equation. FA = -15.08 - 0.0018BE. Its correlation coefficient is -0.h§ and has a confidence level of 95 percent. Williston Basin: Surface elevation, Bouguer and Free-air gravity anomalies are plotted against basement elevation for 1°x 10 area of the Williston Basin in 56 I F I I ' +,h000 0 '2: o C o _ .3 3000- o ‘53 ° . > O 0 H 0 ¢ 2000 2, ° . IL. Regression equation: ° ° o. o 3 SE = 178“ -0.0617BE j 0’ IoooL L ' I l ‘ ~ 1+0! I I I I r U) H a an 5 Id 5 n ...-q “I“ 3 Regression equation: 0 ‘ g PA = 6.35 -0.001lIBE m -20 I I l I I '3 -20 I I I I l g) Correlation coefficient é less than 0.01 , o o -ho._ 5 ° 0 .0 j >. +3 0 g o o a o 0 o to '60P. ° ' '1 n ' o ' a, 0 a ' I. . 3 . ‘ 8 -80 L I I I L . I .1. I. so a I: I, 8 n3 <3 c: <3 Io <3 <3 <3 <3 <3 <3 :3 :3 <3 <3 <3 <3 <3 oI Basement elevation in feet Figure 27. Williston Basin: 1°x 10 relationship 57 Figure 27. The surface elevation-basement elevation relat- ionship (Figure 27a) shows scattered points which are much more pronounced above 2500 feet surface elevation and basement elevations of -3000 feet to -9000 feet. The surface elevation shows an inverse relationship to the basement elevation and the scatter of points is refl- ected in its correlation coefficient of -0.4h. This relationship is eXpressed by the equation, SE = 178# - 0.0617BE and the relationship appears significant as reflected by the confidence level of 98 percent. The Bouguer gravity anomaly-basement elevation relationship has no correlation (calculated correlation coefficient is less than 0.2 and the confidence level is less than 10 percent). The points show a wide and irregular scattering. Hence no relationship is indicated in Figure 27c. In contrast to the lack of correlation in the Bouguer gravity anomaly-basement elevation relationship. the Free-air gravity anomaly-basement elevation relation- ship show a linearity in trend even though the points are somewhat scattered (Figure 27b). The correlation is inverse, i.e.. Free-air gravity anomaly decreases with increasing basement elevations. This relationship is likely to be significant as indicated by Over 98 percent confidence level. The least square line showing the 58 I I I I I I: ~ 4 ° ,H I+000 ,9 c: 9 "'1 9 o ‘9 z 3000 I“ H 9 9 0 O o 2000 .— v " «‘3 I 3.. :5 ‘9 9 In 1000 9 I J, I . I 10 I I I I I ‘9 Q ' '3 g -10 ? 99¢ 52 " ° ” 9 a! L: {10 9 v, 7 ...: g f: -30 JA 1 I I L 80" é: I l I I If ’6 -40— In H 9 a) 9 3 ~60. I: ...: ° 9 g -80 m 0 >3 *3 10 ‘ - '9 5 a, e L: -120- d) .9 9 go I 47 p3 -1LIo.. 4 I 4 l J 9 L : l j I l H H (D O\ 4? N N O O O O O O O O O O O O O O O O 0 Basement elevation in feet Figure 28. Anadarko Basin: 1°x 1° relationship ( Points below -12000 feet neglected) 59 inverse relationship of Free-air gravity anomaly-basement elevation relationship is expressed by the equation, FA = 6.35 — 0.0014BE and has a correlation coefficient of -O.43. The Free-air gravity anomaly of the Williston Basin is generally positive suggesting an isostatically undercompensated region. Anadarko Basin: Within the studied portion of the Anadarko Basin the surface elevation-basement elevation relationship (Figure 28a) shows such a wide scatter that no correlation is possible. The Bouguer gravityanomaly-basement elevation relationship also exhibits a wide scatter (Figure 28c). However. the values less than -80 mgals tend to fall in a group showing an inverse relationship of Bouguer gravity anomaly with basement elevation. This relationship seems significant with 99 percent confidence level and -O.8h correlation coefficient. and is given by the equation. BA = -1h7.82 - 0.0050BE. Widely scattered points make any relationship between Free- air gravity anomaly and basement elevation impossible. Other Basins: No obvious correlation exists between basement elevation and surface elevation, Free-air gravity anomaly or Bouguer gravity anomaly within the Salina Basin (Figure 29A). Forest City Basin (Figure 29B), and Denver and Powder River Basins (Figure 30). The scatter of points is high and the range of basement elevation is limited. As a result it is impossible to establish a correlation With any reasonable certainity. 30001 I 2000 h . ° _ 1000 1 no I 20 P , 3 ‘ o :- v. . e -I -20 ' ‘ 4:0 ‘ 00 I -20 p _. -ho — ° _ -60I- ° . . _ -80 L ' I I O -F-' N O O O O O O 0 Basement elevation in feet A. Salina Basin 0 Free-air anom. mgals Surface elevation ft. 0‘ Bouguer anom. mgals I 2000 i“ e’t ’ _ * ... . ’ ..1000 fi" 1 i l 00 I no :1 I- : -I 20 I I- , ’ I . 00 i “: ' t :* * i “ ' -20 f ‘ -tIo I . 00 l — - -20 ‘ i b 4 fl‘ ’ i-4 -40 ‘ II I... t " II I x 1 -60 I II C>-8o {7 N O O O O O O 0 Basement elevation in feet B. Forest City Basin Figure 29. Salina and Forest City Basins, 1 x 1 relationships. 61 00 nflnmcowpsaou oH on .mcwmmm uo>cmn cum no>wm pmvzom .om mpswam poem as :owpd3oam pamsomwm u-ZOOO --4000 3-6000 -8000 com: owHI.I 00.7: I 03H: I ONHII OCH: .1 stsSm u; Atsmous JenSnoq poem Cw soapm>oam pcmsommm ‘PZOOO d-uooo --6000 -8000 _ _ 000m 1 000: . Iooom sIeBw ut 'moue its-ssag ’AGIS sosyans 4881 u! 62 However. there are indications of an inverse relationship between Bouguer gravity anomaly and basement elevation. Geological Correction For Basin Sedimentary Rocks Assuming that the sedimentary rocks of the basins are less dense than the enclosing basement rocks. the effect of the geological correction to the Bouguer gravity anomaly for the deficiencies of mass within the basins is to increase the Bouguer gravity anomaly with decreasing basement elevation. The geologically corrected Bouguer gravity anomaly based on an assumed density contrast of -0.15 gm/cc between the basement and the basin formations is plotted against basement elevations for Michigan Basin (Figure 31), Illinois Basin (Figure 32) and Williston Basin (Figure 33). The equation of the least square line of the Michigan Basin is given by CBA = -37.00 - 0.0036BE where CBA is the geologically corrected Bouguer anomaly in milligals, with confidence level of 99.9 percent and -0.78 correlation coefficient. The increase in slope of the inverse correlation between Bouguer gravity anomaly and basement elevation is apparent on comparision of this equation with the equation of the least square line for the uncorrected Bouguer gravity anomaly. BA 2 -37.05 - 0.0017BE. The slope of the least square line for Illinois Basin is also increased by an additional 0.0019 in milligals per foot resulting in the equation CBA a -41.35 - 0.000632. 63 A a x H mwsmsowpmaou soaps>oao PsosomwnIszsosm u .swmmm cmwasowsv oswsom vepoopnoo .am muswam poem aw sofipw>oao Psosommm no .0 .0 no o. no no 0 .0 no .0 no no .0 no .0 nu 0. nv 0. .2 o 9.. .... ... ... ... a _ A n _ _ owl mmwmoo.on oo.um: n oam pcosomanuzmsso posmsom copoonuoo .Nm ouswflm Poem a“ sowpw>oao Psosommm O O O O O 0 0 O O o O O 0 0 0 O O O O O 2 O 2 .4 6 8 1 1 . . . . . . 4 _ _ d d 03:. mmmsoo.oI mm.asI n «mo I .sowpssuo scammoHMom O N l sIsSm u; °mous AitAsJB anSnog psqosaaoo O O 65 A a x a .swmmm sovmwaaaz v awnmsowpmaou soapa>oao psmsommanausmna noswsom oopoouuoo .mm onswwm sodpa>oao vcosomam no no no nv no nu nu nv 0. o. n. nu no 0 o. .u .u no nu no “4 o ... .... ... s 3. I . _ _ _ _ con: mmomoo.oI mo.mmI n oa oocovwmsoo.;soapusuo scammeuwmm ~:.oI 0.03 uo>o mmomoo.oI3o.mm «<00 as no” camam .Haaz 03.0: 3.33 9030 mm0000.0:mn.«: u0 mm0300.0I00.3n uoao.emum:.wsom.auoo ~:.0: 0.03 ae>0 mmmaoo.0um~.aa «4mm om wom .vcoocwz.vmmm .>oao emum:.wsom.mem 30.0: 0.03 ao>0 mm:«00.0:nn.0 I <3 as no” :Hmmm .Haaz 00.0: 0.03 ac>0 mmmH00.0:00.ma u <3 cu Nod cwmmm .HHH 03.0: 3.33 ao>0 mmsmoo.0um~.:~ u <3 oH Nod swmwm .soaz mm.0: 0.03 uo>0 mmmH00.0I3H.0 I <3 cm xom msammm 03.0: 0.03 uo>0 mmmH00.0INm.HH I <3 on How .psooowz.vmam .30Ho .mmam:uwm:eoum Aoopmasodmov mm0aoo.0I 0.33 n «m cm xom .pcooeaz.pmem .>mao.mmamI.aoeu.msom 00.0: 0.03 ao>0 mm3a00.0n :03H u mm ca xoa :Hmmm .aaws m3.0 3.33 ao>0 mmm3mo.0+ 0.003 n mm 00 on camam .HHH «3.0: 0.33 ao>0 mm0m00.0: m.amm u mm o« woe swmam .:002 30.0 0.03 ao>0 mmommo.0+ 3N3 u mm omxom .psoovfis.pmmm .30Ho.omsm: .Ho .masm 30.0: 0.33 ao>0 mm3~00.0Im:.«: : u 0 mm3H00.0:00.3m I u 4m oH new :Hmam .5002 00.0: 0.03 ue>0 mmmmoo.0:0:.m:a: n 0 mmH000.0IHm.mm : u «m cm wow magnum .pmdm 00.0: 3.33 ao>0 mmm~00.0:mm.m: I u «m cm xom .psoocwz.pmmm ~0.0: 3.33 uo>0 mmsmoo.0:00.0: I n oae.omam:.soss.wsom .mooo.aon Hw>oH.mmom sowwmsuo .amom meu< muoposmuma uovmaom .N Handy 70 The regression equations. confidence levels and correlation coefficients for the established linear relationships between geological and geophysical parameters are summarized in Table 2. Discussion of Results Midcontinent Area The relationship between Bouguer gravity anomalies and basement elevations in the Midcontinent (Figure 2) is not a simple inverse relationship as has been previously suggested. but is a complex one and varies within the study area. Within the western Mid- continent (Figure 0) area between 96°W longitude and the Rocky Mountain front. there is no obvious correlation between these parameters. This is not unexpected because of the wide range of elevation within the area which will distort the Bouguer gravity anomalies and possible effect of diastrophism associated with the Cordilleran mountain systems. Even in the stable eastern Midcontinent (Figure 3) east of 96°W longitude and west of the Appalachian Basin. the relationship between Bouguer gravity anomalies and basement elevations is not simple. In areas where basement surface is above sea level there is no correlation between the parameters and the areas adjacent to the Appalachian Basin. However. in the eastern area. where basement elevations are less than sea level and 71 excluding areas involved in Paleozoic activities. there is an inverse correlation between Bouguer anomalies and basement elevations (Figures 3 and 5). This relationship can be at least partially explained by the observed direct relationship between surface and basement elev- ations (Figures 8 and 9) using the surface elevation- Bouguer gravity anomaly relationship (Figure 6 and 7). The direct relationship between surface and basement elevations observed in the eastern Midcontinent, where basement elevations are less than sea level. reflects the regional decrease in elevation from cratonic areas where the basement is at higher elevations. This may be due to continued relative vertical movements of Paleozoic basins and arches to the present day and more resistance to erosion of Precambrian and early Paleozoic sediments of the arches. Assuming that the observed inverse relationship between Bouguer gravity anomalies and basement elevations is completely due to surface elevation effect, the regional Free-air anomaly should show no relationship to basement elevation. However, this is not true in the case of 2°x 2° quadrangle averages of Free-air gravity anomaly in the eastern Midcontinent where basement ele- vations are below sea level. These Free-air gravity anomalies show an inverse correlation with basement ele- ‘vations (Figure 14) and the residual Bouguer gravity anomalies have a similar relationship. Therefore. the 72 inverse correlation between Bouguer gravity anomalies and basement elevations observed in the eastern Midcon- tinent, although partially due to surface elevation, is also a result of mass differences within the geological section. The regional Free-air gravity anomalies (Figure 1h) indicate that areas of lowest basement elevation are undercompensated in relation to the areas where the base- ment is near sea level, i.e.. areas of maximum basin development are areas of greater mass. In contrast to this it appears (Figure 10) that the cratonic areas. where basement elevations are above sea level. are also undercompensated. Thus both the highest and lowest base- ment elevations of the eastern Midcontinent are associated with excess mass areas. Lyons (1959), Hinze (1963) and McGinnis (1966) have individually suggested that the inverse relationship of Bouguer gravity anomalies to basement elevations within various basins of the eastern Midcontinent may be the result of elastic deformation of the basement in response to the additional mass of the basic rocks extruded onto or intruded into the basement comlex. This hypothesis is not valid when considering broad areas such as eastern Midcontinent because average Bouguer gravity anomalies equivalent to those obtained over the deepest basins are observed in cratonic areas (Figures 3 and 5). Furthermore, as pointed out. above, both the deepest basin areas and cratonic areas are undercompensated 73 indicating a mass excess. Basins Better relationships of gravity anomalies with basement elevations are exhibited when considering the basins only. although no single correlation exists between Bouguer gravity anomaly and basement elevation. The basins in the eastern and western portions of the Midcontinent show similar inverse but independent rela- tionships (Figure 21). Lack of any single correlation between Bouguer gravity anomaly and basement elevation may be due to the complicating effect of the surface ele- vation on the Bouguer gravity anomaly. However. a possible inverse relationship occurs between Free-air gravity anomaly and basement elevation of sedimentary basins (Figure 24) The relationships between gravity anomalies and basement elevations are clarified by considering individual basins. The Free-air gravity anomalies of the Michigan. Illinois and Williston Basins (Figures 25. 26 and 27) show a distinct inverse correlation with basement elevations. Similarly , an inverse correlation exists between Bouguer gravity anomalies and basement elevations of the Michigan and Illinois Basins (Figure 25 and 26) which are increased or decreased reSpectively by the effect of surface elevation . Parts of Anadarko Basin also exhibits the inverse relationship of Bouguer 7a gravity anomaly with basement elevation. The Bouguer gravity anomalies of the Williston Basin only exhibit an inverse correlation with basement elevations after correcting for the gravitational effect of the sediments (Figure 33). The other basins of the Midcontinent do not exhibit definable relationships between gravity anomalies and basement elevations. This may be a result of a different origin of the basins or subsequent deformation and the limited basement elevation range within some basins which makes it impossible to define the relationship. The Free-air gravity anomalies of the Michigan. Illinois and Williston Basins indicate a relative under- compensation over the center and deepest parts of the basins and an overcompensation along margins. Considering the entire basin, the Michigan and Illinois Basins are essentially in isostatic equilibrium and the Williston Basin is undercompensated perhaps reflecting the higher surface elevations and its proximity to the Cordilleran mountain systems. Recognition of this inverse relationship between Bouguer gravity anomalies and basement elevations in the Michigan and Illinois Basins have led Hinze (1963) and McGinnis (1966) respectively to hypothesize a cause and effect relationship between the source of gravity anomalies and the development of basins. They have suggested. as Lyons (1959) did for basins along the Midcontinent gravity high. that the basins originated Boug. anom in mgals Basement elevation in feet 75 1 I fl Bouguer anomaly profile 00 . -20 L ‘\ I I” -#0 - a -60 I I I i i 3 87° 86° 85° 8u° 83° 82° 00 4. IL I JI : 5 Basement elevation profile -5000" ~10000‘ L l L -15000 ‘ ‘ 5 Figure 34. Bouguer anomaly and basement profiles for Michigan Basin. along hhth parallel Basement elevation 20 , 76 Bouguer anom. in mgals I j profile Bouguer anomaly 00 L -5000' in feet -10000. Basement elevation profile -15000 1 Figure 35. Bouguer anomaly and basement profile for Illinois Basin“ along 38th parallel 77 from elastic deforamation in response to the added mass of basic rocks emplaced in the basement complex in late Precambrian time. McGinnis has expanded on this idea. relating the basins to elastic deformation due to basic intrusives and extrusives emplaced along a Keweenawan rift zone. This theory suggests that similar relation- ship between Bouguer gravity anomalies and basement ele- n vations should be observed over all the basins of the Midcontinent area. Furthermore, if basins originated I by regional elastic deformation associated with crustal loading. there should be a direct correspondence between gravity anomalies and basement elevations. However, gravity and basement profiles of the Michigan Basin (Figure 3h), Illinois Basin (Figure 35), Williston Basin (Figure 36) and Forest City Basin (Figure 37) do not confirm to this conclusion. Recently Hinze, Davidson and Roy (1971) have suggested an alternate theory for the origin of some Midcontinent basins associated with Paleo-rift zones. They point out that the elastic deformation theory of the origin of basins previously suggested encounters serious difficulties in timing. The basins filled with shallow water sediments dev10ped over long period of time. into late Paleozoic time, while the masses associated with rift zones were added to the crust near the end of Precambrian time. The isostatic relaxation time as determined by McConnell (1968) is too short for the 78 .camem coamaaaa; you .Hoaamasn new: macaw hamsocm nosmsom moawmoua pcesomum use .0m assess 000mH1 . — . L p — . . l 0000.7. 0000: 00 r n d 0303 000a owl d oawmonn hameocm hpa>aaw hoswsom Igl- 0NI '1; UI'AGIS eosgzns stsSm u; °mous aenfinog Bouguer anom. in mgals Basement elevation in feet 79 A ~20 -h0 -60 Bouguer anomaly profile 92° -1000 -2000 -300 -400 Figure 37. Bouguer anomaly and basement rofiles for Forest City Basin: along 0th parallel 80 elastic deformation to continue for hundreds of millions of years as observed. Therefore, they suggest that the possible final stage of the continental rifting process involves compression of the crust and slow development of a basin over a paleo-rift zone. According to this theory there is no cause and effect relationship between gravity anomalies and basement elevations, but rather they are both a result of complex continental rifting process where late basin development is centered over a paleo-rift zone which due to basic intrusives and extrus- ives shows up as a gravity high. As a result of this theory there is no necessity for a constant inverse correlation between gravity anomalies and basement elev- ations but only a general correlation of gravity anomalies with basement elevations where basins deve10ped as a last stage of the rifting process. Thus the lack of correla- tion along the Midcontinent gravity high where it traverses the Forest City Basin can be explained by variations in the final stage of the rifting process and variations between other relationships detailed in this study can be explained similarily. Furthermore. not all of the basins of the Midcontinent developed as final stages of a continental rifting process. other origins are quite likely for many basins. Therefore other relationships between gravity anomalies and basement elevations as observed in this study are expected. However, the similarity between the relationships of the Michigan 81 and Illinois and perhaps the Williston Basins suggest a similar origin for the basins. although according to this theory there is no cause and effect relationship between gravity anomalies and basement elevations. CONCLUSION In conclusion. the results of this study substantiate the previous observation that in general basement elevations in the eastern Midcontinent are inversely related to gravity anomalies. both Bouguer and Free-air. and a similar relationship is indicated for the Williston Basin of the western Midcontinent. The Anadarko Basin in part. exhibits this relationship for Bouguer gravity anomaly only. The inverse relationship are particularly well illustrated in the Michigan, Illinois and Williston Basins. Other basins of the Midcontinent do not have definable relationships.IFhiS fact together with the equivalence of average gravity anomalies in the center of the basins and in some cratonic areas suggest that the basins were not developed by elastic deformation in response to the added mass of basic rocks. in the base- ment complex. Thus the relationship is not related by cause and effect. but rather it is suggested that both the increased mass of the basement complex in the center of the basins which produce the gravity anomalies and development of the basins result from stages of the same complex process, perhaps late Precambrian crustal rifting. 82 REFERENCES Bayley. R. W. and Muehlberger. W. R.. 1968. The Basement Rock Map of the United States: U. S. Geol. Surv. Cohee. G. V.. 1962. Tectonic Map of the United States: U. S. Geol. Surv. Eardley. A. 0.. 1951. Structural Geology of North America: Harper and Brothers. New York. 62hp. Goldich. Samuel 0., Lidiak Edward 0., Hedge. Carl E. and Walthall. Frank C., 1966. Geochronology of the Midcontinent Region. United States, part 2. Northern Area: Jour. Geoph. Res. vol 71 pp. 5389-5Q08. Goldich. Samuel 0.. Muehlberger.William R. and Lidiak. Edward G.. 1966. Geochronology of the Midcontinent Region. United States. part 1. ScOpe. Methods and Principles: Jour. Geoph. Res. vol 71. PP- 5375 - 5388: Heiskanen. W. A. and Vening Meinesz. F. A., 1958. The Earth and its Gravity Field: McGraw-Hill Book company pp. #70. Henderson. J. R. and Zietz. 1.. 1958, Interpretation of an Aeromagnetic Survey of Indiana: U.S. Geol. Surv.. Professional paper 316-B. pp. 37. Hinze. William J.. 1963, Regional Gravity and Magnetic Anomaly Maps of the Southern Peninsula of Michigan: Michigan Geol. Surv. Rep. Inv. 1.pp. 26. Hinze. William J., Davidson. Donald M. and Roy, Robert F.. 1971. Continental Rifts: (Abstracts) Inst. Lake Superior Geol. Innes. M. J. 8.. Goodacre. A. K.. Weber, J. R. and McConnell, R. K.. 1967, Structural Implications of the Gravity Field in Hudson Bay and Vicinity: Can. Jour. Earth Sci.. vol h, pp. 977-993- 83 80 King. Philip B.. 1969. The Tectonics-of Middle North America: Hafner Publishing 00.. 203p. Lidiak. E. G., Marvin, R. F.. Thomas. H. H. and Bass, M. N.. 1966. Geochronology of the Midcontinent Region. United States. part 4, Eastern Area: Jour. CeoPh. Res. vol 71. pp. 5027-5438. Lyons, Paul L.. 1959. The Green Leaf Anomaly. A significant Gravity Feature: S osium on Geo h sics in Kansas, State Geol. Surv. o ansas, Bu 137. Martin. Rudolfo. 195u, Gravity Maxima Corresponding with Sedimentary Basin: GeOphysics, vol 19 PP. 89-9“. McConnell, Robert K.. 1968, Viscosity of the Mantle from Relaxation Time Spectra of Isostatic Adjustment: Jour. Geoph. Res. vol 73. no 22. McGinnis. L. D..1966, Crustal Tectonics and Precambrian Basement in the North Eastern Illinois: Ill. Geol. Surv. . Rep. Inv. 219. McGinnis. L. D.. 1970. Tectonics and Gravity Field in the Continental Interior: Jour. Geoph. Res.. vol 75. PP- 317-331. Muehlberger, William R., Hedge. Carl E.. Denison Rodger E. and Marvin Richard F., 1966. Geochronology of the Midcontinent Region. United States. part 3, Southern area: Jour. Geoph. Res. vol 71. pp. 5009-5426. Strange. W. E. and Woollard. G. P.. 1960, The Use of Geologic and Geophysical Parameters in the Evaluation, Interpolation. and Prediction of Gravity: Hawaii Inst. Geoph.. contribution 60-17. Woollard George P.. 1962. Relation of Gravity Anomalies to Surface Elevation. Crustal Structure and Geology: Univ. Wisconsin. Geoph. and Polar Res. Center, Res. Rep. 62-9. Woollard G. P. and Joesting. H.R., 196k, Bouguer Gravity Anomaly Map of the United States. U. S. Geol. Surv. Woollard George P. and Strange W. E.. 1966. The Prediction of Gravity: Geophysical Monograph Series. no. 9. Amer. Geoph. Union. pp. 96-114. 85 General References Rholf. F. James and Sokal. Robert R.. 1969. Statistical Tables: W. H. Freeman and Company. 253p. Sokal. Robert R. and Rholf. F. James. 1969. Biometry: W. H. Freeman and Company. 776p. Thiel. Edward. 1956. Correlation of Gravity Anomalies with the Keweenawan Geology of Wisconsin and Minnesota: Bull. Geol. Soc. Amer.,vol 67. pp. 1079-11000 APPENDIX APPENDIX Data Quadrangle Base. el. Free-air anom. Boug. anom. Surf. e1. Lat. Long. in feet in mgals in mgals in feet 49 105 - 10208 + 011 - 064 2201 49 104 - 11830 + 019 - 055 2169 49 103 - 10313 + 031 - 043 2172 49 101 - 05178 + 021 - 033 1578 49 100 - 03083 + 023 - 032 1617 49 99 - 01509 + 011 - 041 1539 49 98 - 00052 - 001 - 033 0941 49 97 + 00988 + 003 - 031 0988 49 96 + 01161 + 016 - 024 1161 49 95 + 01158 + 013 - 027 1158 49 94 + 01175 + 002 - 038 1175 49 93 + 01276 - 008 - 052 1276 49 92 + 01414 + 009 - 093 1414 49 90 + 01214 - 015 - 056 1214 49 89 + 00643 - 024 - 046 0643 49 88 + 00393 - 004 - 017 0393 49 87 + 00545 - 000 - 019 0545 48 105 - 09877 + 027 - 055 2405 48 104 - 11603 + 020 - 061 2379 48 103 - 12409 - 037 2250 48 102 - 08634 + 014 - 055 2014 The latitude and longitude values are given for the north-western corner of the 1°x 1° quadrangle. 86 87 Quadrangle Base.el. Free-air anom. Boug. anom. Surf. el. Lat. Long. in feet in mgals in mgals in feet 48 101 - 05688 + 006 - 058 1880 48 100 - 03177 + 019 - 039 1699 48 99 - 01367 + 009 - 042 1486 48 98 + 00258 - 001 - 039 1125 48 97 + 00978 + 003 - 030 0978 48 96 + 01381 + 006 - 041 1381 48 95 + 01355 + 008 - 038 1355 48 94 + 01348 - 001 - 047 1348 48 93 + 01411 - 007 - 055 1411 48 92 + 01362 + 043 - 003 1362 48 91 + 00751 + 030 + 004 0751 48 90 + 00318 - 003 - 014 0318 48 89 + 00400 + 004 - 010 0400 48 87 + 00194 - 008 - 015 0194 48 86 + 00427 - 021 - 036 0427 47 105 - 07794 + 023 - 076 2890 47 104 - 07897 + 038 - 058 2825 47 103 - 08591 + 025 - 062 2549 47 102 - 07123 + 007 - 066 2146 47 101 - 04528 + 000 - 063 1854 47 100 - 02383 + 016 - 051 1965 47 99 - 00905 + 005 - 047 1512 47 98 + 00570 - 004 - 043 1148 47 97 + 01056 - 004 - 040 1056 47 96 + 01407 + 012 - 036 1407 47 95 + 01309 + 009 - 036 1309 47 94 + 01266 + 012 - 031 1266 47 93 + 01161 + 033 - 007 1161 47 92 + 01007 + 030 - 004 1007 47 91 + 01050 000 - 036 1050 47 90 + 01237 + 029 -013 1237 47 89 + 01345 + 013 - 033 1345 47 87 + 00328 000 - 020 0594 88 Quadrangle Basa.el. Free-air anom. Boug. anom. Surf. el. Lat. Long. in feet in mgals in mgals ' =in feet 47 86 + 00087 - 003 - 025 0656 47 85 + 00432 - 003 - 029 0751 46 106 - 06525 + 015 - 101 3396 46 105 - 04494 + 023 - 094 3419 46 104 - 05934 + 036 - 069 3064' 46 103 - 06072 + 017 - 073 ' 2628 46 102 - 04723 + 006 - 071 2247 46 101 - 02839 + 006 - 059 1869 46 100 - 01364 + 013 - 051 1873 46 99 - 00167 - 001 - 048 1381 46 98 + 01640 + 009 - 047 1640 46 97 + 01142 + 001 - 038 1142 46 96 + 01207 + 015 - 026 1207 46 95 + 01165 + 016 - 024 1165 46 94 + 01004 + 002 - 032 1004 46 93 + 01040 + 025 - 010 1040 46 92 + 01197 - 001 - 042 1197 46 91 + 01447 + 021 - 028 1447 46 90 + 01549 + 007 - 046 1549 46 89 + 01306 - 007 - 052 1306 46 88 + 00353 - 026 - 051 0735 46 87 - 02197 - 014 - 030 0456 46 86 - 04622 - 002 - 020 0535 46 85 - 04688 - 003 - 029 0751 46 84 - 03169 - 028 - 046 0531 45 106 - 07125 + 021 - 125 4272 45 105 + 00550 + 042 - 108 4386 45 104 + 00403 + 034 - -93 3714 45 103 - 03456 + 009 - 081 2648 45 102 - 02692 - 007 - 083 2228 45 101 - 01195 - 009 - 071 1831 45 100 - 00252 + 005 - 055 1768 45 99 + 00163 + 002 - 046 1411 89 Quadrangle Base. e1. Free-air anom. Boug. anom. Surf. e1. Lat. Long. in feet in mgals in mgals in feet 45 98 + 00559 + 011 - 045 1654 45 97 + 01086 + 015 - 040 1617 45 96 + 01178 + 001 - 039 1178 45 95 + 01040 + 001 - 034 1040 45 94 + 00700 + 027 - 007 1010 45 93 + 00644 - 012 - 047 1040 45 92 + 00958 + 005 - 028 0958 45 91 + 01102 - 020 - 057 1089 45 90 + 00814 - 041 - 079 1102 45 89 + 00713 - 032 - 060 0814 45 88 - 01097 - 024 - 043 0545 45 87 - 05200 - 020 - 032 0361 45 86 - 08608 + 019 - 015 1003 45 85 — 10133 + 001 - 038 1138 45 84 - 07541 - 007 - 031 0692 45 83 - 04236 - 005 - 020 0427 44 105 - 06417 + 037 - 109 4288 44 104 + 02006 + 046 - 093 4088 44 103 - 00969 + 013 - 089 2995 44 102 - 01703 - 001 - 094 2713 44 101 - 00827 - 031 --109 2280 44 100 - 00053 - 031 - 095 1873 44 99 + 00579 - 021 - 074 1555 44 98 + 01041 - 003 - 052 1430 44 97 + 01309 + 002 - 048 1470 44 96 + 01307 + 006 - 045 1503 44 95 + 00656 + 004 - 038 1217 44 94 - 00380 + 004 - 037 1211 44 93 - 00428 - 002 - 044 1227 44 92 - 00256 + 015 - 020 1020 44 91 + 00434 - 025 - 059 1004 44 90 + 00854 - 028 - 061 0978 44 89 + 00398 - 013 - 044 0919 90 Quadrangle Baae. e1. Free-air anom. Boug. anom. Surf. e1. Lat. Long. in feet in mgals in mgals in feet 44 88 - 01697 - 021 - 038 0486 44 87 - 06064 - 007 - 025 0535 44 86 - 09628 + 021 - 010 0922 44 85 - 11594 + 008 — 017 0745 44 84 - 08536 - 001 - 024 0686 44 83 - 05061 - 001 - 022 0604 43 105 + 00877 + 028 - 136 4806 43 104 - 01806 + 022 - 143 4249 43 104 - 01806 + 002 - 143 4249 43 103 - 00217 + 023 - 108 3832 43 102 - 00709 + 026 - 091 3419 43 101 - 00830 + 018 - 079 2831 43 100 - 00411 + 015 - 065 2349 43 99 - 00291 + 002 - 061 1860 43 98 - 00192 + 012 - 042 1572 43 97 - 00134 + 022 - 022 1286 43 96 - 00611 - 008 - 054 1362 43 95 - 01078 + 002 - 039 1191 43 94 - 01131 + 034 - 004 1125 43 93 - 01103 - 021 - 055 0991 43 92 - 01360 + 008 - 025 0974 43 91 - 01399 - 005 - 036 0896 43 90 - 00958 - 006 - 036 0883 43 89 - 01958 - 006 - 035 0860 43 88 - 03198 - 018 — 034 0472 43 87 - 05406 + 001 - 016 0495 43 86 - 06663 + 005 - 023 0823 43 85 - 07272 - 002 - 033 0915 43 84 - 05319 + 003 - 025 0820 43 83 - 03583 - 007 - 028 0610 42 105 - 04013 - 002 - 184 5348 42 104 - 03572 - 009 - 161 4452 42 103 - 02173 + 004 - 128 3875 91 Quadrangle Baee. el. Free-air anom. Boug. anon. Surf. el. Lat. Long. in feet in mgals in mgals in feet 42 102 - 01228 + 014 - 104 3451 42 101 - 00922 + 012 - 089 2963 42 100 - 01178 + 016 - 068 2467 42 99 - 01406 + 006 - 062 2005 42 98 - 01027 + 019 - 037 1640 42 97 - 00881 + 011 - 032 1273 42 96 - 01667 - 004 - 046 1230 42 95 - 02130 + 015 - 026 1214 42 94 - 01805 - 024 - 056 0935 42 93 - 01653 - 016 - 045 0843 42 92 - 02070 - 015 - 040 0735 42 91 — 02773 - 008 - 032 0699 42 90 - 03356 - 006 -030 0712 42 89 - 03547 + 001 - 022 0682 42 88 - 04080 - 004 - 026 0640 42 87 - 03548 - 010 - 035 0732 42 86 - 03250 + 009 - 021 0883 42 85 - 03617 - 008 - 036 0833 42 84 - 02556 - 028 - 051 0666 42 83 - 03244 - 018 - 041 0669 42 82 - 05270 - 002 - 030 0833 41 105 - 05234 - 016 - 186 4990 41 104 - 03872 + 005 - 148 4475 41 103 - 02569 + 006 - 127 3911 41 102 - 01798 - 004 - 114 2338 41 101 - 01094 - 001 - 093 2710 41 100 - 01655 000 - 078 2300 41 99 - 02173 + 004 - 062 1932 41 98 - 01589 - 013 - 067 1585 41 97 - 00694 + 028 - 016 1293 41 96 - 02463 - 022 - 057 1033 41 95 - 02891 - 021 - 058 1083 41 94 - 02420 - 009 - 042 0974 92 Quadrangle Base. el. Free-air anom. Boug. anom. Surf. el. Lat. Long. in feet in mgals in mgals in feet 41 93 - 02013 - 016 - 045 0863 41 92 - 02313 - 020 - 042 0646 41 91 - 03209 - 019 - 040 0630 41 90 - 04577 - 007 - 029 0633 41 89 - 05255 + 011 - 014 0728 41 88 - 05123 - 006 - 029 0686 41 87 - 03771 - 020 - 045 0774 41 86 - 02814 - 017 - 047 0873 41 85 - 02425 + 003 - 028 0919 41 84 ‘ - 02258 - 014 - 047 0981 41 83 - 04127 000 - 037 1079 41 82 - 07666 + 006 - 030 1047 40 105 - 04867 + 005 - 199 5968 40 104 - 03138 + 008 - 167 5118 40 103 - 02139 - 005 - 149 4219 40 102 - 02177 - 009 - 126 3419 40 101 ' 02050 .- 007 - 100 2736 40 100 - 01859 - 001 - 074 2126 40 99 - 02758 000 - 057 1683 40 98 - 02266 + 017 - 031 1417 40 97 - 00900 - 020 - 063 1253 40 96 - 02278 - 020 - 056 1050 40 95 - 02013 - 029 - 060 0922 40 94 - 01880 - 021 - 047 0768 40 93 - 01581 - 009 - 036 0781 40 92 - 01377 - 013 - 036 0676 40 91 - 02792 - 011 - 031 0587 40 90 - 05300 - 005 - 026 0620 40 89 - 07453 - 006 - 028 0640 40 88 - 07173 - 005 - 026 0604 40 87 - 05386 - 005 - 031 0774 40 86 - 03570 - 020 - 051 0899 40 85 - 02722 - 009 - 039 0886 93 Quadrangle Base. el. Free-air anom. Boug. anom. Surf. e1. Lat. Long. in feet in mgals in mgals in feet 40 84 - 02756 + 011 - 022 0961 40 83 - 05245 - 019 - 048 0860 40 82 - 09827 - 021 - 051 0886 39 105 - 01047 - 007 - 199 5633 39 104 - 02567 - 013 - 171 4646 39 103 - 02138 - 003 - 141 4055 39 102 - 02719 - 004 - 119 3383 39 101 - 03034 - 005 - 098 2723 39 100 - 02831 - 004 - 078 2175 39 99 - 02259 + 011 - 050 1775 39 98 - 02600 - 004 - 052 1417 39 97 - 01509 - 021 - 067 1342 39 96 - 01631 - 022 - 058 1060 39 95 - 01170 - 024 - 055 0919 39 94 - 00900 - 024 - 052 0817 39 93 - 00952 - 023 - 050 0784 39 92 - 00903 - 009 - 035 0764 39 91 - 01828 - 001 - 021 0577 39 99 - 06344 + 001 - 016 0486 39 89 - 11078 - 004 - 020 0472 39 88 - 10145 - 003 - 019 0459 39 87 - 06971 - 016 - 038 0636 39 86 - 04692 - 019 - 042 0689 39 85 - 03027 - 026 - 053 0801 39 84 — 03397 - 002 - 032 0879 39 83 - 07291 - 011 - 037 0774 39 82 - 13409 - 017 - 052 1020 38 105 + 01931 + 007 - 205 6204 38 104 + 01452 + 006 - 164 4974 38 103 - 01907 000 - 141 4134 38 102 - 04088 - 015 - 125 3212 38 101 - 04870 - 010 - 102 2687 38 100 - 04630 - 003 - 077 2178 94 Quadrangle Base. e1. Free-air anon. Boug. anon. Surf. el. Lat. Long. in feet in mgals in mgals in feet 38 99 - 03975 - 007 - 065 1693 38 98 - 03684 - 004 - 049 1322 38 97 - 02388 - 062 1150 38 96 - 01411 - 016 - 048 0932 38 95 - 00791 - 013 - 045 0928 38 94 - 00703 - 002 - 039 1089 38 93 - 00847 + 002 - 039 1204 38 92 - 00333 - 007 - 044 1083 38 91 - 00305 + 008 - 020 0807 38 90 - 06378 + 017 + 001 0546 38 89 - 11134 + 011 - 004 0443 38 88 - 10753 + 003 - 012 0443 38 87 - 07909 - 005 - 025 0584 38 86 - 05238 - 021 - 051 0876 38 85 - 05859 + 003 - 031 1004 38 84 - 08536 - 003 - 041 ' 1115 38 83 - 12641 - 002 - 048 1358 38 82 - 19581 + 015 - 058 2149 37 105 + 02459 + 034 - 193 6653 37 104 + 01997 + 022 - 165 5472 37 103 - 02853 - 008 - 148 4091 37 102 - 05191 - 020 - 130 3222 37 101 - 09248 - 021 - 112 2661 37 100 - 10828 - 020 - 090 2054 37 99 - 08795 - 006 - 054 1407 37 98 - 06372 - 009 - 046 1079 37 97 - 03058 + 003 - 029 0928 37 96 - 01519 - 008 - 033 0735 37 ' 95 - 00441 + 009 - 028 1093 37 94 - 00694 + 008 - 035 1273 3? 93 - 01622 + 008 - 023 0912 37 92 - 02614 - 007 - 032 0728 37 91 - 03581 - 003 - 016 0371 95 Quadrangle Base. el. Free-air anon. Boug. anon. Surf. e1. Lat. Long. in feet in ngale 1n mgals in feet 37 90 - 05466 + 005 - 005 0302 37 89 - 08373 + 010 - 005 0440 37 88 - 07577 - 001 - 021 0577 37 87 - 05281 + 003 - 019 0653 37 86 - 05178 - 005 - 038 0961 37 85 - 07345 + 026 - 020 1342 37 84 - 11103 - 007 - 056 1450 37 83 - 13000 - 002 - 071 2018 37 82 + 02638 + 001 - 089 2638 36 105 + 01613 + 005 - 177 1331 36 104 + 00092 - 001 - 149 4334 36 103 - 03266 - 004 - 134 3809 36 102 - 02959 - 011 - 123 3297 36 101 - 08397 - 085 2750 36 100 - 18775 - 010 - 077 1955 36 99 - 20128 + 003 - 048 1499 36 98 - 10825 + 001 - 038 1135 36 97 - 05753 - 002 - 032 0879 36 96 - 05884 - 020 - 043 0666 36 95 - 08422 - 020 - 049 0843 36 94 - 11247 - 040 1000 36 93 - 08562 + 016 - 012 0810 36 92 - 06975 + 008 - 004 0358 36 91 - 06727 + 004 - 004 0236 36 90 - 06305 + 008 - 004 0338 36 89 - 05941 + 003 - 013 0479 36 88 - 05253 + 008 - 018 0751 36 87 - 05009 + 013 - 016 0856 36 86 - 09175 + 009 - 037 1348 36 85 - 08801 - 014 - 055 1207 36 84 + 02546 + 023 - 064 2546 36 83 + 02402 + 004 - 078 2402 36 82 + 01070 - 020 - 056 1070 HICHIGRN STRTE UNIV. LIBRQRIES I1 llll 8420 312931031