Structure of the Bane Dome, Giles County, Virginia

Home , Dome (geology)

VIRGINIA DIVISION OF MINERAL RESOURCES

PUBLICATION 117

STRUCTURE OF THE BANE DOME, GILES COUNTY, VIRGINIA . A GRAVITY TEST

COMMONWEALTH OF VIRGINIA

DEPARTMENT OF MINES, MINERALS AND ENERGY DIVISION OF MINERAL RESOURCES Roberr C. Milici, State Geologist

CHARLOTTES VILLE, VIRGINIA t99l VIRGINIA DIVISION OF MINERAL RESOURCES

PUBLICATION 117

STRUCTURE OF THE BANE DOME, GILES COUNTY, VIRGINIA . A GRAVITY TEST

COMMONWEALTH OF VIRGINIA

DEPARTMENT OF MINES, MINERALS AND ENERGY DIVISION OF MINERAL RESOURCES Robert C. Milici, State Geologist

CHARLOTTESVILLE, VIRGIMA I99l DEPARTMENT OF MINES, MINERALS AND ENERGY RICHMOND, VIRGINIA O. Gene Dishner, Director

Portions of this pubtcation may be quoted if credit is given to the Division of Mineral Resources. CONTENT Page 1 /1^^l^--. I I J 6 8 10 l3 References cited...... 13 15

ILLUSTRATIONS

Generalized geologic map of Giles County, Virginia 2 Geologic cross sections along profiles indicatedin Figure I ...... 4 5 1 9 profiles Gravity and two-dimensional density model corresponding o the geologic cross section from Gresko...... 10 profiles Gravity and two-dimensional density model corresponding to the geologic cross section from Woodward.. 10 Grayity profiles and two-dimensional density model corresponOing to the geolo[ic cross section from ll Graph-s showing the maximum value ofp, the minimum value of p, and the range of p for values of T between 0 11 Average density contoured at an interval of 0.005 gm/cm3 t2 STRUCTURE OF THE BANE DOME, GILES COUNTY, VIRGINH . A GRAVITY TEST

Michael J. Moses and Edwin S. Robinson

ABSTRACT tectonic interpretation asserts that the thrust faulting origi- nates along a major decollement in the relatively incornpetent The Bouguer gravity field, detemrined from 395 meas- Cambrian-age Rome shale. urements in Giles County, Virginia, exhibits a broad positive However, thelocation of theBanedome within theGiles anomaly approximately 12 mgal in amplitude situated over County seismic zone raises the questions about deeper base- the Bane dome, and several smaller anomalies of a few ment tectonics. Seismicity within Giles County is believed to milligals amplitude. For the most part tley are produced by occur on reactivated normal faults which originally were the distribution ofrelatively high density carbonaterocks and formed by rifting during late ltecambrian time (Bollinger lower density clastic rocks within the dome. and others, 1985). They suggestthatthesefaults lie within the These anomalies can be explained by two contrasting basement beneattr the Valley and Ridge province, and due to interprelations of the structure of the Bane dome. One intei- reactivation under the curent compressive stress regime, pretation indicates the relative abundance of high density now appear to be undergoing strike-slip displacement within carbonate rocks transported within the domeby overthrusting. the Giles County seismic zone. Because gravity the anomalies can be entirely explained by Reflection seismic surveys in this area (Figure 1), while sources confined to the Nanows tlrust sheet, this interpreta- clarifying the concept of thin-skinned tectonics, have not tion precludes the existence of significant lateral density completely resolved questions concerning the subsurface contrasts associated with deeper structure beneath the d6col- disfibution of rock units within the dome; nor have these lement zone in ttre Rome Formation. surveys fully clarified the nature and extent of basement The contrasting interpretation indicates a smaller pro- faulting. Because of karst t€rrane and the abundance of high portion of carbonate rocks and a larger proportion of lower velocity carbonate rocks, conventional P-wave seismic re- density clastic rocks cut by imbricate faults in ttre core of the flection surveys give somewhat unclear results. Although dome. This int€rpretation also includes high angle fauls with shear wave seismic reflections (Gresko, 1985) appear to give associated lateral density contrasts in the deeperrocks under- better resolution, they were recorded only along tluee short lying the ddcollement. Because sources within the Nanows survey lines on the northwest flank of the Bane tlome which thrust sheet are insufficient to completely account for the do not cross the axis of the structure. gravity anomalies, the density contrasts associated with deeper In an effort tlr contribute to the resolution of questions structure are required. about the structure of the Bane dome, a gravity survey was The Bouguer gravity _ field can be separated into regional undertaken. A total of 395 gravity stations were established and residual parts. The regional field is attributed to changes in four 7.5-minute quadrangles, Narrows, Peartr$.urg, White in crustal thickness inferred independently from the seismic Gate, and Slaffordsville, which include theregion of the Bane mqsurements. The remaining residual field can be ex- dome. Interpretation of gravity anomalies involves the plained in terms of anomaly sources within the upper l0 km comparison of Bouguer gravity measured along selected of the crust. profiles with gravity profiles calculated for different two-dimensional models. These trvo-dimensional models of density disribution are based upon three geologic cross sections INTRODUCTION that display different interpreiations of the structure of the dome. The discussion of gravity anomalies also addresses The Bane dome is located in Giles County, Virginia in questions concerning separation ofregional and local anoma- th€ Valley and Ridge province of the souttrem Appalachian lies, and the density values used in gravity reduction. Mountains as well as within the Giles County seismic zone. It is a doubly plunging anticline situated in a folded and thrust-faulted rerrane (Figure 1). This clearli defined sfuc- GEOLOGY ture invites speculation. For more than thiee decades the Bane dome has been a focal point in discussions about the The Valley and Ridge province of the sourhern Appala- rylgtive importance of low angle thrust faulting wittrin the chians is characterized by folded and thrust faulted sedimen- Paleozoicsedimeltary sequence compared with high angle tary rocks. The structures within the area tend N60o-65oE basement faulting in the tectonic development of the Jouthern (Schultz and others, 1986), which is the dominant rend in the Appalachians (Rodgers, 1949; Cooper,l9&;perry and oth- southern Appalachians. The Bane dome (Figure l) is a en, 1979). Interpretations of recently acquired seismic doubly plunging anticline located within the Narrows thrust reflection data @dsall, 1974; Gresko, 1985t state ttrar rhe sheet, which is bounded on the surface by ttre Narrows fault structures in the areaare related mainly to tlrrust faulting with on the northwestandby ttre Salwille faulton the southeast. It littleorno involvementof basementrocks. This thin-skinned is bordered to the north by the Wolf Creek and the Butt

Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 VIRGINIA DIVISION OF MINERAL RESOURCES

@ u) -:< \td O a rf 0) $ c .(d z,a c o N z\ () t-\ '- v) 2\ aO f, "'"1 ao ,la (6Y ts '- /n () ts \J- CE OY () = rl ro av t- c.) ra o o () @

a a () X o

() bo

o q)

a 7_\ w*,'r$ h

L) ol (aQ

vii

-9 q) q o tr_ rO d3 X,q o aoy O fo EOQ)^ N N.: |o o -L C)O cH @ .Yrr

Jo N N o F ii< rO PUBLICATION Ii7

Mountain synclines, and along strike to the southwest is the include the Late Ordovician Reedsville, Trenton, Eggleston, Kimberling basin. and Moccasin Formations which consist of limestoni, dolo_ Knowledge of the stratigraphic units in the region of a mite, and shale units. Average density of this sequence is ap_ gravity survey is of central importance in the interprJtadon of proximately 2 .7 gm/cm3-. Younger sedimentary-rocks rang_ gravity anomalies. Variations of Bouguer gravity are related ing in age from Iate Ordovician ro Mississippian do n6t toJhe contrastingdensities of therockunits-present, and !o the appear to exist in the Bane dome, although ttrey are exposed relative abundance of these units. Among ttre rnany sources in the bordering synclines. of information about the stratigraphy in this part of tfre Vattey Several geologic cross sections have been published that and Ridge province ar_e Burrs (1942), Cooper (1961), ani present different interpretations of the structure of the Bane Schultzand others (1986). For purpose. of nt stoOy,'ro"[ dome. For purposes of this study, the three recently compiled density informarion dererm ined 6y Edsall (197 4) and iiolich cross sections in Figure 2 have been selected for further (1974) was used. These rock density valuei were determined analysis. Gresko (1985) modified an earlier cross section of from laboratory measurements on representative wet and dry Perry andothers (1979) toobtain abalancedcross section that samples of rock units exposed in the area. The values incorporates features evident on SH-wave seismic reflection expressed_as grams per cubic centimeter are noted in paren_ profiles recordedover the northwest flank theses following the formations of the Bane dome. named in the discussion He showed that the gravity variation below. over the model was consistent wittr gravity anomaly patterns presented Oldest by Sears in the stratigraphic sequence ofpaleozoic rocks is and Robinson (1971). a sedimentary package dating from Early Cambrian time, The balanced cross section compiled by N. B. Woodward which consists of rhe Unic_oi Sandstone (2'.67),the Hampton and D. R. Gray flMoodward, 1985) is also a modificarion of Shale (2.71), the Erwin Sandsrone (2.59), anO ttre StraOy the earlier cross section of perry and others, which was Dolomire (2.84). Based upon density information in Edsall constrained by ttre gravity dara of Sears and Robinson (1971) the average densiry of this pactage is approximately p. t12!4) and by information from the F. Srader No. 1 test well. 213 gm/cm3 (Gresko, 1985). Thii well was driled to a depth of 440 met€rs at the crest of the Next in the stratigraphic sequence is the Lower _ to Middle Bane dome by the California Co. in 1948. Conodonts from Cambrian Rome Formation whiih consists of mudstone with near the bottom of the well indicate ttrat Rome some minor interbedded shale is in dolomite. Within the relativelv tirust contact with the underlying younger incompetent Rome Formation Knox Group is the ddcolle.ent rone *hei! (Perry and others, 1979). most of the thrust faults in the southern Appalachians appear The third cross section was prepared by M. J. to.originare @erry andothers, 1979). Witirin ttre study area Bartholomew (personal communication, 1987). He modified this formation is fault bounded at tle base (Gresto, i9g5). the earlier cross section of Butts (1933) by ext€nding the rock The dolomitebeds have adensity close io 2.gi gm/cmr, whiie units to grea[er depths in a manner consistent with decoile_ the much more abundant shale facies have an aierage density ment tectonics associated with the incompetent of 2.67 gm/cm3. Rome For- mation. Overlying the Rome Formation is ttre Middle Cambrian The three cross sections in Figure 2 are similar to the Formation, which consisrs of massively !{9nak9r bedded extent that they represent the Bane dome as a structure dolomite. This formarion has the highest density wittrin ttre confined to the Narrows ttrrust sheet. The lower boundary of study area, which is close to 2.g5 gm/cm3. Above it is the this thrust sheet is a zone of decollement in the Rome Nolichucky Forrnation containing dolomite and shale. Formation. These cross sections differ frorn one another in The Upper Cambrian-Lower Ordovician Knox _ Group is regard to folding and faulring within the Nanows thrust sheet. also primarily massively bedded dolomite. This light_^ to Forexample, Gresko ( I 985) included a section of Martinsbure dark-gray dolomite contains biostatigraphically useful cono_ and Moccasin rocks cut by imbricate ttrrusts in the core of th6 dont which were used by perry and ( qZ-9) othe.s f in reevalu- dome. This feature, which is based upon interpretation of th9 earlier inuerprerarion (Cooper, 1964) that lting the Rome seismic reflection profiles, does not appear in the other two Formation at the core of the Bane dome overlies Lower Cam_ cross sections. These latter cross sections display a larger brian Shady Dolomite. The interpretation of Cooper sug_ total volume of high density carbonate rocks than is shown-bv gested basement involvement in the thrust faulting, ho*"uei, Gresko. Furthermore, Gresko includes later structural features in discovery of conodonts in cuttings from a well drilled on the deeper basement rocks that are not seen in the other cross the crest of the dome indicates that the Rome Shale is ycfons. He presents seismic evidence of these high angle underlain by the Knox Group. This lauer interpretation is faults. The present study examines these differencesln rcrirs consistent with thin-skinned ttrrusting and folding unrelated of the gravity anomalies they would be expected tn produce. to local basement tectonics. The Knox Group is poorly exposed within a window at the crest of the dome-. It ij made up of the following units: the Copper Ridge Formation which is GRAVITY mostly dolomite (2.81); the Chepultepec Formarion con_ sisting of dolomite (2.83) and sandsione (i.72); the Kingsport Gravity measurements were taken at 395 sites between Dolomite (2.79); and the Mascot Dolomite (2'.gZ). The Knox February, 1984 andFebruary, 1986. These sites are indicated density of gO 9.jo:lp.hulg.average approximaiety i. gm/cm, in Figure 3. They were located at bench marks, (Kolich, 1974). landmarks where elevations are known to within I foot, or at locations The youngest rocks associated with the Bane dome where elevations can be interpolated to within l0 feet from VIRGINIA DIVISION OF MINERAL RESOURCES

NF tl BAN E DOME SF PF cbr SL Mqr, mcc,p Dch, b, ml

T4 F lO km o- trJ ogse ob ^ eC C oH or pcs L Ot. osz eC C x !-*1 olq \ ^ \-of',0 t"r1 oroq pcg ro km BANE DOME

T2 F L l-rla4

o KM t'I rlTzl ollrl 4 l- 5'L km lOkm

Figure 2. Geologic cross-sections along profilesindicatedinFigure 1 which cross the Bane dome. The top crosssection,following pt6fileC,ir.edriwnfromGresko(1985j. Themiddlecrosssection,followingprofileB,isredrawnfromWoodward(1985). The bottom cross section, following profile A, is redrawn from Bartholomew (personal communicatron, 1987). Symbols are as follows: M-Mississippian, gr-Gieenbrier Formation, mcc-MacCrady Formation, p-Price Formation; D-Devonian, b-Brallier Formation,ch-ChemungFormation,ml-MillboroFormation;S-Silurian;O-Ordovician,c-ChickamaugaFormation,m-Martinsburg Formation; OCk-CambroOrdovician Knox Group; C-Cambrian, bf-broken formation @ome, Elbrook, breccia), c-Conococh- eague Formation, e-Elbrook Formation, hk-Honaker Formation, r-Rome Formation; eC-C-Eocambrian-Cambrian rocks; pcg- prEcambrian Grenville rocks; earthquake hypocenters, o - >5km, . - <5km. SCF = St. Clair fault; NF = Narrows faulq SF = Saltville fault; PF = Pulaski fault. PUBLICATION 117

37"zz'30" r* **I**I ,**a**--j- ** +'** I * ** * x * x 1:;****.ri'***+ *** *.***+***t * ** ***1 **a*-F * ** * **** * :t ** ** 1* * ** :.--****+** + ** +* |* -i +* # t i + *- +* i * + **# **** *.#-* ** * *** * * **l* **** tr***o :l; * 37"r5'oo" ** *** +* ** **.* +o + ***t * Io*o {,.q4 ++ + * o f** .{.**+*+#**** + {*4* **T. Js# * i ***** -"i ftJ--$; **i ++++ {f ** o***f 'o I * o*** *o* ++ ** ** ++ox+ 37bz'3o" ** t** o Boo52'3o" 8o"45'oo" Bo"i7'3o"

Figure 3. Location of gravity observations: (0) benchmark, elevation accurate to better ttran I foot; (+) landmark, elevation accurate within t lfooq (*) site where elevation was interpolated from topographic contours. VIRGINIA DIVISION OF MINERAL RESOURCES

topographic contours. To distinguish between these sites of from a larger data set covering the entire state of virginia, varying elevation accuracy, different symbols are used to which was compiled in the School of Forestry and Wildlife indicate the station accuracy on the map. All gravity readings Resources atVirginia Polytechnic Institute and StateUniver- were made using Lacoste-Romberg gnvimeter No. G-612. sity (Blair Jones, personal communication, 1986). The ter- Gravity measurements were made at approximately 0.25 rain corrections were computed using the elevations within a mile intervals along roads across the Pearisburg, Narrows, 15,000 foot radius of each station. These values were tien Staffordsville, and White Gate 7.5-minute quadrangles ex- checked by computing the Hammer ( 1939) terrain correction ceptwhere difficultaccess on themountains made properele- to theJ ring for various stations. The results of this compari- vation control difficult. These mountain areas, including son gave generally good results (within .05 mgal). For 40 of Walker, Brushy, and SugarRun Mounlains, are primarily to the stations where the tenain was inadequately sampled by the south and west of the Bane dome. the elevation grid, the vertical line approximation method Gravimeter readings were converted to relative gravity produced unacceptable terrain correction values. In those values by using the instrument calibration table provided by cases where the terrain correction exceeded 5 mgal, an the manufacturer. Instrument drift was determined by alternative value determined by the Hammergraphical method occupying the Virginia Tech Gravity Base Station at Derring was used. The terrain corrections for the region range from Hall in Blacksburg, Virginia before and after each day's 0 to 4.88 mgal. The lower values are found in the southeast- survey. The instrument drift was found to be less than 0.1 em portion and the higher values in the central part of the mgal on most days and was removed by applying a time study area. The average terrain correction of 2.3 mgal shows dependent linear drift correction to each reading. Corrections the significance of terrain corrections in this area. for lunar-solar tidal forces were made using the equations of The accuracy of the Bouguer gavity values is largely I-ongman (1959) and a solid earth tidal gravity factor of 1. 16 dependent on the accuracy of the station elevations. As can (Robinson, 1974). be-seen from Equation (2), for a density of 2.67 gm/cm3, an The relative gravity values were converted to observed elevation uncerlainty of I foot produces a Bouguer gravity gravity using the Derring Hall base value of 979719.33 mgal. uncertainty of 0.06 mgal. On the basis of this and other This value was determined from ties to the National Geodetic sources oferror, the Bouguer gravity values at stations where Survey absolute gravity base station in Blacksburg, Virginia elevation is known within one foot are considered accurate to where falling mass measurements were made in July 1987 within + 0.1 mgal. At stations where elevations were infer- and May 1988. polated from contours tle Bouguer gravity precision is closer Gravity data were reduced using the standard formulas to 0.75 mgal. for free air gravity Ag,. (Robinson and Coruh, 1988): A Bouguer gravity map of the study area was prepared using the terrain corrected Bouguer gtavify values included the ABr. = B"u - (& - 0.09406h) (1) in Appendix A. These values were contoured using GCONTOUR routine available from the Statistical Analysis and complete Bouguer gravity Ag, S.obinson and Coruh, System (SAS) Institute, Cary, North Carolina. Theresults in 1988): Figure 4 show that Bouguer gravity varies in the range of -62 mgal to -82 mgal. Anomaly pattems indicate alignments - (2) trending approximately N60'E which is close to the principal ABs = Bob" - G, - 0.09406h + 0.01278ph TC) structural rend in the area. where goon is observed gravity (mgal), gt is normal gravity (mgal), h-is elevation (ft), p is density (gm/cm3), and TC is the tenain correction (mgal). The normal gravity was com- REGIONAL FIELD puted using the GRS-67 formula (lMoollard, 1979). The elevation, h, in feet was taken from the topographic map and The complete Bouguer gravity map indicates the super- a standard density, P = 2.67 gm/cm3, was assumed for position of a broad regional field and a residual field of local Bouguer plate corrections and for tenain conections, TC. anomalies. For purposes of this study the regional field is Two procedures were used to calculate terrain colrec- assumed to result from variation in thickness of the earth's tions necessitated by the rugged relief in the study area. For crust. The basis for this assumption is the seismic survey of some stations the well known but time consuming metlod of crustal thickness reported by James and others (1968). In- Hammer (1939) was used. More suitable for computer cluded in their study is a calculated gravity field that repre- calculations is the vertical line approximation method (S tovall sents the variation in gravity related to crustal thickness. and others, 1989) which involves representing the terrain by The Bane dome is situated on the periphery of the area a square grid of elevation values. The mass of tenain within surveyed by James and others (1968) in a region where their each square grid increment is projected to a vertical line at the calcuiated gravity field indicates a gradient ofapproximately center of that increment. A simple formula is then used to 0.3 mgal/km increasing in the direction of N75oW. Near tle calculate the gravitational attraction of each vertical line Bane dome this regional gradient couldbe closely expressed mass. Theresults are summed to obtain the terrain correction, by a plane polynomial of the form: TC. (3) Tenain corrections were calculated using a FORTRAN g,(x,Y)=Ax+BY+C program and elevations on a 0.33 km by 0.33 km grid covering the study area. These elevations were extracted where g is the regional gravity value at longitude x and PUBLICATION I17

37"22 '3 o " l/fi ,w V:i; D tue '67 \ -65 ru f \\ ql

37" 15' oo"

:&, .66 ,6

80"45'oo" go"s7'30"

Figure 4' Bouguer gravity 2-milligal contour ryp ylth interval. Bouguer gravity values used to prepare this map were computed using ttre standard density of 1.Sl gm/cm3.'- VIRGINIA DIVISION OF MINERAL RESOURCES

latitude y (degrees), and A, B, and C areconsfantcoefficients. values at all points within 1 km were projected to the profile Bouguer gravity values obtained in the present study lines and plotted at those locations. were used to determine the coefficients of a plane polyno- Theoretical variation of gravity over two-dimensional mial. This was done with a software package prepared at the models was calculatedby thewell known method of Talwani Statistical Analysis System (SAS) Institute in Cary, North and others (1959). The models were prepared from the Carolina. In a two step procedure Bouguer gravity values geologic cross sections byreading depths to contactsbetween were first interpolated at points on a square grid by means of rock units at0.25 km intervals along the Gresko (1985) and a bicubic spline interpolation. Then a plane polynomial the Woodward and Gray flMoodward, 1985) profiles, and at function was fitted to the grid values by the method of least 0.40 km intervals along the Bartholomew profile in Figure 2. squares. The following coefficients were found: Density values assigned to the two-dimensional model units A= -17.1434 are consistent with values cited in the earlier discussion of B = 4.8570 stratigaphy. These two-dimensional models and the corre- C = -1636.4214 sponding theoretical gravity profiles are shown in Figures 6, By substituting these values into Equation (3) an expression 7, and 8. for the regional gravity field is obtained. The gravity variation predicted by the Gresko model This regional field exhibits a gradient of approximately Figure 6) compares favorably with the observed residual 0.2 mgalTkm increasing in the direction of N76oW. This anomalies. The broad high of approximately 12 mgal is gradient is remarkably similar to that calculated independ- properly reproduced, and local anomalies ofthe same charac- ently by James and others ( 1968) from seismic measurements ter are evident. The average difference between the observed of crustal thickness. This excellent comparison is the basis and theoretical gravity profiles is -1.6 mgal, and the standard for concluding that tle plane regional gravity field obtained dcviation is 1.9 mgal. Because the actual anomaly sources are from gravity measurements over the Bane dome is produced of finite dimensions, it is not practical to attempt to reproduce for the most part by variation in crustal thickncss. all features by means of a two-dimensional model. The Other efforts to separate a regional field from the B ou guer results show that the main features and the gencral character gravity values included fitting a biquadratic polynomial and of the theoretical gravity profile are consistent with the calculating upward continuations. Patterns of gravity vari- observed profile. ation resulting from these calculations wers too complicated The interpretation of Woodward and Gray figure 7)lso to attribute simply to variation in crustal thickness. It was predicts a broad gavity high over the Bane dome. The concluded that these regional fields resulted from combina- avorage departure of the theoretical gravity profile from the tions of lhis source and upper cruslal anomaly sources. observed residual profile is -1.5 mgal, and thc standard deviation is 2.1 mgal. The largest departure is over the crest of the dome where the theoretical gravity is between 2 and3 UPPER CRUSTAL ANOMALY SOURCES mgal higher than the residual gravity field. Othcrwise, the theoretical gravity profile reproduces the main features of the The principal purpose of this study is to examine gravity residual gravity profile. anomalies associated with the Bane dome. These anomalies The gravity prohle over the Bartholomew model (Figure canbepresentedmoreclearly by subtracting from the Bouguer 8) is scen to be consistent witJr the principal feafures of fte gravity field figure 4) the variation of gravity related to observed residual gravity profile. The average departure is - crustal thickness. The result is the residual gravity anomaly 0.8 mgal, and the standard deviation is 2.2mgal. The largest map in Figure 5. departure occurs near the center of the profile where a local The most prominent feature of the residual field is the high anomaly is not reproduced by the gravity values over the broad gravity high situated directly over the Bane dome, two-dirnensional model. This local high perhaps could be ar which has arelative amplitude of approximately 12 mgal. In tributed to an anomaly source of finite dimensions which is addition, there are several local anomalies that can be attrib- not included on the more general cross section. uted mainly to ttre distribution of high density carboqate The three models used in this study produce similar rocks of the Knox Group andtheHonakerFormation, andthe patterns of gravity variation. Despite the similarities in the contrasting lower density clastic rocks within the dome. The general gravity pattems over the models tlere are some contacts of these rock units, as mapped by Schultz and others important differences between the models. The Gresko (1986) are included in Figure 5 for qualiradve corelation model shows imbricate thrusts of low density Middle Ordo- with gravity anomalies. The geologic cross sections of Figure vician rocks at the core of the dome while the ol.her two 2 show different interpretations of the subsurface distribution models show more high density dolomite at the core. The of these rocks. These geologic interpretations are tested by other imporlant difference between the Gresko model and the comparing residual gravity anomalies observed along these other models is that the Gresko model is the only one to profiles with the theoretical variation of gravity calculated for include structure below the ddcollement in the Rome Forma- two-dimensional models ttrat conform to these three cross tion. The similarities in the overall character of the gravity sections. variations over the three models indicate that low density The residual gravity variation over the geologic cross rock in the core of the dome coupled with deep basement sections is shown in Figures 6,1 , and 8. Gravity values at structure can produce the same overall gravity variations as contour intersections with the profile lines in Figure 5 were high density rock in the core and no basement structure. The plotted and connected by dashed lines. Also, residual gravity results in Figures 6,'7,and 8 show that these two contrasting PUBLICATION 117

37"22'30" }Y J"1+4a

37 " 15' oo"

+4 +2 o' r'1.

3T.or'ro'iV)- 80" 52'30"

Residual ligure 5. gravil.y map with 2-milligal contour interval. Residual gravity values were obtained by subtracting from Bouguer gravity values the conesponding regional gravity values calculated using Equation 3. Shading indicates the surface exposure of the Knox Group according to Schultz and others (1996). 10 VIRGINIA DIVISION OF MINERAL RESOURCES

Ito o,{ to 3. g I 5 5 do J o

E-. E -o u q A -ro E -lo 2 H 2 o a o 2-z 6 g Y -r.l I -.] Fo-6 t- lrJ (L -ej

O-6 1 H -Ej -lo 40 o1o203040 (KM) DISTANCE (KM) DISTAT.ICE

Figure 6. Gravity profiles and a two-dimensional density Figure 7. Gravity profiles and a trvo-dimensional density model corresponding to the geologic cross section from model corresponding to the geologic cross section from Gresko (1985) in Figure 2. The gravity profile shown by the Woodward (1985) in Figure 2. The gravity profile shown by solid line connects residual gavity values at intersections of the solid line connects residual gravity values at intersections contours and line C in Figure 5. The dashed line indicates the of contours and line B in Figwe 5. The dashed line indicates theoretical variation of gravity over the twodimensional the theoretical variation of gravity over thetwo-dimensional density model. Points plotted along the profile represenl density model. Points plotted along the profile represent residual gravity values projected from observation sites within residual gravity values projected from observation sites within 1 km of line C. Numbers on the density model refer to the 1 km of line B. Numbers on the density model refer to the following density values expressed in gm/cm3: | - 2.67;2 - following density values expressed in gmlcm3: | - 2.67:2 - - 2.7 - - 6 - 2.7 6; 7 - 2.85; 8 - 2.78; 9 2.7 0; 3 - 2.80; 4 - 2.85 ; 5 - 2.67 : 6 - 2.80; 7 - 2.7 3; 8 - 2.7 7 . The 2.7 A: 3 3; 4 2.70; 5 2.80; elements of the model do not necessarily conespond to -2.67. The elements of the model do not necessarily corre- specific sratigraphic units. spond to specific sratigraphic unis.

interpretations are consistent with the gravity constraints. arbirary depth T below sea level. Insofar as the average In this study a regional field was separated on the basis density values are consistent with the densities of rocks of independent knowledge of crustal thickness. The residual known to exist in that zone, it can be concluded that gravity gravity fieldremaining after subraction of thatregional field anomalies can be explained by sources within that zone. from the Bouguer gavity field can be explained in terms of A modification of Equation (2) provides the basis for density conrasts within the upper 10 km of the crusl There estimating average density values. In tlnt equation the is no compelling evidence of anomaly sources deeper in the purpose of the term 0.01278ph is to account for the gravita- crust. tional attraction of mass extending from sea level up to the height of ttre gravity station (Robinson and Coruh, 1988). To account for the gravitational atracdon of mass in a zone AVERAGE DENSITY INVERSION extending from a depth T below sea level to height h of the land surface at the gravity station this term can be replaced by Resul8 presented in the previous sections indicate that two t€rms labelled A and B in the following expression (4): Bouguer gravity variation in the region of the Bane dome can AB be explained in t€rms of crustAl thiclness and structure within A8g = 8or. - [e, - 0.09406tr - 0.01278p0T + 0.01278FCI +h) - TC] the upper 10 km of the crugl This conclusion can be tested by means of a density invdrsion analysis. The idea is to use where pn is ttre density assumed for the outer shell of the the gravity measurements o obain estimates of average normal ellipsoid, and p- is the average density in the zone of density in a zone extending from the land surfacetoan the earth reaching from depth T to height h. The purpose of PUBLICATION I17 1l

To the extent that term B in Equation (4) adequately expresses the gravitational attraction of the mass beneath a 9,o gravity station the Bouguer gravity at that station should be g3 the same as the Bouguer gravity at all other stations, if all gravity anomaly sources lie above the depth T. If the depth 5 Tis chosen so thatthezoneabove itdoes notcontain regional do field sources,butdoes contain all otheranomaly sources, then the Bouguer gravity computed for any station using Equation g* (4) as the regional gravity g at that station. q should be the same -'o Forpurposes of this study a standarddensity of p0=2.75 H Emlcms was chosen for the zone above the depth T. Then, 2 average density (p) was calculated for each gravity station using valuesof Tequal to0,2,4,6,8,and 10krn inEquation p o (6). In each calculation the elevation, free air gravity, and 2-z terrain correction at the station were used, and the regional Y gravity g was substituted for Bouguer gravity Ago. Maxi- I-4 mum values of p, minimum values of p, and the ranges of p c-6 are plotted with the corresponding values of T in Figure 9. o_Eul Average density values corresponding o T = 10 km are contoured in Figure 10. -10

ro 20 3. I o.8 DISTANCE (KM} o C) 6 2.s E o.e Figure 8. Gravity profiles and a trrodimensional density o 3 model corresponding the geologic cross section from to E UJ Bartholomew (pers. comm.) in Figure 2. The gnvity profile orl (9 o,4 shown by the solid line connects residual gravity values at > 27 z inlersections of contours and line in Figure The F E A 5. dashed a line indicates the theoretical variation of gravity over the two- z. dimensional density model. Points plotted along the profile UJ F o 2.5 6 0.2 represent residual gravity values projected from observation z sites within I km of line A. Numbers on the density model lrJ refer to the following density values expressed in gm/cm3: I o - 2.67 : 2 - 2.7 0; 3 - 2.80; 4 - 2.85; 5 - 2.67 . The elements of the modeldo notnecessmily conespondto specific statign- 468 "-o z 468 phic units. T(km) T( km) term A is to remove the gravitational attraction of the outer Figure 9. Graphs showing the maximum value of p, the mini- shell of the normal ellipsoid, which extends from its surface mum value of p, and the range of p for values of T between to depth T. The purpose of term B is to replace this mass with 0 and 10 km. Values calculated from the twodimensional a plate reaching from depth T below the ellipsoid surface to density models are plotted using ttre symbols (+) for Figure height h above it. The density of this latter plate can be found 6, ( A) for Figure 7, and (o) for Figure 8. be rearranging Equation (4) to obain:

_ ABg - g;+&-0.09406h -0.01278poT-TC At points along the profiles in Figures 6, 7 , and 8 average density was calculated from the density variation along 0.01278(T + h) vertical lines extending down from the land surface o depttts below sea level of 6 km for ttre Woodward and Gray model Then by substitution from Equation (1) the expression be- and the Bartholomew model, and 10 km for the Gresko comes: model. The maximum and minimum values of average density, and the average density ranges for these profiles are Ag, - Ago - 0.01278p0f - TC plotted in Figure 9. -p= (6) The average density values and density ranges deter- 0.01278(I + h) mined from gravity station dataae seen to be comparable with the similar values and ranges found from the two- dimensional models. These comparisons confirm theconclu- t2 VIRGINIA DIVISION OF MINERAL RESOTIRCES

3 T"z2'3 o "

?1?

;1!. /ak Lt.,

37" l5'oo"

37 " 07 '30 ' -v+ 8 o"sz' 30 " g

Figure 10. Averagc dens ity contoured at an interval of 0.005 gmlcc, 10 km. PUBLICATION 117 13

sion that the residual gravity field @igure 5) can be attributed Geological Guidebook 1, i87 p. to anomaly sources within the upper 10 km of the crust. Figure 10 shows an estimate of the variation in average Cooper, B. N., 1964, Relation of stratigraphy to structure in density in this zone that is associated with the residual gravity the Southern Appalachians, in Tectonics of the Souttrern field. Appalachians, W. D. Lowry, editor: Virginia Polytechnic In- stitute, Departrnent of Geological Sciences, Memoir 1, Blacksburg, Virginia, p. 81-114. CONCLUSION Edsall, R. W., 1974, A seismic reflection study over the Bane A broad gravity anomaly approximately 12 mgal in anticline in Giles County, Virginia [MS Thesis]: Blacksburg, amplitude is associated with the Bane dome. Superposed on Virginia Polytechnic Institute and State University, 111 p. this anomaly are several smaller anomalies with amplitudes of a few milligals. For the most part they are produced by the Gresko, M. J., 1985, Analysis and interpretation of compres- disribution of relatively high density carbonate rocks and sional @-Wave) and shear (SH-Wave) reflection seismic and lower density clastic rocks within the dome. geologic data over the Bane dome, Giles County, Virginia Two contrasting interpretations of the structure of the [Ph.D. Dissert.] : Blacksburg, Virginia Polytechnic Institute Bane dome are consistent with constraints imposed by gav- and State University, 74 p. ity measurements. One interpretation, represented by the geologic cross sections of Woodward and Gray (Woodward, Hammer, S., 1939, Terrain corrections for gravimeter sta- 1985) and Bartholomew (personal communication, 1987) tions: Geophysics, v. 4, p.184-194. indicates relative abundancc of high density carbonate rocks within the dome, and situated entirely in the Narrows thrust. James, D.8., Smith, T. J., and Steinhart, J. S., 1968, Crustal sheet. Because the gravity anomalies can be entirely ex- structure of the Middle Atlantic States: Joumal Geophysical plained by sources confined [o the Narrows thrust sheet, this Research, v. 73, n. 6,p.1983-2007. interprctation precludes the existence of significant lateral dcnsity contrasts associated with deeper structure beneath the Kolich, T. M., 1974, Seismicreflection and refraction studies dccollement zone in the Rome Formation. in the folded Valley and Ridge province at Price Mountain, The contlasting interpretation represented by the cross Montgomery County, Virginia IMS Thesis]: Blacksburg, section of Gresko (1985), indicates a smaller proportion of Virginia Polytechnic Institute and State University, 139 p. carbonate rocks in the Narrows thrust shect. and more lower density clastic rocks. This interpretation includes high angle Longman, I. M., 1959, Formulas for computing the tidal fauls with associated lateral density conlrasts in the deeper accelerations due to the moon and the sun: Journal Geo- rocks underlying the decollement. Because sources within physical Research, v. &, n. 12, p. 2351-2355. l.he Narrows thrust sheet are insufficient to completely account for the gravity anomalies, the density contrasts associ- Mullenax, A. C., 1981, Deformational features within the ated with decper structure are required. Martinsburg Formation in the St. Clair and Narrows thrust The Bouguer gravity field can bc separated into regional sheets, Giles County, Virginia IMS Thesis]: Blacksburg, and residual parts. The regional field can be attributed !o Virginia Polytechnic Institute and State University, 130 p. change in crustal thickness known independently from the seismic measurements of James and others (1968). The Perry, W. G., Harris, A. G., and Harris, L. D., 1979, Cono- remaining residual field can be explained in terms of anomaly dont-based reinterpre[ation of Bane dome - structural reevalu- sources within the upper 10 km of the crust. ation of Allegheny frontal zone: Bulletin American Associa- tion Petroleum Geologists, v. 63, p.6a7-654.

REFERENCES SITED Robinson, E. S., 1974, A reconnaisance of tidal gravity in Soutleastem United S tates: Journal Geophysical Research, Bollinger, G. A.,Teague, A. G., Munsey,J. W., andJohnston, v. 79, n, 29, p. 4418-4424. A. C., 1985, Focal mechanism analyses for Virginia and Eastern Tennessee earthquakes ( 1978- 1984): NUREGiCR- Robinson, E. S., and Coruh, C., 1988, Basic exploration 4288, p. 83. geophysics: New York, John Wiley and Sons, 541 p.

Butts, C., 1933, Geologic map of the Appalachian Valley of Rodgers, J., 1949, Evolution of thought on sFucture of Virginia with Explanatory Text Virginia Geological Sur- Middle and Southem Appalachians: BulletinAmerican As- vey Bulletin 42,56 p. sociation Petroleum Geologists, v. 33, p. l&3-1654.

Butts, C., 1942, Geology of the Appalachian Valley in Schultz, A. P., Stanley, C. B., Gathright, T. M., II, Rader, E. Virginia: Virginia Geological Survey Bulletin 52,568p. K., Bartholomew, M. J.,Lewis, S.8., andEvans, N. H., 1986, Geologic map of Giles County, Virginia: Virginia Division Cooper, B. N., 1961, Grand Appalachian Excursion: Vir- of Mineral Resources Publication 69. ginia Polytechnic Engineering Extension Station Series L4 VIRGINIA DIVISION OF MINERAL RESOURCES

Sears, C. E. andRobinson, E. S., 1971, Relations of Bousuer gravity anomalies to geological structure in the New R'iver disllict of Virginia: Geological Society of America Bulletin, v. 82, p. 2631-2638.

Stovall, R. L., Robinson, E. S., and Bartholomew, M. J., 1988., Gravity anomalies and geology in the Blue Ridge province near Floyd, Virginia, in N.H. Evans, editor, Con- tributions o Virginia Geology - VI: Virginia Division of Mineral Resources Publication 88, p. 6l-82.

Talwani, M., Worzel, J. L., and Landisman, M., 1959, Rapid gravity computations for two.dimensional bodies with appli- cation to the Mendocino submarine fracture zone: Journal Geophysical Research, v. &, p. 49-59.

Woodward, N. B., 1985, Valley and Ridge rhrusr bel[ balanced structural sections, Pennsylvania to Alabama: Appa- lachian Basin Industrial AssociatesAJniversity of Tenneiiee Department of Geological Sciences, Studies in Geology 12, &p.

Woollard, G. P., 1979, The new gravity system - changes in international gravity base values and anomaly values: beo_ physics, v. 44, n. 8, p. 1352-1366. PUBLICATION 117 15

APPENDIX: GRAVITY DATA

Station Longitude Elevation Gravity Free Air Bouguer Terrain Complete Value Anomaly Anomaly Corr. Bouguer Anomalv

10001 37.3336 84.7499 1650.0 979163.r9 -14.38 -70.ffi 2.74 -67.86 r0002 373313 80.1425 1815.0 979754.69 -t.5 I -69.20 0.54 -68.6 10003 37.3264 80.7389 1920.0 979748.62 -3.55 -68.97 0.18 -68.79 10004 37.3279 80.7483 1960.0 979744.25 -4.t7 -70.94 0.42 -70.52 10005 37.3223 80.13& r915.0 979748.r9 -3.52 -68.77 0.17 -68.60 10006 3',1.3268 80.7218 1790.0 979755.87 -8.53 -69.5r 1.31 -68.20 10007 37.320r 80.7184 1880.0 979'750.87 -4.r3 -68.18 0.24 -67.94 10008 37.3262 80.7196 17m.0 979755.8r -8.59 -69.58 2.5r -67.07 10009 31.3197 80.7096 1965.0 979745.N -2.0r -68.96 0.08 -68.88 10010 37.3265 80.7082 1901.0 979749.15 4.22 -68.98 0.12 -68.86 1001 I 37.3201 80.6940 r975.0 979743.25 -2.82 -70.11 0.04 -70.07 10012 37.322r 80.6995 1985.0 979743.56 -r.57 -69.r9 0.08 -69,11 10013 37.3283 80.699r 1880.0 979748.06 -7.88 -7r.93 0.09 -7r.84 10014 37.3296 80.6957 1850.0 979748.31 -10.45 -73.48 0.14 -t)J+ 1 1015 37.3348 80.6882 1855.0 97915r.r9 -7.10 -70.30 0.30 -70.00 r l0r6 37.339r 80.6836 1810.0 979753.25 -t0.21 -71.88 0.& -71.24 1r017 J I.JJ+ I 80.6791 1655.0 979762.44 -r4.6 -71.05 3.79 -67.26 1 1018 31.3270 80.6751 1165.0 919157.56 -9.19 -69.33 3.55 -65.78 I 1019 37.3323 80.6748 1655.0 979762.62 -r4.48 -70.86 3.6 -67.20 11020 37.3363 80.6696 1780.0 979754.8r -tt.47 -72.rr 3.75 -68.36 11021 37.34M 84.6&4 1800.0 979749.69 -14.72 -76.U 3.86 -72.r8 r1022 31.3469 80.6630 2000.0 979741.75 -4.78 -72.92 3.82 -69.10 r1023 3',1.35rr 80.6593 2170.0 979732.62 2.08 -71.85 3.73 -68.r2 rt024 37.3567 80.6624 2190.0 919't3r.62 2.96 -71.65 3.72 -67.93 r1025 37.3620 80.@2 2050.0 979739.94 -2.83 -72.67 3.87 -68.80 rrw6 37.3683 80.6557 24',75.0 979713.44 9.77 -74.56 3.57 -70.99 r1027 37.392 80.6552 2395.0 979718.87 8.55 -'r3.M 3.25 -69J9 I 1028 37.3322 80.7300 1790.0 979755.75 -8.66 -69.& 1.77 -67.87 ILO29 37.3372 80.7219 1870.0 979',754.r2 -3.69 -67.40 0.09 -67.3r 1 1030 37.3596 80.7057 1650.0 979763.06 -r7.32 -73.54 1.60 -7r.94 11031 37.3560 80.7033 n24.0 979758.62 -r4.24 -72.84 1.03 -71.81 r1032 37.3527 80.7049 1795.0 979755.3r -10.50 -71.65 0.74 -70.91 1 1033 3',7.3499 80.7097 1945.0 979749.r2 -2.58 -68.84 0.19 -68.65 l 1034 37.3442 84.7r32 1876.0 9'7915r.8r -5.44 -69.36 0.15 -69.2r 1 1035 37,3397 80.7188 r840.0 979759.37 -r.27 -63.95 0.10 -63.85 1 1036 37.3213 80.6656 1835.0 979748.25 -10.99 -'13.50 0.24 -73.26 I 1037 37.3196 80.6552 1800.0 97975r.r9 -r1.34 -72.66 0.19 -72.47 1 1038 37.3185 80.&72 1725.0 979758.12 -11.46 -70,23 0.73 -69.50 I 1039 37.3194 80.6384 1655.0 979760.44 -r5.73 -72.r1 2.& -69.47 r2u0 37.365r 80.7092 1890.0 979747.62 -10.19 -74.58 r.39 -73.r9 r2Mr 37.3ffi 80.7040 1945.0 979744.r9 -8.45 -74.72 0.67 -74.05 I2M2 37.3718 80.7015 1940.0 979744.69 -9.30 -75.39 1.06 -74.33 r2u3 37.3692 80.6941 1955.0 97975r.75 -0.83 -6',1.43 0.97 -6.46 rzu4 37.3685 80.6838 1943.0 979746.3r -7.39 -73.59 3.81 -69.78 rzu5 37.3714 80.6680 1835.0 979't52.69 -tt.t7 -73.69 3.86 -69.83 720/,6 37.3724 80.6728 1730.0 9'79758.94 -14.80 -73.74 3.89 -69.85 I2U7 373ffi8 80.6729 w5.a 979756.75 -11.88 -72.35 3.81 -68.54 r2u8 3',t.3620 80.6802 1670.0 979761.62 -16.88 -73.7',1 3.67 -70.10 r2u9 37.3578 80.6826 1665.0 919762.06 -16.91 -73.& 2.9r -74.73 r2050 37.3526 80.6937 1605.0 979767.44 -16.24 -70.92 2.93 -67.99 1205r 37,3509 80.6992 1595.0 979766.75 -r't.87 -72.2r 2.87 -69.34 12052 37.35r0 80.6827 1790.0 979756.75 -9.53 -70.51 2.72 -67.79 12053 37.3408 80.672s 1805.0 979753.3r -r0.62 -72.11 3.r2 -68.99 12054 37.3255 80.6215 1845.0 979750.94 -8.30 -71.15 1.77 -69.38 12055 37.3251 80.6381 1700.0 979',t5',1.69 -15.18 -73.10 3.97 -69.13 16 VIRGINIA DIVISION OF MINERAL RESOIIRCES

Longitude Elevation Gravity Free Air Bouguer Terrain Complete Value Anomaly Anomaly Corr. Bouguer Anomaly

12056 37.3322 80.6352 1774.0 979755.06 -rr.22 -7r.53 3.85 -61.68 12057 37.3361 80.6323 1819.0 979752.94 -9.68 -1r.65 ).t I -67.88 12058 37.3398 84.6295 1915.0 979747.81 < ?1 -14.95 3.65 -67 3A 12059 37.3533 80.6408 2540.0 9191tr.25 15.50 -71.03 3.18 -67.85 12060 37.3480 80.6387 2435.0 979717.44 11.82 -7t.t4 r.25 -69.89 tzcrl 31.3410 80.6364 2090.0 979737.25 0.r2 -71.08 J. t+ -67.34 r2M2 37.3319 80.6505 1945.0 979742.62 -7.20 -t).+t U.U) -12.62 r2M3 37.3420 80.&37 2r"t0.0 9'79732.62 3.02 -70.91 1.09 -69.82 rzw 3 t.3++3 80.6558 1923.0 979146.50 -6.33 -71.85 2.97 -68.88 13065 37.3146 80.6363 1680.0 979160.31 -13.44 -70.61 3.46 -67.2r 13066 37.3125 80.6315 n75.4 979754.69 -9.25 -69.73 r.lJ -68.60 13c57 37.301r 80.6393 1830.0 919149.15 -9.02 -7r.37 0.11 -7r.26 13068 37.3139 80.6453 1675.0 9',79'760.94 -12.41 -69.41 2.21 -67.20 13069 37.3108 80.65s7 1860.0 919746.04 -9.95 -73.32 0.06 -'73.26 13070 37.3050 80.6597 1740.0 979755.44 -ri.80 -71.08 0.40 -70.68 13071 373133 80.6744 n45.4 979755.25 -11.51 -70.96 4.24 -tv.tL 13012 37.3073 80.6656 1930.0 919143.37 -5.99 -'7r.74 0.07 -7r.61 13073 31.2989 80.6652 1820.0 919150.69 -8.08 -70.09 r.43 -68.66 13074 37.3W 80.6543 1810.0 919150.62 -9.09 - tu. t) 0.14 -70.61 13075 37.29& 80.&89 1955.0 979741.31 -4.10 -7r.3r 0.05 -71.26 13076 31.3203 80.6301 1695.0 9797 59.r2 -r3.28 -7r.02 3.40 -67.62 r3u7 37.3050 80.&62 1625.0 97976r:75 -16.30 -1r.6 0.69 -70.97 13078 3',1.2909 80.6285 1810.0 919150.r2 -8.65 -70.3r 0.28 -70.03 na79 37.2895 80.6341 1920.0 979143.19 -5.24 -74.65 0.r2 -70.53 13080 37.2909 80.6465 2220.4 979725.37 5.16 -70.47 0.29 -70.18 1308r 37.2890 80.6559 2105.0 979732.r2 1.09 -70.62 0.M -70.58 13082 37.2912 80.ffi4 2115.0 g',t9732.69 2.ffi -69.46 0.03 -69.43 13083 37.2920 80.6755 2165.0 9"79729.94 4.55 -69.2r 0.M -69.17 13084 37.2919 80.6830 2120.0 979733.3r 3.69 -68.53 0.03 -68.50 13085 31.2930 80.6928 1965.0 979144.56 -0.57 -67.52 2.33 -65.r9 13086 3'1.2965 80.7000 1840.0 97975r.00 -5.89 -68.58 3.25 -65.33 13087 3'1.2935 80.7077 1655.0 9'7976r.25 -13.04 -69.42 3.75 -65.67 13088 37.2896 80.7053 1650.0 919162.r9 -11.63 -67.85 3.78 -&.07 13089 31.2848 80.7050 1695.0 9"t9160.r9 -9.40 -6'1.r5 4.30 -62.85 13090 31.2750 80.7093 1830.0 979750.50 -5.46 -67.80 0.08 -67.72 13091 31.2683 80.7118 1690.0 979758.62 -9.56 -67.r4 0.38 -6.76 14092 37.3184 80.7312 1855.0 979752.56 -4.',l9 -61.99 1.56 -ffi.43 14093 37.3109 80.7290 1875.0 97975r.56 -2.97 -66.85 1.05 -65.80 r4w4 37,3062 80.7256 2065.0 919739.81 3.14 -67.2r 0.06 -67.15 14095 37.3063 80.7106 1990.0 919144.25 0.s3 -61.21 0.03 -67.24 I4W6 37.3rc4 80.7385 2010.0 919139.25 3.05 -67.47 0.19 -67.28 r4w7 37.3037 80.7459 2275.0 979728.3',1 rr.46 -66.05 0.32 -65.73 14098 37.3047 80.7383 2020.0 979743.3r 2.41 -66.4r 0.19 -6.22 r4w9 37.3000 80.7395 2015.0 919739.06 +.L I -66.42 0.r7 -6.25 14100 3'7.297r 80.7311 1950.0 979746.69 0.14 -6.29 0.11 -66.18 14101 37.2955 80.7223 1805.0 979754.75 -5.43 -6.93 0.18 -ffi.75 r4r02 37.2962 80.7135 1720.0 9'19758.94 -9.24 -67.84 2.t9 -65.65 15103 37.2636 80.749r 1831.0 979751.25 -3.67 -66.05 0.36 -65.69 15104 37.2630 80.7399 1795.0 9',t9753.87 -4.43 -65.59 0.21 -65.38 15105 37.267r 80.7288 1755.0 979't56.37 -5.70 -65.49 0.17 -65.32 15106 3',1.266s 80.7203 1730.0 9't975',7.75 -6.67 -65.6r 0.30 -65.31 15107 31.2626 80.7229 1755.0 979754.81 -7.20 -6.99 0.40 -6.59 15108 37.2586 80.7283 1785.0 979752.75 -5.56 -66.38 0.16 -6.22 15109 37.2556 80.7337 1855.0 979748.56 -3.17 -6.3'l 0.27 -66"10 l5r 10 37.2538 80.7408 1886.0 979746.31 -L.++A AA -6.69 r.92 -&.77 15111 37.2673 80.7443 19&.0 9't9743.3r 0.90 -6.02 0.13 -65.89 r5rr2 37.27M 80;7396 r935.0 979745.94 0.79 -65.13 0.07 -65.06 t1 PUBLICATION 117 It

Station Latitude I",ongitude Elevation Gravity Free Air Bouguer Terrain Complete Value Anomaly Anomaly Con. Bouguer Anomaly r5113 37.2545 80.7090 1695.0 979755.3r -tt.46 -69.2r 4.2r -65.m 15114 37.2614 80.1r21 1695.0 979756.75 -10.96 -68.71 4.24 -&.47 151 15 37.2738 80.7210 1785.0 979753.94 -6.25 -6"1.06 1.69 -65.37 15116 37.2742 80.7382 1841.0 979750.94 -3.98 -66.7r 0.53 -ffi.r8 151 17 31.2817 80.7396 2145.4 9'79739.r2 r2.79 -&.28 0.10 -60.18 151 18 37.2526 80.6802 1880.0 979743.25 -6.r3 -70.18 3.88 -6.30 15119 37.2556 80.6745 1925.0 979740.N -5.15 -70.73 3.19 -67.54 15r20 37.2ffi2 80.6659 2035.0 979734.25 -0.55 -69.88 3.75 -66.r3 r5tzl 3'1.2633 80.6574 2075.0 9'79732.87 0.90 -69.80 2.45 -67.35 15r22 37.2ffi7 80.6579 2018.0 979736.25 -1.09 -69.84 0.38 -69.46 15r23 37.2658 80.6624 2010.0 979737.00 -1.09 -69.57 0.39 -69.18 t5124 37.2664 80.6750 1860.0 979745.56 -6.63 -70.00 3.9r -66.09 15125 3',7.2&2 80.6845 1775.0 979751.00 -9.19 -69.6 2.n -67.39 15126 31.2ffi 80.6865 n45.4 979752.56 -10.45 -69.90 2.r2 -67.78 15r21 37.2681 80.6932 1695.0 979755.00 -12.71 -70.46 3.07 -67.39 15128 31.2757 80.6901 1665.0 979758.75 -r2.72 -69.45 3.3r -66.r4 15129 37.2771 80.6826 1710.0 979756.50 -r0.74 -69.00 2.78 -ffi.22 15130 3'7.2790 80.6756 1755.0 919753.3r -9.70 -69.49 2.32 -67.r'7 15131 37.2808 80.6718 1780.0 9',19751.75 -8.91 -69.55 2.2r -67.34 15r32 37.2855 80.6638 1870.0 979745.94 -7.r9 -70.90 1.09 -69.81 15 133 37.2'755 80.6541 1970.0 979139.56 -3.23 -70.34 0.35 -69.99 15r34 31.282r 80.6460 1885.0 919143.37 -8.35 -72.57 0.13 -72.44 15135 31.2817 80.6392 1837.0 919747.44 -7.86 -'70.45 0.73 -69J2 15136 37.2785 80.6300 1790.0 979749.69 -10.03 -71.01 0.50 -70.51 r5r3'7 37,2837 80,6265 1755.0 979752.50 -11.45 -7r.u 1.68 -69.56 16138 37.3625 80.7r52 1925.0 919746.44 -8.08 -73.67 0.56 -73.rr 16r39 37.357',1 80.7190 1795.0 979754.8t -rr.94 -73.W 0.61 -72.48 16140 37.3512 80.7209 1665.0 979',762.8',7 -15.16 -71.89 r.02 -'t0.87 t614r 37.3474 80;t265 1615.0 979766.8r -r5.93 -70.95 r.67 -69.28 r6r42 37.343r 80.7314 1595.0 979767.12 -16.56 -70.90 r.72 -69.18 r6r43 37.3400 80.7381 1595.0 979767.44 -16.24 -70.58 r.77 -68.81 16144 3',t.3453 80.7442 1575.0 9',t9765.50 -20.06 -73.72 1.34 -7238 r6145 37.349r 80.74t2 1630.0 979',162.r9 -19.14 -74.67 1.03 -73.& r6146 37.3528 80.7447 1755.0 979753.44 -16.13 -'75.93 1.00 -74.93 r6147 37.3559 80.74r2 1755.0 919152.3'l -r7.20 -76.99 1.18 -75.81 16148 37.3630 80;t426 2005.0 979739.37 -7.62 -75.93 4.42 -7t.5r 16149 37.36/2 80.7461 2090.0 979734.69 -4.32 -75.52 3.90 -7r.62 16150 37.2988 80.74& 2199.0 97973r.8r 8.68 -6.23 0.25 -65.98 16151 3',7.2922 80.7458 2M1.0 979742.W 4.95 -&.58 0.22 -&.36 16152 3',t.2903 80.74r4 2005.0 979744.M 3.63 -&.68 0.17 -&.5r 16153 37.2853 80.7309 1950.0 979744.75 -0.86 -67.29 0.2r -67.08 16154 37.2884 80.7288 1835.0 979752.00 4.42 -6.94 r.24 -65.70 16155 3',1.2676 80.7024 1785.0 979757.87 -1.38 -62.r9 0.45 -6r.74 16156 37.2687 80.6518 1970.0 979740.37 -1.48 -68.59 0.44 -68.15 17r57 37.2694 80.8738 3615.0 979&r.94 54.79 -68.37 0.32 -68.05 17158 37.2731 80.8653 3635.0 9't9639.r2 52.92 -70.92 0.35 -?0.57 r7r59 37.2762 80.8536 3545.0 979&5.69 5r.02 -69.75 0.31 -69.44 17160 37.2',t60 80.8488 3525.0 979&7.50 50.95 -69.r4 0.31 -68.83 t7161 3',t.2708 80.8410 3453.0 979655.56 53.18 [email protected] 0.36 -64.10 17162 3't.294r 80.7543 2218.0 979730.50 9.16 -6.4r 0.34 -ffi.07 r"t163 37.2895 80.7617 2278.0 919',121.25 12.49 -65.r2 0.39 -&.73 t7t& 37.2868 80.7676 2177.0 9',t9',133.00 8.74 -65.43 0.56 -&.87 r7165 37.2826 80.7734 2110.0 979737.M 6.50 -65.38 0.79 -&.59 t7t66 37.2',199 80.7788 2148.0 9',7g',t35.W 8.95 -64.23 0.63 -63.& r7161 37.2757 80.7823 2195.0 919',132.12 10.50 -&.29 0.58 -63.7r 17168 37.2716 80.7864 2205.0 97973r.56 10.87 -&.25 0.44 -63.81 r7169 37.2689 80.7890 2205.0 979732,M 12.3r -62.8r 0.43 -62.38 18 VIRGINIA DIVISION OF MINERAL RESOLIRCES

Longitude Elevation Gravity Free Air Bouguer Terrain Completc Value Anomaly Anomaly Corr. Bouguer Anomalv

r"7170 31.zffi 80.7957 2215.0 979731.52 12.81 -62.65 0.54 -62.rr l7rll 37.2627 80.7956 2235.A 97913A.44 13.51 -62.& 0.45 -62.r9 t7n2 37.2s65 80.7991 2161.0 919133.87 rc.92 -62.70 0.82 -61.88 17r73 37.2527 80.7870 2035.0 9t9139.37 4.57 -&.76 0.79 -63.97 r7r74 3',r.255r 80.7818 2005.0 91974r"31 3.75 -e.56 1.50 -63.06 r7r75 37.2518 80.7122 1940.0 979745.3r 1.58 -u.52 1.50 -63.02 17176 37.2544 80.7&4 1896.0 979747.94 0.06 -&.53 2.86 -6r.67 r7r77 37.258r 80.7566 1864.0 979749.62 -r.26 -&.76 0.50 -&.26 T7178 37.2692 80.7550 1890.0 979748.56 -0.81 -65.20 0.92 [email protected] r7119 31.z',t14 80:t637 1982.0 919744.81 3.21 -&.3r 0.49 -63.82 17180 37.212r 80.7748 2243.0 919128.37 1r.26 -65.16 0.25 -&.9r 17181 37.2139 80.7786 2295.A 979721.r2 14.90 -63.29 0.31 -62.98 18182 37.2559 80.8622 3210.0 979668,94 44.& -&.72 1.69 -63.03 19183 37.3W 80.8635 1650.0 979759.25 -15.51 -1r;72 3.31 -68.41 19184 37.2956 80.8640 1635.0 979759.94 -16.23 -1r.94 3.27 -68.67 19185 31.30/.r 80.8536 1612.0 9't9762.3r -16.96 -71.88 3.20 -68.68 19186 3'7.3070 80.8417 1615.0 97916r.94 -r7.05 -72.07 3.18 -68.89 19187 37.3002 80.8423 1820.0 979752.00 -6.71 -68.78 2.77 -66.01 19188 37.2940 80.8383 2085.0 979137.50 3.65 -67.38 2.5r -&.87 19189 37.2951 80.8304 2r75.0 97913r.00 5.61 -68.49 z.)+ -66.15 19i90 37.2994 80.8267 2122.0 979735"00 4.63 -6'7.66 2.3r -65.35 19191 3',t.3145 80.8397 1585.0 979't62.69 -24.06 -14.06 J.L+ L -70.& 19192 37.3180 80.8320 1570.0 919'7&.r2 -20.03 -73.52 3.45 -70.07 19r93 37.3097 80.8616 1765.0 97975r.06 -r3.82 -73.95 1.00 -72.95 19194 37.3146 80.8486 1s90.0 979161.50 -20.78 -74.95 2.90 -72.45 19195 37.3203 80.8304 1545.0 9797&.94 -2t.5"1 -74.21 3.57 -70.& 19196 37.3238 80.8207 1530.0 9797&.00 -23.92 -16.05 3.61 -72.44 19t91 37.328r 80.8200 1585.0 979162.56 -21.r2 -75.r2 2.28 -72.84 19198 3'1.3284 80.8136 1545.0 919t&.r9 -23.26 -15.94 3.10 -72.80 19r99 37.3209 80.8162 1780.0 979753.75 -10.66 -7r.30 3.23 -68.07 192W 37.3216 80.8089 1910.0 9'79745.37 -6.81 -71.88 5. tz -68.16 1920r 37.3158 80.7942 1895.0 9'19144.44 -9.16 -73.72 2.67 -71.05 19202 37.3214 80.7967 1790.0 97975r.75 -rr.12 -12.10 2.82 -69.88 19203 3't.3299 80.7973 1890.0 979745.00 -10.00 - IlJY 1.45 -72.94 r92M 31.3303 80.8053 1625.0 979760.A6 -r9.86 -15.22 4.85 -70.37 19205 37.3340 80.7947 1600.0 919760.94 -2r.34 -75.85 r.57 -74.28 19206 31.330r 80.7895 1870.0 979147.56 -9.32 -73.03 4.35 -68.68 r9207 31.3312 80.7803 1925.A 919744.8'7 -6.83 -72.42 1.50 -70.92 19208 37.3341 80.7718 1910.0 979745.8r -1.3r -12.38 0.81 -7r.57 19209 31.3382 80.764r 1660.0 979759.06 -18.51 -75.06 t.r2 -73.94 r9210 37.3316 80.7903 1560.0 9797&.81 -22.16 -75.3r 1.58 -73.73 302rr 3"1.3418 80.77r0 1595.0 919763.31 -20.3r -74.65 1.16 -73.49 30212 3',t.3420 80.7818 1570.0 979763.r9 -22.85 -76.33 t.4l -74.92 30213 31.3354 80.7.579 1610.0 9'r9162.87 -18.46 -73.3r 1.53 -7r.78 30214 37.2558 80.8063 2215.0 979730.N t2.t3 -63.34 1.13 -62.2r 30215 31.2554 80.8r42 2337.0 979722.56 16.16 -63.46 0.9s -62.5r 30216 3-7.2553 80.8212 2468.A 919714.44 20.36 -63J2 0.99 -62.73 30217 37.254r 80.8219 2620.0 919'tM.3r 24.53 -&.73 1.08 -63.65 30218 37.2533 80.8414 299r.A 9't9682.44 37.54 -&.36 0.87 -63.49 30219 37.2536 80.8477 3145.0 979672.31 41.90 -65.25 1.18 -&.07 30220 3'7.2558 80.8s58 3380.0 979659.75 5r.44 -63.7r 4.43 -59.28 20001 37.2486 80.7097 1720.0 919754.56 -8.93 -67.52 2.97 -&.55 2cfp2 37.2432 80.7103 1731.0 9191fi.A6 -9.39 -68.37 2.ffi -65.7r 20003 37.2402 80.7087 1726.0 979752.31 -r0.61 -69.4r 2.9r -66.50 20c[4 37.U22 80.7161 1877.0 979745.62 -3.10 -67.05 0.15 -6.90 20005 37.2422 84.7267 1994.0 979739.50 1.78 -66.15 0.07 -66.08 20006 37.2401 80.7362 2150.0 979729.87 6.83 -6.42 0.r2 -66.30 PUBLICATION 117 19

Complete Station Latitude Longitude Elevation GravitY Free Air Bouguer Tenain Value Anomaly Anomaly Con. Bouguer Anornaly

-65.79 20m',7 37.2419 80.7445 2168.0 979729.37 8.02 -65.84 0.05 20008 37.2478 80.7486 2076.0 979735.94 5.93 -64.80 0.16 -&.& 37.2386 80:747t 1991.0 979740.06 3.00 -64.83 0.47 -&.36 20009 -&.72 20010 3'1.2282 80.7402 1789.0 979749.44 -6.62 -67.57 2.85 -6.34 -67.93 0.57 -67.36 2001 1 37.2118 80.7400 1808.0 979747.06 -67.36 2A0n 37.2485 80.7392 1797.4 919745.25 -8.25 -69.47 2.tr -70.09 20013 37.r9'lr 80.7351 1832.0 979738.44 -r0.83 -73.24 3.15 -68.70 20014 3'1.1946 80.7351 1841.0 979738.r9 -9.30 -72.02 5.JZ -"75.37 20015 31.1783 80.7158 2161.0 979714.r9 -1.70 -'75.53 0.16 -74.68 20016 37.1681 80.7M2 1936.0 979'127.N -9.6'l -75.63 0.95 -77.86 20011 37.r&l 80.7001 1983.0 979121.87 -r0.38 -'t7.94 0.08 -78.50 2001 8 37.1618 80.6961 2Mr.0 919716.81 -8.99 -78.52 0.02 200r9 37.1587 80.6938 2050.0 979715.81 -9.20 -79.04 0.02 -'19.02 20024 37.r43-l 80.6906 2087.0 9191rr.8r -8.79 -79.89 0.0 -79.89 -78.63 20021 37.1 388 80.6863 2041.0 979714.69 -9.30 -78.83 0.20 20022 37.1383 80.6809 209r.0 9191r1.44 -7.85 -79.09 0.0 -79.09 20023 37.r3r7 80.6869 2105.0 979712.44 -5.53 -77.25 0.0 -77.25 -76.84 20024 31.1278 80.6941 2116.0 919111.25 -4.75 -16.84 0.0 20025 37.194r 80.1292 1847.0 919737.8r -9.1 l -72.03 2.65 -69.38 20026 31.1499 80.7191 1953.0 979721.r2 -12.08 -'t8.6r 0.23 -78.38 -'t8.82 20421 37.1460 80.7129 1946.0 919121.06 -12.80 -19.09 0.27 20028 37.1368 80.7018 1993.0 9797r',|.54 -l r.00 -78.90 0.72 -78.18 -79.05 20029 37.1 338 80.6934 2100.0 9191r0.94 -7.50 -'79.05 0.0 20030 31.2289 80.1332 1860.0 979144.62 -4.76 -68.13 0.73 -67.40 -65.2r 2003 I 37.231r 80.7208 1143.0 979152.25 -8.14 -67.52 2.3r Aaa 21032 31.2460 80.6280 2000.0 979732.94 -72.36 2.45 -70.3r 21033 37.2483 80.6329 1840.0 919143.31 -8.83 -7r.52 4.08 -67.44 21034 37.2467 80.6358 1860.0 979140.50 -9.82 -'73.r9 3.61 -69.58 21035 37.2446 80.6407 1900.0 979737.t2 -9.44 -74.t7 3.67 -70.50 21036 31.2422 80.6503 2016.0 919734.00 -1.65 -'t0.33 3.& -6.69 2r037 31.2412 80.658s 2127.0 9-19725.75 0.54 -7r.93 0.58 -'tr.35 21038 37.2408 80.6633 2000.0 979133.56 -3.59 -7r.73 r.52 -70,2r 21039 3'7.2406 80.6654 1980.0 979'735.25 -3.79 -71.24 1.69 -69.55 2rM0 37.244r 80.6737 1888.0 979741.62 -6.06 -10.39 4.28 -66.11 2IMI 37.2442 80.6770 1812.0 919142J5 -6.44 -10.22 3.96 -66.26 2ru2 31.2377 80.6786 1931.0 979738.25 -4.46 -10.24 3.93 -66.31 2ru3 37.2355 80.6808 1980.0 979735.50 -2.60 -70.06 4.23 -65.83 2ru4 37.2366 80.6838 1982.0 9',79',734.69 -5.2L -70.75 J.J4+ -67.4r 2ru5 37.2360 80.6866 1955.0 979736.75 -3.70 -70.31 3.38 -66.93 2rM6 37.2306 80.7059 1169.0 919748.06 -9.88 -70.15 3.47 -66.68 2rul 37.2238 80.6966 2000.0 979'133.94 -1.41 -69.54 3.58 -65.96 2rM8 3',7.2181 80.7072 2U0.0 979731.50 -0.08 -69.58 3.49 -66.09 2rM9 37.2160 80.71 l9 1940.0 979736.00 -4.05 -70.14 3.43 -66.7r 2r050 37.2140 80.7r94 2000.0 979733.8r -0.59 -68.13 3.39 -65.34 21051 37.2192 80.7184 19m.0 919739.00 -5.75 -70.48 2.88 -67.60 21052 31.2484 80.6839 1827.0 979146.00 -7.43 -69.67 3.86 -65.81 2r053 37.2412 80.69'/9 18m.0 979746.3r -9.65 -70.98 3.2r -67.77 21054 37.1632 80.6813 2027.0 9797r7.56 -9.62 -78.67 0.04 -78.63 21055 37.1703 80.6102 2002.0 979720.00 -r0.47 -78.67 0.02 -78.65 21056 31.r70r [email protected] 1994.0 979720.W -11.22 -79.t5 0.06 -79.09 2r05'7 31.1728 80.6602 r956.0 919122.W -r2;79 -79.43 0.02 -79.41 21058 31.r7r0 80.6556 1937.0 979723.19 -13.39 -79.38 0.02 -79.36 2rc59 31.1684 80.6524 1939.0 979722.19 -t4.20 -80.26 0.02 -80.24 21060 37.1648 80.6459 1832.0 9't9',tzt.8'l -18.58 -80.99 2.W -'t8.97 2rMr 37.1718 80.6379 1781.0 979737.56 -19.69 -80.37 r.43 -78.94 2rM2 37.1772 80.6294 1767.0 9',79732.50 -2r.01 -81.21 1.72 -79.49 20 VIRGIMA DIVISION OF MINERAL RESOURCES

Station Latitude lnngitude Elevation Gravity Free Air Bouguer Terrain Complete Value Anomaly Anomaly Corr. Bouguer Anomaly

2rM3 37.1638 80.6310 2018^0 979717.ffi -l1.90 -80.65 0.01 -80.64 2rw 37.1526 80.6465 1942.0 97972r.69 -r2.55 -78.7r 0.r2 -78.59 21cr5 37.1458 80.6581 2027.A 979717.94 -8.30 -77.36 0.09 -77.27 zrffi 37.1380 80.6572 2013.0 979716.69 -9.93 -78.51 0.02 -78.49 21c57 37.1331 80.6606 2U7.0 979713.44 -9.98 -79.72 0.0 -79.72 21cf,9 37.1303 80.6347 1899.0 979719.50 -16.90 -81.60 0.i4 -81.46 21070 37.1587 80.6401 1856,0 979726.8r -16.45 -79.68 3.52 -76.16 2207r 37.1332 80.7424 1976.0 979718.r9 - 11.91 -79.23 0.74 -78.49 22W2 37.1350 80.7435 1978.0 979118.69 -rt.22 -78.6r 1.00 -77.6I 22073 37.1379 80.7449 1980.0 9',t9718.3r -11.41 -78.87 a.r7 -78.70 22074 37.1423 80.752r 20ff.A 979715.W -8.r4 -78.32 0.06 -78.26 22075 37.1307 80.7468 2027.A 979715.8r -8.55 -77.6r 0.42 -77.r9 22076 37.1352 80.7593 2012.0 979717.37 -9.34 -77.89 0.09 -77.80 22077 37.1323 80.7ffi 2W7.0 979717.69 -9.50 -77.87 0.24 -77.63 22078 37.1278 80.7744 24A.0 9',79716"3r -8.34 -77.29 0.79 -76.50 22079 37.1279 80.7920 2120.0 9797r0.8r -4.81 -77.03 0.11 -76.92 22080 37.1315 80.7959 zrffi.0 979113.06 0.27 -73.32 0.24 -73.08 2208r 37.1398 80.7980 24ffi.0 979696.62 12.M -7r.77 0.16 -7r.6r 22082 37.1429 80.8ms 2835.0 979672.37 22.r2 -74.46 0.22 -74.24 22083 37.t4/r 80.8075 2560.0 979692.3r 16.20 -7r.02 1.95 -69.07 22084 37.rs06' 80.8055 2180.0 979716.25 4.40 -69.87 1.72 -68.75 22085 37.1567 80.8061 1960.0 97973r.06 -2.42 -69.r9 Lr7 -68.02 22086 37.1594 80.8050 1920.0 979732,0A -5.24 -70.65 r.29 -69.36 22487 37.1594 80.8089 2000.0 979727.62 -2.09 -70.23 1.09 -69.14 22088 37.1549 80.8143 1940.0 979734.81 4.49 -70.58 r.36 -69.22 22089 37.1518 80.8184 1989.0 979728.69 - 1.13 -68.89 1.31 -67.58 22W0 37.1486 80.8240 1958.0 979729.44 -3.29 -70.00 2.0r -67.99 22Wr 37.1472 80.8318 1971.0 9"t9728.87 -2.63 -69.78 1.50 -68.28 2ZW2 37.1430 80.8387 1980.0 979727.37 -3.29 -74.74 1.47 -69.27 22W3 37.1403 80.8469 20n.a 979725.87 -0.84 -69.39 1.27 -68.r2 22W4 37.t376 80.8506 1980.0 979726.8r -2.9r -7437 1.4'l -68.90 22W5 37.r32r 80.8546 20m.0 979726.N -1.84 -69.98 2.22 -67.76 22W6 37.r28r 80.8612 2000.0 979725.69 -r.22 -69.36 2.M -67.32 22W7 37.1310 80.7763 2080.0 979714.3r -5.07 -75.93 0.08 -75.85 22W8 37.1338 80.7'7',l8 zrn.0 979713.50 -3.06 -75.28 0.09 -75.r9 22W9 37.1358 80.7791 2153.0 97971r.56 -1.89 -75.24 0.10 -'75.r4 22rffi 37.1632 80.7934 1908.0 979734.M 4.31 -69.3r 1.31 -68.00 22L0r 37.1637 80.7848 1920.0 979732.8r -5.37 -70.78 2.49 -68.29 22102 37.166.5 80.7801 1900.0 979733.87 -6.18 -70.92 3.30 -67.62 22103 37.1693 8A.776 1900.0 979734.3r -5.75 -70.48 3.25 -67.23 22rU 37.1708 80.7708 19n.0 979733.3r 4.87 -70.28 2.97 -67.3t 22105 37.1740 80.7696 1880.0 979734.50 -7.44 -7r.49 2.44 -69.05 22rM 37.1776 80.7619 19m.0 979734.75 -6.06 -70.79 3.s3 -67.26 22108 37.1838 8A.752,4 1900.0 979735.W -6.00 -74.73 2.40 -68.33 22rW 37.1851 80.t496 1926.0 979733.8r 4.74 -70.36 3.01 -67.35 22TTO 37.1865 80.7480 1880.0 9',19735.75 -8.07 -72.r2 3.68 -68.44 22rrr 37.1899 80.74s6 19m.0 979733.37 -8.56 -73.29 r.28 -72.0r 22t12 37.1920 80.7410 r9m.0 979734.r9 -7.75 -72.48 0.88 -71.60 22t13 37.1923 80.737r 1840.0 979737.62 -9.95 -72.U 2.50 -70.r4 23rr4 37.213r 80.7636 1983.0 979737.s0 1.50 -6.M 0.32 -65.74 23rr5 37.2190 80.7687 22ffi.0 979724.81 8.28 -6.67 0.27 -6.40 23fi.6 37.2204 80.7688 2275.0 979719.3r 9.83 -67.68 0.24 -67.44 231t7 37.2192 80.7727 Bn.0 979714.75 9.50 -69.54 0.52 -69.A2 23trl 37.2187 80.7772 2380.0 9797r1.8r t2.21 -68.88 0.42 -68.46 23rtg 37.2114 8A.7720 1851.0 979743.69 4.73 -67.79 2.r5 -65.& 23r20 37.2084 80.7695 1820.0 979746.r9 -5.15 -67 "r5 r.32 -65.83 PUBLICATION 117 2l

Station Latitude Lnnginrde Elevation Gravity Free Air Bouguer Terrain Complete Value Anomaly Anomaly Con. Bouguer Anomaly

23t2r 37.2127 80.7762 1940.0 979743.37 3.33 -62.77 t.74 -61.03 23r22 37.2123 80.7814 1940.0 979742.87 2.83 -63.27 r.& -6r.63 23r23 37.2W6 80.7842 1960.0 979743.25 5.08 -6r.69 0.96 -ffi.73 23r24 37.2086 80.7914 1915.0 979740.r9 -2.r2 -67.39 1.38 -66.01 23r25 37.2078 80.7975 1900.0 979741.r2 -2.68 -67.42 1.58 -65.84 23126 37.2017 80.80r0 1901.0 979',t40:t5 -2.9',1 -67 J3 1.80 -65.93 23r27 37.2055 80.8074 1980.0 979736.50 1.15 -6.3r 1.22 -65.09 23r28 37.2050 80.8142 2024.0 9't9732.81 r.23 -67.59 r.20 -6.39 23t29 37.2M0 80.8170 2420.0 979732.37 0.79 -68.03 1.16 -6r.87 23r30 37.2012 80.8222 I97I,O 979736.M 4.13 -67.28 1.55 -65.73 2313r 37.1981 80.8231 1920.0 979739.69 -1.30 -6.72 1.80 -&.92 23r32 37.1955 80.8193 1900.0 979740.r2 -1.81 -6.54 1.32 -65.22 23r33 37.t956 80.8142 r92fr.0 979740.37 a32 -65.09 r.51 -63.58 23r34 37.2m3 80.8238 1960.0 979736.25 {.98 -67.76 2.90 -&.86 23135 37.1983 80.82't2 1920.0 979737.69 -3.30 -68.72 2.22 -66.50 23L36 37.1976 80.8305 r92r.0 979737.37 -3.52 -68.97 2.r5 -6r..82 23r37 3',t.1982 80.8358 1920.0 979',t3',t.19 -3.80 -69.22 2.10 -67.r2 23r38 37.1945 80.8383 1q20.4 979736.8r -3.24 -68.65 r.09 -67.56 23r39 37.1923 80.8381 1926.0 979731.n -2.49 -68.11 0.97 -67.L4 23t40 37.1892 80.8349 l!2n.0 979737.94 -2.r2 -67.53 1.87 -65.6 2314r 37.1881 80.8298 2000.0 979734.62 2.@ -ffi.u 1.6 -64.38 23r42 37.1884 80.8247 2080.0 979730.50 5.49 -65.37 0.88 -64.49 23r43 3',t.1872 80.8327 1920.0 979739.r9 4.87 -6.28 3.29 -62.99 23r44 37.1851 80.8255 1940.0 979738.75 1.5r -64.58 1.88 -62.70 23145 37.1800 80.8260 2020.0 979733.56 3.85 -&.97 1.65 -63.32 23146 37.t781 80.8324 2M0.0 979732.r9 4.36 -65.14 4.t4 -61.00 23r47 37.1756 80.8373 2M0.0 979732.44 4.6r -&.89 2.30 -62.59 23r48 37.1698 80.8388 1960.0 9',t9-t33.8r {.60 -67.38 2.88 -64.50 23r49 37.1692 80.8454 1960.0 979735.25 0.83 -65.94 2.67 -63.27 23t50 37.1672 80.8480 2t20.0 979725.44 6.07 -6.16 1.19 -&.97 2315r 37.r&A 80.8514 2100.0 979726.54 5.25 -66.30 r.43 -&.87 23r52 37.rffi1 80.8544 1980.0 979733.W 1.40 -66.06 2.58 -63.48 23r53 37.1601 80.8595 1980.0 979733.r9 1.59 -65.87 4.88 -ffi.99 23t54 37.1620 80.8609 1980.0 979734.r2 2.53 -&.93 2.39 -62.54 23155 37.rffi3 80.8680 1980.0 979734.W 2.40 -65.06 0.97 -&.w 23156 37.1579 80.8667 2000.0 97973r.50 1.78 -6.36 1.31 -65.05 23r57 37.r538 80.8695 1980.0 97973r.75 0.15 -67.3r 3.38 -63.93 23r58 37.1518 80.8719 20n.0 979729.r9 2.29 -6.53 3.26 -63.27 23r59 3',t.r&7 80.8598 2080.0 919726.25 3.r2 -6',1;t5 0.60 -67.15 23lffi 37.1703 80.8599 2080.0 979727.25 4.r2 -6.75 0.54 -6.2r 23161 37.1734 80.8633 23ffi.0 979716.69 14.25 -64.11 0.16 -63.95 23162 37.1721 80.8658 2280.0 9797t7.94 13.62 -&.M 0.18 -63.88 23163 37.1672 80.8703 22n.0 97972r.3r 11.35 -&.29 0.2r -64.08 231& 37.1620 80.8718 2000.0 979733.69 3.9',1 -&.r7 1.38 -62.79 23165 37.1762 80.8567 2rffi.0 9',19725.r2 8.58 -65.01 0.37 -&.& 23166 37.1806 80.8488 214.0.0 979726.62 8.20 -9.7r 0.33 -u.38 23167 37.1786 80.8468 22n.0 97972r.3r r0.41 -65.22 0.38 -64.U 23168 37.r73r 80.8462 20/n.0 979732.3r 5.42 -64.08 2.r9 -61.89 23170 37.t827 80.8451 20/n,0 979732.r2 4.29 -65.21 0.50 -&.71 23r'tr 37.1843 80.8374 2000.0 979135.37 3.78 -&.36 1.27 -63.09 23r72 37.1887 80.8464 2000.0 979733.44 0.91 -67.23 0.65 -66.58 23173 37.1866 80.8519 2M0.4 97973r.25 2.48 -67.02 0.58 -66.44 23174 37.rU4 80.8572 2100.0 979728.44 6.25 -65.30 1.07 -&.23 23t75 37.1796 80.8701 2t72.0 9',79724.6 8.65 -65.35 0.46 -64.89 (a -l

k z

F U. &Yi (h'rl FE =>,,< a, X

Hr=ez q7I l-l O>r{ >z&- -ix rl=n z z ct) z rlr