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Structure of the Bane Dome, Giles County, Virginia

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Structure of the Bane Dome, Giles County, Virginia 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 Copyright 1991, Commonwealth of Virginia 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-di- mensional models. These trvo-dimensional models of den- sity 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) bo 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.
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