The Rome trough, a northeast-trending graben, isthat part of the Cambrian interior rift system thatextends into the central Appalachian foreland basinin eastern North America. On the basis of changesin graben polarity and rock thickness shown fromexploration and production wells, seismic lines, andgravity and magnetic intensity maps, we divide thetrough into the eastern Kentucky, southern WestVirginia, and northern West Virginia segments. Ineastern Kentucky, the master synthetic fault zoneconsists of several major faults on the northwesternside of the trough where the most significant thick-ness and facies changes occur. In southern WestVirginia, however, a single master synthetic fault,called the East-Margin fault, is located on the south-eastern side of the trough. Syndepositional motionalong that fault controlled the concentrated deposi-tion of both the rift and postrift sequences. TheEast-Margin fault continues northward into thenorthern West Virginia segment, apparently withless stratigraphic effect on postrift sequences, and asecond major normal fault, the Interior fault, devel-oped in the northern West Virginia segment. Thesethree rift segments are separated by two basementstructures interpreted as two accommodation zonesextending approximately along the 38th paralleland Burning-Mann lineaments.
75AAPG Bulletin, V. 84, No. 1 (January 2000), P. 75–99.
3Department of Geology and Geography, West Virginia University,Morgantown, West Virginia 26506.
This study was supported by the National Research Center for Coal andEnergy (NRCCE). We thank Columbia Natural Resources (CNR) Inc. forallowing the publication of the seismic lines. Acknowledgment is made to theWest Virginia Geological and Economic Survey for well data and the sonicand density logs for subsurface mapping and synthetic seismic modeling. Wealso benefited from the application of the computer software packages of theMCS (Mapping-Contouring System), SURFACE III, DEAM (Data Editing andManagement), DIG (Digitization), and the programs for synthetic seismicanalysis. This paper benefited from suggestions by Jeffrey A. Karson (DukeUniversity) and journal reviews by William A. Thomas, an anonymousreviewer, and the elected editor Neil F. Hurley. Thanks also go to Sharon L.Crawford, Thomas R. Evans, and Charles A. Meeder (Marathon OilCompany) for their support.
Along-Axis Segmentation and Growth History of theRome Trough in the Central Appalachian Basin1
Dengliang Gao,2 Robert C. Shumaker,3 and Thomas H. Wilson3
Computer-aided interpretation of seismic dataand subsurface geologic mapping indicate that theRome trough experienced several major phases ofdeformation throughout the Paleozoic. From theEarly(?)–Middle Cambrian (pre-Copper Ridge deposi-tion), rapid extension and rifting occurred in associ-ation with the opening of the Iapetus-Theic Oceanat the continental margin. The Late Cambrian–Middle Ordovician phase (Copper Ridge to BlackRiver deposition) was dominated by slow differen-tial subsidence, forming a successor sag basin thatmay have been caused by postrift thermal contrac-tion on the passive continental margin. Faults ofthe Rome trough were less active from the LateOrdovician–Pennsylvanian (post-Trenton deposi-tion), but low-relief inversion structures began toform as the Appalachian foreland started to devel-op. These three major phases of deformation arespeculated to be responsible for the vertical stack-ing of different structural styles and depositionalsequences that may have affected potential reser-voir facies, trapping geometry, and hydrocarbonaccumulation.
INTRODUCTION
The Rome trough (Woodward, 1961; McGuire andHowell, 1963) is one of the major rift elements of theinterior rift system (Harris, 1978) that formed in east-ern North America during the Early and MiddleCambrian in association with the opening andspreading of the Iapetus-Theic Ocean (Thomas,1991) (Figure 1). The Rome trough extends acrossextensive areas of oil and gas production in thecentral Appalachian foreland basin. The Rometrough has aroused interest among structural geolo-gists and petroleum geologists over the past 30 yr(e.g., Woodward, 1961; McGuire and Howell, 1963;Harris, 1975, 1978; Ammerman and Keller, 1979;Kulander and Dean, 1980, 1993; Shumaker, 1986a, b,1993, 1996; Kulander et al., 1987; Thomas, 1991; Patchenet al., 1993; Drahovzal, 1994; Wilson et al., 1994a, b; Gao,1994; Gao and Shumaker, 1996; Harris and Drahovzal,1996; Shumaker and Wilson, 1996; Ryder et al., 1997a,b; Beardsley, 1997). Although the stratigraphy and
structure of the Rome trough have been extensive-ly discussed in previous studies, little has been pub-lished on the along-axis segmentation in troughgeometry and its control on focused sedimentationand hydrocarbon accumulation. In this study, wecompare and contrast the subsurface geology ofdifferent segments on the basis of well, seismic,magnetic, and gravity data. The results reveal com-plexities in the geometry and growth history ofthe trough that have enhanced our understandingof the mechanism for hydrocarbon entrapmentand tectonic evolution of the pre-Appalachian andAppalachian foreland. That understanding, in turn,should provide the basis for a more accurateassessment of the hydrocarbon potential of bothdeeply buried and shallow Paleozoic reservoirsalong the trough.
GEOLOGIC SETTING
The central Appalachian foreland basin is under-lain by a series of continental grabens that are
collectively part of a more extensive interior riftsystem (e.g., Shumaker, 1986a, 1996; Shumakerand Wilson, 1996). This system formed in associa-tion with the late-stage opening of the Iapetus-Theic Ocean at the plate margin during the Earlyand Middle Cambrian (Rankin, 1976; Thomas,1977, 1991; Read, 1989; Ryder et al., 1997a, b).The crustal extension of the interior rift systemproduced a thick sequence of lower Paleozoic sedi-mentary rocks in several grabens (Shumaker,1986a). The Rome trough (McGuire and Howell,1963; Harris, 1975) is one of the elements of theinterior rift system that extends into theAppalachian foreland in eastern Kentucky andwestern West Virginia where it follows the north-east-trending magnetic gradient named the NewYork-Alabama lineament (King and Zietz, 1978).Thousands of meters of sedimentary rocks are pres-ent within the trough, which consists of diverselithologic units (Figure 2) (Schwietering andRoberts, 1988; Ryder, 1992). These sedimentaryrocks can be divided into rift (Rome-Conasauga),passive-margin (Copper Ridge-Black River), and
76 Rome Trough
Figure 1—Tectonic mapshowing the Iapetianstructure of southeasternNorth America and theAppalachian forelandbasin (after Shumaker andWilson, 1996; Shumaker,1996). Rome trough andother grabens of the interior rift system arefrom Shumaker (1986a).Rifts and transform faultsat the plate margin arefrom Thomas (1991). The 38th parallel lineament is based onHeyl (1972), and the Burning-Mann lineamentis based on Shumaker(1986b). Also shown arethe locations of areasshown in Figures 3 and 4.
Gravity maps (Ammerman and Keller, 1979;Kulander et al., 1987) and a total magnetic intensitymap (King and Zietz, 1978) indicate that the Rometrough shows significant along-axis variations indepth of the rift valley and thickness of overlyingPaleozoic sediment infill. Superposition of the totalmagnetic intensity map with major basement struc-tures of the Rome trough (Figure 3) indicates thespatial relationship between magnetic intensityvariations and major basement lineaments. From
south to north, the Rome trough is interrupted by awest-trending basement fault zone, named the 38thparallel lineament, that has been observed at thesurface (Heyl, 1972) and by a north-trending mag-netic gradient, named the Burning-Mann lineament,that extends from the Burning Springs to the MannMountain anticline developed at the surface(Shumaker, 1986b). These two basement linea-ments (Figure 3) divide the Rome trough into theeastern Kentucky, southern West Virginia, andnorthern West Virginia segments.
Shumaker and Wilson (1996) discussed basementstructures of the Appalachian foreland in WestVirginia and their affect on sedimentation. They sug-gested that most of the larger basement faults in theRome trough formed during the PrecambrianGrenville orogeny. Subsequently, the trough experi-enced rifting during the Early(?) and Middle Cam-brian, postrift subsidence (Ryder et al., 1997a), pos-sibly forming a sag basin in the passive-marginsequence (Figure 2) during the Late Cambrian andOrdovician, and, finally, broad regional subsidenceassociated with a foreland basin stage during themiddle and late Paleozoic.
Basement structures of the Rome trough in easternKentucky were discussed in previous studies (e.g.,Black et al., 1976; Ammerman and Keller, 1979; Cableand Beardsley, 1984; Black, 1986; Drahovzal, 1994;Drahovzal and Noger, 1995; Shumaker, 1996). Forexample, Ammerman and Keller (1979) discussed theareal extent and geometry of the Rome trough ineastern Kentucky using gravity and deep drillingdata. Their gravity modeling results indicate thatbasement faults controlled the graben geometryand thickness of the Paleozoic sedimentary rocks inthis part of the Rome trough. Drahovzal and Noger(1995) and Shumaker (1996) mapped the extent ofmajor subsurface faults in eastern Kentucky usingexisting geologic maps, deep well data, and a seriesof widespread regional seismic lines.
The fundamental geological work in WestVirginia dates back to the early 1900s, when theWest Virginia Geological and Economic Surveypublished a series of geologic reports and maps.This was followed by extensive discussions on thestructural geology and stratigraphy in West Virginia(e.g., Neal and Price, 1986; Caramanica, 1988;Schwietering and Roberts, 1988; Zheng, 1990;Shumaker, 1993; Shumaker and Coolen, 1993;Wilson et al., 1994a, b; Gao, 1994; Gao and Shu-maker, 1994, 1996). For example, Shumaker (1993)and Shumaker and Coolen (1993) reported on thestudies on the East-Margin fault and the 38th paral-lel lineament. Wilson et al. (1994a, b) reported onthe study of ref lection seismic data across theRome trough of northern West Virginia. Their seis-mic analyses indicated that basement faults andfault-bounded blocks have been intermittently active
Gao et al. 77
Figure 2—Generalized stratigraphic sequences of thestudy area (after Schwietering and Roberts, 1988; Shu-maker and Wilson, 1996).
during the Paleozoic and have significantly affectedthe deposition of the Paleozoic sedimentary rocks inthe Rome trough of northern West Virginia.
Using more than 4000 shallow wells and severalseismic lines, Gao (1994) and Gao and Shumaker(1996) mapped the subsurface geology in south-western West Virginia and documented the geomet-ric and kinematic relationship of shallow structuresto the East-Margin fault and the 38th parallel andBurning-Mann lineaments. Based on their spatial andtemporal relationship, Gao (1994) and Gao andShumaker (1996) suggested that the 38th paralleland Burning-Mann lineaments represent a possible
oblique (wedge-shape) transfer fault system thataccommodated the complex deformation of thesubsurface structures in southwestern West Virginia.
In this study, using new seismic data and inter-pretation and subsurface mapping techniques, wecompare and contrast structures among the threerift segments to evaluate the along-axis variation ingraben geometry. We propose that the 38th paralleland Burning-Mann lineaments are two possibleaccommodation zones to transfer extension fromone graben segment to the next along the trough.We establish a structural model to emphasize along-rift segmentation and geometric and kinematic
78 Rome Trough
Figure 3—Superimposed magnetic and tectonic map showing the spatial relationship among the total magneticintensity, areal extent of the three major segments of the Rome trough, and associated major basement fault sys-tems. See Figure 1 for location. Magnetic intensity map is from King and Zietz (1978). Basement faults in easternKentucky are from Ammerman and Keller (1979). Basement faults in northern West Virginia are based on Shumak-er and Wilson (1996). Note that the major magnetic intensity lows are spatially related to the three major segmentsof the trough, which are named the eastern Kentucky segment, the southern West Virginia segment, and the north-ern West Virginia segment.
differences among the three rift segments, whichmay have important implications for hydrocarbonexploration along the Rome trough and other riftsystems on a regional basis.
SOUTHERN WEST VIRGINIA SEGMENT
The southern West Virginia segment is definedbetween the 38th parallel and Burning-Mann linea-ments (Figure 3). Because the southern WestVirginia segment is bounded by the 38th paralleland Burning-Mann lineaments to the south andnorth, respectively, a detailed study of the geometryand growth history of that segment, using new seis-mic data and interpretation techniques, provides akey to understand the along-axis variation in grabengeometry and growth history of the Rome trough.
Seismic Analysis
A total of 12 seismic lines were interpreted inthe southern West Virginia segment (Figure 4).
Sonic and density logs of three deep wells (Figure 4)were digitized. Impedance, reflection coefficients,and normal incidence synthetic seismograms werecomputed (Figure 5). The synthetic seismogramsare derived from the convolution between reflec-tion coefficients and a zero-phase wavelet. Thestratigraphic positions of the major ref lectionevents are shown in Figure 5. We identified strati-graphic intervals based on drillers’ logs and lateralcorrelation with other log data (e.g., Ryder, 1992;Drahovzal and Noger, 1995), and the seismic eventpicks are based on synthetic seismic correlationwith actual seismic lines (e.g., Figure 5c).
To better analyze and emphasize lateral and tem-poral variations in structural geometry, we digi-tized two-way traveltimes of major seismic eventsassociated with several key horizons. The digitizedreflections were vertically exaggerated by upwardshifting of sequential horizons coupled with rescal-ing on the vertical axis (Wilson et al., 1994a, b;Shumaker and Wilson, 1996). Although verticalshifts of reflection events eliminate absolute valuesof arrival time, such shifts do not affect absolutedifferences in structural relief along individual
Gao et al. 79
Figure 4—Index map showing the data set in thesouthern West Virginia segment of the Rometrough. See Figure 1 forlocation. The approximatelocations of seismic linesare indicated by bold straightlines. The background structure contour map,which was constructed on the basis of more than1000 shallow wells, showsthe top of the DevonianOnondaga Limestone insouthern West Virginia(from Gao and Shumaker,1996). The Onondaga servesas a reference surface for extrapolating and interpolating deep structures via trend-surfaceanalysis.
-4100
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ft
C.I. = 100 ftLogan County
Boone County
Mingo County
Kan
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N
East-M
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Fault
Line 1
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Line 3Line 4
Line 5
Line 6
Line 7
Line 8
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Line 10
Line 11
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K3462L1469
M805
38°N
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Warf
ield a
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80 Rome Trough
Impe
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reek
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danc
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ck R
iver
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reek
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ref lectors. Figure 6 demonstrates how verticalexaggeration is achieved via vertical shifts andrescaling of the sequential horizons. Original,unshifted reflections generally appear as widelyspaced flat areas. The original plotting scale tendsto mask or attenuate subtle structural and strati-graphic features. By upward shifting and rescalingon the vertical axis, subtle variations associatedwith the anticline (Figure 6a) and the fault (Figure6b) that would otherwise be difficult to recognizeare exaggerated. In the study area, we found nodirect evidence that supports the presence of veloc-ity anomalies, and both seismic lines and sonic logsindicate that major thickness changes within indi-vidual lines are restricted to the deepest, synriftreflections. Most of the shallow intervals show rela-tively minor changes in thickness; hence, in theabsence of a satisfactory time-depth conversiontable, major lateral variations in arrival time can beproperly interpreted as related to structural relief.The distortion, if any, may occur at the basementstructure level due to significant thickness variationof the overlying rift sequence.
To remove structural effects and to emphasizedifferential subsidence, we constructed a series ofsubsidence curves by calculating the differences inarrival times between the adjacent horizons, calledtime difference (Figure 6), that aid in identifyingchanges in the polarity of differential subsidence
and the location of depocenters. Unlike arrivaltime, lateral variation in time difference is notaffected by lateral variations in velocity of the over-lying strata. Basically, lateral variations in time dif-ference represent changes in rock thickness in theabsence of significant lateral variation in intervalvelocity. After careful examination of sonic logs ofseveral wells, and after making several test calcula-tions, we note that average interval velocity is rela-tively constant based on sonic logs from the threedeep wells; therefore, variations in time differencecan be properly interpreted as related to thicknesschanges, and local depocenters can be betterdefined with the help of the differential subsi-dence curves.
Line 1 is located in the central part of the south-ern West Virginia segment, away from both the38th parallel and Burning-Mann lineaments. Thedigitized and processed section (Figure 7a) sug-gests that this segment of the Rome trough is anasymmetric graben with a single East-Margin faultdipping toward the northwest on the southeasternside of the trough. Reflections, interpreted as topsof the Rome, Tomstown, and basement units, diptoward the East-Margin fault, suggesting that thehanging wall rotated clockwise into the footwall.We found more than 0.5 s of lateral variation intime difference (Figure 7b) across the East-Marginfault in the rift sequence (see Rome-Conasauga
Gao et al. 81
Figure 6—Two simplified examples showing the benefits of constructing shifted arrival times and time differencecurves from the original time section. Subtle structural and stratigraphic features such as the (a) anticline and the(b) fault are difficult to recognize in the original time section. By shifting sequential reflections and increasing thevertical scale, the geometry of the anticline (a) and the location of the fault (b) are easily recognizable; furthermore,the time difference curves, calculated by sequentially subtracting shifted arrival times, remove the structural effectto emphasize the differential subsidence associated with the drape anticline (a) and the growth fault (b).
Fault (?)
AoBo
Co
A
C
B
B-A
C-B
0.1
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OriginalTime Section
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Co
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(a) (b)
time difference), but only 0.2 s difference in thepassive-margin sequence (see Conasauga-Trentontime difference) and 0.1 s difference in the fore-land sequence (see Trenton-Onondaga time differ-ence), indicating that a major amount of differen-tial subsidence and deposition across the East-Margin fault occurred during the Early(?) andMiddle Cambrian. The thickest part of the Conasauga-Onondaga interval is located on the hanging walljust northwest of the East-Margin fault, but unlikethe underlying rift sequence, deposition is notrestricted within the trough but extends outsideof the trough. The overlying Onondaga-Bereainterval reveals a shift in the depocenter towardthe southeast (Figure 7), which suggests inver-sion across the East-Margin fault following depo-sition of the Devonian Onondaga Limestone.The relative time difference plot (Figure 7b)indicates that normal displacement of the East-Margin fault occurred intermittently throughoutthe Paleozoic.
Near the 38th parallel lineament is a series ofeast-west–trending basement faults. At the borderbetween the southern West Virginia and easternKentucky segments, lines 2, 3, and 4 show a well-defined basement fault that juxtaposes basementrocks on the south with sedimentary rocks of thetrough on the north (Figure 8). The changes inboth dip polarity (Lee, 1980) and strike of this base-ment fault (Gao, 1994; Gao and Shumaker, 1996)indicate the structural complexity at the border ofthe two rift segments.
Near the Burning-Mann lineament, internal struc-tures of the trough are more complicated. Digitizedreflections from line 5 (Figure 9a) reveal that reflec-tors above the Trenton Limestone dip to the east,whereas reflectors below the Trenton Limestonegenerally dip to the west. The East-Margin faultappears to be a high-angle normal fault dipping tothe northwest with a large normal offset of units inthe rift sequence below the top of the ConasaugaFormation. The southeast-dipping reflections of the
82 Rome Trough
OnondagaTrentonConasauga
East-Margin Fault
Top Berea
SE
East-Margin Fault
Onondaga-Berea
Trenton-OnondagaConasauga-Trenton
SE
(b)
Rel
ativ
e T
ime
Dif
fere
nce
Rel
ativ
e T
WT
(a)
Top Basement
Top Tomstown
Top Rome
Acadian Foreland
Taconic Foreland
Passive Margin
Rift
Sequences
Top OnondagaTop Trenton
Top Conasauga
1.0
(s)
1.0
(s)
LINE 1
Rome-Conasauga
Tomstown-RomeBasement-Tomstown
NW
NW
Figure 7—(a) Digitized andenhanced arrival timesand (b) time differencecurves of line 1 (see Figure 4 for location andscale) showing lateral variations in structure anddifferential subsidence,respectively. Note no absolute values areattached to the verticalaxis because of verticalshift of the arrival timesand time difference curves.See text for explanation.TWT = two-way traveltime.
Rome Formation and the Tomstown Dolomite nearthe East-Margin fault indicate a clockwise rotationof the fault block toward the East-Margin fault dur-ing the deposition of the rift sequence. Variationsin the time differences (Figure 9b) from the base-ment to Tomstown, the Tomstown to Rome, andthe Rome to Conasauga indicate southeastwardthickening locally in the vicinity of the East-Marginfault. The Conasauga to Trenton interval shows anorthwestward thickening into the Rome trough,which would be expected for a sag basin. TheTrenton to Rose Hill and the Rose Hill to Onondagaintervals both show slight thickening toward thenorthwest. These observations suggest that theclockwise rotation of the basement block occurredmainly during the Early(?) and Middle Cambrian.The Onondaga to Huron interval, however, thick-ens toward the southeast across the East-Marginfault, indicating a shift in subsidence polaritytoward the southeast after deposition of theOnondaga. This shift is accompanied by a changefrom carbonate to clastic sedimentary rocks of theAcadian sequence that is generally considered tomark the onset of an orogen in the growth historyof the Appalachian foreland.
Farther to the north, line 6 is located at the bor-der between the southern West Virginia and north-ern West Virginia segments. Here, the trough iscomplicated by a major basement fault in the interi-or of the trough, called the Interior fault (Figure 10).The extension, which occurs largely across the East-Margin fault to the south, probably is distributedbetween the two major faults toward the northernWest Virginia segment north of the Burning-Mannlineament. The Interior fault shows a normal offsetof more than 0.35 s, which is larger than that of theEast-Margin fault (compare the Interior fault andthe East-Margin fault in Figure 10a). The variationsin interval time difference (Figure 10b) suggest acomplex history of differential subsidence duringthe Paleozoic controlled by the Interior fault andthe East-Margin fault. Interestingly, the Interior faultseems to have a longer history of deformation thanthe East-Margin fault; the Interior fault influencedthe deposition from the Cambrian to Devonianwith alternating polarity of differential subsidence,whereas the East-Margin fault had minor effect onsedimentary rocks deposited after the Conasaugawas deposited. These observations indicate thatline 6 shows different internal structure and
Gao et al. 83
NW
0.5
1.0
1.5
2.0
2.5
TW
T (
s)
East-Margin Fault
Top Berea
Top Conasauga
Top Trenton
Top Onondaga
Top Basement
SE
Top Basement
LINE 4 Figure 8—Seismic line 4(see Figure 4 for locationand scale) across the East-Margin fault in thesouthern West Virginiasegment of the Rometrough. Note the East-Margin fault and the discontinuity of reflections across the fault. TWT = two-way traveltime.
growth history of the trough than line 5, whichsuggests that graben geometry and growth historymay have changed from the southern West Virginiasegment to the northern West Virginia segment.
On the northwestern side of the Rome trough(Figure 11), the top of basement dips gently east-ward toward the East-Margin fault; the dip may becaused by the rotational subsidence at the westernmargin of the trough. Increasing dip of the reflec-tions with depth indicates that rotational subsidencelargely occurred during the rift stage, and that differ-ential subsidence or clockwise rotation became lessactive after the formation of the rift basin.
In summary, several points regarding the south-ern West Virginia segment can be drawn from seis-mic analysis. (1) The strike, dip, and throw of theEast-Margin fault change significantly along strikefrom south to north. Between the 38th parallel andBurning-Mann lineaments, the East-Margin faultstrikes to the northeast and dips to the northwest.Toward the south at the segment border, the East-Margin fault swings abruptly to the west near the38th parallel lineament. Toward the north at thesegment border of the Burning-Mann lineament,the graben geometry is complicated by an Interiorfault, and the influence of the East-Margin fault on
deposition decreases from south to north relativeto the Interior fault. (2) Three vertically stackedstructural styles are associated with three verticallystacked rock sequences (Figure 2) that ref lectchanges in the structural history of the study area.These styles include a half graben associated with arift sequence that abruptly appears at the easternmargin of the rift, a relatively broad sag basin asso-ciated with a passive-margin sequence that slightlythickens above the rift, and a foreland sequencethat expands eastward toward the plate margin andis particularly apparent on the seismic expressionof the Acadian clastic wedge.
Structure and Isopach Contouring
A total of 2221 control points were used to con-struct subsurface geologic maps. These datainclude depths to the top of the DevonianOnondaga Limestone extracted from the wells,depths to the top of the Precambrian basement dig-itized from an acoustic basement map (J. Lemon,1993, personal communication), and depths to sev-eral intervening horizons of the Ordovician andCambrian converted from seismic sections.
84 Rome Trough
Onondaga-HuronRose Hill-Onondaga
Trenton-Rose HillConasauga-Trenton
SE
(b)
Top HuronSE
(a)
Top Rome
Top Tomstown
Top Conasauga
Top Rose HillTop Trenton Top Onondaga
Acadian Foreland
Taconic ForelandPassive Margin
Rift
Sequences
Rel
ativ
e T
ime
Dif
fere
nce
Rel
ativ
e T
WT
1.0
(s)
1.0
(s)
LINE 5
Basement-Tomstown
Tomstown-RomeRome-Conasauga
East-Margin Fault
East-Margin Fault
Top Basement
NW
NW
Figure 9—(a) Digitizedand enhanced arrivaltimes and (b) time difference curves of line 5(see Figure 4 for locationand scale) showing lateralvariations in structure anddifferential subsidence,respectively. Note noabsolute values areattached to the vertical axisbecause of vertical shift ofthe arrival times and timedifference curves. See textfor explanation. TWT =two-way traveltime.
To construct the structure and isopach contourmaps at deep structure levels of the Cambrian andOrdovician, arrival times of the seismic sectionswere first converted to depth using interval veloci-ties derived from sonic logs nearest to the seismiclines (e.g., Zheng, 1990). Following the time-depthconversion, we integrated the well data with theseismic data and performed multisurface extrapola-tion and interpolation via thickness trend analysis(Figure 12). This computation may introduce con-siderable error due to a lack of detailed three-dimensional velocity control and structural com-plexities across the whole area, and thus theresultant contour maps provide only low-resolutionconstraints on structures of deeply buried Cam-brian and Ordovician formations. A comparisonwas made with published maps produced by welldata at shallow structural levels to determine thereliability of the interval thickness trend analysis.The comparison demonstrated that maps made viatrend analysis are similar to those made based onlyon well data.
Even allowing for the potential errors inherentin velocity and the sparse well control at depthsbelow the Devonian Onondaga horizon, contourmaps indicate that structural relief generallyincreases with depth, which is consistent withand obvious from seismic sections; however, thehalf graben that is visible below the OrdovicianTrenton Limestone (Figure 12b, c) contrasts withthe asymmetrical anticlines seen in the youngerrocks (Figures 4, 12a) (Gao and Shumaker, 1996).This contrast probably relates to inversion of thetrough. Basement structures are characterized bya structural depression that is bounded by base-ment faults on the southeastern margin of thetrough. The asymmetry of the trough at thebasement level is shown by a di f ference insteepness of dip between the northwestern andsoutheastern margins of the trough. In mapview, the general trend of the graben is to thenortheast, but adjacent to the 38th parallel andBurning-Mann lineaments, it swings to the westand north, respectively (Figure 12c).
Gao et al. 85
SE
Above OnondagaAbove Trenton
Above Conasauga
Above Basement
Interior Fault(b)
East-Margin Fault
SE
(a)
Top BasementInterior Fault
Top Trenton
Top Conasauga
Top Onondaga
Acadian Foreland
Taconic Foreland
Passive Margin
Rift
Sequences
East-Margin Fault
LINE 6R
elat
ive
Tim
e D
iffe
renc
eR
elat
ive
TW
T0.
5 (s
)0.
5 (s
)NW
NW
Figure 10—(a) Digitizedand enhanced arrivaltimes and (b) time difference curves of line 6(see Figure 4 for locationand scale) showing lateralvariations in structure anddifferential subsidence,respectively. Note noabsolute values areattached to the vertical axisbecause of vertical shift ofthe arrival times and timedifference curves. See textfor explanation. TWT =two-way traveltime.
Isopach maps indicate that both the rift se-quence (Figure 12e, f) and the Taconic forelandsequence (Figure 12d) are affected not only by thelimits of the trough itself but also by the 38th par-allel lineament to the south and the Burning-Mannlineament to the north. A northeastward shift ofdepocenter along the trough is suggested by com-paring the thickness patterns of the sequentialisopach maps (Figure 12d–f). Vertical changes inrock thickness patterns and gradients from theoldest sedimentary rocks (Figure 12f) to the suc-cessively younger intervals (Figure 12d, e) proba-bly reflect the transition from the rift to the sagbasin stage.
In summary, subsurface structure and isopachmaps indicate that the southern West Virginia seg-ment of the trough is characterized by the East-Margin fault that developed along the southeasternside of the trough. Across the East-Margin fault, theearly Paleozoic sediments thicken from 1000 ft
(305 m) outside of the trough to more than 20,000ft (6100 m) within the trough (Harris, 1975, 1978;Neal and Price, 1986; Schwietering and Roberts,1988). The asymmetry of the trough cannot beextended along its axis into Kentucky. This changeis spatially associated with the 38th parallel linea-ment and an increased structural complexity with-in the zone itself, as well as by the increased com-plexities in dip and strike of the East-Margin fault(Gao and Shumaker, 1996). We interpret this struc-tural complexity as an indication of an accommoda-tion zone that transfers extension between thesouthern West Virginia segment and the easternKentucky segment of the Rome trough.
EASTERN KENTUCKY SEGMENT
The eastern Kentucky segment is definedbetween the 38th parallel lineament and the
86 Rome Trough
0.5
1.0
1.5
2.0
2.5Deep Reflection Event DippingToward the East-Margin Fault
Western Margin
SE
Top Basement
Top Tomstown
Top Rome
Top Conasauga
Top Trenton
Top OnondagaT
WT
(s)
Flat Reflection Events at Shallow Structural Level
LINE 8
NW
Figure 11—Seismic line 8(see Figure 4 for locationand scale) on the westernflank in the southern WestVirginia segment of theRome trough. Note thebasement top dips towardthe East-Margin fault onthe southeast, indicatingrotation of basement faultblock and asymmetricgeometry of the trough.TWT = two-way traveltime.
Lexington fault zone (Figure 3), which roughly coin-cides with the western overthrusted margin of theGrenville (1.0 Ga) basement (Drahovzal and Noger,1995; Shumaker, 1996). Differing from the southernWest Virginia segment, the eastern Kentucky seg-ment shows an irregular and complicated anomalypattern in both gravity (Ammerman and Keller,1979; Kulander et al., 1987) and magnetic intensi-ties (Figure 3) (King and Zietz, 1978), suggestingmore complicated basement structures than in thesouthern West Virginia segment. Several basementfaults extend to the surface along the northwesternmargin of the trough. Computer modeling of gravityprofiles of the Rome trough in eastern Kentucky byAmmerman and Keller (1979), using data fromdeep wells as constraints, indicates that the troughis bounded on the north by a major gravity gradientthat corresponds to a major basement structurecalled the Kentucky River fault zone that has beenmapped at the surface (Figure 3). A second faultzone called the Irvine-Paint Creek fault zoneextends eastward within the trough from theLexington fault zone (Figure 3).
The Rome trough in eastern Kentucky has beenidentified since the early 1960s on the basis ofstratigraphy and structure in the region. Woodward(1961) first described it as a Lower Cambriancoastal declivity, which may be a fault scarp or asteep coastwise cliff, that is responsible for anabrupt thickening of the Early Cambrian deposits.Along the northwestern boundary of the trough,sedimentary rocks of the Rome Formation thickenfrom approximately 270 ft (82 m) north of thetrough to more than 4560 ft (1390 m) within thetrough (Thomas, 1960; Ammerman and Keller,1979). Harris and Drahovzal (1996) reported that therift sequence (pre-Knox deposition) is 300–600 ft(91–183 m) thick in northernmost Kentucky, butabruptly thicken across a series of extensionalgrowth faults (the Kentucky River fault system) intothe trough where the rift sequence is as thick as10,000 ft (3050 m) in some areas (e.g., Drahovzaland Noger, 1995). Rocks that may be Early Cam-brian but largely are Middle Cambrian in age (Ryderet al., 1997b) in the trough have no equivalents tothe north on the shelf of the rift basin, and theCambrian depocenters are located along the north-western margin of the trough in Kentucky (Harrisand Drahovzal, 1996). Structure maps of thePrecambrian basement surface in eastern Kentucky(Drahovzal and Noger, 1995; Shumaker, 1996) indi-cate that structural relief of the top of the Pre-cambrian basement is greater than 13,000 ft (3965 m)from the northern boundary to the deepest partof the trough, whereas relief along the southernboundary is generally only 7000–8000 ft (2135–2440 m) and locally much less. Webb (1969) sug-gested that the trough was formed by growth
faulting along a cratonic boundary with a fault scarpat its northern boundary. Silberman (1972) con-firmed this interpretation by showing that throwalong the northern boundary is approximately 5000 ft(1525 m) at the top of the Precambrian, decreasingto 3900 ft (1189 m) at the top of the basal sand andto 2060 ft (628 m) at the top of the RomeFormation. The Kentucky River fault zone swingstoward the south to join the Lexington fault zone todelineate what appears to be the northwesternboundary of the Rome trough in Grenville base-ment of that area (Black et al., 1976; Black, 1986;Shumaker, 1996). A few faults extend westward inolder basement to connect with major faults of theWestern Kentucky graben (Figure 1); however, mostof the Rome trough faults converge and apparentlydisappear as they swing southward.
Drahovzal and Noger (1995) and Shumaker(1996) mapped the subsurface extent of faults ineastern Kentucky using proprietary seismic data toconfirm that the surface faults in that area are partof the Cambrian rift system. The asymmetry andarcuate geometry of the fault system in easternKentucky indicate that the northwestern boundaryof the Rome trough in eastern Kentucky is a convex-to-the-northwest fault system (Figure 13a) thatincludes several smaller half grabens with oppositepolarity to the large fault-bounded graben of thesouthern West Virginia segment. The fault alongthe southeastern margin of the trough is diffuseand is generally of small magnitude (Figure 13b)(Ammerman and Keller, 1979; Drahovzal andNoger, 1995; Shumaker, 1996). These observationsindicate that the graben polarity is reversed andthat deposition in the eastern Kentucky segmentwas controlled by several faults along the north-western side of the trough.
NORTHERN WEST VIRGINIA SEGMENT
The northern West Virginia segment refers tothat part of the trough north of the Burning-Mannlineament. Its northern boundary is unknownbecause little subsurface data are published todefine its northern extent. The total magneticintensity map by King and Zietz (1978) shows across-strike magnetic intensity gradient at the lati-tude of 40° in southern Pennsylvania called the40th parallel lineament (Shumaker and Wilson,1996) (Figures 1, 3). In contrast to the southernWest Virginia segment, this segment has a magneticlow on the western side of the trough, and it hasseveral magnetic intensity highs in the middle andthe east of the trough. In addition, the gradient ofthe magnetic intensity across the East-Margin faultis relatively low compared to the southern WestVirginia segment.
Gao et al. 87
C.I.=600 ft
-102
00
-120
00
-138
00
-15000
-11400
-180
00
-174
00
-186
00
-120
00
-156
00
-13200
-15000
-162000
38th Parallel Lineament
Bur
ning
-Man
n L
inea
men
t
East-M
argin
Fault
38°N
82°W
82°W
Tomstown
0 5000025000
0 5 10 15 20 25 km
ft
38°N
(b)
Trenton
-660
0
-630
0
-630
0
-600
0
-600
0-5
400
-570
0
-540
0
-540
0
-5100
-510
0
-510
0
-480
0
-480
0
-450
0
-420
0
-690
0
-720
0
-750
0 -780
0
-8100
-5400
C.I.=300 ft
38°N
82°W
82°W
38th Parallel Lineament
Bur
ning
-Man
n L
inea
men
t
East-M
argin
Fault
-5700 0 5000025000
0 5 10 15 20 25 km
ft
38°N
(a)
Figure 12—Computer-generatedsubsurface structure andisopach maps. These mapsindicate tectonic control bybasement structures on deposition of the Cambrianand Ordovician in the southernWest Virginia segment. Focusedsedimentation was controlledby the East-Margin fault, the38th parallel lineament, andthe Burning-Mann lineament.The structural lows and thickness changes occur in the vicinity of the 38th paralleland Burning-Mann lineaments,which suggest transtensionaldeformation associated with thetwo oblique accommodationzones. (a) Trenton Limestonestructure, (b) TomstownDolomite structure, (c) basementstructure, (d) isopach betweenOnondaga Limestone and Trenton Limestone, (e) isopachbetween Rome Formation and Tomstown Dolomite, (f) isopach between TomstownDolomite and top of basement.
Basement
-23000
-230
00
C.I.=1000 ft
-10000
-130
00
-16000
-19000
-22000
-240
00
-12000
-14000
-19000
-210
00
-230
00
-150
00
38th Parallel Lineament
Bur
ning
-Man
n L
inea
men
t
East-M
argin
Fault
38°N
82°W
82°W
0 5000025000
0 5 10 15 20 25 km
ft
38°N
(c)
C.I.=300 ft
Isopach of Onondaga-Trenton (Taconic Sequence)
1800
2700
3300
2700
1800
210024
00
3000 3000
3600
3600
38th Parallel Lineament
Bur
ning
-Man
n L
inea
men
t
East-M
argin
Fault
38°N
82°W
82°W
4500
0 5000025000
0 5 10 15 20 25 km
ft
38°N
(d)
Figure 12—Continued.
Tomstown (?)-Basement (Lower Rift Sequence)
C.I.=500 ft
3000
2000
4000
5000
60007000
8000500
5500
5000
4500 500
4000
38th Parallel Lineament
Bur
ning
-Man
n L
inea
men
t
East-M
argin
Fault
38°N
82°W
82°W
0 5000025000
0 5 10 15 20 25 km
ft
38°N
(f)
C.I.=300 ft
Isopach of Rome (?)-Tomstown (?) (Upper Rift Sequence)
3000
2400
1500
600
1800
27002100
1200
1800
38th Parallel Lineament
Bur
ning
-Man
n L
inea
men
t
East-M
argin
Fault
38°N
82°W
82°W
0 5000025000
0 5 10 15 20 25 km
ft
38°N
(e)
Figure 12—Continued.
A regional contour map of the top of theGrenville basement (Shumaker, 1996) indicates thatthe trough geometry in the northern West Virginiasegment is somewhat different from that of thesouthern West Virginia segment. The interior of thetrough is broken by a large, tilted horst block that isbounded on the west by the Interior fault, and thetrough becomes wider and the East-Margin fault isless well defined. These changes contrast with thecomparatively unbroken and narrow graben of thesouthern West Virginia segment. Several isopachmaps (Figure 14), such as that of the Devonianblack shale (Harris et al., 1978), Mississippian BigInjun sandstone, and Mississippian Weir sandstone(Zou and Donaldson, 1994) indicate that the north-ern West Virginia segment has different lithofaciesand rock thickness compared to those in the
southern West Virginia segment, and the interven-ing Burning-Mann lineament is associated with anorth-trending linear gradient of rock thicknessshown inseveral published isopach maps (Harris etal., 1978; Zou and Donaldson, 1994; Shumaker andWilson, 1996).
Comparison of seismic lines indicates that theEast-Margin fault in this segment has different geom-etry, internal structure, and deformation historyfrom those observed in the southern West Virginiasegment. For example, a seismic line (Figure 15)across the northern West Virginia segment (T. H.Wilson, 1998, personal communication) indicatesthat the East-Margin fault controlled deposition andbasin geometry during the rifting stage from theEarly(?) to Middle Cambrian, but it was largely inactiveduring the passive-margin and foreland basin stages;
Gao et al. 91
Gra
vity
(m
gal)
-20
-40
-60
2 3
2.652.70
3.0
2.51
2.782.72
2.60
2.85
4 5
0
0
1
2
3
4
5Dep
th (
km)
20 km
Ken
tuck
y R
iver
Fau
lt Z
one
A B
1 6
0 20 mi
Kentucky River Fault Zone
Irvine-Paint Creek Fault Zone
38° N38°N
83°W84°W
37°N 37°N
A
B
Lex
ingt
on L
inea
men
t
38th Parallel Lineament
Southeaster
n Marg
in of Trough
(a)
(b)
Figure 13—Structure map and computed cross sectional modelbased on gravity data along profileAB. Densities for each unit aregiven in grams/cubic centimeter.Wells used for control are 1 = U.S.S.Chemicals well (Scioto County,Ohio), 2 = Inland Gas 538 CoaltonFee, 3 = Inland Gas 533 Fee, 4 =Inland Gas 542 Young, 5 = Signal 1Elkhorn Coal Company, 6 = Signal 1Stratton. The master synthetic faultzone labeled as Kentucky Riverfault zone is located on the northwestern side of the trough(from Ammerman and Keller, 1979).
however, substantial differential subsidenceoccurred along the northwestern margin of thetrough during the passive-margin and foreland basinstages (Figure 15). Thus the northwestern margin inthe northern West Virginia segment of the troughappears to have had a longer history of differentialsubsidence through the passive-margin phase thanthe Eastern-Margin fault. These observations implythat differential subsidence shifted from the easternmargin during the rift stage to the western marginduring the passive-margin and possibly the forelandbasin stages, which contrasts with the growth histo-ry of the southern West Virginia segment where theEast-Margin fault continued to affect sedimentationthrough the passive-margin and foreland basinstages (see Figures 7, 9).
In addition to the deeply buried rift and sagbasin, folds at shallow structural levels also showcontrast in deformation intensity and asymmetrybetween the northern West Virginia and the south-ern West Virginia segments (compare Figures 7 and15). In the northern West Virginia segment, asym-metry polarity of the anticlines at the Rose Hill andthe Onondaga horizons is toward the northwest(Figure 15), whereas in the southern West Virginiasegment, the asymmetry of an anticline at the samehorizons is toward the southeast (Figure 7). Theseobservations suggest that rift segmentation proba-bly continued to impact shallow structures thatformed during the foreland basin stage, and thethree rift segments may have been overprinted todifferent extents by the crustal deformation duringthe foreland basin stage.
DISCUSSION
Accommodation Zones
Total magnetic intensity, Bouguer gravity anoma-ly, reflection seismic, and well data show that theeastern Kentucky, southern West Virginia, andnorthern West Virginia segments of the Rometrough are different in geometry and polarity ofnormal faulting, sedimentation, and folding. Fromsouth to north, these three rift segments are sepa-rated by the 38th parallel and Burning-Mann linea-ments. A regional, comparative analysis supports
92 Rome Trough
OH
Lower Weir
Upper Weir
Mid Weir
PA
WV
C.I.=40 ft
Northern WV Segment
0
40
40
AB
Burning-M
ann Lineam
ent
Southern WV Segment
OH
PA
WVNorthern WV Segment
Southern WV Segment
0
40
0
C.I.=40 ft
Burning-M
ann Lineam
ent
AB
80
800
400
400
OH
PAWVNorthern WV
Segment
400
600
C.I.=200 ft
AB
Burning-M
ann Lineam
ent
Southern WV Segment
East-Margin Fault
20 km
20 mi0
East-Margin Fault
East-Margin Fault
Figure 14—Selected isopach maps of the northern WestVirginia segment (after Shumaker and Wilson, 1996).Note the relationship of the Burning-Mann lineament tochanges in rock thickness and depositional environ-ment. Line AB indicates a seismic line shown in Figure15. (a) Devonian black shale (Harris et al., 1978), (b)Mississippian Big Injun sandstone and Weir sandstonesubcrop (Zou and Donaldson, 1994), (c) MississippianWeir sandstone (Zou and Donaldson, 1994).
the suggestion that the two lineaments representaccommodation zones that transferred the along-axis reversal in geometry and polarity of the threesegments of the Rome trough (Figures 3, 16).
The Burning-Mann lineament is a north-south–trending structure that separates the northern WestVirginia segment from the southern West Virginiasegment. Based on well, seismic, and magneticintensity data, it was previously documented thatthis lineament had a strong effect on the subsurfacegeology in south-central West Virginia, and possiblyrepresents a transfer fault system to accommodatethe complex deformation in the subsurface ofsouth-central West Virginia (Gao, 1994; Gao andShumaker, 1996). Although direct evidence forthroughgoing basement faults along the Burning-Mann lineament is equivocal, it is spatially associat-ed with the detached Burning Springs (Rodgers,1963) and the Mann Mountain anticlines (Perry,1980). Total magnetic intensity maps suggest thepresence of a basement fault zone underneath boththe Burning Springs and the Mann Mountain anti-clines. Presumably, the basement structure served inpart as a locus for the formation of the overlyingdetached folds in the central Appalachian forelandbasin. Seismic data (Shumaker, 1986b) shows a base-ment fault that affects Cambrian strata under the
northern end of the Burning Springs anticline. Root(1996) interpreted strike-slip movement along abasement fault under the Cambridge arch, anextension of the Burning-Mann lineament in Ohio.The intensively deformed Burning Springs and theMann Mountain anticlines in the Paleozoic coverrocks may be attributed in part to transcurrentmovement of the underlying basement fault zonesof the Burning-Mann lineament. The stackedchanges in facies and rock thicknesses along theBurning Springs anticline, as described in thispaper, and in other rock units (Shumaker andWilson, 1996) (see Figure 14) also support thepresence of active basement faults or a basementf lexure under the lineament. Beaumont et al.(1988) suggested that the Burning-Mann lineamentwas a peripheral bulge, but the consistent positionof facies and thickness changes in Phanerozoicrocks through time (Shumaker and Wilson, 1996)suggest the existence of a more fundamentalchange in crustal structure or thickness rather thanan ephemeral bulge.
The 38th parallel lineament (Heyl, 1972),approximately following the 38th parallel of lati-tude for more than 800 mi (1280 km), is an east-trending zone of complex deformation and sedi-mentation. Along the lineament, mullions in fault
Gao et al. 93
Top Basement
Interior Fault East-Margin Fault10 km
A
0
-2000
-4000
-6000
-8000
B
Dep
th (
m)
Top Rose Run
Top Rose Hill
Top Trenton
Top Onondaga
Top Greenbrier
Top Huron
Acadian Foreland
Taconic Foreland
Passive Margin
Rift
Sequences Top Conasauga
W E
Figure 15—A depth-converted seismic line (see Figure 14 for location) across the northern West Virginia segment ofthe Rome trough (T. H. Wilson, 1998, personal communication). Note the East-Margin fault has little effect onpostrift sedimentation after Conasauga deposition, contrasting with the East-Margin fault seen in the southern WestVirginia segment (Figure 7). In contrast, the western margin of the trough experienced consistent differential subsi-dence throughout the early Paleozoic. Asymmetrical anticlines above the Devonian Rose Hill horizon with steeperwestern flanks than eastern flanks, contrast with those mapped in the southern West Virginia segment (Gao, 1994;Gao and Shumaker, 1996).
outcrops show that lateral displacement is a majorcomponent accompanied by scissors and dip-slipmovements (Heyl, 1972). Shumaker (1993) andShumaker and Coolen (1993) reported on the rela-tionship of the 38th parallel lineament to the sub-surface structures in West Virginia. On the basis ofsubsurface geologic mapping in southern WestVirginia, Gao (1994) and Gao and Shumaker (1996)suggested that part of the 38th parallel lineamentpossibly represents another transfer fault system thatwas coupled with the Burning-Mann lineament toaccommodate the complex deformation of the sub-surface structures in south-central West Virginia. Ineastern Kentucky and southern West Virginia, thelineament constitutes the boundary between thetwo rift segments with opposite graben polarity,which is typical of accommodation zones. Both sur-face and subsurface data in this limited section of thelineament indicate the presence of regional east-west–trending faults. The trend of the lineament ismore complicated where it intersects the Rometrough in the border area of eastern Kentucky andWest Virginia. There, a boxlike or rhom-graben(Shumaker, 1996) that is typical of an extensional,transfer zone suggests left-lateral offset between thetwo rift segments. Incoherent seismic reflections atthe Kentucky–West Virginia border also indicatecomplex deformation of Cambrian sedimentaryrocks within the accommodation zone. This in-crease in structural complexity coincides with thewestward bending of the rift trend and the East-Margin fault (Gao, 1994; Gao and Shumaker, 1996)between the southern West Virginia and the easternKentucky segments.
Differing from other accommodation zones com-monly observed along extensional systems that are
orthogonal to the rift axis and parallel to eachother, the east-west–trending 38th parallel and thenorth-south–trending Burning-Mann lineaments areboth oblique to the northeast-trending rift axis ofthe Rome trough and converge toward the south-east. This relationship (Gao, 1994) requires that the38th parallel and Burning-Mann accommodationzones experience transtensional rather than simplestrike-slip displacement during the formation of thetrough (Gao, 1994; Gao and Shumaker, 1996). Thetranstensional deformation is responsible for thenormal displacement component, magnetic intensi-ty gradient, structural lows (e.g., Figure 12c), androck thickness changes (e.g., Figure 12e, f) in thevicinity of the accommodation zones. This trans-tensional deformation along oblique accommoda-tion zones may be an important mechanism toaccommodate the along-axis variation and segmen-tation of rift systems.
Growth History
The Rome trough formed as a result of theEarly(?) and Middle Cambrian faulting associatedwith the late-stage opening of the Iapetus-TheicOcean at the continental margin (Thomas, 1991;Shumaker, 1996; Shumaker and Wilson, 1996). Adetailed study in southern West Virginia indicatesthat the trough has experienced a multiphased tec-tonic evolution and evolved during an episode ofrapid extension in the early Paleozoic following thelate Precambrian Grenville convergent orogeny(Shumaker, 1996; Shumaker and Wilson, 1996). Themajor extensional event is indicated by an abruptchange in thickness of the interval deposited before
94 Rome Trough
Figure 16—A conceptualmodel showing along-axissegmentation and variationin graben geometry andpolarity of the Rometrough. Three segments,denoted as the easternKentucky, southern West Virginia, and northernWest Virginia segments,respectively, are separatedby the 38th parallel and Burning-Mann accommodation zones.Note how the differencesin graben geometry andpolarity of the three majorsegments are accommodatedby the intervening accommodation zonesalong the axis of the Rome trough.
Copper Ridge deposition (Lower and MiddleCambrian) across the southeastern margin of thetrough (see Figures 7, 12e, f). A drill hole in south-ern West Virginia (Donaldson et al., 1975) and therecovered fossils from a cored sequence within thelower one-half of the Rome Formation indicate anage of Middle Cambrian and a possible Late Cam-brian age (Donaldson et al., 1988). Based on thefossil data, Ryder et al. (1997b) considered theRome Formation drilled in southern West Virginiato be Middle Cambrian in age, but a late EarlyCambrian age for the lowermost part of the RomeFormation at that locality cannot be ruled out(Ryder et al., 1997b). They also suggested that theRome Formation at the western margin of theRome trough and in the adjoining area probably isentirely Middle Cambrian in age (Ryder et al.,1997b). This age is different from the ages of othergrabens of the interior rift system to the southeast,such as in Alabama, Virginia, and Tennessee (seeFigure 1) (W. A. Thomas, 1998, personal communi-cation). In addition, radiometric dates at the platemargin (Bartholomew et al., 1991) from igneousrocks of the rift sequence that were deposited onGrenville basement provide an age of approximate-ly 570 Ma, indicating that the major rift event in theRome trough is slightly younger than the earlyspreading at the plate margin (Read, 1989; Shu-maker, 1996), and that major extension associatedwith different grabens of the interior rift systemwas diachronous in association with differentstages of the spreading at the plate margin. Thickinfill of sediments with little igneous rocks withinthe Rome trough, indicated by data from deep welldrilling in the southern West Virginia segment(Donaldson et al., 1988), suggests that the crustalextension was largely accommodated by mechani-cal extension and syntectonic sedimentation. Thisscenario contrasts with successful rifts farther eastand south at the plate margin where continentalcrust completely broke up to form oceanic crust(Thomas, 1991). Although inconsistent stratigraph-ic nomenclature and ages of the Cambrian sedi-mentary rocks make it difficult to achieve adetailed and systematic lateral correlation on aregional basis, age differences among differentgrabens suggest that extension may have occurredat different times and rifting propagated in a sys-tematic manner from spreading center toward theinterior of the craton.
Following the Early(?) and Middle Cambrian rift-ing, a decrease in the rate of differential subsidenceoccurred during the time between deposition ofthe Cambrian Conasauga Formation and the MiddleOrdovician Trenton Limestone, as indicated byanalysis of arrival time differences of seismic reflec-tions (Gao, 1994). This slow subsidence and rever-sal in direction of sedimentary thickening suggest
that a relatively broad, shallow sag basin (Shumaker,1986a, 1996) formed above the rift basin of theRome trough. The mechanism for the formation ofthe sag basin may be attributable to the postriftthermal contraction at the passive continental mar-gin (Shumaker, 1996; Shumaker and Wilson, 1996)and to decreased differential subsidence along themaster synthetic fault and other basement faultsthat have been intermittently reactivated duringthe postrift broad subsidence.
Little differential subsidence had occurred duringthe Late Ordovician–Middle Devonian, but regionalstudies indicate a reversal and thickening of MiddleOrdovician–Lower Silurian rocks into the develop-ing Taconic foreland. Anticlines began to developover the underlying half graben, but their geometryand intensity are different from one rift segment tothe next (e.g., Figures 7, 12a, 15). We suggest thatstructural and depositional inversion probablycould be attributed to Taconic and Acadian oroge-nies as the foreland developed, which, in turn,probably were associated with a major change instress regime that emanated from the plate marginduring the Middle Devonian–Pennsylvanian whenthe Iapetus-Theic Ocean closed during the Acadianand the Alleghanian orogenies.
Implications for Hydrocarbon Exploration
In the southern and northern West Virginiasegments, and to a lesser extent in the easternKentucky segment, exploration has been limitedlargely to post-Ordovician rocks of the forelandsequence. Thus opportunity for exploration existsin the two older (rift and passive-margin) se-quences. The role of accommodation zones inhydrocarbon accumulation lies in the enhancedporosity and complicated structure and stratigra-phy at their intersection along the length of a com-posite graben. The paleotopographic complexity ataccommodation zones favors the formation ofstratigraphic traps due to facies and thicknesschanges at segment borders. Also, accommodationzones are responsible for variations in polarity ofboth half grabens and the overlying inversion struc-tures. Differences in graben geometry, polarity, andmagnitude of extension among the three rift seg-ments may result in changes from one segment tothe next in source rock thickness and distribution,reservoir lithofacies and thickness, fracture porosi-ty and permeability, and geothermal gradient andthermal maturity of sediments. In addition, thick-ness and facies changes are common in the vicinityof the master synthetic fault along one side of a halfgraben. Generally, coarser grained facies, such assand-rich fan deposits, are likely to exist in a nar-row fairway and at point sources in proximity to
Gao et al. 95
the master synthetic fault. Such potential reservoirfacies, coupled with significant enhancement of frac-ture porosity and permeability, could result in signifi-cant hydrocarbon accumulation along the mastersynthetic fault. For example, fan-delta and basin-floorfan deposits have been documented in the vicinity ofthe master synthetic fault on the northwestern sideof the trough in eastern Kentucky (Drahovzal, 1994;Harris and Drahovzal, 1996). Commercial gas wellsthat produce from a fractured interval of the Con-asauga and Rome Formation also have been reportedin eastern Kentucky (Harris and Drahovzal, 1996).The potential reservoir facies are near the northernbounding fault zones, and the fracturing is related tothe faults along the northwestern margin of thetrough in eastern Kentucky (Harris and Drahovzal,1996). In addition, commonly associated with mastersynthetic faults are structural closures that can trapoil and gas at relatively shallow structural levels, suchas in post-Ordovician rocks of the foreland sequencein the southern West Virginia segment of the Rometrough (Gao and Shumaker, 1996). These geometricand spatial relationships make trough margins favor-able sites for the migration and accumulation of oiland gas. Polarity shifts along the Rome trough acrossaccommodation zones probably have affected source
rock thickness, reservoir lithofacies (Figure 14), andthe potential distribution of oil and gas fields (Figure17) in different segments of the Rome trough. Thisalong-axis segmentation and associated accommoda-tion zones may have important implications forassessing hydrocarbon potential of the deeply buriedrift sequence, the intermediate passive-marginsequence, and the shallow foreland sequences alongthe Rome trough.
CONCLUSIONS
(1) The Rome trough can be divided into theeastern Kentucky, southern West Virginia, andnorthern West Virginia segments. The master syn-thetic faults in eastern Kentucky are located on thenorthwestern side of the trough. These master syn-thetic faults shift to the southeastern side of thetrough and coalesce to a single East-Margin fault insouthern West Virginia. The southern West Virginiasegment is characterized by a deep, narrow rift val-ley, and the East-Margin fault controlled the con-centrated sedimentation of the Early(?) and MiddleCambrian deposition. This East-Margin fault hasbeen intermittently reactivated throughout the
96 Rome Trough
0 30 mi
82° 81° 80° 79° 78°
38°
39°
40°
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rn W
V Seg
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Burning-M
ann Accom
omdation Z
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38th Parallel Accommodation Zone
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Anticline
Approximate Extent ofOil and Gas Fields
Figure 17—Areal extent of major oil and gas fields in West Virginiasuperimposed with themajor structural elements, showing spatial relationships of hydrocarbon distribution to rift segments and accommodation zonesalong the Rome trough.
Paleozoic. Northward in northern West Virginia,the trough is complicated by the major Interiorfault, and the East-Margin fault has a differentgrowth history than that in the southern WestVirginia segment.
(2) The 38th parallel and Burning-Mann linea-ments are associated with the boundaries of thethree segments of the Rome trough and may repre-sent two major accommodation zones that transferdifferent extensional polarity and geometry alongthe axis of the trough. Because of their obliqueposition to the trough axis, both are inferred tohave experienced transtensional deformation inassociation with the formation of the trough duringthe Early(?) and Middle Cambrian.
(3) Basically, the Rome trough has experiencedthree major tectonic stages throughout the Paleo-zoic. Stage 1 was characterized by rapid extensionand rifting during the Early(?) and MiddleCambrian in association with the late-stage open-ing of the Iapetus-Theic Ocean. Stage 2 featured aslow subsidence to form a successor sag basinfrom the Late Cambrian to Middle Ordovician, per-haps in association with postrift thermal contrac-tion at the passive continental margin. Stage 3 wasdominated by a major structural inversion from theLate Ordovician through the Pennsylvanian in asso-ciation with the development of the Appalachianforeland basin.
(4) The along-axis segmentation of the Rometrough may have affected the along-axis variation insource rock thickness, thermal maturity, reservoirlithofacies, and trapping mechanism. Thus the seg-mentation model established for the trough mayhave important implications for hydrocarbonexploration on a regional basis.
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98 Rome Trough
Dengliang Gao
Dengliang Gao received a B.S.degree (1983) and an M.S. degree(1986) in geology from Hefei Uni-versity of Technology, People’sRepublic of China. From 1986 to1991, he worked as a lecturer atTongji University, People’s Republicof China, where he won the NationalScience and Technology Progressaward for his research project in1991. In 1992, he went to the UnitedStates and received an M.S. degree in geology from WestVirginia University (1994) and a Ph.D. in geology fromDuke University (1997). After spending one year in the oilindustry at Exxon Production Research Company, hejoined Marathon as a geologist in 1998, focusing on three-dimensional seismic interpretation and visualization. Hiscurrent interests include computer-aided structural andstratigraphic interpretation of three-dimensional seismicusing attribute and visualization technologies.
Robert C. Shumaker
Robert C. Shumaker received anA.B. degree (1953) from BrownUniversity and an M.S. degree andPh.D. (1960) in geology from CornellUniversity. He worked for theHumble Oil and Refining and ExxonCompanies in their research andexploration departments from 1960to 1972. He joined the faculty in theDepartment of Geology and Geogra-phy at West Virginia University in1972 where he was a professor and where he was associat-ed chairperson of the department from 1984 to 1995. Heretired in 1996 and presently is a professor emeritus at WestVirginia University.
Thomas H. Wilson
Thomas H. Wilson received B.A.(1974) and M.S. (1977) degrees inphysics, and a Ph.D. (1980) in geolo-gy from West Virginia University(WVU). He worked with ExxonCompany USA (1981–1983) in theEDPC special seismic problemsgroup and later in the Gulf Atlanticdivision seismic evaluation group. In1983, he joined the WVU Depart-ment of Geology and Geography,where he is a professor. He teaches courses in applied geo-physics and quantitative methods.
