Browsing by Author "Glover, Lynn III"

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    Analysis and interpretation of compressional (P-wave) and shear (SH-wave) reflection seismic and geologic data over the Bane Dome, Giles County, Virginia
    Gresko, Mark J. (Virginia Polytechnic Institute and State University, 1985)
    Approximately 37 km of predominantly 24-fold P-wave Vibroseis data and 16 km of 24-fold SH-wave Vibroseis data were acquired in the southern portion of the folded Appalachians near the Bane Dome in Giles County, Virginia. Data processing techniques included the application of newly developed methods for crossdip removal as well as the determination of statics solutions in the case of time variant shifts within the data traces. Minimum-phase filter deconvolution was also applied for the removal of reverberating energy and multiples recorded on the SH—wave lines. Vp/Vs ratios were used to aid in the determination of lithologies in the absence of bore-hole data. Interpreted thickening of the Lower Cambrian to Upper Precambrian sequence beneath the Bane Dome appears to represent Eocambrian rifting. Faults generated at that time may now be reactivated by the present stress regime, causing earthquake activity in this area. Interpretation of the seismic data supports a duplex structure proposed for the Paleozoic rocks of the Bane Dome Complex within the Narrows thrust sheet of southwestern Virginia.
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    Applications of Roll-Along Electrical Resistivity Surveying in Conjunction with Other Geophysical Methods for Engineering and Environmental Site Characterization
    Sayer, Suzanne (Virginia Tech, 1996-04-29)
    Roll-along electrical resistivity surveying was used with seismic refraction, magnetometer and gravity surveying in geophysical characterization of sites with a specific environmental or engineering problem. Three examples are presented where resistivity surveying provided vital constraints on acquisition and interpretation of other data in chaotic terrane. A commercially resistivity meter was used with prototype equipment designed, assembled, and tested at Virginia Tech. The equipment included a multiconductor cable consisting of interchangeable segments and a circuit allowing selection of numerous electrode configurations. The Sinking Creek Landfill, a 10-acre site, was used for disposal of municipal waste in the early 1970’s. Roll-along resistivity proved to be the most useful geophysical tool in ascertain its internal structure. Wenner configuration resistivity data, sensitive to both conductive leachate and ferrous metals, showed trenches within the landfill displayed in profile. Magnetic field measurements revealed anomalies over some trenches suggesting a method for discriminating between ferrous metal and leachate. Results of a resistivity survey can help planners of a refraction survey avoid low velocity “blind” layers. The Mid County Landfill borrow area is a 26 acre site situated within the Max Meadows Breccia and used for cover material for an adjacent landfill, The engineering problems were to measure the volume of rippable material, but travel time data were somewhat ambiguous. The refraction data interpreted using a) conventional 3-layer analysis b) horizontal 3-layer analysis of single shots, and c)continuous velocity gradient analysis of single end shots were compared with auger refusal depth. The single end horizontal analysis matched auger refusal depths most closely. Roll-along resistivity pseudo-sections made along the refraction lines proved to be effective for qualitatively imaging pinnacles and megaclasts. Excavation of fill material from a 75 acre river terrace in Pembroke exposed an antiform cut by high angle, near surface faults. Geophysical characterization was undertaken to determine the thickness of the alluvial deposit, and the relationship of the faults with structures in the underlying bedrock. Seismic refraction showed the terrace was as much as 134 feet thick. Resistivity pseudosections revealed a resistivity anomaly associated with the graben could be detected for a horizontal distance of several hundred feet. A gravity gradient paralleling the resistivity anomaly extends the feature more than 1000 ft from the exposed structure. Tenuous evidence of a bedrock escarpment beneath the near surface structure is found in a combination of seismic refraction, gravity, and electrical resistivity data. Roll-along resistivity has proved to be key to geophysical interpretation of these three areas. Images displayed on pseudosections reveal lateral inhomogeneity more clearly than could be discerned from seismic, gravity and magnetic data. Roll-along resistivity data can provide information for efficient siting of additional geophysical studies.
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    A comparison of the seismicity and regional geology of the southeastern United States and southeastern Australia
    Hunter, Stephen Allen (Virginia Tech, 1981-05-14)
    The southeastern United States and southeastern Australia are both intraplate regions. They are characterized topographically and geologically by low relief, Paleozoic mountain belts that parallel continental margins formed by rifting. Interestingly, there are also remarkable similarities in the seismicity of the two areas. To characterize the temporal aspects of seismicity, the recurrence relationship, log N (number of earthquakes per year) versus Io (maximum intensity), for southeastern Australia was determined to be: log N = 3.28 - 0.61 Io; III ≤ Io ≤ VII Comparison with a similar relationship for the southeastern United States, log N = 3.01 - 0.59 Io; V ≤ Io ≤ VIII. (Bollinger, G. A., 1973) indicates that both the level of seismic activity and the distribution of earthquakes by size are comparable in the two regions. Additionally, strain release studies for the two regions, while subject to considerable uncertainty, do support that similarity. Note that the intensity X, 1886 Charleston, South Carolina, earthquake was not included in either the recurrence or strain studies. Comparison of the spatial patterns of earthquake epicenters in both regions reveals only weak correlations of the seismicity with topography or with regional geology. There is, however, no apparent correlation with major igneous outcrops or with most major through-going faults. Both regions have seismic zones that are transverse or parallel to the regional geologic trends or tectonic fabrics. The parallel zones are usually associated spatially with severely-faulted regions, suggesting a possible causal relationship. For southeastern Australia, such zones exhibit northeast-trending normal faults of Jurassic age while for the southeastern United States, there are northeast-trending Pennsylvanian-Permian thrust faults. However, no such fault concentrations are found in the transverse seismic zones. Thus, the severely-faulted Source region hypothesis does apply uniformly. Indeed, it may be that, in portions of both of these widely separated geographic regions, the seismic activity is associated with deep crustal features that have no obvious surface expression.
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    Correlation of seismic reflection data with seismicity over the Ramapo, New Jersey, fault zone
    D'Angelo, Richard M. (Virginia Tech, 1985-07-24)
    Reflection seismic data, mylonite reflectivity, gravity data, and earthquake hypocenters have been integrated into a possible explanation for seismicity in the Ramapo fault area. Seven reflection seismic lines were processed using variations in sorting and residual statics. Single VIBROSEIS sweeps were treated as separate sourcepoints. Compressional velocities and densities were determined in the laboratory. Reflection coefficients and gravity models provide evidence for reflections from mylonite zones. Earthquake hypocenters were projected into the vertical seismic sections. The results suggest a correlation between rock volumes containing hypocenters and rock volumes containing mylonite zones. The seismic line furthest from the Taconic suture displays fewer hypocenters and mylonites, in agreement with an assumed model of mylonite development possibly associated with obduction of continental crust. The mylonite zones in the basement may serve as local areas of crustal weakness for seismic activity occurring in the area.
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    Cyclostratigraphy of Late Cambrian cyclic carbonates: An interbasinal field and modelling study, U.S.A.
    Osleger, David Allen (Virginia Tech, 1990)
    An interbasinal study of Late Cambrian cyclic carbonate successions in the Appalachian and Cordilleran passive margins, the Texas cratonic embayment and the southern Oklahoma aulacogen has provided controls on the simultaneous development of peritidal to subtidal meter-scale cycles and the larger scale depositional sequences on which they are superimposed. Fining-upward peritidal cycles grade seaward into coarsening-upward, shallow to deep subtidal cycles that form a continuum across the carbonate platforms and are genetically linked to one another by shared lithofacies. Eustacy appears to exert the dominant control on the simultaneous development of peritidal and subtidal cycles on different carbonate platforms. Based on the recognition of dominant periodicities on power spectra derived from time series of subtidal cycles, high frequency eustatic oscillations may be controlled by Milankovitch astronomical rhythms. Interbasinal correlation of Late Cambrian depositional sequences was performed by graphic correlation and the time-equivalent intervals were correlated lithostratigraphically using isochronous biomere boundaries as time datums. Fischer plots of meter-scale cycles define changes in relative sea level based on the amount of extra accommodation space produced by eustacy beyond that provided by subsidence. Residual eustatic curves derived from subsidence analysis are useful for correlating the longer-term Late Cambrian sea level events and changes in the rate of sea level rise and fall can be used to define shorter-term events. Combining the sea level curves defined by Fischer plots and subsidence analysis with paleobathymetric curves of Late Cambrian cyclic strata suggests that the curves may approximate the form of the eustatic sea level curve. A composite "eustatic” sea level curve for the Late Cambrian was created by qualitatively combining the sea level curves defined by the different techniques for each of the four localities. “Eustatic" sea level curves defined by Fischer plots and subsidence analysis may be used to apply sequence stratigraphic concepts to onedimensional outcrop sections. Combined with systematic changes in the stacking patterns of meter-scale cycles, they can be used to define the internal composition of systems tracts, sequence boundaries, and flooding surfaces of third-order depositional sequences. One- and two-dimensional models of peritidal and subtidal cycle development indicate that peritidal cycle thickness is primarily controlled by accommodation space and deeper subtidal cycle thickness is primarily controlled by sedimentation rate. Whereas lithofacies within peritidal cycles alternate in response to fluctuations in sea level, subtidal cycle development may be related to fluctuations in fairweather and storm wave base that oscillate in harmony with sea level fluctuations.
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    Development of cyclic ramp-to-basin carbonate deposits, lower Mississippian, Wyoming and Montana
    Elrick, Maya (Virginia Tech, 1990-07-01)
    The Lower Mississippian Lodgepole/lower Madison Formations (20-225 m thick) developed along a broad (>700 km) stormdominated cratonic ramp. Three types of shallowing-upward cycles (5th order) are recognized across the ramp-to-basin transition. Peritidal cycles consist of very shallow subtidal facies overlain by algal-laminated tidal flat deposits, which are rarely capped by paleosol/breccia layers. Shallow subtidal cycles consist of stacked ooid grainstone shoal deposits or deeper subtidal facies overlain by ooid-skeletal grainstone caps. Deep subtidal cycles occur along the outer ramp and ramp-slope and consist of sub-storm wave base limestone-argillite, overlain by graded limestone, and are capped by storm-deposited skeletal-ooid grainstone. They pass downslope into rhythmically interbedded limestone and argillite with local deepwater mud mounds; no shallowing-upward cycles occur within the ramp-slope facies. Average cycle periods calculated along the outer ramp range from 30-110 k.y. The cycles likely formed in response to 5th order (20-100 k.y.) sea level oscillations. The cycles are stacked to form three 3rd to 4th order depositional sequences which are defined by regional transgressive-regressive facies trends. The ramp margin wedge (RMW) developed during long-term sea level fall lowstand conditions and consists of cyclic crinoidal bank and oolitic shoal facies which pass downdip into deep subtidal cycles. The transgressive systems tract (TST),which onlapped the ramp during long-term sea level rise, includes thick deep and shallow subtidal cycles; peritidal cycles are restricted to the inner ramp. The highstand systems tract (HST) developed during long-term sea level highstand and fall, and along the ramp is composed of early HST shallow subtidal cycles which are overlain by late HST peritidal cycles; shallow through deep subtidal cycles composed the HST along the ramp-slope.
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    Geologic framework of gravity anomaly sources in the central Piedmont of Virginia
    Keller, Mary Ruth (Virginia Tech, 1983-06-15)
    Bouguer gravity anomalies at 1870 locations on the central Piedmont of Virginia from 37° 37' N to 37° 52' N and 77° 44' W to 78° 23' W display patterns of variation produced by upper crustal density contrasts and thickening of the crust in a WNW direction. No other deep sources are evident. Upper crustal density contrasts are associated with rock units known from geologic mapping. ‘The subsurface distribution of these rock units interpreted from seismic reflection data was confirmed by measured variations in gravity. A two-dimensional model analysis indicates the following average in situ density values for the principal formations: Arvonia Formation-2.77 gm/cc, Columbia Granitoid-2.75 gm/cc (tonalite) and 2.73 gm/cc (pegmatite), Chopawamsic Volcanics- 2.77 gm/cc (felsic units), and 2.79 gm/cc (mafic units}, Catoctin/ Lynchburg-2.815 gm/cc, Maidens Gneiss-2.775 gm/cc, Grenville Basement- 2.71 gm/cc. Gravity and seismic data are consistent with the existence of a major thrust fault at depths between 9 km and 16 km that separates Grenville Basement rocks from younger Catoctin/Lynchburg rocks. The slight eastward dip of this thrust fault beneath the western part of the area increases significantly east of 78° 05' W. Gravity anomalies suggest the existence of several mafic inclusions within the Columbia Granitoid that were not identified by geologic mapping.
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    Geology and Tectonic Significance of the Late Precambrian Eastern Blue Ridge Cover Sequence in Central Virginia
    Wang, Ping (Virginia Tech, 1991)
    The Late Precambrian cover sequence in the Blue Ridge of central Virginia includes rocks of the Moneta Formation and the overlying Lynchburg Group. The Moneta Formation comprises arnphibolites, felsites and biotite gneisses that unconformably overlie the Grenville basement. The Lynchburg Group in central Virginia is divided into three formations. Lynchburg I is made up of massive to thick bedded coarse-grained feldspathic arenites and conglomerates, which are interpreted as slope-apron deposits. Lynchburg IT contains mainly medium to fine grained feldspathic arenites and graphitic schist (black shales) with subordinate conglomeratic rocks. These are believed to be channelized submarine fan turbidites formed in an anoxic environment. Lynchburg ill consists of fine to medium grained feldspathic quartz arenites and a minor amount of conglomeratic rocks, which are considered to be channelized submarine turbidites with a more open marine environment and wider shelf. Three metamorphic facies and two deformation events are recognized in the cover sequence of the study area. The current tectonic models tend to view most of the mafic-ultramafic rocks and the host sedimentary rocks of the Lynchburg as ophiolitic melange, thus creating a suture, of Precambrian to Ordovician age. Detailed field mapping shows that the Lynchburg Group does not have the characteristics of melange and the mafic-ultramafic rocks in it do not resemble ophiolite. Rather, the cover sequence is related to the Late Precambrian Iapetan rifting event. Some tectonomagmatic discriminant diagrams have been used to support the current tectonic model and they are considered one of the most important arguments for ophiolites. These diagrams were tested by plotting samples from Jurassic rift basalts-diabases of eastern North America (ENA). The ENA samples, as well as the post Grenville mafic rocks in the Blue Ridge, tend to plot outside the within-plate field. It is clear that geochemical data alone may give a wrong tectonic classification, and that a knowledge of field relations is of paramount importance for interpretation.
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    Geology of the Ashe Formation between Fries and Galax, Virginia
    Tso, Jonathan Lee (Virginia Polytechnic Institute and State University, 1987)
    Field mapping, structural analysis, and mineral equilibria of rocks between Fries and Galax, Virginia provide information on the tectonic and metamorphic history of the Ashe Formation, a series of late Precambrian gneisses, pelitic schists and amphibolites, and the underlying Cranberry Gneiss. The dominant penetrative foliation of the rocks is composed of an early S₁ foliation that is crenulated and transposed by a later S₂ foliation which strikes N 50° E and dips SE. Metamorphic porphyroblast growth of biotite, garnet, staurolite, and kyanite followed penetrative foliation development, with metamorphic grade increasing stratigraphically upward to form inverted isograds. Thrust faulting along the Fries Fault caused extensive mylonitization of the Cranberry Gneiss and offset of the metamorphic isograds. Textural evidence indicates that mylonitization is the greatest in rocks at the Fries Fault trace, and decreases upward through the Cranberry-Ashe contact. No textural evidence of an additional episode of faulting at the AsheCranberry contact was observed, based on lack of a second episode of mylonite, relatively good preservation of sedimentary clasts in the Ashe, and similarity of metamorphic grade. Mineral equilibria show systematic trends from chlorite-biotite grade to staurolite-kyanite grade. Muscovite compositions show an increase in paragonite component and decrease in celadonite component. Chlorite, biotite, garnet, and staurolite show a systematic increase in Mg/(Mg+ Fe) toward higher grade. AFM topologies indicate that these compositional trends may be explained a series of continuous reactions ( chl + mu=bi + q + V followed by chl + mu + q=gt + bi + V) and discontinuous reactions (gt + chl + mu= st + bi + V followed by st + chl + mu= ky + bi + V). Temperatures inferred from garnet-biotite equilibria are approximately 550 to 600° C in the staurolite zone, with pressures estimated from various equilibria to be between 4.5-7 kb. The inversion of the isograds is interpreted as resulting from overthrusting of the Ashe metasediments during the Paleozoic. The above information can also be applied to the deformation of the Gossan Lead massive sulfide. Structural data from the Bumbarger mine pit indicates that the massive sulfide underwent the same sequence of structural events as the surrounding rocks. During D₂, shearing of gneisses and schists in a matrix of ductile pyrrhotite caused isoclinal folding, fracture, and rotation of the metasediments while preserving the gross sedimentary layering. This information confirms synsedimentary models of the formation of the Gossan Lead.
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    Geology of the late precambrian Flat River complex and associated volcanic rocks near Durham, North Carolina
    McConnell, Keith I. (Virginia Tech, 1974-12-05)
    Disotopic dating of zircons from the Flat River Complex in the Carolina "slate" belt north of Durham, N.C. shows this intrusive complex to be 650 ± m.y. old. Modal analyses of granophyric groundmass compared to experimental data, the presence of vent breccias and related pyroclastic deposits, and consideration of age relations between the intrusive and extrusive rocks indicate that the Flat River was emplaced at very shallow levels (< l km) and acted as the source for most of the volcanic material surrounding the complex. The age determined for the Flat River Complex indicates that deposition of the volcanic rocks began prior to 650 m.y. ago and extends the slate belt volcanicity interval to 130 m.y. (520 to 650 m.y. b.p.) Both subareal and marine depositional environments are represented in the stratigraphic sequence.
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    A geophysical investigation of geologic structure and regional tectonic setting at the Savannah River site, South Carolina
    Domoracki, William Joseph (Virginia Tech, 1995-05-11)
    Time-structure, isochron, and reflection amplitude maps were constructed from 270 km of reprocessed seismic reflection data recorded at the Savannah River Site (SRS), South Carolina. These maps indicate the presence of three major basement blocks bounded by northeast trending faults that penetrate upward from basement into the Atlantic Coastal Plain sediments. Most of these faults are interpreted to be Paleozoic and Mesozoic structures reactivated under compression. The northernmost fault block is bounded on the southeast by the Tinker Creek fault (TCF), a high-angle southeast dipping reverse fault, that appears to be a reactivated splay from a major decollement (Augusta fault?) at 5 km depth. The TCF can be traced with certainty for a distance of 15 km across SRS where it offsets the top of basement and shallower reflections. The throw of the fault, as does its upward penetration into the sedimentary section, increases to the northeast where the shallowest reflections (≈ 20 m) appear to be disturbed. The northwest boundary of this block is unknown. The central fault block, 9 km wide in a NW-SE direction, is bounded on the northwest by the Tinker Creek fault and on the southeast by the Pen Branch fault, the northwest border fault of the concealed Dunbarton Triassic basin, now reactivated as a southeast dipping high-angle reverse fault within the Coastal Plain sediments. The Pen Branch fault (PBF) is the main throughgoing fault in a 3 km wide, 25 km long, fault zone consisting of subparallel fault segments that are commonly down-to-the-northwest north of PBF and down-to-the-southeast south of PBF. The offset of time horizons by the PBF increases to the northeast and the shallowest resolvable horizons appear in time to be deformed across its length. The Steel Creek fault, an antithetic fault to PBF, controls the drainage of Pen Branch, a tributary stream to the Savannah River. The third fault block is bounded by the Pen Branch fault on the northwest. The southeast boundary of this southernmost block is unknown, but could be defined by the Martin fault, a down-to-the-northwest fault located south of SRS that appears to be a major structure. Interpretation of isochron maps and analysis of trend surfaces fitted to reflection time horizons suggest that faulting was ongoing through the Late Cretaceous and was accompanied by tilting and horizontal rotation of the fault blocks. Fault block movements in the Tertiary are uncertain as shallow time horizons cannot be correlated areally with confidence; however, isochrons and reflection horizons correlated across faults reveal deformation, but not offset, of the shallowest resolvable events. The comparison between time-structure and topography reveals areas of positive correlation that suggest either near surface velocity variations or possible Cenozoic uplift. The reprocessed seismic data elucidate the geometry of the Dunbarton basin and the underlying crustal structure. The Dunbarton basin is essentially a half-graben bounded on the northwest by the near vertical Pen Branch fault. The maximum thickness of the basin fill, as determined by seismic modelling, is between 4 and 5 km. Minor intrabasinal faults are imaged within the basin as well as bright discontinuous reflections that probably represent sills. The crystalline crust below SRS is highly reflective and is distinguished by zones of southeast dipping reflections that are correlatable between seismic lines. Two major crustal reflections, tentatively identified as the Augusta fault and a mid crustal decollement, appear to correspond to events imaged in seismic lines collected by COCORP 60 km to the southwest where the latter event was interpreted to be the Appalachian Master decollement. The continuity of deep reflection events along geologic strike from Georgia to South Carolina supports models for evolution of the Appalachian orogen that incorporate large scale, presumably Paleozoic age, thrusts beneath the Piedmont and under the Atlantic Coastal Plain.
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    A geophysical study of the earth's crust in central Virginia with implications for lower crustal reflections and Appalachian crustal structure
    Pratt, Thomas L. (Virginia Polytechnic Institute and State University, 1986)
    Reprocessing of the United States Geological Survey's seismic reflection profile along Interstate 64 (I64) across Virginia with a data extension to 14-sec two-way travel time has provided a stacked section suitable for an integrated interpretation of refraction, earthquake, and blast analyses done by previous workers as well as gravity modelling done in this study. The seismic reflection profile shows a highly reflective upper crust which is consistent with an allochthonous Blue Ridge Province, possibly with underlying thrust sheets, and a basal decollement at about 9 km (3 sec) depth. Immediately east of the Blue Ridge province, the Appalachian structures plunge to up to 12 km (4 sec) depth where their interpretation on the section becomes ambiguous. The Evington Group, Hardware Terrane, and Chopawamsic metavolcanic rocks exposed in the Piedmont Province correspond to numerous reflections which appear to overlie a shallowly (10° to 15°) west-dipping, highly reflective zone dipping from 1.5 sec beneath the Goochland Terrane to 5 sec beneath the Evington Group rocks. Some of the overlying reflections apparently root in this zone which is therefore interpreted to include decollement surfaces along which the overlying rocks were transported. Grenville basement rocks are interpreted to underlie this zone and form autochthonous basement beneath the entire western portion of the profile because relatively few reflections originate from within this region. The Goochland granulite terrane is interpreted as a nappe structure which has overridden a portion of the Chopawamsic metavolcanic rocks. The Goochland terrane is bounded on the east' on the section by a broad zone of east-dipping (20° to 45°) reflections which may penetrate to Moho depths and are possibly correlative with similar events seen on other Appalachian lines. The 164 section contains a layered sequence of reflections at about 9 to 12 sec extending about 70 km west from Richmond, Virginia whose base coincides almost exactly with the Mohorovicic Discontinuity (Moho) interpreted from earlier refraction work. The deep reflections are thus believed to be lower crustal layering forming a 5 to 10 km thick Moho transition zone which is believed to persist across the state. The density contrast of 0.25 gm/cm³ between the lower crust and upper mantle derived from gravity modelling, the seismic transition zone, and the presence of intrusive rocks of lower crust-upper mantle origin at the surface are consistent with partial melting and contamination of the lower crust with upper mantle material. The refraction data and gravity modelling are consistent with a crust which thins from about 52 km beneath the Appalachian mountains to about 35 km beneath Richmond, Virginia, and rethickens by up to 10 km beneath the zone of east-dipping events east of Richmond. The pervasiveness of the zone of east-dipping events on other seismic reflection lines and the continuity of the adjacent Piedmont gravity high suggest that a similar crustal profile occurs along the length of the Appalachians.
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    High resolution sequence stratigraphy of late Mississippian carbonates in the Appalachian Basin
    Al-Tawil, Aus (Virginia Tech, 1998-01-09)
    The late Mississippian carbonates in the Appalachian Basin, U.S.A., were deposited on a huge, south-facing ramp during long-term Mississippian transgression that formed the Mississippian supersequence. The St. Louis- to Glen Dean interval consists of up to twelve fourth-order depositional sequences (300 to 400 k.y. average duration). The sequences (a few meters to over a hundred meters thick) consist of eolianites, lagoonal carbonate muds, ooid shoals, and skeletal banks, and open marine skeletal wackestone and basinal marion the ramp-slope and basin margin. Sequence boundaries are at the top of prograding red-beds, eolianites, and shoal water facies on the ramp, and beneath lowstand sand bodies and quartzose calcisiltite wedges on the ramp margin and slope. Maximum flooding surfaces are difficult to map regionally, therefore it is difficult to separate the TST from the HST of these fourth-order sequences.
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    High-Resolution Sequence Stratigraphy of Late Mississippian (Chesterian) Mixed Carbonates and Siliciclastics, Illinois Basin
    Smith, Langhorne Bullitt (Virginia Tech, 1996-11-01)
    Eight 4th-order (~400 k.y.) disconformity-bounded mixed carbonate-siliciclastic sequences were deposited in the tectonically-active, tide-dominated Illinois basin during the Late Mississippian greenhouse to icehouse transition. Detailed, lithologic cross-sections were constructed through the Chesterian Ste. Genevieve through Glen Dean interval which show an upward change in character from carbonate-dominated sequences bounded by caliche and breccia paleosols to mixed-carbonate siliciclastic sequences bounded by red, slickensided mudrock paleosols and incised valleys. The 4th-order sequences are composed of 5th-order parasequences that can be correlated basin-wide. Parasequences in the basal, dominantly carbonate sequences are composed of patchy ooid grainstone tidal ridge reservoir facies which interfinger with skeletal limestone and are capped by laterally extensive muddy carbonate units. Parasequences in the overlying mixed carbonate siliciclastic interval commonly have basal quartz sandstone valley fill and tidal sand ridge reservoir facies overlain by skeletal limestone and shale-dominated siliciclastics. The sequences can be bundled into sequence pairs and composite sequences. Composite sequences are composed of 4 sequences and are bounded by better developed disconformities that commonly coincide with biostratigraphic zone boundaries. High energy reservoir facies are widespread in transgressive sequence tracts and late highstand sequence tract (where present) and confined to updip areas in the early highstand sequence tracts. Increasing amplitude 4th-order glacio-eustasy produced the sequences and caused the upward increase in incised valleys and deeper water carbonate deposition. Parasequences were produced by 5th-order glacio-eustatic sea-level fluctuations (20-100 k.y.). Sequence pairs and composite sequences were produced by 3rd-order sea-level fluctuations possibly in combination with local tectonics. Spatial and temporal variations in differential subsidence between the eastern and western shelves and the more rapidly subsiding basin interior caused variations in onlap/offlap geometries of sequences and parasequences. Increasingly wetter wet-dry seasonality caused an upward increase in siliciclastic influx and concurrent decrease in ooid deposition. The increasing-amplitude eustasy and progressively more humid climate were caused by the onset of continental glaciation on Gondwana.
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    An Integrated Geophysical Study of the Central Appalachians of Western Virginia and Eastern West Virginia
    Peavy, Samuel Thomas (Virginia Tech, 1997-07-18)
    Over 700 km of industry seismic reflection data in the central Appalachians were reprocessed using both conventional and newly developed processing schemes. A new processing sequence, called dip projection, is introduced. The technique projects crooked-line processed CMPs onto a straight line oriented in the general dip direction for the area. The new stacked sections more closely approximate a dip line and hence are more migration-friendly and interpretable than the crooked-line stacks. Methods of determining the lateral continuity of subsurface density contrasts were also applied to gravity data from the study area. Known collectively as potential field attributes, the analytic signal, the tilt angle, and the gradient of the tilt angle (the potential field wavenumber ) proved valuable in the analysis of the gravity data. Comparison of reflection seismic data from the southern and central Appalachians revealed a dichotomy of seismic reflectivity from east to west. A highly reflective crust beneath the Piedmont in both the central and southern Appalachians contrasts with a general lack of reflectivity beneath the Blue Ridge and Valley and Ridge provinces where coherent reflections are restricted to the upper 3-4 seconds of the data. This difference in reflectivity is interpreted as a fundamental difference in the location and orientation of preexisting zones of weakness between the different crustal regions with respect to the tectonic events affecting the Appalachians since the early Paleozoic. The combination of the results of new methods of seismic and potential fields processing with deep well and geologic information allowed the lateral continuity of two major structures in the central Appalachians to be examined. The Blue Ridge in Virginia was found to overly a duplex of Cambrian-Ordovician carbonates formed in response to stresses during the Alleghanian Orogeny. A large thrust sheet of similar carbonate rocks was interpreted beneath the Nittany Anticlinorium in West Virginia. To the south in Virginia, this thrust sheet is replaced by imbrication of the carbonate package. The change in structural style may be related to the existence of a lateral ramp or it may reflect the overall change in structural style from the central to southern Appalachians.
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    Regional dolomitization of Early Ordovician, Upper Knox Group, Appalachians
    Montañez, Isabel Patricia (Virginia Polytechnic Institute and State University, 1989)
    The Early Ordovician, Upper Knox Group consists of meter-scale shallowing-upward cycles that were deposited on a low-sloping ramp. Cycles formed in response to short term (<100 k.y.) eustatic sea-level fluctuations and typically have well developed tidal flat caps. Cycles are bundled into five transgressive-regressive sequences which correspond to third order (1-10 m.y.) sea-level fluctuations defined by Fischer plots. The Upper Knox Group is 90% dolomite of which greater than 75% predates Middle Ordovician, Knox Unconformity development. Early dolomitization occurred penecontemporaneously with tidal flat progradation during fifth-order (up to 100 k.y.) sea-level falls as indicated by: abundant dolomite in cycles with well-developed tidal flat caps and scarce dolomite in cycles with no or thin laminite caps; decrease in dolomite abundance with distance below tidal flat caps; dolomitized cycles decrease basinward; and dolomite clasts veneer cycle tops and the Knox Unconformity surface. Third-order sea-level fluctuations also strongly controlled early dolomitization as indicated by Fischer plots; limestone, subtidal-dominated cycles correspond to third-order sea level rises and completely dolomitized, peritidal-dominated cycles correspond to third-order sea level falls. "Early" dolomite was metastable and its geochemical composition was modified during initial stabilization by marine brines during progradation of each cycle, and by mixed fresh/marine waters of the Knox aquifer associated with unconformity development. Much "early" dolomite however, remained metastable into the deep burial environment where it was replaced and overgrown by burial fluids as suggested by: covariant trends between crystal size, mole % CaCO₃, Sr²⁺, Mn²⁺ and δ¹⁸O; similar regional trends defined by stable isotope values of "early" dolomites and burial dolomites; and water-rock modeling of trace element and stable isotopic trends. Trace element and stable isotope compositions of least-altered "early" dolomite however, record a memory of a precursor evaporative dolomite. Cathodoluminescent dolomite stratigraphy defines five generations of burial dolomite that can be correlated over 100,000 km². Burial dolomites postdate a regional dissolution event attributed to migration of organic acid-rich fluids through the Knox carbonates. Regional dolomitization occurred coeval with Late Paleozoic deformation and was closely associated with MVT mineralization and hydrocarbon migration. The δ¹⁸O values and trace element contents of burial dolomites in conjunction with fluid inclusions, suggest that burial fluids were warm (135 to 200°C), saline (13 to 22 wt. % NaCl equiv.), ¹⁸O-enriched (+2 to +9 % SMOW) fluids with geochemical compositions similar to present day basinal brines. Mn²⁺ and Fe²⁺ contents of the dolomites suggest a redox control over Mn and Fe fluid chemistry, and in conjunction with regional δ¹³C trends, likely record precipitation from organic acid-rich fluids. Regional trace element and δ¹⁸O trends record a basinal fluid source and regional northwestward flow. Stable isotope values of burial dolomites and fluid inclusions from dolomites and associated minerals, define a prograderetrograde sequence that formed during basinwide, gravity-driven fluid flow which developed in response to Late Paleozoic thrusting and uplift.
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    A sedimentological and structural analysis of the Proterozoic Uncompahgre Group, Needle Mountains, Colorado
    Harris, Charles William (Virginia Polytechnic Institute and State University, 1987)
    Siliciclastic sediments of the Proterozoic Uncompahgre Group can be subdivided into stratigraphic units of quartzite (Q) and pelite (P); these units include a basal, fining- and thinning-upward retrogradational sequence (Q1-P1) that records the transition from an alluvial to a shallow-marine setting. Overlying the basal sequence are three thickening- and coarsening-upward progradational sequences (P2-Q2, P3-Q3 and P4-Q4) that were influenced by tide-, storm- and wave-processes. The progradational units are subdivided into the following facies associations in a vertical sequence. Outer-to inner-shelf mudstones, Bouma sequence beds and storm beds of association A are succeeded by inner-shelf to shoreface cross-stratified sandstones of association B. Conglomerates and cross-bedded sandstones of upper association B represent alluvial braid-delta deposits. Tidal cross-bedded facies of the inner shelf/shoreface (association C) gradationally overlie association B. Interbedded within the tidal facies in upper association C are single pebble layers or <1 m-thick conglomerate beds and trough cross-bedded pebbly sandstones. Single pebble layers could be due to storm winnowing whereas conglomerates and pebbly sandstones may record shoaling to an alluvial/ shoreface setting. A temporally separated storm/alluvial and tidal shelf model best explains the origin and lateral distribution of facies in the progradational sequences. The presence of smaller progradational increments in the mudstone dominated units (P3) and the recurrence of facies associations in the thick quartzite/conglomerate units (Q2, Q3, Q4) suggests that external cyclic factors controlled sedimentation. A composite relative sea level curve integrating glacio-eustatic oscillations and long-term subsidence may account for the evolution of the thick progradational sequences of the Uncompahgre Group. Sedimentary rocks of the Uncompahgre Group have been subjected to polyphase deformation and greenschist facies metamorphism. Phase 1 structures (localized to the West Needle Mountains) include bedding-parallel deformation zones, F₁ folds and an S₁ cleavage. Phase 2 coaxial deformation resulted in the development of upright, macroscopic F₂ folds and an axial-planar crenulation cleavage, S₂. In addition basement-cover contacts were folded. Phase 3 conjugate shearing generated strike-parallel offset in stratigraphic units, a macroscopic F₃ fold, and an S₃ crenulation cleavage. In addition, oblique-slip, reverse faults were activated along basement-cover contacts. The Uncompahgre Group unconformably overlies and is inferred to be parautochthonous upon ca. 1750 Ma gneissic basement that was subjected to polyphase deformation (DB) and amphibolite facies metamorphism. Basement was intruded by ca. 1690 Ma granitoids. Deformation of gneissic and plutonic basement together with cover (DBC) postdates deposition of the Uncompahgre Group. The structural evolution of the Uncompahgre Group records the transition from a ductile, north-directed, fold-thrust belt to the formation of a basement involved “megamullion" structure which was subjected to conjugate strike-slip faulting to accommodate further shortening. DBC deformation may be analogous to the deep foreland suprastructure of an orogenic belt that developed from ca. 1690 to 1600 Ma in the southwestern U.S.A ..
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    Sedimentology and diagenesis of Lower to Middle Cambrian carbonate platform, Shady Dolomite, Virginia
    Barnaby, Roger Joseph (Virginia Polytechnic Institute and State University, 1989)
    Drill cores through the Lower to Middle Cambrian Shady Dolomite carbonate platform (600 to 1200 m thick) in the Austinville, Virginia, region allow the evolution of the carbonate platform from a gently sloping ramp, to a high relief, rimmed shelf to be documented. The Shady Dolomite forms the initial carbonate foundation for the overlying Cambrian-Ordovician carbonate shelf sequence, which persisted for about 30 m.a. until it was destroyed by incipient collision during the Early Ordovician. The Shady Dolomite records several episodes of dolomitization during burial, coeval with late Paleozoic deformation. Rare relict corcs of zone 1 dolomite were replaced and overgrown by zone 2A dolomite, the dominant replacement phase. After replacement dolomitization, the sequence was subjected to dissolution and fracturing, followed by Pb-Zn mineralization, zones 2B, 3 and 4 dolomite cement, sphalerite, quartz, and calcite. Zone 1 dolomite apparently has similar isotopic and trace element composition as zone 2A dolomite. Zone 2A dolomite (δ¹⁸O = -10.2 to -7.0 °/oo PDB; δ¹³C = +1.0 to +1.6 °/oo PDB) is depleted in ¹⁸O and enriched in ¹³C relative to marine cements (δ¹⁸O = -7.5 to -6.1; δ¹³C = +0.2 to +0.8), reflecting precipitation at elevated temperatures from fluids in equilibrium with the host limestone. Zone 2B dolomite cement has identical δ¹⁸O values as zone 2A dolomite, indicating precipitation from similar fluids for the two dolomite generations. The Mn and Fe contents of zones 2A and 2B dolomite likely reflect a pH control over the fluid Mn and Fe chemistry; their similar low total Sr and nonradiogenic ⁸⁷Sr/⁸⁶Sr, imply that Sr was largely derived from the limestone precursor. Zones 3 and 4 dolomite cements (δ¹⁸O = -13.8 to -11.3; δ¹³C = -0.7 to +0.9) are depleted in δ¹⁸O relative to previous dolomites, recording hotter fluids. Zone 3 dolomite is depleted in Fe, due to pyrite precipitation whereas zone 4 dolomite cement has relatively high Mn and Fe contents. Zones 3 and 4 dolomite cements and later calcite are enriched in total Sr and have high ⁸⁷Sr/⁸⁶Sr, indicating late radiogenic Sr-enriched brines. Fluid inclusions indicate that zones 2A and 2B dolomite precipitated from warm (100-175°C), saline (23-26 wt.% NaC1 equiv.) fluids, followed by later hotter (175-225°C) more saline (30-33 wt.% NaCl equiv.) fluids. Pressure solution of the Knox Group dolomites during overthrusting provided much of the Mg²⁺ for dolomitization, this Mg²⁺ was transported by regional gravity-driven fluid flow that developed in response to tectonic uplift.
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    Sedimentology and Regional Implications of Fluvial Quartzose Sandstones of the Lee Formation, Central Appalachian Basin
    Wizevich, Michael Charles (Virginia Tech, 1991)
    Sedimentological analyses, including detailed facies characterization and lateral profiling, demonstrate deposition in a bedload-dominated fluvial system for the quartzose sandstones of Lee Formation. Internal (architectural) elements of the sandstones consist primarily of truncated channel-fIll sequences. Individual channel elements, up to 20 meters thick, contain a complex hierarchy of bedform deposits. The principal internal component of channels were downstream-accreting (mid-channel?) macroforms; channel elements frequently contain deposits of more than one macroform. Reconstruction of the macroforms reveals accretion primarily by superposed bedforms that migrated down a low-angle front. Steeper, giant foresets, transitional along flow with the low-angle facies, indicate that the macroform episodically developed a steep slipface. Uppermost channels within the Rockcastle Member contain macroform elements with components of lateral accretion, interpreted as deposits of alternate bank-attached macroforms. Also recognized within channel-fill deposits are minor-channel, sandy-bedform, gravity-flow (attributed to bank slumping), and channel-bottom elements. The latter element is contained within a facies sequence that suggests rising- to flood- to waning-stage deposition. In general, deposition was probably during relatively high stage; little evidence of low-stage flow was recognized. Subordinate fine-grained facies are interpreted as levee and overbank deposits. Strongly unimodal paleocurrents, lack of facies that suggest low-stage reworking and paucity of lateral-accretion features indicate deposition in a single-channel, low-sinuousity, system (i.e., a low braiding index). Fluvial architecture similar to that found in the Lee Formation has been previously explained by deposition in multi-channel, braided-river systems. However, the internal architecture of sandstone members is also consistent with a single-channel origin. Individual channels were temporarily confined, during which time the passage of several macroforms aggraded the channel. Position of the channel in the alluvial plain was largely controlled by avulsion of the river from fully aggraded channel belts to other areas of the plain. Calculations reveal that avulsion of a single-channel system across a wide alluvial plain is a plausible mechanism for building the sheet-like sandstone bodies of the Lee Formation. Spatial arrangement of individual sandstone members of the Lee Formation was probably controlled by tectonic processes. Episodic thrust-loading in the orogenic belt to the east and subsequent flexure of the crust in the foreland basin caused a step-wise progression of the river system towards the west. Petrographic, sedimentologic and stratigraphic data indicate that source area and climate functioned as the primary controls on the mature composition of sandstones in the Lee Formation. Source areas were composed primarily of quartz-rich sedimentary rocks and were located chiefly to the northeast/north. A east/southeast source area supplied subordinate and low-grade metamorphic rock fragments. Intense weathering, associated with humid tropical climates, acted upon the detritus throughout the sedimentation cycle. Less important controls on composition were tectonics and transport/depositional processes that extended exposure of the sediments to the severe climatic conditions. Quartzose sandstones of the Lee Formation reflect lower rates of tectonic subsidence and greater recycling of sand-sized grains during transportation and temporary deposition on the alluvial plain, relative to lithic time equivalents to the east.
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    Sedimentology and tectonic implications of the Late Proterozoic to Early Cambrian Chilhowee Group in southern and central Virginia
    Simpson, Edward L. (Virginia Polytechnic Institute and State University, 1987)
    Few detailed facies analyses of rift to passive-margin transitions have been undertaken in exhumed orogenic belts. In the central Appalachians, the Chilhowee Group records such an evolution. The Unicoi and basal Hampton Formations record the transition from rifting to opening of the Iapetus Ocean. The majority of the Hampton Formation and the overlying Erwin Formation represent an overall regressive sequence punctuated by five progradational packages that accumulated along a passive margin. The rift to passive·margin phases of sedimentation in the central Appalachians reflect a continuum from fault·influenced to thermotectonic subsidence. Alluvial sediments and intercalated basalts of the lower Unicoi Formation developed in a rift setting. Paleontological data indicate that rifting continued into lower Cambrian time. The upper Unicoi Formation represents the incipient phase of passive-margin sedimentation related to a first-order, sea level rise. Differences in degree of crustal attenuation controlled the distribution of sedimentary environments during transgression. On the most attenuated crust to the east, initial transgressive facies consist of tidal sandwave and sandridge deposits intercalated with proximal and medial braid-pIain deposits. As transgression progressed cratonwards onto less attenuated crust, tidal sedimentation was supplanted by tide- and wave-influenced sedimentation characterized by sandwave complexes, tidal inlets and longshore bedforms. Drowning at the top of the Unicoi Formation is indicated by outer-shelf black mudstones. Deepening may have been enhanced by continued movement along listric faults throughout the incipient phase of passive-margin development. Examination of outcrops of the Hampton and Erwin Formations on different thrust sheets has permitted an across-strike reconstruction of the Early Cambrian Chilhowee shelf in space and time. Progradational packages developed under storm- and fair·weather wave conditions. Coarsening· and thickening-upward sequences on westerly thrust sheets were generated during progradation of shoreface, inner-shelf and outer-shelf environments. Outer-shelf facies predominate on easterly thrust sheets. Intertidal-flat deposits on the most westerly thrust sheet erosively overlie progradational shoreface sediments and developed during transgression in an embayment in which the tidal wave was amplified. More distal transgressive deposits consist of fining- and thinning·upward sequences with glauconitic horizons, and condensed sections in mudstones.