Cahit Çoruh

Short-term paleoclimatic fluctuations expressed in lower Mississippian ramp-slope deposits, southwestern Montana

Lower Mississippian ramp-slope deposits (Paine Member) of southwestern Montana are composed of thin, rhythmically interbedded limestone and argillaceous limestone (argillite). Millimeter-thick graded layers typical of limestone beds represent distal storm deposits, whereas argillite layers containing abundant whole, delicate fossils represent quiet-water deposition during times of little or no storm activity. Spectral analyses of the fluctuating insoluble-residue content (quartz, muscovite-illite, organic matter) indicate a dominant periodicity of 0.6-2.85 ka in the ramp-slope deposits; no spectral peaks corresponding to typical Milankovitch-type periods ({approximately}20-100 ka) were observed. Similar {approximately}2.5 ka paleoclimatic periodicities are recorded in Quaternary continental and alpine glaciers, Quaternary deep-sea sediments, C variations in Holocene tree rings, and Permian deep-water evaporite varves. These short-term paleoclimatic fluctuations may represent one of several harmonics of the precessional (19-23 ka) or obliquity (41 ka) orbital cycles or may be related to variations in solar activity.

Seismic reflection evidence for the evolution of a transcurrent fault system: The Norumbega fault zone, Maine

A seismic reflection profile in east-central Maine reveals a steeply dipping fault zone that terminates in diffraction hyperbolae that are associated with an offset of the Moho. When combined with data from geologic mapping, the seismic data imply Mesozoic or post-Mesozoic dip-slip reactivation of the Norumbega fault zone, hitherto interpreted as a locus of mid- to late Paleozoic dextral offset. This finding highlights the value of seismic reflection data in determining the history of fault displacement and warns of the complexities that arise in attempts to seismically characterize faults in polygenic tectonic regimes.

Paleozoic and Grenvillian Structures in the southern Appalachians: Extended interpretation of seismic reflection data

Interpretive reprocessing of seismic reflection data has elucidated Paleozoic and Grenvillian structures in the southern Appalachians. The seismic data include a 7500-km² grid of ADCOH, Seisdata, and COCORP reflection profiles that traverse the Blue Ridge and Inner Picdmont geologic provinces of North Carolina, South Carolina, and Georgia. Surface geology and potential field data were used to constrain the interpretation. The reprocessed seismic reflection data have delineated the internal and external geometry of the crystalline Blue Ridge-Inner Piedmont allochthon, including the locations of the Blue Ridge master decollement, Haycsville fault, and Brevard fault zone. On the basis of the reprocessed data, all of the major faults within the allochthonous upper crust sole in the Blue Ridge master decollement. Reflections extending to the southeast from beneath the surface location of the Hayesville fault to the Blue Ridge thrust might be the seismic signature of a high strain zone. This implies that internal deformation of the Blue Ridge allochthon associated with the Alleghanian orogeny might have occurred farther to the west than has been previously documented from field studies. Relative amplitude seismic data enabled the discrimination between Blue Ridge-Inner Piedmont crystalline rocks and underlying lower Paleozoic shelf strata, thereby delineating the Blue Ridge thrust. The interpreted geometry constrains the top of the shelf sequence beneath the Blue Ridge to depths of less than 3 km. This relatively shallow depth of the shelf strata together with the presence of duplex structures and bright spots that are imaged within the sequence might imply favorable conditions for hydrocarbon exploration beneath the Blue Ridge. Midcrustal reflections from within the upper-to-Iower crust are interpreted to originate from preserved Grenvillian structures that were reactivated at the basement surface during Late Proterozoic-Early Cambrian extension. Reflection continuity is occasionally disrupted by interpreted post-Grenvillian, pre-Early Cambrian low-density intrusions. Topography at the basement surface, possibly caused by the intrusions, is interpreted to have controlled the formation of some of the structures within the overlying allochthon, including Blue Ridge and Brevard fault zone ramps. Correlation of seismic time-structure contour maps with available gravity data and two-dimensional gravity modeling suggest that anomalies in the gravity field can be attributed to low-density sources within the autochthonous crust. Discontinuous reflection packages from depths of 36–42 km are interpreted to originate from the Mohoroviĉiĉ discontinuity. The reflectors trend about N15°E with a true dip of approximately 15°NW.

Crustal structures and the eastern extent of lower Paleozoic shelf strata within the central Appalachians: A seismic reflection interpretation

Reprocessing of line PR3 proprietary seismic reflection data has delineated Grenvillian, Paleozoic, and Mesozoic structures within the Appalachian foreland, Blue Ridge, and Piedmont of the central Appalachians in Virginia and West Virginia. The eastern portion of PR3 can be correlated along strike with the western portion of line I-64, reprocessed earlier at Virginia Tech. The combined seismic reflection data image the crust from the eastern Valley and Ridge, Blue Ridge, Piedmont, and Atlantic Coastal Plain provinces. Within the Piedmont, large (as much as 10 km wide) reflective structures imaged on both lines PR3 and I-64 are interpreted to be thrust sheets that might be composed of deformed Catoctin, Evington Group, and possibly younger metamorphosed rocks. A concealed extension of the Green Springs mafic mass intrudes a thrust sheet imaged along the PR3 profile. The Blue Ridge-Piedmont allochthon was transported north-west along the Blue Ridge thrust, which ramps upward ∼12 km east of the surface exposure of the Mountain Run Fault. Westward along line PR3, the Blue Ridge thrust maintains an undulating geometry; the maximum thickness of the Blue Ridge allochthon is interpreted to be ∼4.5 km. The Blue Ridge allochthon is generally acoustically transparent and overlies lower Paleozoic shelf strata. The maximum thickness of these strata is ∼8 km. Shelf strata are interpreted to extend in the subsurface 5 km east of the surface exposure of the Mountain Run Fault, the northeastward extension of the Brevard Fault Zone, where they are truncated by the Blue Ridge thrust at a depth of 10.5 km (3.5 s). Various folds and blind thrusts are imaged beneath the Appalachian foreland; however, the foreland has not experienced the same degree of deformation as observed in the eastern provinces. A basement uplift ∼45 km wide is imaged beneath the Valley and Ridge province and is interpreted as having formed prior to Late Cambrian time. Farther west, reflections imaged beneath the Glady Fork anticline in the Appalachian Plateau are interpreted as a positive flower structure associated with wrench fault tectonics. Relatively few deep (>9 km) crustal reflections are imaged along line PR3. The majority of reflections that do exist at these depths are observed beneath the Piedmont and eastern Blue Ridge. The high reflectivity associated with the Grenvillian basement in these areas might be the result of the Paleozoic orogenies and extension related to Late Proterozoic and Mesozoic rifting.

The Southern Appalachian Ultradeep Scientific Drill Hole: Progress of Site Location Investigations and other Recent Developments

The southern Appalachian ultradeep core hole (ADCOH) is designed to test modern ideas about the formation of mountain chains along the edges of continents. The faulting, stratigraphy, lithology and out-crop patterns observed during detailed studies of the surface geology in this region have been interpreted in terms of a series of thrust sheets of diverse ages, which were transported northwestward (Fig. 1) over the Iapetan continental margin of North America (Hatcher 1978). Seismic reflection data and interpretations by Clark and others (1978), Cook and others (1979, 1983) and Harris and Bayer (1979) support this interpretation, and reveal a zone of high reflectivity which dips to the southeast beneath portions of the Blue Ridge and Piedmont physiographic provinces. This zone of reflections is correlative with a similar interval known in the sedimentary rocks of the Appalachian foreland some 150 km northwest of the ADCOH study area, and has been interpreted as a regional decollement, serving as the root zone for major thrust faults such as the Brevard.

Geophysical evidence for an allochthonous Alleghanian(?) granitoid beneath the basement surface of the Coastal Plain near Lumberton, North Carolina

Geophysical data obtained over the Atlantic Coastal Plain near Lumberton, North Carolina, indicate the presence of a granitic pluton buried beneath basement volcanic rocks similar to those of the Carolina slate belt. The volcanic rocks were cored southeast of the city of Lumberton. They consist of interlayered felsic and mafic rocks metamorphosed to lower amphibolite facies. Excellent reflections from a nearby VIBROSEIS line originate from this volcanic sequence, which is interpreted as being >3.5 km in thickness. The volcanic rocks are underlain by an acoustically transparent interval interpreted as being associated with a granitic pluton. This hypothesis is supported by a −35 mgal Bou-guer gravity anomaly and by a relatively high heat flow of 63.5 ± 5.4 mW/m 2 obtained in the drill hole. These are both characteristics of late Paleozoic Alleghanian granitoids in the southeastern United States. Results of gravity modeling suggest that the body is nearly circular in shape (∼45 km diam) and 13 km in thickness. Subhorizontal reflections at 5 to 7 sec on the seismic data are interpreted as being at and below the base of the granitoid at a depth of ∼ 17 km. The deep layering may be stratigraphy or mylonites, and, in either case, the granitoid is allochthonous and did not intrude the layering. High heat flow suggests, from our experience in the Piedmont, that the body is unmetamorphosed and of late Paleozoic age; the thrusting may postdate, or be coeval with, the age of intrusion and suggests that late Paleozoic thrusting in this part of the Coastal Plain is confined to depths below 17 km.

Modeling offset-dependent reflectivity for time-lapse monitoring of water-flood production in thin-layered reservoirs

The objective of this case study is to predict whether 5 years of water-flood production from a thinly layered Gulf of Mexico reservoir will change its seismic amplitude-variation-with-offset (AVO) response in a detectable manner. Density and velocity profiles were computed from in situ wireline logs for 100% oil, gas, and brine saturations and for a 5-year prediction that was based on a fluid-flow and production simulation. Analytical AVO curves for simple half-space models did not match AVO curves extracted from synthetic seismograms computed with a full-waveform layer-stack algorithm. Several different amplitude corrections were tried to reduce the AVO curves from the synthetic data to the analytical ones, but, ultimately, none was deemed satisfactory. Instead, AVO change attributes based on relative changes, polarity changes, or ratios were used. Attributes based on the change of AVO gradient were perceived to be most diagnostic of the water flood, but they were also overly sensitive to interference noise and amplitude correction errors. For field data from the study area, a large decrease in intercept magnitude may be the best indicator of the waterfront.

Alternative processing techniques and data improvement provided by single‐sweep recording

The conventional procedure used to acquire Vibroseis® seismic reflection data is to sum in the field the contributions from several vibrator sources distributed over the source array. An alternative method of recording the data which provides more flexibility in the processing is to record the output from each pad position in the source array rather than summing in the field. Prewhitening these data before summing can improve the signal‐to‐noise (S/N) ratio. If cancellation of surface waves by a source array is not a requirement, then processing each sweep as a separate source point can result in increased lateral resolution. These procedures were applied to seismic data over a buried rift basin in the southeastern United States. The results demonstrate improvement in the S/N ratio and spatial resolution that enable better interpretation of the complex, internal geometry of the basin.

Spectral whitening of impulsive and swept-source shallow seismic data

Two problems that commonly affect shallow seismic data are narrow bandwidth and interference from ground roll. Deconvolution techniques are often attempted to overcome these problems and thus improve resolution, but cannot generally improve resolution without losing signal-to-noise ratio. In this paper, we demonstrate the application of two simple techniques for enhancing shallow seismic data. The first, Vibroseis whitening is accomplished simply by application of an automatic gain control (AGC) to swept source data before they are correlated with the sweep. The second, stretched automatic gain control (SAGC), involves convolution of impulsive source data with a synthetic sweep and application of an automatic gain control before correlation with the sweep. We compare the effectiveness of these techniques with conventional deconvolution techniques, using data acquired during the 1993 non-invasive seismic source comparison at Oak Ridge National Laboratory. Our results indicate that these methods are capable of improving the quality of processed shallow seismic data from both swept sources and impulsive sources.

The Appalachian Ultradeep Core Hole (ADCOH) Project

The principal goal of the Appalachian Ultradeep Core Hole (ADCOH) Project is to study the processes related to the formation and reactivation of large faults in the internal parts of an intact composite crystalline thrust sheet formed by continent-continent collision along an ancient continental margin. Composite crystalline thrust sheets are some of the largest structures in orogenic belts, comparable in size to the large accreted terranes. They are therefore of central importance in the overall processes related to formation of mountain chains and the evolution of continental crust.