Stuart L. Dean

Structure and Tectonics of Central and Southern Appalachian Valley and Ridge and Plateau Provinces, West Virginian and Virginia

The Valley and Ridge and eastern Allegheny Plateau from Pennsylvania into Tennessee is subdivided into distinct allochthonous sheets emplaced over an essentially featureless Precambrian basement surface. Sheet development is related to variations in stratigraphic thickness and lithology, as well as regional basement configuration. Three sheets control central Appalachian structural style. The Massanutten-Blue Ridge sheet is easternmost and is bounded on the west by the (Little) North Mountain-Pulaski fault system. Continuing westward into the plateau, the Martinsburg sheet contains anticlinoria and synclinoria uncut by major thrust faults. Both Massanutten-Blue Ridge and Martinsburg sheets are seated in Upper Ordovician Martinsburg (Reedsville) shales. The underlying Waynesboro sheet is rooted in Lower Cambrian Waynesboro (Rome) shales, and its imbrication has controlled the locations of major anticlinoria and synclinoria in the upper sheet. Within the southern Appalachians, greater imbrication of the lower sheet and increased thrust fault displacement at longitudinal ramps have destroyed the lateral continuity along strike of the central Appalachian Waynesboro and overlying Martinsburg sheets. There, imbrication to the surface has formed the Saltville sheet, continuous into Alabama, as well as the St. Clair-Narrows, Copper Creek, Cove Mountain, and St. Paul-Honaker sheets. Southwest of the trend change, the Holston-Stone Mountain sheet adjacent to the Blue Ridge is rooted primarily in Lower and Middle Cambrian strata and is underlain by the Pulaski sheet. In the westernmost sector of this region, the Pine Mountain and Richlands sheets are seated in Devonian shales. The Pulaski and Massanutten-Blue Ridge sheets, rooted primarily in Upper Ordovician shales, form a continuous structural unit and include Blue Ridge and Piedmont rocks.

Structural chronology of the Alleghanian orogeny in southeastern West Virginia

The trend change between central and southern Appalachian structures is sharply defined in southeastern West Virginia. There, N30°-35°E trends coincident with major central Appalachian folds, such as the Browns Mountain anticline, end abruptly at N60°E-trending southern Appalachian structures along the St. Clair fault. Analysis of local and regional folds, cleavage, bedding-perpendicular stylolite seams and bedding, and fault slickenlines reveals that layer-parallel shortening, directed N10°-30°W, occurred in nonfolded Greenbrier Group (Mississippian) carbonates well into the present Appalachian Plateau area. This structural event is early and is associated with the evolution of southern Appalachian folds and faults south of the St. Clair fault. Central Appalachian folds and mesoscopic structures were superimposed on this early layer-parallel shortening fabric. This structural chronology indicates that southern Appalachian folds and faults predated the development of central Appalachian structures in the region.

Coal-Cleat Domains and Domain Boundaries in the Allegheny Plateau of West Virginia

Regional face cleats cutting Pennsylvanian and Permian coal seams in the Allegheny Plateau of West Virginia can be divided into domains separated by boundaries that are sharply defined along most of their lengths. Domains and domain boundaries are established based on cleat trends, number of regional cleat sets, and relative chronology of cleat-set development. Regional cleats of each set show a common trend, and abutting relationships between multiple sets within a domain describe a progressive history of changing stress fields and cleat development. Boundaries dividing the in-situ directions of horizontal principal stress are coincident with cleat domain boundaries suggesting a common and persistent controlling factor. Uniform trends of face cleats within domains and ab upt changes in cleat signature across domain boundaries can be spatially related to regional basement structures and a depositional hinge line within the coal-bearing and underlying Mississippian rocks. In addition, fold structures in coal and underlying sedimentary rocks across the Allegheny Plateau commonly terminate or change trend abruptly at joint domain boundaries. In some cases, regions of common fold trends in coal bearing rocks are contained within specific domains. Face-cleat trends commonly differ from joint trends in rocks immediately bounding coal seams. However, one domain boundary in coal can be traced into Mississippian rocks through the unconformity at the base of the Pennsylvanian section. In this case, Mississippian and Pennsylvanian joint trends differ. It follows tha joint domain boundaries can extend downward through rocks of different lithologies, as well as coincide with conformable and unconformable stratigraphic boundaries.

Unconformity-bounded seismic reflection sequences define Grenville-age rift system and foreland basins beneath the Phanerozoic in Ohio

Interpretation of reprocessed Ohio Consortium for Continental Reflection Profiling (COCORP) OH-1 seismic reflection profiles indicates four structurally complex Precambrian unconformity-bounded stratigraphic sequences that clarify the relative timing of formation of the Fort Wayne Rift and East Continent Rift System with respect to the Grenville orogeny. Petrographic examination of sparse deep well samples in the region indicates or suggests sedimentary lithologies beneath the Paleozoic sedimentary cover. Other seismic profiles in the region, some with excellent well control, support our proposed model. A generalized model for the latter part of the Grenville orogeny suggests polyphase sedimentation and deformation with multiple episodes of crustal extension and compression. We propose the following events for Ohio and the surrounding region: (1) a major regional unconformity developed on the Eastern Granite-Rhyolite Province and accreted Grenville terranes; (2) western Ohio became the site of extensive fault-bounded rift basins, beginning with the Fort Wayne Rift and extending into west-central Ohio as the East Continent Rift System; (3) westward-advancing thrust sheets followed with deposition of sediments into newly developed basins; (4) continued Grenville thrusting created foreland basins in a westward progression; and (5) a long period of Neoproterozoic to Middle Cambrian erosion removed much of the foreland basin sedimentary sequences prior to Paleozoic deposition. Erosion in the Ohio region did not remove the large volume of rock as in Canada north of Georgian Bay. Other seismic lines in the region suggest that Grenville-age sedimentary basins are preserved beneath the Phanerozoic from Georgian Bay southward. These new findings demonstrate the importance of using fault- and unconformity-bounded seismic sequences to enhance and clarify the relative timing of Proterozoic events in regions where Paleozoic sedimentary cover exists and core samples are sparse or lacking.

Regional tectonics and fracture patterns in the Fall River Formation (Lower Cretaceous) around the Black Hills foreland uplift, western South Dakota and northeastern Wyoming

Abstract The Fall River Formation around the Black Hills uplift is pervasively fractured by layer-perpendicular joints. Systematic joints in the formation maintain consistent orientations over large areas and are commonly abutted by later-formed fractures, resulting in an orthogonal pattern. There are two major systematic sets, trending northeast and northwest, and one minor set trending north-south. The first two sets define two major fracture domains in the study area. The northwest joint set occupies a southern domain where it is the sole systematic fracture set. The northeast joint set is pervasively established throughout the northern domain, where northwest and north-south fracture sets are also developed in well-defined sectors. There is no genetic or spatial relationship between joint sets and local Laramide monoclines or folds of the region. Instead, the stratigraphic record indicates that joint development originated early in the lithification history of Fall River sandstones. Jointing occurred in response to local and regional extensional stresses that pervaded the northern and southern domains as a result of recurrent movement on basement faults that parallel the regional lineament system and surface structural zones throughout the region. Major uplift of the Black Hills and local fold development during Laramide time merely resulted in passive rotation of the early formed systematic and non-systematic joints.

ABSTRACT: INTERPRETATION OF PRECAMBRIAN GEOLOGY ALONG THE STRUCTURALLY RESTORED OHIO CONSORTIUM FOR CONTINENTAL REFLECTION PROFILING (COCORP) SEISMIC LINES

ABSTRACT A structural restoration of the original 1987 Ohio COCORP seismic lines, reprocessed to 3 seconds two-way time, has clarified the configuration and chronological development of the Fort Wayne rift (an arm of the East Continent Rift Basin) and a series of sediment-filled Precambrian foreland basins east of the previously accepted Grenville Front. Grenville deformation post-dated development of the East Continent Rift Basin and associated Precambrian Middle Run Formation sedimentation. Grenville foreland basins developed in an east-to-west sequence during progressive westward thrusting. The foreland basins were filled with sediment derived from the Grenvilleage mountains to the east. Early-formed Grenville basins were then partitioned and partially overthrusted by advancing Grenville thrust sheets. East Continent Rift Basin rift assemblages were also folded and transported westward. Grenville foreland basins are defined by (1) marked impedance contrasts of seismic reflectors between essentially horizontal basin fill and underlying Grenville metamorphic rocks, and (2) abnormally low interval velocities in basin strata. Structural restoration of fault displacements at the basin-fill/Grenville metamorphic rock unconformity reveals a geometry that supports an interpretation of three large foreland basins that were later progressively partitioned as Grenville thrusting advanced from east to west. Total shortening at the top of the Grenville sequence is about 13 percent across Ohio from the West Virginia border to the western boundary of the Grenville tectonic zone in west-central Ohio. Horizontal shortening appears to culminate within the tectonic zone at about 16.5 percent. This new interpretation presents the possibility of a previously unknown regional exploration play in low-velocity foreland-basin-fill sediments and reaffirms the effects that Precambrian structures have had on Paleozoic geology. The fundamental architecture and extent of the Central Ohio Platform and the Appalachian Basin date from the time of East Continent Rift Basin development through Grenville deformation, although shallow well control in Ohio defines the Appalachian Basin only to late Ordovician time. Well control confirms (1) the absence of the Mount Simon Sandstone on basement horst blocks, and (2) presence of anomalously thick Mount Simon and older sediments in grabens, which appear to be related to Precambrian structures. Younger structures and faults of Late Ordovician to Mississippian age, are also related to reactivated Precambrian structures, and define a series of terraces, bounded by monoclines, which step eastward down into the early Appalachian Basin. These faults and structures, and, in part, controlled facies distribution and subsequent hydrocarbon accumulation during the Paleozoic. Drilling has yet to confirm or refute the presence of the Grenville foreland basins. The authors thank Tom McGovern of Lauren Geophysical, Denver, CO, for providing industry-processed COCORP data.

Analysis of Fractures and Tectonic Structures in Core: ABSTRACT

Core analysis applied to characterization of fractured reservoirs should include interpretation of drilling-induced fractures, natural fractures, and tectonic structures. Cored natural fractures may possess geometric and genetic relationships with induced fractures, primary sedimentary features and tectonic structures that show cumulative effects of paleostresses and anisotropies active from initial basin development, through subsequent orogenic-epeirogenic phases, to the present. Drilling-induced fracture frequencies and orientations are related to rock anisotropies, rock mechanical properties, in-situ stresses, drilling stresses and preferred sonic and natural fracture directions. Induced fractures form in the core barrel, at the scribe knives or bit, and before the bit. Those leading the bit are subsequently cored and can be present in the borehole wall. Different types of drilling-induced fractures possess unique developmental histories and orientations. In addition, surface structure geometry on drilling-induced fracture faces, as well as morphology of these fractures, shows distinct relationships to core geometry. In contrast, surface structures and morphology of natural fractures show no geometrical relationships to core parameters. Recorded information should be shown on a fracture-tectonic log designed for easy visualization and ready comparison with other data.

Precambrian Basement Control on Joint Domains in Northwestern Ohio: ABSTRACT

Joint attitudes in Upper Silurian and Lower Devonian carbonates in northwestern Ohio reveal a marked north-south-trending joint domain boundary in Lucas and Wood Counties. East of the domain boundary, first-formed systematic joints trend N45/sup 0/W; west of this boundary, first-formed systematics trend N40/sup 0/E. The line of change in joint trends follows the Lucas County monocline-Bowling Green fault complex and the projected position of the Grenville front from the Canadian shield. Basement well information and gravity and magnetic data indicate a major change in Precambrian rock types across the domain boundary that is coincident with the projected position of the Grenville front. Observed joint patterns are interpreted to result from extensional tectonics associated with the evolution of the Findlay arch and Michigan basin. Recurrent movements on the Bowling Green fault during the early and middle Paleozoic may have been caused by reactivation of Grenville-age faults, which ultimately localized the joint domain boundary along the Lucas County monocline-Bowling Green fault trend.

Fractographic Distinction of Coring-Induced Fractures from Natural Cored Fractures: ABSTRACT

Fracture surface structures (hackle plumes, arrest lines, origins) on coring-induced petal-centerline and disc fractures from three Appalachian Devonian shale cores indicate fracture sequence and propagation directions, relative propagation velocities, and tensile-stress distributions at failure. Surface structures on coring-induced fractures are symmetrically and dimensionally related to the core. In contrast, surface structures on natural fractures, originating away from the core, are asymmetric and oversized. Plume asymmetry shows that stress intensity across natural fractures varied vertically during propagation. Curviplanar petal-centerline fractures are propagated downcore as shown by convex downward arrest lines and hackle plumes that diverge downward about the core axis. Inclined petal sections curve to vertical from core margin toward core center. Some petals continue to spread vertically downcore, forming the centerline section. Petal-centerline fractures can change downcore from one preferred orientation to another, indicating differing orientation of stresses and thus of any fractures induced in a stimulation program. Petal curvature, absence of cored origins, and the 15-cm curvature radius of closely spaced arrest lines show that petal-centerline fractures originated in front of the bits cutting surface. Chipped right-hand core to fracture margins, produced by plucking action of the it, and arrest line-hackle morphology show these fractures were drilled through after propagation. End_Page 482------------------------------ Bed-parallel disc fractures started at bit level, within the core, at bedding irregularities. Hackle plumes indicate that spreading velocity of disc fractures was greatest toward core centers and decreased toward core margins in response to changes in tensile stress intensity. End_of_Article - Last_Page 483------------

Nature and Field Application of Plumose Structures: ABSTRACT

Development of plumose structures in brittle rocks has been investigated by analogy to fracturing experiments on glass and ceramic bodies. Plume morphology shows that structures commonly lumped as plumose are a composite of discrete features, formed at all scales, during fracture propagation. Inclusion hackle forms when an advancing planar fracture front becomes locally distorted at an inhomogeneity. The planar fracture, locally split by the inclusion, does not rejoin in a single plane behind the inclusion. This causes the lagging fracture portion to curve into the leading one forming a steplike tail elongate in the propagation direction. Twist hackle forms when a fracture front abruptly encounters changed stress directions along an extended frontal section. The entire fracture front breaks into individually advancing en echelon twist-hackle faces, each face perpendicular to the new resultant principal tension. Faces diverge and are elongate in the propagation direction. The faces form hackle steps by curving into each other to complete separation. Velocity hackle, uncommo in rocks, forms at a limiting propagation velocity. Plume axes mark areas of greatest tensile stress and lightest propagation velocities. Plume asymmetry indicates intrastratum fracturing stress distributions. Axes consistently at the top or bottom of each stratum in a layered sequence indicate overall downward and upward (perhaps basement induced) propagation directions respectively. Recognizing twist-hackle faces and steps as differently oriented planes produced by a single fracture event eliminates identification and misinterpretation of false fracture sets. End_of_Article - Last_Page 482------------