Byron R. Kulander

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.

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.

Observations on fractography with laboratory experiments for geologists

Abstract Brittle fracture growth proceeds through unique stages, each marked by distinct fractographic features that can only develop during Mode I loading at the propagating crack tip. Fractographic features in any substance can be interpreted in terms of the location of the failure origin, as well as changes in propagation velocities, stress directions and stress magnitudes at the crack tip during failure. Joints in granular pervious rock, however, do not contain fractographic features, commonly formed in glass, that develop at unstable propagation rates under the influence of appreciable amounts of stored strain energy. Yet, features that develop at lower stable rates of propagation are present on fractures in glass and rock implying that the absence of certain fractographic features provides useful information. Simple laboratory experiments, primarily on glass, are discussed to provide geologists an opportunity to recognize and qualitatively interpret fractographic features diagnostic of brittle failure. The exercises demonstrate the changing propagation dynamics that control the morphological evolution of artificially-induced fractures and natural joints in rock.

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.

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------------