Terry Engelder

THE ROLE OF SALT IN FOLD-AND-THRUST BELTS

Davis, D.M. and Engelder, T., 1985. The role of salt in fold-and-thrust belts. In: N.L. Carter and S. Uycda (Editors), Collision Tectonics: Deformation of Continental Lithosphere. Tectonophysics, 119: 67-88. The style of deformation in thin-skinned fold-and-thrust belts is critically dependent upon the resistance to sliding along the detachment between the mass of deforming sediments and the underlying rocks. Evaporites can provide an extremely weak horizon within which a basal detachment can form and along which only a relatively small shear traction can be supported. Fold-and-thrust belts that form atop a salt layer, such as the Appalachian Plateau, the Franklin Mountains in northwestern Canada, and the Jura of the Alps, among others, share several readily observable characteristics. As predicted by a simple mechanical model for fold-and-thrust belts, a detachment in salt permits a thrust belt to have an extremely narrow cross-sectional taper. In addition, predicted orientations of the principal stress axes over a salt dtcollement are consistent with the commonly observed lack of a consistently dominant vergence direction of structures within the thrust belt. Other common attributes of salt-basal thin-skinned deformation include the presence of several widely but regularly spaced folds and abrupt changes in deformational style at the edge of the salt basin.

Natural gas: Should fracking stop?

Extracting gas from shale increases the availability of this resource, but the health and environmental risks may be too high.

The permeability of whole and jointed Barre Granite

Abstract The permeability of whole and jointed Barre granite was measured at pressures up to 2 kbars. Jointed samples were actually split cylinders joined by surfaces with controlled surface roughness. Samples with induced tension fractures were also measured. The permeability of the whole rock ranged from about 10 −6 to 10 −7 darcies. The permeability of the jointed rock ranged from about 8 × 10 −5 darcies at low pressure down to that of the whole rock at high pressures. Permeability was not a simple function of the difference between external confining pressure (P c ) and internal fluid pressure (P f ). Changes in permeability were found to be proportional to (b dP f − a dP c ) where b/a c and P f was also important. Permeability hysteresis and an ultimate decrease in permeability in both whole and jointed rock resulted when internal fluid pressure was cycled. This effect seems to diminish with increasing confining pressure. At a particular P c , the volume flow rate, q, is proportional to (P c − P f ) −n . Increasing the surface roughness of the joints decreased the value of n, which was smallest for the tension fracture and the whole rock. Within the uncertainty of joint aperture measurements, a flat plate model of the joint seem inadequate.

Loading paths to joint propagation during a tectonic cycle: an example from the Appalachian Plateau, U.S.A.

Abstract Based on the timing of joint propagation during the history of burial, lithification, deformation and denudation of clastic rocks within sedimentary basins, four types of joints may be distinguished: tectonic, hydraulic, unloading and release. Tectonic and hydraulic joints form at depth prior to uplift in response to abnormal fluid pressures, whereas unloading and release joints form near the surface in response to thermal-elastic contraction accompanying erosion and uplift. Tectonic joints are distinguished from hydraulic joints in that tectonic compaction is a mechanism for achieving abnormal pore pressures leading to the propagation of the former whereas compaction by overburden loading leads to the abnormal pore pressures in the latter case. The orientation of unloading joints is controlled by either a residual or contemporary tectonic stress whereas the orientation of release joints is controlled by a rock fabric. Examples of some of these joints are found within the Devonian Catskill Delta of the Appalachian Plateau, New York. During the Alleghanian Orogeny tectonic joints (cross-fold joints) formed under abnormal pore pressure as indicated by the observation that joints propagated in the siltstones before they developed in shales and by the cross-cutting relationships of folds, cleavage and joints. This sequence is compatible with oil company hydraulic fracture data which show that the least principal stress within sandstone layers is less than that in the intercalated shale layers. Plumose structures indicate that the joints within siltstones propagated as discontinuous rupture events each of which affected less than a meter of bed length. The discontinuous rupturing is compatible with models for natural hydraulic fracturing. Release joints (strike joints) post-date the Alleghanian Orogeny as indicated by abutting relationships within the deeper parts of the Devonian clastic section. Unloading joints are orthogonal to the contemporary tectonic stress field.

Joint sets that enhance production from Middle and Upper Devonian gas shales of the Appalachian Basin

The marine Middle and Upper Devonian section of the Appalachian Basin includes several black shale units that carry two regional joint sets (J1 and J2 sets) as observed in outcrop, core, and borehole images. These joints formed close to or at peak burial depth as natural hydraulic fractures induced by abnormal fluid pressures generated during thermal maturation of organic matter. When present together, earlier J1 joints are crosscut by later J2 joints. In outcrops of black shale on the foreland (northwest) side of the Appalachian Basin, the east-northeast–trending J1 set is more closely spaced than the northwest-striking J2 set. However, J2 joints are far more pervasive throughout the exposed Devonian marine clastic section on both sides of the basin. By geological coincidence, the J1 set is nearly parallel the maximum compressive normal stress of the contemporary tectonic stress field (SHmax). Because the contemporary tectonic stress field favors the propagation of hydraulic fracture completions to the east-northeast, fracture stimulation from vertical wells intersects and drains J2 joints. Horizontal drilling and subsequent stimulation benefit from both joint sets. By drilling in the north-northwest–south-southeast directions, horizontal wells cross and drain J1 joints, whenever present. Then, staged hydraulic fracture stimulations, if necessary, run east-northeast (i.e., parallel to the J1 set) under the influence of the contemporary tectonic stress field thereby crosscutting and draining J2 joints.

Classification of solution cleavage in pelagic limestones

Spaced cleavage formed by rock dissolution can represent major amounts of shortening parallel to bedding; much so-called fracture cleavage is of this origin. We classify the solution cleavage developed in Mesozoic pelagic limestones of the Umbrian Apennines into four intensity types ( weak, moderate, strong, very strong ) on the basis of qualitative attributes and mean spacing of cleavage surfaces. Shortening can be determined from imbricated chert beds and reaches 50% in rocks with very strong cleavage. In the Umbrian Apennines, solution cleavage is commonly associated with detachment thrusts. We describe an example in which the dissolution mechanism “damaged” the rock beneath a thrust by creating closely spaced discontinuities; fragments bounded by these discontinuities were torn up and incorporated in a nearly chaotic shear zone as the thrust sheet moved forward.

Development of cleavage in limestones of a fold-thrust belt in eastern New York

Abstract Tectonic cleavage has developed in Lower Devonian limestones of the Hudson Valley Fold-Thrust Belt in New York State. Morphology and distribution of cleavage in these limestones is controlled by the amount of clay-quartz matrix present and by strain. Only limestones with greater than 10% clay-quartz matrix developed widespread cleavage. X-ray diffraction analyses indicate that the clay matrix was altered during deformation; the width of the 001 illite peak in cleaved lime wackestone is less than that in uncleaved lime wackestone from the same stratigraphic level. A minimum of 10% clay is necessary to provide interconnectivity between sites of dissolution and the free-fluid system. Preliminary analyses of bulk chemistry indicate that calcite is removed from the local rock system during cleavage development, supporting proposals that circulation of fluid through the rock plays a major role in the development of cleavage. Textural evidence suggests that both pressure solution and free-face dissolution contribute to the removal of ions at grain boundaries. Cross-cutting relations indicate that cleavage was initiated early during the development of the fold-thrust belt, but continued to develop during the late stages of folding.

Factors controlling joint spacing in interbedded sedimentary rocks: integrating numerical models with field observations from the Monterey Formation, USA

Abstract Local tensile stress normal to a joint is reduced in the vicinity of the joint because such stresses are not transmitted across free surfaces. This stress reduction prevents the formation of new joints in the vicinity of existing joints, and thus influences joint spacing. Lateral extent of this stress reduction shadow increases with joint height, which corresponds to bed thickness for many sedimentary rocks. The linear correlation between joint spacing and bed thickness commonly observed in outcrop is a direct result of this relationship. However, other factors in addition to bed thickness influence joint spacing. We evaluate these factors through both a review of the Hobbs model for joint spacing and a 2D finite element simulation of a crack confined to a lithology-controlled mechanical unit. The stress reduction shadow increases in length with incresaing Young’s modulus of the jointing bed, though fracture stress, flaw size, flaw distribution and extensional strain all interact with bed thickness and elastic properties ultimately to control joint spacing. One explanation for the observed decrease in joint spacing with increasing Young’s modulus in outcrops of the Monterey Formation is that beds with higher Young’s moduli fail at lower magnitudes of extensional strain.

Influence of poroelastic behavior on the magnitude of minimum horizontal stress, Sh in overpressured parts of sedimentary basins

In many sedimentary basins of the world the minimum hori- zontal stress, S h , is greater in overpressured zones than in normally pressured zones at equivalent depths. A common explanation is that the frictional slip on listric normal faults keeps the difference between vertical stress, S v and S h within certain bounds, and the difference is smaller under lower effective stress (i.e., higher pore pressure, P p ). However, in the overpressured parts of the central North Sea graben, United Kingdom, and the Sable subbasin of the Scotian Shelf, Canada, conventional friction envelopes underestimate the magnitude of S h . These data instead indicate that S h increases at a rate proportional to but less than the rate of increase of P p , a condition consistent with a P p -induced deformation of the rock called poroelastic behavior. This paper argues that, whereas friction may govern S h in normally pressured basins, poroelastic behavior is responsible for the unusually high S h in the overpres- sured parts of these same basins. Data on the P p and S h gradients from these basins suggest that Δ S h /Δ P p ∼ 0.7.

Disjunctive cleavage formed at shallow depths in sedimentary rocks

Abstract Work of the past decade concerning tectonic cleavage has focused on nine topics: (1) appropriate terminology for cleavage classification, (2) processes involved in cleavage formation, (3) controls on cleavage distribution and morphology, (4) depth dependence of cleavage formation, (5) controls and causes of cleavage initiation, (6) nature of lithologic changes during cleavage development, (7) relation of cleavage to strain, (8) relation of cleavage to stress and (9) chronology of cleavage development. This paper reviews these issues as they apply to the formation of disjunctive cleavage in sedimentary rocks. Discussion is limited to cleavage that formed at shallow crustal conditions in which rock-water interaction, rather than plastic flow, recrystallization, or grain growth, is the dominant deformation process.