Interpreting complex fluvial channel and barform architecture: Carboniferous Central Pennine Province, northern England

The Bashkirian Lower Brimham Grit of North Yorkshire, England, is a fluvio-deltaic sandstone succession that crops out as a complex series of pinnacles, the three-dimensional arrangement of which allows high-resolution architectural analysis of genetically-related lithofacies assemblages. Combined analysis of sedimentary graphic log profiles, architectural panels and palaeocurrent data have enabled three-dimensional geometrical relationships to be established for a suite of architectural elements so as to develop a comprehensive depositional model. Small-scale observations of facies have been related to larger-scale architectural elements to facilitate interpretation of the palaeoenvironment of deposition to a level of detail that has rarely been attempted previously, thereby allowing interpretation of formative processes. Detailed architectural panels form the basis of a semi-quantitative technique for recording the variety and complexity of the sedimentary lithofacies present, their association within recognizable architectural elements and, thus, the inferred spatio-temporal relationship of neighbouring elements. Fluvial channel-fill elements bounded by erosional surfaces are characterized internally by a hierarchy of sets and cosets with subtly varying compositions, textures and structures. Simple, cross-bedded sets represent in-channel migration of isolated mesoforms (dunes); cosets of both trough and planar-tabular cross-bedded facies represent lateral-accreting and downstream-accreting macroforms (bars) characterized by highly variable, yet predictable, patterns of palaeocurrent indicators. Relationships between sandstone-dominated strata bounded by third-order and fifth-order surfaces, which represent in-channel bar deposits and incised channel bases respectively, chronicle the origin of the preserved succession in response to autocyclic barform development and abandonment, major episodes of incision probably influenced by episodic tectonic subsidence, differential tilting and fluvial incision associated with slip on the nearby North Craven Fault system. Overall, the succession represents the preserved product of an upper-delta plain system that was traversed by a migratory fluvial braid-belt system comprising a poorly-confined network of fluvial channels developed between major sandy barforms that evolved via combined lateral-accretion and downstream-accretion

Use of Mohr Diagrams to Predict Fracturing in a Potential Geothermal Reservoir

Inferences have to be made about likely structures and their effects on fluid flow in a geothermal reservoir at the pre-drilling stage. Simple mechanical modelling, using reasonable ranges of values for rock properties, stresses and fluid pressures, is used here to predict the range of possible structures that are likely to exist in the sub-surface and that may be generated during stimulation of a potential geothermal reservoir. In particular, Mohr diagrams are used to show under what fluid pressures and stresses different types and orientations of fractures are likely to be reactivated or generated. The approach enables the effects of parameters to be modelled individually, and for the types and orientations of fractures to be considered. This modelling is useful for helping geoscientists consider, model, and predict the ranges of mechanical properties of rock, stresses, fluid pressures, and the resultant fractures that are likely to occur in the sub-surface. Here, the modelling is applied to folded and thrusted greywackes and slates, which are planned to be developed as an Enhanced Geothermal System beneath Göttingen

Deformation history and basin-controlling faults in the Mesozoic sedimentary rocks of the Somerset coast

Structures in the Triassic and Liassic sedimentary rocks of the Somerset coast indicate spatial and temporal variations in the deformation history. From Kilve to Watchet, there was Mesozoic north-south extension and Tertiary north-south contraction. At Lilstock, about 3 km to the east, there was a more complex history, including: (1) the development of joints, normal faults and veins striking about 060°; (2) approximately north-south extension on 095° striking normal faults, with sinistral transtension; (3) east-west contraction, with sinistral shear on some 095° striking normal faults; (4) dextral reactivation of some 095° striking normal faults; (5) north-south contraction, and thrusts and strike-slip faults, with the reverse-reactivation of the largest 095° striking normal faults. The joints in the district mostly postdate the faults.Two large approximately east-west striking faults are postulated to form the northern edges of the Quantock and Exmoor hills, here called the North Quantocks Fault (NQF) and the North Exmoor Fault (NEF). These were basin-bounding normal faults during the Mesozoic, with maybe more than 1000 m of throw, suggesting that the Bristol Channel Basin is not a half-graben developed above a south-dipping Variscan thrust which underwent Mesozoic extensional reactivation. The NQF and NEF may have been reverse-reactivated during the Alpine contraction. The NQF probably caused variations in the stress history of the Mesozoic sedimentary rocks of the Somerset coast over only a few kilometres along strike. The possible relationships of the NQF and NEF with the Cothelstone Fault are discussed.Several broad orders of relay ramp scale occur, within which are developed smaller antithetic normal faults. Between the north-dipping NQF and NEF, the relay ramp contains south-dipping (antithetic) normal faults with tens to hundreds of metres displacement. In relay ramps between the south-dipping faults are north-dipping normal faults with metre-scale displacements, which in turn have relay ramps with south-dipping normal faults with millimetre-scale displacements.<br/

Structural analyses and fracture network characterisation: seven pillars of wisdom

Seven distinct types of structural analysis can be defined, each with their own data and uses: (1) basic geological descriptions; (2) geometries and topology; (3) age relationships; (4) kinematics; (5) tectonics; (6) mechanics and (7) fluid flow. We illustrate these types of analysis using the example of faults and fractures, which typically form networks of interacting and connected segments. A framework for describing and characterising fault and fracture networks is presented for each of the structural analysis types. We suggest that any structural study be tailored to suit the desired outcome and that this scheme of analysis types should be used as a basis for the development of workflows, for the design of research projects and for testing hypotheses. For example, prediction of fluid flow through a fracture network must begin with the basic geological description of fracture types. Basic geological descriptions should be followed by measuring their geometries and topologies, understanding their age relationships, kinematic and mechanics, and developing a realistic, data-led model for related fluid flow. Missing steps can lead to fundamentally flawed interpretations.</p

Strain and scaling of faults in the chalk at Flamborough Head, U.K.

Analysis has been made of the orientations, displacements and spacings of 1340 extensional faults, with displacements of up to 6 m, along an almost completely exposed 6 km length of cliff. This data set has been used to study how fault populations account for strain in a region and to study relationships between different scales of fault. Strains have been estimated; the maximum and intermediate extensions are sub-horizontal, with approximately equal extension (e ? 0.01) in all horizontal directions. It can be inferred that the minimum extension (maximum compression) was sub-vertical, but that the wide variety of fault orientations and cross-cutting relationships resulted from variable horizontal extensions. Some faults have oblique-slip slickenside lineations, which imply a period of later, dominantly NNW-SSE extension, which possibly developed as the exposure-scale faults linked up E-W-striking larger-scale normal faults, effectively forming a single wide fault zone.Graphs of displacement per unit distance are used to illustrate variations in displacement. The scaling of fault displacement appears to follow a power-law relationship. The differences in orientation between the small-scale and large-scale faults precludes a simple estimation of the total strain over all scales.<br/

Effects of layering and anisotropy on fault geometry

The geometry and orientation of faults are examined using several field examples of small-scale (&lt;1m displacement) fault systems. For isotropic rocks under triaxial compression, faults normally develop in conjugate sets about the maximum compressive stress (1), with dihedral angles usually of about 50°, as predicted by the Coulomb theory of failure. In layered rocks, the geometry of faults varies with the orientation of layering with respect to the stress field. Where 1, is approximately normal to layering or anisotropy, conjugate faults also develop symmetrically about 1. Where rocks have interbedded layers with different mechanical behaviours, however, faults tend to initiate orthogonal to the more brittle layers (i.e. originate as extension fractures sub-parallel to 1), but oblique to the less brittle layers. As the fault steps through the layering, pull-aparts are developed which may reduce the dihedral angle. Where 1 is oblique (c. 25–75°) to anisotropy, one set of faults is developed at a high angle to layering, with another at a low angle, usually showing a ramp-flat geometry. Large dihedral angles (up to 90°) may result and 1, does not bisect this angle. Where 1 is approximately parallel to layering, two cases can be recognized. Where 3 is approximately normal to layering, faults with layer-parallel flats and contractional ramps develop. Where 2 is approximately normal to layering, conjugate faults develop which are symmetrical about 1, the geometry resembling that in isotropic rocks. These observations are in agreement with rock deformation experiments which show the strong effect of anisotropy on fault orientation, but the observations incorporate the effects of layering of materials with different deformation characteristics. <br/

Estimating strain from fault-slip using a line sample

A method is presented that enables data for faults with different orientations and displacements, measured along a single straight line, to be used to estimate the magnitudes and orientations of the principal strain axes. The method combines two well-established techniques. When sampling along a line, the probability of intersecting a fault is affected by its orientation. This sampling bias may be minimized by the use of a weighting, w = 1/cos ?, where ? = angle between the perpendicular to the fault and the sample line. The displacement gradient and Lagrangian strain tensors may then be used to describe the deformation with respect to the undeformed state. The method can also be applied to such structures as veins and stylolites. As an example of the use of the method, 1340 normal faults have been measured along a 6 km length of Cretaceous chalk cliffs at Flamborough Head, Humberside, U.K. Consistent strain estimates have been obtained for different portions of the cliff.<br/

Effects of propagation rate on displacement variations along faults

Field observations indicate that much of the variability in the displacement-distance (d-x) profiles and length-displacement relationships of faults is caused by factors which can affect the propagation of faults. These factors include the interaction and linkage of segments, fault bends, conjugate relationships and lithological variations. Existing models for the d-x profiles of faults do not take these effects into account. Fault development can be modelled assuming faults accumulate displacement by a series of slip events, and using a function (p) to describe the rate of fault propagation. When p is constant during fault development, an approximately linear d-x profile eventually develops. When p decreases, such as when interaction occurs, the d-x profile rises above the linear profile. When p increases, the d3x profile initially falls below the linear profile. Such variations in finite d-x profiles mean that the analysis of finite fault displacement gives little information about the d-x profiles of individual slip events.Variations in p cause variations in r/dMAX ratios (where r is the distance between the maximum displacement point and the fault tip, and dMAX is maximum displacement). Interaction tends to hinder propagation, but displacement continues to increase, causing relatively low r/dMAX ratios. Inelastic deformation can occur at fault tips, especially where strain is concentrated at oversteps, causing steep d-x profiles and low r/dMAX ratios to develop.<br/