Orientation Of Clay Minerals Research Articles

Seismic anisotropy of shales

The seismic properties and preferred clay mineral orientation of a suite of shales are investigated using laboratory velocity measurements as a function of confining pressure, X ray diffraction techniques, and electron microprobe backscatter (BSE) imaging. The velocity measurements indicate that these shales are transversely isotropic with the main symmetry axis perpendicular to bedding. Anisotropy, at elevated pressures caused mainly by preferred orientation of clays (illite) parallel to bedding, ranges from 20% (Vp) and 19% (Vs) for a sample of New Albany Shale to 30% (Vp) and 35% (Vs) for a sample of Chattanooga Shale. The degree of clay mineral alignment in the shales is constrained by “orientation indices” produced using simple X ray diffraction techniques. A strong positive correlation is found between the degree of preferred orientation, as expressed in the orientation indices, and seismic anisotropy. BSE images of the shale fabrics confirm in a qualitative manner the results of the X ray study. To investigate wave propagation in the shales, elastic constants of each sample are calculated and used to produce phase and group (wave) velocity surfaces, which describe variation in velocity as a function of angle to the bedding normal. The calculated velocity surfaces, constrained by independent velocity measurements, display a lack of shear wave splitting at “near‐normal incidence” in even the most anisotropic shales. For the highly anisotropic Chattanooga shales, group velocity surfaces differ significantly from corresponding phase velocity surfaces.

Evolution of structures and fabrics in the Barbados Accretionary Prism. Insights from leg 110 of the Ocean Drilling Program

Detailed analysis of ODP Leg 110 cores provides evidence for a complex structural evolution of the frontal thrust system in the northern Barbados Accretionary Prism. Initial east-directed in-sequence thrusts are biostratigraphically documented, and are overprinted towards the west by out-of-sequence overthrusts, largescale folds and penetrative deformation of the accreted rocks. This structural progression is interpreted to reflect the need for secondary shape adjustments of the accretionary prism in order to maintain a critical taper. At present the basal décollement is located within Lower Miocene strata. Accreted Eocene sediments suggest either fossil frontal off-scraping at a lower stratigraphic level or duplex formation and underplating. The present localization of the basal detachment is provoked by a zone of initial high porosity in the Lower Miocene radiolarian-rich mudstones. Shearing is accompanied by partial porosity breakdown in a 30–40 m wide zone of intense scaly fabrics and stratal disruption that constitutes the Atlantic-Caribbean plate boundary at present. Large displacements are taken up by a relatively narrow zone of contact strain. Initially the preferred orientation of clay minerals (incipient scaly fabrics) may be a flattening fabric, but with progressive rotational flow the scaly planes may act as discrete microshears and provide small-scale flow partitioning. Alternatively, scaly fabrics may be interpreted as brittle microshears. Mud-filled veins are present in the accreted rocks as well as at the oceanic reference site east of the deformation front. Carbonate-filled veins are restricted to the accreted rocks. This indicates persistence of high fluid pressures, but a fundamental change in the type of fluid-rock interaction during frontal accretion tectonics.

Microfabric Analysis of Fine-Grained Clastic Rocks as an Independent Criterion for Determining Depositional Environment: ABSTRACT

SEM examination of clay fabric in siltstones as old as Pliocene has proved useful in distinguishing rapidly deposited muds from hemipelagic clays: turbiditic siltstones are characterized by random orientation of clay minerals and hemipelagic units show preferred (parallel) orientation of clay minerals. Random orientation of clay minerals has been recognized in Paleozoic shales, indicating that postdepositional compaction of shales does not always result in parallel alignment of clay minerals. Therefore, the possibility exists that original depositional fabric of clay minerals is retained in rocks of Paleozoic age, and that clay-fabric analysis would provide an independent tool for interpreting the depositional environment of fine-grained clastic rocks. This study shows that original depositional fabric of clay minerals is preserved in texturally homogeneous, nonbioturbated mudstones as old as Middle Cambrian. Clay fabric may be used in determining whether the mud deposition was rapid or slow (e.g., pelagic). Rapid mud deposition is characterized by a random clay fabric; slow sedimentation is characterized by preferred alignment of clay minerals. Bioturbation and bimodal grain size within a sample (e.g., a clayey siltstone) also are associated with a disordered clay fabric, so this analysis is most confidently used with mudstones that have a homogeneous texture and show no evidence of bioturbation. Therefore, environmental interpretations based on SEM analysis of clay fabric can be used with confidence provided that the caveats of bi odal grain size, bioturbation, and diagenesis are anticipated. End_of_Article - Last_Page 564------------

Clay orientation in some Upper Carboniferous mudrocks, its relationship to quartz content and some inferences about fissility, porosity and compactional history

ABSTRACTThe degree of preferred orientation of clay minerals in a series of fifteen fine‐grained sediments has been precisely measured by transmitted X‐ray goniometry. This varies systematically with quartz content. By contrast it is apparently unrelated to fissility since the one fissile sample (a ‘paper shale’) occurs at the mid‐point of the sample series ranked in order of degree of preferred orientation. In this series fissility corresponds with the presence of fine‐scale lamination. Estimates of compaction strain using the theory of March (1932) and assuming random initial clay orientation are consistent with the view that these samples started life with porosities similar to those of present‐day muddy sediments. On this basis it is concluded that preferred orientation in clay‐rich sediments results almost entirely from compaction strain. The degree to which this can be attained is limited by the presence of nonplaty particles (such as quartz grains) which prevent planar fabric development in their immediate vicinity.

Experiments relevant to the genesis of clay mineral orientation in natural sediments

An abstract is not available for this content so a preview has been provided. Please use the Get access link above for information on how to access this content.

The investigation of and remedial works for unstable slopes beside the A40 Trunk Road near Monmouth

The A40 improvement between Whitchurch and Monmouth entailed the construction of a dual carriageway road with trunk road standards from Ganerew Bridge at about 300 ft above N.D. down a steep sidelong slope beside the river Wye to the flood plain level of the Wye at about 60 ft above N.D. The course of the road approximates to the strike of the rocks which have an easterly dip of about 30° into the Wye valley and which consist of sandstone with subordinate mudstones of the Devonian Brownstones Group. The mudstones have marked clay mineral orientation parallel to the bedding which makes them particularly susceptible to bedding slips. In a length of cut on the up-dip side of the road near Ganerew slips occurred, were investigated, and successful remedial measures were designed and applied. On the sidelong slopes opposite the Wye the road had to cross three ancient landslips involving considerable cut and fill. The fill on one slip at Chapel Farm resulted in reactivation of the entire slipped mass and the ultimate destruction of the farm. On another, the Whipping Green Slip, the intercarriageway cut failed repeatedly. The geometry and piezometric condition of all the slips were investigated thoroughly and remedial measures were prescribed. These involved counterforts, lightweight embankments, and a large berm at the toe of Chapel Farm slip, below which sand drains had to be employed in the Wye alluvium. The object of these remedial measures was to at least restore if not improve the original Factor of Safety which was marginally > 1. Above the road there is a large area of partly slipped rock which it was decided to leave subject to the installation of a permanent monitoring installation. The installation of the monitoring devices is taking place this year.

Clay-Ironstone Concretion Preserving Fabrics Due to Progressive Compaction

Fabrics arising from preferred orientation of clay minerals are presented in a sequence of samples taken from the center of a diagenetic siderite concretion into the enclosing shale sediment. Compactional histories, as deduced directly from estimated porosity values before cementation with siderite, and as deduced from the rotation of mineral flakes to produce the observed fabric, confirm each other. It is concluded that the particular carbonate concretion studied started to grow within the pore space of a flocculated clay sediment soon after deposition. An early period of rapid growth is distinguished from a second, protracted phase which extended effectively until the sediment achieved its present degree of compaction.

Triaxial anisotropy of wave velocity and elastic moduli in slate and their axial concordance with fabric and tectonic symmetry

Orthogonally cut specimens of Penrhyn slate from North Wales are subjected to ultrasonic P and S waves in the laboratory. It is shown that the principal axes of wave velocity and dynamic elasticity are concordant with the axes of the original principal tectonic stresses as determined from an earlier analysis of fabric symmetry. The triaxial velocity anisotropy is interpreted in terms of clay mineral orientation in the slate.

The optical determination of spatial (three-dimensional) orientation of platy clay minerals in soil thin-sections

A technique is described to determine the three-dimensional orientation of platy clay minerals and mineral aggregates in soil thin-sections by means of a polarizing microscope equipped with a universal stage and a Berek compensator. The method utilizes the established geometrical relationships between the basal cleavage and the maximum or minimum apparent birefringence in an arbitrary cross-section of a platy clay mineral. Applying this method, it is unnecessary to know or determine the thickness of the thin-section, the refractive indices or the actual birefringence of the clay minerals involved.

Clay Fabric and its Relation to Structural Properties in Mid-Continent Pennsylvanian Sediments

ABSTRACT Many structural properties of argillaceous sediments correlate closely with clay fabric; the degree of preferred orientation of clay particles. Shaly structure is associated with a significant degree of clay particle orientation, massive structure with a more random clay particle arrangement. Fissile structure occurs in shales where there is a high degree of orientation of clay minerals and other flaky constituents. It is best developed in shales containing a high percentage of organic material and where organic material is concentrated in bands. Organic material and in some cases, carbonate minerals, exert a secondary influence on fabric and structural properties. In ancient sediments that contain a significant percentage of organic material, the preferred orientation of clay minerals generally increases as the percentage of organic matter increases. Sediments with more than 10 percent carbonate minerals commonly show a poor degree of clay particle orientation. Little relation appears to exist between fabric or structural properties and clay mineral composition, except perhaps in sediments with appreciable montmorillonite. Poor particle orientation has been shown to exist in sediments in which montmorillonite is the major clay mineral component. No apparent relation was found between fabric or structural properties and total clay content (above 20 to 30 percent), geologic age, or depth of burial. Variations in orientation of clay particles, from essentially parallel to nearly random, occur within a few vertical inches in some thin stratigraphic sections. This, and the close relation of particle orientation to structural properties suggest that the orientation of clay minerals (fabric) in argillaceous sediments is not dominantly controlled by compaction caused by the weight of overlying sediments. These relations appear to reflect physicochemical conditions in the environment of deposition that influence the mode of clay mineral sedimentation, and/or geochemical factors that may promote particle rearrangement soon after deposition.

A Study of the Relations of Depth, Porosity and Clay Mineral Orientation in Pennsylvanian Shales

ABSTRACT Factors related to depth of burial, porosity, and clay mineral orientation in Pennsylvanian shales of western Kentucky were investigated. An attempt was made to determine some of the major factors affecting porosity and clay mineral orientation. The factors investigated were (1) depth of burial, (2) porosity, (3) preferred clay mineral orientation, (4) clay mineralogy, and (5) grain size. Simple and partial correlation analyses were used to determine statistical correlations among the various shale characters and to determine the relative influence of one or more characters on a particular correlation. Porosity was found to decrease slightly with increasing depth of burial. This negative correlation was found to be strongly influenced by illite content and clay mineral orientation. No correlation was detected between depth of burial and preferred clay mineral orientation. Porosity was found to decrease with an increase in preferred clay mineral orientation and an increase in illite content. Preferred clay mineral orientation was found to increase with increasing clay content, whereas, increasing silt content was accompanied by a decrease in preferred clay mineral orientation. A positive correlation was detected between illite content and depth of burial; chlorite was found to be positively correlated with depth. These relations, alongwith a decrease in chlorite content with increasing illite, suggest the possible alteration of chlorate to illite with increasing depth of burial and age.

Fissility in Argillaceous Rocks: ABSTRACT

A study was undertaken to determine the cause of fissility and non-fissility in argillaceous rocks such as shale and underclay. The orientation of clay flakes in samples of Pennsylvanian shales and underclays was studied by using an X-ray diffractometer. It was found that clay mineral orientation is a factor in determining the degree of fissility of an argillaceous rock. Sedimentation experiments were run in the laboratory to study the influence of various factors upon the development of fissility. Clay materials of various concentration were sedimented in different cation concentrations. Regardless of clay or cation concentration, the clay material flocculated. Various amounts of silt were sedimented on top of the flocculated clay masses. Preferred clay mineral orientation was produced in clay material on which silt was rapidly sedimented. Random orientation was preserved in clay material which had dewatered before a small amount of silt was applied. Samples of clay were also compressed to study the influence of overburden pressure and water content upon the development of fissility. It was found that clay with a lower water content showed less reorientation due to compression than clay with a higher content. This fact suggested that the amount of liquid water relative to rigid water in the clay-water system at the time of application of overburden pressure may be an important factor in facilitating the orientation of clay flakes. It is concluded that preferred orientation of clay flakes commonly found in fissile shales may have resulted from compression of flocculated clay mud under conditions of rapid sedimentation. The random clay mineral orientation present in underclay may have resulted as a flocculated clay accumulated under overburden pressure. Thus the presence of absence of fissility in argillaceous rocks may depend upon the amount of overburden pressure applied to the newly deposited clay material and the amount of liquid and rigid water in the clay at the time of application of pressure. End_of_Article - Last_Page 540------------

Origin of Pennsylvanian Underclays in the Illinois Basin

Experimental evidence suggests the detrital origin of underclays of the Illinois Basin. Flocculated clay materials were deposited in glass tubes and the resulting clay-mineral orientation determined by X-ray techniques. Random orientation in the flocculated clay was preserved in the dried clay, which had dewatered slowly under minimal overburden pressure. Samples were compressed to study the influence of water content and pressure on the preservation of random orientation. Clay with lower water content showed less reorientation due to compression than clay with a higher content, suggesting that the amount of liquid water in proportion to adsorbed water in the system may influence clay-flake orientation. The soil concept in explaining the origin of underclay is discussed. Evidence indicates that the nonbedded character of underclays is due to the random orientation of clay flakes, which could be a primary feature resulting from the slow deposition of flocculated clay during which the clay dewaters under minimal overburden pressure. Observations by the author and others do not support the concept that most underclays in the Illinois Basin are soils subjected to subaerial weathering and reworking by plant rootlets.