Rock Profiles Research Articles

Processes controlling lithium isotopic distribution in contact aureoles: A case study of the Florence County pegmatites, Wisconsin

Li isotopes may be useful tracers of fluid flow in a number of geological environments and case studies of contact aureoles have highlighted the very large Li isotopic fractionation that can be generated in these settings. However, the amount of isotopic fractionation and the distance that Li travels into the country rocks vary greatly from place to place. Seeking to identify the parameters that govern Li distribution in contact aureoles, we apply a combination of Li isotope analyses, 1‐D diffusion and 2‐D advection‐diffusion modeling to two country rock profiles adjacent to Li‐rich pegmatite dikes from the Florence County pegmatite field, Wisconsin. Although less than ∼3 m thick, the pegmatite sheets have a large impact on the Li budget of the country rocks (amphibolites and schists); Li is enriched in adjacent country rocks by up to a factor of 20 over more distant amphibolites and schists. Li from the pegmatite has traveled more than 50 m into the country rocks, and Li isotopes are systematically fractionated with distance from the contacts (with δ7Li varying from +6 at the contact to −7 at 30 m from the contact in one case). These observations are consistent with diffusive fractionation of Li through an advecting grain‐boundary fluid. Both one‐dimensional diffusion and two‐dimensional advection‐diffusion models fail to reproduce the exact Li distribution in the profiles, suggesting that fluid advection, coupled with heterogeneous permeability, plays an important role in determining the final Li distribution within the contact aureoles.

Assessment of rock fracture surface roughness using Riemannian statistics of linear profiles

The mechanical behaviour of rock masses is complex, due partly to the presence of discontinuities within them. Of the geometrical parameters of discontinuities, surface roughness, which encapsulates the topographical features of a rock surface, is known to play a significant role. Here, a new parameter for quantitative roughness determination based on the distribution of unit normal vectors to a rock profile is presented. The analysis of unit normal vectors in terms of directional statistics is customarily performed in Euclidean space using a Cartesian co-ordinate system. Here, the analysis is developed using Riemannian geometry, with Mahalanobis distances being proposed for discrimination between different rock profiles. Statistical parameters on the unit circle are extracted using Riemannian geometry, and from that a roughness parameter, D R1 , is obtained. This parameter corresponds to 1D Riemannian dispersion, and as such D R1 increases as profile roughness increases. D R1 is applied to the analysis of synthetic profiles and some real rock profiles. Conclusions are drawn that demonstrate the advantages of the new method in terms of investigating the scale effect in roughness determination as well as in comparing different profiles. A preliminary study into the correlation between D R1 and the shear strength of a fracture, using analytical and numerical investigation of the strength of profiles comprising symmetric triangular asperities sheared at different normal stress levels, shows a clear relation between Riemannian roughness parameter and profile shear strength.

FieldP–ycurves in weathered rock

In weathered and decomposed rock profiles, the lack of an acceptable analysis procedure for estimating lateral load–displacement response of drilled shafts is compounded by the unavailability of weathered material properties, including the material's lateral subgrade reaction modulus. Such deficiency often leads to the overdesign of the drilled shaft foundation. Six field tests were conducted on drilled shafts to investigate the shape and magnitude of P–y curves in weathered rock material at three locations in North Carolina. The tested shafts were instrumented using dial gages, strain gages, and continuous vertical inclinometers. The measured load versus deflection data are used to study the stiffness response of weathered rock. Measured lateral responses are compared with the results estimated based on a "weak rock" model and a stiff clay model. The comparison shows that Reese's weak rock model overestimated the resistances of the tested shafts while the stiff clay model consistently underestimated the measured shaft resistances. The measured and computed results are analyzed and discussed.Key words: drilled shaft, weathered rock P–y curve, subgrade modulus, ultimate resistances in weathered rock, verification tests.

Chemical and mineralogical heterogeneities of weathered igneous profiles: implications for landslide investigations

Abstract. Landslides in tropical and sub-tropical regions are generally associated with weathered rock profiles which often possess chemical and mineralogical heterogeneities at material- and mineral-scales. Such heterogeneities reach a climax by the occurrences of oxyhydroxide- and clay-rich zones. Weakness and low permeability of these zones makes them ideal for the development of slip zones along which landslides take place. This paper describes the nature and distribution of chemical and mineralogical heterogeneities within weathered profiles developed from felsic igneous rocks in Hong Kong. It sets out the use of integrated geochemical and mineralogical studies to improve understanding of the development of critical heterogeneities and hence to predict their types and presence in a given weathered profile.

Analyses of the morphological changes with time, through denudation and siltation, in ditches of trapezoidal and triangular section

Accounts in the archaeological literature from 1898 onwards of the degradation and siltation of archaeological ditches, generally in the Chalk, are first briefly reviewed. These are often supported by a series of successive measured sections, showing a tendency for a convex outward profile to develop in the degraded rock face. This was termed as ‘trumpet-shaped’ profile by Curwen (Antiquity, 4 (1930) 97), but no theoretical models were proposed. In 1866, Fisher had put forward a theoretical, two-dimensional analysis of the degradation of a single cliff of chalk (a quarry face near Lewes, Sussex) and this work was later generalised by Lehmann (Vierteljahrsschrift der Naturforschende Gesellschaft in Zürich, 78 (1933) 83). A variant of this approach was published by Bakker and Le Heux (Koninklijke Nederlandsche Akademie van Wetenschappen, B 50 (1947) 959, 1154). These theoretical models seem first to have been referred to in an archaeological context by Jewell (A Report of the Research Committee on Archaeological Field Experiments for the British Association for the Advancement of Science, London: The British Association for the Advancement of Science, 1963) in connection with the reporting of the meticulously executed and recorded Experimental Earthwork in the Chalk of Overton Down, Wiltshire. At that time, however, no attempt was made to integrate the field measurements with the available cliff degradation theories. This initiative was taken by Hutchinson (Earth Surf. Proc. Land., 23 (1998) 913) in order to check the validity of the Fisher–Lehmann and Bakker–Le Heux models. For both these models, good agreement between the actual and predicted buried rock profiles was found for a single cliff in chalk. It is clear that in most archaeological ditches, there is mutual interference between the screes developing from the opposite faces. This alters the boundary conditions of the above-mentioned cases of cliff degradation. In this paper, new theoretical, two-dimensional models are developed to cover the main situations found. These were identified from a review of archaeological ditches in southern England and France, and are described as follows: Type I—Wide ditches, with no interference between the opposite screes. Type II—Trapezoidal ditches, with eventual interference between the opposite screes. Type III—Triangular ditches, with immediate interference between the opposite screes. For Type I, the existing models for a single cliff apply. For Types II and III, new models are presented. It should be noted that for Type II, degradation follows initially the Type I model, but switches to Type II when the toes of the opposing screes meet. This produces a point of discontinuity in the buried rock profile where a change of gradient occurs, giving rise to a ‘double trumpet’ shape. This may generally be too subtle a feature to be readily identified archaeologically. For a bulking factor of 1.67 (i.e. c=0.4), the predicted buried rock profiles for Types I, II and III are compared for two specific ditches. Dimensionless plots summarising the relationships between initial ditch face angle, ditch base width and the ultimate amount of ditch crest recession are also given for these three cases. Profiles for other geometries and bulking factors may readily be determined using the equations provided. These make no allowance for the ingress of debris from outside the ditch, for example, from the slope of a rampart, which would further steepen the profiles.

Laterally Loaded Drilled Shafts Embedded in Soft Rock

The current design criterion for laterally loaded drilled shafts embedded in weathered Piedmont rock profiles requires a challenging effort on the part of the engineer. A substantial cost saving could be realized, while maintaining an acceptable and safe performance, if a rational method were developed for the analysis and design of drilled shafts in such a profile. In a current research project, the primary objective is to develop and validate a procedure for design and analysis of laterally loaded drilled shafts embedded in the Piedmont weathered rock profiles. A major component of this research is a field-testing program. Presented are the results of the first in a series of several lateral load tests performed on two drilled shafts 0.762 m (30 in.) in diameter embedded in Piedmont weathered rock. These shafts were instrumented with inclinometers and strain gauges. Field data obtained from the instrumented shafts were used to develop P-y curves. Field testing also encompassed the use of a borehole dilatometer to establish correlations between the rock strength and deformation parameters and potential P-y curves. A comparison is made between backcalculated P-y curves, P-y curves predicted by using Reese’s method, and P-y curves from the rock dilatometer. Loaddeformation results are presented and discussed for all methods used.

Fractal dimension of profiles and surfaces using fuzzy morphological coverings

In this paper, we propose a new method, based on fuzzy morphology coverings, to estimate the fractal dimension of profiles and surfaces. This method is geometrically intuitive and simple to implement. Algorithmically, the method fits a covering to the frames or blocks of the profile or surface using fuzzy morphology. Varying the dimension of the frame or block, estimates of the length or area covered are then used to find the fractal dimension. Validation of the proposed method is performed by comparing its results with known fractal dimensions of mathematical profiles. The method is used to obtain the fractal dimension of rock profiles and surfaces.

Chemical variations in clay minerals of the Archaean Barberton Greenstone Belt (South Africa)

Four rock profiles of shales, carbonates and greywacke-type sediments were sampled from the mafic compositional influenced sedimentary Fig Tree Group and from overlying, more felsic related, sedimentary Moodies Group of the Archaean Barberton Greenstone Belt in South Africa. Clay separates of forty sedimentary rocks were examined by XRD and SEM to study the mineralogical, chemical and morphological variations of the constitutive illite and chlorite. The clay minerals showed a significant K-enrichment in the illites and Mg- and Fe-depletions in the chlorites upwards the stratigraphic sequence. Three independent parameters influenced these variations: (1) a change in the origin of the source rocks; (2) the host-rock lithologies; and (3) a tectono-thermal recrystallization process.

Seasonal pattern of water depletion from soil–rock profiles in a Mediterranean climate in southwestern Oregon

Neutron probe measurements from aluminum access tubes (3.2 m deep) show that conifers and sclerophyllous shrubs deplete water from soil–rock profiles in distinctly different patterns. Measurements were taken during two growing seasons (1993 and 1994) in southwestern Oregon (U.S.A.) under 14-year-old stands of Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco), ponderosa pine (Pinusponderosa Dougl. ex Laws.), Pacific madrone (Arbutusmenziesii Pursh), and whiteleaf manzanita (Arctostaphylosviscida Parry) covers. Both conifer species showed very limited abilities to utilize water from bedrock, even during very dry periods. Ericaceous plants, on the other hand, were very efficient in taking water from the deep rock layers, using nearly all available water and removing about twice as much water as conifers of the same age. Patterns of water depletion suggest that the water resources used by these two types of plants overlap considerably, and competition will be very strong in mixed stands. The ability of ericaceous plants to utilize bedrock water that is unavailable to conifers in late summer will enhance their potential for dominance.

Natural fracture profiles, fractal dimension and joint roughness coefficients

Thirteen natural rock profiles (Barton and Choubey, 1977) are analyzed for their fractal properties. Most of the profiles were found to approximate fractal curves but some also showed features of specific wavelengths and amplitudes superimposed on fractal characteristics. The profiles showed fractal dimensions from 1.1 to 1.5 covering a range of selfsimilar and self-affine curves. The analysis results suggest a negative correlation between fractal dimension,D, and amplitude,A. Joint roughness coefficients (JRC) show a positive correlation with amplitude,A, and a negative correlation with fractal dimension,D. A numerical model of fracture closure is used to investigate the effects of different profile characteristics (D, A and sample size) on the nature of dilation and contact area, using the natural profiles and synthetic fractional Brownian motion profiles. Smooth profiles (low JRC, highD, lowA) display many small contact regions whereas rough fractures (high JRC, lowD, highA) display few large contact areas. The agreement with published experimental data supports the suggested correlations between JRC and the fractal parameters,A andD. It is suggested that observed scale effects in JRC and joint dilation can be explained by small differential strain discontinuities across fractures, which originate at the time of fracture formation.

Mobilization of REE during crustal aging in the Troodos Ophiolite, Cyprus

REE data for secondary minerals from low- and high-temperature alteration environments in the Troodos Ophiolite have been determined using a combined mass spectrometry-neutron activation technique. Smectite- and analcime-REE mimic fresh rock profiles and have concentrations ranging from < 1–5 times chondritic values. Celadonite is depleted in LREE and total REE relative to the rocks. Groundmass-replacing chlorite and epidote are similar to, or enriched in, REE relative to fresh rocks. In contrast, fault-related chlorite and void-filling epidote are LREE-enriched with large positive Eu anomalies and yield comparable REE profiles to those of hydrothermal fluids venting at mid-ocean ridges. The zeolites, clinoptilolite and laumontite, are also LREE-enriched with REE contents significantly lower than rock values. Carbonate is enriched in HREE relative to LREE and concentrations vary from 0.2 to 18 times chondritic values. REE data for these minerals indicate that seawater-REE contents are modified by seawater-rock interaction in volcanic sequences and sheeted dykes in oceanic crust during axial hydrothermal alternation and subsequent crustal aging. The absolute and relative abundances of REE in seawater-derived solutions evolve in response to the precipitation of secondary minerals and the prevailing water/rock ratios. Rocks altered at low temperatures will show no change or a decrease in REE content; the LREE/HREE ratio will reflect the proportion of smectite, celadonite and carbonate in the groundmass assemblage. In contrast, rocks altered at high temperatures will show no change or an increase in REE; the LREE/HREE ratio will reflect the proportion of void-filling and groundmass-replacing chlorite and epidote in the rock.

The role of soil processes in determining mechanisms of slope failure and hillslope development in a humid-tropical forest eastern Puerto Rico

Translational failures, with associated downslope earthflow components and shallow slides, appear to be the primary mechanism of hillslope denudation in the humid tropical forests of the mountains of eastern Puerto Rico. In-situ weathering of quartz diorite and marine-deposited volcaniclastics produces residual soil (saprolite; up to 21 m deep) / weathered rock profiles. Discontinuous zones of contrasting density and permeability particularly in quartz-diorite slopes at 0.5 m, and between 3 and 7 m, create both pathways and impedances for water that can result in excess pore pressures and, ultimately, aid in determining the location of failure planes and magnitudes of slope failures. In combination with relict fractures which create planes of weakness within the saprolite, and the potential significance of tensile stresses in the upper zone of saprolite (hypothesized to be caused by subsurface soil creep), shear failure can then occur during or after periods of heavy rainfall.

Engineering properties of weathered rock profiles in oeninsular Malaysia : Komoo, I Proc 8th Southeast Asian Geotechnical Conference, Kuala Lumpur, Malaysia, 11–15 March 1985V1, P3/81–86. Publ Kuala Lumpur: Organising Committee, 1986

Engineering properties of weathered rock profiles in oeninsular Malaysia : Komoo, I Proc 8th Southeast Asian Geotechnical Conference, Kuala Lumpur, Malaysia, 11–15 March 1985V1, P3/81–86. Publ Kuala Lumpur: Organising Committee, 1986

On the Relationship of Ice-Surface Topography to Bed Topography on the South Polar Plateau

AbstractIce-surface and isopachous maps, and rock-elevation maps, were prepared for the area between the South Pole and the Horlick Mountains from seismic and gravimetric measurements. Basal shear-stress was calculated from these data and surface-slope measurements at 25 sites. The mean is 0.8 bar with an areal standard deviation of 0.5 bar. Forty-five values of average basal shear stress were computed from consecutive 30 km. segments along flow lines. The mean is 0.4 bar with an areal standard deviation of 0.1 bar. These variations are too large for accurate estimates of ice-thickness variations to be made from surface-slope data alone, following Nye’s suggestion that basal shear-stress be treated as constant. From theoretical considerations Nye demonstrated that surface features of wave-length less than 40 km. should disappear within a few months unless related to bed topography. Ice and rock profiles along a 220 km. line do not show an expected theoretical relationship. Surface-elevation measurements, repeated after a 2 yr. interval. showed the surface features with wave-lengths between 5 and 40 km. to be essentially unchanged in amplitude and position. Hence these features are not adequately explained by existing theory.

On the Relationship of Ice-Surface Topography to Bed Topography on the South Polar Plateau

AbstractIce-surface and isopachous maps, and rock-elevation maps, were prepared for the area between the South Pole and the Horlick Mountains from seismic and gravimetric measurements. Basal shear-stress was calculated from these data and surface-slope measurements at 25 sites. The mean is 0.8 bar with an areal standard deviation of 0.5 bar. Forty-five values of average basal shear stress were computed from consecutive 30 km. segments along flow lines. The mean is 0.4 bar with an areal standard deviation of 0.1 bar. These variations are too large for accurate estimates of ice-thickness variations to be made from surface-slope data alone, following Nye’s suggestion that basal shear-stress be treated as constant. From theoretical considerations Nye demonstrated that surface features of wave-length less than 40 km. should disappear within a few months unless related to bed topography. Ice and rock profiles along a 220 km. line do not show an expected theoretical relationship. Surface-elevation measurements, repeated after a 2 yr. interval. showed the surface features with wave-lengths between 5 and 40 km. to be essentially unchanged in amplitude and position. Hence these features are not adequately explained by existing theory.

Lunar Problems in Soil Engineering

In the relatively near future unmanned, and perhaps manned spacecraft will make soft landings on the moon, and ultimately on some of the other planets. Because our knowledge of the intimate small-scale properties of the lunar surface is almost nonexistent, it is desireable to devise surface models and materials in line with current information for the purpose of designing test experiments and forecasting possible future problem areas. This paper considers briefly the state of knowledge of the lunar surface from an engineering viewpoint and outlines a variety of plausible soil or rock profiles to shallow depths. The engineering properties of the soil and rock materials are defined arbitrarily in terms of an idealized mechanical behavior. An attempt is made to establish ranges of material behavior in terms of quantitative properties, and the classes of soil engineering problems that might arise with respect to operations on the lunar surface are examined. Finally, consideration is given to suitable simple experiments that may be conducted from spacecraft on the surface for the purpose of measuring the quantitative properties.

Weathering and Transport of Sediment in the Cheyenne River Basin, Eastern Wyoming

AbstractThe weathering and fluvial transport stages of a sedimentation sequence of the Cheyenne River in Eastern Wyoming are discussed. The river heads in east-central Wyoming, crosses several rock units in its eastward path before reaching the Angostura Reservoir in southwestern South Dakota. The drainage area of the Cheyenne River above the Angostura Reservoir comprises about 9000 square miles.Weathering profiles of the rock units traversed by the river in this drainage area were sampled to determine their respective contributions to the sediment load. Among these formations are the lithologically diverse Chadron, Brule, Fort Union, Lance, Pierre, and Spearfish. The textural and mineralogical analyses of the weathered outcrops were compared with those of sediment samples collected at gauging stations operated by the Water Resources Division of the U.S. Geological Survey.X-ray diffraction studies of the weathered rock profiles and of the suspended sediment indicate a diverse clay mineral assemblage in the 2-1µ diameter size fraction. Kaolinite, illite, and vermiculite occur along with quartz and feldspar. However, the &lt;1µ size fraction of the suspended sediment contained only montmorillonite. The kaolinite content of the 2-1µ size fraction of the suspended sediment increased in the direction of flow, at the expense of vermiculite and ilIite.The computed and observed amounts of sediment contributions by the various rock units show a fairly good fit; the data indicate the importance of defining mineral composition in terms of discrete size separates, cven under 2µ diameter. The increase in kaolinite content in the 2-1µ size fraction of the suspended sediment may be due to bank contribution downstream from the gauging stations. The montmorillonite content of the &lt; 1µ size fraction is unchangcd by transport.

Studies in Tropical Soils II. Some Characteristic Igneous Rock Soil Profiles in British Guiana, South America

1. Five typical British Guiana soil profiles, derived under humid tropical conditions from igneous rocks of different silica contents, are described, and the results of their detailed petrographical, chemical and ecological examination are presented and discussed.2. Each kind of rock has produced a sandy, acidic, nutrient-deficient surface soil, but the katamorphic changes that have occurred during rock alteration, weathering and leaching have been fundamentally different.3. The basic and intermediate igneous rocks have given rise within the zone of alteration to “primary gibbsitic laterite”, covering the parent rock in a thin crusty layer. Within the zone of secondary changes, the laterite has been resilicated, presumably through the agency of ground water containing soluble silica or alkali silicates (partially in the more basic rock profiles, and completely in the intermediate types), into kaolinitic earths, coloured yellow and red by hydrous iron oxides. Residuary gibbsite and iron oxides have accumulated in the more basic profiles as concretionary gravelly masses.