Brittle deformation in pitted pebble conglomerates

The fracture patterns produced in pitted pebble conglomerates from the Alpine Molasse and the Carboniferous of northern Spain, have been studied in relation to the stress concentrations which were produced in the conglomerates during their deformation. The stress distributions which develop around pebble contacts at different stages of their pitting history have been determined from photoelastic experiments. The development of different types of fracture, having dominantly tensile or shear components, and their distribution within the pebbles, are shown to be related to the mineralogy of the pebbles, the strength of the matrix and the amount of deformation the conglomerate has suffered.     

Chemical mass transfer in magmatic processes

Lasaga's (1982) Master Equation for crystal growth is solved for multicomponent systems in situations which allow for coupled diffusion of melt species. The structure of the solution is explored in some detail for the case of a constant diffusion coefficient matrix. Incorporating these results, the growth of plagioclase is modeled in undercooled tholeiitic melts by approximating interface growth rates with (1) a reduced growth rate function and with (2) calculated solid-liquid solution properties obtained from the silicate liquid solution model of Ghiorso et al. (1983; appendix of Ghiorso 1985). For this purpose algorithms are provided for estimating the liquidus temperature or the chemical affinity of a multicomponent solid solution precipitating from a complex melt of specified bulk composition. Compositional trends in initial solids produced by successive degrees of undercooling are opposite to those predicted in the binary system NaAlSi₃O₈-CaAl₂Si₂O₈. Calculations suggest that the solid phase and interface melt compositions rapidly approach a steady state for a given degree of undercooling. Consequently, the overall isothermal growth rate of plagioclase forming from tholeiitic melts appears to be entirely diffusion controlled. In magmatic systems the multicomponent growth equations allow for the formation of oscillatory zoned crystals as a consequence of the couplingr between interface reaction kinetics and melt diffusion. The magnitude of this effect is largely dependent upon the asymmetry of the diffusion coefficient matrix. Methods are described to facilitate the calibration of diffusion matrices from experimental data on multicomponent penetration curves.Experimental results (Lesher and Walker 1986) on steady state Soret concentration profiles resulting from thermal diffusion in MORB and andesitic liquids are analyzed using the theory of multicomponent linear irreversible thermodynamics. Under conditions where the entropy production is minimized, a linear relationship is derived between liquid chemical potentials and temperature. This relationship is utilized to evaluate the validity of the solution model of Ghiorso et al. (1983) in melts up to 300° C above their liquidus. The results indicate that configurational entropies are accurately modeled for MORB and andesite bulk compositions. The modeling fails in two four-component systems tested. Equations are derived which allow the calibration of multicomponent regular solution parameters from steady state Soret arrays. An algorithm is demonstrated which permits the calculation of steady state Soret concentration profiles, given an overall bulk melt composition and temperature gradient. This algorithm uses the liquid solution properties of Ghiorso et al. (1983) and constants obtained from the experimental measurements of Lesher and Walker (1986).     

Regional sedimentological variations in lower triassic fluvial conglomerates (budleigh salterton pebble beds), southwest england: Some implications for palaeogeography and basin evolution

The Budleigh Salterton Pebble Beds (BSPD) are a 20–30 m thick formation of conglomerates and subordinate sandstones which crop out along the western margin of the Wessex Basin. The formation has previously been interpreted as representing a major conduit for southerly-derived (Armorican) detritus and as signalling the start of early Triassic rifting. In this paper the role of the BSPB in the evolution of the Wessex Basin is reassessed. In the south of the outcrop, the lower portions of the BSPB are dominated by extensive (> 50m) sheets or narrow lenses of planar cross-bedded conglomerate (sets 1–3m thick). This reflects deposition from linguoid-shaped bars whose downstream margins were bounded by slipfaces. These accreted at anabranch confluences and as bank-attached bars in relatively confined and channelized gravel-bed streams. Most of the rest of the BSPB consists of couplets of horizontally-bedded conglomerate overlain by large-scale trough cross-bedded sandstone. This style of accretion represents deposition from relatively low-relief gravel bars and large sand dunes in substantial but poorly confined channels. Towards the top of the BSPB in the central parts of the outcrop, there is a change to thinner-bedded units with more lenticular and ribbon-like geometries. This represents deposition in smaller and more flashy streams. The BSPB was deposited either on a braidplain or by an antecedent ‘wet’ alluvial fan. In both settings, the BSPB streams were exotic, draining basins to the south of the Wessex Basin. This implies that the BSPB did not necessarily develop in response to differential subsidence and rifting within the Wessex Basin.     

The interstratified breccias and conglomerates in the Cretaceous flysch of the northern Basque Pyrenees: submarine outflow of diapiric mass

In the Cretaceous flysch of the Northern Pyrenees, polymict conglomerates interbedded in the flysch are described from the vicinity of Orio (near San Sebastián). These contain components derived from rocks of all periods from Paleozoic to Lower Cretaceous. During Late Maastrichtian times a diapir, originated in Keupez evaporites, penetrated the deep-sea Cretaceous flysch which was still undergoing deposition. This resulted in the expulsion of large quantities of diapiric mass. After the solution of the evaporites the pebbles, which had been dragged along with the diapiric masses, slumped into a neighbouring marginal depression where they were redeposited. The reddish strata of Late Maastrichtian and Danian age in this region are interpreted as being out-thrust and transported Keuper shales. It will be shown that the other breccias and conglomerates described by Feuillé and Mathey (1972) from the Late Cretaceous of the Basque Pyrenees probably have the same origin.The comprehensive name “Vascongadian Diapir Zone” is suggested for the diapir field on the northern edge of the Basque sedimentary trough.     

Paragenesis of mineralization within fractured pebbles in Witwatersrand conglomerates

Mineralized conglomerates in the Witwatersrand goldfields consistently exhibit intense fracturing of quartz pebbles. The fractures are coated with quartz, phyllosilicates, organic matter, uraninite, gold, and base metal sulfide minerals, showing that all of these phases were at least partially epigenetic. Samples from five goldfields (Evander, West Rand, Carletonville, Klerksdorp, Welkom) across the Witwatersrand Basin show comparable paragenetic sequences of fracture-fillings. This implies that fracturing and several episodes of mineral deposition were basinwide events, rather than the result of local processes. Fluid inclusion data for the quartz indicates precipitation from these basinwide fluids at relatively low temperatures (<250 °C), which may be consistent with other evidence for mineralization following the Vredefort Event at 2025 Ma.     

Lateral variations in pebble shape in northwest Yorkshire

A simple index of roundness measurement, ($\text{2r}\text{L}\text{) × 100}$), based upon radius of curvature (r) and maximum length (L) is applied to 8500 pebbles sampled from glacial deposits and contemporary river channel deposits in Wensleydale, Yorkshire. The index successfully distinguishes between till and fluvioglacial material but not between till and deposits in features previously described as lateral moraine. When applied to material from the channel of the River Cover, the index demonstrates a rapid increase of roundness in the first 4–5 km below the initial rock outcrop, followed by a further very slight downstream increase in roundness, although spatial variations occur due to increase in slope, waterfalls, potholes and supply of material. The downvalley pattern of roundness of glacial deposits in Wensleydale suggests that one of four postulated moraines is composed of till but that the other three are of fluvioglacial origin. Roundness of material in the glacial deposits increases rapidly in 4 km below the moraine but subsequently shows a slight downvalley increase.     

Sub-sea-floor metamorphism,heat and mass transfer

The ophiolitic rocks of E. Liguria, Italy contain a spilitic metamorphic assemblage sequence, cross-cut by hydrothermal veins, which developed in the oceanic environment. Metamorphic parageneses indicate that temperatures as high as 400°C were realised at depths as shallow as 300 m below the original rock/water interface. The inferred temperature interval was equivalent to a geothermal gradient of 1300°C/km.It is suggested that metamorphism took place in a sub-sea-floor geothermal system, and that such systems are an integral part of the sea-floor spreading process. Modern evidence is provided to support this hypothesis, and to suggest that heavy metal rich solutions discharged from such systems are responsible for the formation of a metal enriched sedimentary component. A unified model of sub-sea-floor metamorphism and mass transfer is proposed, and possible differences between sub-sea-floor and terrestial geothermal systems are discussed. In the light of the model, the origins of certain aspects of bedded cherts found associated with ophiolitic rocks, of ophiolitic massive sulphide deposits and of certain trace element patterns are considered.     

A mass transfer model of bauxite formation

The formation of bauxite due to weathering of a granitic protolith has been simulated by means of a one-dimensional flow and reaction model based on the mass transfer principle. The model couples mineral dissolution and precipitation reactions, speciation in solution, and advective solute transport in a porous medium. A very important aspect of the modeling study is the use of mineral reaction rates determined experimentally in the laboratory. The important effects of solution saturation state and pH have been incorporated into the kinetic rate laws governing the heterogeneous reactions. The values of these parameters have been obtained from the scientific literature to guarantee that realistic reaction rates are used in the simulations. Albite and quartz are the minerals that make up the parent rock in the model. Gibbsite, kaolinite, and a Na-mica (as a surrogate for smectite) are the secondary minerals that have been taken into account. Long-term simulations (>1 Ma) have been run, and the formation of a bauxitic profile, with an upper gibbsite-rich and a lower kaolinite-rich zone, is predicted. In early stages of the process (up to a few hundreds of thousands of years), both gibbsite and kaolinite precipitate directly from solution as a consequence of albite dissolution. In later stages, the bulk of gibbsite precipitation derives from the incongruent dissolution of kaolinite, while kaolinite precipitation is still caused by the dissolution of albite. This is also reflected by the formation of two reaction fronts in the profile. These results are compared with weathering sequences from the Los Pijiguaos bauxite deposit, Venezuela. The overlap between the gibbsite and kaolinite zones and the replacement of kaolinite by gibbsite are consistent with model calculations. Mechanical denudation has to be called upon to explain the limited thicknesses of the weathering profiles in the field. The role of mechanical erosion is supported by the presence of microsedimentary structures in the bauxite and the balance between dissolved and suspended loads in the streams draining the area.     

Elemental distribution of coastal sea and stream sediments in the island-arc region of Japan and mass transfer processes from terrestrial to marine environments

A total of 49 elements have been identified in 338 coastal sea sediment samples collected from an area situated off the Ise-Tokai region of Japan for a nationwide marine geochemical mapping project. The spatial distribution patterns of the elemental concentrations in coastal seas along with the existing geochemical maps in terrestrial areas were used to define the natural geochemical background variation, mass transport, and contamination processes. The elemental concentrations of coastal sea sediments are determined primarily by particle size and regional differences. Most elemental concentrations increase with a decrease in particle size. Some elements such as Ca, Mn, and Yb are found to exist in large quantities in coarse particles containing calcareous shells, Fe–Mn oxides, and felsic volcanic sediments. Regional differences reflect the mass transfer process from terrestrial areas to coastal seas and the influence of the local marine geology. An analysis of variance (ANOVA) reveals that for many elements, the particle size effect is predominant over regional difference. The mean chemical compositions of coastal sea sediments are similar to those of stream sediments in adjacent terrestrial areas and in the upper crust of Japan. This observation supports the fact that coastal sea sediments have certainly originated from terrestrial materials. However, the spatial distributions of elemental concentrations are not always continuous between the land and coastal seas. The scale of mass movement observed in marine geochemical maps occurs at a distance of 20 km from the river mouth. A detailed examination of the spatial distribution patterns of K (K₂O) and Cr concentrations suggests that terrestrial materials supplied through rivers are deposited near the shore initially, and then gravity-driven processes shift the sediments deeper into the basin. Contamination with heavy metals such as Zn, Cd and Pb was observed in coastal bays surrounded by urban and industrial areas. It is noteworthy that the areas with the highest concentration of these elements usually do not occur near the shore (not near the contamination source) but at the center of the bay. Unexpected low concentrations of Zn, Cd and Pb near shore may either be due to a decreased anthropogenic load in the most recent sediments or to dilution by unpolluted flood sediments.     

Characterization of pebble fabrics in modern terrestrial glacigenic sediments

ABSTRACT Pebble fabric data are available from several facies of glacigenic sediments deposited by modern glaciers, where sedimentary processes can be observed or inferred with relatively little ambiguity. Over 100 samples from contemporary environments illustrate fabrics characterizing melt-out till, deformed and undeformed lodgement till, sediment flow deposits and ice slope colluvium. Lodgement till fabric variability is related to the two-layer structure of these sediments; a structureless, friable upper layer with low shear strength and high consolidation coefficient, overlying a very compact material of horizontal platy structure. Fabric strength (assessed by eigenvalue analysis) is weaker and pebble dip is more dispersed in the upper structureless horizon. Stronger fabrics in the lower platy horizon may be primary depositional fabrics which are destroyed by subglacial shearing to give weaker fabrics in the upper horizon. Alternatively, upper horizon fabrics may be characteristic of all recently-deposited lodgement tills, with stronger fabrics developing at depth by dewatering and consolidation. There is a general reduction in fabric strength and an increase in particle dip associated with the transition from melt-out tills, through undeformed and deformed lodgement tills, to sediment flow deposits and ice slope colluvium. There is, however, considerable overlap in the fabric strengths characteristic of sediment flow deposits and deformed lodgement tills. Fabric data from modern glacial sedimentary facies are used to assist in interpreting the mode of deposition of some Quaternary glacial sediments. Relatively strong fabrics characteristic of melt-out tills and undeformed lodgement tills are more likely to be diagnostic of genesis than weaker fabrics associated with deformed sediments.     

从旅游产品角度分析雨花石的利用与开发

雨花石不仅具有特殊的美学观赏价值,而且还具有丰富的文化内涵。主要从旅游产品开发角度阐述雨花石开发为旅游产品的优势以及在开发过程中存在的一些问题并针对问题提出一些建议。     

Calculation of mass transfer coefficients for dolomitization models

Reversible mass transfer associated with alteration of carbonate rocks can be modeled in terms of a pore fluid migrating across a temperature field slowly enough that chemical equilibrium is maintained. These types of models yield both the spatial distribution of the alteration and the absolute amounts of minerals dissolved and precipitated. In order to compute the total mass transfer involved, it is necessary to obtain mass transfer coefficients for each mineral. This paper presents detailed examples of mass transfer in closed systems for the system calcite-dolomite-solution over the temperature range 0–200°C. It is shown that the mass transfer coefficients for the reversible process can be obtained from thermodynamic data and elementary solubility calculations. The connection between these reversible coefficients and the more familiar irreversible mass transfer coefficients is shown to be a simple derivative relation. Calculations are presented for two cases: calcite-ordered dolomite solution and calcite-disordered dolomite solution in both pure water and brines with Ca/Mg ratios similar to seawater. The large mass transfer coefficients obtained for the brines relative tot he pure water is attributed to the fact that the presence of both calcite and dolomite in a system fixes the Ca/Mg cation ratio in the solution as a function of temperature. Dolomitization occurring in nature likely involves both reversible and irreversible processes and accurate interpretation of field data requires the ability to distinguish between these two cases.     

饱和稀疏裂隙岩体三维水流-传热过程中位移和应力的一种半解析计算方法

针对高放射核废深地质处置库近场环境,建立分布热源作用下饱和裂隙岩体三维水流-传热过程中位移和应力的一种半解析计算方法：采用Goodier热弹性位移势和Laplace变换计算由温度梯度产生的温梯位移和应力;考虑单一裂隙的情况,利用经典弹性力学的Boussinesq解和Cerruti解计算为满足边界条件的约束位移和应力,与温梯位移和应力叠加,可得总体热位移和应力;把裂隙面离散为矩形单元集合,采用极坐标系下的解析法计算包含奇点的单元积分,采用数值法计算与分布热源有关和不含奇点的单元积分。与基于裂隙面法向一维热传导假设的一种解析解对比,结果表明,半解析法与解析法的计算结果基本一致,但由于半解析法考虑岩石的三维热传导,因温度时空分布和演变的不同而导致不同的温梯应力。针对一个假想单裂隙岩体三维水流-传热过程,计算温梯位移和应力、约束位移和应力、总体位移和应力;结果表明,裂隙水流-传热可能对位移和应力的分布和演变有显著影响,距离分布热源较近的岩石因升温膨胀受到约束而出现压应力,而距离分布热源较远的岩石则可能因协调收缩受到约束而出现拉应力。     

Development and mass movement processes of the north-eastern Storegga Slide

The Storegga Slide, which occurred ～8100 years ago, is one of the world's largest and best studied exposed submarine landslides. In this study we use novel geomorphometric techniques to constrain the submarine mass movements that have shaped the north-eastern Storegga Slide, understand the link between different forms of failure, and propose a revised development model for this region. According to this model, the north-eastern part of the Storegga Slide has developed in four major events. The first event (event 1) was triggered in water depths of 1500–2000 m. In this event, the surface sediments were removed by debris flows and turbidity currents, and deposited in the Norwegian Sea Basin. Loading of the seabed by sediments mobilised by the debris flows and turbidity currents resulted in the development of an evacuation structure. Loss of support associated with this evacuation structure, reactivation of old headwalls and seismic loading activated spreading in the failure surface of event 1 up to the main headwall (event 2). In some areas, spreading blocks have undergone high displacement and remoulding. Parts of the spreading morphology and the underlying sediment have been deformed or removed by numerous debris flows and turbidity currents (event 3). We suggest that the higher displacement and remoulding of the spreading blocks, and their removal by debris flows and turbidity currents, was influenced by increased pore pressures, possibly due to gas hydrate dissolution/dissociation or by lateral variability in the deposition of contourite drifts in palaoeslide scars. The fourth event entailed a large, blocky debris flow that caused localised compression and transpressive shearing in the southern part of the spreading area.     

Diffusional theory of formation of secondary dispersion halos in ore deposits

A quasi-diffusion theory of origins of mechanical dispersion halos and a discussion of secondary (epigenetic) dispersion halos of ore bodies overlain by unconsolidated sediments are here presented, including the extent of migration of the ore substances by diffusion. --V. P. Sokoloff.