Diffusional mass transfer processes in pitted pebble conglomerates

The diffusional mass transfer of soluble components in pitted pebble conglomerates from the Alpine Molasse and the Carboniferous of northern Spain has been studied using electron microprobes. Two categories of pitted pebble conglomerates are distinguished depending on the presence or absence of an affected zone within the pebble which has suffered volume loss. In these affected zones there is a reduction of mobile components—Ca, Ba, Sr, and sometimes Si, and a concentration of immobile components—Mg, Al, Fe, Mn, Si, K, and Ti. The changes in relative abundances of different elements across these zones are shown to be dependent on the mineralogy of the dissolving pebble. This is interpreted as being partly due to the modification of the stress fields, developed within pebbles due to their composition. Theoretical predictions of the rates of pebble pitting are shown to be in reasonable agreement with natural examples. Most of the deformational structures in these conglomerates were produced by a diffusional mass transfer process such as pressure solution.     

Striated and pitted pebbles as paleostress markers: an example from the central transect of the Betic Cordillera (SE Spain)

Striated and pitted pebbles provide scarce structures that preserve information on the stresses that their host rocks have undergone. This information can be obtained by the measurement of a large number of microfaults with striae and solution marks within a small rock volume. For non-rotational deformation, the statistical procedures for microfault analysis provide a valid tool for determining the overprinting of successive stress ellipsoids, including their axial ratios and the orientations of the main axes. The trends of compressions obtained from striae can be compared with the determinations from the pole of pebble solution pits. However, in complex tectonics settings, the solution pits of several deformation phases are mixed and only striae analysis allows overprinted paleostresses to be accurately distinguished. The analysis of several pebbles from the same outcrop, including five from moderately complex settings, allows determination of the homogeneity of the paleostresses at outcrop scale, the detection of redeposited pebbles, and supports the results of microtectonic analysis for large areas. Solution mark distributions on pebbles depend on the burial and tectonic stresses. Conglomerates from shallow levels, such as those from Quaternary fluvial terraces, only record horizontal compressional solution marks because the minimum vertical stress needed to develop these structures are not reached by burial.In the central Betic Cordillera, striated and pitted pebbles are composed of carbonate surrounded by a matrix containing siliciclastic elements. The study of several outcrops located across a transect of the Cordillera shows a change in the recent stress field. While conglomerates near the Internal–External zone boundary show extensional stresses that may be related to the uplift of the Cordillera since Tortonian times, the outcrops located in the External Zone and up to the mountain front indicate the existence of horizontal NW–SE and NE–SW compressions related to prolate ellipsoids. These two compression directions, which affect conglomerates up to the Quaternary in the same outcrop, may be produced by a local permutation of stress axes, which in general indicates NW–SE compression related to the Eurasia–Africa plate boundary convergence, but which locally may switch to an orthogonal compression.     

Tectonically significant pebble types: Sheared,pocked and second-cycle examples

A history of repeated faulting, within an area, can be obtained from unconformity analysis or sediment-vector sequences (not treated in the present paper) or from studies of gravel tongues. The latter may include sheared pebbles, pocked pebbles, half-rounds, and second-cycle (or multi-cycle) pebbles. Sheared pebbles (and larger fragments) were cut by faulting after deposition, hence represent two episodes of deformation. Pocked pebbles (scarred pebbles) were ground against each other, after deposition, by a second episode. Sheared and pocked pebbles typically develop where sand matrix is not abundant; single pebbles “floating” in sandstone rarely show these features. Quartz sand schist is a fine-grained equivalent of the sheared-pebble gravel or conglomerate. Half-rounds represent transportation of sheared pebbles, after shearing; where half-rounds have been sheared, at least three episodes of deformation are indicated.A second-cycle conglomerate is one in which individual pebbles (or larger pieces) are themselves conglomerates. They likewise indicate at least two episodes of deformation.In some areas, details of displacement can be obtained best from unconformity analysis; in other areas, the special pebble types listed here may provide such details. In a few places, both types of information may be available, thus giving a fuller picture than could be obtained in other ways.The best example of a thick stratigraphic sequence containing these special pebble types (representing repeated deformation), described in the present paper, is located near Bucaramanga, Colombia. A second example from northern New Mexico is described less fully. Brief references to other occurrences, in Wyoming, Montana, Oklahoma and Alabama, are made. It is concluded that, although these pebble types may not be abundant at any one locality, they nevertheless occur at many places, and therefore can provide important information about deformational history.     

Palaeostress and neotectonic analysis of sheared conglomerates: Southwest Alps and Southern Apennines

Geometrical and mechanical characteristics of the deformation of poorly cemented conglomerates are described. Using striated pebbles for analysis of palaeostresses, it is crucial to distinguish radial striation patterns, which result from deformation of the matrix around a rigid pebble, from unidirectional striation patterns that represent shear zones crossing the conglomeratic material. Examples of palaeostress determinations from striations of the latter type are given for extensional settings (Provence) and compressional settings (Southern Apennines, Southwest Alps). Their comparison with fault analyses in brittle rocks that underlie the conglomerates validates their usefulness for palaeostress analyses and suggests that some conglomerates behave as materials containing pre-existing surfaces of mechanical anisotropy that fail by sliding on some suitable oriented surfaces. These examples show that sheared conglomerates can be used for stratigraphic dating of the deformation, for studies of syndepositional deformation and for neotectonic analysis.     

Analysis of solution lineations in pebbles: Kinematical vs. dynamical approaches

Solution lineations in conglomerates, resulting from indentation of non-soluble grains of the matrix into the surface of soluble pebbles, make up a morphological and genetic continuum with gradual transition between orthogonal stylolites, oblique slickolites and parallel striations. The distributions of incidence angles of matrix grains have been analyzed in individual pebbles in order to discern their kinematical or dynamical meaning. As a general rule, they fit theoretical models of flow trajectories determined by the bulk strain (kinematical hypothesis). In contrast, they are not consistent with dynamical hypotheses based upon relationships with stress vectors. In particular, they do not fit the model of frictional sliding, which would give rise to a sharp discontinuity between slickolites parallel to the maximum principal stress σ₁ and true striations parallel to the resolved shear stress τ. Therefore, solution lineations all around a pebble cannot be considered as an analogue of multiple fault slip data, and they should not be generally analysed by methods of stress inversion based upon Bott's principle. Under certain conditions (high pebble solubility; active pressure-solution processes able to accommodate the strain rate; earlier cementation), the solution lineations tend to be parallel to each other and to the maximum shortening/compression axis. They therefore assume a double kinematical and dynanical meaning, and the deformation involves maximum volume reduction.     

THE PROBLEM OF UNDERSTANDING THE NOMENCLATURE OF FACIES

Chert-plus-pyrite pebbles have been known for some time in a number of Witwatersrand conglomerates, but their distribution is highly variable. A wide variety of textures in chert-plus pyrite pebbles are documented here, and these textures place constraints on the origin of such pebbles. Replacement of chert pebbles by pyrite is indicated (pyrite grains and aggregates continue across many pebble boundaries), and both distributional and textural evidence favors a post-burial timing for this process. Significant mobility of sulfur after burial is indicated. Whether this replacement was diagenetic or metamorphic is not certain.     

Unusual intraclastic limestones in Lower Triassic carbonates and their bearing on the aftermath of the end-Permian mass extinction

Flat pebble conglomerates were a common carbonate facies in Cambrian to Early Ordovician open marine settings, but they become extremely rare in these environments after this time. However, the Early Triassic witnessed an anachronistic reappearance of flat pebbles, together with other intraclast types, in a range of carbonate depositional settings. In south China, flat pebble conglomerates are encountered in storm-dominated, platform carbonates to deep basinal settings, while prefossilized bivalve intraclasts and flat pebbles are common in mid-ramp facies of northern Italy. The emplacement mechanisms of the intraclast-bearing beds appear to have been diverse and to have included basinal turbidity flows and storm-generated hyperconcentrated flows: true storm beds, deposited under combined flow conditions, are rare. The cause of the widespread early lithification implied by the Early Triassic intraclasts appears to have been twofold: suppression of bioturbation, allowing the preservation of thin beds, and rapid submarine lithification. Both features appear to be a response to the widespread development of benthic dysoxia/anoxia during and following the end-Permian mass extinction. This event appears to have temporarily recreated the conditions that pertained in Cambro-Ordovician shelf seas. Flat pebble conglomerates may, therefore, constitute a proxy indicator of stressed environmental conditions associated with global anoxic/dysoxic events.     

Strain analysis using deformed pebbles: The influence of initial pebble shape

Several methods exist for the determination of the finite strain ellipsoid from deformed pebble shapes. These methods are critically evaluated and others are proposed on the basis of calculations which predict both the sectional and three-dimensional shape of pebbles in simple deformed simulated conglomerates. In many cases it is found preferable to use an average pebble shape to estimate the tectonic strain and that the harmonic mean of the ratios of axial lengths yields an average pebble shape which is closest to the strain ellipsoid shape.     

云南晋宁梅树村下寒武统砾屑磷块岩的扫描电镜研究

通过扫描电镜观察.发现云南梅树村中谊村段砾屑磷块岩中的磷质砾屑主要由磷质鲕球、磷质微球和磷质雏晶堆积体组成.磷质微晶是磷酸盐的基本晶体.磷质雏晶是在扫描电镜下所能观察到的最小的磷酸盐颗粒,但它们晶形不完全.砾屑磷块岩的形成过程可以分为三个阶段:磷质鲕球、微晶和雏晶形成阶段,磷质砾屑形成阶段,和砾屑磷块岩形成阶段.磷质砾屑的形成环境为浅水近岸,水动力条件不稳定,沉积速率和通量不高.     

Paleogeographic significance of the oldest known oolite pebbles in the Archaean Swaziland Supergroup (South Africa)

In conglomerates of the Archaean Swaziland Supergroup (> 3200 m.y.) pebbles of a characteristic oolite were found, the source rock of which is a well-defined marker horizon at the base of the sedimentary succession. From the pebble finds it can be concluded that this oolite was deposited over a belt several km wide and up to 100 km long. This is comparable in size to recent conditions of oolite formation on the Great Bahama Bank.     

Cataclastic deformation of geological materials in matrices of differing composition: I. Pebbles and conglomerates

Spheres and clylinders of various rock types were embedded in a matrix of crushed rock and the combined samples were then deformed by applying uniaxial compressive loads of up to 4.5 MN. Under these conditions, large confining pressures are built up in the centre of the samples; thus the granular matrix is compressed into relatively hard rock and the objects experience flattening and stretching strains of up to 30%. The rock types used for the objects ranged from weak sandstone and shale to very strong quartzite; matrix materials were crushed adamellite, marble and a marble—salt mixture. The experiments were designed to investigate the relative deformation of the objects and the composite samples and, in particular, the effect of the ductility contrasts (or complete differences) between the objects and the matrix.Spheres and cylinders with a length/diameter ratio of unity are called “pebbles” in the paper. They experience a homogeneous flattening during compression and the ductility contrast controls the load at which yielding occurs and the relative rates at which a pebble and the surrounding matrix deform. The changes in shape of pebble and sample are related linearly and the slope of the straight line graph gives a quantitative estimate of the ductility contrast ; in this way a table of ductility contrasts for the various rock types has been constructed and the relative responses of different pebbles in a granular matrix are nicely illustrated in an artificial deformed conglomerate.Cataclasis is the dominant mode of deformation and much of the large finite strain induced into the objects occurs at surprisingly low applied loads. This suggests that deformed pebbles in natural rock need not necessarily have deformed as a result of tectonic pressures but could have changed shape during diagenesis of the host rocks.     

内蒙古大青山煤田晚古生代砾岩的沉积特征

大青山煤田晚古生代地层中有 1 4层沉积砾岩。其中,晚石炭世地层中赋存 8层,二叠世 6层。通过对这些砾岩层的野外测量、室内岩矿鉴定等综合分析研究,查明晚石炭世的砾岩几乎全是稳定的单成分砾岩,石英砂岩和燧石质砾石的含量高达 93 %以上,砾石最大扁平面均向南倾,倾角在 5° - 1 0°之间,与砾岩伴生的粗粒石英砂岩发育大型低角度双向交错层理,萨胡环境判别参数等特征表明晚石炭世砾岩为滨海环境下的产物。二叠世的砾岩则主要是复杂成分的砾岩,尤其是早二叠世晚期及晚二叠世初期形成的 2层砾岩中砾石种类达 1 5种,以中酸性火山岩和火山碎屑岩砾石为主,并含较多量的变质岩和中酸性侵入岩砾石,砾岩层各种特征表明二叠世砾岩为河流相产物。大青山煤田晚古生代沉积砾岩的形成与北部造山带的隆升密切相关,它们是前陆盆地的沉积产物。     

How pressure solution creep and fracturing processes interact in the upper crust to make it behave in both a brittle and viscous manner

The upper crust has been described as being dominated by brittle deformation along faults, or ductile where folds and cleavage have developed. These two mechanical behaviors are explained by two different mechanisms of deformation: (i) fracture; and (ii) fluid-enhanced deformation (e.g. pressure solution). These two mechanisms operate at two time scales: fast for brittle deformation, slow for pressure solution. Natural observations of relationships between pressure solution and fractures in sandstones, or indented pebbles, and experimental results of pressure solution with an indenter technique indicate that both mechanisms can interact: fracture development increases the kinetics of the pressure solution process, pressure solution relaxes the stress between fracturing events. A simple model of brittle–ductile deformation, applied to indented limestone pebbles, shows that cycles of slow deformation can alternate with short-time fracture.     

芦山—天全地区大溪砾岩岩石学特征及物源区分析

大溪砾岩为一套厚层碳酸盐质砾岩,具有砾石含量高,砾石分选、磨圆较好的特点。据砾石统计、薄片分析、古生物鉴定、古流分析及重矿物组合表明,大溪砾岩中砾石主要来自龙门山区的泥盆系—三叠系,填隙物中的石英碎屑可能主要来自杂岩体。大溪砾岩的砾石成分有向上趋于复杂的特点。     

Cataclastic deformation of geological materials in matrices of differing composition: II. Boudinage

Spheres and clylinders of various rock types were embedded in a matrix of crushed rock and the combined samples were then deformed by applying uniaxial compressive loads of up to 4.5 MN. Under these conditions, large confining pressures are built up in the centre of the samples; thus the granular matrix is compressed into relatively hard rock and the objects experience flattening and stretching strains of up to 30%. The rock types used for the objects ranged from weak sandstone and shale to very strong quartzite; matrix materials were crushed adamellite, marble and a marble—salt mixture. The experiments were designed to investigate the relative deformation of the objects and the composite samples and, in particular, the effect of the ductility contrasts (or complete differences) between the objects and the matrix.Spheres and cylinders with a length/diameter ratio of unity are called “pebbles” in the paper. They experience a homogeneous flattening during compression and the ductility contrast controls the load at which yielding occurs and the relative rates at which a pebble and the surrounding matrix deform. The changes in shape of pebble and sample are related linearly and the slope of the straight line graph gives a quantitative estimate of the ductility contrast ; in this way a table of ductility contrasts for the various rock types has been constructed and the relative responses of different pebbles in a granular matrix are nicely illustrated in an artificial deformed conglomerate.Cataclasis is the dominant mode of deformation and much of the large finite strain induced into the objects occurs at surprisingly low applied loads. This suggests that deformed pebbles in natural rock need not necessarily have deformed as a result of tectonic pressures but could have changed shape during diagenesis of the host rocks.     

仿和田玉籽料的方法及鉴定

和田玉籽料以其洁白温润博得了越来越多人的喜爱,加之其有限的资源和丰厚的利润,市场上出现的和田玉籽料仿制品也越来越多。介绍了和田玉的分类及外部特征,分析了仿和田玉籽料的两种常用方法——滚料磨圆和染色处理做假皮,从其外形、颜色、硬度、密度和光泽等特征及仪器测试方面,讨论研究了和田玉籽料与其仿制品之间的差异,以达到鉴定和田玉籽料真伪的目的。     

Endolithische Wurm-Tunnelbauten (Lapispecus cuniculus n. g. n. sp. undDodecaceria [?] sp.) in Brandungsgeröllen der oberen Kreide im nördlichen Harzvorlande

The well known Santonian conglomerates from the area north of the Harz Mountains contain holes due to leached dolomitic pebbles of Kimmeridgian age, in which well preserved casts of “worm”-burrows have been observed. In smaller pebbles - caused by the lack of space — they were burrowed in narrow windings resembling the Serpulid genusGlomerula. However, their mode of winding differs from that of these Serpulids: The convex arches made by the boring organism must be regarded as a behavioural response when approaching the surface of the pebbles. In the loops of the burrows often a “Spreite” is found, but it is also observed in the convex windings and even parallel to the straight cylindrical burrows. That is in contradiction to the conventional definition of the “Spreite”. The burrows which may belong to the Polychaetes are described asLapispecus cuniculus n. g. n. sp. Other rare burrows have been identified asDodecaceria (?) sp.     

山东省唐王寨上寒武统崮山组沉积环境及竹叶状灰岩的多成因分析

华北地台上寒武统崮山组出现了大量的竹叶状灰岩。通过对山东唐王寨剖面崮山组的系统研究,识别出页岩、疙瘩状泥晶灰岩、泥质条带泥晶灰岩、薄层状泥晶灰岩、生物扰动泥晶灰岩、颗粒质（生物碎屑）泥岩-泥质颗粒岩、含交错层理鲕粒灰岩及竹叶状灰岩8种岩石类型,这些岩石类型组成了页岩盆地、深潮下带及浅潮下带岩石组合。竹叶状灰岩中砾屑和基质的特征及其沉积序列,表明砾屑和基质的来源多样并且在不同的沉积环境中其成因具有多样性。据此总结出崮山组竹叶状灰岩具有4种可能的成因类型：1）竹叶状灰岩中砾屑和基质可能均为原地形成或者仅有短距离的搬运过程;2）竹叶状灰岩中砾屑和基质可能均为近岸形成并经历了长距离的搬运过程,或者竹叶状灰岩的沉积环境经历了海平面的突然升高;3）竹叶状灰岩中基质可能来源于近岸未固结的鲕粒和生物碎屑及原地的灰泥,与原地破碎生成的砾屑和灰泥等混合沉积;4）竹叶状灰岩中砾屑可能来源于远岸的固结的泥晶灰岩,并经搬运作用与原地未固结的灰泥及骨架颗粒等基质混合沉积。     

Sandy gravel accumulation in a fluvial environment

Particle size, pebble shape, pebble fabric, discharge and flow velocity data are used to introduce a model of sandy gravel formation in Welsh gravel-bed rivers. The development of contact-imbrication of the typically very bladed and very platy cobbles and larger pebbles subsequently acts to significantly affect the depositional modes and patterns of small pebbles and sand particles. An important distinction is drawn between sand deposition, which can occur at or below the bed surface, and pebble and cobble deposition, which is merely a surface phenomenon.