Добірка наукової літератури з теми "Silt-sized grains"

Æolian dust deposits are known to be widespread in south-eastern Australia, with the dominant model being that of ‘parna’, an aggregated material comprising clay, calcium carbonate, and silt-sized quartz. Despite a general acceptance of the parna model, there is surprisingly little evidence of silt-sized clay aggregates remaining in parna profiles. To investigate a relatively pure æolian dust deposit within the proposed zone of parna distribution, we examined the various phases of 3 source-bordering sand dunes on the lower Lachlan River floodplain of south-western NSW. In each dune exists an upper-slope phase of coarse brown sand, a mid-slope layer of reddish, clay-enriched sand, and a lower-slope phase of coarse sand dominated by an accumulation of carbonate glaebules. Granulometric analyses of the clay-enriched phase(s) of each dune identified a conspicuous particle population in the 20–60 μm range, and another in the fine-silt/clay range (<10 μm). Mineralogical characterisation revealed an abundance of illite and kaolinite in the upper 2 dune phases, coupled with a minor amount of smectite, further suggesting an allochthonous æolian origin, as the surrounding floodplain is smectite-rich and relatively poor in illite. Micromorphological features within the clay-enriched phase, including abundant argillans and laminar bands of well-sorted fine quartz grains, indicate that this clay is a depositional feature, illuviated from surface horizons and re-deposited at depth. A similar illuvial origin is suggested by the fine crystalline nature of the calcium carbonate accumulation, ubiquitously coating the matrix mineral grains of the lower dune phase. No discrete clay aggregates were identified; however, all the assumed components of parna (silt-sized quartz grains, clay, and calcium carbonate) were identified, spatially separated within each dune. The consistency of these features among the 3 dunes indicates an analogous æolian dust accession, but it is not clear whether the clay component of this dust was transported as coatings on quartz grains, or as silt-sized clay aggregates accompanied by silt-sized quartz grains.

Investigation of a sequence of alternating sand and silt deposits formed in an ephemeral braided stream channel adjacent to the modem Caribou River, Yukon, revealed differences in the palynological spectra of the sediment types. Picea, Betula, Alnus, and Gramineae are more concentrated in the silt units, whereas Cyperaceae, Chenopodium, Lycopodium, and Ericaceae are preferentially concentrated in the sand strata. These distribution patterns reflect the hydrodynamic properties of the grains, in addition to environmental differences. Grains of Picea, Betula, and Alnus settle through still water at the same rates as silt-sized quartz particles and are therefore concentrated in the portion of the deposit derived from the stream's suspended load. Thus, increases in percentages of these palynomorphs with decreasing grain size may not reflect vegetation changes or climatic alterations but may be consequences of the hydrodynamic situation.

It is generally accepted that large areas of southern New South Wales (NSW) and northern Victoria are mantled by soils derived from clayey loess deposits called parna. These parna deposits are believed to have formed during arid phases of the Pleistocene and the mineral constituents are assumed to have been transported as silt- and fine sand-sized pellets of calcareous clay, with some companion quartz grains of a similar size. A common property of parna-derived soils is subplasticity, where the apparent field texture grade becomes more clayey with increasing mechanical working of the bolus. This propensity for subplastic behaviour suggests that parna-derived soils contain stable silt- and fine sand-sized pellets of clay, yet there has been little direct micromorphological evidence of these pellets ever published. In the present study, thin section samples from several parna type-sites in southern NSW were examined micromorphologically to reveal the presence of very well size-sorted quartz grain populations (companion grains) and identifiable prolate clay aggregations of a similar silt to fine sand size. These prolate pellets, comprised of quartz, illite, kaolinite and chlorite, have a very distinctive mosaic-speckled b-fabric and are more widely distributed throughout the soil than the similarly sized and shaped faunal faecal pellets. Where these pelletal aggregations are not evident, such as in the deeper parna deposits, abundant illuviation features suggest that clay particles deposited within the parna, whether as pellets or coatings on grains, have subsequently undergone considerable weathering and a range of pedogenic processes.

This paper presents a qualitative and quantitative study of the microstructure of a compacted silt, carried out using a scanning electron microscope and mercury intrusion pore size distribution measurements. Samples have been statically compacted at three water contents: on the dry side of the standard Proctor optimum water content, at the optimum, and on the wet side. The wet sample has a matrix type structure, with a clayey fraction filling the voids and adhering to the silt-sized grains, whereas the other samples display a structure characterized by a skeleton made of silt grain aggregates linked together by clayey bidges. The distribution of water and air within the microstructure of the compacted soil is described for each of the three compaction states. Key words: compacted soil, microstructure, scanning electron microscope, pore size distribution, mercury intrusion, clay hydration.

AbstractThe mechanical behavior and microstructural evolution of laboratory-prepared, particle-free fresh-water ice and ice with 1 wt.% (~0.43 vol.%) silt-sized particles were investigated under creep with a stress level of 1.45 MPa at −10°C. The particles were present both within the grains and along the grain boundaries. The creep rates of specimens with particles were always higher than those of particle-free ice. Dynamic recrystallization occurred for both sets of specimens, with new grains nucleating along grain boundaries in the early stages of creep. The ice with particles showed a higher nucleation rate. This resulted in a smaller average grain-size for the ice with particles after a given creep strain. Fabric studies indicated that ice with particles showed a more random orientation of c axes after creep to ~10% strain than the particle-free ice.

The partial bleach (R–Γ) procedure of thermoluminescence (TL) dating of unheated sediments has been applied to the 2–4 μm sized feldspar-dominated grains from two components of a 10–11 ka BP glaciolacustrine silt. A previous TL study of the 4–11 μm sized feldspars from a ~12 cm thick summer layer of this varved deposit did not produce the correct age. Here it is shown that the 2–4 μm feldspar grains from a contiguous, 5 mm thick clay-rich winter varve give a satisfactory TL apparent age of 14.2 ± 2.3 ka. On the other hand, and consistent with the previous results, the 2–4 μm feldspar grains from the thicker summer layer yield an incorrect high apparent age of 55 ± 13 ka. These results have implications for general TL dating of waterlaid sediments.

Residual Osl Signals from Modern Greenlandic River SedimentsRipple-laminated sandy deposits at shallow water depths in four rivers on Jameson Land, East Greenland were sampled for optically stimulated luminescence (OSL) dating. Silt-sized grains have significantly higher equivalent doses (~1.1 Gy) than sand grains (~0.1 Gy). This suggests that coarse grain-size fractions are better bleached and more suitable than fine grains for OSL dating of glaciofluvial/fluvial sediments. A sample from a sidebar deposited during the spring flood yielded 1.0 Gy (~500 years) while a subaerial deposit was completely zeroed. The spring flood deposit is considered to be most similar to deglacial conditions and incomplete bleaching of this amount (1 Gy) is generally not a significant source of error for sediments of Pleistocene age. Most samples have rather poor luminescence characteristics and are affected by thermal transfer if preheat temperatures at or above 260°C are used.

The nature of the Pleistocene-Holocene palaeosols in the Gaza Strip, PalestineThe Pleistocene to Holocene succession in the Gaza Strip, Palestine, consists of an alternation of calcareous sandstones and reddish fine-grained deposits (palaeosols). The palaeosols can be subdivided into two main groups based on the sand-sized versus clay- to silt-sized grains: (1) the sandy hamra palaeosols, and (2) the loess and loess-derived palaeosols. The hamra palaeosols can, in turn, also be subdivided into two main types according to their colour and grain size: (1) light brown loamy to sandy hamra palaeosols, and (2) dark brown sandy clay hamra palaeosols. The hamra palaeosols are polygenetic and originated in humid environments. Their red colour results from ferric oxides coating the sand grains, but also by illuviation. The various pedogenitic units and their gradual transition to loess palaeosols are due to different phases of dust accretion. Both groups of palaeosols developed during the last glacial. They are considered to represent different climate environments: hamra palaeosols represent humid climates, whereas the loess and loess-derived palaeosols represent dry and semi-dry climates.

Abstract. Human CO2 emissions may drive the Earth into a next greenhouse state. They can be mitigated by accelerating weathering of natural rock under the uptake of CO2. We disprove the paradigm that olivine weathering in nature would be a slow process, and show that it is not needed to mill olivine to very fine, 10 μm-size grains in order to arrive at a complete dissolution within 1–2 year. In high-energy shallow marine environments olivine grains and reaction products on the grain surfaces, that otherwise would greatly retard the reaction, are abraded so that the chemical reaction is much accelerated. When kept in motion even large olivine grains rubbing and bumping against each other quickly produce fine clay- and silt-sized olivine particles that show a fast chemical reaction. Spreading of olivine in the world's 2% most energetic shelf seas can compensate a year's global CO2 emissions and counteract ocean acidification against a price well below that of carbon credits.

AbstractSequences of quartz-rich coarse (20−63 μm) silt occur in many low- and midlatitude unglaciated arid and semiarid areas and have been termed “desert loess.” The processes by which these deposits are generated have been debated for decades. All hypotheses to explain their origin seek to provide mechanisms for the generation of silt-sized material without glacial grinding, which is the main process involved in the production of coarse silt at high latitudes. Possible mechanisms for the formation of coarse silt in arid regions include derivation from preexisting siltstones, mechanical weathering of silicate rocks, and abrasion of sand grains in active dune environments during intense transport events. Examination of the characteristics of desert loess and field and laboratory experiments to assess the role of dune areas as a source of coarse silt by abrasion and/or resuspension of residual fines suggests that many loess sequences are dominated by locally derived coarse silt. Improvements in the characterization of desert loess particle size, mineralogy, and geochemistry are needed, however, to identify sources and sinks of coarse silt, especially when combined with climatic back-trajectory analysis. Properly scaled experiments and modeling of particle collisions will also help to better quantify the effectiveness of abrasion in the generation of coarse silt in support of field observations.

L’étude de la microstructure des roches est indispensable pour nos enjeux contemporains et futurs en matière d'énergie, d’ingénierie et de construction. D’autre part, cette étude permet de caractériser les processus de déformation géologique ayant conduit à l’état actuel des unités lithologiques. Les roches clastiques à grain fin, communément appelées "shales" en anglais, représentent environ deux-tiers de l’ensemble des roches sédimentaires. Les données 3D relatives aux grains de silt ou clastes inclus dans la matrice argileuse et poreuse de ce type de roche sont peu fréquentes. Ces données sont pourtant cruciales pour comprendre les propriétés anisotropes à l’échelle macroscopique mais aussi pour évaluer l’état de déformation de la matrice rocheuse. Mieux connaitre la microstructure de ces roches permettrait d’être prédictif quant à leurs propriétés mécaniques ou physiques indispensables pour les applications du secteur de l’énergie par exemple. La tomographie à rayons X (XCT) est une technique non destructive permettant d’obtenir une image 3D de la microstructure d'un objet. Une caractérisation géométrique directe des constituants des roches clastiques à grain fin est envisageable grâce à cette technique. Sur la base d'images XCT, cette thèse vise d’abord à développer des aspects méthodologiques pour étudier la fabrique de forme 3D des clastes ainsi que leur distribution spatiale. Ces aspects sont élaborés à partir de la méthode des moments d’inertie qui est appliquée sur les grains segmentés des images 3D numériques. Nous présentons ensuite des applications sur des roches à grain fin possédant une fabrique sédimentaire et sur des roches à grain fin déformées présentant une fabrique d’origine tectonique. Le premier volet applicatif de la thèse s’intéresse à une même unité lithologique ayant enregistrée différentes quantités de déformation. Des échantillons du bassin sud-pyrénéen et des échantillons issus d’un affleurement historique dans les Appalaches centrales ont été collectés. Nous apportons de nouvelles données sur l’évolution de la forme 3D des grains et des pores à l’échelle micrométrique et sur leur agencement dans la matrice rocheuse en fonction de la déformation. Les données obtenues permettent de discuter des mécanismes de déformation à l’échelle du grain des différentes phases minéralogiques. Cependant, la petite taille des échantillons imagés par XCT (≤ 2 mm de diamètre) soulève la question de la représentativité de ces analyses. Sur le chantier sud-pyrénéen, certains échantillons sont étudiés de manière plus approfondie pour vérifier l’homogénéité des résultats. Nous y montrons que les données XCT complètent les mesures indirectes pétrophysiques en permettant de décrire et de localiser les sous-fabriques intégrées dans une mesure globale de la fabrique. Les limites apparaissent lorsque la dimension caractéristique des structures de déformation avoisine la taille de l'échantillon imagé par XCT. Dans le second volet applicatif, des échantillons provenant de systèmes turbiditiques du bassin sud-pyrénéen sont analysés. Ces systèmes, lorsque déformés en tectonique compressive, présentent l’avantage d’enregistrer la même quantité de raccourcissement différemment exprimée dans les unités lithologiques qui les constituent. Les résultats obtenus à partir de la forme des clastes sont comparés à nos mesures magnétiques globales de la fabrique et montrent une bonne cohérence. La méthodologie présentée dans ce travail peut s’étendre à d'autres types de milieux poreux et granulaires pour une meilleure compréhension de l'influence de l'anisotropie structurale sur leurs propriétés macroscopiques et leur comportement mécanique
The study of the microstructure of rocks is essential for our contemporary and future challenges in energy, engineering and construction. Furthermore, this study allows us to characterize the geological deformation processes that led to the current state of geological formations. Fine-grained clastic rocks, commonly called "shales", represent about two-thirds of all sedimentary rocks. 3D data concerning silt-sized grains or clasts embedded in the porous clay-rich matrix of this type of rock are relatively scarce despite the fact that these data are crucial to understand the anisotropic properties of these rocks at the macroscale but also to evaluate the deformation state of the rock matrix. A better understanding of the microstructure of these rocks would allow us to predict their mechanical or physical properties, which are essential for applications in the energy sector, among others. X-ray computed tomography (XCT) is a non-destructive technique providing a 3D image of the microstructure of any object. A direct geometric characterization of the constituents of fine-grained clastic rocks is possible with this technique. Based on XCT images, this thesis aims first to develop methodological aspects to study the 3D shape fabric of silt particles and their spatial distribution. The moments of inertia of segmented grains from 3D digital images are used for this development. We then present applications on fine-grained rocks with a sedimentary fabric and on deformed fine-grained rocks with a tectonic fabric. The first application part of the thesis focuses on the same lithologic unit having experienced different amounts of deformation. Samples from the South Pyrenean Basin and samples from a historical outcrop in the Central Appalachians were collected. We provide new data on the evolution of the 3D shape of grains and pores at the micrometer scale and their arrangement in the rock matrix with respect to the deformation intensity. The obtained data allow discussing the deformation mechanisms at the grain scale of the different mineralogical phases. However, the limited size of the imaged samples by means of XCT (≤ 2 mm diameter) raises the question of the representativeness of these analyses. On the South Pyrenean site, some samples are studied in more detail to evaluate the homogeneity of the results. We show that the XCT data complement the indirect petrophysical measurements by providing access to localized sub-fabrics that are integrated in a bulk measurement of the rock fabric. The limits are reached when the characteristic length of the deformation structures are on the order of the sample size imaged by XCT. In the second application part, samples from turbiditic systems of the South Pyrenean basin are analyzed. These systems, when deformed in compressive tectonic settings, record the same amount of shortening differently expressed in the various siliciclastic matrices. The results obtained from the shape data of the clasts are compared to our bulk magnetic fabric measurements and show a good consistency. The methodology presented in this work can be extended to other types of porous and granular media for a better understanding of the influence of fabric anisotropy on their macroscopic properties and mechanical behavior