Ophiolites   总被引：1，自引：0，他引：1  

ROGER MASON 《Geology Today》1985,1(5):136-140

An ophiolite is an association of igneous and metamorphic rocks, including basalts, dolerites, gabbros and peridotites, which may be a fragment of crust and mantle formed by spreading at a ridge beneath an ancient ocean. This remarkable conclusion is supported by many features of the structure, field relationships, petrology and geochemistry of ophiolites. Although recent research has led to a degree of disillusionment with the idea that ophiolites are fragments of typical ocean floor, these rocks have a good deal to tell us about processes of ocean floor construction and mountain building.  相似文献   

Petrology of glacial sand: implications for the origin and mechanical durability of lithic fragments     

ROGER M. SLATT  NICHOLAS EYLES 《Sedimentology》1981,28(2):171-183

The petrology of first cycle (unmodified) and second cycle (reworked) sand at the termini of eleven valley glaciers eroding coarse- to fine-grained bedrock types is determined in order to evaluate the origin and mechanical durability of lithic sands. First cycle sands are coarse- to medium-grained, poorly sorted, fine-skewed, non-modal lithic sands with an average composition of Q₂₁F₆L₇₃. Grain-size distributions do not vary with composition or source rock types, although sands derived from finer grained source rocks contain more lithic fragments than sands from coarser grained sources. By contrast, second cycle sands are medium-grained, poorly sorted, fine- to coarse-skewed arkosic to lithic sands with an average composition of Q₁₉F₄₀L₄₁, and contain fewer lithic fragments than do first cycle sands. We propose that the origin, mechanical durability and survival potential of lithic fragments are related to the types and abundances of their internal planes of weakness, and the particular stress field of the transporting medium. As a result of abrasion, glacial clasts and lithic fragments are subjected to shear stress so that fractures propagate along intracrystal and intercrystal planes of weakness resulting in a continuous grain-size spectrum of lithic fragments and monomineralic grains irrespective of the type of source rock. Upon subsequent aqueous transport in the proglacial environment lithic fragments are subjected to point-loading during saltation in addition to shear stress. Point-loading produces extensional fractures which preferentially propagate along intercrystal planes of weakness, so the lithic fragments break into smaller monomineralic grains and/or lithic fragments. Lithic fragments of very coarse sand-size are abundant in first cycle sands, thus refuting, at least for glacial sands, the widely held view that grains of this size are deficient in nature. The presence of fewer grains of this size in second cycle sands indicates that very coarse sand-size lithic fragments can survive recycling, but in greatly reduced proportions due to breakage.  相似文献   

Computer program for presentation of grain-size data by the graphic method     

ROGER M. SLATT  DAVID E. PRESS 《Sedimentology》1976,23(1):121-131

A computer program is described for a Hewlett-Packard desk-top Calculator (Model 9820A)-Plotter (Model 9862A) which plots a histogram, frequency polygon and/or cumulative curve, as well as standard statistical parameters and percentages of gravel, sand, silt, and clay from raw weight data from a sediment grain-size analysis. The program utilizes the graphic method for calculating statistical parameters in contrast to other commonly used computer programs derived for the method of moments. This program is advantageous because (1) it is rapid, (2) it allows direct comparison with grain-size data in the literature which have been computed by the graphic method, (3) either a complete presentation of a distribution can be obtained on one sheet of paper or a series of cumulative curves or frequency polygons can be prepared on a single graph which is ready for publication photocopying, (4) automatic extrapolation of data points from a cumulative curve eliminates inconsistencies arising from manual extrapolation, and (5) the desk-top calculator is more convenient to use, eliminates punching of data cards, and can be operated by someone without a knowledge of computers and programming. Disadvantages of the program are: (1) some authors feel critical percentiles are more accurately read off cumulative curves plotted on a probability ordinate than on an arithmetic ordinate, (2) the cumulative curve is a series of straight-line segments, (3) the frequency polygon is not as accurate a representation of grain-size distribution as a frequency curve and (4) size class intervals for the histogram and frequency polygon are standardized at 0.5φ, even though gravel and mud fractions are analysed at whole φ intervals. For most studies, the advantages of the program outweigh the disadvantages.  相似文献   

Tidal-shelf sedimentation: an example from the Scottish Dalradian   总被引：2，自引：0，他引：2  

ROGER ANDERTON 《Sedimentology》1976,23(4):429-458

The Jura Quartzite, a formation of probably late Precambrian metasediments over 5 km thick from the Caledonian belt in Southwest Scotland, has been divided into a coarse and three fine facies. The former comprises cross-bedded sands with some laminated sands and silt horizons, interpreted as the deposits of shallow marine tidal dunes and other bedforms together with some beach units. Deposition from suspension of silt and sand formed climbing dunes while largescale erosion produced flat or channelled surfaces. The fine facies comprise laterally persistent, parallel and cross-laminated sand units from millimetres to decimetres thick, interbedded with muds. The coarse and fine facies can be finely interbedded, the former sometimes filling decimetre deep, straight channels, cut in the latter. The fine facies exhibit structures indicative of deposition from decelerating currents and are interpreted as shallow marine storm deposits. The facies are compared with a model developed from published observations on modern shelf areas. Zones of erosion, large and small dunes, flat bedded sand and mud are considered to be the end product of a wide spectrum of tidal and storm conditions. During severe storms the fair weather tidal dunes may be modified or washed out, new dunes may be initiated downcurrent of the normal dune field while storm-sand layers are deposited in the distal zones. Hence, the nature of the preserved sediment blanket reflects the rare severe storm event rather than normal tidal conditions. The Jura Quartzite was deposited in a tidal gulf intimately connected with an ocean basin. The north-northeast directed palaeocurrent modes are probably roughly parallel to the coastline.  相似文献   

Incorporating Agency Into Climate Change Risk Assessments     

ROGER?N.?JONESEmail author 《Climatic change》2004,67(1):13-36

Human agency has been viewed as a problem for climate change assessments because of its contribution to uncertainty. In this editorial, I outline the advantages of agency in managing climate change risks, describing how those advantages can be placed within a probabilistic framework.  相似文献   

An internally consistent thermodynamic data set for phases of petrological interest   总被引：102，自引：8，他引：94  

T. J. B. HOLL  & R. POWELL 《Journal of Metamorphic Geology》1998,16(3):309-343

The thermodynamic properties of 154 mineral end-members, 13 silicate liquid end-members and 22 aqueous fluid species are presented in a revised and updated data set. The use of a temperature-dependent thermal expansion and bulk modulus, and the use of high-pressure equations of state for solids and fluids, allows calculation of mineral–fluid equilibria to 100  kbar pressure or higher. A pressure-dependent Landau model for order–disorder permits extension of disordering transitions to high pressures, and, in particular, allows the alpha–beta quartz transition to be handled more satisfactorily. Several melt end-members have been included to enable calculation of simple phase equilibria and as a first stage in developing melt mixing models in NCKFMASH. The simple aqueous species density model has been extended to enable speciation calculations and mineral solubility determination involving minerals and aqueous species at high temperatures and pressures. The data set has also been improved by incorporation of many new phase equilibrium constraints, calorimetric studies and new measurements of molar volume, thermal expansion and compressibility. This has led to a significant improvement in the level of agreement with the available experimental phase equilibria, and to greater flexibility in calculation of complex mineral equilibria. It is also shown that there is very good agreement between the data set and the most recent available calorimetric data.  相似文献   

An internally consistent dataset with uncertainties and correlations: 3. Applications to geobarometry, worked examples and a computer program   总被引：31，自引：4，他引：31  

R. POWELL  T.J.B. HOLLAND 《Journal of Metamorphic Geology》1988,6(2):173-204

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Hydraulic characteristics of bioclastic deposits: new possibilities for environmental interpretation using settling velocity fractions   总被引：1，自引：0，他引：1  

PAUL S. KENCH  ROGER F. McLEAN 《Sedimentology》1996,43(3):561-570

This paper examines the hydraulic behaviour of heterogeneous bioclastic sediments using settling velocity fractions. Bioclastic deposits are divided into fractions by splitting hydraulically sorted samples. Compositional analysis of fractions shows that grains exhibit marked variation in composition, shape, size and density which control the hydraulic behaviour of bioclastic deposits. Despite the heterogeneous grain properties of fractions (analogous to sieve fractions) sedimentation analysis shows that they possess a narrow range of settling velocity values. Flume experiments also show that the settling velocity fractions possess a narrow range of threshold velocity values. A threshold relationship is identified in which the velocity required to entrain mixed settling fractions increases from 20.5 to 65-0cm s ^?1 for settling fractions with mean settling values of 5.5 chi (2-2 cm s ^?1) to 1.4 chi (36.0 cm s ^?1), respectively. Heterogeneous settling fractions, due to intergrain effects, were found to possess larger threshold velocities than individual components showing that the composition of bioclastic deposits controls hydraulic behaviour. Thus, caution must be exercised in using the threshold relationship for homogeneous sediments or deposits of markedly different composition. As settling velocity fractions reflect narrow hydraulic (settling and threshold) properties of sediments the mean settling velocity of fractions is considered a good indicator with which to interpret transport and depositional processes. Comparison of settling and sieve-size distributions shows that size distributions do not reflect the hydraulic behaviour of bioclastic deposits and should not be used to interpret environmental processes. The study indicates that examination of settling velocity fractions (for which hydraulic settling and threshold properties are known) coupled with compositional analysis of these fractions will allow much greater environmental interpretation of deposit-forming and energy processes in reef environments.  相似文献   

Evaporitic sediments of Early Archaean age from the Warrawoona Group, North Pole, Western Australia   总被引：2，自引：0，他引：2  

ROGER BUICK  J. S. R. DUNLOP 《Sedimentology》1990,37(2):247-277

Chemical sediments are common and diverse in the c. 3500 Myr old North Pole chert-barite unit in the Warrawoona Group, Western Australia. Although almost all original minerals were replaced during hydrothermal alteration, metamorphism and deformation, pseudomorphic relics of sedimentary and diagenetic textures and structures show that at least six lithofacies were partly or wholly chemical in origin. These contained five main chemical sedimentary components: primary carbonate mud, diagenetic carbonate crystals, primary sulphate crystals, diagenetic sulphate crystals and diagenetic sulphate nodules. All show a wide range of characteristics consistent only with a marine evaporative origin. Diagenetic carbonate and sulphate crystals, once ferroan dolomite and gypsum, were precipitated within volcanogenic lutites high on littoral mudflats. The other evaporative phases were apparently deposited behind a barrier bar composed of stranded pumice rafts. Primary sulphate crystals, once gypsum and now barite, were precipitated in semi-permanent pools immediately behind the bar. Primary carbonate mud, originally calcitic or aragonitic but now silicified, was deposited in nearby channels and on surrounding mudflats. Within these sediments, diagenetic carbonate crystals (formerly ferroan dolomite) and diagenetic sulphate nodules and crystals (once gypsum) grew during later desiccation. The existence of these evaporites, and more like them in the sediments of other Early Archaean cratons, suggests that shallow marine and terrestrial conditions prevailed over a small but significant portion of the early Earth, contrary to some models of global tectonic evolution. Their overall similarity with more recent evaporitic deposits indicates that there was greater conformity between conditions in modern and primeval sea-shore environments than might be expected, given the great age difference. The attitude implicit in many accounts of Earth's early history, that evaporites were either not deposited or not preserved in Archaean sediments, thus seems to be incorrect.  相似文献   

Metamorphism in the Olary Block, South Australia: compression with cooling in a Proterozoic fold belt   总被引：2，自引：1，他引：2  

G.L. CLARKE  M. GUIRAUD  R. POWELL  J.P. BURG 《Journal of Metamorphic Geology》1987,5(3):291-306

Abstract The sedimentary and igneous rocks comprising the lower Proterozoic Olary Block, South Australia, were deformed and metamorphosed during the mid-Proterozoic 'Olarian'Orogeny. The area is divided into three zones on the basis of assemblages in metapelitic rocks, higher grade conditions occurring in the south-east. Mineral assemblages developed during peak metamorphism, which accompanied recumbent folding, include andalusite in Zones I and II and sillimanite in Zone III. Upright folding and overprinting of mineral assemblages occurred during further compression, the new mineral assemblages including kyanite in Zone II and kyanite and sillimanite in Zone III. The timing relationships of the aluminosilicate polymorphs, together with the peak metamorphic and overprinting parageneses, imply an anticlockwise P–T path for the 'Olarian'Orogeny, pressure increasing with cooling from the metamorphic peak.  相似文献