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ABSTRACT Carboniferous speleothem calcites in un-metamorphosed limestones from South Wales exhibit crystal diminution (degrading neomorphism) textures. These textures result from processes which have been considered only important under metamorphic conditions. The textures are interpreted as the result of recrystallization associated with dislocation processes. These processes relate to low temperature creep associated with low strain rates extended over long periods of time. The textures described here occur in unusually large crystals but similar textures have been described in limestones from deep boreholes. Dislocation processes, recovery and recrystallization are likely to be important processes operating during deep burial diagenesis. 相似文献
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JOHN R. GILLIS 《Geographical review》2007,97(2):274-287
ABSTRACT. Islands have long held a central place in Western cultures' mythical geographies. They have been associated for centuries with heroic journeys and holy quests, imagined realms of magical transformations. Islands have also been sites of significant rites of passage, and they continue to perform this function in the modern secular world. Today, popular islomania is expressed in the frequency of seasonal sojourning on European and American archipelagos. No longer destinations of permanent residence, islands now provide access to a sense of temporal and spatial rootedness that is no longer available on mainlands. They loom large on the mental maps even of those who rarely, if ever, visit them. 相似文献
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JOHN WESTERN 《Geographical review》2001,91(4):617-640
ABSTRACT. At the southern tip of Africa, culmination of one of the world'sgreat population shifts is there for the witnessing: a southward movement of Bantu‐speaking Black Africans, previously stalled in South Africa'sEastern Cape first by climatic factors and later by European‐imposed racial segregation. Cape Town, throughout its demographic history a “Coloured” and “White” city, is now Africanizing. Liberation from apartheid has brought rapid African influx, and the metropolitan population has more than doubled in less than twenty years. Sociocultural tensions and sociocultural possibilities ensue. 相似文献
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Low-{{delta}}18O Rhyolites from Yellowstone: Magmatic Evolution Based on Analyses of Zircons and Individual Phenocrysts 总被引:1,自引:0,他引:1
The Yellowstone Plateau volcanic field is one of the largestcenters of rhyolitic magmatism on Earth. Major caldera-formingeruptions are followed by unusual low- 相似文献
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The morphology and dynamics of low amplitude bedwaves upon upper stage plane beds and the preservation of planar laminae 总被引:3,自引:0,他引:3
Experimental studies of subcritical, unidirectional flow over upper stage plane beds of medium grained sand reveal the ubiquitous presence of low amplitude bedwaves. Flow depth was 0·11 m, mean flow velocities were 0·86–1·0 m s?1, shear velocities were 0·058–0·71 m s?1 and dimensionless shear stresses were 0·56–0·87. Bedwaves are asymmetrical in profile and range from 0·75 to 11 mm in height (mainly 2–6 mm), from 0·7 to 1·3 m in wavelength and have mean celerities of 10 mm s?1. Flow records suggest that the bedwaves are associated with accelerating flow over the bedwave crests and flow which decelerates and diverges laterally over the troughs. High resolution bed profiling during aggradation of the bed combined with subsequent box coring illustrates that these bedwaves are responsible for the planar laminae characteristic of upper stage plane beds. Lamina preservation is dependent upon the mean aggradation rate and the sequence of bedwaves of different height crossing any point; individual laminae are more readily preserved at higher aggradation rates where the possibility of reworking by later bedwaves is reduced. Laminae are recognized by small changes in grain size and commonly a fining upwards at the top of laminae which is generated by fine grained material infiltrating a lower lamina in the leeside of a bedwave. 相似文献
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Over the past 100 years, the Isles Dernieres, a low lying barrier island chain along the coast of central Louisiana, Usa , has undergone more than 1 km of northward beach face retreat with the loss of 70% of its surface area. The erosion results from a long term relative sea level rise coupled with day to day wind and wave action that ultimately favours erosion over deposition. At a site in the central Isles Dernieres, 8 days of wind and beach profile measurements during the passage of one winter cold front documented aeolian erosion and deposition patterns under both onshore and offshore winds. For offshore winds, the theoretical erosion rate, based on wind shear velocity, closely matched the measured erosion rate; for onshore winds, the theoretical rate matched the measured rate only after being corrected by a factor that accounted for beach face morphology. In late February 1989, a strong cold front moved into coastal Louisiana. That cold front stalled over the Gulf of Mexico, resulting in 4 days of strong northerly winds at a study site on the Isles Dernieres. During those 4 days, the wind moved sand from the backshore to the upper beach face. When the cold front finally moved out of the area, the wind shifted to the south and decreased in strength. The onshore wind then restored some of the upper beach face sand to the backshore while increased wave activity moved the rest into the nearshore. The theoretical estimate of 1·28 m3 m?1 for the rate of sand transport by the northerly wind compares well with the measured backshore erosion rate of 1·26 m3 m?1, which was determined by comparing beach profiles from the start and end of the period of northerly winds. The theoretical estimate of 0·04 m3 m?1 for the rate of sand transport by the southerly wind, however, is notably less than the measured rate of 0·45 m3 m?1. The large discrepancy between the two rates can be explained by a difference in the shear velocity of the wind between the beach face, where the erosion occurred, and the backshore, where the wind stress was measured. Using an empirical relationship for the wind shear drag coefficient as a function of coastal environment, the theoretical estimate for the rate of sand transport by the southerly wind becomes 0·44 m3 m?1 相似文献