共查询到20条相似文献，搜索用时 31 毫秒
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Haydon Bailey 《Geology Today》2023,39(1):18-26
The unique geological history which resulted in the evolution of the Chiltern Hills to the north of London, The United Kingdom, created the underlying foundations for everything that we see there on the surface today. The roots of the Chiltern Hills lie in their Chalk foundations. To understand the details of the way the chalk acts as an aquifer it is important to understand first the origins of the chalk sediment and how the subsequent geological history of the region has impacted on the rocks preserved today. 相似文献
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David Nowell 《Geology Today》2007,23(4):147-152
The Upper Cretaceous Chalk hills of the South Downs form the southern flank of the Wealden anticline in south-east England, with older Wealden and Purbeck sediments exposed at its core. With prominent chalk escarpments on each side of it, this major structure is up to 70 km wide, and extends eastwards for over 200 km from eastern Hampshire to the area around Boulogne-sur-Mer in Northern France, dissected by the English Channel. 相似文献
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JAKE HANCOCK 《Geology Today》1985,1(5):146-149
Chalk is not the sort of rock that you would expect to yield oil. Although it has a high porosity when first formed, the permeability is low. Even the porosity is normally much reduced under overburdens of more than 1000 m. Yet in the North Sea there are special conditions that allow major reservoirs in the Chalk beneath thousands of metres of Tertiary sediments. 相似文献
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This paper presents results of hydrochemical and isotopic analyses of groundwater (fracture water) and porewater, and physical property and water content measurements of bedrock core at the Chalk River Laboratories (CRL) site in Ontario. Density and water contents were determined and water-loss porosity values were calculated for core samples. Average and standard deviations of density and water-loss porosity of 50 core samples from four boreholes are 2.73 ± 12 g/cc and 1.32 ± 1.24 percent. Respective median values are 2.68 and 0.83 indicating a positive skewness in the distributions. Groundwater samples from four deep boreholes were analyzed for strontium (87Sr/86Sr) and uranium (234U/238U) isotope ratios. Oxygen and hydrogen isotope analyses and selected solute concentrations determined by CRL are included for comparison. Groundwater from borehole CRG-1 in a zone between approximately +60 and −240 m elevation is relatively depleted in δ18O and δ2H perhaps reflecting a slug of water recharged during colder climatic conditions. Porewater was extracted from core samples by centrifugation and analyzed for major dissolved ions and for strontium and uranium isotopes. On average, the extracted water contains 15 times larger concentration of solutes than the groundwater. 234U/238U and correlation of 87Sr/86Sr with Rb/Sr values indicate that the porewater may be substantially older than the groundwater. Results of this study show that the Precambrian gneisses at Chalk River are similar in physical properties and hydrochemical aspects to crystalline rocks being considered for the construction of nuclear waste repositories in other regions. 相似文献
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The Upper Turonian Chalk Rock occurs within a nodular unit within the otherwise generally soft, white chalk that dominates the English Upper Cretaceous. The nodular unit is condensed, and contains a number of hardgrounds that are designated here as the Chalk Rock Formation. The Chalk Rock contains some seven or eight hardgrounds, most of which are lithologically distinctive and can be traced over distances of up to 250 km. Nine beds within the Chalk Rock are named, comprising six hardgrounds and three marl seams. The lowermost widespread hardground appears to be more or less equivalent to the “Spurious Chalk Rock” of the south coast of England. In two areas the thickness of the Chalk Rock is greatly diminished. The most marked area, in west Wiltshire, is located close to the Palaeozoic Mendip Hills and indicates that the Mendip structure has influenced Turonian sedimentation. The other region of thinning is a platform-like area in the eastern Chiltern Hills WNW of London. 相似文献
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《Applied Geochemistry》1987,2(1):81-91
Calcite is a common fracture inflilling mineral in the Grenville gneisses of the Chalk River area, Ontario, Canada. It exhibits a variety of occurrences and textures which suggests calcite has precipitated under different hydrogeochemical conditions that may be identified through a detailed chemical and isotopic investigation of the calcite and associated infilling minerals.The δ18O of these calcites range over 20%. but the δ13C varies over a narrow range of 5%.. None of the calcites analyzed is in isotopic equilibrium with both the δ18O and δ13C of the present day ground water. The lightest δ18O calcites (near 0%. SMOW) are present in sealed fractures and are sometimes associated with laumontite. This suggests that these light calcites formed from hydrothermal solutions (at temperatures less than about 300°C) shortly after the period of metamorphism that formed the gneisses. This interpretation is supported by relatively unradiogenic87Sr/86Sr ratios near 0.709 and δ13C values of −5 to −6%..Most of the Chalk River calcites, however, are considerably heavier in18O and lighter in13C than the hydrothermal end member. This may be the result of low temperature recrystallization of the hydrothermal calcites by meteoric waters under variable water/rock ratios. The13C contents and87Sr/86Sr ratios of these younger, low temperature calcites appear to be partially buffered by the isotopic composition of the original hydrothermal calcite.Pyrite is often associated with the fracture calcites. These pyrites display a wide range in δ34S values of about 70%., which suggests that sulphide precipitation occurred under semi-closed conditions. These data indicate that fracture permeability has been a major control on the isotopic composition of fracture minerals since formation of the gneiss. 相似文献
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Serial variation between Micraster populations from successive zones in the Upper Cretaceous Chalk of Europe is widely cited as evidence for evolution at the species level, whether changes between species are interpreted as gradual or punctuational. That these changes were adaptive and represent an improved functional efficiency with time is also now widely agreed, if not whether the changes were independent of environmental change or a response to it. Dead specimens of Micraster were commonly encrusted by a wide variety of small invertebrates, presumably because they provided islands of hard substrate on an otherwise soft, muddy sea floor. Less commonly, there is evidence that living specimens of Micraster were susceptible to predation by gastropods and other organisms, one of the natural selection pressures favouring adaptation to a burrowing mode of life. 相似文献
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The Goldsworthy Chalk Stones, 14 roughly shaped balls of chalk, each approximately 2 m in diameter, were installed in June 2002. Initial assessments by the artist, Andy Goldsworthy, and the Strange Partners commissioning team concluded that they would last for about 2 years. The balls’ disintegration has been measured as part of an ongoing scientific monitoring programme designed to assess their durability and to study controls on rates of chalk weathering. Preliminary assessment of the first 3 years of data shows that 76% of the total mass loss occurred during year 1, 19% during year 2 and 5% during year 3. This might reflect the fact that winter rainfall amounts decreased substantially over the period of study. The west facing sides of the balls have tended to show greater mass loss than those facing east, which might also be a function of exposure to rain. Judging from their present slow rates of weathering, the balls will last for over 200 years but an exceptionally cold wet winter might cause sudden and massive breakdown. Three models of chalk mass weathering are presented. The ongoing scientific monitoring programme will focus experimental work on four key research themes: process, lithology, environment and stress history. 相似文献
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