共查询到20条相似文献，搜索用时 566 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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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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Frank Dennis John N. Andrews Andrew Parker Jason Poole Manfred Wolf 《Applied Geochemistry》1997,12(6):763-773
The chemical and isotopic composition of groundwater from 52 sites in the London (U.K.) area was determined as part of a project aimed at assessing the spatial variation in the age of Chalk groundwater, and in determining the relationship between fracture and matrix groundwater in this dual porosity system.Systematic changes in groundwater chemistry take place in the downgradient direction in response to several chemical processes. These processes include early concentration by evaporation and congruent dissolution of calcite followed by widespread incongruent dissolution and ion exchange in addition to local oxidation-reduction reactions, gypsum dissolution and saline intrusion. As a result of the above processes, Chalk groundwater follows an evolutionary path from Ca bicarbonate type to Na bicarbonate type.The age of Chalk groundwater was modelled using14C, δ13C,3H, δ2H and δ180. There is a general increase in the groundwater age in a downgradient direction with the oldest water found in N central areas of the basin. Groundwater in the unconfined zones and in areas S of the Greenwich fault is almost entirely of unevolved, modem composition. Carbon-14 modelling suggests that Chalk groundwater in the S basin is generally less than 10000 a old while that in the north is generally between 10000 and 25000 a old. The presence of3H in concentrations of up to 7 TU in groundwater which yields ages of several 1000 a, however, indicates that mechanisms exist for the rapid introduction of recent groundwater to the confined aquifer. Results of palaeorecharge temperature determinations using δ2H, δ180 and noble gas analytical results suggest that significant Devensian recharge did indeed occur in the aquifer.A model of the development of the Chalk recognizes that it is a classic dual porosity aquifer in which groundwater flow occurs predominantly in the fracture system. The upper 50 m of the aquifer was flushed with fresh water during the 2–3 × 106 a of the Quaternary and therefore meteoric water largely replaced the Tertiary and Cretaceous marine water that previously saturated the system. Most processes which control the chemistry of the groundwater occur in the matrix where the surface area is exceptionally high. Although fracture flow dominates the flow regime, diffusion from the matrix into the fracture porosity controls the chemistry of Chalk groundwater. 相似文献