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341.
The waters of Lake Nyos are impounded by a fragile natural dam composed of pyroclastic rocks ejected during the formation of the lake crater (maar). Lateral erosion of this dam has reduced its width from over 500 m to only 45 m. Published whole-rock K-Ar ages of about 100 ka on juvenile basalt from the dam suggests that erosion has been slow and that the dam poses no imminent threat. New apparent 40Ar/39Ar ages of 1.4 to 232 Ma on xenocrystic K-feldspar contained in the basalt show that the xenocrysts, whose source is the 528-Ma crystalline basement, are carriers of inherited radiogenic 40Ar and would cause the whole-rock K-Ar ages to be too old. The best estimate for the age of the maar is provided by a 14C age of 400 ± 100 yr BP on charcoal from the base of the dam. This young age indicates that the dam is eroding at a relatively rapid rate; its failure, perhaps within a few decades, would result in a major flood and imperil thousands of people living downstream in Cameroon and eastern Nigeria. 相似文献
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344.
From an area of brittle shearing and ring complexes in Northern Kordofan, Sudan 28 K-Ar ages were calculated to determine the geochronology of tectonic and magmatic events. The age results for deformational episodes are 560, 340 and 255 Ma, whereas alkaline magma was intruded at 280, 220 and 165 Ma. The incompatibility of age marks leads to the conclusion that magmatic and tectonic activity in this area have to be regarded as independent events. This holds true even when all dated intrusions from the entire region are compared with post-Pan-African continental tectonic features. 相似文献
345.
Radiocarbon dating of thin palaeopodsols buried beneath turf-banked gelifluction lobes at four localities in the low alpine mountain zone in the Jostedalsbreen region, western Norway, show that gelifluction processes were initiated subsequent to the late Subboreal Chronozone. Although large age-depth gradients have been demonstrated from buried palaeosols in southern Norway, evidence is presented that the palaeosols in this study show only moderate age-depth gradients. The age estimates from these buried palaeosols give maximum dates of burial, but the error is not thought to be large. Gelifluction processes were probably initiated close to the time of the climatic deterioration, which led to the formation of the present glaciers during the Subatlantic Chronozone. The processes may have been most active during the peak of the Little Ice Age, during which a periglacial climate was established to low levels in this mountainous region. 相似文献
346.
Changes in the rate of soil erosion in lake catchments can be identified from changes in the rate of sediment accumulation in lakes. Here we compare recently afforested sites with non-afforested sites in the Galloway area of Southwest Scotland. We show that lakes with non-afforested catchments have slow, constant sediment accumulation rates, whereas lakes with recently afforested catchments have changes in accumulation that parallel the known history of afforestation. For Loch Grannoch the sediment accumulation rate increases from 0.1 cm yr?1 to over 2 cm yr?2 during the disturbance period. Data from L. Skerrow, however, suggest that the rate might decline to predisturbance levels after approximately 10 years as the forest canopy closes and drainage channels stabilize. 相似文献
347.
Moss and grass remains associated with a well-developed in situ palaeosol buried beneath a moraine ridge in front of Nigardsbreen (Jostedalsbreen ice cap, southern Norway) have been 14C-dated. Pollen preserved with the plant remains suggests the existence of an agricultural landscape prior to the deposition of the moraine. The calibrated dates and the pollen spectra are in close accord with historically-documented evidence for the timing and palaeoenvironment of the ‘Little Ice Age’ advance of the glacier. Considerable potential is indicated for estimating the maximum ages of moraine ridges and for palaeoenvironmental reconstruction where such documentary evidence does not exist. 相似文献
348.
康定杂岩时代及成因探讨 总被引:21,自引:0,他引:21
康定杂岩是康滇地轴上一套复杂岩系,由紫苏辉长质暗色杂岩、英云闪长质浅色杂岩及复成分混合杂岩构成。岩石结构构构造,铀、钍及稀土特表明,主体为岩浆杂岩,包含大量具层状痕迹的变质杂岩。大量的锆石U-Pb年龄及其它同位素年龄表明,岩浆岩属晋宁-澄江期。变质杂岩时代与河口群相当。 相似文献
349.
苏北榴辉岩的稀土元素地球化学及其成因讨论 总被引:13,自引:0,他引:13
江苏北部的榴辉岩出露于东海群变质岩系中,榴辉岩的产状是:1)呈似层状或透镜状产于东海群片麻岩中;2)呈不规则团块状产于超镁铁岩中,或与超镁铁岩伴生。全岩分析表明,前者相当橄榄拉斑玄琥岩或石英拉斑玄武岩,后者相当橄榄玄武岩或橄榄拉斑玄武岩。 相似文献
350.
Martin Wessels Angela Lenhard Federico Giovanoli Andreas Bollhöfer 《Aquatic Sciences - Research Across Boundaries》1995,57(4):291-304
Sediments in Lake Constance have been dated by radiometric methods and lamination counting. Investigations into the concentrations of lead and zinc in the sediments were carried out to get detailed information about environmental history. Maximum concentrations were found at the beginning of the 1960's. The lead maximum contamination occurred 3 years before the zinc maximum and has an unidentified source. The use of coal, leaded gasoline or the remobilization within the sediment could be excluded to result in the lead maximum. The fluctuations in zinc contamination could be explained with increasing and decreasing pollution (industry, coal burning, building of sewage plants). The greatest fluctuations of both metals occur during periods of great change in the economic history of Germany. 相似文献