Study of weathering processes developed on old piedmont surfaces in Western Spain: new contributions to the interpretation of the “Raña” profiles     

E. Molina Ballesteros  M. Cantano Martín   《Geomorphology》2002,42(3-4)

The Hercynian basement of the Iberian Peninsula was uplifted by the Alpine orogeny during the Tertiary. It gave rise to a set of block mountains and tectonic grabens, one of which is the Ciudad Rodrigo Basin. It is located in Western Spain and forms a westward extension of the great Tertiary Duero Basin. The sediments filling this graben are of continental origin, their ages ranging from Palaeogene to Quaternary. Morphologically, the southern part of this basin forms a set of piedmont surfaces (the “Raña” surfaces) appearing above the terrace system of the present rivers. This paper examines the weathering processes developed over these old piedmont surfaces using micromorphological, XR diffraction, scanning electron microscopy (SEM) and microporosity techniques.The Raña sediments are rich in quartzite pebbles and gravel within a clayey matrix. Once deposited, these materials underwent important in situ weathering processes under somewhat hydromorphic conditions, of which hydrolysis, ferrolysis and xerolysis were the most important. All these processes gave rise to: (1) transformation of most of the clasts of shists and slates into a matrix causing the destruction of the original sedimentary structures and a relative concentration of the resistant lithologies (quartzites and quartz); (2) important changes in the clay fraction, leading to a predominance of kaolinite in the upper levels of the profiles, (3) release of elements from primary minerals, Fe being one of the most important, and (4) redistribution of matter, mainly clay and Fe oxyhydroxides, within the profiles.The coexistence of seasonal periods with pF higher than 4.2 repeated over a long time, together with poor internal and external drainage conditions, are the cause of the special features displayed by Raña deposits.  相似文献   

Morphological variation and reproductive biology of Polaskia chende (Cactaceae) under domestication in Central Mexico     

Martha Cruz  Alejandro Casas 《Journal of Arid Environments》2002,51(4):561

The columnar cactus Polaskia chende, endemic to Central Mexico, occurs in the wild and in silviculturally managed populations where artificial selection for better edible fruits operates by sparing and favouring desirable phenotypes during vegetation clearance. Wild and managed populations were compared in morphology and reproductive biology to analyse if human selection has influenced phenotypic divergence, if it has modified breeding systems and if reproductive isolation exists between them. Fruits from managed populations were larger (6·36±0·17 cm³), heavier (19·62±1·32), sweeter (9·07±0·25 °Brix), and with thinner peel (2·09±0·13 mm) than those from wild populations (5·77±0·18 cm³, 15·73±0·85 g, 8·23±0·31 °Brix, and 2·58±0·14 mm, respectively). Both population types bloom synchronically, show diurnal anthesis, with bees as the most likely pollinators, and have breeding system predominantly self-incompatible. Spatial and temporal barriers to pollen exchange between wild and managed populations are unlikely, and morphological differences would be related with an ongoing process of artificial selection.  相似文献   

Distribution and effectiveness of nivation in Mediterranean mountains: Peñalara (Spain)     

David Palacios  Nuria de Andrs  Enrique Luengo 《Geomorphology》2003,54(3-4):157-178

Snow accumulation is responsible for geomorphic and biogeographic processes taking place in the southern sector of the Peñalara massif in central Spain (40°51′N, 3°57′W; max. altitude 2428 m at Pico de Peñalara). This work compares the intensity of nivation on the eastern slope, leeward of the prevailing westerly winds and heavily eroded by glacial activity during the Pleistocene, to that of the western slope on the windward side, unaffected by glacial erosion and completely covered by a thick weathering mantle. On the eastern slope, nivation is effective only where the weathering mantle is exposed or on morainic formations. It does not occur on the landforms derived from glacial erosion. In contrast, the western side shows almost no evidence of snow action except where catastrophic mass movements have altered the regularity of the slope. During the post-glacial epoch, nivation cirques formed in the scars left by mass wasting. In the last 30 years, spring temperatures have increased and this activity has diminished. The pattern of evolution observed at Peñalara can be extrapolated to other Mediterranean mountains with similar characteristics such as marginal glacial activity during the Pleistocene, unconsolidated formations on the summits caused by chemical weathering, and dry, hot summers that can increase the effectiveness of nivation.  相似文献   

大洋多金属结核中1nm锰矿相的相变及其主要控制因素研究   总被引：1，自引：3，他引：1  

冯旭文  钱江初  张培志 《矿物学报》2003,23(2):109-114

多金属结核样品在室温中放置或加热干燥后 ,其中的 1nm锰矿相会部分或全部地相变为 0 7nm锰矿相 ,这说明 0 7nm锰矿相可能不是多金属结核的原生锰矿相 ,而仅仅是 1nm锰矿相的相变产物。研究结果表明 :1nm锰矿相结构中的Cu2 + 、Co2 + 、Ni2 + 等金属阳离子的含量与其结构稳定性有正相关关系 ,即金属离子含量越高 ,其结构越稳定 ,1nm锰矿相越不易相变为 0 7nm锰矿相 ,反之亦然 ;另外 ,不同金属阳离子稳定 1nm锰矿相结构的能力也不同 ,结核中几种主要金属阳离子对 1nm锰矿相结构的稳定能力从大到小为 :Ni2 + >Cu2 + >Co2 + >Zn2 + >Ca2 + =Mg2 + 。  相似文献   

对天然地基承载力计算公式的理解和应用   总被引：1，自引：0，他引：1  

毛群芳  徐四一 《上海国土资源》2003,(3):26-29

文章就新老国标和上海标准对地基承载力的计算公式进行了分析比较,并认为在上海地区岩土工程勘察报告中提供天然地基承载力设计值fd更适合。  相似文献   

灰色系统理论预测大气氮氧化物污染的应用   总被引：11，自引：0，他引：11  

茅均标  石健  刘刚  李继明 《华东地质学院学报》2003,26(1):38-40

根据中国南方某省所辖城市近年来N0x污染指数的测量数据的统计资料，建立了GM(1，1）灰色预测模型，通过预测探讨了大气中氮氧化物指数的变化趋势。所建模型残差仅1．17％，关联系数为0．9067，精度较高，具有较好的可行性和适应性，可以为管理规则提供决策依据。  相似文献   

WZY-1型钻参仪的研究与应用   总被引：1，自引：0，他引：1  

补家武  补生蓉  李吉春 《探矿工程》2003,30(2):35-37

分析了目前钻参仪未能普遍推广应用的原因。介绍了WZY－1型钻参仪的总体构成、测试参数、性能特点及其分别与钻机和水泵的一体化设计。  相似文献   

灰色系统在地面沉降分析中的应用   总被引：4，自引：0，他引：4  

宫相霖  董荣鑫 《上海国土资源》2003,(3):16-21

该文将灰色系统理论引入地面沉降的研究之中。根据上海地面沉降的历史数据建立了上海地面沉降发展的GM(1,1)模型以及地面沉降与地下水位变化的映射GM(1,2)模型,最后运用所建立的模型对地面沉降的发展态势进行了预测。  相似文献   

内蒙古东部森林沼泽区1∶20万区域化探工作回顾     

叶柱才  王会锋 《物探与化探》2003,27(6):423-424

通过对内蒙古东部森林沼泽区开展1∶20万区域化探4个不同阶段工作方法技术及效果的回顾和总结,认为在不同时期的工作方法技术随着对森林沼泽区这一特殊景观的认识不断加深,其工作方法技术也在不断改进,发现矿点8处、矿化点17处,其中,银锌矿17处、金矿1处,取得了较好的地球化学勘查成果,所完成图幅较客观地反映了工作区区域构造、成岩成矿作用的表层地球化学面貌,大大提高了地球化学信息的可靠性和真实性。  相似文献   

灰色GM(1,1)模型最优维数的研究   总被引：2，自引：1，他引：2  

黄咏庆  张勤  张至峰 《水文地质工程地质》2003,30(1):33-35

灰色GM(1,1)模型的维数对模型的预测精度有很大的影响,本文通过自编的FORTRAN程序,在合适的范围内对GM(1,1)模型的维数进行自动优化,通过对宁波市地面沉降数据的计算分析,得到了C和P的定性关系,确定了优化的评价标准,维数最优化后的模型能反映预测数列变化幅度的大小。  相似文献