The contribution of agricultural erosion to reservoir sedimentation in the Dominican Republic     

《Water Policy》2002,3(6):491-505

It is often assumed that the main cause of contemporary erosion and dam sedimentation in tropical watersheds is the cultivation of steep slopes. This study, using fallout ¹³⁷Cs to estimate net sediment loss since 1963 found erosion in the Nizao watershed of the Dominican Republic to be much lower than often reported for steep tropical hillsides. Because only 17% of the sediment moving into the reservoirs can be attributed to surface agricultural erosion, policies to eliminate upland agriculture will have limited benefits if the principal sources are of natural origin or associated with construction of the dams, and rural roads and trails.  相似文献   

Reconstruction of Alpine Cenozoic paleorelief through the analysis of caves at Siebenhengste (BE,Switzerland)     

《Geodinamica Acta》2002,15(5-6):261-276

The cave region of Siebenhengste, situated north of Lake Thun (Switzerland), contains one of the most important cave systems in the world, which extends from 500 to 2000 m a.s.l. It has a complex multiphase history. The recognized speleogenetic phases are related to spring level and to old valley floors. The six most recent phases were investigated in St. Beatus cave and Bärenschacht. They suggest a progressive Quarternary Aare valley incision to 890, 805, 760, 700, 660, and 558 m a.s.l. that is confirmed by statistical analysis of small caves. U/Th-datings of flowstone allowed a timing of the valley deepening phases: the valley bottom was at 760 m already before 350 ka, the one at 700 m was active between 235 and 160 ka. The cave morphology in the upper part of the cave system was coupled with sedimentological observations. This combination leads to the hypothesis that the uppermost (oldest) cave parts were already created in the Miocene, during and after the last deposition of the Molasse. Ideas about the evolution of the paleorelief suggest that today's Aare valley is a product of glacial erosion, and that the old Aare valley shifted its position several times between the Miocene and today.  相似文献   

Magmatic Gradients in the Cretaceous Caleu Pluton (Central Chile): Injections of Pulses from a Stratified Magma Reservoir     

《Gondwana Research》2002,5(2):307-324

The Caleu pluton (Central Chile) extending over 338 km² and with more than 1, 400 m of vertical relief intrudes the N-S trending Lower Cretaceous volcano-sedimentary and volcanic successions at a depth equivalent to a pressure of 2 kb. The host, stratified volcanic successions, are tilted about 30°–40° E, whereas the pluton shows paleomagnetic evidence of either tilting of <15° E or clockwise rotation by few degrees.A gradient of westward increasing SiO₂ content is recognized across the pluton, giving rise to three N-S elongated zones: Gabbro/Diorite Zone (GDZ), Tonalite Zone (TZ) and Granodiorite Zone (GZ). Biotite and hornblende compositions also exhibit a westward decreasing gradient in Mg/(Mg+Fe), indicating that the more mafic the zone is, the more oxidizing is its crystallization condition. Horizontal inward gradients of progressively less evolved rocks are recognized across GDZ and TZ, whereas no horizontal gradients were found in the GZ. Vertical compositional gradients are recognized in the GDZ and TZ, which consist of an upward increase in SiO₂ and decrease in MgO, FeO, Fe₂O₃, and compatible trace elements. A vertical compositional boundary was recognized along a traverse across the TZ separating two magma pulses with similar trends of compositional variations.The three zones of the Caleu pluton were derived from a common isotopically (Sr-Nd) depleted source. Each zone probably evolved independently, as their compositional characteristics would have not been acquired in situ. The resulting compositional characteristics of the zones would have been developed prior to the intrusion, in a subjacent stratified reservoir placed at about seven kilometers below the pluton.  相似文献   

Evaluating the timing of hydrocarbon generation in the Devonian Duvernay Formation: paleomagnetic,rock magnetic and geochemical evidence     

《Marine and Petroleum Geology》2002,19(3):275-287

The physical and chemical changes associated with the thermal maturation of organic-rich shale have affected the paleomagnetic and rock magnetic characteristics of the Devonian Duvernay Formation in the Western Canada Sedimentary Basin. This formation has several lithofacies that correspond to deposition in platform, slope and deeper water settings under varied redox conditions. Shale, laminated mudstone and some massive mudstone facies show evidence of magnetic changes associated with maturation but wackestone, packstone and some massive mudstone facies appear to be unaffected by the process. Rock magnetic evidence suggests that thermal maturation induces a change in the magnetization carrier from magnetite and hematite to solely magnetite.The packstone and wackestone facies commonly show a reversed characteristic magnetization with a paleopole at 194°E, 70°N (A₉₅=13.2) of Late Cretaceous-age. Shale and laminated mudstone facies in immature areas of the basin have inclination-only characteristic remanent magnetization (ChRM) means that range from 55 to 67 °C, requiring a pre-Cretaceous magnetization age. Shale and laminated mudstone facies in mature areas of the basin have a much steeper ChRM in direction ranging from 77 to 83 °C. Their very steep nature suggests that step demagnetization has not completely removed a drilling-induced remanence in some wells.  相似文献   

冀东秦家峪中元古界高于庄组锰矿成因: 来自矿物学和地球化学的制约          下载免费PDF全文

靳松  马鹏飞  郭华  余文超  许灵通  林茜 《地球科学》2022,47(1):277-289

冀东秦家峪锰矿赋存于中元古界蓟县系高于庄组二段底部含锰岩系内,其成因尚不明确.以秦家峪锰矿ZK58-2钻孔样品为研究对象,通过显微薄片观察、电子探针分析及全岩地球化学分析等方法,探讨了高于庄组锰矿的锰质来源和沉积环境对成矿的贡献.显微薄片观察、电子探针分析表明,原生矿带中含锰矿物主要为菱锰矿、铁镁菱锰矿、钙菱锰矿、锰方...  相似文献   

湖北恩施地区土壤-植物系统中Se元素的 地球化学特征   总被引：4，自引：0，他引：4  

郭宇  鲍征宇  马真真  李慧  魏昌华  严森 《地质通报》2012,31(1):151-155

在对恩施地区典型农业活动区——芭蕉、长平和鱼塘坝三地土壤中Se元素含量进行调查的基础上,通过与富Se土壤质量标准进行对比,发现除长平地区外,其余两地分别属于富Se和高Se地区,并通过土壤理化性质、形态分析和土壤中所生长的植物进行比较,初步研究了Se在土壤-植物体系中迁移转化的规律。结果表明：整体来看,鱼塘坝地区属于超富Se地区,植物中Se的含量也最高,长平地区属于非富Se地区,芭蕉地区则属于中等富Se地区;植物对Se的吸收受一定的土壤理化性质的影响,不同类型的植物对Se的吸收富集相差较大;形态分析研究表明,鱼塘坝地区土壤中生物可利用的Se含量较高,而芭蕉地区土壤中残渣态Se的比例较高,表现为Se的生物可利用性低于鱼塘坝地区。  相似文献   

Arido-eustasy: A new example of non-glacial eustatic sea level change     

《Gondwana Research》2019

Sea level fluctuation is common in the stratigraphical record and is partly explained by periodic polar glaciation. Such fluctuation in the absence of polar glaciation, during the warm periods of Earth's history, is poorly understood and currently under discussion. Here, I reconsider the palaeo-environmental information extracted from the variation in the carbon-isotope record and explain the phenomenon by coupling the levels of marine and fresh-water reservoirs through long-term wet and arid climate modes. The arido-eustasy model produces a standard anti-covariation pattern of marine organic and carbonate carbon-isotope records, which may appear throughout the geochemical record of the last 200 Myr. Under the model, orbitally forced periods of extremely wet climate result in environmental crisis, sea level falls, and carbon cycle perturbations, during which negative excursions in the marine organic carbon-isotope record of sediments containing matter from terrestrial vegetation should be understood as a proxy reporting periods of high precipitation rather than changes in the global carbon reservoir. I propose that the early Toarcian oceanic anoxic event is an example of arido-eustasy.  相似文献   

Recognition of Significant Surface Soil Geochemical Anomalies Via Weighted 3D Shortest-Distance Field of Subsurface Orebodies: A Case Study in the Hongtoushan Copper Mine,NE China     

Wang  Lifang  Wu  Xiangbin  Zhang  Baoyi  Li  Xuefeng  Huang  Anshuo  Meng  Fei  Dai  Pengyao 《Natural Resources Research》2019,28(3):587-607

Natural Resources Research - Quantitative prediction of concealed mineralization is always confronted with difficulties in comprehensive analysis between 2D and 3D data and between qualitative and...  相似文献   

The dispersal of the Gondwana Super-fan System in the eastern Mediterranean: New insights from detrital zircon geochronology     

《Gondwana Research》2014,25(3):1230-1241

We report here new LA-ICPMS detrital zircon U–Pb ages of a quartzite from the autochthon of Peloponnesus (Feneos locality), southern Greece. The rock classifies as a mature quartz arenite and belongs to an original shale–sandstone succession now metamorphosed into a phyllite–quartzite unit. Zircon age clusters at 0.52–0.75, 0.85, 0.95–1.1, 1.75–2 and 2.4–3 Ga point at the Saharan Metacraton and the Transgondwanan Supermountain as contributing sources; the youngest concordant grain is 522 Ma old. Our data collectively suggest deposition during the Cambro-Ordovician in a collisional setting and are in excellent agreement with those of the virtually intact Cambro-Ordovician sandstone–shale sequences of Libya (Murzuq and Kufrah basins) and the Middle East (Israel and Jordan), interpreted to have been deposited in the Gondwana Super-fan System which draped the northern Gondwanan periphery from ~ 525 to 460 Ma. By contrast, re-evaluating the available zircon age-distribution pattern and depositional setting of an analogous sequence forming the autochthon of north-central Crete (Galinos beds) we demonstrate that it was originally deposited in a completely different setting, i.e. in an accretionary/fore-arc complex outboard of the south Laurussian active margin (Pelagonia) during the Late Carboniferous. Comparing similar Cambro-Ordovician metasiliciclastic rocks from north-eastern Crete (Sfaka paragneiss), north-central continental Greece (Vertiskos terrane), north-western Turkey (central Sakarya terrane) and the Romanian Carpathians we show that their detrital zircon distribution patterns testify to an original depositional setting similar to that of Peloponnesus (Feneos), Libya and the Middle East. Using key time-frames from previously published palaeogeographic reconstruction models we are able to trace in space and time the Palaeozoic–Early Mesozoic wondering paths of the aforementioned sequences. Thus, time- and facies-equivalent rocks presently cropping out in the eastern Mediterranean share a common provenance from the Gondwana Super-fan System which was diachronously dispersed between Early Silurian and Early Triassic.  相似文献   

基于深度学习的钨钼找矿靶区预测方法研究     

蔡惠慧  朱伟  李孜轩  刘园园  李龙斌  刘畅 《地球信息科学学报》2019,21(6):928-936

随着矿产勘查工作由浅部矿向深部隐伏矿、由易识别矿向难识别矿发展,找矿难度日益增大,地质专家越来越重视新理论、新方法、新技术的应用。深度学习作为人工智能的前沿领域/技术,对于实现矿产资源预测“智能化预测评价”具有得天独厚的优势。本文以陕西省镇安县西部钨钼矿集区单元素化探异常原始数据为基础,提出了基于深度学习的钨钼矿产评价方法。该方法以归一化地球化学数据作为模型训练数据,通过深度学习中深度自编码网络方法实现异常值提取进而识别重点成矿有利地段,实现矿产资源找矿远景区定性预测。研究结果表明,在对957条单元素化探异常原始数据分类且做好模型标签后,整个过程在计算机的“黑盒子”中自动完成学习和预测,相较于传统预测研究方法,本文方法具有自动化程度高和客观性强的特征。此外,本文利用已知矿点构建训练数据集,采用随机森林方法对预测区进行矿产资源找矿靶区预测圈定,为进一步缩小找矿靶区范围提供科学依据。  相似文献