Recognition of ancient carbonate wind deposits: lessons from a modern analogue, Chrissi Island, Crete   总被引：1，自引：0，他引：1  

PIERRE LE GUERN  ERIC DAVAUD 《Sedimentology》2005,52(5):915-926

Carbonate aeolian deposits are common along arid to semiarid, wind-exposed, present-day coastlines bordered by productive carbonate ramps. Lithified carbonate dunes (aeolianites) have been described around the world in marine terraces of Quaternary age, but these deposits have seldom been identified in the Pre-Quaternary record. Several authors have suggested that this scarcity reflects that these deposits form and are preserved only during icehouse periods characterized by high-amplitude sea-level changes. Others [e.g. McKee and Ward Carbonate Depositional Environments (1983) , AAPG Memoirs, Vol. 33, pp. 131–170] suggest that the scarcity of aeolianites in the Pre-Quaternary record could reflect the ‘great difficulty in recognising wind blown carbonate deposits and in differentiating between them [aeolianites] and other carbonate sands of nearshore environments’. It has been considered that carbonate shoreface/foreshore deposits are very difficult to discriminate petrographically from backshore deposits. This petrographic study of recent sediments from the shoreface to backshore along the northern coast of Chrissi Island, Crete, confirms that carbonate aeolian sands can be very easily misinterpreted as shoreface deposits. Textural examination of thin sections by image analysis techniques indicates, however, that grain orientation patterns differ between facies. Shoreface deposits exhibit a unimodal distribution of grain orientation (flat rose diagram), whereas backshore deposits show a tendency towards a bimodal distribution with a significant proportion of vertical grains. This observation has been confirmed in Pleistocene aeolianites from Tunisia and Western Australia. Grain verticality thus seems to be a reliable criterion for discriminating wind-lain carbonate bodies from shoreface deposits. Vertical grains in aeolian carbonate deposits could reflect gravity effects (e.g. reorientation of grains because of meteoric water percolation and air pull-up). Laboratory experiments conducted on carbonate sands under the action of percolating waters confirm this hypothesis. This reorganization process is preferentially developed in recently deposited and loosely packed sands resulting from grainfall and/or grainflow. In addition, this suggests that the presence of vertical grain orientation might be an indicator of the frequency and intensity of rainfalls during deposition.  相似文献   

Review of paleoseismological and active fault studies in Taiwan in the light of the Chichi earthquake of September 21, 1999   总被引：1，自引：0，他引：1  

Yoko Ota  Yue-Gau Chen  Wen-Shan Chen 《Tectonophysics》2005,408(1-4):63

This paper reviews the research on active and earthquake faults in Taiwan conducted prior and after the 1999 Chichi earthquake. The Chichi earthquake plays as a turning point of the relevant studies, since the 1999 coseismic surface rupture exactly follows preexisting fault scarps, created in turn by previous seismic events along the Chelungpu Fault. This fact indicates that the precise mapping on the other active faults is fundamental to predict the location of surface rupture caused by large future earthquakes. Since 1999, many trenching studies have been carried out along the Chichi earthquake fault. A few of them demonstrates that the penultimate event is as young as probably only 200–430 years old; however, some others show a rather old age of several hundreds years or even older for the last faulting event before 1999. More trenching studies are necessary for such a long fault in order to understand the possible segmentation features and the correlation of the paleoseismic events identified along the entire fault length. In addition, we further discuss the offshore faulting associated with seismic event along the eastern coast of Taiwan, where the multiple Holocene terraces are well known.  相似文献   

河北平原地下水氦氩同位素特征   总被引：1，自引：1，他引：1  

刘存富  晁念英  王佩仪  甘义群  李延河  宋鹤彬 《地球学报》2005,26(Z1):268-271

通过对河北平原地下水氦同位素进行分析比较，根据过剩He(4He exc)、3He/4He比值、δ3He和36Ar/38Ar及40Ar/36Ar值分析认为，河北平源地下水氦氩同位素有7个特征；①地下水中过剩He浓度沿地下水的流向而增高；②地下水中过剩He浓度随着地下水埋深加大而增高；③满城-沧州剖面上过剩He浓度大于石家庄-衡水剖面上的过剩He浓度；④河北平原地下水主要是由大气隆重水补给的；⑤衡水热水过剩He浓度很高（>674.83×10-8cm3STPg-1水）；⑥地下水的36Ar/38Ar比值平均值为5.37，非常接近地球大气的比值（5.35）；⑦地下水的40Ar/36Ar比值从296-412，均比大气氩的40Ar/36Ar比值（295.6）大，这表明40Ar都是放射成因的，且具有“年代积累效应”。  相似文献   

平原区地表水与地下水联合调控模型研究   总被引：2，自引：0，他引：2  

曲兴辉  谷秀英 《水文》2005,25(4):23-25,38

平原区三水转化关系强烈，由地表水和地下水形成了统一的水资源系统。本文研究分析了基于费用准则和供水量准则的地表水与地下水联合调控模型，以合理开发、优化调度本区的水资源．充分发挥该区地表水库和地下含水层空间的联合调蓄作用，提高水资源的利用率，达到最佳的社会效益、环境效益和经济效益。  相似文献   

广东省沿海经济带区域差异探析     

刘志霞  张加恭 《云南地理环境研究》2006,18(6):57-62

广东省沿海经济带的发展对整个广东省的经济有极大的影响。目前，其整体水平高于国内其他省份的沿海经济带，但是其内部区域差异较大。主要表现在经济总量、经济发展水平、产业结构等单要素方面。采用聚类分析法划分了广东省沿海经济带的结构层次，并分析不同层次的特点。进而揭示了广东省沿海经济带巨大的区域差异给整个区域发展所带来的问题。最后，在分析区域差异及其负面影响的基础上，提出广东沿海经济带协调发展的几点对策。  相似文献   

Decentralization of the Nation's Main Street: New Coastal‐Proximity‐Based Portrayals of Population Distribution in the United States, 1950–2000     

Christopher J. Henrie  David A. Plane 《The Professional geographer》2006,58(4):448-459

Almost half a century has passed since Jean Gottmann coined the term “Megalopolis” in reference to the almost continuously urbanized stretch of land spanning the East Coast of the United States from southern New Hampshire to northern Virginia. Because a disproportionate concentration of population resided in this Megalopolis, the northeastern core enjoyed an economic and cultural supremacy, and he termed the Megalopolis “The Main Street of the Nation.” By the later 1960s and 1970s, however, population migration patterns began to reflect the influence exerted by the emergence of a second national core centered on the large metropolitan areas along the Pacific Coast, especially those of the Los Angeles and Bay Area conurbations in California. Although of different character, this burgeoning concentration of population, economic activity, and cultural influence may reflect the development of a West Coast Megalopolis that could soon rival the original Megalopolis of the Northeastern Corridor. Today, the U.S. population distribution is largely a bicoastal one. This article documents the emergence of this bicoastal population distribution. Using historical census data and GIS technology, we present a number of novel ways to graphically portray and examine this population redistribution phenomenon. The United States is not unique in witnessing an increasing share of its inhabitants clustering in coastal zones. Current critical policy concerns about the worldwide vulnerability of coastal populations have focused the need for better coastal population estimates and better mapping methods for portraying population redistribution trends.  相似文献   

冲积平原区土壤中稀土元素分布模式及其示踪作用     

马生明  朱立新  王之峰  刘崇民  胡树起 《物探与化探》2006,30(2):100-103

根据科研生产工作的实际需要,借鉴稀土元素分布模式示踪物源的方法,对冲积平原景观区土壤中稀土元素的分布模式进行了系统研究。结果发现冲积平原区土壤中稀土元素分布模式总体规律大同小异,表明冲积平原区成壤母质特性基本相同。利用稀土元素分布模式的细微差异能够有效判别成壤母质特性的变化。  相似文献   

Analyzing the dynamics of Dafeng coastal wetland based on remote sensing image     

Yuezhen SHI Dongjun XIN 《中国地球化学学报》2006,25(B08):213-213

Dafeng wetland is the largest coastal wetland in Asia, located on the shore of the Yellow Sea. Recently, inning, irrationality utilizing, and water contaminating all threaten it, so the area of the wetland is decreasing and its quality is declining increasingly. Therefore, it is significative to analyze the dynamics of wetland with RS image of Dafeng coastal wetland. To enhance the clustering, fuzzy c-mean algorithm is improved and used to cluster Dafeng coastal wetland. Through the improvement, the problem is solved that assortment is not very good bringing by the difference of sample number. Furthermore, changing information about Dafeng coastal wetland is analyzed by dynamic degree model and the check module of software erdas is used to analyze the dynamics of wetland. Integrating local data with improved fuzzy c-mean algorithm and dynamic degree model mentioned above, dynamics degree of Dafeng coastal wetland＇s area is calculated. The calculating result shows that the area of tour and residence increases greatly while that of bulrush and grassplot decreases largely, that of woodland changes little. The conclusion accords with the fact the main factors include natural and man-made ones, which lead to Dafeng coastal wetland resource change. From nature, coast expansion caused by bedload sediment makes wetland away from sea, disappearing finally. On the other hand, many tour projects are constructed, including training center, reception center, education center, etc; Part of the wetland is utilized as residence, pound, farm, salt field, etc. All of those account for about half of the wetland. Further, fertilizer, pesticide and machine noise contaminate the wetland environment. Finally, problem of management is one of the main restriction factors. Managing manner of traditional agriculture, growth of tourism and expansion of coast are the main driving factors for wetland changes.  相似文献   

Characteristics of natural low pH groundwater in the coastal aquifers near Beihai, China     

Xun ZHOU Rui LI Hua ZHANG Li ZHANG 《中国地球化学学报》2006,25(B08):228-228

Groundwater with low total dissolved solids （less than 200 mg/L） occurs in the unconfined and confined aquifers near Beihai in southern Guangxi, China. Under natural conditions the groundwater ranges in pH from 3,33 to 7,0 with an average value of 5.12. The pH in the unconfined aquifer varies from 3.67 to 7.0 with an average value of 5.17 and the pH in the confined aquifer is in the range 3.33-6.97 with an average value of 5.07. The pH in the groundwater does not show significantly increasing or decreasing trends with time. Fluctuations in pH exist at some of the monitoring wells and the pH in groundwater is a bit higher in the rainy season than in the dry season, Monitoring data show that the pH in rainwater is higher than in groundwater in the unconfined aquifer, whereas the pH in the latter is a bit higher than in the confined aquifer. A slightly decreasing trend in the pH was also found from the inland area to the coastal zone. The occurrence of weakly acidic groundwater in this area is attributed to the recharge from low pH precipitation and the multi-aquifer and leaky groundwater system.  相似文献   

Morphology and stratigraphy of the late Quaternary lower Brazos valley: Implications for paleo-climate, discharge and sediment delivery     

Dennis A. Sylvia  William E. Galloway   《Sedimentary Geology》2006,190(1-4):159-175

A shallow coring and geophysical logging program has recorded the sedimentary fill of the Brazos River valley in the Texas Gulf Coastal Plain. Thermoluminescence dates together with new and recalibrated published radiocarbon dates show the valley fill to include extensive, sandy, buried falling stage and lowstand Oxygen Isotope Stage (OIS) 3 and 2 deposits. These alluvial deposits are punctuated by numerous paleosoil horizons that record alternating periods of cutting, bypass and accumulation. Maximum valley incision and two periods of terrace formation preceded marine lowstand conditions, suggesting significant discordance between preserved fluvial and classical marine system tracts. The latest Pleistocene incision and fill history appears related to cycles of increased discharge and incision, followed by system equilibration and terrace formation. Analysis of the Brazos River incised valley and its contained paleochannels indicates that latest Pleistocene mean annual discharge was as much as four times greater than that of today. This magnitude of discharge in the Brazos would require a two-fold increase in precipitation across the drainage basin. Such an increase is comparable to the present day measured positive El Niño winter precipitation anomaly across the region. Paleochannel geometries and the stratigraphic and sedimentologic data from this investigation support the hypothesis that periods of high-amplitude, El Niño-like climatic perturbations characterized the late Quaternary climate of the south-central and southwestern U.S. This period of high discharge coincides, at least in part, with late OIS 3 progradation of the Brazos delta to the shelf margin, OIS 3 and 2 valley incision across the Texas shelf, and concomitant sand bypass to intraslope basins beyond the shelf edge.  相似文献