Coastal conglomerates around the Hadjer el Khamis inselbergs (western Chad,central Africa): new evidence for Lake Mega‐Chad episodes     

Mathieu Schuster  Philippe Duringer  Jean‐Francois Ghienne  Patrick Vignaud  Alain Beauvilain  Hassan Taisso Mackaye  Michel Brunet 《地球表面变化过程与地形》2003,28(10):1059-1069

This paper reports on a study dealing with the rhyolitic inselbergs of Hadjer el Khamis that formed palaeoislands during Lake Mega‐Chad events. Field observations have shown that: (1) conglomeratic patches of immature to mature clasts are preserved at the feet of the Hadjer el Khamis inselbergs; (2) in cross‐section, their pro?le reveals a well de?ned cliff–platform junction at a constant elevation (325 m). The monolithological clasts show all degrees of roundness, from angular cobbles to well rounded pebbles. This wide range of maturity suggests a coastal origin for these cobbles. The system was permanently fed with angular clasts, which were progressively worn by waves. Cobbles that were wave‐worked for the longest time are the best rounded. The cliff–platform junction is the result of erosion by waves, which attacked and undercut the inselberg cliffs during Lake Mega‐Chad events. Asymmetrical erosion pro?les moreover suggest a wind regime dominated by SW to NE oriented winds. These interpretations have two implications. The elevation of the cliff–platform junction is an indication of the highest water level of Lake Mega‐Chad at 320–325 m, which is in agreement with other observations elsewhere in the basin. The SW to NE oriented winds show that monsoon‐related winds were prevalent during Lake Mega‐Chad events, suggesting the Inter‐Tropical Convergence Zone was located higher in latitude than today. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

山前冲积平原地下水开发利用及保护探讨     

孔波  任召水  辛武允  李凤鸣 《地下水》2003,25(1):26-27

本文以山东省宁阳县城东区地下水开发利用情况为例。科学分析了山前冲积平原大量开采地下水引起的区域地下水环境变化，并提出了遏制地下水环境恶化的措施，采取节流与补源并举，严格控制地下水超采。  相似文献   

豫北平原环境水文地质问题分析     

李中明  程建强 《地质灾害与环境保护》2003,14(3):21-25

通过对豫北平原地下水的综合调查，发现其环境水文地质问题以区域地下水位下降为主，并又导致了泉水消失、湿地减少、地面沉降和地下水污染等次生环境水文地质问题的发生。在环境水文地质问题形成背景和形成条件综合分析的基础上，认为其产生的根本原因是人为活动尤其是地下水的不合理开采破坏了地下水循环系统的天然均衡状态，其演化模式具双向性。最后，提出了减少和防止豫北平原环境水文地质问题的具体措施。  相似文献   

Field measurement and numerical modelling of aeolian mass flux distributions on a sandy beach   总被引：6，自引：0，他引：6  

Steven L. Namikas 《Sedimentology》2003,50(2):303-326

ABSTRACT The vertical and horizontal distributions of aeolian mass flux were measured at Oceano Dunes, California, and these data were used to evaluate a numerical model of saltation. Grain‐size analyses showed that the distributions of the modal sediment size class corresponded closely to those of the total sediment population, and modelling thus focused on replicating the distributions of the mean grain size. Although much previous work has assumed that the mean launch speed of saltating particles varies in proportion to shear velocity, simulations using a constant mean launch speed were found to yield the closest approximations to the mass flux distributions observed in the field. Both exponential and gamma distributions of launch velocity produced realistic simulations, although the latter approach required the inclusion of an additional reptation component to achieve good results. A range of mean launch angles and an equivalent sphere correction were also found to generate comparable results, providing the other input parameters could be varied freely. All the modelling approaches overestimated the proportion of mass flux occurring at the bottom of the vertical distributions, and underestimated the proportion occurring at the upwind end of the horizontal distributions. No theoretical shortcoming that would account for these small, but systematic, discrepancies could be identified, and experimental error thus represents a more plausible explanation. The conclusion that mean grain launch speeds are essentially constant and independent of shear velocity suggests that the additional kinetic energy extracted by grains under more energetic wind conditions is largely transferred to the bed, and that increases in the transport rate are therefore driven primarily by the ejection of additional grains. It is suggested that the kinetic energy of rebounding grains is constrained by the ability of the bed to resist deformation, equivalent to a plastic limit. Hence, grains of larger mass (diameter) rebound from the bed at lower speeds, and follow shorter, lower trajectories, as has been widely observed previously.  相似文献   

300年以来太行山前平原地下水补给演化特征与趋势   总被引：6，自引：5，他引：6       下载免费PDF全文

张光辉  费宇红  王金哲  陈宗宇  聂振龙 《地球学报》2003,24(3):261-266

从近300a来太行山平原典型区地下水入渗补给演化特征、机制和未来50a区域水循环中水分通量演化趋势3个方面，阐明了山前平原地下水补给能力的周期可变性和该区现状资源型缺水的客观性。在此基础上，提出了进一步开展百年尺度水循环演化趋势基础研究的建议。  相似文献   

北方中小滨海城市旅游发展条件分析   总被引：1，自引：0，他引：1  

朱峰  陈伟  韩国圣 《地域研究与开发》2003,22(6):85-88

文章以北方中小滨海城市为研究对象，比较了其作为旅游城市同南方滨海城市在城市景观、适宜旅游期和旅游市场等方面的不同之处，分析了其与大城市相比在旅游发展方面存在的一系列制约因素，认为，北方中小滨海城市的资源和市场条件较好，接待和推广条件不足。  相似文献   

人为活动条件下河北平原第四系地下水系统的演变   总被引：5，自引：0，他引：5  

徐恒力  肖国强  李红 《地质科技情报》2002,21(1):7-13

用地下水系统理论和地下水系统分析方法，从全新的视角对河北平原地下水系统的介质空间、流场、水化学场的结构与特征进行了分析，在此基础上探讨了人为活动条件下地下水系统的演化，从而为该地区水资源评价、开发与管理、环境地质问题的研究与防治提供了新的理论依据。  相似文献   

倾斜细土平原盐渍土分布规律及盐胀作用分析   总被引：1，自引：0，他引：1  

刘南山 《水文地质工程地质》2002,29(4):40-42

本文通过实例分析了倾斜细土平原盐渍土分布规律及盐胀性，盐类分布在平面上呈不连续性和不均匀性，中游以硫酸盐、亚硫酸盐为主，向下游逐渐变为以氯盐、亚氯盐为主，盐胀作用也由强变弱。  相似文献   

Integration and influence of paleoseismic and geologic data for the seismic hazard evaluation in the Catalan coastal ranges, Spain     

Aníbal Ojeda  Kuvvet Atakan  Eullia Masana  Pere Santanach  María-Jos Jimnez  Mariano García-Fernndez 《Soil Dynamics and Earthquake Engineering》2002,22(9-12):911-916

In this study, the influence of paleoseismic and geologic data in the seismic hazard estimation for the Catalan coastal ranges is analysed. We computed the probabilistic seismic hazard using area seismic sources with a Poissonian assumption for the earthquake occurrence. For the computations, a previously published attenuation relationship based on European strong motion data was applied. The resulting hazard estimates show similarities to the previous assessments in the region. These results were then used as a reference for comparison with other new models. In order to analyse the influence of the paleoseismic data three different models were tested. Since the number of faults that are investigated in detail are few, the same area sources that were used in the Poissonian assumption were kept in all three new models. In addition, the new paleoseismic data with faults expressed as line sources were used. In this case, a cyclic earthquake occurrence was assumed. The three models were based on the paleoseismic data with different assumptions on the time elapsed since last event. The time elapsed was set to 0, 10 and 85% of the recurrence interval in each model. The results are presented as maps showing the difference between the three models and the reference model with the Poissonian assumption. The results are given in horizontal peak ground acceleration contour maps for different return periods, also taking into account large return periods as high as 25,000 years. This is done to demonstrate the effect of large recurrence intervals found for some of the active faults. In general, we observe that for short return periods (<1000 years), the Poissonian assumption of earthquake occurrence is probably sufficient and provides a robust estimate of the hazard. However, for longer return periods (>5000 years) the effects of the paleoseismic data become increasingly significant. In order to estimate the true seismic hazard potential of this apparently low seismicity area, long-term behaviour of the possible active faults in the region needs to be investigated systematically.  相似文献   

Evidence for 17th-century tsunamis generated on the Kuril–Kamchatka subduction zone, Lake Tokotan, Hokkaido, Japan     

Yuki Sawai   《Journal of Asian Earth Sciences》2002,20(8)

In the seventeenth century, two tsunamis that were generated by earthquakes on the Kuril–Kamchatka subduction zone inundated the eastern coast of Hokkaido, northern Japan. Stratigraphic evidence for these two tsunamis and related land-level change in coastal Hokkaido consists of two landward-thinning sand layers in the sediments of Lake Tokotan, a coastal lagoon on the Hokkaido coast. The marine origin of these sand layers is indicated by the presence of brackish–marine diatoms. The rarity and high degree of fragmentation of diatom valves suggests that the sands were transported in a short time over a considerable distance. Tsunamis at this site were probably generated by great earthquakes along the Kuril–Kamchatka Trench. Volcanic ash deposits lying just above the sands suggest that tsunamis occurred in the late 17th century. Tsunamis during the historic period are not recorded in Lake Tokotan, which suggests that the sand layers were deposited by tsunamis substantially larger than historic tsunamis.  相似文献