埃及Faiyum盆地沉积物中粒度和磁化率对风沙活动的指示意义

埃及Faiyum盆地是尼罗河下游的泛滥平原,沉积物主要来自尼罗河和撒哈拉沙漠.本文通过对钻孔FYAM(孔深4.6m)沉积物的粒度、磁化率的分析,并结合石英形态、石膏和碳酸盐含量分析,旨在寻找Faiyum盆地沉积物中代表风沙活动的粒度、磁化率特征.研究结果显示,钻孔沉积物中共发现7种粒度频率曲线类型,主要为细粒组(6-20μm)和粗粒组(80-100μm)两个峰值组合而成的单峰或双峰曲线.除去人类活动影响的高磁化率层以及高碳酸盐含量导致的低磁化率层外,其余层位沉积物的磁化率随着曲线类型的不同呈现规律性变化,当80-100μm峰为主峰时,磁化率明显偏低,风成石英颗粒含量升高,代表着风沙活动强烈,当6-20μm的峰为主峰时,磁化率相对较高,风成石英颗粒含量减少,意味着风沙活动减弱,以尼罗河物质输入为主.

湖泊沉积物磁学特征指示的埃及Faiyum盆地全新世以来的环境演变

利用埃及北部Faiyum盆地获得的高取芯率沉积物岩芯,进行沉积物多种磁性参数的测量,结合有机碳、介形虫、粒度等分析,在AMS14C加速器测年的基础上,建立全新世以来湖泊沉积物磁性特征变化的时间序列.结果表明,粒度效应以及沉积后的各种次生作用对沉积物的磁性特征没有明显的影响,磁性变化主要反映了沉积物不同来源组成的相对变化.全新世前沉积物磁性较弱,主要含不完全反铁磁性矿物,与周边沙漠的物质相似,结合其粒度特征,沉积物来源应以近源物质为主.全新世早中期(约10 5.4 ka BP)沉积物磁性变化相对稳定,有机质含量也较高,指示了来自尼罗河较为稳定的物质供应;而大约5.4 ka BP尤其最近约4.2 ka BP以来,磁性的明显变化反映了流域降水减少情况下,来自青尼河物质贡献的相对增加;最近约2.0 ka BP以来沉积物的磁性变化,则更多地与盆地人类活动的强化有关.总体而言,Faiyum盆地全新世以来的环境演变主要受控于全新世以来尼罗河与盆地的水力学联系.即:全新世前盆地未与尼罗河连通时,沉积物主要来源于周边沙漠的风成物质;而受全新世早-中期来流域季风降水增加的影响,泛滥的尼罗河为盆地提供了相对稳定的物质供应,湖泊也处于高湖面;全新世晚期以来,随着流域干旱化的加剧,尼罗河与盆地的连通性开始减弱,来自高磁性的青尼罗河物质贡献开始相对增加.最近约2.0 ka BP以来,虽然仍有人工运河连接尼罗河与盆地,但沉积物磁性的显著变化更多地反映了盆地人类活动的不断强化.

XRF Analysis of Heavy Metals for Surface Soil of Qarun Lake and Wadi El Rayan in Faiyum, Egypt

The environmental pollution with some heavy metals for twenty four surface soil samples collected from Qarun Lake and Wadi El Rayan region in Faiyum, Egypt utilizing X-ray fluorescence (XRF) spectroscopy was measured. The concentrations of 13 elements Cr, Ni, Cu, Zn, Zr, Rb, Y, Ba, Pb, Sr, Ga, V and Nb were determined. The elemental concentrations were compared with the normal values and other studies in different locations from the world. The correlation between elements appears that pollution inside the investigated lake and Wadi result from different sources of contamination present inside them. The results establish a database reference of radioactivity background levels around these regions.

Relevance of AEM and TEM to Detect the Groundwater Aquifer at Faiyum Oasis Area, Faiyum, Egypt

This study investigates the groundwater aquifer located in Fayuim oasis. In this study, two of the electromagnetic measurement methods have been used in determining the hydrological situation in the Fayoum oasis. The first is airborne electromagnetic (AEM) which, sometimes is referred to as Helicopter electromagnetic (HEM) and the second is ground Time-domain Electromagnetic method (TEM). The subsurface consists of four geoelectrical layers with a rough slope towards the center. The third and the fourth layers in the succession are suggested to be the two-groundwater aquifers. The third layer saturates with fresh water overlying saline water which exists in the bottom of the second one. It is worth mentioning that the depth of the fresh water surface undulates between the surface level in two lakes in the study area and 57 meters below the ground, whereas the thickness of the fresh water aquifer varies from 13 to 36 meters. The depth of the saline water surface undulates between 59 and 81 meters below the ground. In general, airborne electromagnetic surveying has the advantage of fast resistivity mapping with high lateral resolution. Groundbased geophysical surveys are often more accurate, but they are definitely slower than airborne surveys. It depends on targets of interest, time, budget, and manpower available by the method or the combination of methods that will be chosen. A combination of different methods is useful to obtain a detailed understanding of the subsurface resistivity distribution.

Structural and Tectonic Evolution of El-Faiyum Depression,North Western Desert,Egypt Based on Analysis of Landsat ETM+,and SRTM Data

False color Landsat ETM＋ （bands 7, 4, 2 in RGB） and SRTM images of the EI-Faiyum depression, Egypt, highlights major NE-SW faults and other lineaments trending NW-SE and N-S. Airborne magnetic data reveal some E-W subsurface faults which are not recognized on Landsat ETM＋ and SRTM images. Ratio images （5/7, 3/1, 4/3） and （5/7, 5/1, 4） were used for lithological discrimination of different rock types. The EI-Faiyum area is occupied by sedimentary rocks of Middle Eocene, Upper Eocene, Oligocene, Lower Miocene, Pliocene, Pleistocene and Holocene. Understanding the structural regime of EI-Faiyum depression in the Western Desert of Egypt is significant in relation to the neotectonics of the Nile Delta and to the development of new communities west of the delta. The area is covered by Late Cretaceous, Middle to Late Eocene deposits, Oligocene deposits （comprising fluviatile sediments and basalt sheets） and Pliocene and Quaternary alluvium and sand deposits. New geological and structural maps were prepared at a scale of 1 ： 100 000 using Landsat ETM＋ and SRTM images coupled with extensive field work. The major structural trends are E-W, ENE, NS and NW--the same trends observed elsewhere in North Africa from NW Sudan to NE Egypt including north and central Sinai. EI-Faiyum depression is a structurally-controlled tectonic basin, marked by northeast-striking faults along its northern margin. These faults are parallel to similar faults of the Syrian arc fold belt, which includes a group of related faults, folds, push-up structures, and basins. Qarun Lake is located in the northern part of EI-Faiyum depression, at a right-step between two strands of the NE striking fault system. We have suggested elsewhere that EI-Faiyum depression is a pull-apart basin related to extension between these mismatched strands of strike-slip faults. The E-W structures are probably related to the relative motion between south Europe and north Egypt and closure of the Neotethys. The continuation of these faults in the present area is only detected from magnetic and seismic studies. The question that needs to be raised is why these old structural features have been reactivated in recent times. Thus, the presence of these fault zones must be heeded in all development plans for EI-Faiyum region.