Metallogenesis of cratonic oolitic ironstone deposits in the Bled el Mass,Azzel Matti,Ahnet and Mouydir basins,Central Sahara,Algeria   总被引：1，自引：0，他引：1  

Dr. Salah Guerrak 《International Journal of Earth Sciences》1987,76(3):903-922

The Bled el Mass, Azzel Matti, Ahnet and Mouydir areas are located in the northwest of the Touareg Shield (Central Sahara, Algeria). Within the Devonian sedimentary formations, nine oolitic ironstone occurrences of EXID type (Extensive Ironstone Deposition) are interbedded.Their mineralogical composition is characterized by four different paragenetic associations: P1 (Chamosite — magnetite — maghemite — goethite); P2 (chamosite — hematite — goethite — calcite); P3 (chamosite — hematitegoethite — quartz); and P4 (chamosite — hematite — goethite). Using textural analysis, four main ironstone facies are distinguished: FOD (ooliths scattered in a detrital groundmass); FOND (ooliths scattered in a non detrital groundmass); FOC (cemented ooliths) and FMC (microconglomeratic facies).Primarily developed in calm conditions by intrasedimentary processes within an iron-rich silicated mud, in lagoons or embayments, ooliths subsequently acquired a detrital character.The ironstone deposition seems to be induced by several pulses of sedimentation through the Devonian and is considered as indicator of sedimentary subcycles. Therefore, the oolitic ferriferous sediments indicate a cratonic sedimentation on the borders of a large epicontinental sea. The source of the iron could be a remote southern continent, probably the Pan-African mobile belt of Nigeria and the Congo Shield.The ironstones of the Central Sahara can be considered as an important branch of the North-African Oolitic Ironstone Belt, extending from Rio de Oro to Libya.

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Zusammenfassung Die Gebiete des Bled el Mass, des Azzel Matti, des Ahnet und des Mouydir, liegen im Nordwesten des Touaregschildes (Zentral Sahara, Algerian). Neun eisenoolithische Horizonte sind hier in die devonischen Sedimente eingeschaltet. Ihr Mineralinhalt kann durch vier verschiedene Paragenesen charakterisiert werden:P1=Chamosit + Magnetit + Maghemit + Goethit; P2= Chamosit + Hämatit + Goethit + Kalzit; P3=Chamosit + Hämatit + Goethit + Quartz; P4=Chamosit + Hämatit + Goethit.Vier vererzte Fazies Typen treten auf: FOD: (die Ooide sind in einer detritischen Marix eingelagert); FOND: (die Ooide sind in einer nichtdetritischen Matrix eingelagert); FOC: (die Ooide sind verfestigt); FMC: microkonglomeratische Fazies.Die Ooide entwickeln sich in ruhigen Bedingungen, in Lagunen oder Meerbusen, durch die Bildung von Konkretionen im Sediment aus einem silikatischen und eisenreichen Schlamm; sie werden dann wie detritische Komponenten aufgenommen und transportiert.Die eisenoolitischen Ablagerungen scheinen durch mehrere Sedimentationsphasen während des Devons entstanden zu sein; so können sie als Zeichen von Sedimentmikrozyklen betrachtet werden. Diese oolitischen Sedimente sind charakteristisch für eine Kratonsedimentation am Rand eines breiten, epikontinentalen Meeres.Der Usprung des Eisens ist in einem südlichen Kontinent zu suchen, wahrscheinlich in dem mobilen panafrikanischen Gebirge von Nigeria, oder auf dem kongolesischen Schild.Diese Erze der Zentralsahara können als ein wichtiger Zweig des Eisengürtels betrachtet werden, der sich in Nordafrika von Rio de Oro bis Libyen erstreckt.

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Résumé Les régions du Bled el Mass, de l'Azzel Matti, de l'Ahnet et du Mouydir sont situées au Nord-Ouest du Bouclier Touareg (Sahara Central, Algérie). Neuf niveaux de minerai de fer oolithique sont interstratifiés dans les formations sédimentaires du Dévonien.Quatre différentes paragenèses caractérisent la composition minéralogique de ces minerais. P1 (chamosite — magnétite — maghémite — goethite); P2 (chamosite — hématite — goethite — calcite); P3 (chamosite — hématite — goethite — quartz) et P4 (chamosite — hématite — goethite). Quatre facies minéralisés ont été en évidence: FOD (oolithes dispersées dans une matrice détrìtique); FOND (oolithes dispersées dans une matrice non détritique); FOC (oolithes cimentées) et FMC (faciès microconglomératique).Développées dans des conditions calmes par concrétionnement intrasédimentaire dans une boue silicatée riche en fer, dans des lagons ou des baies, les oolithes vont acquérir par la suite un comportement détritique.Les dépôts de minerai oolithique semblent avoir été induits par plusieurs pulsations sédimentaires durant le Dévonien et peuvent être ainsi considérés comme des marqueurs de microcycles sédimentaires.Ces sédiments oolithiques sont caractéristiques d'une sédimentation cratonique, sur les bords d'une mer épicontinentale étendue. La source du fer est à rechercher dans un continent situé au Sud, probablement dans la chaîne mobile Pan-Africaine du Nigeria et le Bouclier du Congo.Ces minerais de fer oolithiques du Sahara Central peuvent être considérés comme une branche importante de la ceinture ferrifère Nord africaine, qui s'étend du Rio de Oro à la Libye.

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An Experiment Using the High Resolution Eta and WRF Models to Forecast Heavy Precipitation over India   总被引：1，自引：0，他引：1  

Y. V. Rama Rao  H. R. Hatwar  Ahmad Kamal Salah  Y. Sudhakar 《Pure and Applied Geophysics》2007,164(8-9):1593-1615

In the present study using the Weather Research and Forecasting (WRF) and Eta models, recent heavy rainfall events that occurred (i) over parts of Maharastra during 26 to 27 July, 2005, (ii) over coastal Tamilnadu and south coastal Andhra Pradesh during 24 to 28 October, 2005, and (iii) the tropical cyclone of 30 September to 3 October, 2004/Monsoon Depression of 2 to 5 October 2004, that developed during the withdrawal phase of the southwest monsoon season of 2004 have been investigated. Also sensitivity experiments have been conducted with the WRF model to test the impact of microphysical and cumulus parameterization schemes in capturing the extreme weather events. The results show that the WRF model with the microphysical process and cumulus parameterization schemes of Ferrier et al. and Betts-Miller-Janjic was able to capture the heavy rainfall events better than the other schemes. It is also observed that the WRF model was able to predict mesoscale rainfall more realistically in comparison to the Eta model of the same resolution.  相似文献   

大别造山带毛屋岩体的解体及古地幔性质的厘定   总被引：1，自引：0，他引：1  

匡少平  张宏飞  张本仁  韩吟文 《现代地质》2001,15(4):391-398

根据详细的地球化学 (特别是稀土元素地球化学 )的研究资料 ,提出大别造山带毛屋岩体应该解体。毛屋岩体不是由多幕式镁铁 超镁铁质层状堆晶作用形成的一套岩石系列 ,而是由橄榄岩类、含石榴石方辉岩类、榴辉岩类和绿辉石岩类组成的一个混杂岩体。其中含石榴石方辉岩类与绿辉石岩类之间存在成因上的联系 ,含石榴石方辉岩类的原岩是本区原始地幔经约36 %的部分熔融后剩下的残余 ,而绿辉石岩的原岩是由该熔融体结晶的产物。根据含石榴石方辉岩与绿辉石岩之间的成分制约关系 ,对该区古地幔的稀土元素组成和地幔性质进行了反演 ,结果表明 ,该区原始地幔稀土元素成分与球粒陨石中的相似 ,为轻稀土略亏损的平坦型组成模式 ;该区原始地幔矿物成分为橄榄石 (72 8% )、单斜辉石 (12 4% )和石榴石 (14 8% ) ,属含单辉石榴橄榄岩的地幔。  相似文献   

Investigation on the geological and geotechnical factors leading to a major slope failure at a selected site on a Jordanian highway     

Azm S. Al-homoud  Salah Taqieddin  Safwan Saket 《Natural Hazards》1995,12(3):203-224

During and after the very wet 1991/1992 winter experienced in Jordan, significant slope failures occurred along several sections of the new Irbid-Amman highway in Jordan. The topography over most of the route is hilly, and significant amounts of cut and fill were used to construct the highway. One such slope failure lies at 44+800 km on the highway south of Jerash City at a distance of about 5.0 km from Wadi Zerqa bridge. A landslide occurred downslope of the highway embankment resulting in a collapse of the most of the gabbion wall supporting the embankment. This paper deals with the overall stability of the slope at 44+800 km. It presents the geological and geotechnical studies carried out at this site, identifies the causes and mechanism of instability, and presents appropriate remedial measures. The study concluded that the landslide movement occurred within the colluvium material. It resulted primarily from excess piezometric pressures generated within the slope as a direct consequence of inadequate drainage. It is recommended to reconstruct the entire gabbion wall with its foundation seated on the sandstone that underlies the colluvium and to install a surface drainage system at the site.  相似文献   

Questioning development — growth? destruction? sustainability?     

Ben Wisner  Henry R. Luce 《GeoJournal》1995,35(2):99-104

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Centuries of artificial recharge on the southern edge of the Sierra Nevada (Granada,Spain)   总被引：1，自引：0，他引：1  

A. Pulido-Bosch  Y. Ben Sbih 《Environmental Geology》1995,26(1):57-63

In the Alpujarra (southern Spain), every year between the March and June, an ancestral practice continues in the form of diverting water from the rivers by way of an extensive network of irrigation channels (acequias) to well-defined, highly permeable areas. This practice, known ascareos, constitutes an ancient example of artificial recharge. The objective is to guarantee a supply of drinking water during the dry months, as well as improve the physicochemical characteristics of the water. In addition, this system helps maintain moisture in the immediate environment, and thus has a positive effect on local vegetation.  相似文献   

Studies of the formation process of water-in-oil emulsions   总被引：2，自引：0，他引：2  

Fingas M  Fieldhouse B 《Marine pollution bulletin》2003,47(9-12):369-396

This paper summarizes studies to determine the formation process of water-in-oil emulsions and the stability of such emulsions formed in the laboratory and in a large test tank. These studies have confirmed that water-in-oil mixtures can be grouped into four states: stable emulsions, unstable water-in-oil mixtures, mesostable emulsions, and entrained water. These states are differentiated by rheological properties as well as by differences in visual appearance. The viscosity of a stable emulsion at a shear rate of one reciprocal second is about three orders of magnitude greater than that of the starting oil. An unstable emulsion usually has a viscosity no more than about 20 times greater than that of the starting oil. A stable emulsion has a significant elasticity, whereas an unstable emulsion does not. A mesostable emulsion has properties between stable and unstable, but breaks down within a few days of standing. The usual situation is that emulsions are either obviously stable, mesostable, or unstable. Entrained water, water suspended in oil by viscous forces alone, is also evident. Very few emulsions have questionable stability. Analytical techniques were developed to test these observations.

The type of emulsion produced is determined primarily by the properties of the starting oil. The most important of these properties are the asphaltene and resin content and the viscosity of the oil. The composition and property ranges of the starting oil that would be required to form each of the water-in-oil states are discussed in this paper.  相似文献   

Contaminant concentrations in sport fish from San Francisco Bay, 1997     

Davis JA  May MD  Greenfield BK  Fairey R  Roberts C  Ichikawa G  Stoelting MS  Becker JS  Tjeerdema RS 《Marine pollution bulletin》2002,44(10):1117-1129

In 1997, seven sport fish species were sampled from seven popular fishing areas in San Francisco Bay. Mercury exceeded a human health screening value in 44 of 84 (52%) samples. All collected samples of leopard shark and striped bass exceeded the mercury screening value of 0.23 microg/g wet weight. PCBs exceeded the screening value in 51 of 72 (71%) samples. DDT, chlordane, and dieldrin, had lower numbers of samples above screening values: 16 of 72 (22%) for DDT, 11 of 72 (15%) for chlordanes, and 27 of 72 (37%) for dieldrin. Concentrations of PCBs and other trace organics were highest in white croaker and shiner surfperch, the two species with the highest fat content in their muscle tissue. Fish from one location, Oakland Harbor, had significantly elevated wet weight concentrations of mercury, PCBs, DDTs, and chlordanes compared to other locations. Removal of skin from white croaker fillets reduced lipid concentrations by 27-49% and concentrations of trace organics by 33-40%.  相似文献   

Impact of Climate Change on Agricultural Production in the Sahel – Part 1. Methodological Approach and Case Study for Millet in Niger     

A. Ben Mohamed  N. van Duivenbooden  S. Abdoussallam 《Climatic change》2002,54(3):327-348

In the last 30 years the climate of the West African Sahel has shown various changes, especially in terms of rainfall, of which inter-annual variabilityis very high. This has significant consequences for the poor-resource farmers, whose incomes depend mainly on rainfed agriculture. The West African Sahel is already known as an area characterized by important interaction between climate variability and key socio-economic sectors such as agriculture and water resources. More than 80% of the 55 million population of West African Sahel is rural, involved in agriculture and stock-farming, the two sectors contributing almost 35% of the countries' GDPs. It is thereforeobvious that climate change seriously affects the economies of these countries. Adding to this situation the high rate of population increase(3%), leading to progressive pressure upon ecosystems, and poorsanitary facilities, one comes to the conclusion that Sahelian countries, Niger amongst them, will be highly vulnerable to climate change.This paper investigates the impact of current climate variability and future climate change on millet production for three major millet-producing regions in Niger. Statistical models have been used to predict the effects of climate change on future production on the basis of thirteen available predictors. Based on the analysis of the past 30-years of rainfall and production data, the most significant predictors of the model are (i) seasurface temperature anomalies, (ii) the amount of rainfall in July, August and September, (iii) the number of rainy days and (iv) the wind erosion factor. In 2025, production of millet is estimated to be about 13% lower as a consequence of climate change, translated into a reduction of the total amount of rainfall for July, August and September, combined with an increase in temperature while maintaining other significant predictors at a constant level. Subsequently,various potential strategies to compensate this loss are evaluated, including those to increase water use efficiency and to cultivate varieties that are adapted to such circumstances.  相似文献   

Fracture in Westerly Granite under AE Feedback and Constant Strain Rate Loading: Nucleation, Quasi-static Propagation, and the Transition to Unstable Fracture Propagation   总被引：1，自引：0，他引：1  

Ben D. Thompson  R. Paul Young  David A. Lockner 《Pure and Applied Geophysics》2006,163(5-6):995-1019

New observations of fracture nucleation are presented from three triaxial compression experiments on intact samples of Westerly granite, using Acoustic Emission (AE) monitoring. By conducting the tests under different loading conditions, the fracture process is demonstrated for quasi-static fracture (under AE Feedback load), a slowly developing unstable fracture (loaded at a `slow' constant strain rate of 2.5 × 10<sup>−6</sup> /s) and an unstable fracture that develops near instantaneously (loaded at a `fast' constant strain rate of 5 × 10⁻⁵ /s). By recording a continuous ultrasonic waveform during the critical period of fracture, the entire AE catalogue can be captured and the exact time of fracture defined. Under constant strain loading, three stages are observed: (1) An initial nucleation or stable growth phase at a rate of ~ 1.3 mm/s, (2) a sudden increase to a constant or slowly accelerating propagation speed of ~ 18 mm/s, and (3) unstable, accelerating propagation. In the ~ 100 ms before rupture, the high level of AE activity (as seen on the continuous record) prevented the location of discrete AE events. A lower bound estimate of the average propagation velocity (using the time-to-rupture and the existing fracture length) suggests values of a few m/s. However from a low gain acoustic record, we infer that in the final few ms, the fracture propagation speed increased to 175 m/s. These results demonstrate similarities between fracture nucleation in intact rock and the nucleation of dynamic instabilities in stick slip experiments. It is suggested that the ability to constrain the size of an evolving fracture provides a crucial tool in further understanding the controls on fracture nucleation.  相似文献