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161.
162.
Jacques Bourgois Jacques Azema Peter O. Baumgartner Jean Tournon Alain Desmet Jean Aubouin 《Tectonophysics》1984,108(1-2)
The Pre-Upper Senonian basement of Costa Rica crops out in the Santa Elena and Nicoya peninsulas. From south to north and from base to top the basement includes: the Esperanza, Matapalo and Santa Elena units. The Esperanza unit is Albian-Santonian in age and consists mainly of pillow basalt and massive basalt flows. The Matapalo unit includes Callovian to Cenomanian radiolarite and includes massive basalt flows, basalt, and dolerite basement. The Santa Elena unit contains ultramafic and mafic rocks in which harzburgite is the major component. The most important tectonic features of the Nicoya Complex are the large Santa Elena and Matapalo nappes. Nappe emplacement was from north to south during upper Santonian time. The sedimentary cover of the Nicoya Complex comprises:
- 1. (1) the Campanian El Viejo Formation that consists of shallow-water sediments in the north (Santa Elena Peninsula) and the Campanian-Maastrichtian Sabana Grande Formation of deep-water origin in the South (Nicoya Peninsula);
- 2. (2) Paleocene strata indicating deposition in a deep-water environment comprises the Rivas, Las Palmas and Samara Formations;
- 3. (3) a post-upper Eocene (?) sequence that consists of the shallow-water Barra Honda and Montezuma Formations.
163.
Luk Peeters Bruno Haerens Jan Van der Sluys Alain Dassargues 《Environmental Geology》2004,46(6-7):951-961
The Eisden–Meeswijk region in Belgium has been affected by mining subsidence due to the deep coal mining activities. Groundwater levels in the alluvial plain of the Meuse River are maintained below the ground surface by drainage installations and municipal well fields. A correlation between the water level in the Meuse River and the variation in nitrate and sulphate concentrations in the aquifer has been observed. A transient groundwater model is developed for the period May 1998–May 2002 and advective transport simulations have been carried out using this model. During dry periods, the major groundwater flow is directed towards the Meuse River, thereby feeding the river. During wet periods, however, groundwater flows in the opposite direction. Due to these variations in groundwater flow direction and to the extraction of groundwater, zones of higher solute concentration exist of which the position and extension vary both spatially and temporally. 相似文献
164.
Antonio Tadeu dos Reis Christian Gorini Alain Mauffret Wiktor Weibull 《Comptes Rendus Geoscience》2004,336(2):143-150
The Marseilles (also called Marseilles/Planier) and Grand Rhone sedimentary ridges, offshore the Gulf of Lions, western Mediterranean Sea, are generated by sediment spillover funnelled by the Marseilles and Grand-Rhone canyons, respectively. We show that the Marseilles and Grand-Rhone canyon directions are tectonically conditioned by Plio-Quaternary salt-induced sea-floor relief. Overburden extension in the area created an assemblage of listric faults running parallel to the strike of the continental margin. This salt-induced topography resulted in accentuated fault scarps impacting the sea-floor morphology and forcing submarine valleys to deflect to the east. Gravity-induced extension equally influenced the morphological pattern of the valley overbanking deposits. Sediment spillover occurred on both flanks of the canyons, but because active faulting generated space accommodation for overflow deposits, it inhibited the development of typical levee morphology along the canyons' left flanks. To cite this article: A.T. dos Reis, C. R. Geoscience 336 (2004). 相似文献
165.
Vincent Chaplot Christian Walter Pierre Curmi Alain Hollier-Larousse Henri Robain 《Comptes Rendus Geoscience》2004,336(6):553-560
Geophysical methods have already shown their interest for the continuous characterisation of soils over landscapes, rapidly and, non-intrusively. But in bottomland areas, difficulties are encountered in relating geophysical properties to soil spatial distribution due to large variations in the depth, texture and/or water content of soils. Indeed, respective variations of these parameters can result in ambiguous geophysical responses. For example, a decrease in soil water content, which causes an increase in electrical resistivity, may be offset by an increase in soil clay content, inducing a decrease in resistivity. The objective of this study was to improve the continuous characterisation of soils affected by an excess of water by using a combination of geophysical techniques. Three techniques, the radio-magnetotelluric (RMT), the ground penetrating radar (GPR) and the electrostatic quadrupole (ESQP) were implemented along eight representative transects where soils were extensively described. The soil cover shows a succession from downslope to upslope consisting in fibric Fluvisols, gleyic Fluvisols, and Albefluvisols. None of the geophysical methods allows us to distinguish all soil limits and to estimate the geometry of soil horizons. The ESQP discriminates Fluvisols from Albefluvisols, whereas the RMT above all reveals differences in soil material thickness, which do not permit to discriminate between these soils. In complement, the GPR allows the estimation of the geometry of organic horizons and anthropic structures, such as ditches. Finally, the combination of these three techniques allows us to assess the main features of soil spatial distribution in bottomlands. To cite this article: V. Chaplot et al., C. R. Geoscience 336 (2004). 相似文献
166.
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
167.
The siltation of reservoir causes many problems of management and environmental disturbances that are important to minimize. Solving these problems requires in particular a good knowledge of mud bank instabilities. In partnership with the Laboratoire National d'Hydraulique (LNH), a subsidiary of Electricite de France (E.D.F.), authors realize a study of soft soils sliding entailed by reservoir emptying. Using a geotechnical flat sliding mechanism, the theoretical formulation of the safety factor can be established when the sediment mass is submerged and emergent. The measurement campaign carried out makes it possible to obtain bathymetric and topographic surveys and a geotechnical characterization of the soil for the whole reservoir of Grangent dam, located on the Loire (France). The proposed model appears to agree satisfactorily with the instability of the zones emerged because of the emptying and with the stability of the profile after emptying. The use of this model within the framework of forecasts 相似文献
168.
Analysis of the Major Fe Bearing Mineral Phases in Recent Lake Sediments by EXAFS Spectroscopy 总被引:1,自引:0,他引:1
Extended X-ray absorption fine structure (EXAFS) spectroscopy and chemical analyses were combined to determine the Fe bearing minerals in recent lake sediments from Baldeggersee (Switzerland). The upper section of a laminated sediment core, deposited under eutrophic conditions, was compared to the lower part from an oligotrophic period. Qualitative analysis of FeK EXAFS agreed well with chemical data: In the oligotrophic section Fe(II)–O and Fe(III)–O specieswere present, whereas a significant fraction of Fe(II)–S sulfides was strongly indicated in the eutrophic part. A statistical analysis was performed by least square fitting of normalized reference spectra. The set of reference minerals included Fe(III) oxides and Fe(II) sulfides, carbonates and phosphates. In the oligotrophic regime no satisfying fit was obtained using the set of reference spectra, indicating that siderite (FeCO3) was not present in a significant amount in these carbonate-rich sediments. Simulated EXAFS spectra for a(Cax, Fe1-x)CO3solid solution allowed reconstructing the specificfeatures of the experimental spectra, suggesting that this phase was the dominant Fe carrier in the oligotrophic section of the core. In the eutrophic part, mackinawite was positively identified and represented the dominant Fe(II) sulfide phase. This finding agreed with chemical extraction, which indicated that18–40 mol% of Fe was contained in the acid volatile iron sulfide fraction. EXAFS spectra of the eutrophic section were best fitted by considering the admixture of mackinawite and the Fe–Ca carbonate phase inferred to be predominant in the oligotrophic regime. 相似文献
169.
Christian Coulon M'hamed Megartsi Serge Fourcade Ren C. Maury Herv Bellon Amina Louni-Hacini Joseph Cotten Alain Coutelle Daniel Hermitte 《Lithos》2002,62(3-4):87-110
During the Neogene, a magmatic change from calc-alkaline to alkaline types occurred in all the regions surrounding the western Mediterranean. This change has been studied in Oranie (western Algeria). In this area, potassic to shoshonitic calc-alkaline andesites (with La/Nb ratios in the range 4–6) were mainly erupted between 12 and 9 Ma. They were followed (between 10 and 7 Ma) by basalts displaying geochemical features which are transitional between calc-alkaline and alkaline lavas (La/Nb=1–1.7). After a ca. 3-Ma quiescence period, volcanic activity resumed, with the eruption of OIB-type alkaline basalts (La/Nb=0.5–0.6), from 4 to 0.8 Ma. A combined geochemical approach, using incompatible elements and Sr, Nd and O isotopes, allows us to conclude that the transitional basalts derived from the melting of a heterogeneous mantle source, at the boundary between lithosphere and asthenosphere. We propose that melting of a previously subduction-modified lithospheric mantle occurred between 12 and 10 Ma, in response to the upwelling of hot asthenosphere flowing up into an opening gap above a detached sinking slab. As a result, calc-alkaline magmas were formed. From 10 to 7 Ma, the transitional basalts were generated through melting of the boundary mantle zone between the lithosphere and the upwelling asthenosphere. During that stage, the contribution of the lithospheric source was still predominant. Then, as sinking of the oceanic slab progressed, the increasing uprise of the asthenosphere led to the formation and emplacement (from 4 to 0.8 Ma) of typical within-plate alkaline basalts derived from a plume-modified asthenospheric mantle. 相似文献
170.