Salinité des ressources en eau: intrusion marine et interaction eaux–roches (Maroc occidental)

Résumé

Le flux migratoire de la population vers les grandes villes et la multiplication des activités industrielles et agricoles, lors des ces dernières années (1990–2006), ont provoqué un accroissement des besoins en eaux de la nappe du bassin du Rharb (ouest du Maroc). Cette forte sollicitation a produit une augmentation de la minéralisation et une dégradation de la qualité hydrochimique (salinité) des eaux souterraines. Cet article a pour objectif de suivre l'évolution spatiale de la qualité hydrochimique des eaux souterraines et de comprendre le processus de cette minéralisation en fonction de l'intrusion marine, le gradient hydraulique et l'interaction eaux–réservoirs. Les corrélations hydrochimiques réalisées dans la zone côtière révèlent une contamination par les eaux marines. L'intrusion marine et la caractérisation des complexes pré-Rifains ont été identifiées par les méthodes géophysiques électriques et sismiques.

Citation Zouhri, L., Toto, E. A., Carlier, E. & Debieche, T.-H. (2010) Salinité des ressources en eau: dilution marine et interaction eaux–roches (Maroc occidental). Hydrol. Sci. J. 55(8), 1337–1347.     

La méthode SHYREG débit—application sur 1605 bassins versants en France métropolitaine

Résumé

La méthode SHYREG est une approche développée pour la connaissance régionale de l’aléa pluvial (SHYREG pluie) et hydrologique (SHYREG débit) en tout point du territoire français. Elle est basée sur le couplage d’un générateur stochastique de pluie horaire et d’un modèle hydrologique. Cet article présente les résultats de la mise en ?uvre de la méthode sur 1605 bassins versants répartis sur la France métropolitaine. Sur les fréquences courantes (c.à.d. périodes de retour inférieures à 10 ans), la méthode restitue correctement les quantiles de débit de crue ajustés à une loi statistique sur les observations (loi GEV, selon le critère de Nash-Sutcliffe). Plusieurs critères sont utilisés pour valider l’extrapolation des débits à des fréquences extrêmes: (a) en la confrontant à de longues chroniques de débits observés, (b) en analysant dans le modèle hydrologique la saturation du réservoir de production synonyme de comportement asymptotique avec les pluies, et (c) en étudiant la stabilité de la méthode à travers les critères statistiques.

Editeur Z.W. Kundzewicz; Editeur associé G. Mahé

Citation Aubert, Y., Arnaud, P., Ribstein, P., et Fine, J.-A., 2014. La méthode SHYREG débit, application sur 1605 bassins versants en France Métropolitaine. Hydrological Sciences Journal, 59 (5), 993–1005.     

CARBONATE EROSION RATES IN SOUTHWESTERN WISCONSIN

Between 1977 and 1979 three sets of erosional weight loss tablets were em-placed in the karst of southwestern Wisconsin to measure both potential and actual rates of carbonate erosion. Tablets of Jamaican White Limestone, which has been used in many other karst areas, were used to gauge relative erosion potential while tablets of the local bedrock, Prairie du Chien dolomite, were used to assess absolute rates. Tablets of Yugoslavian limestone were emplaced as part of a world-wide study of comparative rates. Tablets were recovered in 1983 and 1984. Although potential erosion rates are similar to those in other carbonate areas, the sluggish dissolution of the indigenous dolomite retards karst development. Soil cover increases erosion rates and rates at a given depth are consistent over sites of up to 10 m². The highest erosion rates were recorded by the Yugoslavian tablets but betweensite variations caution that results based on a single site may be unrepresentative. Over 37% of all tablets were affected by mechanical weathering processes, probably freeze-thaw action.     

Performance of a transparent Flexible Shear Beam container for geotechnical centrifuge modeling of dynamic problems

A transparent Flexible Shear Beam (FSB) container was designed and constructed to simulate the dynamic response of a stratum of soil under horizontal, one-dimensional (1-D) earthquake shaking in a geotechnical centrifuge. A stack of four rectangular, acrylic frames separated by layers of flexible, high-strength rubber was used to form the transparent container. The fundamental natural frequency of the container was estimated to be similar to a layer of sand in its softened or liquefied state. The suitability of the container in simulating 1-D site response with minimal boundary effects was evaluated by monitoring the uniformity of the induced accelerations and settlements across the soil specimen. Further, the measured lateral displacements were compared with equivalent-linear site response analyses. The new FSB container was found to provide satisfactory boundary conditions for studying complex Soil–Structure-Interaction problems, while simultaneously enabling researchers to visualize deformations of the soil and buried structures during shaking.     

LA–ICP–MS U–Pb定年技术相关问题探讨

LA–ICP–MS U–Pb定年技术是地质科学中被广泛应用的重要手段。发展至今,该技术已相对成熟,但在实际工作中仍需要注意一些关键问题。笔者就该技术的样品准备、定年结果的取舍、铅丢失问题、普通铅问题和定年结果投图与解释等5个方面进行简要探讨。研究认为,对于复杂矿物进行U–Pb定年研究建议不分选出单矿物,而是采用矿物识别定位手段和LA–ICP–MS仪器相结合的技术手段,直接在岩石光片或探针片上进行原地原位微区定年分析,但要注意样品准备过程中可能存在的铅污染问题。在碎屑矿物定年结果选择方面,对于大于1.5 Ga的定年测点,笔者建议采用207Pb/206Pb年龄代表该颗粒的结晶年龄,而对于小于1.5 Ga的定年测点则应采用206Pb/238U年龄。对沉积岩最大沉积年龄的判断和选择主要依靠统计学方法,必要时需要结合地球化学数据和地质背景信息作为辅助判断依据。对于连续分布在谐和线上的年轻样品要提高警惕,需要采用谐和图、加权平均图、CL图像和元素含量等多种手段识别是否存在铅丢失不一致线。针对普通铅校正问题,笔者重点介绍了一种专用于碎屑矿物U–Pb定年的普通铅校正方法,并给出了计算过程。关于对矿物U–Pb定年结果加权平均值数据质量的评价,笔者着重讨论MSWD越接近于1表示数据质量越高的理论基础。总之,应用LA–ICP–MS 技术对矿物进行U–Pb定年研究需要综合考虑多个因素,才能得出准确、可靠和地质意义明确的定年结果。     

Non-metamorphosed autochthonous Kuncha-Naudanda-Heklang Formations and their differences from those of the Kuncha nappe: A multichronological approach

Non-metamorphosed, autochthonous Lesser Himalayan sediments (LHS), which are correlated to the Kuncha and Naudanda Formations, were found in a narrow belt between the Main Boundary Thrust and the Lesser Himalayan Thrust at the base of the Kuncha nappe in southeastern Nepal. The autochthonous Naudanda Formation is comprised of cross-bedded and rippled orthoquartzite and yielded a maximum depositional age of 1795.1 Ma ±5.1 Ma using detrital zircons. Low-grade metamorphosed quartzite in the Kuncha nappe yielded a maximum depositional age of 1867.4 Ma ±3.4 Ma, although it is totally recrystallized. These ages and age distribution patterns of detrital zircon grains indicate that the meta-quartzite of the nappe is originally Naudanda Formation. A zircon fission-track age of the autochthonous Naudanda Formation shows partially annealed age of 864 Ma ±56 Ma, in contrast, that of the Kuncha nappe shows a totally annealed age of 11.9 Ma ±1.6 Ma. These results suggest that the autochthonous LHS have never undergone metamorphism during the Himalayan orogeny. We also discovered a non-metamorphosed Heklang Formation that rests on the Naudanda Formation, and designated it as a sub-type section on the basis of detailed lithostratigraphic study. It is characterized by black and light green slate with dolerite sills and ill-sorted quartzose sandstone, and correlated to the metamorphosed Dandagaon Phyllites in the Kathmandu area. Non-metamorphosed autochthonous formations distributed to the south of the nappe front suggest that they escaped from thermal metamorphism by hot nappe.     

香港西贡粮船湾组火山岩石柱区次生节理研究

在香港世界地质公园西贡火山岩区,白垩纪粮船湾组发育有目前世界已知面积最大的流纹质碎斑熔岩石柱群。在详细的野外地质调查基础上,对粮船湾组火山岩石柱区的次生节理进行了研究,并探讨了柱状节理岩体的构造变形特征及过程。粮船湾组火山岩石柱区次生节理主要包括陡倾的纵节理及缓倾的横节理。前者多具有共轭剪节理的特征,形成于不同方向的挤压构造环境下;后者切割早期构造面理,形成于重力垮塌的构造环境,多发展为正滑断层。粮船湾组火山岩石柱总体上受次生节理改造明显,共轭节理反演的构造应力环境表明石柱区在140 Ma左右经历了快速的构造转换,主挤压应力由近NE-SW向转换为近NW-SE向,可能与莲花山断裂的构造活动相关。     

Neoselachians and Chimaeriformes (Chondrichthyes) from the latest Cretaceous–Paleogene of Sierra Baguales,southernmost Chile. Chronostratigraphic,paleobiogeographic and paleoenvironmental implications

This paper discusses a well-represented fossil record of cartilaginous fishes (Chondrichthyes) from southern South America. The recovered samples allow the recognition of three assemblages with chronostratigraphic and paleogeographic value: i) typical Maastrichtian sharks and rays with affinities to eastern Pacific fauna, including the taxa Ischyrhiza chilensis, Serratolamna serrata, Centrophoroides sp. associated to Carcharias sp., and Dasyatidae indet.; ii) a scarce reworked assemblage of Paleocene–Early Eocene age including the taxa Otodus obliquus and Megascyliorhinus cooperi; iii) a rich assemblage with reworked taxa of Early to Middle Eocene age, together with autochthonous deposited Middle to Late Eocene taxa with close affinities to paleoichthyofaunas recovered from the North Atlantic, represented by Carcharias ‘hopei’, Odontaspis winkleri, Carcharoides catticus, Macrorhizodus praecursor, Carcharocles auriculatus, Striatolamia sp., Striatolamia macrota, Hexanchus agassizi, Notorhynchus sp., Myliobatis sp., Abdounia sp., Pristiophorus sp., Squatina sp., cf. Rhizoprionodon sp., Ischyodus sp., and one new species, Jaekelotodus bagualensis sp. nov. The studied samples include for the first time taxa with well established chronostratigraphic resolutions as well as taphonomic information that help clarifying the age of the fossil-bearing units. In addition, they provide relevant information about the evolution of the Magallanes (=Austral) Basin from the Upper Cretaceous to the Paleogene, suggesting a probable connection with the Quiriquina Basin of south-central Chile during the latest Cretaceous. Finally, the studied assemblages indicate a latitudinal pattern of distribution that provides valuable data on the environmental evolution and temperature of southern South America during the Paleogene.