Les manifestations tectoniques synsédimentaires associées à la compression éocène en Tunisie : implications paléogéographiques et structurales sur la marge Nord-Africaine

In central Tunisia, a synsedimentary tectonic episode has been pointed out through the tectonic movements affecting the Late Palaeocene–Early Eocene successions. This tectonic episode has controlled, to a large extent, the palaeogeographic setting of the area during that period and confirmed the important effect induced by the Pyrenean shortening phase on the edge of the African plate, which obviously has witnessed a common history with the southern part of the European plate. To cite this article: A. El Ghali et al., C. R. Geoscience 335 (2003).     

A lead isotope method for the accurate dating of disturbed geologic systems: numerical demonstrations, some applications and implications

Lead isotopes are compared in two equivalent diagrams. With a range in U/Pb, a closed system that evolved its radiogenic Pb in a single stage yields data that define a line of exactly the same age in the two presentations. This strict reproducibility (within ± a few 10⁶ yr, at most) is the crux of the concept of Pb-isotope synchronism. In contrast, an open system produces data that disperse variably in the two diagrams, yielding ages which are often different by ?10⁷ yr. However, because an age-calculation reconciles uncertainty in the age with the degree of dispersion in the data, a highly disturbed system yields false ages (from both diagrams), which nevertheless overlap within the calculated errors.This dichotomy (tightly reproduced linearity of a closed system vs. loosely correlated dispersion of an open one) is exploited in a new procedure, termed differential Pb correlation. This procedure allows filtering the data, so that an imperfect isochron may be improved or an invisible one may be identified within a dispersion field.Applying the synchronism method to the whole rock Pb isotopic data on the Amîtsoq gneisses results in their resolution into three synchronous lines, yielding the single-stage ages of 4.42, 3.74 and 3.45 Ga. In an attempt to reconcile this observation with geology, the mechanisms which may have produced these lines (multi-stage lineation) are explored. In addition, the possibility that the 4.42-Ga line is an actual isochron, and that vestiges of an ancient terrestrial crust of that age exist, are contemplated in some detail.The utility of the method is further demonstrated by application to meteorites as well as terrestrial rocks. Isochrons of crustal rocks intersect with each other as subgroups in multiple places (that is, at multiple Pb isotope values). This indicates the apparent existence of terrestrial reservoirs distinct in isotopic composition from each other. One of these reservoirs, characterized by a low (U/²⁰⁴Pb) = 4 may represent a depleted complement to the U-enriched source(s) of the ocean-island basalt (OIB). If so, then the so-called “Pb paradox” may no longer exist.     

Evaluating accuracy for two empirical methods in predicting settlement of drilled shafts

Several theoretical, empirical and semi-empirical methods are available in the literature to predict settlement of drilled shafts in sandy soils. In the Arabian Gulf countries, specifically in the United Arab Emirates, equations and procedure from the rest of the world are being used in analysis and design of drilled shafts without proper validation. It is the aim of this study to assess the applicability and evaluate the accuracy of two well known, and commonly used methods for pile prediction in the United Arab Emirates (UAE), namely Vesic (1977) and Poulos (1979), via comparison with data from field pile load tests conducted on shafts drilled in the region. Some of these tests were conducted for the purpose of this study, while others were made available through the courtesy of International Piling Contractors who are active in the region (e.g. Bauer International and Swiss Borings). Pile load test data were analyzed to back-calculate the model parameters related to settlement under different loading stages. Geological data and soil properties were obtained from studies conducted at the relevant sites. An effort is made to correlate soil properties with the prediction models. Statistical analysis is conducted to assess the accuracy of the results obtained from the two methods at different stages of loading via those obtained from pile load tests. Moreover, a detailed parametric study is conducted to assess the effect of the related parameters on the predicted pile settlement and the estimated settlement at different stages of loading. The study concluded with a recommendation of the most appropriate models and procedures to be followed for predicting the settlement of drilled shafts in the UAE, together with useful charts and correlation relations. Results showed that settlement values predicted by Vesic (1977) and Poulos (1979) overestimates the true values. This revised version was published online in July 2006 with corrections to the Cover Date.     

Style and timing of tectonic deformation across the Bou Arada-El Fahs troughs system,Northeast Tunisia: integration in the structural evolution of Atlas fold and thrust belt

The Bou Arada-El Fahs troughs system is a particularly E-W trending collapsed structure with the manifestation of normal-to-strike-slip faults. A combined multi-source and multi-scalar structural and geophysical investigation provides important insights on the geometry and the kinematics evolution of trough systems in the Atlas of Tunisia. New data, as well as a reappraisal of available data show that the studied troughs system is established during three main successive events: the Oligocene-Miocene, the Tortonian, and the Plio-Quaternary events. The goal of this paper is to present the structural evolution of the study area, on a pre-structured substratum. The structural evolution progressed from an extensional event, manifested by the formation of grabens, passing by an episode of reactivation of faults related troughs in strike-slip motion during the Atlas compression accompanied by an en-échelon folding in foot wall and hanging wall. These results acquired and presented in the following order: We will initially present the geological context by integrating and correlating the lithostratigraphic data. We continue by examining the geometry and kinematics of structure-related troughs via the detailed geological mapping, the interpretation of available 2D seismic data, and the interpretation of processed fault slip data. This integrated geological and geophysical study allows a better understanding of the BETS, and makes it possible to propose a new geometrical and kinetic model of the establishment of trough structure in the Tunisian Atlas.     

Causes of the famine among the rural population of the Sahelian zone of the Sudan

Taking the example of W Sudan, an analysis of the climatic incidents leading to the two recent famine disasters is made. On this natural background, the human side is investigated. Focus is directed towards the fragile economic structure of the famine-prone population groups. An early-warning system for famine in the Sahelian Zone is proposed taking grain production and storage, livestock development and the relationship between the prices of livestock and grain as major indicators.     

Fossil cyclic calcrete in the Kuwait Group clastic deposits (Mio-Pleistocene) of Kuwait,Arabian Gulf

An investigation of the upper Kuwait Group clastic sequence (Mio-Pleistocene) exposed along the Jal Az-Zor escarpment in north Kuwait revealed several horizons of fossil calcrete. Field occurrences and textural characteristics of the studied sequence indicated that the fossil calcrete was hosted in cyclic fluviatile muddy sandstones and developed through diagenesis during inter-fluvial periods. Depositional and diagenetic features of the calcretized fluviatile sandstone units suggested that each cycle started with the deposition of a fluviatile muddy host sandstone during a humid period. This was followed by a period of semi-aridity during which calcrete profiles developed. The upper part of the calcrete profiles was weathered and terra rosa type soil was formed during wet seasons. This was succeeded by a severe arid climate and the deposition of desert aeolian sand sheets. Fossil nodular calcrete was only developed within the fluviatile muddy sandstones. The calcretized sandstones were lithified in two stages of diagenesis-an early stage that was responsible for the authigenesis of microcrystalline calcite and the development of calcrete nodules, and a later stage during which macrocrystalline calcite cement was precipitated as an intergranular cement displacing and replacing the clay matrix of the host sediments and filling fractures and cavities in the calcrete nodules. The findings and conclusions of the present study will help in understanding the lithostratigraphic setting of the Kuwait Group clastic sequence in the northern Arabian Gulf region.

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Zusammenfassung Eine Untersuchung der klastischen Sequenz der oberen Kuwait-Gruppe, die entlang der Jal Az-Zor Zone in Nordkuwait aufgeschlossen ist, ergab mehrere Horizonte fossilen Kalksinters. Geländebeobachtungen und texturelle Charakteristik der Sequenz deuten an, daß der fossile Kalksinter in Zyklen eines feinkörnigen fluviatilen Sandsteines eingebettet ist und diagenetisch während interfluviatilen Stadien gebildet wurde. Merkmale der Ablagerungsverhältnisse und der Giagenese des verkalkten fluviatilen Sandsteines sprechen dafür, daß jeder Zyklus mit der Sedimentation des Sandsteines unter humiden Klimabedingungen begann. Auf dieses Stadium folgte eine Phase semiariden Klimas, während dessen sich die Kalksinterprofile entwickelten. Der obere Teil der Kalksinterabschnitte verwitterte in Regenzeiten, dabei bildeten sich »terra rosa«-Böden. Während des folgenden rauhen ariden Klimas wurden wüstenähnliche äclische Sande sedimentiert. Den fossilen knotigen Kalksinter findet man nur innerhalb des fluviatilen Sandsteines. Die Lithifizierung der Sandsteine erfolgte in zwei diagenetischen Phasen: In der frühen Phase wurde auhigener, mikrokristalliner Calcit und die Kalksinterknoten gebildet. Während der späteren Diagenesephase fiel makrokristalliner Calcit als intergranularer Zement aus, der die tonige Ausgangsmatrix substituierte und Klüfte sowie Hohlräume innerhalb der Kalksinterknoten ausfüllte. Beobachtungen und Interpretationen dieser Studie werden das Verständnis um den lithostratigraphischen Zusammenhang der klastischen Sequenz der Kuwait-Gruppe in der Region des nördlichen Arabischen Golfes erleichtern.

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Résumé La série élastique du groupe supérieur du Koweit (MioPleistocène), exposée le long de l'escarpement de Jal Az-Zor (nord du Koweit) montre plusieurs horizons de calcrètes fossiles. Les relations de terrain et les caractères structuraux de cette série montrent que ces calcrètes se sont formées dans des sables argileux fluviatiles cycliques, par un processus de diagenèse intervenu au cours de périodes interfluviales. Les caractères sédimentologiques et diagénétiques de ces roches suggèrent que chaque cycle a débuté par le dépôt, en période humide, des sables argileux fluviatiles et s'est poursuivi en période de semi-aridité par le développement des calcrètes. La partie supérieure des calcrètes a subi une altération atmosphérique avec formation d'un sol de type »terra rossa« au cours des saisons humides. Le cycle se termine par l'installation d'un climat strictement aride, avec dépôt de sables éoliens désertiques. Les calcrètes, nodulaires, se sont formées uniquement dans les sables argileux fluviatiles. Leur lapidification s'est opérée en deux stades: un premier stade responsable de l'apparition de calcite authigène microcristalline et du développement de nodules carbonates; un second stade marqué par la précipitation d'un ciment intergranulaire de calcite macrocristalline qui s'est substitué à la matrice argileuse du sédiment primaire et a rempli les fissures et cavités des nodules. Les résultats de cette étude permettent de mieux comprendre l'édification lithostratigraphique de la série clastique du Groupe du Koweit dans la région septentrionale du Golfe Arabique.

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Dedolomitization of dolocrete deposits in Kuwait,Arabian Gulf

Dedolomitization of a dolocrete profile hosted in Mio-Pleistocene siliciclastic deposits in the area of Kuwait City, Arabian Gulf was investigated. Dolocrete dolomite crystals vary considerably in size, shape and internal structure; however, they are mostly zoned. Zonation is usually due to the alteration of cloudy and clear zones. The cloudy zones, which are mostly formed of disordered metastable dolomite, are more susceptible to dedolomitization than the stable, well ordered clear zones. Two modes of dedolomitization were recognized; the first involves complete dissolution of the metastable dolomite followed by precipitation of intracrystalline cavity-filling calcite. The second is a pseudomorphic replacement of dolomite by calcite. This replacement takes place by the simultaneous dissolution of dolomite and precipitation of calcite in such a manner that the original dolomite fabrics are inherited in the dedolomite. Exposed and near-surface dolocrete profile (less than 5 m deep) are almost completely dedolomitized and altered to secondary calcrete whereas subsurface profiles are slightly dedolomitized. Dedolomitization of the sub surface dolocrete profiles may indicate the effect of flushing by fresh groundwater; which flows from west to east, whereas the alteration of the exposed dolocrete profile could be attributed to be an effect of meteoric water. A new mode of calcrete genesis by dedolomitization and/or complete calcitization of precursor dolocrete is suggested.     

The role of rural-rural migration as a survival strategy in the Sahelian zone of the Sudan —

Taking the example of Burush, N Darfur, Sudan, the authors analyse the role of migration as a survival strategy in the desertification-stricken Sahelian zone. It is being argued that the rual-rual migration plays a greater role than the rual-urban migration among the Berti ethnic group of northern Darfur. Their high mobility within their own area is part of their traditional economic pattern for coping with aridity and high rainfall variability     

Hunger-vulnerable groups within the metropolitan food system of Khartoum

Due to the current civil war and the economic and political deterioration in the Sudan, the majority of the Sudanese are suffering from food shortage. The dislocated southern Sudanese in Khartoum, however, belong to the group most affected. This paper tries to analyse the structure of hunger in the capital city of the Sudan. The formal and informal food supply systems are described. The reasons why the southern Sudanese are specially vulnerable to hunger and the indicators of this vulnerability are discussed.     

Combating famine by grain storage in Western Sudan

Traditional grain storage used to be an effective mechanism to combat famine in the rural areas of W Sudan. Of late, this mechanism has failed to function and a severe famine took place in 1984–1985. This paper endeavours to highlight this crisis, show the traditional methods of grain storage and point out to ways of retrieving grain storage systems to start again on a less vulnerable basis. An assessment of grain storage requirements under the new conditions where both market and subsistence economy are effective in rural areas is made for the different ecological zones of W Sudan.