Structural styles and tectonic evolution of the Kolia-Boboti sedimentary Basin,Kédougou-Kéniéba inlier,eastern Senegal

New structural data obtained on the Birimian terranes of the Kolia-Boboti sedimentary Basin, the eastern part of the Dialé-Daléma Supergroup in the Kédougou-Kéniéba inlier show two major phases of Eburnean compressional deformation: (1) a D1 phase of thrusting tectonics affected the Lower Birimian B1 tourmalinized sediments. This first tectonic phase is characterized by isoclinals overturned to recumbent folds P1 with N040° 20°NE trending axis, associated with axial plane schistosity S₀S₁ which is mainly transposed in the bedding; (2) a D2 phase of compressional (D2a) and transpressional (D2b) tectonics is responsible for the crossfolds P2a-P2b exhibiting curved axes. These P2 folds are associated with the major schistosity S₂, north-south to SW-NE trending, mainly dipping to the south-east. The S₂ schistosity is mostly displayed in the large shear zones corridors where it steeply dips locally toward the north-west. A north-west vergence thrusting phase (D2c) of flats and ramps, associated with reverse folds, represents the last Eburnean event. This geometrical feature is characteristic of a “positive flower structure”. These different Eburnean compressional phases are separated by extensional deformation which is characterized by sedimentary deposits and volcanic flows.     

Contribution of rock magnetism to stratigraphy and palaeoenvironment of the Karaïn cave infill,Antalya, Turkey

The magnetic properties of the infill of the Karaïn cave in Turkey were compared to the results of a sedimentological study on three longitudinal sections. The sediments of this site, namely cavity E, cover a period of ca. 500 ka and correspond to a large part of the Middle and Upper Pleistocene. The evolution observed for the various magnetic parameters is mainly related to climatic changes which followed one another during the infilling history of the cave. Two preconditions are necessary to understand the environmental magnetism, namely both the identification and the quantification of different magnetic phases (components) in the sediments, expressed by the ratio of the various magnetic parameters and the analysis of the magnetization curves. The results obtained through the study of the different magnetic parameters (low-field initial bulk (κ_lf) and mass (χ_lf) magnetic susceptibility, frequency dependence of the magnetic susceptibility (χ_fd), anhysteretic remanent magnetization (ARM), hysteresis parameters, anhysteretic susceptibility, ...) on 171 samples allowed us: 1 — to follow the nature, size and source of the grains at each level, and 2 — to identify six alternating levels, having different magnetic susceptibility signatures during warm, damp periods, in which the contribution of prehistorical hominidae (Homo Sapiens or Homo Neandertalensis) and animals, could have modified the original sedimentary structures.This study, in agreement with the sedimentological results, suggests a significant difference between sandy or sandy-silty levels (3 and 5) and those rich in clays and concretions. The sandy or sandy-silty levels are mainly characterized by a low amount of antiferromagnetic type grains, likely goethite and hematite, dominated by single-domain size (SD) and a small content of ferrimagnetic type grains, likely magnetite of multidomain size (MD), inherited from the host rock. Antiferromagnetic grains did not develop and were a priori drifted by the wind. They could be contemporaneous with a cold climate.The clays and clayey-silty levels are characterized by a large amount of superparamagnetic (SP) and SD grains, and a high content of low coercivity magnetic grains (magnetite, maghemite and Ti-magnetite). The saturation remanent magnetization (SIRM) of clayey levels is 5 to 10 times higher than that of sandy levels. The clayey levels are contemporaneous with a humid climate, which favoured the formation of secondary iron oxides, of stalagmitic floors and calcite concretions and of the emplacement of local pedogenesis phenomena, mainly at the top of the infill, i.e. in Level 6.     

Spatial and temporal variability of groundwater quality of an Algerian aquifer: the case of Soummam Wadi

ABSTRACT

Five-year monitoring of physicochemical parameters was performed with two campaigns in low and high water periods of the Lower Soummam catchment. Data from 18 wells were processed by multivariate statistical tools in order to identify the principal factors influencing groundwater chemistry. Two matrices of 14 and 8 physicochemical parameters with 18 groundwater samples collected in wells were obtained. The correlation matrix showed strong associations between nine variables: K⁺, Ca²⁺, Na⁺, SO₄^2?, Cl^?, Mg²⁺, NO₂^?, Zn²⁺ and Sr²⁺. Principal component analysis and factor analysis showed that the cumulated variance of high and low water periods was of 83.19% and 78.55%, respectively. The variables assigned to the mineralization effect or to pollution indicators were presented by the factor analysis. The bivariate plots confirmed a mineralization model, ascribed to dissolution of geological materials, and to high levels of saline contamination attributed to leakages from sanitary systems. They also showed an increase “upstream to downstream” of the mineralization, visualization of temporal variations, and a dilution process identification of the natural mineralization during the recharge of the aquifer.

EDITOR D. Koutsoyiannis; ASSOCIATE EDITOR X. Chen     

Apatite solubility in carbonatitic liquids and trace element partitioning between apatite and carbonatite at high pressure

We have measured apatite solubility in calcic carbonatitic liquids and determined apatite/melt partition coefficients for a series of trace elements, including the rare earth elements (REE), high field strength elements (HFSE), Rb, Sr, U-Th-Pb. Experiments were performed between 4 and 6 GPa, from 1200 to 1380 °C, using the multianvil apparatus. Trace element concentrations were determined by laser ablation ICP-MS and electron microprobe. In addition, a specific protocol was designed to measure carbon concentration in the apatites, using the electron microprobe. Two starting apatite samples were used in order to test for the effect of apatite chemistry on partitioning behavior.Apatite solubility is lower in calcitic melts by a factor 3-5 compared to dolomitic melts (3-5.5 vs. 10-18 wt.% P₂O₅ in melt). We interpret this difference in terms of solubility product in the liquid and propose an empirical model for apatite saturation that takes into account melt calcium content. We conclude that calcitic melts that may form by melting of carbonated eclogites could be saturated with residual apatite, contrary to dolomitic melts formed in carbonated peridotites.Compatibility behavior of the REE depends on apatite silica content: REE are compatible in apatites containing 3.5-5 wt.% SiO₂, with values between 1.5 and 4, whereas REE are incompatible in apatites containing 0.2 wt.% SiO₂. HFSE, U, Th, and Y are compatible in silica-rich apatite, with while . Strontium is always retained in the melt, with of the order of 0.5. Lead appears to be incompatible in apatite, although this finding is weakened by almost complete Pb loss to sample container. High silica concentration favors REE incorporation in apatite by allowing for charged balanced coupled substitution. Sulfur and carbonate may also favor REE incorporation in apatite. Our results allow to reconcile previously published experimental determinations of REE partitioning. We use our experimentally determined partition coefficients to investigate the impact of residual apatite during partial melting of recycled carbonated material (eclogite + sediments) and discuss how the chemical characteristics of the produced liquids can be affected by residual apatite.     

Hull/mooring/riser coupled dynamic analysis and sensitivity study of a tanker-based FPSO

A computer program is developed for hull/mooring/riser coupled dynamic analysis of a tanker-based turret-moored FPSO (Floating Production Storage and Offloading) in waves, winds, and currents. In this computer program, the floating body is modeled as a rigid body with six degrees of freedom. The first- and second-order wave forces, added mass, and radiation damping at various yaw angles are calculated from the second-order diffraction/radiation panel program WAMIT. The wind and current forces for various yaw angles of FPSO are modeled following the empirical method suggested by OCIMF (Oil Company International Marine Forum).

The mooring/riser dynamics are modeled using a rod theory and finite element method (FEM), with the governing equations described in a generalized coordinate system. The dynamics of hull, mooring lines, and risers are solved simultaneously at each time step in a combined matrix for the specified connection condition. For illustration, semi-taut chain-steel wire-chain mooring lines and steel catenary risers are employed and their effects on global FPSO hull motions are investigated. To better understand the physics related to the motion characteristics of a turret-moored FPSO, the role of various hydrodynamic contributions is analyzed and assessed including the effects of hull and mooring/riser viscous damping, second-order difference-frequency wave-force quadratic transfer functions, and yaw-angle dependent wave forces and hydrodynamic coefficients. To see the effects of hull and mooring/riser coupling and mooring/riser damping more clearly, the case with no drag forces on those slender members is also investigated. The numerical results are compared with MARIN's wave basin experiments.     

Contribution de la gravimétrie à l'étude de la structure du bassin de Tadla (Maroc) : Implications hydrogéologiques

This study is based on the analysis and the interpretation of the gravity data of the Tadla basin. Its purpose is to increase the knowledge of this basin structure. A residual anomaly map was first calculated from the Bouguer anomaly data witch are strongly affected by a regional gradient. The computed map provides information on the ground density variation but it does not bring enough of new elements. Data filtering allows us to emphasize the structures affecting the basin. We chose the horizontal gradient coupled to the upward continuation techniques that permit to highlight news structures and to give information on their dip. The elaborated structural map of the study area constitutes a useful document for rationalizing the future groundwater exploration in the Tadla basin. To cite this article: A. Najine et al., C. R. Geoscience 338 (2006).     

Ordering in double carbonates and implications for processes at subduction zones

We have studied cation ordering in dolomite in situ as a function of pressure, temperature, and experimental time using the multi-anvil apparatus and synchrotron radiation. Starting with ordered dolomite, we observe the onset of disordering taking place at 950°C, while complete disordering is achieved at 1,070 (±20)°C, for pressures ranging between 3.37 and 4.05 GPa. Pressure does not appear to have significant effect on the order/disorder transition over the investigated range. We find that dolomite can reach its equilibrium ordering state above 900°C within duration of laboratory experiment (few hours), both from disordered state and from ordered state. In addition, we have reversed the dolomite breakdown reaction [magnesite + aragonite = dolomite] between 4.5 and 5.5 GPa, by monitoring diffraction peak intensity. We also have determined that dolomite is stable up to 7.4 GPa at 1,100°C. We confirm some earlier studies where a change in slope (dP/dT) has been observed, but we find a non-zero slope in the low pressure range. Combining the values of entropy obtained from dolomite degree of ordering with enthalpy values deduced from our bracketing of [magnesite + aragonite = dolomite] equilibrium, we model the location of dolomite breakdown in the P–T space as a function of cation ordering. By comparing previous conflicting studies, we show that, although kinetics of order/disorder is fast, disequilibrium dolomite breakdown is possible. Our modeling shows that subducted disordered dolomite present in carbonated sediments could be decomposed to [magnesite + aragonite] at lower pressure (3.5 GPa) than usually considered (>5 GPa). This 2-GPa (60 km) difference is valid on a fast subduction path and is possible if disorder inherited from sedimentation is preserved. On a slow subduction path, however, dolomite breakdown is encountered at about 250 km depth, which is 100 km deeper than currently considered.     

Passive earth pressure on embedded vertical plate anchors in sand

Passive earth pressure on embedded anchor plates constitutes a viable resisting force for the design of underground structures. In the current practice, these forces are empirically calculated, ignoring the effects of the depth of embedment and the level of consolidation of the surrounding soil, which takes place during plate installation on the in situ stress levels. Accordingly, wide discrepancies between predicted and measured pullout capacities of these plates were reported in the literature. Numerical model was developed using finite-element technique and the constitutive law of Mohr?CCoulomb to simulate the case of a retaining wall partially supported by an embedded anchor plate in sand. The results produced in this investigation showed that the passive earth pressure acting on anchor plates increases due to the increase of angle of shearing resistance and the overconsolidation ratio of sand, and it decreases due to an increase of the embedment depth of anchor. Design theories were developed for the case of embedded anchor plate in overconsolidated sand. The theories developed will satisfy the design needed in terms of allowable pullout load and/or displacement.     

The stability of hydrous phases beyond antigorite breakdown for a magnetite-bearing natural serpentinite between 6.5 and 11 GPa

Phase relations for a natural serpentinite containing 5 wt% of magnetite have been investigated using a multi-anvil apparatus between 6.5 and 11 GPa and 400–850 °C. Post-antigorite hydrous phase assemblages comprise the dense hydrous magnesium silicates (DHMSs) phase A (11.3 wt% H₂O) and the aluminous phase E (Al-PhE, 11.9 wt% H₂O). In addition, a ferromagnesian hydrous silicate (11.1 wt% H₂O) identified as balangeroite (Mg,Fe)₄₂Si₁₆O₅₄(OH)₄₀, typically described in low pressure natural serpentinite, was found coexisting with Al-PhE between 650 and 700 °C at 8 GPa. In the natural antigorite system, phase E stability is extended to lower pressures (8 GPa) than previously reported in simple chemical systems. The reaction Al-phase E?=?garnet?+?olivine?+?H₂O is constrained between 750 and 800 °C between 8 and 11 GPa as the terminal boundary between hydrous mineral assemblages and nominally anhydrous assemblages, hence restricting water transfer into the deep mantle to the coldest slabs. The water storage capacity of the assemblage Al-PhE?+?enstatite (high-clinoenstatite)?+?olivine, relevant for realistic hydrated slab composition along a relatively cold temperature path is estimated to be ca. 2 wt% H₂O. Attempts to mass balance run products emphasizes the role of magnetite in phase equilibria, and suggests the importance of ferric iron in the stabilization of hydrous phases such as balangeroite and aluminous phase E.     

Ground calcium carbonate (GCC) from the Barre de Ghomrassene in Southeast Tunisia: a suitable raw material for paint industry

The main scope of the present work is to investigate the potential of using ground calcium carbonate (GCC) from the Barre de Ghomrassene (BDG) in Southeast Tunisia in paint industry. In order to evaluate the performance of the material and the formulated paint films, representative raw specimens of main deposits of naturel GCCs in the Mediterranean basin and Middle East countries such as extra-white limestone from the Abiod Formation (Feriana region in West-Central Tunisia), A??gedi?i Formation (Ni?de Group, South Central Turkey), and Samalut Formation (Elminea, South Cairo, Egypt) were considered. Samples were ground to specific surface of about 3000 cm²/g and were subjected to detailed characterization including chemical and mineral composition, physical, and chromaticity characteristics. Results show that the GCC from the lower part of the Barre de Ghomrassene is, in most, dark and grainstone in texture, which limit its use in paint and coating application. Elsewhere, it is marked by high purity degree (generally more than 97% calcite); high lightness (more than 86.5, 95.2 in average) with relatively low chromaticity (a*?<?3.3, b*?<?9.1); low oil intake (17.4 g/100 GCC); very low electrolyte levels; good pH buffering (close to 9); accepted ranges of density and abrasion (2.65–2.7 and 10–18, respectively); good grindability; low levels of harmful components such as MgO, SiO₂, Fe₂O₃, and acid insoluble residue (less than 0.1% each); and improved rheological properties. The BDG seems to be a suitable filler for paint when mixed with water, styrene acrylic, and common additives. The formulated paint films meet all standard requirements, in that they have very good opacity, matt visual dualgloss 20/60°, high luminescence (L*?=?96.4), suitable hardness (145 s), good adhesion (B5), and sufficient impact resistance (1.5 kg m). The performances of these films are analogous to those based on Abiod and Samalut formations (natural carbonate), but they are slightly less lighter than those based on A??gedi?i Formation (metamorphic carbonate). Hence, the studied GCC can be used to substitute them in particular for local GCCs from the Abiod Formation, which are limited and over exploited.