Journal of African Earth Sciences (and the Middle East)最新文献

Landsat TM data were used for lithological discrimination and mapping in the little-known, semiarid 900 km² area around Caraculo station and the middle course of the Rio Giraul (Namibe Province, SW Angola) following two main procedures. The first of these was based on visual evaluation of three-band composites, band-ratio composites and Principal Component Analysis. The second method relied on the extraction of spectral signatures, and their use to obtain automatic classifications. Satisfactory results were reached with the first procedure, thus allowing - with limited support of ground information — the draft of a lithological map, while the second method was not systematically efficient, even for confirmation of data acquired with the first procedure.

Image interpretation suggests that an extensive but hithertoun differentiated metasedimentary complex consisting of a heterogeneous supracrustal sequence should be subdivided into at least two units. Field observations proved that one of these is marked by a notable frequency of marbles and the other is characterized by a widespread occurrence of amphibolitic bodies. Moreover, a belt of undetermined (thermally metamorphosed ?) metamorphic rocks is interposed between them.

The distinction of so far unidentified units, though restricted to interpretation of processed Landsat TM data, has significant geological implications also in the regional context and will be helpful in guiding future work with conventional geological methods.

Combined gravity and aeromagnetic geophysical surveys were carried out in the Khulais area of western Saudi Arabia to determine the thickness of the sedimentary cover and the depth to the basement. The results suggested the existence of an extensive sedimentary basin structure (20 × 7 km) with a maximum thickness of 1000 m. Similar depth estimations were reported from electrical sounding results.

This is an example of an integrated application of geophysical methods to provide hydrogeological information with an acceptable balance between accuracy and cost.

The Triassic series in the southern part of Tunisia, only known from the oil exploration wells in the Kirchaou area (sandy series from Early Scythian to Ladinian) and in its Djeffara/Dahar-North outcrops (alternating series from Ladinian to Rhaetian) is now subdivided into 10 Depositional Sequences (DS) sensu Vail et al. (1987), T₁ to T₁₀ corresponding approximately to most of the 3^rd order eustatic cycle from the Haq et al. curve (1987). Within the DS succession, 3 basal discontinuities (or sequence boundaries) appear to be important in Tunisia: the base of T₂, correlated to the “H Discordanz” from the “Germanic Triassic” (intrascythian); the base of T₆ (intracarnian), well-marked by the Sidi Stout angular discordance from Dahar North; the base of T₈ (intranoirian additional DS, previously recognized in Germany) pointed by the cartographic discordance of Low Stand conglomerates. The shallow-water facies from the carbonate Transgressive Systems Tracts of 7 Tunisian DS contain macrofauna, such as Myophoria, and particularly 6 successive benthonic Foraminifera assemblages, F₁ to F₆ giving chronostratigraphic informations as complement of Palynomorphs and Conodonts. Belonging to the genus Meandrospira, Triadodiscus, Aulotortus, Lamelliconus, Endothyranella, Pilamminella, Agathammina, Gandinella etc.…, most of them, commonly used as zone-indices in Europe, are new in Tunisia.

The main feature of this young (9.5 Ma) off-shore Nigerian basin is rapid sedimentation of shales leading to very high overpressure development. The ratio of total fluid pressure to sedimentary load is estimated to be between about 0.9 to 0.95 at the bottom of well A, while the main tectonic activity appears to be gravitational, characterized by the development of growth faults. To model these conditions, where slant faults could have played an important role in the development of overpressure, a 2-D fluid-flow/ compaction model must reproduce the geometry of the basin according to the development of faults and in relation to their possible effects as pathways for fluid migration.

For this reason, the 2-D model is designed to respect, as much as possible, the geometry of the system. A significant improvement over previous uses of the 2-D fluid-flow/compaction code is the ability to account for inclined layers around faults, instead of only horizontal layers. As a consequence, the basin geometry is much better reproduced. This improvement requires a fine grid of points and a fault throw less than the thickness of the smallest layer offset by the fault, so that no layer is “missing” at any position along the fault. The evolution of excess pressure or temperature with time, especially when fault occur, is consistent with results obtained synthetic tests and using flat layers around faults. Finally, the model reproduces correctly the measured fluid data. To obtain such overpressure, the fracturing must be higher than 0.9 to 0.95, as estimated by previous studies, and the faults cannot be always open.

The Jbel Saghro is one of the two late Cenozoic, large volume strato-volcano of the so-called Anti-Atlas. Saghro (east) and Siroua (west) lie within a zone of local extension but in a regional context of compression, related to a major E-W fault system that defines the southern rim of the Haut-Atlas. The tectonic regime is trans-tensional.

Lavas composition range from olivine nephelinites to phonolithes, but major and trace element criteria permit distinctions between one group which includes only phonolites versus a separate more diverse group which includes nephelinites and phonolites. K-Ar dating of 22 whole rock samples demonstrate a temporal-spatial progression from early volcanism in the centre of the area (10-6.4 Ma) to activity focused in the northern part of the complex (5.5-2.8 Ma; includes the youngest eruption). The sequence is successively more evolved from nephelinite through tephrite to phonolite, but the younger, northern sequence is confined to olivine-nephelinites.

Geological, petrological and geochemical studies indicate two distinct types of granitoid rocks: an older quartz diorite to granodiorite assemblage and younger granitoids, the latter occurring in two phases. The older granitoids have a calc-alkaline character, with high MgO, (FeO)_t, TiO₂ and P₂O₅ and low SiO₂, K₂O and Rb. Their major and trace elements compositions, together with a low ⁸⁷Sr/⁸⁶Sr (0.7029 ± 0.0008) denote I-type affinites. The second and third phase granitoids range from calc-alkaline to alkaline respectively. The second phase granitoids have a peraluminous chemistry and high Sr, Ba, CaO, MgO, Al₂O₃ and Ti₂O, and low SiO₂, K₂O, Zr, Nb and Rb relative to the third phase granitoids. The corundum normative nature and field observations suggest that it was formed by partial melting of the lower crust (S-type). The third phase granitoids are characterized by high SiO₂, Rb, Y, Zr and low MgO, CaO, Sr and Ba values than younger granitoid phase II. They show A-type granitoids which are generated from below or within existing continental crust.

The surveyed area straddles the s.s. West-African and The Nigeria-Beninian plates (Fig. 1). It corresponds to the northern part of the Volta Basin, the external and internal structural units, and the suture zone of the Dahomeyide Pan-African Orogen. The main gravity anomalies are sometimes correlated with geological outcropping formations. Numerous gravity gradient changes and discontinuities are noticed. They testify to the structural and geological complexity of this region. This complexity suggests two main hypothese: that of aggregation of micro-continents during the Pan-African; and that of a “gravity smoothing” following the great tangential movements.

The Ninakri complex (Côte d'Ivoire) is made up of two small blocks which intrude into the Birimian formations of the West African Craton. The rocks range from nepheline monzosyenites, aegirine and/or riebeckite syenites to micromonzonites. Their mineralogical and geochemical composition (major and trace) element) show that they belong to A-type granitoids. The zircon typology data and the rocks' content in incompatible elements indicate that they originate in an hypersolvus alkaline to subalkaline magma coming from a partial melting of an attenuated continental crust. This magma was crystallized under low water pressure and low O₂ fugacity. The comparison with others anorogenic syenitic massifs in the Côte d'Ivoire and West Africa (Nigeria, Cameroon, Niger) suggest that these syenites were intruded along paleosutures at the end of the Eburnean Orogen between 1900 to 1600 Ma.

A sedimentological study using grain-size distribution analysis investigated the processes of deposition that occurred during the preglacial Pleistocene age of the QI palynozone in the Sdom-2 borehole. The QI palynozone of this borehole is found in a continuous very well sorted and coars-grained sand column.

The lithological and textural characteristics of this sand unit reflect processes of deposition that commonly prevail in environments of regional sand blankets. The sand was probably transported by aeolian processes and by braided stream river systems of the Bijou-Creek type.