Journal of African Earth Sciences (1983)最新文献

A new gravimetric reconnaissance survey of the West-Congolian fold belt between the Congo and Lower Zaire has enabled the definition of four major gravimetric domains, which coincide with the recognized structural units. The interpretation of the gravimetric pattern in terms of recent geological data is consistent with the interpretation that the present day structure of the belt resulted from Pan-African tectogenesis (600 Ma), which produced geological structures with eastward vergence, towards the Zaïrian craton. In the proposed model, the different crustal segments would have been welded together in Pan-African time, and the existence of a suture zone further west, below the Mesozoïc to Recent sedimentary cover, is postulated.

The River Nile in Sudan, was during the Tertiary, a series of closed lake basins. Each basin occupying one of the major Sudanese rift systems (Salama, 1985a). In this paper evidence is presented for the presence of the buried saline Sudd Lake in Bahr El Arab rift. The thick Tertiary sediments filling the deep grabens were eroded from the elevated blocks; Jebel Marra, Darfur Dome, Nuba Mountains and the Nile-Congo Divide. The thick carbonate deposits existing at the faulted boundaries of Bahr El Arab defines the possible boundaries between the fresh and saline water bodies. The widespread presence of kanker nodules in the sediments was a result of continuous efflorescence, leaching and evaporative processes. The highly saline zone in the central part of the Sudd was formed through the same processes with additional sulphate being added by the oxidation of the hydrogen sulphide gases emanating from the oil fields.

Analyses of rocks, minerals, fossil bone and hot spring waters from Kanam and Kanjera, Kenya, are presented. The data show evidence of rare earth element mobility and uptake. Spring waters from both localities are rich in bicarbonate and fluoride; those from Kanjera are also unusually rich in zirconium. All analysed rock specimens are enriched in rare earths relative to chondrites and are further enriched in light rare earths relative to the heavy rare earths. There has been significant uptake of rare earth and other trace elements by bone apatite and travertime, in contrast to the behaviour of trona.

This study is based on a structural and sedimentary analysis of high resolution seismic profiles carried out on the southern margin of the Alboran sea.

In the eastern area, recent tectonic movements have strongly affected the deposits. The Alboran Ridge was structured in a complex antiform at the end of the Pliocene; its present disposition, in horst and subsiding trough structures with a thick infilling, is the result of Quaternary to Recent recurrent faulting. In the western area, near Gibraltar Strait, alternate stages of erosion and construction are apparent in the sequences of the marginal plateau, particularly in the northern part, which progrades towards the Strait. Hydrodynamic patterns and glacio-eustatic variations of sea level have played a prominent role in the genesis of sedimentary bodies.

Neither sedimentation nor tectonics seem to indicate a general Miocene-Pliocene boundary but the Pliocene-Quaternary transition is marked, on the ridge and marginal areas, by a major discontinuity; it can be correlated with onshore neotectonic data.

The Bouguer gravity coverage in the offshore Safaga area of the Egyptian Red Sea shelf, between 26°09′–27°00′N and 34°00′–34°33E′, was investigated by vertical second derivative, analytic upward continuation and subsurface modeling methods to study the basement setup and to investigate probable relationships to the Red Sea rifting.

The second derivative approach delineated narrow Tertiary sedimentary tracts bound by normal Red Sea faulting and showed three traces of probable transverse faulting. The upward continuation indicated broad deep-seated gravity effect which may be related to the upper mantle. Both the derivative and continuation methods showed that the observed transverse gravity gradients may be transverse faults which constitute continuation to mapped shear faults (of repeated movement since the Precambrian) in the continental Safaga. The deep-seated gravity anomaly was found to pass to the well-known positive gravity belt centered over the axial Red Sea zone. The model incident to arching, thinning of crust and subsequent collapse of marginal zones may fit the offshore Safaga area.

The present work confirms the basic ideas of Gupta [1983 Geophysics48, 357–360] and Abdelrahman et al. [1985 Geophysics50, 262–263] on the effectiveness of the least-squares method in determining depths to a vertical step (thin plate approximation) and a horizontal cylinder from the second vertical derivative maps by finding a solution of a non-linear equation in the form of f(z) = 0. Analysis of the second vertical derivative anomaly map in an area of the Gulf of Suez region of Egypt by this present approach revealed a depth to the center of a long NW-SE trending salt body approximated by a horizontal cylinder to be in a very good agreement with that obtained from seismic depth maps.

A variety of techniques for optically and digitally processing the Landsat multispectral scanner data were applied to scence number 1109-07493 of the Gulf of Suez Area. The results of each of these techniques were compared in order to assess the usefulness of each technique in the interpretation of the Landsat images. The best overall results for this scene were achieved using optically enhanced digital images. The high-bandpass color-infrared image was nearly as good. The intensity, hue and saturation images showed increased contrast between the various geological units, but lacked the fine detail necessary for certain aspects of geological interpretation. These results should equally apply to images of similar ground features but will probably differ when the features are significantly different.

In the Permo-Carboniferous basin of Béchar-Abadla (SW Oran, Algeria), on the northern margin of the Saharan Platform, the final infilling is essentially continental.

The microflora recognised in part of these upper terrigenous deposits is composed principally of spores. These show strong similarity with those in the basins of western Europe.

This study completes the palaeontological inventory so far discussed in the basin and confirms previous age determinations.

Comprehensive measures of radiometric age have been made on proterozoic rock samples belonging to various structural units in the People's Republic of the Congo (Mayumbian belt, Niari synclinorium, Sembé-Ouesso sequences). Having gathered them in tables, we have been able to date certain major tectonic events. The calc-alkalic magmatism associated with the Mayumbian belt (1400 Ma) is linked to the oceanic closure (Kibarian orogenesis) and the ultimate metamorphic episode is dated at 600 Ma (Pan-African). The Niari synclinorium sedimentary sequences, which were deposited from 1000 Ma on, have undergone Pan-African metamorphism. Moreover, we notice a systematic rejuvenation attributed to the South Atlantic proto ocean opening. At last, the Sembe-Ouesso sequences have been subjected to an epimetamorphic episode around 630 Ma, probably also linked to the Pan-African orogenesis.