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.

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.

The Lamja Sandstone of the Upper Benue Basin, northeastern Nigeria, is dated Coniacian on the basis of a fossil microfloral assemblage. The assemblage is constituted by Droseridites senonicus, Cretacaeiporites scabratus, Ephedripites costaliferous, E. multicostatus, Gleichenidites senonicus, Tricolpopollenites retiformis, Triorites africaensis and ? Steevesipollenites binodosus. The dating eliminates the uncertainty that has hitherto plagued the formation's precise age and indicates that the sandstone is in part laterally equivalent to the underlying Sukuliye and Numanha Formations. This study also confirms the hypothesis that the folded Lamja Sandstone was partly deposited by a regressing sea.

The Nkogam massif is exposed over an area of about 140 km². It is located on the ‘Cameroun Line’ and is one of the tertiary massives along this structure. It is composed of three units: (1) basalts; (2) ignimbrites; and (3) acid plutonic rocks. The first two are alkalic, the third one is peraluminous. Rb/Sr radiometric analysis on whole rocks yields an age of 67 Ma for the granites. The fact that two parallel isochrons are obtained shows possible slightly different origins.