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

This paper provides an overview of recent ideas concerning the tectonic framework and evolution of the southern African subcontinent from the Early Archaean to the present. Mid-Archaean crustal growth of ancient continental fragments resulted from episodic terrane accretion, a process that culminated in the juxtaposition of the Kaapvaal and Zimbabwe granitoid-greenstone terranes during the Limpopo orogeny at ∼2.7 Ga. Whilst intracratonic sedimentary basins developed on this stable cratonic block from the late Archaean to Mid-Protorozoic (3.0–2.4 Ga), crustal growth occurred during the successive Eburnian (∼2.0–1.7 Ga), Kibaran (∼1.2−1.0 Ga) and Pan-African (∼500 Ma) global accretionary events. The Palaeozoic evolution of southern Africa following the assembly of the Gondwana supercontinent is dominated by the development of the Cape and Karoo basins and their subsequent inversion to produce the Cape Fold Belt (∼280−230 Ma). Mesozoic fragmentation of Gondwana (∼180−130 Ma) was associated with continental rift volcanism and fault reactivation. Minor tectonic activity continues to the present day.

Wadi El Miyah area is located in the central part of the Eastern Desert of Egypt. It is covered mainly by Basement rocks of Precambrian age, overlain on its western side by Nubian sandstones. The basement rocks comprise igneous and metamorphic rocks. The different types of rocks were studied photogeologically and checked in the field radiometrically and magnetically from the air to differentiate between these rock types and to delineate the contacts between them. All the information and results obtained were used to improve and correct the geological map previously constructed for the same area. It was found that the different structural features interpreted photogeologically could be correlated with the aeroradiometric results. Besides the centers of high structural intensities in the structural contour map were found to be comparable with the centers of high radioactivity on the aeroradiometric map.

Photogeological interpretation indicated that each type of rocks has its characteristics and the contacts between the main rock types are clear using the criteria of topographic expression, photographic tone, surface texture and drainage density.

Radioactivity of the rock exposures was found to vary with the different rock types and each group of the same rock type has nearly the same background aeroradioactivity. The serpentinites and the geosynclinical volcanics have the lowest radioactivity level, (0.83–6.00 Ur) the geosynclinal sediments have moderate radioactivity level (3.33–5.00 Ur), the synorogenic plutonites have high radioactivity level (2.5–12.67 Ur) and the late orogenic plutonites have very high level of radioactivity (4.17–25.00 Ur). The aeroradiometric contacts were quite clear when there is marked difference in the radiometric level between the studied rock exposures and the surrounding rocks.

The aeromagnetic survey showed that the NE-SW and WNW-ESE tectonic trends are the prominent structural trends in the studied area. They represent faults and dykes. There are other trends namely the NNE-SSW, N-S, NW-SE which affect nearly all types of rocks in the Basement complex.

Qualitative and quantitative analysis of benthic foraminiferal fauna reveals the succession of five benthic assemblages in the Paleocene El Haria Formation of El Kef section (Tunisia). The eugubina Zone is characterized by Anomalinoides acuta, A. praecuta, Cibicidoides alleni, C. susanaensis and Bulimina quadrata, developed in a bathyal environment of relatively low oxygen conditions. The pseudobulloides Zone presents a “Velasco-type” fauna, with Gavelinella beccariiformis, Osangularia plummerae, Cibicidoides alleni, C. susanaensis, N. truempyi and textulariids. Microfauna suggests an oxic environment in the upper bathyal stage. A regressive sea movement becomes perceptible and benthic fauna indicates an outer shelf paleobathymetry during the interval comprised between the trinidadensis and the pusilla Zones. The assemblage, developed in oxic conditions, is characterized by Alabamina midwayensis, Anomalinoides praeacuta and Cibicidoides susanaensis. The fourth assemblage, observed during the pseudomenardii and velascoensis (lower part) Zones, is typical of the continental shelf (middle to inner neritic; “Midway-type” fauna) with Alabamina midwayensis, Eponides elevatus, Anomalinoides midwayensis, buliminids and lenticulinids. The upper part of the section (velascoensis Zone) presents an inner neritic assemblage dominated by Stainforthia troosteri, Haplophragmoides excavata, Eponides elevatus and lenticulinids.

These El Kef data are compared with Paleocene foraminiferal data of field sections of central and southern Tunisia. Statistical analysis (correspondence analysis) confirms the lowering of the sea level from the K/T boundary to Early Eocene and indicates precisely the paleogeographic scheme of the Paleocene deposits of Tunisia, with the restricted Gulf of Gafsa in the south, the emerged island of Kasserine in the west and an unstable shelf cut up into deep basins in the east and in the north.

The significance of any trace element in an environment largely depends on the abundance of such element in the bedrock as well as its mobility/solubility in an aqueous solution. On the basis of these parameters, the trace elements lead (Pb) and copper (Cu) in the various rock units of the Ganawuri Granite Complex of Joss Plateau have been studied and their environmental significance evaluated.

The results of the study indicate that their levels of concentration are higher than their crustal abundance (i.e. Pb - 30 - 65; Cu = 10 – 50 ppm) and can, therefore, be considered hazardous and of environmental significance. The element copper with high mobility can be very hazardous in drinking water, plants etc. lead of this concentration can be very hazardous in drinking water in the presence of CO₂.

Radon 222 was measured continuously in a groundwater system in Nigeria for one year by γ-spectroscopy. The mean Rn concentration for a particular month is 15,1 Bq/l with a standard deviation of 1.0 Bq/l while the mean for the whole year is 14.8 Bq/l with a standard deviation of 1.4 Bq/l. All the values for the year lie essentially within ±δ of the mean. Rainfall appears to be the only major factor causing variations.

The structural history of the eastern portion of the Murchison greenstone belt can be explained in terms of three deformational events. Early high-angle faults, formed during gravity slumping of unstable ensimatic crust, provided a regional synformal framework for the preservation of linear homoclinal and synclinal greenstone belt assemblages. A second stage of deformation was initiated by the emplacement of discrete diapric gneiss plutons into and around the greenstone belt margins, leading to the development of the “granite/greenstone pattern”. Cleavage, lineation, flattening of early open folds, and reactivation of early faults developed concomitantly, and with increasing intensity, throughout the second stage of deformation. Syntectonic granitoids were emplaced into the greenstone belt assemblages along subvertical upthrusts and high-angled reverse faults. These faults are somewhat conformable with the regional lithological trends and developed between the first and second stages of deformation. A third and final deformation event is registered in the sinistral ductile shear zones of the Letaba shear zone along the northern flank of the Letaba schist belt. Kinkband, crenulation, and chevron folds are related to this final event of deformation.