Journal of African Earth Sciences (1983)最新文献

The potential safe yields of aquifers in the rocks and saprolites of third-order drainage basins on the Pre-Cambrian Basement Complex of southwestern Nigeria are determined and presented using the Total Potential Groundwater discharge (Q_TP) model of Butler (1957) and Meyboom (1961). The safe yield values range from 22 1 min⁻¹ to 2156 1 min⁻¹. These values compare with those from the Basement Complex in some other parts of Nigeria but most of them appear very low when compared with yields of between 640 and 1740 1 min⁻¹ reported for sedimentary rocks within the same region. The relatively high values of between 642 and 2156 1 min⁻¹ reported in this study were obtained from aquifers located within the highly fissured quartzitic rocks and their associated coarse grained saprolites.

The potential safe yield values from the basins may thus show that apart from those areas underlain by quartzites and such other highly fissured rocks overlain by coarse grained saprolites e.g. porphyritic/coarse grained granites (and these are not widespread on the Basement Complex), the Basement Complex is an area of poor groundwater yield.

The pyrite framboids enclosed in the phosphate deposits of Abu-Tartur area, Western Desert, Egypt, occur as discrete individuals and as clustered aggregates. They are intimately associated with the organic matter contained in the phosphates. The morphology and other characteristics of these framboids are presented.

A biogenic mode of formation is suggested for such framboids. Bacterial action on soft organic rich sediments resulted in formation of local concentration of hydrogen sulfide. By the increase in activity of the latter in presence of Fe²⁺ and organic matter, a hydrophobic sulfide gel might have formed. It is considered possible that precipitation of the sulfide around bacteria in the gel would promote formation of globular pyrite rather than other forms, and clustering of the globules may be a result of precipitation of the sulfide around bacterial growths. However, the possibility of forming the framboids as an internal consequence of pyrite crystallization by increasing forces of surface tension is not completely excluded.

The Paleozoic sediments are present over the greatest part of the SaharanPlatform, from the border of Morocco, Rio de Oro and Mauritania in the W, to the Libyan frontier in the E. They outcrop in the Ougarta chains, the Gourara and the Touat, the Northern border of the Reguibat Shield and constitute a sedimentary girdle around the Touareg Shield, namely the Azzel Matti, the Ahnet, the Mouydir, the Ajjers Tassilis, the Tafassasset Tassilis and the Ouan Ahaggar Tassilis.

In all these formations, numerous oolitic ironstones occur, particularly in Ordovician, Devonian and Lower Carboniferous rocks. We can distinguish essentially two types of oolitic ironstones (OIS): OIS with constant thin beds and horizontal extension: they will be named EXID type (Extensive Iron Deposition), and OIS of local extension and irregular thickness: they will be named LOID type (Local Iron Deposition).

The EXID type is located in the Ordovician, the Middle Devonian and in the very Lower Carboniferous (Tournaisian) sedimentary rocks. The LOID type occurs in the Lower and Upper Devonian rocks and appears as the economically interesting type.

From the Zemmour to the Fezzan, a real Paleozoic oolitic iron belt appears 3000 km long, determining a N Gondwanian oolitic iron province, formed during cold to temperate climates.

A geochemical study on the Albian and Turonian shales from the southern Benee trough of Nigeria was carried out to establish their geochemical fingerprints.

The results show that these shale units are geochemically dissimilar both in major element oxides, trace and some rare earth elements. The element pairs which differentiate them best are MgO/Fe₂O₃, MgO/MnO, CaO/P₂O₅, CaO/Sr, K₂O/Rb, Zr/P₂O₅, Pb/Th, Sr/Rb and Zr/Y.

The Albian shales are chemically fairly similar to the North American Albian Mowry shale (sediments of shallow restricted inland seas) whereas the Turonian shales compare favourably well with the average normal shales.

Changes in source rock characteristics, paleo-depositional environments and diagenesis are collectively thought to have caused the variation in geochemistry of these shales. The paleogeographic significance of the chemical variation is discussed.

A soft foraminiferal graphitic marble band of 3 m thickness overlying and grading into a grey laminated marble band up to 40 m in thickness interbanded with low grade eugeosynclinal metasediments and metavolcanics is exposed in the Wadi Heimur Area to the east of Lake Nasser. It has been considered previously as Pre-Cambrian (El Shazly et al. 1973, 1977 and Hunting 1967). The graphitic marble contains rare arenaceous Mississippian foraminifera comprising Ammobaculites, Lituotubella cl., Palaeospiroplectammina cf. mellina, Trochammina and Endothyra.

The REE (rare earth element) contents of the bulk, heavy mineral fraction and various size fractions (0.250-0.037 mm) of recent sediments of the Niger Delta vary considerably. ΣREE varies from 26 to 5140 ppm in the bulk samples, 54–77,400, in the heavy mineral fraction, while the 0.125 mm size is the most enriched. All samples are enriched in La, the lower and the intermediate REE (compared to the average shale). There is a correlation of REE with mineralogy.

The central delta sediments, with an average SiO₂ content of 82% and kaolinite as the dominant clay, have a low REE content. The heavy minerals contribute a large fraction of the REE content. The pH and Eh conditions favour the leaching of REE. The eastern delta, with high smectite content and an average SiO₂ of 58%, has the highest ΣREE and pronounced negative Eu and Ce anomalies. The higher REE content in eastern and coastal delta sediments is attributed to the basic conditions, the organic matter and REE exchange between sea water and the smectitic clays.

Rocks of the Chilwa Alkaline Igneous Province fall into three principal groups, namely: (1) syenite-quartz syenite-granite, (2) syenite-nepheline syenite, and (3) carbonatite-nephelinite-nepheline syenite. Dykes of all the principal rock types are abundant and there are numerous small agglomerate-filled volcanic vents. The nephelinites, which are minor, are the only basic silicate rocks found in the province, which is characterized by its overwhelmingly felsic character. The petrochemistry indicates that three fundamental magmas of trachytic, phonolitic and nephelinitic/carbonatitic composition are required to encompass all the rock types present. It is suggested that these three magma types were produced by partial melting at different depths of strongly metasomatized lithosphere, the whole igneous event having been preceded by a long period of focused metasomatism which produced anomalous lithosphere mantle, and lower crust, leading to isostatic uplift, rift faulting, and eventually volcanism. The trachytic melt was generated towards the base of the crust from rocks akin to fenites, and the phonolitic melt also directly, but at greater depths. It is suggested that although the carbonatite may have formed by immiscible separation from carbonate-rich nephelinite, it may also have been generated directly from a relatively carbonate-rich zone in the upper mantle concentrated by gradual upward creep of low viscosity carbonate melt.

The analysis of Eburnean strain fields in the Upper (?) Birrimian Comoé Unit shows four great stages of tectogenesis:

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  an early cleavage subparallel to stratification which results from synkinematic granitoïd intrusions;

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  a folding (N40°E to N60°E);

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  a transcurrent faulting period, essentially ductile N-S sinistral;

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  N120°E schistosity, and a N110°E transcurrent ductile sinistral (?) faulting phase.

The importance of the sinistral N-S faulting system and, to a lesser extent, the N120°E schistosity direction, is shown. The probable rôle of these directions (particularly N-S) from Liberian to Upper Cretaceous is shown by the within-plate tectonic occurrences in the West-African craton.

A study of the groundwater resources and aquifers underlying Lagos metropolis has been made. This is based on well-logs, pumping tests, well-production and water quality data. The data were obtained during construction and use of recent boreholes of the Lagos State mini water works programme.

The sub-surface geology indicates a complex lithology of alternating sequence of sand and clay deposits. Three aquifer horizons were delineated. The first, a water table aquifer (average thickness of 8 m) is mostly exploited through dug wells. The other two, confined aquifers (thickness of 10–25 and 10–35 m respectively), are harnessed through boreholes and are the basis of the mini water works.

Average values of transmissivity (T) and storage coefficients (S) are 3.53 × 10⁻³ m² s⁻¹ and 2.95 × 10⁻⁴ respectively for the second and 17.44 × 10⁻³ m² s⁻¹ and 3.29 × 10⁻⁴ for the third. The range of transmissitivity, the storage coefficient and also permeability indicate considerable inhomogeneity in the aquifers.

Chemical quality determinations show that the harnessed aquifers waters are generally soft. They are suitable for use in domestic, agricultural and industrial processes, if the excessive iron occurring is removed.

The Oligo-Miocene succession of Eil is the product of a depositional regression and constitutes a 120–150 m thick depositional sequence that prograded seaward for at least 20–25 km. Its time-transgressive stratigraphy is documented physically by well exposed tangential clinoforms (previously considered as evidence of a tectonic coastal flexure) and biostratigraphically by the occurrence of calcareous nannoplankton, planktonic and benthonic foraminifera, and a rich coral fauna. The upper boundary of the sequence is indicated by a reefal toplap, which constitutes the flat surface of the Nogal Plateau. Age (Chattian to Burdigalian) and toplap relationships of the sequence indicate clearly that progradation took place after the Late Oligocene flooding which followed the strong fall of sea-level during the Chattian.

Because of the horizontal geometry of the entire sedimentary system, it has been possible to make a clear environmental reconstruction and a facies model with original water depths. A worldwide Tertiary facies—the Lepidocyclina beds— was confined to the front of the reef, at depths ranging from 35–40 to 120–130 m.