Journal of African Earth Sciences (1983)最新文献

Detailed clay mineralogic and chemical analyses of Tertiary subsurface sediments of the Agbada and Akata Formations, from two wells on the Calabar Flank of the Niger Delta, have been systematically studied with a view to understanding clay mineral burial diagenesis. Five principal clay minerals, smectite, illite, kaolinite, chlorite and various proportions of mixed-layer illite/smectite were identified. Seven major oxides (SiO₂, Al₂O₃, MgO, Fe₂O₃, CaO, Na₂O, K₂O) were analysed for with an atomic absorption spectrophotometer, with a view to ascertain any depth related variations. The geothermal gradient of the two wells (Uruan-1 and Uda-1) was also calculated.

The results appear to suggest a transformation from smectite to a mixed-layer illite/smectite (I/S) phase. The transformation first goes to a random I/S phase, and then to ordered I/S and back to random I/S, even though postulated conditions for a complete transformation to illite did exist. It would therefore seem, from this case study, that neither temperature nor the availability of potassium is the principal factor controlling the transformation. Kaolinite and chlorite distribution does not exhibit any systematic trend that could be related to burial diagenesis.

These results provoked an extensive literature review on the subject, and key ideas discerned are summarized here. The prognosis? In the author's opinion, we still have a lot to learn about the factors that control the mechanics and reaction extent of clay mineral burial diagenesis.

An interactive programme for microcomputers has been developed and has been used for rapid interpretation of electrical resistivity soundings (conventional Schlumberger configuration) and frequency domain electromagnetic conductivity measurements at low induction numbers (vertical and horizontal loop configuration; Geonics EM 343).

Layer model interpretation can be improved by simultaneously calculating the electrical resistivity sounding response and the electromagnetic conductivity response. Whenever discrepancies occur between the measured electromagnetic data and the calculated data the resistivity layer-model interpretation has to be matched, until a model has been found that fits both the electrical and the electromagnetic measurements. In this way, equivalence in interpretation of electrical soundings can be reduced.

Calculation of ‘imaginary EM 34-3 sounding curves’ makes it possible for the surveyor to predetermine the optimal configuration of the electromagnetic system of measuring variations in thickness or conductivity of a target layer. An example is given.

Two recent case histories are discussed in which equivalence was eliminated: one concerning the delineation of a pollution plume (The Netherlands), the second concerning the determination of depth to basement (Sudan). For groundwater surveys in basement areas of Africa the combined application of electrical resistivity soundings and an electromagnetic system operating at low induction numbers can be very useful; the survey target in general is shallow and the electrical resistivities of the crystalline bedrock are high.

The migmatites of the High Dam Western Quarry are part of a crystalline massif south of Aswan and comprise granitoid and trondhjemitic varieties. Both comprise leucocratic (leucosome) and melanocratic (melanosome) fractions. The granitoid migmatites are more abundant and their mineralogy is similar to that of biotite gneisses whereas that of the trondhjemitic migmatites is similar to that of the hornblende gneisses of Aswan. The migmatites show stromatic, ptygmatic and phlebitic structures. The plagioclase is normally zoned and usually carries fine inclusions of sphene, apatite and iron oxide that lie in planes parallel to the long axes or the twin planes of these plagioclase crystals. K-feldspar megacrysts show intensive perthitization of exsolution origin and some include xenomorphic quartz crystals between the idiomorphic cores and their outer rims. Porphyritic, myrmekitic, graphic and rapakivi textures are common in the migmatites. These features suggest crystallization of the leucosomes from silicate melts formed by partial melting in small closed systems in water-undersaturated conditions through high grade regional metamorphism up to the K-feldspar-sillimanite-almandine sub-facies.

Paleomagnetic measurements on samples of late Precambrian-early Paleozoic volcanic and intrusive rocks of the Egyptian Eastern Desert show a variety of magnetic components. A stable, hematite-carried direction from the Dokhan Volcanic Formation indicates an age of about 680 Ma if the magnetization is primary and was acquired prior to structural tilting, or an age of about 500–550 Ma if it post-dates tilting. Several lines of evidence led us to prefer the latter interpretation. A high temperature component found in samples from a Dokhan xenolith within a Younger Granite pluton may also date at 500 Ma. Other directions found in the xenolith and in Younger Granites are provisionally attributed to late Mesozoic and Cenozoic overprinting.

The ‘Continental Terminal’ in the Senegalo-Mauritanian basin is a Cenozoic and detrital formation, presenting signs of an intense ferralitic alteration with formation of ferruginous concretions and crustings, neo-formation of kaolinite and significant silica movements.

Sedimentary structures are generally obliterated by alteration in the formation's summit. However, some fossil layers which have undergone epigenesis by geothite make it possible to establish the sea origin of the eocene and miocene deposits in this ‘Continental Terminal’. A better idea of Cenozoic transgressions and regressions can be achieved by a reconstitution of fossil river beds through alterations on the edge of the African continent.

The new elements in the ‘Continental Terminal’ and the Senegalo-Mauritanian Cenozoic paleoclimates are: The ‘Continental Terminal’ clearly represents an alteration fringe developed at the expense of marine formations (Tessier et al. 1975 Actes 9ème Congr. Int. Sédim., Nice, pp. 207–211), but this concept cannot be generalized to all of the coastal Cenozoic or interior Iullemmeden Nigerian basins.

The ferrallitic alterations mostly occurred in the Pliocene period after the sinking of the basin, as in the Miocene margino-littoral facies, and are still highly dominant. The ferruginous crusting can be seen in this period and also during the lower Pleistocene, because of the latitudinal migration of the basin northwards starting from the upper Cretaceous period.

Magnetic, very low frequency electromagnetic (VLF) and electrical resistivity methods have been applied in mapping buried bedrock relief for groundwater exploration in the Precambrian terrains of Ikare, southwestern Nigeria.

Quantitative interpretation of the magnetic data involved initial modelling and Euler deconvolution using elementary magnetic sources to identify applicable geological models in the spatial domain. A depth estimate of the top of the buried valley of 23 m was obtained. The application of power spectral deconvolution yielded an estimated depth of up to 67 m on the bedrock surface. The VLF data indicated the existence of subsurface lineament in the area while obtained resistivity of the sedimentary fill amounted to 260 ohm-m.

The results of the three techniques have successfully delineated a buried river channel for groundwater extraction in a crystalline basement terrain.

The Cape Coast granite complex of the West African Craton in southern Ghana has been dated as 2051 ±45 Ma by whole rock RbSr;. Mineral dates are ∼ 20006 for hornblende by KAr, 1974 ± 28 Ma for biotite by RbSr and ∼ 1900 Ma for biotite by KAr. A few biotite dates are somewhat younger. The low whole-rock ⁸⁷Sr/⁸⁶Sr initial ratio, 0.7015 ± 0.0002, indicates that the granites were created shortly before 2050 Ma. The successively younger mineral dates result from uplift, erosion, and cooling following the Eburnian cratonization event.

Dahomeyan metamorphic rocks of southeastern Ghana give biotite RbAr dates of 330–542 Ma, mostly 508–542 Ma, due to the Pan-African orogeny. Hornblende KAr dates are somewhat older, at 561–611 Ma.

Gneisses from Teshie, Numgua, and Tema Beach are parts of a reworked older basement at least 2841 ± 468 Ma old with an initial ratio of 0.7041 ± 0.0014. Gneisses from Ho and Dzigakope also appear to be pre-Eburnian with RbSr dates of 2176 ± 44 Ma and about 2800 Ma, respectively, but this is not as well established. Nine whole-rock samples of Dahomeyan gneiss scatter about an isochron of 515 ± 16 Ma with an initial ratio of 0.7056 ± 0.0002.

An intrusive carbonatite from Kpong gives concordant whole-rock RbSr, biotite RbSr, and biotite KAr dates of 593 ± 250, 545 ± 11 and 572 ± 15 Ma, respectively, but it also gives discordant and geologically contradictory plagioclase KAr and Rb dates of 665 ± 20 and 975 ± 167 Ma, respectively. Excess Ar and radiogenic Sr in xenocrysts unrelated to the groundmass are suspected.

The present investigation is mainly devoted to the critical analysis of the available exploratory data obtained from more than 35 deep holes, with the aim of delineating the geo-seismic characteristic features of the different rock units existing within the conspicuous Abu Gharadig Cretaceous Basin. Such a study reveals that the base Tertiary, which is separating the strongly deformed Cretaceous reflectors from the undeformed Tertiary ones, has a medium positive reflectivity, whereas the top Alamein ‘dolomites’ (top Aptian) and the top Massajid (top Middle Jurassic) reflectors have strong positive reflectivity of the first order. Moreover, the top Apollonia (top Middle Eocene) and the top Abu Roash (top Coniacian) are found to show up with a medium of strong positive reflectivity of second order. Also, the top Dabaa (top Oligocene), top Bahariya (top Lower Cenomanian) and top Kharita (top Albian) reflectors are considered as weak variable reflectors of the third order. Implications as to the structural setting of the basin are introduced here in view of such acoustic characteristics when applied to some seismic lines in the area studied.

The first deposition of near-shore sandstones on basement rock in the northern Wadi Qena area occurred during Lower Carboniferous times. Paleozoic deposits are overlain by similar sandstones of Cretaceous age with a large but inconspicuous hiatus between them representing erosion and non-deposition. A Cenomanian transgression inundated sandstones in the north as well as basement rock in the south. The Cenomanian and Turonian sea deposited marine and near-shore material in the Wadi Qena area, and only to the south of it were fluviatile beds laid down. The extant Red Sea Hills, at that time, represented a high and formed a peninsula extending in northerly direction into the shelf sea of the Tethys ocean. The sea withdrew in or after Coniacian times and the following erosion removed almost all Coniacian marine deposits. During Campanian times, the sea returned and also flooded the Red Sea Hill peninsula. Phosphoritic marls, at times of unrest, were redeposited as phosphorite sands in the south. Carbonate deposition followed, ending in Maastrichtian times. An erosional phase during the Cretaceous-Tertiary transition removed most of these chalks and limestones from the Wadi Qena area, and Paleocene and Eocene seas deposited limestone and mmarly chalk before a final regression marked the closure of the Tethys ocean.