Journal of African Earth Sciences (1983)最新文献

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.

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.

Talc-kyanite-yoderite-quartz schist and associated rocks belonging to the Proterozoic Usagaran System occurring along the western edge of the Mozambique Orogenic Belt (450–600 Ma) were studied using petrographic, X-ray diffraction, electron-microprobe and fluid inclusion methods. The main rock types studied in the area include talc-kyanite-yoderite-quartz schist, piemontite quartzite, epidote-phogopite quartzite, kyanite-quartz-biotite schist and biotite gneiss.

Fluid inclusion studies on the selected rock types indicate the presence of usually two-phased H₂O-rich and CO₂-rich fluids with a range of filling from 0.6 to 0.95. Some CO₂-rich fluids may be one-phased (liquid) at room temperature with their degree of filling ranging from 0.4 to 1.0. Most of the CO₂-rich inclusions show negative crystal shapes.

Fluid inclusions trapped in kyanite in the talc-kyanite-yoderite-quartz schist with isolated negative crystal shapes are considered primary. The presence of CO₂-rich fluids indicates low water fugacity during the formation of the talc-kyanite assemblage, and so pressure was probably lower. Primary fluid inclusions could be trapped at pressures between 5.2 and 5.6 kb and temperatures ranging from 540 to 570°C; this gives the possible P-T range of the peak of the first phase of progressive metamorphism.

The geologic history and stratigraphy of Kenya is represented and discussed. Sedimentary units are more than 12,000 m in deeper parts of basins. Sedimentation rates were high throughout the late Paleozoic and Mesozoic.

The whole sedimentary sequence in Kenya have been tectonically classified into four basins, namely Mandera Basin in northeast Kenya, Lamu Embayment along the cost, and the Anza Graben and Gregory rift basins inland (BEICIP 1984). Each of these have been affected by rifting at one time or another.

Source rocks, reservoir rocks, traps and caprocks are not very well studied but from the geology they are expected to be well developed. More than 25 wells have been drilled; although no commercial discovery had been reported so far there still is a chance of a find in Kenya. The Anza Graben, a Cretaceous/Tertiary rift basin, is the basin recommended in this paper. However, other basins need further study.

Seafloor spreading and transform faulting processes are also likely to be operative during continental rifting events. Continental lines of old weakness oriented at high angles to the direction of continental rifting may be reactivated by transform faulting. These older continental transform faults, which predate and accomodate the rifting, will continue to propagate as younger oceanic transform faults as the rift develops into seas and oceans. This model is applied to the East African Rift which is postulated to be a continental spreading rift that is accommodated by east-northeast continental transform lineaments that are reactivated older crustal defects of appropriate orientation. At least five continental transform lineaments can be tentatively identified by empirical best fits to oceanic transform directions of the South Atlantic Ocean and to various continental African northeast-trending structures: (1) Cape Town-Maputo (CT-LM); (2) Orange River-Beira (OR-B); (3) Luderitz-Lindi (L-Li); (4) Walvis Bay-Mombasa-Mogadishu (WB-M-Mo); and (5) Luanda-Afar (Lu-Af). As these postulated lineaments are perennial deep seated crustal defects they may also control the development of mineral deposits.

A calcareous sandstone sequence that forms part of the Eze-Aku Formation (Reyment) features an assemblage of ichnofossils in a section exposed on a quarry face. The traces encountered include horizontal burrows, some of which are preserved as casts, lined with organic matter and belonging to the ichnogenera Gyrolithes, Pholeus and Arthrophycus. Three other types of trace fossils are described as horizontal crawling trails, flat impressions and cylindrical shafts without formal names. Gyrolithes are by far the dominant traces in this sequence. The degree of bioturbation is high in every horizon. Body fossils are very rare except for scattered occurrence of fragments of calcitic pelecypod shells.

Ichnological and lithological considerations suggest that the sediments were deposited in an aerated shallow shelf environment which supported an assemblage of decapods, worms and other shallow water marine benthos. Deposition was generally below wave base under a continuous but relatively slow rate of sedimentation.

The Maastrichtian oolitic ironstone examined caps the mesas of the Agbaja Plateau, which covers an area approximately 150 km² and is situated NW of Lokoja, near the confluence of the Benue and Niger rivers, in the Cretaceous Middle Niger Basin of Nigeria. This basin is one of six depositional (Mesozoic to Recent) basins in Nigeria and is a shallow trough filled with Campanian to Maastrichtian, marine to fluviatile strata on the Pre-Cambrian basement.

Four major lithofacies of ironstone may be discerned on the basis of semi-quantitative X-ray diffraction analyses of borehole samples. These are from oxidized to more dehydrated and reduced conditions: (1) goethite + kaolinite; (2) hematite + geothite + kaolinite; (3) maghemite/magnetite + goethite + kaolinite; (4) siderite + magnetite + kaolinite. The maghemite/magnetite + geothite + kaolinite facies is dominant.

The ironstone was deposited during a major transgression. It overlies either a carbonaceous mudstone/shale or clay/sandstone, which is underlain by a fluviatile sandstone. Primary oxidized ironstone minerals were formed in nearshore fluviatile environment, and transported into deeper water. Through re-working and diagenesis, ironstone deposition continued under estuarine and reducing conditions with evidence of low energy wave action at the top of the ironstone.

Gravity and magnetic measurements were recorded while making a geotraverse from the Red Sea at Port Sudan to the River Nile at Abu Hamad. Much of the region is poorly known geologically and the geophysical interpretations have been constrained by new observations along and near to the traverse line. There are close correlations between gravity, magnetics and many of the major geological features of the region. Western, central and eastern blocks can be distinguished on the basis of combined geology and geophysics. The largely metasedimentary western block shows flat geophysical profiles, whereas the batholith which composes most of the central block shows minor anomalities related to its inhomogenous primary composition and to zones of later N-S shearing. The eastern block is composed largely of low-grade metavolcanic rocks but has a local basement of higher grade rocks, and there are numerous intrusions of granite and gabbro, with ophiolitic lenses within the NE-trending Nakasib shear zone. The strong geophysical anomalies over the Nakasib zone are in keeping with interpretation of this zone as a reworked oceanic suture. Other strong anomalies relate to the presence of basic intrusions and the distribution of basic basement rocks. The regional gravity profile is similar to those measured elsewhere on the flanks of the Red Sea and reflects thinning of the lithosppheric units as the Red Sea axis is approached.

The discovery of Upper Miocene marine layers in a water-borehole in Abidjan, to the north of the great tectonical fault zone dividing lengthwise the terrestrial part of the Ivory Coast sedimentary basin, incited us to study their microfauna, palynoflora and nannoflora in detail. To our knowledge it is for the first time that fossils of the two last named groups of such age have been studied in Ivory Coast and none of the characteristic species of the three groups illustrated up to now. The study has been carried out within the geological and sedimentological framework of the borehole.

Apatite, either detrital or authigenic, or both, occurs in the Precambrian arkoses and Cambrian subarkoses, grits and quartz-arenites in Israel, Sinai, southern Jordan and northwest Arabia. However, no apatite is found in the sandstones that overlie the sub-Carboniferous unconformity (often superceded by a sub-Cretaceous unconformity) throughout the Middle East.

Within the Precambrian-Cambrian sequence, apatite distribution is not uniform and varies between 0–80% of transparent heavy minerals. The detrital apatite was derived from the acid igneous terrain that supplied the sands. Authigenic apatite was formed by recrystallization of the detrital apatite.

Until its abrupt stratigraphic termination, apatite accompanies the ultrastable minerals zircon-tourmalinerutile without displaying, like the semi- and nonstable heavy minerals, vertical trends of gradual disappearnce. Therefore its disappearance cannot be explained by repeated reworking and transportation which, by themselves, are not known to result in the complete elimination of a heavy mineral from an assemblage.

Numerous heavy-mineral studies, and especially the experimental work of Nickel (Contr. Sedimentol.1, 1–68, 1973) have shown that apatite dissolves under conditions of low (< 6) pH, which may develop in well-leached humic soils, pedalfers and laterosols, but which are not known in intrastratal groundwaters. The apatite distribution suggests, together with clay-mineral indications, that pedogenesis of this type developed with the spread of terrestrial floras over the Arabo-Nubian land surfaces during the Late Paleozoic. Various lines of evidence, including paleomagnetic data, indicate that the present-day Near East area was in a temperate-humid zone during the Cambrian. After drifting through subpolar latitudes during the Ordovician-Silurian-Devonian, it moved again into a tropical-humid zone toward the Late Paleozoic. This coincided with the appearance of plant fossils in the clastic section, and the disappearance of apatite. Since the dissolution of apatite is inhibited in the presence of carbonate or Ca²⁺ ions, its removal must already have been completed before the Permian, when carbonate deposition became gradually dominant.