Journal of African Earth Sciences (1983)最新文献

Two sets of data are used to estimate heat flow density in Morocco: 36 determinations were made in shallow boreholes (100–500 m) in equilibrium conditions and 44 determinations in oil exploration boreholes (1000–5000 m).

These two sets of data are apparently consistent and are used to define the gross feature of heat flow in northern Morocco: Anti-Atlas, located at the West African craton boundary, is affected by a low heat flow (40 ± 5 m Wm⁻²) close to the average value obtained in this craton (37 ±8 m Wm⁻²); Mesetas, formed by a paleozoic and mesozoic basement, are characterized by a “normal” heat flow density (around 60 m Wm⁻²); northeastern part of Morocco (Eastern Rif, Middle Atlas and Horst chain) is affected by a higher heat flow density ranging from 80 m Wm⁻² to 110 m Wm⁻². It is suggested that this anomaly is directly connected with the recent quaternary volcanic activity.

From the N.S. linear structure of the Itombwe syncline arise some problems in central Africa. Some authors considered it as a Kibarian structure and others as an isolated fold belt of Pan-African orogeny. This ambiguity is the result, in its greatest part, of the scarceness of field work and the lack of dating on the rocks of this syncline. A study on the central area showed the existence of two lithostructural units. The western area is correlated with a Kibarian structure while the eastern area had some similarities with the tectonic Pan-African troughts from NE of Zaïre and west of Uganda. The existence of an area folded during the Pan-African orogeny and in the same environment as the great African lakes area must be considered.

Integration of petrographic and stable isotopic analyses of dolomite from the reefal facies of the Lower Cretaceous Shuaiba Formation of Abu Dhabi, indicates two types of dolomites: host (very fine to medium crystalline dolomite formed in a meteoric-marine mixing zone) and baroque (white, coarsely crystalline dolomite). The baroque dolomite is characterized by coarsely crystalline mosaics of anhedral to subhedral crystals with undulose extinction, curved crystal faces and cleavage planes, and abundant inclusions. Baroque dolomite from this formation was precipitated in veins and narrow solution channels and is commonly composed of coarse bladed crystals, sometimes fan-shaped toward the center, precipitated as void filling cements that partially occlude porosity. Analysis of the formation of water chemistry and stable isotope for the dolomite of the Shuaiba Formation reveals that it probably formed by mixing seawater or meteoric water in the subsurface, with interstitial brines derived from halite-bearing evaporites. Also, oxygen isotope analyses (δ ¹⁸O PDB values range from −4.8 to −9.6% and δ ¹³C PDB values range from 2.4 to 4.9%) suggest that baroque dolomite formed at temperatures ranging from 67 to 112°C.

Radar imageries revealed three major geomorphological units and a series of long, short and arcuate lineaments. The different lineament systems seem to delineate blocks containing folded structures. The metamorphosed terrain is easily recognisable as a highly deformed area. Including the metamorphosed rocks, three lithologic units have been recognised and demarcated in the area; consequently a radar-geological map at 1:50,000 scale of area 20,000 km² based on interpretation of radar imagery and a brief period of field work has been produced from this study.

The radar imagery studies have disclosed regional lineaments of tectonic origin, and indicate that an important tectonic episode occurred in the Oban massif and post the Precambrian period.

The electrical anisotropy of the Basement Complex area of the University of Ife has been established using apparent resistivity maps.

The electrical anisotropy and hence inhomogeneity in the area is primarily due to foliation; structural trends of concealed basement rocks deduced from resistivity maps correlate significantly with strikes of foliation/geologic boundary.

It follows therefore that strikes of foliation of concealed solid rocks, in which the predominant structural feature is foliation or where the fracture/joint directions are generally in line with strikes of foliation, can be reasonably determined from surface geophysical measurements.

Petrological, geochemical and K/Ar radiometrical whole rock age data of volcanic rocks from SW Egypt and N Sudan in the area of Gebel Kamil-Lake Nasser-Laqia Arbain-Nile are presented. These rocks are (i) trachytic-phonolitic rocks and (ii) alkali olivine basalts occurring mostly as volcanic plugs. Their petrographic and their major and trace element patterns are typical for continental within-plate volcanism. Two groups of ages were determined: Triassic (240, 233 Ma) with predominantly silicic, differentiated magmas, and Tertiary (60-20 Ma) with predominantly basaltic magmas. Major element abundances of the basalts show a regional trend from west to east from low to high fractionation irrespective of the age of the rock. Their occurrence can be correlated with two W-E uplift zones which are important for the structural development of the large sedimentary basins of the Libyan desert.

New geochemical and RbSr isotopic age data are presented for three suites of plutonic rocks from the central and western parts of the Wadi Shuqub quadrangle. The syn-tectonic diorite-tonalite are the products of early arc magmatism and the post-tectonic biotite-granite is the result of intra-arc plutonism. These plutonic suites have an intrunsive relationship with the layered metavolcanic and metasedimentary rocks of the Baish and Bahah groups, which have contributed to the development of an oceanic crust.

The syn-tectonic tonalite, which forms a large batholith of tonalite-quartz diorite in the western part of the Wadi Shuqub quadrangle, yields an age of 854 ± 10 Ma (2σ) with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70159 ± 0.00004 and an MSWD value of 1.97. The late-tectonic diorite-tonalite stock yields an age of 815 ± 13 Ma (2σ) with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7029 ± 0.00005 (2σ) and an MSWD value of 1.81. The post-tectonic suite of biotite-granite from the Gabalat area defines a poorly fitted Rb-Sr isochron age of 552 ± 20 Ma with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.71327 ± 0.003 (2σ) and an MSWD value of 4.47.

The syn- to late-tectonic tonalite and diorite-tonalite are of low-K calc-alkaline affinity and are characterized by low abundances of K, Rb/Sr, and high Sr, CaO, MgO, TiO₂, and FeO^t (total Fe as FeO is denoted as FeO_t. These two suites with low initial ⁸⁷Sr/⁸⁶Sr ratios are likely to be derived from a mantle differentiation or partial melting of the basic rocks above the subduction zone. The post-tectonic biotite-granite is enriched in SiO₂, K, Rb, Rb/Sr and depleted in Sr, CaO, MgO and FeO_t and is of dominantly high-K calc-alkaline affinity. This suite of biotite-granite with high initial ⁸⁷Sr/⁸⁶Sr ratio appears to have been derived from lower crustal material. Limited trace element data on diorite-tonalite and biotite-granite show that the biotite-granite is enriched in Rb, Pb, Zn, Sn, U, Th, Zr, and Y and depleted in Sr, Ba, Cu and Ni compared with diorite-tonalite.

Only specific desert environments are significant suppliers of the millions of tons of dust blown out annually from the Sahara to its fringes and across the seas. The major dustsource is weathered debris detrained by fluvial transport and redeposited on alluvial fans, in wadis and terminal basins. Rocky gravel and wet sebha surfaces are the only effective dust traps in the desert. Thick loess mantles accumulate on the fringes of the desert after medium or long distance transport where the rate of accretion exceeds 50 g m⁻² per year over long a period. At lower deposition rates the dust becomes incorporated and assimilated by the local ground soil (or ocean sediment). Such additions have affected distant areas in southern Europe and Africa and have contributed to their fertility. More desert loess-derived or affected soils are likely to be recognized in the future.

Geophysical studies have revealed extensive rift systems throughout Central Africa, among them the White Nile Rift runnng SE-NW across central Sudan. Few earthquakes have been detected teleseismically in areas of Sudan away from the East African or Red Sea Rift systems. Relocations presented here confirm three earthquakes to have occurred at shallow depth at the NW extreme of the White Nile Rift, around Jebel Dumbeir, Kordofan province. Ambiguity in a teleseismic P-wave first-motion solution for the 9 October 1966 0648GMT event is resolved by use of the P/pP/sP relative amplitude method. The fault plane is near-vertical, striking to 005–025° with a downthrow to the SE and a minor sinistral strike-slip component. This is consistent with ground deformation and location of an aftershock. Its downthrow direction appears to reflect subsidence of the White Nile Rift and central Sudan. Its strike is common with present-day activity in the Ethiopian Rift, suggesting that a near ESE-WSW stress regime persists across central and southern Sudan.

The Eocene Formi Formation in southeastern Nigeria consists, in its type area, of the following four lithologic units, in ascending order: (1) silty to fine calcareous sandstone; (2) grey to dark shale with interacaltions of siltstone; (3) silty to fine argillaceous sandstone; and (4) fine to coarse pebbly sandstone. A palaeoecological study of the fauna of these units is presented. The most significant fossils are the foraminifera comprising chiefly of calcareous benthonic forms. The dominant families are Miliolidae and Nonionidae. Arenaceous forms are absent, while planktonic forms, which are rare, are represented by the generaGlobigerina and Globorotalia. The ostracod assemblage is mainly dominated by the shallow water genera, Togoina, Buntonia, Loxoconcha, Paracypris, Bythocypis, Costa, Basslerites and Cytherella. Based on an integrated analysis of lithologic and faunal data, the following depositional environments are suggested for the Ameki Formation, in ascending order: subtidal and intertidal zones of the shelf environment; barrier ridge, outer lagoon, inner lagoon and beach ridge of the near-shore environment. The water temperature of the Ameki sea was partly influenced by the Bengeula Current.