The major element chemistry and Sr-Nd isotope composition of three basaltic glasses and two whole rock samples dredged from the Nereus Deep, in the central Red Sea, were used to constrain the nature of the magmatism in the Nereus axial valley. The glass and whole rock compositions show the tholeiitic character of magmas and their affinity with MORE. The chemical composition of the most magnesian glass (mg# = 69.9) would suggest an origin from a melting event in the mantle at a pressure in the range 8-10 kbar. Magma differentiation modelling using the major element chemistry of the more evolved glasses indicates mineral-melt equilibration at moderate pressure, followed by most of the fractionation under low pressure conditions. The observed Ti content and Sr isotope composition suggest the occurrence of additional processes besides fractionation, and could reflect different degrees of partial melting of a slightly heterogeneous mantle source. The Sr-Nd isotope composition reveals the time-integrated depleted nature of the mantle, and overlaps the fields of MORE and basalts from the southern Red Sea axial ridge. The Sr isotope data on glasses resemble more closely the compositional range reported for the Indian Ocean ridges, and suggest a dominant asthenospheric source, with minor enriched components, (C) 1998 Elsevier Science Limited.
A numerical model is presented to assess the probable tsunami impact of future earthquakes occurring along the Eastern Mediterranean Ridge, and their effect on Alexandria, Egypt. On the 8th of August 1303 a major earthquake of magnitude about eight caused a large tsunami that killed many people around Alexandria, where ships were carried over buildings and settled on land.Calculations were done with an initial condition of continuous water flow normal to the shore line. This tsunamigenic event was examined to study the effect of location, direction, travel time and height towards the Egyptian Coast. Computed tsunami features such as travel times and run-up height distribution are given; which are useful in the evaluation of the tsunami hazard.
An eight point Rb/Sr isochron determination on whole rocks yielded an age of 1670 M.a. for the mylonites from the Sassandra couloir of ductile deformation, which extends N-S. A similar age is proposed for the mylonite from a zone of parallel trend stretching NNW-SSE in the Reguibat dome (Mauritania). A correlation is suggested with the Guri fault of the Guayana shield in South America.
Teleseismic Broadband seismograms of P-waves from the May 1990 southern Sudan and the December, 2005 Lake Tanganyika earthquakes; the western branch of the East African Rift System at different azimuths have been investigated on the basis of magnitude spectra. The two earthquakes are the largest shocks in the East African Rift System and its extension in southern Sudan. Focal mechanism solutions along with geological evidences suggest that the first event represents a complex style of the deformation at the intersection of the northern branch of the western branch of the East African Rift and Aswa Shear Zone while the second one represents the current tensional stress on the East African Rift. The maximum average spectral magnitude for the first event is determined to be 6.79 at 4 s period compared to 6.33 at 4 s period for the second event. The other source parameters for the two earthquakes were also estimated. The first event had a seismic moment over fourth that of the second one. The two events are radiated from patches of faults having radii of 13.05 and 7.85 km, respectively. The average displacement and stress drop are estimated to be 0.56 m and 1.65 MPa for the first event and 0.43 m and 2.20 MPa for the second one. The source parameters that describe inhomogeneity of the fault are also determined from the magnitude spectra. These additional parameters are complexity, asperity radius, displacements across the asperity and ambient stress drop. Both events produce moderate rupture complexity. Compared to the second event, the first event is characterized by relatively higher complexity, a low average stress drop and a high ambient stress. A reasonable explanation for the variations in these parameters may suggest variation in the strength of the seismogenic fault which provides the relations between the different source parameters. The values of stress drops and the ambient stresses estimated for both events indicate that these earthquakes are of interplate type.
Two earthquakes occurred in the Congo Basin in March and April 1998. These two events are reported to have comparable size (5.5 mb) and nearly the same epicentral locations, far from the western branch of the East African Rift System (EARS). The P-wave groups of these two earthquakes as recorded at FURI station have nearly identical waveforms, which is indicative of similar rupture process. The epicentral distribution in the area has no apparent trend and the reported seismicity is relatively sporadic. The results of the fault mechanism study show that the two earthquakes have a similar rupture process with slight variation. The duration of the main shock (event 980426) is 3.3 s and its scalar seismic moment, Mo, is 2.6×1017 Nm. The style of the dominant deformation for both events is thrust type, which is not commonly observed in the African Plate. A compressive stress environment may be created by plate-boundary forces of the mid Atlantic Ridge and the EARS.
At the northwest end of the Lake Turkana Basin (northern Kenya Rift), intensive fieldwork conducted on the Plio-Pleistocene fluvio-lacustrine Nachukui Formation by the National Museums of Kenya and the West Turkana Archaeological Project (WTAP), led to the discovery of more than 50 archaeological sites aged between 2.4 and 0.7 Ma. Among them is the Lokalalei archaeological site complex, which includes the two oldest archaeological sites (2.34 Ma) found in the Kenyan segment of the East African Rift System. The environmental background of the two sites was described as a succession of ephemeral streams with floodplain palaeosols in which the archaeological sites are situated, bordering the western bank of a large axial meandering river flowing southward. The Lokalalei 1 (LA1) and Lokalalei 2C (LA2C) archaeological sites are of extreme importance in terms of knowledge of hominins’ knapping activities. The stratigraphic position of the LA1 and LA2C sites as well as implications on the technical differences between the two sites have been successively discussed by Roche et al., 1999, Brown and Gathogo, 2002 and Delagnes and Roche, 2005. In terms of stratigraphic position, Lokalalei 2C was estimated to be slightly higher in the section (i.e. younger) than Lokalalei 1. An alternative stratigraphic correlation was proposed by Brown and Gathogo (2002), who suggested that LA2C site should have been approximately 100,000 years younger than LA1.
In August 2002, there was high seismic activity in Afar concentrated at the plateau margin of the northern Ethiopian rift east of Mekele, near the western part of the Danakil microplate. The spatial and temporal distributions of this seismic activity over four weeks indicate the NNW propagation of the Gulf of Aden rift across the Afar Depression towards the western Ethiopian plateau. Fault plane solutions for six larger earthquakes from the August 2002 sequence are estimated from moment tensor inversion of local broadband waveform data. The results show only normal faulting on NNW trending and NE dipping faults, which agree with tectonics of the area and distribution of aftershocks. No strike-slip component is observed in any of our fault plane solutions or those of other workers including Harvard CMT solutions in the region. Such motion would be indicative of oblique-slip deformation between the Nubian plate and the Danakil microplate consistent with counter-clockwise rotation of the microplate. Hypocentral depths of well-constrained events are 5–7 km, which is the approximate elastic plate thickness in the Main Ethiopian rift, possibly indicating the depth to the brittle–ductile transition zone in this part of the Afar Depression. The shallowness of the depth estimates agree with the macroseismic reports available from a wide area in northern Ethiopia. Potential future shallow crustal deformation may cause significant loss of human life and damage to property in the densely populated highland region around Mekele unless measures are taken in improving building standards. The b-value for this sequence is estimated to be 0.66 using a least squares fit, while it is 0.67 ± 0.16 from a maximum-likelihood approach. This estimated b-value is low or the frequency of occurrence of relatively larger magnitude events is high indicating that it is a highly stressed region as evidenced by the recent increase of the seismicity in the area.
The 2007 drilling season by the Tanzania drilling project (TDP) reveals a much more expanded Upper Cretaceous sequence than was recognized previously in the Lindi region of southern Tanzania. This TDP expedition targeted recovery of excellently preserved microfossils (foraminifera and calcareous nannofossils) for Late Cretaceous paleoclimatic, paleoceanographic and biostratigraphic studies. A total of 501.17 m of core was drilled at six Upper Cretaceous sites (TDP Sites 21, 22, 23, 24, 24B and 26) and a thin Miocene–Pleistocene section (TDP Site 25). Microfossil preservation at all these sites is good to excellent, with foraminifera often showing glassy shells and consistently good preservation of small and delicate nannofossil taxa. In addition to adding to our knowledge of the subsurface geology, new surface exposures were mapped and the geological map of the region is revised herein.
The Cretaceous deposits in central Tunisia blocks were studied by sequence stratigraphy, 2D seismic interpretation calibrated to the well and associated outcrop data. The constructing and comparing histories of the northern and southern blocks of the Gafsa master fault was the establishment of platform to basin stratigraphic configuration based on the major unconformity surfaces.Three important basin zones mark subsurface structures: Gafsa to the south, Souinia-Majoura to the northeast and Sidi Aïch-Mèjel Bel Abbès to the northwest. Basin depocenters and upthrown blocks are bounded by the N120° Gafsa and Majoura and N180° Sidi Ali Ben Aoun wrench fault salt-intruded tectonic corridors and subdivided by the associated N60° and N90° trending second-order fault corridors. The Mèjel Bel Abbès block is characterized by brittle structures associated with a deep asymmetric geometry that is organized into depressions and uplifts. Halokinesis of Triassic salt began in the Jurassic and continued during the Cretaceous periods. During extensional deformations, salt movement controlled sedimentation distribution and location of pre-compressional structures. During compressional deformations, salt remobilization accentuated the folded uplifts. The Triassic salt facies constitutes a level of decollement at the base of the Mesozoic deposits during the later displacements.The coeval dextral strike-slip motion along the three northwest–southeast bounding master faults (Gafsa, Sehib-Alima and Majoura-Mech) suggests a pull-apart opening of the Gafsa basin. Synchronous movements of the Gafsa first-order dextral strike-slip fault with the Sidi Ali Ben Aoun sinistral wrench fault caused formation of tectonic obstacles that are shown first by the sealed structures, then by development of the local compressive stress that caused formation of the south overturned folds and the syncline depressions. The transcurrent fault systems caused formation of Turonian and Senonian unconformities and hiatuses on the Sidi Aïch and Souinia-Majoura uplifts that corresponded to the Kasserine Islets. Sedimentary distribution and structural features indicate two events of regional deformation. The Albian–Cenomanian and Turonian transtensional displacements are demonstrated by depressions and grabens, with divergent reflectors striking against dipping growth fault. The Coniacian–Maastrichtian transpressional movements are marked by reverse faults and folds associated with pinch out structures and stratigraphic unconformities.
The Nsuze Group (∼3000 Ma) in South Africa is comprised of volcanics and sediments that rest unconformably on older continental crust. Nsuze volcanics, which range in composition from basalt to rhyolite and include a significant proportion of andesites, have tholelitic to calc-alkaline geochemical affinities. Incompatible element distributions in Nsuze volcanics are similar to those in volcanics associated with continental-margin arcs and exhibit a typical subduction zone component (SZC) (enrichment in LILE and relative to Nb-Ta). The volcanics can be related to each other by shallow fractional crystallization of average Nsuze basalt, together with up to 10% assimilation of granite with a composition similar to that of the underlying Mpuluzi granite.Geological data are consistent with an origin for the Nsuze Group in an intracratonic rift. The SZC in Nsuze volcanics may have been inherited from an older subcontinental lithospheric source.
The mafic to ultramafic rocks of the Barberton greenstone belt, South Africa, form a pseudostratigraphy comparable to that of Phanerozoic ophiolites. This Archaean complex, referred to here as the Jamestown Ophiolite Complex, consists of a high temperature tectono-metamorphic peridotite overlain by an intrusive extrusive igneous section, which in turn is capped by a chert-shale sequence. There is a complete range from komatiitic to tholeiitic compositions within single intrusive units. Crustal contamination and magma mixing is evident from field and geochemical data.Pillow structures, 40Ar/39Ar ages and oxygen isotope analysis suggest that hydrothermal interaction with the Archaean ocean severely hydrated and chemically altered the entire simatic section during its formation. As a consequence, only a ‘ghost’ igneous geochemistry is preserved. This regional open-system alteration may have increased the MgO content of the igneous rocks by as much as 13%, and the most primitive liquids, from which the extrusive sequence evolved, were ‘picritic’ in character. Rocks with a komatiitic chemistry were derived during crystal accumulation from picritic-crystal mushes (predominantly olivine-clinopyroxene) and/or by metasomatism during one or more subsequent episodes of hydration-dehydration.In contrast to Phanerozoic ophiolites, the Jamestown complex is relatively thin (≦3 km), which implies that locally at least the ca 3.5 Ga oceanic crust was also thin. This is consistent with the regionally extensive metasomatic alteration, and is compatible with theoretical and experimental models predicting higher Archaean heat transfer from the mantle concentrated within Archaean oceans.
Travel times from earthquakes recorded at two seismic networks were used to derive an average P wavespeed model for the crust and upper mantle to depths of 320 km below southern Africa. The simplest model (BPI1) has a Moho depth of 34 km, and an uppermost mantle wavespeed of 8.04 km/s, below which the seismic wavespeeds have low positive gradients. Wavespeed gradients decrease slightly around 150 km depth to give a ‘knee’ in the wavespeed-depth model, and the wavespeed reaches 8.72 km/s at a depth of 320 km. Between the Moho and depths of 270 km, the seismic wavespeeds lie above those of reference model IASP91 of Kennett [Research School of Earth Sciences, Australian National University, Canberra, Australia (1991)] and below the southern African model of Zhao et al. [Journal of Geophysical Research 104 (1999) 4783]. At depths near 300 km all three models have similar wavespeeds. The mantle P wavespeeds for southern Africa of Qiu et al. [Geophysical Journal International 127 (1996) 563] lie close to BPI1 at depths between 40 and 140 km, but become lower at greater depths. The seismic wavespeeds in the upper mantle of model BPI1 agree satisfactorily with those estimated from peridotite xenoliths in kimberlites from within the Kaapvaal craton.The crustal thickness of 34 km of model BPI1 is systematically lower than the average thickness of 41 km computed over the same region from receiver functions. This discrepancy can be partly explained by an alternative model (BPI2) in which there is a crust–mantle transition zone between depths of 35 and 47 km, below which seismic wavespeed increases to 8.23 km/s. A low-wavespeed layer is then required at depths between 65 and 125 km.
The coal deposits of southern Africa (Botswana, Malawi, Mozambique, Namibia, South Africa, Swaziland, Tanzania, Zambia and Zimbabwe) are reviewed. The coal seams formed during two periods, the Early Permian (Artinskian–Kungurian) and the Late Permian (Ufimian–Kazanian). The coals are associated with non-marine terrestrial clastic sedimentary sequences, most commonly mudrock and sandstones, assigned to the Karoo Supergroup. The Early Permian coals are most commonly sandstone-hosted while the younger coals typically occur interbedded with mudstones. The sediments were deposited in varying tectono-sedimentary basins such as foreland, intracratonic rifts and intercratonic grabens and half-grabens. The depositional environments that produced the coal-bearing successions were primarily deltaic and fluvial, with some minor shoreline and lacustrine settings. Coals vary in rank from high-volatile bituminous to anthracite and characteristically have a relatively high inertinite component, and medium- to high-ash content. In countries where coal is mined, it is used for power generation, coking coal, synfuel generation, gasification and for (local) domestic household consumption.
Existing tectonic models for the evolution of the Usagaran Orogen place much significance on Palaeoproterozoic K–Ar and Rb–Sr ages. Laser 40Ar/39Ar data from single mica grains from the Isimani Suite near the western margin of the orogen indicate that excess 40Ar is common in micas and this casts considerable doubt on tectonic models that are based on previously published K–Ar ages. Biotites lying within a well-developed S2 foliation (previously constrained at 1999–1991 Ma) yield ages up to 3.3 Ga and contain a significant excess 40Ar component that is variable at an intra- and inter-sample scale. Textural evidence indicates that muscovite grew or recrystallized after the synkinematic biotites and they also record younger 40Ar/39Ar ages with individual steps from 524 to 1055 Ma. It is shown that the mica age variation does not reflect different periods of growth but the preferential partitioning of excess 40Ar into the biotite. The muscovite data also have a component of excess 40Ar and the youngest muscovite 40Ar/39Ar date (535.4 ± 2.3 Ma) indicates a maximum age for greenschist facies metamorphism. This date corresponds to thermal activity associated with the East African Orogen. Greenschist facies deformation (D4 and D5) is interpreted to have been coincident with this thermal event and indicates localized tectonic activity associated with Gondwanan amalgamation. The data are also consistent with greenschist facies deformation, metamorphism and deposition of the Usagaran Konse Group being of Neoproterozoic–Early Palaeozoic age. These new data therefore preclude a solely Palaeoproterozoic tectonic history for the Usagaran Orogen and indicate a complex thermal-tectonic reworking in the Neoproterozoic–Early Palaeozoic.
Karoo-equivalent rocks in the Tuli Basin of Zimbabwe are described, with a focus on the dinosaur-bearing Mpandi Formation, which correlates with the Elliot Formation (Late Triassic–Early Jurassic) in the main Karoo Basin. Isolated exposures of the Mpandi Formation along the banks of the Limpopo River consist of red silty claystones and siltstones that preserve root traces, small carbonate nodules, and hematite-coated prosauropod bones. These fine-grained facies accumulated on an ancient semi-arid floodplain. Widespread exposures of quartz-rich sandstone and siltstone representing the upper Mpandi Formation crop out on Sentinel Ranch. These strata preserve carbonate concretions and silicified root casts, and exhibit cross-bedding indicative of deposition via traction currents, presumably in stream channels. Prosauropod fossils are also preserved in the Sentinel Ranch exposures, with one particularly noteworthy site characterized by a nearly complete and articulated Massospondylus individual.An unconformity caps the Mpandi Formation in the study area, and this stratigraphically significant surface rests on a laterally-continuous zone of pervasive silicification interpreted as a silcrete. Morphologic, petrographic, and geochemical data indicate that the Mpandi silcrete formed by intensive leaching near the ground surface during prolonged hiatus. Chert clasts eroded from the silcrete are intercalated at the base of the overlying Samkoto Formation (equivalent to the Clarens Formation in the main Karoo Basin), which in turn is overlain by the Tuli basalts. These basalts, which are part of the Karoo Igneous Province, yield a new 40Ar/39Ar plateau age of 186.3 ± 1.2 Ma.
Deformed conglomeratic clasts exposed along the Neoproterozoic Nakasib Suture and the Oko Shear Zone are used to calculate three-dimensional (3D) tectonic strain associated with the latter to quantify strain associated with post-accretionary deformational belts in the Arabian–Nubian Shield. The Nakasib Suture is a NE-trending fold and thrust belt that is sinistrally offset (∼10 km) by the cross-cutting NNW- to NW-trending strike-slip faults of the Oko Shear Zone. The Nakasib Suture was formed as a result of collision between the Haya terrane and the Gebeit terrane at ∼750 Ma ago. The Oko Shear Zone was subsequently formed as a result of an E–W directed shortening of the Arabian–Nubian Shield due to collision between East and West Gondwana at ∼670–610 Ma ago. This analysis indicates the following: (1) The Nakasib Suture is dominated by flattening strain with the flattening plane of the associated strain ellipsoid oriented at 21°/77°SE. This flattening deformation is interpreted to be associated with nappe emplacement from north to south. (2) Some regions along the Nakasib Suture are characterized by constriction strain that might be due to refolding of the early nappes about NE-trending axes. (3) The Oko Shear Zone is characterized by constriction strain, with the XY plane of the strain ellipsoid oriented at 171°/68°E. The strain ellipsoid associated with the Oko Shear Zone manifests superimposition of E–W shortening on the NE-trending fold and thrust belt associated with the Nakasib Suture. (4) The tectonic strain of the Oko Shear Zone, superimposed over the structures of the Nakasib Suture, is characterized by a strain ellipsoid whose flattening plane is oriented at 21°/49°W. The strain ellipsoid of the tectonic strain has a major axis with a quadratic elongation of 3.6 and an orientation of 357°/25°, an intermediate axis with a quadratic elongation of 1.2 and an orientation of 231°/30°, and a minor axis with a quadratic elongation of 0.25 and an orientation of 115°/18°. This suggests that the post-accretionary deformation of the Arabian–Nubian Shield was superimposed as a NW–SE directed shortening that created early N–S shortening zones and late NW-trending sinistral strike-slip faults.
The legendary wealth in gold of ancient Egypt seems to correspond with an unexpected high number of gold production sites in the Eastern Desert of Egypt and Nubia. This contribution introduces briefly the general geology of these vast regions and discusses the geology of the different varieties of the primary gold occurrences (always related to auriferous quartz mineralization in veins or shear zones) as well as the variable physico-chemical genesis of the gold concentrations. The development of gold mining over time, from Predynastic (ca. 3000 BC) until the end of Arab gold production times (about 1350 AD), including the spectacular Pharaonic periods is outlined, with examples of its remaining artefacts, settlements and mining sites in remote regions of the Eastern Desert of Egypt and Nubia. Finally, some estimates on the scale of gold production are presented.
The Mission Paléoanthropologique Franco-Tchadienne (MPFT) found a new species of Orycteropodidae (Mammalia, Tubulidentata) in the Kollé fossiliferous sector, northern Chad. After Orycteropus abundulafus [Journal of Vertebrate Paleontology 20 (1) (2000) 205–209; Lehmann, T., Vignaud, P., Likius A., Brunet M., in press. A new Orycteropodidae (Mammalia, Tubulidentata) in the Mio-Pliocene of Northern Chad. Zool. J. Linnean Soc.], this specimen is the second complete skeleton of fossil aardvark found in the Djurab desert. It is the first complete representative of an Orycteropus species found in the Pliocene of Africa. In regard to the Miocene fossil aardvarks, this new taxon, Orycteropus djourabensis nov. sp., shows more affinities with the extant O. afer. The main differences are the larger teeth and the shorter hand in the fossil form. Kossom Bougoudi and Kollé represent a chronological series that gives a unique opportunity for studying the evolution of the African Tubulidentata around the Mio-Pliocene boundary (5.5-4 My). The new species is distinct from the older Chadian Orycteropodid from KB and it embodies the taxonomic turnover that took place within the order Tubulidentata around this boundary in Africa. Moreover, this new species is the oldest known Orycteropus species that clearly belongs to the modern forms including the extant aardvark.
The Asu River Group (middle-late Albian), exposed in the Nkalagu area of the southern Benue trough, consists of two formations, viz: the Abakaliki Formation at the base overlain by the Ebonyi Formation (new name). The Abakaliki Formation is composed mainly of illitic and kaolinitic black shales and mudstones interbedded with black micritic limestone, siltstone adn minor feldspathic sandstones. The limestone is composed of grapestones, infralimeclasts and brown algal remains. These together with the pyritic and fossiliferous chale, suggest oscillatory deposition of argillaceous sediment in a shallow but anoxic subsiding bottom and marine facies.Rapid alternations of sandstone, siltstone, shales, mudstones, oolitic and serpulid grainstones and packstone-wackestone indicative of transitional environment of deposition characterise the Ebonyi Formation. The thick horizontal strata and interference ripple marks suggest lower flow regime of deposition while the thin evaporite laminite indicate locally short-term aridity during the Cretaceous in the southern Benue trough.
The Middle Jurassic (Bathonian) rocks of the Ras El Abd area include three main shell concentrations: a molluscan shell bed, the lower rhynchonellid bed, and the upper rhynchonellid bed. Analysis of the taphonomic signatures indicates that; the molluscan shell bed represents a proximal tempestite, the lower rhynchonellid bed corresponds to a primary biogenic concentration sensu [Fürsich, F.T., Oschmann, W., 1993. Shell beds as tools in basin analysis: the Jurassic of Kachchh, western India. Journal of the Geological Society 150, 169–185], and the upper rhynchonellid bed a proximal storm-flow concentration. The shell concentrations formed below fair-weather wave-base in shallow, relatively high energy environments.
Outcrops in the Cretaceous Abeokuta Group of the Dahomey (Benin) Basin in southwestern Nigeria are described. Based on detailed sedimentological logging, 13 lithofacies were delineated. They are: massive and loose sand; cross-bedded loose sand; normally graded sandstone (upward fining sandstone); laminated silt; clay; laminated sandstone; clay with sandstone injections and clasts (sandstone boulders); clayey sand with sandstone boulders; silty clay with intraformational folds and breccias; highly contorted conglomerate/clayey silt; and channel infills. The association of the lithofacies and their sedimentary structures support that these lithologies were deposited by turbidity current, grain flow, debris flow, liquefied flow and fluidized flow. Since these depositional features are diagnostic of slope environments, the Cretaceous Abeokuta Group was concluded to have been deposited in slope environments.
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.
Abu Hamamid (AH) Neoproterozoic (Sm/Nd model age of ∼770 ± 20 Ma) mafic–ultramafic intrusion lies along a NE–SW fracture zone in the Shadli Metavolcanic Belt, south Eastern Desert, Egypt. AH intrusion is concentrically zoned with cumulate clinopyroxene-bearing dunite core mantled by olivine clinopyroxenite, hornblende clinopyroxenite and hornblende gabbroic rim. The observed crystallization sequence is olivine (+spinel)-clinopyroxene–hornblende. Orthopyroxene is an extremely rare phase in the core rocks. Clinopyroxene is Ca-rich diopside and spinel shows wide range of Cr# (38–85) and Fe3+# (22–95) ratios. Olivine ranges from Fo74 to Fo81. The ferromagnesian minerals from the AH intrusion show a consistent decrease in the Mg# of olivine (81–74), clinopyroxene (89–81), and hornblende (87–66) from core to rim. The gradational contact between the different rock types of the AH mafic–ultramafic rocks, their cumulate nature, the recognition of small-scale layering together with the systematic modal and compositional variations of rock-forming minerals all point to generation by fractional crystallization from a common parental magma. Petrography and mineral compositional data suggest that the AH complex crystallized from fractionated hydrous tholeiitic magma with no significant crustal contamination.AH intrusion shares many important field, petrographic and mineralogical features with Phanerozoic Alaskan-type intrusions formed above subduction zones, suggesting that the AH rocks formed in a similar tectonic environment. Clinopyroxene and spinel chemistry support the subduction-related (island-arc) tectonic environment of origin for the AH complex. This revives interests in models involving subduction-related (island-arc), and possibly plume-interaction origin for the Shadli Metavolcanic Belt.
Abu Zawal area is located north of the Qena-Safaga road. It consists of igneous and metamorphic rocks of late Precambrian age.It has been subjected to geological, radiometric and structural investigations to reveal the structural setting of the area and its relation to the trends of mineralization, as well as the distribution of the radioactivity in the different rock units. Each rock units has been characterized statistically by a certain radiometric background through the radiometric survey traverses. Application of certain statistical inference tests found the relation between the different rock types of Abu Zawal area and their equivalents in other studied areas.It is found that least radioactivity is associated with basic metavolcanic rocks, while the highest values are associated with the late orogenic plutonites and related rocks, while granodiorites have moderate levels of radioactivity. The study suggests that the late orogenic plutonites of Abu Zawal area may be younger than plutons of similar composition in the north of the Eastern Desert.
Phosphorites in Egypt occur in the Eastern Desert, the Nile Valley and the Western Desert at Abu Tartur area and present in Duwi Formation as a part of the Middle Eastern to North African phosphogenic province of Late Cretaceous to Paleogene age (Campanian–Maastrichtian). The Maghrabi-Liffiya phosphorite sector is considered as the most important phosphorite deposits in the Abu Tartur area due to its large reserve thickness and high-grade of lower phosphorite bed beside high content of REE. Back scattered electron (BSE) images show framboidal pyrite filling the pores of the phosphatic grains, suggesting diagenetic reducing conditions during phosphorites formation.Electron Probe Micro Analyzer (EPMA) chemical mapping was conducted to examine the variation and distributions of selected elements (P, F, La, Fe, Yb, Si, Ce, W, Eu, S, Ca, Y and Er) within the shark teeth, coprolites and bone fragments. In the teeth W, S, Fe are concentrated along the axis of the teeth, the bone fragments show high concentration of W, Yb, Er and Eu, whereas coprolites are nearly homogenous in composition contains S, Er with some Si as micro-inclusions. Fluorapatite is considered as main phosphate mineral whereas pyrite occurs as pore-filling within the phosphatic grains and cement materials. Maghrabi-Liffiya samples show a wide range in the P2O5 content, between 19.8 wt.% and 29.8 wt.% with an average of 24.6 wt.% and shows low U content ranging from 15 ppm to 34 ppm with an average of 22 ppm. The total REE content in nine samples representing the Maghrabi-Liffiya ranges from 519 to 1139 ppm with an average of about 879 ppm. The calculation of LREE (La–Gd) show indeed a marked enrichment relative to the HREE (Tb–Lu) where LREE/HREE ratio attains 8.4 indicating a strong fractionation between the LREE and HREE. Chondrite-normalized REE patterns of the studied phosphorite samples show a negative Eu anomaly.
In the Lake Léré region, southern Chad, Neoproterozoic terrains are distributed in four lithostructural groups that reveal the geotectonic evolution of a part of the Pan-African orogenic domain. The first group includes basaltic volcanic rocks and fine-grained detrital sedimentary rocks of pre-tectonic basins that were emplaced in an extensional regime, close to a volcanic arc. The second and third groups include calc-alkaline gabbroic intrusions emplaced at an upper crustal level and a midcrustal tonalite, respectively, that are interpreted to be the roots of an active margin volcanic arc. These first three groups experienced WNW to ESE compression, and may belong to a fore-arc basic—volcanic arc—back-arc basin system that was accreted eastward to the Palaeoproterozoic Adamaoua-Yadé Block. The fourth group includes post-tectonic granite plutons invading the older groups. This paper documents the accretion processes in the southern margin of the Saharan Metacraton.
A suite of sediments and FeMn oxide crusts from the Romanche Fracture Zone (RFZ) in the equatorial Atlantic has been investigated by radiochemical techniques to determine the rates of metal accumulation in the deposits. This approach was used to assess any possible hydrothermal inputs to the deposits.The metal accumulation rate data for the sediments indicate that there is no significant hydrothermal influence in the deposits. The concentration of metals can be accounted for largely in terms of normal oceanic sedimentation processes. This was confirmed by metal accumulation rate data of the crusts that represent hydro-genetic deposit formed by slow accumulation of trace-enriched oxides directly from the water column.
The Acheulian archaeological site Gesher Benot Ya’aqov (GBY) is located along the shoreline of a Pleistocene Lake, the GBY Lake, within the Jordan Rift valley. The uniqueness of the site is expressed in its complex sedimentary sequence, rich fossil assemblages, and in the first appearance of lithic technologies not previously seen in Eurasia. The site is reliably dated at the Matuyama–Brunhes geomagnetic polarity chron boundary (∼780 kA), which occurs within the lake sediments.The sedimentary sequence at GBY exhibits a pronounced cyclicity. A single first order cycle begins with a lower conglomerate followed by lacustrine beds, overlain by an upper conglomerate that closes the cycle. Second-order cyclicity is apparent in shifts from littoral to deeper water facies, with five such cycles, identified within the first order cycle. The diatom and ostracod distributions indicate a freshwater–oligohaline environment of deposition for the lower part of the section and oligohaline–meio-mesohaline waters in the upper part of the section. In addition, the C and O isotopes and the Sr/Ca and Mg/Ca ratios of the ostracod (Candona neglecta) shells exhibit three geochemical zones supporting the paleoecological biozonation. They reflect a cooler period for the lower part of the sequence (interval I), changing into warmer climate at the Matuyama–Brunhes boundary and upward (interval II). Further gradual temperature increase in the upper interval III caused the retraction and desiccation of the GBY Lake.
The Kibaran aged Irumide belt and the Pan African aged Lufilian arc intersect in central Zambia. The Irumide belt is a thrust belt comprising northwesterly verging structures in the north, upright structures in the central zone and southeasterly verging structures in the south. Tectonic transport, as deduced from regional stretching lineations, changes across the central upright zone. To the north of this zone, movement is to the northwest; to the south of the zone, movement is to the southeast. This divergence of structures about a central upright zone is recognized throughout the belt. The Lufilian arc comprises a northeasterly verging thrust belt involving large basement thrust sheets forming domal culimations throughoutregion. These thrusts climb up-section towards the northeast and have telescoped the Katangan stratigraphy. In the Copperbelt area of the arc, the Irumide and Lufilian structures are separated by a marked unconformity. However in the Mubalashi area, south of the Copperbelt, there is aa coincidence of strike of Lufilian and Irumide structures which, in the past, has made their separation difficult. The structures can be separated on the basis of stretching lineations associated with the deformation. In the ENE striking Lufilian structures stretching lineations are seen to be sub-horizontal, suggesting a lateral ramp relationship to the main Lufilian deformation. Similar striking Irumide structures have a steeply plunging down dip lineation.The intersection of these two belts represents the junction of two different tectonic systems operating in Africa during the Late Proterozoic.
An aeromagnetic profile across the Benue Trough of Nigeria has been interpreted. An attempt to interpret the anomalies solely in terms of a metamorphic basement of variable topography proved unsuccessful as such an interpretation leads to a basement of too high a magnetisation, too thick a sedimentary cover and outcrops of the basement in areas not compatible with the known geology of the area. The anomalies in the profile were best accounted for in terms of basic intrusive bodies occurring either predominantly within the Cretaceous sediments or within the metamorphic basement or both.
Waters from Wadi Aggai display a magnesian bicarbonate chemical facies, related to a dominantly dolomitic basement. Large travertine deposition occurred during the humid episodes of the Plio-Quaternary. Current travertine deposition is strongly reduced, the only functional sites being located in the upper parts of the watershed. This is due to the general climatic trend towards aridity and, more importantly, the environmental disequilibrium due to human impact. Such an impact is inferred from changes in water quality, due to deforestation and resulting soil erosion, disposal of sewage and waste materials, and use of fertilisers.
Whilst groundwater vulnerability mapping and the delineation of resource and source protection zones have become an appropriate set of management tools for Britain as incorporated in European policy, much of semi-arid Africa is still dealing with more pressing issues centred on water supply coverage. There are a number of fundamental differences with Britain which disallow conventional vulnerability mapping and land zonation in much of Africa. Firstly, the scale of groundwater occurrence in weathered basement aquifers does not encourage vulnerability mapping to be undertaken at a field scale, whereas the Karoo and some of the larger areas of unconsolidated sedimentary aquifers could more readily be zoned according to aquifer vulnerability. Secondly, analysis needs to disregard the productivity (or recharge potential) of the aquifer so that poorly productive but socially important aquifers can be assessed. Such practice also avoids the need to identify a value for effective rainfall, a problematical value in semi-arid climates given current uncertainties. Thirdly, it is difficult to protect the many small and dispersed groundwater supply sources typical of many African communities when compared with the fewer and larger sources characteristic of Britain and Europe. Some aspects of European groundwater management practice have been transferred to the African context, notably by South Africa, but there are other practices that should not be attempted. Lessons from experience in South Africa highlight capacity to implement as a key inhibiting factor. Examples of vulnerability assessment and land zonation in a variety of African settings indicate only limited success so far with standard vulnerability assessment and land zonation techniques. Alternative means of quantifying the problem of the optimum proximity of the pit latrine from the well are highlighted, with a minimum separation of 10 m suggested for typical weathered basement rocks.
Ichnological and sedimentological analyses in the Eastern Cape allowed the first description of a Cochlichnus-dominated ichnofossil site from the mid- to Upper Permian Middleton Formation (Karoo Supergroup) in South Africa. The locality is within the uppermost Pristerognathus Assemblage Zone, a biostratigraphic interval characterized by a low vertebrate biodiversity at the turn of the mid- to Late Permian. Our field data indicates that the surficial bioturbation of very fine to fine-grained sand layers resulted from life activities of shallow infaunal and epifaunal invertebrates (possibly annelids, aquatic oligochaetes, nematodes, insect larvae) and fish. The morphology of the trails, their relationship to the substrate and the behaviour inferred from them indicate that the tracemakers developed a strategy that facilitated the optimization of low food resources in a permanently submerged freshwater setting. Combined ichnological and sedimentological evidence suggests a low-energy, freshwater lacustrine depositional environment, where occasional higher energy currents brought nutrients. Data also imply that colonization of these erratic event beds by opportunistic sediment-feeders was short-lived and followed by longer intervals of lower energy deposition under possibly poorly oxygenated conditions. We propose that these event beds as well as the sporadic red mudstones of the Middleton Formation may have formed during short-term, higher storm-frequency and dryer periods, signalling changes in the otherwise humid climate in this part of the main Karoo Basin during the mid- to Late Permian.Highlights► Simple, low diversity, low abundance, shallow infaunal and epifaunal grazing traces. ► Ichnofossils represent an impoverished, permanently subaqueous Mermia ichnofacies. ► Associated with lacustrine event beds at the turn of the Middle to Late Permian. ► Middleton Formation lakes were deep enough for oxygen-stratification to occur.
We here summarize the evolution of the greater Red Sea–Gulf of Aden rift system, which includes the Gulfs of Suez and Aqaba, the Red Sea and Gulf of Aden marine basins and their continental margins, and the Afar region. Plume related basaltic trap volcanism began in Ethiopia, NE Sudan (Derudeb), and SW Yemen at ∼31 Ma, followed by rhyolitic volcanism at ∼30 Ma. Volcanism thereafter spread northward to Harrats Sirat, Hadan, Ishara-Khirsat, and Ar Rahat in western Saudi Arabia. This early magmatism occurred without significant extension, and continued to ∼25 Ma. Much of the Red Sea and Gulf of Aden region was at or near sea level at this time. Starting between ∼29.9 and 28.7 Ma, marine syn-tectonic sediments were deposited on continental crust in the central Gulf of Aden. At the same time the Horn of Africa became emergent. By ∼27.5–23.8 Ma a small rift basin was forming in the Eritrean Red Sea. At approximately the same time (∼25 Ma), extension and rifting commenced within Afar itself. At ∼24 Ma, a new phase of volcanism, principally basaltic dikes but also layered gabbro and granophyre bodies, appeared nearly synchronously throughout the entire Red Sea, from Afar and Yemen to northern Egypt. This second phase of magmatism was accompanied in the Red Sea by strong rift-normal extension and deposition of syn-tectonic sediments, mostly of marine and marginal marine affinity. Sedimentary facies were laterally heterogeneous, being comprised of inter-fingering siliciclastics, evaporite, and carbonate. Throughout the Red Sea, the principal phase of rift shoulder uplift and rapid syn-rift subsidence followed shortly thereafter at ∼20 Ma. Water depths increased dramatically and sedimentation changed to predominantly Globigerina-rich marl and deepwater limestone.
The Blue Nile Basin is located in the Central Plateau of Ethiopia. The basin consists of Precambrian basement, Palaeozoic and Mesozoic sedimentary rocks and Tertiary volcanic rocks. The sedimentary successions and Adigrat Sandstone reach a maximum thickness of 3000 and 800 m, respectively. The formation is composed of mudstone, finely laminated siltstone, very fine-grained cross-bedded sandstone, coarse to medium-grained sandstone, massive to crudely cross-bedded gravely sandstone and massive to crudely-bedded conglomerate. In the Blue Nile Basin, the Adigrat Sandstone was deposited in alluvial fan, fluviatile and lacustrine depositional environments. The formation has a complex diagenetic history and cemented by silica, carbonate, kaolinite and hematite with minor amounts of dolomite, illite, chlorite and feldspar overgrowths. Depositional environment, burial history and diagenetic processes are the major factors, which control the porosity and permeability of the Adigrat Sandstone. Primary porosity is preserved due to framework grain stability. Dissolution of carbonate cements created a certain amounts of secondary porosity in medium to coarse-grained sandstones, siltstones and mudstones facies. The porosity and permeability reach up to 20.4% and 710 mD, respectively. The medium-coarse-grained sandstones are porous and potential for oil and gas reservoir, whilst low-permeability siltstones and mudstones are possible gas reservoir.
The Adigrat Sandstone in northern Ethiopia was sampled for chemical and mineralogical analysis. The samples were analysed for major elements, rare earth and other trace elements by XRF and ICP-MS. The mineralogy was studied using petrographic microscopes and XRD. The results were used to infer the provenance and geotectonic setting of the depositional basin.Quartz (average of 91%), with subordinate amount of clays and K-feldspar, dominates the mineralogy of the Adigrat Sandstone indicating that the sandstone is highly recycled and matured. The apparent source material was a quartzose sedimentary rock, which is represented in the study area by the Palaeozoic sediments of the region. However, the low values of Eu/Eu*, Ta/Ta*, Cr/Th and high values of LREE/HREE, La/Sc, Th/Sc, Th/Co, and the REE pattern, demonstrate that its ultimate sources were silicic to intermediate (granites, granodiorites and granitic gneiss) rocks of the nearby basement. The dominance of quartz in the mineralogy together with enrichment in the immobile elements like La, Zr, Hf and others, suggest that the tectonic setting of the depositional basin formed part of a passive margin.
Over 35,000 onshore and offshore gravity stations have been compiled in order to test isostatic models against geologic structures over a part of the Afro–Arabian shield. The area of Ethiopia covers an important part of this system because it contains the major section of the ≈5000 km Afro–Arabian rift and includes the transition between the Arabo-Nubian-Shield (ANS) and the Mozambique Belt (MB).Isostatic residual anomalies have been calculated using both Airy and Vening-Meinesz (flexural rigidity D = 1022 Nm) models. The isostatic residual anomalies outline the major Precambrian belts, the Cenozoic rifts and associated major structures. Positive residual anomalies associated with the main Ethiopian Rift (MER) and Kenyan rift systems could be the expressions of an axial intrusive body and swarms of local faults and fractures. The residual anomalies indicate relative stability in the MER and increased tectonic activity in the areas of the Red Sea, Gulf of Aden and Afar. Near-zero isostatic residuals flank the MER and Kenya rifts and are found within the Danakil Alps and some plateau regions.The small mean isostatic residual anomaly (about 8 mGal) and the isostatic analysis show a slight positive bias indicating under compensation. The undercompensation may imply that there are upper crustal features that are not compensated regionally (probably supported by the rigidity of the lithosphere) and isostatic disequilibrium in the region. Therefore, the high topography of Ethiopia and East African plateau is partly compensated by thicker crust (broad negative isostatic regional anomaly) and partly by dynamic forces.The results of the qualitative interpretation form the basis of continuing three-dimensional gravity modelling and quantitative analysis that also integrates data from eastern Sudan.
The Adola belt forms the transition zone between the Neoproterozoic Arabian-Nubian shield and the Mozambique belt. In an attempt to assess its plate tectonic evolution, representative suites of magmatic rocks were analyzed for major and trace elements (including rare earth elements - REE). Based largely on REE and other trace element data, four tectonic environments are distinguished. These are: 1. i) continental basement (Awata gneiss); 2. ii) island arc (Daba and Bursano meta-igneous complexes); 3. iii) back-arc (Megado and Kenticha meta-igneous-sedimentary rocks); and 4. iv) MORB (Reji amphibolite). Boninitic meta-andesites mark the onset of back-arc tholeiitic volcanism in the Megado rocks. The recognition of MORB-type chemical characteristics from Reji makes the Adola region an important site in the East African orogen, in which a component of Neoproterozoic oceanic crust is preserved. The rock units that characterize the different tectonic settings from narrow belts up to 15 km wide signifying shortening during collision between east and west Gondwanaland. The extensive shortening and late-stage strike-slip components recognized in the Adola belt are consistent with the Adola region being geographically close to the Mozambique orogenic front during the Neoproterozoic. The results support the application of collision and tectonic escape models to the East African orogen.
A large part of the central Sinai peninsula, covering mainly the El-Tih and Egma plateaux, was evaluated using geophysical data. The Bouguer and aeromagnetic anomaly maps were critically analyzed and correlated to investigate regional tectonism, structural elements and major sedimentary basins characterizing this region. The regional Bouguer anomalies reflect density variations at great depth as well as crustal thickening towards the Gulf of Aqaba. Parameters of the interpreted major and minor faults, which are oriented mainly NNE, NE and NW, were calculated. The boundary faults along the Gulf of Aqaba downthrow by about 6 km to the east, with some of these faults showing surface expression. Residual gravity and magnetic maps reflect a central high anomaly zone trending NE, but its northern part deviates E-W suggesting either a shallow and wide basic/ultrabasic dyke or uplifted basement. Depth calculations of the magnetic and gravity anomaly profiles indicate that the shallow sources have depths down to 1.0 km, however, the depth to the basement surface is ∼3.5 km. Significantly low Bouguer and magnetic anomalies reflect a major sedimentary basin containing a thick layer of clastic Palaeozoic-Mesozoic sediments, which overlie basement rocks, and are topped by subhorizontal non-clastic Tertiary sediments. The results of this paper should be of considerable help in delineating some exploration concepts of hydrocarbon and groundwater resources for developing this part of Sinai.
Airborne magnetic data and field observations were used to define structural trends in some parts of North Central Nigeria. The aeromagnetic anomalies of the Jurassic Younger Granite complexes are distinctive with relatively high amplitudes and short wave-lengths and are directly associated with the outcrop patterns of the intrusions. The mosaic pattern of magnetic anomalies may be used to identify the Younger Granite ring complexes. The aeromagnetic data also contain evidence of linear structural discontinuities. some of these linear features are long and extend through the survey area. They are identified by a pattern of contour offsets and probably reflect deep crustal features cutting some of the Younger Granite known to be mineralised. In particular, a major fault is mapped which cuts the Ririwai complex to the East. This fault trending NW-SE, appears to extend southwards through the area. The lineaments probably acted as conduit through which mineralized fluid flowed. The regional trends of structures are revealed on the aeromagnetic map, these being mainly northeast-southwest and northwest-southeast. The Northwest-southeast trends are superimposed on the northeast-southwest trends. This overprinting of one magnetic anomaly trend over the other suggests that the Northeast-southwest lineaments are older than the northwest-southeast lineament patterns Three known tin-bearing ring complexes show strong magnetic anomalies which make them more prominent than the other complexes on the coloured magnetic anomaly map. This characteristic should assist in locating new provinces for primary tin deposits.
The interpretation of regional aeromagnetic data is very useful for structural interpretations. In this study, we have used three techniques to locate geological contacts and major faults, namely: the Euler deconvolution, the analytic signal and the local wavenumber methods. These three techniques have been tested on synthetic models for accuracy and sensitivity, to determine the best parameters that yield a realistic interpretation. The results obtained from each method on synthetic data, with or without random noise, are compared and discussed here. The three methods were applied to the interpretation of aeromagnetic data from Eastern Hoggar (Algeria). This interpretation led to the establishment of a structural map of the Eastern Hoggar with the eastern boundary of the Issalane terrane suggested to be the suture zone between the central Hoggar and the Saharan metacraton.
This article outlines geomorphological and tectonic elements of the Afar Depression, and discusses its evolution. A combination of far-field stress, due to the convergence of the Eurasian and Arabian plates along the Zagros Orogenic Front, and uplift of the Afar Dome due to a rising mantle plume reinforced each other to break the lithosphere of the Arabian–Nubian Shield. Thermal anomalies beneath the Arabian–Nubian Shield in the range of 150 °C–200 °C, induced by a rising plume that mechanically and thermally eroded the base of the mantle lithosphere and generated pulses of prodigious flood basalt since ∼30 Ma. Subsequent to the stretching and thinning the Afar Dome subsided to form the Afar Depression. The fragmentation of the Arabian–Nubian Shield led to the separation of the Nubian, Arabian and Somalian Plates along the Gulf of Aden, the Red Sea and the Main Ethiopian Rift. The rotation of the intervening Danakil, East-Central, and Ali-Sabieh Blocks defined major structural trends in the Afar Depression. The Danakil Block severed from the Nubian plate at ∼20 Ma, rotated anti-clockwise, translated from lower latitude and successively moved north, left-laterally with respect to Nubia. The westward propagating Gulf of Aden rift breached the Danakil Block from the Ali-Sabieh Block at ∼2 Ma and proceeded along the Gulf of Tajura into the Afar Depression. The propagation and overlap of the Red Sea and the Gulf of Aden along the Manda Hararo–Gobaad and Asal–Manda Inakir rifts caused clockwise rotation of the East-Central Block. Faulting and rifting in the southern Red Sea, western Gulf of Aden and northern Main Ethiopian Rift superimposed on Afar. The Afar Depression initiated as diffused extension due to far-field stress and area increase over a dome elevated by a rising plume. With time, the lithospheric extension intensified, nucleated in weak zones, and developed into incipient spreading centers.
Africa’s landscape is dominated by a manifold of second-order epeirogenic structures superimposed on a first-order bimodal topography. Bivariate regression analysis of Africa’s surface topography shows that this is a complexly folded surface with regionally elevated areas in southern and eastern Africa, and a topographically low northern and western Africa. The apparent spatial relationships between these features are analysed using anomaly correlation between surface topography and free-air gravity anomalies. Occurrences of positively correlated features between gravity and topography in Africa are found to be limited to second-order epeirogenic features. Geophysical modelling and geologic evidence indicate that Africa’s bimodal topography is genetically distinct from these second-order features, and linked to sources as deep as the sublithospheric mantle. The age, measured and modelled elevation of the bimodal topography require that topographic uplift of south-central Africa be episodic. We infer from our findings together with relative sea-level changes, that the near-bimodality of Africa’s topography is an ancient feature inherited at least from upper Paleozoic times. Our reconstructed paleotopography suggests that Africa was largely a low-lying continent dominated by its cratons, and that basement distribution disregards the present-day uplift patterns of Africa.
An exploration programme over the Blaauwbank group of gold deposits in the Witwatersberg Goldfield, which lies to the south and in the footwall of the Bushveld Complex, South Africa, has shown that these deposits possess many similarities and some differences to turbidite-hosted gold deposits.The dominant control to this hydrothermal mineralization is structural, with the associated quartz veins being localized along bedding planes, fold axes and a variety of semi-ductile to brittle faults. The deposits of the Witwatersberg Goldfield are also preferentially located in the Klapperkop Quartzite Member and overlying shales of the Timeball Hill Formation, Transvaal Supergroup. The rocks do not appear to form a particularly favourable protore but rather represent a favourable depositional site. The competency contrast of the shales, siltstones and sandstones in this stratigraphic interval is regarded as being important in facilitating vein formation, and the zone is also characterized by a high Fe2+/Fe3+ ratio and high C content, which may have reduced the mineralizing fluids and caused gold to precipitate. Fluid inclusion data show the quartz veins formed at 250–330 °C and pressures of 1.3–1.75 kb, from CaCl2-rich brines.Comparison with structural studies to the north and with the Sabie-Pilgrim’s Rest Goldfield to the east suggest that the mineralization probably predates the peak of metamorphism related to the intrusion of the Bushveld Complex.
Radiometric analyses and ages for 96 monazite grains from the five subunits of the Kwabonambi and Sibaya Formations (Cenozoic Era; Maputaland Group) package of heavy-mineral rich sands on the KwaZulu-Natal coastal plain are presented. Although the ages range from 445 to 2366 Ma, and the different sub units and magnetic fractions show slight variations, there is a clearly defined pattern. The larger, and older, subgroup of grains (800–1299 Ma) encompasses, but shows a greater age range than, the rocks of the Natal Structural and Metamorphic Province; while the younger subgroup (400–699 Ma) can be correlated with the Saldanhian or Mozambican Orogeny.The presence of monazite grains with ages that are clearly not primarily related to stratigraphic units present within the modern (post Cretaceous) drainage basin, but reflect their initial provenance, indicates that the derivation history of the heavy minerals in the heavy mineral-rich Cenozoic sediments of the Kwazulu-Natal coastal plain is more complex than generally accepted. A polycyclic history is therefore envisaged with an intermediate sedimentary host (the Natal Group and/or Karoo Supergroup) for most, if not all of the heavy mineral grains in the Kwabonambi and Sibaya Formations.
In the south-western part of the Karoo basin, southward-dipping monoclines apparently had an influence on sedimentation patterns in fluvial deposits of the Late Permian-Early Triassic Beaufort Group. Palaeocurrents in this area are generally directed towards the northeast but deviate sharply towards the east on the steep flanks and towards the north on the gently dipping flanks of all five monoclines examined. Lithofacies also show statistically significant differences on the two flanks of the DR-3 monocline which is associated with a major uranium deposit. Structural evidence suggests that the monoclines probably formed after compaction of the sediments, therefore, an earlier phase in their development must have been responsible for the characteristic sedimentation patterns. It is suggested that shallow synclines, related to compressional paroxysms of the Cape orogeny, deflected streams parallel to their axial trends. As uranium occurs preferentially in thick channel sandstones, their association with monoclines can therefore be used as an exploration tool in the Karoo.
Africa displays a variety of continental margin structures, tectonic styles and sedimentary records. The comparative review of two representative segments: the equatorial western Africa and the SW Africa margins, helps in analysing the main controlling factors on the development of these margins. Early Cretaceous active rifting south of the Walvis Ridge resulted in the formation of the SW Africa volcanic margin that displays thick and wide intermediate igneous crust, adjacent to a thick unstretched continental crust. The non-volcanic mode of rifting north of the Walvis ridge, led to the formation of the equatorial western Africa margin, characterised by a wide zone of crustal stretching and thinning, and thick, extensive, syn-rift basins. Contrasting lithologies of the early post-rift (salt vs shale) determined the style of gravitational deformation, whilst periods of activity of the decollements were controlled by sedimentation rates. Regressive erosion across the prominent shoulder uplift of SW Africa accounts for high clastic sedimentation rate during Late Cretaceous to Eocene, while dominant carbonate production on equatorial western Africa shelf suggests very little erosion of a low hinterland. The early Oligocene long-term climate change had contrasted response in both margins. Emplacement of the voluminous terrigenous Congo deep-sea fan reflects increased erosion in equatorial Africa, under the influence of wet climate, whereas establishment of an arid climate over SW Africa induced a drastic decrease of denudation rate, and thus reduced sedimentation on the margin. Neogene emplacement of the African superswell beneath southern Africa was responsible for renewed onshore uplift on both margins, but it accelerated erosion only in the Congo catchment, due to wetter climatic conditions. Neogene high sedimentation rate reactivated gravitational tectonics that had remained quiescent since late Cretaceous.
Evidence of Late Pleistocene and Holocene palaeoenvironmental change in the northern hinterland of the KwaZulu-Natal province, eastern South Africa, is preserved within a widespread sequence of colluvium and intercalated palaeosols, comprising the Masotcheni Formation. Palaeosol profile characteristics and micromorphological features are described for the full stratigraphic succession, preserved at the St Paul's donga reference site and other comparative exposures, permitting the interpretation of pedogenic processes active on hillslopes during the polycyclic accretion of colluvial sediments blanketing many hillslopes in the region. The importance of soil erosion and the transport of pre-weathered sediment and pedorelict aggregates is highlighted. The 'eluilluviation' particle translocation process, commonly synchronous with periodic Fe mobilization and reprecipitation,was active at differing intensities in all of the palaeosols studied. The palaeoclimatic implications of the aforementioned processes during the past similar to 120 ka are placed in chronological context using a series of radiocarbon dates from palaeosol organic matter and soil carbonates and infra red stimulated luminescence (IRSL) dating of the parent colluvium. The dating framework permits generalized interpretation of the periodicity of erosional and colluvial events as well as the period of existence of palaeosol landsurfaces, which is of relevance when interpreting the age of similar soils on the current landsurface.
Geophysical and geological evidence suggest that the asthenospheric structure beneath the presently active east African rifts resembles that of the Basin and Range tectonic province of the western United States. Both regions are underlain by anomalously thin lithosphere riding on hot asthenospheric pillows that extend laterally for as much as 1500 km normal to structural trend. The continental rifts associated with these terranes do not appear to be leading to plate separation, and young oceanic rifts such as the Red Sea probably evolved from narrower, more restricted mantle thermal anomalies. Low-angle normal faults are interpreted to control the physiography and structural style in the east African rifts in a fashion analogous to that proposed for the Basin and Range. The limited horizontal extension that has occurred in east Africa, however, requires that mechanical thinning of the lithosphere must play a subsidiary role to thermal crosion from below. The upward migration of mantle isotherms has converted the base of the lithosphere to material with asthenospheric properties beneath thousands of square kilometers of crust. The presence of a broad zone of thin, warm lithosphere in both these tectonic provinces may help to distribute extensional strains over correspondingly broad regions. This in turn is expected to result in the development of numerous detachment surfaces (and corresponding surficial rifts), none of which ever completely thin the lithosphere or evolve to oceanic-style rifts.
The Karoo Supergroup in southern Africa occurs in the areally extensive Main Karoo and Kalahari basins as well as in a number of subsidiary basins in South Africa, Namibia, Botswana, Zimbabwe and Mozambique. The main Karoo basin constitutes a retro-arc foreland basin, while the rest are intracratonic sag basins or rift basins. Pre-erosion thicknesses of the sedimentary succession range from over 10,000 m in the southern foredeep part of the Main Karoo basin and the Cabora Bassa rift basin, to less than 1000 m in most of the subsidiary basins. A lithostratigraphical subdivision into groups, formations and members has now been accepted for most of the basins, but in some, traditional, non-lithostratigraphical terms remain in use. Non-marine vertebrate and plant fossils are common in many of the basins and the former have been used to subdivide the Beaufort Group in the main basin into eight assemblage zones. A palynological biozonation has proved feasible for the Permian strata in some of the Karoo basins. The Karoo Supergroup ranges in age from Late Carboniferous to Early Jurassic. The strata were deposited in glacial, deep marine (including turbidite), shallow marine, deltaic, fluvial, lacustrine and aeolian environments.