Journal of African Earth Sciences

Published by Elsevier
Print ISSN: 0899-5362
The record of arid climatic events in continental sedimentation is subject to the intensity of the climatic mechanism which generated them and to the nature of the environment which recorded them. In them Sahel, where the environmental equilibrium is particularly precarious, all types of arid climatic events are likely to be recorded. But the consequences of events of small global intensity (exceptional climatic events and local climatic anomalies) merge with the effects of the ⪡average climate⪢. On the other hand, climatic events on a global scale (climatic crises and climatostratigraphic events) are clearly pointed out, due to the large breaks in environmental equilibrium that they initiate.RésuméL'enregistrement des évènements climatiques arides dans la sédimentation continentale est fonction de l'intensité des mécanismes climatiques qui les prodiusent et de la nature du milieu qui les enregistre. Au Sahel, oùl l'environnement est en équilibre précaire, tous les types d'évenements climatiques arides sont susceptibles d'être enregistrés. Mais les conséquences des évènements de faible intensité á l'échelle globale (évènements climatiques exceptionnels et anomalies climatiques) se confondent avec les effets du climat sahelien moyen et n'ont pas de valeur stratigraohiques Par contre, les évènements enregistrables à l'ćhelle globale (crises climatiques et évènements climatostratigraphiques) sont généralement mis en évidence dans les séries géologiques, en raison des importantes ruptures d'équilibre qu'ils provoquent dans l'environnement du Sahel.
An orientation survey was undertaken in northwest Ngamiland to evaluate, among other criteria, an optimum size fraction for trace element analysis in the Kalahari sandveld regolith samples for a regional geochemical mapping project in the area. The most prominent geomorphological features of the region are the longitudinal rolling sand dunes and floodplain sediments adjacent to the unique, mid-continent Okavango Delta. Most of the dune sands were deposited in the Late Quaternary, and they show evidence of both pluvial and and environments. Presently the a lab dunes support varied shrub/grass vegetation with intermittent tree cover. Extensive forest fires have affected the region for generations, and the occurrences of remnants of large trees indicate that the region may have also supported large forests at some stage.
Phosphate rock has been known in Africa since at least 1873, when it was discovered by Philippe Thomas in Algeria. Further discoveries were made in fairly rappid succession elsewhere in northern Africa, notably in Tunisia, Egypt and, in the early part of the 19th century, in Morocco, Mauritania and South Africa. Phosphate rock of diverse types is now known to occur in most other African countries, in rocks ranging from Late Precambrian (Late Proterozoic) to Pleistocene in age and of both sedimentary and igneous origin. Resources are unequally distributed, however, deposits of Lower Tertiary age in northern Africa accounting for the bulk of the resources as well as most of the commercial production of phosphate rock in Africa.There is considerable need for systematic and comprehensive geological reference data on known deposits and occurrences, particularly when the mineral potential of individual countries or regions is assessed. Several major reference works on phosphate rock are now available and include a bibliography of world resources published in 1983. As such, the present bibliography for the period 1979–1988 is a useful supplement, comprising a compilation of the more recent literature on the phosphate geology and resources of Africa. As previously, the bibliography is arranged by continent and under individual countries. References to literature are then given in alphabetical order by author.
An integrated chronicle of major events leading to the growth of the pre-2.0 Ga Indian Craton, which is the aim of this paper, is an essential requirement to constrain the possibility of Neoarchaean unification between Africa and India. The primordial sialic crust that eventually developed into the early Indian Craton segregated from the mantle before 3.8 Ga. Intially there were two seperate Indian blocks, the northern (NIB) and the southern (SIB), and they possibly amalgamated before 2.5 Ga. Rapid and extensive crustal growth at ca 3.1, 2.5 and 2.0 Ga, in conjunction with a related rise in relative sea level due to ocean basin volume reduction, kept the continental freeboard at a moderate level. The 2.5 Ga event was the greatest in magnitude and is likely to have led to the formation of an Indian supercontinent. Four sedimentary basins, one in the NIB and three in the SIE, developed on the typical Archaean tonalite-trondhjemite-granodiorite basement, through rifting induced by mantle upwelling. Continental freeboard was lowered as a consequence and transgressions generally followed. Rifting persisted in all the pre-2.0 Ga basins, except one (Bastar) in the SIE, which only underwent a Wilson cycle as the two blocks collided. All the SIE basins were closed by 2.0 Ga, while the basin in the NIB, which only developed at ca 2.5 Ga, still persisted. Neoarchaean continuity between the Central Indian Tectonic Zone and the Limpopo Belt appears likely from all major aspects, but for the deformation history, which still remains elusive.
Drilling penetrated pre-Mesozoic crystalline basement beneath abbreviated sedimentary sequences overlying fault blocks in the south-eastern Gulf of Mexico. At Site 538A located on Catoche Knoll, a foliated, regional metamorphic association of variably mylonitic felsic gneisses and interlayered amphibolite is intruded by post-tectonic diabase dikes. Hornblende from the amphibolite records 40Ar/39Ar plateau dates of c. 500 Ma, indicating early Paleozoic metamorphism. Biotite from the gneiss yields a plateau age of c. 350 Ma which likely dates a superposed late Paleozoic thermal event. A whole-rock diabase sample records a crystallization date of 190.4 ± 3.6 Ma. A phyllitic metasedimentary sequence was penetrated at Site 537 drilled 30 km northwest of Catoche Knoll. Whole-rock phyllite samples display variably discordant 240Ar/39Ar age spectra, but plateau segments clearly document an early Paleozoic metamorphism at c. 500 Ma. The age and lithologic character of the basement terrane penetrated at Sites 537 and 538A suggest that the drilled fault block are underlain by attenuated fragments of continental crust of “Pan-African” affinity. This support pre-Mesozoic tectonic reconstructions which locate Yucatan in the present Gulf recess during amalgamation of Pangea.
Data from 48 points of electrical soundings (maximum AB = 1 100 m) around the Abakaliki lead-zinc and brine prospects in south-eastern Nigeria were interpreted. Results derived by both direct and inverse methods portray the significant lithostratigraphic and structural setting of the region.The core of a NE-SW anticlinal structure is composed of a thick sandstone formation overlain by marine shales along the flancks of the anticline.Shally portions with higher conductivity represent reservoirs of connate brines which could be characteristic of a potential source for the brine springs.These results, when correlated with recent geochemical data, confirm the existence of two major genetical relationships between the lead-zinc mineralization and the connate brines (constituting the mineralizing fluids on the one hand and the sandstone formation serving as potential source for the metals) on the other hand.
(A) Location of the town of Nalut along the Jabal Nafusah escarpment, near the Tunisian border, northeastern Libya. (B) Simplified bedrock geology of the Nalut area, including the outcrop belt of the Cabao and Chicla formations (the Cabao Formation is a time transgressive unit (?TithonianNeocomian) which underlies the Chicla Formation). Figure modified from Smith et al. (in press).
An isolated theropod dinosaur tooth discovered in 1984 near Nalut in northwestern Libya is the only known dinosaur record from the Aptian–Albian (Lower Cretaceous) Chicla Formation in the Jabal Nafusah region of the country. The tooth was re-examined in an effort to better ascertain its taxonomic affinities. A stepwise discriminant function analysis compared the Libyan tooth to the dentitions of 24 other theropods and classified the tooth with cranial material from the Late Cretaceous of India previously referred to the abelisaurid Indosuchus. The temporal and paleogeographic “distance” separating the Libyan specimen from “Indosuchus” indicates that the former cannot pertain to that genus. However, the results of the analysis and synapomorphies of Abelisauridae present on the Libyan crown indicate that it can be referred to the clade, thus contributing to the growing record of Abelisauridae in the Cretaceous of mainland Africa.
Combined investigations on the clay minerals and the microfloras of the backshore to shallow marine offshore strata of the Abu Ballas Formation (Aptian of southern Egypt), give evidence of warm and semi-arid climatic conditions. The sedimentary successions studied, predominantly consist of alternating pelitic and psammitic siliciclastic deposits. Their clay-mineral association with kaolinite, high-cristallinity illite, illite-dominated randomly interstratified illite-smectite and chlorite is inherited from continental areas under erosion. The Abu Ballas clay minerals reflect only subordinate pedogenetic chemical alteration which suggests a restricted humidity and the absence of a major vegetation. They were transported into the marine environment by periodic river systems. Aeolian processes played a minor role. The Abu Ballas microfloras are overwhelmingly dominated by terrestrial pollen and spores. Marine phytoplankton is extremely rare. Important changes in the local Jurassic to Early Cretaceous microfloras include a decline of ferns and Araucariaceae and, starting with Barremian-Aptian time, the appearance and rise in frequency of early angiosperms and of ephedroids. This seems to indicate a paleoclimatic trend towards less humidity and rising aridity which may be supported by other Abu Ballas fossils such as the lung-fish Ceratodus and the palm fruit Hyphaeneocarpon aegyptiaca Vaudois-Miéja and Lejal-Nicol, 1987.
An attempt is made to analyse how magmatism in continental rifts is controlled by the geometry of rifting. Distribution of volcanics in some rifts is compatible with the simple shear model of rift evolution. Using data on the Ethiopian and Red Sea Rifts, it is demonstrated that at the early stages of rifting, magmatic activity concentrates in belts 5-10 km wide coinciding with the break-away and/or hinge zones of the rift. Comparison of two belts exposed at different erosional levels-the Wonji belt (Ethiopian Rift) and the magmatic belt of the Red Sea eastern coastal plain, with addition of data on xenoliths in basalts allows the construction of a composite vertical section of the magmatic belt. It appears that processes similar to spreading and crustal accretion in oceanic spreading centres may be active in continental rifts. However, the geometry of accretion and composition of the accreting material are different from the ones in oceanic settings. A model scheme of 'magmatic' rift evolution in 5 stages is presented. (1) Formation of a low angle fault, uplift of the asthenosphere, decompression and partial melting. (2) Migration of melt along the low angle detachment, formation of sub-crustal reservoir, eruption of plateau basalts on the upper plate. (3) Further concentration of extension and magmatic activity in the break-away and hinge zones of the rift. At that time, magmatic accretion of the crust, compensating for extension becomes effective. (4) Blocking of magmatic conduits by underplating and formation of new ruptures in the axial zone of the rift. (5) Eruption of MORB through the axial ruptures.
This paper proposes an alternative high-order, non-genetic classification of the basement rocks of medial New Zealand. More than 90% of the rocks in the Median Tectonic Zone are plutonic and can be included in part of a newly defined Carboniferous to Early Cretaceous, ca 10,200 km2 composite regional batholith - the Median Batholith. The plutonic rocks of the batholith intrude the volcanic and sedimentary rocks of the Brook Street and Takaka Terranes (Eastern and Western Provinces, respectively). Emerging matches between the chronology of magmatism in the Median Batholith and batholiths in the Western Province also support probable in situ growth of most of the batholith. The internal and external contacts, and shape, of the batholith have been strongly modified by post-plutonic Cretaceous and Late Cenozoic tectonism, particularly within 50 km of the Alpine Fault. The Median Batholith represents a significant but previously little-recognised 250 Ma record of magmatism along the continental margin of South Gondwana, and invites comparison with other Cordilleran batholiths.
The Adobha Belt consists largely of metamorphosed volcanic and sedimentary sequences and syntectonic granitoids. Identification of the ancient tectonic setting of the belt has been attempted using trace element (including rare earth element) geochemical data on igneous suites. The Himbol volcanics of the Nakfa Terrane represent a calc-alkaline island-arc setting, while the Uogame basalts and associated gabbros characterise a tholeiitic, transitional MORB setting. The chemistry and petrographic features of the basalts and gabbros clearly suggest that they are parts of a dismembered ophiolite. Other authors' work shows that further to the west, the Hager Terrane belongs to a calc-alkaline island-arc setting. The Uogame ophiolitic suite thus portrays a MORB crust sandwiched between two island-arc terranes. Based on the fact that the MORB-type crust occurs within a high strain zone that extends for several hundreds of kilometres along strike, it is concluded here that it represents a terrane-bounding Neoproterozoic ophiolite belt. It is further suggested that this belt might outline a suture that signifies the collision between the island-arcs.
Erosion surfaces and major palaeosols in Quaternary deposits of the eastern Jifarah Plain, allow subdivision of the sediment sequence into six aggradational units Q1–Q6. Comparison of Quaternary sequences in varying continental facies across northern Libya shows that the erosion surfaces and major palaeosols can be traced throughout the region and that they constitute major correlatable events. Correlation also confirms the presence of a seventh aggradational unit between units Q4 and Q5 in upland areas. The depositional units display an irregular but characteristic geometry which reflects changing basin architecture. In upland regions the deposits are typically linear wadi fill and coalesced fan apron passing out into more sheet-like deposits in lowland regions where valley fills are connected across interfluve areas by sheet-wash colluvial and aeolian deposits. Following overall vertical accretion through sequences Q1 to Q4, a major phase of downcutting and terrace formation commenced with the pre-Q5 erosional surface in the early Holocene. Age of the older units is as yet in doubt but there is evidence to assign unit Q1 to the early Pleistocene, unit Q2 to the late Riss and units Q3 and Q4 to the Wurm.
Glacially-crushed feldspar in the BTI-AI2 horizon showing sub-parallel crushing planes and only minor solution etching.This grain was probably redistributed locally by aeolian processes,
Glacially-crushed feldspar showing minorsolution etching in the BT1-Clox horizon.
A scanning electron microscopy study of sand grains in a compound paleosol in the Mutonga Dune Field, on Mount Kenya, yielded data confirming an aeolian origin for some sand-sized particles. The data support aeolian deposition and local reworking of glacially-crushed grains of quartz and feldspar, as well as polished and rounded grains of volcanic glass. Recovered pollen from the buried paeosol confirms a drier and cool paleoclimate circa 1500 yr BP following the climatic optimum period.
The airborne magnetic survey data of Gabal Gattar area has been qualitatively and quantitatively interpreted through the application of a couple of geophysical interpretation techniques, with the aim of structurally mapping an area of uncovered Precambrian shield where structural conditions are significantly reflected on the aeromagnetic contour patterns, trends and intensities. These techniques involved the separation of magnetic anomalies using filtering of analytical downward continuation of the aeromagnetic data and statistical analysis of the azimuth of magnetic and geologic trends using the overlap technique of frequency distribution.The study revealed that Gabal Gattar area has been affected by five significant tectonic trends that are responsible for its tectonic development. These are the NE, NNE, NS, NW and ENE trends. In addition, interpretation of the integrated geophysical and geological information has resulted in the construction of the basement tectonic map of the area which shows its gross structural framework.
A compilation of the structural history of the southern Red Sea is presented and a new conceptual model for its evolution is suggested. According to this model, three major transverse, pre-rift structures control the “rough” and “smooth” propagations of the spreading axes of the Red Sea and Gulf of Aden and the general pattern of this rift system. In the southern Red Sea, between 15° 30″N and 20°N, the crustal structure enables “smooth” propagation of spreading because of parallel ancient deep-seated structures. This condition changes at the southern end, where a transverse structure forms a terminus to the “smooth” propagation and the spreading axis “jumps” westward and splits into nine secondary axes; eight axial ranges in the Afar Depression and one south of the transverse structure in the Red Sea itself. The formation of the Afar Depression and the extremely elevated marginal plateaus with intensive fissure volcanic activity expresses the “rough” propagation southward of the spreading axis. The Afar Depression itself is composed of attenuated, partly oceanized, continental crust.
Most of the statistics given in this overview of the history and current status of Geoscience Education in South Africa pertain to the more geologically inclined disciplines; while the report does mention the extent to which earth sciences are taught in mining and geography departments, no detailed information about these activities are given. There are 13 active geoscience departments countrywide (eleven at universities and two at technical institutions) teaching a wide range of geological topics, some at a highly specialised level. There are just over 100 academic staff members engaged in teaching, supported by 65 technical and administrative staff. Of the teaching staff, 89% have Ph.D. degrees, and most are engaged in active research. About 150 three-year B.Sc., slightly fewer B.Sc.(Hons.), graduates, plus 10 geological technicians pass through the system every year, with most finding employment in the mining industry. Approximately 120 M.Sc. and 60 Ph.D. candidates are currently registered at the universities, about 40% of whom graduate in any particular year.
The continental Sende Formation of northeastern Benin dated by means of silicified wood as Lower Cretaceous overlies the Kandi Formation unconformably. Trace fossils (Arthrophycus and Cruziana) place the Kandi Formation in the Lower Paleozoic. These traces indicate two marine transgressions, one during the middle to Late Ordovician, the other during Lower Silurian, supported by fluvial-marine sandstones from deltaic, marginal-littoral and epicontinental environments. This proposed stratigraphic and paleogeographic model contradicts the former idea of the "Kandi basin" as being a Mesozoic, continental deposit. The Ordovician transgression previously, described in northern Niger, thus extended 1000 km further south, thereby implying a new interpretation of the geological history of West Africa. Close ichnofaunistic relationships are found between the study area and the North African domain, thus supporting the proposal of an Ordovician transgression across the Sahara. These results should be seen in the context of a circumgondwanan province along this northeastern margin.
Kalahari Group sediments cover 2.5 × 106 km2 of central southern Africa. This paper analyses their deposition and development by integrating geological and geomorphological evidence. The post-Gondwanaland tectonic evolution of the sub-continent, drainage development, climatic change and post depositional modification are all identified as important for the development of the Kalahari Group. It is concluded that local, regional and sub-continent wide influences have all played important roles. Consequently, caution should be implemented when attempting to correlate sedimentary units in different areas.RésuméLes dépôts du Groupe Kalahari couvrent 2,5 × 106 km2 du centre de l'Afrique australe. Cet article fait une analyse géologique et géomorphologique de leur origine et de leur genèse. L'évolution tectonique post-Gondowana du sous-continent, le développement des réseaux hydrographiques, les variation climatiques et les modification post-dépositionnelles sont les facteurs les plus importants. Ceux-ci ont joué l'échelle locale, régionale et globale. Il faut donc être prudent dans les corrélations entre ensembles sédimentaires de régions différents.
In addition to their importance as stratigraphical levels of enhanced Au concentration, the unconformities of the Central Rand Group of South Africa also form natural lithostratigraphical formation boundaries. With a view to stimulating research of these unconformities, two hypotheses on the origins of two types of unconformities are reviewed in this paper and criteria for their distinction are proposed. Progressive unconformities are syntectonic and diachronous in nature, and develop in response to source-area orogeny. The result of such tectonism is a basinward shift of the locus of erosion and a progradation, manifested by a conformable coarsening-upward succession in distal, downstream areas of net aggradation. However, in proximal areas net erosion prevails, and parts of such coarsening-upward successions are truncated. Progressive unconformities typically result from localised source-area tectonism and will therefore be restricted to the proximal regions of a particular sub-basin. Some unconformities develop due to eustatic changes and form during relative sea level falls. Because of differential subsidence within different parts of the composite Witwatersrand Basin, some such unconformities would be restricted to certain sub-basins. Both types of unconformities discussed form natural local lithostratigraphical boundaries. However, long-range correlation of formation boundaries is a practice alien to lithostratigraphy.
The Oranjekom Complex, a small mafic to anorthositic layered intrusion situated on the eastern margin of the Namaqua Mobile Belt, forms part of a number of such intrusions that were intruded early in the evolution of the belt. These have subsequently been deformed and isochemically metamorphosed to amphibole-bearing rocks. Petrographic and geochemical evidence indicates that the magmas were alumina-rich and that plagioclase was the primary liquidus phase, but in situ differentiation trends are shown mainly by the mafic phases. This implies that plagioclase crystals, although present in the magma, remained partially in suspension, whereas the mafic minerals accumulated. This is supported by both major and trace element character of the rocks. The Rb-Sr and Sm-Nd isotopic systems indicate that the Oranjekom Complex was intruded and metamorphosed at ca. 1100 Ma. This was followed by further metamorphism at ca. 944 Ma. The isotopic character indicates a depleted mantle source, that gives an age slightly older but within error of the estimated time of intrusion. Further data is required to precisely constrain the age of intrusion of these important markers in the evolution of the Namaqua Orogen. The isotope data indicate that there was limited crustal interaction indicating a thin (possibly juvenile) crust at that time.
Late Palaeozoic glacigene deposits forming the base of the Gondwana megasequence are present along the entire length of the Tethyan margin of Gondwana. The lithology of the deposits in these sequences was controlled largely by the prevailing climate and this influence was imposed also upon early diagenesis. The change from the cold, glacial climate to the cool temperate, postglacial environment is reflected by a change from a mineralogically immature composition (arkosic chloritic) of the glacial periglacial sediments, to a mature, kaolinite pyrite quartz dominated lithofacies of the deglaciation deposits. A typical feature of the latter period is the appearance of black, kaolinitic lutites, usually with a high organic C content. From the many occurrences, only a few examples of typical deglaciation sequences are discussed in this paper, including Tanzania, southern Oman, the Lesser Himalaya, northwestern Australia and southwest China. Microflora and fauna demonstrate that all these deglaciation sequences are of Late Asselian to Early Sakmarian age, indicating that deglaciation along the Tethyan margin of Gondwana was, within the limit of the dating methods, a synchronuous event. A peak in sea level in the Late Sakmarian/Early Artinskian, experienced in all but one of the sections described, supports this statement. The high content of organic matter in the deglaciation deposits was caused by the sudden increase of bioproduction stimulated by higher temperatures and a high CO2 concentration in the atmosphere. A high input of organic material, combined with glacially overdeepened basin geometries, were responsible for the dominance of euxinic conditions during that period. The swift and synchronuous climatic amelioration cannot be explained by shifting pole positions, but only by rapid and substantial global warming.
Four major groups of fish are represented by fragmentary remains from South Africa's Lower Bokkeveld Group of Early to Middle Devonian age: the Acanthodii, Chondrichthyes, Placodermi and Osteichthyes. These represent the oldest known occurrences of these groups in southern Africa, as well as an important addition to the very meagre record of earlier Devonian fish from the Malvinokaffric Province of southwestern Gondwana. Bokkeveld fish material comes from the Gydo (Late Emsian) and Tra Tra (Middle Eifelian) Formations of the Western Cape and Eastern Cape Provinces. The cosmopolitan marine acanthodian Machaeracanthus is represented only by isolated fin spines which may belong to two different species on the basis of their external ornamentation, cross-sectional outline and internal histology. The elasmobranchs are represented by four elements: (1) a flattened chondrocranium which bears affinity to the Late Devonian-Carboniferous symmoriid (protacrodont) 'cladodont' sharks. It is probably the earliest known (Emsian) shark chondrocranium; (2) an isolated, primitive scapulocoracoid with a very short coracoidal ridge; (3) ankylosed and isolated radials, interpreted as parts of pterygial plates of a paired fin of an unknown chondrichthyan bearing affinity to the Middle Devonian Zamponiopteron from Bolivia; and (4) isolated barlike structures, perhaps gill arch or a jaw elements, thought to be from the same taxon as (3). The placoderms are represented by an incomplete trunk armour and fragmentary, finely ornamented plates of a primitive antiarch. The Osteichthyes are represented by a single large scale of an unidentified dipnoan from the Eifelian of the Cedarberg range, as well as a probable sarcopterygian dermal plate from the Emsian of the Prince Albert area. These are among the earliest sarcopterygian remains recorded from the Malvinokaffric Province.
The structure of the external, fold-and-thrust region of the Neoproterozoic Katangan orogenic belt of Central Africa is characterised by thrust sheets, gravitationally transported towards the foreland region in the north, and associated with prominent megabreccias. The thrust sheets contain a more or less complete succession of the Katangan sedimentary rocks. The megabreccias form tabular and wedge-shaped, mostly chaotic bodies, underlying the allochthonous sheets and composed of subangular to rounded fragments of Katangan provenance. Fragments range from millimetres to many kilometres in size and are set in an abudant clay matrix with dolomitic cement. It is commonly accepted in the literature that the Katangan megabreccias are of entirely tectonic origin; friction acting beneath the advancing thrust sheets is thought to be the main process responsible for fragmentation and abrasion of clasts. However, the analysis of stratigraphical, textural and structural features presented in this paper implies a sedimentary genesis of the Katangan megabreccias. It is shown that they resulted from subaqueous re-sedimentation by mass gravity flows of clastic materials eroded from the emergent tops and fronts of the advancing thrust sheets, in front of which olistostrome wedges were deposited. The deposition of each individual olistostrome body stopped when the approaching thrust block overrode it due to the acceleration of gravity gliding. These processes, and their products, define the Katangan foreland region. The olistostrome sedimentary model has significant implications for our understanding of the structural and tectonostratigraphic evolution of this sector of the Katangan belt.
The use of charophytes as biomarkers is discussed with emphasis on the differences in study methods for cosmopolitan and ecotype species. A first extensive inventory of Quaternary deposits of charophytes in Africa north of the equator comprising 18 sites from Senegal to the Sudan is drawn up with data on spatial and temporal distribution. The existence of relatively deep cold lakes in the Holocene is shown by the frequent presence of specimens of cold flora no longer present in Africa today. All the original data show the complementary nature of the study of fossil Charophyta for the multidisciplinary reconstitution of palaeoenvironments.RésuméDans le présent article les principes de l'utilisation de Charophytes comme biomarqueurs sont discutés en soulignant la différence des méthodes d'étude selon qu'il s'agit d'espéces cosmopolites ou espèces écotypiques. Le premier inventaire des gisements à Charophytes du Quaternaire d'Afrique nord-équatoriale comprenant 18 localités réparties du Sénégal au Soudan est dressé et leurs répartitions spatiale et temporelle sont indiquées. L'existence de lacs permanents relativement profonds et froids à l'Holocène est attestée par la présence fréquente d'éléments de flore froide, actuellement disparus du continent africain. La synthèse des données souligne la complémentarité de l'étude des Charophytes fossiles pour la reconstitution pluridisciplinaire des paléoenvironnments.
A scale of confinement in the recent paralic environments of western Africa can be proposed based on the distribution of foraminiferal assemblages. It is thus possible to describe the gradual evolution from an oceanic environment, where marine species are dominant, to a continental environment characterized by the disappearance of foraminifers and the development of thecamoebians. The peculiar environments of athalassic salt lakes are not considered here. This scale, based on four characteristic assemblages and extended to 90 species can be applied to recent or fossil paralic ecosystems. However, it must be used with caution as its significance may change with hydrological properties.
The Karoo Basin of South Africa was one of several contemporaneous intracratonic basins in southwestern Gondwana that became active in the Permo-Carboniferous (280 Ma) and continued to accumulate sediments until the earliest Jurassic, 100 million years later. At their maximum areal extent, during the early Permian, these basins covered some 4.5 million km2. The present outcrop area of Karoo rocks in southern Africa is about 300 000 km2 with a maximum thickness of some 8000 m.The economic importance of these sediments lies in the vast reserves of coal within the Ecca Group rocks of northern and eastern Transvaal and Natal, South Africa. Large reserves of sandstone-hosted uranium and molybdenum have been proven within the Beaufort Group rocks of the southern Karoo trough, although they are not mineable in the present market conditions.Palaeoenvironmental analysis of the major stratigraphic units of the Karoo succession in South Africa demonstrates the changes in depositional style caused by regional and localized tectonism within the basin. These depocentres were influenced by a progressive aridification of climate which was primarily caused by the northward drift of southwestern Gondwana out of a polar climate and accentuated by the meteoric drying effect of the surrounding land masses. Changing palaeoenvironments clearly influenced the rate and direction of vertebrate evolution in southern Gondwana as evidenced by the numerous reptile fossils, including dinosaurs, which are found in the Karoo strata of South Africa, Lesotho, Namibia and Zimbabwe.During the Late Carboniferous the southern part of Gondwana migrated over the South Pole resulting in a major ice sheet over the early Karoo basin and surrounding highlands. Glacial sedimentation in upland valleys and on the lowland shelf resulted in the Dwyka Formation at the base of the Karoo Sequence. After glaciation, an extensive shallow sea covered the gently subsiding shelf, fed by large volumes of meltwater. Marine clays and muds accumulated under cool climatic conditions (Lower Ecca Group) including the distinctive Mesosaurus-bearing carbonaceous shales of the Whitehill Formation.Subduction of the palaeo-Pacific plate reslted in an extensive chain of mountains which deformed and later truncated the southern rim of the main Karoo Basin. Material derived from these “Gondwanide” mountains as well as from the granitic uplands to the north-east, accumulated in large deltas that prograded into the Ecca sea (Upper Ecca Group). The relatively cool and humid climate promoted thick accumulations of peat on the fluvial and delta plains which now constitute the major coal reserves of southern Africa.As the prograding deltas coalesced, fluvio-lacustrine sediments of the Beaufort Group were laid down on broad gently subsiding alluvial plains. The climate by this time (Late Permian) had warmed to become semi-arid with highly seasonal rainfall. Vegetation alongside the meander belts and semi-permanent lakes supported a diverse reptilian fauna dominated by therapsids or “mammal-like reptiles”. Pulses of uplift in the southern source areas combined with possible orographic effects resulted in the progadation of two coarse-grained alluvial fans into the central parts of the basin (Katberg Sandstone Member and Molteno Formation).In the upper Karoo Sequence, progressive aridification and tectonic deformation of the basin through the late Triassic and early Jurassic led to the accumulation, in four separate depositories, of “redbeds” which are interpreted as fluvial and flood-fan, playa and dune complexes (Elliot Formation). This eventually gave way to westerly wind-dominated sedimentation that choked the remaining depositories with fine-grained dune sand. The interdune areas were damp and occasionally flooded and provided a habitat for small dinosaurs and the earliest mammals. During this time (Early Jurassic), basinwide volcanic activity began as a precursor to the break-up of Gondwana in the late Jurassic and continued until the early Cretaceous. This extrusion of extensive flood basalts (Drakensberg Group) onto the Clarens landscape eventually brought Karoo sedimentation to a close.
Folding, refolding and subsequent faulting of cratonic basement and overlying Ventersdorp and Transvaal strata in SE Botswana provide a model for understanding deformation in the Kaapvaal Craton in late Archaean and early Proterozoic times. Folding of the Archaean basement generated E-W trending antiforms and synforms the latter developing into depositional basins during the Proterozoic. Subsidence of the Transvaal Basin also caused folding of the Bushveld Complex into an E-W trending synform. Before deposition of Waterberg strata commenced, existing E-W trends wre refolded about NNW axes and huge volumes of material both from the complex and its wall rocks were removed by erosion. Crustal extension in the middle Proterozoic caused formation of rifts in which Waterberg deposits accumulated. Compressive forces in post-Karoo time caused attenuation of existing fold trends and further erosion of the antiforms.
The sediments in two of South Africa's major Archaean greenstone belts, the Barberton and Pietersburg greenstone belts, span an age range of some 800 million years. Both greenstone belts represent remnants of extensive fold and thrust belts with complex, but different polyphase tectonic histories. The oldest sediments were deposited between circa 3470 and 3490 M.a. on oceanic like crust preserved in the Barberton belt, possibly at the same time as sedimentation on similar oceanic crust preserved in the Pietersburg belt. Thereafter, the geologic evolution of these two belts diverged considerably. In the Barberton belt, there is clear evidence that the oceanic crust and sediments were obducted onto an intra-arc basin environment within 50 million years of its formation. The sequence was later further imbricated by northwest directed thrust stacking between 3300-3200 M.a. Basin development during both periods of thrusting took place in close proximity to active “calc-alkaline” arc systems. Deformation of the sediments within these basins took place while the same sediments were being deposited. Sedimentation took place predominantly in environments, ranging from submarine mid-fans below the photic zone to tidal flats and deltaic plains. The sediments represent a polyhistory successor-type basin: early basins developed along a complex subduction related plate boundary; these basins later evolved into foreland depositories along and within collisional environments of an accretionary orogen. Late in the history of the Barberton greenstone belt (circa 3100 M.a.), the rocks were in places thermally reactivated and probably subjected to extensional processes; these processes overlapped in time with the main episodes of economic gold mineralization, and are of “early Witwatersrand-basin” age.
The continental margin of eastern North America was initiated when West Africa and North America were rifted apart in Triassic-Early Jurassic time. Cooling of the crust and its thinning by rifting and extension caused subsidence. Variation in amounts of subsidence led to formation of five basins. These are listed from south to north. (1) The Blake Plateau Basin, the southernmost, is the widest basin and the one in which the rift-stage basement took longest to form. Carbonate platform deposition was active and persisted until the end of Early Cretaceous. In Late Cretaceous, deposition slowed while subsidence persisted, so a deep water platform was formed. Since the Paleocene the region has undergone erosion. (2) The Carolina Trough is narrow and has relatively thin basement, on the basis of gravity modeling. The two basins with thin basement, the Carolina Trough and Scotian Basin, also show many salt diapirs indicating considerable deposition of salt during their early evolution. In the Carolina Trough, subsidence of a large block of strata above the flowing salt has resulted in a major, active normal fault on the landward side of the basin. (3) The Baltimore Canyon Trough has an extremely thick sedimentary section; synrift and postrift sediments exceed 18 km in thickness. A Jurassic reef is well developed on the basin's seaward side, but post-Jurassic deposition was mainly non-carbonate. In general the conversion from carbonate to terrigenous deposition, characteristics of North American Basins, occurred progressively earlier toward the north. (4) The Georges Bank Basin has a complicated deep structure of sub-basins filled with thick synrift deposits. This may have resulted from some shearing that occurred at this offset of the continental margin. Postrift sediments apparently are thin compared to other basins—only about 8 km. (5) The Scotian Basin, off Canada, contains Jurassic carbonate rocks, sandstone, shale and coal covered by deltaic deposits and Upper Cretaceous deeper water chalk and shale.RésuméLa marge continentale de l'Est de l'Amérique du Nord s'est constituée lorsque l'Afrique de l'Ouest et l'Amérique du Nord se sont séparées à l'époque Trias-Jurassique inférieur. Le refroidissement de la croûte et son amincissement par rifting et extension ont entraîné la subsidence. Des variations dans la valeur de cette subsidence ont abouti à la formation de cinq bassins, à savoir du Sud au Nord: (1) le bassin du Blake Plateau, le plus au Sud, qui est le plus large et un de ceux où le stade de rifting a pris le plus de temps à se développer. Les dépôts du type plate-form carbonatée ont été importants et ont persisté jusqu'à la fin du Crétacé inférieur. Au Crétacé supérieur, les dépôts se sont ralentis alors que la subsidence se poursuivait, d'où la formation d'une plate-forme en eau profonde. Depuis le Paléocène la région a été soumise à l'érosion. (2) La fosse de Caroline, étroite et au socle relativement mince d'après les modèles gravimétriques. Les deux bassins à socle mince, la fosse de Caroline et le Scotian Basin, montrent également de nombreux diapirs de sel indiquant une accumulation considérable de sel pendant le début de leur évolution. Dans la fosse de Caroline, la subsidence d'un vaste bloc de strates au-dessus du sel mobile a abouti à la formation d'une faille normale active majeure sur la bordure côté continent du bassin. (3) La fosse du Baltimore Canyon, qui possède une séquence sédimentaire extrêmement épaisse. Les sédiments syn- et post-rift dépassent 18 km d'épaisseur. Un récif jurassique est bien développé sur la bordure côté océan du bassin, mais les dépôts post-jurassiques sont principalement non carbonatés. En général le passage de la sédimentation carbonatée à terrigène, caractéristique des bassins nord-américains, s'est produite progressivement de plus en plus tôt en allant vers le Nord. (4) Le bassin du Georges Bank qui présente une structure profonde complexe de sous-bassins remplis d'épais dépôts syn-rift. Ceci pourrait être le résultat d'un cisaillement qui se serait produit à cet endroit où la marge continentale présente un décalage. Les sédiments post-rift y semblent minces en comparaison de ceux des autres bassins, environ 8 km seulement. (5) Le Scotian basin, au large du Canada, qui contient des roches jurassiques, calcaires, grès, shales et charbon, recouvertes par des dépôts deltaïques et des sédiments crétacé supérieur d'eau plus profonde, craie et shales.
Field, petrological, geochemical, isotopic and geophysical data have been assembled to determine the nature and extent of Archæan Kraaipan granite-greenstone rocks on the western edge of the Kaapvaal Craton, southern Africa.The Kraaipan greenstone belts, consisting of metamorphosed mafic volcanic rocks and interlayered metasediments (mainly banded iron formations, jaspilites and ferruginous cherts), occur poorly exposed beneath cover sequences comprising mainly Neoarchæan Ventersdorp Supergroup volcanic rocks and a blanket of Tertiary-Recent Kalahari sediments. A variety of granitoid rocks intruded the Kraaipan greenstones, which, on the basis of whole rock PbPb dating of banded iron formations, have yielded an age of 3410+61/-64 Ma. The earliest granitic rocks, which comprise tonalites and trondhjemitic gneisses, were dated using the single grain Pb evaporation technique on zircons, and yielded minimum ages ranging from 3162±8 to 3070±7 Ma in the study area. This, coupled with 3250-3030 Ma ages reported for gneisses in the Kimberley and other areas on the western edge of the Kaapvaal Craton, suggests a prolonged evolution for the basement gneisses which were also disturbed between 2940 and 2816 Ma ago, probably during episodes of migmatisation. Potassium-rich granitoids, also dated using the single grain Pb evaporation method, range in age from 2880±2 to 2846±22 Ma and extend from the Schweizer-Reneke area in the south to the Botswana border and beyond in the north. Geophysical evidence (aeromagnetic and Bouguer gravity data) suggest that the intrusions may be interconnected and might have been emplaced episodically across the study area. A close spatial relationship exists between these granodiorites and adamellites, and known Au mineralisation present in the Kraaipan-Madibe areas in the north and the Amalia area in the south. This suggests a possible genetic link which could be of significance in mineral exploration. Lastly, a late granitoid pluton, the Mosita Adamellite, yielded a Pb evaporation age of 2749±3 Ma and is the youngest intrusive body recorded in the Kraaipan granite-greenstone terrane. Its presence beneath Kalahari sand cover is defined by Bouguer gravity data. The Kraaipan granite-greenstone terrane, with a prominent north-south trend, appears to represent an Archæan crustal segment that may have accreted episodically on to the western edge of the Kaapvaal Craton. In a manner similar to the Murchison granite-greenstone terrane in the northeastern part of the craton, the region may also have constituted an important potential source of placer Au mineralisation found in the Witwatersrand Basin.RésuméLes données de terrain, pétrologiques, géochimiques, isotopiques et géophysiques ont été rassemblées afin de déterminer la nature et l'étendue de l'ensemble archéen de roches vertes et granites de Kraaipan, sur le bord occidental du craton du Kaapvaal en Afrique du Sud. La ceinture verte de Kraaipan consiste en une série métamorphique de roches volcaniques mafiques interstratifiées avec des métasédiments, principalement des itabirites, des jaspilites et des cherts ferrugineux. Elle affleure mal sous une série de couverture comprenant essentiellement les roche volcaniques tardi-archéennes du Supergroupe de Ventersdorp et une pellicule de sédiments tertiaires à récents du Kalahari. Divers granitoïdes intrudent les roches vertes de Kraaipan qui, sur base de datations PbPb roche totale des itabirites, ont des âges de 3410+61/-64 Ma. Les granitoïdes les plus précoces, des gneiss tonalitiques et trondhjemitiques, ont été datés par la technique d'évaporation du Pb sur monocristaux de zircon. Ces données fournissent des âges dinimums s'étalant de 3162±8 à 3070±7 Ma dans la région étudiée. Ceci, couplé avec les âges de 3250-3030 Ma connus pour les gneiss de Kimberley et d'autres régions du bord occidental du craton du Kaapvaal, suggère une évolution prolongée des gneiss du socle. Ces derniers ont été perturbés entre 2940 et 2816 Ma, probablement lors d'épisodes migmatitiques. Des granitoïdes potassiques, datés par la même méthode d'évaporation du Pb sur monocristaux de zircon, s'étalent entre 2880±2 et 2846±22 Ma depuis la région de Schweizer-Reneke dans le sud jusqu'à la frontière du Botswana et au-delá, au nord. Les données géophysiques (aéromagnétiques et gravité Bouguer) suggèrent que les intrusions sont interconnectées et se sont mises en place par épisode dans la région étudiée. Une relation spatiale étroite existe entre ces granodiorites et adamellites et les minéralisations aurifères connues dans les régions de Kraaipan-Madibe au nord et dans la région d'Amalia au sud. Un lien génétique est possible, ce qui est de grande importance pour la prospection minérale. Enfin, un pluton granitique tardif, l'adamellite de Mosita, a fourni un âge d'évaporation Pb de 2749±3 Ma et constitue le corps intrusif le plus jeune reconnu dans la ceinture verte de Kraaipan. Sa présence sous la couverture sableuse du Kalahari est définie par les données gravimétriques Bouguer. Le terrane de roches vertes et granites de Kraaipan, avec son orientation marquée nord-sud, semble représenter un segment crustal archéen qui se serait accrêtéépisodiquement sur le bord occidental du craton de Kaapvaal. De même que le terrain similaire de Murchison dans le nord-est du craton, la région étudiée pourrait constituer une source potentiellement importante pour la minéralisation aurifère en placer du bassun du Witwatersrand.
The widespread Cardium (Cerastoderma) fauna of Quaternary lakes of the Sahara gave rise to the “Saharian seas” hypothesis. Present Egyptian (Fayum, Siwa) and Algerian Holocene salt lakes show an evolution from a fresh water fauna to a Cardium, Potamids and Foraminifera fauna in spite of their considerable distance from the Mediterranean sea and their altitude above sea level. The inevitable conclusion is that thalassoid or so-called margino-littoral faunas cannot be used as indicative of sea connection, or even proximity, in ancient deposits.RésuméLa faune à Cardium des lacs quaternaires du Sahara a donné naissance à l'hypothèse des Mers Sahariennes qui doit être maintenant abandonnée. En effet, les lacs salés actuels (Fayoum, Siwa-Egypte) et récents (Algérie) ont montré une évolution progressive d'une faune d'eau douce à la faune á Cardium, Potamides et Foraminifères, malgré leur distance et leur altitude vis-à-vis de la Méditerranée. La conclusion nécessaire est qu'il faut cesser d'utiliser les faunes thalassoïdes dites margino-littorales pour caractériser une relation directe ou de proximité avec les anciennes mers.
New data from the recent IPOD drilling of DSDP Site 534 in the Blake-Bahama Basin give a definitive age for the spreading-center shift involved in the breakup of the North American Atlantic margin. A basal Callovian age is determined for the Blake Spur magnetic anomaly marking this spreading-center shift that signals the birth of the modern North Atlantic Ocean. This is some 20 Ma younger than previously thought. One implication of this result is that this spreading-center shift starting North Atlantic breakup is now of an age which could be assigned to the spreading-center shift needed to end the spreading in the Gulf of Mexico. It is suggested that this might be one and the same event. Another implication of this younger age for the Blake Spur event is that very high spreading rates are now required for the Jurassic outer magnetic quiet zone along the North American margin. This association of a relatively high spreading rate with a magnetic quiet zone is similar to that for the Middle Cretaceous and implies a link between the processes controlling plate spreading, which are in the upper mantle, and the processes controlling the magnetic field, which are in the outer core. The cylces of fast and slow spreading and quiet and reversing magnetic field have a period of 60–100 Ma. A theory of pulsation tectonics involving the cyclic eruption of plumes of hot mantle material from the lowermost mantle could explain the correlation. Plumes carry heat away from the core/mantle boundary and later reach the asthenosphere and lithosphere to induce faster spreading. The pulse of fast spreading in the Jurassic apparently caused the breakup of the North Atlantic. Other pulses of fast spreading appear to correlate with major ocean openings on various parts of the globe, implying that this might be a prevalent process. Rifting of passive margins may be controlled by the more fundamental global processes described by the theory of pulsation tectonics.
As a result of field observations carried out in Morocco, together with microscopic observations and laboratory experimentation, we have been able to show that many continental carbonated formations are of biological origin. Bacteria absorb and retain calcium, and progressively build crystalline structures of calcium carbonate. Incrusted films on present-day pebbles are among the more simple constructions. They are constituted of microflora (algae, mycellium, bacteria) incrusted with calcium carbonate. They enable us to understand the formation of fossil oncolithic pebbles and of zoned crusts. Zoned crusts are amalgamated filaments, incrusted with limestone by this process, in the same way as stromatolites. An experimental reproduction of travertine was performed using revived fossil bacteria, whose DNA and proteins were demonstrated by means of biological tests. In the carbonated deposits of Saharan paleolakes, paleo-microbiological variations are observed: diatomic deposits are cemented by calcium carbonate of bacterian origin (bacilli and cocci).
The mangrove swamps of West African Coast belong to the Atlantic type which is characterized by a small number of species. They colonize tidal environments which are dissected by numerous meandering tidal channels and are presently subject to a low rate of sediment accumulation.The mangrove vegetation exhibits a characteristic zonation pattern that basically reflects the adaptation of the various species to saline conditions. The typical zonation sequence is: Rhizophora racemosa (or Rh. mangle), Rh. mangle + Avicennia africana, Avicennia, flooded tanne, barren tanne, herbaceous tanne. The tannes are generated by aridic climatic conditions, heavy soil and water salt content, and are, in a way a peculiar feature of mangrove swamps in West Africa.The sediment colonized by the mangroves is relatively homogenous. Mineralogically, they are dominated by quartz and clay to which are associated halite, pyrite and jarosite. The clay suite is mainly composed of smectite and kaolinite. Smectite is predominant in the inlet areas and is replaced inland by kaolinite.Chemically, the sediments contain very low amounts of Ca, bases and trace elements. The mangrove swamp floodwaters have a chemical composition similar to that of seawater. It is dominated by sodium and chloride.Morphologically, the ripening of the soils appears with a chestnut mash colour horizon and buttery consistency in relation with the decomposition of fibrous roots of Rhizophora and also with pale yellow jarosite mottles in the top horizons of the tanne profiles due to the oxidation of pyrine.The two main properties of the mangrove soils of West Africa are acidity and salinity; the first is related to the high content of sulphur and the second to the sea influence.The acidity has to be connected mainly to the Rhizophora vegetation whose the root system is a real trap for catching the pyrites resulting from the reduction of the sulphates of sea water by the sulphate reducing bacteria, in a reduced environment rich in organic matter and iron.The salinity is mainly related to the sea water which is flooding the mangrove or which flows through the water table up to the tanne. It is mainly sodic chloride.From the geochemical point of view, the disequilibrium between mangrove and tanne is appearing by a high increase in the tanne area of silica in one hand due to the dissolution, partly of the quartz, but mainly of diatomaceous frustules, and increase of magnesium, on the other hand, due to the clay mineral weathering.RésuméLes mangroves sont présentes sur tout le littoral ouest africain, et plus particulièrement, du Sénégal à la Siérra Léone où elles couvrent une superficie d'environ 3 millions d'heactares. Elle font partie de la mangrove de type atlantique représentée par un petit nombre d'espèces de palétuviers dont Rhizophora racemosa, Rh. mangle, Rb. harrissonii, Avicennia africana et Laguncularia racemosa.Le caractère commun à toutes ces mangroves est la présence, à l'arrière des palétuviers de zones nues, les 〈tannes〉 résultant de la sursalure des eaux et des sols, malgré des différence de pluviométrie importantes (500 mm au Sénegal, 3000 mm en Guinée).Ceci est essentiellement dû à une saison sèche longue (6 mois et plus).L'installation des mangroves sur le littoral ouest africain date de la transgression nuakchottienne, (5500 B.P.) et depuis, elles ont évolué sous l'influence de la sécheresse climatique.Le sédiment colonisé par les mangroves est relativement homogène et a dominance de quartz et d'argile auxquels sont associés la halite, la pyrite, la jarosite et le gypse. La fraction argileuse est représentée par la kaolinite et la smectite, l'une étant fournie par le continent et l'autre étant apportée par la mer.Du point de vue chimique, le sédiment de mangrove est caractérisé par ses très faibles teneurs en Ca et en éléments traces et les nappes phréatiques ont une composition chimique proche de celle de l'eau de mer.La pédogenèse des sols de mangroves est liée à 2 facteurs, d'une part à la pyrite, provenant de la réduction des sulfates de l'eau de mer en sulfures, sous l'action des bactéries sulfato-réductrices et dont l'oxydation conduit à la formation de jarosite et à l'acidité de ces sols, d'autre part aux sels solubles liés à l'action de la marée et principalement au chlorure de sodium. Les sols sont tous sulfatés acides et salés.Du point de vue géochimique, le déséquilibre entre la mangrove et le tanne se manifeste par une augmentation importante de la silice dans le tanne, resultant, en partie de la dissolution du quartz, mais aussi des frustules de diatomées. L'acidité de la nappe phréatique dans le tanne est aussi a l'origine de l'altération de certains minéraux argileux et de la néogenése d'autres.
Three zones of thrust faulting, each zone with its own structural characteristics, have been identified in rocks of the Palaeozoic Table Mountain Group, just south of Uniondale, in the Cape Fold Belt. From north to south these zones consist of (1) shallow dipping sets of thrusts spaced, on average 10 m apart, (2) closely spaced (average 4 m apart) moderate to steeply-dipping thrusts associated with mesofolds, and (3) a zone mainly of backthrusts, spaced on average 2 m apart. All three zones formed during a single compressional event which took place during the Late Palaeozoic. Thrusting is interpreted to have commenced in zone (1) where thrusts developed in a break-back sequence, followed by thrusting in zones (2) and (3) where fore- and backthrusting were probably coeval. Upright fold structures in zone (2) controlled the steep orientation of thrust planes in this zone. The intensity of thrusting is so great that the 'Cedarberg Shale Formation', generally regarded as a regionally reliable stratigraphic marker in the Cape Fold Belt, has been eliminated in the study area. In addition, the scale of deformation seen locally in all units of the Table Mountain Group brings into sharp focus potential problems in respect of the recognition, thicknesses and correlation of lithostratigraphical units in the Cape Fold Belt.
Carbonate dykes, exposed within the Barberton greenstone belt, display geochemical signatures similar to altered carbonatite. The trace element signature normalised to primordial mantle, and the chondrite-normalised REE trends of the Ulundi Dyke display geochemical similarities to carbonatites. In addition, stable isotope results from the Ulundi Dyke (δ13C(whole rock) and δ18O(whole rock) range from -3.7 to -4.9‰ and 12.8 to 13.2‰, respectively) are similar to values found for samples of wall rock and vein carbonate from Archaean Au-quartz-carbonate-sulphide vein systems studied in the Barberton greenstone belt. Although the data do not plot in the field of primary igneous carbonatite, they are similar to data of deuterically-altered carbonatite. These associations strengthen the deduction that Iode-Au mineralised fractures and shear zones in the Barberton greenstone belt were open to mantle-tapping fundamental faults.
This paper compares Karoo deposits within the Lower Beaufort (Late Permian) time interval from southern to central Africa. Facies aspects are summarized for selected sequences and depositional environments assessed in connection with the palaeogeography. The comparison shows that thickness of Lower Beaufort sequences varies greatly; sequences are over a kilometre thick at the southern tip, but decrease drastically to the north, northwest and northeast, and is commonly absent from the western part of the subcontinent. Depositional environments are continental except for small estuarine intervals from a sequence in Tanzania. The commonest lithologies comprise mudstones, siltstones, arkoses and carbonates. In spite of the dominance of fluvial facies, the records preserved by intervals of lacustrine sequences suggest that large lakes were major features of the palaeogeography, and that lacustrine environments may have been dominant deposition environments.
This paper models the physico-chemical conditions of a Neoarchæan to Palæoproterozoic marine basin in which the sedimentary sequence of BIF, Fe and Mn ores of the Lake Superior-type formed. The model is based on Eh-pH diagram stability fields for Fe, silica and Mn solubilities (taken from the literature) and on field observations of the lithological sequences. BIF formation took place in epicontinental marine basins with free access to the ocean. The main Fe source for BIF formation was ocean enriched with about 6–10 ppm ferrous Fe of hydrothermal geochemical affinity. Land-derived Fe influxes into the BIF-forming basins certainly contributed, but the lack of clastic sedimentation precludes estimation of element budgets. The main silica source for formation of chert layers is sea water. If silica was precipitated by evaporation, the silica concentration of the BIF-forming sea must have been close to saturation (15–20 ppm). Biogenic silica concentration from a possible silica undersaturated sea may not be excluded. These inferred BIF-forming conditions fit the global occurrence of Lake Superior-type BIF in general, whereas special sedimentary environments were probably responsible for the formation of highly enriched laminated Fe ore at the Maremane Dome and in the Sishen-Kathu mining district in Griqualand West, and for the FeMn ores in the Kalahari field. Formation of laminated Fe ore in the Maremane Dome and in the Sishen-Kathu areas were restricted to local deeps within the BIF basins, caused by karst collapse in the underlying Campbellrand dolomites. In such deeps, increased pH values relative to the normal BIF-forming sea caused sufficiently increased silica solubility, resulting in the almost exclusive sedimentation of colloidal Fe precipitates.
A diverse assemblage of terrestrial plant palynomorphs is reported from Koingnaas, a site in the Namaqualand region of the Northern Cape Province, South Africa. In excess of one hundred palynomorph types are recognised from three peaty clay horizons contained within a fluviatile sediment suite filling a palæochannel of Tertiary age. No stratigraphically significant differences occur between the three horizons. Angiosperm pollen dominates the assemblage, with a lesser contribution of gymnosperm pollen, and spores make up a significant although small component. The presence of podocarp pollen, angiosperm pollen with modern arboreal counterparts and fossilised wood suggests that the palæoflora consisted of a forested temperate to subtropical environment. The Koingnaas assemblage contains little evidence of the modern regional flora, but links with the Cape Floristic Region are indicated by pollen related to the Proteaceae and possibly the ‘palæoendemic’ Bruniaceae. Previously, the palæochannel sediments were thought to be Pliocene, but an early Tertiary age is preferred for the Koingnaas site, supported by similarities with the Palæocene Arnot Pipe assemblage and the position of the palæochannel relative to early Tertiary palæodrainage patterns.
The main result of geological work of our group in Egypt and Sudan was the subdivision of the so-called Nubian Sandstone in its type-area Nubia and in the surroundings of Nubia. More than 20 formations have been identified, ranging in age from Cambrian to Paleocene and comprising an impressive number of facies variations. The only characteristic these strata have in common is their position near the northern edge of the African craton. This position caused similiarities within the continental facies and it also influenced marine sedimention, which in general took place in shallow to very shallow seas. The former Nubian Sandstone is strata deposited on both sides of shallow transgressions or in shallow, purely continental basins. Variations of facies were mainly influenced by changes of climate. Except for shallow marine carbonates of Early Tertiary age present in a larger area in northwestern Sudan, all sediments formenly called Nubian Sandstone are of clastic nature including thick sequences of paleosols.
The African continent is the second largest land mass in the world with an area of over 30,000,000 km2. The continent is known to contain appreciable quantities of minerals and metals of great economic value.This paper reviews the major known and potential solid mineral resources of the African continent. It provides some relevant statistics that gives a quantitative picture of their distribution and an evaluation of their future potentials. A brief analysis of selected minerals of great importance to the economies of African countries and with significant potentials with regards to the future of the continent is also undertaken. Finally, the perspectives of the African mining industry is reviewed.
Geochronological and paleomagnetism data for southern West African craton and Guyana shield in South America, are concordant and suggest the existence of a large unit grouping them during Archean and Lower Proterozoic times. The paleomagnetism data allow to put on a single line, the Zednes (Mauritania), Sassandra (Ivory Coast) and Guri (Venezuela) fault zones, the mylonites of which were dated 1670 Ma. This age reflects the end of the eburnean-transamazonian shearing tectonic, which affected the large West Africa-Guyana unit. This line separates the western Archean domain from the eastern lower Proterozoic one; thence it is possible to correlate the Sasca (Ivory Coast) and Pastora (Venezuela) areas. Archean relics have been found in mobile pan-african-bresiliano zones which surround the Precambrian cratons; this fact suggests the existence of still more extended Archean craton than defined above.
The area between Manzalah Lake and the southern Galala Plateau in northeast Egypt constitutes the Galalas, Cairo-Suez, southern Nile Delta and northern Nile Delta structural provinces. The northern Galala Fault separates the Galalas Province from the Cairo-Suez Province and is considered to be the westward extension of the Themed Fault in central Sinai. The pre-Eocene rocks are affected by northeast to east-northeast-orientated folds and reverse faults, as well as east-west-orientated oblique-slip faults with dextral and normal components. Some folds and reverse faults are interpreted to have been formed by northwest to north-northwest-orientated compression related to the Syrian Arc movement, whereas the others by the secondary northwest orientated shortening, which accompanied dextral strike-slip component along the planes of the east-west-orientated faults. The east-west-orientated faults were initially formed during the Late Triassic/Early Jurassic extension related to the drifting of the African/Arabian Plate away from the Eurasian Plate as a result of opening of the Neotethyan Sea. The Neotethyan began to close due to convergence between the two plates, leading to the Syrian Arc deformation. This deformation mildly started in Late Cenomanian and followed by a more intensive phase in Conacian/Santonian. It mildly continued in the Maastrichtian, Early Palæocene and Late Palæocene/Early Eocene. The southward thinning of the pre-Eocene rocks controlled the intensity and style of deformation. Two deformational mechanisms are proposed for the Nile Delta hinge zone. The first is related to Late Oligocene—Early Miocene north-northwest-orientated Alpine compression. The second is related to northward gravitational sliding of the post-Oligocene shale and sandstone over Cretaceous-Eocene carbonates.
Rift-basin sedimentation appears to be closely related to the influence of phenomena such tectonics, volcanism, and climate. The continent-scale structure of the East African Rift offers numerous examples of sediments types, sedimentary bodies or sedimentary environments showing clearly their dependence on these phenomena. Thus the distribution and the longevity of the troughs which compose the rift depend directly on tectonics and volcanic activity. Evolution of these troughs towards "sedimentary basins" is related to the permanence of the tectonic depression thus authorizing the deposition of a sedimentary sequence. Examples from the Afar Depression or the Gregory Rift show clearly cases of interruptions or removals of sedimentation related to the migration of axes of tectonic activity. Looking at sedimentary formations of Recent age in the East African Rift, it appears that the variety of tectonic, volcanic and climatic conditions along the 4000 km of horsts and grabens influences the sedimentation which can be in teh form of clastic, organic, evaporitic or metalliferous deposits. Such facies are associated to form sedimentary bodies or environments such as piedmont accumulations, alluvial fans and fan deltas, fluvio-deltaic systems, and lacustrine basins. Examples of Recent to Modern environments have been chosen in the Eastern and Western Branches of the East African Rift, Afar Depression, Ethiopian and Gregory Rifts, Kivu Trough, Tanganyika and Malawi Troughs. They clearly illustrate the close correlations existing between sedimentation, tectonics, volcanism and climate at different scales of time and space in such a complicated structure as the East African Rift.
Two syenite bodies from south-western Nigeria considered to be pre to syntectonic (Iwo) and late tectonic (Okeiho) with respect to the dominant N-S trending structures that characterize the Nigerian Percambrian basement were dated by the U-Pb zircon method. However, they yield Pan-African emplacement ages of respectively which are indistinguishable within limits of the experimental errors. This suggests that the Pan-African deformation in the region was either migratory or heterogeneous in nature. Comparison of these results with data from other parts of the Pan-African domain east of the West African craton provides further evidence of the synchronous nature of the peak of Pan-African magmatism (630-600) in the area.
Samples collected from 4 sites on the northwestern and eastern African margins were used to test the reliability of marine sedimentary record of continental environmental variations, during the last Glacial and Interglacial climatic cycle. On the northwestern margin which is passive and stable (between Cape Verde and Cape Blanc), climatic variations are marked by parameters such as sedimentary facies, sedimentary dynamics, sedimentation rates or faunal assemblages. These parameters are controlled by climatic changes that modify continental environments (erosion conditions, rate of terrigenous supplies) and marine environments as well (sea-level, currents and biogenic sediment productivity). On the opposite, in the Gulf of Aden, 3 sites show the extent to which tectonics may affect the record of environment modifications due to climatic changes. In the East of the Gulf, on the Sukra margin that is passive but with young and still active structures, the continental slope is uneven with tectonic basins acting as sediment traps. Here, several parameters like sedimentation rates become unreliable for they no longer reflect the importance of terrigenous inputs nor that of primary productivity. Further to the West, the deep narrow trough of Alula Fartak and the epicontinental domain belonging to the Assal rift (Ghubbet el Kharab), are part of highly active tectonic and volcanic margins. Continental environment variations cease to be recorded through sedimentological parameters which are closely related to morpho-structural and volcanic factors.
The break up of Gondwana and the final separation of South America and Africa resulted in the formation of sedimentary basins along the coast of West Africa. These basins were infilled with Cretaceous and Tertiary sediments. The paleoflora (spores and pollen grains) of these sediments was studied as a follow-up to an earlier general study of the microfauna (Foraminifera and Ostracodes). It is interesting to note that the African intertropical microflora assemblages are significantly different from those of north African basins but bear close ressemblance to those of South America and to a lesser extent to those of India and Borneo. The literature on the palynology of Cretaceous-Tertiary sediments of West Africa, from English and French sources, has been reviewed and synthesized with a view to describe the paleofloral succession as defined by the appearance, predominance and disappearance of pollen species which correspond approximately with established stratigraphic subdivisions. A palynostratigraphic scale is proposed ranging from Neocomian to Pliocene, showing the marker-pollen and spore species for intertropical Africa. Some of these species also occur in virtually all the nowadays intertropical zone.
The geological history in southern Senegal and Guinea results in the existence, on the western margin of the West African craton, of a Pan-African orogenic belt which is capped in part with late Proterozoic and Paleozoic terranes. In addition to geological features, the gravity signature and deduced crustal model bear evidence of an eastern crustal block corresponding to the old rigid craton and a denser and thicker western block related to the reactivated basement province. The discontinuity in density between both is interpreted as the Pan-African suture which dips westward beneath the reactivated block. The short wavelength gravity highs superimposed to the gravity gradient in the central domain are interpreted as west-dipping wedge-shaped dense bodies squeezed at depth along the suture. These may reflect either remains of oceanic crust or granulite facies rocks derived from the crustal overthrusting process. Finally using both geological and geophysical materials, the Pan-African belt of southern Mauritanides and northern Rokelides appears to be consistent with a continental collision-basement reactivation model.
Implicit in the African Renaissance is the synergy between government, the private sector, the educated minority and the disadvantaged majority. For this concept to work, belief and commitment must arise first from the African individual, whatever his or her potential contribution may be. The geosciences in South Africa provide a currently vibrant example of such cooperation, which has the potential to contribute significantly to the upliftment of the country and its neighbouring states. Based largely on personal interviews with various role players, from the Presidency of South Africa, through ministerial levels, the corporate sector and down to the individual, we present a spectrum of viewpoints and initiatives which are starting to result in practical implementation of the African revival. An end to conflict and xenophobia, the entrenchment of democratic government and corporate expression of the entrepreneurial spirit are essential to provide the framework within which the individual African can become a “Renaissance Man or Woman”.
The succession of spore-pollen assemblages during the Cretaceous and Tertiary, as defined in each of the basin from Senegal to Angola, gives the possibility to consider the intertropical African flora evolution for the past 120 M.a. During the Early Cretaceous, xeric-adapted gymnosperms and various ferns were predominant the flora which nevertheless comprises previously unknown early angiosperm pollen. During the Middle Cretaceous, gymnospers were gradually replaced by angiosperms; these became more and more abundant, along with the diversification of new genera and species. During the Paleocene, the radiation of the monocotyledons (mainly that of the palm-trees) as well as a greater diversification among the dicotyledons and ferms are noteworthy. Since gymnosperms had almost disappeared by the Eocene, the diversification of the dicotyledons went on until the neogene, when all extinct pollen types are already present. These important modifications of the vegetation reflect evolutionary trends as well as climatic changes during the Cretaceous: the climate, firstly hot, dry and perhaps arid, did probably induced salt deposition, and later became gradually more humid under oceanic influences which arose in connection with the Gondwana break-up.
Top-cited authors
Mohamed Abdelsalam
  • Oklahoma State University - Stillwater
Bob Stern
  • University of Texas at Dallas
Harald Fritz
  • Karl-Franzens-Universität Graz
Kamal A. Ali
  • United Arab Emirates University
Timothy M. Kusky
  • China University of Geosciences