Climate and crop yields in Northern Africa (Senegal,Sudan, Tunisia)

The study attempts to relate crop yields to climatic events in the Sahelian Zone of Africa. The examples are taken from Senegal, Sudan and Tunisia. Theinvestigation was centered to examine closely the relations between weather and crop yields, that means first of all to analyze the influence of the inter-annual variations of the weather on the yields. The aim was to reveal the dependency of the temporal variability of the yields (1960–1980) on the climatic parameters during the different phenological stages of the plants. This analysis made it possible to develop models of prognosis for different cultivated plants (food crops basically). In general suitable models of forecasts could be established for all food crops. An early and exact prediction of harvest losses or surpluses could reduce negative effects for the respective population in unfavourable, and induce food storage in favourable years.     

Precambrian metallogeny in West Africa

The West African Shield is composed of a diverse assemblage of crystalline rocks that range in age from approximately 500 to more than 3000 million jears. These Precambrian rocks constitute the principal source of mineral resources for the subcontinent. The distribution patterns and origin of Precambrian mineral deposits are examined in relation to their geotectonic setting and host- or source rock associations as a prelude to the understanding of regional controls of Precambrian mineralization in the West African Shield.Occurrences of chrome, asbestos, talc and nickel are associated with ultramafic rocks within greenstone and related schist belts. Mafic volcanic piles and related sediments within greenstone, schist and orogenic belts are hosts to banded iron formations, manganese and vein-type gold deposits. Sedimentary deposits of gold and diamonds are closely associated with molasse-facies sediments of orogenic belts whereas magnetitehematite quartzites occur as low-grade iron ores within gneissic belts. Pan-African orogenic granites and pegmatites show a close genetic association with tin, niobium and tantalum deposits.The Archean schist belts within the West African Shield are the most diversely mineralized relative to other geotectonic domains. Compared to the Precambrian hields of southern Africa and elsewhere, the variability in the type and intensity of mineralization is attributed to the considerable differences in lithologic types, proportions and thicknesses.

---

Zusammenfassung Der Westafrikanische Schild besteht aus einer mannigfaltigen Ansammlung von kristallinen Gesteinen, deren Alter zwischen 500 m. und etwas mehr als 3000 m. Jahre liegen. Diese präkambrischen Gesteine sind die Quelle für manche Mineralien auf dem Subkontinent. Das Verbreitungsschema und die Herkunft der präkambrischen Minerallagerstätten werden in Beziehung zu ihrer geotektonischen Bildung und den Muttergesteinen untersucht. Dies zeigt die regionale Kontrolle der präkambrischen Mineralisation im Westafrikanischen Schild.Vorkommen von Chrom, Asbest, Talk und Nickel sind mit ultramafischen Gesteinen innerhalb der Grünsteine und anderen verwandten der Schiefergürtel verbunden. Gebänderte Eisenerzformationen, Mangan und Gold (Gang-Typ) befinden sich in den Lagerstätten mafisch vulkanischer Herkunft und zugehöriger Sedimente im Grünstein-Schiefer- und Orogen-Gürtel. Sedimentäre Lagerstätten von Gold und Diamanten sind eng mit Molasse-Fazies der Sedimente der orogenen Gürtel verbunden. Magnetit-Hämatit Quarzite kommen als arme Eisenerze im Gürtel der Gneise vor. Pan-afrikanische orogene Granite und Pegmatite zeigen einen engen genetischen Zusammenhang von Zinn, Niob und Tantal Lagerstätten.Der archaische Grünstein-Gürtel innerhalb des Westafrikanischen Schildes ist im Verhältnis zu anderen geotektonischen Einheiten am stärksten mineralisiert. Verglichen mit dem präkambrischen Schild des südlichen Afrika und anderen Kratonen sind die Varietäten im Typ und in der Intensität der Mineralisation von der unterschiedlichen Lithologie und den Mächtigkeitsverhältnissen geprägt.

---

Résumé Le socle de l'Afrique occidentale se compose d'une abondante association de roches cristallines dont l'âge va de 500 à plus de 3000 millions d'années. Ces roches précambriennes constituent la principale source des ressources minérales pour le subcontinent. Les schémas de distribution et l'origine des gîtes minéraux précambriens sont étudiés en relation avec leur formation géotectonique, leur association avec la roche voisine ou la roche-mère comme introduction à la compréhension des facteurs régionaux controlant la minéralisation précambrienne dans le socle de l'Afrique occidentale.L'occurrence du chrome, de l'asbeste, du talc et du nickel est liée aux roches ultramafiques dans les roches vertes et autres roches apparentées des zones schisteuses (ceintures schisteuses). Les formations rubanées de minerai de fer, le manganèse et l'or (type filonien) se trouvent en liaison avec les accumulations volcaniques mafiques et les sédiments apparentés dans les roches vertes, schistes et ceintures orogéniques. Les gîtes sédimentaires d'or et de diamant sont liés étroitement au facies molassique des sédiments des ceintures orogéniques, tandis que les quartzites à magnétite et hématite, de qualité médiocre comme minerai de fer, se présentent, dans les ceintures de gneiss. Les granites et pegmatites de l'orogenèse panafricaine montrent une étroite relation génétique avec les gisements de Sn, Niob et Tantale.La ceinture archéenne de roches vertes du bouclier de l'Afrique occidentale est le plus souvent minéralisée là où elle est en liaison avec d'autres unités géotectoniques. Comparées au bouclier précambrien de l'Afrique du Sud et d'ailleurs, la variabilité dans le type et l'intensité de la minéralisation sont attribuées aux notables différences dans les types lithologiques, leur proportion et leur puissance.

---

500 3000 . . , , . , , — . , , . — , , - — , . , . , , . , , .

     

Inequality and development in rural West Africa

Inequality seems to be a feature of all societies, but some are probably more equal than others. It is in Third World countries however where inequalities between particular areas, groups and individuals are most marked. The paper first traces the growing interest in both the study of inequality and the rural household as a focus of analysis. Some of the problems of measuring inequality are then considered and various indicators of inequality are examined with reference to empirical evidence from rural W Africa. The paper concludes that recent development strategies have often directly or indirectly led to greater rural inequality and makes suggestions as to how future projects might achieve greater impact in reducing the gap between rich and poor. Paper presented at Commonwealth Geographical Bureau, Workshop on Spatial Inequalities in the Developing World, Kano, Nigeria 16–20 September 1985     

Climate and cropping systems in West Africa

The importance of the water balance in determining cropping patterns in West Africa is examined. Crops in each country were ranked in order of area occupied. Crop diversity is reduced when the period of water surplus is very short or very long. The fraction of the cropped area occupied by a particular crop is related to the length of the period of water surplus. Four basic cropping patterns are identified.     

The precambrian of West Africa

Consequent upon continued active field investigations and the advent of radioactive dating of samples from the area, the time is thought opportune to attempt a synthesis of Precambrian correlations throughout West Africa. Some isotope age determinations would, at first glance, appear to necessitate radically new concepts of stratigraphic successions and correlations. For example, the Ahaggar Suggarian and the Anti-Atlas Precambrian I suggest age values consonant with Upper or then Middle Precambrian times; the Older and Younger Granites of Nigeria appear to have been emplaced in Lower Palaeozoic and Jurassic times respectively. These and other examples trend towards younger ages than had hitherto been surmised for many stratigraphic units. However, it is thought premature at this time to place too much emphasis on isotope age values obtained for these ancient rocks of West Africa.Field studies to date have been more successful in establishing successions rather than correlations, whereas isotope age determinations have achieved little in either respect.Several well-known series, once placed in the Precambrian, e.g. Falemian, Akjoujt, Bakel, are now thought to be rather Cambro-Ordovician in age. On the other hand, for other familiar units, e.g. Ahnet Purple Series, Buem Formation, the stratigraphic position remains problematical.

---

Zusammenfassung Nach fortgesetzten aktiven Feldforschungen und mit dem Erscheinen von radioaktiven Datumsbestimmungen von Proben aus dem Gebiet, scheint es an der Zeit, eine Synthese der präkambrischen Verhältnisse in ganz Westafrika zu unternehmen.Einige isotopische Altersbestimmungen scheinen auf den ersten Blick eine vollkommen neue Auffassung der stratigraphischen Sukzessionen und Korrelationen nötig zu machen. So lassen zum Beispiel das Ahaggar Suggarien und das Anti-Atlas Präkambrium I Alterswerte vermuten, die mit dem oberen und mittleren Präkambrium übereinstimmen; die älteren und jüngeren Granite in Nigeria scheinen im unteren Paläozoikum bzw. im Jura übergelagert worden zu sein. Diese und andere Beispiele zeigen einen Trend zu jüngerem Alter, als bisher für viele stratigraphische Einheiten angenommen wurde. Jedoch wird es für verfrüht gehalten, den isotopischen Alterswerten für diese alten Gesteine in Westafrika eine zu große Bedeutung beizumessen.Feldstudien waren bisher erfolgreicher bei der Herstellung von Sukzessionen als bei der Herstellung von Korrelationen, während die isotopischen Altersbestimmungen für beides wenig erbracht haben.Einige wohlbekannte Reihen, z. B. Falemien, Akjoujt und Bakel, die zuvor ins Präkambrium gelegt wurden, werden nun dem Alter nach eher dem Kambro-Ordovicium zugeordnet. Andererseits bleibt für andere bekannte Reihen, z. B. die Ahnet Purple Series und de Buem Formation, die stratigraphische Position problematisch.

---

Résumé Par suite des recherches actives et prolongées sur le terrain et de la venue d'échantillons radio-actifs datés de cette région, le temps semble opportun pour tenter une synthèse de corrélations précambriennes de toute l'Afrique Occidentale.A première vue, quelques déterminations d'âge absolu semblent nécessiter de nouvelles idées de successions et de corrélations stratigraphiques. Par example, l'Ahagar Suggarien et l'Anti-Atlas Précambrien I suggèrent des valeurs d'âge conforme au temps Précambrien supérieur ou moyen; les Vieux Granites (Older Granites) et les Granites Jeunes (Younger Granites) du Nigéria semblent avoir été placés respectivement dans le Paléozoique inférieur et le Jurassique. Ces examples-ci, et encore d'autres, tendent à indiquer pour beaucoup d'unités stratigraphiques des âges plus jeunes qu'on n'a supposés jusqu'à présent. Mais il est encore prématuré d'appuyer trop sur les valeurs d'âge absolu obtenus pour ces anciens terrains de l'Afrique Occidentale.Jusqu'à présent, des recherches sur le terrain ont réussi à établir des successions plutôt que des corrélations, pendant que les déterminations d'âge absolu n'ont eu que peu de succès des deux côtés.Quelques séries bien connues, qui jadis étaient placées dans le Précambrien, p. ex. le Falmien, la Série de l'Akjoujt et les Quartzites de Bakel sont à présent considérés d'être plutôt de l'âge Cambro-Ordovicien. Par contre, la position stratigraphique reste problématique pour d'autres unités bien connues, comme pour la Série Pourprée de l'Ahnet, la formation de Buem, et d'autres.

---

. , , , . .

     

Dust emission and transport in Mali, West Africa

Vertical dust fluxes were measured in the Inland Delta region of the Niger River, Mali, West Africa, during April-June, 1989 and 1990. Measurements of dust flux represent, for the most part, non-dust storm conditions or ‘dust haze’ periods. The observed concentration versus height relationships are similar to data presented by other investigators. The relationship between wind shear velocity (u_*, m s^?1) and vertical dust flux (F, μg m^?2 s^?1) can be described by a relationship in which F is proportional to u_*⁴. However, there is considerable scatter within the data set which is attributable to textural controls and surface conditions. The vertical dust fluxes measured in Mali are compared to dust fluxes measured in Texas, USA, and Yukon Territories, Canada. The significantly different values for the constant of proportionality (a) in the F α a u_*⁴ relationship for these geographically diverse areas is a function of surficial controls on the release of sediments to the air stream. Dust concentrations measured in Mali were found to be uniformly high and in general exceed WHO health standards for acceptable total suspended particulate loadings. As a result background dust may be considered a long term stress on health for the people of this region of Mali.     

Slim pickings: Fairtrade cotton in West Africa

This paper examines the extent to which certified fairtrade cotton programs in West Africa present an alternative to the conventional cotton economy. Two fairtrade programs operating in Burkina Faso and Mali serve as case studies. The paper argues that fairtrade cotton fails to offer an alternative to conventional cotton because it works within the same commodity chain that impoverishes farmers in the first place. Cotton grower organizations seek more power within the conventional cotton sector to increase incomes and improve the living standards of all cotton growers. They are also active at the international level to eliminate the inequities of international trade. It is in these arenas that cotton growers are struggling to improve their incomes and livelihoods. Fairtrade does not address these fundamental inequities and power relations. The slim pickings of these programs are further evident in the very small amount of cotton produced and marketed as fair trade. Plans to expand production from less than 1% to 10% by 2012 are unrealistic in light of the introduction of genetically modified cotton and the limited market demand for fairtrade cotton. Despite these limitations, fairtrade cotton programs are producing some positive effects, notably women’s participation in cash crop cultivation, higher cotton quality, and the diffusion of organic farming techniques. An innovative direct marketing agreement linking the National Cotton Growers’ Union of Burkina Faso with the US women’s apparel company Victoria’s Secret indicates that alternative trading relations can be constructed outside the conventional commodity chain.     

Sustainability of groundwater in Mali,West Africa

The following paper describes the goals and some preliminary work in the Bani sustainability study, an ongoing project in Mali, West Africa. Rural communities in Mali are increasingly relying on hand-pumps, which tap groundwater resources, as a means of obtaining potable water. The long-term sustainable yield of groundwater resources is not known but can be evaluated in sustainability study. In 2005, a groundwater sustainability study was established along the Bani River of Mali. The Bani study collected groundwater levels that were used in a conceptual groundwater flow model—the Bani model—to develop an understanding of current aquifer conditions and to make limited predictions of sustainability under various future scenarios. The Bani model showed the climatic parameters of recharge (derived from precipitation) and evapotranspiration to influence simulated groundwater levels and groundwater volume available, while increased pumping rates, due to population growth, showed little effect. When considered in the context of the actual Bani sustainability study area, the change in groundwater levels resulting from climatic parameters may have negative implications, especially during several consecutive years of decreased precipitation, such as drought, or if downward trends anticipated for precipitation continue.     

Metal enrichment in the groundwater of the arid environment in South Africa

This paper presents the enrichment tendency and spatial distribution of metals in the groundwater which is pumped out from the granitic aquifers in South Africa. Groundwater is the sole source of water supply for the local community in the study area (Namaqualand), and hence, it was necessary to understand the controlling geochemical processes and interrelationship of metals in the groundwater. The geochemical association of metals has been assessed based on the geostatistical methods. The results show that geochemical processes such as oxidation, leaching, and evaporation besides water–rock interaction are very important in controlling metal enrichment in the groundwater from highly mineralized rocks. The metal enrichment index for selected toxic metals in groundwater increases in the order of Cd > U > Cr > Pb. The observed enrichment trend could be considered as a result of mineralization of basement rocks which is facilitated by active geochemical process in the arid environment. The lack of aquifer flushing due to negligible recharge helps the metals to concentrate at shallow groundwater zones supported by severe evaporation process.     

ICL/IPL regional activities in West Africa

Landslides account for huge amount of property damage in virtually every part of the world. The 2011 Iva Valley debris flow in Enugu, Nigeria; the 2010 landslide in Bududa region, 275?km east of Kampala, Uganda; the 2010 debris avalanche in Benue state, Nigeria; the 2008 landslide in Congo; and the 2007 landslide in western Cameroon with a combined human death of over 4,000 remain a modern day illustration of the degree of threat to public safety catastrophic landslides symbolize. Landslides do not only destroy lives and resources, they also deface and devalue historical, cultural, and entertainment facilities critical to the survival of man. Taming their aggression and ruinous impacts, thereby rescuing the environment from potential crisis, should, in point of fact, become a priority. In West Africa, landslides cause losses worth several millions of dollars annually. Nigeria remains one of the most vulnerable places. In both the southern and northern divide of the country, there is a mix of sedimentary, igneous, and metamorphic rocks which provide a good basis for the application of geology to urban development and hazard zonation in Nigeria. Quite often, similar engineering geologic problems including landslides and rockfalls, foundation problems in montmorilonite-rich clayey soils and problems associated with abandoned mines have been observed in both divides. The ICL/IPL program attempts to evaluate the engineering geologic problems in these two zones and provide case histories as illustrations. Experience gained in one area may be applied regionally to tackle problems in any other area with similar geology.     

西非铁矿资源现状及潜力分析

西非是世界上铁矿资源最为丰富的地区之一,铁矿石总资源量可达649.5亿吨,占全球8%,主要分布于马恩地盾和雷圭巴特地盾,以几内亚、塞拉利昂、利比里亚和毛里塔尼亚四国最为丰富,成因类型以BIF型为主。公开资料显示,目前有43家公司投资开发73个西非铁矿项目,其中中国庆华集团、宝武集团、赢联盟集团、中国铝业和中投资本公司等五家中资企业拥有权益资源量达217.5亿吨,形成与西非本土8国和西方7国公司三足鼎立的局势。基于标普数据库和各公司最新年报统计,发现近10年西非铁矿石年均产量为0.2亿吨,世界占比不足0.1%。目前大部分矿山处于暂停开发或暂时搁置状态,仅有21个铁矿山处于活跃状态。其中,中资企业控制的五个项目中也只有唐克里里铁矿处于开发状态。基于西非地质和航磁资料,利用证据权模型,预测在90%、50%和10%置信水平下,西非地区未发现铁矿床数量分别为63个、92个和324个,铁矿石预测资源量分别为264.6亿吨、386.4亿吨和1360.8亿吨。     

Means of increasing productivity in extensive grassland farming in arid areas of Africa

The agricultural and pastoral development of the huge dry areas of the world are subject of this paper. The author describes the possible stages of development of the pastoral industry in accordance with the general economic development of a country or region.The development of the pastoral industry is much related to the availability of fodder during the year and over the years. Typical of the pastoral industry in the dry areas are the different ways of fodder compensation. The author points out that each development stage of a ranch has its typical way of fodder compensation in accordance with the general economic development stage of the region.     

The West Waterberg tonstein,South Africa

What is thought to be the first recorded occurrence of a tonstein in the coalfields of South Africa is described from the Volkrust Shale Formation of the Permain Karoo Sequence in the West Waterberg Coalfield. Now, virtually pure kaolin, petrological, mineralogical and geochemical evidence points to its origin being an airfall tuff of acid composition. The concentrations of Ba, Sr, and Pb in the tonstein are unusually high. It is thought the elements were released from the ash and precipitated as phosphates of mixed composition. The phosphate distribution is not uniform and is apparently related to the fragmental texture of the rock.     

Metallogenic portfolio of the West Africa craton

The West African Craton hosts major resources of gold, iron ore, aluminium ore, diamonds, phosphates and manganese. This portfolio of ore deposits is linked to the formation of Archean–Paleoproterozoic greenstone belts, Jurassic rifting and extended periods of Mesozoic to Cenozoic weathering and erosion. We give a brief overview of the temporal and spatial distribution patterns of West African ore deposits with emphasis on the main commodity types. The oldest ore forming processes generated major resources in iron ore and gold in the Kénéma–Man and Reguibat Shields during the Neo-Archean. The majority of gold, porphyry copper, lead–zinc and sedimentary manganese deposits formed during the Paleoproterozoic, dominantly within the Baoulé-Mossi domain. At the same time diamond-bearing kimberlites developed in Ghana. Another distinct diamond event has been recognized in the Mesozoic of the Kénéma–Man shield. Isolated occurrences of IOCG's as well as copper–gold and gold formed in Pan-African/Variscan belts. During the Neoproterozoic, the majority of mineralization consists of sedimentary iron ore and phosphate deposits located within intracratonic basins. During the Phanerozoic aluminium ore, phosphates and mineral sands concentrated along the margins of the coastal and intracratonic basins.     

Uranium-bearing phoscrete from Mali, West Africa

Many radioactive anomalies have been found in Mali. The occurrence of the continental U-bearing phoscrete differs considerably from that of marine sedimentary phoscrete which is being used as the raw material in the manufacturing of phosphoric acid. The results of some chemical and physical measurements on the U-bearing phoscrete will be briefly explained here and the genesis will be inferred.     

Hydrogeochemical and multi-tracer investigations of arsenic-affected aquifers in semi-arid West Africa

The semi-arid Sahel regions of West Africa rely heavily on groundwater from shallow to moderately deep(100 m b.g.l.)crystalline bedrock aquifers for drinking water production.Groundwater quality may be affected by high geogenic arsenic(As)concentrations(10μg/L)stemming from the oxidation of sulphide minerals(pyrite,arsenopyrite)in mineralised zones.These aquifers are still little investigated,especially concerning groundwater residence times and the influence of the annual monsoon season on groundwater chemistry.To gain insights on the temporal aspects of As contamination,we have used isotope tracers(noble gases,~3H,stable water isotopes(~2 H,~(18)O))and performed hydrochemical analyses on groundwater abstracted from tube wells and dug wells in a small study area in southwestern Burkina Faso.Results revealed a great variability in groundwater properties(e.g.redox conditions,As concentrations,water level,residence time)over spatial scales of only a few hundred metres,characteristic of the highly heterogeneous fractured underground.Elevated As levels are found in oxic groundwater of circum-neutral pH and show little relation with any of the measured parameters.Arsenic concentrations are relatively stable over the course of the year,with little effect seen by the monsoon.Groundwater residence time does not seem to have an influence on As concentrations,as elevated As can be found both in groundwater with short(50 a)and long(10~3 a)residence times as indicated by ~3He/~4He ratios spanning three orders of magnitude.These results support the hypothesis that the proximity to mineralised zones is the most crucial factor controlling As concentrations in the observed redox/pH conditions.The existence of very old water portions with residence times10~3 years already at depths of50 m b.g.l.is a new finding for the shallow fractured bedrock aquifers of Burkina Faso,suggesting that overexploitation of these relatively low-yielding aquifers may be an issue in the future.     

西部非洲水井的施工工艺及其改进措施

介绍在西部非洲干旱缺水地区的水井施工工艺，并且对其设备、安装和施工工艺等提出了改进措施，从而提高了效率，降低了成本。     

Rejuvenation of dry paleochannels in arid regions in NE Africa: a geological and geomorphological study

Although the River Nile Basin receives annually ca. 1600 billion cubic meters of rainfall, yet some countries within the Basin are suffering much from lack of water. The great changes in the physiography of the Nile Basin are well displayed on its many high mountains, mostly basement rocks that are overlain by clastic sediments and capped by volcanics in eastern and western Sudan. The central part of the Nile Basin is nearly flat including volcanics in the Bayuda Mountains and volcanic cones and plateaus in southwestern Egypt. The high mountains bordering the Nile Basin range in elevation from 3300 to 4600 m.a.s.l. in the Ethiopian volcanic plateau in the east to ca. 3070 m.a.s.l. in the western Gebel Marra, and 1310 m.a.s.l. in the Ennedi Mountains in northwestern Sudan. In central Sudan, the Nile Valley rises approximately 200–300 m.a.s.l. In Egypt, the River Nile is bounded by the Red Sea Mountains in the east, assuming ca. 1000–2600 m.a.s.l., mostly of basement rocks, which are covered to the north of Aswan by Phanerozoic sediments sloping to the west, passing by the Nile Valley and continuing through the Western Desert. The Phanerozoic cover on both sides of the Nile is known as the Eastern and Western Limestone Plateaus. These plateaus assume elevations varying from 300 to 350 m.a.s.l. near the eastern bank of the Nile to 400–500 m.a.s.l. south Luxor at Esna and west of Aswan. The nearly flat Sahara west of the Nile Valley rises gradually westward until it reaches Gebel Uweinat in the triple junction between Egypt, Sudan, and Libya. Gebel Uweinat has an elevation of 1900 m.a.s.l. sloping northward towards the Gilf Kebir Plateau, which is 1100 m.a.s.l. The high mountains and plateaus in the southern and western Egypt slope gradually northward where the Qattara Depression is located near the Mediterranean coast. The depression is ?134 m.b.s.l., which is the lowest natural point in Africa. All these physiographic features in Sudan and Egypt are related to (i) the separation of South America from Africa, which started in the Late Paleozoic and continued up to the Cretaceous, giving rise to several generally E–W-oriented tectonic features inside Africa, (ii) the uplift of the Red Sea Mountains and their continuation inside Africa resulted in the East African Rift System (EARS), (iii) the Guinea–Nubia Lineament crossing Africa from the Atlantic to the Red Sea where many havoc trends, mostly E–W-trending faults, and uplifted basement features pierce the overlying sediments, (iv) parallel and longitudinal structures associated with volcanic plateaus and cones extend from west Sudan (Gebel Marra) to Ethiopian Plateau, passing by volcanics and plume features in between and the basins in east Africa were subjected to wrench related inversions, and (v) the Sudd linear E–W area stretching more than 1000 km between Gebel Marra in the west, passing by South Sudan and reaching southwestern Ethiopia. Here, fluviatile and subsurface waters led to ponds, lakes, and wet areas that are hard to exploit. The impact of these features led to the present south to north River Nile, but passing by many changes in the direction of its many tributaries and slope reversal of some of the major extinct rivers, either sectors of the main Nile or the rivers once flowed into the main river. The paleoclimatic changes during the Quaternary period: wet and dry have a great effect on the physiographic features and slope reversal of the Nile Basin drainage system.     

Kimberlites of the Man craton, West Africa

The Man craton in West Africa is an Archaean craton formerly joined to the Guyana craton (South America) that was rifted apart in the Mesozoic. Kimberlites of the Man craton include three Jurassic-aged clusters in Guinea, two Jurassic-aged clusters in Sierra Leone, and in Liberia two clusters of unknown age and one Neoproterozoic cluster recently dated at 800 Ma.

All of the kimberlites irrespective of age occur as small pipes and prolific dykes. Some of the Banankoro cluster pipes in Guinea, the Koidu pipes in Sierra Leone and small pipes in the Weasua cluster in Liberia contain hypabyssal-facies kimberlite and remnants of the so-called transitional-facies and diatreme-facies kimberlite. Most of the Man craton kimberlites are mineralogically classified as phlogopite kimberlites, although potassium contents are relatively low. They are chemically similar to mica-poor Group 1A Southern African examples.

The Jurassic kimberlites are considered to represent one province of kimberlites that track from older bodies in Guinea (Droujba 153 Ma) to progressively younger kimberlites in Sierra Leone (Koidu, 146 Ma and Tongo, 140 Ma). The scarcity of diatreme-facies kimberlites relative to hypabyssal-facies kimberlites and the presence of the so-called transitional-facies indicate that the pipes have been eroded down to the interface between the root and diatreme zones. From this observation, it is concluded that extensive erosion (1–2 km) has occurred since the Jurassic. In addition to erosion, the presence of abundant early crystallizing phlogopite is considered to have had an effect on the relatively small sizes of the Man craton kimberlites.