Matière organique et transformations structurales superficielles dans un sol ferrallitique rouge de la zone forestière du Cameroun

Les sols ferrallitiques modaux de la zone forestière du sud Cameroun présentent, dans la partie supérieure du profil, un horizon plus compact résultant de la dégradation de la structure initiale en microagrégats et d'une illuviation de l'argile. Cette étude vise à établir les relations entre les différenciations morphologiques et organiques se produisant dans cet horizon. Il semble que la matière organique agisse directement en développant une acidité qui favorise le lessivage de l'argile lorsqu'elle est faible, l'altération de l'hématite et des argiles lorsqu'elle est plus élevé

Geochemistry and Geochronology of the Nlonako Alkaline Complex, Central Africa

The Nlonako Anorogenic Complex (NAC) is located in the southern continental sector of the Cameroon Line (CL). It intrudes the Cretaceous sediments and the Pan-african granitogneissic basement and is characterized by an association of diversified plutonic and volcanic rocks. Geochemical studies of major, trace and rare earth elements reveal an alkaline series which corresponds to a gabbro-diorite-monzonite-syenite-granite type. This series comprises two suites: a metaluminous suite made of basic and intermediate rocks, fayalite alkali granite and syenites; and a peralkaline suite including rhyolite and other granites. Both suites are derived from a single, hot and dry magma of basaltic composition. The evolution of the magma is dominated by fractional crystallization of mantle material and was progressively affected by crustal contamination at the end of the differentiation process. The conventional whole-rock Rb/Sr method was performed on three syenites and five granites. The ages obtained range between 44.6 and 49.7 Ma, confirming the Tertiary time of emplacement of the NAC.Keywords: Nlonako Anorogenic Complex (NAC), alkaline series, magma, fractional crystallization, crustal contamination, tertiary age

Cartographie des zones à risques d’inondation en zone soudano-sahélienne : cas de Maga et ses environs dans la région de l’extrême-nord Cameroun

Une étude des facteurs du milieu mis à contribution dans le processus d’inondation de la zone de Maga et ses environs a démontré que les conditions naturelles de ce milieu sont très favorables au processus d’inondation. En effet, sur le plan morphologique, la plaine du Logone est bordée de part et d’autre par des reliefs élevés qui empêchent l’écoulement des nombreux cours d’eau qui convergent donc dans la plaine. Sur le plan pédologique, la texture des sols est dominée par les sables et les argiles. Ainsi, le sol sableux favorisent une remontée rapide de la nappe phréatique tandis que les sols très argileux induisent une imperméabilisation de ces derniers ; d’où un ruissèlement intense. De plus, ces sols sont très pauvres en matières organique et l’essentiel des nutriments indispensables pour assurer une bonne production agricole est apportée par les inondations ; d’où la délicatesse de l’approche visant la réduction des effets de ce phénomène dommageable sur les populations. Sur le plan climatique, les précipitations sont relativement faibles, mais tombent très brutalement sur de très courtes périodes. Enfin, les pressions anthropiques sans cesse croissantes sur le milieu amplifient les conséquences des inondations sur les populations. La télédétection et les SIG ont été mis à contribution pour la cartographie des zones à risque d’inondation dans le site d’étude. C’est ainsi que 32,31% de la superficie totale de la zone de Maga et ses environs sont soumis à un risque fort d’inondation, 19,75% à un risque moyen et 47, 94% à un risque faible. Les solutions efficace envisageables pour la prévention des risques dans la zone concernent l’aménagement de la digue de retenu d’eau de Maga et la lutte contre l’érosion des berges de cours d’eaux de la région de l’Extrême Nord du Cameroun.Mots-clés: risque d’inondation, région soudano-sahélienne, Maga, prévention des risques. Mapping of flood risk areas in sudano-sahelian zone : Maga and surrounding area case in the far north region of CameroonA study of environmental factors of flooding in Maga area and its surroundings has shown that the natural conditions of this environment is very favorable to the flooding process. Indeed, morphologically, the plain of the Logone is lined on both sides by high reliefs that prevent the flow of many rivers thus converge on the plain. On the soil map, soil texture is dominated by sands and clays. Thus, the sandy soil favor a rapid rise in the water table while the heavy clay soils induce waterproofing of the latter; where intense trickling. In addition, these soils are very poor in organic matter, and most of the nutrients essential for good agricultural production is provided by the floods, where the delicacy of the approach to reducing the effects of this damaging phenomenon populations. In terms of climate, rainfall is relatively low; but fall very sharply over very short periods. Finally, the ever-increasing human pressure on the environment amplify the impact of floods on the population. Remote sensing and GIS have been involved in the mapping of areas at risk of flooding on the site Thus, 32.31% of the total area of Maga area and its surroundings are subjected to a high risk of flooding, 19.75% medium risk and 47, 94% at low risk. The effective options for risk prevention in the area related to the layouts of the dam of retained water Maga and the fight against the erosion of the banks of rivers in the region of the Far North of Cameroon.Keywords: flood risk, sudano-sahelian region, Maga, risk prevention

Trace element differentiation in ferruginous accumulation soil patterns under tropical rainforest of southern Cameroon, the role of climatic change

Regions under tropical rainforest cover, such as central Africa and Brazil are characterised by degradation and dismantling of old ferricrete structures. In southern Cameroon, these processes are relayed by present-day ferruginous accumulation soil facies, situated on the middle and the lower part of hill slopes. These facies become progressively harder towards the surface, containing from bottom to top, mainly kaolinite, kaolinite-goethite and Al-rich goethite-hematite, and are discontinuous to the relictic hematite-dominated ferricrete that exist in the upper part of the hill slope. These features were investigated in terms of geochemical differentiation of trace elements. It appears that, in contrast to the old ferricrete facies, the current ferruginous accumulations are enriched in transitional trace elements (V, Cr, Co, Y, Sc) and Ph, while alkali-earth elements are less differentiated. This recent chemical accumulation is controlled both by intense weathering of the granodiorite bedrock and by mobilisation of elements previously accumulated in the old ferricrete. The observed processes are clearly linked to the present-day humid climate with rising groundwater tables. They slowly replace the old ferricretes formed during Cretaceous time under more seasonal climatic conditions, representing an instructive case of continuos global change. (C) 2002 Elsevier Science B.V. All rights reserved

Old ferricrete landscape dismantling in Central Africa rain forest zone: formation of the present downslope iron accumulations.

Present downslope iron accumulations were investigated in the rainforest zone in southern Cameroon. Six clay and Fe-hydroxide dominated patterns have been identified and occur on the lower part of hill slopes. They can be subdivided in three different sequences, related to gentle, moderate or steep slopes. They are discontinuous with respect to the dismantling zone of the old ferricrete cap formed at Cretaceous period. They show a gradual development from a soft Fe-crust (carapace) to a vesicular facies that will, with time, cover the whole landscape again