MORPHOLOGY, MINERALOGY AND GEOCHEMISTRY OF A LATERITIC SOIL SEQUENCE DEVELOPED ON MICASCHIST IN THE ABONG-MBANG REGION, SOUTHEAST CAMEROON

Abstract

The aim of the study was to highlight the morphological, mineralogical and geochemical characteristics of soils developed on micaschists without garnet and on interfluve with plan tabular summit which dominate the landscape below 800 m above sea level in the rainforest zone of southern Cameroon. Morphologically, these lateritic soils are more than 10 m thick. They are constituted of height major horizons regrouped into indurated and loose materials. Indurated material predominates in the median part of the profiles in the form of gleabular ferruginous horizons and downstream above the water table in the form of toeslope ferruginous accumulation horizons. These horizons, which can reach 4 to 5 m thick on the slopes, are constituted mainly of goethite and hematite, these two iron oxides being always associated to kaolinite. In this mineral paragenesis, the presence of goethite is the result of the disruption of kaolinite-hematite association which characterizes the primary iron duricrust under hydratation, redox processes confirmed by Eh values between 0.09 and 0.12 volt and biological activities. The released iron may be exported and partially retained in the lower part of the interfluve and accumulate in the form of toeslope ferruginous accumulation horizons bearing goethite and in the weathering horizon where it forms iron rich nodules in the alloteritic horizon. In the loose material, composed of weathering horizons at the base of the profiles, loose clayey horizons at the surface and also fine earth between nodules, pebbles or iron duricrust blocks of the glaebular ferruginous horizons, kaolinite is the predominant mineral. Iron oxides, although weakly represented, remain in sufficient quantity to print vivid colorations to these horizons. In the profile, this results, geochemically, in the occurrence of high content, of iron, but also of aluminium and silicon, combined in the form of kaolinite, as attests by the atomic ratio S i A l generally between 0.9 and 1.3. These lateritic soils are characterized firstly by the presence of muscovite which is an indication of autochthony of soils developed on micaschists without garnet and, secondly by the abundance of impregnated ferruginous nodules in the alloteritic horizon, which marks a specific continuous nodulation different from observations already made on lateritic soils developed on other rocks in south Cameroon Plateau.