MINERALOGICAL STUDY OF WEATHERING ON THE BANA COMPLEX, WESTERN PART OF CAMEROON

The Bana complex is composed of various rocks from volcanics to plutonics located in the western part of Cameroon at latitude 5° 08'N and longitude 10° 20'E. The climate of this region is essentially sudanese tropical type characterized by alternation of humid and arid seasons. The annual precipitation in this area amounts to 1500-2500mm. Under these circumstance, the Bana complex is extensively weathered. The basement rock is composed of gneiss (orthogneiss) which is intruded and covered by later plutonic rocks from gabbro to granite and volcanic rocks, from basalt to rhyolite, respectively. Many weathering profiles selected in the granite as well as in volcanic rocks were investigated in detail. Mineralogical characteristics such as chemical composition and micro-morphological variation of the weathering products were examined by means of X-ray diffraction (XRD), microprobe analysis (EPMA), X-ray fluorescent (XRF), scanning and transmission electron microscopy (SEM and TEM). Halloysite and kaolinite are the most predominant weathering products regardless type of the host rocks. Detailed mineralogical investigation on the selected profiles revealed the transformation sequence of kaolin minerals as weathering proceed. That is, the lower parts are rich in halloysite whereas the upper parts are rich in kaolinite with a small amount of halloysite. The obtained mineralogical sequence was further confirmed by micro-morphological investigation of these minerals, i.e., from spherical halloysite to finally platy kaolinite. Spherical halloysite associated with short tubular crystal is characteristic in the bottom part whereas irregular platy crystals predominate in the upper parts. These morphological variation together with crystal size reasonably explain the formation of kaolin minerals and crystallographical transformation sequence from spherical halloysite, short tubular halloysite, long tubular halloysite and finally platy kaolinite was established. Etch pits developed on quartz and K-feldspar grains are commonly observed. However, shape and size of the etch pits are quite complicated suggesting none monotonous micro-weathering condition. Etch pits found in the upper part are much rougher than those of the lower part. Based on the results obtained, weathering condition prevailed the Bana complex were discussed mainly from the stand point of clay mineralogy

Risk Link to the Volcanic Activity of the Mt. Cameroon in Cameroon

The Mt. Cameroon is a large volcanic horst belonging to the majure tectonic line (Cameroon volcanic line). The geographical morphology of the mountain is controlled by regional tectonics. The Mt. Cameroon is consisted of precambrian metamorphic basement covered with Cretaceous to recent sediments distributed mainly in the Douala and Riodel Rey basin. The oldest lava could be of upper Miocene age. The Mt. Cameroon has erupted six times in the 20th century. The 1982 eruption took place inside the crater of an ancient cone. The lavas are picrites (with forsteritic olivine phenocrysts), alkali basalts (with salitic augite phenocrysts), hawaiites (with labrador-bytownite plagioclase phenocrysts) and mugearites (with scarce kaersutite phenocrysts and microlitic phlogopite or nosean). According to Deruelle et al.(1987), the Mt. Cameroon lava series is typically alkaline with no tholeiitic or transitional trend. Risk related to volcanism is actual for the human constructions especially along the axis of the horst. The severe volcanic risk has been limited to explosion, lava flows and lahar. It is possibility that explosion, laver flow and earthquarke take place anywhere on the volcanic mountain. Nevertheless, these explosions are most likely to occur in the delimited sector which is shown in a map presented in this paper. Even if we could divert the course of the lava flows to certain safer direction, flows caused by sudden earthquarke and explosion may cause serious damages

Application of geostatistical methods to estimate the mineral contents in the alluvial clay deposit, Monoum plain, West Cameroon

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Method of choosing soils as CEB or pavement layers based on geological and environmental information and mode of treatment

During research on the geological, mineralogical, mechanical and geotechnical nature of thick layers of soils developed on the rocks of the lower zone of the southern slopes of Mount Bambouto (West Cameroon), a couple of observations were made. The mechanical behavior of soils as construction materials is linked to the nature of the parent rock and its environment of formation. Four different types of rock have been noted: trachybasalt, orthogneiss, anatexite, and biotite-hornblende granitoid. These rocks in a hot and humid mountain climate, exposed to the monsoon wind, have weathered to give soils with exploitable characteristics in compressed earth bricks (CEB) and pavement layers. This article proposes a method of choosing a soil for pavement layers and for the production of fired earth bricks / earth bricks treated or not with hydraulic binders, based on the nature of the bedrock and the climatic environment. Several formulation models have been defined. The performance, fairness, accuracy and adequacy of these models with real values were made by metric evaluation. So, it is: • Preliminary method for quickly and easily formulating masonry units in clay soil. • Preliminary method for quickly and easily formulating pavement layers in clay soil. • Simple and reliable method for choosing clay soils as construction materials from elementary geological information

Effect of Soil Geomechanical Properties and Geo-Environmental Factors on Landslide Predisposition at Mount Oku, Cameroon

In this work, we explored a novel approach to integrate both geo-environmental and soil geomechanical parameters in a landslide susceptibility model. A total of 179 shallow to deep landslides were identified using Google Earth images and field observations. Moreover, soil geomechanical properties of 11 representative soil samples were analyzed. The relationship between soil properties was evaluated using the Pearson correlation coe cient and geotechnical diagrams. Membership values were assigned to each soil property class, using the fuzzy membership method. The information value method allowed computing the weight value of geo-environmental factor classes. From the soil geomechanical membership values and the geo-environmental factor weights, three landslide predisposition models were produced, two separate models and one combined model. The results of the soil testing allowed classifying the soils in the study area as highly plastic clays, with high water content, swelling, and shrinkage potential. Some geo-environmental factor classes revealed their landslide prediction ability by displaying high weight values. While the model with only soil properties tended to underrate unstable and stable areas, the model combining soil properties and geo-environmental factors allowed a more precise identification of stability conditions. The geo-environmental factors model and the model combining geo-environmental factors and soil properties displayed predictive powers of 80 and 93%, respectively. It can be concluded that the spatial analysis of soil geomechanical properties can play a major role in the detection of landslide prone areas, which is of great interest for site selection and planning with respect to sustainable development at Mount Oku