New Discriminant Functions and Geochemistry of the Mamfe Cretaceous Formations (South West Cameroon)

Major and trace element geochemistry have been used to unravel the tectonic setting, source rock composition, and depositional environment of sedimentary rocks in the Mamfe formation. Field studies reveal both sub tabular and tabular outcrops indicating a post tectonic sedimentary activity for the subtabualar outcrops. Major element geochemistry reveals a moderate to high proportion (50-75wt %) of silica for the analyzed samples. New discriminant diagrams constructed for usage of adjusted major elements shows samples plotting on collision, arc and rift. Another discriminant plot for adjusted major element combined with trace elements shows samples plotting on active and passive tectonic setting.  Ratios of highly immobile trace elements such as Cr/Th, Th/Sc, Th/Co, and La/Sc conclude a felsic source rock for the studied rocks of the Mamfe formation. Trace elements ratios for redox conditions and marine-continental discrimination such as Ni/Co, U/Th, V/Cr, Th/U, and Y/Ho show that the sedimentary rocks of the   formation were deposited in a shallow oxygenated continental fluvio-lacustrine environment

Morphotectonic of Regolith Landsurfaces in Cameroon (Central Africa): Weighting the Evidences of Late Cretaceous Uplift and Climatic Deep Weathering and Stripping

International audienceIn the last few decades, a range of regolith-related issues highlighted their usefulness on assessing landform dynamics for better understanding and quantifying earth surface processes. A model for the evolution of weathered landsurfaces in Cameroon is developed using available data on their morphology and petrogeochemical patterns, tectonic evidences, geophysical anomalies and sedimentological data. On deformed orogens, the model demonstrates the pivotal role of uplift, consequent volcanic cover and rheology-dependent gravitational collapses in inducing large scale compartmentalization. In areas of tectonic quiescence, it enhances a well known predominance of deep weathering and striping resulting in morphogenetic sequences similar to that of climate-dependent dynamics on the West African craton. These schemes in accordance with regional correlations allow assuming four regolith landsurfaces recognizable with their actual relicts and their specific petrogeochemical patterns. These are the high glacis, the widespread Intermediate ferricrete, the African bauxitic surface, and the modern bauxites developed on Neogene volcanics across the so-called Cameroon Volcanic Line. These findings provide a unique geomorphic record resulting in interplay between late Cretaceous epeirogeny, heterogeneity of uplifted basement, and climate variability. It stands as an input in understanding the dynamics of the complex geomorphic system of the African tropical belt, and specifically in Central Africa where no tectonic input were considered in old issues. It is also a basis for assessing denudation rates and their comparison with weathering-limited steady cratonic denudation of West Africa, and to investigate Cenozoic drainage rearrangement and stabilization as a consequence of uplift

Morphotectonic of Regolith Landsurfaces in Cameroon (Central Africa): Weighting the Evidences of Late Cretaceous Uplift and Climatic Deep Weathering and Stripping

International audienceIn the last few decades, a range of regolith-related issues highlighted their usefulness on assessing landform dynamics for better understanding and quantifying earth surface processes. A model for the evolution of weathered landsurfaces in Cameroon is developed using available data on their morphology and petrogeochemical patterns, tectonic evidences, geophysical anomalies and sedimentological data. On deformed orogens, the model demonstrates the pivotal role of uplift, consequent volcanic cover and rheology-dependent gravitational collapses in inducing large scale compartmentalization. In areas of tectonic quiescence, it enhances a well known predominance of deep weathering and striping resulting in morphogenetic sequences similar to that of climate-dependent dynamics on the West African craton. These schemes in accordance with regional correlations allow assuming four regolith landsurfaces recognizable with their actual relicts and their specific petrogeochemical patterns. These are the high glacis, the widespread Intermediate ferricrete, the African bauxitic surface, and the modern bauxites developed on Neogene volcanics across the so-called Cameroon Volcanic Line. These findings provide a unique geomorphic record resulting in interplay between late Cretaceous epeirogeny, heterogeneity of uplifted basement, and climate variability. It stands as an input in understanding the dynamics of the complex geomorphic system of the African tropical belt, and specifically in Central Africa where no tectonic input were considered in old issues. It is also a basis for assessing denudation rates and their comparison with weathering-limited steady cratonic denudation of West Africa, and to investigate Cenozoic drainage rearrangement and stabilization as a consequence of uplift

Differentiated Neogene bauxitization of volcanic rocks (westernCameroon): Morpho-geological constraints on chemical erosion (vol 194,104685, 2020)

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Lateritic landsurface-regolith differentiation on Pan-African granitic basement of Adamaoua highland, Cameroon

International audienceThe aim of this paper is to describe and discuss the laterization pattern and the structuration mode of landsurfaces formed over the Pan-African granitic basement of Adamaoua highland. Paleocene to mid-Eocene epeirogeny compartmentalized the regional topography of Adamaoua on which lateritic landsurfaces have been formed over granitoids of comparable geochemistry. Regional scale geomorphological observations and petrogeochemical characterization of lateritic duricrusted regolith allow to distinguish three distinct lateritic landsurfaces, which are compared to lateritic surfaces from West Africa. Regolith of the upper and middle landsurfaces incorporate composite lateritic duricrusts including old bauxites and ferricretes, which are petrologicalheritages from former regolith of etch-plain type Paleogene African Surface. On Pan-African granitic basement of Adamaoua, the upper and middle landsurfaces formed on granitoids before mid-Miocene basaltic-andesitic outpourings that locally cover them, while the lower landsurface also formed on grantic rocks has been mostly shaped over late Neogene. The geochemical compositions of lateritic duricrusts of upper and middle landsurfaces are clearly distributed between aluminous, ferruginous and kaolinite poles that typifies lateritic regolith of thebauxitic and (Fe-rich) intermediate etch-plains surfaces similarly as lateritic regolith of the Paleogene West- African sequence. Geochemical compositions of lateritic regolith on lower landsurface are mostly distributed between silica, kaolinite and iron, suggesting that more or less evolved or duricrusted horizons have been exposed by differential erosion of this landsurface. Lateritic regoliths of the three landsurfaces are also typified and differentiated by their index of laterization (IOL) and fractionation patterns of some trace (Cr, V, Zr, Ti, Nb, Th, Y, Ta, Ga) and rare earth elements (HREE, Eu/Eu*, (Gd/Yb)N). Though Neogene volcanic epirogenic uplift effects on landscape cannot be totally excluded, erosion and down-wasting of Paleogene landscape mostlyevolved to a degraded lateritic etch-plain and late formation of a lower pediplain on Pan-African basement of Adamaoua highland under long-lasting per-humid climatic conditions over Cenozoic

Erosion mode and history of Eastern Adamaoua landscapes (Cameroon): Superimposed lateritic weathering of granites and basalts

International audienceLandscapes of eastern Adamaoua highlands have been shaped by successive weathering and erosion processes of Pan-African crystalline rocks and Neogene volcanics cover. Basalts have outpoured through major inherited structural discontinuities mostly in shallow incisions of lateritic pediplains previously shaped on the granitic basement. That has resulted in formation of a singular composite lateritic weathering profile on basalt superimposed to a truncated profile composed of mottled clays and saprolite on granite. The composite profile studied here is little evolved, mostly kaolinised on granite while basalt is weakly to moderately lateritised owing to differences in silica and iron oxide contents of the two contrasted parent rocks. During lateritic weathering, Cu, Ni, and Co are enriched in profile on granite but depleted on basalt, while Zr/Ti is relatively constant. Behavior and fractionation of REE are comparable except higher Eu* and Ce* anomaly on granite profile than on weathered basalt, owing to parent minerals differences, mostly feldspars in granite versus plagioclase and Fe-Mg minerals in basalt. The contrast in Eu* is also linked to differences in the Index Of Laterization (IOL) in weathered horizons of the two parent rocks. Beyond quite classical litho-dependent geochemical differences in the composite profile, persistence of per-humid climate and good drainage over Neogene have sustained rock-weathering and local surface erosion processes upon volcanic outpours and their contact surface with pediplains formed on Pan-African granitoids of Adamaoua highland