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

• Epirogeny maintained per-humid climate for laterization on Adamaoua, West Africa. • Three stepped lateritic landsurfaces differentiated on granitoids of Adamaoua. • Eocene and Mid-Miocene volcanisms bound periods of lateritic surfaces formation. • Lateritic remains from etch-plain type Paleosurface in landsurfaces regolith. • Pediplain formed downslope the first two etch-plain type landsurfaces. The aim of this paper is to describe and discuss the laterization pattern and the structuration mode of land-surfaces 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 petro-geochemical 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 petrological heritages 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 the bauxitic 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 mostly evolved 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. [ABSTRACT FROM AUTHOR]