Variations minéralogiques et structurales accompagnant le passage «sols rouges - sols jaunes» dans un bassin versant caractéristique de la zone de contact forêt-savane de l'Afrique occidentale (Booro Borotou, Côte-d'Ivoire). / Mineralogical and structural changes associated with the transition «Red soils - Yellow soils» in a watershed at the border between the forest and the savannah zones of Western Africa (Booro

Abstract

Mineralogical and structural changes associated with the transition «Red soils -Yellow soils » in a watershed at the border between the forest and the savannah zones of Western Africa (Booro Borotou, Ivory Coast) Most of the mineralogical, textural and structural changes occurring in the soil cover of a small watershed (136 ha) located in an area at the border between the forest and savannah zones had been previously attributed to the functioning and the development of different types of geochemical systems. The present study deals with the system that works in the upper part of the soil cover (A and В horizons) and transforms red clayey ferrallitic soils (located upslope in the catena) into yellow less clayey ferruginous ones (located downwards). It impoverishes the soils first in hematite, then in goethite and in kaolinite. Important structural and textural changes are orderly associated with the observed impoverishments in iron oxides and clay minerals. The mineralogy of the sand and the kaolinitic fractions of the different soils points to an inherited heterogeneity of the soil cover along the slopes : the soils located upslope have always been more clayey and more weathered than the soils located downslope. It is therefore postulated that at the beginning, the transforming system developed within a soil cover that, in terms of mineralogy, texture and structure, differed from the one where it works presently. At the present stage of its development, the system affects mainly the red soils located upslope.