Genesis and classification of soils developed on gabbro in the high reliefs of Maroua region, North Cameroon

Abstract

The purpose of this work was to examine the genesis, properties and classification ofsoils resulting from the weathering of gabbro rock in the high reliefs of Maroua in theFar North Region of Cameroon. The studied soils were ~ 2 m thick, made of fourhorizons which consisted of coarse saprolite, fine saprolite, loose loamy clayey horizonand humiferous horizon. From petrographical view point, at the bottom of the soilprofile, the preservation of the bedrock structure was marked by numerous remnantsof altered plagioclases shapes. The groundmass was characterized by a double spacedfine, ranging to equal, enaulic c/f related distribution pattern. It was yellowish,characterized by weakly separated granular microstructure in the fine saprolite andhad a speckled and cloudy limpidity in the loose loamy clayey horizon. Secondaryminerals consisted of montmorilonite, kaolinite, goethite, quartz, gibbsite,lepidocrocite, sepiolite, feldspar and calcite. Globally, Si/Al ratio ranged between 2.85and 3.24. The chemical index of alteration ranged from 50.95 to 55.27 % while themineralogical index of alteration values were between 1.90 and 10.54 %.Physicochemically, soil pH varied from slightly acidic to slightly above neutral. Soilorganic carbon contents were low to very low. Exchangeable bases contents were high,mostly represented by Ca2+ and Mg2+. The CEC of soils and the CEC of clay were alsohigh, ranging respectively between 53.68 and 82.88 cmol(+).kg-1, and 116.80 and181.38 cmol(+).kg-1. The studied soils were classified as dystric haplustepts clayeyisohyperthermic. They were developed in situ by the collapse of primary mineralstructures from the bottom of the coarse saprolite, due to leaching as a result ofbisiallitisation and monosiallitisation. This is accompanied by a progressiveferruginization of materials, confirmed by the densification under the microscope ofgoethitic brown veil from the base to the loamy clay horizon and the increase in ironcontents from the bedrock to the humiferous surface horizon.