Dissolution et precipitation des carbonates dans des Sols (Calcrètes) en zone aride, bassin de Ksabi, moyenne moulouya, maroc: Consequence sur la desertification

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Abstract
Dissolution and Precipitation of Carbonates in Arid Soils (Calcretes), Ksabi Basin, Morocco; Consequences on the Desertification. In an arid region of North‐Eastern Morocco (Ksabi basin, central Moulouya), formation and conditions of carbonate accumulation in coarse alluvium were studied. All levels of pleistocene terraces were affected by those accumulations, among them the ones of pedological origin (calcretes) were caracterized. Three categories were chosen, typical of a terrace level belonging to recent, middle, or older Pleistocene, respectively. Macroscopic and microscopic studies of coarse calcareous elements, and of carbonated precipitates in all horizons of each profile, showed that coarse calcerous elements were partly dissolved. The study of the dissolving process outlines a seven stage‐distribution showing an interdependence: one between the seven stages and the age of the terrace level; the other one between the seven stages and the pedological horizon. Those seven stages characterize the dissolution depth in coarse elements superior face: (1) no dissolution; (2) dissolution depth between 0 and 0.1 mm; (3) dissolution depth between 0.1 and 0.5 mm; (4) dissolution depth between 0.5 and 1 mm; (5) dissolution length between 1 and 4 mm; (6) less than half the pebble heigh, in place, is dissolved; (7) more than half the pebble height, in place, is dissolved. The dissolving process of calcite crystals (sparite and microsparite) forms triangular shaped cavities. In recent formations, carbonates precipitated into the textural voids of the original sediment (primary voids) and the ones precipitated into the voids left by dissolving process (secondary voids), differ only by their structure: pseudocolumnar‐massive into primary voids; massive into secondary voids. In middle and older Pleistocene formations, carbonated precipitates differ in shape and colour according to localizations: massive and red‐brown in primary voids, and crystallized and whitish in secondary voids. The type of precipitated carbonates in secondary voids was determined by the presence or absence of a clay‐iron layer enveloping the coarse elements; in the first case microsparite and sparite, and in the second case micrite developed. Calcin was a pallissadic and whitish carbonated precipitate located in the basis of coarse elements. Irregular crystallizations of carbonates resulted from successive dissolving‐precipitating processes associated with soil watering. In this area, soil water availability depended on cyclic climatic variations during the Quaternary and on evolution of porosity in soil horizons. Both influenced the pedo‐climate and condition for dissolution processes and forms of precipitate. The vegetation was directly affected by those pedo‐climatic and pedo‐morphologic effects; water availability, differing according to calcrete evolution, was often limited because of soil hardering, thus causing the lead desertificaiton.