Genèse des carbonates terrestres dans la dépression de Quarzazate (Maroc méridional).

The carbonate encrusting process and its consequent products (crusts, tufa and travertine) in the southern Morocco (Ouarzazate) are studied in this thesis. A detailed inventory of the carbonate deposits is given. The dynamic aspects of carbonate precipitation and dissolution are considered from an environmental point of view. The main factors influencing their genesis are discussed on the basis of a multidisciplinary investigation. Among the accumulated carbonates, caliches are the most abundant. Their genesis invokes a changing environment where sedimentation has the same importance as the biogenic and pedogenic processes. Dated by the Uranium/Thorium method, these caliches are older than 300 ka. They indicate that the encrusting process is ancient. These deposits have become progressively rare during the Quaternary period except for the important hydrothermal travertine of Skoura. Only a few accumulations are still forming nowadays (Ait Arbi and Ifesfas). Their absence in present day water, especially in rivers is interpreted as indicating an environmental change. It is hypothesised that a desequilibrium affects the incrusting process. A comparative study between rivers and sources in different settings is carried out. The saturation degree of water, its chemical composition, especially in elements that may have particular origins (potassium, nitrate, sulfate, etc.) and its bacteriological characteristics are investigated in the thesis. Results seem to indicate that differences exist between the calcite - precipitating waters of some sources (not affected by human activities) and the sterile waters of rivers (Dades and M'goun). The latter are polluted by chemical fertilizers or turbulent, such that no carbonate deposits are permitted. The conclusion is that the reduced incrusting phenomena indicate an environmental change. A progressively drier climate and progressively reduced discharge make the deposits more limited in space and rare. The human impact on the quality of water must be considered to explain the limited extend of present day encrusting processes