Abstract
A carbonate crust profile has developed from Triassic basalt flows in the Moroccan Middle Atlas. The study of pore distribution of these supergene formations shows a good agreement between calculated (from density measurements) and directly measured values of porosity (mercury porosimetry). Weathering of basalt develops clayey layers (saprolite and soil) displaying increases in porosity (macroporosity essentially). In the Quaternary, these very porous and permeable layers permitted easy meteoric water circulation and a profile calcitization by per descensum. processes. Carbonate precipitation occurs by epigenesis replacement of silicate minerals. In the friable saprolite, carbonate replaces directly the basaltic primary minerals, while this replacement operates on the weathering products (smectites) and residual primary minerals in the coherent saprolite, indicating the very fast progression of the calcitization front. At the weathering front, within the coherent saprolite, both smectitic weathering and carbonatation compete as functions of time and fluid chemical composition.
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