Porosity analysis in a carbonate crust profile developed on Triassic basalt from the Middle Atlas (Morocco)

A carbonate crust profile has developed from Triassic basalt Rows in the Mo roccan Middle Atlas. The study of pore distribution of these supergene form ations shows a good agreement between calculated (from density measurements ) and directly measured values of porosity (mercury porosimetry). Weatherin g of basalt develops clayey layers (saprolite and soil) displaying increase s in porosity (macroporosity essentially). In the Quaternary, these very po rous and permeable layers permitted easy meteoric water circulation and a p rofile calcitization by per descensum processes. Carbonate precipitation oc curs by epigenesis replacement of silicate minerals. In the friable saproli te, carbonate replaces directly the basaltic primary minerals, while this r eplacement operates on the weathering products (smectites) and residual pri mary minerals in the coherent saprolite, indicating the very fast progressi on of the calcitization front. At the weathering front, within the coherent saprolite, both smectitic weathering and carbonatation compete as function s of time and fluid chemical composition. (C) 1999 Academie des sciences.