The Calcare-di-base is the only significant carbonate facies of the upper Miocene evaporites of Sicily. It was deposited within an exceedingly saline but marine water body adjacent to older, exposed carbonate terranes which surround the depositional basin. This somewhat dolomitic limestone is commonly cavernous or brecciated at outcrop and contains numerous zones of evident halite dissolution. It also interfingers with thin gypsum stringers in many areas and overlies diatomites of variable thickness. Based on petrographic and isotopic studies, it appears that this limestone was produced by diagenetic processes from an origininal aragonitic mud containing displacive halite hoppers and massive halite zones. Regional variation in carbon-13 (^dgrC¹³ from 0 to 49 pp PDB) and oxygen-18 (^dgrO¹⁸ from +6 to -5 ppt PDB) can be tied to variations in the mineral content of the original sediment and to later diagenetic waters and their organic content. The very negative carbon-13 values and the presence of native sulfur are indicative of the calcitization of gypsum, the by-product of the life processes of sulfate-reducing bacteria. These bacteria utilize part of the available and commonly copious organic matter associated with both the carbonate and with the diatomite. Inversion of the original aragonite to calcite has resulted in expulsion of strontium from the carbonate crystal lattice and the formation of celestite (SrSO₄) which now fills the voids left by the dissolution of halite, and the associate pore spaces.

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