The origin of "evaporitive" limestones; an example from the Messinian of Sicily (Italy)

Abstract

A good example of an extensive evaporative carbonate deposit which has altered little since formation is found in the Messinian (Upper Miocene) evaporites of Sicily. In this study, these Messinian carbonates (the "Calcare di Base") are examined through a combination of field observations, together with petrologic, mineralogic, and stable isotope studies. These evaporitic carbonates are found in massive, nearly featureless beds (both calcite and aragonite), commonly composed of automorphic breccias and/or weakly layered micrite. Additionally fine-scale features are observed within the carbonates such as small algal stromatolites and algally controlled laminae. Petrologic study of thin sections reveals ubiquitous algal filaments and grumeleuse to pelletal micrites as the primary components. This suggests, by analogy to modern hypersaline environments, that the original calcium carbonate precipitated as algally controlled aragonite in a hypersaline environment. Displacive and interbedded halite, still present within the deposit in the subsurface in some areas, has been leached in others, leaving residual breccias, cubic voids, and halite pseudomorphs. The carbonate has been partially calcitized during one or more phases of early diagenesis. The diangeetic systems which affected these sediments were not wholly open. The halite was removed in some areas early in the sediment''s history, but after partial lithification (probably synsedimentary), as indicated by breccias containing well-preserved halite voids. These breccias lie within a nonhalite-bearing micritic matrix. Later alteration has removed more of the halite, leaving cubic void spaces, porous layers, and halite pseudomorphs filled with calcite, celestite, and strontianite. The presence, in some areas, of considerable amounts of unaltered aragonite and, in other regions, of secondary strontium minerals indicate that the systems which affected these sediments were not open ones. Based on the oxygen-isotope values obtained from the unaltered aragonites, it appears that the large volumes of calcium carbonate in this deposit were derived from a combination of continual influx of seawater, together with a probable contribution from continental sources coming into a restricted hypersaline environment (at the margins of the basin). The isotopic composition of the altered carbonates suggests that meteoric waters participated in the diagenesis, while secondary bacterial processes (oxidation, sulfate reduction, and methanogenesis) contributed carbonate ions delpeted in carbon-12 to the final product.