| By using lithological and geochemical methods, the depositional environment of stromatolitic limestone of pre-salt Barra Velha Formation in Santos Basin, offshore Brazil, was analyzed. It is concluded that the dominating fabric of stromatolitic limestone consist of shrub, finger-like and spherulitic stromatolites, which relate to depositional environment evolutionarily. From a macroscopic view, many stromatolites occur in aggregation with overlaid and adjacent pattern. The main consitution of stromatolite are made of sparry calcite precipitated directly from lake water and microbial-mediated micrite. The stromatolites, although not produced by microorganism directly, are influenced significantly by microorganism. That the high contents of magnesium and silicon, and the positive depletion in carbon and oxygen isotopes, which have been measured from stromatolitic samples, indicate a saline environment with intensive evaporation, mirroring a closed lacustrine system. Stromatolite aggregates with several centimeters in height are widespread, which implies the depth of water favored stromatolite growth did not last long, perhaps on a millennium scale or even shorter, and that stromatolites grown with different and alternate height vertically shows that the frequent fluctuation in water depth was common during that period. |
