The objectives of this study were to understand the stages of crack-fill calcite in these concretions that can help in a better understanding of these secondary structures in NE Iran as well as other parts of the world. Petrography and stable isotopes signatures reveal that the septarian concretions are multiphase and display several stages of cementation (G2 to G4). The earliest carbonate matrix (septarian body, called G1) with δ13 C and δ18 O values of 0.01‰ and -21‰ PDB, respectively, precipitated from seawater during shallow burial and anaerobic microbial oxidation of organic matter. The formation temperature of G1 is calculated to be 15.5º C. The Presence of authigenic pyrite with G1 supports the interpreted environment of precipitation. Calcite from the walls to the central parts of the cracks (G2 to G4) show increasing crystal sizes, clarity and depletion of δ18 O values. The formation temperature of G4 is calculated to be 54ºC. δ13 C values in three generations of calcite show no major differences. These data reveal that the cracks are filled by calcite during burial, which the last generation formed at higher temperatures than the temperature of the nodules

Keywords