Petrographic and Geochemical Evidence for Paragenetic Sequence Interpretation of the Lower Cretaceous Limestones in the Eastern Binalood Mountain Range، NE Iran

The Lower Cretaceous sediments (mainly limestones with a few horizons of sandstones and conglomerates) are present south of Mashhad along the Binalood Mountain range. Petrographic studies reveal that the carbonate rocks were deposited on a shallow carbonate platform of ramp type including open-marine, shoal, lagoon and tidal-flat facies belts. Diagenetic processes include compaction, cementation, micritization, replacement (silicification), neomorphism and fracturing. Oxygen and carbon isotope signatures of the limestones range from -11.2 to -2.1‰ and 0.2 to 4.3‰ PDB respectively. This variation reveals meteoric and burial diagenetic environments. Decreasing trend in the value of oxygen isotopes with increasing the amount of Fe and Mn as well as decreasing Na and Sr also indicate that these rocks may have been exposed to the meteoric diagenetic environment too. The calculated temperature for the ambient water in which calcite was deposited is about 26? C