The Cenomanian heterozoan carbonates in the north-central Alborz, north-east Kelardasht, north Iran

Detailed field surveys, petrographic investigation and SEM and EDS analyses have been used to evaluate Cenomanian glauconitic heterozoan carbonates in north-east Kelardasht, north-central Alborz, north Iran. Lithofacies and microfacies analyses led to recognition of six microfacies types related to the inner-, mid- and outer-ramp facies belts of a carbonate ramp. The heterozoan nature of these carbonates is inferred from a predominance of echinoderms associated with calcispheres, planktonic foraminifers, a lack of ooid grains, and a low carbonate production rate, together with a high content of glauconite grains and prevailing high-Mg calcite mineralogy. Petrographic and SEM studies reveal that glauconite filling skeletal grains retains the shape and morphology of host grains, signifying an authigenic origin at low sedimentation rates and slightly reducing conditions. SEM images show cauliflower and rosette structures associated with well-developed lamellae indicating an authigenic origin of evolved glauconite grains. Our findings are compatible with a nutrient-rich waters and palaeoecological stress related to relative sea-level rise and eutrophic conditions, which contributed to the generation of these heterozoan carbonates despite the hot greenhouse conditions during the Cenomanian in the north-central Alborz Mountains.