Biostratigraphy and paleoecology of the Qom Formation in Chenar area (northwestern Kashan), Iran

A study of large benthic foraminifera from the 147 in-thick Qom Formation in the Chenar area (northwestern Kashan) is reported. One hundred and twelve thin sections were prepared and the distribution of benthic foraminifera was analyzed to reconstruct paleoenvironmental conditions. Study of these sections led to the identification of 28 genera and 38 species. On the basis of the recognized foraminifera. the section is comparable to Lepidocyclina- Operculina- Ditrupa Assemblage zone, and the age of the Qom Formation in the studied section is assigned to the Oligocene. Evidence of sea level changes is observed from bottom to top of the studied section. On the basis of large benthic foraminifera assemblages and microfacies features, three major depositional environments (inner shelf, middle shelf and outer shelf) were defined. The inner shelf facies is characterized by wackstone-packstone, dominated by miliolid and small perforate foraminifera. The middle shelf facies is represented by packstone-grainstone with diverse assemblage of large perforate benthic foraminifera. Outer shelf facies is dominated by large perforate benthic foraminifera as well as planktic foraminifera. The distribution of the Oligocene large benthic foraminifera in the studied area indicates that shallow marine carbonate sediments of the Qom Formation were deposited in a photic zone of tropical to subtropical environments. Finally the correlation between the study area and some other sections of Central Iran indicates that sedimentation of the Qom Formation is continued from Late Rupelian to Citation in northwest and Late Rupelian to Aquitanian in southeast direction.