Biostratigraphy of the Qom Formation in the Northeast and Southeast of Semnan

In this study, sediments of the Qom Formation were investigated from the two stratigraphical sections in the northeast (Attari section) and southeast (Garmab section) of Semnan. The Attari section that consists mainly of limestone, sandy limestone, sandstone, calcareous shale and marl has a thickness measured 280 meters. In this section, the Qom Formation lies nonconformably on basic subvolcanic rocks and is disconformably overlain by the Upper Red Formation. Garmab section, with 97.5 m thick is composed of limestone and lies disconformably between Tuff at the base and Upper Red Formations at the top. The study of 167 samples taken from the studied sections led to the identification of 31 genera and 36 species of benthonic foraminifera, 1 genus and species of planktonic foraminifera, 3 genera and species of red algae and 2 genera and species of bryozoa. Among them, benthonic foraminifera have more variety and abundance, so the benthonic foraminifera from the measured sections are used to determine the age of successions and justify their correlation. Based on the identified foraminifera, the Attari section is comparable to biozonation introduced by Adams & Bourgeois (1967). It is analogous to Borelis melo group-Meandropsina iranica Assemblage Zone (1). According to the distribution of the index foraminifera, the age of sediments belonging to studied formation in Attari section is Burdigalian. The age of sediments of the Qom Formation in Garmab section based on the occurrence of Triloculina tricarinata, Triloculina trigonula and Globigerinoides triloba is probably Aquitanian.