Biostratigraphy, biofacies and paleoenvironment of Lower Eocene strata at northwestern Shahrud

Paleontological studies of the Ziarat and lower part of Karaj Formations in the Mojen area have been done. The study section, northwest of Shahrud, consists of limestone, sandy limestone, marly limestone, marl and gypsiferous marl with a thickness of about 101 meters. At the study section, the Ziarat Formation underlain conformably by Fajan Formation. A total of 48 species of foraminifera (distributed among 30 genera) were recognized in this study. Therefore, Ziarat Formation can be dated as Ypresian and Karaj Formation can be dated as Lutetian. One biozone (Cuvillierina eocenica range zone) was regionally established and five informal biofacies identified as follows: 1) Miliolidae – Rotalidae – bioclast, 2) Alveolina – Miliolidae – Orbitolites, 3) Alveolina – Nummulites, 4) Nummulites – bioclast, 5) Nummulitidae – Discocyclinidae. Based on these 5 biofacies, deposition may have taken place in a carbonate ramp. The stratigraphic position and index foraminifera reveal that just E1 (Ypresian) of the Ziarat Formation is present at the Mojen section. Abundance of large size flosculinized Alveolina implies a warm humid climatic condition during deposition of the Ziarat Formation (Ypresian) in the Mojen section.