Biostratigraphy and Paleoenvironments of the Oligocene Deposits (Qom Formation) in the Neyzar Area (Southeast of Salafchegan), Iran

Abstract. The Qom Formation deposits, located at 12 km southeast of Salafchegan (N: 34° 21′ 26″ and E: 50° 32′ 14″), have a thickness of 110 m. The formation includes thin, and medium-to-thick bedded limestone, as well as shale, overlying the Lower Red Formation (early Oligocene) above an erosional unconformity. Its upper boundary is covered by alluvium sediments. Biostratigraphic distributions of benthic Foraminifera were used to determine the age and paleoenvironmental conditions of the Qom Formation. Two assemblage zones were recognized. Assemblage zones 1 and 2 were indicative of a Rupelian–Chattian age. The inner shelf (restricted and semi-restricted lagoons) and middle shelf (open marine) settings were formed on an open shelf platform. Euphotic conditions were dominant during the early and late Rupelian in the studied area. During the middle Rupelian, photic conditions were variable between euphotic and mesophotic to oligophotic. During the early and middle Chattian, photic conditions varied between oligophotic, mesophotic to oligophotic, and euphotic. During the late Chattian, mesophotic-oligophotic conditions were dominant. Additionally, a high level of salinity (40–50 and > 50 psu) was present in the studied area during the early Rupelian. The level of salinity varied from normal (30–40 psu) to hypersaline (40–50 psu) during the late Rupelian. Salinity during early and late Chattian subages was normal (30–40 psu). However, hypersaline (40–50 and > 50 psu) and normal (30–40 psu) conditions were present in the middle Chattian. Eutrophic to mesotrophic-oligotrophic conditions were found in the early Rupelian age. However, mesotrophy-oligotrophy dominated during the middle and late Rupelian and Chattian ages. In addition, the paleo-water depth of the Qom Sea fluctuated from < 10 m to > 20 m. Foralgal and foramol associations are dominant in the studied area during the Rupelian-Chattian ages. The general depositional environment of the Qom Formation is associated with seagrass meadows.