Sedimentary facies and depositional environments of the Oligocene–early Miocene marine Qom Formation, Central Iran Back-Arc Basin, Iran (northeastern margin of the Tethyan Seaway)

Abstract

The Qom Formation was deposited at the northeastern margin of the Tethyan Seaway. To analyze the sedimentary facies and depositional environments, the marine Qom Formation deposits in the Qom-Natanz region (Central Iran Back-Arc Basin) were studied. The Qom Formation is the main reservoir and source rock of hydrocarbons in central Iran. Studying the Qom Formation in Khurabad (SE Qom) and Natanz sections revealed that the study sections are dominated mainly by larger benthic foraminifera, coralline red algae, and corals. The Qom Formation is “Rupelian–Burdigalian” and “Rupelian?–Aquitanian” in age in the Khurabad and Natanz sections, respectively. Based on component distributions, texture, and field investigations, 14 different facies were identified and interpreted. A homoclinal ramp is the suggested model for the deposition of the Qom Formation in the study sections. Accordingly, open marine (middle ramp), lagoon (inner ramp), and peritidal flat/beach as major depositional environments were identified. The middle ramp was characterized by the association of perforate foraminifera, coralline red algae, bryozoans, echinoids, as well as planktonic foraminifera. The distal part of the inner ramp was characterized mainly by the co-occurrence of perforate and imperforate foraminifera. Coral patch reefs were also present, but there is no reef complex with sigmoidal geometries. The proximal part of the inner ramp was characterized mainly by imperforate foraminifera with low diversity. The terrigenous facies were more probably deposited in the high-stressed shoreface to the shallow inner ramp. However, the evaporative facies were deposited in a saltern environment. The abundant presence of larger benthic foraminifera (perforate and imperforate) and coralline red algae, as well as the presence of z-corals, emphasize a tropical–subtropical palaeoenvironment.