The Oligo-Miocene Qom Formation (Iran): Re-examination of biostratigraphy and age interpretations in the Sanandaj–Sirjan and Central Iran basins (NE margin of the Tethyan Seaway)

Abstract The depositional history of the Qom Formation, along the northeastern margin of the Tethyan Seaway, is recognized as essential to understanding the connection between the Eastern Tethys (the proto-Indian Ocean) and the Western Tethys regions (the proto-Mediterranean Sea). This paper re-examines the Qom Formation’s biostratigraphy and age interpretations of four stratigraphic sections (Abadeh, Zefreh, Chalheghareh, and Qom) and proposes revisions. These sections were previously studied within a project dealing with the palaeogeographic and palaeobiogeographic reconstruction of the Tethyan Seaway during the Late Oligocene to Early Miocene. Those biostratigraphic interpretations subdivided the time interval for deposition of the Qom Formation into Rupelian, Chattian, Aquitanian, and Burdigalian. However, some age-based interpretations were inconsistent with the confirmed age ranges of seven planktic foraminiferal species, as well as with the last occurrence of true-Nummulites spp. and the first appearance of Borelis melo curdica. These revised interpretations show that: a) the basal 38 m of the Abadeh section must be attributed to Rupelian; b) in the Qom section, 500 m of deposits previously interpreted as Burdigalian, must be attributed to the Aquitanian; and c) in the Zefreh section, the lower 30 m likely were deposited in the Chattian, and all deposits (107 m) previously interpreted as Burdigalian can be attributed to the Aquitanian.