The Tethyan Seaway during the early to middle Miocene – New data and a review

Abstract

The Tethyan Seaway was the connection between the Eastern and Western Tethys which became restricted and finally closed during the Early and Middle Miocene. The growing Zagros Mountains split the seaway into two entities, the Iranian Gateway in the northeast and the Mesopotamian Gateway in the southwest. The reconstruction of the seaway is based on sedimentological and paleontological data of the Qom, Asmari and Gachsaran formations predominantly on biostratigraphy. This paper presents an evaluation and new data on the occurrence of the Qom Formation in the Zanjan area (Sheikh Jaber section) of Iran. The Qom Formation there is dominated by marls and sandstones and subordinated limestones. Unusual for the Qom Formation is the high share of volcanic rocks (basalts, tuffs, tuffitic marls, lapilli, coarse volcanoclastic components in all types of lithologies) which indicate that the Qom Formation in the Zanjan area belongs either to the Sanandaj-Sirjan Basin or the Urumieh-Dokhtar Basin. Planktonic foraminifers and mollusks, in particular, pteropods, indicate a late Burdigalian to Langhian age for the studied section. The mollusk fauna exhibits a pure Mediterranean character and no overlap with the Indo-Pacific fauna occurs. Both the fauna and the overlying continental deposits of the Upper Red Formation preclude a marine connection along the Iranian Gateway in the Langhian between the Proto-Mediterranean and Indian Ocean, the mollusk fauna also obviates a connection between the Proto-Mediterranean/Indian Ocean and the Eastern Paratethys. The sediments in the Mesopotamian Gateway are represented by the Asmari Formation, which is similar in facies and stratigraphy to the Qom Formation, and the Gachsaran Formation dominated by evaporites and shallow marine sediments. This gateway may have not been completely blocked in the Langhian, but the very shallow water connection was paleoceanographically ineffective and not passable for biota such as mollusks.