Deeper-water gypsum formation constrained by micropaleontology and stable isotopes in the Burdigalian, northern Tethyan Seaway (Qom Formation)

The Qom Formation was deposited in the Central Basin of Iran, representing the northern part of the Tethyan Seaway during the Burdigalian (Lower Miocene). This study focuses on the deep marine marls of member e of the Qom succession in the Dochah section. This study integrates lithostratigraphy, calcareous nannofossils, oxygen and carbon isotopes of planktonic foraminifera, and oxygen isotopes measured on gypsum crystals. Deposition of evaporites started in the lower Burdiglian in the basin, increasing upwards. During middle to late Burdigalian, salinity increased in the basin due to tectonic activity and a relatively warm and definitely arid climate. The negative water budget resulted in precipitation and sedimentation of gypsum and halite. The oxygen isotope data measured on gypsum crystals indicate a primary, syn-depositional origin for these evaporite minerals. In addition, the oxygen measured on the planktonic foraminifera (average: -4.18‰ VPDB) indicate that the biota lived in a surface water with relatively normal salinity. We concluded that the evaporites were formed on the sea-bottom due to an increasing bottom water salinity under increased water column stratification. The Qom Basin shows similarities to the deep Mediterranean basin during the Messinian Salinity Crisis, where largest part of the evaporites precipitated under water from a deeper-water brine with increasing salinity in a stratified water column. However, the high diversity of calcareous nannoplankton coexistent with planktic foraminifera observed in the Qom succession is interpreted as reflecting high-frequency, low-amplitude sea-level fluctuations within the Milankovitch band with establishment of a temporary connection to the open-marine realm. These oscillations alternated between more open-marine conditions and short-lived stressed intervals, during which basin restriction, enhanced evaporation and episodic evaporite deposition occurred. Subsequently, thin evaporite layers that formed on the seafloor, were later fragmented and dispersed within the marls during diagenesis. Overall, this study provides new insights into the detailed paleoenvironmental evolution of the northern part of Tethyan seaway.

Keywords: Qom Formation, Paleosalinity, Tethyan Seaway, Evaporites, Nannoplankton