Age determination of the oligocene Qom formation and interpretation of palaeoenvironments in the Qom back-arc basin (Northern neotethys) using benthic foraminifera

Abstract

One outcrop of the Qom Formation was measured for age determination and interpretation of palaeoenvironmen-tal conditions based on benthic foraminifera and the vertical distribution of microfacies and microtaphofacies in the Bijegan area of Iran. The study area is located about 20 km northeast of Delijan. There the Qom Formation is 162 m thick and dominantly contains thin, medium, thick bedded, and massive limestone. The formation overlies the Lower Red Formation in the Bijegan area. The upper boundary of the Qom Formation is formed by unconformably overlying recent alluvial sedi-ments. Three assemblage zones were identified. Assemblage zone 1 indicates a Rupelian age while assemblage zones 2 and 3 were deposited during the Chattian. The correlation between the curve of seawater depth changes and the distribution curve of microtaphofacies in the study area is indicative of a high energy environment (under the effect of the fair-weather wave base [FWWB] or storm wave base [SWB]) throughout the Rupelian-Chattian. Salinities are interpreted to have oscil-lated between normal marine (30–40 psu) and hypersaline (>50 psu) during deposition. The trophic conditions were mainly mesotrophic to oligotrophic. Euphotic conditions prevailed during the Rupelian and Chattian. In the Bijegan area, the Qom Formation formed in euphotic to meso-oligophotic conditions. Lagoonal and open marine environments characterise the lower section of the studied sequence (Rupelian and lower Chattian stages) and open marine conditions were dominant in the upper section of the study sequence (upper Chattian stage). Correlation of the study area with five other Rupelian-Chattian outcrops shows high sedimentation rate in the Qohrud area during the Rupelian and in the Mashhad Ardehal area during the Chattian. Throughout the Oligocene, the activity of local faults presumably affected sedimentation rates, sedimentary environments, and sea-level change in the Qohrud, Vidoje, Naragh, Bijegan, Mashhad Ardehal and Neizar areas. © 2020 E. Schweizerbart’sche Verlagsbuchhandlung, Stuttgart, Germany.