Microfacies and depositional model of the Qom Formation in the Khurabad area (southeastern Qom)

Deposition of the Qom Formation took place in three NW–SE-trending basins: Sanandaj–Sirjan fore-arc basin, Urumieh–Dokhtar intra-arc basin and Central Iran back-arc basin. The Qom Formation deposits in southeastern Qom have been sampled and studied in order to study the microfacies and their depositional conditions. Outcrops of the Qom Formation in the study area, with 260-m thickness, consist mainly of medium to thick bedded and massive limestones, marly limestone, marl, evaporate deposits and siliciclastic deposits (siltstone, sandstone, congelomerate). Study section is unconformably underlain by the Eocene volcanic rocks and overlain by the Upper Red Formation (URF). On the basis of recognized foraminifera and their vertical distribution, the Qom Formation is Rupelian-Burdigalian in age in the study area. Fourteen microfacies were recognized based on field investigations and microscopic studies. Based on recognized microfacies and field investigations and absence of reefal, oolitic and bioclastic barriers, the Qom Formation in southeastern Qom deposited on an open shelf. This shelf, is generally dividable into two depositional environments including inner shlf (lagoon) and middle shelf (open marine). However, outer shelf deposites which is characterized by the presence of planktonic foraminifera and/or light-independent, heterotroph organisms, as well as absence of larger benthic foraminifera are not observed in the study area.