Biostratigraphy of the Tirgan Formation in the Kopet-Dagh basin:Stratigraphic sections of the Tirgan village and Amirabad

The Lower Cretaceous successions of the NE Iran in the sedimentary Kopet-Dagh basin are result from transgression of the Berriasian-Barremian marine, which are deposited after first erosional cycle (Late Jurassic-Early Cretaceous). In this research, two stratigraphic sections of these successions, including the Tirgan Formation, described and studied in detail, and is provided a biostratigraphic framework for them. The new paleontological results of the Tirgan Formation are based on the benthic foraminifera. Some of them are reported for the first time (e.g. Conorbinella azerbaidjanica, Cuneolina composaurii, Feurtillia gracilis and Orbitolinopsis cf. nikolovi). Feurtillia gracilis taxon range zone offers the Valanginian age for the base parts of the Tirgan Formation as well genera and species; Balkhania balkhanica, Dictyoconus pachymarginalis and Orbitolinopsis cf. nikolovi confirm Late Barremian-Aptian age for the upper parts of this Formation. This study shows a diachrony in the age of base of the Tirgan Formation, so that transgressive successions start with Valanginian age (or probably Berriasian) in the Tirgan village section, however the Amirabad section shows Late Barremian age at the beginning of transgression.