Lithostratigraphy and biostratigraphy of Dalichai Formation in the Bāghi stratigraphic section, northwest of Esfarāyen, based on ammonitic fauna

The Binalud Mountains in the northeast of Iran is a limited structural unit between the central Iran, Kopet-Dagh, and Alborz Mountains, mainly considered as a southeastern extension of Alborz Mountains (Lammerer et al., 1983; Eftekhar-Nezhad & Behroozi, 1991; Alavi, 1992; Seyed-Emami & Schairer, 2011). Jurassic successions, which have very well extended outcrops in the eastern Alborz and Binalud Mountains, were the main topic of the publications by Seyed-Emami & Schairer (2010), Wilmsen et al. (2009a,b), Raoufian et al. (2011, 2014), Seyed-Emami and Raoufian (2017), Seyed-Emami et al. (2018). Dalichai Formation in the eastern Alborz and Binalud Mountains comprises a thickness of marl and marly limestone, with an abundance of ammonite fauna which is the most important characteristic of the formation in the Binalud Mountains. This fauna in the Alborz and Binalud has long been considered in different papers (e.g., Shafeizad et al., 2005; Seyed-Emami et al., 2008, 2013, 2015, 2018; Raoufian et al., 2011, 2014). In this research, an outcrop of Dalichai Formation in the Bāghi stratigraphic section, northwest of Esfarayen city, was lithostratigraphycally and biostratigraphically studied.

The base of the measured stratigraphic section is located in 37°11′45.34″ and 57°16′21.89″, 29 km northwest of Esfarayen city (Esfarayen to Jajarm route). After field studies, reviews of lithofacies in the field, and drawing the log, 540 in situ ammonite specimens were obtained from the section. They are kept in the Museum of the Geology Department at the Ferdowsi University of Mashhad. The biometric details of ammonite specimens, including the dimensions of the specimens, size, depth, and diameter of the umbilical area, the number, arrangement, and shape of ribs, marginal keels, nodes, suture lines, and apertural shape, have been studied. The studied ammonitic fauna are typically Northwest Tethyan and belongs to the Submediterranean Province with episodic Mediterranean and Subboreal affinities (Seyed-Emami et al., 2013). It reflects the relationship between these regions with other parts of the Paleotethys sidelines during the Jurassic time.

In the Baghi section, the Dalichai Formation unconformably overlies the siliciclastic Shemshak Formation and gradationally underlies the cliff-forming carbonate rocks of Lar Formation. The Dalichai Formation, with a thickness of 655 meters, is divided into ten separate informal members and is mainly composed of marls, limestone, and marly limestone. One of the most important features of the Dalichai Formation in this section is the abundance of ammonite fauna, especially in the lower half of the section. Due to the specific color, lithology, and abundance of ammonite fauna, the second member is a key member.Paleontological studies in this research is led to the determination of 96 species, 26 subgenera, 39 genera, 20 subfamilies, 14 families, and 6 superfamilies belonging to 3 suborders of Ammonitida. According to the mentioned assemblage, 17 standard biozones are recognized, consisting of Grantiana, Parkinsoni, Zigzag, Retrocostatum, Bullatus, Gracilis, Anceps, Coronatum, Athleta, Lamberti, Transversarium, Bifurcatus, Bimammatum, Planula, Platynota, Hypselocyclum, and Acanthicum. These biozones show the age of late Bajocian – late Kimmeridgian for the Dalichai Formation. The studied ammonitic assemblages are closely related to the other Middle-Late Jurassic fauna from Iran and further peri-Mediterranean regions, which correlates with the Sub-Mediterranean province.