Lithostratigraphy and Biostratigraphy of Dalichai Formation in South of Babol (Northern Alborz)

The Middle and Upper Jurassic strata of the Alborz are named the Dalichai and Lar formations. The Dalichai Formation is introducing primary unites of Alborz Jurassic marine deposites and lithologically consist of a sequence of greyish limestones and marls that specially exposed in the south of Alborz Mountains. Most of the studies on the Dalichai Formation mainly focus on the ammonite fauna and their biostratigraphical importance (e.g. Seyed-Emami et al. 1985, 1989, 1995, 2013, 2015; Seyed-Emami and Schairer 2010, 2011a, 2011b; Dietze et al. 2014; Seyed-Emami and Raoufian 2017, Majidifard 2003, 2015; Schairer et al. 1991 and Raoufian et al. 2014, 2019). Ammonites are the most important fossils in the Dalichai Formation, they are of fundamental importance in determining the age and conditions of its paleoenvironment. In most area of the Alborz, the Dalichai Formation has marl, marly limestone, spary and thin bedded limestone with intercalation of marl and marly shale. In the sections studied this Formation consists of marl, marly limestone, sandy limestone and limestone. The aim of this study is to give an overview of the Oppeliidae fauna of the Dalichai Formation in Northern Alborz. The family of Oppeliidae has a high frequency as one of the Bathonian and Callovian index families in Alborz (e.g. Seyed-Emami et al. 2013). In the studied sections, this family has a frequency of 21.5%.

Lithostratigraphy and biostratigraphy of Middle Jurassic Dalichai Formation in Northern Alborz ranges have been studied for the first time. Three stratigraghical sections of Dalichai Formation in Northern Alborz ranges include Keriush in south of Babol, Kanefejar in south of Babol and Vana in Haraz road have been chosen for the paleontological studies. After field studies due to the abundance and importance of ammonite fauna in these sections, detailed sampling of ammonite fauna within the layers was done. After cleaning and preparing the ammonite samples and taking photograph of them, these samples were identified up to the species level and based on the international standard zonation (Cariou & Hantzpergue, 1997); zones of ammonites have been identified.

The studied ammonites come from three localities in northern Alborz Mountains. These are: 1. Keriush: 65 km south of Babol (N 36° 2'53.61", E 52°40'22.23"). The thickness of the Dalichai Formation at this section is 21/3 meters and can be subdivided into four members. The age of the Dalichai Formation is Upper Bajocian to Upper Callovian. 2. Kanefejar: 60 km south of Babol (N 36° 3'15.59", E 52°36'26.74"). The thickness of the Dalichai Formation is measured at this section 18/9 meters and can be subdivided, from bottom to top, into four members. The age of the Dalichai Formation is Upper Bajocian to Upper Callovian. 3. Vana: 75 km south of Amol (N 35°56'31.48", E 52°15'40.36"). The thickness of the Dalichai Formation is 70 meters and can be subdivided into four members. The age of the Dalichai Formation is Upper Bajocian to Upper Callovian. In this region, Dalichai Formation follows discontinuously on the dark, siliciclastic Shemshak Formation and is overlain gradationally by the light and cliff-forming carbonates of the Lar Formation. The biostratigraphy of the Dalichai Formation is discussed with special emphasis on the ammonite fauna (Oppeliidae family).

Palaeobiogeographically the middle Jurassic ammonite fauna in this area is closely related to that of the northwest-Tethyan, with relations to Subboreal / Submediterranean provinces. The study of ammonite fauna in this region led to identification of 48 genus, 18 subgenus and 26 species. Existing ammonite fauna indicates 8 ammonite biozones and Late Bajocian-Late Callovian ages for Dalichai Formation in Northern parts of Alborz ranges. The thickness of the formation in Northern parts of Alborz represents a much lower thickness of this formation than the southern parts of Alborz. The thickness changes of the Dalichai Formation in the northern and southern parts of the Alborz Mountains can be due to the tectonic conditions of the area and the existence of an extensional phase during its formation.