Stratigraphy and reconstruction of the sedimentary environment of the Dalichai and Lar formations in the Yengejeh section (southeast of Zanjan)

Abstract Stratigraphic and sedimentology studies have been carried out on sixty-nine rock samples related to the one hundred fifteen meters thickness of the Dalichai and Lar formations in the southeast of Zanjan. The carbonate layers of the Dalichai Formation with seventy-eight meters thickness overlay the strata of the Shemshak Group/Formation (the boundary has been covered) and are overlaid by thirty-seven meters thickness of carbonate layers of the Lar Formation with gradual boundary in this area. There are some sedimentological changes in the succession across the Dalichai-Lar formations boundary such as color changes, bedding thickness changes from medium to thick, disappearance of Thalassinoides facies, chert nodules, and Ammonite/Belemnite facies and finally changes in microfacies from mudstone to the wackestone/packstone (deeper to the shallower). The Lar Formation is the youngest formation in this area that has been covered by the Quaternary sediments. Five lithofacies have been identified based on field and microscopic studies: 1- Pelloidal packstone with intraclasts, 2- Pellet/oncoid wackestone/packstone, 3- Mudstone with Thalassinoides, 4- Bositra buchi pelagic packstone and 5- Pelagic mudstone that shows basin and outer ramp as depositional environments for the Dalichai and Lar formations in this section. Field and facies studies show a deepening upward trend to the middle parts of the section (upper parts of the Dalichai Formation) and followed by shallowing upward trend to the upper parts (Lar Formation) of the section. Keywords: Stratigraphy, Sedimentary environment, Dalichai Formation, Lar Formation, Zanjan.     Introduction      Several studies have been carried out on the stratigraphy and sedimentary environment of the Dalichai and Lar formations in different parts of the Alborz sedimentary basin, which show that the Dalichai Formation is lithologically composed of marls, shales, and limestones, and the  Lar Formation include thick-bedded fossiliferous limestones. Microfacies studies on the Dalichai Formation show that this formation was deposited in a deep marine ramp environment. Separation of the boundary between Dalichai and Lar formations has been discussed by many geologists, so for this purpose, in addition to fieldwork studies and lithological boundary, geochemical data have also been used for this purpose in some studies. In this research, a suitable outcrop in the southeast of Zanjan was examined for the purpose of studying stratigraphy, separating the boundary between the Dalichai and Lar formations, and reconstructing the sedimentary environment of these formations.   Material & Methods     During the initial field studies, the best and most complete section was selected for stratigraphic study and sampling. During the field studies, features such as changes in lithology composition and the thickness of succession, fossil content, the color of the weathered surface of rocks and rock texture were considered. Sixty-nine samples were collected from the Dalichai and Lar formations. Thirty samples are related to Dalichai Formation and the rest are related to Lar Formation. Also, two samples were collected from the Shemshak Formation/Group. Thin sections were prepared from all the samples. The classification of Dunham (1962) was used in determining and naming the microfacies, and the environmental interpretation was based on Flügel and Munnecke (2010). The proposed sedimentary model for these deposits was presented based on the combination of field and microscopic data.   Discussion of Results & Conclusions     The lithology of the Dalichai Formation in the western Alborz is mainly limestones and argillaceous limestones (Adabi and Abarghani 2001), but towards the central and eastern Alborz, it is observed as shale and marl (Hashemi-Yazdi et al. 2020). The Dalichai Formation in the studied section consists of about 78 meters of blue and grey medium-bedded limestones with mudstone texture. A collection of ammonite and belemnite fossils are observed in the upper parts of the Dalichai Formation, which are mainly in the form of external molds and in most cases are filled with secondary silica. The identification of the boundary between the Dalichai and Lar formations is based on the presence of cherty sequences and thick-bedded limestone layers, which is clearly visible in the studied section. The stratigraphic and sedimentological studies on the Dalichai and Lar formations in the Yengje section led to the identification of five facies, which are as follows from shallow to the deep marine environment: pelloidal packstone with intraclast, pellet-oncoid wackestone-packstone, mudstone with Thalassinoides, Bositra buchi pelagic packstone and finally pelagic mudstone. The identified microfacies indicate the deposition of the Dalichai Formation in the studied area in an open marine environment belonging to the outer ramp and deep basin during the Middle Jurassic. The dominant presence of micrite and little amounts of cement in the carbonate facies also probably indicates a low-energy sedimentary environment during the deposition of carbonates of the Dalichai Formation in the studied area. The presence of semi-rounded intraclasts shown in situ intra-basin transport can be affected by the activities of the Middle Cimmerian orogenic phase in the region. This facies can indicate the beginning of a transgression, and then, the facies of the deep basin were deposited. Following that, as a result of the sea level fall, the facies change to wackestone to packstone with pellets and ancoid related to the outer ramp of the Lar Formation