Reconstruction of Jeirud Formation depositional environment in Aro Section (Central Alborz)

In this paper it is attempted to reconstruct the depositional environment of the Jeirud Formation in Aro section. The Jeirud Formation (Late Devonian) is composed of about 45 meters of clastic sedimentary units in the southwest of Firoozkuh city, southern Alborz area. To carry out this research a field work was done during which a precise and detailed sedimentary log of the section was prepared and 52 samples from different lithologies of the formation were collected. Thin sections were prepared from all conglomeratic and sandstone samples, besides some thin sections were prepared from mudrocks and just 1 sample of paleosoil and then all were petrographically studied. In the study area, both basal and top boundary of the Jeirud Formation is disconformable with Mila and Mobarak formations respectively. All facies in the section include clastics (conglomerates, sandstones, mudstones and a paleosoil horizon). The facies were classified based on Miall classification so that conglomeratic facies comprise Gcm and Gmm; sandstone facies comprise Sh, Sp and Sm and mudrocks include Fl and Fm facies. Combined field and lab studies resulted in identification of two sedimentary facies associations including channel fill and flood plain facies associations. The reconstructed sedimentary environment of the Jeirud Formation in the Aro area represents a braided river depositional environment. The lower parts of this formation represent a finning-upward sequence which indicates deposition within a braided river channel subenvironment (conglomerates and sandstones) and the upper part is dominated with mudrocks with a paleosoil horizon representing deposition in a flood plain subenvironment.