Lithostratigraphy and Biostratigraphy of the Dalichai Formation West of Shahroud (Eastern Alborz)

Middle Jurassic strata are well developed at west of shahroud (SE Alborz). In the investigated area two sections at kuh-e Parikhan and kuh-e Tepal have been studied in detail. The Dalichai Formation has a maximum thickness of 235 m consisting of an alternation of bluish-grey limestones and marls, being subdivided into 8 members. The lower boundary with the siliciclastics of the underlying shemshak Formation is sharp and discontinuous, whereas the upper boundary to the light and cliff-forming carbonates of the Lar Formation is rather inconspicuous and transitional. The Dalichai Formation in both sections is fossiliferous. Summing up 1985 fossil specimens were collected, among which the ammonites with 1749 specimens clearly predominate. The fossil-content is mainly concentrated within few meters of red and nodular limestone in the upper part of the section. Based on the ammonite fauna the age of the Dalichai Formation in the investigated area ranges from Upper Bajocian to lower Oxfordian. Pleobigeographically the fauna shows close relationship to western Europe and sub-mediterranean region. This indicates a paleogeographic position of the area at the northern Tethys during the Middle Jurassic. The depositional environment ranges from deeper shelve to deeper open marine.