Stratigraphical investigations on the Shal Formation in Talesh Region, NW Alborz Mountains, Iran

The sedimentary successions corresponding to the Shal Formation (Upper Bajocian-Tithonian) which is located at east of the village of Shal in the Alborz Mountains are described in detail from stratigraphical points of view. In this framework, the ammonite succession, sampled bed-by-bed, is described as well. The Shal Formation is measured 22 m in thick in the study stratigraphic section but at the type-section it is measured 51 m and is consisting of bioclastic glauconitic nodular sandy limestones and limestones with numerous fossiliferous beds. The lower boundary of the Shal Formation is an unconformity due to the Mid-Cimmerian tectonic event (Seyed-Emami Alavi-Naini, 1990; Fürsich et al., 2009a) and the marine transgression of the Shal Formation across the underlying Shemshak Group (UpperTriassic-Lower Bajocian; Fürsich et al., 2009b) is diachronous. The upper boundary to the light-coloured Kolur Formation is gradational. Three lithofacies are differentiated in the Shal Formation. The fauna of the succession is composed mainly of ammonites, followed by bivalve, belemnite, sponge and brachiopoda. Ammonites occur in abundance throughout the sequence, mainly in the lower and middle parts of the formation. Six ammonite families are represented in the fauna (Phylloceratidae, Haploceratidae, Oppeliidae, Parkinsoniidae, Reineckeiidae, Perisphinctidae). Based on ammonites, the Shal Formation ranges from the Upper Bajocian to Tithonian in the studied stratigraphic section.