Application of shell beds in the sequence stratigraphic analysis of the Aitamir Formation in the Sheikh and the Bibahreh synclines, west Kopet-Dagh

One of the stratigraphic units of the Kopet-Dagh Basin is Aitamir Formation. This formation has been studied at four locations including Sheikh and Sorkhezoo in the Sheikh Syncline and Gadganloo and Paskooh in the Bibahreh syncline. The Aitamir Formation consists of fine glauconitic sandstone، siltstone، shale and carbonate rocks. This formation in studied area contains shell beds or shell concentrations. Three types of shell concentrations can be distinguished according to differences in stratigraphic، sedimentologic، palaeoecological، and taphonomic features. These three types of shell concentrations are found in distinct positions within the depositional sequence. Ammonite-dominated concentration (S3) in the early TST has a distinct basal erosional surface (ravinement surface)، and the shells are completely disarticulated، fragmented and abraded with a chaotic orientation، suggesting reworking of previous deposited sediments، and redeposition in a semi-restricted lagoon environment. Shell concentration at the end of the TST is a hiatus type or condensed back lap shell bed. It is characterized by (S1 S2 G1G3 G4P2P4 and P5) high percentage of whole shell، low abrasion and fragmentation، poorly sorted and diversity of fauna. This concentration was accumulated during periods of high rates of production of biogenic hard parts and low sedimentation rates، in an offshore environment. Shell concentrations at the top of the HST (top lap shell bed) (G2، P3) are multiple-event type. Taphonomic features of this concentration are very similar to the early TST concentrations. One distinct difference in characteristic of these two types is lateral extension، so that TST concentration is commonly seen as a bed، whereas the late HST concentration is a lenticular، patchy or pavement form.