Biostratigraphy of the Baghamshah Formation based on calcareous nannofossils in the Deheshk section (North of Lut Block)

The studied section is located in the eastern part of the Lut block (Iran). Lut Block represents an important structural unit of the Central and Eastern Iran. The block extends for about 900 Km in the north-south direction. It is separated from the Flysch Zone of eastern Iran by the Nehbandan fault in the east, whereas the western boundary with Central Iran is the Nayband fault and Shotori Mountains. Stocklin in 1968 believes that the Lut block could be divided into two parts (eastern and western). Camel mountain range (Shotori Mountain range) is regarded as the boundary between two parts. Based on petrological properties, it is shown that the mentioned parts are completely different from each other. In addition, various seismotectonic movements were seen in two parts that make the difference between the two parts. Dasht-e-Lut includes volcanic rocks of the Tertiary system with approximately 2000 m thickness. The volcanic rocks cover more than half of the Lut block.

Seventy-two samples for the study of the calcareous nannofossils were collected from the argillaceous limestone, pale- green marl at several different stratigraphic positions. Calcareous nannofossils were prepared in 72 smear slides using the standard techniques described by Bown and Young (1998), and studied under an Olympus Bx50 light microscope in parallel light and crossed Nicols with a magnification of 100x. Digital images of nannofossil specimens were taken with a Sony digital camera. All calcareous nannofossil specimens were identified by the taxonomic schemes of Perch-Nielsen, (1985) and Bown & Young (1998). The biostratigraphic data were interpreted by the nannofossil zonation of Sissing (1977), commonly used for the lower and upper Cretaceous in the Tethyan area.

In the studied samples, 55 species were identified. In most slides, the most common components of the nannofossil assemblages are the Watznaueria, Nannoconus, Conusphaera and Cyclagelosphaera genera. In the studied interval, the CC1 – CC5 calcareous nannofossil zones of Sissingh (1977) were recognized in the Deheshk section in eastern Iran. These biozones cover the Early Berriasian to Early Barremian in this section. The appearance of species like, N. colomii, N. dolomiticus, N. globulus, N. steinmannii minor and N. steinmannii steinmannii with Lithraphidites carniolensis, Polycostella senaria and Polycostella beckmannii at the base of this section was remarked and corresponded to the CC1 Calcareous Nannofossil Zone (Sissingh, 1977). The proposed biozones are arranged from the base to the top and they are Nannoconus steinmannii Zone (CC1), Stradneria crenulata Zone (CC2), Calcicalathina oblongata Zone (CC3), Cretarhabdus loriei Zone (CC4) and Lithraphidites bollii zone (CC5). Biostratigraphically, nannofossil assemblages suggest that the studied succession belongs to the CC1 Zone to CC5 Zone of Sissingh (1977) biozonation from the Tethyan province. According to this study and on the bases of calcareous nannofossils, the age of Baghmeshah Formation in the Deheshk section is Early Berriasian to Early Barremian.

In this study, 55 species belong to 26 genera of calcareous nannofossils were identified. Based on the present species and their comparison to the Tethyan calcareous nannofossils, the age of the Early Berriasian to Early Barremian is suggested for the studied succession that equivalent to CC1 - CC5 Zones of Sissingh (1977) biozonation.