Biostratigraphy of the Chamanbid Formation in Ghoroneh section at the Binalud Mountains based on Calcareous Nannofossils

In this research, Calcareous Nannofossils of the Chamanbid Formation in Ghoroneh section have been studied. The Ghoroneh stratigraphic section with 750 meters thickness is located at the Binalud Mountains, about 43 kilometers far in northwestward direction from Neyshabour city. In this study, 64 samples from Chamanbid Formation have been studied. Samples were prepared following a standard smear slide method (Bown, 1999). Calcareous nannofossils nomenclature follows the taxonomic schemes of Perch-Nielson (1985). In general, 27 genera and 49 species of the calcareous nannofossils have been recognized in this section. Calcareous nannofossils recorded in the Mesozoic strata are believed to be an appropriate means for biostratigraphic studies. Abbreviations used in this study are the FO (first occurrence) and the LO (last occurrences). Based on the zone maker of index taxa and their accociated assemblages, biozones NJ9-NJ18 from biozonation scheme of Bown et al. (1988) with the age of Early Bajocian to Jurassic-Cretaceous boundary and CC1 zone from Sissingh (1977) belongs to Early Berriasian, have been suggested for the studied sequence and the biozones are as follows:
Watznaueria britannica Zone: The first nannofossil unit recorded in this study is the NJ9 zone.This bio zone is recorded from the FO Watznaueria britannica to the FO of Stephanolithion speciosum. The age of this zone is Lower Bajocian.

Stephanolithion speciosum Zone: The second nannofossil unit recorded in this study is the NJ10 zone. This biozone is recorded from the FO Stephanolithion speciosum to the FO of Pseudoconus enigma. The age of this zone is Lower Bajocian to Upper Bajocian.

Pseudoconus enigma Zone (NJ11(: This zone spans the interval from the FO of Pseudoconus enigma to the FO of Ansulasphaera helvetica. The age of this zone is Upper Bajocian to Upper Bathonian

Ansulasphaera helvetica Zone )NJ12(: This zone spans the interval from the FO of Ansulasphaera helvetica to the FO of Stephanolithion bigotii bigotii. The age of this zone is Upper Bathonian to Callovian.

Stephanolithion bigotii bigotii Zone )NJ13(: This zone spans the interval from the FO of Stephanolithion bigotii bigotii to the FO of Stephanolithion bigotii maximum. The age of this zone is Callovian.

Stephanolithion bigotii maximum Zone: The NJ14 zone spans the interval from the FO of Stephanolithion bigotii maximum to the LO of Stephanolithion bigotii maximum. The age of this zone is Callovian to Lower Oxfordian.

Cyclagelosphaera margerelii Zone )NJ15(: This zone spans the interval from the LO of Stephanolithion bigotii maximum to the FO of Stephanolithion brevispinus. The age of this zone is Lower Oxfordian  to Upper Kimmeridgian.

Stephanolithion brevispinus Zone (Nj16): This zone spans the interval from the FO of Stephanolithion brevispinus to the FO of Stephanolithion atmetos. The age of this zone is Upper Kimmeridgian to Lower Volgian.

Stephanolithion atmetos Zone (NJ17): This zone spans the interval from the FO of Stephanolithion atmetos to the LO of Stephanolithion atmetos. The age of this zone is Lower Volgian to Middle Volgian.

Watznaueria fossacincta Zone: The NJ18 zone spans the interval from the LO of Stephanolithion atmetos  to ?. The age of this zone is Middle Volgian to Jurassic/Cretaceous boundary. The detail study of Chamanbid Formation at the Binalud Mountains, based on calcareous nannofossils, enables the subdivision of the studied deposits into biozones NJ9-NJ18 from biozonation scheme of Bown et al. (1988) with the age of Early Bajocian to Jurassic-Cretaceous boundary and CC1 zone from Sissingh (1977) belongs to early Berriasian, have been suggested for the studied sequence.