Bioevents of calcareous nannofossils from Triquetrorhabdulus carinatus Zone to Helicosphaera ampliaperta Zone in Oligo-Miocene succession of the Navab anticline southwest flank

One of the most extensive Tertiary succession in Central Iran is the marine strata of the Qom Formation, which was studied based on paleontology studies. The type section of the Qom Formation has been studied in the areas around Qom city (Bozorgnia, 1965). One of the most important achievements obtained from the formation is evaluating stage boundary (Oligocene-Miocene interval) and biostratigraphy of it. In this section the Qom Formation consists of 346 m thick which mainly consists of marl, marly limestone, and limestones.

In this study, 78 samples from the Gurpi Formation interval have been studied. The samples were prepared following the standard smear slide method (Bown and Young 1998). All slides were examined under the polarized light microscope at ×1000 magnification. The nomenclature of calcareous nannofossil follows the taxonomic schemes of Perch-Nielsen (1985), Martini (1971), and Okada & Bukry (1980).

Calcareous nannofossils are among the most important fossil indicators for determining relative age, biostratigraphy studies, paleoecology studies, and correlation of sedimentary strata of various geological periods in different parts of the world (Hewaidy et al. 2014; Faris et al. 2016). A succession of the Qom Formation in southwestern flank Navab anticline was selected to study biostratigraphy based on calcareous nannofossils. In this section, the Qom Formation with 346 m thickness mainly consists of marl and marly limestones. For introducing index species, calcareous nannofossil assemblages, and biozones, slides have been studied at the Qom Formation, which led to the recognition of 29 species of calcareous nannofossils. In this study, four biozones were detected in the formation. Bio zones including Triquetrorhabdulus carinatus Zone (CN1a-b), Discoaster druggii Zone (CN1c), Sphenolithus belemnos Zone (CN2), and Helicosphaera ampliaperta Zone (CN3) with late Oligocene (late Chattian) to early/middle Miocene (Burdigalian-Langian) in age. According to the first and last occurrence of index species, the following biozones are identified:1. Triquetrorhabdulus carinatus Zone (CN1a-b)This biozone is recorded from the LO Helicosphaera recta or Sphenolithus ciperoensis to the FO of Discoaster druggii. The age of this zone is late Oligocene (Chattian) to early Miocene. The thickness of this zone has been measured at approximately 14 m. In this zone, some associated fossil assemblages are Coccolithus miopelagicus, Discoaster deflandrei, Helicosphaera obliqua, Helicosphaera euphratis, Helicosphaera intermedia, Pontosphaera multipora, Pyrocyclus orangensis, Reticulofenestra bisecta, Reticulofenestra locker, Sphenolithus capricornutus, Sphenolithus calyculus, Sphenolithus delphix, Sphenolithus conicus, Sphenolithus compactus.2. Discoaster druggii Zone (CN1c)The second zone is recorded from the FO of Discoaster druggii to the LO of Triquetrorhabdulus carinatus. The age of this zone is the early Miocene. The thickness of this zone is 187.68 m. Some fossil assemblages in this zone include Braarudosphaera bigelowii, Cyclicargolithus floridanus, Coccolithus pelagicus, Coccolithus miopelagicus, Discoaster druggii, Discoaster deflandrei, Helicosphaera ampliaperta, Helicosphaera granulate, Helicosphaera obliqua, Helicosphaera euphratis, Helicosphaera intermedia, Helicosphaera scissura, Pontosphaera multipora, Pyrocyclus orangensis, Sphenolithus conicus, Sphenolithus compactus, Sphenolithus dissimilis, Sphenolithus moriformis, Triquetrorhabdulus carinatus.3. Sphenolithus belemnos Zone (CN2)This zone spans the interval from the LO of Triquetrorhabdulus carinatus to the last occurrence of Sphenolithus belemnos. The thickness of this zone is 81/32 m. The age of this zone is the early Miocene. Some fossil assemblages in this zone include Braarudosphaera bigelowii, Cyclicargolithus floridanus, Coccolithus pelagicus, Coccolithus miopelagicus, Discoaster druggii, Discoaster deflandrei, Helicosphaera ampliaperta, Helicosphaera granulata, Helicosphaera obliqua, Helicosphaera euphratis, Helicosphaera intermedia, Helicosphaera scissura, Pontosphaera multipora, Sphenolithus belemnos, Sphenolithus conicus, Sphenolithus compactus, Sphenolithus moriformis.4. Helicosphaera ampliaperta Zone (CN3)The last nannofossil unit recorded in this study is the CN3. This zone spans the interval from the LO of Sphenolithus belemnos to the LO of Helicosphaera ampliaperta. The thickness of this zone is 63 m. The age of this zone is early/middle Miocene (Burdigalian-Langian). Some fossil assemblages in this zone include: Braarudosphaera bigelowii, Cyclicargolithus floridanus, Coccolithus pelagicus, Discoaster deflandrei, Discoaster druggii, Helicosphaera ampliaperta, Helicosphaera obliqua, Helicosphaera euphratis, Helicosphaera intermedia, Helicosphaera scissura, Pontosphaera multipora, Sphenolithus compactus, Sphenolithus moriformis.Consequently, according to the identified biozones, the age of the section is late Oligocene (Late Chattian) to early/middle Miocene (Burdigalian-Langian).