Investigation of distribution pattern and biostratigraphy of calcareous nannofossils in the upper part of Qom Formation in Siah-Kuh section

The Qom Formation is widely distributed in the Qom back-arc, arc, and fore-arc basins (Reuter et al., 2007). The Siah-Kuh section has the best outcrop of the Qom Formation in back-arc basin and is located northeast of type section. Despite of several studies that having been carried out on the biostratigraphy of the Qom Formation, no comprehensive agreement is still present for its dating, especially upper part of the formation. Therefore, the aim of the present work is to document, through a high-resolution study, the stratigraphic occurrence of calcareous nannofossils into “e” and “f” members of the Qom Formation at the Siah-Kuh section in the north side of the Qom sedimentary basin (south of Garmsar city).

In the present study, the upper part of the Qom Formation (“e” to “f” members) with a thickness of 351 m consists of green to gray marlstones, green calcareous marlstones and argillaceous limestone that overlies the thick-bedded gypsum of the “d” member. A total of 121 samples obtained from the top of “d” member to marlstones and marly limestones succession of “e” and “f” members. The collected samples prepared using the simple smear slide and Gravity techniques that described by Bown & Young (1998). Slides were studied using an Olympus BX53 light microscope at 1250X magnification inside of the PPL, XPL, XPL+GP, XPL+QP areas and species images were taken using an Olympus DP73 camera. In the present study, the Martini (1971; NN zones) zonation pattern is used as the standard zonation scheme. However, the zonal marker of Okada & Bukry (1980; CN zones) and Backman et al. (2012; CNM zones) used for high-resolution biostratigraphic study.As well as, the semi-quantitative analysis was utilized to reconstructing distribution pattern of calcareous nannofossil taxa. The preservation, species abundance and slide abundance of species was determined by counting the number of specimens on the 46 smear slide following Lupi & Wise (2006), and Self-Trail (2011).

The investigation of calcareous nannofossil assemblages led to the identification of 38 species belonging to 15 genera. Based on the index taxa, the Discoaster druggii Zone (NN2) to Helicosphaera ampliaperta Zone (NN4) of Martini (1971) are distinguished from the studied interval of the Qom Formation. The established biozones can be correlated with CN1c-CN2-CN3 zones of Okada & Bukry (1980) and CNM4-CNM5-CNM6 zones of Backman et al., (2012), that is confirmed the Burdigalian-early Langhian age for the studied interval from the “e” and “f” members of the Qom Formation in Siah-Kuh section.The semi-quantitative analysis shows that the preservation of nannofossil specimens is poor to good and richness of nannofossil assemblages (Slide abundance) is frequent (F) to Abundant (A). The significant decreases in abundance of some species such as Helicosphaera ampliaperta, Helicosphaera euphratis, and Cyclicargolithus floridanus etc. has been observed towards the Burdigalian-Langhian boundary. Although, the calcareous nannofossil species have a good to moderate abundance from the base of the "e" member to the below of the boundary.

The studied interval of “e” and “f” members, spanning from NN2 to NN4 zones of Martini (1971) and CNM4 to CNM6 zones of Backman et al., (2012). The recognition of these biozones confirms the Burdigalian-early Langhian age of sediments in the Siah-Kuh section.The Burdigalian-Langhian boundary at the studied interval is marked by an important decreases in the abundance of Helicosphaera ampliaperta, Helicosphaera euphratis and Cyclicargolithus floridanus which is followed by continuously recording of Sphenolithus heteromorphus. Above the boundary, Helicosphaera carteri species have been observed dominantly.Acknowledgment The authors thanks to Professor Marie Pierre Aubry (University of Rutgers, USA) and Professor Jeremy Young (University College of London, UK) for their advices and who checked determinations of calcareous nannofossils. We would like to acknowledge the Exploration Directorate of NIOC (National Iranian Oil Company) for laboratorial facilities provided. This paper is extracted from the research project No. 3/39428 of Ferdowsi University of Mashhad that is necessary to the gratitude.