Investigation of calcareous nannofossils of the Jahati anticline in Coastal Makran of Iran

The Makran area is one of the largest accretionary wedges on Earth, located in SE Iran and South Pakistan. It extends about 1000 km between two, nearly N–S-running, transform fault systems (Dolati and Burg, 2013). Less biostratigraphic studies carried out in this area, therefore this research aims to investigate the Neogene strata of Coastal Makran Zone on the bases of calcareous nannofossil in the Jahati anticline. The calcareous nannofossils are among the best planktonic microfossils that used to determine relative ages of the Mesozoic and especially Cenozoic strata (Bown & Young, 1998). In this research, 81 samples were collected from intervals of mudstone, marl and marly limestone in the Jahati anticline (Gati-Jahati series) for calcareous nannofossil studies. To avoid small sized coccoliths, the samples were processed by simple smear slide and gravity methods as described by Bown and Young (1998). The slides were examined using aNikonE200 light microscope with cross-polarized light at a magnification of 1000X. For the systematic studies of calcareous nannofossil, Perch-Nielsen (1985) was used. The standard calcareous nannofossil zonations of Martini (1971), Okada & Bukry (1980) and Backman et al. (2012) were followed for the Neogene strata. In this research, many index nannofossils were identified, however, standard nannofossil biozonation was not performed, because the studied stratigraphic sections were thick and sometimes distance between coolected samples were wide. Therefore, the age determination was only based on index species, calcareous nannofossil assemblages and index bio-events.
The calcareous nannofossil study on the basal part of this section indicated co-occurrence of Sphenolithus heteromorphus and Helicosphaera ampliaperta species. According to the studies so far conducted (e.g. Perch-Nielsen, 1985; Fornaciari & Rio, 1996, Bown, 1998), presence of S. heteromorphus index species indicates the NN4-5 biozones and the LO of H. ampliaperta species was also considered as upper boundary of the NN4 zone (Perch-Nielsen, 1985; Bown, 1998). Then, at the middle part of the section, the nannofossil studies led to recognition of species Discoaster braarudii, Discoaster kugleri and Discoaster quinqueramus appearance, the FO of the rare Ceratolithus acutus, first appearance of Coccolithus pelagicus forms with bar, the FO of Discoaster asymmetricus, the FO of Pseudoemiliania lacunosa, and then the appearance of Gephyrocapsa spp. small from bottom to top of this part of the section, respectively.
At the upper part of the studied section, the frequency of Discoaster surculus, Discoaster pentaradiatus as well as the LO of Discoaster brouweri are low or sometimes rare, however some species of Discoaster genus was noted. Some evidences such as disappearance of various species of Discoaster genus, appearance of various and large species with high frequency of Gephyrocapsa genus such as Gephyrocapsa caribbeanica, Gephyrocapsa oceanica and Gephyrocapsa spp. (>4μm) are indicative of Pliocene to Pleistocene transition event at the upper part of the section. Finally, the FO of Emiliania huxleyi species at the uppermost parts of the section was recorded and continued to the top of the section. The present research is the first published study on the marine-clastic complete sequence of the Neogene of the Coastal Makran Zone of Iran. The calcareous nannofossil studies led to identification of 39 fair to well preserved species belonging to 11 genera. Based on index taxa, the basal part of the studied section was assigned to Helicosphaera ampliaperta zone (NN4) of Martini (1971) and the uppermost part to the Emiliania huxleyi zone (NN21), ranging in age from the late Early Miocene (late Burdigalin- early Langhian) to the Late Pleistocene. Also, the recognized biozones in this study are correlated to standard zonations of Backman et al. (2012) and Okada & Bukry (1980).