Nannostratigraphy of Afzalabad flyschoids (North Birjand)

The studied section is located in east Iran, Lut block (a part of flysch basin east of Iran) and west of Afzalabad (north Birjand city). The mentioned flyschoids are more than 4200 m thick. The studied section consists of two units including 200 m shale and sandstone of Paleocene age (abbreviated as Pef in the geological map) and 300 m of shale and sandstone, marl, sandstone and limestone of Eocene (as Ef in the geological map).

290 samples were taken from this sequence. The upper and lower parts of the mentioned sequence were sampled over a smaller distance. In order to avoid weathering, samples were often taken from a depth of 50cm. Most of the samples were from shale and marl. Most samples were prepared using smear slide method and others by gravity method. Nannofossils were studied using Polarizing Light Microscope Olympus (BH2 model) in PPL and XPL lights, and most of them were photographed.

In order to determination calcareous nannofossils of this sequence, (Romein, 1979; Perch-Nielsen, 1985; Bown & Young, 1985) reports were used. According to these studies, 21 genera and 56 species of calcareous nannofossils have been identified in the mentioned sequence. Martini, 1971 biozonation is amongst common biozonations for Paleogene and Neogene. Recently other complementary studies were conducted by Agnini et al., 2014. They have adapted their biozonation to that made by Martini, 1971 using some minor changes. Biozonations introduced in this sequence include:Discoaster binodosus Zone (NP11)
This biozone has been defined based on Martini, 1971 biozonation from the last appearance of Tribrachiatus contortus to the first appearance of Discoaster lodoensis. The age of this biozone is early Eocene (Ypresian). It is worth mentioning that Agnini et al., 2014 have defined their CNE3 biozone, which is started a bit before NP11, at lower edge by the first appearance of Tribrachiatus orthostylus and its upper edge exactly matches that of NP11. In our studied sequence the first appearance of Tribrachiatus orthostylus occurs at the base of the sequence and the first appearance of Discoaster lodoensis was seen 230 m from the sequence base which is the upper boundary of biozone NP11 of Martini, 1971. The age of this biozone is early Eocene (Ypresian).
Tribrachiathus Orthostylus ZONE (NP12)
According to Martini, 1971 biozonation, this biozone continues from the first appearance of Discoaster lodoensis to the last appearance of Tribrachiatus orthostylus. The age of this biozone is early Eocene (Periabonian). In the studied sequence the first appearance of Discoaster lodoensis was seen as the base of NP12 biozone and the upper boundary of this biozone was determined based on the last appearance of Tribrachiatus orthostylus.
Isthmolithus recurvus ZONE (NP19)
Based on NP19 biozone (Martinin, 1971), the lower boundary of this biozone in this sequence was also determined by the first appearance of Istmolithus recurvus and the upper boundary of this biozone was determined by the first appearance of Sphnolithus pseudoradians. The age of this biozone is Late Eocene (Priabonian). The thickness of this biozone is 100 m in Afzalabad sequence.

In Afzalabad flyschoid (north Birjand), 21 genera and 56 species of calcareous nannofossils were identified. Based on the identified species, NP11 and NP12 biozons (Martini, 1971) were identified for lower part of this sequence which nearly matches CNE3 and CNE4 biozons from Agnini et al., 2014 biozonation. In the upper part of the studied sequence NP19 biozone (Martini, 1971) was determined. Agnini et al., 2014 biozonation measures were not seen in this section. According to the identified biozons, the age of studied sequence, Ypresian (early Eocene) to Periabonian (Late Eocene) is suggested for the studied thickness. Considering the presence of NP11 biozone in the upper part of Pef and the lower part of Ef, the pass of mentioned units is continuous and gradual.
Keywords: Nannostratigraphy; Flyschoid; Afzalabad; Birjand.
References
Agnini, C., Fornaciari, E., Raffi, I., Catanzariti, R., Palike, H., Backman, J., & Rio, D., 2014. Biozonation and biochronology of Paleogene calcareous nannofossils from low and middle latitudes. Newsletters on Stratigraphy, 47 (2): 131-181.
Martini, E., 1971. Standard Tertiary and Quaternary Calcareous Nannoplankton Zonation. In: Farinacci, A., (ed.), Proceeding of 2nd International Conference of Planktonic Microfossils, Roma, 2: 739-785.
Perch-Nielsen, K., 1985. Cenozoic calcareous nannofossils. In: Bolli, H.M., Saunders, J.H., & Perch-Nielsen, K., (eds.), Plankton Stratigraphy, Cambridge University Press, 427-554.
Romein, A.J.T., 1979. Lineagesin early Paleogene Calcareous Nannoplankton Utchret Microplaentol. Utrecht micropaleontological bulletins, 22: 1-230.