Systematic determination and diagnostic characters of Ranikothalia nuttalli in the late Paleocene–early Eocene deposits in Padagi stratigraphic section, Sefid-Abeh Basin, Sistan Suture Zone

Abstract

Of the most important characteristics of the Ranikothalia genus, as large benthic foraminifera, are the existence of ornamentation in the external part of the shell especially around the central umbilicus knob, marginal mesh channels (MMC) in the marginal chord, trabeculate system with channels, V-shape plate in the floor of chambers and accumulation of septum that differentiated this genus from Nummulites genus and placing it in the Palaeonummulitinae sub-family. The Ilerdian transgression near the P5 Zone (Pelagic P5) is a stratal key surface for the determination of the Paleocene-Eocene boundary in the neritic deposits. Ranikothalia nuttalli as an index species is used for recognition of the Paleocene–Eocene boundary in the Neo-Tethys shallow water successions. This species, in the Padagi section, resembling the Pakistan Indus Basin, accompanied with Miscellanea miscella, Discocyclina sp., Discocyclina sp., Assilina sp. that indicated SBZ5/6 and suggested lower Ilerdian (earliest Eocene) age. Keywords: Ranikothalia , Ilerdian, systematic, Sistan Suture Zone Introduction Large benthic foraminifera, such as Fusulinidae, Oritoididae, Alveolinidae, Rotaliidae and Nummulitidae, are important fossil and extant forms that with respect to systematic and biostratigraphic widespread, are studied and some biozonation schemes based on them are suggested. Nummulitidae (such as Ranikothalia) are a group of large benthic foraminiferas developed in the Paleocene and Eocene of the Tethys basin. Larger benthic foraminiferas biozonation is represented by Serra-Kiel et al. (1998) which based on these group of foraminifera Paleocene and Eocene series are divided into 20 shallow benthic zones (SBZ1–SBZ20). The Paleocene–Eocene boundary is considered between SBZ4–SBZ5 (Scheibner & Speijer 2009). Ranikothalia nuttalli as an index species is used for recognition of the Paleocene–Eocene boundary in the Neo-Tethys shallow water successions. This species, in the Padagi section, resembles the Pakistan Indus Basin, accompanied with Miscellanea miscella, Discocyclina sp., Discocyclina sp., Assilina sp. that indicated SBZ5/6 and suggested the lower Ilerdian (earliest Eocene) age. The purpose of this study is to identify and introduce the Ranikothalia nuttali species and investigate its expansion in the adjacent sedimentary basin in the Neo-Tethys Ocean. Material & Method To do research and biostratigraphic investigations of the Paleocene–Eocene deposits in the Sefid-Abeh basin, the Padagi section, ten rock samples are collected that 60 thin sections prepared from them. The systematic determination of Ranikothalia, in the level of genus and species, is done by using binocular microscope, and then based on identified Ranikothalia nutalli, the age of the Padagi section is suggested. Discussion of Results & Conclusions Studies of different genera of Nummulitidae in the thin sections and isolated forms led to the identification of different species of Ranikothalia especially Ranikothalia nutalli accompanied by other large benthic foraminiferas suggested the late Paleocene–early Eocene age for the Padagi section in the Sefid-Abeh basin. Ranikothalia nutalli as a specific species for identification of the Paleocene–Eocene boundary in the Neo-Tethys shallow water succession is suggested. This specific species, in the Padagi section, is seen with Miscellanea miscella and the different species of Assilina and Discocyclina, such as the Indus basin, that show SBZ 5/6 and the earliest Eocene (lower Ilerdian) age. The most important characteristics of Ranikothalia genus, as large benthic foraminifera, are the existence of ornamentation in the external part of the shell especially around the central umbilicus knob, marginal mesh channels (MMC) in the marginal chord, trabeculate system with channels, V-shape plate in the floor of chambers and accumulation of septum that differentiated this genus from Nummulites genus and placing it in the Palaeonummulitinae sub-family. The Ilerdian transgression in near the P5 Zone (Pelagic P5) is a stratal key surface for the determination of the Paleocene–Eocene boundary in the neritic deposits. Ranikothalia nuttalli as an index species is used for recognition of the Paleocene–Eocene boundary in the Neo-Tethys shallow water successions. With regards to affinity and similarity of larger benthic foraminifera assemblages (widespread of Ranikothalia and Miscellanea) in the Sefid-Abeh basin (Padagi section) with Indus basin (in Pakistan and India) we can concluded that palaeobiogeographically, the mentioned basin is related to the eastern Neotethys.

Keywords

• ranikothalia
• ilerdian
• systematic
• sistan suture zone