Biostratigraphy of the Pabdeh Formation based on planktonic foraminifera in the Tang-e-Chogan section in Interior Fars, Zagros Basin

Abstract The biostratigraphy of the marly-limestone Pabdeh Formation has been studied in a section located in the Interior Fars (folded Zagros). Five assemblage biozones of plankton foraminifera with Early Eocene to Late Eocene age were identified in the studied section. Biozone 1 with a thickness of two meters has been identified at the base of the studied succession and includes Morozovellavela scoensis and Morozovella conicotruncata species defining the Early Eocene age. Biozone 2 with a thickness of 59 meters is the Early Eocene in age and is associated with the first occurrence of Morozovella aragonensis at the base and the last occurrence of Chiloguembelina wilcoxensis at the top. Biozone 3 with a thickness of 66 meters is defined by the first occurrence of Globigerinatheka kugleri at the base and the occurrence of Guembelitrioides nuttalli at the top and represents the beginning of the part of the Middle Eocene. The presence of Orbulinoides beckmanni indicates the Middle Eocene for biozone 4 with a thickness 88 meters. Biozone 5 with a thickness of 163 meters indicates the Middle–Late Eocene for the top of studied succession according to the first and last occurrence of Hantkenina alabamensis. Keywords: Pabdeh Formation, Planktonic foraminifera, Early Eocene–Late Eocene, Biostratigraphy     Introduction The Laramide orogeny event is one of the tectonic events affecting the geology of Iran, which played a significant role in the formation of independent Cenozoic sedimentary basins of Iran (Aghanabati 2004). The subduction of Neotethys under the Iranian Plate began in the Upper Jurassic–Lower Cretaceous and was accompanied by the evolution of the present-day tectonic structure of Zagros in the Upper Paleocene–Lower Eocene (Heidari 2008). The Zagros sedimentary basin has three sedimentary units: Khuzestan Plain, folded Zagros or external Zagros, high Zagros or internal Zagros (Darvishzadeh 1991). After the Late Cretaceous movements, in the Early Paleogene, the whole Zagros was covered by an progressive sea. In the coastal areas of this sea the Sachun Formation, in shallow areas the Jahrom Formation and in deep areas the Pabdeh Formation  have been deposited (Aghanabati 2004). The Pabdeh Formation is limited to the shales layers of the Gurpi Formation with the Maastrichtian age (in Fars and Khuzestan) to the Paleocene age (in Lorestan) at the lower boundary, and is limited to the limestones layers of the Asmari Formation with Oligocene–Miocene age at the upper boundary. The studied section is located in the Tang-e-Chogan area 35 km of northeast of Kazeron in the Interior Fars Basin in the folded Zagros. This section is on the axial line of the Dashtak anticline in the geographical position 51° 36′ 48/22′′ to 51° 36′ 58/09′′ East and 29° 47′ 28/52′′ to 29° 47′ 69/59′′ North. The lithology of the Pabdeh Formation in the Tang-e-Chogan section with a thickness of 378 meters is composed of marls, marly limestones, and medium to thick bedded limestones. In its lower boundary, there are gray shales of the Gurpi Formation (Upper Cretaceous) with disconformities (Moghaddasi et al. 2020) and the upper boundary is overlain by the limestones of the Asmari Formation (Oligocene–Miocene). Considering the special position of the Pabdeh Formation in the petroleum system of the Zagros Basin, it is important to study this formation. For this reason, in this research, the stratigraphy and biostratigraphy of this formation based on plankton foraminifera have been investigated.   Discussion of Results & Conclusions The biostratigraphy of the Pabdeh Formation in the studied section is according to the biozonation of planktonic foraminifera by Toumarkine and Luterbacher (1985), Bolli et al. (1985), Berggren and Pearson (2005), Wade et al. (2011), and Bown et al. (2020). Five assemblage biozones of plankton foraminifera with the Early Eocene to Late Eocene age were identified in the studied section. Biozone 1 with a thickness of two meters has been identified at the base of the studied succession and includes Morozovella velascoensis and Morozovella conicotruncata species defining the Early Eocene age. Biozone 2 with a thickness of 59 meters is Early Eocene in age and is associated with the first occurrence of Morozovella aragonensis at the base and the last occurrence of Chiloguembelina wilcoxensis at the top and is consistent to the base of the biozone E5 of Berggren and Pearson (2005), Wade et al. (2011). Assemblage biozone 2 includes Morozovellaaequa, Morozovella formosa, Morozovella gracilis, Chiloguembelina wilcoxensis, Morozovella aragonensis, Morozovella caucasica, Morozovella subbotinae, Alicantinasp., Subbotinasp., Subbotinacf. eocanea, Pseudohastigerinamicra, Planorotalite pseudoscitula, Clavigerinella akersi, Parasubbotina pseudowilsoni, Hantkenina sp. Biozone 3 with a thickness 66 meters includes assemblage of Guembelitrioides nuttalli, Globigerina sp., Hantkenina sp., Uvigerina havanensis, Hantkenina mexicana, Globigerinatheka kugleri, Acarinina bullbrooki, Morozovella cf. aragonensis, Planorotalites pseudoscitula, Globorotalia renzi, Globorotalia sp. ,Hantkenina longispina, Morozovelloides cf. crassatus, Hantkenina alabamensis, Pseudohastigerina cf. micra, Turborotalia cerroazulensis, Globigerina velascoensis, Subbotina eocaena. It is defined by the first occurrence of Globigerinatheka kugleri at the base and the occurrence of Guembelitrioides nuttalli at the top and represents the beginning of the part of the Middle Eocene and is in accordance of the base of the biozone E8 of Wade et al. (2011). Biozone 4 with a thickness 88 meters includes Globigerina sp., Hantkenina sp., Hantkenina mexicana, Acarinina bullbrooki, Hantkenina longispina, Hantkenina alabamensis, Turborotalia cerroazulensis, Subbotina eocaena, Orbulinoides beckmanni, Pseudohastegina cf. micra, Globigerina pseudoeocaena, Pseudohastegrina micra, Hantkenina aff. Dumblei. According to biozone P13 of Berggren and Van Couvering (1974) and Berggren and Pearson (2005) and biozone E12 of Wade et al. (2011) the presence of Orbulinoides beckmanni indicates the Middle Eocene for biozone 4. Biozone 5 indicates the Middle–Late Eocene for the top of the studied section according to the first and last occurrence of Hantkenina alabamensis. Biozone 5 with a thickness of 163 meters Globigerina sp., Hantkenina sp., Hantkenina mexicana, Acarinina bullbrooki, Hantkenina longispina, Hantkenina alabamensis, Turborotalia cerroazulensis, Globigerina pseudoeocaena, Globigerina sp., Planorotalites pseudoscitula The biostratigraphy of the Pabdeh Formation in the Tang-e-Chogan section indicates five assemblage biozones of plankton foraminifera with the Early Eocene to Late Eocene age. These biozones include 21 genera and 24 species of the following planktonic and benthic foraminifera: Acarinina bullbrooki, Alicantina sp., Chiloguembelina wilcoxensis, Clavigerinella akersi, Globorotalia renzi, Globigerina pseudoeocaena, Globigerina sp., Globigerinatheka kugleri, Guembelitrioides nuttalli, Hantkenina alabamensis, Hantkenina longispina, Hantkenina mexicana, Hantkenina sp., Kathina sp., Lenticulina sp., Morozovella aequa, Morozovella aragonensis, Morozovella caucasica, Morozovella conicotruncata, Morozovella formosa, Morozovella gracilis, Morozovella subbotinae, Morozovellavela scoensis, Morozovelloides cf. crassatus, Orbulinoides beckmanni, Parasubbotina pseudowilsoni, Planorotalites pseudoscitula, Pseudohastegrina micra, Streptochilus sp., Subbotina eocaena, Subbotina sp., Turborotalia cerroazulensis, Uvigerina havanensis