Biostratigraphy of the Oligocene deposits of Pabdeh Formation based on planktonic foraminifera in Bandar Abbas area

Abstract

Abstract In this research, the biostratigraphy of Oligocene deposits of the Pabdeh Formation in Bandar Abbas region is considered. For this purpose, the planktonic foraminifera was studied and identified. Based on the index forms of these microfossils, biozones were recognized and compared to global biozonation. Investigation of the stratigraphic distribution of these microfossils in the four studied surface and subsurface sections (Champeh, Safid, Band-e Lengeh, and Taftan) led us to the identification of the Eocene/Oligocene boundary and four biozones of planktonic foraminifera (Zone E16, Hantkenina alabamensis Highest-occurrence Zone, Zone O1, Pseudohastigerina naguewichiensis Highest-occurrence Zone, Zone O2, Turborotalia ampliapertura Highest-occurrence Zone, Zone O3, Dentoglobigerina sellii Partial-range Zone) in this region. Based on the foraminiferal assemblages and their biozonation, the age of Oligocene deposits of the Pabdeh Formation is Rupelian. Also, the lateral extension of Oligocene deposits of the Pabdeh Formation in the studied area has been considered. Keywords: Pabdeh Formation, Biozonation, Oligocene, Planktonic foraminifera, Bandar Abbas Hinterland. Introduction Folded Zagros with about 2,000 km long is divided into Lurestan, Dezful Embayment, Izeh Zone, Abadan Plain, Fars, and Bandar Abbas hinterland. ThePabdeh Formation mainly is composed of shales and marls with a marine origin, comprises alternation of dark to light gray shale beds and thin to thick limestones with good preservation of planktonic foraminifera, developed in southwest Lorestan, Khuzestan and Fars Provinces (Aghanabati 2011). James and Wynd (1965) introduced a type section of this formation in Tang-e Pabdeh and Wynd (1965) presented biofacies and biostratigraphic zonations of this formation for the first time. In this research, the Oligocene deposit of Pabdeh Formation has been studied in terms of biostratigraphy and standard biozonation. Material & Methods In order to study of planktonic foraminifera in Bandar Abbas hinterland, four stratigraphic sections including Champeh outcrop section and Safid, Taftan and Band-e Lengeh wells were investigated. In total, 938 samples were studied for foraminiferal biostratigraphy. Planktonic foraminiferal taxa were identified from the washed residues of 53 marl samples in the Champeh section. Other samples were studied in thin sections. Identification of planktonic foraminifera and standard biozonation carried out based on Berggren et al. (1995), Berggren and Pearson (2005), Wade et al. (2011), and Berggren et al. (2018). Discussion of Results & Conclusions In studied sections, the stratigraphic distribution of planktonic foraminifera permitted us to recognize the Eocene/Oligocene boundary and four planktonic foraminiferal biozones (E16 to O3). We recognized the Eocene/Oligocene boundary and Hantkenina alabamensis Highest-occurrence Zone (E16) in studied sections according to the extinction of hantkeninids. Pseudohastigerina naguewichiensis Highest-occurrence Zone (O1): The lower boundary of this Zone is the latest occurrence of Hantkenina alabamensis and the upper boundary is based on the last occurrence of Pseudohastigerina naguewichiensis. Turborotalia ampliapertura Highest-occurrence Zone (O2): The lower boundary of this zone is the last occurrence of Pseudohastigerina naguewichiensis and the upper boundary is the highest occurrence of Turborotalia ampliapertura. Globigerina sellii Partial-range Zone (O3): The lower boundary of this biozone is the highest occurrence of Turborotalia ampliapertura but the upper boundary is not observed in the studied sections. In Taftan well, the Eocene/Oligocene boundary was defined based on the extinction of Turborotalia cerroazulensis because of the absence of hantkeninids. Therefore, P17 is introduced for upper Eocene instead of E16. Also, the key species Pseudohastigerina naguewichiensis was not observed in Band-e Lengeh and Taftan wells. This species shows the upper boundary of O1 and the lower boundary of O2 zones. Thus, no distinction can be made between the O1 and O2. These two biozones are comparable with Small globigerinids-Haplophragmium slingeri-Zeauvigerina assemblage zone of Wynd (1965). The study area is located in Lengeh trough. In shallow parts, the Asmari Formation was deposited on the Pabdeh Formation (Champeh section and Safid well), while at the same time, sedimentation of Pabdeh Formation continued in the deeper parts of the basin (Band-e Lengeh and Taftan wells). In Bandar Abbas hinterland, the thickness of Oligocene deposits of the Pabdeh Formation has increased relatively from northwest to southeast, so there is the lowest thickness in the Safid well and the highest thickness in the Band-e Lengeh well. Generally, the Oligocene deposits of Pabdeh Formation are thicker and belong to Rupelian in Bandar Abbas hinterland. Towards the Fars area, these deposits are the same in age but their thicknesses are reduced. Towards the Izeh area, the deposits are younger and continued until Chattian, but the thickness is less than the study area.

Keywords

• pabdeh formation
• biozonation
• oligocene
• planktonic foraminifera
• bandar abbas hinterland