Integrated biostratigraphy and benthic foraminifera of the middle-upper Eocene deposits of Urtsadzor section (Southern Armenia)


Abstract: The Urtsadzor section, typical for the Paleocene-Eocene of Southern Armenia, is composed of hemipelagic to shelf succession. Planktonic foraminifera (PF), larger benthic foraminifera (LBF), and nannofossils were studied from Priabonian sediments of Urtsadzor section that enabled direct correlation of the biohorizons of these three groups. We present here the results of the microfossil study from the Bartonian and Priabonian parts of the section, with a special focus on larger and smaller benthic foraminifera (SBF). The 350 m thick studied interval corresponds to the planktonic foraminiferal zones P12/E10-11 to P15/E15, the nannofossil zones NP16 to NP19, the LBF zones SBZ17 to SBZ19, the SBF zones Heterolepa eocaena - Cibicidoides landjaricum, Cibicidoides truncanus, Planulina costata. The SBZ17 zone is characterized by the lowest and highest occurrences (LO and HO) of Nummulites ptukhiani and corresponds to P12 and NP16-17/CNE14-15 zones. The upper boundary of SBZ17 and that of the SBZ18A/B subzones are missing. The SBZ18C subzone, identified by the LO of Heterostegina reticulata multifida accompanied by the LOs of Pellatispira,Biplanispira, and Nummulites hormoensis, lies within P15/E14 and NP18-19 zones. The LBF is represented by 40 species of nummulitids, orthophragminids, pellatispirids, as well as Silvestriella, Fabiania, Chapmanina, and Sphaerogypsina. The SBF assemblages contain more than 50 species. The transition from SBZ18 to SBZ19 in the studied section includes the following bioevents: 1. The highest common occurrence of N. maximus, HO of D. discus, and LOs of D. pratti minor, A. alpina, and Silvestriella; 2. The lowermost rare occurrence of Spiroclypeus, LO of H. reticulata mossanensis; 3. The lowermost common occurrence of N. fabianii and Spiroclypeus. The Bartonian-Priabonian boundary in this section, defined by the LO of Chiasmolithus oamaruensis, falls to the lower part of P14 and E14 zones, Cibicidoides truncanus zone of SBF, and between SBZ17 and 18C zones of LBF.

Keywords: Benthic foraminifera, nannofossils, planktonic foraminifera, zonal biostratigraphy, middle-late Eocene, Armenia

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