Middle Eocene to earliest Oligocene dinoflagellate cysts from southern Armenia: biostratigraphy and palaeoecology

Abstract

The results of a palynological study of the middle Eocene-lowermost Oligocene strata of the Shagap and Landzhar outcrop sections (southern Armenia) are presented. A series of stratigraphically important dinoflagellate cyst events were recognized in the uppermost Lutetian-lowermost Rupelian parts of the sections, including the lowermost occurrences of the key-species Rhombodinium draco, Schematophora speciosa, Hemiplacophora semilunifera, Hapsocysta kysingensis, Thalassiphora? cf. rota, Rhombodinium porosum, Glaphyrocysta semitecta, Reticulatosphaera actinocoronata, Stoveracysta ornata-group and Wetzeliella gochtii. Based on the first-order calibrations with calcareous nannoplankton and foraminifers, R. draco first occurs at the Lutetian-Bartonian transition, while the lowermost occurrence of R. actinocoronata is very close to the Bartonian-Priabonian boundary. The analysis of the ratios of palynogroups and dinoflagellate eco-groups permits to interpret the environmental conditions during the accumulation of regional formations: (1) a major marine transgression started at the beginning of the Lutetian and the gradual deepening of the marine basin occurred during the middle-late Lutetian (Arpa Formation); (2) the marine transgression, accompanied by deepening, continued during the early Bartonian, but towards the end of the Bartonian slightly shallower depositional conditions were established (Azatek Formation); (3) the sea continued to gradually become shallower during the Priabonian (Urtsadzor Formation) and, finally, the maximum of regression was observed at the Eocene-Oligocene transition, ending with the establishment of brackish environments at the beginning of Rupelian (Shagap Formation).

Keywords:

• Middle Eocene-earliest Oligocene
• dinoflagellate cysts
• palynomorphs
• biostratigraphy
• palaeosettings
• Armenia

Acknowledgments

We are grateful to our Armenian colleagues F.A. Hayrapetyan, L.G. Sahakyan, T.E. Grigoryan and K.B. Meliksetyan from the Institute of Geological Sciences of the Armenian National Academy of Sciences for the organization of the fieldwork in 2013–2019. G.N. Aleksandrova (GIN RAS) is thanked for the chemical preparation of palynological samples. Thanks to Henk Brinkhuis (Utrecht University) for fruitful discussions on the Eocene dinoflagellate cysts. Claus Heilmann-Clausen, Peter Bijl and an anonymous reviewer are thanked for their careful and very constructive reviews, which improved the paper. We also thank James Riding for his very helpful editorial suggestions. The fieldwork in southern Armenia was funded by the Russian Foundation for Basic Research (RFFI project No. 15-55-05102 and 18-55-05017) and the State Committee for Science of the Republic of Armenia (project SCS 18RF-090). The research of A.I. Iakovleva and E. A. Shcherbinina was supported by the Russian State program of the Geological Institute of Russian Academy of Sciences. The research of E.Y. Zakrevskaya was carried out within the framework of the state assignment to SGM RAS 1021061009468-8-1.5.1 (Vernadsky State Geological Museum).

Disclosure statement

No potential conflict of interest was reported by the authors.