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).