Micropaleontological data (dinocysts, nannoplankton, spores and pollen) and biostratigraphy of the alkun formation, North Ossetia

摘要

The initially obtained data characterize organic-walled phytoplankton (dinocysts), nannoplankton, spores, and pollen from the Upper Argun, Alkun, and Lower Assa deposits of the Fiagdon and Mairamadag sections situated on the southeast of North Ossetia (North Caucasus) in the Alkun Formation stratotype area. The lithology of the formation is described, and its position in the Oligocene-Miocene succession is considered. According to the analyzed data on organic-walled phytoplankton from the Fiagdon and Mairamadag sections, the transitional late Oligocene-early Miocene dinoflora from the top of the Argun Formation and the lower part of the Alkun Formation is in general of the Miocene aspect. Based on the dinocysts, deposits from the upper part of the Alkun Formation coupled with the Assa Formation are attributed to the lower Miocene. The entire interval of the studied deposits corresponds to the acme subzone Def of the Ebu Zone in the Mediterranean dinocyst zonation by D. Zevenboom, where it spans the top of the Chattian and lower part of the Aquitanian Stage. Nannoplankton assemblages from the Alkun deposits are tentatively correlated with the nannoplankton assemblage of Zone NN1 in the standard zonation by E. Martini. It is impossible to date precisely the Alkun Formation based on nannoplankton, as the respective deposits are lacking index species and characteristic taxa of the standard zonation. The dominance of the cosmopolitan species Cyclicargolithus floridanus is a peculiar feature of nannoplankton assemblages from the Alkun Formation. Palynological data are used to consider the dynamics of the evolution of the vegetation cover and climate. Four episodes of relative cooling are distinguished against a background of warm-temperate climatic conditions in general. The most significant episodes occurred in the mid-Alkun time and during the early phase of the Assa time.