On the Paleogene-Neogene boundary and age of the Alkun Formation in the North Caucasus and Ciscaucasia

The problems related to age of the lower part of the Caucasian regional stage (Alkun Formation) and Paleogene-Neogene boundary in the North Caucasus and Ciscaucasia are discussed with analysis and generalization of the data on nannoplankton and dinocyst assemblages from Alkun sediments in their different areas. The new data obtained from the section of the Alkun Formation in the stratotypical area of the Caucasian regional stage (valley of the Kuban River middle reaches, Karamurzinskii village), including finds of the index species Triquetrorhabdulus carinatus and typical species of the zonal nannoplankton assemblage, confirm the belonging of Alkun sediments to Zone NN1. The composition and peculiar features of the Karamurzinskii nannoflora and dinocyst assemblages indicate their Neogene age. It is established that the Alkun sediments in the Kuban, North Ossetia, and Belorechensk areas are characterized by similar nannoplankton and dinocyst assemblages, which implies their synchronism in these localities. The dinocyst assemblages from the Alkun Formation exhibit significant similarity with their counterparts from the uppermost part of the lower Krosno Subformation and lower part of the upper Menilite Subformation of the Ukrainian Carpathians and from lower layers of the Chernobaevka Formation of the northern Black Sea region. The analysis of available data reveals that the Paleogene-Neogene boundary in the North Caucasus and Ciscaucasia corresponds to a narrow interval spanning from the upper layers of the Batalpasha Formation and its analogs to the lower layers of the Alkun Formation. Thus, it is shown that the Caucasian regional stage belongs practically in its entire volume, probably except for its basal layers, the age of which needs to be specified, to the lower Miocene. New data on hydrology and bionomy of the basin, vegetation, and climate of the Alkunian time are also considered.