THE KUYALNYKIAN REGIONAL STAGE OF THE EUXINIAN AREA OF THE EASTERN PARATETHYS AND THE POSSIBILITY OF DETERMINING THE NEOGENE-QUATERNARY BOUNDARY (PIACENZIAN-GELASIAN) IN THE SOUTHERN UKRAINIAN DEPOSITS
Authors: Vernyhorova Juliia 
Published: July 2024

The Kuyalnykian regional stage is a stage of evolution of the Euxine Basin of the Eastern Paratethys with its own conditions for the existence of biota. During that time, the estuarine-marine conditions in southern Ukraine prevailed, leading to the development of various geological formations. However, to date, the Kuyalnykian does not have the official status of a regional stratigraphic unit, either in the Neogene stratigraphic scheme of the Eastern Paratethys or in the Neogene stratigraphic scheme of southern Ukraine, also there is no generalised (formal) description of it as a separate regional stratigraphic unit. The lowering of the Neogene–Quaternary boundary from 1.8 to 2.595 Ma led to a discussion about tracing it in the middle of the Kuyalnykian deposits. This study provides a comprehensive analysis of lithofacies, biostratigraphy (molluscs, ostracods, nannofossils, small mammals), palaeomagnetic and paleogeographic data to obtain a holistic view of the stratigraphy of the Kuyalnykian deposits in southern Ukraine. As a result, we acquired a generalized description and regional stratigraphic scheme of the Kuyalnykian, introduced new local stratigraphic units, obtained criteria for tracing the Neogene-Quaternary boundary, and clarified which part of the Kuyalnykian deposits are of Pliocene age and remain on the pre-Quaternary geological maps, and which of them are already of Pleistocene age and should be transferred to the Quaternary geological maps. The Euxine Basin of the Eastern
Paratethys during Kuyalnykian was characterised by euryhaline brackish water, freshwater mollusc assemblages, brackish water and freshwater ostracods assemblages that existed in estuarine-marine environments. The Kuyalnykian deposits are divided into two regional substages and be determinated in the Northern Peri-Black Sea Region, Northern Azov Region, the northern part of the Crimean Peninsula and the Kerch Peninsula. We established the following lithostratigraphic units of the Kuyalnykian for southern Ukraine: the Zamorske Formation in the central and eastern part of the Kerch Peninsula; The Mysove Formation in the northwestern part of the Kerch Peninsula and the eastern part of the Crimean Peninsula is partly Kuyalnykian; the upper part of the Bekhtery Formation on the Left Bank of the Lower Dnieper in the Skadovsk and Holoprystan districts belongs to the lower regional substage of the Kuyalnykian; the Kuyalnyk Formation in the Northern Peri-Black Sea region and Northern Azov region, in the north of the Crimean Peninsula belongs to the upper regional substage of the Kuyalnykian. The age of the Kuyalnykian deposits has been determined based on palaeomagnetic and faunal data. The lower boundary of the Kuyalnykian regional stage is 3.6 Ma which coincides with the boundary of the Hilbert-Gauss palaeomagnetic epochs. The boundary between the Lower and Upper substages is 2.59 Ma (the Gauss-Matuyama boundary). The upper boundary of the Kuyalnykian is set at different stratigraphic levels; it dates 1.780 Ma (the upper boundary of the Olduvai Subchron) in the Indol structural facies zone and also ~1.500 Ma (perhaps above the Gils Subchron, based on small mammal data) or ~0.990 Ma (above the Jaramillo Subchron, based on palaeomagnetic data) in the Western and Eastern structural facies zones of the Northern Black Sea Region, the Northern Azov Region, and the Sivash structural facies zone. This diachrony of the upper Kuyalnykian boundary is explained by environmental difference: while in more marine and “deep water” conditions (Indol structural facies zone) the Kuyalnykian molluscs have already changed to “Gurian (sensu lato)”, in relatively remote estuaries (the Northern Black Sea Region, Northern Azov Region) the Kuyalnykian fauna still “survived”. In addition, it should be noted that the palaeomagnetic dates obtained from studied sections of the Northern Black Sea region are not “absolute” and therefore may be reinterpreted in the future.