GENUS DIDACNA MOLLUSCS — BIOSTRATIGRAPHICAL BASIS OF CORRELATION OF THE PONTO-CASPIAN NEOPLEISTOCENE EVENTS

Abstract

The mollusks of the genus Didacna Eichwald playing a key role in stratigraphical and paleogeographical reconstructions for the Caspian Sea are used by authors for the same purposes in the Black Sea region and in the Manych depression. The work purpose is the drawing up the regional biostratigraphical schemes of the Caspian, Pont and Manych Neopleistocene on the basis of the analysis of spatial and temporary distribution of species of one genus Didacna and correlation of events of the Ponto-Caspian Seas on this biostratigraphical base. The object of research is the Neopleistocene deposits and the mollusks of the Caspian and Black Sea regions. The subject of research is the spatial-temporal distribution of mollusk shells of the genus Didacna as a basis of correlation of the basins of the Caspian Sea and the Black Sea in the Neopleistocene. The use of species of one genus for the creation of three regional biostratigraphical schemes is a basis for correlation of deposits and events of the Pont and the Caspian Sea. The material is collected by authors for many years of field and laboratory studying of Neopleistocene deposits and malacofauna of the PontoCaspian region. Researches captured all coasts and a number of sea cores of the Caspian Sea, the Manych depression, Taman and Kerch peninsulas, the coast and a shelf of Bulgaria, partially the Caucasian coast of the Black Sea. The main method is malacofaunistic method. It includes the analyses of taxonomical structure, taphonomy, phylogeny, biostratigraphical distribution, historical development, and biogeography of mollusks. The study focuses on the bivalve genus Didacna, an indexefossil genus for the modern Caspian Sea and an endemic fossil for the Quaternary Ponto-Caspian basins. The genus is known for its high evolutionary rates at the species and subspecies levels. The study of peculiarities and patterns in the spatial-temporal distribution of mollusk shells of the genus Didacna in the deposits of the Neoleistocene of the Ponto-Caspian region showed, that the molluscan fauna represent a complex hierarchical system of faunal assemblages with different taxonomic composition and at different taxonomic levels: faunas, complexes, subcomplexes, which become the basis for establishment of stratigraphical subdivision and paleogeographical reconstructions. On the basis of the biostratigraphical position of the Caspian Didacnas in the Manych and Pont Neopleistocene the correlation of the late Bakinian transgressive stage of the Caspian Sea with a late Chaudian transgressive stage of the Pont is established; two stages of early Khazarian transgression of the Caspian Sea are correlated with two Euxinian – Uzunlarian basins of the Pont; the Girkanian transgressive basin of the Caspian Sea is correlated with the second half of the Karangat transgression of the Pont; early Khvalynian transgressive stage of the Caspian Sea is correlated with Pontian New Euxinian transgression.