Biostratigraphy of the Middle and Upper Pleistocene of the Caspian Region

Biostratigraphy of the Caspian Pleistocene is based on changes in evolutionary patterns and ecological assemblage change of the mollusc genus Didacna Eichwald. The study of peculiarities and patterns in the spatial-temporal distribution of Didacna Eichwald in the deposits of the middle and upper Pleistocene of the 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, associations, distinguished following particular criteria. Molluscan fauna, namely distinguished faunal units at different hierarchical levels, were used to build a regional biostratigraphic (ecostratigraphic) scheme of the Middle and Upper Pleistocene (Neopleistocene according to Russian stratigraphic scheme) of the Caspian, supplementing and specifying the existing schemes. Caspian Middle and Upper Pleistocene (Neopleistocene) represent a Didacna biozone - deposits encompassing the entire stratigraphic interval of this taxon distribution. Based on temporal distribution of faunas, the zone is subdivided into subzones, which become the biostratigraphic basis for establishment of horizons. Interval-zones, characterized by faunal complexes, are used for distinguishing subhorizons. Subcomplexes are used to define beds. Mollusc associations show the facies diversity of these subdivisions. All distinguished stratigraphic units are related to paleogeographic events at variable hierarchical levels (transgression, stage, phase) in the development of the basin.