New types of Pliocene ostracods of South-Eastern Altai

The Chuyskaya and Kurayskaya depressions are the largest in the system of intermountaindepressions of Southeast Altai formed in the Cenozoic in place of the Earlier Paleogene denudation plain. Lacustrine and alluvialdeposits of the Neogene Chuysko-Kuraysky paleobasin of significant power (more than 1000m) include variable fresh-water ostracodfauna. Ostracod complexes are extracted from sediments of the Koshagachskaya (the Late Oligocene - the Lower Miocene),Tuerykskaya (the Middle Miocene - the Middle Pliocene), Kyzylgirskaya (the Lower - the Middle Pliocene) and Bekenskaya (theUpper Pliocene) formations. A review and comparison of the systems was made, the dependence of species composition on physical andgeographical conditions of the paleobasin is traced. The Early Miocene phase is characterized by the Koshagachsky (Aral) ostracodcomplex and restores the conditions of a warm, shallow paleobasin. The Middle-Late Miocene phase is characterized by the Tuerykskycomplex with species typical to a paleobasin of the lake type. The Early and the Middle Pliocene stage of sedimentation is characterizedby the Upper Tueryksky (Pavlodar) complex of ostracods (from lacustrine facies) and the Kyzylgirsky complex (from coastal lacustrinefacies). The Late Pliocene stage is characterized by the Bekensky complex with species typical for this time of sedimentation. Thestratigraphic division of the Pliocene sedimentary formations of this area is complicated by their extreme facial variability. In theostracod fauna the Pliocene species Denticulocythere, Advenocypris appeared. The Kyzylgirsky complex has endemic species ofostracods. Endemic species are allocated for Candona, Advenocypris, Turkmenella, Potamocypris genera. The Kyzylgirskaya formationsediments are known in the north-western part of the Chuyskaya depression, where they can be traced in some outcrops and describedby the fauna of ostracods, and mollusks. In the central part of the Chuyskaya depression the Kyzylgirsky ostracod complex was firstallocated in the sediments penetrated by Well 1. The complex shows the previously known species, as well as new Candona andPotamocypris, which are found in the sediments of the Kyzylgirskaya and Bekenskaya suites of the north-western part of the Chuyskayadepression, thus allowing correlation of these deposits. The biostratigraphic correlation of the sections of the Neogene intermountaindepressions of Altai and paleogeographic reconstruction of the main stages of sedimentation can be used in allocation of stratigraphicunits. The results of the analysis of species composition of ostracod complexes can be used to correlate the coeval deposits of adjacentareas.

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