A NEW BIOSTRATIGRAPHIC LEVEL OF PLIOCENE IN WEST SIBERIA AND THE AGE OF THE LOWER-MIDDLE MIOCENE STRATOTYPE OF THE BESHCHEUL HORIZON

Abstract

The thorough study of main Neogene sections in scarps of the Irtysh River valley north of Omsk allows substantial corrections in stratigraphy of the West Siberian Plain. We have established the Isakovka assemblage, marking a new stage in the development of small mammals in southern West Siberia. This assemblage fills the gap in faunal evolution between the Cherlak and Peshnev assemblages. Based on the stratigraphic position of the Isakovka assemblage, the stratotype of the Beshcheul Horizon, previously referred to the Middle Miocene, is placed into the Lower Pliocene. The Isakovka Formation is defined as a new stratigraphic unit in the Pliocene succession. The type flora of the Middle Miocene Isakovka floral assemblage should be referred to the Lower Pliocene.