THE OLIGOCENE EARLY MIOCENE FLORAL ASSEMBLAGES FROM WESTERN SIBERIA

The results of research show that the main geologic events Western Sib eria at the end of the Eocene occurred in a succession of negative and positive tectonic movements that brought about regression of the basi n. The climate cooling caused the extinction of subtropical flora and the replacement of the evergreen forests by the coniferous. In the Oli gocene, the climate was the principal factor in biota evolution, and t he vegetation developed in two stages. During the first stage, i.e., i n the early-middle Oligocene (the Atlym-Novomikhailovskoe time), the c oniferous dominated in the mixed coniferous-broad-leaved forests. In t he course of the second stage, i.e., in the late Oligocene-early Mioce ne (the Turtass and Abrosimovka time), the Turtass flora included subt ropical plants. The continental sedimentation was typical of both stag es. According to paleobotanical data, there were no significant reorga nizations in the vegetal biota at the late Oligocene-early Miocene int erface. Consequently, the palynological data are insufficient to draw a boundary between the Turtass and Abrosimovka formations.