STRATIGRAPHY OF THE UPPER CENOZOIC VOLCANOSEDIMENTARY SEQUENCE IN THE EAST SAYAN MOUNTAINS ADJACENT TO THE BAIKAL REGION

Abstract

The large-scale geological mapping in the southeastern area of the East Sayan Mountains permitted the subdivision of the Upper Cenozoic volcanosedimentary sequence into three units: the Sorok (lower-middle Miocene, 24-12 Ma), Khirpisa (upper Miocene, 11-6 Ma), and Khirbes (Pliocene, 5-1.6 Ma) formations. The K-Ar dates for basaltic lavas are in agreement with the age determinations by palynological assemblages recovered from the sedimentary lenses intercalated with lavas. The factorial analysis of spore and pollen spectra revealed two stages in the evolution of vegetal assemblages: first, the stage of gradual degradation of the Turgai-type flora during the warm humid climate of the Miocene and, second, the stage of the boreal flora evolution that succeeded the former after a sudden cooling and aridization of climate. This crisis in flora development occurred in the late Miocene 10 to 5 Ma ago.