Palynological and diatom analyses of sediments from the Late Cenozoic Amalat valley (Western Transbaikalia)

Sections of interlayered sediments and basaltic lavas have been studied in drill cores from the erosional valley of the Amalat paleoriver and its trib utaries. Deposition in the valley started in the Middle Miocene about 14 Ma ago and ceased in mid-quaternary time about 0.6 Ma ago. Palynologic and di atom age constraints of the sediments are in line with K-Ar dating of volca nics, The composition of diatoms attests to repeated appearance and drying of deep lakes between 14 and 9 Ma ago. A Factor analysis of spore and pollen spectra in the sediments shows a two- stage evolution of vegetation on the Vitim plateau in the Late Cenozoic: 1) progressive changes of Miocene Turgai-type flora in a warm humid climate b etween 14 and 9 Ma ago and 2) transition to Boreal nora under sharp cooling and aridization since 9-5 Ma ago. The two stages generally correspond to s imilar evolution stages of the flora in southeastern East Sayan. The Miocen e flora in the territory between Northern Baikal and southeastern East Saya n differed from that on the Vitim plateau. These differences gradually disa ppeared since the Late Miocene, whereas the flora of southeastern East Saya n still preserved its specific features.