Cenozoic sediments from the Selenga-Vitim Downwarp and Baikal Rift Zone spatially combined in the Uro section

Abstract

Research subject. Cenozoic stratified sedimentary formations exposed by well 517 in the Uro area.Aim. To study a section of spatially combined deposits from the Selenga-Vitim Downwarp and Baikal Rift Zone.Methods. Lithological and stratigraphic analysis, lithogeochemical, paleontological studies.Results. Oligocene deposits of the Kularikta Formation from the Selenga-Vitim Downwarp found for the first time, unconformably overlain by deposits of the Baikal-type basins of the Baikal Rift Zone (Miocene-Lower Pliocene Tankhoi Formation, Upper Pliocene-Eopleistocene Anosov Formation, and Quaternary polyfacies stratum). Four palynocomplexes are characteristic of the section: PC I of the Early Oligocene shows mixed forests of spruce, pine, hemlock with an admixture of alder, birch, and broad-leaved deciduous trees with an insignificant participation of evergreens that existed in a warm temperate climate; PC II of the Late Oligocene reflects expansion of areas of coniferous vegetation in the transition to a moderate climate; PC III of the Early Miocene demonstrates mesophilic coniferous forests and a significant role of broad-leaved tree species in warmer and more humid climatic conditions; PC IV of the Late Miocene–Early Pliocene displays vegetation close to taiga with rare relics of coniferous and deciduous plants under conditions of increasing cooling with a decrease in total moisture supply. A lack of pollen grains in the transitional deposits from the Kularikta to the Tankhoi Formation of the late Oligocene–early Miocene corresponds to conditions of a paleoclimatic pessimum. A member of lacustrine sediments with diatoms was found in the Tankhoi Formation, 50 species of diatoms are identified, among which the planktonic species Alveolophora antiqua is absolutely dominant in a paleowater body of significant depths and a large volume of water mass.Conclusion. The Uro section combines the Oligocene deposits of the Selenga-Vitim Downwarp and Miocene-Quaternary deposits of the Baikal Rift Zone.