The Barguzin Valley belongs to the central part of the Baikal Rift Zone. It is located northwest of the Svyatoy Nos Peninsula and Barguzin Bay; the Barguzin Range detaches it from Lake Baikal. From east and southeast, the slope of the Ikat mountain ridges surrounds the basin (Fig. 1). We first published the data on the fossil diatom flora of the Barguzin Valley in (Hassan et al., 2019), where the results of the investigation of diatoms from the cores 532 and 531 revealed four local diatom zones that reflected the development stages of flora in the southern part of the Barguzin Valley from the end of the Middle Miocene to the beginning of the Pliocene. This work presents the results of the study diatoms from the core 545, which was obtained in 2012 near the Uro settlement (southwesten part of the valley) by the Baikal Branch of Joint Stock Company "Urangeologorazvedka". The study aimed to search for the most ancient diatoms in the Baikal region. Using light and scanning electron microscopy, we investigated species diversity of diatoms and their distribution in the core. In total, we identified 36 species, among which 8 were extinct. Planktonic diatoms dominated along the entire section. Their species composition has changed. Based on the appearance or disappearance of indicator species, we determined two local diatom zones (Fig. 2). Diatom zone 1 was determined from the dominance of the oval Alveolophora sp. 1 valves in the depth interval (170.0-160.0 m). This is a planktonic species with an unidentified taxonomic status, which will be described as novel to science. It was 90-99% of the total number of diatoms. The maximum number of

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From the analysis of spatial-temporal distribution of sediments in the south of East Siberia, we present evidence on two stages in the evolution of neotectonic structure: Cretaceous-Paleogene and Neogene-Quaternary. The former was exhibited by formation of the South Baikal orogen accompanied by flank development of the Predbaikal and Selenga-Vitim foredeeps, the latter by the subsidence of the South Baikal basin, its filling of sediments and water simultaneously with the development of the whole Baikal Rift System.

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