The Vendian age of the Oselok Group (foothills of the Sayan Mountains, Uda and Biryusa river basins) is substantiated based on tillites discovered at the group base, on trace fossils and impressions of multicellular soft-bodied organisms found in the overlying Ozerki and Bol'shaya Aisa members, and on secular delta(13)C variations. In the Uda River basin, the lower part of the Oselok Group, which corresponds to the Marnya Formation and the Uda Formation lower part, is divided into 13 members of four sequences. Glacial deposits at the base of the Ulyakha Sequence (lower one) overlie different subdivisions of the Karagassy Group with stratigraphic discordance, filling in the deep erosion incisions. Tillites are of two types, corresponding to diamictites with subordinate breccias of supra- and subglacial channels, fractures and pavements (Ulyakha type), or to faceted-boulder breccias associated with glaciofluvial and eolian sandstones (Plity type). Rock fragments (tillstones) in diamictites and breccias bear marks of glacial abrasion (faceted forms, striation and polish), while the glacier bed show signs of glacial erosion and dislocations. In addition to dominant tillstones derived from the Karagassy Group and Nersa basic complex that intruded the group, erratic material is represented by granite gneisses and crystalline schists of the basement. Tillites of the Ulyakha Member are attributed to the supraglacial lodgment till, whereas tillites of the Plity Member consist of drift deposited by glacial meltwaters and of partially redeposited rudaceous material, which was buried under sand dunes. Four phases of glacier advance and subsequent deposition of cap dolomite (Ozerki Member) are distinguished. Glacial deposits of the Marnya Formation are correlated with lithologically similar or associative deposits characteristic of several regional subdivisions. These are an upper part of the Aleshinskii Formation (Taseeva Group) and basal strata of the Pod"em and Stolbovaya formations (Chapa Group) in the Yenisei Ridge, the Goloustnaya Formation (Baikal Group) in the Lake Baikal region, and the Vanavara and Nepa formations in inner areas of the Siberian platform. Directions of erosion forms in the bedrocks and orientation of clasts and glacial striation suggest the glacier advance from the south northward. Structural elements of meltwater deposits and glaciofluvial stream beds are oriented west- and southwestward, and these trends presumably point to the peripheral and intracratonic glaciation centers. According to data of delta(13)C chemostratigraphy, the discovered tillites originated during the Early Vendian glaciation correlative with the Early Varangerian (Laplandian) glaciations in the East European and other continental blocks of the Precambrian. Trace fossils abundant in the Ozerki Members of postglacial coastal-marine sandstones and impressions of soft-bodied Metazoa found in tempestites of the Bol'shaya Aisa Member represent the Early and Late Vendian Ediacarian biota.
