Structure, Age, and Formation Stages of the First River Terraces in the Selenga Drainage Basin

New and generalized published data on the structure, age, and formation stages of deposits of the mantle and alluvial genetic complexes of the first terrace of rivers in the Selenga drainage basin are presented. The sections of the terrace of the Bryanka River in the Bryansk Depression, the Arshan River in the Khilok–Chikoi Depression, and the Shivert-Gol River in the Boro-Gol River basin in Mongolia are described. Information on the terrace structure and on the composition and absolute (radiocarbon) age of deposits is obtained. The first terrace 4–9 (15) m high above low water level is distinguished in the Selenga drainage basin. Large differences in the structure and composition of the terrace deposits depend on the morphology of the river valleys, water discharges, and the structural and tectonic conditions of the river basins. It has been established that the accumulation of sediments of the first river terrace in the Selenga basin began at the end of the Late Pleistocene. The channel, floodplain, and oxbow alluvium facies are distinguished. Channel alluvium was accumulated 30–15 cal kyr BP, and the accumulation of floodplain alluvium occurred during the Late Glacial and Early Holocene (15–8 cal kyr BP). The alluvium is overlain by Holocene sediments of the mantle genetic complex of different composition, genesis, and age reaching 3.5 m in thickness. It has been revealed that the first terrace of rivers in the Selenga drainage basin was formed in the Early Holocene (11.3–8 cal kyr BP) as a result of incising of the rivers during high floods. The chronological stages of sedimentation and soil formation have been identified. There was a long stage of soil formation 7–2 (0) cal kyr BP after the alluvium accumulation stop.