The sequence of sediment accumulation of the first and second terraces of the Tarbagataika River and changes in the stages of incision and filling, as well as sedimentation and soil formation over the last 14 kyr in the river valley were revealed. The dynamics of the sedimentation stages and of the phases of their stabilization and pedogenesis was determined by landscape-climatic changes in the study area. The period of geomorphologic formation of above-floodplain terraces was determined. The floodplain sedimentation regime was changed by the subaerial regime on the second terrace of the Tarbagataika River approximately 8 kyr BP. The accumulation of cover deposits on the first terrace began approximately 3.4 kyr BP. Deposits of the second terrace of the Tarbagataika River were formed during the Late Glacial and the Holocene during the last 14 kyr. They are composed of alluvial sediments that accumulated 14.0–8.0 kyr BP and of cover deposits, which accumulated 8.0–0 kyr BP. The first terrace of the Tarbagataika River of the Holocene age is composed of alluvial deposits, which were accumulated 7.0–3.4 kyr BP, and of deposits of the covering complex, which were formed over the last 3.4 kyr. The phases of intensification of erosion-accumulative processes related to increased water discharge were identified. The main stages of soil formation in the study area were determined. The data we obtained are in good agreement with the previously identified stages of pedogenesis in the Selenga middle mountains and adjacent regions of South-Eastern Siberia and Mongolia.

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.