Late Pleistocene deposit including a bone-bearing horizon of mammoth fauna has been described in the series of sections along the lower reaches of the Semiyulyakh River (in the Tirekhtyakh River basin, the right tributary of the Indigirka River). The combination of the unit position between the lower and upper ice complex, radiocarbon date, and fossil complexes testify to the age corresponding to the beginning of MIS 3 (Late Neopleistocene). The lower and upper layers were formed in different environments. The lower layer was formed in the gradually degraded channels of a small river. The upper layer accumulated on a floodplain during meandering of a rather large river. Lenses with autochthonous detritus in the top of the lower layer contain the remains of mammoth fauna, including those with preserved soft tissues. The studied buried deposits are considered as natural resource for which paleoecological reconstructions are highly significant. The richness of the bone-bearing horizon could be explained by the spatial overlap of the in situ and the allochthonous deposits formed as a result of a number of crisis events and regular mortality under standard conditions. The necessary factors for the formation of both types of deposits were intensive meandering and, as a result, active thermal erosion of the sides and bottoms of the valley. The supposed source of the allochthonous deposit was the lower ice complex (MIS 4), mechanical barriers were formed under conditions of slowing down the flow velocity at the bends of overgrown secondary channels. At the same time, the in-situ deposit of mammoth fauna was formed as a result of the formation of marshy traps in under the permafrost melting conditions.