Abstract: This article presents the results of a comprehensive geochronological study of organic-rich sediments lying at the base of the sand suite in the well-known reference Belyi Yar II section on the left bank of the Irkut river (Tunka depression, Southwestern Cisbaikalia). It was found that in the 1960s, a pack of lake-marsh sediments from the lower part of the section was assigned to the upper Eopleistocene, and the overlying sands to the Samarovo period of the Middle Pleistocene. Later, based on the first 14C datings, the stratigraphy of these sediments was revised. The lower part of the sediments was attributed to the Karginsky time, and the overlying sands to the Sartanian glaciation. However, the questions concerning the genesis, age and conditions of formation of these sediments remain controversial. In particular, the individual dates of 14C differ greatly from one another; therefore, they require further verification. A renewed study of the lower part of the Belyi Yar II section discovered peat and gyttjas deposits of alluvial origin. It was shown that their numerical age is beyond the ¹⁴C dating method limits. Paleobotanical data showed that the gittya was formed in conditions of a relatively warm and humid climate, and the overlying layers accumulated in the cooling and more xerophytic condition. Evaluation of the ²³⁰Th/U isochronous age of these organogenic deposits, combined with palynological data, suggested that the bottom of the sand layer in the Belyi Yar II section was not formed during the Karginsky time (MIS 3), as was thought previously, but during MIS 5. The gittya could have formed during MIS-5c, whereas the overlying peat layers formed, possibly, at the transition to MIS-5b.