The main goal of this study was to elucidate the chronostratigraphic position of the sediments of lacustrine-bog stratum from the Ch-14 section located in the sub-latitudinal sector of the Chernaya River in the north of the Bolshezemelskaya tundra. This study presents data on their numerical age and time of their formation, obtained by the parallel ²³⁰Th/U and ¹⁴С dating of organic-rich layers. According to the results of spore-pollen analysis obtained earlier, the lacustrine-bog sediments were attributed to the Byzovaya (Middle Valdai) interstadial, together with the lake and alluvial deposits from the Ch-3 and Ch-4 sections in the same sector of the Chernaya River. The ²³⁰Th/U and ¹⁴C age data are in good agreement with each other, especially for the lower boundary of the lacustrine-bog sediments of the Ch-14 section. The time range of the lacustrine-bog stratum accumulation is estimated to be approximately 13.7-13.3 to ~12.0 thousand years ago. Thus, its formation (10.5-6.5 m layer) did not occur in the Middle Valdai, but in the Late Valdai time from the beginning of Allerød to the beginning of the Younger Dryas cooling. It has been suggested that a certain delay of late glacial climatic events took place in the Bolshezemelskaya tundra compared with their age-related estimation by the Greenland isotope-oxygen stages. For the first time, the isochronous approximation approach was applied to obtain the ²³⁰Th/U age of such young terrestrial organic-rich deposits. Earlier this approach was usually used in dating similar sediments not younger than MIS-3. The geochronological results presented in this paper allow us to recommend the ²³⁰Th/U method for determining the age of late glacial organic-rich deposits and climate events correlated with them.