Abstract: A mineralogical-geochemical study of globular phyllosilicates (GPS) of the glauconite-illite series of the Dolgokta Formation from the stratigraphic well Chunkinskaya Well 282 was conducted, and their Rb–Sr age was determined. The suitability of the mineral for geochronological studies was estimated based on the Mössbauer spectroscopy data and the model of cation distribution in the octahedral sheet of GPS. The mineralogical and crystallochemical characteristics indicate epigenetic defects in the glauconite crystal structure; therefore, its Rb–Sr isochronous age (1300 ± 7 Ma) is “rejuvenated” and reflects a partial recrystallization event. The ⁸⁷Sr/⁸⁶Sr ratio in dolomites of the Dolgokta and Kuyumba formations varies from 0.70602 to 0.72230 and thereby confirms the epigenetic recrystallization of rocks. The model Rb–Sr age of glauconite, which was estimated taking the enrichment in radiogenic ⁸⁷Sr into account, is within 1340–1400 Ma. The age estimate defines the upper limit of the stratigraphic distribution of the Mesoproterozoic fossil Tappania that was found in the Yurubchen and Dolgokta formations of the Baikit Anteclise.