Abstract— <p>For the first time, a Sr-chemostratigraphic study was conducted, and the Pb–Pb age was determined for carbonate rocks of the Burlya Formation of the Selenga Group in Western Transbaikalia. Limestones and dolomites from the Formation were examined in several sections within the Morskoy Ridge: the Cheremshanka River, the Burlya River, the Bilyuta River, and the Kika River. The ⁸⁷Sr/⁸⁶Sr ratios in the least altered limestones and dolomites of the lower part fall within similar ranges: 0.70561–0.70591 (Cheremshanka), 0.70557–0.70584 (Burlya), and for the upper part, 0.70628–0.70642 (Burlya) and 0.70639–0.70664 (Kika). The similarity of the ⁸⁷Sr/⁸⁶Sr ratios in both rock types indicates good preservation of primary isotopic signatures and the cogenetic formation of limestones and dolomites. The obtained ⁸⁷Sr/⁸⁶Sr values suggest that the carbonate sediments of the Burlya Formation were deposited during the Late Riphean, roughly 830–790 Ma. The Pb–Pb age of the carbonate rocks, 830 ± 60 Ma (MSWD = 7), aligns with the Sr-isotope data and the Late Riphean age of the tholeiitic metabasalts of the Meteshikha Complex, which intrudes the Selenga Group 50 km west of the studied sections. The Burlya Formation of the Selenga zone and the Nyandoni Formation of the Katera zone are the earliest carbonate formations in Western Transbaikalia.</p>