The former stratigraphic chart of the Cambrian in the Turukhansk–Irkutsk–Olekma region of the Siberian Platform was compiled in 1986 and approved in 1988. Large amounts of data were obtained from new wells drilled in the western and northern parts of the region during the preceding 20 years. Modifications necessitated by new data entail an indefinitely continuous series of revised facies zonation, recognition of new facies areas, regions and zone, as well as definition of a regional early Mayan stratigraphic hiatus. A new local stratigraphic division is the Olenchima Member (base of the Evenk Formation) underlain by a hiatus. The term Kochumdek Formation is no longer used in the Bakhta region and its lithologically distinct subformations (Yasenga, Moktakon, Mara, and Abakun) were raised to formation rank. The Olenchima Formation is no longer used in the Baikit zone; but instead, the Litvintsevo Formation was established here by correlation.

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