Permian biostratigraphy of the northern Okhotsk region (Northeast Asia)

Fossil biota and detailed biostratigraphy of the Permian northeastern Okhotsk and Ayan-Yuryakh sedimentary basins located in the northern Okhotsk region are characterized. The biota core is represented by bivalves associated with gastropods, brachiopods, and other faunal groups. It is shown that a new regional stratigraphic scale of northeastern Russia based on brachiopods and bivalves may be used for subdivision of Permian sections in these basins. Fossil assemblages have many features in common with their counterparts from the Verkhoyansk region; at the same time, there are endemic species and the big group of species characteristic of the Omolon basin. The upper part of the Khivach regional horizon is subdivided into subzones based on the evolutionary lineages of the Maitaia and Intomodesma genera. Changes in the taxonomic composition of coeval communities in different basins are shown to be largely controlled by the basin depths and facies environments. Impoverished fauna of the Ayan-Yuryakh basin was less diverse in taxonomic aspect, represented almost entirely by bivalves (mostly Inoceramus-like) accompanied by rare gastropods. The discovered taxa are figured in majority, and table of their stratigraphic distribution is presented.