Biostratigraphy and deposition environments of the Middle-Late Miocene volcanosedimentary section in the Dzhilinda basin, Western Transbaikalia

We report results of an integrated study of volcanosedimentary rocks of the Middle-Late Miocene Dzhilinda Formation consisting of the prevolcanic sedimentary Lower Dzhilinda and volcanosedimentary Upper Dzhilinda subformations. The section was studied in three wells drilled near Lake Mukhal (Khoigot paleovalley, Vitim upland). The Dzhilinda sediments contain ubiquitous Alveolophora jouseana (Moiss.) Moiss. typical of Miocene environments. Deposition occurred in two main stages represented by different water (diatoms) and land (spores and pollen) plant communities. The upsection successive changes in diatom species are attendant with changes in pollen and spore assemblages. The K-Ar ages of lavas indicate that the mostly sedimentary lower section of the Dzhilinda Formation deposited between 12 and 14 Ma and the more volcanic upper section, with a lacustrine lens at base, formed at about 10.8–9.5 Ma. The isotope dating of volcanic rocks agrees with the ages inferred from the diatom and spore-pollen analyses. The prevolcanic Lower Dzhilinda subformation deposited during the Middle Miocene climate optimum. The stratigraphy, lithology and facies of sediments suggest that the Dzhilinda deposition was associated with the development of a deep freshwater lake in the conditions of active tectonism and volcanism.