Vol 27, No 3 (2019)

The results of mineralogical and geochemical studies of the metaclastic rocks of the Grebnev and Magdagachi sequences of the Chalovskaya Group, one of the key lithostratigraphic units of the Argun continental massif in the eastern part of the Central Asian Fold Belt, as well as the results of the U–Pb dating of detrital zircons from these rocks by the LA-ICP-MS method, are presented. It was found that the age of the youngest zircons is ~478 Ma (Early Ordovician) in the metasandstones of the Grebnev Sequence and ~448 Ma (Late Ordovician), in the metasiltstones of the Magdagachi Sequence. Together with the previously determined age of the metadacites of the Isagachi Sequence (476 ± 8 Ma), this suggests the Ordovician age of the Chalovskaya Group, dated previously as Late Precambrian. The available data indicate that the deposits of the Chalovskaya Group accumulated in a setting of an active continental margin or an island arc against the background of intense magmatic activity. Considering that the Early Paleozoic suprasubduction magmatism is manifested in all continental massifs of the eastern part of the Central Asian Fold Belt (Argun, Mamyn, Bureya, Jiamusi, Khanka), one cannot exclude that these continental massifs have a common geodynamic history.

Some decisions of the Third Interdepartmental Regional Stratigraphic Meeting on Precambrian, Paleozoic, and Mesozoic of Northeastern Russia (St. Petersburg, 2002) are analyzed in light of the latest data obtained. The regional stratigraphic units (“horizons”) of the nonmarine Cretaceous recognized at this meeting are in fact not the basic subdivisions, but rather specialized biostratigraphic units, namely, “beds with flora.” The ages of some of these units are specified; notably, the beds with Arman flora are Turonian–Coniacian, the beds with Amka flora are Coniacian, and the beds with Arkagala flora are Santonian–Campanian. We recommend keeping the previous names for the Penzhina, Barykov, and Koryak phases of floral development and for the corresponding stratigraphic units. An updated version of the nonmarine Cretaceous stratigraphic chart of the Okhotsk–Chukotka structure-facies region is proposed for discussion.

The section of Maikop deposits on the Belaya River upstream of the town of Maikop is characteristic of Western and Central Ciscaucasia; it is well exposed and well-studied and can be considered as the type section of the Maikop Group. The section shows a continuous transition from the underlying white marl of the Belaya Glina Formation to dark clay lying at the base of the Maikop Group. The lower Oligocene part of the section exposes an almost uninterrupted outcrop of the Pshekha Formation and shows unconformities at the base and top of the Polbian Bed. These are overlain by the lower non-carbonate and upper carbonate subformations of the Morozkina Balka Formation and the Batalpashinsk and undivided Upper Oligocene Septarian + Zelenchuk formations. This paper contains a detailed lithological description of the section with a summary of its fossils. The composition of the studied microphytoplankton and animal remains, including nannoplankton, planktonic and benthic foraminifers, mollusks, and ichthyofauna is described and paleomagnetic study results are presented. The study of palynology and dinocysts from the same series of samples was previously completed. The history of the study of the Maikop Group and its subdivision into formations are discussed. This paper discusses the possibility of using the data for correlation and reconstruction of the depositional environment.