Vol 24, No 6 (2016)

The structure of Riphean deposits developed on the western slope of the Anabar Massif is described with analysis of their depositional environments, distribution of stromatolite assemblages and organic-walled and silicified microfossils through sections, and evolution of views on stratigraphic significance of some of these assemblages. The investigation included complex mineralogical, geochemical, structural, and isotopic‒geochronological study of globular phyllosilicates (GPS) of the glauconite‒illite series from paleontologically well substantiated Riphean sequences (Ust’-Il’ya and Yusmastakh formations of the Billyakh Group) of the Anabar Massif in the Kotuikan River basin. Isotopic dating of monomineral size and density fractions of GPS from the Billyakh Group was performed in combination with simulation of the distribution of octahedral cations and comparison of the results obtained with Mössbauer spectrometry data. The applied approach is based on an assumption that the formation and transformation of Rb‒Sr and K‒Ar systems in GPS are synchronous with stages in their structural evolution, which are determined by the geological and geochemical processes during depositional history. Such an approach combined with the mineralogical and structural analysis contributes to correct interpretation of stratigraphic significance of isotopic data. The results obtained provide grounds for the conclusion that isotopic dates of GPS from the Ust’-Il’ya (Rb‒Sr, 1485 ± 13 Ma; K‒Ar, 1459 ± 20 Ma) and Yusmastakh (Rb‒Sr, 1401 ± 10 Ma; K‒Ar, 1417 ± 44 Ma) formations mark the stage of early diagenesis of sediments and are suitable for estimating the age of formations in question.

The section on the outskirts of the town of Kantemirovka is proposed to serve as a reference one for Paleogene deposits of the Voronezh region. Observing the requirements of the current Stratigraphic Code of Russia, it is suggested to introduce the Middle Eocene Sergeevka and Tishki formations, which were defined previously by V.P. Semenov as mappable lithostratigraphic units instead of the Kiev Formation (regional stage) in the Paleogene stratigraphic scale of Ukraine, into the regional Paleogene stratigraphic scale of the Voronezh anteclise. The structure of the Sergeevka Formation and hypostratotype of the Tishki Formation defined in this section are considered with the analysis of the distribution of nannofossils, foraminifers, and diatoms in corresponding deposits. Benthic and planktonic foraminifers are used for defining regional biostratigraphic units, which are correlated between each other, with zones of the standard stratigraphic nannofossil scale, and units of the Middle Eocene Kiev regional stage of northern Ukraine. The boundary between the formations under consideration is universally marked by the replacement of carbonate sediments by terrigenous‒siliceous facies, disappearance of calcareous fossils, and development of a new biota. These features allow the boundary between these formations to be conditionally correlated with that between the Keresta and Kuma regional stages of the North Caucasus region.

The succession of short-term natural events at the thermal humid maximum in the Middle Ages (10th–12th centuries) within the forest zones of European Russia was established. The archaeological excavation in the most ancient part of the town of Yaroslavl opened a thick cultural horizon, where fragments of lacustrine deposits dating back to the 11th–12th centuries were preserved. The most ancient fragments of the construction, found in lacustrine deposits, date back to the first third of the 13th century. An analysis of the structural and textural peculiarities of these deposits made it possible to reconstruct types of hydrogeological regime in the Timerevo paleolake and a paleostrait between this paleolake and Lake Nero. Thus, the waterway between the central part of the Principality of Rostov and early Yaroslavl (the first Russian settlement at the Great Volga Waterway) was revealed. The probable reasons for a rapid warming event followed by a cooling one in the Middle Ages are discussed.