In the 90-m river bank of the middle reaches of the Icha River, thick strata of volcano-sedimentary deposits represented by pyroclastic rocks and siliceous tuffites with layers of diatomites were studied. Representative diatom assemblages assigned to the Middle Miocene Denticulopsis hyalina Zone (14.9–13.1 Ma) indicate relatively deep-water conditions of the marine basin. Comparison with the data on diatom assemblages from reference sections of Western Kamchatka indicates that the deposits of the Tyrkachin section belong to the upper part of the Kakert Formation. The petrographic composition and lithological structure of volcanogenic formations (members 1, 2, and 4) suggest that they are the product of pyroclastic flows associated with volcanic activity in central Kamchatka. It is assumed that Members 1 and 2 were formed in subaerial conditions, as indicated by the complete absence of marine microfossils, signs of primary volcanic stratification, and the absence of layering. The sediments of Member 4 with eroded contacts and pronounced layering apparently accumulated in a subaqueous setting.

The study of several Paleogene sections in the south of the Russian Plate allowed the refinement of the taxonomic list of diatom and silicoflagellate assemblages. Using an integrated biostratigraphic approach, the stratigraphic range of the Bipalla oamaruensis diatom Zone was defined. The high degree of taxonomic diversity of the studied assemblages is associated with the shallow-water environments of the marginal part of the basin inhabited by various ecological groups of diatoms. The zone was established in the Tishki and Kasyanovka formations and, based on direct comparison, corresponds to the Bartonian–Lower Priabonian Cyrtophormis alta–Etmosphaera polysiphonia–Theocyrtis andriashevi radiolarian interval-zones. The Priabonian age of the Kasyanovka Formation is also confirmed by dinocyst data. The Bartonian-Priabonian boundary approximately coincides with the first appearance of the silicoflagellate Corbisema hexacantha.

Diatoms play a major role in carbon and silicon cycles, and thus diatom-bearing sediments represent an archive of past climatic and environmental settings. In shallow-marine and freshwater environments, the accumulation of diatom frustules forms a sedimentary rock called diatomite. While most global-scale studies of diatom-bearing sediments focus on deep-sea sites, shallow-marine and freshwater diatomites are studied mainly at a regional level. To address this problem, we present a global-scale compilation of diatomite occurrences spanning the Palaeogene (∼66 to ∼23 Ma). This period was characterized by initial extreme warmth, followed by prolonged cooling, disrupted by short-term climatic events called hyperthermals and by a number of palaeoceanographic and palaeogeographic changes. The aim of this compilation is to determine the response of diatom production to Palaeogene environmental fluctuations by examining the influence of climate, tectonic activity, and ocean circulation on diatomite deposition. Although climatic factors appear to have had an indirect impact, our study suggests that palaeogeographic and palaeoceanographic changes were key drivers of diatomite deposition during the Palaeogene, particularly from the Early Eocene Climatic Optimum (∼53 to ∼49 Ma) onwards. In fact, our compilation suggests the absence of diatomite deposition in epicontinental seas between ∼46 and ∼44 Ma, whereas diatomites did not begin to accumulate in open-ocean environments until ∼43.5 Ma. Moreover, we observe that regional climate and volcano-tectonic activity have had an impact on the deposition of freshwater diatomites.

Data on diatom algae and silicoflagellates were obtained for the first time from the lower part of the stratotype of the Liman Formation (Liman Mount area), which allowed dating of this part of the Liman Formation as far back as the end of the Middle–beginning of the Late Miocene (Denticulopsis praedimorpha Zone, 12.9–11.5 Ma and Thalassiosira yabei Zone, 11.5–10.0 Ma by diatoms) and revealed the marine genesis of host deposits. The upper part of the stratotype of the Liman Formation (Pyata Cape area) that hosts the continental deposits with an assemblage of freshwater diatoms, with a certain degree of confidence may be referred to the Late Miocene (?)–Eopleistocene (Calabrian Stage).

The diatom assemblage of the Veshenskaya Formation studied in the borehole P-321 of the Cis-Donets monocline (southern part of the Russian Plate) is assigned to the lower Ypresian Moisseevia uralensis Zone. According to the available data, this zone corresponds to the Deflandrea oebisfeldensis and Dracodinium (=Wetzeliella) astra dinocyst zones and, accordingly, to the nanoplankton NP10 Zone. The presence of the zonal silicoflagellate species Naviculopsis foliacea confirms the Early Eocene age of the Veshenskaya Formation. The taxonomic similarity of diatoms and silicoflagellates of the region under consideration with silicofossil assemblages of Northern Europe suggests a stable link between these paleobasins at the beginning of the Early Eocene.

Eocene silicofossils (diatoms and silicoflagellates), dinoflagellate cysts, and continental palynomorphs of two boreholes in the Cis-Donetsk and Pavlovsk–Kantemirovka structural-facies zones of the Donets Syncline were studied. The Veshenskaya Formation, characterized by the dinocyst assemblage of the Stenodinium meckelfeldense Zone, is assigned to the early Ypresian. The Tishki and Kasyanоvka formations, with assemblage of the Bipalla oamaruensis diatom zone and first appearance of silicoflagellate Corbisema hexacantha, as well as a dinocyst assemblage with Enneadocysta pectiniformis, are dated as the late Lutetian–early Priabonian. The microfossil assemblages are typical of shallow-water marginal parts of the epicontinental paleobasin.

— This work presents the results of integrated micropaleontological and lithogeochemical analysis of Upper Oligocene continental deposits of the Turtas Formation from the Zyryanskaya-1 borehole (southwestern part of the Tyumen Region, Western Siberia). The predominance of freshwater planktonic diatoms at the subordinate significance of benthic species, as well as the presence of gemmuloscleres of sponges, indicates the coastal sedimentation conditions in a freshwater basin varying in depth from 3 to 50 m. The studied palynocomplex is similar to the spore-and-pollen spectra of the Turtas Formation of the Lower Ob Region, but differs from palynocomplexes of the same formation spread over the southern regions of Western Siberia in a low amount of Taxodiaceae/Cupressaceae and Betulaceae pollen, as well as the absence of Fagaceae pollen and Pseudokomewuia freshwater dinocysts. The revealed differences can be explained both by facies features and by variations in the volume of the Turtas Formation in the currently known localities. The data on the microelemental composition of samples, as well as geochemical and petrogeochemical modules, indicate that the accumulation of the Turtas Formation occurred in the coastal areas of a low-mineralized basin under relatively warm humid climatic conditions.

The Turtas Formation (the Upper Oligocene of Western Siberia), uncovered by the Zyryanka-1 Borehole, contain diverse assemblage of microfossils. The ecological composition (freshwater diatoms, spicules and gemmuloscleres of freshwater sponges, spores and pollen, as well as zygospores of Zygnematales algae) indicates the nonmarine nature of the Early Turtas Basin. In contrast to the assemblage of the Zhuravka horizon (southern regions of Western Siberia), the absence of Fagaceae and dinocysts of the genus Pseudokomewuia, as well as low numbers of Taxodiaceae–Cupressaceae and Betulaceae, are probably explained by local characteristics.

The stratigraphic subdivisions of the Paleogene included in the updated regional stratigraphic scheme of the Ulyanovsk-Syzran facial district with a wide development of biosiliceous deposits, are described and paleontologically characterized. The biostratigraphic subdivision of the reference sections using diatoms and dinocysts is discussed. The recently recognized stratigraphic units, Smyshlyaevka and Karanino within the Kamyshin (Upper Paleocene) and Proleyka (Lower Eocene) regional stages are described.

An integrated micropaleontological study of deposits enriched in biogenic silica was conducted on material drilled in the Zyryanka and Uspenka fields of the southwestern Tyumen oblast (Western Siberia). The data obtained on palynology and diatoms allowed the assignment of the Zyryanka Field to the Turtas Formation (lower part of the Chattian Stage of the Oligocene) and the deposits of the Uspenka Field to be dated as Pleistocene. The absence in the Zyryanka Field diatom assemblage of in situ marine taxa and the mass occurrence of freshwater species, gemmuloscleres of freshwater sponges, and zygospores of Zygnematales algae suggest that a nonmarine basin existed in the early Turtas time. A considerable proportion of redeposited diatoms and dinocysts in the Quaternary deposits of the Uspenka Field suggest strong erosional processes during the time of deposition.

A compilation of published and new diatom data from 14 stratigraphic sections of Central Russia, Transuralia, and adjacent regions across the Paleocene-Eocene Thermal Maximum (PETM) is presented. It is shown that in response to the rapid warming during the PETM, diatoms show evolutionary patterns displayed also by other microplankton groups, in particular dinocysts and nannofossils. Species and genera evolution becomes intensified, with the appearance of short-lived species of unusual morphologies and invasion of thermophilic migrants from open ocean tropical regions to the intracontinental basins of North Eurasia. Moreover, diatom assemblages are characterized by reappearance of Late Cretaceous and Early Paleocene relics that subsequently went extinct in the Early Eocene.

A representative assemblage of diatoms and silicoflagellates found in a excellent state of preservation is described from terrigenous cherty deposits of the Upper Cretaceous, which are exposed in a quarry near the Kologrivovka village in Saratov Region. The assemblage is correlative with diatoms of the Costopyxis antiqua Zone of the lower Campanian and with silicoflagellates of the Cornua trifurcata Zone, the upper one in the lower Campanian. According to the taxonomic composition, dominated by diatom genera Cortinocornus, Costopyxis, Hemiaulus, Paralia, Pseudopodosira , and Stephanopyxis , the assemblage was typical of a highly productive neritic zone with an active hydrological regime. A perceptible content of silicoflagellates among the diatoms suggests the normal salinity of the sedimentation basin. The comparable sedimentation settings in marginal parts of the Early Campanian epicontinental basin on the south of the East European platform can also be inferred based on the data from the lithological analysis.

Из терригенно-кремнистых отложений верхнего мела, вскрытых карьером в районе с. Кологривовка Саратовской области, выделен уникальный по сохранности и представительности комплекс диатомовых и силикофлагеллат. Он соответствует комплексу зоны Сostopyxis antiqua по диатомеям нижнего кампана и комплексу зоны Cornua trifurcata по силикофлагеллатам (верхняя часть нижнего кампана), установленным ранее для аркто-бореальной области. Структура изученных комплексов с доминированием диатомовых Cortinocornus, Costopyxis, Hemiaulus, Paralia, Pseudopodosira, Stephanopyxis типична для высокопродуктивной неритической зоны с активным гидрологическим режимом. Заметное присутствие силикофлагеллат говорит об условиях нормальной солености бассейна. Сходные условия седиментации в краевой части внутриконтинентального бассейна южной части Восточно-Европейской платформы в раннекампанское время реконструируются и по данным литологического анализа.

During the Paleocene and Eocene the extratropical Central Eurasia was the main seaway link of the contiguous south to north seaway system connecting the Tethys and Arctic oceans. The seaway extended from North Pakistan and India to North Siberia through a system of inland seas and straits and acted as a kind of heat transfer to the Arctic during this time. Before the emergence of the latitudinal Alpine-Himalayan orogenic belt, the Tethys and its marginal seas to the north formed a continuous shelf area. The closest linkage of the water masses and exchange of biota between the Tethys and the Arctic ocean existed during the Thanetian and Ypresian. Latest Paleocene - Early Eocene heat transfer developed both by water-and atmospheric currents at the mid-latitudes of Central and Eastern Eurasia. From the end of the Paleocene or earliest Eocene onwards this system was complicated by latitudinally oriented straits that ensured the connection of the Northern Peri-Tethys with the Atlantic through the North Sea basin. The combination of two sea systems controlled the climatic history of the Central Asia regions from the Late Paleocene until the Late Eocene. During the Bartonian and Priabonian the West Siberian inland sea was isolated completely from the Arctic Basin during the last phase of marine sedimentation. It was connected with the Turan sea through the Turgai strait. Azolla beds accumulated periodically during eustatic sea level lowstands in fresh-water surface waters, and in dysoxic bottom waters inhabited stunted benthos during the Bartonian - earliest Late Eocene. During the Late Eocene eustatic rise of sea level the input of saline water from the Peri-Tethys increased and the West Siberian basin became normally marine again. The monsoonal subtropical climate during Ypresian - Early Lutetian was transformed to subtropical seasonal with semi-arid features during the Late Lutetian - Early Priabonian, up to subtropical to warm-temperate with alternations of humid and arid phases in the Late Priabonian.