Biofacies analysis of Upper Cretaceous deposits in the Ust-Yenisei region: Implications of palynomorphs

The results of palynomorph biofacies analysis in the Upper Cretaceous deposits of the Ust-Yenisei region are presented. The established facies confinement and indicative features of separate palynomorph groups are used, along with identified dinocyst morphotypes and taxa, in paleogeographic reconstructions. Seven palynomorph associations characterizing continental, coastal-marine, shallow-and deep-water facies are distinguished based on quantitative proportions between morphological groupings and individual taxa. As boundaries between distinguishable biostratigraphic and facies subdivisions do not coincide, dinocysts were likely insignificantly dependent in distribution on facies in the West Siberian epicontinental basin at least. On the other hand, distribution trends of particular dinocyst morphotypes and other microphytofossils are correlative with transgressive-regressive cycles and can be used for reconstruction of paleoenvironments.     

Sedimentary cover of the Lomonosov Ridge: Stratigraphy,structure, deposition history,and ages of seismic facies units

Seismic and geological information on the Lomonosov Ridge is considered with reference to drilling data obtained during the ACEX-302 expedition. A new zonation proposed for the composite section of the ridge sedimentary cover and based on marine microfossils (silicoflagellates and dinocysts) from several boreholes is correlated with biostratigraphic zones of Paleogene sections in northern West Siberia. Principal stages of the Arctic basin development in the Aptian-Cenozoic are defined in onshore and Lomonosov Ridge sections. Synchronous formation of sedimentary sequences in the ridge, an element of the Arctic basin, shelves, and epicontinental seas is established for the period under consideration.     

Dinocyst biostratigraphy of the Lower Cretaceous in North Siberia

The described scheme of the Lower Cretaceous (Berriasian-Barremian) stratigraphic subdivisions is elaborated based on palynological study of sections in the Khatanga depression, Ust-Yenisei region, Pur-Taz interfluve, and around the Ob River latitudinal segment, the North Siberia. Stratigraphic distribution of microphytoplankton studied in detail is used to distinguish 10 biostratigraphic units in the rank of dinocysts zones. Stratigraphic position of the zones is determined with confidence using data on the Lower Cretaceous reference sections in the Khatanga depression, which were principal ones by constructing the Boreal standard zonation. In majority, boundaries of the dinocysts beds are of a high correlation potential and can be regarded as reliable stratigraphic markers, as they are recognizable not only in Siberia, but also in northern Europe and America.     

Palynostratigraphy of Oligocene-Miocene continental deposits in Southwestern Siberia

Data on spores, pollen, and dinoflagellate cysts studied in composite section of Oligocene-Miocene deposits in southern part of West Siberia are presented. Eleven biostratigraphic units distinguished in the section are ranked as palynozones and beds with palynological assemblages. Palynological data substantiate age of deposits and specify ranges and boundaries of palynozones. Based on dinocyst assemblages first studied in sediments of the Zhuravka and Abrosimovo horizons (upper Oligocene, lower Miocene), the Pseudokomewuia Beds are included into local stratigraphic scheme. According to results of comparative analysis, similar and distinctive features of Oligocene-Miocene dinocyst assemblages from West Siberia, China and North America are elucidated. Based on palynological data, the local stratigraphic scheme of higher resolution is suggested for subdivision of Oligocene and Miocene deposits in southern part of West Siberia (Baraba and Kulunda lithofacies regions).     

To the problem of the Oligocene-Miocene boundary position in the northern Caucasus (in support of the Caucasian regional stage)

Stratigraphic position of the Oligocene-Miocene (Paleogene-Neogene) boundary in the northern Caucasus and, in this connection, of the Alkun Formation and the Caucasian regional stage in geological sections of Maikop deposits, primarily in those of the Belaya and Kuban rivers, is considered. Arguments in support of the Caucasian regional stage are presented and necessity of distinguishing the Karadzhalganian regional stage is called in question. Stratigraphic implications of the Alkun Formation, the regional reference horizon traceable in many sections of the northern Caucasus, are discussed. It is substantiated based on nannoplankton, dinocysts, foraminifers, and distinctive lithological features of deposits that the Oligocene-Miocene boundary is confined to the Alkun Formation at the Belaya River.     

Problems of Oxfordian and Kimmeridgian stratigraphy in northern Central Siberia (Nordvik Peninsula section)

The Oxfordian–Lower Hauterivian section of the Nordvik Peninsula (northern Central Siberia) is a reference for developing zonal scales for various fossil groups and improving the Boreal zonal standard. In the middle 1950s–late 1980s, it was studied extensively by geologists, stratigraphers, lithologists, and experts on various fossil groups. These studies yielded rich fossil and microfossil collections and a set of parallel zonal scales for various faunal groups. Recently, a new detailed ammonite zonation of the Oxfordian and Kimmeridgian units of this section has been proposed. These results contradict the previous biostratigraphic data on ammonites, foraminifers, and palynomorphs. In the present paper, all the biostratigraphic data on the Oxfordian and Kimmeridgian units of the Nordvik Peninsula (Cape Urdyuk-Khaya) and northern Central Siberia undergo a comprehensive analysis and comparison with those on the Boreal Realm. The ammonite-constrained stratigraphic position of the lower Upper Jurassic in the Cape Urdyuk-Khaya section is interpreted as Upper Oxfordian or Middle Oxfordian. In our view, this difference in the understanding is due to the misidentification of some Oxfordian ammonite forms. The zones based on other fossil groups (foraminifers, dinocysts) which were distinguished in the Upper Oxfordian and Kimmeridgian sections of the Nordvik Peninsula are well traceable circumarctically. Their stratigraphic position in various regions of the Northern Hemisphere is constrained by ammonites and bivalves. However, if we use the last alternative ammonite zonation of this section part, hardly explicable contradictions will appear in interregional foraminiferal and dinocyst correlations.     

Palynological characteristics of Upper Cretaceous and Paleogene deposits on the west of the Sambian Peninsula (Kaliningrad region), Part 2

The results of studying dinocysts in the Upper Cretaceous-Lower Paleogene succession of the Kaliningrad region are considered. Distinguished in the succession are seven biostratigraphic units in the rank of the Palaeohystrichophora infusorioides, Chatangiella vnigrii, Cerodinium diebelii, Alisocysta margarita, Deflandrea oebisfeldensis, Areosphaeridium diktyoplokum, and Rhombodinium perforatum beds and one Charlesdowniea clathrata angulosa Zone. The Lyubavas Formation has not been distinguished on the west of the Sambian Peninsula. Ages of the Sambia, Alka, and Prussian formations are verified.     

Provincialism in trends and high frequency changes in the northwest North Atlantic during the Holocene

In the present paper, we report on micropaleontological (dinocysts) and isotopic (¹⁸O and ¹³C in foraminifers) analyses performed in Holocene sediments from fifteen cores raised from the central and northwest North Atlantic. Sea-surface temperature (SST), sea-surface salinity (SSS), thus potential density, and sea-ice cover are reconstructed based on dinocyst assemblages. After proper calibration, oxygen isotope data on the mesopelagic foraminifer Neogloboquadrina pachyderma left coiled (Npl) are converted into potential density values deeper in the water column, thus allowing documentation of vertical density gradients and identification of intervals favourable for winter convection to occur with formation of intermediate Labrador Sea Water (LSW). The most important findings from this study include: (1) the existence of an early-mid Holocene thermal optimum with positive anomalies up to 6 °C above present along the main SW–NE axis of the North Atlantic Current, but no significant SST maximum at most sites along eastern Canadian margins; (2) the evidence for larger than present amplitude of annual SSTs during the early Holocene, thus for a stronger seasonality; (3) minimum sea-ice cover from 11 500 to 6000 cal years BP, and a slight increase of sea-ice variability, and average seasonal duration of 0.5 to 1 month per year afterwards; (4) variable SSS during the entire Holocene, suggesting changes in the routing and rates of freshwater–meltwater discharges from the Arctic and eastern Canada; (5) the setting of conditions compatible with LSW production after 8 ka only, and likely a more steady production during the late Holocene; (6) an overall trend for a potential density increase of the Labrador Sea, throughout the Holocene, matching a decreasing trend eastward, thus suggesting a progressive enhancement of the western branch of the Atlantic Meridional Overturning with respect to its northeastern route; and (7) indication of maximum production and fast dispersal of LSW in the entire North Atlantic during recent times only, as suggested by linearly-converging δ¹⁸O-values of Npl from all sites, towards its modern relatively homogeneous composition ( 2.5/2.6‰). The overall picture of the Holocene North Atlantic arising from this study is that of a basin marked by a strong regionalism with large discrepancies in hydrographical trends and high frequency oscillations, at least partly controlled by freshwater–meltwater routes and rates of export from the Arctic.     

Organic-walled phytoplankton from Paleogene sections of Kamchatka

Organic-walled phytoplankton assemblages from Eocene and Oligocene reference sections of western Kamchatka are analyzed. They are close in taxonomic composition to coeval assemblages of northern Japan and Sakhalin that is a good opportunity to correlate regional units and verify their age. The euryhaline species Trinovantedinium boreale that is associated with Paralecaniella indentata and Micrhystridium preferring shallow low-salinity waters, on the one hand, and taxa dwelling in normal-salinity open sea environments, on the other, are dominant in most assemblages, which are examined. The suggested variant of bed succession with dinocyst assemblages may be valid for Japan, Sakhalin, Kamchatka, and the Bering Sea.     

Palynological characteristics of Upper Cretaceous and Paleogene deposits on the west of the Sambian Peninsula (Kaliningrad region), Part 1

The results of studying dinocysts in the Upper Cretaceous-Lower Paleogene succession of the Kaliningrad region are considered. Distinguished in the succession are seven biostratigraphic units in the rank of the Palaeohystrichophora infusorioides, Chatangiella vnigrii, Cerodinium diebelii, Alisocysta margarita, Deflandrea oebisfeldensis, Areosphaeridium diktyoplokum, and Rhombodinium perforatum beds and one Charlesdowniea clathrata angulosa Zone.