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Russian Geology and Geophysics

2015 year, number 6

1.
P-T-t RECONSTRUCTIONS OF SOUTH YENISEI RIDGE METAMORPHIC HISTORY (Siberian craton): PETROLOGICAL CONSEQUENCES AND APPLICATION TO THE SUPERCONTINENTAL CYCLES

I.I. Likhanov1, V.V. Reverdatto1, P.S. Kozlov2, S.V. Zinoviev1,3, V.V. Khiller2
1V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
2A.N. Zavaritsky Institute of Geology and Geochemistry, Ural Branch of the Russian Academy of Sciences, Pochtovyi per. 7, Yekaterinburg, 620151, Russia
3Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia
Keywords: Metamorphism, deformations, geothermobarometry, Th-U-Pb monazite dating, Yenisei Ridge, Siberian craton, Nuna and Rodinia supercontinents

Abstract >>
Studies of gneisses from the Yenisei regional shear zone (YRSZ) provide the first evidence for Mesoproterozoic tectonic events in the geologic history of the South Yenisei Ridge and allowed the recognition of several stages of deformation and metamorphism spanning from Late Paleoproterozoic to Vendian. The first stage (~1.73 Ga), corresponding to the period of granulite-amphibolite metamorphism at P = 5.9 kbar and T = 635 ºC, marks the final amalgamation of the Siberian craton to the Paleo-Mesoproterozoic Nuna supercontinent. During the second stage, corresponding to a hypothesized breakup of Nuna as a result of crustal extension, these rocks underwent Mesoproterozoic dynamic metamorphism ( P = 7.4 kbar and T = 660 ºC) with three peaks at 1.54, 1.38, and 1.25 Ga and the formation of high-pressure blastomylonite rocks in shear zones. Late-stage deformations during the Mesoproterozoic tectonic activity in the region, related to the Grenville-age collision processes and assembly of Rodinia, took place at 1.17-1.03 Ga. The latest pulse of dynamic metamorphism (615-600 Ma) marks the final stage of the Neoproterozoic evolution of the Yenisei Ridge, which is associated with the accretion of island-arc terranes to the western margin of the Siberian craton. The overall duration of identified tectonothermal processes within the South Yenisei Ridge during the Riphean (~650 Ma) is correlated with the duration of geodynamic cycles in the supercontinent evolution. A similar succession and style of tectonothermal events in the history of both the southern and the northern parts of the Yenisei Ridge suggest that they evolved synchronously within a single structure over a prolonged time span (1385-600 Ma). New data on coeavl events identified on the western margin of the Siberian craton contradict the hypothesis of a mantle activity lull (from 1.75 to 0.7 Ga) on the southwestern margins of the Siberian craton during the Precambrian. The synchronous sequence and similar style of tectonic events on the periphery of the large Precambrian Laurentia, Baltica, and Siberia cratons suggest their spatial proximity over a prolonged time span (1550-600 Ma). The above conclusion is consistent with the results of modern paleomagnetic reconstructions suggesting that these cratons represented the cores of Nuna and Rodinia within the above time interval.

DOI: http://dx.doi.org/10.1016/j.rgg.2015.05.001



2.
EARLY MESOZOIC LAMPROITES AND MONZONITOIDS OF SOUTHEASTERN GORNY ALTAI: GEOCHEMISTRY, Sr-Nd ISOTOPE COMPOSITION, AND SOURCES OF MELTS

V.I. Krupchatnikov1, V.V. Vrublevskii2, N.N. Kruk3,4
1Gorny Altai Expedition, Maloeniseiskoe Village, Altai Territory, 659370, Russia
2Tomsk State University, pr. Lenina 36, Tomsk, 634050, Russia
3V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
4Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia
Keywords: Lamproites, lamprophyres, monzonitoids, magmatic sources, mantle plume, Altai-Sayan folded area, Gorny Altai

Abstract >>
Small intrusions of lamprophyres and lamproites (Chuya complex) and K-monzonitoids (Tarkhata and Terandzhik complexes) are widespread in southeastern Gorny Altai. Geochronological (U-Pb and Ar-Ar) isotope studies show their formation in the Early-Middle Triassic (~234-250 Ma). Lamproites have been revealed within two magmatic areas and correspond in geochemical parameters to the classical Mediterranean and Tibet orogenic lamproites. According to isotope data ((87Sr/86Sr) T = 0.70850-0.70891, (143Nd/144Nd) T = 0.512157-0.512196, 206Pb/204Pb = 17.95-18.05) and Th/La and Sm/La values, the Chuya lamproites and lamprophyres melted out from the enriched lithospheric mantle with the participation of DM, EM1, EM2, and SALATHO. The monzonitoid series of the Tarkhata and Terandzhik complexes are similar in petrographic and geochemical compositions but differ significantly in Sr-Nd isotope composition: The Tarkhata monzonitoids are close to the Chuya lamproites, whereas the Terandzhik ones show a higher portion of DM ((87Sr/86Sr) T = 0.70434-0.70497, (143Nd/144Nd) T = 0.512463-0.512487) in their source, which suggests its shallower depth of occurrence and the higher degree of its partial melting as compared with the derivates of the Chuya and Tarkhata complexes. The studied rock associations tentatively formed in the postcollisional setting under the impact of the Siberian superplume.

DOI: http://dx.doi.org/10.1016/j.rgg.2015.05.002



3.
MAJOR MINERALS OF ABNORMALLY HIGH-GRADE ORES OF THE TOMTOR DEPOSIT (Arctic Siberia)

E.V. Lazareva1, S.M. Zhmodik1,2, N.L. Dobretsov3,2, A.V. Tolstov1, B.L. Shcherbov1, N.S. Karmanov1, E.Yu. Gerasimov4, A.V. Bryanskaya5
1V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
2Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia
3A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
4G.K. Boreskov Institute of Catalysis, pr. Akademika Lavrenteva 5, Novosibirsk, 630090, Russia
5Institute of Cytology and Genetics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Lavrenteva 10, Novosibirsk, 630090, Russia
Keywords: Pyrochlore, minerals of the crandallite group, monazite, halloysite, REE deposit, carbonatites

Abstract >>
The Tomtor massif of Paleozoic ultramafic alkaline rocks and carbonatites is located in the northern part of the Sakha Republic (Yakutia). The massif (its total area is ~250 km 2) is ~20 km in diameter; it has a rounded shape and a concentrically zoned structure. The core of the massif consists of carbonatites surrounded by a discontinuous ring of ultramafic rocks and foidolites. The outer part is composed of alkali and nepheline syenites. A weathering crust formed after all the rocks; the thickest crust formed after the carbonatites, which are enriched in phosphates and REE. Four horizons are recognized from the top: kaolinite-crandallite, siderite, goethite, and francolite. The highest-grade ores are observed in the bedded deposit which fills depressions in the sagging weathering crust of the carbonatite massif. The ores are thin-bedded and cryptogranular, with high Nb, Y, Sc, and REE contents (on average, 4.5% Nb 2O 5, 7-10% REE 2O 3, 0.75% Y 2O 3, and 0.06% Sc 2O 3). The highest-grade ores are natural Nb and REE concentrates. The total REE content of some horizons is >10%. The morphologic features of the highest-grade phosphate ores from the northern part of the Burannyi site were studied. The major ore-forming minerals are minerals of the pyrochlore group, crandallite group (goyazite), and monazite-Ce. The pyrochlore group minerals in the ores occur mainly as crystals that were completely replaced by barium-strontium pyrochlore and/or plumbopyrochlore but retained the original faces; also, they occur as numerous conchoidal fragments. The grains of the pyrochlore group minerals sometimes have a zonal structure: The core consists of relics of unaltered pyrochlore, and the rim is replaced. Goyazite occurs predominantly as colloform grains. According to SEM and TEM data, monazite occurs in the ores as ~50 nm particles, which cover the outer part of halloysite tubes (800-3000 nm long and 300 nm in diameter) as a dense layer and make up peculiar biomorphic aggregates. The mineralogical data, the occurrence of biomorphic aggregates, and the close association of organic remains with ore minerals suggest that the high-grade ores of the Tomtor deposit, including the Burannyi site, resulted from a hydrothermal-sedimentary process with a presumably important role of biogenic concentration of REE phosphates.

DOI: http://dx.doi.org/10.1016/j.rgg.2015.05.003



4.
EARLY PRECAMBRIAN HIGH-GRADE METAMORPHOSED TERRIGENOUS ROCKS OF GRANULITE-GNEISS TERRANES OF THE SHARYZHALGAI UPLIFT (southwestern Siberian craton)

O.M. Turkina1,2, V.P. Sukhorukov1,2
1V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
2Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia
Keywords: Early Precambrian, metasedimentary rocks, trace elements, provenances, Sm-Nd isotope composition, growth and recycling of crust, Sharyzhalgai uplift

Abstract >>
We present results of geochemical and Sm-Nd isotope studies of high-grade metaterrigenous rocks of the Kitoi and northwestern Irkut terranes of the Sharyzhalgai uplift on the Siberian Platform in comparison with paragneisses of the southeastern Irkut terrane. The metasedimentary rocks of the first region are high-alumina garnet-sillimanite-cordierite-bearing paragneisses; their protoliths were mostly mudstones and pelitic mudstones by major-element composition. The low-alumina biotite gneisses of the Kitoi terrane formed, most likely, from magmatic protoliths similar in petrochemical features to intraplate volcanics. The major factor controlling the composition of the studied high-alumina paragneisses is precipitation of most of incompatible trace elements in the clay fraction of sediments, as evidenced from the positive correlation between trace-element and Al2O3 contents. The Cr and Ni contents, showing a positive correlation with MgO and no correlation with Al2O3, are an indicator of the contribution of the mafic-source material to the formation of high-alumina rocks. The contribution of a mafic source-derived material to the formation of terrigenous rocks increases in passing from Kitoi to northwestern Irkut terrane. The high-alumina and garnet-biotite paragneisses of the southeastern Irkut terrane are similar in trace-element patterns to the analogous rocks of the Kitoi terrane and northwestern part of the Irkut terrane but show higher Th contents and a distinct negative Eu anomaly related to the change in the composition of the felsic source. The participation of felsic potassic igneous rocks in the formation of the southeastern terrigenous sediments is consistent with their deposition after the Neoarchean collision processes (metamorphism and granite magmatism), whereas sedimentation in the Kitoi and northwestern Irkut terranes preceded them. The Sm-Nd isotope characteristics indicate that the latter sediments formed mostly as a result of the erosion of the Paleo-Mesoarchean crust, whereas the metasediments of the southeastern Irkut terrane formed with the participation of Paleoproterozoic juvenile rocks. Thus, the variations in the trace-element and isotope compositions of the high-grade metamorphosed terrigenous rocks reflect recycling and growth of the continental crust of the Sharyzhalgai uplift during the Neoarchean-Paleoproterozoic transition.

DOI: http://dx.doi.org/10.1016/j.rgg.2015.05.004



5.
CHANGE IN THE VISCOSITY OF KIMBERLITE AND BASALTIC MAGMAS DURING THEIR ORIGIN AND EVOLUTION (prediction)

E.S. Persikov1, P.G. Bukhtiyarov1, A.G. Sokol2
1Institute of Experimental Mineralogy, Russian Academy of Sciences, ul. Akademika Osipyana 4, Chernogolovka, Moscow Region, 142432, Russia
2Novosibirsk State University, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
Keywords: Viscosity, model, kimberlite, basalt, water, evolution, temperature, pressure, composition of melts, mantle and crust, crystals, bubbles

Abstract >>
Based on the analysis of experimental data on the viscosity of mafic to ultramafic magmatic melts with the use of our structure-chemical model for the calculation and prediction of the viscosity of magmas, we have first predicted that diamond-carrying kimberlite magma must ascend from mantle to crust with considerable acceleration. The viscosity of kimberlite magma decreases by more than three times during its genesis, evolution, and ascent from mantle to crust despite the significant decrease in the temperature of the ascending kimberlite magma (~150 ºC) and its partial crystallization and degassing. In the case of partial melting (<1 wt.%) of carbonated peridotite in the mantle at depths of 250-350 km, high-viscosity (~35 Pas) kimberlite melts can be generated at ~8.5 GPa and ~1350 ºC, the water content in the melt being up to ~8 wt.%, C(OH) = 02 wt.%, and C(H2O) = 06 wt.%. On the other hand, during the formation of kimberlite pipes, dikes, and sills, the viscosity of near-surface kimberlite melts is much lower (~10 Pas) at ~50 MPa and 1200 ºC, the volume contents of crystals (Vcr) and the fluid phase (bubbles) (Vfl) are 35 and 5 vol.%, respectively, and the water content in magma, C(OH), is 0.5 wt.%. On the contrary, the viscosity of basaltic magmas increases by more than two orders of magnitude during their ascent from mantle to crust. The basaltic magmas which can be generated in the asthenosphere at depths of ~100 km have the minimum viscosity (up to ~2.3 Pas) at ~4.0 GPa, 1350 ºC, C(OH) ≈ 3 wt.%, and C(H2O) ≈ 5 wt.%. However, at the final stage of evolution (e.g., during volcanic eruptions), the viscosity of basaltic magma is considerably higher (600 Pas) at ~10 MPa, 1180 ºC, Vcr ≈ 30 vol.%, Vfl ≈ 15 vol.%, and C(OH) ≈ 0.5 wt.%.

DOI: http://dx.doi.org/10.1016/j.rgg.2015.05.005



6.
COMPUTER RECONSTRUCTION OF THE PHYSICOCHEMICAL CONDITIONS OF SULFIDE FORMATION FOR THE KRASNOV AND ASHADZE HYDROTHERMAL SYSTEMS (Mid-Atlantic Ridge)

Yu.V. Laptev
V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
Keywords: Sphalerite, nonideal solid solution, sulfide formation, hydrothermal systems

Abstract >>
Computer-based reconstruction of the physicochemical conditions of formation of the Krasnov (16º N) and Ashadze (13º N) submarine systems in the Mid-Atlantic Ridge (MAR) has been performed using the equilibrium-thermodynamic approach to study samples from these sites. In the first case, a sphalerite-pyrite-barite association was considered, and in the second case, a sphalerite-pyrite association. In the modeling conducted, the composition of the sphalerite solid solution corresponding to the nonideal mixing of ZnS and FeS was used as a correlation parameter with the total composition of the Fe-Zn-Ba-S-H2O-NaCl-HCl hydrothermal system depending on temperature (200300 ºC) and a given pressure of 100 bar. The calculation results predict that at an iron content of 0.170.36 wt.% in sphalerite, the minimum formation temperatures of the equilibrium sphalerite-pyrite-barite association should correspond to the interval of 280-300 ºC (Krasnov site). As the iron content in sphalerite increases to 4.1513.28 wt.%, the occurrence of barite in the systems studied becomes impossible and the formation temperatures of the sphalerite-pyrite association become equal to or higher than 300 ºC (Ashadze site).

DOI: http://dx.doi.org/10.1016/j.rgg.2015.05.006



7.
P-T-X PARAMETERS OF METAMORPHOGENE AND HYDROTHERMAL FLUIDS, ISOTOPY AND AGE OF THE BOGUNAI GOLD DEPOSIT, SOUTHERN YENISEI RIDGE (Russia)

M.A. Ryabukha1, N.A. Gibsher1, A.A. Tomilenko1, T.A. Bulbak1, M.O. Khomenko1, A.M. Sazonov2
1V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
2Institute of Mining, Geology, and Geotechnologies of Siberian Federal University, pr. Svobodnyi 79, Krasnoyarsk, 660041, Russia
Keywords: Quartz, sulfides, gold, ore-forming fluid, hydrocarbons, δ34S, Ar-Ar age

Abstract >>
Fluid inclusions in quartz, sulfides from quartz veins, and quartz, garnet, plagioclase, and orthoclase from granulites of the Bogunai gold deposit located in the granulites of the Angara-Kan block of the Yenisei Ridge were studied by thermobarometry, gas chromatography, chromato-mass-spectrometry, Raman spectroscopy, and mass spectrometry with inductively coupled plasma. The formation temperatures (850-950 ºC) and pressures (8.59.0 kbar) of minerals of the granulite metamorphic facies are much higher than the crystallization temperatures (220420 ºC) and pressures (0.11.6 kbar) of gold-quartz veins of the Bogunai deposit. These veins formed with the participation of H2OCO2hydrocarbon fluids with a salt (predominantly MgCl2) concentration of 219 wt.% NaCl equiv. The gas phase of fluid inclusions from quartz, pyrite, chalcopyrite, galena, and sphalerite contains not only H2O, CO2, CH4, and N2 but also the first found compounds of sulfur (CS2, O2S, COS, C2H6S2) and nitrogen (C3H7N, C3H7NO, C4H8N2O) and numerous hydrocarbons of different classes (paraffins, arenes, naphthenes, alcohols, aldehydes, ketones, carbonic acids, and furans). The age of the Krasnoyarsk mineralized zone, one of the sites of the Bogunai deposit, is 466 3.2-461.6 3.1 Ma, which is almost 1400 Ma younger than the age of granulite metamorphism and 255 Ma younger than the age of diaphthoresis but is close to the age of the Lower Kan granitoid massif (455.7 3.4 Ma). The sulfur isotope ratios (δ34S) of sulfides (pyrite, chalcopyrite, sphalerite, and galena) are close to the mantle values, 0.8 to 3.5, and are in the range of the granitoid values, which indicates the crustal source of the fluid sulfur. Gold of the Bogunai deposit accumulated with the participation of H2OCO2hydrocarbon fluids generated both in deep-fault zones and in granitoid intrusions.

DOI: http://dx.doi.org/10.1016/j.rgg.2015.05.007



8.
LITHOLOGIC AND FACIES CHARACTERISTICS OF THE LOWER CAMBRIAN USOLKA FORMATION AND ITS AGE ANALOGS IN THE CIS-YENISEI SEDIMENTARY BASIN, WEST SIBERIA

S.V. Saraev
A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
Keywords: Sedimentology, paleogeography, Lower Cambrian, Cis-Yenisei sedimentary basin, West Siberian geosyneclise

Abstract >>
Based on the correlation between Lower Cambrian sections of deep wells on the left bank of the Yenisei River and outcrops in the westernmost Yenisei Ridge, the sedimentologic characteristics of the Usolka Formation and coeval deposits are compared. Three paleogeographic regions (subbasins) are distinguished in the Cis-Yenisei basin in Usolka time. The leading role in the sediment genesis was played by the region of barrier reefs stretching northwestward and northward as a ~70 km wide band to their joint with the reefs of the Igarka-Norilsk facies region in the north. The salt basin east of the barrier reef ridge was a bay of the East Siberian salt basin, separated from the latter by the Yenisei paleoislands (at the place of the Yenisei Ridge). Sedimentation in that basin was controlled by evaporite processes, the supply of terrigenous material from the islands and carbonate debris from the opposite side (barrier reef), and the predominance of storm processes. The subbasin west of the barrier reefs evolved under different scenario. Since the Early Cambrian, a trough has been forming here, which was initially characterized by a regime of starvation and the first appearance of carbonaceous rocks (Lower Churbiga Subformation), regarded as Domanic oil source rocks, in the general Cambrian section. On the estimation of the prospects for the distribution of reservoirs whose origin depends on the facies regularities, the leading role is played by the barrier reef region. Importantly, the Lower Cambrian section of the Cis-Yenisei basin contains two large adjacent paleogeomorphologic structures and the corresponding facies zones of a starved prereef depression with the Domanic oil source rocks of the Lower Churbiga Subformation at the early stage of evolution and a potentially productive barrier reef system with a steep slope toward the basin. The favorable combination of these factors suggests that the Cis-Yenisei basin has a high petroleum potential.

DOI: http://dx.doi.org/10.1016/j.rgg.2015.05.008



9.
CALLOVIAN AND UPPER JURASSIC FORAMINIFERAL AND AMMONITE BIOSTRATIGRAPHY OF THE SHAIM PETROLEUM REGION (West Siberia)

L.K. Levchuk, B.L. Nikitenko, S.V. Meledina
A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
Keywords: Biostratigraphy, Callovian, Oxfordian, Kimmeridgian, Volgian, foraminifers, ammonites, West Siberia, Shaim petroleum region

Abstract >>
The results of biostratigraphic analysis of foraminiferal assemblages were used to identify a nearly complete succesion of biostratigraphic units in the Callovian-Upper Jurassic section of the Shaim region. The finds of the ammonites from different parts of the Callovian-Volgian sections helped to correlate the identified foraminiferal biostratigraphic units to the General Stratigraphic Scale. Analysis of variations in the taxonomic composition and structure of foraminiferal assemblages allowed refinement of paleontological characteristics of the foraminiferal biostratigraphic units for different facies conditions in the southwest of the West Siberian Basin.

DOI: http://dx.doi.org/10.1016/j.rgg.2015.05.009



10.
DIATOM CHARACTERISTICS OF THE FAR EAST SILICEOUS ORGANOGENIC DEPOSITS

A.S. Avramenko1,2, M.V. Cherepanova1, V.S. Pushkar2,3, S.B. Yarusova4
1Institute of Biology and Soil Science, Far Eastern Branch, Russian Academy of Sciences, pr. 100-letiya Vladivostoka, 159, Vladivostok, 690022, Russia
2Far East Geological Institute, Far Eastern Branch, Russian Academy of Sciences, pr. 100-letiya Vladivostoka, 159, Vladivostok, 690022, Russia
3Far Eastern Federal University, ul. Sukhanova 8, Vladivostok, 690091, Russia
4Institute of Chemistry, Far Eastern Branch, Russian Academy of Sciences, pr. 100-letiya Vladivostoka 159, 690022, Vladivostok, Russia
Keywords: Diatomite, freshwater deposits, diatoms, Quaternary system

Abstract >>
Three Far East diatomites, Puzanov (Kunashir Island, Kuril Islands), Sergeevskii, and Terekhovka (Southern Primorye), are characterized in detail. Rock-forming taxa are identified (Puzanov Aulacoseira subarctica (O. Müll.) Hawort and Stephanodiscus niagarae var. pusanovae Genkal et Cherepanova; Sergeevskii Staurosira construens var. venter (Ehr.) Grun., Aulacoseira italica (Ehr.) Sim., and representatives of the genus Cymbella with large valves (up to 175 μm in length); Terekhovka Aulacoseira praegranulata var. praeislandica f. praeislandica (Sim.) Moiss.). Morphometric analysis of valves of the dominant taxa showed a low variability of valve parameters for the Puzanov diatomite and a high one for the Sergeevskii and Terekhovka diatomites. The lake environments of the diatoms forming the diatomites were reconstructed based on the elemental composition of diatom valves and the ecological structure of diatom paleocommunities. The high oxygen and low silicon concentrations in valves of the Puzanov diatomite, most likely, indicate that the diatoms were part of plankton communities formed in a large deep freshwater lake. Significant concentrations of silicon in valves of the Terekhovka diatomite, in contrast, suggest that the diatoms occurred in benthic ecotopes in a shallow lake. The age of the deposits was refined by detailed studies of Aulacoseira valves, using light and scanning electron microscopes. For example, the presence of Aulacoseira taxa of the prae group in the Terekhovka diatomite confirms its Pliocene age, and the presence of valves of the present species A. italica in the Sergeevskii diatomite points to its Late Pliocene age. The identified features of the diatomites permit their use in practice.

DOI: http://dx.doi.org/10.1016/j.rgg.2015.05.010