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Russian Geology and Geophysics

2018 year, number 6

1.
C AND Sr ISOTOPE CHEMOSTRATIGRAPHY OF VENDIAN-LOWER CAMBRIAN CARBONATE SEQUENCES IN THE CENTRAL SIBERIAN PLATFORM

B.B. Kochnev1,2, B.G. Pokrovsky3, A.B. Kuznetsov4, V.V. Marusin1,2
1A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
2Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia
3Geological Institute, Russian Academy of Sciences, Pyzhevsky per. 7, Moscow, 119017, Russia
4Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences, nab. Makarova 2, St. Petersburg, 199034, Russia
Keywords: Венд, нижний кембрий, эдиакарий, изотопы углерода и стронция, карбонатные осадочные породы, изотопная хемостратиграфия, внутренние районы Сибирской платформы, Vendian, Lower Cambrian, Ediacaran, carbon and strontium isotope compositions, carbonate sediments, δ13C chemostratigraphy, craton interior

Abstract >>
We propose a detailed δ13С curve for the Vendian and Lower Cambrian (Tommotian) strata of the central Siberian Platform. Two positive carbon isotope excursions identified near the base of the Yuryakh Formation (up to 5.5‰) and in the lower Bilir Formation (up to 5 ‰) are assigned to the lowermost and middle Tommotian, respectively. This correlation is supported by paleontological data, specific 87Sr/86Sr values (0.70845-0.70856), and similar C isotope record in coeval Early Cambrian basins. The documented minor vertical oscillations (a few meters) of these isotope excursions relative to the formation boundaries in remote boreholes is presumably caused by the spatiotemporal migration of facies. A high-amplitude negative δ13С excursion (-8 to -11 ‰) in the upper Nepa Regional Stage putatively corresponds to the global Shuram-Wonoka negative carbon isotope excursion (Middle Ediacaran). Carbonates of the lower Nepa Regional Stage (Besyuryakh Formation) demonstrate positive δ13С values (up to 5 ‰) and minimum 87Sr/86Sr ratios of 0.70796-0.70832. The C and Sr isotope record of the Nepa Regional Stage provides its robust correlation with the Dal’nyaya Taiga and Zhuya Groups of the Patom Foredeep. Micropaleontological data herein reported and glacial diamictites documented at the base of the Vendian sedimentary cover both in the central Patom Foredeep and on its periphery suggest a full stratigraphic volume of the Ediacaran System in the most stratigraphically complete sections of the central Siberian Platform.



2.
SEDIMENTOLOGY AND SEQUENCE STRATIGRAPHY OF A RETROGRADATIONAL FAN-DELTA SYSTEM WITHIN THE LOWER TRIASSIC IN THE MABEI AREA, JUNGGAR BASIN (northwestern China)

B. Peng1,2,3, Z. Jin1,2, J. Wang1,2, H. Jia1,2, X. Zhu1,2, T. Chang1,2, K. Yuan1,2
1State Key Laboratory of Petroleum Resource and Prospecting, Beijing, 102249, China
2College of Geosciences, China University of Petroleum, Beijing, 102249, China
3Shanxi Provincial Land Engineering Construction Group Co. Ltd, Xi’an, 710075, China
Keywords: Ретроградационные конусы выноса, сиквенс-стратиграфия, нижний триас, район Мабей, бассейн Джунгар, Retrogradational fan deltas, sequence stratigraphy, Lower Triassic, Mabei area, Junggar Basin

Abstract >>
The sedimentology and sequence stratigraphy of a retrogradational fan-delta system within Lower Triassic in the Mabei area of Junggar Basin in northwestern China were investigated using seismic, well log, and core data, complemented by the modern deposition and a flume tank experiment. The Lower Triassic in the Mabei area is dominated by fan deltas, which are composed of fan-delta plain (including subaerial debris flow, braided channel, conglomerate overbank, and floodplain), fan-delta front (including conglomerate shoal and sandy shoal), and prodelta. The braided channels form during the low flood period. The conglomerate overbanks form during the flood period and occupy most part of the fan-delta plain. The conglomerate shoals and sandy shoals form by a sheet flow prograding into lakes and occur as a sheet. The braided channels, conglomerate shoals, and sandy shoals are easy to form high-porosity reservoirs. One long-term base level cycles (LSC1), three middle-term base level cycles (MSC1, MSC2, and MSC3), and fifteen short-term base level cycles are identified. MSC1 is dominated by the fan-delta plain; MSC2 is dominated by the fan-delta front; and MSC3 is dominated by the fan-delta front and prodelta. The stratigraphy shows a proximal-middle-distal trend demonstrating an overall retrogradation stacking pattern. The sequence architecture is controlled by an interplay between lake level changes and sediment supply. The quick rise in the lake level and the creation of accommodation outpacing the rate of sediment supply result in a retrogradational fan-delta system.



3.
JURASSIC SEDIMENTS IN THE IRKUT BASIN AND SOUTHWESTERN TRANSBAIKALIA: CORRELATIONS BASED ON PALEOBOTANICAL AND GEOCHRONOLOGICAL DATA

A.V. Arzhannikova1, A.O. Frolov1, S.G. Arzhannikov1, E.I. Demonterova1, A.V. Ivanov1, M. Jolivet2, M.N. Rubtsova1, A.L. Dorozhko3
1Institute of the Earth’s Crust, Siberian Branch of the Russian Academy of Sciences, ul. Lermontova 128, Irkutsk, 664033, Russia
2Laboratoire de Gйosciences Rennes, CNRS-UMR 6118, Universite Rennes 1, Rennes, France
3Sergeev Institute of Environmental Geoscience, Russian Academy of Sciences, Ulanskii per. 13, str. 2, Moscow, 101000, Russia
Keywords: Юрские осадочные отложения, палеоботанические исследования, геохронологическое датирование, Забайкалье, Иркутский бассейн, Jurassic sediments, paleobotanic studies, geochronology, Transbaikalia, Irkut basin

Abstract >>
The Jurassic growth of mountain ranges along the southern edge of the Siberian platform occurred in an active tectonic setting related to the closure of the Mongol-Okhotsk Ocean. The oceanic subduction and subsequent continent collision events induced compressive deformations at the platform boundary. Understanding the paleogeography related to the Mesozoic closure of the Mongol-Okhotsk Ocean requires dating and correlation of the Jurassic Prisayan Formation in the Irkut basin and Tugnuyskaya Formation in southwestern Transbaikalia. This work presents structural and paleobotanic results within both formations. 40Ar/39Ar dating of underlying volcanics from the upper member of the Ichetuyskaya Formation is used to refine the age of the sediment series and provide probable correlation. The results show that the Tugnuyskaya Formation initiated at the end of the Middle Jurassic-beginning of the Late Jurassic and was not coeval with the Prisayan Formation, whose upper fine-grained members were deposited in the early Middle Jurassic. 40Ar/39Ar dating of volcanics from the upper member of the Ichetuyskaya Formation yielded a Middle Jurassic age of 167.7 ± 1.2 Ma (Bajocian to Bathonian). The paleogeographic data analysis based on facies and mineralogical composition of sediments and on a study of source areas from Sm-Nd data and the U-Pb ages of detrital zircon from the deposits in the southern Irkut basin indicates that the deposition of the Prisayan Formation was followed by the intensification of relief building along the southern edge of the Siberian Platform. Our geochronological data show that active tectonic deformations in southwestern Transbaikalia evidenced in the volcanoclastic Ichetuyskaya Formation in the Tugnuy basin also occurred during the Middle Jurassic. The uppermost sediments of the Tugnuy basin were deposited at the end of the Middle Jurassic-Late Jurassic in a quiet tectonic setting with low relief and lacustrine-boggy depositional environments.



4.
RADIOCARBON CHRONOLOGY OF OCCUPATION OF THE CHICHA SITE AND BAYESIAN STATISTICS FOR THE ASSESSMENT OF A DISCONTINUOUS TRANSITION FROM LATE BRONZE TO EARLY IRON AGE (West Siberia)

J. SchneeweiЯ1, F. Becker2, V.I. Molodin3,4, H. Parzinger5, Zh.V. Marchenko4,6, S.V. Svyatko7
1Institute for the History of Material Culture, Russian Academy of Sciences, Dvortsovaya nab. 18, St. Petersburg, 191186, Russia
2Freie Universitдt Berlin, Department of Earth Sciences, Institute of Geographical Sciences, Physical Geography, Malteserstr. 74-100, 12249 Berlin, Germany
3Altai State University, pr. Lenina 61, Barnaul, 656049, Russia
4Institute of Archaeology and Ethnography, Siberian Branch of the Russian Academy of Sciences, pr. Lavrentieva 17, Novosibirsk, 630090, Russia
5Stiftung Preussischer Kulturbesitz, The president, Von-der-Heydt-Str. 16-18, 10785 Berlin, Germany
6Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia
714CHRONO Centre for Climate, the Environment and Chronology, Queen’s University Belfast, 42 Fitzwilliam Street, Belfast BT9 6AX, Northern Ireland, UK
Keywords: Радиоуглеродная хронология, байесовская статистика, поселение Чича, Западная Сибирь, Radiocarbon chronology, Bayesian statistics, Chicha settlement, West Siberia

Abstract >>
The Late Bronze to Iron Age transition in West Siberia involved significant cultural and political changes, which were reflected in the emergence of protourban settlements in the forest-steppe zone. The transition is roughly dated to the turn from the 2nd to the 1st mil. BC, mainly based on archaeological artefacts and on the general sociocultural development. A more precise radiocarbon dating for this transition was problematic mainly because of the general lack of data for West Siberia. This paper analyzes the chronology of the Late Bronze to Iron Age transition at Chicha, a reference site in the region for this period. The assessment of a possible hiatus between the Late Bronze Age and the following transitional period at Chicha is the key issue of our paper, as its presence may have far reaching cultural implications. To increase the precision of the radiocarbon chronology with focus on the possible hiatus, the samples from well-defined stratigraphic contexts were analyzed using Bayesian modeling incorporating stratigraphic information. The performed chronological modeling supports existing archaeological hypothesis of a hiatus at the very end of the 2nd mil. BC, just before the regular emergence of a new protourban settlement at the site of an abandoned Late Bronze settlement. A sensitivity analysis including the simulated hiatus confirms the reliability of the model. A complete population change seems to be plausible. The protourban settlement of Chicha, typical of the transitional period, did not exist very long. With the emergence of the early nomadic cultures in the South Siberian steppes at the beginning of the 1st mil. BC, it most probably stopped functioning. The cultural development of that time, former believed to be continuous, was deeply interrupted several times.



5.
METACARBONATE-TERRIGENOUS COMPLEX OF THE DERBA BLOCK (East Sayan): PETROGEOCHEMICAL AND ISOTOPE PARAMETERS, METAMORPHISM, AND TIME OF FORMATION

A.D. Nozhkin1,2, O.M. Turkina1,2, N.V. Dmitrieva1, A.V. Travin1, I.I. Likhanov1
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: Карбонатно-терригенные породы, геохимия, U-Pb (La-ICP-MS), Ar-Ar датирование, метаморфизм, Дербинский блок, Terrigenous carbonate rocks, geochemistry, U-Pb (LA-ICP-MS) and Ar-Ar dating, metamorphism, Derba block

Abstract >>
The Derba block is one of the largest Precambrian terranes of the Sayan-Yenisei accretionary belt in the southwestern margin of the Siberian Platform. It is composed of metamorphosed terrigenous-carbonate rocks of the Sayan Group, injected by granitoids. The geochemical features of gneiss-schist associations indicate the low maturity of their sedimentary protoliths corresponding in composition mainly to graywackes and terrigenous-carbonate rocks (marls). According to the results of U-Pb (LA-ICP-MS) dating of detrital zircons from gneisses and schists, the sedimentary protolith formed in the Vendian. Neoproterozoic subduction complexes were probably the major provenance for terrigenous material, and Early Precambrian rocks made a limited contribution. The Ar-Ar and U-Pb isotope data testify to nearly coeval and multistage events of metamorphism (up to the amphibolite facies) and granitoid magmatism (~510-500 and 480-465 Ma) in the Derba block. These processes were reflective of the Early Caledonian orogenic processes in the structures of the Central Asian Orogenic Belt. The similarity in the composition, time of sedimentation, and provenances of metaterrigenous-carbonate complexes of the Derba block (Sayan Group), West Sangilen block of the Tuva-Mongolian massif (Erzin and Moren complexes), and the Khamar-Daban terrane (Slyudyanka Group) suggests that these structures were a single Vendian continental margin with lateral variations in depositional environments and the sources of terrigenous material.



6.
THE P-T PATH OF METAMORPHISM AND AGE OF MIGMATITES FROM THE NORTHWESTERN IRKUT BLOCK (Sharyzhalgai uplift of the Siberian Platform)

V.P. Sukhorukov1,2, O.M. Turkina1,2
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: Мигматиты, ITD, PT-тренд, симплектиты, PERPLEX, U-Pb возраст циркона, палеопротерозой, Шарыжалгайский выступ, фундамент Сибирской платформы, Мigmatites, ITD, P-T path, symplectites, PERPLEX, U-Pb zircon age, Paleoproterozoic, Sharyzhalgai uplift, Siberian Platform basement

Abstract >>
We study migmatized aluminous gneisses in the northwest of the Irkut granulite complex in the southeastern Sharyzhalgai uplift of the Siberian Platform basement. Migmatized gneisses with the mineral assemblage Grt + Sil + Bt + Kfs + Pl + Qz (+ Crd + Opx + Spl) contain a leucosome and widespread cordierite-bearing (+ orthopyroxene, quartz, and spinel) symplectites developed after garnet and sillimanite. Study of the microstructural relationships of minerals and modeling using the PERPLEX 672 software have shown a retrograde P - T path of metamorphism for the metasedimentary gneisses, close to the isothermal decompression (ITD). The parameters of the peak of metamorphism are T = 850-870 ºC and P ≥ 7 kbar. The weighted average age of zircon from the metasedimentary gneisses (1856 ± 13 Ma, SHRIMP) corresponds to the time of metamorphism. The decompression type of retrograde metamorphism of the rocks in the northwest of the Irkut block indicates their formation in the crust extension and thinning setting. The presence of domal structures in the section of the Irkut block on the shore of Lake Baikal suggests that the dome tectogenesis was involved in the exhumation processes. The Paleoproterozoic metamorphism and granite formation were associated with the same stage of collision processes, when the compression setting was changed by an extension one (1.88-1.85 Ga).



7.
MAGNETIC VISCOSITY EFFECT ON TEM DATA OF AN ARRAY WITH A FIXED TRANSMITTER LOOP

N.O. Kozhevnikov1,2, E.Yu. Antonov1
1A.A. Trofimuk Institute of Petroleum Geology and Geophysics, 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: Метод переходных процессов, магнитная вязкость, двухслойная модель, установка с закрепленным источником, TEM method, magnetic viscosity, two-layer model, fixed source array

Abstract >>
In this paper, we present and discuss the results of modeling of the induction processes observed at different distances from the center of a transmitter loop in studies of a two-layer earth in which the magnetic susceptibility of the top layer or the base depends on frequency. Profiling graphs (the offset is plotted on the abscissa, and the EMF induced in the receiver coil at a fixed time is plotted on the ordinate) show that the polarity of the EMF changes as the offset increases. For the model with a magnetic layer of any thickness, the polarity of the EMF reverses immediately after the receiver crosses the loop wire. For the model with a magnetic base, the offset at which the EMF polarity reversal occurs is the larger the greater the thickness of the layer. For both models, the EMF at a fixed time depends on the thickness of the layer and the offset. Particularly strong dependence of the EMF on layer thickness or offset is observed at the loop side. Therefore, measurements near the wire make it possible to determine the thickness of the layer in the case where it is much less than the length of the loop side. The overall effect of magnetic relaxation and transient eddy currents leads to the fact that small changes in the layer thickness or offset can lead to a dramatic change in the transient response.



8.
DETAILED PALEOSEISMOLOGICAL RESEARCH ON THE FLANK OF THE LAKE IMANDRA DEPRESSION (Kola region): NEW APPROACHES AND RESULTS

S.B. Nikolaeva1, A.A. Nikonov2, S.V. Shvarev2,3, M.V. Rodkin4,5
1Geological Institute of the Kola Science Center, Russian Academy of Sciences, ul. Fersmana 14, Apatity, Murmansk, Region, 184209, Russia
2Schmidt Institute of Physics of the Earth, Russian Academy of Sciences, ul. Bol. Gruzinskaya 10, Moscow, 123242, Russia
3Institute of Geography, Russian Academy of Sciences, Staromonetnyi per. 29, Moscow, 119017, Russia
4Institute of Earthquake Prediction Theory and Mathematical Geophysics, Russian Academy of Sciences, ul. Profsoyuznaya 84/32, Moscow, 117997, Russia
5Institute of Marine Geology and Geophysics, Far Eastern Branch of the Russian Academy of Sciences, ul. Nauki 1B, Yuzhno-Sakhalinsk, 693022, Russia
Keywords: Палеосейсмодеформации, палеосейсмичность, палеоземлетрясения, сейсмотектоническая зона, Кольский регион, Фенноскандинавский щит, Кольская АЭС, Палеосейсмодеформации, палеосейсмичность, палеоземлетрясения, сейсмотектоническая зона, Кольский регион, Фенноскандинавский щит, Кольская АЭС, Paleoseismic deformations, paleoseismicity, paleoearthquake, seismotectonic zone, Kola region, Fennoscandian Shield, Kola Nuclear Power Plant

Abstract >>
We present results of detailed paleoseismological research at the key site on the flank of the Lake Imandra depression (Kola region, northeast of the Fennoscandian Shield). Study of various groups of paleoseismic deformations in the fault zone and application of new methods and approaches made it possible to recognize a segment of a large seismotectonic zone where violent earthquakes occurred repeatedly at the end of the Late Glacial and in the Holocene. The dates of earthquakes and the location of their foci are determined.