The evolution of the southern margin of the East European Craton based on seismic and potential field data

This paper presents an integrated geophysical study of the southern margin of the East European Craton (EEC) in the Karpinksy Swell-North Caucasus area. It presents new interpretations of deep refraction and wide-angle reflection “deep seismic sounding” (DSS) data as well as conventional seismic and CDP profiling and new analyses of potential field data, including three-dimensional gravity and magnetic modelling. An integrated model of the physical properties and structure of the Earth's crust and, partially, upper mantle displays distinct features that are related to tectonic history of the study area. The Voronezh Massif (VM), the Ukrainian Shield and Rostov Dome (RD) of the EEC as well as the Donbas Foldbelt (DF), Karpinsky Swell (KS), Scythian Plate (SP) and Precaspian Basin (PCB) constitute the geodynamic ensemble that developed on the southern margin of the continent Baltica. There proposed evolutionary model comprises a stage of rifting during the middle to late Devonian, post-rift extension and subsidence during Carboniferous–early Permian times (synchronous with and related to the southward displacement of the Rostov Dome and extension in a palaeo-Scythian back-arc basin), and subsequent Mesozoic and younger evolution. A pre-Ordovician, possibly Riphean (?), mafic magmatic complex is inferred on a near vertical reflection seismic cross-section through the western portion of the Astrakhan Dome in the southwest part of the Precaspian Basin. This complex combined with evidence of a subducting slab in the upper mantle imply the presence of pre-Ordovician (Riphean?) island arc, with synchronous extension in a Precaspian back-arc basin is suggested. A middle Palaeozoic back-arc basin ensemble in what is now the western Karpinsky Swell was more than 100 km to the south from its present location. The Stavropol High migrated northwards, dislocating and moving fragments of this back-arc basin sometime thereafter. Linear positive magnetic anomalies reflect the position of associated faults, which define the location of the eastern segment of the Karpinsky Swell. These faults, which dip northward, are recognised on crustal DSS profiles crossing the Donbas Foldbelt and Scythian Plate. They are interpreted in terms of compressional tectonics younger than the Hercynian stage of evolution (i.e., post-Palaeozoic).     

The first finding of reliable Jurassic radiolarians in the Crimea

Radiolarians of Leugeonidae Yang et Wang, 1990, which represent a morphologically distinctive group of spherical radiolarians of the Spumellaria order, were found for the first time in Crimea and reliably confirm the Jurassic age of the finding. The nodules, which host the Jurassic radiolarians, were collected by A.S. Alekseev in 1983 in the terrigenous sequence of the Lozovskaya tectonic zone. The radiolarian assemblage in the nodules includes Levileugeo ordinarius Yang et Wang, Triactoma jonesi Pessagno, Pseudocrucella aff. prava Blome, Paronella kotura Baumgartner, P. ex gr. mulleri Pessagno, and Praeconocaryomma sp. The Levileugeo genus is easily identified due to its unique hexagonal element, which is typical only of the Jurassic, in particular, Upper Bajocian–Lower Tithonian radiolarians.     

The Vendian Vorogovka Group of Yenisei Ridge: Chemostratigraphy and results of U–Pb dating of detrital zircons

Application of the method of isotope stratigraphy showed that the ⁸⁷Sr/⁸⁶Sr ratio in the least altered carbonate rocks of the Severnaya Rechka and Sukhaya Rechka formations in the Vorogovka Group of Yenisei Ridge ranges from 0.70813 to 0.70828, and δ¹³C varies from –0.7 to +1.8‰. Comparison of these data with similar data for the Late Precambrian sequences from other regions provides evidence for the Vendian age of the Vorogovka Group. This is supported by the results of LA ICP-MS U–Pb dating of detrital zircons from sandstone of the Severnaya Rechka Formation, which is the basal level of the group: the mid-weight age of the youngest zircon population is 584 ± 3 Ma.     

The Vendian passive continental margin in the southern Siberian Craton: geochemical and isotopic (Sr,Sm–Nd) evidence and U–Pb dating of detrital zircons by the LA-ICP-MS method

Geochemical and isotopic (Sm–Nd and Sr) studies of deposits of the Baikal and Oselok Groups in the southern Siberian Craton and LA-ICP-MS U–Pb dating of detrital zircons show that they accumulated in passive continental-margin settings in the Vendian. The time limits of sedimentation were assessed on the basis of Sr chemostratigraphy of carbonate deposits of the Baikal Group and LA-ICP-MS U–Pb dating of detrital zircons in first-cycle terrigenous deposits of the Oselok Group. The main provenances for rocks of these groups were constant. These were rocks of the cover and basement of the Siberian Craton. Tuffite horizons in upper portions of the groups are the only sign of Late Vendian activation of this block, which is reflected in changes of geochemical indices of terrigenous rocks and their younger Sm–Nd model ages.     

Global Climate Change and the Temperature Regime of the Mesozoic Marine Environment in Northeastern Asia

A combination of the facies-geodynamic and radiolarian methods allowed us to determine the morphometric and temperature conditions for the initial accumulation of the Mesozoic siliceous-volcanic allochthonous complexes from the tectonic slabs of the Okhotsk–Koryak Orogenic Belt, which involves different marginal marine and ocean water areas and heterogenous marine rises. The radiolarian analysis made it possible to estimate approximately the paleolatitudinal position of these complexes and to reveal the pattern of the Mesozoic climate change in the Pacific and worldwide: from the warm Triassic through the Jurassic cooling to the Cretaceous optimum; the stages of warming demonstrate the dependence on global superplume episodes.     

Sr isotopic characteristic of neoproterozoic carbonate sediments from the southern Yenisei Ridge

This paper presents the first Sr isotopic data for the Late Precambrian carbonate rocks of the southern Yenisei Ridge. Their geochemical study allowed estimation of the degree of secondary alterations and gave the possibility to reveal rocks with a less disturbed Rb-Sr isotopic system. The Sr isotopic data indicated Neoproterozoic sedimentation of the rocks about 1070–750 Ma ago. Sr and C isotopic data showed that carbonate rocks of the Sukhoi Pit, Tungusik, and Shirokino groups are Late Riphean and could be comparable with sedimentary sequences of three Precambrian key sections of the Northern Eurasia: the subsequent Derevnino, Burovaya, and Shorikha formations from the Turukhansk Uplift, the Lakhanda Group from the Uchur-Maya region, and the Karatav Group from the South Urals. All studied carbonate rocks are older than 750 Ma and, according to the International Stratigraphic Chart, accumulated prior to global glaciations in the Cryogenian. This is evident from sedimentological study indicating the absence of tillite horizons in the studied sections. δ¹³C values in the sections vary from +0.4 up to +5.3‰, which testifies to the absence of periods of great cold.     

Allochthonous Mesozoic marine sequences of northeastern Asia and western North America: Correlation of the stratigraphic levels and geodynamic depositional settings

The problem of correlating the isolated outcrops of allochthonous Middle Mesozoic deposits developed in northeastern Asia and western North America includes several aspects: (1) the stratigraphic subdivision of the sequences using the radiolarian assemblages and taking into account the complex nappe-thrust structure of the region; (2) the analysis of their facies composition with reconstructing the geodynamic depositional settings; (3) the search for features in common (with respect to the previous two aspects) in all these localities. The first widely applied radiolarian analysis revealed that the Middle Mesozoic marine rocks from separate nappes and slices of tectono-stratigraphic sections in East Asia enclose 20 different-age (Norian to Hauterivian) radiolarian assemblages. The correlation of these assemblages revealed the wide lateral distribution of their host Middle Mesozoic marine sequences in the regions under consideration. The significance of the presented materials is determined by the following facts: (1) using the radiolarian analysis, the Middle Mesozoic marine sequences of northeastern Asia are first subdivided into stage and substage units with defining their analogs in western North America; (2) despite the discrete distribution of Mesozoic allochthons, it is shown that the defined stratigraphic units are widespread in northeastern Asia; (3) it is established that these allochthons consist of rocks formed in different geodynamic depositional settings: ocean floor, island arc, fore-arc, and marginal-sea domains. This spectrum of heterogeneous rocks is traceable practically through the entire northeastern Asia region.     

Upper cretaceous radiolarians of the East European platform and their biostratigraphic significance

Radiolarian assemblages from the Upper Cretaceous carbonate-cherty deposits of the East European platform are analyzed. Biostratigraphic subdivisions ranked as radiolarian beds are distinguished in sediments of the Moscow syneclise, Voronezh anteclise, and the Ul’yanovsk-Saratov depression. The correlation between biostratigraphic beds established in three tectonic structures and variants of their coordination with radiolarian subdivisions in southern and northern areas of Russia are considered. The key importance of radiolarians for the subdivision and correlation of the Upper Cretaceous cherty deposits is demonstrated.     

Upper Cretaceous deposits in the northwest of Saratov oblast,Part 1: Litho- and biostratigraphic analysis of the Vishnevoe section

The work is dedicated to multidisciplinary study of Upper Cretaceous deposits exposed at the day surface in a ravine near the village of Vishnevoe, the Petrovsk district of Saratov oblast. The exposed section includes deposits of the Bannovka, Mozzhevelovyi Ovrag, Mesino-Lapshinovka, Rybushka, Ardym, Lokh formations and of the Borisoglebsk sequence first distinguished in the Volga River basin. Age ranges of the formations studied are confirmed or defined more precisely (the Ardym and Lokh formations) based on fossil faunas of cephalopods, bivalves, radiolarians, planktonic and benthic foraminifers. The middle-upper Coniacian range of the Borisoglebsk sequence is substantiated. Distribution of brachiopods, sponges, radiolarians, ostracodes and calcareous nannoplankton in the section is established. Radiolarian assemblages are used to distinguish biostratigraphic subdivisions corresponding in rank to faunal beds. Based on nannofossil assemblages, zones and subzones of standard zonations after Perch-Nielsen (1985) and Burnett (1998) are established. Stratigraphic ranges of certain radiolarian, ostracode and calcareous nannoplankton taxa are verified.