Home – Home – Jornals – Russian Geology and Geophysics 2011 number 10

Chronostratigraphic space is defined as information about the geologic history and paleobiosphere that elucidates the Earth's evolution in the interaction of different layers of the geosphere. Stratigraphic subdivisions act as an information-carrying medium. Elementary units in the Phanerozoic chronostratigraphic space are biostratigraphic zones, whereas in the Proterozoic space, sequenthemes. The Vendian sequentheme has a unique paleontological characterstic, which continuously expands and is refined. Its lower boundary determines the top of cryogenic suberatheme and is marked by deposition of the last diamictite (Varangerian, Marinoan) and the largest postglacial transgression. In the chronostratigraphic space, the Vendian is surely Neoproterozoic but constitutes a single acrochrone of the biosphere evolution with the Phanerozoic. The Vendian stage divides and connects two different-sized intervals in the evolution of Geomerida: a long interval of time (from the origin of life to the beginning of the Vendian) marked by a predominance of prokaryotic ecosystems and a relatively short interval with the incredible biodiversity of modern-type ecosystems dominated by eukaryotes.

The hierarchical structure and conceptual bases of the International Stratigraphic Сhart (ISS) were created during the first eight sessions of the International Geological Congress (IGC) (1878-1900) by the systematization and unification of mainly the West European regional stratigraphic references, except for the Permian and, partly, Carboniferous systems with stratotypes in the Russian territory. In the next age, the structure was refined and supplemented on the basis of the same principles. Since the middle 1980s, a radical ISS reforming has been performed in the framework of the International Commission on Stratigraphy (ICS), which revealed the contradictory tendencies in the development of modern stratigraphy. They are expressed first of all in the principal difference between the new methodology of the "rational" substantiation of global stratigraphic references adopted by the ICS and the real practice of the development of their regional equivalents based not on the choice of chronometric markers for global correlations but on the revealing of natural geohistorical stages in the evolution of ecosystems, using the total set of paleontological, sedimentological, and isotope-chronometric data. The new approach is based mainly on the concept of the self-sufficiency of limiting types, i.e., monotaxon markers for graduating the ICS, which replace the stratotypes of subunits, their integral complex characteristics reflecting the trends and periodicity of the Earth's evolution. The negative consequences of this "rational" substantiation and revision of the stratigraphic references of the ICS are particularly obvious by the example of innovations in the subdivision of the Cambrian and Ordovician systems on the comparison of the new series and stages adopted by the ISC with the global ecosystem rebuildings in the Early Paleozoic.

The present-day notions of the Paleogene history, paleogeography, and paleobiogeography of Central Asia middle latitudes are based on studies carried by A.L. Yanshin in the second quarter of the 20th century. Here, main phases in the geologic history of the West Siberian and Turan Plates and Turgai depression are considered. In the Paleocene and Eocene, these regions were key links of a continuous meridional marine communication system connecting the Tethys and Arctic Oceans. Before the emergence of the latitudinal Alpine-Himalayan orogenic belt, the Tethys and its marginal seas formed a united shelf area. The closest linkage of the water bodies and biota exchange between them existed in the Thanetian and in the first half of the Ypresian. There was intense heat transfer from tropical to higher latitudes through the entire system of epeiric seas and straits. From the end of the Paleocene, this system was supplemented and complicated by latitudinal straits that ensured the connection of the seas of the Northern Peri-Tethys with the Northern Sea basin and the Atlantic. The combination of two sea systems determined the climatic history of this region from the Paleocene till the Late Eocene. The Arctic Basin influenced mainly the structure of cold-water benthos, and the Tethys, the composition of planktonic biota in the photic zone. During the Bartonian and Priabonian, in the last phase of marine sedimentation, the West Siberian epeiric sea was completely isolated from the Arctic Basin and connected only with the Turan sea through the Turgai strait. The Azolla beds accumulated during the low stand of the World Ocean in desalted surface waters and in disoxic bottom waters inhabited by depauperated benthos. At the Eocene/Oligocene boundary, the drainage of the Tavda-Chegan sea was followed by the formation of an N-S-directed river network in the vast areas of West Siberia, Turgai, and the northern cis-Aral region. The climate was unstable, moderately warm to subtropical, with variable humidity. The formation of the Turgai ecotype of mesophytic conifer-broadleaved flora was completed by the end of the Early Oligocene.

Seismostratigraphic analysis of Paleogene deposits in northeastern Ustyurt was carried out. Four seismostratigraphic complexes have been recognized: Paleocene-Ypresian, Lutetian-Bartonian (Tasaran unit), Bartonian-Priabonian (Saksaul'skaya-Chegan), and Oligocene. We have established for the first time the clinoform structure of the Tasaran and Saksaul'skaya-Chegan complexes, which explains various concepts of their age. Seismostratigraphic analysis must be an integral part of stratigraphic research, which will significantly increase the reliability of correlation between geologic sequences.

Recent studies of the continental sources of aerosol production, transport, and deposition to the ocean (natural sink) allowed us to recognize the possibility of long-range high-altitude (transoceanic) transport of aerosol dust of specific composition and properties. The dust consists of fine (micro- and nanosized) particles (94%, less than 2 ?m) originated in the arid (undrained) tropical and subtropical regions extending into the oceans and are similar in composition to deep-sea (pelagic) red clays. Satellite and aircraft observation data were used to track trajectories of long-range (and transoceanic) transport of dust clouds. These data were coupled with direct shipboard measurements on ice core records and data on nuclear explosions and volcanic eruptions.
Several zones of arid sedimentation were identified based on climatic conditions (shortage of water), conditions of dust production, wind-blown transport at different altitudes, and deposition onto the ocean surface. The main transport occurs at two altitude scales (fr om land to the cloud top and above) (5-7 km) wh ere wind speed of 300 km/h will be critical for a long-range transport.
Three types of transport are identified based on the particle dynamics, composition and properties: 1 - local (0-10 km from the source); 2 - regional (100-1000 km); and 3 - global (over 1000 km). The finer particles are the product of local-scale transport with a total flux of 1.6 billion t/yr, which is almost equal to the net influx of the riverine terrigenous material to the pelagic zones of the oceans (outside the marginal filters). There are four main sources of aerosol dust, which is transported and deposited over arid oceanic regions. The arid oceanic regions account for about 1/3 of the modern ocean surface. During glacial periods, the sea-level drop of 100-120 m caused a significant increase in the size of arid regions as a result of the exposure of the shelf areas, which is equal to the area of the African continent. This caused 3-5 times higher dust emission and a decrease in the heat flux and in the transparency of the atmosphere. Comparison of ice core records and deep drilling data provide a basis for studies of the ancient arid sedimentation.

The first bacterial (bacillus-like) form in phosophorites was described by B. Renault and C.E. Bertrand fr om coprolites of vertebrates in the bituminous shales from the Autun region, France, in 1895. In 1990, B. Renault revealed coccoid bacteria from the same deposits. In 1983, D. Soudry and Y. Champetier described for the first time the capsules of cyanobacterial threads from Campanian phosphorites of the Negev desert, Israel. Glycocalics was first found in the Upper Jurassic-Lower Cretaceous phosphorites of the Egor'evskoe deposit on the Russian Platform, wh ere it coexists with coccoid bacteria.

Diverse stromatolitic buildings are recognized in the Meso-Neoproterozoic basins in southern East Siberia: simple (bioherms and biostromes), individual, barrier, shore reefs and reef-like structures, and large reef-like banks. These data and results of macro- and microscopic studies of stromatolites point to the leading role of microbial communities in the production of primary carbonate material. Most of the carbonate mud was supplied to the deep-water shelf zones, slopes, and plains of the Meso-Neoproterozoic East Siberian basins from shallow-water shelves. Analysis of the materials from almost all Precambrian carbonate complexes shows that the microbial communities have played the leading role in the generation of carbonate material since the Late Mesoarchean. Both the shallow- and deep-water Precambrian carbonate sedimentary systems have much in common with the Phanerozoic ones. Their diversity and development are controlled by a number of factors, which exert different effects in geodynamically different basins, and the carbonate systems of different ranks are fine indicators of various events in the basin evolution, from the low-amplitude eustatic fluctuations of the sea level to the global epochs of flooding and highstand of supercontinents.
The absence of lime-producing organisms in the Precambrian determined the similar composition of carbonate rocks and sometimes the indistinct structure-morphologic differentiation of the Precambrian organogenic buildings.
The evolution of reef systems in the Precambrian was determined first of all by the evolution of the lithosphere, which periodically led to the formation and cessation of development of basins favorable for the mass evolution of microbial communities, and by the structural complication of microbial communities producing stromatolites. The epochs of stromatolite formation reduction and destruction of sea basins correlate well with the global epochs of the formation and highstand of supercontinents.

Results of isotope (Sr, C, and O) and geochemical studies of the carbonate deposits of the Yenisei Group of the Azyr-Tal Ridge (Kuznetsk Alatau) are presented. Using the Sr and C isotope chemostratigraphy, the age restrictions for these deposits have been made. It is shown that the sediments of the Charyshtag, Bidzha, and Martyukhina Formations and the lower part of the Sorna Formation accumulated in the Late Vendian and Early Cambrian, at 580-530 Ma, and those of the upper part of the Sorna Formation, at 525-517 Ma. Their successive accumulation in the same sedimentary basin is not confirmed by the performed geochemical studies. The deposits of the upper part (members 2 and 3) of the Sorna Formation accumulated in a sedimentary basin of other type as compared with the rest underlying deposits of the Yenisei Group. For example, at least two types of shallow-water sea basins are recognized within Kuznetsk Alatau in the Late Vendian: shelf, localized within a block with a passive tectonic regime (Charyshtag, Bidzha, and Martyukhina Formations and lower part of the Sorna Formation), and oceanic, where accumulation proceeded within oceanic islands with underwater hydrothermal activity (upper part of the Sorna Formation).

Estimating the paleobasin depth is one of the most difficult problems in studying the evolution of sedimentation. Considerable depth variations in ocean and shelf paleobasins are due not only to their origin but also to the evolution of their large constituents owing to the differentiation of the dissected bottom. The Early Paleozoic volcano sedimentary and sedimentary rocks of Gorny Altai have been studied from this standpoint. They formed in the paleoceanic mounts of the Early Cambrian Kuznetsk-Altai island arc, in the Late Cambrian-Early Ordovician Altai segment of the Paleoasian Ocean, and in the outer zone of the Late Ordovician Altai shelf basin. Geological, geochemical, and lithofacies data permitted expert estimates of the absolute vertical depths of individual paleobasin fragments with siliceous sedimentation, calibrated at the final stage by bioindicator methods. The upper parts of the paleoceanic mounts in the Early Cambrian Kuznetsk-Altai island arc are presumed to occur at depths of 300-400 m; those of the paleoceanic mounts in the Late Cambrian-Early Ordovician Altai segment of the Paleoasian ocean, at depths of 500-1200 m; and the outer edge of the outer zone of the Late Ordovician Altai shelf basin, at depths of 150-500 m.

We summarize the results of the integrated lithological and ichnofacies studies of the Callovian-Oxfordian sediments in the West Siberian sedimentary basin on the basis of core samples from more than 200 wells. In the sections of various structure-facies zones, ten ichnofossils of the ichnofacies Skolithos, Cruziana, and Zoophycos have been identified and spatial variations in the species composition of ichnofacies assemblages have been traced. It has been found that lateral and vertical variations in the ichnofossil distribution agree with the regional structural patterns of the Vasyugan Horizon and are related to the Callovian-Oxfordian cyclic turnovers of the basin depositional systems.

An integrated sedimentological study of the Vasyugan Horizon in the Aleksandrovskoe arch area was performed, during which its depositional environments were reconstructed and a series of paleogeographic maps was plotted. It has been established that shallow-marine, marginal-marine, transitional, and continental depositional environments existed here at various stages. Most of the arch area is characterized by sections of the transitional Naunak-Vasyugan and Vasyugan-Naunak types; the westernmost part, by sections of the Vasyugan type; and the easternmost part, by sections of the Naunak type. The sand beds of the J₁ horizon are low-quality reservoirs, whereas those on the western slope and the northern pericline of the Aleksandrovskoe arch are of better quality.

Seismic gravity modeling along the Eurobridge-96-Eurobridge-97 DSS geotransects has shown that the Central Belarus suture zone formed in the Paleoproterozoic as a result of the subduction and collision processes at the convergent junction of the Fennoscandian and Sarmatian segments of the East European Platform. The influence of pre-Riphean processes on the subsequent tectonic events occurring on the territory of Belarus during the Phanerozoic has been established.

In the Phanerozoic, the sea depth in epeiric sedimentary basins showed considerable variations, often accompanied by regression. In periods of regression and erosion, the subaerially exposed shelf and the adjacent parts of the marine basins gave rise to numerous nonstructural (stratigraphic) hydrocarbon traps. Sea depth variations with a magnitude of up to 100-200 m and 1-3 myr long (third-order cycles) are usually attributed to the eustatic fluctuations of the sea level. To estimate their possible range, a model is proposed which describes the water depth variations as a function of eustatic fluctuations in tectonically subsiding carbonate platforms. We take into account the crustal isostatic response to the changing water load and the finite time necessary for soil and karst formation in the exposed shelf or its upper part. This model allowed analyzing data on the reference sections of the North Timan shallow-water sediments. According to the analysis, the third-order sea level changes in the Middle Carboniferous, Late Carboniferous, and Early Permian did not exceed several tens of meters. During the same period, shorter fluctuations (~100 kyr) occurred owing to the waxing and waning of large Gondwanan ice sheets. In the first half of the Bashkirian Age (Early Pennsylvanian), regression took place in the East European and North American cratons and then shallow-water sedimentation resumed. This regression is usually attributed to a considerable sea level fall. In some other areas, slow shallow-water sedimentation continued throughout the Bashkirian. This suggests that the Bashkirian regression was due to the crustal uplifts. Short-term uplifts can be explained by ascending convective currents beneath the asthenosphere. In southern North America, they brought an active fluid into the lithosphere. This caused rapid eclogitization-related crustal subsidence in the Arkoma and Anadarko basins as well as intense lithospheric weakening and shortening in the Ouachita Fold Belt.

Structural and isopach maps of seismogeologic megacomplexes were constructed on the basis of an integrated interpretation of seismic prospecting, GIS, and deep-drilling data. The tectonic evolution of the northwestern part of the Kaimysovy arch and adjacent areas of the Yugan megadepression in the Mesozoic-Cenozoic was analyzed, and the main stages of formation of structures of different orders were established. An analysis of geological and geophysical data shows that the Larlomkiny and Pervomaiskoe-Vesennii swells have radically different formation histories despite their similar modern tectonic structure.
The results of the investigations suggest that most of the local uplifts in the Bazhenovka Formation, in particular, the Larlomkiny structure bearing an Upper Jurassic oil pool, formed in the Neocomian as a result of the inherited growth of erosional-tectonic salients (ETS) of the pre-Jurassic basement, and then developed only slightly. The large tectonic structures of the first order - the Kaimysovy arch and the Yugan megadepression and the Pervomaiskoe-Vesennii swell - a big trap associated with one of the largest oil fields of southeastern West Siberia, completed their formation in the Cenozoic.