Home – Home – Jornals – Russian Geology and Geophysics 2001 number 4

The recent decades have become a "golden age" for lithology and marine geology. The principal achievements include investigation and mapping of oceanic bottom sediments and studies of the sedimentary material in the atmosphere, rivers, seas, and oceans, in ice and snow; development of a new approach to the role of biogenic processes; analysis of endogenic material supplied to the mid-oceanic ridges; and changes in the view of sedimentary material of the land provenance and its transformation at the river-sea boundary. Another achievement is the concept of rapid ("avalanche") sedimentation at several levels, responsible for concentration of 93% of the sedimentary material of the Earth in limited basins, which become major reservoirs of hydrocarbons. The regions of avalanche sedimentation alternate with those of sedimentation gaps.
The lithology is gaining a more solid quantitative basis, using such dynamic parameters as flux and vectors of sedimentary material.
It has been found out that sedimentation in oceans follows its specific laws, as the oceanic crust differs from the continental crust. Modern lithological studies employ remote sensing methods (seismic stratigraphy, magnetometry, multiray sea beams, side-view sonars, etc.), as well as mapping and special sampling from natural bottom outcrops at depths of to 6000 m with the use of submersibles.
Deep-sea drilling experiments (since 1968) and data processing performed by the joint efforts of international teams became an important step forward. As a result, the amount of data obtained in the course of the recent 30-40 years is hundreds or thousands of times as great as that accumulated through the whole previous history of the science, though lithologists, unfortunately, use very few of these data. The recent results led to numerous discoveries and creation of the mobilistic lithology, or the lithology of lithospheric plates, and required the synthetic use of the data of tectonics, geophysics, marine geology and geochemistry, petrology and petrochemistry, oceanology and climatology.

Marine and oceanic carbonate accumulation in the Vendian-Cambrian was a biochemogenic process, i.e., was caused mainly by the vital activity of organisms that formed a geochemical environment favoring carbonate sedimentation. In the early Paleozoic, this process became completely biogenic. Later on, till the end of the Paleozoic, carbonates accumulated mainly in vast shelf seas, and in the Mesozoic, in ocean shoals, reefs, and pelagic zones. In the Cenozoic, pelagic zones and, to a lesser extent, reefs became the main accumulators of carbonates. It is shown that the carbonate accumulation process depended on the atmosphere composition.

Modern notions of phosphorite genesis are reviewed, with emphasis on the role of A. L. Yanshin's studies. We have recognized three stages in the development of the theory of phosphorite genesis: biogenic; orthochemogenic (according to A. V. Kazakov's theory); and modern, biochemogenic-diagenetic. We considered the features of phosphorites belonging to major genetic types, analyzed the global distribution of the resources of marine phosphorites with time, and revealed two main phosphorite accumulation epochs – Vendian-Cambrian and Cretaceous-Paleogene. The structure-tectonic conditions of phosphorite deposition proceeding in global tectonomagmatic epochs are considered. It is shown that the largest deposits are confined to shelves (land margins).
The phosphorite deposition zones are connected with the ocean, partially isolated, or occur in the internal continental areas. These are mainly shallow-water zones with enhanced bioproductivity, peneplain provenance of the terrigenous sediment component, and arid climate.
nderstanding of phosphorite genesis requires considering the peculiarities of various types of phosphorites and changes of the process of phosphate mineralization with evolution of geologic processes. The most probable mechanisms and conditions of phosphorite genesis are outlined.

The recent concepts on the processes of marine phosphorite genesis are discussed. Modern and ancient phosphorites show significant differences in facies and paleogeographic aspects. The most important factor for phosphorite formation is the process of replacement of biogenic material by phosphate, which to a certain degree allows matching the hypotheses of phosphorite genesis in spite of the difference in the geologo-mineralogical and structural parameters of marine phosphorites.

The idea of the relation between phosphorite genesis and weathering is based on the corresponding concepts of N. S. Shatskii. V. P. Kazarinov accepted this idea and substantiated correlation of the epochs of intense phosphorite formation with the epochs of deep chemical weathering. G. I. Bushinskii had a similar, though not quite identical, opinion that phosphorus involved in the formation of phosphorite-bearing deposits and basins is supplied by rivers of the nearby plains in the regions of humid climate. Today many researchers hold to the idea of the significant importance of weathering in phosphorite genesis. This idea has received further support for many global epochs of phosphorite formation (Early Ordovician, Late Jurassic, Early Cretaceous, Late Cretaceous, Paleocene, Eocene, Miocene, etc.) and is treated as reasonably justified. At the same time, some epochs of deep chemical weathering (e.g., Late Devonian) are characterized by high contents of dispersed phosphorus in rocks, which, however, did not participate in the formation of commercial phosphorite deposits.

Academician A. L. Yanshin, who paid much attention to studies and use of mineral products of agrochemical purpose, appreciated the works on bog phosphates started in the early 1980s. He treated them and other mineral accumulations in peats as a serious basis for improving the structure of soils and increasing the fertility of Siberian lands.
In recent years, high contents of bog phosphates, bog carbonates (alm), and sapropel have been found in lowland peat deposits in Siberia, which is the largest peat territory of the world.
Prediction studies including analysis of the composition of peats, their substrate, underground waters feeding peat deposits, and other characteristics provided insignt into the geochemical processes of bog lithogenesis and the character and sequence of peat mineral formation.
Assessment-prospecting and exploration works have been carried out in the revealed promising areas and some objects. The assessed and predicted resources of bog phosphates, alm, and sapropel indicate that West Siberia is rich in organic mineral products, which might compensate for a deficiency of phosphorus and calcium in Siberian soils and improve their structure. The lines for judicious usage of resources are suggested.

Support is given to a two-stage model for the salinity of the cover of the Siberian Platform. Cambrian and Devonian halmeic formations are recognized in the section and traced laterally. Lower and Middle Paleozoic halmeic basins were limited by intermediate zones providing a persistent regime of salt accumulation and supply of mother solutions from the vast seas covering the territory of the Patom-Vitim Upland and Upper-Yana region. Parameters are given, and paleostructural conditions of accumulation of these formations are characterized. In some regions and districts of the Lena-Tunguska petroliferous province it has been observed that the industrial production of subsalt reservoirs depends directly on the regional persistence and thickness of the saliferous screen. The largest deposits occur in zones of thick terrigenous sequences overlapped by regional carbonate-saliferous confining beds. Hydrocarbon deposits have not been revealed in the Middle Paleozoic complex yet, though oil, gas, and bitumen occurrences exist there and the geologostructural conditions (reservoirs, brachyanticlinal traps, etc.) are favorable for hydrocarbon accumulation. Taking into account regular relationships in the Vendian-Cambrian complex, we suppose that the Devonian halmeic formation is also appropriate for accumulation and preservation of hydrocarbons in subsalt and intersalt reservoirs.
The recognized structural features of Paleozoic halmeic formations, regional confining beds, permitted us to specify the earlier schemes of petroleum-geological regionalization of the Siberian Platform. In the Vendian-Cambrian complex, the main petroliferous belt is the Nepa-Botuobiyan and Baikit anteclises separated by the Katanga Saddle. In addition, the Berezovo, Kempendyai, and Ygyatta troughs, Suntara uplift, and favorable structures of the central regions of the Angara-Lena trough are estimated as highly promising. The Devonian subsalt and intersalt reservoirs are of greatest practical interest in the favorable zones of the West Vilyui potentially petroliferous region. The Atyyakh, Uchugei, and Kedepchik structural stages are top-priority objects for seismic prospecting and deep drilling there.

The bottom deposits of a wedge-like complex in the Northern Ob' region were subjected to sedimentation analysis. On the basis of core material, several lithofacies have been recognized within the Achimov sequence: 1) massive and stratified sandstones; 2) gradational stratified and finely cross-bedded sandstones; 3) massive poorly graded siltstones with clay intraclasts; 4) gradational siltstones and mudstones. Differences in composition and structure of the recognized lithofacies were governed by the specific dynamics of sedimentation. The massive and stratified sandstones (lithofacies 1) making up a considerable volume of sand members distinguished within the Achimov sequence are interpreted as deposits of high-density sandy turbidite flows. The gradational stratified and finely cross-bedded sandstones (lithofacies 2), whose structure corresponds to the Bouma cycle, are considered classic sandy turbidites that were formed by low-density sandy turdidite flows between channels on an underwater slope and at its base or in underwater channels at the final stage of their filling. The finely bedded gradational siltstones and mudstones (lithofacies 3) are interpreted as fine-grained turbidites and hemipelagites formed by low-density fine-grained turbidite flows and hemipelagic sedimentation. The structure of gradational siltstones and mudstones is well described in terms of the Stow sequence. The massive poorly graded siltstones with clay intraclasts (lithofacies 4) are deposits of fine-grained debris flows having developed within the slope apron and between channels on underwater fans.
Sedimentation analysis shows an important role of processes of gravitational transportation of sediments on accumulation of the Achimov sequence, which is in agreement with the idea that the lower part of the Neocomian sedimentary complex has a wedge-like structure.

The evolution of biosphere included the following processes: (1) emergence of new ecologically specialized groups (guilds), providing a more efficient use, transfer, and transformation of matter and energy in ecosystems; (2) areal expansion of life throughout the Earth (gradual transition of the biosphere from discrete to continual on exploration of new bionomic zones and biotopes; (3) complication of the trophic structure of ecosystems (from simple Archean autotrophic-heterotrophic procaryotic systems to the modern global ecosystem); (4) variations in the spatial and energetic indices of biogeochemical cycles. In this context, the Ordovician can be regarded as one of the greatest critical stages in the biosphere evolution. In the Ordovician, the emergence of new taxa (ecologic guilds) with better trophic adaptability in benthic associations and settling of pelagic zones in euphotic sea areas resulted in dramatic changes in marine ecosystems, which predetermined further evolution of marine biotas. The chief evolutionary strategy of Precambrian marine organisms was to improve adaptation to physicochemical environmental settings by complication of biological organization and separation of metabolic and reproductive functions within a body. In the Early Cambrian, main phyla of marine invertebrates emerged, and multistage trophic realationships between autotrophs and heterotrophs, with division of ecologic functions, began to form. Adaptation to the biotic environment became as evolutionarily important as adaptation to abiotic conditions. Starting in the Ordovician, the ecologic mechanisms of organism interaction became the key factor of the evolutionary strategy in biota associations owing to the gradual stabilization of the abiotic indices in sea basins.
New edificator groups first appeared in abundance in the Ordovician and reached their acme in the Middle Ordovician: articulate brachiopods and sessile colonial (tabulates, tetracorals, heliolitoids, and stromatoporates), aggregated (crinoids), and colonial-aggregated (bryozoans) filter-feeding organisms with carcass skeletons. This resulted in a breakdown of biotopes and complication and heterogeneity of food webs. The lowest trophic level was dominated by ostracods, first small hydrobiontic universal eaters simultaneously belonging to several trophic levels and capable of a deeper transformation of organic matter. In the Ordovician, the pelagic zone became a constant rather than a facultative, as before, habitat for zooplanktonic and nektonic organisms: graptolites, radiolarians, conodontoforids, nautiloids, meroplankton (mainly larvae of colonial organisms and brachiopods), pelagic trilobites, ostracods, and early primitive fishes. In the Ordovician, a spatial rearrangement of the lowest trophic level - major producers - took place. This had a dramatic effect on the stage and lateral structure of trophic chains. Until the early Middle Ordovician, bottom cyanobacterial associations, or meadows, were widespread in Late Precambrian and Early Paleozoic epicontinental seas and were main photosynthesizing producers. At the Early-Middle Ordovician boundary, the areas of these meadows decreased, and phytoplankton became the main producer. The global ecologic event was accompanied by the greatest (in the Phanerozoic) burst of the diversity of Ordovician marine biotas followed by rapid stabilization. Later the stability was maintained by a phylogenetic succession of ecologically equivalent taxa supplemented by replacement of some ecologic guilds at critical borderlines.
Thus, in the Ordovician, marine ecosystems became multistage, their trophic structure became more complex, and a global closed biogeochemical cycle formed for the first time throughout the sea area. The Ordovician global biotic events matched large-scale geologic events (abrupt climatic changes, maximum range of transgressions and regressions of epicontinental seas, changes in the Mg and Ca balance in marine sediments, increase in the content of oxygen in the Earth's atmosphere and hydrosphere, and formation of the ozone screen). It is supposed that the appearance of the ozone screen and increase in the content of oxygen in sea water had a determining impact on the settling of the pelagic zone by heterotrophs and formation of coherent (ecologically complete) benthic ecosystems. At the initial metastable stage of development of the ozone screen, dramatic fluctuations of biodiversity in bottom and pelagic associations were determined by profound changes in spatial parameters of sea shelves, the main habitat of life, which were caused by eustatic fluctuations of the World Ocean level. The Late Ordovician extinction of marine biotas resulted from an abrupt shrinkage of the shelf habitat caused by a lowering of the World Ocean, which, in turn, resulted from the fixation of great volumes of water in continental glaciers after the Ordovician transgression maximum.

The warm biospheres of the Late Cretaceous and Early Triassic interglacials existed alternately in the humid or arid state. The stages of humid and arid climate differed considerably in their paleogeography and sedimentation regimes and showed dissimilar climatic zonation. The Late Cretaceous humid belts covered up to 75% of the land. The climate humidity was caused by the opening of oceans, large-scale transgression, and formation of large shelf and epicontinental seas, as well as by the fact that the land occupied a small area and included intracontinental lowland peneplain. The global spread of warm humid climate in the Late Cretaceous was favored by the existence of the Tethys and circum-global western currents in the tropic latitudes of the Northern Hemisphere. The Early Triassic climate was mostly arid (arid and semiarid belts occupied up to 80% of the land) because of the existence of the Pangea supercontinent, with its high hypsometric level, marginal and intracontinental mountain systems, and elevated plateaus separated by undrained areas. Investigation of geological, geochemical, and biological consequences of humid and arid climate stages may provide better understanding of the geologic and climatic history of the Earth.

Spore-and-pollen associations from moraine, lacustrine, and glaciolacustrine deposits of the last Quaternary (Sartan) Glaciation are considered. Three types of plant formations existing at the glaciation maximum (20-18 ka BP)

Based on geodynamic data, we consider paleogeographic positions of basins where sediments rich in organic matter (black-shale facies) accumulated in three Devonian epochs. A brief description is given to black-shale formations (series, members, and horizons) and the position of zones of their accumulation in the basins and in the cycles of sedimentation. We study the evolution of the geologic positions of these zones and their dependence on the arrangement of continents and the sequence of transgressive events.
In the Early Devonian, when the continents were located relatively close to each other, black-shale sedimentation basins existed on their periphery, in the area transitive between shoal and pelagic zones. In the Middle Devonian, such basins also appeared in pre-orogeny zones, and black-shale formations accumulated in more shallow-water settings as compared with the Early Devonian black shales. In the Late Devonian, the shelf areas expanded, and the number of black-shale zones and their areas considerably increased. In stratigraphic sections, black-shale facies occur in the middle parts of sedimentation cycles, which are the most transgressive and subsided.

Sedimentary basins have been characterized from the viewpoint of their genesis, and relevant physical mechanisms have been proposed. These mechanisms are involved with mantle convection and diapirism, which provide vertical and horizontal movements forming sedimentary basins.

The highest and longest ridges of the East Yakutian mountainous province exist on its periphery as the Upper Yana system in the west and the Chersky and Moma systems in the east. Northward, the ridges give way to the Primor'ye lowlands that grade into the Laptev Sea shelf separated from the Eurasian ocean basin by the continental slope. The ridges of the Upper Yana system make an asymmetric arch dislocated by younger normal faults. The system of Chersky and Moma ridges is likewise an arch cut with strike-parallel rift basins (Moma, Upper Selennyakh, etc.). The Cenozoic fill of the basins adjacent to the ridges provides evidence that rapid uplifting of the latter started in the Oligocene. Doming in the Late Miocene-early Pliocene was accompanied by compression, which produced imbricated thrusts with horizontal displacements of up to several kilometers. In the Late Pliocene-Early Pleistocene, the crust of the region experienced a large-scale extension responsible for the opening of the Moma rift and normal fautling in the Upper Yana ridge.
he growth of mountain ridges in Eastern Yakutia may be related to the interaction of the Eurasian and North American plates in the Cenozoic and the opening of the Arctic Eurasian ocean. Multiple changes in the position of the plates' poles through the Cenozoic caused alternation of extension and compression regimes. The opening of the Eurasian basin and the formation of mountain ridges in Eastern Yakutia were, apparently, nearly coeval geodynamic events.

Different stages of evolution of geological structures are recorded in time dynamics of their gravity field. The regularities of the crust evolution on the territory of Belarus were investigated through claculation and geological interpretetation of the gravity field dynamics in a succession of density models that reflect the main stages of the tectonic history of the region: Early Archean-Late Archean-Early Proterozoic-Riphean-Vendian-Late Paleozoic (Devonian-Carboniferous).