Ancient Platforms Research Articles

TECTONIC CONDITIONS FOR OIL AND GAS GENERATION AND DISTRIBUTION ON ANCIENT PLATFORMS

This paper deals with the influence of tectonic factors on teh distribution and generation of oil and gas accumulations in oil‐and gas‐bearing basins ancient platforms. Threee main platform evolutionary stages, with their corresponding structural‐formational associations, may be recognized; the petroleum potential of each stage is different.Important factors are: the intensity of subsidence; the areal distribution of individual formations, and the presence of sulphate‐saltis in teh section, formation regional seals.The influence of inversion processes in the development of “giant”l gas‐condensate accumulations is emphasized, adn the petroleum‐bearing basins of ancient platforms are classified according to their oil and gas reserve.

Deep geothermal structure and mantle heat flow along the Barents Sea-East Alps geotraverse

The deep geothermal structure of the continental crust along the Barents Sea-Eastern Alps profile is investigated. Numerical geothermal models are constructed. The mantle heat flows and the Moho temperatures are computed for different tectonic blocks of the crust. Within ancient platform areas, the mantle heat flow varies from 15–28 mW m −2 in the Baltic shield to 6–14 mW m −2 in the stable Russian plate. In the Baltic syncline, the mantle heat flow is about 25–33 mW m −2. The maximum mantle heat flow value of 40 m W m −2 characterizes the Central European Platform. Mantle heat flow values vary from 15 to 35 mW m −2 in the Hercynian part of Central Europe. For the thermal model, the seismic velocity data are used. The Moho temperature values vary from 400°C in the ancient platform area to 900°–1000°C in the Alpine zone. In the northern Baltic shield T M = 600° C.

Preferred strike of Precambrian fold structures on ancient platforms

Preferred strike of Precambrian fold structures on ancient platforms

Bahama carbonate platforms — The deep and the past

Carbonate deposition prevailed for 150 Ma in the Bahama—Florida segment of the West Atlantic margin and provides an interesting test for actualism. Present-day facies patterns on the banks are controlled by the backward decrease of waves and tides and the concomitant increase of temperature and salinity variation. In the troughs, influx of sand and rubble from gravity flows varies with topography and distance from shallow-water sources and allows one to define facies belts: a rhythmic sequence of ooze and graded beds on the basin floors, subdivided into a basin-margin belt of coarse, thick turbidites and basin interior with fine turbidites; slope facies change with increase in height and declivity from accretionary to by-pass to erosional regimes. Stratigraphic history of the Bahamas is not simply a projection of the “Holo-Scene” back in time. Both long-term natural evolution (decrease in subsidence, upbuilding of the banks, submarine erosion) and outside factors (climate, eustacy) have caused significant changes. Since the Jurassic, the Bahamas seem to have evolved from a clastics-evaporite province to a single carbonate-evaporite platform and finally to an array of platforms and troughs. During the platform-trough stage, the rate of upbuilding of the platforms decreased, submarine canyon erosion increased. Platform flanks steepened as they grew higher and changed from accretionary to by-pass to erosional slopes. A change imposed by extraneous factors occurred in the Pliocene, when the Great Bahama Bank changed from a giant reef-rimmed atoll to a flat platform covered by oolites and peloid sands. The Bahamas share both long-term trends as well as random changes by extraneous factors with other platforms in the geologic record. Compared to ancient platforms, however, they are unusually long-lived and at a more advanced stage of growth, they are deeply dissected by erosion, their flanks are unusually high and steep and the troughs very narrow. The Neogene platform sequence is strongly controlled by eustatic sea-level fluctuations.

Aulacogens of ancient platforms: Problems of their origin and tectonic development

Riphean and Palaeozoic linear graben-like platform depressions — aulacogens —jointly with linear swell uplifts in a platform cover, intraplatform folded zones and Meso-Cenozoic epi-platform rift zones are different morphological manifestations of a single group of linear platform structures. Aulacogens are characterized by the long duration of their development and ability for regeneration. Their existence is certain, mainly since the Late Proterozoic (Riphean). Usually they inherit more ancient linear attenuated tectonic zones of platform basement or “adjust” to them. Aulacogens pass through common stages of development forming a single “cycle” or in many cases, even several similar though not identical “cycles”. The “cycle” starts with the incipience (or regeneration) of a linear graben-like depression limited by normal faults and after a more or less deep and long subsidence, accompanied often by basic or alkaline-basic volcanism, it is completed in many cases by an inversion stage. At this stage horst-like uplifts appear limited by thrust and overthrusts in the aulacogen basement, while linear swell-like uplifts or their systems and even linear folded structures occur in its cover. The early stages of an aulacogen evolution occur under horizontal extension conditions, whereas the inversion stage takes place under a compressional regime. “Cycles” of aulacogen development resemble (in attenuated, miniature form) the development “cycle” of adjacent geosynclinal areas, coincide with them in time and many “through” and “penetrating” aulacogens are their direct offsets. The Late Proterozoic “cycle” of aulacogen evolution corresponds to a “pre-plate” stage of the platform history (“aulacogen stage” by Bogdanov) and is characterized by the widest development of a network of differently oriented aulacogens over the whole area of ancient platforms which have been subjected to some general horizontal extension (“crawling away”) and only partially compensated by compression during the inversion stage. The subsequent aulacogen development in Palaeozoic and on Laurasian platforms —partly in Meso-Cenozoic — was spatially and genetically connected with the history of geosynclinal (later orogenic) belts bordering these platforms. With regard to Gondwana platforms (and later the North American and Siberian), the Mesozoic saw the development of many aulacogens or rift zones ranging parallel to transversally with respect to the recent margins of platforms. This process was connected with the break up of the Gondwana supercontinent and the formation of “secondary” oceanic depressions. As a rule, it has not been completed by inversion, but took place under extension conditions and probably some general expansion of the earth activated during the Meso-Cenozoic.

Reefal Margins of Pliocene-Pleistocene of Great Bahama Bank: ABSTRACT

End_Page 896------------------------------Examination of 16 shallow core borings from the Great Bahama Bank reveals that coral-coralline algal deposits of Pliocene-Pleistocene age line the margins of the bank. These reefal deposits extend up to 5 km bankward from both windward and leeward edges of the platform. Those along leeward margins are framestones and grew in relatively deep water. Those along the windward margins are a mixture of framestone and bafflestone formed in water of various depths probably including low intertidal. A similar assymetric distribution of depositional textures may be indicative of windward versus leeward margins on ancient platforms. The margins of the bank evolved upward through the Pliocene-Pleistocene. This evolution may be divided into three stages. In stage I, the lowermost, discrete depositional units average 8 m in thickness and contain an abundance of species of corals now extinct, including Stylophora affinis. In stage III, depositional units average 3 m in thickness and corals such as S. affinis are rare or absent. Stage III is marked by accumulation of nonskeletal sands (beach and eolian dune deposits) along inner-bankward parts of the margins. The distribution of reefal sediments was reduced to a narrow belt similar to that of the present. The change from stage I to II is of uniform time, apparently coinciding with the initiation of major Northern Hemisphere glaciation at the beginning of the late Plio ene; that from stage II to III is more variable, occurring from the middle to late Pleistocene. End_of_Article - Last_Page 897------------

Tectonic Evolution of Chinese Petroleum Basins

Petroleum basins in China are closely related to the tectonic regimes in different stages of geological evolution. The Mesozoic and Cenozoic basins were controlled by the deformation of the lithosphere of the Chinese plate in the course of subductions y neighboring plates situated in opposite directions. The crustal position may be a decisive factor for the different styles of basin development. Paleozoic (including Sinian and parts of Triassic) basins, on the other hand, might have been related to the fragmentation and reunification as well as the accretion of the ancient platform. The superposition of basins of different regimes or different generations brought about a complex set of depositional and structural characteristics influencing both constructively and destructivelythe mode of distribution of oil and gas.

Structural knots - important elements of structural boundaries of ancient platforms

A special type of structure of ancient platforms is the structural knot, formed by deep-seated tectonic zones, in contrast to other structures of the platform boundaries, structural knots are nonlinear, more or less isometric, and are characterized by the absence of linear orientation of the internal structural elements. The structural and metallogenic characteristics of the Colorado and the Uchur-Maya structural knots, and their similarities and differences, are presented. Structural knots represent marginal zones of ancient platforms that have undergone intense tectonomagmatic activation. Their most important feature is the variety of mineral resources which, in all probability, has been determined by the superimposition of the metallogenic features of the platforms, mobile belts, and intense ore-forming processes associated with tectonomagmatic activation. Most characteristic are ore (molybdenum, polymetallic, etc.), anthracite, and in some cases, oil deposits. The richness in mineral deposits of the w...

Sources of ore material in endogenic deposits of lithophile rare elements

According to the geological setting and localisation la different structures, deposits of lithophile rare elements may be placed in three groups: deposits on ancient platforms sod shields, deposits of folded regions, and deposits in zones of tectonic-magmatic reactivation. We discuss the role of crustal and subcrustal sources of ore-forming components is each group. —Authors.

Geological conditions of petroleum formation and accumulations on ancient platforms

Geological conditions of petroleum formation and accumulations on ancient platforms

Petroleum capacity of ancient platforms

Petroleum capacity of ancient platforms

Development of main structures of the siberian platform: history and dynamics

The location of the regions with maximum subsidence within the Siberian platform and the age volumes of the structural stages have been determined in relation to the location and periods of development of the adjacent geosynclines. The western and eastern parts of the platform differ in the character of the structures and magmatism, in the volume and age of the structural stages, as well as in the asynchronism of the main tectonic stages, which is connected with the boundary location of the platform between the Atlantic and Pacific Ocean segments of the earth's crust. The Siberian platform sharply differs from other ancient platforms of the northern hemisphere by the intensity of magmatism and style of tectonic deformation in the Meso-Cenozoic, and it is similar in this respect to the southern ancient platforms. The subcrustal processes in the adjacent geosynclines have played a leading role in the development of the platform structure; but the release of the stresses during the epochs of the magmatism and rift formation took place directly on the platform itself.

Principal types of basins of ancient platforms and the problem of their origin

Principal types of basins of ancient platforms and the problem of their origin

Geologic Structural Synthesis of Pacific Area As Basis for Establishing Regularities of Distribution of Mineral Resources: ABSTRACT

Geologic studies of the Pacific Ocean and its margins have advanced in the Soviet Union. Some of the results are presented on the unique published maps. (a) Tectonic Map of Eurasia, scale 1:5,000,000 (1966); (b) Geological Map of the Pacific Mobile Belt and the Pacific, scale 1:10,000,000 (1973); and (c) Tectonic Map of the Pacific Segment of the Earth, scale 1:10,000,000 (1970). Moreover, many maps and books have been published which deal with the geology and mineral resources of the northwestern segment of the Pacific including the eastern USSR. A geologic map synthesizes the data on stratigraphy, age, and composition of sedimentary, volcanic, and intrusive rocks on the Pacific continental margins, in the island arcs, and on the Pacific Ocean floor. The arrangement of geologic complexes of different age and character reflects various styles of tectonic systems on the inner structure: ancient platform--perioceanic belt--ocean. A regular rejuvenation of geologic complexes from continental nuclei toward the ocean is indicated. The map also shows that the distribution of sediment types on the Pacific floor is determined by such factors as climatic zonation, position of zones of upwelling and subsidence of waters, and depth of the ocean; whereas, in the perioceanic zones, a tectonic factor influencing intensity of supply of terrig nous and volcanic material is especially important. The tectonic map presents the structural formation of the Pacific area which is divided into four principal categories: folding zones of the ocean margins, recent geosynclinal zones, thalassogenes (large parts of the oceanic floor), and the East Pacific mobile belt. The structural analysis, on the whole, shows that plate tectonics fails to explain the constitution of the Pacific area. This hypothesis does not conform to the concept of the Circum-Pacific belt as the earth's structural zone of planetary importance. However, horizontal movements of rather large blocks and sheets of the Earth's crust are real. The geologic-structural synthesis creates the basis for general metallogenic constructions. Further studies on concentric zonation in the arrangement of ore deposits around the Pacific Ocean appear promising. The zone adjacent to the ocean is rich in gold and copper; the next continentward zone is characterized by rich deposits of tin and tungsten (in addition to gold) in Asia, and lead and zinc in North America. The ophiolitic belts are characterized by peculiar complexes of metals. Study of morphologic and geodynamic properties of ore-controlling faults poses a special problem. The ocean floor is characterized by extensive accumulations of manganese-polymetallic ores and locally by metasomatic phosphorites. The geologic-structural synthesis is important for prediction of oil and gas prospects. About 140 actual and possible gas and oil basins have been recorded in the Pacific margins. Most of them are associated with depressions and basins within folded zones and oceanic depressions. The basins in the perioceanic sector are composed mostly of Cenozoic deposits; the main productive strata are Miocene to Pliocene in age. Oil and gas already have been detected in about half of the basins. End_of_Article - Last_Page 1455------------

Oil and Gas Content of Sedimentary Basins in Circum-Pacific Belt: ABSTRACT

The Circum-Pacific belt, more than 50,000 km long and from 600 to 5,000 km wide, is a complex of crustal structures which were developed at different times since late Precambrian, have different makeup, and are confined to common ring zone. About 40% of all basins known in world are within Circum-Pacific belt. The sedimentary basins End_Page 1432------------------------------ within belt are of different size, age, and relief. Some contain oil and gas and account for almost one third of world oil production. These basins contain thick sedimentary sequences which form lens-like bodies. Sedimentary basins in which sediments are more than 3-3.5 km thick generally contain oil and gas. Tectogenesis is leading process in origin and formation of sedimentary basins and their transformation to oil and gas basins. Therefore, when classifying them, one should be guided by tectonic setting of basins. All basins in Circum-Pacific belt can be divided into five groups. The first group includes basins at junction of Circum-Pacific belt with ancient platforms. Such basins are composed of thick Mesozoic, Cenozoic, and some Paleozoic sequences and contain oil and gas. The second group is connected with intermountain areas. These basins usually are on continental-type crust and contain Mesozoic and Cenozoic sequences 3-5 or more km thick. This is most numerous group including more than 70 basins. The third group includes pericontinental folded basins at junction of folded continental structures and oceanic floor. They are mainly along American Pacific Coast and contain great thicknesses of predominantly Cenozoic deposits. The fourth group includes perioceanic basins connected with island arcs. These basins may be between an island-arc uplift and an oceanic floor or in a deep-sea depression. The fifth group includes intraplatform basins, which are rare and are chiefly within East Australian Paleozoic folded belt. The main criterion for oil and gas content is thickness of sedimentary rocks. The function of thickness is degree of katagenetic transformation of dispersed organic matter in subaqueous part of sedimentary section. All sedimentary basins more than 3.5 km thick contain oil and gas fields, irrespective of their hypsometric position. The thicknesses of rock sequences, rather than faults, control oil and gas content. No distinct relation has been observed between oil and gas accumulation and position of lithospheric plates defined by the new global tectonics concept. End_of_Article - Last_Page 1433------------

Structure of the earth's crust on the territory of the U.S.S.R.

The compilation of statistical data for 269 seismic crustal sections (total length: 81,000 km) which are available in the U.S.S.R. has shown that the preliminary conclusions drawn on relations between the elevation of the surface relief and Bouguer anomalies on one hand and crustal thickness (depth to the M-discontinuity) on the other hand are not fulfilled for the continental part of the U.S.S.R. The level of isostatic compensation has been found to be much deeper than the base of the earth's crust due to density inhomogeneities of the crust and upper mantle down to a depth of 150 km. The results of seismic investigations have revealed a great diversity of relations between shallow geological and deep crustal structures: Changes in the relief of the M-discontinuity have been found within the ancient platforms which are conformable with the Precambrian structures and which can exceed 20 km. In the North Caspian syneclise, extended areas devoid of the “granitic” layer have been discovered for the first time in continents. The crust was found to be thicker in the syneclises and anteclises of the Turanian EpiHercynian plate. In the West Siberian platforms these relations are reversed to a great extent. Substantial differences in crustal structure and thickness were found in the crust of the Palaeo zoides and Mesozoides. Regions of substantial neotectonic activity in the Tien-Shan Palaeozoides do not greatly differ in crustal thickness if compared to the Kazakhstan Palaeozoides which were little active in Cenozoic time. The same is true for the South Siberian Palaeozoides. The Alpides of the southern areas in the U.S.S.R. display a sharply differing surface relief and a strongly varying crustal structure. Mountains with roots (Greater Caucasus, Crimea) and without roots (Kopet-Dagh, Lesser Caucasus) were found there. The Cenozoides of the Far East are characterized by a rugged topography of the M-discontinuity, a thinner crust and a less-pronounced “granitic” layer. A relatively small thickness of the crust was discovered in the Baikal rift zone. The effective thickness of the magnetized domains of the crust as well as other calculations show that the temperature at the depth of the M-discontinuity (i.e., at depths of 40–50 km) is not higher than 300–400° C for most parts of the U.S.S.R.

Magnetic Anomalies of Precambrian Shields and some Problems of their Geological Interpretation

The predominance of extended zones of linear anomalies as well as of curved, ring-shaped, and other complicated anomalies, is characteristic of the magnetic field over ancient shields and platforms. The zones of linear anomalies reflect large tectonic features of the ancient geosynclines—deep faults and the associated structural facies zones. Curved anomaly zones characterize regions of deep-seated folding which are peculiar to ancient shields and platforms.Three large groups of deep structures may be distinguished in terms of the magnetic effects they produce. The first group includes domes (arches) composed essentially of nonmagnetic granitoids surrounded by stable magnetically active metamorphic zones. Structures of this group are most extensively developed in the inner zones of ancient geosynclines.A characteristic feature of the second group is magnetic anomalies over migmatite domes, whose minima extend over bordering synclines composed of nonmagnetic rocks. The trend of individual magnetic horizons is concordant with the direction of banding in the migmatites.The third group includes deep structures whose synclinal and anticlinal forms are composed of rocks showing an increased magnetization. Mapping of such structures in the magnetic field presents considerable difficulties.The autocorrelation functions of anomalous magnetic fields over ancient shields indicate the intricate nature of the fields. A zero-correlation radius of a high-frequency component ranges from 5 to 10 km; that of a long-wave component amounts to about 40 km.Two layers corresponding to the sources of anomalies are identified in the crustal cross section. The depths to lower boundaries of the bodies responsible for the local magnetic anomalies have shown that most of the sources are confined to a surface layer which is not more than 10 km thick. The source of regional magnetic anomalies is attributable to magnetic inhomogeneities of the earth's crust, lying at depths from 10 to 30 km.

Basic trends in the evolution of continents

The purpose of this paper is to formulate the most important trends in the evolution of the continental earth's crust and to propose a hypothetic model of deep processes determining this evolution. On the continents endogenous processes (tectonic, metamorphic, magmatic) combine in quite fixed patterns constituting different endogenous regimes. Two extreme regimes are that of a geosyncline and that of an ancient platform. There are many intermediate regimes. The most general trend of evolution of continents is from a geosynclinal regime to a platform one with temporary and local events of tectonic and magmatic activation. Geological history displays the periodical recurrence of the entire complex of endogenous processes. There are regularities in the distribution in space and time of such endogenous or tectonic cycles. Geosynclinal (holomorphic) folding is due to processes within a geosyncline provoked by the force of gravity. There are correlations between endogenous regimes and the magnitude of heat flow. The model proposed proceeds from the progressive radioactive heating of the earth. This process is complicated by the periodical formation of partly molten layers moving upwards and vanishing one after the other. From such a layer (asthenosphere) basaltic asthenoliths ascend to the crust, thus provoking its periodical heating and regional metamorphism. The data available indicate that superficial endogenous regimes have their roots deep in the mantle, which is in contradiction with the idea of large horizontal displacements taking place in tectonosphere (i.e., lithosphere and asthenosphere).

Oil-gas potential of Late Precambrian deposits

The available materials on the distribution of Precambrian nonmetamorphosed sedimentary formations and the naphthide occurrences associated with them show that oil and gas potential of the Late Proterozoic deposits can and should be regarded as an important and potentially profitable matter of great scientific and economic significance. This paper reviews distribution and geochemical properties of the Precambrian rock in Asia, Africa, Australia, Europe and the Americas. The main emphasis is on distribution in the USSR. The practical solution to the problem will expand the potential oil and gas reserves of the sedimentary mantel, both in known oil and gas basins of the USSR, and new basins located in the areas of ancient platforms.

Carbonate mud; production by epibiont growth on Thalassia testudinum

ABSTRACT Epibiotic growth of coralline red algae and serpulid worms on the marine turtle grass Thalassia testudinum is sufficient to produce carbonate mud at rates comparable to the rates of accumulation of ancient platform carbonates.