Petroliferous basins at continental margins of Paleozoic paleoseas: Communication 1. Petroliferous basins at margins of Iapetus and Panthalassa

Although large marine basins governing the fabric of our planet in the Paleozoic disappeared later (whether or not they were oceans is a debatable issue), sedimentary basins formed at continental margins at that time played a crucial role as depositories of various fossil minerals, including ores, salts, phosphorites, coal, bauxites, and construction materials. Many of these basins are oil- and gas-bearing structures. Their oldest representatives are confined to margins of Proterozoic/Paleozoic paleoseas (Iapetus and Panthalassa), whereas other basins appeared after opening of the Central Asian, Uralian, and Rheic (Paleotethys) deep-marine basins. Study of specific features of the sedimentary cover of such basins, rock composition therein, rocks and associated oil- and gas-bearing systems revealed that the Paleozoic planet was divided into two parts: Gondwana, with the major portion confined to high latitudes of the Southern Hemisphere; and other smaller near-equatorial continents. This pattern significantly governed the composition and mode of post-sedimentary transformations of natural reservoirs, as well as age and spatial distribution of the major hydrocarbon (HC) source sequences. Most Paleozoic oil- and gas-bearing basins make up specific belts because of their confinement to continental margins in paleoseas of that time.     

Source rocks in sedimentary basins of continental margins in the early paleozoic

During the Paleozoic, epochs with the relatively cold climate alternated with epochs marked by significant warming. Moreover, cooling epochs were characterized by the substantial sea level fall, while warming was accompanied by its rapid rise. In many basins located at margins of Laurentia, Baltica, and the North China continental block, such an alternation is reflected in the structure of sedimentary sequences and the lateral/vertical distribution of reservoirs, confining beds, and source rocks. Despite the fact that sediments with high concentrations of sapropelic OM accumulated in different periods, their distribution areas on continents and their margins became highly reduced during cold epochs, when these sediments filled mostly rift troughs and foreland basins. After the colonization of land by higher plant communities in the Carboniferous and Permian sediments deposited during cold epochs, the humic material became an important constituent of OM in the source rock sequences.     

Oil source rocks at mesozoic and cenozoic continental margins: Communication 2. Oil source rocks at continental margins in the second half of the cretaceous and in the Cenozoic

Oil source rocks represent sequences with the C_org content ranging from 3–5 to 15–20%. Sedimentary sections of large petroliferous basins usually include one or two such sequences, which generated liquid and gaseous hydrocarbons (HCs) during their long-term subsidence to the elevated temperature zone. The middle episode of the Late Cretaceous was marked by the accumulation of sediments with a high C_org content in different areas of the World Ocean. However, truly unique settings favorable for accumulation of the sapropelic organic matter (OM) appeared at continental margins that primarily faced the Tethys Ocean. The La Luna Formation is one of the best known source rock sequences responsible for the generation of liquid HCs in basins of the Caribbean region. In the Persian Gulf, the Kazhdumi Formation composed of marls and clayey limestones is considered the main oil-generating sequence. In the Paleogene after closure of the Tethys, the Pacific continental margins became the main domains that accumulated source rocks. The maximal deposition of sapropelic OM in this region corresponded to the early-middle Eocene. In the Neogene, the accumulation of source sediments was associated with deltas and submarine fans of large rivers and with upwelling zones. In basins of the Californian borderland, the main oil-generating sequences are represented by siliceous rocks of the Monterey Formation. They were deposited in a regional upwelling zone related to the cold California Current.     

Oil source rocks at the mesozoic and cenozoic continental margins: Communication 1. Oil source rocks at continental margins in the Triassic-Jurassic and Neocomian-Aptian

The presence of rocks capable of generating hydrocarbons (HC) in the section of sedimentaryrock basins is an essential criterion for their qualification as structures with oil and gas pools. Although organic matter (OM) is always present as dissemination in genetically different sediments, it is believed that rocks enriched with OM of the sapropel series (2 to 3% C_org) can generate a significant amount of liquid HC. However, rock sequences with the C_org ranging from 3–5 to 15–20% are considered oil source formations. The rock section of large petroliferous basins usually includes one or two source sequences, which generated liquid and gaseous HCs after submergence to high temperature and pressures zones. In the basin confined to the Arctic slope of Alaska, one of the main producers of liquid HC is represented by the Upper Triassic clays and limestones of the Shublik Formation. In the Barents Sea and North Sea basins, such rocks are represented by the Spekk Formation and the Kimmeridge Clay, respectively; in the West Siberian basin, by the Bazhenovo Formation; in the Persian Gulf, by the Fahlian, Sargelu, and Garau formations; in basins of the Caribbean region, by marls and clayey limestones of the La Luna Formation. In perioceanic basins of the South Atlantic, the major source sequences are represented by the Neocomian and Barremian clays and marls. The source rocks are identified as the Lagoa Feia Formation in the Campos and Santos basins. They are cognized as the Black Marlstone or Bukomazi Formation in the Lower Congo, Kwanzaa-Cameroon, and Angola basins.     

Graptolite shales in the continental margins of the Paleozoic Yapetus and Paleothetys (Rheic) Oceans

Graptolite shales are a type of fossil shales that contain a large number of graptolite imprints and remains. These deposits are characterized by high TOC contents (C_org = 2–18%). Based on the data of many studies, graptolite shales are one of the main hydrocarbon sources that formed oil and gas fields in Paleozoic deposits around the world, e.g., the Silurian graptolite shales make up to 9–15% of all hydrocarbons that form the oil and gas fields in the largest petroleum basins.     

Hydrocarbon source rocks in sedimentary basins of continental margins in the Middle-Late Paleozoic

The second half of the Paleozoic was marked by amalgamation of large continental blocks. The collision between the Laurentia and Baltica continents in the Devonian culminated in the formation of Laurussia. This event was followed by accretion of the Siberian and Kazakhstan continental blocks after the closure of the Uralian marine basin in the terminal Carboniferous-initial Permian. These processes were responsible for the formation of the Pangea supercontinent at the end of the Permian Period. They were accompanied by climate changes reflected in the alternation of warming and cooling epochs. One of these cooling epochs was terminated by large-scale glaciation of Gondwana at the end of the Carboniferous Period. Nevertheless, the most significant process, which drastically changed the existing paleogeographic situation, was colonization of continents by plants and animals, and, thus, accumulation of coaliferous formations in them. The lacustrine and sea basins also accumulated humic and mixed humic/sapropel organic matter (OM) in addition to pure sapropelic sediments.     

Early paleozoic granitoids in the Lesser Khingan terrane, Central Asian Foldbelt: Age, geochemistry, and geodynamic interpretations

The U-Pb zircon dates obtained for the Sutara (480 ± 4 Ma), Kabalinskii (471 ± 10 Ma), and Durilovskii (461 ± 5 Ma) massifs reliably confirm an Early Proterozoic orogenic event, which took place after granulite metamorphism at approximately 500 Ma (Wilde et al., 2003) in the Lesser Khingan (Jiamusi) terrane. The rocks emplaced most shortly after the main metamorphic event are the granites of the Sutara Massif and leucogranites of the Kabalinskii Massif, whose geochemistry is close to that of collision granites. The quartz diorites and subalkaline granites of the Durilovskii Massif, whose geochemistry suggests their origin in a postcollision environment with the participation of an enriched mantle source, were emplaced longer after metamorphic event and after the aforementioned massifs.     

被动陆缘洋陆转换带和岩石圈伸展破裂过程分析及其对南海陆缘深水盆地研究的启示

作为伸展陆壳和正常洋壳之间重要的过渡和衔接,洋陆转换带(ocean-continent transition,简写为OCT)蕴含有丰富的地壳岩石圈伸展破裂过程的信息。文中通过系统的资料调研,在总结OCT研究历史、现状和发展趋势的基础上,阐明了OCT的现代概念、类型及其识别标志;详细介绍了以OCT为基础而建立的被动陆缘地壳岩石圈结构构造单元划分方案、表层沉积盆地构造地层格架及重要的构造变革界面特征;分析了大型拆离断层在地壳岩石圈薄化、地幔剥露过程中的控制作用;揭示了陆缘变形集中、迁移和叠合的规律,建立了被动陆缘岩石圈伸展、薄化、剥露和裂解模式。最后,论文对比了国际非岩浆型被动大陆边缘与我国南海OCT的研究,介绍了南海OCT和陆缘深水超深水盆地研究的新发现,提出深入研究南海OCT将为南海陆缘构造演化、洋盆扩张过程和深水超深水盆地的成因机制研究提供新的启示。     

Mechanisms of continental crust formation in the Central Asian Foldbelt

Geological and isotopic study of rocks occurring in the Early and Late Baikalian, Caledonian, Hercynian, and Indosinian fold regions of Central Asia is carried out. The juvenile crust formation occurred in these fold regions have determined the systematic differences in isotopic compositions of the crust. In the course of the subsequent (postaccretion) evolution, the crust of these domains underwent multiple reworking. These processes were accompanied by variations in the Nd isotopic compositions of the crust, which, in turn, are recorded in the isotopic compositions of granites and felsic volcanics as products of crust melting. Three types of crust differing in Nd isotopic composition and structure and, as a consequence, in formation mechanisms, are distinguished. The isotopically homogeneous crust is a source of igneous rocks with constant model Nd isotopic age (T_Nd(DM2st)) irrespective of the age of the crustal igneous rocks. These are the isotopic provinces, the crust of which remained isolated from addition of alien materials during postaccretion evolution. The axial zone of the Hercynides in the Central Asian Foldbelt is an example. The isotopically heterogeneous layered crust consists of fragments differing in isotopic composition. The products of its melting are characterized by widely scattered ɛ_Nd(T) and (T_Nd(DM2st). The appearance of alien sources of melt is considered in terms of underplating. This mechanism develops either due to subduction of the juvenile oceanic lithosphere beneath the mature continental lithosphere at convergent boundaries or as a result of plume-lithosphere interaction. The first mechanism operated during the formation of granitoids pertaining to the Tuva-Mongolia microcontinent. The second mechanism was responsible for the formation of batholiths in the zonal Hangay, Barguzin, and Mongolia-Transbaikalia magmatic fields. The isotopically heterogeneous mixed crust is composed of fragments differing in isotopic composition, which are tectonically mixed, resulting in the formation of an isotopically uniform reservoir in the domain of magma generation. As a result, the products of melting acquire isotopic parameters substantially distinct from the juvenile rocks of the corresponding structural zone. The formation of such a crust is related to the tectonic delamination, which provides for juxtaposition and a high degree of tectonic mingling of heterogeneous fragments at deep levels. The Caledonides of the Central Asian Foldbelt are characterized by such a mechanism of crust formation.     

Accretion of oceanic plateaus at continental margins: Numerical modeling

The accretion of oceanic plateaus has played a significant role in continental growth during Earth's history, which is evidenced by the presence of oceanic island basalts (OIB) and plume-type ophiolites in many modern orogens. However, oceanic plateaus can also be subducted into the deeper mantle, as revealed by seismic tomography. The controlling factors of accretion versus subduction of oceanic plateaus remain unclear. Here, we investigate the dynamics of oceanic plateau accretion at active continental margins using a thermo-mechanical numerical model. Three major factors for the accretion of oceanic plateaus are studied: (1) a thinned continental margin of the overriding plate, (2) “weak” layers in the oceanic lithosphere, and (3) a young oceanic plateau. For a large oceanic plateau, the modes of oceanic plateau accretion can be classified into one-sided and two-sided subduction–collisional regimes, which mainly depend on the geometry of the continental margin (normal or thinned). For smaller-sized seamounts, accretion occurs only if all three factors are satisfied, of which a thinned continental margin is the most critical. Possible geological analogues for the two-sided subduction–collisional mode include the Taiwan orogenic belt and subduction of the Ontong Java Plateau. The accretion model for small oceanic plateaus applies to the Nadanhada Terrane in Northeast China.     

Sedimentary basins: Regularities in their formation and classification principles. Communication 2. Sedimentary rock basins

The article is dedicated to sedimentary rock basins. It is postulated that sedimentary rock basins represent an element of the Earth’s sedimentary shell characterized by the general thermobaric regime, type of fluidodynamics, and combination of epigenetic mineral resources. Fluidodynamic regimes of three types are recognized in the upper part of the crust: infiltration, elision, and exfiltration. The dominant type of fluidodynamics is reflected in the most widespread mineralogical-geochemical zonality and association of epigenetic mineral deposits. These properties allow us to define the infiltration and elision types of rock basin in the sedimentary shell. The structural features of different-type rock basins are characterized with the emphasis on their relations with geotectonics.     

主动大陆边缘海沟-斜坡盆地沉积体系分析——以缅甸若开海岸盆地某工区为例

本文以国内少见的发育在主动大陆边缘的特殊盆地类型海沟-斜坡盆地作为研究对象,通过建立井震结合的不同沉积体系的地质-地球物理识别模板,用二维地震资料对研究区目的层的沉积体系进行了解析。结果显示,研究区发育窄的滨浅海、小规模的三角洲、浊积扇、下切谷、半深海陆坡和半深海平原等沉积体系。滨浅海、三角洲和浊积扇沉积体系发育的砂体是主要的储集体。海沟-斜坡盆地的规模较小,以生气为主,可能形成中小型断块油气藏。     

Seaward dipping reflectors and the continent-ocean boundary at passive continental margins

The definition of the continent-ocean boundary at passive continental margins has proved to be an elusive task. Even the relatively direct method of seismic refraction experiments has yielded results that cannot always be interpreted unequivocally. Multichannel seismic reflection profiles on many passive margins have revealed the presence of remarkable suites of arcuate reflectors, dipping seaward to form a wedge-shaped structure. Their general characteristics and velocity structure suggest that they may be largely volcanic in nature, but there is no agreed upon model for their origin. Nevertheless it is generally thought that they lie at or close to the boundary between continent and ocean, and as such they are extremely important structural markers that may yield important evidence concerning the structure and evolution of passive margins.     

Sedimentary basins,regularities of their formation and classification principles: Communication 1. Sedimentation basins

Two types of sedimentary basins are proposed: sedimentation basins and rock formation basins (rock basins). Such an approach reflects a complete cycle of the sedimentary process ranging from the stage of material mobilization and transport to the stage of accumulation of sediments, their transformation into sedimentary rocks, and formation of associated mineral deposits. Sedimentation basins are divided into lakes, intracontinental and marginal seas, and oceans, where phase differentiation of matter proceeds in different manners. It is shown that eupelagic and miopelagic clays, analogues of which are missing from Paleozoic sequences, represent the main indicator of recent sedimentation in the Word Ocean. It is stated that each sedimentation basin is characterized by a specific association of sedimentary mineral deposits.     

Lithospheric extension from rifting to continental breakup at magma-poor margins: rheology,serpentinisation and symmetry

The symmetry or asymmetry of the process of continental breakup has been much debated over the last 20 years, with various authors proposing asymmetric simple shear models, others advocating more symmetric, pure shear models and some combinations of the two. The unroofing of vast expanses of sub-continental mantle at non-volcanic margins has led some authors to argue in favour of simple shear models, but supporting evidence is lacking. Subsidence evidence from conjugate margin pairs is equivocal, and the detailed crustal and lithospheric structure of such pairs not generally well enough known to draw firm conclusions. In the Porcupine Basin, where the final stages of break-up are preserved, the development of structural asymmetry is demonstrable, and apparently related to late stage coupling of the crust to the mantle following the complete embrittlement of the crust. This agrees with theoretical modelling results, which predict that asymmetric models can develop only on a lithospheric scale when the crust and mantle are tightly coupled. However, whether such asymmetry is maintained during continued exhumation of the mantle is unclear.     

Composition, sources, and mechanism of continental crust growth in the Lake zone of the Central Asian Caledonides: I. Geological and geochronological data

Data on the composition, inner structure, and age of volcanic and siliceous-terrigenous complexes and granitoids occurring in association with them in the Caledonian Lake zone in Central Asia are discussed in the context of major relations and trends in the growth of the Caledonian continental crust in the Central Asian Foldbelt (CAFB). The folded structures of the Lake zone host basalt, basalt-andesite, and andesite complexes of volcanic rocks that were formed in distinct geodynamic environments. The volcanic rocks of the basalt complex are noted for high concentrations of TiO₂ and alkalis, occur in association with fine-grained siliceous siltstone and siliceous-carbonate rocks, are thus close to oceanic-island complexes, and were likely formed in relation to a mantle hotspot activity far away from erosion regions supplying terrigenous material. The rocks of the basalt-andesite and andesite complexes have lower TiO₂ concentrations and moderate concentrations of alkalis and contain rock-forming amphibole. These rocks are accompanied by rudaceous terrigenous sediments, which suggests their origin in island-arc environments, including arcs with a significantly dissected topography. These complexes are accompanied by siliceous-terrigenous sedimentary sequences whose inner structure is close to those of sediments in accretionary wedges. The folded Caledonides of the Lake zone passed through the following evolutionary phases. The island arcs started to develop at 570 Ma, their evolution was associated with the emplacement of layered gabbroids and tonalitetrondhjemite massifs, and continued until the onset of accretion at 515–480 Ma. The accretion was accompanied by the emplacement of large massifs of the tonalite-granodiorite-plagiogranite series. The postaccretionary evolutionary phase at 470–440 Ma of the Caledonides was marked by intrusive subalkaline and alkaline magmatism. The Caledonides are characterized by within-plate magmatic activity throughout their whole evolutionary history, a fact explained by the accretion of Vendian-Cambrian oceanic structures (island arcs, oceanic islands, and back-arc basins) above a mantle hotspot. Indicators of within-plate magmatic activity are subalkaline high-Ti basalts, alkaline-ultrabasic complexes with carbonatites and massifs of subalkaline and alkaline gabbroids, nepheline syenites, alkaline granites, subalkaline granites, and granosyenites. The mantle hotspot likely continued to affect the character of the lithospheric magmatism even after the Caledonian folded terrane was formed.