Biofacies of Upper Jurassic and Lower Cretaceous sediments of central West Siberia

Paleontological study of Upper Jurassic and Lower Cretaceous sediments recovered by boreholes in the Agan-Vakh and Nadym-Vengapur interfluves clarified environments of their deposition. As is shown, influx of siliciclastic material to central areas of the West Siberian sea basin varied through time. Taxonomic composition and ecological structure of nektonic and benthic fossil assemblages are analyzed and considered in terms of environmental factors such as hydrodynamics, aeration, temperature, and salinity of seawater.     

中扬子北缘上侏罗统-白垩系沉积岩地球化学特征与构造背景分析

沉积岩中的微量元素和稀土元素特征对沉积环境、沉积物的物源和构造背景具有十分重要的示踪与指示作用。对采自中扬子北缘晚侏罗世-白垩纪地层的泥质岩样品中微量元素和稀土元素进行了测试与分析,结果表明,研究区微量元素中的深源元素普遍低于地壳粘土岩中的平均值,说明沉积物的来源为陆源物质。而典型陆源元素Zr、Hf的平均含量远高于地壳粘土岩中的平均含量,反映了大量的陆源物质的快速堆积。上侏罗统与下白垩统泥质岩的微量元素特征相似,而上白垩统则发生了明显的变化,表现为深源性微量元素含量较上白垩统明显降低,而陆源性微量元素含量显著增加,反映早晚白垩世之间沉积背景发生了变化。纵向上,稀土元素的变化规律显示,上白垩统与下白垩统的稀土元素特征具明显的差异,表现为Eu的负异常明显减小,Ce的负异常增大,轻稀土富集程度降低,反映了沉积作用分异度较低的特点。La-Th-Sc、Th-Co-Zr／10和Th-Sc—Zr／10构造环境判别图及La／Th-Hf和Th／Sc-Sc比值图表明,中扬子北缘晚侏罗世．白垩纪沉积岩物源区的构造背景以大陆岛弧型为主,可能也存在类似被动大陆边缘构造背景。     

The Meso-Cenozoic tectonics and petroleum potential of West Siberia

The relationship between the petroleum potential of the West Siberian province and the Mesozoic to Cenozoic tectonic processes is analyzed. The studies were based on structural and isopach maps of seismogeologic megacomplexes compiled from generalized geological and geophysical data on the province at the Trofimuk Institute of Petroleum Geology and Geophysics as well as on the results of interpretation of regional seismic CDP (common depth point) profiles. The main stages of formation of structures of different ranks and faults have been established. It is shown that the petroleum potential of the province was determined mainly by its structure and tectonic processes at the Cenozoic stage of evolution. At that time, the Koltogory–Urengoi megatrench formed, which became the main zone of hydrocarbon generation, as well as large positive structures—petroleum accumulation zones. Also, disjunctions originated, which served as channels for hydrocarbon migration from the oil source rocks of the Bazhenovo Formation to the main Neocomian and Aptian–Albian–Cenomanian petroleum reservoirs of the province.     

Callovian and Upper Jurassic foraminiferal and ammonite biostratigraphy of the Shaim petroleum region (West Siberia)

The results of biostratigraphic analysis of foraminiferal assemblages were used to identify a nearly complete succesion of biostratigraphic units in the Callovian-Upper Jurassic section of the Shaim region. The finds of the ammonites from different parts of the Callovian-Volgian sections helped to correlate the identified foraminiferal biostratigraphic units to the General Stratigraphic Scale. Analysis of variations in the taxonomic composition and structure of foraminiferal assemblages allowed refinement of paleontological characteristics of the foraminiferal biostratigraphic units for different facies conditions in the southwest of the West Siberian Basin.     

Paleomagnetism and magnetostratigraphy of Upper Cretaceous and Cretaceous-Paleogene boundary intervals,southern Kulunda basin (West Siberia)

The study presents new paleomagnetic data on the Upper Cretaceous and Cretaceous-Paleogene boundary intervals of the southern Kulunda basin (Alei area), which were obtained from core samples collected from a 305-m-thick section penetrated in two wells. The paleomagnetic sections of each well were compiled and correlated based on the characteristic remanent magnetization (ChRM). Paleomagnetic, geological, stratigraphic, and paleontological data were used to compile the Upper Cretaceous and Cretaceous-Paleogene magnetostratigraphic section of the southern Kulunda basin. The magnetostratigraphic section consists of five magnetozones, one normal polarity zone, and four reversed polarity zones spanning the Upper Cretaceous and Lower Paleogene. The lower part of the Gan’kino Horizon, showing normal polarity, forms a single normal polarity magnetozone N. The upper part of the Gan’kino Horizon comprises two reversed polarity magnetozones (R₁km and R₂mt). The Talitsa and Lyulinvor Formations of Lower Paleogene age correspond to two reversed polarity magnetozones (R₁zl and R₂i). The compiled Upper Cretaceous and Lower Paleogene magnetostratigraphic section was correlated with the geomagnetic polarity time scale. Two options were considered for correlating the lower normal polarity part of the section with geomagnetic polarity time scale of Gradstein.     

Chalk thickness trends and the role of tectonic processes in the Upper Cretaceous of southern England

A series of six thickness maps created at a formation scale for the Chalk of the Southern and Transitional Chalk provinces of SE England reinforce the difficulty in determining the controls on Chalk deposition. However, at the broad scale, they do appear to show that thickness patterns in the Cenomanian to Turonian chalks of the West Melbury Marly Chalk, the Zig Zag Chalk and the Holywell Nodular Chalk show correspondence with the underlying Mesozoic extensional basin structure. The major exception to this is the south Dorset area which was uplifted in the Early Cretaceous as an eastern extension to the Cornubian Ridge. The younger New Pit Chalk and Lewes Nodular Chalk show a switch toward thicker successions on the London Platform and thinner, more uniform successions across the Mesozoic basins to the south. This change may indicate some initial basin inversion starting in the mid Turonian which caused a shift in the main locus of Chalk deposition toward East Anglia. The work potentially suggests multiple control-modes shaping the geometry of Chalk deposits, involving an interplay of: 1) long-lived basin-defining faults and structural blocks acting to shape large-scale thickness trends through differential compaction and interaction with relative sea level change; 2) smaller scale structures that may function to more effectively dissipate stress created by intra-Cretaceous tectonic events, producing more localised/sub-regional thickness and facies variations; 3) early basin inversion reflecting the broader basin-scale response to intra-Cretaceous tectonics, potentially responsible for regional shifts in patterns of sedimentation.     

Lithostratigraphic and tectonic features of the formation of hydrocarbon pools on the shelf of the Russian eastern Arctic

Chronostratigraphic, geologic, and tectonic preconditions of the formation of hydrocarbon pools on the shelf of the Russian eastern Arctic are considered. Based on study of the Upper Cretaceous section at Cape Derevyannye Gory (Novaya Sibir Island), it is shown that the Arctic shelf comprises thick deltaic sequences that can accommodate solid and liquid hydrocarbons.     

Late Jurassic and early Cretaceous Ostreidae in Arctic seas of Siberia

Four arctic species of Liostrea related to the European L. delta (Smith) form a distinct evolving lineage in the Anabar-Khatanga basin during Late Jurassic Early Cretaceous time These are, in ascending order: Liostrea ex. gr. delta (Early Kimmeridgian), L. plastics (Trautschold) (Kimmeridgian), L. praeanabarensis n. sp. (described) (Volgian), and L. anabarensis Bodyl. (Early Cretaceous). Their morphology is reviewed. The lineage is characterized by small, oval to falcate shells with strong posterior curvature, convex left valves, and flat to slightly concave right valves. Widely, subevenly spaced concentric lamellae cover the surface. Some species develop small dorsoposterior auricles. Population analysis of two species demonstrates considerable variability in valve outline and development of the attachment scar and auricle.. Chronologically successive species show up to 10 percent morphologic overlap, demonstrating their genetic relationship. Principal evolutionary trends in progressively Younger species are 1) reduction of attachment scar size, and presumably of duration of attachment during ornogeny; 2) elongation and upward curving of the posterior shell margin — change from oval to falcate outline; 3) development and subsequent loss of a dorsoposterior auricle. Elongation of the valve takes place only with reduction of attachment scar size in all species. These trends are adaptations to changing marine environments within the Anabar- Khatanga basin after its geographic and environmental restriction during the Late Jurassic. The lineage appears endemic to the basin. The adaptive value of many morphologic features is discussed at length.—E. J. Kauffman.     

Sedimentation Model of Rocks of the Pokur Formation: Basis for the Prediction of Filtration-Volumetric Characteristics (Upper Cretaceous,West Siberia)

The HC-rich Upper Cretaceous rocks in the West Siberian petroliferous basin comprise genetically heterogeneous sediments, resulting in the specificity of morphology, structure, and properties of natural reservoirs therein. One of the possible techniques for more reliable predictions of reservoir rock characteristics is the utilization of results of sedimentation modeling based on the complex interpretation of borehole and seismic data. The paper presents an example of the realization of such approach for refining the structure of oil-and-gas pools in the Pokur Formation of the Russko-Chasel megaswell.     

The Jurassic/Cretaceous boundary in northern Siberia and Boreal-Tethyan correlation of the boundary beds

There is no international consensus regarding the GSSP for the Berriasian, the basal stage of the Cretaceous System. Any of the events discussed by the international expert community can be regarded as a marker of the Jurassic/Cretaceous boundary: a phylogenetic change of taxa, paleomagnetic reversal, or isotopic excursion. However, the problem of identification of this level in Boreal sections can be solved only using a combination of data obtained by paleontological and nonpaleontological methods of stratigraphy (bio-, chemo-, magnetostratigraphy, etc.). With any of the accepted markers, the Jurassic/Cretaceous boundary in Siberian sections will be within the upper part of the regional Bazhenovo Horizon. The least interval of the uncertainty of the position of this boundary in Siberian sections will be ensured by the selection of one of two markers: biostratigraphic (base of the Pseudosubplanites grandis Subzone) or magnetostratigraphic (base of the M18r magnetozone).     

Stratigraphy,sedimentology and tectonic evolution of the Upper Cretaceous/Paleogene succession in north Eastern Desert,Egypt

The stratigraphy, sedimentology and syn-depositional tectonic events (SdTEs) of the Upper Cretaceous/Paleogene (K–P) succession at four localities in north Eastern Desert (NED) of Egypt have been studied. These localities are distributed from south-southwest to north-northeast at Gebel Millaha, at North Wadi Qena, at Wadi El Dakhal, and at Saint Paul Monastery. Lithostratigraphically, four rock units have been recorded: Sudr Formation (Campanian–Maastrichtian); Dakhla Formation (Danian–Selandian); Tarawan Formation (Selandian–Thanetian) and Esna Formation (Thanetian–Ypresian). These rock units are not completely represented all over the study area because some of them are absent at certain sites and others have variable thicknesses. Biostratigrapgically, 18 planktonic foraminiferal zones have been recorded. These are in stratigraphic order: Globotruncana ventricosa Zone (Campanian); Gansserina gansseri, Contusotruncana contusa, Recimguembelina fructicosa, Pseudohastigerina hariaensis, Pseudohastigerina palpebra and Plummerita hantkenenoides zones (Maastrichtian); Praemurica incostans, Praemurica uncinata, Morozovella angulata and Praemurica carinata/Igorina albeari zones (Danian); Igorina albeari, Globanomanlina pseudomenradii/Parasubbotina variospira, Acarinina subsphaerica, Acarinina soldadoensis/Globanomanlina pseudomenardii and Morozovella velascoensis zones (Selandian/Thantian); and Acarinina sibaiyaensis, Pseudohastigerina wilcoxensis/Morozovella velascoensis zones (earliest Ypresian). Sedimentologically, four sedimentary facies belts forming southwest gently-dipping slope to basin transect have been detected. They include tidal flats, outer shelf, slumped continental slope and open marine hemipelagic facies. This transect can be subdivided into a stable basin plain plus outer shelf in the extreme southwestern parts; and an unstable slope shelf platform in the northeastern parts. The unstable slope shelf platform is characterized by open marine hemipelagic, fine-grained limestones and fine siliciclastic shales (Sudr, Dakhla, Tarawan and Esna formations). The northeastern parts are marked by little contents of planktonic foraminifera and dolomitized, slumped carbonates, intercalated with basinal facies. Tectonically, four remarkable syn-depositional tectonic events (SdTEs) controlled the evolution of the studied succession. These events took place strongly within the Campanian–Ypresian time interval and were still active till Late Eocene. These events took place at: the Santonian/Campanian (S/C) boundary; the Campanian/Maastrichtian (C/M) boundary; the Cretaceous/Paleogene (K/P) boundary; and the Middle Paleocene–Early Eocene interval. These tectonic events are four pronounced phases in the tectonic history of the Syrian Arc System (SAS), the collision of the Afro-Arabian and Eurasian plates as well as the closure of the Tethys Sea.     

渤海海域侏罗—白垩纪沉积与构造演化

覆盖区侏罗—白垩系分布、变形特征及构造演化对理解华北克拉通破坏过程具有重要意义。根据编制的地层分布图和地震资料解释,研究了渤海海域侏罗—白垩纪时期沉积、构造变形及演化特征。渤海海域燕山期构造变形与板块俯冲引起的地幔上拱有关。早-中侏罗世,库拉—伊泽奈崎板块北西向俯冲,地层展布继承了印支期古构造格局,呈近东西向,属于坳陷成盆期。晚侏罗—早白垩世,库拉—伊泽奈崎板块北北西向俯冲,火山活动强烈,为热拱断陷期。受郯庐断裂左行活动影响,地层展布具有明显分带性,多呈北东—南西向和北西西—南东东向。晚白垩世,太平洋板块北西向俯冲挤压,岩浆冷凝,进入萎缩隆褶期。     

Facies and depositional processes in an Upper Jurassic to Lower Cretaceous pelagic sedimentary sequence, Antarctica

The Nordenskjöld Formation (?Oxfordian-Berriasian age) is exposed on the east coast of the Antarctic Peninsula, where it consists of interbedded ash layers and biosiliceous mudstones which accumulated under anaerobic to dysaerobic bottom waters. The mudstones were deposited by pelagic settling and the ash layers by pelagic settling from suspension or as fallout from subaerial eruption columns. The lower part of the succession accumulated in a basinal setting under anaerobic bottom waters and is characterized by parallel bedding. Mudstones deposited in this setting preserve abundant zooplankton faecel pellets. Compaction of these pellets has given rise to a bedding parallel fissility. The upper part of the succession accumulated under dysaerobic bottom waters in a slope setting. The sequence is wavy bedded and contains abundant evidence of post-depositional sediment instability and resedimentation, much of which was caused by tectonic activity. Discrete slide masses are absent from the slope sequence and it appears that slope processes were dominated by creep. Examination of the mudstones shows that as levels of dissolved oxygen in bottom waters increase, pelleted mudstones give way to structureless mudstones before visible bioturbation is noted.     

Palynostratigraphy of the Lower-Middle Jurassic deposits,borehole section Vostok 4 (Southeast of West Siberia)

This work presents the results of palynological investigation of the Vostok 4 borehole section situated in the Azharma region of the Ob-Irtysh facies area, where drilling has penetrated through the Urmanskaya, Ilansk, Peshkovskaya, and Tyumen formations. Six palynoassemblages are distinguished in the section based on the analyzed distribution of spores, pollen of terrestrial plants, and microphytoplankton. According to their succession, the section is subdivided into biostratigraphic units ranked as beds with palynomorphs. Comparative analysis and correlation with the Siberian palynostratigraphic standard scale of the Lower-Middle Jurassic, where succession of palynoassemblages is coordinated with ammonite zones, substantiate the Pliensbachian-Bajocian age of the deposits studied. Diverse microphytoplankton (dinoflagellate cysts, acritarchs, prasinophytes, zignematacean and colonial algae) occurs throughout the section. The facies environment of sedimentation is reconstructed based on quantitative proportions of microphytoplankton taxa.     

Jurassic and Cretaceous stratigraphy of the Anabar area (Arctic Siberia,Laptev Sea coast) and the Boreal zonal standard

Recent integrated studies of Mesozoic reference sections of the Anabar area (northern Middle Siberia, Laptev Sea coast) and the reinterpretation of all the previous data on a modern stratigraphic basis permit considerable improvement of the bio- and lithostratigraphic division and facies zoning of Jurassic and Cretaceous sediments in the region. Analysis of abundant paleontological data allows the development or considerable improvement of zonal scales for ammonites, belemnites, bivalves, foraminifers, ostracods, dinocysts, and terrestrial palynomorphs from several Jurassic and Cretaceous intervals. All the zonal scales have been calibrated against one another and against regional ammonite scale. Reference levels of different scales useful for interregional correlation have been defined and substantiated based on the analysis of lateral distribution of fossils in different regions of the Northern Hemisphere. It provides the possibilities to propose and consider parallel zonal scales within the Boreal zonal standard for the Jurassic and Cretaceous periods. A combination of these scales forms an integrated biostratigraphic basis for a detailed division of Boreal-type sediments regardless of the place of their formation and for the comparison with the international stratigraphic standard as far as a possible use of a set of reference levels for correlation.