2015 Vol. 40, No. 4
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2015, 40(4): 597-605.
doi: 10.3799/dqkx.2015.047
Abstract:
The reef carbonate rocks of well Xike-1 are analyzed for mineral compositions to explore the development process of reef and paleo-ocean environmental changes. It is found that carbonate minerals in the core of well Xike-1 are mainly composed of calcite (low-Mg and high-Mg), aragonite and dolomite. Mineral characteristics suggest that there is an important stratigraghic or environmental change interface at the core depth of 35.4 m. Five dolomite layers have been identified in the core(0-748 m)including one layer (289.3-312.3 m core depth) in the Yinggehai Formation of Pliocene, three layers (373.3-412.7 m, 424.7-450.6 m and 469.70-564.96 m respectively) in Huangliu Formation of Upper Miocene, and one layer (615.20-636.96 m) in the Meishan Formation of Middle Miocene. The biggest value of dolomite contents varies from 79.3% to 100%, indicating dolomitization difference in different dolomite layers. The development of the reef is mainly controlled by paleo-ocean environment changes and dolomitization of island reef is associated with sea-level changes controlled by paleoclimate events, such as the formation of ice sheet in Arctic in Pliocene, Messinian event in Late Miocene and expansion of ice sheet in Antarctic in Middle Miocene. The five dolomite layers should have been developed during a long-term stable environment mainly by the lagoon seepage-refluction model and possibly by mixed-water model.
The reef carbonate rocks of well Xike-1 are analyzed for mineral compositions to explore the development process of reef and paleo-ocean environmental changes. It is found that carbonate minerals in the core of well Xike-1 are mainly composed of calcite (low-Mg and high-Mg), aragonite and dolomite. Mineral characteristics suggest that there is an important stratigraghic or environmental change interface at the core depth of 35.4 m. Five dolomite layers have been identified in the core(0-748 m)including one layer (289.3-312.3 m core depth) in the Yinggehai Formation of Pliocene, three layers (373.3-412.7 m, 424.7-450.6 m and 469.70-564.96 m respectively) in Huangliu Formation of Upper Miocene, and one layer (615.20-636.96 m) in the Meishan Formation of Middle Miocene. The biggest value of dolomite contents varies from 79.3% to 100%, indicating dolomitization difference in different dolomite layers. The development of the reef is mainly controlled by paleo-ocean environment changes and dolomitization of island reef is associated with sea-level changes controlled by paleoclimate events, such as the formation of ice sheet in Arctic in Pliocene, Messinian event in Late Miocene and expansion of ice sheet in Antarctic in Middle Miocene. The five dolomite layers should have been developed during a long-term stable environment mainly by the lagoon seepage-refluction model and possibly by mixed-water model.
2015, 40(4): 606-614.
doi: 10.3799/dqkx.2015.048
Abstract:
Reef-bank carbonate is one of the most important oil and gas reservoirs in deep-water area of the South China Sea. By macroscopic description of the well Xike-1 core (0-576.5 m) and microscopic analysis, combining with paleontology, paleomagnetism, high-resolution core scanning and large amounts of test result analysis, the carbonate reef system is studied in detail in this paper. The research indicates that this well has recorded 6 exposure events, and 2 drowning events since Late Miocene. Based on analyses of paleontology, paleomagnetism and lithium isotopes, 8 third-order sequences can be recognized, among which Late Miocene Huangliu Formation and Quaternary Ledong Formation are the main reef growth phase. By detailed petrography study, the rock types, diagenetic types and pore types of the Quaternary reef carbonate are systematically depicted, and the diagenetic environments are rebuilt.
Reef-bank carbonate is one of the most important oil and gas reservoirs in deep-water area of the South China Sea. By macroscopic description of the well Xike-1 core (0-576.5 m) and microscopic analysis, combining with paleontology, paleomagnetism, high-resolution core scanning and large amounts of test result analysis, the carbonate reef system is studied in detail in this paper. The research indicates that this well has recorded 6 exposure events, and 2 drowning events since Late Miocene. Based on analyses of paleontology, paleomagnetism and lithium isotopes, 8 third-order sequences can be recognized, among which Late Miocene Huangliu Formation and Quaternary Ledong Formation are the main reef growth phase. By detailed petrography study, the rock types, diagenetic types and pore types of the Quaternary reef carbonate are systematically depicted, and the diagenetic environments are rebuilt.
2015, 40(4): 615-632.
doi: 10.3799/dqkx.2015.049
Abstract:
There are two pending key problems in research on western boundary faults (WBF) of South China Sea (SCS). One is itself structural-geometric linkage of the WBF of the SCS, that is whether there is a united dynamic boundary along the WBF of the SCS. The other one is dynamic consanguinity with Ailaoshan-Red River tectonic belt and Lupar line in region, that is, as a dynamic boundary, between which blocks the united dynamic boundary exists. For the sake of these two problems, the authors comparatively analyzed the structural geometry and structural kinetics of the major sections of the WBF, such as the Ailao-Red River (ARR)-East Vietnam (EV)-Wan'na (WN)-Lupar line (AEWL for short), based on the comprehensive analyses of geological, geophysical and geochemical data of the western margin area of the SCS. The AEWL is considered a mutual structural-geometric link-up system of strike-slip faults.The AEWL can be divided into three first-order segments: northern segment i.e.the ARR strike-slip fault zone; middle segment from mouth of Red River to the southwestern end of Southwest sub-basin of the SCS, through the EV, characterised by strike-slip and pull-apart; and southern segment along the east boundary fault of Wan'an basin and southward linking with Lupar line in northwestern Kalimantan Island, characterised by an extension-contract-type dextral strike-slip duplex system.Each first-order segment can be further divided into several second-order segments.Then combined with comparatively analyses on geological evolution history of blocks in the region and the analyses on the Cenozoic spreading model of the SCS lithosphere, a dynamic model of the WBF of the SCS during the formation process of the SCS was instituted, and a geological model, which suggests the ARR ran along the No.1 fault of eastern boundary fault of Yinggehai basin after entering the SCS, through Zhongjian ridge-EV-WN-Lupar line, and stopped at the west end of Bacu subduction-collision zone which results from southward subduction of paleo-SCS, was proposed. The reasonableness of the geological model was examined through the results of elasticity-medium-model numerical simulation.
There are two pending key problems in research on western boundary faults (WBF) of South China Sea (SCS). One is itself structural-geometric linkage of the WBF of the SCS, that is whether there is a united dynamic boundary along the WBF of the SCS. The other one is dynamic consanguinity with Ailaoshan-Red River tectonic belt and Lupar line in region, that is, as a dynamic boundary, between which blocks the united dynamic boundary exists. For the sake of these two problems, the authors comparatively analyzed the structural geometry and structural kinetics of the major sections of the WBF, such as the Ailao-Red River (ARR)-East Vietnam (EV)-Wan'na (WN)-Lupar line (AEWL for short), based on the comprehensive analyses of geological, geophysical and geochemical data of the western margin area of the SCS. The AEWL is considered a mutual structural-geometric link-up system of strike-slip faults.The AEWL can be divided into three first-order segments: northern segment i.e.the ARR strike-slip fault zone; middle segment from mouth of Red River to the southwestern end of Southwest sub-basin of the SCS, through the EV, characterised by strike-slip and pull-apart; and southern segment along the east boundary fault of Wan'an basin and southward linking with Lupar line in northwestern Kalimantan Island, characterised by an extension-contract-type dextral strike-slip duplex system.Each first-order segment can be further divided into several second-order segments.Then combined with comparatively analyses on geological evolution history of blocks in the region and the analyses on the Cenozoic spreading model of the SCS lithosphere, a dynamic model of the WBF of the SCS during the formation process of the SCS was instituted, and a geological model, which suggests the ARR ran along the No.1 fault of eastern boundary fault of Yinggehai basin after entering the SCS, through Zhongjian ridge-EV-WN-Lupar line, and stopped at the west end of Bacu subduction-collision zone which results from southward subduction of paleo-SCS, was proposed. The reasonableness of the geological model was examined through the results of elasticity-medium-model numerical simulation.
2015, 40(4): 633-644.
doi: 10.3799/dqkx.2015.050
Abstract:
The dolomites in the well Xike-1 drilled in Xisha Islands, South China Sea, are mainly found in the Upper Miocene Huangliu Formation and secondly found in the 2nd member of Pliocene Yinggehai Formation and in the Middle Miocene Meishan Formation. The major dolomite layers commonly develop below the exposed boundary that are dipped and dyed with brown ferric minerals. While the dolomite crystals are found to be bimodal with micrite and silty-fine dolomite by means of the casting thin sections, scanning electron microscope and cathodoluminescence. The hypidiomorphic micrite dolomites with flat crystal planes are the result of the dolomitization from the micrite matrix. Meanwhile, the silty-fine crystalline dolomites are clean and bright cement or dolomite crystals with misted heart and bright side, and the latter is generated from the so-called overdolomitization. It's thought that the cement dolomites are with the same composition to the misted heart of the overdolomitization dolomites due to their alike optical properties from the cathodoluminescence. It is indicated by means of the microelement and C, O isotopic measurements that the dolomite is with less iron and manganese composition, positive values of δ18OPDB and δ13CPDB. The δ18OPDB values range from 2.293‰ to 5.072‰, and the δ13CPDB are 1.214‰-3.051‰. The similar distribution of dolomite layers and the similar C, O isotopic values of dolomites in the wells Xike-1 and Xichen-1 might indicate the same or similar cause of the dolomites in these two wells in Xisha area. The dolomitization model of infiltration and circumfluence may be applied to that in the wells Xike-1 and Xichen-1. The highly permeable reef carbonates in Xisha area enabled the infiltration and circumfluence of the fluid with a high Mg/Ca ratio, which was affected by the frequent sea-level changes, the evaporitic environment in the atolls and the upwelling of the thermal fluid during the latest Miocene to Early Pliocene periods.
The dolomites in the well Xike-1 drilled in Xisha Islands, South China Sea, are mainly found in the Upper Miocene Huangliu Formation and secondly found in the 2nd member of Pliocene Yinggehai Formation and in the Middle Miocene Meishan Formation. The major dolomite layers commonly develop below the exposed boundary that are dipped and dyed with brown ferric minerals. While the dolomite crystals are found to be bimodal with micrite and silty-fine dolomite by means of the casting thin sections, scanning electron microscope and cathodoluminescence. The hypidiomorphic micrite dolomites with flat crystal planes are the result of the dolomitization from the micrite matrix. Meanwhile, the silty-fine crystalline dolomites are clean and bright cement or dolomite crystals with misted heart and bright side, and the latter is generated from the so-called overdolomitization. It's thought that the cement dolomites are with the same composition to the misted heart of the overdolomitization dolomites due to their alike optical properties from the cathodoluminescence. It is indicated by means of the microelement and C, O isotopic measurements that the dolomite is with less iron and manganese composition, positive values of δ18OPDB and δ13CPDB. The δ18OPDB values range from 2.293‰ to 5.072‰, and the δ13CPDB are 1.214‰-3.051‰. The similar distribution of dolomite layers and the similar C, O isotopic values of dolomites in the wells Xike-1 and Xichen-1 might indicate the same or similar cause of the dolomites in these two wells in Xisha area. The dolomitization model of infiltration and circumfluence may be applied to that in the wells Xike-1 and Xichen-1. The highly permeable reef carbonates in Xisha area enabled the infiltration and circumfluence of the fluid with a high Mg/Ca ratio, which was affected by the frequent sea-level changes, the evaporitic environment in the atolls and the upwelling of the thermal fluid during the latest Miocene to Early Pliocene periods.
2015, 40(4): 645-652.
doi: 10.3799/dqkx.2015.051
Abstract:
Reef carbonate rocks of well Xike-1 are analyzed for major constituents, trace and rare earth elements in this study, aimed to explore the formation, evolution and environmental change recorded in reef carbonates. The results show that there is an important interface of formation or environmental mutation at depth of 36 m, where geochemical parameters change significantly. The major constituents can be divided into following three components: the lithogenic carbonate components, consisting of CaO and K2O, corresponding to calcites; the enriched components by dolomitization, including MgO, Na2O and P2O5, which indicates restricted lagoon environment; the enriched components by reef-builders, composed of SiO2, Al2O3, TiO2, Fe2O3 and MnO, potentially including a small amount of volcanic components. Contents of most trace and rare earth elements in well Xike-1 are low compared to those of other endmembers such as carbonate rocks of Quaternary around the world, upper crustal, and shales. The contents of redox-sensitive trace elements (RSE) in well Xike-1 are low, indicating oxic environment through most of development process of reefs. There are alternations of oxidation and reduction environment, with wide fluctuations of RSE contents. In addition, there is no obvious correlation between mineral compositions and geochemical parameter distribution, such as trace elements, REEs and their characteristic parameters, indicating that dolomitization and diagenesis did not result in the significant changes of trace elements, REEs, and their eigenvalues.
Reef carbonate rocks of well Xike-1 are analyzed for major constituents, trace and rare earth elements in this study, aimed to explore the formation, evolution and environmental change recorded in reef carbonates. The results show that there is an important interface of formation or environmental mutation at depth of 36 m, where geochemical parameters change significantly. The major constituents can be divided into following three components: the lithogenic carbonate components, consisting of CaO and K2O, corresponding to calcites; the enriched components by dolomitization, including MgO, Na2O and P2O5, which indicates restricted lagoon environment; the enriched components by reef-builders, composed of SiO2, Al2O3, TiO2, Fe2O3 and MnO, potentially including a small amount of volcanic components. Contents of most trace and rare earth elements in well Xike-1 are low compared to those of other endmembers such as carbonate rocks of Quaternary around the world, upper crustal, and shales. The contents of redox-sensitive trace elements (RSE) in well Xike-1 are low, indicating oxic environment through most of development process of reefs. There are alternations of oxidation and reduction environment, with wide fluctuations of RSE contents. In addition, there is no obvious correlation between mineral compositions and geochemical parameter distribution, such as trace elements, REEs and their characteristic parameters, indicating that dolomitization and diagenesis did not result in the significant changes of trace elements, REEs, and their eigenvalues.
2015, 40(4): 653-659.
doi: 10.3799/dqkx.2015.052
Abstract:
The reef-carbonate rock types and reef types are described based on the core observation and petrologic characteristic analysis of reef-carbonate rocks in well Xike-1. Analyses of 543 casting lamellas and observation of 214.89 m core samples indicate that the reef-carbonate rock types are composed mainly of wackstone, packstone and framestone, and secondly of bindstone, grainstone, floatstone and rudstone. Vertical variations of rock types are characterized by dominant grain stone at a depth of 0-10 m, bioclastic calcarenite at a depth of 10-22 m, and wackstone, packstone and framestone at a depth of 22-214.89 m, respectively. It is concluded that the reef types are frame reefs and main reef-building organisms consisting of corals and a few coralline algae by the quantitative analysis results. Frame reefs develop at the depth of 0-214.89 m, and cluster and segment reefs only develop at the depth of 83-98 m.
The reef-carbonate rock types and reef types are described based on the core observation and petrologic characteristic analysis of reef-carbonate rocks in well Xike-1. Analyses of 543 casting lamellas and observation of 214.89 m core samples indicate that the reef-carbonate rock types are composed mainly of wackstone, packstone and framestone, and secondly of bindstone, grainstone, floatstone and rudstone. Vertical variations of rock types are characterized by dominant grain stone at a depth of 0-10 m, bioclastic calcarenite at a depth of 10-22 m, and wackstone, packstone and framestone at a depth of 22-214.89 m, respectively. It is concluded that the reef types are frame reefs and main reef-building organisms consisting of corals and a few coralline algae by the quantitative analysis results. Frame reefs develop at the depth of 0-214.89 m, and cluster and segment reefs only develop at the depth of 83-98 m.
2015, 40(4): 660-670.
doi: 10.3799/dqkx.2015.053
Abstract:
By means of paleontology and sedimentology, we conducted a study on specific intervals of wells BY7-1-1 and L29, which located in the deep-water area in northern South China Sea. The results from analyses of foraminifera, algae sporopollen and sedimentology, revealed that the sedimentary environment of Baiyun deep-water area in northern South China Sea during the Late Eocene belonged to littoral and neritic facies. This finding also confirmed the time when marine sediments first deposited in northern South China Sea. The distributions of marine strata during Eocene in northern South China Sea were mainly located in Taixi Basin and Taixinan Basin, and in Late Eocene the area of marine strata spread into Baiyun Sag of Pearl River Mouth Basin. The distribution of marine strata during Eocene in souther part of South China Sea was dissimilar to the north. The marine strata were widespread in Zengmu Basin, Beikang Basin, Liyue Basin and Palawan Basin etc.. The distribution of marine strata during Eocene in South China Sea is the combine effects of both the spreading of South China Sea and the shrinking of Proto-South China Sea. The marine strata distribution in E-M Eocene was quite different from Late Eocene with the area of marine strata enlarged as a whole. This reflects the aggravation of tension and fault movement in that period. The marine strata of Eocene in the South China Sea are provided with high petroleum petential. In some basins of this area it turns out to be great source rocks and reservoirs. Meanwhile, the discovery of Late Eocene marine strata in Baiyun Sag has significant reference value to the deep-water marine oil and gas exploration of South China Sea.
By means of paleontology and sedimentology, we conducted a study on specific intervals of wells BY7-1-1 and L29, which located in the deep-water area in northern South China Sea. The results from analyses of foraminifera, algae sporopollen and sedimentology, revealed that the sedimentary environment of Baiyun deep-water area in northern South China Sea during the Late Eocene belonged to littoral and neritic facies. This finding also confirmed the time when marine sediments first deposited in northern South China Sea. The distributions of marine strata during Eocene in northern South China Sea were mainly located in Taixi Basin and Taixinan Basin, and in Late Eocene the area of marine strata spread into Baiyun Sag of Pearl River Mouth Basin. The distribution of marine strata during Eocene in souther part of South China Sea was dissimilar to the north. The marine strata were widespread in Zengmu Basin, Beikang Basin, Liyue Basin and Palawan Basin etc.. The distribution of marine strata during Eocene in South China Sea is the combine effects of both the spreading of South China Sea and the shrinking of Proto-South China Sea. The marine strata distribution in E-M Eocene was quite different from Late Eocene with the area of marine strata enlarged as a whole. This reflects the aggravation of tension and fault movement in that period. The marine strata of Eocene in the South China Sea are provided with high petroleum petential. In some basins of this area it turns out to be great source rocks and reservoirs. Meanwhile, the discovery of Late Eocene marine strata in Baiyun Sag has significant reference value to the deep-water marine oil and gas exploration of South China Sea.
2015, 40(4): 671-676.
doi: 10.3799/dqkx.2015.054
Abstract:
Diagenesis and pore characteristics of Quaternary reef-carbonate adjacent typical exposed surfaces in well Xike-1 have been illuminated by means of core observation, thin-section rock identification and image analysis. The results show that the rock types around the typical exposed surface in the depth of 68.75-70.24 m include packestones and grainstone. And associated diagenesis includes dissolution, cementation, micritization and neomorphism. The pore types identified include intergranular pore, intergranular dissoved pore, intragranular pore, intragranular dissoved pore, skeleton growth pore, skeleton growth dissolution pore, modic pore and intergranular dissolved pore. Drapstone shape of the carbonate mineral, calcite filling in the pore and common dissolution of calcite indicate that the depth section was reformed strongly by the atmospheric water leaching.
Diagenesis and pore characteristics of Quaternary reef-carbonate adjacent typical exposed surfaces in well Xike-1 have been illuminated by means of core observation, thin-section rock identification and image analysis. The results show that the rock types around the typical exposed surface in the depth of 68.75-70.24 m include packestones and grainstone. And associated diagenesis includes dissolution, cementation, micritization and neomorphism. The pore types identified include intergranular pore, intergranular dissoved pore, intragranular pore, intragranular dissoved pore, skeleton growth pore, skeleton growth dissolution pore, modic pore and intergranular dissolved pore. Drapstone shape of the carbonate mineral, calcite filling in the pore and common dissolution of calcite indicate that the depth section was reformed strongly by the atmospheric water leaching.
2015, 40(4): 677-687.
doi: 10.3799/dqkx.2015.055
Abstract:
Carbonatite reef-bank system is one of the most important reservoirs in South China Sea. Well Xike-1, as a full-coring scientific drilling on the Paracel Islands, provides excellent conditions for the detailed study of the carbonatite reef-bank system. A deep study on carbonatite reef-bank system is made in the paper, using the macro-description and micro-analysis of cores obtained from well Xike-1, as well as high-resolution core scanning and many other tests results. The research indicates that 6 exposed surfaces, 2 flooding surfaces are recognized since Middle Miocene, and the well Xike-1 can be divided into 9 third-order sequences. The Late Miocene Huangliu Formation and Quaternary Ledong Formation are taken as the main reef-forming intervals, according to the characteristics of flooding surface and exposed surface, reef can be generalized into 2 types: submerged growth unit and exposed growth unit, the former can be subdivided into hard-based and soft-based, the latter can be subdivided into rapid-submerge and slow-submerge. Distinctive reef complexes are assembled vertically: drowned reef, aggradational reef, exposed reef. Results can be of huge value for the exploration of carbonatite reef-bank reservoirs, as well as the study of growing process of the reef in South China Sea.
Carbonatite reef-bank system is one of the most important reservoirs in South China Sea. Well Xike-1, as a full-coring scientific drilling on the Paracel Islands, provides excellent conditions for the detailed study of the carbonatite reef-bank system. A deep study on carbonatite reef-bank system is made in the paper, using the macro-description and micro-analysis of cores obtained from well Xike-1, as well as high-resolution core scanning and many other tests results. The research indicates that 6 exposed surfaces, 2 flooding surfaces are recognized since Middle Miocene, and the well Xike-1 can be divided into 9 third-order sequences. The Late Miocene Huangliu Formation and Quaternary Ledong Formation are taken as the main reef-forming intervals, according to the characteristics of flooding surface and exposed surface, reef can be generalized into 2 types: submerged growth unit and exposed growth unit, the former can be subdivided into hard-based and soft-based, the latter can be subdivided into rapid-submerge and slow-submerge. Distinctive reef complexes are assembled vertically: drowned reef, aggradational reef, exposed reef. Results can be of huge value for the exploration of carbonatite reef-bank reservoirs, as well as the study of growing process of the reef in South China Sea.
2015, 40(4): 688-696.
doi: 10.3799/dqkx.2015.056
Abstract:
The coral in 0-748 m of the well Xike-1 is less abundant and simple, 6 families including 16 genera and 1 undefined genus are identified. Some Quaternary genera such as Endopsammia and Heliopora are only found in the strata above 216 m depth and disappear below, which indicate that the strata above 216 m depth is Quaternary and the strata below is older. In term of time, the palaeosedimentary environment has been apparent changing and the ability of wave or wind resistance of the biology has been strengthening. It is lagoonal environment in Miocene, and then gradually changes to the inner reef flat facies. In Pliocene, the inner and outer reef flat environments alternately occur. Until the Quaternary, the palaeosedimentary environment may be outer reef flat facies.
The coral in 0-748 m of the well Xike-1 is less abundant and simple, 6 families including 16 genera and 1 undefined genus are identified. Some Quaternary genera such as Endopsammia and Heliopora are only found in the strata above 216 m depth and disappear below, which indicate that the strata above 216 m depth is Quaternary and the strata below is older. In term of time, the palaeosedimentary environment has been apparent changing and the ability of wave or wind resistance of the biology has been strengthening. It is lagoonal environment in Miocene, and then gradually changes to the inner reef flat facies. In Pliocene, the inner and outer reef flat environments alternately occur. Until the Quaternary, the palaeosedimentary environment may be outer reef flat facies.
2015, 40(4): 697-710.
doi: 10.3799/dqkx.2015.057
Abstract:
The fossil species, lithofacies types and genetic facies types of the reef-bank depositional system of Yinggehai Formation and Ledong Formation since the Pliocene have been systematically analyzed through the observation of microscopic thin slice of well Xike-1 in Xisha Islands. The results show that coral and red algae are the main reef-building organisms in the Ledong Formation, while red algae is the dominant reef-forming organism in the Yinggehai Formation. Reef-dwelling organisms in the Yinggehai and Ledong formations include primarily foraminifer, echinoderm and green algae, of which foraminifer is the most important, and green algae only develops prosperously at intervals. Ten lithofacies types have been identified according to the fossil species and their contents, and content relationship of micrite, calcsparite and intergranular pore. On this basis, reef-bank system could be divided into three genetic facies groups including reef, biodetritus beach and lagoon. Reef includes three genetic facies types, namely, reef base, reef core and reef cap, among which significant exposures can be observed at reef cap, leading to the formation of secondary pores, such as intragranular dissolved pore, moldic pore, etc.. The backreef bank only develops in well Xike-1, in which there are more content of bioclast and intergranular pore, and minor micrite in the backreef inner bank close to reef core, whereas the opposite is observed in backreef outer bank near the lagoon. Large amounts of micrite limestones with little bioclast are mainly deposited in lagoon environment. Reef-bank system only develops in transgressive system tract and highstand system tract, in which genetic facies have relatively stable vertical stack relationships. The transgressive system tract commonly forms the retrogradational genetic facies group series in which parasequence ends with genetic facies having more micrite representing the flooding surface, but highstand system tract mainly develops the progradational genetic facies group series whose parasequence ends with reef cap corresponding to exposed surface.
The fossil species, lithofacies types and genetic facies types of the reef-bank depositional system of Yinggehai Formation and Ledong Formation since the Pliocene have been systematically analyzed through the observation of microscopic thin slice of well Xike-1 in Xisha Islands. The results show that coral and red algae are the main reef-building organisms in the Ledong Formation, while red algae is the dominant reef-forming organism in the Yinggehai Formation. Reef-dwelling organisms in the Yinggehai and Ledong formations include primarily foraminifer, echinoderm and green algae, of which foraminifer is the most important, and green algae only develops prosperously at intervals. Ten lithofacies types have been identified according to the fossil species and their contents, and content relationship of micrite, calcsparite and intergranular pore. On this basis, reef-bank system could be divided into three genetic facies groups including reef, biodetritus beach and lagoon. Reef includes three genetic facies types, namely, reef base, reef core and reef cap, among which significant exposures can be observed at reef cap, leading to the formation of secondary pores, such as intragranular dissolved pore, moldic pore, etc.. The backreef bank only develops in well Xike-1, in which there are more content of bioclast and intergranular pore, and minor micrite in the backreef inner bank close to reef core, whereas the opposite is observed in backreef outer bank near the lagoon. Large amounts of micrite limestones with little bioclast are mainly deposited in lagoon environment. Reef-bank system only develops in transgressive system tract and highstand system tract, in which genetic facies have relatively stable vertical stack relationships. The transgressive system tract commonly forms the retrogradational genetic facies group series in which parasequence ends with genetic facies having more micrite representing the flooding surface, but highstand system tract mainly develops the progradational genetic facies group series whose parasequence ends with reef cap corresponding to exposed surface.
2015, 40(4): 711-717.
doi: 10.3799/dqkx.2015.058
Abstract:
In this study, petrographic and diagenetic characteristics of Quaternary reef-carbonates in well Xike-1 have been illuminated according to the observation analyses of polarizing microscope, alizarine red S and potassium ferricyanide composite staining, scanning electron microscope(SEM) and cathode luminescence. It is found that the diagenesis types include predominately weak compaction, neomorphism, micritization, dissolution and cementation. The diagenetic environments have been identified using the typical petrographic occurrences of cements and the cathode luminescence features. Among which, meniscus and pendant calcite indicate meteoric diagenetic environment and fibrous-needle aragonite indicates marine diagenetic environment. Diagenetic environments can be divided into 5 zones vertically including significant typical meteoric environment at depth of 0-27 m; the complex diagenetic environment of predominately meteoric and secondly marine at depth of 27-42 m; meteoric environment with weak influence of marine environment at depth of 42-105 m; relatively distinct evidences of both meteoric and marine environments at depth of 105-172 m; obvious marine environment at depth of 172.00-214.89 m.
In this study, petrographic and diagenetic characteristics of Quaternary reef-carbonates in well Xike-1 have been illuminated according to the observation analyses of polarizing microscope, alizarine red S and potassium ferricyanide composite staining, scanning electron microscope(SEM) and cathode luminescence. It is found that the diagenesis types include predominately weak compaction, neomorphism, micritization, dissolution and cementation. The diagenetic environments have been identified using the typical petrographic occurrences of cements and the cathode luminescence features. Among which, meniscus and pendant calcite indicate meteoric diagenetic environment and fibrous-needle aragonite indicates marine diagenetic environment. Diagenetic environments can be divided into 5 zones vertically including significant typical meteoric environment at depth of 0-27 m; the complex diagenetic environment of predominately meteoric and secondly marine at depth of 27-42 m; meteoric environment with weak influence of marine environment at depth of 42-105 m; relatively distinct evidences of both meteoric and marine environments at depth of 105-172 m; obvious marine environment at depth of 172.00-214.89 m.
2015, 40(4): 718-724.
doi: 10.3799/dqkx.2015.059
Abstract:
This paper presents our study of the calcareous algae in the well Xike-1, Xisha Islands, and it is found that there are 3 families and 7 genera in the Quaternary sedimentary deposit mainly including Rhodophyta Corallinaceae Jania, Amphiroa, Corallina, Lithothamnium, Lithoporella and Rhodophyta Chlorophyta Codiaceae Halimeda and Dasycladaceae Cymopolia. Analysis of the stratigraphic distribution of the calcareous algae in the well Xike-1 shows that it was forereef facies in the early Quaternary and then became the outer reef flat facies alternated with the inner reef flat facies in the middle Quaternary. In the later Quaternary, it developed into the backreef beach facies mixed with brief lagoon facies. Calcareous algae played the role of providing calcareous skeleton debris and building reef framework.
This paper presents our study of the calcareous algae in the well Xike-1, Xisha Islands, and it is found that there are 3 families and 7 genera in the Quaternary sedimentary deposit mainly including Rhodophyta Corallinaceae Jania, Amphiroa, Corallina, Lithothamnium, Lithoporella and Rhodophyta Chlorophyta Codiaceae Halimeda and Dasycladaceae Cymopolia. Analysis of the stratigraphic distribution of the calcareous algae in the well Xike-1 shows that it was forereef facies in the early Quaternary and then became the outer reef flat facies alternated with the inner reef flat facies in the middle Quaternary. In the later Quaternary, it developed into the backreef beach facies mixed with brief lagoon facies. Calcareous algae played the role of providing calcareous skeleton debris and building reef framework.
2015, 40(4): 725-732.
doi: 10.3799/dqkx.2015.060
Abstract:
The oxygen stable isotopes in carbonate sediment of well Xike-1 cannot be used to calibrate strata ages due to the destructive effect of recrystallization and dolomitization, while the paired δ13C records can be used to calibrate Xike-1 strata age since 200 ka, because of the similar patterns with those of the South China Sea and other oceans. Accordingly, the 0-50 m sediment of the well corresponds to the oxygen isotope stages (MIS) 1-7, with the MIS 1/2 boundary (14 ka) at 5 m, the MIS 2/3 boundary (29 ka) at 11.70 m, the MIS 3/4 boundary (57 ka) at 13.90 m, the MIS 4/5 boundary (71 ka) at 16.8 m, the MIS 5/6 boundary (129 ka) at 23.8 m and the MIS 6/7 boundary at 35.65 m respectively. Moreover, the δ13C records exhibit low values in glacial periods and high values in interglacial periods. The U-Th dating of coral from 25.21 m is 131.062±2.320 ka. It is concluded from the dating that there are no sediments younger than 5 ka existing on the Shidao Island of Xisha, and the transition from the interglacial to glacial periods with downward movement of global sea-level lead to the exposition and erosion of the local carbonate platform, which indicates that the abrupt changes of the δ13C records of the carbonate sediments from Xike-1 are responses to global climate variations.
The oxygen stable isotopes in carbonate sediment of well Xike-1 cannot be used to calibrate strata ages due to the destructive effect of recrystallization and dolomitization, while the paired δ13C records can be used to calibrate Xike-1 strata age since 200 ka, because of the similar patterns with those of the South China Sea and other oceans. Accordingly, the 0-50 m sediment of the well corresponds to the oxygen isotope stages (MIS) 1-7, with the MIS 1/2 boundary (14 ka) at 5 m, the MIS 2/3 boundary (29 ka) at 11.70 m, the MIS 3/4 boundary (57 ka) at 13.90 m, the MIS 4/5 boundary (71 ka) at 16.8 m, the MIS 5/6 boundary (129 ka) at 23.8 m and the MIS 6/7 boundary at 35.65 m respectively. Moreover, the δ13C records exhibit low values in glacial periods and high values in interglacial periods. The U-Th dating of coral from 25.21 m is 131.062±2.320 ka. It is concluded from the dating that there are no sediments younger than 5 ka existing on the Shidao Island of Xisha, and the transition from the interglacial to glacial periods with downward movement of global sea-level lead to the exposition and erosion of the local carbonate platform, which indicates that the abrupt changes of the δ13C records of the carbonate sediments from Xike-1 are responses to global climate variations.
2015, 40(4): 733-743.
doi: 10.3799/dqkx.2015.061
Abstract:
Using high-resolution 2D seismic data, this study reveals the presence of a deep-water contourite depositional system that developed in vicinity of the Yi'tong Shoal (700-1 000 m depth), on the northern margin of the South China Sea. The development of a moat north of the Yi'tong Shoal indicates eastward flowing bottom currents, which might belong to the South China Sea Intermediate Water Circulation. Due to the coriolis deflection, an elongated-mounded drift is developed to the north of the moat. A marginal trough, a plastered drift and a deposition zone developed south of the Yi'tong Shoal, indicative of relatively weak hydrodynamics in the south side. A shadow zone downstream of the Yi'tong Shoal was generated where the currents slowed down, developing a depositional tail. Seismic stratigraphy shows that the initial development of the deep-water contourite depositional system was developed from the early Late Miocene (11.6 Ma).
Using high-resolution 2D seismic data, this study reveals the presence of a deep-water contourite depositional system that developed in vicinity of the Yi'tong Shoal (700-1 000 m depth), on the northern margin of the South China Sea. The development of a moat north of the Yi'tong Shoal indicates eastward flowing bottom currents, which might belong to the South China Sea Intermediate Water Circulation. Due to the coriolis deflection, an elongated-mounded drift is developed to the north of the moat. A marginal trough, a plastered drift and a deposition zone developed south of the Yi'tong Shoal, indicative of relatively weak hydrodynamics in the south side. A shadow zone downstream of the Yi'tong Shoal was generated where the currents slowed down, developing a depositional tail. Seismic stratigraphy shows that the initial development of the deep-water contourite depositional system was developed from the early Late Miocene (11.6 Ma).
2015, 40(4): 744-762.
doi: 10.3799/dqkx.2015.062
Abstract:
In order to better understand the evolution and mechanism of Cenozoic sedimentary basins in the South China Sea, we employed tectono-stratigraphic analysis on the sedimentary basins in the South China Sea. Reviewing the previous studies on the tectonics and sedimentation in the South China Sea and its peripheral regions, we used a compilation of newly acquisition and reprocessed seismic reflection profiles covering the large basins and got a clear result of the basins in the South China Sea with different basin structures, evolutions and mechanism. Considering the nature of the basin developed in the South China Sea, we suggested in our paper that the basins in the South China Sea can be divided into two sedimentary basin groups by a tectonic boundary, which is characterized by a transform boundary of the Red River fault extended south along the Vietnamese margin and joined with the trench along the Lupar line. These two basin groups are collsion-extrusion tectonic province basin group and Proto-South China Sea slab pull tectonic province basin group. The Proto-South China Sea tectonic province basin group is strongly controlled by the southward subduction of Proto-South China Sea. These basins include Beibuwan, Qiongdongnan, Pearl River Mouth, Zengmu, Beikang, Brunei-Sabah, Liyue basins and so on.However, the evolution of the collision-extrusion tectonic province basin group is controlled by the extrusion toward the southeast and rotation of Indochina block, which is as a result of the indentation of rigid India into Asia. This kind of basins mainly includes Yinggehai, Zhongjiannan, Mekong, Wan'an basins and so on. Considering the peripheral plate dynamic events, the tectonics of these basins and significant boundaries refined on the seismic profiles, the evolution of the South China Sea and its peripheral regions is reconstructed in our paper.
In order to better understand the evolution and mechanism of Cenozoic sedimentary basins in the South China Sea, we employed tectono-stratigraphic analysis on the sedimentary basins in the South China Sea. Reviewing the previous studies on the tectonics and sedimentation in the South China Sea and its peripheral regions, we used a compilation of newly acquisition and reprocessed seismic reflection profiles covering the large basins and got a clear result of the basins in the South China Sea with different basin structures, evolutions and mechanism. Considering the nature of the basin developed in the South China Sea, we suggested in our paper that the basins in the South China Sea can be divided into two sedimentary basin groups by a tectonic boundary, which is characterized by a transform boundary of the Red River fault extended south along the Vietnamese margin and joined with the trench along the Lupar line. These two basin groups are collsion-extrusion tectonic province basin group and Proto-South China Sea slab pull tectonic province basin group. The Proto-South China Sea tectonic province basin group is strongly controlled by the southward subduction of Proto-South China Sea. These basins include Beibuwan, Qiongdongnan, Pearl River Mouth, Zengmu, Beikang, Brunei-Sabah, Liyue basins and so on.However, the evolution of the collision-extrusion tectonic province basin group is controlled by the extrusion toward the southeast and rotation of Indochina block, which is as a result of the indentation of rigid India into Asia. This kind of basins mainly includes Yinggehai, Zhongjiannan, Mekong, Wan'an basins and so on. Considering the peripheral plate dynamic events, the tectonics of these basins and significant boundaries refined on the seismic profiles, the evolution of the South China Sea and its peripheral regions is reconstructed in our paper.
