2019 Vol. 37, No. 3
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2019, 37(3): 443-454.
doi: 10.14027/j.issn.1000-0550.2018.148
Abstract:
Based on core observations, thin section identification, geochemical analysis, and regional geological background research, this paper systematically studied the genesis of dolomite in the Sinian Dengying Formation in the Sichuan Basin. The petrological characteristics show that Dengying dolomite can be divided into four types, including micritic to powder crystal dolomite, grain dolomite, microbial dolomite, and carneule. Its structural type is micritic crystal to powder crystal, indicating that dolomitization occurred earlier, during the penecontemporaneous stage. Geochemical analysis showed that the average δ13C of Dengying dolomite is close to that of the Sinian original seawater, and the δ18O value is characterized by a medium-high negative bias. The order degree and the Fe and Mn contents are relatively low, but the Na content is relatively high. The rare earth element partition curve of the dolomite is similar to that of the seawater, indicating that dolomization was mainly affected by sea-source fluids. Further analysis shows that the ancient lands and submarine uplifts around the basin were able to easily trap the seawater in the basin and keep it concentrated. The Ca2+-rich algae in the basin could increase the Mg2+/Ca2+ ratio of the seawater indirectly. Meanwhile, the evaporation in the basin was strong, caused by the dry-hot palaeoclimate during the Dengying period, and the dolomite was often accompanied with evaporite minerals such as gypsum, which could promote dolomitization. Based on the above characteristics, this paper considers that the genesis of dolomite from the Dengying Formation in the Sichuan Basin was evaporative seawater seepage-reflux dolomitization under the participation of microorganisms.
Based on core observations, thin section identification, geochemical analysis, and regional geological background research, this paper systematically studied the genesis of dolomite in the Sinian Dengying Formation in the Sichuan Basin. The petrological characteristics show that Dengying dolomite can be divided into four types, including micritic to powder crystal dolomite, grain dolomite, microbial dolomite, and carneule. Its structural type is micritic crystal to powder crystal, indicating that dolomitization occurred earlier, during the penecontemporaneous stage. Geochemical analysis showed that the average δ13C of Dengying dolomite is close to that of the Sinian original seawater, and the δ18O value is characterized by a medium-high negative bias. The order degree and the Fe and Mn contents are relatively low, but the Na content is relatively high. The rare earth element partition curve of the dolomite is similar to that of the seawater, indicating that dolomization was mainly affected by sea-source fluids. Further analysis shows that the ancient lands and submarine uplifts around the basin were able to easily trap the seawater in the basin and keep it concentrated. The Ca2+-rich algae in the basin could increase the Mg2+/Ca2+ ratio of the seawater indirectly. Meanwhile, the evaporation in the basin was strong, caused by the dry-hot palaeoclimate during the Dengying period, and the dolomite was often accompanied with evaporite minerals such as gypsum, which could promote dolomitization. Based on the above characteristics, this paper considers that the genesis of dolomite from the Dengying Formation in the Sichuan Basin was evaporative seawater seepage-reflux dolomitization under the participation of microorganisms.
2019, 37(3): 455-465.
doi: 10.14027/j.issn.1000-0550.2018.145
Abstract:
In studying the black fine-grained rocks from the Permian Lucaogou Formation of Santanghu Basin, which has long been considered to consist mostly of mudstone, tuff, and carbonate rocks, a series of special laminar sedimentary rocks were discovered. These rocks are neither composed of terrigenous clastic sediments nor intraclast, and they are different from hydrothermal sedimentary rocks (exhalative rocks). Our research team named the series magmatic-hydrothermal explosion rock (MHER). MHER is formed by deep derived magmatic-hydrothermal material (and fluid), which usually forms via a vapor-liquid-solid mechanism, upwelling along the exhalite channels, erupting to the bottom of lake floor and then depositing with the surrounding hydrothermal fluids and water. MHER presents laminated sediments, distributed randomly, and dominantly composed of unstable minerals and metastable minerals with low-grade texture maturity. Based on the observation of a large number of cores and thin sections, in combination with electronic probes and other relative data, this paper classifies explosion rocks into four types according to their different mineral compositions:calcite explosion rock, dolomite explosion rock, diopside explosion rock, and felsic explosion rock. Petrology and mineralogy features of all types are described. MHER developed in an uncompensated lake of an intracontinental rift. The clastics of MHER were more likely derived from magmatic-hydrothermal material or fluid at a variety of depths in the Earth's interior, rather than weathered source rocks or hydrothermal precipitation. This paper also discusses the differences between MHER, sedimentary clastic rock, pyroclastic rock, and exhalaite. Rock-forming material sources, distribution scale, and forming mechanisms can distinguish MHER from the rocks mentioned above.
In studying the black fine-grained rocks from the Permian Lucaogou Formation of Santanghu Basin, which has long been considered to consist mostly of mudstone, tuff, and carbonate rocks, a series of special laminar sedimentary rocks were discovered. These rocks are neither composed of terrigenous clastic sediments nor intraclast, and they are different from hydrothermal sedimentary rocks (exhalative rocks). Our research team named the series magmatic-hydrothermal explosion rock (MHER). MHER is formed by deep derived magmatic-hydrothermal material (and fluid), which usually forms via a vapor-liquid-solid mechanism, upwelling along the exhalite channels, erupting to the bottom of lake floor and then depositing with the surrounding hydrothermal fluids and water. MHER presents laminated sediments, distributed randomly, and dominantly composed of unstable minerals and metastable minerals with low-grade texture maturity. Based on the observation of a large number of cores and thin sections, in combination with electronic probes and other relative data, this paper classifies explosion rocks into four types according to their different mineral compositions:calcite explosion rock, dolomite explosion rock, diopside explosion rock, and felsic explosion rock. Petrology and mineralogy features of all types are described. MHER developed in an uncompensated lake of an intracontinental rift. The clastics of MHER were more likely derived from magmatic-hydrothermal material or fluid at a variety of depths in the Earth's interior, rather than weathered source rocks or hydrothermal precipitation. This paper also discusses the differences between MHER, sedimentary clastic rock, pyroclastic rock, and exhalaite. Rock-forming material sources, distribution scale, and forming mechanisms can distinguish MHER from the rocks mentioned above.
2019, 37(3): 466-476.
doi: 10.14027/j.issn.1000-0550.2018.151
Abstract:
The Waiotapu geothermal system is located in the centre of the Taupo Volcanic Zone of New Zealand, famous for numerous silicic volcanics. Through X-ray diffraction, electron probe, SEM, and the laser Raman test, we examine the petrological and mineralogical features and the genesis of special structures from acid volcanic rock in the period of condensing consolid-supergene reformation. We determined that there are three types of the deposits in the crater:1) Spherulite rhyolite, with the following mineral composition:α-cristobalite, po-tridymite, and orthoclase; Plagioclase phenocryst; Hypersthene and titanomagnetite (ilmenite). 2) Rhyolitic ignimbrite:Plagioclase phenocryst; Plastic glass. 3) Sulfur:Powdered sulphur; Felsic fine-grained detrital rock. Comprehensively, spherulite rhyolite is formed by a volcanic glass-rich rhyolite devitrified from volcanic glass under the effect of a hydrotherm, which is a sign of supergene reformation in acidic volcanic rocks. Rhyolitic ignimbrite is formed by the crystallization of rhyolitic magma during the plastic flow process after they reach the surface, which is a sign of cooling until the rock becomes a condensing consolid in acidic volcanic rocks. According to the sedimentary characteristics, a pattern of supergene reformation in acid-volcanic rock has been established, which provides a modern sample for the study of condensing consolid-supergene reformation of acid-volcanic rock.
The Waiotapu geothermal system is located in the centre of the Taupo Volcanic Zone of New Zealand, famous for numerous silicic volcanics. Through X-ray diffraction, electron probe, SEM, and the laser Raman test, we examine the petrological and mineralogical features and the genesis of special structures from acid volcanic rock in the period of condensing consolid-supergene reformation. We determined that there are three types of the deposits in the crater:1) Spherulite rhyolite, with the following mineral composition:α-cristobalite, po-tridymite, and orthoclase; Plagioclase phenocryst; Hypersthene and titanomagnetite (ilmenite). 2) Rhyolitic ignimbrite:Plagioclase phenocryst; Plastic glass. 3) Sulfur:Powdered sulphur; Felsic fine-grained detrital rock. Comprehensively, spherulite rhyolite is formed by a volcanic glass-rich rhyolite devitrified from volcanic glass under the effect of a hydrotherm, which is a sign of supergene reformation in acidic volcanic rocks. Rhyolitic ignimbrite is formed by the crystallization of rhyolitic magma during the plastic flow process after they reach the surface, which is a sign of cooling until the rock becomes a condensing consolid in acidic volcanic rocks. According to the sedimentary characteristics, a pattern of supergene reformation in acid-volcanic rock has been established, which provides a modern sample for the study of condensing consolid-supergene reformation of acid-volcanic rock.
2019, 37(3): 477-490.
doi: 10.14027/j.issn.1000-0550.2018.157
Abstract:
A set of clastic-carbonate rocks developed in the Maerkansu of West Kunlun during the Late Carboniferous recently revealed that it has a great potential for rhodochrosite prospecting and has attracted significant attention. The clastic-carbonate lithofacies division and sedimentary environment analyses were carried out by measuring the manganese-bearing rock series profile, thin section identification, electron probe, scanning electron microscope, geochemical analysis, and strontium isotope testing of the carbonate rocks in the Muhu manganese mine. Our study indicates that the manganese-bearing rock series consists of eight clastic-carbonate rock facies:marlite, arenaceous limestone, microcrystalline manganese carbonate, microcrystalline-micrite limestone, gravelly limestone, gravelly arenaceous limestone, calcareous sandstone, calcareous conglomerate, and combinations of three rock facies. Based on the geochemical and strontium isotope characteristics of the ore body and its roof and floor rocks, the ancient seawater temperature averaged 22.68℃, which belongs to the subtropical zone. In all, the manganese-bearing rocks are clastic shore facies and shallow sea-land shelf facies deposits with shallow seawater. The roof and floor rocks of the ore body were in a relatively shallow oxidation-weak reduction environment with seawater, while the rhodochrosite was formed in shallow sea-land shelf sedimentary depressions with relatively deep seawater under a reduced sedimentary environment, which is related to submarine volcanic hydrothermal activity.
A set of clastic-carbonate rocks developed in the Maerkansu of West Kunlun during the Late Carboniferous recently revealed that it has a great potential for rhodochrosite prospecting and has attracted significant attention. The clastic-carbonate lithofacies division and sedimentary environment analyses were carried out by measuring the manganese-bearing rock series profile, thin section identification, electron probe, scanning electron microscope, geochemical analysis, and strontium isotope testing of the carbonate rocks in the Muhu manganese mine. Our study indicates that the manganese-bearing rock series consists of eight clastic-carbonate rock facies:marlite, arenaceous limestone, microcrystalline manganese carbonate, microcrystalline-micrite limestone, gravelly limestone, gravelly arenaceous limestone, calcareous sandstone, calcareous conglomerate, and combinations of three rock facies. Based on the geochemical and strontium isotope characteristics of the ore body and its roof and floor rocks, the ancient seawater temperature averaged 22.68℃, which belongs to the subtropical zone. In all, the manganese-bearing rocks are clastic shore facies and shallow sea-land shelf facies deposits with shallow seawater. The roof and floor rocks of the ore body were in a relatively shallow oxidation-weak reduction environment with seawater, while the rhodochrosite was formed in shallow sea-land shelf sedimentary depressions with relatively deep seawater under a reduced sedimentary environment, which is related to submarine volcanic hydrothermal activity.
2019, 37(3): 491-501.
doi: 10.14027/j.issn.1000-0550.2018.153
Abstract:
The Late Mesozoic Chuxiong Basin is located at the intersection of the western margin of the Yangtze platform and the Sanjiang orogenic belt in Yunnan province, SW China. Abundant strata-bound copper deposits and occurrences within multiple Cretaceous sandstones or mudstones have attracted considerable interest since the late 1970s. A significant amount of care was previously paid to the ore genesis of the sandstone-hosted copper deposits, but sedimentary environments and sources of their host rocks were overlooked, which seriously affects our understanding of their tectonic setting and ore-forming mechanism. Systematic geological surveying, tunnels, and drillhole logging demonstrate that the Upper Cretaceous Matoushan and Jiangdihe Formations were deposited in a braided river channel and marginal bank subfacies environment with a relatively dry climate. Detrital fragments of sandstone, conglomerate composition, and paleocurrent data demonstrate that the Chuxiong Basin developed in a southward facing paleogeography with a mixed source consisting of northern source rocks and basement uplift during the Late Cretaceous. Vast basic volcanics, granitoids, clastic rocks, carbonates, and minor metamorphic rocks should be exposed at the northern source area. The Upper Cretaceous Jiangdihe Formation also experienced the uplift and collapse events caused by syn-sedimentary compressional tectonics, and the sandstone-and mudstone-hosted copper mineralization was coeval with the growing tectonic events during the syn-sedimentation processes of the Late Cretaceous Jiangdihe Formation. This is a vital controlling factor of the major copper transport and deposition of the Guihua ore field in the Chuxiong Basin.
The Late Mesozoic Chuxiong Basin is located at the intersection of the western margin of the Yangtze platform and the Sanjiang orogenic belt in Yunnan province, SW China. Abundant strata-bound copper deposits and occurrences within multiple Cretaceous sandstones or mudstones have attracted considerable interest since the late 1970s. A significant amount of care was previously paid to the ore genesis of the sandstone-hosted copper deposits, but sedimentary environments and sources of their host rocks were overlooked, which seriously affects our understanding of their tectonic setting and ore-forming mechanism. Systematic geological surveying, tunnels, and drillhole logging demonstrate that the Upper Cretaceous Matoushan and Jiangdihe Formations were deposited in a braided river channel and marginal bank subfacies environment with a relatively dry climate. Detrital fragments of sandstone, conglomerate composition, and paleocurrent data demonstrate that the Chuxiong Basin developed in a southward facing paleogeography with a mixed source consisting of northern source rocks and basement uplift during the Late Cretaceous. Vast basic volcanics, granitoids, clastic rocks, carbonates, and minor metamorphic rocks should be exposed at the northern source area. The Upper Cretaceous Jiangdihe Formation also experienced the uplift and collapse events caused by syn-sedimentary compressional tectonics, and the sandstone-and mudstone-hosted copper mineralization was coeval with the growing tectonic events during the syn-sedimentation processes of the Late Cretaceous Jiangdihe Formation. This is a vital controlling factor of the major copper transport and deposition of the Guihua ore field in the Chuxiong Basin.
2019, 37(3): 502-510.
doi: 10.14027/j.issn.1000-0550.2018.173
Abstract:
Compared with traditional grain-size test methods, research on the application of the dynamic image and thin-section measuring methods in underground sedimentary rock grain-size analysis is currently relatively weak. In this paper, taking 12 sandstone samples from Well X100 in the Lunnan area, Tarim basin and from underwater distributary channels of Triassic, we compared and analyzed the results obtained using the dynamic image and thin-section measuring methods. The content of each grain gradation, grain-size curves, and grain-size parameters are included in the testing results. Then calibration research of the testing results obtained from the thin-section measuring method was carried out. This paper aims to make the two methods more widely applicable to grain-size analysis in the future. The research results show that, compared with results obtained from the thin-section measuring method, the content of coarser sand measured using the dynamic image method is higher and that of finer sand is correspondingly smaller. The clay-to-silt content and medium sand content represent the boundary line which differentiates the two methods. The sharp contrast of detecting particle amount and lithologic characteristics are the main reasons for the difference, upon which the measurement range, measurement principle, and particle shape also exert some influence. The calibration method for the thin-section measuring method proposed here is based on rock type and makes the correlation coefficients (R2) of the testing results (content of fine sand to coarse sand and grain diameters) by the post-calibrated thin-section measuring method with the dynamic image method increase from 0.25-0.80 to 0.88-0.95, achieving the primary purpose of calibration and improving the accuracy of the thin-section measuring method.
Compared with traditional grain-size test methods, research on the application of the dynamic image and thin-section measuring methods in underground sedimentary rock grain-size analysis is currently relatively weak. In this paper, taking 12 sandstone samples from Well X100 in the Lunnan area, Tarim basin and from underwater distributary channels of Triassic, we compared and analyzed the results obtained using the dynamic image and thin-section measuring methods. The content of each grain gradation, grain-size curves, and grain-size parameters are included in the testing results. Then calibration research of the testing results obtained from the thin-section measuring method was carried out. This paper aims to make the two methods more widely applicable to grain-size analysis in the future. The research results show that, compared with results obtained from the thin-section measuring method, the content of coarser sand measured using the dynamic image method is higher and that of finer sand is correspondingly smaller. The clay-to-silt content and medium sand content represent the boundary line which differentiates the two methods. The sharp contrast of detecting particle amount and lithologic characteristics are the main reasons for the difference, upon which the measurement range, measurement principle, and particle shape also exert some influence. The calibration method for the thin-section measuring method proposed here is based on rock type and makes the correlation coefficients (R2) of the testing results (content of fine sand to coarse sand and grain diameters) by the post-calibrated thin-section measuring method with the dynamic image method increase from 0.25-0.80 to 0.88-0.95, achieving the primary purpose of calibration and improving the accuracy of the thin-section measuring method.
2019, 37(3): 511-518.
doi: 10.14027/j.issn.1000-0550.2018.161
Abstract:
Single mineral geochemistry is an important approach for sedimentary provenance analysis and is widely used. Whether grain size compositions of clastic minerals (i.e., hydrodynamic sorting process) affect sedimentary provenance interpretation is not yet clear, and not enough attention has been given to this issue. In this paper, 680 detrital garnets (0.068-0.557 mm) from the Tertiary sedimentary rocks in the northern Qaidam basin were studied, and the provenance of the detrital garnets with different grain sizes was interpreted based on major element geochemical data. Fe2+ and Mn2+ contents are higher when the grain size of the garnet is between 0.063 mm and 0.125 mm, so the result of sedimentary provenance interpretation is more likely to be from the metamorphism of lower-level amphibolite facies. In the other grains, Ca2+ and Mg2+ contents are higher, and the resulting explanation of provenance is the broad result of low-grade metamorphic facies, intermediate-acidic igneous rocks and eclogite, etc. Different geochemical compositions caused by the difference of grain size of the clastic garnet show the grain size of the detrital garnets, which will affect the geochemistry-based provenance interpretation. Therefore, in the case of single mineral geochemistry-based provenance analysis, concentrating on detrital minerals with certain grain size compositions (0.063-0.125 mm or 0.125-0.25 mm) can eliminate the influence of the hydrodynamic sorting process and help obtain more accurate provenance interpretation results.e
Single mineral geochemistry is an important approach for sedimentary provenance analysis and is widely used. Whether grain size compositions of clastic minerals (i.e., hydrodynamic sorting process) affect sedimentary provenance interpretation is not yet clear, and not enough attention has been given to this issue. In this paper, 680 detrital garnets (0.068-0.557 mm) from the Tertiary sedimentary rocks in the northern Qaidam basin were studied, and the provenance of the detrital garnets with different grain sizes was interpreted based on major element geochemical data. Fe2+ and Mn2+ contents are higher when the grain size of the garnet is between 0.063 mm and 0.125 mm, so the result of sedimentary provenance interpretation is more likely to be from the metamorphism of lower-level amphibolite facies. In the other grains, Ca2+ and Mg2+ contents are higher, and the resulting explanation of provenance is the broad result of low-grade metamorphic facies, intermediate-acidic igneous rocks and eclogite, etc. Different geochemical compositions caused by the difference of grain size of the clastic garnet show the grain size of the detrital garnets, which will affect the geochemistry-based provenance interpretation. Therefore, in the case of single mineral geochemistry-based provenance analysis, concentrating on detrital minerals with certain grain size compositions (0.063-0.125 mm or 0.125-0.25 mm) can eliminate the influence of the hydrodynamic sorting process and help obtain more accurate provenance interpretation results.e
2019, 37(3): 519-531.
doi: 10.14027/j.issn.1000-0550.2018.155
Abstract:
The giant thick Cretaceous stratum that is widely distributed in Southern China is regarded as fluvio-lacustrine sediment and contains a wealth of information about paleoclimate and paleoenvironmental change. The climate during the Cretaceous period represents one of the "greenhouse states" of Earth's history. Study of Cretaceous climate change can inform current and further greenhouse climate research. This paper reports a preliminary study on the magnetic properties of three Cretaceous Shaxian Formation sections located in Shaxian and Yong'an, Sanming city, Fujian Province. Rock magnetic parameters (including magnetic susceptibility, frequency-dependent magnetic susceptibility, remnant magnetization, hystersis loop, and temperature-dependent magnetization), the diffuse reflectance spectrum (DRS), and chroma parameters (lightness L*, redness a*, yellowness b*) were measured and analyzed to discuss the variations in magnetic properties, their environmental significance, and the origin of red pigments. The results show that:1) The main magnetic minerals of the red and yellow beds are hematite and goethite, respectively. Both contain paramagnetic minerals and a few ferromagnetic minerals. The content of these weakly magnetic minerals is low, so the numerical value of the magnetic susceptibility is low. 2) The hematite content in the siltstone is higher than in the sandstone. 3) The redness and yellowness are controlled by the hematite and goethite content, respectively. Hematite and goethite were formed before the diagenetic stage. The details about the formation stage need further study. Hematite was mainly formed in the provenance area and can carry the detrital remnant magnetization, which is important to the magnetostratigraphy. 4) Red bed hematite suggests hot weather, and yellow bed goethite suggests local wet conditions. Because the magnetic properties were controlled by many factors, further research is needed to determine the environmental significance of the magnetic parameters.
The giant thick Cretaceous stratum that is widely distributed in Southern China is regarded as fluvio-lacustrine sediment and contains a wealth of information about paleoclimate and paleoenvironmental change. The climate during the Cretaceous period represents one of the "greenhouse states" of Earth's history. Study of Cretaceous climate change can inform current and further greenhouse climate research. This paper reports a preliminary study on the magnetic properties of three Cretaceous Shaxian Formation sections located in Shaxian and Yong'an, Sanming city, Fujian Province. Rock magnetic parameters (including magnetic susceptibility, frequency-dependent magnetic susceptibility, remnant magnetization, hystersis loop, and temperature-dependent magnetization), the diffuse reflectance spectrum (DRS), and chroma parameters (lightness L*, redness a*, yellowness b*) were measured and analyzed to discuss the variations in magnetic properties, their environmental significance, and the origin of red pigments. The results show that:1) The main magnetic minerals of the red and yellow beds are hematite and goethite, respectively. Both contain paramagnetic minerals and a few ferromagnetic minerals. The content of these weakly magnetic minerals is low, so the numerical value of the magnetic susceptibility is low. 2) The hematite content in the siltstone is higher than in the sandstone. 3) The redness and yellowness are controlled by the hematite and goethite content, respectively. Hematite and goethite were formed before the diagenetic stage. The details about the formation stage need further study. Hematite was mainly formed in the provenance area and can carry the detrital remnant magnetization, which is important to the magnetostratigraphy. 4) Red bed hematite suggests hot weather, and yellow bed goethite suggests local wet conditions. Because the magnetic properties were controlled by many factors, further research is needed to determine the environmental significance of the magnetic parameters.
2019, 37(3): 532-540.
doi: 10.14027/j.issn.1000-0550.2018.160
Abstract:
Analysis of the hydrochemical characteristics and origin of deep sandy gravel brine in Mahai Basin is based on mathematical statistics, cluster analysis, and ion characteristic coefficients. The spatial distribution characteristics of brine ion concentrations shows that the ion concentrations of pH, Cl-, TDS, and Na+ have a narrow range of change and uniform distribution, while the ion concentrations of K+, Rb+, Br-, Mg2+, Li+, Sr2+, I-, B2O3, Ca2+, Cs+, NO3-, and SO42- exhibit significant variation and uneven distribution. Cluster analysis indicates that Cl-, TDS, Na+, K+, Mg2+, Li+, and SO42- piled, which shows the evolution of brine is related to the joining of acid liquid. K+, which has correlation with Mg2+ and Li+, shows an increasing trend from west to east in the variety graph of ion contents. The characteristic coefficients and higher contents of trace ions suggest that the brine has multifocal features of evaporation residue and gypsum dissolution. The Na+, K+, and Mg2+//Cl--H2O four-element system mesostationary phase, as well as the analysis of ionic characteristic coefficients in brine, indicate that the hydrochemical type is the single chloride-type, which reflects the single provenance and may have a relationship with the stochastic underlying salt strata of the Pleistocene.
Analysis of the hydrochemical characteristics and origin of deep sandy gravel brine in Mahai Basin is based on mathematical statistics, cluster analysis, and ion characteristic coefficients. The spatial distribution characteristics of brine ion concentrations shows that the ion concentrations of pH, Cl-, TDS, and Na+ have a narrow range of change and uniform distribution, while the ion concentrations of K+, Rb+, Br-, Mg2+, Li+, Sr2+, I-, B2O3, Ca2+, Cs+, NO3-, and SO42- exhibit significant variation and uneven distribution. Cluster analysis indicates that Cl-, TDS, Na+, K+, Mg2+, Li+, and SO42- piled, which shows the evolution of brine is related to the joining of acid liquid. K+, which has correlation with Mg2+ and Li+, shows an increasing trend from west to east in the variety graph of ion contents. The characteristic coefficients and higher contents of trace ions suggest that the brine has multifocal features of evaporation residue and gypsum dissolution. The Na+, K+, and Mg2+//Cl--H2O four-element system mesostationary phase, as well as the analysis of ionic characteristic coefficients in brine, indicate that the hydrochemical type is the single chloride-type, which reflects the single provenance and may have a relationship with the stochastic underlying salt strata of the Pleistocene.
2019, 37(3): 541-549.
doi: 10.14027/j.issn.1000-0550.2018.169
Abstract:
The seasonal variation of coastal waters off eastern Shandong Peninsula contributes significantly to the special sedimentary dynamic environment. Based on temperature, turbidity, and concentration of suspended matter observed during the summer and winter of 2007 and tidal currents and suspended matter observed during the winter of 2017, the seasonal variation for the distribution of suspended matter was analyzed, and the annual suspended matter net flux to the south through eastern Shandong Peninsula in winter was calculated. The sedimentary dynamic environment of the mud wedge was further studied, combined with the distribution of tidal and residual currents in winter. The results show that the distribution of suspended matter in the shelf waters off eastern Shandong peninsula is mostly affected by seasonal variations of the coastal currents, the Yellow Sea cold water mass, and the Yellow Sea warm current. Its concentration in winter is several times larger than in summer. Suspended matter off eastern Shandong Peninsula is restricted near the bottom by vertical water stratification and the Yellow Sea cold water mass in summer. A large amount of suspended matter is transported southward, driven by the strong coastal currents due to strong winter winds, while some suspended matter can also be transported northward by the Yellow Sea warm current. The annual suspended matter net flux in winter through the shelf waters off eastern Shandong Peninsula accounts for 3.22%-9.10% of the annual suspension matter net flux outward through the Bohai Strait, and also accounts for 6.84%-19.38% of the annual suspension matter net flux outward through the Bohai Strait in winter.
The seasonal variation of coastal waters off eastern Shandong Peninsula contributes significantly to the special sedimentary dynamic environment. Based on temperature, turbidity, and concentration of suspended matter observed during the summer and winter of 2007 and tidal currents and suspended matter observed during the winter of 2017, the seasonal variation for the distribution of suspended matter was analyzed, and the annual suspended matter net flux to the south through eastern Shandong Peninsula in winter was calculated. The sedimentary dynamic environment of the mud wedge was further studied, combined with the distribution of tidal and residual currents in winter. The results show that the distribution of suspended matter in the shelf waters off eastern Shandong peninsula is mostly affected by seasonal variations of the coastal currents, the Yellow Sea cold water mass, and the Yellow Sea warm current. Its concentration in winter is several times larger than in summer. Suspended matter off eastern Shandong Peninsula is restricted near the bottom by vertical water stratification and the Yellow Sea cold water mass in summer. A large amount of suspended matter is transported southward, driven by the strong coastal currents due to strong winter winds, while some suspended matter can also be transported northward by the Yellow Sea warm current. The annual suspended matter net flux in winter through the shelf waters off eastern Shandong Peninsula accounts for 3.22%-9.10% of the annual suspension matter net flux outward through the Bohai Strait, and also accounts for 6.84%-19.38% of the annual suspension matter net flux outward through the Bohai Strait in winter.
2019, 37(3): 550-564.
doi: 10.14027/j.issn.1000-0550.2019.005
Abstract:
We examine the relationship between gravel and transport distance in the modern Qingshuihe delta plain, north of Bosteng Lake, as well as the relationship between gravel development and the shoreline in fan deltas. We determine the geological parameters and reconstruct the ancient lake shoreline based on similarity analysis of the tectonic setting, climate, fan delta along length, sedimentary slope, and gravel composition of modern and ancient fan deltas. The distance from the sedimentary source to the lake is less than ca 32 km, and the gravel diameter is ca 2.9 cm on the shoreline. The early fan delta occurs in the middle of the Mahu Depression, in the first section of the Baikouquan Formation, and reservoirs developed in this area. There are more than two occurrences of lake level rise in the second section of the Baikouquan Formation. The largest lake area is in the third section of the Baikouquan Formation, and we examine its palaeogeography characteristics. We show the important role that gravel can play in analyzing the relationship between fan deltas and lake shorelines in a lacustrine basin.
We examine the relationship between gravel and transport distance in the modern Qingshuihe delta plain, north of Bosteng Lake, as well as the relationship between gravel development and the shoreline in fan deltas. We determine the geological parameters and reconstruct the ancient lake shoreline based on similarity analysis of the tectonic setting, climate, fan delta along length, sedimentary slope, and gravel composition of modern and ancient fan deltas. The distance from the sedimentary source to the lake is less than ca 32 km, and the gravel diameter is ca 2.9 cm on the shoreline. The early fan delta occurs in the middle of the Mahu Depression, in the first section of the Baikouquan Formation, and reservoirs developed in this area. There are more than two occurrences of lake level rise in the second section of the Baikouquan Formation. The largest lake area is in the third section of the Baikouquan Formation, and we examine its palaeogeography characteristics. We show the important role that gravel can play in analyzing the relationship between fan deltas and lake shorelines in a lacustrine basin.
2019, 37(3): 565-578.
doi: 10.14027/j.issn.1000-0550.2018.136
Abstract:
Several different types of tempestites are found in the Xiaoerbulak Formation, Aksu area, Tarim Basin. Tempestites and bioherms have a symbiotic relationship or macroscopic mix growth pattern, with various types of microbial composition in microcosmic aspects. However, little research has performed on this phenomenon. Based on outcrop section observations and thin section identifications, 4 classes and 8 types of bioherm-tempestite sedimentary successions are recognized in the Sugaitblak and Yutixi areas, and the sedimentary model is established. As a result, corresponding locations and characteristics are mainly controlled by "three bases", including mean sea level, fair wave base, and storm wave base, "four stages", including quiet stage, strong stage, weakening stage, and dying-out stage, and "five flow regime", including storm eddy, lower storm back-flow (split into storm turbidity currents and storm debris flow), and upper storm back-flow (split into storm overflow and storm tide). The position of the ancient storm center can also affect the storm deposit characteristics. At last, based on previous research, the origin of the stormy sea with the scarcity of metazoan skeletons is discussed.
Several different types of tempestites are found in the Xiaoerbulak Formation, Aksu area, Tarim Basin. Tempestites and bioherms have a symbiotic relationship or macroscopic mix growth pattern, with various types of microbial composition in microcosmic aspects. However, little research has performed on this phenomenon. Based on outcrop section observations and thin section identifications, 4 classes and 8 types of bioherm-tempestite sedimentary successions are recognized in the Sugaitblak and Yutixi areas, and the sedimentary model is established. As a result, corresponding locations and characteristics are mainly controlled by "three bases", including mean sea level, fair wave base, and storm wave base, "four stages", including quiet stage, strong stage, weakening stage, and dying-out stage, and "five flow regime", including storm eddy, lower storm back-flow (split into storm turbidity currents and storm debris flow), and upper storm back-flow (split into storm overflow and storm tide). The position of the ancient storm center can also affect the storm deposit characteristics. At last, based on previous research, the origin of the stormy sea with the scarcity of metazoan skeletons is discussed.
2019, 37(3): 579-588.
doi: 10.14027/j.issn.1000-0550.2018.159
Abstract:
Through detailed outcrop observations and meticulous measurement of the Upper Permian Xidalongkou and Shuixigou profiles on the northeast periphery of Bogda Mountain, the stratigraphic and sedimentary facies characteristics of the study area were carefully analyzed, from which a sedimentary evolution model was established. The results show that, during the Upper Permian on the northeast periphery of Bogda Mountain, the Quanzijie, Wutonggou, and Guodikeng Formations developed from the bottom up. During the Quanzijie Formation deposition period, in the Xidalongkou and Shuixigou profiles, alluvial fan and meandering river sedimentary facies were developed. Inside the alluvial fan glutenite body, there is a large amount of sandstone and oil shale conglomerates, which are the product of the process whereby the Middle Permian Hongyanchi and Lucaogou Formations in the periphery of Bogda Mountain were eroded and then deposited in front of the mountain. During the Wutonggou Formation deposition period, the ancient Bogda Mountain rose up and became the main sediment source, during which, inside the Xidalongkou and Shuixigou profiles, braided river sedimentary facies and meandering river sedimentary facies were developed; more volcanic rock gravels can be found in the braided river glutenite body. During the Guodikeng Formation deposition period, the Xidalongkou and Shuixigou profiles mainly developed meandering river floodplain sedimentary facies, in which fuchsia silty mudstone and mudstone were the major depositions.
Through detailed outcrop observations and meticulous measurement of the Upper Permian Xidalongkou and Shuixigou profiles on the northeast periphery of Bogda Mountain, the stratigraphic and sedimentary facies characteristics of the study area were carefully analyzed, from which a sedimentary evolution model was established. The results show that, during the Upper Permian on the northeast periphery of Bogda Mountain, the Quanzijie, Wutonggou, and Guodikeng Formations developed from the bottom up. During the Quanzijie Formation deposition period, in the Xidalongkou and Shuixigou profiles, alluvial fan and meandering river sedimentary facies were developed. Inside the alluvial fan glutenite body, there is a large amount of sandstone and oil shale conglomerates, which are the product of the process whereby the Middle Permian Hongyanchi and Lucaogou Formations in the periphery of Bogda Mountain were eroded and then deposited in front of the mountain. During the Wutonggou Formation deposition period, the ancient Bogda Mountain rose up and became the main sediment source, during which, inside the Xidalongkou and Shuixigou profiles, braided river sedimentary facies and meandering river sedimentary facies were developed; more volcanic rock gravels can be found in the braided river glutenite body. During the Guodikeng Formation deposition period, the Xidalongkou and Shuixigou profiles mainly developed meandering river floodplain sedimentary facies, in which fuchsia silty mudstone and mudstone were the major depositions.
2019, 37(3): 589-600.
doi: 10.14027/j.issn.1000-0550.2018.141
Abstract:
In order to investigate the effect of lithofacies paleogeography on the reservoir, the lithofacies and lithofacies paleogeography characteristics of the Majiagou submember 51-2 in the central-southern Ordos Basin were studied by various methods, including core observation, thin sections identification, and multi-mineral logging interpretation, as well as single factor analysis and multi factor comprehensive mapping. Then the effect of lithofacies paleogeography on the reservoir was analyzed by combining with reservoir distribution, which we obtained by logging interpretation and the data of actual gas production. The result shows that the main rock types of the Majiagou submember 51-2 mainly include dolostone (dolomicrite, gypsum and salt corrosion breccia dolomicrite, karst breccia dolomicrite, karst breccia calcitic crystal powder dolostone, calcitic dolostone, gypsiferous dolostone, argillaceous calcitic dolostone, and argillaceous dolostone) and limestone (dolomitic limestone and argillaceous dolomitic limestone), as well as partial evaporite (gypsum and halite). The lithofacies paleogeography environment mainly includes the evaporate platform, which can be divided into dolomitic flat and gypsiferous dolomitic flat, as well as a restricted platform, which can be divided into salt lake, gypsum lake, and gypsiferous lake. The reservoir can be affected by lithofacies paleogeography to some extent. Dolomitic flat and gypsiferous flat, as well as partial gypsiferous and gypsum lake are all the favorable areas for the reservoir.
In order to investigate the effect of lithofacies paleogeography on the reservoir, the lithofacies and lithofacies paleogeography characteristics of the Majiagou submember 51-2 in the central-southern Ordos Basin were studied by various methods, including core observation, thin sections identification, and multi-mineral logging interpretation, as well as single factor analysis and multi factor comprehensive mapping. Then the effect of lithofacies paleogeography on the reservoir was analyzed by combining with reservoir distribution, which we obtained by logging interpretation and the data of actual gas production. The result shows that the main rock types of the Majiagou submember 51-2 mainly include dolostone (dolomicrite, gypsum and salt corrosion breccia dolomicrite, karst breccia dolomicrite, karst breccia calcitic crystal powder dolostone, calcitic dolostone, gypsiferous dolostone, argillaceous calcitic dolostone, and argillaceous dolostone) and limestone (dolomitic limestone and argillaceous dolomitic limestone), as well as partial evaporite (gypsum and halite). The lithofacies paleogeography environment mainly includes the evaporate platform, which can be divided into dolomitic flat and gypsiferous dolomitic flat, as well as a restricted platform, which can be divided into salt lake, gypsum lake, and gypsiferous lake. The reservoir can be affected by lithofacies paleogeography to some extent. Dolomitic flat and gypsiferous flat, as well as partial gypsiferous and gypsum lake are all the favorable areas for the reservoir.
2019, 37(3): 601-609.
doi: 10.14027/j.issn.1000-0550.2018.142
Abstract:
Studying the quality and distribution law of the Xiaoerblak Formation (Fm.) is significant for guiding the exploration of Cambrian sub-salt area. The quality and scale of the reservoir are the main factors causing the restriction of further exploration in the Xiaoerblak Fm. of the Lower Cambrian, Tarim Basin. This study takes the Sugaiteblak Section in the Keping area as an example. Based on the detailed description, physical properties analysis, and three-dimensional (3D) reservoir geological modeling in the Xiaoerblak Fm., we found that microbial dolomite (including laminate, thrombolite, stromatolite, and microbial framework dolomite), dolarenite, and muddy to micritic dolomite are the main rock types indicating the sedimentary characteristics of the ramp. Vugs, intergranular dissolved pores (or intercrystalline pores), and framework pores are the main reservoir spaces, which have an obvious correlation with lithofacies. Among them, dolarenite and microbial framework dolomite are the most favorable reservoir rocks. Generally speaking, the reservoir has characteristics of middle-high porosity and middle-low permeability. The scale development of the reservoir was controlled by a high frequency sequence and sedimentary microfacies, and microbial mounds and shoals in the middle ramp around the ancient uplift are the favorable facies zones.
Studying the quality and distribution law of the Xiaoerblak Formation (Fm.) is significant for guiding the exploration of Cambrian sub-salt area. The quality and scale of the reservoir are the main factors causing the restriction of further exploration in the Xiaoerblak Fm. of the Lower Cambrian, Tarim Basin. This study takes the Sugaiteblak Section in the Keping area as an example. Based on the detailed description, physical properties analysis, and three-dimensional (3D) reservoir geological modeling in the Xiaoerblak Fm., we found that microbial dolomite (including laminate, thrombolite, stromatolite, and microbial framework dolomite), dolarenite, and muddy to micritic dolomite are the main rock types indicating the sedimentary characteristics of the ramp. Vugs, intergranular dissolved pores (or intercrystalline pores), and framework pores are the main reservoir spaces, which have an obvious correlation with lithofacies. Among them, dolarenite and microbial framework dolomite are the most favorable reservoir rocks. Generally speaking, the reservoir has characteristics of middle-high porosity and middle-low permeability. The scale development of the reservoir was controlled by a high frequency sequence and sedimentary microfacies, and microbial mounds and shoals in the middle ramp around the ancient uplift are the favorable facies zones.
2019, 37(3): 610-622.
doi: 10.14027/j.issn.1000-0550.2018.147
Abstract:
Quantitative description of braided river delta sedimentary facies of the Oficina Formation in MPE-3 block of the Orinoco heavy oil belt in Venezuela Basin is difficult, which is the key geological factor restricting the efficient development of horizontal wells in this block. Based on the study of regional sedimentary background, it shows that the braided river delta deposits dominates this area which is determined by core description and analysis of laboratory data,mainly developing the subfacies of braided river delta plain and delta front and microfacies such as the mid-channel bar, composite mid-channel bar, braided channel, splay/overflow, flood plain, underwater distributary channel, interdistributary bay, sand sheet and tidal sand bar. Quantitative characterization of major sedimentary microfacies in the study area has been achieved by using three methods:seismic and logging data, analogy of modern sedimentary surveys and empirical formulas. The results suggest that the width of braided channel ranges from 1 km to 5 km, and the width of underwater distributary channel is about 1 km. Empirical formulas between the width and length of the heart beach, the width of the heart beach and the braided channel, the width and depth of the single channel of the underwater distributary channel are established, and the evolution law of the braided river delta during the deposition of the Oficina Formation is further clarified. The research results provide an important geological basis for improving the recovery degree of super heavy oil reservoirs in the study area.
Quantitative description of braided river delta sedimentary facies of the Oficina Formation in MPE-3 block of the Orinoco heavy oil belt in Venezuela Basin is difficult, which is the key geological factor restricting the efficient development of horizontal wells in this block. Based on the study of regional sedimentary background, it shows that the braided river delta deposits dominates this area which is determined by core description and analysis of laboratory data,mainly developing the subfacies of braided river delta plain and delta front and microfacies such as the mid-channel bar, composite mid-channel bar, braided channel, splay/overflow, flood plain, underwater distributary channel, interdistributary bay, sand sheet and tidal sand bar. Quantitative characterization of major sedimentary microfacies in the study area has been achieved by using three methods:seismic and logging data, analogy of modern sedimentary surveys and empirical formulas. The results suggest that the width of braided channel ranges from 1 km to 5 km, and the width of underwater distributary channel is about 1 km. Empirical formulas between the width and length of the heart beach, the width of the heart beach and the braided channel, the width and depth of the single channel of the underwater distributary channel are established, and the evolution law of the braided river delta during the deposition of the Oficina Formation is further clarified. The research results provide an important geological basis for improving the recovery degree of super heavy oil reservoirs in the study area.
2019, 37(3): 623-632.
doi: 10.14027/j.issn.1000-0550.2018.150
Abstract:
Analysis of connectivity among lithologic reservoirs could provide the basis for finding new oil and gas enrichment areas. Taking the Chang 8 reservoir in the Maling area of the Ordos Basin as an example, and according to sand body and the interlayer development, the fluid connectivity among reservoirs was determined by the total hydrocarbon chromatogram of its crude oil. This paper discusses the connectivity among Chang 8 reservoirs in the Maling area by combining dynamic and static data. Study results show that the sand body can be divided into three superposition models, which are the isolated, contacted, and tangency models. The tangency sand body model has good connectivity. A new formula for the connectivity probability of the sand body is proposed by taking the interlayer development into consideration and using this formula to calculate the connectivity probability of the sand body in the study area. Dynamic data analysis can be used to divide the Chang 8 reservoir into three fluid connected systems, each of which is connected to each other, and the interconnectivity among reservoirs in different systems is poor. The effectiveness of this method was verified according to the latest exploration and evaluation results, and this method has certain guiding significance for the analysis of the connectivity among the ultra-low permeability lithologic reservoirs and the next step for exploration and deployment.
Analysis of connectivity among lithologic reservoirs could provide the basis for finding new oil and gas enrichment areas. Taking the Chang 8 reservoir in the Maling area of the Ordos Basin as an example, and according to sand body and the interlayer development, the fluid connectivity among reservoirs was determined by the total hydrocarbon chromatogram of its crude oil. This paper discusses the connectivity among Chang 8 reservoirs in the Maling area by combining dynamic and static data. Study results show that the sand body can be divided into three superposition models, which are the isolated, contacted, and tangency models. The tangency sand body model has good connectivity. A new formula for the connectivity probability of the sand body is proposed by taking the interlayer development into consideration and using this formula to calculate the connectivity probability of the sand body in the study area. Dynamic data analysis can be used to divide the Chang 8 reservoir into three fluid connected systems, each of which is connected to each other, and the interconnectivity among reservoirs in different systems is poor. The effectiveness of this method was verified according to the latest exploration and evaluation results, and this method has certain guiding significance for the analysis of the connectivity among the ultra-low permeability lithologic reservoirs and the next step for exploration and deployment.
2019, 37(3): 633-647.
doi: 10.14027/j.issn.1000-0550.2018.158
Abstract:
The eastern deep-water area in Qiongdongnan Basin (QDNB) is in the early stages of exploration stage, and some drilling samples collected from this area are severely contaminated by the oil-based mud. Little research regarding the organic geochemistry of the Oligocene source rock deposited in the marine or transitional environments in this area has been performed so far. In this paper, effective oil-cleaning experiments and organic geochemical analysis were carried out for Oligocene mudstone cuttings contaminated by oil-based mud in the eastern deep-water area, coupled with organic geochemical and petrological data of mudstone cuttings in the shallow-water area, and the geochemical characteristics, paleoenvironment, controlling factors, and developmental model of Oligocene source rocks from different sedimentary facies in the eastern QDNB were analyzed. Two distinct models of delta front source rock and neritic source rock were developed according to their organic matter sources and geochemical characteristics. Transitional delta front source rock has fair to good quality and gas-prone type organic matter, and its biomarkers are characterized by pristane/phytane bigger than 3, (nC21+nC22/nC28+nC29) smaller than 1.2, high ratios of oleanane/C30 hopane, T-bicadinane/C30 hopane, low ratios of C23 tricyclic terpane/C30 hopane, C27 sterane/C29 sterane, gammacerane/C30 hopane, and C35 homohopane/C34 homohopane. There exists a strong correlation between total organic carbon (TOC) and total sulfur, which means water in the delta front subface was of oxidation fresh or brackish condition. Organic matter abundance in the delta front source rock is heavily controlled by terrigenous organic matter input. Neritic source rock has fair quality and an organic matter type prone to generating both oil and gas, and the biomarkers are characterized by pristane/phytane smaller than 3, (nC21+nC22/nC28+nC29) bigger than 1.5, low ratios of oleanane/C30 hopane, T-bicadinane/C30 hopane, high ratios of C23 tricyclic terpane/C30 hopane, C27sterane/C29 sterane, gammacerane/C30 hopane, and C35 homohopane/C34 homohopane. However, there exists no correlation between TOC and total sulfur, which means water in the neritic face was of the weak oxidation brackish or saline condition. Organic matter abundance in the neritic source rock is controlled by aquatic organic matter input and influenced by the water environment. Littoral source rock is generally of poor to medium quality and is seldom influenced by organic matter sources and water condition, and the source rock depositional model is considered to be a destructive model.
The eastern deep-water area in Qiongdongnan Basin (QDNB) is in the early stages of exploration stage, and some drilling samples collected from this area are severely contaminated by the oil-based mud. Little research regarding the organic geochemistry of the Oligocene source rock deposited in the marine or transitional environments in this area has been performed so far. In this paper, effective oil-cleaning experiments and organic geochemical analysis were carried out for Oligocene mudstone cuttings contaminated by oil-based mud in the eastern deep-water area, coupled with organic geochemical and petrological data of mudstone cuttings in the shallow-water area, and the geochemical characteristics, paleoenvironment, controlling factors, and developmental model of Oligocene source rocks from different sedimentary facies in the eastern QDNB were analyzed. Two distinct models of delta front source rock and neritic source rock were developed according to their organic matter sources and geochemical characteristics. Transitional delta front source rock has fair to good quality and gas-prone type organic matter, and its biomarkers are characterized by pristane/phytane bigger than 3, (nC21+nC22/nC28+nC29) smaller than 1.2, high ratios of oleanane/C30 hopane, T-bicadinane/C30 hopane, low ratios of C23 tricyclic terpane/C30 hopane, C27 sterane/C29 sterane, gammacerane/C30 hopane, and C35 homohopane/C34 homohopane. There exists a strong correlation between total organic carbon (TOC) and total sulfur, which means water in the delta front subface was of oxidation fresh or brackish condition. Organic matter abundance in the delta front source rock is heavily controlled by terrigenous organic matter input. Neritic source rock has fair quality and an organic matter type prone to generating both oil and gas, and the biomarkers are characterized by pristane/phytane smaller than 3, (nC21+nC22/nC28+nC29) bigger than 1.5, low ratios of oleanane/C30 hopane, T-bicadinane/C30 hopane, high ratios of C23 tricyclic terpane/C30 hopane, C27sterane/C29 sterane, gammacerane/C30 hopane, and C35 homohopane/C34 homohopane. However, there exists no correlation between TOC and total sulfur, which means water in the neritic face was of the weak oxidation brackish or saline condition. Organic matter abundance in the neritic source rock is controlled by aquatic organic matter input and influenced by the water environment. Littoral source rock is generally of poor to medium quality and is seldom influenced by organic matter sources and water condition, and the source rock depositional model is considered to be a destructive model.
2019, 37(3): 648-660.
doi: 10.14027/j.issn.1000-0550.2018.144
Abstract:
Previous studies on Site 4B suggested the presence of methane seepage in this area. In this study, we systematically analyzed the particle size distribution, minerals, and chemical compositions of the sediments at an interval of 15 cm.
The particle size distribution varied significantly at a depth of 98 cm, which is ascribed to the changed hydrodynamic conditions. Significantly increased particle size was discovered at depths of 161 cm, 186 cm, and 213 cm. According to the sedimentation rate, these abnormally increased particle sizes occurred 24.15 ka, 27.47 ka, and 31.05 ka, which may be related to methane seeping events.The chemical compositions of the sediments changed with the particle size variation. At depths lower than 98 cm, the concentrations of CaO, Na2O, and Sr increased with depth, while that of SiO2, Al2O3, Fe2O3, Px dramatically decreased. At depths larger than 98 cm, on the contrary, these elements did not change significantly with depth. The positive correlations between CaO, Na2O, and Sr, as well as the negative correlations between these elements and SiO2 indicated that the carbonates (containing Ca and Sr) were affected by secondary processes, e.g., the production of authigenic carbonate from methane seepage.
The variation of the Mg/Ca and Sr/Ca ratios as a function of depth suggests that the sediment might be affected by mud volcanic activity before 15.78 ka. The mud volcanic activity arrived at a maximum at 35.96 ka, where the highest Mg/Ca and Sr/Ca ratios were measured. Furthermore, we calculated the relative contents of aragonite, high-Mg calcite, biogenic calcite, and detrital materials by using Bayon's model. The results indicate that the detrital materials likely originated from a stable source, while the calcite and aragonite were authigenic carbonates produced by microbial activities related to methane seepage. Combined with previous studies on pyrite and the particle characteristics of the sediments, we postulated that the methane seepage was related to the mud volcano activity before 15.78 ka. These periodic mud volcanic activities brought the deep-sourced methane to a shallow depth. Then the anaerobic oxidation of methane produced authigenic carbonates (including high-Mg calcite and aragonite). Strong methane seepage was observed at 24.15 ka, 27.47 ka, and 31.05 ka, where the maximum seepage occurred at 31.05 ka. At <98 cm (after 15.78 ka), however, the sediments were affected by both terrigenous inputs and methane seepage.
This study investigated the timing and intensity of methane seepagw in Site-4B using Mg/Ca and Sr/Ca ratios for the first time. It is important for unveiling the methane seeping history in the Northern SCS.
Previous studies on Site 4B suggested the presence of methane seepage in this area. In this study, we systematically analyzed the particle size distribution, minerals, and chemical compositions of the sediments at an interval of 15 cm.
The particle size distribution varied significantly at a depth of 98 cm, which is ascribed to the changed hydrodynamic conditions. Significantly increased particle size was discovered at depths of 161 cm, 186 cm, and 213 cm. According to the sedimentation rate, these abnormally increased particle sizes occurred 24.15 ka, 27.47 ka, and 31.05 ka, which may be related to methane seeping events.The chemical compositions of the sediments changed with the particle size variation. At depths lower than 98 cm, the concentrations of CaO, Na2O, and Sr increased with depth, while that of SiO2, Al2O3, Fe2O3, Px dramatically decreased. At depths larger than 98 cm, on the contrary, these elements did not change significantly with depth. The positive correlations between CaO, Na2O, and Sr, as well as the negative correlations between these elements and SiO2 indicated that the carbonates (containing Ca and Sr) were affected by secondary processes, e.g., the production of authigenic carbonate from methane seepage.
The variation of the Mg/Ca and Sr/Ca ratios as a function of depth suggests that the sediment might be affected by mud volcanic activity before 15.78 ka. The mud volcanic activity arrived at a maximum at 35.96 ka, where the highest Mg/Ca and Sr/Ca ratios were measured. Furthermore, we calculated the relative contents of aragonite, high-Mg calcite, biogenic calcite, and detrital materials by using Bayon's model. The results indicate that the detrital materials likely originated from a stable source, while the calcite and aragonite were authigenic carbonates produced by microbial activities related to methane seepage. Combined with previous studies on pyrite and the particle characteristics of the sediments, we postulated that the methane seepage was related to the mud volcano activity before 15.78 ka. These periodic mud volcanic activities brought the deep-sourced methane to a shallow depth. Then the anaerobic oxidation of methane produced authigenic carbonates (including high-Mg calcite and aragonite). Strong methane seepage was observed at 24.15 ka, 27.47 ka, and 31.05 ka, where the maximum seepage occurred at 31.05 ka. At <98 cm (after 15.78 ka), however, the sediments were affected by both terrigenous inputs and methane seepage.
This study investigated the timing and intensity of methane seepagw in Site-4B using Mg/Ca and Sr/Ca ratios for the first time. It is important for unveiling the methane seeping history in the Northern SCS.
