This study investigates the Middle Jurassic rocks of the Labino-Malkin zone(Caucasus),represented by ooidal ironstones,biogenic limestones,and nodular phosphorites.All rocks contain ooids/pisoids.Based on the mineral ...This study investigates the Middle Jurassic rocks of the Labino-Malkin zone(Caucasus),represented by ooidal ironstones,biogenic limestones,and nodular phosphorites.All rocks contain ooids/pisoids.Based on the mineral composition,nine types of ooids and three possible scenarios for their formation have been identified.The mineral composition of ooids indicates the variable conditions of their deposition and repeated changes in physical and chemical conditions of the environment,including fluctuations in theredox regime.The access of oxygen in the bottom environment as a factor of increasing oxic conditions was associated with the regression of the sea and/or the inflow of meteoric waters.The main sources of ooids are the drift of material from the continent and convecting metal-bearing meteoric and/or deep solutions.展开更多
The role of microorganisms in the formation of giant ooids is one of the areas of long-term controversy in ooidal research,but it has not been confirmed conclusively.Abundant giant ooids developed in the Zhangxia Form...The role of microorganisms in the formation of giant ooids is one of the areas of long-term controversy in ooidal research,but it has not been confirmed conclusively.Abundant giant ooids developed in the Zhangxia Formation of the Cambrian Miaolingian Series in North China.Giant ooids in the study area were examined by using Polarized Light Microscopy and Field Emission Environmental Scanning Electron Microscopy.The nuclei of the ooids consist of micritic pellets or radial ooids with diameters less than 2 mm and are formed in a weak-agitating seawater environment.Their cortices are concentric,and are characterized by the alternations of the dark laminae of micritic calcite or Girvanella filaments and light laminae of microsparry calcite.In the environments of inter-bank sea with the alternating development of medium and low energy and chiefly weak-agitating conditions,giant ooids were formed under the joint action of Girvanella filamentous growth,biologically-induced calcification and/or biologically-influenced calcification and inorganic calcium carbonate precipitation.The microfossils of Girvanella are distributed in inner and outer cortices of giant ooids,especially dense in the latter.This distinctly indicates that microbes play a significant role in the formation of giant ooids,and also provides a vital example for discussing the microbial origin of giant ooids.展开更多
Calcified biofiims are preserved as thin micritic coatings in the Cambrian oolitic limestone of two sections in North and South China. Standard petrographic examination revealed that the biofilms were developed during...Calcified biofiims are preserved as thin micritic coatings in the Cambrian oolitic limestone of two sections in North and South China. Standard petrographic examination revealed that the biofilms were developed during the early diagenetic stage immediately after the freshly deposited ooids, proceeding in the continuous sequence of depositional processes. The biofilm outlines are highly irregular, with steep sides, tower-like structures and overhanging projections; the internal fabric of the biofilms is composed of roughly laminated micrite aggregates with channel-like structures. Biofilms exhibit a strong fluorescent reaction. Detailed SEM examination suggests that the biofilms are biotically dominated by cyanobacteria. Our study demonstrates that microbial colonies, such as biofilms, can develop on ooid cortices and influence the formation and microstructures of those ooids.展开更多
Detailed micro-meso to macroscopic structural analyses reveal two deformation phases in the western limb of the Hazara-Kashmir Syntaxis（HKS）. Bulk top to NW shearing transformed initially symmetrical NNE-SSW trendin...Detailed micro-meso to macroscopic structural analyses reveal two deformation phases in the western limb of the Hazara-Kashmir Syntaxis（HKS）. Bulk top to NW shearing transformed initially symmetrical NNE-SSW trending meso to macroscopic folds from asymmetric to overturned ones without changing their trend. Sigmoidal en-echelon tension gashes developed during this deformation,that were oblique to bedding parallel worm burrows and bedding planes themselves. Strain analyses of deformed elliptical ooids using the Rf/Ф method constrain the internal strain patterns of the NNE-SSW structures. The principal stretching axis（S3） defined by deformed elliptical ooids is oriented N27°E at right angles to WNW-ESE shortening. The deformed elliptical ooids in sub-vertical bedding vertical planes contain ooids that plunge -70° SE due to NW-directed tectonic transport. Finite strain ratios are1.45（Rxy） parallel to bedding plane and 1.46（Ryz） for the vertical plane. From these 2D strain values, we derive an oblate strain ellipsoidal in 3D using the Flinn and Hsu/Nadai techniques. Strains calculated from deformed elliptical ooids average-18.10% parallel to bedding and-18.47% in the vertical plane.However, a balanced cross-section through the study area indicates a minimum of--28% shortening.Consequently, regional shortening was only partially accommodated by internal deformation.展开更多
On the basis of systematic study of sedimentary facies, microfacies and geochemistry of measured strata sections and wells, the ooid shoals of the Cambrian Miaolingian Zhangxia Formation in the southern Ordos Basin ar...On the basis of systematic study of sedimentary facies, microfacies and geochemistry of measured strata sections and wells, the ooid shoals of the Cambrian Miaolingian Zhangxia Formation in the southern Ordos Basin are composed of both high-and low-energy ooid shoals. The characteristics of the high-energy shoals are sparry cementation, with moderate to well sorting, large grain size, high ooid content and low micrite, weak micritization, and well-preserved internal textures of the ooids. Macroscopically, the high-energy ooid shoals display a thick-bedded aggradational stacking pattern in upward coarsening and thickening sequences. The low-energy ooid shoals are mainly made up of micritic cementation, with moderate to poor sorting, relatively small grain size, lower ooid content and higher micrite, strong micritization, and poorly-preserved internal textures of the ooids. Macroscopically, the low-energy ooid shoals show a thin-bedded, interbedded stacking pattern in upward fining and thinning sequences. The sedimentological evidence and carbon isotope data show that the differences of internal fabric and sedimentary evolution of both high-and low-energy ooid shoals are closely related to the sea-level change.展开更多
文摘This study investigates the Middle Jurassic rocks of the Labino-Malkin zone(Caucasus),represented by ooidal ironstones,biogenic limestones,and nodular phosphorites.All rocks contain ooids/pisoids.Based on the mineral composition,nine types of ooids and three possible scenarios for their formation have been identified.The mineral composition of ooids indicates the variable conditions of their deposition and repeated changes in physical and chemical conditions of the environment,including fluctuations in theredox regime.The access of oxygen in the bottom environment as a factor of increasing oxic conditions was associated with the regression of the sea and/or the inflow of meteoric waters.The main sources of ooids are the drift of material from the continent and convecting metal-bearing meteoric and/or deep solutions.
基金financially supported by the National Natural Science Foundation of China(Giants No:41872111 and 41902115)。
文摘The role of microorganisms in the formation of giant ooids is one of the areas of long-term controversy in ooidal research,but it has not been confirmed conclusively.Abundant giant ooids developed in the Zhangxia Formation of the Cambrian Miaolingian Series in North China.Giant ooids in the study area were examined by using Polarized Light Microscopy and Field Emission Environmental Scanning Electron Microscopy.The nuclei of the ooids consist of micritic pellets or radial ooids with diameters less than 2 mm and are formed in a weak-agitating seawater environment.Their cortices are concentric,and are characterized by the alternations of the dark laminae of micritic calcite or Girvanella filaments and light laminae of microsparry calcite.In the environments of inter-bank sea with the alternating development of medium and low energy and chiefly weak-agitating conditions,giant ooids were formed under the joint action of Girvanella filamentous growth,biologically-induced calcification and/or biologically-influenced calcification and inorganic calcium carbonate precipitation.The microfossils of Girvanella are distributed in inner and outer cortices of giant ooids,especially dense in the latter.This distinctly indicates that microbes play a significant role in the formation of giant ooids,and also provides a vital example for discussing the microbial origin of giant ooids.
基金financial support from the Major Basic Research Project of the Ministry of Science and Technology of China(Grant:2013CB835002)the Natural Science Foundation of China(Grants:41272036)+1 种基金the China Postdoctoral Science Foundation(Grant:2014M552481)the"Sanqin Scholarship"project of the Shaanxi Authority
文摘Calcified biofiims are preserved as thin micritic coatings in the Cambrian oolitic limestone of two sections in North and South China. Standard petrographic examination revealed that the biofilms were developed during the early diagenetic stage immediately after the freshly deposited ooids, proceeding in the continuous sequence of depositional processes. The biofilm outlines are highly irregular, with steep sides, tower-like structures and overhanging projections; the internal fabric of the biofilms is composed of roughly laminated micrite aggregates with channel-like structures. Biofilms exhibit a strong fluorescent reaction. Detailed SEM examination suggests that the biofilms are biotically dominated by cyanobacteria. Our study demonstrates that microbial colonies, such as biofilms, can develop on ooid cortices and influence the formation and microstructures of those ooids.
基金Financial support by the Department of Geology, University of Peshawar
文摘Detailed micro-meso to macroscopic structural analyses reveal two deformation phases in the western limb of the Hazara-Kashmir Syntaxis（HKS）. Bulk top to NW shearing transformed initially symmetrical NNE-SSW trending meso to macroscopic folds from asymmetric to overturned ones without changing their trend. Sigmoidal en-echelon tension gashes developed during this deformation,that were oblique to bedding parallel worm burrows and bedding planes themselves. Strain analyses of deformed elliptical ooids using the Rf/Ф method constrain the internal strain patterns of the NNE-SSW structures. The principal stretching axis（S3） defined by deformed elliptical ooids is oriented N27°E at right angles to WNW-ESE shortening. The deformed elliptical ooids in sub-vertical bedding vertical planes contain ooids that plunge -70° SE due to NW-directed tectonic transport. Finite strain ratios are1.45（Rxy） parallel to bedding plane and 1.46（Ryz） for the vertical plane. From these 2D strain values, we derive an oblate strain ellipsoidal in 3D using the Flinn and Hsu/Nadai techniques. Strains calculated from deformed elliptical ooids average-18.10% parallel to bedding and-18.47% in the vertical plane.However, a balanced cross-section through the study area indicates a minimum of--28% shortening.Consequently, regional shortening was only partially accommodated by internal deformation.
基金supported by the National Natural Science Foundation of China (Nos. 42272014, 41290260 and 41872034)。
文摘On the basis of systematic study of sedimentary facies, microfacies and geochemistry of measured strata sections and wells, the ooid shoals of the Cambrian Miaolingian Zhangxia Formation in the southern Ordos Basin are composed of both high-and low-energy ooid shoals. The characteristics of the high-energy shoals are sparry cementation, with moderate to well sorting, large grain size, high ooid content and low micrite, weak micritization, and well-preserved internal textures of the ooids. Macroscopically, the high-energy ooid shoals display a thick-bedded aggradational stacking pattern in upward coarsening and thickening sequences. The low-energy ooid shoals are mainly made up of micritic cementation, with moderate to poor sorting, relatively small grain size, lower ooid content and higher micrite, strong micritization, and poorly-preserved internal textures of the ooids. Macroscopically, the low-energy ooid shoals show a thin-bedded, interbedded stacking pattern in upward fining and thinning sequences. The sedimentological evidence and carbon isotope data show that the differences of internal fabric and sedimentary evolution of both high-and low-energy ooid shoals are closely related to the sea-level change.
