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
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
As a type of non-laminated microbial carbonates, dendrolites are dominated by isolated dendritic clusters of calcimicrobes and are distinct from stromatolites and thrombolites. The dendrolites in the upper part of the...As a type of non-laminated microbial carbonates, dendrolites are dominated by isolated dendritic clusters of calcimicrobes and are distinct from stromatolites and thrombolites. The dendrolites in the upper part of the Miaolingian Zhangxia Formation at Anjiazhuang section in Feicheng city of Shandong Province, China, provide an excellent example for further understanding of both growth pattern and forming mechanism of dendrolites. These dendrolites are featured by sedimentary fabrics and composition of calcified microbes as follows.(1) The strata of massive limestones, composed of dendrolites with thickness of more than one hundred meters, intergrade with thick-bedded to massive leiolites, formimg the upper part of a third-order depositional sequence that constitutes a forced regressive systems tract.(2) A centimeter-sized bush-like fabric(shrub) typically produced by calcified microbes is similar to the mesoclot in thrombolites but distinctive from clotted fabrics of thrombolites. This bushlike fabric is actually constituted by diversified calcified microbes like the modern shrub as a result of gliding mobility of filamentous cyanobacteria. Such forms traditionally include: the Epiphyton group(which actually has uncertain biological affinity), the Hedstroemia group which closely resembles modern rivulariacean cyanobacteria,and the possible calcified cyanobacteria of the Lithocodium–Bacinella group.(3) Significantly, dense micrite of leiolite is associated with sponge fossils and burrows, and is covered by microstromatolite. The Lithocodium–Bacinella group is a controversial group of interpreted calcified cyanobacteria in the Cambrian that has also been widely observed and described in the Mesozoic. Therefore, dendrolites with symbiosis of leiolites in the studied section provide an extraordinary example for further understanding of growing style of bush-like fabrics(shrubs) of the dendrolites dominated by cyanobacterial mats. Furthermore, the present research provides some useful thinking approaches for better understanding of the history of the Early Paleozoic skeletal reefs and the microbe–metazoan transitions of the Cambrian.展开更多
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.
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funded by the National Natural Science Foundation of China(Grant No. 41472090)。
文摘As a type of non-laminated microbial carbonates, dendrolites are dominated by isolated dendritic clusters of calcimicrobes and are distinct from stromatolites and thrombolites. The dendrolites in the upper part of the Miaolingian Zhangxia Formation at Anjiazhuang section in Feicheng city of Shandong Province, China, provide an excellent example for further understanding of both growth pattern and forming mechanism of dendrolites. These dendrolites are featured by sedimentary fabrics and composition of calcified microbes as follows.(1) The strata of massive limestones, composed of dendrolites with thickness of more than one hundred meters, intergrade with thick-bedded to massive leiolites, formimg the upper part of a third-order depositional sequence that constitutes a forced regressive systems tract.(2) A centimeter-sized bush-like fabric(shrub) typically produced by calcified microbes is similar to the mesoclot in thrombolites but distinctive from clotted fabrics of thrombolites. This bushlike fabric is actually constituted by diversified calcified microbes like the modern shrub as a result of gliding mobility of filamentous cyanobacteria. Such forms traditionally include: the Epiphyton group(which actually has uncertain biological affinity), the Hedstroemia group which closely resembles modern rivulariacean cyanobacteria,and the possible calcified cyanobacteria of the Lithocodium–Bacinella group.(3) Significantly, dense micrite of leiolite is associated with sponge fossils and burrows, and is covered by microstromatolite. The Lithocodium–Bacinella group is a controversial group of interpreted calcified cyanobacteria in the Cambrian that has also been widely observed and described in the Mesozoic. Therefore, dendrolites with symbiosis of leiolites in the studied section provide an extraordinary example for further understanding of growing style of bush-like fabrics(shrubs) of the dendrolites dominated by cyanobacterial mats. Furthermore, the present research provides some useful thinking approaches for better understanding of the history of the Early Paleozoic skeletal reefs and the microbe–metazoan transitions of the Cambrian.
