Magnetic susceptibility (MS) data were obtained from 11 sections of the Doushantuo (Edicaran) cap carbonate that directly overlies the Nantuo glacial diamictite in the southeastern margin of the Yangtze plat- form. Th...Magnetic susceptibility (MS) data were obtained from 11 sections of the Doushantuo (Edicaran) cap carbonate that directly overlies the Nantuo glacial diamictite in the southeastern margin of the Yangtze plat- form. The MS data revealed two regionally correlatable peaks at the bottom and top of the cap carbonate, sepa- rated by an interval of low values. The lower MS peak coincides with high percentage of insoluble siliciclastic residues that are compositionally identical to the matrix of the underlying diamictite, suggesting its origin con- trolled mainly by detrital components during the first phase of cap carbonate deposition at the end of the glacia- tion. The upper MS peak is associated with high clay content and iron sulfides, and can be interpreted as either derived from enhanced greenhouse weathering that could have brought more terrigenous components into the ocean, or the result of ocean anoxia at the late stage of cap carbonate deposition that could led to formation of abundant iron sulfides. The regionally consistent MS curves from the cap carbonates provided the first geo- physical record for the rapid climate change from icehouse to greenhouse conditions in the aftermath of the Neoproterozoic “snowball Earth” event.展开更多
The well-preserved Mesoproterozoic succession in the North China platform consists mainly of three iithological associations including peritidal quartz sandstone, shallow marine and lagoonal dark to black shales, and ...The well-preserved Mesoproterozoic succession in the North China platform consists mainly of three iithological associations including peritidal quartz sandstone, shallow marine and lagoonal dark to black shales, and shallow epeiric carbonates, with a total thickness of up to 8 000 m. In addition to well-documented microplants, macroalgae, and microbial buildups, abundant microbially induced sedimentary structures （MISS） and mat-related sediments have been recognized in these rocks. Intensive microbial mat layers and MISS are especially well preserved in the carbonates of the upper Gaoyuzhuang （高于庄） （ca. 1.5 Ga） and lower Wumishan （雾迷山） （ca. 1.45 Ga） formations, indicating diversified microbial activities and a high organic production. In these petrified biomats, putative microbial fossils （both coccoidal and filamentous） and framboidal pyrites have been identified. The abundance of authigenic carbonate minerals in the host rocks, such as, acicular aragonites, rosette barites, radial siderites, ankerites, and botryoidal carbonate cements, suggests authigenic carbonate precipitation from anaerobic oxidation of methane （AOM） under anoxic/euxinic conditions. Warm climate and anoxic/euxinic conditions in the Mesoproterozoic oceans may have facilitated high microbial productivity and organic burial in sediments. Although authigenic carbonate cements may record carbonate precipitation from anaerobic methane oxidation, gas blister （or dome） structures may indicate gas release from active methanogenesis during shallow burial. Bituminous fragments in mat-related carbonates also provide evidence for hydrocarbon generation. Under proper conditions, the Mesoproterozoic mat-rich carbonates will have the potential for hydrocarbon generation and serve as source rocks. On the basis of petrified biomats, a rough estimation suggests that the Mesoproterozoic carbonates of the North China platform might have a hydrocarbon production potential in the order of 10 ×10^8 t.展开更多
基金*This work was supported by the National Natural Science Foundation (Grant No.40032010B)
文摘Magnetic susceptibility (MS) data were obtained from 11 sections of the Doushantuo (Edicaran) cap carbonate that directly overlies the Nantuo glacial diamictite in the southeastern margin of the Yangtze plat- form. The MS data revealed two regionally correlatable peaks at the bottom and top of the cap carbonate, sepa- rated by an interval of low values. The lower MS peak coincides with high percentage of insoluble siliciclastic residues that are compositionally identical to the matrix of the underlying diamictite, suggesting its origin con- trolled mainly by detrital components during the first phase of cap carbonate deposition at the end of the glacia- tion. The upper MS peak is associated with high clay content and iron sulfides, and can be interpreted as either derived from enhanced greenhouse weathering that could have brought more terrigenous components into the ocean, or the result of ocean anoxia at the late stage of cap carbonate deposition that could led to formation of abundant iron sulfides. The regionally consistent MS curves from the cap carbonates provided the first geo- physical record for the rapid climate change from icehouse to greenhouse conditions in the aftermath of the Neoproterozoic “snowball Earth” event.
基金supported by the SINOPEC project (G0800-06-ZS-319)the National Natural Science Foundation of China (No. 40621002)the Ministry of Education of China (IRT00546, B0711)
文摘The well-preserved Mesoproterozoic succession in the North China platform consists mainly of three iithological associations including peritidal quartz sandstone, shallow marine and lagoonal dark to black shales, and shallow epeiric carbonates, with a total thickness of up to 8 000 m. In addition to well-documented microplants, macroalgae, and microbial buildups, abundant microbially induced sedimentary structures （MISS） and mat-related sediments have been recognized in these rocks. Intensive microbial mat layers and MISS are especially well preserved in the carbonates of the upper Gaoyuzhuang （高于庄） （ca. 1.5 Ga） and lower Wumishan （雾迷山） （ca. 1.45 Ga） formations, indicating diversified microbial activities and a high organic production. In these petrified biomats, putative microbial fossils （both coccoidal and filamentous） and framboidal pyrites have been identified. The abundance of authigenic carbonate minerals in the host rocks, such as, acicular aragonites, rosette barites, radial siderites, ankerites, and botryoidal carbonate cements, suggests authigenic carbonate precipitation from anaerobic oxidation of methane （AOM） under anoxic/euxinic conditions. Warm climate and anoxic/euxinic conditions in the Mesoproterozoic oceans may have facilitated high microbial productivity and organic burial in sediments. Although authigenic carbonate cements may record carbonate precipitation from anaerobic methane oxidation, gas blister （or dome） structures may indicate gas release from active methanogenesis during shallow burial. Bituminous fragments in mat-related carbonates also provide evidence for hydrocarbon generation. Under proper conditions, the Mesoproterozoic mat-rich carbonates will have the potential for hydrocarbon generation and serve as source rocks. On the basis of petrified biomats, a rough estimation suggests that the Mesoproterozoic carbonates of the North China platform might have a hydrocarbon production potential in the order of 10 ×10^8 t.
