Cryogenian Datangpo Formation was deposited during the interglacial time between the Sturtian and Marinoan ice ages. We studied nitrogen isotope compositions and contents of Mo of the black shales from the basal Datan...Cryogenian Datangpo Formation was deposited during the interglacial time between the Sturtian and Marinoan ice ages. We studied nitrogen isotope compositions and contents of Mo of the black shales from the basal Datangpo Formation in northeastern Guizhou, South China, for an attempt to reconstruct the marine redox change and nitrogen cycle during the interglacial time. Based on lithostratigraphy as well as geochemical profiles, the basal black shales can be divided into four intervals： Interval 1 has the lowest δ^（15）N value（＋5.0‰）; in interval 2, δ^（15）N values vary between ＋6.4‰ and ＋7.4‰（the first peak）; interval 3 records stable values of δ^（15）N around ＋6‰; and interval 4 is characterized by its higher δ^（15）N values, between ＋6.7‰ and ＋7.8‰（the second peak）. The values of enrichment factor of Mo decrease from 56.8 to 2.6 with the ascending stratigraphic trend. It indicated that immediately after the Sturtian glaciations, the marine seawater above the transitional zone between the shelf to slope of the southern margin of the Yangtze Platform was stratified, with shallow seawater being oxic but deep water being sulfidic. Subsequently, high denitrification rates prevailed in expanded suboxic areas in spite of a short emergence of an oxic condition in the surface seawater, and the deep seawaters were still anoxic or even euxinic.展开更多
In recent years,much attention has been paid to the development environment,biogenetic compositions and hydrocarbon generation characteristics of ancient source rocks in the deep strata of the Sichuan Basin because oi...In recent years,much attention has been paid to the development environment,biogenetic compositions and hydrocarbon generation characteristics of ancient source rocks in the deep strata of the Sichuan Basin because oil and gas exploration extends continuously to the deep and ultra-deep strata and a giant gas field with the explored reserves of more than 1×10^(12)m^(3)was discovered in the Middle and Upper ProterozoiceLower Paleozoic strata in the stable inherited paleo-uplift of the central Sichuan Basin.Based on the previous geological research results,outcrop section of the Datangpo Fm,Nanhua System,at the southeastern margin of the Sichuan Basin was observed and the samples taken from the source rocks were tested and analyzed in terms of their organic geochemistry and organic petrology.It is shown that high-quality black shale source rocks of the Datangpo Fm are developed in the tensional background at the southeastern margin of the Sichuan Basin between two glacial ages,i.e.,Gucheng and Nantuo ages in the Nanhua Period.Their thickness is 16e180 m and mineral compositions are mainly clay minerals and clastic quartz.Besides,shale in the Datangpo Fm is of high-quality sapropel type source rock with high abundance at an overmature stage,and it is characterized by low pristane/phytane ratios(0.32-0.83),low gammacerane abundance,high-abundance tricyclic terpane and higher-content C_(27)and C_(29)gonane,indicating that biogenetic compositions are mainly algae and microbes in a strong reducing environment with low salinity.It is concluded that the Datangpo Fm source rocks may be developed in the rift of Nanhua System in central Sichuan Basin.Paleo-uplifts and paleo-slopes before the Caledonian are the favorable locations for the accumulation of dispersed liquid hydrocarbons and paleo-reservoirs derived from the Datangpo Fm source rocks.In addition,scale accumulation zones of dispersed organic matter cracking gas and paleo-reservoirs originated from the Datangpo Fm source rocks may be discovered in the stable area inside the Sichuan Basin.展开更多
Palaeoclimatic and palaeoenvironmental reconstructions of the Cryogenian Period have attracted attention in relation to the debated“Snowball Earth”hypothesis and the early evolution of metazoan life.The carbon cycle...Palaeoclimatic and palaeoenvironmental reconstructions of the Cryogenian Period have attracted attention in relation to the debated“Snowball Earth”hypothesis and the early evolution of metazoan life.The carbon cycle and redox conditions of the Sturtian-Marinoan non-glacial interval have been subjected to much controversy in the past decades because of the lack of a high-resolution stratigraphic correlation scheme.As one of the typical Sturtian-Marinoan interglacial deposits,the Datangpo Formation was widely distributed in South China with shales continuously deposited.The previous zircon dating data of the Datangpo Formation provide important ages for global constrain of the Sturtian-Marinoan non-glacial interval.Here we present a high-resolution straitigraphic study of the organic carbon isotopes of the Datangpo Formation from a drill core section in northern Guizhou Province.Based on measured episodicδ^(13)C_(org) perturbations,three positive shifts and three negative excursions are identified.Aδ^(13)C_(org)-based chemostratigraphic correlation scheme is proposed herein that works well for the Datangpo Formation regionally.Meanwhile,theδ^(13)C_(org) vertical gradients changed dynamically throughout the formation.This discovery implies that a significant ocean circulation overturn might have occurred in the upper Datangpo Formation,coinciding with the potential oxygenation.展开更多
基金supported by the National Basic Research Program of China (No. 2013CB835004)NSFC programs (Nos. 41102018, 41230102)
文摘Cryogenian Datangpo Formation was deposited during the interglacial time between the Sturtian and Marinoan ice ages. We studied nitrogen isotope compositions and contents of Mo of the black shales from the basal Datangpo Formation in northeastern Guizhou, South China, for an attempt to reconstruct the marine redox change and nitrogen cycle during the interglacial time. Based on lithostratigraphy as well as geochemical profiles, the basal black shales can be divided into four intervals： Interval 1 has the lowest δ^（15）N value（＋5.0‰）; in interval 2, δ^（15）N values vary between ＋6.4‰ and ＋7.4‰（the first peak）; interval 3 records stable values of δ^（15）N around ＋6‰; and interval 4 is characterized by its higher δ^（15）N values, between ＋6.7‰ and ＋7.8‰（the second peak）. The values of enrichment factor of Mo decrease from 56.8 to 2.6 with the ascending stratigraphic trend. It indicated that immediately after the Sturtian glaciations, the marine seawater above the transitional zone between the shelf to slope of the southern margin of the Yangtze Platform was stratified, with shallow seawater being oxic but deep water being sulfidic. Subsequently, high denitrification rates prevailed in expanded suboxic areas in spite of a short emergence of an oxic condition in the surface seawater, and the deep seawaters were still anoxic or even euxinic.
文摘In recent years,much attention has been paid to the development environment,biogenetic compositions and hydrocarbon generation characteristics of ancient source rocks in the deep strata of the Sichuan Basin because oil and gas exploration extends continuously to the deep and ultra-deep strata and a giant gas field with the explored reserves of more than 1×10^(12)m^(3)was discovered in the Middle and Upper ProterozoiceLower Paleozoic strata in the stable inherited paleo-uplift of the central Sichuan Basin.Based on the previous geological research results,outcrop section of the Datangpo Fm,Nanhua System,at the southeastern margin of the Sichuan Basin was observed and the samples taken from the source rocks were tested and analyzed in terms of their organic geochemistry and organic petrology.It is shown that high-quality black shale source rocks of the Datangpo Fm are developed in the tensional background at the southeastern margin of the Sichuan Basin between two glacial ages,i.e.,Gucheng and Nantuo ages in the Nanhua Period.Their thickness is 16e180 m and mineral compositions are mainly clay minerals and clastic quartz.Besides,shale in the Datangpo Fm is of high-quality sapropel type source rock with high abundance at an overmature stage,and it is characterized by low pristane/phytane ratios(0.32-0.83),low gammacerane abundance,high-abundance tricyclic terpane and higher-content C_(27)and C_(29)gonane,indicating that biogenetic compositions are mainly algae and microbes in a strong reducing environment with low salinity.It is concluded that the Datangpo Fm source rocks may be developed in the rift of Nanhua System in central Sichuan Basin.Paleo-uplifts and paleo-slopes before the Caledonian are the favorable locations for the accumulation of dispersed liquid hydrocarbons and paleo-reservoirs derived from the Datangpo Fm source rocks.In addition,scale accumulation zones of dispersed organic matter cracking gas and paleo-reservoirs originated from the Datangpo Fm source rocks may be discovered in the stable area inside the Sichuan Basin.
基金Special thanks to Erik Tihelka for improving the English.This study was supported by the National Natural Science Foundation of China(41602126)the China Geological Survey(DD20160018,DD20221661)+1 种基金the Second Tibetan Plateau Scientific Expedition and Research Program(2019QZKK0706)Liu Bao-jun Academician Research Funds subsidized by Chengdu Center of China Geological Survey.
文摘Palaeoclimatic and palaeoenvironmental reconstructions of the Cryogenian Period have attracted attention in relation to the debated“Snowball Earth”hypothesis and the early evolution of metazoan life.The carbon cycle and redox conditions of the Sturtian-Marinoan non-glacial interval have been subjected to much controversy in the past decades because of the lack of a high-resolution stratigraphic correlation scheme.As one of the typical Sturtian-Marinoan interglacial deposits,the Datangpo Formation was widely distributed in South China with shales continuously deposited.The previous zircon dating data of the Datangpo Formation provide important ages for global constrain of the Sturtian-Marinoan non-glacial interval.Here we present a high-resolution straitigraphic study of the organic carbon isotopes of the Datangpo Formation from a drill core section in northern Guizhou Province.Based on measured episodicδ^(13)C_(org) perturbations,three positive shifts and three negative excursions are identified.Aδ^(13)C_(org)-based chemostratigraphic correlation scheme is proposed herein that works well for the Datangpo Formation regionally.Meanwhile,theδ^(13)C_(org) vertical gradients changed dynamically throughout the formation.This discovery implies that a significant ocean circulation overturn might have occurred in the upper Datangpo Formation,coinciding with the potential oxygenation.
