贵州习科1井奥陶-志留系之交的碳同位素化学地层学

Carbon isotope chemostratigraphy of the Ordovician-Silurian transition interval of the Xike-1 drillcore in Guizhou, China

奥陶系赫南特阶碳同位素正漂移(HICE)事件已为人熟知,但受限于样品质量和地层对比精度,目前尚存在一些争议。本次研究对黔北习水县习科1井中的五峰组至龙马溪组底部共22个岩芯样品进行了总有机碳(TOC)和δ13Corg分析,识别出HICE正漂移事件,并将其与华南和国外其他地区的同期事件进行了对比。对华南HICE事件记录的分析表明,五峰组和龙马溪组两个页岩气有利层段与该HICE事件存在较为稳定的层位关系。五峰组页岩气有利层段的结束层位大致对应于HICE事件的起始层位,即笔石Paraorthograptus pacificus带(WF3)顶部至Metabolograptus extraordinarius带(WF4)底部;而龙马溪组页岩气有利层段的起始层位大致对应于HICE事件的结束层位,即笔石Metabolograptus persculptus带(LM1)中部至Akidograptus ascensus带(LM2)底部。在两个有利层段间夹有一段显著正漂移事件的这种模式可在化石材料较为缺乏的剖面和钻井中为有利层段的识别提供较好的参考。分析对比结果还表明,华南HICE事件的特殊性与奥陶纪末冰期期间扬子海的区域氧化还原条件波动相关。在冰盛期,扬子海处于弱还原环境,海底埋藏的有机质大量分解并返回海水中,使得海水δ13CDIC下降,这是华南扬子区HICE事件正漂幅度明显低于全球广海沉积环境的主要控制因素。

The Hirnantian isotope carbon excursion(HICE)has been well known around the world,which is largely based on samples from outcrops but rarely on fresh samples from a drillcore.The HICE event records in South China exhibit significant differences compared with those from sections in other continents,and the reason for the differences remains a debate.In this study,22 samples from the Ordovician-Silurian transition interval(from the upper Wufeng Formation to the basal Longmaxi Formation)of the Xike-1 drillcore in northern Guizhou are analyzed for total organic carbon(TOC)andδ13Corg.Theδ13Corg values show an apparently significant positive excursion,which can be well correlated with the HICE event observed in the outcrop sections in South China and other regions in the world.A comparison of the present and published data from South China indicates that theδ13Corg HICE event constantly occurs between two shale-gas-favorable layers in the Ordovician-Silurian transition,i.e.,one in the Wufeng Formation and another in the lower Longmaxi Formation.The upper boundary of the first shale-gas-favorable interval is coincident with the start of the HICE event,which lies from the topmost Paraorthograptus pacificus Biozone(WF3)to the lowermost Metabolograptus extraordinarius Biozone(WF4).The lower boundary of the second shale-gas-favorable interval coincides with the end of the HICE event,which lies from the middle Metabolograptus persculptus Biozone(LM1)to the basal Akidograptus ascensus Biozone(LM2).These patterns provide a useful reference for the recognition and stratigraphic correlation of the shale-gas-favorable layers in the Ordovician-Silurian transition in South China,especially for the outcrops or drillcores with poor biostratigraphical constraint.Comparison of the HICE records in South China and other continents has revealed a significant difference in the magnitude of the excursion,which is much less prominent in the former.We thus suggest that this is related to the end-Ordovician glaciation and the consequent regional redox fluctuation in the Yangtze Sea.During the maximum glacial stage,the Yangtze Sea was under a weak-reduction condition without dissolved sulfide,and the buried organic matter enormously degenerated,which released abundant 12C back to the seawater and led to the decrease in seawaterδ13CDIC and consequently in a lower HICE peak value.