云南永善肖滩早寒武世早期碳氧同位素记录

CABBON AND OXYGEN ISOTOPIC RECORD OF THE EARLY CAMBR IAN FR OM THE XIAOTAN SECTION , YUNNAN , SOUTH CHINA

本文对采自云南永善肖滩剖面下寒武统１００余件碳酸盐岩样品的碳氧同位素进行了分析研究。在从灯影组白岩哨段上部至筇竹寺组玉案山段碳酸盐岩碳同位素变化曲线上，梅树村组小歪头山段底部Ｃ值表现出显著负飘移。早寒武世梅树村期，碳同位素值缓慢增大，至大海段达到最大值。在筇竹寺组石岩头段底部，Ｃ值表现为明显负飘移。在玉案山段，碳同位素又开始一个新的正飘移旋回。根据同位素资料，肖滩剖面下寒武统梅树村组小歪头山段至大海段地层相当于前托莫特阶地层，筇竹寺组石岩头段与托莫特阶地层对应，玉案山段与呵特达班阶下部地层对比。小歪头山段底部标志着寒武纪沉积已经开始。

Thecarbonaterock.plesanalyzed.herein were taken from the Xiaotan. section on the bank of the Jinshajiaag River, nor&eastem Yunaan, Soutb China. The early Cambdan cropping out at this section disonformably overlies the dolonuita of the Baiyanshao Member of the Precambrian Dengying Formadon. lts stratigraphical succession ondsb of the Xiaowaitoushan Member, the Zhongyicun Member and the Dember Member. of tbe Mdshucun Formation, the Shiyantou Member and the Yu, anshan Membw of the Qiongzhusi Fonmadon . Inmajor features the profile shows and 8‰, sharp drop in through the bounda-ry of the Baiyanshao Member and the Xiaowaitousban Member,followed by a slow rise in .values from about -l2‰, to nearlyz+.6‰,throughout the early CambrianMeishucun Formation ,The valuse show a pronounced decrease to about -5.4‰ through the boundary between the Dahai and Shiyantou Members, and then are. characterized by generally negative valuseraningbetwem -11.32‰and -0.06‰in the overlying Qiongzhusi Formation, Obviously, this curve presents four four carboa-isotope cy-cles includtiagthe rise throughthe Xiaowaitoushan Membeand.lower andrmtddle parts of the Zhongyicun member, the proaounod positive excursion theough the up- per Zhongyicun Member and the Dahai Member, the negadve excunion through the Shiyantou member and rising trend through the Yu , anshan Member. The pattern in the Meishucun Formation is believedtobeprnadly an origbal oceanic signal,while the values in the Qiongzhusi Formationa were pardally afected bydiagenesis.This isotopic patternmay be conelakdwiththatoftheadjacent MeishucunCprofile and many others around the we prder to equate the lowerrisewithin the Meishucun Formation from the Xiaotan section to the pre-Yomrmotian rise, and the upper reladvely deplekd values of C within the Shiyantou Member to that of Tormmotian and relatively rising valuse of within the overlying the overlying yu,anshanMember to the lower Atdbanian C profile in Siberia. The coneladoas also suggest that the Pnxambdan/Cambrian bound. exist betwam the Xiaowaitoushan and Bdyaoshao Memtbers