末前寒武系年代地层研究:问题,进展与建议

在讨论当前全球末前寒武系和中国震旦系年代地层划分中存在问题的基础上 ,根据三峡地区震旦纪和寒武纪生物地层学、层序地层学和年代地层学的综合研究 ,作者认为 :( 1 )中国传统的震旦系底界和后来建立的陡山沱阶和灯影峡阶的底界是不合适的 ,前者位于不整合面上 ,而后二者的底界分别位于相对动荡的低位域的开始和由海退所形成的高位域的开始 ,不仅横向变化大 ,而且很难保存和发现可供对比的生物和化学标志。 ( 2 )建议将中国震旦系的底界上移到陡山沱组第 2段近底部 ,即靠近陡山沱组第 2段黑色页岩与第 1段灰白色白云岩之间的界线 ,以出现分异度高、个体较大、构造复杂的疑源类为标志 ,该界面与南沱冰期后所形成的海侵生物面基本一致 ;同时主张将原灯影峡阶的底界要么下移到陡山沱组第 4段产庙河生物群的黑色页岩底部 ,要么上移到灯影组产文德生物群的石板滩段黑色页岩夹炭质灰岩底部 ,因为这两个界面分别与南沱冰期后形成的层序 2和层序 3的最大海泛面基本一致。 ( 3)根据同位素年代学研究 ,指出陡山沱组第 4段 ,即庙河生物群的 Rb-Sr等时线年龄为 ( 6 85± 9) Ma,而水井沱组底部含三叶虫的黑色页岩的 Rb-Sr等时线年龄为 ( 5 30± 8) Ma

TERMINAL PRE-CAMBRIAN CHRONOSTRATIGRAPHY: PROBLEM,PROGRESS AND PROPOSAL

On the basisof the discussion of global terminal pre- Cambrian and Sinian subdivision in association with integrated sequence- ,bio- ,and chemo- stratigraphic study of the Sinian and lowest Cambrian from the Yangtze Gorgesarea itissuggested the following points.Traditional Sinian basalboundary and later proposed basal boundaries of the Doushantuo and Dengyingxia stages,appear to be not suitable because the former boundary is lain on the unconformable surface,and the latter two boundaries are separately situated at the beginning of relatively unrest LST and HST.Both bio- and chemo- stratigraphic marks are difficult to be preserved in these levelswith greatchange of lateralfacies.The basalboundary ofthe Sinian would be moved upward to nearthe base of Member2 ofthe Doushantuo Formation,i.e.nearthe boundary between the black shale at the base of the Member 2 and greyish white dolomite at the base of Member 1 .This boundary is characterized by the appearance of aritarch assemblage consisting of very large and complex forms and basically coincided with the Nantuo postglacial transgressive bio- surface.The base of the Dengyingxia Stage would be moved downward to the base of the Member 4 of the Doushantuo Formation consisting of black shale with the Miaohe biota or upward to the base of Shibantan Member( the Dengying Formation) composing of black siliceous shale and thin bedded siliceous limestone with Vendian biota,because these two boundaries are close to the flooding surfaces of sequence 2 and 3formed by the Nantuo postglacial transgression respectively.Isotopic geochoronological re- study indicates ( 685± 9) Ma of Rb- Sr isochron age for the Member4 of the Doushantuo Formation with the Miaohe biota and( 5 30± 8) Ma of Rb- Sr isochron age for black shale at the base of the Shuijingtuo Formation with trilobite fossils.