侏罗系、白垩系界线问题及对中国北方陆相界线的思考

Jurassic-Cretaceous boundary and its terrestrial issue in Northern China

侏罗系、白垩系间是显生宙唯一还没有定义系级GSSP的界线。生物演替在Thithonian期与Berriasian期之间不存在明显变化,很难找到可以进行全球对比的标志。作为中生代生物年代地层学主要划分依据的菊石类生物,亦受生物区系影响而难作为侏罗系、白垩系界线的标志。微体化石具有优势。近期,国际上趋于将瓮虫类Calpionella alpina带之底、钙质超微化石Nannoconus steinmannii minor和N.kamptneri minor二者的始现面作为侏罗系、白垩系界线,对应于极性带M19n.2n内部,年龄为145 Ma。国际地层划分与对比以海相为标准。中国侏罗系、白垩系以陆相地层发育为特征,与海相对比颇具难度。以陆相生物为标志的界线划分产生了地区性的生物年代地层格架,也形成了地方生物地层与国际年代地层对比的错位,导致中国陆相侏罗系、白垩系界线成为地学界争议的问题。本文认为,在综合考虑不同学科研究结果的同时,问题解决的实质是客观认识陆相生物对比的缺陷,结合考虑非生物的标准,如磁性地层、旋回地层和同位素测年的精度,获得超越相区的对比结果。基于陆相生物特点,着眼于宏观演化阶段来识别界线位置是问题解决的初期阶段。燕辽生物群与热河生物群是代表侏罗纪和白垩纪的两大生物演化类群,其间年龄可为土城子组顶部的139 Ma。这是为全方位研究而铺设的临时工作界线。今后将土城子组作为侏罗纪、白垩纪过渡地层而加大研究力度,有可能在该组顶部或其内部发现界线生物标志和同层位绝对年龄值,或许会使其向目前国际推荐年龄(145Ma)更靠近一步;也不排除GSSP未定的国际侏罗系、白垩系界线年龄会向年轻方向变化,趋于139Ma的Valanginian阶底界位置。

The Jurassic/Cretaceous is the only Phanerozoic boundary that does not yet have an accepted global boundary definition.In contrast to most geological systems,there are nearly no ＂global events＂within the uppermost Jurassic and lowermost Cretaceous.It has been difficult to find any significant widespread biostratigraphic,geochemical or other markers for inter-regional correlation within this interval.The ammonite definition for the base of the Cretaceous appears to fall within the middle of relatively long ammonite zones.The current markers tend to integrate regional calpionellid zones,calcareous nannofossil datums,ammonite zonations,and magnetostratigraphy.The J/K boundary is at the base of Calpionella alpina zone and FAD of Nannoconus steinmannii minor and N.kamptneri minor.It is in the Chron M19 n.2nand of 145 Ma age.The international stratigraphical division and correlation are based on marine standard.The Jurassic and Cretaceous strata in northern China are mostly of terrestrial origin.It appears a difficulty to define the boundary.In the previous biostratigraphic work,the J/K boundary was referred to a higher position of much younger age,which caused a big controversy between local biostratigraphy and international age. The pronounced provincialism of terrestrial fauna and flora obstructs global correlation.The solution of persistent debates on the J/K boundary in China should be the integrated stratigraphic markers which might provide the most useful global correlation.Owing to the obscure of terrestrial fossil evidences to indicate the boundary,the macro biotic evolution is suggested as a potential workable J/K boundary indicator.The Yanliao Biota and Jehol Biota are two big fossil groups representing Jurassic and Cretaceous respectively.If it is the case,the transition beds of two biotas would be down to the Tuchengzi Formation.The boundary age could alternatively be 139 Ma on the top or 145 Ma in the middle of the formation.Only accurate indicators or markers would be obtained from the Tuchengzi Formation and/or GSSP being accepted,the J/K boundary in China tends to be completely located.