燕辽地区土城子组古生物组合与陆相侏罗系-白垩系界线年代地层

TERRESTRIAL FOSSIL ASSEMBLAGES OF TUCHENGZI FORMATION AND JURASSIC-CRETACEOUS BOUNDARY CHRONOSTRATIGRAPHY IN YANLIAO AREA

土城子组是一套广泛发育于燕辽地区的陆相红色碎屑沉积岩系,为中国陆相侏罗系-白垩系(J/K)界线研究的典型。由于陆相化石记录的不完备,以及生物地层划分与同位素年代学的矛盾,使该地层时代归属不定。该组所含生物门类比较丰富,但化石层位较少,可以识别出叶肢介、介形类、孢粉等化石组合,但是前人对其时代意见不一。国内外学者在土城子组及其上下地层获得大量同位素测年资料,指示该组年龄处于156-139 Ma之间,属于晚侏罗世-早白垩世,因此J/K界线就划定在土城子组内部。尽管传统的生物地层学研究积淀正被日渐积累的年代地层学数据引发各种挑战,但毋庸置疑的是,年代地层单元和地质历史的划分必然需要有可供区域对比的标志,生物演化始终是地球发展阶段划分的依据。在土城子组生物地层研究精度尚待提高之时,本文尝试提出一粗略的J/K界线划分方案,以晚侏罗世叶肢介Pseudograpta-Monilestheria-Sinograpta(P-M-S)组合、介形类Cetacella substriata-Mantelliana alta-Darwinula bapanxiaensis(C-M-D)组合与早白垩世叶肢介Yanshanoleptestheria-Pingquania-Lingyuanella(Y-P-L)组合、介形类Djungarica yangshulingensis-Mantelliana reniformis-Stenestroemia yangshulingensis(D-M-S)组合为界线划分依据。近期孢粉的研究结果显示,J/K界线可以Cicatricosisporites-Lygodioisporites-Jiaohepollis组合的出现为标志。根据化石组合发育特征分析,J/K界线位置大致在土城子组2段与3段之间,年龄为~145 Ma。建议将北票巴图营乡剖面作为对比标准。由此提出下白垩统底部"巴图营阶"的工作模型,为中国陆相白垩纪年代地层的建阶工作提供参照。

The Jurassic and Cretaceous strata in northern China are mostly of terrestrial origin.It presents a difficulty in defining the Jurassic-Cretaceous(J/K)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 the international standard.Recently,large quantity measurements of isotopic ages reveal that the J/K boundary is possibly within the Tuchengzi Formation.The formation is a group of terrestrial reddish sedimentary deposits,and divided into 3 members.It is the type sequence of terrestrial Jurassic-Cretaceous transition in northern China.The stratigraphic age of the formation is in disagreement,owing to more-or-less limited preservation of fossils and a contradiction between biostratigraphic and isotopic data.Certain fossil groups have been found in scattered beds,dominated by conchostracans,ostracodes,dinosaurs,spore and pollen.Furthermore,the pronounced provincialism of the terrestrial fauna and flora obstructs global correlation.Paleontologists have already produced many publications after hard work,and they gave different stratigraphic divisions.A large amount of isotopic dating provides reliable data showing that the duration of the Tuchengzi Formation is from 156 Ma to 139 Ma,covering the late Jurassic to early Cretaceous period.The J/K boundary lies within this interval.Huge chronostratigraphic data challenge the traditional biostratigraphic results.The chronostratigraphic unit,otherwise,has to be indicated by a biotic boundary marker.Before a biostratigraphic division of high precision has been worked out,we attempt to make an option for the J/K boundary marker according to the data in our possession.After recognizing the fossil assemblages,the location of the J/K boundary is between the Jurassic conchostracan Pseudograpta-Monilestheria-Sinograpta(P-M-S)and ostracode Cetacella substriata-Mantelliana alta-Darwinula bapanxiaensis(C-M-D)assemblages,and Cretaceous conchostracan Yanshanoleptestheria-Pingquania-Lingyuanella(Y-P-L)and ostracode Djungarica yangshulingensis-Mantelliana reniformis-Stenestroemia yangshulingensis(D-M-S)assemblages.It is also indicated by the first appearance of spore and pollen Cicatricosisporites-Lygodioisporites-Jiaohepollis(C-L-J)assemblage.Lithologically,the boundary is between the members 2 and 3 of the Tuchengzi Formation and of^145 Ma in age.The Batuying sections in Beipiao are recommended as the type sequence of the formation.Based on the occurrence of the earliest Cretaceous fossil assemblages in the member 3,we propose a"Batuying Stage"as the lowermost terrestrial chronostratigraphic unit of the Cretaceous in China.The duration of the stage is 145–139 Ma,correlating roughly to the international Berriasian Stage.The members 1–2 are referred to the uppermost"5 th stage"which approximately corresponds to the Kimmeridgian–Tithonian stages of the Upper Jurassic.The present suggestion is only to provide a consultation for the terrestrial Cretaceous chronostratigraphic study.