TERRESTRIAL FOSSIL ASSEMBLAGES OF TUCHENGZI FORMATION AND JURASSIC-CRETACEOUS BOUNDARY CHRONOSTRATIGRAPHY IN YANLIAO AREA
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Abstract:
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 posi-tion of much younger age, which caused a big controversy between local biostratigraphy and the international standard. Re-cently, 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. Further-more, the pronounced provincialism of the terrestrial fauna and flora obstructs global correlation. Paleontologists have al-ready 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 Cetacel-lasubstriata-Mantelliana alta-Darwinula bapanxiaensis (C-M-D) assemblages, and Cretaceous conchostracan Yanshano-leptestheria-Pingquania-Lingyuanella (Y-P-L) and ostracode Djungarica yangshulingensis-Mantelliana reniform-is-Stenestroemia yangshulingensis (D-M-S) assemblages. It is also indicated by the first appearance of spore and pollen Ci-catricosisporites-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 pro-pose 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 upper-most “5th 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.
