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STANDARD STRATO-SEQUENCE OF TERRESTRIAL JURASSIC—CRETACEOUS BOUNDARY STRATA IN LUANPING,NORTHERN HEBEI

Tian Shugang~1 Li Peixian~1 Liu Yongqing~1 Pang Qiqing~2 Niu Shaowu~3 1 Institute of Geology,CAGS,Beijing 100037; 2 Shijiazhuang Economic College,Shijiazhuang,Hebei 065000; 3 Tianjin Institute of Geology and Mineral Resources,Tianjin 300170  
Based on discovery of sedimento-successive sections of the Jurassic-Cretaceous transitional stra- ta,a boundary stratotype candidate has been preliminarily erected in the Luanping basin of northern Hebei.By multi-course studies,some achievements have gained as follows.Two formations named originally,the Dabeigou Fm and the Dadianzi Fm,have been reviewed and divided to 7 litho-units systematically in which 4 mark-beds distinguished and named.A chrono-stratigraphic sequence and contents of two stages,the Dabeigouan and the Yixianian,have been completed intensively.The transitional strata have been divided into 5 cycles and proved that they deposited in a successive lacustrine conditions.Seven ostracod zones and 6 conchostracan zones have been di- vided strictly according to occurrences and evolutions of fossils.Development of the fauna indicates that it consists of 4 evolutionary stages and the J—K bound could be put at the initial of the earlier Johul Fauna(earlier Yixiani- an).Sequence-stratigraphy is applied firstly,5 sequences,13 subsequences and 136 parasequences have been distinguished,an isochrononic framework of strata constructed and basin developments in that period was discussed. Moreover,an O-isotope abnormal value appears at the boundary where some isotopic ages have been measured (i.e.zircon ~(206)Pb/~(238)U).The value of the uppermost Zhangjiakou Fm is 135.4±1.6 Ma and two from the mid- dle Dabeigou Fm are 133.9±2.5 Ma and 130.1±2.5 Ma.The boundary section point is located at the bottom of bed 9 of the Zhangjiagou section and marked by is the first-occurrence of ostracod species Cypridea stenolanga.
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