Stratotype Section And Ostracod Biostratigraphy Of The Dabeigou Formation From The Yushuxia Section In Luanping Basin,Northern Heibei

The strata in western Liaoning and northern Hebei is the typical layer bearing the famous Jehol Biota. But, there exists interruption of sedimentation under the Yixian Formation or the Xiguayuan Formation, which is the mainly layer consists of the Jehol Biota. This results in great difficulty in researching the early evolution of the Jehol Biota and the early cretaceous strata of China. Luckily, the geological workers have found a well-espoused section that has continues sediment and yields abundant fossils in the Luanping Basin. This research is based on the former foundation, and does some further studies on the lithostratigraphy, biostratigraphy and sedimentary facies. Based on the investigation from the field outcrops and the analysis of thin section, we divide the Dabeigou Formation of the Yushuxia Section into 3 members and 46 layers, and establish the sedimentary sequence of the Dabeigou Formation in centimeter level(2~5 cm). And recognizing the fan delta facies with rough sediment in first member and the mid-deep to deep lake facies with fine sediment in the second and the third member of the Dabeigou Foramtion. Based on the systematic collecting ostracod fossils from the third member of the Dabeigou Formation and the newly found ostracod fossil from the upper part of the second member, 9 genus and 17 species of ostracod are identified. Besides, they can be divided into 1 zones: Luanpingella-Eoparacypris- Pseudoparacypridopsis and 2 sub-zones: Luanpingella postacuta and Pseudoparacypridopsis mountfeildensis. We have found abundant insect fossils: Ephemeropsis trisetaris, which is the one of the typical fossils of the Jehol Biota, from the middle-lower part of the second member to the third member of the Dabeigou Formtian on the Yushuxia Section. And according to the former workers, lots of the fossils that have closing relation with the Jehol Biota, such as Peipiaosteus sp., Peipiaosteus fengningensis Bai, 1983, Liaoningornis sp. Gao, 2014, Protopteryx fengningensis Zhang, 2000, have been found in the Dabeigou Formation in this section or other section of the northern Hebei. Thus, we tend to think that the fossils of the Dabeigou Formation should belong to the early state of the Jehol Biota. According to the analysis of some important species of the ostracods, conchostracans and sporopollen from the Dabeigou Formation in the geological time aspect, and consider the restriction of the geological time of the Jehol Biota as well as the isotope dating results, we assign geological time of the Dabeigou Formation to the Early Cretaceous temporarily. The above outcomes will have important manning in improving the strato-typed section of the Dabeigou Formation, and establishing the dividing and comparing standard of continental strata in north-east Asia.