NEW OBSERVATION ON THE ORDOVICIAN AND SILURIAN ROCKS IN XICHUAN(EAST QINLING),WESTERN HENAN,CENTRAL CHINA

Preliminary investigation on the Upper Ordovician and lower Silurian Llandovery in Xichuan,western Henan reveal new observations.1)The clastic Upper Ordovician in the research area should be called the Manziying Formation as a whole(e.g.at Wangguangou of Shangji town),while those localities with both limestones and mudstones include the Yangzhuangcun,Shiyanhe and the Liujiapo formations in ascending order(e.g.at Shiyanhe of Dashiqiao town).2)According to the conodonts conodont and brachiopods biostratigraphic data,the Shiyanhe Formation(with the Rongatrypabrachiopod fauna)and the lower Zhangwan Formation(with the Cathaysiorthis brachiopod fauna)may be of respectively,the middle Katian(Late Ordovician)and early-middle Rhuddanian(Llandovery,Silurian)age and the top Katian the Hirnantian and the basal Rhuddanian(lasted for about 2Myr)are probably missing in the area studied.3)The Ordovician-Silurian boundary is placed between the Manziying and the Zhangwan formations,i.e.at a disconformity between these two formations.The discovery of the Cathaysiorthis fauna indicates that the rocks older than the Pristiograptus leei biozone were deposited in the research area.4)The Zuoqu Formation may be of Early Ordovician age,and hence the entire Middle Ordovician is missing there,i.e.without deposition for about 10 Myr in this part of the South China paleoplate.5)During the Late Ordovician,the conodont fauna of North China and Mid-Continental(North American)type and the central to eastern Asian brachiopod fauna developed,but transitioned to typical shelly fauna of South China type at the beginning of Silurian.Such paleobiogeographic transition shows that South China,North China,North America,and central-eastern Asia might have been all around the paleoequator during the Ordovician-Silurian transition,and biotic exchanges were facilitated by equatorial ocean currents.