Graptolite Biostratigraphy And Carbon Isotope Stratigraphy Of The Upper Ordovician - Lower Silurian In Changning, Sichuan, China

The outcrop section of the Upper Ordovician Wufeng Formation– the Lower Silurian Lungmachi Formation was measured in Changning Shuangh e of Sichuan province, Southwest China. And the whole outcrop section was sampled systematically for graptolites fossils. About 1000 graptolite specimens were collected and identified, and 31 species belonging to 16 genera are described herein. Based on the graptolite data and according to the new researches domestic and abroad, the biozones of the Upper Ordovician Wufeng Formation– the Lower Silurian Lungmachi Formation are revised.In the section of Changning Shuanghe, 9 graptolite zones were recognized in the Katian of the Upper Ordovician–the Llandovery of Lower Silurian, that are : Dicellograptus complexus and Paraorthograptus pacificus zones in the Katian (Lower subzone and Tangyagraptus typicus subzone were identified in P. pacificus); Normalograptus extraordinarius and Normalograptus persculptus zones in Hirnantian; Akidograptus ascensus, Parakidograptus acuminatus and Coronogratpus cyphus zones in the Rhuddanian of Lower Silurian; Demirastrites triangulatus and Stimulograptus sedgwickii zones in the Aeronian of Lower Silurian.The Changning Shuanghe Section was densely sampled for organic carbon isotope analysis. According to the organic carbon isotope analysis results, two positive excursions ofδ¹³C are apparently recognized. The first peak appears at the middle of the N.extraordinary zone before Hirnantian, the second one appears at the middle of Hirnantian. In this paper, a detailed comparison is made between the organic isotope curves and both the domectic sections such as Wangjiawan section, Fenxiang section and Huanghuachang section and those abroad sections such as the Dob's Linn section in Scotland, the Arctic section in Canada, the Anticosti Island section in Canada and the Baltica section in Estonia. Although the trend of the organic carbon isotope curves are different somewhat locally, however, two positive excursion peaks of the carbon isotope around the Hirnantian are recorded globally. The two peaks of the organic isotope curves in Changning Shuanghe are corresponded with the two glacier events in the Gandwana at the late Ordovician. This provides a powerful evidence for the global glacier events at the Late Ordovician and the biological extinction and recovery events.