Abstract:The Middle—Late Ordovician Pagoda limestone in the Jiaodingshan area of western Upper Yangtze platform was used as the studying object and continuous fossil samples were obtained in the Pagoda Formation. Fifteen genera and twenty species were identified and three conodont zones were divided which could be correlated within China and globally. In ascending order, they are Amorphognathus tvaerensis zone, Hamarodus brevirameus zone, and Protopanderodus insculptus zone. It is found that there is one or two conodont zones absence in the lower Pagoda Formation compared with typical continuous conodont zones in the adjacent areas. Besides, high resolution of carbon and oxygen isotopic samples were collected in the Pagoda Formation. The positive drift anomaly is found in the δ13C isotope sequence of the Pagoda Formation, which contains three secondary peaks, ranging from 2.00‰ to 3.00‰. The positive excursions also occur in the eastern and southern parts of the Upper Yangtze platform. The conodont Hamarodus brevirameus made its first appearance at the beginning of this excursion and the excursion spans two conodont zones (H. brevirameus zone and the lower part of the P. inscrulptus zone). It could be well correlated within the Upper Yangtze platform and also could be compared with the positive drift anomaly events of the Qilang Formation with three secondary peaks in the Kalpin area, the western Tarim Basin, where the strata thickness is larger and the deposition rate is higher. The results of carbon and oxygen isotopes and conodonts demonstrate that there is hiatus in the lower part of Pagoda Formation, which is less than that in the Middle Yangtze platform, and it is speculated that there is an unconformity contact relationship in the Pagoda Formation with the underlying strata.
