Using chemostratigraphy and biostratigraphy methods to define the unconformity underlying the Upper Ordovician Pagoda Formation in the Upper Yangtze Platform

Abstract

The discovery of hydrocarbon in the Pagoda Formation in South China has received attention from the petroleum industry. This research paper identifies the discontinuity surface underlying the Pagoda Formation. The Upper Ordovician strata were correlated in different regions using the conodont biostratigraphy, and δ13C and δ18O isotope stratigraphy. The biostratigraphic result illustrates that three conodont zones can be established in each study section, and one to three conodont zones are missing in the lower Pagoda Formation and the strata underlying the Pagoda Formation of the Upper Yangtze Platform. Moreover, the chemostratigraphic result highlighted that the development of the lower δ13C isotope positive excursion events in the Xiaogangwan and Liangcun areas was less complete than that in the Jiaodingshan area. Combined with the cyclostratigraphic results, the development of the δ13C isotope positive excursion is not complete. The development of the δ13C excursion event is more complete in the south and east than in the center and north. This demonstrates that the Pagoda Formation began its deposition at different times and the transgression process may have advanced from the southeast to the north and that the contact between the Pagoda Formation and the underlying formation is not conformable. It is proposed that the Late Ordovician in the South China Plate was affected by the tectonic movement and the cooling paleoclimate at that time, which resulted in the discontinuity between the Pagoda Formation and the underlying strata. The unconformity could be the channel for hydrocarbon migration or for the high-quality oil and gas reservoirs being developed and preserved under the unconformity surface.