CHANGES IN THE SEDIMENTARY FACIES DURING THE ORDOVICIAN-SILURIAN TRANSITION IN SOUTH CHINA

The sedimentary facies and paleogeography of South China through the Ordovician-Silurian transition has been studied for many decades. In this period, the land-sea distribution pattern is one of the most important reasons for the formation of the Wufeng and Lungmachi black shales, which are the major petroleum source beds in South China, and it also affects the processes of a series of bioevents, including the end-Ordovician mass extinction and the subsequent recovery and radiation in the early Silurian. Based on a unified and latest biostratigraphic framework, a big data set of lithostratigraphic and biostratigraphic information from more than 400 sections in South China were collected and systematically revised through the GBDB database (Geobiodiversity Database, http://www.geobiodiversity.com), in order to reconstruct the distribution of sedimentary facies from the late Katian to Rhuddanian.

The Wufeng Formation was widely distributed over most of the Yangtze Platform in South China, ranging from the Dicellograptus complexus Zone of the upper Katian to the Metabolograptus extraordinarius Zone of the lower Hirnantion. It indicates an anoxic, semi-enclosed environment. In the border areas of the Yangtze Platform, many other types of coeval sediments were deposited. During the mid-Hirnantian, the major deposited rocks on the Yangtze Platform changes from the Wufeng black shales to the Kuanyinchiao limestones, and the dominated fauna changed from the planktonic graptolites to the benthic shelly fauna, which reflects the impact of the global sea level drop caused by the formation of the continental glaciers in the Late Ordovician South Pole. Meanwhile, the sedimentary environment changed from the anoxic, limited, semi-enclosed sea to the normal shallow marine environment. From the late Hirnantian through the Rhuddanian to the late Aeronian, most of the depositional area in South China was covered with black graptolite shales again, such as the Lungmachi Formation, the Kaochiapien Formation and so on. All these formations are similar on the whole, but with slightly differences in the grain size of the rocks and the richness of the fossils. The major driving factor for the widely distributed lower Silurian black shales in South China is the global sea-level rise.