RECONSTRUCTING THE DROWNED RAMP DURING EARLY TO MIDDLE ORDOVICIAN IN THE YANGTZE REGION, SOUTH CHINA: IMPLICATIONS FOR THE GREAT ORDOVICIAN BIODIVERSIFICATION EVENT (GOBE)

During Early-Middle Ordovician in South China, the Yangtze Platform was changed from a shelf to a ramp due to drowning event, coincided with biodiversity acmes of many taxa, also known as the Great Ordovician Biodiversification Event (GOBE) in South China. The GOBE is one of the most significant and rapid radiations of marine organisms during geological history, especially at lower taxonomic levels, associated with ecological revolutions indicated by the replacement of Cambrian evolutionary fauna by Paleozoic evolutionary fauna. Both the intrinsic and extrinsic factors are taken into considerations in order to explain the origin of GOBE. The extrinsic factors, however, should be more important at larger scale (i.e., paleocontinental scale). In South China, numerous researches focused on the GOBE, showing the earliest biodiversity acme of brachiopods during Early-Middle Ordovician and other unique features. Although some corresponding sedimentary responses have been observed, the correlations with ramp background have never been discussed in detailed.

Thus six research sections have been selected, including Daoba, Xiangshuidong, Daling, Gudongkou and Nanba sections, and they can be correlated with each other based on the biostratigraphical framework.The mixed siliciclastic and carbonate deposits of the Meitan, the Dawan and the Zitai formations are widely distributed in the Yangtze Platform after drowning. More specifically, in this research, four siliciclastic lithofacies and fifteen carbonate lithofacies are identified. The results show that the morphology of ramp after drowning are high in northwest and low in southeast, dipping downward from north to south, with the terrigenous sediments coming from western platform, being controlled by eustatic sea-level and tectonic movements. The brachiopods, moreover, attains its first diversity peak in South China or even around the world during Ordovician, and this diversity peak comprises three onshore migrations, which is probably facilitated by a series of new niches provided by the gentle morphology of ramp. Since the brachiopods occupied the shallow-water environment, the Whiterock fauna (trilobites) migrated toward deep water, have been suggested as avoiding the competitions with brachiopods, and gradually replaces the Ibex fauna (trilobites) in the process of migration. Thus the episodic radiations of organisms in South China are more or less controlled by the paleoenvironmental background.