FIRST REEF RECOVERY AFTER THE END-ORDOVICIAN EXTINCTION IN SOUTH CHINA: MID-AERONIAN CORAL-STROMATOPOROID REEFS FROM NORTHERN GUIZHOU

Metazoan reefs were significantly affected by the end-Ordovician extinction event. With a rapid isotopic recovery to normal sea water levels as well as global re-warming and rising sea levels, the Rhuddanian Stage (earliest Silurian) witnessed a post-extinction return of shelly biota with a relatively low diversity. The initial resurgence of metazoan reefs was reported in Rhuddanian strata from Canada and the Baltic region. In contrast, initial recovery of coral–stromatoporoid reefs in South China occurred later, in the mid-Aeronian. Good examples are known from the upper Xiangshuyuan Formation in Shiqian, North Guizhou, but investigated in detail only recently. The main reef body exposed at Xiangshuyuan section is about 18.5 m high and 150 m in diameter, growing directly upon crinoidal grainstones. Laterally, the reef grades into surrounding bioclastic limestones. The massive reef limestones are characterized by a dense framework composed mostly of corals and stromatoporoids, containing an accessory fauna of abundant bryozoans, sponges and crinoids, as well as some brachiopods, trilobites, molluscs and calcimicrobes. Heliolites, Favosites and Halysites are the dominant framework builders. The reef body is capped by siltstone/shales of the Leijiatun Formation, representing deepening of the sedimentary environment due to transgression and is interpreted to have produced turbid shallow-marine environments. Thus, transgression and siliciclastic input are interpreted to have ended the reef growth.