REEF COLLAPSE IN THE MID-CAPITANIAN NEAR THE NORTHERN CONNECTING CHANNEL BETWEEN THE TETHYS AND PANTHALASSA: LITHOSTRATIGRAPHY OF THE TOPMOST IWAIZAKI LIMESTONE (CAPITANIAN) IN NE JAPAN

Permian biota suffered two-step mass extinction; the biodiversity declined for the first time from the Capitanian (Late Guadalupian) before the G-L boundary (ca. 260Ma) when the world sea-level reached the lowest in the Phanerozoic, suggesting a cause-effect relationship between cooling and extinction. Fossiliferous Capitanian carbonate sequences have been analyzed in detail in various low-latitude sections. The Wordian-Capitanian Iwaizaki limestone in NE Japan represents a shelf carbonate partly with reef, deposited on the northeastern part of Greater South China. In contrast to the reef facies in the middle Iwaizaki limestone, the topmost part records the collapse history of reef. The uppermost Iwaizaki limestone yields Capitanian large fuzuline (Lepidolina) and rugose coral (Waagenophyllum). In order to clarify extinction-relevant environmental changes during the Capitanian, we analyzed lithostratigraphy of the topmost Iwaizaki limestone (ca. 50m thick), and the following results were obtained. 1) The uppermost part consists of interbedded dark limestone/mudstone, in which mudstone gradually became dominant upsection. 2) Lepidolina became extinct in the middle of the uppermost part, clearly below the G-L boundary, and 3) various warm water-adapted shallow marine animals disappeared step by step during the Capitanian. Such a gradual disappearance pattern of tropical organisms is consistent with the appearance of cooling. This is a new constraint to the Capitanian extinction first obtained from the northern connecting channel between the Tethys and Panthalassa.