Upper Triassic reef corals from the Sambosan Accretionary Complex, Kyushu, Japan

Reef-building and dwelling scleractinian corals attained worldwide distribution in the Late Triassic and are best known from the former Tethys but some taxa also occur in the collage of displaced terranes now comprising the circum-Pacific rim. Among these are Upper Triassic corals from Japan, which have received little systematic study during the past 40 years but hold keys for resolving crucial questions about the depositional history and paleogeography of this region. Ten Upper Triassic coral taxa are here described from limestone rocks of the Sambosan Accretionary Complex, of Japan—a tectonically mobile belt extending from the Okinawa Islands to south of Tokyo on the island of Honshu. This belt contains the remains of reefs and carbonate sediment deposited on volcanic atolls formed at unknown tropical paleogeographic positions. The paleogeographic relationship of the Sambosan Accretionary Complex (SAC) relative to the Tethys and terranes of the circum-Pacific is not well constrained. The Upper Triassic corals of Japan occur within isolated carbonate blocks and extensive breccia deposits of the SAC. Corals and other organisms contain mixed Carnian and Norian taxa, many of which are known to build reefs and appear to have been derived from reef facies. Here we describe and discuss solitary and colonial, potentially reef-building corals from the SAC that come from the island of Kyushu and from Nara and Kochi Prefectures. Three new species described for the first time are: Retiophyllia tosaensis n. sp., Margarosmilia mizukamia n. sp., and Guembelastraea kanmerae n. sp. Other corals are taxonomically reevaluated from previous work: Craspedophyllia japonica n. sp., Thamnasteriamorpha okudai n. sp., Khytrastrea ominensis (Okuda and Yamagiwa), Craspedophyllia ramosa Roniewicz, Protoheterastraea konosensis (Kanmera), and Seriastraea furukawai (Kanmera). Two additional taxa, Retiophyllia cf. R. frechi Roniewicz and Volzeia cf. V. badiotica (Volz) show relationships with Carnian to Norian corals of the former Tethys. Results, when compared with previous taxa, increase knowledge of the composition of Upper Triassic corals of Japan. It shows a high degree of endemism among the Triassic corals of the SAC with some paleogeographic connection to the western Tethys and Pamir Mountains and Timor. The presence of many reef-building coral taxa and reef-type carbonate microfacies along with their paleogeographic distribution suggests a location in the southwestern Panthalassa.