Upper Triassic reef-limestone blocks of southwestern Japan: New data from a Panthalassan seamount

摘要

Norian-Rhaetian reef-limestone outcrops belonging to the Sambosan Accretionary Complex located near and within Inaba Cave, western Shikoku Island (Japan), are described in detail for the first time. This patch-reef complex is inferred to have formed within an atoll-type carbonate system accumulated over a mid-oceanic seamount surrounded by deep-water radiolarian cherts in the Panthalassic Ocean during the Late Triassic. Because most Upper Triassic reef studies are from the former Tethys Ocean, counterparts from the Panthalassic Ocean, such as this study, are fundamental in resolving sedimentological, palaeontological and palaeobiogeographical issues related to Late Triassic reefs. The microfacies, palaeontology and palaeoecology of the Inaba reef limestones were investigated. The reef-boundstone facies is characterised by abundant coralline sponges that, in association with microbial crusts, constitute the main framebuilders. Some phaceloid and/or dendroid corals occur, but these groups are poorly represented, as are algae. Microproblematica and foraminifers exhibit rich associations, acting as secondary reef builders and/or reef dwellers. The surrounding setting comprises biodetrital sponge-coral rudstone and well-preserved megalodont rudstone-floatstone. The sedimentary contact between reef and lagoon facies is observed for the first time within Inaba Cave. Important similarities with the coeval Upper Triassic reefs of the southern classic Peri-Tethys area and especially with the Omani seamounts are recognised, suggesting a more southern-Hemisphere origin for Upper Triassic Japanese reefs than predicted by previous reef studies.