Reconstruction of the lost Triassic stratigraphy of shallow marine formations in the Mino terrane : Link between deep and shallow marine formations in accretionary complex

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Although pelagic deep-sea radiolarian chert of Permian to Early Jurassic age and pelagic shallow-marine limestone of Permian age occur as accreted oceanic rocks in the Mino terrane, no shallow-marine limestone of Triassic to Jurassic age are known to occur in the terrane. This might be resulted from the subsidence of the oceanic islands and plateaus on which the Permian shallow-marine sediments were accumulated. On the other hand, Permian limestones in the Gujo-hachiman and Akasaka areas have cave deposits with mixed-Triassic conodont fauna, and deep-sea cherts contain clastic grains derived from shallower areas. These rocks indicate existence of the Triassic shallow-marine limestone. We examined limestone interbedded with chert in the Takayama, Gujo-hachiman, Kamiaso, Miyama, Neo and Ibuki areas. They are (1)clastic limestone interbedded with Permian chert, (2)clastic limestone interbedded with chert of unknown age, and (3)pelagic deep-sea limestone interbedded with Triassic chert; we could not find clastic limestone interbedded with Triassic chert, which indicate the existence of Triassic shallow-marine limestone. White chert interbedded with Triassic chert might have been originally clastic limestone, but our microscope observation and age determination provide no evidence. We made similar preliminary examinations in the Samarka terrane, Fareast Russia, which is the northern ]extension of the Mino terrane. Triassic limestone at Taho and Kamura in the Chichibu terrane have horizons indicating hiatus and chemical disolution ; similar fenomena might occur in limestone of the Mino terrane.