Abstract
We report a new discovery of Lower Triassic (Spathian) chert and siliceous claystone at two localities within the Mino terrane in the Mt. Funabuseyama area, central Japan. The two localities were thought to be underlain by the Hashikadani Formation, which is characterized by upper Lower to uppermost Permian siliceous rocks, representing deep-water facies that formed on the lower slope of an oceanic seamount and the surrounding ocean floor in the Panthalassa Ocean.
The Lower Triassic siliceous rocks occur close to upper Middle to upper Upper Permian chert and siliceous claystone. The neighboring Permian and Triassic siliceous rocks show identical facing directions. The close proximity of the exposures, lithologic affinities, and identical facing indicate that the Lower Triassic siliceous rocks rest upon the Upper Permian Hashikadani siliceous rocks and were primarily accumulated as part of a Permo-Triassic siliceous rock succession.
The inferred Permo-Triassic siliceous rock succession comprises (1) upper Middle to upper Upper Permian grayish chert with minor black chert in the upper part (ca. 20 m thick; Follicucullus scholasticus-F. ventricosus Zone to Neoalbaillella ornithoformis Zone) ; (2) uppermost Permian gray chert, including alternating black chert, greenish gray siliceous claystone, and black claystone (ca. 6-7 m thick; Neoalbaillella optima Zone) ; and (3) upper Lower Triassic rocks characterized by an upward increase in the proportion of grayish chert along with gray siliceous claystone and black claystone (thickness unknown; Spathian).
These new findings indicate that the stratigraphy and age of the Hashikadani Formation should be revised. Our data suggest that the accumulation of Permian siliceous sediments around a Panthalassic mid-oceanic seamount persisted until at least the late Early Triassic, across the Permo-Triassic boundary. The revised Hashikadani Formation has potential as an archive of long-term environmental changes in a mid-oceanic realm of the Panthalassa Ocean.
