Panthalassan Seamount-Associated Permian-Triassic Boundary Siliceous Rocks, Mino Terrane, Central Japan

Abstract

We describe the lithology and age of an intact section (NF 1212R) and two reference sections of Panthalassan seamount-associated Permian-Triassic boundary (PTB) siliceous rocks. The sections occupy the upper part of the Hashikadani Formation of the Mino terrane in the Mt. Funabuseyama area, central Japan. Section NF 1212R comprises a lower unit of gray chert (ca. 1.7 m thick), a middle unit of dark gray to black chert (ca. 0.8 m) with a pyrite-rich layer at the top (ca. 0.1 m), and an upper unit of black claystone with thin, intermittent beds of black to dark gray chert (ca. 1.2 m), in ascending order. The chert of the lower and middle units is rich in radiolarian remains with minor siliceous sponge spicules. The black chert of the middle unit is carbonaceous and includes tiny pyrite grains. The black claystone consists of microcrystalline quartz and clay minerals rich in carbonaceous matter. The chert of the upper unit is also carbonaceous and rich in radiolarian remains. The lower and middle units are correlated with the Neoalbaillella optima Zone (Changhsingian). The basal part of the upper unit is referable to the Hindeodus parvus Zone (basal Griesbachian), and the major part of the upper unit is possibly correlated with the middle to upper Dienerian. We position the PTB at the sharp lithologic boundary between the upper Upper Permian chert and lower Lower Triassic black claystone. The examined PTB siliceous rocks are stratigraphically attributed to the upper part of the Hashikadani Formation, reconstructed as an oceanic rock unit characterized by Lower Permian to Lower Triassic siliceous rocks that accumulated upon the lower flank of a mid-oceanic seamount in a pelagic realm of the Panthalassa Ocean. Our results present the world's first record of deep-marine PTB siliceous rocks associated with a Panthalassan seamount.