This paper presents the litho- and biostratigraphy of the Permian-Triassic boundary (PTB) siliceous rocks that accumulated on the lower flank of a seamount in a mid-oceanic realm of the Panthalassa Ocean. The study section correlates with the upper part of the Lower Permian to Lower Triassic Hashikadani Formation of the Mino terrane in the Mt. Funabuseyama area, central Japan.
The lithologic succession comprises a lower unit (ca. 6.2 m thick) dominated by gray chert with small amounts of black chert and black claystone, middle unit (ca. 0.7 m) of black chert with subordinate black claystone and gray chert including pyrite nodules near the top, and upper unit (ca. 0.9 m) characterized by black claystone with thin black chert beds. The lithologic change from the middle unit to the upper unit is sharp and abrupt.
The lower and middle units correlate to the upper Wuchiapingian to Changhsingian (Neoalbaillella ornithoformis to N. optima zones). The upper unit includes the lower Induan, as the black chert bed in its middle part yields Hindeodus parvus. These beds also yield Triassospongocyrtis? sp., a primitive Mesozoic-type Nassellaria, with several types of the sphaeroid spumellarians.
The Permian radiolarians exhibit a marked loss in diversity across the lithologic boundary between the middle and upper units. We hypothesize that this lithologic boundary marks the main extinction horizon of the Permian radiolarians. Also a profound faunal turnover of radiolarians occurred at this level.
On the basis of the Global Boundary Stratotype Section and Point (GSSP) designation, it is reasonable to position the PTB at the bottom of the H. parvus-yielding chert bed in the upper unit. Nevertheless placing the PTB at this level remains tentative, because age-diagnostic fossils are absent from the lower part of the upper unit. Alternatively, with the emphasis upon correlation with the recently reported PTB section of the Hashikadani Formation, the PTB of the study section is preferably positioned at the sharp lithologic boundary between the middle and upper units. Given this alternative, the PTB corresponds to the main extinction event of Permian radiolarians, which, in turn, is thought to correspond to significant environmental changes.