Stratigaraphic age of the ejecta deposit from the Sakahogi section in the Mino Terrane, Central Japan

Onoue, Tetsuji*; Sato, Honami*; Nakamura, Tomoki*; Hatsukawa, Yuichi; Osawa, Takahito  ; Toh, Yosuke   ; Koizumi, Mitsuo 

We report that an Upper Triassic deep-sea deposit (bedded chert) in Japan, deposited within the Paleo-Pacific (Panthalassa) pelagic basin, contains microspherules, Ni-rich spinels, and iridium anomaly that are characteristic of distal impact ejecta. These indicators of extraterrestrial impact were discovered from a claystone layer in an Upper Triassic bedded chert succession of the Sakahogi section, Inuyama area, central Japan. The age of the impact-related claystone is constrained by the presence of radiolarians and conodonts. We recognized 18 radiolarian species across the claystone. The claystone layer lies between pelagic chert layers of the radiolarian zones Capnodoce-Trialatus and Trialatus robustus-Lysemelas olbia. The last occurrence of Trialatusrobustus, which indicates the Lacian (Early Norian), is a good indicator of the ejecta deposit. Capnodoce serisa and Xiphosphaerafistulata and the co-occurrence of these taxa clearly indicate the late Lacian to early Alaunian (Middle Norian) age for the examined part of the Sakahogi section. Small numbers of conodonts from the claystone include Mockina medionorica, indicating an early Alaunian age (Mockina medionorica Zone). Therefore, the Lacian-Alaunian boundary in the Sakahogi section can belocated around the Capnodoce-Trialatus and Trialatus robustus-Lysemelas olbia Zone boundary.