09:45 〜 10:00
[MIS13-04] 三畳紀-ジュラ紀境界における放散虫・コノドント化石群集の絶滅・変遷の再検討
★招待講演
キーワード：三畳紀-ジュラ紀境界、放散虫、コノドント、層状チャート、パンサラッサ海
The end-Triassic mass extinction (ETE) is one of the five largest mass extinction events in Phanerozoic Earth history. The crisis was closely linked to eruptions of the Central Atlantic Magmatic Province (CAMP), which is commonly invoked as the trigger for climatic perturbations during the last 300 kyrs of the Triassic and lead to the mass extinction. Although previous studies have noted significant extinction of major pelagic groups (e.g. radiolarians and conodonts) during the ETE, a limited number of studies have reported on integrated biostratigraphic research of radiolarians and conodonts in the Panthalassa Ocean. Here, we present a high-resolution radiolarian-conodont stratigraphic distribution of an upper Norian to lowest Hettangian bedded chert succession, including the Triassic-Jurassic boundary (TJB) interval in the Katsuyama-B section, Inuyama area, Mino Belt, central Japan. This section is inferred to be a lateral extension of the Katsuyama (section UF) section, which accumulated in a pelagic deep-sea environment in a low to middle latitudinal zone of the Panthalassa Ocean.
The Katsuyama-B section is approximately 12.6 m thick, and composed mainly of the red and purple bedded cherts. The purple bedded chert interval (ca. 1.8 m) is intercalated with in the upper part of studied section. The biostratigraphic analysis confirmed that the radiolarian species in this section characterize from the TR8B (Praemesosaturnalis pseudokahleri) Zone to the JR0B (Bipedis horiae) Zone established by Sugiyama (1997). We also recognized three conodont zones proposed by Rigo et al. (2018) in the studied section: the upper Norian Misikella hernsteini zone, the lower Rhaetian Misikella posthernsteini zone, and the upper Rhaetian Misikella ultima zone. Our biostratigraphic studies revealed that some Rhaetian radiolarian and conodont fauna occurred in the earliest Jurassic. Furthermore, our biostratigraphic analysis revealed an unusually abundant occurrence of a previously unidentified Mesosaturnalis species with a thickness 0.6 m across the TJB in the studied section. This finding suggests that this Mesosaturnalis species, which occurs highly abundant in a short interval across the Triassic–Jurassic boundary, is likely to be an excellent indicator for the TJB. Since, no study related to this radiolarian species was published from previously TJB section (e.g. Katsuyama and Kurusu) in Japan until now, the stratigraphic interval across the TJB in Japan are likely to be missing.
The Katsuyama-B section is approximately 12.6 m thick, and composed mainly of the red and purple bedded cherts. The purple bedded chert interval (ca. 1.8 m) is intercalated with in the upper part of studied section. The biostratigraphic analysis confirmed that the radiolarian species in this section characterize from the TR8B (Praemesosaturnalis pseudokahleri) Zone to the JR0B (Bipedis horiae) Zone established by Sugiyama (1997). We also recognized three conodont zones proposed by Rigo et al. (2018) in the studied section: the upper Norian Misikella hernsteini zone, the lower Rhaetian Misikella posthernsteini zone, and the upper Rhaetian Misikella ultima zone. Our biostratigraphic studies revealed that some Rhaetian radiolarian and conodont fauna occurred in the earliest Jurassic. Furthermore, our biostratigraphic analysis revealed an unusually abundant occurrence of a previously unidentified Mesosaturnalis species with a thickness 0.6 m across the TJB in the studied section. This finding suggests that this Mesosaturnalis species, which occurs highly abundant in a short interval across the Triassic–Jurassic boundary, is likely to be an excellent indicator for the TJB. Since, no study related to this radiolarian species was published from previously TJB section (e.g. Katsuyama and Kurusu) in Japan until now, the stratigraphic interval across the TJB in Japan are likely to be missing.