[M-IS25_28AM1] 遠洋域の進化

[MIS25-03] 三畳紀後期カーニアン～ノーリアン前期におけるアンモナイト・コノドント・放散虫化石層序

*山下 大輔¹、安田 知佳²、佐藤 峰南³、尾上 哲治⁴ (1.鹿児島大学大学院理工学研究科、2.国際石油開発、3.九州大学大学院理学府、4.熊本大学大学院自然科学研究科)

キーワード：三畳紀後期, カーニアン?ノーリアン前期, アンモナイト・コノドント・放散虫化石層序, 三宝山帯, 美濃帯, パンサラサ海

The chronology for the Triassic pelagic deposits in the Panthalassa Ocean is based on the radiolarian zonation, which is well studied in the Middle and Upper Triassic bedded chert successions in the Japanese accretionary complex. Although accurate calibration for the chronostratigraphic stages and substages are established basically by means of ammonites and conodonts, most of the Japanese radiolarian zones were calibrated through correlation with zonal schemes in other regions, and have not been calibrated with ammonoid and conodont biostratigraphy. Here we present the results of Late Triassic (Carnian-early Norian) conodont biostratigraphy from the two pelagic sections in the Jurassic accretionary complex of southwest Japan. Samples for this study were collected from the Sakahogi section of a bedded chert sequence in central Japan and the Nakijin Formation of a pelagic limestone sequence in the northern tip of the Okinawa Island. We found 56 platform conodonts from 36 samples in the Sakahogi section, where the radiolarian biostratigraphy have previously been investigated. The biostratigraphy of the Carnian-Norian sequence of the Nakijin Formation is based primarily on ammonites, since the rare occurrence of conodonts minimizes the stratigraphic potential of these groups. However, our study revealed that the clastic limestones intercalated within the Nakijin Formation contain rich conodonts assemblages. Based on detailed study of the conodont biostratigraphy from the interval of the Carnian and the early Norian in the Sakahogi section and the Nakijin Formation, three conodont zones are recognized in ascending order as follows: lower Carnian Paragondolella praelindae - Metapolygnathus polygnathiformis zone, upper Carnian Metapolygnathus lindae - Metapolygnathus primitius zone, and lower Norian Epigondolella quadrata zone. This result is consistent with the presence of the lower to upper Carnian ammonites assemblages in the Nakijin Formation.