Early Triassic Conodonts from the Tahogawa Member of the Taho Formation, Ehime Prefecture, Southwest Japan

Abstract

The Triassic Taho Formation crops out in Taho, Shirokawa Town, Seiyo City, Ehime Prefecture, Southwest Japan. The formation is newly divided into the Tahogawa and Sakuragatouge members. The Tahogawa Member consists mainly of partly dolomitized light-, dark-, brownish-gray and black bedded limestones, and yields abundant Early to Middle Triassic conodonts, ammonoids, gastropods, bivalves, brachiopods and fish remains. The Sakuragatouge Member is dominated by brownish gray lenticular conglomeratic limestone and massive gray and white limestones that contain Middle to Late Triassic conodonts, gastropods, bivalves, echinoids and fish remains. In the Tahogawa Member, conodont assemblages are composed of 13 genera and 45 species, including three new species: Neospathodus arcus Maekawa sp. nov., Novispathodus shirokawai Maekawa sp. nov., and Nv. tahoensis Maekawa sp. nov. The member is divided into five conodont zones in ascending order: the Ns. dieneri Zone, the Ns. cristagalli Zone, the Nv. ex gr. waageni Zone, the Nv. pingdingshanensis Zone and the Nv. brevissimus Zone. The Nv. ex gr. waageni Zone contains three conodont subzones (the Eurygnathodus costatus Subzone, the Guangxidella bransoni Subzone and the Scythogondolella milleri Subzone), and the Nv. brevissimus Zone includes the Icriospathodus collinsoni Subzone. The conodont biostratigraphy of the member indicates a late Dienerian to early Spathian age, and correlates with that of Tethyan and Panthalassean sections. The Induan-Olenekian boundary is indicated by the first occurrence of Nv. ex gr. waageni. The Smithian-Spathian boundary is immediately below or intercalated within the Nv. pingdingshanensis Zone.