Latest Olenekian (Early Triassic) Ammonoids from the Uppermost Part of the Osawa Formation (Inai Group) in the South Kitakami Belt, Northeast Japan

Abstract

The Osawa Formation of the South Kitakami Belt is correlated with the upper Olenekian (Lower Triassic) based on its rich ammonoid fauna. Although two ammonoid zones, the Subcolumbites Zone below and Arnautoceltites Zone above, were established in the formation, its uppermost part has been almost barren of ammonoids. An “Olenekian–Anisian mixed” ammonoid fauna found from the uppermost Osawa Formation at Asadanuki, comprising eight genera, is the only one representing the uppermost part. It is unique in the generic composition and new ammonoid zone, the Eodanubites Zone, is proposed in the uppermost part of the Osawa Formation. The fauna consists of typical Olenekian genera such as Pseudosageceras, Ceccaisculitoides, Procarnites and Eodanubites, the Olenekian–Anisian genus Leiophyllites and Keyserlingitidae, and “Anisian” genera Japonites and Procladiscites. The last two genera, however, actually range down into the uppermost Olenekian, and therefore, based on the ammonoids, the Eodanubites Zone is considered to be latest Olenekian in age. This fauna is similar to the uppermost Olenekian Ziyun fauna in Guizhou, South China, and the Eodanubites Zone is correlated with the uppermost Olenekian Deslicairites Beds of Romania and the Neopopanoceras haugi Zone of Nevada.