Stratigraphy of the late Mesozoic Tetori Group in the Hakusan Region, central Japan : an overview

Abstract

The late Mesozoic Tetori Group is distributed in central Japan. Although the stratigraphy of this Group is thought to be quite important to study the fossil assemblage including various vertebrate remains collected from the Group, it is rather complicated. We review in the present paper the geological studies on the Tetori Group around Mt. Hakusan to give accurate information especially on the stratigraphy of this Group. The Tetori Group consists of the Kuzuryu, Itoshiro and Akaiwa subgroups in ascending order. The formations compose these subgroups in the reviewed districts are as follows. Kuzuryu River district, Fukui Prefecture Kuzuryu Subgroup. Shimoyama, Oidani, Tochimochiyama, Kaizara and Yambarazaka formations. Itoshiro Subgroup. Yambara, Ashidani, Obuchi and Itsuki formations. Akaiwa Subgroup. Akaiwa and Kitadani formations. Tedori River district, Ishikawa Prefecture Itoshiro Subgroup. Gomijima and Kuwajima formations. Akaiwa Subgroup. Akaiwa and Myodani formations. Shokawa district, Gifu Prefecture Kuzuryu Subgroup. Ushimaru, Akahoke and Mitarai formations. Itoshiro Subgroup. Otaniyama, Okurodani and Amagodani formations. Akaiwa Subgroup. Okura and Bessandani formations. Following formations are thought to be almost the same horizon: the upper part of the Kaizara Formation and the lower part of the Mitarai Formation; the Kuwajima, Itsuki and Okurodani formations; the lower part of the Akaiwa Formation along the Tedori and Kuzuryu rivers and the Okura Formation. Only five formations are correlated to the geologic time units based on the index fossils: the Kaizara (Bathonian to Oxfordian), Yambarazaka (Oxfordian), Mitarai (Callovian to ?Tithonian), Kitadani and Myodani (upper Barremian to lower Aptian) formations. To summarize, the Kuzuryu, Itoshiro and Akaiwa subgroups in these districts are roughly correlated to the Middle to Upper Jurassic, Upper Jurassic to lower Lower Cretaceous and Lower Cretaceous, respectively. The formations from which vertebrate remains have been collected are: the Otaniyama, Itsuki, Kuwajima, Okurodani and Amagodani formations of the Itoshiro Subgroup, and the Akaiwa and Kitadani formations of the Akaiwa Subgroup.