Revision of the stratigraphy of the Toyora and Toyonishi Groups in the Ouchi-Kikugawa area, Yamaguchi Prefecture, west Japan

Abstract

The stratigraphy of the Toyora（Lower to Middle Jurassic）and Toyo- nishi（Upper Jurassic to Lower Cretaceous）Groups in the Ouchi-Kiku- gawa area, south of the Tabe Fault, in the western part of Yama- guchi Prefecture is revised. The revised succession is, in ascending order, the Higashinagano, Nishinakayama and Utano Formations in the Toyora Group, the Kiyosue and Yoshimo Formation in the Toyonishi Group. The Kiyosue Formation is further subdivided into the Nakao Siltstone and Nanami Sandstone Members. The Utano Formation has been thought to be marine in the type Toyora area, while non-marine in the studied area. However, marine facies, characteristic in the Utano Formation, is ascertained in the formation hitherto classified as the “Nishinakayama Formation”in the studied Ouchi-Kikugawa area. The deposits referred to the non-marine sequence of the“Utano Forma- tion”by previous authors are re-classified into the Nakao Siltstone Member of the Kiyosue Formation. Newly established stratigraphy suggests that the age of the so-called “Utano flora” , reported from the Kiyosue Formation as well as the “Kiyosue flora” , of the studied area is younger than Bathonian, con- trarily to the previous age assignment older than Bathonian. The rela- tionship of these two florae should be explored in the future.