Geological age of the Ayukawa Formation (Oshika Group) in the South Kitakami Belt, Northeast Japan based on the ammonoids

Four ammonoid species, Olcostephanus atherstoni, O. cf. stephanophorus, Busnardoites aff. campylotoxus and Neocomites? sp. are described from the uppermost part of the Futawatashi Shale Member of the Ayukawa Formation distributed in Ajishima Island, Oshika Area, South Kitakami Belt, Northeast Japan. The age of this ammonoid fauna likely corresponds to the middle part (Neocomites neocomiensiformis Zone) of the early Valanginian, Early Cretaceous. According to our taxonomic re-examination of ammonoids previously reported from the Kobitawatashi Sandstone and Shale Member, and from the lower part of the Futawatashi Shale Member, the boundary between the Kobitawatashi and Futawatashi members correlates approximately with the Berriasian/Valanginian boundary. The starting time of the intense volcanic activity in the Oshika Area, represented by the deposition of the Yamadori Formation overlying unconformably the Ayukawa Formation, is considered to be late Valanginian to early Hauterivian.