[B-PT05] Biotic History

[BPT05-04] Tephrochronological study of the Yezo Group (uppermost Albiane-basal Campanian) in Hokkaido, Japan

*西 弘嗣¹、高嶋 礼詩¹、林 圭一²、桑原 里²、折橋 裕二³、山中 寿朗⁴ (1.東北大学学術資源研究公開センター 東北大学総合学術博物館、2.北海道立総合研究機構地質調査所、3.弘前大学理工学部地球環境防災学科、4.東京海洋大学)

キーワード：火山灰層序学、白亜系、対比

The Yezo Group consists of siliciclastic marine sequence that deposited in the forearc region of the active Asian continental margin in the northwest Pacific during the Cretaceous interval. This group is the most important for the Cretaceous stratigraphy in Japan because the detailed geological ages of the group have been determined by numerous bio- and chemo-stratigraphic studies that is associated with many radiometric dating of tuff beds. Here we report the results of tephrostratigraphy in the middle to upper parts of the Yezo Group (the uppermost Albian -basal Campanian interval) based on the trace-element composition of apatite contained in tuffs, and improve the regional stratigraphic correlation of the Cretaceous sequences in the northwest Pacific region.

Our results demonstrate that the geochemical analysis of apatite grains are very useful for correlation of individual tuff beds in the Yezo Group. In particular, three widely distributed tuff beds are recognized, and they are intercalated in the lower Cenomanian, Turonian/Coniacian boundary and the Santonian/Campanian boundary. These tuff beds give the suggestion for local biostratigraphic problem where the datum plane of age-diagnostic fossils is sometimes diachronous across areas, which is attributed to the sporadic occurrences of fossils and/or different depositional settings.