Abstract
The upper part of the Yezo Group, which is widely exposed in southern central Hokkaido, consists of the Saku, Kashima, and Hakobuchi formations, in ascending order. Although the Yezo Group has been well studied in central and northwestern Hokkaido, little is known about the stratigraphy of the group in southern central Hokkaido. To clarify the geological age and microfossil faunal assemblage of the Yezo Group in this region, we studied the geology, litho-biostratigraphy, and microfossil-biostratigraphy of the group in the southern Oyubari and Tomiuchi-Azumi areas. Based on the stratigraphic distribution of benthonic and planktonic foraminifera in the Saku and Kashima formations in the studied areas, we establish four new benthonic foraminiferal zones, as follows (in ascending order): the Haplophragmoides obesus Assemblage Zone (upper Turonian), the Silicosigmoilina futabaensis Interval Zone (upper Turonian? to upper Coniacian), the Nuttallinella florealis-Notoplanulina japonica Concurrent Zone (upper Coniacian to Santonian), and the Lenticulina spp. Interval Zone (lower Campanian). The benthonic foraminiferal assemblage indicates the sequence was deposited at middle to upper bathyal depths from the late Turonian to early Campanian.
