Cretaceous radiolaria from the Lower Yezo Group, central Hokkaido, Japan

The thick Cretaceous forearc basin sediments are well developed in central Hokkaido, northern Japan. This sequence is called the Yezo Supergroup, which is subdivided into four groups: the Lower Yezo, the Middle Yezo, the Upper Yezo and the Hakobuchi Group in ascending order. Biostratigraphy based on megafossils and microfossils, has been clarified for the last three groups. On the other hand, the age of the Lower Yezo Group has not been precisely known, because megafossils occur only sporadically and microfossils are relatively poorly known through the group. Recent geochemical studies indicated that an Oceanic Anoxic Event is recorded in the group. Therefore, in order to determine the age of the Lower Yezo Group, authors have clarified the biostratigraphy of the group, based on radiolarian fossils. The geological ages of the Lower Yezo Group are determined as follows. The age of the base of the Lower Yezo Group is Barremian, Early Cretaceous. The upper part of the Lower Yezo Group is between Aptian to Late Albian. This correlation is concordant with that indicated by the ammonoid and foraminiferal data. This results shows that there are no sedimentation gap between the upper part of the Sorachi Group and the Lower Yezo Group in the southern Central Hokkaido.