Integrated megafossil-foraminiferal biostratigraphy of the Santonian to lower Campanian (Upper Cretaceous) succession in northwestern Hokkaido, Japan

The Upper Yezo Group (Coniacian to lower Campanian) in the area of Haboro to Kotanbetsu in northwestern Hokkaido, Japan, yields abundant mega- and microfossils at many levels. In this paper, an integrated analysis of the ammonites, inoceramids, and Foraminifera has been carried out to determine the detailed biostratigraphy of the Santonian to the lower Campanian succession of northern Pacific, with reference to the proposals of the Subcommission on Cretaceous Stratigraphy (SCS) of the International Union of Geological Sciences in 1983 and 1995. The Santonian/Campanian stage boundary in the Japanese stratigraphic scale is defined on the basis of the first occurrence ofMenabites mazenotiorSubmortonicerascf.condamyi. This horizon is located very close to the boundary between theInoceramus amakusensisandI.(Platyceramus)japonicusZones, which is also situated between the last occurrence ofMarginotruncana pseudolinneianaand the first occurrence of the typical form ofGlobotruncana arca. On the other hand, the first occurrence ofAspidolithus parcus parcusis above that ofG. arca, and both horizons are located in theI.(P.)japonicusZone in south-central Hokkaido. A K-Ar age of 82.2±0.6Ma, reported by Shibata & Uchiumi (1995), is situated in the lower part of theI.(P.)japonicusZone, which agrees well with the Santonian/Campanian boundary age of 83.5Ma. Unfortunately, the marker species selected in 1995 by the SCS to definine the base of the Campanian Stage, such asPlacenticeras bidorsatum,Gonioteuthis granulataquadrataandMarsupites testudinarius, have not yet been found in the Cretaceous deposits of Japan.