Towards an Integrated Ammonoid-, Inoceramid-and Foraminiferal Biostratigraphy of the Cenomanian and Turonian (Cretaceous) in Hokkaido

Abstract

A zonal scheme of the Cenomanian and Turonian stages is established on the basis of bio-stratigraphic sequences of ammonids and inoceramids along the provincial standard and supplementary sections in the Ikushumbetsu, Oyubari and some other areas in Hokkaido. Owing to the recent advances in palaeontology, the biostratigraphic zones in Hokkaido are much refined and fairly well correlated with the current standard zones (Table 1) in Europe and North America.
While the stratigraphic sections of shallow-sea sediments are observable in the Ikushumbetsu and contiguous areas, those of off-shore and muddy sediments predominate in the Oyubari and certain other areas. The foraminiferal samples have been collected mainly in the latter areas, where some of the ammonoid and inoceramid zonal indices may also occur. Thus the zonal scheme by means of planktonic and benthic foraminifera can be directly tied up with that of ammonoids and inoceramids. The results obtained are summarized in Table 2, which evidently shows much improvement on previous schemes. There remain, however, several unsettled problems which should be worked out further, especially as regards the stage boundaries.