日本の白亜系の有孔虫化石帯区分と国際対比について  [in Japanese] Cretaceous foraminiferal zonations of Japan and their international correlation  [in Japanese]

Dual biostratigraphic zonal schemes based on both planktonic and benthonic foraminifers had been established for the Middle to Upper Cretaceous sequence developed in the meridional zone of Hokkaido by MAIYA and TAKAYANAGI (1977). After that, many authors have been carrying out the investigation of Cretaceous foraminiferal biostratigraphy in the eastern part of Hokkaido and the Kanto region (MATSUMOTO et al., 1981; KAIHO, 1984; OBATA et al. 1982; YAMADA, 1984). Consequently, in addition to the foraminiferal zones by MAIYA and TAKAYANAGI (1977), three planktonic and four benethonic zones are newly proposed for the Cretaceous sequences of Japan. They are as follows- the Globigerinelloides ferreolensis-Globigerinelloides macrocameratus Zone (upper Aptian), Ticinella zone (lower Albian), Rugoglobigerina Zone (Maastrichtian) in respect to planktonic zones and the Verneulinoides plexus neocomiensis-Lenticulina heiermani Zone (Barremian), Lenticulina nodosa-Falsoguttulina obatai Zone - (uppermost Barremian to lower Aptian), Gyroidinoides infracretaceous-Planulina suturalis Zone (upper Aptian) and Spiroplectammina grzybowski-Silicosigmoilina futabaensis Zone (Maastrichtian) in terms of benthonic zones. Thus, on the basis of the first and last occurrence and abundance of a given species, a sequence of lower to uppermost Cretaceous (Barremian to Maastrichtian) marine strata of Japan is biostratigraphically subdivided into ten planktonic zones and ten benthonic ones. The correlation of these zones with those of mega fossils (inocerami and ammonites) shows a bio- and chrono-stratigraphically good coincidence. It is confirmed that the zonal schemes based on both planktonic and benthonic foraminifers are very useful not only for regional correlation of Cretaceous sidiments with in Japan, but also for inter-regional correlation between the Cretaceous sequences exposed at distant parts of the European type region and Japan. In addition, the vertical changes of the calcareous foraminiferal dominance and the composition of the faunal assemblages themselves suggest that the Cretaceous depositional environments were under a closed and stagnant condition during the Barremian and Cenomanian through the Turonian stages, and under an aerobic condition in the face of open sea during the Aptian through Albian and Coniacian through Santonian stages.