Cretaceous and Paleocene Foraminifera from northern Hokkaido, Japan

This paper provides the descriptions of foraminiferal assemblages from the Nakatombetsu area, situated in the northern part of the meridional zone of Hokkaido, Japan, giving their systematics and stratigraphic distribution. On the basis of five events in the intra-regional stratigraphic ranges of the selected benthic foraminiferal species, a sequence of Campanian to Paleocene strata is biostratigraphically divided into three benthic foraminiferal zones, one of which is subdivided into four subzones. These three zones are named, in upward sequence, as follows : The Haplophragmoides walteri-Lenticulina obirashibensis Assemblage-zone (subdivided into the Pleurostomella subnodosa-Dorothia crassa Assemblage-subzone, Pseudobaggina hokkaidoana-Dentalina cylindroides Assemblage-subzone, Haplophragmoides walteri-Lenticulina obirashibensis-Dentalina cylindroides Assemblage-subzone, and Haplophragmoides-Silicosigmoilina-Bathysiphon Assemblage-subzone), Reophax clavulina-Cribrostomoides sp. A Assemblage-zone, and Cyclammina sp. A-Praebulimina angulata Assemblage-zone. The discovery of nine planktonic foraminiferal species in the Hakobuchi Group, which has been considered to be the Upper Cretaceous System, has revealed for the first time the existence of the Paleocene Series in this region. A marked change in the benthic foraminiferal components is recognized below and above the Reophax clavulina-Cribrostomoides sp. A Assemblage-zone, which is correlated with the upper Maastrichtian. The flourishing of agglutinated foraminifera, equated to the "Flysch type agglutinated foraminifera" (Gradstein and Berggren, 1981), in the lower part of the Heitarozawa Formation indicates a disparate depositional environment. Additionally, the name "Silicosigmoilina fauna" is proposed for the Upper Cretaceous to Lower Tertiary benthic foraminiferal fauna in the North Pacific region, in contrast with the "Rzehakina fauna" in Europe. The Silicosigmoilina fauna indicates the extent of the Paleo-North Pacific Ocean. A total of 126 species and two subspecies belonging to 62 genera are discriminated in the present study. Of these, one genus, 14 species and one subspecies are proposed as new to science