Abstract
In subsurface sections of the Yufutsu-Umaoi district, Hokkaido, northern Japan, three Eocene benthic foraminiferal assemblage zones were defined in the Ishikari Group and the overlying Poronai Formation. They are in ascending order : Evolutinella subamakusaensis-Haplophragmoides crassiformis Assemblage Zone, Globocassidulina globosa-Cribroelphidium sorachiense Assemblage Zone, and Bulimina schwageri-Angulogerina hannai Assemblage Zone. Assemblages characterizing each zone indicate the littoral to the inner sublittoral, middle sublittoral, and outer sublittoral paleobathymetric zones, respectively. A foraminiferal fauna in the upper bathyal zone was also identified based on reinterpretation of previous studies. It is composed of calcareous species such as Gyroidina yokoyamai and Plectofrondicularia packardii. Abundant occurrences of agglutinated foraminifera in shallower paleoenvironment suggest brackish and related stratified-water paleoenvironments caused by freshwater input into an embayment called the "Poronai Sea". Such stratified conditions in coastal shallow marine areas may have formed oxygen-depleted zones as suggested in the previous study. These data and their paleoenvironmental implications are expected to furnish a basis for further consideration on geohistory of the Paleogene formations and also on the Eocene foraminiferal fauna of the northwestern Pacific.
