有孔虫化石からみた始新統幌内層および漸新統紅葉山層の地史（再検討）

Abstract

The middle to upper Eocene Poronai and overlying Oligocene Momijiyama formations in Hokkaido are the major Paleogene strata studied by many workers in Japan. Recent progress in biostratigraphy and foraminiferal paleoecology requires a thorough revision of the regional geohistory interpretations.Depositional environments of the Poronai and Momijiyama formations are discussed based of the re-interpretation of the published foraminiferal data sets. Transgression, which is recognized in the lower sequences of the Poronai Formation in the Yubari, Ashibetsu and Ikushunbetsu areas, reached a maximum water depth in the upper bathyal facies. In addition, a temporal regression, which occurs in the middle part of the Poronai Formation in the Yubari area, is related to the eustatic sea level drop in the beginning of Late Eocene. Conversely, shallower water depths are inferred in the Yufutsu-Umaoi and Hobetsu districts. Based on these paleobathymetric interpretations and the distributions of the mega-fossil zones, the marine transgression during the deposition of the Poronai Formation was from northeast to southwest.Stratigraphic relationship between the Poronai Formation and the underlying Ishikari Group, which has been controversial, is demonstrated to be unconformable mainly due to the existence of transgressive phase at the basal part of the Poronai Formation.