有孔虫化石からみた北海道西部の中部始新統～下部漸新統の地史(再検討)

抄録

Geohistorical implications of several middle Eocene-lower Oligocene formations distributed in the western Hokkaido are restudied mainly from published data of foraminiferal assemblages. These formations are correlative to the Poronai and Momijiyama formations.
Foraminiferal assemblages from the Utsunai and Magaribuchi formations in northernmost part of Hokkaido indicate paleobathymetric change from the upper bathyal to sublittoral zones. Those from the lower part of the Sankebetsu Formation in Chikubetsu-Haboro area indicate the shallower paleobathymetric zones than the upper bathyal zone, and those from the Shimokine and overlying Tappu formations indicate transgression from none-marine condition to the upper bathyal zone.
These paleobathymetric changes and the geologic ages determined by calcareous nannofossil biostratigraphy show that the transgression which generated the Poronai Sea occurred from east to west and reached maximum depths in the late Eocene (CP 15b). Previous interpretation that paleo-topography of the Poronai Sea as an embayment can not be accepted, because there is no evidence that shallower bathymetric zones existed in the eastern part of this sea.