Oligocene shallow marine foraminifera from the subsurface of southern Hokkaido

Abstract

Foraminiferal faunas from the Upper Oligocene Minaminaganuma Formation recovered from boreholes of the Yufutsu Oil and Gas Field and MITI Umaoi located in southern Hokkaido are characterized exclusively by the large elphidiids Elphidium mabutii and Cribroelphidium ombetsuense. They indicate cold water temperatures and an inner to middle sublittoral paleobathymetry. The total organic carbon (TOC) content and hydrogen index (HI) values obtained by CHN Corder and Rock-Eval pyrolysis indicate the significant deposition of organic carbon of phytoplankton origin in the inner sublittoral condition. Elphidium mabutii and C. ombetsuense show ubiquitous distribution suggesting high tolerance of these two species against environmental stresses such as fluctuations in primary productivity (nutritional condition). This newly recognized shallow marine fauna is located between the Eocene-Oligocene Poronai-Momijiyama fauna (agglutinated foraminifera dominant) and the Miocene Takinoue fauna (Ammonia dominant). A comparison between the thickness of strata and the range of paleobathymetric changes in the study area suggests that an equilibrium between basin subsidence and sediment accumulation existed to keep this area under shallow marine conditions during a certain period in the Late Oligocene, which preceded the rifting of the Japan Sea during the Early Miocene.