Miocene-Pliocene foraminifera recovered from three subsurface sections in the Yufutsu Oil and Gas Field, southern Hokkaido, are studied in detail to infer paleoceanographic and paleobathymetric implications and to clarify the history of the basin. Foraminiferal faunas indicate a progressive increase in bathymetry from a brackish shallow marine to a bathyal condition during the Middle Miocene. The basin then came under the spell of volcanism and nearly 1000 m of basalt-basaltic andesite flows accumulated until the top of the volcano emerged out of the sea. After the cessation of volcanic activity, the basin subsided and cold bathyal conditions prevailed in which diatomaceous-siliceous sediment was accumulated during the Late Miocene. The periodic episodes of subsidence are inferred to have been related to the genesis of the Japan Sea. The basin witnessed a major hiatus during the Late Miocene-Early Pliocene. During the Late Pliocene, coarse clastic sediments accumulated in the region in a cold bathyal condition of deposition. The clastic sediment is thought to have derived from the eastern upland where the Upper Cretaceous and Paleogene sedimentary rocks were exposed. It is supposed that the hiatus in the Late Miocene-Early Pliocene is a result of an upheaval of central Hokkaido, which unstabilized the sediment and changed bottom current condition.
The Early to Middle Miocene microfauna of the region is similar to those of the Japan Sea region, whereas the Late Miocene fauna is different in abundance of agglutinated foraminifera. Such faunal differences between the study area and Japan Sea region of Honshu in the Late Miocene are mainly due to the variable distances from the proto-Tsugaru Strait that let carbonate-saturated Pacific seawater into the Japan Sea.