Geology and planktonic foraminiferal biostratigraphy of Cretaceous strata distributed along the Shuparo River, Hokkaido, Japan

A continuous sequence of the lower and middle parts of the Cretaceous Yezo Group is exposed in the northern part of the Oyubari area, central Hokkaido. This group, striking in a north-south direction, consists chiefly of turbidite and interbedded hemipelagic mudstone sequences and intercalates a prominent olistostrome unit in its lower part. The lower and middle parts of the Yezo Group in the studied area comprise the following five formations in ascending order: the Tomitoi Formation (sandstone-dominant alternating beds of sandstone and mudstone), the Shuparogawa Formation (thinly alternating beds of sandstone and mudstone, including Orbitolina-Praecaprotina limestone olistolith in its middle part). The Maruyama Formation (debris flow deposits and tuffaceous sandstone), the Hikagenosawa Formation (laminated mudstone and massive mudstone), and the Takinosawa Formation (mudstone-dominant alternative beds of mudstone and sandstone). These formations yield the following planktonic foraminiferal species which are useful for interregional correlation: Globigerinelloides duboisi, Hedbergella gorbachikae, Ticinella primula, Biticinella breggiensis, Totalipora subticinensis, Rotalipora brotzeni, Rotalipora cushmani, and Helvetoglobotruncana helvetica. Based on the strataigraphic distribution of these age-diagnostic species, the Tomitoi Formation and the middle part of the Shuparogawa Formation are correlated with the late Aptian Stage, the upper part of the Shuparogawa Formation and the lower part of the Hikagenosawa Formation with the Albian Stage, the middle part of the Hikagenosawa Formation and the lowermost part of the Takinosawa Formation with the Cenomanian stage, and the lower part of the Takinosawa Formation with the Turonian Stage.