Megafossil biostratigraphy of the Upper Cretaceous Yezo Group in the northern Hobetsu area, Hokkaido, Japan

Abstract

This paper describes the lithostratigraphy and megafossil biostratigraphy of the Upper Cretaceous Yezo Group in the northern Hobetsu area, central Hokkaido, northern Japan. The Upper Cretaceous in this area is restricted to the Kashima Formation, which is characterized by dark-grey bioturbated mudstone. The N–S-trending Osawa Fault cuts the central part of the study area, separating deposits of different ages. Based on age-diagnostic ammonoids and inoceramid bivalves, the strata west of the fault range in age from Late Turonian to Coniacian, whereas those east of the fault are Santonian.
The diachronous nature of the base of the Kashima Formation (“Upper Yezo Group”) in the Oyubari area is also confirmed in the present study area. The timing of deposition of the Kashima Formation in the northern Hobetsu area is the same as, or earlier than, that in the northern and middle part of the Oyubari area.