Stratigraphy and fossil assemblage of the Upper Cretaceous in the Teshionakagawa area, Hokkaido, northern Japan

Abstract

The Cretaceous Yezo Supergroup, divided into the Middle Yezo, Upper Yezo, and Hakobuchi Groups in ascending order, is widely exposed in the Teshionakagawa area, northern Hokkaido. The Middle Yezo Group (part) consists of the following three formations in ascending order : (1) the Sakotandake Formation composed of alternating beds of sandstone and mudstone, (2) the Sakugawa Formation composed of siltstone and sandy siltstone, and (3) the Saku Formation composed of conglomerate and alternating beds of sandstone and mudstone. The Saku Formation is subdivided into the Chirashinaigawa Sandstone Member (new name), Kyowa Mudstone Member (new name), and Sakubashi Alternating beds of sandstone and mudstone Member (new name) in ascending order. The Upper Yezo Group consists of the following three formations in ascending order : (1) the Nishichirashinai Formation mainly composed of siltstone, (2) the Omagari Formation composed of conglomerate and alternating beds of sandstone and mudstone, and (3) the Osoushinai Formation mainly composed of sandy siltstone. The Hakobuchi Group consists of the Hakobuchi Formation, being mainly composed of sandstone and conglomerate. The stage boundaries were assigned to the following horizons based on age indicative ammonoid and inoceramid species : (1) the upper part of the Sakugawa Formation (Cenomanian/Turonian), (2) adjacent to the boundary of the Saku-Nishichirashinai Formation (Turonian/Coniacian), (3) the Nishichirashinai Formation (Coniacian/Santonian), and (1) the upper part of the Omagari Formation (Santonian/Campanian) . Field observation and molluscan occurrence suggest that the Hakobuchi Group overlays conformably the Upper Yezo Group. The numbers of individuals and species of ammonoids and inoceramids from the Coniacian and the Santonian in the Teshionakagawa area are remarkably few compared with other areas. It would be related with a local massive influx of coarse-grained sediments (Omagari Formation) into the low oxygenated mud bottom at that time.