Abstract
This paper describes litho- and biostratigraphy of the Upper Cretaceous in the Ashibetsu Lake area, Hokkaido, Japan. The Cretaceous deposits in this area are represented by offshore facies, and are divided into the Takinosawa Formation composed of alternating beds of sandstone and mudstone (upper part of the Middle Yezo Group), and the Kashima Formation composed of mudstone (lower part of the Upper Yezo Group). Each Formation is divided into two units respectively : unit Mh and Mi below, and unit Ua and Ub above. The Cretaceous succession ranges from the Upper Turonian to Santonian, based on age indicative ammonoids and inoceramids. The Turonian/Coniacian boundary is assigned to the middle part of the unit Ua, and the Coniacian/Santonian boundary to the uppermost part of the unit Ub. The species richness and abundance of ammonoids decrease in units Mi and Ua. It is considered that during the late Turonian-early Coniacian time, ammonoid assemblages were affected by the spread of the oxygen-depleted conditions in the bottom water.
