Fossil bivalve assemblages and depositional environments of the upper part of the Cretaceous Yezo Supergroup, Kotanbetsu-Haboro area, Hokkaido, Japan

The upper part of the Cretaceous Upper Yezo Group, which is distributed throughout the Kotanbetsu-Haboro area, Hokkaido, north Japan, contains a regressive sequence from outer shelf to shoreface sediments and is sedimentologically divided, from top to bottom, into four facies: Facies 1 (hummocky cross-stratified sandstone), Facies 2 (bioturbated fine-grained sandstone), Facies 3 (intensely bioturbated sandy mudstone), and Facies 4 (massive mudstone). The strata commonly yield bivalve fossils. With the exception of inoceramids, the bivalves are classified into three fossil assemblages: the Parvamussium-Nucula, Thetis-Nucula, and Nucula-Heterotrigonia assemblage, respectively. These assemblages are mostly autochthonous and include remarkable benthic fauna such as Propeamussiidae, Lucinacea, Nuculacea, and Tellinidae that were adapted to live under oxygen-poor conditions. In particular, the bivalves of Facies 3 inhabited favorable surroundings in terms of a rich supply of organic material and a suitable depositional environment.