The mid-Cretaceous Goshoura Group, non-marine to shallow marine fossiliferous clastic deposits, is typically exposed on Goshoura Island, Amakusa, western Kyushu, Japan. Based on precise sedimentological analyses, the group is subdivided into the following three formations, the Eboshi, Enokuchi and Karakizaki Formations in ascending order. These formations are herein redefined, and the Eboshi and Enokuchi Formations are newly subdivided into members, i.e., the Eboshi Formation is composed of the Tanoshiri, Hobashiraiwa and Arakuchizaki Members, and the Enokuchi Formation consists of Hokahira and Gannohana Members. Geological age of the group is determined by age-diagnostic ammonite species, e.g., the Late Albian Mortoniceras cf. rostratum from the Hokahira Member, and the Early Cenomanian Graysonites adkinsi from the overlying Gannohana Member.
Fifty-eight bivalve species belonging to 42 genera are identified from the group. These bivalve fossils are grouped into 7 fossil assemblages whose occurrences are closely linked with depositional facies. Several species occur as in life position in situ. Diversified Pterotrigonia are abundant in marine bivalve assemblages. Brackish-water bivalves such as Tetoria shishijimensis, Pulsidis higoensis and Crassostrea kawauchidensis repeatedly occur from tidal flat deposits at variable stratigraphic levels. The faunal diversity of the brackish-water assemblage increases from the estuarine deposits of the Eboshi Formation to the tidal-flat deposits on the shoreface of the Karakizaki Formation. These results in the Goshoura Group provide a new paleoecological scheme of the mid-Cretaceous bivalve fauna in the Far East Realm.