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10. Geology of the Upper Cretaceous Himenoura Group in the western part of Amakusa-shimojima Island, Kumamoto, Kyushu, Japan

Author

Yamaguchi, Hiroyuki, Komatsu, Toshifumi, Sato, Michitaka, Hasegawa, Shiro, and Nishi, Hiroshi

Subjects
Campanian, Amakusa-shimojima, stratigraphy, Kumamoto, Himenoura Group, Upper Cretaceous, bivalve
Abstract

The Upper Cretaceous Himenoura Group is composed of non-marine to marine fossiliferous clastic deposits, and is widely exposed on Shimojima, Amakusa Islands, western Kyushu, Japan. The Himenoura Group in this area is divided into the Hamasato, Yokohama, Ikusagaura, and Sakitsu Formations in ascending order. These formations are newly defined on the basis of the present study, and the Ikusagaura Formation is subdivided into the Kurosezaki Sandstone and Mudstone, Kotakahama Conglomerate and Sandstone, and Shimo Sandstone and Mudstone Members. The Hamasato and Yokohama Formations consist mainly of mudstone, sandstone, and gravelly sandstone containing abundant shallow marine and brackish-water molluscan fossils. The Ikusagaura and Sakitsu Formations are composed of alternating beds of sandstone and mudstone, thick sandstone, and conglomerate. The Ikusagaura Formation yields abundant brackish-water bivalves. The Sakitsu Formation is dominated by coarse sediment, and rarely contains marine bivalves. The Himenoura Group yielded over 26 bivalve species belonging to 21 genera. The brackish-water bivalves are characterized by Crassostrea, Corbula, Mesochione, and Leptosolen, and the shallow marine bivalves consist of well-preserved Glycymeris, Loxo, Apiotrigonia, Inoceramus, and Sphenoceramus. The geological age of the group is determined from age-diagnostic inoceramid species. The Hamasato Formation contains Sphenoceramus orientalis (Sokolow) and S. nagaoi (Matsumoto and Ueda), indicating lower Campanian depostion, while S. schmidti (Michael) and S. sachalinensis (Sokolow) are found together in the upper parts of the Yokohama Formation, indicating middle Campanian deposition. The Ikusagaura Formation yields the brackish-water bivalve Mesochione trigonalis, corresponding to the middle to upper Campanian (-Maastrichtian?).

Published
2008

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