Stratigraphy and detailed age of the Upper Cretaceous Himenoura Group in the eastern part of Amakusa-Kamishima Island, Kumamoto, Japan

Abstract

The Upper Cretaceous Himenoura Group is exposed in the eastern part of the Amakusa-Kamishima Island, Kumamoto, Japan, where it is divided into the Hinoshima and Amura Formations, in ascending order. Inoceramids, ammonoids, and microfossils (e.g., radiolarians and foraminifers) are abundant in this area. The Hinoshima Formation yields Dicarinella canaliculata and abundant Marginotruncana pseudolinneiana (foraminifers), Inoceramus amakusensis (inoceramid), and Texanites kawasakii (ammonoid), indicating a Santonian age. The Amura Formation yields a Campanian foraminifera assemblage composed mainly of Grobotruncana linneiana and G. cf. arca; these species are also abundant in the upper part of the Amura Formation. Therefore, the Santonian—Campanian boundary is probably situated in the lower part of the Amura Formation. In the Hinoshima and Amura Formations, the radiolarian assemblage is characterized by Amphipyndax stocki, Dictyomitra formosa, and D. koslovae, indicating the Santonian to Middle Campanian.