Abstract
This study reports on a newly discovered radiolarian assemblage from the Sakamoto Formation in the Shiburi belt, which is an element of the Kurosegawa belt, in the Sakamoto area, Kumamoto Prefecture, western Kyushu. The assemblage, which contains Bagotum kimbroughi Whalen and Carter, B. modestum Pessagno and Whalen, B. maudense Pessagno and Whalen, Broctus ruesti Yeh sensu Goric̆an et al. (2006), B. selwynensis Pessagno and Whalen, Parahsuum simplum Yao, P. ovale Hori and Yao, P. edenshawi (Carter), P. izeense (Pessagno and Whalen), Droltus sanignacioensis Whalen and Carter, Zhamoidellum yehae Dumitrica, and Stichocapsa biconica Matsuoka, indicates an Early Jurassic (Pliensbachian) age and provides the first age dating of the Sakamoto Formation in the Shiburi belt.
Over the last 10 years, Lower Jurassic formations have been discovered in four areas of the Kurosegawa belt in western Kyushu. The radiolarian assemblage plays an important role in undertaking correlations among these formations. For example, the Sakamoto Formation in the Shiburi belt correlates exactly with the Hirasawatsudani Formation in the Sakamoto belt, lithostratigraphically and chronologically. In addition, according to chronologic relationships in western Kyushu, it is suggested that deposition of the Lower Jurassic formations in the Kurosegawa belt was synchronous with the Early Jurassic development of accretionary complexes in the Outer Zone of Southwest Japan.
