Late Jurassic radiolarian fauna from the Ikenohara Formation of the Kurosegawa Belt in the Toyo - Izumi area, Kumamoto Prefecture,Kyushu, Japan.

Abstract

Abundant well-preserved radiolarians were obtained from the Kurosegawa Belt in the Toyo - Izumi Area, Kumamoto Prefecture, Kyushu. Radiolarian-bearing rocks were collected from the Upper Jurassic Ikenohara Formation, which is composed mainly of mudstone. The samples GSJ R76489 and GSJ R76490 include 330 and 329 radiolarian species, respectively. The radiolarian faunas of both samples are very similar in composition. Based on the radiolarian zonation of Matsuoka (1995a), the radiolarian fauna represents the Kilinora spiralis Zone and is assigned to Oxfordian in age. This radiolarian fauna includes greater numbers of spumellarians and quite differs from radiolarian faunas reported from the Jurassic accretionary complex of Japan. This evidence may imply that the depositional environment of the Ikenohara Formation is different from that of the Jurassic accretionary complex, and the Ikenohara Formation possibly deposited in a trench slope environment.