Radiolarians of the Stylocapsa (?) spiralis Zone (uppermost Middle to lower Upper Jurassic) extracted from a borehole core sample in the Chichibu Composite Belt, Hamakita City, Shizuoka Prefecture, Southwest Japan

Age-determinable radiolarian assemblage was extracted from a borehole core in Hamakita City, east of Lake Hamana-ko, Shizuoka Prefecture, Southwest Japan. Rocks of the borehole core are derived from a melange belonging to the accretionary complex of the Chichibu Composite Belt. The core consists of blocks of mafic volcanic rocks, limestone, chert, sandstone and so on in a scaly mudstone matrix. The radiolaria-bearing sample is the alternation of thin beds of sandstone and siltstone, which occurs as a block in a scaly mudstone matrix. Radiolarian assemblage from the sample belongs to the Stylocapsa (?) spiralis Zone, which indicates latest Middle to early Late Jurassic age (latest Callovian to Oxfordian) according to the radiolarian zonation in Japan and the western Pacific of Matsuoka (1995). On the basis of the radiolarian age of the alternation, we conclude that the formative age of the melange is the latest Middle to early Late Jurassic or later.