Middle jurassic radiolarians from calcareous nodules within silty sandstone float boulder derived from the Kaizara Formation of the Tetori group in central Japan

This is the first report on radiolarians present in the Kaizara Formation of the Kuzuryu Subgroup, Tetori Group, located in the Shimoyama-Kaizara area, in central Japan. Previous works based on ammonoid biostratigraphy date the Kaizara Formation in the late Bathonian to early Callovian. The radiolarian fossils were extracted from two calcareous nodules included in silty sandstone that was collected as a float boulder on a river bed draining into the Taniyama River, a tributary of the Kuzuryu River. Although its precise stratigraphic horizon is unknown, the location of collection and characteristic features, namely the presence of calcareous nodules, make it a probable derivate of the Kaizara Formation. Several other microfossils and bioclasts were also retrieved from the same radiolarian-bearing nodules: benthic foraminifers, sponge spicules, prodissoconchs of pelecypods, and echinoderm fragments. According to the Unitary Associations Zones (UAZ.) of Baumgartner et al. (1995), the presence of Dictyomitrella ? kamoensis, Striatojaponocapsa conexa, Stichocapsa naradaniensis and Williriedellum carpathicum places the radiolarian age between the late Bathonian and early Callovian (Middle Jurassic) of UAZ. 7. In addition, the radiolarian zonation for Japan and the western Pacific proposed by Matsuoka (1995a) suggests the age to fall in the interval ranging from the Callovian to Oxfordian (Middle-Late Jurassic) based on the co-occurrence of Stichocapsa naradaniensis and Striatojaponocapsa conexa. According to the UAZ. of Baumgartner et al. (1995), the radiolarian age of the Kaizara Formation is shown to range from the late Bathonian to early Callovian, based on the consistency between the radiolarian age and the ammonoid age.