Abstract
The western Kii Peninsula which is situated in the Outer Zone of the Southwest Japan, is generally characterized by E-W trend zonal structure of the Sambagawa, Chichibu and Shimanto Belts from north to south. However, the zonal structure is incomplete in the central part of Kii Peninsula and rocks of the Chichibu Belt are not cropped on the land surface. In this paper, geology of the Hanazono Formation in the Koyasan area where the rocks of the Chichibu Belt are absent is described, and radiolarian fossils from the Hanazono Formation and the adjacent Yukawa and Miyama Formations are reported. The Hanazono Formation is in fault contact with the Sambagawa Metamorphic Rocks through the Aridagawa Tectonic Line to the north, and is in fault contact with the Yukawa or Miyama Formations through the Yanase Fault or the Yukawa Thrust to the south. The Hanazono Formation is divided into 5 units and they are called Hn1, Hn2, Hn3, Hn4 and Hn5 Units, respectively. Hn1 Unit is situated in the northern-most part of the Hanazono Formation, and characterized by obvious foliation as compared with Hn2 to Hn5 Units. Hn1 Unit is in fault contact with Hn2-Hn5 Units through the Kamiya Fault. The Hanazono Formation mainly consists of shale with sandstone, felsic tuff, chert, red shale, limestone and basalt, and is characterized by mixed rock which includes the blocks of sandstone, chert and basalt in shale matrix. Radiolarian fossils were obtained from chert, felsic tuff, shale and tuffaceous shale of the Hanazono, Yukawa and Miyama Formations, and 4 radiolarian fossil assemlages are recognized, that is, Holocryptocαnium bαrbui Assemblage，Dictyomitra formosa Assemblage, Dictyomitra koslovae Assemblage and Amphipyndax tylotus Assemblage. According to the previous studies, Holocryptocanium barbui Assemblage is correlated to late Albian to Cenomanian, Dictyomitra formosa Assemblage to Turonian to Coniacian, Dictyomitra koslovae Assemblage to Santonian to early Campanian, and Amphipyndax tylotus Assemblage to late Campanian. Judging from the lithologic characters and imbricated structure, the Hanazono Formation is considered to be constructed as an accretionary complex. The radiolarian fossils show that the construction age of the Hn1 Unit was Turonian to Coniacian, Hn2 Unit was Turonian to late Campanian, and Hn3 and Hn4 Units were Turonian to latest Campanian and Hn5 Unit was late Albian to latest Campanian.
