Permian Fusulinids from Idani District, Shimizu-machi, Arita-gun, Wakayama Prefecture, Southwest Japan

We have studied the geology in the Idani district, Shimizu-machi, Aritagun, Wakayama Prefecture, Southwest Japan, since 1968. Recently we found several fusulinids in this district, so that we certify the existence of the Palaeozoic formations. In this paper we intend to report these fusulinids and discuss the geologic significance of the Palaeozoic formations. The geology in this district was investigated and reported by HIRAYAMA and TANAKA in 1955, and the formation that distributes in this district was named the Idani formation by them. They reported Thamnasteria sp., Stromatopora japonica and others from the limestones, so they have decided that the Idani formation is referable to the Upper Jurassic. This formation mainly consists of black mudstone, with subordinate sandstone, chert and limestone. The strike of the formation is generally ENE-WSW and it inclines to the north. According to them, the Idani formation is overturned to the north and overlies conformably the Yukawa formation which occupies the area to the south of the Idani formation and is referable to the Upper Cretaceous. The northern border of the formation is in contact with the Futagawa formation (Upper Cretaceous in fault. Along the fault a small mass of serpentine is observed, so that they explained the fault as the Butsuzo tectonic line. From the data above mentioned, they considered that the Idani formation might be equivalent to the Nishikawa formation in Shikoku and the Yura formation in the western part of Wakayama Prefecture in the Shimanto terrain. KATTO (1965) supposed that the south limit of the Idani formation was cut with fault and considered it to be the Butsuzo tectonic line, so he maintained that the Idani formation should be a member of the Chichibu terrain. According to NAKAZAWA (1970), the Yukawa formation which occupies the area to the south of this district has a monoclinic structure as a whole dipping north-ward and the sedimentary top of the formation looks northward; the above fact proves the formation to be in normal order. He guessed that the Idani formation and the Yukawa formation were in fault contact with each other, and, as KATTO previously supposed, he also considered the fault to be the Butsuzo tectonic line and the Idani formation to belong to the Chichibu terrain. In 1968, the fault which exists betweeen the Idani and Futagawa formations has been named the Shimizu-Itao fault by SAKA. Along this fault, SAKA and IMAI (1970) discovered a small mass of granodiorite which is remarkably crushed and partly mylonitized. According to them, this granodiorite is notably similar to that of Nabae Peninsula, Yuasa-machi, Arita-gun, Wakayama Prefecture and moreover resembles the Mitaki igneous rocks discovered in the Kurosegawa zone in Shikoku (Ichikawa et al., 1956). Consequently, they considered that the granodiorite of this district might be identified with the Mitaki Igneous rocks. As for the palaeontological study of the Idani formation, NOHDA and SETOGUCHI (1967) described several Torinosu pelecypods such as Myophorella cfr. obsoleta, Grammatodon takiensis, Entolium yatsujiensis and others from mudstone near the starting point of the Murokawa Forest road in Idani, and two conodonts such as Gladigondolella abneptis and Hindeodella sp. from a crystalline limestone exposed along the Murokawa Forest road in Idani. They considered that these fossils are proofs that the formation was generated an Upper Jurassic age. Recently KOIKE et al. (1970), however, expressed that these conodonts might be compared with the Triassic. In this study, we discovered fusulinids from the north bank of the Arita River (Locality 1) and the Takora Valley (Locality 2). These localities are situated in the southwestern part of this district. Locality 1: On the north bank of the Arita River, the formation yielding the fusulinids consists of mudstone, sandstone and schalstein, with subordinate limestone and chert. This formation and the Mesozoic formation to the north are in fault with fractured zone; the latter one includes several Torinosu lime-stone lenses yielding stromatoporoids. Though the south boundary is obscure due to the Arita River, we found the Torinosu limestones yielding stromatoporoids and Hexacorals on the opposite bank of the Arita River. The Mesozoic formation bearing these limestones corresponds to the southwestern extension of the mudstone yielding the Torinosu pelecypods described by NOHDA and SETOGUCHI (1967). Therefore, we guess that the present formation and the Mesozoic formation are probably in fault contact. The strike of the formation is generally ENE-WSW and it inclines to the north; however, it is remarkably disturbed. The thickness of this formation is about 80m. We considered that the formation was squeezed out tectonically as a lenticular body along the fault within the Mosozoic. The following fusulinids and minor foraminifera are found from a small limestone lens in this formation. Lepidolina multiseptata gigantea L. multiseptata shiraiwensis Yabeina sp. indet. Chusenella sp. indet. Pachyphloia sp. indet. The faunal assemblage of the above mentioned fossils is very similar to that known from the Kuma formation in Kyushu. (KANMERA, 1954), the Maizuru group in Kinki (NOGAMI, 1958) and the Toba group in Shima (YAMAGIWA and SAKA, in printing), so that the age of this formation is undoubtedly in the Upper Permian. Locality 2: In the Takora Valley, the northern border of the formation yielding fusulinids is in contact with th Mesozoic formation including the Torinosu pelecypods described by NOHDA and SETOGUCHI (1967) in fault. The southern border is cut by a fault and comes into contact with the Mesozoic formation containing conodonts, stromatoporoids and Hexacorals reported by HIRAYAMA and TANAKA (1955), and NOHDA and SEOGUCHI (1967). This formation is composed of sandstone and mudstone with limestone, chert and schalstein. We discovered two fusulinids such as Schwagerina cfr. etoi and Triticites aff. plummeri from a small limestone lens attending schalstein of this formation. By the above mentioned fossils, its age belongs to the Lower Permian. The strike of the formation is generally ENE-WSW. It inclines to the north. The thickness is about 50m. In this study, there are found not only the Mesozoic but also the Palaeozoic formations in this district. Therefore, we guess that the geological members consist of a composite formation including the Palaeozoic and the Mesozoic (Jurassic and Triassic?). Through the survey, we discovered that the southern border of this composite formation was in contact with the Yukawa formation in the Shimanto terrain in fault. From the above geological data these two conclusions are arrived at: (1) this fault correponds to the Butsuzo tectonic line and (2) this composite formation belongs to the Chichibu terrain. As above mentioned, SAKA and IMAI (1970) discovered a small mass of granodiorite along the Shimizu-Itao fault which exist between the composite formation and the Futagawa formation. It is very interesting that the lithologic character of this granodiorite is notably similar to a member of the Mitaki igneous rocks discovered from the Kurosegawa zone in Shikoku (ICHIKAWA et al., 1956)