Biostratigraphy of the Akiyoshi Limestone Group,

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Fusulinacean Biostratigraphy of the Akiyoshi Limestone Group, Part I 35 subzone strike N10°W, gently dipping toward southwest. They are composed of micritic constituents, but limestones with sparry calcite cements are intercalated at the basal partofthis subzone. Additionally, limestones ofblack to dark brown color occur in this basal horizon, though the origin of the black to dark brown limestones has not been determined. 2-2. Pseudofusulinella hidaensis Subzone. The base of the Pseudofusulinella hidaensis Subzone is marked by the first occur rence of Pseudofusulinella hidaensis (Kanuma) and this diagnostic species is associated with Fusulina cf. shikokuensis Ishii. Limestones of this subzone are estimated to be 2 m in thickness, and have a general NW strike and gentle dip toward the southwest. They are characterized by micrite, but are partly intercalated by limestones with the sparry calcite cements. 3. Obsoletes obsoletus Zone. The diagnostic species of the Obsoletes obsoletus Zone is Obsoletes obsoletus (Schellwien). In theJigoku-dani area, this zone is recognized as an interval from the hori zon with the first occurrence of the genus Obsoletes or the genus Protriticites, to the horizon with Montiparus () sp. A or Montiparus sp. A. It is subdivided into two subzones, the lower, the Protriticites toriyamai Subzone with primitive species of Protriticites, and the upper, the Protriticites matsumotoi Subzone characterized by Protriticites matsu motoi (Kanmera). 3-1. Protriticites toriyamai Subzone. In this subzone, primitive species of Protriticites, such as Protriticites toriyamai Ota and Protriticites masamichii Ota, are associated with the diagnostic species, Obsoletes obsoletus (Schellwien). These primitive species of Protriticites are very important to determine the upper limit of the Middle Carboniferous. This subzone was named after the characteristic occurrences of Protriticites toriyamai Ota. Maximum thickness of this zone is estimated as 63 m. Limestones of this zone have a wide variety of matrices from micrites to sparry calcites though they are somewhat coarser in grain size. Black to dark brown limestones previously mentioned are observed in this zone. 3-2. Protriticites matsumotoi Subzone. The Protriticites matsumotoi Subzone is defined by the first occurrence of Protriticites matsumotoi (Kanmera). The other constituent species of this subzone are Obsoletes obsoletus (Schellwien), Protriticites yanagidai Ota and Quasifusulinoides () sp. This zone has an estimated maximum thickness of about 31 m. Limestones of this subzone strike NW and have a gentle dip toward the southwest. This zone is mostly limestones with micrite matrix, but those with sparry calcite cements are intercalated in the lower part of this subzone. The grains of sparry calcite cements are rather coarse in size. Black to dark brown limestones are common in this zone. 4. Montiparus sp. A Zone. The Montiparus sp. A Zone is characterized by the occurrence of Montiparus sp. A.
36 Yasuhiro Ota However, limestones along some traverses, did not yield Montiparus sp. A, but were characterized by Montiparus () sp. A. Montiparus () sp. A is closely similar to Montiparus sp. A, except for some features. Limestones of this zone are estimated to be about 34 m in maximum thickness. They generally show a NW strike, dipping toward southwest direction, with an occasional NE strike, dipping toward northwest direction. The main constituent of limestones is micrite. Black to dark brown limestones are also present in this zone. The following fusuhnaceans also occur in this zone: Quasifusulina longissima (Moller), Quasifusulina sp. A, Triticites hidensis Igo, Triticites cf. obai Toriyama, and Triticites saurini Igo. 5. Triticitesyayamadakensis Zone. The Triticites yayamadakensis Zone is defined by the occurrence of Triticites yayamadakensis Kanmera. In the Jigoku-dani area, Triticites yayamadakensis Kanmera is easily recognized, and is a good index fossil. The estimated thickness of this zone is 124m. This zone is subdivided into the following three subzones by characteristic fusuhnaceans in ascending order. 5-1. Triticites saurini Subzone. This subzone is characterized by abundance of Triticites saurini Igo. The base of this zone is demarcated by the first occurrence of Triticitesyayamadakensis Kanmera or Triticites yayamadakensis evectus Kanmera. The following fusuhnaceans are also in association with Triticites cf. bonus Chen and Wang, T. hidensis Igo, T. michiae Toriyama, T. aff. simplex (Schellwien). Triticites yayamadakensis evectus was first described by Kanmera (1958) from the Yayamadake Limestone, Kyushu, Japan. He pointed out the difference of the stratigraphic horizon between Triticites yayamadakensis yayamadakensis Kanmera and Triticitesyayamadakensis evectus Kanmera. However, the difference was not confirmed in the Jigoku-dani area. The thickness of this subzone attains 15 m at most. Lime stones of this subzone show general strike of about N14°W and dip about 18° toward SW. Limestones are mainly composed of micritic matrices, and black to dark brown limestones are intercalated in some horizons. 5-2. Schwagerina sp. A Subzone. The Schwagerina sp. A Subzone is characterized by the occurrence of Schwagerina sp. A and Triticitesyayamadakensis Kanmera. The lowest boundary of this subzone is demarcated by the first occurrence of the genus Schwagerina. The occurrence of Schwagerina sp. A was also ascertained from the AK Traverse in front of the Akiyoshidai Museum of Natural History and this species possibly has a wide distribution in the Akiyoshi Limestone Group. The general strike and dip of limestones in this subzone are unknown. However, from the conformable relationship with the underlying limestones, they are common to those of the underlying subzone. Furthermore, the limestone sequence of this subzone intercalates limestones with sparry calcite cements as were seen in the underlying Triticites saurini Subzone. Limestones of this subzone yield the following fusuhnaceans such as Triticites biconicus Toriyama, Triticites aff.
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Biostratigraphy of the Akiyoshi Limestone Group,

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