Biostratigraphy of the Akiyoshi Limestone Group,

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Fusulinacean Biostratigraphy of the Akiyoshi Limestone Group, Part I 41 following fusulinacean zones are successively discriminated from the top to the bottom along the slope: 1. Fusulinella biconica Zone, 2. Fusulina cf. shikokuensis Zone: Fusulinella cf. obesa Subzone, Pseudofusulinella hidaensis Subzone, 3. Obsoletes obsoletus Zone: Protriticites toriyamai Subzone, Protriticites matsumotoi Subzone, 4. Montiparus sp. A Zone, 5. Triticites yayamadakensis Zone: Triticites saurini Subzone, Schwagerina sp. A Subzone, Triticites biconicus Subzone, 6. Schwagerina () cf. satoi Zone, 7. Pseudoschwager ina muongthensis Zone, 8. Pseudofusulina vulgaris globosa Zone, and 9. Pseudofusulina aff. ambigua Zone. In this chapter, the author presents a correlation chart (Fig. 17) of each zone in the Jigoku-dani area with Middle Carboniferous to Early Permian biostratigraphic units established by workers in different areas of the Akiyoshi Limestone Group. 1. Fusulinella biconica Zone. The Fusulinella biconica Zone was characterized by the occurrence of Fusulinella biconica (Hayasaka). This zone was first introduced by Toriyama (1954a) with chronostratigraphic position of the Akiyoshian Series for this zone. According to M. Ota (1977), the lower part of Fusulinella biconica Zone is characterized by rather primitive species of Fusulinella, namely, Fusulinella simplicata Toriyama. Fusulinella biconica (Hayasaka) typified the middle part of the zone and Fusulinella itoi Ozawa, the upper part. Ueno (1989) divided the Fusulinella Zone into the following two zones, the lower, the Fusulinella biconica Zone and the upper, the Fusulinella taishakuensis Zone. In Jigoku-dani, the Fusulinella biconica Zone is definitively located at the top of the slope of the Jigoku-dani area, but the Fusulinella taishakuensis Zone was not con firmed in this area. This zone corresponds to Fusulinella Zone in other areas. 2. The Fusulina cf. shikokuensis Zone is characterized by the first occurrence of subfamily Fusulininae, especially by Fusulina cf. shikokuensis Ishii. This zone is divided into two subzones, the lower, the Fusulinella cf. obesa Subzone, and the upper, the Pseudofusulina hidaensis Subzone. The Fusulinella cf. obesa Subzone, is characterized by Fusulinella cf. obesa Sheng. This subzone probably corresponds to the Kurikian Series that was proposed by Kanmera (1952) from the fusulinacean biostratigraphy of the Yayamadake Limestone in the outer zone of Kyushu. Furthermore, this subzone is stratigraphically correlative with the Beedeina akiyoshiensis Zone by M. Ota (1977). According to him, the Beedeina akiyoshiensis Zone is typically developed in the Shishide-dai area, the northeastern part of Akiyoshi-dai with distinguishable litho- and biofacies from the subjacent zone. In Jigoku-dani, limestones of this subzone do not contain reef building organisms and are considered to have accumulated in a lagoonal environment within the Akiyoshi organic reef complex. Beedeina akiyoshiensis (Toriyama) has not been found yet. The difference of the fusulinacean assemblage seems to be dependent on the dif ference of the bio- and lithofacies of limestones. This subzone is correlated to the Fusulina Zone elsewhere. The upper subzone, the Pseudofusulinella hidaensis Subzone, is characterized by the
42 Yasuhiro Ota occurrence of Pseudofusulinella hidaensis (Kanuma). This zone was first introduced by Ozawa and Kobayashi (1990) in the Akiyoshi Limestone Group. However, at present, the occurrence of this species is confined to particular areas. In the Jigokudani area, Pseudofusulinella hidaensis (Kanuma) is found in some localities, but its distribution could not be traced widely. This subzone is probably correlated with the upper part of the upper Fusulina-Fusulinella Zone. 3. Obsoletes obsoletus Zone The Obsoletes obsoletus Zone is defined by the occurrence of Obsoletes obsoletus (Schellwien). This zone is subdivided into two subzones, the lower Protriticites toriyamai Subzone and the upper Protriticites matsumotoi Subzone. The lower, Protriticites toriyamai Subzone comprises primitive species of Protriti cites, namely Protriticites toriyamai Ota and Protriticites masamichii Ota. This subzone seems to be also characterized by the first appearance of very finely perforated walls in genus. Distribution of this subzone is restricted in its stratigraphic distribution. The Protriticites toriyamai Subzone is considered to be the basal zone of the Hikawan that was first introduced at Hikawa Valley, Kyushu, by Kanmera (1952). Addi tionally, this subzone is equivalent to the lower part of Obsoletes-Protriticites Zone or Obsoletes Zone in other areas. The upper, Protriticites matsumotoi Subzone is characterized by the occurrence of Protriticites matsumotoi (Kanmera) with Obsoletes obsoletus (Schellwien). The basal boundary of the Protriticites matsumotoi Subzone with the underlying Protriticites tori yamai Subzone is marked by the first occurrence of Protriticites matsumotoi (Kanmera). Protriticites matsumotoi (Kanmera) was first described by Kanmera (1955) under the generic name of Triticites (s. 1.) and this species is a diagnostic one of the Hikawan. Here, the author treats this species as the genus Protriticites by its having the "Protriticites-tyot wall" only in the outer volution. This subzone may be equivalent to a lower part of Hikawan and the upper part of the Obsoletes-Protriticites Zone, and can be correlated with the Triticites (s. 1.) matsumotoi Zone by M. Ota (1977). In the AK area and some other regions, the occurrence of the genus Quasifusulinoides was reported from the upper part ofProtriticites Zone and/or the overlying strata. In the Jigoku-dani area, however, the upper part of the present subzone yields Quasifusuli noides () sp., but no well-preserved specimen of Quasifusulinoides were obtained. The upper part ofthis zone is possibly correlative with the Quasifusulinoides Zone in other areas. 4. Montiparus sp. A Zone. The Montiparus sp. A Zone is characterized by the occurrence of the genus Montiparus, and the basal boundary of this zone is drawn by the first occurrence of Montiparus sp. A or Montiparus () sp. A. The upper limit of this zone is defined by the first occurrence of Triticites yayamadakensis Kanmera and T.yayamadakensis evectus Kanmera. The genus Montiparus was first introduced by Rozovskaya (1948) as a subgenus of Triticites (s. 1.) with the name of Triticites montiparus Ehr. em. Moell.
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Biostratigraphy of the Akiyoshi Limestone Group,

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