Biostratigraphy of the Akiyoshi Limestone Group,

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Fusulinacean Biostratigraphy of the Akiyoshi Limestone Group, Part I 37 simplex (Schellwien), Triticites suzukii (Ozawa), and Schwagerina sp. B. 5-3. Triticites biconicus Subzone. The Triticites biconicus Subzone is characterized by the abundant occurrences of Triticitesyayamadakensis Kanmera and the characteristicoccurrence of Triticites biconicus Toriyama. Therefore, the fusulinacean constituents of this subzone clearly distin guish them from those of other subzones. Triticitesyayamadakensis Kanmera from this subzone has a rather advanced form, such as larger shell and thicker spirotheca, than those in the lower horizon. Limestones of this subzone have a general strike of about N10°W and gently dip to the south ward. The basal part of this subzone mainly consists of limestones with sparry calcite cements. Matrices of limestones gradually change from sparry calcites to micrites upward in the section. However, the uppermost part of this subzone contains coarse grains of fossil fragments of frame builders. In the Jigoku-dani area, this zone is the upper limit of distribution of black to dark brown limestones. The following fusuhnaceans are recognized in this subzone: Quasifusulina sp. B, Triticites cf. isaensis Toriyama and Triticites kuroiwaensis Toriyama. 6-1. Schwagerina () cf. satoi Zone. The lowest part of the Schwagerina () cf. satoi Zone is defined by the horizon from the first occurrence of Schwagerina () cf. satoi (Ozawa) or Pseudofusulina () sp. A to the first occurrence of Pseudoschwagerina muongthensis (Deprat) or Pseudoschwagerina sp. The estimated thickness of this zone is 76 m. The following fusuhnaceans are also recognized in this zone: Ozawainella aff. magna Sheng, Nankinella sp. A, Schubertella lata Lee and Chen, Triticites haydeni (Ozawa), T. michiae Toriyama, T cf. obai Toriyama, T. cf. pseudosimplex Chen, T. aff. simplex (Schellwien), T. aff. suzukii (Ozawa), Schwagerina cf. stabilis (Rauser), "Pseudofusulina" cf. bacca Morikawa and Isomi, "Psf" aff.fusiformis (Schellwien), and Rugosofusulina arctica (Schellwien). The fusulinacean index fossil, Schwagerina () cf. satoi (Ozawa) resembles the specimen that was first described as Schellwienia satoi by Ozawa (1925). As it has been already pointed out by Ozawa (1925) and Toriyama (1958), Schwagerina satoi (Ozawa) is intermediate in form between Schwagerina and Pseudoschwagerina. Schwagerina () cf. satoi (Ozawa) from the Jigoku-dani area also has an intermediate form between Schwagerina and Pseudoschwagerina morikawai Igo and the present form is similar to the microspheric form of Pseudoschwagerina. The question whether the present species is referable to the inflated schwagerinids is uncertain. This species, however, is very important for regional correlation. Fusuhnaceans of this zone are distinguished from those of the underlying zone by their rather advanced forms in having larger shell, and thicker spirotheca. Weakly developed phrenothecae occur in some specimens. The phrenothecae are generally observed in Pseudofusulina, but the present specimens are found in a lower horizon than the horizon of Pseudofusulina as known before in the Akiyoshi Limestone Group. More detailed studies are required to solve the problem of the "Pseudofusulina" Horizon.
38 Yasuhiro Ota Limestones of this zone are mainly composed of micritic matrices. However, the upper part of the underlying zone contains some fragments offossils of reef frame builders. No evidence of change in lithofacies of the lower Triticites biconicus Subzone to the lower part of the Schwagerina () cf. satoi Zone suggest a remarkable hiatus. 7. Pseudoschwagerina muongthensis Zone. The lowest part of the Pseudoschwagerina muongthensis Zone in the Jigoku-dani area is marked by the first occurrence of the genus Pseudoschwagerina, namely, Pseudo schwagerina muongthensis (Deprat) or Pseudoschwagerina sp. In the Jigoku-dani area, well preserved specimens of Pseudoschwagerina were not obtained. Along the AK Traverse near the Akiyoshi-dai Museum of Natural History, microspheric forms of Pseudoschwagerina are obtained from a stratigraphically slightly lower position com pared with that of the megalospheric forms. Along the northern JI Traverse, JI Traverse 5, the microspheric form of Pseudoschwagerina also occurs in a lower horizon about 2 m below the megalospheric form. However, along the other JI Traverses, the microspheric form has not yet been confirmed. Therefore, the boundary on the underlying zone is tentatively drawn between the horizons of microspheric form of Pseudoschwagerina sp. and megalospheric form of Pseudoschwagerina muongthensis (De prat). The estimated maximum thickness of this zone is 32 m-K Limestones in the lower part of this zone, consist of rather coarse constituents and intraclasts. The upper part of the underlying Schwagerina () cf. satoi Zone is dominated by micritic limestones. The changes of lithofacies is present between the upper part of Schwagerina () satoi Zone and the lowest part of the Pseudoschwagerina muongthensis Zone. However, each zone is conformably distributed with nearly the same strike and dip. Therefore, a visible gap is not assumed between the two zones in the field. On the other hand, limestones gradually become micritic in texture as the stratigra phy is studied from the lower to the upper part in the present zone. This means that the energy of the environment decreased. The recognized species of this zone are as follows: Staffella moelleri Ozawa, Schubertella kingi Dunbar, Schwagerina okafujii Toriyama, Schwagerina primigena Nogami, Schwagerina cf. princeps (Ehrenberg), Schwagerina sp. A, Schwagerina sp. B, Schwagerina sp. C, Schwagerina sp. D, Schwagerina () aff. kueichihensis (Chen), Pseudofusulina regularis (Schellwien) and Pseudoschwagerina () sp. 8. Pseudofusulina vulgaris globosa Zone. The Pseudofusulina vulgaris globosa Zone is characterized by the abundant occur rence of Pseudofusulina vulgaris globosa (Schellwien) and the association of Paraschwagerina sp. In this zone the author could not directly determine the strike and dip of the limestone. The present zone is considered to conformably overlie the Pseudoschwagerina muongthensis Zone. Limestones in this zone are estimated to be at most 83 m in thickness. They are characterized by micritic texture. However, the lowest part of this zone is composed of sparry calcite cements with rather coarse grains. The grain size of limestones near the top of the Pseudoschwagerina muongthensis
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Biostratigraphy of the Akiyoshi Limestone Group,

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