Biostratigraphy of the Akiyoshi Limestone Group,

Yasuhiro Ota
Historical review
-I. Geology of the Akiyoshi Limestone
The geology of the Akiyoshi Limestone and surrounding areas was first investi
gated by Ozawa (1923), who established the biostratigraphy by the use of fusulina
cean zones.
He discovered the inverted succession of the fusulinacean zones and
recognized the inverted sequences as an autochthonous recumbent fold caused by lat
eral movement from south to north. Ozawa's interpretation was developed by
Kobayashi (1935) and he proposed the Akiyoshi Phase for the first of the Mesozoic
orogenic movements. Toriyama (1954a, b, 1958) reexamined the geology and
fusulinacean paleontology of the Akiyoshi Limestone Group and the surrounding
non-calcareous sedimentary rocks. He studied the Carboniferous and Permian
fusulinaceans in detail and established refined fusulinacean zones on the Carbonif
erous and Permian limestones. The fusulinacean zones by Toriyama (1963, 1967,
1978) had been treated as a standard of the Carboniferous and Permian biostratigra
phy in Japan. Along with the biostratigraphic work, Toriyama showed his interpre
tation on the geologic structure of the Akiyoshi Limestone Group. Later, Hasegawa
(1958, 1963) and Murata (1961) showed different interpretations on the inverted
structure of the Akiyoshi Limestone Group.
Concurrently, a sedimentological study with the biostratigraphy, for the purpose
of the paleoenvironmental examination had been started by M. Ota and others.
Eto (1967) examined the bio- and litho- facies of the lower part of the Akiyoshi
Limestone Group in the Okubo area. He analyzed biofacies of limestones and re
constructed the sediment depositional process of limestones on the volcanic seamount.
In 1968, M. Ota first suggested for the paleoenvironments of the Akiyoshi
Limestone Group that it was formed as an organic reef complex on the basaltic seamount
like an atoll of the present ocean. This is an epoch-making study from the
viewpoint of paleoenvironmental analysis on the Akiyoshi Limestone Group. The
detailed lithology of the basal pyroclastic rocks was examined and described by
Yanagida, M. Ota, Sugimura and Haikawa (1971). They showed the sedimentary
sequence of the Akiyoshi Limestone in the Shishide-dai area with a columnar section
and description of the biostratigraphy of the lowest part of the Akiyoshi Limestone
Group. Schwan and M. Ota (1977) carefully reexamined the geologic structures of
the Akiyoshi Limestone Group and surrounding non-calcareous rocks. They sug
gested lateral and rotating pressures caused by gravity movement for the partly in
verted succession of the Akiyoshi Limestone Group.
The new interpretation for the tectonics of limestones and the surrounding noncalcareous
rocks, based on the plate tectonic theory of the new global tectonics, was
shown by Kanmera and Nishi (1983) and Sano and Kanmera (1988). They con
sidered that the Akiyoshi organic reef was developed on a seamount of basaltic rock
and then accreted to the terrigenous sediments of the Permian Tsunemori Formation