Biostratigraphy of the Akiyoshi Limestone Group,

Yasuhiro Ota
toriyamai Ota. From viewpoints of their morphological characters and affinities,
Protriticites yanagidai Ota represents a primitive stage in the phylogenetic line between
Protriticites yanagidai Ota and Protriticites matsumotoi (Kanmera), whereas Protriticites
masamichii Ota is likely to be a transitional species to Montiparus matsumotoi injlatus,
reported by Watanabe (1991).
Based on these fusulinacean assemblages and phylogenetic considerations, it is
concluded that the variation of elements among fusulinacean assemblages is caused by
change of lithofacies in development of the Akiyoshi organic reef complex. It has a
large influence in recognition of the biostratigraphic units. The palcoenvironmental
analysis of the Akiyoshi organic reef complex is indispensable for establishment of the
reexamined biostratigraphy.
Introduction
The study area is located in the Akiyoshi Terrane of Southwest Japan where the
Middle Carboniferous to Lower Permian Akiyoshi Limestone Group is widely dis
tributed. The Akiyoshi Limestone Group which contains well-preserved fusulinaceans
and many other well-preserved mega-fossils is considered to have originally
formed as an organic reef complex upon a basaltic mound. This paper describes the
fusulinacean faunas and discusses the elements of the newly discriminated fusulina
cean zones. It also examines the Middle Carboniferous to Early Permian fusulina
cean phylogenetic transition in the Jigoku-dani area and two other related areas on
the Akiyoshi limestone plateau.
The principal survey area, Jigoku-dani, is located in the northwestern part of the
Akiyoshi Plateau. Karst topography characterized by lapie field, is well developed
and a valley with a NE-SW trend is located in the middle of the area. Middle Car
boniferous to Lower Permian limestones with a number of well-preserved fusulinaceans,
are widely distributed. Consequently, this area was first selected for the ex
amination of the transition of the fusulinacean assemblages during Middle Carbo
niferous to Early Permian. M. Ota (1977) suggested that the general strikes and
dips of the Akiyoshi Limestone Group in the area are nearly horizontal. The author
set a starting point at an altitude of 355 m, and carried out field observation and
sampling of materials with a measuring tape. The measured traverses (JI Traverse)
were mainly drawn by crossing the general trend of the strike and sometimes drawn
by lines parallel to strike. Limestone samples were carefully collected along the
measured traverses.
The second investigated AK area was selected in front of the Akiyoshi-dai Muse
um of Natural History. In this area, there are nearly complete successions of Car
boniferous to Lower Permian limestones with abundant fusulinaceans.
also characterized by a lapie field.
This area is
The third investigated area, Mt. Maruyama is located in the Isa Quarry, Mine
City. A principal traverse was set and measured along the eastern slope of Mt.