Carboniferous Foraminiferal Biostratigraphy of the Lower Part of the Akiyoshi Limestone Group, Southwest Japan

Based on the detailed field survey and laboratory examination, the lower part of the Akiyoshi Limestone Group exposed in the Okubo area is divided into the following 10 foraminifer zones in descending order: Fusulinella biconica Zone, Profusulinella beppensis Zone, Pseudostaffella antiqua Zone, Pseudostaffella minuta Zone, Millerella yowarensis Zone, Eostaffella ikensis Zone, Eostatffella mosquensis Zone, Mediocris mediocris Zone, Endothyra n. sp. A Zone, Endothyra Zone. These biozones of foraminifers range from the Early Visean of the late Early Carboniferous to the Late Carboniferous Moscovian in European and Russian standard. According to the distribution of biozones and observation of the laminae, limestones of the Okubo area show a homoclinical structure, dipping south with completeIy reversed stratigraphical succession, and are essentially successive in both northern and southern slopes along a conspicuous valley of E-W trend in this area. As far as the Okubo area is concerned the normal order of the limestone succession has long been interpreted, and then the result of this study is considerably reverse to the previous recognition. Foraminifers of the lower part of the Akiyoshi Limestone Group can be divided into two groups by their stratigraphic distributions. Most species of the first group are stratigraphically restricted in the occurrence below the Millerella yowarensis Zone. The remarkable faunal change at the level, including the Eostaffella ikensis and M. yowarensis Zones, is recognized not only within families, such as Archaediscidae and Palaeotextulariidae, but also within genera such as Tetrataxis and Palaeotextularia. Despite of the abundant occurrence of various kinds of fossils, there remained some different opinions among researchers on the age determination of the lower part of the Akiyoshi Limestone Group. The different interpretations have been caused by various field evidences based on some isolated localities. Therefore the careful faunal correlation among independent locations are strictly required for establishing the summarized biostratigraphy. Summarizing the biostratigraphic characters on the lower part of the Akiyoshi Limestone Group, it is confirmed that the mid-Carboniferous boundary is apparently recognized at the base of the Millerella yowarensis Zone.