| Abstract |
The Akiyoshi limestone Group is world-famous for fusulinacean biostratigraphy, but zonation of the lower part of the group is still incomplete. The Okubo area along the southern margin of the Akiyoshi... limestone plateau has been regarded as one of the best fields where the lower part of the Akiyoshi limestone Group is well exposed. Despite many previous works, the biostratigraphic sequence and geologic age and structure of this area still undetermined. Specific components and their ranges of the foraminiferal fauna, together with vertical and horizontal changes of the bio-and litho-facies, have been examined in detail. The following 10 biozones are discriminated in descending order: Fusulinella biconica zone, Profusulinella beppensis zone, Pseudostaf fella anti qua zone, Pseudostaf fella mi nut a zone, Millerella yowarensis zone, Eostaffella ikensis zone, Eostaf fella mosquensis zone, Mediocris mediocris zone, Endostaf fella sp. A zone, and Endothyra zone. The lower part of the Millerella yowarensis zone, comprising the "Uzura" fauna, is correlated with the lower Serpukhovian in Russia, the lower Namurian in Western Europe, and the Upper Chesterian in North America. The age of the Endothyra zone, the lowest part of the Akiyoshi limestone Group of this area, is probably Early Visean. The type locality of the OTA's Zaphrentoides sp. zone can be set in to the lowest part of the Mediocris mediocris zone. The age of the lowest part of this zone is probably referable to late Middle Visean on the basis of foraminifers. The biozones of this area are distributed in parallel to one another from south to north ascending order, and trend in the WSW-ENE directon. The general trend of laminae of limestones well accords with that of biozones. Moreover, the laminae dip south with 30 to 60 degrees. Unlike the current interpretation, these evidence indicate a completely reversed monoclinal structure of the Akiyoshi limestone Group of this area. This result requires reexamination not only of other areas along the southern margin but also of relevant areas of the Akiyoshi limestone plateau.show more
|
Export Mendeley
1404_p163