Restudy on the geologic structure of the Akiyoshi Limestone Group, Southwest Japan

Abstract

Based on the fact that the fusulinacean fossil zones are in reverse order at Kaerimizu doline and other localities in the Akiyoshi area, OZAWA (1923) discovered a large scale of recumbent folds in the Akiyoshi limestone Group. Since then the Akiyoshi area has been regarded as one of the most important places to study the geotectonic movement taken place at an interval between Late Paleozoic and Late Triassic, and to study fusulinacean foraminifers and their zonation. Accordingly numerous works have been appeared after OZAWA, such as SUGIYAMA (1939), KOBAYASHI (1941), HANZAWA (1944), TORIYAMA (1954, 1957, 1958), HASEGAWA (1958), YABE (1958), MURATA (1963), OTA (1968, 1971), etc. Some of them were lying stress on the geotectonic problems while others on the biostratigraphical ones.So far as the micropaleontological and biostratigraphical studies of the Akiyoshi limestone Group are concerned, the results obtained. by various authors are not so different from each other. However, it is remarkable that opinions are very much diverse on the geotectinic structure of the Akiyoshi limestone Group and the non-calcareous Paleozoic formations developed around the former, in spite of the fact that their geologic structures have important bearing on the consideration of the geotectonic history of Southwest Japan.In order to make clear the true nature of geologic structures of the Akiyoshi limestone Group, detailed field survey has been carried out in the Kaerimizu area, northeastern part of the plateau, where the typical limestone sequence of reverse order is observed. As the results of our field studies (in scale of 1 : 100 to 1 : 500) we have confirmed ten fusulinacean zones in the Kaerimizu area among twenty fossil zones established by TORIYAMA and OTA (1971) throughout whole Akiyoshi limestone Group.Fossil zones in the Akiyoshi limestone Group. The zones with asterisk are found in the Kaerimizu area. (The Plα2 zone is included in the Plα1 zone in the Kaerimizu area.)Among ten Permian fusulinacean zones found in the Kaerimizu area mentioned above, we have found that the Misellina claudiae zone is the best key bed, because M. claudiae (DEPRAT) has a characteristic features by which it is easily distinguishable from other species, and it concentrates in a limited horizon, though only 2 m in thickness. It is noted, moreover, that the limestone of Misellina claudiae zone consists mostly of micritic limestone, showing a considerable contrast in facies with the limestones of the zones above and below.During the field studies were going on, the Akiyoshi-dai Science Museum conceived a project for subsurface biostratigraphy in the Kaerimizu area, because it is absolutely necessary to confirm the order of subsurface succession of fossil zones in the area. The Kaerimizu boring well No. 1 was thus settled about 200 m north of Kaerimizu doline where is 225.83 m above the sea-level (N. Long. 34°16'1 “; E. Lat. 131°18'58”). Because of poor budget for the project in the Science Museum, the drilling works have been carried out by ourselves, and No. 1 well reached at the depth of 250.56 m (about 25 m below the sea-level) in February, 1972.Through detailed micropaleontological study on the materials obtained from the cores of No. 1 boring well, the following facts have been confirmed as shown in the columnar section (Fig. 3).In the depth from 0 to 162 m the Misellina claudiae, Parafusulina kaerirnizensis, Afghanella schencki, Neoschwagerina craticulifera, Verbeekina verbeeki, and Colania douvillei zones are found successively without any stratigraphical break ; namely, the fusulinacean zones in this part are completely reverse in order.