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.
The four nations of Indonesia, Japan, Malaysia and Singapore carried out jointly the hydrographic survey of the shipping route through the Straits of Malacca and Singapore with a view to securing navigational safety in the Straits. (1) Preliminary hydrographic survey. In 1969, a preliminary survey was conducted on four sounding lines with the spacing of 1 kilometre along the shipping route through the Straits. The purpose was to obtain necessary information for determining those sea areas where detailed hydrographic survey would be required, as well as to investigate performances of various survey instruments to be used in the detailed survey. Incidentally, the survey revealed that sandwaves developed in the area and that there existed 21 uncharted shoals of less than 23-metre deep which might present danger to huge vessels' navigation. (2) 1st detailed hydrographic survey. Of the areas where detailed survey was necessitated by the results of the preliminary survey, those in the Main Strait and Phillip Channel and their adjoining waters were survey in detail in 1970. As the result, 30 uncharted shoals were revealed to exist. It was also found that discrepancies existed between the geodetic systems of Malaysia and Indonesia, i.e. about 60 metres in south-north and about 400 metres in east-west directions. (3) 2nd detailed hydrographic survey. In 1972, the 2nd detailed hydrographic survey was conducted in the area western continuation to the survey area of the 1st detailed survey and in the areas off Cape Rachado. Neither any dangerous uncharted shoal nor data indicating shift of sandwaves formerly disclosed by the preliminary survey was found by the survey. However, a sunken wreck with a least depth of 14.9 metres over it was found amid the shipping route.