Submarine geology off the Miyako and Yaeyama islands.
Since 1967 the Hydrographic Department of Japan has been conducting a surveying project for the preparation of the Basic Map of the Continental Shelves (scale 1 : 200000), covering submarine topography, geology, geomagnetism and gravity. The present paper describes the results on submarine topography and geology in the area off the Miyako and Yaeyama Islands, based on the surveying data obtained in 1976. Two cruises were made for the survey by the Syoyo (1900 gr. tons), in April/May (designated by OK), and in June/July (designated by MI) (Figure 3). The ship’s positions were fixed mainly by a hybrid system using Loran C and NNSS. Soundings were made by both a deep-sea-water type echo-sounder (type NS-16) and a shallow-water type echo-sounder (type NS-77). The submarine geological structure was probed by an air-gun seismic reflection profiling system with an acoustic frequency of 60 to 300 Hz. The survey area is located around the southeastern part of the Nansei Islands which、stretch from Kyusyu to Taiwan (Figure 1). The main topographical features in this area are the Okinawa Trough in the northwestern part, and a ridge (called the Ryukyu Ridge temporally), in the south-eastern part extending in a NE-SW direction (Figure 4). One continental slope of the Ridge spreads down toward the Okinawa Trough and has several submarine scarps, trending to the NE-SW and ENE-WSW in echelon, parallel to the Ridge. Along the base of this slope is distributed a row of knolls. The other slopes spread down toward the Nansei Islands Trench with comparatively gentle gradient (Table 2). Submarine scarps, however, run locally off Haderuma Island trending to the ENE-WSW; and submarine canyons are distributed in the southern offing between the Isigaki and Miyako Islands. The acoustic stratigraphy in the survey area can be divided into Cm-, Bm- and Am formations, in an ascending order from the basement. The Cm formation is the basement of the Ryukyu Ridge and is distinctly found in the southeastern slope between the Tarama Saddle and Iriomote Island. It becomes indistinct northward from the Miyako Islands, especially in the northern part of the southeastern slope. The Bm formation is distributed almost wholly over the area. Although it is covered by Am formation, a number of outcrops appear. The Am formation is thinly sedimented on the upper part of the Ridge, although thick on the Okinawa Trough (Figure 5). Process of the geological development can be summarized as follows. The Cm formation was sedimented in the Plioceneor Pre-pliocene, forming land or shallow water areas. Then, the Bm formation, presumably a Simaziri group in the Pliocene,sedimented widely in the area: thinly on the upper part of the Ridge and thickly on the continental slopes. Tectonic movement began the formation of the Okinawa Trough and the Ryukyu Ridge in this age, and caused the Trough to submerge, and the Ridge to emerge. Thereafter the Ridge tilted southeastward. With this movement the faults which run in the NE-SW or ENE-WSW direction were formed in the northwestern part of the Ridge. This movement also caused the axis of the Ridge to describe an arc with the northwestern slope inside. During this period the Tarama and Isigaki canyons were formed, but a row of knolls were also formed along the northwestern base of the Ridge. The movement continued and the Ridge was divided by faults into several blocks, some of which run in the NE-SW or ENE-WSW direction, some in the NW-SE, and others in the NNW-SSE or NNE-SSW. As a result of this movement, several of submerged blocks formed the present planation surface in the shallow water area, others in the Am sedimented area, presumably in the Quaternary. The insular shelves were formed by a lowering of the sea level during a glacial epoch of the Quaternary. Thereafter the block of the Isigaki and Iriomote Islands was raised, in contrast to that of Miyako Island and the Miyako Sone which has been continued to tilt up to the present. An Am formation has been sedimented in the Okinawa Trough, which still had a tendency towards submergence during these periods.
| Main Authors: | , , |
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| Format: | Journal Contribution biblioteca |
| Language: | Japanese |
| Published: |
1979
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| Online Access: | http://hdl.handle.net/1834/16294 |
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