To clarify the regional distribution and characteristics of the sedimentary depos-

Some small basins have developed in the northern Philippine Sea along the northern and southern sides of the Daito Ridge (DR) : the Kita-Daito Basin (KDB) , Amami-Sankaku Basin (ASB) and Minami-Daito Basin (MDB) . From the interpretation of multichannel reflection seismic profiles crossing these basins, their noticeable differences of geological character can be recognized. There are several E-W trending normal faults in the main part of the KDB which suggest that this basin was formed by seafloor spreading. A seismic section crossing the OkiDaito Ridge (ODR) and DR presents a reflector, possibly indicating subduction of the slab containing the ODR. The subduction probably caused back-arc spreading of the KDB, which finally ceased upon the collision of both ridges. Based on the trace of characteristic reflectors in seismic sections and correlations with several drill holes, sediments are divided into Units I through V, ranging from Lower Eocene to Holocene. The Choju Seamounts occupy the middle part of the MDB, dividing the basin into the eastern main basin and the western small subbasin (MinamiDaito Nishi Sub Basin; MDNSB) . The seismic character of Unit II (≒ Middle to Upper Eocene) is noticeably different among the ASB, the main basin of MDB, and the MDNSB, indicating different sources of sediments. The thickness of Unit II in the ASB increases towards the KyushuPalau Ridge (KPR) , indicating that sediments were supplied by the KPR. Unit II in the main basin of MDB has no direct relationship with the KPR, and should have had another source, such as the DR or the ODR. The thickness of Unit II in the MDNSB increases towards the DR, suggesting an equivalent origin of the carbonate layer at the top of the DR.

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The natural gas resources, probably of disolved-in-water type, exist in the Tertiary Shimajiri Group of Southern Okinawa Islands.From the detailed study of surface geology and biostratigraphy, writers have proposed following stratigraphic succession.Writers have also studied Shimajiri Group from the economic point of view, and the following facts were recognized.1) The thickness of the Shimajiri Group attains to about 2500 meters in this area. Shimajiri Group chiefly consists of massive siltstone, and is open sea deposits with a montonous cycle of sedimentation.2) From the investigation of geostructural and biostratigraphical evidence, this area are divided into three blocks: northern, middle, and southern districts. Each block is bordered by the fault, NW-SE in direction. Geological structures of northern and southern districts indicate monoclinal.On the other hand, in middle district complicated several foldings are recognized.3) The sedimentary environment of Shimajiri Group indicate a relatively stable bathyal to outer neritic condition, and is most favorable as the mother rock of natural gas, but the very fine sandstone might have been a little permeable as the reservoir rocks.

Abstract:By the middle of