Submarine geology off San-in.

Abstract

Since 1967 the Hydrographic Department of Japan has been conducting a surveying project for the preparation of the Basic Maρof the Sea on the Continental Shelves (scale 1: 200, 000), covering topography, geology, magnetism and gravity. The present paper describes the results on submarine topography and geology in the area off San-in being based on the surveying data in 1975 and 1976. Three cruises were made for the survey (Fig. 4): by the Shoyo (1900 gr. tons) in October May/June, 1975 for 4000 miles (designated YA) and in May/June, 1976 for 2600 miles (designated Si). Ships' positions were fixed mainly by Loran C, Decca and NNSS. Soundings were made by medium-deep water type echo-sounder (type NS-16) and shallow water type echo-sounders (types NS-39 and -77). Submarine geological structure was probed by air-gun seismic profiling system with acoustic frequency of 60 to 300 Hz. Main topographical features in the surveyed area are a remarkably wide continental shelf in the western part (YA-region) and a marginal plateau (Oki Spur) in the eastern part (Si-region). The continental shelf in YA-region have a width of about 120 km and is divided into two flats by a group of shoals locating northward from Mi Sima. Another flat is developed between the mainland (Honsyu) and the Oki Is. The Oki Spur extends northward lingulately from the Oki Is. for about 140 km with width of about 60-70 km. (Figs. 2 and 5, Tab. 1) The acoustic stratigraphy in the surveyed area can be divided into E-, D- , C-, B-and A-layers in ascending order from the basement. A-layer is found on the sedimentary basins and distributes almost wholly over the marginal plateau, excepting west of Dozen of the Oki Is. and offing of Ota. B-and C-layers also distribute widely, but the former is lacked at the continental shelf between Honsyu and the Old Is. Although D-layer is found under the Oki Spur, its extension is narrower than those of the other layers. In the other regions, D-layer may be contained in the lower part of C-layer. E-layer,being adopted as the acoustic basement, appears as opaque around the Oki Is., the Oki Spur and around Mi Sima, while it appears as transparent along the coast of Si-region and offing of YA-region. The trends of geological structure are NE-SW and ENE-WSW directions parallel to the coast of Honsyu and the Oki Is., and NNE-SSW and E-W directions parallel to Tusima. On the continental shelf in the surveyed area, two sedimentary basins are found:one off Nagato with NNE-SSW direction and the other off Hamada with NE-SW direction. Burried platform of acoustic basement and dome-like seabottoms are seen on the Oki Spur (Fig. 3). Strata warp downward, causing slumping widely,on the lower parts of continental slopes (Fig. 6). The process of geological development can be summarized as follows: 1) After the deposition of E-layer, the basement platforms existing on the Oki Spur, around the Oki Is. and north of Mi Sima were formed. 2) Tectonic movement began after the formation of the basement, so that the lineation with NNW-SSE direction was formed at the western edge of Mi Sima, while undulation might be larger on the Oki Spur, forming small depressions where D-layer deposited. The succeeding movement became more activein the Oki Spur and caused large undulations of the basement. Depressions thus formed as well as the basement platforms were covered by C-layer. Off Hamada, on the contrary, the undulation was rather mild along the coast, producing several basins on which C-layer deposited. 3) Diastrophism became gradually quiet and continuous sedimentation developed B-layer especially on the basins and depressions which had been formed in the preceding age． 4) Quiet age still continued after the deposition of B-layer. Gentle uplifting in N-S direction took place broadly over the Oki Spur. 5) Continental shelf eroded repeatedly due to the alternation of lowering of the sea level through the glacial ages. The marginal plateau extending westward from the Oki Is. was carved and then submerged below the sea level. Finally through the erosion and sedimentation in the last glacial age, the continental shelf was developed to form the present topograhpy.