Submarine geology of the continental borderland west of Hokkaido.
The westward beyond the continental shelf off the Japan Sea side of Hokkaido is the continental borderland which includes many banks, basins and troughs. The western margin of the continental borderland is the Okushiri Ridge, the western flank of which is the continental slope joining to deep basin of the Japan Sea. Reflection survey by continuous seismic profiler was carried out along all sounding lines. On the records of reflection survey, the lowest formation was the acoustical Basement which was acoustically opaque portion. The Basement distribute at north of the Cape Shakotan and west of the Cape Ofuyu; they are just both sides of the continuation of the Ishikari Lowland. Another distributions are seen in the two zones of WNW-ESE trend; Mashike-Otaru Bank and Teur-iIs.-Yagishiri Is. Musashi Bank. The lower acoustically opaque layer called the E Formation and correlated to the Masuhoro Stage (Mid-Miocene〕is superposed on the Basement. The sea of the E Formation was attained to the maximum extent. There were palaeo-islands in the environs of the Cape Ofuyu, Otaru Bank and Musashi Bank. The D Formation superposed on the E Formation is remarkable acoustically transparent layers. In the D Stage, the uplifts and subsidences were occurred in many places. Generally, the deposition of the D Formation had been continued over whole area especially in the Ishikari Basin and its environs, but following events had occurred; the subsidence and deposition along the inside of the Rishiri-Shokanbetsu Zone which is nearly sited at shelf edge, remarkable subsidence in the Ishikari Basin, and the depression of the Teuri Trough. The D Formation is presumably correlated to the Wakkanai Stage (Late Miocene to early Pliocene)． After the deposition of the D Formation, tectonic movements had occurred in many places; elevation of the Tengu-no-hana Zone, depression of the Rishiri Trough, elevation of the Okushiri Ridge, uplifts of the Shakotan Spur and Yoichi Spur, and general elevation of the Northern Musashi Bank including Rebun Bank. These movements were named the Tengu-no-hana Disturbance which resulted the differentiation and reducing of sedimentary basins. Upper acoustically opaque layers are divided into the C Formation correlated to the Koema Stage (Pliocene) and the B Formation to the Sarabetsu Stage (Pleistocene). They filled the differentiated basins where the C Formation was conformable at central part of basins and abutted to underposed sediments at marginal part of basins. However, the general subsidence occurred in the B Stage; the B Formation overlapped the underlying sediments Post the B Stage, movements completed the submarine configurations in today, and the faults and foldings of NNW -SSE to NNE-SSW trend. The tectonic movement was called the Musashi Bank Disturbance． Relieves after the Musashi Bank Disturbance have been buried by the horizontal deposition of the A Formation. During the A Stage, the Rishiri Shokanbetsu Zone has been developed and the Musashi Bank has changed to subside. In the end of the A Stage, it occurred the 140 m lowering of sea level in Wurm Glacial Age, and the truncations of banks and continental shelf. Since then, the Northern Musashi Bank and Tengu-no-hana have been tilting towards east, the continental shelf towards west and the Otaru Bank towards NW.
| Main Authors: | , , , , , |
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| Format: | Journal Contribution biblioteca |
| Language: | Japanese |
| Published: |
1973
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| Online Access: | http://hdl.handle.net/1834/16338 |
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