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AbstractThe investigated area includes the continental shelf and continental slope off the coast from Kuji to the Nemuro Peninsula. The survey was carried out by continuous seismic profiler and echo sounder, spacing normally two nautical miles. The acoustical strata are divided into the As, Bs, Cs, Ds, and Es Formations in descending order off the northern part of Sanriku District and southern coast of Hokkaido ; each formation shows relationship of disconformity to lower formation. On the other hand, the AK, BK, CK, DK, Upper EK and Lower EK Formations are recognized off Tokachi, Kushiro and Nemuro Districts. Each formation superposes lower formation with disconformity. The disconformities between the Upper and Lower EK Formation, and the CK and DK Formations are remarkable. The ages of acoustical strata were estimated as follows ; Sanriku-Hidaka Districts As ... Quaternary Bs ... Pliocene and late Miocene Cs ...late Miocene Ds…middle Miocene Es ... early Miocene and pre-Neogene Kushiro District Ak ... Quaternary Bk ... same as above Ck ...Pliocene Dk ...same as above Ek ... Miocene EK. (lower) ... pre-Neogene General strike of the geological structures in the area is divided into three groups, namely N-S (NNE-SSW to NNW-SSE), N W-SE (WNW-ESE), NE-SW (ENE-WSW). The first predominates on the continental shelf and the marginal plateau off the northern part of Sanriku and Oshima Districts. The second predominates on the area of the continental shelf in the environs of the southern part of Hidaka Range and the deep sea terrace including Hiroo spur. The third predominates on the continental shelf and slope area off Kushiro and Nemuro bordered by the Kushiro submarine Canyon. The authors assumed the process of development of geological structures as follows ; In the continental shelf area off the northern part of Sanriku District, there occured a crustal movement accompanied with uplifting after the deposition of the Ds Formation and a movement occurred after the deposition of Cs Formation again. A crustal movement of folding accompanied with uplifting after the deposition of the Cs and lower part of Bs Formations an effect on the area of the outer edge of marginal plateau along the Oshima Peninsula. In the area off Hidaka District along the west side of Hidaka Range, there was a crustal movement of uplift accompanied with folding after the deposition of the Cs Formation. Consequently,the Hidaka Trough area became a synclinal sedimentary basin where the Bs and As Formations successively deposited. On the other hand, in the area located eastward of the Hidaka Range, there was a conspicuous crustal movement after the deposition of the Lower EK Formation, then the Hiroo spur off Tokachi District and the folding of the continental shelf area of Kushiro and Nemuro Districts were originally formed. After the deposition of the Dk Formation in the area between them, the crustal movement occurred again to form the original shape of the Kushiro Submarine Canyon and the deep sea terraces. The Ck, Bk, and Ak Formations successively deposited in the area of the deep sea terraces accompanying with some crustal movement.
JournalReport of Hydrographic and Oceanographic Researches