著者

加賀美 英雄 満塩 大洸 野田 耕一郎

出版者

城西大学

雑誌

城西大学研究年報. 自然科学編 (ISSN:09149775)

巻号頁・発行日

vol.18, pp.9-28, 1994-03

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Tributaries of the Shimanto River have a north-south trending stream starting from the watershed of the Shikoku karst plateau, a part of the Shikoku coastal mountains, but do not end at the nearby coast of Tosa Bay. Because a small ridge (Rokusen-san, 507 m) along the coast of Okitsu Peninsula prevented the tributaries to cross over, the Shimanto River made a westward detour into the mountainous region for more than 100 km. This study shows new evidences on eastward flow of the ancient Shimanto River : 1) the inclosed meander of Matsuba River comes close to the coast, that of Ihoso River almost contacts to the same contour line of the coastal valley of Iyoki River, and that of Yusuhara River continues to the coastal valley of Iyoki River without interruption : 2) the lower gravel bed of 50 cm thick is found at Hirose meander to show a eastward dip-strike. The gravel bed correlates to the pre-terrace deposit : 3) a mudstone bed of lake-deposit origin is found in an abandoned meander at Satokawa. It deposited after closure of eastward flow by uplifting of the coastal ridges. The measured dip-strike of the bed indicates eastward flow. Uplifting of Okitsu Peninsula was the main reason for westward migration of the Shimanto River. The uplifting zone was formed between earthquake faults, which was clearly shown between the Tosa basin and Nakamura basin during the last Nankaido Earthquake in 1946. Often the weak zone is associated with protrusion of igneous rocks such as Kashiwa-shima/Okino-Shima uplifting zone and Ashizuri uplifting zone. Rejuvenation of the Shikoku peneplain started at round 0.7 Ma B.P. and the uplifting of the Okitsu Peninsula occurred sometime later.