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Stratigraphical and Sedimentological Studies of the Paleogene System of the Shimanto Complex in the Shishikui-cho Area in Tokushima Prefecture, the Northeastern Part of the Muroto Peninsula

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  • 室戸半島北東部,徳島県ししくい町周辺の四万十累層群古第三系――層位学的・たい積学的検討
  • 室戸半島北東部, 徳島県宍喰町周辺の四万十累層群古第三系:層位学的・堆積学的検討
  • ムロト ハントウ ホクトウブ トクシマケン シシクイチョウ シュウヘン ノ シ

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Abstract

The Paleogene system of the Shimanto supergroup in the shishikui-cho area is divided into the Kaifu and Naharigawa formations, separated by the Shishikui fault. The Kaifu formation is composed mainly of flysch-like alternation of shale and sandstone, shale, and green tuffaceous shale in the lower, and massive thick sandstone, flysch-like alternation of sandstone and shale, and shale in the upper. The total thickness is about 3,000 m. The Naharigawa formation is composed of massive sandstone, flysch-like alternation of sandstone and shale, shale, and conglomerate. The total thickness is about 4-,000 m. The age of the Naharigawa formation is assigned to Oligocene from molluscan fossils. The kaifu formation is probably Eocene in age. The conglomerates of the Naharigawa formation are composed mainly of quartz, chert, acidic igneous rocks, sandstone and shale. Some of the conglomerates contain orthoquartzite graveles at the rate of 4-7%. Massive sandstones of the Kaifu and Naharigawa formations are composed of quartz (about 44%), feldspar (about 25%), rock fragments (about 11 %) and matrix (about 15 %). These sandstones are distinct from those of the Cretaceous Mugi formation distributed to the north of the Kaifu formation. 214 paleocurrent indications were measured at 59 localities. The main paleocurrent was from east to west (longitudinal), but in some parts, lateral current from southeast to northwest was dominant. The conglomerates containing orthoquartzite gravels are dominant in the horizons of such lateral current. This indicates the existence of the “Kuroshio Paleoland” to the south of the basin, which is considered to have been occupied by granitic and acidic volcanic rocks, along with pre-Cambrian orthoquartzite, as judged from gravels and sand grains.

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地質学雑誌. 82(6):383-394 (1976)

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