群馬県西部における後期中新世の陥没構造 : 五輪岩付近の上部中新統の地質学的研究

概要

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The upper Miocene series mainly consisting of pyroclastic rocks are well developed in the Gorin-iwa area, western part of Gunma prefecture, Japan. Stratigraphic succession of the studied area is shown in Table 1. The lacustrine Sakamoto formation unconformably overlies the mostly marine Yoshii and the Itahana formations, partly with a distinct relationship of high-angle abutting. Basal conglomerate of the Sakamoto formation is containing poorly sorted, angular to rounded pebbles and boulders. Angular clasts are amphibolite, rhyolite, chert and mudstone in lithology, which must have been derived either directly from the underlying formations or from the pre-Neogene series. Whereas rounded clasts may have been reworked from the conglomerate layer within the underlying Itahana formation. These facts may strongly indicate that there existed highly rugged, steep mountains along the margin of the basin where the Sakamoto formation was deposited. The authers further believe that the basin was caused by a collapse, although actual fault system has not been detected as yet.
日本地質学会の論文
1979-03-30