群馬県北西部の吾妻川中流域に分布する新第三系 : 特に中新世後期の陥没盆地について
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概要
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The stratigraphic succession of the Neogene system in the investigated area is as follows; [table] Strata of the middle Miocene Sawatari Formation were deposited in marine condition. On the other hand, the Negoya welded tuff Member and the Agatsuma Formation were deposited on land. The total thickness of them is over 1500 meters. The Agatsuma Formation abuts on the basement rocks with high angled unconformity plane. Its unconformity line is straight and strike NNE-SSW and WNW-ESE in direction. Poorly-sorted conglomerates which are mainly composed of angular boulder gravel at attaining 11 meters in diameter are often developed in the base of the Agatsuma Formation. On the basis of these facts, it is suposed that the Agatsuma Formation was deposited in a kind of the collapsed basin (Fujita,1973). This basin is considered to be contructed by severe faulting which had existed before the deposition of the Agatsuma Formation. Moreover, many of the faults extending NNE-SSW and WNW-ESE directions are developed near the margin of this basin. The lower member of the Agatsuma Formation was often deposited in waters. The depressed amount of the collapsed basin was about 300 meters in maximum. Many intrusive rocks composed of the volcanic rocks and hypabyssal to plutonic rocks are intruded into the Neogene system developed in the investigated area. The former rocks are composed largely of andesites with minor amounts of rhyolites and basalts, and the periods of intrution are divided into the pre-deposition of the Negoya welded tuff Member and the post-one of the Agatsuma Formation. The latter is composed of diorites, quartzdiorites and porphylitic granodiorites.
- 地学団体研究会の論文
- 1986-07-25
著者
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