群馬県北部に分布する新第三系について : 特に中之条北部で発見された陥没構造とその意義
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概要
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The Neogenic Sarugakyo group, distributed at the northern part of Gunma Prefecture, is divided into the Tochikubo, Akasaka and Daido formations in ascending order. The Tochikubo formation is composed of tuffaceous sandstones and andesitic pyroclastics with lavas, which are partly welded. The Akasaka formation is divided into the lower and the upper parts. The lower part of the formation is composed of rhyolitic pyroclastics, tuffaceous sandstones and conglomerates. Ill-sorted basal conglomerates consisting angular to subangular gravels are developed in the lowest of the formation. Some molluscan fossils occur in the sandstones. The upper part is composed of acidic tuffs, andesitic welded tuff and tuffaceous sandstones, and partly contains iron sand beds. The Daido formation is composed of tuffaceous sandstones, mudstones, conglomerates and andesitic tuff breccias, and partly contains lignites. The Tochikubo and the Akasaka formations are correlated to the Kassezawa formation and are considered to be the middle Miocene. This group is presumed to have deposited under shallow sea to terrestrial environment from some fossils and lithofacies. The Kirigakubo welded tuff formation unconformablly covers the Sarugakyo group and is composed of rhyolitic to dacitic welded tuff and is unconformablly overlain by the formation consisting andesite lavas and tuff breccias. It is considered that the basal conglomerates of the Akasaka formation have deposited in the peripheries and bottoms of the collapse basin, which was formed by depression caused by the fracturing after deposition of the Tochikubo formation. Relation between the lower part of the Akasaka formation and the Tochikubo formation is abut unconformity or faulting. Width of the collapse basin is about 5 km and 3 km in NS and EW directions respectively. Maximum depth of the basin may be about 500 meters. Peripheries of the basin display a polygon enclosed by the straight lines, and accompanied with numerous faults of NWW-SEE and NNE-SSW directions. The Akasaka and Daido formations show semi-dome structure. Andesite dikes and sheets such as the Arikawadake, Namezawa and Nakutafuji bodies intrude the Sarugakyo group and the Kirigakubo welded tuff formation.
- 1981-09-25
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