長野県北部荒倉山周辺の層序および地質構造
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概要
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In the area around Mt. Arakurayama the Cenozoic strata ranging from the middle Miocene to the Pleistocene are developed with a total thickness over 8,000 m. Stratigraphy and geologic structure of the strata are summarized as follows. 1. The strata are classified into five formations in ascending order, namely the Aoki, the Ogawa, the Shigarami, the Sarumaru and the Toyono formations as shown in Table 1. The middle Miocene Aoki formation and the upper Miocene Ogawa formation consist of alternation of sandstone and mudstone. The Pliocene Shigarami formation is composed mostly of thick series of mudstone, conglomerate, sandstone and andesite. The Sarumaru formation, the uppermost Pliocene to lower Pleistocene, consists of sandstone, conglomerate and acidic tuff. The middle Pleistocene Toyono formation consists of andesitic pyroclastics and lake deposits, and covers the lower formations with clino-unconformity. 2. The andesitic rocks of the Arakurayama member, the middle part of the Shigarami formation, consist of augite andesite, hornblende andesite and two-pyroxene andesite. In the former two types of andesite hornblende megacrysts and gabbroic inclusions are included. The Arakurayama member is underlain by the lower members with clino-unconformity in the western part, but with conformity in the eastern part. The erosion loss of the sediments is estimated stratigraphically to be over 1,800 m. 3. The principal geologic structures are trending in N-S to NNE-SSW directions in the southern part, and in an ENE-WSW direction in the northeastern part. Two orders of folds are distinguished, while the faults are classified into three types, namely a strike fault, oblique faults and transverse faults. 4. The Takafu syncline is an elongate basin extending from SSW to NNE, and the axis of the Orihashi syncline extends from WSW to ENE, and plunges to ENE. Consequently these two synclines are not parallel but oblique to each other. In the domain of the Takafu syncline the Ogawa formation and the lower part of the Shigarami formation are most extensive. The sedimentary basins of these formations are inferred to have migrated toward the north during an age from the late Miocene to the Pliocene to display a mode of the so called "sedimentary imbricated structure". On the other hand, in the domains of the Orihashi syncline and the Kitago syncline, the middle and upper parts of the Shigarami formation and the Sarumaru formation are developed, and the sedimentary basins are considered to have migrated east-northeastward. These sedimentary basins were generated with the intense volcanism of the middle Pliocene Arakurayama member.
- 日本地質学会の論文
- 1976-12-30
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