北部フォッサ・マグナ日影向斜地域の地質

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Large-scale and well-ordered folds are famous in the Neogene formations of northern Nagano Prefecture. The writers studied the stratigraphy and geologic structure of the Neogene formations in the Hikage Syncline, one of the large folds mentioned above. The results are summarized as follows. 1. The Neogene sediments in the Hikage Syncline attain about 5,000 min thickness, and are composed mainly of sandstones, mudstones and conglomerates. Some beds of pyroclastic rocks ranging from basic to intermediate rocks in chemical composition are found intercalated. 2. These sediments are divided, in descending order, into the following members upon the basis of rock facies. Hikages and stone and conglomerate member; Tanogashira mudstone member/Azumezawa conglomerate member; Idoirisawa mudstone member/Togakushi pyroclastics member; Hiratokozawa mudstone member; Kumanoiri sandstone and mudstone member. Among them the Tanogashira and Azumezawa and also the Idoirisawa and the Togakushi are nearly contemporaneous but heteropic in facies. 3. The age of the sediments, as a whole, ranges from the middle Miocene to the early Pleistocene. 4. The Hikage Syncline is a large syncline about 30 km long, and its axis runs from NNE to SSW. The Syncline is symmetric in its southernmost part, where the limbs on both sides of the axis are steep. It is, however, asymmetric in the northern half, where the axis runs from NE to SW. 5. Sedimentation was most active, in this area, during and after the Pliocene. It represents the prolongation or the succeeding phase of the northward migration of sedimentary basins which commenced in the south as the Komiji Syncline and migrated to the Takafu Syncline. The Komiji, the Takafu and the Hikage Synclines, from south to north, are nearly equal in scale, and are arranged en echeion with parallel axes.
日本地質学会の論文
1976-12-30

北部フォッサ・マグナ日影向斜地域の地質

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