18. 静岡縣庵原郡兩河内村附近の地質構造
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In this paper the writer describes the stratigraphy and geologic structure of the western Ibara district, northeast of Sizuoka city, which lies on the upper course of the river Okitu. Since the district in, question is near the southwest end of the so-called. Itoigawa-Sizuoka tectonic line, in order to study its geologic structure, it is necessary to understand the geologic nature of the whole of this tectonic region. For convenience in describing the succession of strata, the writer divides this districts into three tectonic provinces, all bounded by meridional faults the three provinces, being the Ryuso, the Ryokoti, and the Takane mountainlands. Stratigraphically, the Ryuso mountainland, which is the most western of these provinces, consists, in descending order, of Alluvial deposits, namely, the Ryuso alternation of sandstone and mudstone beds, the Takayama coarse black sandstone beds, and the Kaibusi mudstone beds, all of which, excepting the Alluvial deposits, are stratified conformably, and are intensely folded and overturned, as shown in the geologic section, Fig.10. The eastern. margin of this province is bounded by reversed faults(Kaibusi-Isabu fault and Katase-Maesawa fault) with the Maesawa fractured zone. The Ryokoti mountainland consists stratigraphically, in descending order, of Alluvial deposits, river terrace deposits, the Saginota gravel beds, the Kawaaino conglomerate beds, the Hinokimura mudstone beds, the Tozurasawa pyroclastic beds, the Do coarse sandstone beds, and the Sugiyama mudstone beds. The Kawaaino and the other older beds in this province, form an asymmetrical syncline, with its axis trending N 30° W. The eastern margin of this tectonic province is bounded by the Nakakoti-Sodesi faulted zone. The most eastern province, the Takane mountainland, consists of the Seikenzi group and the Kogoti mud beds, the former being subdivided into two main parts, the lower Takane pyroclastic beds and the upper, the Takane sandstone beds. The Seikenzi group which, monoclinally, dips westward may be contemporaneous with the Hamaisidake group exposed in the eastern Ibara district, the geologic age of which may be Lower Pliocene, paiaeontologically, speaking. A geologic history of the respective provinces as inferred from this study, follows:- The Ryuso mountainland. 1. Deposition of the Kaibusi mudstone beds. Marine conditions were calm, but water deep(Miocene). 2. Deposition of the Takayama sandstone. Volcanisms were active during the early part of this stage, Clastic materials of this sandstone derived from the western Mesozoic land. 3. Flysch type deposits of the Ryuso beds(Miocene). 4. Intense folding(probably late Miocene). 5. Faulting and probably folding(later half of Miocene). 6. Denudation(latest Pliocene). 7. Dissection(post Pleistocene). The Kogoti mountainland. 1. Deposition of the Sugiyama mudstone beds(Miocene). 2. Deposition of the "Do" sandstone and conglomerate. 3. Volcanic activity of the Tozurasawa igneous mass. 4. Deposition of the Hinokimura mudstone. 5. Deposition of the Kawaaino conglomerate. 6. Formation of an asymmetrical syncline(later half of Miocene). 7. Denudation(latest Pliocene). 8. Dissection(post Pleistocene). The Takane mountainland. 1. Deposition of the Kogoti mudstone beds(late Miocene). 2. Deposition of the Takane pyroclastic formations. Volcanism was active during this stage. 3. Deposition of sandy materials of the Takane formation(Pliocene). 4. Faulting and folding(later half of Pliocene). 5. Denudation(latest Pliocene). 6. Dissection(post Pleistocene).
- 1944-08-15
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