塩原化石湖の地史に就て

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Since first treated by A. G. Nathorst in 1883, Shiobara Lake Deposits have been investigated and reported on by many persons, but the data concerning their stratigraphy and sedimentation are few. The present witers, paying attention to this point, will attempt to describe the subdivision and facies changes of the lake deposits and their relations with Takahara volcanoe adjoining southward, and state the geological history of the lake from the begining of its basin to its extinction. 1. The Shiobara Group overlies unconformably the basement rock of Fukuwata and Kanomatazawa Groups, and moreover it has been found, as the result of the recent investigation, to cover with uncomformity Furumachi agglomerate (newly named) which is also of the older Takahara Volcanic Period. The Furumachi agglomerate is related unconformably to Fukuwata and Kanomatozawa Groups. 2. The lake deposits are divided into two classes, namely pelitic formation which occupies the central area of the lake and psephitic and psammitic formation occupying the marginal area, these being a set of lacustrine facies, begin with basal conglomerate. In general, the former is pelitic rock with lamina in it and the latter is coarse grained rock chiefly constituted with basemental and pyloclastic materials. These two are contemporaneous heteropic facies. 3. Newer Takahara volcanic materials lie in the lake deposits, and to the two formations of the pyloclastic materials which are closely related to the formation process of the lake deposits, the writers here propose Amayu mud-flow and Nakashiobara agglomerate. Especially the latter forms the basement rock on the western side of the area of the lake deposits, so by this agglomerate the pelitic rock can be divided into the upper and lower formations. 4. The lower formation (Sumaki siltstone formation) was built in the first order lake-basin (the eastern area) and separated bv the Amayu mud-flow into the upper and lower members (Su and SL) The upper formattion (Miyajima mudstone formation) is also divided by the M_M bed, showing the time-rock unit into Mu, M_M and M_L, these are further divided into Mu 1-3, and M_L1-3, according to their rock facies. Su is deposited near the middle part of the eastern area and M_L and Mu are deposited near the middle of the western area and both of them are present in laminated beds, which constitute the typical formational facies of pelitic rock. 5. Amayu mud-flow dammed up the south-eastern part of the area and raised the lake level at the time of Sumaki siltstone formation and deposited the laminated pelitic rock of Su member. Nakashiobara agglomerate, as far as the lake deposits were concerned, staged the largest volcanic activity. Its eruptive materials flowed into and acumulated in the lake basin, bringing a remarkable change of the shape of the lake basin. This built a flexure structure in the middle of the area or near Nakashiobara. The central part of the lake area moved from east to west and formed the second period lake basin with the western part as its center. 5. The lake became a narrow ditch between the mass of the accumulated debris of the Nakashiobara agglomerate eruption and the mass of the basement rock to the west of the area. The writers consider that the Shiobara Fossil Lake perhaps had its greatest depth then. In the latter part of this period M_L and Mu accumulated laminae, together with volcanic fine materials, diatom relics and pelitic sediments, and in the area centering on Siran valley lot of plant fossils, insects and other animal remains were at last accumlated. 7. The new slope of the mass made by the accumulation of Nakashiobara agglomerate in the south of the area extends in a tongue form from north to south, and down at the lake bottom in the western area the slope decreases almost to level. Over this new slope aquatic sliding was apt to happen by slipping or creeping with the progress of sedimentation often conbined by the later volcanic action. The new slope is regarded to be identical with the angle of repose of the coarse grained materials. Thus an intra-formational disturbance was caused in the lake deposis. The anomalies of intra-formational disturbance generelly reveal analogous forms in the same or similar rock facies and these seems to be simultaneity as regards time. 8. The lamination recalls us valved clay and it carries rich plant fossils, which constitute the typical characteristic of the Shiobara Fossil Lake, we can see neither change of species through horizons, seasonal variation as seen in valved clay, nor change owing to the amount of rainfall. 9. The formation of the lake basin, as is stated by H. Yabe (1928-9) and H. Niino (1933), is considered to have been caused by faults, but whether by continuous faults or by more than two of fault-groups is not known. The older Takahara eruptive materials participated in this faulting movement. The lake deposits, reserved on the kettle shaped lake basin bordered by the fault line, reveal strikes and dips adaptsd for the basement form. The dip gradually becomes low angle from the margin to the central area and in the latter becomes almost horizontal. Irregular strikes or dips, which we see occasionally, are reflected by local intra-fromational disturbance controlled by reliefs of the basemeut. The sediments also show the universal character of shifting from coarse grained materials to fine grained, and the shifting generally occurs suddenly. The lake basin is gradually entombed, but in addition to the change of the lake basin and lake area caused by the Amayu mud-flow and Nakashiobara agglomerate in the sedimentation, the volcanic materials which were thrown or flowed into the lake basin reached a considerably great quantity. Thus the lake deposits were formed and changed in close connection with the volcanic activity, and at the end of sedimentation the lake itself disappeared, leaving the top conglomerate behind. 10. Terrace deposits, composing the terrace, are made unconformably over the lake deposits. On the top of these, talus deposits, calcareous sinters and alluvium are deposited. 11. The geological age of the lake deposits is considered probably to belong to I_2-J_1, judging from the plant fossils, geotectonics and from the correlations between the pyroclastic materials of the lake deposits and those of their adjoining area.
横浜国立大学の論文
1956-10-05

塩原化石湖の地史に就て

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横浜国立大学 | 論文

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