The Subsurface Geological Structure of the Alluvial Plain of Muroran, Hokkaido
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概要
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The general features of the Quaternary sediments and the post-Tertiary valleys buried in the Alluvial plain of Muroran based upon the data from the 94 bore holes, standard sounding tests and geoelectrical observations are summarized below. 1. The Quaternary sediments in the present area can be classified into six units : a-I, a-II, b, c, d-I and d-II, from the younger to the older. 2. a-I unit (Alluvium) consists of soil, clay, silt, fine and medium grained sand and peat which comprises the major part of this unit. The unit is 2-4 meters in thickness. The peat bed is 0.5-4 meters thick and occupies the larger part of the area. 3. a-II unit is the Alluvial river deposits and consists mainly of round or subround boulders and pebbles of andesite, 2-4 meters in thickness, and is distributed along the Washibetsu and Chiribetsu-rivers. 4. b unit, the major part of the Quaternary sediments in this area (Dilluvium?) consists mainly of pumiceous, fine and medium grained sand intercalated with silt layers containing semicar-bonaceous matters and indeterminable shell fragments. The age of this unit may be Dilluvium judged from the subsurface stratigraphical features and the N value of the standard sounding test which exceeds 15. The maximum thickness exceeds 24.5 meters in the southern part of the present area, where, the lower limit was not confirmed at the depth of 30 meters from the ground surface. 5. c unit (Dilluvium?) consists mainly of clay and silt. It overlies the basement Tertiary formations with unconformity, and is conformable with b unit. The average thickness is 3-8 meters but attains 10.3 meters in maximum. 6. d-I and d-II units are the weathered part of the Dilluvial Washibetsu-dake agglomerate. The former unit consists mainly of pumiceous silt and the latter mainly of volcanic breccia. In the present area, these units, totaling 4-8 meters in average thickness are distributed in the northern hilly area. 7. One of the notable features of the subsurface relief of the upper bedding plane of the basement Tertiary in the present area is the Y shaped buried valley, the main branch of which extends in NE-SW direction along the Itanki-hama coastal line and opens to the Pacific Ocean. The height of the valley bottom is 20 meters below sea-level, and its depth from the ground surface exceeds 25 meters. Another feature is the subsurface flat plain situated between the two branches of Y shaped buried valley, here, the base of the valley is 0-10 meters in height, and its depth from the ground surface is 3-10 meters. The pre-peat bed subsurface relief is much different from that of the basement Tertiary. The outstanding feature of this subsurface relief is the buried narrow ridge in the eastern part of this area extending in nearly N-S direction and the soft sediments (a-I unit) are less than one meter in thickness and distributed without the development of the peat-bed. Another feature is the broad subsurface flat plain in the northern part of this area, its average height is about one meter above sea-level, and its average depth is 1-2 meters from the ground surface.
- 東北大学の論文
- 1973-02-28