グリーン・タフ積成盆の発生期にまつわる二,三の問題 : 山陰・松江湾入部を一例として

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The Neogene Tertiary system, so-called Green Tuff Formation, is widely distributed in the Matsue basin, which is one of the Neogene Tertiary sedimentary basins in Shimane prefecture. The lower Green Tuff Formation, composed chiefly of andesitic and dacitic volcanic rocks, and abuts against the basement rocks. Some particular basal conglomerate beds of the lower Green Tuff Formation develop the marginal area of the present basin, and their characteristic features are shown as follows: (1) They consist mainly of the debris and the fine rock fragments derived from the adjacent basement rocks. (2) The volcanic rocks subsequent to the accumulations of the present basal conglomerates abut against the conglomerates. (3) They are distributed in the marginal parts of the present basin. (4) The basement rocks are unconformablly covered with the present basal conglomerate beds at high angle (70°-80°). The present beds seem to be accumulated originally with an incline of 30 degrees or less. (5) They are usually massive and are poorly sorted by water. (6) The rock fragments in them show angular and subangular form, and are irregular in size ranging from granular to boulder. (7) Some reversal grading-structures in them are recognized at a few outcrops. The above-mentioned basal conglomerate is newlly named "Talus Basal Conglomerate", because its litho-facies approximately similar to that of talus deposits. The talus basal conglomerates seem to have been deposited under the existence of some steep cliffs in the environments of the sedimentary basin in its earliest stage. The sedimentary basin may have resulted from depression caused by faulting. In the investigated region, the talus basal conglomerate beds show their developmentat two horizons. The lowerbeds are recognized only in the south region. Although the upper beds cover unconformablly the basement rocks in the north, they develop nearly conformablly on the volcanic rocks abutting against the lower beds in the south. Judging from these geological evidences, the depression activities by faulting may surely be considered to have occurred at least two times: the first time in the south and the second time in the north. Furthermore, it can be clearly said that the depression took place prior to the volcanic eruptions in the present sedimentary basin.
日本地質学会の論文
1973-07-01

グリーン・タフ積成盆の発生期にまつわる二,三の問題 : 山陰・松江湾入部を一例として

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