大荒沢層の積成機構にかんする問題

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The area along the Okoku Railroad Line is the type area of the "Green Tuff" In this area the "Green Tuff" are divided into two parts ; the lower division is called the Oarasawa Formation, the upper the Oishi Formation. The Oarasawa Formation which is the forerunner of the Neogene volcanism in the "Uetsu Geosynclinal Basin" consists of altered andesite or its pyroclastics and covers the pre-Tertiary rocks with unconformity-The pre-Tertiary rocks in this area consists of slate, sandstone, chert, schalstein, limestone, biotite schists, hornfels and granodiorite. The pre-Tertiary sedimentary rocks are intruded by granodiorite and altered to various kinds of schists or hornfels by contact metamorphism at the western part of Wakasennin The Oishi Formation which covers with unconformity the Oarasawa Formation consists of fine green coloured acidic or intermediate tuff breccia and pumiceous tuff intercalated with black siltstane. Peculiar facies just like volcanic canglomerate of altered andesite (propyrite) which are exposed along the highway at the northern part of the Yuda Dum covers with unconformity the pre-Tertiary grandiorite without basal conglomerate. The basal conglomerate of the Oarasalva Formation is developed only around the Takearasawa, a branch of the Wakagawa. It is within one meter in thickness and composed of rounded pebbles of chert and granodiorite and cemented by arkosic sandstone Contrary to the pebbles of the basal conglomerate, the. rounded blocks in the "volcanic conglomerate" ranges from 5 to 20 cm in diameter and consists almost wholly of propyrite The matrix of this "volcanic conglomerate" show the aspects of volcanic breccia or tuff breccia of altered andesite (propyrite) Angular fragments in this matrix are smaller than 5 cm and cemented with finer ashes of the same altered andesite There is a gradual transition between the "volcanic conglomerate" and tuff breccia and both facies intertongue several times vertically and in an extensive way horizontally If the rounded or subrounded blocks of more than 5 cm are removd from this "volcaic conglomerate" the matrix itself is analogous to volcanic breccia or tuff breccia seen in the Oarasawa Formation and should be considered to have originated from a pyroclastic viscous flow because the angular fragments of propyrite are intermingled at random in both size and orientation and shown no stratification The rounded or subraunded blacks, even of smaller angular fragments in the matrix, usually do not touch one another, and are separated by varying distances The rounded or subrounded blocks have Smoothly curved surface, sometimes, with slickenside on its surface and coated by thin films of chlorite There is no positive evidence of water wearing so that the matrix of the "volcanic conglomerate" originated from the pyroclastic viscous flow and the rounded blocks in it are quite different from the pebbles of the basal conglomerate both in lithology and sizes Under the micrascope the petrographic character of each rounded block, angular fragments in the matrix and propyrite lava itself show homogeneity. The concentric structures parallel to the external shape or porous or vesicular structure which indicate an origin as of volcanic bomb are absent in all the observed blocks. Autobrecciated facies of propyrite grade into volcanic breccia or tuff breccia in the lowest part of the Oarasawa Formation. Curtis (1954) and Carozzi (1960) have described on the interesting mechanism of autobrecciation of andesite lava. The viscous magma which supplied the andesite lava undergoes vesiculation along joints or fractures at or near the surface as a reduction of confining pressure. For a brief instant, confining pressure drops to almost zero along dilated joints, and gasses in the vesicles expand rapidly in the direction of reduction of pressure that they cause spalling of the viscous lava along both joint surfaces Continued spalling occurs by additional movements and thus the autobrecciation becomes completed. Autobrecciated lava, volcanic breccia and tuff breccia of the Oarasawa Formation may have been produced by the same mechanism as proposed by Curtis and Carozzi. The uniformity in the petrographic characters of the rounded blocks and smaller angular fragments in the matrix strongly suggests similar producting mechanism. Carozzi further described as follows. "Following autobrecciation, further movement of the mass results in additional pulverization by attrition whereas the larger blocks became somewhat rounded." Rounded or subrounded propyrite blocks produced by mutual abrasion was observed during the course of the geological survey around the southern part of Hirosaki The geological occurrence of them in this area are quite similar to the rounded blocks in the "volcanic conglomerate" in the lower part of the Oarasawa Formation not only in geological horizon but also in petrographic characters. Although positive evidence for mutual abrasion could not obtained from the observations on the rounded blocks in the Oarasawa Formation, it is most reasonable to assume that the rounded blocks of propyrite were produced by mutual abrasion after autobrecciation at same depth and then transported into the succeeding pyroclastic breccia flow. Sheared chlorite films around the rounded blocks and the slickenside on their surfaces apparently suggest mutual abrasion The facies allied to the "volcanio conglomerate" in the Oarasawa Formation are also observed at many localities in the "Green Tuff Region" in Northeast Honshu and are mostly limited to the lower half of the "Green Tuff". It is difficult to determine the structural pattern of Northeast Honshu, because the genetical relation between pre-Tertiary orogenensis and Tertiary orogenensis is not yet clarified. The general structural trend inferred from the axis of foldings and the zones of faults in the Tertiary sediments in Northeastern Honshu is prevailing NNE-SSW while the direction of NW-SE trend, which links the culmination of the geanticlines or the basins prevailing to the completion of the geanticlinal upheaval may be a manifestation of the movements of the pre-Tertiary segment under the Neogene sediments Especially, the lower part of the "Green Tuft" have been certainly been affected by the structural movements of the pre-Tertiary segments The first intense extrusion or fissure eruption of andesite (later altered to propyrite) may have been controlled by these two structural patterns, and then, result in the same mechanism of accumulation of propyritic pyroclastics simultaneously over large areas in the "Uetsu Geosynclinal Basin".

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