瀬戸内南岸沖積平野の地質学的研究
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概要
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Stratigraphical and geological studies were made of the Alluvial deposits distributed in the plains bordering the northern coastal area of Shikoku or the southern coastal region of Setouchi. The deposits covering the high-, middle-, and low level terraces and the alluvial plains were studied from the viewpoints of stratigraphy, geomorphology, sedimentology, paleontology of the cores drilled in many parts of the different plains, electrical prospecting, data obtained from the many deep and shallow wells drilled for the purpose of developing underground water, sedimentological studies on the cores obtained from the wells drilled, submarine configuration as related with the topographical features of the buried valleys, and others. Geomorphologically three terraces were distinguished in the foreland of the Sanuki Mountain Range, namely, high-, middle-, and low. These three terraces are covered with deposits consisting of sand and gravel in which sometimes drift woods are found. Also incipient sedimentary structures as scouring, weak cross-laminations and others were observed in the deposite covering the terraces as well as in the Alluvial deposits covering the plains. The terraces are in three levels, the higher of 300-100 meters, the middle one of 100-60 meters and the lower of 60-10 meters in respective heights. The deposits covering them are characteristically of weathered gravels of different kinds and thickness, thus enabling distinction between them. There is a remarkably thrust fault between the lower and middle terraces, thus proving that the Rokko Movement, the name of which is taken from the characteristic thrust fault movements in the Rokko Mountains near Kobe, had considerable effect in the present area. The Alluvial deposits developed and distributed in the Takamatsu-, Saijo-, Marugame-, and other plains befronting the Sanuki Mountains were deposited in buried valleys on the one hand and on the basement rocks consisting of the Ryoke Granite, Izumi Sandstone of Cretaceous age and also partly on the Mitoyo Group, a lacustrine deposit of Pliocene age. These alluvial deposits show considerable variation in vertical and horizontal distribution, particularly in their thickness and compositions, although there is a general tendency of the deposits changing from thick gravels and sands in the lower, upwards into silty or muddy sediments then again in sandy and gravelly deposits. Sedimentary cycles are developed below the plain surface. At some places two sedimentary cycles can be distinguished although at other places only one. These diffreences in number of sedimentary cycles have relation with the thickness of the alluvial deposits at those localities and also with the crustal movements of local nature. In other words, there is good correspondence between the thick-ness of the deposits, number of sedimentary cycles, local crustal movements and relation with the positions of the buried valleys. The buried valleys developed under the plain show that the courses of the present drainages do not correspond to those of the previous ones on the one hand and to that the significant and abrupt change in the courses have intimate relation with the development of the major and minor faults of roughly N-S direction, by which their courses have been displaced, and also the minor faults of approximatelly E-W direction which are related to the depth of incision of the buried valleys and to their general trends. These faults, particularly the ones of N-S direction have relation with the thickness variation of the Alluvial deposits covering the plains in that there is a distinct flexure-like structure within the buried valleys, and by which the thickness of the alluvial deposits show abrupt change in their thickness. The sediments filling the buried vallyes and making up the plains of the southern border of the Setouchi sea are of lacustrine and marine origin, being of the former in the lower part and of the latter in the middle to upper parts, although the uppermost part seems to be deposited during a regressive phase. The kinds of sediments related intimately with the thickness and extension in east-west direction of the lacustrine and subsiding conditions, then to the transgressive to more or less stable phase, then to the regressive phase corresponding respectively to the local crustal movements, which seem to be still active to the present day. As shown in the correlation table the three terraces in different heights and the sediments filling the buried valleys and making up the different plains have analogous features throughout southwestern Japan, and that these features can be correlated with deposits of similar age distributed in Central and Northern Japan. That is to say, the Alluvial deposits where developed regardless of area always begin with a thick gravel facies intermixed wih sandy sediments, thus pointing to a trans-gressive phase or to lacustrine deposits of marginal facies and to a gradually subsiding stage. This phase is succeeded as shown in the present area by silty sediments which may contain fossils of marine diatoms or shells frequently intermixed with pollen remains, and all of which suggest continued subsidence followed with a rather stable phase, after which again there occurred gradual emergence corresponding to the regressive phase and deposition of gravels and sandy sediments. This just mentioned feature can be recognized widely in the Alluvial age of the Japanese Islands. However, in the present area the most outstanding difference compared with other areas of the same age is the significant local crustal movement. This movement shows or at least suggests that the stability of the marginal areas in Southwest Japan were subjected to movements not so pronounced in other parts of the Japanese Islands.
- 東北大学の論文
- 1972-03-10