日本海南東部の海底地質

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The submarine geology of the sea area extending from the Oga Peninsula, Akita Prefecture at the north southwards to off the San'in Region in the southeastern part of the Japan Sea was studied. The data from the mentioned sea area were collected by the surveying ships of the Hydrographic Division, Maritime Safety Agency, the Ministry of Transportation, supplemented in part with the data of the surveying ships of the Regional Fisheries Experimental Stations. During the surveys, echo soundings and bottom-penetrating soundings were made in parallel with dredge and coring operations. The precise localities of each items mentioned above were plotted on the newly constructed configuration maps (Figs. 2 and 3). Particular emphasis was given to the descriptions, analyses of the bottom data, bottom sediments, configuration and interpretation of the continental shelf of the area investigated, the continental slope, banks, marginal plateau, basins and troughs, and the submarine canyons and valleys. Based on the morphological divisions (Fig. 4), the submarine geology of the southeastern part of the Japan Sea can be summarized as follows. 1. Bordering the land, the continental shelf stretches through the investigated area (Figs. 5, 6, 7, 9 and 10). Flat submarine terraces and valleys incised into them are found on the surfaces of the continental shelf (Table 1). Rock bottom and coarse sediment areas are found with relation to the distribution of the terraces or valleys, and along the shelf-break, some of them show the locations of the ancient surf zone (Figs. 11, 13, 17 and 18). This surf zone seems to have been developed mainly during the fluctuating sea level controlled by glacial eustasy. The depth of the shelf-break is about 140 meters in Northeast Honshu and partly 200 meters in Southwest Japan, and there the regional displacement of the shelf topography is due to the crustal movement of the basement (Fig. 19). 2. The remarkable marginal plateau extending off Southwest Japan consists of several flat surfaces which are developed at levels deeper than the continental shelf (Figs. 4 and 20). At least, some terraces on the plateau which are thought to have originally been of the same level seem to have been levelled during the Pliocene to the Middle Pleistocene age, and thereafter dislocated to different levels by the differential movement of the structural block. 3. The oldest rock forming the banks in the southeastern part of the Japan Sea is probably the gneissose granites of the Yamato Rise (Table 2). On the rise and banks off Southwest Japan there are developed Late Mesozoic igneous rocks with welded tuff and ash flow (Figs. 35, 37 and 38). The bank tops off Northeast Japan are covered mainly with Late Tertiary sedimentary rocks (Fig. 34). Many banks are situated side by side and form the range parallel to the Honshu arc, therefore it is inferred that the bank-ranges or ridges indicate same geanticlinal structure analogous with the mountain ranges of the mainland. It seems that the geomorphological features of the ridges were brought into existence since the end of the Tertiary on the whole. Indications of structural movement had already commenced and continued through the Tertiary, especially in Southwest Japan where certain areas had emerged from the sea during geological development. 4. The formation of the depressions of basins and troughs (Fig. 42) are closely related to the building of the bank-ranges (Figs. 45 and 46), and conspicuous depressions since their birth have been filled by sediments measuring several hundred meters in thickness. The submarine fans or deltas developed along the continental slope, and floor of the basins tend to become deeper from the land side to the sea area, in general (Fig. 43 and 44). It seems that the chief source of the detrital sediments was from land, and an abundant supply of the sediments was through the submarine canyons. 5. There are many kinds of submarine canyons, such as slightly indenting the continental shelf, straightly cutting down the slope of the bank or the shelf, and engraving the floor of basins and troughs (Fig. 49). At the terminal part of the canyons there are developed submarine fans. The deep-sea channel of a few hundred meters in depth meanders strongly, and along the stream there are found the development of submarine natural level and delta-like topography. Generally the bottom sediments of the submarine canyons and the surrounding area are coarser than in other areas (Fig. 47 and Table 4). It seems that the submarine valleys such as incising the shelf or its slope were formed by erosion on the land, and the others engraving the basin floor and having long length were formed mainly by the turbidity currents. In conclusion, the framework of the present major geomorphological features, such as bank-ranges, basins and troughs, and the continental margin, became arranged since the end of the Tertiary, probably during the Early Pleistocene age, and the minor features on the shelf were developed mainly during the intermittent rise and fall of sea level in relation with the phenomena of the glacial age.
東北大学の論文
1968-11-30

日本海南東部の海底地質

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概要

東北大学 | 論文

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