日本における段丘・低地研究の現状と問題点--日本第4紀地図の作成を通して (日本第4紀研究の諸問題--第4紀地図の作成過程から<シンポジウム>)

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The purpose of this paper is to review recent progress and problems in the study of terraces and lowlands in Japan, as revealed during the preparation of Quaternary Maps of Japan (1:1, 000, 000, 1987), published in celebration of the 30th anniversary of Japan Association for Quaternary Research. Special attention is paid to a comparison of recent research with that undertaken about 30 years ago when the Association was established.Mapping of terraces and lowlands has progressed rapidly by means of interpreting aerial photographs, ca. 1:40, 000 or 1:20, 000 (or larger) in scale, which now cover all of the Japanese islands. Distribution of terraces and lowlands shows marked regional and local variation more than expected, mainly reflecting the different nature of Quaternary vertical movements. Extensive terraces have developed in the areas which are underlain by thick Neogene and lower Pleistcene sediments, in other words, the areas that changed from subsidence to uplift some time during the Quaternary. At the same time, there are still great differences in the number and accuracy of the papers which discussed these landforms. With the development of techniques for detailed mapping of terraces, the reconsideration of their origin and significance becomes necessary in some cases; for example, “low relief erosion surfaces” of Noto Peninsula are now considered to be remnants of a flight of marine terraces, and it requires a considerable change of interpretation of paleogeography since the middle Pleistocene. Small remnants of marine terraces have been found along the complicated shoreline of ria coasts such as Sanriku, eastern Shikoku, and others, indicating that the formation of the original configuration of the ria type shoreline should be dated back to the middle or late Pleistcene.One of the important advances in the study of terraces and lowlands over these 30 years is the increased information on radiometric dates of late Quaternary terraces that became available in the 1970s with the use of 14C, U-series and fission-track methods as well as the recently developed ESR method. Dating of terraces in the coastal areas made it possible to establish the sequence of late Quaternary sea level fluctuations and their worldwide correlation. Higher sea levels of ca. 120, 100, 80 and 60ka have been reconstructed in South Kanto, Kikai Island of the northern Ryukyus (with younger high sea level of ca. 40ka), and Hateruma Island of the south Ryukyus, where only three older high sea levels were recognized because of the low uplift rate of this island. Morphostratigraphic correlation of the main terrace, associated with major seal level rise, to the main stage of the last interglacial of ca. 120-125ka has now been confirmed in the areas mentioned above and in some other areas in Japan by different methods, including tephrochronological method. The ages of higher sea levels of the middle Pleistcene are also identified in some areas. Numerous radiocarbon dates of Holocene deposits underlain by coastal lowlands have been obtained since the 1960s and used as essential data for the reconstruction of various relative sea level curves since the maximum lowering of the sea level of ca. 15, 000yBP. However, separation of the tectonic factor from relative sea level curves is still difficult, and determination of the height of paleo sea level is still problematical.Simultaneity of depositional flurial terraces in central Japan, confirmed by widespread dated tephra, strongly suggests that they represent climatic terraces which were formed in the cold period of ca. 2.5-3.0 and 5.0-5.5ka. The correlation of terraces of the inland area with those of the coastal areas is not yet well established, however.
日本第四紀学会の論文

日本における段丘・低地研究の現状と問題点--日本第4紀地図の作成を通して (日本第4紀研究の諸問題--第4紀地図の作成過程から<シンポジウム>)

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