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GEOMORPHIC DEVELOPMENT SINCE THE LATEST PLEISTOCENE OF THE DRAINAGE AREA OF THE OBITSU AND THE YORO RIVERS, BOSO PENINSULA, JAPAN

1982; Association of Japanese Geographers; Volume: 55; Issue: 2 Linguagem: Inglês

10.4157/grj.55.113

2185-1735

Kaoru Kashima,

Geology and Paleoclimatology Research

The Obitsu and the Yoro rivers are located in the central part of the Boso Peninsula. Meandsring valleys and many strath river terraces are developed well in the upper to middle courses of both rivers. On the basis of the topographic classification of those terracces by the aerial photo interpretation, the past developmental process of valley incision can be deduced from the distribution characters of those terraces. The past developmental process of the Alluvium deposition in the lower courses of both rivers can be reconstructed by the analysis of coring data and by the investigation about the continuity between the profiles of buried valley floor below the alluvial plain and those of river terraces. The aim of this paper is to investigate the chronological relation between the developmental process of valley incision and that of the Alluvium deposition, and to discuss the effect of sea level change on the development of the river valley in the upper to middle courses. The results obtained by this investigation on the geomorphic development of both rivers are summarized as follows. 1. In the upper to middle courses of both rivers, valley incision has continued at least since ca. 20, 000 Y. B. P., because no fill top terraces can be found along the courses. There are typical topographies which can be thought to be made by the past cuttings of meandering channels. Those cuttings had been formed frequently in the first half of the Holocene. This evidence shows that the lateral erosion of the river had been dominant over the down cutting in this period and the valley meander could have grown easily. In the latter half of the Holocene, however, the cutting of the meandering channel has hardly been made so that the deepning of the valley has been predominant, leaving the former meandering pattern. 2. In the lower course of each river, it is assumed that the sedimentation of the Alluvium had began at the end of the Pleistocene estimated from the radiometric datings of the lowest part of the Alluvium and the correlations between the profiles of burisd valley floors below the alluvial plain and those of river terraces. In the first half of the Holocene, the Alluvium had rapidly deposited with the rise of sea level, and the depositional area of the Alluvium had d the spred inland. The Alluvium deposition has been over about 6, 000_??_5, 000 y. B. P. an surface of the alluvial plain has gradually emergenced from the inland part of the plain. This retreast of the dePositional area of the Alluvium is thought to be caused by a little fall of sea level since ca. 6, 000_??_5 000 y. B. P.. 3. The correlation of river terraces from the upper to lower courses shows that the change of the character of valley incision in the upper to middle course had begun at the same stage retreat of the depositional area of the Alluvium had started in the inland region of when the the plain. This phenomenon is considered to show that the change of base level of erosion, which have been caused by the sea level change since the Late Plesistocene, has effected on the process of valley incision in the upper to middle courses, because the pattern of crustal move ment in this area is thought to have hardly changed since the Late Pleistocene Juding from the distribution characters of the river terraces.