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HOLOCENE GEOMORPHIC DEVELOPMENT IN DRAINAGE AREAS OF RIVER MARUYAMA AND RIVER WADA, BOSO PENINSULA, JAPAN

1985; Association of Japanese Geographers; Volume: 58; Issue: 11 Linguagem: Inglês

10.4157/grj1984a.58.11_734

2185-1735

Kaoru Kashima,

Earthquake and Disaster Impact Studies

River Maruyama and River Wada are located in southern part of Boso Peninsula, Along both rivers, One Pleistocene terrace and three Holocene terrace series can be observed. Upper Holocene terrace (Maruyama I terrace) is distributed widely from the upper to lower course. By field observations of terrace deposits, Maruyama I terrace is thought to be the fluvial fill-top terrace. Only in the lower course, drowned valley had been formed and marine silt and clay with shell fragements had been deposited from about 10, 000 y. B. P. till 0, 000y. B. P., But, before the emergence of Maruyama I terrace, this drowned valley had been filled by fluvial materials from both rivers. Mid and Lower Holocene terraces (Maruyama IT and Maruyama III terraces) are distributed from the upper to lower courses. But distribution area of each terrace is smaller than that of Maruyama I terrace. From theupp er to middle course Maruyama II and III terraces are thought to be fluvial strath terraces because terrace deposits are composed of granule and sand whose thickness is less than 2m. But in the lower course those terrace are thought to be fluvial fill-top terrace because thickness of those terrace deposits is more than 5m. Those evidences show that the terrace topography of this area had been formed by three cyclic changes of depositional and erosional processes of both rivers in the Holocene. And those changes were persumed to be caused by rapid sea level rise in early Holocene and seismo-tectonical movement during the Holocene.