キーワード：秩父盆地，中新世，構造運動，基盤，礫岩，ファブリック

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Ａ礫岩層堆積期の秩父盆地周辺 の 模式概念図		Ｂ・Ｃ・Ｄ・Ｅ礫岩層堆積期の秩父 盆地周辺の模式概念図
Ｆ・Ｇ礫岩層堆積期の秩父盆地 周辺の模式概念図		Ｈ・Ｉ礫岩層堆積期の秩父盆地 周辺の模式概念図

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Tectonic Movement of the Basement Rocks Inferred
from the Conglomerates in the Miocene Series
of the Chichibu Basin, Central Japan

Osamu TAKAHASHI,
Yukiko OKO,
Atsushi ISHII
and
Saburo OKAMURA
Department of Astronomy and Earth Sciences, Tokyo Gakugei University, Tokyo 184 , Japan

Abstract
　　　In the Miocene series of the Chichibu Basin, Kanto Mountains, central Japan, conglomerates are intercalated at several stratigraphic horizons. In this study, we describe the following results of the field survey and the analyses of the conglomerates from the viewpoint of depositional environments and directions of sediment transportation.
　　　Composition: The conglomerates are composed mainly of sandstone gravels in the western part of the basin, of chert ones in the eastern or the northeastern part, and accompanied by plutonic ones in the northern marginal area of the basin.
　　　Roundness: The angular gravels are more abundant in the southern and eastern marginal areas of the basin than in the northern and western ones. Grain size: The grain size increases from the center to each margin of the basin. In particular, there are some boulder size gravels in the basal conglomerates along the north and west margins of the basin, and the southern marginal area of the basin.
　　　Paleo - current direction: The fabric of the conglomerates shows that the paleo - currents flew nearly parallel to the present dip direction of each stratum. From these results, the tectonic history of the Chichibu Basin is inferred as follows:
　　　As the Miocene series deposited in the basin, the northern and the western hinterlands of the basin were elevated continuously and / or periodically. Almost at the same time, the southern hinterland was uplifted accompanied by the movement of the Hakutai Fault. Nearly at the end of deposition, rapid upheaval of the eastern hinterland began. The episodes of the sedimentation were ended by the uplifting of surrounded hinterland and the marine left southeastward of the basin.