Abstract
This paper shows that the Tamugiyama Basin was formed during the Latest Pleistocene by repeated uplift and collapse fault activities. The Tamugiyama Collapse Basin is the youngest one, has ever been reported collapse basins in the Japanese Islands. The Tamugiyama Collapse Basin is located in the northern part of Uonuma Hills, and is attacked by the 2004 Niigata Chuetsu earthquake. The lozenge-shaped basin, about one to two kilometers wide, is surrounded by hills composed of the Pliocene basement. The Tamugiya Formation within this collapse basin is in high-angle abut unconformity contact with Pliocene basement. Talus deposits are recognized on the unconformities. The deposits are resulted from fault scarp and its ancient slope failures. The Tamugiyama Formation is composed of irregularly alternating beds of gravels, sands and silts, intercalated with the Aira Tn tephra bed (AT) about 26,000〜29,000 years ago. The formation contains peat layers, which characterize static slack-water environments. The Kowakita Formation, underlying the Tamugiyama Formation, consists of fluvial gravels carried by the Uono River, therefore it suggests that there was once the ancient Uono River running through this area. The rapid change in sedimentary environments, from the Kowakita formation to the Tamugiyama Formation, results from uplift, faulting and collapse caused by tectonic block movements in this area. The Uono River had changed its course by fault activity and uplift just before the Tamugiyama Collapse Basin was formed, not by subsidence of the present Uono River course. The Yamamotoyama tectonic block including this study area has been the most active uplift block in the Shinano Valley since the Late Pleistocene. The 2004 Niigata Chuetsu earthquake occurred in areas characterized by the same geological setting.
