加治丘陵西部の飯能層と関東産地の接峰面について

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(1) The Hanno Formation distributed at the western Kaji Hill, west of the Tachikawa Fault is 60 m thick and composed of the lower silt bed and the upper medium-sized gravel bed. These features are characteristic of distal facies of the fluvial fan, which will contradict a proximal model supplied from the Tama River. It was known that the Hanno Formation was equivalent to the Kurotaki Formation, the basal unit of the Kazusa Group observed in the Boso Peninsula at the other side of the Kanto Basin. An unconformity between the lower silt bed and the upper gravel bed of the Hanno Formation is clearly identified. (2) The summit level map of the Kanto Mountains is made by using computer program of Shiono et al. It is found that a continuous hillslope develops from the summit of Mt. Kumotori down to the Kanto Plain. At four sites on the hillslope, reddishly weathered basements are observed, three of which are covered by the Hanno Formation. Because of this evidence, we conclude that the hillslope is a peneplain formed immediately before deposition of the Hanno Formation. We call it the Pre-Hanno Plane. (3) The Hanno gravel bed, the upper unit of the Hanno Formation at the western Kaji Hill is characterized by granite and hornfels gravels. There were discussions whether they were supplied from Koma/Iruma River or from Tama River. The summit level map indicates that the Tama River is the only conduit for transportation of these gravels from the Kofu granitic body. (4) The trough-like depressions on the 200 m contour of the summit level map are observed along the Tama/Asa River and Iruma River. They may indicate either large amounts of erosion or a depositional basin for receiving various sediments. Regarding Tama River, it represents huge amounts of erosion. But for Iruma River, it indicates a depositional basin so as to receive thick deposits from Koma, Iruma and Tama Rivers. (5) It was pointed that Hansin Earthquake of January 17,1995 occurred at one of the micro-plate boundaries. The Kanto Mountains is also one of the micro-plates bounded by Fossa Magna, a plate boundary between Eurasia and American plates. From the land-form study, we conclude that uplifting of the Kanto Mountains started at the end of Pliocene or early Pleistocene and deposited red-conglomerate facies of the Hanno Formation at the foot of the Pre-Hanno Plane.

加治丘陵西部の飯能層と関東産地の接峰面について

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