THE DEBRIS-MOUNDS OF THE “TSURUGIZAWA CIRQUE” IN THE JAPANESE NORTHERN ALPS

Abstract

The Tsurugizawa gorge locates in the center of the Tateyama-Tsurugidake Range in the Japanese Northern Alps, Central Honshyu. The Tsurugizawa river flows to northward from Mt. Tsurugigozen which is head of so-called glacial cirque, then turns east or north-eastward in the middle reaches, and finally drains into the Kurobe river at the Jujikyo junction. The upper-most area of this gorge is characterized by the so-called “Tsurugizawa Cirque” as previously reported by Dr. Imamura and others. A large quantity of debris exists in the head area of the gorge, which is usually designated as “moraine”. The diameter of some debris exceeds five meters. The distribution of the quantity of debris is not uniform along the stream valley in the area. That is, a large quantity of debris accumulates on the right bank of the stream, building up debris-mounds which form rampart, tongue, stage, bank and knob. The main purposes of this study are to make clear the pushed out direction of debris relating to the topography of valley bottom and the mechanism of accumulation of debris. In this study, the author utilizes “sedimentary fabric” in order to clear the transported direction of debris. Prior to this investigation, many observations on the sedimentary fabric of recently formed taluses in the Tanzawa Mountains were made to elucidate the relationship between the sedimentary fabric and the mechanism of debris movement. The major results of the field survey are as follows. The debris in the studied area are composed of two kinds. The upper area is covered with diorite, transported from south-west. The lower area is covered with granite, transported from south. The line of demarcation between the two kinds of rocks is shown by a line from st. 72 to st. 77. Debris accumulated in the valley bottom are weathered to a considerable degree, but debris recently pushed out from side slope are less weathered than the former. The sedimentary fabric of the weathered debris shows approximately the same direction in its arrangement as that of the valley bottom. In the both area, the upper segments of weathered debris mounds are partly covered with a large quantity of fresh debris pushed out from the side slope. Especially, in the lower area, the fresh debris are supplied from slopes facing north-west and or north. In the valley of the upper area, it is certain that there existed at least two periods of debris formation in the past because both debris-mounds of rampart type and tongue type had deposited on debris mound of stage type.