Abstract
In the present paper, geological and topographical characteristics of the upper stream of the Kurobe River, (one of the representative river basins in the high mountains of Central Japan), are decribed and the relationship between the fluvial topographic changes and climatic changes since the latest Pleistocene is discussed.
The stratigraphical and topographical relationships between old gravel beds, a huge accumulation of detritus, and andestic lava flows which issued from the Jijidake volcano are important in estimating the fluvial topographic change in this area.
The gravel beds, which are found in the old river bed, have a maximum thickness of 80 meters and are fairly well stratified. Trey can be divided into roughly four characteristic rock facies, i. e. from Lg4 to Lg1 (bottom to top). These are unconformably overlain by the andestic lava flows. The age of activity of the Jiiidake volcano may be correlated to the third stage of the Tateyama volcano, viz. 32, 000-40, 000 y. B.P. Consequently, the depositional period of the old gravel beds may be assigned to the Altwürm Stadial (Wv3). Large deposits of small-sized gravels indicates a cool/cold climatic condition during that period.
The dissected lava flows are overlain by detritus deposits which are widely distributed along the main stream of the Kurobe River and and the Iwagokekodani (a branch of the Kurobe River) and which form very broad, gentle slopes. The maximum thickness of the deposits is more than 100 meters. The period of a large accumulation of detritus may be correlated to the maximum phase of the Wurm Ice Age (Wh2), around 20, 000 y. B.P. Lowering of the timber line and a decrease in precipitation accompanied by a very cold climate might have accelerated the production and accumulation of detritus.
There was heavy river erosion both before and after the detritus accumulation period. The erosion before this period is estimated to have started within the Göttweigel Interstadial. This estimation is based on the large-scale erosion that took place and that resulted in the formation of wide river beds. The erosion that occurred after the detritus accumulation period is assigned to the Holocene. Original depositional form of detritus has been subjected to intense river erosion by running water and has formed remarkable topography that looks like river terraces.
Generally speaking, the fluvial topographic changes occurring since the latest Pleistocene in the upper stream of the Kurobe River coincide quite closely with the climatic changes.
