2018 Volume 127 Issue 1 Pages 73-87
This study classifies composite small-scale fans along the eastern foot of Mt. Ikeda into five categories, and estimates their formation ages to discuss factors controlling fan development. Besides, it describes microtopography of fan surfaces and surface geology (sedimentary units), focusing on one of the typical terraced small-scale fans to discuss small-fan forming processes. Methods are interpretation of aerial photographs for terraces division, field observations with microtopography measurements, and radiocarbon dating for overlying sediments of terrace deposits. The results show that the five terrace surfaces are mainly formed by a few debris flow deposits, and the estimated formation ages obtained with radiocarbon dating fall into the general classification of the Last Glacial Maximum (LGM) and the early Holocene. However, the difference between each formation periods is 1,000-3,000 years because estimated formation ages are about 20,000 years (I surface) and about 17,000 years ago (I surface) in the LGM, about 9,000 years (III surface), and about 8,000 years ago (IV surface) in the early Holocene. Geomorphic development of the study area might be difficult to explain using a simple formation model of river terraces with time scales of 104-105 years, expressed by dynamic river fluctuations under the full influence of global climatic changes. This speculation is highly suggestive for further investigations on the timing of debris flow sedimentations forming alluvial fans in this area.