Abstract
A lot of alluvial fans in the Kofu Basin can be devided into the following two types by their location; the piedmont-type fans and the floor-type fans. This study aims to make clear the processes and ages of their formation through the analysis of micro-landforms.
The conclusions are as follows:
1) A piedmont-type fan is mainly built up by debris flows or mud flows. This type fan displays the micro-landforms such as debris-flow lobes or mud flow levees which are formed by massive transportation.
2) A floor-type fan is mainly formed by water flows and has slightly elevated microforms as channel bars, natural levees, and crevasses sprays.
3) The particle sizes and micro-landforms of piedmont-type fan are significantly affected by the geological condition of its source area. This type fan constituting mainly of boulders and cobbles forms debris-flow lobes of “the trepang type”. On the other hand, the mud-flow levees of “the bank type” are developed on a piedmont-type fan composed of pebble, granule, sand and silt.
4) A floor type fan is divided into the upper part and lower part. The former is formed by flood of turbulent flow and the latter by over-bank flood or back-water dammed up at gorges. Different distribution patterns and shapes of micro-landforms are observed in the two parts.
