Gentle Slopes and Fluvial Terraces around Mt. Shokanbetsudake, Hokkaido

Abstract

This paper discusses the paleoenvironment of eastern slopes of Mt. Shokanbetsudake, Hokkaido, on the basis of evidences of gentle slopes and fluvial terraces.The fluvial terraces are classified into five surfaces (terrace I to V in descending order) and the gentle slopes into two surfaces (gentle slope I and II). In most case, gentle slope I and gentle slope II develop behind terrace II and behind terrace IV, respectively.The gentle slopes are mainly composed of lower angular gravel several meters thick and upper fine textured materials several meters thick, which were produced by reworking of the lower layer. Long axis direction of the angular gravel is consistent with the maximum slope direction. The facts indicate that the gentle slopes were developed by gelifiuction under a periglacial environment.The age of gentle slope I formation is estimated to be older than 40, 000 or 40, 000-20, 000yr B. P. as inferred by the C-14 ages of peat and charcoal buried in the deposits of gentle slope I and terrace III. Terrace II and III are considered to have developed in this period, and the feature of their deposits suggests that they were formed under a colder climatic environment than present.On the other hand, the gentle slope II is inferred to have developed in a short period of cold climate between 20, 000yr B. P. and 5, 000yr B. P.