Fossil Periglacial Phenomena on Karst in the late Quaternary in the Akka Karst, Northeastern Japan

Abstract

Fossil periglacial phenomena on karst in the late Quaternary in the Akka Karst, northeastern Japan, were investigated tephrochronologically. The limestone bedrock in the study area is overlain by several late Quaternary tephras. The tephra overlying a slope on karst indicates that the slope has been stabilized since the tephra fell. On the other hand, the absence of a tephra layer, which is usually found at the bottom of dolines or at the foot of slopes, implies slope instability after the falling of the tephra. On the karstic plateau, there are some fossil periglacial deposits which were presumably formed around 50 ka BP and during 30 to 10 ka BP. Pleistocene tephras are absent on the gentle slopes as well as on the steep slopes, whereas Holocene tephras are present on the gentle slopes. Therefore, Pleistocene tephras on the slopes in the Akka Karst were mainly removed by periglacial processes, such as solifluction and frost shattering of limestone bedrock, in the cold phases of the Last Glacial Stage. The periods of slope evolution by periglacial processes are considered to be around 50 ka BP and from 30 to 10 ka BP, which are in accord with those of mass movement in the Central Kitakami Mountains, and of involution and solifluction lobes in the Northern Kitakami Lowland. The high content of biotite, which derived from granitic rocks, in soils in the bottom of dolines indicates that bare ground affected by periglacial processes was distributed over the summits of granitic mountains adjacent to the Akka Limestone during the late Pleistocene. Accordingly, allogenic deposits on karst in the study area have helped to reconstruct environmental conditions in non-limestone areas as well as in limestone areas.