Fossil Solifluction Lobes and Steps in the Upper Kitakami River Basin, Northeastern Japan

Abstract

The author observed buried fossil-forms of solifluction lobes and steps at two localities in the upper Kitakami River basin, northeastern Japan (fig. 1). In this paper their forms and formation period are discussed with paleoenvironment.Solifluction lobes and steps consist of two scoria layers (T2S, T1S) and loamy volcanic ash containing three buried soil layers (BS3 BS2, and BS1), which fell on the area during the late Last Glacial period (Fig. 2).T2S, T1S and BS3 show the overriding or upturn structures, reflecting down slope movement of front of the solifluction lobes and steps (Fig. 3). Furthermore at the base of scoria layer that occur to overriding can be recognized to buried phenomena of previous land surface soil (Fig. 3, 5).The period of their formation are estimated between ca, 26, 500 and 13, 000y. B. P. by the deformed features of scoria layers or buried soil layers and their ages. This period corresponds to that of formation of involution in this area, and age of mass movement in the Kitakami mountains (Fig. 7).Existences of the fossil forms as solifluction lobes and steps indicate that dense forest vegetation under periglacial environment did not existed in this area between ca. 26, 500 and 13, 000y. B. P..