抄録
A number of ancient sand dunes in a periglacial environment of the late Pleistocene are widely developed in the Tokachi plain, east Hokkaido. A detailed stratigraphical and tephrochronologic studies on these sand dunes, which were constructed by reworked Eniwa-a (E-a) pumice-fall deposits, showed that geologic succession of the dune is divided into five stages (Fig. 2 and Table 1).
The E-a dune deposit is characterized by a) development of laminations associated with windblown ripples, b) presence of a thin loam in the deposit, c) abundance of well sorted (So=1.2-1.7) and fine-graind pumiceous sands (Md=0.3-0.8mm), and d) covering by the paleosols and younger volcanic ash soils. Dating on the charcoal piece discovered from a thin loam in the deposit procured an age of 13, 100±1, 200yrs.B.P. (Fig. 2).
A dune field on the late Pleistocene terraces (Kamisatsunai I Plain), about 35 square km in area, has blown from the northwest area to the southeast and has formed more than 180 sand dunes. As shown in Fig. 7, the volume of the E-a dune sands are calculated the maximum value 2.6×106m3 par square km.
Lee-slope beds dipping 5-20 degrees predominate within these dunes, and the orientation of several lee-slopes is reflected modestly the influence of prevailing northwesterly winds (Fig. 8).
It is probable that the agency and condition that caused the formation and fixing of ancient sand dune in a periglacial area are closely related to the distinctive deposits, such as the pumiceous sands in this district, and the fluctuation of climate during the late Pleistocene.
