抄録
In the coastal region of the northern end of Hokkaido, thick gravel deposits called the Keihoku formation unconformably covers the Neogene Tertiary deposits. The bottom of this formation is especially sunken in the subsurface of the Sarobetsu coastal Plain. It is situated at approximately 60m below sea level. While the highest outcrop of this formation is found at the hill immediately north of this plain. It is about 60m above sea level. The formation is observed little tilting or folding. Accordingly it is estimated to be about 120m in thickness in all, if it was not largely eroded away in the past.
In the Keihoku formation, peat deposits are rather widely traceable especially in the lower horizon, in which dominant seeds of Menyanthes, elytra of beetles, and small branches of willow and birch trees are generally found. According to the result of pollen analysis made by N. Fuji, the climatic condition of this deposits is indicated to be cooler than the present days.
The Keihoku formation is unconformably covered by the pumice fall deposits, called the Wankonosawa pumiceous deposits, which simultaneously cover the gravel beds of the lower terrace. The age of the pumiceous deposits is estimated to be the latest Pleistocene. These pumiceous deposits are also covered by the another ash called the Hotoku volcanic ash. The age of the ash may be at least the Pre-Subboreal age, because the ash is locally highly disturbed by cryoturbation. The extension and isopach of these two volcanic products, shows their origin to be the Rishiri volcano, now situated off the Sarobetsu coast. The age of the Keihoku formation is unknown for us at the present, but is undoubtedly older than the age of the Middle terrace, because it is cut by the terracing of that age.
In all probability, it may be correlated to the Nopporo formation or the Obihiro formation composed of heavy-beded gravel beds, and may eventually represent the early Pleistocene.
