北海道白滝遺跡と周辺地域のテフラ

Abstract

Eleven marker tephras were found at and around a famous paleolithic site in the Shirataki Basin, eastern Hokkaido. The following conclusions are the result mainly of a petrographic analysis, i.e., ratio of glass and minerals, composition of heavy minerals, shape of volcanic glass, and refractive indices of glass, orthopyroxene and hornblende.1) Three widespread tephras were identified in the Shirataki Basin; Tarumae-a pumice (Ta-a) that erupted from Tarumae volcano, western Hokkaido at 1739AD, Kussharo-Shoro ash (Kc-Sr) that erupted from the Kussharo caldera, eastern Hokkaido at ca. 32ka, and Shikotsu-1 pumice (Spfa-1) that erupted from the Shikotsu caldera, western Hokkaido at ca. 40 ka.2) Shirataki-1 pumice (Sit1) is identified with Daisetsu-Ohachidaira pumice (Ds-Oh). Ds-Oh erupted from Daisetsu volcano, central Hokkaido, prior to the eruption of a large pyroclastic flow associated with the formation of the Ohachidaira caldera at ca. 30 ka. Sit1 =Ds-Oh spread widely in the Shirataki Basin and shows a horizon just below the Shirataki stone implements. Thus Ds-Oh must be an important key bed for correlating and chronologically arranging paleolithic cultures in the eastern Hokkaido.3) Shirataki-5 pumice (Sit5) overlies the middle terrace surface in the Shirataki Basin. Sit5 is identified with Toetoko ash (TT) distributed around Lake Saroma. On the basis of stratigraphy by means of pyroclastic flow deposits from the Kussharo caldera and palynological evidence, Sit5=TT seems to be dated back to 150-200 ka. Sit5 is probably distributed in the wide area of the Okhotsk region of Hokkaido. Although the source vent has not been determined yet, Sit5 should be a valuable marker tephra indicating the Middle Pleistocene.