樽前c_1火山灰の分布・層序と強磁性鉱物の化学組成 : とくに十勝平野の十勝c_1火山灰との関連性についての検討

抄録

In the Tokachi Plain of the south-eastern Hokkaido, a number of the volcanic ashes ranging from the late Pleistocene to Holocene are widely distributed. The central part of the plain is chiefly covered with Holocene ashes which include the Tarumai "b" (Ta-b), "d" (Ta-d) and Tokachi "c_1", "c_2" (To-c_1,c_2). Unlike Ta-b and Ta-d, the Tarumai "c_1" (Ta-c_1) ash extends over the Sapporo-Tomakomai Lowland and across the Hidaka district and reaches to the west-faced foothill of the Hidaka Mountains, but has not been reported on the east side of the mountains where the Tokachi Plain is located. Since radiocarbon analysis dated Ta-c_1: and To-c_1 ashes at 2500-3000 yrs. B. P., which is also supported with the remains of pottery and stone implements buried in these ashes, and they are morphologically alike as well, To-c_1 has been suspected to be identical with Ta-c_1. This study aims at reinvestigating the origins of Ta-c_1 and To-c_1 ashes. Soil profiles were examined in terms of the stratigraphy of To-c_1 and Ta-c_1. Both of them, always lie between Ta-b (erupted in 1667 AD) and To-c_2 (erupted ca. 4000 yrs. B.P.), having wide distribution pattern in the Tokachi Plain. To-c_1 was, however, missing on the east-faced foot slopes of the Tokachi valcano. Nine samples from Ta-c_1 and To-c_1 ashes were analysed for median diameter (Md), heavy mineral content, and chemical components of ferromagnetic minerals. Md and heavy mineral content were less in the east part of the plain than the west. Both ashes showed similar chemical composition of ferromagnetic minerals and are classified into the andesitic ash on the basis of V/(Fe+Ti) and Zn/(Fe+Ti) ratios and SiO_2 content (SHOJI et al., 1975). Examination of Ta-c_1 and To-c_1: ashes in terms of the above mentioned characteristics suggested that both of them are derived from the Tarumai volcano and so-called To-c_1 is identical with Ta-c_1.