国後島南部および色丹島における北海道起源の完新世広域テフラの同定

概要

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Five Holocene tephras originating from Hokkaido volcanoes underlie coastal lowlands on Kunashiri and Shikotan islands, Southern Kuril Islands. These Hokkaido tephras were identified based on the refractive index and major element composition of glass shards in the tephra. They are: Tarumae-a (Ta-a, AD 1739), Komagatake-c2 (Ko-c2, AD 1694), Mashu-b (Ma-b, 774-976 cal BP), Tarumae-c (Ta-c, 2500-2800 cal BP), and Mashu-d1 (Ma-d1, 3267-3368 cal BP). Ko-c2 and Ta-c are the most widely distributed ash fall deposits, and provide valuable time markers for examining the stratigraphy of these islands, including possible paleotsunami deposits. On Kunashiri Island, 12 tephras that probably originated from at least three volcanoes located on Kunashiri Island are interspersed with the five Hokkaido tephras. These tephras are classified into three groups according to the combination of TiO_2 and K_2O abundance in glass shards. Many of them contain glass shards showing relatively low refractive index (n=1.480-1.490) and high SiO_2 content (77-79%) in comparison with the widespread tephras from Hokkaido Island.
2009-12-31