北陸の海成段丘における三瓶木次テフラの同定とその意義

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A distal tephra laryer occuring in the coastal areas of the Hokuriku district, Central Japan, is characterized and identified with the Sambe Kisuki tephra (SK). It occurs as airfall and water-laid ash associated with overlying drift pumice within the marine terrace deposits.The tephra is characterized by 1) biotite-bearing rhyolitic ash, 2) pumiceous glass with an unusually low refractive index, ranging from 1.494 to 1.498, and with higher concentration of potassium than of sodium, 3) stratigraphic position underlain by the Aso-4 and Kikai-Tozurahara tephra layers.The stratigraphic relationships between SK and well-dated marker tephra layers suggest that the tephra erupted between 80ka and 90ka from Sambe volcano, ca. 400km west of the area.Paleomagnetic measurement of the tephra shows normal polarity, in contrast to the result of NAKAJIMA and MIURA (1983), who demonstrated reversed polarity as evidence of the Blake event.This tephra is recognized as a useful tool for elucidating paleocurrents as well as formation ages of terrace deposits in the Hokuriku area, where no marker had been recognized for the earlier half of Late Pleistocene.(1) The drift pumice balls of SK suggest the presence of an ocean current running eastwards along the Japan Sea coast at that time.(2) The marine terrace deposits distributed at an elevation of 30-40m above sea level in the Hiradoko upland in the Noto peninsula and the Awara sand bed in the Kaetu upland, on the northern part of the Echizen coast, have an age of around 80ka. They can be correlated with the Obaradai terrace deposits in the southern Kanto district.(3) The raised marine terraces intercalating SK and Aso-4 ashes suggest that the rate of uplift may have been higher than was previously considered.

北陸の海成段丘における三瓶木次テフラの同定とその意義

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