Stratigraphy and Ages of Aira Caldera Deposits in Shinjima (Moeshima), Kagoshima Prefecture, West Japan

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The present innermost part of Kagoshima Bay corresponds to Aira Caldera, which appeared with a large eruption about 25, 000yrs BP. Shinjima (Moeshima) Island, our study site, is located in the center of Aira Caldera. According to historical records, this island was uplifted from the caldera bottom during the “An'ei volcanic eruption” of Sakurajima in 1779-1780. Therefore, the geology of Shinjima Island ought to record a part of the environmental changes in Aira Caldera.As a reexamination, geological survey and dating of the Shinjima Island were carried out. The Pleistocene to Holocene series of Sinjima Island is classified into the Susaki Pumice Bed, Moeshima Silt Bed, Moeshima Shirasu Bed, Moeshima Shell Bed, Shinjima Shirasu Bed, descended (air-falled) volcanic ash layers from Sakurajima volcano, and surface soil; they are overlapping in that order. The silt bed on the southern coast was identified as corresponding to the Moeshima Silt Bed on the northern coast on the basis of agreement of the lithologic features and formative ages. The Moeshima Silt Bed was formed in the period between 14, 600 and 7, 800yrs BP. A thick pumice bed on the southern coast was distinguished from the Moeshima Shirasu Bed on the northern coast on the basis of stratigraphic relationship. The former was identified below the Moeshima Silt Bed and named the Susaki Pumice Bed. The formative periods of Susaki Pumice Bed and Moeshima Shirasu Bed were estimated to be about 16±4ka by fission-track dating (before about 14, 600 14C yrs BP) and about 7, 800 to 6, 000 14C yrs BP., respectively. The origins of both were subaqueous eruption on deep caldera bottom. Subaqueous eruption resulted in deformation of the previous strata and an unconformity between the previous and subsequent strata.The period when Aira Caldera was submerged under seawater was specified to be between about 11, 700 and 11, 000yrs BP. The outer and inner areas became a seawater environment at about the same time with a rapid rise in sea level, because the south wall of the Aira Caldera opened at that time.The Moeshima Shell Bed was formed during the period from about 6, 000 yrs BP to 2, 300yrs BP. Progression from the Moeshima Shell Bed with outer sea species to the Shinjima Shirasu Bed with inner bay species occurred in successive steps. The innermost bay changed to a more close condition with the decreasing of outer seawater after about 2, 300yrs BP.

Stratigraphy and Ages of Aira Caldera Deposits in Shinjima (Moeshima), Kagoshima Prefecture, West Japan

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