Abstract
The 50-m-thick uppermost Pleistocene to Holocene succession exposed on Shinjima Island represents part of the sedimentary succession that was deposited in the Aira caldera. The succession was uplifted in 1780 due to magma intrusion, providing an opportunity to study post-caldera volcanism and environmental changes in the Aira caldera. The stratigraphy and origin of this succession, however, remain controversial. Based on careful revision of the classification and stratigraphic relationships of the constituent facies, here we propose the division of the succession into the Lower Shinjima Silt Bed, Shinjima Pumice, Southern Shinjima Pumice, Upper Shinjima Silt Bed, and Moeshima Shell Bed, in ascending stratigraphic order. The Lower and Upper Silt Beds are composed mainly of greenish-brown silt and contain shallow-marine fossil assemblages, representing a 100-200 m-deep inner bay environment. The Moeshima Shell Bed comprises lahar (debris and hyper-concentrated flows) deposits derived from Sakurajima volcano. The Shinjima Pumice and overlying Southern Shinjima Pumice, which are sandwiched between the Lower and Upper Silt Beds, are submarine eruption-fed density current deposits of rhyolitic composition, derived from the Wakamiko caldera over a brief interval of 12-13 cal ka BP. Tephras of known ages from the Sakurajima volcano and Yonemaru maar have been identified in the Upper Shinjima Silt Bed, and the Moeshima Shell Bed also contains pumices derived from the Sakurajima tephras. Numerous shell and wood fragments and organic particles have been collected from the Lower and Upper Sjinjima Silt Beds, Southern Shinjima Pumice, and Moeshima Shell Bed, and ages have been obtained by 14C dating. These chronological data suggest that the age of the Shinjima succession ranges from 13-2 cal ka BP.
