We identified four pyroclastic flow deposits in central Hokkaido as belonging to the same flow deposit which erupted from the Tokachimitsumata basin with a circular topographic moat (10×14 km). A dense network survey of gravimetric data revealed a negative depression profile of a Bouguer anomaly in the basin. The surface elevation and thickness of the Muka pyroclastic flow deposit adjacent to the basin gradually increase toward the basin moat. These findings suggest that the Muka deposit was derived from the basin, even though the volume of this deposit is substantially less than that estimated for the caldera. Field surveys, petrographic analyses, and K-Ar dating were also conducted on the other three deposits (i.e. the Meto tuff bed, Kuttari pyroclastic flow deposit, and Biotite dacite tuff-breccia), which had similar petrographic characteristics to those of the Muka deposit. The strong correlation between the four deposits can be inferred by the following: the K-Ar ages of feldspar minerals from pumices of the four deposits are identical (1 Ma), the pumices of the four deposits exhibit very similar mineral assemblages and volcanic-glass and mineral chemistry, and, like the Muka deposit, the surface elevation, thickness and welding degree of the three deposits appear to increase toward the basin. The Muka, Meto, and Kuttari deposits began forming a pumice fall deposit, indicating a plinian phase for the first eruption. It can thus be inferred that these four deposits have arisen from a large-scale eruption that occurred 1 Ma, which formed the caldera. The total volume of ejecta was approximately
>130 km
3. We have named the fall deposit, flow deposit, and basin the Tokachimitsumata pyroclastic fall deposit (Tkm-pfa), Tokachimitsumata pyroclastic flow deposit (Tkm-pfl), and Tokachimitsumata caldera, respectively.
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