Flute-like scours at the base of Plinian pumice-fall deposits at Ohachidaira caldera, Hokkaido, Japan

Abstract We describe scours and related erosional features at the boundary between the sub-Plinian and overlying Plinian units of the 34-ka eruption at the Ohachidaira caldera and discuss their formation mechanisms. These scours are observed at a proximal section ~ 1.8 km from the caldera center (~ 0.9 km from the rim) and show asymmetric cross-sectional shapes that closely resemble flute marks typically found in subaqueous sedimentary rocks; they cut into underlying strata of the immediately preceding sub-Plinian fallout deposits with surrounding beds variably disturbed along the scours. The scours are overlain by a thin ash lens, in turn overlain by pumice lapilli of the Plinian phase. Another characteristic erosional feature is shear deformation of the top ash layer of the sub-Plinian deposits; this ash layer is locally bent, folded, split, or fragmented. We interpret these features, based upon their field characteristics, to have been formed by dilute turbulent pyroclastic currents that occurred at the beginning of the Plinian phase. No pyroclastic current deposits or erosional features were observed at the base of the Plinian sequence beyond 1 km from the caldera rim, suggesting short runout distances of the dilute pyroclastic currents; such small currents might have resulted from a local collapse along the margin of the Plinian column.