Abstract
We describe flute-like marks 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 and are filled with 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. These erosional features appear to be coeval with the basal part of the Plinian unit. 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. Observations allow the estimation of timescales for formation of the flute-like marks to be at ~ 0.5–5 min, which is rarely obtained from flute marks in sedimentary rocks. The only deposit left by the erosive currents is a thin fine ash lens that occurs at the bases of the flute-like marks, and 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.