The Sedimentology of Pyroclastic Flow Lift-Off: The 18 May 1980 Mt. St. Helens Singe Zone Deposit

Abstract:

Pyroclastic density currents (PDCs) reverse buoyancy to form buoyant plumes after sufficient entrainment and heating of ambient atmosphere and deposition of suspended sediment. The deposits associated with buoyancy reversal depend on input from the feeding flow. It is expected that fines are swept away by the buoyant plume, and coarse sediment might be deposited preferentially during reversal; in an extreme case, one might expect deposit thickening at the point of buoyancy reversal as the coarse sediment rains out of the lofting plume. Alternatively, a thinner, finer grained deposit would be expected from a flow that deposited the bulk of its coarse sediment load prior to buoyancy reversal. The PDC associated with the 18 May 1980 Mt. St. Helens lateral blast downed trees as it traversed the ground (blowdown zone); at the distal extent of the blowdown zone, trees were burned, yet remained standing, in an area thought to represent a well-constrained case of PDC buoyancy reversal (singe zone). We present a sedimentological analysis of preserved deposits from the blowdown and singe zones to investigate this transition. The PDC deposit is poorly sorted, gray, sandy ash, generally <15 cm thick, and is locally normally graded. The unit includes lapilli sized clasts, with fragments of pumice, lithics, and, locally, wood fragments. Flow velocities at the blowdown/singe zone transition varied by a factor of ~2, and, where the flow velocities were slower, deceleration occurred within the blowdown zone prior to lift-off. In the singe zone, the deposit is structureless, and thins from <10 cm to <5 cm thick over a distance of 0.5-1 km. The grain-size distribution and sorting of the unit within the singe zone is broadly similar to deposits in the blowdown zone, but either become better sorted where deceleration in the blowdown zone preceded lift-off, or fine at the point of lift-off before becoming fines poor further downstream. These data indicate that the flow experienced only partial buoyancy reversal, and a dense undercurrent continued to move through the singe zone. However, in areas where tree blowdown orientations indicate that the buoyant plume was fed from multiple directions, the singe zone deposits fine towards the center of the area of focusing, and the <63 μm size fraction increases to as much as ~20 wt. % of the deposit.