Shallow-marine records of pyroclastic surges and fallouts over water in Jeju Island, Korea, and their stratigraphic implications

Abstract

Explosive volcanism results in a wide range of volcaniclastic deposits in many of Earth's subaerial and subaqueous environments. In this paper, we introduce a unique, shallow-marine volcaniclastic deposit from Jeju Island, Korea, for which the materials were transported to the water surface by pyroclastic clouds and then settled from the surface as they were entrained in the water. The deposition occurred under alternating currents and still waters, which is most plausibly attributed to tidal processes. Mud flasers or drapes intercalated in the deposit, which indicate periods of slack water during tidal cycles, suggest that the deposit accumulated in a very short period of a fortnight or a month, about a million times faster than the adjacent sedimentary strata. Because of the unusually high sedimentation rate, the volcaniclastic deposit could record the "usual" fair-weather processes in the depositional site at a resolution that is almost never provided by ordinary sedimentary deposits. This finding highlights the biases in Earth's stratigraphic records and teaches us that volcanic deposits, commonly regarded as the products of catastrophic events, can in some cases record more faithfully the ordinary and usual processes that nonvolcanic deposits cannot.