Distinguishing between primary and secondary volcaniclastic deposits

C Sohn,Y K Sohn

doi:10.1038/s41598-019-48933-4

Abstract

The distinction between primary and secondary volcaniclastic deposits, which are currently defined as the “direct” products of volcanic eruptions and the “reworked” products of the former, respectively, is the first step to interpreting volcaniclastic deposits, particularly the genetic connection with active volcanism. The distinction appears straightforward, but is not always applicable to natural deposits. During the 3.7 ka BP eruption of the Songaksan tuff ring, Jeju Island, Korea, there was an invasion of typhoon. The tuff ring was partly submerged underwater and affected by wave activity for over a day, resulting in a peculiar volcaniclastic deposit composed of both vent-derived (primary) and substrate-derived (reworked or secondary) volcaniclastic particles. We propose a new term “reprocessed” for a category of volcaniclastic deposits or particles, which originated directly from volcanic eruption but was deposited finally by nonvolcanic processes. Here we show that both reprocessed and reworked particles can coexist in the same volcaniclastic deposit, making it impossible to differentiate it into either a primary or a secondary deposit according to the current definition of volcaniclastic deposits. We thus define the secondary volcaniclastic deposits as comprising either or both of reprocessed and reworked volcaniclastic particles.

Highlights

Volcaniclastic deposits form by various processes of volcanic and sedimentary nature, comprising a range of particles that underwent diverse fragmentation, transport, and depositional processes[1,2,3,4]
Whether such current- or wave-worked deposits can be covered by the restrictive definition of primary volcaniclastic deposits because the final deposition of the deposits was accomplished by a process that is completely unrelated with a volcanic eruption
In order to solve this problem, we proposes some modification of the current definition of volcaniclastic deposits based on a study of a deposit in Jeju Island, Korea, that can be described as both primary

Summary

Introduction

Volcaniclastic deposits form by various processes of volcanic and sedimentary nature, comprising a range of particles that underwent diverse fragmentation, transport, and depositional processes[1,2,3,4]. White and Houghton[8] supports the latter definition, stating that “all deposits that do not involve interim storage of particles are primary deposits, regardless of whether transport occurs through air, water, granular debris, or some combination thereof.” According to this definition, all volcaniclastic deposits that were deposited by nonvolcanic processes, such as stream flows, waves, ocean currents, winds, and etc, are primary unless the constituent particles experienced temporary deposition and reworking before final deposition. The tuff ring formed mostly above high tide level by pyroclastic surges and fall[15], but contains three interbeds of horizontally laminated, low- to high-angle cross-stratified, and hummocky to swaly cross-stratified volcaniclastic deposits in the middle of the tuff sequence (Fig. 2) up to an altitude of ~5.5 m, i.e., ~4.5 m above the high tide level These interbeds, named units R1, R2, and R4, are interpreted to have formed by wave activity in a swash to surf zone when the sea level rose several meters above normal high-tide level during a storm event[16]. We focus on unit R2 because it occurs along an interface between two tuff sequences that have contrasting accidental componentry and juvenile tephra composition, thereby making it possible to assess the relative proportions of the vent-derived (i.e., primary) and substrate-derived (i.e., reworked or secondary) particles in the deposit

Methods

Findings

Conclusion