Stratigraphy, petrochemistry and Quaternary depositional record of the Songaksan tuff ring, Jeju Island, Korea

Abstract

The Songaksan tuff ring (STR) is one of several recent hydrovolcanic centers on Jeju Island, Korea, which provides an excellent example of proximal-to-distal facies changes in wet pyroclastic surge deposits. A multidisciplinary study has been carried out on the STR and adjacent lithostratigraphic units to constrain absolute age, geochemical characteristics, and Quaternary depositional history. A number of rock units were identified inside the crater of the STR, including Scoria deposit I, trachybasalt lava, Scoria deposit II, and a late-stage basaltic tuff, indicative of a rather complex sequence of magmatic and phreatomagmatic eruptions after the construction of the tuff ring. Petrochemical analysis shows that the STR was generated from different magma batches that fractionated from a homogenous magma chamber, and the early erupted magma was more evolved and volatile-rich. Reworking of the STR commenced shortly after the hydromagmatic eruption in a high-energy nearshore environment, resulting in deposition of the Hamori Formation. The formation is composed of planar-stratified and low- to high-angle cross-stratified tuffaceous (pebbly) sandstones and occurs up to an altitude of about 4 m above present sea level. 14C dating of molluscan shells beneath the formation indicates that it began to be deposited after about 4000 yr BP. Detailed sedimentary logging reveals that the formation consists of several stratal packages bounded by laterally persistent and distinct lithologic boundaries, probably formed by millennial-scale sea-level fluctuations. Occurrence of another hydrovolcanic sequence (the Sinyangri Formation) on the opposite side of Jeju Island, having similar sedimentary characteristics and ages, suggests that the sea-level fluctuations as seen in the Hamori Formation have affected a wide area of Jeju Island, probably related to the high-frequency sea-level oscillations during the post-6 ka BP regression period in the East Asian region. It can be concluded that the formation of the STR was possible because of the Holocene transgression, which made the present coastal areas water-saturated and adequate for hydrovolcanic eruptions. The STR in turn contributed to record high-frequency sea-level fluctuations during the Holocene via acting as a local and short-lived but affluent source of loose sediment.