Volcanic debris-avalanche and epiclastic deposits in the Seongeup area, Jeju Island, Korea

Abstract

It has been reported that the Seongeup area was largely covered with basaltic lavas. However, detailed field surveys reveal that the area is mainly covered with volcanic epiclastic and debris-avalanche deposits. The volcano-stratigraphy of the Seongeup area is divided into the Pyoseolli Basalt Group, Seongeup epiclastic sediments, basaltic dykes, aa rubble flows and volcanic debrisavalanche deposits with the associated cinder cones and lavas in ascending order. The Pyoseolli Basalt Group occurs only along the channel bottom of Cheonmicheon and on lower, broad hills in the southeastern part of the area. The Seongeup epioclastic sediments, which consists of conglomerates, conglomeratic sandstones, conglomeratic sandy mudstones and granule-bearing muddy sandstones, are widespread in the study area. Four volcanic debrisavalanche deposits (VDADs) are described: the Jwabomi and the Southwest Donggeomunoreum VDADs in the northeastern part, and the Mojioreum and the Ttarabi VDADs in the southwestern part. The VDADs are characterized by hummocky topography, jigsaw-fractured debris blocks and disaggregated clasts. The study area was a fluvial plain during the deposition of the Seongeup epiclastic sediments. After the depositon of the sediments, this area was uplifted by uprising magma, and formed the northeastern part of the long axial ridge of Jeju Island. The Bukchon-Pyoseon Line is probably a fault.