Characteristics And Interpretation Of Subsurface Diatreme Deposits From Western Jeju Island

Boring for groundwater development was carried out at a site in Sindo-ri, Daejeong-eup, Jeju Island. The boring was initially planned to reach the U Formation supposed to be located at about 150 m below the surface. The U Formation was not, however, recovered although the bore hole reached a depth of 310 m. Instead, a core, named the Sindo core, showing unique lithology, was acquired, which is composed of lava flows (ca. 160 m thick) and volcaniclastic deposits (ca. 150 m thick). The volcaniclastic deposits are divided into three depositional units I, II, and III. Unit I (ca. 15 m thick) comprises well bedded and clast- or matrix-supported reworked volcaniclastic deposits, in which the clasts have different shapes, compositions, colors and roundness. This unit is interpreted to have been deposited by surface runoff and intermittent floods. Unit II (ca. 60 m thick) consists of bedded basaltic (lapilli) tuff ([L]Ts), cross-bedded (lapilli) tuff ([L]Tcs), and massive lapilli tuff (LTm). This unit has bomb-sags, accretionary lapilli, and jigsaw-fit structures of lithic clasts, and is interpreted to have formed by hydrovolcanic activity. Unit III (ca. 70 m thick) is composed of steeply inclined (30-60 degrees) ([L]Tis), horizontal or low-angle stratified ([L]Ts), massive ([L]Tm) and disturbed basaltic (lapilli) tuff (Tc). The facies [L]Tis constituting the majority of Unit III has stratification that dips in excess of the angle of repose and in opposite directions within short core depths repeatedly. This unit is thus interpreted as diatreme deposits produced by collapse of unstable materials from the vent walls and crater rims into the volcanic conduit probably caused by volcanic tremors or shockwaves. The thickness of Units II and III (totally ca. 130 m) is much larger than the average thickness (generally 20-40 m) of the volcaniclastic deposits (Seoguipo Formation) in the nearby area because Unit III was deposited in the deeply excavated, funnel-like structure inside a crater, i.e., a volcanic conduit. The Sindo core is therefore interpreted to have penetrated the inner part of a hydrovolcanic crater and may provide a glimpse into the subvolcanic structures of hydromagmatic volcanoes in the subsurface of Jeju Island and help understand the volcanic history of Jeju Island.