Stratigraphic analysis of the drilling core in Woljong-ri coastal area, Jeju Island

Abstract

Jeju Island has been formed by repeated volcanic eruptions from Pleistocene (about 1.8 Ma) to Holocene, which resulted in extensive accumulations of lava flow units overlying hydrovolcanic and marine deposits (Seogwipo Formation). This study focuses on the subsurface stratigraphy and lithological characteristics of volcanic successions in the northeastern coastal area (Woljeong-ri) of Jeju Island. The main material is a 130 m long, borehole core, which was described in terms of lithofacies, texture, structure, and further analyzed by XRF, XRD, SEM, and well-logging. Major lithological units are acicular feldspar olivine basalts (Volcanic rocks I and II above 98.7 m in core depth (MD)) and fossiliferous sandy to silty sedimentary deposits (Sedimentary rock I between 65.7 and 67.7 m, and Sedimentary rock II below 98.7 m in core depth). In the volcanic successions, 34 lava flow units (0.2 to 20 m in thickness) are recognized by abrupt changes in lithofacies which classified into 5 types based on vesicle occurrence, mineral composition and rock color: highly porous basalt (HPB), moderately porous basalt (MPB), poorly porous basalt (PPB), glassy basalt (GLB), and fractured basalt (FRB). Individual lava flow unit generally has a lithofacies change of GLB-MPB-PPB-HPB-GLB in ascending order, suggesting the subaerial pahoehoe lavas. However, the result of XRF analysis on rock samples from 20 different lava flow units shows a scattered distribution over the ranges of basalt and basaltic andesite without clear differentiation among the flow units. In the lowermost part of Volcanic rock I (48.7-65.7 m in core depth), a hyaloclastite unit of FRB resulting from lava-seawater interaction occurs overlying the sedimentary unit with abundant marine shell fragments, which is strongly suggestive of a relative sea-level rise or marine transgression. They can be used as key beds for regional stratigraphic correlation in coastal area of Jeju Island.