Origin And Characteristics Of Glassy Breccias From The Boreholes In The Woljeong-Haengwon Area Of Northeastern Jeju Island, Korea

This study focuses on the lithological characteristics and origin of fractured glassy breccias interlayered with thick lava-flow units from boreholes in the Woljeong-Haengwon coastal area, NNE Jeju Island, Borehole cores and optical televiewer (OTV) logs were analyzed in terms of mineralogy, lithology, and texture to characterize the stratigrphy and lithofacies. Major lithological units of the borehole cores are divided into basaltic lava flows, sedimentary deposits, paleosoil, and glassy breccias. The glassy breccia units consist of unsorted breccias with low pore contents. The color of breccias is mostly black owing to high proportion of volcanic glass, except for the outer light brown rim covered by palagonite. In OTV log images, glassy breccia units are composed not only of glassy breccias, but of matrix which is mixture of palagonite and volcanic glass. These lithologic features are interpreted as hyaloclastites which were formed by quench fragmentation along the contact between the lava flow and the seawater. Based on the size, shape, color, and developmental proportion of glassy fragments over the matrix, 3 types of lithofacies (i.e. semi-fractured basalt (SFB), clast-supported breccia (CSB) and matrix-supported breccia (MSB)) are classified in the boreholes, and are interpreted as the topset, foreset, and bottomset of the lava-fed delta, respectively. The paleo-coastline of the lava-fed delta has moved seaward from 1.0 similar to 1.5 km inland to the current location by progradation of the lava-fed delta.