Depositional environment of the upper middle Miocene strata in the southwestern Ulleung Basin, Korea

This study is to interpret the sedimentary environments using some core data from two exploration wells adjacent to the Donghae Gas Field in the southwestern Ulleung Basin, Korea. Based on sedimentological and ichnological characteristics, the cores were classified into nine facies (Mm: homogeneous mudstone, Ml: faintly laminated mudstone, MS1, 2: interbedded sandstone and mudstone, Sm: homogenous sandstone, Sh: horizontally stratified sandstone, Sm-b: bioturbated massive sandstone, Sc: cross-bedded sandstone, G: matrix-to clastsupported conglomerate). The facies can be grouped into seven facies associations. The study section is interpreted mostly as a deltaic environment from shoreface to offshore, and it can be divided into prodelta, outer delta front, and inner delta front. FA1, composed of mostly mud deposits with thin sandy layers, is accumulated in a prodelta environment. FA2 is sand deposits interbedded with mud, and FA3 are deposits of fine mature sands. Both of them are to have been deposited in an outer delta front environment. In this environment, alternating sediments of mudstone and sandstone were formed by continuous reactivation of deltaic sediments at the steep slope. In an outer delta front, close to land, fine-grained sandstones are observed with frequent Skolithos ichnofacies. This ichnofacies indicates a high-energy environment, possibly in a lower to middle shoreface environment. FA4 is fine to medium sandy deposits containing rare trace fossils. FA5 and FA7 are medium to coarse sandy deposits with various cross-beddings. FA6 is composted of muds with sandy layers and medium to coarse sandy deposits. Inner delta front deposits are FA4, FA5, FA6, FA7. In this environment they consists of medium to pure coarse sandy sediments, which are interpreted as distributary channel deposits.