동해 울릉분지 남서해역 퇴적물의 음향특성 및 퇴적환경

Abstract

High-resolution seismic profiles and core samples were acquired in order to present the geoacoustic characteristics and to investigate the sedimentary environments of the southwestern part of Ulleung Basin in the East Sea. The inner shelf consists of fine-grained sediments that are interpreted as recent sediments deposited under present environmental conditions. The Ulleung Basin is also dominated by fine-grained sediments of hemi-pelagic mud. In the mid-shelf, the materials are mixed with fine-grained sediments and relict coarse sediments. The outer shelf is composed of very coarse-grained sediments that are considered to be relict sediments deposited during the last glacial periods when the sea level was lower than the present. In situ velocities are calculated from laboratory velocity data for classification of precise geoacoustic provinces. These are corrected the effect of temperature using Kim et al. (2004)'s average value of fluid mud. The study area is divided into five different geoacoustic provinces based on in situ geoacoustic properties: (1) Province IA is hemi-pelagic mud partially mixed with intermittent sandy sediments which are originated from the outer shelf by slide/slump or mass flow (1427 m/s, 8.4Φ, 1.24 g/cm3, and 84%), (2) Province IB is composed of muddy sediments that are deposited during Holocene (1442 m/s, 7.9Φ, 1.35 g/cm3, and 78%), (3) Province II is consisted of muddy sand sediments that are the boundary between recent and relict sediments (1477 m/s, 6.1Φ, 1.53 g/cm3, and 68%), (4) Province III is dominated by coarse-grained relict sediments deposited during Pleistocene (1560 m/s, 3.8Φ, 1.77 g/cm3, and 54%), (5) Province IV is comprised of more coarser sediments than Province III (1668 m/s, 3.0Φ, 1.91 g/cm3, and 46%). Compressional wave velocity, mean grain size, and bulk density increase from Province IA to Province IV, whereas porosity decreases from Province IA to Province IV. Variability of physical and acoustic properties tend to follow the general trend of mean grain size. High-resolution seismic profiles reveal three seismic units (Unit A1, Unit A2, and Unit B) of eastern boundary of the Korea Strait shelf mud (KSSM). Unit A1 occurs with KSSM that is underlain by pre-Holocene deposit. Unit A2 between transgressive surface and maximum flooding surface consists of sandy sediments deposited during the post-glacial transgression (15,000~6,000 yr B.P.). Unit A1 is composed of the fine-grained sediments deposited during the last 6,000 yrs when sea level was close to the present level and its distribution is restricted to the inner shelf along the shore line. Unit B is considered to be mass flow deposits in the slope of Ulleung Basin. Through the comparison of geoacoustic provinces and seismic data, Unit A1 and Province IB accord with distribution of recent Holocene sediments. Also Unit A1 and B correspond with Province II of the boundary between recent and relict sediments. And pre-Holocene deposits coincide with Province III that are composed of Pleistocene relict sediments. Consequentially, geoacoustic provinces represent not only textural and physical properties but also sedimentary environments.