남해 대륙붕 현생 니질퇴적체의 퇴적특성

Abstract

Characterization of Holocene inner shelf mud deposit the South Sea of Korea

Bae, Sung-Ho

Department of Energy Resources Engineering, Graduate School Pukyong National University

Abstract

High-resolution seismic profiles and core sediment sampling have been conducted to investigate the acoustic characteristics and physical properties of Holocene mud deposit in the Yeosu Sound inner shelf of Korea. The major sediment source is the Seomjin River. The sediments are transported through the Yeosu Sound. Approximately 1000 line-km data of Chirp sonar and sparker were used. Along with seismic profiling, 40 piston core samples were collected. High-resolution seismic profiles show the Holocene mud deposits dominated in the study area, ranging from 10 to 40 m in thickness, and gas-related acoustic anomalies were also identified in the Yeosu Sound and the south of Oenaro Island. The acoustic anomalies were classified into two types (TypeⅠandⅡ). The types were divided by the location of acoustic anomaly. Type Ⅰ, dominating in the western Yeosu Sound, is characterized by parallel and/or subparallel pattern of acoustic blanking and turbidity to the seafloor. The acoustic turbidity of Type Ⅱ arises to the top of seabed. The Type Ⅰ predominates the sound and Type Ⅱ occurs regionally. The late Quaternary deposits in the study area can be divided into three sequences (Unit Ⅰ, Unit Ⅱ, Unit Ⅲ). The seismic units Ⅱ and Ⅲ are interpreted as the inner-shelf transgressive sand sheet and the deltaic-estuarine complex, respectively. The highstand systems tract (Unit Ⅰ) overlying the maximum flooding surface is recent mud deposits formed during the recent highstand of sea level. Core samples were analyzed for sediment texture (grain size, sand, silt and clay contents), physical properties (porosity, water content, bulk density, grain density and shear strength), and acoustic properties (compressional wave velocity and attenuation). Electrical resistivity was also measured. Twenty-three piston core samples were employed for the measurement. The mud sediments in the study area are vertically homogenous in texture. The mean grain size generally decreases from the mouth of Yeosu Sound to seaward. The sound velocity decreases from 1505 to 1485 m/s, then increases to 1520 m/s southeastward due to Pleistocene sand ridges. Kim et al. (1992) suggested the grading effect of the Seomjin River discharge transported through the Yeosu Peninsula and Namhae Island resulted in variation of the physical and acoustic properties. The exceptionally low velocity (1338 ~ 1372 m/s) between 120 cm and 490 cm subbottom depth in the YS24 core indicates the possibility of penetration of shallow gas zone. Probably the degassying cracks generated by escaping gas result in lowering the velocity. The cracks were identified well by X-radiograph images.