Holocene evolution of the Nakdong River delta, Busan, Korea

Abstract

This study presents the Holocene evolution of the Nakdong River delta in the southeastern coast of the Korean Peninsula. Two borehole sediments (KND-1 and KND-3) were taken from mid- and down-stream areas of the Nakdong River delta respectively. Sedimentary facies of the borehole sediments were classified based on grain-size compositions, sedimentary structures, geochemical compositions such as total organic and inorganic carbons and nitrogen contents. AMS 14C datings of 6 samples were accomplished for age controls. Based on the sedimentary facies distribution, the borehole sediments are interpreted to represent six depositional environments: fluvial, estuarine, shallow marine, prodetla, delta front, and delta plain. The fluvial environment was characterized by coarse sandy sediments containing a large amount of gravels and relatively low contents of total carbon and nitrogen. Sediments of the estuarine environment were dominated by sand including a lot of shell fragments and charcoals and showing sand-mud couplets. The shallow marine environment was characterized by a finning upward trend from medium silt to clay with aboundant shell-rich layer. The total carbon and nitrogen contents increased upward. The prodelta environment was characterized by a coarsening upward trend from clay to very fine silt. Charcoal fragments is generally present and no shell fragments were found within this environment. The total carbon and nitrogen contents decreased upward. Sediments of the delta front environment mainly consisted of silt with sand-mud couplets containing a few charcoal fragments. The delta plain environment, which occurs in the upper part of the borehole, was characterized by fine sand sediments containing charcoal, and aboundant mica fragments. The total carbon and nitrogen contents of the delta plain environment were relatively low. The sequencial changes of the sediment environments indicate that the Holocene evolution of Nakdong River delta can be divided into three stages by sea-level changes: 1) lowstand fluvial-dominated stage when sea-level was 120-130 m below its present level at the LGM, 2) transgressive fluvial-to-marine stage during 15 ka-7 ka BP when the sea-level rose rapidly, 3) highstand delta progradation stage since 7 ka.