The Heuksan mud belt on the tide-dominated shelf of Korea: a supply-driven depositional system?

摘要

A 20-50-km-wide, approx. 200-km-long and 50-m-thick mud deposit around Heuksan Island, southwest Korea, extends from the mouth of the Geum River to the shelf off Jeju Island. The deposit formed in association with postglacial sea-level rise. However, the substantial thickness and rapid seaward progradation of the deposit over a period of only about 3-4 thousand years, as postulated by previous investigators, are difficult to reconcile with the rapid sea-level rise during the early to mid-Holocene. To resolve this discrepancy, the origin and stratigraphic evolution of the Heuksan mud belt were reexamined using high-resolution seismic profiles and three drill cores penetrating down to the basement. A sedimentary facies analysis with AMS C-14 age control facilitated reconstruction of the postglacial evolution of the deposit. The seismic profiles reveal that the Heuksan mud deposit can be divided into three depositional events clearly defined by distinct spatiotemporal bounding surfaces. The lower unit contains retrograding tidal flat sequences formed during an earlier transgression commencing at ca.16.0 ka and terminating at 11.6 ka. The middle unit consists of rapidly prograding mud clinoforms deposited from 11-6.5 ka. This unit extends 80 km onto the distal shelf, and is characterized by sedimentation rates of 5.0 cm/year. From 6.5-6 ka, progradation was interrupted for a 500 year period during which the deposit retreated by 35 km due to a decrease in the sedimentation rate to 0.18 cm/year. The abrupt stratigraphic break was evidently triggered by a sudden change in sediment supply, resulting in rapid retreat of the unit despite continuing local sea-level rise. Renewed sedimentation commenced at about 6.0 ka, being initially characterized by clinoform progradation, followed by autoretreat from about 2.5 ka onward due to a decreasing sediment supply. The Heuksan mud belt may thus represent an example of a shelf depositional system driven predominantly by the rate of sediment supply, rather than by sea-level rise.