Sedimentary structure and origin of a mud-cored pseudo-tidal sand ridge, eastern Yellow Sea, Korea

Abstract

Using high-resolution seismic profiles, sidescan sonar images, and piston cores, we have examined an elongate ridge located off the Tae-An Peninsula of Korea in the eastern Yellow Sea. This ridge, 50 km long, 30 m high, and up to 12 km wide, is one of many similar longitudinal ones which are approximately parallel to the direction (NE-SW) of strong tidal currents. Bathymetric profiles across the ridge are all asymmetric: the eastern ridge flank is narrow and dips steeply towards the adjoining rocky valley. Below the eastern ridge flank, less steeply sloping subbottom reflectors are truncated by a seafloor-parallel erosion surface on which is draped a 45-cm-thick unit of muddy sand. A piston core taken here indicates that the subbottom reflectors are due to interlayered coarse sediments. The more gently sloping western ridge flank contains sand waves (ca. 100 m wavelength, ENE-WSW trend) on which megaripples (10-15 m wavelength, WNW-ESE trend) are superimposed, indicating recent bottom current activity. in particular the reflectors beneath the westernmost ridge show westward progradation over an acoustically transparent unit of thin sand sheets. However, subbottom profiles collected over the ridge crest do not show conspicuous reflectors. Only a few discontinuous, faint reflectors are observed. The lack of conspicuous reflectors below the ridge crest area may reflect more thorough bottom-current-generated sediment homogenization as well as poor penetration of acoustic energy; due to the current-scoured/rippled seafloor. We interpret the ridge as an early Holocene tidal mud tongue blanketed by recently reworked relict (palimpsest) sands. The 'pseudo-tidal sand ridge' has prograded westward. The fine-mud tongue sediments might have been derived mostly from the Korean Peninsula via the Han River at a time when sea level was lower than present. (C) 1998 Elsevier Science B.V. All rights reserved.