ABSTRACT

A shelly sand ridge, 860 m long, 30-60 m wide, and up to 1.6 m high, parallels the shoreline on a high tidal mudflat in Gomso Bay, which opens to the eastern Yellow Sea. The ridge is composed entirely of fine to medium sand and shell material, with some subangular gravel, and overlies tidal mud discordantly. The lithostratigraphy of the bay deposits and ¹⁴C dates of shells from vibracores suggest that the shelly sand ridge, together with mid to lower sand flats, has accumulated on the tidal mud since 1800 yr B.P. This geologically recent development suggests that ridge evolution has been independent of regional sea-level change. River influence also seems to be negligible because of the apparently meager riverine sediment input into the bay.

Judging from a series of aerial photographs spanning the past two decades (1967-1989), broad sand shoals on the mid to lower flat have moved landward and have become the shelly sand ridge. Vertical sections from a trench cut across the ridge also attest to its landward migration, with gently landward-dipping interbeds of sand and shell. Repeated measurements of the morphological change of the ridge over a two-year period (1990-1992) indicate that net transport was landward, on the order of 8 m/yr, except for during the typhoon season. The migration rate in the stormy winter monsoon season was generally two or three times greater than that of the remaining season. However, in the summer of 1992, a typhoon entering the Yellow Sea vigorously shifted the ridge landward, causing a displace ent of up to 11 m in a few days. This suggests that strong waves associated with storms or typhoons have predominantly driven the shelly sand ridge and offshore sands onto the tidal flat.