BIOGENIC SEDIMENTARY STRUCTURES ON A KOREAN MUD FLAT - SPRING-NEAP VARIATIONS

Biogenic sedimentary structures created by dominant benthic animals of a mud flat were investigated in a small embayment at Panweol on the w est coast of Korea. Burrows were replicated using the resin casting me thod on a sufficiently large scale to evaluate their variations statis tically. The size and shape of burrows and other traces varied with lo cations and tidal phases. Burrows of the upper-intertidal crabs Helice tridens sheni and Ilyoplax dentimerosa were T- or I-shaped, vertical and deep, whereas those of the mid-intertidal crab Macrophthalmus japo nicus were U-shaped, oblique and shallow. The upper-intertidal polycha ete Perinereis aibuhitensis, a subsurface feeder, had sinuous and high ly-branched burrows while the mid-intertidal one Periserrula leucophry na, a surface feeder, had vertical burrows without subsurface branches . Variations of crab burrows with tidal phases were distinct: The burr ows of both H. tridens sheni and M. japonicus were significantly deepe r and wider during neap tides than during spring tides. Sediment mound s were common in the upper intertidal zone, while crawling and feeding traces such as trails, trackways and cheliped scrapings were more pro minent in the mid-intertidal zone. These traces were also observed in the upper intertidal zone during spring tides when surface sediments b ecame watery. The size of the sediment mounds varied, being largest du ring neap tides. These biogenic sedimentary structures observed at Pan weol were distinctly different from those found near Inchon only 50 km away, probably due to differences in hydrology and sedimentology.