Spatio-temporal zonation by ichnocoenoses combined with sedimentary facies in the Yeochari tidal flat (Ganghwa Island), the Han River estuary of Korea

Abstract

Ichnocoenoses analyses supplemented with sedimentary facies analysis have been performed to identify seasonal patterns and ichnocoenoses distributions in the Yeochari tidal flats, the Ganghwa Island of Han-River estuary. The Yeochari intertidal flats can be divided into upper and middle tidal flats based on tide levels along a transect line. The latter flats can be further subdivided into higher and lower middle flats, and sand shoal, according to sedimentary facies, bioturbation index (BI), and distribution of ichnocoenoses. The upper tidal flats consist mainly of bioturbated muds with a dominant ichnocoenosis Psilonichnus, both in winter and summer, suggesting a stable environment in habitat conditions. The higher middle flats comprising alternating flaser and bioturbated sand-muds represent ichnocoenoses, Rosselia and Rhizocorallium, in winter and summer, respectively. In the lower middle flats, crossand parallel-laminated sand facies are dominant in winter, whereas, in summer, the alternating sand-mud facies with cross-laminated sands are predominant. The representative ichnocoenoses in the lower flats display Cylindrichnus, Scolicia and Echiurus burrows, which are commonly found in wave-dominated environments. However, Psilonichnus as well as mixed facies in summer indicate coexistence of tides with wave. No seasonal changes have been recognized in the outer sand shoals both in the ichno-characters and sedimentary facies. The dominant ichnocoenosis is cryptic bioturbation in the sand shoal, the ichnocoenosis being preserved in the wave-dominated sequences evidently in the lower middle flats of the Yeochari. Ichnocoenoses analysis in tidal-flat environments reveales that Rosselia, Rhizocorallium, Scolicia, Cylindrichnus and Echiurus are the key ichnocoenoses for identifying specific seasonal changes in sedimentation, and defining the middle intertidal flats and/or mixed tidal flats, commonly difficult to recognize by sedimentary facies alone. This study shows that ichnocoenosis analysis is a useful tool to characterize or define intertidal flats being subject to stark seasonal changes in sedimentation likely driven by the monsoon climate. More importantly, this may further provide a tool to differentiate between tideinfluenced and fully wave-dominated successions in ancient tidal-flat analogues.