Open-coast tidal flats

Abstract

Recent research advances highlight the importance of open-coast tidal-flat depositional system in both modern and ancient coastal environments. The system is unique in its wave- and tide-dominated physical setting, notably distinct from the tide-dominated barred tidal flats and the wave-dominated shorefaces. Interactions of waves and tides over different time scales produce not only cyclic morphologic variations in terms of erosion and deposition, but also rhythmic depositional units consisting of storm-generated sand-dominated layers (SDLs) and post-storm mud-dominated layers (MDLs). Ancient deposits of the open-coast tidal flats can be distinguished by abundance of storm-generated structures and scarcity of tidal-channel deposits from those of barred tidal flats, and by abundance of the structures created by combined flows or the interactions of waves and tides from those of (tidal) shorefaces. Difference is also remarkable between muddy and sandy open-coast tidal flats. Muddy open-coast tidal flats tend to develop along mega-river deltas and the adjacent chenier plains, have a general accretional convex-up profile with clear zonation, and produce aggradational fining-upward intertidal successions. Sandy open-coast tidal flats are common in the open-mouth estuaries of small rivers and the adjacent strand plains, usually develop an erosional concave-up profile with common presence of inner swash bars having the coarsest sediment near the high water, and produce coarsening-upward retrogradational successions. The vertical successions of sandy open-coast tidal flats generally contain more storm-generated beds volumetrically than those of muddy open-coast tidal flats. Notably, there are some accretional sandy open-coast tidal flats, lying in between the above two settings. A new spectrum of coastal morphodynamic settings is therefore proposed to change from the tide-dominated barred tidal flats, the wave-influenced and tide-dominated muddy open-coast tidal flats, the wave- and tide-dominated accretional sandy open-coast tidal flats, the wave-dominated erosional sandy open-coast tidal flats, to the wave-dominated tidal shorefaces.