Intertidal areas contain a number of subenvironments, many of which generate distinctive types of internal structures. Experimental studies conducted over a 5-year period in intertidal areas in Willapa Bay, Washington, indicate the origin of different types of structures. The experiments consisted of establishing a datum by scattering fine particles of lead over the surface in six subenvironments and repetitively coring the experimental plots at daily, seasonal, or yearly intervals. The subenvironments studied include the upper and middle accretionary bank of a tidal runoff channel, the uppermost bank of a tidal river, muddy tidal flats covered by (1) Zostera and (2) low mounds of blue-green algae, and sandy tidal flat.

The internal structure in each subenvironment depends on the dominant processes and on the rate of sedimentation. The middle accretionary bank of the runoff channel accreted at a rate of more than 5 cm per month during the summer of 1976; the stratification reflects the semidiurnal ebb and flood of the tides. On the upper accretionary banks of the runoff channel and on the tidal river, the sediment responds more to seasonal variations, accreting during the summer and eroding during the winter. On the uppermost bank of the tidal river, these processes were recorded over a 5-year period in 8 cm of alternating mud (summer) and fine sand (winter) laminae.

Very little net accumulation of sediment occurred on the tidal flats. The Zostera-covered muddy flat and the sandy flat are dominated by bioturbational processes and no lamination is preserved. On the algal mounds, the binding of the sediment by algal filaments and the inhibition of faunal activity by oxygen depletion combine to produce well-defined thin laminations. Repeated sedimentation and algal growth produce stratification similar to the upper accretionary bank of the tidal river.

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