ABSTRACT

In Boundary Bay, on the south side of the Fraser Delta, sandy tidal flats extend 2.5 mi seaward. Four main sedimentologic-hydrologic divisions exist -- the salt marsh, and the high, intermediate and low tidal flats. Each division is characterized by distinct drainage and sedimentary structures plus a diagnostic floral and faunal assemblage.

The salt marsh, bounded landward by an artificial dike, is incised by meandering tidal creeks and contains fresh-water ponds. The seaward portion of the marsh is covered with a dense growth of halophytes whereas normal terrestrial plants dominate on the landward portion. Poorly stratified, silty and sandy peat is accumulating throughout the area. In the western part of the Bay the marsh is prograding over the high flats, whereas to the east it has receded at least 0.75 mi since 4350 yrs B.P.

The high tidal flats, bounded landward by the salt marsh, have an irregular, incomplete drainage system. In the winter sand covers the high flats, but in summer blue-green algal mats spread over the surface, particularly on the upper landward side. Thus, a varve-like stratification is produced.

In contrast, the intermediate tidal flats have a well-developed dendritic drainage system; they are devoid of vegetation and are composed principally of fine to medium-grained sand which is continually being re-worked by shifting channels and fauna.

The low tidal flats are incised by deeper, more stable channels, in which high flow velocities produce dunes. Eel-grass meadows flank the sides of the channels and the shell beds are interpreted as lag deposits.

The average rate of sedimentation for the last 4350 years is 0.42 mm/yr. The index of re-working on the high tidal flats is 12.